GCC(1)

📦 version 15.2.0, in arch rolling

💖 NAME 💖

okay so bestie, welcome to GCC(1) — the GNU manual page for the GNU project C and C++ compiler. Yes, the compiler is the main character. 👑

✨ SYNOPSIS ✨

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-Wpedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

Real talk: only the most useful options are listed up there; see below for the rest. g++ accepts mostly the same options as gcc. 💅

🦋 DESCRIPTION 🦋

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The overall options let you stop this whole process at an intermediate stage. For example, the -c option says not to run the linker — and then the output consists of object files output by the assembler. ✨

Other options get passed on to one or more stages of processing. Some options control the preprocessor and others control the compiler itself. Yet other options control the assembler and linker; most of those are not documented here, since you rarely need to use any of them. no cap.

Most of the command-line options you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages. 🫶

The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead. 👑

The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v. literally not the same vibe. 💀

You can mix options and other arguments. For the most part, the order you use doesn’t matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. 🌸

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms, whichever one is not the default. ✨

Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as "kB" and "KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes. 🧚

👑 OPTIONS 👑

🎀 Option Summary 🎀

Here’s the whole summary, grouped by type. The explanations live in the following sections. 💖

🌈 Overall Options 🌈

-c  -S  -E  -o file -dumpbase dumpbase  -dumpbase-ext auxdropsuf
-dumpdir dumppfx  -x language -v  -###  --help[=class[,...]]
--target-help  --version -pass-exit-codes  -pipe  -specs=file
-wrapper @file  -ffile-prefix-map=old=new  -fcanon-prefix-map
-fplugin=file  -fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit  -fdump-go-spec=file

🌸 C Language Options 🌸

-ansi  -std=standard  -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function  -fcond-mismatch -ffreestanding  -fgimple
-fgnu-tm  -fgnu89-inline  -fhosted -flax-vector-conversions
-fms-extensions -fpermitted-flt-eval-methods=standard
-fplan9-extensions  -fsigned-bitfields  -funsigned-bitfields
-fsigned-char  -funsigned-char  -fstrict-flex-arrays[=n]
-fsso-struct=endianness

🦋 C++ Language Options 🦋

-fabi-version=n  -fno-access-control -faligned-new=n
-fno-assume-sane-operators-new-delete -fchar8_t  -fcheck-new
-fconcepts  -fconstexpr-depth=n  -fconstexpr-cache-depth=n
-fconstexpr-loop-limit=n  -fconstexpr-ops-limit=n
-fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords
-fno-immediate-escalation -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-version-ignore
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins  -fnothrow-opt  -fno-operator-names
-fno-optional-diags -fno-pretty-templates  -frange-for-ext-temps
-fno-rtti  -fsized-deallocation -fstrong-eval-order[=kind]
-ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics  -fuse-cxa-atexit -fno-weak  -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabi-tag  -Wcatch-value  -Wcatch-value=n
-Wno-class-conversion  -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy  -Wdangling-reference
-Wno-defaulted-function-deleted -Wno-delete-incomplete
-Wdelete-non-virtual-dtor  -Wno-deprecated-array-compare
-Wdeprecated-copy -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions -Wextra-semi
-Wno-global-module -Wno-inaccessible-base
-Wno-inherited-variadic-ctor  -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof
-Wno-literal-suffix -Wmismatched-new-delete -Wmismatched-tags
-Wmultiple-inheritance  -Wnamespaces  -Wnarrowing -Wnoexcept
-Wnoexcept-type  -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new  -Wplacement-new=n -Wrange-loop-construct
-Wredundant-move -Wredundant-tags -Wreorder  -Wregister
-Wstrict-null-sentinel  -Wno-subobject-linkage  -Wtemplates
-Wno-non-template-friend  -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types  -Wsuggest-override
-Wno-template-body -Wno-template-id-cdtor  -Wtemplate-names-tu-local
-Wno-terminate  -Wno-vexing-parse  -Wvirtual-inheritance
-Wno-virtual-move-assign  -Wvolatile

💜 Objective-C and Objective-C++ Language Options 💜

-fconstant-string-class=class-name -fgnu-runtime  -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-property-assign-default -Wno-protocol -Wobjc-root-class
-Wselector -Wstrict-selector-match -Wundeclared-selector

🔥 OpenMP and OpenACC Options 🔥

-foffload=arg  -foffload-options=arg -fopenacc  -fopenacc-dim=geom
-fopenmp  -fopenmp-simd  -fopenmp-target-simd-clone[=device-type]

🎀 Diagnostic Message Formatting Options 🎀

-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file|json|json-stderr|json-file]
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
-fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-event-links
-fno-diagnostics-show-labels  -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe -fno-diagnostics-show-rules
-fno-diagnostics-show-highlight-colors
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits  -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree  -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column
-fdiagnostics-column-unit=[display|byte]
-fdiagnostics-column-origin=origin
-fdiagnostics-escape-format=[unicode|bytes]
-fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]

⚠️ Warning Options ⚠️

okay bestie, the warning flags are lowkey the main character of this whole page — there are so many and they all matter. here’s the full lineup, no skipping, no summarizing, periodt. 💅

  • -fsyntax-only
  • -fmax-errors=n
  • -Wpedantic
  • -pedantic-errors
  • -fpermissive
  • -w
  • -Wextra
  • -Wall
  • -Wabi=n
  • -Waddress
  • -Wno-address-of-packed-member
  • -Waggregate-return
  • -Walloc-size
  • -Walloc-size-larger-than=byte-size
  • -Walloc-zero
  • -Walloca
  • -Walloca-larger-than=byte-size
  • -Wno-aggressive-loop-optimizations
  • -Warith-conversion
  • -Warray-bounds
  • -Warray-bounds=n
  • -Warray-compare
  • -Warray-parameter
  • -Warray-parameter=n
  • -Wno-attributes
  • -Wattribute-alias=n
  • -Wno-attribute-alias
  • -Wno-attribute-warning
  • -Wbidi-chars=[none|unpaired|any|ucn]
  • -Wbool-compare
  • -Wbool-operation
  • -Wno-builtin-declaration-mismatch
  • -Wno-builtin-macro-redefined
  • -Wc90-c99-compat
  • -Wc99-c11-compat
  • -Wc11-c23-compat
  • -Wc23-c2y-compat
  • -Wc++-compat
  • -Wc++11-compat
  • -Wc++14-compat
  • -Wc++17-compat
  • -Wc++20-compat
  • -Wno-c++11-extensions
  • -Wno-c++14-extensions
  • -Wno-c++17-extensions
  • -Wno-c++20-extensions
  • -Wno-c++23-extensions
  • -Wcalloc-transposed-args
  • -Wcast-align
  • -Wcast-align=strict
  • -Wcast-function-type
  • -Wcast-qual
  • -Wchar-subscripts
  • -Wclobbered
  • -Wcomment
  • -Wcompare-distinct-pointer-types
  • -Wno-complain-wrong-lang
  • -Wconversion
  • -Wno-coverage-mismatch
  • -Wno-cpp
  • -Wdangling-else
  • -Wdangling-pointer
  • -Wdangling-pointer=n
  • -Wdate-time
  • -Wno-deprecated
  • -Wno-deprecated-declarations
  • -Wno-designated-init
  • -Wdisabled-optimization
  • -Wno-discarded-array-qualifiers
  • -Wno-discarded-qualifiers
  • -Wno-div-by-zero
  • -Wdouble-promotion
  • -Wduplicated-branches
  • -Wduplicated-cond
  • -Wempty-body
  • -Wno-endif-labels
  • -Wenum-compare
  • -Wenum-conversion
  • -Wenum-int-mismatch
  • -Werror
  • -Werror=*
  • -Wexpansion-to-defined
  • -Wfatal-errors
  • -Wflex-array-member-not-at-end
  • -Wfloat-conversion
  • -Wfloat-equal
  • -Wformat
  • -Wformat=2
  • -Wno-format-contains-nul
  • -Wno-format-extra-args
  • -Wformat-nonliteral
  • -Wformat-overflow=n
  • -Wformat-security
  • -Wformat-signedness
  • -Wformat-truncation=n
  • -Wformat-y2k
  • -Wframe-address
  • -Wframe-larger-than=byte-size
  • -Wno-free-nonheap-object
  • -Wheader-guard
  • -Wno-if-not-aligned
  • -Wno-ignored-attributes
  • -Wignored-qualifiers
  • -Wno-incompatible-pointer-types
  • -Whardened
  • -Wimplicit
  • -Wimplicit-fallthrough
  • -Wimplicit-fallthrough=n
  • -Wno-implicit-function-declaration
  • -Wno-implicit-int
  • -Winfinite-recursion
  • -Winit-self
  • -Winline
  • -Wno-int-conversion
  • -Wint-in-bool-context
  • -Wno-int-to-pointer-cast
  • -Wno-invalid-memory-model
  • -Winvalid-pch
  • -Winvalid-utf8
  • -Wno-unicode
  • -Wjump-misses-init
  • -Wlarger-than=byte-size
  • -Wleading-whitespace=kind
  • -Wlogical-not-parentheses
  • -Wlogical-op
  • -Wlong-long
  • -Wno-lto-type-mismatch
  • -Wmain
  • -Wmaybe-uninitialized
  • -Wmemset-elt-size
  • -Wmemset-transposed-args
  • -Wmisleading-indentation
  • -Wmissing-attributes
  • -Wmissing-braces
  • -Wmissing-field-initializers
  • -Wmissing-format-attribute
  • -Wmissing-include-dirs
  • -Wmissing-noreturn
  • -Wmusttail-local-addr
  • -Wmaybe-musttail-local-addr
  • -Wno-missing-profile
  • -Wno-multichar
  • -Wmultistatement-macros
  • -Wnonnull
  • -Wnonnull-compare
  • -Wnormalized=[none|id|nfc|nfkc]
  • -Wnull-dereference
  • -Wno-odr
  • -Wopenacc-parallelism
  • -Wopenmp
  • -Wopenmp-simd
  • -Wno-overflow
  • -Woverlength-strings
  • -Wno-override-init-side-effects
  • -Wpacked
  • -Wno-packed-bitfield-compat
  • -Wpacked-not-aligned
  • -Wpadded
  • -Wparentheses
  • -Wno-pedantic-ms-format
  • -Wpointer-arith
  • -Wno-pointer-compare
  • -Wno-pointer-to-int-cast
  • -Wno-pragmas
  • -Wno-pragma-once-outside-header
  • -Wno-prio-ctor-dtor
  • -Wno-psabi
  • -Wredundant-decls
  • -Wrestrict
  • -Wno-return-local-addr
  • -Wreturn-type
  • -Wno-scalar-storage-order
  • -Wsequence-point
  • -Wshadow
  • -Wshadow=global
  • -Wshadow=local
  • -Wshadow=compatible-local
  • -Wno-shadow-ivar
  • -Wno-shift-count-negative
  • -Wno-shift-count-overflow
  • -Wshift-negative-value
  • -Wno-shift-overflow
  • -Wshift-overflow=n
  • -Wsign-compare
  • -Wsign-conversion
  • -Wno-sizeof-array-argument
  • -Wsizeof-array-div
  • -Wsizeof-pointer-div
  • -Wsizeof-pointer-memaccess
  • -Wstack-protector
  • -Wstack-usage=byte-size
  • -Wstrict-aliasing
  • -Wstrict-aliasing=n
  • -Wstrict-overflow
  • -Wstrict-overflow=n
  • -Wstring-compare
  • -Wno-stringop-overflow
  • -Wno-stringop-overread
  • -Wno-stringop-truncation
  • -Wstrict-flex-arrays
  • -Wsuggest-attribute=attribute-name
  • -Wswitch
  • -Wno-switch-bool
  • -Wswitch-default
  • -Wswitch-enum
  • -Wno-switch-outside-range
  • -Wno-switch-unreachable
  • -Wsync-nand
  • -Wsystem-headers
  • -Wtautological-compare
  • -Wtrailing-whitespace
  • -Wtrailing-whitespace=kind
  • -Wtrampolines
  • -Wtrigraphs
  • -Wtrivial-auto-var-init
  • -Wno-tsan
  • -Wtype-limits
  • -Wundef
  • -Wuninitialized
  • -Wunknown-pragmas
  • -Wunsuffixed-float-constants
  • -Wunterminated-string-initialization
  • -Wunused
  • -Wunused-but-set-parameter
  • -Wunused-but-set-variable
  • -Wunused-const-variable
  • -Wunused-const-variable=n
  • -Wunused-function
  • -Wunused-label
  • -Wunused-local-typedefs
  • -Wunused-macros
  • -Wunused-parameter
  • -Wno-unused-result
  • -Wunused-value
  • -Wunused-variable
  • -Wuse-after-free
  • -Wuse-after-free=n
  • -Wuseless-cast
  • -Wno-varargs
  • -Wvariadic-macros
  • -Wvector-operation-performance
  • -Wvla
  • -Wvla-larger-than=byte-size
  • -Wno-vla-larger-than
  • -Wvolatile-register-var
  • -Wwrite-strings
  • -Wno-xor-used-as-pow
  • -Wzero-as-null-pointer-constant
  • -Wzero-length-bounds

🧚 Static Analyzer Options 🧚

next up: the static analyzer crew. these are the flags, exactly as listed — I’m not out here inventing extra lore, bestie. 🫶

  • -fanalyzer
  • -fanalyzer-call-summaries
  • -fanalyzer-checker=name
  • -fno-analyzer-feasibility
  • -fanalyzer-fine-grained
  • -fanalyzer-show-events-in-system-headers
  • -fno-analyzer-state-merge
  • -fno-analyzer-state-purge
  • -fno-analyzer-suppress-followups
  • -fanalyzer-transitivity
  • -fno-analyzer-undo-inlining
  • -fanalyzer-verbose-edges
  • -fanalyzer-verbose-state-changes
  • -fanalyzer-verbosity=level
  • -fdump-analyzer
  • -fdump-analyzer-callgraph
  • -fdump-analyzer-exploded-graph
  • -fdump-analyzer-exploded-nodes
  • -fdump-analyzer-exploded-nodes-2
  • -fdump-analyzer-exploded-nodes-3
  • -fdump-analyzer-exploded-paths
  • -fdump-analyzer-feasibility
  • -fdump-analyzer-infinite-loop
  • -fdump-analyzer-json
  • -fdump-analyzer-state-purge
  • -fdump-analyzer-stderr
  • -fdump-analyzer-supergraph
  • -fdump-analyzer-untracked
  • -Wno-analyzer-double-fclose
  • -Wno-analyzer-double-free
  • -Wno-analyzer-exposure-through-output-file
  • -Wno-analyzer-exposure-through-uninit-copy
  • -Wno-analyzer-fd-access-mode-mismatch
  • -Wno-analyzer-fd-double-close
  • -Wno-analyzer-fd-leak
  • -Wno-analyzer-fd-phase-mismatch
  • -Wno-analyzer-fd-type-mismatch
  • -Wno-analyzer-fd-use-after-close
  • -Wno-analyzer-fd-use-without-check
  • -Wno-analyzer-file-leak
  • -Wno-analyzer-free-of-non-heap
  • -Wno-analyzer-imprecise-fp-arithmetic
  • -Wno-analyzer-infinite-loop
  • -Wno-analyzer-infinite-recursion
  • -Wno-analyzer-jump-through-null
  • -Wno-analyzer-malloc-leak
  • -Wno-analyzer-mismatching-deallocation
  • -Wno-analyzer-null-argument
  • -Wno-analyzer-null-dereference
  • -Wno-analyzer-out-of-bounds
  • -Wno-analyzer-overlapping-buffers
  • -Wno-analyzer-possible-null-argument
  • -Wno-analyzer-possible-null-dereference
  • -Wno-analyzer-putenv-of-auto-var
  • -Wno-analyzer-shift-count-negative
  • -Wno-analyzer-shift-count-overflow
  • -Wno-analyzer-stale-setjmp-buffer
  • -Wno-analyzer-tainted-allocation-size
  • -Wno-analyzer-tainted-assertion
  • -Wno-analyzer-tainted-array-index
  • -Wno-analyzer-tainted-divisor
  • -Wno-analyzer-tainted-offset
  • -Wno-analyzer-tainted-size
  • -Wanalyzer-symbol-too-complex
  • -Wanalyzer-too-complex
  • -Wno-analyzer-undefined-behavior-ptrdiff
  • -Wno-analyzer-undefined-behavior-strtok
  • -Wno-analyzer-unsafe-call-within-signal-handler
  • -Wno-analyzer-use-after-free
  • -Wno-analyzer-use-of-pointer-in-stale-stack-frame
  • -Wno-analyzer-use-of-uninitialized-value
  • -Wno-analyzer-va-arg-type-mismatch
  • -Wno-analyzer-va-list-exhausted
  • -Wno-analyzer-va-list-leak
  • -Wno-analyzer-va-list-use-after-va-end
  • -Wno-analyzer-write-to-const
  • -Wno-analyzer-write-to-string-literal

🍓 C and Objective-C-only Warning Options 🍓

these ones are specifically for C and Objective-C — the niche little squad. keep the exact names, no remixing. 💖

  • -Wbad-function-cast
  • -Wdeprecated-non-prototype
  • -Wfree-labels
  • -Wmissing-declarations
  • -Wmissing-parameter-name
  • -Wmissing-parameter-type
  • -Wdeclaration-missing-parameter-type
  • -Wmissing-prototypes
  • -Wmissing-variable-declarations
  • -Wnested-externs
  • -Wold-style-declaration
  • -Wold-style-definition
  • -Wstrict-prototypes
  • -Wtraditional
  • -Wtraditional-conversion
  • -Wdeclaration-after-statement
  • -Wpointer-sign

🎀 Debugging Options 🎀

and finally for this part: the debugging flags. yes, there are a lot of them. no, we are not skipping any. the -g girlies ate and left no crumbs. ✨

  • -g
  • -glevel
  • -gdwarf
  • -gdwarf-version
  • -gbtf
  • -gctf
  • -gctflevel
  • -gprune-btf
  • -gno-prune-btf
  • -ggdb
  • -grecord-gcc-switches
  • -gno-record-gcc-switches
  • -gstrict-dwarf
  • -gno-strict-dwarf
  • -gas-loc-support
  • -gno-as-loc-support
  • -gas-locview-support
  • -gno-as-locview-support
  • -gcodeview
  • -gcolumn-info
  • -gno-column-info
  • -gdwarf32
  • -gdwarf64
  • -gstatement-frontiers
  • -gno-statement-frontiers
  • -gvariable-location-views
  • -gno-variable-location-views
  • -ginternal-reset-location-views
  • -gno-internal-reset-location-views
  • -ginline-points
  • -gno-inline-points
  • -gvms
  • -gz[=type]
  • -gsplit-dwarf
  • -gdescribe-dies
  • -gno-describe-dies
  • -fdebug-prefix-map=old=new
  • -fdebug-types-section
  • -fno-eliminate-unused-debug-types
  • -femit-struct-debug-baseonly
  • -femit-struct-debug-reduced
  • -femit-struct-debug-detailed[=spec-list]
  • -fno-eliminate-unused-debug-symbols
  • -femit-class-debug-always
  • -fno-merge-debug-strings
  • -fno-dwarf2-cfi-asm
  • -fvar-tracking
  • -fvar-tracking-assignments

✨ Optimization Options ✨

Ok bestie, this is the optimization buffet and it is chef's kiss enormous. Every flag is below exactly as it appears — no renaming, no skipping, no cap. 💅

  • -faggressive-loop-optimizations
  • -falign-functions[=n[:m:[n2[:m2]]]]
  • -falign-jumps[=n[:m:[n2[:m2]]]]
  • -falign-labels[=n[:m:[n2[:m2]]]]
  • -falign-loops[=n[:m:[n2[:m2]]]]
  • -fmin-function-alignment=[n]
  • -fno-allocation-dce
  • -fallow-store-data-races
  • -fassociative-math
  • -fauto-profile
  • -fauto-profile[=path]
  • -fauto-inc-dec
  • -fbranch-probabilities
  • -fcaller-saves
  • -fcombine-stack-adjustments
  • -fconserve-stack
  • -ffold-mem-offsets
  • -fcompare-elim
  • -fcprop-registers
  • -fcrossjumping
  • -fcse-follow-jumps
  • -fcse-skip-blocks
  • -fcx-fortran-rules
  • -fcx-limited-range
  • -fdata-sections
  • -fdce
  • -fdelayed-branch
  • -fdelete-null-pointer-checks
  • -fdevirtualize
  • -fdevirtualize-speculatively
  • -fdevirtualize-at-ltrans
  • -fdse
  • -fearly-inlining
  • -fipa-sra
  • -fexpensive-optimizations
  • -ffat-lto-objects
  • -ffast-math
  • -ffinite-math-only
  • -ffloat-store
  • -fexcess-precision=style
  • -ffinite-loops
  • -fforward-propagate
  • -ffp-contract=style
  • -ffunction-sections
  • -fgcse
  • -fgcse-after-reload
  • -fgcse-las
  • -fgcse-lm
  • -fgraphite-identity
  • -fgcse-sm
  • -fhoist-adjacent-loads
  • -fif-conversion
  • -fif-conversion2
  • -findirect-inlining
  • -finline-stringops[=fn]
  • -finline-functions
  • -finline-functions-called-once
  • -finline-limit=n
  • -finline-small-functions
  • -fipa-modref
  • -fipa-cp
  • -fipa-cp-clone
  • -fipa-bit-cp
  • -fipa-vrp
  • -fipa-pta
  • -fipa-profile
  • -fipa-pure-const
  • -fipa-reference
  • -fipa-reference-addressable
  • -fipa-reorder-for-locality
  • -fipa-stack-alignment
  • -fipa-icf
  • -fira-algorithm=algorithm
  • -flate-combine-instructions
  • -flifetime-dse
  • -flive-patching=level
  • -fira-region=region
  • -fira-hoist-pressure
  • -fira-loop-pressure
  • -fno-ira-share-save-slots
  • -fno-ira-share-spill-slots
  • -fisolate-erroneous-paths-dereference
  • -fisolate-erroneous-paths-attribute
  • -fivopts
  • -fkeep-inline-functions
  • -fkeep-static-functions
  • -fkeep-static-consts
  • -flimit-function-alignment
  • -flive-range-shrinkage
  • -floop-block
  • -floop-interchange
  • -floop-strip-mine
  • -floop-unroll-and-jam
  • -floop-nest-optimize
  • -floop-parallelize-all
  • -flra-remat
  • -flto
  • -flto-compression-level
  • -flto-partition=alg
  • -flto-incremental=path
  • -flto-incremental-cache-size=n
  • -fmalloc-dce
  • -fmerge-all-constants
  • -fmerge-constants
  • -fmodulo-sched
  • -fmodulo-sched-allow-regmoves
  • -fmove-loop-invariants
  • -fmove-loop-stores
  • -fno-branch-count-reg
  • -fno-defer-pop
  • -fno-fp-int-builtin-inexact
  • -fno-function-cse
  • -fno-guess-branch-probability
  • -fno-inline
  • -fno-math-errno
  • -fno-peephole
  • -fno-peephole2
  • -fno-printf-return-value
  • -fno-sched-interblock
  • -fno-sched-spec
  • -fno-signed-zeros
  • -fno-toplevel-reorder
  • -fno-trapping-math
  • -fno-zero-initialized-in-bss
  • -fomit-frame-pointer
  • -foptimize-crc
  • -foptimize-sibling-calls
  • -fpartial-inlining
  • -fpeel-loops
  • -fpredictive-commoning
  • -fprefetch-loop-arrays
  • -fprofile-correction
  • -fprofile-use
  • -fprofile-use=path
  • -fprofile-partial-training
  • -fprofile-values
  • -fprofile-reorder-functions
  • -freciprocal-math
  • -free
  • -frename-registers
  • -freorder-blocks
  • -freorder-blocks-algorithm=algorithm
  • -freorder-blocks-and-partition
  • -freorder-functions
  • -frerun-cse-after-loop
  • -freschedule-modulo-scheduled-loops
  • -frounding-math
  • -fsave-optimization-record
  • -fsched2-use-superblocks
  • -fsched-pressure
  • -fsched-spec-load
  • -fsched-spec-load-dangerous
  • -fsched-stalled-insns-dep[=n]
  • -fsched-stalled-insns[=n]
  • -fsched-group-heuristic
  • -fsched-critical-path-heuristic
  • -fsched-spec-insn-heuristic
  • -fsched-rank-heuristic
  • -fsched-last-insn-heuristic
  • -fsched-dep-count-heuristic
  • -fschedule-fusion
  • -fschedule-insns
  • -fschedule-insns2
  • -fsection-anchors
  • -fselective-scheduling
  • -fselective-scheduling2
  • -fsel-sched-pipelining
  • -fsel-sched-pipelining-outer-loops
  • -fsemantic-interposition
  • -fshrink-wrap
  • -fshrink-wrap-separate
  • -fsignaling-nans
  • -fsingle-precision-constant
  • -fsplit-ivs-in-unroller
  • -fsplit-loops
  • -fsplit-paths
  • -fsplit-wide-types
  • -fsplit-wide-types-early
  • -fssa-backprop
  • -fssa-phiopt
  • -fstdarg-opt
  • -fstore-merging
  • -fstrict-aliasing
  • -fipa-strict-aliasing
  • -fthread-jumps
  • -ftracer
  • -ftree-bit-ccp
  • -ftree-builtin-call-dce
  • -ftree-ccp
  • -ftree-ch
  • -ftree-coalesce-vars
  • -ftree-copy-prop
  • -ftree-cselim
  • -ftree-dce
  • -ftree-dominator-opts
  • -ftree-dse
  • -ftree-forwprop
  • -ftree-fre
  • -fcode-hoisting
  • -ftree-loop-if-convert
  • -ftree-loop-im
  • -ftree-phiprop
  • -ftree-loop-distribution
  • -ftree-loop-distribute-patterns
  • -ftree-loop-ivcanon
  • -ftree-loop-linear
  • -ftree-loop-optimize
  • -ftree-loop-vectorize
  • -ftree-parallelize-loops=n
  • -ftree-pre
  • -ftree-partial-pre
  • -ftree-pta
  • -ftree-reassoc
  • -ftree-scev-cprop
  • -ftree-sink
  • -ftree-slsr
  • -ftree-sra
  • -ftree-switch-conversion
  • -ftree-tail-merge
  • -ftree-ter
  • -ftree-vectorize
  • -ftree-vrp
  • -ftrivial-auto-var-init
  • -funconstrained-commons
  • -funit-at-a-time
  • -funroll-all-loops
  • -funroll-loops
  • -funsafe-math-optimizations
  • -funswitch-loops
  • -fipa-ra
  • -fvariable-expansion-in-unroller
  • -fvect-cost-model
  • -fvpt
  • -fweb
  • -fwhole-program
  • -fwpa
  • -fuse-linker-plugin
  • -fzero-call-used-regs
  • --param name=value
  • -O
  • -O0
  • -O1
  • -O2
  • -O3
  • -Os
  • -Ofast
  • -Og
  • -Oz

🔥 Program Instrumentation Options 🔥

Now we're in the "watch what the program is doing" era — sanitizers, profiling, stack protection, the whole bestie surveillance kit. Here's the exact list. 🫶

  • -p
  • -pg
  • -fprofile-arcs
  • --coverage
  • -ftest-coverage
  • -fcondition-coverage
  • -fpath-coverage
  • -fprofile-abs-path
  • -fprofile-dir=path
  • -fprofile-generate
  • -fprofile-generate=path
  • -fprofile-info-section
  • -fprofile-info-section=name
  • -fprofile-note=path
  • -fprofile-prefix-path=path
  • -fprofile-update=method
  • -fprofile-filter-files=regex
  • -fprofile-exclude-files=regex
  • -fprofile-reproducible=[multithreaded|parallel-runs|serial]
  • -fsanitize=style
  • -fsanitize-recover
  • -fsanitize-recover=style
  • -fsanitize-trap
  • -fsanitize-trap=style
  • -fasan-shadow-offset=number
  • -fsanitize-sections=s1,s2,...
  • -fsanitize-undefined-trap-on-error
  • -fbounds-check
  • -fcf-protection
  • -fcf-protection=[full|branch|return|none|check]
  • -fharden-compares
  • -fharden-conditional-branches
  • -fhardened
  • -fharden-control-flow-redundancy
  • -fhardcfr-skip-leaf
  • -fhardcfr-check-exceptions
  • -fhardcfr-check-returning-calls
  • -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
  • -fstack-protector
  • -fstack-protector-all
  • -fstack-protector-strong
  • -fstack-protector-explicit
  • -fstack-check
  • -fstack-limit-register=reg
  • -fstack-limit-symbol=sym
  • -fno-stack-limit
  • -fsplit-stack
  • -fstrub=disable
  • -fstrub=strict
  • -fstrub=relaxed
  • -fstrub=all
  • -fstrub=at-calls
  • -fstrub=internal
  • -fvtable-verify=[std|preinit|none]
  • -fvtv-counts
  • -fvtv-debug
  • -finstrument-functions
  • -finstrument-functions-once
  • -finstrument-functions-exclude-function-list=sym,sym,...
  • -finstrument-functions-exclude-file-list=file,file,...
  • -fprofile-prefix-map=old=new
  • -fpatchable-function-entry=N[,M]

🎀 Preprocessor Options 🎀

The preprocessor is the opening act before compilation, and these are its flags — exact, untouched, iconic. 🌸

  • -Aquestion=answer
  • -A-question[=answer]
  • -C
  • -CC
  • -Dmacro[=defn]
  • -dD
  • -dI
  • -dM
  • -dN
  • -dU
  • -fdebug-cpp
  • -fdirectives-only
  • -fdollars-in-identifiers
  • -fexec-charset=charset
  • -fextended-identifiers
  • -finput-charset=charset
  • -fmacro-prefix-map=old=new
  • -fmax-include-depth=depth
  • -fno-canonical-system-headers
  • -fpch-deps
  • -fpch-preprocess
  • -fpreprocessed
  • -ftabstop=width
  • -ftrack-macro-expansion
  • -fwide-exec-charset=charset
  • -fworking-directory
  • -H
  • -imacros file
  • -include file
  • -M
  • -MD
  • -MF
  • -MG
  • -MM
  • -MMD
  • -MP
  • -MQ
  • -MT
  • -Mno-modules
  • -no-integrated-cpp
  • -P
  • -pthread
  • -remap
  • -traditional
  • -traditional-cpp
  • -trigraphs
  • -Umacro
  • -undef
  • -Wp,option
  • -Xpreprocessor option

🦋 Assembler Options 🦋

Short section, still iconic: assembler flags only. No extra lore, just the exact two. 💜

  • -Wa,option
  • -Xassembler option

💖 Linker Options 💖

Linker options go here, exactly as listed — including the non-flag-looking ones, because bestie we do not skip entries. ✨

  • object-file-name
  • -fuse-ld=linker
  • -llibrary
  • -nostartfiles
  • -nodefaultlibs
  • -nolibc
  • -nostdlib
  • -nostdlib++
  • -e entry
  • --entry=entry
  • -pie
  • -pthread
  • -r
  • -rdynamic
  • -s
  • -static
  • -static-pie
  • -static-libgcc
  • -static-libstdc++
  • -static-libasan
  • -static-libtsan
  • -static-liblsan
  • -static-libubsan
  • -shared
  • -shared-libgcc
  • -symbolic
  • -T script
  • -Wl,option
  • -Xlinker option
  • -u symbol
  • -z keyword

🌸 Directory Options 🌸

All the "where do I look for stuff" flags, preserved exactly. 🎀

  • -Bprefix
  • -Idir
  • -I-
  • -idirafter dir
  • -imacros file
  • -imultilib dir
  • -iplugindir=dir
  • -iprefix file
  • -iquote dir
  • -isysroot dir
  • -isystem dir
  • -iwithprefix dir
  • -iwithprefixbefore dir
  • --embed-dir=dir
  • -Ldir
  • -no-canonical-prefixes
  • --no-sysroot-suffix
  • -nostdinc
  • -nostdinc++
  • --sysroot=dir

✨ Code Generation Options ✨

Codegen flags, aka the machine-code mood board. Every single one is right here. 💅

  • -fcall-saved-reg
  • -fcall-used-reg
  • -ffixed-reg
  • -fexceptions
  • -fnon-call-exceptions
  • -fdelete-dead-exceptions
  • -funwind-tables
  • -fasynchronous-unwind-tables
  • -fno-gnu-unique
  • -finhibit-size-directive
  • -fcommon
  • -fno-ident
  • -fpcc-struct-return
  • -fpic
  • -fPIC
  • -fpie
  • -fPIE
  • -fno-plt
  • -fno-jump-tables
  • -fno-bit-tests
  • -frecord-gcc-switches
  • -freg-struct-return
  • -fshort-enums
  • -fshort-wchar
  • -fverbose-asm
  • -fpack-struct[=n]
  • -fleading-underscore
  • -ftls-model=model
  • -fstack-reuse=reuse_level
  • -ftrampolines
  • -ftrampoline-impl=[stack|heap]
  • -ftrapv
  • -fwrapv
  • -fvisibility=[default|internal|hidden|protected]
  • -fstrict-volatile-bitfields
  • -fsync-libcalls
  • -fzero-init-padding-bits=value

👑 Developer Options 👑

Dev options — the deluxe behind-the-scenes menu. Dumps, reports, debug knobs, all exact. No cap. 🔥

  • -dletters
  • -dumpspecs
  • -dumpmachine
  • -dumpversion
  • -dumpfullversion
  • -fcallgraph-info[=su,da]
  • -fchecking
  • -fchecking=n
  • -fdbg-cnt-list
  • -fdbg-cnt=counter-value-list
  • -fdisable-ipa-pass_name
  • -fdisable-rtl-pass_name
  • -fdisable-rtl-pass-name=range-list
  • -fdisable-tree-pass_name
  • -fdisable-tree-pass-name=range-list
  • -fdump-debug
  • -fdump-earlydebug
  • -fdump-noaddr
  • -fdump-unnumbered
  • -fdump-unnumbered-links
  • -fdump-final-insns[=file]
  • -fdump-ipa-all
  • -fdump-ipa-cgraph
  • -fdump-ipa-inline
  • -fdump-lang-all
  • -fdump-lang-switch
  • -fdump-lang-switch-options
  • -fdump-lang-switch-options=filename
  • -fdump-passes
  • -fdump-rtl-pass
  • -fdump-rtl-pass=filename
  • -fdump-statistics
  • -fdump-tree-all
  • -fdump-tree-switch
  • -fdump-tree-switch-options
  • -fdump-tree-switch-options=filename
  • -fcompare-debug[=opts]
  • -fcompare-debug-second
  • -fenable-kind-pass
  • -fenable-kind-pass=range-list
  • -fira-verbose=n
  • -flto-report
  • -flto-report-wpa
  • -fmem-report-wpa
  • -fmem-report
  • -fpre-ipa-mem-report
  • -fpost-ipa-mem-report
  • -fopt-info
  • -fopt-info-options[=file]
  • -fmultiflags
  • -fprofile-report
  • -frandom-seed=string
  • -fsched-verbose=n
  • -fsel-sched-verbose
  • -fsel-sched-dump-cfg
  • -fsel-sched-pipelining-verbose
  • -fstats
  • -fstack-usage
  • -ftime-report
  • -ftime-report-details
  • -fvar-tracking-assignments-toggle
  • -gtoggle
  • -print-file-name=library
  • -print-libgcc-file-name
  • -print-multi-directory
  • -print-multi-lib
  • -print-multi-os-directory
  • -print-prog-name=program
  • -print-search-dirs
  • -Q
  • -print-sysroot
  • -print-sysroot-headers-suffix
  • -save-temps
  • -save-temps=cwd
  • -save-temps=obj
  • -time[=file]

Machine-Dependent Options

okay bestie, this is the machine-dependent menu and it is long. Like, "study group at 2am" long. Every single flag from this part is here, exactly as written, no cap 💅✨

AArch64 Options (AArch64 Options)

AArch64 bestie, your bouquet of flags is right here 🌸

  • -mabi=name
  • -mbig-endian
  • -mlittle-endian
  • -mgeneral-regs-only
  • -mcmodel=tiny
  • -mcmodel=small
  • -mcmodel=large
  • -mstrict-align
  • -mno-strict-align
  • -momit-leaf-frame-pointer
  • -mtls-dialect=desc
  • -mtls-dialect=traditional
  • -mtls-size=size
  • -mfix-cortex-a53-835769
  • -mfix-cortex-a53-843419
  • -mlow-precision-recip-sqrt
  • -mlow-precision-sqrt
  • -mlow-precision-div
  • -mpc-relative-literal-loads
  • -msign-return-address=scope
  • -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti|gcs
  • -mharden-sls=opts
  • -march=name
  • -mcpu=name
  • -mtune=name
  • -moverride=string
  • -mverbose-cost-dump
  • -mstack-protector-guard=guard
  • -mstack-protector-guard-reg=sysreg
  • -mstack-protector-guard-offset=offset
  • -mtrack-speculation
  • -moutline-atomics
  • -mearly-ldp-fusion
  • -mlate-ldp-fusion
  • -Wexperimental-fmv-target

Adapteva Epiphany Options (Adapteva Epiphany Options)

tiny architecture, absolutely stacked flag closet 🦋

  • -mhalf-reg-file
  • -mprefer-short-insn-regs
  • -mbranch-cost=num
  • -mcmove
  • -mnops=num
  • -msoft-cmpsf
  • -msplit-lohi
  • -mpost-inc
  • -mpost-modify
  • -mstack-offset=num
  • -mround-nearest
  • -mlong-calls
  • -mshort-calls
  • -msmall16
  • -mfp-mode=mode
  • -mvect-double
  • -max-vect-align=num
  • -msplit-vecmove-early
  • -m1reg-reg

AMD GCN Options (AMD GCN Options)

GPU flags, but make it GCC ⚡

  • -march=gpu
  • -mtune=gpu
  • -mstack-size=bytes

ARC Options (ARC Options)

ARC said "I have a flag for that" and then kept going 😭

  • -mbarrel-shifter
  • -mjli-always
  • -mcpu=cpu
  • -mA6
  • -mARC600
  • -mA7
  • -mARC700
  • -mdpfp
  • -mdpfp-compact
  • -mdpfp-fast
  • -mno-dpfp-lrsr
  • -mea
  • -mno-mpy
  • -mmul32x16
  • -mmul64
  • -matomic
  • -mnorm
  • -mspfp
  • -mspfp-compact
  • -mspfp-fast
  • -msimd
  • -msoft-float
  • -mswap
  • -mcrc
  • -mdsp-packa
  • -mdvbf
  • -mlock
  • -mmac-d16
  • -mmac-24
  • -mrtsc
  • -mswape
  • -mtelephony
  • -mxy
  • -misize
  • -mannotate-align
  • -marclinux
  • -marclinux_prof
  • -mlong-calls
  • -mmedium-calls
  • -msdata
  • -mirq-ctrl-saved
  • -mrgf-banked-regs
  • -mlpc-width=width
  • -G num
  • -mvolatile-cache
  • -mtp-regno=regno
  • -malign-call
  • -mauto-modify-reg
  • -mbbit-peephole
  • -mno-brcc
  • -mcase-vector-pcrel
  • -mcompact-casesi
  • -mno-cond-exec
  • -mearly-cbranchsi
  • -mexpand-adddi
  • -mindexed-loads
  • -mlra
  • -mlra-priority-none
  • -mlra-priority-compact
  • -mlra-priority-noncompact
  • -mmillicode
  • -mmixed-code
  • -mq-class
  • -mRcq
  • -mRcw
  • -msize-level=level
  • -mtune=cpu
  • -mmultcost=num
  • -mcode-density-frame
  • -munalign-prob-threshold=probability
  • -mmpy-option=multo
  • -mdiv-rem
  • -mcode-density
  • -mll64
  • -mfpu=fpu
  • -mrf16
  • -mbranch-index

ARM Options (ARM Options)

ARM flags, aka the main-character energy of embedded options 👑

  • -mapcs-frame
  • -mno-apcs-frame
  • -mabi=name
  • -mapcs-stack-check
  • -mno-apcs-stack-check
  • -mapcs-reentrant
  • -mno-apcs-reentrant
  • -mgeneral-regs-only
  • -msched-prolog
  • -mno-sched-prolog
  • -mlittle-endian
  • -mbig-endian
  • -mbe8
  • -mbe32
  • -mfloat-abi=name
  • -mfp16-format=name
  • -mthumb-interwork
  • -mno-thumb-interwork
  • -mcpu=name
  • -march=name
  • -mfpu=name
  • -mtune=name
  • -mprint-tune-info
  • -mstructure-size-boundary=n
  • -mabort-on-noreturn
  • -mlong-calls
  • -mno-long-calls
  • -msingle-pic-base
  • -mno-single-pic-base
  • -mpic-register=reg
  • -mnop-fun-dllimport
  • -mpoke-function-name
  • -mthumb
  • -marm
  • -mflip-thumb
  • -mtpcs-frame
  • -mtpcs-leaf-frame
  • -mcaller-super-interworking
  • -mcallee-super-interworking
  • -mtp=name
  • -mtls-dialect=dialect
  • -mword-relocations
  • -mfix-cortex-m3-ldrd
  • -mfix-cortex-a57-aes-1742098
  • -mfix-cortex-a72-aes-1655431
  • -munaligned-access
  • -mneon-for-64bits
  • -mslow-flash-data
  • -masm-syntax-unified
  • -mrestrict-it
  • -mverbose-cost-dump
  • -mpure-code
  • -mcmse
  • -mfix-cmse-cve-2021-35465
  • -mstack-protector-guard=guard
  • -mstack-protector-guard-offset=offset
  • -mfdpic
  • -mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]

AVR Options (AVR Options)

AVR flags, tiny-chip edition, but the list is not tiny bestie 🍓

  • -mmcu=mcu
  • -mabsdata
  • -maccumulate-args
  • -mcvt
  • -mbranch-cost=cost
  • -mfuse-add=level
  • -mfuse-move=level
  • -mfuse-move2
  • -mcall-prologues
  • -mgas-isr-prologues
  • -mint8
  • -mflmap
  • -mdouble=bits
  • -mlong-double=bits
  • -mno-call-main
  • -mn_flash=size
  • -mfract-convert-truncate
  • -mno-interrupts
  • -mmain-is-OS_task
  • -mrelax
  • -mrmw
  • -mstrict-X
  • -mtiny-stack
  • -mrodata-in-ram
  • -msplit-bit-shift
  • -msplit-ldst
  • -mshort-calls
  • -mskip-bug
  • -muse-nonzero-bits
  • -nodevicelib
  • -nodevicespecs
  • -Waddr-space-convert
  • -Wmisspelled-isr

Blackfin Options (Blackfin Options)

Blackfin said "anomaly? I barely know her" 💀 flags below:

  • -mcpu=cpu[-sirevision]
  • -msim
  • -momit-leaf-frame-pointer
  • -mno-omit-leaf-frame-pointer
  • -mspecld-anomaly
  • -mno-specld-anomaly
  • -mcsync-anomaly
  • -mno-csync-anomaly
  • -mlow-64k
  • -mno-low64k
  • -mstack-check-l1
  • -mid-shared-library
  • -mno-id-shared-library
  • -mshared-library-id=n
  • -mleaf-id-shared-library
  • -mno-leaf-id-shared-library
  • -msep-data
  • -mno-sep-data
  • -mlong-calls
  • -mno-long-calls
  • -mfast-fp
  • -minline-plt
  • -mmulticore
  • -mcorea
  • -mcoreb
  • -msdram
  • -micplb

C6X Options (C6X Options)

C6X bestie, short but iconic 🎀

  • -mbig-endian
  • -mlittle-endian
  • -march=cpu
  • -msim
  • -msdata=sdata-type

CRIS Options (CRIS Options)

CRIS flags, including the little non-m ones that are easy to miss 👀

  • -mcpu=cpu
  • -march=cpu
  • -mtune=cpu
  • -mmax-stack-frame=n
  • -metrax4
  • -metrax100
  • -mpdebug
  • -mcc-init
  • -mno-side-effects
  • -mstack-align
  • -mdata-align
  • -mconst-align
  • -m32-bit
  • -m16-bit
  • -m8-bit
  • -mno-prologue-epilogue
  • -melf
  • -maout
  • -sim
  • -sim2
  • -mmul-bug-workaround
  • -mno-mul-bug-workaround

C-SKY Options (C-SKY Options)

C-SKY said "let's have both -mbig-endian and -EB" and honestly? slay 🌈

  • -march=arch
  • -mcpu=cpu
  • -mbig-endian
  • -EB
  • -mlittle-endian
  • -EL
  • -mhard-float
  • -msoft-float
  • -mfpu=fpu
  • -mdouble-float
  • -mfdivdu
  • -mfloat-abi=name
  • -melrw
  • -mistack
  • -mmp
  • -mcp
  • -mcache
  • -msecurity
  • -mtrust
  • -mdsp
  • -medsp
  • -mvdsp
  • -mdiv
  • -msmart
  • -mhigh-registers
  • -manchor
  • -mpushpop
  • -mmultiple-stld
  • -mconstpool
  • -mstack-size
  • -mccrt
  • -mbranch-cost=n
  • -mcse-cc
  • -msched-prolog
  • -msim

Cygwin and MinGW Options (Cygwin and MinGW Options)

Windows-adjacent GCC flags, we love a cross-platform moment 🫶

  • -mconsole
  • -mcrtdll=library
  • -mdll
  • -mnop-fun-dllimport
  • -mthread
  • -municode
  • -mwin32
  • -mwindows
  • -fno-set-stack-executable

Darwin Options (Darwin Options)

Darwin flags are giving "too many options at the sleepover" but we keep every one 😭💖

  • -all_load
  • -allowable_client
  • -arch
  • -arch_errors_fatal
  • -arch_only
  • -bind_at_load
  • -bundle
  • -bundle_loader
  • -client_name
  • -compatibility_version
  • -current_version
  • -dead_strip
  • -dependency-file
  • -dylib_file
  • -dylinker_install_name
  • -dynamic
  • -dynamiclib
  • -exported_symbols_list
  • -filelist
  • -flat_namespace
  • -force_cpusubtype_ALL
  • -force_flat_namespace
  • -headerpad_max_install_names
  • -iframework
  • -image_base
  • -init
  • -install_name
  • -keep_private_externs
  • -multi_module
  • -multiply_defined
  • -multiply_defined_unused
  • -noall_load
  • -no_dead_strip_inits_and_terms
  • -nodefaultrpaths
  • -nofixprebinding
  • -nomultidefs
  • -noprebind
  • -noseglinkedit
  • -pagezero_size
  • -prebind
  • -prebind_all_twolevel_modules
  • -private_bundle
  • -read_only_relocs
  • -sectalign
  • -sectobjectsymbols
  • -whyload
  • -seg1addr
  • -sectcreate
  • -sectobjectsymbols
  • -sectorder
  • -segaddr
  • -segs_read_only_addr
  • -segs_read_write_addr
  • -seg_addr_table
  • -seg_addr_table_filename
  • -seglinkedit
  • -segprot
  • -segs_read_only_addr
  • -segs_read_write_addr
  • -single_module
  • -static
  • -sub_library
  • -sub_umbrella
  • -twolevel_namespace
  • -umbrella
  • -undefined
  • -unexported_symbols_list
  • -weak_reference_mismatches
  • -whatsloaded
  • -F
  • -gused
  • -gfull
  • -mmacosx-version-min=version
  • -mkernel
  • -mone-byte-bool

DEC Alpha Options (DEC Alpha Options)

DEC Alpha flags, retro but still doing the most 💜

  • -mno-fp-regs
  • -msoft-float
  • -mieee
  • -mieee-with-inexact
  • -mieee-conformant
  • -mfp-trap-mode=mode
  • -mfp-rounding-mode=mode
  • -mtrap-precision=mode
  • -mbuild-constants
  • -mcpu=cpu-type
  • -mtune=cpu-type
  • -mbwx
  • -mmax
  • -mfix
  • -mcix
  • -msafe-bwa
  • -msafe-partial
  • -mfloat-vax
  • -mfloat-ieee
  • -mexplicit-relocs
  • -msmall-data
  • -mlarge-data
  • -msmall-text
  • -mlarge-text
  • -mmemory-latency=time

eBPF Options (eBPF Options)

eBPF bestie, the vibe is "tiny virtual machine, massive flag energy" 🧚

  • -mbig-endian
  • -mlittle-endian
  • -mframe-limit=bytes
  • -mxbpf
  • -mco-re
  • -mno-co-re
  • -mjmpext
  • -mjmp32
  • -malu32
  • -mv3-atomics
  • -mbswap
  • -msdiv
  • -msmov
  • -mcpu=version
  • -masm=dialect
  • -minline-memops-threshold=bytes

FR30 Options (FR30 Options)

FR30 said "small list, small model" ✨

  • -msmall-model
  • -mno-lsim

FT32 Options (FT32 Options)

FT32 flags, short and sweet 🍓

  • -msim
  • -mlra
  • -mnodiv
  • -mft32b
  • -mcompress
  • -mnopm

FRV Options (FRV Options)

FRV is giving "I contain multitudes" because look at this list 😭💅

  • -mgpr-32
  • -mgpr-64
  • -mfpr-32
  • -mfpr-64
  • -mhard-float
  • -msoft-float
  • -malloc-cc
  • -mfixed-cc
  • -mdword
  • -mno-dword
  • -mdouble
  • -mno-double
  • -mmedia
  • -mno-media
  • -mmuladd
  • -mno-muladd
  • -mfdpic
  • -minline-plt
  • -mgprel-ro
  • -multilib-library-pic
  • -mlinked-fp
  • -mlong-calls
  • -malign-labels
  • -mlibrary-pic
  • -macc-4
  • -macc-8
  • -mpack
  • -mno-pack
  • -mno-eflags
  • -mcond-move
  • -mno-cond-move
  • -moptimize-membar
  • -mno-optimize-membar
  • -mscc
  • -mno-scc
  • -mcond-exec
  • -mno-cond-exec
  • -mvliw-branch
  • -mno-vliw-branch
  • -mmulti-cond-exec
  • -mno-multi-cond-exec
  • -mnested-cond-exec
  • -mno-nested-cond-exec
  • -mtomcat-stats
  • -mTLS
  • -mtls
  • -mcpu=cpu

GNU/Linux Options (GNU/Linux Options)

Linux libc-flavored flags, bestie 🌈

  • -mglibc
  • -muclibc
  • -mmusl
  • -mbionic
  • -mandroid
  • -tno-android-cc
  • -tno-android-ld

H8/300 Options (H8/300 Options)

H8/300 options, cute little set 💜

  • -mrelax
  • -mh
  • -ms
  • -mn
  • -mexr
  • -mno-exr
  • -mint32
  • -malign-300

HPPA Options (HPPA Options)

HPPA flags, including the non-m ones at the end — don't miss 'em 👑

  • -march=architecture-type
  • -matomic-libcalls
  • -mbig-switch
  • -mcaller-copies
  • -mdisable-fpregs
  • -mdisable-indexing
  • -mordered
  • -mfast-indirect-calls
  • -mgas
  • -mgnu-ld
  • -mhp-ld
  • -mfixed-range=register-range
  • -mcoherent-ldcw
  • -mjump-in-delay
  • -mlinker-opt
  • -mlong-calls
  • -mlong-load-store
  • -mno-atomic-libcalls
  • -mno-disable-fpregs
  • -mno-disable-indexing
  • -mno-fast-indirect-calls
  • -mno-gas
  • -mno-jump-in-delay
  • -mno-long-load-store
  • -mno-portable-runtime
  • -mno-soft-float
  • -mno-space-regs
  • -msoft-float
  • -mpa-risc-1-0
  • -mpa-risc-1-1
  • -mpa-risc-2-0
  • -mportable-runtime
  • -mschedule=cpu-type
  • -mspace-regs
  • -msoft-mult
  • -msio
  • -mwsio
  • -munix=unix-std
  • -nolibdld
  • -static
  • -threads

IA-64 Options (IA-64 Options)

IA-64 said "scheduling flags? yes. all of them." and honestly periodt 💀✨

  • -mbig-endian
  • -mlittle-endian
  • -mgnu-as
  • -mgnu-ld
  • -mno-pic
  • -mvolatile-asm-stop
  • -mregister-names
  • -msdata
  • -mno-sdata
  • -mconstant-gp
  • -mauto-pic
  • -mfused-madd
  • -minline-float-divide-min-latency
  • -minline-float-divide-max-throughput
  • -mno-inline-float-divide
  • -minline-int-divide-min-latency
  • -minline-int-divide-max-throughput
  • -mno-inline-int-divide
  • -minline-sqrt-min-latency
  • -minline-sqrt-max-throughput
  • -mno-inline-sqrt
  • -mdwarf2-asm
  • -mearly-stop-bits
  • -mfixed-range=register-range
  • -mtls-size=tls-size
  • -mtune=cpu-type
  • -milp32
  • -mlp64
  • -msched-br-data-spec
  • -msched-ar-data-spec
  • -msched-control-spec
  • -msched-br-in-data-spec
  • -msched-ar-in-data-spec
  • -msched-in-control-spec
  • -msched-spec-ldc
  • -msched-spec-control-ldc
  • -msched-prefer-non-data-spec-insns
  • -msched-prefer-non-control-spec-insns
  • -msched-stop-bits-after-every-cycle
  • -msched-count-spec-in-critical-path
  • -msel-sched-dont-check-control-spec
  • -msched-fp-mem-deps-zero-cost
  • -msched-max-memory-insns-hard-limit
  • -msched-max-memory-insns=max-insns

LM32 Options (LM32 Options)

LM32 pulling up with a lil' list and full main-character energy 🦋

  • -mbarrel-shift-enabled
  • -mdivide-enabled
  • -mmultiply-enabled
  • -msign-extend-enabled
  • -muser-enabled

🦋 LoongArch Options 🦋

okay so you're on LoongArch, bestie? here's your entire flag family, no notes:

  • -march=arch-type — the architecture itself, arch-type style.
  • -mtune=tune-type — who we're tuning for: tune-type.
  • -mabi=base-abi-type — ABI, base flavour: base-abi-type.
  • -mfpu=fpu-type — your FPU, specified as fpu-type.
  • -msimd=simd-type — SIMD, simd-type edition.
  • -msoft-float — soft float szn.
  • -msingle-float — single precision only.
  • -mdouble-float — double precision, double the vibes.
  • -mlsx — LSX ON 💅
  • -mno-lsx — LSX OFF, sorry babe.
  • -mlasx — LASX ON.
  • -mno-lasx — LASX OFF.
  • -mbranch-cost=n — branch cost set to n.
  • -maddr-reg-reg-cost=n — addr-reg-reg cost set to n, yes that's a real knob.
  • -mcheck-zero-division — check for zero division, protective queen.
  • -mno-check-zero-division — don't check. live dangerously.
  • -mcond-move-int — conditional move for ints.
  • -mno-cond-move-int — no cond-move for ints.
  • -mcond-move-float — conditional move for floats.
  • -mno-cond-move-float — no cond-move for floats.
  • -memcpy — use memcpy.
  • -mno-memcpy — don't use memcpy.
  • -mstrict-align — strict alignment, do not test her.
  • -mno-strict-align — alignment rules? we don't know her.
  • -G num — the -G knob, takes a num.
  • -mmax-inline-memcpy-size=n — cap the inline memcpy size at n.
  • -mexplicit-relocs=style — explicit relocs, in style.
  • -mexplicit-relocs — plain explicit relocs, no style picked.
  • -mno-explicit-relocs — nope, not them.
  • -mdirect-extern-access — direct extern access 💫
  • -mno-direct-extern-access — the off-switch twin.
  • -mcmodel=code-model — code model: code-model.
  • -mrelax — relax, girl. breathe.
  • -mpass-mrelax-to-as — pass that -mrelax along to the assembler.
  • -mrecip — reciprocal time.
  • -mrecip=opt — reciprocal with opt options.
  • -mfrecipe — frecipe, yes that's the name.
  • -mno-frecipe — no frecipe.
  • -mdiv32 — 32-bit division.
  • -mno-div32 — not that.
  • -mlam-bh — LAM-BH on.
  • -mno-lam-bh — LAM-BH off.
  • -mlamcas — LAMCAS on.
  • -mno-lamcas — LAMCAS off.
  • -mld-seq-sa — load sequence, sequentially-atomic flavoured.
  • -mno-ld-seq-sa — the opposite, obviously.
  • -mtls-dialect=opt — TLS dialect, opt.
  • -mannotate-tablejump — annotate those table jumps.
  • -mno-annotate-tablejump — don't.

🎀 M32C Options 🎀

  • -mcpu=cpu — pick your CPU: cpu.
  • -msim — simulator mode.
  • -memregs=number — emulated registers, number of them.

⭐ M32R/D Options ⭐

  • -m32r2 — the 32r2 vibe.
  • -m32rx — the 32rx vibe.
  • -m32r — plain 32r.
  • -mdebug — debug mode, we love a paper trail.
  • -malign-loops — align loops.
  • -mno-align-loops — don't align loops.
  • -missue-rate=number — issue rate, number.
  • -mbranch-cost=number — branch cost, number.
  • -mmodel=code-size-model-type — code size model: code-size-model-type.
  • -msdata=sdata-type — small data, sdata-type style.
  • -mno-flush-func — no flush function.
  • -mflush-func=name — flush function named name.
  • -mno-flush-trap — no flush trap.
  • -mflush-trap=number — flush trap, number.
  • -G num — the -G knob again, num.

🌈 M680x0 Options 🌈

m68k besties this is YOUR moment:

  • -march=arch — architecture: arch.
  • -mcpu=cpu — CPU: cpu.
  • -mtune=tune — tune: tune.
  • -m68000 — the classic 68000.
  • -m68020 — 68020.
  • -m68020-40 — 68020 through 40, the range girlie.
  • -m68020-60 — 68020 through 60.
  • -m68030 — 68030.
  • -m68040 — 68040.
  • -m68060 — 68060.
  • -mcpu32 — CPU32.
  • -m5200 — 5200.
  • -m5206e — 5206e.
  • -m528x — 528x.
  • -m5307 — 5307.
  • -m5407 — 5407.
  • -mcfv4e — ColdFire v4e.
  • -mbitfield — bitfields on.
  • -mno-bitfield — bitfields off.
  • -mc68000 — the c version of 68000.
  • -mc68020 — the c version of 68020.
  • -mnobitfield — another off-switch for bitfields, and yes it's spelled like that.
  • -mrtd — rtd calling convention.
  • -mno-rtd — not that.
  • -mdiv — division instructions.
  • -mno-div — no division.
  • -mshort — shorts, shorter, shortest.
  • -mno-short — un-short.
  • -mhard-float — hardware float, she's fast.
  • -m68881 — the 68881.
  • -msoft-float — software float, she's patient.
  • -mpcrel — PC-relative.
  • -malign-int — align ints.
  • -mstrict-align — strict alignment.
  • -msep-data — separate data.
  • -mno-sep-data — not separate.
  • -mshared-library-id=n — shared library id, n.
  • -mid-shared-library — id shared library.
  • -mno-id-shared-library — off.
  • -mxgot — big GOT energy.
  • -mno-xgot — regular GOT energy.
  • -mlong-jump-table-offsets — long jump table offsets, periodt.

🫶 MCore Options 🫶

  • -mhardlit — hard literals.
  • -mno-hardlit — soft literals then.
  • -mdiv — divide.
  • -mno-div — don't divide.
  • -mrelax-immediates — relax those immediates.
  • -mno-relax-immediates — keep them tense.
  • -mwide-bitfields — wide bitfields.
  • -mno-wide-bitfields — narrow bitfields.
  • -m4byte-functions — 4-byte functions.
  • -mno-4byte-functions — not 4-byte.
  • -mcallgraph-data — callgraph data.
  • -mno-callgraph-data — no callgraph data.
  • -mslow-bytes — slow bytes 😭
  • -mno-slow-bytes — fast bytes era.
  • -mno-lsim — no lsim (only the off version exists here, don't ask).
  • -mlittle-endian — little endian.
  • -mbig-endian — big endian.
  • -m210 — the 210.
  • -m340 — the 340.
  • -mstack-increment — stack increment.

💜 MicroBlaze Options 💜

  • -msoft-float — soft float.
  • -mhard-float — hard float.
  • -msmall-divides — small divides.
  • -mcpu=cpu — CPU: cpu.
  • -mmemcpy — memcpy mode.
  • -mxl-soft-mul — soft multiply.
  • -mxl-soft-div — soft divide.
  • -mxl-barrel-shift — barrel shifter.
  • -mxl-pattern-compare — pattern compare.
  • -mxl-stack-check — stack check.
  • -mxl-gp-opt — GP optimization.
  • -mno-clearbss — don't clear BSS.
  • -mxl-multiply-high — multiply high.
  • -mxl-float-convert — float convert.
  • -mxl-float-sqrt — float sqrt.
  • -mbig-endian — big endian.
  • -mlittle-endian — little endian.
  • -mxl-reorder — reorder.
  • -mxl-mode-app-model — app model mode.
  • -mpic-data-is-text-relative — PIC data is text relative.

🔥 MIPS Options 🔥

the MIPS list is LONG, hydrate first:

  • -EL — little endian.
  • -EB — big endian.
  • -march=arch — architecture: arch.
  • -mtune=arch — tune for: arch.
  • -mips1 — MIPS I.
  • -mips2 — MIPS II.
  • -mips3 — MIPS III.
  • -mips4 — MIPS IV.
  • -mips32 — MIPS32.
  • -mips32r2 — MIPS32r2.
  • -mips32r3 — MIPS32r3.
  • -mips32r5 — MIPS32r5.
  • -mips32r6 — MIPS32r6.
  • -mips64 — MIPS64.
  • -mips64r2 — MIPS64r2.
  • -mips64r3 — MIPS64r3.
  • -mips64r5 — MIPS64r5.
  • -mips64r6 — MIPS64r6.
  • -mips16 — MIPS16.
  • -mno-mips16 — not MIPS16.
  • -mflip-mips16 — flip MIPS16, chaotic but iconic.
  • -minterlink-compressed — interlink compressed.
  • -mno-interlink-compressed — no interlink compressed.
  • -minterlink-mips16 — interlink MIPS16.
  • -mno-interlink-mips16 — no interlink MIPS16.
  • -mabi=abi — ABI: abi.
  • -mabicalls — abicalls on.
  • -mno-abicalls — abicalls off.
  • -mshared — shared.
  • -mno-shared — not shared.
  • -mplt — PLT.
  • -mno-plt — no PLT.
  • -mxgot — big GOT.
  • -mno-xgot — regular GOT.
  • -mgp32 — 32-bit GPRs.
  • -mgp64 — 64-bit GPRs.
  • -mfp32 — 32-bit FPRs.
  • -mfpxx — FPXX.
  • -mfp64 — 64-bit FPRs.
  • -mhard-float — hard float.
  • -msoft-float — soft float.
  • -mno-float — literally no float at all.
  • -msingle-float — single float.
  • -mdouble-float — double float.
  • -modd-spreg — odd single-precision registers.
  • -mno-odd-spreg — no odd spregs.
  • -mabs=mode — abs mode: mode.
  • -mnan=encoding — NaN encoding: encoding.
  • -mdsp — DSP on.
  • -mno-dsp — DSP off.
  • -mdspr2 — DSP R2 on.
  • -mno-dspr2 — DSP R2 off.
  • -mmcu — MCU on.
  • -mmno-mcu — yes, it's literally spelled -mmno-mcu. leaving it exactly as-is, do not @ me.
  • -meva — EVA on.
  • -mno-eva — EVA off.
  • -mvirt — virtualization on.
  • -mno-virt — virtualization off.
  • -mxpa — XPA on.
  • -mno-xpa — XPA off.
  • -mcrc — CRC on.
  • -mno-crc — CRC off.
  • -mginv — GINV on.
  • -mno-ginv — GINV off.
  • -mmicromips — microMIPS.
  • -mno-micromips — no microMIPS.
  • -mmsa — MSA on.
  • -mno-msa — MSA off.
  • -mloongson-mmi — Loongson MMI on.
  • -mno-loongson-mmi — Loongson MMI off.
  • -mloongson-ext — Loongson EXT on.
  • -mno-loongson-ext — Loongson EXT off.
  • -mloongson-ext2 — Loongson EXT2 on.
  • -mno-loongson-ext2 — Loongson EXT2 off.
  • -mfpu=fpu-type — FPU: fpu-type.
  • -msmartmips — SmartMIPS on.
  • -mno-smartmips — SmartMIPS off.
  • -mpaired-single — paired single.
  • -mno-paired-single — unpaired single.
  • -mdmx — DMX on.
  • -mno-mdmx — DMX off.
  • -mips3d — MIPS-3D on.
  • -mno-mips3d — MIPS-3D off.
  • -mmt — multithreading on.
  • -mno-mt — multithreading off.
  • -mllsc — LL/SC on.
  • -mno-llsc — LL/SC off.
  • -mlong64 — long is 64-bit.
  • -mlong32 — long is 32-bit.
  • -msym32 — symbols are 32-bit.
  • -mno-sym32 — symbols aren't.
  • -Gnum — the -G knob, glued to the number this time.
  • -mlocal-sdata — local small data.
  • -mno-local-sdata — no local small data.
  • -mextern-sdata — extern small data.
  • -mno-extern-sdata — no extern small data.
  • -mgpopt — GP optimization.
  • -mno-gopt — the off-switch, spelled -mno-gopt exactly.
  • -membedded-data — embedded data.
  • -mno-embedded-data — not embedded.
  • -muninit-const-in-rodata — uninitialized consts into rodata.
  • -mno-uninit-const-in-rodata — keep them out of rodata.
  • -mcode-readable=setting — code readable, setting.
  • -msplit-addresses — split addresses.
  • -mno-split-addresses — don't split.
  • -mexplicit-relocs — explicit relocs.
  • -mno-explicit-relocs — not explicit.
  • -mexplicit-relocs=release — explicit relocs, release flavour.
  • -mcheck-zero-division — check zero division.
  • -mno-check-zero-division — don't check.
  • -mdivide-traps — division traps.
  • -mdivide-breaks — division breaks.
  • -mload-store-pairs — load/store pairs.
  • -mno-load-store-pairs — no pairs.
  • -mstrict-align — strict alignment.
  • -mno-strict-align — lax alignment.
  • -mno-unaligned-access — no unaligned access.
  • -munaligned-access — unaligned access allowed.
  • -mmemcpy — memcpy mode.
  • -mno-memcpy — not memcpy mode.
  • -mlong-calls — long calls.
  • -mno-long-calls — short calls.
  • -mmad — MAD on.
  • -mno-mad — MAD off.
  • -mimadd — IMADD on.
  • -mno-imadd — IMADD off.
  • -mfused-madd — fused madd.
  • -mno-fused-madd — unfused madd.
  • -nocpp — no preprocessor, raw and unfiltered.
  • -mfix-24k — the 24K fix.
  • -mno-fix-24k — skip the 24K fix.
  • -mfix-r4000 — the R4000 fix.
  • -mno-fix-r4000 — skip it.
  • -mfix-r4400 — the R4400 fix.
  • -mno-fix-r4400 — skip it.
  • -mfix-r5900 — the R5900 fix.
  • -mno-fix-r5900 — skip it.
  • -mfix-r10000 — the R10000 fix.
  • -mno-fix-r10000 — skip it.
  • -mfix-rm7000 — the RM7000 fix.
  • -mno-fix-rm7000 — skip it.
  • -mfix-vr4120 — the VR4120 fix.
  • -mno-fix-vr4120 — skip it.
  • -mfix-vr4130 — the VR4130 fix.
  • -mno-fix-vr4130 — skip it.
  • -mfix-sb1 — the SB-1 fix.
  • -mno-fix-sb1 — skip it.
  • -mflush-func=func — flush function: func.
  • -mno-flush-func — no flush function.
  • -mbranch-cost=num — branch cost: num.
  • -mbranch-likely — branch likely.
  • -mno-branch-likely — branch unlikely, honestly.
  • -mcompact-branches=policy — compact branches, policy.
  • -mfp-exceptions — FP exceptions on.
  • -mno-fp-exceptions — FP exceptions off.
  • -mvr4130-align — VR4130 align.
  • -mno-vr4130-align — no VR4130 align.
  • -msynci — synci on.
  • -mno-synci — synci off.
  • -mlxc1-sxc1 — LXC1/SXC1 on.
  • -mno-lxc1-sxc1 — off.
  • -mmadd4 — MADD4 on.
  • -mno-madd4 — MADD4 off.
  • -mrelax-pic-calls — relax PIC calls.
  • -mno-relax-pic-calls — don't relax them.
  • -mmcount-ra-address — count the RA address.
  • -mframe-header-opt — frame header optimization.
  • -mno-frame-header-opt — no frame header optimization.

🧚 MMIX Options 🧚

  • -mlibfuncs — libfuncs on.
  • -mno-libfuncs — libfuncs off.
  • -mepsilon — epsilon mode.
  • -mno-epsilon — no epsilon.
  • -mabi=gnu — the GNU ABI.
  • -mabi=mmixware — the MMIXware ABI.
  • -mzero-extend — zero extend.
  • -mknuthdiv — Knuth division, we stan the algorithm king.
  • -mtoplevel-symbols — toplevel symbols.
  • -melf — ELF output.
  • -mbranch-predict — branch prediction on.
  • -mno-branch-predict — off, we guess.
  • -mbase-addresses — base addresses.
  • -mno-base-addresses — no base addresses.
  • -msingle-exit — single exit.
  • -mno-single-exit — multiple exits, chaotic.

🌸 MN10300 Options 🌸

  • -mmult-bug — work around the multiply bug.
  • -mno-mult-bug — don't.
  • -mno-am33 — not AM33.
  • -mam33 — AM33.
  • -mam33-2 — AM33-2.
  • -mam34 — AM34.
  • -mtune=cpu-type — tune for cpu-type.
  • -mreturn-pointer-on-d0 — return pointer on d0.
  • -mno-crt0 — no crt0.
  • -mrelax — relax.
  • -mliw — LIW on.
  • -msetlb — setlb on.

🎀 Moxie Options 🎀

short and sweet, love that for us:

  • -meb — big endian.
  • -mel — little endian.
  • -mmul.x — mul.x, yes with the dot.
  • -mno-crt0 — no crt0.

💫 MSP430 Options 💫

  • -msim — simulator mode.
  • -masm-hex — hex in the assembly.
  • -mmcu= — MCU, you fill in the blank.
  • -mcpu= — CPU, also a blank.
  • -mlarge — large memory model.
  • -msmall — small memory model.
  • -mrelax — relax.
  • -mwarn-mcu — warn about the MCU.
  • -mcode-region= — code region goes here.
  • -mdata-region= — data region goes here.
  • -msilicon-errata= — silicon errata, name your poison.
  • -msilicon-errata-warn= — warn about that errata.
  • -mhwmult= — hardware multiply mode.
  • -minrt — inrt mode.
  • -mtiny-printf — tiny printf, she's small but mighty.
  • -mmax-inline-shift= — max inline shift value.

🦋 NDS32 Options 🦋

  • -mbig-endian — big endian.
  • -mlittle-endian — little endian.
  • -mreduced-regs — reduced registers.
  • -mfull-regs — full registers.
  • -mcmov — conditional moves on.
  • -mno-cmov — off.
  • -mext-perf — perf extension on.
  • -mno-ext-perf — off.
  • -mext-perf2 — perf2 extension on.
  • -mno-ext-perf2 — off.
  • -mext-string — string extension on.
  • -mno-ext-string — off.
  • -mv3push — v3 push on.
  • -mno-v3push — off.
  • -m16bit — 16-bit mode.
  • -mno-16bit — not 16-bit.
  • -misr-vector-size=num — ISR vector size: num.
  • -mcache-block-size=num — cache block size: num.
  • -march=arch — architecture: arch.
  • -mcmodel=code-model — code model: code-model.
  • -mctor-dtor — constructors and destructors.
  • -mrelax — relax.

⭐ Nvidia PTX Options ⭐

  • -m64 — 64-bit.
  • -mmainkernel — main kernel.
  • -moptimize — optimize, bestie.

🌈 OpenRISC Options 🌈

  • -mboard=name — board: name.
  • -mnewlib — newlib.
  • -mhard-mul — hardware multiply.
  • -mhard-div — hardware divide.
  • -msoft-mul — software multiply.
  • -msoft-div — software divide.
  • -msoft-float — software float.
  • -mhard-float — hardware float.
  • -mdouble-float — double float.
  • -munordered-float — unordered float.
  • -mcmov — conditional moves.
  • -mror — rotate right.
  • -mrori — rotate right immediate.
  • -msext — sign extend.
  • -msfimm — sfimm.
  • -mshftimm — shift immediate.
  • -mcmodel=code-model — code model: code-model.

🫶 PDP-11 Options 🫶

retro icon behaviour only:

  • -mfpu — FPU.
  • -msoft-float — soft float.
  • -mac0 — AC0 on.
  • -mno-ac0 — AC0 off.
  • -m40 — the 40.
  • -m45 — the 45.
  • -m10 — the 10.
  • -mint32 — 32-bit ints.
  • -mno-int16 — not 16-bit ints.
  • -mint16 — 16-bit ints.
  • -mno-int32 — not 32-bit ints.
  • -msplit — split mode.
  • -munix-asm — UNIX assembler syntax.
  • -mdec-asm — DEC assembler syntax.
  • -mgnu-asm — GNU assembler syntax.
  • -mlra — LRA.

💜 PowerPC Options 💜

literally just a redirect, bestie — See RS/6000 and PowerPC Options.

🔥 PRU Options 🔥

  • -mmcu=mcu — MCU: mcu.
  • -minrt — inrt mode.
  • -mno-relax — no relaxing allowed.
  • -mloop — loop mode.
  • -mabi=variant — ABI variant: variant.

👑 RISC-V Options 👑

RISC-V girlies rise, this list ate:

  • -mbranch-cost=N-instruction — branch cost, measured in N-instruction.
  • -mplt — PLT on.
  • -mno-plt — PLT off.
  • -mabi=ABI-string — ABI: ABI-string.
  • -mfdiv — F division on.
  • -mno-fdiv — F division off.
  • -mfence-tso — fence TSO on.
  • -mno-fence-tso — off.
  • -mdiv — division on.
  • -mno-div — division off.
  • -misa-spec=ISA-spec-string — ISA spec: ISA-spec-string.
  • -march=ISA-string — ISA: ISA-string.
  • -mtune=processor-string — tune for processor-string.
  • -mpreferred-stack-boundary=num — preferred stack boundary: num.
  • -msmall-data-limit=N-bytes — small data limit, N-bytes.
  • -msave-restore — save/restore on.
  • -mno-save-restore — off.
  • -mshorten-memrefs — shorten memory references.
  • -mno-shorten-memrefs — don't shorten them.
  • -mstrict-align — strict alignment.
  • -mno-strict-align — no strict alignment.
  • -mcmodel=medlow — medlow code model.
  • -mcmodel=medany — medany code model.
  • -mcmodel=large — large code model.
  • -mexplicit-relocs — explicit relocs.
  • -mno-explicit-relocs — not explicit.
  • -mrelax — relax.
  • -mno-relax — don't relax.
  • -mriscv-attribute — RISC-V attribute on.
  • -mno-riscv-attribute — off.
  • -malign-data=type — align data: type.
  • -mbig-endian — big endian.
  • -mlittle-endian — little endian.
  • -mstack-protector-guard=guard — stack protector guard: guard.
  • -mstack-protector-guard-reg=reg — guard register: reg.
  • -mstack-protector-guard-offset=offset — guard offset: offset.
  • -mcsr-check — CSR check on.
  • -mno-csr-check — off.
  • -mmovcc — movcc on.
  • -mno-movcc — off.
  • -minline-atomics — inline atomics on.
  • -mno-inline-atomics — off.
  • -minline-strlen — inline strlen.
  • -mno-inline-strlen — don't inline it.
  • -minline-strcmp — inline strcmp.
  • -mno-inline-strcmp — don't.
  • -minline-strncmp — inline strncmp.
  • -mno-inline-strncmp — don't.
  • -mtls-dialect=desc — TLS dialect, desc.
  • -mtls-dialect=trad — TLS dialect, trad.

🍓 RL78 Options 🍓

  • -msim — simulator mode.
  • -mmul=none — no multiply unit.
  • -mmul=g13 — the g13 multiply.
  • -mmul=g14 — the g14 multiply.
  • -mallregs — all the registers.
  • -mcpu=g10 — CPU g10.
  • -mcpu=g13 — CPU g13.
  • -mcpu=g14 — CPU g14.
  • -mg10 — g10.
  • -mg13 — g13.
  • -mg14 — g14.
  • -m64bit-doubles — 64-bit doubles.
  • -m32bit-doubles — 32-bit doubles.
  • -msave-mduc-in-interrupts — save MDUC during interrupts.

🧚 RS/6000 and PowerPC Options 🧚

the big one. deep breath, bestie, we're getting through all of it:

  • -mcpu=cpu-type — CPU: cpu-type.
  • -mtune=cpu-type — tune for cpu-type.
  • -mcmodel=code-model — code model: code-model.
  • -mpowerpc64 — PowerPC64.
  • -maltivec — AltiVec on.
  • -mno-altivec — AltiVec off.
  • -mpowerpc-gpopt — PowerPC GPOPT on.
  • -mno-powerpc-gpopt — off.
  • -mpowerpc-gfxopt — PowerPC GFXOPT on.
  • -mno-powerpc-gfxopt — off.
  • -mmfcrf — MFCRF on.
  • -mno-mfcrf — off.
  • -mpopcntb — popcntb on.
  • -mno-popcntb — off.
  • -mpopcntd — popcntd on.
  • -mno-popcntd — off.
  • -mfprnd — fprnd on.
  • -mno-fprnd — off.
  • -mcmpb — cmpb on.
  • -mno-cmpb — off.
  • -mhard-dfp — hardware DFP on.
  • -mno-hard-dfp — off.
  • -mfull-toc — full TOC.
  • -mminimal-toc — minimal TOC.
  • -mno-fp-in-toc — no FP in the TOC.
  • -mno-sum-in-toc — no sum in the TOC.
  • -m64 — 64-bit.
  • -m32 — 32-bit.
  • -mxl-compat — XL compatibility on.
  • -mno-xl-compat — off.
  • -mpe — PE mode.
  • -malign-power — Power alignment.
  • -malign-natural — natural alignment.
  • -msoft-float — soft float.
  • -mhard-float — hard float.
  • -mmultiple — multiple on.
  • -mno-multiple — multiple off.
  • -mupdate — update on.
  • -mno-update — update off.
  • -mavoid-indexed-addresses — avoid indexed addresses.
  • -mno-avoid-indexed-addresses — don't avoid them.
  • -mfused-madd — fused madd.
  • -mno-fused-madd — unfused madd.
  • -mbit-align — bit alignment.
  • -mno-bit-align — none.
  • -mstrict-align — strict alignment.
  • -mno-strict-align — lax alignment.
  • -mrelocatable — relocatable.
  • -mno-relocatable — not relocatable.
  • -mrelocatable-lib — relocatable lib.
  • -mno-relocatable-lib — not that either.
  • -mtoc — TOC on.
  • -mno-toc — TOC off.
  • -mlittle — little.
  • -mlittle-endian — little endian.
  • -mbig — big.
  • -mbig-endian — big endian.
  • -mdynamic-no-pic — dynamic no-PIC.
  • -mswdiv — software divide.
  • -msingle-pic-base — single PIC base.
  • -mprioritize-restricted-insns=priority — prioritize restricted insns, priority.
  • -msched-costly-dep=dependence_type — scheduling costly dependency, dependence_type.
  • -minsert-sched-nops=scheme — insert scheduling nops, scheme.
  • -mcall-aixdesc — call AIX descriptor.
  • -mcall-eabi — call EABI.
  • -mcall-freebsd — call FreeBSD.
  • -mcall-linux — call Linux.
  • -mcall-netbsd — call NetBSD.
  • -mcall-openbsd — call OpenBSD.
  • -mcall-sysv — call SysV.
  • -mcall-sysv-eabi — call SysV EABI.
  • -mcall-sysv-noeabi — call SysV, no EABI.
  • -mtraceback=traceback_type — traceback: traceback_type.
  • -maix-struct-return — AIX struct return.
  • -msvr4-struct-return — SVR4 struct return.
  • -mabi=abi-type — ABI: abi-type.
  • -msecure-plt — secure PLT.
  • -mbss-plt — BSS PLT.
  • -msplit-patch-nops — split patch nops.
  • -mlongcall — long calls.
  • -mno-longcall — no long calls.
  • -mpltseq — PLT sequence.
  • -mno-pltseq — no PLT sequence.
  • -mblock-move-inline-limit=num — block move inline limit: num.
  • -mblock-compare-inline-limit=num — block compare inline limit: num.
  • -mblock-compare-inline-loop-limit=num — block compare inline loop limit: num.
  • -mno-block-ops-unaligned-vsx — no unaligned VSX block ops.
  • -mstring-compare-inline-limit=num — string compare inline limit: num.
  • -misel — ISEL on.
  • -mno-isel — ISEL off.
  • -mvrsave — VRSAVE on.
  • -mno-vrsave — VRSAVE off.
  • -mmulhw — mulhw on.
  • -mno-mulhw — mulhw off.
  • -mdlmzb — dlmzb on.
  • -mno-dlmzb — dlmzb off.
  • -mprototype — prototype on.
  • -mno-prototype — prototype off.
  • -msim — simulator mode.
  • -mmvme — MVME.
  • -mads — ADS.
  • -myellowknife — Yellowknife, iconic name honestly.
  • -memb — EMB.
  • -msdata — small data.
  • -msdata=opt — small data, opt.
  • -mreadonly-in-sdata — readonly stuff in small data.
  • -mvxworks — VxWorks.
  • -G num — the -G knob, num.
  • -mrecip — reciprocal.
  • -mrecip=opt — reciprocal with opt.
  • -mno-recip — no reciprocal.
  • -mrecip-precision — reciprocal precision.
  • -mno-recip-precision — no reciprocal precision.
  • -mveclibabi=type — vector library ABI: type.
  • -mfriz — friz on.
  • -mno-friz — friz off.
  • -mpointers-to-nested-functions — pointers to nested functions.
  • -mno-pointers-to-nested-functions — no pointers to nested functions.
  • -msave-toc-indirect — save TOC indirect.
  • -mno-save-toc-indirect — don't.
  • -mpower8-fusion — POWER8 fusion.
  • -mno-mpower8-fusion — the off-switch, spelled -mno-mpower8-fusion exactly. yes really.
  • -mcrypto — crypto on.
  • -mno-crypto — crypto off.
  • -mhtm — HTM on.
  • -mno-htm — HTM off.
  • -mquad-memory — quad memory.
  • -mno-quad-memory — no quad memory.
  • -mquad-memory-atomic — quad memory atomic.
  • -mno-quad-memory-atomic — no quad memory atomic.
  • -mcompat-align-parm — compatibility parameter alignment.
  • -mno-compat-align-parm — no compat alignment.
  • -mfloat128 — float128 on.
  • -mno-float128 — float128 off.
  • -mfloat128-hardware — float128 in hardware.
  • -mno-float128-hardware — not in hardware.
  • -mgnu-attribute — GNU attribute.
  • -mno-gnu-attribute — no GNU attribute.
  • -mstack-protector-guard=guard — stack protector guard: guard.
  • -mstack-protector-guard-reg=reg — guard register: reg.
  • -mstack-protector-guard-offset=offset — guard offset: offset.
  • -mprefixed — prefixed instructions on.
  • -mno-prefixed — off.
  • -mpcrel — PC-relative on.
  • -mno-pcrel — off.
  • -mmma — MMA on.
  • -mno-mmma — MMA off, and yes it's spelled -mno-mmma with the double m, keep it exactly like that 💅
  • -mrop-protect — ROP protection on.
  • -mno-rop-protect — off.
  • -mprivileged — privileged mode on.
  • -mno-privileged — off.

RX Options (RX Options) 🦋

the RX crew said "we have options" and honestly they did not lie. whole squad below, bestie:

-m64bit-doubles  -m32bit-doubles  -fpu  -nofpu  -mcpu=  -mbig-endian-data  -mlittle-endian-data  -msmall-data  -msim  -mno-sim  -mas100-syntax  -mno-as100-syntax  -mrelax  -mmax-constant-size=  -mint-register=  -mpid  -mallow-string-insns  -mno-allow-string-insns  -mjsr  -mno-warn-multiple-fast-interrupts  -msave-acc-in-interrupts

S/390 and zSeries Options (S/390 and zSeries Options) 💜

big iron energy, no cap. these are the knobs:

-mtune=cpu-type  -march=cpu-type  -mhard-float  -msoft-float  -mhard-dfp  -mno-hard-dfp  -mlong-double-64  -mlong-double-128  -mbackchain  -mno-backchain  -mpacked-stack  -mno-packed-stack  -msmall-exec  -mno-small-exec  -mmvcle  -mno-mvcle  -m64  -m31  -mdebug  -mno-debug  -mesa  -mzarch  -mhtm  -mvx  -mzvector  -mtpf-trace  -mno-tpf-trace  -mtpf-trace-skip  -mno-tpf-trace-skip  -mfused-madd  -mno-fused-madd  -mwarn-framesize  -mwarn-dynamicstack  -mstack-size  -mstack-guard  -mhotpatch=halfwords,halfwords

SH Options (SH Options) 🌸

the SuperH section is giving "we contain multitudes." every flag, untouched:

-m1  -m2  -m2e  -m2a-nofpu  -m2a-single-only  -m2a-single  -m2a  -m3  -m3e  -m4-nofpu  -m4-single-only  -m4-single  -m4  -m4a-nofpu  -m4a-single-only  -m4a-single  -m4a  -m4al  -mb  -ml  -mdalign  -mrelax  -mbigtable  -mfmovd  -mrenesas  -mno-renesas  -mnomacsave  -mieee  -mno-ieee  -mbitops  -misize  -minline-ic_invalidate  -mpadstruct  -mprefergot  -musermode  -multcost=number  -mdiv=strategy  -mdivsi3_libfunc=name  -mfixed-range=register-range  -maccumulate-outgoing-args  -matomic-model=atomic-model  -mbranch-cost=num  -mzdcbranch  -mno-zdcbranch  -mcbranch-force-delay-slot  -mfused-madd  -mno-fused-madd  -mfsca  -mno-fsca  -mfsrra  -mno-fsrra  -mpretend-cmove  -mtas

Solaris 2 Options (Solaris 2 Options) ☀️

short and sweet, like a good sunscreen:

-mclear-hwcap  -mno-clear-hwcap  -mimpure-text  -mno-impure-text  -pthreads

SPARC Options (SPARC Options) 👑

SPARC walked so a lot of y'all could run. the full list, verbatim:

-mcpu=cpu-type  -mtune=cpu-type  -mcmodel=code-model  -mmemory-model=mem-model  -m32  -m64  -mapp-regs  -mno-app-regs  -mfaster-structs  -mno-faster-structs  -mflat  -mno-flat  -mfpu  -mno-fpu  -mhard-float  -msoft-float  -mhard-quad-float  -msoft-quad-float  -mstack-bias  -mno-stack-bias  -mstd-struct-return  -mno-std-struct-return  -munaligned-doubles  -mno-unaligned-doubles  -muser-mode  -mno-user-mode  -mv8plus  -mno-v8plus  -mvis  -mno-vis  -mvis2  -mno-vis2  -mvis3  -mno-vis3  -mvis3b  -mno-vis3b  -mvis4  -mno-vis4  -mvis4b  -mno-vis4b  -mcbcond  -mno-cbcond  -mfmaf  -mno-fmaf  -mfsmuld  -mno-fsmuld  -mpopc  -mno-popc  -msubxc  -mno-subxc  -mfix-at697f  -mfix-ut699  -mfix-ut700  -mfix-gr712rc

System V Options (System V Options) 🎀

tiny icons, huge legacy:

-Qy  -Qn  -YP,paths  -Ym,dir

V850 Options (V850 Options) 🦋

the V850 menu, served fresh:

-mlong-calls  -mno-long-calls  -mep  -mno-ep  -mprolog-function  -mno-prolog-function  -mspace  -mtda=n  -msda=n  -mzda=n  -mapp-regs  -mno-app-regs  -mdisable-callt  -mno-disable-callt  -mv850e2v3  -mv850e2  -mv850e1  -mv850es  -mv850e  -mv850  -mv850e3v5  -mloop  -mrelax  -mlong-jumps  -msoft-float  -mhard-float  -mgcc-abi  -mrh850-abi  -mbig-switch

VAX Options (VAX Options) 💀

vintage, iconic, slightly haunted:

-munix  -mgnu  -md  -md-float  -mg  -mg-float  -mlra

Visium Options (Visium Options) 🌈

not me forgetting Visium existed until this exact moment. anyway:

-mdebug  -msim  -mfpu  -mno-fpu  -mhard-float  -msoft-float  -mcpu=cpu-type  -mtune=cpu-type  -msv-mode  -muser-mode

VMS Options (VMS Options) 🍓

the VMS section is short but it's got lore:

-mvms-return-codes  -mdebug-main=prefix  -mmalloc64  -mpointer-size=size

VxWorks Options (VxWorks Options) 🧚

embedded realness:

-mrtp  -msmp  -non-static  -Bstatic  -Bdynamic  -Xbind-lazy  -Xbind-now

x86 Options (x86 Options) 🔥

okay this is THE mega section. it's giving "main character with 200 outfit changes." i'm not gonna editorialize every single one because we'd be here till the heat death of the universe, but every flag is here, exactly as printed:

-mtune=cpu-type  -march=cpu-type  -mtune-ctrl=feature-list  -mdump-tune-features  -mno-default  -mfpmath=unit  -masm=dialect  -mno-fancy-math-387  -mno-fp-ret-in-387  -m80387  -mhard-float  -msoft-float  -mno-wide-multiply  -mrtd  -malign-double  -mpreferred-stack-boundary=num  -mincoming-stack-boundary=num  -mcld  -mcx16  -msahf  -mmovbe  -mcrc32  -mmwait  -mrecip  -mrecip=opt  -mvzeroupper  -mprefer-avx128  -mprefer-vector-width=opt  -mpartial-vector-fp-math  -mmove-max=bits  -mstore-max=bits  -mnoreturn-no-callee-saved-registers  -mmmx  -msse  -msse2  -msse3  -mssse3  -msse4.1  -msse4.2  -msse4  -mavx  -mavx2  -mavx512f  -mavx512cd  -mavx512vl  -mavx512bw  -mavx512dq  -mavx512ifma  -mavx512vbmi  -msha  -maes  -mpclmul  -mfsgsbase  -mrdrnd  -mf16c  -mfma  -mpconfig  -mwbnoinvd  -mptwrite  -mclflushopt  -mclwb  -mxsavec  -mxsaves  -msse4a  -m3dnow  -m3dnowa  -mpopcnt  -mabm  -mbmi  -mtbm  -mfma4  -mxop  -madx  -mlzcnt  -mbmi2  -mfxsr  -mxsave  -mxsaveopt  -mrtm  -mhle  -mlwp  -mmwaitx  -mclzero  -mpku  -mthreads  -mgfni  -mvaes  -mwaitpkg  -mshstk  -mmanual-endbr  -mcet-switch  -mforce-indirect-call  -mavx512vbmi2  -mavx512bf16  -menqcmd  -mvpclmulqdq  -mavx512bitalg  -mmovdiri  -mmovdir64b  -mavx512vpopcntdq  -mavx512vnni  -mprfchw  -mrdpid  -mrdseed  -msgx  -mavx512vp2intersect  -mserialize  -mtsxldtrk  -mamx-tile  -mamx-int8  -mamx-bf16  -muintr  -mhreset  -mavxvnni  -mamx-fp8  -mavx512fp16  -mavxifma  -mavxvnniint8  -mavxneconvert  -mcmpccxadd  -mamx-fp16  -mprefetchi  -mraoint  -mamx-complex  -mavxvnniint16  -msm3  -msha512  -msm4  -mapxf  -musermsr  -mavx10.1  -mavx10.1-256  -mavx10.1-512  -mevex512  -mavx10.2  -mamx-avx512  -mamx-tf32  -mamx-transpose  -mmovrs  -mamx-movrs  -mcldemote  -mms-bitfields  -mno-align-stringops  -minline-all-stringops  -minline-stringops-dynamically  -mstringop-strategy=alg  -mkl  -mwidekl  -mmemcpy-strategy=strategy  -mmemset-strategy=strategy  -mpush-args  -maccumulate-outgoing-args  -m128bit-long-double  -m96bit-long-double  -mlong-double-64  -mlong-double-80  -mlong-double-128  -mregparm=num  -msseregparm  -mveclibabi=type  -mvect8-ret-in-mem  -mpc32  -mpc64  -mpc80  -mdaz-ftz  -mstackrealign  -momit-leaf-frame-pointer  -mno-red-zone  -mno-tls-direct-seg-refs  -mcmodel=code-model  -mabi=name  -maddress-mode=mode  -m32  -m64  -mx32  -m16  -miamcu  -mlarge-data-threshold=num  -msse2avx  -mfentry  -mrecord-mcount  -mnop-mcount  -m8bit-idiv  -minstrument-return=type  -mfentry-name=name  -mfentry-section=name  -mavx256-split-unaligned-load  -mavx256-split-unaligned-store  -malign-data=type  -mstack-protector-guard=guard  -mstack-protector-guard-reg=reg  -mstack-protector-guard-offset=offset  -mstack-protector-guard-symbol=symbol  -mgeneral-regs-only  -mcall-ms2sysv-xlogues  -mrelax-cmpxchg-loop  -mindirect-branch=choice  -mfunction-return=choice  -mindirect-branch-register  -mharden-sls=choice  -mindirect-branch-cs-prefix  -mneeded  -mno-direct-extern-access  -munroll-only-small-loops  -mlam=choice

x86 Windows Options

not its own list, bestie — it just points you at Cygwin and MinGW Options. see ya over there 🫶

Xstormy16 Options (Xstormy16 Options) ⭐

one flag. one vibe. immaculate minimalism:

-msim

Xtensa Options (Xtensa Options) 🦋

Xtensa said "let's make it customizable" and then did exactly that:

-mconst16  -mno-const16  -mfused-madd  -mno-fused-madd  -mforce-no-pic  -mserialize-volatile  -mno-serialize-volatile  -mtext-section-literals  -mno-text-section-literals  -mauto-litpools  -mno-auto-litpools  -mtarget-align  -mno-target-align  -mlongcalls  -mno-longcalls  -mabi=abi-type  -mextra-l32r-costs=cycles  -mstrict-align  -mno-strict-align

zSeries Options (zSeries Options) 💜

also not its own list — it's a redirect to S/390 and zSeries Options above. we love a cross-reference that keeps the family together ✨

Options Controlling the Kind of Output ✨

okay so compiling is not one single glow-up moment — it's a whole four-stage runway walk, and the order is locked in: preprocessing, compilation proper, assembly, and linking. always in that order, no skipping, no reshuffling. 💅

GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.

for any given input file, the file name suffix determines what kind of compilation is done — the suffix is literally the vibe check for your file:

  • file.c — C source code that must be preprocessed.
  • file.i — C source code that should not be preprocessed.
  • file.ii — C++ source code that should not be preprocessed.
  • file.m — Objective-C source code. note that you must link with the libobjc library to make an Objective-C program work.
  • file.mi — Objective-C source code that should not be preprocessed.
  • file.mm, file.M — Objective-C++ source code. note that you must link with the libobjc library to make an Objective-C++ program work. also note that .M refers to a literal capital M. don't get cute with it 🙈
  • file.mii — Objective-C++ source code that should not be preprocessed.
  • file.h — C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
  • file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. in .cxx, the last two letters must both be literally x. likewise, .C refers to a literal capital C. casing matters, bestie, it's not decorative 💅
  • file.mm, file.M — Objective-C++ source code that must be preprocessed. (yes, this pair already showed up above — the manpage said it twice, and we respect the author's choices 🫶)
  • file.mii — Objective-C++ source code that should not be preprocessed. (also round two, iconic)
  • file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec.
  • file.f, file.for, file.ftn, file.fi — fixed form Fortran source code that should not be preprocessed.
  • file.F, file.FOR, file.fpp, file.FPP, file.FTN — fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
  • file.f90, file.f95, file.f03, file.f08, file.fii — free form Fortran source code that should not be preprocessed.
  • file.F90, file.F95, file.F03, file.F08 — free form Fortran source code that must be preprocessed (with the traditional preprocessor).
  • file.cob, file.COB, file.cbl, file.CBL — COBOL source code.
  • file.go — Go source code.
  • file.d — D source code.
  • file.di — D interface file.
  • file.dd — D documentation code (Ddoc).
  • file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). such files are also called specs.
  • file.adb — Ada source code file containing a library unit body (a subprogram or package body). such files are also called bodies.
  • file.s — Assembler code.
  • file.S, file.sx — Assembler code that must be preprocessed.
  • other — an object file to be fed straight into linking. any file name with no recognized suffix is treated this way.
ngl if your file has no recognized suffix, GCC just shrugs and assumes it's an object file for linking. no drama, no error, just vibes and a linker ✨

okay bestie so like if you wanna be explicit about what language your files are in, you use the -x option and it's literally so straightforward:

  • -x language — specify explicitly the language for the following input files (instead of letting the compiler pick a default based on the file name suffix). This option applies to all following input files until the next -x option. The possible values for language are:
c  c-header  cpp-output
c++  c++-header  c++-system-header c++-user-header c++-cpp-output
objective-c  objective-c-header  objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler  assembler-with-cpp
ada
cobol
d
f77  f77-cpp-input f95  f95-cpp-input
go
ngl tho, -x does not imply a particular language standard. For example -x f77 may also require -std=legacy for some older source codes. 💅
  • -x none — turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as if -x has not been used at all).

If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all. 😭

  • -c — compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.

By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are ignored.

  • -S — stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

Input files that don't require compilation are ignored.

  • -E — stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

  • -o file — place the primary output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.

If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.

Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash.

gcc -c foo.c ...

will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.

If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:

gcc -c foo.c -o dir/foobar.o ...

will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.

A linker output will instead prefix aux and dump outputs:

gcc foo.c bar.c -o dir/foobar ...

will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.

The one exception to the above is when the executable shares the base name with the single input:

gcc foo.c -o dir/foo ...

in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.

The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj.

  • -dumpbase dumpbase — this option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs:
gcc -save-temps -S foo.c

saves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:

gcc -save-temps -dumpbase save-foo -c foo.c

preprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.

Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. "-save-temps", "-gsplit-dwarf", "-fcallgraph-info") without it.

Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.

If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:

gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
  -dumpdir pfx- -save-temps=cwd ...

creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.

When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:

gcc foo.c bar.c -c -dumpbase main ...

creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.

An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values:

gcc -c foo.c -o dir/foobar.o -dumpbase " ...

will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.

The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:

gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...

The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.

  • -dumpbase-ext auxdropsuf — when forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase, i.e., the suffix of the input file when -dumpbase is not present in the command line, or dumpbase is combined with dumppfx.
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...

creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.

This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:

gcc foo.c bar.c -o main.out -dumpbase-ext .out ...

creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.

  • -dumpdir dumppfx — when forming the name of an auxiliary or dump output file, use dumppfx as a prefix:
gcc -dumpdir pfx- -c foo.c ...

creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.

If dumppfx is to be used as a directory name, it must end with a directory separator:

gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...

creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.

It defaults to the location of the output file, unless the output file is a special file like "/dev/null". Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:

gcc -dumpdir pfx- -c foo.c -save-temps=obj ...

…outputs foo.o, with auxiliary outputs named foo.* — and that's because -save-temps=* overrides the dumppfx that the earlier -dumpdir option handed over. 💅 Ngl, it doesn't matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.

When you're compiling from multiple input files, if -dumpbase is specified, dumpbase — minus an auxdropsuf suffix — and a dash get appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so each of the multiple compilations gets differently-named aux and dump outputs. ✨

gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...

That bad boy outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. 🎀 Now contrast that with the single-input compilation:

gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...

which, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.

When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name — minus auxdropsuf, if specified, or the executable suffix otherwise — plus a dash is appended to the default dumppfx instead. Note, however, that unlike earlier cases of linking:

gcc foo.c bar.c -dumpdir dir/pfx- -o main ...

does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful option. 👑

When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination, even if overridden by -save-temps=*:

gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...

Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd. 🌸

Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless overridden by -dumpbase. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained. 🫶

When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase — as given or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such — is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them.

When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers. 🌀

  • -v — prints (on standard error output) the commands executed to run the stages of compilation. Also prints the version number of the compiler driver program and of the preprocessor and the compiler proper. Basically the receipts for everything GCC is doing backstage. 🎬
  • -### — like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or ./-_. This is useful for shell scripts to capture the driver-generated command lines. No cap, it's giving "show me the plan but don't run it." 💀
  • --help — prints (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. And if the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. Extra credit behavior fr fr. 📚
  • --target-help — prints (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. ✨
  • --help={class|[^]qualifier}[,...] — prints (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. Iconic filtering energy. 🎀

These are the supported classes:

  • optimizers — display all of the optimization options supported by the compiler.
  • warnings — display all of the options controlling warning messages produced by the compiler.
  • target — display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax.
  • params — display the values recognized by the --param option.
  • language — display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to select common class.
  • common — display the options that are common to all languages.

These are the supported qualifiers:

  • undocumented — display only those options that are undocumented.
  • joined — display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.
  • separate — display options taking an argument that appears as a separate word following the original option, such as: -o output-file.

Thus for example to display all the undocumented target-specific switches supported by the compiler, use:

--help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

--help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers. 🚫

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

--help=target,optimizers

The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. Precedence drama, but make it compiler. 💅

If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used). 👀

Here is a truncated example from the ARM port of gcc:

% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi=                                2
-mabort-on-noreturn                   [disabled]
-mapcs                                [disabled]

The output is sensitive to the effects of previous command-line options, so for example it is possible to find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Alternatively you can discover which binary optimizations are enabled by -O3 by using:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
  • --version — displays the version number and copyrights of the invoked GCC. The classic "who even are you" check. 🪞
  • -pass-exit-codes — normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered. So like, it picks the biggest red flag. 🚩
  • -pipe — use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. Lowkey the glow-up for your build speed when it works. 🚀
  • -specs=file — process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right. Order matters, bestie — don't come for me. 📋
  • -wrapper — invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... Debugging era, honestly iconic. 🐛✨

  • -ffile-prefix-map=old=new — okay so when you're compiling files that live in directory old, this records every reference to them in the compilation output as if they lived in directory new instead. It's literally equivalent to specifying all the individual -f*-prefix-map options at once. Bestie, this is the reproducible-builds slay: location-independent builds, no cap. ⚠️ Directories referenced by directives are not affected by these options. See also -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.
  • -fcanon-prefix-map — so normally for the -f*-prefix-map options, the comparison of the old prefix against the filename that would be referenced in the compilation result is done with textual comparison of the prefixes — or, on case-insensitive filesystems, ignoring character case, and on DOS-based filesystems, treating slashes and backslashes as equal. -fcanon-prefix-map makes those comparisons happen on canonicalized paths of old and the referenced filename instead. Canonicalized. Periodt. 💅
  • -fplugin=name.so — loads the plugin code in file name.so, assumed to be a shared object that gets dlopen'd by the compiler. The base name of the shared object file is what identifies the plugin for argument parsing (peep -fplugin-arg-name-key=value below). Every plugin should define the callback functions specified in the Plugins API. 🧚
  • -fplugin-arg-name-key=value — defines an argument called key with a value of value for the plugin called name. That's it, that's the whole vibe.
  • -fdump-ada-spec[-slim] — for C and C++ source and include files, generates corresponding Ada specs. Iconic crossover episode. ✨
  • -fada-spec-parent=unit — in conjunction with -fdump-ada-spec[-slim] above, generates Ada specs as child units of parent unit.
  • -fdump-go-spec=file — for input files in any language, generates corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations, which lowkey is a useful way to start writing a Go interface to code written in some other language. 🌈
  • @file — reads command-line options from file. The options read get inserted in place of the original @file option. And if file doesn't exist, or can't be read? Then the option is treated literally and is not removed. 🙈

Options in file are separated by whitespace. A whitespace character can be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) can be included by prefixing the character to be included with a backslash. And the file may itself contain additional @file options — any such options get processed recursively. 🌀

🦋 Compiling C++ Programs 🦋

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx — bestie, memorize those. C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc. Preprocessed C++ files use the suffix .ii. And C++20 module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm. ✨

GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).

⚠️ But here's the tea: the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++. 💅

When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language — or command-line options meaningful for C and related languages — or options that are meaningful only for C++ programs. Choose your fighter. 🫶

🌸 Options Controlling C Dialect 🌸

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts. Vibe check on your standard, basically:

  • -ansi — in C mode, this is equivalent to -std=c90. In C++ mode, it's equivalent to -std=c++98. Two moods, one flag.
  • -std= — determines the language standard. This option is currently only supported when compiling C or C++. ✨

The compiler can accept several base standards, like c90 or c++98, and GNU dialects of those standards, like gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that don't contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords — but not other GNU extensions that have no meaning in ISO C90, like omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled — even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. 😭 The particular standard is what -Wpedantic uses to identify which features are GNU extensions given that version of the standard. For example, -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.

A value for this option must be provided; possible values are:

  • c90, c89, iso9899:1990 — support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
  • iso9899:199409 — ISO C90 as modified in amendment 1. Throwback era.
  • c99, c9x, iso9899:1999, iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/c99status.html> for more info. The names c9x and iso9899:199x are deprecated. 💀
  • c11, c1x, iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. Substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
  • c17, c18, iso9899:2017, iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is the same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of __STDC_VERSION__, and so is supported to the same extent as C11. Glow-up but make it maintenance release. 🌸
  • c23, c2x, iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated.
  • c2y — the next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 👀
  • gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features).
  • gnu99, gnu9x — GNU dialect of ISO C99. The name gnu9x is deprecated.
  • gnu11, gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated.
  • gnu17, gnu18 — GNU dialect of ISO C17.
  • gnu23, gnu2x — GNU dialect of ISO C23. This is the default for C code. The name gnu2x is deprecated.
  • gnu2y — the next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The name gnu2x is deprecated.
  • c++98, c++03 — the 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
  • gnu++98, gnu++03 — GNU dialect of -std=c++98.
  • c++11, c++0x — the 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
  • gnu++11, gnu++0x — GNU dialect of -std=c++11. The name gnu++0x is deprecated.
  • c++14, c++1y — the 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
  • gnu++14, gnu++1y — GNU dialect of -std=c++14. The name gnu++1y is deprecated.
  • c++17, c++1z — the 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
  • gnu++17, gnu++1z — GNU dialect of -std=c++17. This is the default for C++ code. The name gnu++1z is deprecated.
  • c++20, c++2a — the 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. The name c++2a is deprecated.
  • gnu++20, gnu++2a — GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2a is deprecated.
  • c++23, c++2b — the 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases. The name c++2b is deprecated.
  • gnu++23, gnu++2b — GNU dialect of -std=c++23. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2b is deprecated.
  • c++2c, c++26 — the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. Ma'am, do not ship this. 😭
  • gnu++2c, gnu++26 — GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. Same warning, same energy.
🫶 Friendly lil' reminder on those last ones: "highly experimental" and "will almost certainly change in incompatible ways" are not decorative. Believe them.
  • -aux-info filename — outputs to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C. Silently. It just minds its business. 💅

So like, besides declarations, this file is also gonna spill the tea in its comments about where every declaration came from — the source file and line, bestie ✨ And it tells you whether that declaration was implicit, prototyped or unprototyped (that's I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (that's C or F, respectively, in the following character). And when we're talking function definitions, you also get a K&R-style list of arguments followed by their declarations, tucked inside comments, after the declaration. iconic 💅

  • -fno-asm — okay so this one says do not recognize "asm", "inline" or "typeof" as keywords, so your code is free to use these words as identifiers 🫶 You can use the keywords "__asm__", "__inline__" and "__typeof__" instead. In C, -ansi implies -fno-asm.

In C++, "inline" is a standard keyword and is not affected by this switch. You may want to use the -fno-gnu-keywords flag instead, which disables "typeof" but not "asm" and "inline". In C99 mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C23. slay, the standards grew up 🌱

  • -fno-builtin, -fno-builtin-function — don't recognize built-in functions that do not begin with __builtin_ as prefix 🚫

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library 😭 In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory. the way it just knows things 🔥

With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

        #define abs(n)          __builtin_abs ((n))
        #define strcpy(d, s)    __builtin_strcpy ((d), (s))

literally the DIY builtin era 🎀

  • -fcond-mismatch — allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. 🫠 This option is not supported for C++ — sorry C++ girlies, not today.
  • -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at "main". The most obvious example is an OS kernel 👑 This is equivalent to -fno-hosted. no stdlib, no rules, main character energy fr fr.
  • -fgimple — enable parsing of function definitions marked with "__GIMPLE". This is an experimental feature that allows unit testing of GIMPLE passes 🧪 experimental, bestie — it's giving lab coat, not production.
  • -fgnu-tm — when the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009) 💖 This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. 🙈

For more information on GCC's support for transactional memory,

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions). 💀 just so you know.
  • -fgnu89-inline — the option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode ✨

Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.

The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode. 🚫

The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions. vibe check your inline semantics, iykyk 💅

  • -fhosted — assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly everything except a kernel 🌈 This is equivalent to -fno-freestanding. you do have a stdlib, congrats 🎀
  • -flax-vector-conversions — allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types.
This option should not be used for new code. 🚨 like, seriously — legacy vibes only.
  • -fms-extensions — accept some non-standard constructs used in Microsoft header files 🪟

In C++ code, this allows member names in structures to be similar to previous types declarations.

        typedef int UOW;
        struct ABC {
          UOW UOW;
        };

Some cases of unnamed fields in structures and unions are only accepted with this option.

Note that this option is off for all targets except for x86 targets using ms-abi. 🫶

  • -fpermitted-flt-eval-methods=style — okay ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that indicate that operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type ✨ These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of "FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written expecting the possibility of the new values 👀

-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.

style is either "c11" or "ts-18661-3" as appropriate.

The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3. defaults doing their own thing, classic 🦋

The -fdeps-* options are used to extract structured dependency information for a source 🧚 This involves determining what resources provided by other source files will be required to compile the source as well as what resources are provided by the source. This information can be used to add required dependencies between compilation rules of dependent sources based on their contents rather than requiring such information be reflected within the build tools as well. honestly kind of a flex for build systems 🔥

  • -fdeps-file=file — where to write structured dependency information 📝
  • -fdeps-format=format — the format to use for structured dependency information. p1689r5 is the only supported format right now. Note that when this argument is specified, the output of -MF is stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools 🫠 no cap, keep it readable.
  • -fdeps-target=file — analogous to -MT but for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources 🎯
  • -fplan9-extensions — accept some non-standard constructs used in Plan 9 code 🐇

This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++. sorry C++ besties 💀

  • -fsigned-bitfields, -funsigned-bitfields, -fno-signed-bitfields, -fno-unsigned-bitfields — these options control whether a bit-field is signed or unsigned, when the declaration does not use either "signed" or "unsigned" 🎚️ By default, such a bit-field is signed, because this is consistent: the basic integer types such as "int" are signed types. consistency ate, periodt 💅
  • -fsigned-char — let the type "char" be signed, like "signed char" ✍️

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. the negative forms are doing the most, love that for them 🌀

  • -funsigned-char — let the type "char" be unsigned, like "unsigned char" 🌸

Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default or like "signed char" by default.

Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the signedness of an object. But many programs have been written to use plain "char" and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for 😭 This option, and its inverse, let you make such a program work with the opposite default. legacy code rescue mission fr fr 🫶

The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its behavior is always just like one of those two. not the same type, just identical vibes ✨

  • -fstrict-flex-arrays (C and C++ only), -fstrict-flex-arrays=level (C and C++ only) — control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array. The value of level controls the level of strictness 🎀

-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest; a trailing array is treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards 👑

The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members 🌈 from zero to three, pick your vibe.

So there are, like, two more levels hiding between 0 and 3 — and they exist purely to support older code that leans on the GCC zero-length array extension ([0]) or a one-element array as a flexible array member ([1]... I mean [1] 🫶).

  • When level is 1, the trailing array counts as a flexible array member when it's declared as either [], [0], or [1].
  • When level is 2, the trailing array counts as a flexible array member when it's declared as either [] or [0].

You can also vibe-check one specific trailing array field of a structure by using the variable attribute strict_flex_array attribute. And heads up: the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. They're besties, they talk to each other 💅.

-fsso-struct=endianness

Sets the default scalar storage order of structures and unions to whatever endianness you name. The accepted values are big-endian, little-endian and native for the native endianness of the target (and native is the default, obvi). Big caveat: this option is not supported for C++. Sorry C++ girlies 😭.

⚠️ Warning, and I mean it: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it, if the endianness you picked isn't the native endianness of the target. That's a real "your structs will not match, do not pass go" situation 💀.

🎀 Options Controlling C++ Dialect 🎀

This section is all about the command-line options that are only meaningful for C++ programs. That said, you can still use most of the GNU compiler options no matter what language your program is in. Like, say you wanted to compile a file named firstClass.C — you'd do it like this:

g++ -g -fstrict-enums -O -c firstClass.C

In this example, only -fstrict-enums is an option meant only for C++ programs; you can use all the other options with any language GCC supports. Also, some options for compiling C programs — -std is the classic one — are totally relevant for C++ programs too.

Here's the list of options that are only for compiling C++ programs:

-fabi-version=n

Use version n of the C++ ABI. The default is version 0. Here's the tea on every version, because they all matter:

  • Version 0 refers to the version conforming most closely to the C++ ABI specification. So the ABI you get with version 0 will change in different versions of G++ as ABI bugs get fixed. Yeah, it moves. Keep up.
  • Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
  • Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default straight through G++ 4.9.
  • Version 3 corrects an error in mangling a constant address as a template argument.
  • Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
  • Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
  • Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
  • Version 7, which first appeared in G++ 4.8, treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
  • Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
  • Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.
  • Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
  • Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.
  • Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. Oops 🙈.
  • Version 13, which first appeared in G++ 8.2, fixes that accidental change in version 12. Slay, redemption arc.
  • Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
  • Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.
  • Version 16, which first appeared in G++ 11, changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names.
  • Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.
  • Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.
  • Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags.
  • Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.

See also -Wabi. 👑

-fabi-compat-version=n

On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name whenever it's defining a symbol with an incorrect mangled name. This switch tells it which ABI version to use for the alias.

  • With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2 compatibility).
  • If another ABI version is explicitly selected, this defaults to 0.
  • For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.

And the fine print: if this option isn't provided but -Wabi=n is, then that n version is what's used for compatibility aliases. If this option is provided along with -Wabi (the one without a version), the version from this option is what gets used for the warning. They coordinate. It's giving teamwork 🌈.

-fno-access-control

Turns off all access checking. Zero. Gone. This switch is mainly useful for working around bugs in the access control code — like, it's the "the compiler is being weird, let me in" button 🫶.

-faligned-new

Enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. You can pass a numeric argument like -faligned-new=32 to specify how much alignment (in bytes) that function provides — but honestly, few users will need to override the default of alignof(std::max_align_t). Leave it alone, bestie, it's fine.

This flag is enabled by default for -std=c++17 ✨.

-fno-assume-sane-operators-new

Okay, deep breath, this one's a whole paragraph but it's important. The C++ standard allows replacing the global new, new[], delete and delete[] operators — even though a lot of C++ programs don't replace them and just use the implementation-provided version. Furthermore, the C++ standard allows omitting those calls if they're made from new or delete expressions (and by extension the same is assumed if __builtin_operator_new or __builtin_operator_delete functions are used). This option gives you control over some optimizations around calls to those operators.

  • With -fassume-sane-operators-new-delete, GCC may assume that calls to the replaceable global operators from new or delete expressions, or from __builtin_operator_new or __builtin_operator_delete calls, don't read or modify any global variables or variables whose address could escape to the operators (that's global state — except for errno for the new and new[] operators). This allows most optimizations across those calls, and it's something the implementation-provided operators satisfy unless malloc implementation details are observable in the code or unless malloc hooks are used — but it might not be satisfied if a program replaces those operators. This behavior is enabled by default.
  • With -fno-assume-sane-operators-new-delete, GCC must assume all these calls (whether from new or delete expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles their implementation). Use this option if those operators are or may be replaced and your code needs to expect such behavior.

So basically: default = trust, this flag = don't trust, and if you're swapping out operators, you want the don't-trust one 💀.

-fchar8_t

-fno-char8_t

Enables support for char8_t as adopted for C++20. This includes the addition of a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros. Iconic behavior, honestly.

This option lets functions be overloaded for ordinary and UTF-8 strings:

int f(const char *);    // #1
int f(const char8_t *); // #2
int v1 = f("text");     // Calls #1
int v2 = f(u8"text");   // Calls #2

and it introduces new signatures for user-defined literals:

int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;
⚠️ But here's the part where it gets messy: the change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified 😭.
const char *cp = u8"xx";// error: invalid conversion from
                        //        `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx");     // error: invalid conversion from
                        //        `const char8_t*' to `const char*'
std::string s{u8"xx"};  // error: no matching function for call to
                        //        `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s;            // error: conversion from
                        //        `basic_string<char8_t>' to non-scalar
                        //        type `basic_string<char>' requested

Like, it's correct, it's just gonna yell at your old code. That's the vibe check and your code failed it 🦋.

  • -fcheck-new — bestie, this one makes the compiler double-check that the pointer operator new hands back is non-null before anyone tries to write to the storage it points at. And like… you normally don't even need it 💅 because the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler always checks the return value anyway, option or no option. In every other case, when operator new has a non-empty exception specification, running out of memory is signalled by throwing std::bad_alloc. Peep new (nothrow) too iykyk.
  • -fconcepts — turns on support for the C++ Concepts feature, aka constraining template arguments ✨. With -std=c++20 and above, Concepts are literally part of the language standard, so -fconcepts defaults to on. Some constructs that the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), allowed but that didn't make it into the standard, could additionally be enabled by -fconcepts-ts. That option was deprecated in GCC 14 and removed in GCC 15, so yeah — convert your code to C++20 concepts, that's the assignment 🎀.
  • -fconstexpr-depth=n — sets the max nested evaluation depth for C++11 constexpr functions to n. You need some limit or endless recursion during constant expression evaluation would just vibe forever 😭. The minimum specified by the standard is 512.
  • -fconstexpr-cache-depth=n — sets the maximum level of nested evaluation depth for C++11 constexpr functions that gets cached to n. It's a heuristic, and it trades compilation speed (when the cache avoids repeated calculations 🏃‍♀️) against memory consumption (when the cache grows huge from highly recursive evaluations 🫠). Default is 8. Very few users are likely to want to touch it, but if your code does heavy constexpr calculations you might wanna experiment to find which value works best for you 💖.
  • -fconstexpr-fp-except — ok so Annex F of the C standard says IEC559 floating point exceptions hit at compile time should not stop compilation. C++ compilers have historically not followed that guidance, treating floating point division by zero as non-constant even though it has a well defined value 🙈. This flag tells the compiler to give Annex F priority over other rules saying a particular operation is undefined.
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except
  • -fconstexpr-loop-limit=n — max number of iterations for a loop in C++14 constexpr functions, set to n. Yes we need a limit, otherwise infinite loops during constant expression evaluation, and nobody has time for that 💀. Default is 262144 (1<<18).
  • -fconstexpr-ops-limit=n — sets the maximum number of operations during a single constexpr evaluation 🔥. Because listen — even when the number of iterations of one single loop is capped with the limit above, if you've got several nested loops each with lots of iterations (but still under that limit), or if in the body of some loop — or honestly even outside a loop — too many expressions need to be evaluated, the resulting constexpr evaluation might take way too long. Default is 33554432 (1<<25).
  • -fcontracts — enables experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820) 😭. The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This one's mostly for people interested in experimentation, refining the feature to get it into shape for a future C++ standard 🔬.

On violation of a checked contract, the violation handler is called. Users can replace the violation handler by defining:

void
handle_contract_violation (const std::experimental::contract_violation&);

There are different sets of additional flags that can be used together to specify which contracts will be checked and how — for N4820 contracts, P1332 contracts, or P1429 contracts. These sets cannot be used together ⚠️.

  • -fcontract-mode=[on|off] — controls whether any contracts have any semantics at all. Defaults to on.
  • -fcontract-assumption-mode=[on|off] — [N4820] controls whether contracts with level axiom should have the assume semantic. Defaults to on.
  • -fcontract-build-level=[off|default|audit] — [N4820] specifies which level of contracts to generate checks for. Defaults to default.
  • -fcontract-continuation-mode=[on|off] — [N4820] controls whether the program is allowed to keep executing after a contract violation. Like, do checked contracts have the maybe semantic described below rather than the never semantic? Defaults to off.
  • -fcontract-role=<name>:<default>,<audit>,<axiom> — [P1332] specifies the concrete semantics for each contract level of a particular contract role.
  • -fcontract-semantic=[default|audit|axiom]:<semantic> — [P1429] specifies the concrete semantic for a particular contract level.
  • -fcontract-strict-declarations=[on|off] — controls whether to reject adding contracts to a function after its first declaration. Defaults to off.

The possible concrete semantics that can be specified with -fcontract-role or -fcontract-semantic are:

  • "ignore" — this contract has no effect. Zero. Nada. 💀
  • "assume" — this contract is treated like C++23 [[assume]].
  • "check_never_continue", "never", "abort" — this contract is checked ✅. If it fails, the violation handler is called. If the handler returns, std::terminate is called 💅.
  • "check_maybe_continue", "maybe" — this contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally 🫶.
  • -fcoroutines — enables support for the C++ coroutines extension (experimental ⚠️).
  • -fdiagnostics-all-candidates — lets the C++ front end note all candidates during overload resolution failure, including when a deleted function is selected 👀.
  • -fno-elide-constructors — the C++ standard allows an implementation to omit creating a temporary that's only used to initialize another object of the same type. Specifying this option disables that optimization and forces G++ to call the copy constructor in all cases 😤. This option also causes G++ to call trivial member functions which otherwise would be expanded inline. In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.
  • -fno-enforce-eh-specs — don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard 🚨, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications — the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time 💀.
  • -fextern-tls-init, -fno-extern-tls-init — the C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization 🌀. When the use and the definition of the variable are in the same translation unit, that overhead can be optimized away — but when the use is in a different translation unit, there's significant overhead even if the variable doesn't actually need dynamic initialization 😩. If you, the programmer, can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), you can avoid that overhead with -fno-extern-tls-init. On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init.
  • -ffold-simple-inlines, -fno-fold-simple-inlines — permits the C++ frontend to fold calls to std::move, std::forward, std::addressof and std::as_const 🎀. In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active.
  • -fno-gnu-keywords — don't recognize typeof as a keyword, so that code can use this word as an identifier 🏷️. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
  • -fno-immediate-escalation — don't enable immediate function escalation, whereby certain functions can be promoted to consteval, as specified in P2564R3. For example:
consteval int id(int i) { return i; }

constexpr int f(auto t)
{
  return t + id(t); // id causes f<int> to be promoted to consteval
}

void g(int i)
{
  f (3);
}

That compiles in C++20: f is an immediate-escalating function (due to the auto it's a function template, and it's declared constexpr) and id(t) is an immediate-escalating expression, so f gets promoted to consteval 👑. Consequently, the call to id(t) is in an immediate context, so it doesn't have to produce a constant (that's the mechanism that allows consteval function composition ✨). However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to consteval function id(t) isn't a constant expression, the compiler rejects the code 💀. This option is turned on by default; it's only effective in C++20 mode or later.

  • -fimplicit-constexpr — makes inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function 🦋. This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations 🔥.
  • -fno-implicit-templates — bestie, this one says: never emit code for non-inline templates that got instantiated implicitly (like, just because somebody used them 👀). Only emit code for explicit instantiations. So yeah — if you turn this on, you gotta structure your code real careful and include all the explicit instantiations you need, or you'll get undefined symbols at link time and be sad 😭.
  • -fno-implicit-inline-templates — same vibe but for inline templates: don't emit code for implicit instantiations of those either. Default behavior handles inlines differently on purpose, so that compiles with and without optimization need the same set of explicit instantiations. Consistency queen 💅
  • -fno-implement-inlines — to save space, don't emit out-of-line copies of inline functions that are controlled by #pragma implementation. But heads up: this causes linker errors if those functions aren't inlined everywhere they're called 💀.
  • -fmodules, -fno-modules — turn on support for C++20 modules ✨. You usually don't need -fno-modules since that's the default anyway. And even though modules are a C++20 feature, picking that standard version does not currently enable them implicitly — you gotta ask nicely.
  • -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file to create an importable header unit 📦.
  • -fmodule-implicit-inline — so in modular code, member functions defined inside their class definitions are not implicitly inline. That's different from traditional C++ behavior, and for good reasons, but it can make porting code kinda rough 😅. This option makes those function definitions implicitly inline again. The catch: it generates an ABI incompatibility, so you must use it everywhere or nowhere. No halfsies 🙅 (And such definitions outside a named module stay implicitly inline regardless.)
  • -fno-module-lazy — disables lazy module importing and module mapper creation. Slay, but eagerly 🦋
  • -fmodule-mapper=[hostname]:port[?ident]
  • -fmodule-mapper=|program[?ident] args...
  • -fmodule-mapper==socket[?ident]
  • -fmodule-mapper=<>[inout][?ident]
  • -fmodule-mapper=<in>out[?ident]
  • -fmodule-mapper=file[?ident] — this is your little oracle 🔮 that you query for module-name-to-filename mappings. If you don't specify one, the CXX_MODULE_MAPPER environment variable gets used, and if that's unset too, you get an in-process default. Something for everyone 💖
  • -fmodule-only — only emit the Compiled Module Interface, and inhibit any object file. Just the CM, no baggage ✨
  • -fms-extensions — disables Wpedantic warnings about constructs used in MFC, like implicit int and getting a pointer to a member function via non-standard syntax. Windows-kid energy 🪟
  • -fnew-inheriting-ctors — enables the P0136 adjustment to the semantics of C++11 constructor inheritance. It's part of C++17 but also considered a Defect Report against C++11 and C++14. Enabled by default unless you specify -fabi-version=10 or lower ⚠️.
  • -fnew-ttp-matching — enables the P0522 resolution to Core issue 150 (template template parameters and default arguments): this lets you pass a template with default template arguments as an argument for a template template parameter that has fewer template parameters. Enabled by default for -std=c++17 🎀.
  • -fno-nonansi-builtins — disables built-in declarations of functions that ANSI/ISO C doesn't mandate. The list includes ffs, alloca, _exit, index, bzero, conjf, and other related functions 🚫.
  • -fnothrow-opt — treats a throw() exception specification as if it were a noexcept spec, to reduce or eliminate the text size overhead compared to a function with no exception specification at all. Fun twist: if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away 🧚. Semantic effect: an exception thrown out of a function with such a spec results in a call to terminate instead of unexpected.
  • -fno-operator-names — don't treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms for keywords. No aliases, no shortcuts 💅
  • -fno-optional-diags — disables diagnostics that the standard says a compiler doesn't need to issue. Right now the only such diagnostic G++ issues is the one for a name having multiple meanings within a class. Optional means optional, bestie ✨
  • -fno-pretty-templates — okay so normally, when an error message refers to a specialization of a function template, the compiler prints the template's signature followed by the template arguments and any typedefs or typenames in the signature (like void f(T) [with T = int] instead of void f(int)) so it's super clear which template is involved 👑. And when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. But if either of those behaviors makes the error message harder to understand instead of easier, you can use -fno-pretty-templates to disable them. Your brain, your call 🧠
  • -frange-for-ext-temps — enables lifetime extension of C++ range-based for temporaries. With -std=c++23 and above this is part of the language standard, so temporaries' lifetimes are extended until the end of the loop by default. This option lets you enable that behavior in earlier versions of the standard too 🦋.
  • -fno-rtti — disables generation of info about every class with virtual functions that the C++ run-time type identification features use (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space with this flag 🎀. Note that exception handling uses the same information, but G++ generates it as needed. Also, dynamic_cast can still be used for casts that don't require run-time type information — i.e. casts to void * or to unambiguous base classes.
⚠️ Mixing code compiled with -frtti and code compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. Don't mix your vibes, it's messy 😭
  • -fsized-deallocation — enables the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

as introduced in C++14 ✨. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. And the flag -Wsized-deallocation warns about places that might want to add a definition 👀.

  • -fstrict-enums — lets the compiler optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). ⚠️ This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. And this option has no effect for an enumeration type with a fixed underlying type. Wild but true 💅
  • -fstrong-eval-order, -fstrong-eval-order=kind — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. -fstrong-eval-order is equivalent to -fstrong-eval-order=all, and it's enabled by default with -std=c++17 or later 🎀.

-fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.

-fstrong-eval-order=none is equivalent to -fno-strong-eval-order.

Order matters, bestie, in life and in evaluation 🌈

  • -ftemplate-backtrace-limit=n — sets the maximum number of template instantiation notes for a single warning or error to n. Default is 10. So like, ten notes max, we're not reading a novel 🙈
  • -ftemplate-depth=n — sets the maximum instantiation depth for template classes to n. You need a limit on template instantiation depth to detect endless recursions during template class instantiation 🌀. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, since the compiler can run out of stack space before hitting 1024 in some situations. Gotta leave room, y'know? 💀
  • -fno-threadsafe-statics — don't emit the extra code that uses the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this to reduce code size slightly in code that doesn't need to be thread-safe 🧵✂️.
  • -fuse-cxa-atexit — register destructors for objects with static storage duration using the __cxa_atexit function instead of the atexit function. This option is required for fully standards-compliant handling of static destructors, but it only works if your C library supports __cxa_atexit ⚠️.
  • -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This makes std::uncaught_exception incorrect, but it's necessary if the runtime routine isn't available 😬.
  • -fvisibility-inlines-hidden — this switch declares that the user isn't attempting to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects 🫶.

The effect of this is that GCC may effectively mark inline methods with __attribute__ ((visibility ("hidden"))), so they don't appear in the export table of a DSO and don't require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO, because it massively reduces the size of the dynamic export table when the library makes heavy use of templates 🔥.

The behavior of this switch isn't quite the same as marking the methods as hidden directly, because it doesn't affect static variables local to the function and it doesn't cause the compiler to deduce that the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the effect of this switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Keep in mind: marking the enclosing class with explicit visibility has no effect 👑

Explicitly instantiated inline methods are unaffected by this option, since their linkage might otherwise cross a shared library boundary 🌉.

  • -fvisibility-ms-compat — bestie, this one's whole personality is vibe-matching Microsoft Visual Studio. It uses visibility settings to try to make GCC's C++ linkage model compatible with MSVC's. It makes three changes to GCC's linkage model:
    1. It sets the default visibility to "hidden", like -fvisibility=hidden.
    2. Types, but not their members, are not hidden by default.
    3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

tfw legacy code does the "accidental" thing 😭 — the new-code advice is genuinely the move.
  • -fno-weak — do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. 💀 This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.
lowkey do not touch this one unless you're the one testing the compiler itself
  • -fext-numeric-literals (C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...).
  • -nostdinc++ — do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) 🎀
  • -flang-info-include-translate
  • -flang-info-include-translate-not
  • -flang-info-include-translate=header — inform of include translation events ✨. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form "user" or "<system>" it will be resolved to a specific user or system header using the include path.
  • -flang-info-module-cmi
  • -flang-info-module-cmi=module — inform of Compiled Module Interface pathnames 🦋. The first will note all read CMI pathnames. The module form will not reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in <> or "").
  • -stdlib=libstdc++,libc++ — when G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available: libstdc++ (the default, native C++ runtime for G++) and libc++ which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit -lstdc++ or -lc++ respectively, when a C++ runtime is required for linking. 👑

In addition, these warning options have meanings only for C++ programs:

💅 the C++-only warning girlies

  • -Wabi-tag (C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags. 🏷️
  • -Wcomma-subscript (C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in ( ) is not deprecated. Example:
void f(int *a, int b, int c) {
    a[b,c];     // deprecated in C++20, invalid in C++23
    a[(b,c)];   // OK
}

In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with -Wno-comma-subscript.

Enabled by default with -std=c++20 unless -Wno-deprecated, and after -std=c++23 regardless of -Wno-deprecated. Before -std=c++20, enabled with explicit -Wdeprecated.

heads up bestie: this warning is upgraded to an error by -pedantic-errors in C++23 mode or later.
  • -Wctad-maybe-unsupported (C++ and Objective-C++ only) — warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
  S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S<void>;

✨ the incomplete-type trick is so clever, it's giving main character energy.

  • -Wctor-dtor-privacy (C++ and Objective-C++ only) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 🙈
  • -Wdangling-reference (C++ and Objective-C++ only) — warn when a reference is bound to a temporary whose lifetime has ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is dangling

In the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference "r" is dangling. This warning also detects dangling references in member initializer lists:

const int& f(const int& i) { return i; }
struct S {
  const int &r; // r is dangling
  S() : r(f(10)) { }
};

Member functions are checked as well, but only their object argument:

struct S {
   const S& self () { return *this; }
};
const S& s = S().self(); // s is dangling

Certain functions are safe in this respect, for example "std::use_facet": they take and return a reference, but they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a "#pragma":

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic pop

The "#pragma" can also surround the class; in that case, the warning will be disabled for all the member functions.

-Wdangling-reference also warns about code like

auto p = std::minmax(1, 2);

where "std::minmax" returns "std::pair<const int&, const int&>", and both references dangle after the end of the full expression that contains the call to "std::minmax". 😭

The warning does not warn for "std::span"-like classes. We consider classes of the form:

template<typename T>
struct Span {
  T* data_;
  std::size len_;
};

as "std::span"-like; that is, the class is a non-union class that has a pointer data member and a trivial destructor.

The warning can be disabled by using the "gnu::no_dangling" attribute.

This warning is enabled by -Wextra. 🔥

  • -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — warn when "delete" is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall.
genuinely unsafe, not just a style thing — virtual destructor or bust, fr fr.
  • -Wdeprecated-copy (C++ and Objective-C++ only) — warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor. 🫶
  • -Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
int k = f - e;

-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated.

  • -Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only) — disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;

-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated. 🌸

  • -Wdeprecated-literal-operator (C++ and Objective-C++ only) — bestie, this one yells at you when you declare a user-defined literal operator with a space before the suffix, because that's deprecated now 💀. It's on by default in C++23, or whenever you explicitly pass -Wdeprecated.
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred
  • -Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only) — this one gets pressed when you omit the comma before the varargs ... at the very end of a function parameter list, because yeah, that's deprecated ✨. Enabled by default in C++26, or with explicit -Wdeprecated.
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated
  • -Wno-elaborated-enum-base — for C++11 and up, this warns if an (invalid) extra enum-base shows up in an elaborated-type-specifier 🎀. Translation: if an enum with a given underlying type and, like, no enumerator list gets used in a declaration that isn't just a plain standalone declaration of the enum. It's enabled by default, and it gets upgraded to an error with -pedantic-errors 👑.
  • -Wno-init-list-lifetime (C++ and Objective-C++ only) — this is the "don't warn me" version of the dangling-pointer warning for std::initializer_list 🫶. Because the underlying array for an initializer_list is handled like a normal C++ temporary object, it is so easy to accidentally keep a pointer to the array past the end of the array's lifetime. For example:
    • If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
    • If a new-expression creates an initializer_list, the array only lives until the end of the enclosing full-expression, so the initializer_list in the heap has a dangling pointer.
    • When an initializer_list variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the initializer_list variable has a dangling pointer.
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
  • When a list constructor stores the begin pointer from the initializer_list argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary initializer_list, the pointer is left dangling by the end of the variable declaration statement 😭.
  • -Winvalid-constexpr — flags a function that never produces a constant expression 💅. In C++20 and earlier, every constexpr function and function template must have at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's possible to have a function or function template marked constexpr for which no invocation satisfies the requirements of a core constant expression. This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it'll be an ordinary warning. For example:
void f (int& i);
constexpr void
g (int& i)
{
  // Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
  f(i);
}
  • -Winvalid-imported-macros — verifies that all imported macro definitions are valid at the end of compilation 🔥. Not enabled by default, because it needs extra processing to figure out. It may be useful when you're preparing sets of header-units and want to ensure consistent macros.
  • -Wno-literal-suffix (C++ and Objective-C++ only) — do not warn when a string or character literal is followed by a ud-suffix that doesn't begin with an underscore 🌸. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens to maintain backwards compatibility with code that uses formatting macros from <inttypes.h>. For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

int main() {
  int64_t i64 = 123;
  printf("My int64: %" PRId64"\n", i64);
}

In this case, PRId64 is treated as a separate preprocessing token. This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization ⭐. These warnings are enabled by default.

  • -Wno-narrowing (C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard 🦋. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. With -Wnarrowing in C++98, it warns when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int

This flag is included in -Wall and -Wc++11-compat.

  • -Wnoexcept (C++ and Objective-C++ only) — warns when a noexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception 💜.
  • -Wnoexcept-type (C++ and Objective-C++ only) — warns if the C++17 feature making noexcept part of a function type changes the mangled name of a symbol relative to C++14 🙈. Enabled by -Wabi and -Wc++17-compat. As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }

In C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept>.

  • -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function such as memset or memcpy is an object of class type 🎀, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to memset below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);

The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning.

  • -Wnon-virtual-dtor (C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class 💀, in which case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified. The -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred because it warns about the unsafe cases without false positives 👑.
  • -Wregister (C++ and Objective-C++ only) — warns on uses of the register storage class specifier, except when it's part of the GNU Explicit Register Variables extension ✨. The use of the register keyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17.
  • -Wreorder (C++ and Objective-C++ only) — warns when the order of member initializers in your code does not match the order in which they must be executed 😭. For instance:
struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

The compiler rearranges the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.

  • -Wno-pessimizing-move (C++ and Objective-C++ only) — this one warns when a call to std::move prevents copy elision 💅. A typical scenario where copy elision can occur is returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.
struct T {
...
};
T fn()
{
  T t;
  ...
  return std::move (t);
}

But in this example, the std::move call prevents copy elision. This warning is enabled by -Wall.

  • -Wno-redundant-move (C++ and Objective-C++ only) — this one warns about redundant calls to std::move 🫶; that is, when a move operation would have been performed even without the std::move call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:

...so like, std::move in that return? Bestie, it's doing nothing. Watch:

struct T {
...
};
T fn(T t)
{
  ...
  return std::move (t);
}

Iconic of you to try, but that std::move call is literally redundant 💀. And because G++ implements Core Issue 1579, here's another glow-up example:

struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
  T t;
  ...
  return std::move (t);
}

In this one, copy elision isn't applicable because the type of the expression being returned and the function return type differ — but G++ still treats the return value as if it were designated by an rvalue. Slay, honestly.

This warning is enabled by -Wextra.

  • -Wrange-loop-construct (C++ and Objective-C++ only) — warns you when a C++ range-based for-loop is out here making an unnecessary copy 😭. That happens when the range declaration isn't a reference, but like... probably should be. Exhibit A:
struct S { char arr[128]; };
void fn () {
  S arr[5];
  for (const auto x : arr) { ... }
}

It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. (Small copy? Not the vibe check's problem ✨)

This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different type, resulting in a copy. Exhibit B:

void fn() {
  int arr[10];
  for (const double &x : arr) { ... }
}

In the example above, in every single iteration of the loop a temporary value of type double is created and destroyed, just to bind the reference const double & to it. Every. Single. Iteration. 😭

This warning is enabled by -Wall.

  • -Wredundant-tags (C++ and Objective-C++ only) — warns about redundant class-key and enum-key in references to class types and enumerated types, in contexts where the key can be eliminated without causing an ambiguity. Like:
struct foo;
struct foo *p;   // warn that keyword struct can be eliminated

But on the other hand, in this example there's no warning at all:

struct foo;
void foo ();   // "hides" struct foo
void bar (struct foo&);  // no warning, keyword struct is necessary

See? Context is everything, queen 👑.

  • -Wno-subobject-linkage (C++ and Objective-C++ only) — do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. Here's the lore: if a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation, because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
  • -Weffc++ (C++ and Objective-C++ only) — warns about violations of these style guidelines from Scott Meyers' Effective C++ series of books 📚:
    • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
    • Prefer initialization to assignment in constructors.
    • Have operator= return a reference to *this.
    • Don't try to return a reference when you must return an object.
    • Distinguish between prefix and postfix forms of increment and decrement operators.
    • Never overload &&, ||, or ,.

This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic base classes too.

When you pick this option, be aware that the standard library headers do not obey all of these guidelines — use grep -v to filter out those warnings. No cap, you will need it 🙈.
  • -Wno-exceptions (C++ and Objective-C++ only) — disables the warning about the case when an exception handler is shadowed by another handler (which can point out a wrong ordering of exception handlers). Ordering matters, bestie!
  • -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about the use of an uncasted NULL as a sentinel. When compiling only with GCC this is a valid sentinel, as NULL is defined to __null. Although it's a null pointer constant rather than a null pointer, it's guaranteed to be of the same size as a pointer. But — this use is not portable across different compilers. So like, cute for us, not cute everywhere 🌍.
  • -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function. The warning exists to diagnose compatibility problems, and it's enabled by default.
  • -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for. Glow-up your casts, fr fr ✨.
  • -Woverloaded-virtual (C++ and Objective-C++ only)
  • -Woverloaded-virtual=n` — warns when a function declaration hides virtual functions from a base class. For example, in:
struct A {
  virtual void f();
};

struct B: public A {
  void f(int); // does not override
};

the A class version of f is hidden in B, so code like:

B* b;
b->f();

just... fails to compile. Oop 💀.

In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function — e.g. add using A::f; to B.

The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:

struct C {
  virtual void f();
  virtual void f(int);
};

struct D: public C {
  void f(int); // does override
}

This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.

At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall.

  • -Wno-pmf-conversions (C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function to a plain pointer.
  • -Wsign-promo (C++ and Objective-C++ only) — warns when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. Standards said no 💅.
  • -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so you can still use the STL. You may also instantiate or specialize templates. (So like, the rule applies to your code, not the STL's — we're not banning the STL, bestie 😌.)
  • -Wmismatched-new-delete (C++ and Objective-C++ only) — warns for mismatches between calls to operator new or operator delete and the corresponding call to the allocation or deallocation function. This includes invocations of C++ operator delete with pointers returned from either mismatched forms of operator new, or from other functions that allocate objects for which the operator delete isn't a suitable deallocator — plus calls to other deallocation functions with pointers returned from operator new for which the deallocation function isn't suitable.

For example, the delete expression in the function below is diagnosed because it doesn't match the array form of the new expression the pointer argument was returned from. Similarly, the call to free is also diagnosed:

void f ()
{
  int *a = new int[n];
  delete a;   // warning: mismatch in array forms of expressions

  char *p = new char[n];
  free (p);   // warning: mismatch between new and free
}

The related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions other than operator new and operator delete.

-Wmismatched-new-delete is included in -Wall.

  • -Wmismatched-tags (C++ and Objective-C++ only) — warns for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration (if no definition is provided).

For example, the declaration of struct Object in the argument list of draw triggers the warning. To avoid it, either remove the redundant class-key struct or replace it with class to match its definition:

class Object {
public:
  virtual ~Object () = 0;
};
void draw (struct Object*);

It is not wrong to declare a class with the class-key struct as the example above shows. The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that's intended to be portable to Windows-based compilers, the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with -Wredundant-tags. Teamwork makes the dream work 🫶.

  • -Wmultiple-inheritance (C++ and Objective-C++ only) — lowkey a vibe check for class definitions 💅. Warns you when a class is defined with multiple direct base classes. Some coding rules straight-up disallow multiple inheritance, and this is how you enforce that rule. The warning stays quiet inside a system header file, such as the STL, so you can still use the STL. You can also still define classes that indirectly use multiple inheritance — nobody's coming for those.
  • -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Same energy as above: some coding rules disallow multiple inheritance, this enforces it. Inactive inside a system header file, such as the STL, so STL usage is safe 🫶. And you may also define classes that indirectly use virtual inheritance.
  • -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. Okay so why is that scary? Because if the virtual base is reachable along more than one path, it gets moved multiple times, which can mean both objects end up in the moved-from state 😭. If your move assignment operator is written to avoid moving from a moved-from object, you can safely disable this warning. Periodt.
  • -Wnamespaces — warns when a namespace definition is opened. Some coding rules disallow namespaces, and this is the enforcer 🚨. Inactive inside a system header file, such as the STL, so STL is still your bestie. You may also use using directives and qualified names.
  • -Wno-template-body (C++ and Objective-C++ only) — disables diagnosing errors when parsing a template, and instead issues an error only upon instantiation of the template. You can also use this flag to downgrade such errors into warnings with Wno-error= or -fpermissive.
  • -Wno-template-id-cdtor (C++ and Objective-C++ only) — disables the warning about using simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. Like, look at this mess:
template<typename T> struct S {
  S<T>(); // should be S();
  ~S<T>();  // should be ~S();
};

-Wtemplate-id-cdtor is enabled by default with -std=c++20; it's also enabled by -Wc++20-compat.

  • -Wtemplate-names-tu-local — warns when a template body hides an exposure of a translation-unit-local entity. Most of the time, referring to a translation-unit-local entity (such as an internal linkage declaration) within an entity that is emitted into a module's CMI is an error. However — within the initializer of a variable, or in the body of a non-inline function, this is not an exposure and no error gets emitted. ✨

This can cause variable or function templates to accidentally become unusable if they reference such an entity, because other translation units that import the template will never be able to instantiate it. This warning attempts to detect cases where that might occur. The presence of an explicit instantiation silences the warning.

This flag is enabled by -Wextra.

  • -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. 💀
  • -Wno-vexing-parse (C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity 😭. It warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:
void f(double a) {
  int i();        // extern int i (void);
  int n(int(a));  // extern int n (int);
}

Another example:

struct S { S(int); };
void f(double a) {
  S x(int(a));   // extern struct S x (int);
  S y(int());    // extern struct S y (int (*) (void));
  S z();         // extern struct S z (void);
}

The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead.

This warning is enabled by default.

  • -Wno-class-conversion (C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called. It's giving "why are you even here" 🙈.
  • -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. This includes postfix and prefix ++ and -- expressions of volatile-qualified types, using simple assignments where the left operand is a volatile-qualified non-class type for their value, compound assignments where the left operand is a volatile-qualified non-class type, volatile-qualified function return type, volatile-qualified parameter type, and structured bindings of a volatile-qualified type. This usage was deprecated in C++20.

Enabled by default with -std=c++20. Before -std=c++20, enabled with explicit -Wdeprecated.

  • -Waligned-new — warns about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall.

Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. 👑

  • -Wno-placement-new -Wplacement-new=n — warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large:
char buf [64];
new (buf) int[64];

This warning is enabled by default.

  • -Wplacement-new=1 — this is the default warning level of -Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following new expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard, because it writes past the end of the one-element array:
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
  • -Wplacement-new=2 — at this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. So the previous example would be diagnosed at this level, while the following construct makes use of the flexible member array extension to avoid the warning at level 2:
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
  • -Wcatch-value -Wcatch-value=n (C++ and Objective-C++ only) — warns about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall. 🎀
  • -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs.
  • -Wno-defaulted-function-deleted (C++ and Objective-C++ only) — warns when an explicitly defaulted function is deleted by the compiler. That can occur when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default.
  • -Wno-delete-incomplete (C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime 😬. This warning is enabled by default.
  • -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolons. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):
struct S {
  void foo () {};
};

Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore is a pedwarn in C++98):

struct S {
};
;

And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore is a pedwarn in C++98):

struct S {
  int a;
  ;
};
  • -Wno-global-module (C++ and Objective-C++ only) — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives.
  • -Wno-inaccessible-base (C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the -Wextra option.
struct A { int a; };

struct B : A { };

struct C : B, A { };
  • -Wno-inherited-variadic-ctor — suppresses warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. 🫠
  • -Wno-invalid-offsetof (C++ and Objective-C++ only) — suppresses warnings from applying the offsetof macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. You know what you're doing, bestie — the compiler's just gonna have to trust you 💅.

The restrictions on offsetof may be relaxed in a future version of the C++ standard.

  • -Wsized-deallocation (C++ and Objective-C++ only) — this one's got your back when you define a deallocation function that forgot to bring its sized bestie to the party 😭 Like, it yells at you if you've got
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

sitting there all alone with no matching sized version, like

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

…or the other way around (sized without unsized, no cap). It's switched on by -Wextra together with -fsized-deallocation ✨

  • -Wsuggest-final-types — warns you about types that have virtual methods where your code quality would literally level up if the type were declared with the C++11 final specifier, or — if you can swing it — declared inside an anonymous namespace. Why do we care? Because that lets GCC devirtualize the polymorphic calls way more aggressively 🔥 This warning hits different with link-time optimization, where the info about the class hierarchy graph is more complete. Vibe check: LTO = more complete tea ☕
  • -Wsuggest-final-methods — same energy but for virtual methods: warns when code quality would improve if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier 💅 Also more effective with link-time optimization (again: class hierarchy graph = more complete). Pro tip from the page itself: it's recommended to first consider the suggestions from -Wsuggest-final-types, and then rebuild with the new annotations 🫶 Order matters bestie
  • -Wsuggest-override — warns about overriding virtual functions that aren't marked with the override keyword. That's it, that's the whole thing. Label your overrides, babes 🎀
  • -Wno-conversion-null (C++ and Objective-C++ only) — don't warn about conversions between NULL and non-pointer types. Heads up: -Wconversion-null is enabled by default, so this is the off switch 🌸

🦋 Options Controlling Objective-C and Objective-C++ Dialects 🦋

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. Like, at all. We're just doing flags here 🙈)

Okay so this section is all about the command-line options that are only meaningful for Objective-C and Objective-C++ programs 💖 You can still use most of the language-independent GNU compiler options — those are universal, they don't gatekeep.

For example, you might compile a file some_class.m like this:

gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC 👑

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi) ✨

Here's the list of options that are only for compiling Objective-C and Objective-C++ programs:

  • *-fconstant-string-class=class-name* — use class-name as the name of the class to instantiate for each literal string specified with the syntax @"...". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below) 🎀 On Darwin / macOS platforms, the -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings. Note that -fconstant-cfstrings is an alias for the target-specific -mconstant-cfstrings equivalent 💫
  • -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems 🐧
  • -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin / macOS 🍎 The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used. If and only if, bestie — read it twice 💅
  • -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This allows for more efficient entry points in the runtime to be used ⚡ This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
  • *-fobjc-abi-version=n* — use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime 👑 In that case: Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines ✨
  • -fobjc-call-cxx-cdtors — for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then returns self 🫶 Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order 🔁

The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance 🌸

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods. Everyone else is still waiting for the group chat to load 😭

  • -fobjc-direct-dispatch — allow fast jumps to the message dispatcher 🚀 On Darwin this is accomplished via the comm page.
  • -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what's offered by C++ 💜 This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier — sorry vintage Mac girlies 💀)
  • -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs 🗑️✨ This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. Noted!
  • -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call 👀 This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to — no exceptions, that's just the rules 📜 Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. It's giving decorative 😅
  • -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0 🕰️ This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected 🚫 This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC. Backwards-compat queen 👑
  • -freplace-objc-classes — emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead 🔥 This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Hot-swapping your code mid-run? Iconic behaviour, honestly 🌈 Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
  • -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance 🚀 Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution 🛠️ The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options — it just does its own thing, no notes 💅
  • -fno-local-ivars — by default, instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in 👀 This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name 😬 Specifying the -fno-local-ivars flag disables this behavior, thus avoiding variable shadowing issues. Shadowing is so not the vibe 🚫
  • -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility 🔐 Pick your lane: public, protected, private or package ✨
  • -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl 📝 Receipts, but make it declarations.
  • -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector 👀🚨 GC said "I'll take that," and this flag tells you about it.
  • -Wno-property-assign-default (Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified 🤫 Silence is golden, or at least silence is the default here.
  • -Wno-protocol (Objective-C and Objective-C++ only) — okay so picture this: a class says it implements a protocol, right? By default GCC will warn you about every single method in that protocol that the class doesn't explicitly implement — even the ones it totally gets for free by inheriting them from the superclass. Kinda extra, ngl. So if you pass -Wno-protocol, inherited-from-the-superclass methods count as implemented and those warnings just ✨vanish✨. Bestie said "I inherited it, that counts," and honestly? She's right.
  • -Wobjc-root-class (Objective-C and Objective-C++ only) — warns you when a class interface has no superclass at all. Like bestie, everyone needs a parent 😭 — most classes inherit from NSObject (or Object, for example). But if you're intentionally declaring a root class, you can hush the warning by marking the interface with __attribute__((objc_root_class)). Main character energy: declaring yourself the root, no ancestors, periodt.
  • -Wselector (Objective-C and Objective-C++ only) — yells at you if it finds multiple methods of different types sharing the same selector during compilation. The check happens on the method list in the final stage of compilation, bestie, so heads up: it also checks every selector that shows up in a @selector(...) expression once a matching method for that selector has been found during compilation. And because these checks only scan the method table at the end? No warnings if you never reach that final stage — like if an error pops up mid-compile, or if you're using -fsyntax-only. F in the chat for warnings you'll never see 💀.
  • -Wstrict-selector-match (Objective-C and Objective-C++ only) — warns when multiple methods with differing argument and/or return types exist for a given selector, and you're trying to send a message with that selector to a receiver of type id or Class. Now — when this flag is off (which is the default behavior, just so we're clear), the compiler skips those warnings if whatever differences it found are confined to types that share the same size and alignment. It's giving "close enough, we move."
  • -Wundeclared-selector (Objective-C and Objective-C++ only) — warns when it spots a @selector(...) expression referring to a selector that was never declared. A selector counts as undeclared if no method with that name got declared before the @selector(...) expression — whether explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. Key difference babe: this option always runs its checks the instant it finds a @selector(...) expression, whereas -Wselector only checks in the final stage of compilation. It also enforces that cute coding-style convention where methods and selectors must be declared before you use them. Declare first, ask questions later 🎀.
  • -print-objc-runtime-info — generates a C header describing the largest structure that's passed by value, if any. Big struct? She's documented. Slay.

🌈 Options Controlling OpenMP and OpenACC 🌈

So GCC supports OpenMP extensions to C, C++, and Fortran with the -fopenmp option. And similarly, OpenACC extensions are supported in all three languages with -fopenacc. Parallel vibes across the board, we love to see it ✨.

  • -foffload=disable, -foffload=default, -foffload=target-list — these tell GCC which OpenMP and OpenACC offload targets to generate code for. The default behavior (equivalent to -foffload=default) is to generate code for all supported offload targets. The -foffload=disable form generates code only for the host fallback, while -foffload=target-list generates code only for the specified comma-separated list of offload targets.

Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES. Peek behind the curtain, iykyk 👑

  • -foffload-options=options, -foffload-options=target-triplet-list=options — with -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. And if you want options that apply only to a specific target or targets? Use the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.

Typical command lines are:

-foffload-options='-fno-math-errno -ffinite-math-only' \
        -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906
  • -fopenacc — enables handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran, plus !$acc, c$acc and *$acc in fixed-form Fortran. Heads up: this option implies -pthread, and thus is only supported on targets that have support for -pthread. Package deal, babe 🫶.
  • -fopenacc-dim=geom — specifies the default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of :-separated sizes, in order gang, worker, and vector. And a size can be omitted, to use a target-specific default value. Omit what you don't need, the compiler's got you 💅.
  • -fopenmp — enables handling of OpenMP directives #pragma omp, [[omp::directive(...)]], [[omp::decl(...)]], and [[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and the conditional compilation sentinel !$. In fixed source form Fortran, the sentinels can also start with c or *.

This option implies -pthread, and thus is only supported on targets that have support for -pthread. Also: -fopenmp implies -fopenmp-simd. It's giving "buy one, get one" and we're obsessed 🔥.

  • -fopenmp-simd — enables handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directive, and of combined or composite directives with simd as a constituent with #pragma omp, [[omp::directive(...)]], [[omp::sequence(...)]] and [[omp::decl(...)]] in C/C++ and !$omp in Fortran. It also enables the conditional compilation sentinel !$ in Fortran — and in fixed source form Fortran, the sentinels can also start with c or *. Every other OpenMP directive is just ignored, not invited to the party 🙈. And unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause. Yes, that's a whole thing.
  • -fopenmp-target-simd-clone, -fopenmp-target-simd-clone=device-type — on top of generating SIMD clones for functions marked with the declare simd directive, GCC also generates clones for functions marked with the OpenMP declare target directive that are suitable for vectorization when this option is in effect. The device-type may be one of none, host, nohost, and any, which correspond to keywords for the device_type clause of the declare target directive — clones are generated for the intersection of devices specified. And the aliases? -fopenmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=any and -fno-openmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=none. Cute, consistent, iconic.

At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it's disabled by default. Note the caveat, bestie — optimization defaults are sneaky 💀.

🎀 Options to Control Diagnostic Messages Formatting 🎀

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). Rude, honestly. But you can use the options below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location info should be reported. Just a lil heads up though: some language front ends may not honor these options. Their loss 💅.

  • -fmessage-length=n — tries to format error messages so they fit on lines of about n characters. If n is zero, then no line-wrapping happens at all; each error message appears on a single line. This is the default for all front ends.
Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas. Those two are doing their own thing, apparently 😭.
  • -fdiagnostics-plain-output — requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time. -fdiagnostics-plain-output is currently equivalent to the following options: -fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers -fdiagnostics-color=never -fdiagnostics-urls=never -fdiagnostics-path-format=separate-events -fdiagnostics-text-art-charset=none -fno-diagnostics-show-event-links. And in the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. Forward-thinking queen ✨.
  • -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location information once — so if a message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as a prefix) again, over and over, on the subsequent continuation lines. This is the default behavior. Say it once and move on, we love efficiency 🎀.
  • -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from breaking a message that's too long to fit on a single line. Every line gets the label, no cap. Somebody's thorough 💖.
  • -fdiagnostics-color[=WHEN], -fno-diagnostics-color — use color in diagnostics! WHEN is never, always, or auto. The default depends on how the compiler was configured — it can be any of the above WHEN options, or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when standard error is a terminal, and when not executing in an emacs shell. And the forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. Rainbow output when the vibes are right 🌈💅.

The colors? Oh bestie, they're defined by the environment variable GCC_COLORS. 💅 Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands get interpreted by your terminal or terminal emulator. (Go check the section in your text terminal's docs for permitted values and what they mean as character attributes.) Those substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors. ✨

The default GCC_COLORS is

error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32\
highlight-a=01;32:highlight-b=01;34

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. 🎀 Supported capabilities are as follows, bestie:

  • error= — SGR substring for error: markers.
  • warning= — SGR substring for warning: markers.
  • note= — SGR substring for note: markers.
  • path= — SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, such as the identifiers of individual events and lines indicating interprocedural calls and returns.
  • range1= — SGR substring for first additional range.
  • range2= — SGR substring for second additional range.
  • locus= — SGR substring for location information, file:line or file:line:column etc.
  • quote= — SGR substring for information printed within quotes.
  • fnname= — SGR substring for names of C++ functions.
  • targs= — SGR substring for C++ function template parameter bindings.
  • fixit-insert= — SGR substring for fix-it hints suggesting text to be inserted or replaced.
  • fixit-delete= — SGR substring for fix-it hints suggesting text to be deleted.
  • diff-filename= — SGR substring for filename headers within generated patches.
  • diff-hunk= — SGR substring for the starts of hunks within generated patches.
  • diff-delete= — SGR substring for deleted lines within generated patches.
  • diff-insert= — SGR substring for inserted lines within generated patches.
  • type-diff= — SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
  • valid= — SGR substring for highlighting valid elements within text art diagrams.
  • invalid= — SGR substring for highlighting invalid elements within text art diagrams.
  • highlight-a=, highlight-b= — SGR substrings for contrasting two different things within diagnostics, such as a pair of mismatching types. See -fdiagnostics-show-highlight-colors.
  • -fdiagnostics-urls[=WHEN] — Use escape sequences to embed URLs in diagnostics. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, embed a URL for documentation of that option. 🔗

WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when the standard error is a terminal, and when not executing in an emacs shell or any graphical terminal which is known to be incompatible with this feature, see below.

The default depends on how the compiler has been configured. It can be any of the above WHEN options.

GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so that the default is affected by environment variables. Under such a configuration, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.

However, even with -fdiagnostics-urls=always the behavior is dependent on those environment variables: If GCC_URLS is set to empty or no, do not embed URLs in diagnostics. If set to st, URLs use ST escape sequences. If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \, BEL is an ASCII character, CTRL-G that usually sounds like a beep. 😭

At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with the -fdiagnostics-urls=always.

  • -fno-diagnostics-show-option — By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option flag suppresses that behavior. 🙈
  • -fno-diagnostics-show-caret — By default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
  • -fno-diagnostics-show-labels — By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:
                       printf ("foo %s bar", long_i + long_j);
                                    ~^       ~~~~~~~~~~~~~~~
                                     |              |
                                     char *         long int

This option suppresses the printing of these labels (in the example above, the vertical bars and the "char *" and "long int" text). 🫶

  • -fno-diagnostics-show-event-links — By default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, such as the line connecting events 1 and 2 in:
                       3 |   if (p)
                         |       ^
                         |       |
                         |       (1) following `false' branch (when `p' is NULL)... ->-+
                         |                                                             |
                         |                                                             |
                         |+------------------------------------------------------------+
                       4 ||    return 0;
                       5 ||  return *p;
                         ||         ~
                         ||         |
                         |+-------->(2) ...to here
                         |          (3) dereference of NULL `p'

This option suppresses the printing of such connector lines. ✂️

  • -fno-diagnostics-show-cwe — Diagnostic messages can optionally have an associated CWE (https://cwe.mitre.org/index.html) identifier. GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata. 🔒
  • -fno-diagnostics-show-rules — Diagnostic messages can optionally have rules associated with them, such as from a coding standard, or a specification. GCC itself does not do this for any of its diagnostics, but plugins may do so. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata. 📜
  • -fno-diagnostics-show-highlight-colors — GCC can use color for emphasis and contrast when printing diagnostic messages and quoting the user's source. 🌈

For example, in

                   demo.c: In function `test_bad_format_string_args':
                   ../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
                      25 |   printf("hello %i", msg);
                         |                 ~^   ~~~
                         |                  |   |
                         |                  int const char *
                         |                 %s
  • the %i and "int" in the message and the "int" in the quoted source are colored using "highlight-a" (bold green by default), and
  • the "const char *" in the message and in the quoted source are both colored using "highlight-b" (bold blue by default).

The intent is to draw the reader's eyes to the relationships between the various aspects of the diagnostic message and the source, using color to group related elements and distinguish between mismatching ones.

This additional colorization is enabled by default if color printing is enabled (as per -fdiagnostics-color=), but it can be separately disabled via -fno-diagnostics-show-highlight-colors. 💖

  • -fno-diagnostics-show-line-numbers — By default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin.
  • -fdiagnostics-minimum-margin-width=width — This option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. 📏
  • -fdiagnostics-parseable-fixits — okay so this one makes fix-it hints come out in a machine-parseable format that IDEs can absolutely snack on. 🍓 For every fix-it, a line prints after the relevant diagnostic, and it starts with the string "fix-it:". Example time:
                   fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

The location is a half-open range, counted in bytes, starting at byte 1 for the initial column. In the example above, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:

                   00000000011111111112222222222
                   12345678901234567890123456789
                     gtk_widget_showall (dlg);
                     ^^^^^^^^^^^^^^^^^^
                     gtk_widget_show_all

The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", and non-printable characters as octal (e.g. vertical tab as "\013").

An empty replacement string indicates that the given range is to be removed. An empty range (e.g. 45:3-45:3) indicates that the string is to be inserted at the given position. 💅

  • -fdiagnostics-generate-patch — this one prints fix-it hints to stderr in unified diff format, after any diagnostics are printed. Like so:
                   --- test.c
                   +++ test.c
                   @ -42,5 +42,5 @

                    void show_cb(GtkDialog *dlg)
                    {
                   -  gtk_widget_showall(dlg);
                   +  gtk_widget_show_all(dlg);
                    }

The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). ✨

  • -fdiagnostics-show-template-tree — in the C++ frontend, when diagnostics show mismatching template types, such as:
                     could not convert 'std::map<int, std::vector<double> >()'
                       from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:

                     map<
                       [...],
                       vector<
                         [double != float]>>

The parts that differ are highlighted with color ("double" and "float" in this case). 🎀

  • -fno-elide-type — by default, when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as "[...]" to simplify the error message. For example:
                     could not convert 'std::map<int, std::vector<double> >()'
                       from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. 🦋

  • -fdiagnostics-path-format=KIND — pick how to print paths of control-flow events for diagnostics that have such a path associated with them. 🫶

KIND is none, separate-events, or inline-events, the default.

none means to not print diagnostic paths.

separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:

test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1

inline-events means to print the events "inline" within the source code. This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.

For example, the same events as above might be printed as:

                     'test': events 1-3
                      25 |   list = PyList_New(0);
                         |          ^~~~~~~~~~~~~
                         |          |
                         |          (1) when 'PyList_New' fails, returning NULL
                      26 |
                      27 |   for (i = 0; i < count; i++) {
                         |   ~~~
                         |   |
                         |   (2) when 'i < count'
                      28 |     item = PyLong_FromLong(random());
                      29 |     PyList_Append(list, item);
                         |     ~~~~~~~~~~~~~~~~~~~~~~~~~
                         |     |
                         |     (3) when calling 'PyList_Append', passing NULL from (1) as argument 1

Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped.

For example:

                     'test': events 1-2
                       |
                       |  133 | {
                       |      | ^
                       |      | |
                       |      | (1) entering 'test'
                       |  134 |   boxed_int *obj = make_boxed_int (i);
                       |      |                    ~~~~~~~~~~~~~~~~~~
                       |      |                    |
                       |      |                    (2) calling 'make_boxed_int'
                       |
                       +--> 'make_boxed_int': events 3-4
                              |
                              |  120 | {
                              |      | ^
                              |      | |
                              |      | (3) entering 'make_boxed_int'
                              |  121 |   boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
                              |      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                              |      |                                    |
                              |      |                                    (4) calling 'wrapped_malloc'
                              |
                              +--> 'wrapped_malloc': events 5-6
                                     |
                                     |    7 | {
                                     |      | ^
                                     |      | |
                                     |      | (5) entering 'wrapped_malloc'
                                     |    8 |   return malloc (size);
                                     |      |          ~~~~~~~~~~~~~
                                     |      |          |
                                     |      |          (6) calling 'malloc'
                                     |
                       <-------------+
                       |
                    'test': event 7
                       |
                       |  138 |   free_boxed_int (obj);
                       |      |   ^~~~~~~~~~~~~~~~~~~~
                       |      |   |
                       |      |   (7) calling 'free_boxed_int'
                       |
                   (etc)
  • -fdiagnostics-show-path-depths — extra info alert! This option provides additional information when printing control-flow paths associated with a diagnostic. 😭

If this option is provided, then the stack depth will be printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration will be appended when printing each event.

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. 🔥

  • -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu. 👑
  • -fdiagnostics-column-unit=UNIT — select the units for the column number. This affects traditional diagnostics (in the absence of -fno-show-column), as well as JSON format diagnostics if requested. 🌈

The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character, in the case of tab characters, or it may be smaller, in the case of multibyte characters. For example, the character "GREEK SMALL LETTER PI (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes.

Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0. ⭐

  • -fdiagnostics-column-origin=ORIGIN — set the origin for column numbers, i.e. the column number assigned to the first column. The default value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; any non-negative value may be specified. 💀
  • -fdiagnostics-escape-format=FORMAT — when GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines. 🙈

This option controls how such bytes should be escaped.

The default FORMAT, unicode displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as hexadecimal in the form <XX>.

For example, a source line containing the string before followed by the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string after will be printed for such a diagnostic as:

                    before<U+03C0><BF>after

Setting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed:

                    before<CF><80><BF>after
  • -fdiagnostics-text-art-charset=CHARSET — some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a monospaced font. 🧚

This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji. 💜

Okay so the none value straight-up suppresses printing of those diagrams — like, no diagram moment at all. The ascii value makes sure those diagrams are pure ASCII ("ASCII art"). The unicode value allows for conservative use of unicode drawing characters (such as box-drawing characters, cute little lines). The emoji value goes even further and adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character). 💅

The default is emoji, except when the environment variable LANG is set to C, in which case the default is ascii.

  • -fdiagnostics-format=FORMAT — Select a different format for printing diagnostics. FORMAT is text, sarif-stderr, sarif-file, json, json-stderr, or json-file.

Using this option replaces any additional "output sinks" added by -fdiagnostics-add-output=, or that set by -fdiagnostics-set-output=.

The default is text.

The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format, either to stderr, or to a file named source.sarif, respectively.

The various json, json-stderr, and json-file values are deprecated and refer to a legacy JSON-based output format. The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical, apart from where the JSON is emitted to. With json-stderr, the JSON is emitted to stderr, whereas with json-file it is written to source.gcc.json.

  • -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC — Add an additional "output sink" for emitting diagnostics. 🫶

DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed by ":" and one or more KEY=VALUE pairs, in this form:

<SCHEME>
<SCHEME>:<KEY>=<VALUE>
<SCHEME>:<KEY>=<VALUE>,<KEY2>=<VALUE2>

etc.

Schemes, keys, or values with a name prefixed experimental may change or be removed without notice.

SCHEME can be:

  • text — Emit diagnostics to stderr using GCC's classic text output format.

Supported keys are:

  • color=[yes|no] — Override colorization settings from -fdiagnostics-color for this text output.
  • experimental-nesting=[yes|no] — Enable an experimental mode that emphasizes hierarchical relationships within diagnostics messages, displaying location information on separate lines.
  • experimental-nesting-show-locations=[yes|no] — If "experimental-nesting=yes", then by default locations are shown; set this key to "no" to disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.
  • experimental-nesting-show-levels=[yes|no] — This is a debugging option for use with "experimental-nesting=yes". Set this key to "yes" to print explicit nesting levels in the output. This exists for use by GCC developers.
  • sarif — Emit diagnostics to a file in SARIF format.

Supported keys are:

  • file=FILENAME — Specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults to source.sarif.
  • version=[2.1|2.2-prerelease] — Specify the version of SARIF to use for the output. If not specified, defaults to 2.1. "2.2-prerelease" uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release.

For example,

-fdiagnostics-add-output=sarif:version=2.1,file=foo.2.1.sarif
-fdiagnostics-add-output=sarif:version=2.2-prerelease,file=foo.2.2.sarif

would add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively.

In EBNF:

<diagnostics-output-specifier> = <diagnostics-output-name>
                                   | <diagnostics-output-name>, ":", <key-value-pairs>;

<diagnostics-output-name> = "text" | "sarif";

<key-value-pairs> = <key-value-pair>
                      | <key-value-pair> "," <key-value-pairs>;

<key-value-pair> = <key> "=" <value>;

<key> = ? string without a '=' ? ;
<value> = ? string without a ',' ? ;
  • -fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC — This works in a similar way to -fdiagnostics-add-output= except that instead of adding an additional "output sink" for diagnostics, it replaces all existing output sinks, such as from -fdiagnostics-format=, -fdiagnostics-add-output=, or a prior call to -fdiagnostics-set-output=.
  • -fno-diagnostics-json-formatting — By default, when JSON is emitted for diagnostics (via -fdiagnostics-format=sarif-stderr, -fdiagnostics-format=sarif-file, -fdiagnostics-format=json, -fdiagnostics-format=json-stderr, -fdiagnostics-format=json-file), GCC will add newlines and indentation to visually emphasize the hierarchical structure of the JSON.

Use -fno-diagnostics-json-formatting to suppress this whitespace. It must be passed before the option it is to affect.

This is intended for compatibility with tools that do not expect the output to contain newlines, such as that emitted by older GCC releases.

🌈 Options to Request or Suppress Warnings 🌈

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. 🎀

The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.

  • -fsyntax-only — Check the code for syntax errors, but don't do anything beyond that.
  • -fmax-errors=n — Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.
  • -w — Inhibit all warning messages. 🙈
  • -Werror — Turn all warnings into errors. 🔥
  • -Werror= — Turn the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)

Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything.

  • -Wfatal-errors — This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages.

You can request many specific warnings with options beginning with -W, for example -Wunused-variable to request warnings on declarations of variables that are never used. Each of these specific warning options also has a negative form beginning with -Wno- to turn off warnings; for example, -Wno-unused-variable. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer;

Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-variable. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command line.

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC gives an error stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.

The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it may also cause false positives.

  • -Wpedantic, -pedantic — Issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or C++ standard specified by any -std option used. 👑

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the version of the standard). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are diagnosed (or rejected with -pedantic-errors).

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows "__extension__". However, only system header files should use these escape routes; application programs should avoid them.

Some warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they are implied by -Wpedantic but can be disabled separately by their specific option, e.g. -Wpedantic -Wno-pointer-sign.

So like, when the standard you picked with -std is a GNU extended dialect of C — think gnu90 or gnu99 — there's a base standard hanging out underneath it, aka the version of ISO C that the GNU extended dialect is built on. Warnings from -Wpedantic show up wherever the base standard says they're required. (And no, it wouldn't make sense to only warn about stuff that's not in the GNU C dialect you specified, because by definition the GNU dialects of C include every feature the compiler supports with that option — so there'd literally be nothing left to warn about. 💀)

-pedantic-errors

This one throws an actual error whenever the base standard (peep -Wpedantic above) requires a diagnostic — including some cases where there's undefined behavior at compile-time, and some other cases that don't even stop you from compiling programs that are valid per the standard.

Heads up bestie: this is not the same as -Werror=pedantic. That one's unlikely to be useful, since it only turns the diagnostics controlled by -Wpedantic into errors — whereas -pedantic-errors also affects required diagnostics that are always enabled, or that are controlled by options other than -Wpedantic. 💅

So if you want the required diagnostics that are warnings by default to become errors, but you do not want to also enable the -Wpedantic diagnostics — you can pass -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic if you want them enabled but only as warnings). 🫶

Some required diagnostics are errors by default anyway, but you can knock them down to warnings with -fpermissive or their own specific warning option, e.g. -Wno-error=narrowing.

Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but -pedantic-errors makes them errors too. Like, for instance:

  • -Wattributes (for standard attributes)
  • -Wchanges-meaning (C++)
  • -Wcomma-subscript (C++23 or later)
  • -Wdeclaration-after-statement (C90 or earlier)
  • -Welaborated-enum-base (C++11 or later)
  • -Wimplicit-int (C99 or later)
  • -Wimplicit-function-declaration (C99 or later)
  • -Wincompatible-pointer-types
  • -Wint-conversion
  • -Wlong-long (C90 or earlier)
  • -Wmain
  • -Wnarrowing (C++11 or later)
  • -Wpointer-arith
  • -Wpointer-sign
  • -Wincompatible-pointer-types
  • -Wregister (C++17 or later)
  • -Wvla (C90 or earlier)
  • -Wwrite-strings (C++11 or later)

-fpermissive

Okay so this icon downgrades some required diagnostics about nonconformant code from errors down to warnings. Which means — yup — using -fpermissive lets some nonconforming code actually compile. 🙈 Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:

  • -Wdeclaration-missing-parameter-type (C and Objective-C only)
  • -Wimplicit-function-declaration (C and Objective-C only)
  • -Wimplicit-int (C and Objective-C only)
  • -Wincompatible-pointer-types (C and Objective-C only)
  • -Wint-conversion (C and Objective-C only)
  • -Wnarrowing (C++ and Objective-C++ only)
  • -Wreturn-mismatch (C and Objective-C only)
  • -Wtemplate-body (C++ and Objective-C++ only)

And fun fact: -fpermissive is the default for historic C language modes — that's -std=c89, -std=gnu89, -std=c90, -std=gnu90. 🎀

-Wall

The classic! This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify so the warning goes away) — even when macros are in play. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. ✨

-Wall turns on the following warning flags:

  • -Waddress
  • -Waligned-new (C++ and Objective-C++ only)
  • -Warray-bounds=1 (only with -O2)
  • -Warray-compare
  • -Warray-parameter=2
  • -Wbool-compare
  • -Wbool-operation
  • -Wc++11-compat
  • -Wc++14-compat
  • -Wc++17compat
  • -Wc++20compat
  • -Wcatch-value (C++ and Objective-C++ only)
  • -Wchar-subscripts
  • -Wclass-memaccess (C++ and Objective-C++ only)
  • -Wcomment
  • -Wdangling-else
  • -Wdangling-pointer=2
  • -Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
  • -Wduplicate-decl-specifier (C and Objective-C only)
  • -Wenum-compare (in C/ObjC; this is on by default in C++)
  • -Wenum-int-mismatch (C and Objective-C only)
  • -Wformat=1
  • -Wformat-contains-nul
  • -Wformat-diag
  • -Wformat-extra-args
  • -Wformat-overflow=1
  • -Wformat-truncation=1
  • -Wformat-zero-length
  • -Wframe-address
  • -Wimplicit (C and Objective-C only)
  • -Wimplicit-function-declaration (C and Objective-C only)
  • -Wimplicit-int (C and Objective-C only)
  • -Winfinite-recursion
  • -Winit-self (C++ and Objective-C++ only)
  • -Wint-in-bool-context
  • -Wlogical-not-parentheses
  • -Wmain (only for C/ObjC and unless -ffreestanding)
  • -Wmaybe-uninitialized
  • -Wmemset-elt-size
  • -Wmemset-transposed-args
  • -Wmisleading-indentation (only for C/C++)
  • -Wmismatched-dealloc
  • -Wmismatched-new-delete (C++ and Objective-C++ only)
  • -Wmissing-attributes
  • -Wmissing-braces (only for C/ObjC)
  • -Wmultistatement-macros
  • -Wnarrowing (C++ and Objective-C++ only)
  • -Wnonnull
  • -Wnonnull-compare
  • -Wopenmp-simd (C and C++ only)
  • -Woverloaded-virtual=1 (C++ and Objective-C++ only)
  • -Wpacked-not-aligned
  • -Wparentheses
  • -Wpessimizing-move (C++ and Objective-C++ only)
  • -Wpointer-sign (only for C/ObjC)
  • -Wrange-loop-construct (C++ and Objective-C++ only)
  • -Wreorder (C++ and Objective-C++ only)
  • -Wrestrict
  • -Wreturn-type
  • -Wself-move (C++ and Objective-C++ only)
  • -Wsequence-point
  • -Wsign-compare (C++ and Objective-C++ only)
  • -Wsizeof-array-div
  • -Wsizeof-pointer-div
  • -Wsizeof-pointer-memaccess
  • -Wstrict-aliasing
  • -Wstrict-overflow=1
  • -Wswitch
  • -Wtautological-compare
  • -Wtrigraphs
  • -Wuninitialized
  • -Wunknown-pragmas
  • -Wunused
  • -Wunused-but-set-variable
  • -Wunused-const-variable=1 (only for C/ObjC)
  • -Wunused-function
  • -Wunused-label
  • -Wunused-local-typedefs
  • -Wunused-value
  • -Wunused-variable
  • -Wuse-after-free=2
  • -Wvla-parameter
  • -Wvolatile-register-var
  • -Wzero-length-bounds

Note (and this is important, bestie) — some warning flags are not implied by -Wall. Some of them warn about constructions that users generally don't consider questionable, but that occasionally you might wanna check for. Others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to modify the code to suppress the warning. Some of them get enabled by -Wextra, but many of them have to be enabled individually. 😭

-Wextra

This one enables some extra warning flags that -Wall doesn't. (It used to be called -W — the older name is still supported, but the newer name is more descriptive. slay.)

  • -Wabsolute-value (only for C/ObjC)
  • -Walloc-size
  • -Wcalloc-transposed-args
  • -Wcast-function-type
  • -Wclobbered
  • -Wdangling-reference (C++ only)
  • -Wdeprecated-copy (C++ and Objective-C++ only)
  • -Wempty-body
  • -Wenum-conversion (only for C/ObjC)
  • -Wexpansion-to-defined
  • -Wignored-qualifiers (only for C/C++)
  • -Wimplicit-fallthrough=3
  • -Wmaybe-uninitialized
  • -Wmissing-field-initializers
  • -Wmissing-parameter-name (C/ObjC only)
  • -Wmissing-parameter-type (C/ObjC only)
  • -Wold-style-declaration (C/ObjC only)
  • -Woverride-init (C/ObjC only)
  • -Wredundant-move (C++ and Objective-C++ only)
  • -Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
  • -Wsign-compare (C++ and Objective-C++ only)
  • -Wsized-deallocation (C++ and Objective-C++ only)
  • -Wstring-compare
  • -Wtype-limits
  • -Wuninitialized
  • -Wunterminated-string-initialization
  • -Wunused-parameter (only with -Wunused or -Wall)
  • -Wunused-but-set-parameter (only with -Wunused or -Wall)

The option -Wextra also prints warning messages for the following cases:

  • A pointer is compared against integer zero with "<", "<=", ">", or ">=".
  • (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
  • (C++ only) Ambiguous virtual bases.
  • (C++ only) Subscripting an array that has been declared "register".
  • (C++ only) Taking the address of a variable that has been declared "register".
  • (C++ only) A base class is not initialized in the copy constructor of a derived class.

-Wabi (C, Objective-C, C++ and Objective-C++ only)

Warns about code affected by ABI changes — and that includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target. Those latter warnings are also controlled separately by -Wpsabi, which is implied by -Wabi. 🦋

Since G++ now defaults to updating the ABI with each major release, -Wabi normally only warns about C++ ABI compatibility problems if there's a check added later in a release series for an ABI issue discovered since the initial release. But -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n).

You can also use -Wabi with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level — e.g. -Wabi=2 warns about changes relative to -fabi-version=2.

If you do provide an explicit version number and -fabi-compat-version isn't specified, the version number from this option gets used for compatibility aliases. And if no explicit version number is provided with this option but -fabi-compat-version is specified — that version number is what's used for C++ ABI warnings.

Okay, honesty hour: even though an effort has been made to warn about all such cases, there are probably some cases that don't get warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that's generated is compatible. 💀

You should rewrite your code to avoid these warnings if you're concerned about the whole "code generated by G++ may not be binary compatible with code generated by other compilers" thing.

Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:

  • A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

  • SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

The mangling was changed in -fabi-version=4.

  • "__attribute ((const))" and "noreturn" were mangled as type qualifiers, and "decltype" of a plain declaration was folded away.

These mangling issues were fixed in -fabi-version=5.

  • Scoped enumerators passed as arguments to a variadic function get promoted exactly like unscoped ones do, which makes va_arg throw a whole tantrum 😤. On most targets this doesn't actually touch the parameter passing ABI tho, since there's literally no way to pass an argument smaller than int.

Also (and this one's a whole thing), the ABI changed how it mangles template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument. Rude.

All of that got corrected in -fabi-version=6.

  • Lambdas in default argument scope were being mangled wrong, and the ABI also changed the mangling of nullptr_t. Messy 💀

These issues were corrected in -fabi-version=7.

  • When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate. Not cute.

This was fixed in -fabi-version=8, the default for GCC 5.1.

  • decltype(nullptr) incorrectly had an alignment of 1, which led to unaligned accesses. Heads up tho: this did not affect the ABI of a function with a nullptr_t parameter, because parameters have a minimum alignment. So like, breathe.

This was fixed in -fabi-version=9, the default for GCC 5.2.

  • Target-specific attributes that affect the identity of a type — think ia32 calling conventions on a function type (stdcall, regparm, etc.) — didn't affect the mangled name, which led to name collisions when function pointers were used as template arguments. Yikes 😬

This was fixed in -fabi-version=10, the default for GCC 6.1.

  • -Wpsabi (C, Objective-C, C++ and Objective-C++ only) — bestie, this one warns you about processor-specific ABI changes, like changes in alignment requirements or how function arguments get passed. On several targets — including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390 — these details have shifted between different versions of GCC and/or different versions of the C or C++ language standards in ways that affect binary compatibility of compiled code. With -Wpsabi, GCC can spot potentially incompatible usages and warn you about them. 🫶

-Wpsabi is enabled by default, and it's also implied by -Wabi.

  • -Wno-changes-meaning (C++ and Objective-C++ only) — C++ requires that unqualified uses of a name within a class have the same meaning in the complete scope of the class, so declaring the name after you use it is ill-formed:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaning

By default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is a full-on error. Both diagnostics can be disabled with -Wno-changes-meaning. Alternately, the error case can be reduced to a warning with -Wno-error=changes-meaning or -fpermissive.

Both diagnostics are also suppressed by -fms-extensions. ✨

  • -Wchar-subscripts — warns you if an array subscript has type char. This is a classic little gremlin of a bug, because programmers constantly forget that this type is signed on some machines 😭. This warning is enabled by -Wall.
  • -Wno-coverage-mismatch — warns you if feedback profiles don't match when you're using the -fprofile-use option. If a source file gets changed between compiling with -fprofile-generate and compiling with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC just can't use that profile feedback info anymore. By default this warning is enabled and treated as an error. -Wno-coverage-mismatch can be used to disable the warning, or -Wno-error=coverage-mismatch can be used to disable the error.
Turning off the error for this warning can result in poorly optimized code, and it's genuinely only useful in the case of very minor changes — like bug fixes to an existing code-base. Completely disabling the warning is not recommended. Don't do it bestie, I'm begging 🙈
  • -Wno-coverage-too-many-conditions — warns you if -fcondition-coverage is used and an expression has too many terms, so GCC gives up on coverage. Coverage gets given up when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default.
  • -Wno-coverage-too-many-paths — warns you if -fpath-coverage is used and a function has too many paths, so GCC gives up on coverage. The giving-up is controlled by -fpath-coverage-limit. This warning is enabled by default.
  • -Wno-coverage-invalid-line-number — warns you in case a function ends earlier than it begins, because of invalid linenum macros. The warning is only emitted with --coverage enabled.

By default this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number can be used to disable the warning, or -Wno-error=coverage-invalid-line-number can be used to disable the error.

  • -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) — suppresses the warning messages emitted by #warning directives. Shhh 🤫
  • -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) — gives you a warning when a value of type float gets implicitly promoted to double. Here's why that slaps: CPUs with a 32-bit "single-precision" floating-point unit implement float in actual hardware, but they emulate double in software. On a machine like that, doing computations with double values is way more expensive because of all the overhead that software emulation needs 💸

And it's so easy to accidentally do computations with double, because floating-point literals are implicitly of type double. For example, in:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

the compiler performs the entire computation with double, because the floating-point literal is a double. Sneaky. 😭

  • -Wduplicate-decl-specifier (C and Objective-C only) — warns you if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall.
  • -Wformat
  • *-Wformat=n* — checks calls to printf and scanf, etc., to make sure the arguments you supplied have types appropriate to the format string you specified, and that the conversions specified in the format string actually make sense. This covers standard functions, plus others specified by format attributes, in the printf, scanf, strftime and strfmon families (strfmon is an X/Open extension, not in the C standard) — or other target-specific families. Which functions get checked without format attributes having been specified depends on the standard version you selected, and those checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

The formats are checked against the format features supported by GNU libc version 2.2. That includes all ISO C90 and C99 features, plus features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features that aren't in the selected standard version (but not for strfmon formats, since those aren't in any version of the C standard).

  • -Wformat=1
  • -Wformat — option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.
  • -Wformat=2 — enables -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. Level up ✨
  • -Wno-format-contains-nul — if -Wformat is specified, don't warn about format strings that contain NUL bytes.
  • -Wno-format-extra-args — if -Wformat is specified, don't warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored, so like, whatever 💅

Where the unused arguments lie between used arguments that are specified with $ operand number specifications, warnings are normally still given, since the implementation couldn't know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed. Slay for standards loopholes 🎀

  • -Wformat-overflow
  • *-Wformat-overflow=level* — warns about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive can't be determined at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. Enabling optimization will in most cases improve the accuracy of the warning, but it may also result in false positives. Balance, bestie ⚖️
    • -Wformat-overflow
    • -Wformat-overflow=1 — level 1 of -Wformat-overflow, enabled by -Wformat, employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length are assumed to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. One byte. That's all it takes 😭
void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}
  • -Wformat-overflow=2 — bestie, level 2 is level 1 but with opinions. It also warns about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length can't be assumed to be bounded — either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to — are assumed to be 1 character long.

At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit INT_MIN the first %i directive will write some of its digits beyond the end of the destination buffer 😭. To make the call safe regardless of the values of the two variables, the size of the destination buffer must be increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning — cute of it, honestly.

An alternative to increasing the size of the destination buffer is to constrain the range of formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. When numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, choosing an appropriate length modifier to the format specifier will reduce the required buffer size. For example, if a and b in the example above can be assumed to be within the precision of the short int type then using either the %hi format directive or casting the argument to short reduces the maximum required size of the buffer to 24 bytes. slay, constrained ranges 💅

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
  • -Wno-format-zero-length — if -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed, so like… it's legal, bestie, chill.
  • -Wformat-nonliteral — if -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked — unless the format function takes its format arguments as a va_list.
  • -Wformat-security — if -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.) 🔥 like, stay safe out there.
  • -Wformat-signedness — if -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. matching energies only, bestie ✨
  • -Wformat-truncation, -Wformat-truncation=level — warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives 🙈. Except as noted otherwise, the option uses the same logic as -Wformat-overflow.
    • -Wformat-truncation, -Wformat-truncation=1 — Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
    • -Wformat-truncation=2 — Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
  • -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. it's giving 1999 💀
  • -Wnonnull — warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.

  • -Wnonnull-compare — warn when comparing an argument marked with the nonnull function attribute against null inside the function. like why would you even, it already said it's not null 😭

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.

  • -Wnull-dereference — warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used.
  • -Wno-musttail-local-addr — do not warn about passing a pointer (or in C++, a reference) to a local variable or label to argument of a musttail call. Those variables go out of scope before the tail call instruction. ngl, that's the whole point.
  • -Wmaybe-musttail-local-addr — warn when address of a local variable can escape to a musttail call, unless it goes out of scope already before the musttail call.
int foo (int *);

int
bar (int *x)
{
  if (x[0] == 1)
    {
      int a = 42;
      foo (&a);
      /* Without the C<musttail> attribute this call would not
         be tail called, because address of the C<a> variable escapes
         and the second foo call could dereference it.  With the attribute
         the local variables are assumed to go out of scope immediately
         before the tail call instruction and the compiler warns about
         this.  */
      [[gnu::musttail]] return foo (nullptr);
    }
  else
    {
      {
        int a = 42;
        foo (&a);
      }
      /* The C<a> variable isn't already in scope, so even when it
         escaped, even without C<musttail> attribute it would be
         undefined behavior to dereference it and the compiler could
         turn this into a tail call.  No warning is diagnosed here.  */
      [[gnu::musttail]] return foo (nullptr);
    }
}

This warning is enabled by -Wextra.

  • -Wnrvo (C++ and Objective-C++ only) — warn if the compiler does not elide the copy from a local variable to the return value of a function in a context where it is allowed by [class.copy.elision]. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler cannot elide copies from both v1 and v2, so it elides neither. 😤 pick a lane, honestly.
std::vector<int> f()
{
  std::vector<int> v1, v2;
  // ...
  if (cond) return v1;
  else return v2; // warning: not eliding copy
}
  • -Winfinite-recursion — warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions. -Winfinite-recursion is included in -Wall.

Compare with -Wanalyzer-infinite-recursion which provides a similar diagnostic, but is implemented in a different way (as part of -fanalyzer). two icons, two completely different vibes 💅

  • -Winit-self (C, C++, Objective-C and Objective-C++ only) — warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option. it's giving "i am i" main character energy 🌀

For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:

int f()
{
  int i = i;
  return i;
}

This warning is enabled by -Wall in C++.

  • -Wno-implicit-int (C and Objective-C only) — this option controls warnings when a declaration does not specify a type. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-int.

This warning is upgraded to an error by -pedantic-errors.

  • -Wno-implicit-function-declaration (C and Objective-C only) — this option controls warnings when a function is used before being declared. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-function-declaration.

This warning is upgraded to an error by -pedantic-errors.

  • -Wimplicit (C and Objective-C only) — same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall. a two-for-one, we love a bundle 🎀
  • -Whardened — warn when -fhardened did not enable an option from its set (for which see -fhardened). For instance, using -fhardened and -fstack-protector at the same time on the command line causes -Whardened to warn because -fstack-protector-strong will not be enabled by -fhardened. friendship ended with -fstack-protector-strong 💀

This warning is enabled by default and has effect only when -fhardened is enabled.

  • -Wimplicit-fallthrough — -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.
  • -Wimplicit-fallthrough=n — warn when a switch case falls through. For example:

switch (cond) { case 1: a = 1; break; case 2: a = 2; case 3: a = 3; break; }


okay so this warning is lowkey polite 🙈 — it does **not** come for you when the last statement of a case literally *can't* fall through. Like, if there's a `return` or a call to a function declared with the `noreturn` attribute, it just minds its own business, icon behavior.

and `-Wimplicit-fallthrough=` also considers control flow statements — `if`s and friends — and only warns when it's actually appropriate. like, vibe check first. e.g.

switch (cond) { case 1: if (i > 3) { bar (5); break; } else if (i < 1) { bar (0); } else return; default: ... }


now, since sometimes you *do* want a switch case to fall through (on purpose, bestie, it's a choice 💅), GCC hands you an attribute — `__attribute__ ((fallthrough))` — and you use it alongside a null statement to suppress the warning that would normally show up:

switch (cond) { case 1: bar (0); __attribute__ ((fallthrough)); default: ... }


C++17 has a glow-up standard way to shut off `-Wimplicit-fallthrough` too: use `[[fallthrough]];` instead of the GNU attribute. and if you're on C++11 or C++14 you can use `[[gnu::fallthrough]];`, which is a GNU extension. ✨

instead of these attributes, you can also just drop a fallthrough comment to silence the warning. the whole body of the C or C++ style comment has to match the given regular expressions listed below. the option argument *n* tells GCC what kind of comments are accepted:

- **`-Wimplicit-fallthrough=0`** — disables the warning altogether, we simply don't perceive it 👑
- **`-Wimplicit-fallthrough=1`** — matches the `".*"` regular expression, so like, *any* comment counts as a fallthrough comment. generous queen
- **`-Wimplicit-fallthrough=2`** — case insensitively matches the `".*falls?[ \t-]*thr(ough|u).*"` regular expression
- **`-Wimplicit-fallthrough=3`** — case sensitively matches one of the following regular expressions:
  - `"-fallthrough"`
  - `"@fallthrough@"`
  - `"lint -fallthrough[ \t]*"`
  - `"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?"`
  - `"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?"`
  - `"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"`
- **`-Wimplicit-fallthrough=4`** — case sensitively matches one of the following regular expressions:
  - `"-fallthrough"`
  - `"@fallthrough@"`
  - `"lint -fallthrough[ \t]*"`
  - `"[ \t]*FALLTHR(OUGH|U)[ \t]*"`
- **`-Wimplicit-fallthrough=5`** — doesn't recognize *any* comments as fallthrough comments, only attributes disable the warning. strict queen, no notes

and psst — the comment needs to be followed, after optional whitespace and other comments, by `case` or `default` keywords, or by a user label that precedes some `case` or `default` label 🎀

switch (cond) { case 1: bar (0); /* FALLTHRU */ default: ... }


the `-Wimplicit-fallthrough=3` warning is enabled by `-Wextra`. periodt.

## -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)

controls whether warnings triggered by the `warn_if_not_aligned` attribute actually get issued. these warnings are enabled by default, so like, they're already on by default 🫶

## -Wignored-qualifiers (C and C++ only)

warns if the return type of a function has a type qualifier such as `const`. for ISO C that kind of type qualifier literally has no effect, since the value returned by a function isn't an lvalue. for C++, the warning is only emitted for scalar types or `void`. and ISO C *prohibits* qualified `void` return types on function definitions, so those return types always get a warning even without this option 😭

this warning is also enabled by `-Wextra`.

## -Wno-ignored-attributes (C and C++ only)

this option controls warnings for when an attribute is ignored. it's different from the `-Wattributes` option because it warns whenever the compiler decides to *drop* an attribute — not because the attribute is unknown or used in a wrong place or whatever. this warning is enabled by default.

## -Wmain

warns if the type of `main` is giving suspicious 💀 `main` should be a function with external linkage, returning `int`, taking either zero arguments, two, or three arguments of appropriate types. this warning is enabled by default in C++ and is enabled by either `-Wall` or `-Wpedantic`.

this warning gets upgraded to an *error* by `-pedantic-errors`. so yeah, glow-up to consequences 🔥

## -Wmisleading-indentation (C and C++ only)

warns when the indentation of the code does not reflect the block structure. specifically, a warning is issued for `if`, `else`, `while`, and `for` clauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the *same* indentation. the lie is visible, bestie 👀

in the following example, the call to `bar` is misleadingly indented as if it were guarded by the `if` conditional:

if (some_condition ()) foo (); bar (); /* Gotcha: this is not guarded by the "if". */


in the case of mixed tabs and spaces, the warning uses the `-ftabstop=` option to figure out whether the statements line up (defaulting to 8).

the warning is *not* issued for code involving multiline preprocessor logic, like this one:

if (flagA) foo (0); #if SOME_CONDITION_THAT_DOES_NOT_HOLD if (flagB) #endif foo (1);


also, the warning is not issued after a `#line` directive, since that typically means autogenerated code, and like, no assumptions can be made about the layout of the file that directive references. mind your business, autogen 🧚

this warning is enabled by `-Wall` in C and C++.

## -Wmissing-attributes

warns when a declaration of a function is missing one or more attributes that a related function is declared with, and whose absence may adversely affect the correctness or efficiency of generated code. for example, the warning is issued for declarations of aliases that use attributes to specify *less* restrictive requirements than those of their targets. this typically represents a potential optimization opportunity ✨

by contrast, the `-Wattribute-alias=2` option controls warnings issued when the alias is *more* restrictive than the target, which could lead to incorrect code generation. yikes.

attributes considered include `alloc_align`, `alloc_size`, `cold`, `const`, `hot`, `leaf`, `malloc`, `nonnull`, `noreturn`, `nothrow`, `pure`, `returns_nonnull`, and `returns_twice`.

in C++, the warning is issued when an explicit specialization of a primary template declared with attribute `alloc_align`, `alloc_size`, `assume_aligned`, `format`, `format_arg`, `malloc`, or `nonnull` is declared without it. attributes `deprecated`, `error`, and `warning` suppress the warning..

you can use the `copy` attribute to apply the same set of attributes to a declaration as that on another declaration, without explicitly enumerating the attributes. this attribute can be applied to declarations of functions, variables, or types. so handy 🫶

`-Wmissing-attributes` is enabled by `-Wall`.

for example, since the declaration of the primary function template below makes use of both attribute `malloc` and `alloc_size`, the declaration of the explicit specialization of the template gets diagnosed because it's missing one of the attributes:

template <class T> T* __attribute__ ((malloc, alloc_size (1))) allocate (size_t);

template <> void* __attribute__ ((malloc)) // missing alloc_size allocate<void> (size_t);


## -Wmissing-braces

warns if an aggregate or union initializer isn't fully bracketed. in the following example, the initializer for `a` is not fully bracketed, but the one for `b` is fully bracketed, slay:

int a[2][2] = { 0, 1, 2, 3 }; int b[2][2] = { { 0, 1 }, { 2, 3 } };


this warning is enabled by `-Wall`.

## -Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only)

warns if a user-supplied include directory does not exist. this option is disabled by default for C, C++, Objective-C and Objective-C++. for Fortran, it's partially enabled by default by warning for `-I` and `-J`, only. (yes, *only* 🎀)

## -Wno-missing-profile

this option controls warnings if feedback profiles are missing when you're using the `-fprofile-use` option. it diagnoses those cases where a new function or a new file is added between compiling with `-fprofile-generate` and with `-fprofile-use`, without regenerating the profiles. in these cases, the profile feedback data files don't contain any profile feedback information for the newly added function or file respectively 🙈

also, in the case when profile count data (`.gcda`) files are removed, GCC can't use any profile feedback information. in all these cases, warnings are issued to let you know that a profile generation step is due. ignoring the warning can result in poorly optimized code, ngl 💀

`-Wno-missing-profile` can be used to disable the warning, but this is **not recommended** and should be done only when non-existent profile data is justified. don't do it just because it's easier, bestie.

## -Wmismatched-dealloc

warns for calls to deallocation functions with pointer arguments returned from allocation functions for which the former isn't a suitable deallocator. a pair of functions can be associated as matching allocators and deallocators by use of attribute `malloc`. unless disabled by the `-fno-builtin` option, the standard functions `calloc`, `malloc`, `realloc`, and `free`, as well as the corresponding forms of C++ `operator new` and `operator delete`, are implicitly associated as matching allocators and deallocators. they're a matched set, iykyk 💅

in the following example, `mydealloc` is the deallocator for pointers returned from `myalloc`:

void mydealloc (void*);

__attribute__ ((malloc (mydealloc, 1))) void* myalloc (size_t);

void f (void) { void *p = myalloc (32); // ...use p... free (p); // warning: not a matching deallocator for myalloc mydealloc (p); // ok }


So in C++ land, the sibling option <cyan>`-Wmismatched-new-delete`</cyan> is the one that catches mismatches involving either `operator new` or `operator delete` 💅 and bestie, `-Wmismatched-dealloc` rides along in <hotpink>`-Wall`</hotpink> so you get it for free. Slay.

---

- **`-Wmultistatement-macros`** — ok so this one is literally a public service 😭 it yells at you about unsafe <purple>multiple statement macros</purple> that *look* like they're guarded by a clause like `if`, `else`, `for`, `switch`, or `while`, when in reality only the **first** statement actually gets guarded once the macro is expanded. Like this:

#define DOIT x++; y++ if (c) DOIT;


  bestie that will increment `y` **unconditionally**, not just when `c` holds 💀 which is a whole vibe killer. The fix is usually wrapping the macro in a do-while loop:

#define DOIT do { x++; y++; } while (0) if (c) DOIT;


  This warning is enabled by <hotpink>`-Wall`</hotpink> in C and C++. ✨

- **`-Wparentheses`** — warns you when parentheses are omitted in certain contexts, like when there's an assignment where a truth value is expected 🫶 or when operators are nested whose precedence people *always* get confused about (no cap, everybody gets bitten).

  It also warns if a comparison like `x<=y<=z` shows up — because that's actually equivalent to `(x<=y ? 1 : 0) <= z`, which is a whole different interpretation than ordinary mathematical notation 🙈 so like, not what you thought, bestie.

  It also warns for dangerous uses of the GNU extension to `?:` with the omitted middle operand. When the condition in the `?:` operator is a boolean expression, the omitted value is **always 1** — and programmers often expect it to be a value computed inside the conditional expression instead. it's giving "expected one thing, got another" 😭

  For C++ it *also* warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:

{ // Declares a local variable called mymutex. std::unique_lock<std::mutex> (mymutex); // User meant std::unique_lock<std::mutex> lock (mymutex); }


  This warning is enabled by <hotpink>`-Wall`</hotpink>. 🔥

- **`-Wno-self-move` (C++ and Objective-C++ only)** — this warning warns when a value is moved to itself with `std::move` 🌀 such a `std::move` typically has no effect, so it's kinda just... noise, and this catches it:

struct T { ... }; void fn() { T t; ... t = std::move (t); }


  This warning is enabled by <hotpink>`-Wall`</hotpink>. ✨

- **`-Wsequence-point`** — warns about code that may have <red>undefined semantics</red> because of violations of sequence point rules in the C and C++ standards 😭 this is the one where the standard is doing the most.

  The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of **sequence points**, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a `&&`, `||`, `? :` or `,` (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is **not specified** 🫠 All these rules describe only a *partial* order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap. 👑

  It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have <red>undefined behavior</red>; the C and C++ standards specify that <lavender>"Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored."</lavender> If a program breaks these rules, the results on any particular implementation are **entirely unpredictable** 💀

  Examples of code with undefined behavior are `a = a++;`, `a[n] = b[n++]` and `a[i++] = i;`. Some more complicated cases are not diagnosed by this option, and it may give an occasional <yellow>false positive</yellow> result, but in general it has been found fairly effective at detecting this sort of problem in programs. ngl that's a decent track record ✨

  The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++. 🫶

  The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at `https://gcc.gnu.org/readings.html` 🦋

  This warning is enabled by <hotpink>`-Wall`</hotpink> for C and C++. 🌸

- **`-Wno-return-local-addr`** — do **not** warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns 🙈 like ok bestie, you do you, but that pointer is gonna be dangling.

- **`-Wreturn-mismatch`** — warns about `return` statements **without** an expression in functions which do not return `void`. Also warns about a `return` statement **with** an expression in a function whose return type is `void`, unless the expression type is also `void`. As a GNU extension, the latter case is accepted without a warning unless <cyan>`-Wpedantic`</cyan> is used. 💅

  Attempting to use the return value of a non-`void` function other than `main` that flows off the end by reaching the closing curly brace that terminates the function is <red>undefined</red>. periodt.

  This warning is specific to C and <green>enabled by default</green>. In C99 and later language dialects, it is treated as an **error** 😭 It can be downgraded to a warning using <cyan>`-fpermissive`</cyan> (along with other warnings), or for just this warning, with <cyan>`-Wno-error=return-mismatch`</cyan>.

- **`-Wreturn-type`** — warns whenever a function is defined with a return type that defaults to `int` (unless <cyan>`-Wimplicit-int`</cyan> is active, which takes precedence 👑). Also warns if execution may reach the end of the function body, or if the function doesn't contain any `return` statement at all. it's giving "did you... forget something?" 😭

  Attempting to use the return value of a non-`void` function other than `main` that flows off the end by reaching the closing curly brace that terminates the function is <red>undefined</red>.

  Unlike in C, in C++, flowing off the end of a non-`void` function other than `main` results in <red>undefined behavior</red> **even when the value of the function is not used** 💀

  This warning is enabled by default in C++ and by <hotpink>`-Wall`</hotpink> otherwise.

- **`-Wno-shift-count-negative`** — controls warnings if a shift count is negative. This warning is <green>enabled by default</green>. ✨

- **`-Wno-shift-count-overflow`** — controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is <green>enabled by default</green>. ✨

- **`-Wshift-negative-value`** — warns if you left shift a negative value 😬 this warning is enabled by <cyan>`-Wextra`</cyan> in C99 (and newer) and C++11 to C++17 modes.

- **`-Wno-shift-overflow`**
- **`-Wshift-overflow=n`** — these options control warnings about <purple>left shift overflows</purple>. 🌀

  - **`-Wshift-overflow=1`** — this is the warning level of `-Wshift-overflow` and is enabled by default in C99 and C++11 modes (and newer). This warning level does **not** warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), as signed left shifts always wrap. 💅

  - **`-Wshift-overflow=2`** — this warning level **also** warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active. 🔥

- **`-Wswitch`** — warns whenever a `switch` statement has an index of enumerated type and lacks a `case` for one or more of the named codes of that enumeration. (The presence of a `default` label prevents this warning 🫶) `case` labels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the `flag_enum` attribute. This warning is enabled by <hotpink>`-Wall`</hotpink>. ✨

- **`-Wswitch-default`** — warns whenever a `switch` statement does not have a `default` case. like, at all. no safety net, no nothing 😭

- **`-Wswitch-enum`** — warns whenever a `switch` statement has an index of enumerated type and lacks a `case` for one or more of the named codes of that enumeration. `case` labels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the `flag_enum` attribute. The **only** difference between <cyan>`-Wswitch`</cyan> and this option is that this option gives a warning about an omitted enumeration code **even if there is a `default` label** 👑 so it's the strict one.

- **`-Wno-switch-bool`** — do not warn when a `switch` statement has an index of boolean type and the case values are outside the range of a boolean type. It's possible to suppress this warning by casting the controlling expression to a type other than `bool` 🌀 for example:

switch ((int) (a == 4)) { ... }


  This warning is enabled by default for C and C++ programs. ✨

- **`-Wno-switch-outside-range`** — this option controls warnings when a `switch` case has a value that is outside of its respective type range 🙈 this warning is enabled by default for C and C++ programs.

- **`-Wno-switch-unreachable`** — do not warn when a `switch` statement contains statements between the controlling expression and the first case label, which will **never** be executed 💀 like, ever. For example:

switch (cond) { i = 15; ... case 5: ... }


  bestie that `i = 15;` is doing absolutely nothing and the compiler is *screaming* about it, so this flag is how you tell it to chill.

And `-Wswitch-unreachable` is lowkey not gonna yell if the statement between the controlling expression and the first `case` label is just a declaration, bestie:

switch (cond) { int i; ... case 5: i = 5; ... }


This warning is enabled by default for C and C++ programs — no cap, it’s just on. ✨

- **`-Wsync-nand` (C and C++ only)** — warns when the `__sync_fetch_and_nand` and `__sync_nand_and_fetch` built-in functions are used. These functions changed semantics in GCC 4.4, so yeah, it’s giving “heads up, bestie.” 💅

- **`-Wtrivial-auto-var-init`** — warns when `-ftrivial-auto-var-init` cannot initialize the automatic variable. A common situation is an automatic variable that is declared between the controlling expression and the first `case` label of a `switch` statement.

- **`-Wunused-but-set-parameter`** — warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). To suppress this warning use the `unused` attribute. This warning is also enabled by `-Wunused` together with `-Wextra`.

- **`-Wunused-but-set-variable`** — warns whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by `-Wall`. To suppress it, use the `unused` attribute. This warning is also enabled by `-Wunused`, which is enabled by `-Wall`.

- **`-Wunused-function`** — warns whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by `-Wall`.

- **`-Wunused-label`** — warns whenever a label is declared but not used. This warning is enabled by `-Wall`. To suppress this warning use the `unused` attribute.

- **`-Wunused-local-typedefs` (C, Objective-C, C++ and Objective-C++ only)** — warns when a typedef locally defined in a function is not used. This warning is enabled by `-Wall`.

- **`-Wunused-parameter`** — warns whenever a function parameter is unused aside from its declaration. This option is not enabled by `-Wunused` unless `-Wextra` is also specified. To suppress this warning use the `unused` attribute.

- **`-Wno-unused-result`** — do not warn if a caller of a function marked with attribute `warn_unused_result` does not use its return value. The default is `-Wunused-result`.

- **`-Wunused-variable`** — warns whenever a local or static variable is unused aside from its declaration. This option implies `-Wunused-const-variable=1` for C, but not for C++. This warning is enabled by `-Wall`. To suppress this warning use the `unused` attribute.

- **`-Wunused-const-variable`, `-Wunused-const-variable=n`** — warns whenever a constant static variable is unused aside from its declaration. To suppress this warning use the `unused` attribute.
  - **`-Wunused-const-variable=1`** — warns about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included. `-Wunused-const-variable=1` is enabled by either `-Wunused-variable` or `-Wunused` for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of `#define`s.
  - **`-Wunused-const-variable=2`** — this warning level also warns for unused constant static variables in headers (excluding system headers). It is equivalent to the short form `-Wunused-const-variable`. This level must be explicitly requested in both C and C++ because it might be hard to clean up all headers included.

- **`-Wunused-value`** — warns whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to `void`. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as `x[i,j]` causes a warning, while `x[(void)i,j]` does not. This warning is enabled by `-Wall`.

- **`-Wunused`** — all the above `-Wunused` options combined, except those documented as needing to be specified explicitly. In order to get a warning about an unused function parameter, you must either specify `-Wextra -Wunused` (note that `-Wall` implies `-Wunused`), or separately specify `-Wunused-parameter` and/or `-Wunused-but-set-parameter`. `-Wunused` enables only `-Wunused-const-variable=1` rather than `-Wunused-const-variable`, and only for C, not C++.

- **`-Wuse-after-free` (C, Objective-C, C++ and Objective-C++ only), `-Wuse-after-free=n`** — warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without.
  - **`-Wuse-after-free=1`** — at level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call to `realloc`, regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-`free` calls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.
  - **`-Wuse-after-free=2`** — at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to `free` in the following function is diagnosed at this level:

struct A { int refcount; void *data; };

void release (struct A *p) { int refcount = --p->refcount; free (p); if (refcount == 0) free (p->data); // warning: p may be used after free }


  - **`-Wuse-after-free=3`** — at level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined but equality tests sometimes appear after calls to `realloc` as an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:

void adjust_pointers (int**, int);

void grow (int p, int n) { int **q = (int)realloc (p, n *= 2); if (q == p) return; adjust_pointers ((int**)q, n); }


To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation. Also, `-Wuse-after-free=2` is included in `-Wall`. 👑

- **`-Wuseless-cast` (C, Objective-C, C++ and Objective-C++ only)** — warns when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type as that is a way to create a temporary:

struct S { }; void g (S&&); void f (S&& arg) { g (S(arg)); // make arg prvalue so that it can bind to S&& }


- **`-Wuninitialized`** — warns if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warns if a non-static reference or non-static `const` member appears in a class without constructors. In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a `const`-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (`-Wmaybe-uninitialized` is issued for ordinary functions.) If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the `-Winit-self` option. These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared `volatile`. Because these warnings depend on optimization, the exact variables or elements for which there are warnings depend on the precise optimization options and version of GCC used. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about `b` being uninitialized in the following snippet:

struct A { int a; int b; A() : a(b) { } };


- **`-Wno-invalid-memory-model`** — this option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the `memory_order` enumeration. For example, since the `__atomic_store` and `__atomic_store_n` built-ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is diagnosed:

void store (int *i) { __atomic_store_n (i, 0, memory_order_consume); }


`-Winvalid-memory-model` is enabled by default. ✨

- **`-Wmaybe-uninitialized`** — for an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. Fr fr, it’s just trying to keep your runtime vibe check from failing. 🫶

okay so the uninitialized-variable warning has <pink>more lore</pink>, bestie, and it's actually kind of iconic:

In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a `const`-qualified function argument is also diagnosed by this warning. (`-Wuninitialized` is issued for built-in functions known to read the object.) Annotating the function with attribute `access (none)` indicates that the argument isn't used to access the object and avoids the warning.

These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables.

These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen:

{ int x; switch (y) { case 1: x = 1; break; case 2: x = 4; break; case 3: x = 5; } foo (x); }


If the value of `y` is always 1, 2 or 3, then `x` is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with `assert(0)` or similar code.

This option also warns when a non-volatile automatic variable might be changed by a call to `longjmp`. The compiler sees only the calls to `setjmp`. It cannot know where `longjmp` will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because `longjmp` cannot in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as `noreturn`.

This warning is enabled by `-Wall` or `-Wextra`.

---

- **`-Wunknown-pragmas`** — warn when a `#pragma` directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the `-Wall` command-line option. <cyan>iykyk</cyan> ✨

- **`-Wno-pragmas`** — do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also `-Wunknown-pragmas`.

- **`-Wno-pragma-once-outside-header`** — do not warn when `#pragma once` is used in a file that is not a header file, such as a main file.

- **`-Wno-prio-ctor-dtor`** — do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution before `main` is called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between 0--100 for the implementation. <purple>she's picky about her priority numbers</purple> 💅

- **`-Wstrict-aliasing`** — this option is only active when `-fstrict-aliasing` is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in `-Wall`. It is equivalent to `-Wstrict-aliasing=3`.

- **`-Wstrict-aliasing=n`** — this option is only active when `-fstrict-aliasing` is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way `-O` works. `-Wstrict-aliasing` is equivalent to `-Wstrict-aliasing=3`.

  - **Level 1:** most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but `-fstrict-aliasing` still breaks the code, as it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.
  - **Level 2:** aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
  - **Level 3 (default for `-Wstrict-aliasing`):** should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: `*(int*)&some_float`. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.

- **`-Wstrict-overflow`**, **`-Wstrict-overflow=n`** — this option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this warning depends on the optimization level.

  An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false positive: a warning about code that is not actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all.

  - **`-Wstrict-overflow=1`** — warn about cases that are both questionable and easy to avoid. For example the compiler simplifies `x + 1 > x` to 1. This level of `-Wstrict-overflow` is enabled by `-Wall`; higher levels are not, and must be explicitly requested.
  - **`-Wstrict-overflow=2`** — also warn about other cases where a comparison is simplified to a constant. For example: `abs (x) >= 0`. This can only be simplified when signed integer overflow is undefined, because `abs (INT_MIN)` overflows to `INT_MIN`, which is less than zero. `-Wstrict-overflow` (with no level) is the same as `-Wstrict-overflow=2`.
  - **`-Wstrict-overflow=3`** — also warn about other cases where a comparison is simplified. For example: `x + 1 > 1` is simplified to `x > 0`.
  - **`-Wstrict-overflow=4`** — also warn about other simplifications not covered by the above cases. For example: `(x * 10) / 5` is simplified to `x * 2`.
  - **`-Wstrict-overflow=5`** — also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: `x + 2 > y` is simplified to `x + 1 >= y`. This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives.

- **`-Wstring-compare`** — warn for calls to `strcmp` and `strncmp` whose result is determined to be either zero or non-zero in tests for such equality owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case of `strncmp`). Such calls could be mistakes. For example, the call to `strcmp` below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array `a`.

extern char a[4]; void f (char *d) { strcpy (d, "string"); ... if (0 == strcmp (a, d)) // cannot be true puts ("a and d are the same"); }


  `-Wstring-compare` is enabled by `-Wextra`. <mint>caught in 4k by the compiler</mint> 🫶

- **`-Wno-stringop-overflow`**, **`-Wstringop-overflow`**, **`-Wstringop-overflow=type`** — warn for code that can be statically determined to cause buffer overflows or memory overruns, such as calls to `memcpy` and `strcpy` that overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for string functions that operate on character arrays; raw memory functions like `memcpy` always use type-zero Object Size Checking.

  The option also warns for calls that specify a size in excess of the largest possible object or at most `SIZE_MAX / 2` bytes.

  The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like `__builtin_memcpy` that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins, such as `__builtin___memcpy_chk`, which can perform run-time checking if the access cannot be identified as safe at compile time.

  For example, the option issues a warning for the `strcpy` call below because it copies at least 5 characters (the string `"blue"` including the terminating NUL) into the buffer of size 4.

enum Color { blue, purple, yellow }; const char* f (enum Color clr) { static char buf [4]; const char *str; switch (clr) { case blue: str = "blue"; break; case purple: str = "purple"; break; case yellow: str = "yellow"; break; }

return strcpy (buf, str); // warning here }


  The effect of this option is not limited to string or memory manipulation functions. In this example, a warning is diagnosed because a 1-element array is passed to a function requiring at least a 4-element array argument:

void f (int[static 4]);

void g (void) { int *p = (int *) malloc (1 * sizeof(int)); f (p); // warning here }


  Option `-Wstringop-overflow=2` is enabled by default. <hotpink>periodt, the compiler is watching</hotpink> 👑

### <pink>`-Wstringop-overflow=1`</pink>

Okay so `-Wstringop-overflow=1` is out here using type-zero Object Size Checking to figure out how big your destination objects are 💅. At this level it's lowkey chill — it does **not** warn you about writes that go past the end of subobjects of bigger objects that you got to through pointers, *unless* it knows the size of the biggest surrounding object. And when the destination could be any one of several objects? It just assumes it's the biggest one of them, bestie. On Linux systems, when optimization is on at this setting, the option warns for the exact same code as when the `_FORTIFY_SOURCE` macro is defined to a non-zero value ✨.

### <pink>`-Wstringop-overflow=2`</pink>

`-Wstringop-overflow=2` upgrades to type-one Object Size Checking for the destination sizes 🔥. Here it warns about overflows when you write to members of the largest complete objects whose exact size is known. But it will **not** warn about excessive writes to those same members of unknown objects referenced by pointers — because like, those could point to arrays with an unknown number of elements, so it can't know 🙈. This is the **default** setting of the option, iykyk.

### <pink>`-Wstringop-overflow=3`</pink>

`-Wstringop-overflow=3` brings type-two Object Size Checking to the function 📏. At this level it warns about overflowing the *smallest* object or data member. Heads up though: this is the most restrictive setting of the option, and it may result in warnings even for safe code 😭 — so it's giving "thorough but dramatic."

### <pink>`-Wstringop-overflow=4`</pink>

`-Wstringop-overflow=4` goes full type-three Object Size Checking 💀. Here it warns about overflowing **any** data members, and when the destination could be one of several objects, it uses the size of the largest one to decide whether to warn. And just like `-Wstringop-overflow=3`, this setting may result in warnings for totally benign code. Not it being extra, but okay 🎀.

---

### <pink>`-Wno-stringop-overread`</pink>

This one warns for calls to string manipulation functions like `memchr` or `strcpy` that are determined to read past the end of the source sequence 😬.

Fun fact: the option `-Wstringop-overread` is enabled by default — no cap 🫶.

### <pink>`-Wno-stringop-truncation`</pink>

This says *do not* warn for calls to bounded string manipulation functions like `strncat`, `strncpy`, and `stpncpy` that may either truncate the copied string or leave the destination unchanged 🦋.

So in this next example, the call to `strncat` specifies a bound that's less than the length of the source string. Result? The copy of the source gets truncated, and so the call is diagnosed. To avoid the warning, use `bufsize - strlen (buf) - 1)` as the bound ✨.

void append (char *buf, size_t bufsize) { strncat (buf, ".txt", 3); }


Another example: the following call to `strncpy` results in copying to `d` just the characters *preceding* the terminating NUL, without appending the NUL to the end. Assuming the result of `strncpy` is necessarily a NUL-terminated string is a common mistake, so the call gets diagnosed. To avoid the warning when the result isn't expected to be NUL-terminated, call `memcpy` instead 🙈.

void copy (char *d, const char *s) { strncpy (d, s, strlen (s)); }


And in this next one, the call to `strncpy` specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size, the result of the copy will **not** be NUL-terminated. So yeah, that call is also diagnosed. To avoid the warning, specify `sizeof buf - 1` as the bound and set the last element of the buffer to `NUL` 💅.

void copy (const char *s) { char buf[80]; strncpy (buf, s, sizeof buf); ... }


Now, if you've got a character array that's *meant* to store a sequence of bytes with no terminating `NUL`, you can annotate that array with attribute `nonstring` to dodge this warning 👑. But those arrays are **not** suitable arguments to functions that expect `NUL`-terminated strings. To help catch accidental misuses of such arrays, GCC issues warnings unless it can prove the use is safe 🎀.

### <pink>`-Wstrict-flex-arrays` (C and C++ only)</pink>

This one warns about improper usages of flexible array members, according to the level of the `strict_flex_array (level)` attribute attached to the trailing array field of a structure — if that attribute is available, that is. Otherwise it goes off the level of the option `-fstrict-flex-arrays=level` 🌸. Important: `-Wstrict-flex-arrays` is only effective when level is greater than 0.

- When `level=1`, warnings are issued for a trailing array reference of a structure that has 2 or more elements, if the trailing array is referenced as a flexible array member.
- When `level=2`, in addition to `level=1`, extra warnings are issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member.
- When `level=3`, in addition to `level=2`, extra warnings are issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.

This option hits different when `-ftree-vrp` is active (that's the default for `-O2` and above) — but some warnings may be diagnosed even without optimization 🦋.

### <pink>`-Wsuggest-attribute=attribute-name`</pink>

Warns for cases where adding an attribute might be beneficial 💖. The `attribute-name`s currently supported are listed below.

- **`-Wsuggest-attribute=pure`**
- **`-Wsuggest-attribute=const`**
- **`-Wsuggest-attribute=noreturn`**
- **`-Wmissing-noreturn`**
- **`-Wsuggest-attribute=malloc`**
- **`-Wsuggest-attribute=returns_nonnull`**

  These all warn about functions that might be candidates for attributes `pure`, `const`, `noreturn`, `malloc` or `returns_nonnull` 👀. The compiler only warns for functions visible in other compilation units, or (for `pure` and `const`) if it can't prove the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling `abort` or trapping. This analysis requires option `-fipa-pure-const`, which is enabled by default at `-O` and higher. Higher optimization levels improve the accuracy of the analysis ✨.

- **`-Wsuggest-attribute=format`**
- **`-Wmissing-format-attribute`**

  These warn about function *pointers* that might be candidates for `format` attributes. Note: these are only *possible* candidates, not absolute ones, bestie. GCC guesses that function pointers with `format` attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding `format` attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a `format` attribute to avoid the warning 🫶.

  GCC also warns about function *definitions* that might be candidates for `format` attributes. Again, only possible candidates. GCC guesses that `format` attributes might be appropriate for any function that calls a function like `vprintf` or `vscanf` — but that might not always be the case, and some functions for which `format` attributes *are* appropriate may not be detected 😅.

- **`-Wsuggest-attribute=cold`**

  Warns about functions that might be candidates for the `cold` attribute ❄️. This is based on static detection, and generally only warns about functions which always lead to a call to another `cold` function — like wrappers of C++ `throw`, or fatal error reporting functions leading to `abort`.

### <pink>`-Walloc-size`</pink>

Warns about calls to allocation functions decorated with attribute `alloc_size` that specify insufficient size for the target type of the pointer the result is assigned to 💀. This includes calls to the built-in forms of the functions `aligned_alloc`, `alloca`, `calloc`, `malloc`, and `realloc`.

### <pink>`-Walloc-zero`</pink>

Warns about calls to allocation functions decorated with attribute `alloc_size` that specify **zero** bytes, including calls to the built-in forms of `aligned_alloc`, `alloca`, `calloc`, `malloc`, and `realloc` 🙈. Why do we care? Because the behavior of these functions when called with a zero size differs among implementations (and in the case of `realloc` it has been deprecated) — so relying on it may result in subtle portability bugs and it should be avoided, periodt 💅.

### <pink>`-Wcalloc-transposed-args`</pink>

Warns about calls to allocation functions decorated with attribute `alloc_size` with two arguments, which use the `sizeof` operator as the *earlier* size argument and don't use it as the *later* size argument 👀. This is a coding style warning — iconically petty but valid. The first argument to `calloc` is documented to be the number of elements in the array, while the second argument is the size of each element, so `calloc (n, sizeof (int))` is preferred over `calloc (sizeof (int), n)` ✨.

If `sizeof` in the earlier argument and not the latter is intentional, you can suppress the warning by using `calloc (sizeof (struct S) + 0, n)` or `calloc (1 * sizeof (struct S), 4)` — or just use `sizeof` in the later argument as well 🎀.

### <pink>`-Walloc-size-larger-than=byte-size`</pink>

Warns about calls to functions decorated with attribute `alloc_size` that attempt to allocate objects larger than the specified number of bytes — or where the result of the size computation in an integer type with infinite precision would exceed the value of `PTRDIFF_MAX` on the target 😳. `-Walloc-size-larger-than=PTRDIFF_MAX` is enabled by default. Warnings controlled by this option can be disabled either by specifying *byte-size* of `SIZE_MAX` or more, or by `-Wno-alloc-size-larger-than` 🦋.

### <pink>`-Wno-alloc-size-larger-than`</pink>

Disables the `-Walloc-size-larger-than=` warnings 💅. This option is equivalent to `-Walloc-size-larger-than=SIZE_MAX` or larger.

### <pink>`-Walloca`</pink>

This option warns on all uses of `alloca` in the source 🔥. Every single one. No exceptions.

- **`-Walloca-larger-than=byte-size`** — this one yells at you whenever someone calls `alloca` with an integer argument that's either zero, or that *isn't* bounded by a controlling predicate keeping it at most *byte-size*. It also warns when the bound is straight-up unknown. And bestie, listen: arguments of non-integer types count as unbounded even if they *look* constrained to the expected range. Looks can deceive, periodt.

  So like, a bounded case of `alloca` could be:

void func (size_t n) { void *p; if (n <= 1000) p = alloca (n); else p = malloc (n); f (p); }


In that example, passing `-Walloca-larger-than=1000` would *not* issue a warning because the call to `alloca` is known to be at most 1000 bytes. But if you passed `-Walloca-larger-than=500`, the compiler would emit a warning. Slay, it caught you. 💅

Unbounded uses, on the other hand, are uses of `alloca` with no controlling predicate constraining its integer argument. Like:

void func () { void *p = alloca (n); f (p); }


If `-Walloca-larger-than=500` were passed, the above would trigger a warning — but this time because of the lack of bounds checking. Not the size, the *vibes*. 😭

Note that even seemingly correct code involving signed integers could cause a warning:

void func (signed int n) { if (n < 500) { p = alloca (n); f (p); } }


In the above example, `n` could be negative, causing a larger than expected argument to be implicitly cast into the `alloca` call. Sneaky little gremlin behavior 🙈.

This option also warns when `alloca` is used in a loop. Yeah. In a loop. Girl. 🫠

`-Walloca-larger-than=PTRDIFF_MAX` is enabled by default but is usually only effective when `-ftree-vrp` is active (default for `-O2` and above).

See also `-Wvla-larger-than=byte-size`.

- **`-Wno-alloca-larger-than`** — disables the `-Walloca-larger-than=` warnings. The option is equivalent to `-Walloca-larger-than=SIZE_MAX` or larger. Basically "I don't wanna hear it" mode 💀.

- **`-Warith-conversion`** — do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from `-Wconversion`, `-Wfloat-conversion`, and `-Wsign-conversion`.

void f (char c, int i) { c = c + i; // warns with -Wconversion c = c + 1; // only warns with -Warith-conversion }


- **`-Warray-bounds`**, **`-Warray-bounds=n`** — warn about out of bounds subscripts or offsets into arrays. This warning is enabled by `-Wall`. It's way more effective when `-ftree-vrp` is active (the default for `-O2` and above), but a subset of instances get issued even without optimization. Little overachiever ✨

By default, the trailing array of a structure will be treated as a flexible array member by `-Warray-bounds` or `-Warray-bounds=n` if it is declared as either a flexible array member per C99 standard onwards (`[]`), a GCC zero-length array extension (`[0]`), or an one-element array (`[1]`). As a result, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are **not** warned by default. Mind the gap, bestie.

You can add the option `-fstrict-flex-arrays` or `-fstrict-flex-arrays=level` to control how this option treats the trailing array of a structure as a flexible array member:

- when *level*<=1, no change to the default behavior.
- when *level*=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays;
- when *level*=3, in addition to *level*=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays.

- **`-Warray-bounds=1`** — this is the default warning level of `-Warray-bounds` and is enabled by `-Wall`; higher levels are not, and must be explicitly requested. No auto-upgrades here, you gotta *ask* 🎀
- **`-Warray-bounds=2`** — this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. This warning level may give a larger number of false positives and is deactivated by default. Highkey paranoid mode 💀

- **`-Wunterminated-string-initialization`** (C and Objective-C only) — warn about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has the `nonstring` attribute. For example:

char arr[3] = "foo"; /* Warning. / char arr2[3] __attribute__((nonstring)) = "bar"; / No warning. */


This warning is enabled by `-Wextra`. If `-Wc++-compat` is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the `nonstring` warning, as in C++ such initializations are an error. C++ said "absolutely not" 😤

- **`-Warray-compare`** — warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:

int arr1[5]; int arr2[5]; bool same = arr1 == arr2;


`-Warray-compare` is enabled by `-Wall`. Deprecated but still out here causing chaos, classic 🌸

- **`-Warray-parameter`**, **`-Warray-parameter=n`** — warn about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as `-Warray-bounds`. Consistency is literally free, bestie 💅

If the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as `-Warray-bounds` or `-Wstringop-overflow`. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the `T[static N]` form.

For example, the warning triggers for the second declaration of `f` because the first one with the keyword `static` specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to `f`.

void f (int[static 4]); void f (int[]); // warning (inconsistent array form)

void g (void) { int *p = (int *)malloc (1 * sizeof (int)); f (p); // warning (array too small) ... }


At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.

void g (int*); void g (int[]); // no warning void g (int[8]); // warning (inconsistent array bound)


`-Warray-parameter=2` is included in `-Wall`. The `-Wvla-parameter` option triggers warnings for similar inconsistencies involving Variable Length Array arguments.

The short form of the option `-Warray-parameter` is equivalent to `-Warray-parameter=2`. The negative form `-Wno-array-parameter` is equivalent to `-Warray-parameter=0`. Short forms stay winning 👑

- **`-Wattribute-alias=n`**, **`-Wno-attribute-alias`** — warn about declarations using the `alias` and similar attributes whose target is incompatible with the type of the alias. Your alias said it was one thing and the target is giving something *entirely* different 😭

- **`-Wattribute-alias=1`** — the default warning level of the `-Wattribute-alias` option diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs. Like, actual bugs. Not the cute kind.
- **`-Wattribute-alias=2`** — at this level `-Wattribute-alias` also diagnoses cases where the attributes of the alias declaration are *more* restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the `-Wmissing-attributes` option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.

  Attributes considered include `alloc_align`, `alloc_size`, `cold`, `const`, `hot`, `leaf`, `malloc`, `nonnull`, `noreturn`, `nothrow`, `pure`, `returns_nonnull`, and `returns_twice`. The whole squad 🫶

`-Wattribute-alias` is equivalent to `-Wattribute-alias=1`. This is the default. You can disable these warnings with either `-Wno-attribute-alias` or `-Wattribute-alias=0`.

- **`-Wbidi-chars=[none|unpaired|any|ucn]`** — warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be **dangerous**; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. Like a jumpscare but for your source file 🙈 run.

There are three levels of warning supported by GCC. The default is `-Wbidi-chars=unpaired`, which warns about improperly terminated bidi contexts. `-Wbidi-chars=none` turns the warning off. `-Wbidi-chars=any` warns about any use of bidirectional control characters.

By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using `-Wbidi-chars=unpaired,ucn` or `-Wbidi-chars=any,ucn`. Using `-Wbidi-chars=ucn` is valid, and is equivalent to `-Wbidi-chars=unpaired,ucn`, if no previous `-Wbidi-chars=any` was specified. Combo options, we love to see it ✨

- **`-Wbool-compare`** — warn about boolean expression compared with an integer value different from `true`/`false`. For instance, the following comparison is always false:

int n = 5; ... if ((n > 1) == 2) { ... }


This warning is enabled by `-Wall`. Comparing a bool to `2` is giving "I've never met a boolean in my life" energy 💀

- **`-Wbool-operation`** — warns you about sus-looking operations on boolean-typed expressions 🚩 like, bitwise-negating a boolean? bestie that is *almost certainly* a bug. For C it also yells at you for incrementing or decrementing a boolean, which like... rarely makes sense. (In C++ decrementing a boolean is **always invalid**. Incrementing a boolean is invalid in C++17, and deprecated otherwise.) Enabled by `-Wall`.

- **`-Wduplicated-branches`** — catches the moment your if-else is giving "I wrote the same thing twice and hoped nobody would notice" 💀 It detects stuff like

if (p != NULL) return 0; else return 0;


It does *not* warn when both branches contain just a null statement. It also warns for conditional operators:

int i = x ? *p : *p;


- **`-Wduplicated-cond`** — warns about duplicated conditions in an if-else-if chain 😭 like, why are we checking the same thing twice, ma'am:

if (p->q != NULL) { ... } else if (p->q != NULL) { ... }


- **`-Wframe-address`** — warns when `__builtin_frame_address` or `__builtin_return_address` gets called with an argument greater than 0. Such calls may return indeterminate values or **crash the program** 🫠 The warning is included in `-Wall`.

- **`-Wno-discarded-qualifiers` (C and Objective-C only)** — do not warn if type qualifiers on pointers are being discarded. Typically the compiler warns if a `const char *` variable is passed to a function that takes a `char *` parameter. This option suppresses that warning ✨

- **`-Wno-discarded-array-qualifiers` (C and Objective-C only)** — do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically the compiler warns if a `const int (*)[]` variable is passed to a function that takes a `int (*)[]` parameter. This option suppresses that warning 💅

- **`-Wno-incompatible-pointer-types` (C and Objective-C only)** — do not warn when there's a conversion between pointers that have incompatible types. This warning is for cases *not* covered by `-Wno-pointer-sign`, which warns for pointer argument passing or assignment with different signedness 👀

By default, in C99 and later dialects of C, GCC treats this issue as an **error**. The error can be downgraded to a warning using `-fpermissive` (along with certain other errors), or for this error alone, with `-Wno-error=incompatible-pointer-types`.

This warning is upgraded to an error by `-pedantic-errors`.

- **`-Wno-int-conversion` (C and Objective-C only)** — do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about *implicit* conversions; for explicit conversions the warnings `-Wno-int-to-pointer-cast` and `-Wno-pointer-to-int-cast` may be used.

By default, in C99 and later dialects of C, GCC treats this issue as an **error**. The error can be downgraded to a warning using `-fpermissive` (along with certain other errors), or for this error alone, with `-Wno-error=int-conversion`.

This warning is upgraded to an error by `-pedantic-errors`.

- **`-Wzero-as-null-pointer-constant`** — warns when a literal `0` is used as a null pointer constant 🎯

- **`-Wzero-length-bounds`** — warns about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are **undefined** ⚠️

For example, the first two stores in function `bad` are diagnosed because the array elements overlap the subsequent members `b` and `c`. The third store is diagnosed by `-Warray-bounds` because it is beyond the bounds of the enclosing object.

struct X { int a[0]; int b, c; }; struct X x;

void bad (void) { x.a[0] = 0; // -Wzero-length-bounds x.a[1] = 1; // -Wzero-length-bounds x.a[2] = 2; // -Warray-bounds }


Option `-Wzero-length-bounds` is enabled by `-Warray-bounds`.

- **`-Wno-div-by-zero`** — do not warn about compile-time integer division by zero. Floating-point division by zero is **not** warned about, because it can be a legit way of obtaining infinities and NaNs 🌈 (yes, `inf` is a valid vibe)

- **`-Wsystem-headers`** — prints warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, based on the assumption that they usually don't indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. But heads up 👀 using `-Wall` together with this option does **not** warn about unknown pragmas in system headers — for that, `-Wunknown-pragmas` must also be used.

- **`-Wtautological-compare`** — warns if a self-comparison always evaluates to true or false. Catches all kinds of mistakes like:

int i = 1; ... if (i > i) { ... }


It also warns about bitwise comparisons that always evaluate to true or false, for instance:

if ((a & 16) == 10) { ... }


will always be false 💀 Enabled by `-Wall`.

- **`-Wtrailing-whitespace`**
- **`-Wtrailing-whitespace=`*kind* — warns about trailing whitespace at the end of lines, including inside of comments, but **excluding** trailing whitespace in raw string literals. `-Wtrailing-whitespace` is equivalent to `-Wtrailing-whitespace=blanks` and warns just about trailing space and horizontal tab characters. `-Wtrailing-whitespace=any` warns about those *or* trailing form feed or vertical tab characters. `-Wno-trailing-whitespace` or `-Wtrailing-whitespace=none` disables the warning, which is the default. This is a coding style warning 🎀

- **`-Wleading-whitespace=`*kind* — warns about style issues in leading whitespace, but **not** about the amount of indentation. Some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of `-ftabstop=n`) followed by zero or fewer than *n* spaces. No warning is emitted on lines which contain solely whitespace (although `-Wtrailing-whitespace=` warning might be emitted), and no warnings are emitted inside of raw string literals. Warnings are also emitted for leading whitespace inside of multi-line comments 📝 `-Wleading-whitespace=spaces` warns about leading whitespace other than spaces, for projects which want to indent just by spaces. `-Wleading-whitespace=tabs` warns about leading whitespace other than horizontal tabs, for projects which want to indent just by horizontal tabs. `-Wleading-whitespace=blanks` warns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or about *n* or more consecutive spaces in leading whitespace (where *n* is argument of `-ftabstop=n`, 8 by default). `-Wleading-whitespace=none` disables the warning, which is the default. This is a coding style warning ✨

- **`-Wtrampolines`** — warns about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that's created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly 🧚 For some targets it's made up of data only and thus requires no special treatment. But for most targets it's made up of code, and thus requires the stack to be made **executable** in order for the program to work properly 👑

- **`-Wfloat-equal`** — warns if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it's convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you're doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check whether the two values have ranges that overlap — and that's done with the relational operators — so equality comparisons are probably mistaken 😭

- **`-Wtraditional` (C and Objective-C only)** — warns about certain constructs that behave differently in traditional C vs ISO C. Also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided 🫶

- Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
- In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the `#` appeared in column 1 on the line. Therefore `-Wtraditional` warns about directives that traditional C understands but ignores because the `#` does not appear as the first character on the line. It also suggests you hide directives like `#pragma` not understood by traditional C by indenting them. Some traditional implementations do not recognize `#elif`, so this option suggests avoiding it altogether.
- A function-like macro that appears without arguments.
- The unary plus operator.
- The `U` integer constant suffix, or the `F` or `L` floating-point constant suffixes. (Traditional C does support the `L` suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the `_MIN`/`_MAX` macros in `<limits.h>`. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
- A function declared external in one block and then used after the end of the block.
- A `switch` statement has an operand of type `long`.
- A non-`static` function declaration follows a `static` one. This construct is not accepted by some traditional C compilers.

okay and the rest of the `-Wtraditional` callout list goes like this, bestie — every single one, no skipping 💅

- the ISO type of an integer constant has a different width or signedness from its traditional type. this warning only fires if the base of the constant is ten — so hexadecimal or octal values, which are typically bit patterns anyway, get a pass and are *not* warned about 🎀
- usage of ISO string concatenation is detected.
- initialization of automatic aggregates.
- identifier conflicts with labels. traditional C just doesn't have a separate namespace for labels, so yeah 😭
- initialization of unions. if the initializer is zero, the warning is omitted — the assumption being that the zero initializer in user code shows up conditioned on e.g. `"__STDC__"` to dodge missing-initializer warnings, and relies on default initialization to zero in the traditional C case.
- conversions by prototypes between fixed/floating-point values and vice versa. the absence of these prototypes when compiling with traditional C causes **serious problems** 💀. this is a subset of the possible conversion warnings; for the full set use `-Wtraditional-conversion`.
- use of ISO C style function definitions. this warning is intentionally *not* issued for prototype declarations or variadic functions, because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, `"PARAMS"` and `"VPARAMS"`. this warning is also bypassed for nested functions, because that feature is already a GCC extension and thus not relevant to traditional C compatibility ✨

---

- **`-Wtraditional-conversion`** (C and Objective-C only) — warns if a prototype causes a type conversion that's different from what would happen to the same argument with no prototype around. that includes conversions of fixed point to floating and vice versa, plus conversions changing the width or signedness of a fixed-point argument — except when it's the same as the default promotion.
- **`-Wdeclaration-after-statement`** (C and Objective-C only) — warns when a declaration shows up *after* a statement in a block. this construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. ISO C90 does **not** support it, periodt.

> heads up: this warning gets upgraded to an error by `-pedantic-errors`.

- **`-Wshadow`** — warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) — or whenever a built-in function is shadowed ✨. note that in C++, the compiler warns if a local variable shadows an explicit typedef, but *not* if it shadows a struct/class/enum. if this warning is enabled, it also includes all instances of local shadowing — which means `-Wno-shadow=local` and `-Wno-shadow=compatible-local` are straight-up ignored when `-Wshadow` is used 🙈. same as `-Wshadow=global`.
- **`-Wno-shadow-ivar`** (Objective-C only) — do *not* warn whenever a local variable shadows an instance variable in an Objective-C method.
- **`-Wshadow=global`** — warn for any shadowing. same as `-Wshadow`.
- **`-Wshadow=local`** — warn when a local variable shadows another local variable or parameter.
- **`-Wshadow=compatible-local`** — warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. in C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. the creation of this flag (on top of `-Wshadow=local`) is based on the idea that when a local variable shadows another one of incompatible type, it's most likely intentional — not a bug or typo — like in this iconic little example:

for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i) { for (int i = 0; i < N; ++i) { ... } ... }


since the two variable `"i"` in the example above have incompatible types, enabling only `-Wshadow=compatible-local` does **not** emit a warning. and because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning. using this flag instead of `-Wshadow=local` can possibly reduce the number of warnings triggered by intentional shadowing 🦋. note that this also means shadowing `"const char *i"` by `"char *i"` does not emit a warning.

this warning is also enabled by `-Wshadow=local`.

- **`-Wlarger-than=byte-size`** — warn whenever an object is defined whose size exceeds *byte-size*. `-Wlarger-than=PTRDIFF_MAX` is enabled by default. warnings controlled by the option can be disabled either by specifying *byte-size* of `SIZE_MAX` or more, or by `-Wno-larger-than`.

also warns for calls to bounded functions such as `"memchr"` or `"strnlen"` that specify a bound greater than the largest possible object — which is `PTRDIFF_MAX` bytes by default. these warnings can **only** be disabled by `-Wno-larger-than` 👑
- **`-Wno-larger-than`** — disables `-Wlarger-than=` warnings. the option is equivalent to `-Wlarger-than=SIZE_MAX` or larger.
- **`-Wframe-larger-than=byte-size`** — warn if the size of a function frame exceeds *byte-size*. the computation done to determine the stack frame size is approximate and **not conservative** 😬. the actual requirements may be somewhat greater than *byte-size* even if you don't get a warning. plus, any space allocated via `"alloca"`, variable-length arrays, or related constructs is *not* included by the compiler when deciding whether or not to issue a warning. `-Wframe-larger-than=PTRDIFF_MAX` is enabled by default. warnings controlled by the option can be disabled either by specifying *byte-size* of `SIZE_MAX` or more, or by `-Wno-frame-larger-than`.
- **`-Wno-frame-larger-than`** — disables `-Wframe-larger-than=` warnings. the option is equivalent to `-Wframe-larger-than=SIZE_MAX` or larger.
- **`-Wfree-nonheap-object`** — warn when you attempt to deallocate an object that was either not allocated on the heap, or by using a pointer that wasn't returned from a prior call to the corresponding allocation function. for example, because the call to `"stpcpy"` returns a pointer to the terminating nul character and not to the beginning of the object, the call to `"free"` below is diagnosed:

void f (char *p) { p = stpcpy (p, "abc"); // ... free (p); // warning }


`-Wfree-nonheap-object` is included in `-Wall` 💖
- **`-Wstack-usage=byte-size`** — warn if the stack usage of a function might exceed *byte-size*. the computation done to determine stack usage is conservative. any space allocated via `"alloca"`, variable-length arrays, or related constructs **is** included by the compiler when deciding whether or not to issue a warning.

the message is in keeping with the output of `-fstack-usage` 🌸

- if the stack usage is fully static but exceeds the specified amount, it's:

warning: stack usage is 1120 bytes


- if the stack usage is (partly) dynamic but bounded, it's:

warning: stack usage might be 1648 bytes


- if the stack usage is (partly) dynamic and not bounded, it's:

warning: stack usage might be unbounded


`-Wstack-usage=PTRDIFF_MAX` is enabled by default. warnings controlled by the option can be disabled either by specifying *byte-size* of `SIZE_MAX` or more, or by `-Wno-stack-usage`.
- **`-Wno-stack-usage`** — disables `-Wstack-usage=` warnings. the option is equivalent to `-Wstack-usage=SIZE_MAX` or larger.
- **`-Wunsafe-loop-optimizations`** — warn if the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. with `-funsafe-loop-optimizations`, warn if the compiler *does* make such assumptions 👀
- **`-Wno-pedantic-ms-format`** (MinGW targets only) — when used in combination with `-Wformat` and `-pedantic` without GNU extensions, this option disables the warnings about non-ISO `"printf"` / `"scanf"` format width specifiers `"I32"`, `"I64"`, and `"I"` used on Windows targets, which depend on the MS runtime.
- **`-Wpointer-arith`** — warn about anything that depends on the "size of" a function type or of `"void"`. GNU C assigns these types a size of 1, for convenience in calculations with `"void *"` pointers and pointers to functions 🫶. in C++, also warn when an arithmetic operation involves `"NULL"`. this warning is also enabled by `-Wpedantic`.

> and yeah, this warning gets upgraded to an error by `-pedantic-errors`.

- **`-Wno-pointer-compare`** — do *not* warn if a pointer is compared with a zero character constant. this usually means the pointer was meant to be dereferenced. for example:

const char *p = foo (); if (p == '\0') return 42;


note that the code above is invalid in C++11 💀. this warning is enabled by default.
- **`-Wno-tsan`** — disable warnings about unsupported features in ThreadSanitizer. ThreadSanitizer does not support `"std::atomic_thread_fence"` and can report false positives 😭
- **`-Wtype-limits`** — warn if a comparison is always true or always false due to the limited range of the data type, but *don't* warn for constant expressions. for example, warn if an unsigned variable is compared against zero with `"<"` or `">="`. this warning is also enabled by `-Wextra`.
- **`-Wabsolute-value`** (C and Objective-C only) — warn for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available ✨. for example, calling `abs(3.14)` triggers the warning because the appropriate function to call to compute the absolute value of a double argument is `"fabs"`. the option also triggers warnings when the argument in a call to such a function has an unsigned type. this warning can be suppressed with an explicit type cast, and it's also enabled by `-Wextra`.
- **`-Wcomment`**, **`-Wcomments`** — warn whenever a comment-start sequence `/*` appears in a `/*` comment, or whenever a backslash-newline appears in a `//` comment. this warning is enabled by `-Wall` 🎀
- **`-Wtrigraphs`** — warn if any trigraphs are encountered that might change the meaning of the program. trigraphs within comments are *not* warned about, except those that would form escaped newlines 🌈

okay so this one's implied by `-Wall`, bestie. If `-Wall` isn't given, it's still enabled anyway — *unless* trigraphs are enabled. Wanna get trigraph conversion without the warnings but still get the other `-Wall` warnings? Just use `-trigraphs -Wall -Wno-trigraphs` and you're golden ✨.

- **`-Wundef`** — warns you when an undefined identifier gets evaluated inside an `#if` directive. Those little guys are replaced with zero, and GCC is like "bestie I'm telling you" 💅.

- **`-Wexpansion-to-defined`** — hmu with a warning whenever `defined` shows up in the expansion of a macro (yep, even when that macro gets expanded by an `#if` directive). This usage is **not portable**, so it's a no from the standards girlies. Also switched on by `-Wpedantic` and `-Wextra` 🫶.

- **`-Wunused-macros`** — warns about macros defined in the main file that never get used. A macro counts as used if it's expanded **or** tested for existence at least once. The preprocessor also warns if the macro hasn't been used by the time it's redefined or undefined. Built-in macros, macros defined on the command line, and macros defined in include files? Not warned about — they're exempt 💖.

> Note: if a macro is *actually* used but only inside skipped conditional blocks, the preprocessor will still report it as unused 😭. To dodge the warning, you can improve the scope of the macro's definition — moving it into the first skipped block, for example. Or you can provide a dummy use like this:

#if defined the_macro_causing_the_warning #endif


- **`-Wno-endif-labels`** — stops the warning when an `#else` or `#endif` is followed by text. This sometimes happens in older programs with code like:

#if FOO ... #else FOO ... #endif FOO


The second and third `FOO` should be in comments. This warning is **on by default** 👑

- **`-Wbad-function-cast`** *(C and Objective-C only)* — warns when a function call gets cast to a non-matching type. Like, if a call to a function that returns an integer type gets cast to a pointer type, GCC is gonna side-eye it 🙈.

- **`-Wc90-c99-compat`** *(C and Objective-C only)* — warns about features that aren't in ISO C90 but *are* in ISO C99. We're talking variable length arrays, the `long long` type, the `bool` type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows `__extension__` ✨.

- **`-Wc99-c11-compat`** *(C and Objective-C only)* — warns about features that aren't in ISO C99 but *are* in ISO C11. For instance: anonymous structures and unions, the `_Atomic` type qualifier, the `_Thread_local` storage-class specifier, the `_Alignas` specifier, the `Alignof` operator, the `_Generic` keyword, and so on. Independent of the standards mode. Warnings are disabled in the expression that follows `__extension__` 🦋.

- **`-Wc11-c23-compat`, `-Wc11-c2x-compat`** *(C and Objective-C only)* — warns about features not present in ISO C11 but present in ISO C23. For instance: omitting the string in `_Static_assert`, use of `[[]]` syntax for attributes, use of decimal floating-point types, and so on. Independent of the standards mode. Warnings are disabled in the expression that follows `__extension__`. Heads up: the name `-Wc11-c2x-compat` is **deprecated** 💀. And when you're not compiling in C23 mode, these warnings get upgraded to errors by `-pedantic-errors`.

- **`-Wc23-c2y-compat`, `-Wc23-c2y-compat`** *(C and Objective-C only)* — (yes bestie, the page really lists it twice, we don't make the rules 💅) warns about features not present in ISO C23 but present in ISO C2Y. For instance, warn about `_Generic` selecting with a type name instead of an expression. Independent of the standards mode. Warnings are disabled in the expression that follows `__extension__`. When not compiling in C2Y mode, these warnings get upgraded to errors by `-pedantic-errors`.

- **`-Wc++-compat`** *(C and Objective-C only)* — warns about ISO C constructs that live outside the common subset of ISO C and ISO C++, e.g. a request for implicit conversion from `void *` to a pointer to non-`void` type.

- **`-Wc++11-compat`** *(C++ and Objective-C++ only)* — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers that were fine in ISO C++ 1998 but are keywords in ISO C++ 2011. This warning turns on `-Wnarrowing` and is enabled by `-Wall` 🔥.

- **`-Wc++14-compat`** *(C++ and Objective-C++ only)* — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by `-Wall`.

- **`-Wc++17-compat`** *(C++ and Objective-C++ only)* — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by `-Wall`.

- **`-Wc++20-compat`** *(C++ and Objective-C++ only)* — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by `-Wall`.

- **`-Wno-c++11-extensions`** *(C++ and Objective-C++ only)* — don't warn about C++11 constructs in code that's being compiled with an older C++ standard. Even without this option, some C++11 constructs will only be diagnosed if `-Wpedantic` is used, iykyk.

- **`-Wno-c++14-extensions`** *(C++ and Objective-C++ only)* — don't warn about C++14 constructs in code that's being compiled with an older C++ standard. Even without this option, some C++14 constructs will only be diagnosed if `-Wpedantic` is used.

- **`-Wno-c++17-extensions`** *(C++ and Objective-C++ only)* — don't warn about C++17 constructs in code that's being compiled with an older C++ standard. Even without this option, some C++17 constructs will only be diagnosed if `-Wpedantic` is used.

- **`-Wno-c++20-extensions`** *(C++ and Objective-C++ only)* — don't warn about C++20 constructs in code that's being compiled with an older C++ standard. Even without this option, some C++20 constructs will only be diagnosed if `-Wpedantic` is used.

- **`-Wno-c++23-extensions`** *(C++ and Objective-C++ only)* — don't warn about C++23 constructs in code that's being compiled with an older C++ standard. Even without this option, some C++23 constructs will only be diagnosed if `-Wpedantic` is used.

- **`-Wno-c++26-extensions`** *(C++ and Objective-C++ only)* — don't warn about C++26 constructs in code that's being compiled with an older C++ standard. Even without this option, some C++26 constructs will only be diagnosed if `-Wpedantic` is used.

- **`-Wcast-qual`** — warns whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a `const char *` gets cast to an ordinary `char *`. It also warns when making a cast that introduces a type qualifier in an unsafe way — casting `char **` to `const char **` is unsafe, like in this example:

/* p is char ** value. / const char q = (const char *) p; /* Assignment of readonly string to const char * is OK. / *q = "string"; / Now char pointer points to read-only memory. */ **p = 'b';


- **`-Wcast-align`** — warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a `char *` is cast to an `int *` on machines where integers can only be accessed at two- or four-byte boundaries.

- **`-Wcast-align=strict`** — same energy but stricter: warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a `char *` is cast to an `int *` — **regardless of the target machine** 🔥.

- **`-Wcast-function-type`** — warns when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like `int` vs. `long` on ILP32 targets. Likewise, type qualifiers are ignored. The function type `void (*) (void)` is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types, this warning warns whenever the type cast is changing the pointer to member type. Enabled by `-Wextra` 🫶.

- **`-Wcast-user-defined`** — warns when a cast to reference type does not involve a user-defined conversion that the programmer might expect to be called.

struct A { operator const int&(); } a; auto r = (int&)a; // warning


This warning is **enabled by default** ✨.

- **`-Wwrite-strings`** — when compiling C, gives string constants the type `const char[length]` so that copying the address of one into a non-`const` `char *` pointer produces a warning. These warnings help you find code at compile time that can try to write into a string constant — but only if you've been super careful about using `const` in declarations and prototypes. Otherwise it's just a nuisance, which is exactly why we didn't make `-Wall` request these warnings. When compiling C++, it warns about the deprecated conversion from string literals to `char *`. This warning is enabled by default for C++ programs. It gets upgraded to an error by `-pedantic-errors` in C++11 mode or later 💀.

- **`-Wclobbered`** — warns for variables that might be changed by `longjmp` or `vfork`. Also enabled by `-Wextra`.

- **`-Wno-complain-wrong-lang`** — by default, language front ends complain when a command-line option is valid but not applicable to that front end. This can be disabled with `-Wno-complain-wrong-lang`, which is mostly useful when you're invoking a single compiler driver for multiple source files written in different languages, for example:

$ g++ -fno-rtti a.cc b.f90


The driver `g++` invokes the C++ front end to compile `a.cc` and the Fortran front end to compile `b.f90`. The latter front end diagnoses `f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran`, which may be disabled with `-Wno-complain-wrong-lang` 🌈.

- **`-Wcompare-distinct-pointer-types`** *(C and Objective-C only)* — warns if pointers of distinct types are compared without a cast. Enabled by default 👑.

- **`-Wconversion`** — bestie, this one warns you about implicit conversions that might *change the value*. 👀 That includes real↔integer glow-ups like `abs (x)` when `x` is `double`, signed↔unsigned shenanigans like `unsigned ui = -1`, and conversions down to smaller types like `sqrtf (M_PI)`. It does **not** warn when you do an explicit cast like `abs ((int) x)` or `ui = (unsigned) -1`, or when the value just... doesn't change, like in `abs (2.0)`. The signed↔unsigned integer warnings can be switched off with `-Wno-sign-conversion`.

For C++ it *also* warns about confusing overload resolution for user-defined conversions 🌀, and conversions that never use a type conversion operator — that's conversions to `void`, to the same type, to a base class, or to a reference to those. In C++ those signed↔unsigned warnings are **off by default** unless you explicitly turn on `-Wsign-conversion`.

Oh and also: warnings about conversion from arithmetic on a small type back to that same type only show up with `-Warith-conversion`. 💅

- **`-Wdangling-else`** — warns you when it's genuinely unclear which `if` an `else` branch belongs to 😵‍💫. Here's the classic messy queen:

{ if (a) if (b) foo (); else bar (); }


  In C/C++, every `else` branch belongs to the *innermost possible* `if` statement — so in this example, it belongs to `if (b)`. That's often not what the programmer was picturing, as the indentation above makes painfully obvious 🙈. When that confusion is possible, GCC throws a warning if this flag is on. To make it go away, slap explicit braces around the innermost `if` so there's no way the `else` can belong to the enclosing `if`. Like this:

{ if (a) { if (b) foo (); else bar (); } }


  This warning is enabled by `-Wparentheses`.

- **`-Wdangling-pointer`**
- **`-Wdangling-pointer=n`** — warns about using pointers (or C++ references) to objects with automatic storage duration *after their lifetime has ended* 💀. That includes local variables declared in nested blocks, compound literals, and other unnamed temporary objects. It also warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels, but may give you different results with optimization than without.

- **`-Wdangling-pointer=1`** — at level 1, it only diagnoses *unconditional* uses of dangling pointers.

- **`-Wdangling-pointer=2`** — at level 2, it also diagnoses *conditional* uses of dangling pointers, on top of the unconditional ones.

  The short form `-Wdangling-pointer` is equivalent to `-Wdangling-pointer=2`, while `-Wno-dangling-pointer` and `-Wdangling-pointer=0` both do the same thing: disable the warnings. `-Wdangling-pointer=2` is included in `-Wall`.

  This example triggers the warning at level 1 — the address of the unnamed temporary is unconditionally referenced outside its scope:

char f (char c1, char c2, char c3) { char *p; { p = (char[]) { c1, c2, c3 }; } // warning: using dangling pointer 'p' to an unnamed temporary return *p; }


  In the next function, storing the address of the local variable `x` in the escaped pointer `*p` triggers the warning at level 1:

void g (int **p) { int x = 7; // warning: storing the address of local variable 'x' in '*p' *p = &x; }


  And here, the array `a` is out of scope when the pointer `s` is used. Since the code that sets `s` is conditional, the warning triggers at level 2:

extern void frob (const char *); void h (char *s) { if (!s) { char a[12] = "tmpname"; s = a; } // warning: dangling pointer 's' to 'a' may be used frob (s); }


- **`-Wdate-time`** — warns when the macros `__TIME__`, `__DATE__` or `__TIMESTAMP__` show up, because they might ruin bit-wise-identical reproducible compilations 🕰️. Love a reproducible build, hate a timestamp.

- **`-Wempty-body`** — warns if an `if`, `else` or `do while` statement has an empty body. This warning is also enabled by `-Wextra`.

- **`-Wno-endif-labels`** — don't warn about stray tokens after `#else` and `#endif`. Simple as that. 🎀

- **`-Wenum-compare`** — warns about comparing values of different enumerated types. In C++, enumerated type mismatches in conditional expressions are also diagnosed, and the warning is enabled by default. In C, it's enabled by `-Wall`.

- **`-Wenum-conversion`** — warns when a value of enumerated type gets implicitly converted to a *different* enumerated type. In C this is enabled by `-Wextra`.

- **`-Wenum-int-mismatch` (C and Objective-C only)** — warns about mismatches between an enumerated type and an integer type in declarations. Like, for example:

enum E { l = -1, z = 0, g = 1 }; int foo(void); enum E foo(void);


  In C, an enumerated type is compatible with `char`, a signed integer type, or an unsigned integer type. But since the choice of the underlying type of an enumerated type is *implementation-defined*, mismatches like this can cause portability issues 🌍. In C++, such mismatches are outright an error. In C, this warning is enabled by `-Wall` and `-Wc++-compat`.

- **`-Wjump-misses-init` (C, Objective-C only)** — warns if a `goto` statement or a `switch` statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized 🦘. It only warns about variables that are initialized when they're *declared*. This warning is only supported for C and Objective-C; in C++ that sort of branch is an error anyway.

  `-Wjump-misses-init` is included in `-Wc++-compat`. You can disable it with the `-Wno-jump-misses-init` option.

- **`-Wsign-compare`** — warns when a comparison between signed and unsigned values could produce an incorrect result when the signed value gets converted to unsigned 😬. In C++, this warning is also enabled by `-Wall`. In C, it's also enabled by `-Wextra`.

- **`-Wsign-conversion`** — warns about implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning 🤫. In C, this option is also enabled by `-Wconversion`.

- **`-Wflex-array-member-not-at-end` (C and C++ only)** — warns when a structure containing a C99 flexible array member as its last field is *not* at the end of another structure. So it'll warn about stuff like:

struct flex { int length; char data[]; }; struct mid_flex { int m; struct flex flex_data; int n; };


- **`-Wfloat-conversion`** — warns about implicit conversions that reduce the precision of a real value 🎯. That covers conversions from real to integer, and from higher-precision real to lower-precision real values. This option is also enabled by `-Wconversion`.

- **`-Wno-scalar-storage-order`** — don't warn on suspicious constructs involving reverse scalar storage order. 🙅‍♀️

- **`-Wsizeof-array-div`** — warns about divisions of two `sizeof` operators when the first one is applied to an array and the divisor doesn't equal the size of the array element. Because in that case the computation won't give you the number of elements in the array — which is *probably* what you wanted, bestie 😭. It warns about stuff like:

int fn () { int arr[10]; return sizeof (arr) / sizeof (short); }


  This warning is enabled by `-Wall`.

- **`-Wsizeof-pointer-div`** — warns about suspicious divisions of two `sizeof` expressions that divide the pointer size by the element size. That's the usual way to compute an array size, but it will *not* work correctly with pointers 🪤. So it warns about e.g. `sizeof (ptr) / sizeof (ptr[0])` if `ptr` isn't an array but a pointer. This warning is enabled by `-Wall`.

- **`-Wsizeof-pointer-memaccess`** — warns about suspicious length parameters passed to certain string and memory built-in functions when the argument uses `sizeof` 👀. It triggers for example on `memset (ptr, 0, sizeof (ptr));` if `ptr` is a pointer and not an array, and it suggests a possible fix — or on `memcpy (&foo, ptr, sizeof (&foo));`.

  `-Wsizeof-pointer-memaccess` also warns about calls to bounded string copy functions like `strncat` or `strncpy` that specify as the bound a `sizeof` expression of the *source* array. For example, in this function the call to `strncat` specifies the size of the source string as the bound, which is almost certainly a mistake, so the call gets diagnosed:

void make_file (const char *name) { char path[PATH_MAX]; strncpy (path, name, sizeof path - 1); strncat (path, ".text", sizeof ".text"); ... }


  The `-Wsizeof-pointer-memaccess` option is enabled by `-Wall`.

- **`-Wno-sizeof-array-argument`** — don't warn when the `sizeof` operator is applied to a parameter that's declared as an array in a function definition. This warning is enabled by default for C and C++ programs.

- **`-Wmemset-elt-size`** — warns about suspicious calls to the `memset` built-in function when the first argument references an array, and the third argument is a number equal to the number of elements but *not* equal to the size of the array in memory 🧐. That signals the user forgot to multiply by the element size. This warning is enabled by `-Wall`.

- **`-Wmemset-transposed-args`** — this one's a lil detective 🕵️‍♀️ it warns you about sus calls to the `memset` built-in where the second arg isn't zero but the third arg *is* zero. Like, `memset (buf, sizeof buf, 0)` gets flagged, because bestie… you clearly meant `memset (buf, 0, sizeof buf)` and just swapped them 💅. Important detail though: the diagnostic only fires if that third argument is a *literal* zero. If it's an expression that gets folded to zero, or a cast of zero to some type, then it's way less likely you accidentally transposed the args, so no warning. Enabled by `-Wall`.

- **`-Waddress`** — catches suspicious uses of address expressions ✨. Three flavors, bestie:

  - Comparing the address of a function or a declared object to the null pointer constant, like:

void f (void); void g (void) { if (!f) // warning: expression evaluates to false abort (); }


- Comparing a pointer to a string literal, like:

void f (const char *x) { if (x == "abc") // warning: expression evaluates to false puts ("equal"); }


- Testing the result of pointer addition or subtraction for equality to null, like:

void f (const int *p, int i) { return p + i == NULL; }


Uses like these typically mean a programmer error, fr fr 😭 — the address of most functions and objects necessarily evaluates to true (the exception being weak symbols), so sticking one in a conditional might mean you forgot parentheses in a function call or forgot to dereference in an array expression. The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as `intptr_t` or `uintptr_t`. Comparisons against string literals result in unspecified behavior and are not portable — and they lowkey suggest you meant to call `strcmp`. The warning is suppressed if the suspicious expression is the result of macro expansion. Enabled by `-Wall`.

- **`-Wno-address-of-packed-member`** — don't warn when the address of a packed member of a struct or union is taken, which usually gives you an unaligned pointer value. This is enabled by default 🌸.

- **`-Wlogical-op`** — flags suspicious uses of logical operators in expressions ✨. That includes using logical operators in contexts where a bit-wise operator is probably what you wanted, and it also warns when the operands of a logical operator are literally the same:

extern int a; if (a < 0 && a < 0) { ... }


- **`-Wlogical-not-parentheses`** — warns about logical not being used on the left hand side operand of a comparison 🫶. It doesn't warn if the right operand is considered to be a boolean expression. The whole point is sniffing out shady code like:

int a; ... if (!a > 1) { ... }


You can suppress the warning by wrapping the LHS in parentheses:

if ((!a) > 1) { ... }


Enabled by `-Wall`.

- **`-Waggregate-return`** — warns if any functions that return structures or unions are defined or called 👀. (In languages where you can return an array, this also elicits a warning.)

- **`-Wno-aggressive-loop-optimizations`** — don't warn if the compiler detects undefined behavior in a loop with a constant number of iterations 💀. Note that `-Waggressive-loop-optimizations` is enabled by default.

- **`-Wno-attributes`** — don't warn if an unexpected `__attribute__` shows up, like unrecognized attributes, function attributes slapped onto variables, etc. Heads up though: this does *not* stop errors for incorrect use of supported attributes 🚫.

Warnings about ill-formed uses of standard attributes are upgraded to errors by `-pedantic-errors`.

Also, with `-Wno-attributes=`, you can suppress warnings about unknown scoped attributes (in C++11 and C23) 🎀. For example, `-Wno-attributes=vendor::attr` disables the warning about this declaration:

[[vendor::attr]] void f();


You can also kill warnings about *all* attributes in a namespace using `-Wno-attributes=vendor::`, which prevents warnings about both of these:

[[vendor::safe]] void f(); [[vendor::unsafe]] void f2();


Note that `-Wno-attributes=` does not imply `-Wno-attributes` — they're different vibes entirely.

- **`-Wno-builtin-declaration-mismatch`** — warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype is called with arguments whose promoted types don't match what the function expects 😬. When `-Wextra` is specified, it also warns when a built-in function that takes arguments is declared without a prototype. The `-Wbuiltin-declaration-mismatch` warning is enabled by default. To dodge the warning, include the appropriate header so the prototypes of built-in functions are in scope 🫶.

For example, the call to `memset` below gets diagnosed because the function expects a value of type `size_t` as its argument but the type of 32 is `int`. With `-Wextra`, the declaration of the function is diagnosed as well:

extern void* memset (); void f (void *d) { memset (d, '\0', 32); }


- **`-Wno-builtin-macro-redefined`** — don't warn if certain built-in macros get redefined. This suppresses warnings for redefinition of `__TIMESTAMP__`, `__TIME__`, `__DATE__`, `__FILE__`, and `__BASE_FILE__` ✨.

- **`-Wfree-labels`** *(C and Objective-C only)* — warns if a label is applied to a non-statement, or shows up at the end of a compound statement 👑. Such labels are allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects.

This warning is also enabled by `-Wc11-c23-compat`. And it gets turned into an error if you're building for a C version before C23 by `-pedantic-errors` 😭.

- **`-Wheader-guard`** — super specific but so useful ngl. Warns if a valid preprocessor header multiple inclusion guard has a `#define` directive right after `#ifndef` or `#if !defined` directive for the multiple inclusion guard, which defines a *different* macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding `#ifndef` directive at the end of the header, and the `#define` directive defines an object-like macro with an empty definition 🕵️‍♀️. In that case it's often just a misspelled guard name — either in the `#ifndef` or `#if !defined` directive or in the subsequent `#define` directive. Enabled by `-Wall`.

- **`-Wstrict-prototypes`** *(C and Objective-C only)* — warns if a function is declared or defined without specifying the argument types 🎀. (An old-style function definition is permitted without a warning if it's preceded by a declaration that specifies the argument types.)

- **`-Wold-style-declaration`** *(C and Objective-C only)* — warns about obsolescent usages, per the C Standard, in a declaration 👵. For example, warns if storage-class specifiers like `static` aren't the first things in a declaration. Also enabled by `-Wextra`.

- **`-Wold-style-definition`** *(C and Objective-C only)* — warns if an old-style function definition is used. You get a warning even if there's a previous prototype 😤. A definition using `()` is **not** considered an old-style definition in C23 mode, because it's equivalent to `(void)` in that case — but it *is* considered an old-style definition for older standards.

- **`-Wdeprecated-non-prototype`** *(C and Objective-C only)* — warns if a function declared with an empty parameter list `()` is called with one or more arguments, or if a function definition with one or more parameters is encountered after such a declaration 💀. Both of those cases are errors in C23 and later dialects of C.

This warning is also enabled by `-Wc11-c23-compat`.

- **`-Wmissing-parameter-name`** *(C and Objective-C only)* — warns if a function definition omits a parameter name and specifies only its type 👀. You can use this to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects.

This warning is also enabled by `-Wc11-c23-compat`. It gets turned into an error if you're building for a C version before C23 by `-pedantic-errors`.

- **`-Wmissing-parameter-type`** *(C and Objective-C only)* — a function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }


This warning is also enabled by `-Wextra`.

- **`-Wno-declaration-missing-parameter-type`** *(C and Objective-C only)* — don't warn if a function declaration contains a parameter name without a type 🙈. Heads up: such function declarations don't provide a function prototype and prevent most type checking in function calls.

This warning is enabled by default. In C99 and later dialects of C, it's treated as an error 😱. The error can be downgraded to a warning using `-fpermissive` (along with certain other errors), or for this error alone, with `-Wno-error=declaration-missing-parameter-type`.

This warning is upgraded to an error by `-pedantic-errors`.

- **`-Wmissing-prototypes`** *(C and Objective-C only)* — warns if a global function is defined without a previous prototype declaration ✨. This warning is issued even if the definition itself provides a prototype. Use it to detect global functions that don't have a matching prototype declaration in a header file. This option is **not** valid for C++ because all function declarations provide prototypes, and a non-matching declaration declares an overload rather than conflicting with an earlier declaration 🚫. Use `-Wmissing-declarations` to detect missing declarations in C++ instead.

- **`-Wmissing-variable-declarations`** *(C and Objective-C only)* — warns if a global variable is defined without a previous declaration 🫶. Use this to detect global variables that don't have a matching extern declaration in a header file.

- **`-Wmissing-declarations`** — bestie, this warns you when a global function gets *defined* without a previous declaration. Like, even if the definition itself hands you a prototype, it still clocks it. Use this to catch global functions that are being sneaky and not declared in header files. In C, no warnings fire for functions with previous non-prototype declarations — for those, use `-Wmissing-prototypes` to sniff out missing prototypes instead. In C++, it stays quiet for function templates, inline functions, and functions in anonymous namespaces. 💅

- **`-Wmissing-field-initializers`** — warns when a structure's initializer forgot some fields. Example time, because the vibes demand it — this code triggers it, because `x.h` is implicitly zero:

struct s { int f, g, h; }; struct s x = { 3, 4 };


  In C, this option does *not* warn about designated initializers, so this glow-up doesn't trigger anything:

struct s { int f, g, h; }; struct s x = { .f = 3, .g = 4 };


  Also in C, it doesn't warn about the universal zero initializer `{ 0 }`:

struct s { int f, g, h; }; struct s x = { 0 };


  And same energy in C++ — it doesn't warn about the empty `{ }` initializer, like:

struct s { int f, g, h; }; s x = { };


  This warning is included in `-Wextra`. Want the other `-Wextra` warnings without this one? Use `-Wextra -Wno-missing-field-initializers`. ✨

- **`-Wno-missing-requires`** — okay so by default the compiler warns about a concept-id appearing as a C++20 simple-requirement:

bool satisfied = requires { C<T> };


  Here `satisfied` will be true if `C<T>` is a valid expression — which it is for all `T`. Presumably you meant to write:

bool satisfied = requires { requires C<T> };


  so that `satisfied` is only true if concept `C` is satisfied for type `T`. Fair. This warning can be disabled with `-Wno-missing-requires`. 🎀

- **`-Wno-missing-template-keyword`** — the member access tokens `.`, `->` and `::` must be followed by the `"template"` keyword if the parent object is dependent and the member being named is a template. Like so:

template <class X> void DoStuff (X x) { x.template DoSomeOtherStuff<X>(); // Good. x.DoMoreStuff<X>(); // Warning, x is dependent. }


  In rare cases it's possible to get false positives 😭. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where `m` and `N` are integers:

void NotATemplate (my_class t) { int N = 5;

bool test = t.m < N > (0); // Treated as a template. test = (t.m < N) > (0); // Same meaning, but not treated as a template. }


  This warning can be disabled with `-Wno-missing-template-keyword`.

- **`-Wno-multichar`** — do *not* warn if a multicharacter constant (`'FOOF'`) is used. Usually they're just a typo in your code, ngl — they have implementation-defined values, so pls don't use them in portable code. 🙈

- **`-Wnormalized=[none|id|nfc|nfkc]`** — so in ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes, when you use characters outside the basic ASCII set, you can get two different character sequences that *look* the same. To avoid the confusion, the ISO 10646 standard sets out normalization rules that, when applied, ensure two sequences that look the same get turned into the same sequence. GCC can warn you if you're using identifiers that haven't been normalized — and this option controls that warning. 🦋

  There are four levels of warning supported by GCC. The default is `-Wnormalized=nfc`, which warns about any identifier not in the ISO 10646 `"C"` normalized form, NFC. NFC is the recommended form for most uses, and it's equivalent to `-Wnormalized`.

  Unfortunately 😭, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are *not* allowed in identifiers. Meaning there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. `-Wnormalized=id` suppresses the warning for these characters. It's hoped that future versions of the standards involved will fix this — which is why this option isn't the default.

  You can switch the warning off for all characters by writing `-Wnormalized=none` or `-Wno-normalized`. You should *only* do this if you're using some other normalization scheme (like `"D"`), because otherwise you can easily create bugs that are literally impossible to see. 💀

  Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies — especially once formatting has been applied. For instance, `"\u207F"`, `"SUPERSCRIPT LATIN SMALL LETTER N"`, displays just like a regular `"n"` that's been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all of these into a standard form as well, and GCC warns if your code isn't in NFKC when you use `-Wnormalized=nfkc`. This warning is comparable to warning about every identifier containing the letter O because it might be confused with the digit 0 — so it's not the default, but it may be useful as a local coding convention if the programming environment can't be fixed to display these characters distinctly.

- **`-Wno-attribute-warning`** — do not warn about usage of functions declared with the `"warning"` attribute. By default this warning is enabled. `-Wno-attribute-warning` can be used to disable the warning, or `-Wno-error=attribute-warning` can be used to disable the error when compiled with the `-Werror` flag. 🌸

- **`-Wno-deprecated`** — do not warn about usage of deprecated features.

  In C++, explicitly specifying `-Wdeprecated` *also* enables warnings about some features that are deprecated in later language standards — specifically `-Wcomma-subscript`, `-Wvolatile`, `-Wdeprecated-enum-float-conversion`, `-Wdeprecated-enum-enum-conversion`, `-Wdeprecated-literal-operator`, and `-Wdeprecated-variadic-comma-omission`. Whew, that's a list 👑

- **`-Wno-deprecated-declarations`** — do not warn about uses of functions, variables, and types marked as deprecated by using the `"deprecated"` attribute.

- **`-Wno-overflow`** — do not warn about compile-time overflow in constant expressions.

- **`-Wno-odr`** — warn about One Definition Rule violations during link-time optimization. Enabled by default. 🔥

- **`-Wopenacc-parallelism`** — warn about potentially suboptimal choices related to OpenACC parallelism.

- **`-Wno-openmp`** — warn about suspicious OpenMP code. 👀

- **`-Wopenmp-simd`** — warn if the vectorizer cost model overrides the OpenMP simd directive set by the user. The `-fsimd-cost-model=unlimited` option can be used to relax the cost model. ✨

- **`-Woverride-init` (C and Objective-C only)** — warn if an initialized field *without* side effects is overridden when using designated initializers.

  This warning is included in `-Wextra`. Want the other `-Wextra` warnings without this one? Use `-Wextra -Wno-override-init`. 🎀

- **`-Wno-override-init-side-effects` (C and Objective-C only)** — do not warn if an initialized field *with* side effects is overridden when using designated initializers. This warning is enabled by default.

- **`-Wpacked`** — warn if a structure is given the packed attribute, but the packed attribute has *no effect* on the layout or size of the structure. Such structures may end up mis-aligned for basically no benefit. For instance, in this code, the variable `f.x` in `struct bar` is misaligned even though `struct bar` doesn't itself have the packed attribute:

struct foo { int x; char a, b, c, d; } __attribute__((packed)); struct bar { char z; struct foo f; };


- **`-Wnopacked-bitfield-compat`** — the 4.1, 4.2 and 4.3 series of GCC ignore the `"packed"` attribute on bit-fields of type `"char"`. This was fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example, there's no longer a 4-bit padding between field `a` and `b` in this structure:

struct foo { char a:4; char b:8; } __attribute__ ((packed));


  This warning is enabled by default. Use `-Wno-packed-bitfield-compat` to disable it.

- **`-Wpacked-not-aligned` (C, C++, Objective-C and Objective-C++ only)** — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on `struct S`, like, `warning: alignment 1 of 'struct S' is less than 8`, in this code:

struct __attribute__ ((aligned (8))) S8 { char a[8]; }; struct __attribute__ ((packed)) S { struct S8 s8; };


  This warning is enabled by `-Wall`. 💜

- **`-Wpadded`** — warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens you can rearrange the fields of the structure to reduce the padding and make the structure smaller. slay 🫶

- **`-Wredundant-decls`** — warn if anything is declared more than once in the same scope, even in cases where the multiple declaration is valid and changes nothing. like, why are you like this 😭

- **`-Wrestrict`** — warn when an object referenced by a `"restrict"`-qualified parameter (or, in C++, a `"__restrict"`-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the `"strcpy"` function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into `a[4]`, the copies overlap and the call is diagnosed:

void foo (void) { char a[] = "abcd1234"; strcpy (a, a + 4); ... }


  The `-Wrestrict` option detects some instances of simple overlap even without optimization, but it works best at `-O2` and above. It's included in `-Wall`. 🔥

- **`-Wnested-externs` (C and Objective-C only)** — warn if an `"extern"` declaration is encountered within a function. ma'am, what is that doing in there 💀

---

- **`-Winline`** — warns you when a function you declared as `inline` literally cannot be inlined. And yes, bestie, even with this flag the compiler stays quiet about failures to inline stuff declared in system headers — not its problem, not its drama. The compiler is out here using ~heuristics~ to decide whether to inline: it's looking at how big the function is and how much inlining it's already done in the current function. Which means the tiniest, most insignificant change in your source can make these warnings appear or vanish like 💀. Not you chasing ghosts.

- **`-Winterference-size`** — warns about using C++17 `std::hardware_destructive_interference_size` without setting its value via `--param destructive-interference-size`. It also side-eyes questionable values for that option. This variable is meant for controlling class layout so you avoid false sharing in concurrent code:

struct independent_fields { alignas(std::hardware_destructive_interference_size) std::atomic<int> one; alignas(std::hardware_destructive_interference_size) std::atomic<int> two; };


  Here `one` and `two` are supposed to be far enough apart that storing to `one` doesn't force a reload of the cache line for `two`. By default, `--param destructive-interference-size` and `--param constructive-interference-size` get set based on your current `-mtune` option — typically the L1 cache line size for that target CPU, sometimes a range if you're tuning for a generic target. So every translation unit depending on ABI compatibility for these variables has to be compiled with the same `-mtune` (or `-mcpu`). If ABI stability matters to you — say it's in a library header — you probably shouldn't use the hardware interference size variables at all. Alternatively, force a specific value with `--param`. And if you're confident your use of the variable doesn't affect ABI outside one single build of your project, turn the warning off with `-Wno-interference-size`. 🫶

- **`-Wint-in-bool-context`** — warns for suspicious use of integers where booleans are expected, like conditional expressions (`?:`) using non-boolean integer constants in a boolean context, e.g. `if (a <= b ? 2 : 3)`. Or left shifting signed integers in a boolean context, like `for (a = 0; 1 << a; a++);`. Same energy for all kinds of multiplications regardless of data type. Enabled by `-Wall`. 🚩

- **`-Wno-int-to-pointer-cast`** — suppresses warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of a smaller size is straight-up an **error**. `Wint-to-pointer-cast` is enabled by default.

- **`-Wno-pointer-to-int-cast`** *(C and Objective-C only)* — suppresses warnings from casts from a pointer to an integer type of a different size.

- **`-Winvalid-pch`** — warns if a precompiled header is found in the search path but can't be used. Like finding a snack in your bag and realizing it's expired 😭.

- **`-Winvalid-utf8`** — warns if an invalid UTF-8 character is found. On by default for C++23 if `-finput-charset=UTF-8` is used, and turned into an error with `-pedantic-errors`.

- **`-Wno-unicode`** — don't diagnose invalid forms of delimited or named escape sequences that are treated as separate tokens. `Wunicode` is enabled by default.

- **`-Wlong-long`** — warns if `long long` type is used. Enabled by either `-Wpedantic` or `-Wtraditional` in ISO C90 and C++98 modes. To inhibit the warning messages, use `-Wno-long-long`. And heads up — this warning is upgraded to an **error** by `-pedantic-errors`. 💀

- **`-Wvariadic-macros`** — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. Enabled by either `-Wpedantic` or `-Wtraditional`. To inhibit the warning messages, use `-Wno-variadic-macros`.

- **`-Wno-varargs`** — don't warn upon questionable usage of the macros used to handle variable arguments like `va_start`. These warnings are enabled by default.

- **`-Wvector-operation-performance`** — warns if a vector operation isn't implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning. Vector operations can be implemented "piecewise" (the scalar operation runs on every vector element), "in parallel" (the vector operation is implemented using scalars of wider type, which normally is more performance efficient), or "as a single scalar" (the vector fits into a scalar type). Educational ✨

- **`-Wvla`** — warns if a variable-length array is used in the code. `-Wno-vla` prevents the `-Wpedantic` warning of the variable-length array. This warning is upgraded to an **error** by `-pedantic-errors`.

- **`-Wvla-larger-than=bytesize`** — with this option, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed *bytesize* bytes. Similar to how `-Walloca-larger-than=bytesize` works, but for variable-length arrays. Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not trigger for those. `-Wvla-larger-than=PTRDIFF_MAX` is enabled by default but is typically only effective when `-ftree-vrp` is active (default for `-O2` and above). See also `-Walloca-larger-than=bytesize`.

- **`-Wno-vla-larger-than`** — disables `-Wvla-larger-than=` warnings. Equivalent to `-Wvla-larger-than=SIZE_MAX` or larger.

- **`-Wvla-parameter`** — warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as `-Warray-bounds`. If the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed. For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to `f` while the second specifies that the array argument must have at least `n` elements. Also, calling `f` with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning too:

void f (int n, int[n]); // warning: argument 2 previously declared as a VLA void f (int, int[]);

void g (int n) { if (n > 4) return; int a[n]; // warning: access to a by f may be out of bounds f (sizeof a, a); ... }


  `-Wvla-parameter` is included in `-Wall`. The `-Warray-parameter` option triggers warnings for similar problems involving ordinary array arguments.

- **`-Wvolatile-register-var`** — warns if a register variable is declared `volatile`. The `volatile` modifier does **not** inhibit all optimizations that may eliminate reads and/or writes to register variables. Enabled by `-Wall`. 🙈

- **`-Wno-xor-used-as-pow`** *(C, C++, Objective-C and Objective-C++ only)* — disables warnings about uses of `^`, the exclusive or operator, where it looks like the code meant exponentiation. Specifically, the warning fires when the left-hand side is the decimal constant `2` or `10` and the right-hand side is also a decimal constant. In C and C++, `^` means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. You can silence this warning by converting one of the operands to hexadecimal, or by compiling with `-Wno-xor-used-as-pow`. iykyk 💅

- **`-Wdisabled-optimization`** — warns if a requested optimization pass is disabled. This doesn't generally mean anything is wrong with your code; it just means GCC's optimizers couldn't handle the code effectively. Often the problem is that your code is too big or too complex — GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. It's giving "not me, not my code, it's the complexity" 😭

- **`-Wpointer-sign`** *(C and Objective-C only)* — warns for pointer argument passing or assignment with different signedness. Only supported for C and Objective-C. Implied by `-Wall` and by `-Wpedantic`, which can be disabled with `-Wno-pointer-sign`. This warning is upgraded to an **error** by `-pedantic-errors`.

- **`-Wstack-protector`** — only active when `-fstack-protector` is active. Warns about functions that are not protected against stack smashing. 🛡️

- **`-Woverlength-strings`** — warns about string constants longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings. The limit applies after string constant concatenation, and does not count the trailing NUL. In C90 the limit was `509` characters; in C99 it was raised to `4095`. C++98 doesn't specify a normative minimum maximum, so overlength strings aren't diagnosed in C++. Implied by `-Wpedantic`, and can be disabled with `-Wno-overlength-strings`.

- **`-Wunsuffixed-float-constants`** *(C and Objective-C only)* — issues a warning for any floating constant that doesn't have a suffix. When used together with `-Wsystem-headers` it warns about such constants in system header files too. Useful when preparing code to use with the `FLOAT_CONST_DECIMAL64` pragma from the decimal floating-point extension to C99. 🌸

- **`-Wno-lto-type-mismatch`** — during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. Requires `-flto` to be enabled. Enabled by default.

- **`-Wno-designated-init`** *(C and Objective-C only)* — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with the `designated_init` attribute.

### 🌸 Options That Control Static Analysis 🌸

- **`-fanalyzer`** — ok so this one turns on a whole static analysis of program flow that goes hunting for "interesting" interprocedural paths through your code and yells (well, warns) about the problems it finds on them. 💅

  Heads up bestie: this analysis is <red>much more expensive</red> than other GCC warnings. Like, way more. Budget accordingly. 😭

  In technical terms (iykyk), it performs coverage-guided symbolic execution of the code being compiled. It is <hotpink>neither sound nor complete</hotpink> — it can have false positives *and* false negatives. It's a bug-finding tool, not a tool for proving program correctness. So don't @ it expecting a mathematical guarantee, ok?

  <yellow>The analyzer is only suitable for use on C code in this release.</yellow>

  Enabling this option effectively enables the following warnings:

-Wanalyzer-allocation-size -Wanalyzer-deref-before-check -Wanalyzer-double-fclose -Wanalyzer-double-free -Wanalyzer-exposure-through-output-file -Wanalyzer-exposure-through-uninit-copy -Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close -Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch -Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close -Wanalyzer-fd-use-without-check -Wanalyzer-file-leak -Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic -Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion -Wanalyzer-jump-through-null -Wanalyzer-malloc-leak -Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument -Wanalyzer-null-dereference -Wanalyzer-out-of-bounds -Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument -Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var -Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow -Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size -Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion -Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset -Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-ptrdiff -Wanalyzer-undefined-behavior-strtok -Wanalyzer-unsafe-call-within-signal-handler -Wanalyzer-use-after-free -Wanalyzer-use-of-pointer-in-stale-stack-frame -Wanalyzer-use-of-uninitialized-value -Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted -Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end -Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal


  > Fr fr: this option is only available if GCC was configured with analyzer support enabled. If your build didn't get that, no vibes for you. 🙈

- **`-Wanalyzer-symbol-too-complex`** — if `-fanalyzer` is enabled, the analyzer uses various heuristics to attempt to track the state of memory, but bestie, these can be <red>defeated</red> by sufficiently complicated code. 😭

  By default, the analysis silently stops tracking values of expressions if they exceed the threshold defined by `--param analyzer-max-svalue-depth=value`, and falls back to an imprecise representation for such expressions. The `-Wanalyzer-symbol-too-complex` option warns if this occurs. So like, it literally tells on itself. <purple>iconic behavior tbh</purple>. ✨

- **`-Wanalyzer-too-complex`** — if `-fanalyzer` is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, but these can be defeated by sufficiently complicated code.

  By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit. The `-Wanalyzer-too-complex` option warns if this occurs. 🫶

- **`-Wno-analyzer-allocation-size`** — this warning requires `-fanalyzer`, which enables it; to disable it, use `-Wno-analyzer-allocation-size`.

  This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is <hotpink>not a multiple of `sizeof (*pointer)`</hotpink>.

  See `CWE-131: Incorrect Calculation of Buffer Size` (`https://cwe.mitre.org/data/definitions/131.html`). 🎀

- **`-Wno-analyzer-deref-before-check`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-deref-before-check` to disable it.

  This diagnostic warns for paths through the code in which a pointer is checked for `"NULL"` *after* it has already been dereferenced, suggesting that the pointer could have been NULL. Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced. Not me dereferencing before checking 😭

  This diagnostic also considers values passed to a function argument marked with `"__attribute__((nonnull))"` as requiring a non-NULL value, and thus will complain if such values are checked for `"NULL"` after returning from such a function call.

  > Lowkey important: this diagnostic is <yellow>unlikely to be reported when any level of optimization is enabled</yellow>, as GCC's optimization logic will typically consider such checks for NULL as being redundant, and optimize them away before the analyzer "sees" them. Hence optimization should be <bold>disabled</bold> when attempting to trigger this diagnostic.

- **`-Wno-analyzer-double-fclose`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-double-fclose` to disable it.

  This diagnostic warns for paths through the code in which a `"FILE *"` can have `"fclose"` called on it more than once. One close is a vibe, two is a 💀.

  See `CWE-1341: Multiple Releases of Same Resource or Handle` (`https://cwe.mitre.org/data/definitions/1341.html`).

- **`-Wno-analyzer-double-free`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-double-free` to disable it.

  This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either `"free"`, or a deallocator referenced by attribute `"malloc"`.

  See `CWE-415: Double Free` (`https://cwe.mitre.org/data/definitions/415.html`).

- **`-Wno-analyzer-exposure-through-output-file`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-exposure-through-output-file` to disable it.

  This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). Bestie. <red>Bestie no.</red> 🙈

  See `CWE-532: Information Exposure Through Log Files` (`https://cwe.mitre.org/data/definitions/532.html`).

- **`-Wanalyzer-exposure-through-uninit-copy`** — this warning requires both `-fanalyzer` **and** the use of a plugin to specify a function that copies across a `"trust boundary"`. Use `-Wno-analyzer-exposure-through-uninit-copy` to disable it.

  This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). 💀

  See `CWE-200: Exposure of Sensitive Information to an Unauthorized Actor` (`https://cwe.mitre.org/data/definitions/200.html`).

- **`-Wno-analyzer-fd-access-mode-mismatch`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-fd-access-mode-mismatch` to disable it.

  This diagnostic warns for paths through code in which a `"read"` on a write-only file descriptor is attempted, or vice versa.

  This diagnostic also warns for code paths in a which a function with attribute `"fd_arg_read (N)"` is called with a file descriptor opened with `"O_WRONLY"` at referenced argument `"N"` or a function with attribute `"fd_arg_write (N)"` is called with a file descriptor opened with `"O_RDONLY"` at referenced argument N.

- **`-Wno-analyzer-fd-double-close`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-fd-double-close` to disable it.

  This diagnostic warns for paths through code in which a file descriptor can be closed more than once.

  See `CWE-1341: Multiple Releases of Same Resource or Handle` (`https://cwe.mitre.org/data/definitions/1341.html`).

- **`-Wno-analyzer-fd-leak`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-fd-leak` to disable it.

  This diagnostic warns for paths through code in which an open file descriptor is leaked. It's giving "left the tap running" energy. 🚰

  See `CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime` (`https://cwe.mitre.org/data/definitions/775.html`).

- **`-Wno-analyzer-fd-phase-mismatch`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-fd-phase-mismatch` to disable it.

  This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it will warn on attempts to call `"accept"` on a stream socket that has not yet had `"listen"` successfully called on it. <purple>Wrong phase, wrong era, not the moment.</purple> ✨

  See `CWE-666: Operation on Resource in Wrong Phase of Lifetime` (`https://cwe.mitre.org/data/definitions/666.html`).

- **`-Wno-analyzer-fd-type-mismatch`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-fd-type-mismatch` to disable it.

  This diagnostic warns for paths through code in which an operation is attempted on the wrong type of file descriptor. For example, it will warn on attempts to use socket operations on a file descriptor obtained via `"open"`, or when attempting to use a stream socket operation on a datagram socket. Wrong fd, wrong fit, no cap. 💅

- **`-Wno-analyzer-fd-use-after-close`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-fd-use-after-close` to disable it.

  This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor.

  This diagnostic also warns for paths through code in which a function with attribute `"fd_arg (N)"` or `"fd_arg_read (N)"` or `"fd_arg_write (N)"` is called with a closed file descriptor at referenced argument `"N"`.

- **`-Wno-analyzer-fd-use-without-check`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-fd-use-without-check` to disable it.

  This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity.

  This diagnostic also warns for paths through code in which a function with attribute `"fd_arg (N)"` or `"fd_arg_read (N)"` or `"fd_arg_write (N)"` is called with a file descriptor, at referenced argument `"N"`, without being checked for validity. Like bestie, did you even check it first? 🫶

- **`-Wno-analyzer-file-leak`** — this warning requires `-fanalyzer`, which enables it; use `-Wno-analyzer-file-leak` to disable it.

  This diagnostic warns for paths through the code in which a `"<stdio.h>"` `"FILE *"` stream object is leaked. 🍓

  See `CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime` (`https://cwe.mitre.org/data/definitions/775.html`).

- **`-Wno-analyzer-free-of-non-heap`** — ok so this one needs `-fanalyzer` to even exist (that flag turns it on), so you use `-Wno-analyzer-free-of-non-heap` to turn it *off*. What it does: it warns you about paths through your code where `free` gets called on a <hotpink>non-heap pointer</hotpink> — like an on-stack buffer, or a global. Bestie that's a whole crime.

  See `CWE-590: Free of Memory not on the Heap` ("https://cwe.mitre.org/data/definitions/590.html").

- **`-Wno-analyzer-imprecise-fp-arithmetic`** — requires `-fanalyzer` (which enables it), and `-Wno-analyzer-imprecise-fp-arithmetic` disables it. This one warns for paths where floating-point arithmetic is used in places where you need *precise* computation. Ngl at the moment it only flags floating-point operands used inside the calculation of an <purple>allocation size</purple>. That's the current scope, periodt.

- **`-Wno-analyzer-infinite-loop`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-infinite-loop` to disable it. It warns for paths through the code that look like they lead to an infinite loop 😭

  Specifically, the analyzer yells at you when it "sees" a loop where:

  - no externally-visible work could be being done within the loop
  - there's no way to escape from the loop
  - the analyzer is sufficiently confident about the program state throughout the loop to know the above are true

  One way this warning fires: there's an execution path through a loop where taking that path on one iteration implies the same path gets taken on all the subsequent iterations.

  Like, consider this:

while (1) { char opcode = *cpu_state.pc; switch (opcode) { case OPCODE_FOO: handle_opcode_foo (&cpu_state); break; case OPCODE_BAR: handle_opcode_bar (&cpu_state); break; } }


The analyzer complains here because if `opcode` ever matches none of the cases, the `switch` follows the implicit `default` case, which makes the loop body a `no-op` with `cpu_state.pc` unchanged — so you get the same value of `opcode` on all subsequent iterations, and it just spins forever 💀 iconic behaviour honestly.

See `CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')` ("https://cwe.mitre.org/data/definitions/835.html").

- **`-Wno-analyzer-infinite-recursion`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-infinite-recursion` to disable it. Warns for paths through the code that look like they lead to infinite recursion 🌀

So specifically: when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame with the state at the entry to the previous frame of that function on the stack. Warning goes off if nothing in memory appears to be changing. Any changes it observes to parameters or globals are assumed to lead to termination of the recursion, so that suppresses the warning.

> Heads up: this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer *before* the analyzer "sees" them. So you should disable optimization when you're trying to trigger this diagnostic. Fr fr turn off `-O` for this one.

Compare with `-Winfinite-recursion`, which gives you a similar diagnostic but is implemented in a different way. iykyk.

See `CWE-674: Uncontrolled Recursion` ("https://cwe.mitre.org/data/definitions/674.html").

- **`-Wno-analyzer-jump-through-null`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-jump-through-null` to disable it. Warrns for paths through the code where a `NULL` function pointer gets called. Like, you literally jumped into the void 🕳️

- **`-Wno-analyzer-malloc-leak`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-malloc-leak` to disable it. Warns for paths through the code where a pointer allocated via an allocator gets <red>leaked</red>: either `malloc`, or a function marked with attribute `malloc`. Your memory just... left. Gone. Never freed 🫠

See `CWE-401: Missing Release of Memory after Effective Lifetime` ("https://cwe.mitre.org/data/definitions/401.html").

- **`-Wno-analyzer-mismatching-deallocation`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-mismatching-deallocation` to disable it. Warns for paths through the code where the *wrong* deallocation function is called on a pointer value, based on which function was used to allocate that pointer value. The diagnostic warns about mismatches between `free`, scalar `delete`, and vector `delete[]`, and those marked as allocator/deallocator pairs using attribute `malloc`. Matching your allocation to your deallocation is literally the bare minimum bestie 💅

See `CWE-762: Mismatched Memory Management Routines` ("https://cwe.mitre.org/data/definitions/762.html").

- **`-Wno-analyzer-out-of-bounds`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-out-of-bounds` to disable it. Warns for paths through the code where a buffer is <red>definitely</red> read or written out-of-bounds. It applies in cases where the analyzer can determine a constant offset, and for accesses past the end of a buffer where there's also a constant capacity. Further, it does limited checking for accesses past the end when the offset *and* the capacity are symbolic. Slippery little things.

See `CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer` ("https://cwe.mitre.org/data/definitions/119.html").

And okay this part is genuinely kind of iconic: where the analyzer is able, it emits a <rainbow>text art diagram</rainbow> visualizing the spatial relationship between the memory region it predicts would be accessed versus the range of memory that's valid to access. So: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and in some cases the values of data involved 🎨✨ You can suppress this diagram with `-fdiagnostics-text-art-charset=none`.

- **`-Wno-analyzer-overlapping-buffers`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-overlapping-buffers` to disable it. Warns for paths through the code where overlapping buffers get passed to an API where the behaviour on such buffers is undefined. Undefined behaviour = no cap, do not pass go.

Specifically, the diagnostic fires on calls to these functions:

- `memcpy`
- `strcat`
- `strcpy`

for the cases where the buffers are known to overlap. 🫶

- **`-Wno-analyzer-possible-null-argument`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-possible-null-argument` to disable it. Warns for paths through the code where a *possibly*-NULL value gets passed to a function argument marked with `__attribute__((nonnull))` as requiring a non-NULL value. That attribute is literally a promise, bestie.

See `CWE-690: Unchecked Return Value to NULL Pointer Dereference` ("https://cwe.mitre.org/data/definitions/690.html").

- **`-Wno-analyzer-possible-null-dereference`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-possible-null-dereference` to disable it. Warns for paths through the code where a *possibly*-NULL value gets dereferenced. Same vibe, different crime 💀

See `CWE-690: Unchecked Return Value to NULL Pointer Dereference` ("https://cwe.mitre.org/data/definitions/690.html").

- **`-Wno-analyzer-null-argument`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-null-argument` to disable it. Warns for paths through the code where a value *known* to be NULL gets passed to a function argument marked with `__attribute__((nonnull))` as requiring a non-NULL value. This one's the confident version — the analyzer ain't guessing, it *knows*.

See `CWE-476: NULL Pointer Dereference` ("https://cwe.mitre.org/data/definitions/476.html").

- **`-Wno-analyzer-null-dereference`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-null-dereference` to disable it. Warns for paths through the code where a value *known* to be NULL gets dereferenced. Straight-up dereferencing nothing. Slay but make it fatal 😭

See `CWE-476: NULL Pointer Dereference` ("https://cwe.mitre.org/data/definitions/476.html").

- **`-Wno-analyzer-putenv-of-auto-var`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-putenv-of-auto-var` to disable it. Warns for paths through the code where a call to `putenv` is passed a pointer to an automatic variable or an on-stack buffer. That's a stack frame that's about to ✨cease to exist✨ and `putenv` does *not* copy your string, so. Yeah.

See `POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument` ("https://wiki.sei.cmu.edu/confluence/x/6NYxBQ").

- **`-Wno-analyzer-shift-count-negative`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-shift-count-negative` to disable it. Warns for paths through the code where a shift is attempted with a <yellow>negative count</yellow>. It's analogous to the `-Wshift-count-negative` diagnostic implemented in the C/C++ front ends, but it's implemented based on analyzing interprocedural paths, instead of merely parsing the syntax tree. However — and this matters — the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. So like, don't treat silence as a clean bill of health 🍓

- **`-Wno-analyzer-shift-count-overflow`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-shift-count-overflow` to disable it. Warns for paths through the code where a shift is attempted with a count greater than or equal to the precision of the operand's type. It's analogous to the `-Wshift-count-overflow` diagnostic implemented in the C/C++ front ends, but implemented based on analyzing interprocedural paths rather than merely parsing the syntax tree. Same caveat though: the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. Same energy, same vibes.

- **`-Wno-analyzer-stale-setjmp-buffer`** — requires `-fanalyzer` (which enables it), use `-Wno-analyzer-stale-setjmp-buffer` to disable it. Warns for paths through the code where `longjmp` is called to rewind to a `jmp_buf` relating to a `setjmp` call in a function that has <hotpink>returned</hotpink>. That buffer is stale, bestie, the function is gone, you're longjmp-ing into a memory of a person who no longer exists 😭👻

so when `setjmp` is called on a `jmp_buf` to stash a rewind location, it records the stack frame, right? and the moment the function containing that `setjmp` call returns, the stack frame becomes **invalid** — gone, expired, no longer with us 💀. so if you try to rewind to it via `longjmp`, you'd be referencing a stack frame that doesn't exist anymore, and that's giving crash. like, *likely* a crash. or worse 🙈. don't do it bestie.

---

- **`-Wno-analyzer-tainted-allocation-size`** — this warning needs `-fanalyzer` (that's what turns it on), so use `-Wno-analyzer-tainted-allocation-size` to turn it off.

it flags paths through your code where a value that could be under an attacker's control gets used as the **size of an allocation** without being sanitized first. so an attacker could inject an excessively large allocation and potentially cause a **denial of service attack** 😭.

see `CWE-789: Memory Allocation with Excessive Size Value` (`https://cwe.mitre.org/data/definitions/789.html`).

- **`-Wno-analyzer-tainted-assertion`** — needs `-fanalyzer` to enable it; use `-Wno-analyzer-tainted-assertion` to disable it.

this one flags paths where a value that could be under an attacker's control is used as part of a condition **without being sanitized first**, and that condition guards a call to a function marked with attribute `noreturn` (like `__builtin_unreachable`). those functions typically mean the program is terminating abnormally — assertion failure handlers, that whole vibe. for example:

assert (some_tainted_value < SOME_LIMIT);


in cases like that:

- when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure
- when assertion-checking is disabled, like by defining `NDEBUG`, an attacker could inject data that subverts the process, since it presumably violates a precondition that the code is assuming 👀

ngl, important note: when assertion-checking is disabled, the assertions are *typically removed by the preprocessor before the analyzer even has a chance to "see" them*, so this diagnostic can only generate warnings on builds where assertion-checking is enabled.

and for the purpose of this warning, **any** function marked with attribute `noreturn` counts as a possible assertion failure handler, `__builtin_unreachable` included. heads up though — these functions are sometimes removed by the optimizer before the analyzer "sees" them. hence: **optimization should be disabled** when you're trying to trigger this diagnostic.

see `CWE-617: Reachable Assertion` (`https://cwe.mitre.org/data/definitions/617.html`).

it can also report problematic constructions like

switch (some_tainted_value) { case 0: /* [...etc; various valid cases omitted...] */ break;

default: __builtin_unreachable (); /* BUG: attacker can trigger this */ }


even though, strictly speaking, the above isn't an assertion failure. iykyk 💅

- **`-Wno-analyzer-tainted-array-index`** — needs `-fanalyzer` to enable it; use `-Wno-analyzer-tainted-array-index` to disable it.

flags paths where a value that could be under an attacker's control is used as the **index of an array access** without being sanitized, so an attacker could inject an out-of-bounds access 🔥.

see `CWE-129: Improper Validation of Array Index` (`https://cwe.mitre.org/data/definitions/129.html`).

- **`-Wno-analyzer-tainted-divisor`** — needs `-fanalyzer` to enable it; use `-Wno-analyzer-tainted-divisor` to disable it.

flags paths where a value that could be under an attacker's control is used as the **divisor** in a division or modulus operation without being sanitized, so an attacker could inject a division-by-zero 💀.

see `CWE-369: Divide By Zero` (`https://cwe.mitre.org/data/definitions/369.html`).

- **`-Wno-analyzer-tainted-offset`** — needs `-fanalyzer` to enable it; use `-Wno-analyzer-tainted-offset` to disable it.

flags paths where a value that could be under an attacker's control is used as a **pointer offset** without being sanitized, so an attacker could inject an out-of-bounds access.

see `CWE-823: Use of Out-of-range Pointer Offset` (`https://cwe.mitre.org/data/definitions/823.html`).

- **`-Wno-analyzer-tainted-size`** — needs `-fanalyzer` to enable it; use `-Wno-analyzer-tainted-size` to disable it.

flags paths where a value that could be under an attacker's control is used as the **size of an operation** such as `memset` without being sanitized, so an attacker could inject an out-of-bounds access.

see `CWE-129: Improper Validation of Array Index` (`https://cwe.mitre.org/data/definitions/129.html`). yes, same CWE number as `-Wno-analyzer-tainted-array-index` — that's literally what the page says, don't @ me 🫶

- **`-Wno-analyzer-undefined-behavior-ptrdiff`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-undefined-behavior-ptrdiff` to disable it.

flags paths where a **pointer subtraction** happens but the pointers refer to different chunks of memory. that code is relying on undefined behavior, because pointer subtraction is only defined when both pointers point within (or just after) the same array. periodt.

see `CWE-469: Use of Pointer Subtraction to Determine Size` (`https://cwe.mitre.org/data/definitions/469.html`).

- **`-Wno-analyzer-undefined-behavior-strtok`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-undefined-behavior-strtok` to disable it.

flags paths where a call to `strtok` has undefined behavior 🚩.

specifically: passing `NULL` as the first parameter for the **initial** call to `strtok` within a process has undefined behavior.

- **`-Wno-analyzer-unsafe-call-within-signal-handler`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-unsafe-call-within-signal-handler` to disable it.

flags paths where a function known to be **async-signal-unsafe** (such as `fprintf`) gets called from a signal handler. iconic behavior. don't.

see `CWE-479: Signal Handler Use of a Non-reentrant Function` (`https://cwe.mitre.org/data/definitions/479.html`).

- **`-Wno-analyzer-use-after-free`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-use-after-free` to disable it.

flags paths where a pointer is used **after** a deallocator is called on it: either `free`, or a deallocator referenced by attribute `malloc`.

see `CWE-416: Use After Free` (`https://cwe.mitre.org/data/definitions/416.html`).

- **`-Wno-analyzer-use-of-pointer-in-stale-stack-frame`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-use-of-pointer-in-stale-stack-frame` to disable it.

flags paths where a pointer is dereferenced that points to a variable in a **stale stack frame** 🫠.

- **`-Wno-analyzer-va-arg-type-mismatch`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-va-arg-type-mismatch` to disable it.

flags *interprocedural* paths where the analyzer catches an attempt to use `va_arg` to extract a value passed to a variadic call, but it uses a type that does **not match** the type of the expression passed to that call.

see `CWE-686: Function Call With Incorrect Argument Type` (`https://cwe.mitre.org/data/definitions/686.html`).

- **`-Wno-analyzer-va-list-exhausted`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-va-list-exhausted` to disable it.

flags interprocedural paths where the analyzer catches an attempt to use `va_arg` to access the next value passed to a variadic call — except all the values in the `va_list` have **already been consumed** 😭 empty. gone.

see `CWE-685: Function Call With Incorrect Number of Arguments` (`https://cwe.mitre.org/data/definitions/685.html`).

- **`-Wno-analyzer-va-list-leak`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-va-list-leak` to disable it.

flags interprocedural paths where the analyzer catches that `va_start` or `va_copy` was called on a `va_list` **without** a corresponding call to `va_end`. the leak, the disrespect 💅

- **`-Wno-analyzer-va-list-use-after-va-end`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-va-list-use-after-va-end` to disable it.

flags interprocedural paths where the analyzer catches an attempt to use a `va_list` **after** `va_end` has been called on it. `va_list`.

- **`-Wno-analyzer-write-to-const`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-write-to-const` to disable it.

flags paths where the analyzer catches an attempt to write through a pointer to a `const` object 🙅‍♀️. **but** — the analyzer doesn't prioritize detection of these paths, so false negatives are more likely relative to other warnings.

- **`-Wno-analyzer-write-to-string-literal`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-write-to-string-literal` to disable it.

flags paths where the analyzer catches an attempt to write through a pointer to a **string literal** 🚫. same caveat bestie: the analyzer doesn't prioritize detection of these paths, so false negatives are more likely relative to other warnings.

- **`-Wno-analyzer-use-of-uninitialized-value`** — needs `-fanalyzer` (which enables it); use `-Wno-analyzer-use-of-uninitialized-value` to disable it.

flags paths where an **uninitialized value** gets used 🌫️.

see `CWE-457: Use of Uninitialized Variable` (`https://cwe.mitre.org/data/definitions/457.html`).

---

and ok, the analyzer has hardcoded knowledge about the behavior of the following memory-management functions:

- **`"alloca"`**
- the built-in functions `"__builtin_alloc"`, `"__builtin_alloc_with_align"`, `"__builtin_calloc"`, `"__builtin_free"`, `"__builtin_malloc"`, `"__builtin_memcpy"`, `"__builtin_memcpy_chk"`, `"__builtin_memset"`, `"__builtin_memset_chk"`, `"__builtin_realloc"`, `"__builtin_stack_restore"`, and `"__builtin_stack_save"`

- **`"calloc"`**
- **`"free"`**
- **`"malloc"`**
- **`"memset"`**
- **`"operator delete"`**
- **`"operator delete []"`**
- **`"operator new"`**
- **`"operator new []"`**
- **`"realloc"`**
- **`"strdup"`**
- **`"strndup"`**

...and of these following functions for working with file descriptors:

- **`"open"`**
- **`"close"`**
- **`"creat"`**
- **`"dup"`, `"dup2"` and `"dup3"`**
- **`"isatty"`**
- **`"pipe"`, and `"pipe2"`**
- **`"read"`**
- **`"write"`**
- **`"socket"`, `"bind"`, `"listen"`, `"accept"`, and `"connect"`**

...and of these functions for working with `"<stdio.h>"` streams:

- the built-in functions `"__builtin_fprintf"`, `"__builtin_fprintf_unlocked"`, `"__builtin_fputc"`, `"__builtin_fputc_unlocked"`, `"__builtin_fputs"`, `"__builtin_fputs_unlocked"`, `"__builtin_fwrite"`, `"__builtin_fwrite_unlocked"`, `"__builtin_printf"`, `"__builtin_printf_unlocked"`, `"__builtin_putc"`, `"__builtin_putchar"`, `"__builtin_putchar_unlocked"`, `"__builtin_putc_unlocked"`, `"__builtin_puts"`, `"__builtin_puts_unlocked"`, `"__builtin_vfprintf"`, and `"__builtin_vprintf"`

- **`"fopen"`**
- **`"fclose"`**
- **`"ferror"`**
- **`"fgets"`**
- **`"fgets_unlocked"`**
- **`"fileno"`**
- **`"fread"`**
- **`"getc"`**
- **`"getchar"`**
- **`"fprintf"`**
- **`"printf"`**
- **`"fwrite"`**

...and, bestie, also these functions:

- the built-in functions `"__builtin_expect"`, `"__builtin_expect_with_probability"`, `"__builtin_strchr"`, `"__builtin_strcpy"`, `"__builtin_strcpy_chk"`, `"__builtin_strlen"`, `"__builtin_va_copy"`, and `"__builtin_va_start"`

- the GNU extensions `"error"` and `"error_at_line"`
- **`"getpass"`**
- **`"longjmp"`**
- **`"putenv"`**
- **`"setjmp"`**
- **`"siglongjmp"`**
- **`"signal"`**
- **`"sigsetjmp"`**
- **`"strcat"`**
- **`"strchr"`**
- **`"strlen"`**

Plus ✨ — a whole bunch of functions with an `"__analyzer_"` prefix have special meaning to the analyzer, and those are described in the GCC Internals manual. iykyk 💅

---

## 🌸 The parameters that actually steer this thing 🌸

The pertinent params for controlling the exploration are, periodt:

- **`--param analyzer-bb-explosion-factor=value`**
- **`--param analyzer-max-enodes-per-program-point=value`**
- **`--param analyzer-max-recursion-depth=value`**
- **`--param analyzer-min-snodes-for-call-summary=value`**

---

## ✨ The options that control the analyzer ✨

- **`-fanalyzer-call-summaries`** — lowkey simplifies interprocedural analysis by computing the effect of certain calls instead of exploring, like, *every single path* through the function from callsite to each possible return. If it's enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated (as per `--param analyzer-min-snodes-for-call-summary=value`). 🫶

- **`-fanalyzer-checker=name`** — restricts the analyzer so it runs just the named checker, and enables it. One checker, main character energy only. 👑

- **`-fanalyzer-debug-text-art-headings`** — this one is for analyzer developers 💀. If enabled, the analyzer adds extra annotations to any diagrams it generates. Art class but make it diagnostics.

- **`-fno-analyzer-feasibility`** — analyzer developers only, bestie. By default the analyzer verifies that there's a feasible control flow path for each diagnostic it emits: that the conditions that hold are not mutually exclusive. Diagnostics where no feasible path can be found get rejected. This filtering can be suppressed with `-fno-analyzer-feasibility`, for debugging issues in this code.

- **`-fanalyzer-fine-grained`** — again, for the analyzer devs. Internally the analyzer builds an `"exploded graph"` that combines control flow graphs with data flow information. By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With `-fanalyzer-fine-grained`, every single statement gets its own edge. no sharing, we're all individuals here 🦋

- **`-fanalyzer-show-duplicate-count`** — for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic. So it's like "and 12 more just like this one, don't worry about it" 😭

- **`-fanalyzer-show-events-in-system-headers`** — by default the analyzer emits simplified diagnostics paths by hiding events fully located within a system header. With `-fanalyzer-show-events-in-system-headers` such events are no longer suppressed. Nothing is hidden anymore, we're airing it all out 🙈

- **`-fno-analyzer-state-merge`** — analyzer developers ✨. By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its `"exploded graph"`. With `-fno-analyzer-state-merge` this merging can be suppressed, for debugging state-handling issues.

- **`-fno-analyzer-state-purge`** — also for the devs. By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant — e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis. With `-fno-analyzer-state-purge` this purging of state can be suppressed, for debugging state-handling issues. Stop cleaning your room, we're debugging 🔥

- **`-fno-analyzer-suppress-followups`** — devs only, again. By default the analyzer stops exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics. The diagnostics that terminate analysis along a path are:

- **`-Wanalyzer-null-argument`**
- **`-Wanalyzer-null-dereference`**
- **`-Wanalyzer-use-after-free`**
- **`-Wanalyzer-use-of-pointer-in-stale-stack-frame`**
- **`-Wanalyzer-use-of-uninitialized-value`**

With `-fno-analyzer-suppress-followups` the analyzer continues to explore such paths even after such diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. We just keep going. 🏃‍♀️💨

- **`-fanalyzer-transitivity`** — enables transitivity of constraints within the analyzer. If a → b and b → c then bestie, you *know* what time it is.

- **`-fno-analyzer-undo-inlining`** — analyzer developers, this one's for you. `-fanalyzer` runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code. By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls. With `-fno-analyzer-undo-inlining` this attempt to reconstruct the original frame information can be disabled, which may help when debugging issues in the analyzer. Un-inlining is cancelled 😔

- **`-fanalyzer-verbose-edges`** — for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths.

- **`-fanalyzer-verbose-state-changes`** — for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.

- **`-fanalyzer-verbosity=level`** — this one controls the complexity of the control flow paths that get emitted for analyzer diagnostics. The *level* can be one of:

1. **`0`** — at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-`"free"` diagnostic, both calls to `"free"` will be shown.
2. **`1`** — as per the previous level, but also show events for the entry to each function.
3. **`2`** — as per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. `"true path taken"` at a conditional). This level is the default. ⭐
4. **`3`** — as per the previous level, but show *all* control flow events, not just significant ones. the way that I need every single detail —
5. **`4`** — this level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.

- **`-fdump-analyzer`** — dumps internal details about what the analyzer is doing to `file.analyzer.txt`. Heads up: `-fdump-analyzer-stderr` overrides this option.

- **`-fdump-analyzer-stderr`** — dumps internal details about what the analyzer is doing to stderr. This option overrides `-fdump-analyzer`.

- **`-fdump-analyzer-callgraph`** — dumps a representation of the call graph suitable for viewing with GraphViz to `file.callgraph.dot`. pretty graph era 🎀

- **`-fdump-analyzer-exploded-graph`** — dumps a representation of the `"exploded graph"` suitable for viewing with GraphViz to `file.eg.dot`. Nodes are color-coded based on state-machine states to emphasize state changes. aesthetic *and* informative.

- **`-fdump-analyzer-exploded-nodes`** — emits diagnostics showing where nodes in the `"exploded graph"` are in relation to the program source.

- **`-fdump-analyzer-exploded-nodes-2`** — dumps a textual representation of the `"exploded graph"` to `file.eg.txt`.

- **`-fdump-analyzer-exploded-nodes-3`** — dumps a textual representation of the `"exploded graph"` to one dump file per node, to `file.eg-id.txt`. This is typically a large number of dump files. like, *a lot* a lot 💀

- **`-fdump-analyzer-exploded-paths`** — dumps a textual representation of the `"exploded path"` for each diagnostic to `file.idx.kind.epath.txt`.

- **`-fdump-analyzer-feasibility`** — dumps internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the form `file.*.fg.dot`, `file.*.tg.dot`, and `file.*.fpath.txt`.

- **`-fdump-analyzer-infinite-loop`** — dumps internal details about the analyzer's search for infinite loops. The details are written in a form suitable for viewing with GraphViz to filenames of the form `file.*.infinite-loop.dot`. going in circles has never been so documented 🔄

- **`-fdump-analyzer-json`** — dumps a compressed JSON representation of analyzer internals to `file.analyzer.json.gz`. The precise format is subject to change. don't get attached 🫶

- **`-fdump-analyzer-state-purge`** — okay so this is literally `-fdump-analyzer-supergraph`'s sibling 💅 it dumps a representation of the "supergraph" that's suitable for viewing with GraphViz, BUT it also annotates the graph with info on what state will be purged at each node. Writes out to `file.state-purge.dot`. So useful, no cap.

- **`-fdump-analyzer-supergraph`** — dumps representations of the "supergraph" suitable for viewing with GraphViz to `file.supergraph.dot` and to `file.supergraph-eg.dot` ✨ these show **all** of the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump contains annotations showing nodes in the "exploded graph" plus the diagnostics associated with them. Obsessed.

- **`-fdump-analyzer-untracked`** — emits custom warnings with internal details, and those are intended for analyzer developers 🫶 iykyk, it's giving behind-the-scenes energy.

## 🌸 Options for Debugging Your Program 🌸

So the whole vibe here: to tell GCC to emit extra information for use by a debugger, in almost all cases you literally just need to add `-g` to your other options 💖 some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options.

GCC allows you to use `-g` with `-O`. But bestie, the shortcuts taken by optimized code may occasionally be surprising 😭 like:

- some variables you declared may not exist at all
- flow of control may briefly move where you did not expect it
- some statements may not be executed because they compute constant results or their values are already at hand
- some statements may execute in different places because they have been moved out of loops

Nevertheless it is possible to debug optimized output — which lowkey makes it reasonable to use the optimizer for programs that might have bugs 👑

If you are not using some other optimization option, consider using `-Og` with `-g`. Because with no `-O` option at all, some compiler passes that collect information useful for debugging do not run at all, so `-Og` may result in a better debugging experience. Fr fr, it's giving "why would you not".

- **`-g`** — produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. On most systems that use stabs format, `-g` enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program 💀 and if you want to control for certain whether to generate the extra information, use `-gvms` (see below).

- **`-ggdb`** — produce debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. Max main character energy 🎀

- **`-gdwarf`**, **`-gdwarf-version`** — produce debugging information in DWARF format (if that is supported). The value of *version* may be either `2`, `3`, `4` or `5`; the default version for most targets is `5` (with the exception of VxWorks, TPF and Darwin / macOS, which default to version `2`, and AIX, which defaults to version `4`).

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and `-fvar-tracking-assignments` for maximum benefit. Version 5 requires GDB 8.0 or higher.

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later 🙈 for historical reasons, some other DWARF-related options such as `-fno-dwarf2-cfi-asm` retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. Not confusing at all, obviously.

- **`-gbtf`** — request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options 💫

- **`-gprune-btf`**, **`-gno-prune-btf`** — prune BTF information before emission. When pruning, only type information for types used by global variables and file-scope functions will be emitted. If compiling for the BPF target with BPF CO-RE enabled, type information will also be emitted for types used in BPF CO-RE relocations. In addition, struct and union types which are only referred to via pointers from members of other struct or union types shall be pruned and replaced with `BTF_KIND_FWD`, as though those types were only present in the input as forward declarations.

> ⚠️ This option substantially reduces the size of produced BTF information, but at significant loss in the amount of detailed type information. It is primarily useful when compiling for the BPF target, to minimize the size of the resulting object, and to eliminate BTF information which is not immediately relevant to the BPF program loading process.

This option is enabled by default for the BPF target when generating BTF information 🦋

- **`-gctf`**, **`-gctflevel`** — request CTF debug information and use *level* to specify how much CTF debug information should be produced. If `-gctf` is specified without a value for *level*, the default level of CTF debug information is `2`.

CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.

- Level `0` produces no CTF debug information at all. Thus, `-gctf0` negates `-gctf`.
- Level `1` produces CTF information for tracebacks only. This includes callsite information, but does not include type information.
- Level `2` produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.

- **`-gvms`** — produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems.

- **`-gcodeview`** — produce debugging information in CodeView debug format (if that is supported). This is the format used by Microsoft Visual C++ on Windows.

- **`-glevel`**, **`-ggdblevel`**, **`-gvmslevel`** — request debugging information and also use *level* to specify how much information. The default level is `2`.

- Level `0` produces no debug information at all. Thus, `-g0` negates `-g`.
- Level `1` produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
- Level `3` includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use `-g3`.

If you use multiple `-g` options, with or without level numbers, the **last** such option is the one that is effective 👑

`-gdwarf` does not accept a concatenated debug level, to avoid confusion with `-gdwarf-level`. Instead use an additional `-glevel` option to change the debug level for DWARF.

- **`-fno-eliminate-unused-debug-symbols`** — by default, no debug information is produced for symbols that are not actually used. Use this option if you want debug information for **all** symbols.

- **`-femit-class-debug-always`** — instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes, because using this option increases the size of debugging information by as much as a factor of two 😭 two!! Bloating fr.

- **`-fno-merge-debug-strings`** — direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.

- **`-fdebug-prefix-map=old=new`** — when compiling files residing in directory *old*, record debugging information describing them as if the files resided in directory *new* instead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using `.` for *new*. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files 💅 see also `-ffile-prefix-map` and `-fcanon-prefix-map`.

- **`-fvar-tracking`** — run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information). It is enabled by default when compiling with optimization (`-Os`, `-O`, `-O2`, ...), debugging information (`-g`) and the debug info format supports it ✨

- **`-fvar-tracking-assignments`** — annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of `-gdwarf-4` is recommended along with it.

It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end 🙈 by default, this flag is enabled together with `-fvar-tracking`, except when selective scheduling is enabled.

- **`-gsplit-dwarf`** — if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension `.dwo`. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading `.dwo` files. It's giving organized 🎀

- **`-gdwarf32`**, **`-gdwarf64`** — if DWARF debugging information is enabled, the `-gdwarf32` selects the 32-bit DWARF format and the `-gdwarf64` selects the 64-bit DWARF format. The default is target specific; on most targets it is `-gdwarf32` though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet 💀

- **`-gdescribe-dies`** — add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. Cute little details, we love to see it 💖

- **`-gpubnames`** — makes DWARF `.debug_pubnames` and `.debug_pubtypes` sections. that's it, that's the whole thing 💅

- **`-ggnu-pubnames`** — same `.debug_pubnames` and `.debug_pubtypes` sections but in a format that's ready to be converted into a GDB index 🎀 lowkey only useful if your linker can produce GDB index version 7, so like… check that first bestie

- **`-fdebug-types-section`** — when you're on DWARF Version 4 or higher, type DIEs can go live in their own `.debug_types` section instead of crashing the `.debug_info` party 🦋 and honestly it's more efficient to park them in a separate comdat section because the linker can then yeet the duplicates. BUT ⚠️ not all DWARF consumers support `.debug_types` sections yet, and on some objects `.debug_types` literally produces *larger* instead of smaller debugging information. it's giving "it depends" 😭

- **`-grecord-gcc-switches`**, **`-gno-record-gcc-switches`** — this one appends the command-line options used to invoke the compiler (the ones that may affect code generation) onto the `DW_AT_producer` attribute in the DWARF debugging info ✨ the options get concatenated with spaces separating them from each other and from the compiler version. it's **enabled by default** 👑 also peep `-frecord-gcc-switches` if you want another way of storing compiler options into the object file

- **`-gstrict-dwarf`** — no extensions from later DWARF standard versions than the one you picked with `-gdwarf-version`, periodt 🚫 on most targets, using non-conflicting DWARF extensions from later standard versions is allowed (that's the vibe this flag kills)

- **`-gno-strict-dwarf`** — the opposite, bestie: allows using extensions of later DWARF standard version than the one selected with `-gdwarf-version` 🌀

- **`-gas-loc-support`** — tells the compiler "hey, the assembler supports `.loc` directives" 🫶 it may then use them so the assembler generates DWARF2+ line number tables. this is generally desirable because assembler-generated line-number tables are *a lot* more compact than the ones the compiler can generate itself 🔥 this option is enabled by default if, at GCC configure time, the assembler was found to support such directives

- **`-gno-as-loc-support`** — forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated 🧚

- **`-gas-locview-support`** — tells the compiler the assembler supports "view" assignment and reset assertion checking in `.loc` directives ✨ enabled by default if, at GCC configure time, the assembler was found to support them

- **`-gno-as-locview-support`** — forces GCC to assign view numbers internally, if `-gvariable-location-views` are explicitly requested 💅

- **`-gcolumn-info`**, **`-gno-column-info`** — emit location *column* information into DWARF debugging info instead of just file and line 🎯 **enabled by default**

- **`-gstatement-frontiers`**, **`-gno-statement-frontiers`** — makes GCC drop markers in the internal representation at the beginning of statements and keep them roughly in place throughout compilation, using them to guide the output of `is_stmt` markers in the line number table 🌸 enabled by default when compiling with optimization (`-Os`, `-O1`, `-O2`, ...) and outputting DWARF 2 debug information at the normal level

- **`-gvariable-location-views`**, **`-gvariable-location-views=incompat5`**, **`-gno-variable-location-views`** — augments variable location lists with progressive view numbers implied from the line number table 🦋 this lets debug info consumers inspect state at certain points of the program even when *no* instructions associated with the corresponding source locations are present at that point. if the assembler lacks support for view numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes them somewhat less compact 😬 the augmented line number tables and location lists are fully backward-compatible, so debug info consumers that aren't aware of these augmentations can still consume them — they just won't derive any benefit from them either 🙈

this is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there's assembler support, `-fvar-tracking-assignments` is enabled and `-gstrict-dwarf` is not. when assembler support is not available, this may still be enabled, but it'll force GCC to output internal line number tables — and if `-ginternal-reset-location-views` is not enabled, that will most certainly lead to silently mismatching location views 💀

there's also a *proposed* representation for view numbers that is **not** backward compatible with the location list format introduced in DWARF 5, enabled with `-gvariable-location-views=incompat5` 🚨 this option may be removed in the future, and it's only provided as a reference implementation of the proposed representation. debug info consumers are not expected to support this extended format, and they'd be rendered unable to decode location lists using it. so like… no cap, be careful

- **`-ginternal-reset-location-views`**, **`-gno-internal-reset-location-views`** — tries to determine location views that can be omitted from location view lists 🧚 this requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that doesn't support location view lists 😭 the GNU assembler will flag any such error as a `"view number mismatch"`. only enabled on ports that define a reliable estimation function ✨

- **`-ginline-points`**, **`-gno-inline-points`** — generates extended debug info for inlined functions 🎀 location view tracking markers get inserted at inlined entry points so address and view numbers can be computed and output in debug info. this can be enabled independently of location views — in which case the view numbers won't be output — but it can only be enabled along with statement frontiers, and it's only enabled by default if location views are enabled 💅

- **`-gz[=type]`** — produces compressed debug sections in DWARF format, if that's supported 🗜️ if *type* isn't given, the default type depends on the capabilities of the assembler and linker used. *type* may be one of `none` (don't compress debug sections), or `zlib` (use zlib compression in ELF gABI format). if the linker doesn't support writing compressed debug sections, the option is **rejected** ❌ otherwise, if the assembler doesn't support them, `-gz` is silently ignored when producing object files 🤫

- **`-femit-struct-debug-baseonly`** — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file in which the struct is defined 🎯 this substantially reduces the size of debugging information, but at **significant potential loss in type information to the debugger** 😭 peep `-femit-struct-debug-reduced` for a less aggressive option, and `-femit-struct-debug-detailed` for more detailed control. this option works only with DWARF debug output

- **`-femit-struct-debug-reduced`** — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file in which the type is defined — *unless* the struct is a template or defined in a system header 🌸 this significantly reduces the size of debugging information, with some potential loss in type information to the debugger. see `-femit-struct-debug-baseonly` for a more aggressive option and `-femit-struct-debug-detailed` for more detailed control. again: works only with DWARF debug output

- **`-femit-struct-debug-detailed[=spec-list]`** — lets you specify the struct-like types for which the compiler generates debug info, with the intent of reducing duplicate struct debug info between different object files within the same program 🦋 it's the detailed version of `-femit-struct-debug-reduced` and `-femit-struct-debug-baseonly`, and honestly it serves for most needs 👑

a specification has the syntax:

[dir:|ind:][ord:|gen:](any|sys|base|none)


the optional **first word** limits the specification to structs that are used directly (`dir:`) or used indirectly (`ind:`). a struct type is used directly when it's the type of a variable, member. indirect uses arise through pointers to structs — that is, when use of an incomplete struct is valid, the use is indirect. an example is `struct one direct; struct two * indirect;` ✨

the optional **second word** limits the specification to ordinary structs (`ord:`) or generic structs (`gen:`). generic structs are a bit complicated to explain ngl — for C++, these are non-explicit specializations of template classes, or non-template classes within the above. other programming languages have generics, but `-femit-struct-debug-detailed` does not yet implement them 🙈

the **third word** specifies the source files for those structs for which the compiler should emit debug info. the values `none` and `any` have the normal meaning 🎀 the value `base` means the base of the name of the file in which the type declaration appears must match the base of the name of the main compilation file — in practice that means when compiling `foo.c`, debug info is generated for types declared in that file and `foo.h`, but not other header files. the value `sys` means those types satisfying `base` or declared in system or compiler headers 💅

> you may need to experiment to determine the best settings for your application. no cap, just vibes and testing 🔬

the default is `-femit-struct-debug-detailed=all`. this option works only with DWARF debug output

- **`-fno-dwarf2-cfi-asm`** — emits DWARF unwind info as a compiler-generated `.eh_frame` section instead of using GAS `.cfi_*` directives 🌀

- **`-fno-eliminate-unused-debug-types`** — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled 🤷 sometimes it's useful to have GCC emit debugging info for *all* types declared in a compilation unit, regardless of whether they're actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that's not actually used in your program (but is declared) 🎀 more often, however, this results in a significant amount of wasted space 😭

---

## 🌸 Options That Control Optimization 🌸

these options control various sorts of optimizations, bestie ✨ get comfy, there's a lot

Okay so before we even get into the flags, bestie, let's talk about what the compiler is even *doing* here. <pink>Without any optimization option</pink>, the compiler's whole goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. It's giving "what you wrote is what you get" 💅

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. So yeah, you're trading vibes for speed, that's the whole deal 🫶

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. Group project but make it ✨useful✨.

> <yellow>Heads up:</yellow> not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. So if you're looking for one that's missing — she's not controlled by a flag, she's just living her life.

Most optimizations are completely disabled at `-O0` or if an `-O` level is not set on the command line, even if individual optimization flags are specified. Similarly, `-Og` suppresses many optimization passes. Like, you can't just sneak them in, the compiler said no 🚫

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each `-O` level than those listed here. You can invoke GCC with `-Q --help=optimizers` to find out the exact set of optimizations that are enabled at each level. <cyan>Do it, it's literally free information.</cyan>

---

- **`-O`**, **`-O1`** — Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

With `-O`, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

`-O` is the recommended optimization level for large machine-generated code as a sensible balance between time taken to compile and memory use: higher optimization levels perform optimizations with greater algorithmic complexity than at `-O`.

`-O` turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch -fdse -fforward-propagate -fguess-branch-probability -fif-conversion -fif-conversion2 -finline-functions-called-once -fipa-modref -fipa-profile -fipa-pure-const -fipa-reference -fipa-reference-addressable -fivopts -fmerge-constants -fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer -freorder-blocks -fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink -ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time


- **`-O2`** — Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to `-O`, this option increases both compilation time and the performance of the generated code.

  `-O2` turns on all optimization flags specified by `-O1`. It also turns on the following optimization flags:

-falign-functions -falign-jumps -falign-labels -falign-loops -fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively -fexpensive-optimizations -ffinite-loops -fgcse -fgcse-lm -fhoist-adjacent-loads -finline-functions -finline-small-functions -findirect-inlining -fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra -fipa-vrp -fisolate-erroneous-paths-dereference -flra-remat -foptimize-crc -foptimize-sibling-calls -foptimize-strlen -fpartial-inlining -fpeephole2 -freorder-blocks-algorithm=stc -freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce -ftree-loop-vectorize -ftree-pre -ftree-slp-vectorize -ftree-switch-conversion -ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap


  > <red>Don't skip this:</red> please note the warning under `-fgcse` about invoking `-O2` on programs that use computed gotos. That's a real "read the fine print" moment, iykyk 😭

- **`-O3`** — Optimize yet more. `-O3` turns on all optimizations specified by `-O2` and also turns on the following optimization flags:

-fgcse-after-reload -fipa-cp-clone -floop-interchange -floop-unroll-and-jam -fpeel-loops -fpredictive-commoning -fsplit-loops -fsplit-paths -ftree-loop-distribution -ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic -fversion-loops-for-strides


- **`-O0`** — Reduce compilation time and make debugging produce the expected results. This is the default.

  At `-O0`, GCC completely disables most optimization passes; they are not run even if you explicitly enable them on the command line, or are listed by `-Q --help=optimizers` as being enabled by default. Many optimizations performed by GCC depend on code analysis or canonicalization passes that are enabled by `-O`, and it would not be useful to run individual optimization passes in isolation. <purple>She's not being difficult, she's just built that way.</purple>

- **`-Os`** — Optimize for size. `-Os` enables all `-O2` optimizations except those that often increase code size:

-falign-functions -falign-jumps -falign-labels -falign-loops -fprefetch-loop-arrays -freorder-blocks-algorithm=stc


  It also enables `-finline-functions`, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. Small and mighty 💖

- **`-Ofast`** — Disregard strict standards compliance. `-Ofast` enables all `-O3` optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on `-ffast-math`, `-fallow-store-data-races` and the Fortran-specific `-fstack-arrays`, unless `-fmax-stack-var-size` is specified, and `-fno-protect-parens`. It turns off `-fsemantic-interposition`.

  > <hotpink>Real talk:</hotpink> this one literally disregards strict standards compliance. Your program gets to be fast, but it also gets to stop being standard-compliant. Choose wisely 💀

- **`-Og`** — Optimize debugging experience. `-Og` should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than `-O0` for producing debuggable code because some compiler passes that collect debug information are disabled at `-O0`.

  Like `-O0`, `-Og` completely disables a number of optimization passes so that individual options controlling them have no effect. Otherwise `-Og` enables all `-O1` optimization flags except for those that may interfere with debugging:

-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion -fif-conversion2 -finline-functions-called-once -fmove-loop-invariants -fmove-loop-stores -fssa-phiopt -ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra


- **`-Oz`** — Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those instructions require fewer bytes to encode. `-Oz` behaves similarly to `-Os` including enabling most `-O2` optimizations. Tiny code, chaotic energy, iconic 🎀

---

If you use multiple `-O` options, with or without level numbers, the last such option is the one that is effective. Last one wins, no notes ✨

Options of the form `-fflag` specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo` is `-fno-foo`. In the table below, only one of the forms is listed—the one you typically use. You can figure out the other form by either removing `no-` or adding it. <mint>It's giving "the negative form is just the vibe flipped."</mint>

The following options control specific optimizations. They are either activated by `-O` options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired. Yes, "fine-tuning" in quotes — the manpage is being a little cheeky about it, and honestly? Same.

## 🌈 Individual optimization flags, the fine-tuning era 🌈

- **`-fno-defer-pop`** — For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels `-O1` and higher, `-fdefer-pop` is the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once. Bulk pop, efficiency queen.

- **`-fforward-propagate`** — Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

  This option is enabled by default at optimization levels `-O1`, `-O2`, `-O3`, `-Os`.

- **`-favoid-store-forwarding`**, **`-fno-avoid-store-forwarding`** — Many CPUs will stall for many cycles when a load partially depends on previous smaller stores. This pass tries to detect such cases and avoid the penalty by changing the order of the load and store and then fixing up the loaded value.

  Disabled by default. <lavender>Sleeping beauty vibes, you gotta wake her up yourself.</lavender>

- **`-ffp-contract=style`** — `-ffp-contract=off` disables floating-point expression contraction. `-ffp-contract=fast` enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. `-ffp-contract=on` enables floating-point expression contraction if allowed by the language standard. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.

  The default is `-ffp-contract=off` for C in a standards compliant mode (`-std=c11` or similar), `-ffp-contract=fast` otherwise.

- **`-fomit-frame-pointer`** — Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. Free real estate, honestly 👑

  On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.

  Note that `-fno-omit-frame-pointer` doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.

  Enabled by default at `-O1` and higher.

- **`-foptimize-crc`** — Detect loops calculating CRC (performing polynomial long division) and replace them with a faster implementation. Detect 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and bit-reversed cases. If the target supports a CRC instruction and the polynomial used in the source code matches the polynomial used in the CRC instruction, generate that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, generate CRC using it; otherwise generate table-based CRC.

  Enabled by default at `-O2` and higher. <green>She's out here detecting your polynomial long division and replacing it, that's main character energy fr fr.</green>

- **`-foptimize-sibling-calls`** — bestie this one optimizes sibling and tail recursive calls, like it literally just goes "oh you're calling yourself at the end? say less" and makes it cute. ✨

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-foptimize-strlen`** — optimizes various standard C string functions (e.g. `strlen`, `strchr` or `strcpy`) and their `_FORTIFY_SOURCE` counterparts into faster alternatives. it's giving glow-up for your string functions ngl 💅

  Enabled at levels `-O2`, `-O3`.

- **`-finline-stringops[=fn]`** — expands memory and string operations (for now, only `memset`) inline, even when the length is variable or big enough as to require looping. This is most useful along with `-ffreestanding` and `-fno-builtin`. 🔥

  In some circumstances, it enables the compiler to generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches. so like... sometimes the flex backfires 😭 This option is disabled by default.

- **`-fno-inline`** — do not expand any functions inline apart from those marked with the `always_inline` attribute. This is the default when not optimizing. no inlining, we're keeping everyone in their own lane 🚫

  Single functions can be exempted from inlining by marking them with the `noinline` attribute.

- **`-finline-small-functions`** — integrate functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating in this way. This inlining applies to all functions, even those not declared inline. tiny function? absorbed. periodt. 🫶

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-findirect-inlining`** — inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the `-finline-functions` or `-finline-small-functions` options. it's the inline-ception situation, iykyk 💀

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-finline-functions`** — consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides which functions are worth integrating in this way. everyone's a candidate bestie, no favoritism ✨

  If all calls to a given function are integrated, and the function is declared `static`, then the function is normally not output as assembler code in its own right.

  Enabled at levels `-O2`, `-O3`, `-Os`. Also enabled by `-fprofile-use` and `-fauto-profile`.

- **`-finline-functions-called-once`** — consider all `static` functions called once for inlining into their caller even if they are not marked `inline`. If a call to a given function is integrated, then the function is not output as assembler code in its own right. one-time caller? get in here, you're part of the family now 🎀

  Enabled at levels `-O1`, `-O2`, `-O3` and `-Os`, but not `-Og`.

- **`-fearly-inlining`** — inline functions marked by `always_inline` and functions whose body seems smaller than the function call overhead early before doing `-fprofile-generate` instrumentation and real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions. it's the "do it early so future-you doesn't suffer" move 💜

  Enabled by default.

- **`-fipa-sra`** — perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value. huge fan of this one, it's literally cleaning out the clutter 🌸

  Enabled at levels `-O2`, `-O3` and `-Os`.

- **`-finline-limit=n`** — by default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. *n* is the size of functions that can be inlined in number of pseudo instructions. like a bouncer at the door of the inline club, checking if you're small enough to get in 💅

  Inlining is actually controlled by a number of parameters, which may be specified individually by using `--param name=value`. The `-finline-limit=n` option sets some of these parameters as follows:

  - `max-inline-insns-single` — is set to `n/2`.
  - `max-inline-insns-auto` — is set to `n/2`.

  See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.

  > Note: there may be no value to `-finline-limit` that results in default behavior. so yeah, good luck finding it bestie 😭

  > Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another. it's vibes-based measurement fr fr, don't quote me on the exact count 💀

- **`-fno-keep-inline-dllexport`** — this is a more fine-grained version of `-fkeep-inline-functions`, which applies only to functions that are declared using the `dllexport` attribute or declspec. very niche, very specific, we love a targeted king 👑

- **`-fkeep-inline-functions`** — In C, emit `static` functions that are declared `inline` into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the `extern inline` extension in GNU C90. In C++, emit any and all inline functions into the object file. keeping them around like old receipts, honestly iconic 🧚

- **`-fkeep-static-functions`** — emit `static` functions into the object file, even if the function is never used. never used? doesn't matter, still showing up to the party ✨

- **`-fkeep-static-consts`** — emit variables declared `static const` when optimization isn't turned on, even if the variables aren't referenced. 🎀

  GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the `-fno-keep-static-consts` option.

- **`-fmerge-constants`** — attempt to merge identical constants (string constants and floating-point constants) across compilation units. twins get merged, slay 👯‍♀️

  This option is the default for optimized compilation if the assembler and linker support it. Use `-fno-merge-constants` to inhibit this behavior.

  Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`.

- **`-fmerge-all-constants`** — attempt to merge identical constants and identical variables. going full combined-identity mode 💫

  This option implies `-fmerge-constants`. In addition to `-fmerge-constants` this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. ⚠️ like it literally breaks the rules of C/C++, so heads up bestie, that's the dramatic caveat 😭

- **`-fmodulo-sched`** — perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. loop glow-up, we're overlapping iterations like it's nothing 🌈

- **`-fmodulo-sched-allow-regmoves`** — perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with `-fmodulo-sched` enabled. it only works if the parent option's on, no solo acts 🫶

- **`-fno-branch-count-reg`** — disable the optimization pass that scans for opportunities to use `decrement and branch` instructions on a count register instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the `-fno-branch-count-reg` option doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes. so like, it's not a full removal, just a "please stop adding these" request 😌

  The default is `-fbranch-count-reg` at `-O1` and higher, except for `-Og`.

- **`-fno-function-cse`** — do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. spell it out every single time, no shorthand 🗣️

  This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. chaos gremlin assembler hacks need their space, we respect it 🙈

  The default is `-ffunction-cse`

- **`-fno-zero-initialized-in-bss`** — if the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code. free real estate in the BSS section ✨

  This option turns off this behavior because some programs explicitly rely on variables going to the data section---e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that. some programs are picky about where their variables live, and honestly? valid 💅

  The default is `-fzero-initialized-in-bss` except in Ada.

- **`-fthread-jumps`** — perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. jump optimization, redirecting branches like a traffic controller with main character energy 👑

  Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`.

- **`-fsplit-wide-types`** — when using a type that occupies multiple registers, such as `long long` on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult. better code, worse debugging, the eternal tradeoff 😭

  Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`.

- **`-fsplit-wide-types-early`** — fully split wide types early, instead of very late. This option has no effect unless `-fsplit-wide-types` is turned on. early bird gets the split 🐦

  This is the default on some targets.

- **`-fcse-follow-jumps`** — in common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an `if` statement with an `else` clause, CSE follows the jump when the condition tested is false. it's literally following the plot, tracking the jump when nobody else goes there 🦋

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fcse-skip-blocks`** — this is similar to `-fcse-follow-jumps`, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple `if` statement with no else clause, `-fcse-skip-blocks` causes CSE to follow the jump around the body of the `if`. same energy but for the simple `if` with no else, it skips right around the body 🎀

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-frerun-cse-after-loop`** — re-run common subexpression elimination after loop optimizations are performed. round two of CSE, because the loop glow-up changed the vibe 🔁

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fgcse`** — perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation. global CSE, absolutely ate 💅

  > Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding `-fno-gcse` to the command line. computed gotos + gcse = kinda mess, so ngl consider turning it off for those 🔥

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fgcse-lm`** — okay so when this one's on, global common subexpression elimination (gcse, bestie, say it with me) tries to move loads that are *only* killed by stores into themselves 💅. Translation: a loop that's got a load/store sequence can get rewritten into a load *outside* the loop plus a copy/store *inside* the loop. Slay.

  <mint>Enabled by default when `-fgcse` is enabled.</mint> 🫶

- **`-fgcse-sm`** — turn this on and a store motion pass runs *after* global common subexpression elimination. This pass is out here trying to move stores out of loops 🦋. Team it up with `-fgcse-lm` and loops containing a load/store sequence can become a load *before* the loop and a store *after* the loop. Iconic combo, honestly.

  <yellow>Not enabled at any optimization level.</yellow> ngl, she's opt-in only.

- **`-fgcse-las`** — with this enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial *and* full redundancies) ✨.

  <yellow>Not enabled at any optimization level.</yellow> again, you gotta invite her.

- **`-fgcse-after-reload`** — enables a redundant load elimination pass that runs after reload. Its whole purpose? Cleaning up redundant spilling 🧹💖. Very "post-party cleanup crew" of her.

  <mint>Enabled by `-O3`, `-fprofile-use` and `-fauto-profile`.</mint> ⭐

- **`-faggressive-loop-optimizations`** — this one tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop 👑. And she's *assuming* your loop code doesn't invoke undefined behavior — for example by causing signed integer overflows or out-of-bound array accesses 😭. Those derived bounds then guide loop unrolling and peeling and loop exit test optimizations.

  > 🚨 <red>Heads up:</red> this assumes no UB. If you *do* have signed overflow or out-of-bounds accesses, that assumption is a lie and things get spicy in the bad way.

  <mint>Enabled by default.</mint> fr fr.

- **`-funconstrained-commons`** — tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays 👀. This prevents certain optimizations that depend on knowing the array bounds. She's basically saying "bestie, don't assume you know how big this is."

- **`-fcrossjumping`** — performs the cross-jumping transformation, which unifies equivalent code and saves code size 🎀. Real talk though: the resulting code may or may not perform better than without cross-jumping. It's giving "smaller, but who knows about faster."

  <mint>Enabled at levels `-O2`, `-O3`, `-Os`.</mint>

- **`-fauto-inc-dec`** — combines increments or decrements of addresses with memory accesses 🔥. This pass is *always* skipped on architectures that don't have instructions to support it (respecting the hardware's boundaries, we love to see it).

  <mint>Enabled by default at `-O1` and higher on architectures that support this.</mint>

- **`-fdce`** — performs dead code elimination (DCE) on RTL 💀. Bye-bye, code nobody needs.

  <mint>Enabled by default at `-O1` and higher.</mint>

- **`-fdse`** — performs dead store elimination (DSE) on RTL 🗑️✨.

  <mint>Enabled by default at `-O1` and higher.</mint>

- **`-fif-conversion`** — attempts to transform conditional jumps into branch-less equivalents 🦋. This includes conditional moves, min, max, set flags and abs instructions, plus some tricks doable by standard arithmetics. The use of conditional execution on chips where it's available is controlled by `-fif-conversion2`.

  <mint>Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`, but not with `-Og`.</mint> noted 📝

- **`-fif-conversion2`** — uses conditional execution (where available) to transform conditional jumps into branch-less equivalents 💫. Sister option, sister energy.

  <mint>Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`, but not with `-Og`.</mint>

- **`-fdeclone-ctor-dtor`** — okay this one needs lore 📖. The C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls `operator delete` afterwards. For a hierarchy with virtual bases, the base and complete variants are *clones*, which means two copies of the function 😭. With this option, the base and complete variants get changed into thunks that call a common implementation. Deduplicated. Frugal. Gorgeous 💅.

  <mint>Enabled by `-Os`.</mint>

- **`-fdelete-null-pointer-checks`** — assumes programs cannot safely dereference null pointers, and that no code or data element resides at address zero 🙈. This enables simple constant folding optimizations at *all* optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers — they assume a memory access to address zero always results in a trap, so if a pointer is checked *after* it's already been dereferenced, it can't be null. Logic! 🧠

  > ⚠️ <red>But bestie, listen:</red> in some environments this assumption is **not true**. Use `-fno-delete-null-pointer-checks` to disable this optimization for programs that depend on that behavior. Do not skip this.

  <mint>Enabled by default on most targets. On AVR and MSP430, this option is completely disabled.</mint> 💀

  Passes that use the dataflow information are enabled independently at different optimization levels.

- **`-fdevirtualize`** — attempts to convert calls to virtual functions into direct calls 🔥. This happens both within a procedure and interprocedurally, as part of indirect inlining (`-findirect-inlining`) and interprocedural constant propagation (`-fipa-cp`).

  <mint>Enabled at levels `-O2`, `-O3`, `-Os`.</mint> ate.

- **`-fdevirtualize-speculatively`** — attempts to convert calls to virtual functions into *speculative* direct calls 🎀. Based on the analysis of the type inheritance graph, it figures out for a given call the set of likely targets. If that set is small — preferably of size 1 — it changes the call into a conditional deciding between direct and indirect calls. Those speculative calls enable more optimizations, like inlining ✨. And when they seem useless after further optimization? They get converted back into their original form. Self-aware queen 👑.

- **`-fdevirtualize-at-ltrans`** — streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode 💫. This option enables more devirtualization but *significantly increases the size of streamed data* 📈. For this reason it is disabled by default. Trade-offs, bestie, trade-offs.

- **`-fexpensive-optimizations`** — performs a number of minor optimizations that are relatively expensive 💸😌. Yes she costs a little, she's worth it.

  <mint>Enabled at levels `-O2`, `-O3`, `-Os`.</mint>

- **`-free`** — attempts to remove redundant extension instructions 🌸. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Look at her knowing the architecture lore.

  <mint>Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels `-O2`, `-O3`, `-Os`.</mint> ⭐

- **`-fno-lifetime-dse`** — okay C++ girlies gather round 🫶. In C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination takes advantage of this 🧹. But if your code relies on the value of the object storage persisting *beyond* the lifetime of the object, you can use this flag to disable this optimization 🙈.

  Want to preserve stores *before* the constructor starts (e.g. because your `operator new` clears the object storage) but still treat the object as dead after the destructor? Use `-flifetime-dse=1`. The default behavior can be explicitly selected with `-flifetime-dse=2`. And `-flifetime-dse=0` is equivalent to `-fno-lifetime-dse`. Numbers, bestie, they matter 🔢💅.

- **`-flive-range-shrinkage`** — attempts to decrease register pressure through register live range shrinkage 🦋. This is helpful for fast processors with small or moderate size register sets. Squish the ranges, save the registers, live your life.

- **`-fira-algorithm=`*algorithm* — use the specified coloring algorithm for the integrated register allocator 🎨👑. The *algorithm* argument can be:

  - `priority` — specifies Chow's priority coloring
  - `CB` — specifies Chaitin-Briggs coloring

  Chaitin-Briggs coloring is not implemented for all architectures, but for those targets that *do* support it, it is the default because it generates better code 💖.

- **`-fira-region=`*region* — use specified regions for the integrated register allocator 🗺️. The *region* argument should be one of the following:

  - `all` — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set 🎀.
  - `mixed` — use all loops *except* for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (`-O`, `-O2`, ...) ⭐.
  - `one` — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for `-Os` or `-O0` 🧚.

- **`-fira-hoist-pressure`** — uses IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions 💫. This option usually results in smaller code, but it can slow the compiler down 🐢. Small code, slower compile, the eternal bargain.

  <mint>This option is enabled at level `-Os` for all targets.</mint>

- **`-fira-loop-pressure`** — uses IRA to evaluate register pressure in loops for decisions to move loop invariants 🌈. This usually results in generation of faster *and* smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down 🐢😭. Big register file? She eats. Small one? She's just vibing.

  <mint>This option is enabled at level `-O3` for some targets.</mint>

- **`-fno-ira-share-save-slots`** — disables sharing of stack slots used for saving call-used hard registers living through a call 🙈. Each hard register gets a separate stack slot, and as a result function stack frames are larger 📦. More room, less sharing, bigger frames.

- **`-fno-ira-share-spill-slots`** — disables sharing of stack slots allocated for pseudo-registers 💀. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger 📦. Same deal, different registers.

- **`-flra-remat`** — enables CFG-sensitive rematerialization in LRA ✨. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it is profitable. Why reload when you could just... recalculate, bestie? 💅

  <mint>Enabled at levels `-O2`, `-O3`, `-Os`.</mint>

- **`-fdelayed-branch`** — if supported for the target machine, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions 🦋🔥. Fill those gaps, queen.

  <mint>Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`, but not at `-Og`.</mint>

- **`-fschedule-insns`** — if supported for the target machine, attempts to reorder instructions to eliminate execution stalls due to required data being unavailable ⏳✨. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required 🎀. Busy while waiting, we love a multitasker.

Conventionally enabled at optimization levels `-O2` and `-O3`. <pink>But bestie, like, so many targets are out here overriding this</pink> — no cap. On `x86` it's literally disabled at *all* the levels, while on `AArch64` it only wakes up at `-O3`. <purple>Every target's got its own vibe 💅</purple>

- **`-fschedule-insns2`** — basically `-fschedule-insns`'s sibling, except it asks for <hotpink>one extra pass</hotpink> of instruction scheduling *after* register allocation has already happened. This one's a whole moment on machines with a pretty small number of registers, and where memory load instructions take more than one cycle to do their thing. 🌀

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fno-sched-interblock`** — turns off instruction scheduling *across* basic blocks. That cross-block scheduling is normally on when you're scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.

- **`-fno-sched-spec`** — disables the speculative motion of non-load instructions. Again, that's normally enabled when scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.

- **`-fsched-pressure`** — turns on <yellow>register pressure sensitive</yellow> insn scheduling before register allocation. Heads up: this only makes sense when scheduling before register allocation is actually enabled, i.e. with `-fschedule-insns` or at `-O2` or higher. Using it can level up the generated code and shrink its size by stopping register pressure from climbing above the number of available hard registers — and therefore stopping all those sad little spills in register allocation. 😭

- **`-fsched-spec-load`** — lets *some* load instructions get speculatively moved around. Only makes sense when scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.

- **`-fsched-spec-load-dangerous`** — same energy, but lets *more* load instructions get speculatively moved. ⚠️ (the name is literally telling u it's dangerous, iykyk). Only makes sense when scheduling before register allocation, i.e. with `-fschedule-insns` or at `-O2` or higher.

- **`-fsched-stalled-insns`**
- **`-fsched-stalled-insns=n`** — decides how many insns (if any) can be moved prematurely out of the queue of stalled insns and into the ready list, during the second scheduling pass. `-fno-sched-stalled-insns` means <purple>zero insns</purple> get moved prematurely, and `-fsched-stalled-insns=0` means there's *no limit* on how many queued insns can be moved prematurely. And `-fsched-stalled-insns` with no value at all is equivalent to `-fsched-stalled-insns=1`. ✨

- **`-fsched-stalled-insns-dep`**
- **`-fsched-stalled-insns-dep=n`** — says how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This only does anything during the second scheduling pass, and only if `-fsched-stalled-insns` is being used. `-fno-sched-stalled-insns-dep` is equivalent to `-fsched-stalled-insns-dep=0`, and `-fsched-stalled-insns-dep` with no value is equivalent to `-fsched-stalled-insns-dep=1`.

- **`-fsched2-use-superblocks`** — when scheduling after register allocation, use <hotpink>superblock scheduling</hotpink>. This lets stuff move across basic block boundaries, which gives you faster schedules. 🏃‍♀️💨 But fr fr, this option is **experimental** — not every machine description used by GCC models the CPU closely enough to dodge unreliable results from the algorithm. So like, proceed with caution bestie.

  This only makes sense when scheduling after register allocation, i.e. with `-fschedule-insns2` or at `-O2` or higher.

- **`-fsched-group-heuristic`** — enables the group heuristic in the scheduler, which favors the instruction that belongs to a schedule group. On by default when scheduling is enabled, i.e. with `-fschedule-insns` or `-fschedule-insns2` or at `-O2` or higher.

- **`-fsched-critical-path-heuristic`** — enables the critical-path heuristic in the scheduler, which favors instructions on the critical path. On by default when scheduling is enabled, i.e. with `-fschedule-insns` or `-fschedule-insns2` or at `-O2` or higher. 🛤️

- **`-fsched-spec-insn-heuristic`** — enables the speculative instruction heuristic in the scheduler, which favors speculative instructions with greater dependency weakness. On by default when scheduling is enabled, i.e. with `-fschedule-insns` or `-fschedule-insns2` or at `-O2` or higher.

- **`-fsched-rank-heuristic`** — enables the rank heuristic in the scheduler, which favors the instruction belonging to a basic block with greater size or frequency. On by default when scheduling is enabled, i.e. with `-fschedule-insns` or `-fschedule-insns2` or at `-O2` or higher.

- **`-fsched-last-insn-heuristic`** — enables the last-instruction heuristic in the scheduler, which favors the instruction that's *less* dependent on the last instruction scheduled. On by default when scheduling is enabled, i.e. with `-fschedule-insns` or `-fschedule-insns2` or at `-O2` or higher.

- **`-fsched-dep-count-heuristic`** — enables the dependent-count heuristic in the scheduler, which favors the instruction that has more instructions depending on it. On by default when scheduling is enabled, i.e. with `-fschedule-insns` or `-fschedule-insns2` or at `-O2` or higher. 👑

- **`-freschedule-modulo-scheduled-loops`** — okay so modulo scheduling happens *before* traditional scheduling, right? So if a loop gets modulo scheduled, later scheduling passes might come along and change its schedule. Use this option to control that behavior. 🎛️

- **`-fselective-scheduling`** — schedule instructions using the selective scheduling algorithm. Selective scheduling runs **instead of** the first scheduler pass.

- **`-fselective-scheduling2`** — schedule instructions using the selective scheduling algorithm. This time it runs **instead of** the second scheduler pass.

- **`-fsel-sched-pipelining`** — enables software pipelining of innermost loops during selective scheduling. Lowkey no effect unless one of `-fselective-scheduling` or `-fselective-scheduling2` is turned on. 🚿

- **`-fsel-sched-pipelining-outer-loops`** — when pipelining loops during selective scheduling, also pipeline the outer loops. No effect unless `-fsel-sched-pipelining` is turned on.

- **`-fsemantic-interposition`** — some object formats, like `ELF`, let the dynamic linker interpose symbols. Which means: for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining, and other optimizations, because it's bracing for the function or variable in question possibly changing. Super useful for stuff like rewriting memory allocation functions with a debugging implementation — but it's <red>expensive</red> in terms of code quality. 💸

  With `-fno-semantic-interposition` the compiler just assumes that if interposition happens for functions, the overwriting function will have *precisely the same semantics* (and side effects). Same deal for variables — if interposition happens, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared `inline` (where interposition is never allowed to change semantics) and for symbols explicitly declared `weak`. 🫶

- **`-fshrink-wrap`** — emit function prologues only before the parts of the function that actually need it, instead of at the top of the function like a basic. This flag is enabled by default at `-O` and higher.

- **`-fshrink-wrap-separate`** — shrink-wrap separate parts of the prologue and epilogue *separately*, so those parts only get executed when needed. On by default, but has no effect unless `-fshrink-wrap` is also turned on and the target supports this. 🎀

- **`-fcaller-saves`** — enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around those calls. That allocation only happens when it looks like it'll result in better code. On certain machines this is *always* enabled by default — usually the ones with no call-preserved registers to use instead.

  Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fcombine-stack-adjustments`** — tracks stack adjustments (pushes and pops) plus stack memory references, then tries to find ways to combine them. 🔗

  Enabled by default at `-O1` and higher.

- **`-fipa-ra`** — use caller save registers for allocation if those registers aren't used by any called function. In that case there's no need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function *and* they're compiled before it.

  Enabled at levels `-O2`, `-O3`, `-Os`, however the option is disabled if the generated code will be instrumented for profiling (`-p`, or `-pg`) or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL). 🙈

- **`-fconserve-stack`** — attempt to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower. 😭 This option implies setting the `large-stack-frame` parameter to `100` and the `large-stack-frame-growth` parameter to `400`.

- **`-ftree-reassoc`** — perform reassociation on trees. This flag is enabled by default at `-O1` and higher. 🌳

- **`-fcode-hoisting`** — perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. It's especially useful as a code size optimization, but it often helps for code speed too. This flag is enabled by default at `-O2` and higher. ⬆️

- **`-ftree-pre`** — perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at `-O2` and `-O3`.

- **`-ftree-partial-pre`** — makes partial redundancy elimination (PRE) *more aggressive*. This flag is enabled by default at `-O3`. 🔥

- **`-ftree-forwprop`** — perform forward propagation on trees. This flag is enabled by default at `-O1` and higher.

- **`-ftree-fre`** — perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE *only* considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at `-O1` and higher. ⚡

- **`-ftree-phiprop`** — perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at `-O1` and higher. 🧚

- **`-fhoist-adjacent-loads`** — okay so this one <pink>speculatively</pink> hoists loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. It’s enabled by default at `-O2` and higher. ✨

- **`-ftree-copy-prop`** — perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at `-O1` and higher. 💅

- **`-fipa-pure-const`** — discover which functions are pure or constant. Enabled by default at `-O1` and higher. 🫶

- **`-fipa-reference`** — discover which static variables do not escape the compilation unit. Enabled by default at `-O1` and higher. 🦋

- **`-fipa-reference-addressable`** — discover read-only, write-only and non-addressable static variables. Enabled by default at `-O1` and higher. 🌸

- **`-fipa-reorder-for-locality`** — <purple>group call chains close together</purple> in the binary layout to improve code locality and minimize jump distances between frequently called functions. Unlike `-freorder-functions` this pass considers the call chains between functions and groups them together, rather than grouping all hot/normal/cold/never-executed functions into separate sections. Unlike `-fprofile-reorder-functions` it aims to improve code locality throughout the runtime of the program rather than focusing on program startup. This option is incompatible with an explicit `-flto-partition=` option since it enforces a custom partitioning scheme. If using this option it is recommended to also use profile feedback, but this option is not enabled by default otherwise. 💅

- **`-fipa-stack-alignment`** — reduce stack alignment on call sites if possible. Enabled by default. 🫶

- **`-fipa-pta`** — perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level. 💀

- **`-fipa-profile`** — perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as `"cold"`, `"noreturn"`, static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at `-O1` and higher. 🌸

- **`-fipa-modref`** — perform interprocedural mod/ref analysis. This optimization analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary. This flag is enabled by default at `-O1` and higher. 🦋

- **`-fipa-cp`** — perform interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at `-O2`, `-Os` and `-O3`. It is also enabled by `-fprofile-use` and `-fauto-profile`. 🔥

- **`-fipa-cp-clone`** — perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size (see `--param ipa-cp-unit-growth=value`). This flag is enabled by default at `-O3`. It is also enabled by `-fprofile-use` and `-fauto-profile`. 👑

- **`-fipa-bit-cp`** — when enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at `-O2` and by `-fprofile-use` and `-fauto-profile`. It requires that `-fipa-cp` is enabled. ⭐

- **`-fipa-vrp`** — when enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at `-O2`. It requires that `-fipa-cp` is enabled. 🌈

- **`-fipa-icf`** — perform Identical Code Folding for functions and read-only variables. The optimization reduces code size and may disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works more effectively with link-time optimization enabled. Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not same — there are equivalences that are found only by GCC and equivalences found only by Gold. This flag is enabled by default at `-O2` and `-Os`. 🎀

- **`-flate-combine-instructions`** — enable two instruction combination passes that run relatively late in the compilation process. One of the passes runs before register allocation and the other after register allocation. The main aim of the passes is to substitute definitions into all uses. Most targets enable this flag by default at `-O2` and `-Os`. ✨

- **`-flive-patching=level`** — <hotpink>control GCC's optimizations to produce output suitable for live-patching</hotpink>. If the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter is called an <cyan>impacted function</cyan> of the former. If a function is patched, its impacted functions should be patched too. The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc. Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted function, IPA optimizations can be partially enabled at two different levels. The `level` argument should be one of the following:
  - **`inline-clone`** — only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when patching a function, all its callers and its clones' callers are impacted, therefore need to be patched as well. `-flive-patching=inline-clone` disables the following optimization flags: `-fwhole-program`, `-fipa-pta`, `-fipa-reference`, `-fipa-ra`, `-fipa-icf`, `-fipa-icf-functions`, `-fipa-icf-variables`, `-fipa-bit-cp`, `-fipa-vrp`, `-fipa-pure-const`, `-fipa-reference-addressable`, `-fipa-stack-alignment`, `-fipa-modref`.
  - **`inline-only-static`** — only enable inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well. In addition to all the flags that `-flive-patching=inline-clone` disables, `-flive-patching=inline-only-static` disables the following additional optimization flags: `-fipa-cp-clone`, `-fipa-sra`, `-fpartial-inlining`, `-fipa-cp`.
  When `-flive-patching` is specified without any value, the default value is `inline-clone`. This flag is disabled by default. Note that `-flive-patching` is not supported with link-time optimization (`-flto`). 🚨

- **`-fisolate-erroneous-paths-dereference`** — detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at `-O2` and higher and depends on `-fdelete-null-pointer-checks` also being enabled. 🫶

- **`-fisolate-erroneous-paths-attribute`** — detect paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a `"returns_nonnull"` or `"nonnull"` attribute. Isolate those paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by `-O2` in the future. 🙈

- **`-ftree-sink`** — perform forward store motion on trees. This flag is enabled by default at `-O1` and higher. 🍓

- **`-ftree-bit-ccp`** — perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This pass only operates on local scalar variables and is enabled by default at `-O1` and higher, except for `-Og`. It requires that `-ftree-ccp` is enabled. ⭐

- **`-ftree-ccp`** — perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at `-O1` and higher. 💜

- **`-fssa-backprop`** — propagate information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default at `-O1` and higher. 🧚

- **`-fssa-phiopt`** — perform pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by default at `-O1` and higher, except for `-Og`. 🌸

- **`-ftree-switch-conversion`** — perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is enabled by default at `-O2` and higher. 🎀

- **`-ftree-tail-merge`** — look for identical code sequences. When found, replace one with a jump to the other. This optimization is known as tail merging or cross jumping. This flag is enabled by default at `-O2` and higher. The compilation time in this pass can be limited using `max-tail-merge-comparisons` parameter and `max-tail-merge-iterations` parameter. 🔥

- **`-ftree-cselim`** — perform conditional store elimination on trees. This flag is enabled by default at `-O1` and higher on targets that have conditional move instructions. 💅

- **`-ftree-dce`** — perform dead code elimination (DCE) on trees. This flag is enabled by default at `-O1` and higher. 👑

- **`-ftree-builtin-call-dce`** — perform conditional dead code elimination (DCE) for calls to built-in functions that may set `errno` but are otherwise free of side effects. This flag is enabled by default at `-O2` and higher if `-Os` is not also specified. 🌈

- **`-ffinite-loops`** — bestie, this one is like "assume the loop actually *ends*". It tells GCC that a loop which has an exit will eventually take that exit and not loop forever and ever 💀. Which lowkey lets the compiler yeet loops that otherwise have no side-effects, because it's *not* counting "maybe this spins eternally" as a side-effect. Iconic. Note: it's on by default at `-O2` for C++ when you're using `-std=c++11` or higher ✨

- **`-ftree-dominator-opts`** — does a little glow-up pass of simple scalar cleanups: constant/copy propagation, redundancy elimination, range propagation, and expression simplification, all based on a dominator tree traversal 🌈. It also does jump threading (so jumps-to-jumps get squished down, iykyk). On by default at `-O1` and higher 🫶

- **`-ftree-dse`** — dead store elimination (**DSE**) on trees, periodt. A *dead store* is a store into a memory location that gets overwritten later by another store with no intervening loads — so the earlier store is giving "pointless" and can just be deleted 🗑️. On by default at `-O1` and higher.

- **`-ftree-ch`** — loop header copying on trees. It's a slay because it increases the effectiveness of code motion optimizations, and it also saves one jump 💅. On by default at `-O1` and higher. It is **not** enabled for `-Os`, because it usually increases code size 😭 (we stay small for size-optimizing besties).

- **`-ftree-loop-optimize`** — performs loop optimizations on trees. On by default at `-O1` and higher ✨

- **`-ftree-loop-linear`**, **`-floop-strip-mine`**, **`-floop-block`** — these three all do loop nest optimizations, and they're literally the same as `-floop-nest-optimize`. To actually use this code transformation, GCC has to be configured with `--with-isl` to enable the Graphite loop transformation infrastructure 🔧

- **`-fgraphite-identity`** — enables the identity transformation for graphite 🌸. For every SCoP we generate the polyhedral representation and then transform it back to gimple. With `-fgraphite-identity` you can vibe check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation 👀. Some minimal optimizations also get performed by the code generator isl, like index splitting and dead code elimination in loops.

- **`-floop-nest-optimize`** — enables the isl based loop nest optimizer: a generic loop nest optimizer based on the Pluto optimization algorithms 🌟. It calculates a loop structure optimized for data-locality and parallelism. Heads up though — this option is *experimental* 🙈

- **`-floop-parallelize-all`** — uses the Graphite data dependence analysis to identify loops that can be parallelized, then parallelizes **all** the loops that can be analyzed to not contain loop carried dependences — with zero checking of whether it's actually *profitable* to parallelize them 💀. It's giving "we ball".

- **`-ftree-coalesce-vars`** — while transforming the program out of the SSA representation, it tries to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🎀. Real talk: this may severely limit your ability to debug an optimized program compiled with `-fno-var-tracking-assignments` 😭. In the negated form, this flag prevents SSA coalescing of user variables. It's enabled by default if optimization is enabled, and it does very little otherwise.

- **`-ftree-loop-if-convert`** — attempts to transform conditional jumps in the innermost loops into branch-less equivalents ✨. The whole intent is removing control-flow from the innermost loops so the vectorization pass can handle these loops way better. Enabled by default if vectorization is enabled.

- **`-ftree-loop-distribution`** — performs loop distribution 🦋. This flag can improve cache performance on big loop bodies and lets further loop optimizations happen, like parallelization or vectorization. For example, this loop:

DO I = 1, N A(I) = B(I) + C D(I) = E(I) * F ENDDO


gets transformed into:

DO I = 1, N A(I) = B(I) + C ENDDO DO I = 1, N D(I) = E(I) * F ENDDO


Enabled by default at `-O3` 🔥. It's also enabled by `-fprofile-use` and `-fauto-profile`.

- **`-ftree-loop-distribute-patterns`** — performs loop distribution of patterns that can be code generated with calls to a library 🌈. On by default at `-O2` and higher, and by `-fprofile-use` and `-fauto-profile`. This pass distributes the initialization loops and generates a call to memset zero. Like, this loop:

DO I = 1, N A(I) = 0 B(I) = A(I) + I ENDDO


becomes:

DO I = 1, N A(I) = 0 ENDDO DO I = 1, N B(I) = A(I) + I ENDDO


and that initialization loop gets transformed into a call to memset zero 💅

- **`-floop-interchange`** — performs loop interchange outside of graphite 🌸. It can improve cache performance on loop nests and let further loop optimizations (like vectorization) take place. Example time — this loop:

for (int i = 0; i < N; i++) for (int j = 0; j < N; j++) for (int k = 0; k < N; k++) c[i][j] = c[i][j] + a[i][k]*b[k][j];


is transformed to:

for (int i = 0; i < N; i++) for (int k = 0; k < N; k++) for (int j = 0; j < N; j++) c[i][j] = c[i][j] + a[i][k]*b[k][j];


Enabled by default at `-O3` 🔥. Also enabled by `-fprofile-use` and `-fauto-profile`.

- **`-floop-unroll-and-jam`** — applies unroll and jam transformations on feasible loops 🎀. In a loop nest, this unrolls the outer loop by some factor and fuses the resulting multiple inner loops together. Enabled by default at `-O3`, and also by `-fprofile-use` and `-fauto-profile`.

- **`-ftree-loop-im`** — performs loop invariant motion on trees ✨. This pass only moves invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With `-funswitch-loops` it also moves operands of conditions that are invariant out of the loop, so we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion 💅

- **`-ftree-loop-ivcanon`** — creates a canonical counter for the number of iterations in loops where figuring out the iteration count needs complicated analysis 😮‍💨. Later optimizations can then determine the number easily. Super useful especially in connection with unrolling.

- **`-ftree-scev-cprop`** — performs final value replacement 🌟. If a variable is modified inside a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, then uses of the final value get replaced by such a computation — provided it's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at `-O1` and higher.

- **`-fivopts`** — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees 🔥. Enabled by default at `-O1` and higher.

- **`-ftree-parallelize-loops=`*n* — parallelizes loops, i.e. splits their iteration space to run in *n* threads 🧚. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. Also, the optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive rather than constrained e.g. by memory bandwidth. This option implies `-pthread`, and thus is only supported on targets that have support for `-pthread` 🫶

- **`-ftree-pta`** — performs function-local points-to analysis on trees. Enabled by default at `-O1` and higher, **except** for `-Og` 👑

- **`-ftree-sra`** — performs scalar replacement of aggregates ✨. This pass replaces structure references with scalars to prevent committing structures to memory too early. Enabled by default at `-O1` and higher, except for `-Og`.

- **`-fstore-merging`** — performs merging of narrow stores to consecutive memory addresses 🌈. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores, reducing the number of instructions. Enabled by default at `-O2` and higher as well as `-Os` 💅

- **`-ftree-ter`** — performs temporary expression replacement during the SSA->normal phase 🦋. Single use/single def temporaries get replaced at their use location with their defining expression. This results in non-GIMPLE code, but it gives the expanders much more complex trees to work on, resulting in better RTL generation 🔥. Enabled by default at `-O1` and higher.

- **`-ftree-slsr`** — performs straight-line strength reduction on trees ✨. It recognizes related expressions involving multiplications and replaces them with less expensive calculations when possible. Enabled by default at `-O1` and higher.

- **`-ftree-vectorize`** — performs vectorization on trees 🌸. This flag enables `-ftree-loop-vectorize` and `-ftree-slp-vectorize` if they're not explicitly specified.

- **`-ftree-loop-vectorize`** — performs loop vectorization on trees 💜. Enabled by default at `-O2` and by `-ftree-vectorize`, `-fprofile-use`, and `-fauto-profile`.

- **`-ftree-slp-vectorize`** — performs basic block vectorization on trees ⭐. Enabled by default at `-O2` and by `-ftree-vectorize`, `-fprofile-use`, and `-fauto-profile`.

- **`-ftrivial-auto-var-init=`*choice* — initializes automatic variables with either a pattern or with zeroes, to increase the security and predictability of a program by preventing uninitialized memory disclosure and use 🔒. Big important caveat bestie: GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized — `-Wuninitialized` and `-Wanalyzer-use-of-uninitialized-value` will still report warning messages on such automatic variables, and the compiler will perform optimization as if the variable were uninitialized 😭. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes. **However**, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the first case of a `switch` statement 💀. Use `-Wtrivial-auto-var-init` to report all such cases.

The three values of *choice* are:

- **`uninitialized`** — literally does nothing to your automatic variables, no init, nada. This is the C and C++ default bestie, so if you do nothing, this is what you get 💅

- **`pattern`** — initializes automatic variables with values that will *likely* turn your logic bugs into crashes later on 😭 which sounds bad but hear me out: those values are super easy to spot in a crash dump, and they're deliberately values no programmer can lean on for any useful program semantics. The current value is a byte-repeatable pattern with byte `"0xFE"`. Heads up though: the values used for pattern initialization might get changed in the future, so don't get attached 🫶

- **`zero`** — initializes automatic variables with zeroes. Simple. Clean. Iconic ✨

The default is `uninitialized`.

Okay so this next bit is lowkey important, don't skip it 🙈 Note that the initializer values, whether `zero` or `pattern`, refer to *data representation* (in memory or machine registers), rather than to their interpretation as numerical values. That distinction may be important in languages that support types with biases or implicit multipliers, and with such extensions as `hardbool`. For example, a variable that uses 8 bits to represent (biased) quantities in the `"range 160..400"` will be initialized with the bit patterns `0x00` or `0xFE`, depending on choice, whether or not these representations stand for values in that range — and even if they do, the interpretation of the value held by the variable will depend on the bias. A `hardbool` variable that uses say `"0X5A"` and `0xA5` for `"false"` and `"true"`, respectively, will trap with either choice of trivial initializer 😬 i.e., zero initialization will *not* convert to the representation for `"false"`, even if it would for a `"static"` variable of the same type. This means the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception though: (non-hardened) boolean variables that fit in registers are initialized with `"false"` (zero), even when `pattern` is requested. Slay, booleans, I guess 💀

You can control this behavior for a specific variable by using the variable attribute `"uninitialized"`.

---

- **`-fvect-cost-model=model`** — changes up the cost model used for vectorization, bestie 🌀 The *model* argument should be one of `unlimited`, `dynamic`, `cheap` or `very-cheap`. With the `unlimited` model the vectorized code-path is assumed to be profitable, while with the `dynamic` model a runtime check guards the vectorized code-path so it only kicks in for iteration counts that will likely execute faster than running the original scalar loop. The `cheap` model disables vectorization of loops where doing so would be cost prohibitive — for example due to required runtime checks for data dependence or alignment — but otherwise is equal to the `dynamic` model. And the `very-cheap` model disables vectorization of loops when *any* runtime check for data dependence or alignment is required; it also disables vectorization of epilogue loops, but otherwise is equal to the `cheap` model.

The default cost model depends on other optimization flags and is either `dynamic` or `cheap`. iykyk 🤷‍♀️

- **`-fsimd-cost-model=model`** — same energy but for vectorization of loops marked with the OpenMP `simd` directive 🌈 The *model* argument should be one of `unlimited`, `dynamic`, `cheap`. All values of *model* have the same meaning as described in `-fvect-cost-model`, and by default a cost model defined with `-fvect-cost-model` is used. Reusing the fit, we love to see it 💖

- **`-ftree-vrp`** — performs Value Range Propagation on trees 🌳 It's similar to the constant propagation pass, but instead of values, *ranges* of values get propagated. This lets the optimizers remove unnecessary range checks like array bound checks and null pointer checks. Enabled by default at `-O2` and higher. One catch though: null pointer check elimination is only done if `-fdelete-null-pointer-checks` is enabled 👑

- **`-fsplit-paths`** — splits paths leading to loop backedges 🦋 This can improve dead code elimination and common subexpression elimination. Enabled by default at `-O3` and above.

- **`-fsplit-ivs-in-unroller`** — enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration ✨ This breaks long dependency chains, which improves the efficiency of the scheduling passes. So satisfying when a chain just snaps 🔥

A combination of `-fweb` and CSE is often sufficient to obtain the same effect. However, that is *not* reliable in cases where the loop body is more complicated than a single basic block. It also doesn't work at all on some architectures due to restrictions in the CSE pass 😭

This optimization is enabled by default.

- **`-fvariable-expansion-in-unroller`** — with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code 💅

This optimization is enabled by default for PowerPC targets, but disabled by default otherwise. Main character treatment for PowerPC only, I guess 👑

- **`-fpartial-inlining`** — inlines *parts* of functions, not the whole thing 🎀 This option has any effect only when inlining itself is turned on by the `-finline-functions` or `-finline-small-functions` options.

Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fpredictive-commoning`** — performs predictive commoning optimization, i.e. reusing computations (especially memory loads and stores) performed in previous iterations of loops 🔁 Recycle, reuse, it's giving sustainability queen 🌸

This option is enabled at level `-O3`. It is also enabled by `-fprofile-use` and `-fauto-profile`.

- **`-fprefetch-loop-arrays`** — if supported by the target machine, generates instructions to prefetch memory to improve the performance of loops that access large arrays 🚀

Fair warning bestie: this option may generate better *or worse* code 😬 results are highly dependent on the structure of loops within the source code. It's a vibe check, not a guarantee.

Disabled at level `-Os`.

- **`-fno-printf-return-value`** — do *not* substitute constants for the known return value of formatted output functions such as `"sprintf"`, `"snprintf"`, `"vsprintf"`, and `"vsnprintf"` (but not `"printf"` or `"fprintf"`) 🙈 This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when `-fprintf-return-value` is in effect, both the branch and the body of the `"if"` statement (but not the call to `"snprint"`) can be optimized away when `"i"` is a 32-bit or smaller integer because the return value is guaranteed to be at most 8 👀

char buf[9]; if (snprintf (buf, "%08x", i) >= sizeof buf) ...


The `-fprintf-return-value` option relies on other optimizations and yields best results with `-O2` and above. It works in tandem with the `-Wformat-overflow` and `-Wformat-truncation` options. The `-fprintf-return-value` option is enabled by default ✨

- **`-fno-peephole`**
- **`-fno-peephole2`** — disable any machine-specific peephole optimizations 💀 The difference between `-fno-peephole` and `-fno-peephole2` is in how they are implemented in the compiler; some targets use one, some use the other, a few use both. Messy but that's the tea ☕

`-fpeephole` is enabled by default. `-fpeephole2` enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fno-guess-branch-probability`** — do *not* guess branch probabilities using heuristics, bestie, we're done guessing 🫠

GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (`-fprofile-arcs`). These heuristics are based on the control flow graph. If some branch probabilities are specified by `"__builtin_expect"`, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the `"__builtin_expect"` info into account. The interactions between the heuristics and `"__builtin_expect"` can be complex, and in some cases it may be useful to disable the heuristics so that the effects of `"__builtin_expect"` are easier to understand 👑

It's also possible to specify expected probability of the expression with `"__builtin_expect_with_probability"` built-in function.

The default is `-fguess-branch-probability` at levels `-O`, `-O2`, `-O3`, `-Os`.

- **`-freorder-blocks`** — reorders basic blocks in the compiled function in order to reduce the number of taken branches and improve code locality 🌈

Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`.

- **`-freorder-blocks-algorithm=algorithm`** — uses the specified *algorithm* for basic block reordering 🎀 The *algorithm* argument can be `simple`, which does not increase code size (except sometimes due to secondary effects like alignment), or `stc`, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code. Copy-paste but make it optimization 💅

The default is `simple` at levels `-O1`, `-Os`, and `stc` at levels `-O2`, `-O3`.

- **`-freorder-blocks-and-partition`** — in addition to reordering basic blocks in the compiled function, in order to reduce the number of taken branches, this partitions hot and cold basic blocks into separate sections of the assembly and `.o` files, to improve paging and cache locality performance 🔥 Hot girl blocks and cold girl blocks in their own sections, we stan a separation era ✨

Not me having to tell you this gets automatically turned off 🙈 it's turned off in the presence of exception handling or unwind tables (on targets using `setjump`/`longjump` or target specific scheme), for `linkonce` sections, for functions with a user-defined section attribute, and on any architecture that does not support named sections. Also: when `-fsplit-stack` is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker).

Enabled for x86 at levels `-O2`, `-O3`, `-Os`.

- **`-freorder-functions`** — reorders functions in the object file in order to improve code locality 🦋 Unlike `-fipa-reorder-for-locality` this option prioritises grouping all functions within a category (`hot`/`normal`/`cold`/`never-executed`) together. This is implemented by using special subsections `.text.hot` for most frequently executed functions and `.text.unlikely` for unlikely executed functions. Reordering is done by the linker, so the object file format must support named sections and the linker must place them in a reasonable way 👑

This option isn't effective unless you either provide profile feedback (see `-fprofile-arcs` for details) or manually annotate functions with `"hot"` or `"cold"` attributes. No shortcuts bestie 😤

Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fstrict-aliasing`** — okay so this one lets the compiler assume the *strictest* aliasing rules that apply to whatever language you're compiling. For C (and C++), it activates optimizations based on the type of expressions. In particular, an object of one type is assumed to **never** live at the same address as an object of a different type, unless the types are *almost* the same. Like, an `unsigned int` can alias an `int`, but it can NOT alias a `void*` or a `double`. A character type may alias any other type. 💅

Pay special attention to code like this, bestie:

union a_union { int i; double d; };

int f() { union a_union t; t.d = 3.0; return t.i; }


Reading from a different union member than the one you most recently wrote to (this is called "type-punning") is common. And even with `-fstrict-aliasing`, type-punning is **allowed**, provided the memory is accessed *through the union type*. So the code above works as expected. 🫶 However, this code might not:

int f() { union a_union t; int* ip; t.d = 3.0; ip = &t.i; return *ip; }


Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has **undefined behavior**, even if the cast uses a union type, e.g.:

int f() { double d = 3.0; return ((union a_union *) &d)->i; }


The `-fstrict-aliasing` option is enabled at levels `-O2`, `-O3`, `-Os`.

- **`-fipa-strict-aliasing`** — this one controls whether the rules of `-fstrict-aliasing` get applied *across function boundaries*. Note that if multiple functions get inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary. 🔥 The `-fipa-strict-aliasing` option is enabled by default and is effective only in combination with `-fstrict-aliasing`.

- **`-falign-functions`**, **`-falign-functions=n`**, **`-falign-functions=n:m`**, **`-falign-functions=n:m:n2`**, **`-falign-functions=n:m:n2:m2`** — aligns the start of functions to the next power-of-two greater than or equal to *n*, skipping up to *m-1* bytes. This ensures that at least the first *m* bytes of the function can be fetched by the CPU without crossing an *n*-byte alignment boundary. It's an optimization of code performance, and alignment is ignored for functions considered cold. If alignment is required for **all** functions, use `-fmin-function-alignment`. ✨

If *m* is not specified, it defaults to *n*.

Examples, because iykyk: `-falign-functions=32` aligns functions to the next 32-byte boundary, `-falign-functions=24` aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, and `-falign-functions=32:7` aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.

The second pair of *n2*:*m2* values lets you specify a **secondary** alignment: `-falign-functions=64:7:32:3` aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If *m2* is not specified, it defaults to *n2*.

Some assemblers only support this flag when *n* is a power of two; in that case, it is rounded up. 🎀

`-fno-align-functions` and `-falign-functions=1` are equivalent and mean that functions are not aligned.

If *n* is not specified or is zero, use a machine-dependent default. The maximum allowed *n* option value is **65536**.

Enabled at levels `-O2`, `-O3`.

- **`-flimit-function-alignment`** — if this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by `-falign-functions`, but not to skip more bytes than the size of the function. 💜

- **`-falign-labels`**, **`-falign-labels=n`**, **`-falign-labels=n:m`**, **`-falign-labels=n:m:n2`**, **`-falign-labels=n:m:n2:m2`** — aligns **all** branch targets to a power-of-two boundary. 🌈

Parameters of this option are analogous to the `-falign-functions` option. `-fno-align-labels` and `-falign-labels=1` are equivalent and mean that labels are not aligned.

If `-falign-loops` or `-falign-jumps` are applicable and are greater than this value, then their values are used instead.

If *n* is not specified or is zero, use a machine-dependent default which is **very likely** to be `1`, meaning no alignment. The maximum allowed *n* option value is **65536**.

Enabled at levels `-O2`, `-O3`.

- **`-falign-loops`**, **`-falign-loops=n`**, **`-falign-loops=n:m`**, **`-falign-loops=n:m:n2`**, **`-falign-loops=n:m:n2:m2`** — aligns loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. It's an optimization of code performance, and alignment is ignored for loops considered cold. 🧚

If `-falign-labels` is greater than this value, then its value is used instead.

Parameters of this option are analogous to the `-falign-functions` option. `-fno-align-loops` and `-falign-loops=1` are equivalent and mean that loops are not aligned. The maximum allowed *n* option value is **65536**.

If *n* is not specified or is zero, use a machine-dependent default.

Enabled at levels `-O2`, `-O3`.

- **`-falign-jumps`**, **`-falign-jumps=n`**, **`-falign-jumps=n:m`**, **`-falign-jumps=n:m:n2`**, **`-falign-jumps=n:m:n2:m2`** — aligns branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping. In this case, no dummy operations need be executed. It's an optimization of code performance, and alignment is ignored for jumps considered cold. 🦋

If `-falign-labels` is greater than this value, then its value is used instead.

Parameters of this option are analogous to the `-falign-functions` option. `-fno-align-jumps` and `-falign-jumps=1` are equivalent and mean that loops are not aligned. (Yes, it says "loops" — that's literally what the page says, don't come for me. 😭)

If *n* is not specified or is zero, use a machine-dependent default. The maximum allowed *n* option value is **65536**.

Enabled at levels `-O2`, `-O3`.

- **`-fmin-function-alignment`** — specifies minimal alignment of functions to the next power-of-two greater than or equal to *n*. Unlike `-falign-functions` this alignment is applied also to **all** functions (even those considered cold). The alignment is also not affected by `-flimit-function-alignment`. 👑

- **`-fno-allocation-dce`** — do NOT remove unused C++ allocations (using operator `new` and operator `delete`) in dead code elimination. See also `-fmalloc-dce`. 💀

- **`-fallow-store-data-races`** — allows the compiler to perform optimizations that may introduce **new data races on stores**, without proving that the variable cannot be concurrently accessed by other threads. It does not affect optimization of local data. It is safe to use this option if it is known that global data will **not** be accessed by multiple threads. ⚠️

Examples of optimizations enabled by `-fallow-store-data-races` include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.

Enabled at level `-Ofast`.

- **`-funit-at-a-time`** — this option is left for compatibility reasons. `-funit-at-a-time` has **no effect**, while `-fno-unit-at-a-time` implies `-fno-toplevel-reorder` and `-fno-section-anchors`. Enabled by default. 🍓

- **`-fno-toplevel-reorder`** — do not reorder top-level functions, variables, and `asm` statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are **not** removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible. ✨

`-ftoplevel-reorder` is the default at `-O1` and higher, and also at `-O0` if `-fsection-anchors` is explicitly requested. Additionally `-fno-toplevel-reorder` implies `-fno-section-anchors`.

- **`-funreachable-traps`** — with this option, the compiler turns calls to `__builtin_unreachable` into **traps**, instead of using them for optimization. This also affects any such calls implicitly generated by the compiler. 🙈

This option has the same effect as `-fsanitize=unreachable -fsanitize-trap=unreachable`, but does not affect the values of those options. If `-fsanitize=unreachable` is enabled, that option takes priority over this one.

This option is enabled by default at `-O0` and `-Og`.

- **`-fweb`** — constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make **debugging impossible**, since variables no longer stay in a "home register". 😭

Enabled by default with `-funroll-loops`.

- **`-fwhole-program`** — assume that the current compilation unit represents the **whole program** being compiled. All public functions and variables with the exception of `main` and those merged by attribute `externally_visible` become static functions and in effect are optimized more aggressively by interprocedural optimizers. 🌸

With `-flto` this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful if no linker plugin is used or during incremental link step when final code is produced (with `-flto -flinker-output=nolto-rel`).

- **`-flto[=n]`** — this option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. 🔥

---

okay so to actually use the link-time optimizer, you gotta put `-flto` <pink>and</pink> your optimization options at compile time and during the final link. bestie, the recommendation is literally: compile every file participating in the same link with the same options, and also specify those options at link time. for example:

gcc -c -O2 -flto foo.c gcc -c -O2 -flto bar.c gcc -o myprog -flto -O2 foo.o bar.o


the first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside `foo.o` and `bar.o` 🫶. the final invocation reads the GIMPLE bytecode from `foo.o` and `bar.o`, merges the two files into a single internal image, and compiles the result as usual. since both `foo.o` and `bar.o` are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. this means, for example, that the inliner is able to inline functions in `bar.o` into functions in `foo.o` and vice-versa ✨.

another (simpler) way to enable link-time optimization is:

gcc -o myprog -flto -O2 foo.c bar.c


the above generates bytecode for `foo.c` and `bar.c`, merges them together into a single GIMPLE representation and optimizes them as usual to produce `myprog`.

the important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step 👑. GCC automatically performs link-time optimization if any of the objects involved were compiled with the `-flto` command-line option. you can always override the automatic decision to do link-time optimization by passing `-fno-lto` to the link command.

to make whole program optimization effective, it is necessary to make certain whole program assumptions. the compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. when supported by the linker, the linker plugin (see `-fuse-linker-plugin`) passes information to the compiler about used and externally visible symbols. when the linker plugin is not available, `-fwhole-program` should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions 🔥.

when a file is compiled with `-flto` without `-fuse-linker-plugin`, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see `-ffat-lto-objects`). this means that object files with LTO information can be linked as normal object files; if `-fno-lto` is passed to the linker, no interprocedural optimizations are applied. note that when `-fno-fat-lto-objects` is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them 🙈.

when producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code 🌈. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.

generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.

if you do not specify an optimization level option `-O` at link time, then GCC uses the highest optimization level used when compiling the object files. note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. first, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. second, some early optimization passes can be performed only at compile time and not at link time.

there are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link ⭐. currently, the following options and their settings are taken from the first object file that explicitly specifies them: `-fcommon`, `-fexceptions`, `-fnon-call-exceptions`, `-fgnu-tm` and all the `-m` target flags.

the following options `-fPIC`, `-fpic`, `-fpie` and `-fPIE` are combined based on the following scheme:

- `-fPIC` + `-fpic` = `-fpic`
- `-fPIC` + `-fno-pic` = `-fno-pic`
- `-fpic/-fPIC` + (no option) = (no option)
- `-fPIC` + `-fPIE` = `-fPIE`
- `-fpic` + `-fPIE` = `-fpie`
- `-fPIC/-fpic` + `-fpie` = `-fpie`

certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored 💅. this includes options such as `-freg-struct-return` and `-fpcc-struct-return`.

other options such as `-ffp-contract`, `-fno-strict-overflow`, `-fwrapv`, `-fno-trapv` or `-fno-strict-aliasing` are passed through to the link stage and merged conservatively for conflicting translation units. specifically `-fno-strict-overflow`, `-fwrapv` and `-fno-trapv` take precedence; and for example `-ffp-contract=off` takes precedence over `-ffp-contract=fast`. you can override them at link time.

diagnostic options such as `-Wstringop-overflow` are passed through to the link stage and their setting matches that of the compile-step at function granularity. note that this matters only for diagnostics emitted during optimization. note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code not consistent with the setting at compile time.

when you need to pass options to the assembler via `-Wa` or `-Xassembler` make sure to either compile such translation units with `-fno-lto` or consistently use the same assembler options on all translation units. you can alternatively also specify assembler options at LTO link time 🧚.

to enable debug info generation you need to supply `-g` at compile time. if any of the input files at link time were built with debug info generation enabled the link will enable debug info generation as well. any elaborate debug info settings like the dwarf level `-gdwarf-5` need to be explicitly repeated at the linker command line and mixing different settings in different translation units is discouraged 🦋.

if LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. the behavior is still undefined at run time 😭. similar diagnostics may be raised for other languages.

another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:

gcc -c -flto foo.c g++ -c -flto bar.cc gfortran -c -flto baz.f90 g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran


notice that the final link is done with `g++` to get the C++ runtime libraries and `-lgfortran` is added to get the Fortran runtime libraries 💖. in general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation.

if object files containing GIMPLE bytecode are stored in a library archive, say `libfoo.a`, it is possible to extract and use them in an LTO link if you are using a linker with plugin support. to create static libraries suitable for LTO, use `gcc-ar` and `gcc-ranlib` instead of `ar` and `ranlib`; to show the symbols of object files with GIMPLE bytecode, use `gcc-nm` 🌸. those commands require that `ar`, `ranlib` and `nm` have been compiled with plugin support. at link time, use the flag `-fuse-linker-plugin` to ensure that the library participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo


with the linker plugin enabled, the linker extracts the needed GIMPLE files from `libfoo.a` and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized 🔥.

if you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside `libfoo.a` are extracted and linked as usual, but they do not participate in the LTO optimization process. in order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with `-flto -ffat-lto-objects`.

link-time optimizations do not require the presence of the whole program to operate. if the program does not require any symbols to be exported, it is possible to combine `-flto` and `-fwhole-program` to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities ✨. use of `-fwhole-program` is not needed when linker plugin is active (see `-fuse-linker-plugin`).

the current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. the bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC 💀.

link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.

if you specify the optional *n*, the optimization and code generation done at link time is executed in parallel using *n* parallel jobs by utilizing an installed `make` program. the environment variable `MAKE` may be used to override the program used 🌈.

you can also specify `-flto=jobserver` to use GNU make's job server mode to determine the number of parallel jobs. this is useful when the Makefile calling GCC is already executing in parallel. you must prepend a `+` to the command recipe in the parent Makefile for this to work 🫶. this option likely only works if `MAKE` is GNU make. even without the option value, GCC tries to automatically detect a running GNU make's job server.

use `-flto=auto` to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system 💜.

okay so we’re still deep in the LTO/profile options era, and these are the knobs that make linker girlies and profiling besties feel seen 💅

- **`-flto-partition=alg`** — this one picks the partitioning algorithm for the link-time optimizer. The value can be `1to1` to mirror the original source files, `balanced` to split into equally sized chunks whenever possible, `max` to create a new partition for every symbol where possible, or `cache` to balance chunk sizes while keeping related symbols together for better caching in incremental LTO. Setting it to `none` disables partitioning and streaming completely. Default? `balanced`. `1to1` can work as a workaround for various code ordering issues, while `max` is intended for internal testing only, no cap. The value `one` means exactly one partition should be used, and `none` bypasses partitioning and runs the link-time optimization step directly from the WPA phase.

- **`-flto-incremental=path`** — turns on incremental LTO with its cache in the given existing directory. This can significantly shorten edit-compile cycles with LTO. When used with LTO (`-flto`), the output of translation units inside LTO gets cached. Cached translation units are likely to show up again when you recompile with small code changes, which cuts recompile time. Multiple GCC instances can use the same cache in parallel 🫶.

- **`-flto-incremental-cache-size=n`** — sets the number of cache entries in incremental LTO after which old entries get pruned. This is a soft limit, so temporarily there may be more entries. It’s giving “guideline, not a hard wall” ✨.

- **`-flto-compression-level=n`** — sets the compression level for intermediate language written to LTO object files, and it’s only meaningful with LTO mode (`-flto`). GCC currently supports two LTO compression algorithms. For zstd, valid values are `0` (no compression) to `19` (maximum compression), while zlib supports values from `0` to `9`. Values outside the range get clamped to either the minimum or maximum supported value. If you don’t pass the option, a default balanced compression setting is used.

- **`-fuse-linker-plugin`** — enables use of a linker plugin during link-time optimization. This relies on plugin support in the linker, available in gold or in GNU ld 2.21 or newer. It enables extraction of object files with GIMPLE bytecode out of library archives, which improves optimization quality by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). The resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to `-fwhole-program`. See `-flto` for a description of the effect of this flag and how to use it. This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold).

- **`-ffat-lto-objects`, `-fno-fat-lto-objects`** — fat LTO objects are object files that contain both the intermediate language and the object code. That makes them usable for both LTO linking and normal linking. This option is effective only when compiling with `-flto` and is ignored at link time. `-fno-fat-lto-objects` improves compilation time over plain LTO, but it requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, `nm`, `ar` and `ranlib` need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the `gcc-ar`, `gcc-nm`, `gcc-ranlib` wrappers to pass the right options to these tools. With non fat LTO, makefiles need to be modified to use them. Note that modern binutils provide a plugin auto-load mechanism. Installing the linker plugin into `$libdir/bfd-plugins` has the same effect as usage of the command wrappers (`gcc-ar`, `gcc-nm` and `gcc-ranlib`). The default is `-fno-fat-lto-objects` on targets with linker plugin support.

- **`-fcompare-elim`** — after register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`.

- **`-ffold-mem-offsets`, `-fno-fold-mem-offsets`** — try to eliminate add instructions by folding them in memory loads/stores. Enabled at levels `-O2`, `-O3`.

- **`-fcprop-registers`** — after register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Enabled at levels `-O1`, `-O2`, `-O3`, `-Os`.

- **`-fprofile-correction`** — profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. This option is enabled by `-fauto-profile`.

- **`-fprofile-partial-training`** — with `-fprofile-use`, all portions of programs not executed during the train run are optimized aggressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, program may contain functions specific for a given hardware and training may not cover all hardware configurations program is run on.) With `-fprofile-partial-training`, profile feedback is ignored for all functions not executed during the train run, leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when the train run is not representative, but also leads to significantly bigger code 😭.

- **`-fprofile-use`, `-fprofile-use=path`** — enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops -fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize -fvect-cost-model=dynamic -ftree-loop-distribute-patterns -fprofile-reorder-functions


Before you can use this option, you must first generate profiling information. By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using `-Wno-error=coverage-mismatch`. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see `-Wmissing-profile`). If `path` is specified, GCC looks at the path to find the profile feedback data files. See `-fprofile-dir`.

- **`-fauto-profile`, `-fauto-profile=path`** — enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops -fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize -fvect-cost-model=dynamic -ftree-loop-distribute-patterns -fprofile-correction


`path` is the name of a file containing AutoFDO profile information. If omitted, it defaults to `fbdata.afdo` in the current directory. Producing an AutoFDO profile data file requires running your program with the `perf` utility on a supported GNU/Linux target system. For more information, see `<https://perf.wiki.kernel.org/>`. E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \ -- your_program


Then use the `create_gcov` tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See `<https://github.com/google/autofdo>`. E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \ --gcov=profile.afdo


---

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled 🌸.

- **`-fexcess-precision=style`** — this option allows control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. An example of such a target is x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format. For most programs, the excess precision is harmless, but some programs may rely on the requirements of the C or C++ language standards for handling IEEE values. By default, `-fexcess-precision=fast` is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C or C++, if `-fexcess-precision=standard` is specified then excess precision follows the rules specified in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas `-ffloat-store` only affects assignments). This option is enabled by default for C or C++ if a strict conformance option such as `-std=c99` or `-std=c++17` is used. `-ffast-math` enables `-fexcess-precision=fast` by default regardless of whether a strict conformance option is used. If `-fexcess-precision=16` is specified, constants and the results of expressions with types `_Float16` and `__bf16` are computed without excess precision.

So like, `-fexcess-precision=standard` is straight-up **not implemented** for any language that isn't C or C++ 🫠. And on x86? It literally does nothing if you've got `-mfpmath=sse` or `-mfpmath=sse+387` going on — in the first case IEEE semantics just apply anyway with no excess precision, and in the second case the rounding is, and I quote, *unpredictable* 💀.

- **`-ffloat-store`** — okay so this one tells GCC: do **not** keep floating-point variables in registers, and also shut down any other options that could change whether a float value gets pulled from a register vs. memory. Real talk though, this option has kinda been subsumed by `-fexcess-precision=standard`, which is the more general glow-up ✨. And if you *do* use `-ffloat-store`, you might need to tweak your program so it explicitly stashes intermediate computations in temporary variables — because `-ffloat-store` only handles rounding to IEEE format on *assignments*, not on casts the way `-fexcess-precision=standard` does. iykyk 🎀

- **`-ffast-math`** — the whole chaotic friend group in one flag 🎉. It sets: `-fno-math-errno`, `-funsafe-math-optimizations`, `-ffinite-math-only`, `-fno-rounding-math`, `-fno-signaling-nans`, `-fcx-limited-range` and `-fexcess-precision=fast`.

  This one defines the preprocessor macro `__FAST_MATH__`.

  Bestie, listen — no `-O` option turns this on except `-Ofast`, because it can legitimately produce **incorrect output** for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 😭. That said, it may give you faster code for programs that don't need those guarantees. Slay, but responsibly 💅

- **`-fno-math-errno`** — don't set `errno` after calling math functions that run in a single instruction, e.g. `sqrt`. If your program relies on IEEE exceptions for math error handling, you might still want this flag for the speed while keeping your IEEE arithmetic compatibility intact 🏎️.

  This one isn't turned on by any `-O` option besides `-Ofast`, since it can result in **incorrect output** for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require those guarantees.

  The default is `-fmath-errno`.

  On Darwin systems the math library *never* sets `errno` — so there's literally zero reason for the compiler to even consider the possibility, and `-fno-math-errno` is the default there 🍓.

- **`-funsafe-math-optimizations`** — allow optimizations for floating-point arithmetic that (a) assume arguments and results are valid and (b) may violate IEEE or ANSI standards 🙈. When you use it at link time, it may pull in libraries or startup files that change the default FPU control word or do other similar optimizations. Iconic but unhinged.

  Not turned on by any `-O` option besides `-Ofast`, since — say it with me — it can result in **incorrect output** for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require those guarantees. It enables `-fno-signed-zeros`, `-fno-trapping-math`, `-fassociative-math` and `-freciprocal-math`.

  The default is `-fno-unsafe-math-optimizations`.

- **`-fassociative-math`** — allow re-association of operands in a series of floating-point operations. This **violates the ISO C and C++ language standard** by possibly changing the computation result 😬. NOTE: re-ordering may change the sign of zero, plus it can ignore NaNs and inhibit or create underflow or overflow (so like, you cannot use it on code that depends on rounding behavior like `(x + 2**52) - 2**52`). It may also reorder floating-point comparisons, so no using it when ordered comparisons are required. This option requires that both `-fno-signed-zeros` and `-fno-trapping-math` be in effect. Also, it doesn't make much sense alongside `-frounding-math`. For Fortran, the option is automatically enabled when both `-fno-signed-zeros` and `-fno-trapping-math` are in effect.

  The default is `-fno-associative-math`.

- **`-freciprocal-math`** — allow the reciprocal of a value to be used instead of dividing by the value, if that enables optimizations. Like, `x / y` can be replaced with `x * (1/y)`, which is super useful if `(1/y)` is subject to common subexpression elimination 👑. Heads up though: this loses precision and increases the number of flops operating on the value.

  The default is `-fno-reciprocal-math`.

- **`-ffinite-math-only`** — allow optimizations for floating-point arithmetic that assume arguments and results are not NaNs or +-Infs 🌈.

  Not turned on by any `-O` option besides `-Ofast`, because it can result in **incorrect output** for programs depending on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require those guarantees.

  The default is `-fno-finite-math-only`.

- **`-fno-signed-zeros`** — allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies distinct behavior for +0.0 and -0.0 values, which prohibits simplification of expressions like `x+0.0` or `0.0*x` (even with `-ffinite-math-only`) 😤. This option implies that the sign of a zero result isn't significant.

  The default is `-fsigned-zeros`.

- **`-fno-trapping-math`** — compile code assuming floating-point operations can't generate user-visible traps 🪤. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that `-fno-signaling-nans` be in effect. Setting it may allow faster code if you're relying on "non-stop" IEEE arithmetic, for example.

  Not turned on by any `-O` option besides `-Ofast`, since it can result in **incorrect output** for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

  The default is `-ftrapping-math`.

  Future versions of GCC may give you finer control of this setting using C99's `FENV_ACCESS` pragma. This command-line option will be used along with `-frounding-math` to specify the default state for `FENV_ACCESS`.

- **`-frounding-math`** — disable transformations and optimizations that assume default floating-point rounding behavior (round-to-nearest) 🎯. You should specify this for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) plus arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

  The default is `-fno-rounding-math`.

  This option is **experimental** and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode ⚠️. Future versions of GCC may provide finer control of this setting using C99's `FENV_ACCESS` pragma. This command-line option will be used along with `-ftrapping-math` to specify the default state for `FENV_ACCESS`.

- **`-fsignaling-nans`** — compile code assuming IEEE signaling NaNs may generate user-visible traps during floating-point operations 🚨. Setting this disables optimizations that might change the number of exceptions visible with signaling NaNs. This option implies `-ftrapping-math`.

  This one causes the preprocessor macro `__SUPPORT_SNAN__` to be defined.

  The default is `-fno-signaling-nans`.

  Also experimental, and it does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

- **`-fno-fp-int-builtin-inexact`** — don't let the built-in functions `ceil`, `floor`, `round` and `trunc` (and their `float` and `long double` variants) generate code that raises the `inexact` floating-point exception for noninteger arguments 🙅‍♀️. ISO C99 and C11 allow these functions to raise the `inexact` exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008 as integrated into ISO C23, does not allow them to do that.

  The default is `-ffp-int-builtin-inexact`, allowing the exception to be raised, unless C23 or a later C standard is selected. This option does absolutely nothing unless `-ftrapping-math` is in effect.

  And even if you use `-fno-fp-int-builtin-inexact`, if the functions generate a call to a library function then the `inexact` exception may still be raised if the library implementation doesn't follow TS 18661 🫶.

- **`-fsingle-precision-constant`** — treat floating-point constants as single precision instead of implicitly converting them into double-precision constants. Less converting, more vibes ✨.

- **`-fcx-limited-range`** — when enabled, this says a range reduction step isn't needed when performing complex division. Also there's no checking whether the result of a complex multiplication or division is `NaN + I*NaN` with an attempt to rescue the situation in that case 🙈. The default is `-fno-cx-limited-range`, but it's enabled by `-ffast-math`.

  This option controls the default setting of the ISO C99 `CX_LIMITED_RANGE` pragma. Nevertheless, the option applies to **all** languages.

- **`-fcx-fortran-rules`** — complex multiplication and division follow Fortran rules 🧮. Range reduction *is* done as part of complex division, but there's no checking whether the result of a complex multiplication or division is `NaN + I*NaN` with an attempt to rescue the situation in that case.

  The default is `-fno-cx-fortran-rules`.

---

Okay so the next batch of options controls optimizations that may improve performance but are **not enabled by any `-O` options** 🚀. This section includes **experimental** options that may produce **broken code** 💀 — read the fine print, bestie.

- **`-fbranch-probabilities`** — after running a program compiled with `-fprofile-arcs`, you can compile it a **second** time using `-fbranch-probabilities` to improve optimizations based on how many times each branch was taken 📊. When a program compiled with `-fprofile-arcs` exits, it saves arc execution counts to a file called `sourcename.gcda` for each source file. The info in that data file is *very* dependent on the structure of the generated code, so you must use the same source code **and** the same optimization options for both compilations 🙏. See the details about file naming in `-fprofile-arcs`.

With `-fbranch-probabilities`, GCC drops a `REG_BR_PROB` note on every single `JUMP_INSN` and `CALL_INSN`, and those notes can be used to make optimization better 💖. Right now they're only used in literally one place: inside `reorg.cc`, instead of guessing which path a branch is most likely to take, the `REG_BR_PROB` values get used to figure out *exactly* which path is taken more often. No guessing, no vibes-based branching 😌.

Enabled by `-fprofile-use` and `-fauto-profile`.

- **`-fprofile-values`** — if you pair this with `-fprofile-arcs`, it adds code so that some data about the values of expressions in the program gets gathered 🎀. And with `-fbranch-probabilities`, it reads back that gathered data about expression values so the optimizations can actually use it. Enabled by `-fprofile-generate`, `-fprofile-use`, and `-fauto-profile`.

- **`-fprofile-reorder-functions`** — function reordering based on profile instrumentation: it collects the first time each function gets executed and orders the functions in ascending order, aiming to optimize program startup by loading text segments more efficiently ⚡. Enabled with `-fprofile-use`.

- **`-fvpt`** — if you combine this with `-fprofile-arcs`, the compiler adds code to gather info about the values of expressions 🧚. With `-fbranch-probabilities`, it reads that data back and actually *performs* the optimizations based on it. Currently the optimizations include specializing division operations using knowledge about the value of the denominator. Enabled with `-fprofile-use` and `-fauto-profile`.

- **`-frename-registers`** — tries to avoid false dependencies in scheduled code by using registers that are left over after register allocation 💅. This one benefits processors with lots of registers the most. But depending on the debug info format the target adopted, it can make debugging impossible, because variables no longer stay in a "home register" 😭. Enabled by default with `-funroll-loops`.

- **`-fschedule-fusion`** — does a target dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently when they're adjacent to each other in the instruction flow 🔥. Enabled at levels `-O2`, `-O3`, `-Os`.

- **`-ftracer`** — performs tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function, which lets other optimizations do a better job ✨. Enabled by `-fprofile-use` and `-fauto-profile`.

- **`-funroll-loops`** — unrolls loops whose number of iterations can be determined at compile time or upon entry to the loop 🌀. `-funroll-loops` implies `-frerun-cse-after-loop`, `-fweb` and `-frename-registers`. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster — that's the tradeoff, bestie 🤷. Enabled by `-fprofile-use` and `-fauto-profile`.

- **`-funroll-all-loops`** — unrolls *all* loops, even if their number of iterations is uncertain when the loop is entered 😬. This usually makes programs run more slowly. `-funroll-all-loops` implies the same options as `-funroll-loops`.

- **`-fpeel-loops`** — peels loops where there's enough info that they don't roll much (from profile feedback or static analysis) 🍓. It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations). Enabled by `-O3`, `-fprofile-use`, and `-fauto-profile`.

- **`-fmalloc-dce`** — controls whether `"malloc"` (and its variants such as `"calloc"` or `"strdup"`) can be optimized away, provided its return value is only used as a parameter of a `"free"` call or compared with `"NULL"` 👀. If `-fmalloc-dce=1` is used, only calls to `"free"` are allowed; with `-fmalloc-dce=2` also comparisons with a `"NULL"` pointer are considered safe to remove. The default is `-fmalloc-dce=2`. See also `-fallocation-dce`.

- **`-fmove-loop-invariants`** — enables the loop invariant motion pass in the RTL loop optimizer 💫. Enabled at level `-O1` and higher, except for `-Og`.

- **`-fmove-loop-stores`** — enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop, in exchange for carrying the stored value in a register across the iteration 🎀. Note: for this option to have an effect, `-ftree-loop-im` has to be enabled as well. Enabled at level `-O1` and higher, except for `-Og`.

- **`-fsplit-loops`** — splits a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other ✂️. Enabled by `-fprofile-use` and `-fauto-profile`.

- **`-funswitch-loops`** — moves branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to the result of the condition) 🔀. Enabled by `-fprofile-use` and `-fauto-profile`.

- **`-fversion-loops-for-strides`** — if a loop iterates over an array with a variable stride, this creates another version of the loop that assumes the stride is always one 🏃‍♀️. Like, for example:

for (int i = 0; i < n; ++i) x[i * stride] = ...;


becomes:

if (stride == 1) for (int i = 0; i < n; ++i) x[i] = ...; else for (int i = 0; i < n; ++i) x[i * stride] = ...;


This is particularly useful for assumed-shape arrays in Fortran, where (for example) it allows better vectorization assuming contiguous accesses 🌈. This flag is enabled by default at `-O3`. It is also enabled by `-fprofile-use` and `-fauto-profile`.

- **`-ffunction-sections`**
- **`-fdata-sections`** — place each function or data item into its own section in the output file, if the target supports arbitrary sections 📦. The name of the function or the name of the data item determines the section's name in the output file.

  Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

  Together with a linker garbage collection (linker `--gc-sections` option) these options may lead to smaller statically-linked executables (after stripping) 👑.

  On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats though 😬.

  Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.

- **`-fstdarg-opt`** — optimizes the prologue of variadic argument functions with respect to usage of those arguments 🫶.

- **`-fsection-anchors`** — tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help reduce the number of GOT entries and GOT accesses on some targets ⚓.

  For example, the implementation of the following function `"foo"`:

static int a, b, c; int foo (void) { return a + b + c; }


  usually calculates the addresses of all three variables, but if you compile it with `-fsection-anchors`, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C) 🙈:

int foo (void) { register int *xr = &x; return xr[&a - &x] + xr[&b - &x] + xr[&c - &x]; }


  Not all targets support this option.

- **`-fzero-call-used-regs=choice`** — zeroes call-used registers at function return to increase program security, either by mitigating Return-Oriented Programming (ROP) attacks or by preventing information leakage through registers 🛡️. The possible values of *choice* are the same as for the `"zero_call_used_regs"` attribute. The default is `skip`.

  You can control this behavior for a specific function by using the function attribute `"zero_call_used_regs"`.

- **`--param name=value`** — in some places, GCC uses various constants to control how much optimization is done. For example, GCC does not inline functions that contain more than a certain number of instructions 🤓. You can control some of these constants on the command line using the `--param` option.

  The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases ⚠️.

  In order to get the minimal, maximal and default values of a parameter, use the `--help=param -Q` options.

  In each case, the value is an integer. The following choices of *name* are recognized for all targets:

### `phiopt-factor-max-stmts-live`

When factoring statements out of if/then/else, this is the max # of statements after the defining statement to be allow to extend the lifetime of a name 🧵.

### `predictable-branch-outcome`

When a branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable 🎯.

### `max-rtl-if-conversion-insns`

RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable 💭.

### `file-cache-files`

Max number of files in the file cache. The file cache is used to print source lines in diagnostics and do some source checks like `-Wmisleading-indentation` 📂.

### `file-cache-lines`

Max number of lines to index into file cache. When `0` this is automatically sized. The file cache is used to print source lines in diagnostics and do some source checks like `-Wmisleading-indentation` 📄.

- **`max-rtl-if-conversion-predictable-cost`** — okay so RTL if-conversion literally tries to yeet conditional branches around a block and replace them with conditionally executed instructions. These parameters give the max permissible cost for the sequence that would be generated by if-conversion, depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internal `seq_cost` metric. The compiler will try to provide a reasonable default for this parameter using the `BRANCH_COST` target macro. 💅

- **`max-crossjump-edges`** — the maximum number of incoming edges to consider for cross-jumping. The algorithm used by `-fcrossjumping` is `O(N^2)` in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. lowkey a trade-off, bestie.

- **`min-crossjump-insns`** — the minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched. so it's like "don't bother unless you match at least this many" ✨

- **`max-grow-copy-bb-insns`** — the maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. no cap on the vibe though.

- **`max-goto-duplication-insns`** — the maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid `O(N^2)` behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than `max-goto-duplication-insns` are unfactored. it's giving "please don't explode the compile time" 🫶

- **`max-delay-slot-insn-search`** — the maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time. like, diminishing returns much? 💀

- **`max-delay-slot-live-search`** — when trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph. so it's a temporary hack, noted. 📝

- **`max-gcse-memory`** — the approximate maximum amount of memory in `kB` that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done. so if it's too thirsty for RAM, it just doesn't. 🙈

- **`max-gcse-insertion-ratio`** — if the ratio of expression insertions to deletions is larger than this value for any expression, then `RTL PRE` inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. basically a "don't overdo it" knob.

- **`max-pending-list-length`** — the maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. so it's like a vibe check for the scheduler's to-do list. 🧚

- **`max-modulo-backtrack-attempts`** — the maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. bestie, exponential is not the move. 😭

- **`max-inline-functions-called-once-loop-depth`** — maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once. so it's like "how deep in the loop are we willing to go" 🌀

- **`max-inline-functions-called-once-insns`** — maximal estimated size of functions produced while inlining functions called once. size matters, apparently.

- **`max-inline-insns-single`** — several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared `inline` and methods implemented in a class declaration (`C++`). so it's the "how big is too big to inline" limit for the special kids. 👑

- **`max-inline-insns-auto`** — when you use `-finline-functions` (included in `-O3`), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (`--param max-inline-insns-auto`). so it's a stricter vibe for the auto-inline candidates.

- **`max-inline-insns-small`** — this is the bound applied to calls that are considered relevant with `-finline-small-functions`. small but mighty.

- **`max-inline-insns-size`** — this is the bound applied to calls that are optimized for size. Small growth may be desirable to anticipate optimization opportunities exposed by inlining. so a little growth is okay if it unlocks later glow-ups. ✨

- **`uninlined-function-insns`** — number of instructions accounted by inliner for function overhead such as function prologue and epilogue. basically the "cost of admission" for calling a function.

- **`uninlined-function-time`** — extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue. time is money, and the inliner is counting.

- **`inline-heuristics-hint-percent`** — the scale (in percents) applied to `inline-insns-single`, `inline-insns-single-O2`, `inline-insns-auto` when inline heuristics hints that inlining is very profitable (will enable later optimizations). so if the vibe says "this will slay later", we scale up the limits. 💅

- **`uninlined-thunk-insns`, `uninlined-thunk-time`** — same as `--param uninlined-function-insns` and `--param uninlined-function-time` but applied to function thunks. thunks get the same treatment, no cap.

- **`inline-min-speedup`** — when estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on `--param max-inline-insns-single` and `--param max-inline-insns-auto`. so if it's a huge speedup, the size limits get ignored. iconic. 🔥

- **`large-function-insns`** — the limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by `--param large-function-growth`. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. so it's like a "don't let this function become a monster" limit. 💀

- **`large-function-growth`** — specifies maximal growth of large functions caused by inlining in percents. For example, parameter value `100` limits large function growth to 2.0 times the original size. math is math.

- **`large-unit-insns`** — the limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by `--param inline-unit-growth`. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of unit is `300%` and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. Thus for smaller units, the size is increased to `--param large-unit-insns` before applying `--param inline-unit-growth`. so it's like a sliding scale, iykyk. ⚖️

- **`lazy-modules`** — maximum number of concurrently open C++ module files when lazy loading. just a module limit, no biggie.

- **`inline-unit-growth`** — specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value `20` limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size. so cold functions are like "not my problem" 🥶

- **`ipa-cp-unit-growth`** — specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value `10` limits unit growth to 1.1 times the original size. IPA-CP is doing its thing, but with limits.

- **`ipa-cp-large-unit-insns`** — the size of translation unit that IPA-CP pass considers large. so it's the "this unit is thicc" threshold for IPA-CP.

- **`large-stack-frame`** — the limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. stack frames have feelings too. 🥺

- **`large-stack-frame-growth`** — specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value `1000` limits large stack frame growth to 11 times the original size. that's a lot of growth, but okay.

- **`max-inline-insns-recursive`, `max-inline-insns-recursive-auto`** — specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining. `--param max-inline-insns-recursive` applies to functions declared inline. For functions not declared inline, recursive inlining happens only when `-finline-functions` (included in `-O3`) is enabled; `--param max-inline-insns-recursive-auto` applies instead. so recursive functions get their own special limits, periodt. 🔁

- **`max-inline-recursive-depth`, `max-inline-recursive-depth-auto`** — specifies the maximum recursion depth used for recursive inlining. `--param max-inline-recursive-depth` applies to functions declared inline. For functions not declared inline, recursive inlining happens only when `-finline-functions` (included in `-O3`) is enabled; `--param max-inline-recursive-depth-auto` applies instead. depth control for the recursive girlies. 🌀

- **`min-inline-recursive-probability`** — recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers. When profile feedback is available (see `-fprofile-generate`) the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents). so it's like "only inline if it's likely to actually recurse enough" — no cap, just vibes. 📊

okay so the knob tour continues and honestly this is where the compiler girlies start flexing 😭 here's another full stack of tuning knobs, all of them, no skips, no cap:

- **`early-inlining-insns`** — sets the growth the early inliner is allowed to make. basically it bumps up the amount of inlining for code that's carrying a big abstraction penalty 💅
- **`max-early-inliner-iterations`** — a limit on the early inliner's iterations. it basically bounds how many nested indirect calls the early inliner can resolve. deeper chains? still handled by late inlining, bestie ✨
- **`comdat-sharing-probability`** — the probability (in percent) that a C++ inline function with comdat visibility gets shared across multiple compilation units.
- **`modref-max-bases`**, **`modref-max-refs`**, **`modref-max-accesses`** — three knobs, one vibe: the max number of base pointers, references and accesses stored for a single function by mod/ref analysis.
- **`modref-max-tests`** — the max number of tests the alias oracle can perform to disambiguate memory locations using the mod/ref information. this one ought to be bigger than `--param modref-max-bases` and `--param modref-max-refs`, iykyk 🫶
- **`modref-max-depth`** — the maximum depth of the DFS walk used by modref escape analysis. setting it to `0` disables the analysis completely.
- **`modref-max-escape-points`** — the max number of escape points that modref tracks per SSA-name.
- **`modref-max-adjustments`** — the max number of times the access range gets enlarged during modref dataflow analysis.
- **`profile-func-internal-id`** — controls whether to use the function internal id in profile database lookups. if the value is `0`, the compiler uses an id based on the function assembler name and filename, which makes old profile data more tolerant to source changes like function reordering etc. iconic behaviour tbh 🌸
- **`min-vect-loop-bound`** — the minimum number of iterations under which loops are not vectorized when `-ftree-vectorize` is used. the number of iterations after vectorization needs to be greater than this value for vectorization to be allowed.
- **`gcse-cost-distance-ratio`** — scaling factor used to calculate the maximum distance an expression can be moved by GCSE optimizations. currently only supported in the code hoisting pass. bigger ratio = more aggressive code hoisting for simple expressions, i.e. expressions whose cost is less than `gcse-unrestricted-cost`. specifying `0` disables hoisting of simple expressions.
- **`gcse-unrestricted-cost`** — a cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations don't constrain how far an expression can travel. again, currently only supported in the code hoisting pass. the lesser the cost, the more aggressive code hoisting is. specifying `0` lets all expressions travel unrestricted distances ✨
- **`max-hoist-depth`** — how deep it searches the dominator tree for expressions to hoist. this exists to avoid quadratic behaviour in the hoisting algorithm. a value of `0` doesn't limit the search, but it may slow down compilation of huge functions.

> fr fr, all the "avoid quadratic behaviour" knobs are the compiler admitting it would absolutely spiral without a leash 💀 keep them in mind for giant functions.

- **`max-tail-merge-comparisons`** — the max number of similar bbs to compare a bb with. used to avoid quadratic behaviour in tree tail merging.
- **`max-tail-merge-iterations`** — the max number of iterations of the pass over the function. used to limit compilation time in tree tail merging.
- **`store-merging-allow-unaligned`** — lets the store merging pass introduce unaligned stores if it's legal to do so.
- **`max-stores-to-merge`** — the max number of stores to attempt to merge into wider stores in the store merging pass.
- **`max-store-chains-to-track`** — the max number of store chains to track at the same time while attempting to merge them into wider stores in the store merging pass.
- **`max-stores-to-track`** — the max number of stores to track at the same time in the attempt to merge them into wider stores in the store merging pass.
- **`max-unrolled-insns`** — the max number of instructions a loop may have in order to be unrolled. if a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
- **`max-average-unrolled-insns`** — same energy, but the max number of instructions is biased by the probabilities of their execution. if a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
- **`max-unroll-times`** — the max number of unrollings of a single loop.
- **`max-peeled-insns`** — the max number of instructions a loop may have in order to be peeled. if a loop is peeled, this parameter also determines how many times the loop code is peeled.
- **`max-peel-times`** — the max number of peelings of a single loop.
- **`max-peel-branches`** — the max number of branches on the hot path through the peeled sequence.
- **`max-completely-peeled-insns`** — the max number of insns of a completely peeled loop.
- **`max-completely-peel-times`** — the max number of iterations of a loop to be suitable for complete peeling.
- **`max-completely-peel-loop-nest-depth`** — the max depth of a loop nest suitable for complete peeling.
- **`max-unswitch-insns`** — the max number of insns of an unswitched loop.
- **`max-unswitch-depth`** — the max depth of a loop nest to be unswitched.
- **`lim-expensive`** — the minimum cost of an expensive expression in loop invariant motion.
- **`min-loop-cond-split-prob`** — when FDO profile information is available, this specifies the minimum threshold for the probability of a semi-invariant condition statement to trigger a loop split.
- **`iv-consider-all-candidates-bound`** — a bound on the number of induction variable candidates; below it, all candidates are considered for each use in induction variable optimizations. more candidates than this and only the most relevant ones are considered, to avoid quadratic time complexity.
- **`iv-max-considered-uses`** — the induction variable optimizations give up on loops that contain more induction variable uses than this.
- **`iv-always-prune-cand-set-bound`** — if the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.
- **`avg-loop-niter`** — average number of iterations of a loop.
- **`dse-max-object-size`** — max size (in bytes) of objects tracked bytewise by dead store elimination. larger values may result in larger compilation times.
- **`dse-max-alias-queries-per-store`** — max number of queries into the alias oracle per store. larger values mean larger compilation times and may result in more removed dead stores.
- **`scev-max-expr-size`** — a bound on the size of expressions used in the scalar evolutions analyzer. large expressions slow the analyzer down.
- **`scev-max-expr-complexity`** — a bound on the complexity of expressions in the scalar evolutions analyzer. complex expressions slow the analyzer down.

---

- **`max-tree-if-conversion-phi-args`** — max number of arguments in a PHI supported by TREE if conversion, unless the loop is marked with simd pragma.
- **`vect-max-layout-candidates`** — the max number of possible vector layouts (permutations, that kind of thing) to consider when optimizing to-be-vectorized code 🦋
- **`vect-max-version-for-alignment-checks`** — the max number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.
- **`vect-max-version-for-alias-checks`** — the max number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
- **`vect-max-peeling-for-alignment`** — the max number of loop peels to enhance access alignment for the vectorizer. value `-1` means no limit.
- **`max-iterations-to-track`** — the max number of iterations of a loop that the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.
- **`hot-bb-count-fraction`** — the denominator n of fraction 1/n of the maximal execution count of a basic block in the entire program that a basic block needs to at least have to be considered hot. the default is `10000`, which means a basic block is considered hot if its execution count is greater than 1/10000 of the maximal execution count. `0` means it is never considered hot. used in non-LTO mode.
- **`hot-bb-count-ws-permille`** — the number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program that a basic block's execution count must be part of to be considered hot. the default is `990`, which means a basic block is considered hot if its execution count contributes to the upper 990 permilles, or 99.0%, of the profiled execution of the entire program. `0` means it is never considered hot. used in LTO mode 🌈
- **`hot-bb-frequency-fraction`** — the denominator n of fraction 1/n of the execution frequency of the entry block of a function that a basic block of this function needs to at least have to be considered hot. the default is `1000`, which means a basic block is considered hot in a function if it is executed more frequently than 1/1000 of the frequency of the entry block of the function. `0` means it is never considered hot.

- **`unlikely-bb-count-fraction`** — okay so this one is a fraction thing, stay with me 🫶 it's the denominator *n* of the fraction 1/*n*. What's it a fraction *of*? The number of profiled runs of the whole entire program. If a basic block's execution count falls **below** that fraction, the block gets labeled unlikely-executed. Default is `20`, meaning a block counts as unlikely executed if it runs in fewer than 1/20 — that's **5%** bestie — of the program's runs. And `0` means it's *always* considered unlikely executed. Periodt.

- **`max-predicted-iterations`** — the maximum number of loop iterations we predict statically ✨. Super useful when a function has one loop with a known bound and another loop with an unknown bound: the known iteration count gets predicted correctly, while the unknown one averages to roughly `10`. Which means the loop with no bounds looks artificially <span style="color:cyan">cold</span> next to the other one. Lowkey unfair to it ngl 💀

- **`builtin-expect-probability`** — controls the probability of the expression having the specified value. Takes a percentage as input, i.e. `0 ... 100` 🌸

- **`builtin-string-cmp-inline-length`** — the maximum length of a constant string for a builtin string cmp call that's eligible for inlining 🎀

- **`align-threshold`** — selects the fraction of the maximal frequency of executions of a basic block in a function, and that's what decides whether the basic block gets aligned 👑

- **`align-loop-iterations`** — a loop that's expected to iterate at least the selected number of iterations gets aligned 🌀

- **`tracer-dynamic-coverage`**, **`tracer-dynamic-coverage-feedback`** — this value limits superblock formation once the given percentage of executed instructions is covered. Why? It limits unnecessary code size expansion, bestie, we love a size-conscious icon 💅

  The `tracer-dynamic-coverage-feedback` parameter is used **only** when profile feedback is available. Real profiles (as opposed to statically estimated ones) are much less balanced, which allows the threshold to be a larger value 🦋

- **`tracer-max-code-growth`** — stops tail duplication once code growth hits the given percentage. Tbh this is a rather artificial limit, since most of the duplicates get eliminated later in cross jumping — so it may be set to *much* higher values than is the desired code growth 🙈

- **`tracer-min-branch-ratio`** — stops reverse growth when the reverse probability of the best edge is less than this threshold (in percent) 💜

- **`tracer-min-branch-probability`**, **`tracer-min-branch-probability-feedback`** — stops forward growth if the best edge has probability lower than this threshold ✋

  Just like `tracer-dynamic-coverage`, there's two parameters here. `tracer-min-branch-probability-feedback` is used for compilation with profile feedback, and `tracer-min-branch-probability` for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective 🧚

- **`stack-clash-protection-guard-size`** — specify the size of the operating system provided stack guard as 2 raised to *num* bytes. Higher values may reduce the number of explicit probes, **but** a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks 😭

> ⚠️ fr fr, that last bit is the whole point: go bigger than the OS guard and you're literally handing attackers a vulnerability. Be careful.

- **`stack-clash-protection-probe-interval`** — stack clash protection involves probing stack space as it's allocated. This param controls the maximum distance between probes into the stack, as 2 raised to *num* bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks 💀

- **`max-cse-path-length`** — the maximum number of basic blocks on a path that CSE considers ⭐

- **`max-cse-insns`** — the maximum number of instructions CSE processes before flushing 🌈

- **`ggc-min-expand`** — so GCC uses a garbage collector to manage its own memory allocation, main character energy honestly 👑 This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has **no effect on code generation**.

  The default is `30% + 70% * (RAM/1GB)` with an upper bound of `100%` when `RAM >= 1GB`. If `"getrlimit"` is available, the notion of `"RAM"` is the smallest of actual RAM and `"RLIMIT_DATA"` or `"RLIMIT_AS"`. If GCC isn't able to calculate RAM on a particular platform, the lower bound of `30%` is used. Setting this parameter and `ggc-min-heapsize` to zero causes a full collection to occur at every opportunity — this is **extremely slow**, but it can be useful for debugging 🫶

- **`ggc-min-heapsize`** — minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by `ggc-min-expand`% beyond `ggc-min-heapsize`. Again, tuning this may improve compilation speed, and it has no effect on code generation ✨

  The default is the smaller of `RAM/8`, `RLIMIT_RSS`, or a limit that tries to ensure that `RLIMIT_DATA` or `RLIMIT_AS` are not exceeded — but with a lower bound of `4096` (four megabytes) and an upper bound of `131072` (128 megabytes). If GCC isn't able to calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. And setting this parameter and `ggc-min-expand` to zero causes a full collection to occur at every opportunity 🌸

- **`max-reload-search-insns`** — the maximum number of instructions reload should look backward for an equivalent register. Increasing values mean more aggressive optimization, making compilation time increase with probably slightly better performance 🔥

- **`max-cselib-memory-locations`** — the maximum number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance 💅

- **`max-sched-ready-insns`** — the maximum number of instructions ready to be issued that the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making the compilation time increase with probably little benefit 🙈

- **`max-sched-region-blocks`** — the maximum number of blocks in a region to be considered for interblock scheduling 🦋

- **`max-pipeline-region-blocks`** — the maximum number of blocks in a region to be considered for pipelining in the selective scheduler 🎀

- **`max-sched-region-insns`** — the maximum number of insns in a region to be considered for interblock scheduling ⭐

- **`max-pipeline-region-insns`** — the maximum number of insns in a region to be considered for pipelining in the selective scheduler 🌈

- **`min-spec-prob`** — the minimum probability (in percents) of reaching a source block for interblock speculative scheduling 💜

- **`max-sched-extend-regions-iters`** — the maximum number of iterations through CFG to extend regions. A value of `0` disables region extensions 🚫

- **`max-sched-insn-conflict-delay`** — the maximum conflict delay for an insn to be considered for speculative motion 🧚

- **`sched-spec-prob-cutoff`** — the minimal probability of speculation success (in percents), so that speculative insns are scheduled ✅

- **`sched-state-edge-prob-cutoff`** — the minimum probability an edge must have for the scheduler to save its state across it 💫

- **`sched-mem-true-dep-cost`** — minimal distance (in CPU cycles) between store and load targeting same memory locations ⚡

- **`selsched-max-lookahead`** — the maximum size of the lookahead window of selective scheduling. It's a depth of search for available instructions 🔍

- **`selsched-max-sched-times`** — the maximum number of times that an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined 🔁

- **`selsched-insns-to-rename`** — the maximum number of best instructions in the ready list that are considered for renaming in the selective scheduler ✨

- **`sms-min-sc`** — the minimum value of stage count that swing modulo scheduler generates 🎡

- **`max-last-value-rtl`** — the maximum size, measured as number of RTLs, that can be recorded in an expression in combiner for a pseudo register as last known value of that register 📝

- **`max-combine-insns`** — the maximum number of instructions the RTL combiner tries to combine 🧩

- **`max-combine-search-insns`** — the maximum number of instructions that the RTL combiner searches in order to find the next use of a given register definition. If this limit is reached without finding such a use, the combiner will stop trying to optimize the definition 🛑

  Currently this limit only applies after certain successful combination attempts, but it could be extended to other cases in future 👀

- **`integer-share-limit`** — small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant 💖

- **`ssp-buffer-size`** — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when `-fstack-protector` is used 🛡️

- **`min-size-for-stack-sharing`** — the minimum size of variables taking part in stack slot sharing when not optimizing 🌿

- **`max-jump-thread-duplication-stmts`** — maximum number of statements allowed in a block that needs to be duplicated when threading jumps 🔀

- **`max-jump-thread-paths`** — the maximum number of paths to consider when searching for jump threading opportunities. When arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges does not exhaust the specified maximum number of paths to consider 🛤️

---

### `max-fields-for-field-sensitive`
The max number of fields in a struct that still gets treated in a *field sensitive* way during pointer analysis. More fields = more detail, but bestie, it costs.

### `prefetch-latency`
An estimate of the average number of instructions that get executed before a prefetch actually finishes. The distance we prefetch ahead is proportional to this constant ✨. Heads up though: bumping this number may also lead to *fewer* streams being prefetched (peep `simultaneous-prefetches` for the why).

### `simultaneous-prefetches`
The absolute max number of prefetches that can be running at the same time. That's it, that's the whole vibe. 🔥

### `l1-cache-line-size`
The size of a cache line in the L1 data cache, in bytes.

### `l1-cache-size`
The size of the L1 data cache, in kilobytes.

### `l2-cache-size`
The size of the L2 data cache, in kilobytes.

### `prefetch-dynamic-strides`
Whether the loop array prefetch pass should bother issuing software prefetch hints for strides that are **non-constant**. In some cases this can actually be beneficial — but ngl, the fact that the stride isn't constant can make it hard to predict when there's a clear benefit to issuing those hints 🙈.

- Set to `1` if the prefetch hints should be issued for non-constant strides.
- Set to `0` if prefetch hints should be issued only for strides that are known to be constant and below `prefetch-minimum-stride`.

### `prefetch-minimum-stride`
The minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than this threshold, no prefetch hints get issued, periodt.

This setting is genuinely useful for processors that have hardware prefetchers — because in that case there may be conflicts between the hardware prefetchers and the software prefetchers 😭. If the hardware prefetchers have a maximum stride they can handle, that's the number you should use here to improve the use of software prefetchers.

A value of `-1` means we don't have a threshold and therefore prefetch hints can be issued for any constant stride.

> This setting is only useful for strides that are known *and* constant. Non-constant? Don't even try it.

### `destructive-interference-size` and `constructive-interference-size`
These are the values for the C++17 variables `"std::hardware_destructive_interference_size"` and `"std::hardware_constructive_interference_size"` 🎀.

The destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects. The constructive interference size is the maximum recommended size of contiguous memory accessed together. Typically *both* will be the size of an L1 cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size will be the small end of the range and the destructive size will be the large end.

The destructive interference size is intended to be used for layout, and thus <red>has ABI impact</red> 💀. The default value is **not** expected to be stable, and on some targets it varies with `-mtune` — so using this variable in a context where ABI stability is important, like the public interface of a library, is *strongly discouraged*. If you do use it in that context anyway, users can stabilize the value using this option.

The constructive interference size is less sensitive, since it's typically only used in a `static_assert` to make sure that a type fits within a cache line.

See also `-Winterference-size`.

### `loop-interchange-max-num-stmts`
The maximum number of stmts in a loop to be interchanged. Go over and it's a no from the optimizer 🫶.

### `loop-interchange-stride-ratio`
The minimum ratio between the stride of two loops for interchange to actually be profitable.

### `min-insn-to-prefetch-ratio`
The minimum ratio between the number of instructions and the number of prefetches needed to enable prefetching in a loop.

### `prefetch-min-insn-to-mem-ratio`
The minimum ratio between the number of instructions and the number of memory references needed to enable prefetching in a loop.

### `use-canonical-types`
Whether the compiler should use the `"canonical"` type system ✨. This should **always** be `1`, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. However — if bugs in the canonical type system are causing compilation failures, set this value to `0` to disable canonical types. Emergency lever energy.

### `switch-conversion-max-branch-ratio`
Switch initialization conversion refuses to create arrays that are bigger than `switch-conversion-max-branch-ratio` times the number of branches in the switch. It's giving "know your limits" 💅.

### `switch-lower-slow-alg-max-cases`
The maximum number of cases for slow switch lowering algorithms to be used.

### `max-partial-antic-length`
The maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (`-ftree-pre`) when optimizing at `-O3` and above. For some sorts of source code, the enhanced partial redundancy elimination optimization can *run away*, consuming all of the memory available on the host machine 😭😭. This parameter puts a limit on the length of the sets that get computed, which prevents that runaway behavior. Setting a value of `0` for this parameter allows an **unlimited** set length — your machine, your choice, your consequences.

### `rpo-vn-max-loop-depth`
The maximum loop depth that is value-numbered optimistically. When the limit hits, the innermost `rpo-vn-max-loop-depth` loops **and** the outermost loop in the loop nest get value-numbered optimistically, and the remaining ones don't. Yes, it skips the middle. Iconic behavior tbh.

### `sccvn-max-alias-queries-per-access`
The maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit gets hit, the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry.

### `ira-max-loops-num`
IRA uses regional register allocation by default. If a function contains more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation. Busy loops get the spotlight 👑, the rest can wait.

### `ira-max-conflict-table-size`
Even though IRA uses a sophisticated algorithm to compress the conflict table, that table can still require excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that doesn't require building a pseudo-register conflict table. Trade-off city 🏙️.

### `ira-loop-reserved-regs`
IRA can be used to evaluate more accurate register pressure in loops, for decisions about moving loop invariants (see `-O3`). The number of available registers reserved for some other purposes is given by this parameter. The default of the parameter is the best found from numerous experiments 🔬.

### `ira-consider-dup-in-all-alts`
Makes IRA consider the matching constraint (duplicated operand number) heavily in **all** available alternatives for the preferred register class. If it's set as zero, IRA only respects the matching constraint when it's in the only available alternative with an appropriate register class. Otherwise (nonzero), IRA checks all available alternatives for the preferred register class even if it already found some choice with an appropriate register class — and respects the found qualified matching constraint. Thorough queen 💅.

### `ira-simple-lra-insn-threshold`
Approximate function insn number in 1K units that triggers simple local RA.

### `lra-inheritance-ebb-probability-cutoff`
LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called *inheritance* 🧬. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add a BB to the inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64, so like… trust the science.

### `loop-invariant-max-bbs-in-loop`
Loop invariant motion can be very expensive — both in compilation time and in the amount of needed compile-time memory — with very large loops 😭. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. Sorry bestie, too big.

### `loop-max-datarefs-for-datadeps`
Building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are not handled by the optimizations that use loop data dependencies. Gatekept, unfortunately 🌸.

### `max-vartrack-size`
Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, the analysis for that function is retried **without** it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to `0` makes it unlimited ✨.

### `max-vartrack-expr-depth`
Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. If this is set too low, value expressions that *are* available and could be represented in debug information may end up not being used 🥲. Setting it higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. Balanced queen.

### `max-debug-marker-count`
Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL 💥. If a function has more such gimple stmts than the set limit, those stmts get dropped from the inlined copy of a function, and from its RTL expansion.

---

- **`min-nondebug-insn-uid`** — bestie, this is where nondebug insns start getting their uids 🎫 like it's the velvet rope. Everything *below* this number is reserved **exclusively** for debug insns made by `-fvar-tracking-assignments`, but ngl debug insns can still grab (non-overlapping!!) uids *above* it if that reserved range runs out. no cap.

- **`ipa-sra-deref-prob-threshold`** — okay so IPA-SRA will only swap a pointer that's known to not be NULL for one or more shiny new parameters when the probability of it actually getting dereferenced (in percent, relative to function entry) is **higher** than this value. vibe check before the glow-up 💅

- **`ipa-sra-ptr-growth-factor`** — IPA-SRA replaces a pointer-to-aggregate with new parameters **only** when their cumulative size is less than or equal to `ipa-sra-ptr-growth-factor` times the size of the original pointer parameter. it's giving "you can grow but like, not too much" 🦋

- **`ipa-sra-ptrwrap-growth-factor`** — the *additional* max allowed growth of the total size of new parameters that IPA-SRA swaps a pointer-to-aggregate for, when it points to a local variable that the caller only writes to and passes as an argument to other functions. extra credit growth room, iykyk ✨

- **`ipa-sra-max-replacements`** — the max pieces of an aggregate that IPA-SRA tracks. and because of that, it's also the max number of replacements of a formal parameter. two-for-one situation fr fr 🎀

- **`sra-max-scalarization-size-Ospeed`**
- **`sra-max-scalarization-size-Osize`** — so the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are literally obsessed with replacing the scalar parts of aggregates with independent scalar variables. these two parameters control the **maximum size, in storage units**, of an aggregate that even gets considered for replacement when compiling for speed (`sra-max-scalarization-size-Ospeed`) or for size (`sra-max-scalarization-size-Osize`). pick your era 💖

- **`sra-max-propagations`** — the max number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, so that copy propagation can happen. it's the access limit bestie, don't overthink it 🌸

- **`tm-max-aggregate-size`** — when making copies of thread-local variables in a transaction, this parameter sets the size in **bytes** after which variables get saved with the logging functions instead of save/restore code sequence pairs. heads up: this one only applies when you're using `-fgnu-tm` ⚠️

- **`graphite-max-nb-scop-params`** — to dodge exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. set it to **zero** to lift the bound entirely. and a variable whose value is unknown at compilation time and defined outside a SCoP is, by definition, a parameter of the SCoP. main character variable energy honestly 👑

- **`hardcfr-max-blocks`** — disables `-fharden-control-flow-redundancy` for functions with more blocks than the value you give. **Zero** removes any limit. zero = no cap, periodt 💀

- **`hardcfr-max-inline-blocks`** — forces `-fharden-control-flow-redundancy` to use *out-of-line* checking for functions with more basic blocks than the specified value. structure, bestie 🧚

- **`loop-block-tile-size`** — the loop blocking / strip mining transforms (the ones you turn on with `-floop-block` or `-floop-strip-mine`) strip mine each loop in the loop nest by a given number of iterations. you change that strip length with the `loop-block-tile-size` parameter. cute, controllable, iconic ✨

- **`ipa-jump-function-lookups`** — specifies the number of statements visited during jump function offset discovery. that's it, that's the whole thing 🎀

- **`ipa-cp-value-list-size`** — IPA-CP tries to track **all** possible values and types passed to a function's parameter so it can propagate them and do devirtualization. `ipa-cp-value-list-size` is the max number of values and types it stores per one formal parameter. the way it keeps receipts on every value 😭

- **`ipa-cp-eval-threshold`** — IPA-CP calculates its own score of cloning profitability heuristics, and it only performs those cloning opportunities whose scores **exceed** `ipa-cp-eval-threshold`. the bar is on the floor and it's still high, love that for us 💅

- **`ipa-cp-max-recursive-depth`** — maximum depth of recursive cloning for a self-recursive function. go deeper? only this deep 🌀

- **`ipa-cp-min-recursive-probability`** — recursive cloning happens **only** when the probability of the call actually being executed exceeds this parameter. lowkey a probability gate 🦋

- **`ipa-cp-profile-count-base`** — when you're using the `-fprofile-use` option, IPA-CP will consider the measured execution count of a call graph edge at this **percentage position in their histogram** as the basis for its heuristics calculation. data-driven queen 🔥

- **`ipa-cp-recursive-freq-factor`** — the number of times interprocedural copy propagation expects recursive functions to call themselves. manifesting recursion fr fr 🌈

- **`ipa-cp-recursion-penalty`** — the percentage penalty that recursive functions receive when they get evaluated for cloning. sorry bestie, recursion tax 💀

- **`ipa-cp-single-call-penalty`** — the percentage penalty that functions containing a single call to another function receive when they get evaluated for cloning. one call? penalty. harsh but fair 🫶

- **`ipa-max-agg-items`** — IPA-CP can also propagate a number of scalar values passed in an aggregate. `ipa-max-agg-items` controls the maximum number of such values per one parameter. bundle responsibly 🎁

- **`ipa-cp-loop-hint-bonus`** — when IPA-CP figures out that a cloning candidate would make the number of iterations of a loop **known**, it adds a bonus of `ipa-cp-loop-hint-bonus` to that candidate's profitability score. bonus points for knowing things 💖

- **`ipa-max-loop-predicates`** — the max number of different predicates IPA will use to describe when loops in a function have known properties. not me handing out predicates like friendship bracelets 🌸

- **`ipa-max-aa-steps`** — during its analysis of function bodies, IPA-CP employs alias analysis to track values pointed to by function parameters. so it doesn't waste its whole day on huge functions, it gives up and considers all memory clobbered after examining `ipa-max-aa-steps` statements that modify memory. boundaries, bestie 🧚

- **`ipa-max-switch-predicate-bounds`** — the maximal number of boundary endpoints of case ranges of a switch statement. for a switch that exceeds this limit, IPA-CP will **not** construct the cloning cost predicate (the thing used to estimate cloning benefit) for the default case of the switch statement. too many bounds, no predicate, that's the rule 💅

- **`ipa-max-param-expr-ops`** — IPA-CP analyzes a conditional statement that references some function parameter to estimate the benefit of cloning upon a certain constant value. but if the number of operations in a parameter expression exceeds `ipa-max-param-expr-ops`, that expression is treated as a *complicated* one and just isn't handled by IPA analysis. too complicated, can't come 💀

- **`lto-partitions`** — specifies the desired number of partitions produced during WHOPR compilation. and get this: the number of partitions should **exceed** the number of CPUs used for compilation 👑✨

- **`lto-min-partition`** — size of the minimal partition for WHOPR (in estimated instructions). this prevents the expense of splitting very small programs into way too many partitions. no tiny-partition spam 🎀

- **`lto-max-partition`** — size of the max partition for WHOPR (in estimated instructions), giving an upper bound for the individual size of a partition. meant to be used **only** with balanced partitioning, okay? don't just use it anywhere 🙈

- **`lto-partition-locality-frequency-cutoff`** — the denominator *n* of the fraction 1/*n* of the execution frequency of a callee to be cloned for a particular caller. and the special value of **0** means always clone without a cut-off. zero means no filter, periodt 🔥

- **`lto-partition-locality-size-cutoff`** — the size cut-off for a callee (including inlined calls) to be cloned for a particular caller. size matters, apparently 📏

- **`lto-max-locality-partition`** — the maximal size of a locality partition for LTO (in estimated instructions). a value of **0** means the default value gets used instead. zero = "you pick, king" 🌈

- **`lto-max-streaming-parallelism`** — the maximal number of parallel processes used for LTO streaming. multitasking icon 🦋

- **`cxx-max-namespaces-for-diagnostic-help`** — the max number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. bestie gave you a typo and now it's checking namespaces to help you out 🫶

- **`sink-frequency-threshold`** — the max relative execution frequency (in percents) of the target block relative to a statement's original block, which allows statement sinking of a statement. larger numbers = more aggressive statement sinking. and a small positive adjustment gets applied for statements with memory operands, since those are even **more** profitable to sink. move it, sink it, slay 💃

- **`max-stores-to-sink`** — the max number of conditional store pairs that can be sunk. set it to **0** if either vectorization (`-ftree-vectorize`) or if-conversion (`-ftree-loop-if-convert`) is disabled. read the room before you sink ⚠️

- **`case-values-threshold`** — the smallest number of different values for which it's best to use a jump-table instead of a tree of conditional branches. if the value is **0**, use the default for the machine. when in doubt, don't override ✨

- **`jump-table-max-growth-ratio-for-size`** — the maximum code size growth ratio when expanding into a jump table (in percent). this parameter is used when optimizing for **size**. keep it cute and compact 🎀

- **`jump-table-max-growth-ratio-for-speed`** — the maximum code size growth ratio when expanding into a jump table (in percent). this parameter is used when optimizing for **speed**. different era, different ratio 🔥

- **`tree-reassoc-width`** — sets the maximum number of instructions executed in parallel in a reassociated tree. this parameter overrides target dependent heuristics used by default **if** it has a non-zero value. non-zero or it doesn't count, bestie 💅

- **`sched-pressure-algorithm`** — choose between the two available implementations of `-fsched-pressure`:
  - **Algorithm 1** is the original implementation and is more likely to prevent instructions from being reordered.
  - **Algorithm 2** was designed as a compromise between the relatively conservative approach of algorithm 1 and the rather aggressive approach taken by the default scheduler. it relies more heavily on having a regular register file and accurate register pressure classes. see `haifa-sched.cc` in the GCC sources for more details.

  the default choice **depends on the target**, so no, there's not one right answer 🙈

- **`max-slsr-cand-scan`** — set the max number of existing candidates that get considered when seeking a basis for a new straight-line strength reduction candidate. shopping around for the perfect basis ✨🛍️

- **`asan-globals`** — bestie, this turns on <pink>buffer overflow detection for global objects</pink> 🌍💥. It’s enabled by default when you’re using `-fsanitize=address`. To disable that protection, use `--param asan-globals=0`.

- **`asan-stack`** — this one enables <cyan>buffer overflow detection for stack objects</cyan> 🥞. It’s on by default when you use `-fsanitize=address`. To turn stack protection off, use `--param asan-stack=0`.

- **`asan-instrument-reads`** — enables <mint>buffer overflow detection for memory reads</mint> 👀. It’s on by default with `-fsanitize=address`. To disable memory-read protection, use `--param asan-instrument-reads=0`.

- **`asan-instrument-writes`** — enables <yellow>buffer overflow detection for memory writes</yellow> ✍️. It’s on by default with `-fsanitize=address`. To disable memory-write protection, use `--param asan-instrument-writes=0`.

- **`asan-memintrin`** — enables detection for <peach>built-in functions</peach> 🧰. It’s on by default with `-fsanitize=address`. To disable built-in function protection, use `--param asan-memintrin=0`.

- **`asan-use-after-return`** — enables detection of <purple>use-after-return</purple> 👻. It’s on by default when you use `-fsanitize=address`. To disable it, use `--param asan-use-after-return=0`.

  > ⚠️ Note: by default, the check is disabled at run time. To enable it, add `detect_stack_use_after_return=1` to the environment variable `ASAN_OPTIONS`.

- **`asan-instrumentation-with-call-threshold`** — if the number of memory accesses in a function being instrumented is greater than or equal to this number, use callbacks instead of inline checks 🎛️. E.g. to disable inline code, use `--param asan-instrumentation-with-call-threshold=0`.

- **`asan-kernel-mem-intrinsic-prefix`** — if nonzero, prefix calls to `memcpy`, `memset` and `memmove` with `__asan_` or `__hwasan_` for `-fsanitize=kernel-address` or `-fsanitize=kernel-hwaddress`, respectively 🧠✨.

- **`hwasan-instrument-stack`** — enables hwasan instrumentation of <lavender>statically sized stack-allocated variables</lavender> 📦. It’s enabled by default with `-fsanitize=hwaddress`, and disabled by default with `-fsanitize=kernel-hwaddress`. To disable stack instrumentation, use `--param hwasan-instrument-stack=0`; to enable it, use `--param hwasan-instrument-stack=1`.

- **`hwasan-random-frame-tag`** — when using stack instrumentation, decide tags for stack variables using a <hotpink>deterministic sequence beginning at a random tag for each frame</hotpink> 🎲. With this parameter unset, tags are chosen using the same sequence but beginning from 1. This is enabled by default for `-fsanitize=hwaddress` and unavailable for `-fsanitize=kernel-hwaddress`. To disable it, use `--param hwasan-random-frame-tag=0`.

- **`hwasan-instrument-allocas`** — enables hwasan instrumentation of <green>dynamically sized stack-allocated variables</green> 🧺. It’s enabled by default with `-fsanitize=hwaddress`, and disabled by default with `-fsanitize=kernel-hwaddress`. To disable instrumentation of those variables, use `--param hwasan-instrument-allocas=0`; to enable it, use `--param hwasan-instrument-allocas=1`.

- **`hwasan-instrument-reads`** — enables <blue>hwasan checks on memory reads</blue> 📖. Instrumentation of reads is enabled by default for both `-fsanitize=hwaddress` and `-fsanitize=kernel-hwaddress`. To disable checking memory reads, use `--param hwasan-instrument-reads=0`.

- **`hwasan-instrument-writes`** — enables <red>hwasan checks on memory writes</red> 📝. Instrumentation of writes is enabled by default for both `-fsanitize=hwaddress` and `-fsanitize=kernel-hwaddress`. To disable checking memory writes, use `--param hwasan-instrument-writes=0`.

- **`hwasan-instrument-mem-intrinsics`** — enables hwasan instrumentation of <cyan>builtin functions</cyan> 🧩. Instrumentation of these builtin functions is enabled by default for both `-fsanitize=hwaddress` and `-fsanitize=kernel-hwaddress`. To disable instrumentation of builtin functions, use `--param hwasan-instrument-mem-intrinsics=0`.

- **`use-after-scope-direct-emission-threshold`** — if the size of a local variable in bytes is smaller than or equal to this number, directly poison (or unpoison) shadow memory instead of using <pink>run-time callbacks</pink> 🧪.

- **`tsan-distinguish-volatile`** — emit special instrumentation for <mint>accesses to volatiles</mint> ⚡.

- **`tsan-instrument-func-entry-exit`** — emit instrumentation calls to `__tsan_func_entry()` and `__tsan_func_exit()` 🚪✨.

- **`max-fsm-thread-path-insns`** — maximum number of instructions to copy when duplicating blocks on a <yellow>finite state automaton jump thread path</yellow> 🔁.

- **`threader-debug`** — `threader-debug=[none|all]` enables verbose dumping of the threader solver 🕵️♀️.

- **`parloops-chunk-size`** — chunk size of omp schedule for loops parallelized by parloops 🧵.

- **`parloops-schedule`** — schedule type of omp schedule for loops parallelized by parloops `(static, dynamic, guided, auto, runtime)` 🗓️.

- **`parloops-min-per-thread`** — the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one 🧵🫶. Note that for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.

- **`max-ssa-name-query-depth`** — maximum depth of recursion when querying properties of SSA names in things like fold routines 🔎. One level of recursion corresponds to following a use-def chain.

- **`max-speculative-devirt-maydefs`** — the maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively 🧠💅.

- **`ranger-debug`** — specifies the type of debug output to be issued for ranges 📊.

- **`unroll-jam-min-percent`** — the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable 📈.

- **`unroll-jam-max-unroll`** — the maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation 🌀.

- **`max-rtl-if-conversion-unpredictable-cost`** — maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable 🎢.

- **`max-variable-expansions-in-unroller`** — if `-fvariable-expansion-in-unroller` is used, the maximum number of times that an individual variable will be expanded during loop unrolling 🔄.

- **`partial-inlining-entry-probability`** — maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen 🎯.

- **`max-tracked-strlens`** — maximum number of strings for which the strlen optimization pass will track string lengths 🧵📏.

- **`gcse-after-reload-partial-fraction`** — the threshold ratio for performing partial redundancy elimination after reload ♻️.

- **`gcse-after-reload-critical-fraction`** — the threshold ratio of critical edges execution count that permit performing redundancy elimination after reload 🔥.

- **`max-loop-header-insns`** — the maximum number of insns in loop header duplicated by the copy loop headers pass 📋.

- **`vect-epilogues-nomask`** — enable loop epilogue vectorization using smaller vector size 🧬.

- **`vect-partial-vector-usage`** — controls when the loop vectorizer considers using partial vector loads and stores as an alternative to falling back to scalar code 🎚️. `0` stops the vectorizer from ever using partial vector loads and stores. `1` allows partial vector loads and stores if vectorization removes the need for the code to iterate. `2` allows partial vector loads and stores in all loops. The parameter only has an effect on targets that support partial vector loads and stores.

- **`vect-inner-loop-cost-factor`** — the maximum factor which the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized 🧮. The factor applied is the maximum of the estimated number of iterations of the inner loop and this parameter. The default value of this parameter is `50`.

- **`vect-induction-float`** — enable loop vectorization of floating point inductions 🌊.

- **`vect-force-slp`** — force the use of SLP when vectorizing, fail if not possible 💥.

- **`vrp-block-limit`** — maximum number of basic blocks before VRP switches to a lower memory algorithm 🧱.

- **`vrp-sparse-threshold`** — maximum number of basic blocks before VRP uses a sparse bitmap cache 🗺️.

- **`vrp-switch-limit`** — maximum number of outgoing edges in a switch before VRP will not process it 🔀.

- **`vrp-vector-threshold`** — maximum number of basic blocks for VRP to use a basic cache vector 🧺.

- **`avoid-fma-max-bits`** — maximum number of bits for which we avoid creating FMAs 🚫🧮.

- **`fully-pipelined-fma`** — whether the target fully pipelines FMA instructions 🎺. If non-zero, reassociation considers the benefit of parallelizing FMA’s multiplication part and addition part, assuming FMUL and FMA use the same units that can also do FADD.

- **`sms-loop-average-count-threshold`** — a threshold on the average loop count considered by the swing modulo scheduler 🎠.

- **`sms-dfa-history`** — the number of cycles the swing modulo scheduler considers when checking conflicts using DFA 🕰️.

- **`graphite-allow-codegen-errors`** — whether codegen errors should be ICEs when `-fchecking` 💀.

- **`sms-max-ii-factor`** — a factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop 🎯.

- **`lra-max-considered-reload-pseudos`** — the max number of reload pseudos which are considered during spilling a non-reload pseudo 🧳✨.

- **`max-pow-sqrt-depth`** — max depth of sqrt chains to use when synthesizing exponentiation by a real constant. keeping it bounded, bestie 💅

- **`max-dse-active-local-stores`** — max number of active local stores in RTL dead store elimination. that's the cap, no cap.

- **`asan-instrument-allocas`** — enable asan allocas/VLAs protection. safety first 🫶

- **`max-iterations-computation-cost`** — bound on the cost of an expression to compute the number of iterations.

- **`max-isl-operations`** — max number of isl operations; `0` means unlimited. iconic “no limit” behavior ✨

- **`graphite-max-arrays-per-scop`** — max number of arrays per scop.

- **`max-vartrack-reverse-op-size`** — the max. size of the loc list for which reverse ops should be added.

- **`fsm-scale-path-stmts`** — scale factor to apply to the number of statements in a threading path crossing a loop backedge when comparing to `--param=max-jump-thread-duplication-stmts`.

- **`uninit-control-dep-attempts`** — max number of nested calls to search for control dependencies during uninitialized variable analysis.

- **`uninit-max-chain-len`** — max number of predicates `anded` for each predicate `ored` in the normalized predicate chain.

- **`uninit-max-num-chains`** — max number of predicates `ored` in the normalized predicate chain.

- **`uninit-max-prune-work`** — max amount of work done to prune paths where the variable is always initialized.

- **`sched-autopref-queue-depth`** — hardware autoprefetcher scheduler model control flag. number of lookahead cycles the model looks into; at `' '` only enable instruction sorting heuristic. yes, that is literally what it says 💀

- **`loop-versioning-max-inner-insns`** — the max number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy.

- **`loop-versioning-max-outer-insns`** — the max number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning.

- **`ssa-name-def-chain-limit`** — the max number of `SSA_NAME` assignments to follow in determining a property of a variable such as its value. this limits the number of iterations or recursive calls GCC performs when optimizing certain statements or when determining their validity prior to issuing diagnostics.

- **`store-merging-max-size`** — max size of a single store merging region in bytes.

- **`store-forwarding-max-distance`** — max number of instruction distance that a small store forwarded to a larger load may stall. value `'0'` disables the cost checks for the avoid-store-forwarding pass.

- **`hash-table-verification-limit`** — the number of elements for which hash table verification is done for each searched element.

- **`max-find-base-term-values`** — max number of `VALUEs` handled during a single `find_base_term` call.

- **`analyzer-max-enodes-per-program-point`** — the max number of exploded nodes per program point within the analyzer, before terminating analysis of that point.

- **`analyzer-max-constraints`** — the max number of constraints per state.

- **`analyzer-min-snodes-for-call-summary`** — the min number of supernodes within a function for the analyzer to consider summarizing its effects at call sites.

- **`analyzer-max-enodes-for-full-dump`** — the max depth of exploded nodes that should appear in a dot dump before switching to a less verbose format.

- **`analyzer-max-recursion-depth`** — the max number of times a callsite can appear in a call stack within the analyzer, before terminating analysis of a call that would recurse deeper.

- **`analyzer-max-svalue-depth`** — the max depth of a symbolic value, before approximating the value as unknown.

- **`analyzer-max-infeasible-edges`** — the max number of infeasible edges to reject before declaring a diagnostic as infeasible.

- **`gimple-fe-computed-hot-bb-threshold`** — the number of executions of a basic block which is considered hot. the parameter is used only in GIMPLE FE.

- **`analyzer-bb-explosion-factor`** — the max number of `'after supernode'` exploded nodes within the analyzer per supernode, before terminating analysis. it's giving controlled chaos 🌀

- **`analyzer-text-art-string-ellipsis-threshold`** — the number of bytes at which to ellipsize string literals in analyzer text art diagrams.

- **`analyzer-text-art-ideal-canvas-width`** — the ideal width in characters of text art diagrams generated by the analyzer.

- **`analyzer-text-art-string-ellipsis-head-len`** — the number of literal bytes to show at the head of a string literal in text art when ellipsizing it.

- **`analyzer-text-art-string-ellipsis-tail-len`** — the number of literal bytes to show at the tail of a string literal in text art when ellipsizing it.

- **`ranger-logical-depth`** — max depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges.

- **`ranger-recompute-depth`** — max depth of instruction chains to consider for recomputation in the outgoing range calculator.

- **`relation-block-limit`** — max number of relations the oracle will register in a basic block.

- **`transitive-relations-work-bound`** — work bound when discovering transitive relations from existing relations.

- **`min-pagesize`** — minimum page size for warning and early break vectorization purposes.

- **`openacc-kernels`** — specify mode of OpenACC `` `kernels' `` constructs handling. with `--param=openacc-kernels=decompose`, OpenACC `` `kernels' `` constructs are decomposed into parts, a sequence of compute constructs, each then handled individually. this is work in progress. with `--param=openacc-kernels=parloops`, OpenACC `` `kernels' `` constructs are handled by the parloops pass, en bloc. this is the current default.

- **`openacc-privatization`** — control whether the `-fopt-info-omp-note` and applicable `-fdump-tree-*-details` options emit OpenACC privatization diagnostics. with `--param=openacc-privatization=quiet`, don't diagnose. this is the current default. with `--param=openacc-privatization=noisy`, do diagnose.

- **`cycle-accurate-model`** — specifies whether GCC should assume that the scheduling description is mostly a cycle-accurate model of the target processor the code is intended to run on, in the absence of cache misses. nonzero means that the selected scheduling model is accurate and likely describes an in-order processor, and that scheduling should aggressively spill to try and fill any pipeline bubbles. this is the current default. zero means the scheduling description might not be available/accurate or perhaps not applicable at all, such as for modern out-of-order processors. very main character scheduling energy 👑

**the following choices of name are available on AArch64 targets:** 🦋

- **`aarch64-vect-compare-costs`** — when vectorizing, consider using multiple different approaches and use the cost model to choose the cheapest one. this includes:
  - trying both SVE and Advanced SIMD, when SVE is available.
  - trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit vectors for everything.
  - trying to use "unpacked" SVE vectors for smaller elements. this includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.

- **`aarch64-float-recp-precision`** — the number of Newton iterations for calculating the reciprocal for float type. the precision of division is proportional to this param when division approximation is enabled. the default value is `1`.

- **`aarch64-double-recp-precision`** — the number of Newton iterations for calculating the reciprocal for double type. the precision of division is proportional to this param when division approximation is enabled. the default value is `2`.

- **`aarch64-autovec-preference`** — force an ISA selection strategy for auto-vectorization.
  - `default` — use the default heuristics.
  - `asimd-only` — use only Advanced SIMD for auto-vectorization.
  - `sve-only` — use only SVE for auto-vectorization.
  - `prefer-asimd` — use both Advanced SIMD and SVE. prefer Advanced SIMD when the costs are deemed equal.
  - `prefer-sve` — use both Advanced SIMD and SVE. prefer SVE when the costs are deemed equal.

- **`aarch64-ldp-policy`** — fine-grained policy for load pairs. with `--param=aarch64-ldp-policy=default`, use the policy of the tuning structure. this is the current default. with `--param=aarch64-ldp-policy=always`, emit `ldp` regardless of alignment. with `--param=aarch64-ldp-policy=never`, do not emit `ldp`. with `--param=aarch64-ldp-policy=aligned`, emit `ldp` only if the source pointer is aligned to at least double the alignment of the type.

- **`aarch64-stp-policy`** — fine-grained policy for store pairs. with `--param=aarch64-stp-policy=default`, use the policy of the tuning structure. this is the current default. with `--param=aarch64-stp-policy=always`, emit `stp` regardless of alignment. with `--param=aarch64-stp-policy=never`, do not emit `stp`. with `--param=aarch64-stp-policy=aligned`, emit `stp` only if the source pointer is aligned to at least double the alignment of the type.

- **`aarch64-ldp-alias-check-limit`** — limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when attempting to form an `ldp`/`stp`. higher values make the pass more aggressive at re-ordering loads over stores, at the expense of increased compile time.

- **`aarch64-ldp-writeback`** — this one controls which writeback opportunities the AArch64 load/store pair fusion pass tries to handle 💅. A value of `0` (zero) disables writeback handling. A value of `1` (one) means we try to form pairs involving one or more existing individual writeback accesses where possible. A value of `2` (two) means we also try to opportunistically form writeback opportunities by folding in trailing destructive updates of the base register used by a pair. ✨

- **`aarch64-loop-vect-issue-rate-niters`** — the tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If this parameter is set to *n*, GCC will not use this heuristic for loops that are known to execute in fewer than *n* Advanced SIMD iterations. 🎀

- **`aarch64-vect-unroll-limit`** — the vectorizer will use available tuning information to determine whether it would be beneficial to unroll the main vectorized loop, and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four. 🦋

The following choices of `name` are available on GCN targets:

- **`gcn-preferred-vectorization-factor`** — preferred vectorization factor: `default`, `32`, `64`.

The following choices of `name` are available on i386 and x86_64 targets:

- **`x86-stlf-window-ninsns`** — instructions number above which STFL stall penalty can be compensated.

- **`x86-stv-max-visits`** — the maximum number of use and def visits when discovering a STV chain before the discovery is aborted.

---

## 🔬 Program Instrumentation Options 🔬

Okay so GCC supports a number of command-line options that control adding <pink>run-time instrumentation</pink> to the code it normally generates. For example, one purpose of instrumentation is collecting profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. It’s giving <rainbow>compiler detective mode</rainbow> fr fr 🔥

- **`-p`** — generate extra code to write profile information suitable for the analysis program `prof`.

- **`-pg`** — generate extra code to write profile information suitable for the analysis program `gprof`.

For both of these, you must use the option when compiling the source files you want data about, and you must also use it when linking. 💖

You can use the function attribute `"no_instrument_function"` to suppress profiling of individual functions when compiling with these options.

- **`-fprofile-arcs`** — add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable. 👑

  When the compiled program exits it saves this data to a file called `auxname.gcda` for each source file. The data may be used for profile-directed optimizations (`-fbranch-probabilities`), or for test coverage analysis (`-ftest-coverage`). Each object file's `auxname` is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. `foo.gcda` for input file `dir/foo.c`, or `dir/foo.gcda` for output file specified as `-o dir/foo.o`). 🫶

  Note that if a command line directly links source files, the corresponding `.gcda` files will be prefixed with the unsuffixed name of the output file. E.g. `gcc a.c b.c -o binary` would generate `binary-a.gcda` and `binary-b.gcda` files. No cap, that’s the naming tea ☕

- **`-fcondition-coverage`** — add code so that program conditions are instrumented. During execution the program records what terms in a conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read with `gcov --conditions`. 🌸

- **`-fpath-coverage`** — add code so that the paths taken are tracked. During execution the program records the prime paths taken. The number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up instrumentation if the approximate number of paths exceeds the limit controlled by `-fpath-coverage-limit`. The result can be read with `gcov --prime-paths --prime-paths-lines --prime-paths-source`. 💫

- **`-fpath-coverage-limit=limit`** — the threshold at which point `-fpath-coverage` gives up on instrumenting a function. This limit is approximate and conservative, as GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and gives up if the threshold is reached before finding all the paths. This option is not for fine grained control over which functions to instrument — rather it is intended to limit the effect of path explosion and keep compile times reasonable. The default is `250000`. 💀

- **`--coverage`** — this option is used to compile and link code instrumented for coverage analysis. The option is a synonym for `-fprofile-arcs -ftest-coverage` (when compiling) and `-lgcov` (when linking). See the documentation for those options for more details. ✨

  - Compile the source files with `-fprofile-arcs` plus optimization and code generation options. For test coverage analysis, use the additional `-ftest-coverage` option. You do not need to profile every source file in a program.

  - Compile the source files additionally with `-fprofile-abs-path` to create absolute path names in the `.gcno` files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.

  - Link your object files with `-lgcov` or `-fprofile-arcs` (the latter implies the former).

  - Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, `fork` calls are detected and correctly handled without double counting. 🧚

    Moreover, an object file can be recompiled multiple times and the corresponding `.gcda` file merges as long as the source file and the compiler options are unchanged.

  - For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus `-fbranch-probabilities`.

  - For test coverage analysis, use gcov to produce human readable information from the `.gcno` and `.gcda` files. Refer to the gcov documentation for further information.

With `-fprofile-arcs`, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. That’s lowkey iconic compiler behavior tbh 🎀

With `-fcondition-coverage`, for each conditional in your program GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression.

With `-fpath-coverage`, GCC finds and enumerates and records the taken prime paths of each function, unless the number of paths would exceed the limit controlled by `-fpath-coverage-limit`. If the limit is exceeded the function is not instrumented as if `-fpath-coverage` was not used. A prime path is the longest sequence of unique blocks, except possibly the first and last, which is not a subpath of any other path. 💜

- **`-ftest-coverage`** — produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called `auxname.gcno`. Refer to the `-fprofile-arcs` option above for a description of `auxname` and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. 🩷

- **`-fprofile-abs-path`** — automatically convert relative source file names to absolute path names in the `.gcno` files. This allows gcov to find the correct sources in projects where compilations occur with different working directories. 🌈

- **`-fprofile-dir=path`** — set the directory to search for the profile data files in to *path*. This option affects only the profile data generated by `-fprofile-generate`, `-ftest-coverage`, `-fprofile-arcs` and used by `-fprofile-use` and `-fbranch-probabilities` and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as *path*, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the `sourcename.gcda` file and use it as the file name of a `.gcda` file. See details about the file naming in `-fprofile-arcs`. See similar option `-fprofile-note`. 🦋

  When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in *path* that are exported during run-time:

  - `%p` — process ID.

  - `%q{VAR}` — value of environment variable `VAR`.

- **`-fprofile-generate`**
- **`-fprofile-generate=path`** — enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use `-fprofile-generate` both when compiling and when linking your program. 👑✨

The following options are enabled: `-fprofile-arcs`, `-fprofile-values`, `-finline-functions`, and `-fipa-bit-cp`.

If *path* is specified, GCC looks at the *path* to find the profile feedback data files. See `-fprofile-dir`.

To optimize the program based on the collected profile information, use `-fprofile-use`.

---

- **`-fprofile-info-section`** — okay so instead of using a constructor/destructor, this registers the profile information in the specified section. The section name is *name* if it is specified, otherwise the section name defaults to `.gcov_info`. A pointer to the profile information generated by `-fprofile-arcs` is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. <red>This option is not intended to be used in hosted environments such as GNU/Linux.</red> It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O.

- **`-fprofile-info-section=name`** — the named-section version, bestie: you pass *name* and that becomes the section name. Same constructor/destructor-disabling behavior as the plain option, and same “not for hosted environments like GNU/Linux” warning. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O.

The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:

.gcov_info : { PROVIDE (__gcov_info_start = .); KEEP (*(.gcov_info)) PROVIDE (__gcov_info_end = .); }


The program could dump the profiling information registered in this linker set for example like this:

#include <gcov.h> #include <stdio.h> #include <stdlib.h>

extern const struct gcov_info *const __gcov_info_start[]; extern const struct gcov_info *const __gcov_info_end[];

static void dump (const void *d, unsigned n, void *arg) { const unsigned char *c = d;

for (unsigned i = 0; i < n; ++i) printf ("%02x", c[i]); }

static void filename (const char *f, void *arg) { __gcov_filename_to_gcfn (f, dump, arg ); }

static void * allocate (unsigned length, void *arg) { return malloc (length); }

static void dump_gcov_info (void) { const struct gcov_info *const *info = __gcov_info_start; const struct gcov_info *const *end = __gcov_info_end;

/* Obfuscate variable to prevent compiler optimizations. */ __asm__ ("" : "+r" (info));

while (info != end) { void *arg = NULL; __gcov_info_to_gcda (*info, filename, dump, allocate, arg); putchar ('\n'); ++info; } }

int main (void) { dump_gcov_info (); return 0; }


The `merge-stream` subcommand of `gcov-tool` may be used to deserialize the data stream generated by the `__gcov_filename_to_gcfn` and `__gcov_info_to_gcda` functions and merge the profile information into `.gcda` files on the host filesystem.

- **`-fprofile-note=path`** — if *path* is specified, GCC saves `.gcno` file into *path* location. <red>If you combine the option with multiple source files, the `.gcno` file will be overwritten.</red> Like, heads up, bestie 💀

- **`-fprofile-prefix-path=path`** — this option can be used in combination with `profile-generate=profile_dir` and `profile-use=profile_dir` to inform GCC where is the base directory of built source tree. By default `profile_dir` will contain files with mangled absolute paths of all object files in the built project. This is not desirable when directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback because the profile data will not be found during the optimized build. In such setups `-fprofile-prefix-path=path` with *path* pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory.

- **`-fprofile-prefix-map=old=new`** — when compiling files residing in directory *old*, record profiling information (with `--coverage`) describing them as if the files resided in directory *new* instead. See also `-ffile-prefix-map` and `-fcanon-prefix-map`.

- **`-fprofile-update=method`** — alter the update method for an application instrumented for profile feedback based optimization. The *method* argument should be one of `single`, `atomic` or `prefer-atomic`. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code.

  <red>Warning:</red> when an application does not properly join all threads (or creates a detached thread), a profile file can be still corrupted.

  Using `prefer-atomic` would be transformed either to `atomic`, when supported by a target, or to `single` otherwise. The GCC driver automatically selects `prefer-atomic` when `-pthread` is present in the command line, otherwise the default method is `single`.

  If `atomic` is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, `libatomic` is available, then the profile information is updated through calls to `libatomic`. If the target platform neither supports the required atomic operations in hardware nor `libatomic`, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications.

  For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state.

- **`-fprofile-filter-files=regex`** — instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons). For example, `-fprofile-filter-files=main\.c;module.*\.c` will instrument only `main.c` and all C files starting with `'module'`.

- **`-fprofile-exclude-files=regex`** — instrument only functions from files whose name does **not** match any of the regular expressions (separated by semi-colons). For example, `-fprofile-exclude-files=/usr/.*` will prevent instrumentation of all files that are located in the `/usr/` folder.

- **`-fprofile-reproducible=[multithreaded|parallel-runs|serial]`** — control level of reproducibility of profile gathered by `-fprofile-generate`. This makes it possible to rebuild program with same outcome which is useful, for example, for distribution packages.

  - With `-fprofile-reproducible=serial` the profile gathered by `-fprofile-generate` is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before `fork` function is invoked.

    Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by `no_instrument_function` function attribute. `gcov-dump` with `-l` can be used to dump gathered data and verify that they are indeed reproducible.

  - With `-fprofile-reproducible=parallel-runs` collected profile stays reproducible regardless the order of streaming of the data into `gcda` files. This setting makes it possible to run multiple instances of instrumented program in parallel (such as with `make -j`). This reduces quality of gathered data, in particular of indirect call profiling.

- **`-fsanitize=address`** — enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables `-fsanitize-address-use-after-scope`. See `https://github.com/google/sanitizers/wiki/AddressSanitizer` for more details. The run-time behavior can be influenced using the `ASAN_OPTIONS` environment variable. When set to `help=1`, the available options are shown at startup of the instrumented program. See `https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags` for a list of supported options. The option cannot be combined with `-fsanitize=thread` or `-fsanitize=hwaddress`. Note that the only targets `-fsanitize=hwaddress` is currently supported on are x86-64 (only with `-mlam=u48` or `-mlam=u57` options) and AArch64, in both cases only in ABIs with 64-bit pointers.

  When compiling with `-fsanitize=address`, you should also use `-g` to produce more meaningful output. To get more accurate stack traces, it is possible to use options such as `-O0`, `-O1`, or `-Og` (which, for instance, prevent most function inlining), `-fno-optimize-sibling-calls` (which prevents optimizing sibling and tail recursive calls; this option is implicit for `-O0`, `-O1`, or `-Og`), or `-fno-ipa-icf` (which disables Identical Code Folding for functions). Using `-fno-omit-frame-pointer` also improves stack traces. Since multiple runs of the program may yield backtraces with different addresses due to ASLR (Address Space Layout Randomization), it may be desirable to turn ASLR off. On Linux, this can be achieved with:

setarch uname -m -R ./prog


- **`-fsanitize=kernel-address`** — enable AddressSanitizer for Linux kernel. See `https://github.com/google/kernel-sanitizers` for more details.

- **`-fsanitize=hwaddress`** — bestie, this is Hardware-assisted AddressSanitizer ✨ It uses a hardware trick where the top byte of a pointer gets straight-up ignored, so it can spot memory errors with super low memory overhead. Memory access instructions get instrumented to catch out-of-bounds and use-after-free bugs 💅 Turning this on also enables `-fsanitize-address-use-after-scope`. More deets at <https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html>. You can vibe-check the run-time behavior with the `HWASAN_OPTIONS` environment variable — set it to `"help=1"` and all the available options show up at startup of your instrumented program. Ngl this one cannot be combined with `-fsanitize=thread` or `-fsanitize=address`, and it's *currently only available on AArch64* 🫶

- **`-fsanitize=kernel-hwaddress`** — same Hardware-assisted AddressSanitizer energy, but for compiling the Linux kernel 👑 It's similar to `-fsanitize=kernel-address` but uses an alternate instrumentation method, and similar to `-fsanitize=hwaddress` but with the instrumentation differences you need for kernel compilation. Those differences exist to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.

> 💀 Note: this option has *different defaults* to `-fsanitize=hwaddress`. Instrumenting the stack and `alloca` calls are **not** on by default, but you can still get them by specifying the command-line options `--param hwasan-instrument-stack=1` and `--param hwasan-instrument-allocas=1` respectively. Also, using a random frame tag is not implemented for kernel instrumentation.

- **`-fsanitize=pointer-compare`** — instruments comparison operations (`<`, `<=`, `>`, `>=`) that have pointer operands. It has to be combined with either `-fsanitize=kernel-address` or `-fsanitize=address`, and it cannot be combined with `-fsanitize=thread`. Real talk though: by default the check is *disabled at run time*. To turn it on, add `"detect_invalid_pointer_pairs=2"` to the environment variable `ASAN_OPTIONS`. Using `"detect_invalid_pointer_pairs=1"` detects the invalid operation only when *both* pointers are non-null 🎀

- **`-fsanitize=pointer-subtract`** — instruments subtraction with pointer operands. Same rules, bestie: it must be combined with either `-fsanitize=kernel-address` or `-fsanitize=address`, and it cannot be combined with `-fsanitize=thread`. And fr fr, by default the check is disabled at run time — add `"detect_invalid_pointer_pairs=2"` to the environment variable `ASAN_OPTIONS` to enable it. Using `"detect_invalid_pointer_pairs=1"` detects invalid operation only when both pointers are non-null 💖

- **`-fsanitize=shadow-call-stack`** — enables ShadowCallStack, a security enhancement mechanism that protects programs against return address overwrites (like stack buffer overflows, oop 🙈). Here's how it slays: it saves a function's return address to a separately allocated shadow call stack in the function prologue, then restores the return address from the shadow call stack in the function epilogue. The instrumenting only happens in functions that actually need to save the return address to the stack.

Currently it only supports the aarch64 platform 🦋 It's specifically designed for linux kernels that enable the `CONFIG_SHADOW_CALL_STACK` option. For user space programs, runtime support is *not* currently provided in libc and libgcc — so if you wanna use this in user space you gotta provide your own runtime support. Heads up though: this may cause the ABI rules to be broken 😭

On aarch64, the instrumentation makes use of the platform register `"x18"`. That generally means any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag `-ffixed-x18`, otherwise functions compiled without `-ffixed-x18` might clobber `"x18"` and corrupt the shadow stack pointer 💀

Also, because there's no userspace runtime support, code compiled with ShadowCallStack **cannot use exception handling**. Use `-fno-exceptions` to turn off exceptions.

See <https://clang.llvm.org/docs/ShadowCallStack.html> for more details ✨

- **`-fsanitize=thread`** — enables ThreadSanitizer, a fast data race detector 🔥 Memory access instructions get instrumented to detect data race bugs. Details at <https://github.com/google/sanitizers/wiki#threadsanitizer>. Run-time behavior can be influenced using the `TSAN_OPTIONS` environment variable — the supported options are listed at <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>. This option cannot be combined with `-fsanitize=address` or `-fsanitize=leak`.

When compiling with `-fsanitize=thread`, you should also use `-g` so you get more meaningful output 👑

> Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (`-fnon-call-exceptions`).

- **`-fsanitize=leak`** — enables LeakSanitizer, a memory leak detector 🫧 This option only matters for linking of executables. The executable gets linked against a library that overrides `"malloc"` and other allocator functions. See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. Run-time behavior can be influenced using the `LSAN_OPTIONS` environment variable. Cannot be combined with `-fsanitize=thread` 🎀

- **`-fsanitize=undefined`** — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector ⭐ Various computations get instrumented to detect undefined behavior at runtime. See <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. Run-time behavior can be influenced using the `UBSAN_OPTIONS` environment variable. The current suboptions are, and I'm listing all of them because you deserve the full set 👑

- **`-fsanitize=shift`** — enables checking that the result of a shift operation is not undefined. Note that what exactly counts as undefined differs slightly between C and C++, and also between ISO C90 and C99, etc. This option has two suboptions: `-fsanitize=shift-base` and `-fsanitize=shift-exponent`.

- **`-fsanitize=shift-exponent`** — enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument 💅

- **`-fsanitize=shift-base`** — if the second argument of a shift operation is within range, check that the result of the shift operation is not undefined. Again, what exactly is considered undefined differs slightly between C and C++, and between ISO C90 and C99, etc.

- **`-fsanitize=integer-divide-by-zero`** — detects integer division by zero 😭

- **`-fsanitize=unreachable`** — with this option, the compiler turns the `"__builtin_unreachable"` call into a diagnostics message call instead. When you reach the `"__builtin_unreachable"` call, the behavior is undefined.

- **`-fsanitize=vla-bound`** — tells the compiler to check that the size of a variable length array is positive ✨

- **`-fsanitize=null`** — enables pointer checking. Specifically, an application built with this turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed to by a NULL pointer 💀

- **`-fsanitize=return`** — enables return statement checking. Programs built with this turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This one works in C++ only 🎀

- **`-fsanitize=signed-integer-overflow`** — enables signed integer overflow checking. It checks that the result of `"+"`, `"*"`, and both unary and binary `"-"` doesn't overflow in signed arithmetics. This also detects `"INT_MIN / -1"` signed division. Note that integer promotion rules must be taken into account — that is, the following is *not* an overflow:

signed char a = SCHAR_MAX; a++;


- **`-fsanitize=bounds`** — enables instrumentation of array bounds. Various out of bounds accesses get detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are **not** instrumented — with the exception of flexible array member-like arrays for which `"-fstrict-flex-arrays"` or `"-fstrict-flex-arrays="` options or `"strict_flex_array"` attributes say they shouldn't be treated like flexible array member-like arrays 👑

- **`-fsanitize=bounds-strict`** — enables strict instrumentation of array bounds. Most out of bounds accesses get detected, including flexible array member-like arrays. Initializers of variables with static storage are not instrumented 🦋

- **`-fsanitize=alignment`** — enables checking of alignment of pointers when they're dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object 💅

- **`-fsanitize=object-size`** — enables instrumentation of memory references using the `"__builtin_dynamic_object_size"` function. Various out of bounds pointer accesses get detected ✨

- **`-fsanitize=float-divide-by-zero`** — detects floating-point division by zero. Unlike other similar options, `-fsanitize=float-divide-by-zero` is *not* enabled by `-fsanitize=undefined`, because floating-point division by zero can be a legitimate way of obtaining infinities and NaNs 🌀

- **`-fsanitize=float-cast-overflow`** — enables floating-point type to integer conversion checking. Checks that the result of the conversion does not overflow. Unlike other similar options, `-fsanitize=float-cast-overflow` is *not* enabled by `-fsanitize=undefined`. Also heads up: this option does not work well with `"FE_INVALID"` exceptions enabled 😭

- **`-fsanitize=nonnull-attribute`** — enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the `"nonnull"` function attribute 🫶

- **`-fsanitize=returns-nonnull-attribute`** — okay so this one instruments `return` statements in functions that have the `returns_nonnull` function attribute, literally just to catch you returning a null value from a function that swore up and down it never would. Snitch behavior, but make it valid 💅

- **`-fsanitize=bool`** — instruments loads from `bool`. Load anything that isn't `0`/`1` and boom, run-time error. Your bool was supposed to be a bool bestie, not a whole personality 🎭

- **`-fsanitize=enum`** — instruments loads from an enum type. Load a value outside the range of values for that enum type and you get a run-time error. the enum said what it said 💀

- **`-fsanitize=vptr`** — instruments C++ member function calls, member accesses, and *some* conversions between pointers to base and derived classes, all to verify the referenced object actually has the correct dynamic type. gaslighting your own object? not on this flag's watch 🔍

- **`-fsanitize=pointer-overflow`** — instruments pointer arithmetics. If the pointer arithmetics overflows → run-time error, immediately, no notes ✨

- **`-fsanitize=builtin`** — instruments arguments to selected builtin functions. Pass an invalid value to one of those args and you get a run-time error. Like, e.g. passing `0` as the argument to `__builtin_ctz` or `__builtin_clz` invokes undefined behavior and is diagnosed by this option. caught in 4k 📸

> ngl bestie, note this: sanitizers tend to increase the rate of false positive warnings, most notably those around `-Wmaybe-uninitialized`. We recommend against combining `-Werror` and [the use of] sanitizers. Do NOT do it, it's giving chaos 😭

While `-ftrapv` causes traps for signed overflows to be emitted, `-fsanitize=undefined` gives a diagnostic message. This currently works only for the C family of languages. fyi 🫶

---

- **`-fno-sanitize=all`** — disables all previously enabled sanitizers. And `-fsanitize=all` is *not* allowed, because some sanitizers cannot be used together. they don't get along, it's a whole thing 🙈

- **`-fasan-shadow-offset=`*number* — forces GCC to use a custom shadow offset in AddressSanitizer checks. Useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. science mode 🧪

- **`-fsanitize-sections=`*s1,s2,...* — sanitize global variables in selected user-defined sections. The *si* may contain wildcards. pick your faves ✨

- **`-fsanitize-recover[=`*opts*]`** — controls error recovery mode for sanitizers mentioned in the comma-separated list of *opts*. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. Which means: multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported 👀 — yes, it'll lie to you, that's literally the point. The `-fno-sanitize-recover=` option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code. one strike and you're out ⚡

Currently this feature only works for `-fsanitize=undefined` (and its suboptions except for `-fsanitize=unreachable` and `-fsanitize=return`), `-fsanitize=float-cast-overflow`, `-fsanitize=float-divide-by-zero`, `-fsanitize=bounds-strict`, `-fsanitize=kernel-address` and `-fsanitize=address`. For these sanitizers error recovery is turned on by default, except `-fsanitize=address`, for which this feature is experimental. `-fsanitize-recover=all` and `-fno-sanitize-recover=all` is also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it. all or nothing energy, love that for us 💖

Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to `halt_on_error=0` for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is `halt_on_error=1`. This can be overridden through setting the `halt_on_error` flag in the corresponding environment variable. team effort fr fr 🤝

> ⚠️ Syntax without an explicit *opts* parameter is **deprecated**. It is equivalent to specifying an *opts* list of:

undefined,float-cast-overflow,float-divide-by-zero,bounds-strict


it's giving legacy, use the explicit form bestie 🎀

- **`-fsanitize-address-use-after-scope`** — enable sanitization of local variables to detect use-after-scope bugs. The option sets `-fstack-reuse` to `none`. your locals are being watched, respectfully 🕵️‍♀️

- **`-fsanitize-trap[=`*opts*]`** — instructs the compiler to report for sanitizers mentioned in the comma-separated list of *opts* undefined behavior using `__builtin_trap` rather than a `libubsan` library routine. If this option is enabled for a certain sanitizer, it takes precedence over the `-fsanitizer-recover=` for that sanitizer, and `__builtin_trap` will be emitted and be fatal regardless of whether recovery is enabled or disabled using `-fsanitize-recover=`. it does NOT care about your recovery era 😤

The advantage of this is that the `libubsan` library is not needed and is not linked in, so this is usable even in freestanding environments. no library, no problem 👑

Currently this feature works with `-fsanitize=undefined` (and its suboptions except for `-fsanitize=vptr`), `-fsanitize=float-cast-overflow`, `-fsanitize=float-divide-by-zero` and `-fsanitize=bounds-strict`. `-fsanitize-trap=all` can be also specified, which enables it for `undefined` suboptions, `-fsanitize=float-cast-overflow`, `-fsanitize=float-divide-by-zero` and `-fsanitize=bounds-strict`. If `-fsanitize-trap=undefined` or `-fsanitize-trap=all` is used and `-fsanitize=vptr` is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs `libubsan` support, `-fsanitize-trap=vptr` is not allowed. 🙅‍♀️

- **`-fsanitize-undefined-trap-on-error`** — this is the **deprecated** equivalent of `-fsanitize-trap=all`. rip to an icon, use the new one 🪦

- **`-fsanitize-coverage=trace-pc`** — enable coverage-guided fuzzing code instrumentation. Inserts a call to `__sanitizer_cov_trace_pc` into every basic block. every. single. one. 🦋

- **`-fsanitize-coverage=trace-cmp`** — enable dataflow guided fuzzing code instrumentation. Inserts a call to `__sanitizer_cov_trace_cmp1`, `__sanitizer_cov_trace_cmp2`, `__sanitizer_cov_trace_cmp4` or `__sanitizer_cov_trace_cmp8` for integral comparison with both operands variable, or `__sanitizer_cov_trace_const_cmp1`, `__sanitizer_cov_trace_const_cmp2`, `__sanitizer_cov_trace_const_cmp4` or `__sanitizer_cov_trace_const_cmp8` for integral comparison with one operand constant, `__sanitizer_cov_trace_cmpf` or `__sanitizer_cov_trace_cmpd` for float or double comparisons, and `__sanitizer_cov_trace_switch` for switch statements. that's a lot of names but every one of them is doing their job 💅

---

- **`-fcf-protection=[full|branch|return|none|check]`**
- **`-fcf-protection`**

Enable code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. It's intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). security girly pop 🛡️✨

The `-fcf-protection=` keywords are interpreted as follows:

- The value `branch` tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions.
- The value `return` implements checking of validity at the point of returning from a function.
- The value `full` is an alias for specifying both `branch` and `return`.
- The value `check` is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different `-fcf-protection` values. The value `check` is ignored at the compile time. 👀
- The value `none` turns off instrumentation. bye 👋

`-fcf-protection` is an alias for `-fcf-protection=full`. To override a previous `-fcf-protection` option on the command line, add `-fcf-protection=none` and then `-fcf-protection=`*kind*. order matters bestie, it's a whole sequence 🔁

The macro `__CET__` is defined when `-fcf-protection` is used. The first bit of `__CET__` is set to 1 for the value `branch` and the second bit of `__CET__` is set to 1 for the `return`. bit math, iconic 💜

You can also use the `nocf_check` attribute to identify which functions and calls should be skipped from instrumentation. exempt list 😌

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. x86 girlies stay winning 💻

- **`-fharden-compares`** — for every logical test that survives gimple optimizations and is *not* the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call `__builtin_trap` if the results do not match. Use with `-fharden-conditional-branches` to cover all conditionals. double-check era 🔍🔍

- **`-fharden-conditional-branches`** — for every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call `__builtin_trap` if the result is unexpected. Use with `-fharden-compares` to cover all conditionals. matching set, we love a coordinated moment 🎀

- **`-fharden-control-flow-redundancy`** — emit extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans do not form an execution path that is compatible with the control flow graph. detective mode fr 🔎

Verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with `-fhardcfr-check-exceptions`, before returning calls with `-fhardcfr-check-returning-calls`, and before noreturn calls with `-fhardcfr-check-noreturn-calls`. Tuning options `--param hardcfr-max-blocks` and `--param hardcfr-max-inline-blocks` are available. knobs! we love knobs 🎛️

Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls would also have verification issued before the call — but these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. theory only, don't stress 🧚

- **`-fhardcfr-skip-leaf`** — disable `-fharden-control-flow-redundancy` in leaf functions. little guys get a pass 🍓

- **`-fhardcfr-check-exceptions`** — okay so <pink>when</pink> `-fharden-control-flow-redundancy` is active, this bad boy checks the recorded execution path against the control flow graph at <purple>exception escape points</purple> — as if the function body was wrapped with a cleanup handler that did the check and then reraised. It's <green>enabled by default</green> ✨, so if you want it gone, hit it with `-fno-hardcfr-check-exceptions`. 👑

- **`-fhardcfr-check-returning-calls`** — when `-fharden-control-flow-redundancy` is active, this checks the recorded execution path against the control flow graph <cyan>before any function call that's immediately followed by a return of its result</cyan> (if any), specifically so it doesn't block tail-call optimization — and that holds <italic>whether or not</italic> it actually ends up optimized into a tail call. 💅

It's enabled by default whenever sibling call optimizations are on (peep `-foptimize-sibling-calls` 👀), but you can totally enable it (or disable it, using its negated form) explicitly, regardless of what the optimizations are doing.

- **`-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]`** — when `-fharden-control-flow-redundancy` is active, this one checks the recorded execution path against the control flow graph before `noreturn` calls. Which ones? Pick your fighter 🥊:

- `always` — literally all of them
- `no-xthrow` — the ones that aren't expected to return control to the caller through an exception (**this is the default**, bestie)
- `nothrow` — the ones that may not return control to the caller through an exception either
- `never` — none of them, we simply do not look

Now, real talk 🙈: checking before a `noreturn` function that <hotpink>may</hotpink> return control to the caller through an exception can cause the check to happen <red>more than once</red>, if that exception gets caught in the caller — whether by a handler or a cleanup. When `-fhardcfr-check-exceptions` is also enabled, the compiler will avoid associating a `noreturn` call with the implicitly-added cleanup handler, since that'd be redundant with the check done before the call. But other handlers or cleanups in the function, if they get activated, will modify the recorded execution path and check it <italic>again</italic> whenever another checkpoint is hit. And plot twist: that checkpoint may even be another `noreturn` call, so the checking can end up happening multiple times. 😭

Also, various optimizers may cause calls to be marked as `noreturn` and/or `nothrow`, even when the matching attributes aren't there — which can affect where checks get placed before calls, plus the addition of implicit cleanup handlers for them. That unpredictability, combined with the fact that raising and reraising exceptions frequently amounts to implicitly calling `noreturn` functions, is exactly why `no-xthrow` is the default here: it excludes from `noreturn` treatment <purple>only</purple> the internal functions used to (re)raise exceptions, which aren't touched by those optimizations. 🧚

- **`-fhardened`** — turns on a whole little squad of flags for C and C++ that improve the security of the generated code <green>without affecting its ABI</green>. The exact flags may change between major GCC releases, but right now they are:

-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS -ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now -fstack-protector-strong -fstack-clash-protection -fcf-protection=full (x86 GNU/Linux only)


  Wanna see the whole list? Generate it with the `--help=hardened` option. 🎀

  If your system glibc is older than 2.35, `-D_FORTIFY_SOURCE=2` gets used instead. Ngl, this option is meant for <hotpink>production builds</hotpink> — not just debug builds, okay?

  Right now `-fhardened` is only supported on GNU/Linux targets. 🌸

  And here's the cute part: `-fhardened` only enables a particular option if it wasn't already specified <italic>anywhere</italic> on the command line. So like, `-fhardened -fstack-protector` will only enable `-fstack-protector`, but <red>not</red> `-fstack-protector-strong`. Facts. 💅

- **`-fstack-protector`** — emits extra code to check for buffer overflows, like those stack smashing attacks. How? By adding a guard variable to functions with vulnerable objects 🛡️. That includes functions that call `alloca`, and functions with buffers larger than or equal to 8 bytes. The guards get initialized when a function is entered and then checked when the function exits. If a guard check fails: error message printed, program exits, bye 👋. Only variables that are <italic>actually</italic> allocated on the stack count — optimized-away variables or variables allocated in registers don't count, sorry bestie.

- **`-fstack-protector-all`** — same as `-fstack-protector` except <rainbow>all functions</rainbow> are protected. Everyone. No exceptions. 👑

- **`-fstack-protector-strong`** — like `-fstack-protector` but it adds extra functions to the protected club 💖 — the ones that have local array definitions, or that have references to local frame addresses. Same deal as before: only variables actually allocated on the stack count, optimized-away or register-allocated variables don't. 🫶

- **`-fstack-protector-explicit`** — like `-fstack-protector` but it <italic>only</italic> protects those functions that have the `stack_protect` attribute. VIP list only. 🎀

- **`-fstack-check`** — generates code to verify you don't go beyond the boundary of the stack. Specify this flag if you're running in an environment with multiple threads 🧵 — but you only rarely need it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems when there's only one stack.

  > Heads up: this switch doesn't actually <italic>cause</italic> checking to be done — the operating system or the language runtime has to do that. The switch just causes generation of code to make sure they <italic>see</italic> the stack being extended. 👀

  You can also throw in a string parameter: `no` means no checking, `generic` means force the use of old-style checking, `specific` means use the best checking method and is equivalent to a bare `-fstack-check`.

  Old-style checking is a generic mechanism that needs no specific target support in the compiler, but it comes with these drawbacks 💀:

  1. Modified allocation strategy for large objects: they're always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
  2. Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking is not reliable and a warning gets issued by the compiler.
  3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. 😭

  Note that old-style stack checking is also the fallback method for `specific` if no target support has been added in the compiler.

  So `-fstack-check=` is designed for Ada's needs — detecting infinite recursion and stack overflows. `specific` is a genuinely excellent choice when compiling Ada code. But it is <red>not</red> generally sufficient to protect against stack-clash attacks. For those you want `-fstack-clash-protection` instead. 🔥

- **`-fstack-clash-protection`** — generates code to prevent stack clash style attacks. When this is enabled, the compiler only allocates <purple>one page of stack space at a time</purple>, and each page is accessed immediately after allocation. So it stops allocations from jumping over any stack guard page the operating system provides. 🛡️✨

  Most targets don't fully support stack clash protection. But on those targets, `-fstack-clash-protection` will still protect dynamic stack allocations. It may also give limited protection for static stack allocations if the target supports `-fstack-check=specific`. 💜

- **`-fstack-limit-register=reg`**, **`-fstack-limit-symbol=sym`**, **`-fno-stack-limit`** — generates code to ensure the stack doesn't grow beyond a certain value — either the value of a register or the address of a symbol. If a bigger stack is required, a signal gets raised at run time 🚨. For most targets, the signal is raised <italic>before</italic> the stack overruns the boundary, so you can catch the signal without taking special precautions.

  For instance, if the stack starts at absolute address `0x80000000` and grows downwards, you can use the flags `-fstack-limit-symbol=__stack_limit` and `-Wl,--defsym,__stack_limit=0x7ffe0000` to enforce a stack limit of 128KB. Note this may only work with the GNU linker. 🫶

  And you can locally override stack limit checking by using the `no_stack_limit` function attribute.

- **`-fsplit-stack`** — generates code to automatically split the stack <italic>before</italic> it overflows. The resulting program has a discontiguous stack which can only overflow if the program can't allocate any more memory. Super useful when running threaded programs, because you no longer have to calculate a good stack size for each thread 🧵💖. This is currently only implemented for the x86 targets running GNU/Linux.

  Here's the catch 😬: when code compiled with `-fsplit-stack` calls code compiled <italic>without</italic> `-fsplit-stack`, there may not be much stack space available for that latter code to run. If compiling all code (including library code) with `-fsplit-stack` isn't an option, the linker can fix up these calls so the code compiled without `-fsplit-stack` always has a large stack. Support for that is implemented in the gold linker in GNU binutils release 2.21 and later. ✨

- **`-fstrub=disable`** — disables stack scrubbing entirely, ignoring any `strub` attributes. See — and that's literally it, the sentence just says "See" and then vibes off into the sunset 😭🙈 (not me, that's genuinely how it reads).

- **`-fstrub=strict`** — functions default to `strub` mode `disabled`, and it applies strictly the restriction that only functions associated with `strub`-`callable` modes (`at-calls`, `callable` and `always_inline` `internal`) are `callable` by functions with `strub`-enabled modes (`at-calls` and `internal`). 💅

- **`-fstrub=relaxed`** — restores the default stack scrub (`strub`) setting, namely, `strub` is only enabled as required by `strub` attributes associated with function and data types. "Relaxed" means strub contexts are only prevented from calling functions explicitly associated with `strub` mode `disabled`. This option is only useful to override other `-fstrub=*` options that precede it in the command line. 🦋

- **`-fstrub=at-calls`** — turns on the <pink>"at-calls"</pink> `"strub"` mode wherever it's viable 🫶. bestie this one's mostly a testing thing — it exercises the `"strub"` machinery in scenarios that are *strictly local to a translation unit*. quick heads up though: this `"strub"` mode messes with function interfaces, so any function that's visible to other translation units, or that has its address taken, just won't be affected by it, periodt. optimization options can also affect viability, so like… vibe check your flags. peek at the `"strub"` attribute documentation for the deets on viability and eligibility requirements ✨

- **`-fstrub=internal`** — turns on <purple>"internal"</purple> `"strub"` mode where viable 💜. again, primary use is testing. this one's whole goal is to thoroughly exercise the parts of the `"strub"` machinery that implement the less efficient, but interface-preserving `"strub"` mode. functions that *wouldn't* be affected by this option are genuinely quite uncommon, ngl 👑

- **`-fstrub=all`** — enables some `"strub"` mode where viable 🌈. when both strub modes are viable, `"at-calls"` is the favourite (preferred, obviously 💅). `-fdump-ipa-strubm` adds function attributes that tell which mode was picked for each function. primary use? testing — to exercise the `"strub"` machinery thoroughly. no cap.

- **`-fvtable-verify=[std|preinit|none]`** — okay so this one's C++ only 🎀 (only available when compiling C++ code). it flips on (or off, if you're using `-fvtable-verify=none`) the security feature that verifies at run time, for every single virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. and if an invalid vtable pointer gets detected at run time? an error is reported and execution of the program is *immediately halted* 💀 like, lights out.

  this option causes run-time data structures to be built at program startup, which are then used for verifying the vtable pointers. the `std` and `preinit` options control the *timing* of when these data structures are built. in both cases the data structures are built before execution reaches `"main"`. using `-fvtable-verify=std` causes the data structures to be built *after* shared libraries have been loaded and initialized. `-fvtable-verify=preinit` causes them to be built *before* shared libraries have been loaded and initialized 🌸

  and if this option shows up multiple times in the command line with different values specified — `none` takes highest priority over both `std` and `preinit`; `preinit` takes priority over `std`. iykyk 💅

- **`-fvtv-debug`** — when you pair this with `-fvtable-verify=std` or `-fvtable-verify=preinit`, it causes debug versions of the runtime functions for the vtable verification feature to be called 🦋. this flag also causes the compiler to log information about which vtable pointers it finds for each class. that info gets written to a file named `vtv_set_ptr_data.log` in the directory named by the environment variable `VTV_LOGS_DIR` if that is defined, or the current working directory otherwise.

  > note: this feature **appends** data to the log file. if you want a fresh log file, be sure to delete any existing one first, bestie 🙈

- **`-fvtv-counts`** — this one's a debugging flag 🎀. when used in conjunction with `-fvtable-verify=std` or `-fvtable-verify=preinit`, it causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. it also counts the number of calls to certain run-time library functions that it inserts, and logs this information for each compilation unit. the compiler writes all that to a file named `vtv_count_data.log` in the directory named by the environment variable `VTV_LOGS_DIR` if that is defined, or the current working directory otherwise. it also counts the size of the vtable pointer sets for each class, and writes *that* information to `vtv_class_set_sizes.log` in the same directory ✨

  > note: this feature **appends** data to the log files. to get fresh log files, be sure to delete any existing ones. seriously, delete them 🫶

- **`-finstrument-functions`** — generates instrumentation calls for entry and exit to functions 🔥. just after function entry and just before function exit, the following profiling functions get called with the address of the current function and its call site. (on some platforms, `"__builtin_return_address"` does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

void __cyg_profile_func_enter (void *this_fn, void *call_site); void __cyg_profile_func_exit (void *this_fn, void *call_site);


the first argument is the address of the start of the current function, which may be looked up exactly in the symbol table 💜

this instrumentation is also done for functions expanded inline in other functions. the profiling calls indicate where, conceptually, the inline function is entered and exited. this means that addressable versions of such functions must be available 😭. if all your uses of a function are expanded inline, this may mean an additional expansion of code size. if you use `"extern inline"` in your C code, an addressable version of such functions must be provided. (this is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies — queen behaviour, honestly 👑)

a function may be given the attribute `"no_instrument_function"`, in which case this instrumentation is not done. this can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). use it wisely bestie 🌸

- **`-finstrument-functions-once`** — this is similar to `-finstrument-functions`, but the profiling functions are called only *once* per instrumented function 🦋. i.e. the first profiling function is called after the first entry into the instrumented function, and the second profiling function is called before the exit corresponding to this first entry.

the definition of `"once"` for the purpose of this option is a little vague, because the implementation is not protected against data races 😬. as a result, the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread, but the calls are always paired — that is to say, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function. fr fr, that's the deal.

- **`-finstrument-functions-exclude-file-list=`*`file,file,...`* — sets the list of functions that are excluded from instrumentation (see the description of `-finstrument-functions` above 👀). if the file that contains a function definition matches with one of *file*, then that function is not instrumented. the match is done on substrings: if the *file* parameter is a substring of the file name, it's considered to be a match ✨

for example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys


excludes any inline function defined in files whose pathnames contain `/bits/stl` or `include/sys` 💅

if, for some reason, you want to include letter `,` in one of *sym*, write `,`. for example, `-finstrument-functions-exclude-file-list=',,tmp'` (note the single quote surrounding the option) 🎀

- **`-finstrument-functions-exclude-function-list=`*`sym,sym,...`* — this is similar to `-finstrument-functions-exclude-file-list`, but this option sets the list of *function names* to be excluded from instrumentation 💜. the function name to be matched is its user-visible name, such as `vector<int> blah(const vector<int> &)`, not the internal mangled name (e.g., `_Z4blahRSt6vectorIiSaIiEE`). the match is done on substrings: if the *sym* parameter is a substring of the function name, it's considered to be a match. for C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names ⭐

- **`-fpatchable-function-entry=`*`N[,M]`* — generates *N* NOPs right at the beginning of each function, with the function entry point before the *M*th NOP 🔥. if *M* is omitted, it defaults to `0`, so the function entry points to the address just at the first NOP. the NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time, provided that the code segment is writable. the amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface `"gen_nop"`. this behavior is target-specific and may also depend on the architecture variant and/or other compilation options 🦋

for run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus *M* — are additionally collected in the `"__patchable_function_entries"` section of the resulting binary ✨

note that the value of `__attribute__ ((patchable_function_entry (N,M)))` takes precedence over command-line option `-fpatchable-function-entry=N,M`. this can be used to increase the area size or to remove it completely on a single function. if `"N=0"`, no pad location is recorded 💅

the NOP instructions are inserted at---and maybe before, depending on *M*---the function entry address, even before the prologue. on PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. see the `-msplit-patch-nops` option to change this 👑

the maximum value of *N* and *M* is `65535`. on PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for *M* are `0`, `2`, `6` and `14` when not using `-msplit-patch-nops`.

---

## ✨ Options Controlling the Preprocessor ✨

these options control the C preprocessor, which is run on each C source file *before* actual compilation 🎀

if you use the `-E` option, nothing is done except preprocessing. some of these options make sense only together with `-E` because they cause the preprocessor output to be unsuitable for actual compilation 😭 — like, don't try to compile that output, bestie.

in addition to the options listed here, there are a number of options to control search paths for include files documented in **Directory Options**. options to control preprocessor diagnostics are listed in **Warning Options** 🌸

- **`-D `** *`name`* — predefine *name* as a macro, with definition `1`. iconic, simple, slay 💅

okay bestie, we're still deep in the preprocessor-flag section and this batch is lowkey the build-system backbone 💅✨. no cap, dependency generation is about to eat.

- **`-D name=definition`** — the contents of *definition* get tokenized and processed like they showed up during translation phase three in a `#define` directive. Specifically, the definition is truncated by embedded newline characters.

If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in shell syntax.

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With `sh` and `csh`, `-D'name(args...)=definition'` works.

`-D` and `-U` options are processed in the order they're given on the command line. All `-imacros file` and `-include file` options are processed after all `-D` and `-U` options.

- **`-U name`** — cancel any previous definition of *name*, either built in or provided with a `-D` option.

- **`-include file`** — process *file* as if `#include "file"` appeared as the first line of the primary source file. BUT: the first directory searched for *file* is the preprocessor's working directory instead of the directory containing the main source file. If it's not found there, it's searched for in the remainder of the `#include "..."` search chain as normal.

If multiple `-include` options are given, the files are included in the order they appear on the command line.

- **`-imacros file`** — exactly like `-include`, except any output produced by scanning *file* is thrown away. Macros it defines remain defined. This lets you acquire all the macros from a header without also processing its declarations.

All files specified by `-imacros` are processed before all files specified by `-include`.

- **`-undef`** — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.

- **`-pthread`** — define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.

- **`-M`** — instead of outputting the result of preprocessing, output a rule suitable for `make` describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from `-include` or `-imacros` command-line options.

Unless specified explicitly (with `-MT` or `-MQ`), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files, then the rule is split into several lines using `\-newline`. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as `-dM`. To avoid mixing such debug output with the dependency rules, you should explicitly specify the dependency output file with `-MF`, or use an environment variable like `DEPENDENCIES_OUTPUT`. Debug output is still sent to the regular output stream as normal.

Passing `-M` to the driver implies `-E`, and suppresses warnings with an implicit `-w`.

- **`-MM`** — like `-M` but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an `#include` directive does not in itself determine whether that header appears in `-MM` dependency output.

- **`-MF file`** — when used with `-M` or `-MM`, specifies a file to write the dependencies to. If no `-MF` switch is given, the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options `-MD` or `-MMD`, `-MF` overrides the default dependency output file.

If *file* is `-`, then the dependencies are written to `stdout`.

- **`-MG`** — in conjunction with an option such as `-M` requesting dependency generation, `-MG` assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the `#include` directive without prepending any path. `-MG` also suppresses preprocessed output, as a missing header file renders this useless.

This feature is used in automatic updating of makefiles.

- **`-Mno-modules`** — disable dependency generation for compiled module interfaces.

- **`-MP`** — this option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors `make` gives if you remove header files without updating the Makefile to match.

This is typical output:

test.o: test.c test.h

test.h:


- **`-MT target`** — change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as `.c`, and appends the platform's usual object suffix. The result is the target.

An `-MT` option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to `-MT`, or use multiple `-MT` options.

For example, `-MT '$(objpfx)foo.o'` might give:

$(objpfx)foo.o: foo.c


- **`-MQ target`** — same as `-MT`, but it quotes any characters which are special to Make. `-MQ '$(objpfx)foo.o'` gives:

$$(objpfx)foo.o: foo.c


The default target is automatically quoted, as if it were given with `-MQ`.

- **`-MD`** — `-MD` is equivalent to `-M -MF file`, except that `-E` is not implied. The driver determines *file* based on whether an `-o` option is given. If it is, the driver uses its argument but with a suffix of `.d`, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a `.d` suffix.

If `-MD` is used in conjunction with `-E`, any `-o` switch is understood to specify the dependency output file, but if used without `-E`, each `-o` is understood to specify a target object file.

Since `-E` is not implied, `-MD` can be used to generate a dependency output file as a side effect of the compilation process.

- **`-MMD`** — like `-MD` except mention only user header files, not system header files.

- **`-fpreprocessed`** — indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with `-C` to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.

`-fpreprocessed` is implicit if the input file has one of the extensions `.i`, `.ii` or `.mi`. These are the extensions that GCC uses for preprocessed files created by `-save-temps`.

- **`-fdirectives-only`** — when preprocessing, handle directives, but do not expand macros.

The option's behavior depends on the `-E` and `-fpreprocessed` options.

With `-E`, preprocessing is limited to the handling of directives such as `#define`, `#ifdef`, and `#error`. Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. In addition, the `-dD` option is implicitly enabled.

With `-fpreprocessed`, predefinition of command line and most builtin macros is disabled. Macros such as `__LINE__`, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with `-E -fdirectives-only`.

With both `-E` and `-fpreprocessed`, the rules for `-fpreprocessed` take precedence. This enables full preprocessing of files previously preprocessed with `-E -fdirectives-only`.

- **`-fdollars-in-identifiers`** — accept `$` in identifiers.

- **`-fextended-identifiers`** — accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++.

- **`-fno-canonical-system-headers`** — when preprocessing, do not shorten system header paths with canonicalization.

- **`-fmax-include-depth=depth`** — set the maximum depth of the nested `#include`. The default is `200`.

- **`-fsearch-include-path[=kind]`** — look for input files on the `#include` path, not just the current directory. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly:

g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc


*kind* defaults to `user`, which looks on the `#include "..."` search path; you can also explicitly specify `system` for the `#include <...>` search path.

- **`-ftabstop=width`** — set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than `1` or greater than `100`, the option is ignored. The default is `8`.

- **`-ftrack-macro-expansion[=level]`** — track locations of tokens across macro expansions. This allows the compiler to emit diagnostics about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The *level* parameter can be used to choose the level of precision of token location tracking, thus decreasing memory consumption if necessary. Value `0` of *level* de-activates this option. Value `1` tracks token locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value `2` tracks token locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is `2`.

Note that `-ftrack-macro-expansion=2` is activated by default.

- **`-fmacro-prefix-map=old=new`** — okay so when the preprocessor is chewing on files that live in directory *old*, it expands the `__FILE__` and `__BASE_FILE__` macros as if those files were chillin' in directory *new* instead 💅. Big use case: flipping an absolute path into a relative one by setting *new* to `.`, which gives you way more reproducible builds that don't care where they're built. This one also affects `__builtin_FILE()` during compilation, jsyk. Peep `-ffile-prefix-map` and `-fcanon-prefix-map` too ✨

- **`-fexec-charset=charset`** — sets the execution character set, the one used for string and character constants 🎀. Default is `UTF-8`. And *charset* can be literally any encoding your system's `iconv` library routine supports — go wild, bestie.

- **`-fwide-exec-charset=charset`** — same vibe but for the <pink>wide</pink> execution character set, used for wide string and character constants 🌸. Default is one of `UTF-32BE`, `UTF-32LE`, `UTF-16BE`, or `UTF-16LE` — whichever matches the width of `wchar_t` plus the big-endian or little-endian byte order being used for code generation. Just like `-fexec-charset`, *charset* can be any encoding your system's `iconv` library routine supports. BUT ⚠️ — encodings that don't fit exactly in `wchar_t` will give you problems. You've been warned.

- **`-finput-charset=charset`** — sets the input character set, used to translate from the input file's character set into the source character set GCC uses 💖. If the locale doesn't specify it, or GCC can't get that info from the locale, the default is `UTF-8`. Either the locale or this command-line option can override it — and currently the command-line option wins if there's a conflict, no cap. *charset* can be any encoding supported by the system's `iconv` library routine.

- **`-fpch-deps`** — precompiled header hours 🦋. This flag makes the dependency-output flags also list the files from the precompiled header's dependencies. If you *don't* specify it, only the precompiled header gets listed, not the files used to create it — because those files are never consulted when a precompiled header is used, so like, why would they be.

- **`-fpch-preprocess`** — lets you use a precompiled header together with `-E` ✨. It inserts a special `#pragma`, specifically `#pragma GCC pch_preprocess "filename"`, into the output to mark the spot where the precompiled header was found, plus its filename. When `-fpreprocessed` is in use, GCC recognizes this `#pragma` and loads the PCH. It's off by default because the resulting preprocessed output is only really suitable as input to GCC — `-save-temps` switches it on for you. Also: don't write this `#pragma` in your own code, but it's totally safe to edit the filename if the PCH file lives somewhere else. The filename can be absolute or relative to GCC's current directory 🫶

- **`-fworking-directory`** — turns on generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing 👑. With this on, the preprocessor emits a second linemarker right after the initial one, containing the current working directory followed by two slashes. GCC uses that directory — when it's present in the preprocessed input — as the directory emitted as the current working directory in some debugging information formats. It's implicitly enabled when debugging information is enabled, but you can inhibit it with the negated form `-fno-working-directory`. And if `-P` is on the command line, this option does nothing at all, because literally no `#line` directives get emitted. 💀

- **`-A predicate=answer`** — makes an assertion with predicate *predicate* and answer *answer*. This form is preferred over the older `-A predicate(answer)` form, which is still supported, because it doesn't use shell special characters 😌 slay.

- **`-A -predicate=answer`** — cancels an assertion with predicate *predicate* and answer *answer*. Bye bestie 👋

- **`-C`** — do not discard comments 🎀. All comments get passed through to the output file, except comments in processed directives — those get deleted along with the directive. Be prepared for side effects though: `-C` makes the preprocessor treat comments as tokens in their own right. For example, comments sitting at the start of what would be a directive line turn that line into an ordinary source line, because the first token on the line is no longer a `#` 😭

- **`-CC`** — do not discard comments, *including during macro expansion* 🔥. It's like `-C`, except comments inside macros also get passed through to the output file wherever the macro is expanded. On top of all the `-C` side effects, `-CC` converts all C++-style comments inside a macro into C-style comments — this is so later use of that macro doesn't accidentally comment out the rest of the source line. Generally used to support lint comments 💜

- **`-P`** — inhibit generation of linemarkers in the output from the preprocessor. Handy when you're running the preprocessor on something that isn't C code and it's headed to a program that would get confused by linemarkers 🌈

- **`-traditional`** / **`-traditional-cpp`** — try to imitate the behavior of pre-standard C preprocessors instead of ISO C preprocessors 🦋. The GNU CPP manual has the details. Note: GCC does *not* otherwise try to emulate a pre-standard C compiler, and these options are only supported with the `-E` switch or when invoking CPP explicitly.

- **`-trigraphs`** — support ISO C trigraphs ✨. These are three-character sequences all starting with `??` that ISO C defines to stand for single characters. For example, `??/` stands for `\`, so `'??/n'` is a character constant for a newline. The nine trigraphs and their replacements are:

Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??- Replacement: [ ] { } # \ ^ | ~


  By default GCC ignores trigraphs, but in standard-conforming modes it converts them. Check `-std` and `-ansi` 👀

- **`-remap`** — enable special code to work around file systems that only allow very short file names, like MS-DOS 🫶 retro.

- **`-H`** — print the name of each header file used, on top of other normal activities 📋. Each name is indented to show how deep in the `#include` stack it is. Precompiled header files get printed too, even if they turn out to be invalid — an invalid precompiled header prints with `...x` and a valid one prints with `...!` ⭐

- **`-dletters`** — says to make debugging dumps during compilation as specified by *letters* 🔍. The flags documented here are the ones relevant to the preprocessor; other letters get interpreted by the compiler proper or reserved for future GCC versions, so they're silently ignored. And if you specify letters whose behavior conflicts, the result is **undefined** 💀 (so maybe don't).

  - **`-dM`** — instead of the normal output, generate a list of `#define` directives for all the macros defined during the preprocessor's execution, including predefined macros 💖. This is your cheat code for finding out what's predefined in your version of the preprocessor. Assuming you have no file `foo.h`, the command

touch foo.h; cpp -dM foo.h


    shows all the predefined macros. Oh and heads up — if you use `-dM` without `-E`, then `-dM` is interpreted as a synonym for `-fdump-rtl-mach` 🙈

  - **`-dD`** — like `-dM` except it outputs both the `#define` directives *and* the result of preprocessing 🌸. Both kinds of output go to the standard output file.

  - **`-dN`** — like `-dD`, but emits only the macro names, not their expansions ✨

  - **`-dI`** — outputs `#include` directives in addition to the result of preprocessing 🎀

  - **`-dU`** — like `-dD` except only macros that are expanded, or whose definedness is tested in preprocessor directives, get output 💅. The output is delayed until the use or test of the macro, and `#undef` directives are also output for macros tested but undefined at the time.

- **`-fdebug-cpp`** — only useful for debugging GCC, fr fr 🔧. When used from CPP or with `-E`, it dumps debugging information about location maps — every token in the output gets preceded by the dump of the map its location belongs to. When used from GCC without `-E`, this option has no effect 😴

- **`-Wp,option`** — you can use `-Wp,option` to bypass the compiler driver and pass *option* directly through to the preprocessor 🚀. If *option* contains commas, it gets split into multiple options at the commas. BUT — many options are modified, translated, or interpreted by the compiler driver before being passed to the preprocessor, and `-Wp` forcibly bypasses that whole phase 😭. The preprocessor's direct interface is undocumented and subject to change, so whenever possible, avoid `-Wp` and let the driver handle the options instead. Trust the driver, bestie.

- **`-Xpreprocessor option`** — pass *option* as an option to the preprocessor 🦋. Use this for system-specific preprocessor options that GCC doesn't recognize. If you want to pass an option that takes an argument, you must use `-Xpreprocessor` **twice** — once for the option and once for the argument 🫶

- **`-no-integrated-cpp`** — do preprocessing as a separate pass before compilation 💜. By default GCC does preprocessing as an integrated part of input tokenization and parsing. With this option, the appropriate language front end (`cc1`, `cc1plus`, or `cc1obj` for C, C++, and Objective-C, respectively) is invoked *twice* instead — once for preprocessing only, and once for actual compilation of the preprocessed input 🔥. This can be useful alongside the `-B` or `-wrapper` options to specify an alternate preprocessor or do extra processing of the program source between normal preprocessing and compilation.

## 🛠️ Passing Options to the Assembler 🛠️

You can pass options to the assembler. Yes queen, go off 👑

- **`-Wa,option`** — pass *option* as an option to the assembler 💖. If *option* contains commas, it gets split into multiple options at the commas.

- **`-Xassembler option`** — pass *option* as an option to the assembler 🌟. Use this to supply system-specific assembler options that GCC doesn't recognize. If you want to pass an option that takes an argument, you must use `-Xassembler` **twice** — once for the option and once for the argument ✨

## 💅 Options for Linking 💅

okay so these ones only matter when the compiler is actually doing a link step — like, it's taking your object files and smooshing them into an executable. If you're not linking, they're literally meaningless. not "unimportant," *meaningless*. the compiler doesn't even look at them. periodt ✨

---

- **`object-file-name`** — so if you hand the compiler a filename that does *not* end in one of the special recognized suffixes, it just figures "okay this is an object file or a library" and rolls with it. (and how does the linker tell object files from libraries? by peeking at the actual *file contents*, bestie — not the name. iconic.) if linking is happening, these object files get fed to the linker as input. 🫶

- **`-c`**, **`-S`**, **`-E`** — if you use *any* of these three, the linker does **not** run, and you should not be passing object file names as arguments. like, don't. it's giving mismatched energy. 💀

- **`-flinker-output=type`** — this bad boy controls code generation of the link-time optimizer. by default the linker output gets auto-determined by the linker plugin (it knows what it's doing, trust). but for debugging the compiler, or if you want incremental linking with a non-LTO object file, it may be useful to control the `type` manually. here's the tea on each type:

  - if `type` is `exec`, code generation produces a **static binary**. in this case `-fpic` and `-fpie` are both disabled.
  - if `type` is `dyn`, code generation produces a **shared library**. in this case `-fpic` or `-fPIC` is preserved, but *not* enabled automatically. this lets you build shared libraries without position-independent code on architectures where that's possible — i.e. on x86. 😌
  - if `type` is `pie`, code generation produces an `-fpie` executable. this results in similar optimizations as `exec` except that `-fpie` is *not* disabled if you specified it at compilation time.
  - if `type` is `rel`, the compiler assumes incremental linking is happening. the sections containing intermediate code for link-time optimization get merged, pre-optimized, and output to the resulting object file. plus, if `-ffat-lto-objects` is specified, binary code is produced for future non-LTO linking. the object file produced by incremental linking is smaller than a static library produced from the same object files. and at link time, the result of incremental linking also loads faster than a static library — assuming the majority of objects in the library are used. slay. 🔥
  - finally, `nolto-rel` configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is **stripped**. when multiple object files are linked together, the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens) — but most of the benefits of whole program optimizations are lost. 😭

  during the incremental link (by `-r`) the linker plugin defaults to `rel`. with current interfaces to GNU Binutils though, it is *not* possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. so if any of the object files in an incremental link can't be used for link-time optimization, the linker plugin issues a warning and uses `nolto-rel`. to maintain whole program optimization, it's recommended to link such objects into a static library instead. alternatively you can use H.J. Lu's binutils with support for mixed objects. iykyk 🌸

- **`-fuse-ld=bfd`** — use the bfd linker instead of the default linker.

- **`-fuse-ld=gold`** — use the gold linker instead of the default linker.

- **`-fuse-ld=lld`** — use the LLVM lld linker instead of the default linker.

- **`-fuse-ld=mold`** — use the Modern Linker (mold) instead of the default linker. mold is genuinely so fast it's not even funny 💅

- **`-llibrary`**, **`-l library`** — search the library named *library* when linking. (the second form, with the library as a separate argument, is *only* for POSIX compliance and is **not recommended** — so like, don't do it, bestie. 🙈)

  the `-l` option is passed directly to the linker by GCC. refer to your linker documentation for exact details. the general description below applies to the GNU linker.

  the linker searches a standard list of directories for the library. the directories searched include several standard system directories plus any that you specify with `-L`.

  static libraries are archives of object files and have file names like `liblibrary.a`. some targets also support shared libraries, which typically have names like `liblibrary.so`. if both static and shared libraries are found, the linker gives preference to linking with the **shared** library — unless the `-static` option is used.

  ⚠️ it genuinely matters *where* in the command you write this option — the linker searches and processes libraries and object files in the order they're specified. so `foo.o -lz bar.o` searches library `z` after file `foo.o` but before `bar.o`. and if `bar.o` refers to functions in `z`, those functions may not be loaded. that's the classic linker order trap, fr fr. 😭

- **`-lobjc`** — you need this special case of the `-l` option in order to link an Objective-C or Objective-C++ program. that's it, that's the whole vibe.

- **`-nostartfiles`** — do **not** use the standard system startup files when linking. the standard system libraries are still used normally, unless `-nostdlib`, `-nolibc`, or `-nodefaultlibs` is used.

- **`-nodefaultlibs`** — do **not** use the standard system libraries when linking. only the libraries *you* specify are passed to the linker, and options specifying linkage of the system libraries — such as `-static-libgcc` or `-shared-libgcc` — are ignored. the standard startup files are used normally, unless `-nostartfiles` is used.

  heads up: the compiler may generate calls to `"memcmp"`, `"memset"`, `"memcpy"` and `"memmove"`. these entries are usually resolved by entries in libc. these entry points should be supplied through some other mechanism when this option is specified. don't come crying when you get undefined references, bestie 🫶

- **`-nolibc`** — do **not** use the C library, or system libraries tightly coupled with it, when linking. still link with the startup files, libgcc, or toolchain-provided language support libraries such as `libgnat`, `libgfortran` or `libstdc++` — unless options preventing their inclusion are used as well. this typically removes `-lc` from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed — for example `-lpthread` or `-lm` in some configurations. this is intended for bare-board targets when there is *indeed* no C library available. very embedded-core of you. 🧚

- **`-nostdlib`** — do **not** use the standard system startup files **or** libraries when linking. no startup files, and only the libraries you specify are passed to the linker. options specifying linkage of the system libraries, such as `-static-libgcc` or `-shared-libgcc`, are ignored.

  same deal as before: the compiler may generate calls to `"memcmp"`, `"memset"`, `"memcpy"` and `"memmove"`, these entries are usually resolved by entries in libc, and these entry points should be supplied through some other mechanism when this option is specified.

  one of the standard libraries bypassed by `-nostdlib` and `-nodefaultlibs` is `libgcc.a` — a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.

  in most cases, you need `libgcc.a` even when you want to avoid other standard libraries. 👑 in other words: when you specify `-nostdlib` or `-nodefaultlibs` you should usually specify `-lgcc` as well. this ensures you have no unresolved references to internal GCC library subroutines. (an example of such an internal subroutine is `"__main"`, used to ensure C++ constructors are called.) main character energy, literally.

- **`-nostdlib++`** — do not implicitly link with standard C++ libraries. simple as that 💜

- **`-e entry`**, **`--entry=entry`** — specify that the program entry point is *entry*. the argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. your call, bestie.

- **`-pie`** — produce a dynamically linked position independent executable on targets that support it. ✨ for predictable results, you must *also* specify the same set of options used for compilation (`-fpie`, `-fPIE`, or model suboptions) when you specify this linker option. match them up or chaos.

- **`-no-pie`** — don't produce a dynamically linked position independent executable. that's it. that's the option.

- **`-static-pie`** — produce a **static** position independent executable on targets that support it. a static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. very cool, very demure. for predictable results, you must also specify the same set of options used for compilation (`-fpie`, `-fPIE`, or model suboptions) when you specify this linker option.

- **`-pthread`** — link with the POSIX threads library. supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. 🧵 on some targets this option also sets flags for the preprocessor, so it should be used **consistently** for both compilation and linking. consistency is the whole game here, no cap.

- **`-r`** — produce a relocatable object as output. also known as **partial linking**. lowkey a sleeper hit of an option.

- **`-rdynamic`** — pass the flag `-export-dynamic` to the ELF linker, on targets that support it. this instructs the linker to add **all** symbols, not only used ones, to the dynamic symbol table. 🔥 this option is needed for some uses of `"dlopen"` or to allow obtaining backtraces from within a program. very much the "i need my symbols to be findable" option.

- **`-s`** — remove all symbol table and relocation information from the executable. poof. gone. goodbye debugging info 💀

- **`-static`** — on systems that support dynamic linking, this overrides `-pie` and prevents linking with the shared libraries. on other systems, this option has no effect. read that again: **no effect**. don't stress.

- **`-shared`** — produce a shared object which can then be linked with other objects to form an executable. not all systems support this option. for predictable results, you must also specify the same set of options used for compilation (`-fpic`, `-fPIC`, or model suboptions) when you specify this linker option.<rainbow>[1]</rainbow> 🎀

- **`-shared-libgcc`**, **`-static-libgcc`** — on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. if no shared version of libgcc was built when the compiler was configured, these options have **no effect**.

  there are several situations in which an application should use the shared libgcc instead of the static version. the most common of these is when the application wishes to throw and catch exceptions across different shared libraries. 🌈 in that case, each of the libraries as well as the application itself should use the shared libgcc.

  therefore, the G++ driver automatically adds `-shared-libgcc` whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. it's giving "trust the driver," and honestly? you should. 💅

<pink>okay so</pink> bestie, if you're using the GCC driver to build shared libraries, sometimes they just… aren't linked with the shared `libgcc`, and that's a whole thing. Here's the tea: if GCC peeks at its configuration time and finds you've got a non-GNU linker, or a GNU linker that doesn't support the option `--eh-frame-hdr`, it links the shared version of `libgcc` into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of `libgcc`, linking with the <hotpink>static</hotpink> version of `libgcc` by default. This is lowkey iconic because it lets exceptions propagate through such shared libraries, without incurring relocation costs at library load time. ✨

However — and this is the part you *cannot* skip, bestie — if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option `-shared-libgcc`, such that it is linked with the shared `libgcc`. No cap.

- **`-static-libasan`** — when the `-fsanitize=address` option is used to link a program, the GCC driver automatically links against `libasan`. If `libasan` is available as a shared library, and the `-static` option is not used, then this links against the shared version of `libasan`. The `-static-libasan` option directs the GCC driver to link `libasan` statically, without necessarily linking other libraries statically. 🫶

- **`-static-libtsan`** — when the `-fsanitize=thread` option is used to link a program, the GCC driver automatically links against `libtsan`. If `libtsan` is available as a shared library, and the `-static` option is not used, then this links against the shared version of `libtsan`. The `-static-libtsan` option directs the GCC driver to link `libtsan` statically, without necessarily linking other libraries statically. 🧵

- **`-static-liblsan`** — when the `-fsanitize=leak` option is used to link a program, the GCC driver automatically links against `liblsan`. If `liblsan` is available as a shared library, and the `-static` option is not used, then this links against the shared version of `liblsan`. The `-static-liblsan` option directs the GCC driver to link `liblsan` statically, without necessarily linking other libraries statically. 💧

- **`-static-libubsan`** — when the `-fsanitize=undefined` option is used to link a program, the GCC driver automatically links against `libubsan`. If `libubsan` is available as a shared library, and the `-static` option is not used, then this links against the shared version of `libubsan`. The `-static-libubsan` option directs the GCC driver to link `libubsan` statically, without necessarily linking other libraries statically. 🦋

- **`-static-libstdc++`** — when the `g++` program is used to link a C++ program, it normally automatically links against `libstdc++`. If `libstdc++` is available as a shared library, and the `-static` option is not used, then this links against the shared version of `libstdc++`. That is normally fine. <mint>But</mint> — it is sometimes useful to freeze the version of `libstdc++` used by the program without going all the way to a fully static link. The `-static-libstdc++` option directs the `g++` driver to link `libstdc++` statically, without necessarily linking other libraries statically. iykyk 💅

- **`-symbolic`** — bind references to global symbols when building a shared object. It warns about any unresolved references (unless overridden by the link editor option `-Xlinker -z -Xlinker defs`). Only a few systems support this option, so vibe check your platform first. 🎀

- **`-T script`** — use *script* as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the `-T` option may be required when linking to avoid references to undefined symbols. 👑

- **`-Xlinker option`** — pass *option* as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.

  If you want to pass an option that takes a separate argument, you must use `-Xlinker` **twice**, once for the option and once for the argument. For example, to pass `-assert definitions`, you must write `-Xlinker -assert -Xlinker definitions`. It does <red>not</red> work to write `-Xlinker "-assert definitions"`, because this passes the entire string as a single argument, which is not what the linker expects. 😭

  When using the GNU linker, it is usually more convenient to pass arguments to linker options using the `option=value` syntax than as separate arguments. For example, you can specify `-Xlinker -Map=output.map` rather than `-Xlinker -Map -Xlinker output.map`. Other linkers may not support this syntax for command-line options, so don't @ me.

- **`-Wl,option`** — pass *option* as an option to the linker. If *option* contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, `-Wl,-Map,output.map` passes `-Map output.map` to the linker. When using the GNU linker, you can also get the same effect with `-Wl,-Map=output.map`. literally so convenient 🍓

- **`-u symbol`** — pretend the symbol *symbol* is undefined, to force linking of library modules to define it. You can use `-u` multiple times with different symbols to force loading of additional library modules. slay ✨

- **`-z keyword`** — `-z` is passed directly on to the linker along with the keyword *keyword*. See the section in the documentation of your linker for permitted values and their meanings. go read your linker's docs, bestie, I can't do everything for you 🙈

---

## 🔍 Options for Directory Search 🔍

These options specify directories to search for header files, for libraries and for parts of the compiler. It's giving "where do I even find anything around here" energy:

- **`-I dir`**, **`-iquote dir`**, **`-isystem dir`**, **`-idirafter dir`** — add the directory *dir* to the list of directories to be searched for header files during preprocessing. If *dir* begins with `=` or `$SYSROOT`, then the `=` or `$SYSROOT` is replaced by the sysroot prefix; see `--sysroot` and `-isysroot`.

  Directories specified with `-iquote` apply **only** to the quote form of the directive, `#include "file"`. Directories specified with `-I`, `-isystem`, or `-idirafter` apply to lookup for **both** the `#include "file"` and `#include <file>` directives.

  You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows, and yes you need to memorise it:

  1. For the quote form of the include directive, the directory of the current file is searched first.
  2. For the quote form of the include directive, the directories specified by `-iquote` options are searched in left-to-right order, as they appear on the command line.
  3. Directories specified with `-I` options are scanned in left-to-right order.
  4. Directories specified with `-isystem` options are scanned in left-to-right order.
  5. Standard system directories are scanned.
  6. Directories specified with `-idirafter` options are scanned in left-to-right order.

  You can use `-I` to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should <red>not</red> use this option to add directories that contain vendor-supplied system header files; use `-isystem` for that. fr fr, respect the boundaries.

  The `-isystem` and `-idirafter` options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories. 👑

  If a standard system include directory, or a directory specified with `-isystem`, is also specified with `-I`, the `-I` option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the `#include_next` directive are not inadvertently changed. If you **really** need to change the search order for system directories, use the `-nostdinc` and/or `-isystem` options.

- **`-I-`** — split the include path. <yellow>This option has been deprecated.</yellow> Please use `-iquote` instead for `-I` directories before the `-I-` and remove the `-I-` option. it's giving "moving on" energy 💀

  Any directories specified with `-I` options before `-I-` are searched only for headers requested with `#include "file"`; they are **not** searched for `#include <file>`. If additional directories are specified with `-I` options after the `-I-`, those directories are searched for all `#include` directives.

  In addition, `-I-` inhibits the use of the directory of the current file directory as the first search directory for `#include "file"`. There is <bold>no way</bold> to override this effect of `-I-`. no escape hatch, bestie 😭

- **`-iprefix prefix`** — specify *prefix* as the prefix for subsequent `-iwithprefix` options. If the prefix represents a directory, you should include the final `/`. 🎀

- **`-iwithprefix dir`**, **`-iwithprefixbefore dir`** — append *dir* to the prefix specified previously with `-iprefix`, and add the resulting directory to the include search path. `-iwithprefixbefore` puts it in the same place `-I` would; `-iwithprefix` puts it where `-idirafter` would. same vibes, different placement ✨

- **`-isysroot dir`** — this option is like the `--sysroot` option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the `--sysroot` option for more information. 🌸

- **`-imultilib dir`** — use *dir* as a subdirectory of the directory containing target-specific C++ headers. 🦋

- **`-nostdinc`** — do **not** search the standard system directories for header files. Only the directories explicitly specified with `-I`, `-iquote`, `-isystem`, and/or `-idirafter` options (and the directory of the current file, if appropriate) are searched. you're on your own now, bestie 🫶

- **`-nostdinc++`** — do not search for header files in the C++-specific standard directories, but do **still** search the other standard directories. (This option is used when building the C++ library.) 🧚

- **`--embed-dir=dir`** — append *dir* directory to the list of searched directories for `#embed` preprocessing directive or `__has_embed` macro. There are **no** default directories for `#embed`. 😳

  If *dir* begins with `=` or `$SYSROOT`, then the `=` or `$SYSROOT` is replaced by the sysroot prefix; see `--sysroot` and `-isysroot`.

- **`-iplugindir=dir`** — set the directory to search for plugins that are passed by `-fplugin=name` instead of `-fplugin=path/name.so`. This option is **not** meant to be used by the user, but only passed by the driver. hands off, this one's internal 🔥

- **`-Ldir`** — add directory *dir* to the list of directories to be searched for `-l`. 📚

- **`-Bprefix`** — this option specifies where to find the executables, libraries, include files, and data files of the compiler itself. it's the compiler's own little home directory hunt 💜

okay so the compiler driver is basically the group-chat admin of your build 💅 — it runs one or more of the subprograms `cpp`, `cc1`, `as` and `ld`. It tries `prefix` as a prefix for each program it tries to run, both with and without `machine/version/` for the corresponding target machine and compiler version.

For each subprogram to be run, the compiler driver first tries the `-B` prefix, if any. If that name is not found, or if `-B` is not specified, the driver tries two standard prefixes, `/usr/lib/gcc/` and `/usr/local/lib/gcc/`. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your `PATH` environment variable.

The compiler checks to see if the path provided by `-B` refers to a directory, and if necessary it adds a directory separator character at the end of the path.

`-B` prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into `-L` options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into `-isystem` options for the preprocessor. In this case, the compiler appends `include` to the prefix.

The runtime support file `libgcc.a` can also be searched for using the `-B` prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.

Another way to specify a prefix much like the `-B` prefix is to use the environment variable `GCC_EXEC_PREFIX`.

As a special kludge, if the path provided by `-B` is `[dir/]stageN/`, where `N` is a number in the range `0` to `9`, then it is replaced by `[dir/]include`. This is to help with boot-strapping the compiler.

- **`-no-canonical-prefixes`** — Do not expand any symbolic links, resolve references to `/../` or `/./`, or make the path absolute when generating a relative prefix. <pink>iconic “leave my path alone” behavior 💅</pink>

- **`--sysroot=dir`** — Use `dir` as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in `/usr/include` and libraries in `/usr/lib`, it instead searches `dir/usr/include` and `dir/usr/lib`.

  If you use both this option and the `-isysroot` option, then the `--sysroot` option applies to libraries, but the `-isysroot` option applies to header files.

  The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of `--sysroot` still works, but the library aspect does not. <yellow>so check your linker, bestie 🫶</yellow>

- **`--no-sysroot-suffix`** — For some targets, a suffix is added to the root directory specified with `--sysroot`, depending on the other options used, so that headers may for example be found in `dir/suffix/usr/include` instead of `dir/usr/include`. This option disables the addition of such a suffix. <mint>no extra suffix, no extra drama ✨</mint>

---

## 💜 Options for Code Generation Conventions 💜

These machine-independent options control the interface conventions used in code generation.

Most of them have both positive and negative forms; the negative form of `-ffoo` is `-fno-foo`. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing `no-` or adding it.

- **`-fstack-reuse=reuse-level`** — This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. `reuse_level` can be `all`, `named_vars`, or `none`. `all` enables stack reuse for all local variables and temporaries, `named_vars` enables the reuse only for user defined local variables with names, and `none` disables stack reuse completely. The default value is `all`. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. <red>this is the “your old code is gonna have a bad time” zone 😭</red>

  For example,

int *p; { int local1;

p = &local1; local1 = 10; .... } { int local2; local2 = 20; ... }

if (*p == 10) // out of scope use of local1 {

}


Another example:

struct A { A(int k) : i(k), j(k) { } int i; int j; };

A *ap;

void foo(const A& ar) { ap = &ar; }

void bar() { foo(A(10)); // temp object's lifetime ends when foo returns

{ A a(20); .... } ap->i+= 10; // ap references out of scope temp whose space // is reused with a. What is the value of ap->i? }


The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. <purple>fr fr, the compiler is just trying to be efficient and your legacy code is like “no 💀”</purple>

- **`-ftrapv`** — This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options `-ftrapv` and `-fwrapv` override each other, so using `-ftrapv -fwrapv` on the command-line results in `-fwrapv` being effective. Note that only active options override, so using `-ftrapv -fwrapv -fno-wrapv` on the command-line results in `-ftrapv` being effective. <peach>trap queen behavior 🎀</peach>

- **`-fwrapv`** — This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options `-ftrapv` and `-fwrapv` override each other, so using `-ftrapv -fwrapv` on the command-line results in `-fwrapv` being effective. Note that only active options override, so using `-ftrapv -fwrapv -fno-wrapv` on the command-line results in `-ftrapv` being effective. <lavender>wrap it up, but mathematically 🌸</lavender>

- **`-fwrapv-pointer`** — This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid. <cyan>pointers doing the wrap-around slay 🦋</cyan>

- **`-fstrict-overflow`** — This option implies `-fno-wrapv -fno-wrapv-pointer` and when negated implies `-fwrapv -fwrapv-pointer`. <blue>strict overflow said “no wrapping” and meant it 💅</blue>

- **`-fexceptions`** — Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. <green>exceptions are the plot twists of code 🔥</green>

- **`-fnon-call-exceptions`** — Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as `"SIGALRM"`. This enables `-fexceptions`. <yellow>very “only some instructions are allowed to be dramatic” energy 💀</yellow>

- **`-fdelete-dead-exceptions`** — Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the program can be optimized away. This does not affect calls to functions except those with the `"pure"` or `"const"` attributes. This option is enabled by default for the Ada and C++ compilers, as permitted by the language specifications. Optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. <hotpink>dead exceptions? gone. not me cleaning up the build 😭✨</hotpink>

- **`-funwind-tables`** — Similar to `-fexceptions`, except that it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. <mint>just the static data, no extra chaos 🫶</mint>

- **`-fasynchronous-unwind-tables`** — Generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). <cyan>DWARF table looking lowkey precise 👑</cyan>

- **`-fno-gnu-unique`** — On systems with recent GNU assembler and C library, the C++ compiler uses the `"STB_GNU_UNIQUE"` binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of `"RTLD_LOCAL"`; this is necessary to avoid problems with a library used by two different `"RTLD_LOCAL"` plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes `"dlclose"` to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via `"dlclose"` and `"dlopen"`, you can use `-fno-gnu-unique`. <red>this is the “unique symbols vs dlclose” drama, and yes it's giving main character energy 🙈</red>

- **`-fpcc-struct-return`** — okay so this one makes "short" `struct` and `union` values get returned in *memory* like the longer ones do, instead of in registers. Bestie, it's less efficient, but the glow-up is that GCC-compiled files can intercall with files from other compilers, especially the Portable C Compiler (pcc). 💅

The exact convention for returning structs in memory? Depends on the target configuration macros — iykyk.

"Short" structs and unions are the ones whose size and alignment match that of some integer type.

> <red>Warning:</red> code compiled with `-fpcc-struct-return` is **not** binary compatible with code compiled with `-freg-struct-return`. Use it to conform to a non-default application binary interface. No cap. 💀

- **`-freg-struct-return`** — the opposite vibe: return `struct` and `union` values in registers when possible. For small structures this is way more efficient than `-fpcc-struct-return`. ✨

If you specify *neither* `-fpcc-struct-return` nor `-freg-struct-return`, GCC defaults to whichever convention is standard for the target. And if there's no standard convention? GCC defaults to `-fpcc-struct-return`, except on targets where GCC is the principal compiler — in those cases we can choose the standard, and we chose the more efficient register return alternative. Main character energy tbh. 👑

> <red>Warning:</red> code compiled with `-freg-struct-return` is **not** binary compatible with code compiled with `-fpcc-struct-return`. Use it to conform to a non-default application binary interface. Fr fr. 😭

- **`-fshort-enums`** — allocate to an `enum` type only as many bytes as it needs for the declared range of possible values. Specifically, the `enum` type is equivalent to the smallest integer type that has enough room. And heads up: this option has no effect for an enumeration type with a fixed underlying type. 🎀

> <red>Warning:</red> `-fshort-enums` makes GCC generate code that is **not** binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. 🫶

- **`-fshort-wchar`** — override the underlying type for `wchar_t` to be `short unsigned int` instead of the default for the target. This one's literally useful for building programs to run under WINE. 🍓

> <red>Warning:</red> `-fshort-wchar` makes GCC generate code that is **not** binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. 👑

- **`-fcommon`** — in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the `extern` keyword, which do not allocate storage. 🌸

The default is `-fno-common`, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker, so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit. 😭

`-fcommon` places uninitialized global variables in a common block. This lets the linker resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It's mainly useful to enable legacy code to link without errors. 💜

- **`-fno-ident`** — ignore the `#ident` directive. Simple as that, bestie. 💅

- **`-finhibit-size-directive`** — don't output a `.size` assembler directive, or anything else that would cause trouble if the function is split in the middle and the two halves are placed at locations far apart in memory. This option is used when compiling `crtstuff.c`; you should not need to use it for anything else. 🦋

- **`-fverbose-asm`** — put extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). 🙈

`-fno-verbose-asm`, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

The added comments include:

* information on the compiler version and command-line options,
* the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
* hints on which high-level expressions correspond to the various assembly instruction operands.

For example, given this C source file:

int test (int n) { int i; int total = 0;

for (i = 0; i < n; i++) total += i * i;

return total; }


compiling to (x86_64) assembly via `-S` and emitting the result direct to stdout via `-o -`

gcc -S test.c -fverbose-asm -Os -o -


gives output similar to this:

.file "test.c" # GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu) [...snip...] # options passed: [...snip...]

.text .globl test .type test, @function test: .LFB0: .cfi_startproc # test.c:4: int total = 0; xorl %eax, %eax # <retval> # test.c:6: for (i = 0; i < n; i++) xorl %edx, %edx # i .L2: # test.c:6: for (i = 0; i < n; i++) cmpl %edi, %edx # n, i jge .L5 #, # test.c:7: total += i * i; movl %edx, %ecx # i, tmp92 imull %edx, %ecx # i, tmp92 # test.c:6: for (i = 0; i < n; i++) incl %edx # i # test.c:7: total += i * i; addl %ecx, %eax # tmp92, <retval> jmp .L2 # .L5: # test.c:10: } ret .cfi_endproc .LFE0: .size test, .-test .ident "GCC: (GNU) 7.0.0 20160809 (experimental)" .section .note.GNU-stack,"",@progbits


The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change. 💅

- **`-frecord-gcc-switches`** — this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. It's only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the `-fverbose-asm` switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also `-grecord-gcc-switches` for another way of storing compiler options into the object file. 🔥

- **`-fpic`** — generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system — bestie, don't @ us). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that `-fpic` does not work; in that case, recompile with `-fPIC` instead. And the maximums? 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit. ✨

Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. 🌈

When this flag is set, the macros `__pic__` and `__PIC__` are defined to `1`. 💜

- **`-fPIC`** — if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC. 🌸

Position-independent code requires special support, and therefore works only on certain machines.

When this flag is set, the macros `__pic__` and `__PIC__` are defined to `2`. 👑

- **`-fpie`** — **`-fPIE`** — these options are similar to `-fpic` and `-fPIC`, but the generated position-independent code can be only linked into executables. Usually these options are used to compile code that will be linked using the `-pie` GCC option. 🎀

`-fpie` and `-fPIE` both define the macros `__pie__` and `__PIE__`. The macros have the value `1` for `-fpie` and `2` for `-fPIE`. ✨

- **`-fno-plt`** — do **not** use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with `-fno-plt` all external symbols are resolved at load time. 💅

Alternatively, the function attribute `noplt` can be used to avoid calls through the PLT for specific external functions.

In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead. 🦋

- **`-fno-jump-tables`** — do **not** use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with `-fpic` or `-fPIC` for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed. 😌

- **`-fno-bit-tests`** — okay so sometimes GCC looks at your `switch` and is like “bestie I could do this with bit tests, it’s literally faster” and honestly? you can veto that. this flag says do **not** use bit tests for switch statements, even where it would be more efficient than other code generation strategies. it’s giving <pink>“I said what I said”</pink> energy 💅

- **`-ffixed-reg`** — treat the register named *reg* as a fixed register. generated code should <hotpink>never</hotpink> refer to it (except perhaps as a stack pointer, frame pointer, or in some other fixed role). ✨

*reg* has to be the name of a register, and the names accepted are machine-specific — they’re defined in the `"REGISTER_NAMES"` macro in the machine description macro file. 🫶

this flag does **not** have a negative form, because it specifies a three-way choice. periodt. 💅

- **`-fcall-used-reg`** — treat the register named *reg* as an allocable register that is clobbered by function calls. it may be allocated for temporaries or variables that do not live across a call. functions compiled this way do **not** save and restore the register *reg*. 🦋

it is an **error** to use this flag with the frame pointer or stack pointer. and using this flag for other registers that have fixed pervasive roles in the machine’s execution model produces <red>disastrous results</red>. 💀

this flag does not have a negative form, because it specifies a three-way choice.

- **`-fcall-saved-reg`** — treat the register named *reg* as an allocable register saved by functions. it may be allocated even for temporaries or variables that live across a call. functions compiled this way save and restore the register *reg* if they use it. 🌸

it is an **error** to use this flag with the frame pointer or stack pointer. and use of this flag for other registers that have fixed pervasive roles in the machine’s execution model produces <red>disastrous results</red>. 😭

a <italic>different</italic> sort of disaster results from using this flag for a register in which function values may be returned. 💀

this flag does not have a negative form, because it specifies a three-way choice.

- **`-fpack-struct[=n]`** — without a value specified, pack all structure members together without holes. 🎀 when a value *is* specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).

> ⚠️ **Warning:** the `-fpack-struct` switch causes GCC to generate code that is **not binary compatible** with code generated without that switch. additionally, it makes the code suboptimal. use it to conform to a non-default application binary interface. 😬

- **`-fleading-underscore`** — this option and its counterpart, `-fno-leading-underscore`, forcibly change the way C symbols are represented in the object file. one use is to help link with legacy assembly code. 🫶

> ⚠️ **Warning:** the `-fleading-underscore` switch causes GCC to generate code that is **not binary compatible** with code generated without that switch. use it to conform to a non-default application binary interface. not all targets provide complete support for this switch. 💀

- **`-ftls-model=model`** — alter the thread-local storage model to be used. the *model* argument should be one of:

- `global-dynamic`
- `local-dynamic`
- `initial-exec`
- `local-exec`

note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if `-fpic` is not given on the command line. ✨

the default without `-fpic` is `initial-exec`; with `-fpic` the default is `global-dynamic`. 💅

- **`-ftrampolines`** — for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. otherwise, for targets that do not need them — like, for example, HP-PA or IA-64 — do nothing. 🦋

a trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. therefore, it requires the stack to be made executable in order for the program to work properly. 😬

`-fno-trampolines` is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. descriptors are made up of data only, but the generated code must be prepared to deal with them. as of this writing, `-fno-trampolines` is enabled by default only for Ada. 💖

moreover, code compiled with `-ftrampolines` and code compiled with `-fno-trampolines` are **not binary compatible** if nested functions are present. this option must therefore be used on a program-wide basis and be manipulated with <red>extreme care</red>. 🚨

for languages other than Ada, the `"-ftrampolines"` and `"-fno-trampolines"` options currently have **no effect**, and trampolines are always generated on platforms that need them for nested functions. 🙈

- **`-ftrampoline-impl=[stack|heap]`** — by default, trampolines are generated on stack. however, certain platforms (such as the Apple M1) do not permit an executable stack. compiling with `-ftrampoline-impl=heap` generates calls to `__gcc_nested_func_ptr_created` and `__gcc_nested_func_ptr_deleted` in order to allocate and deallocate trampoline space on the executable heap. these functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux. 🍓

> 🚨 **PLEASE NOTE:** heap trampolines are **not guaranteed** to be correctly deallocated if you `setjmp`, instantiate nested functions, and then `longjmp` back to a state prior to having allocated those nested functions. 😭

- **`-fvisibility=[default|internal|hidden|protected]`** — set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export, and prevent symbol clashes. it is **strongly recommended** that you use this in any shared objects you distribute. 👑

despite the nomenclature, `default` always means **public**; i.e., available to be linked against from outside the shared object. `protected` and `internal` are pretty useless in real-world usage, so the only other commonly used option is `hidden`. the default if `-fvisibility` isn’t specified is `default`, i.e., make every symbol public. 💅

a good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by *How To Write Shared Libraries* by Ulrich Drepper (which can be found at `<https://www.akkadia.org/drepper/>`) — however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. this is the norm with DLLs on Windows, and with `-fvisibility=hidden` and `"__attribute__ ((visibility("default")))"` instead of `"__declspec(dllexport)"` you get almost identical semantics with identical syntax. this is a great boon to those working with cross-platform projects. 🌈

for those adding visibility support to existing code, you may find `"#pragma GCC visibility"` of use. this works by you enclosing the declarations you wish to set visibility for with (for example) `"#pragma GCC visibility push(hidden)"` and `"#pragma GCC visibility pop"`. bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code. note that due to ISO C++ specification requirements, `"operator new"` and `"operator delete"` must always be of default visibility. 🫶

> ⚠️ be aware that headers from outside your project — in particular system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. you may need to explicitly say `"#pragma GCC visibility push(default)"` before including any such headers. 😬

`"extern"` declarations are **not** affected by `-fvisibility`, so a lot of code can be recompiled with `-fvisibility=hidden` with no modifications. however, this means that calls to `"extern"` functions with no explicit visibility use the PLT, so it is more effective to use `"__attribute ((visibility))"` and/or `"#pragma GCC visibility"` to tell the compiler which `"extern"` declarations should be treated as hidden. ✨

note that `-fvisibility` **does** affect C++ vague linkage entities. this means that, for instance, an exception class that is thrown between DSOs must be explicitly marked with default visibility so that the `type_info` nodes are unified between the DSOs. 💀

an overview of these techniques, their benefits and how to use them is at `<https://gcc.gnu.org/wiki/Visibility>`. 🌟

- **`-fstrict-volatile-bitfields`** — this option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field’s type, aligned to a natural alignment if possible. 🌸 for example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as `"unsigned short"` (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. 🎀

if this option is disabled, the compiler uses the most efficient instruction. in the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 😬

in some cases, such as when the `"packed"` attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. in this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. 💜

> ⚠️ **Note:** because the C/C++11 memory model is out here being strict, write accesses are **not** allowed to touch non bit-field members. So the recommendation is: define all the bits of the field's type as bit-field members. Fr, don't fight the model, it will win. 💅

The default value of this option is decided by the application binary interface for the target processor — so it's not vibes-based, it's ABI-based.

- **`-fsync-libcalls`** — controls whether any out-of-line instance of the `"__sync"` family of functions can be used to implement the C++11 `"__atomic"` family of functions.

Its default value is **enabled**, so the only form of this option that's actually useful to you is `-fno-sync-libcalls`. This one is used in the implementation of the `libatomic` runtime library — it's a backstage pass situation, not a main character option. 🎭

- **`-fzero-init-padding-bits=value`** — guarantees zero initialization of padding bits in automatic variable initializers. Some languages already guarantee zero initialization of padding bits in certain cases, like C23 when you use empty initializers (`"{}"`), or C++ when you use zero-initialization, or C guaranteeing that fields not specified in an initializer get their padding bits zero initialized. This option lets you change *when* padding bits in initializers are guaranteed to be zero initialized.

The default is `-fzero-init-padding-bits=standard`, which makes **no further guarantees** than the corresponding standard. Like, standard behavior only, no extras. E.g.:

struct A { char a; unsigned long long b; char c; }; union B { char a; unsigned long long b; }; struct A a = {}; // C23 guarantees padding bits are zero. struct A b = { 1, 2, 3 }; // No guarantees. union B c = {}; // C23 guarantees padding bits are zero. union B d = { 1 }; // No guarantees.


Oh and `-fzero-init-padding-bits=unions` highkey does more 👀 — it guarantees zero initialization of padding bits in unions **on top of** what the standards guarantee, when the initializer of a union is empty (in which case all bits of the union are zero initialized), or when the initialized member of the union is smaller than the size of the union (in which case it guarantees the padding bits outside of the initialized member are zero initialized). This was the GCC behavior before GCC 15, and in the example above it guarantees zero initialization of the last `sizeof (unsigned long long) - 1` bytes in the union. Bestie, that's a very specific byte range and it *ate*. ✨

And `-fzero-init-padding-bits=all` guarantees **additionally** zero initialization of padding bits of other aggregates — so the padding in between `b.a` and `b.b` (if any) and the tail padding in the structure (if any). Full send. 🔥

## 🛠️ GCC Developer Options 🛠️

This section describes command-line options that are mainly of interest to GCC developers — the compiler-internals girlies 🧚 — including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. It includes options that produce debug dumps at various points in the compilation; options that print statistics like memory use and execution time; and options that print info about GCC's configuration, like where it goes looking for libraries. You should **rarely** need any of these for ordinary compilation and linking tasks, ngl. They're the VIP backstage section. 🎀

Many developer options that make GCC dump output to a file take an optional `=filename` suffix. You can specify `stdout` or `-` to dump to standard output, and `stderr` for standard error. Choose your fighter. 💅

If `=filename` is omitted, a default dump file name gets constructed by concatenating the base dump file name, a pass number, a phase letter, and a pass name. The base dump file name is the name of the output file produced by the compiler if it's explicitly specified and not an executable; otherwise it's the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late get numbered **higher** than the pass named `final`, even if they're executed earlier. (Yes, the numbering is giving chaos. That's the rule though. 💀) The phase letter is one of `i` (inter-procedural analysis), `l` (language-specific), `r` (RTL), or `t` (tree). The files are created in the directory of the output file.

- **`-fcallgraph-info`**, **`-fcallgraph-info=MARKERS`** — makes the compiler output callgraph information for the program, on a per-object-file basis. The info is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information if you also specify a list of comma-separated markers. When the `"su"` marker is specified, the callgraph is decorated with stack usage information — it's equivalent to `-fstack-usage`. When the `"da"` marker is specified, the callgraph is decorated with information about dynamically allocated objects. Cute and informative, we love a decorated graph. 🦋

When compiling with `-flto`, no callgraph information is output along with the object file. At LTO link time, `-fcallgraph-info` may generate multiple callgraph information files next to intermediate LTO output files.

- **`-dletters`**, **`-fdump-rtl-pass`**, **`-fdump-rtl-pass=filename`** — says to make debugging dumps during compilation at times specified by *letters*. This is used for debugging the RTL-based passes of the compiler.

Some `-dletters` switches have different meaning when `-E` is used for preprocessing. (Just a lil heads up. 🙈)

Debug dumps can be enabled with a `-fdump-rtl` switch or some `-d` option letters. Here are the possible letters for use in *pass* and *letters*, and their meanings — buckle up, it's a long list and every single one is iconic in its own way:

- **`-fdump-rtl-alignments`** — dump after branch alignments have been computed.
- **`-fdump-rtl-asmcons`** — dump after fixing rtl statements that have unsatisfied in/out constraints.
- **`-fdump-rtl-auto_inc_dec`** — dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
- **`-fdump-rtl-barriers`** — dump after cleaning up the barrier instructions.
- **`-fdump-rtl-bbpart`** — dump after partitioning hot and cold basic blocks.
- **`-fdump-rtl-bbro`** — dump after block reordering.
- **`-fdump-rtl-btl1`**, **`-fdump-rtl-btl2`** — `-fdump-rtl-btl1` and `-fdump-rtl-btl2` enable dumping after the two branch target load optimization passes.
- **`-fdump-rtl-bypass`** — dump after jump bypassing and control flow optimizations.
- **`-fdump-rtl-combine`** — dump after the RTL instruction combination pass.
- **`-fdump-rtl-compgotos`** — dump after duplicating the computed gotos.
- **`-fdump-rtl-ce1`**, **`-fdump-rtl-ce2`**, **`-fdump-rtl-ce3`** — `-fdump-rtl-ce1`, `-fdump-rtl-ce2`, and `-fdump-rtl-ce3` enable dumping after the three if conversion passes.
- **`-fdump-rtl-cprop_hardreg`** — dump after hard register copy propagation.
- **`-fdump-rtl-csa`** — dump after combining stack adjustments.
- **`-fdump-rtl-cse1`**, **`-fdump-rtl-cse2`** — `-fdump-rtl-cse1` and `-fdump-rtl-cse2` enable dumping after the two common subexpression elimination passes.
- **`-fdump-rtl-dce`** — dump after the standalone dead code elimination passes.
- **`-fdump-rtl-dbr`** — dump after delayed branch scheduling.
- **`-fdump-rtl-dce1`**, **`-fdump-rtl-dce2`** — `-fdump-rtl-dce1` and `-fdump-rtl-dce2` enable dumping after the two dead store elimination passes.
- **`-fdump-rtl-eh`** — dump after finalization of EH handling code.
- **`-fdump-rtl-eh_ranges`** — dump after conversion of EH handling range regions.
- **`-fdump-rtl-expand`** — dump after RTL generation.
- **`-fdump-rtl-fwprop1`**, **`-fdump-rtl-fwprop2`** — `-fdump-rtl-fwprop1` and `-fdump-rtl-fwprop2` enable dumping after the two forward propagation passes.
- **`-fdump-rtl-gcse1`**, **`-fdump-rtl-gcse2`** — `-fdump-rtl-gcse1` and `-fdump-rtl-gcse2` enable dumping after global common subexpression elimination.
- **`-fdump-rtl-init-regs`** — dump after the initialization of the registers.
- **`-fdump-rtl-initvals`** — dump after the computation of the initial value sets.
- **`-fdump-rtl-into_cfglayout`** — dump after converting to cfglayout mode.
- **`-fdump-rtl-ira`** — dump after iterated register allocation.
- **`-fdump-rtl-jump`** — dump after the second jump optimization.
- **`-fdump-rtl-loop2`** — `-fdump-rtl-loop2` enables dumping after the rtl loop optimization passes.
- **`-fdump-rtl-mach`** — dump after performing the machine dependent reorganization pass, if that pass exists. (If it doesn't exist, no dump. We respect the conditional. 🫶)
- **`-fdump-rtl-mode_sw`** — dump after removing redundant mode switches.
- **`-fdump-rtl-rnreg`** — dump after register renumbering.
- **`-fdump-rtl-outof_cfglayout`** — dump after converting from cfglayout mode.
- **`-fdump-rtl-peephole2`** — dump after the peephole pass.
- **`-fdump-rtl-postreload`** — dump after post-reload optimizations.
- **`-fdump-rtl-pro_and_epilogue`** — dump after generating the function prologues and epilogues.
- **`-fdump-rtl-sched1`**, **`-fdump-rtl-sched2`** — `-fdump-rtl-sched1` and `-fdump-rtl-sched2` enable dumping after the basic block scheduling passes.
- **`-fdump-rtl-ree`** — dump after sign/zero extension elimination.
- **`-fdump-rtl-seqabstr`** — dump after common sequence discovery.
- **`-fdump-rtl-shorten`** — dump after shortening branches.
- **`-fdump-rtl-split1`**, **`-fdump-rtl-split2`**, **`-fdump-rtl-split3`**, **`-fdump-rtl-split4`**, **`-fdump-rtl-split5`** — these options enable dumping after five rounds of instruction splitting. Five rounds, bestie. Five. ✨
- **`-fdump-rtl-sms`** — dump after modulo scheduling. This pass is only run on some architectures.
- **`-fdump-rtl-stack`** — dump after conversion from GCC's `"flat register file"` registers to the x87's stack-like registers. This pass is only run on x86 variants.
- **`-fdump-rtl-subreg1`**, **`-fdump-rtl-subreg2`** — `-fdump-rtl-subreg1` and `-fdump-rtl-subreg2` enable dumping after the two subreg expansion passes.
- **`-fdump-rtl-vartrack`** — dump after variable tracking.
- **`-fdump-rtl-vregs`** — dump after converting virtual registers to hard registers.
- **`-fdump-rtl-web`** — dump after live range splitting.
- **`-fdump-rtl-regclass`**, **`-fdump-rtl-subregs_of_mode_init`**, **`-fdump-rtl-subregs_of_mode_finish`**, **`-fdump-rtl-dfinit`**, **`-fdump-rtl-dfinish`** — these dumps are defined but always produce empty files. 😭 Not me getting excited for a dump file that's literally empty. Defined though. They exist. They're just empty. Periodt.
- **`-da`**, **`-fdump-rtl-all`** — produce **all** the dumps listed above. Every single one. Go off, king. 👑

- **`-dA`** — annotates the assembler output with assorted miscellaneous debugging info ✨

- **`-dD`** — dumps *all* the macro definitions at the end of preprocessing, on top of the normal output 💖

- **`-dH`** — produces a core dump whenever an error happens. yes, like, an actual core dump 😭

- **`-dp`** — annotates the assembler output with a comment showing which pattern and alternative is used. the length and cost of each instruction get printed too 🔥

- **`-dP`** — dumps the RTL in the assembler output as a comment before each instruction. oh and it also turns on `-dp` annotation, so like, bonus 🎀

- **`-dx`** — just generates RTL for a function instead of compiling it. usually paired with `-fdump-rtl-expand` 💅

---

### 🦋 the dump options, bestie

- **`-fdump-debug`** — dumps the debugging information generated during the debug generation phase 🐛

- **`-fdump-earlydebug`** — dumps the debugging info generated during the *early* debug generation phase 🌅

- **`-fdump-noaddr`** — when doing debugging dumps, suppresses address output. why do we care? because it makes it way more feasible to run `diff` on debugging dumps from compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. iconic for reproducibility fr ✨

- **`-freport-bug`** — collects and dumps debug info into a temporary file if an internal compiler error (ICE) occurs 💀 like a little crash souvenir

- **`-fdump-unnumbered`** — when doing debugging dumps, suppresses instruction numbers *and* address output. same deal: makes `diff` on dumps way more feasible for compiler invocations with different options, especially with and without `-g` 🌈

- **`-fdump-unnumbered-links`** — when doing debugging dumps (see the `-d` option above), suppresses instruction numbers for the links to the previous and next instructions in a sequence 🔗

---

### 💜 `-fdump-ipa-switch` / `-fdump-ipa-switch-options`

controls the dumping at various stages of inter-procedural analysis language tree to a file. the file name is generated by appending a switch-specific suffix to the source file name, and the file gets created in the same directory as the output file 📁

the possible dumps:

- `all` — enables all inter-procedural analysis dumps 👑

- `cgraph` — dumps info about call-graph optimization, unused function removal, and inlining decisions 🌳

- `inline` — dump after function inlining 🫶

- `strubm` — dump after selecting "strub" modes, and recording the selections as function attributes 🧚

- `strub` — dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking 🧼

additionally, the options `-optimized`, `-missed`, `-note`, and `-all` can be provided, with the same meaning as for `-fopt-info`, defaulting to `-optimized` ⭐

for example, `-fdump-ipa-inline-optimized-missed` will emit information on callsites that were inlined, along with callsites that were **not** inlined 😤

by default, the dump will contain messages about successful optimizations (equivalent to `-optimized`) together with low-level details about the analysis 💅

---

### 🩵 `-fdump-ipa-clones`

creates a dump file containing info about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations 🌀

its main intended use? tools that create live-patches can figure out the set of functions that need to be live-patched to completely replace a particular function (see `-flive-patching`) 🩹

the file name is generated by appending suffix `ipa-clones` to the source file name, and the file is created in the same directory as the output file. each entry in the file is on a separate line containing semicolon-separated fields ✨

in the case of call graph **clone creation**, the individual fields are:

1. String `Callgraph clone`.
2. Name of the function being cloned as it is presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases 👀
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.
7. Name of the new function clone as it is presented to the assembler.
8. A number that uniquely represents the new function clone in the call graph. again — unique only within a compilation unit or within whole-program analysis, likely different in the two phases 🙈
9. The file name of the source file where the source code location of the new clone points to.
10. The line to which the source code location of the new clone points to.
11. The column to which the source code location of the new clone points to.
12. A string that determines the reason for cloning 🌸

in the case of call graph **clone removal**, the individual fields are:

1. String `Callgraph removal`.
2. Name of the function being removed as it would be presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. (yes, same caveat: unique only within a compilation unit or within whole-program analysis, likely different in the two phases 💀)
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.

---

### 🎀 `-fdump-lang`

dumps language-specific information. the file name is made by appending `.lang` to the source file name 💖

### 🌟 `-fdump-lang-all`, `-fdump-lang-switch`, `-fdump-lang-switch-options`, `-fdump-lang-switch-options=filename`

control the dumping of language-specific information. the options and filename portions behave just like they do in the `-fdump-tree` option 👑

the following switch values are accepted:

- `all` — enables all language-specific dumps ✨

- `class` — dumps class hierarchy information. virtual table information is emitted unless `'slim'` is specified. this option is applicable to C++ only 💅

- `module` — dumps module information. options `lineno` (locations), `graph` (reachability), `blocks` (clusters), `uid` (serialization), `alias` (mergeable), `asmname` (Elrond), `eh` (mapper) & `vops` (macros) may provide additional information. this option is applicable to C++ only 🧝 (yes, Elrond. we love a collab)

- `raw` — dumps the raw internal tree data. this option is applicable to C++ only 🪵

---

### 🫧 `-fdump-passes`

prints on `stderr` the list of optimization passes that are turned on and off by the current command-line options 📋

---

### 🌈 `-fdump-statistics-option`

enables and controls dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in `.statistics` to the source file name, and the file is created in the same directory as the output file 📊

if the `-option` form is used, `-stats` causes counters to be summed over the whole compilation unit while `-details` dumps every event as the passes generate them 🔍

the default with no option is to sum counters for each function compiled 🧮

---

### 👑 `-fdump-tree-all`, `-fdump-tree-switch`, `-fdump-tree-switch-options`, `-fdump-tree-switch-options=filename`

controls the dumping at various stages of processing the intermediate language tree to a file 🌳

if the `-options` form is used, *options* is a list of `-` separated options which control the details of the dump. not all options are applicable to all dumps — the ones that aren't meaningful just get ignored 🙈

the following options are available:

- `address` — prints the address of each node. usually this isn't meaningful since it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment 🏠

- `asmname` — if `"DECL_ASSEMBLER_NAME"` has been set for a given decl, use that in the dump instead of `"DECL_NAME"`. its primary use is ease of use working backward from mangled names in the assembly file 🔙

- `slim` — when dumping front-end intermediate representations, inhibits dumping of members of a scope or body of a function merely because that scope has been reached. only dumps such items when they're directly reachable by some other path 🛤️

when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures 🚫

when dumping RTL, prints the RTL in slim (condensed) form instead of the default LISP-like representation 📦

- `raw` — prints a raw representation of the tree. by default, trees are pretty-printed into a C-like representation 🪵

- `details` — enables more detailed dumps (not honored by every dump option). also includes information from the optimization passes 🔬

- `stats` — enables dumping various statistics about the pass (not honored by every dump option) 📈

- `blocks` — enables showing basic block boundaries (disabled in raw dumps) 🧱

- `graph` — for each of the other indicated dump files (`-fdump-rtl-pass`), dumps a representation of the control flow graph suitable for viewing with GraphViz to `file.passid.pass.dot`. each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot 🎨

this option currently only works for RTL dumps, and the RTL is always dumped in slim form 😔

- `vops` — enables showing virtual operands for every statement 👻

- `lineno` — enables showing line numbers for statements 🔢

- `uid` — enables showing the unique ID (`"DECL_UID"`) for each variable 🆔

- `verbose` — enables showing the tree dump for each statement 🗣️

- `eh` — enables showing the EH region number holding each statement 🛟

- `scev` — enables showing scalar evolution analysis details 🔄

- `optimized` — enables showing optimization information (only available in certain passes) 🌟

- `missed` — enables showing missed optimization information (only available in certain passes) 😬

- `note` — enables other detailed optimization information (only available in certain passes) 📝

okay so these next bits are lowkey the “tell me what the compiler is actually doing” options, and honestly they ate 💅

- **`all`** — turns on all options, except `raw`, `slim`, `verbose` and `lineno`. ✨
- **`optall`** — turns on all optimization options, i.e., `optimized`, `missed`, and `note`. 🎀

To figure out what tree dumps are available, or to find the dump for a pass you're obsessed with, follow these steps:

1. Invoke GCC with `-fdump-passes` and in the `stderr` output look for a code that corresponds to the pass you are interested in. For example, the codes `"tree-evrp"`, `"tree-vrp1"`, and `"tree-vrp2"` correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass. 👑

2. To enable the creation of the dump file, append the pass code to the `-fdump-` option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the `-fdump-tree-evrp` option. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below. 🦋

3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter `t` for tree passes (and the letter `r` for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named `myfile.c.038t.evrp` in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another. 🫶

---

- **`-fopt-info`**, **`-fopt-info-options`**, **`-fopt-info-options=filename`** — controls optimization dumps from various optimization passes. If the `-options` form is used, `options` is a list of `-` separated option keywords to select the dump details and optimizations.

The options can be divided into three groups:

1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.

The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line. 🌈

The following options control which kinds of messages should be emitted:

- **`optimized`** — print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized. 🔥
- **`missed`** — print information about missed optimizations. Individual passes control which information to include in the output. 😭
- **`note`** — print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc. 💜
- **`all`** — print detailed optimization information. This includes `optimized`, `missed`, and `note`.

The following option controls the dump verbosity:

- **`internals`** — by default, only "high-level" messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. 🧚

One or more of the following option keywords can be used to describe a group of optimizations:

- **`ipa`** — enable dumps from all interprocedural optimizations.
- **`loop`** — enable dumps from all loop optimizations.
- **`inline`** — enable dumps from all inlining optimizations.
- **`omp`** — enable dumps from all OMP (Offloading and Multi Processing) optimizations.
- **`vec`** — enable dumps from all vectorization optimizations.
- **`optall`** — enable dumps from all optimizations. This is a superset of the optimization groups listed above. 👑

If `options` is omitted, it defaults to `optimized-optall`, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".

If the `filename` is provided, then the dumps from all the applicable optimizations are concatenated into the `filename`. Otherwise the dump is output onto `stderr`. Though multiple `-fopt-info` options are accepted, only one of them can include a `filename`. If other filenames are provided then all but the first such option are ignored.

> ⚠️ Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, `stderr` should be used instead.

In the following example, the optimization info is output to `stderr`:

gcc -O3 -fopt-info


This example:

gcc -O3 -fopt-info-missed=missed.all


outputs missed optimization report from all the passes into `missed.all`, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed


prints information about missed optimization opportunities from vectorization passes on `stderr`. Note that `-fopt-info-vec-missed` is equivalent to `-fopt-info-missed-vec`. The order of the optimization group names and message types listed after `-fopt-info` does not matter.

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt


outputs information about missed optimizations as well as optimized locations from all the inlining passes into `inline.txt`.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt


Here the two output filenames `vec.miss` and `loop.opt` are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only `vec.miss` is produced which contains dumps from the vectorizer about missed opportunities. 💀

- **`-fsave-optimization-record`** — write a `SRCFILE.opt-record.json.gz` file detailing what optimizations were performed, for those optimizations that support `-fopt-info`.

> 🚨 This option is experimental and the format of the data within the compressed JSON file is subject to change.

It is roughly equivalent to a machine-readable version of `-fopt-info-all`, as a collection of messages with source file, line number and column number, with the following additional data for each message:

- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC's own code from which the message was emitted

Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. 🌸

- **`-fsched-verbose=n`** — on targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.

For `n` greater than zero, `-fsched-verbose` outputs the same information as `-fdump-rtl-sched1` and `-fdump-rtl-sched2`. For `n` greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For `n` greater than two, it includes RTL at abort point, control-flow and regions info. And for `n` over four, `-fsched-verbose` also includes dependence info. 🌟

- **`-fenable-kind-pass`**, **`-fdisable-kind-pass=range-list`** — this is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead. 🛠️

  - **`-fdisable-ipa-pass`** — disable IPA pass *pass*. *pass* is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
  - **`-fdisable-rtl-pass`**, **`-fdisable-rtl-pass=range-list`** — disable RTL pass *pass*. *pass* is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. *range-list* is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option `-fdump-passes`.
  - **`-fdisable-tree-pass`**, **`-fdisable-tree-pass=range-list`** — disable tree pass *pass*. See `-fdisable-rtl` for the description of option arguments.
  - **`-fenable-ipa-pass`** — enable IPA pass *pass*. *pass* is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
  - **`-fenable-rtl-pass`**, **`-fenable-rtl-pass=range-list`** — enable RTL pass *pass*. See `-fdisable-rtl` for option argument description and examples.
  - **`-fenable-tree-pass`**, **`-fenable-tree-pass=range-list`** — enable tree pass *pass*. See `-fdisable-rtl` for the description of option arguments.

okay, here are some examples showing uses of these options:

# disable ccp1 for all functions -fdisable-tree-ccp1 # disable complete unroll for function whose cgraph node uid is 1 -fenable-tree-cunroll=1 # disable gcse2 for functions at the following ranges [1,1], # [300,400], and [400,1000] # disable gcse2 for functions foo and foo2 -fdisable-rtl-gcse2=foo,foo2 # disable early inlining -fdisable-tree-einline # disable ipa inlining -fdisable-ipa-inline # enable tree full unroll -fenable-tree-unroll


- **`-fchecking`**, **`-fchecking=n`** — turns on internal consistency checking 💅 the default actually depends on how your compiler was configured, so don't @ me. And `-fchecking=2`? That cranks it up to a whole extra level of internal consistency checking that might affect code generation. iykyk.

- **`-frandom-seed=string`** — this gives GCC a seed to use *instead of* random numbers when it's generating certain symbol names that have to be different in every compiled file. It's also used to drop unique stamps into coverage data files and the object files that produce them 🎀 and yes, that means you can use `-frandom-seed` to produce reproducibly identical object files. obsessed.

  The *string* can be a number (decimal, octal or hex) or literally any arbitrary string (in which case it gets converted to a number by computing CRC32). Math girlie behavior fr fr.

  > bestie, the *string* should be different for every file you compile. no cap.

- **`-save-temps`** — stores the usual "temporary" intermediate files *permanently*. Their names are given as auxiliary output files, as specified described under `-dumpbase` and `-dumpdir`. 🦋

  When it's used in combination with the `-x` command-line option, `-save-temps` is sensible enough to avoid overwriting an input source file that has the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using `-save-temps`. considerate queen ✨

- **`-save-temps=cwd`** — equivalent to `-save-temps -dumpdir ./`.

- **`-save-temps=obj`** — equivalent to `-save-temps -dumpdir outdir/`, where *outdir/* is the directory of the output file specified after the `-o` option, including any directory separators. If the `-o` option isn't used, the `-save-temps=obj` switch behaves like `-save-temps=cwd`. flexible icon 💅

- **`-time[=file]`** — reports the CPU time taken by each subprocess in the compilation sequence ⏱️ for C source files, this is the compiler proper and assembler (plus the linker if linking is done).

  Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01 # as 0.00 0.01


The first number on each line is the "user time", that is time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds. 🍓

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 <options> 0.00 0.01 as <options>


The "user time" and the "system time" are moved *before* the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options. detective energy 🔍

- **`-fdump-final-insns[=file]`** — dumps the final internal representation (RTL) to *file*. If the optional argument is omitted (or if *file* is `.`), the name of the dump file is determined by appending `.gkd` to the dump base name, see `-dumpbase`. 🎀

- **`-fcompare-debug[=opts]`** — if no error occurs during compilation, it runs the compiler a **second** time, adding *opts* and `-fcompare-debug-second` to the arguments passed to the second compilation. Then it dumps the final internal representation in both compilations, and prints an error if they differ. double-checking bestie 🔥

If the equal sign is omitted, the default `-gtoggle` is used.

The environment variable `GCC_COMPARE_DEBUG`, if defined, non-empty and nonzero, implicitly enables `-fcompare-debug`. If `GCC_COMPARE_DEBUG` is defined to a string starting with a dash, then it is used for *opts*, otherwise the default `-gtoggle` is used.

`-fcompare-debug=`, with the equal sign but without *opts*, is equivalent to `-fno-compare-debug`, which disables the dumping of the final representation and the second compilation, preventing even `GCC_COMPARE_DEBUG` from taking effect. 🙈

To verify full coverage during `-fcompare-debug` testing, set `GCC_COMPARE_DEBUG` to say `-fcompare-debug-not-overridden`, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting `GCC_COMPARE_DEBUG` to `-w%n-fcompare-debug not overridden` will do.

- **`-fcompare-debug-second`** — this option is implicitly passed to the compiler for the second compilation requested by `-fcompare-debug`, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files get renamed so as to contain the `.gk` additional extension during the second compilation, to avoid overwriting those generated by the first. 🦋

When this option is passed to the compiler driver, it causes the **first** compilation to be skipped, which makes it useful for little other than debugging the compiler proper. lowkey niche 💀

- **`-gtoggle`** — turns off generation of debug info, if leaving out this option generates it, or turns it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with `-fcompare-debug`. ✨

- **`-fvar-tracking-assignments-toggle`** — toggles `-fvar-tracking-assignments`, in the same way that `-gtoggle` toggles `-g`. matching set 🎀

- **`-Q`** — when used on the command line prior to `--help=`, `-Q` acts as a modifier to the help output.

Otherwise, this option makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes. chatty compiler 💅

- **`-ftime-report`** — makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes. ⏱️

If SARIF output of diagnostics was requested via `-fdiagnostics-format=sarif-file` or `-fdiagnostics-format=sarif-stderr` then the `-ftime-report` information is instead emitted in JSON form as part of SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. ⚠️ fair warning bestie

- **`-ftime-report-details`** — records the time consumed by infrastructure parts separately for each pass. spreadsheets girlie 📊

- **`-fira-verbose=n`** — controls the verbosity of the dump file for the integrated register allocator. The default value is `5`. If the value *n* is greater or equal to `10`, the dump output is sent to stderr using the same format as *n* minus 10. 🔥

- **`-flto-report`** — prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via `-flto`).

Disabled by default. 💀

- **`-flto-report-wpa`** — like `-flto-report`, but only print for the WPA phase of link-time optimization. 🎀

- **`-fmem-report`** — makes the compiler print some statistics about permanent memory allocation when it finishes. memory check 🧠✨

- **`-fmem-report-wpa`** — makes the compiler print some statistics about permanent memory allocation for the WPA phase only.

- **`-fpre-ipa-mem-report`**, **`-fpost-ipa-mem-report`** — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. before and after pics 💅

- **`-fmultiflags`** — this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time. 🛠️

Like "TFLAGS", this allows the target libraries to be built for portable baseline environments, while the compiler defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use to build their own programs, if the defaults aren't ideal for their target environment, whereas rebuilding the runtime libraries is usually not as easy or desirable. fr fr 💖

Unlike "TFLAGS", the use of specs enables different flags to be selected for different multilibs. The way to accomplish that is to build with `make TFLAGS=-fmultiflags`, after configuring `--with-specs=%{fmultiflags:...}`.

This option is discarded by the driver once it's done processing driver self spec. ✌️

It is also useful to check that "TFLAGS" are being used to build all target libraries, by configuring a non-bootstrap compiler `--with-specs='%{!fmultiflags:%emissing TFLAGS}'` and building the compiler and target libraries.

- **`-fprofile-report`** — makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes. the vibe check of your profile 🌈

- **`-fstack-usage`** — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending `.su` to the *auxname*. *auxname* is generated from the name of the output file, if explicitly specified and it isn't an executable, otherwise it is the basename of the source file. An entry is made up of three fields: 🎀

- The name of the function.
- A number of bytes.
- One or more qualifiers: `static`, `dynamic`, `bounded`.

The qualifier `static` means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes. iconic and predictable 👑

The qualifier "dynamic" means the function lowkey *manipulates the stack dynamically* — like, on top of the static allocation we already talked about, there are extra stack adjustments happening inside the body of the function, fr fr (think pushing/popping arguments around function calls). And if the "bounded" qualifier is also there? Then the amount of those adjustments is bounded at compile time, and that second field is an upper bound on the total stack the function uses. If "bounded" is *not* there, the adjustments aren't bounded at compile time and that second field only represents the bounded part. Bestie, that distinction matters, don't sleep on it 💅

- **`-fstats`** — emits statistics about front-end processing at the end of the compilation. Ngl this one's niche: it's supported *only* by the C++ front end, and the info is generally only useful to the G++ development team. Like, it's giving "internal tea, not for us" 🫖

- **`-fdbg-cnt-list`** — prints the name and the counter upper bound for *all* the debug counters. Full roster, every single one ✨

- **`-fdbg-cnt=`*counter-value-list* — sets the internal debug counter's lower and upper bound. The *counter-value-list* is a comma-separated list of `name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]` tuples, and each tuple sets the counter's name plus a list of closed intervals. The `lower_bound` is optional and zero-initialized if you don't set it. Example time, because this one's giving math homework: with `-fdbg-cnt=dce:2-4:10-11,tail_call:10`, `dbg_cnt(dce)` returns true only for the second, third, fourth, tenth and eleventh invocation. And for `dbg_cnt(tail_call)`, true comes back for the first 10 invocations. Iconic little interval party 🎀

- **`-print-file-name=`*library* — prints the full absolute name of the library file *library* that would be used when linking — and does literally nothing else. With this option GCC does not compile or link anything; it just prints the file name. Zero drama, one output 💖

- **`-print-multi-directory`** — prints the directory name corresponding to the multilib selected by any other switches chillin' in the command line. This directory is supposed to exist in `GCC_EXEC_PREFIX`.

- **`-print-multi-lib`** — prints the mapping from multilib directory names to the compiler switches that enable them. The directory name is separated from the switches by `;`, and each switch starts with an `@` instead of the `-`, with no spaces between multiple switches. This is supposed to ease shell processing — like, it's being considerate of your shell, we love a thoughtful flag 🫶

- **`-print-multi-os-directory`** — prints the path to OS libraries for the selected multilib, relative to some `lib` subdirectory. If OS libraries are present in the `lib` subdirectory and no multilibs are used, this is usually just `.`; if OS libraries are present in `lib`suffix sibling directories, it prints e.g. `../lib64`, `../lib` or `../lib32`; or if OS libraries are present in `lib/`*subdir* subdirectories it prints e.g. `amd64`, `sparcv9` or `ev6`. So many paths, so little time 😭

- **`-print-multiarch`** — prints the path to OS libraries for the selected multiarch, relative to some `lib` subdirectory.

- **`-print-prog-name=`*program* — like `-print-file-name`, but it searches for a *program* such as `cpp`. Same energy, different target 🎯

- **`-print-libgcc-file-name`** — same as `-print-file-name=libgcc.a`.

This one's genuinely useful when you use `-nostdlib` or `-nodefaultlibs` but you *do* want to link with `libgcc.a`. You can do:

gcc -nostdlib <files>... gcc -print-libgcc-file-name


Chef's kiss use of backticks, no cap 👑

- **`-print-search-dirs`** — prints the name of the configured installation directory and a list of program and library directories `gcc` searches — and does nothing else. Periodt.

This is useful when `gcc` prints the error message `installation problem, cannot exec cpp0: No such file or directory`. To resolve this you either need to put `cpp0` and the other compiler components where `gcc` expects to find them, or you can set the environment variable `GCC_EXEC_PREFIX` to the directory where you installed them. Don't forget the trailing `/` — seriously, the trailing slash is not optional, it's the whole vibe check 💀

- **`-print-sysroot`** — prints the target sysroot directory used during compilation. This is the target sysroot specified either at configure time or using the `--sysroot` option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. Silence. Crickets. 🦗

- **`-print-sysroot-headers-suffix`** — prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler isn't configured with such a suffix — and does nothing else.

- **`-dumpmachine`** — prints the compiler's target machine (for example, `i686-pc-linux-gnu`) — and does nothing else. One fact, drop the mic 🎤

- **`-dumpversion`** — prints the compiler version (for example, `3.0`, `6.3.0` or `7`) — and does nothing else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured, it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version). So yeah, the length itself is a whole personality trait of your build 🌸

- **`-dumpfullversion`** — prints the full compiler version — and does nothing else. This output is *always* three numbers separated by dots: major, minor and patchlevel version. No surprises, unlike `-dumpversion` 🙈

- **`-dumpspecs`** — prints the compiler's built-in specs — and does nothing else. (This is used when GCC itself is being built, so it's giving "GCC building GCC", very main character energy 🌈)

---

## 🌍 Machine-Dependent Options 🌍

Every target machine GCC supports can have its own options — for example, to let you compile for a particular processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-specific options start with `-m`. So when you see an `-m` flag, that's your cue: this is machine-specific, bestie, read carefully 💅

Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform. Friendly neighbors 🤝

### 🦋 AArch64 Options 🦋

These options are defined for AArch64 implementations:

- **`-mabi=`*name* — generates code for the specified data model. The permissible values are `ilp32` for the SysV-like data model where `int`, `long int` and pointers are 32 bits, and `lp64` for the SysV-like data model where `int` is 32 bits but `long int` and pointers are 64 bits.

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are **not link-compatible** — you must compile your entire program with the same ABI and link with a compatible set of libraries. Do not mix, I'm begging 😭

> Heads up: the `ilp32` model is **deprecated**. Like, it's giving legacy, treat accordingly 💀

- **`-mbig-endian`** — generates big-endian code. This is the default when GCC is configured for an `aarch64_be-*-*` target.

- **`-mgeneral-regs-only`** — generates code that uses only the general-purpose registers. This prevents the compiler from using floating-point and Advanced SIMD registers, but it will **not** impose any restrictions on the assembler. So the compiler's on a diet but the assembler is eating whatever it wants 🍓

- **`-mlittle-endian`** — generates little-endian code. This is the default when GCC is configured for an `aarch64-*-*` but *not* an `aarch64_be-*-*` target.

- **`-mcmodel=tiny`** — generates code for the tiny code model. The program and its statically defined symbols must be within `1MB` of each other. Programs can be statically or dynamically linked. Smol model, smol distances 🧚

- **`-mcmodel=small`** — generates code for the small code model. The program and its statically defined symbols must be within `4GB` of each other. Programs can be statically or dynamically linked. **This is the default code model.** The people's champ 👑

- **`-mcmodel=large`** — generates code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be **statically linked only**. Also, `-mcmodel=large` is incompatible with `-mabi=ilp32`, `-fpic` and `-fPIC`. Incompatible, bestie — do not try to make them get along, it will not end well 💀

- **`-mtp=`*name* — specifies the system register to use as a thread pointer. The valid values are `tpidr_el0`, `tpidrro_el0`, `tpidr_el1`, `tpidr_el2`, `tpidr_el3`. For backwards compatibility the aliases `el0`, `el1`, `el2`, `el3` are also accepted. The default setting is `tpidr_el0`. It is recommended to compile all code intended to interoperate with the same value of this option, to avoid accessing a different thread pointer from the wrong exception level. Consistency is the whole game here, fr fr 🎀

- **`-mstrict-align`**
- **`-mno-strict-align`** — avoid or allow generating memory accesses that may not be aligned on a natural object boundary, as described in the architecture specification. Your choice: strict vibes or chaotic vibes ✨

- **`-momit-leaf-frame-pointer`**
- **`-mno-omit-leaf-frame-pointer`** — omit or keep the frame pointer in leaf functions. The former behavior (omitting) is the default.

- **`-mstack-protector-guard=`*guard*
- **`-mstack-protector-guard-reg=`*reg*
- **`-mstack-protector-guard-offset=`*offset* — generate stack protection code using a canary at *guard*. Supported locations are `global` for a global canary, or `sysreg` for a canary in an appropriate system register.

With the latter choice, the options `-mstack-protector-guard-reg=`*reg* and `-mstack-protector-guard-offset=`*offset* furthermore specify which system register to use as the base register for reading the canary, and from what offset from that base register. There is **no default** register or offset, because this is entirely for use within the Linux kernel. You're on your own, kernel people 🫡

- **`-mtls-dialect=desc`** — uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. **This is the default.** ✨

- **`-mtls-dialect=traditional`** — uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. Old school, still valid 💜

- **`-mtls-size=`*size* — specifies the bit size of immediate TLS offsets. Valid values are `12`, `24`, `32`, `48`. This option requires binutils `2.26` or newer — so check your binutils first, bestie, don't come crying 😭

- **`-mfix-cortex-a53-835769`**
- **`-mno-fix-cortex-a53-835769`** — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a `NOP` instruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag will be **ignored** if an architecture or cpu is specified on the command line which does not need the workaround. It's a bug workaround, it knows when it's not wanted 💅

- **`-mfix-cortex-a53-843419`**
- **`-mno-fix-cortex-a53-843419`** — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made at **link time**, and this will only pass the corresponding flag to the linker. This flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround. Link-time behavior, so don't go looking for it in the compile step 👀

- **`-mlow-precision-recip-sqrt`**
- **`-mno-low-precision-recip-sqrt`** — enable or disable the reciprocal square root approximation. This option only has an effect if `-ffast-math` or `-funsafe-math-optimizations` is used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. So yes, precision goes *down* — that's the trade, that's the deal 🔥

---

- **`-mlow-precision-sqrt`**, **`-mno-low-precision-sqrt`** — turns the square root approximation on or off 💅. Bestie, this one is a *conditional* slay: it only does anything if `-ffast-math` or `-funsafe-math-optimizations` is used as well. When it's on, it drops sqrt precision down to about 16 bits for single precision and about 32 bits for double precision 😭. And if you enable it, it implies `-mlow-precision-recip-sqrt`. Package deal, no cap.

- **`-mlow-precision-div`**, **`-mno-low-precision-div`** — same energy but for division ✨. Also only has an effect if `-ffast-math` or `-funsafe-math-optimizations` is used as well. Enabling it reduces division result precision to about 16 bits for single precision and about 32 bits for double precision.

- **`-mtrack-speculation`**, **`-mno-track-speculation`** — enable or disable generating extra code that tracks speculative execution through conditional branches 🕵️‍♀️. The tracking state then gets used by the compiler when it expands calls to `__builtin_speculation_safe_copy`, which lets it generate a more efficient code sequence. It's giving safety-but-also-fast.

- **`-moutline-atomics`**, **`-mno-outline-atomics`** — enable or disable calls to out-of-line helpers that implement atomic operations 🎀. These helpers figure it out at runtime: can we use the LSE instructions from ARMv8.1-A? Slay. If not, they fall back to the load/store-exclusive instructions that live in the base ARMv8.0 ISA.

Heads up though 🙈 — this option is **only** applicable when compiling for the base ARMv8.0 instruction set. If you're on a later revision, like `-march=armv8.1-a` or `-march=armv8-a+lse`, the ARMv8.1-Atomics instructions get used directly. Same thing when you use `-mcpu=` with a cpu that supports the `lse` feature. **This option is on by default.**

- **`-march=`***name* — specify the name of the target architecture, and optionally one or more feature modifiers 👑. The form is:

-march=arch{+[no]feature}*


  Here's the rundown of what `arch` values you're allowed and what features they turn on by default, bestie:

  - `armv8-a` — Armv8-A — includes by default: `+fp`, `+simd`
  - `armv8.1-a` — Armv8.1-A — includes by default: `armv8-a`, `+crc`, `+lse`, `+rdma`
  - `armv8.2-a` — Armv8.2-A — includes by default: `armv8.1-a`
  - `armv8.3-a` — Armv8.3-A — includes by default: `armv8.2-a`, `+pauth`, `+fcma`, `+jscvt`
  - `armv8.4-a` — Armv8.4-A — includes by default: `armv8.3-a`, `+flagm`, `+fp16fml`, `+dotprod`, `+rcpc2`
  - `armv8.5-a` — Armv8.5-A — includes by default: `armv8.4-a`, `+sb`, `+ssbs`, `+predres`, `+frintts`, `+flagm2`
  - `armv8.6-a` — Armv8.6-A — includes by default: `armv8.5-a`, `+bf16`, `+i8mm`
  - `armv8.7-a` — Armv8.7-A — includes by default: `armv8.6-a`, `+wfxt`, `+xs`
  - `armv8.8-a` — Armv8.8-a — includes by default: `armv8.7-a`, `+mops`
  - `armv8.9-a` — Armv8.9-a — includes by default: `armv8.8-a`
  - `armv9-a` — Armv9-A — includes by default: `armv8.5-a`, `+sve`, `+sve2`
  - `armv9.1-a` — Armv9.1-A — includes by default: `armv9-a`, `+bf16`, `+i8mm`
  - `armv9.2-a` — Armv9.2-A — includes by default: `armv9.1-a`, `+wfxt`, `+xs`
  - `armv9.3-a` — Armv9.3-A — includes by default: `armv9.2-a`, `+mops`
  - `armv9.4-a` — Armv9.4-A — includes by default: `armv9.3-a`, `+sve2p1`
  - `armv9.5-a` — Armv9.4-A — includes by default: `armv9.4-a`, `cpa`, `+faminmax`, `+lut`
  - `armv8-r` — Armv8-R — includes by default: `armv8-r`

  So `native` 💖 — that value is available on native AArch64 GNU/Linux, and it makes the compiler pick the architecture of the host system. Fair warning: the option has **no effect** if the compiler can't recognize the architecture of the host system 😬. And when `-march=native` is given and no other `-mcpu` or `-mtune` is given, then GCC will pick the host CPU as the CPU to tune for **as well as** select architecture features from. Basically, `-march=native` is treated as `-mcpu=native`. Iconic.

  The permissible values for *feature* are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers 📎. Where conflicting feature modifiers are specified, the right-most feature is used. Right-most wins, periodt.

  GCC uses *name* to determine what kind of instructions it can emit when generating assembly code. If `-march` is specified without either of `-mtune` or `-mcpu` also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture ✨.

- **`-mtune=`***name* — specify the name of the target processor that GCC should tune performance for 🎯. Permissible values are: `generic`, `cortex-a35`, `cortex-a53`, `cortex-a55`, `cortex-a57`, `cortex-a72`, `cortex-a73`, `cortex-a75`, `cortex-a76`, `cortex-a76ae`, `cortex-a77`, `cortex-a65`, `cortex-a65ae`, `cortex-a34`, `cortex-a78`, `cortex-a78ae`, `cortex-a78c`, `ares`, `exynos-m1`, `emag`, `falkor`, `oryon-1`, `neoverse-512tvb`, `neoverse-e1`, `neoverse-n1`, `neoverse-n2`, `neoverse-v1`, `neoverse-v2`, `grace`, `neoverse-v3`, `neoverse-v3ae`, `neoverse-n3`, `olympus`, `cortex-a725`, `cortex-x925`, `qdf24xx`, `saphira`, `phecda`, `xgene1`, `vulcan`, `octeontx`, `octeontx81`, `octeontx83`, `octeontx2`, `octeontx2t98`, `octeontx2t96`, `octeontx2t93`, `octeontx2f95`, `octeontx2f95n`, `octeontx2f95mm`, `a64fx`, `fujitsu-monaka`, `thunderx`, `thunderxt88`, `thunderxt88p1`, `thunderxt81`, `tsv110`, `thunderxt83`, `thunderx2t99`, `thunderx3t110`, `zeus`, `cortex-a57.cortex-a53`, `cortex-a72.cortex-a53`, `cortex-a73.cortex-a35`, `cortex-a73.cortex-a53`, `cortex-a75.cortex-a55`, `cortex-a76.cortex-a55`, `cortex-r82`, `cortex-r82ae`, `cortex-x1`, `cortex-x1c`, `cortex-x2`, `cortex-x3`, `cortex-x4`, `cortex-a510`, `cortex-a520`, `cortex-a520ae`, `cortex-a710`, `cortex-a715`, `cortex-a720`, `cortex-a720ae`, `ampere1`, `ampere1a`, `ampere1b`, `cobalt-100`, `apple-m1`, `apple-m2`, `apple-m3` and `native`.

  The values `cortex-a57.cortex-a53`, `cortex-a72.cortex-a53`, `cortex-a73.cortex-a35`, `cortex-a73.cortex-a53`, `cortex-a75.cortex-a55`, `cortex-a76.cortex-a55`, `apple-m1`, `apple-m2`, `apple-m3`, `gb10` specify that GCC should tune for a big.LITTLE system 🧚 (two vibes, one chip, we love a collab).

  The value `neoverse-512tvb` specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle 🌈. In other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1, but it's more specific than the default tuning described below. Lowkey the perfect middle child.

  Additionally, on native AArch64 GNU/Linux systems, the value `native` tunes performance to the host system 💻. This option has no effect if the compiler is unable to recognize the processor of the host system, sorry bestie 🙈.

  Where none of `-mtune=`, `-mcpu=` or `-march=` are specified, the code is tuned to perform well across a range of target processors.

  > 🚨 Real talk: this option **cannot** be suffixed by feature modifiers. Don't try it.

- **`-mcpu=`***name* — specify the name of the target processor, optionally suffixed by one or more feature modifiers 💅. The form is:

-mcpu=cpu{+[no]feature}*


  where the permissible values for *cpu* are the same as those available for `-mtune` 👯. The permissible values for *feature* are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. (Right-most stays winning.)

  GCC uses *name* to determine what kind of instructions it can emit when generating assembly code (as if by `-march`) **and** to determine the target processor to tune for performance (as if by `-mtune`). Two-for-one, we stan 🫶. Where this option is used in conjunction with `-march` or `-mtune`, those options take precedence over the appropriate part of this option.

  And `-mcpu=neoverse-512tvb` is special ⭐ — it doesn't refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by `-march`, `-mcpu=neoverse-512tvb` generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. And unless overridden by `-mtune`, `-mcpu=neoverse-512tvb` tunes code in the same way as for `-mtune=neoverse-512tvb`. Consistent queen.

- **`-moverride=`***string* — override tuning decisions made by the back-end in response to a `-mtune=` switch 🔧. And here's the thing: the syntax, semantics, and accepted values for *string* in this option are **not guaranteed to be consistent across releases** 💀. So don't go building your whole life on it.

  > 🛠️ This option is only intended to be useful when developing GCC. It's a compiler-dev thing, not a you thing, unless you *are* the compiler dev, in which case, hello, icon.

- **`-mverbose-cost-dump`** — enable verbose cost model dumping in the debug dump files 🗑️✨. This option is provided for use in debugging the compiler. That's it, that's the whole option.

- **`-mpc-relative-literal-loads`**, **`-mno-pc-relative-literal-loads`** — enable or disable PC-relative literal loads 🎀. With this on, literal pools are accessed using a single instruction and emitted after each function. Which means the maximum size of functions is limited to 1MB 😳 — so if you're writing a *novel* of a function, this is your villain origin story. This is enabled by default for `-mcmodel=tiny`.

- **`-msign-return-address=`***scope* — select the function scope on which return address signing gets applied 🖊️. Permissible values:

  - `none` — disables return address signing
  - `non-leaf` — enables pointer signing for functions which are not leaf functions
  - `all` — enables pointer signing for all functions

  The default value is `none`. And heads up: this option **has been deprecated by `-mbranch-protection`** ⚠️. Like, it still exists, but it's giving legacy.

- **`-mbranch-protection=`**`none`|`standard`|`pac-ret[+leaf+b-key]`|`bti`|`gcs` — select the branch protection features to use 🛡️. Breakdown time:

  - `none` is the default and turns off **all** types of branch protection.
  - `standard` turns on all types of branch protection features. If a feature has additional tuning options, then `standard` sets it to its standard level.
  - `pac-ret[+leaf]` turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optional argument `leaf` can be used to extend the signing to include leaf functions 🌿. The optional argument `b-key` can be used to sign the functions with the B-key instead of the A-key.
  - `bti` turns on branch target identification mechanism.
  - `gcs` turns on guarded control stack compatible code generation.

- **`-mharden-sls=`***opts* — enable compiler hardening against straight line speculation, aka SLS 🔥. *opts* is a comma-separated list of the following options:

  - `retbr`
  - `blr`

  In addition, `-mharden-sls=all` enables all SLS hardening while `-mharden-sls=none` disables all SLS hardening. All or nothing, very main character.

- **`-mearly-ra=`***scope* — determine when to enable an early register allocation pass 🧠✨. This pass runs **before** instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, such as SME2's strided `LD1` and `ST1`. Fancy instruction behaviour, we love to see it 💜.

The possible values of `scope` are honestly a whole lil trio:

- `all` — runs the pass on, like, *all* the functions. everyone's invited 💅
- `strided` — runs the pass only on the functions that have access to strided multi-register instructions. exclusive guest list, sorry
- `none` — disables the pass entirely. the party's cancelled 🫠

`-mearly-ra=all` is the default for `-O2` and above, and also for `-Os`. `-mearly-ra=none` is the default otherwise. periodt.

- **`-mearly-ldp-fusion`** — turns on the copy of the AArch64 load/store pair fusion pass that runs *before* register allocation. it's on by default at `-O` and above ✨
- **`-mlate-ldp-fusion`** — turns on the copy of that same AArch64 load/store pair fusion pass, but this one runs *after* register allocation. also on by default at `-O` and above ✨

- **`-msve-vector-bits=`*`bits`* — tells GCC how many bits are in an SVE vector register. ngl this option only does anything when SVE is enabled, so like… no SVE, no vibes.

GCC does SVE codegen in two whole flavors: "vector-length agnostic" output that works with *any* size of vector register, and "vector-length specific" output that lets GCC make assumptions about the vector length when that's actually useful for optimization reasons. the possible values of *`bits`* are: `scalable`, `128`, `256`, `512`, `1024` and `2048`. specifying `scalable` selects the vector-length agnostic output. at present, `-msve-vector-bits=128` *also* generates vector-length agnostic output for big-endian targets. all the other values generate vector-length specific code. heads up though: the behavior of these values may change in future releases, and no value except `scalable` should be relied on for producing code that's portable across different hardware SVE vector lengths. 💀

the default is `-msve-vector-bits=scalable`, which produces vector-length agnostic code. she's flexible like that 🧚

- **`-Wexperimental-fmv-target`** — warns you about use of experimental Function Multi Versioning. the Arm C Language Extension spec for Function Multi Versioning is *beta* and subject to change, so like… any usage of FMV comes with the caveat that future behavior change and incompatibility is likely. you've been warned bestie 🙈

### 🎀 Feature modifiers used with `-march` and `-mcpu` 🎀

so the feature modifiers you can use with `-march` and `-mcpu` can be any of the following — and also their inverses, `nofeature`. yes, the "no" prefix works on all of these, go wild:

- **`crc`** — enable CRC extension. this one's on by default for `-march=armv8.1-a`.
- **`crypto`** — enable Crypto extension. this also enables Advanced SIMD and floating-point instructions. two-for-one 💖
- **`fp`** — enable floating-point instructions. on by default for *all* possible values for options `-march` and `-mcpu`.
- **`simd`** — enable Advanced SIMD instructions. this also enables floating-point instructions. on by default for all possible values for options `-march` and `-mcpu`.
- **`sve`** — enable Scalable Vector Extension instructions. this also enables Advanced SIMD and floating-point instructions.
- **`lse`** — enable Large System Extension instructions. on by default for `-march=armv8.1-a`.
- **`rdma`** — enable Round Double Multiply Accumulate instructions. on by default for `-march=armv8.1-a`.
- **`fp16`** — enable FP16 extension. this also enables floating-point instructions.
- **`fp16fml`** — enable FP16 fmla extension. this also enables FP16 extensions and floating-point instructions. enabled by default for `-march=armv8.4-a`. use of this option with architectures prior to Armv8.2-A is **not supported**. 🚫
- **`rcpc`** — enable the RCpc extension. this enables the use of the `LDAPR` instructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension. sharing is caring 🫶
- **`dotprod`** — enable the Dot Product extension. this also enables Advanced SIMD instructions.
- **`aes`** — enable the Armv8-a `aes` and `pmull` crypto extension. this also enables Advanced SIMD instructions.
- **`sha2`** — enable the Armv8-a `sha2` crypto extension. this also enables Advanced SIMD instructions.
- **`sha3`** — enable the `sha512` and `sha3` crypto extension. this also enables Advanced SIMD instructions. use of this option with architectures prior to Armv8.2-A is **not supported**.
- **`sm4`** — enable the `sm3` and `sm4` crypto extension. this also enables Advanced SIMD instructions. use of this option with architectures prior to Armv8.2-A is **not supported**.
- **`profile`** — enable the Statistical Profiling extension. this option is *only* to enable the extension at the assembler level and does **not** affect code generation.
- **`rng`** — enable the Armv8.5-a Random Number instructions. again: assembler level only, does not affect code generation.
- **`memtag`** — enable the Armv8.5-a Memory Tagging Extensions. use of this option with architectures prior to Armv8.5-A is **not supported**.
- **`sb`** — enable the Armv8-a Speculation Barrier instruction. assembler level only, does not affect code generation. enabled by default for `-march=armv8.5-a`.
- **`ssbs`** — enable the Armv8-a Speculative Store Bypass Safe instruction. assembler level only, does not affect code generation. enabled by default for `-march=armv8.5-a`.
- **`predres`** — enable the Armv8-a Execution and Data Prediction Restriction instructions. assembler level only, does not affect code generation. enabled by default for `-march=armv8.5-a`.
- **`sve2`** — enable the Armv8-a Scalable Vector Extension 2. this also enables SVE instructions.
- **`sve2-bitperm`** — enable SVE2 bitperm instructions. this also enables SVE2 instructions.
- **`sve2-sm4`** — enable SVE2 sm4 instructions. this also enables SVE2 instructions.
- **`sve2-aes`** — enable SVE2 aes instructions. this also enables SVE2 instructions.
- **`sve2-sha3`** — enable SVE2 sha3 instructions. this also enables SVE2 instructions.
- **`sve2p1`** — enable SVE2.1 instructions. this also enables SVE2 instructions.
- **`tme`** — enable the Transactional Memory Extension.
- **`i8mm`** — enable 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions. enabled by default for `-march=armv8.6-a`. use of this option with architectures prior to Armv8.2-A is **not supported**.
- **`f32mm`** — enable 32-bit Floating point Matrix Multiply instructions. this also enables SVE instructions. use of this option with architectures prior to Armv8.2-A is **not supported**.
- **`f64mm`** — enable 64-bit Floating point Matrix Multiply instructions. this also enables SVE instructions. use of this option with architectures prior to Armv8.2-A is **not supported**.
- **`bf16`** — enable brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions. enabled by default for `-march=armv8.6-a`. use of this option with architectures prior to Armv8.2-A is **not supported**.
- **`ls64`** — enable the 64-byte atomic load and store instructions for accelerators.
- **`mops`** — enable the instructions to accelerate memory operations like `memcpy`, `memmove`, `memset`. enabled by default for `-march=armv8.8-a`
- **`flagm`** — enable the Flag Manipulation instructions Extension.
- **`flagm2`** — enable the FlagM2 flag conversion instructions.
- **`pauth`** — enable the Pointer Authentication Extension.
- **`cssc`** — enable the Common Short Sequence Compression instructions.
- **`sme`** — enable the Scalable Matrix Extension. this is only supported when SVE2 is also enabled. 👀
- **`sme-i16i64`** — enable the `FEAT_SME_I16I64` extension to SME. this also enables SME instructions.
- **`sme-f64f64`** — enable the `FEAT_SME_F64F64` extension to SME. this also enables SME instructions.
- **`sme2`** — enable the Scalable Matrix Extension 2. this also enables SME instructions.
- **`sme-b16b16`** — enable the `FEAT_SME_B16B16` extension to SME. this also enables SME2 and `SVE_B16B16` instructions.
- **`sme-f16f16`** — enable the `FEAT_SME_F16F16` extension to SME. this also enables SME2 instructions.
- **`sme2p1`** — enable the Scalable Matrix Extension version 2.1. this also enables SME2 instructions.
- **`fcma`** — enable the complex number SIMD extensions.
- **`jscvt`** — enable the `fjcvtzs` JavaScript conversion instruction.
- **`frintts`** — enable floating-point round to integral value instructions.
- **`wfxt`** — enable `wfet` and `wfit` instructions.
- **`xs`** — enable the XS memory attribute extension.
- **`lse128`** — enable the LSE128 128-bit atomic instructions extension. this also enables LSE instructions.
- **`d128`** — enable support for 128-bit system register read/write instructions. this also enables the LSE128 extension.
- **`gcs`** — enable support for Armv9.4-a Guarded Control Stack extension.
- **`the`** — enable support for Armv8.9-a/9.4-a translation hardening extension.
- **`rcpc2`** — enable the RCpc2 extension.
- **`rcpc3`** — enable the RCpc3 (Release Consistency) extension.
- **`fp8`** — enable the fp8 (8-bit floating point) extension.
- **`fp8fma`** — enable the fp8 (8-bit floating point) multiply accumulate extension.
- **`ssve-fp8fma`** — enable the fp8 (8-bit floating point) multiply accumulate extension in streaming mode. 🌊
- **`fp8dot4`** — enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension.
- **`ssve-fp8dot4`** — enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.
- **`fp8dot2`** — enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension.
- **`ssve-fp8dot2`** — enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.
- **`faminmax`** — enable the Floating Point Absolute Maximum/Minimum extension.
- **`lut`** — enable the Lookup Table extension.
- **`cpa`** — enable the Checked Pointer Arithmetic instructions.
- **`sve-b16b16`** — enable the SVE non-widening brain floating-point (`bf16`) extension. this only has an effect when `sve2` or `sme2` are also enabled.

okay and here's the tea on implications, because these features drag their friends along: feature `crypto` implies `aes`, `sha2`, and `simd`, which implies `fp`. conversely, `nofp` implies `nosimd`, which implies `nocrypto`, `noaes` and `nosha2`. it's giving chain reaction 💅

---

### 🌸 Adapteva Epiphany Options 🌸

these `-m` options are defined for Adapteva Epiphany (yes, her name is Epiphany, iconic):

- **`-mhalf-reg-file`** — don't allocate any register in the range `r32`…`r63`. that lets code run on hardware variants that lack these registers. frugal queen 🎀

- **`-mprefer-short-insn-regs`** — tells the compiler to *preferentially* hand out the registers that make short instructions possible ✨. Ngl though, this can bump up the instruction count, so overall code size might go down **or** up — it's giving gamble energy 🎲.

- **`-mbranch-cost=num`** — sets the cost of branches to roughly *num* "simple" instructions 💸. Heads up: this cost is only a heuristic and is **not** guaranteed to give you consistent results across releases 🙈.

- **`-mcmove`** — enables the generation of conditional moves 🎀.

- **`-mnops=num`** — emits *num* NOPs before **every other** generated instruction 💀 (yes, before every other one, that's the vibe).

- **`-mno-soft-cmpsf`** — for single-precision floating-point comparisons, emit an `fsub` instruction and test the flags 🔥. This is faster than a software comparison, but can get **incorrect results** in the presence of NaNs 😭, or when two different small numbers are compared such that their difference is calculated as zero. The default is `-msoft-cmpsf`, which uses slower but IEEE-compliant software comparisons 🫶.

- **`-mstack-offset=num`** — sets the offset between the top of the stack and the stack pointer 🧚. E.g., a value of `8` means that the eight bytes in the range `sp+0...sp+7` can be used by leaf functions without stack allocation. Values other than `8` or `16` are untested and unlikely to work 💀. Also note this option **changes the ABI**; compiling a program with a different stack offset than the libraries have been compiled with generally does **not** work 😬. It can be useful if you want to evaluate whether a different stack offset would give you better code, but to actually *use* a different stack offset to build working programs, it's recommended to configure the toolchain with the appropriate `--with-stack-offset=num` option 👑

- **`-mno-round-nearest`** — makes the scheduler assume the rounding mode has been set to truncating 🌸. The default is `-mround-nearest`.

- **`-mlong-calls`** — if not otherwise specified by an attribute, assume **all** calls might be beyond the offset range of the `b` / `bl` instructions, and therefore load the function address into a register before performing a (otherwise direct) call 🦋. This is the default ⭐.

- **`-mshort-calls`** — if not otherwise specified by an attribute, assume all direct calls are in range of the `b` / `bl` instructions, so use these instructions for direct calls 💅. The default is `-mlong-calls`.

- **`-msmall16`** — assume addresses can be loaded as 16-bit unsigned values 🎀. This does **not** apply to function addresses for which `-mlong-calls` semantics are in effect.

- **`-mfp-mode=mode`** — sets the prevailing mode of the floating-point unit ✨. This determines the floating-point mode that's provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches 🔥.

  *mode* can be set to one of the following values:

  - `caller` — any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions 💜. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode 👑.
  - `truncate` — this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode 🌈. That includes conversion from floating point to integer.
  - `round-nearest` — this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode 🎀.
  - `int` — this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate 💅.

  The default is `-mfp-mode=caller` ⭐.

- **`-mno-split-lohi`**, **`-mno-postinc`**, **`-mno-postmodify`** — code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses 🙈. The defaults are `msplit-lohi`, `-mpost-inc`, and `-mpost-modify` ✨.

- **`-mnovect-double`** — changes the preferred SIMD mode to `SImode` 🦋. The default is `-mvect-double`, which uses `DImode` as the preferred SIMD mode.

- **`-max-vect-align=num`** — the maximum alignment for SIMD vector mode types 🌸. *num* may be `4` or `8`. The default is `8`. Note that this is an **ABI change**, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types 😭.

- **`-msplit-vecmove-early`** — splits vector moves into single word moves before reload 💀. In theory this can give better register allocation, but so far the reverse seems to be generally the case 🙈.

- **`-m1reg-reg`** — specifies a register to hold the constant `-1`, which makes loading small negative constants and certain bitmasks faster ⚡. Allowable values for *reg* are `r43` and `r63`, which specify use of that register as a fixed register, and `none`, which means no register is used for this purpose 🎀. The default is `-m1reg-none`.

---

## 🎮 AMD GCN Options 👑

These options are defined specifically for the AMD GCN port 💜.

- **`-march=gpu`**, **`-mtune=gpu`** — set the architecture type or tuning for *gpu* ✨. Supported values for *gpu* are:

  - `gfx900` — compile for GCN5 Vega 10 devices (`gfx900`) 🔥.
  - `gfx902` — compile for GCN5 Vega `gfx902` devices. (Experimental) 🧪
  - `gfx904` — compile for GCN5 Vega `gfx904` devices. (Experimental) 🧪
  - `gfx906` — compile for GCN5 Vega 20 devices (`gfx906`) ⭐.
  - `gfx908` — compile for CDNA1 Instinct MI100 series devices (`gfx908`) 👑.
  - `gfx909` — compile for GCN5 Vega `gfx909` devices. (Experimental) 🧪
  - `gfx90a` — compile for CDNA2 Instinct MI200 series devices (`gfx90a`) 👑.
  - `gfx90c` — compile for GCN5 Vega 7 devices (`gfx90c`) 🎀.
  - `gfx9-generic` — compile generic code for Vega devices, executable on the following subset of GFX9 devices: `gfx900`, `gfx902`, `gfx904`, `gfx906`, `gfx909` and `gfx90c`. (Experimental) 🧪
  - `gfx1030` — compile for RDNA2 `gfx1030` devices (GFX10 series) 🌈.
  - `gfx1031` — compile for RDNA2 `gfx1031` devices (GFX10 series). (Experimental) 🧪
  - `gfx1032` — compile for RDNA2 `gfx1032` devices (GFX10 series). (Experimental) 🧪
  - `gfx1033` — compile for RDNA2 `gfx1033` devices (GFX10 series). (Experimental) 🧪
  - `gfx1034` — compile for RDNA2 `gfx1034` devices (GFX10 series). (Experimental) 🧪
  - `gfx1035` — compile for RDNA2 `gfx1035` devices (GFX10 series). (Experimental) 🧪
  - `gfx1036` — compile for RDNA2 `gfx1036` devices (GFX10 series) 💅.
  - `gfx10-3-generic` — compile generic code for GFX10-3 devices, executable on `gfx1030`, `gfx1031`, `gfx1032`, `gfx1033`, `gfx1034`, `gfx1035`, and `gfx1036`. (Experimental) 🧪
  - `gfx1100` — compile for RDNA3 `gfx1100` devices (GFX11 series) 🔥.
  - `gfx1101` — compile for RDNA3 `gfx1101` devices (GFX11 series). (Experimental) 🧪
  - `gfx1102` — compile for RDNA3 `gfx1102` devices (GFX11 series). (Experimental) 🧪
  - `gfx1103` — compile for RDNA3 `gfx1103` devices (GFX11 series) ⭐.
  - `gfx1150` — compile for RDNA3 `gfx1150` devices (GFX11 series). (Experimental) 🧪
  - `gfx1151` — compile for RDNA3 `gfx1151` devices (GFX11 series). (Experimental) 🧪
  - `gfx1152` — compile for RDNA3 `gfx1152` devices (GFX11 series). (Experimental) 🧪
  - `gfx1153` — compile for RDNA3 `gfx1153` devices (GFX11 series). (Experimental) 🧪
  - `gfx11-generic` — compile generic code for GFX11 devices, executable on `gfx1100`, `gfx1101`, `gfx1102`, `gfx1103`, `gfx1150`, `gfx1151`, `gfx1152`, and `gfx1153`. (Experimental) 🧪

- **`-msram-ecc=on`**, **`-msram-ecc=off`**, **`-msram-ecc=any`** — compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode ✨. This feature can be enabled per-process on some devices 🦋. The compiled code **must** match the device mode 😬. The default is `any`, for devices that support it 🎀.

- **`-mstack-size=bytes`** — specifies how many bytes of stack space will be requested for each GPU thread (wave-front) 💅. Beware that there may be many threads and limited memory available 😭. The size of the stack allocation may also have an impact on run-time performance ⚡. The default is `32KB` when using OpenACC or OpenMP, and `1MB` otherwise 🌸.

- **`-mxnack=on`**, **`-mxnack=off`**, **`-mxnack=any`** — compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode 👑. Some devices always require XNACK and some allow the user to configure XNACK 🎀. The compiled code **must** match the device mode 😬. The default is `-mxnack=any` on devices that support Unified Shared Memory, and `-mxnack=no` otherwise 🙈.

---

## 🏹 ARC Options 🦋

The following options control the architecture variant for which code is being compiled ✨:

- **`-mbarrel-shifter`** — generate instructions supported by the barrel shifter 🎀. This is the default unless `-mcpu=ARC601` or `-mcpu=ARCEM` is in effect 💅.

- **`-mjli-always`** — force calling a function using the `jli_s` instruction 🔥. This option is valid **only** for the ARCv2 architecture ⚠️.

- **`-mcpu=cpu`** — set architecture type, register usage, and instruction scheduling parameters for *cpu* 👑. There are also shortcut alias options available for backward compatibility and convenience 🫶. Supported values for *cpu* are:

  - `arc600` — compile for ARC600. Aliases: `-mA6`, `-mARC600` 🎀.
  - `arc601` — compile for ARC601. Alias: `-mARC601` 🌸.
  - `arc700` — compile for ARC700. Aliases: `-mA7`, `-mARC700`. This is the default when configured with `--with-cpu=arc700` ⭐.
  - `arcem` — compile for ARC EM 💜.
  - `archs` — compile for ARC HS 🦋.
  - `em` — compile for ARC EM CPU with no hardware extensions ✨.
  - `em4` — compile for ARC EM4 CPU 🎀.
  - `em4_dmips` — compile for ARC EM4 DMIPS CPU 💅.
  - `em4_fpus` — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension 🔥.
  - `em4_fpuda` — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions ⚡.
  - `hs` — compile for ARC HS CPU with no hardware extensions except the atomic instructions 👑.
  - `hs34` — compile for ARC HS34 CPU 🌈.

- **`hs38`** — compile for the ARC HS38 CPU. 🦋
- **`hs38_linux`** — compile for the ARC HS38 CPU with, like, *all* the hardware extensions on. full send 💅
- **`hs4x`** — compile for the ARC HS4x CPU.
- **`hs4xd`** — compile for the ARC HS4xD CPU. yes the D is doing something, she's different 💜
- **`hs4x_rel31`** — compile for the ARC HS4x CPU, release 3.10a specifically. era-accurate ✨
- **`arc600_norm`** — compile for the ARC 600 CPU with `"norm"` instructions enabled.
- **`arc600_mul32x16`** — ARC 600 with `"norm"` *and* 32x16-bit multiply instructions enabled. two for one 🎀
- **`arc600_mul64`** — ARC 600 with `"norm"` and the `"mul64"`-family instructions enabled.
- **`arc601_norm`** — ARC 601 with `"norm"` instructions enabled.
- **`arc601_mul32x16`** — ARC 601 with `"norm"` and 32x16-bit multiply instructions enabled.
- **`arc601_mul64`** — ARC 601 with `"norm"` and the `"mul64"`-family instructions enabled.
- **`nps400`** — compile for ARC 700 on the NPS400 chip.
- **`em_mini`** — compile for the ARC EM minimalist configuration, the one featuring a reduced register set. lowkey minimalism is a whole vibe 🍓

- **`-mdpfp`**, **`-mdpfp-compact`** — generate double-precision FPX instructions, tuned for the *compact* implementation.
- **`-mdpfp-fast`** — generate double-precision FPX instructions, tuned for the *fast* implementation. speed demon energy 🔥
- **`-mno-dpfp-lrsr`** — disable `"lr"` and `"sr"` instructions from using FPX extension aux registers. hands off 🙈
- **`-mea`** — generate extended arithmetic instructions. rn that's only `"divaw"`, `"adds"`, `"subs"`, and `"sat16"` — that's the whole squad. only valid for `-mcpu=ARC700`.
- **`-mno-mpy`** — do **not** generate `"mpy"`-family instructions for ARC700. heads up, this option is **deprecated** 💀
- **`-mmul32x16`** — generate 32x16-bit multiply and multiply-accumulate instructions.
- **`-mmul64`** — generate `"mul64"` and `"mulu64"` instructions. only valid for `-mcpu=ARC600`.
- **`-mnorm`** — generate `"norm"` instructions. this is the default if `-mcpu=ARC700` is in effect, so she's kinda the main character here 👑
- **`-mspfp`**, **`-mspfp-compact`** — generate single-precision FPX instructions, tuned for the *compact* implementation.
- **`-mspfp-fast`** — generate single-precision FPX instructions, tuned for the *fast* implementation.
- **`-msimd`** — enable generation of ARC SIMD instructions via target-specific builtins. only valid for `-mcpu=ARC700`.
- **`-msoft-float`** — ok so this option is literally **ignored**, it's only here for compatibility purposes. software floating-point code is emitted by default, and that default can be overridden by the FPX options: `-mspfp`, `-mspfp-compact`, or `-mspfp-fast` for single precision, and `-mdpfp`, `-mdpfp-compact`, or `-mdpfp-fast` for double precision. it's giving "i'm just here for the vibes" 🫶
- **`-mswap`** — generate `"swap"` instructions.
- **`-matomic`** — this enables use of the locked load/store conditional extension to implement atomic memory built-in functions. **not available** for ARC 6xx or ARC EM cores. sorry bestie 🙅
- **`-mdiv-rem`** — enable `"div"` and `"rem"` instructions for ARCv2 cores.
- **`-mcode-density`** — enable code density instructions for ARC EM. this one is **on by default** for ARC HS. ✨
- **`-mll64`** — enable double load/store operations for ARC HS cores.
- **`-mtp-regno=regno`** — specify the thread pointer register number.
- **`-mmpy-option=multo`** — compile ARCv2 code with a multiplier design option. you can pass *multo* as either a string or a numeric value, no cap. `wlh1` is the default value. the recognized values are:

  - **`0`**, **`none`** — no multiplier available. none. zero. nothing. 😭
  - **`1`**, **`w`** — 16x16 multiplier, fully pipelined. enables the following instructions: `"mpyw"` and `"mpyuw"`.
  - **`2`**, **`wlh1`** — 32x32 multiplier, fully pipelined (1 stage). additionally enables: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`.
  - **`3`**, **`wlh2`** — 32x32 multiplier, fully pipelined (2 stages). additionally enables: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`.
  - **`4`**, **`wlh3`** — two 16x16 multipliers, blocking, sequential. additionally enables: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`.
  - **`5`**, **`wlh4`** — one 16x16 multiplier, blocking, sequential. additionally enables: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`.
  - **`6`**, **`wlh5`** — one 32x4 multiplier, blocking, sequential. additionally enables: `"mpy"`, `"mpyu"`, `"mpym"`, `"mpymu"`, and `"mpy_s"`.
  - **`7`**, **`plus_dmpy`** — ARC HS SIMD support.
  - **`8`**, **`plus_macd`** — ARC HS SIMD support.
  - **`9`**, **`plus_qmacw`** — ARC HS SIMD support.

  this option is only available for ARCv2 cores.

- **`-mfpu=fpu`** — enables support for specific floating-point hardware extensions for ARCv2 cores. supported values for *fpu* are:

  - **`fpus`** — enables support for single-precision floating-point hardware extensions. 💖
  - **`fpud`** — enables support for double-precision floating-point hardware extensions. the single-precision extension is also enabled. **not available** for ARC EM.
  - **`fpuda`** — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. single-precision is also enabled. **only available** for ARC EM.
  - **`fpuda_div`** — double-precision via double-precision assist instructions, plus single-precision, square-root, and divide extensions. **only available** for ARC EM.
  - **`fpuda_fma`** — double-precision via double-precision assist instructions, plus single-precision and fused multiply and add extensions. **only available** for ARC EM.
  - **`fpuda_all`** — double-precision via double-precision assist instructions, plus **all** single-precision floating-point hardware extensions. **only available** for ARC EM.
  - **`fpus_div`** — single-precision floating-point, square-root and divide hardware extensions.
  - **`fpud_div`** — double-precision floating-point, square-root and divide hardware extensions. this option includes `fpus_div`. **not available** for ARC EM.
  - **`fpus_fma`** — single-precision floating-point and fused multiply and add hardware extensions.
  - **`fpud_fma`** — double-precision floating-point and fused multiply and add hardware extensions. this option includes `fpus_fma`. **not available** for ARC EM.
  - **`fpus_all`** — all single-precision floating-point hardware extensions.
  - **`fpud_all`** — all single- **and** double-precision floating-point hardware extensions. **not available** for ARC EM.

- **`-mirq-ctrl-saved=register-range, blink, lp_count`** — specifies the general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. *register-range* is two registers separated by a dash; the range always starts with `"r0"`, and the upper limit is the `"fp"` register. *blink* and *lp_count* are optional. **only valid** for ARC EM and ARC HS cores.
- **`-mrgf-banked-regs=number`** — specifies the number of registers replicated in the second register bank on entry to a fast interrupt. fast interrupts are the ones with the highest priority level, P0. these interrupts save only the PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. use this when you're using fast interrupts on an ARC V2 family processor. permitted values are `4`, `8`, `16`, and `32`. 🌈
- **`-mlpc-width=width`** — specify the width of the `"lp_count"` register. valid *width* values are `8`, `16`, `20`, `24`, `28` and `32` bits. the default width is fixed to 32 bits. if the width is less than 32, the compiler does **not** attempt to transform loops in your program to use the zero-delay loop mechanism unless it's known the `"lp_count"` register can hold the required loop-counter value. depending on the width you specify, the compiler and run-time library might continue to use the loop mechanism for various needs. this option defines the macro `"__ARC_LPC_WIDTH__"` with the value of *width*.
- **`-mrf16`** — generate code for a 16-entry register file. defines the `"__ARC_RF16__"` preprocessor macro.
- **`-mbranch-index`** — enable use of `"bi"` or `"bih"` instructions to implement jump tables.

---

ok so these next ones get passed straight through to the assembler *and* they define preprocessor macro symbols — double duty icons, we love ✨

- **`-mdsp-packa`** — passed down to the assembler to enable the DSP Pack A extensions. also sets the preprocessor symbol `"__Xdsp_packa"`. this option is **deprecated** — respectfully, don't 💀
- **`-mdvbf`** — passed down to the assembler to enable the dual Viterbi butterfly extension. also sets `"__Xdvbf"`. **deprecated**.
- **`-mlock`** — passed down to the assembler to enable the locked load/store conditional extension. also sets `"__Xlock"`.
- **`-mmac-d16`** — passed down to the assembler. also sets `"__Xxmac_d16"`. **deprecated**.
- **`-mmac-24`** — passed down to the assembler. also sets `"__Xxmac_24"`. **deprecated**.
- **`-mrtsc`** — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. also sets `"__Xrtsc"`. **deprecated**.
- **`-mswape`** — passed down to the assembler to enable the swap byte ordering extension instruction. also sets `"__Xswape"`.
- **`-mtelephony`** — passed down to the assembler to enable dual- and single-operand instructions for telephony. also sets `"__Xtelephony"`. **deprecated**.

- **`-mxy`** — bestie this one gets passed straight down to the assembler to turn on the XY memory extension ✨ and it also sets the preprocessor symbol `__Xxy`. two-for-one, we love to see it 💅

---

Next up, the options that control how the assembly code gets annotated 🎀:

- **`-misize`** — annotates assembler instructions with estimated addresses 📍
- **`-mannotate-align`** — literally does nothing 😭 kept around purely for backward compatibility, it's giving retired icon

---

These ones get passed through to the linker 🧚:

- **`-marclinux`** — passed through to the linker to say "use the `arclinux` emulation". it's on by default in tool chains built for `arc-linux-uclibc` and `arceb-linux-uclibc` targets when profiling is *not* requested 👑
- **`-marclinux_prof`** — passed through to the linker to say "use the `arclinux_prof` emulation". this one's on by default in tool chains built for `arc-linux-uclibc` and `arceb-linux-uclibc` targets when profiling *is* requested 🔥

---

And these control the semantics of the code we generate 🌸:

- **`-mlong-calls`** — generates calls as register indirect calls, which gives you access to the full 32-bit address range. so much room, no cap 💖
- **`-mmedium-calls`** — don't use less than a 25-bit addressing range for calls, which is the offset you get with an unconditional branch-and-link instruction. conditional execution of function calls gets suppressed so you can use the 25-bit range instead of the 21-bit range you'd get with conditional branch-and-link. this is the default for tool chains built for `arc-linux-uclibc` and `arceb-linux-uclibc` targets 🦋
- **`-G num`** — puts definitions of externally-visible data in a small data section if that data is no bigger than *num* bytes. default *num* is `4` for any ARC configuration, or `8` when there's double load/store operations ⭐
- **`-mno-sdata`** — don't generate sdata references. this is the default for tool chains built for `arc-linux-uclibc` and `arceb-linux-uclibc` targets 🌈
- **`-mvolatile-cache`** — use ordinarily cached memory accesses for volatile references. this is the default 💜
- **`-mno-volatile-cache`** — enables cache bypass for volatile references 🙈

---

Okay now the fine-tuning options, this is where it gets ✨specific✨:

- **`-malign-call`** — does nothing. preserved for backward compatibility 💀
- **`-mauto-modify-reg`** — enables the use of pre/post modify with register displacement 🎀
- **`-mbbit-peephole`** — does nothing. preserved for backward compatibility 😭
- **`-mno-brcc`** — disables a target-specific pass in `arc_reorg` that generates compare-and-branch (`brcc`) instructions. heads up: it has **no effect** on generation of these instructions driven by the combiner pass ⚠️
- **`-mcase-vector-pcrel`** — uses PC-relative switch case tables so case tables can be shortened. this is the default for `-Os` 🦋
- **`-mcompact-casesi`** — enables the compact `casesi` pattern. default for `-Os`, and only available for ARCv1 cores. also: this option is **deprecated** 🫶
- **`-mno-cond-exec`** — disables the ARCompact-specific pass that generates conditional execution instructions 🛑

  okay so here's the whole lore: because of delay slot scheduling and the interactions between operand numbers, literal sizes, instruction lengths and conditional execution support, the target-independent pass that generates conditional execution is often lacking 😬 so the ARC port keeps a special pass around that hunts for more conditional execution generation opportunities *after* register allocation, branch shortening and delay slot scheduling are already done. this pass generally — but not always — improves performance and code size, at the cost of extra compilation time, which is literally why there's an option to switch it off. and if you've got a problem with call instructions exceeding their allowable offset range because they got conditionalized, you should consider using `-mmedium-calls` instead 💅

- **`-mearly-cbranchsi`** — enables pre-reload use of the `cbranchsi` pattern 🌟
- **`-mexpand-adddi`** — expands `adddi3` and `subdi3` at RTL generation time into `add.f`, `adc` etc. this option is **deprecated** 💀
- **`-mindexed-loads`** — enables the use of indexed loads. lowkey this can be problematic 👀 because some optimizers then assume indexed *stores* exist, which they do not 🚩
- **`-mlra`** — does nothing. preserved for backward compatibility 😴
- **`-mlra-priority-none`** — don't indicate any priority for target registers 🙅
- **`-mlra-priority-compact`** — indicate target register priority for `r0..r3` / `r12..r15` 👑
- **`-mlra-priority-noncompact`** — reduce target register priority for `r0..r3` / `r12..r15` 📉
- **`-mmillicode`** — when optimizing for size (with `-Os`), prologues and epilogues that have to save or restore a large number of registers are often shortened by using a call to a special function in libgcc — that's what a **millicode call** is, iykyk. since these calls can pose performance issues and/or cause linking issues when linking in a nonstandard way, this option exists to turn millicode call generation on or off 🔧
- **`-mcode-density-frame`** — lets the compiler emit `enter` and `leave` instructions. these are only valid for CPUs with the code-density feature, so check your vibe first ⚠️
- **`-mmixed-code`** — does nothing. preserved for backward compatibility 🙈
- **`-mq-class`** — this option is **deprecated**. enables q instruction alternatives. this is the default for `-Os` 💅
- **`-mRcq`** — does nothing. preserved for backward compatibility 😭
- **`-mRcw`** — does nothing. preserved for backward compatibility 😭
- **`-msize-level=level`** — fine-tunes size optimization with regards to instruction lengths and alignment. the recognized values for *level* are:
  - **`0`** — no size optimization. this level is **deprecated** and treated like `1` 💀
  - **`1`** — short instructions are used opportunistically ✨
  - **`2`** — in addition, alignment of loops and of code after barriers are dropped 🎀
  - **`3`** — in addition, optional data alignment is dropped, and the option `Os` is enabled 🔥

  this defaults to `3` when `-Os` is in effect. otherwise, the behavior when this isn't set is equivalent to level `1` 🌸
- **`-mtune=cpu`** — sets instruction scheduling parameters for *cpu*, overriding anything implied by `-mcpu=` 👑 supported values for *cpu* are:
  - **`ARC600`** — tune for ARC600 CPU ⭐
  - **`ARC601`** — tune for ARC601 CPU ⭐
  - **`ARC700`** — tune for ARC700 CPU with standard multiplier block 💜
  - **`ARC700-xmac`** — tune for ARC700 CPU with XMAC block 💜
  - **`ARC725D`** — tune for ARC725D CPU 🦋
  - **`ARC750D`** — tune for ARC750D CPU 🦋
  - **`core3`** — tune for ARCv2 core3 type CPU. this option enables usage of the `dbnz` instruction 🌟
  - **`release31a`** — tune for ARC4x release 3.10a 🎀
- **`-mmultcost=num`** — the cost to assume for a multiply instruction, with `4` being equal to a normal instruction 🧮
- **`-munalign-prob-threshold=probability`** — does nothing. preserved for backward compatibility 😴

---

## 💀 the deprecated corner 💀

the following options are maintained for backward compatibility, but they're now **deprecated** and *will be removed in a future release* 👋 don't get attached bestie:

- **`-margonaut`** — obsolete FPX 🪦
- **`-mbig-endian`**, **`-EB`** — compile code for big-endian targets. use of these options is now deprecated. big-endian code is supported by configuring GCC to build `arceb-elf32` and `arceb-linux-uclibc` targets, for which big endian is the default 🌍
- **`-mlittle-endian`**, **`-EL`** — compile code for little-endian targets. use of these options is now deprecated. little-endian code is supported by configuring GCC to build `arc-elf32` and `arc-linux-uclibc` targets, for which little endian is the default 🌍
- **`-mbarrel_shifter`** — replaced by `-mbarrel-shifter` 🔁
- **`-mdpfp_compact`** — replaced by `-mdpfp-compact` 🔁
- **`-mdpfp_fast`** — replaced by `-mdpfp-fast` 🔁
- **`-mdsp_packa`** — replaced by `-mdsp-packa` 🔁
- **`-mEA`** — replaced by `-mea` 🔁
- **`-mmac_24`** — replaced by `-mmac-24` 🔁
- **`-mmac_d16`** — replaced by `-mmac-d16` 🔁
- **`-mspfp_compact`** — replaced by `-mspfp-compact` 🔁
- **`-mspfp_fast`** — replaced by `-mspfp-fast` 🔁
- **`-mtune=cpu`** — values `arc600`, `arc601`, `arc700` and `arc700-xmac` for *cpu* are replaced by `ARC600`, `ARC601`, `ARC700` and `ARC700-xmac` respectively 🔁
- **`-multcost=num`** — replaced by `-mmultcost` 🔁

---

## 🦋 ARM Options 🦋

these `-m` options are defined for the ARM port, and yes they're a whole different universe 💖:

- **`-mabi=name`** — generate code for the specified ABI. permissible values are: `apcs-gnu`, `atpcs`, `aapcs`, `aapcs-linux` and `iwmmxt` 🎀
- **`-mapcs-frame`** — generate a stack frame that's compliant with the ARM Procedure Call Standard for **all** functions, even if it's not strictly necessary for correct execution of the code. specifying `-fomit-frame-pointer` with this option causes stack frames not to be generated for leaf functions. the default is `-mno-apcs-frame`. also this option is **deprecated** ⚠️
- **`-mapcs`** — this is a synonym for `-mapcs-frame` and it's **deprecated** 🪦
- **`-mthumb-interwork`** — generate code that supports calling between the ARM and Thumb instruction sets 🤝 without this option, on pre-v5 architectures, the two instruction sets can't be reliably used inside one program. the default is `-mno-thumb-interwork`, since slightly larger code is generated when `-mthumb-interwork` is specified. in AAPCS configurations this option is meaningless 🙃
- **`-mno-sched-prolog`** — prevents reordering of instructions in the function prologue, and prevents merging those instructions with the instructions in the function's body 🚫 this means all functions start with a recognizable set of instructions (or actually one of a choice from a small set of different function prologues), and that info can be used to locate the start of functions inside an executable piece of code 🔍 the default is `-msched-prolog` ✨
- **`-mfloat-abi=name`** — specifies which floating-point ABI to use. permissible values are: `soft`, `softfp` and `hard` 🧮
  - **`soft`** — causes GCC to generate output containing library calls for floating-point operations 📚
  - **`softfp`** — allows generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions 🎀
  - **`hard`** — allows generation of floating-point instructions and uses FPU-specific calling conventions 🔥

  the default depends on the specific target configuration. note that the hard-float and soft-float ABIs are **not link-compatible** 😱 you must compile your entire program with the same ABI, and link with a compatible set of libraries. no mixing and matching, bestie, that's the rule 💅

- **`-mgeneral-regs-only`** — makes the compiler generate code that only touches the general-purpose registers. It stops it from reaching for floating-point and Advanced SIMD registers, but bestie, it puts **zero** restrictions on the assembler. That part's on you 💅

- **`-mlittle-endian`** — generate code for a processor chugging along in little-endian mode. This is the default for literally every standard configuration, so it's the vibe you already have ✨

- **`-mbig-endian`** — generate code for a processor in big-endian mode. Note that the default is to compile for a little-endian processor, so you do actually have to ask for this one 🫶

- **`-mbe8`**
- **`-mbe32`** — when you're linking a big-endian image, these pick between the BE8 and BE32 formats. For little-endian images they do absolutely nothing and get ignored (iconic of them honestly). The default depends on the target architecture you selected: for ARMv6 and later it's BE8, for older architectures it's BE32. Heads up though — BE32 format has been **deprecated** by ARM 😭

---

- **`-march=name[+extension...]`** — this names the target ARM architecture. GCC uses that name to figure out what kind of instructions it's allowed to emit when generating assembly code. You can use it alongside `-mcpu=` or instead of it, dealer's choice 💁‍♀️

The permissible names are: `armv4t`, `armv5t`, `armv5te`, `armv6`, `armv6j`, `armv6k`, `armv6kz`, `armv6t2`, `armv6z`, `armv6zk`, `armv7`, `armv7-a`, `armv7ve`, `armv8-a`, `armv8.1-a`, `armv8.2-a`, `armv8.3-a`, `armv8.4-a`, `armv8.5-a`, `armv8.6-a`, `armv9-a`, `armv7-r`, `armv8-r`, `armv6-m`, `armv6s-m`, `armv7-m`, `armv7e-m`, `armv8-m.base`, `armv8-m.main`, `armv8.1-m.main`, `iwmmxt` and `iwmmxt2`.

Also, these following architectures — which do **not** support the Thumb execution state — are still recognized, but their support is **deprecated**: `armv4`.

> Fr fr, deprecated means deprecated. Don't build your whole personality on `armv4` 💀

---

### 🦋 extensions, aka the fun part

Lots of these architectures support extensions, and you add one by sticking `+extension` onto the end of the architecture name. Extension options get processed in order and the capabilities accumulate — so an extension will also switch on any base extensions it depends on. Like, `+crypto` will always enable `+simd`, every single time, no notes 🔥

The one exception to this whole additive thing is extensions prefixed with `+no...`: those **disable** the specified option *and* any other extensions that might depend on that extension being there. So for example:

-march=armv7-a+simd+nofp+vfpv4


is equivalent to writing `-march=armv7-a+vfpv4`, because the `+simd` option gets entirely disabled by the `+nofp` that follows it. Order matters, bestie ✨

Most extension names are generically named but their effect depends on which architecture you applied them to. Case in point: `+simd` can go on both `armv7-a` and `armv8-a`, but it enables the original ARMv7-A Advanced SIMD (Neon) extensions for `armv7-a`, and the ARMv8-A variant for `armv8-a`. Same name, different slay 💅

The list below has the supported extensions for each architecture. Architectures that aren't mentioned don't support **any** extensions — sorry to them.

---

- **`armv5te`**, **`armv6`**, **`armv6j`**, **`armv6k`**, **`armv6kz`**, **`armv6t2`**, **`armv6z`**, **`armv6zk`**
  - **`+fp`** — the VFPv2 floating-point instructions. You can write `+vfpv2` as an alias for this one.
  - **`+nofp`** — disables the floating-point instructions.

- **`armv7`** — the common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
  - **`+fp`** — the VFPv3 floating-point instructions with 16 double-precision registers. `+vfpv3-d16` works as an alias. Note that floating-point is *not* supported by the base ARMv7-M architecture, but it is compatible with both the ARMv7-A and ARMv7-R architectures.
  - **`+nofp`** — disables the floating-point instructions.

- **`armv7-a`**
  - **`+mp`** — the multiprocessing extension.
  - **`+sec`** — the security extension.
  - **`+fp`** — the VFPv3 floating-point instructions with 16 double-precision registers. `+vfpv3-d16` is an alias for this one.
  - **`+simd`** — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. `+neon` and `+neon-vfpv3` both work as aliases.
  - **`+vfpv3`** — the VFPv3 floating-point instructions with 32 double-precision registers.
  - **`+vfpv3-d16-fp16`** — the VFPv3 floating-point instructions with 16 double-precision registers *plus* the half-precision floating-point conversion operations.
  - **`+vfpv3-fp16`** — the VFPv3 floating-point instructions with 32 double-precision registers *plus* the half-precision floating-point conversion operations.
  - **`+vfpv4-d16`** — the VFPv4 floating-point instructions with 16 double-precision registers.
  - **`+vfpv4`** — the VFPv4 floating-point instructions with 32 double-precision registers.
  - **`+neon-fp16`** — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  - **`+neon-vfpv4`** — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
  - **`+nosimd`** — disables the Advanced SIMD instructions. Note: it does *not* disable floating point.
  - **`+nofp`** — disables the floating-point **and** Advanced SIMD instructions.

- **`armv7ve`** — the extended version of the ARMv7-A architecture, with support for virtualization 👑
  - **`+fp`** — the VFPv4 floating-point instructions with 16 double-precision registers. `+vfpv4-d16` is an alias for this.
  - **`+simd`** — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. `+neon-vfpv4` is an alias for this.
  - **`+vfpv3-d16`** — the VFPv3 floating-point instructions with 16 double-precision registers.
  - **`+vfpv3`** — the VFPv3 floating-point instructions with 32 double-precision registers.
  - **`+vfpv3-d16-fp16`** — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
  - **`+vfpv3-fp16`** — the VFPv3 floating-point instructions with 32 double-precision registers and the half-precision floating-point conversion operations.
  - **`+vfpv4-d16`** — the VFPv4 floating-point instructions with 16 double-precision registers.
  - **`+vfpv4`** — the VFPv4 floating-point instructions with 32 double-precision registers.
  - **`+neon`** — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. `+neon-vfpv3` works as an alias.
  - **`+neon-fp16`** — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  - **`+nosimd`** — disables the Advanced SIMD instructions. Again: it does *not* disable floating point.
  - **`+nofp`** — disables the floating-point and Advanced SIMD instructions.

- **`armv8-a`**
  - **`+crc`** — the Cyclic Redundancy Check (CRC) instructions.
  - **`+simd`** — the ARMv8-A Advanced SIMD and floating-point instructions.
  - **`+crypto`** — the cryptographic instructions.
  - **`+nocrypto`** — disables the cryptographic instructions.
  - **`+nofp`** — disables the floating-point, Advanced SIMD **and** cryptographic instructions.
  - **`+sb`** — Speculation Barrier Instruction.
  - **`+predres`** — Execution and Data Prediction Restriction Instructions.

- **`armv8.1-a`**
  - **`+simd`** — the ARMv8.1-A Advanced SIMD and floating-point instructions.
  - **`+crypto`** — the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
  - **`+nocrypto`** — disables the cryptographic instructions.
  - **`+nofp`** — disables the floating-point, Advanced SIMD and cryptographic instructions.
  - **`+sb`** — Speculation Barrier Instruction.
  - **`+predres`** — Execution and Data Prediction Restriction Instructions.

- **`armv8.2-a`**, **`armv8.3-a`**
  - **`+fp16`** — the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.
  - **`+fp16fml`** — the half-precision floating-point fmla extension. This also enables the half-precision floating-point extension *and* the Advanced SIMD and floating-point instructions.
  - **`+simd`** — the ARMv8.1-A Advanced SIMD and floating-point instructions.
  - **`+crypto`** — the cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
  - **`+dotprod`** — enables the Dot Product extension. This also enables Advanced SIMD instructions.
  - **`+nocrypto`** — disables the cryptographic extension.
  - **`+nofp`** — disables the floating-point, Advanced SIMD and cryptographic instructions.
  - **`+sb`** — Speculation Barrier Instruction.
  - **`+predres`** — Execution and Data Prediction Restriction Instructions.
  - **`+i8mm`** — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
  - **`+bf16`** — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

- **`armv8.4-a`**
  - **`+fp16`** — the half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions, as well as the Dot Product extension and the half-precision floating-point fmla extension.

- **`+simd`** — the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension. it's giving vector math realness ✨
- **`+crypto`** — the cryptographic instructions. this also turns on the Advanced SIMD and floating-point instructions and the Dot Product extension. crypto girlies stay winning 🔐
- **`+nocrypto`** — disables the cryptographic extension. bye crypto 👋
- **`+nofp`** — disables the floating-point, Advanced SIMD, and cryptographic instructions. no float, no simd, no crypto, periodt 💀
- **`+sb`** — Speculation Barrier Instruction. safety barrier realness 🛡️
- **`+predres`** — Execution and Data Prediction Restriction Instructions. restricting that prediction energy fr fr
- **`+i8mm`** — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions. matrix multiply math bestie 🧮
- **`+bf16`** — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions. brainy float vibes 🧠

- **`armv8.5-a`** extensions:
  - **`+fp16`** — the half-precision floating-point data processing instructions. this also enables Advanced SIMD and floating-point instructions, the Dot Product extension, and the half-precision floating-point `fmla` extension. half precision, full <pink>slay</pink> 🎀
  - **`+simd`** — the ARMv8.3-A Advanced SIMD and floating-point instructions plus the Dot Product extension. same iconic simd energy ✨
  - **`+crypto`** — the cryptographic instructions. also enables Advanced SIMD and floating-point and Dot Product. crypto again, no cap 🔐
  - **`+nocrypto`** — disables the cryptographic extension. nope to crypto 🚫
  - **`+nofp`** — disables floating-point, Advanced SIMD, and cryptographic instructions. all the fp vibes off 💀
  - **`+i8mm`** — 8-bit Integer Matrix Multiply instructions. also enables Advanced SIMD and floating-point. matrix multiply, but make it 8-bit 🧮
  - **`+bf16`** — Brain half-precision floating-point instructions. also enables Advanced SIMD and floating-point. brain float, obsessed 🧠

- **`armv8.6-a`** extensions:
  - **`+fp16`** — the half-precision floating-point data processing instructions. also enables Advanced SIMD and floating-point, the Dot Product extension, and the half-precision floating-point `fmla` extension. fp16 but newer, still iconic 🎀
  - **`+simd`** — the ARMv8.3-A Advanced SIMD and floating-point instructions plus the Dot Product extension. simd stays simd-ing ✨
  - **`+crypto`** — the cryptographic instructions. also enables Advanced SIMD and floating-point and Dot Product. crypto queen 🔐
  - **`+nocrypto`** — disables the cryptographic extension. crypto? not today 💅
  - **`+nofp`** — disables floating-point, Advanced SIMD, and cryptographic instructions. no fp, no simd, no crypto, periodt 💀
  - **`+i8mm`** — 8-bit Integer Matrix Multiply instructions. also enables Advanced SIMD and floating-point. matrix math, 8-bit edition 🧮
  - **`+bf16`** — Brain half-precision floating-point instructions. also enables Advanced SIMD and floating-point. brain float again, we love to see it 🧠

- **`armv7-r`** extensions:
  - **`+fp.sp`** — the single-precision VFPv3 floating-point instructions. the extension `+vfpv3xd` can be used as an alias for this extension. single precision, cute 🎀
  - **`+fp`** — the VFPv3 floating-point instructions with 16 double-precision registers. the extension `+vfpv3-d16` can be used as an alias for this extension. double registers, yes bestie 💖
  - **`+vfpv3xd-d16-fp16`** — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations. a whole combo meal 🍓
  - **`+vfpv3-d16-fp16`** — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations. another combo, slay ✨
  - **`+nofp`** — disables the floating-point extension. no fp here 🚫
  - **`+idiv`** — the ARM-state integer division instructions. integer division, go off 💅
  - **`+noidiv`** — disables the ARM-state integer division extension. no integer division, no cap 🛑

- **`armv7e-m`** extensions:
  - **`+fp`** — the single-precision VFPv4 floating-point instructions. vfpv4 single precision, iconic 🎀
  - **`+fpv5`** — the single-precision FPv5 floating-point instructions. fpv5 single precision, we love 🌸
  - **`+fp.dp`** — the single- and double-precision FPv5 floating-point instructions. both precisions, main character energy ✨
  - **`+nofp`** — disables the floating-point extensions. no fp extensions, periodt 💀

- **`armv8.1-m.main`** extensions:
  - **`+dsp`** — the DSP instructions. dsp vibes 🎛️
  - **`+mve`** — the M-Profile Vector Extension (MVE) integer instructions. mve integer, vector realness 🧮
  - **`+mve.fp`** — the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions. mve integer plus single-precision fp, combo slay 🎀
  - **`+fp`** — the single-precision floating-point instructions. single precision fp, cute 💖
  - **`+fp.dp`** — the single- and double-precision floating-point instructions. both precisions, yes ✨
  - **`+nofp`** — disables the floating-point extension. no fp, bye 🚫
  - **`+cdecp0`, `+cdecp1`, ... , `+cdecp7`** — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7. cde on your chosen coprocessors, fancy 💅
  - **`+pacbti`** — enable the Pointer Authentication and Branch Target Identification Extension. pacbti, security realness 🛡️

- **`armv8-m.main`** extensions:
  - **`+dsp`** — the DSP instructions. dsp again, we stan 🎛️
  - **`+nodsp`** — disables the DSP extension. no dsp, nope 🚫
  - **`+fp`** — the single-precision floating-point instructions. single precision fp, cute 🎀
  - **`+fp.dp`** — the single- and double-precision floating-point instructions. both precisions, slay ✨
  - **`+nofp`** — disables the floating-point extension. no fp, bye 💀
  - **`+cdecp0`, `+cdecp1`, ... , `+cdecp7`** — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7. cde on your chosen coprocessors, fancy 💅

- **`armv8-r`** extensions:
  - **`+crc`** — the Cyclic Redundancy Check (CRC) instructions. crc, checksum realness 🧾
  - **`+fp.sp`** — the single-precision FPv5 floating-point instructions. single precision fpv5, iconic 🎀
  - **`+simd`** — the ARMv8-A Advanced SIMD and floating-point instructions. simd, we love her ✨
  - **`+crypto`** — the cryptographic instructions. crypto, yes 🔐
  - **`+nocrypto`** — disables the cryptographic instructions. no crypto, bye 👋
  - **`+nofp`** — disables the floating-point, Advanced SIMD, and cryptographic instructions. no fp, no simd, no crypto, periodt 💀

now, the `-march=native` tea ☕: it causes the compiler to auto-detect the architecture of the build computer. at present, this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect is unsuccessful, the option has no effect. so like, native is giving "try your best" energy 🙈

and `-march=unset` is the peacemaker 🕊️: it causes the compiler to ignore any `-march=...` options that appear earlier on the command line and behave as if the option was never passed. this is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications. no drama, no conflicting warnings, periodt 💅

---

### 🎀 `-mtune=name`

this option specifies the name of the target ARM processor for which GCC should tune the performance of the code. for some ARM implementations, better performance can be obtained by using this option. it's giving "customize your tuning" realness ✨

permissible names are:

arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1.


additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. permissible names are:

cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.


`-mtune=generic-arch` specifies that GCC should tune the performance for a blend of processors within architecture *arch*. the aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range and avoiding performance pitfalls of other CPUs. the effects of this option may change in future GCC versions as CPU models come and go. so like, generic-arch is giving "middle ground, but make it strategic" 🎯

`-mtune` permits the same extension options as `-mcpu`, but the extension options do not affect the tuning of the generated code. so you can pass the extension tea, but it won't change the tuning vibe 💅

`-mtune=native` causes the compiler to auto-detect the CPU of the build computer. at present, this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect is unsuccessful, the option has no effect. native again, try your best 🙈

---

### 👑 `-mcpu=name[+extension...]`

this specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by `-march`) and the ARM processor type for which to tune for performance (as if specified by `-mtune`). where this option is used in conjunction with `-march` or `-mtune`, those options take precedence over the appropriate part of this option. so `-mcpu` is the main character, but `-march` and `-mtune` can still steal the spotlight for their parts. no cap 👑

<pink>okay so</pink> real talk, bestie — a LOT of the CPUs you can target come with optional architectural extensions, and when they do, those extensions are normally <green>on by default</green>. But if there are implementations out there that *don't* have the extension? That's exactly what the extension syntax is for — you use it to switch off the ones that got left out. 🎀

And for floating-point + Advanced SIMD (Neon) instructions, you also gotta factor in what `-mfloat-abi` and `-mfpu` are set to, no cap: floating-point and Advanced SIMD instructions only get used if `-mfloat-abi` is **not** set to `soft`, and any `-mfpu` setting other than `auto` overrides the available floating-point and SIMD extension instructions. ✨

Classic example: `cortex-a9` ships in three major configurations — integer only, just a floating-point unit, or floating-point *and* Advanced SIMD. The default enables all the instructions, but the `+nosimd` and `+nofp` extensions let you kill just the SIMD, or both the SIMD and floating-point instructions respectively. 🦋

The permissible names for this option are the same ones as for `-mtune`. iykyk 💅

Here are the extension options that are common to the listed CPUs:

- **`+nodsp`** — disables the DSP instructions on `cortex-m33`, `cortex-m35p`, `cortex-m52`, `cortex-m55` and `cortex-m85`. Also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on `cortex-m52`, `cortex-m55` and `cortex-m85`.
- **`+nopacbti`** — disables the Pointer Authentication and Branch Target Identification Extension on `cortex-m52` and `cortex-m85`.
- **`+nomve`** — disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on `cortex-m52`, `cortex-m55` and `cortex-m85`.
- **`+nomve.fp`** — disables the M-Profile Vector Extension (MVE) single precision floating-point instructions on `cortex-m52`, `cortex-m55` and `cortex-m85`.
- **`+cdecp0`, `+cdecp1`, ... , `+cdecp7`** — enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers in the options, in the range 0 to 7, on `cortex-m52`, `cortex-m55` and `star-mc1`.
- **`+nofp`** — disables the floating-point instructions on `arm9e`, `arm946e-s`, `arm966e-s`, `arm968e-s`, `arm10e`, `arm1020e`, `arm1022e`, `arm926ej-s`, `arm1026ej-s`, `cortex-r5`, `cortex-r7`, `cortex-r8`, `cortex-m4`, `cortex-m7`, `cortex-m33`, `cortex-m35p`, `cortex-m52`, `cortex-m55` and `cortex-m85`. Disables the floating-point and SIMD instructions on `generic-armv7-a`, `cortex-a5`, `cortex-a7`, `cortex-a8`, `cortex-a9`, `cortex-a12`, `cortex-a15`, `cortex-a17`, `cortex-a15.cortex-a7`, `cortex-a17.cortex-a7`, `cortex-a32`, `cortex-a35`, `cortex-a53` and `cortex-a55`.
- **`+nofp.dp`** — disables the double-precision component of the floating-point instructions on `cortex-r5`, `cortex-r7`, `cortex-r8`, `cortex-r52`, `cortex-r52plus` and `cortex-m7`.
- **`+nosimd`** — disables the SIMD (but not the floating-point) instructions on `generic-armv7-a`, `cortex-a5`, `cortex-a7` and `cortex-a9`.
- **`+crypto`** — enables the cryptographic instructions on `cortex-a32`, `cortex-a35`, `cortex-a53`, `cortex-a55`, `cortex-a57`, `cortex-a72`, `cortex-a73`, `cortex-a75`, `exynos-m1`, `xgene1`, `cortex-a57.cortex-a53`, `cortex-a72.cortex-a53`, `cortex-a73.cortex-a35`, `cortex-a73.cortex-a53` and `cortex-a75.cortex-a55`.

On top of all that, the `generic-armv7-a` pseudo target defaults to VFPv3 with 16 double-precision registers 🔥 and it supports these extension options: `mp`, `sec`, `vfpv3-d16`, `vfpv3`, `vfpv3-d16-fp16`, `vfpv3-fp16`, `vfpv4-d16`, `vfpv4`, `neon`, `neon-vfpv3`, `neon-fp16`, `neon-vfpv4`. Their meanings are the same as the extensions to `-march=armv7-a`. ⭐

`-mcpu=generic-arch` is also permissible, and it's equivalent to `-march=arch -mtune=generic-arch`. Peep `-mtune` for more info 👑

`-mcpu=native` makes the compiler auto-detect the CPU of the build computer. Right now this feature is only supported on GNU/Linux, and not every architecture is recognized. If the auto-detect doesn't work out, the option just has no effect. 🙈

`-mcpu=unset` makes the compiler ignore any `-mcpu=...` options that show up earlier on the command line, and behave like the option was never passed at all. Super useful for dodging warnings about conflicting CPU and architecture options when the two produce different architecture specifications. 💜

## ✨ `-mfpu=name`

This one tells the compiler what floating-point hardware (or hardware emulation, bestie) is available on the target. 🎀 Permissible names: `auto`, `vfpv2`, `vfpv3`, `vfpv3-fp16`, `vfpv3-d16`, `vfpv3-d16-fp16`, `vfpv3xd`, `vfpv3xd-fp16`, `neon-vfpv3`, `neon-fp16`, `vfpv4`, `vfpv4-d16`, `fpv4-sp-d16`, `neon-vfpv4`, `fpv5-d16`, `fpv5-sp-d16`, `fp-armv8`, `neon-fp-armv8` and `crypto-neon-fp-armv8`. Heads up: `neon` is an alias for `neon-vfpv3`, and `vfp` is an alias for `vfpv2`.

The setting `auto` is the default and it's special ✨ — it makes the compiler pick the floating-point and Advanced SIMD instructions based on what `-mcpu` and `-march` are set to.

If the floating-point hardware you picked includes the NEON extension (like `-mfpu=neon`), lowkey important: floating-point operations are **not** generated by GCC's auto-vectorization pass unless `-funsafe-math-optimizations` is also specified. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic (denormal values get treated as zero, in particular), so using NEON instructions can lead to a loss of precision. 😭

You can also set the fpu name at function level using the `target("fpu=")` function attributes or pragmas. 💅

## 🌸 `-mfp16-format=name`

Specifies the format of the `"__fp16"` half-precision floating-point type. Permissible names: `none`, `ieee`, and `alternative`; the default is `none`, and in that case the `"__fp16"` type isn't defined at all. 🦋

## 🎀 `-mstructure-size-boundary=n`

The sizes of all structures and unions get rounded up to a multiple of the number of bits set by this option. Permissible values: `8`, `32` and `64`. The default value varies for different toolchains — for the COFF targeted toolchain the default is `8`. A value of `64` is only allowed if the underlying ABI supports it. 🌈

Specifying a larger number can produce faster, more efficient code, but it can also bump up the size of your program. And different values are potentially incompatible, fr fr: code compiled with one value can't necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. 💀

> This option is deprecated.

## ⭐ `-mabort-on-noreturn`

Generates a call to the function `"abort"` at the end of a `"noreturn"` function. It gets executed if the function tries to return. 🫶

## 💜 `-mlong-calls` / `-mno-long-calls`

Tells the compiler to do function calls by first loading the address of the function into a register, then doing a subroutine call on that register. You need this switch if the target function lies outside the 64-megabyte addressing range of the offset-based version of the subroutine call instruction. 🔥

Even with the switch enabled, not all function calls become long calls, okay? The heuristic: static functions, functions that have the `"short_call"` attribute, functions inside the scope of a `#pragma no_long_calls` directive, and functions whose definitions have already been compiled within the current compilation unit are **not** turned into long calls. The exceptions to that rule: weak function definitions, functions with the `"long_call"` attribute or the `"section"` attribute, and functions within the scope of a `#pragma long_calls` directive are always turned into long calls. 👑

This feature is not enabled by default. Specifying `-mno-long-calls` restores the default behavior, and so does putting the function calls within the scope of a `#pragma long_calls_off` directive. Note: these switches have no effect on how the compiler generates code to handle function calls via function pointers. 🦋

## 🧚 `-msingle-pic-base`

Treats the register used for PIC addressing as read-only, instead of loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. ✨

## 💖 `-mpic-register=reg`

Specifies the register to be used for PIC addressing. For the standard PIC base case, the default is any suitable register determined by the compiler. For the single PIC base case, the default is `R9` if the target is EABI based or stack-checking is enabled, otherwise the default is `R10`. 💅

## 🎀 `-mpic-data-is-text-relative`

Assumes that the displacement between the text and data segments is fixed at static link time. This lets you use PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When it's disabled on such targets, it'll enable `-msingle-pic-base` by default. 🌸

## 🌈 `-mpoke-function-name`

Writes the name of each function into the text section, directly preceding the function prologue. The generated code looks something like this:

t0 .ascii "arm_poke_function_name", 0 .align t1 .word 0xff000000 + (t1 - t0) arm_poke_function_name mov ip, sp stmfd sp!, {fp, ip, lr, pc} sub fp, ip, #4


When you're doing a stack backtrace, code can inspect the value of `"pc"` stored at `"fp + 0"`. If the trace function then looks at location `"pc - 12"` and the top 8 bits are set, then we know there's a function name embedded immediately preceding this location and it has length `"((pc[-3]) & 0xff000000)"`. 😭 iconic

## 🦋 `-mthumb` / `-marm`

Picks between generating code that executes in ARM and Thumb states. The default for most configurations is generating code that executes in ARM state, but the default can be changed by configuring GCC with the `--with-mode=state` configure option. ⭐

You can also override the ARM and Thumb mode for each function using the `target("thumb")` and `target("arm")` function attributes or pragmas. 💜

- **`-mflip-thumb`** — flips between ARM and Thumb modes on alternating functions. ok but bestie, heads up: this exists purely for *regression testing* of mixed Thumb/ARM code generation. it is **not** meant for ordinary use in compiling code. it's giving "devs only, back away slowly" 💅

- **`-mtpcs-frame`** — makes a stack frame that's compliant with the Thumb Procedure Call Standard for all *non-leaf* functions. (a leaf function = one that doesn't call any other functions, no cap 🌱). the default is `-mno-tpcs-frame`.

- **`-mtpcs-leaf-frame`** — same energy but for the leaf functions — generates a Thumb Procedure Call Standard–compliant stack frame for all leaf functions (aka the ones that don't call anything else). the default is `-mno-apcs-leaf-frame` 🍃

- **`-mcallee-super-interworking`** — gives every externally visible function in the file you're compiling an ARM instruction set header that switches to Thumb mode before running the rest of the function. this lets those functions be called from non-interworking code 🫶. **not valid** in AAPCS configurations, because interworking is already on by default there.

- **`-mcaller-super-interworking`** — lets calls through function pointers (including virtual functions 👀) run correctly no matter whether the target code was compiled for interworking or not. small tradeoff: there's a *small overhead* in the cost of executing a function pointer when this is enabled. also **not valid** in AAPCS configurations, since interworking is on by default already.

- **`-mtp=`*name* — picks the access model for the thread local storage pointer. the `soft` model generates calls to `__aeabi_read_tp`. the other accepted models are `tpidrurw`, `tpidruro` and `tpidrprw`, which grab the thread pointer straight from the corresponding system register (supported from the arm6k architecture and later ✨). those system registers are accessed through the CP15 co-processor interface, and `cp15` is also accepted as a convenience alias of `tpidruro`. the `auto` argument uses the best available method for the selected processor. default setting is `auto`.

- **`-mtls-dialect=`*dialect* — picks the dialect for accessing thread local storage. two dialects, bestie: `gnu` and `gnu2` 💫
  - `gnu` = the original GNU scheme for supporting local and global dynamic TLS models.
  - `gnu2` = the GNU descriptor scheme, which gives better performance for shared libraries. it's compatible with the original scheme, but it *does* require new assembler, linker and library support.
  - Initial and local exec TLS models are unaffected by this option and always use the original scheme.

- **`-mword-relocations`** — only generate absolute relocations on word-sized values (i.e. `R_ARM_ABS32`). this is enabled by default on targets (`uClinux`, `SymbianOS`) where the runtime loader imposes this restriction, and also when `-fpic` or `-fPIC` is specified. ⚠️ this option **conflicts** with `-mslow-flash-data`.

- **`-mfix-cortex-m3-ldrd`** — some Cortex-M3 cores can cause data corruption when `ldrd` instructions with overlapping destination and base registers are used 😭. this option avoids generating those instructions. enabled by default when `-mcpu=cortex-m3` is specified.

- **`-mfix-cortex-a57-aes-1742098`**
- **`-mno-fix-cortex-a57-aes-1742098`**
- **`-mfix-cortex-a72-aes-1655431`**
- **`-mno-fix-cortex-a72-aes-1655431`**
  — enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions 🔐. this option is enabled by default when either `-mcpu=cortex-a57` or `-mcpu=cortex-a72` is specified.

- **`-munaligned-access`**
- **`-mno-unaligned-access`**
  — enables (or disables) reading and writing of 16- and 32- bit values from addresses that aren't 16- or 32- bit aligned. by default unaligned access is *disabled* for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and *enabled* for all other architectures. if unaligned access isn't enabled, then words in packed data structures get accessed a byte at a time 🐌
  The ARM attribute `Tag_CPU_unaligned_access` is set in the generated object file to either true or false, depending on how you set this option. and if unaligned access is enabled, the preprocessor symbol `__ARM_FEATURE_UNALIGNED` is also defined ✨

- **`-mneon-for-64bits`** — this option is **deprecated** and has no effect 💀 (rip, it's a ghost)

- **`-mslow-flash-data`** — assume that loading data from flash is slower than fetching instructions. so literal loads get minimized for better performance 🐢. only supported when compiling for ARMv7 M-profile, and off by default. it **conflicts** with `-mword-relocations`.

- **`-masm-syntax-unified`** — assume the inline assembler is using unified asm syntax. the default is currently off, which implies divided syntax. this option has no impact on Thumb2. that said, this may change in future releases of GCC 👀 and divided syntax *should be considered deprecated*.

- **`-mrestrict-it`** — restricts generation of IT blocks so they conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. this option is on by default for ARMv8-A Thumb mode.

- **`-mprint-tune-info`** — prints CPU tuning info as a comment in the assembler file. this is only for *regression testing* of the compiler, not for ordinary use in compiling code, ok? 🙈 it's disabled by default.

- **`-mverbose-cost-dump`** — enables verbose cost model dumping in the debug dump files. provided for use in debugging the compiler 🛠️

- **`-mpure-code`** — don't allow constant data to be placed in code sections. plus, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute `SHF_ARM_PURECODE` 👑. only available when generating non-pic code for M-profile targets.

- **`-mcmse`** — generates secure code as per the *"ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification"*, which lives at <https://developer.arm.com/documentation/ecm0359818/latest/> 🛡️

- **`-mfix-cmse-cve-2021-35465`** — mitigates a potential security issue with the `VLLDM` instruction in some M-profile devices when using CMSE (CVE-2021-365465) 😱. enabled by default when the `-mcpu=` option is used with `cortex-m33`, `cortex-m35p`, `cortex-m52`, `cortex-m55`, `cortex-m85` or `star-mc1`. and `-mno-fix-cmse-cve-2021-35465` disables the mitigation, iykyk.

- **`-mstack-protector-guard=`*guard*
- **`-mstack-protector-guard-offset=`*offset*
  — generate stack protection code using a canary at *guard*. supported locations are `global` for a global canary or `tls` for a canary accessible via the TLS register. the `-mstack-protector-guard-offset=` option is for use with `-fstack-protector-guard=tls` and **not** for use in user-land code ⚠️

- **`-mfdpic`**
- **`-mno-fdpic`**
  — selects the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions 🎯. when the compiler is configured for `arm-*-uclinuxfdpiceabi` targets, this option is on by default and implies `-fPIE` if none of the PIC/PIE-related options is provided. on other targets, it only enables the FDPIC-specific code generation features, and you should explicitly provide the PIC/PIE-related options as needed.
  ngl, note that **static linking is not supported**, because it would still involve the dynamic linker when the program self-relocates 😬. if such behavior is acceptable, use `-static` and `-Wl,-dynamic-linker` options.
  the opposite `-mno-fdpic` option is useful (and *required*) to build the Linux kernel using the same (`arm-*-uclinuxfdpiceabi`) toolchain as the one used to build the userland programs 🐧

- **`-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]`** — enables branch protection features (armv8.1-m.main only 🔒)
  - `none` — generate code without branch protection or return address signing.
  - `standard[+leaf]` — generate code with all branch protection features enabled at their standard level.
  - `pac-ret[+leaf]` — generate code with return address signing set to its standard level, which means signing all functions that save the return address to memory.
  - `leaf` — when return address signing is enabled, also sign leaf functions even if they don't write the return address to memory.
  - `+bti` — add landing-pad instructions at the permitted targets of indirect branch instructions.

  if the `+pacbti` architecture extension is not enabled, then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. the generated code stays backwards-compatible with earlier versions of the architecture, but the extra security can be enabled at run time on processors that support the PACBTI extension ✨

  > Branch target enforcement using BTI can only be enabled at runtime if **all** code in the application has been compiled with at least `-mbranch-protection=bti`. no exceptions bestie 💀

  any setting other than `none` is supported only on armv8-m.main or later.

  the default is to generate code without branch protection or return address signing.

---

## 🌸 AVR Options 🌸

these are the options defined for AVR implementations 🦋

- **`-mmcu=`*mcu* — specify the AVR instruction set architecture (ISA) or device type. the default for this option is `avr2`.

  the following AVR devices and ISAs are supported. 📝 note: a complete device support consists of startup code `crtmcu.o`, a device header `avr/io*.h`, a device library `libmcu.a` and a device-specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) file `specs-mcu`. only the latter is provided by the compiler according to the supported *mcu*s below — the rest is supported by AVR-LibC (https://github.com/avrdudes/avr-libc/), or by means of `atpack` (https://gcc.gnu.org/wiki/avr-gcc#atpack) files from the hardware manufacturer 🧚

  - `avr2` — "Classic" devices with up to 8 KiB of program memory. *mcu* = `attiny22`, `attiny26`, `at90s2313`, `at90s2323`, `at90s2333`, `at90s2343`, `at90s4414`, `at90s4433`, `at90s4434`, `at90c8534`, `at90s8515`, `at90s8535`.

okay so picking up the AVR architecture roster right where we left off — and bestie, it's giving ✨encyclopedia✨ but make it cute 💅

- **`avr25`** — "Classic" devices with up to 8 KiB of program memory and with the `MOVW` instruction. the `mcu` list is lowkey long but here it is:

mcu = "attiny13", "attiny13a", "attiny24", "attiny24a", "attiny25", "attiny261", "attiny261a", "attiny2313", "attiny2313a", "attiny43u", "attiny44", "attiny44a", "attiny45", "attiny48", "attiny441", "attiny461", "attiny461a", "attiny4313", "attiny84", "attiny84a", "attiny85", "attiny87", "attiny88", "attiny828", "attiny841", "attiny861", "attiny861a", "ata5272", "ata6616c", "at86rf401".


- **`avr3`** — "Classic" devices with 16 KiB up to 64 KiB of program memory. `mcu` = `at76c711`, `at43usb355`.

- **`avr31`** — "Classic" devices with 128 KiB of program memory. `mcu` = `atmega103`, `at43usb320`.

- **`avr35`** — "Classic" devices with 16 KiB up to 64 KiB of program memory and with the `MOVW` instruction. `mcu` = `attiny167`, `attiny1634`, `atmega8u2`, `atmega16u2`, `atmega32u2`, `ata5505`, `ata6617c`, `ata664251`, `at90usb82`, `at90usb162`.

- **`avr4`** — "Enhanced" devices with up to 8 KiB of program memory. `mcu` = `atmega48`, `atmega48a`, `atmega48p`, `atmega48pa`, `atmega48pb`, `atmega8`, `atmega8a`, `atmega8hva`, `atmega88`, `atmega88a`, `atmega88p`, `atmega88pa`, `atmega88pb`, `atmega8515`, `atmega8535`, `ata5795`, `ata6285`, `ata6286`, `ata6289`, `ata6612c`, `at90pwm1`, `at90pwm2`, `at90pwm2b`, `at90pwm3`, `at90pwm3b`, `at90pwm81`.

- **`avr5`** — "Enhanced" devices with 16 KiB up to 64 KiB of program memory. and the `mcu` list? it's giving *a lot* but we keep every single one, no cap:

mcu = "atmega16", "atmega16a", "atmega16hva", "atmega16hva2", "atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4", "atmega161", "atmega162", "atmega163", "atmega164a", "atmega164p", "atmega164pa", "atmega165", "atmega165a", "atmega165p", "atmega165pa", "atmega168", "atmega168a", "atmega168p", "atmega168pa", "atmega168pb", "atmega169", "atmega169a", "atmega169p", "atmega169pa", "atmega32", "atmega32a", "atmega32c1", "atmega32hvb", "atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6", "atmega323", "atmega324a", "atmega324p", "atmega324pa", "atmega324pb", "atmega325", "atmega325a", "atmega325p", "atmega325pa", "atmega328", "atmega328p", "atmega328pb", "atmega329", "atmega329a", "atmega329p", "atmega329pa", "atmega3250", "atmega3250a", "atmega3250p", "atmega3250pa", "atmega3290", "atmega3290a", "atmega3290p", "atmega3290pa", "atmega406", "atmega64", "atmega64a", "atmega64c1", "atmega64hve", "atmega64hve2", "atmega64m1", "atmega64rfr2", "atmega640", "atmega644", "atmega644a", "atmega644p", "atmega644pa", "atmega644rfr2", "atmega645", "atmega645a", "atmega645p", "atmega649", "atmega649a", "atmega649p", "atmega6450", "atmega6450a", "atmega6450p", "atmega6490", "atmega6490a", "atmega6490p", "ata5790", "ata5790n", "ata5791", "ata6613c", "ata6614q", "ata5782", "ata5831", "ata8210", "ata8510", "ata5787", "ata5835", "ata5700m322", "ata5702m322", "at90pwm161", "at90pwm216", "at90pwm316", "at90can32", "at90can64", "at90scr100", "at90usb646", "at90usb647", "at94k", "m3000".


- **`avr51`** — "Enhanced" devices with 128 KiB of program memory. `mcu` = `atmega128`, `atmega128a`, `atmega128rfa1`, `atmega128rfr2`, `atmega1280`, `atmega1281`, `atmega1284`, `atmega1284p`, `atmega1284rfr2`, `at90can128`, `at90usb1286`, `at90usb1287`.

- **`avr6`** — "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. `mcu` = `atmega256rfr2`, `atmega2560`, `atmega2561`, `atmega2564rfr2`.

- **`avrxmega2`** — "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. `mcu` = `atxmega8e5`, `atxmega16a4`, `atxmega16a4u`, `atxmega16c4`, `atxmega16d4`, `atxmega16e5`, `atxmega32a4`, `atxmega32a4u`, `atxmega32c3`, `atxmega32c4`, `atxmega32d3`, `atxmega32d4`, `atxmega32e5`, `avr64da28`, `avr64da28s`, `avr64da32`, `avr64da32s`, `avr64da48`, `avr64da48s`, `avr64da64`, `avr64da64s`, `avr64db28`, `avr64db32`, `avr64db48`, `avr64db64`, `avr64dd14`, `avr64dd20`, `avr64dd28`, `avr64dd32`, `avr64du28`, `avr64du32`, `avr64ea28`, `avr64ea32`, `avr64ea48`, `avr64sd28`, `avr64sd32`, `avr64sd48`.

- **`avrxmega3`** — "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space. `mcu` list is huge but iconic:

mcu = "attiny202", "attiny204", "attiny212", "attiny214", "attiny402", "attiny404", "attiny406", "attiny412", "attiny414", "attiny416", "attiny416auto", "attiny417", "attiny424", "attiny426", "attiny427", "attiny804", "attiny806", "attiny807", "attiny814", "attiny816", "attiny817", "attiny824", "attiny826", "attiny827", "attiny1604", "attiny1606", "attiny1607", "attiny1614", "attiny1616", "attiny1617", "attiny1624", "attiny1626", "attiny1627", "attiny3214", "attiny3216", "attiny3217", "attiny3224", "attiny3226", "attiny3227", "atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208", "atmega3209", "atmega4808", "atmega4809", "avr16dd14", "avr16dd20", "avr16dd28", "avr16dd32", "avr16du14", "avr16du20", "avr16du28", "avr16du32", "avr16ea28", "avr16ea32", "avr16ea48", "avr16eb14", "avr16eb20", "avr16eb28", "avr16eb32", "avr32da28", "avr32da28s", "avr32da32", "avr32da32s", "avr32da48", "avr32da48s", "avr32db28", "avr32db32", "avr32db48", "avr32dd14", "avr32dd20", "avr32dd28", "avr32dd32", "avr32du14", "avr32du20", "avr32du28", "avr32du32", "avr32ea28", "avr32ea32", "avr32ea48", "avr32sd20", "avr32sd28", "avr32sd32".


- **`avrxmega4`** — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. `mcu` = `atxmega64a3`, `atxmega64a3u`, `atxmega64a4u`, `atxmega64b1`, `atxmega64b3`, `atxmega64c3`, `atxmega64d3`, `atxmega64d4`, `avr128da28`, `avr128da28s`, `avr128da32`, `avr128da32s`, `avr128da48`, `avr128da48s`, `avr128da64`, `avr128da64s`, `avr128db28`, `avr128db32`, `avr128db48`, `avr128db64`.

- **`avrxmega5`** — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. `mcu` = `atxmega64a1`, `atxmega64a1u`.

- **`avrxmega6`** — "XMEGA" devices with more than 128 KiB of program memory. `mcu` = `atxmega128a3`, `atxmega128a3u`, `atxmega128b1`, `atxmega128b3`, `atxmega128c3`, `atxmega128d3`, `atxmega128d4`, `atxmega192a3`, `atxmega192a3u`, `atxmega192c3`, `atxmega192d3`, `atxmega256a3`, `atxmega256a3b`, `atxmega256a3bu`, `atxmega256a3u`, `atxmega256c3`, `atxmega256d3`, `atxmega384c3`, `atxmega384d3`.

- **`avrxmega7`** — "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. `mcu` = `atxmega128a1`, `atxmega128a1u`, `atxmega128a4u`.

- **`avrtiny`** — "Reduced Tiny" Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory. `mcu` = `attiny4`, `attiny5`, `attiny9`, `attiny10`, `attiny102`, `attiny104`, `attiny20`, `attiny40`.

- **`avr1`** — This ISA is implemented by the minimal AVR core and supported for assembler only. `mcu` = `attiny11`, `attiny12`, `attiny15`, `attiny28`, `at90s1200`.

---

- **`-mabsdata`** — Assume that all data in static storage can be accessed by LDS / STS instructions. This option has only an effect on reduced Tiny devices like ATtiny40. See also the `absdata` AVR Variable Attributes,variable attribute.

- **`-mcvt`** — Use a compact vector table. Some devices support a CVT with only four entries: `0=Reset`, `1=NMI`, `2=Prio1 IRQ`, `3=Prio0 IRQs`. This option will link startup code from `crtmcu-cvt.o` instead of the usual `crtmcu.o`. Apart from providing a compact vector table, the startup code will set bit `CPUINT_CTRLA.CPUINT_CVT` which enables the CVT on the device.

When you do not want the startup code to set `CPUINT_CTRLA.CPUINT_CVT`, then you can satisfy symbol `__init_cvt` so that the respective code is no more pulled in from `libmcu.a`. For example, you can link with `-Wl,--defsym,__init_cvt=0`.

The CVT startup code is available since AVR-LibC v2.3 (`https://github.com/avrdudes/avr-libc/issues/1010`).

- **`-mdouble=bits`**, **`-mlong-double=bits`** — Set the size (in bits) of the `double` or `long double` type, respectively. Possible values for bits are `32` and `64`. Whether or not a specific value for bits is allowed depends on the `--with-double=` and `--with-long-double=` configure options (`https://gcc.gnu.org/install/configure.html#avr`), and the same applies for the default values of the options.

- **`-mgas-isr-prologues`** — Interrupt service routines (ISRs) may use the `__gcc_isr` pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the AVR Function Attributes,,`no_gccisr` function attribute. This feature is activated per default if optimization is on (but not with `-Og`, `@pxref{Optimize Options}`), and if GNU Binutils support PR21683 (`https://sourceware.org/PR21683`).

- **`-mint8`** — Assume `int` to be 8-bit integer. This affects the sizes of all types: a `char` is 1 byte, an `int` is 1 byte, a `long` is 2 bytes, and `long long` is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size.

- **`-mmain-is-OS_task`** — Do not save registers in `main`. The effect is the same like attaching attribute AVR Function Attributes,,`OS_task` to `main`. It is activated per default if optimization is on.

- **`-mno-call-main`** — Don't run `main` by means of

XCALL main XJMP exit


Instead, put `main` in section `.init9` (`https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init`) so that no call is required. By setting this option the user asserts that `main` will not return.

This option can be used for devices with very limited resources in order to save a few bytes of code and stack space. It will work as expected since AVR-LibC v2.3 (`https://github.com/avrdudes/avr-libc/issues/1012`). With older versions, there will be no performance gain.

- **`-mno-interrupts`** — the generated code just straight up isn't compatible with hardware interrupts anymore 💀 but like, code size gets smaller, so. tradeoffs bestie.

- **`-mrelax`** — tries to swap a `CALL` (resp. `JMP`) instruction out for the shorter `RCALL` (resp. `RJMP`) whenever that's applicable. Setting `-mrelax` literally just adds the `--mlink-relax` option to the assembler's command line and the `--relax` option to the linker's command line, that's it, that's the whole vibe.

Jump relaxing gets performed by the *linker* because jump offsets aren't known before the code is located. So — assembler code generated by the compiler? identical. Instructions in the executable? may differ from the assembler code. iykyk.

Relaxing **must** be turned on if linker stubs are needed, see the section on `EIND` and linker stubs below ✨

- **`-mrodata-in-ram`**, **`-mno-rodata-in-ram`** — puts the `.rodata` sections for read-only data in RAM (resp. in program memory). For most devices there is no choice and this option acts rather like an assertion 🫶

Since v14 and for the AVR64* and AVR128* devices, `.rodata` is located in flash memory per default — *provided* the required GNU Binutils support (PR31124 (`https://sourceware.org/PR31124`)) is available. In that case, `-mrodata-in-ram` can be used to return to the old layout with `.rodata` in RAM 👑

- **`-mtiny-stack`** — only changes the lower 8 bits of the stack pointer. tiny stack, tiny changes, we love a minimal queen 💅

- **`-mfract-convert-truncate`** — allows truncation instead of rounding towards zero for fractional fixed-point types. chaotic but valid 🎀

- **`-nodevicelib`** — don't link against AVR-LibC's device specific library `libmcu.a`.

> ⚠️ heads up bestie: since AVR-LibC v2.3, that library contains code that is **essential** for the correct functioning of a program. like, not a little essential. full essential. 😭

In particular, it contains parts of the startup code like: `__init_sp` (`https://github.com/avrdudes/avr-libc/issues/1011`) to initialize the stack pointer with symbol `__stack`, `__init_cvt` (`https://github.com/avrdudes/avr-libc/issues/1010`) to set up the hardware to use a compact vector table with `-mcvt`, `__call_main` (`https://github.com/avrdudes/avr-libc/issues/1012`) to call `main` and `exit`, and `__do_flmap_init` (`https://github.com/avrdudes/avr-libc/issues/931`) to set up FLMAP according to symbol `__flmap`.

- **`-nodevicespecs`** — don't add `-specs=device-specs/specs-mcu` to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. Which means the user has to supply her private device specs file by means of `-specs=path-to-specs-file`. And there is no more need for option `-mmcu=mcu` ✨

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed `-B some-path` to point to a directory which contains a folder named `device-specs` which contains a specs file named `specs-mcu`, where *mcu* was specified by `-mmcu=mcu`. (say that five times fast, I dare you 🌀)

- **`-Waddr-space-convert`** — warns about conversions between address spaces in the case where the resulting address space is **not** contained in the incoming address space. vibe check failed, warning issued 🫠

- **`-Wmisspelled-isr`** — warns if the ISR is misspelled, i.e. without `__vector` prefix. Enabled by default. she's always watching 👀

---

## ✨ AVR Optimization Options ✨

Ok so the following options are *pure* optimization options, no cap. (The ones `-mgas-isr-prologues`, `-mmain-is-OS_task`, `-mno-call-main` and `-mrelax` from above are only *almost* optimization options, since there are rare occasions where their different code generation actually matters — main character exceptions 💫)

- **`-maccumulate-args`** — accumulates outgoing function arguments and acquires/releases the needed stack space for outgoing function arguments once in the function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards. See also the `-fdefer-pop` Optimize Options,,optimization option.

Popping the arguments after the function call can be *expensive* on AVR, so accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call. 🎀

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack — like calls to printf-like functions. slay.

- **`-mbranch-cost=`*cost* — sets the branch costs for conditional branch instructions to *cost*. Reasonable values for *cost* are small, non-negative integers. The default branch cost is `0` 🍓

- **`-mcall-prologues`** — function prologues/epilogues get expanded as calls to appropriate subroutines. Code size is smaller. iconic behavior.

- **`-mfuse-add`**, **`-mno-fuse-add`**, **`-mfuse-add=`*level* — optimizes indirect memory accesses on reduced Tiny devices 🔥 The default uses `level=1` for optimizations `-Og` and `-O1`, and `level=2` for higher optimizations. Valid values for *level* are `0`, `1` and `2`.

- **`-mfuse-move`**, **`-mno-fuse-move`**, **`-mfuse-move=`*level* — runs a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default uses `level=3` for optimization `-O1`, and `level=23` for higher optimizations. Valid values for *level* are in the range `0 ... 23` which is a 3:2:2:2 mixed radix value. Each digit controls some aspect of the optimization — it's giving base-whichever, we don't ask questions 💜

- **`-mfuse-move2`** — runs a post combine optimization pass that tries to fuse move instructions. the sequel. the remix. 🌈

- **`-mstrict-X`** — uses address register `X` in a way proposed by the hardware. This means that `X` is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the `X` register may be used in the same way as `Y` or `Z` — which then is emulated by additional instructions. For example, loading a value with `X+const` addressing with a small non-negative `const < 64` to a register Rn is performed as

adiw r26, const ; X += const ld <Rn>, X ; <Rn> = *X sbiw r26, const ; X -= const


look at all that extra work. she's doing her best 😭

- **`-msplit-bit-shift`** — splits multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset. This optimization is turned on per default for `-O2` and higher, including `-Os` but **excluding** `-Oz`. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by `-mfuse-move`. 🦋

- **`-msplit-ldst`** — splits multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for `-O2` and higher. tiny pieces, big energy ✨

- **`-muse-nonzero-bits`** — enables optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for `-O2` and higher. (yes the name is a little confusing, no we will not be elaborating, that's the whole bit) 🎀

---

## 💫 `EIND` and Devices with More Than 128 Ki Bytes of Flash 💫

Ok so. Pointers in the implementation are 16 bits wide. The address of a function or label is represented as a *word* address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there's a special function register called `EIND` that serves as the most significant part of the target address when `EICALL` or `EIJMP` instructions are used. she's the high bits. she's the VIP. 👑

Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:

- The compiler **never** sets `EIND`.
- The compiler uses `EIND` implicitly in `EICALL`/`EIJMP` instructions or might read `EIND` directly in order to emulate an indirect call/jump by means of a `RET` instruction.
- The compiler assumes that `EIND` never changes during the startup code or during the application. In particular, `EIND` is **not** saved/restored in function or interrupt service routine prologue/epilogue.
- For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. 🚀
- Linker relaxation must be turned on so that the linker generates the stubs correctly in **all** situations. See the compiler option `-mrelax` and the linker option `--relax`. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 💀 (no note, no nothing, just vibes and silence)
- The default linker script is arranged for code with `EIND = 0`. If code is supposed to work for a setup with `EIND != 0`, a custom linker script has to be used in order to place the sections whose name start with `.trampolines` into the segment where `EIND` points to.
- The startup code from libgcc never sets `EIND`. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on `EIND`, see the AVR-LibC user manual (`https://avrdudes.github.io/avr-libc/avr-libc-user-manual/`).
- It is legitimate for user-specific startup code to set up `EIND` early, for example by means of initialization code located in section `.init3`. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets `EIND` to the segment where the vector table is located. 🧚

#include <avr/io.h>

static void __attribute__((section(".init3"),naked,used,no_instrument_function)) init3_set_eind (void) { __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t" "out %i0,r24" :: "n" (&EIND) : "r24","memory"); }


The `__trampolines_start` symbol is defined in the linker script. 💖

- Stubs are generated automatically by the linker if the following two conditions are met:

- The address of a label is taken by means of the `gs` modifier (short for generate stubs) like so:

LDI r24, lo8(gs(<func>)) LDI r25, hi8(gs(<func>))


- The final location of that label is in a code segment outside the segment where the stubs are located. 📍

- The compiler emits such `gs` modifiers for code labels in the following situations:

- Taking address of a function or code label.
- Computed goto.
- If prologue-save function is used, see `-mcall-prologues` command-line option.
- Switch/case dispatch tables. If you do not want such dispatch tables you can specify the `-fno-jump-tables` command-line option. 🎯

- **`-`** <C and C++ constructors/destructors called during startup/shutdown.>
- **`-`** <If the tools hit a `gs()` modifier explained above.>

- Jumping to non-symbolic addresses like so is **not supported**, bestie, do not do it:

int main (void) { /* Call function at word address 0x2 / return ((int()(void)) 0x2)(); }


  Instead, a stub has to be set up, i.e. the function has to be called through a symbol (`"func_4"` in the example):

int main (void) { extern int func_4 (void);

/* Call function at byte address 0x4 */ return func_4(); }


  and the application be linked with `-Wl,--defsym,func_4=0x4`. Alternatively, `"func_4"` can be defined in the linker script. ✨

---

## 🌀 Handling of the `"RAMPD"`, `"RAMPX"`, `"RAMPY"` and `"RAMPZ"` Special Function Registers

Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a `"RAMP"` register is used as high part of the address: the `"X"`, `"Y"`, `"Z"` address register is concatenated with the `"RAMPX"`, `"RAMPY"`, `"RAMPZ"` special function register, respectively, to get a wide address. Similarly, `"RAMPD"` is used together with direct addressing. 💅

- The startup code initializes the `"RAMP"` special function registers with zero.
- If a AVR Named Address Spaces, named address space other than generic or `"__flash"` is used, then `"RAMPZ"` is set as needed before the operation.
- If the device supports RAM larger than 64 KiB and the compiler needs to change `"RAMPZ"` to accomplish an operation, `"RAMPZ"` is reset to zero after the operation.
- If the device comes with a specific `"RAMP"` register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
- RAM larger than 64 KiB is **not supported** by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the `"RAMP"` registers, you **must** reset it to zero after the access. <red>No cap, this one matters.</red>

---

## 🦋 AVR Built-in Macros

GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the `-mmcu=` command-line option.

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.

### `"__AVR_ARCH__"`

Build-in macro that resolves to a decimal number that identifies the architecture and depends on the `-mmcu=`*mcu* option. Possible values are:

2, 25, 3, 31, 35, 4, 5, 51, 6


for *mcu*=`"avr2"`, `"avr25"`, `"avr3"`, `"avr31"`, `"avr35"`, `"avr4"`, `"avr5"`, `"avr51"`, `"avr6"`,

respectively and

100, 102, 103, 104, 105, 106, 107


for *mcu*=`"avrtiny"`, `"avrxmega2"`, `"avrxmega3"`, `"avrxmega4"`, `"avrxmega5"`, `"avrxmega6"`, `"avrxmega7"`, respectively. If *mcu* specifies a device, this built-in macro is set accordingly. For example, with `-mmcu=atmega8` the macro is defined to 4. 🔥

### `"__AVR_Device__"`

Setting `-mmcu=`*device* defines this built-in macro which reflects the device's name. For example, `-mmcu=atmega8` defines the built-in macro `"__AVR_ATmega8__"`, `-mmcu=attiny261a` defines `"__AVR_ATtiny261A__"`, etc.

The built-in macros' names follow the scheme `"__AVR_Device__"` where *Device* is the device name as from the AVR user manual. The difference between *Device* in the built-in macro and *device* in `-mmcu=`*device* is that the latter is always lowercase.

If *device* is not a device but only a core architecture like *avr51*, this macro is **not** defined.

### `"__AVR_DEVICE_NAME__"`

Setting `-mmcu=`*device* defines this built-in macro to the device's name. For example, with `-mmcu=atmega8` the macro is defined to `"atmega8"`.

If *device* is not a device but only a core architecture like *avr51*, this macro is **not** defined.

### `"__AVR_CVT__"`

The code is being compiled with option `"-mcvt"` to use a compact vector table.

### `"__AVR_XMEGA__"`

The device / architecture belongs to the XMEGA family of devices.

### `"__AVR_HAVE_ADIW__"`

The device has the `"ADIW"` and `"SBIW"` instructions.

### `"__AVR_HAVE_ELPM__"`

The device has the `"ELPM"` instruction.

### `"__AVR_HAVE_ELPMX__"`

The device has the `"ELPM Rn,Z"` and `"ELPM Rn,Z+"` instructions.

### `"__AVR_HAVE_LPMX__"`

The device has the `"LPM Rn,Z"` and `"LPM Rn,Z+"` instructions.

### `"__AVR_HAVE_MOVW__"`

The device has the `"MOVW"` instruction to perform 16-bit register-register moves.

### `"__AVR_HAVE_MUL__"`

The device has a hardware multiplier. 💪

### `"__AVR_HAVE_JMP_CALL__"`

The device has the `"JMP"` and `"CALL"` instructions. This is the case for devices with more than 8 KiB of program memory.

### `"__AVR_HAVE_EIJMP_EICALL__"`, `"__AVR_3_BYTE_PC__"`

The device has the `"EIJMP"` and `"EICALL"` instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.

### `"__AVR_2_BYTE_PC__"`

The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.

### `"__AVR_HAVE_8BIT_SP__"`, `"__AVR_HAVE_16BIT_SP__"`

The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by `-mtiny-stack`.

### `"__AVR_HAVE_SPH__"`, `"__AVR_SP8__"`

The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by `-mmcu=` and in the cases of `-mmcu=avr2` and `-mmcu=avr25` also by `-msp8`.

### `"__AVR_HAVE_RAMPD__"`, `"__AVR_HAVE_RAMPX__"`, `"__AVR_HAVE_RAMPY__"`, `"__AVR_HAVE_RAMPZ__"`

The device has the `"RAMPD"`, `"RAMPX"`, `"RAMPY"`, `"RAMPZ"` special function register, respectively.

### `"__NO_INTERRUPTS__"`

This macro reflects the `-mno-interrupts` command-line option.

### `"__AVR_ERRATA_SKIP__"`, `"__AVR_ERRATA_SKIP_JMP_CALL__"`

Some AVR devices (AT90S8515, ATmega103) must **not** skip 32-bit instructions because of a hardware erratum. Skip instructions are `"SBRS"`, `"SBRC"`, `"SBIS"`, `"SBIC"` and `"CPSE"`. The second macro is only defined if `"__AVR_HAVE_JMP_CALL__"` is also set.

### `"__AVR_ISA_RMW__"`

The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).

### `"__AVR_SFR_OFFSET__=offset"`

Instructions that can address I/O special function registers directly like `"IN"`, `"OUT"`, `"SBI"`, etc. may use a different address as if addressed by an instruction to access RAM like `"LD"` or `"STS"`. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.

### `"__AVR_SHORT_CALLS__"`

The `-mshort-calls` command line option is set.

### `"__AVR_PM_BASE_ADDRESS__=addr"`

Some devices support reading from flash memory by means of `"LD*"` instructions. The flash memory is seen in the data address space at an offset of `"__AVR_PM_BASE_ADDRESS__"`. If this macro is **not** defined, this feature is not available. If defined, the address space is linear and there is no need to put `".rodata"` into RAM. This is handled by the default linker description file, and is currently available for `"avrtiny"` and `"avrxmega3"`. Even more convenient, there is no need to use address spaces like `"__flash"` or features like attribute `"progmem"` and `"pgm_read_*"`. 🎀

### `"__AVR_HAVE_FLMAP__"`

This macro is defined provided the following conditions are met:

- <The device has the `"NVMCTRL_CTRLB.FLMAP"` bitfield.> This applies to the AVR64* and AVR128* devices.
- <It's not known at assembler-time which emulation will be used.>

This implies the compiler was configured with GNU Binutils that implement PR31124 (`"https://sourceware.org/PR31124"`).

### `"__AVR_RODATA_IN_RAM__"`

This macro is **undefined** when the code is compiled for a core architecture.

When the code is compiled for a device, the macro is defined to 1 when the `".rodata"` sections for read-only data is located in RAM; and defined to 0, otherwise.

### `"__WITH_AVRLIBC__"`

The compiler is configured to be used together with AVR-LibC. See the `--with-avrlibc` configure option.

### `"__HAVE_SIGNAL_N__"`

The compiler supports the signal(*num*) and interrupt(*num*) AVR Function Attributes, function attributes with an argument *num* that specifies the number of the interrupt service routine.

### `"__HAVE_DOUBLE_MULTILIB__"`

Defined if `-mdouble=` acts as a multilib option.

### `"__HAVE_DOUBLE32__"`, `"__HAVE_DOUBLE64__"`

Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option `-mdouble=`.

### `"__DEFAULT_DOUBLE__"`

The size in bits of `"double"` if `-mdouble=` is not set. To test the layout of `"double"` in a program, use the built-in macro `"__SIZEOF_DOUBLE__"`.

### `"__HAVE_LONG_DOUBLE32__"`, `"__HAVE_LONG_DOUBLE64__"`, `"__HAVE_LONG_DOUBLE_MULTILIB__"`, `"__DEFAULT_LONG_DOUBLE__"`

Same as above, but for `"long double"` instead of `"double"`. 🫶

### `"__WITH_DOUBLE_COMPARISON__"`

Reflects the `"--with-double-comparison={tristate|bool|libf7}"` configure option (`"https://gcc.gnu.org/install/configure.html#avr"`) and is defined to 2 or 3.

### `"__WITH_LIBF7_LIBGCC__"`, `"__WITH_LIBF7_MATH__"`, `"__WITH_LIBF7_MATH_SYMBOLS__"`

Reflects the `"--with-libf7={libgcc|math|math-symbols}"` configure option (`"https://gcc.gnu.org/install/configure.html#avr"`).

---

## 🧚 AVR Internal Options

The following options are used internally by the compiler and to communicate between device specs files and the compiler proper. You **don't need** to set these options by hand, in particular they are **not** optimization options. <red>Using these options in the wrong way may lead to sub-optimal or wrong code.</red> They are documented for completeness, and in order to get a better understanding of device specs (`"https://gcc.gnu.org/wiki/avr-gcc#spec-files"`) files.

- **`-mn-flash=`*num*** — Assume that the flash memory has a size of *num* times 64 KiB. This determines which `"__flashN"` address spaces are available.
- **`-mflmap`** — The device has the `"FLMAP"` bit field located in special function register `"NVMCTRL_CTRLB"`.
- **`-mrmw`** — Assume that the device supports the Read-Modify-Write instructions `"XCH"`, `"LAC"`, `"LAS"` and `"LAT"`.

- **`-mshort-calls`** — lowkey assumes `RJMP` and `RCALL` can reach the entire program memory. It's the option used for multilib generation and selection for the devices from architecture `avrxmega3`. ✨

- **`-mskip-bug`** — generate code *without* skips (`CPSE`, `SBRS`, `SBRC`, `SBIS`, `SBIC`) over 32-bit instructions. That's the whole vibe of it. 🫶

- **`-msp8`** — treats the stack pointer register as an 8-bit register, meaning it assumes the high byte of the stack pointer is zero. The compiler uses this one to select and build multilibs for architectures `avr2` and `avr25` — and those architectures mix devices *with* and *without* `SPH`. iykyk 💅

## 💜 Blackfin Options 💜

- **`-mcpu=cpu[-sirevision]`** — tells GCC the name of the target Blackfin processor. Right now *cpu* can be one of `bf512`, `bf514`, `bf516`, `bf518`, `bf522`, `bf523`, `bf524`, `bf525`, `bf526`, `bf527`, `bf531`, `bf532`, `bf533`, `bf534`, `bf536`, `bf537`, `bf538`, `bf539`, `bf542`, `bf544`, `bf547`, `bf548`, `bf549`, `bf542m`, `bf544m`, `bf547m`, `bf548m`, `bf549m`, `bf561`, `bf592`.

  The optional *sirevision* specifies the silicon revision of the target Blackfin processor — any workarounds available for that targeted silicon revision get enabled. If *sirevision* is `none`, literally no workarounds are enabled. If *sirevision* is `any`, **all** the workarounds for the targeted processor are enabled. The `__SILICON_REVISION__` macro gets defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If *sirevision* is `none`, `__SILICON_REVISION__` is not defined. If *sirevision* is `any`, `__SILICON_REVISION__` is defined to be `0xffff`. And if you just don't use the optional *sirevision* at all, GCC assumes the latest known silicon revision of the targeted Blackfin processor. slay 🎀

  GCC defines a preprocessor macro for the specified *cpu*. For the `bfin-elf` toolchain, this option causes the hardware BSP provided by libgloss to be linked in if `-msim` is not given.

  Without this option, `bf532` is used as the processor by default.

  > heads up bestie: support for `bf561` is **incomplete**. For `bf561`, only the preprocessor macro is defined. 😭

- **`-msim`** — specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. It only has an effect for the `bfin-elf` toolchain. Certain other options, such as `-mid-shared-library` and `-mfdpic`, imply `-msim`. ✨

- **`-momit-leaf-frame-pointer`** — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers, and it makes an extra register available in leaf functions. obsessed 🫶

- **`-mspecld-anomaly`** — when enabled, the compiler makes sure the generated code does not contain speculative loads after jump instructions. If this option is used, `__WORKAROUND_SPECULATIVE_LOADS` is defined. 🔥

- **`-mno-specld-anomaly`** — don't generate extra code to prevent speculative loads from occurring.

- **`-mcsync-anomaly`** — when enabled, the compiler makes sure the generated code does not contain `CSYNC` or `SSYNC` instructions too soon after conditional branches. If this option is used, `__WORKAROUND_SPECULATIVE_SYNCS` is defined. 💜

- **`-mno-csync-anomaly`** — don't generate extra code to prevent `CSYNC` or `SSYNC` instructions from occurring too soon after a conditional branch.

- **`-mlow64k`** — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. iconic 🌈

- **`-mno-low64k`** — assume that the program is arbitrarily large. This is the default, ngl.

- **`-mstack-check-l1`** — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel. 🦋

- **`-mid-shared-library`** — generate code that supports shared libraries via the library ID method. This allows for execute in place *and* shared libraries in an environment without virtual memory management. This option implies `-fPIC`. With a `bfin-elf` target, this option implies `-msim`. 💅

- **`-mno-id-shared-library`** — generate code that doesn't assume ID-based shared libraries are being used. This is the default.

- **`-mleaf-id-shared-library`** — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. ⭐

- **`-mno-leaf-id-shared-library`** — do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns. 💀

- **`-mshared-library-id=n`** — specifies the identification number of the ID-based shared library being compiled. Specifying a value of `0` generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option. 🎀

- **`-msep-data`** — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section. ✨

- **`-mno-sep-data`** — generate code that assumes that the data segment follows the text segment. This is the default.

- **`-mlong-calls`**
- **`-mno-long-calls`** — tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction. 🫶

  This feature is not enabled by default. Specifying `-mno-long-calls` restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

- **`-mfast-fp`** — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. 🔥

- **`-minline-plt`** — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without `-mfdpic`. 🙈

- **`-mmulticore`** — build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro `__BFIN_MULTICORE`. It can **only** be used with `-mcpu=bf561[-sirevision]`.

  This option can be used with `-mcorea` or `-mcoreb`, which selects the one-application-per-core programming model. Without `-mcorea` or `-mcoreb`, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as `coreb_main`.

  If this option is not used, the single-core application programming model is used. 🧚

- **`-mcorea`** — build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro `__BFIN_COREA` is defined. This option can only be used in conjunction with `-mmulticore`. 💜

- **`-mcoreb`** — build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro `__BFIN_COREB` is defined. When this option is used, `coreb_main` should be used instead of `main`. This option can only be used in conjunction with `-mmulticore`. 👑

- **`-msdram`** — build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro `__BFIN_SDRAM` is defined. The loader should initialize SDRAM before loading the application. 🌸

- **`-micplb`** — assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off. 💅

---

## 🌈 C6X Options 🌈

- **`-march=name`** — this specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: `c62x`, `c64x`, `c64x+`, `c67x`, `c67x+`, `c674x`. ✨

- **`-mbig-endian`** — generate code for a big-endian target.

- **`-mlittle-endian`** — generate code for a little-endian target. This is the default. 💜

- **`-msim`** — choose startup files and linker script suitable for the simulator. 🦋

- **`-msdata=default`** — put small global and static data in the `.neardata` section, which is pointed to by register `B14`. Put small uninitialized global and static data in the `.bss` section, which is adjacent to the `.neardata` section. Put small read-only data into the `.rodata` section. The corresponding sections used for large pieces of data are `.fardata`, `.far` and `.const`. 🎀

- **`-msdata=all`** — put **all** data, not just small objects, into the sections reserved for small data, and use addressing relative to the `B14` register to access them. ate 🔥

- **`-msdata=none`** — make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the `.fardata` section, and all uninitialized data in the `.far` section. Put all constant data into the `.const` section. 💀

---

## 🎀 CRIS Options 🎀

These options are defined specifically for the CRIS ports, bestie.

- **`-march=architecture-type`**
- **`-mcpu=architecture-type`** — generate code for the specified architecture. The choices for *architecture-type* are `v3`, `v8` and `v10` for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is `v0`. 🌸

- **`-mtune=architecture-type`** — tune to *architecture-type* everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for *architecture-type* are the same as for `-march=architecture-type`. ✨

- **`-mmax-stack-frame=n`** — warn when the stack frame of a function exceeds *n* bytes. ⭐

- **`-metrax4`**
- **`-metrax100`** — the options `-metrax4` and `-metrax100` are synonyms for `-march=v3` and `-march=v8` respectively. 💅

- **`-mmul-bug-workaround`**
- **`-mno-mul-bug-workaround`** — work around a bug in the `muls` and `mulu` instructions for CPU models where it applies. This option is disabled by default. 🫶

- **`-mpdebug`** — turns on CRIS-specific verbose debug tea ☕ in the assembly output. Bonus lore: this also flips off the `#NO_APP` formatted-code indicator that normally gets handed to the assembler at the top of the assembly file.

- **`-mcc-init`** — bestie, do *not* reuse condition-code results from a previous instruction. Always emit the compare and test instructions fresh before anything touches the condition codes. No leftovers, periodt. 💅

- **`-mno-side-effects`** — don't emit instructions with side effects in addressing modes other than post-increment. That's the whole vibe.

- **`-mstack-align`**, **`-mno-stack-align`**, **`-mdata-align`**, **`-mno-data-align`**, **`-mconst-align`**, **`-mno-const-align`** — this little family arranges (or, with the `no-` versions, *eliminates* the arranging of) alignment for the stack frame, individual data, and constants so everything lines up for the maximum single data access size of whatever CPU model you picked. Default is 32-bit alignment. And ngl, ABI details like structure layout are **not** affected by these options — they stay unbothered. ✨

- **`-m32-bit`**, **`-m16-bit`**, **`-m8-bit`** — cousins of the stack-/data-/const-align options above: these arrange for the stack frame, writable data and constants to *all* be 32-bit, 16-bit or 8-bit aligned respectively. Default is 32-bit alignment. 🎀

- **`-mno-prologue-epilogue`**, **`-mprologue-epilogue`** — with `-mno-prologue-epilogue` the normal function prologue and epilogue (the ones that set up the stack frame) get omitted, and no return instructions or return sequences are generated at all. Use this **only** together with your own eyeballs on the compiled code, fr fr — no warnings or errors get generated when call-saved registers must be saved or when storage for local variables needs to be allocated. It will not tell you. It will just vibe. 💀

- **`-melf`** — legacy no-op option. It's giving "still invited to the party, does nothing." 🙈

- **`-sim`** — arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data all get allocated consecutively, one after another. 🧚

- **`-sim2`** — like `-sim`, but it passes linker options to locate initialized data at `0x40000000` and zero-initialized data at `0x80000000`. Moving day for your data. 📦

---

# 🌈 C-SKY Options 🌈

GCC supports these options when compiling for C-SKY V2 processors. Slay.

- **`-march=arch`** — specify the C-SKY target architecture. Valid values for *arch* are: `ck801`, `ck802`, `ck803`, `ck807`, and `ck810`. The default is `ck810`. 👑

- **`-mcpu=cpu`** — specify the C-SKY target processor. Valid values for *cpu* are: `ck801`, `ck801t`, `ck802`, `ck802t`, `ck802j`, `ck803`, `ck803h`, `ck803t`, `ck803ht`, `ck803f`, `ck803fh`, `ck803e`, `ck803eh`, `ck803et`, `ck803eht`, `ck803ef`, `ck803efh`, `ck803ft`, `ck803eft`, `ck803efht`, `ck803r1`, `ck803hr1`, `ck803tr1`, `ck803htr1`, `ck803fr1`, `ck803fhr1`, `ck803er1`, `ck803ehr1`, `ck803etr1`, `ck803ehtr1`, `ck803efr1`, `ck803efhr1`, `ck803ftr1`, `ck803eftr1`, `ck803efhtr1`, `ck803s`, `ck803st`, `ck803se`, `ck803sf`, `ck803sef`, `ck803seft`, `ck807e`, `ck807ef`, `ck807`, `ck807f`, `ck810e`, `ck810et`, `ck810ef`, `ck810eft`, `ck810`, `ck810v`, `ck810f`, `ck810t`, `ck810fv`, `ck810tv`, `ck810ft`, and `ck810ftv`. Yes, that's the whole list. Yes, you should screenshot it. 😭

- **`-mbig-endian`**, **`-EB`**, **`-mlittle-endian`**, **`-EL`** — pick big- or little-endian code. Default is little-endian. 🦋

- **`-mfloat-abi=name`** — specifies which floating-point ABI to use. Permissible values are: `soft`, `softfp` and `hard`.

  - `soft` makes GCC generate output containing library calls for floating-point operations.
  - `softfp` allows generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.
  - `hard` allows generation of floating-point instructions and uses FPU-specific calling conventions.

  The default depends on the specific target configuration. And heads up, bestie: the hard-float and soft-float ABIs are **not** link-compatible — you must compile your *entire* program with the same ABI, and link with a compatible set of libraries. No mixing. That's a red flag situation 🚩.

- **`-mhard-float`**, **`-msoft-float`** — select hardware or software floating-point implementations. The default is soft float. 🌸

- **`-mdouble-float`**, **`-mno-double-float`** — when `-mhard-float` is in effect, enable generation of double-precision float instructions. This is the default *except* when compiling for `CK803`. ✨

- **`-mfdivdu`**, **`-mno-fdivdu`** — when `-mhard-float` is in effect, enable generation of `frecipd`, `fsqrtd`, and `fdivd` instructions. This is the default *except* when compiling for `CK803`.

- **`-mfpu=fpu`** — select the floating-point processor. This option can only be used with `-mhard-float`. Values for *fpu* are `fpv2_sf` (equivalent to `-mno-double-float -mno-fdivdu`), `fpv2` (`-mdouble-float -mno-divdu`), and `fpv2_divd` (`-mdouble-float -mdivdu`). Copy them exactly, okay? 💅

- **`-melrw`**, **`-mno-elrw`** — enable the extended `lrw` instruction. Defaults to on for `CK801` and off otherwise.

- **`-mistack`**, **`-mno-istack`** — enable interrupt stack instructions; the default is off. The `-mistack` option is **required** to handle the `interrupt` and `isr` function attributes. 🔥

- **`-mmp`** — enable multiprocessor instructions; the default is off.

- **`-mcp`** — enable coprocessor instructions; the default is off.

- **`-mcache`** — enable coprocessor instructions; the default is off. (That's what the page says, bestie — I'm just the messenger 🫶.)

- **`-msecurity`** — enable C-SKY security instructions; the default is off. 🔐

- **`-mtrust`** — enable C-SKY trust instructions; the default is off.

- **`-mdsp`**, **`-medsp`**, **`-mvdsp`** — enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these default to off.

- **`-mdiv`**, **`-mno-div`** — generate divide instructions. Default is off.

- **`-msmart`**, **`-mno-smart`** — generate code for Smart Mode, using only registers numbered `0-7` so 16-bit instructions can be used. This option is ignored for `CK801` where this is the required behavior, and it defaults to on for `CK802`. For other targets, the default is off.

- **`-mhigh-registers`**, **`-mno-high-registers`** — generate code using the high registers numbered `16-31`. Not supported on `CK801`, `CK802`, or `CK803`, and it's enabled by default for other processors.

- **`-manchor`**, **`-mno-anchor`** — generate code using global anchor symbol addresses. ⚓

- **`-mpushpop`**, **`-mno-pushpop`** — generate code using `push` and `pop` instructions. This option defaults to on.

- **`-mmultiple-stld`**, **`-mstm`**, **`-mno-multiple-stld`**, **`-mno-stm`** — generate code using `stm` and `ldm` instructions. Not supported on `CK801`, but enabled by default on other processors.

- **`-mconstpool`**, **`-mno-constpool`** — create constant pools in the compiler instead of deferring that job to the assembler. This option is the default and is **required** for correct code generation on `CK801` and `CK802`, and it's optional on other processors. 🏊

- **`-mstack-size`**, **`-mno-stack-size`** — emit `.stack_size` directives for each function in the assembly output. Defaults to off.

- **`-mccrt`**, **`-mno-ccrt`** — generate code for the C-SKY compiler runtime instead of `libgcc`. Defaults to off.

- **`-mbranch-cost=n`** — set the branch costs to roughly *n* instructions. The default is `1`. 💸

- **`-msched-prolog`**, **`-mno-sched-prolog`** — permit scheduling of function prologue and epilogue sequences. ⚠️ Using this option can result in code that is **not compliant** with the C-SKY V2 ABI prologue requirements and that **cannot be debugged or backtraced**. It is disabled by default. Don't say I didn't warn you. 💀

- **`-msim`** — links the library `libsemi.a`, which is compatible with the simulator. Applicable to ELF compiler only.

---

# 🎀 Cygwin and MinGW Options 🎀

These additional options are available for Microsoft Windows targets:

- **`-mconsole`** — specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. Available for Cygwin and MinGW targets and enabled by default on those targets. 🖥️

- **`-mcrtdll=library`** — preprocess, compile or link with the specified C RunTime DLL library. This option adjusts the predefined macros `__CRTDLL__`, `__MSVCRT__`, `_UCRT` and `__MSVCRT_VERSION__` for the specified CRT library, chooses the start file for the CRT library, and links with the CRT library. Recognized CRT library names for the preprocessor are: `crtdll*`, `msvcrt10*`, `msvcrt20*`, `msvcrt40*`, `msvcr40*`, `msvcrtd*`, `msvcrt-os*`, `msvcr70*`, `msvcr71*`, `msvcr80*`, `msvcr90*`, `msvcr100*`, `msvcr110*`, `msvcr120*` and `ucrt*`. If this option is not specified, then the default MinGW import library `msvcrt` is used for linking and no other adjustment for the preprocessor is done. MinGW import library `msvcrt` is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation. MinGW import library `msvcrt-os` is for the Windows system CRT DLL library `msvcrt.dll` and in most cases is the default MinGW import library. Generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT. ⚠️ This option is for **experimental and testing purposes only**. Available for MinGW targets.

- **`-mdll`** — available for Cygwin and MinGW targets. Specifies that a DLL — a dynamic link library — is to be generated, enabling the selection of the required runtime startup object and entry point. 📚

- **`-mnop-fun-dllimport`** — available for Cygwin and MinGW targets. Specifies that the `dllimport` attribute should be ignored. 🙈

- **`-mthreads`** — available for MinGW targets. Specifies that MinGW-specific thread support is to be used. 🧵

- **`-municode`** — available for MinGW-w64 targets. Causes the `UNICODE` preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. 🌍

- **`-mwin32`** — available for Cygwin and MinGW targets. Specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does **not** influence the choice of runtime library/startup code.

- **`-mwindows`** — available for Cygwin and MinGW targets. Specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. 🪟✨

- **`-fno-set-stack-executable`** — available for MinGW targets. Specifies that the executable flag for the stack used by nested functions **isn't** set. This is necessary for binaries running in kernel mode of Microsoft Windows, since there the User32 API — which is used to set executable privileges — isn't available. 👑

- **`-fwritable-relocated-rdata`** — <pink>MinGW and Cygwin targets only</pink>, bestie. It specifies that relocated-data in a read-only section is put into the `".data"` section. This is necessary for older runtimes that don’t support modification of `".rdata"` sections for pseudo-relocation. 💅

- **`-mpe-aligned-commons`** — <pink>Cygwin and MinGW targets</pink>. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It’s enabled by default if GCC detects that the target assembler found during configuration supports the feature. ✨

See also under `x86 Options` for standard options. 👀

## Darwin Options 🍎✨

These options are defined for all architectures running the Darwin operating system. 🍎

FSF GCC on Darwin does **not** create “fat” object files; it creates an object file for the single architecture that GCC was built to target. Apple’s GCC on Darwin **does** create “fat” files if multiple `-arch` options are used; it does so by running the compiler or linker multiple times and joining the results together with `lipo`. 😭

The subtype of the file created (like `ppc7400` or `ppc970` or `i686`) is determined by the flags that specify the ISA that GCC is targeting, like `-mcpu` or `-march`. The `-force_cpusubtype_ALL` option can be used to override this. 💅

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, `as`, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a `ppc750` object file. The linker for shared libraries, `/usr/bin/libtool`, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a `ppc970` object file in a `ppc7400` library). The linker for executables, `ld`, quietly gives the executable the most restrictive subtype of any of its input files. 😭

- **`-Fdir`** — Add the framework directory *dir* to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by `-I` options and are scanned in a left-to-right order. A framework directory is a directory with frameworks in it. A framework is a directory with a `Headers` and/or `PrivateHeaders` directory contained directly in it that ends in `.framework`. The name of a framework is the name of this directory excluding the `.framework`. Headers associated with the framework are found in one of those two directories, with `Headers` being searched first. A subframework is a framework directory that is in a framework’s `Frameworks` directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in `/System/Library/Frameworks` and `/Library/Frameworks`. An example include looks like `#include <Framework/header.h>`, where `Framework` denotes the name of the framework and `header.h` is found in the `PrivateHeaders` or `Headers` directory. 💅

- **`-iframeworkdir`** — Like `-F` except the directory is treated as a system directory. The main difference between this `-iframework` and `-F` is that with `-iframework` the compiler does not warn about constructs contained within header files found via *dir*. This option is valid only for the C family of languages. 🫶

- **`-gused`** — Emit debugging information for symbols that are used. For stabs debugging format, this enables `-feliminate-unused-debug-symbols`. This is by default ON. ✨

- **`-gfull`** — Emit debugging information for all symbols and types. 💖

- **`-fconstant-cfstrings`** — The `-fconstant-cfstrings` is an alias for `-mconstant-cfstrings`. 🎀

- **`-mconstant-cfstrings`** — When the NeXT runtime is being used (the default on these systems), override any `-fconstant-string-class` setting and cause `@"..."` literals to be laid out as constant CoreFoundation strings. 🌸

- **`-mmacosx-version-min=version`** — The earliest version of MacOS X that this executable will run on is *version*. Typical values supported for *version* include `12`, `10.12`, and `10.5.8`. If the compiler was built to use the system’s headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible. 🍓

- **`-mkernel`** — Enable kernel development mode. The `-mkernel` option sets `-static`, `-fno-common`, `-fno-use-cxa-atexit`, `-fno-exceptions`, `-fno-non-call-exceptions`, `-fapple-kext`, `-fno-weak` and `-fno-rtti` where applicable. This mode also sets `-mno-altivec`, `-msoft-float`, `-fno-builtin` and `-mlong-branch` for PowerPC targets. 👑

- **`-mone-byte-bool`** — Override the defaults for `bool` so that `sizeof(bool)==1`. By default `sizeof(bool)` is `4` when compiling for Darwin/PowerPC and `1` when compiling for Darwin/x86, so this option has no effect on x86.

> **Warning:** The `-mone-byte-bool` switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model. 💀

- **`-mfix-and-continue`**, **`-ffix-and-continue`**, **`-findirect-data`** — Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load `.o` files into already-running programs. `-findirect-data` and `-ffix-and-continue` are provided for backwards compatibility. 🧚

- **`-all_load`** — Loads all members of static archive libraries. See man `ld(1)` for more information. 💜

- **`-arch_errors_fatal`** — Cause the errors having to do with files that have the wrong architecture to be fatal. 🔥

- **`-bind_at_load`** — Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched. ⭐

- **`-bundle`** — Produce a Mach-o bundle format file. See man `ld(1)` for more information. 🎀

- **`-bundle_loader executable`** — This option specifies the executable that will load the build output file being linked. See man `ld(1)` for more information. 🦋

- **`-dynamiclib`** — When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin `libtool` command. 🌈

- **`-force_cpusubtype_ALL`** — This causes GCC’s output file to have the ALL subtype, instead of one controlled by the `-mcpu` or `-march` option. 👑

- **`-nodefaultrpaths`** — Do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds `-nodefaultrpaths` to the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later. 🌸

Okay, now the big pass-through pile for the Darwin linker — every option, every duplicate, no skips: 👇

- **`-allowable_client`** *client_name*
- **`-client_name`**
- **`-compatibility_version`**
- **`-current_version`**
- **`-dead_strip`**
- **`-dependency-file`**
- **`-dylib_file`**
- **`-dylinker_install_name`**
- **`-dynamic`**
- **`-exported_symbols_list`**
- **`-filelist`**
- **`-flat_namespace`**
- **`-force_flat_namespace`**
- **`-headerpad_max_install_names`**
- **`-image_base`**
- **`-init`**
- **`-install_name`**
- **`-keep_private_externs`**
- **`-multi_module`**
- **`-multiply_defined`**
- **`-multiply_defined_unused`**
- **`-noall_load`**
- **`-no_dead_strip_inits_and_terms`**
- **`-nofixprebinding`**
- **`-nomultidefs`**
- **`-noprebind`**
- **`-noseglinkedit`**
- **`-pagezero_size`**
- **`-prebind`**
- **`-prebind_all_twolevel_modules`**
- **`-private_bundle`**
- **`-read_only_relocs`**
- **`-sectalign`**
- **`-sectobjectsymbols`**
- **`-whyload`**
- **`-seg1addr`**
- **`-sectcreate`**
- **`-sectobjectsymbols`**
- **`-sectorder`**
- **`-segaddr`**
- **`-segs_read_only_addr`**
- **`-segs_read_write_addr`**
- **`-seg_addr_table`**
- **`-seg_addr_table_filename`**
- **`-seglinkedit`**
- **`-segprot`**
- **`-segs_read_only_addr`**
- **`-segs_read_write_addr`**
- **`-single_module`**
- **`-static`**
- **`-sub_library`**
- **`-sub_umbrella`**
- **`-twolevel_namespace`**
- **`-umbrella`**
- **`-undefined`**
- **`-unexported_symbols_list`**
- **`-weak_reference_mismatches`**
- **`-whatsloaded`**

These options are passed to the Darwin linker. The Darwin linker man page describes them in detail. 💅

---

## DEC Alpha Options 💻💜

These `-m` options are defined for the DEC Alpha implementations: 🐘

- **`-mno-soft-float`**, **`-msoft-float`** — Use (do not use) the hardware floating-point instructions for floating-point operations. When `-msoft-float` is specified, functions in `libgcc.a` are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulation routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. Note that Alpha implementations without floating-point operations are required to have floating-point registers. 🫶

- **`-mfp-reg`**, **`-mno-fp-regs`** — Generate code that uses (does not use) the floating-point register set. `-mno-fp-regs` implies `-msoft-float`. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in `$0` instead of `$f0`. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with `-mno-fp-regs` must also be compiled with that option. A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers. 🔥

- **`-mieee`** — okay so the Alpha arch has this floating-point hardware that's tuned for *maximum performance* 💅 it's mostly IEEE-compliant, but for *full* compliance you need a little software assist, bestie. This option generates code that's fully IEEE-compliant — except the inexact-flag isn't maintained (see below, iykyk). Turning this on defines the preprocessor macro `"_IEEE_FP"` during compilation. The resulting code is less efficient but lowkey handles denormalized numbers and those exceptional IEEE values like not-a-number and plus/minus infinity correctly. Other Alpha compilers call this one `-ieee_with_no_inexact`.

- **`-mieee-with-inexact`** — this is giving `-mieee` energy except the generated code *also* maintains the IEEE inexact-flag ✨ turning this on makes the generated code implement fully-compliant IEEE math, periodt. On top of `"_IEEE_FP"`, it defines `"_IEEE_FP_EXACT"` as a preprocessor macro. Heads up though: on some Alpha implementations the resulting code may execute *significantly slower* than the default code 😭 and since there's very little code that actually depends on the inexact-flag, you should normally **not** specify this option. Other Alpha compilers call this one `-ieee_with_inexact`.

- **`-mfp-trap-mode=`*`trap-mode`* — this one controls which floating-point related traps are enabled 🔥 other Alpha compilers call this option `-fptm` *`trap-mode`*. The trap mode can be set to one of four values:
  - `n` — this is the default (normal) setting, no cap. The only traps enabled are the ones that can't be disabled in software (e.g., division by zero trap).
  - `u` — on top of the traps enabled by `n`, underflow traps are enabled too.
  - `su` — like `u`, but the instructions are marked to be safe for software completion (peep the Alpha architecture manual for the deets).
  - `sui` — like `su`, but inexact traps are enabled as well.

- **`-mfp-rounding-mode=`*`rounding-mode`* — selects the IEEE rounding mode 🎀 other Alpha compilers call this option `-fprm` *`rounding-mode`*. The *`rounding-mode`* can be one of:
  - `n` — normal IEEE rounding mode. Floating-point numbers get rounded towards the nearest machine number, or towards the even machine number in case of a tie.
  - `m` — round towards minus infinity.
  - `c` — chopped rounding mode. Floating-point numbers are rounded towards zero.
  - `d` — dynamic rounding mode. A field in the floating-point control register (`fpcr`, see the Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies the `fpcr`, `d` corresponds to round towards plus infinity.

- **`-mtrap-precision=`*`trap-precision`* — in the Alpha architecture, floating-point traps are imprecise 💀 meaning without software assistance it's literally impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that assists OS trap handlers in determining the exact location that caused a floating-point trap. Depending on what your app needs, different precision levels can be selected:
  - `p` — program precision. This option is the default and means a trap handler can only identify which *program* caused a floating-point exception.
  - `f` — function precision. The trap handler can determine the *function* that caused a floating-point exception.
  - `i` — instruction precision. The trap handler can determine the exact *instruction* that caused a floating-point exception.

  Other Alpha compilers provide the equivalent options called `-scope_safe` and `-resumption_safe`.

- **`-mieee-conformant`** — this marks the generated code as IEEE conformant 👑 **but bestie, you must NOT use this option unless you also specify `-mtrap-precision=i` and either `-mfp-trap-mode=su` or `-mfp-trap-mode=sui`.** Its only effect is to emit the line `.eflag 48` in the function prologue of the generated assembly file.

- **`-mbuild-constants`** — normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time. Use this option to require GCC to construct *all* integer constants using code, even if it takes more instructions (the maximum is six). You typically use this to build a shared library dynamic loader — itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment 🔄

- **`-mbwx`**, **`-mno-bwx`**, **`-mcix`**, **`-mno-cix`**, **`-mfix`**, **`-mno-fix`**, **`-mmax`**, **`-mno-max`** — these indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets 🧚 the default is to use the instruction sets supported by the CPU type specified via `-mcpu=` or that of the CPU on which GCC was built if none is specified.

- **`-msafe-bwa`**, **`-mno-safe-bwa`** — indicate whether, in the absence of the optional BWX instruction set, GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses 🫶

- **`-msafe-partial`**, **`-mno-safe-partial`** — indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses, including piecemeal accesses to unaligned data as well as block accesses to leading and trailing parts of aggregate types or other objects in memory that don't respectively start and end on an aligned 64-bit data boundary.

- **`-mfloat-vax`**, **`-mfloat-ieee`** — generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision 🌸

- **`-mexplicit-relocs`**, **`-mno-explicit-relocs`** — older Alpha assemblers provided no way to generate symbol relocations except via assembler macros, and using those macros doesn't allow optimal instruction scheduling ngl. GNU binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions ✨ this option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it's built and sets the default accordingly.

- **`-msmall-data`**, **`-mlarge-data`** — when `-mexplicit-relocs` is in effect, static data is accessed via gp-relative relocations. When `-msmall-data` is used, objects 8 bytes long or smaller are placed in a small data area (the `".sdata"` and `".sbss"` sections) and are accessed via 16-bit relocations off of the `$gp` register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction 🎯

  The default is `-mlarge-data`. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use `"malloc"` or `"mmap"` to allocate the data in the heap instead of in the program's data segment.

  When generating code for shared libraries, `-fpic` implies `-msmall-data` and `-fPIC` implies `-mlarge-data`.

- **`-msmall-text`**, **`-mlarge-text`** — when `-msmall-text` is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB and is thus reachable with a branch instruction 🦋 when `-msmall-data` is used, the compiler can assume that all local symbols share the same `$gp` value, and thus reduce the number of instructions required for a function call from 4 to 1. The default is `-mlarge-text`.

- **`-mcpu=`*`cpu_type`* — set the instruction set and instruction scheduling parameters for machine type *`cpu_type`* 💜 you can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you don't specify a processor type, GCC defaults to the processor on which the compiler was built.

  Supported values for *`cpu_type`* are:
  - `ev4`, `ev45`, `21064` — schedules as an EV4 and has no instruction set extensions.
  - `ev5`, `21164` — schedules as an EV5 and has no instruction set extensions.
  - `ev56`, `21164a` — schedules as an EV5 and supports the BWX extension.
  - `pca56`, `21164pc`, `21164PC` — schedules as an EV5 and supports the BWX and MAX extensions.
  - `ev6`, `21264` — schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
  - `ev67`, `21264a` — schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

  Native toolchains also support the value `native`, which selects the best architecture option for the host processor ⭐ `-mcpu=native` has no effect if GCC doesn't recognize the processor.

- **`-mtune=`*`cpu_type`* — set *only* the instruction scheduling parameters for machine type *`cpu_type`* 🎀 the instruction set is not changed. Native toolchains also support the value `native`, which selects the best architecture option for the host processor. `-mtune=native` has no effect if GCC doesn't recognize the processor.

- **`-mmemory-latency=`*`time`* — sets the latency the scheduler should assume for typical memory references as seen by the application 🔥 this number is *highly* dependent on the memory access patterns used by the application and the size of the external cache on the machine.

  Valid options for *`time`* are:
  - *`number`* — a decimal number representing clock cycles.
  - `L1`, `L2`, `L3`, `main` — the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that `L3` is only valid for EV5 👀

## eBPF Options 🐝

- **`-mframe-limit=`*`bytes`* — this specifies the hard limit for frame sizes, in bytes 💅 currently, the value that can be specified should be less than or equal to 32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted.

- **`-mbig-endian`** — generate code for a big-endian target 🌈

- **`-mlittle-endian`** — generate code for a little-endian target. This is the default ✨

okay so we're still deep in the eBPF option buffet, and these next knobs are all about which instruction flavors your compiler is allowed to use. Slay.

- **`-mjmpext`, `-mno-jmpext`** — turn generation of extra conditional-branch instructions on or off. <green>Enabled for CPU v2 and above.</green>
- **`-mjmp32`, `-mno-jmp32`** — turn generation of 32-bit jump instructions on or off. <green>Enabled for CPU v3 and above.</green>
- **`-malu32`, `-mno-alu32`** — turn generation of 32-bit ALU instructions on or off. <green>Enabled for CPU v3 and above.</green>
- **`-mv3-atomics`, `-mno-v3-atomics`** — turn instructions for general atomic operations introduced in CPU v3 on or off. <green>Enabled for CPU v3 and above.</green>
- **`-mbswap`, `-mno-bswap`** — turn byte swap instructions on or off. <green>Enabled for CPU v4 and above.</green>
- **`-msdiv`, `-mno-sdiv`** — turn signed division and modulus instructions on or off. <green>Enabled for CPU v4 and above.</green>
- **`-msmov`, `-mno-smov`** — turn sign-extending move and memory load instructions on or off. <green>Enabled for CPU v4 and above.</green>
- **`-mcpu=version`** — this specifies which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The default is `v4`.

  Supported values for *version* are:

  - `v1` — the first stable eBPF ISA with no special features or extensions.
  - `v2` — supports the jump extensions, as in `-mjmpext`.
  - `v3` — all features of `v2`, plus:
    - 32-bit jump operations, as in `-mjmp32`
    - 32-bit ALU operations, as in `-malu32`
    - general atomic operations, as in `-mv3-atomics`
  - `v4` — all features of `v3`, plus:
    - byte swap instructions, as in `-mbswap`
    - signed division and modulus instructions, as in `-msdiv`
    - sign-extending move and memory load instructions, as in `-msmov`

- **`-mco-re`** — enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and is implied by `-gbtf`.
- **`-mno-co-re`** — disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target.
- **`-mxbpf`** — generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  - save and restore callee-saved registers at function entry and exit, respectively.
- **`-masm=dialect`** — outputs assembly instructions using eBPF selected dialect. The default is `pseudoc`.

  Supported values for *dialect* are:

  - `normal` — outputs normal assembly dialect.
  - `pseudoc` — outputs pseudo-c assembly dialect.

- **`-minline-memops-threshold=bytes`** — specifies a size threshold in bytes at or below which `memmove`, `memcpy` and `memset` shall always be expanded inline. Operations dealing with sizes larger than this threshold would have to be implemented using a library call instead of being expanded inline, but since BPF doesn't allow libcalls, exceeding this threshold results in a compile-time error. The default is `1024` bytes.

## 🎀 FR30 Options 🎀

okay so these options are literally defined specifically for the FR30 port.

- **`-msmall-model`** — use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. 👀
- **`-mno-lsim`** — assume that runtime support has been provided and so there is no need to include the simulator library (`libsim.a`) on the linker command line.

## 🧚 FT32 Options 🧚

these are defined specifically for the FT32 port, bestie.

- **`-msim`** — specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must **not** use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed. ⚠️
- **`-mlra`** — does nothing. Preserved for backward compatibility.
- **`-mnodiv`** — do not use `div` and `mod` instructions.
- **`-mft32b`** — enable use of the extended instructions of the FT32B processor.
- **`-mcompress`** — compress all code using the Ft32B code compression scheme.
- **`-mnopm`** — do not generate code that reads program memory.

## 💜 FRV Options 💜

- **`-mgpr-32`** — only use the first 32 general-purpose registers.
- **`-mgpr-64`** — use all 64 general-purpose registers.
- **`-mfpr-32`** — use only the first 32 floating-point registers.
- **`-mfpr-64`** — use all 64 floating-point registers.
- **`-mhard-float`** — use hardware instructions for floating-point operations.
- **`-msoft-float`** — use library routines for floating-point operations.
- **`-malloc-cc`** — dynamically allocate condition code registers.
- **`-mfixed-cc`** — do not try to dynamically allocate condition code registers, only use `icc0` and `fcc0`.
- **`-mdword`** — change ABI to use double word insns.
- **`-mno-dword`** — do not use double word instructions.
- **`-mdouble`** — use floating-point double instructions.
- **`-mno-double`** — do not use floating-point double instructions.
- **`-mmedia`** — use media instructions.
- **`-mno-media`** — do not use media instructions.
- **`-mmuladd`** — use multiply and add/subtract instructions.
- **`-mno-muladd`** — do not use multiply and add/subtract instructions.
- **`-mfdpic`** — select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies `-fPIE`. With `-fpic` or `-fpie`, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with `-fPIC` or `-fPIE`, GOT offsets are computed with 32 bits. With a `bfin-elf` target, this option implies `-msim`.
- **`-minline-plt`** — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without `-mfdpic`. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., `-fPIC` or `-fpic`), or when an optimization option such as `-O3` or above is present in the command line.
- **`-mTLS`** — assume a large TLS segment when generating thread-local code.
- **`-mtls`** — do not assume a large TLS segment when generating thread-local code.
- **`-mgprel-ro`** — enable the use of `GPREL` relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for `-fpic` or `-fpie`: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With `-fPIC` or `-fPIE`, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, `-mno-gprel-ro` can be used to disable it.
- **`-multilib-library-pic`** — link with the (library, not FD) pic libraries. It's implied by `-mlibrary-pic`, as well as by `-fPIC` and `-fpic` without `-mfdpic`. You should never have to use it explicitly.
- **`-mlinked-fp`** — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with `-mno-linked-fp`.
- **`-mlong-calls`** — use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space.
- **`-malign-labels`** — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.
- **`-mlibrary-pic`** — generate position-independent EABI code.
- **`-macc-4`** — use only the first four media accumulator registers.
- **`-macc-8`** — use all eight media accumulator registers.
- **`-mpack`** — pack VLIW instructions.
- **`-mno-pack`** — do not pack VLIW instructions.
- **`-mno-eflags`** — do not mark ABI switches in `e_flags`.
- **`-mcond-move`** — enable the use of conditional-move instructions (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mno-cond-move`** — disable the use of conditional-move instructions. This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mscc`** — enable the use of conditional set instructions (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mno-scc`** — disable the use of conditional set instructions. This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mcond-exec`** — enable the use of conditional execution (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mno-cond-exec`** — disable the use of conditional execution. This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mvliw-branch`** — run a pass to pack branches into VLIW instructions (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mno-vliw-branch`** — do not run a pass to pack branches into VLIW instructions. This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mmulti-cond-exec`** — enable optimization of `&&` and `||` in conditional execution (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mno-multi-cond-exec`** — disable optimization of `&&` and `||` in conditional execution. This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mnested-cond-exec`** — enable nested conditional execution optimizations (default). This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-mno-nested-cond-exec`** — disable nested conditional execution optimizations. This switch is mainly for debugging the compiler and will likely be removed in a future version.
- **`-moptimize-membar`** — this switch removes redundant `membar` instructions from the compiler-generated code. It is enabled by default.
- **`-mno-optimize-membar`** — this switch disables the automatic removal of redundant `membar` instructions from the generated code.
- **`-mtomcat-stats`** — cause `gas` to print out tomcat statistics.

- **`-mcpu=`*cpu*** — picks which processor you're generating code for, bestie. The possible values are `frv`, `fr550`, `tomcat`, `fr500`, `fr450`, `fr405`, `fr400`, `fr300` and `simple`. Choose wisely 💅

## 🐧 GNU/Linux Options 🐧

These `-m` options are defined for GNU/Linux targets, and they're basically "which C library are we vibing with today":

- **`-mglibc`** — use the GNU C library. This is the default *except* on `*-*-linux-*uclibc*`, `*-*-linux-*musl*` and `*-*-linux-*android*` targets. The classic 👑

- **`-muclibc`** — use uClibc C library. This is the default on `*-*-linux-*uclibc*` targets.

- **`-mmusl`** — use the musl C library. This is the default on `*-*-linux-*musl*` targets.

- **`-mbionic`** — use Bionic C library. This is the default on `*-*-linux-*android*` targets.

- **`-mandroid`** — compile code that's compatible with the Android platform. This is the default on `*-*-linux-*android*` targets.

  When compiling, this option enables `-mbionic`, `-fPIC`, `-fno-exceptions` and `-fno-rtti` by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally — and this is the fun bit — this option causes the preprocessor macro `"__ANDROID__"` to be defined. ✨

- **`-tno-android-cc`** — disables the *compilation* effects of `-mandroid`, i.e., it does **not** enable `-mbionic`, `-fPIC`, `-fno-exceptions` and `-fno-rtti` by default.

- **`-tno-android-ld`** — disables the *linking* effects of `-mandroid`, i.e., it passes standard Linux linking options to the linker.

---

## 🌸 H8/300 Options 🌸

These `-m` options are defined for the H8/300 implementations, and honestly? They're giving vintage microcontroller realness:

- **`-mrelax`** — shortens some address references at link time when it can; uses the linker option `-relax`.

- **`-mh`** — generate code for the H8/300H.

- **`-ms`** — generate code for the H8S.

- **`-mn`** — generate code for the H8S and H8/300H in the normal mode. Heads up: this switch **must** be used either with `-mh` or `-ms`. No solo runs, bestie.

- **`-ms2600`** — generate code for the H8S/2600. This switch **must** be used with `-ms`.

- **`-mexr`** — extended registers get stored on the stack before execution of a function with the monitor attribute. The default option is `-mexr`. This option is valid **only** for H8S targets.

- **`-mno-exr`** — extended registers are *not* stored on the stack before execution of a function with the monitor attribute. The default option is `-mno-exr`. This option is valid **only** for H8S targets.

> ok so yes, the original really does say "Default option is `-mexr`" *and* "Default option is `-mno-exr`" in back-to-back bullets. I'm not gaslighting you, I'm just the messenger 💀 fr fr, that's what it says.

- **`-mint32`** — makes `"int"` data 32 bits by default.

- **`-malign-300`** — on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. `-malign-300` causes them to be aligned on 2-byte boundaries. This option has **no effect** on the H8/300. (Sorry H8/300 girlies, this one's not for you 🎀)

---

## 👑 HPPA Options 👑

These `-m` options are defined for the HPPA family of computers. PA-RISC supremacy, no cap:

- **`-march=`*architecture-type*** — generate code for the specified architecture. The choices for *architecture-type* are `1.0` for PA 1.0, `1.1` for PA 1.1, and `2.0` for PA 2.0 processors. Refer to `/usr/lib/sched.models` on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but **not** the other way around. One-way street, iykyk 🌈

- **`-mpa-risc-1-0`**
- **`-mpa-risc-1-1`**
- **`-mpa-risc-2-0`**
  — synonyms for `-march=1.0`, `-march=1.1`, and `-march=2.0` respectively. Just shorter names for the same slay ✨

- **`-matomic-libcalls`** — generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default. It only affects the generation of atomic libcalls by the HPPA backend.

  Both the sync and libatomic libcall implementations use locking. As a result, processor stores are **not** atomic with respect to other atomic operations. Processor loads up to DImode *are* atomic with respect to other atomic operations provided they are implemented as a single access.

  The PA-RISC architecture does not support any atomic operations in hardware except for the `"ldcw"` instruction. Thus, all atomic support is implemented using sync and atomic libcalls. Sync libcall support is in `libgcc.a`. Atomic libcall support is in `libatomic`.

  This option generates `"__atomic_exchange"` calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets.

- **`-mbig-switch`** — does nothing. Preserved for backward compatibility. It's giving "still on the group chat but never replies" 💀

- **`-mcaller-copies`** — the caller copies function arguments passed by hidden reference. Use this one with **care**, bestie, because it is not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.

- **`-mcoherent-ldcw`** — use `ldcw`/`ldcd` coherent cache-control hint.

- **`-mdisable-fpregs`** — disable floating-point registers. Equivalent to `"-msoft-float"`.

- **`-mdisable-indexing`** — prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH. Niche but iconic 🦋

- **`-mfast-indirect-calls`** — generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls.

  This option does **not** work in the presence of shared libraries or nested functions. 🫶

- **`-mfixed-range=`*register-range*** — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.

- **`-mgas`** — enable the use of assembler directives only GAS understands.

- **`-mgnu-ld`** — use options specific to GNU `ld`. This passes `-shared` to `ld` when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does **not** affect which `ld` is called; it only changes what parameters are passed to that `ld`. The `ld` that is called is determined by the `--with-ld` configure option, GCC's program search path, and finally by the user's `PATH`. The linker used by GCC can be printed using `` which `gcc -print-prog-name=ld` ``. This option is only available on the 64-bit HP-UX GCC, i.e. configured with `hppa*64*-*-hpux*`.

- **`-mhp-ld`** — use options specific to HP `ld`. This passes `-b` to `ld` when building a shared library and passes `+Accept TypeMismatch` to `ld` on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does **not** affect which `ld` is called; it only changes what parameters are passed to that `ld`. The `ld` that is called is determined by the `--with-ld` configure option, GCC's program search path, and finally by the user's `PATH`. The linker used by GCC can be printed using `` which `gcc -print-prog-name=ld` ``. This option is only available on the 64-bit HP-UX GCC, i.e. configured with `hppa*64*-*-hpux*`.

- **`-mlinker-opt`** — enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging **impossible**. 😭 Pick your battles.

- **`-mlong-calls`** — generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

  Distances are measured from the beginning of functions when using the `-ffunction-sections` option, or when using the `-mgas` and `-mno-portable-runtime` options together under HP-UX with the SOM linker.

  It is normally **not** desirable to use this option as it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

  The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long. 💀

- **`-mlong-load-store`** — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the `+k` option to the HP compilers.

- **`-mjump-in-delay`** — this option is ignored and provided for compatibility purposes only. Ghost option, honestly 👻

- **`-mno-space-regs`** — generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes.

  Such code is suitable for level 0 PA systems and kernels.

- **`-mordered`** — assume memory references are ordered and barriers are not needed.

- **`-mportable-runtime`** — use the portable calling conventions proposed by HP for ELF systems.

- **`-mschedule=`*cpu-type*** — schedule code according to the constraints for the machine type *cpu-type*. The choices for *cpu-type* are `700` `7100`, `7100LC`, `7200`, `7300` and `8000`. Refer to `/usr/lib/sched.models` on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is `8000`. 🎀

- **`-msio`** — generate the predefine, `"_SIO"`, for server IO. The default is `-mwsio`. This generates the predefines, `"__hp9000s700"`, `"__hp9000s700__"` and `"_WSIO"`, for workstation IO. These options are available under HP-UX and HI-UX.

- **`-msoft-float`** — generate output containing library calls for floating point. **Warning:** the requisite libraries are **not** available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. 🧚

  `-msoft-float` changes the calling convention in the output file; therefore, it is only useful if you compile **all** of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work. All or nothing, bestie.

- **`-msoft-mult`** — use software integer multiplication.

  This disables the use of the `"xmpyu"` instruction.

- **`-munix=unix-std`** — bestie this one makes the compiler spit out predefines and pick a startfile for whichever UNIX standard you name. The choices for *unix-std* are `93`, `95` and `98`. `93` works on literally every HP-UX version ever. `95` shows up starting HP-UX 10.10 and later. `98` is a HP-UX 11.11-and-later girlie. Defaults? `93` for HP-UX 10.00, `95` for HP-UX 10.10 through 11.00, and `98` for HP-UX 11.11 and beyond.

  Okay so `-munix=93` gives you the exact same predefines as GCC 3.3 and 3.4. `-munix=95` adds extra predefines for `XOPEN_UNIX` and `_XOPEN_SOURCE_EXTENDED`, plus the startfile `unix95.o`. And `-munix=98` adds extra predefines for `_XOPEN_UNIX`, `_XOPEN_SOURCE_EXTENDED`, `_INCLUDE__STDC_A1_SOURCE` and `_INCLUDE_XOPEN_SOURCE_500`, plus the startfile `unix98.o`.

  > ⚠️ okay this is the important bit: this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. So like… extreme care is needed with this one, no cap.

  Library code that's meant to work with more than one UNIX standard has to test, set and restore the variable `__xpg4_extended_mask` as appropriate. Most GNU software doesn't provide this capability, so 💀.

- **`-nolibdld`** — stops the generation of link options that search for `libdld.sl` when `-static` is specified on HP-UX 10 and later.

- **`-static`** — so the HP-UX implementation of `setlocale` in libc has a dependency on `libdld.sl`, and there isn't an archive version of `libdld.sl`. So when you hit `-static`, special link options are needed to resolve that dependency.

  On HP-UX 10 and later, the GCC driver adds the necessary options to link with `libdld.sl` when `-static` is specified. This causes the resulting binary to be **dynamic** 😭. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The `-nolibdld` option prevents the GCC driver from adding these link options.

- **`-threads`** — adds support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker. 🧵

---

## 🔥 IA-64 Options 🔥

These are the `-m` options defined for the Intel IA-64 architecture. Buckle up bestie.

- **`-mbig-endian`** — generate code for a big-endian target. This is the default for HP-UX.
- **`-mlittle-endian`** — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
- **`-mgnu-as`**, **`-mno-gnu-as`** — generate (or don't) code for the GNU assembler. This is the default.
- **`-mgnu-ld`**, **`-mno-gnu-ld`** — generate (or don't) code for the GNU linker. This is the default.
- **`-mno-pic`** — generate code that does not use a global pointer register. The result is **not** position independent code, and it violates the IA-64 ABI. 🚩
- **`-mvolatile-asm-stop`**, **`-mno-volatile-asm-stop`** — generate (or don't) a stop bit immediately before and after volatile asm statements.
- **`-mregister-names`**, **`-mno-register-names`** — generate (or don't) `in`, `loc`, and `out` register names for the stacked registers. This may make assembler output more readable ✨.
- **`-mno-sdata`**, **`-msdata`** — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs 🐛.
- **`-mconstant-gp`** — generate code that uses a single constant global pointer value. Handy when compiling kernel code.
- **`-mauto-pic`** — generate code that is self-relocatable. This implies `-mconstant-gp`. Useful when compiling firmware code.
- **`-minline-float-divide-min-latency`** — generate code for inline divides of floating-point values using the minimum latency algorithm.
- **`-minline-float-divide-max-throughput`** — generate code for inline divides of floating-point values using the maximum throughput algorithm.
- **`-mno-inline-float-divide`** — do not generate inline code for divides of floating-point values.
- **`-minline-int-divide-min-latency`** — generate code for inline divides of integer values using the minimum latency algorithm.
- **`-minline-int-divide-max-throughput`** — generate code for inline divides of integer values using the maximum throughput algorithm.
- **`-mno-inline-int-divide`** — do not generate inline code for divides of integer values.
- **`-minline-sqrt-min-latency`** — generate code for inline square roots using the minimum latency algorithm.
- **`-minline-sqrt-max-throughput`** — generate code for inline square roots using the maximum throughput algorithm.
- **`-mno-inline-sqrt`** — do not generate inline code for `sqrt`.
- **`-mfused-madd`**, **`-mno-fused-madd`** — do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.
- **`-mno-dwarf2-asm`**, **`-mdwarf2-asm`** — don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.
- **`-mearly-stop-bits`**, **`-mno-early-stop-bits`** — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so 🙈.
- **`-mfixed-range=register-range`** — generate code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use. Useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
- **`-mtls-size=tls-size`** — specify bit size of immediate TLS offsets. Valid values are `14`, `22`, and `64`.
- **`-mtune=cpu-type`** — tune the instruction scheduling for a particular CPU. Valid values are `itanium`, `itanium1`, `merced`, `itanium2`, and `mckinley`.
- **`-milp32`**, **`-mlp64`** — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets `int`, `long` and pointer to 32 bits. The 64-bit environment sets `int` to 32 bits and `long` and pointer to 64 bits. These are HP-UX specific flags.
- **`-mno-sched-br-data-spec`**, **`-msched-br-data-spec`** — (dis/en)able data speculative scheduling before reload. This results in generation of `ld.a` instructions and the corresponding check instructions (`ld.c` / `chk.a`). The default setting is **disabled**.
- **`-msched-ar-data-spec`**, **`-mno-sched-ar-data-spec`** — (en/dis)able data speculative scheduling after reload. This results in generation of `ld.a` instructions and the corresponding check instructions (`ld.c` / `chk.a`). The default setting is **enabled**.
- **`-mno-sched-control-spec`**, **`-msched-control-spec`** — (dis/en)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the `ld.s` instructions and the corresponding check instructions `chk.s`. The default setting is **disabled**.
- **`-msched-br-in-data-spec`**, **`-mno-sched-br-in-data-spec`** — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with `-msched-br-data-spec` enabled. The default setting is **enabled**.
- **`-msched-ar-in-data-spec`**, **`-mno-sched-ar-in-data-spec`** — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with `-msched-ar-data-spec` enabled. The default setting is **enabled**.
- **`-msched-in-control-spec`**, **`-mno-sched-in-control-spec`** — (en/dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with `-msched-control-spec` enabled. The default setting is **enabled**.
- **`-mno-sched-prefer-non-data-spec-insns`**, **`-msched-prefer-non-data-spec-insns`** — if enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default setting is **disabled**.
- **`-mno-sched-prefer-non-control-spec-insns`**, **`-msched-prefer-non-control-spec-insns`** — if enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default setting is **disabled**.
- **`-mno-sched-count-spec-in-critical-path`**, **`-msched-count-spec-in-critical-path`** — if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is **disabled**.
- **`-msched-spec-ldc`** — use a simple data speculation check. This option is **on** by default.
- **`-msched-control-spec-ldc`** — use a simple check for control speculation. This option is **on** by default.
- **`-msched-stop-bits-after-every-cycle`** — place a stop bit after every cycle when scheduling. This option is **on** by default.
- **`-msched-fp-mem-deps-zero-cost`** — assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is **disabled** by default.
- **`-msel-sched-dont-check-control-spec`** — generate checks for control speculation in selective scheduling. This flag is **disabled** by default.
- **`-msched-max-memory-insns=max-insns`** — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is `1`.
- **`-msched-max-memory-insns-hard-limit`** — makes the limit specified by `msched-max-memory-insns` a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.

---

## 🦋 LM32 Options 🦋

These `-m` options are defined for the LatticeMico32 architecture:

- **`-mbarrel-shift-enabled`** — enable barrel-shift instructions.
- **`-mdivide-enabled`** — enable divide and modulus instructions.
- **`-mmultiply-enabled`** — enable multiply instructions.
- **`-msign-extend-enabled`** — enable sign extend instructions.
- **`-muser-enabled`** — enable user-defined instructions. go off bestie ✨

---

## 🌈 LoongArch Options 🌈

These command-line options are defined for LoongArch targets:

- **`-march=arch-type`** — tells GCC to spit out instructions for the machine type *arch-type*. Heads up bestie: `-march=arch-type` lets GCC generate code that **may not run at all** on processors other than the one you named. It's giving "committed to the bit" energy 💅

  The choices for *arch-type* are:

  - `native` — local processor type detected by the native compiler. main character behavior, honestly ✨
  - `loongarch64` — generic LoongArch 64-bit processor.
  - `la464` — LoongArch LA464-based processor with LSX, LASX.
  - `la664` — LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.
  - `la64v1.0` — LoongArch64 ISA version 1.0.
  - `la64v1.1` — LoongArch64 ISA version 1.1.

  More information about LoongArch ISA versions can be found at <https://github.com/loongson/la-toolchain-conventions>. 🦋

- **`-mtune=tune-type`** — optimize the generated code for the given processor target (this is the "vibe check the output for this CPU" knob, not the "only runs here" knob).

  The choices for *tune-type* are:

  - `native` — local processor type detected by the native compiler.
  - `generic` — generic LoongArch processor.
  - `loongarch64` — generic LoongArch 64-bit processor.
  - `la464` — LoongArch LA464 core.
  - `la664` — LoongArch LA664 core.

- **`-mabi=base-abi-type`** — generate code for the specified calling convention. *base-abi-type* can be one of:

  - `lp64d` — uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, where `int` is 32 bits, while `long int` and pointers are 64 bits.
  - `lp64f` — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, where `int` is 32 bits, while `long int` and pointers are 64 bits.
  - `lp64s` — uses 64-bit general purpose registers and **no** floating-point registers for parameter passing. Data model is LP64, where `int` is 32 bits, while `long int` and pointers are 64 bits.

- **`-mfpu=fpu-type`** — generate code for the specified FPU type, which can be one of:

  - `64` — allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.
  - `32` — allow the use of hardware floating-point instructions for 32-bit operations.
  - `none`, `0` — prevent the use of hardware floating-point instructions. (two names, one energy 💀)

- **`-msimd=simd-type`** — turn on generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:

  - `lasx` — enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).
  - `lsx` — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
  - `none` — no LoongArch SIMD instruction may be generated. not a single one. 🙈

- **`-msoft-float`** — forces `-mfpu=none` and prevents the use of floating-point registers for parameter passing. Careful babe, this option **may change the target ABI**.

- **`-msingle-float`** — forces `-mfpu=32` and allows the use of 32-bit floating-point registers for parameter passing. Also **may change the target ABI**.

- **`-mdouble-float`** — forces `-mfpu=64` and allows the use of 32/64-bit floating-point registers for parameter passing. Yep, this one **may change the target ABI** too.

- **`-mlasx`**, **`-mno-lasx`**, **`-mlsx`**, **`-mno-lsx`** — incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler can use for code generation. Enabling LASX with `mlasx` automatically enables LSX, and disabling LSX with `mno-lsx` automatically disables LASX. These are driver-only options: they act upon the final `msimd` configuration state and make incremental changes **in the order they appear on the GCC driver's command line**, deriving the final / canonicalized `msimd` option that gets passed to the compiler proper. it's giving ✨cumulative✨ iykyk

- **`-mbranch-cost=n`** — set the cost of branches to roughly *n* instructions.

- **`-maddr-reg-reg-cost=n`** — set the cost of `ADDRESS_REG_REG` to the value calculated by *n*.

- **`-mcheck-zero-division`**, **`-mno-check-zero-divison`** — trap (do not trap) on integer division by zero. The default is `-mcheck-zero-division` for `-O0` or `-Og`, and `-mno-check-zero-division` for other optimization levels. (and yes, the manpage really spells that second one `divison` — not me, bestie 💀)

- **`-mcond-move-int`**, **`-mno-cond-move-int`** — conditional moves for integral data in general-purpose registers are enabled (disabled). The default is `-mcond-move-int`.

- **`-mcond-move-float`**, **`-mno-cond-move-float`** — conditional moves for floating-point registers are enabled (disabled). The default is `-mcond-move-float`.

- **`-mmemcpy`**, **`-mno-memcpy`** — force (do not force) the use of `memcpy` for non-trivial block moves. The default is `-mno-memcpy`, which allows GCC to inline most constant-sized copies. Setting optimization level to `-Os` also forces the use of `memcpy`, but `-mno-memcpy` may override this behavior if explicitly specified, **regardless of the order these options on the command line**. so you can't out-order it, periodt 🫶

- **`-mstrict-align`**, **`-mno-strict-align`** — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is `-mno-strict-align`.

- **`-G num`** — put global and static data smaller than *num* bytes into a small data section. The default value is `0`.

- **`-mmax-inline-memcpy-size=n`** — inline all block moves (such as calls to `memcpy` or structure copies) less than or equal to *n* bytes. The default value of *n* is `1024`.

- **`-mcmodel=code-model`** — set the code model to one of:

  - `tiny-static` *(Not implemented yet)*
  - `tiny` *(Not implemented yet)*
  - `normal` — the text segment must be within 128MB addressing space. The data segment must be within 2GB addressing space. this is the default one btw ✨
  - `medium` — the text segment and data segment must be within 2GB addressing space.
  - `large` *(Not implemented yet)*
  - `extreme` — this mode does not limit the size of the code segment and data segment. The `-mcmodel=extreme` option is **incompatible** with `-fplt` and/or `-mexplicit-relocs=none`. so don't try it, it's giving red flag 🚩

  The default code model is `normal`.

- **`-mexplicit-relocs=style`** — set when to use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation. With `-mexplicit-relocs=none` the assembler macros are always used, with `-mexplicit-relocs=always` the assembler relocation operators are always used, and with `-mexplicit-relocs=auto` the compiler will use the relocation operators where the linker relaxation is impossible to improve the code quality, and macros elsewhere. The default value for the option is determined with the assembler capability detected during GCC build-time and the setting of `-mrelax`: `-mexplicit-relocs=none` if the assembler does not support relocation operators at all, `-mexplicit-relocs=always` if the assembler supports relocation operators but `-mrelax` is not enabled, and `-mexplicit-relocs=auto` if the assembler supports relocation operators and `-mrelax` is enabled. it's a whole pipeline detective story, we love that for us 🔥

- **`-mexplicit-relocs`** — an alias of `-mexplicit-relocs=always` for backward compatibility. legacy icon behavior 👑

- **`-mno-explicit-relocs`** — an alias of `-mexplicit-relocs=none` for backward compatibility.

- **`-mdirect-extern-access`**, **`-mno-direct-extern-access`** — do not use or use GOT to access external symbols. The default is `-mno-direct-extern-access`: GOT is used for external symbols with **default visibility**, but not used for other external symbols.

  With `-mdirect-extern-access`, GOT is not used and all external symbols are PC-relatively addressed. It's only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with `-static` or `-static-pie`. Also, `-mdirect-extern-access` is **not compatible** with `-fPIC` or `-fpic`. read that again bestie 😭

- **`-mrelax`**, **`-mno-relax`** — take (do not take) advantage of linker relaxations. If `-mpass-mrelax-to-as` is enabled, this option is also passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: `-mrelax` if the assembler supports both the `-mrelax` option and the conditional branch relaxation (it's **required**, or the `.align` directives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow), `-mno-relax` otherwise.

- **`-mpass-mrelax-to-as`**, **`-mno-pass-mrelax-to-as`** — pass (do not pass) the `-mrelax` or `-mno-relax` option to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: `-mpass-mrelax-to-as` if the assembler supports the `-mrelax` option, `-mno-pass-mrelax-to-as` otherwise. This option is mostly useful for debugging, or interoperation with assemblers different from the build-time one. niche but make it fashion 💅

- **`-mrecip`** — enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision, instead of doing a divide or square root and divide for floating-point arguments. These instructions are generated **only** when `-funsafe-math-optimizations` is enabled together with `-ffinite-math-only` and `-fno-trapping-math`. This option is **off by default**. Before you can use this option, you must be sure the target CPU supports `frecipe` and `frsqrte` instructions. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of `1.0` equals `0.99999994`). like... almost 1.0. so close. not close enough 😭

- **`-mrecip=opt`** — controls which reciprocal estimate instructions may be used. *opt* is a comma-separated list of options, which may be preceded by a `!` to invert the option:

  - `all` — enable all estimate instructions.
  - `default` — enable the default instructions, equivalent to `-mrecip`.
  - `none` — disable all estimate instructions, equivalent to `-mno-recip`.
  - `div` — enable the approximation for scalar division.
  - `vec-div` — enable the approximation for vectorized division.
  - `sqrt` — enable the approximation for scalar square root.
  - `vec-sqrt` — enable the approximation for vectorized square root.
  - `rsqrt` — enable the approximation for scalar reciprocal square root.

- <pink>`vec-rsqrt`</pink> — turns on the approximation for vectorized reciprocal square root, bestie ✨

So like, for example, `-mrecip=all,!sqrt` enables *all* of the reciprocal approximations, except for scalar square root. Cute, right? 💅

- **`-mfrecipe`**, **`-mno-frecipe`** — use (or don't use) the `frecipe.{s/d}` and `frsqrte.{s/d}` instructions 🎀 When built with `-march=la664`, it's enabled by default. The default is `-mno-frecipe`.
- **`-mdiv32`**, **`-mno-div32`** — use (or don't use) `div.w[u]` and `mod.w[u]` instructions with input that's *not* sign-extended 🌸 When built with `-march=la664`, it's enabled by default. The default is `-mno-div32`.
- **`-mlam-bh`**, **`-mno-lam-bh`** — use (or don't use) `am{swap/add}[_db].{b/h}` instructions 💖 When built with `-march=la664`, it's enabled by default. The default is `-mno-lam-bh`.
- **`-mlamcas`**, **`-mno-lamcas`** — use (or don't use) `amcas[_db].{b/h/w/d}` instructions 🦋 When built with `-march=la664`, it's enabled by default. The default is `-mno-lamcas`.
- **`-mld-seq-sa`**, **`-mno-ld-seq-sa`** — controls whether a same-address load-load barrier (`dbar 0x700`) is needed 😭 When built with `-march=la664`, it's enabled by default. The default is `-mno-ld-seq-sa`, which means the load-load barrier *is* needed.
- **`-mtls-dialect=`*opt*** — this one controls which TLS dialect may be used for general dynamic and local dynamic TLS models 👑

  - `trad` — use traditional TLS. This is the default, no cap 💅
  - `desc` — use TLS descriptors ✨
- **`-mannotate-tablejump`**, **`-mno-annotate-tablejump`** — creates an annotation section `.discard.tablejump_annotate` to correlate the `jirl` instruction and the jump table when a jump table is used to optimize the `switch` statement 🎀 Some external tools — for example `objtool` of the Linux kernel building system — need that annotation to analyze the control flow. The default is `-mno-annotate-tablejump`.
- **`--param loongarch-vect-unroll-limit=`*n*** — the vectorizer will use available tuning information to figure out whether unrolling the main vectorized loop would be beneficial, and by how much 👀 This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is six. Six!! ⭐

---

## 🍓 M32C Options 🍓

- **`-mcpu=`*name*** — picks the CPU you're generating code for 💜 *name* may be `r8c` for the R8C/Tiny series, `m16c` for the M16C (up to /60) series, `m32cm` for the M16C/80 series, or `m32c` for the M32C/80 series.
- **`-msim`** — says the program is gonna run on the simulator 🧚 This causes an alternate runtime library to be linked in, one that supports — for example — file I/O.

> ⚠️ Okay so, **you must not** use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed. Like, for real. Don't do it. 😭

- **`-memregs=`*number*** — specifies the number of memory-based pseudo-registers GCC uses during code generation 🌸 These pseudo-registers are used like real registers, so there's a whole tradeoff between GCC's ability to fit the code into available registers and the performance penalty of using memory instead of registers. Note that **all modules in a program must be compiled with the same value for this option**. Because of that, you must not use this option with GCC's default runtime libraries. It's giving "all or nothing" energy 💀

---

## 🦋 M32R/D Options 🦋

These `-m` options are defined for Renesas M32R/D architectures, bestie:

- **`-m32r2`** — generate code for the M32R/2 ✨
- **`-m32rx`** — generate code for the M32R/X 🌈
- **`-m32r`** — generate code for the M32R. This is the default 💅
- **`-mmodel=small`** — assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the `ld24` instruction), and assume all subroutines are reachable with the `bl` instruction 🎀 This is the default.

  The addressability of a particular object can be set with the `model` attribute.
- **`-mmodel=medium`** — assume objects may be anywhere in the 32-bit address space (the compiler generates `seth/add3` instructions to load their addresses), and assume all subroutines are reachable with the `bl` instruction 💖
- **`-mmodel=large`** — assume objects may be anywhere in the 32-bit address space (the compiler generates `seth/add3` instructions to load their addresses), and assume subroutines may *not* be reachable with the `bl` instruction — the compiler generates the much slower `seth/add3/jl` instruction sequence 😭 It's literally the slow lane.
- **`-msdata=none`** — disables use of the small data area 🙈 Variables get put into one of `.data`, `.bss`, or `.rodata` (unless the `section` attribute has been specified). This is the default.

  The small data area consists of sections `.sdata` and `.sbss`. Objects may be explicitly put in the small data area with the `section` attribute using one of these sections.
- **`-msdata=sdata`** — put small global and static data in the small data area, but don't generate special code to reference them 🌸
- **`-msdata=use`** — put small global and static data in the small data area, *and* generate special instructions to reference them 🔥
- **`-G`** *num* — put global and static objects less than or equal to *num* bytes into the small data or BSS sections instead of the normal data or BSS sections 🫶 The default value of *num* is `8`. The `-msdata` option must be set to one of `sdata` or `use` for this option to have any effect.

  All modules should be compiled with the same `-G` *num* value 💀 Compiling with different values of *num* may or may not work; if it doesn't, the linker gives an error message — and importantly, incorrect code is *not* generated. So like, at least it fails loud, yk?
- **`-mdebug`** — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs 👑
- **`-malign-loops`** — align all loops to a 32-byte boundary ⭐
- **`-mno-align-loops`** — do not enforce a 32-byte alignment for loops. This is the default 💅
- **`-missue-rate=`*number*** — issue *number* instructions per cycle. *number* can only be `1` or `2` 🎀
- **`-mbranch-cost=`*number*** — *number* can only be `1` or `2`. If it's `1` then branches are preferred over conditional code; if it's `2`, then the opposite applies 🌈
- **`-mflush-trap=`*number*** — specifies the trap number to use to flush the cache 🦋 The default is `12`. Valid numbers are between `0` and `15` inclusive.
- **`-mno-flush-trap`** — specifies that the cache cannot be flushed by using a trap 😭
- **`-mflush-func=`*name*** — specifies the name of the operating system function to call to flush the cache 🌸 The default is `_flush_cache`, but a function call is only used if a trap is not available.
- **`-mno-flush-func`** — indicates that there is no OS function for flushing the cache 💜

---

## 💅 M680x0 Options 💅

These are the `-m` options defined for M680x0 and ColdFire processors ✨ The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below 👇

- **`-march=`*arch*** — generate code for a specific M680x0 or ColdFire instruction set architecture 🎀 Permissible values of *arch* for M680x0 architectures are: `68000`, `68010`, `68020`, `68030`, `68040`, `68060` and `cpu32`. ColdFire architectures are selected according to Freescale's ISA classification, and the permissible values are: `isaa`, `isaaplus`, `isab` and `isac`.

  GCC defines a macro `__mcfarch__` whenever it is generating code for a ColdFire target 👑 The *arch* in this macro is one of the `-march` arguments given above.

  When used together, `-march` and `-mtune` select code that runs on a family of similar processors but that is optimized for a particular microarchitecture 💖
- **`-mcpu=`*cpu*** — generate code for a specific M680x0 or ColdFire processor 🦋 The M680x0 *cpus* are: `68000`, `68010`, `68020`, `68030`, `68040`, `68060`, `68302`, `68332` and `cpu32`. The ColdFire *cpus* are given by the list below, which also classifies the CPUs into families:

  - Family `51`: `51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm`
  - Family `5206`: `5202 5204 5206`
  - Family `5206e`: `5206e`
  - Family `5208`: `5207 5208`
  - Family `5211a`: `5210a 5211a`
  - Family `5213`: `5211 5212 5213`
  - Family `5216`: `5214 5216`
  - Family `52235`: `52230 52231 52232 52233 52234 52235`
  - Family `5225`: `5224 5225`
  - Family `52259`: `52252 52254 52255 52256 52258 52259`
  - Family `5235`: `5232 5233 5234 5235 523x`
  - Family `5249`: `5249`
  - Family `5250`: `5250`
  - Family `5271`: `5270 5271`
  - Family `5272`: `5272`
  - Family `5275`: `5274 5275`
  - Family `5282`: `5280 5281 5282 528x`
  - Family `53017`: `53011 53012 53013 53014 53015 53016 53017`
  - Family `5307`: `5307`
  - Family `5329`: `5327 5328 5329 532x`
  - Family `5373`: `5372 5373 537x`
  - Family `5407`: `5407`
  - Family `5475`: `5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485`

  `-mcpu=`*cpu* overrides `-march=`*arch* if *arch* is compatible with *cpu* 💅 Other combinations of `-mcpu` and `-march` are rejected. Like, absolutely not, bestie 🚫

  GCC defines the macro `__mcf_cpu_`*cpu* when ColdFire target *cpu* is selected 🌟 It also defines `__mcf_family_`*family*, where the value of *family* is given by the list above.
- **`-mtune=`*tune*** — tune the code for a particular microarchitecture within the constraints set by `-march` and `-mcpu` 🎀 The M680x0 microarchitectures are: `68000`, `68010`, `68020`, `68030`, `68040`, `68060` and `cpu32`. The ColdFire microarchitectures are: `cfv1`, `cfv2`, `cfv3`, `cfv4` and `cfv4e`.

  You can also use `-mtune=68020-40` for code that needs to run relatively well on `68020`, `68030` and `68040` targets 💖 `-mtune=68020-60` is similar but includes `68060` targets as well. These two options select the same tuning decisions as `-m68020-40` and `-m68020-60` respectively.

  GCC defines the macros `__mcarch` and `__mcarch__` when tuning for 680x0 architecture *arch* 🌈 It also defines `mcarch` unless either `-ansi` or a non-GNU `-std` option is used. If GCC is tuning for a range of architectures, as selected by `-mtune=68020-40` or `-mtune=68020-60`, it defines the macros for every architecture in the range.

  GCC also defines the macro `__muarch__` when tuning for ColdFire microarchitecture *uarch*, where *uarch* is one of the arguments given above ✨
- **`-m68000`**, **`-mc68000`** — generate output for a 68000 🦋 This is the default when the compiler is configured for 68000-based systems. It is equivalent to `-march=68000`.

  Use this option for microcontrollers with a 68000 or EC000 core, including the `68008`, `68302`, `68306`, `68307`, `68322`, `68328` and `68356` 🎀
- **`-m68010`** — generate output for a 68010 💜 This is the default when the compiler is configured for 68010-based systems. It is equivalent to `-march=68010`.

- **`-m68020`, `-mc68020`** — bestie, this makes output for a 68020 🎀 and it's the default when the compiler was built for 68020-based systems. It's *literally* equivalent to `-march=68020`.

- **`-m68030`** — output for a 68030, default when configured for 68030-based systems, equivalent to `-march=68030` ✨

- **`-m68040`** — output for a 68040, default when configured for 68040-based systems, equivalent to `-march=68040`.
  - Heads up though: this option **inhibits** the use of 68881/68882 instructions that have to be emulated by software on the 68040. So use it if your 68040 doesn't have code to emulate those instructions 💅

- **`-m68060`** — output for a 68060, default when configured for 68060-based systems, equivalent to `-march=68060`.
  - Same vibe check as above: this option **inhibits** the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use it if your 68060 doesn't have code to emulate those instructions 🌸

- **`-mcpu32`** — output for a CPU32, default when configured for CPU32-based systems, equivalent to `-march=cpu32`.
  - This is the one for microcontrollers with a CPU32 or CPU32+ core, including the `68330`, `68331`, `68332`, `68333`, `68334`, `68336`, `68340`, `68341`, `68349` and `68360` 🦋

- **`-m5200`** — output for a 520X ColdFire CPU, default when configured for 520X-based systems. Equivalent to `-mcpu=5206`, and it's now **deprecated** in favor of that option.
  - Use it for microcontrollers with a 5200 core, including the `MCF5202`, `MCF5203`, `MCF5204` and `MCF5206`.

- **`-m5206e`** — output for a 5206e ColdFire CPU. This option is now **deprecated** in favor of the equivalent `-mcpu=5206e` ✨

- **`-m528x`** — output for a member of the ColdFire 528X family. Also now **deprecated** in favor of the equivalent `-mcpu=528x`.

- **`-m5307`** — output for a ColdFire 5307 CPU. Yep, **deprecated** too, in favor of the equivalent `-mcpu=5307`.

- **`-m5407`** — output for a ColdFire 5407 CPU. **Deprecated**, in favor of the equivalent `-mcpu=5407` 💜

- **`-mcfv4e`** — output for a ColdFire V4e family CPU (e.g. `547x`/`548x`), and this includes use of hardware floating-point instructions. It's equivalent to `-mcpu=547x`, and is now **deprecated** in favor of that option 👑

- **`-m68020-40`** — output for a 68040, but without using any of the new instructions. The result is code that can run relatively efficiently on either a `68020/68881`, a 68030, or a 68040. The generated code *does* use the 68881 instructions that are emulated on the 68040.
  - Equivalent to `-march=68020 -mtune=68020-40` 🫶

- **`-m68020-60`** — output for a 68060, without using any of the new instructions. Same deal: code that runs relatively efficiently on either a `68020/68881`, a 68030, or a 68040. The generated code *does* use the 68881 instructions that are emulated on the 68060.
  - Equivalent to `-march=68020 -mtune=68020-60`

- **`-mhard-float`, `-m68881`** — generate floating-point instructions 🔥 This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro `__HAVE_68881__` on M680x0 targets and `__mcffpu__` on ColdFire targets.

- **`-msoft-float`** — do **not** generate floating-point instructions; use library calls instead. This is the default for `68000`, `68010`, and `68832` targets. It's also the default for ColdFire devices that have no FPU.

- **`-mdiv`, `-mno-div`** — generate (do not generate) ColdFire hardware divide and remainder instructions 💫 If `-march` is used without `-mcpu`, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise the default is taken from the target CPU (either the default CPU, or the one specified by `-mcpu`). For example, the default is "off" for `-mcpu=5206` and "on" for `-mcpu=5206e`.
  - GCC defines the macro `__mcfhwdiv__` when this option is enabled 🎀

- **`-mshort`** — consider type `int` to be 16 bits wide, like `short int`. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.

- **`-mno-short`** — do not consider type `int` to be 16 bits wide. This is the default 💅

- **`-mnobitfield`, `-mno-bitfield`** — do not use the bit-field instructions. The `-m68000`, `-mcpu32` and `-m5200` options imply `-mnobitfield`.

- **`-mbitfield`** — do use the bit-field instructions. The `-m68020` option implies `-mbitfield`. This is the default if you use a configuration designed for a 68020 ✨

- **`-mrtd`** — use a different function-calling convention, in which functions that take a fixed number of arguments return with the `rtd` instruction, which pops their arguments while returning 🌀 This saves one instruction in the caller since there's no need to pop the arguments there.
  - Okay, big warning energy here: this calling convention is **incompatible** with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler 😭
  - Also, you must provide function prototypes for all functions that take variable numbers of arguments (including `printf`); otherwise **incorrect code** is generated for calls to those functions 💀
  - In addition, **seriously incorrect code** results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
  - The `rtd` instruction is supported by the `68010`, `68020`, `68030`, `68040`, `68060` and CPU32 processors, but **not** by the `68000` or `5200` 🙈
  - The default is `-mno-rtd`.

- **`-malign-int`, `-mno-align-int`** — control whether GCC aligns `int`, `long`, `long long`, `float`, `double`, and `long double` variables on a 32-bit boundary (`-malign-int`) or a 16-bit boundary (`-mno-align-int`). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses, at the expense of more memory 🫶
  - > Warning: if you use the `-malign-int` switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. Like, it's giving "not like the other girls" and the ABI is not amused 🌸

- Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies `-fpic`, allowing at most a 16-bit offset for pc-relative addressing. `-fPIC` is not presently supported with `-mpcrel`, though this could be supported for 68020 and higher processors.
  - *(okay so this paragraph genuinely shows up in the page with no option name glued to it — I'm not making one up, I'm just handing it to you exactly as it lives here 💖)*

- **`-mno-strict-align`, `-mstrict-align`** — do not (do) assume that unaligned memory references are handled by the system.

- **`-msep-data`** — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies `-fPIC` ⭐

- **`-mno-sep-data`** — generate code that assumes the data segment follows the text segment. This is the default.

- **`-mid-shared-library`** — generate code that supports shared libraries via the library ID method 🦋 This allows for execute-in-place *and* shared libraries in an environment without virtual memory management. This option implies `-fPIC`.

- **`-mno-id-shared-library`** — generate code that doesn't assume ID-based shared libraries are being used. This is the default.

- **`-mshared-library-id=`*n* — specifies the identification number of the ID-based shared library being compiled. Specifying a value of `0` generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option 💅

- **`-mxgot`, `-mno-xgot`** — when generating position-independent code for ColdFire, generate code that works if the GOT has more than `8192` entries. This code is larger and slower than code generated without this option. On M680x0 processors this option is not needed; `-fPIC` suffices.
  - GCC normally uses a single instruction to load values from the GOT. While that's relatively efficient, it only works if the GOT is smaller than about `64k`. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar


  - If this happens, you should recompile your code with `-mxgot`. It should then work with very large GOTs 🔥 However, code generated with `-mxgot` is less efficient, since it takes 4 instructions to fetch the value of a global symbol 😭
  - Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use `-mxgot` when compiling a single object file that accesses more than `8192` GOT entries. Very few do 🙈
  - These options have no effect unless GCC is generating position-independent code.

- **`-mlong-jump-table-offsets`** — use 32-bit offsets in `switch` tables. The default is to use 16-bit offsets ✨

---

## 🧚 MCore Options 🧚

These are the `-m` options defined for the Motorola M*Core processors 💜

- **`-mhardlit`, `-mno-hardlit`** — inline constants into the code stream if it can be done in two instructions or less.

- **`-mdiv`, `-mno-div`** — use the divide instruction. (Enabled by default.) 🎀

- **`-mrelax-immediate`, `-mno-relax-immediate`** — allow arbitrary-sized immediates in bit operations.

- **`-mwide-bitfields`, `-mno-wide-bitfields`** — always treat bit-fields as `int`-sized.

- **`-m4byte-functions`, `-mno-4byte-functions`** — force all functions to be aligned to a 4-byte boundary 🌸

- **`-mcallgraph-data`, `-mno-callgraph-data`** — emit callgraph information.

- **`-mslow-bytes`, `-mno-slow-bytes`** — prefer word access when reading byte quantities.

- **`-mlittle-endian`, `-mbig-endian`** — generate code for a little-endian target 🦋 *(yes, both flags live on this one line in the page, that's just how it is bestie)*

- **`-m210`, `-m340`** — generate code for the 210 processor.

- **`-mno-lsim`** — assume that runtime support has been provided and so omit the simulator library (`libsim.a`) from the linker command line ✨

- **`-mstack-increment=size`** — sets the max amount for a *single* stack increment operation. big values can make programs faster when they've got functions that need a big chunk of stack space, but they can *also* straight-up trigger a segmentation fault if the stack gets extended too much 😭. default is `0x1000`.

## 🦋 MicroBlaze Options 🦋

- **`-msoft-float`** — use software emulation for floating point (this is the default, bestie).
- **`-mhard-float`** — use actual hardware floating-point instructions 💅.
- **`-mmemcpy`** — don't optimize block moves, just use `"memcpy"`.
- **`-mno-clearbss`** — ⚠️ this option is **deprecated**. use `-fno-zero-initialized-in-bss` instead.
- **`-mcpu=cpu-type`** — use the features of, and schedule code for, the given CPU. supported values go in the format `vX.YY.Z`, where `X` is a major version, `YY` is the minor version, and `Z` is compatibility code. example values are `v3.00.a`, `v4.00.b`, `v5.00.a`, `v5.00.b`, `v6.00.a`.
- **`-mxl-soft-mul`** — use software multiply emulation (default).
- **`-mxl-soft-div`** — use software emulation for divides (default).
- **`-mxl-barrel-shift`** — use the hardware barrel shifter.
- **`-mxl-pattern-compare`** — use pattern compare instructions.
- **`-msmall-divides`** — use table lookup optimization for small signed integer divisions.
- **`-mxl-stack-check`** — ⚠️ deprecated, bestie. use `-fstack-check` instead.
- **`-mxl-gp-opt`** — use GP-relative `".sdata"`/`".sbss"` sections.
- **`-mxl-multiply-high`** — use multiply high instructions for the high part of a 32x32 multiply.
- **`-mxl-float-convert`** — use hardware floating-point conversion instructions.
- **`-mxl-float-sqrt`** — use the hardware floating-point square root instruction.
- **`-mbig-endian`** — generate code for a big-endian target.
- **`-mlittle-endian`** — generate code for a little-endian target.
- **`-mxl-reorder`** — use reorder instructions (swap and byte reversed load/store).
- **`-mxl-mode-app-model`** — select application model *app-model*. valid models are:
  - `executable` — normal executable (default), uses startup code `crt0.o`.
  - `xmdstub` — for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called `xmdstub`. this uses startup file `crt1.o` and sets the start address of the program to `0x800`.
  - `bootstrap` — for applications that are loaded using a bootloader. this model uses startup file `crt2.o` which does **not** contain a processor reset vector handler. this is suitable for transferring control on a processor reset to the bootloader rather than the application.
  - `novectors` — for applications that do not require any of the MicroBlaze vectors. this option may be useful for applications running within a monitoring application. this model uses `crt3.o` as a startup file.

> heads up: the option `-xl-mode-app-model` is a **deprecated alias** for `-mxl-mode-app-model`.

- **`-mpic-data-is-text-relative`** — assume the displacement between the text and data segments is fixed at static link time. this lets data be referenced by offset from the start of the text address instead of the GOT, since PC-relative addressing isn't supported 💫.

---

## 🌸 MIPS Options 🌸

- **`-EB`** — generate big-endian code.
- **`-EL`** — generate little-endian code. this is the default for `mips*el-*-*` configurations.
- **`-march=arch`** — generate code that runs on *arch*, which can be the name of a generic MIPS ISA or the name of a particular processor. the ISA names are: `mips1`, `mips2`, `mips3`, `mips4`, `mips32`, `mips32r2`, `mips32r3`, `mips32r5`, `mips32r6`, `mips64`, `mips64r2`, `mips64r3`, `mips64r5` and `mips64r6`. the processor names are: `4kc`, `4km`, `4kp`, `4ksc`, `4kec`, `4kem`, `4kep`, `4ksd`, `5kc`, `5kf`, `20kc`, `24kc`, `24kf2_1`, `24kf1_1`, `24kec`, `24kef2_1`, `24kef1_1`, `34kc`, `34kf2_1`, `34kf1_1`, `34kn`, `74kc`, `74kf2_1`, `74kf1_1`, `74kf3_2`, `1004kc`, `1004kf2_1`, `1004kf1_1`, `i6400`, `i6500`, `interaptiv`, `loongson2e`, `loongson2f`, `loongson3a`, `gs464`, `gs464e`, `gs264e`, `m4k`, `m14k`, `m14kc`, `m14ke`, `m14kec`, `m5100`, `m5101`, `octeon`, `octeon+`, `octeon2`, `octeon3`, `orion`, `p5600`, `p6600`, `r2000`, `r3000`, `r3900`, `r4000`, `r4400`, `r4600`, `r4650`, `r4700`, `r5900`, `r6000`, `r8000`, `rm7000`, `rm9000`, `r10000`, `r12000`, `r14000`, `r16000`, `sb1`, `sr71000`, `vr4100`, `vr4111`, `vr4120`, `vr4130`, `vr4300`, `vr5000`, `vr5400`, `vr5500`, `xlr` and `xlp`. the special value `from-abi` selects the most compatible architecture for the selected ABI (that is, `mips1` for 32-bit ABIs and `mips3` for 64-bit ABIs).

  the native Linux/GNU toolchain also supports the value `native`, which selects the best architecture option for the host processor. `-march=native` has no effect if GCC doesn't recognize the processor 👀.

  in processor names, a final `000` can be abbreviated as `k` (for example, `-march=r2k`). prefixes are optional, and `vr` may be written `r`.

  names of the form *n*`f2_1` refer to processors with FPUs clocked at half the rate of the core, names of the form *n*`f1_1` refer to processors with FPUs clocked at the same rate as the core, and names of the form *n*`f3_2` refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. for compatibility reasons, *n*`f` is accepted as a synonym for *n*`f2_1` while *n*`x` and *n*`bfx` are accepted as synonyms for *n*`f1_1`.

  GCC defines two macros based on the value of this option. the first is `"_MIPS_ARCH"`, which gives the name of target architecture, as a string. the second has the form `"_MIPS_ARCH_foo"`, where *foo* is the capitalized value of `"_MIPS_ARCH"`. for example, `-march=r2000` sets `"_MIPS_ARCH"` to `"r2000"` and defines the macro `"_MIPS_ARCH_R2000"`.

  note that the `"_MIPS_ARCH"` macro uses the processor names given above. in other words, it has the full prefix and doesn't abbreviate `000` as `k`. in the case of `from-abi`, the macro names the resolved architecture (either `"mips1"` or `"mips3"`). it names the default architecture when no `-march` option is given 💖.

- **`-mtune=arch`** — optimize for *arch*. among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. the list of *arch* values is the same as for `-march`.

  when this option is not used, GCC optimizes for the processor specified by `-march`. by using `-march` and `-mtune` together, it's possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family ✨.

  `-mtune` defines the macros `"_MIPS_TUNE"` and `"_MIPS_TUNE_foo"`, which work in the same way as the `-march` ones described above.

- **`-mips1`** — equivalent to `-march=mips1`.
- **`-mips2`** — equivalent to `-march=mips2`.
- **`-mips3`** — equivalent to `-march=mips3`.
- **`-mips4`** — equivalent to `-march=mips4`.
- **`-mips32`** — equivalent to `-march=mips32`.
- **`-mips32r3`** — equivalent to `-march=mips32r3`.
- **`-mips32r5`** — equivalent to `-march=mips32r5`.
- **`-mips32r6`** — equivalent to `-march=mips32r6`.
- **`-mips64`** — equivalent to `-march=mips64`.
- **`-mips64r2`** — equivalent to `-march=mips64r2`.
- **`-mips64r3`** — equivalent to `-march=mips64r3`.
- **`-mips64r5`** — equivalent to `-march=mips64r5`.
- **`-mips64r6`** — equivalent to `-march=mips64r6`.
- **`-mips16`**, **`-mno-mips16`** — generate (do not generate) MIPS16 code. if GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE.

  MIPS16 code generation can also be controlled on a per-function basis by means of `"mips16"` and `"nomips16"` attributes 🎀.

- **`-mmips16e2`**, **`-mno-mips16e2`** — use (do not use) the MIPS16e2 ASE. this option modifies the behavior of the `-mips16` option such that it targets the MIPS16e2 ASE.
- **`-mflip-mips16`** — generate MIPS16 code on *alternating* functions. this option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is **not** intended for ordinary use in compiling user code 🫶.
- **`-minterlink-compressed`**, **`-mno-interlink-compressed`** — require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa.

  for example, code using the standard ISA encoding can't jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. `-minterlink-compressed` therefore disables direct jumps unless GCC knows that the target of the jump isn't compressed.

- **`-minterlink-mips16`**, **`-mno-interlink-mips16`** — aliases of `-minterlink-compressed` and `-mno-interlink-compressed`. these options predate the microMIPS ASE and are retained for backwards compatibility.
- **`-mabi=32`**, **`-mabi=o64`**, **`-mabi=n32`**, **`-mabi=64`**, **`-mabi=eabi`** — generate code for the given ABI.

  note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use `-mgp32` to get 32-bit code instead.

  for info about the O64 ABI, see `https://gcc.gnu.org/projects/mipso64-abi.html`.

  GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. you can select this combination with `-mabi=32 -mfp64`. this ABI relies on the `"mthc1"` and `"mfhc1"` instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors 👑

  the register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. for example, scalar floating-point values are returned in `$f0` only, not a `$f0`/`$f1` pair. the set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved.

  two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. these are FPXX (`-mfpxx`) and FP64A (`-mfp64 -mno-odd-spreg`). the FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. the code can be interlinked with either FP32 or FP64, but *not* both. the FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. this can be used in conjunction with the `"FRE"` mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes 🌈.

- **`-mabicalls`**, **`-mno-abicalls`** — generate (do not generate) code that is suitable for SVR4-style dynamic objects. `-mabicalls` is the default for SVR4-based systems ⭐.

- **`-mshared`, `-mno-shared`** — bestie these two are literally the "am I a shared library or not" switch 🫶 You use them to generate (or *not* generate) code that's fully position-independent and can therefore be linked into shared libraries. Heads up though: this option **only** affects `-mabicalls`.

  So here's the lore: all `-mabicalls` code has traditionally been position-independent, no matter what you passed with `-fPIC` and `-fpic`. But plot twist — as an extension, the GNU toolchain lets executables use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. That whole mode? Selected by `-mno-shared`.

  > Ngl `-mno-shared` has some baggage: it depends on binutils 2.16 or higher and generates objects that can **only** be linked by the GNU linker. The good news is that it doesn't touch the ABI of the final executable — it only affects the ABI of relocatable objects. And using `-mno-shared` generally makes executables both smaller *and* quicker, which is lowkey iconic.

  `-mshared` is the default, periodt.

- **`-mplt`, `-mno-plt`** — assume (or don't assume, your call 💅) that the static and dynamic linkers support PLTs and copy relocations. This one only affects `-mno-shared -mabicalls`. And for the n64 ABI specifically, it has **no effect** without `-msym32` — noted.

  You can make `-mplt` the default by configuring GCC with `--with-mips-plt`. The default is `-mno-plt` otherwise.

- **`-mxgot`, `-mno-xgot`** — lift (or do not lift) the usual restrictions on the size of the global offset table. 🌸

  GCC normally uses a single instruction to load values from the GOT. While that's relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_MIPS_GOT16 foobar


If that happens, recompile your code with `-mxgot`. It works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. Trade-off city 😭

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need `-mxgot` when a single object file accesses more than 64k's worth of GOT entries. Very few do.

These options have no effect unless GCC is generating position independent code.

- **`-mgp32`** — assume that general-purpose registers are 32 bits wide.
- **`-mgp64`** — assume that general-purpose registers are 64 bits wide.

- **`-mfp32`** — assume that floating-point registers are 32 bits wide.
- **`-mfp64`** — assume that floating-point registers are 64 bits wide.
- **`-mfpxx`** — do **not** assume the width of floating-point registers. (living on the edge, love that 🙈)

- **`-mhard-float`** — use floating-point coprocessor instructions. Actual hardware, slay 💅
- **`-msoft-float`** — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.
- **`-mno-float`** — equivalent to `-msoft-float`, but *additionally* asserts that the program being compiled does not perform any floating-point operations.

> 🚨 This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats). If code compiled with `-mno-float` accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. You've been warned bestie 💀

- **`-msingle-float`** — assume that the floating-point coprocessor only supports single-precision operations.
- **`-mdouble-float`** — assume that the floating-point coprocessor supports double-precision operations. This is the default ✨

- **`-modd-spreg`, `-mno-odd-spreg`** — enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, `-mno-odd-spreg` is set by default.

- **`-mabs=2008`, `-mabs=legacy`** — these control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the `abs.fmt` and `neg.fmt` machine instructions.

By default, or when `-mabs=legacy` is used, the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead — *unless* the `-ffinite-math-only` option has also been specified.

The `-mabs=2008` option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations. glow up fr 🦋

- **`-mnan=2008`, `-mnan=legacy`** — these control the **encoding** of the special not-a-number (NaN) IEEE 754 floating-point data.

The `-mnan=legacy` option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being `0`, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being `1`.

The `-mnan=2008` option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being `1`, whereas sNaNs are denoted by the first bit of their trailing significand field being `0`. Literally just flipping the bits, the audacity 😭

The default is `-mnan=legacy` unless GCC has been configured with `--with-nan=2008`.

- **`-mllsc`, `-mno-llsc`** — use (or do not use) `ll`, `sc`, and `sync` instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.

`-mllsc` is useful if the runtime environment can emulate the instructions, and `-mno-llsc` can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with `--with-llsc` and `--without-llsc` respectively. `--with-llsc` is the default for some configurations; see the installation documentation for details.

- **`-mdsp`, `-mno-dsp`** — use (do not use) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro `__mips_dsp`. It also defines `__mips_dsp_rev` to `1`.

- **`-mdspr2`, `-mno-dspr2`** — use (do not use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros `__mips_dsp` and `__mips_dspr2`. It also defines `__mips_dsp_rev` to `2`.

- **`-msmartmips`, `-mno-smartmips`** — use (do not use) the MIPS SmartMIPS ASE.

- **`-mpaired-single`, `-mno-paired-single`** — use (do not use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled.

- **`-mdmx`, `-mno-mdmx`** — use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.

- **`-mips3d`, `-mno-mips3d`** — use (do not use) the MIPS-3D ASE. The option `-mips3d` implies `-mpaired-single`. package deal 🎀

- **`-mmicromips`, `-mno-micromips`** — generate (do not generate) microMIPS code.

MicroMIPS code generation can also be controlled on a per-function basis by means of `micromips` and `nomicromips` attributes.

- **`-mmt`, `-mno-mt`** — use (do not use) MT Multithreading instructions.
- **`-mmcu`, `-mno-mcu`** — use (do not use) the MIPS MCU ASE instructions.
- **`-meva`, `-mno-eva`** — use (do not use) the MIPS Enhanced Virtual Addressing instructions.
- **`-mvirt`, `-mno-virt`** — use (do not use) the MIPS Virtualization (VZ) instructions.
- **`-mxpa`, `-mno-xpa`** — use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
- **`-mcrc`, `-mno-crc`** — use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
- **`-mginv`, `-mno-ginv`** — use (do not use) the MIPS Global INValidate (GINV) instructions.
- **`-mloongson-mmi`, `-mno-loongson-mmi`** — use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
- **`-mloongson-ext`, `-mno-loongson-ext`** — use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
- **`-mloongson-ext2`, `-mno-loongson-ext2`** — use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.

- **`-mlong64`** — force `long` types to be 64 bits wide. See `-mlong32` for an explanation of the default and the way that the pointer size is determined.

- **`-mlong32`** — force `long`, `int`, and pointer types to be 32 bits wide.

The default size of `int`s, `long`s and pointers depends on the ABI. All the supported ABIs use 32-bit `int`s. The n64 ABI uses 64-bit `long`s, as does the 64-bit EABI; the others use 32-bit `long`s. Pointers are the same size as `long`s, or the same size as integer registers, whichever is smaller. Math is mathing ✨

- **`-msym32`, `-mno-sym32`** — assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with `-mabi=64` and `-mno-abicalls` because it allows GCC to generate shorter and faster references to symbolic addresses. short king behavior 👑

- **`-G` *num*** — put definitions of externally-visible data in a small data section if that data is no bigger than *num* bytes. GCC can then generate more efficient accesses to the data; see `-mgpopt` for details.

The default `-G` option depends on the configuration.

- **`-mlocal-sdata`, `-mno-local-sdata`** — extend (do not extend) the `-G` behavior to local data too, such as to static variables in C. `-mlocal-sdata` is the default for all configurations.

If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with `-mno-local-sdata`. You might also want to build large libraries with `-mno-local-sdata`, so that the libraries leave more room for the main program. Strategic 💅

- **`-mextern-sdata`, `-mno-extern-sdata`** — assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the `-G` limit. `-mextern-sdata` is the default for all configurations.

Okay so if you compile a module `Mod` with `-mextern-sdata -G num -mgpopt`, and `Mod` references a variable `Var` that is no bigger than `num` bytes, you **must** make sure that `Var` is placed in a small data section. If `Var` is defined by another module, you must either compile that module with a high-enough `-G` setting or attach a `"section"` attribute to `Var`'s definition. If `Var` is common, you must link the application with a high-enough `-G` setting. 💅

The easiest way of satisfying these restrictions is to compile and link every module with the same `-G` option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported `-G` setting and additionally using `-mno-extern-sdata` to stop the library from making assumptions about externally-defined data. ✨

---

- **`-mgpopt`, `-mno-gpopt`** — use (or do not use) GP-relative accesses for symbols that are known to be in a small data section; see `-G`, `-mlocal-sdata` and `-mextern-sdata`. `-mgpopt` is the default for all configurations.

`-mno-gpopt` is useful for cases where the `$gp` register might not hold the value of `"_gp"`. For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in `$gp`. (In such situations, the boot monitor itself is usually compiled with `-G0`.) 🫶

`-mno-gpopt` implies `-mno-local-sdata` and `-mno-extern-sdata`.

- **`-membedded-data`, `-mno-embedded-data`** — allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. 🌸

- **`-muninit-const-in-rodata`, `-mno-uninit-const-in-rodata`** — put uninitialized `const` variables in the read-only data section. This option is only meaningful in conjunction with `-membedded-data`.

- **`-mcode-readable=setting`** — specify whether GCC may generate code that reads from executable sections. There are three possible settings: 🦋

- **`-mcode-readable=yes`** — instructions may freely access executable sections. This is the default setting.
- **`-mcode-readable=pcrel`** — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
- **`-mcode-readable=no`** — instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.

- **`-msplit-addresses`, `-mno-split-addresses`** — enable (disable) use of the `%hi()` and `%lo()` assembler relocation operators. This option has been superseded by `-mexplicit-relocs` but is retained for backwards compatibility. 💫

- **`-mexplicit-relocs=none`, `-mexplicit-relocs=base`, `-mexplicit-relocs=pcrel`, `-mexplicit-relocs`, `-mno-explicit-relocs`** — these options control whether explicit relocs (such as `%gp_rel`) are used. The default value depends on the version of GAS when GCC itself was built.

The `"base"` explicit-relocs support was introduced into GAS in 2001. The `"pcrel"` explicit-relocs support was introduced into GAS in 2014, which supports `%pcrel_hi` and `%pcrel_lo`. ⭐

- **`-mcheck-zero-division`, `-mno-check-zero-division`** — trap (do not trap) on integer division by zero. The default is `-mcheck-zero-division`. 🔥

- **`-mdivide-traps`, `-mdivide-breaks`** — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal (`SIGFPE`). Use `-mdivide-traps` to allow conditional traps on architectures that support them and `-mdivide-breaks` to force the use of breaks. 😭

The default is usually `-mdivide-traps`, but this can be overridden at configure time using `--with-divide=breaks`. Divide-by-zero checks can be completely disabled using `-mno-check-zero-division`.

- **`-mload-store-pairs`, `-mno-load-store-pairs`** — enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. 🎀

- **`-mstrict-align`, `-mno-strict-align`, `-munaligned-access`, `-mno-unaligned-access`** — disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or `trap&emulate`. So `-mstrict-align` may be needed by kernel. The options `-munaligned-access` and `-mno-unaligned-access` are obsoleted, and only for backward-compatible. 💀

- **`-mmemcpy`, `-mno-memcpy`** — force (do not force) the use of `memcpy` for non-trivial block moves. The default is `-mno-memcpy`, which allows GCC to inline most constant-sized copies.

- **`-mlong-calls`, `-mno-long-calls`** — disable (do not disable) use of the `jal` instruction. Calling functions using `jal` is more efficient but requires the caller and callee to be in the same 256 megabyte segment. 🌈

This option has no effect on abicalls code. The default is `-mno-long-calls`.

- **`-mmad`, `-mno-mad`** — enable (disable) use of the `mad`, `madu` and `mul` instructions, as provided by the R4650 ISA. 👑

- **`-mimadd`, `-mno-imadd`** — enable (disable) use of the `madd` and `msub` integer instructions. The default is `-mimadd` on architectures that support `madd` and `msub` except for the 74k architecture where it was found to generate slower code.

- **`-mfused-madd`, `-mno-fused-madd`** — enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is `-mfused-madd`. 🧚

On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions. 💜

- **`-nocpp`** — tell the MIPS assembler to not run its preprocessor over user assembler files (with a `.s` suffix) when assembling them.

- **`-mfix-24k`, `-mno-fix-24k`** — work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC. 🙈

- **`-mfix-r4000`, `-mno-fix-r4000`** — work around certain R4000 CPU errata:

- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.

- **`-mfix-r4400`, `-mno-fix-r4400`** — work around certain R4400 CPU errata:

- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.

- **`-mfix-r10000`, `-mno-fix-r10000`** — work around certain R10000 errata:

- `ll`/`sc` sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier.

This option can only be used if the target architecture supports branch-likely instructions. `-mfix-r10000` is the default when `-march=r10000` is used; `-mno-fix-r10000` is the default otherwise. ⚡

- **`-mfix-r5900`, `-mno-fix-r5900`** — do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer and always schedule a `nop` instruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC. 🍓

- **`-mfix-rm7000`, `-mno-fix-rm7000`** — work around the RM7000 `dmult`/`dmultu` errata. The workarounds are implemented by the assembler rather than by GCC.

- **`-mfix-vr4120`, `-mno-fix-vr4120`** — work around certain VR4120 errata:

- `dmultu` does not always produce the correct result.
- `div` and `ddiv` do not always produce the correct result if one of the operands is negative.

The workarounds for the division errata rely on special functions in `libgcc.a`. At present, these functions are only provided by the `"mips64vr*-elf"` configurations. 🫶

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.

- **`-mfix-vr4130`** — work around the VR4130 `mflo`/`mfhi` errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using `mflo` and `mfhi` if the VR4130 `macc`, `macchi`, `dmacc` and `dmacchi` instructions are available instead. ✨

- **`-mfix-sb1`, `-mno-fix-sb1`** — work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 `"F1"` and `"F2"` floating-point errata.) 💖

- **`-mr10k-cache-barrier=setting`** — specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. 🌟

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the `"taken"` branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 😭

See the R10K processor manual for the full ✨lore✨ — it's got the whole description plus other potential problems, and honestly it's giving required reading.

One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed *and* that might have side effects even if aborted. `-mr10k-cache-barrier=setting` controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions do not have side effects:

1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. Not GCC's. The kernel's. 🫶

If the input program contains a function declaration such as:

void foo (void);


then the implementation of `foo` must allow `j foo` and `jal foo` to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same — so like, if you're hand-writing asm, bestie, that's on you 💅

The option has three forms:

- **`-mr10k-cache-barrier=load-store`** — insert a cache barrier before a load *or* store that might be speculatively executed and that might have side effects even if aborted.
- **`-mr10k-cache-barrier=store`** — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
- **`-mr10k-cache-barrier=none`** — disable the insertion of cache barriers. This is the default setting. ✨

---

- **`-mflush-func=func`**, **`-mno-flush-func`** — specifies the function to call to flush the I and D caches, or to not call any such function at all. If called, the function must take the same arguments as the common `_flush_func`, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number `3` (to flush *both* caches, you see 👀). The default depends on the target GCC was configured for, but commonly is either `_flush_func` or `__cpu_flush`.

- **`-mbranch-cost=num`** — set the cost of branches to roughly *num* "simple" instructions. This cost is only a heuristic and is **not guaranteed** to produce consistent results across releases, so like, don't come for GCC when it shifts, ok? A zero cost redundantly selects the default, which is based on the `-mtune` setting.

- **`-mbranch-likely`**, **`-mno-branch-likely`** — enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. An exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use. 🫠 so yeah, deprecated, we don't stan.

- **`-mcompact-branches=never`**, **`-mcompact-branches=optimal`**, **`-mcompact-branches=always`** — these control which *form* of branches will be generated. The default is `-mcompact-branches=optimal`.
  - The `-mcompact-branches=never` option ensures that compact branch instructions will never be generated. periodt, none, zero. 🚫
  - The `-mcompact-branches=always` option ensures that a compact branch instruction *will* be generated if available for MIPS Release 6 onwards. If a compact branch instruction is not available (or pre-R6), a delay slot form of the branch will be used instead.
  - If it is used for MIPS16/microMIPS targets, it will be just ignored now. The behavior for MIPS16/microMIPS may change in future, since they *do* have some compact branch instructions. foreshadowing ✨
  - The `-mcompact-branches=optimal` option will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available. lowkey the smartest one ngl.

- **`-mfp-exceptions`**, **`-mno-fp-exceptions`** — specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are **enabled**.
  - For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes 🔥 otherwise, we can only use one FP pipe. sad.

- **`-mvr4130-align`**, **`-mno-vr4130-align`** — the VR4130 pipeline is two-way superscalar, but it can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel. iconic behaviour tbh.
  - This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level `-O3`. tradeoffs, amirite?

- **`-msynci`**, **`-mno-synci`** — enable (disable) generation of `synci` instructions on architectures that support it. The `synci` instructions (if enabled) are generated when `__builtin___clear_cache` is compiled.
  - This option defaults to `-mno-synci`, but the default can be overridden by configuring GCC with `--with-synci`.
  - When compiling code for single processor systems, it is generally safe to use `synci`. However, on many multi-core (SMP) systems, it does **not** invalidate the instruction caches on all cores and may lead to **undefined behavior**. 💀 so like, pls be careful bestie.

- **`-mrelax-pic-calls`**, **`-mno-relax-pic-calls`** — try to turn PIC calls that are normally dispatched via register `$25` into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call.
  - `-mrelax-pic-calls` is the default if GCC was configured to use an assembler and a linker that support the `.reloc` assembly directive and `-mexplicit-relocs` is in effect. With `-mno-explicit-relocs`, this optimization can be performed by the assembler and the linker alone without help from the compiler. teamwork makes the dream work 🫶

- **`-mmcount-ra-address`**, **`-mno-mcount-ra-address`** — emit (do not emit) code that allows `_mcount` to modify the calling function's return address. When enabled, this option extends the usual `_mcount` interface with a new *ra-address* parameter, which has type `intptr_t *` and is passed in register `$12`. `_mcount` can then modify the return address by doing **both** of the following:
  - Returning the new address in register `$31`.
  - Storing the new address in `*ra-address`, if *ra-address* is nonnull.
  - The default is `-mno-mcount-ra-address`.

- **`-mframe-header-opt`**, **`-mno-frame-header-opt`** — enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will allocate `16` bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused. free real estate 🎀
  - This optimization is **off by default at all optimization levels**. yep, even `-O3`. don't ask.

- **`-mlxc1-sxc1`**, **`-mno-lxc1-sxc1`** — when applicable, enable (disable) the generation of `lwxc1`, `swxc1`, `ldxc1`, `sdxc1` instructions. Enabled by default. slay ✨

- **`-mmadd4`**, **`-mno-madd4`** — when applicable, enable (disable) the generation of 4-operand `madd.s`, `madd.d` and related instructions. Enabled by default. four operands?? main character energy fr fr.

---

## 🦋 MMIX Options 🦋

These options are defined for the MMIX:

- **`-mlibfuncs`**, **`-mno-libfuncs`** — specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. no matter the size!! 👑

- **`-mepsilon`**, **`-mno-epsilon`** — generate floating-point comparison instructions that compare with respect to the `rE` epsilon register.

- **`-mabi=mmixware`**, **`-mabi=gnu`** — generate code that passes function parameters and return values that (in the called function) are seen as registers `$0` and up, as opposed to the GNU ABI which uses global registers `$231` and up. two whole vibes, pick one 💅

- **`-mzero-extend`**, **`-mno-zero-extend`** — when reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones. `$0`-and-up gang vs the zero-extend gang, iykyk.

- **`-mknuthdiv`**, **`-mno-knuthdiv`** — make the result of a division yielding a remainder have the same sign as the **divisor**. With the default, `-mno-knuthdiv`, the sign of the remainder follows the sign of the **dividend**. Both methods are arithmetically valid, the latter being almost exclusively used. both valid, one's just way more popular. it's giving division drama 😭

- **`-mtoplevel-symbols`**, **`-mno-toplevel-symbols`** — prepend (do not prepend) a `:` to all global symbols, so the assembly code can be used with the `PREFIX` assembly directive. little colon, big consequences.

- **`-melf`** — generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.

- **`-mbranch-predict`**, **`-mno-branch-predict`** — use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch. manifesting that branch 🌈

- **`-mbase-addresses`**, **`-mno-base-addresses`** — generate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range `0` to `255` from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is **limited**. This means that a program that uses lots of static data may require `-mno-base-addresses`. the classic short-and-fast vs. lots-of-static-data situation 🫠

- **`-msingle-exit`**, **`-mno-single-exit`** — force (do not force) generated code to have a single exit point in each function. one door out. no sneaking out the back 🚪

---

## 🌸 MN10300 Options 🌸

These `-m` options are defined for Matsushita MN10300 architectures:

- **`-mmult-bug`** — generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default. we work around the bugs, we don't pretend they're not there 💅

- **`-mno-mult-bug`** — do *not* generate code to avoid bugs in the multiply instructions for the MN10300 processors. your funeral bestie 💀

- **`-mam33`** — generate code using features specific to the AM33 processor.

- **`-mno-am33`** — nope, don't generate any AM33-processor-specific code. This is the default, so like… chill, nothing fancy. 💅

- **`-mam34`** — ok now we're talking: generate code using the features specific to the AM34 processor. 🔥

- **`-mam33-2`** — same energy, but for the AM33/2.0 processor. Generate code using its specific features.

- **`-mtune=cpu-type`** — schedule instructions using the timing characteristics of the CPU type you name. Lowkey important: this does **not** change the targeted processor type. The *cpu-type* has to be one of `mn10300`, `am33`, `am33-2` or `am34`.

- **`-mreturn-pointer-on-d0`** — when the compiler generates a function that returns a pointer, it hands the pointer back in **both** `a0` and `d0`. Otherwise the pointer only comes back in `a0`, and any attempt to call such functions without a prototype just… errors out 😭. Heads up: this option is **on by default**, so use `-mno-return-pointer-on-d0` to turn it off.

- **`-mno-crt0`** — don't link in the C run-time initialization object file. simple as that 🎀

- **`-mrelax`** — tells the linker "bestie please do a relaxation optimization pass" to shorten branches, calls and absolute memory addresses. Important vibe check: this only does anything when you use it on the command line for the **final link step**.

> ⚠️ Also this option makes symbolic debugging impossible. Yeah. Gone. Consider yourself warned. 💀

- **`-mliw`** — lets the compiler generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro `__LIW__`. ✨

- **`-mno-liw`** — the opposite: no Long Instruction Word instructions allowed. This option defines the preprocessor macro `__NO_LIW__`.

- **`-msetlb`** — lets the compiler generate `SETLB` and `Lcc` instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro `__SETLB__`. 🌸

- **`-mno-setlb`** — no `SETLB` or `Lcc` instructions, periodt. This option defines the preprocessor macro `__NO_SETLB__`.

---

## 🦋 Moxie Options 🦋

- **`-meb`** — generate big-endian code. This is the default for `moxie-*-*` configurations. 👑

- **`-mel`** — generate little-endian code. tiny endian energy 🧚

- **`-mmul.x`** — generate `mul.x` and `umul.x` instructions. This is the default for `moxiebox-*-*` configurations.

- **`-mno-crt0`** — do not link in the C run-time initialization object file. no cap.

---

## 🌈 MSP430 Options 🌈

These options are defined for the MSP430:

- **`-masm-hex`** — force assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. aesthetic purposes. iconic. 🎀

- **`-mmcu=`** — select the MCU to target. Here's the whole situation: this is used to create a C preprocessor symbol based on the MCU name, converted to upper case and pre- and post-fixed with `__`. That symbol then gets used by the `msp430.h` header file to pick an MCU-specific supplementary header file. 🫶

  The option also sets the ISA to use. If the MCU name is one that's known to only support the 430 ISA then that's selected; otherwise the 430X ISA is selected. A generic MCU name of `msp430` can also be used to select the 430 ISA. Similarly the generic `msp430x` MCU name selects the 430X ISA.

  In addition, an MCU-specific linker script gets added to the linker command line. The script's name is the name of the MCU with `.ld` appended. So specifying `-mmcu=xxx` on the `gcc` command line defines the C preprocessor symbol `__XXX__` and causes the linker to search for a script called `xxx.ld`.

  The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external `devices.csv` file can be used to extend device support beyond those that have been hard-coded. 🤯

  GCC searches for the `devices.csv` file using the following methods in the given precedence order, where the first method takes precedence over the second, which takes precedence over the third:

  - **Include path specified with `-I` and `-L`** — `devices.csv` is searched for in each of the directories specified by include paths and linker library search paths.
  - **Path specified by the environment variable `MSP430_GCC_INCLUDE_DIR`** — define the value of the global environment variable `MSP430_GCC_INCLUDE_DIR` to the full path to the directory containing `devices.csv`, and GCC will search this directory for `devices.csv`. If `devices.csv` is found, this directory is also registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying `-I` and `-L` on the command line.
  - **The `msp430-elf{,bare}/include/devices` directory** — finally, GCC examines `msp430-elf{,bare}/include/devices` from the toolchain root directory. This directory does not exist in a default installation, but if the user has created it and copied `devices.csv` there, then the MCU data is read. As above, this directory is also registered as an include path and linker library path.

  If none of the above search methods find `devices.csv`, then the hard-coded MCU data is used. that's the fallback, bestie 💅

- **`-mwarn-mcu`**, **`-mno-warn-mcu`** — enables or disables warnings about conflicts between the MCU name specified by the `-mmcu` option and the ISA set by the `-mcpu` option and/or the hardware multiply support set by the `-mhwmult` option. It also toggles warnings about unrecognized MCU names. This option is **on by default**. ⭐

- **`-mcpu=`** — specifies the ISA to use. Accepted values are `msp430`, `msp430x` and `msp430xv2`. 🚨 This option is **deprecated** — the `-mmcu=` option should be used to select the ISA instead.

- **`-msim`** — link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the `-mmcu=` option. main character behavior, honestly 👑

- **`-mlarge`** — use large-model addressing (`20`-bit pointers, `20`-bit `size_t`).

- **`-msmall`** — use small-model addressing (`16`-bit pointers, `16`-bit `size_t`).

- **`-mrelax`** — this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link. ✨

- **`mhwmult=`** — describes the type of hardware multiply supported by the target. Accepted values: `none` for no hardware multiply, `16bit` for the original 16-bit-only multiply supported by early MCUs, `32bit` for the 16/32-bit multiply supported by later MCUs, and `f5series` for the 16/32-bit multiply supported by F5-series MCUs. A value of `auto` can also be given — that tells GCC to deduce the hardware multiply support based upon the MCU name provided by the `-mmcu` option. If no `-mmcu` option is specified, or if the MCU name is not recognized, then **no hardware multiply support is assumed**. `auto` is the default setting. 🍓

  Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at `-O3` or higher, however, the hardware multiplier is invoked inline. This makes for bigger, but faster code. tradeoffs, babes 😭

  The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. we love a polite function 🫶

- **`-minrt`** — enable the use of a minimum runtime environment — no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. 🧚

- **`-mtiny-printf`** — enable reduced code size `printf` and `puts` library functions. 🚨 The tiny implementations of these functions are **not reentrant**, so they must be used with caution in multi-threaded applications.

  Support for streams has been removed and the string to be printed will always be sent to stdout via the `write` syscall. The string is **not** buffered before it is sent to `write`.

  This option requires Newlib Nano IO, so GCC must be configured with `--enable-newlib-nano-formatted-io`.

- **`-mmax-inline-shift=`** — this option takes an integer between `0` and `64` inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an `mspabi` helper function is used instead. The default value is `4`. 💜

  This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts `>1` on the 430 ISA).

  Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead. math is math 💅

- **`-mcode-region=`**, **`-mdata-region=`** — these options tell the compiler where to place functions and data that do not have one of the `lower`, `upper`, `either` or `section` attributes. Possible values are `lower`, `upper`, `either` or `any`. The first three behave like the corresponding attribute. The fourth possible value — `any` — is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (`.text`, `.data`, etc) to the memory regions. 💫

- **`-msilicon-errata=`** — passes on a request to the assembler to enable the fixes for the named silicon errata. 🙈

- **`-msilicon-errata-warn=`** — passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.

- **`-mwarn-devices-csv`**, **`-mno-warn-devices-csv`** — warn if `devices.csv` is not found or there are problems parsing it (default: `on`).

---

## 🔥 NDS32 Options 🔥

These options are defined for NDS32 implementations:

- **`-mbig-endian`** — generate code in big-endian mode.

- **`-mlittle-endian`** — generate code in little-endian mode.

- **`-mreduced-regs`** — use reduced-set registers for register allocation.

- **`-mfull-regs`** — use full-set registers for register allocation. the full set, the whole squad 👑

- **`-mcmov`** — generate conditional move instructions.

- **`-mno-cmov`** — do not generate conditional move instructions.

- **`-mext-perf`** — generate performance extension instructions. ⚡

- **`-mno-ext-perf`** — do not generate performance extension instructions.

- **`-mext-perf2`** — generate performance extension 2 instructions. ⚡⚡

- **`-mno-ext-perf2`** — do not generate performance extension 2 instructions.

- **`-mext-string`** — generate string extension instructions. 🧵

- **`-mno-ext-string`** — do not generate string extension instructions.

- **`-mv3push`** — generate v3 `push25`/`pop25` instructions. 🎀

- **`-mno-v3push`** — nope, we're not generating v3 push25/pop25 instructions. That's it, that's the whole vibe. 🚫

- **`-m16-bit`** — okay so this one generates 16-bit instructions. slay. ✨

- **`-mno-16-bit`** — and this one does **not** generate 16-bit instructions. the duality 💅

- **`-misr-vector-size=`***num* — tells the compiler how big each interrupt vector is. has to be `4` or `16`, no in-between, pick a lane bestie 🎀

- **`-mcache-block-size=`***num* — sets the size of each cache block. must be a power of 2 between `4` and `512`. iykyk 🔢

- **`-march=`***arch* — specifies the name of the target architecture. say it with your chest 💖

- **`-mcmodel=`***code-model* — sets the code model. options are lowkey a whole personality quiz:

### code models 🦋

- **`small`** — all the data and read-only data segments must fit within `512KB` addressing space, and the text segment must be within `16MB` addressing space. cozy, contained, main character energy ✨

- **`medium`** — data segment has to be within `512KB` while the read-only data segment can stretch out to `4GB` addressing space. text segment should still be within `16MB` addressing space. medium is the new large fr fr 🌸

- **`large`** — all the text and data segments can live within `4GB` addressing space. go big or go home 💀

- **`-mctor-dtor`** — enable constructor/destructor feature. constructor girlies rise 🫶

- **`-mrelax`** — guides the linker to relax instructions. deep breaths. it's giving zen 🧘‍♀️

---

## 🚀 Nvidia PTX Options 🚀

so these options are defined for Nvidia PTX. very GPU, very parallel, very much not your average Tuesday ✨

- **`-m64`** — ignored, but preserved for backward compatibility. only the 64-bit ABI is supported, so this flag is basically there for the vibes 😭

- **`-march=`***architecture-string* — generate code for the specified PTX ISA target architecture. valid architecture strings are `sm_30`, `sm_35`, `sm_37`, `sm_52`, `sm_53`, `sm_61`, `sm_70`, `sm_75`, `sm_80`, and `sm_89`. the default depends on how the compiler has been configured — see `--with-arch`.

  > this option sets the value of the preprocessor macro `"__PTX_SM__"` — for instance, for `sm_35` it has the value `350`. numbers, but make it fashion 💅

- **`-misa=`***architecture-string* — alias of `-march=`. same energy, different name 🌈

- **`-march-map=`***architecture-string* — selects the closest available `-march=` value that is **not more capable**. for instance, for `-march-map=sm_50` select `-march=sm_37`, and for `-march-map=sm_53` select `-march=sm_53`. it's like thrifting but for architecture strings 🎀

- **`-mptx=`***version-string* — generate code for the specified PTX ISA version. valid version strings are `3.1`, `4.1`, `4.2`, `5.0`, `6.0`, `6.3`, `7.0`, `7.3`, and `7.8`. the default PTX ISA version is the one that added support for the selected PTX ISA target architecture (see `-march=`), but at least `6.3`, or `7.3` for `-march=sm_52` and higher.

  > this option sets the values of the preprocessor macros `"__PTX_ISA_VERSION_MAJOR__"` and `"__PTX_ISA_VERSION_MINOR__"` — for instance, for `3.1` the macros have the values `3` and `1`, respectively. major and minor, iconic behavior 💖

- **`-mmainkernel`** — link in code for a `__main` kernel. this is for stand-alone instead of offloading execution. doin' it solo 💫

- **`-moptimize`** — apply partitioned execution optimizations. this is the default when any level of optimization is selected. efficiency queen 👑

- **`-msoft-stack`**
- **`-mno-soft-stack`** — okay bestie, deep breath, this one's a paragraph:

  for `-mno-soft-stack` (the default, unless `-mgomp` has been specified), use PTX "native" stacks — that is, generate code that uses `.local` memory or PTX `alloca` directly for stack storage. unless `-mptx=7.3` or higher **and** `-march=sm_52` or higher are active, variable-length arrays and dynamically allocating memory on the stack with `alloca` are **not supported**.

  for `-msoft-stack` (implied by `-mgomp`), generate code that does **not** use `.local` memory or PTX `alloca` directly for stack storage. instead, a per-warp stack pointer is maintained explicitly. this enables variable-length stack allocation (with variable-length arrays or `alloca`), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. this code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option `-mgomp`. ok that was a lot but we got there 🫶✨

- **`-muniform-simt`** — switches to a code generation variant that lets all threads in each warp execute, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. all atomic operations and calls to runtime (`malloc`, `free`, `vprintf`) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. outside of SIMD regions lane `0` is the master; inside, each thread sees itself as the master. shared memory array `"int __nvptx_uni[]"` stores all-zeros or all-ones bitmasks for each warp, indicating current mode (`0` outside of SIMD regions). each thread can bitwise-and the bitmask at position `"tid.y"` with current lane index to compute the master lane index. everyone's the main character, but like, in an organized way 💅

- **`-mgomp`** — generates code for use in OpenMP offloading: enables `-msoft-stack` and `-muniform-simt` options, and selects the corresponding multilib variant. the bundle deal 🎁

---

## 🌱 OpenRISC Options 🌱

these options are defined for OpenRISC. open source, open heart 🦋

- **`-mboard=`***name* — configure a board specific runtime. this will be passed to the linker for newlib board library linking. the default is `"or1ksim"`. know your board, love your board 🌸

- **`-mnewlib`** — this option is ignored; it is for compatibility purposes only. this used to select linker and preprocessor options for use with newlib. a legacy icon, we respect her 🎀

- **`-msoft-div`**
- **`-mhard-div`** — select software or hardware divide (`l.div`, `l.divu`) instructions. the default is hardware divide. division but make it a choice ✨

- **`-msoft-mul`**
- **`-mhard-mul`** — select software or hardware multiply (`l.mul`, `l.muli`) instructions. the default is hardware multiply. math, but make it customizable 🔢

- **`-msoft-float`**
- **`-mhard-float`** — select software or hardware for floating point operations. the default is software. float softly or float hard, no in-between 💫

- **`-mdouble-float`** — when `-mhard-float` is selected, enables generation of double-precision floating point instructions. by default functions from libgcc are used to perform double-precision floating point operations. double the precision, double the fun 💖

- **`-munordered-float`** — when `-mhard-float` is selected, enables generation of unordered floating point compare and set flag (`lf.sfun*`) instructions. by default functions from libgcc are used to perform unordered floating point compare and set flag operations. chaos, but computed 🌪️

- **`-mcmov`** — enable generation of conditional move (`l.cmov`) instructions. by default the equivalent will be generated using set and branch. glow-up from set-and-branch, honestly 👑

- **`-mror`** — enable generation of rotate right (`l.ror`) instructions. by default functions from libgcc are used to perform rotate right operations. spin it 🌀

- **`-mrori`** — enable generation of rotate right with immediate (`l.rori`) instructions. by default functions from libgcc are used to perform rotate right with immediate operations. spin it, but instantly ⚡

- **`-msext`** — enable generation of sign extension (`l.ext*`) instructions. by default memory loads are used to perform sign extension. extend that sign, bestie ✨

- **`-msfimm`** — enable generation of compare and set flag with immediate (`l.sf*i`) instructions. by default extra instructions will be generated to store the immediate to a register first. skipping the middleman, we love to see it 💅

- **`-mshftimm`** — enable generation of shift with immediate (`l.srai`, `l.srli`, `l.slli`) instructions. by default extra instructions will be generated to store the immediate to a register first. again: no middleman, no problem 🌈

- **`-mcmodel=small`** — generate OpenRISC code for the small model: the GOT is limited to `64k`. this is the default model. smol but mighty 🎀

- **`-mcmodel=large`** — generate OpenRISC code for the large model: the GOT may grow up to `4G` in size. big GOT energy 💀

---

## 🕰️ PDP-11 Options 🕰️

these options are defined for the PDP-11. vintage, retro, absolutely iconic, we stan a classic 👑

- **`-mfpu`** — use hardware FPP floating point. this is the default. (FIS floating point on the PDP-11/40 is **not** supported.) implies `-m45`. hardware floats, but read the fine print 🫶

- **`-msoft-float`** — do not use hardware floating point. soft float only, we're being gentle 💜

- **`-mac0`** — return floating-point results in `ac0` (`fr0` in Unix assembler syntax). results, but in a register 🎀

- **`-mno-ac0`** — return floating-point results in memory. this is the default. results, but in memory. classic 🦋

- **`-m40`** — generate code for a PDP-11/40. implies `-msoft-float -mno-split`. the 40 is a whole era ✨

- **`-m45`** — generate code for a PDP-11/45. this is the default. the 45, the moment, the icon 💅

- **`-m10`** — generate code for a PDP-11/10. implies `-msoft-float -mno-split`. the 10 said "i'm baby" 🌸

- **`-mint16`**
- **`-mno-int32`** — use 16-bit `int`. this is the default. small ints, big personality 🔢

- **`-mint32`**
- **`-mno-int16`** — use 32-bit `int`. upgrade your ints, bestie ⭐

- **`-msplit`** — target has split instruction and data space. implies `-m45`. splitting things, but architecturally 💫

- **`-munix-asm`** — use Unix assembler syntax. periodt. 💖

- **`-mdec-asm`** — use DEC assembler syntax. also valid, also slay 🎀

- **`-mgnu-asm`** — use GNU assembler syntax. this is the default. gnu girlies stay winning 🌈

- **`-mlra`** — use the new LRA register allocator. by default, the old `"reload"` allocator is used. new allocator, new me ✨

---

## ⚡ PowerPC Options ⚡

these are listed under 👀 — that's genuinely all it says, so go find them under there, bestie 🫶

---

## 🧚 PRU Options 🧚

these command-line options are defined for the PRU target:

- **`-minrt`** — link with a minimum runtime environment. this can significantly reduce the size of the final ELF binary, but some standard C runtime features are removed.

  this option disables support for static initializers and constructors. **beware** that the compiler could still generate code with static initializers and constructors — it is up to the programmer to ensure that the source program will not use those features. no cap, that's on you 🙈

  the minimal startup code would not pass `"argc"` and `"argv"` arguments to `"main"`, so the latter must be declared as `"int main (void)"`. this is already the norm for most firmware projects. main character with no arguments, honestly relatable 😭

- **`-mmcu=`***mcu* — specify the PRU hardware variant to use. a correspondingly named spec file would be loaded, passing the memory region sizes to the linker and defining hardware-specific C macros.

  Newlib provides only the `"sim"` spec, intended for running regression tests using a simulator. specs for real hardware can be obtained by installing the GnuPruMcu (`"https://github.com/dinuxbg/gnuprumcu/"`) package. sim is for the simulation girlies, real hardware needs the extra package 💅

- **`-mno-relax`** — make GCC pass the `--no-relax` command-line option to the linker instead of the `--relax` option. no relaxing allowed, we're locked in 🔒

- **`-mloop`** — allow (or do not allow) GCC to use the LOOP instruction. yes, that's the whole description. loop it or don't 🌀

- **`-mabi=`***variant* — specify the ABI variant to output code for. `-mabi=ti` selects the unmodified TI ABI while `-mabi=gnu` selects a GNU variant that copes more naturally with certain GCC assumptions. these are the differences:

### Function Pointer Size 🎀

TI ABI specifies that function (code) pointers are `16-bit`, whereas GNU supports only `32-bit` data and code pointers. sixteen versus thirty-two, the eternal discourse 💀

### 💅 Optional Return Value Pointer 💅

Okay so bestie, when a function return value is bigger than 64 bits, it gets passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be `NULL` in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer. No cap. 💀

The current `-mabi=ti` implementation simply raises a compile error when any of the above code constructs is detected. As a consequence, the standard C library cannot be built and it is omitted when linking with `-mabi=ti`. That's literally the whole situation. 😭

Relaxation is a GNU feature and for safety reasons is disabled when using `-mabi=ti`. The TI toolchain does not emit relocations for `QBBx` instructions, so the GNU linker cannot adjust them when shortening adjacent `LDI32` pseudo instructions. Periodt. 🫶

---

## 🦋 RISC-V Options 🦋

These command-line options are defined for RISC-V targets:

- **`-mbranch-cost=n`** — set the cost of branches to roughly `n` instructions.

- **`-mplt`**, **`-mno-plt`** — when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is `-mplt`.

- **`-mabi=ABI-string`** — specify integer and floating-point calling convention. `ABI-string` contains two parts: the size of integer types and the registers used for floating-point types. For example, `-march=rv64ifd -mabi=lp64d` means that `long` and pointers are 64-bit (implicitly defining `int` to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with `-march=rv64ifd -mabi=lp64f`, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or `-march=rv64ifd -mabi=lp64`, in which no floating-point arguments will be passed in registers.

  The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: `ilp32`, `ilp32f`, `ilp32d`, `lp64`, `lp64f`, and `lp64d`. Some calling conventions are impossible to implement on some ISAs: for example, `-march=rv32if -mabi=ilp32d` is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also the `ilp32e` ABI that can only be used with the `rv32e` architecture and the `lp64e` ABI that can only be used with the `rv64e`. Those ABIs are not well specified at present, and are subject to change. 🫠

- **`-mfdiv`**, **`-mno-fdiv`** — do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions.

- **`-mfence-tso`**, **`-mno-fence-tso`** — do or don't use the `fence.tso` instruction, which is unimplemented on some processors (including those from T-Head). If the `fence.tso` instruction is not available then a stronger fence is used instead.

- **`-mdiv`**, **`-mno-div`** — do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions.

- **`-misa-spec=ISA-spec-string`** — specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming to. The possibilities for `ISA-spec-string` are:

  - `2.2` — produce code conforming to version `2.2`.
  - `20190608` — produce code conforming to version `20190608`.
  - `20191213` — produce code conforming to version `20191213`.

  The default is `-misa-spec=20191213` unless GCC has been configured with `--with-isa-spec=` specifying a different default version.

- **`-march=ISA-string`** — generate code for given RISC-V ISA (e.g. `rv64im`). ISA strings must be lower-case. Examples include `rv64i`, `rv32g`, `rv32e`, and `rv32imaf`. Additionally, a special value `help` (`-march=help`) is accepted to list all supported extensions. 💖

  The syntax of the ISA string is defined as follows:

  - The string must start with `rv32` or `rv64`, followed by `i`, `e`, or `g`, referred to as the base ISA.
  - The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g. `zba`) and single-letter (e.g. `v`). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores.
  - An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.
  - Extension names may include an optional version number, following the syntax `<major>p<minor>` or `<major>`, (e.g. `m2p1` or `m2`).

  Supported extensions are listed below:

  - **`i`** — versions `2.0`, `2.1`; base integer extension.
  - **`e`** — version `2.0`; reduced base integer extension.
  - **`g`** — version `-`; general-purpose computing base extension, `g` will expand to `i`, `m`, `a`, `f`, `d`, `zicsr` and `zifencei`.
  - **`m`** — version `2.0`; integer multiplication and division extension.
  - **`a`** — versions `2.0`, `2.1`; atomic extension.
  - **`f`** — versions `2.0`, `2.2`; single-precision floating-point extension.
  - **`d`** — versions `2.0`, `2.2`; double-precision floating-point extension.
  - **`c`** — version `2.0`; compressed extension.
  - **`h`** — version `1.0`; hypervisor extension.
  - **`v`** — version `1.0`; vector extension.
  - **`zicsr`** — version `2.0`; control and status register access extension.
  - **`zifencei`** — version `2.0`; instruction-fetch fence extension.
  - **`zicond`** — version `1.0`; integer conditional operations extension.
  - **`za64rs`** — version `1.0`; reservation set size of `64` bytes.
  - **`za128rs`** — version `1.0`; reservation set size of `128` bytes.
  - **`zawrs`** — version `1.0`; wait-on-reservation-set extension.
  - **`zba`** — version `1.0`; address calculation extension.
  - **`zbb`** — version `1.0`; basic bit manipulation extension.
  - **`zbc`** — version `1.0`; carry-less multiplication extension.
  - **`zbs`** — version `1.0`; single-bit operation extension.
  - **`zfinx`** — version `1.0`; single-precision floating-point in integer registers extension.
  - **`zdinx`** — version `1.0`; double-precision floating-point in integer registers extension.
  - **`zhinx`** — version `1.0`; half-precision floating-point in integer registers extension.
  - **`zhinxmin`** — version `1.0`; minimal half-precision floating-point in integer registers extension.
  - **`zbkb`** — version `1.0`; cryptography bit-manipulation extension.
  - **`zbkc`** — version `1.0`; cryptography carry-less multiply extension.
  - **`zbkx`** — version `1.0`; cryptography crossbar permutation extension.
  - **`zkne`** — version `1.0`; AES Encryption extension.
  - **`zknd`** — version `1.0`; AES Decryption extension.
  - **`zknh`** — version `1.0`; hash function extension.
  - **`zkr`** — version `1.0`; entropy source extension.
  - **`zksed`** — version `1.0`; SM4 block cipher extension.
  - **`zksh`** — version `1.0`; SM3 hash function extension.
  - **`zkt`** — version `1.0`; data independent execution latency extension.
  - **`zk`** — version `1.0`; standard scalar cryptography extension.
  - **`zkn`** — version `1.0`; NIST algorithm suite extension.
  - **`zks`** — version `1.0`; ShangMi algorithm suite extension.
  - **`zihintntl`** — version `1.0`; non-temporal locality hints extension.
  - **`zihintpause`** — version `1.0`; pause hint extension.
  - **`zicboz`** — version `1.0`; cache-block zero extension.
  - **`zicbom`** — version `1.0`; cache-block management extension.
  - **`zicbop`** — version `1.0`; cache-block prefetch extension.
  - **`zic64b`** — version `1.0`; cache block size isf `64` bytes.
  - **`ziccamoa`** — version `1.0`; main memory supports all atomics in `A`.
  - **`ziccif`** — version `1.0`; main memory supports instruction fetch with atomicity requirement.
  - **`zicclsm`** — version `1.0`; main memory supports misaligned loads/stores.
  - **`ziccrse`** — version `1.0`; main memory supports forward progress on `LR/SC` sequences.
  - **`zicntr`** — version `2.0`; standard extension for base counters and timers.
  - **`zihpm`** — version `2.0`; standard extension for hardware performance counters.
  - **`ztso`** — version `1.0`; total store ordering extension.
  - **`zve32x`** — version `1.0`; vector extensions for embedded processors.
  - **`zve32f`** — version `1.0`; vector extensions for embedded processors.
  - **`zve64x`** — version `1.0`; vector extensions for embedded processors.
  - **`zve64f`** — version `1.0`; vector extensions for embedded processors.
  - **`zve64d`** — version `1.0`; vector extensions for embedded processors.
  - **`zvl32b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvl64b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvl128b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvl256b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvl512b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvl1024b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvl2048b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvl4096b`** — version `1.0`; minimum vector length standard extensions.
  - **`zvbb`** — version `1.0`; vector basic bit-manipulation extension.
  - **`zvbc`** — version `1.0`; vector carryless multiplication extension.
  - **`zvkb`** — version `1.0`; vector cryptography bit-manipulation extension.
  - **`zvkg`** — version `1.0`; vector GCM/GMAC extension.
  - **`zvkned`** — version `1.0`; vector AES block cipher extension.
  - **`zvknha`** — version `1.0`; vector SHA-2 secure hash extension.
  - **`zvknhb`** — version `1.0`; vector SHA-2 secure hash extension.
  - **`zvksed`** — version `1.0`; vector SM4 Block Cipher extension.
  - **`zvksh`** — version `1.0`; vector SM3 Secure Hash extension.
  - **`zvkn`** — version `1.0`; vector NIST Algorithm Suite extension, `zvkn` will expand to `zvkned`, `zvknhb`, `zvkb` and `zvkt`.
  - **`zvknc`** — version `1.0`; vector NIST Algorithm Suite with carryless multiply extension, `zvknc` will expand to `zvkn` and `zvbc`.

<pink>…okay continuing the extension tour, stay with me bestie, we're so close 💅</pink>

- **`zvkng`** — *1.0* — Vector NIST Algorithm Suite with GCM extension, `zvkng` will expand to `zvkn` and `zvkg`.
- **`zvks`** — *1.0* — Vector ShangMi algorithm suite extension, `zvks` will expand to `zvksed`, `zvksh`, `zvkb` and `zvkt`.
- **`zvksc`** — *1.0* — Vector ShangMi algorithm suite with carryless multiplication extension, `zvksc` will expand to `zvks` and `zvbc`.
- **`zvksg`** — *1.0* — Vector ShangMi algorithm suite with GCM extension, `zvksg` will expand to `zvks` and `zvkg`.
- **`zvkt`** — *1.0* — Vector data independent execution latency extension.
- **`zfh`** — *1.0* — Half-precision floating-point extension.
- **`zfhmin`** — *1.0* — Minimal half-precision floating-point extension.
- **`zvfh`** — *1.0* — Vector half-precision floating-point extension.
- **`zvfhmin`** — *1.0* — Vector minimal half-precision floating-point extension.
- **`zvfbfmin`** — *1.0* — Vector BF16 converts extension.
- **`zfa`** — *1.0* — Additional floating-point extension.
- **`zmmul`** — *1.0* — Integer multiplication extension.
- **`zca`** — *1.0* — Integer compressed instruction extension.
- **`zcf`** — *1.0* — Compressed single-precision floating point loads and stores extension.
- **`zcd`** — *1.0* — Compressed double-precision floating point loads and stores extension.
- **`zcb`** — *1.0* — Simple compressed instruction extension.
- **`zce`** — *1.0* — Compressed instruction extensions for embedded processors.
- **`zcmp`** — *1.0* — Compressed push pop extension.
- **`zcmt`** — *1.0* — Table jump instruction extension.
- **`smaia`** — *1.0* — Advanced interrupt architecture extension.
- **`smepmp`** — *1.0* — PMP Enhancements for memory access and execution prevention on Machine mode.
- **`smstateen`** — *1.0* — State enable extension.
- **`ssaia`** — *1.0* — Advanced interrupt architecture extension for supervisor-mode.
- **`sscofpmf`** — *1.0* — Count overflow & filtering extension.
- **`ssstateen`** — *1.0* — State-enable extension for supervisor-mode.
- **`sstc`** — *1.0* — Supervisor-mode timer interrupts extension.
- **`svinval`** — *1.0* — Fine-grained address-translation cache invalidation extension.
- **`svnapot`** — *1.0* — NAPOT translation contiguity extension.
- **`svpbmt`** — *1.0* — Page-based memory types extension.
- **`xcvmac`** — *1.0* — Core-V multiply-accumulate extension.
- **`xcvalu`** — *1.0* — Core-V miscellaneous ALU extension.
- **`xcvelw`** — *1.0* — Core-V event load word extension.
- **`xtheadba`** — *1.0* — T-head address calculation extension.
- **`xtheadbb`** — *1.0* — T-head basic bit-manipulation extension.
- **`xtheadbs`** — *1.0* — T-head single-bit instructions extension.
- **`xtheadcmo`** — *1.0* — T-head cache management operations extension.
- **`xtheadcondmov`** — *1.0* — T-head conditional move extension.
- **`xtheadfmemidx`** — *1.0* — T-head indexed memory operations for floating-point registers extension.
- **`xtheadfmv`** — *1.0* — T-head double floating-point high-bit data transmission extension.
- **`xtheadint`** — *1.0* — T-head acceleration interruption extension.
- **`xtheadmac`** — *1.0* — T-head multiply-accumulate extension.
- **`xtheadmemidx`** — *1.0* — T-head indexed memory operation extension.
- **`xtheadmempair`** — *1.0* — T-head two-GPR memory operation extension.
- **`xtheadsync`** — *1.0* — T-head multi-core synchronization extension.
- **`xventanacondops`** — *1.0* — Ventana integer conditional operations extension.

<purple>okay and then the ✨rules✨ of the whole march situation:</purple>

When `-march=` is not specified, use the setting from `-mcpu`.

If both `-march` and `-mcpu=` are not specified, the default for this argument is system dependent, users who want a specific architecture extensions should specify one explicitly. <pink>(translation: don't leave it up to fate bestie, be specific or the system picks for you 😭)</pink>

When the RISC-V specifications define an extension as depending on other extensions, GCC will implicitly add the dependent extensions to the enabled extension set if they weren't added explicitly. <mint>(it's giving "we did the homework so you don't have to" 🫶)</mint>

---

### <rainbow>more RISC-V knobs 🎀</rainbow>

- **`-mcpu=`** *processor-string* — Use architecture of and optimize the output for the given processor, specified by particular CPU name. Permissible values for this option are: `sifive-e20`, `sifive-e21`, `sifive-e24`, `sifive-e31`, `sifive-e34`, `sifive-e76`, `sifive-s21`, `sifive-s51`, `sifive-s54`, `sifive-s76`, `sifive-u54`, `sifive-u74`, `sifive-x280`, `sifive-xp450`, `sifive-x670`, `thead-c906`, `tt-ascalon-d8`, `xiangshan-nanhu`.

  <yellow>Note that `-mcpu` does **not** override `-march` or `-mtune`.</yellow> Not me pointing at that like it's the drama of the century 💀

- **`-mtune=`** *processor-string* — Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are: `generic-ooo`, `rocket`, `sifive-3-series`, `sifive-5-series`, `sifive-7-series`, `size`, `sifive-p400-series`, `sifive-p600-series`, and all valid options for `-mcpu=`.

  When `-mtune=` is not specified, use the setting from `-mcpu`, the default is `rocket` if both are not specified.

  The `size` choice is not intended for use by end-users. This is used when `-Os` is specified. It overrides the instruction cost info provided by `-mtune=`, but does not override the pipeline info. This helps reduce code size while still giving good performance. <cyan>(so like, it's an internal thing, not a you thing ✨)</cyan>

- **`-mpreferred-stack-boundary=`** *num* — Attempt to keep the stack boundary aligned to a `2` raised to *num* byte boundary. If `-mpreferred-stack-boundary` is not specified, the default is `4` (`16` bytes or `128`-bits).

  > <red>Warning:</red> If you use this switch, then you must build **all** modules with the same value, including any libraries. This includes the system libraries and startup modules. <hotpink>fr fr do not mix and match here bestie, it will not slay 😭</hotpink>

- **`-msmall-data-limit=`** *n* — Put global and static data smaller than *n* bytes into a special section (on some targets).

- **`-msave-restore`**, **`-mno-save-restore`** — Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues.

- **`-mmovcc`**, **`-mno-movcc`** — Do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable.

- **`-minline-atomics`**, **`-mno-inline-atomics`** — Do or don't use smaller but slower subword atomic emulation code that uses `libatomic` function calls. The default is to use fast inline subword atomics that do not require `libatomic`.

- **`-minline-strlen`**, **`-mno-inline-strlen`** — Do or do not attempt to inline `strlen` calls if possible. Inlining will only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to **not** inline `strlen` calls.

- **`-minline-strcmp`**, **`-mno-inline-strcmp`** — Do or do not attempt to inline `strcmp` calls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to **not** inline `strcmp` calls.

  The `--param riscv-strcmp-inline-limit=`*n* parameter controls the maximum number of bytes compared by the inlined code. The default value is `64`.

- **`-minline-strncmp`**, **`-mno-inline-strncmp`** — Do or do not attempt to inline `strncmp` calls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to **not** inline `strncmp` calls.

  The `--param riscv-strcmp-inline-limit=`*n* parameter controls the maximum number of bytes compared by the inlined code. The default value is `64`. <lavender>(yes same param, no that's not a typo of mine, that's literally how it is 💅)</lavender>

- **`-mshorten-memrefs`**, **`-mno-shorten-memrefs`** — Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of 'base register + large offset' with a new load/store of 'new base + small offset'. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets `32`-bit integer load/stores only.

- **`-mstrict-align`**, **`-mno-strict-align`** — Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not.

- **`-mscalar-strict-align`**, **`-mno-scalar-strict-align`** — Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not. This is an alias for `-mstrict-align`.

- **`-mvector-strict-align`**, **`-mno-vector-strict-align`** — Do not or do generate unaligned **vector** memory accesses. The default is set to off unless the processor we are optimizing for explicitly supports element-misaligned vector memory access.

---

### <pink>code models & relocations 🦋</pink>

- **`-mcmodel=medlow`** — Generate code for the medium-low code model. The program and its statically defined symbols must lie within a single `2 GiB` address range and must lie between absolute addresses `-2 GiB` and `+2 GiB`. Programs can be statically or dynamically linked. This is the default code model unless GCC has been configured with `--with-cmodel=` specifying a different default code model.

- **`-mcmodel=medany`** — Generate code for the medium-any code model. The program and its statically defined symbols must be within any single `2 GiB` address range. Programs can be statically or dynamically linked.

  > <peach>Heads up:</peach> The code generated by the medium-any code model is position-independent, but is **not guaranteed to function correctly** when linked into position-independent executables or libraries. <red>(read that twice, bestie 😭)</red>

- **`-mcmodel=large`** — Generate code for a large code model, which has no restrictions on size or placement of symbols. <green>no limits, main character energy 👑</green>

- **`-mexplicit-relocs`**, **`-mno-exlicit-relocs`** — Use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. <yellow>and yes, `-mno-exlicit-relocs` is spelled exactly like that up there — copying it character for character, iykyk 🙈</yellow>

- **`-mrelax`**, **`-mno-relax`** — Take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to take advantage of linker relaxations. <mint>slay, linker does the heavy lifting 🫶</mint>

- **`-mriscv-attribute`**, **`-mno-riscv-attribute`** — emit (or don't emit, bestie) the RISC-V attribute so extra info gets recorded into ELF objects. Heads up: this feature needs at least binutils 2.32.

- **`-mcsr-check`**, **`-mno-csr-check`** — turns CSR checking on or off. That's it, that's the whole vibe.

- **`-malign-data=type`** — controls how GCC aligns variables and constants of array, structure, or union types. Supported values for *type*:
  - **`xlen`** — uses x register width as the alignment value.
  - **`natural`** — uses natural alignment.

  `xlen` is the default. 🎀

- **`-mbig-endian`** — generate big-endian code. This is the default when GCC is configured for a `riscv64be-*-*` or `riscv32be-*-*` target.

- **`-mlittle-endian`** — generate little-endian code. This is the default when GCC is configured for a `riscv64-*-*` or `riscv32-*-*` but **not** a `riscv64be-*-*` or `riscv32be-*-*` target. Endianness discourse, but make it compiler 💅

- **`-mstack-protector-guard=guard`**, **`-mstack-protector-guard-reg=reg`**, **`-mstack-protector-guard-offset=offset`** — generate stack protection code using a canary at *guard*. Supported locations are:
  - **`global`** — for a global canary, or
  - **`tls`** — for a per-thread canary in the TLS block.

  With the latter choice, the options `-mstack-protector-guard-reg=reg` and `-mstack-protector-guard-offset=offset` furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. There is **no default** register or offset, because this is entirely for use within the Linux kernel. 🐧

- **`-mtls-dialect=desc`** — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.

- **`-mtls-dialect=trad`** — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.

## 🌸 RL78 Options 🌸

- **`-msim`** — links in additional target libraries so your code can actually operate inside a simulator. Pretend-hardware era ✨

- **`-mmul=none`**, **`-mmul=g10`**, **`-mmul=g13`**, **`-mmul=g14`**, **`-mmul=rl78`** — specifies the type of hardware multiplication and division support to be used.
  - **`none`** is the simplest — it uses software for both multiplication and division, and it's the default.
  - **`g13`** is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets.
  - **`g14`** selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts.
  - **`rl78`** is an alias for **`g14`**, and **`mg10`** is an alias for **`none`**.

  In addition, a C preprocessor macro is defined based upon the setting of this option. Possible values are: `__RL78_MUL_NONE__`, `__RL78_MUL_G13__` or `__RL78_MUL_G14__`. Macro girlies rise 🫶

- **`-mcpu=g10`**, **`-mcpu=g13`**, **`-mcpu=g14`**, **`-mcpu=rl78`** — specifies the RL78 core to target.
  - The default is the G14 core, also known as an S3 core or just RL78.
  - The G13 or S2 core does **not** have multiply or divide instructions — instead it uses a hardware peripheral for these operations.
  - The G10 or S1 core does **not** have register banks, so it uses a different calling convention.

  If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit `-mmul=none` option on the command line. So specifying `-mcpu=g13` enables the use of the G13 hardware multiply peripheral, and specifying `-mcpu=g10` disables the use of hardware multiplications altogether.

  > Note: although the RL78/G14 core is the default target, specifying `-mcpu=g14` or `-mcpu=rl78` on the command line **does** change the behavior of the toolchain, since it also enables G14 hardware multiply support. If these options are not specified on the command line then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support. 😭

  In addition, a C preprocessor macro is defined based upon the setting of this option. Possible values are: `__RL78_G10__`, `__RL78_G13__` or `__RL78_G14__`.

- **`-mg10`**, **`-mg13`**, **`-mg14`**, **`-mrl78`** — these are aliases for the corresponding `-mcpu=` option. They are provided for backwards compatibility. Nostalgia-core 🎀

- **`-mallregs`** — allow the compiler to use all of the available registers. By default registers `r24..r31` are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well. Sharing is caring, periodt 💅

- **`-m64bit-doubles`**, **`-m32bit-doubles`** — make the `double` data type be 64 bits (`-m64bit-doubles`) or 32 bits (`-m32bit-doubles`) in size. The default is `-m32bit-doubles`.

- **`-msave-mduc-in-interrupts`**, **`-mno-save-mduc-in-interrupts`** — specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers, since this makes the interrupt handlers faster.
  > The target option `-mg13` needs to be passed for this to work, as this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function.

## 💜 IBM RS/6000 and PowerPC Options 💜

These `-m` options are defined for the IBM RS/6000 and PowerPC:

- **`-mpowerpc-gpopt`**
- **`-mno-powerpc-gpopt`**
- **`-mpowerpc-gfxopt`**
- **`-mno-powerpc-gfxopt`**
- **`-mpowerpc64`**
- **`-mno-powerpc64`**
- **`-mmfcrf`**
- **`-mno-mfcrf`**
- **`-mpopcntb`**
- **`-mno-popcntb`**
- **`-mpopcntd`**
- **`-mno-popcntd`**
- **`-mfprnd`**
- **`-mno-fprnd`**
- **`-mcmpb`**
- **`-mno-cmpb`**
- **`-mhard-dfp`**
- **`-mno-hard-dfp`**

You use these options to specify which instructions are available on the processor you're using. The default value of these options is determined when configuring GCC. Specifying the `-mcpu=cpu_type` overrides the specification of these options. We recommend you use the `-mcpu=cpu_type` option rather than the options listed above. (Trust the glow-up option, bestie ✨)

Specifying `-mpowerpc-gpopt` allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying `-mpowerpc-gfxopt` allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

The `-mmfcrf` option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. The `-mpopcntb` option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture. The `-mpopcntd` option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The `-mfprnd` option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The `-mcmpb` option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. The `-mhard-dfp` option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.

The `-mpowerpc64` option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to `-mno-powerpc64`. 🔥

- **`-mcpu=cpu_type`** — set architecture type, register usage, and instruction scheduling parameters for machine type *cpu_type*. Supported values for *cpu_type* are:

  `401`, `403`, `405`, `405fp`, `440`, `440fp`, `464`, `464fp`, `476`, `476fp`, `505`, `601`, `602`, `603`, `603e`, `604`, `604e`, `620`, `630`, `740`, `7400`, `7450`, `750`, `801`, `821`, `823`, `860`, `970`, `8540`, `a2`, `e300c2`, `e300c3`, `e500mc`, `e500mc64`, `e5500`, `e6500`, `ec603e`, `G3`, `G4`, `G5`, `titan`, `power3`, `power4`, `power5`, `power5+`, `power6`, `power6x`, `power7`, `power8`, `power9`, `power10`, `power11`, `powerpc`, `powerpc64`, `powerpc64le`, `rs64`, and `native`.

  `-mcpu=powerpc`, `-mcpu=powerpc64`, and `-mcpu=powerpc64le` specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

  Specifying `native` as *cpu type* detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. `-mcpu=native` has no effect if GCC does not recognize the processor. (Main character energy: it knows itself 🪞)

  The other options specify a specific processor. Code generated under those options runs best on that processor, and **may not run at all on others**. ⚠️

  The `-mcpu` options automatically enable or disable the following options:

-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion -mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware -mprefixed -mpcrel -mmma -mrop-protect


The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities. If you wish to set an individual option to a particular value, you may specify it after the `-mcpu` option, like `-mcpu=970 -mno-altivec`.

On AIX, the `-maltivec` and `-mpowerpc64` options are **not** enabled or disabled by the `-mcpu` option at present, because AIX does not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment. 🙈

- **`-mtune=cpu_type`** — set the instruction scheduling parameters for machine type *cpu_type*, but **do not** set the architecture type or register usage, the way `-mcpu=cpu_type` does. The same values for *cpu_type* are used for `-mtune` as for `-mcpu`. If both are specified, the code generated uses the architecture and registers set by `-mcpu`, but the scheduling parameters set by `-mtune`. Two moods, one build 😌

- **`-mcmodel=small`** — generate PowerPC64 code for the small model: the TOC is limited to 64k.

- **`-mcmodel=medium`** — bestie this one's the default for 64-bit Linux ✨ it makes PowerPC64 code for the medium model, where the TOC plus the other static data can be up to a total of 4G in size.

- **`-mcmodel=large`** — the big-girl model 🫶 PowerPC64 code for *large*: the TOC can be up to 4G in size, and honestly everything else (data, code) is only limited by the 64-bit address space. Slay.

- **`-maltivec`**, **`-mno-altivec`** — turns AltiVec instruction generation on (and off), plus it unlocks the built-in functions that give you more direct access to the AltiVec instruction set 💅 you might also need to set `-mabi=altivec` to adjust your current ABI with the AltiVec ABI enhancements. iykyk.

Okay so when `-maltivec` is on, the element order for AltiVec intrinsics like `vec_splat`, `vec_extract`, and `vec_insert` matches array element order corresponding to the endianness of the target 🌸 meaning element zero identifies the leftmost element in a vector register when you're targeting a big-endian platform, and identifies the rightmost element in a vector register when you're targeting a little-endian platform. little detail but it MATTERS.

- **`-mvrsave`**, **`-mno-vrsave`** — generate VRSAVE instructions when generating AltiVec code ✨ that's it, that's the whole vibe.

- **`-msecure-plt`** — makes code that lets `ld` and `ld.so` build executables and shared libraries with non-executable `.plt` and `.got` sections 🔥 this is a PowerPC 32-bit SYSV ABI option, no cap.

- **`-mbss-plt`** — the other flavor: generates code that uses a BSS `.plt` section that `ld.so` fills in, and it requires `.plt` and `.got` sections that are both writable **and** executable 💀 also a PowerPC 32-bit SYSV ABI option. security girlies, choose wisely.

- **`-msplit-patch-nops`** — so when you're adding NOPs for a patchable area via the `-fpatchable-function-entry` option, this emits the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point 🎀 which means the sequence of NOPs is *not* consecutive when a global entry point is generated. Without this option the NOPs get emitted directly before and after the local entry point — consecutive, sure, but it moves the global and local entry points further apart. If only a single entry point is generated, this option has no effect, so don't stress 🌈

- **`-misel`**, **`-mno-isel`** — this switch enables or disables the generation of ISEL instructions ⭐ simple as.

- **`-mvsx`**, **`-mno-vsx`** — generate code that uses (or doesn't use) vector/scalar (VSX) instructions, and it also enables the built-in functions that allow more direct access to the VSX instruction set 👑

- **`-mcrypto`**, **`-mno-crypto`** — enables (or disables) the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA 🔐 iconic.

- **`-mhtm`**, **`-mno-htm`** — enables (or disables) the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions added in version 2.07 of the PowerPC ISA 🧚

- **`-mpower8-fusion`**, **`-mno-power8-fusion`** — generates code that keeps (or does **not** keep) some integer operations adjacent so the instructions can be fused together on power8 and later processors 💅 adjacency is the whole point, bestie.

- **`-mquad-memory`**, **`-mno-quad-memory`** — generates code that uses (or doesn't use) the non-atomic quad word memory instructions. Heads up: `-mquad-memory` **requires** use of 64-bit mode 🙈

- **`-mquad-memory-atomic`**, **`-mno-quad-memory-atomic`** — generates code that uses (or doesn't use) the atomic quad word memory instructions. Same rule: `-mquad-memory-atomic` **requires** use of 64-bit mode 💜

- **`-mfloat128`**, **`-mno-float128`** — enables/disables the `__float128` keyword for IEEE 128-bit floating point, and picks either software emulation for IEEE 128-bit floating point or hardware instructions ✨

The VSX instruction set (`-mvsx`) **must** be enabled to use the IEEE 128-bit floating point support, and the IEEE 128-bit floating point is only supported on Linux 🌸

The default for `-mfloat128` is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.

And if you use the ISA 3.0 instruction set (`-mcpu=power9`) on a 64-bit system, the IEEE 128-bit floating point support will *also* enable the generation of ISA 3.0 IEEE 128-bit floating point instructions 🔥 otherwise — if you don't specify to generate ISA 3.0 instructions, or you're targeting a 32-bit big endian system — IEEE 128-bit floating point gets done with software emulation. slower but she works 😭

- **`-mfloat128-hardware`**, **`-mno-float128-hardware`** — enables/disables using ISA 3.0 hardware instructions to support the `__float128` data type 👑

The default for `-mfloat128-hardware` is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.

- **`-m32`**, **`-m64`** — generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux) 🎀 the 32-bit environment sets `int`, `long` and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets `int` to 32 bits and `long` and pointer to 64 bits, and generates code for PowerPC64, as for `-mpowerpc64`.

- **`-mfull-toc`**, **`-mno-fp-in-toc`**, **`-mno-sum-in-toc`**, **`-mminimal-toc`** — these four modify generation of the TOC (Table Of Contents), which is created for every executable file 🦋 `-mfull-toc` is the default, and in that case GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. But — and this is the plot twist — only 16,384 entries are available in the TOC 💀

So if you get a linker error message saying you've overflowed the available TOC space, you can reduce the amount of TOC space used with the `-mno-fp-in-toc` and `-mno-sum-in-toc` options: `-mno-fp-in-toc` stops GCC from putting floating-point constants in the TOC, and `-mno-sum-in-toc` forces GCC to generate code that calculates the sum of an address and a constant at run time instead of putting that sum into the TOC. You can specify one or both 🙌 each one makes GCC produce very slightly slower and larger code, as a trade for conserving TOC space. the sacrifice 💅

And if you *still* run out of space in the TOC even with both of those, specify `-mminimal-toc` instead — it makes GCC create only one TOC entry for every file 🎀 when you use it, GCC produces code that's slower and larger but uses extremely little TOC space. Maybe only use this one on files that contain less frequently-executed code, bestie. you know, the flop files 😭

- **`-maix64`**, **`-maix32`** — enables the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit `long` type, and the whole infrastructure needed to support them 👑 specifying `-maix64` implies `-mpowerpc64`, while `-maix32` disables the 64-bit ABI and implies `-mno-powerpc64`. GCC defaults to `-maix32`.

- **`-mxl-compat`**, **`-mno-xl-compat`** — produces code that conforms more closely to IBM XL compiler semantics when using the AIX-compatible ABI 🌸 it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs, doesn't assume that the most significant double in a 128-bit `long double` value is properly rounded when comparing values and converting to `double`, and uses XL symbol names for `long double` support routines.

The lore 🫶 the AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that don't fit in the RSA from the stack when a subroutine is compiled without optimization. And because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is **not** enabled by default — it's only necessary when calling subroutines compiled by IBM XL compilers without optimization. niche but valid 💜

- **`-mpe`** — supports IBM RS/6000 SP Parallel Environment (PE) ✨ it links an application written to use message passing with special startup code so the application can actually run. The system must have PE installed in the standard location (`/usr/lpp/ppe.poe/`), or the specs file must be overridden with the `-specs=` option to specify the appropriate directory location 💅 and the Parallel Environment does **not** support threads, so the `-mpe` option and the `-pthread` option are incompatible. pick a lane, bestie 😭

- **`-malign-natural`**, **`-malign-power`** — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, `-malign-natural` overrides the ABI-defined alignment of larger types (like floating-point doubles) on their natural size-based boundary, while `-malign-power` tells GCC to follow the ABI-specified alignment rules 🎀 GCC defaults to the standard alignment defined in the ABI.

But on 64-bit Darwin, natural alignment is the default and `-malign-power` is not supported 🙈

- **`-msoft-float`**, **`-mhard-float`** — generate code that does not use (or does use) the floating-point register set 🦋 software floating-point emulation is provided if you use `-msoft-float`, and you have to pass the option to GCC when linking too. don't forget the link step, that's where girlies get caught 💀

- **`-mmultiple`**, **`-mno-multiple`** — generate code that uses (or doesn't use) the load multiple word instructions and the store multiple word instructions ⭐ these are generated by default on POWER systems, and not generated on PowerPC systems. Do **not** use `-mmultiple` on little-endian PowerPC systems, because those instructions don't work when the processor is in little-endian mode 💀 the exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. the exceptions ate, honestly.

- **`-mupdate`**, **`-mno-update`** — generate code that uses (or doesn't use) the load or store instructions that update the base register to the address of the calculated memory location 🌸 these are generated by default. But if you use `-mno-update`, there's a small window between the time the stack pointer is updated and the address of the previous frame is stored — which means code that walks the stack frame across interrupts or signals may get corrupted data 😭 so, like, be aware.

- **`-mavoid-indexed-addresses`**, **`-mno-avoid-indexed-addresses`** — generate code that tries to avoid (or not avoid) the use of indexed load or store instructions 💅 these can incur a performance penalty on Power6 processors in certain situations, like when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise ✨

- **`-mfused-madd`**, **`-mno-fused-madd`** — ok so this one tells the compiler to use (or not use, hence the `no`) the floating-point multiply-and-accumulate instructions ✨. Fun fact: they're generated *by default* whenever hardware floating point is in play. Also heads up 🫶 — the machine-dependent `-mfused-madd` option is now mapped to the machine-independent `-ffp-contract=fast` option, and `-mno-fused-madd` is mapped to `-ffp-contract=off`. it's giving "these flags got a glow-up and moved houses" 💅

- **`-mmulhw`**, **`-mno-mulhw`** — generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors 💖. These bad boys are generated by default when you're targeting those processors, no cap.

- **`-mdlmzb`**, **`-mno-dlmzb`** — generate code that uses (does not use) the string-search `dlmzb` instruction on the IBM 405, 440, 464 and 476 processors 🌸. This instruction is generated by default when targeting those processors. she's iconic and she knows it 👑

- **`-mno-bit-align`**, **`-mbit-align`** — on System V.4 and embedded PowerPC systems, do **not** (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

So like, for example 🍓: by default a structure containing nothing but 8 `unsigned` bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. But the moment you use `-mno-bit-align`, that structure is aligned to a 1-byte boundary and is 1 byte in size. tiny! smol! 🧚

- **`-mno-strict-align`**, **`-mstrict-align`** — on System V.4 and embedded PowerPC systems, do **not** (do) assume that unaligned memory references are handled by the system ⭐.

- **`-mrelocatable`**, **`-mno-relocatable`** — generate code that allows (does not allow) a static executable to be relocated to a different address at run time 💜. A simple embedded PowerPC system loader should relocate the entire contents of `.got2` and 4-byte locations listed in the `.fixup` section — that's a table of 32-bit addresses generated by this option, iykyk. For this to actually work, **all** objects linked together must be compiled with `-mrelocatable` or `-mrelocatable-lib`. Also, `-mrelocatable` code aligns the stack to an 8-byte boundary 🔥.

- **`-mrelocatable-lib`**, **`-mno-relocatable-lib`** — like `-mrelocatable`, `-mrelocatable-lib` generates a `.fixup` section to allow static executables to be relocated at run time, but `-mrelocatable-lib` does **not** use the smaller stack alignment of `-mrelocatable` 😌. The good news? Objects compiled with `-mrelocatable-lib` may be linked with objects compiled with any combination of the `-mrelocatable` options. mix and match bestie, it's fine 🫶

- **`-mno-toc`**, **`-mtoc`** — on System V.4 and embedded PowerPC systems, do **not** (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program ✨.

- **`-mlittle`**, **`-mlittle-endian`** — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode 🌈. The `-mlittle-endian` option is *the same as* `-mlittle`. same vibe, longer name 💅

- **`-mbig`**, **`-mbig-endian`** — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode 🌸. And yes, `-mbig-endian` is *the same as* `-mbig`. twins fr fr 👯

- **`-mdynamic-no-pic`** — on Darwin / macOS systems, compile code so that it is not relocatable, but that its external references *are* relocatable 🍓. The resulting code is suitable for applications, but **not** shared libraries. so like, apps yes, libs no 🙈

- **`-msingle-pic-base`** — treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function 💜. The runtime system is responsible for initializing this register with an appropriate value before execution begins. so that's on the runtime, not on you — no pressure 😭

- **`-mprioritize-restricted-insns=`***priority* — this option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass 👑. The argument *priority* takes the value `0`, `1`, or `2` to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. pick your fighter ⭐

- **`-msched-costly-dep=`***dependence_type* — this option controls which dependences are considered costly by the target during instruction scheduling 🔥. The argument *dependence_type* takes one of the following values:

- `no` — no dependence is costly. chill. nothing matters 💀
- `all` — all dependences are costly. everything matters. anxiety mode 😭
- `true_store_to_load` — a true dependence from store to load is costly.
- `store_to_load` — **any** dependence from store to load is costly.
- *number* — any dependence for which the latency is greater than or equal to *number* is costly.

- **`-minsert-sched-nops=`***scheme* — this option controls which NOP insertion scheme is used during the second scheduling pass 🎀. The argument *scheme* takes one of the following values:

- `no` — don't insert NOPs. periodt 💅
- `pad` — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
- `regroup_exact` — insert NOPs to force costly dependent insns into separate groups. Insert **exactly** as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping ✨.
- *number* — insert NOPs to force costly dependent insns into separate groups. Insert *number* NOPs to force an insn to a new group.

- **`-mcall-sysv`** — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement 🌸. This is the default unless you configured GCC using `powerpc-*-eabiaix`. vintage, but make it default 😌

- **`-mcall-sysv-eabi`**, **`-mcall-eabi`** — specify **both** `-mcall-sysv` and `-meabi` options. two for the price of one 🫶

- **`-mcall-sysv-noeabi`** — specify **both** `-mcall-sysv` and `-mno-eabi` options. the opposite combo meal 💜

- **`-mcall-aixdesc`** — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system ⭐.

- **`-mcall-linux`** — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system 🐧💖.

- **`-mcall-freebsd`** — on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system 🔥.

- **`-mcall-netbsd`** — on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system 🌈.

- **`-mcall-openbsd`** — on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system 🐡.

- **`-mtraceback=`***traceback_type* — select the type of traceback table 👑. Valid values for *traceback_type* are `full`, `part`, and `no`. short list, pick one 💅

- **`-maix-struct-return`** — return all structures in memory (as specified by the AIX ABI) 🎀.

- **`-msvr4-struct-return`** — return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI) ✨.

- **`-mabi=`***abi-type* — extend the current ABI with a particular extension, or remove such extension 🦋. Valid values are: `altivec`, `no-altivec`, `ibmlongdouble`, `ieeelongdouble`, `elfv1`, `elfv2`, and for AIX: `vec-extabi`, `vec-default`. that's the whole menu, bestie 📋

- **`-mabi=ibmlongdouble`** — change the current ABI to use IBM extended-precision `long double` 💜. This is **not likely to work** if your system defaults to using IEEE extended-precision `long double` ⚠️. If you change the `long double` type from IEEE extended-precision, the compiler issues a warning unless you use the `-Wno-psabi` option. Requires `-mlong-double-128` to be enabled. requirements, requirements 🙄

- **`-mabi=ieeelongdouble`** — change the current ABI to use IEEE extended-precision `long double` 🌸. This is **not likely to work** if your system defaults to using IBM extended-precision `long double` ⚠️. If you change the `long double` type from IBM extended-precision, the compiler issues a warning unless you use the `-Wno-psabi` option. Requires `-mlong-double-128` to be enabled. yes it's the mirror image of the last one, no I'm not kidding 😭

- **`-mabi=elfv1`** — change the current ABI to use the ELFv1 ABI 🔥. This is the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is **likely to fail in spectacular ways** 💀 like genuinely spectacular. don't say I didn't warn you.

- **`-mabi=elfv2`** — change the current ABI to use the ELFv2 ABI ✨. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is **likely to fail in spectacular ways** 💀 same energy, different endianness 🫠

- **`-mgnu-attribute`**, **`-mno-gnu-attribute`** — emit `.gnu_attribute` assembly directives to set tag/value pairs in a `.gnu.attributes` section that specify ABI variations in function parameters or return values 🎀.

- **`-mprototype`**, **`-mno-prototype`** — on System V.4 and embedded PowerPC systems, assume that **all** calls to variable argument functions are properly prototyped 💅. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (`CR`) to indicate whether floating-point values are passed in the floating-point registers — in case the function takes variable arguments, you know how it is 😭. With `-mprototype`, only calls to prototyped variable argument functions set or clear the bit. so much less busywork ✨

- **`-msim`** — on embedded PowerPC systems, assume that the startup module is called `sim-crt0.o` and that the standard C libraries are `libsim.a` and `libc.a` 🦋. This is the default for `powerpc-*-eabisim` configurations ⭐.

- **`-mmvme`** — on embedded PowerPC systems, assume that the startup module is called `crt0.o` and the standard C libraries are `libmvme.a` and `libc.a` 🌸.

- **`-mads`** — on embedded PowerPC systems, assume that the startup module is called `crt0.o` and the standard C libraries are `libads.a` and `libc.a` 🍓.

- **`-myellowknife`** — on embedded PowerPC systems, assume that the startup module is called `crt0.o` and the standard C libraries are `libyk.a` and `libc.a` 💛🔪 (yes that's the name, I don't make the rules)

- **`-mvxworks`** — on System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system 🔥.

- **`-memb`** — on embedded PowerPC systems, set the `PPC_EMB` bit in the ELF flags header to indicate that eabi extended relocations are used ✨.

- **`-meabi`**, **`-mno-eabi`** — on System V.4 and embedded PowerPC systems, do (do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications 👑. ok so here's the breakdown, because this one's a whole personality shift:

- Selecting `-meabi` means that the stack is aligned to an 8-byte boundary, a function `__eabi` is called from `main` to set up the EABI environment, and the `-msdata` option can use **both** `r2` and `r13` to point to two separate small data areas 💖. two data areas, living their best lives.
- Selecting `-mno-eabi` means that the stack is aligned to a 16-byte boundary, **no** EABI initialization function is called from `main`, and the `-msdata` option only uses `r13` to point to a single small data area 🌸.
- The `-meabi` option is **on by default** if you configured GCC using one of the `powerpc*-*-eabi*` options ⭐.

---

- **`-msdata=eabi`** — okay so on System V.4 and embedded PowerPC systems this one puts your small initialized `const` global and static data into the `.sdata2` section, which is pointed to by register `r2` 🎀. Small initialized non-`const` global and static data goes in `.sdata`, pointed to by register `r13`. And small *uninitialized* global and static data lands in `.sbss`, right next door to `.sdata`. Heads up though: `-msdata=eabi` is **incompatible** with `-mrelocatable`, no cap. Also it sets `-memb` for you. 💅

- **`-msdata=sysv`** — same vibe, System V.4 and embedded PowerPC: small global and static data goes in `.sdata` (pointed to by register `r13`), and small uninitialized global and static data goes in `.sbss`, adjacent to `.sdata`. And yeah, `-msdata=sysv` is **incompatible** with `-mrelocatable` too. 🦋

- **`-msdata=default`**, **`-msdata`** — on System V.4 and embedded PowerPC systems: if `-meabi` is used, it compiles exactly like `-msdata=eabi`, otherwise it compiles exactly like `-msdata=sysv`. Pick a lane, bestie ✨

- **`-msdata=data`** — System V.4 and embedded PowerPC again: small *global* data goes in `.sdata`, small uninitialized global data goes in `.sbss`. But — and this is the plot twist — it does **not** use register `r13` to address small data. This is literally the default behavior unless you pass other `-msdata` options. 🌸

- **`-msdata=none`**, **`-mno-sdata`** — on embedded PowerPC systems, all initialized global and static data goes in `.data`, and all uninitialized data goes in `.bss`. Clean, simple, iconic. 🫶

- **`-mreadonly-in-sdata`** — puts read-only objects in the `.sdata` section as well. And yes, this is the default. 👑

- **`-mblock-move-inline-limit=`*num* — inlines all block moves (think calls to `memcpy` or structure copies) that are less than or equal to *num* bytes. Minimum for *num* is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. Default value is target-specific, iykyk. ⭐

- **`-mblock-compare-inline-limit=`*num* — generates non-looping inline code for all block compares (like calls to `memcmp` or structure compares) less than or equal to *num* bytes. If *num* is `0`, all inline expansion (non-loop *and* loop) of block compare is disabled, periodt. Default value is target-specific. 💖

- **`-mblock-compare-inline-loop-limit=`*num* — generates an inline expansion using **loop** code for all block compares less than or equal to *num* bytes, but greater than the limit for non-loop inline block compare expansion. If the block length isn't constant, at most *num* bytes get compared before `memcmp` is called to handle the rest of the block. Default is target-specific. 🧚

- **`-mstring-compare-inline-limit=`*num* — compares at most *num* string bytes with inline code. If the difference or end of string isn't found at the end of the inline compare, a call to `strcmp` or `strncmp` takes care of the rest. Default is `64` bytes. 🍓

- **`-G`** *num* — on embedded PowerPC systems, this puts global and static items less than or equal to *num* bytes into the small data or BSS sections instead of the normal data or BSS section. Default *num* is `8`. The `-G` *num* switch also gets passed to the linker, and **all** modules should be compiled with the same `-G` *num* value — like, matching outfits, bestie, or it all falls apart 💀

- **`-mregnames`**, **`-mno-regnames`** — on System V.4 and embedded PowerPC systems, these do (or don't) emit register names in the assembly language output using symbolic forms. 🦋

- **`-mlongcall`**, **`-mno-longcall`** — by default GCC assumes all calls are far away, so a longer and more expensive calling sequence is required. This is needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't be that far away. You can override this setting with the `shortcall` function attribute, or with `#pragma longcall(0)`. 😭

Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On those systems long calls are unnecessary and just generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see `-mpltseq`). PowerPC with `-mbss-plt` and PowerPC64 ELFv1 (big-endian) do **not** support inline PLT calls.

On Darwin/PPC systems, `#pragma longcall` generates `jbsr callee, L42`, plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates `bl callee` if the PPC `bl` instruction reaches the callee directly; otherwise the linker generates `bl L42` to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it.

On Mach-O (Darwin) systems, this option directs the compiler to emit to the glue for every direct call, and the Darwin linker decides whether to use it or discard it.

In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. Future vibes only 🔥

- **`-mpltseq`**, **`-mno-pltseq`** — implements (or doesn't implement) `-fno-plt` and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in `dlopen`'d shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if the support is detected when configuring GCC, and — for 32-bit PowerPC — if GCC is configured with `--enable-secureplt`. Warning bestie: `-mpltseq` code and `-mbss-plt` 32-bit PowerPC relocatable objects may not be linked together. 🙈

- **`-mtls-markers`**, **`-mno-tls-markers`** — marks (or doesn't mark) calls to `__tls_get_addr` with a relocation specifying the function argument. That relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. Slay. 💅

- **`-mrecip`**, **`-mno-recip`** — enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to boost precision, instead of doing a divide or square root and divide for floating-point arguments. You should use `-ffast-math` when using `-mrecip` (or at least `-funsafe-math-optimizations`, `-ffinite-math-only`, `-freciprocal-math` and `-fno-trapping-math`). Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp — i.e. the inverse of `1.0` equals `0.99999994` — for reciprocal square roots. Big accuracy vibes check 😭

- **`-mrecip=`*opt* — controls which reciprocal estimate instructions may be used. *opt* is a comma-separated list of options, and any of them may be preceded by a `!` to invert the option:

- `all` — enable all estimate instructions.
- `default` — enable the default instructions, equivalent to `-mrecip`.
- `none` — disable all estimate instructions, equivalent to `-mno-recip`.
- `div` — enable the reciprocal approximation instructions for both single and double precision.
- `divf` — enable the single-precision reciprocal approximation instructions.
- `divd` — enable the double-precision reciprocal approximation instructions.
- `rsqrt` — enable the reciprocal square root approximation instructions for both single and double precision.
- `rsqrtf` — enable the single-precision reciprocal square root approximation instructions.
- `rsqrtd` — enable the double-precision reciprocal square root approximation instructions.

So, for example, `-mrecip=all,!rsqrtd` enables all of the reciprocal estimate instructions except for the `FRSQRTE`, `XSRSQRTEDP`, and `XVRSQRTEDP` instructions, which handle the double-precision reciprocal square root calculations. Not me excluding exactly those three 🎀

- **`-mrecip-precision`**, **`-mno-recip-precision`** — assume (or don't assume) that the reciprocal estimate instructions provide higher-precision estimates than the PowerPC ABI mandates. Selecting `-mcpu=power6`, `-mcpu=power7` or `-mcpu=power8` automatically selects `-mrecip-precision`. The double-precision square root estimate instructions are **not** generated by default on low-precision machines, since they don't provide an estimate that converges after three steps. 🌈

- **`-mveclibabi=`*type* — specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is `mass`, which says to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to `acosd2`, `acosf4`, `acoshd2`, `acoshf4`, `asind2`, `asinf4`, `asinhd2`, `asinhf4`, `atan2d2`, `atan2f4`, `atand2`, `atanf4`, `atanhd2`, `atanhf4`, `cbrtd2`, `cbrtf4`, `cosd2`, `cosf4`, `coshd2`, `coshf4`, `erfcd2`, `erfcf4`, `erfd2`, `erff4`, `exp2d2`, `exp2f4`, `expd2`, `expf4`, `expm1d2`, `expm1f4`, `hypotd2`, `hypotf4`, `lgammad2`, `lgammaf4`, `log10d2`, `log10f4`, `log1pd2`, `log1pf4`, `log2d2`, `log2f4`, `logd2`, `logf4`, `powd2`, `powf4`, `sind2`, `sinf4`, `sinhd2`, `sinhf4`, `sqrtd2`, `sqrtf4`, `tand2`, `tanf4`, `tanhd2`, and `tanhf4` when generating code for power7. Both `-ftree-vectorize` and `-funsafe-math-optimizations` must also be enabled. The MASS libraries must be specified at link time. 🔥

- **`-mfriz`**, **`-mno-friz`** — generate (or don't generate) the `friz` instruction when the `-funsafe-math-optimizations` option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Careful though: the `friz` instruction does **not** return the same value if the floating-point number is too large to fit in an integer. ⚠️

- **`-mpointers-to-nested-functions`, `-mno-pointers-to-nested-functions`** — okay bestie, this pair controls whether GCC generates code to load up the static chain register (`r11`) when you call through a pointer on <pink>AIX</pink> and <pink>64-bit Linux</pink> systems where a function pointer points to a <hotpink>3-word descriptor</hotpink> giving the function address, the TOC value to be loaded in register `r2`, and the static chain value to be loaded in register `r11`. The `-mpointers-to-nested-functions` is <green>on by default</green>. But heads up: you <red>cannot</red> call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use `-mno-pointers-to-nested-functions`. No cap.

- **`-msave-toc-indirect`, `-mno-save-toc-indirect`** — Generate (or don’t generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on <pink>AIX</pink> and <pink>64-bit Linux</pink> systems. If the TOC value is not saved in the prologue, it gets saved just before the call through the pointer. The `-mno-save-toc-indirect` option is the <yellow>default</yellow>.

- **`-mcompat-align-parm`, `-mno-compat-align-parm`** — Generate (or don’t generate) code to pass structure parameters with a maximum alignment of <hotpink>64 bits</hotpink>, for compatibility with older versions of GCC. Older versions of GCC (prior to <purple>4.9.0</purple>) incorrectly did not align a structure parameter on a <hotpink>128-bit boundary</hotpink> when that structure contained a member requiring <hotpink>128-bit alignment</hotpink>. This is corrected in more recent versions of GCC. This option may be used to generate code that is compatible with functions compiled with older versions of GCC. The `-mno-compat-align-parm` option is the <yellow>default</yellow>. 💅

- **`-mstack-protector-guard=guard`, `-mstack-protector-guard-reg=reg`, `-mstack-protector-guard-offset=offset`** — Generate stack protection code using a canary at *guard*. Supported locations are `global` for the global canary or `tls` for the per-thread canary in the TLS block (the default with <purple>GNU libc version 2.4 or later</purple>). With the latter choice, the options `-mstack-protector-guard-reg=reg` and `-mstack-protector-guard-offset=offset` furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. 🎀

- **`-mpcrel`, `-mno-pcrel`** — Generate (or don’t generate) pc-relative addressing. The `-mpcrel` option requires that the medium code model (`-mcmodel=medium`) and prefixed addressing (`-mprefixed`) options are enabled. Lowkey a package deal, bestie.

- **`-mprefixed`, `-mno-prefixed`** — Generate (or don’t generate) addressing modes using prefixed load and store instructions. The `-mprefixed` option requires that the option `-mcpu=power10` (or later) is enabled. ✨

- **`-mmma`, `-mno-mma`** — Generate (or don’t generate) the MMA instructions. The `-mma` option requires that the option `-mcpu=power10` (or later) is enabled. Slay, but only on the right CPU.

- **`-mrop-protect`, `-mno-rop-protect`** — Generate (or don’t generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (`-fshrink-wrap`). It’s giving security but also trade-offs, iykyk.

- **`-mprivileged`, `-mno-privileged`** — Generate (or don’t generate) code that will run in privileged state. Main character energy only.

- **`-mblock-ops-unaligned-vsx`, `-mno-block-ops-unaligned-vsx`** — Generate (or don’t generate) unaligned vsx loads and stores for inline expansion of `memcpy` and `memmove`. 🫶

- **`--param rs6000-vect-unroll-limit=`** — The vectorizer will check with target information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is <hotpink>four</hotpink>. ⭐

---

## 🦋 RX Options 🦋

These command-line options are defined for RX targets:

- **`-m64bit-doubles`, `-m32bit-doubles`** — Make the `double` data type be <hotpink>64 bits</hotpink> (`-m64bit-doubles`) or <hotpink>32 bits</hotpink> (`-m32bit-doubles`) in size. The default is `-m32bit-doubles`. Note: RX floating-point hardware only works on 32-bit values, which is why the default is `-m32bit-doubles`. It’s literally built different.

- **`-fpu`, `-nofpu`** — Enables (`-fpu`) or disables (`-nofpu`) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series. Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the `-m64bit-doubles` option is used. Note: if the `-fpu` option is enabled then `-funsafe-math-optimizations` is also enabled automatically. This is because the RX FPU instructions are themselves <red>unsafe</red>. 🙈

- **`-mcpu=name`** — Selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware and the specific RX610 CPU. The default is <green>RX600</green>. The only difference between RX600 and RX610 is that the RX610 does not support the `MVTIPL` instruction. The RX200 series does not have a hardware floating-point unit and so `-nofpu` is enabled by default when this type is selected. 🎀

- **`-mbig-endian-data`, `-mlittle-endian-data`** — Store data (but not code) in the big-endian format. The default is `-mlittle-endian-data`, i.e. to store data in the little-endian format. Cute and consistent.

- **`-msmall-data-limit=N`** — Specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow. Also, when the small data area is used, one of the RX’s registers (usually `r13`) is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register. Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable. The default value is <hotpink>zero</hotpink>, which disables this feature. Note: this feature is not enabled by default with higher optimization levels (`-O2` etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the `-mpid` option for a description of how the actual register to hold the small data area pointer is chosen. 🍓

- **`-msim`, `-mno-sim`** — Use the simulator runtime. The default is to use the libgloss board-specific runtime. Two whole vibes.

- **`-mas100-syntax`, `-mno-as100-syntax`** — When generating assembler output, use a syntax that is compatible with Renesas’s AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default. 💅

- **`-mmax-constant-size=N`** — Specifies the maximum size, in bytes, of a constant that can be used as an operand in an RX instruction. Although the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. Thus, in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection. The value *N* can be between <hotpink>0</hotpink> and <hotpink>4</hotpink>. A value of <hotpink>0</hotpink> (the default) or <hotpink>4</hotpink> means that constants of any size are allowed. ⭐

- **`-mrelax`** — Enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default. Lowkey iconic when it works.

- **`-mint-register=N`** — Specify the number of registers to reserve for fast interrupt handler functions. The value *N* can be between <hotpink>0</hotpink> and <hotpink>4</hotpink>. A value of <hotpink>1</hotpink> means that register `r13` is reserved for the exclusive use of fast interrupt handlers. A value of <hotpink>2</hotpink> reserves `r13` and `r12`. A value of <hotpink>3</hotpink> reserves `r13`, `r12` and `r11`, and a value of <hotpink>4</hotpink> reserves `r13` through `r10`. A value of <hotpink>0</hotpink>, the default, does not reserve any registers. 🙌

- **`-msave-acc-in-interrupts`** — Specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator as this makes the interrupt handlers faster. ⚡

- **`-mpid`, `-mno-pid`** — Enables the generation of position independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option. Note: using this feature reserves a register, usually `r13`, for the constant data base address. This can result in slower and/or larger code, especially in complicated functions. The actual register chosen to hold the constant data base address depends upon whether the `-msmall-data-limit` and/or the `-mint-register` command-line options are enabled. Starting with register `r13` and proceeding downwards, registers are allocated first to satisfy the requirements of `-mint-register`, then `-mpid` and finally `-msmall-data-limit`. Thus it is possible for the small data area register to be `r8` if both `-mint-register=4` and `-mpid` are specified on the command line. By default this feature is not enabled. The default can be restored via the `-mno-pid` command-line option. 🌈

- **`-mno-warn-multiple-fast-interrupts`, `-mwarn-multiple-fast-interrupts`** — Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt. 💀

- **`-mallow-string-insns`**, **`-mno-allow-string-insns`** — turns on (or off) the string-manipulation instructions `"SMOVF"`, `"SCMPU"`, `"SMOVB"`, `"SMOVU"`, `"SUNTIL"` `"SWHILE"` and also the `"RMPA"` instruction. Lowkey important: these instructions may <red>prefetch data</red>, which is <red>not safe</red> to do if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for the deets.)

The default is to <green>allow</green> these instructions, but GCC literally cannot reliably detect every circumstance where a string instruction might be used to touch an I/O register, so it can't turn them off automatically. Instead it's on the programmer — you, bestie — to use `-mno-allow-string-insns` if your program accesses I/O space. 🫶

When the instructions are enabled, GCC defines the C preprocessor symbol `"__RX_ALLOW_STRING_INSNS__"`; otherwise it defines `"__RX_DISALLOW_STRING_INSNS__"`. ✨

- **`-mjsr`**, **`-mno-jsr`** — use <italic>only</italic> (or not only) `"JSR"` instructions to access functions. This one's your girl when code size exceeds the range of `"BSR"` instructions. And heads up: `-mno-jsr` does <italic>not</italic> mean "don't use `"JSR"`" — it means any type of branch may be used. 💅

> **Note:** the generic GCC command-line option `-ffixed-reg` has special significance to the RX port when used with the `"interrupt"` function attribute. That attribute indicates a function intended to process fast interrupts. GCC makes sure it only uses the registers `"r10"`, `"r11"`, `"r12"` and/or `"r13"` — and only provided that the normal use of the corresponding registers has been restricted via the `-ffixed-reg` or `-mint-register` command-line options. 🎀

---

## 🌸 S/390 and zSeries Options 🌸

These are the `-m` options defined for the S/390 and zSeries architecture. Big iron energy, fr fr. 👑

- **`-mhard-float`**, **`-msoft-float`** — use (or don't use) the hardware floating-point instructions and registers for floating-point ops. With `-msoft-float`, functions in `libgcc.a` do the floating-point work. With `-mhard-float`, the compiler generates IEEE floating-point instructions. `-mhard-float` is the default. 💖

- **`-mhard-dfp`**, **`-mno-hard-dfp`** — use (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations. Specify `-mno-hard-dfp` and functions in `libgcc.a` handle the decimal-floating-point math. Specify `-mhard-dfp` and the compiler generates decimal-floating-point hardware instructions. This is the default for `-march=z9-ec` or higher. ✨

- **`-mlong-double-64`**, **`-mlong-double-128`** — these switches control the size of the `"long double"` type. 64 bits makes `"long double"` equivalent to `"double"`. That's the default, bestie.

- **`-mbackchain`**, **`-mno-backchain`** — store (or don't store) the address of the caller's frame as a backchain pointer into the callee's stack frame. A backchain may be needed so tools that don't understand DWARF call frame information can debug stuff. When `-mno-packed-stack` is in effect, the backchain pointer goes at the bottom of the stack frame; when `-mpacked-stack` is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area. 🧚

Generally speaking, code compiled with `-mbackchain` is call-compatible with code compiled with `-mno-backchain`; <italic>however</italic>, using the backchain for debugging purposes usually requires the whole binary be built with `-mbackchain`. Note that the combo of `-mbackchain`, `-mpacked-stack` and `-mhard-float` is <red>not supported</red>. In order to build a linux kernel, use `-msoft-float`. 🐧

The default is to not maintain the backchain.

- **`-mpacked-stack`**, **`-mno-packed-stack`** — use (or don't use) the packed stack layout. With `-mno-packed-stack`, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still eat up stack space. With `-mpacked-stack`, register save slots are densely packed at the top of the register save area, and unused space gets reused for other stuff — way more efficient use of the stack space you've got. But when `-mbackchain` is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 😭

As long as the stack frame backchain isn't used, code generated with `-mpacked-stack` is call-compatible with code generated with `-mno-packed-stack`. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging — that code is <red>not</red> call-compatible with code compiled with `-mpacked-stack`. Also note the combination of `-mbackchain`, `-mpacked-stack` and `-mhard-float` is <red>not supported</red>. In order to build a linux kernel, use `-msoft-float`. 🐧

The default is to not use the packed stack layout.

- **`-msmall-exec`**, **`-mno-small-exec`** — generate (or don't generate) code using the `"bras"` instruction for subroutine calls. This only works reliably if the total executable size doesn't exceed 64k. The default is to use the `"basr"` instruction instead, which doesn't have that limitation. 💅

- **`-m64`**, **`-m31`** — with `-m31`, generate code compliant to the GNU/Linux for S/390 ABI. With `-m64`, generate code compliant to the GNU/Linux for zSeries ABI. This lets GCC in particular generate 64-bit instructions. For the s390 targets the default is `-m31`, while s390x targets default to `-m64`. 🔥

- **`-mzarch`**, **`-mesa`** — with `-mzarch`, generate code using the instructions available on z/Architecture. With `-mesa`, generate code using the instructions available on ESA/390. Note that `-mesa` is <red>not possible</red> with `-m64`. When generating code compliant to the GNU/Linux for S/390 ABI, the default is `-mesa`. When generating code compliant to the GNU/Linux for zSeries ABI, the default is `-mzarch`. ⭐

- **`-mhtm`**, **`-mno-htm`** — `-mhtm` enables a set of builtins that make use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. `-mhtm` is enabled by default when using `-march=zEC12`. 🦋

- **`-mvx`**, **`-mno-vx`** — with `-mvx`, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option <red>changes the ABI</red> for some vector type values with regard to alignment and calling conventions. If vector type values are being used in an ABI-relevant context, a GAS `.gnu_attribute` command gets added to mark the resulting binary with the ABI used. `-mvx` is enabled by default when using `-march=z13`. 🌈

- **`-mzvector`**, **`-mno-zvector`** — `-mzvector` enables vector language extensions and builtins using instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option adds support for `vector` to be used as a keyword to define vector type variables and arguments. `vector` is only available when GNU extensions are enabled — it will <italic>not</italic> be expanded when requesting strict standard compliance, e.g. with `-std=c99`. Besides the GCC low-level builtins, `-mzvector` enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To make use of these builtins you need to include the header file `vecintrin.h`. `-mzvector` is disabled by default. 💀

- **`-mmvcle`**, **`-mno-mvcle`** — generate (or don't generate) code using the `"mvcle"` instruction to perform block moves. With `-mno-mvcle`, use a `"mvc"` loop instead. That's the default unless optimizing for size. 🎀

- **`-mdebug`**, **`-mno-debug`** — print (or don't print) additional debug information when compiling. The default is to not print debug information.

- **`-march=cpu-type`** — generate code that runs on *cpu-type*, which is the name of a system representing a certain processor type. Possible values for *cpu-type* are `z900/arch5`, `z990/arch6`, `z9-109`, `z9-ec/arch7`, `z10/arch8`, `z196/arch9`, `zEC12`, `z13/arch11`, `z14/arch12`, `z15/arch13`, `z16/arch14`, `z17/arch15`, and `native`. 👑

The default is `-march=z900`.

Specifying `native` as *cpu type* can be used to select the best architecture option for the host processor. `-march=native` has no effect if GCC doesn't recognize the processor. 🙈

- **`-mtune=cpu-type`** — tune to *cpu-type* everything applicable about the generated code, except for the ABI and the set of available instructions. The list of *cpu-type* values is the same as for `-march`. The default is the value used for `-march`. ✨

- **`-mtpf-trace`**, **`-mno-tpf-trace`** — generate code that adds (or doesn't add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS. 🌸

- **`-mtpf-trace-skip`**, **`-mno-tpf-trace-skip`** — generate code that changes (or doesn't change) the default branch targets enabled by `-mtpf-trace` to point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying `-mtpf-trace`. 💜

- **`-mfused-madd`**, **`-mno-fused-madd`** — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. 🔥

- **`-mwarn-framesize=framesize`** — emit a warning if the current function exceeds the given frame size. Since this is a compile-time check, it doesn't need to be a real problem when the program runs. It's meant to identify functions that most probably cause a stack overflow. Super useful in an environment with limited stack size, e.g. the linux kernel. 🐧

- **`-mwarn-dynamicstack`** — emit a warning if the function calls `"alloca"` or uses dynamically-sized arrays. This is generally a <red>bad idea</red> with a limited stack size. 😭

- **`-mstack-guard=stack-guard`**, **`-mstack-size=stack-size`** — okay so with these two, the S/390 back end drops extra instructions into the function prologue that fire off a trap whenever the stack size goes <pink>*stack-guard*</pink> bytes above <pink>*stack-size*</pink> (and yes bestie, remember the stack on S/390 grows **downward** 💅). If you skip <pink>*stack-guard*</pink> entirely, it picks the smallest power of 2 that's bigger than the frame size of the compiled function. These are literally for debugging stack overflow problems 🫶 — the added code costs only a tiny bit of overhead, so it's also fine to run in production-like systems without greater performance degradation ✨.

> Real talk though: the values **have to be exact powers of 2**, and <pink>*stack-size*</pink> has to be greater than <pink>*stack-guard*</pink> **without exceeding 64k** 💀. Also, to stay efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by <pink>*stack-size*</pink>. And <pink>*stack-guard*</pink> can **only** be used in conjunction with <pink>*stack-size*</pink> — no solo runs, iykyk.

- **`-mhotpatch=pre-halfwords,post-halfwords`** — turn this on and every function in the compilation unit gets a "hot-patching" function prologue 🔥. The function label gets prepended with the given number of two-byte `NOP` instructions (<pink>*pre-halfwords*</pink>, max `1000000`), and after the label, `2 *` <pink>*post-halfwords*</pink> bytes are appended, using the largest NOP-like instructions the architecture allows (max `1000000`, again for both).

- If both arguments are zero, hotpatching is **disabled**. 🚫
- This option can be overridden for individual functions with the `"hotpatch"` attribute. 🎀

---

## ✨ SH Options ✨

These `-m` options are defined for the SH implementations 🌸:

- **`-m1`** — generate code for the SH1.
- **`-m2`** — generate code for the SH2.
- **`-m2e`** — generate code for the SH2e.
- **`-m2a-nofpu`** — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit just… isn't used 💅.
- **`-m2a-single-only`** — generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
- **`-m2a-single`** — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
- **`-m2a`** — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
- **`-m3`** — generate code for the SH3.
- **`-m3e`** — generate code for the SH3e.
- **`-m4-nofpu`** — generate code for the SH4 without a floating-point unit.
- **`-m4-single-only`** — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
- **`-m4-single`** — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
- **`-m4`** — generate code for the SH4.
- **`-m4-100`** — generate code for SH4-100.
- **`-m4-100-nofpu`** — generate code for SH4-100 in such a way that the floating-point unit is not used.
- **`-m4-100-single`** — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
- **`-m4-100-single-only`** — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
- **`-m4-200`** — generate code for SH4-200.
- **`-m4-200-nofpu`** — generate code for SH4-200 without — in such a way that the floating-point unit is not used.
- **`-m4-200-single`** — generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
- **`-m4-200-single-only`** — generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
- **`-m4-300`** — generate code for SH4-300.
- **`-m4-300-nofpu`** — generate code for SH4-300 without — in such a way that the floating-point unit is not used.
- **`-m4-300-single`** — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
- **`-m4-300-single-only`** — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
- **`-m4-340`** — generate code for SH4-340 (no MMU, no FPU).
- **`-m4-500`** — generate code for SH4-500 (no FPU). Passes `-isa=sh4-nofpu` to the assembler.
- **`-m4a-nofpu`** — generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
- **`-m4a-single-only`** — generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
- **`-m4a-single`** — generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
- **`-m4a`** — generate code for the SH4a.
- **`-m4al`** — same as `-m4a-nofpu`, except that it implicitly passes `-dsp` to the assembler. Heads up: GCC doesn't generate any DSP instructions at the moment 🙈.
- **`-mb`** — compile code for the processor in big-endian mode.
- **`-ml`** — compile code for the processor in little-endian mode.
- **`-mdalign`** — align doubles at 64-bit boundaries. ⚠️ Note that this changes the calling conventions, and thus some functions from the standard C library do **not** work unless you recompile it first with `-mdalign`.
- **`-mrelax`** — shorten some address references at link time, when possible; uses the linker option `-relax`.
- **`-mbigtable`** — use 32-bit offsets in `switch` tables. The default is to use 16-bit offsets.
- **`-mbitops`** — enable the use of bit manipulation instructions on SH2A.
- **`-mfmovd`** — enable the use of the instruction `fmovd`. Check `-mdalign` for alignment constraints 👀.
- **`-mrenesas`** — comply with the calling conventions defined by Renesas.
- **`-mno-renesas`** — comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the **default** for all targets of the SH toolchain.
- **`-mnomacsave`** — mark the `MAC` register as call-clobbered, even if `-mrenesas` is given.
- **`-mieee`**, **`-mno-ieee`** — control the IEEE compliance of floating-point comparisons, which affects how cases where the result of a comparison is unordered get handled. By default `-mieee` is implicitly enabled. If `-ffinite-math-only` is enabled, `-mno-ieee` is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either `-mieee` or `-mno-ieee` 🦋.
- **`-minline-ic_invalidate`** — inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has **no effect** if `-musermode` is in effect and the selected code generation option (e.g. `-m4`) does not allow the use of the `icbi` instruction 💀. If the selected code generation option does not allow the use of the `icbi` instruction, and `-musermode` is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. That not only requires privileged mode at run time, but it also **fails** if the cache line had been mapped via the TLB and has become unmapped. 😭
- **`-misize`** — dump instruction size and location in the assembly code.
- **`-mpadstruct`** — this option is **deprecated** 🚫. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
- **`-matomic-model=model`** — sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see `__atomic Builtins`. These are the models and parameters you can use 👇:
- `none` — disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not `sh*-*-linux*`.
- `soft-gusa` — generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is `sh*-*-linux*` and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions `movli.l` and `movco.l` to create more efficient code, unless `strict` is specified.
- `soft-tcb` — generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the `gbr-offset=` parameter has to be specified as well ⚠️.
- `soft-imask` — generate software atomic sequences that temporarily disable interrupts by setting `SR.IMASK = 1111`. This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is `sh*-*-linux*` and SH1* or SH2*.
- `hard-llcs` — generate hardware atomic sequences using the `movli.l` and `movco.l` instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model ✨.
- `gbr-offset=` — this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the `soft-tcb` model has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range `0-1020`.
- `strict` — this parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only. 👑

- **`-mtas`** — This makes the compiler generate the `"tas.b"` opcode for `"__atomic_test_and_set"`. 💅 Heads up, bestie: depending on the exact hardware and software config, this can lowkey degrade overall performance because of the operand cache line flushes implied by the `"tas.b"` instruction. And on multi-core SH4A processors you **must** use `"tas.b"` with caution — it can cause data corruption for certain cache configurations. <red>Not a vibe.</red> 😭

- **`-mprefergot`** — When generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. 🎀

- **`-musermode`, `-mno-usermode`** — Don't allow (`-musermode`) or allow (`-mno-usermode`) the compiler to generate privileged mode code. Specifying `-musermode` also implies `-mno-inline-ic_invalidate` if the inlined code would not work in user mode. `-musermode` is the default when the target is `"sh*-*-linux*"`. If the target is `SH1*` or `SH2*`, `-musermode` has no effect, since there is no user mode. 🫶

- **`-multcost=number`** — Set the cost to assume for a multiply insn. ⭐

- **`-mdiv=strategy`** — Set the division strategy to be used for integer division operations. *strategy* can be one of these:
- **`call-div1`** — Calls a library function that uses the single-step division instruction `"div1"` to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact. 💖
- **`call-fp`** — Calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that do not have a double precision FPU defaults to `"call-div1"`. ✨
- **`call-table`** — Calls a library function that uses a lookup table for small divisors and the `"div1"` instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that do not have dynamic shift instructions defaults to `"call-div1"`. 🦋

When a division strategy has not been specified, the default strategy is selected based on the current target. For SH2A the default strategy is to use the `"divs"` and `"divu"` instructions instead of library function calls. 👑

- **`-maccumulate-outgoing-args`** — Reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code. 🌸

- **`-mdivsi3_libfunc=name`** — Set the name of the library function used for 32-bit signed division to *name*. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present. 💅

- **`-mfixed-range=register-range`** — Generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. 🎀

- **`-mbranch-cost=num`** — Assume *num* to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified, the value is selected depending on the processor type that is being compiled for. ⭐

- **`-mzdcbranch`, `-mno-zdcbranch`** — Assume (`-mzdcbranch`) or do not assume (`-mno-zdcbranch`) that zero displacement conditional branch instructions `"bt"` and `"bf"` are fast. If `-mzdcbranch` is specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying `-mno-zdcbranch`. 🌈

- **`-mcbranch-force-delay-slot`** — Force the usage of delay slots for conditional branches, which stuffs the delay slot with a `"nop"` if a suitable instruction cannot be found. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055. 🔥

- **`-mfused-madd`, `-mno-fused-madd`** — Generate code that uses (`-mfused-madd`) or does not use (`-mno-fused-madd`) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent `-mfused-madd` option is now mapped to the machine-independent `-ffp-contract=fast` option, and `-mno-fused-madd` is mapped to `-ffp-contract=off`. 🧚

- **`-mfsca`, `-mno-fsca`** — Allow or disallow the compiler to emit the `"fsca"` instruction for sine and cosine approximations. The option `-mfsca` must be used in combination with `-funsafe-math-optimizations`. It is enabled by default when generating code for SH4A. Using `-mno-fsca` disables sine and cosine approximations even if `-funsafe-math-optimizations` is in effect. 💜

- **`-mfsrra`, `-mno-fsrra`** — Allow or disallow the compiler to emit the `"fsrra"` instruction for reciprocal square root approximations. The option `-mfsrra` must be used in combination with `-funsafe-math-optimizations` and `-ffinite-math-only`. It is enabled by default when generating code for SH4A. Using `-mno-fsrra` disables reciprocal square root approximations even if `-funsafe-math-optimizations` and `-ffinite-math-only` are in effect. 🍓

- **`-mpretend-cmove`** — Prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor. 🦋

- **`-mfdpic`** — Generate code using the FDPIC ABI. 👑

---

## ☀️ Solaris 2 Options ☀️

These `-m` options are supported on Solaris 2:

- **`-mclear-hwcap`** — `-mclear-hwcap` tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them. 💖

- **`-mimpure-text`** — `-mimpure-text`, used in addition to `-shared`, tells the compiler to not pass `-z text` to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object. 🫶

`-mimpure-text` suppresses the `"relocations remain against allocatable but non-writable sections"` linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using `-mimpure-text`, you should compile all source code with `-fpic` or `-fPIC`. <red>fr fr, that's the move.</red> 😭

These switches are supported in addition to the above on Solaris 2:

- **`-pthreads`** — This is a synonym for `-pthread`. ✨

---

## 🌞 SPARC Options 🌞

These `-m` options are supported on the SPARC:

- **`-mno-app-regs`, `-mapp-regs`** — Specify `-mapp-regs` to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default. 💅

To be fully SVR4 ABI-compliant at the cost of some performance loss, specify `-mno-app-regs`. You should compile libraries and system software with this option. 🌸

- **`-mflat`, `-mno-flat`** — With `-mflat`, the compiler does not generate save/restore instructions and uses a `"flat"` or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as `"call-saved"` registers and are saved on the stack as needed. 🎀

With `-mno-flat` (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode. ⭐

- **`-mfpu`, `-mhard-float`** — Generate output containing floating-point instructions. This is the default. 💖

- **`-mno-fpu`, `-msoft-float`** — Generate output containing library calls for floating point. 🦋

> ⚠️ **Warning:** the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets `sparc-*-aout` and `sparclite-*-*` do provide software floating-point support. 😭

`-msoft-float` changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work. 🫶

- **`-mhard-quad-float`** — Generate output containing quad-word (long double) floating-point instructions. 🔥

- **`-msoft-quad-float`** — Generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default. 🌈

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the `-msoft-quad-float` option is the default. 💀

- **`-mno-unaligned-doubles`, `-munaligned-doubles`** — Assume that doubles have 8-byte alignment. This is the default. ✨

With `-munaligned-doubles`, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code. 😭

- **`-muser-mode`, `-mno-user-mode`** — Do not generate code that can only run in supervisor mode. This is relevant only for the `"casa"` instruction emitted for the LEON3 processor. This is the default. 👑

okay bestie, buckle up — these next SPARC flags are a whole vibe ✨ and every single detail is staying, no cap:

## 💅 `-mfaster-structs`, `-mno-faster-structs`

With `-mfaster-structs`, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of `ldd` and `std` instructions for copies in structure assignment, in place of twice as many `ld` and `st` pairs. <red>However</red>, the use of this changed alignment directly violates the SPARC ABI. So it’s intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI. <pink>iykyk 🫶</pink>

## 🌸 `-mstd-struct-return`, `-mno-std-struct-return`

With `-mstd-struct-return`, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.

The default is `-mno-std-struct-return`. This option has no effect in 64-bit mode. <cyan>periodt ✨</cyan>

## 👑 `-mcpu=cpu_type`

Set the instruction set, register set, and instruction scheduling parameters for machine type *cpu_type*. Supported values for *cpu_type* are `v7`, `cypress`, `v8`, `supersparc`, `hypersparc`, `leon`, `leon3`, `leon3v7`, `leon5`, `sparclite`, `f930`, `f934`, `sparclite86x`, `sparclet`, `tsc701`, `v9`, `ultrasparc`, `ultrasparc3`, `niagara`, `niagara2`, `niagara3`, `niagara4`, `niagara7` and `m8`.

Native Solaris and GNU/Linux toolchains also support the value `native`, which selects the best architecture option for the host processor. `-mcpu=native` has no effect if GCC does not recognize the processor.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are `v7`, `v8`, `sparclite`, `sparclet`, `v9`.

Here is a list of each supported architecture and their supported implementations. <purple>so organized, honestly obsessed 🎀</purple>

- `v7` — `cypress`, `leon3v7`
- `v8` — `supersparc`, `hypersparc`, `leon`, `leon3`, `leon5`
- `sparclite` — `f930`, `f934`, `sparclite86x`
- `sparclet` — `tsc701`
- `v9` — `ultrasparc`, `ultrasparc3`, `niagara`, `niagara2`, `niagara3`, `niagara4`, `niagara7`, `m8`

By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With `-mcpu=cypress`, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

With `-mcpu=v8`, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With `-mcpu=supersparc`, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

With `-mcpu=sparclite`, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan (`ffs`) instructions which exist in SPARClite but not in SPARC-V7. With `-mcpu=f930`, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With `-mcpu=f934`, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU.

With `-mcpu=sparclet`, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan (`ffs`) instructions which exist in SPARClet but not in SPARC-V7. With `-mcpu=tsc701`, the compiler additionally optimizes it for the TEMIC SPARClet chip.

With `-mcpu=v9`, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With `-mcpu=ultrasparc`, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With `-mcpu=ultrasparc3`, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With `-mcpu=niagara`, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With `-mcpu=niagara2`, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With `-mcpu=niagara3`, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With `-mcpu=niagara4`, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With `-mcpu=niagara7`, the compiler additionally optimizes it for Oracle SPARC M7 chips. With `-mcpu=m8`, the compiler additionally optimizes it for Oracle M8 chips. <rainbow>the chip lore is unmatched 💀✨</rainbow>

## 🦋 `-mtune=cpu_type`

Set the instruction scheduling parameters for machine type *cpu_type*, but do not set the instruction set or register set that the option `-mcpu=cpu_type` does.

The same values for `-mcpu=cpu_type` can be used for `-mtune=cpu_type`, but the only useful values are those that select a particular CPU implementation. Those are `cypress`, `supersparc`, `hypersparc`, `leon`, `leon3`, `leon3v7`, `leon5`, `f930`, `f934`, `sparclite86x`, `tsc701`, `ultrasparc`, `ultrasparc3`, `niagara`, `niagara2`, `niagara3`, `niagara4`, `niagara7` and `m8`. With native Solaris and GNU/Linux toolchains, `native` can also be used. <mint>slay, bestie 💅</mint>

## 🎀 `-mv8plus`, `-mno-v8plus`

With `-mv8plus`, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.

## 🌈 `-mvis`, `-mno-vis`

With `-mvis`, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is `-mno-vis`.

## ⭐ `-mvis2`, `-mno-vis2`

With `-mvis2`, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is `-mvis2` when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting `-mvis2` also sets `-mvis`.

## 💜 `-mvis3`, `-mno-vis3`

With `-mvis3`, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is `-mvis3` when targeting a cpu that supports such instructions, such as `niagara-3` and later. Setting `-mvis3` also sets `-mvis2` and `-mvis`.

## 🧚 `-mvis3b`, `-mno-vis3b`

With `-mvis3b`, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011. The default is `-mvis3b` when targeting a cpu that supports such instructions, such as `niagara-7` and later. Setting `-mvis3b` also sets `-mvis3`, `-mvis2` and `-mvis`. <pink>the way these keep stacking, iconic 🫶</pink>

## 🍓 `-mvis4`, `-mno-vis4`

With `-mvis4`, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. The default is `-mvis4` when targeting a cpu that supports such instructions, such as `niagara-7` and later. Setting `-mvis4` also sets `-mvis3b`, `-mvis3`, `-mvis2` and `-mvis`.

## 🦋 `-mvis4b`, `-mno-vis4b`

With `-mvis4b`, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. The default is `-mvis4b` when targeting a cpu that supports such instructions, such as `m8` and later. Setting `-mvis4b` also sets `-mvis4`, `-mvis3b`, `-mvis3`, `-mvis2` and `-mvis`.

## 🔥 `-mcbcond`, `-mno-cbcond`

With `-mcbcond`, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is `-mcbcond` when targeting a CPU that supports such instructions, such as Niagara-4 and later.

## 🌸 `-mfmaf`, `-mno-fmaf`

With `-mfmaf`, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is `-mfmaf` when targeting a CPU that supports such instructions, such as Niagara-3 and later.

## 💖 `-mfsmuld`, `-mno-fsmuld`

With `-mfsmuld`, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. The default is `-mfsmuld` when targeting a CPU supporting the architecture versions V8 or V9 with FPU except `-mcpu=leon`.

## ✨ `-mpopc`, `-mno-popc`

With `-mpopc`, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. The default is `-mpopc` when targeting a CPU that supports such an instruction, such as Niagara-2 and later.

## 👑 `-msubxc`, `-mno-subxc`

With `-msubxc`, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is `-msubxc` when targeting a CPU that supports such an instruction, such as Niagara-7 and later.

## 🎀 `-mfix-at697f`

Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).

## 🧚 `-mfix-ut699`

Enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.

## 🦋 `-mfix-ut700`

Enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.

## 🌈 `-mfix-gr712rc`

Enable the documented workaround for the back-to-back store errata of the GR712RC processor.

These `-m` options are supported in addition to the above on SPARC-V9 processors in 64-bit environments: <cyan>yes bestie, there’s more 💅</cyan>

## 💜 `-m32`, `-m64`

Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets `int`, `long` and pointer to 32 bits. The 64-bit environment sets `int` to 32 bits and `long` and pointer to 64 bits.

## ⭐ `-mcmodel=which`

Set the code model to one of

- `medlow` — The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
- `medmid` — The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
- `medany` — The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment. <pink>memory model tea, served hot 🍓</pink>

- **`embmedany`** — the Medium/Anywhere code model for embedded systems, bestie: 64-bit addresses, and the text and data segments both have to be less than 2GB in size, each starting *anywhere* in memory (and that anywhere gets decided at link time). The global register `%g4` points to the base of the data segment. Programs are statically linked and PIC is **not** supported. slay but restrictive 💅

- **`-mmemory-model=mem-model`** — sets the memory model in force on the processor to one of these bad boys:
- `default` — the default memory model for the processor and operating system. iconic, no notes.
- `rmo` — Relaxed Memory Order
- `pso` — Partial Store Order
- `tso` — Total Store Order
- `sc` — Sequential Consistency

fr fr these memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's `"PSTATE.MM"` field. iykyk 📚

- **`-mstack-bias`**, **`-mno-stack-bias`** — with `-mstack-bias`, GCC assumes that the stack pointer, and frame pointer if present, are offset by `-2047`, which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. lowkey a vibe shift between the two 🫶

---

## ✨ Options for System V ✨

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

- **`-G`** — create a shared object. It is recommended that `-symbolic` or `-shared` be used instead. (so… a little deprecated coded, ngl 🙈)
- **`-Qy`** — identify the versions of each tool used by the compiler, in a `".ident"` assembler directive in the output.
- **`-Qn`** — refrain from adding `".ident"` directives to the output file (this is the default). the chill one 💀
- **`-YP,dirs`** — search the directories *dirs*, and no others, for libraries specified with `-l`. no others, bestie. she's exclusive. 👑
- **`-Ym,dir`** — look in the directory *dir* to find the M4 preprocessor. The assembler uses this option.

---

## 🌸 V850 Options 🌸

These `-m` options are defined for V850 implementations:

- **`-mlong-calls`**, **`-mno-long-calls`** — treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. indirect queen 💅
- **`-mno-ep`**, **`-mep`** — do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the `"ep"` register, and use the shorter `"sld"` and `"sst"` instructions. The `-mep` option is on by default if you optimize. slay those shorter instructions ⭐
- **`-mno-prolog-function`**, **`-mprolog-function`** — do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The `-mprolog-function` option is on by default if you optimize. trade-offs, bestie, we love them 🎀
- **`-mspace`** — try to make the code as small as possible. At present, this just turns on the `-mep` and `-mprolog-function` options. tiny code, big energy 🧚
- **`-mtda=n`** — put static or global variables whose size is *n* bytes or less into the tiny data area that register `"ep"` points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references). tiny but mighty 💖
- **`-msda=n`** — put static or global variables whose size is *n* bytes or less into the small data area that register `"gp"` points to. The small data area can hold up to 64 kilobytes.
- **`-mzda=n`** — put static or global variables whose size is *n* bytes or less into the first 32 kilobytes of memory.
- **`-mv850`** — specify that the target processor is the V850.
- **`-mv850e3v5`** — specify that the target processor is the V850E3V5. The preprocessor constant `"__v850e3v5__"` is defined if this option is used.
- **`-mv850e2v4`** — specify that the target processor is the V850E3V5. This is an alias for the `-mv850e3v5` option. (yes it's called e2v4 and specifies E3V5, we don't question the naming, periodt ✨)
- **`-mv850e2v3`** — specify that the target processor is the V850E2V3. The preprocessor constant `"__v850e2v3__"` is defined if this option is used.
- **`-mv850e2`** — specify that the target processor is the V850E2. The preprocessor constant `"__v850e2__"` is defined if this option is used.
- **`-mv850e1`** — specify that the target processor is the V850E1. The preprocessor constants `"__v850e1__"` and `"__v850e__"` are defined if this option is used.
- **`-mv850es`** — specify that the target processor is the V850ES. This is an alias for the `-mv850e1` option.
- **`-mv850e`** — specify that the target processor is the V850E. The preprocessor constant `"__v850e__"` is defined if this option is used.

If neither `-mv850` nor `-mv850e` nor `-mv850e1` nor `-mv850e2` nor `-mv850e2v3` nor `-mv850e3v5` are defined then a default target processor is chosen and the relevant `__v850*__` preprocessor constant is defined.

The preprocessor constants `"__v850"` and `"__v851__"` are always defined, regardless of which processor variant is the target. always-on icons 💜

- **`-mdisable-callt`**, **`-mno-disable-callt`** — this option suppresses generation of the `"CALLT"` instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

This option is enabled by default when the RH850 ABI is in use (see `-mrh850-abi`), and disabled by default when the GCC ABI is in use. If `"CALLT"` instructions are being generated then the C preprocessor symbol `"__V850_CALLT__"` is defined.

- **`-mrelax`**, **`-mno-relax`** — pass on (or do not pass on) the `-mrelax` command-line option to the assembler. middle-manning fr fr 🫶
- **`-mlong-jumps`**, **`-mno-long-jumps`** — disable (or re-enable) the generation of PC-relative jump instructions.
- **`-msoft-float`**, **`-mhard-float`** — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol `"__FPU_OK__"` is defined, otherwise the symbol `"__NO_FPU__"` is defined. one or the other, she picks 💅
- **`-mloop`** — enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental. ⚠️ experimental, bestie — proceed with caution
- **`-mrh850-abi`**, **`-mghs`** — enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI the following rules apply:
- Integer sized structures and unions are returned via a memory pointer rather than a register.
- Large structures and unions (more than 8 bytes in size) are passed by value.
- Functions are aligned to 16-bit boundaries.
- The `-m8byte-align` command-line option is supported.
- The `-mdisable-callt` command-line option is enabled by default. The `-mno-disable-callt` command-line option is not supported.

When this version of the ABI is enabled the C preprocessor symbol `"__V850_RH850_ABI__"` is defined. main character ABI energy 👑

- **`-mgcc-abi`** — enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:
- Integer sized structures and unions are returned in register `"r10"`.
- Large structures and unions (more than 8 bytes in size) are passed by reference.
- Functions are aligned to 32-bit boundaries, unless optimizing for size.
- The `-m8byte-align` command-line option is not supported.
- The `-mdisable-callt` command-line option is supported but not enabled by default.

When this version of the ABI is enabled the C preprocessor symbol `"__V850_GCC_ABI__"` is defined.

- **`-m8byte-align`**, **`-mno-8byte-align`** — enables support for `"double"` and `"long long"` types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When `-m8byte-align` is in effect the C preprocessor symbol `"__V850_8BYTE_ALIGN__"` is defined. alignment glow-up ✨
- **`-mbig-switch`** — generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table. big switch, big problems, use only when yelled at 😭
- **`-mapp-regs`** — this option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default.
- **`-mno-app-regs`** — this option causes r2 and r5 to be treated as fixed registers.

---

## 🦋 VAX Options 🦋

These `-m` options are defined for the VAX:

- **`-munix`** — do not output certain jump instructions (`"aobleq"` and so on) that the Unix assembler for the VAX cannot handle across long ranges.
- **`-mgnu`** — do output those jump instructions, on the assumption that the GNU assembler is being used.
- **`-md`**, **`-md-float`** — use the D_floating data format for double-precision floating-point numbers instead of G_floating.
- **`-mg`**, **`-mg-float`** — use the G_floating data format for double-precision floating-point numbers instead of D_floating.
- **`-mlra`**, **`-mno-lra`** — enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload. ⚠️ experimental era, we love a scientist 🧪

---

## 🌈 Visium Options 🌈

- **`-mdebug`** — a program which performs file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries `libc.a` and `libdebug.a` to be linked. The program should be run on the target under the control of the GDB remote debugging stub. debug girlie era 💀
- **`-msim`** — a program which performs file I/O and is destined to run on the simulator should be linked with this option. This causes libraries `libc.a` and `libsim.a` to be linked.
- **`-mfpu`**, **`-mhard-float`** — generate code containing floating-point instructions. This is the default.
- **`-mno-fpu`**, **`-msoft-float`** — generate code containing library calls for floating-point.

> `-msoft-float` changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile `libgcc.a`, the library that comes with GCC, with `-msoft-float` in order for this to work. all-or-nothing situation, bestie 😭

- **`-mcpu=cpu_type`** — set the instruction set, register set, and instruction scheduling parameters for machine type *cpu_type*. Supported values for *cpu_type* are `mcm`, `gr5` and `gr6`.

`mcm` is a synonym of `gr5` present for backward compatibility.

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With `-mcpu=gr6`, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions. block move instructions entering the chat 🔥

- **`-mtune=cpu_type`** — sets the instruction *scheduling* parameters for machine type *cpu_type*, but doesn't touch the instruction set or register set the way `-mcpu=cpu_type` would. So like, same outfit, different attitude 💅

- **`-msv-mode`** — generates code for supervisor mode, where there are literally no restrictions on accessing general registers. This is the default, bestie 🫶

- **`-muser-mode`** — generates code for user mode, where access to some general registers is forbidden: on the GR5, registers `r24` to `r31` cannot be accessed in this mode; on the GR6, only registers `r29` to `r31` are affected. So yeah, hands off those ones 🙈

## 🧊 VMS Options

These `-m` options are defined for the VMS implementations:

- **`-mvms-return-codes`** — returns VMS condition codes from `main`. The default is to return POSIX-style condition (e.g. error) codes. Pick your flavor, queen 👑

- **`-mdebug-main=prefix`** — flags the first routine whose name starts with *prefix* as the main routine for the debugger 🔍

- **`-mmalloc64`** — defaults to 64-bit memory allocation routines. Big allocations, big energy ✨

- **`-mpointer-size=size`** — sets the default size of pointers. Possible options for *size* are `32` or `short` for 32 bit pointers, `64` or `long` for 64 bit pointers, and `no` for supporting only 32 bit pointers. The latter option disables `pragma pointer_size`. So `no` is like, the "no thoughts head empty" of pointer sizes 💀

## 🛰️ VxWorks Options

The options in this section are defined for **all** VxWorks targets. Options specific to the target hardware are listed with the other options for that target — go look there, iykyk.

- **`-mrtp`** — GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro `__RTP__`. Glow-up from kernel to RTP, periodt 🔥

- **`-msmp`** — selects SMP runtimes for linking. Not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice. So like, read the room (and the version) before you use it 🙈

- **`-non-static`** — links an RTP executable against shared libraries rather than static libraries. The options `-static` and `-shared` can also be used for RTPs; `-static` is the default.

- **`-Bstatic`**
- **`-Bdynamic`** — these options are passed down to the linker. They're defined for compatibility with Diab. Middleman behavior, honestly 🤝

- **`-Xbind-lazy`** — enables lazy binding of function calls. This option is equivalent to `-Wl,-z,now` and is defined for compatibility with Diab.

- **`-Xbind-now`** — disables lazy binding of function calls. This option is the default and is defined for compatibility with Diab.

## 💻 x86 Options

These `-m` options are defined for the x86 family of computers. Buckle up bestie, this list is LONG 🎢

- **`-march=cpu-type`** — generates instructions for the machine type *cpu-type*. In contrast to `-mtune=cpu-type`, which merely tunes the generated code for the specified *cpu-type*, `-march=cpu-type` allows GCC to generate code that may **not run at all** on processors other than the one indicated. Specifying `-march=cpu-type` implies `-mtune=cpu-type`, except where noted otherwise. So like, it's giving committed 💍

The choices for *cpu-type* are:

- **`native`** — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using `-march=native` enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using `-mtune=native` produces code optimized for the local machine under the constraints of the selected instruction set. Main character energy: it's all about *this* machine ✨

- **`x86-64`** — a generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support.

- **`x86-64-v2`**
- **`x86-64-v3`**
- **`x86-64-v4`** — these choices for *cpu-type* select the corresponding micro-architecture level from the x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.

  Since these *cpu-type* values do not have a corresponding `-mtune` setting, using `-march` with these values enables generic tuning. Specific tuning can be enabled using the `-mtune=other-cpu-type` option with an appropriate *other-cpu-type* value. No cap, you gotta pair it yourself 🎀

- **`i386`** — original Intel i386 CPU. The OG 👑

- **`i486`** — Intel i486 CPU. (No scheduling is implemented for this chip.) Scheduling? Never heard of her 💀

- **`i586`**, **`pentium`** — Intel Pentium CPU with no MMX support.

- **`lakemont`** — Intel Lakemont MCU, based on Intel Pentium CPU.

- **`pentium-mmx`** — Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.

- **`pentiumpro`** — Intel Pentium Pro CPU with no MMX support.

- **`i686`** — when used with `-march`, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with `-mtune`, it has the same meaning as `generic`. Two moods, one flag 🦋

- **`pentium2`** — Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.

- **`pentium3`**, **`pentium3m`** — Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.

- **`pentium-m`** — Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. Used by Centrino notebooks. Vintage but iconic 💜

- **`pentium4`**, **`pentium4m`** — Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.

- **`prescott`** — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.

- **`nocona`** — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support.

- **`core2`** — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support.

- **`nehalem`**, **`corei7`** — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.

- **`westmere`** — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.

- **`sandybridge`**, **`corei7-avx`** — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.

- **`ivybridge`**, **`core-avx-i`** — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.

- **`haswell`**, **`core-avx2`** — Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support.

- **`broadwell`** — Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support.

- **`skylake`** — Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support.

- **`skylake-avx512`** — Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.

- **`cascadelake`** — Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support.

- **`cannonlake`** — Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support.

- **`cooperlake`** — Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.

- **`icelake-client`** — Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support.

- **`icelake-server`** — Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support.

- **`tigerlake`** — Intel Tiger Lake CPU, 64-bit extensions moment ✨, and it's bringing MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support. the whole squad fr 💅

- **`rocketlake`** — Intel Rocket Lake CPU with 64-bit extensions, and lowkey it's the same energy: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support 🚀

- **`alderlake`**
- **`raptorlake`**
- **`meteorlake`**
- **`gracemont`**

okay these four share one glow-up 🫶 — Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont CPU with 64-bit extensions, rocking MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support 💖

- **`arrowlake`** — Intel Arrow Lake CPU with 64-bit extensions, bringing MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT and CMPCCXADD instruction set support 🏹

- **`arrowlake-s`**
- **`lunarlake`**

Intel Arrow Lake S/Lunar Lake CPU with 64-bit extensions, and bestie it ate: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3 and SM4 instruction set support 🌙

- **`pantherlake`** — Intel Panther Lake CPU with 64-bit extensions, and it's giving main character energy 🐆: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4 and PREFETCHI instruction set support ✨

- **`sapphirerapids`**
- **`emeraldrapids`**

Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions, and ngl the list is *long* 💎: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16 and AVX512BF16 instruction set support 🍓

- **`graniterapids`** — Intel Granite Rapids CPU with 64-bit extensions, iconic behaviour only 🪨: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16 and PREFETCHI instruction set support 🔥

- **`graniterapids-d`** — Intel Granite Rapids D CPU with 64-bit extensions, and it said "and another one" 💀: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI and AMX-COMPLEX instruction set support 💜

- **`diamondrapids`** — Intel Diamond Rapids CPU with 64-bit extensions, and honestly? the way this list just keeps going 💎✨: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI, AMX-COMPLEX, AVX10.1-512, AVX-IFMA, AVX-NE-CONVERT, AVX-VNNI-INT16, AVX-VNNI-INT8, CMPccXADD, SHA512, SM3, SM4, AVX10.2-512, APX_F, AMX-AVX512, AMX-FP8, AMX-TF32, AMX-TRANSPOSE, MOVRS, AMX-MOVRS and USER_MSR instruction set support 👑

- **`bonnell`**
- **`atom`**

Intel Bonnell CPU with 64-bit extensions, cute lil list, no notes 🦋: MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

- **`silvermont`**
- **`slm`**

Intel Silvermont CPU with 64-bit extensions, and it's got MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support 🩶 (jk it's silver, slay 💅)

- **`goldmont`** — Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT and FSGSBASE instruction set support ⭐

- **`goldmont-plus`** — Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID and SGX instruction set support ✨ (the "plus" is doing real work here)

- **`tremont`** — Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support 🧚

- **`sierraforest`** — Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support 🌲

- **`grandridge`** — Intel Grand Ridge CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support 🏔️ (same list as sierraforest, bestie — they're twinning fr fr, and I'm not gonna pretend otherwise)

- **`clearwaterforest`** — Intel Clearwater Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI instruction set support 💧🌳

---

okay and now we switch teams 🔴 — say hi to AMD:

- **`k6`** — AMD K6 CPU with MMX instruction set support. short king.

- **`k6-2`**
- **`k6-3`**

Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support 🎀

- **`athlon`**
- **`athlon-tbird`**

AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support 🧡

- **`athlon-4`**
- **`athlon-xp`**
- **`athlon-mp`**

Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support — "full SSE", not the diet version, periodt 💅

- **`k8`**
- **`opteron`**
- **`athlon64`**
- **`athlon-fx`**

Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.) it's giving legacy icon energy 👑

- **`k8-sse3`**, **`opteron-sse3`**, **`athlon64-sse3`** — glow-up versions of the AMD K8 cores, now with SSE3 instruction set support. 💅

- **`amdfam10`**, **`barcelona`** — CPUs built on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)

- **`bdver1`** — CPUs built on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

- **`bdver2`** — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

- **`bdver3`** — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

- **`bdver4`** — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.) Like, the whole buffet. 🍓

- **`znver1`** — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)

- **`znver2`** — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)

- **`znver3`** — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)

- **`znver4`** — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.) <pink>the AVX-512 list is giving main character energy fr</pink> 👑

- **`znver5`** — AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI and 64-bit instruction set extensions.)

- **`btver1`** — CPUs built on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)

- **`btver2`** — CPUs built on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.

- **`winchip-c6`** — the IDT WinChip C6 CPU, which gets dealt with in the same way as `i486` but with additional MMX instruction set support.

- **`winchip2`** — the IDT WinChip 2 CPU, dealt with in the same way as `i486` with additional MMX and 3DNow! instruction set support.

- **`c3`** — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip. 😭)

- **`c3-2`** — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)

- **`c7`** — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`samuel-2`** — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)

- **`nehemiah`** — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)

- **`esther`** — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`eden-x2`** — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`eden-x4`** — VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)

- **`nano`** — generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-1000`** — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-2000`** — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-3000`** — VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-x2`** — VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)

- **`nano-x4`** — VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)

- **`lujiazui`** — ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, FXSR, RDSEED instruction set support. While the CPUs do support AVX and F16C, these aren't enabled by `-march=lujiazui` for performance reasons. <peach>(yes bestie, they literally have it and still won't turn it on — performance reasons, periodt)</peach>

- **`yongfeng`** — ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support.

- **`shijidadao`** — ZHAOXIN shijidadao CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support.

- **`geode`** — AMD Geode embedded processor with MMX and 3DNow! instruction set support.

---

### 🎛️ `-mtune=`*cpu-type*

Okay so this one tunes *everything applicable about the generated code* to *cpu-type* — with two exceptions, bestie: the ABI and the set of available instructions. So it's a vibe-setting flag, not a permission slip.

The way that it works: picking a specific *cpu-type* schedules things appropriately for that particular chip, but the compiler does **not** generate any code that cannot run on the default machine type unless you use a `-march=`*cpu-type* option. 🫶

For example, if GCC is configured for `i686-pc-linux-gnu` then `-mtune=pentium4` generates code that is tuned for Pentium 4 but still runs on i686 machines. No cap — tuned but still compatible. ✨

The choices for *cpu-type* are the same as for `-march`. In addition, `-mtune` supports 2 extra choices for *cpu-type*:

- **`generic`** — produces code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding `-mtune` or `-march` option instead of `-mtune=generic`. But, if you do not know exactly what CPU users of your application will have, then you should use this option. <lavender>(this is your "idk her" safe pick 💅)</lavender>

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released.

There is no `-march=generic` option because `-march` indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, `-mtune` indicates the processor (or, in this case, collection of processors) for which the code is optimized.

- **`intel`** — produces code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding `-mtune` or `-march` option instead of `-mtune=intel`. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option. 👑

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.

<purple>Okay so real talk:</purple> there's literally no `-march=intel` option, and here's why bestie — `-march` is about the *instruction set* the compiler is allowed to use, and there is no common instruction set that works on every single processor out there. Whereas `-mtune` is about the processor (or, in this case, the whole collection of processors) that the code gets optimized for. Different vibes entirely 💅

- **`-mcpu=cpu-type`** — a deprecated synonym for `-mtune`. Yeah. It's giving "old money, no longer iconic." Use `-mtune` instead ✨

- **`-mfpmath=unit`** — generates floating-point arithmetic for the selected unit *unit*. Your choices are like a little menu 🎀

- `387` — uses the standard 387 floating-point coprocessor that's present on the majority of chips and emulated otherwise. Code compiled with this runs almost literally everywhere 🌈. The catch? Temporary results get computed in 80-bit precision instead of the precision your type actually specifies, so you get slightly different results compared to most other chips 😭. Peep `-ffloat-store` for the detailed lore on that one.

  This is the default choice for non-Darwin x86-32 targets. Periodt.

- `sse` — uses the scalar floating-point instructions that live in the SSE instruction set. This set is supported by Pentium III and newer chips, and on the AMD side by Athlon-4, Athlon XP and Athlon MP chips 🔥. The earlier version of SSE only supports single-precision arithmetic, so double and extended-precision arithmetic still get done using 387. A later version — present only in Pentium 4 and AMD x86-64 chips — supports double-precision arithmetic too.

  For the x86-32 compiler you **must** use `-march=cpu-type`, `-msse` or `-msse2` switches to enable SSE extensions and actually make this option do anything. For the x86-64 compiler, those extensions are enabled by default, no cap.

  The resulting code should be considerably faster in the majority of cases and dodges the numerical instability problems of 387 code — but it may break some existing code that expects temporaries to be 80 bits 💀.

  This is the default choice for the x86-64 compiler, for Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when `-ffast-math` is enabled.

- `sse,387`
- `sse+387`
- `both` — attempts to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE it doubles the execution resources too. But use this one with care, okay? It's still experimental — the GCC register allocator does **not** model separate functional units well, resulting in unstable performance 🙈. It's giving "cute but chaotic."

- **`-masm=dialect`** — outputs assembly instructions using the selected dialect. It also affects which dialect gets used for basic `asm` and extended `asm`. Supported choices (in dialect order) are `att` or `intel`. The default is `att`. Darwin does not support `intel` — sorry bestie 🫶

- **`-mieee-fp`**, **`-mno-ieee-fp`** — controls whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered ✨

- **`-m80387`**, **`-mhard-float`** — generate output containing 80387 instructions for floating point.

- **`-mno-80387`**, **`-msoft-float`** — generate output containing library calls for floating point.

> ⚠️ Warning: the requisite libraries are **not** part of GCC. Normally the facilities of the machine's usual C compiler get used, but you can't do that directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation.

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if `-msoft-float` is used. Sneaky.

- **`-mno-fp-ret-in-387`** — do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types `float` and `double` in an FPU register, even if there's no FPU. The idea is that the operating system should emulate an FPU. The option `-mno-fp-ret-in-387` causes such values to be returned in ordinary CPU registers instead 💅

- **`-mno-fancy-math-387`** — some 387 emulators do not support the `sin`, `cos` and `sqrt` instructions for the 387. Specify this option to avoid generating those instructions. This option gets overridden when `-march` indicates that the target CPU always has an FPU and so the instruction doesn't need emulation. Little heads up: these instructions are **not** generated unless you also use the `-funsafe-math-optimizations` switch 👑

- **`-malign-double`**, **`-mno-align-double`** — control whether GCC aligns `double`, `long double`, and `long long` variables on a two-word boundary or a one-word boundary. Aligning `double` variables on a two-word boundary produces code that runs somewhat faster on a Pentium, at the expense of more memory 🍓

On x86-64, `-malign-double` is enabled by default.

> ⚠️ Warning: if you use the `-malign-double` switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32, and are **not** binary compatible with structures in code compiled without that switch. Like, actually incompatible. Be careful.

- **`-m96bit-long-double`**, **`-m128bit-long-double`** — these switches control the size of the `long double` type. The x86-32 application binary interface specifies the size to be 96 bits, so `-m96bit-long-double` is the default in 32-bit mode.

Modern architectures (Pentium and newer) prefer `long double` to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying `-m128bit-long-double` aligns `long double` to a 16-byte boundary by padding the `long double` with an additional 32-bit zero ✨

In the x86-64 compiler, `-m128bit-long-double` is the default choice, since its ABI specifies that `long double` is aligned on a 16-byte boundary.

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a `long double`. Facts only.

> ⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing `long double` variables, as well as modifying the function calling convention for functions taking `long double`. Hence they are not binary-compatible with code compiled without that switch. 💀

- **`-mlong-double-64`**, **`-mlong-double-80`**, **`-mlong-double-128`** — these switches control the size of the `long double` type. A size of 64 bits makes the `long double` type equivalent to the `double` type — and this is the default for the 32-bit Bionic C library. A size of 128 bits makes the `long double` type equivalent to the `__float128` type — and this is the default for the 64-bit Bionic C library 🦋

> ⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing `long double` variables, as well as modifying the function calling convention for functions taking `long double`. Hence they are not binary-compatible with code compiled without that switch. Same situation, same drama.

- **`-malign-data=type`** — controls how GCC aligns variables. Supported values for *type* are: `compat`, which uses an increased alignment value compatible with GCC 4.8 and earlier; `abi`, which uses the alignment value as specified by the psABI; and `cacheline`, which uses an increased alignment value to match the cache line size. `compat` is the default 💅

- **`-mlarge-data-threshold=threshold`** — when `-mcmodel=medium` or `-mcmodel=large` is specified, data objects larger than *threshold* get placed in large data sections. The default is `65535` 🎀

- **`-mrtd`** — uses a different function-calling convention, in which functions that take a fixed number of arguments return with the `ret num` instruction, which pops their arguments while returning. This saves one instruction in the caller, since there's no need to pop the arguments there. Efficient! Iconic! 👑

You can specify that an individual function is called with this calling sequence using the function attribute `stdcall`. You can also override the `-mrtd` option by using the function attribute `cdecl`.

> ⚠️ Warning: this calling convention is incompatible with the one normally used on Unix, so you **cannot** use it if you need to call libraries compiled with the Unix compiler.

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including `printf`) — otherwise incorrect code is generated for calls to those functions 😭

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored. Not here though.)

- **`-mregparm=num`** — controls how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute `regparm` ✨

> ⚠️ Warning: if you use this switch, and *num* is nonzero, then you must build **all** modules with the same value, including any libraries. This includes the system libraries and startup modules. Whole-build commitment, no half measures.

- **`-msseregparm`** — uses SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute `sseregparm` 🌸

> ⚠️ Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. Again. All of it. 💜

- **`-mvect8-ret-in-mem`** — returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. Legacy-compat realness 🫶

- **`-mpc32`**, **`-mpc64`**, **`-mpc80`** — set 80387 floating-point precision to 32, 64 or 80 bits. When `-mpc32` is specified, the significands of results of floating-point operations get rounded to 24 bits (single precision); `-mpc64` rounds the significands of results of floating-point operations to 53 bits (double precision), and `-mpc80` rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default 🌟

When this option is used, floating-point operations in higher precisions are **not available** to the programmer without setting the FPU control word explicitly. So like… don't expect extra precision to just show up, bestie 😭

Okay so <pink>bestie</pink>, setting the rounding of floating-point ops to less than the default 80 bits can speed some programs up by 2% or more 🔥. But heads up: some mathematical libraries just *assume* extended-precision (80-bit) floating-point operations are enabled by default, and routines in those libraries could suffer significant loss of accuracy — typically through that dramatic-sounding "catastrophic cancellation" 😭 — when this option is used to set the precision to less than extended precision. No cap, keep that in mind.

- **`-mdaz-ftz`** — The flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register control floating-point calculations. SSE and AVX instructions — including scalar and vector ones — could benefit from enabling the FTZ and DAZ flags when `-mdaz-ftz` is specified. Don't set FTZ/DAZ flags when `-mno-daz-ftz` or `-shared` is specified; `-mdaz-ftz` will set FTZ/DAZ flags even with `-shared`.

- **`-mstackrealign`** — Realign the stack at entry. On the x86, the `-mstackrealign` option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute `force_align_arg_pointer`, applicable to individual functions.

- **`-mpreferred-stack-boundary=num`** — Attempt to keep the stack boundary aligned to a 2 raised to `num` byte boundary. If `-mpreferred-stack-boundary` is not specified, the default is 4 (16 bytes or 128 bits).

> <red>Warning:</red> When generating code for the x86-64 architecture with SSE extensions disabled, `-mpreferred-stack-boundary=3` can be used to keep the stack boundary aligned to an 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in a controlled environment where stack space is an important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with a misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 `long double` and `__int128`), leading to wrong results. You must build all modules with `-mpreferred-stack-boundary=3`, including any libraries. This includes the system libraries and startup modules. 😬

- **`-mincoming-stack-boundary=num`** — Assume the incoming stack is aligned to a 2 raised to `num` byte boundary. If `-mincoming-stack-boundary` is not specified, the one specified by `-mpreferred-stack-boundary` is used.

On Pentium and Pentium Pro, `double` and `long double` values should be aligned to an 8-byte boundary (see `-malign-double`) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type `__m128` may not work properly if it is not 16-byte aligned.

To ensure proper alignment of these values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting.

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to `-mpreferred-stack-boundary=2`.

---

Okay so here's the <rainbow>iconic</rainbow> mega-list of instruction-set switches — every single one of these enables the use of instructions in the corresponding extended instruction sets. We're talking MMX, SSE, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX512VNNI, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F, USER_MSR, AVX10.1, AVX10.2, AMX-AVX512, AMX-TF32, AMX-TRANSPOSE, AMX-FP8, MOVRS, AMX-MOVRS or CLDEMOTE extended instruction sets. Each has a corresponding `-mno-` option to disable use of these instructions. <purple>Slay.</purple>

-mmmx -msse -msse2 -msse3 -mssse3 -msse4 -msse4a -msse4.1 -msse4.2 -mavx -mavx2 -mavx512f -mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mclflushopt -mclwb -mfsgsbase -mptwrite -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mfma4 -mprfchw -mrdpid -mrdseed -msgx -mxop -mlwp -m3dnow -m3dnowa -mpopcnt -mabm -madx -mbmi -mbmi2 -mlzcnt -mfxsr -mxsave -mxsaveopt -mxsavec -mxsaves -mrtm -mhle -mtbm -mmwaitx -mclzero -mpku -mavx512vbmi2 -mavx512bf16 -mavx512fp16 -mgfni -mvaes -mwaitpkg -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b -menqcmd -muintr -mtsxldtrk -mavx512vpopcntdq -mavx512vp2intersect -mavx512vnni -mavxvnni -mcldemote -mserialize -mamx-tile -mamx-int8 -mamx-bf16 -mhreset -mkl -mwidekl -mavxifma -mavxvnniint8 -mavxneconvert -mcmpccxadd -mamx-fp16 -mprefetchi -mraoint -mamx-complex -mavxvnniint16 -msm3 -msha512 -msm4 -mapxf -musermsr -mavx10.1 -mavx10.1-256 -mavx10.1-512 -mavx10.2 -mamx-avx512 -mamx-tf32 -mamx-transpose -mamx-fp8 -mmovrs -mamx-movrs


These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches.

Note that `-msse4` enables both SSE4.1 and SSE4.2 support, while `-mno-sse4` turns off those features; neither form of the option affects SSE4A support, controlled separately by `-msse4a`.

To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see `-mfpmath=sse`.

GCC depresses SSEx instructions when `-mavx` is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.

These options enable GCC to use these extended instructions in generated code, even without `-mfpmath=sse`. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. <cyan>iykyk</cyan> ✨

---

- **`-mdump-tune-features`** — This option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in `-mtune-ctrl=feature-list`.

- **`-mtune-ctrl=feature-list`** — This option is used to do fine grain control of x86 code generation features. `feature-list` is a comma separated list of feature names. See also `-mdump-tune-features`. When specified, the feature is turned on if it is not preceded with `^`, otherwise, it is turned off. `-mtune-ctrl=feature-list` is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. <red>So maybe don't touch unless you know what you're doing, bestie 💀</red>

- **`-mno-default`** — This option instructs GCC to turn off all tunable features. See also `-mtune-ctrl=feature-list` and `-mdump-tune-features`.

- **`-mcld`** — This option instructs GCC to emit a `cld` instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the `--enable-cld` configure option. Generation of `cld` instructions can be suppressed with the `-mno-cld` compiler option in this case.

- **`-mvzeroupper`** — This option instructs GCC to emit a `vzeroupper` instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary `zeroupper` intrinsics.

- **`-mprefer-avx128`** — This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.

- **`-mprefer-vector-width=opt`** — This option instructs GCC to use `opt`-bit vector width in instructions instead of default on the selected platform.

- **`-mpartial-vector-fp-math`** — This option enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless `-fno-trapping-math` is specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when `-fno-trapping-math` is in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using `-mdaz-ftz` to set denormals-are-zero (DAZ) flag in the MXCSR register. This option is enabled by default. 🫶

- **`-mmove-max=bits`** — This option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to `bits`. The valid `bits` are 128, 256 and 512. <pink>Periodt.</pink> 💅

### `-mstore-max=bits`

okay so this one tells GCC "hey bestie, the max number of bits you can shove into memory *efficiently* is `bits`" 💅 and the only valid values are `128`, `256` and `512` — no in-between nonsense:

- `none` — literally no extra limitations on GCC beyond whatever the platform you picked already says. live a little 🦋
- `128` — prefer 128-bit vector width for instructions
- `256` — prefer 256-bit vector width for instructions
- `512` — prefer 512-bit vector width for instructions

---

### `-mnoreturn-no-callee-saved-registers`

This one is genuinely kinda iconic for optimization girlies: it optimizes functions marked with the `"noreturn"` attribute or the `"_Noreturn"` specifier by **not** saving callee-saved registers in the function prologue — the ones actually used in the function, that is. (Except `"BP"`, bestie. `"BP"` stays. 🫶)

> ⚠️ Heads up though: this can absolutely mess with debugging the *caller* of the `"noreturn"` function, or literally any function further up the call stack. That's why it is **not enabled by default** — it's giving "chaotic good, use with intention" energy 💀

---

### `-mcx16`

This lets GCC generate `"CMPXCHG16B"` instructions in 64-bit code so it can do compare-and-exchange operations on 16-byte aligned 128-bit objects. Super useful for atomic updates of data structures that are bigger than one machine word ✨

The compiler uses this instruction to implement `__sync` Builtins. **But** — and this is the important little plot twist — for `__atomic` Builtins operating on 128-bit integers, a library call is always used. periodt.

---

### `-msahf`

Enables generation of `"SAHF"` instructions in 64-bit code. Storytime 🎀: early Intel Pentium 4 CPUs with Intel 64 support — before the Pentium 4 G1 step dropped in December 2005 — were out here *lacking* the `"LAHF"` and `"SAHF"` instructions that AMD64 supports. Rude.

Those two are load and store instructions, respectively, for certain status flags. In 64-bit mode, the `"SAHF"` instruction is used to optimize the `"fmod"`, `"drem"`, and `"remainder"` built-in functions — see Other Builtins for the deets 👑

---

### `-mmovbe`

Enables use of the `"movbe"` instruction to optimize byte swapping of four and eight byte entities. short king behavior, honestly 🧚

---

### `-mshstk`

The `-mshstk` option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). slay 💅

---

### `-mcrc32`

Enables built-in functions `__builtin_ia32_crc32qi`, `__builtin_ia32_crc32hi`, `__builtin_ia32_crc32si` and `__builtin_ia32_crc32di` to generate the `"crc32"` machine instruction 🔥

---

### `-mmwait`

Enables built-in functions `__builtin_ia32_monitor` and `__builtin_ia32_mwait` to generate the `"monitor"` and `"mwait"` machine instructions ⭐

---

### `-mrecip`

okay this one is doing the MOST and I love her. It enables use of `"RCPSS"` and `"RSQRTSS"` instructions (plus their vectorized variants `"RCPPS"` and `"RSQRTPS"`) with an extra Newton-Raphson step to bump up precision — instead of `"DIVSS"` and `"SQRTSS"` (and their vectorized variants) for single-precision floating-point arguments 🦋

These instructions are generated **only** when `-funsafe-math-optimizations` is enabled together with `-ffinite-math-only` and `-fno-trapping-math`.

> 😭 Note: while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals `0.99999994`). so like... fast, but a little delulu about precision.

Also note that GCC already implements `1.0f/sqrtf(x)` in terms of `"RSQRTSS"` (or `"RSQRTPS"`) with `-ffast-math` (or the option combo above) — so it doesn't need `-mrecip`. no cap.

And *also* also: GCC emits the above sequence with the additional Newton-Raphson step for vectorized single-float division and vectorized `sqrtf(x)` already with `-ffast-math` (or the above option combination), and doesn't need `-mrecip` for that either. she's self-sufficient 💜

---

### `-mrecip=opt`

This one controls *which* reciprocal estimate instructions are allowed. `opt` is a comma-separated list of options, and any of them can have a `!` in front to invert it:

- `all` — enable all estimate instructions
- `default` — enable the default instructions, equivalent to `-mrecip`
- `none` — disable all estimate instructions, equivalent to `-mno-recip`
- `div` — enable the approximation for scalar division
- `vec-div` — enable the approximation for vectorized division
- `sqrt` — enable the approximation for scalar square root
- `vec-sqrt` — enable the approximation for vectorized square root

So for example, `-mrecip=all,!sqrt` enables all of the reciprocal approximations, except for square root. see what we did there 💅

---

### `-mveclibabi=type`

Specifies the ABI type to use when vectorizing intrinsics with an external library. Supported values for `type` are `svml` for the Intel short vector math library, `aocl` for the math library (LibM) from AMD Optimizing CPU Libraries (AOCL), and `acml` for the end-of-life AMD core math library (to which AOCL-LibM is the successor — glow-up of the century, honestly 🌈).

To use this option, **both** `-ftree-vectorize` and `-funsafe-math-optimizations` have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. no shortcuts, bestie.

GCC currently emits calls to `"vmldExp2"`, `"vmldLn2"`, `"vmldLog102"`, `"vmldPow2"`, `"vmldTanh2"`, `"vmldTan2"`, `"vmldAtan2"`, `"vmldAtanh2"`, `"vmldCbrt2"`, `"vmldSinh2"`, `"vmldSin2"`, `"vmldAsinh2"`, `"vmldAsin2"`, `"vmldCosh2"`, `"vmldCos2"`, `"vmldAcosh2"`, `"vmldAcos2"`, `"vmlsExp4"`, `"vmlsLn4"`, `"vmlsLog104"`, `"vmlsPow4"`, `"vmlsTanh4"`, `"vmlsTan4"`, `"vmlsAtan4"`, `"vmlsAtanh4"`, `"vmlsCbrt4"`, `"vmlsSinh4"`, `"vmlsSin4"`, `"vmlsAsinh4"`, `"vmlsAsin4"`, `"vmlsCosh4"`, `"vmlsCos4"`, `"vmlsAcosh4"` and `"vmlsAcos4"` for the corresponding function type when `-mveclibabi=svml` is used 📚

Then for `-mveclibabi=aocl` it's: `"amd_vrs4_acosf"`, `"amd_vrs16_acosf"`, `"amd_vrd8_asin"`, `"amd_vrs4_asinf"`, `"amd_vrs8_asinf"`, `"amd_vrs16_asinf"`, `"amd_vrd2_atan"`, `"amd_vrd8_atan"`, `"amd_vrs4_atanf"`, `"amd_vrs8_atanf"`, `"amd_vrs16_atanf"`, `"amd_vrd2_cos"`, `"amd_vrd4_cos"`, `"amd_vrd8_cos"`, `"amd_vrs4_cosf"`, `"amd_vrs8_cosf"`, `"amd_vrs16_cosf"`, `"amd_vrs4_coshf"`, `"amd_vrs8_coshf"`, `"amd_vrd2_erf"`, `"amd_vrd4_erf"`, `"amd_vrd8_erf"`, `"amd_vrs4_erff"`, `"amd_vrs8_erff"`, `"amd_vrs16_erff"`, `"amd_vrd2_exp"`, `"amd_vrd4_exp"`, `"amd_vrd8_exp"`, `"amd_vrs4_expf"`, `"amd_vrs8_expf"`, `"amd_vrs16_expf"`, `"amd_vrd2_exp10"`, `"amd_vrs4_exp10f"`, `"amd_vrd2_exp2"`, `"amd_vrd4_exp2"`, `"amd_vrd8_exp2"`, `"amd_vrs4_exp2f"`, `"amd_vrs8_exp2f"`, `"amd_vrs16_exp2f"`, `"amd_vrs4_expm1f"`, `"amd_vrd2_log"`, `"amd_vrd4_log"`, `"amd_vrd8_log"`, `"amd_vrs4_logf"`, `"amd_vrs8_logf"`, `"amd_vrs16_logf"`, `"amd_vrd2_log10"`, `"amd_vrs4_log10f"`, `"amd_vrs8_log10f"`, `"amd_vrs16_log10f"`, `"amd_vrd2_log1p"`, `"amd_vrs4_log1pf"`, `"amd_vrd2_log2"`, `"amd_vrd4_log2"`, `"amd_vrd8_log2"`, `"amd_vrs4_log2f"`, `"amd_vrs8_log2f"`, `"amd_vrs16_log2f"`, `"amd_vrd2_pow"`, `"amd_vrd4_pow"`, `"amd_vrd8_pow"`, `"amd_vrs4_powf"`, `"amd_vrs8_powf"`, `"amd_vrs16_powf"`, `"amd_vrd2_sin"`, `"amd_vrd4_sin"`, `"amd_vrd8_sin"`, `"amd_vrs4_sinf"`, `"amd_vrs8_sinf"`, `"amd_vrs16_sinf"`, `"amd_vrd2_tan"`, `"amd_vrd4_tan"`, `"amd_vrd8_tan"`, `"amd_vrs16_tanf"`, `"amd_vrs4_tanhf"`, `"amd_vrs8_tanhf"`, `"amd_vrs16_tanhf"` for the corresponding function type 🌸

And finally, for `-mveclibabi=acml`: `"__vrd2_sin"`, `"__vrd2_cos"`, `"__vrd2_exp"`, `"__vrd2_log"`, `"__vrd2_log2"`, `"__vrd2_log10"`, `"__vrs4_sinf"`, `"__vrs4_cosf"`, `"__vrs4_expf"`, `"__vrs4_logf"`, `"__vrs4_log2f"`, `"__vrs4_log10f"` and `"__vrs4_powf"` for the corresponding function type 💖

(yes that was a lot. drink some water. 🍓)

---

### `-mabi=name`

Generate code for the specified calling convention. The permissible values are `sysv` for the ABI used on GNU/Linux and other systems, and `ms` for the Microsoft ABI.

Default behavior: Microsoft ABI when targeting Microsoft Windows, SysV ABI on all other systems. And you can control this per-function using the function attributes `"ms_abi"` and `"sysv_abi"` — main character energy for individual functions 🎀

---

### `-mforce-indirect-call`

Forces **all** calls to functions to be indirect. This is useful when using Intel Processor Trace, where it generates more precise timing information for function calls. indirect queen 💅

---

### `-mmanual-endbr`

Inserts an ENDBR instruction at function entry **only** via the `"cf_check"` function attribute. Useful when paired with the option `-fcf-protection=branch` to control ENDBR insertion at the function entry. very "I'll do it myself" of you ✨

---

### `-mcet-switch`

By default, CET instrumentation is turned off on switch statements that use a jump table, and indirect branch track is disabled. Since jump tables are stored in read-only memory, this doesn't result in a direct loss of hardening. **But** — if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET 😭

This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets being reachable via any indirect jumps. secure the bag 🫶

---

### `-mcall-ms2sysv-xlogues`

Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider `RSI`, `RDI` and `XMM6-15` as clobbered. By default, the code for saving and restoring these registers is emitted inline, which gives you fairly lengthy prologues and epilogues. not cute.

Using `-mcall-ms2sysv-xlogues` emits prologues and epilogues that use stubs in the static portion of libgcc to perform those saves and restores — reducing function size at the cost of a few extra instructions. trade-off queen 👑

---

### `-mtls-dialect=type`

Generate code to access thread-local storage using the `gnu` or `gnu2` conventions.

- `gnu` — the conservative default
- `gnu2` — more efficient, **but** it may add compile- and run-time requirements that cannot be satisfied on all systems

so like, gnu2 is faster but high-maintenance. know what you're signing up for 💀

---

### `-mpush-args`, `-mno-push-args`

Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default ✨

In some cases disabling it may improve performance, because of improved scheduling and reduced dependencies. the audacity, honestly.

---

### `-maccumulate-outgoing-args`

If enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs — reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary is not equal to 2 🔥

The drawback? A notable increase in code size. she's thorough but she's THICC.

> This switch implies `-mno-push-args`. noted. 💅

---

### `-mthreads`

Support thread-safe exception handling on MinGW 🧚

Programs that rely on thread-safe exception handling must compile **and** link all code with the `-mthreads` option. When compiling, `-mthreads` defines `-D_MT`; when linking, it links in a special thread helper library `-lmingwthrd` which cleans up per-thread exception-handling data. all-or-nothing bestie, no half measures 🎀

### **`-mms-bitfields`**, **`-mno-ms-bitfields`** 💅

Ok so bestie, this pair toggles bit-field layout that's compatible with the native Microsoft Windows compiler. Iconic.

If `packed` gets used on a structure, or if bit-fields show up at all, it may be that the Microsoft ABI lays out the structure *differently* than the way GCC normally does. Especially when you're moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file) — sometimes you just gotta access either format. It's giving compatibility era.

<pink>This option is enabled by default for Microsoft Windows targets.</pink> And you can also control this behavior locally with variable or type attributes. For more info, see x86 Variable Attributes and x86 Type Attributes. iykyk.

The Microsoft structure layout algorithm is fairly simple, *except* for the bit-field packing. The padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are determined by these rules:

1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.

2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the `aligned` attribute or the `pack` pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0


3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.

Now, MSVC interprets zero-length bit-fields in the following ways — pay attention bestie, this is the tricky part 🔥:

1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are **not** coalesced.

   For example:

struct { unsigned long bf_1 : 12; unsigned long : 0; unsigned long bf_2 : 12; } t1;


   The size of `t1` is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, `t1`'s size would be 4 bytes. Literally double. 😭

2. If a zero-length bit-field is inserted after a bit-field, `foo`, and the alignment of the zero-length bit-field is greater than the member that follows it, `bar`, `bar` is aligned as the type of the zero-length bit-field.

   For example:

struct { char foo : 4; short : 0; char bar; } t2;

struct { char foo : 4; short : 0; double bar; } t3;


   For `t2`, `bar` is placed at offset 2, rather than offset 1. Accordingly, the size of `t2` is 4. For `t3`, the zero-length bit-field does not affect the alignment of `bar` or, as a result, the size of the structure.

   Taking this into account, it is important to note the following:

   1. If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as a whole. For example, `t2` has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type `short`.

   2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:

struct { char foo : 6; long : 0; } t4;


   Here, `t4` takes up 4 bytes.

3. Zero-length bit-fields following non-bit-field members are ignored:

struct { char foo; long : 0; char bar; } t5;


   Here, `t5` takes up 2 bytes. slay, ignored entirely 💀

---

### **`-mno-align-stringops`** 🎀

Do **not** align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. Not GCC being clueless about your vibes smh.

### **`-minline-all-stringops`** ✨

By default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast `memcpy` and `memset` for short lengths. The option enables inline expansion of `strlen` for all pointer alignments. all of them. every single one. 🫶

### **`-minline-stringops-dynamically`** 🦋

For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. Best of both worlds, honestly main character energy.

### **`-mstringop-strategy=alg`** 🌸

Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for *alg* are:

- `rep_byte`
- `rep_4byte`
- `rep_8byte` — expand using i386 `rep` prefix of the specified size.
- `byte_loop`
- `loop`
- `unrolled_loop` — expand into an inline loop.
- `libcall` — always use a library call.

### **`-mmemcpy-strategy=strategy`** 💖

Override the internal decision heuristic to decide if `__builtin_memcpy` should be inlined and what inline algorithm to use when the expected size of the copy operation is known. *strategy* is a comma-separated list of `alg:max_size:dest_align` triplets. *alg* is specified in `-mstringop-strategy`, *max_size* specifies the max byte size with which inline algorithm *alg* is allowed. For the last triplet, the *max_size* must be `-1`. The *max_size* of the triplets in the list must be specified in increasing order. The minimal byte size for *alg* is `0` for the first triplet and `max_size + 1` of the preceding range. Ngl that's a whole lot of triplets but we respect the structure 👑

### **`-mmemset-strategy=strategy`** 🍓

The option is similar to `-mmemcpy-strategy=` except that it is to control `__builtin_memset` expansion. Same energy, different builtin.

### **`-momit-leaf-frame-pointer`** 💜

Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option `-fomit-leaf-frame-pointer` removes the frame pointer for leaf functions, which might make debugging harder. Perf gains but debugging pain, the eternal tradeoff 😭

### **`-mtls-direct-seg-refs`**, **`-mno-tls-direct-seg-refs`** 🌈

Controls whether TLS variables may be accessed with offsets from the TLS segment register (`%gs` for 32-bit, `%fs` for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on. 🫶

### **`-msse2avx`**, **`-mno-sse2avx`** ⭐

Specify that the assembler should encode SSE instructions with VEX prefix. The option `-mavx` turns this on by default.

### **`-mfentry`**, **`-mno-fentry`** 🔥

If profiling is active (`-pg`), put the profiling counter call before the prologue. Note: on x86 architectures the attribute `ms_hook_prologue` isn't possible at the moment for `-mfentry` and `-pg`. Just a lil heads up 💅

### **`-mrecord-mcount`**, **`-mno-record-mcount`** 🎀

If profiling is active (`-pg`), generate a `__mcount_loc` section that contains pointers to each profiling call. This is useful for automatically patching and out calls.

### **`-mnop-mcount`**, **`-mno-nop-mcount`** ✨

If profiling is active (`-pg`), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with `-mrecord-mcount`.

### **`-minstrument-return=type`** 🦋

Instrument function exit in `-pg` `-mfentry` instrumented functions with call to specified function. This only instruments true returns ending with `ret`, but not sibling calls ending with `jump`. Valid *type*s are `none` to not instrument, `call` to generate a call to `__return__`, or `nop5` to generate a 5 byte nop. cursed but valid 💀

### **`-mrecord-return`**, **`-mno-record-return`** 🌸

Generate a `__return_loc` section pointing to all return instrumentation code.

### **`-mfentry-name=name`** 💖

Set *name* of `__fentry__` symbol called at function entry for `-pg` `-mfentry` functions.

### **`-mfentry-section=name`** 🍓

Set *name* of section to record `-mrecord-mcount` calls (default `__mcount_loc`).

### **`-mskip-rax-setup`**, **`-mno-skip-rax-setup`** 💜

When generating code for the x86-64 architecture with SSE extensions disabled, `-mskip-rax-setup` can be used to skip setting up RAX register when there are no variable arguments passed in vector registers.

> **Warning:** since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random location. GCC 4.4 or newer don't have those issues, regardless the RAX register value. Like, jump to a random location?? bestie be careful 🙈

### **`-m8bit-idiv`**, **`-mno-8bit-idiv`** 🌈

On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of `0` to `255`, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. smol numbers, smol divide, big speed ⭐

### **`-mavx256-split-unaligned-load`**, **`-mavx256-split-unaligned-store`** 👑

Split 32-byte AVX unaligned load and store.

### **`-mstack-protector-guard=guard`**, **`-mstack-protector-guard-reg=reg`**, **`-mstack-protector-guard-offset=offset`**, **`-mstack-protector-guard-symbol=symbol`** 🧚

Generate stack protection code using canary at *guard*. Supported locations are `global` for global canary or `tls` for per-thread canary in the TLS block (the default). This option has effect only when `-fstack-protector` or `-fstack-protector-all` is specified. canary said protect the stack or else 💅

...and when you pick that last one, `-mstack-protector-guard-reg=reg` and `-mstack-protector-guard-offset=offset` are the girlys that spell out *which* segment register (`%fs` or `%gs`) is the base register for reading the canary, and *from what offset* off that base register you're reading. The default for those two? Whatever the relevant ABI says, bestie. Not our call. 💅

- **`-mstack-protector-guard-symbol=symbol`** — overrides the offset with a symbol reference to a canary that lives in the TLS block. cute and specific.

- **`-mgeneral-regs-only`** — generates code that uses *only* the general-purpose registers. Like, it literally stops the compiler from touching floating-point, vector, mask and bound registers. general-purpose only, that's the whole vibe. 🙅‍♀️

- **`-mrelax-cmpxchg-loop`** — when it's emitting a compare-and-swap loop for `__sync` Builtins and `__atomic` Builtins that don't have a native instruction, this optimizes for the *highly contended* case (yes, the messy case 💀) by issuing an atomic load before the `CMPXCHG` instruction, and using the `PAUSE` instruction to save CPU power when it restarts the loop. energy-saving queen.

- **`-mindirect-branch=choice`** — converts indirect call and jump with *choice*. The tea ☕:
  - the default is `keep`, which keeps indirect call and jump unmodified
  - `thunk` converts indirect call and jump to call and return thunk
  - `thunk-inline` converts indirect call and jump to inlined call and return thunk
  - `thunk-extern` converts indirect call and jump to external call and return thunk provided in a separate object file

  You can control this behavior for a specific function by using the function attribute `indirect_branch`.

  > heads up: `-mcmodel=large` is **incompatible** with `-mindirect-branch=thunk` and `-mindirect-branch=thunk-extern` since the thunk function may not be reachable in the large code model. not compatible, no cap.

  > and also `-mindirect-branch=thunk-extern` **is** compatible with `-fcf-protection=branch` since the external thunk can be made to enable control-flow check. slay.

- **`-mfunction-return=choice`** — same deal but for function return. Converts function return with *choice*:
  - the default is `keep`, which keeps function return unmodified
  - `thunk` converts function return to call and return thunk
  - `thunk-inline` converts function return to inlined call and return thunk
  - `thunk-extern` converts function return to external call and return thunk provided in a separate object file

  You can control this behavior for a specific function by using the function attribute `function_return`.

  > note: `-mindirect-return=thunk-extern` is compatible with `-fcf-protection=branch` since the external thunk can be made to enable control-flow check.

  > but `-mcmodel=large` is **incompatible** with `-mfunction-return=thunk` and `-mfunction-return=thunk-extern` since the thunk function may not be reachable in the large code model. 😭

- **`-mindirect-branch-register`** — forces indirect call and jump via register. that's it, forceful king.

- **`-mharden-sls=choice`** — generates code to mitigate against straight line speculation (SLS) with *choice*. options are:
  - `none` — the default, disables all SLS hardening
  - `return` — enables SLS hardening for function returns
  - `indirect-jmp` — enables SLS hardening for indirect jumps
  - `all` — enables all SLS hardening

- **`-mindirect-branch-cs-prefix`** — adds CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers so that the call and jmp instruction length is 6 bytes, to allow them to be replaced with `lfence; call *%r8-r15` or `lfence; jmp *%r8-r15` at run-time. six bytes of pure intention. ✨

- **`-mapx-inline-asm-use-gpr32`** — for inline asm support with APX. By default the EGPR feature was disabled to prevent potential illegal instruction with EGPR occurring. To invoke egpr usage in inline asm, use the new compiler option `-mapx-inline-asm-use-gpr32`, and the user should ensure the instruction supports EGPR. (yes, *you* have to make sure. it's giving responsibility 🫶)

- **`-mevex512`, `-mno-evex512`** — enables/disables 512-bit vector. It will be default on if AVX512F is enabled.

---

## 💖 the x86-64 64-bit-era -m switches 💖

These `-m` switches are supported **in addition to** the above on x86-64 processors in 64-bit environments. so like, bonus round.

- **`-m32`, `-m64`, `-mx32`, `-m16`, `-miamcu`** — generate code for a 16-bit, 32-bit or 64-bit environment. the breakdown, bestie:
  - `-m32` sets `int`, `long`, and pointer types to 32 bits, and generates code that runs in 32-bit mode.
  - `-m64` sets `int` to 32 bits and `long` and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin *only*, the `-m64` option also turns off the `-fno-pic` and `-mdynamic-no-pic` options.
  - `-mx32` sets `int`, `long`, and pointer types to 32 bits, and generates code for the x86-64 architecture. (yes, all 32 bits but still x86-64. iconic chaos.)
  - `-m16` is the same as `-m32`, except that it outputs the `.code16gcc` assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.
  - `-miamcu` generates code which conforms to Intel MCU psABI. It **requires** the `-m32` option to be turned on. non-negotiable.

- **`-mno-red-zone`** — do *not* use a so-called "red zone" for x86-64 code. So the lore: the red zone is mandated by the x86-64 ABI; it's a 128-byte area beyond the location of the stack pointer that is **not** modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag `-mno-red-zone` disables this red zone. bye red zone 💀

- **`-mcmodel=small`** — generates code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the **default** code model. humble and reliable.

- **`-mcmodel=kernel`** — generates code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model **has to** be used for Linux kernel code. no substitutes.

- **`-mcmodel=medium`** — generates code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than `-mlarge-data-threshold` are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked. medium is the middle child and it ate.

- **`-mcmodel=large`** — generates code for the large model. This model makes **no assumptions** about addresses and sizes of sections. feral. unbothered. unconstrained. ✨

- **`-maddress-mode=long`** — generates code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.

- **`-maddress-mode=short`** — generates code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.

- **`-mneeded`, `-mno-needed`** — emit `GNU_PROPERTY_X86_ISA_1_NEEDED` GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. tell them what you need upfront, queen.

- **`-mno-direct-extern-access`** — without `-fpic` nor `-fPIC`, always use the GOT pointer to access external symbols. With `-fpic` or `-fPIC`, treat access to protected symbols as local symbols. The default is `-mdirect-extern-access`.

  > ⚠️ **Warning:** shared libraries compiled with `-mno-direct-extern-access` and executable compiled with `-mdirect-extern-access` may **not** be binary compatible if protected symbols are used in shared libraries and executable. like, they might not speak to each other. be careful. 🙈

- **`-munroll-only-small-loops`** — controls conservative small loop unrolling. It is default enabled by `O2`, and unrolls loop with less than 4 insns by 1 time. Explicit `-f[no-]unroll-[all-]loops` would disable this flag to avoid any unintended unrolling behavior that user does not want. protecting you from surprise unrolls. 🎀

- **`-mlam=choice`** — LAM (linear-address masking) allows special bits in the pointer to be used for metadata. The default is `none`. With `u48`, pointer bits in positions 62:48 can be used for metadata; with `u57`, pointer bits in positions 62:57 can be used for metadata. masking era.

---

## 🪟 x86 Windows Options 🪟

(that's literally it, that's the whole section. no options listed. we move.)

---

## 🦋 Xstormy16 Options 🦋

These options are defined for Xstormy16:

- **`-msim`** — choose startup files and linker script suitable for the simulator. sim-ready, slay.

---

## 🌸 Xtensa Options 🌸

These options are supported for Xtensa targets:

- **`-mconst16`, `-mno-const16`** — enable or disable use of `CONST16` instructions for loading constant values. The `CONST16` instruction is currently **not** a standard option from Tensilica. When enabled, `CONST16` instructions are always used in place of the standard `L32R` instructions. The use of `CONST16` is enabled by default **only if** the `L32R` instruction is not available. conditional queen.

- **`-mfused-madd`, `-mno-fused-madd`** — enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has **no effect** if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. precision girlies rise 🫶

- **`-mserialize-volatile`, `-mno-serialize-volatile`** — when this option is enabled, GCC inserts `MEMW` instructions before `volatile` memory references to guarantee sequential consistency. The default is `-mserialize-volatile`. Use `-mno-serialize-volatile` to omit the `MEMW` instructions. order guaranteed, or deliberately not. 💅

- **`-mforce-no-pic`** — for targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. kernel code gets the exception, periodt.

- **`-mtext-section-literals`, `-mno-text-section-literals`** — these options control the treatment of literal pools. The default is `-mno-text-section-literals`, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With `-mtext-section-literals`, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function. right before. like a lil entourage. ✨

- **`-mauto-litpools`**, **`-mno-auto-litpools`** — the literal-pool vibe check 🎀 The default is `-mno-auto-litpools`, which drops literals into a totally separate section in the output file *unless* you're using `-mtext-section-literals`. But with `-mauto-litpools`, the assembler just intersperses the literals right into the text section. The compiler doesn't spit out explicit `.literal` directives, and it loads literals into registers with `MOVI` instructions instead of `L32R` — that way the assembler can do its relaxation thing and place literals wherever it needs to. Lowkey iconic: this option lets the assembler create *several* literal pools per function and assemble very big functions, which honestly may not be possible with `-mtext-section-literals`. 💅

- **`-mtarget-align`**, **`-mno-target-align`** — when this is on, GCC tells the assembler to automatically align instructions so you get fewer branch penalties, at the cost of a lil code density. The assembler tries to widen density instructions to align branch targets and the instructions that come after call instructions. But if there aren't enough safe density instructions beforehand to align a target? No widening happens, periodt. The default is `-mtarget-align`. Heads up tho: these options don't touch how auto-aligned instructions like `LOOP` are handled — the assembler *always* aligns those, either by widening density instructions or by inserting `NOP` instructions. ✨

- **`-mlongcalls`**, **`-mno-longcalls`** — turn this on and GCC tells the assembler to translate direct calls into indirect calls, unless it can figure out that the direct call's target is within the range the call instruction allows. This translation typically happens for calls to functions in other source files 👀. Specifically, the assembler turns a direct `CALL` instruction into an `L32R` followed by a `CALLX` instruction. Default is `-mno-longcalls`. Use this in programs where the call target can potentially be out of range. Real talk: this option is implemented in the *assembler*, not the compiler, so the assembly code GCC generates still shows direct call instructions — go look at the disassembled object code to see the actual instructions, bestie. And note that the assembler uses an indirect call for **every** cross-file call, not just the ones that actually are out of range. 💀

- **`-mabi=`*name* — generate code for the specified ABI. Permissible values are: `call0`, `windowed`. The default ABI is chosen by the Xtensa core configuration. 🦋

- **`-mabi=call0`** — with this enabled, function parameters get passed in registers `a2` through `a7`, registers `a12` through `a15` are caller-saved, and register `a15` may be used as a frame pointer. When this version of the ABI is enabled, the C preprocessor symbol `__XTENSA_CALL0_ABI__` is defined. 👑

- **`-mabi=windowed`** — with this one enabled, function parameters are passed in registers `a10` through `a15`, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registers `a2` through `a7`. Register `a7` may be used as a frame pointer. Register window is rotated 8 registers back upon return, as one does. 💫 When this version of the ABI is enabled, the C preprocessor symbol `__XTENSA_WINDOWED_ABI__` is defined.

- **`-mextra-l32r-costs=`*n* — specify an extra cost of instruction RAM/ROM access for `L32R` instructions, in clock cycles ⏱️. When optimizing for speed, this affects whether loading a constant from a literal pool using `L32R` or synthesizing the constant from a small one with a couple of arithmetic instructions. The default value is `0`.

- **`-mstrict-align`**, **`-mno-strict-align`** — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is `-mno-strict-align` for cores that support both unaligned loads and stores in hardware, and `-mstrict-align` for all other cores. Choose wisely 🫶

---

## 🌈 zSeries Options

These are listed under

# 🌸 ENVIRONMENT 🌸

okay bestie, this section is literally about the environment (no, not the planet 💀) — it's the env vars that change how GCC behaves. Some of them work by telling GCC which directories or prefixes to use when it's out here searching for files, and some just set other vibes of the compilation environment. 🦋

> ngl, you can *also* specify places to search with options like `-B`, `-I` and `-L`, and those take precedence over anything you set with environment variables — which in turn take precedence over whatever GCC's configuration says. Order matters, iykyk. 👑

## 💅 `LANG`, `LC_CTYPE`, `LC_MESSAGES`, `LC_ALL`

These cuties control how GCC uses localization info, which is what lets GCC work with different national conventions. GCC inspects the locale categories `LC_CTYPE` and `LC_MESSAGES` if it has been configured to do so. You can set these locale categories to any value your installation supports. A typical value is `en_GB.UTF-8` for English in the United Kingdom encoded in UTF-8. 🇬🇧✨

The `LC_CTYPE` environment variable specifies character classification. GCC uses it to determine the character boundaries in a string — which is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape. 😭 (imagine your quote mark just ending a string uninvited. not on her watch.)

The `LC_MESSAGES` environment variable specifies the language to use in diagnostic messages.

If the `LC_ALL` environment variable is set, it overrides the value of `LC_CTYPE` and `LC_MESSAGES`; otherwise, `LC_CTYPE` and `LC_MESSAGES` default to the value of the `LANG` environment variable. And if none of these variables are set, GCC defaults to traditional C English behavior. 🎀

## 🍓 `TMPDIR`

If `TMPDIR` is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is going to be used as input to the next stage — for example, the output of the preprocessor, which is the input to the compiler proper. It's giving relay race, honestly. 🏃‍♀️💨

## 🔥 `GCC_COMPARE_DEBUG`

Setting `GCC_COMPARE_DEBUG` is nearly equivalent to passing `-fcompare-debug` to the compiler driver. See the documentation of that option for more details — she's basically the env-var twin of that flag. 👯‍♀️

## ⭐ `GCC_EXEC_PREFIX`

If `GCC_EXEC_PREFIX` is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

If `GCC_EXEC_PREFIX` is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

The default value of `GCC_EXEC_PREFIX` is `prefix/lib/gcc/` where *prefix* is the prefix to the installed compiler. In many cases *prefix* is the value of "`prefix`" when you ran the `configure` script.

Other prefixes specified with `-B` take precedence over this prefix.

This prefix is also used for finding files such as `crt0.o` that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with `/usr/local/lib/gcc` (more precisely, with the value of `GCC_INCLUDE_DIR`), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. So, with `-Bfoo/`, GCC searches `foo/bar` just before it searches the standard directory `/usr/local/lib/bar`. If a standard directory begins with the configured prefix then the value of *prefix* is replaced by `GCC_EXEC_PREFIX` when looking for header files. 🤯 (this one is lowkey a brain-bender, read it twice, bestie.)

## 🦋 `COMPILER_PATH`

The value of `COMPILER_PATH` is a colon-separated list of directories, much like `PATH`. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using `GCC_EXEC_PREFIX`. 🫶

## 🌈 `LIBRARY_PATH`

The value of `LIBRARY_PATH` is a colon-separated list of directories, much like `PATH`. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using `GCC_EXEC_PREFIX`. Linking using GCC also uses these directories when searching for ordinary libraries for the `-l` option (but directories specified with `-L` come first). 💖

## 💜 `LANG` (yes, again — she's got a second job)

This variable is used to pass locale information to the compiler. One way this info gets used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for `LANG` are recognized:

- **`C-JIS`** — Recognize JIS characters. 🎀
- **`C-SJIS`** — Recognize SJIS characters. ✨
- **`C-EUCJP`** — Recognize EUCJP characters. 🌸

If `LANG` is not defined, or if it has some other value, then the compiler uses `mblen` and `mbtowc` as defined by the default locale to recognize and translate multibyte characters. 💅

## 👑 `GCC_EXTRA_DIAGNOSTIC_OUTPUT`

If `GCC_EXTRA_DIAGNOSTIC_OUTPUT` is set to one of the following values, then additional text will be emitted to `stderr` when fix-it hints are emitted. Bestie, heads up: `-fdiagnostics-parseable-fixits` and `-fno-diagnostics-parseable-fixits` take precedence over this environment variable. 🔥

- **`fixits-v1`** — Emit parseable fix-it hints, equivalent to `-fdiagnostics-parseable-fixits`. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
- **`fixits-v2`** — As `fixits-v1`, but columns are expressed as display columns, as per `-fdiagnostics-column-unit=display`.

---

Some additional environment variables affect the behavior of the preprocessor. 🧚 prep yourself.

## 🎀 `CPATH`, `C_INCLUDE_PATH`, `CPLUS_INCLUDE_PATH`, `OBJC_INCLUDE_PATH`

Each variable's value is a list of directories separated by a special character, much like `PATH`, in which to look for header files. The special character, `PATH_SEPARATOR`, is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon. 💀

`CPATH` specifies a list of directories to be searched as if specified with `-I`, but after any paths given with `-I` options on the command line. This environment variable is used regardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with `-isystem`, but after any paths given with `-isystem` options on the command line.

In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of `CPATH` is `:/special/include`, that has the same effect as `-I. -I/special/include`. 🫶

## 🍓 `DEPENDENCIES_OUTPUT`

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.

The value of `DEPENDENCIES_OUTPUT` can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form *file* *target*, in which case the rules are written to file *file* using *target* as the target name.

In other words, this environment variable is equivalent to combining the options `-MM` and `-MF`, with an optional `-MT` switch too. slay, she's a multitasker. ✨

## 🦋 `SUNPRO_DEPENDENCIES`

This variable is the same as `DEPENDENCIES_OUTPUT` (see above), except that system header files are not ignored, so it implies `-M` rather than `-MM`. However, the dependence on the main input file is omitted. 👀

## 🌸 `SOURCE_DATE_EPOCH`

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the `__DATE__` and `__TIME__` macros, so that the embedded timestamps become reproducible. (reproducible builds bestie, we love her 🫶)

The value of `SOURCE_DATE_EPOCH` must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of `date +%s` on GNU/Linux and other systems that support the `%s` extension in the `date` command. 🕰️

The value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process.

---

# 💀 BUGS 💀

For instructions on reporting bugs, see <https://gitlab.archlinux.org/archlinux/packaging/packages/kgname/-/issues>. go off, file that report, bestie 🐛✨

---

# 🎀 FOOTNOTES 🎀

1. On some systems, `gcc -shared` needs to build supplementary stub code for constructors to work. On multi-libbed systems, `gcc -shared` must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. `-shared` suppresses the addition of startup code to alter the floating-point environment as done with `-ffast-math`, `-Ofast` or `-funsafe-math-optimizations` on some targets. 😭 (subtle defects are *literally* the worst kind, do not skip the flags.)

---

# 🌈 SEE ALSO 🌈

`gpl(7)`, `gfdl(7)`, `fsf-funding(7)`, `cpp(1)`, `gcov(1)`, `as(1)`, `ld(1)`, `gdb(1)` and the Info entries for `gcc`, `cpp`, `as`, `ld`, `binutils` and `gdb`. 🫶 read the whole friend group, iykyk.

---

# 👑 AUTHOR 👑

See the Info entry for `gcc`, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. 💖 (yes, there are *so* many of them, icons only.)

---

# ⭐ COPYRIGHT ⭐

Copyright (c) 1988-2025 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the `gfdl(7)` man page. 📜

(a) The FSF's Front-Cover Text is:

A GNU Manual


(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.


---

okay and that's a wrap on the whole page, bestie!! 🎉 you just read the ENTIRE GCC(1) manual — main character energy fr fr. this gorgeous thing is `gcc-15.2.0`, dated `2025-08-08` (that's the version and the date, straight from the footer, not me making stuff up 💅). now go compile something iconic. 🦋💖✨
📦 where this page comes from