GCC(1)

GCC(1) — GNU project C and C++ compiler ✨

bestie, welcome to the gcc(1) manual. this is part 1 of 131, so we're just getting started. no cap. 🫶

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NAME 💖

gcc — GNU project C and C++ compiler. periodt. 👑

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SYNOPSIS 🦋

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

Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.

---

DESCRIPTION ✨

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

Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.

Most of the command-line options that 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.

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.

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OPTIONS 👑

Option Summary 📋

Here is a summary of all the options, grouped by type. Explanations are 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]

---

ok so warning options bestie. this is the section where GCC decides whether to be your supportive bestie or your brutally honest bestie 💅 these are the flags that make the compiler notice things about your code. some of them are on by default, some you gotta ask for, some you gotta ask it to stop doing (hi, all the -Wno- ones 👋). either way, here's the whole roster, no skips, no cuts, we're doing all of them.

🌸 Warning Options 🌸

• -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

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still with me? good, because we're like 10% through this list 😭 keep going:

• -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

okay that was a LOT. hydrate. breathe. we're not done 💀

---

🔮 Static Analyzer Options 🔮

next up: the static analyzer, aka GCC's little detective hat 🕵️ she goes digging through your code looking for trouble before it ever runs. here's every knob, switch, and dump-flag she takes:

• -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 exclusive bestie — C and Objective-C only, so if you're writing C++ don't even look, it's not for you 🙈

• -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 now, the moment your debugger has been waiting for: debug info 🎀 these are the flags that tell GCC how much little breadcrumb trail to leave behind in your binary so that later-you (or gdb) can actually figure out what past-you was thinking. there are a lot of these and they're all listed right here, no cap:

• -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

---

that's the debugging crew, every single one of them accounted for 💖 and bestie, we are still deep in this manpage — there's more, obviously, there's always more, so stay tuned.

🌸 Optimization Options 🌸

okay bestie, deep breath, because this list is long and it's giving "I have 250 best friends and I love them all". Every single one of these is a real flag, spelled exactly like this, and I'm not skipping a single one because we don't do that here 💅

• -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

and THAT'S the whole optimization roster, we ate, no crumbs left 🍓

---

💖 Program Instrumentation Options 💖

next up: the flags that watch your program while it runs, like little fairy godmothers with clipboards 🧚 keep them exactly as written, bestie.

• -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 literally the opening act before the main show, and these are its flags. iconic behavior, honestly.

• -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 🎀

just two little besties here and they're both "pass this straight through to the assembler" energy. short and slay.

• -Wa,option
• -Xassembler option

---

🦋 Linker Options 🦋

the linking era!! this is where all your object files hold hands and become one executable 🫶

• 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 🌈

where does gcc look for stuff? these flags, bestie. they're the map 🗺️

• -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 👑

the moment where source code actually becomes instructions. main character energy, truly 👑

• -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 🧚

last block of this part and it's the nerdy one (affectionate). this is where gcc dumps its whole diary for you 🔥

• -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, deep breath 💖 this is the part where GCC stops being a generalist and gets hyper-specific about your chip. every architecture out there rolls up with its own little posse of flags, and we are listing every single one because we don't skip. no cap. 🎀

---

🌸 AArch64 Options 🌸

the 64-bit ARM girlies are here and they brought receipts ✨

• -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 🧚

the parallel-computing cutie with the tiniest stack offsets 💜

• -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 💅

GPU besties, this one's short and sweet fr fr

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

---

🎀 ARC Options 🎀

okay this list is long but we're not skipping a single one, periodt

• -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 🦋

the OGs, the icons, the ones you already know 👑

• -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 🍓

tiny microcontroller energy, huge option list 🌈

• -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 💜

• -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 ✨

short, sweet, ate 💅

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

---

🌈 CRIS Options 🌈

• -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 🎀

• -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 🦋

windows girlies, we see you 🫶

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

---

🍓 Darwin Options 🍓

macOS besties, this list is a whole meal (yes, some entries show up twice in the original — we're keeping them exactly as-is 💖)

• -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 💅

the retro icon serving vintage vibes 🌸

• -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 🌈

kernel-level main character energy fr fr 💜

• -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 🧚

• -msmall-model
• -mno-lsim

---

✨ FT32 Options ✨

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

---

🎀 FRV Options 🎀

• -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 🦋

libc picking time bestie, choose your fighter 🫶

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

---

🌸 H8/300 Options 🌸

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

---

👑 HPPA Options 👑

• -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 💜

Itanium besties, this is a lot and you know it 😭

• -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 🧚

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

✨ LoongArch Options ✨

okay so LoongArch walked in and said "here's my entire personality," and honestly? respect. 💅 every one of these is a real flag, keep them exactly as written:

• -march=arch-type
• -mtune=tune-type
• -mabi=base-abi-type
• -mfpu=fpu-type
• -msimd=simd-type
• -msoft-float
• -msingle-float
• -mdouble-float
• -mlsx
• -mno-lsx
• -mlasx
• -mno-lasx
• -mbranch-cost=n
• -maddr-reg-reg-cost=n
• -mcheck-zero-division
• -mno-check-zero-division
• -mcond-move-int
• -mno-cond-move-int
• -mcond-move-float
• -mno-cond-move-float
• -memcpy
• -mno-memcpy
• -mstrict-align
• -mno-strict-align
• -G num
• -mmax-inline-memcpy-size=n
• -mexplicit-relocs=style
• -mexplicit-relocs
• -mno-explicit-relocs
• -mdirect-extern-access
• -mno-direct-extern-access
• -mcmodel=code-model
• -mrelax
• -mpass-mrelax-to-as
• -mrecip
• -mrecip=opt
• -mfrecipe
• -mno-frecipe
• -mdiv32
• -mno-div32
• -mlam-bh
• -mno-lam-bh
• -mlamcas
• -mno-lamcas
• -mld-seq-sa
• -mno-ld-seq-sa
• -mtls-dialect=opt
• -mannotate-tablejump
• -mno-annotate-tablejump

---

🌸 M32C Options 🌸

short list, tiny icon, big vibes:

• -mcpu=cpu
• -msim
• -memregs=number

---

🦋 M32R/D Options 🦋

• -m32r2
• -m32rx
• -m32r
• -mdebug
• -malign-loops
• -mno-align-loops
• -missue-rate=number
• -mbranch-cost=number
• -mmodel=code-size-model-type
• -msdata=sdata-type
• -mno-flush-func
• -mflush-func=name
• -mno-flush-trap
• -mflush-trap=number
• -G num

---

💖 M680x0 Options 💖

the 68000 family said "let me serve," and she DID:

• -march=arch
• -mcpu=cpu
• -mtune=tune
• -m68000
• -m68020
• -m68020-40
• -m68020-60
• -m68030
• -m68040
• -m68060
• -mcpu32
• -m5200
• -m5206e
• -m528x
• -m5307
• -m5407
• -mcfv4e
• -mbitfield
• -mno-bitfield
• -mc68000
• -mc68020
• -mnobitfield
• -mrtd
• -mno-rtd
• -mdiv
• -mno-div
• -mshort
• -mno-short
• -mhard-float
• -m68881
• -msoft-float
• -mpcrel
• -malign-int
• -mstrict-align
• -msep-data
• -mno-sep-data
• -mshared-library-id=n
• -mid-shared-library
• -mno-id-shared-library
• -mxgot
• -mno-xgot
• -mlong-jump-table-offsets

---

🎀 MCore Options 🎀

• -mhardlit
• -mno-hardlit
• -mdiv
• -mno-div
• -mrelax-immediates
• -mno-relax-immediates
• -mwide-bitfields
• -mno-wide-bitfields
• -m4byte-functions
• -mno-4byte-functions
• -mcallgraph-data
• -mno-callgraph-data
• -mslow-bytes
• -mno-slow-bytes
• -mno-lsim
• -mlittle-endian
• -mbig-endian
• -m210
• -m340
• -mstack-increment

---

🧚 MicroBlaze Options 🧚

• -msoft-float
• -mhard-float
• -msmall-divides
• -mcpu=cpu
• -mmemcpy
• -mxl-soft-mul
• -mxl-soft-div
• -mxl-barrel-shift
• -mxl-pattern-compare
• -mxl-stack-check
• -mxl-gp-opt
• -mno-clearbss
• -mxl-multiply-high
• -mxl-float-convert
• -mxl-float-sqrt
• -mbig-endian
• -mlittle-endian
• -mxl-reorder
• -mxl-mode-app-model
• -mpic-data-is-text-relative

---

👑 MIPS Options 👑

bestie. BESTIE. this is the big one. MIPS has so many flags and she's not sorry about it. stay with me, we're going all the way down. 🔥

• -EL
• -EB
• -march=arch
• -mtune=arch
• -mips1
• -mips2
• -mips3
• -mips4
• -mips32
• -mips32r2
• -mips32r3
• -mips32r5
• -mips32r6
• -mips64
• -mips64r2
• -mips64r3
• -mips64r5
• -mips64r6
• -mips16
• -mno-mips16
• -mflip-mips16
• -minterlink-compressed
• -mno-interlink-compressed
• -minterlink-mips16
• -mno-interlink-mips16
• -mabi=abi
• -mabicalls
• -mno-abicalls
• -mshared
• -mno-shared
• -mplt
• -mno-plt
• -mxgot
• -mno-xgot
• -mgp32
• -mgp64
• -mfp32
• -mfpxx
• -mfp64
• -mhard-float
• -msoft-float
• -mno-float
• -msingle-float
• -mdouble-float
• -modd-spreg
• -mno-odd-spreg
• -mabs=mode
• -mnan=encoding
• -mdsp
• -mno-dsp
• -mdspr2
• -mno-dspr2
• -mmcu
• -mmno-mcu
• -meva
• -mno-eva
• -mvirt
• -mno-virt
• -mxpa
• -mno-xpa
• -mcrc
• -mno-crc
• -mginv
• -mno-ginv
• -mmicromips
• -mno-micromips
• -mmsa
• -mno-msa
• -mloongson-mmi
• -mno-loongson-mmi
• -mloongson-ext
• -mno-loongson-ext
• -mloongson-ext2
• -mno-loongson-ext2
• -mfpu=fpu-type
• -msmartmips
• -mno-smartmips
• -mpaired-single
• -mno-paired-single
• -mdmx
• -mno-mdmx
• -mips3d
• -mno-mips3d
• -mmt
• -mno-mt
• -mllsc
• -mno-llsc
• -mlong64
• -mlong32
• -msym32
• -mno-sym32
• -Gnum
• -mlocal-sdata
• -mno-local-sdata
• -mextern-sdata
• -mno-extern-sdata
• -mgpopt
• -mno-gopt
• -membedded-data
• -mno-embedded-data
• -muninit-const-in-rodata
• -mno-uninit-const-in-rodata
• -mcode-readable=setting
• -msplit-addresses
• -mno-split-addresses
• -mexplicit-relocs
• -mno-explicit-relocs
• -mexplicit-relocs=release
• -mcheck-zero-division
• -mno-check-zero-division
• -mdivide-traps
• -mdivide-breaks
• -mload-store-pairs
• -mno-load-store-pairs
• -mstrict-align
• -mno-strict-align
• -mno-unaligned-access
• -munaligned-access
• -mmemcpy
• -mno-memcpy
• -mlong-calls
• -mno-long-calls
• -mmad
• -mno-mad
• -mimadd
• -mno-imadd
• -mfused-madd
• -mno-fused-madd
• -nocpp
• -mfix-24k
• -mno-fix-24k
• -mfix-r4000
• -mno-fix-r4000
• -mfix-r4400
• -mno-fix-r4400
• -mfix-r5900
• -mno-fix-r5900
• -mfix-r10000
• -mno-fix-r10000
• -mfix-rm7000
• -mno-fix-rm7000
• -mfix-vr4120
• -mno-fix-vr4120
• -mfix-vr4130
• -mno-fix-vr4130
• -mfix-sb1
• -mno-fix-sb1
• -mflush-func=func
• -mno-flush-func
• -mbranch-cost=num
• -mbranch-likely
• -mno-branch-likely
• -mcompact-branches=policy
• -mfp-exceptions
• -mno-fp-exceptions
• -mvr4130-align
• -mno-vr4130-align
• -msynci
• -mno-synci
• -mlxc1-sxc1
• -mno-lxc1-sxc1
• -mmadd4
• -mno-madd4
• -mrelax-pic-calls
• -mno-relax-pic-calls
• -mmcount-ra-address
• -mframe-header-opt
• -mno-frame-header-opt

---

⭐ MMIX Options ⭐

• -mlibfuncs
• -mno-libfuncs
• -mepsilon
• -mno-epsilon
• -mabi=gnu
• -mabi=mmixware
• -mzero-extend
• -mknuthdiv
• -mtoplevel-symbols
• -melf
• -mbranch-predict
• -mno-branch-predict
• -mbase-addresses
• -mno-base-addresses
• -msingle-exit
• -mno-single-exit

---

🌈 MN10300 Options 🌈

• -mmult-bug
• -mno-mult-bug
• -mno-am33
• -mam33
• -mam33-2
• -mam34
• -mtune=cpu-type
• -mreturn-pointer-on-d0
• -mno-crt0
• -mrelax
• -mliw
• -msetlb

---

🍓 Moxie Options 🍓

tiny, cute, four flags, she's an icon:

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

💜 MSP430 Options 💜

• -msim
• -masm-hex
• -mmcu=
• -mcpu=
• -mlarge
• -msmall
• -mrelax
• -mwarn-mcu
• -mcode-region=
• -mdata-region=
• -msilicon-errata=
• -msilicon-errata-warn=
• -mhwmult=
• -minrt
• -mtiny-printf
• -mmax-inline-shift=

---

🫶 NDS32 Options 🫶

• -mbig-endian
• -mlittle-endian
• -mreduced-regs
• -mfull-regs
• -mcmov
• -mno-cmov
• -mext-perf
• -mno-ext-perf
• -mext-perf2
• -mno-ext-perf2
• -mext-string
• -mno-ext-string
• -mv3push
• -mno-v3push
• -m16bit
• -mno-16bit
• -misr-vector-size=num
• -mcache-block-size=num
• -march=arch
• -mcmodel=code-model
• -mctor-dtor
• -mrelax

---

🎀 Nvidia PTX Options 🎀

three flags. minimal. clean. she's giving less-is-more:

• -m64
• -mmainkernel
• -moptimize

---

🌸 OpenRISC Options 🌸

• -mboard=name
• -mnewlib
• -mhard-mul
• -mhard-div
• -msoft-mul
• -msoft-div
• -msoft-float
• -mhard-float
• -mdouble-float
• -munordered-float
• -mcmov
• -mror
• -mrori
• -msext
• -msfimm
• -mshftimm
• -mcmodel=code-model

---

🦋 PDP-11 Options 🦋

vintage bestie behavior, and we love her for it:

• -mfpu
• -msoft-float
• -mac0
• -mno-ac0
• -m40
• -m45
• -m10
• -mint32
• -mno-int16
• -mint16
• -mno-int32
• -msplit
• -munix-asm
• -mdec-asm
• -mgnu-asm
• -mlra

---

🔥 PowerPC Options 🔥

okay so PowerPC is not gonna list her whole self here, she's sending you to her other page:

See RS/6000 and PowerPC Options.

---

✨ PRU Options ✨

• -mmcu=mcu
• -minrt
• -mno-relax
• -mloop
• -mabi=variant

---

🌈 RISC-V Options 🌈

RISC-V said "I also have a lot to say" and honestly same energy as MIPS:

• -mbranch-cost=N-instruction
• -mplt
• -mno-plt
• -mabi=ABI-string
• -mfdiv
• -mno-fdiv
• -mfence-tso
• -mno-fence-tso
• -mdiv
• -mno-div
• -misa-spec=ISA-spec-string
• -march=ISA-string
• -mtune=processor-string
• -mpreferred-stack-boundary=num
• -msmall-data-limit=N-bytes
• -msave-restore
• -mno-save-restore
• -mshorten-memrefs
• -mno-shorten-memrefs
• -mstrict-align
• -mno-strict-align
• -mcmodel=medlow
• -mcmodel=medany
• -mcmodel=large
• -mexplicit-relocs
• -mno-explicit-relocs
• -mrelax
• -mno-relax
• -mriscv-attribute
• -mno-riscv-attribute
• -malign-data=type
• -mbig-endian
• -mlittle-endian
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mcsr-check
• -mno-csr-check
• -mmovcc
• -mno-movcc
• -minline-atomics
• -mno-inline-atomics
• -minline-strlen
• -mno-inline-strlen
• -minline-strcmp
• -mno-inline-strcmp
• -minline-strncmp
• -mno-inline-strncmp
• -mtls-dialect=desc
• -mtls-dialect=trad

---

💅 RL78 Options 💅

• -msim
• -mmul=none
• -mmul=g13
• -mmul=g14
• -mallregs
• -mcpu=g10
• -mcpu=g13
• -mcpu=g14
• -mg10
• -mg13
• -mg14
• -m64bit-doubles
• -m32bit-doubles
• -msave-mduc-in-interrupts

---

👑 RS/6000 and PowerPC Options 👑

and now, the moment you've been waiting for, the main character herself. RS/6000 and PowerPC said "let me cook" and cooked a whole feast. no skips, we read every single one. 🫡

• -mcpu=cpu-type
• -mtune=cpu-type
• -mcmodel=code-model
• -mpowerpc64
• -maltivec
• -mno-altivec
• -mpowerpc-gpopt
• -mno-powerpc-gpopt
• -mpowerpc-gfxopt
• -mno-powerpc-gfxopt
• -mmfcrf
• -mno-mfcrf
• -mpopcntb
• -mno-popcntb
• -mpopcntd
• -mno-popcntd
• -mfprnd
• -mno-fprnd
• -mcmpb
• -mno-cmpb
• -mhard-dfp
• -mno-hard-dfp
• -mfull-toc
• -mminimal-toc
• -mno-fp-in-toc
• -mno-sum-in-toc
• -m64
• -m32
• -mxl-compat
• -mno-xl-compat
• -mpe
• -malign-power
• -malign-natural
• -msoft-float
• -mhard-float
• -mmultiple
• -mno-multiple
• -mupdate
• -mno-update
• -mavoid-indexed-addresses
• -mno-avoid-indexed-addresses
• -mfused-madd
• -mno-fused-madd
• -mbit-align
• -mno-bit-align
• -mstrict-align
• -mno-strict-align
• -mrelocatable
• -mno-relocatable
• -mrelocatable-lib
• -mno-relocatable-lib
• -mtoc
• -mno-toc
• -mlittle
• -mlittle-endian
• -mbig
• -mbig-endian
• -mdynamic-no-pic
• -mswdiv
• -msingle-pic-base
• -mprioritize-restricted-insns=priority
• -msched-costly-dep=dependence_type
• -minsert-sched-nops=scheme
• -mcall-aixdesc
• -mcall-eabi
• -mcall-freebsd
• -mcall-linux
• -mcall-netbsd
• -mcall-openbsd
• -mcall-sysv
• -mcall-sysv-eabi
• -mcall-sysv-noeabi
• -mtraceback=traceback_type
• -maix-struct-return
• -msvr4-struct-return
• -mabi=abi-type
• -msecure-plt
• -mbss-plt
• -msplit-patch-nops
• -mlongcall
• -mno-longcall
• -mpltseq
• -mno-pltseq
• -mblock-move-inline-limit=num
• -mblock-compare-inline-limit=num
• -mblock-compare-inline-loop-limit=num
• -mno-block-ops-unaligned-vsx
• -mstring-compare-inline-limit=num
• -misel
• -mno-isel
• -mvrsave
• -mno-vrsave
• -mmulhw
• -mno-mulhw
• -mdlmzb
• -mno-dlmzb
• -mprototype
• -mno-prototype
• -msim
• -mmvme
• -mads
• -myellowknife
• -memb
• -msdata
• -msdata=opt
• -mreadonly-in-sdata
• -mvxworks
• -G num
• -mrecip
• -mrecip=opt
• -mno-recip
• -mrecip-precision
• -mno-recip-precision
• -mveclibabi=type
• -mfriz
• -mno-friz
• -mpointers-to-nested-functions
• -mno-pointers-to-nested-functions
• -msave-toc-indirect
• -mno-save-toc-indirect
• -mpower8-fusion
• -mno-mpower8-fusion
• -mcrypto
• -mno-crypto
• -mhtm
• -mno-htm
• -mquad-memory
• -mno-quad-memory
• -mquad-memory-atomic
• -mno-quad-memory-atomic
• -mcompat-align-parm
• -mno-compat-align-parm
• -mfloat128
• -mno-float128
• -mfloat128-hardware
• -mno-float128-hardware
• -mgnu-attribute
• -mno-gnu-attribute
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mprefixed
• -mno-prefixed
• -mpcrel
• -mno-pcrel
• -mmma
• -mno-mmma
• -mrop-protect
• -mno-rop-protect
• -mprivileged
• -mno-privileged

💖 RX Options (RX Options)

okay so these are the RX knobs, and there are a lot of them, so buckle up 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)

main character energy, big iron edition:

-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)

-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)

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

---

⭐ SPARC Options (SPARC Options)

-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)

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

---

🦋 V850 Options (V850 Options)

-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)

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

---

🎀 Visium Options (Visium Options)

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

---

🍓 VMS Options (VMS Options)

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

---

💅 VxWorks Options (VxWorks Options)

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

---

🔥 x86 Options (x86 Options)

okay this one is the big one, the moment, the icon, the legend — look how many there are:

-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

See Cygwin and MinGW Options. (no cap, that's literally all it says here 💀)

---

✨ Xstormy16 Options (Xstormy16 Options)

-msim

---

🧚 Xtensa Options (Xtensa Options)

-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

See S/390 and zSeries Options. (they said what they said, just scroll up bestie 😌)

---

✨ Options Controlling the Kind of Output ✨

okay so the way compilation works is a whole four-stage pipeline and it ALWAYS goes in this exact order: preprocessing, compilation proper, assembly, and linking. literally always, no skips.

GCC can preprocess and compile several files — either into several assembler input files, or into one assembler input file. then each assembler input file produces an object file, and then linking swoops in and combines all the object files (the freshly compiled ones AND the ones you passed as input) into one executable file. it's giving assembly line, it's giving teamwork, we love to see it 💅

---

🎀 the file suffix situation

for any given input file, the file name suffix is what decides what kind of compilation happens. so like, pay attention to your extensions, they're doing the heavy lifting:

• 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. heads up: you must link with the libobjc library to make an Objective-C program actually 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.
• 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 (this is the 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. note that in .cxx, the last two letters must both be literally x. likewise, .C refers to a literal capital C.
• file.mm, file.M — Objective-C++ source code that must be preprocessed.
• file.mii — Objective-C++ source code that should not be preprocessed.
• 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 gets treated this way. (mystery file? straight to linking. iconic 🦋)

Okay so you can also tell gcc the input language straight up with the -x option, bestie: 💅

• -x language — Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. 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

No cap, -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.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. And honestly? It does not 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.*. Slay. ✨

When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs. It's giving "everyone gets their own name tag". 🏷️

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

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. Contrast with the single-input compilation:

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

that, 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. Iykyk. 💅

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. No cap, that's the whole tea. 🫖

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 — Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. Bestie, it's literally the "what is actually happening" button. 👑
• -### — 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. So useful for scripting, no cap. 🐚
• --help — Print (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. 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. The way this just gives you all the lore. 📖
• --target-help — Print (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. Target-specific era. 🎯
• --help={class|[^]qualifier}[,...] — Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. 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.

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 — Display the version number and copyrights of the invoked GCC. Gotta know what era you're in. 🕰️
• -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. Dramatic exit codes, honestly. 💀
• -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. Pipes are iconic, temporary files are not. 🚰
• -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. Customization era. 🎀
• -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 .... Wrapped and slaying. 🎁

• -ffile-prefix-map=old=new — okay so when you're compiling files that live in the directory old, this records every reference to them in the compile output as if those files were chilling in directory new instead. ✨ Specifying this one is literally the same as specifying all the individual -f*-prefix-map options. Bestie, this is how you get reproducible builds that don't care where you built them — location independent, slay. One thing tho: 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 for the -f*-prefix-map options, normally the comparison of the old prefix against the filename that would normally be referenced in the compile result is done with a textual comparison of the prefixes — or ignoring character case on case insensitive filesystems, and treating slashes and backslashes as equal on DOS based filesystems. The -fcanon-prefix-map makes those comparisons happen on canonicalized paths of old and the referenced filename instead. 💅
• -fplugin=name.so — loads the plugin code in file name.so, which is 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 purposes (peep -fplugin-arg-name-key=value below). Every plugin should define the callback functions specified in the Plugins API, no cap.
• -fplugin-arg-name-key=value — defines an argument called key with a value of value for the plugin called name. 🎀
• -fdump-ada-spec[-slim] — for C and C++ source and include files, this generates the corresponding Ada specs. 😌
• -fada-spec-parent=unit — works together with -fdump-ada-spec[-slim] above to generate Ada specs as child units of parent unit. 🌸
• -fdump-go-spec=file — for input files in any language, this generates corresponding Go declarations in file. It generates Go "const", "type", "var", and "func" declarations, which lowkey might be 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 that get read are inserted right in place of the original @file option. 🫶 If file doesn't exist, or can't be read, then the option is treated literally and not removed.

Options inside file are separated by whitespace. A whitespace character can be included in an option by wrapping the entire option in either single or double quotes. Any character (yes, even a backslash) can be included by prefixing the character you want with a backslash. And the file can itself contain additional @file options — any of those get processed recursively. ✨

---

💖 Compiling C++ Programs 💖

Okay bestie, listen. C++ source files conventionally use one of these suffixes: .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx. 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 exact same way you would for compiling C programs (usually with the name gcc). 👑

However (and this is the part people trip on), 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 you're precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed under the name c++. 💅

When you compile C++ programs, you may specify many of the same command-line options 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. Periodt.

---

🎀 Options Controlling C Dialect 🎀

The following options control the dialect of C (or languages derived from C, like C++, Objective-C and Objective-C++) that the compiler accepts:

• -ansi — in C mode, this is equivalent to -std=c90. In C++ mode, it's equivalent to -std=c++98. ✨
• -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 — like 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 used by -Wpedantic 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.
• 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). Peep 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. This standard is 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. 🌸
• 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. 💀
• 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. 💀

• -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.

Okay so besides just the declarations themselves, that file also spills the tea in comments: it tells you the origin of every single declaration (source file and line 🗺️), whether that declaration was implicit, prototyped or unprototyped (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 (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, right after the declaration. 💅

---

• -fno-asm — bestie, this one tells GCC to stop recognizing asm, inline and typeof as keywords, so your code is free to use those words as identifiers instead. No cap. You can still reach for the keywords __asm__, __inline__ and __typeof__ whenever you need them. In C, -ansi implies -fno-asm.

In C++, inline is a standard keyword and is not affected by this switch, periodt. If that's the vibe you're going for, you may want -fno-gnu-keywords instead, which disables typeof but not asm and inline. And 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. ✨

• -fno-builtin, -fno-builtin-function — don't recognize built-in functions that don't begin with __builtin_ as prefix. Okay so here's the lore: 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.

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 — so 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))

• -fcond-mismatch — allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. Heads up though: this option is not supported for C++. 🙈
• -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.
• -fgimple — enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. 🧪
• -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). 🚫

• -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. 🔍

• -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.
• -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. fr fr, just don't. 🙅
• -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 — 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. ✨

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. 🧩

• -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. 👑
• -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++. 💅

• -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. 🌸
• -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. 🔁

• -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. 🫶

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. periodt.

• -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. 🦋

There are two more levels chillin' between 0 and 3, and they exist purely to hold space for older code that leans on the GCC zero-length array extension ([0]) or a one-element array acting as a flexible array member ([1]). So when level is 1, the trailing array gets treated as a flexible array member whether it's declared as [], [0], or [1]. And when level is 2, the trailing array counts as a flexible array member only when it's declared as [] or [0]. Bestie, it's literally just vibes scaling down.

You can also control this behavior for 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 a duo, iykyk. 💅

• -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 (which is the default). This option is not supported for C++. 🫶

⚠️ Warning: the -fsso-struct switch makes GCC spit out code that is not binary compatible with code generated without it, if the endianness you picked isn't the native endianness of the target. So like… be careful, okay?

---

🌈 Options Controlling C++ Dialect 🌈

This section is all about the command-line options that are only meaningful for C++ programs. But ngl, you can still use most of the GNU compiler options no matter what language your program is in. For example, you might compile a file firstClass.C like this:

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

In this example, only -fstrict-enums is an option meant only for C++ programs — the rest you can use with any language GCC supports. Slay, flexible much?

Some options for compiling C programs, like -std, are also relevant for C++ programs. She's versatile like that. ✨

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. 👑
  • Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. (Yes, she moves. It's giving chaos but it's fine.)
  • 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 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. (oop, an accidental moment 😭)
  • Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.
  • 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 when defining a symbol with an incorrect mangled name. This switch specifies 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.

If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning.

• -fno-access-control — turn off all access checking. This switch is mainly useful for working around bugs in the access control code. Not her being the "just ignore the rules" queen 💅
• -faligned-new — enable support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument such as -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t).

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

• -fno-assume-sane-operators-new — so the C++ standard allows replacing the global new, new[], delete and delete[] operators, 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 are 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 allows control over some optimizations around calls to those operators. With -fassume-sane-operators-new-delete option 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 (global state; except for errno for the new and new[] operators). This allows most optimizations across those calls and is something that the implementation provided operators satisfy unless malloc implementation details are observable in the code or unless malloc hooks are used, but might not be satisfied if a program replaces those operators. This behavior is enabled by default. With -fno-assume-sane-operators-new-delete option 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 code needs to expect such behavior.
• -fchar8_t
• -fno-char8_t — enable 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.

This option enables functions to 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 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;

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

• -fcheck-new — okay so this one makes the compiler double-check that the pointer operator new hands back is actually not null before it tries to write into that storage. Bestie, this check is usually doing the most for no reason: the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler checks the return value anyway, even without this flag. In literally every other case — when operator new has a non-empty exception specification — running out of memory is signalled by throwing std::bad_alloc. Also see new (nothrow). 💅
• -fconcepts — turns on support for the C++ Concepts feature, which is how you constrain template arguments. With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on. ✨

Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but that didn't make it into the standard, could additionally be enabled by -fconcepts-ts. The option -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15 — so fr fr, users are expected to convert their code to C++20 concepts. 🫶

• -fconstexpr-depth=n — sets the max nested evaluation depth for C++11 constexpr functions to n. You need a limit here so endless recursion during constant expression evaluation actually gets caught instead of vibing forever. The minimum the standard specifies is 512.
• -fconstexpr-cache-depth=n — sets the max level of nested evaluation depth for C++11 constexpr functions that'll be cached to n. It's a heuristic that trades off compile speed (when the cache dodges repeated calculations, slay) against memory consumption (when the cache grows huge from super recursive evaluations, yikes). Default is 8. Very few users are gonna wanna touch this, but if your code does heavy constexpr calculations you might wanna experiment to find what value works best for you. 🔥
• -fconstexpr-fp-except — 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 this, instead 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 — sets the max number of iterations for a loop in C++14 constexpr functions to n. You need a limit so infinite loops get detected during constant expression evaluation. Default is 262144 (1<<18). 🌸
• -fconstexpr-ops-limit=n — sets the max number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited by the option above, if there are several nested loops and each one has many iterations but still under that limit, or if in the body of some loop — or even outside a loop — way too many expressions need to be evaluated, the resulting constexpr evaluation might take 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 functionality is intended mostly for people interested in experimentation towards 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 to let the program continue executing after a contract violation. That is, 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.
• "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. no cap, it shows you everybody. 👀
• -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 it 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.

bestie this one is giving "I know better than the standard" energy. it will absolutely bite you if you're wrong. 😭

• -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 definition of the variable are in the same translation unit, this 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 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), they can avoid this overhead with the -fno-extern-tls-init option. 🌈

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 — do not 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 — do not 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);
}

compiles in C++20: f is an immediate-escalating function (because of the auto it's a function template and is declared constexpr) and id(t) is an immediate-escalating expression, so f is promoted to consteval. Consequently, the call to id(t) is in an immediate context, so it doesn't have to produce a constant (that is the mechanism allowing consteval function composition). However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to consteval function id(t) is not a constant expression, the compiler rejects the code. 😤

This option is turned on by default; it is only effective in C++20 mode or later.

• -fimplicit-constexpr — make 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 — okay so this one is giving “I do it myself” energy: never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. 💅
• -fno-implicit-inline-templates — Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations. ✨
• -fno-implement-inlines — To save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called. 😭
• -fmodules, -fno-modules — Enable support for C++20 modules. The -fno-modules is usually not needed, as that is the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. iykyk 🦋
• -fmodule-header, -fmodule-header=user, -fmodule-header=system — Compile a header file to create an importable header unit. 🎀
• -fmodule-implicit-inline — Member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.) 💀
• -fno-module-lazy — Disable lazy module importing and module mapper creation. 🛑
• -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] — An oracle to query for module name to filename mappings. If unspecified the CXX_MODULE_MAPPER environment variable is used, and if that is unset, an in-process default is provided. 🔮
• -fmodule-only — Only emit the Compiled Module Interface, inhibiting any object file. just the interface, bestie ✨
• -fms-extensions — Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax. 🪟
• -fnew-inheriting-ctors — Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified. 👑
• -fnew-ttp-matching — Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for -std=c++17. 🌈
• -fno-nonansi-builtins — Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions. no extras, periodt 💅
• -fnothrow-opt — Treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification. 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. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. important vibe shift 😭
• -fno-operator-names — Do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. 🚫
• -fno-optional-diags — Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class. 🤫
• -fno-pretty-templates — When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it’s clear which template is involved. 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. If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them. error messages but make them less pretty? okay bestie 🎀
• -frange-for-ext-temps — Enable lifetime extension of C++ range based for temporaries. With -std=c++23 and above this is part of the language standard, so lifetime of the temporaries is extended until the end of the loop by default. This option allows enabling that behavior also in earlier versions of the standard. 🦋
• -fno-rtti — Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.

Mixing code compiled with -frtti with that 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. 💀

• -fsized-deallocation — Enable 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. The flag -Wsized-deallocation warns about places that might want to add a definition. ✨

• -fstrict-enums — Allow the compiler to 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. This option has no effect for an enumeration type with a fixed underlying type. do not lie to your enum, bestie 👑
• -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 is 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. 🔥

• -ftemplate-backtrace-limit=n — Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. 📝
• -ftemplate-depth=n — Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed 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, as the compiler can run out of stack space before hitting 1024 in some situations. recursion go brrr but also no 😭
• -fno-threadsafe-statics — Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option 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 with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but 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 causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available. incorrect but necessary sometimes 🙈
• -fvisibility-inlines-hidden — This switch declares that the user does not attempt 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 that they do not appear in the export table of a DSO and do not 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 as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or 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 the 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. Marking the enclosing class with explicit visibility has no effect.

Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. DSO glow-up, no cap 💖

• -fvisibility-ms-compat — bestie, this one tries to use visibility settings to make GCC's C++ linkage model play nice with Microsoft Visual Studio's. ✨

Here's the tea on what it changes in GCC's linkage model:

1. It sets the default visibility to "hidden", like -fvisibility=hidden does.
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're permitted when this option is not used.

In new code it's highkey better to use -fvisibility=hidden and export those classes that are meant to be externally visible. Unfortunately it's 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.

• -fno-weak — do not use weak symbol support, even if the linker provides it. By default, G++ uses weak symbols if they're 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++. 💀
• -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:

• -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's 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.

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>;

• -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.
• -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's calling out that it's ✨deprecated✨ to declare a user-defined literal operator with a space before the suffix. Like, the space is giving 2014. It's on by default in C++23, or whenever you say -Wdeprecated out loud.

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) — snitches on you when you skip the comma before the varargs ... at the end of a function parameter list, which is deprecated behavior. On by default in C++26, or with an explicit -Wdeprecated. the comma is not optional, it's iconic.

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 yells when an (invalid!!) extra enum-base gets used in an elaborated-type-specifier. Translation: when an enum with a given underlying type and no enumerator list shows up in a declaration that isn't just a standalone declaration of the enum. It's enabled by default, and it gets upgraded to a full error with -pedantic-errors. no pressure tho 💀

---

• -Wno-init-list-lifetime (C++ and Objective-C++ only) — this one says "pls don't warn me" about std::initializer_list uses that are probably gonna leave you holding a dangling pointer. The tea ☕: the underlying array of an initializer_list is handled like a normal C++ temporary object, so it's so easy to accidentally keep a pointer to that array past the end of its lifetime. The ways this goes wrong:
  • 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 you get back 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 sitting in the heap has a dangling pointer.
  • when an initializer_list variable gets assigned from a brace-enclosed initializer list, the temporary array made for the right side of the assignment only lives until the end of the full-expression — so by the next statement, that initializer_list variable has a dangling pointer. not the vibe.

// 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, that does not extend the array's lifetime — 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 — warns when a function literally never produces a constant expression. In C++20 and earlier, for every constexpr function and function template, there had to be at least one set of function arguments in at least one instantiation such that invoking the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed that restriction ✨, so now you can have a function or function template marked constexpr where no invocation satisfies the requirements of a core constant expression. slay, I guess.

This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, and then it's just 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 — checks that all imported macro definitions are valid at the end of compilation 🧚. It's not on by default, since it needs extra processing to figure out. Lowkey useful when you're preparing sets of header-units and you want to make sure the macros stay consistent.

---

• -Wno-literal-suffix (C++ and Objective-C++ only) — don't warn when a string or character literal is followed by a ud-suffix that doesn't start with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens, to keep backwards compatibility with code using 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);
}

Here, 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, bestie.

These warnings are enabled by default.

---

• -Wno-narrowing (C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions get diagnosed by default, as the standard requires. A narrowing conversion from a constant produces an error, and one 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, warn when a narrowing conversion prohibited by C++11 happens inside { }, 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. the compiler is being a little know-it-all, respectfully. 🙈

---

• -Wnoexcept-type (C++ and Objective-C++ only) — warns if the C++17 feature that makes noexcept part of a function type changes the mangled name of a symbol compared to C++14. Enabled by -Wabi and -Wc++17-compat.

Example time:

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>. the glow-up is real. ✨

---

• -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function like memset or memcpy is an object of class type, and when writing into that object might bypass the class's 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 memset call below is undefined because it modifies a non-trivial class object and is therefore diagnosed. The safe way to initialize or clear storage of such types is using the appropriate constructor or assignment operator, if one's available. pls don't raw-dog the memory.

std::string str = "abc";
memset (&str, 0, sizeof str);

-Wclass-memaccess 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, either 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 the 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 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. rip register, you had a good run 🪦

---

• -Wreorder (C++ and Objective-C++ only) — warns when the order of member initializers written in the code doesn't match the order they must actually be executed in. 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, and emits a warning about it. This warning is enabled by -Wall. the compiler said "I'll fix it but I'm telling everyone." 💀

---

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this warning fires when a call to std::move prevents copy elision. A classic scenario where copy elision can happen is returning from a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, isn't 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. bestie, you played yourself. 😭

This warning is enabled by -Wall.

---

• -Wno-redundant-move (C++ and Objective-C++ only) — this warning calls out redundant calls to std::move; that is, when a move operation would've 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, like returning a local variable, where copy elision isn't applicable. Consider:

okay so in that example the std::move call is literally doing nothing — it's redundant, bestie, and G++ is gonna say so. 💅 And because G++ implements Core Issue 1579, there's a whole other flavour of this:

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

Here, the std::move call is redundant. Because G++ implements Core Issue 1579, another example is:

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

See what's going on? Copy elision can't apply here, because the type of the expression being returned and the function return type are different — and yet G++ treats that return value as if it were designated by an rvalue. 🙈

This warning is enabled by -Wextra.

---

-Wrange-loop-construct (C++ and Objective-C++ only)

This one yells at you when a C++ range-based for-loop is making an unnecessary copy. It happens when the range declaration isn't a reference… but like, it probably should be. 👀 Example:

struct S { char arr[128]; };
void fn () {
  S arr[5];
  for (const auto x : arr) { ... }
}

Good news tho: it does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. 🫶

It 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. Like:

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

In the example above, in every iteration of the loop a temporary value of type double is created and destroyed, to which the reference const double & is bound. 💀

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. It's giving "you didn't need that word, bestie":

struct foo;
struct foo *p;   // warn that keyword struct can be eliminated

On the other hand, in this example there is no warning:

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

---

-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 tea ☕: 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, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.

---

-Weffc++ (C++ and Objective-C++ only)

Warns about violations of the following style guidelines from Scott Meyers' Effective C++ series of books. It's the vibes-based lint of the C++ world ✨:

• 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 bases classes too.

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. 😭

---

-Wno-exceptions (C++ and Objective-C++ only)

Disable the warning about the case when an exception handler is shadowed by another handler — which can point out a wrong ordering of exception handlers. (Which is lowkey a bug, so like… your call. 🙈)

---

-Wstrict-null-sentinel (C++ and Objective-C++ only)

Warn about the use of an uncasted NULL as sentinel. When compiling only with GCC this is a valid sentinel, as NULL is defined to __null. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. 🌍🚫

---

-Wno-non-template-friend (C++ and Objective-C++ only)

Disable 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 is enabled by default. 👑

---

-Wold-style-cast (C++ and Objective-C++ only)

Warn 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

Warn 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, and code like:

B* b;
b->f();

fails to compile. 💀

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)

Disable the diagnostic for converting a bound pointer to member function to a plain pointer. Bye 👋

---

-Wsign-promo (C++ and Objective-C++ only)

Warn 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. The standard said "actually no," and we all moved on. 💅

---

-Wtemplates (C++ and Objective-C++ only)

Warn 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 one can still use the STL. One may also instantiate or specialize templates. So like… templates are fine, you just can't declare new ones. rules are rules bestie 🎀

---

-Wmismatched-new-delete (C++ and Objective-C++ only)

Warn 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, as well as 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)

Warn 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 is 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. 🪟✨

• -Wmultiple-inheritance (C++ and Objective-C++ only) — bestie, this one yells at you when a class is defined with multiple direct base classes 👀 Some coding rules literally forbid multiple inheritance, and this flag is how you enforce that rule fr. The warning stays inactive inside a system header file (hi STL 💖) so you can still use the STL no problem. Also, you can totally define classes that indirectly use multiple inheritance — that's fine, it's only the direct ones getting the side-eye.
• -Wvirtual-inheritance — this warns when a class is defined with a virtual direct base class 🦋 Same energy as above: some coding rules disallow multiple inheritance and this enforces it. Inactive inside system headers like the STL, so STL stays usable. And again, classes that indirectly use virtual inheritance are fine.
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. And 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, then go ahead and disable this warning, you're fine.
• -Wnamespaces — warns when a namespace definition is opened. Some coding rules don't allow namespaces and this is how you enforce it. Inactive inside system headers like the STL (STL stays safe 💅). And you may also use using directives and qualified names — those don't trip it.
• -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 a simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. Like, for example:

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 is also enabled by -Wc++20-compat.

• -Wtemplate-names-tu-local — warns when a template body hides an exposure of a translation-unit-local entity 🌸 In most cases, referring to a translation-unit-local entity (like an internal linkage declaration) inside an entity that gets emitted into a module's CMI is an error. BUT — inside the initializer of a variable, or in the body of a non-inline function, that's not an exposure and no error is emitted, periodt.

So here's the drama: 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 tries to detect cases where that might happen. Good news: 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. like, immediately. no buffer. 💀
• -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 for 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 anyway 🙈
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. And bestie, the list is long, so buckle up: 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 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, like constructing an object in a buffer that's 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 a structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, 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) it warns about polymorphic class types that are caught by value. With -Wcatch-value=2 it warns about all class types caught by value. With -Wcatch-value=3 it warns about all types that are not caught by reference. And -Wcatch-value is enabled by -Wall 💖
• -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs. that's the whole vibe, bestie 🌸
• -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, and we're listing them all because we don't gatekeep 🫶

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 it 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 it's 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. And heads up: the warning for ambiguous virtual bases is enabled by the -Wextra option, not this one. ✨

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 😭 (the ellipsis really said "not my problem")
• -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. So this flag is for users who are fully aware they're writing nonportable code and who have deliberately chosen to ignore the warning about it 💅

Also, the restrictions on offsetof may be relaxed in a future version of the C++ standard 👀

• -Wsized-deallocation (C++ and Objective-C++ only) — bestie, this one is a lil' snitch for mismatched deallocation. It yells at you when you define an unsized deallocation function

void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

but there's no definition of the corresponding sized deallocation function

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

— or the other way around, same energy, still a problem. It's enabled by -Wextra along with -fsized-deallocation. 💅

• -Wsuggest-final-types — warns about types with virtual methods where your code quality would be so much better if the type were declared with the C++11 final specifier, or — if you can swing it — just declared in an anonymous namespace. Why do we care? Because it lets GCC go full main-character and devirtualize the polymorphic calls way more aggressively. ✨ Heads up: this warning hits different with link-time optimization, where the info about the class hierarchy graph is more complete. 👑
• -Wsuggest-final-methods — the method edition of the above. It warns about virtual methods where code quality would improve if the method were declared with the C++11 final specifier, or, if possible, if its type were declared in an anonymous namespace or with the final specifier. Same deal: more effective with link-time optimization since the class hierarchy graph info is more complete. And the recommended workflow, bestie, is to first consider suggestions from -Wsuggest-final-types and then rebuild with new annotations. Order matters, iykyk. 🌸
• -Wsuggest-override — warns about overriding virtual functions that aren't marked with the override keyword. Just say override. It's free. 💅
• -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. Note that -Wconversion-null is enabled by default, so this is you asking for peace and quiet. 🙈

---

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

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. Sorry bestie, we do options here. 💖)

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 alongside them. Like, 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, but you can use the other options with any language GCC supports. ✨

Also important: since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (like -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (like -Wabi). So the vibes overlap, fr fr.

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. The classic. The default. The moment. 💜
• -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. 👑
• -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. Heads up though: 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. Currently this option is only supported 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. 🫶

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. Zoom zoom. 🔥
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what C++ offers. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. It's 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. 😭
• -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. 💅
• -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. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. 🙈
• -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 your Objective-C code can be compiled with older versions of GCC. Time travel but for compilers. ⭐
• -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 needing to restart the program itself. 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. 🌈
• -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. No more name collisions bestie. ✨
• -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. 💅
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. Free documentation, kind of. 🍓
• -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector. 👀
• -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, but make it consensual. 🙈
• -Wno-protocol (Objective-C and Objective-C++ only) — okay so bestie, if a class says it implements a protocol, GCC is gonna warn you about every method in that protocol the class doesn't implement. Default vibe: it warns about every method that isn't explicitly implemented in the class, even if the implementation is actually inherited from the superclass 😤. But flip on -Wno-protocol and suddenly methods inherited from the superclass count as implemented, so no warning for them. slay 💅
• -Wobjc-root-class (Objective-C and Objective-C++ only) — warns you if a class interface has no superclass. Like, most classes are gonna inherit from NSObject (or Object), hello?? 🫶 So when you're declaring classes that are meant to be root classes, you can shut the warning up by marking their interfaces with __attribute__((objc_root_class)). iconic behavior tbh.
• -Wselector (Objective-C and Objective-C++ only) — warns if it finds multiple methods of different types for the same selector during compilation. The check runs on the list of methods in the final stage of compilation. Also, it checks each selector that shows up in a @selector(...) expression, and whether a corresponding method for that selector was found during compilation. But here's the tea ☕ — because these checks only scan the method table at the very end of compilation, you get no warnings if the final stage never happens. Like if an error pops up during compilation, or if you're using -fsyntax-only. no final stage, no receipts 💀
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warns when it finds multiple methods with differing argument and/or return types for a given selector, while you're trying to send a message with that selector to a receiver of type id or Class. When this flag is off (which is the default, fyi), the compiler just skips those warnings if any differences it found are confined to types that share the same size and alignment. lowkey chill of it.
• -Wundeclared-selector (Objective-C and Objective-C++ only) — warns if there's a @selector(...) expression referring to an undeclared selector. A selector counts as undeclared if no method with that name was declared before the @selector(...) expression — either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option runs its checks the second a @selector(...) expression shows up, whereas -Wselector only does its checks in the final stage of compilation. It also enforces that coding-style convention that methods and selectors must be declared before being used. main character energy, honestly ✨
• -print-objc-runtime-info — generates a C header describing the largest structure that is passed by value, if there is one. that's it, that's the whole thing 🎀

---

🦋 Options Controlling OpenMP and OpenACC 🦋

GCC supports OpenMP extensions to C, C++, and Fortran with the -fopenmp option. Similarly, OpenACC extensions are supported in all three languages with -fopenacc. bestie, parallelism is right there 💜

• -foffload=disable
• -foffload=default
• -foffload=target-list — tells GCC which OpenMP and OpenACC offload targets to generate code for. The default behavior (same as -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. cute little peek behind the curtain 🫣

• -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. If you wanna target just one specific target (or a few), use the -foffload-options=target-list=options form instead. 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, so it's only supported on targets that have support for -pthread. no pthread, no party 🚫
• -fopenacc-dim=geom — sets 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. You can omit a size to use a target-specific default value. pick and choose, queen 🎀
• -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, so it's only supported on targets that have support for -pthread. And -fopenmp implies -fopenmp-simd. double whammy ✨

• -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 !$omp in Fortran. It additionally enables the conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the sentinels can also start with c or *. Every other OpenMP directive is just ignored, periodt. And unless -fopenmp is also specified, the loop region binds to the current task region, independent of the specified bind clause. iykyk 💅
• -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 can be one of none, host, nohost, and any, which correspond to keywords for the device_type clause of the declare target directive; clones get generated for the intersection of devices specified. -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.

At -O2 and higher (but not -Os or -Og), this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it's disabled by default. noted 📝

---

🌈 Options to Control Diagnostic Messages Formatting 🌈

Traditionally, diagnostic messages have been formatted with zero regard for the output device's aspect (like its width, etc.). Rude, honestly. 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 gets reported. Just a lil warning though: some language front ends may not honor these options 🙈

• -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 shows up 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. not those, bestie 🚫

• -fdiagnostics-plain-output — requests that diagnostic output look as plain as humanly possible, which is useful when you're running dejagnu or other utilities that need to parse diagnostics output and would prefer it stay 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. so keep an eye out 👀
• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location info once — meaning if the message is too long to fit on a single physical line and has to be wrapped, the source location won't get emitted (as a prefix) over and over again on the subsequent continuation lines. This is the default behavior. thank god 🙏
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the same source location info (as a prefix) for the physical lines that result from breaking a message that's too long to fit on one line. consistency queen ✨
• -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. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. pretty in pink or plain and simple, your call 💖

okay so the colors? 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 the terminal or terminal emulator — so like, go read the section in your text terminal's docs for the permitted values and what they mean as character attributes, bestie ✨

These substring values are integers in decimal representation and you can concatenate them with semicolons 🫶 common ones 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
• 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 🌈 and setting GCC_COLORS to the empty string disables colors, periodt.

Supported capabilities are as follows 💖

• 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 is for the IDE girlies 💅 — it emits fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line gets printed right after the relevant diagnostic, and it starts with the string "fix-it:". For example:

                   fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. So 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

And the escaping rules, bestie, write these down 📝: 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 means that the given range is to be removed. An empty range (e.g. "45:3-45:3") means that the string is to be inserted at the given position. ✨

-fdiagnostics-generate-patch

Print fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:

                   --- 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 (peep -fdiagnostics-color). 🌈

-fdiagnostics-show-template-tree

In the C++ frontend, when printing diagnostics showing 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

This one lets you specify 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. Like:

                     '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

This option provides additional information when printing control-flow paths associated with a diagnostic. 🫶

If this is 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

Select 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.

the <code>none</code> value straight up ghosts the diagrams — no picture, no nothing. <code>ascii</code> keeps them in pure ASCII (yes, the "ASCII art" of legend 💀). <code>unicode</code> lets it be a lil classy and use unicode drawing characters conservatively (box-drawing vibes, iykyk). And <code>emoji</code> goes even further and allows emoji in the output — like emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character. 🎀

The default is <code>emoji</code>, EXCEPT when the environment variable <code>LANG</code> is set to <code>C</code>, in which case the default is <code>ascii</code>. So if you're getting boring output, check your <code>LANG</code>, bestie, that's the plot twist. 👑

• -fdiagnostics-format=FORMAT — pick a whole different format for printing diagnostics. <code>FORMAT</code> is one of <code>text</code>, <code>sarif-stderr</code>, <code>sarif-file</code>, <code>json</code>, <code>json-stderr</code>, or <code>json-file</code>.

Using this option replaces any additional "output sinks" added by <code>-fdiagnostics-add-output=</code>, or the one set by <code>-fdiagnostics-set-output=</code>. It's not additive, it's a full takeover, main character energy. ✨

The default is <code>text</code>.

The <code>sarif-stderr</code> and <code>sarif-file</code> formats both emit diagnostics in SARIF Version 2.1.0 format — one to stderr, the other to a file named <code>source.sarif</code>, respectively.

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

• -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC — adds an additional "output sink" for emitting diagnostics. More sinks, more drama, we love to see it. 💅

<code>DIAGNOSTICS-OUTPUT-SPEC</code> should specify a scheme, optionally followed by <code>":"</code> and one or more <code>KEY=VALUE</code> 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. Consider yourself warned, bestie. 🙈

<code>SCHEME</code> can be:

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

Supported keys are:

• color=[yes|no] — Override the colorization settings from <code>-fdiagnostics-color</code> for this text output specifically.
• experimental-nesting=[yes|no] — Turn on an experimental mode that hypes up hierarchical relationships within diagnostics messages, showing location info on separate lines.
• experimental-nesting-show-locations=[yes|no] — If <code>experimental-nesting=yes</code>, then by default locations are shown; set this key to <code>no</code> to stop printing those 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 <code>experimental-nesting=yes</code>. Set this key to <code>yes</code> 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 you don't specify it, it defaults to <code>source.sarif</code>.
• version=[2.1|2.2-prerelease] — Specify which version of SARIF to use for the output. If not specified, defaults to <code>2.1</code>. <code>2.2-prerelease</code> uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release. Play responsibly. ⚠️

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 one writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively. Two files, two vibes. 🦋

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 — works basically the same way as <code>-fdiagnostics-add-output=</code>, except that instead of adding another "output sink" for diagnostics, it replaces all existing output sinks — like the ones from <code>-fdiagnostics-format=</code>, <code>-fdiagnostics-add-output=</code>, or a prior call to <code>-fdiagnostics-set-output=</code>. Clean slate, periodt.
• -fno-diagnostics-json-formatting — By default, when JSON is emitted for diagnostics (via <code>-fdiagnostics-format=sarif-stderr</code>, <code>-fdiagnostics-format=sarif-file</code>, <code>-fdiagnostics-format=json</code>, <code>-fdiagnostics-format=json-stderr</code>, <code>-fdiagnostics-format=json-file</code>), GCC adds newlines and indentation to visually emphasize the hierarchical structure of the JSON. Pretty JSON, we love her. 🌸

Use <code>-fno-diagnostics-json-formatting</code> to suppress all that whitespace. 👑 It must be passed before the option it is to affect — order matters, bestie.

This is intended for compatibility with tools that don't expect the output to contain newlines, such as the output 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. It's the compiler going "bestie… are you sure?" 😭

The following language-independent options don't 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. Just vibes and vibes alone, no actual output artifacts.
• -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 <code>-Wfatal-errors</code> is also specified, then <code>-Wfatal-errors</code> takes precedence over this option.
• -w — Inhibit all warning messages. Silence. 🤫
• -Werror — Turn all warnings into errors. Everything is a big deal now. 💀
• -Werror= — Turn the specified warning into an error. The specifier for a warning is appended; for example <code>-Werror=switch</code> turns the warnings controlled by <code>-Wswitch</code> into errors. This switch takes a negative form, to be used to negate <code>-Werror</code> for specific warnings; for example <code>-Wno-error=switch</code> makes <code>-Wswitch</code> warnings not be errors, even when <code>-Werror</code> is in effect. Pick your battles. ✨

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

Note that specifying <code>-Werror=foo</code> automatically implies <code>-Wfoo</code>. However, <code>-Wno-error=foo</code> 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. One strike and you're out. 🎀

You can request many specific warnings with options beginning with <code>-W</code>, for example <code>-Wunused-variable</code> to request warnings on declarations of variables that are never used. Each of these specific warning options also has a negative form beginning with <code>-Wno-</code> to turn off warnings; for example, <code>-Wno-unused-variable</code>. 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 <code>-Wall</code> and <code>-Wextra</code>, turn on other options, such as <code>-Wunused</code>, which may turn on further options, such as <code>-Wunused-variable</code>. It's options all the way down, literally. 🌀 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., <code>-Wunknown-warning</code>), GCC gives an error stating that the option is not recognized. However, if the <code>-Wno-</code> form is used, the behavior is slightly different: no diagnostic is produced for <code>-Wno-unknown-warning</code> unless other diagnostics are being produced. This allows the use of new <code>-Wno-</code> options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. Sneaky but intentional. 🙈

The effectiveness of some warnings depends on optimizations also being enabled. For example, <code>-Wsuggest-final-types</code> 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. Can't win 'em all. 😭

• -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 <code>-std</code> option used. She's strict but she's fair. 👑

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require <code>-ansi</code> or a <code>-std</code> 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 <code>-pedantic-errors</code>).

<code>-Wpedantic</code> does not cause warning messages for use of the alternate keywords whose names begin and end with <code>__</code>. This alternate format can also be used to disable warnings for non-ISO <code>__intN</code> types, i.e. <code>__intN__</code>. Pedantic warnings are also disabled in the expression that follows <code>"__extension__"</code>. However, only system header files should use these escape routes; application programs should avoid them. Don't be out here using the cheat codes in your own code, bestie. 🚫

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

okay so real quick, back to the -std thing from before — when the standard you picked with -std is a GNU extended dialect of C, like gnu90 or gnu99, there's a matching base standard — the version of ISO C that the GNU extended dialect is built on top of. Warnings from -Wpedantic show up wherever that base standard says they're required. (And bestie, it literally wouldn't make sense to only warn about stuff that's not in the GNU C dialect you picked, because by definition the GNU dialects of C include every feature the compiler supports with that option, so there'd be nothing left to warn about. Make it make sense 💅)

• -pedantic-errors — throw an error whenever the base standard (go check -Wpedantic, it's right there) requires a diagnostic — in some cases where there's undefined behavior at compile-time, and in some other cases that don't even stop compilation of programs that are valid per the standard. And no, this is NOT the same as -Werror=pedantic: that option is unlikely to be useful, because it only turns into errors the diagnostics controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic. It's giving main character, not side character ✨

If you want the required diagnostics that are warnings by default to be errors instead, but you don't also want to switch on the -Wpedantic diagnostics, you can specify -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic to enable them but only as warnings). Choose your fighter 🫶

Some required diagnostics are errors by default, but can be reduced to warnings using -fpermissive or their specific warning option, e.g. -Wno-error=narrowing.

Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but are also made errors by -pedantic-errors. 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 — downgrade some required diagnostics about nonconformant code from errors to warnings. So 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)

Heads up bestie: the -fpermissive option is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90).

• -Wall — this switches on all the warnings about constructions that some users consider questionable, and that are easy to avoid (or easy to modify so the warning goes away), even when macros are in the mix. It also enables some language-specific warnings described over in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

-Wall turns on the following warning flags, and yes I'm listing all of them, no skimming allowed 💖:

• -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 that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally don't consider questionable, but which occasionally you might wish to 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 are enabled by -Wextra but many of them must be enabled individually. iykyk ✨

• -Wextra — this enables some extra warning flags that aren't enabled by -Wall. (Fun lil history moment: this option used to be called -W. The older name is still supported, but the newer name is more descriptive. glow-up era 🌈)
  • -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.

---

ABI-era warnings, hold onto your seats 🎀

• -Wabi (C, Objective-C, C++ and Objective-C++ only) — warn about code affected by ABI changes. This includes code that may not be compatible with the vendor-neutral C++ ABI, plus 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, normally -Wabi warns only 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. -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n). fr fr it's giving time traveler energy ⏳

-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.

If an explicit version number is provided and -fabi-compat-version isn't specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.

Although an effort has been made to warn about all such cases, there are probably some cases that are not 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. Bestie, it's not perfect, nobody's perfect 😭

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

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 doesn't 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 that get passed as arguments to a variadic function are promoted exactly like unscoped enumerators are, and that makes va_arg big mad 😤. On most targets this genuinely does not affect the parameter passing ABI, because there's literally no way to pass an argument smaller than int.

Also (buckle up) the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.

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

• Lambdas in default argument scope were being mangled incorrectly, and the ABI changed the mangling of nullptr_t.

These got corrected in -fabi-version=7. ✨

• When mangling a function type with function-cv-qualifiers, the un-qualified function type was wrongly treated as a substitution candidate. oop.

Fixed in -fabi-version=8, which is the default for GCC 5.1.

• decltype(nullptr) had an alignment of 1, which was just wrong and led to unaligned accesses 🙈. Note that this did not affect the ABI of a function with a nullptr_t parameter, since parameters have a minimum alignment anyway.

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.) — did not affect the mangled name, which caused name collisions whenever function pointers got used as template arguments. yikes.

Fixed in -fabi-version=10, the default for GCC 6.1. 👑

---

• -Wpsabi (C, Objective-C, C++ and Objective-C++ only) — -Wpsabi turns on warnings about processor-specific ABI changes, like changes in alignment requirements or in how function arguments get passed. On a bunch of 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 inside a class mean the same thing across the complete scope of that class, so declaring the name after you've used 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 both declarations have the same type, while the B2 case is an error. Both diagnostics can be switched off with -Wno-changes-meaning. Or, if you'd rather, the error case can be downgraded to a warning with -Wno-error=changes-meaning or -fpermissive.

Both diagnostics are also suppressed by -fms-extensions.

• -Wchar-subscripts — Warns if an array subscript has type char. This is a super common source of bugs, because programmers keep forgetting that this type is signed on some machines 😭. This warning is enabled by -Wall.
• -Wno-coverage-mismatch — Warns 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 can't use the profile feedback info. By default this warning is enabled and it's treated as an error. -Wno-coverage-mismatch can disable the warning, or -Wno-error=coverage-mismatch can disable the error. Disabling the error for this warning can leave you with poorly optimized code, so it's only useful for very minor changes like bug fixes to an existing code-base. Completely disabling the warning is not recommended.
• -Wno-coverage-too-many-conditions — Warns 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 if -fpath-coverage is used and a function has too many paths, so GCC gives up on coverage. Whether it gives up is controlled by -fpath-coverage-limit. This warning is enabled by default.
• -Wno-coverage-invalid-line-number — Warns when a function ends earlier than it begins, because of an invalid linenum macro. The warning is emitted only with --coverage enabled.

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

• -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) — Suppresses warning messages emitted by #warning directives. 🤫
• -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. CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware but emulate double in software. On a machine like that, doing computations with double values is much more expensive because of all the overhead that software emulation needs.

It's genuinely easy to do computations with double by accident, because floating-point literals are implicitly of type double. For example, in:

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

the compiler does the entire computation with double, because that floating-point literal is a double. 💀

• -Wduplicate-decl-specifier (C and Objective-C only) — Warns 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 supplied have types appropriate for the format string you specified, and that the conversions 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 (that last one 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.

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 might 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 together 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 at all.

• -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.

• -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 says such arguments are ignored, so like, whatever.

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

• -Wformat-overflow
• -Wformat-overflow=level — Warns about calls to formatted input/output functions like 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's 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. 🙃
  • -Wformat-overflow, -Wformat-overflow=1 — Level 1 of -Wformat-overflow, enabled by -Wformat, takes a conservative approach and 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') that the function appends 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.

void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}

-Wformat-overflow=2 — level 2 is the full paranoia setting, and honestly? she's valid. Level 2 also drags calls that might overflow the destination buffer, given an argument of sufficient length or magnitude. 💀

At level 2, unknown numeric arguments get assumed to be the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments — the ones 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 by the character array they may point to — are assumed to be 1 character long.

At level 2, the call in the example above gets diagnosed <em>again</em>, but this time because with a equal to a 32-bit INT_MIN, that first %i directive is gonna write some of its digits past the end of the destination buffer. 😭 To make the call safe no matter what those two variables hold, the destination buffer size has to go up to at least 34 bytes. GCC is nice like that — it drops the minimum buffer size in an informational note right after the warning. 🫶

An alternative to growing the destination buffer is to constrain the range of formatted values. Max length of string arguments can be bounded by specifying the precision in the format directive. And when numeric arguments of format directives can be assumed bounded by less than the precision of their type, picking an appropriate length modifier for the format specifier shrinks 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 drops the maximum required buffer size to 24 bytes. ✨

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 says zero-length formats are allowed, so like, let them live. 💅
• -Wformat-nonliteral — if -Wformat is specified, also warn when the format string isn't a string literal and therefore can't 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 could be security problems. Right now this warns about calls to printf and scanf functions where the format string isn't a string literal and there are no format arguments, like printf (foo);. 🚨 This may be a security hole if the format string came from untrusted input and contains %n. (Currently it's a subset of what -Wformat-nonliteral warns about, but bestie, future warnings may get added to -Wformat-security that aren't in -Wformat-nonliteral.)
• -Wformat-signedness — if -Wformat is specified, also warn when the format string wants an unsigned argument but the argument is signed, and vice versa. 👀
• -Wformat-truncation
• -Wformat-truncation=level — warn about calls to formatted input/output functions like snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive can't be figured out at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. Turning on optimization will in most cases improve how accurate the warning is, but it can also hand you false positives, ngl. Except as noted otherwise, the option uses the same logic -Wformat-overflow does.
  • -Wformat-truncation
  • -Wformat-truncation=1 — level 1 of -Wformat-truncation, enabled by -Wformat, takes the conservative route and only warns about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
  • -Wformat-truncation=2 — level 2 also warns 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 and we are not doing that again.
• -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. -Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
• -Wnull-dereference — warn if the compiler spots paths that trigger erroneous or undefined behavior by dereferencing a null pointer. 💀 This option is only active when -fdelete-null-pointer-checks is active, which gets enabled by optimizations on most targets. How precise the warnings are depends on the optimization options you used.
• -Wno-musttail-local-addr — do not warn about passing a pointer (or in C++, a reference) to a local variable or label as an argument of a musttail call. Those variables go out of scope before the tail call instruction.
• -Wmaybe-musttail-local-addr — warn when the 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 doesn't elide the copy from a local variable to the return value of a function in a context where [class.copy.elision] allows it. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler can't elide copies from both v1 and v2, so it elides neither. 😤

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 it needs optimization to detect infinite recursion in calls between two or more functions. -Winfinite-recursion is included in -Wall.

Compare with -Wanalyzer-infinite-recursion, which gives a similar diagnostic but is implemented a whole different way (as part of -fanalyzer).

• -Winit-self (C, C++, Objective-C and Objective-C++ only) — warn about uninitialized variables that get initialized with themselves. 🙈 Heads up: this option can only be used together with the -Wuninitialized option.

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 doesn't 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. 🫶
• -Whardened — warn when -fhardened didn't enable an option from its set (go look at -fhardened for that). For instance, using -fhardened and -fstack-protector at the same time on the command line makes -Whardened warn, because -fstack-protector-strong will not be enabled by -fhardened.

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;
  }

Bestie, hear me out: this warning is not gonna come for you when the last statement of a case literally can't fall through. Like if there's a return up in there, or a call to a function declared with the noreturn attribute? Chill. No warning. 💅

And -Wimplicit-fallthrough= also keeps tabs on control flow statements — yes, the ifs and everything — and only yells when it's actually appropriate. For example:

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

Okay but sometimes you want a switch case to fall through, right? Totally valid. So GCC gives you an attribute, __attribute__ ((fallthrough)), that you use along with a null statement to shush the warning that would normally pop off:

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

C++17 comes through with a standard way to suppress the -Wimplicit-fallthrough warning, using [[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 any of those attributes, you can also just drop a fallthrough comment in there to silence the warning. The whole body of the C or C++ style comment has to match the regular expressions listed below. The option argument n tells GCC what kind of comments are accepted:

• -Wimplicit-fallthrough=0 disables the warning altogether.
• -Wimplicit-fallthrough=1 matches ".*" regular expression, any comment is used as fallthrough comment.
• -Wimplicit-fallthrough=2 case insensitively matches ".*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.

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.

---

✨ OPTIONS, continued ✨

• -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — controls whether warnings triggered by the warn_if_not_aligned attribute get issued. These warnings are enabled by default, bestie, so if you're seeing them, that's why. 🙈
• -Wignored-qualifiers (C and C++ only) — warns if the return type of a function has a type qualifier like const. For ISO C, that type qualifier does literally nothing, 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 catch a warning even without this option. 😭

This warning is also enabled by -Wextra.

• -Wno-ignored-attributes (C and C++ only) — this option controls warnings when an attribute is ignored. It's different from the -Wattributes option because it warns whenever the compiler decides to drop an attribute — not that the attribute is unknown, used in the wrong place, etc. 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.

Heads up: this warning gets upgraded to an error by -pedantic-errors. 💀

• -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 does not use braces, followed by an unguarded statement with the same indentation.

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".  */

When it's mixed tabs and spaces chaos, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic like this next example:

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

And it's not issued after a #line directive either, since that typically means autogenerated code, and you can't make any assumptions about the layout of the file that the directive references. 🤷‍♀️

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. 😱

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.

-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 is diagnosed because it is 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 is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. 🎀

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 is partially enabled by default by warning for -I and -J, only.
• -Wno-missing-profile — this option controls warnings if feedback profiles are missing when 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 do not 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 cannot use any profile feedback information. In all these cases, warnings are issued to inform you that a profile generation step is due. 😭

Ignoring the warning can result in poorly optimized code. -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.

• -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. In the following example mydealloc is the deallocator for pointers returned from myalloc. ✨

void mydealloc (void*);

so like, here's the whole thing in code so you can see it, bestie:

__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
}

In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete.

Option -Wmismatched-dealloc is included in -Wall.

---

• -Wmultistatement-macros — okay this one is a genuine jumpscare 💀 it yells at you about unsafe multiple-statement macros that look like they're guarded by a clause like if, else, for, switch, or while, but where only the first statement actually gets guarded once the macro expands. sneak attack fr fr.

Like, look at this:

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

that will increment y unconditionally, not just when c holds. not the vibe. The fix is usually to wrap the macro in a do-while loop, like so:

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

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

• -Wparentheses — warns you when parentheses are missing in certain contexts, like when there's an assignment in a spot where a truth value is expected, or when operators are nested in a way that people constantly get confused about. we've all been there 🫶

It also warns if a comparison like x<=y<=z shows up, because that's equivalent to (x<=y ? 1 : 0) <= z, which is a whole different interpretation from what ordinary mathematical notation would have you believe. math lied to you 😭

Also warns for dangerous uses of the GNU extension to ?: with the middle operand omitted. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead. expectation vs reality, truly

For C++ this 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 -Wall.

• -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. bestie you are moving into your own apartment, nothing happened 💅

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

This warning is enabled by -Wall.

• -Wsequence-point — warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. undefined behavior, my beloved nightmare.

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 undefined behavior; the C and C++ standards specify that "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." If a program breaks these rules, the results on any particular implementation are entirely unpredictable. chaos, no cap.

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 false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs. not perfect, but she's trying ✨

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. iykyk 📚

This warning is enabled by -Wall for C and C++.

• -Wno-return-local-addr — don't warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. that pointer is a ghost, bestie 👻
• -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 -Wpedantic 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 undefined. do not do this, I am begging 🙈

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

• -Wreturn-type — warns whenever a function is defined with a return type that defaults to int (unless -Wimplicit-int 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.

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 undefined.

Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used. yeah. even then.

This warning is enabled by default in C++ and by -Wall otherwise.

• -Wno-shift-count-negative — controls warnings if a shift count is negative. This warning is enabled by default.
• -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 enabled by default.
• -Wshift-negative-value — warns if you're left shifting a negative value. why though 😭 This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes.
• -Wno-shift-overflow, -Wshift-overflow=n — these options control warnings about left shift overflows.
  • -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 -Wall.
• -Wswitch-default — warns whenever a switch statement does not have a default case. cover all your bases bestie ✨
• -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 -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label. stricter. louder. iconic. 👑
• -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 is 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. dead code energy 💀 For example:

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

-Wswitch-unreachable is genuinely so chill about one specific thing: it does not warn if the statement sitting between the controlling expression and the first case label is literally just a declaration. No cap:

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

And bestie, this warning is enabled by default for C and C++ programs. it's giving baseline. 💅

• -Wsync-nand (C and C++ only) — yells at you whenever __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions get used. heads up: these functions changed semantics in GCC 4.4, so like… the vibe shifted and the compiler wants you to know. 😭
• -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init can't initialize the automatic variable. a super common situation is an automatic variable declared between the controlling expression and the first case label of a switch statement. lowkey cursed placement tbh.
• -Wunused-but-set-parameter — warns whenever a function parameter gets assigned to but is otherwise unused (aside from its declaration, obviously). to shut this warning up, use the unused attribute. this one's also enabled by -Wunused together with -Wextra. 🫶
• -Wunused-but-set-variable — warns whenever a local variable gets assigned to but is otherwise unused (again, aside from its declaration). this warning is enabled by -Wall. to suppress it, use the unused attribute. it's also enabled by -Wunused, which is itself enabled by -Wall. the whole chain, iconic.
• -Wunused-function — warns whenever a static function is declared but not defined, or a non-inline static function is just… sitting there unused. enabled by -Wall. 💀
• -Wunused-label — warns whenever a label is declared but never used. 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 defined locally inside a function goes unused. enabled by -Wall.
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration. important detail: this option is not enabled by -Wunused unless -Wextra is also specified. to suppress it, use the unused attribute.
• -Wno-unused-result — do not warn if a caller of a function marked with attribute warn_unused_result doesn't 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++. enabled by -Wall. to suppress it, 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 it, 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 that got 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++ it's not an error since const variables take the place of #defines. ✨
  • -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (system headers excluded, bestie). it's 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 the headers you included. 😭
• -Wunused-value — warns whenever a statement computes a result that's explicitly not used. to suppress it, 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 like x[i,j] causes a warning, while x[(void)i,j] does not. enabled by -Wall. slay.
• -Wunused — all the above -Wunused options combined, except the ones documented as needing to be specified explicitly. 👑 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. this 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 actually reallocated memory. this includes double-free calls as well as uses in arithmetic and relational expressions. although it's undefined behavior, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level. 🙈
  • -Wuse-after-free=2 — at level 2, on top of 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 figure out whether the call relocated the object to a different address. they get diagnosed at a separate level to help gradually transition legacy code over 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. that approach obviates needing to adjust the stored pointers after reallocation. chef's kiss. 🎀

• -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. ngl, this warning does not occur when a class object is converted to a non-reference type, because that's 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++, it also warns if a non-static reference or non-static const member appears in a class without constructors. 😳 on top of that, 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 wanna 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. and because these warnings depend on optimization, the exact variables or elements you get warned about depend on the precise optimization options and version of GCC you're using. fr fr. note that there may be no warning about a variable that's 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 even 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's 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. the compiler is literally doing its best detective work here. 🔍✨

In addition, if you pass a pointer (or in C++, a reference) to an uninitialized object into a const-qualified function argument, this warning calls it out too. (And -Wuninitialized gets issued for built-in functions that are known to read the object.) If you annotate the function with the attribute access (none), that says "bestie I am NOT using this argument to touch the object," and it dodges the warning. 🫶

These warnings can only even happen in optimizing compilation, because otherwise GCC just isn't keeping track of the state of your variables. Periodt.

And they're optional for a reason: GCC might not be able to figure out when your code is actually correct even though it looks sus. Here's one example of how that goes down:

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

If y is always 1, 2 or 3, then x is literally always initialized — but GCC doesn't know that. To silence the warning, you need a default case with assert(0) or something similar.

This option also yells at you when a non-volatile automatic variable might get changed by a call to longjmp. The compiler only sees the calls to setjmp. It has zero way of knowing where longjmp will get called from — honestly a signal handler could call it at literally any point in your code. So yeah, you might get a warning even when there's genuinely no problem, because longjmp can't actually be called at the place that would cause the issue.

Some of those 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 — Warns when a #pragma directive shows up that GCC just does not understand. And if you use this command-line option, warnings get issued even for unknown pragmas hiding in system header files. That does NOT happen if the warnings are only turned on by the -Wall command-line option. Sneaky difference, ik.
• -Wno-pragmas — Do not warn about pragma misuses — think incorrect parameters, invalid syntax, or pragmas beefing with each other. 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, like a main file. 🎀
• -Wno-prio-ctor-dtor — Do not warn if a priority from 0 to 100 is used for a constructor or destructor. The constructor and destructor attributes let you assign a priority to the constructor/destructor to control the order it runs before main is called or after it returns. The priority values must be greater than 100, because the compiler reserves priority values between 0–100 for the implementation itself.
• -Wstrict-aliasing — This one is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules the compiler is using for optimization. It doesn't catch all cases, but it does try to catch the more common pitfalls. It's included in -Wall. And it's equivalent to -Wstrict-aliasing=3.
• -Wstrict-aliasing=n — Also only active when -fstrict-aliasing is active. Warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels = higher accuracy (fewer false positives). Higher levels also = more effort, kinda like how -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
  • Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code, since it has very few false negatives. However, it has many false positives. Warns for all pointer conversions between possibly incompatible types, even if they're 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 doesn't occur. Note: it does not warn about every case where the code might overflow — only cases where the compiler actually implements some optimization. So this warning depends on the optimization level. 👑

An optimization that assumes signed overflow doesn't occur is perfectly safe if the values of the variables involved are such that overflow never actually happens. So this warning can easily give a false positive: a warning about code that is genuinely not a problem. To help you focus on the important stuff, several warning levels are defined. No warnings get issued for the use of undefined signed overflow when estimating how many iterations a loop requires — in particular when determining whether a loop will even 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 only reported at the highest warning level because this simplification applies to so many comparisons, so this warning level gives a very large number of false positives. 😭

• -Wstring-compare — Warns for calls to strcmp and strncmp whose result is determined to be either zero or non-zero in tests for that equality, because the length of one argument is greater than the size of the array the other argument is stored in (or the bound, in the case of strncmp). Calls like that could be mistakes. For example, the call to strcmp below gets diagnosed because its result is necessarily non-zero no matter what's inside 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.

• -Wno-stringop-overflow
• -Wstringop-overflow
• -Wstringop-overflow=type — Warns for code that can be statically determined to cause buffer overflows or memory overruns, like 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. That argument is only meaningful 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 it can detect a small subset of simple buffer overflows even without optimization in calls to GCC built-in functions like __builtin_memcpy that correspond to the standard functions. Either way, 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 can't 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 gets 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. ✨

🌊 -Wstringop-overflow

ok so this one comes in LEVELS bestie, and the level you pick decides exactly how nosy the compiler gets about your destination objects. let's go one at a time, no skipping 💅

-Wstringop-overflow=1

The -Wstringop-overflow=1 option uses type-zero Object Size Checking to figure out the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. ✨

-Wstringop-overflow=2

The -Wstringop-overflow=2 option uses type-one Object Size Checking to figure out the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers, since they may point to arrays containing unknown numbers of elements. This is the default setting of the option. 👑

-Wstringop-overflow=3

The -Wstringop-overflow=3 option uses type-two Object Size Checking to figure out the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code. 😭

-Wstringop-overflow=4

The -Wstringop-overflow=4 option uses type-three Object Size Checking to figure out the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to -Wstringop-overflow=3 this setting of the option may result in warnings for benign code. 🙈

---

🌸 -Wno-stringop-overread

Warn for calls to string manipulation functions such as memchr, or strcpy that are determined to read past the end of the source sequence. 🔍

Option -Wstringop-overread is enabled by default.

---

✂️ -Wno-stringop-truncation

Do not warn for calls to bounded string manipulation functions such as strncat, strncpy, and stpncpy that may either truncate the copied string or leave the destination unchanged.

In the following example, the call to strncat specifies a bound that is less than the length of the source string. As a result, the copy of the source will be 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);
}

As 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, and so the call is diagnosed. To avoid the warning when the result is not expected to be NUL-terminated, call memcpy instead.

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

In the following example, 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. Therefore, the 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);
  ...
}

In situations where a character array is intended to store a sequence of bytes with no terminating NUL such an array may be annotated with attribute nonstring to avoid this warning. Such arrays, however, are not suitable arguments to functions that expect NUL-terminated strings. To help detect accidental misuses of such arrays GCC issues warnings unless it can prove that the use is safe. 🚨

---

🦋 -Wstrict-flex-arrays (C and C++ only)

Warn 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 it's available, otherwise according to the level of the option -fstrict-flex-arrays=level. -Wstrict-flex-arrays is effective only when level is greater than 0.

When level=1, warnings are issued for a trailing array reference of a structure that have 2 or more elements if the trailing array is referenced as a flexible array member.

When level=2, in addition to level=1, additional 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, additional 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 is more effective when -ftree-vrp is active (the default for -O2 and above) but some warnings may be diagnosed even without optimization. 🔥

---

🎀 *-Wsuggest-attribute=attribute-name*

Warn for cases where adding an attribute may 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 — 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 (in the case of pure and const) if it cannot prove that 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 — Warn about function pointers that might be candidates for format attributes. Note these are only possible candidates, not absolute ones. 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, these are only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected.

• -Wsuggest-attribute=cold — Warn about functions that might be candidates for cold attribute. This is based on static detection and generally only warns about functions which always leads to a call to another cold function such as wrappers of C++ throw or fatal error reporting functions leading to abort.

---

💜 -Walloc-size

Warn 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, including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc.

---

🍓 -Walloc-zero

Warn about calls to allocation functions decorated with attribute alloc_size that specify zero bytes, including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc has been deprecated) relying on it may result in subtle portability bugs and should be avoided. 😬

---

🔄 -Wcalloc-transposed-args

Warn about calls to allocation functions decorated with attribute alloc_size with two arguments, which use sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning. The first argument to calloc is documented to be number of elements in array, while the second argument is 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, the warning can be suppressed by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4) or using sizeof in the later argument as well. 💅

---

📏 *-Walloc-size-larger-than=byte-size*

Warn 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 the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-alloc-size-larger-than.

---

🚫 -Wno-alloc-size-larger-than

Disable -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger.

---

🗑️ -Walloca

This option warns on all uses of alloca in the source. 😱

• -Walloca-larger-than=byte-size — bestie, this one yells at you when you call alloca with an integer argument that's either zero or that isn't reined in by a controlling predicate keeping it at most byte-size. It also side-eyes calls to alloca where the bound value is just... unknown. And non-integer-typed arguments? Treated as unbounded, no matter how well-behaved they look. 💀

Like, here's a bounded alloca moment that's actually fine:

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 warn, because the call to alloca is known to be at most 1000 bytes. But if you passed -Walloca-larger-than=500 instead? Yeah, the compiler would emit a warning. 😭

Unbounded uses, on the other hand, are alloca calls with literally no controlling predicate constraining that 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.

And ngl, even code that looks correct with signed integers can cause a warning:

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

In that example, n could be negative, which causes a larger than expected argument to be implicitly cast into the alloca call. Sneaky. 🙈

This option also warns when alloca is used in a loop.

-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 — turns off -Walloca-larger-than= warnings. It's equivalent to -Walloca-larger-than=SIZE_MAX or larger. ✨
• -Warith-conversion — do warn about implicit conversions from arithmetic operations even when converting the operands to the same type can't 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 — warns about out of bounds subscripts or offsets into arrays. 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 flagged even without optimization. 👑

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's declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or an one-element array ([1]). So as a result, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default.

You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treats a 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.
• -Warray-bounds=2 — this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. This level may give a larger number of false positives and is deactivated by default. 🙃

• -Wunterminated-string-initialization (C and Objective-C only) — warns 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, because in C++ such initializations are an error. 💅

• -Warray-compare — warns 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.

• -Warray-parameter
• -Warray-parameter=n — warns about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enables detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds. 🫶

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. ✨

• -Wattribute-alias=n
• -Wno-attribute-alias — warns about declarations using the alias and similar attributes whose target is incompatible with the type of the alias. 🦋
  • -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.
  • -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.

-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] — warns about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. These characters can flip left-to-right writing direction into right-to-left (and vice versa), which causes confusion between the logical order and the visual order. And this is genuinely dangerous — for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 😭🚨

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. 💜

• -Wbool-compare — warns 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. 🔥

• -Wbool-operation — bestie, this one clocks suspicious stuff you're doing to boolean expressions. Like, bitwise-negating a bool? That's almost certainly a bug in your program, no cap. And for C it also yells at you for incrementing or decrementing a boolean, which like... rarely ever makes sense 💀 (in C++, decrementing a boolean is always invalid; incrementing a boolean is invalid in C++17 and deprecated in everything else).

This warning is enabled by -Wall.

• -Wduplicated-branches — this one's like "girl why did you write the same thing twice" 🫶. It warns when an if-else has identical branches, catching cases 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 — the if-else-if chain vibe check ✨. Warns about duplicated conditions in an if-else-if chain. For instance, warn for this code:

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. Those calls may return indeterminate values or crash your 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 can be used to suppress such a 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 can be used to suppress such a warning.
• -Wno-incompatible-pointer-types (C and Objective-C only) — do not warn when there is 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, as it can be a legitimate way of obtaining infinities and NaNs. 💅
• -Wsystem-headers — prints warning messages for constructs found in system header files. Warnings from system headers are normally suppressed, on the assumption that they usually do not 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. However, note that using -Wall in conjunction 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. This warning detects various mistakes such as:

int i = 1;
...
if (i > i) { ... }

This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:

if ((a & 16) == 10) { ... }

will always be false.

This warning is 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), 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 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. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is 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 is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are 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 to see whether the two values have ranges that overlap; and this is 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 and 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.
• The ISO type of an integer constant has a different width or signedness from its traditional type. Ngl this one only fires if the base of the constant is ten. Like, hexadecimal or octal values — which usually represent bit patterns — get a pass, no warning, they're fine 💅
• Usage of ISO string concatenation is detected. caught you bestie 👀
• Initialization of automatic aggregates.
• Identifier conflicts with labels. Traditional C doesn't have a separate namespace for labels, so it's giving name collision energy 😭
• Initialization of unions. But if the initializer is zero, the warning is skipped entirely. Why? Because the assumption is that the zero initializer in user code is conditioned on e.g. __STDC__ to dodge missing initializer warnings, and it relies on default initialization to zero in the traditional C case. Iconic reasoning tbh ✨
• Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems 🚨. Heads up though: this is only a subset of the possible conversion warnings — for the full set use -Wtraditional-conversion.
• Use of ISO C style function definitions. And the way this one is intentional about not firing for prototype declarations or variadic functions, because those ISO C features show up in your code when you're using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. It's also bypassed for nested functions because that feature is already a GCC extension, so it's not relevant to traditional C compatibility. Periodt.

---

-Wtraditional-conversion (C and Objective-C only)

Warn if a prototype causes a type conversion that's different from what would happen to the same argument in the absence of a prototype. 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)

Warn when a declaration shows up after a statement in a block. This construct, which is known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. 🙈

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

-Wshadow

Warn 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 includes also all instances of local shadowing. This means that -Wno-shadow=local and -Wno-shadow=compatible-local are 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 (in addition to -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 — as shown in the following 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. 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. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means that 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 warn 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

Disable -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 do not get a warning 😭. In addition, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when determining 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

Disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or larger.

-Wfree-nonheap-object

Warn when attempting to deallocate an object that was either not allocated on the heap, or by using a pointer that was not 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 the stack usage is conservative. Any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when determining 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

Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger.

-Wunsafe-loop-optimizations

Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations, warn if the compiler makes 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++, warn also when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic. ✨

This warning is 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 that 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 do not 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 is 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. 🌈

This option is implied by -Wall. If -Wall isn't given, it's still on unless trigraphs are enabled. Want the trigraph conversion but not the warning, while keeping the rest of the -Wall warnings? Do -trigraphs -Wall -Wno-trigraphs. 🎀

• -Wundef — warns you when an undefined identifier gets evaluated inside an "#if" directive. Those identifiers get replaced with zero, bestie, so like… probably not what you meant. 💀
• -Wexpansion-to-defined — yells whenever defined shows up in the expansion of a macro (yes, including when the macro gets expanded by an #if directive). That usage is not portable. This warning is also turned on by -Wpedantic and -Wextra.
• -Wunused-macros — snitches on macros defined in the main file that nobody ever 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 gets redefined or undefined. 👀

Built-in macros, macros defined on the command line, and macros defined in include files? Not warned about. They're exempt, they're the main characters. ✨

Note: if a macro is actually used, but only inside skipped conditional blocks, the preprocessor will report it as unused. To dodge that warning, you might improve the scope of the macro's definition — like moving it into the first skipped block. Or you could just give it a dummy use with something like:

        #if defined the_macro_causing_the_warning
        #endif

• -Wno-endif-labels — don't warn whenever an "#else" or an "#endif" is followed by text. This sometimes happens in older programs with code that looks like:

        #if FOO
        ...
        #else FOO
        ...
        #endif FOO

Those second and third "FOO"s should be in comments, ngl. 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, warn if a call to a function returning an integer type gets cast to a pointer type. 🚩
• -Wc90-c99-compat (C and Objective-C only) — warns about features that aren't in ISO C90 but are in ISO C99. For instance: 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 not in ISO C99 but present 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. This option is independent of the standards mode. Warnings are disabled in the expression that follows "__extension__".
• -Wc11-c23-compat (C and Objective-C only)
• -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. This option is independent of the standards mode. Warnings are disabled in the expression that follows "__extension__". Heads up: the name -Wc11-c2x-compat is deprecated. 😭

When you're not compiling in C23 mode, these warnings get upgraded to errors by -pedantic-errors.

• -Wc23-c2y-compat (C and Objective-C only)
• -Wc23-c2y-compat (C and Objective-C only) — warns about features not present in ISO C23 but present in ISO C2Y. For instance: "_Generic" selecting with a type name instead of an expression. This option is independent of the standards mode. Warnings are disabled in the expression that follows "__extension__".

When you're 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 a 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 in ISO C++ 1998 that became 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) — do not warn about C++11 constructs in code being compiled with an older C++ standard. Even without this option, some C++11 constructs will only be diagnosed if -Wpedantic is used.
• -Wno-c++14-extensions (C++ and Objective-C++ only) — do not warn about C++14 constructs in code 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) — do not warn about C++17 constructs in code 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) — do not warn about C++20 constructs in code 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) — do not warn about C++23 constructs in code 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) — do not warn about C++26 constructs in code 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 in a way that removes a type qualifier from the target type. Like, warn if a "const char *" gets cast to an ordinary "char *". 🫣

It also warns when you make a cast that introduces a type qualifier in an unsafe way. For example, casting "char **" to "const char **" is unsafe, as 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 gets 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 — 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. No mercy, no exceptions. 💅
• -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 the 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. Type qualifiers are ignored too. The function type "void (*) (void)" is special and matches everything — which you can use to suppress this warning. In a cast involving pointer to member types, this warning fires whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra. 🔥
• -Wcast-user-defined — warns when a cast to reference type doesn't 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, this 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, at compile time, code that might 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 literally why -Wall doesn't 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.

This warning is 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". This warning is 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. You can turn that off 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. That 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. This warning is enabled by default. 🧐
• -Wconversion — bestie, this one clocks every implicit conversion that could change a value. We're talking: real ↔ integer swaps (like abs (x) when x is a double 😭), signed ↔ unsigned glow-ups (like unsigned ui = -1, which is giving chaos), and shrinking into smaller types (like sqrtf (M_PI), so smol). It will NOT snitch on explicit casts though — abs ((int) x) and ui = (unsigned) -1 are fine, and neither is anything where the value literally doesn't change, like abs (2.0). Also, the signed/unsigned integer conversion warnings? You can mute those with -Wno-sign-conversion.

For C++, it also flags confusing overload resolution for user-defined conversions, plus conversions that never use a type conversion operator: conversions to void, to the same type, to a base class, or to a reference to them. Heads up: in C++ the signed/unsigned conversion warnings are off by default unless you explicitly turn on -Wsign-conversion.

Oh and btw, warnings about conversion from arithmetic on a small type back to that same type are only given with -Warith-conversion. iykyk.

• -Wdangling-else — the classic "wait, which if does that else belong to??" situation. GCC warns when there's potential for confusion. Behold the crime scene:

{
  if (a)
    if (b)
      foo ();
  else
    bar ();
}

In C/C++, every else branch belongs to the innermost possible if statement — which here is if (b). Not what the programmer intended, clearly, judging by the indentation they chose 🙈. When this confusion is possible, GCC drops a warning if this flag is on. Want it gone? Add explicit braces around the innermost if so the else has literally no other option:

{
  if (a)
    {
      if (b)
        foo ();
      else
        bar ();
    }
}

This warning is enabled by -Wparentheses.

• -Wdangling-pointer, -Wdangling-pointer=n — warns when you use pointers (or C++ references) to objects with automatic storage duration after their lifetime 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. Enabled at all optimization levels, but results may differ with optimization versus without.
  • -Wdangling-pointer=1 — at level 1, it only diagnoses unconditional uses of dangling pointers.
  • -Wdangling-pointer=2 — at level 2, it diagnoses conditional uses too, on top of 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 entirely. Also -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;
}

Next one: storing the address of local variable x into 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, array a is out of scope by the time pointer s gets used. Because the code setting s is conditional, this one 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 whenever the macros __TIME__, __DATE__ or __TIMESTAMP__ show up, because they can mess with bit-wise-identical reproducible compilations ✨.
• -Wempty-body — warns if there's an empty body in an if, else or do while statement. Also enabled by -Wextra.
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif.
• -Wenum-compare — warns about comparing values of different enumerated types. In C++, enumerated type mismatches in conditional expressions get diagnosed too, and the warning is on 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. Enabled by -Wextra in C.
• -Wenum-int-mismatch (C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. 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 underlying type for an enumerated type is implementation-defined, mismatches like this can cause portability drama. In C++, such mismatches are 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 a variable's initialization, or jumps backward to a label after the variable has been initialized. It only cares about variables initialized at their declaration. And ngl, this warning is only supported for C and Objective-C — in C++ this kind of branch is an error anyway.

-Wjump-misses-init is included in -Wc++-compat. You can turn it off with -Wno-jump-misses-init.

• -Wsign-compare — warns when comparing signed and unsigned values could produce an incorrect result once 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 it. In C, this option is enabled by -Wconversion too.
• -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 like, it warns about:

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 real → integer conversions, and higher-precision real → lower-precision real conversions. Also enabled by -Wconversion.
• -Wno-scalar-storage-order — do not 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. In that case the computation will not give you the number of elements in the array, which is probably what you wanted. Like:

int fn ()
{
  int arr[10];
  return sizeof (arr) / sizeof (short);
}

Enabled by -Wall.

• -Wsizeof-pointer-div — warns for suspicious divisions of two sizeof expressions that divide pointer size by element size. That's the usual way to compute array size, but bestie, it does not work correctly with pointers. It warns about stuff like sizeof (ptr) / sizeof (ptr[0]) when ptr is a pointer and not an array. Enabled by -Wall.
• -Wsizeof-pointer-memaccess — warns about suspicious length parameters to certain string and memory built-in functions when the argument uses sizeof. Triggers for example on memset (ptr, 0, sizeof (ptr)); when ptr is a pointer and not an array (it even suggests a possible fix ✨), or on memcpy (&foo, ptr, sizeof (&foo));.

It also warns about calls to bounded string copy functions like strncat or strncpy that specify a sizeof expression of the source array as the bound. In this next function, the strncat call passes the size of the source string as the bound — 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");
  ...
}

-Wsizeof-pointer-memaccess is enabled by -Wall.

• -Wno-sizeof-array-argument — do not warn when the sizeof operator is applied to a parameter declared as an array in a function definition. This warning is on 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 means the user forgot to multiply by the element size. Enabled by -Wall.

---

• -Wmemset-transposed-args — okay so bestie, this one's for when you call the memset built-in and it's giving suspicious 💀. Specifically: second argument isn't zero and third argument is zero. Like the call memset (buf, sizeof buf, 0) gets diagnosed because clearly you meant memset (buf, 0, sizeof buf) and just… swapped them 🙈. BUT — and this matters — the diagnostic only fires if that third argument is a literal zero. If it's some expression that folds down to zero, or a cast of zero to some type? Way less likely you transposed the args, so no warning, no drama. This warning is enabled by -Wall ✨.
• -Waddress — warns you about suspicious uses of address expressions, and ngl these are always a little embarrassing. These include 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 ();
}

Also comparisons of a pointer to a string literal, like:

void f (const char *x)
{
  if (x == "abc")   // warning: expression evaluates to false
    puts ("equal");
}

And tests of the results of pointer addition or subtraction for equality to null, like:

void f (const int *p, int i)
{
  return p + i == NULL;
}

Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception being weak symbols 👀), so using them in a conditional might mean you're missing parentheses in a function call, or missing a 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 really suggest you meant to call strcmp. The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress warning is enabled by -Wall. 💅

• -Wno-address-of-packed-member — do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default. 🎀
• -Wlogical-op — warns about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is probably what you wanted. Also warns when the operands of a logical operator are literally the same:

extern int a;
if (a < 0 && a < 0) { ... }

Like bestie. The same thing twice. We see it. 😭

• -Wlogical-not-parentheses — warns about logical not used on the left hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. Its purpose is to detect suspicious code like the following:

int a;
...
if (!a > 1) { ... }

It is possible to suppress the warning by wrapping the LHS into parentheses:

if ((!a) > 1) { ... }

This warning is 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 — do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations. -Waggressive-loop-optimizations is enabled by default.
• -Wno-attributes — do not warn if an unexpected __attribute__ is used, such as unrecognized attributes, function attributes applied to variables, etc. 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.

Additionally, using -Wno-attributes=, it's possible to suppress warnings about unknown scoped attributes (in C++11 and C23). For example, -Wno-attributes=vendor::attr disables warning about the following declaration:

[[vendor::attr]] void f();

It's also possible to disable warning about all attributes in a namespace using -Wno-attributes=vendor:: which prevents warning about both of these declarations:

[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();

Note that -Wno-attributes= does not imply -Wno-attributes. 🫶

• -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 those expected by the function. 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 avoid the warning, include the appropriate header to bring the prototypes of built-in functions into scope.

For example, the call to memset below is diagnosed by the warning 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 — do not warn if certain built-in macros are 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 occurs 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. It's turned into an error if building for a C version before C23 by -pedantic-errors. 👑

• -Wheader-guard — 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 empty definition. In such case, it often is just a misspelled guard name — either in the #ifndef or #if !defined directive, or in the subsequent #define directive. This warning is enabled by -Wall. 💀 (the classic typo arc)
• -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 preceded by a declaration that specifies the argument types.)
• -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static aren't the first things in a declaration. This warning is also enabled by -Wextra. ✨
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. A warning is given 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 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, specifying only its type. This can be used 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's turned into an error if 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) — do not warn if a function declaration contains a parameter name without a type. Such function declarations do not 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 this option 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 conflict with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++. 🧚
• -Wmissing-variable-declarations (C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use this option to detect global variables that don't have a matching extern declaration in a header file. 🌈
• -Wmissing-declarations — okay so this one yells at you if a global function gets defined without a previous declaration. It does this even if the definition itself gives you a prototype. Use this option to catch global functions that aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. 💅
• -Wmissing-field-initializers — bestie this warns if a structure's initializer has some fields missing. For example, the following code causes such a warning, 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 the following modification does not trigger a warning:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C this option does not warn about the universal zero initializer { 0 }:

struct s { int f, g, h; };
struct s x = { 0 };

Likewise, in C++ this option does not warn about the empty { } initializer, for example:

struct s { int f, g, h; };
s x = { };

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. ✨

• -Wno-missing-requires — lowkey 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 the user meant to write:

bool satisfied = requires { requires C<T> };

so satisfied is only true if concept C is satisfied for type T. This warning can be disabled with -Wno-missing-requires. 🦋

• -Wno-missing-template-keyword — the way that 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.

template <class X>
void DoStuff (X x)
{
  x.template DoSomeOtherStuff<X>(); // Good.
  x.DoMoreStuff<X>(); // Warning, x is dependent.
}

In rare cases it is 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 — not me do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as they have implementation-defined values, and should not be used in portable code. 💀
• -Wnormalized=[none|id|nfc|nfkc] — okay so in ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However, sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if you are using identifiers that have not been normalized; this option controls that warning.

There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is 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. That is, 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 is hoped that future versions of the standards involved will correct this, which is why this option is not 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 are 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 has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. 🌈

• -Wno-attribute-warning — bestie do not warn about usage of functions declared with 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 -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. 💜
• -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 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. To get 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 be mis-aligned for little benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not 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 is 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 this warning. 🧚

• -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 it is possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller. 💅
• -Wredundant-decls — warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. 🙈
• -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 works best at -O2 and above. It is included in -Wall. 🔥

• -Wnested-externs (C and Objective-C only) — warn if an extern declaration is encountered within a function. 🍓
• -Winline — bestie, this one snitches on you 💅. It warns whenever a function you declared as inline … can't actually be inlined. Rude, right? BUT (and this is the plot twist) even with this option on, the compiler will not warn you about failures to inline functions declared in system headers. So like, system headers stay un-snitched-on, iykyk.

The thing is, the compiler is out here using a whole variety of heuristics to decide whether or not to inline a function — not a vibe check, actual math 🧮. For example, it takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. So yeah, seemingly insignificant changes in your source program can make the -Winline warnings appear or disappear out of nowhere. Not me chasing a warning that ghosts me every time I touch an unrelated line 😭.

• -Winterference-size — warns about your use of C++17 std::hardware_destructive_interference_size when you didn't specify its value with --param destructive-interference-size. It'll also warn about questionable values for that option, because it cares about you 🫶.

This variable is intended to be used for controlling class layout, so you can 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 intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line. Iconic spacing, honestly ✨.

By default, --param destructive-interference-size and --param constructive-interference-size are set based on the current -mtune option — typically to the L1 cache line size for the particular target CPU, and sometimes to a range if you're tuning for a generic target. So ⚠️ all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu). No mixing, bestie, that's a whole mess.

If ABI stability is important — like, say, if the use is in a header for a library — you should probably not use the hardware interference size variables at all. Sorry queen, that's just the truth. Alternatively, you can force a particular value with --param.

And if you're confident that your use of the variable does not affect ABI outside a single build of your project, you can turn off the warning with -Wno-interference-size. Confidence is free 💅.

• -Wint-in-bool-context — warns you about suspicious use of integer values where boolean values are expected 👀. Like conditional expressions (?:) using non-boolean integer constants in boolean context, e.g. if (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, like for (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is 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 smaller size is an error, so heads up ⚠️. 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 cannot be used. It's giving "found your ex in the search path but you can't even use them" 💀.
• -Winvalid-utf8 — warns if an invalid UTF-8 character is found. This warning is on by default for C++23 if -finput-charset=UTF-8 is used, and it gets turned into an error with -pedantic-errors. So keep those bytes valid, bestie 🦋.
• -Wno-unicode — don't diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens. Wunicode is enabled by default.
• -Wlong-long — warns if the long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.

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. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
• -Wno-varargs — do not warn upon questionable usage of the macros used to handle variable arguments like va_start. These warnings are enabled by default, so this is your off-switch 🔇.
• -Wvector-operation-performance — warns if a vector operation is not implemented via the SIMD capabilities of the architecture. Mainly useful for the performance tuning girlies 📈. A vector operation can be implemented:
  • "piecewise" — which means the scalar operation is performed on every vector element.
  • "in parallel" — which means the vector operation is implemented using scalars of wider type, which normally is more performance efficient.
  • "as a single scalar" — which means the vector fits into a scalar type.
• -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=byte-size — if this option is used, 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 byte-size bytes. This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays.

Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. Sneaky 👀.

-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=byte-size.

• -Wno-vla-larger-than — disables -Wvla-larger-than= warnings. The option is 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. Very much the friend who notices when your stories don't line up 🧐.

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 one specifies that the array argument must have at least n elements. In addition, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.

        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. So don't trust it like that, bestie 😭. This warning is 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 appears the code meant exponentiation. Specifically, the warning occurs 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. Different languages, different lore 🌈.

This warning can be silenced by converting one of the operands to hexadecimal as well as by compiling with -Wno-xor-used-as-pow.

• -Wdisabled-optimization — warns if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to 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 not you, it's the size of the function 💅.
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is 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 — this option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. Protection check, bestie 🛡️.
• -Woverlength-strings — warns about string constants that are 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 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.

This option is 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 does not have a suffix. When used together with -Wsystem-headers it warns about such constants in system header files too, no mercy. This can be 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 the link-time optimization, do not 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 — bestie, this is the moment. 👑 This turns on a static analysis of program flow that goes hunting for "interesting" interprocedural paths through your code and yells at you with warnings about the problems it finds on them. It is so extra about it too — this analysis is much more expensive than other GCC warnings, like it's the main character and your compile time is just paying rent. 💅

Technically speaking, it does coverage-guided symbolic execution of the code you're compiling. And ngl, it is neither sound nor complete — it can have false positives and false negatives. It's a bug-finding tool, not a tool for proving your program is correct, so don't come for it when it's wrong. 🫶 The analyzer is only suitable for use on C code in this release.

Turning this on effectively enables all these 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

Heads up bestie: this option is only available if GCC was configured with analyzer support enabled. No support, no slay. 😭

• -Wanalyzer-symbol-too-complex — so when -fanalyzer is on, the analyzer uses a bunch of heuristics to try to track the state of memory, but sufficiently complicated code can just defeat them entirely. 💀

By default the analysis silently stops tracking values of expressions once they exceed the threshold defined by --param analyzer-max-svalue-depth=value, and falls back to an imprecise representation for those expressions. This option is the one that actually warns you when that happens. 👀

• -Wanalyzer-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to try to explore the control flow and data flow in the program, but again — sufficiently complicated code can defeat them. 🙈

By default the analysis silently stops if the code is too complicated for the analyzer to fully explore and it hits an internal limit. This option warns you when that 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 where a pointer to a buffer gets assigned to point at a buffer with a size that is not a multiple of sizeof (*pointer). 😬

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 where a pointer is checked for "NULL" after it has already been dereferenced — which is giving "bestie, this pointer could have been NULL the whole time." 🚩 Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced.

It also considers values passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value, so it will complain if such values are checked for "NULL" after returning from such a function call.

Also — this diagnostic is unlikely to be reported when any level of optimization is enabled, because GCC's optimization logic will typically treat those NULL checks as redundant and optimize them away before the analyzer even "sees" them. So optimization should be disabled when you're trying 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 where a "FILE *" can have fclose called on it more than once. 😭

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 where 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 where a security-sensitive value is written to an output file — like writing a password to a log file. 🫣

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 where uninitialized values are copied across a security boundary. Think 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 where a read on a write-only file descriptor is attempted, or vice versa. 🔄

It also warns for code paths where 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 where 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 where an open file descriptor 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-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 where an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it'll warn on attempts to call accept on a stream socket that has not yet had listen successfully called on it. ⏳

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 where an operation is attempted on the wrong type of file descriptor. For example, it'll 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. 🧐

• -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 where a read or write is called on a closed file descriptor. 💀

It also warns for paths through code where 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 where a file descriptor is used without being checked for validity. 🚩

It also warns for paths through code where 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.

• -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 where 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 — bestie, this one needs -fanalyzer to even exist (that's what turns it on), and you use -Wno-analyzer-free-of-non-heap to switch it back off.

What it's clocking: any path through your code where free gets called on a pointer that was never on the heap. We're talking an on-stack buffer, a global, that kind of chaos. 💀

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; use -Wno-analyzer-imprecise-fp-arithmetic to disable it.

This one snitches on code paths where floating-point arithmetic shows up in places that genuinely need precise computation. Heads up though: right now it only warns when floating-point operands are used inside the calculation of an allocation size. That's the whole scope, periodt.

• -Wno-analyzer-infinite-loop — requires -fanalyzer, which enables it; use -Wno-analyzer-infinite-loop to disable it.

It warns about code paths that look like they're heading straight into an infinite loop. Not me being dramatic — the analyzer literally only says something when it "sees" a loop where:

• no externally-visible work could be being done inside the loop
• there's no way to escape the loop
• the analyzer is confident enough about the program state throughout the loop to know the two things above are actually true

One way this fires: there's an execution path through a loop where taking it on one iteration means you'll take the exact same path on every iteration after. 😭

Like, look at 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 will complain here because if opcode ever matches none of the cases, the switch falls through to the implicit default case, which makes the body of the loop a total "no-op" — cpu_state.pc never changes, so you keep getting the same opcode on every iteration, and boom: infinite loop. 💅

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 about code paths that look like they lead to infinite recursion.

Here's the tea: when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame against the state at the entry to the previous frame of that function on the stack. If nothing in memory appears to be changing → warning. But if it spots any change to parameters or globals, it assumes the recursion will terminate, and the warning gets suppressed. 🙈

Lowkey important: this diagnostic is likely to miss cases of infinite recursion that the optimizer converts into iteration before the analyzer ever "sees" them. So if you're trying to trigger it, turn optimization off.

Compare it with -Winfinite-recursion, which gives you a similar diagnostic but is implemented a whole different way.

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.

This one warns for paths through the code where a NULL function pointer gets called. Straight up.

• -Wno-analyzer-malloc-leak — requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to disable it.

Catches code paths where a pointer allocated via an allocator gets leaked — that's either malloc, or any function marked with the attribute malloc.

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 code paths where the wrong deallocation function gets called on a pointer value — judged by which function was used to allocate it in the first place. It'll flag mismatches between free, scalar delete, and vector delete[], plus anything marked as an allocator/deallocator pair using the attribute malloc.

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 code paths where a buffer is definitely read or written out-of-bounds. It applies when the analyzer can determine a constant offset, and for accesses past the end of a buffer with a constant capacity. It also does limited checking for accesses past the end when both the offset and the capacity are symbolic.

See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer ("https://cwe.mitre.org/data/definitions/119.html").

And when it can, the analyzer will drop a whole text art diagram 💅 visualizing the spatial relationship between the memory region it predicts would be accessed versus the range of memory that's valid to access. That means: whether they overlap, are touching, are close or far apart; which one comes 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. If you don't want the art, suppress it 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 code paths where overlapping buffers get passed to an API whose behavior on such buffers is undefined. Rude.

Specifically, it fires on calls to these functions:

• memcpy
• strcat
• strcpy

...for 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 code paths where a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) — you know, the one that swears it requires a non-NULL value. 😬

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 code paths where a possibly-NULL value gets dereferenced.

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 code paths where a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.

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 code paths where a value known to be NULL is dereferenced.

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 code paths where a call to putenv is handed a pointer to an automatic variable or an on-stack buffer. Bestie, that pointer is not going to outlive the scope, and putenv does not copy the string. 🫠

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 code paths where a shift is attempted with a negative count. It's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends — but this one is implemented by analyzing interprocedural paths, not just parsing the syntax tree. One caveat: the analyzer doesn't prioritize detection of these paths, so false negatives are more likely here than with other warnings.

• -Wno-analyzer-shift-count-overflow — requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-overflow to disable it.

Warns for code paths where a shift is attempted with a count greater than or equal to the precision of the operand's type. Analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but again — implemented by analyzing interprocedural paths rather than merely parsing the syntax tree. And again, the analyzer doesn't prioritize detection of such paths, so false negatives are more likely relative to other warnings.

• -Wno-analyzer-stale-setjmp-buffer — requires -fanalyzer, which enables it; use -Wno-analyzer-stale-setjmp-buffer to disable it.

Warns for code paths where longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has already returned. 💀

When setjmp gets called on a jmp_buf to write down a rewind location, bestie, it records the stack frame. And that stack frame goes ✨invalid✨ the literal second the function holding the setjmp call returns. So if you try to rewind back to it with longjmp, you're pointing at a stack frame that does not exist anymore — which is basically a guaranteed crash (or worse 💀). Not the vibe, don't do it.

---

• -Wno-analyzer-tainted-allocation-size — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-tainted-allocation-size to turn it back 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. Which means an attacker could inject a comically large allocation and potentially cause a denial of service attack. ma'am, that's a DoS 😭.

See CWE-789: Memory Allocation with Excessive Size Value (https://cwe.mitre.org/data/definitions/789.html).

• -Wno-analyzer-tainted-assertion — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-tainted-assertion to turn it back off.

It flags paths through your code where a value that could be under an attacker's control gets used as part of a condition without being sanitized first, and that condition guards a call to a function marked with attribute noreturn (think __builtin_unreachable). Those functions usually mean the program is terminating abnormally, like an assertion failure handler would. Example incoming:

assert (some_tainted_value < SOME_LIMIT);

And in those cases:

• 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. yikes.

Heads up though: when assertion-checking is disabled, the assertions are usually already removed by the preprocessor before the analyzer ever gets a chance to "see" them, so this diagnostic can only generate warnings on builds where assertion-checking is enabled.

For the purpose of this warning, any function marked with attribute noreturn counts as a possible assertion failure handler, __builtin_unreachable included. And ngl, those functions sometimes get removed by the optimizer before the analyzer "sees" them either — so you should disable optimization when you're trying to trigger this diagnostic.

See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).

It can also call out problematic constructions like this one:

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 thing above isn't an assertion failure at all. 🙈

• -Wno-analyzer-tainted-array-index — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-tainted-array-index to turn it back off.

It flags paths through your code where a value that could be under an attacker's control gets used as the index of an array access without being sanitized first — so an attacker could inject an out-of-bounds access. we do not stan.

See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).

• -Wno-analyzer-tainted-divisor — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-tainted-divisor to turn it back off.

It flags paths through your code where a value that could be under an attacker's control gets used as the divisor in a division or modulus operation without being sanitized first — so an attacker could inject a division-by-zero. math girlies in shambles 💅.

See CWE-369: Divide By Zero (https://cwe.mitre.org/data/definitions/369.html).

• -Wno-analyzer-tainted-offset — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-tainted-offset to turn it back off.

It flags paths through your code where a value that could be under an attacker's control gets used as a pointer offset without being sanitized first — 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 — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-tainted-size to turn it back 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 operation like memset without being sanitized first — 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-undefined-behavior-ptrdiff — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-undefined-behavior-ptrdiff to turn it back off.

It flags paths through your code where a pointer subtraction happens but the pointers are referring 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 — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-undefined-behavior-strtok to turn it back off.

It flags paths through your code where a call is made to strtok with undefined behavior.

Specifically: passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior. So like, don't 🙈.

• -Wno-analyzer-unsafe-call-within-signal-handler — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-unsafe-call-within-signal-handler to turn it back off.

It flags paths through your code where a function known to be async-signal-unsafe (like fprintf) gets called from a signal handler. bad idea energy.

See CWE-479: Signal Handler Use of a Non-reentrant Function (https://cwe.mitre.org/data/definitions/479.html).

• -Wno-analyzer-use-after-free — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-use-after-free to turn it back off.

It flags paths through your code where a pointer gets used after a deallocator was called on it: either free, or a deallocator referenced by attribute malloc. the ghosting went too far bestie 👻.

See CWE-416: Use After Free (https://cwe.mitre.org/data/definitions/416.html).

• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to turn it back off.

It flags paths through your code where a pointer gets dereferenced, and that pointer points to a variable sitting in a stale stack frame.

• -Wno-analyzer-va-arg-type-mismatch — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-va-arg-type-mismatch to turn it back off.

It flags interprocedural paths through your code where the analyzer catches an attempt to use va_arg to pull out a value passed to a variadic call, but the type it uses doesn't match the type of the expression that was passed to the call. mismatch era 😭.

See CWE-686: Function Call With Incorrect Argument Type (https://cwe.mitre.org/data/definitions/686.html).

• -Wno-analyzer-va-list-exhausted — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-va-list-exhausted to turn it back off.

It flags interprocedural paths through your code where the analyzer catches an attempt to use va_arg to reach the next value passed to a variadic call — except all the values in the va_list have already been consumed. empty. nothing left. gone. 🫠

See CWE-685: Function Call With Incorrect Number of Arguments (https://cwe.mitre.org/data/definitions/685.html).

• -Wno-analyzer-va-list-leak — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-va-list-leak to turn it back off.

It flags interprocedural paths through your code where the analyzer catches that va_start or va_copy was called on a va_list without a matching call to va_end. close the loop, bestie 🔁.

• -Wno-analyzer-va-list-use-after-va-end — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-va-list-use-after-va-end to turn it back off.

It flags interprocedural paths through your code 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 — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-write-to-const to turn it back off.

It flags paths through your code where the analyzer catches an attempt to write through a pointer to a const object. the disrespect 😭. But fair warning: the analyzer doesn't prioritize detecting these paths, so false negatives are more likely here than for other warnings.

• -Wno-analyzer-write-to-string-literal — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-write-to-string-literal to turn it back off.

It flags paths through your code where the analyzer catches an attempt to write through a pointer to a string literal. Same disclaimer as above though: the analyzer doesn't prioritize detecting these paths, so false negatives are more likely relative to other warnings.

• -Wno-analyzer-use-of-uninitialized-value — this one needs -fanalyzer, which turns it on; use -Wno-analyzer-use-of-uninitialized-value to turn it back off.

It flags paths through your code where an uninitialized value gets used. girl where did that value even come from 💀.

See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).

---

And okay so, the analyzer also 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 bestie, we're also talking about the 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"

plus the following 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 of the following 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"

In addition, various functions with an __analyzer_ prefix have special meaning to the analyzer, described in the GCC Internals manual. (iykyk 💅)

---

🎛️ Pertinent parameters for controlling the exploration are:

• --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 following options control the analyzer ✨

• -fanalyzer-call-summaries — ok so this one simplifies the interprocedural analysis by computing the effect of certain calls, instead of exploring all the paths through the function from callsite to each possible return. lowkey a glow-up for your compile times.

If 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 to run just the named checker, and enables it. one checker, one moment, very main character of it 💅
• -fanalyzer-debug-text-art-headings — this option is intended for analyzer developers. If enabled, the analyzer will add extra annotations to any diagrams it generates. (it's giving art class, honestly 🎨)
• -fno-analyzer-feasibility — this option is intended for analyzer developers.

By default the analyzer verifies that there is a feasible control flow path for each diagnostic it emits: that the conditions that hold are not mutually exclusive. Diagnostics for which no feasible path can be found are rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code.

• -fanalyzer-fine-grained — this option is intended for analyzer developers.

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, each statement gets its own edge. no sharing, no merging, every statement its own little moment ✨

• -fanalyzer-show-duplicate-count — this option is intended 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. (it literally counts the clones for you 🫶)
• -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 stays hidden, bestie 🙈
• -fno-analyzer-state-merge — this option is intended for 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 — this option is intended for analyzer developers.

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.

• -fno-analyzer-suppress-followups — this option is intended for analyzer developers.

By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics. because one drama is enough, we don't need the whole group chat reacting 💀

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 will continue 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.

• -fanalyzer-transitivity — this option enables transitivity of constraints within the analyzer. (math girlies rise 🧚)
• -fno-analyzer-undo-inlining — this option is intended for analyzer developers.

-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 be of help when debugging issues in the analyzer.

• -fanalyzer-verbose-edges — this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. the tea is piping hot and fully detailed ☕
• -fanalyzer-verbose-state-changes — this option is intended 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 option controls the complexity of the control flow paths that are emitted for analyzer diagnostics.

The level can be one of:

1. 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. As per the previous level, but also show events for the entry to each function.
3. 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. ✨ the one you're already on, bestie ✨

4. As per the previous level, but show all control flow events, not just significant ones.
5. 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. -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. GraphViz said slay 📊
• -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 of files, do not run this on a whim 😭
• -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. yes it's an infinite loop AND it has its own dot file, iconic behavior 🌀
• -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change. (so don't build a whole pipeline on it, bestie, the format said "no commitment" 🙈)
• -fdump-analyzer-state-purge — bestie this is the same vibe as -fdump-analyzer-supergraph, it dumps a representation of the "supergraph" that's suitable for viewing with GraphViz, except it also annotates the graph with info about what state is gonna get purged at each node. The graph gets written to file.state-purge.dot. 💅
• -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. 🦋
• -fdump-analyzer-untracked — emits custom warnings with internal details that are intended for analyzer developers. iykyk, it's giving inside-baseball 🎀

---

🐛 Options for Debugging Your Program 🐛

To tell GCC to emit extra info for a debugger to use, in almost all cases you only 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 lets you use -g with -O. The shortcuts taken by optimized code may occasionally be surprising, and honestly it's giving chaos: 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. This 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. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. 🫶

• -g — produces 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. If you want to control for certain whether to generate the extra information, use -gvms (see below). 👑

• -ggdb — produces 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. 🌸
• -gdwarf, -gdwarf-version — produces 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. 💀

• -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 — produces debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems. 🦋
• -gcodeview — produces 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. 😭
• -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. 🦋
• -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. 🎀
• -gpubnames — makes DWARF sections called .debug_pubnames and .debug_pubtypes. slay, name indexes for your debugger bestie 🔍
• -ggnu-pubnames — same .debug_pubnames and .debug_pubtypes sections but in a format that can be turned into a GDB index. only useful if your linker can produce GDB index version 7, iykyk. otherwise it's giving nothing 💅
• -fdebug-types-section — with DWARF Version 4 or higher, type DIEs can live in their own .debug_types section instead of being crammed into .debug_info. putting them in a separate comdat section is more efficient bc the linker can then remove duplicates ✨ BUT — not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types actually produces larger instead of smaller debugging info 😭 so like. vibe check your toolchain first.
• -grecord-gcc-switches, -gno-record-gcc-switches — this one appends the command-line options you used to invoke the compiler (the ones that may affect code generation) to the DW_AT_producer attribute in your DWARF debug info. the options get concatenated with spaces separating them from each other and from the compiler version 🫶 enabled by default. also see -frecord-gcc-switches if you want another way to store compiler options into the object file 👑
• -gstrict-dwarf — says no to using extensions from a later DWARF standard version than whatever you picked with -gdwarf-version. on most targets, non-conflicting DWARF extensions from later standard versions are allowed, so this is you being strict on purpose 💅
• -gno-strict-dwarf — the opposite bestie: allow using extensions of later DWARF standard versions than selected with -gdwarf-version 🌈
• -gas-loc-support — tells the compiler the assembler supports .loc directives. then it may use them so the assembler generates DWARF2+ line number tables. this is generally desirable bc assembler-generated line-number tables are a LOT more compact than the ones the compiler makes itself ✨ 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. no assembler help, we do it ourselves 💪
• -gas-locview-support — informs the compiler that 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 info into DWARF debugging info, not just file and line. enabled by default 🎀
• -gstatement-frontiers, -gno-statement-frontiers — this makes GCC create 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 if 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 causes 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 🙃

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 isn't available, this may still be enabled, but it forces 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 a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5. you can turn it on with -gvariable-location-views=incompat5. heads up: this option may be removed in the future, it's only provided as a reference implementation of the proposed representation, and debug info consumers aren't expected to support this extended format — they'd be rendered unable to decode location lists using it 💀

• -ginternal-reset-location-views, -gno-internal-reset-location-views — tries to figure out 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 are inserted at inlined entry points, so that 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] — produce 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 — emit 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 info, but at significant potential loss in type information to the debugger 😭 see -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 — emit 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 info, 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. works only with DWARF debug output.
• -femit-struct-debug-detailed[=spec-list] — specify the struct-like types for which the compiler generates debug info. the intent is to reduce duplicate struct debug info between different object files within the same program 👯 this option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.

a specification has the syntax

[dir:|ind:][ord:|gen:](any|sys|base|none)

the optional first word limits the spec 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 spec to ordinary structs (ord:) or generic structs (gen:). generic structs are a bit complicated to explain. 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 doesn't 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 that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. in practice, this means that 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 🔬

the default is -femit-struct-debug-detailed=all.

this option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit 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 debug 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 wanna cast a value to a type that isn't 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 🔥

Okay so optimization — the part everyone either obsesses over or ignores completely, no in-between. 💅

When you don't pass any optimization option at all, the compiler's entire goal is to keep compilation cheap and make debugging do exactly what you expect. Statements stay independent: if you stop the program at a breakpoint between statements, you can then assign a new value to any variable, or straight-up move the program counter to any other statement in the function, and you get exactly the results you expect from the source code. No surprises, no gaslighting. 🎀

Flip on optimization flags and the compiler starts trying to improve performance and/or code size — but the cost is compilation time and possibly your ability to debug the thing. Trade-offs, bestie. 😭

The compiler optimizes based on what it knows about your program. Compiling multiple files at once into a single output file mode lets it use info gained from all of the files while compiling each one. Group project but everyone actually pulls their weight. ✨

Not every optimization is controlled directly by a flag, okay? Only the optimizations that have a flag are listed in this section.

Most optimizations are completely disabled at -O0, or if an -O level isn't set on the command line — even if you specify individual optimization flags yourself. Same energy: -Og suppresses many optimization passes too.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than the ones listed here. Want the receipts? Invoke GCC with -Q --help=optimizers to find out the exact set of optimizations enabled at each level. 🕵️‍♀️

---

-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 — it's a sensible balance between the time taken to compile and memory use, because 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. 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

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. Bestie, read it. 🙈

-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. So like, no, you can't just cherry-pick. 💀

-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. Tiny and cute. 🦋

-Ofast — the menace 💅

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.

Standards compliance? Never met her. 😭 She's fast though. She is fast.

-Og — Optimize debugging experience 🧚

-Og should be the optimization level of choice for the standard edit-compile-debug cycle — it offers a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It's 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.

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If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. Last one wins, periodt. 👑

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Okay so 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. 🫶

The following options control specific optimizations. They're either activated by -O options or 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. Niche behavior, honestly iconic. ✨

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-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. Stack hoarding but make it efficient. 🍓

-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. 🚫

-ffp-contract=style

• -ffp-contract=off disables floating-point expression contraction.
• -ffp-contract=fast enables floating-point expression contraction, such as forming 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. 👑

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.

• -foptimize-sibling-calls — bestie, this one optimizes sibling and tail recursive calls 💫 so your recursion doesn't eat the whole stack, iconic behavior honestly.

Turned on at levels -O2, -O3, -Os.

• -foptimize-strlen — it takes various standard C string functions (like strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts and rewrites them into faster alternatives. the glow-up is real ✨

Enabled at levels -O2, -O3.

• -finline-stringops[=fn] — expands memory and string operations inline (for now, only memset), even when the length is variable or big enough that it would normally require looping. most useful when you're rolling with -ffreestanding and -fno-builtin 🫶

In some circumstances, it lets the compiler generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and it can grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches. 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 you're not optimizing at all. no notes 💅

Single functions can be exempted from inlining by marking them with the noinline attribute.

• -finline-small-functions — integrates functions into their callers when their body is smaller than expected function call code (so the overall size of the program gets smaller). the compiler heuristically decides which functions are simple enough to be worth integrating this way. and listen — this inlining applies to all functions, even those not declared inline 👑

Enabled at levels -O2, -O3, -Os.

• -findirect-inlining — inlines indirect calls too, the ones discovered to be known at compile time thanks to previous inlining. this option only does anything when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

• -finline-functions — considers all functions for inlining, even if they're not declared inline. the compiler heuristically decides which functions are worth integrating this way 🌸

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 — considers all static functions called once for inlining into their caller, even if they're 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.

Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.

• -fearly-inlining — inlines functions marked by always_inline plus functions whose body seems smaller than the function call overhead, and it does this early, before doing -fprofile-generate instrumentation and the real inlining pass. doing it this way makes profiling significantly cheaper and usually makes inlining faster on programs that have large chains of nested wrapper functions. the way that it's just doing the little ones first?? obsessed 💖

Enabled by default.

• -fipa-sra — performs interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference with parameters passed by value. slay 🎀

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 that limit. n is the size of functions that can be inlined in number of pseudo instructions.

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 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. 💀

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 another. so like, don't get too attached to the number, bestie 🙈

• -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.
• -fkeep-inline-functions — in C, emits 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++, it emits any and all inline functions into the object file. keeping them around just in case, main character energy fr 💜
• -fkeep-static-functions — emits static functions into the object file, even if the function is never used. it's giving "you never know when you'll need me" 🌈
• -fkeep-static-consts — emits 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 — attempts to merge identical constants (string constants and floating-point constants) across compilation units. twins!! merging them fr 🦋

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 — attempts to merge identical constants and identical variables.

This option implies -fmerge-constants. In addition to -fmerge-constants it 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. ⚠️ be careful bestie

• -fmodulo-sched — performs swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. choreography but for instructions ✨
• -fmodulo-sched-allow-regmoves — performs 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.
• -fno-branch-count-reg — disables 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 it's not a full undo, just a "stop adding more" 💅

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.

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.

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.

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.

The default is -fzero-initialized-in-bss except in Ada.

• -fthread-jumps — performs 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. she's following the threads 🧵

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, splits the registers apart and allocates them independently. This normally generates better code for those types, but may make debugging more difficult. tradeoffs, we love a tradeoff 😭

Enabled at levels -O1, -O2, -O3, -Os.

• -fsplit-wide-types-early — fully splits wide types early, instead of very late. This option has no effect unless -fsplit-wide-types is turned on. no cap, it needs its bestie to do anything.

This is the default on some targets.

• -fcse-follow-jumps — in common subexpression elimination (CSE), scans 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. she said "I'll go around" 🚶♀️💨

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.

Enabled at levels -O2, -O3, -Os.

• -frerun-cse-after-loop — re-runs common subexpression elimination after loop optimizations are performed. round two, and this time she knows things 🎀

Enabled at levels -O2, -O3, -Os.

• -fgcse — performs a global common subexpression elimination pass. This pass also performs global constant and copy propagation. global girlie behavior fr 💖

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.

Enabled at levels -O2, -O3, -Os.

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• -fgcse-lm — when this one's on, global common subexpression elimination tries to move loads that are only killed by stores into themselves. Which sounds unhinged but hear me out bestie: it lets a loop that's got a load/store sequence get rewritten as a load outside the loop plus a copy/store inside the loop. Iconic behaviour honestly. 💅

Enabled by default whenever -fgcse is enabled.

• -fgcse-sm — with this on, a store motion pass runs after global common subexpression elimination, and it tries to move stores out of loops. Pair it with -fgcse-lm and loops containing a load/store sequence can become a load before the loop and a store after the loop. Chef's kiss. 🎀

Not enabled at any optimization level.

• -fgcse-las — when this is enabled, the global common subexpression elimination pass yeets redundant loads that come after stores to the same memory location. Both partial and full redundancies, no cap.

Not enabled at any optimization level.

• -fgcse-after-reload — turns on a redundant load elimination pass that runs after reload, and its whole job is cleaning up redundant spilling. Little tidy-up era. ✨

Enabled by -O3, -fprofile-use and -fauto-profile.

• -faggressive-loop-optimizations — tells the loop optimizer to use language constraints to derive bounds for how many iterations a loop runs. This assumes your loop code doesn't invoke undefined behavior — like, no signed integer overflows, no out-of-bound array accesses, none of that. The iteration-count bounds then guide loop unrolling, peeling, and loop exit test optimizations.

Enabled by default.

Lowkey a vibe check on your code: if you do rely on signed overflow or OOB access, this option is assuming you don't, and that's on you, not on it.

• -funconstrained-commons — tells the compiler that variables declared in common blocks (e.g. Fortran) might later get overridden with longer trailing arrays. Which blocks certain optimizations that depend on knowing the array bounds. Rip to those optimizations. 😭
• -fcrossjumping — performs cross-jumping transformation, which unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping — no promises, bestie.

Enabled at levels -O2, -O3, -Os.

• -fauto-inc-dec — combines increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that don't have instructions supporting it, so no stress if your arch can't. Enabled by default at -O1 and higher on architectures that support this.
• -fdce — performs dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher.
• -fdse — performs dead store elimination (DSE) on RTL. Enabled by default at -O1 and higher.
• -fif-conversion — attempts to transform conditional jumps into branch-less equivalents. That means conditional moves, min, max, set flags, abs instructions, plus some tricks you can do with standard arithmetics. The use of conditional execution on chips where it's available is controlled by -fif-conversion2.

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

• -fif-conversion2 — uses conditional execution (where available) to transform conditional jumps into branch-less equivalents.

Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.

• -fdeclone-ctor-dtor — okay so 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 — meaning two copies of the function. 🙈 With this option, the base and complete variants get changed into thunks that call a common implementation. So much less duplication.

Enabled by -Os.

• -fdelete-null-pointer-checks — assumes programs can't safely dereference null pointers, and that no code or data element lives 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 null pointer checks — they assume a memory access to address zero always traps, so if a pointer is checked after it's already been dereferenced, it can't be null.

Heads up though: in some environments that assumption is just not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

This option is enabled by default on most targets. On AVR and MSP430, this option is completely disabled.

Passes that use the dataflow information are enabled independently at different optimization levels.

• -fdevirtualize — attempts to convert calls to virtual functions into direct calls. It does this both within a procedure and interprocedurally, as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
• -fdevirtualize-speculatively — attempts to convert calls to virtual functions into speculative direct calls. Based on analysis of the type inheritance graph, it figures out for a given call the set of likely targets. If that set is small — preferably size 1 — it changes the call into a conditional deciding between direct and indirect calls. The speculative calls enable more optimizations, like inlining. And when they seem useless after further optimization, they get converted back into their original form. It's giving glow-up that can be reversed. 🦋
• -fdevirtualize-at-ltrans — streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This enables more devirtualization but significantly increases the size of streamed data, which is why it's disabled by default.
• -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive. Cute little extra credit.

Enabled at levels -O2, -O3, -Os.

• -free — attempts to remove redundant extension instructions. Especially helpful for x86-64, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Slay.

Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os.

• -fno-lifetime-dse — 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 once 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 that optimization. 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.
• -flive-range-shrinkage — attempts to decrease register pressure through register live range shrinkage. Helpful for fast processors with small or moderate size register sets. Main character energy for tiny register files. 👑
• -fira-algorithm=algorithm — use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring isn't implemented for all architectures, but for those targets that do support it, it's 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 when deciding whether to hoist expressions. This option usually results in smaller code, but it can slow the compiler down.

Enabled at level -Os for all targets.

• -fira-loop-pressure — uses IRA to evaluate register pressure in loops when deciding to move loop invariants. This usually generates faster and smaller code on machines with large register files (>= 32 registers), but again, it can slow the compiler down.

Enabled at level -O3 for some targets.

• -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 its own separate stack slot, and as a result function stack frames get larger. 🫣
• -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers. Each pseudo-register that doesn't get a hard register gets its own separate stack slot, and as a result function stack frames get larger. Same energy, different slots.
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. Basically: why reload when you can just redo it. 🔥

Enabled at levels -O2, -O3, -Os.

• -fdelayed-branch — if supported for the target machine, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions.

Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.

• -fschedule-insns — if supported for the target machine, attempts to reorder instructions to eliminate execution stalls caused by required data being unavailable. This helps machines with slow floating point or memory load instructions by letting other instructions be issued until the result of the load or floating-point instruction is actually required. Filling the dead air, fr fr. 🌸

Conventionally enabled at optimization levels -O2 and -O3. BUT. bestie, so many targets just override this whole vibe. Like on x86, it's disabled at literally all levels (🙈 rude), while on AArch64, it's only enabled at -O3. The way that varies is giving chaos but okay, that's the tea. 🫖

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• -fschedule-insns2 — so this is like -fschedule-insns's little sibling ✨ but it asks for an extra pass of instruction scheduling after register allocation has already happened. It's especially useful on machines that have 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 scheduling is normally ON when you're scheduling before register allocation, aka with -fschedule-insns or at -O2 or higher. no interblock for you 💀
• -fno-sched-spec — disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-pressure — enables register pressure sensitive insn scheduling before register allocation. Okay but heads up: this only makes sense if scheduling-before-register-allocation is even on, i.e. with -fschedule-insns or at -O2 or higher. Using this can genuinely improve the generated code and shrink its size 🎀 by preventing register pressure from creeping above the number of available hard registers, which would cause spills later in register allocation. Nobody wants spills, fr fr.
• -fsched-spec-load — allows speculative motion of some load instructions. Only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — ⚠️ the name is not a joke bestie — allows speculative motion of more load instructions. Again, 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 — defines how many insns (if any 👀) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass.
  • -fno-sched-stalled-insns means literally no insns get moved prematurely.
  • -fsched-stalled-insns=0 means there's no limit on how many queued insns can be moved prematurely. ✨ unlimited edition ✨
  • -fsched-stalled-insns with no value at all is the same as -fsched-stalled-insns=1.
• -fsched-stalled-insns-dep
• -fsched-stalled-insns-dep=n — defines 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. Note: this only does anything during the second scheduling pass, and only if -fsched-stalled-insns is actually 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 superblock scheduling 🌈. This lets motion happen across basic block boundaries, which results in faster schedules. But it's experimental!! Not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. So like, proceed with vibes and caution. 🫶

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. This one favors the instruction that belongs to a schedule group. It's enabled by default whenever scheduling is on, 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 sitting on the critical path. Enabled by default when scheduling is on, 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. Enabled by default when scheduling is on, 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. Enabled by default when scheduling is on, 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. Enabled by default when scheduling is on, 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 (very main character of it tbh). Enabled by default when scheduling is on, 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. If a loop gets modulo scheduled, later scheduling passes might 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, but this time selective scheduling runs instead of the second scheduler pass. 💅
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling. Heads up: this option has 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 outer loops. And again, this has no effect unless -fsel-sched-pipelining is turned on.
• -fsemantic-interposition — so some object formats, like ELF, let the dynamic linker interpose symbols. What that means is: for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because the function or variable in question might change 😳. This feature is useful for stuff like rewriting memory allocation functions with a debugging implementation, but it's expensive in terms of code quality. With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. This flag has no effect for functions explicitly declared inline (where it's never allowed for interposition 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 dumping them at the top of the function. 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 are only executed when needed. This option is on by default, but it has no effect unless -fshrink-wrap is also turned on AND the target supports this. (so many conditions bestie 😭)
• -fcaller-saves — enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore those registers around such calls. This allocation only happens when it seems to result in better code. ✨

This option is always enabled by default on certain machines, usually the ones that have 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, and 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 you don't 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 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 — attempts 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 — performs reassociation on trees. This flag is enabled by default at -O1 and higher.
• -fcode-hoisting — performs code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible ⏰. This is 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 — performs 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 — performs forward propagation on trees. This flag is enabled by default at -O1 and higher.
• -ftree-fre — performs full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions 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 — performs hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O1 and higher. 🌸

🦋 -fhoist-adjacent-loads

okay so imagine an if-then-else where both branches are loading stuff. If those loads come from adjacent locations in the same structure and your target architecture actually has a conditional move instruction, GCC will speculatively hoist those loads up out of the branches. ✨ This one is on by default at -O2 and higher.

🎀 -ftree-copy-prop

does copy propagation on trees, bestie. This pass deletes the copy operations that literally do nothing useful. Enabled by default at -O1 and higher. 💅

🧚 -fipa-pure-const

figures out which functions are pure or constant. Enabled by default at -O1 and higher.

🍓 -fipa-reference

figures out which static variables never escape the compilation unit. Enabled by default at -O1 and higher.

🌸 -fipa-reference-addressable

figures out which static variables are read-only, write-only, or non-addressable. Enabled by default at -O1 and higher. ✨

👑 -fipa-reorder-for-locality

Groups call chains close together in the binary layout so the code locality is better and the jump distances between frequently called functions are as tiny as possible. 🫶

Now, the receipts on how it's different from its cousins:

• Unlike -freorder-functions, this pass looks at the call chains between functions and groups them together — it does not just bucket everything into separate hot/normal/cold/never-executed sections.
• Unlike -fprofile-reorder-functions, it's trying to improve code locality across the whole runtime of the program, not just obsessing over startup.

Heads up: this option is incompatible with an explicit -flto-partition= option, because it enforces its own custom partitioning scheme.

If you're using this, it's recommended that you also use profile feedback. But it 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. Warning: this option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level, bestie. 😭

⭐ -fipa-profile

Performs interprocedural profile propagation. Functions that are only called from cold functions get marked as cold too. It also identifies functions that get executed exactly once — think "cold", "noreturn", static constructors or destructors. Then cold functions, plus the loop-less parts of functions that execute once, get optimized for size. Enabled by default at -O1 and higher. ✨

🦋 -fipa-modref

Performs interprocedural mod/ref analysis. This optimization analyzes the side effects of functions — like which memory locations get modified or referenced — and it lets GCC optimize better across the function call boundary. Enabled by default at -O1 and higher. 💖

🎀 -fipa-cp

Performs interprocedural constant propagation. It analyzes the program to figure out when the values being passed to functions are constants, and then optimizes accordingly. If your app passes constants into functions, this can be a substantial performance glow-up. Enabled by default at -O2, -Os and -O3. It's also enabled by -fprofile-use and -fauto-profile. 👑

🍓 -fipa-cp-clone

Performs function cloning to make interprocedural constant propagation stronger. When it's on, interprocedural constant propagation will clone a function when an externally visible function can be called with constant arguments. But — and this is the important part — 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's also enabled by -fprofile-use and -fauto-profile. 💀

🌸 -fipa-bit-cp

When enabled, it performs 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, it performs interprocedural propagation of value ranges. This flag is enabled by default at -O2. It requires that -fipa-cp is enabled. ✨

💜 -fipa-icf

Performs Identical Code Folding for functions and read-only variables. The optimization reduces code size, but it may disturb unwind stacks, because it replaces a function with an equivalent one that has 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 so the optimizations are not the same — there are equivalences that are found only by GCC, and equivalences that are found only by Gold.

This flag is enabled by default at -O2 and -Os. 🎀

🌈 -flate-combine-instructions

Enables two instruction combination passes that run pretty late in the compilation process. One of them runs before register allocation, and the other one runs after register allocation. The main aim of these passes is to substitute definitions into all uses.

Most targets enable this flag by default at -O2 and -Os. ⭐

🔥 -flive-patching=level

Controls GCC's optimizations so the output is suitable for live-patching. This one is a whole situation, so let's break it down.

If the compiler's optimization uses a function's body — or information extracted from its body — to optimize or change another function, then that latter function is called an impacted function of the former. So if a function gets patched, its impacted functions should be patched too. Periodt.

The impacted functions get determined by the compiler's interprocedural optimizations. For example, a caller is impacted when you inline a function into its caller, or when you clone a function and change its caller to call this new clone, or when you extract a function's pureness/constness information to optimize its direct or indirect callers, etc.

Usually, the more IPA optimizations you enable, the larger the number of impacted functions for each function. So in order to control the number of impacted functions and more easily compute the list of impacted functions, 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 they 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 they 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

Detects paths that trigger erroneous or undefined behavior because they dereference a null pointer. It isolates those paths away from the main control flow and turns the statement with the erroneous or undefined behavior into a trap. 🪤 This flag is enabled by default at -O2 and higher, and it depends on -fdelete-null-pointer-checks also being enabled.

🎀 -fisolate-erroneous-paths-attribute

Detects paths that trigger erroneous or undefined behavior because a null value is being used in a way that's forbidden by a "returns_nonnull" or "nonnull" attribute. It isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap.

This is not currently enabled, but it may be enabled by -O2 in the future. 👀

🍓 -ftree-sink

Performs forward store motion on trees. This flag is enabled by default at -O1 and higher. ✨

🌸 -ftree-bit-ccp

Performs sparse conditional bit constant propagation on trees and propagates pointer alignment information. This pass only operates on local scalar variables, and it's enabled by default at -O1 and higher — except for -Og. It requires that -ftree-ccp is enabled.

🧚 -ftree-ccp

Performs 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

Propagates information about uses of a value up the definition chain so the definitions can be simplified. 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

Performs 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

Performs conversion of simple initializations in a switch into initializations from a scalar array. This flag is enabled by default at -O2 and higher. ✨

👑 -ftree-tail-merge

Looks for identical code sequences. When it finds one, it replaces one of them 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

Performs conditional store elimination on trees. This flag is enabled by default at -O1 and higher on targets that have conditional move instructions. 💖

🎀 -ftree-dce

Performs dead code elimination (DCE) on trees. This flag is enabled by default at -O1 and higher. 💀✨

🍓 -ftree-builtin-call-dce

Performs 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 that a loop with an exit will actually take that exit eventually, and not just vibe in there forever. Which means the compiler gets to yeet loops that otherwise have zero side-effects, because it no longer has to consider "what if this just spins until the heat death of the universe" as a possibility. ✨

This option is enabled by default at -O2 for C++ with -std=c++11 or higher.

• -ftree-dominator-opts — does a whole lil glow-up 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 (which is the "reduce jumps to jumps" thing, so you're not jumping into another jump, which is giving messy). This flag is enabled by default at -O1 and higher. 💅
• -ftree-dse — dead store elimination (DSE) on trees, no cap. A dead store is when you write to a memory location and then some other store just overwrites it later with no loads in between. Rude? Maybe. But in that case the earlier store can simply be deleted. This flag is enabled by default at -O1 and higher. 🗑️
• -ftree-ch — loop header copying on trees, and it's honestly kind of iconic. It's beneficial because it makes code motion optimizations way more effective, plus it saves one jump. This flag is enabled by default at -O1 and higher. It is not enabled for -Os, since it usually increases code size (and -Os said "we're being mindful of size today"). 🎀
• -ftree-loop-optimize — performs loop optimizations on trees. This flag is enabled by default at -O1 and higher. 🌀
• -ftree-loop-linear, -floop-strip-mine, -floop-block — these three are all doing loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. So like, it's not just there by default, you have to have set it up. 🛠️
• -fgraphite-identity — enables the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops. It's giving "round trip just to see what happens." 🌈
• -floop-nest-optimize — enables the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. Heads up bestie: this option is experimental. ⚠️
• -floop-parallelize-all — uses the Graphite data dependence analysis to identify loops that can be parallelized. It parallelizes all the loops that can be analyzed to not contain loop carried dependences, without checking that it is profitable to parallelize the loops. So like, it just goes for it. 🏃‍♀️
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, it attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries. Big warning though: this may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents SSA coalescing of user variables. This option is 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 to branch-less equivalents. The whole intent is to remove control-flow from the innermost loops so the vectorization pass has an easier time handling these loops. This is enabled by default if vectorization is enabled. 🔀
• -ftree-loop-distribution — performs loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop

        DO I = 1, N
          A(I) = B(I) + C
          D(I) = E(I) * F
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = B(I) + C
        ENDDO
        DO I = 1, N
           D(I) = E(I) * F
        ENDDO

This flag is enabled by default at -O3. It is 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. This flag is enabled 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. For example, the loop

        DO I = 1, N
          A(I) = 0
          B(I) = A(I) + I
        ENDDO

is transformed to

        DO I = 1, N
           A(I) = 0
        ENDDO
        DO I = 1, N
           B(I) = A(I) + I
        ENDDO

and the initialization loop is transformed into a call to memset zero. 🧼

• -floop-interchange — performs loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. For example, the 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];

This flag is enabled by default at -O3. It is 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. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🍓
• -ftree-loop-im — performs loop invariant motion on trees. This pass moves only 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 that 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 for which determining the number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling. 🔢
• -ftree-scev-cprop — performs final value replacement. If a variable is modified in 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, it replaces uses of the final value by such a computation — provided it is 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 — parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. 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. This flag is 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. This flag is 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 to reduce the number of instructions. This is 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 are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on, resulting in better RTL generation. This is enabled by default at -O1 and higher. 🔁
• -ftree-slsr — performs straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at -O1 and higher. ➗
• -ftree-vectorize — performs vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified. ⚡
• -ftree-loop-vectorize — performs loop vectorization on trees. This flag is enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile. 🔂
• -ftree-slp-vectorize — performs basic block vectorization on trees. This flag is 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. GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized, and -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. Using -Wtrivial-auto-var-init to report all such cases.

The three values of choice are:

• uninitialized — bestie, this one does literally nothing 💅. It doesn't initialize any automatic variables, which is exactly C and C++'s default. Chill. Hands off. No notes.
• pattern — okay this one is spicy 🔥. It initializes automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and — key detail — are not values that programmers can rely on for useful program semantics. The current value is a byte-repeatable pattern with byte "0xFE". And heads up: the values used for pattern initialization might be changed in the future. So don't get attached. 😭
• zero — initializes automatic variables with zeroes. Predictable, calm, lowkey iconic ✨.

The default is uninitialized.

---

Big-brain moment incoming 🧠✨: 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. This 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. So basically, 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. 🫠

You can control this behavior for a specific variable by using the variable attribute uninitialized.

---

• -fvect-cost-model=model — alters the cost model used for vectorization. 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. With the dynamic model, a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing 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. 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.

• -fsimd-cost-model=model — same vibe, 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.
• -ftree-vrp — performs Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This lets the optimizers remove unnecessary range checks like array bound checks and null pointer checks. Enabled by default at -O2 and higher. 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, thus improving efficiency of the scheduling passes. 🏃‍♀️💨

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 does not 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. Enabled by default for PowerPC targets, but disabled by default otherwise.
• -fpartial-inlining — inline parts of functions ✂️. 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. Enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -fprefetch-loop-arrays — if supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays. 👑

Heads up though: this option may generate better or worse code; results are highly dependent on the structure of loops within the source code.

Disabled at level -Os.

• -fno-printf-return-value — do not substitute constants for known return value of formatted output functions such as sprintf, snprintf, vsprintf, and vsnprintf (but not printf of 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. 🙈

-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.

• -fno-guess-branch-probability — do not guess branch probabilities using heuristics.

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 is 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 — reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality. Enabled at levels -O1, -O2, -O3, -Os.
• -freorder-blocks-algorithm=algorithm — use 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.

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 number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance. 🥶🔥

This optimization is automatically 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. 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 — reorder 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 object file format must support named sections and 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.

Enabled at levels -O2, -O3, -Os.

• -fstrict-aliasing — okay bestie, this one lets the compiler assume the strictest aliasing rules that apply to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type.

Pay special attention to code like this:

union a_union {
  int i;
  double d;
};

int f() {
  union a_union t;
  t.d = 3.0;
  return t.i;
}

The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. 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 — controls whether rules of -fstrict-aliasing are applied across function boundaries. Note that if multiple functions gets 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 — align 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. This is 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: -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, -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 allows you to 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 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 — align 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 — align 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. This is 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 — align 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. This is 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.

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 — specify 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 — allow 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. 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, bestie, you gotta pass -flto and your optimization options at compile time and at the final link. The move is: compile every file that's gonna be in the same link with the same options, and then say those options again at link time. Like so ✨

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 save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. Then the final invocation reads the GIMPLE bytecode back out of foo.o and bar.o, merges the two files into a single internal image, and compiles the result like normal. Because foo.o and bar.o are merged into one image, all the interprocedural analyses and optimizations in GCC work across the two files as if they were literally one file. Which means — for example — the inliner can inline functions in bar.o into functions in foo.o and vice-versa. Cross-file inlining, we love to see it 💅

Another (simpler) way to turn on link-time optimization:

gcc -o myprog -flto -O2 foo.c bar.c

That one generates bytecode for foo.c and bar.c, merges them into a single GIMPLE representation, and optimizes them as usual to produce myprog. Iconic.

The important thing to keep in mind: to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically does link-time optimization if any of the objects involved were compiled with the -flto command-line option. And you can always override that automatic decision by passing -fno-lto to the link command. 🫶

Now, for whole program optimization to be effective, you gotta make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime stuff outside of the link-time optimized unit. When the linker supports it, the linker plugin (see -fuse-linker-plugin) passes info to the compiler about used and externally visible symbols. When the linker plugin isn't available, you should use -fwhole-program to let the compiler make those assumptions, which leads to more aggressive optimization decisions. 🔥

When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is bigger than a regular object file, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). Which means object files with LTO info can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations get applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster, but you can't do a regular, non-LTO link on them. Tradeoffs, bestie. 🎀

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. So you can mix and match object files and libraries with GIMPLE bytecodes and final object code — GCC automatically picks which files to optimize in LTO mode and which files to link without further processing. Very main character of it. 👑

Generally, options specified at link time override those specified at compile time, although in some cases GCC tries to infer link-time options from the settings used to compile the input files.

If you don't specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. But note that it's generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons:

1. Compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time.
2. Some early optimization passes can be performed only at compile time, not at link time.

There are some code generation flags GCC preserves when generating bytecodes, because 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 this 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. Don't fight it. 😭

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 though. ✨

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. And 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. Pick a lane, babe. 🦋

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's 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. Multi-language LTO? Ate. 💅

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it's possible to extract and use them in an LTO link if you're 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're not using a linker with plugin support and/or you don't 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. Best of both worlds, periodt. 🎀

Link-time optimizations don't require the presence of the whole program to operate. If the program doesn't require any symbols to be exported, it's 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 the 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. No cap, keep your versions matched. 💀

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. Parallelism, we are obsessed. ⚡

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. Auto-magic. ✨

• -flto-partition=alg — this is the ✨vibe selector✨ for how the link-time optimizer chops stuff up into partitions. Pick your algorithm:
  • 1to1 — partitions that mirror the original source files, one-to-one, iconic
  • balanced — splits into equally sized chunks (whenever possible) 💅
  • max — makes a whole new partition for every symbol where possible
  • cache — balances chunk sizes while keeping related symbols together, so incremental LTO caching hits different 🧚
  • none — no cap, disables partitioning and streaming completely

The default is balanced, bestie. 1to1 can be used as a workaround for various code ordering issues, but the max partitioning is intended for internal testing only — not for you, sorry. 🙈

Also there's one (spelled one), which says exactly one partition should be used, while 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 living in the given existing directory. It can significantly shorten edit-compile cycles with LTO, no cap.

When you use it 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 means recompile time goes 📉.

Multiple GCC instances can use the same cache in parallel. Sharing is caring, periodt. 🫶

• -flto-incremental-cache-size=n — how many cache entries incremental LTO holds before it starts pruning the old ones. It's a soft limit, so temporarily there may be more entries. Not a hard rule, just a vibe. 💫
• -flto-compression-level=n — the compression level for intermediate language written to LTO object files. Only meaningful when you're in LTO mode (-flto), iykyk. GCC currently supports two LTO compression algorithms:
  • for zstd, valid values are 0 (no compression) to 19 (maximum compression)
  • for zlib, values from 0 to 9

Values outside the range get clamped to either the minimum or maximum of the supported values. If you don't pass the option at all, a default balanced compression setting is used. 🎀

• -fuse-linker-plugin — enables the use of a linker plugin during link-time optimization. This relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer. ✨

It enables the 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 info 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 — 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, this identifies arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, it eliminates 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, it performs 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's not practical to train all possible hot paths in the program. (For example, a program may contain functions specific for a given hardware and training may not cover all hardware configurations the 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. Tradeoffs, bestie. ⚖️
• -fprofile-use
• -fprofile-use=path — enables profile feedback-directed optimizations, plus 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. No skipping steps, babe. 📋

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 — enables sampling-based feedback-directed optimizations, plus 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 one lets you control 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. 💅

• -fexcess-precision=standard is NOT implemented for languages other than C or C++ 💀 — like, it just doesn't exist there, bestie. And on the x86? It does nothing if you've got -mfpmath=sse or -mfpmath=sse+387 going on. With the former, IEEE semantics just apply, no excess precision, periodt ✨. With the latter, the rounding is unpredictable 🙈 — genuinely, we don't know what it'll do.
• -ffloat-store — do not store floating-point variables in registers, and shut down other options that might mess with whether a floating-point value comes from a register or from memory 🫶. Ngl this one has pretty much been subsumed by -fexcess-precision=standard, which is way more general. But if you do use -ffloat-store, you might need to edit your program to explicitly stash intermediate computations in temporary variables 😭 — because -ffloat-store only handles rounding to IEEE format on assignments, not on casts, whereas -fexcess-precision=standard does both. iykyk 💅
• -ffast-math — this one's the whole squad rolling in at once, it sets: -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast 🌈.

It also makes the preprocessor macro "__FAST_MATH__" get defined ✨.

It is not turned on by any -O option besides -Ofast, since it can produce incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 💀. It may, however, give you faster code for programs that don't need those guarantees. Main character speed energy but at a cost, bestie 👑

• -fno-math-errno — don't set "errno" after calling math functions that run in a single instruction, like "sqrt" 🍓. If your program relies on IEEE exceptions for math error handling, you might want this flag for speed while still keeping IEEE arithmetic compatibility.

Again: not turned on by any -O option besides -Ofast, because incorrect output is possible for programs depending on an exact IEEE/ISO implementation for math functions 😭. Faster code for programs that don't need those guarantees, though.

The default is -fmath-errno.

On Darwin systems, the math library never sets "errno" — so there's literally no reason for the compiler to even consider that it might, 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 used at link time, it may pull in libraries or startup files that change the default FPU control word or do other similar optimizations.

Not turned on by any -O option besides -Ofast, since incorrect output is possible for programs depending on an exact IEEE/ISO implementation for math functions. But faster code for programs that don't need 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 you genuinely cannot use it on code that relies on rounding behavior like "(x + 2**52) - 2**52" 💀. It may also reorder floating-point comparisons, so it's a no-go when ordered comparisons are required. This option needs both -fno-signed-zeros and -fno-trapping-math to be in effect. And honestly? 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 that value, if it enables optimizations 🦋. For example "x / y" can be swapped for "x * (1/y)", which slaps 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 just assume arguments and results are not NaNs or +-Infs 💫.

Not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs depending on an exact IEEE/ISO implementation for math functions. Faster code for programs that don't need those guarantees, though 🌸.

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 then 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 that floating-point operations cannot 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 depending on an exact IEEE/ISO implementation for math functions 💀.

The default is -ftrapping-math.

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 -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) 🎀. 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) and arithmetic transformations that are unsafe when sign-dependent rounding modes are around.

The default is -fno-rounding-math.

⚠️ This option is experimental and does not currently guarantee to disable all GCC optimizations 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 that IEEE signaling NaNs may generate user-visible traps during floating-point operations 😭. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. It implies -ftrapping-math.

This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined ✨.

The default is -fno-signaling-nans.

⚠️ This option is experimental and 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", plus 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, which allows the exception to be raised — unless C23 or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect 💅.

Even with -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 to double-precision constants 🌸.
• -fcx-limited-range — when enabled, this states that a range reduction step is not 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.

---

The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code 💀.

• -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 absolutely must use the same source code and the same optimization options for both compilations, bestie 🫶. See details about the file naming in -fprofile-arcs.

With -fbranch-probabilities, GCC drops a REG_BR_PROB note on every JUMP_INSN and CALL_INSN. bestie those lil notes are straight-up glow-ups for optimization 💅 but rn they're only used in 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 exactly figure out which path gets taken more often. no vibes-based guessing here, we're doing math 💖

Enabled by -fprofile-use and -fauto-profile.

---

• -fprofile-values — if you pair it with -fprofile-arcs, it adds code so that some data about the values of expressions in your program gets gathered. 📊

And with -fbranch-probabilities, it reads back all that gathered data about expression values and uses it for optimizations.

Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

• -fprofile-reorder-functions — function reordering, but make it profile-instrumented 🎀 it collects the first time each function gets executed and orders those functions in ascending order, aiming to optimize program startup by loading the text segments more efficiently. iconic behavior tbh ✨

Enabled with -fprofile-use.

• -fvpt — if you combine it with -fprofile-arcs, this one tells the compiler to add code that gathers info about the values of expressions. 🕵️‍♀️

And with -fbranch-probabilities, it reads back the gathered data and actually does the optimizations based on it. rn the optimizations include specializing division operations using what it knows about the denominator's value. slay.

Enabled with -fprofile-use and -fauto-profile.

• -frename-registers — tries to dodge false dependencies in scheduled code by making use of registers that are left over after register allocation. this optimization is highkey a gift for processors with lots of registers. 💜 BUT — depending on the debug info format your target uses, it can make debugging literally 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 right next to each other, because the target machine can execute them more efficiently when they're adjacent in the instruction flow. besties sitting together in class energy fr fr 🫶

Enabled at levels -O2, -O3, -Os.

• -ftracer — performs tail duplication to enlarge superblock size. this transformation simplifies the control flow of the function, letting other optimizations do a better job. teamwork makes the dream work ✨

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). heads up tho: this option makes code bigger, and it may or may not make it run faster. it's giving chaotic 😭

Enabled by -fprofile-use and -fauto-profile.

• -funroll-all-loops — unrolls ALL the 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 for which there's enough information 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 like "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, comparisons with the "NULL" pointer are also 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 actually have an effect, -ftree-loop-im has to be enabled too.

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, it 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 that 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, ngl.

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. 🙈

⚠️ 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 gets done. for example, GCC doesn't inline functions that contain more than a certain number of instructions. you can control some of these constants on the command line with 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 a probability lower than this threshold (in percent), then it's 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 the 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 — so RTL if-conversion wants to yeet conditional branches around a block and swap them for conditionally executed instructions, right? This param sets the max permissible cost for the sequence if-conversion would spit out, depending on whether the branch is statically determined to be predictable or not. Units are the same as the GCC internal seq_cost metric, bestie. The compiler will try to give you a reasonable default using the BRANCH_COST target macro. 💅
• max-crossjump-edges — the max number of incoming edges to consider for cross-jumping. The algorithm behind -fcrossjumping is O(N^2) in the number of edges incoming to each block. Bigger values = more aggressive optimization, and compile time goes up with probably a tiny improvement in executable size. 🙈
• min-crossjump-insns — the minimum number of instructions that have to match at the end of two blocks before cross-jumping happens to them. This value gets ignored when all instructions in the block being cross-jumped from are matched. periodt.
• max-grow-copy-bb-insns — the max code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. ✨
• max-goto-duplication-insns — the max number of instructions to duplicate to a block that jumps to a computed goto. To dodge O(N^2) behavior in a bunch of passes, GCC factors computed gotos early in the compilation process, then unfactors them as late as possible. Only computed jumps at the end of a basic block with no more than max-goto-duplication-insns get unfactored. no cap.
• max-delay-slot-insn-search — the max number of instructions to consider when hunting for an instruction to fill a delay slot. If more than this arbitrary number of instructions get searched, the time savings from filling the delay slot are minimal, so like, stop searching bestie. Bigger values = more aggressive optimization, compile time goes up with probably small improvement in execution time. 😭
• max-delay-slot-live-search — when trying to fill delay slots, the max number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value = more aggressive optimization, and compile time increases. This parameter should be removed when the delay slot code gets rewritten to maintain the control-flow graph. iykyk. 💀
• max-gcse-memory — the approximate max amount of memory in "kB" that can be allocated to perform the global common subexpression elimination optimization. If more memory than specified is needed, the optimization just doesn't happen. 🫶
• max-gcse-insertion-ratio — if the ratio of expression insertions to deletions is bigger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. slay.
• max-pending-list-length — the max 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. it's giving wasteful ngl. 🍓
• max-modulo-backtrack-attempts — the max number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. 🔥
• 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.
• max-inline-functions-called-once-insns — maximal estimated size of functions produced while inlining functions called once.
• max-inline-insns-single — several parameters control the tree inliner used in GCC. This number sets the max 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++). ✨
• 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 get investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto). 💅
• max-inline-insns-small — this is the bound applied to calls that are considered relevant with -finline-small-functions.
• 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.
• uninlined-function-insns — number of instructions accounted by inliner for function overhead such as function prologue and epilogue.
• uninlined-function-time — extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue.
• 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). 👑
• uninlined-thunk-insns
• uninlined-thunk-time — same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks.
• 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. main character energy fr fr. 🌈
• 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. 😭
• 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.
• 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. 🧚
• lazy-modules — maximum number of concurrently open C++ module files when lazy loading. 💜
• 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. 🌸
• 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-large-unit-insns — the size of translation unit that IPA-CP pass considers large.
• large-stack-frame — the limit specifying large stack frames. While inlining, the algorithm tries not to grow past this limit too much. 🙈
• 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. 💀
• max-inline-insns-recursive
• max-inline-insns-recursive-auto — specifies the max 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.

• max-inline-recursive-depth
• max-inline-recursive-depth-auto — specifies the max 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.

• min-inline-recursive-probability — recursive inlining is profitable only for functions having deep recursion on average, and can hurt for functions 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 the function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents). 🎀

• early-inlining-insns — how much growth the early inliner is allowed to make ✨ in effect, it bumps up how much inlining happens for code with a big abstraction penalty.
• max-early-inliner-iterations — caps how many times the early inliner iterates. This 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 — these three specify the maximal number of base pointers, references and accesses stored for a single function by mod/ref analysis.
• modref-max-tests — the maximal 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, okay? not optional fr.
• 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 maximum number of escape points tracked by modref per SSA-name.
• modref-max-adjustments — the maximum number the access range gets enlarged during modref dataflow analysis.
• profile-func-internal-id — controls whether to use the function internal id in profile database lookup. If the value is 0, the compiler uses an id based on function assembler name and filename, which makes old profile data more tolerant to source changes like function reordering and stuff 👑
• 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 the value you specify here to allow vectorization.
• gcse-cost-distance-ratio — a scaling factor in the calculation of the maximum distance an expression can be moved by GCSE optimizations. Currently only supported in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e. the expressions that cost 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 stop constraining the distance an expression can travel. 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 — the depth of search in the dominator tree for expressions to hoist. Used to avoid quadratic behavior in the hoisting algorithm. A value of 0 doesn't limit the search, but it may slow down compilation of huge functions 😭
• max-tail-merge-comparisons — the maximum amount of similar bbs to compare a bb with. Used to avoid quadratic behavior in tree tail merging.
• max-tail-merge-iterations — the maximum amount of iterations of the pass over the function. Used to limit compilation time in tree tail merging.
• store-merging-allow-unaligned — allow the store merging pass to introduce unaligned stores if it's legal to do so 💅
• max-stores-to-merge — the maximum number of stores to attempt to merge into wider stores in the store merging pass.
• max-store-chains-to-track — the maximum number of store chains to track at the same time while trying to merge them into wider stores in the store merging pass.
• max-stores-to-track — the maximum number of stores to track at the same time while attempting to merge them into wider stores in the store merging pass.
• max-unrolled-insns — the maximum number of instructions a loop may have to be unrolled. If a loop does get unrolled, this parameter also determines how many times the loop code is unrolled.
• max-average-unrolled-insns — the maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
• max-unroll-times — the maximum number of unrollings of a single loop.
• max-peeled-insns — the maximum number of instructions a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled.
• max-peel-times — the maximum number of peelings of a single loop.
• max-peel-branches — the maximum number of branches on the hot path through the peeled sequence.
• max-completely-peeled-insns — the maximum number of insns of a completely peeled loop.
• max-completely-peel-times — the maximum number of iterations of a loop to be suitable for complete peeling.
• max-completely-peel-loop-nest-depth — the maximum depth of a loop nest suitable for complete peeling.
• max-unswitch-insns — the maximum number of insns of an unswitched loop.
• max-unswitch-depth — the maximum depth of a loop nest to be unswitched.
• lim-expensive — the minimum cost of an expensive expression in the loop invariant motion.
• min-loop-cond-split-prob — when FDO profile information is available, min-loop-cond-split-prob 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 candidates for induction variables; below it, all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered, to dodge quadratic time complexity.
• iv-max-considered-uses — the induction variable optimizations give up on loops that contain more induction variable uses.
• 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 — maximum 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 — maximum number of queries into the alias oracle per store. Larger values result in 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 the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer down.
• max-tree-if-conversion-phi-args — maximum number of arguments in a PHI supported by TREE if conversion, unless the loop is marked with simd pragma.
• vect-max-layout-candidates — the maximum number of possible vector layouts (like permutations) to consider when optimizing to-be-vectorized code.
• vect-max-version-for-alignment-checks — the maximum 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 maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
• vect-max-peeling-for-alignment — the maximum number of loop peels to enhance access alignment for the vectorizer. A value of -1 means no limit.
• max-iterations-to-track — the maximum number of iterations of a loop that the brute-force algorithm for analyzing the number of iterations of the loop tries to evaluate.
• hot-bb-count-fraction — the denominator n of the 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 in order 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's 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 in order 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's never considered hot. Used in LTO mode.
• hot-bb-frequency-fraction — the denominator n of the 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 in order to be considered hot. The default is 1000, which means a basic block is considered hot in a function if it's executed more frequently than 1/1000 of the frequency of the entry block of the function. 0 means it's never considered hot.
• unlikely-bb-count-fraction — okay so this one is the denominator n of the fraction 1/n of the total profiled runs of the whole program, and it's the cutoff below which a basic block's execution count has to fall for that block to get labeled "unlikely executed" 💀. Default is 20, which means a block counts as unlikely executed if it runs in fewer than 1/20, aka 5%, of the program's runs. And a value of 0? That means it's always considered unlikely executed. always. no cap.
• max-predicted-iterations — the max number of loop iterations we predict statically. Super handy when a function has one loop with a known bound and another loop with a totally unknown bound — the known iteration count gets predicted correctly, but the unknown one just averages out to roughly 10, which makes that unbounded loop look artificially cold next to the other one 🥶.
• builtin-expect-probability — controls the probability of an expression having the specified value. Takes a percentage (so 0 ... 100) as input, bestie.
• 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 — pick the fraction of the maximal frequency of executions of a basic block in a function, and that's what aligns the basic block.
• align-loop-iterations — a loop expected to iterate at least the selected number of iterations gets aligned.
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value caps superblock formation once the given percentage of executed instructions is covered, which limits unnecessary code size expansion 📈.

The tracer-dynamic-coverage-feedback parameter is only used when profile feedback is available. Real profiles (as opposed to the statically estimated ones) are way less balanced, which lets the threshold be a larger value.

• tracer-max-code-growth — stops tail duplication once code growth hits the given percentage. It's honestly a pretty artificial limit, because most of those duplicates get eliminated later in cross jumping anyway, so you can set it to much higher values than the code growth you actually want 💅.
• tracer-min-branch-ratio — stop reverse growth when the reverse probability of the best edge dips below this threshold (in percent).
• tracer-min-branch-probability, tracer-min-branch-probability-feedback — stop forward growth if the best edge has a probability lower than this threshold.

Just like tracer-dynamic-coverage, there are two parameters here: tracer-min-branch-probability-feedback is for compilation with profile feedback, and tracer-min-branch-probability is for compilation without. The value for the profile-feedback one needs to be more conservative (higher) to actually 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 OS provided guard will leave your code vulnerable to stack clash style attacks 😭 so watch it.
• stack-clash-protection-probe-interval — stack clash protection is all about probing stack space as it gets allocated. This param controls the max distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but again, a value larger than the operating system provided guard leaves the code vulnerable to stack clash style attacks 🙈.
• max-cse-path-length — the max number of basic blocks on a path that CSE considers.
• max-cse-insns — the max number of instructions CSE processes before flushing.
• ggc-min-expand — GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the GC's heap should be allowed to expand between collections. Tuning it may improve compilation speed; it has no effect on code generation 👑

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 can't calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero makes a full collection happen at every opportunity — which is extremely slow, but can be useful for debugging 🔍.

• ggc-min-heapsize — minimum size of the garbage collector's heap before it even bothers to collect garbage. The first collection happens 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 💖.

Default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure RLIMIT_DATA or RLIMIT_AS aren't exceeded — but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC can't calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity.

• max-reload-search-insns — the max number of instructions reload should look backward for an equivalent register. Increasing values mean more aggressive optimization, which makes compilation time go up with probably slightly better performance.
• max-cselib-memory-locations — the max number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making compilation time increase with probably slightly better performance.
• max-sched-ready-insns — the max 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 compilation time increase with probably little benefit 🫠.
• max-sched-region-blocks — the max number of blocks in a region to be considered for interblock scheduling.
• max-pipeline-region-blocks — the max number of blocks in a region to be considered for pipelining in the selective scheduler.
• max-sched-region-insns — the max number of insns in a region to be considered for interblock scheduling.
• max-pipeline-region-insns — the max 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 max number of iterations through CFG to extend regions. A value of 0 disables region extensions.
• max-sched-insn-conflict-delay — the max 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 get 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 a store and a load targeting the same memory locations.
• selsched-max-lookahead — the max size of the lookahead window of selective scheduling. It's basically a depth of search for available instructions 🔎.
• selsched-max-sched-times — the max number of times that an instruction gets 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 max 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 max size, measured as number of RTLs, that can be recorded in an expression in combiner for a pseudo register as the last known value of that register.
• max-combine-insns — the max number of instructions the RTL combiner tries to combine.
• max-combine-search-insns — the max number of instructions 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 stops trying to optimize the definition.

Currently this limit only applies after certain successful combination attempts, but it could be extended to other cases in the future 👀.

• integer-share-limit — small integer constants can use a shared data structure, which reduces the compiler's memory usage and boosts its speed. This sets the max 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 max 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 a struct can have and still get treated in a field-sensitive way during pointer analysis. bestie, go over this and the analysis gets fuzzy 🫠
• prefetch-latency — a lil estimate of how many instructions get executed, on average, before a prefetch actually finishes. the distance we prefetch ahead is proportional to this constant ✨ heads up tho: bumping this number can also mean fewer streams get prefetched (peep simultaneous-prefetches).
• simultaneous-prefetches — the max number of prefetches that can be running at the same time. no more, no less 💅
• l1-cache-line-size — the size of a cache line in the L1 data cache, measured 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 drop software prefetch hints for strides that are non-constant. sometimes that's a win, but honestly if the stride is non-constant it's lowkey hard to predict when there's a clear benefit to issuing those hints 🙈

set to 1 if prefetch hints should be issued for non-constant strides. set to 0 if prefetch hints should only be issued for strides known to be constant and below prefetch-minimum-stride.

• prefetch-minimum-stride — the minimum constant stride, in bytes, at which we start using prefetch hints. if the stride is less than this threshold, no prefetch hints get issued, periodt.

this one's genuinely useful for processors that have hardware prefetchers, because in that case there can be conflicts between the hardware prefetchers and the software ones 😭 if the hardware prefetchers have a maximum stride they can handle, that's the value to use here, to improve how the software prefetchers get used.

a value of -1 means there's no threshold, so prefetch hints can be issued for any constant stride.

this setting is only useful for strides that are known and constant, fr fr.

• destructive-interference-size, constructive-interference-size — 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 that gets accessed together. typically both are the size of an L1 cache line for the target, in bytes. for a generic target covering a range of L1 cache line sizes, usually the constructive interference size is the small end of the range and the destructive size is the large end 👑

the destructive interference size is meant to be used for layout, which means it has ABI impact. 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 matters — like the public interface of a library — is strongly discouraged 🚩 if you do use it there, 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 a type fits within a cache line.

see also -Winterference-size.

• loop-interchange-max-num-stmts — the maximum number of stmts in a loop that can be interchanged.
• 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. should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++ ✨ but hey, if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. we do what we must 🙈
• switch-conversion-max-branch-ratio — switch initialization conversion refuses to create arrays bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. no bloat allowed 💅
• switch-lower-slow-alg-max-cases — the maximum number of cases before the slow switch lowering algorithms get 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 literally run away, consuming all the memory available on the host machine 😭 this parameter caps the length of the sets that get computed, which stops that runaway behavior. setting a value of 0 for this parameter allows an unlimited set length (chaos, but your call).
• rpo-vn-max-loop-depth — the maximum loop depth that gets 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. main character loops only 🎀
• sccvn-max-alias-queries-per-access — the max number of alias-oracle queries we perform when hunting 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 🔥
• ira-max-conflict-table-size — even tho 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 need to build a pseudo-register conflict table. tradeoffs, bestie.
• 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 matching constraints (duplicated operand number) heavily in all available alternatives for the preferred register class. if it's set as zero, it means IRA only respects the matching constraint when it's in the only available alternative with an appropriate register class. otherwise, it means IRA will check all available alternatives for the preferred register class, even if it already found some choice with an appropriate register class, and respect the found qualified matching constraint.
• 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 the region to do this optimization. the parameter defines a minimal fall-through edge probability in percentage, used to add BB to inheritance EBB in LRA. the default value was chosen from numerous runs of SPEC2000 on x86-64 🏆
• 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 babe, 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 get considered for data dependence analysis. these large loops are not handled by the optimizations that use loop data dependencies.
• max-vartrack-size — sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. if this limit gets exceeded with variable tracking at assignments enabled, 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 zero 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 this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow.
• 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, such stmts get dropped from the inlined copy of a function, and from its RTL expansion.
• min-nondebug-insn-uid — ok so this is the uid you start handing out to nondebug insns 🎀. Everything <em>below</em> this number is reserved, like VIP-only, exclusively for the debug insns that -fvar-tracking-assignments creates. But bestie, if that reserved range runs out? Debug insns are allowed to grab (non-overlapping!!) uids above it too. no cap.
• ipa-sra-deref-prob-threshold — IPA-SRA will only swap out a pointer that it knows for sure isn't NULL for one or more brand new parameters if the probability (in percent, measured relative to function entry) that it gets dereferenced is higher than this parameter. Otherwise it's not worth the girl math 💅.
• ipa-sra-ptr-growth-factor — IPA-SRA replaces a pointer-to-an-aggregate with one or more new parameters <em>only</em> when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of that original pointer parameter. growth limit, periodt.
• ipa-sra-ptrwrap-growth-factor — this one's the extra max allowed growth of the total size of the new parameters that ipa-sra swaps in for a pointer-to-aggregate, but <em>specifically</em> when that pointer points to a local variable that the caller only writes to and then passes along as an argument to other functions. special case, iconic 🦋.
• ipa-sra-max-replacements — the max pieces of an aggregate that IPA-SRA will keep track of. and because of that, it's also the max number of replacements allowed for a formal parameter. two-for-one 💖.
• sra-max-scalarization-size-Ospeed, sra-max-scalarization-size-Osize — so the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are out here trying to replace scalar parts of aggregates with independent scalar variables. These parameters set the max size, in storage units, of an aggregate that even gets considered for replacement when you're compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize). pick your fighter 🫶.
• sra-max-propagations — the maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per <em>one</em> local variable, all so copy propagation can happen. bestie's got a whole list going 📝.
• tm-max-aggregate-size — when you're making copies of thread-local variables inside a transaction, this parameter is the size in bytes after which variables get saved using the logging functions instead of save/restore code sequence pairs. and ngl, this one only applies when you're using -fgnu-tm 💀.
• graphite-max-nb-scop-params — to avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. a value of zero can be used to lift that bound entirely. and fyi, a variable whose value is unknown at compilation time and defined outside a SCoP <em>is</em> a parameter of the SCoP ✨.
• hardcfr-max-blocks — disables -fharden-control-flow-redundancy for functions with a larger number of blocks than the specified value. set it to zero and any limit just... vanishes 🧚.
• hardcfr-max-inline-blocks — forces -fharden-control-flow-redundancy to use out-of-line checking for functions with a larger number of basic blocks than the specified value. no inlining for the big ones, sorry 🎀.
• loop-block-tile-size — loop blocking / strip mining transforms (enabled 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 💅.
• ipa-jump-function-lookups — specifies the number of statements visited during jump function offset discovery. that's it, that's the whole vibe 🔍.
• ipa-cp-value-list-size — IPA-CP is out here trying to track every possible value and type 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 of a function. it's giving organized 🌸.
• 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. above the bar or bye 👑
• ipa-cp-max-recursive-depth — the maximum depth of recursive cloning for a self-recursive function. how deep does the recursion go? this deep, and not deeper 💜.
• ipa-cp-min-recursive-probability — recursive cloning happens <em>only</em> when the probability of the call actually being executed exceeds this parameter. otherwise it's a no from me 🙈.
• ipa-cp-profile-count-base — when you're using the -fprofile-use option, IPA-CP will treat the measured execution count of a call graph edge at <em>this</em> percentage position in their histogram as the basis for its heuristics calculation. find your reference point bestie 📊.
• ipa-cp-recursive-freq-factor — the number of times interprocedural copy propagation expects recursive functions to call themselves. girl, same 🔁.
• ipa-cp-recursion-penalty — the percentage penalty that recursive functions receive when they get evaluated for cloning. recursive? taxed 💀.
• ipa-cp-single-call-penalty — the percentage penalty that functions containing a single call to another function receive when they're evaluated for cloning. one call and you're already paying up 🫶.
• ipa-max-agg-items — IPA-CP can also propagate a number of scalar values that get passed in an aggregate. ipa-max-agg-items controls the maximum number of those values per one parameter 🎀.
• ipa-cp-loop-hint-bonus — when IPA-CP figures out that a cloning candidate would make the number of iterations of a loop <em>known</em>, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of that candidate. bonus points for knowing things ✨.
• ipa-max-loop-predicates — the maximum number of different predicates IPA will use to describe when loops in a function have known properties. only so many receipts allowed 🧾.
• ipa-max-aa-steps — during its analysis of function bodies, IPA-CP employs alias analysis to track values pointed to by function parameters. but to avoid spending forever on huge functions, it gives up and considers all memory clobbered after examining ipa-max-aa-steps statements that modify memory. it's got a time limit bestie ⏰.
• ipa-max-switch-predicate-bounds — the maximal number of boundary endpoints of case ranges of a switch statement. for any switch exceeding this limit, IPA-CP will <em>not</em> construct the cloning cost predicate (the thing used to estimate cloning benefit) for the default case of the switch statement. default case left out 😭.
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter to estimate the benefit for cloning upon a certain constant value. but if the number of operations in a parameter expression exceeds ipa-max-param-expr-ops, that expression gets treated as a complicated one and IPA analysis won't handle it. too messy, moving on 💅.
• lto-partitions — specify the desired number of partitions produced during WHOPR compilation. and note: the number of partitions should exceed the number of CPUs used for compilation. more partitions than CPUs, okurrr 💻.
• lto-min-partition — the 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 need to over-slice 🍓.
• lto-max-partition — the size of the max partition for WHOPR (in estimated instructions), to provide an upper bound for the individual size of a partition. meant to be used only with balanced partitioning. don't use it solo 🎀.
• 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. the special value of 0 dictates that you always clone without a cut-off. zero means no chill 🔥.
• lto-partition-locality-size-cutoff — the size cut-off for a callee (including inlined calls) to be cloned for a particular caller. size limit for the cloning crew 💜.
• lto-max-locality-partition — the maximal size of a locality partition for LTO (in estimated instructions). a value of 0 results in the default value being used. so just leave it at 0 if you want the vibe 🦋.
• lto-max-streaming-parallelism — the maximal number of parallel processes used for LTO streaming. multitasking queen 👑.
• cxx-max-namespaces-for-diagnostic-help — the maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. "did you mean..." but make it thorough 💖.
• sink-frequency-threshold — the maximum relative execution frequency (in percents) of the target block relative to a statement's original block that still allows statement sinking of that statement. larger numbers mean more aggressive statement sinking. and psst — a small positive adjustment gets applied for statements with memory operands, because those are even more profitable to sink 😌.
• max-stores-to-sink — the maximum number of conditional store pairs that can be sunk. set it to 0 if <em>either</em> vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. zero sinks for you 🫶.
• 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. let the machine decide, honestly 🌈.
• 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 🎀.
• 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 🔥.
• tree-reassoc-width — sets the maximum number of instructions executed in parallel in a reassociated tree. this parameter overrides the target dependent heuristics used by default, but only if it has a non-zero value. zero = no override bestie 💅.
• sched-pressure-algorithm — choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely one 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 of 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 📚. oh and the default choice depends on the target ✨.
• max-slsr-cand-scan — sets the maximum number of existing candidates that get considered when seeking a basis for a new straight-line strength reduction candidate. window shopping for candidates 🛍️.
• asan-globals — buffer overflow detection for global objects?? yes bestie 💖 it's on by default when you're using -fsanitize=address. wanna turn it off? --param asan-globals=0.
• asan-stack — same energy but for stack objects 🌸 enabled by default with -fsanitize=address. kill it with --param asan-stack=0.
• asan-instrument-reads — buffer overflow detection for memory reads 👀 on by default with -fsanitize=address. --param asan-instrument-reads=0 to disable.
• asan-instrument-writes — buffer overflow detection for memory writes ✍️ on by default with -fsanitize=address. disable with --param asan-instrument-writes=0.
• asan-memintrin — detection for built-in functions 💅 on by default with -fsanitize=address. --param asan-memintrin=0 turns it off.
• asan-use-after-return — catches use-after-return 🫠 enabled by default with -fsanitize=address. disable with --param asan-use-after-return=0.

heads up bestie: by default the check is disabled at run time. to actually turn it on, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS. iykyk 😭

• asan-instrumentation-with-call-threshold — if the number of memory accesses in the function being instrumented is greater or equal to this number, use callbacks instead of inline checks. like, 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 — hwasan instrumentation of statically sized stack-allocated variables 🎀 enabled by default when using -fsanitize=hwaddress, and disabled by default when using -fsanitize=kernel-hwaddress. turn it off with --param hwasan-instrument-stack=0, turn it on with --param hwasan-instrument-stack=1.
• hwasan-random-frame-tag — when using stack instrumentation, picks tags for stack variables using a deterministic sequence beginning at a random tag for each frame 🌈 with this parameter unset, tags are chosen using the same sequence but beginning from 1. enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress 💀 disable it with --param hwasan-random-frame-tag=0.
• hwasan-instrument-allocas — hwasan instrumentation of dynamically sized stack-allocated variables ✨ enabled by default when using -fsanitize=hwaddress and disabled by default when using -fsanitize=kernel-hwaddress. disable it with --param hwasan-instrument-allocas=0, enable with --param hwasan-instrument-allocas=1.
• hwasan-instrument-reads — hwasan checks on memory reads 📖 instrumentation of reads is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. to stop checking memory reads, --param hwasan-instrument-reads=0.
• hwasan-instrument-writes — hwasan checks on memory writes ✍️ on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. --param hwasan-instrument-writes=0 to disable.
• hwasan-instrument-mem-intrinsics — hwasan instrumentation of builtin functions 🫶 on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. --param hwasan-instrument-mem-intrinsics=0 to disable.
• use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks 🧪
• tsan-distinguish-volatile — emit special instrumentation for accesses to volatiles ⚡
• tsan-instrument-func-entry-exit — emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit() 🔥
• max-fsm-thread-path-insns — the maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path 🧵
• 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. lowkey important note: 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 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. this 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 — the deepest a chain of square roots can go when GCC is cooking up exponentiation by a real constant. don't go crazy in there ✨
• max-dse-active-local-stores — the max number of active local stores chillin' in RTL dead store elimination.
• asan-instrument-allocas — turns on the asan allocas/VLAs protection. safety girlies stay winning 💅
• max-iterations-computation-cost — the ceiling on how much an expression is allowed to cost when GCC is figuring out the number of iterations.
• max-isl-operations — the most isl operations allowed. 0 means unlimited, go feral bestie 🦋
• graphite-max-arrays-per-scop — the most arrays you can have per scop.
• max-vartrack-reverse-op-size — the max size of a loc list where reverse ops should actually get added.
• fsm-scale-path-stmts — a scale factor, applied to the number of statements in a threading path that crosses a loop backedge, when it's being compared against --param=max-jump-thread-duplication-stmts.
• uninit-control-dep-attempts — the deepest it'll nest calls while hunting for control dependencies during uninitialized variable analysis.
• uninit-max-chain-len — the most predicates that can get anded for each predicate that's ored in the normalized predicate chain.
• uninit-max-num-chains — the most predicates that can be ored in the normalized predicate chain.
• uninit-max-prune-work — the most work GCC will do pruning paths where the variable is always initialized.
• sched-autopref-queue-depth — hardware autoprefetcher scheduler model control flag, iykyk. it's how many lookahead cycles the model peers into; at ' ' it only enables the instruction sorting heuristic. 👀
• loop-versioning-max-inner-insns — the max number of instructions an inner loop can have before the loop versioning pass decides it's too big to copy.
• loop-versioning-max-outer-insns — the max number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy — and it doesn't even count instructions in inner loops that directly benefit from versioning. generous fr 💖
• ssa-name-def-chain-limit — the max number of SSA_NAME assignments to follow when determining a property of a variable, like its value. this caps how many iterations or recursive calls GCC does when optimizing certain statements, or when checking their validity before it drops a diagnostic.
• store-merging-max-size — max size of one single store merging region, in bytes.
• store-forwarding-max-distance — the max instruction distance a small store forwarded to a larger load is allowed to stall. value '0' disables the cost checks for the avoid-store-forwarding pass.
• hash-table-verification-limit — how many elements get hash table verification done, per each searched element.
• max-find-base-term-values — the 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 inside the analyzer before it just gives up and terminates analysis of that point 💀
• analyzer-max-constraints — the max number of constraints per state.
• analyzer-min-snodes-for-call-summary — the minimum number of supernodes that have to be inside a function before the analyzer even considers summarizing its effects at call sites.
• analyzer-max-enodes-for-full-dump — the max depth of exploded nodes that should show up in a dot dump before it switches 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 it terminates analysis of a call that would recurse any deeper.
• analyzer-max-svalue-depth — the max depth of a symbolic value, before it just approximates the value as unknown.
• analyzer-max-infeasible-edges — the max number of infeasible edges it'll reject before declaring a diagnostic infeasible.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block that counts as hot. this param is used only in GIMPLE FE.
• analyzer-bb-explosion-factor — the max number of 'after supernode' exploded nodes within the analyzer per supernode, before it terminates analysis. 💥
• analyzer-text-art-string-ellipsis-threshold — the number of bytes at which string literals get ellipsized in analyzer text art diagrams. yes, GCC does text art now, slay 🎀
• analyzer-text-art-ideal-canvas-width — the ideal width, in characters, of the text art diagrams the analyzer generates.
• analyzer-text-art-string-ellipsis-head-len — how many literal bytes to show at the head of a string literal in text art when ellipsizing it.
• analyzer-text-art-string-ellipsis-tail-len — how many literal bytes to show at the tail of a string literal in text art when ellipsizing it.
• ranger-logical-depth — the max depth of logical expression evaluation that ranger will look through when evaluating outgoing edge ranges.
• ranger-recompute-depth — the max depth of instruction chains to consider for recomputation in the outgoing range calculator.
• relation-block-limit — the max number of relations the oracle will register in a basic block.
• transitive-relations-work-bound — the work bound when discovering transitive relations from existing relations.
• min-pagesize — minimum page size, used for warning and early break vectorization purposes.
• openacc-kernels — picks the mode for handling OpenACC kernels constructs. with --param=openacc-kernels=decompose, OpenACC kernels constructs get decomposed into parts — a sequence of compute constructs — and each one is then handled individually. this is work in progress, jsyk. with --param=openacc-kernels=parloops, OpenACC kernels constructs are handled by the parloops pass, en bloc. this is the current default. ✨
• openacc-privatization — controls whether the -fopt-info-omp-note and applicable -fdump-tree-*-details options emit OpenACC privatization diagnostics. with --param=openacc-privatization=quiet, don't diagnose — and this is the current default. with --param=openacc-privatization=noisy, do diagnose. 🗣️
• cycle-accurate-model — says whether GCC should assume the scheduling description is mostly a cycle-accurate model of the target processor the code is meant to run on, assuming no cache misses. nonzero means the selected scheduling model is accurate and probably describes an in-order processor, so 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 maybe not applicable at all — like for modern out-of-order processors. 🌀

The following choices of name are available on AArch64 targets:

• aarch64-vect-compare-costs — when vectorizing, consider using multiple different approaches and let the cost model pick 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 — forces an ISA selection strategy for auto-vectorization. the options are:
  • 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 — a 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 — a 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 — the limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when it's trying 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 — bestie, this one controls which writeback opportunities we even try to handle in the AArch64 load/store pair fusion pass. Value zero = writeback handling is off, no cap. One = we try to form pairs that involve one or more existing individual writeback accesses where possible. Two = we also try to opportunistically form writeback opportunities by folding in trailing destructive updates of the base register that a pair uses. It's giving "how ambitious are we feeling today" energy. 💅
• aarch64-loop-vect-issue-rate-niters — okay so for some AArch64 CPUs the tuning 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 you set this param to n, GCC will not use this heuristic for loops that are known to execute in fewer than n Advanced SIMD iterations. Lowkey a "know your audience" setting. ✨
• aarch64-vect-unroll-limit — 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 on how much the vectorizer will unroll the main loop. Default value is four. Periodt. 🎀

The following choices of name are available on GCN targets:

• gcn-preferred-vectorization-factor — preferred vectorization factor: default, 32, 64. That's the whole menu, girl, pick your fighter. 🦋

The following choices of name are available on i386 and x86_64 targets:

• x86-stlf-window-ninsns — instructions number above which the STFL stall penalty can be compensated. (Yes, "instructions number". We said what we said. 🙈)
• x86-stv-max-visits — the maximum number of use and def visits when discovering a STV chain before the discovery is just... aborted. It gives up. We've all been there. 💀

---

✨ Program Instrumentation Options ✨

GCC supports a bunch of command-line options that control adding run-time instrumentation to the code it normally generates. Like, GCC is out here adding little spy cameras to your program (affectionate). 💖

For example, one purpose of instrumentation is to collect 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. 🚨 And there's also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. Very versatile, very main character. 👑

🩷 The profiling girlies

• -p
• -pg — generate extra code to write profile information suitable for the analysis program prof (that's for -p) or gprof (that's for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. Both. No skipping, bestie. ✋

You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. (A little "not you, babe" for one specific function. 🙈)

• -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. Constructor inception, honestly. 🌈

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. See? The naming lore goes deep. 💅

• -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. Chef's kiss coverage. 🎀
• -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,

ngl that trailing comma is just sitting there in the original, and we are nothing if not faithful. 🫶

• -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. Efficiency queen. 👑

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. That's the definition, bestie, memorize it, it's on the quiz. 📝

• -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. (Yes, optimization really said "and I took that personally" 😭)
• -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.

Parallel-run girlies, this is your moment. Spread those profiles across folders like confetti. 🎉

• -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. Both times. Do not skip the link step, I'm begging. 🙏

The following options are enabled</p>: -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

💖 -fprofile-info-section=name

Register the profile information in the specified section instead of using a constructor/destructor, okay? The section name is name if it's specified, otherwise the section name defaults to .gcov_info — that's the little default it falls back on, no cap. A pointer to the profile information generated by -fprofile-arcs gets placed in the specified section for each translation unit. Bestie, this option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. ✨

Heads up: this option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O. So like, tiny little embedded cuties only, not your big Linux vibe. 🧚

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. ⚠️ But if you combine the option with multiple source files, the .gcno file will be overwritten — so like, one file only bestie, otherwise she's gone. 💀

---

🌈 -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 the built source tree. By default profile_dir will contain files with mangled absolute paths of all object files in the built project. That's not the vibe when the 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. Iconic behavior, honestly. ✨

---

🎀 -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. ✨

⚠️ Warning: When an application does not properly join all threads (or creates a detached thread), a profile file can still be corrupted. Like, she's still not safe bestie, no matter how careful you think you were. 💀

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. bye girl, not today 💅

---

⭐ -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. fr fr, super useful for distro girlies. 📦

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. trade-offs, bestie, it's giving balance 💅

---

💜 -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. kernel girlies, this one's for you 🫶

• -fsanitize=hwaddress — bestie, this turns on Hardware-assisted AddressSanitizer, which is giving efficiency ✨. It uses a hardware ability to ignore the top byte of a pointer so it can catch memory errors with a low memory overhead. Memory access instructions get instrumented to detect 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>. The run-time behavior can be influenced using the HWASAN_OPTIONS environment variable. When set to "help=1", the available options are shown at startup of the instrumented program. 💅

Okay so heads up: this option cannot be combined with -fsanitize=thread or -fsanitize=address, and it's currently only available on AArch64. No cap.

• -fsanitize=kernel-hwaddress — for compiling the Linux kernel, specifically. It's similar to -fsanitize=kernel-address but uses an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences that are necessary for compiling the Linux kernel. 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 the -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default but are still possible by specifying the command-line options --param hwasan-instrument-stack=1 and --param hwasan-instrument-allocas=1 respectively. Using a random frame tag is not implemented for kernel instrumentation.

• -fsanitize=pointer-compare — instruments comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread.

Note: by default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null.

• -fsanitize=pointer-subtract — instruments subtraction with pointer operands. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread.

Note: by default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. 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 used to protect programs against return address overwrites (e.g. stack buffer overflows.) It works by saving a function's return address to a separately allocated shadow call stack in the function prologue and restoring the return address from the shadow call stack in the function epilogue. Instrumentation only occurs in functions that need to save the return address to the stack. 🔥

Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For the user space programs, runtime support is not currently provided in libc and libgcc. Users who want to use this feature in user space need to provide their own support for the runtime. It should be noted that this may cause the ABI rules to be broken. 😭

On aarch64, the instrumentation makes use of the platform register "x18". This generally means that 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 so corrupt the shadow stack pointer. 💀

Also, because there is 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 are instrumented to detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options. The option cannot be combined with -fsanitize=address, -fsanitize=leak. ✨

When compiling with -fsanitize=thread, you should also use -g to produce more meaningful output. Fr fr, do it.

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 is linked against a library that overrides "malloc" and other allocator functions. See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with -fsanitize=thread. 🫶
• -fsanitize=undefined — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to detect undefined behavior at runtime. See <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. The run-time behavior can be influenced using the UBSAN_OPTIONS environment variable. Current suboptions are: 👑
  • -fsanitize=shift — this option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent.
  • -fsanitize=shift-exponent — this option 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 a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.
  • -fsanitize=integer-divide-by-zero — detect integer division by zero.
  • -fsanitize=unreachable — with this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message call instead. When reaching the "__builtin_unreachable" call, the behavior is undefined. 🙈
  • -fsanitize=vla-bound — this option instructs the compiler to check that the size of a variable length array is positive.
  • -fsanitize=null — this option enables pointer checking. Particularly, the application built with this option 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 by a NULL pointer.
  • -fsanitize=return — this option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.
  • -fsanitize=signed-integer-overflow — this option enables signed integer overflow checking. We check that the result of "+", "*", and both unary and binary "-" does not overflow in the signed arithmetics. This also detects "INT_MIN / -1" signed division. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — this option enables instrumentation of array bounds. Various out of bounds accesses are 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 — this option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array member-like arrays. Initializers of variables with static storage are not instrumented.
• -fsanitize=alignment — this option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object.
• -fsanitize=object-size — this option enables instrumentation of memory references using the "__builtin_dynamic_object_size" function. Various out of bounds pointer accesses are detected.
• -fsanitize=float-divide-by-zero — detect floating-point division by zero. Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. 🌈
• -fsanitize=float-cast-overflow — this option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. This option does not work well with "FE_INVALID" exceptions enabled.
• -fsanitize=nonnull-attribute — this option 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 — this one instruments return statements inside functions tagged with the returns_nonnull function attribute, so if one of them tries to hand back a null value it gets caught. bestie said "i promise i return a real pointer" and this is the vibe check that holds her to it 💅
• -fsanitize=bool — instruments loads from bool. if the value that comes back is literally anything other than 0/1, boom, run-time error. yes, even bool can be a liar 🙈
• -fsanitize=enum — instruments loads from an enum type. if the value loaded is outside the range of values for that enum type, run-time error, no cap. your enum said it only had those options and the load showed up with something else entirely 😭
• -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. it's giving "are you really who you say you are" energy 🔥
• -fsanitize=pointer-overflow — instruments pointer arithmetics. if the pointer arithmetic overflows, run-time error gets issued. pointer math said "i can do this" and then could not 🫶
• -fsanitize=builtin — instruments the arguments to selected builtin functions. pass an invalid value to one of those arguments and you get a run-time error. e.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior, and this option diagnoses it. iconic catch ✨

---

ngl, sanitizers lowkey increase the rate of false positive warnings, most notably the ones hanging around -Wmaybe-uninitialized. so we recommend against combining -Werror and [the use of] sanitizers. pick a struggle bestie 💀

also, while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives you a diagnostic message instead. this currently works only for the C family of languages.

---

• -fno-sanitize=all — turns off all the previously enabled sanitizers. note that -fsanitize=all is not allowed, because some sanitizers literally cannot be used together. they're not all friends, and we respect that 🌸
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks. super useful when you're experimenting with different shadow memory layouts in Kernel AddressSanitizer. customizing the shadow, main character behavior 👑
• -fsanitize-sections=s1,s2,... — sanitizes global variables in selected user-defined sections. heads up: si may contain wildcards. slay, wildcards 🦋
• -fsanitize-recover[=opts] — so -fsanitize-recover= controls error recovery mode for the sanitizers named in the comma-separated list of opts. enabling this for a sanitizer component makes it attempt to keep running the program as if no error ever happened. meaning: multiple runtime errors can get reported in a single program run, and the exit code of the program may even indicate success while errors were being reported. the -fno-sanitize-recover= option flips that: only the first detected error is reported and then the program exits with a non-zero exit code. 💅

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. also -fsanitize-recover=all and -fno-sanitize-recover=all are accepted: the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it. 🎀

even if recovery mode is turned on the compiler side, it needs to be enabled on the runtime library side too, 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. you can override this by setting the halt_on_error flag in the corresponding environment variable. 🧚

syntax without an explicit opts parameter is deprecated. it's equivalent to specifying an opts list of: undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

• -fsanitize-address-use-after-scope — enables sanitization of local variables to detect use-after-scope bugs. this option sets -fstack-reuse to none. periodt ✨
• -fsanitize-trap[=opts] — the -fsanitize-trap= option tells the compiler to report undefined behavior for the sanitizers named in the comma-separated list of opts using __builtin_trap rather than a libubsan library routine. if this is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer — __builtin_trap gets emitted and is fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover=. 💀

the advantage here? the libubsan library isn't needed and isn't linked in, so this is usable even in freestanding environments. no dependencies, just vibes 🫶

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 also be specified, which enables it for the undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. and if -fsanitize-trap=undefined or -fsanitize-trap=all is used while -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored, since that instrumentation always needs libubsan support — and -fsanitize-trap=vptr is not allowed. 🙈

• -fsanitize-undefined-trap-on-error — deprecated equivalent of -fsanitize-trap=all. she's old, she's retired, still shows up to the function 💜
• -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation. inserts a call to __sanitizer_cov_trace_pc into every basic block. every. single. one. 🦋
• -fsanitize-coverage=trace-cmp — enables 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 where both operands are 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, plus __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. it's giving thorough 💅
• -fcf-protection=[full|branch|return|none|check]
• -fcf-protection — enables code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (like 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 threats like Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). 🛡️

the -fcf-protection= keywords are interpreted like this:

• 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.

-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. ✨

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 return. 👑

you can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. not everything wants the security guard, and that's valid 🫶

currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. 🎀

• -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 it with -fharden-conditional-branches to cover all conditionals. double-checking everything, paranoid queen 💅
• -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 it with -fharden-compares to cover all conditionals. together they eat 🔥
• -fharden-control-flow-redundancy — emits 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. 🦋

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. 🌸

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. iykyk 🧚

• -fhardcfr-skip-leaf — disables -fharden-control-flow-redundancy in leaf functions. sometimes the leaves just wanna be left alone 🍓
• -fhardcfr-check-exceptions — okay so when -fharden-control-flow-redundancy is active, this one checks the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. This one is enabled by default, bestie 💚 — use -fno-hardcfr-check-exceptions to disable it.
• -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before any function call that's immediately followed by a return of its result, if any — so it doesn't prevent tail-call optimization, whether or not it ultimately gets optimized to a tail call. ✨

This option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but you can enable it (or disable it, using its negated form) explicitly, regardless of the optimizations. no cap, you get to choose 💅

• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before noreturn calls. Which ones? Pick your fighter: all of them (always), those that aren't expected to return control to the caller through an exception (no-xthrow, the default), those that may not return control to the caller through an exception either (nothrow), or none of them (never). 🎀

Checking before a noreturn function that may return control to the caller through an exception can cause checking to be performed more than once, if the exception is 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 it would be redundant with the check performed before the call — but other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit. And the checkpoint may even be another noreturn call, so checking may end up performed multiple times. 😭 it's giving "check, check, and check again"

Various optimizers may cause calls to be marked as noreturn and/or nothrow, even in the absence of the corresponding attributes, which may affect the placement of checks before calls, as well as the addition of implicit cleanup handlers for them. This unpredictability, plus the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, have made no-xthrow the default setting for this option: it excludes from the noreturn treatment only internal functions used to (re)raise exceptions, that are not affected by these optimizations. the default really said "let me be sensible about this" 🧚

• -fhardened — enables a whole set of flags for C and C++ that improve the security of the generated code without affecting its ABI. The precise flags enabled may change between major releases of GCC, but right now they're:

-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)

The list of options enabled by -fhardened can be generated using the --help=hardened option. 🔍

When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead. noted, bestie

This option is intended to be used in production builds, not merely in debug builds. 👑

Currently, -fhardened is only supported on GNU/Linux targets.

-fhardened only enables a particular option if it wasn't already specified anywhere on the command line. For instance, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. it knows better than to fight you 💅

• -fstack-protector — emit 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 are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered — optimized away variables or variables allocated in registers don't count. 💀
• -fstack-protector-all — like -fstack-protector, except that all functions are protected. no exceptions, everyone gets the guard 🫶
• -fstack-protector-strong — like -fstack-protector but it includes additional functions to be protected --- those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count. ✨
• -fstack-protector-explicit — like -fstack-protector but it only protects those functions which have the stack_protect attribute. opt-in energy, we respect it 🎀
• -fstack-check — generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems if there is only one stack. 🧵

Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended. important!! it's a setup girlie, not the bouncer

You can additionally specify 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 bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the compiler, but it comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are 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 is topped by a particular function, stack checking is not reliable and a warning is 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.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. stack-clash is a whole different beast, don't get it twisted 🔥

• -fstack-clash-protection — generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time, and each page is accessed immediately after allocation. So it prevents allocations from jumping over any stack guard page provided by the operating system. 🛡️✨

Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific. limited but trying, okay

• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit — generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it's possible to 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 that this may only work with the GNU linker. 🎀

You can locally override stack limit checking by using the no_stack_limit function attribute. 💅

• -fsplit-stack — generate code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, since it's no longer necessary to calculate a good stack size to use for each thread. This is currently only implemented for the x86 targets running GNU/Linux. 🧵✨

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. gold linker said "I got you" 🫶

• -fstrub=disable — disable stack scrubbing entirely, ignoring any strub attributes. See
• -fstrub=strict — functions default to strub mode disabled, and apply 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). strict mode really said "know your place" 💅
• -fstrub=relaxed — restore the default stack scrub (strub) setting, namely, strub is only enabled as required by strub attributes associated with function and data types. Relaxed means that 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 "at-calls" "strub" mode wherever it's actually viable, bestie. The main reason this exists? Testing. It flexes the "strub" machinery in scenarios that are strictly local to one translation unit. Heads up though: this "strub" mode messes with function interfaces, so any function that other translation units can see, or that has its address taken, just... won't be touched by this option. Optimization options can affect viability too. Go read the "strub" attribute docs for the whole tea on viability and eligibility requirements. 💅
• -fstrub=internal — enables "internal" "strub" mode where viable. Again, primarily a testing thing. This one is meant 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? Genuinely rare. 😭
• -fstrub=all — enables some "strub" mode where viable. When both modes are viable, "at-calls" wins. -fdump-ipa-strubm adds function attributes that tell you which mode got picked for each function. Primary use: testing, to thoroughly exercise the "strub" machinery. 🧚
• -fvtable-verify=[std|preinit|none] — only available when you're compiling C++ code, no cap. It switches on (or off, if you pass -fvtable-verify=none) the security feature that verifies at run time, for every single virtual call, that the vtable pointer you're calling through is actually valid for the type of the object, and hasn't been corrupted or overwritten. If an invalid vtable pointer gets detected at run time, an error is reported and the program's execution is halted immediately. Like, right then. Done. 💀

This option makes run-time data structures get built at program startup, and those are what verify the vtable pointers. The std and preinit values control when those data structures get built. In both cases they're built before execution reaches main. Using -fvtable-verify=std builds them after shared libraries have been loaded and initialized. -fvtable-verify=preinit builds them before shared libraries have been loaded and initialized.

If this option shows up multiple times on the command line with different values, none takes highest priority over both std and preinit; and preinit takes priority over std.

• -fvtv-debug — when you pair it with -fvtable-verify=std or -fvtable-verify=preinit, it makes debug versions of the runtime functions for the vtable verification feature get called. This flag also makes the compiler log info about which vtable pointers it finds for each class. That info goes into a file named vtv_set_ptr_data.log, in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise.

Note: this feature appends data to the log file. Want a fresh log? Delete any existing one first, fr fr. 🫶

• -fvtv-counts — a debugging flag, bestie. Paired with -fvtable-verify=std or -fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it runs into 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 all of that for each compilation unit. The compiler writes this to a file named vtv_count_data.log, in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class and writes that to vtv_class_set_sizes.log in the same directory.

Note: this feature appends data to the log files. For fresh log files, delete any existing ones first. ✨

• -finstrument-functions — generates instrumentation calls for entry and exit to functions. Just after a function enters and just before it exits, the following profiling functions get called with the address of the current function and its call site. (On some platforms __builtin_return_address doesn't work beyond the current function, so the call site info might 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 you can look up exactly in the symbol table. 👑

This instrumentation also happens for functions expanded inline inside other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. Which means: addressable versions of such functions must be available. If every use you have of a function gets expanded inline, this might mean extra code size. And if you use extern inline in your C code, an addressable version of such functions must be provided. (Normally that's the case anyway, but if you get lucky and the optimizer always expands them inline, you might have gotten away without providing static copies. 🙈)

A function can be given the attribute no_instrument_function, and then this instrumentation just doesn't happen. Useful for, like, the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions can't safely be called — maybe signal handlers, if the profiling routines generate output or allocate memory.

• -finstrument-functions-once — similar to -finstrument-functions, but the profiling functions get called only once per instrumented function. So the first profiling function is called after the first entry into the instrumented function, and the second one is called before the exit corresponding to that first entry.

The definition of "once" here is a little vague, because the implementation isn't protected against data races. So basically, the implementation only guarantees the profiling functions are called at least once per process and at most once per thread — but the calls are always paired, meaning if a thread calls the first function, it'll call the second one too, unless it never reaches the exit of the instrumented function. 😭

• -finstrument-functions-exclude-file-list=file,file,... — sets the list of functions excluded from instrumentation (peep the description of -finstrument-functions). If the file containing a function definition matches one of file, that function doesn't get instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it counts as 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 the 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,... — same energy as -finstrument-functions-exclude-file-list, but this one sets the list of function names to exclude from instrumentation. The name to be matched is the user-visible name, like vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The match is on substrings: if the sym parameter is a substring of the function name, it's 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 sitting before the Mth NOP. If M is omitted it defaults to 0, so the function entry points to the address just at the first NOP. These NOP instructions reserve extra space, which you can use to patch in whatever instrumentation you want at run time — provided 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 the command-line option -fpatchable-function-entry=N,M. You can use this 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 that.

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 runs on every C source file before the actual compilation. 💅

If you use the -E option, nothing happens except preprocessing. Some of these options only make sense together with -E, because they make the preprocessor output unsuitable for actual compilation. 🫶

On top of the options listed here, there are a bunch of options controlling search paths for include files documented in Directory Options. Options controlling preprocessor diagnostics are listed in Warning Options.

• -D name — predefine name as a macro, with definition 1. 🎀
• -D name=definition — bestie, this is the macro-defining icon 💅. Whatever you put in definition gets tokenized and processed exactly like it showed up during translation phase three in a #define directive. Heads up though: the definition gets truncated by embedded newline characters, so no sneaky multi-line business 🚫.

If you're calling the preprocessor from a shell or shell-like program, you might need the shell's quoting syntax to protect characters like spaces that mean something to the shell. Shell syntax is a whole other language, fr.

Wanna 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 quote the whole option, okay? With sh and csh, -D'name(args...)=definition' works. ✨

Important ordering tea: -D and -U options are processed in the order they are given on the command line. And all -imacros file and -include file options are processed after all -D and -U options. Periodt.

• -U name — cancel any previous definition of name, whether it was built in or came from a -D option. Undefining is self-care 🫶
• -include file — process file as if #include "file" appeared as the first line of the primary source file. But plot twist 🎀: 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 gets searched for in the remainder of the #include "..." search chain like normal.

If you give multiple -include options, the files are included in the order they appear on the command line.

• -imacros file — literally exactly like -include, except that any output produced by scanning file is thrown away. The macros it defines stay defined though. This lets you acquire all the macros from a header without also processing its declarations — so efficient, we love a minimal queen 👑

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, no cap.
• -pthread — define additional macros required for using the POSIX threads library. Use this option consistently for both compilation and linking, bestie, don't do one and not the other 😭. 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. Dependency manifest energy 📋

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with the object file suffix and with any leading directory parts removed. If there are many included files, the rule gets 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. Sneaky but iconic.

• -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. So don't come for the brackets, they're innocent 💀

• -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. Classic dash behaviour, we stan 🌈

• -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. Makefile girlies rise up 🔥

• -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. Genius little hack, ngl.

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. That result is the target.

An -MT option sets the target to be exactly the string you specify. Want multiple targets? Specify them as a single argument to -MT, or use multiple -MT options. Your call 💅

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. So -MQ '$(objpfx)foo.o' gives

        $$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ. Automatic queen behaviour 👑

• -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. Same flag, different meaning, main character duality 🦋

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 though, so 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. Glorified tokenizer era 💀

-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. Sit down, this one has layers 🎀

• 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. Yes, really. Dollar sign girlies, this one's for you 💸
• -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. Don't nest infinitely, bestie, it's giving chaos 😭
• -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. C++20 modules girlies, this one is your moment ✨

• -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. Tab drama, handled 🎀
• -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 ⚠️ — trade-offs, babe.

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. You didn't ask, but you got it anyway 🫶

• -fmacro-prefix-map=old=new — okay so bestie, when you're preprocessing files that live in directory old, this makes the "__FILE__" and "__BASE_FILE__" macros act like the files lived in directory new instead. 💅 You can turn an absolute path into a relative one by using . for new, which lowkey gives you more reproducible builds that are location independent. This option also does its thing to "__builtin_FILE()" during compilation. Also peep -ffile-prefix-map and -fcanon-prefix-map. ✨
• -fexec-charset=charset — sets the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's "iconv" library routine. 🎀
• -fwide-exec-charset=charset — sets the wide execution character set, used for wide string and character constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the width of "wchar_t" and the big-endian or little-endian byte order being used for code generation. As with -fexec-charset, charset can be any encoding supported by the system's "iconv" library routine; however, you will have problems with encodings that do not fit exactly in "wchar_t". 😭
• -finput-charset=charset — sets the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine. 🦋
• -fpch-deps — when using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used. 💀
• -fpch-preprocess — this option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma GCC pch_preprocess "filename"" in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH. 🎀

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps. ✨

You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory. 🫶

• -fworking-directory — enables generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no "#line" directives are emitted whatsoever. 😭
• -A predicate=answer — makes an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. ✨
• -A -predicate=answer — cancels an assertion with the predicate predicate and answer answer. 🙈
• -C — do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive. 👀

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. 💀

• -CC — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded. 🎀

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line. 😭

The -CC option is generally used to support lint comments. 💅

• -P — inhibits generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers. ✨
• -traditional, -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. 🦋

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly. So don't get it twisted. 💅

• -trigraphs — supports ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined by ISO C 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. See the -std and -ansi options. 👑

• -remap — enables special code to work around file systems which only permit very short file names, such as MS-DOS. 💾
• -H — prints the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! . 🎀
• -dletters — says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. 😭
  • -dM — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is 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. ✨

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. 💅

• -dD — like -dM except that 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 emit only the macro names, not their expansions. 🎀
• -dI — outputs #include directives in addition to the result of preprocessing. 🦋
• -dU — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are 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 — this option is only useful for debugging GCC. When used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is 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 is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. 🫶
• -Xpreprocessor option — pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not 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 — perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation. 🎀

💅 Passing Options to the Assembler 💅

Okay so you can pass options to the assembler. Here's how bestie: ✨

• -Wa,option — pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas. 🦋
• -Xassembler option — pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not 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 show up when the compiler is actually linking object files into an executable output file. If the compiler isn't doing a link step, they're literally meaningless. bye. 💅

• object-file-name — if a file name doesn't end in a special recognized suffix, it's treated as naming an object file or a library. (The linker tells object files apart from libraries by looking at the file contents, no cap.) If linking happens, these object files get fed to the linker as input. 🫶
• -c, -S, -E — if you use any one of these, the linker does not run, so pls don't pass object file names as arguments. 🙈
• -flinker-output=type — this one controls code generation of the link-time optimizer. By default the linker output is auto-determined by the linker plugin, but for debugging the compiler, or if you want incremental linking with a non-LTO object file, you might lowkey want to control the type manually. here's the menu bestie:
  • 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 gives similar optimizations to exec, except that -fpie isn't disabled if it was specified at compilation time.
  • If type is rel, the compiler assumes incremental linking is happening. The sections containing intermediate code for link-time optimization are 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 most of the objects in the library actually get used.
  • 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 get linked together, the resulting code is better optimized than with link-time optimizations disabled (cross-module inlining happens, for example), 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's not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. 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. 🌈

• -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. 🔥
• -llibrary, -l library — search the library named library when linking. (The second alternative, with the library as a separate argument, exists only for POSIX compliance and is not recommended.)

The -l option is passed straight to the linker by GCC. Refer to your linker documentation for the 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.

Where you write this option in the command matters: 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. If bar.o refers to functions in z, those functions may not be loaded. 💀

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify get passed to the linker, and options specifying linkage of the system libraries — like -static-libgcc or -shared-libgcc — are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. When this option is specified, these entry points should be supplied through some other mechanism. 👑

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. Still links 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 once absence of a C library is assumed — for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there's genuinely no C library available. 🍓
• -nostdlib — do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify get passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

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.

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.) 🧚

• -nostdlib++ — do not implicitly link with standard C++ libraries.
• -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.
• -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.
• -no-pie — don't produce a dynamically linked position independent executable.
• -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. 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. 🌸
• -r — produce a relocatable object as output. Also known as partial linking.
• -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This tells the linker to add all symbols to the dynamic symbol table, not just used ones. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program.
• -s — remove all symbol table and relocation information from the executable.
• -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.
• -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.[1]
• -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 where 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. 💜

If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. 😭 If GCC finds, at its configuration time, that you have a non-GNU linker, or a GNU linker that does not support 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 static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time. 💅

However, 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, bestie. However, 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. ❄️
• -symbolic — bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option. 🙈
• -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 not 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.

• -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. 🌈
• -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. 💪
• -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. 📖

---

🌸 Options for Directory Search 🌸

These options specify directories to search for header files, for libraries and for parts of the compiler. iykyk 💖

• -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:

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 not use this option to add directories that contain vendor-supplied system header files; use -isystem for that. 🙅‍♀️

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. This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I- option. ⚠️

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 no way to override this effect of -I-. 😤

• -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. 🎀
• -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.
• -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. 🔌
• -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. 🦋

okay so real talk: the compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. 💅 And 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. multitasking queen behavior, honestly.

For each subprogram it's about to run, the driver first tries the -B prefix, if there is one. If that name isn't found, or if -B wasn't specified at all, the driver tries two standard prefixes: /usr/lib/gcc/ and /usr/local/lib/gcc/. And if neither of those lands on a file name that actually exists, the unmodified program name is searched for using the directories specified in your PATH environment variable. classic fallback arc.

The compiler also checks to see if the path provided by -B actually refers to a directory, and if necessary it adds a directory separator character at the end of the path. slay, attention to detail. 🎀

-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's not found there, the two standard prefixes above are tried, and that is all. If it's not found by those means, the file is left out of the link. 😭 devastating.

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.

And then — 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. iconic hack behavior ngl.

• -no-canonical-prefixes — Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. leave it exactly how it is bestie 🫶
• --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. 💀

• --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.

---

🎀 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. iykyk.

• -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. 🫠

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.

• -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. 🔥
• -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.
• -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.
• -fstrict-overflow — This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer.
• -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.
• -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. ⚠️
• -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.
• -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. 💅
• -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). 👑
• -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. 💀🙈
• -fpcc-struct-return — bestie, this one makes "short" struct and union values get returned in memory just like the longer ones, instead of in registers. It's less efficient, sure, but the glow-up is that files compiled by GCC can talk to files compiled by other compilers — especially the Portable C Compiler (pcc). Intercallability, we love to see it. 🫶

The precise convention for returning structures in memory depends on the target configuration macros.

"Short" structures and unions, for the record, are those whose size and alignment match that of some integer type.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. 💀

• -freg-struct-return — returns struct and union values in registers when possible. For small structures this is highkey 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. 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 choice, honestly. 👑

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface. 😭

• -fshort-enums — allocates to an enum type only as many bytes as it actually needs for the declared range of possible values. Specifically, the enum type becomes equivalent to the smallest integer type that has enough room. Tiny enum energy. 🎀

This option has no effect for an enumeration type with a fixed underlying type.

Warning: the -fshort-enums 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. 💅

• -fshort-wchar — overrides the underlying type for wchar_t to be short unsigned int instead of the default for the target. This option is useful for building programs to run under WINE. 🍷

Warning: the -fshort-wchar 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. 💀

• -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. Big difference, fr fr.

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. Rude, but fair. 🙈

The -fcommon places uninitialized global variables in a common block. This allows the linker to 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 is mainly useful to enable legacy code to link without errors. Legacy code getting its moment, we support it. 🦋

• -fno-ident — ignore the #ident directive. That's it. Not me just ignoring things. 🙈
• -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. Stay in your lane, bestie. 🚧
• -fverbose-asm — puts 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). Niche queen behavior. 👑

-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. So don't go parsing them, okay? 🧚

• -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 — generates 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 — and no, the dynamic loader is not part of GCC, it's part of the operating system. 🫶

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. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit. Slay, x86. 💅)

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. Iconic, honestly.

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 this one is basically "do not use bit tests for switch statements," even in the places where bit tests would be more efficient than the other code generation strategies. Efficient? Don't care. No bit tests. 💅
• -ffixed-reg — treat the register named reg as a fixed register. Generated code should never refer to it (except maybe as a stack pointer, a frame pointer, or in some other fixed role). reg has to be the name of a register, and the register names that get accepted are machine-specific — they're defined in the "REGISTER_NAMES" macro in the machine description macro file. Heads up bestie, this flag does not have a negative form, because it specifies a three-way choice. 🎀
• -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 disastrous results. 😭

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 using this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 💀

And a different sort of disaster results from using this flag for a register in which function values may be returned. Not the same disaster, a whole other one. 🙈

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 (and it 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. And ngl, 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 or local-exec. Bestie, 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 extreme care. 😭

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 be 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. 💅

⚠️ IMPORTANT NOTE, bestie, do not skip this one: because the C/C++11 memory model is out here being strict, write accesses are literally not allowed to touch non bit-field members. So the recommended move is to define all the bits of the field's type as bit-field members. it's giving "just do it this way or suffer" 💅

And the default value of this option? That's not a GCC vibe check, that's decided by the application binary interface for the target processor. ABI said what it said. 🎀

---

• -fsync-libcalls — this is the switch that controls whether any out-of-line instance of the "__sync" family of functions is allowed to be used to implement the C++11 "__atomic" family of functions. ✨

The default value is enabled, so honestly the only form of this option that does literally anything useful for you is -fno-sync-libcalls. This option gets used in the implementation of the libatomic runtime library. iykyk 🦋

• -fzero-init-padding-bits=value — this one guarantees zero initialization of padding bits in automatic variable initializers. Lowkey iconic.

Certain languages 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, which guarantees that fields not specified in an initializer have 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 already does. For example:

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.

-fzero-init-padding-bits=unions guarantees zero initialization of padding bits in unions on top of what the standards guarantee — that's when the initializer of a union is empty (then 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 that 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. slay 🌸

-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). 👑

---

🛠️ GCC Developer Options 🛠️

okay so this section is for the compiler girlies specifically 💅 — the command-line options that are primarily of interest to GCC developers, including options that support compiler testing and investigation of compiler bugs and compile-time performance problems. That means 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 searches for libraries. If you're just doing ordinary compilation and linking tasks, you should rarely need any of these. fr fr.

Many developer options that cause GCC to 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. ✨

If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, a phase letter, and the pass name. The base dump file name is the name of the output file produced by the compiler if it was explicitly specified and it's 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 are numbered higher than the pass named final, even if they're executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files get 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 comes out in the common VCG format. It can be decorated with extra per-node and/or per-edge info if you also specify a list of comma-separated markers. When the "su" marker is specified, the callgraph gets decorated with stack usage information — it's equivalent to -fstack-usage. When the "da" marker is specified, the callgraph gets decorated with information about dynamically allocated objects. 🦋

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. not me learning that the hard way 🙈

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:

• -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.
• -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. 💀 iconic behaviour honestly.
• -da, -fdump-rtl-all — produce all the dumps listed above. go big or go home 👑
• -dA — okay so this one's little but mighty: it annotates the assembler output with miscellaneous debugging information. ✨
• -dD — dumps all the macro definitions at the end of preprocessing, on top of the normal output. so you get your usual output PLUS the full macro receipts 💅
• -dH — literally produces a core dump whenever an error occurs. it's giving dramatic and honestly? respect. 💀
• -dp — annotates the assembler output with a comment showing which pattern and alternative is used. it also prints the length and cost of each instruction. receipts for every single instruction, no cap.
• -dP — dumps the RTL in the assembler output as a comment before each instruction. also turns on the -dp annotation. double feature, she ate 🫶
• -dx — just generates RTL for a function instead of compiling it. usually you'd use it with -fdump-rtl-expand. RTL only, no compile, we're just vibing.
• -fdump-debug — dumps the debugging information generated during the debug generation phase. 🐛✨
• -fdump-earlydebug — same energy, but for the early debug generation phase. early bird gets the dump 🌅
• -fdump-noaddr — when you're doing debugging dumps, this suppresses address output. why tho? because it makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. diff-able dumps, iconic.
• -freport-bug — collects and dumps debug information into a temporary file if an internal compiler error (ICE) occurs. your ICE gets its own lil temp file, how thoughtful 💖
• -fdump-unnumbered — when doing debugging dumps, suppresses instruction numbers and address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g. comparing dumps is so back.
• -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. no numbers, just vibes 🔗
• -fdump-ipa-switch, -fdump-ipa-switch-options — these control 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 following dumps are possible:
  • all — enables all inter-procedural analysis dumps. everything, all at once ✨
  • cgraph — dumps information 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. both the wins and the losses, we love a full picture 😭

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 with information about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use is that tools that create live-patches can determine 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 the 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. 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.
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. 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.

so many fields bestie, but every single one matters, no skipping 💅

• -fdump-lang — dumps language-specific information. the file name is made by appending .lang to the source file name. very on brand 🦋
• -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — these control the dumping of language-specific information. the options and filename portions behave as described in the -fdump-tree option. the following switch values are accepted:
  • all — enable all language-specific dumps. the whole buffet ✨
  • class — dump class hierarchy information. virtual table information is emitted unless 'slim' is specified. this option is applicable to C++ only.
  • module — dump 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 the Elrond is real, iykyk 🧚
  • raw — dump 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. a lil guest list for your passes 👑
• -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. math girlies rise 📊
• -fdump-tree-all, -fdump-tree-switch, -fdump-tree-switch-options, -fdump-tree-switch-options=filename — these control 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; those that are not meaningful are ignored. the following options are available:
  • address — print the address of each node. usually this is not meaningful as 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, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they are directly reachable by some other path.

when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

when dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation. condensed queen 💅

• raw — print a raw representation of the tree. by default, trees are pretty-printed into a C-like representation.
• details — enable more detailed dumps (not honored by every dump option). also include information from the optimization passes.
• stats — enable dumping various statistics about the pass (not honored by every dump option).
• blocks — enable showing basic block boundaries (disabled in raw dumps).
• graph — for each of the other indicated dump files (-fdump-rtl-pass), dump 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. one plot, all the functions, so satisfying 🌈

• vops — enable showing virtual operands for every statement.
• lineno — enable showing line numbers for statements.
• uid — enable showing the unique ID (DECL_UID) for each variable.
• verbose — enable showing the tree dump for each statement.
• eh — enable showing the EH region number holding each statement.
• scev — enable showing scalar evolution analysis details.
• optimized — enable showing optimization information (only available in certain passes).
• missed — enable showing missed optimization information (only available in certain passes).
• note — enable other detailed optimization information (only available in certain passes).

• all — turns on everything, except raw, slim, verbose and lineno. it's giving "give me the whole menu but hold those four" 💅
• optall — turns on all the optimization options, i.e. optimized, missed, and note. the optimization girlies only, no extras 🎀

---

ok so bestie, if you're tryna figure out what tree dumps even exist or find the dump for a pass you're obsessed with, here's the lil ritual 🧚:

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. lowkey iconic that they're numbered fr fr ✨
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. your call, queen 👑
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. so like, don't hardcode those numbers bestie, i'm begging 😭

---

🌸 -fopt-info 🌸

-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. yes it's a lil word salad, we'll get through it together 🫶

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. so order matters, periodt 💅

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. slay ✨
• missed — print information about missed optimizations. Individual passes control which information to include in the output. the "aww you almost had it" report 😭
• note — print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc. the yapper mode 🗣️
• all — print detailed optimization information. This includes optimized, missed, and note. the full tea ☕

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. ngl this is GCC-dev-coded, the rest of us are just vibing 🙈

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. the "all of the above" option 🍓

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". no thinking required, it just does the thing 💖

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. so like, pick a fave, only one file gets the crown 👑

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. not me warning you about this twice, it's important 😤

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. mix and match, it's a vibe 🌈

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. look at her go 💅

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. first one wins, the rest are ghosted 💀

---

✨ -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. so don't build your whole life on it bestie, it's giving "we might change this" 🫠

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. it's giving ✨family tree✨ of messages 🌸

---

🎀 -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 output 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. so the higher the number the more it yaps, no cap 🗣️🔥

---

💜 -fenable-kind-pass / -fdisable-kind-pass=range-list 💜

-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. so like, unless you're literally hacking on GCC itself, this is not your lane bestie 🙈

• -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 — 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 — 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 — enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
• -fenable-tree-pass — enable tree pass pass. See -fdisable-rtl for the description of option arguments.

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 bestie ✨ and the default lowkey depends on how your compiler was configured. If you go -fchecking=2 you get even more internal consistency checking, and heads up — that extra checking can actually affect code generation. 👀
• -frandom-seed=string — this one hands GCC a seed to use instead of random numbers when it's generating certain symbol names that have to be different in every single compiled file. It also gets used to drop unique stamps into coverage data files and the object files that produce them 🎀 So if you want object files that come out reproducibly identical, this is your girl — use -frandom-seed.

The string can be a number (decimal, octal or hex) or literally any arbitrary string — and if it's a string, it gets converted to a number by computing CRC32. 🧮

Real talk though: the string should be different for every file you compile. No cap. 🙈

• -save-temps — keeps the usual "temporary" intermediate files around permanently instead of yeeting them 💾 They get named as auxiliary output files, exactly as specified under -dumpbase and -dumpdir.

When you pair it with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that happens to share an extension with an intermediate file. If you want that intermediate file anyway, you can get it by renaming the source file before you use -save-temps. 💅

• -save-temps=cwd — same energy as -save-temps -dumpdir ./.
• -save-temps=obj — same energy as -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file you specified after the -o option, directory separators and all. If you didn't use -o, then -save-temps=obj just behaves like -save-temps=cwd. 🦋
• -time[=file] — reports the CPU time that each subprocess in the compilation sequence ate up ⏱️ For C source files, that's the compiler proper and the assembler (plus the linker, if linking is happening).

Leave out the output file and 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" — the time spent executing the program itself. The second number is "system time" — the time spent executing operating system routines on behalf of the program. Both numbers are in seconds, bestie. 🌸

Now, if you do specify an output file, the output gets appended to that named file instead, and it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

Notice the glow-up: the "user time" and the "system time" got moved before the program name, and the options passed to the program are on display too — so later you can tell exactly what file was being compiled, and with which options. 👑

• -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. If you leave out that optional argument (or if file is "."), the name of the dump file gets figured out by appending .gkd to the dump base name — see -dumpbase for that. 🍓
• -fcompare-debug[=opts] — if no error happens during compilation, this runs the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to that second compilation. It dumps the final internal representation in both compilations, and prints an error if they differ. 💀

Skip the equal sign and the default -gtoggle gets used.

The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty and nonzero, implicitly enables -fcompare-debug. And if GCC_COMPARE_DEBUG is defined to a string starting with a dash, then that string is used for opts; otherwise the default -gtoggle is used.

-fcompare-debug= — equal sign present, opts absent — is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, which prevents 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). And if you only want a warning instead of that rejection, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do it. ✨

• -fcompare-debug-second — this option gets implicitly passed to the compiler for that second compilation that -fcompare-debug asks for, along with options to silence warnings, and omitting other options that would make the compiler produce output to files or to standard output as a side effect. Dump files and preserved temporary files get renamed so they carry the .gk additional extension during the second compilation, which avoids overwriting the ones the first compilation generated. 🌈

And 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. 😭

• -gtoggle — turns off the generation of debug info if leaving this option out would generate it, or turns it on at level 2 otherwise. The position of this argument in the command line does not matter at all — it takes effect after all other options are processed, and it only does so once, no matter how many times you give it. This is mainly intended to be used with -fcompare-debug. 🎀
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in exactly the same way that -gtoggle toggles -g. 🪞
• -Q — when it's 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. 👑

• -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 the SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr, because they're being written out at a slightly different place within the compiler. 🫠

• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass. 🔍
• -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 than 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's 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 prints for the WPA phase of link-time optimization. 🌸
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes. 🧠
• -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. 🦋
• -fmultiflags — this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from the ones 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 lets the target libraries be built for portable baseline environments while the compiler itself defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use when building their own programs, if those defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. 😤

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 the driver self spec. 🗑️

It's 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 the consistency of the (estimated) profile and the effect of individual passes. 📊
• -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. That auxname is generated from the name of the output file, if one was explicitly specified and it isn't an executable — otherwise it's 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 the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit, and no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes. 💖

Okay so the qualifier "dynamic" means the function is out here manipulating the stack dynamically — like, on top of the static allocation we just talked about, it's also doing stack adjustments inside the body of the function, for example to push/pop 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 of the total amount of stack the function uses. If "bounded" is not there, then the amount of adjustments is not bounded at compile time and the second field only represents the bounded part. periodt 💅

• -fstats — emit statistics about front-end processing at the end of the compilation. Lowkey this one is supported only by the C++ front end, and the info is generally only useful to the G++ development team. iykyk 🫶
• -fdbg-cnt-list — print the name and the counter upper bound for all debug counters.
• -fdbg-cnt=counter-value-list — set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...] tuples, which sets the name of the counter and a list of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, 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. For dbg_cnt(tail_call) true is returned for the first 10 invocations. the way that's so specific, obsessed ✨
• -print-file-name=library — print the full absolute name of the library file library that would be used when linking and don't do anything else, bestie. With this option, GCC does not compile or link anything; it just prints the file name.
• -print-multi-directory — print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib — print the mapping from multilib directory names to 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. shell girlies stay winning 🐚
• -print-multi-os-directory — print 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 libsuffix sibling directories this 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.
• -print-multiarch — print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
• -print-prog-name=program — like -print-file-name, but searches for a program such as cpp.
• -print-libgcc-file-name — same as -print-file-name=libgcc.a.

This is 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`

• -print-search-dirs — print the name of the configured installation directory and a list of program and library directories gcc searches and don't do anything else.

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 /. fr fr do not forget it 💀

• -print-sysroot — print the target sysroot directory that is 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.
• -print-sysroot-headers-suffix — print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix and don't do anything else.
• -dumpmachine — print the compiler's target machine (for example, i686-pc-linux-gnu) and don't do anything else.
• -dumpversion — print the compiler version (for example, 3.0, 6.3.0 or 7) and don't do anything 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).
• -dumpfullversion — print the full compiler version and don't do anything else. The output is always three numbers separated by dots, major, minor and patchlevel version.
• -dumpspecs — print the compiler's built-in specs and don't do anything else. (This is used when GCC itself is being built.)

---

🦋 Machine-Dependent Options 🦋

Each target machine supported by GCC 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. cute little naming scheme, slay 💅

Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.

🌸 AArch64 Options 🌸

These options are defined for AArch64 implementations:

• -mabi=name — generate code for the specified data model. Permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a 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.

The ilp32 model is deprecated. she's giving legacy, we don't love that 🫠

• -mbig-endian — generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler.
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
• -mcmodel=tiny — generate 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.
• -mcmodel=small — generate 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 moment she's the default... main character energy ✨
• -mcmodel=large — generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. The -mcmodel=large option is incompatible with -mabi=ilp32, -fpic and -fPIC. not them beefing 💀
• -mtp=name — specify 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.
• -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.
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior is the default.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using 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 base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. kernel girlies know 🐧

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
• -mtls-dialect=traditional — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
• -mtls-size=size — specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer.
• -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.
• -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.
• -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. precision girlies, choose wisely 🎀
• -mlow-precision-sqrt, -mno-low-precision-sqrt — okay bestie, this one toggles the square root approximation ✨. It only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. If enabled, it implies -mlow-precision-recip-sqrt. 💅
• -mlow-precision-div, -mno-low-precision-div — same vibe but for division approximation 🍓. This option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation, -mno-track-speculation — enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to __builtin_speculation_safe_copy to permit a more efficient code sequence to be generated. 👑
• -moutline-atomics, -mno-outline-atomics — enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.

This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same applies when using -mcpu= when the selected cpu 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. This option has the form -march=arch{+[no]feature}*. slay ✨

The table below summarizes the permissible values for arch and the features that they enable by default:

• armv8-a : Architecture Armv8-A : Includes by default +fp, +simd
• armv8.1-a : Architecture Armv8.1-A : Includes by default armv8-a, +crc, +lse, +rdma
• armv8.2-a : Architecture Armv8.2-A : Includes by default armv8.1-a
• armv8.3-a : Architecture Armv8.3-A : Includes by default armv8.2-a, +pauth, +fcma, +jscvt
• armv8.4-a : Architecture Armv8.4-A : Includes by default armv8.3-a, +flagm, +fp16fml, +dotprod, +rcpc2
• armv8.5-a : Architecture Armv8.5-A : Includes by default armv8.4-a, +sb, +ssbs, +predres, +frintts, +flagm2
• armv8.6-a : Architecture Armv8.6-A : Includes by default armv8.5-a, +bf16, +i8mm
• armv8.7-a : Architecture Armv8.7-A : Includes by default armv8.6-a, +wfxt, +xs
• armv8.8-a : Architecture Armv8.8-a : Includes by default armv8.7-a, +mops
• armv8.9-a : Architecture Armv8.9-a : Includes by default armv8.8-a
• armv9-a : Architecture Armv9-A : Includes by default armv8.5-a, +sve, +sve2
• armv9.1-a : Architecture Armv9.1-A : Includes by default armv9-a, +bf16, +i8mm
• armv9.2-a : Architecture Armv9.2-A : Includes by default armv9.1-a, +wfxt, +xs
• armv9.3-a : Architecture Armv9.3-A : Includes by default armv9.2-a, +mops
• armv9.4-a : Architecture Armv9.4-A : Includes by default armv9.3-a, +sve2p1
• armv9.5-a : Architecture Armv9.4-A : Includes by default armv9.4-a, cpa, +faminmax, +lut
• armv8-r : Architecture Armv8-R : Includes by default armv8-r

The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system. 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 the architecture features from. That is, -march=native is treated as -mcpu=native.

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.

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 for which GCC should tune the performance of the code. Permissible values for this option 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.

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 is more specific than the default tuning described below.

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.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

This option cannot be suffixed by feature modifiers. 🦋

• -mcpu=name — specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -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.

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 for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-mcpu=neoverse-512tvb is special in that it does not 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. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb. 🌈

• -moverride=string — override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases.

This option is only intended to be useful when developing GCC. 🙈

• -mverbose-cost-dump — enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler. 🔥
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny. 👑
• -msign-return-address=scope — select the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none. This option has been deprecated by -mbranch-protection. 💀
• -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti|gcs — select the branch protection features to use. 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 (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. ⭐

• -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. 🧚

The possible values of scope are:

• all — runs the pass on all the functions, no skips, no exceptions 💅
• strided — only runs the pass on functions that have access to strided multi-register instructions
• none — disables the pass entirely

-mearly-ra=all is the default for -O2 and above, and also for -Os. Everywhere else, -mearly-ra=none is the default.

• -mearly-ldp-fusion — turns on the copy of the AArch64 load/store pair fusion pass that runs before register allocation. It's already enabled by default at -O and above, so like, it's on unless you say otherwise. ✨
• -mlate-ldp-fusion — turns on the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Also enabled by default at -O and above. Two passes, bestie, one before and one after, they're a duo 🫶
• -msve-vector-bits=bits — sets the number of bits in an SVE vector register. Ngl this option does absolutely nothing unless SVE is enabled, so don't come for me.

GCC supports two whole forms of SVE code generation:

• "vector-length agnostic" output — works with any size of vector register
• "vector-length specific" output — lets GCC make assumptions about the vector length when that's useful for optimization reasons

The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Picking scalable selects vector-length agnostic output. At present, -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. Every other value generates vector-length specific code. The behavior of these values may change in future releases, and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths. 🚨

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. Periodt.

• -Wexperimental-fmv-target — warns you about use of experimental Function Multi Versioning. The Arm C Language Extension specification for Function Multi Versioning is beta and subject to change, so any usage of FMV comes with the caveat that future behavior change and incompatibility is likely. It's giving "we told you so" energy, so heads up. 🙈

---

🌈 -march and -mcpu Feature Modifiers 🌈

Feature modifiers used with -march and -mcpu can be any of the following, plus their inverses nofeature. So yeah, every single one of these has a no version that turns it back off. Balance, bestie. ✨

• crc — Enable CRC extension. This is on by default for -march=armv8.1-a.
• crypto — Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
• fp — Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• simd — Enable Advanced SIMD instructions. This also enables floating-point instructions. This is 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. This is on by default for -march=armv8.1-a.
• rdma — Enable Round Double Multiply Accumulate instructions. This is 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. This option is 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.
• 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. This option is only to enable the extension at the assembler level and 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. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• ssbs — Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• predres — Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to enable the extension at the assembler level and does not affect code generation. This option is 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. This option is 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. This option is 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. This option is 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.

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. It's a whole chain reaction, bestie, flip one and the dominos fall. 💅

---

🦋 Adapteva Epiphany Options 🦋

These -m options are defined for Adapteva Epiphany:

• -mhalf-reg-file — Don't allocate any register in the range r32...r63. That allows code to run on hardware variants that lack these registers. 🎀
• -mprefer-short-insn-regs — bestie it literally prefers to hand out the registers that make short instructions possible, okay? The tea tho: this can bump up your instruction count, so overall code size might go down OR up. It's giving chaos. 💅
• -mbranch-cost=num — sets the cost of branches to roughly num "simple" instructions. Heads up babes, this is only a heuristic — it is NOT guaranteed to give you consistent results across releases. No cap. 🙈
• -mcmove — turns ON the generation of conditional moves. slay.
• -mnops=num — emits num NOPs before every other generated instruction. Every other one. iconic behavior honestly. ✨
• -mno-soft-cmpsf — for single-precision floating-point comparisons, this emits an fsub instruction and tests the flags. It's faster than a software comparison BUT 😭 it can get incorrect results when NaNs are in the mix, or when two different small numbers get compared and their difference comes out as zero. The default is -msoft-cmpsf, which is slower but IEEE-compliant software comparison. Slower but correct, periodt.
• -mstack-offset=num — sets the offset between the top of the stack and the stack pointer. Like, a value of 8 means the eight bytes in the range sp+0...sp+7 can be used by leaf functions without any 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 were compiled with generally does NOT work. It can be useful if you just 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, the recommendation is to configure the toolchain with the appropriate --with-stack-offset=num option instead. k thx 🫶
• -mno-round-nearest — makes the scheduler assume the rounding mode has been set to truncating. The default is -mround-nearest.
• -mlong-calls — unless an attribute says otherwise, assumes all calls might be beyond the offset range of the b / bl instructions, so it loads the function address into a register before doing an (otherwise direct) call. This is the default. 👑
• -mshort-calls — unless an attribute says otherwise, assumes all direct calls are in range of the b / bl instructions, so it just uses those instructions for direct calls. The default is -mlong-calls.
• -msmall16 — assumes addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses, where -mlong-calls semantics are in effect. iykyk.
• -mfp-mode=mode — sets the prevailing mode of the floating-point unit. This decides the floating-point mode that's provided and expected at function call and return time. Making this mode match the one you predominantly need at function start can make your programs smaller and faster by dodging unnecessary mode switches. main character optimization energy.

mode can be set to one of the following values:

• caller — any mode at function entry is valid, and it's 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 drop into different programs with different prevailing FPU modes, and the convenience of using a single object file outweighs the size and speed overhead for any extra mode switching that might be needed — compared with what a more specific choice of prevailing FPU mode would need.
• 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. theory vs reality bestie 😭
• -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. gpu girlies rise up 🦋

• -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 bestie: 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 let the user 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 — generates instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — forces a function call using the jli_s instruction. This option is valid only for ARCv2 architecture. no ARCv2, no entry, periodt 🚫
• -mcpu=cpu — sets the 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, obviously 💅
• hs38_linux — ARC HS38 CPU but with every hardware extension switched on. all of them. the whole buffet 🍓
• hs4x — compile for the ARC HS4x CPU.
• hs4xd — compile for the ARC HS4xD CPU.
• hs4x_rel31 — compile for the ARC HS4x CPU, release 3.10a specifically.
• arc600_norm — ARC 600 CPU with "norm" instructions enabled.
• arc600_mul32x16 — ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
• arc600_mul64 — ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
• arc601_norm — ARC 601 CPU with "norm" instructions enabled.
• arc601_mul32x16 — ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
• arc601_mul64 — ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
• nps400 — compile for ARC 700 on the NPS400 chip.
• em_mini — compile for the ARC EM minimalist configuration, which comes with a reduced register set (she's on a budget, respect it 💀).

---

• -mdpfp, -mdpfp-compact — generate double-precision FPX instructions, tuned for the compact implementation.
• -mdpfp-fast — generate double-precision FPX instructions, but tuned for the fast implementation. speed demon behavior 🏎️
• -mno-dpfp-lrsr — disables "lr" and "sr" instructions from using FPX extension aux registers. hands off the aux registers bestie 🙈
• -mea — generate extended arithmetic instructions. right now only "divaw", "adds", "subs", and "sat16" are supported. 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 ✨
• -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 — okay so this option is literally ignored, it's only here for compatibility purposes. software floating-point code is emitted by default anyway, 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. girl just sits there doing nothing 💀
• -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.
• -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 specify the option using either a string or a numeric value for multo. wlh1 is the default value. the recognized values are:
  • 0, none — no multiplier available. not a single one. broke behavior 😭
  • 1, w — 16x16 multiplier, fully pipelined. enables these 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. the 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 floating-point extension is also enabled. not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point extension is also enabled. this option is only available for ARC EM.
  • fpuda_div — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point, square-root, and divide extensions are also enabled. only available for ARC EM.
  • fpuda_fma — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. the single-precision floating-point and fused multiply and add hardware extensions are also enabled. only available for ARC EM.
  • fpuda_all — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. all single-precision floating-point hardware extensions are also enabled. only available for ARC EM.
  • fpus_div — enables support for single-precision floating-point, square-root and divide hardware extensions.
  • fpud_div — enables support for double-precision floating-point, square-root and divide hardware extensions. this one includes the fpus_div option. not available for ARC EM.
  • fpus_fma — enables support for single-precision floating-point and fused multiply and add hardware extensions.
  • fpud_fma — enables support for double-precision floating-point and fused multiply and add hardware extensions. includes fpus_fma. not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions. the whole collection 💅
  • fpud_all — enables support for 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 and restores on interrupt entry and exit. register-range is given as two registers separated by a dash. the register range *always starts with "r0", and the upper limit is the "fp" register. *blink and lp_count are optional. this option is only valid for ARC EM and ARC HS cores.
• *-mrgf-banked-regs=number* — specifies how many registers get 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, so they can avoid memory transactions during the interrupt entry and exit sequences. use this when you're using fast interrupts in 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 values for width 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 doesn't try to transform loops in your program to use the zero-delay loop mechanism unless it's known that the "lp_count" register can hold the required loop-counter value. depending on the width you pick, the compiler and run-time library might keep using the loop mechanism for various needs anyway. this option defines the macro "__ARC_LPC_WIDTH__" with the value of width.
• -mrf16 — tells the compiler to generate code for a 16-entry register file. this option defines the "__ARC_RF16__" preprocessor macro.
• -mbranch-index — enable use of "bi" or "bih" instructions to implement jump tables.

---

The following options get passed straight through to the assembler, and they also define preprocessor macro symbols.

• -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.
• -mdvbf — passed down to the assembler to enable the dual Viterbi butterfly extension. also sets the preprocessor symbol "__Xdvbf". this option is deprecated.
• -mlock — passed down to the assembler to enable the locked load/store conditional extension. also sets the preprocessor symbol "__Xlock".
• -mmac-d16 — passed down to the assembler. also sets the preprocessor symbol "__Xxmac_d16". this option is deprecated.
• -mmac-24 — passed down to the assembler. also sets the preprocessor symbol "__Xxmac_24". this option is deprecated.
• -mrtsc — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. also sets the preprocessor symbol "__Xrtsc". this option is deprecated.
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. also sets the preprocessor symbol "__Xswape".
• -mtelephony — passed down to the assembler to enable dual- and single-operand instructions for telephony. also sets the preprocessor symbol "__Xtelephony". this option is deprecated.
• -mxy — hands this down to the assembler so it flips on the XY memory extension. It also sets the preprocessor symbol __Xxy, so like, everyone's on the same page 💫

The next ones are all about how your assembly code gets annotated:

• -misize — annotates assembler instructions with estimated addresses. Little breadcrumbs of where stuff lives ✨
• -mannotate-align — does absolutely nothing. It's just vibing here for backward compatibility, bestie 💀

The following ones get passed straight through to the linker:

• -marclinux — passed through to the linker to specify use of 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 specify use of the arclinux_prof emulation. On by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested 🎀

---

💅 the ones that control what your generated code actually means 💅

• -mlong-calls — generates calls as register indirect calls, which means you get access to the full 32-bit address range. Full range, no cap 🌈
• -mmedium-calls — says don't use anything 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 that 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 into a small data section if that data is no bigger than num bytes. The default num is 4 for any ARC configuration, or 8 when there are double load/store operations. Small data, big energy 🍓
• -mno-sdata — don't generate sdata references at all. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets 🙈
• -mvolatile-cache — uses ordinarily cached memory accesses for volatile references. This is the default 💜
• -mno-volatile-cache — enables cache bypass for volatile references. Skips the cache, main character move 🦋

---

🌸 the fine-tuning-the-codegen girlies 🌸

• -malign-call — does nothing. Preserved for backward compatibility 💀
• -mauto-modify-reg — enables 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. It has no effect on generation of these instructions when they're driven by the combiner pass. Two different pipelines, bestie 🫶
• -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. This is the default for -Os, and it's only available for ARCv1 cores. Heads up: this option is deprecated 💀
• -mno-cond-exec — disables the ARCompact-specific pass that generates conditional execution instructions.

Okay so the tea here: because of delay slot scheduling and the way operand numbers, literal sizes, instruction lengths and conditional execution support all interact, the target-independent pass for generating conditional execution is often just… not enough. 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, and it costs extra compilation time, which is literally why there's an option to switch it off. And if you ever hit a problem where call instructions exceed their allowable offset range because they got conditionalized, you should consider using -mmedium-calls instead. Noted 📝

• -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. Fair warning: this can be problematic, because some optimizers will then assume that indexed stores exist too — and they do not. Rude awakening territory 😭
• -mlra — does nothing. Preserved for backward compatibility.
• -mlra-priority-none — doesn't indicate any priority for target registers.
• -mlra-priority-compact — indicates target register priority for r0..r3 / r12..r15.
• -mlra-priority-noncompact — reduces 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 lot of registers often get shortened by calling a special function in libgcc. That's what a millicode call is. But because these calls can cause performance issues and/or linking issues when you're linking in a nonstandard way, this option exists so you can turn millicode call generation on or off 🎀
• -mcode-density-frame — lets the compiler emit enter and leave instructions. These are only valid for CPUs that have the code-density feature. No code-density, no enter/leave, periodt ✋
• -mmixed-code — does nothing. Preserved for backward compatibility.
• -mq-class — this option is deprecated. It 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 gets 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, if it's not set, the behavior 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, where 4 is equal to a normal instruction.
• -munalign-prob-threshold=probability — does nothing. Preserved for backward compatibility.

---

🕰️ the deprecated era — still here, but on borrowed time 🕰️

The following options are maintained for backward compatibility, but they're now deprecated and will be removed in a future release. So like… don't get attached 💔

• -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 — the 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, bestie. Different architecture, same chaotic energy 💖

• -mabi=name — generates code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
• -mapcs-frame — generates a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even if that's not strictly necessary for the code to execute correctly. If you specify -fomit-frame-pointer along with this option, stack frames won't be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated 🫠
• -mapcs — this is a synonym for -mapcs-frame and is deprecated.
• -mthumb-interwork — generates 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 gets generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless. Nothing. Nada. 💀
• -mno-sched-prolog — prevents the reordering of instructions in the function prologue, and prevents merging those instructions with the instructions in the function's body. So every function starts 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.
  • Specifying soft makes GCC generate output containing library calls for floating-point operations.
  • softfp allows the 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, this one's important: 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, no matching, no exceptions, fr fr 🚨

---

• -mgeneral-regs-only — okay so this one makes the compiler emit code that touches only the general-purpose registers 💅 it stops GCC from reaching for the floating-point and Advanced SIMD registers, but bestie listen, it does not put any restrictions on the assembler. The assembler is gonna do what it does, no cap.
• -mlittle-endian — generate code for a processor running in little-endian mode. This is the default for all standard configurations, so it's the lowkey main character 🎀
• -mbig-endian — generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor. so yeah, you have to ask for it ✨
• -mbe8
• -mbe32 — when you're linking a big-endian image, these pick between BE8 and BE32 formats. For little-endian images the option has no effect and is literally ignored, periodt. The default depends on the selected target architecture: for ARMv6 and later architectures the default is BE8, and for older architectures the default is BE32. And heads up — BE32 format has been deprecated by ARM 🙈
• -march=name[+extension...] — this bad boy specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it's allowed to emit when generating assembly code. You can use it together with, or instead of, the -mcpu= option 🦋

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, the following architectures, which lack support for the Thumb execution state, are recognized but their support is deprecated: armv4.

---

Many of the architectures support extensions 🌸 you add them by appending +extension to the architecture name. Extension options get processed in order and the capabilities just accumulate, it's giving stacking buffs. An extension will also enable any necessary base extensions it depends on — like, the +crypto extension will always enable the +simd extension, iykyk.

The exception to this additive construction is for extensions prefixed with +no...: those disable the specified option and any other extensions that might depend on that extension being there. So it's not just "add stuff", it's also "nah, take it back" 💀

For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4, since the +simd option is entirely disabled by the +nofp option that follows it. The order matters, bestie 👑

Most extension names are generically named but have an effect that depends on the architecture you applied them to. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but it'll enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a. Same name, different vibe check 🌈

The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions.

armv5te / armv6 / armv6j / armv6k / armv6kz / armv6t2 / armv6z / armv6zk

• +fp — The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this extension.
• +nofp — Disable 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. The extension +vfpv3-d16 can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.
• +nofp — Disable 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. The extension +vfpv3-d16 can be used as an alias for this extension.
• +simd — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can be used as aliases for this extension.
• +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-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 — Disable the Advanced SIMD instructions (does not disable floating point).
• +nofp — Disable 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. The extension +vfpv4-d16 can be used as an alias for this extension.
• +simd — The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can be used as an alias for this extension.
• +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. The extension +neon-vfpv3 can be used as an alias for this extension.
• +neon-fp16 — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
• +nosimd — Disable the Advanced SIMD instructions (does not disable floating point).
• +nofp — Disable 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 — Disable the cryptographic instructions.
• +nofp — Disable 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 — Disable the cryptographic instructions.
• +nofp — Disable 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 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 — Enable the Dot Product extension. This also enables Advanced SIMD instructions.
• +nocrypto — Disable the cryptographic extension.
• +nofp — Disable 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. two-for-one bestie 💅
• +crypto — the cryptographic instructions. lowkey a bundle: this one also switches on the Advanced SIMD and floating-point instructions and the Dot Product extension. package deal ✨
• +nocrypto — disable the cryptographic extension. not today 💀
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions. all of them. gone. bye 🫶
• +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. (yes it says "Brain", we do not question the manpage, we love her as she is 😭)

---

armv8.5-a

• +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions, plus the Dot Product extension and the half-precision floating-point fmla extension. it's giving overachiever 🌸
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — disable the cryptographic extension.
• +nofp — disable the floating-point, Advanced SIMD and cryptographic 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.

armv8.6-a

• +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions, the Dot Product extension and the half-precision floating-point fmla extension.
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — disable the cryptographic extension.
• +nofp — disable the floating-point, Advanced SIMD and cryptographic 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.

armv7-r

• +fp.sp — the single-precision VFPv3 floating-point instructions. the extension +vfpv3xd can be used as an alias for this extension. alias era 💅
• +fp — the VFPv3 floating-point instructions with 16 double-precision registers. the extension +vfpv3-d16 can be used as an alias for this extension.
• +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
• +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
• +nofp — disable the floating-point extension.
• +idiv — the ARM-state integer division instructions.
• +noidiv — disable the ARM-state integer division extension.

armv7e-m

• +fp — the single-precision VFPv4 floating-point instructions.
• +fpv5 — the single-precision FPv5 floating-point instructions.
• +fp.dp — the single- and double-precision FPv5 floating-point instructions.
• +nofp — disable the floating-point extensions.

armv8.1-m.main

• +dsp — the DSP instructions.
• +mve — the M-Profile Vector Extension (MVE) integer instructions.
• +mve.fp — the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- and double-precision floating-point instructions.
• +nofp — disable the floating-point extension.
• +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.
• +pacbti — enable the Pointer Authentication and Branch Target Identification Extension.

armv8-m.main

• +dsp — the DSP instructions.
• +nodsp — disable the DSP extension.
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- and double-precision floating-point instructions.
• +nofp — disable the floating-point extension.
• +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.

armv8-r

• +crc — the Cyclic Redundancy Check (CRC) instructions.
• +fp.sp — the single-precision FPv5 floating-point instructions.
• +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
• +crypto — the cryptographic instructions.
• +nocrypto — disable the cryptographic instructions.
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.

---

-march=native makes the compiler auto-detect the architecture of the build computer. ok but rn? this feature is only supported on GNU/Linux, and not all architectures are recognized. and if the auto-detect is unsuccessful, the option just... has no effect. no drama, no error, just vibes 🙈

-march=unset makes the compiler ignore any -march=... options that appear earlier on the command line, and behave as if the option was never passed. it's for when you're dodging warnings about conflicting CPU and architecture options and the two of them produce different architecture specifications. the peacemaker option fr fr 🫶

---

-mtune=name`

this one picks the target ARM processor that GCC should tune the performance of the code for. for some ARM implementations you can genuinely get better performance by using this option. ✨

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.

and 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 says GCC should tune the performance for a blend of processors within architecture arch. the whole point is generating code that runs well on the current most popular processors — balancing the optimizations that benefit some CPUs in the range while avoiding the performance pitfalls of the other CPUs. and heads up bestie: the effects of this option may change in future GCC versions as CPU models come and go. 🦋

-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code. read that twice.

-mtune=native makes the compiler 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. same energy as -march=native, no cap 💀

---

-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 you'd specified it with -march — and the ARM processor type to tune for performance, as if you'd specified it with -mtune. one option doing two jobs, main character energy 🎀

where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. so -mcpu doesn't get the final say. noted. 💅

Okay so bestie, real talk: a bunch of the supported CPUs have optional architectural extensions. Usually those extensions are enabled by default. If implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. And for floating-point plus Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions. Periodt.

For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD or both the SIMD and floating-point instructions respectively.

Permissible names for this option are the same as those for -mtune.

The following extension options are common to the listed CPUs:

• +nodsp — Disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Also disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nopacbti — Disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
• +nomve — Disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nomve.fp — Disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +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 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 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.

Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a.

-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more information.

-mcpu=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.

-mcpu=unset causes the compiler to ignore any -mcpu=... 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.

• -mfpu=name — This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: 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. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2.

The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.

If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision.

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.

• -mfp16-format=name — Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the "__fp16" type is not defined.
• -mstructure-size-boundary=n — The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value 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 can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot 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 — Generate a call to the function abort at the end of a noreturn function. It is executed if the function tries to return.
• -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 64-megabyte addressing range of the offset-based version of subroutine call instruction.

Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the short_call attribute, functions that are 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 this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are 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, as does placing 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 — 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.
• -mpic-register=reg — Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10.
• -mpic-data-is-text-relative — Assume that the displacement between the text and data segments is fixed at static link time. This permits using 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 disabled on such targets, it will enable -msingle-pic-base by default.
• -mpoke-function-name — Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to 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 performing 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 that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000).

• -mthumb, -marm — Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate 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 by using the target("thumb") and target("arm") function attributes or pragmas.

• -mflip-thumb — switches between ARM and Thumb modes on alternating functions. Bestie, this is a regression-testing option only — it's there to test mixed Thumb/ARM codegen and is NOT meant for normal compiling. Don't be putting this in your build flags, I'm begging 😭
• -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, iykyk 🍃) Default is -mno-tpcs-frame.
• -mtpcs-leaf-frame — same energy, but for all leaf functions. (Still a function that calls nobody 💅) 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. She's built different ✨
• -mcaller-super-interworking — lets calls through function pointers (including virtual functions 👑) run correctly whether or not the target code was compiled for interworking. Downside: there's a small overhead cost every time you execute a function pointer when this is enabled. Also not valid in AAPCS configurations, same reason — interworking is on by default.
• *-mtp=name* — sets the access model for the thread local storage pointer. The soft model generates calls to __aeabi_read_tp. Other accepted models are tpidrurw, tpidruro and tpidrprw, which fetch the thread pointer straight from the corresponding system register (supported from the arm6k architecture and later). Those system registers get 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 processor you picked. Default setting is auto. No cap 🫶
• *-mtls-dialect=dialect* — picks the dialect for accessing thread local storage. Two dialects: gnu and gnu2. gnu selects the original GNU scheme for supporting local and global dynamic TLS models. gnu2 selects the GNU descriptor scheme, which performs better for shared libraries. The GNU descriptor scheme is compatible with the original scheme but does require new assembler, linker and library support — plan accordingly bestie. 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). Enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes that restriction, and when -fpic or -fPIC is specified. Conflicts with -mslow-flash-data. 💀
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when ldrd instructions have overlapping destination and base registers 😭 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. 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, 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 this option's setting. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined.

• -mneon-for-64bits — this option is deprecated and has no effect. It's giving ghost 👻
• -mslow-flash-data — assume 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 it's off by default. It conflicts with -mword-relocations.
• -masm-syntax-unified — assume inline assembler is using unified asm syntax. Default is currently off, which implies divided syntax. This option has no impact on Thumb2. But heads up: that may change in future releases of GCC. Divided syntax should be considered deprecated, fr fr.
• -mrestrict-it — restricts generation of IT blocks to conform to ARMv8-A rules. IT blocks can only contain a single 16-bit instruction from a select set of instructions. On by default for ARMv8-A Thumb mode. ✨
• -mprint-tune-info — prints CPU tuning information as a comment in the assembler file. This one's only for regression testing of the compiler, not intended for ordinary use in compiling code. Disabled by default.
• -mverbose-cost-dump — enables verbose cost model dumping in the debug dump files. Provided for use in debugging the compiler, iykyk 🛠️
• -mpure-code — do not allow constant data to be placed in code sections. Also, 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 — generate secure code as per the ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification, which can be found at https://developer.arm.com/documentation/ecm0359818/latest/. 🔐✨
• -mfix-cmse-cve-2021-35465 — mitigate against a potential security issue with the VLLDM instruction in some M-profile devices when using CMSE (CVE-2021-365465). Enabled by default when the option -mcpu= is used with cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85 or star-mc1. The option -mno-fix-cmse-cve-2021-35465 can be used to disable the mitigation.
• *-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 option -mstack-protector-guard-offset= is for use with -fstack-protector-guard=tls and not for use in user-land code. 🐤
• -mfdpic, -mno-fdpic — select 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.

Note that static linking is not supported because it would still involve the dynamic linker when the program self-relocates 😭 If that behavior is acceptable to you, 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]] — enable branch protection features (armv8.1-m.main only). Let's break it down bestie:
  • 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 turned on 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.

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 options are 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 file specs-mcu (https://gcc.gnu.org/wiki/avr-gcc#spec-files). 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.

avr25

"Classic" devices with up to 8 KiB of program memory and with the MOVW instruction. 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. 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 = 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 — bestie, this one says: assume that all data in static storage can be reached by LDS / STS instructions. It only actually does anything on reduced Tiny devices like the ATtiny40, okay? 👀 Also peep 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. What this does is link startup code from crtmcu-cvt.o instead of the usual crtmcu.o. And besides handing you a compact vector table, the startup code sets bit CPUINT_CTRLA.CPUINT_CVT, which is what actually enables the CVT on the device. ✨

When you do not want the startup code to set CPUINT_CTRLA.CPUINT_CVT, 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. It's 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 an 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.

Heads up bestie: 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. no cap on that one ⭐

• -mno-interrupts — the code it makes is not compatible with hardware interrupts, bestie. the upside? the code size is smaller. that's the trade, periodt.
• -mrelax — try to swap out a CALL (or JMP) instruction for the shorter RCALL (or RJMP) one whenever that's applicable. setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line. 💅

okay so the jump relaxing itself is done by the linker, because the jump offsets aren't known until the code is actually located. which means: the assembler code the compiler spits out stays the same, but the instructions in the executable may differ from the instructions in that assembler code. lowkey wild, ik.

you must turn relaxing on if linker stubs are needed — see the section on EIND and linker stubs down below. 👇

• -mrodata-in-ram, -mno-rodata-in-ram — put the .rodata sections (that's your read-only data) in RAM or in program memory respectively. for most devices there's literally no choice and this option acts more like an assertion than anything else. 🙈

since v14, and for the AVR64* and AVR128* devices, .rodata lives in flash memory by default — provided the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is available. in that case you can use -mrodata-in-ram to go back to the old layout with .rodata in RAM.

• -mtiny-stack — only change the lower 8 bits of the stack pointer. that's it. that's the whole thing. ✨
• -mfract-convert-truncate — allow truncation to be used instead of rounding towards zero for fractional fixed-point types.
• -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 a program to function correctly. 😭

specifically, 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
• __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. you, the user, are taking responsibility for supplying the sub-processes (compiler proper, assembler and linker) with appropriate command line options. which means you have to supply your own private device specs file via -specs=path-to-specs-file. the good news? there's no more need for the option -mmcu=mcu. 🎀

this option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point to a directory containing a folder named device-specs, which in turn contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu.

• -Waddr-space-convert — warn about conversions between address spaces when the resulting address space is not contained in the incoming address space.
• -Wmisspelled-isr — warn if the ISR is misspelled, i.e. without the __vector prefix. enabled by default, bestie. 👑

---

✨ AVR Optimization Options ✨

so these next ones are pure optimization options. meanwhile -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from up above are only almost optimization options, because there are rare occasions where their different code generation actually matters. iykyk. 💅

• -maccumulate-args — accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once, in the function prologue/epilogue. without this option, outgoing arguments get 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 don't need to be removed from the stack after such a function call.

this option can lead to reduced code size for functions that make several calls to functions that get their arguments on the stack — like calls to printf-like functions. 🍓

• *-mbranch-cost=cost* — set 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 are expanded as calls to appropriate subroutines. code size is smaller. slay.
• -mfuse-add, -mno-fuse-add, *-mfuse-add=level* — optimize 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* — run 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. 🧚
• -mfuse-move2 — run a post combine optimization pass that tries to fuse move instructions.
• -mstrict-X — use address register X in the way the hardware proposes. meaning 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 gets 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

• -msplit-bit-shift — split 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 — split multi-byte loads and stores into several byte loads and stores. this optimization is turned on per default for -O2 and higher.
• -muse-nonzero-bits — enable 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. 💖

---

✨ EIND and Devices with More Than 128 Ki Bytes of Flash ✨

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.

to make indirect jumps possible 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 whenever the EICALL or EIJMP instructions are used.

indirect jumps and calls on these devices are handled as follows by the compiler, and they come with some limitations, bestie — read carefully:

• the compiler never sets EIND.
• the compiler uses EIND implicitly in EICALL/EIJMP instructions, or it 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, and the stub contains a direct jump to the desired address. 🦋
• linker relaxation must be turned on so 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 just aborts — without relaxation and without a helpful error message. 😭
• the default linker script is arranged for code with EIND = 0. if your code is supposed to work for a setup with EIND != 0, you have to use a custom linker script in order to place the sections whose name starts with .trampolines into the segment where EIND points to.
• the startup code from libgcc never sets EIND. heads up: 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's completely 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 the 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 these 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 don't want such dispatch tables you can specify the -fno-jump-tables command-line option.
• C and C++ constructors/destructors called during startup/shutdown. lowkey just vibing in the background
• if the tools hit a gs() modifier explained above. yeah that one, you know the one
• Jumping to non-symbolic addresses like this? Straight up not supported, bestie, the compiler said no:

int main (void)
{
    /* Call function at word address 0x2 */
    return ((int(*)(void)) 0x2)();
}

Instead, you gotta set up a stub — like, the function has to be called through a symbol (here it's "func_4", in the example):

int main (void)
{
    extern int func_4 (void);

    /* Call function at byte address 0x4 */
    return func_4();
}

and then the application gets linked with -Wl,--defsym,func_4=0x4. Or, alternatively, "func_4" can be defined in the linker script. Either way, slay.

---

💫 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers 💫

Some AVR devices have memories bigger than the 64 KiB range that you can reach with 16-bit pointers. To get to memory locations outside that 64 KiB range, the content of a RAMP register gets used as the high part of the address: the X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to build a wide address. Similarly, RAMPD gets used together with direct addressing. iykyk.

• 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. Cleanup queen behavior.
• 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. No exceptions, no cap.

---

🌈 AVR Built-in Macros 🌈

GCC defines several built-in macros so that your 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. They literally just know.

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. Math but make it silicon.

__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, 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. Using these options in the wrong way may lead to sub-optimal or wrong code. 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. Consider this your ✨lore✨ drop.

• -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 — assume that RJMP and RCALL can reach literally the entire program memory, no big deal, they're that girl. This one gets used for multilib generation and selection for devices from the avrxmega3 architecture. 🦋
• -mskip-bug — generate code with no skips (CPSE, SBRS, SBRC, SBIS, SBIC) over 32-bit instructions. dodging the bug like it's an ex in the group chat 💅
• -msp8 — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. The compiler uses this to select and build multilibs for the avr2 and avr25 architectures. Those architectures mix devices that have SPH with devices that do not. ✨

---

🖤 Blackfin Options 🖤

• -mcpu=cpu[-sirevision] — names the target Blackfin processor. rn 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. that's the whole roster, bestie, no cap.

The optional sirevision names the silicon revision of the target Blackfin processor, and any workarounds available for that targeted silicon revision get enabled. If sirevision is none, no workarounds get enabled. If sirevision is any, all the workarounds for the targeted processor get enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, __SILICON_REVISION__ is not defined at all. If sirevision is any, __SILICON_REVISION__ is defined to be 0xffff. And if you just don't use this optional sirevision, GCC assumes the latest known silicon revision of the targeted Blackfin processor. 🫶

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, unless -msim is 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. that's it. 💀

• -msim — says the program is gonna run on the simulator, which causes the simulator BSP provided by libgloss to be linked in. Only does anything for the bfin-elf toolchain. Certain other options, like -mid-shared-library and -mfdpic, imply -msim. 😌
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This skips the instructions that save, set up and restore frame pointers, and frees up an extra register in leaf functions. main character optimization energy 👑
• -mspecld-anomaly — when enabled, the compiler makes sure the generated code doesn't contain speculative loads after jump instructions. If you use this option, __WORKAROUND_SPECULATIVE_LOADS is defined. ✨
• -mno-specld-anomaly — don't generate the extra code that prevents speculative loads from occurring.
• -mcsync-anomaly — when enabled, the compiler makes sure the generated code doesn't contain CSYNC or SSYNC instructions too soon after conditional branches. If you use this option, __WORKAROUND_SPECULATIVE_SYNCS is defined.
• -mno-csync-anomaly — don't generate the extra code that prevents CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
• -mlow64k — when enabled, the compiler is free to take advantage of knowing that the entire program fits into the low 64k of memory. cute and compact 🎀
• -mno-low64k — assume the program is arbitrarily large. This is the default.
• -mstack-check-l1 — do stack checking using information that the uClinux kernel places into L1 scratchpad memory.
• -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 with no 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 this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. go faster, bestie 🔥
• -mno-leaf-id-shared-library — don't assume the code being compiled won't link against any ID shared libraries. Slower code gets 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 it's no more space- or time-efficient than just omitting this option.
• -msep-data — generate code that lets the data segment live in a different area of memory from the text segment. This allows execute in place in an environment without virtual memory management, by eliminating relocations against the text section. 🦋
• -mno-sep-data — generate code that assumes the data segment follows the text segment. This is the default.
• -mlong-calls
• -mno-long-calls — tells the compiler to do function calls by first loading the address of the function into a register and then doing a subroutine call on that register. You need this switch if the target function lies outside the 24-bit addressing range of the offset-based version of the subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. And ngl, 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), all in the interest of performance. sacrifice for speed, periodt 💅
• -minline-plt — enable inlining of PLT entries in function calls to functions that aren't known to bind locally. It does nothing without -mfdpic.
• -mmulticore — build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts that support multicore to be used, and it defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision].

This option can be used with -mcorea or -mcoreb, which select the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In that model, the main function of Core B should be named coreb_main.

If this option isn't 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 together 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 you use this option, coreb_main should be used instead of main. This option can only be used together 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 figure out what kind of instructions it can emit when generating assembly code. The permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x. that's the list, memorize it bestie 💖
• -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.
• -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. 🎀

• *-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 everything applicable about the generated code to architecture-type, 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. watch that frame, bestie 👀
• -metrax4
• -metrax100 — -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively. iconic shortcuts ✨
• -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 the CRIS-specific verbose debug tea ☕ in the assembly code. Bonus side quest: it also switches off the #NO_APP formatted-code indicator that normally gets handed to the assembler at the very top of the assembly file. iykyk ✨
• -mcc-init — do not reuse condition-code results from the previous instruction, bestie. Always emit fresh compare and test instructions before anything touches the condition codes. no shortcuts, no leftovers 💅
• -mno-side-effects — don't emit instructions with side effects in addressing modes other than post-increment. periodt.
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — the plain versions arrange for the stack frame, individual data, and constants to be aligned to the maximum single data access size of whichever CPU model you picked; the no- versions eliminate those arrangements. The default is to arrange for 32-bit alignment. ABI deets like structure layout are not affected by these, so breathe 🫶
• -m32-bit, -m16-bit, -m8-bit — basically the stack-/data-/const-align options above but ✨themed✨: they 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 that set up the stack frame are just… gone 🙈 and no return instructions or return sequences get generated in the code.

Use this one only while visually inspecting the compiled code — literally no warnings or errors are generated when call-saved registers must be saved, or when storage for local variables needs to be allocated. It will not tell you. It's giving silent treatment 💀

• -melf — legacy no-op option. it does nothing, iconic behavior, we respect it 👑
• -sim — arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively. all lined up cute 🌸
• -sim2 — like -sim, but it passes linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000. fancy addresses bestie 💖

---

🦋 C-SKY Options 🦋

GCC supports these options when compiling for C-SKY V2 processors. yes, that's a whole vibe 🌈

• -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.

• -mbig-endian, -EB, -mlittle-endian, -EL — select big- or little-endian code. The default is little-endian. pick your lane 🎀
• -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 the 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. Also heads up: 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, bestie.

• -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).
• -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. don't forget it or they simply will not work 🙈

• -mmp — enable multiprocessor instructions; the default is off.
• -mcp — enable coprocessor instructions; the default is off.
• -mcache — enable coprocessor instructions; the default is off.
• -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 options 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 anyway), 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. This option is not supported on CK801, CK802, or CK803, and is 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. Defaults to on.
• -mmultiple-stld, -mstm, -mno-multiple-stld, -mno-stm — generate code using "stm" and "ldm" instructions. This option isn't supported on CK801 but is enabled by default on other processors.
• -mconstpool, -mno-constpool — create constant pools in the compiler instead of deferring it 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. bestie. please 😭

• -msim — links the library libsemi.a which is in compatible with 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. It 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. It specifies that the "dllimport" attribute should be ignored.
• -mthreads — available for MinGW targets. It specifies that MinGW-specific thread support is to be used. 🧵
• -municode — available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. 🌈
• -mwin32 — available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but it does not influence the choice of runtime library/startup code.
• -mwindows — available for Cygwin and MinGW targets. It 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. It 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, as there the User32 API — which is used to set executable privileges — isn't available.

---

• -fwritable-relocated-rdata — MinGW and Cygwin targets only, bestie 💅 This one specifies that relocated-data chillin' in a read-only section gets put into the ".data" section instead. It's a necessary thing for older runtimes that don't support modification of ".rdata" sections for pseudo-relocation. So like, required, not a vibe choice.
• -mpe-aligned-commons — Cygwin and MinGW targets 🫶 This 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 it 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. Periodt.

Okay so here's the tea ☕: 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, and it does that by running the compiler or linker multiple times and joining the results together with lipo. Two totally different energies, iykyk.

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. And the -force_cpusubtype_ALL option can be used to override that. 💅

The Darwin tools all behave differently when you hit them with an ISA mismatch, and honestly it's giving chaos:

• 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. Not allowed, nope.
• The linker for shared libraries, /usr/bin/libtool, fails and prints an error if it's 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. Quietly. Ominous.

---

• -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. Everything. All of it.
• -fconstant-cfstrings — The -fconstant-cfstrings is an alias for -mconstant-cfstrings. That's it, that's the whole thing 💅
• -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. One option, so many friends 🫶
• -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. Not a casual choice, bestie.

• -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.

---

And now, the entire squad of options that get handed straight to the Darwin linker. Every single one, no skipping 👑:

• -all_load
• -arch_errors_fatal
• -bind_at_load
• -bundle
• -bundle_loader executable
• -dynamiclib
• -force_cpusubtype_ALL
• -nodefaultrpaths
• -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

Hold up, some of these need their own lil' moment too, so here they are with the deets 💖:

• -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.

And as for that big block above — -allowable_client client_name on through -whatsloaded — those 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. Yes, even the ones that can't do the math 🙈

• -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. No mix-and-matching, bestie 🫶

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 architecture has floating-point hardware that's built for maximum performance 💅, and it's mostly IEEE-compliant... but bestie, "mostly" is doing a LOT of heavy lifting there. For full compliance you need software help. This option makes GCC spit out fully IEEE-compliant code — except the inexact-flag is not maintained (there's more on that below 👀). Flip this on and the preprocessor macro _IEEE_FP gets defined during compilation. The tradeoff? The code is less efficient, but it can correctly handle denormalized numbers and those unhinged exceptional IEEE values like not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.
• -mieee-with-inexact — this is like -mieee but the generated code also maintains the IEEE inexact-flag. Turning this on means the generated code implements fully-compliant IEEE math, periodt. On top of _IEEE_FP, the macro _IEEE_FP_EXACT is also defined as a preprocessor macro. Heads up though: on some Alpha implementations the resulting code may run significantly slower than the default code. And since basically very little code 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 it -fptm trap-mode. The trap mode can be set to one of four values:
  • n — this is the default (normal) setting. The only traps enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
  • u — in addition to the traps enabled by n, underflow traps are enabled as well.
  • su — like u, but the instructions are marked to be safe for software completion (see the Alpha architecture manual for the deets).
  • sui — like su, but inexact traps are enabled as well.
• -mfp-rounding-mode=rounding-mode — picks 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 get 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 — so in the Alpha architecture, floating-point traps are imprecise 😭. That means 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 operating system trap handlers in figuring out the exact location that caused a floating-point trap. Depending on what your application needs, you can pick different levels of precision:
  • p — program precision. This option is the default, and it 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 — marks the generated code as IEEE conformant. 🚨 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 option to build a shared library dynamic loader — itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment. iconic behavior honestly 🫶
• -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 the -mcpu= option, 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 do not 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 gave you no way to generate symbol relocations except via assembler macros, and using those macros does not allow optimal instruction scheduling 😔. 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 is 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 do not 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 does not 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 does not 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. 💖
• -mjmpext, -mno-jmpext — okay so this pair turns extra conditional-branch instructions on or off. It's auto-enabled for CPU v2 and above. ✨
• -mjmp32, -mno-jmp32 — vibe check on 32-bit jump instructions. Toggle them here. Enabled for CPU v3 and above. 💅
• -malu32, -mno-alu32 — literally controls 32-bit ALU instructions. Enabled for CPU v3 and above. 🦋
• -mv3-atomics, -mno-v3-atomics — the way that this handles general atomic operations introduced in CPU v3. Enabled for CPU v3 and above. 🔥
• -mbswap, -mno-bswap — byte swap instructions? Yes bestie. Enabled for CPU v4 and above. 🌸
• -msdiv, -mno-sdiv — signed division and modulus instructions. Enabled for CPU v4 and above. 💖
• -msmov, -mno-smov — sign-extending move and memory load instructions. Enabled for CPU v4 and above. 🎀
• -mcpu=version — this one picks the eBPF ISA version. Newer versions may not be supported by all kernels. The default is v4. No cap. Supported values for version are:
  • v1 — The first stable eBPF ISA with no special features or extensions. iconic OG
  • v2 — Supports the jump extensions, as in -mjmpext. slay
  • 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. periodt
• -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. iykyk
• -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. main character energy
• -masm=dialect — Outputs assembly instructions using eBPF selected dialect. The default is pseudoc. Supported values for dialect are:
  • normal — Outputs normal assembly dialect. clean
  • pseudoc — Outputs pseudo-c assembly dialect. cute
• -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. don't test it fr fr

🌸 FR30 Options 🌸

These options are defined specifically for the FR30 port. bestie note

• -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. smol but risky
• -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. no sim needed

🦋 FT32 Options 🦋

These options are defined specifically for the FT32 port.

• -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. warning bestie 🚨
• -mlra — Does nothing. Preserved for backward compatibility. iconic useless queen
• -mnodiv — Do not use div and mod instructions. no math pls
• -mft32b — Enable use of the extended instructions of the FT32B processor. upgrade vibe
• -mcompress — Compress all code using the Ft32B code compression scheme. zip it
• -mnopm — Do not generate code that reads program memory. memory? never heard of her

🔥 FRV Options 🔥

• -mgpr-32 — Only use the first 32 general-purpose registers. less is more
• -mgpr-64 — Use all 64 general-purpose registers. all the regs
• -mfpr-32 — Use only the first 32 floating-point registers. floaty but limited
• -mfpr-64 — Use all 64 floating-point registers. full float
• -mhard-float — Use hardware instructions for floating-point operations. hardware slay
• -msoft-float — Use library routines for floating-point operations. software era
• -malloc-cc — Dynamically allocate condition code registers. dynamic queen
• -mfixed-cc — Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0". fixed vibes
• -mdword — Change ABI to use double word insns. double trouble
• -mno-dword — Do not use double word instructions. single only
• -mdouble — Use floating-point double instructions. double float
• -mno-double — Do not use floating-point double instructions. no double float
• -mmedia — Use media instructions. media era
• -mno-media — Do not use media instructions. no media
• -mmuladd — Use multiply and add/subtract instructions. math combo
• -mno-muladd — Do not use multiply and add/subtract instructions. no combo
• -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. big ABI energy
• -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. inline slay
• -mTLS — Assume a large TLS segment when generating thread-local code. big TLS
• -mtls — Do not assume a large TLS segment when generating thread-local code. small TLS
• -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. relocation tea
• -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. never touch this fr
• -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. frame pointer always
• -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. long calls
• -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. align era
• -mlibrary-pic — Generate position-independent EABI code. PIC queen
• -macc-4 — Use only the first four media accumulator registers. four only
• -macc-8 — Use all eight media accumulator registers. eight is great
• -mpack — Pack VLIW instructions. pack it up
• -mno-pack — Do not pack VLIW instructions. no pack
• -mno-eflags — Do not mark ABI switches in e_flags. no flags
• -mcond-move — Enable the use of conditional-move instructions (default). default slay This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mno-cond-move — Disable the use of conditional-move instructions. no cond move This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mscc — Enable the use of conditional set instructions (default). default scc This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mno-scc — Disable the use of conditional set instructions. no scc This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mcond-exec — Enable the use of conditional execution (default). default exec This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mno-cond-exec — Disable the use of conditional execution. no exec This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mvliw-branch — Run a pass to pack branches into VLIW instructions (default). default branch pack This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mno-vliw-branch — Do not run a pass to pack branches into VLIW instructions. no branch pack This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mmulti-cond-exec — Enable optimization of "&&" and "||" in conditional execution (default). default multi cond This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mno-multi-cond-exec — Disable optimization of "&&" and "||" in conditional execution. no multi cond This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mnested-cond-exec — Enable nested conditional execution optimizations (default). default nested This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -mno-nested-cond-exec — Disable nested conditional execution optimizations. no nested This switch is mainly for debugging the compiler and will likely be removed in a future version. future gone
• -moptimize-membar — This switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by default. membar cleanup
• -mno-optimize-membar — This switch disables the automatic removal of redundant "membar" instructions from the generated code. no cleanup
• -mtomcat-stats — Cause gas to print out tomcat statistics. stats era

...and the last FRV one, saving the best for last:

• -mcpu=cpu — picks the processor type your code gets generated for. The possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. That's the whole menu, no substitutions 🍓

---

🐧 GNU/Linux Options 🐧

These -m options are defined for GNU/Linux targets, and honestly bestie, they're just "which libc are we vibing with today" energy:

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets. So basically it's the main character everywhere else ✨
• -muclibc — use the 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 the 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. And finally — the lil cherry on top — this option causes the preprocessor macro "__ANDROID__" to be defined. 🫶

• -tno-android-cc — turns off the compilation effects of -mandroid, i.e. it does not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. Not me undoing the whole vibe 😭
• -tno-android-ld — turns off 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:

• -mrelax — shorten some address references at link time, when possible; it 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. This switch must be used either with -mh or -ms. No solo runs, it's giving duo energy 💅
• -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
• -mexr — extended registers are 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.
• -mint32 — make 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 instead. Heads up: this option has no effect on the H8/300. 🙈

---

👑 HPPA Options 👑

These -m options are defined for the HPPA family of computers:

• -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. That directionality is not a suggestion, it's a law of physics fr fr.
• -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. Same thing, shorter name, iconic ✨

• -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. So, all atomic support is implemented using sync and atomic libcalls. Sync libcall support lives in libgcc.a. Atomic libcall support lives 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 retired legend, still on the guest list 💀
• -mcaller-copies — the caller copies function arguments passed by hidden reference. This option should be used with care, as 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 — stop the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.
• -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. This lets GCC emit code that performs faster indirect calls. 🏎️

Heads up: 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 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 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 that this makes symbolic debugging impossible. Pick your fighter 💀
• -mlong-calls — generate code that uses long call sequences. This ensures 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. It's just here for the lore 📜
• -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.

• -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 unix-std choices are 93, 95 and 98. 93 works on literally every HP-UX version. 95 shows up on 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 up.
  • -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.
  • -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 so this is the important part, no skipping: this option changes the interfaces for various library routines. It also messes with the operational behavior of the C library. So like… extreme care is needed here. Not a vibe-lightly situation.

Library code that's meant to run 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. 🙈

• -nolibdld — suppresses the generation of link options that go searching for libdld.sl when the -static option 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 is no archive version of libdld.sl. Which means when -static is specified, you need special link options 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 can be used to stop the GCC driver from adding these link options.

• -threads — adds support for multithreading with the dce thread library under HP-UX. This one sets flags for both the preprocessor and the linker. ✨

---

🦋 IA-64 Options 🦋

These are the -m options defined for the Intel IA-64 architecture.

• -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. This is useful when compiling kernel code.
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. This is 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 it does not always do so. 🙃
• -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.
• -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. 💅

---

🎀 LoongArch Options 🎀

These command-line options are defined for LoongArch targets:

• -march=arch-type — bestie this tells GCC what machine type to generate instructions for, periodt. Heads up tho: -march=arch-type lets GCC generate code that may not run at all on processors other than the one you indicated. iykyk.

The choices for arch-type are:

• native — local processor type detected by the native compiler. it's giving "just look at my own machine bestie" ✨
• 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. she's the overachiever fr fr 👑
• la64v1.0 — LoongArch64 ISA version 1.0.
• la64v1.1 — LoongArch64 ISA version 1.1.

More info about LoongArch ISA versions can be found at <https://github.com/loongson/la-toolchain-conventions>. go read it bestie 🦋

• -mtune=tune-type — optimizes the generated code for the given processor target. tuning is self-care 💅

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 — generates 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 — generates 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. no floaty math for u 🙈
• -msimd=simd-type — enables 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). she's doing the MOST 🎀
  • lsx — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
  • none — no LoongArch SIMD instruction may be generated.
• -msoft-float — forces -mfpu=none and prevents the use of floating-point registers for parameter passing. > This option may change the target ABI. so like, be aware bestie 😭
• -msingle-float — forces -mfpu=32 and allows the use of 32-bit floating-point registers for parameter passing. > This option may change the target ABI.
• -mdouble-float — forces -mfpu=64 and allows the use of 32/64-bit floating-point registers for parameter passing. > This option may change the target ABI.
• -mlasx
• -mno-lasx
• -mlsx
• -mno-lsx — incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that can be used by the compiler for code generation. Enabling LASX with mlasx automatically enables LSX, and diabling LSX with mno-lsx automatically disables LASX (yes it says "diabling", that's a typo in the original, we don't fix, we just vibe 💀). These driver-only options 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 is passed to the compiler proper. so the order you type them in? that's the order they apply. iconic behavior fr.
• -mbranch-cost=n — sets the cost of branches to roughly n instructions.
• -maddr-reg-reg-cost=n — sets 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. (again, note the original's spelling: -mno-check-zero-divison — one i short and we keep it exactly as-is 🫶)
• -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 the 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 appear on the command line. so explicit wins, no matter what, 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.
  • 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. they do NOT mix bestie 💀

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, 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, -mexplicit-relocs=auto if the assembler supports relocation operators and -mrelax is enabled. it's giving ✨conditional✨ and honestly, valid.
• -mexplicit-relocs — an alias of -mexplicit-relocs=always for backward compatibility.
• -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 is only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with -static or -static-pie. > -mdirect-extern-access is not compatible with -fPIC or -fpic. don't try it bestie, she will not cooperate 🙅‍♀️

• -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.
• -mrecip — this option 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 sure the target CPU supports frecipe and frsqrte instructions. (yes, "you must sure" — we do not edit the original's grammar, we just respect it 💜) 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). so like… faster but slightly more delulu about the truth 😭
• -mrecip=opt — this option 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.
• vec-rsqrt — turn on the approximation for vectorized reciprocal square root 💅

So like, for example, -mrecip=all,!sqrt gives you all the reciprocal approximations except the scalar square root one. Slay.

• -mfrecipe, -mno-frecipe — use (or don't use) the frecipe.{s/d} and frsqrte.{s/d} instructions. When you build with -march=la664 this is on by default, but the actual default is -mno-frecipe.
• -mdiv32, -mno-div32 — use (or don't use) the div.w[u] and mod.w[u] instructions when the input isn't sign-extended. When you build with -march=la664 it's on by default; the actual default is -mno-div32.
• -mlam-bh, -mno-lam-bh — use (or don't use) the am{swap/add}[_db].{b/h} instructions. Building with -march=la664 turns this on by default, but the default default is -mno-lam-bh.
• -mlamcas, -mno-lamcas — use (or don't use) the amcas[_db].{b/h/w/d} instructions. Same story: on by default when you build with -march=la664, but the default is -mno-lamcas.
• -mld-seq-sa, -mno-ld-seq-sa — this one's about whether a same-address load-load barrier (dbar 0x700) is needed. Building with -march=la664 enables it by default, but the default is -mno-ld-seq-sa, meaning the load-load barrier is needed.
• -mtls-dialect=opt — controls which TLS dialect is allowed for the general dynamic and local dynamic TLS models. The options are:
  • trad — use traditional TLS. This is the default.
  • desc — use TLS descriptors.
• -mannotate-tablejump, -mno-annotate-tablejump — makes an annotation section called .discard.tablejump_annotate so you can correlate the jirl instruction with the jump table whenever a jump table is used to optimize a switch statement. Some external tools — like objtool from the Linux kernel build system — need that annotation to analyze the control flow. Default is -mno-annotate-tablejump.
• --param loongarch-vect-unroll-limit=n — the vectorizer uses whatever tuning info it has to decide whether unrolling the main vectorized loop is even worth it, and by how much. This parameter sets the upper bound on how much it'll unroll the main loop. Default value is six. 🌀

---

✨ M32C Options ✨

• -mcpu=name — pick which CPU the code is generated for. name can 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 links in an alternate runtime library that supports stuff like file I/O. > You must not use this option when generating programs that run on real hardware — you have to supply your own runtime library for whatever I/O functions you need. No cap, it will not go well.
• -memregs=number — sets how many memory-based pseudo-registers GCC uses during code generation. These pseudo-registers act like real registers, so there's a whole tradeoff between GCC's ability to fit code into the available registers and the performance hit of using memory instead of registers. Heads up: 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. 💀

---

🌈 M32R/D Options 🌈

These -m options are defined for Renesas M32R/D architectures:

• -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 — assumes all objects live in the lower 16MB of memory (so their addresses can be loaded with the ld24 instruction), and assumes 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 — assumes objects may be anywhere in the 32-bit address space (so the compiler generates seth/add3 instructions to load their addresses), and assumes all subroutines are reachable with the bl instruction.
• -mmodel=large — assumes objects may be anywhere in the 32-bit address space (compiler generates seth/add3 instructions to load addresses), and assumes subroutines may not be reachable with the bl instruction — so the compiler generates the much slower seth/add3/jl instruction sequence. 🐌
• -msdata=none — disables use of the small data area. Variables get put into one of .data, .bss, or .rodata (unless the section attribute was specified). This is the default.

The small data area is made up of the sections .sdata and .sbss. Objects can be explicitly dropped into 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 that are 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 has to be set to either sdata or use for this option to do literally anything.

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 throws an error message — incorrect code is not generated. 🫠

• -mdebug — makes the M32R-specific code in the compiler print out some statistics that might help when debugging programs.
• -malign-loops — align all loops to a 32-byte boundary.
• -mno-align-loops — don't 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, branches are preferred over conditional code; if it's 2, the opposite applies.
• -mflush-trap=number — specifies the trap number to use for flushing 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 isn't available.
• -mno-flush-func — indicates that there's 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's 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 is optimized for one 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 table 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. 🙅

GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family comes from the table 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 too. 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's 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's equivalent to -march=68010.
• -m68020, -mc68020 — bestie, this makes output for a 68020 💖. It's the default when the compiler is configured for 68020-based systems, and it's literally equivalent to -march=68020.
• -m68030 — generates output for a 68030 ✨. Default when the compiler's configured for 68030-based systems, and it's equivalent to -march=68030.
• -m68040 — generates output for a 68040 🌸. Default when the compiler's set up for 68040-based systems, and it's equivalent to -march=68040.

Okay so this one inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040 🙈. Use it if your 68040 does not have code to emulate those instructions.

• -m68060 — generates output for a 68060 🦋. Default when the compiler is configured for 68060-based systems, equivalent to -march=68060.

And this one inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060 💅. Use it if your 68060 does not have code to emulate those instructions.

• -mcpu32 — generates output for a CPU32 👑. Default when the compiler is configured for CPU32-based systems, equivalent to -march=cpu32.

Use this for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360 ⭐.

• -m5200 — generates output for a 520X ColdFire CPU 💜. Default when the compiler is configured for 520X-based systems. It's equivalent to -mcpu=5206, and ngl it's now deprecated in favor of that option.

Use it for a microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206 🌈.

• -m5206e — generates output for a 5206e ColdFire CPU ✨. This option is now deprecated in favor of the equivalent -mcpu=5206e.
• -m528x — generates output for a member of the ColdFire 528X family 🎀. Now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generates output for a ColdFire 5307 CPU 💖. Now deprecated in favor of the equivalent -mcpu=5307.
• -m5407 — generates output for a ColdFire 5407 CPU 🦋. Now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generates output for a ColdFire V4e family CPU (e.g. 547x/548x) 🌸. This includes use of hardware floating-point instructions. It's equivalent to -mcpu=547x, and it's now deprecated in favor of that option.
• -m68020-40 — generates output for a 68040, without using any of the new instructions 👑. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

The option is equivalent to -march=68020 -mtune=68020-40 💅.

• -m68020-60 — generates output for a 68060, without using any of the new instructions ✨. So the code can run relatively efficiently on either a 68020/68881 or 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 (or don't 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 bestie, listen up, this one has receipts 💀. 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.

• 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.
• -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 this is 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 👑.
• -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 — okay so this sets the max amount for a single stack increment operation. Big values can make programs with functions needing lots of stack space run faster, but bestie, they can also trigger a segmentation fault if the stack gets extended too much. The default value is 0x1000. 💅

🧚 MicroBlaze Options 🧚

• -msoft-float — use software emulation for floating point. This is the default, bestie. ✨
• -mhard-float — use hardware floating-point instructions. slay.
• -mmemcpy — do not optimize block moves, use memcpy. no cap.
• -mno-clearbss — this option is deprecated. Use -fno-zero-initialized-in-bss instead. 🚫
• -mcpu=cpu-type — use features of, and schedule code for, the given CPU. Supported values are 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. periodt.
• -mxl-soft-mul — use software multiply emulation. default, bestie. 🎀
• -mxl-soft-div — use software emulation for divides. default. 💜
• -mxl-barrel-shift — use the hardware barrel shifter. iconic.
• -mxl-pattern-compare — use pattern compare instructions. 🔥
• -msmall-divides — use table lookup optimization for small signed integer divisions. iykyk.
• -mxl-stack-check — this option is deprecated. Use -fstack-check instead. 💀
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections. 🌈
• -mxl-multiply-high — use multiply high instructions for high part of 32x32 multiply. ✨
• -mxl-float-convert — use hardware floating-point conversion instructions. 🦋
• -mxl-float-sqrt — use 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. 👀

Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model. so don't use it, bestie. 💀

• -mpic-data-is-text-relative — assume that the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from start of text address instead of GOT since PC-relative addressing is not supported. no cap.

🌸 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 does not 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 nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_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 does not 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 is 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 cannot 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 is not 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 information 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 — generate (or don't generate) code that's fully position-independent, so it can get linked into shared libraries, bestie. This one only touches -mabicalls, k?

okay so here's the lore: all -mabicalls code has traditionally been position-independent, no matter what -fPIC or -fpic are doing. but — 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. 👑

-mno-shared depends on binutils 2.16 or higher, and it generates objects that can only be linked by the GNU linker. However — plot twist — the option doesn't affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. ate. 💅

-mshared is the default. 🎀

• -mplt, -mno-plt — assume (or don't assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32.

You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.

• -mxgot, -mno-xgot — lift (or don't 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 bigger and the linker starts yelling at you with an error like:

relocation truncated to fit: R_MIPS_GOT16 foobar

If that happens, you should recompile your code with -mxgot. This 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.

note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -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. mystery vibe, honestly.
• -mhard-float — use floating-point coprocessor instructions.
• -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. accidentally?? it WILL break. 😭

• -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 options 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. ✨

• -mnan=2008, -mnan=legacy — these options 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. the way they just flip it?? iconic.

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

• -mllsc, -mno-llsc — use (or don't 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 (or don't 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 (or don't 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 (or don't use) the MIPS SmartMIPS ASE.
• -mpaired-single, -mno-paired-single — use (or don't use) paired-single floating-point instructions.

This option requires hardware floating-point support to be enabled.

• -mdmx, -mno-mdmx — use (or don't 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 (or don't use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
• -mmicromips, -mno-micromips — generate (or don't 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 (or don't use) MT Multithreading instructions.
• -mmcu, -mno-mcu — use (or don't use) the MIPS MCU ASE instructions.
• -meva, -mno-eva — use (or don't use) the MIPS Enhanced Virtual Addressing instructions.
• -mvirt, -mno-virt — use (or don't use) the MIPS Virtualization (VZ) instructions.
• -mxpa, -mno-xpa — use (or don't use) the MIPS eXtended Physical Address (XPA) instructions.
• -mcrc, -mno-crc — use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions.
• -mginv, -mno-ginv — use (or don't use) the MIPS Global INValidate (GINV) instructions.
• -mloongson-mmi, -mno-loongson-mmi — use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
• -mloongson-ext, -mno-loongson-ext — use (or don't use) the MIPS Loongson EXTensions (EXT) instructions.
• -mloongson-ext2, -mno-loongson-ext2 — use (or don't 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 ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs. Pointers are the same size as longs, or the same size as integer registers, whichever is smaller.

• -msym32, -mno-sym32 — assume (or don't 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. ⚡
• -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 (or don't 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. 🫶

• -mextern-sdata, -mno-extern-sdata — assume (or don't 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.

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. And if Var is common, you must link the application with a high-enough -G setting. No cap, these rules are not vibes-based, they will bite you. 💀

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. But bestie, maybe you wanna build a library that supports several different small data limits? Iconic, honestly. 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. That's the move. 💅

• -mgpopt, -mno-gpopt — Use (or don't use, queen) GP-relative accesses for symbols that are known to be in a small data section. Check -G, -mlocal-sdata and -mextern-sdata for the whole lore. -mgpopt is the default for all configurations, periodt.

-mno-gpopt is lowkey useful when 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.) 🫶

Heads up: -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. It's giving cascade.

• -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. Slow-ish but thrifty, we love a frugal queen. 🌸
• -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. It does literally nothing on its own, so don't be out here solo-using it. 🙈
• -mcode-readable=setting — Specify whether GCC may generate code that reads from executable sections. There are three possible settings, bestie:
  • -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. It's giving legacy, it's giving "still here for the elders". 👑
• -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. Yes, it depends on when your GCC was born, astrology but for toolchains. 🔮

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, so by default we do care about dividing by zero, tyvm. 🚨
• -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. Turning off safety checks? Bold. Do it knowingly, bestie.

• -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. So it's on, but it's also kinda waiting for the right moment. Main character energy, honestly. 💜
• -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. They're the older sisters of the group chat, still in the gc but not saying much. 🎀
• -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. Inlining is the vibe, forcing memcpy is not. 🌈
• -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. They must be neighbors, fr fr. 256 megabytes of proximity or nothing. 🏠

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. Certified R4650 ISA moment. ✨
• -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. The 74k said "not today" and honestly, respect. 💀
• -mfused-madd, -mno-fused-madd — Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is -mfused-madd. Math go brrr. 🔥

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. So it's giving "numerically identical, unless you're on an R8000, in which case pay attention bestie." ⚠️

• -nocpp — Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them. Straight up "no preprocessing today, thanks." 🙅
• -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. Not GCC's problem, the assembler handles it. 🛠️
• -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. Yeah, deadlock. That's the scary one. 😭

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. Hardware said "loops? maybe. maybe not." 🌀
• -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. Again: assembler's job, not ours. 💅

• -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. GCC dodging instructions like it's avoiding an ex. 🙈
• -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. The R10K really said "oops, my bad, but the damage is done." 💀

---

This problem only affects kernel stores and, depending on the system, kernel loads. 💀 Like, picture it: a speculatively-executed store loads the target memory into cache and marks that cache line as dirty, even if the store itself gets aborted later. Then if a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. 💥 See the R10K processor manual for the full tea, including other potential problems.

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 does 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 make sure that speculative accesses to these regions are indeed safe. 🫡

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. 🤝

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 — tells GCC which function to call to flush the I and D caches, or to not call any such function at all. If it is called, that function must take the same arguments as the common _flush_func, which is: the address of the memory range for which the cache is being flushed, the size of that memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but it's commonly either _flush_func or __cpu_flush.
• -mbranch-cost=num — sets the cost of branches to roughly num "simple" instructions. Ngl this cost is only a heuristic, so it's not guaranteed to produce consistent results across releases. 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. One exception: for the MIPS32 and MIPS64 architectures, and the processors that implement those architectures, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. 🚫
• -mcompact-branches=never, -mcompact-branches=optimal, -mcompact-branches=always — these control which form of branches gets generated. The default is -mcompact-branches=optimal.
  • -mcompact-branches=never ensures that compact branch instructions will never be generated.
  • -mcompact-branches=always ensures that a compact branch instruction will be generated if one is available for MIPS Release 6 onwards. If a compact branch instruction isn't available (or it's pre-R6), a delay slot form of the branch is used instead.
  • If it's used for MIPS16/microMIPS targets, it's just ignored right now. The behavior for MIPS16/microMIPS may change in the future, since they do have some compact branch instructions. 👀
  • -mcompact-branches=optimal 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.
• -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're emitting 64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP pipe. 🎯
• -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.
  • 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's enabled by default at optimization level -O3.
• -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's 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. ⚠️
• -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 any help from the compiler. 💅
• -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.
  • This optimization is off by default at all optimization levels. 🙈
• -mlxc1-sxc1, -mno-lxc1-sxc1 — when applicable, enable (disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default. ✨
• -mmadd4, -mno-madd4 — when applicable, enable (disable) the generation of 4-operand madd.s, madd.d and related instructions. Enabled by default.

---

🌈 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.
• -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.
• -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.
• -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. 📐
• -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.
• -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.
• -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.
• -msingle-exit, -mno-single-exit — force (do not force) generated code to have a single exit point in each function.

---

🎀 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.
• -mno-mult-bug — do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
• -mam33 — generate code using features specific to the AM33 processor.
• -mno-am33 — don't generate code that uses AM33-processor-specific features. This is the default, bestie.
• -mam33-2 — ok so now we do generate code using features specific to the AM33/2.0 processor. glow up ✨
• -mam34 — same energy but for the AM34 processor.
• *-mtune=cpu-type* — use the timing characteristics of the CPU type you name when scheduling instructions. Important: this does not change the targeted processor type, it just tunes the vibe. The cpu-type has to be one of mn10300, am33, am33-2 or am34.
• -mreturn-pointer-on-d0 — when we generate a function that returns a pointer, return that pointer in both a0 and d0. Otherwise the pointer only comes back in a0, and trying to call functions like that without a prototype gives you errors. Also, heads up: this option is on by default, so use -mno-return-pointer-on-d0 if you want it off.
• -mno-crt0 — do not link in the C run-time initialization object file. that's it, that's the whole thing 🫶
• -mrelax — tells the linker "hey bestie, do a relaxation optimization pass" so branches, calls and absolute memory addresses get shortened. This only does anything when you use it on the command line for the final link step.

Warning: this option makes symbolic debugging impossible. like, gone. 💀

• -mliw — let the compiler generate Long Instruction Word instructions if the target is AM33 or later. This is the default. It defines the preprocessor macro __LIW__.
• -mno-liw — do not let the compiler generate Long Instruction Word instructions. This one defines the preprocessor macro __NO_LIW__.
• -msetlb — let the compiler generate the SETLB and Lcc instructions if the target is AM33 or later. Default again. It defines the preprocessor macro __SETLB__.
• -mno-setlb — do not let the compiler generate SETLB or Lcc instructions. It 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, big vibes 🎀
• -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. (yes, again, she's back 💅)

---

🌸 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 purely aesthetic purposes. slay, honestly.
• -mmcu= — pick the MCU to target. This creates a C preprocessor symbol based on the MCU name, converted to upper case and wrapped with __ on both ends. 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 what gets selected; otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA, and the generic msp430x MCU name selects the 430X ISA.

On top of all that, an MCU-specific linker script gets added to the linker command line. The script's name is the MCU name with .ld appended. So specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and makes the linker go looking for a script called xxx.ld.

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. But — an external devices.csv file can be used to extend device support beyond what's hard-coded.

GCC searches for the devices.csv file using the following methods, in this precedence order, where the first method beats the second, which beats 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 — set the global environment variable MSP430_GCC_INCLUDE_DIR to the full path of the directory containing devices.csv, and GCC will search that 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 checks msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if the user created it and copied devices.csv there, then the MCU data is read. Same 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.

• -mwarn-mcu
• -mno-warn-mcu — enables or disables warnings about conflicts between the MCU name given by -mmcu and the ISA set by -mcpu and/or the hardware multiply support set by -mhwmult. 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.

Deprecated alert!! This option is deprecated. Use the -mmcu= option to select the ISA instead. 🚨

• -msim — link to the simulator runtime libraries and linker script. Overrides any scripts that would have been selected by the -mmcu= option.
• -mlarge — use large-model addressing (20-bit pointers, 20-bit size_t). big girl pointers 💅
• -msmall — use small-model addressing (16-bit pointers, 16-bit size_t). smol but mighty.
• -mrelax — this option is passed to the assembler and linker, and lets the linker perform certain optimizations that can't happen until the final link.
• mhwmult= — describes the type of hardware multiply supported by the target. Accepted values are 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 on the MCU name provided by the -mmcu option. If no -mmcu option is specified, or if the MCU name isn't 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. However — when compiling at -O3 or higher, the hardware multiplier is invoked inline. That makes for bigger, but faster, code. tradeoff queen ✨

The hardware multiply routines disable interrupts while they're 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. 🫶

• -minrt — enable the use of a minimum runtime environment — no static initializers, no 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.

Careful bestie: 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= — takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions that 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 is used instead. math girlie behavior 🧮

• -mcode-region=
• -mdata-region= — these options tell the compiler where to place functions and data that don't 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.
• -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 — bestie, do NOT emit v3 push25/pop25 instructions. That's it, that's the whole vibe. 🚫
• -m16-bit — ok so generate 16-bit instructions ✨
• -mno-16-bit — and this one says nope, no 16-bit instructions for you 🙅
• -misr-vector-size=num — set the size of each interrupt vector. It's gotta be 4 or 16, no in-betweens, the compiler is NOT flexible on this 💅
• -mcache-block-size=num — sets the size of each cache block, and it must be a power of 2 sitting somewhere between 4 and 512. Math girlies rise 🎀
• -march=arch — name the target architecture, queen. That's the whole job 👑
• -mcmodel=code-model — picks your code model, and the options are literally this trio:
  • small — every data and read-only data segment has to live inside a 512KB addressing space, and the text segment has to stay within 16MB addressing space. Cozy and tiny 🍓
  • medium — now the data segment must be within 512KB while the read-only data segment can stretch out to 4GB addressing space. Text segment should still be hanging out within 16MB addressing space. Glow-up but make it contained ✨
  • large — all the text AND data segments can be within 4GB addressing space. Go big or go home, no cap 🌈
• -mctor-dtor — turns on the constructor/destructor feature 🎀
• -mrelax — gives the linker a lil nudge to relax instructions. Softer, calmer, lowkey 💆

---

🌸 Nvidia PTX Options 🌸

These ones are defined for Nvidia PTX, bestie:

• -m64 — literally ignored, it's just preserved for backward compatibility. Only the 64-bit ABI is supported, so this flag is giving participation trophy energy 🏆
• -march=architecture-string — generate code for the PTX ISA target architecture you name. The 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 was configured, so go peek at --with-arch 👀

This option sets the value of the preprocessor macro "__PTX_SM__" — like, for sm_35 it has the value 350. Numbers go brrr 🔥

• -misa=architecture-string — it's an alias of -march=. Same girl, different name 💅
• -march-map=architecture-string — picks the closest available -march= value that is not more capable. For instance, for -march-map=sm_50 you select -march=sm_37, and for -march-map=sm_53 you select -march=sm_53. It's the "dress code says no, find the closest thing in your closet" option 🙈
• -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 (check -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. Two for one deal, we love that 🫶

• -mmainkernel — link in code for a __main kernel. This is for stand-alone instead of offloading execution 🧚
• -moptimize — apply partitioned execution optimizations. This is the default whenever any level of optimization is selected, so it's basically already thriving 💅
• -msoft-stack, -mno-soft-stack — okay so here's the tea ☕

With -mno-soft-stack (which is the default, unless -mgomp has been specified), you use PTX "native" stacks — meaning generate code that uses ".local" memory or PTX "alloca" directly for stack storage. But heads up: 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. Read that twice, bestie 😭

With -msoft-stack (implied by -mgomp), you 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. iykyk ✨

• -muniform-simt — switches to a code generation variant that lets you execute all threads in each warp, 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. Main character energy for every single lane, honestly 👑
• -mgomp — generate code for use in OpenMP offloading: it enables the -msoft-stack and -muniform-simt options, and selects the corresponding multilib variant. Combo move, slay 💅

---

🌈 OpenRISC Options 🌈

These options are defined for OpenRISC:

• -mboard=name — configure a board specific runtime. This gets passed to the linker for newlib board library linking. The default is "or1ksim" 🎀
• -mnewlib — this option is ignored; it's for compatibility purposes only. It used to select linker and preprocessor options for use with newlib. Retired icon, still showing up to the function 💀
• -msoft-div, -mhard-div — pick software or hardware divide ("l.div", "l.divu") instructions. The default is hardware divide ✨
• -msoft-mul, -mhard-mul — pick software or hardware multiply ("l.mul", "l.muli") instructions. The default is hardware multiply 💖
• -msoft-float, -mhard-float — pick software or hardware for floating point operations. The default is software 🌸
• -mdouble-float — when -mhard-float is selected, this enables generation of double-precision floating point instructions. By default, functions from libgcc are used to perform double-precision floating point operations 👀
• -munordered-float — when -mhard-float is selected, this 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 🦋
• -mcmov — enable generation of conditional move ("l.cmov") instructions. By default the equivalent will be generated using set and branch 💫
• -mror — enable generation of rotate right ("l.ror") instructions. By default, functions from libgcc are used to perform rotate right operations 🌀
• -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 🔄
• -msext — enable generation of sign extension ("l.ext*") instructions. By default, memory loads are used to perform sign extension ⭐
• -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 🙈
• -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 💅
• -mcmodel=small — generate OpenRISC code for the small model: the GOT is limited to 64k. This is the default model 🍓
• -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:

• -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) It implies -m45 ✨
• -msoft-float — do not use hardware floating point 🚫
• -mac0 — return floating-point results in ac0 (fr0 in Unix assembler syntax) 💖
• -mno-ac0 — return floating-point results in memory. This is the default 🧠
• -m40 — generate code for a PDP-11/40. It implies -msoft-float -mno-split ⭐
• -m45 — generate code for a PDP-11/45. This is the default 👑
• -m10 — generate code for a PDP-11/10. It implies -msoft-float -mno-split 🦋
• -mint16, -mno-int32 — use 16-bit "int". This is the default 🌸
• -mint32, -mno-int16 — use 32-bit "int" 💅
• -msplit — the target has split instruction and data space. It implies -m45 ✂️
• -munix-asm — use Unix assembler syntax 🐧
• -mdec-asm — use DEC assembler syntax 🎀
• -mgnu-asm — use GNU assembler syntax. This is the default 🐃
• -mlra — use the new LRA register allocator. By default, the old "reload" allocator is used. New allocator just dropped 🔥

---

💜 PowerPC Options 💜

These are listed under

---

🫶 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. Trade-offs, bestie, they're real 😭

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. Not me being responsible for my own consequences 🙈

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, so honestly, it ate 💅

• -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 📦

• -mno-relax — make GCC pass the --no-relax command-line option to the linker instead of the --relax option 🙅
• -mloop — allow (or do not allow) GCC to use the LOOP instruction 🔁
• -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. Tiny pointers vs. chunky pointers, pick your fighter 🥊

🫶 Optional Return Value Pointer 🫶

ok so picture this: a function returns something bigger than 64 bits. How do you even hand that back? You sneak a hidden pointer in as the first argument of the function. classic.

The TI ABI, though, says that pointer is allowed to be NULL when the caller isn't even using the returned value. GNU does not play that game — GNU always passes a valid return value pointer and always expects one.

The current -mabi=ti implementation is just like… no. It straight up raises a compile error the moment it detects any of those code constructs. Which means the standard C library literally cannot be built, and it gets omitted when linking with -mabi=ti. 💀

Relaxation is a GNU feature and, for safety reasons, it's disabled when you use -mabi=ti. The TI toolchain doesn't emit relocations for QBBx instructions, so the GNU linker can't adjust them when it shortens adjacent LDI32 pseudo instructions. iykyk.

---

🌈 RISC-V Options 🌈

These command-line options are defined for RISC-V targets, bestie:

• -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, and 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 extension are listed below:

• i — supported version: 2.0, 2.1 — Base integer extension.
• e — supported version: 2.0 — Reduced base integer extension.
• g — supported version: - — General-purpose computing base extension, g will expand to i, m, a, f, d, zicsr and zifencei.
• m — supported version: 2.0 — Integer multiplication and division extension.
• a — supported version: 2.0, 2.1 — Atomic extension.
• f — supported version: 2.0, 2.2 — Single-precision floating-point extension.
• d — supported version: 2.0, 2.2 — Double-precision floating-point extension.
• c — supported version: 2.0 — Compressed extension.
• h — supported version: 1.0 — Hypervisor extension.
• v — supported version: 1.0 — Vector extension.
• zicsr — supported version: 2.0 — Control and status register access extension.
• zifencei — supported version: 2.0 — Instruction-fetch fence extension.
• zicond — supported version: 1.0 — Integer conditional operations extension.
• za64rs — supported version: 1.0 — Reservation set size of 64 bytes.
• za128rs — supported version: 1.0 — Reservation set size of 128 bytes.
• zawrs — supported version: 1.0 — Wait-on-reservation-set extension.
• zba — supported version: 1.0 — Address calculation extension.
• zbb — supported version: 1.0 — Basic bit manipulation extension.
• zbc — supported version: 1.0 — Carry-less multiplication extension.
• zbs — supported version: 1.0 — Single-bit operation extension.
• zfinx — supported version: 1.0 — Single-precision floating-point in integer registers extension.
• zdinx — supported version: 1.0 — Double-precision floating-point in integer registers extension.
• zhinx — supported version: 1.0 — Half-precision floating-point in integer registers extension.
• zhinxmin — supported version: 1.0 — Minimal half-precision floating-point in integer registers extension.
• zbkb — supported version: 1.0 — Cryptography bit-manipulation extension.
• zbkc — supported version: 1.0 — Cryptography carry-less multiply extension.
• zbkx — supported version: 1.0 — Cryptography crossbar permutation extension.
• zkne — supported version: 1.0 — AES Encryption extension.
• zknd — supported version: 1.0 — AES Decryption extension.
• zknh — supported version: 1.0 — Hash function extension.
• zkr — supported version: 1.0 — Entropy source extension.
• zksed — supported version: 1.0 — SM4 block cipher extension.
• zksh — supported version: 1.0 — SM3 hash function extension.
• zkt — supported version: 1.0 — Data independent execution latency extension.
• zk — supported version: 1.0 — Standard scalar cryptography extension.
• zkn — supported version: 1.0 — NIST algorithm suite extension.
• zks — supported version: 1.0 — ShangMi algorithm suite extension.
• zihintntl — supported version: 1.0 — Non-temporal locality hints extension.
• zihintpause — supported version: 1.0 — Pause hint extension.
• zicboz — supported version: 1.0 — Cache-block zero extension.
• zicbom — supported version: 1.0 — Cache-block management extension.
• zicbop — supported version: 1.0 — Cache-block prefetch extension.
• zic64b — supported version: 1.0 — Cache block size isf 64 bytes.
• ziccamoa — supported version: 1.0 — Main memory supports all atomics in A.
• ziccif — supported version: 1.0 — Main memory supports instruction fetch with atomicity requirement.
• zicclsm — supported version: 1.0 — Main memory supports misaligned loads/stores.
• ziccrse — supported version: 1.0 — Main memory supports forward progress on LR/SC sequences.
• zicntr — supported version: 2.0 — Standard extension for base counters and timers.
• zihpm — supported version: 2.0 — Standard extension for hardware performance counters.
• ztso — supported version: 1.0 — Total store ordering extension.
• zve32x — supported version: 1.0 — Vector extensions for embedded processors.
• zve32f — supported version: 1.0 — Vector extensions for embedded processors.
• zve64x — supported version: 1.0 — Vector extensions for embedded processors.
• zve64f — supported version: 1.0 — Vector extensions for embedded processors.
• zve64d — supported version: 1.0 — Vector extensions for embedded processors.
• zvl32b — supported version: 1.0 — Minimum vector length standard extensions
• zvl64b — supported version: 1.0 — Minimum vector length standard extensions
• zvl128b — supported version: 1.0 — Minimum vector length standard extensions
• zvl256b — supported version: 1.0 — Minimum vector length standard extensions
• zvl512b — supported version: 1.0 — Minimum vector length standard extensions
• zvl1024b — supported version: 1.0 — Minimum vector length standard extensions
• zvl2048b — supported version: 1.0 — Minimum vector length standard extensions
• zvl4096b — supported version: 1.0 — Minimum vector length standard extensions
• zvbb — supported version: 1.0 — Vector basic bit-manipulation extension.
• zvbc — supported version: 1.0 — Vector carryless multiplication extension.
• zvkb — supported version: 1.0 — Vector cryptography bit-manipulation extension.
• zvkg — supported version: 1.0 — Vector GCM/GMAC extension.
• zvkned — supported version: 1.0 — Vector AES block cipher extension.
• zvknha — supported version: 1.0 — Vector SHA-2 secure hash extension.
• zvknhb — supported version: 1.0 — Vector SHA-2 secure hash extension.
• zvksed — supported version: 1.0 — Vector SM4 Block Cipher extension.
• zvksh — supported version: 1.0 — Vector SM3 Secure Hash extension.
• zvkn — supported version: 1.0 — Vector NIST Algorithm Suite extension, zvkn will expand to zvkned, zvknhb, zvkb and zvkt.
• zvknc — supported version: 1.0 — Vector NIST Algorithm Suite with carryless multiply extension, zvknc will expand to zvkn and zvbc.

• zvkng — 1.0 — Vector NIST Algorithm Suite with GCM extension, and heads up bestie, 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. the way that is literally the whole description 💅
• 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.

okay so, housekeeping ✨ When -march= is not specified, use the setting from -mcpu.

And if both -march and -mcpu= are not specified, the default for this argument is system dependent, so bestie, users who want a specific architecture extensions should specify one explicitly. fr fr, don't leave it to fate. 🙈

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. it's giving helpful, honestly. 💖

---

🦋 OPTIONS, but make it RISC-V processor tea 🦋

• -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. 👑

bestie note: Note that -mcpu does not override -march or -mtune. it simply does not. it will not. periodt. 💀

• -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. lowkey iconic of it. ✨

• -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).

⚠️ 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. no cap, do not mix and match, that's a one-way ticket to chaos. 😭

• -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. (yes, same param name, no I did not typo it, that's just the vibe 🫶)

• -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. (a whole alias, we love a shortcut 💜)
• -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.
• -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.

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. read that twice bestie. 😭

• -mcmodel=large — Generate code for a large code model, which has no restrictions on size or placement of symbols. no restrictions?? main character energy fr. 👑
• -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. (and yes, the manpage really spells it -mno-exlicit-relocs, I am simply copying the queen exactly as she is 💅)
• -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. slay, less instructions, more chill. ✨
• -mriscv-attribute, -mno-riscv-attribute — emit (or like, don't emit) the RISC-V attribute that records extra info into ELF objects. Heads up bestie: this feature needs at least binutils 2.32, no cap.
• -mcsr-check, -mno-csr-check — turns CSR checking on or off. simple as that 💅
• *-malign-data=type* — controls how GCC aligns variables and constants of array, structure, or union types. The supported values for type are:
  • xlen — uses the x register width as the alignment value.
  • natural — uses natural alignment.

xlen is the default, periodt.

• -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-*-* target, but not for a riscv64be-*-* or riscv32be-*-* target.
• *-mstack-protector-guard=guard*
  • *-mstack-protector-guard-reg=reg*
  • *-mstack-protector-guard-offset=offset* — generate stack protection code using a canary at guard. The supported locations are global for a global canary, or tls for a per-thread canary living in the TLS block.

With that latter choice, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset further specify which register to use as the base register for reading the canary, and what offset from that base register to read it at. There is no default register or offset, because this is entirely for use within the Linux kernel. iykyk 🧚

• -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 to support operation within a simulator.
• -mmul=none
• -mmul=g10
• -mmul=g13
• -mmul=g14
• -mmul=rl78 — specifies the type of hardware multiplication and division support to be used. The simplest one is none, which uses software for both multiplication and division — and it's the default. The g13 value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The g14 value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value rl78 is an alias for g14, and the value mg10 is an alias for none.

In addition, a C preprocessor macro is defined based on the setting of this option. The possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__.

• -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 that gets 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, bestie: 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 that did not have hardware multiply and divide support.

In addition, a C preprocessor macro is defined based on the setting of this option. The possible values are: __RL78_G10__, __RL78_G13__ or __RL78_G14__.

• -mg10
• -mg13
• -mg14
• -mrl78 — these are aliases for the corresponding -mcpu= option. They're provided for backwards compatibility.
• -mallregs — allow the compiler to use all of the available registers. By default the registers r24..r31 are reserved for use in interrupt handlers. With this option enabled, these registers can be used in ordinary functions as well. slay ✨
• -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, because 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 if 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 -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above. 👑

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. The 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.

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. it's giving teamwork 💅
• -mcmodel=small — generate PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — bestie, this one makes PowerPC64 code for the medium model: your TOC and other static data can be up to a total of 4G in size. And it's literally the default for 64-bit Linux, so you're probably already living in it. 💅
• -mcmodel=large — the large model era ✨: the TOC itself may be up to 4G, and everything else (data and code) is only limited by the 64-bit address space. Go big, no cap.
• -maltivec, -mno-altivec — generate code that uses (or does not use) AltiVec instructions, and also unlock the built-in functions that let you poke the AltiVec instruction set directly. You may also need -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements. 🎀

When -maltivec is on, the element order for AltiVec intrinsics like vec_splat, vec_extract, and vec_insert matches array element order for the target's endianness. So element zero identifies the leftmost element in a vector register on a big-endian platform, and the rightmost element on a little-endian platform. Order matters, bestie. 🦋

• -mvrsave, -mno-vrsave — generate VRSAVE instructions when you're generating AltiVec code. That's it, that's the vibe.
• -msecure-plt — generate code that lets ld and ld.so build executables and shared libraries with non-executable .plt and .got sections. It's a PowerPC 32-bit SYSV ABI option. 🔒
• -mbss-plt — generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. Also a PowerPC 32-bit SYSV ABI option. 👑
• -msplit-patch-nops — when 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. That makes the sequence of NOPs not consecutive when a global entry point is generated. Without this option, the NOPs are emitted directly before and after the local entry point, which makes them consecutive but moves the global and local entry points further apart. If only a single entry point is generated, this option has no effect. (Not me reading that twice. 😭)
• -misel, -mno-isel — this switch enables or disables the generation of ISEL instructions. Simple as. ✨
• -mvsx, -mno-vsx — generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the built-in functions that allow more direct access to the VSX instruction set. 🧚
• -mcrypto, -mno-crypto — enable (disable) the use of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA. 🔐
• -mhtm, -mno-htm — enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.
• -mpower8-fusion, -mno-power8-fusion — generate code that keeps (does not keeps) some integer operations adjacent so the instructions can be fused together on power8 and later processors. (Yes, "does not keeps" — I'm just relaying what the page says, don't come for me. 💀)
• -mquad-memory, -mno-quad-memory — generate code that uses (does not use) the non-atomic quad word memory instructions. Heads up: the -mquad-memory option requires use of 64-bit mode. 🌈
• -mquad-memory-atomic, -mno-quad-memory-atomic — generate code that uses (does not use) the atomic quad word memory instructions. And yep, -mquad-memory-atomic also requires use of 64-bit mode. ⭐
• -mfloat128, -mno-float128 — enable/disable the __float128 keyword for IEEE 128-bit floating point, and use 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 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.

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 do not specify to generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE 128-bit floating point will be done with software emulation. 🫶

• -mfloat128-hardware, -mno-float128-hardware — enable/disable 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 modify generation of the TOC (Table Of Contents), which gets created for every executable file. 🎀

The -mfull-toc option is selected by default. 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. However, only 16,384 entries are available in the TOC.

If you receive a linker error message saying you have 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 prevents GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. (Translation: emergency mode, use sparingly bestie. 😭)

• -maix64, -maix32 — enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the 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 — produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when comparing values and converting to double. Use XL symbol names for long double support routines. 🌸

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 do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. 🦋

• -mpe — support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with special startup code to enable the application to 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. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible. Pick a struggle. 💀
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.

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 (uses) the floating-point register set. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking. 🧚
• -mmultiple, -mno-multiple — generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. ⚠️
• -mupdate, -mno-update — generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use -mno-update, there is a small window between the time that 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. > Bestie this is a real caution, not a vibe — corrupted data fr fr. 😭
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as 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 — bestie, this is your switch for whether the code uses those floating-point multiply-and-accumulate instructions. -mfused-madd says yes use them, -mno-fused-madd says no don't. They get generated by default whenever hardware floating point is in play. Important glow-up note: the machine-dependent -mfused-madd is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd maps to -ffp-contract=off. iykyk 💅
• -mmulhw, -mno-mulhw — do (or don't) generate code that uses the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These are on by default when you're targeting those processors. ✨
• -mdlmzb, -mno-dlmzb — do (or don't) generate code that uses the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This one is also generated by default when targeting those processors. 🌸
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (do) force structures and unions containing bit-fields to be aligned to the base type of the bit-field.

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 second you use -mno-bit-align, that same structure is aligned to a 1-byte boundary and is 1 byte in size. shrinkage era fr fr 😭

• -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 (or 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, which is a table of 32-bit addresses generated by this option. 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 so static executables can be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. versatile queen 👑
• -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. twinsies 💜
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig.
• -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. no cap 🎀
• -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 like, that's on the runtime, not on gcc, bestie 💅
• -mprioritize-restricted-insns=priority — this option controls the priority 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.
• -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.
  • all — all dependences are costly.
  • 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.
  • 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. the default era 🦋
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options. two for one, we love a bundle 💖
• -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi options.
• -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. hiii that's us 🌈
• -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.
• -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.
• -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. heads up bestie, this one can get messy 😭
• -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.
• -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. not the spectacular failure 💀
• -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 warning, same drama 🙈
• -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. With -mprototype, only calls to prototyped variable argument functions set or clear the bit.
• -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. yellowknife is such a name, obsessed ⭐
• -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. Selecting -meabi means 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. Selecting -mno-eabi means 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 — PowerPC besties, this one's for you ✨ On System V.4 and embedded PowerPC systems, put small initialized const global and static data in the .sdata2 section, which is pointed to by register r2. Put small initialized non-const global and static data in the .sdata section, which is pointed to by register r13. Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. Heads up though: the -msdata=eabi option is incompatible with the -mrelocatable option. The -msdata=eabi option also sets the -memb option. 💅
• -msdata=sysv — On System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to by register r13. Put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. And again, sorry bestie: the -msdata=sysv option is incompatible with the -mrelocatable option.
• -msdata=default, -msdata — On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise compile code the same as -msdata=sysv. She's flexible like that 💁‍♀️
• -msdata=data — On System V.4 and embedded PowerPC systems, put small global data in the .sdata section. Put small uninitialized global data in the .sbss section. Do not use register r13 to address small data however. This is the default behavior unless other -msdata options are used.
• -msdata=none, -mno-sdata — On embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section. Classic. Simple. No notes. 🫶
• -mreadonly-in-sdata — Put read-only objects in the .sdata section as well. This is the default. 👑
• -mblock-move-inline-limit=num — Inline all block moves (such as calls to memcpy or structure copies) less than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific. 🔥
• -mblock-compare-inline-limit=num — Generate non-looping inline code for all block compares (such as 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. The default value is target-specific.
• -mblock-compare-inline-loop-limit=num — Generate an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length is not constant, at most num bytes will be compared before memcmp is called to compare the remainder of the block. The default value is target-specific.
• -mstring-compare-inline-limit=num — Compare at most num string bytes with inline code. If the difference or end of string is not found at the end of the inline compare a call to strcmp or strncmp will take care of the rest of the comparison. The default is 64 bytes. ✨
• -G num — On embedded PowerPC systems, put 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. By default, num is 8. The -G num switch is also passed to the linker. Important: All modules should be compiled with the same -G num value. Don't mix and match bestie, it's giving chaos 😭
• -mregnames, -mno-regnames — On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using symbolic forms. Symbolic form besties, we love to see it 💜
• -mlongcall, -mno-longcall — By default assume that all calls are far away so that a longer and more expensive calling sequence is required. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call is generated if the compiler knows the call cannot be that far away. This setting can be overridden by the shortcall function attribute, or by #pragma longcall(0).

Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is 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 a 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. so much machinery for one lil jump 🧚

On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it.

In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. foreshadowing ✨

• -mpltseq, -mno-pltseq — Implement (do not 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 are enabled by default if the support is detected when configuring GCC, and, in the case of 32-bit PowerPC, if GCC is configured with --enable-secureplt. Notable: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together.
• -mtls-markers, -mno-tls-markers — Mark (do not mark) calls to __tls_get_addr with a relocation specifying the function argument. The relocation allows the linker to reliably associate function call with argument setup instructions for TLS optimization, which in turn allows GCC to better schedule the sequence. teamwork makes the dream work 🤝
• -mrecip, -mno-recip — This option 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. You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Note: while the throughput of the sequence is generally 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) for reciprocal square roots. she's fast but she's messy 💀
• -mrecip=opt — This option 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 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. subtract the ones you don't want, iconic ✨

• -mrecip-precision, -mno-recip-precision — Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. 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 do not provide an estimate that converges after three steps. three steps or it's not invited to the function 💅
• -mveclibabi=type — Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is mass, which specifies 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. look at that function list bestie, she ate 🔥
• -mfriz, -mno-friz — Generate (do not 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. Warning bestie: the friz instruction does not return the same value if the floating-point number is too large to fit in an integer. so like... know what you're doing 😭
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — bestie, this one is about the static chain register, aka r11. Turn it on and you generate code that loads up that static chain register whenever you're calling through a pointer on AIX and 64-bit Linux systems where a function pointer points at a 3-word descriptor that hands over the function address, the TOC value destined for register r2, and the static chain value destined for register r11. -mpointers-to-nested-functions is on by default 💖.

🚨 Real talk though: if you use -mno-pointers-to-nested-functions, you cannot call through pointers to nested functions, or through pointers to functions compiled in other languages that use the static chain. Like, at all. Not a vibe. Don't do it.

• -msave-toc-indirect, -mno-save-toc-indirect — Generate (or don't generate) code to save the TOC value into the reserved stack location in the function prologue, but only if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value isn't saved in the prologue, it gets saved just before the call through the pointer instead. -mno-save-toc-indirect is the default ✨.
• -mcompat-align-parm, -mno-compat-align-parm — Generate (or don't generate) code that passes structure parameters with a maximum alignment of 64 bits, purely for compatibility with older versions of GCC 🫶.

Okay so storytime: older versions of GCC (anything before 4.9.0) would incorrectly not align a structure parameter on a 128-bit boundary when that structure had a member needing 128-bit alignment. That's fixed in more recent versions of GCC. This option is the time machine — use it to generate code that plays nice with functions compiled by those older GCCs.

-mno-compat-align-parm is the default 💅.

• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — Generate stack protection code with the canary sitting at guard. The supported locations are global for a global canary, or tls for a per-thread canary living in the TLS block (that's the default with GNU libc version 2.4 or later) 🌸.

If you pick tls, then -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally let you specify which register to use as the base register for reading the canary, and what offset from that base register to read it at. The defaults for those two are whatever the relevant ABI says, so, iykyk 💜.

• -mpcrel, -mno-pcrel — Generate (or don't generate) pc-relative addressing. Heads up: -mpcrel requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled 👑.
• -mprefixed, -mno-prefixed — Generate (or don't generate) addressing modes that use prefixed load and store instructions. -mprefixed 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 ⭐.
• -mrop-protect, -mno-rop-protect — Generate (or don't generate) ROP protection instructions when the target processor actually supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap) 🦋.
• -mprivileged, -mno-privileged — Generate (or don't generate) code that will run in privileged state 👑.
• -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 checks in with target information to figure out whether unrolling the main vectorized loop would be beneficial, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll that main loop. The default value is four 🌈.

---

🌸 RX Options 🌸

These command-line options are defined for RX targets, bestie 💖:

• -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. And here's why — RX floating-point hardware only works on 32-bit values, which is exactly why the default is -m32bit-doubles 💅.
• -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 though, so the FPU hardware is not used for doubles if you go and use the -m64bit-doubles option. F in the chat for 64-bit doubles 😭.

⚠️ Note: if the -fpu option is enabled, then -funsafe-math-optimizations is also enabled automatically. Why? Because the RX FPU instructions are themselves unsafe, periodt 💀.

• -mcpu=name — Selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, plus the specific RX610 CPU. The default is RX600 👑.

The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction. That's it. That's the whole diff 🫶.

The RX200 series doesn't have a hardware floating-point unit, so -nofpu is enabled by default when this type is selected 🌸.

• -mbig-endian-data, -mlittle-endian-data — Store data (but not code, bestie, just data) in the big-endian format. The default is -mlittle-endian-data, i.e. storing data in the little-endian format 🦋.
• -msmall-data-limit=N — Specifies the maximum size in bytes of global and static variables that can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited, and it is up to the programmer to make sure that area doesn't overflow 🙈.

Also, when the small data area is used, one of the RX's registers (usually r13) is reserved for pointing at this area, so it's no longer available for use by the compiler. That could result in slower and/or larger code if variables get pushed onto the stack instead of chilling in this register 😭.

Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, because they're assigned to other sections in the output executable 💜.

The default value is zero, 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's up to the programmer to experiment and discover whether this feature benefits 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 ⭐.
• -mas100-syntax, -mno-as100-syntax — When generating assembler output, use a syntax 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. Even though 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. So in some circumstances it can be beneficial to restrict the size of constants used in instructions. Constants that are too big get placed into a constant pool instead and referenced via register indirection 🌈.

The value N can be between 0 and 4. A value of 0 (the default) or 4 means constants of any size are allowed 💖.

• -mrelax — Enable linker relaxation. Linker relaxation is a process where the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default ✨.
• -mint-register=N — Specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means register r13 is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves r13 and r12. A value of 3 reserves r13, r12 and r11, and a value of 4 reserves r13 through r10. A value of 0, 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, since that 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 lets the location of constant data be determined at run time without requiring the executable to be relocated — a big glow-up for 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 on 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. So it's entirely 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 while compiling a file. The default is to issue a warning for each extra fast interrupt handler found, because the RX only supports one such interrupt 💀.
• -mallow-string-insns, -mno-allow-string-insns — turns the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL, SWHILE on or off, and also the RMPA instruction, bestie. The tea here is that these instructions might prefetch data, which is not safe if you're touching an I/O register. (see section 12.2.7 of the RX62N Group User's Manual for the receipts 💀).

The default is to allow these instructions, but lowkey GCC cannot reliably detect every single circumstance where a string instruction might get used to access an I/O register, so it can't just turn them off automatically. Instead it's on you, the programmer to pass -mno-allow-string-insns if your program accesses I/O space. no cap.

When the instructions are enabled, GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__; otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__. ✨

• -mjsr, -mno-jsr — use only (or not only) JSR instructions to access functions. You'd reach for this when code size exceeds the range of BSR instructions. And heads up: -mno-jsr does not mean "never use JSR", it means any type of branch may be used. okay? okay. 🫶

📌 Note: the generic GCC command-line option -ffixed-reg has special significance to the RX port when it's used with the interrupt function attribute. That attribute flags a function meant 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. let's go 💅

• -mhard-float, -msoft-float — use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to do the floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default. ⭐
• -mhard-dfp, -mno-hard-dfp — use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used for decimal-floating-point operations. When -mhard-dfp is specified, 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. A size of 64 bits makes the long double type equivalent to the double type. This is the default.
• -mbackchain, -mno-backchain — store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that don't understand DWARF call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored 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.

In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, using the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. And ngl, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float. 🌸

The default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space gets reused for other purposes, which allows for more efficient use of the available stack space. 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. it's giving ✨optimization✨

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. But watch out: 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 purposes. That code is not call-compatible with code compiled with -mpacked-stack. Also, the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. 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 do not generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the basr instruction instead, which does not have this limitation. periodt. 💅
• -m64, -m31 — when -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, 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 the s390x targets default to -m64. 👑
• -mzarch, -mesa — when -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible 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 — the -mhtm option 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 — when -mvx is specified, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. This option changes the ABI for some vector type values with regard to alignment and calling conventions. In case vector type values are being used in an ABI-relevant context, a GAS .gnu_attribute command will be added to mark the resulting binary with the ABI used. -mvx is enabled by default when using -march=z13.
• -mzvector, -mno-zvector — the -mzvector option 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 not be expanded when requesting strict standard compliance, e.g. with -std=c99. On top of the GCC low-level builtins, -mzvector enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. In order to make use of these builtins, the header file vecintrin.h needs to be included. -mzvector is disabled by default. 🦋
• -mmvcle, -mno-mvcle — generate (or do not generate) code using the mvcle instruction to perform block moves. When -mno-mvcle is specified, use a mvc loop instead. This is the default unless optimizing for size.
• -mdebug, -mno-debug — print (or do not 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 does not 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 (does not 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 (does not 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 (does not 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. Because this is a compile-time check, it doesn't need to be a real problem when the program runs. It's intended to identify functions that most probably cause a stack overflow. It's useful to be used 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 bad idea with a limited stack size. 💀
• -mstack-guard=stack-guard, -mstack-size=stack-size — okay so if you pass these two, the S/390 back end sprinkles extra instructions into the function prologue that go 👀 trap 👀 if the stack size ends up being stack-guard bytes above stack-size (and yes bestie, remember the stack on S/390 grows downward, so like, opposite of your fave glow-up trajectory 💅). If you skip stack-guard, the compiler picks the smallest power of 2 that's bigger than the frame size of the compiled function. These options are literally made for debugging stack overflow problems, and the extra emitted code only adds a lil bit of overhead, so you can lowkey use them in production-like systems without major performance degradation ✨. The values you give have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. And for efficiency, the extra code assumes the stack starts at an address aligned to the value given by stack-size. Also stack-guard can only be used in conjunction with stack-size — they're a package deal, no solo acts 🫶.
• -mhotpatch=pre-halfwords,post-halfwords — turn this on and a "hot-patching" function prologue gets generated for all functions in the compilation unit 🔥. The function label gets prepended with the given number of two-byte NOP instructions (pre-halfwords, max 1000000), and after the label, 2 * post-halfwords bytes are appended using the largest NOP-like instructions the architecture allows (also max 1000000).

If both arguments are zero, hotpatching is disabled.

And plot twist: 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 is not 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. And ngl, 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. ⚠️ Heads up: 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 the handling of cases where the result of a comparison is unordered. 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. This 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. So like... don't 💀.
• -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. The following models and parameters are supported:
  • 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 — makes the compiler emit the tas.b opcode for __atomic_test_and_set. but bestie, heads up 🫶 depending on your particular hardware + software config this can degrade overall performance, because of the operand cache line flushes that the tas.b instruction implies. And on multi-core SH4A processors, tas.b must be used with caution since it can result in data corruption for certain cache configurations. 😭 not the vibe.
• -mprefergot — when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. iykyk ✨
• -musermode
• -mno-usermode — don't allow (allow) 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*. And if the target is SH1* or SH2*, -musermode has no effect, because there is no user mode. 💀
• -multcost=number — set the cost to assume for a multiply insn.
• -mdiv=strategy — set the division strategy used for integer division operations. strategy can be one of:
  • call-div1 — calls a library function that uses the single-step division instruction div1 to do 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, so you 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. no cap.
• -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. Super useful when compiling kernel code. A register range is specified as two registers separated by a dash. You can specify multiple register ranges 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 being compiled for.
• -mzdcbranch
• -mno-zdcbranch — assume (do not assume) that zero displacement conditional branch instructions bt and bf are fast. If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. Enabled by default when generating code for SH4 and SH4A. You can explicitly disable it with -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. You can enable it to work around hardware bugs as found in the original SH7055. 🐛
• -mfused-madd
• -mno-fused-madd — generate code that uses (does not use) 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. -mfsca must be used in combination with -funsafe-math-optimizations. It's 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. -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. 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.

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🌞 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. So instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. 🙈 do the right thing bestie

These switches are supported in addition to the above on Solaris 2:

• -pthreads — this is a synonym for -pthread. 💅

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🌈 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 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. 💀 trap handler overhead is giving "why are we like this"

• -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. 👑
• -mfaster-structs, -mno-faster-structs — with -mfaster-structs the compiler just decides that structs get 8-byte alignment, bestie. That lets it copy struct assignments using pairs of ldd and std instructions instead of literally twice as many ld/st pairs. It's giving efficiency 💅. But — and this is the part you need to hear — that changed alignment directly violates the SPARC ABI. So it's only meant for targets where the dev is fully aware that the code they get out is not in line with the rules of the ABI. Not a vibe you want by accident, fr fr. 🙈
• -mstd-struct-return, -mno-std-struct-return — with -mstd-struct-return, the compiler drops checking code into functions that return structures or unions, so it can spot size mismatches between the two sides of a function call, per the 32-bit ABI.

The default is -mno-std-struct-return. Also heads up: this option does nothing in 64-bit mode. 🦋

• -mcpu=cpu_type — sets 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 picks the best architecture option for the host processor. -mcpu=native has no effect if GCC doesn't recognize the processor. 😭

Default instruction scheduling parameters get used for values that select an architecture and not an implementation. Those are v7, v8, sparclite, sparclet, v9.

Okay so here's the full roster of each supported architecture and the implementations it supports, bestie 👑

• 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 it was 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. That's also the appropriate pick 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 — the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, 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. 🔥

• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set the way -mcpu=cpu_type does.

You can use the same values for -mcpu=cpu_type here with -mtune=cpu_type, but the only ones that are actually useful 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. 🧚

• -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. 🌈
• -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 — enables the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
• -mfix-ut699 — enables the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor. 🛠️
• -mfix-ut700 — enables the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — enables the documented workaround for the back-to-back store errata of the GR712RC processor.

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Okay so — these -m options are supported in addition to the above, on SPARC-V9 processors in 64-bit environments 👑

• -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 these, bestie:
  • 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. 🦋
• embmedany — the Medium/Anywhere code model for embedded systems 💫 64-bit addresses, and both the text and data segments have to be under 2GB in size, and they can each start anywhere in memory (this gets figured out at link time, bestie). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported. 🫶
• -mmemory-model=mem-model — sets the memory model in force on the processor. Pick one of these cuties:
  • default — the default memory model for the processor and operating system.
  • rmo — Relaxed Memory Order.
  • pso — Partial Store Order.
  • tso — Total Store Order.
  • sc — Sequential Consistency.

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's PSTATE.MM field. 📚

• -mstack-bias, -mno-stack-bias — with -mstack-bias, GCC assumes the stack pointer, and the frame pointer if there is one, are offset by -2047, and that offset has to be added back when making stack frame references. This is the default in 64-bit mode ✨. Otherwise, assume no such offset is present.

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💅 Options for System V 💅

These extra 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. (iykyk, the vibes are just better that way 💁)
• -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).
• -YP,dirs — search the directories dirs, and no others, for libraries specified with -l.
• -Ym,dir — look in the directory dir to find the M4 preprocessor. The assembler uses this option.

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🌸 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. 🎯
• -mno-ep, -mep — do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times, to copy the pointer into the ep register and use the shorter sld and sst instructions. The -mep option is on by default if you optimize. ✨
• -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 they 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.
• -mspace — try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options. 🐜
• -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).
• -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.
• -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. 💯

• -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.
• -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. 🔥
• -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. 🙈
• -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. 👑

• -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.
• -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. ⚠️
• -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.

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🦋 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. 🧪

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⭐ 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. 🐛
• -msim — a program which performs file I/O and is destined to run on the simulator should be linked with 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. > bestie this is a whole-or-nothing situation, no half-measures 💀

• -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. 🌈

• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but it does not set the instruction set or register set that -mcpu=cpu_type would. so it's like giving the code a glow-up without changing what it's even allowed to do 💅
• -msv-mode — generate code for supervisor mode, where there are no restrictions on access to general registers. this is the default, bestie, you're already here 👑
• -muser-mode — generate 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. some registers are just in their "do not perceive me" era 🙈

---

💖 VMS Options 💖

these -m options are defined for the VMS implementations:

• -mvms-return-codes — return VMS condition codes from main. the default is to return POSIX-style condition (e.g. error) codes. so if you want the old-school VMS behavior, you gotta ask for it ✨
• -mdebug-main=prefix — flag the first routine whose name starts with prefix as the main routine for the debugger. it's giving "hey debugger, look here first" 🫶
• -mmalloc64 — default to 64-bit memory allocation routines. big allocations, big energy 💜
• -mpointer-size=size — set the default size of pointers. possible options for size are:
  • 32 or short — for 32 bit pointers
  • 64 or long — for 64 bit pointers
  • no — for supporting only 32 bit pointers

that last option disables pragma pointer_size. so no take-backs, no growth spurts, just 32 bits forever 🔥

---

🌸 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.

• -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__. switching lanes, no cap 🦋
• -msmp — select 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 before you use this one 😭
• -non-static — link 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 are defined for compatibility with Diab. just relaying the message, fr fr 📬
• -Xbind-lazy — enable lazy binding of function calls. this option is equivalent to -Wl,-z,now and is defined for compatibility with Diab. yes, "lazy" enabling "now", the naming is a whole situation 💀
• -Xbind-now — disable 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.

• -march=cpu-type — generate 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.

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: "this machine, specifically" 💅
• 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. so you can have both, you just have to ask nicely ✨

• i386 — original Intel i386 CPU.
• i486 — Intel i486 CPU. (no scheduling is implemented for this chip.) it's giving vintage, respect the elders 🎀
• 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.
• 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. the notebook girlies know 🌸
• 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 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, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support. okay this list is genuinely never ending and I respect it 💅
• rocketlake — Intel Rocket 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, 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 — Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont 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, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support.

bestie yes those four names all point at the same CPU description, that's just how they did it, I don't make the rules 🫶

• arrowlake — Intel Arrow Lake 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, 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, 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, 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, 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, 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, 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.

ok side note, AVX512-FP16 versus AVX512FP16 up there — yes those are two different spellings in the original and no I am not "fixing" them, that's not my job, my job is vibes ✨

• diamondrapids — Intel Diamond Rapids CPU with 64-bit extensions, 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.

and yeah MOVBE shows up twice in that one. I know. I see it. We're leaving it. 💀

---

• bonnell
• atom — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
• silvermont
• slm — Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support.
• 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.
• 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.
• 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.

notice how sierraforest and grandridge are literally the same list. besties. twins. matching outfits. obsessed. 👑

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• k6 — AMD K6 CPU with MMX instruction set support.
• 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.
• 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.)

also yes, 3dNOW! in one place and 3DNow! in another. lowercase d. capital D. both canon. we move. 🦋

• k8-sse3, opteron-sse3, athlon64-sse3 — glow-up versions of the AMD K8 cores 💅 but now with SSE3 instruction set support. Same energy, extra spice.
• amdfam10, barcelona — CPUs based 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 based 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.)
• 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.) Zen is that girl, no notes.
• 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.) okay the AVX-512 wall of text?? iconic actually 💀
• 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 based 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 based 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. (notice it says "includes", bestie — different vibe from "supersets", that's not a typo 💅)
• winchip-c6 — IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
• winchip2 — IDT WinChip 2 CPU, dealt in 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. heads up bestie, that's not a bug, that's on purpose 👀
• 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 is the vibe knob, not the what-are-you-allowed-to-say knob 💅

Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. So like: while picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. Fr fr, here's the 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. so it's giving Pentium 4 energy but it's still invited to the i686 party ✨

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

• generic — Produce 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.

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. (translation: it's a moving target, bestie, don't @ me about it later 😭)

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 — Produce 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. same deal as generic — this one shifts under your feet when you update GCC 🌸

Okay so real talk: there is no -march=intel option, bestie, and here's why — -march is telling the compiler which instruction set it's allowed to use, and there just isn't one common instruction set that works on every single processor. Whereas -mtune is about which processor (or, in this case, which little collection of processors) the code gets optimized for. Different jobs, different vibes. 💅

---

• -mcpu=cpu-type — a deprecated synonym for -mtune. It's giving "we don't do that anymore," so like… don't. 🙈
• -mfpmath=unit — makes floating-point arithmetic happen for the chosen unit. The choices for unit are:
  • 387 — uses the standard 387 floating-point coprocessor that's chilling on the majority of chips, and gets emulated everywhere else. Code compiled with this runs pretty much anywhere, iconic. The catch? Temporary results get computed in 80-bit precision instead of whatever precision the type actually specifies, so you get slightly different results compared to most other chips. Want the full tea on that, go read -ffloat-store.

This is the default choice for non-Darwin x86-32 targets.

• sse — uses the scalar floating-point instructions hanging out in the SSE instruction set. SSE 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 gets done with 387. A later version, only present in Pentium 4 and AMD x86-64 chips, supports double-precision too. 🦋

For the x86-32 compiler you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option actually do something. For the x86-64 compiler, those extensions are already enabled by default. No cap.

The resulting code should be considerably faster in the majority of cases and it 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, 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 — tries 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, bestie, because it's still experimental — the GCC register allocator does not model separate functional units well, so you get unstable performance. It's giving chaos. 💀

• -masm=dialect — outputs assembly instructions using the selected dialect. It also affects which dialect is 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.
• -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 — generates output containing 80387 instructions for floating point.
• -mno-80387
• -msoft-float — generates 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 have to 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 still get emitted even if -msoft-float is used.

• -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 at all. The idea being that the operating system should emulate an FPU.

The option -mno-fp-ret-in-387 makes those values get 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, meaning the instruction doesn't need emulation. Oh and heads up: these instructions are not generated unless you also use the -funsafe-math-optimizations switch. ✨
• -malign-double
• -mno-align-double — controls 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. Trade-offs, bestie.

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. 🚨

• -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.

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, and also modifies the function calling convention for functions taking long double. So 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. This is the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 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, and also modifies the function calling convention for functions taking long double. So they are not binary-compatible with code compiled without that switch. (Yes, same warning, same energy. 🫶)

• -malign-data=type — controls how GCC aligns variables. Supported values for type are compat, which uses increased alignment value compatible with GCC 4.8 and earlier; abi, which uses alignment value as specified by the psABI; and cacheline, which uses 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, where 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! 💅

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 can't 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 gets 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.)

• -mregparm=num — controls how many registers get 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. All of them. Every single one. No exceptions. 🚨

• -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. Same deal as above, bestie.

• -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. It's giving legacy support. 🙈
• -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. Periodt. 👑

okay so cranking the rounding of floating-point operations down to less than the default 80 bits can legitimately speed some programs up by 2% or more — free speed, right? 💅 But bestie, hear me out: some mathematical libraries literally assume that extended-precision (80-bit) floating-point operations are turned on by default. So routines in those libraries could suffer a significant loss of accuracy — usually through the whole "catastrophic cancellation" thing 😭 — the moment you use this option to set precision below extended precision.

---

• -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 — scalar and vector — can benefit from having FTZ and DAZ enabled when you pass -mdaz-ftz. The catch: don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is specified, and heads up that -mdaz-ftz will set the FTZ/DAZ flags even with -shared. 🙈
• -mstackrealign — realign the stack at entry, periodt. On x86, -mstackrealign generates an alternate prologue and epilogue that realigns the run-time stack if it needs to. This is what lets you mix legacy code keeping 4-byte stack alignment with modern code keeping 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, which you can apply to individual functions. ✨
• -mpreferred-stack-boundary=num — try to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary isn't specified, the default is 4 (16 bytes or 128 bits).

⚠️ Warning: when generating code for x86-64 with SSE extensions disabled, you can use -mpreferred-stack-boundary=3 to keep the stack boundary aligned to an 8-byte boundary. But since the x86-64 ABI requires 16-byte stack alignment, this is ABI incompatible and only meant for a controlled environment where stack space is a serious limitation. This option leads to wrong code when functions compiled with 16-byte stack alignment (like functions from a standard library) get called with a misaligned stack. In that case, SSE instructions may lead to misaligned memory access traps. On top of that, variable arguments are handled incorrectly for 16-byte aligned objects (including x87 long double and __int128), which gives you wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries — that 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 isn't specified, the one specified by -mpreferred-stack-boundary is used instead.

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or they'll suffer significant run time performance penalties 😭. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it isn't 16-byte aligned.

To make sure these values are properly aligned on the stack, the stack boundary has to be as aligned as whatever value is required by anything stored on the stack. And every function must be generated so it keeps the stack aligned. So 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. Recommendation: libraries that use callbacks should always use the default setting. 🫶

This extra alignment does eat extra stack space and generally increases code size. Code that's sensitive to stack space usage — embedded systems, operating system kernels, that whole vibe — may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

• the giant instruction-set squad 👑 — these switches all work the same way, so here's the full lineup, verbatim:

-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 switches enable the use of instructions in the 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 one has a matching -mno- option to disable use of those instructions. 🎀

These extensions are also available as built-in functions: see x86 Built-in Functions for details on 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, which is controlled separately by -msse4a. iykyk 💅

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. 🫶

• -mdump-tune-features — tells GCC to dump the names of the x86 performance tuning features and default settings. Those names are what you feed into -mtune-ctrl=feature-list. 👑
• -mtune-ctrl=feature-list — fine grain control over 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 isn't preceded with ^, otherwise it's 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. 💀

• -mno-default — tells GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features.
• -mcld — tells GCC to emit a cld instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to pick between autoincrement or autodecrement mode. While the ABI specifies the DF flag should be cleared on function entry, some operating systems violate this spec by not clearing the DF flag in their exception dispatchers 😭. The exception handler can get 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 that case.
• -mvzeroupper — tells GCC to emit a vzeroupper instruction before a transfer of control flow out of the function, to minimize the AVX to SSE transition penalty and also remove unnecessary zeroupper intrinsics. ✨
• -mprefer-avx128 — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
• -mprefer-vector-width=opt — tells GCC to use an opt-bit vector width in instructions instead of the default on the selected platform.
• -mpartial-vector-fp-math — lets GCC generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements live 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 so they 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 the denormals-are-zero (DAZ) flag in the MXCSR register. 🦋

This option is enabled by default.

• -mmove-max=bits — tells GCC to set the maximum number of bits that can be moved from memory to memory efficiently to bits. The valid bits values are 128, 256 and 512. 🎀
• -mstore-max=bits — bestie, this one tells GCC the max number of bits it can efficiently store to memory, and you set that number to bits. The only valid bits are 128, 256 and 512 — no vibes-based in-between values, okay? 👑
  • none — literally no extra limitations on GCC beyond whatever the selected platform already imposes.
  • 128 — prefer a 128-bit vector width for instructions.
  • 256 — prefer a 256-bit vector width for instructions.
  • 512 — prefer a 512-bit vector width for instructions.
• -mnoreturn-no-callee-saved-registers — this one optimizes functions that have the noreturn attribute or the _Noreturn specifier by not saving callee-saved registers in the function prologue, even the ones used inside the function — except for the BP register, which still gets saved. Sounds iconic, but here's the tea: it can mess with debugging of the caller of the noreturn function, or any function further up the call stack. That's why it is not enabled by default. ⚠️
• -mcx16 — lets GCC generate CMPXCHG16B instructions in 64-bit code, so it can do compare-and-exchange on 16-byte aligned 128-bit objects. Perfect for atomic updates of data structures that are bigger than one machine word. The compiler uses this instruction to implement __sync Builtins. But lowkey important: for __atomic Builtins operating on 128-bit integers, a library call is always used instead. 💅
• -msahf — enables generation of SAHF instructions in 64-bit code. Story time: early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step dropped in December 2005, were missing the LAHF and SAHF instructions that AMD64 supports. Those are load and store instructions (respectively) for certain status flags. In 64-bit mode, SAHF is used to optimize the fmod, drem, and remainder built-in functions — see Other Builtins for details.
• -mmovbe — enables use of the movbe instruction to optimize byte swapping of four- and eight-byte entities. ✨
• -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). slay.
• -mcrc32 — this enables the built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di so they generate the crc32 machine instruction.
• -mmwait — enables the built-in functions __builtin_ia32_monitor and __builtin_ia32_mwait so they generate the monitor and mwait machine instructions.
• -mrecip — okay so this one lets GCC use RCPSS and RSQRTSS instructions (plus their vectorized besties RCPPS and RSQRTPS) with an extra Newton-Raphson step to boost precision, instead of using DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments. But! These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. And yeah, the throughput of the sequence is higher than the non-reciprocal instruction's throughput, but the precision can drop by up to 2 ulp — like, the inverse of 1.0 equals 0.99999994. 💀

Also, note that GCC already implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) with -ffast-math (or that option combination above), and it doesn't need -mrecip.

And also note: GCC emits that sequence with an additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the option combination above), and again, doesn't need -mrecip.

• -mrecip=opt — this controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, and any of them can be preceded by a ! 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. cute, right? 🎀

• -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). 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.

When -mveclibabi=svml is used, GCC currently emits calls to these for the corresponding function type:

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

When -mveclibabi=aocl is used, it emits these for the corresponding function type:

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

And when -mveclibabi=acml is used, it emits these for the corresponding function type:

__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

• -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? It uses the Microsoft ABI when targeting Microsoft Windows, and the SysV ABI on literally every other system. You can control this per-function using the function attributes ms_abi and sysv_abi. 🌈
• -mforce-indirect-call — forces all calls to functions to be indirect. This is useful with Intel Processor Trace, where it generates more precise timing information for function calls. ✨
• -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.
• -mcet-switch — by default, CET instrumentation is turned off on switch statements that use a jump table, and indirect branch track is disabled. Now, since jump tables live in read-only memory, that doesn't cause 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 CET instrumentation on to enable indirect branch track for switch statements with jump tables, which leads to the jump targets being reachable via any indirect jumps. 🔥
• -mcall-ms2sysv-xlogues — because of differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function has to treat RSI, RDI and XMM6-15 as clobbered. By default the code for saving and restoring those registers is emitted inline, which makes for pretty lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to do those saves and restores instead — so function size goes down, at the cost of a few extra instructions. trade-offs, bestie 🫶
• -mtls-dialect=type — generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that can't be satisfied on all systems. ⚠️
• -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually just as 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.
• -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 because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary isn't equal to 2. The drawback? A notable increase in code size. Heads up: this switch implies -mno-push-args. 💀
• -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. 💖
• -mms-bitfields, -mno-ms-bitfields — turn on/off the bit-field layout that plays nice with the native Microsoft Windows compiler 💖

Okay so here's the tea: if packed is used on a struct, or if bit-fields are in the mix, the Microsoft ABI might lay that structure out differently than GCC's usual way. And that matters big time when you're passing packed data between functions compiled with GCC and the native Microsoft compiler — whether that's via a function call or just chillin' as data in a file — because then you might need to read it in either format.

This option is on by default for Microsoft Windows targets. You can also control this behavior locally with variable or type attributes — see x86 Variable Attributes and x86 Type Attributes for the deets.

The Microsoft structure layout algorithm is pretty simple ngl, except for the bit-field packing part. The padding and alignment of members, plus whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

1. Structure members are stored sequentially in the order they're declared: the first member gets the lowest memory address, and the last member gets the highest.
2. Every data object has an alignment requirement. For everything except structures, unions, and arrays, the alignment requirement is either the size of the object or the current packing size (set 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 gets an offset such 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 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 like this:

1. If a zero-length bit-field is inserted between two bit-fields that would normally be coalesced, they don't get coalesced. Example bestie:

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 you removed that zero-length bit-field, t1's size would be 4 bytes. Wild, right? 😭

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, then bar is aligned as the type of the zero-length bit-field. Like so:

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, not offset 1. So the size of t2 is 4. For t3, the zero-length bit-field doesn't affect the alignment of bar or, as a result, the size of the structure.

Taking all this into account, 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 isn't 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.

• -mno-align-stringops — don't align the destination of inlined string operations ✨ this switch cuts down code size and boosts performance when the destination is already aligned, but GCC just doesn't know that. Rude of it honestly.
• -minline-all-stringops — by default GCC only inlines string operations when the destination is known to be aligned to at least a 4-byte boundary. This turns up the inlining and grows your code size, but it may improve performance for code that leans on fast memcpy and memset for short lengths. This option also enables inline expansion of strlen for all pointer alignments. 💅
• -minline-stringops-dynamically — for string operations of unknown size, it uses run-time checks: inline code for small blocks, and a library call for large blocks.
• -mstringop-strategy=alg — override the internal decision heuristic for which algorithm to use when inlining string operations. The allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte

Expand using the 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 for whether __builtin_memcpy should be inlined, and which 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, and 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.
• -mmemset-strategy=strategy — just like -mmemcpy-strategy= except it controls __builtin_memset expansion. 🎀
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions 🦋 this dodges the instructions to save, set up, and restore frame pointers, and frees up an extra register in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions too, which might make debugging harder — so heads up.
• -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 has to be added. Whether this is even valid depends on the operating system and whether it maps the segment to cover the entire TLS area.

For systems using the GNU C Library, the default is on.

• -msse2avx, -mno-sse2avx — says the assembler should encode SSE instructions with the 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. 🙈
• -mrecord-mcount, -mno-record-mcount — if profiling is active (-pg), generate a __mcount_loc section containing pointers to each profiling call. Super 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. 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 a call to the specified function. This only instruments true returns ending with ret, not sibling calls ending with jump. Valid types are none to not instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop.
• -mrecord-return, -mno-record-return — generate a __return_loc section pointing to all return instrumentation code.
• -mfentry-name=name — set the name of the __fentry__ symbol called at function entry for -pg -mfentry functions.
• -mfentry-section=name — set the name of the section used 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 the RAX register when there are no variable arguments passed in vector registers.

Warning: since the RAX register is used to avoid unnecessarily saving vector registers on the stack when passing variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location 😭 GCC 4.4 or newer doesn't have those issues, regardless of the RAX register value.

• -m8bit-idiv, -mno-8bit-idiv — on some processors, like Intel Atom, 8-bit unsigned integer divide is way faster than 32-bit/64-bit integer divide. This option generates a run-time check: if both the 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. Iconic optimization behavior honestly.
• -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 a canary at guard. Supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (the default). This option only does anything when -fstack-protector or -fstack-protector-all is specified. 🛡️

With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset get even more specific and tell you which segment register (%fs or %gs) to use as the base register for reading the canary, and from what offset off that base register. The defaults for those two? They're whatever the relevant ABI says they are. 💅

• -mstack-protector-guard-symbol=symbol — overrides the offset with a symbol reference to a canary sitting in the TLS block. 🎀
• -mgeneral-regs-only — generates code that uses only the general-purpose registers. This stops the compiler from touching floating-point, vector, mask and bound registers. It's giving minimalism, fr fr. ✨
• -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 by issuing an atomic load before the CMPXCHG instruction, and using the PAUSE instruction to save CPU power when restarting the loop. Bestie said "let's not cook the CPU" 😭🔥
• -mindirect-branch=choice — converts indirect call and jump with choice. 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, because the thunk function may not be reachable in the large code model. 💀

Also: -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check. 🫶

• -mfunction-return=choice — 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 that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check. 🌈

And note that -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. No detours, we're going direct. 💅
• -mharden-sls=choice — generates code to mitigate against straight line speculation (SLS) with choice. The default is none, which disables all SLS hardening. return enables SLS hardening for function returns. indirect-jmp enables SLS hardening for indirect jumps. all enables all SLS hardening. Pick your fighter. 👑
• -mindirect-branch-cs-prefix — adds a 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, letting them be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time. Sneaky little byte-counting moment, we love to see it. 🧚
• -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 from 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. You've been warned, bestie. 🙈
• -mevex512 — enables 512-bit vector. It will be default on if AVX512F is enabled.
• -mno-evex512 — disables 512-bit vector. Same default story: it will be default on if AVX512F is enabled.

---

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. Okay so this next batch is the 64-bit girlies 💖

• -m32 — generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.
• -m64 — the -m64 option 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 — the -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.
• -m16 — the -m16 option is the same as -m32, except for 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 — the -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on.
• -mno-red-zone — do not use a so-called "red zone" for x86-64 code. 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. It's giving "we don't do shortcuts here". 💀
• -mcmodel=small — generate 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. The main character of code models ✨
• -mcmodel=kernel — generate 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. Kernel girlies, this one's for you 👑
• -mcmodel=medium — generate 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.
• -mcmodel=large — generate code for the large model. This model makes no assumptions about addresses and sizes of sections. Chaotic neutral energy, lowkey iconic 🌸
• -maddress-mode=long — generate 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 — generate 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 — emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary.
• -mno-needed — the off switch for the above. ✨
• -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. Bestie. Do not play with this one casually. 😭

• -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. Respecting your boundaries, we love that 🫶
• -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. Metadata girlies rise up 💜

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x86 Windows Options

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Xstormy16 Options

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. Simulator core, no notes. 🦋

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Xtensa Options

These options are supported for Xtensa targets:

• -mconst16 — enable use of CONST16 instructions for loading constant values.
• -mno-const16 — 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 — enable use of fused multiply/add and multiply/subtract instructions in the floating-point option.
• -mno-fused-madd — 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. Numbers girlies, this one's for you 🔥
• -mserialize-volatile — when this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile.
• -mno-serialize-volatile — use this to omit the MEMW instructions. ✨
• -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. Rules are made to be broken (for kernel code specifically) 😌
• -mtext-section-literals — with this, 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.
• -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. So like… the default is the tidy one, and the other one is the "keep your friends close" one. 🎀
• -mauto-litpools, -mno-auto-litpools — bestie, these two control how we treat literal pools. The default is -mno-auto-litpools, which drops the literals into a separate section in the output file, unless you're also using -mtext-section-literals. But with -mauto-litpools the literals get interspersed into the text section by the assembler instead. The compiler doesn't spit out explicit .literal directives here, and it loads literals into registers using MOVI instructions instead of L32R, which lets the assembler do relaxation and place those literals wherever it needs them. The way this option lets the assembler create several literal pools per function and assemble very big functions? Iconic. That might not even be possible with -mtext-section-literals. ✨
• -mtarget-align, -mno-target-align — turn this on and GCC tells the assembler to automatically align instructions to cut down branch penalties, at the cost of some code density. The assembler tries to widen density instructions to align branch targets and the instructions that follow call instructions. If there aren't enough preceding safe density instructions to align a target, then no widening happens, periodt. Default is -mtarget-align. Heads up though: these options don't touch auto-aligned instructions like LOOP, because the assembler always aligns those — either by widening density instructions or by inserting NOP instructions. 💅
• -mlongcalls, -mno-longcalls — enable this and GCC instructs the assembler to translate direct calls into indirect calls, unless it can figure out that the target of a direct call is within the range the call instruction allows. This translation usually kicks in for calls to functions in other source files. Specifically, the assembler turns a direct CALL instruction into an L32R followed by a CALLX instruction. The default is -mno-longcalls. Use this one in programs where the call target could potentially be out of range. Ngl, this is implemented in the assembler, not the compiler, so the assembly code GCC generates still shows direct call instructions — go peek at the disassembled object code to see the actual instructions. And note: the assembler uses an indirect call for every cross-file call, not just the ones that are really out of range. 😭
• -mabi=name — generate code for the specified ABI. Permissible values are call0 and windowed. The default ABI is chosen by the Xtensa core configuration. 🎀
• -mabi=call0 — turn this on and function parameters are 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. Slay. 🌸
• -mabi=windowed — turn this on and 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. The register window gets rotated 8 registers back upon return. 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, measured 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 is the move. 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. Vibe check passed. ✅

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👑 zSeries Options 👑

These are listed under

💖 ENVIRONMENT 💖

okay bestie, welcome to the vibes layer ✨ this whole section is about the environment variables that change how GCC does its thing. some of them are just like "here's a directory to look in", some are "here's a prefix, go wild", and some are lowkey about the whole compilation environment. it's giving backstage passes 🎀

Note that you can also specify places to search using options such as -B, -I and -L. and those take precedence over places specified using environment variables — which in turn take precedence over those specified by the configuration of GCC. so the order of the group chat is: options first, env vars second, built-in config last. periodt 💅

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🌍 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL 🌍

These environment variables control the way that GCC uses localization information, 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. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8 🫶

• LC_CTYPE — specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape. it's literally doing char-by-char detective work 🔍
• LC_MESSAGES — specifies the language to use in diagnostic messages. aka which language your errors yell at you in 😭

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. If none of these variables are set, GCC defaults to traditional C English behavior. so LC_ALL is the main character here 👑

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🗂️ 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 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 a lil relay race and the baton is a temp file 🏃‍♀️

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🐛 GCC_COMPARE_DEBUG 🐛

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of this option for more details. iykyk 💅

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🛣️ 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. you do you boo 🌸

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. Thus, 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. it's giving swap-the-prefix sleight of hand 🎩✨

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🔎 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. bestie said "here's my backup list" 📋

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📚 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). so -L is the VIP entrance and LIBRARY_PATH is general admission 💅

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🈺 LANG (yes, again, bestie) 🈺

This variable is used to pass locale information to the compiler. One way in which this information is 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. default behavior ate, no notes 💀

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🩹 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. -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.

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Some additional environment variables affect the behavior of the preprocessor. 🧚

🧵 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. iconic behavior honestly ✨

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📝 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. combo meal energy 🍓

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☀️ 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. okay so it's the same but the drama is dialed up 😭

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⏰ 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. no more "why does my binary say it was built in 2019" 😭

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. fr fr, set it properly 🫶

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💀 BUGS 💀

For instructions on reporting bugs, see <https://bugzilla.redhat.com/>. go tell them everything bestie, they wanna hear it 🎀

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🦋 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.

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👀 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. the whole friend group, all linked 💜

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👑 AUTHOR 👑

See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. shoutout to the contributors, they really carried 🌈

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📜 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.

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and that's the whole manpage bestie, you made it to the end 🎉 this was gcc-15.2.0, dated 2025-08-08 — go forth and compile something iconic 💅✨