GCC(1)

👑 GCC(1) 👑

okay bestie, settle in 💖 this is the manual page for gcc, the GNU project C and C++ compiler, and it is long. grab a snack 🍓, hydrate 💧, and let's get into it.

---

🌸 NAME 🌸

gcc — GNU project C and C++ compiler ✨

---

✨ SYNOPSIS ✨

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

okay so real talk: only the most useful options are listed up there 👆 see below for the remainder. and g++ accepts mostly the same options as gcc, so no stress 🫶

---

💅 DESCRIPTION 💅

okay so when you invoke GCC, it normally does preprocessing, compilation, assembly and linking — the whole pipeline, the full main-character arc 👑. The "overall options" let you stop this process at an intermediate stage, like hitting pause mid-glow-up. For example, the -c option says do not run the linker. Then the output consists of object files output by the assembler. iconic behavior fr fr.

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. lowkey you'll probably never touch them and that's fine 💅.

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. no gatekeeping here 🌈.

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. it's giving "same energy, different name" 😌.

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. do not merge them, I am begging 🙈.

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. so like… vibes are flexible, but -L and -l are keeping receipts 🧾.

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. so if you're looking for the other one, just flip it, bestie 🔄.

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. yes this is the unit-lore section, we love a standards moment 📏✨

---

🎀 OPTIONS 🎀

🌈 Option Summary 🌈

okay so here's the whole menu — a summary of all the options, grouped by type. Explanations are in the following sections, so don't panic, we're getting there 💖

💫 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 -foffload=arg  -foffload-options=arg -fopenacc
-fopenacc-dim=geom -fopenmp  -fopenmp-simd
-fopenmp-target-simd-clone[=device-type]
-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
-fargs-in-order=n  -fchar8_t  -fcheck-new -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-ts -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 -fno-rtti
-fsized-deallocation -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-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-id-cdtor -Wno-terminate  -Wno-vexing-parse
-Wvirtual-inheritance -Wno-virtual-move-assign  -Wvolatile
-Wzero-as-null-pointer-constant

💫 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

💫 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]
-fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe
-fno-diagnostics-show-rule -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]

🦋 and that's just the summary bestie — the explanations are coming in the following sections, so stay tuned 💜

✨ Warning Options ✨

bestie. sit down. hydrate. because this is the main character list of the whole manpage and it is long. these are all the flags that make gcc 👀 look at your code and go "girl…" — every single one of them, no skips, no shortcuts, we are doing this properly 💅

here's the full lineup:

• -fsyntax-only
• -fmax-errors=n
• -Wpedantic
• -pedantic-errors
• -fpermissive
• -w
• -Wextra
• -Wall
• -Wabi=n
• -Waddress
• -Wno-address-of-packed-member
• -Waggregate-return
• -Walloc-size
• -Walloc-size-larger-than=byte-size
• -Walloc-zero
• -Walloca
• -Walloca-larger-than=byte-size
• -Wno-aggressive-loop-optimizations
• -Warith-conversion
• -Warray-bounds
• -Warray-bounds=n
• -Warray-compare
• -Warray-parameter
• -Warray-parameter=n
• -Wno-attributes
• -Wattribute-alias=n
• -Wno-attribute-alias
• -Wno-attribute-warning
• -Wbidi-chars=[none|unpaired|any|ucn]
• -Wbool-compare
• -Wbool-operation
• -Wno-builtin-declaration-mismatch
• -Wno-builtin-macro-redefined
• -Wc90-c99-compat
• -Wc99-c11-compat
• -Wc11-c23-compat
• -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
• -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
• -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
• -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-prio-ctor-dtor
• -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=[pure|const|noreturn|format|malloc]
• -Wswitch
• -Wno-switch-bool
• -Wswitch-default
• -Wswitch-enum
• -Wno-switch-outside-range
• -Wno-switch-unreachable
• -Wsync-nand
• -Wsystem-headers
• -Wtautological-compare
• -Wtrampolines
• -Wtrigraphs
• -Wtrivial-auto-var-init
• -Wno-tsan
• -Wtype-limits
• -Wundef
• -Wuninitialized
• -Wunknown-pragmas
• -Wunsuffixed-float-constants
• -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-length-bounds

---

🔬 Static Analyzer Options 🔬

okay so this next group is the detective squad 🕵️♀️ — the -fanalyzer family and all its little siblings. they go digging through your code for the drama, and there's a whole bunch of -Wno-analyzer-* flags for when you're like "no bestie, I know, I know, let it go 🙈". every one of them, coming right up:

• -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-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 — C and Objective-C only, no C++ invited to this party 🎀. if you're a C girlie these are your besties:

• -Wbad-function-cast
• -Wmissing-declarations
• -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 🐛

last group for this part, bestie — the debug era ✨. this is where you tell gcc to bake all that juicy debug info into your binary so your debugger can actually see what's going on, and it's giving very much "let me tell you everything" energy. all of them, in order:

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

✨ Optimization Options ✨

okay bestie, deep breath 🫶 this is the big one — every single knob GCC has for making your code faster, smaller, or frankly just weirder. No descriptions on this roster, just the whole squad lined up. Let's read 'em out. 💅

• -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-stack-alignment
• -fipa-icf
• -fira-algorithm=algorithm
• -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
• -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-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-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, no cap 😭💖 it's giving infinite scroll fr fr.

---

💖 Program Instrumentation Options 💖

okay so this cluster is all about watching what your program actually does — profiling, coverage, sanitizers, hardening, stack stuff. Very "main character who reads her own logs" energy. ✨

• -p
• -pg
• -fprofile-arcs
• --coverage
• -ftest-coverage
• -fcondition-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=[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 era 💅 all the #define-adjacent behavior, macro tracing, and the flags that decide which flavor of C your headers live in. it's giving "I read the include guards for fun".

• -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
• -flarge-source-files
• -fmacro-prefix-map=old=new
• -fmax-include-depth=depth
• -fno-canonical-system-headers
• -fpch-deps
• -fpch-preprocess
• -fpreprocessed
• -ftabstop=width
• -ftrack-macro-expansion
• -fwide-exec-charset=charset
• -fworking-directory
• -H
• -imacros file
• -include file
• -M
• -MD
• -MF
• -MG
• -MM
• -MMD
• -MP
• -MQ
• -MT
• -Mno-modules
• -no-integrated-cpp
• -P
• -pthread
• -remap
• -traditional
• -traditional-cpp
• -trigraphs
• -Umacro
• -undef
• -Wp,option
• -Xpreprocessor option

---

🎀 Assembler Options 🎀

short and sweet, two of them, icons only. 👑

• -Wa,option
• -Xassembler option

---

🌈 Linker Options 🌈

the linking era — where all your little .o girlies finally meet and become one executable. plus a whole lotta static/shared drama. 💅

• 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 your headers and libs, bestie? this is the whole search-path mood board. 🗺️

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

---

🌸 Code Generation Options 🌸

the "how exactly do I want the machine code to look" section 💜 registers, structs, PIC/PIE, visibility, all of it.

• -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

---

💜 Developer Options 💜

and finally, the deep lore — the debug, dump, report and "I am literally a GCC developer" flags. if you're here, you're not like the other girls, you're compiling GCC itself. 😭👑

• -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, buckle UP, because this is the part where every chip family on earth gets its own lil menu of flags. no cap, this is the most "we contain multitudes" section of the whole page. one bullet per option, exactly like the source, because we do not skip. 💅

---

🧚 AArch64 Options

• -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
• -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

---

🌸 Adapteva Epiphany Options

• -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

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

---

✨ ARC Options

• -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

• -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

• -mmcu=mcu
• -mabsdata
• -maccumulate-args
• -mbranch-cost=cost
• -mfuse-add=level
• -mcall-prologues
• -mgas-isr-prologues
• -mint8
• -mflmap
• -mdouble=bits
• -mlong-double=bits
• -mn_flash=size
• -mno-interrupts
• -mmain-is-OS_task
• -mrelax
• -mrmw
• -mstrict-X
• -mtiny-stack
• -mrodata-in-ram
• -mfract-convert-truncate
• -mshort-calls
• -mskip-bug
• -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

• -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

---

🍎 Darwin Options

• -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

bestie i need you to know that -sectobjectsymbols, -segs_read_only_addr and -segs_read_write_addr each show up twice in the source list. i'm not making the rules, i'm just the messenger 💀 keeping every single one, in order, like a good girl.

---

🔥 DEC Alpha Options

• -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
• -mfloat-vax
• -mfloat-ieee
• -mexplicit-relocs
• -msmall-data
• -mlarge-data
• -msmall-text
• -mlarge-text
• -mmemory-latency=time

---

🐝 eBPF Options

• -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

• -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

• -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 bestie, LoongArch time — it's giving very-now architecture energy, and here's every knob it's got:

• -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
• -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
• -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

---

💜 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

---

🍓 M32C Options 🍓

• -mcpu=cpu
• -msim
• -memregs=number

---

🔥 M680x0 Options 🔥

the retro-motorola section, no cap, absolute legends:

• -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 👑

the iconic one, the classic, the way it ate every SGI-era lunch 💅 buckle up, there's like a million:

• -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 💫

short n sweet fr:

• -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

---

🌸 Nios II Options 🌸

• -G num
• -mgpopt=option
• -mgpopt
• -mno-gpopt
• -mgprel-sec=regexp
• -mr0rel-sec=regexp
• -mel
• -meb
• -mno-bypass-cache
• -mbypass-cache
• -mno-cache-volatile
• -mcache-volatile
• -mno-fast-sw-div
• -mfast-sw-div
• -mhw-mul
• -mno-hw-mul
• -mhw-mulx
• -mno-hw-mulx
• -mno-hw-div
• -mhw-div
• -mcustom-insn=N
• -mno-custom-insn
• -mcustom-fpu-cfg=name
• -mhal
• -msmallc
• -msys-crt0=name
• -msys-lib=name
• -march=arch
• -mbmx
• -mno-bmx
• -mcdx
• -mno-cdx

---

💜 Nvidia PTX Options 💜

tiny list, big GPU energy:

• -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 🍓

the actual fossil, and we respect her:

• -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 🌈

See RS/6000 and PowerPC Options.

---

✨ PRU Options ✨

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

---

👑 RISC-V Options 👑

the open-source girlboss of ISAs, obsessed with her tbh:

• -mbranch-cost=N-instruction
• -mplt
• -mno-plt
• -mabi=ABI-string
• -mfdiv
• -mno-fdiv
• -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
• -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 🦋

okay this one is enormous, bestie, brace yourself, she's the main character of embedded-and-HPC:

• -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
• -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

Bestie, if you're cooking for an RX chip, these are the little dials you get to spin:

-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

(and yeah, -mcpu=, -mmax-constant-size= and -mint-register= are just sitting there with a trailing = and no value, that's literally how the page prints them, no cap 🙈)

---

💖 S/390 and zSeries Options

Mainframe girlies, this is your section:

-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

SuperH hive, rise up ✨:

-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 besties, short and sweet:

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

---

🦋 SPARC Options

Okay SPARC is giving range, look at this list:

-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 -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 -mlra -mno-lra

---

🌈 System V Options

Tiny but mighty:

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

---

🍓 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

Four flags, all iconic:

-mg -mgnu -munix -mlra

---

⭐ Visium Options

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

---

🫶 VMS Options

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

---

💅 VxWorks Options

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

---

🔥 x86 Options

Okay this is the list, bestie. It's enormous. It's iconic. We're doing all of it, no skipping, no summarising, periodt. 💀

-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 -mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw
-mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul
-mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite
-mprefetchwt1 -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 -mavx5124fmaps
-mavx512vnni -mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -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 -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

Whew. Slay. 💅

---

🧚 x86 Windows Options

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

---

🌸 Xstormy16 Options

One (1) flag. It's doing its best:

-msim

---

💜 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. That's the whole thing, bestie — it's a redirect and honestly? Respect. 🫶

---

🎀 Options Controlling the Kind of Output 🎀

Okay so here's the tea on how compiling actually flows. Compilation can involve up to four stages, and they always go in this order:

1. preprocessing
2. compilation proper
3. assembly
4. linking

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 linking combines all the object files — the ones just freshly compiled and the ones you handed it as input — into one executable file. Chef's kiss. 😘

For any given input file, the file name suffix determines what kind of compilation gets done, so let's vibe check every single one:

• 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 work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. Heads up: you must link with the libobjc library to make an Objective-C++ program work. And 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 (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.go — Go source code.
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
• file.s — Assembler code.
• file.S, file.sx — Assembler code that must be preprocessed.
• other — An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.

Not me noticing file.mm / file.M and file.mii each show up twice in this list. That's not a typo on my end, bestie — the page really does say it both ways. We report, we don't edit. 🙈

And if you wanna be loud and proud about which language you're feeding in, you can specify the input language explicitly with the -x option:

• -x language — okay bestie, this is you explicitly saying what language the following input files are, instead of letting the compiler guess from 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  objc-cpp-output
objective-c++  objective-c++-header  objective-c++-cpp-output
objc++-cpp-output
assembler  assembler-with-cpp
ada  adascil  adawhy
d
f77  f77-cpp-input  f95  f95-cpp-input
go
modula-2  modula-2-cpp-output
rust
lto

• -x none — turn off any specification of a language, so subsequent files are handled according to their file name suffixes, just like if -x had not been used at all. periodt.

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. Heads up: 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. 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 ...

okay so picture this bestie: -save-temps=* overrides the dumppfx that the earlier -dumpdir option gave you, so you get foo.o as your main output, with auxiliary outputs named foo.*. And it literally does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. The dump outputs? Those get named foo.c.*. 💅

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

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

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

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

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

When you're compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes that dumpbase- transformation from above (like 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 gets appended to the default dumppfx instead. But heads up — note that unlike earlier cases of linking:

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

...this does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The whole 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, so you avoid simply discarding a meaningful option. 🫶

When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in the 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 ...

Here the 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 gets 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 overriden 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. okay? iconic. 👑

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. it's giving receipts ✨
• -### — 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.
• --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. And if the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. bestie loves a full info dump 💖
• --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.
• --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.

So 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. the way that it just ✨ announces itself ✨
• -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.
• -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. 🦋
• -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.
• -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 .... not me wrapping everything in gdb like it's a lil gift 🎀

• -ffile-prefix-map=old=new — okay so when you're compiling files that live in directory old, bestie, every reference to them in the result of the compilation gets written down as if they actually lived in directory new instead. Specifying this one is literally equivalent to specifying all the individual -f*-prefix-map options at once. It's giving reproducible-builds-that-are-location-independent energy ✨. Heads up 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 old prefix against the filename that would normally be referenced in the result of the compilation is done using textual comparison of the prefixes, or ignoring character case for 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. fancy fr fr 🦋
• -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.
• -fplugin-arg-name-key=value — defines an argument called key with a value of value for the plugin called name. cute lil handshake 🤝
• -fdump-ada-spec[-slim] — for C and C++ source and include files, generates corresponding Ada specs. slay for the Ada girlies 💜
• -fada-spec-parent=unit — works together with -fdump-ada-spec[-slim] from above, and generates Ada specs as child units of parent unit.
• -fdump-go-spec=file — for input files in any language, generates corresponding Go declarations in file. This spits out Go const, type, var, and func declarations, which may be a genuinely useful way to start writing a Go interface to code that was written in some other language. main character crossover episode 🐹
• @file — reads command-line options from file. The options that get read are inserted in place of the original @file option. If file does not exist, or cannot be read, then the option will be treated literally, and not removed. Options inside file are separated by whitespace. A whitespace character may be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) may be included by prefixing the character to be included with a backslash. And plot twist: the file may itself contain additional @file options, and any such options will be processed recursively 😭 it's giving inception

---

✨ Compiling C++ Programs ✨

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; and preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as you would for compiling C programs (usually with the name gcc). the way it just knows 😌

However — and this is the important part, bestie — the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++. two names, one icon 💅

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

---

🎀 Options Controlling C Dialect 🎀

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

• -ansi — in C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword. rip to your vibes if you liked those 💀

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.

The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi. no cap, you need both 🫶

The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used.

• -std= — determines the language standard. This option is currently only supported when compiling C or C++. periodt ✨

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as 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 — supports 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). See https://gcc.gnu.org/c99status.html for more information. 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 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 (expected to be published in 2024). The support for this version is experimental and incomplete. The name c2x is deprecated.
• 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. This is the default for C code 👑
• gnu23, gnu2x — 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++2b, c++23 — bestie this is the next revision of the ISO C++ standard, planned for 2023. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. So like… vibe check before you ship it 💀
• gnu++2b, gnu++23 — the GNU dialect of -std=c++2b. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. Same warning, same energy 🫶
• 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. Yes we're already talking about 2026, she's planning ahead ✨
• gnu++2c, gnu++26 — the GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. No cap 🎀

---

• -aux-info filename — okay so this one outputs prototyped declarations to the filename you give it, for all functions declared and/or defined in a translation unit, header file ones included. And ngl? It's silently ignored in any language other than C. Like it just… doesn't happen 🙈

Besides the declarations, the file also tells you, in comments, the origin of each declaration (source file and line), whether the 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). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration. So many little letters, she's thorough 💅

• -fno-asm — do not recognize asm, inline or typeof as a keyword, so your code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. In C, -ansi implies -fno-asm.

In C++, inline is a standard keyword and is not affected by this switch. You may want to use the -fno-gnu-keywords flag instead, which disables typeof but not asm and inline. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the asm keyword, since typeof is a standard keyword in ISO C23. She's doing the math per-standard, iconic 🧚

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

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

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

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

• -fcond-mismatch — allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++. Sorry C++ girlies, not today 💀
• -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted. Main character energy but make it no main 👑
• -fgimple — enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes. Very niche, very cute 🎀
• -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). Bestie that sentence just ends, I'm not fixing it, I'm reporting it fr fr 😭

• -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. Two little macros, your inline detective squad 🔍

• -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. Basically the default girlie life 💖
• -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. Ma'am that's a red flag, please 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. Not me learning x86 has special privileges 💅

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

Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES. Peep the list, bestie ✨

• -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. You can specify options that apply only to a specific target or targets by using the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.

Typical command lines are

-foffload-options='-fno-math-errno -ffinite-math-only' -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906

• -fopenacc — enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran and !$acc, c$acc and *$acc in fixed-form Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. Accelerated girlie behavior 🔥
• -fopenacc-dim=geom — specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of ':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A size can be omitted, to use a target-specific default value. Pick your dimensions, queen 👑
• -fopenmp — enable handling of OpenMP directives #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 *. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. Parallel processing, she ate that 🌈
• -fopenmp-simd — enable handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directive, and of combined or composite directives with simd as 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 *. Other OpenMP directives are ignored. Unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause. Just the simd bits, nothing else, stay in your lane bestie 🦋
• -fopenmp-target-simd-clone
• -fopenmp-target-simd-clone=device-type — so normally GCC makes SIMD clones for functions marked with the declare simd directive, right? But with this option on, it ALSO makes clones for functions marked with the OpenMP declare target directive, as long as they're the kind that can actually get vectorized. Bestie, the device-type can be none, host, nohost, or any, and those line up exactly with the keywords for the device_type clause of the declare target directive — clones get generated for the intersection of the devices you specified. Oh and fyi: -fopenmp-target-simd-clone is the same thing as -fopenmp-target-simd-clone=any, and -fno-openmp-target-simd-clone is the same as -fopenmp-target-simd-clone=none. ✨

At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost. Otherwise? Disabled by default. No cap. 💅

• -fpermitted-flt-eval-methods=style — okay so ISO/IEC TS 18661-3 defines some new permissible values for FLT_EVAL_METHOD, and what they say is that operations and constants with a semantic type that's an interchange or extended format should be evaluated to the precision and range of that type. These new values are a superset of what C99/C11 allow, and C99/C11 doesn't even specify what the other positive values of FLT_EVAL_METHOD mean. Which means, and this is the tea ☕, code that conforms to C11 may not have been written expecting these new values to be possible at all.

-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, whichever fits your vibe.

The default when you're in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when you're 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. What does that even mean? It means determining what resources provided by other source files will be required to compile the source, plus what resources the source itself provides. This info can be used to add required dependencies between compilation rules of dependent sources based on their contents, instead of requiring that information be reflected within the build tools as well. It's giving organized. 🗂️

• -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. Heads up bestie: when this argument is specified, the output of -MF is stripped of some information (namely C++ modules) so that it doesn't use extended makefile syntax that most tools wouldn't understand anyway. 🙈
• -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. And the catch: 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 doesn't use either signed or unsigned. By default, such a bit-field is signed, because that's consistent: the basic integer types like int are signed types. Consistent queen behavior. 👑
• -fsigned-char — let the type char be signed, like signed char.

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char. The way these flip around is so real. 🦋

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

Every kind of machine has a default for what char should be. It's 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 so many programs were 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. Slay, backwards compatibility. 💖

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. It's literally its own thing.

• -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: all trailing arrays of structures are treated as flexible array members.

The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case, a trailing array is treated as a flexible array member only when it's declared as a flexible array member per C99 standard onwards.

The possible values of level are the same as for the strict_flex_array attribute.

You can control this behavior for a specific trailing array field of a structure by using the variable attribute strict_flex_array attribute.

Also, the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. They're besties. 🫶

• -fsso-struct=endianness — set the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian, and native for the native endianness of the target (which is the default). And ngl, this option is not supported for C++.

⚠️ Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it, if the specified endianness is not the native endianness of the target. Like, do not mix them. Fr fr. 💀

---

🌸 Options Controlling C++ Dialect 🌸

This section describes the command-line options that are only meaningful for C++ programs. But lowkey, you can also use most of the GNU compiler options regardless of 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; you can use the other options with any language supported by GCC. Iconic. ✨

Some options for compiling C programs, such as -std, are also relevant for C++ programs.

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 get fixed. (Chaotic? A little. But that's just how it is.)

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. (Not the accidental change bestie 😭)

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 appeard in G++ 13, fixes manglings of lambdas that have additional context.

Version 19, which first appeard in G++ 14, fixes manglings of structured bindings to include ABI tags.

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 — bestie, this one just turns off ALL the access checking. It’s mainly for working around bugs in the access control code. Use it when the compiler’s being extra and you need a workaround, okay? 💅
• -faligned-new — ✨ Enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. You can pass a numeric argument like -faligned-new=32 to specify how much alignment (in bytes) that function provides, but few users will need to override the default of alignof(std::max_align_t).

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

• -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 — Check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that operator new only returns 0 if it is declared throw(), in which case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also new (nothrow).
• -fconcepts
• -fconcepts-ts — Enable support for the C++ Concepts feature for constraining 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 didn’t make it into the standard, can additionally be enabled by -fconcepts-ts. The option -fconcepts-ts was deprecated in GCC 14 and may be removed in GCC 15; users are expected to convert their code to C++20 concepts.

• -fconstexpr-depth=n — Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.
• -fconstexpr-cache-depth=n — Set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. 🔥
• -fconstexpr-fp-except — Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, 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 that a particular operation is undefined.

constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except

• -fconstexpr-loop-limit=n — Set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).
• -fconstexpr-ops-limit=n — Set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25).
• -fcontracts — Enable 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 those 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] — Control whether any contracts have any semantics at all. Defaults to on.
• -fcontract-assumption-mode=[on|off] — [N4820] Control whether contracts with level axiom should have the assume semantic. Defaults to on.
• -fcontract-build-level=[off|default|audit] — [N4820] Specify which level of contracts to generate checks for. Defaults to default.
• -fcontract-continuation-mode=[on|off] — [N4820] Control whether to allow the program to 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] Specify the concrete semantics for each contract level of a particular contract role.
• -fcontract-semantic=[default|audit|axiom]:<semantic> — [P1429] Specify the concrete semantic for a particular contract level.
• -fcontract-strict-declarations=[on|off] — Control whether to reject adding contracts to a function after its first declaration. Defaults to off.

The possible concrete semantics for 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 — Enable support for the C++ coroutines extension (experimental). 🦋
• -fdiagnostics-all-candidates — Permit the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected.
• -fno-elide-constructors — The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline.

In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.

• -fno-enforce-eh-specs — Don’t generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.

⚠️ bestie, that last one is giving “I know better than the standard” energy. It violates the C++ standard and can cause undefined behavior at runtime if you throw an unexpected exception. Be careful! 💀

• -fextern-tls-init, -fno-extern-tls-init — okay so the C++11 and OpenMP standards let thread_local and threadprivate variables have dynamic (runtime) initialization, right? And to make that work, every single use of such a variable has to go through a lil wrapper function that does the init if it needs doing 🫶. When the use and the definition live in the same translation unit, that overhead can just get optimized away, no biggie. But when the use is in a different translation unit?? 💀 huge overhead, even if the variable literally doesn't need dynamic initialization at all. So if you the programmer can be 100% sure that no use of the variable in a non-defining TU needs to trigger dynamic init — either because the variable is statically initialized, or because a use in the defining TU will run before any uses in another TU — then bestie you can yeet that overhead with -fno-extern-tls-init. On targets that support symbol aliases, the default is -fextern-tls-init. On targets that don't support symbol aliases, the default is -fno-extern-tls-init.
• -ffold-simple-inlines, -fno-fold-simple-inlines — lets the C++ frontend fold calls to std::move, std::forward, std::addressof and std::as_const. And no, it's not the same as inlining — unlike inlining, this means no debug information gets generated for those calls at all. Since these functions are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active. periodt ✨
• -fno-gnu-keywords — do NOT recognize typeof as a keyword, so your code can use that word as an identifier 💅. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
• -fno-immediate-escalation — don't enable immediate function escalation, the thing where certain functions can get promoted to consteval, as specified in P2564R3. Like, 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);
}

This compiles in C++20: f is an immediate-escalating function (because of the auto it's a function template, and it's declared constexpr) and id(t) is an immediate-escalating expression, so f gets promoted to consteval 👑. Consequently, the call to id(t) is in an immediate context, so it doesn't have to produce a constant (that's literally the mechanism allowing consteval function composition, iykyk). BUT 😭 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 just rejects the code. This option is turned on by default; it is only effective in C++20 mode or later.

• -fimplicit-constexpr — makes inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function 🌸. You can use this in C++14 mode or later. This can result in initialization changing from dynamic to static, plus other optimizations. slay.
• -fno-implicit-templates — never emit code for non-inline templates that get instantiated implicitly (i.e. by use); only emit code for explicit instantiations. And bestie, heads up 🚨: if you use this option you must take care to structure your code to include all the necessary explicit instantiations, otherwise you're getting undefined symbols at link time. not fun.
• -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. Warning though: this causes linker errors if those functions are not inlined everywhere they're called 🙈.
• -fmodules-ts, -fno-modules-ts — enable support for C++20 modules 🦋. The -fno-modules-ts is usually not needed, since that's the default. And even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. ik, wild.
• -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 — no in-between, fr fr. (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 CMI, nothing else 💖.
• -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. we don't need the nagging here 🎀.
• -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 🫶.
• -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. And get this: 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. noted 💀.
• -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 — so 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 🌈. And when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. But if either of these behaviors makes it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them. your call bestie 💅.
• -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.
• -fstrong-eval-order — 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 🔥. Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17.
• *-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. (Because 10 notes is already a lot to read, bestie.)
• -ftemplate-depth=n — sets the max instantiation depth for template classes to n. bestie, why do we even need a ceiling? because endless recursion during template class instantiation is a real thing and we need to catch it 😭. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (this got changed to 1024 in C++11). the default value is 900, because sometimes the compiler runs out of stack space before it even hits 1024. so like… 900 is the vibe. ✨
• -fno-threadsafe-statics — do not emit the extra code that uses the routines specified in the C++ ABI for thread-safe initialization of local statics. you'd use this to shave a lil bit off the code size in code that doesn't need to be thread-safe. lowkey a niche flex but ok 🎀
• -fuse-cxa-atexit — registers destructors for objects with static storage duration with the __cxa_atexit function instead of the atexit function. this option is required for fully standards-compliant handling of static destructors, but heads up — it only works if your C library supports __cxa_atexit. no support, no slay 🙈
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. this makes std::uncaught_exception incorrect 😭 but it's necessary if the runtime routine isn't available. tradeoffs, bestie, tradeoffs.
• -fvisibility-inlines-hidden — this switch declares that the user is not attempting to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

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

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.

• -fvisibility-ms-compat — this flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio. yes, that Microsoft Visual Studio 👀

the flag makes these changes to GCC's linkage model:

1. it sets the default visibility to hidden, like -fvisibility=hidden.
2. types, but not their members, are not hidden by default.
3. the One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

in new code it's better to use -fvisibility=hidden and export those classes that are intended 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 it's provided by the linker. 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. it may be removed in a future release of G++. not me warning you twice 💀
• -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, those suffixes are treated as C++11 user-defined literal numeric suffixes. it's 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. it's 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. pick your fighter 💅

---

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 doesn't have that ABI tag. see C++ Attributes for more info 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 with -std=c++23 regardless of -Wno-deprecated.

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. it's giving… locked out of your own house 🏠🔒
• -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. 😭 like bestie the references are GONE the second that expression ends, do not touch them.

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 — meaning: the class is a non-union class that has a pointer data member and a trivial destructor. ✨ That's the vibe check for "span-like", and those get a pass.

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

This warning is enabled by -Wall.

---

• -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 a 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.

• -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.

• -Wno-elaborated-enum-base — For C++11 and above, warn if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with -pedantic-errors. 🔥
• -Wno-init-list-lifetime (C++ and Objective-C++ only) — Do not warn about uses of std::initializer_list that are likely to result in dangling pointers. Since the underlying array for an initializer_list is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example:
  • If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. 💀
  • If a new-expression creates an initializer_list, the array only lives until the end of the enclosing full-expression, so the initializer_list in the heap has a dangling pointer.
  • When an initializer_list variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the initializer_list variable has a dangling pointer.

// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior

• When a list constructor stores the begin pointer from the initializer_list argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary initializer_list, the pointer is left dangling by the end of the variable declaration statement. 😭

• -Winvalid-constexpr — Warn when a function never produces a constant expression. In C++20 and earlier, for every constexpr function and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's possible to have a function or a function template marked constexpr for which no invocation satisfies the requirements of a core constant expression.

This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it will be an ordinary warning. For example:

void f (int& i);
constexpr void
g (int& i)
{
  // Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
  f(i);
}

• -Winvalid-imported-macros — Verify all imported macro definitions are valid at the end of compilation. This is not enabled by default, as it requires additional processing to determine. It may be useful when preparing sets of header-units to ensure consistent macros. 🫶
• -Wno-literal-suffix (C++ and Objective-C++ only) — Do not warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from <inttypes.h>. For example:

#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>

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

In this case, PRId64 is treated as a separate preprocessing token. ✨

This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.

These warnings are enabled by default.

• -Wno-narrowing (C++ and Objective-C++ only) — For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. 🧚

With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.

int i = { 2.2 }; // error: narrowing from double to int

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

• -Wnoexcept (C++ and Objective-C++ only) — Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception. 👑
• -Wnoexcept-type (C++ and Objective-C++ only) — Warn if the C++17 feature making noexcept part of a function type changes the mangled name of a symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat.

As an example:

template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }

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

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

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

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

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified. The -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred because it warns about the unsafe cases without false positives. 💅
• -Wregister (C++ and Objective-C++ only) — Warn on uses of the register storage class specifier, except when it is part of the GNU Explicit Register Variables extension. The use of the register keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. ⭐
• -Wreorder (C++ and Objective-C++ only) — okay so bestie, this one yells at you when the order of your member initializers in the code doesn't match the order they actually get executed in. Because the compiler is gonna run them in declaration order whether you like it or not. Like, look at this mess:

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 to that effect. This warning is enabled by -Wall.

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this one warns when a call to std::move is out here blocking copy elision. Rude, honestly. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type. Like:

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

But in this example, the std::move call prevents copy elision. 💀

This warning is enabled by -Wall.

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

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);
}

In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ, yet G++ treats the 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 calls out a C++ range-based for-loop that's making an unnecessary copy 🫶. This can happen when the range declaration is not a reference, but probably should be. For example:

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

It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes.

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

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. For example:

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. 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 📚:
  • 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.
• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns 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) — disables warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default.
• -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for. glow up your casts bestie 💅
• -Woverloaded-virtual (C++ and Objective-C++ only)
• -Woverloaded-virtual=n — warns when a function declaration hides virtual functions from a base class. For example, in:

struct A {
  virtual void f();
};

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

the A class version of f is hidden in B, 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) — disables the diagnostic for converting a bound pointer to member function to a plain pointer.
• -Wsign-promo (C++ and Objective-C++ only) — warns when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. the standard said "no" and that's that 🤷
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates.
• -Wmismatched-new-delete (C++ and Objective-C++ only) — warns for mismatches between calls to operator new or operator delete and the corresponding call to the allocation or deallocation function. This includes invocations of C++ operator delete with pointers returned from either mismatched forms of operator new, or from other functions that allocate objects for which the operator delete isn't a suitable deallocator, 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
   }

btw the sibling option -Wmismatched-dealloc is the one that clocks mismatches between allocation and deallocation functions that are not operator new and operator delete. different job, same energy 💅

also: -Wmismatched-new-delete is riding along inside -Wall bestie, so it's on by default unless you tell it to chill.

---

• -Wmismatched-tags (C++ and Objective-C++ only) — okay so this one yells at you when you declare structs, classes, and class templates (plus their specializations) with a class-key that doesn't match the definition, or the first declaration if there's no definition at all. like, pick a lane, babe.

for example, the declaration of struct Object in the argument list of draw triggers the warning. to dodge it, either yeet the redundant class-key struct or swap it for class so it matches its definition:

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

and like, to be clear: it is NOT wrong to declare a class with the class-key struct — the example above proves it. the -Wmismatched-tags option is just here to help you achieve a consistent style of class declarations. and for code that's meant to be portable to Windows-based compilers, this warning helps prevent unresolved references caused by the difference in the mangling of symbols declared with different class-keys. you can run this option on its own or in conjunction with -Wredundant-tags. 🎀

• -Wmultiple-inheritance (C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes. some coding rules disallow multiple inheritance and this is how you enforce that rule, no cap. the warning is inactive inside a system header file, like the STL, so you can still use the STL. and you may also define classes that indirectly use multiple inheritance — that's fine too. 🦋
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. again, some coding rules disallow multiple inheritance and this enforces it. warning is inactive inside a system header file such as the STL, so STL usage stays unbothered. and you may also define classes that indirectly use virtual inheritance. ✨
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. this is DANGEROUS, bestie, like genuinely: 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 the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. 💀
• -Wnamespaces — warns when a namespace definition is opened. some coding rules disallow namespaces and this is how you enforce that rule. warning is inactive inside a system header file, such as the STL, so the STL stays usable. you may also use using directives and qualified names. 🫶
• -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. 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. 💅

• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. like, throwing straight into the void fr fr. 😭
• -Wno-vexing-parse (C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity. this 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. so like, it's dead code, no need to scream about it. 🍓
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. this includes postfix and prefix ++ and -- expressions of volatile-qualified types, using simple assignments where the left operand is a volatile-qualified non-class type for their value, compound assignments where the left operand is a volatile-qualified non-class type, volatile-qualified function return type, volatile-qualified parameter type, and structured bindings of a volatile-qualified type. this usage was deprecated in C++20.

enabled by default with -std=c++20. ✨

• -Wzero-as-null-pointer-constant (C++ and Objective-C++ only) — warns when a literal 0 is used as a null pointer constant. this can be useful to facilitate the conversion to nullptr in C++11. like bestie, we have nullptr now, it's 2011, catch up 💅
• -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, such as constructing an object in a buffer that is smaller than the type of the object. for example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large:

char buf [64];
new (buf) int[64];

this warning is enabled by default.

• -Wplacement-new=1 — this is the default warning level of -Wplacement-new. at this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. for example, the following new expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard, because it writes past the end of the one-element array:

struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();

• -Wplacement-new=2 — at this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of a structure whose type is an array of a single element and whose size is less than the size of the object being constructed. so 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) warn about polymorphic class types that are caught by value. with -Wcatch-value=2 warn about all class types that are caught by value. with -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall. 🎀
• -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs. iykyk. 🦋
• -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 after in-class function definitions. extra semis are giving nothing, bestie, let them go. 💅
• -Wno-global-module (C++ and Objective-C++ only) — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives. 🌸
• -Wno-inaccessible-base (C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. the warning is enabled by default. note that the warning for ambiguous virtual bases is enabled by the -Wextra option.

struct A { int a; };

struct B : A { };

struct C : B, A { };

• -Wno-inherited-variadic-ctor — suppresses warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. the ellipsis stays behind, it does not come with you 🫶
• -Wno-invalid-offsetof (C++ and Objective-C++ only) — suppresses warnings from applying the offsetof macro to a non-POD type. according to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. in existing C++ implementations, however, offsetof typically gives meaningful results. this flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

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

• -Wsized-deallocation (C++ and Objective-C++ only) — warns about a definition of an unsized deallocation function

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

without a definition of the corresponding sized deallocation function

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

or vice versa. enabled by -Wextra along with -fsized-deallocation. 👑

• -Wsuggest-final-types — okay bestie, this one is a glow-up suggestion generator 💅 It warns about types with virtual methods where your code quality would be improved if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. Why do we care? Because that lets GCC more aggressively devirtualize the polymorphic calls — speed, honey. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. 🔥
• -Wsuggest-final-methods — same energy, but for virtual methods: it warns where code quality would be improved if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. Also more effective with link-time optimization, where the information about the class hierarchy graph is more complete. And here's the pro strat: it is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. Do it in that order, bestie, trust. ✨
• -Wsuggest-override — warns about overriding virtual functions that are not marked with the override keyword. Just write override, it's free and it slays. 🎀
• -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. Heads up: -Wconversion-null is enabled by default, so this is you turning it off. 🫶

---

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

NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. We're doing flags only, bestie — the language lore is a whole other fic.

This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:

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

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

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi). Options stacking like outfits, we love to see it.

Here is a 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 cause @"..." 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, so it's giving "the usual."
• -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. Conditional queen. ⭐
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. 🫶
• -fobjc-abi-version= n — use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. Pick your era, babe. 💅
• -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 return 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. So check your OS era before you come for me. 🙈

• -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 is offered by C++. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier). ⚠️
• -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. Your faves could never. 🌸
• -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. It's giving "does nothing but look cute." 💀
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC. Retro compatibility era, iconic. 👑
• -freplace-objc-classes — emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. 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. Note the underscore difference, bestie, don't get got. 😭
• -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, we're healing. ✨
• -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. 💅
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. Receipts, generated. 🧚
• -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. 🫶
• -Wno-protocol (Objective-C and Objective-C++ only) — if a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them. Inherited counts as done, periodt. 🌈
• -Wobjc-root-class (Objective-C and Objective-C++ only) — warn if a class interface lacks a superclass. Most classes will inherit from NSObject (or Object) for example. When declaring classes intended to be root classes, the warning can be suppressed by marking their interfaces with __attribute__((objc_root_class)). Root class energy is valid, just label it, babe. ⭐
• -Wselector (Objective-C and Objective-C++ only) — bestie, this one calls out the compiler like "umm hello??" when it finds multiple methods of different types for the same selector during compilation. The check happens on the method list in the final stage of compilation, okay? And there's also a check for each selector that shows up in a @selector(...) expression where a corresponding method for that selector was found during compilation. But here's the tea: because these checks scan the method table only at the end of compilation, the warnings just... don't happen if the final stage never gets reached. Like if an error is found during compilation, or if you're using -fsyntax-only. So no final stage, no warnings, periodt. 💅
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warns when multiple methods with differing argument and/or return types are found for a given selector, specifically when you're trying to send a message using this selector to a receiver of type id or Class. When this flag is off (which is the default behavior, jsyk), the compiler skips those warnings if any differences found are confined to types that share the same size and alignment. So like, same size, same alignment, same vibe — no warning. 🫶
• -Wundeclared-selector (Objective-C and Objective-C++ only) — yells at you when a @selector(...) expression referring to an undeclared selector is found. A selector counts as undeclared if no method with that name has been declared before the @selector(...) expression — either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. And the way this option works? It always performs its checks as soon as a @selector(...) expression is found, whereas -Wselector only does its checks in the final stage of compilation. It also enforces that whole coding style convention that methods and selectors must be declared before being used. Iconic, honestly. 👑
• -print-objc-runtime-info — generates a C header describing the largest structure that is passed by value, if any. That's it, that's the whole thing. 🎀

---

✨ Options to Control Diagnostic Messages Formatting ✨

So, historically, diagnostic messages have been formatted with zero regard for the output device's aspect (like its width, ...). Rude, honestly. But you can use the options below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location information gets reported. 💖

Heads up though: some language front ends may not honor these options. Not everyone's invited to the party, I guess. 😭

• -fmessage-length=n — tries to format error messages so they fit on lines of about n characters. If n is zero, then no line-wrapping happens at all; each error message appears on a single line. This is the default for all front ends. 📏

Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas. Those are doing their own thing. 🙈

• -fdiagnostics-plain-output — requests that diagnostic output look as plain as possible, which is lowkey useful when you're running dejagnu or other utilities that need to parse diagnostics output and prefer it stay more stable over time. Right now, -fdiagnostics-plain-output is 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

And fr fr, 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. Future-proofing, bestie. 🦋

• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once — so if the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted as a prefix again and again and again on subsequent continuation lines. This is the default behavior. 💅
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as a prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line. Every. Single. Line. 😤
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — use color in diagnostics! 🌈 WHEN is never, always, or auto. The default depends on how the compiler has been 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 the 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.

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

The default GCC_COLORS is

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

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors entirely. 💀

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.

• -fdiagnostics-urls[=WHEN] — uses escape sequences to embed URLs in diagnostics! 🔗 For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, it embeds a URL for documentation of that option. Slay.

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, bestie. 👀

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 — and this is the plot twist — 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. Sometimes you just wanna be mysterious, yk? 🙈
• -fno-diagnostics-show-caret — bestie, by default every diagnostic comes with the original source line AND a little caret ^ pointing at the column, like a tiny finger going it's right there 💅. This option says no thanks, no caret, no source line. (Fun fact: that source line gets truncated to n characters if you passed -fmessage-length=n. And when the output is going to a terminal, the width gets capped at whatever the COLUMNS environment variable says — or, if that's not set, at the terminal's actual width.) Slay, but quietly.
• -fno-diagnostics-show-labels — so when GCC prints source code (via -fdiagnostics-show-caret), diagnostics can label ranges of that source with relevant info — like what type each expression actually is. Behold the vibe:

            printf ("foo %s bar", long_i + long_j);
                         ~^       ~~~~~~~~~~~~~~~
                          |              |
                          char *         long int

This option kills those labels (in the example above: the vertical bars and the "char *" and "long int" text). Gone. Not invited.

• -fno-diagnostics-show-cwe — diagnostic messages can optionally have an associated CWE ("https://cwe.mitre.org/index.html") identifier attached 🎀. GCC itself only provides that metadata for some of the -fanalyzer diagnostics. GCC plugins may also hand over diagnostics carrying that metadata. By default, if the info is there, it gets printed with the diagnostic. This option suppresses printing it. Mysterious metadata, begone.
• -fno-diagnostics-show-rules — diagnostics can also optionally have rules associated with them, like from a coding standard or a spec 📜. GCC itself does not do this for any of its diagnostics, but plugins might. By default, if that info exists, it's printed with the diagnostic. This option suppresses it. Not today, rulebook.
• -fno-diagnostics-show-line-numbers — when GCC prints source code (via -fdiagnostics-show-caret), there's normally a left margin showing line numbers. This option yeets that left margin entirely. No numbers, just vibes.
• -fdiagnostics-minimum-margin-width=width — controls the minimum width of that left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. Six. That's the number. Remember it.
• -fdiagnostics-parseable-fixits — emits fix-it hints in a machine-parseable format, perfect for IDEs to snack on 🤖. For each fix-it, a line gets printed right after the relevant diagnostic, starting with the string "fix-it:". Like so:

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

The location is a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. So in that example, bytes 3 through 20 of line 45 of "test.c" are getting replaced with the given string:

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

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

An empty replacement string means the given range is to be removed. An empty range (e.g. "45:3-45:3") means the string is to be inserted at that position. Insertion rights, unlocked ✨

• -fdiagnostics-generate-patch — prints fix-it hints to stderr in unified diff format, after all the diagnostics are printed. Example incoming:

        --- 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 exact same rules as for diagnostics (see -fdiagnostics-color). Patch era 💖

• -fdiagnostics-show-template-tree — in the C++ frontend, when printing diagnostics about mismatching template types, like:

          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, like:

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

The parts that differ are highlighted with color ("double" and "float" in this case). Tree branch realness 🌸

• -fno-elide-type — by default, when the C++ frontend prints diagnostics about mismatching template types, common parts of the types get printed as "[...]" to keep the error message simple. Like:

          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. No more [...] erasure, bestie.

• -fdiagnostics-path-format=KIND — specifies 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 is inline-events.

none means: do not print diagnostic paths.

separate-events means: print a separate "note" diagnostic for each event within the diagnostic. 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: print the events "inline" within the source code. This view tries 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 get 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. Stack frame lore 📚

For example:

          'test': events 1-2
            |
            |  133 | {
            |      | ^
            |      | |
            |      | (1) entering 'test'
            |  134 |   boxed_int *obj = make_boxed_int (i);
            |      |                    ~~~~~~~~~~~~~~~~~~
            |      |                    |
            |      |                    (2) calling 'make_boxed_int'
            |
            +--> 'make_boxed_int': events 3-4
                   |
                   |  120 | {
                   |      | ^
                   |      | |
                   |      | (3) entering 'make_boxed_int'
                   |  121 |   boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
                   |      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   |      |                                    |
                   |      |                                    (4) calling 'wrapped_malloc'
                   |
                   +--> 'wrapped_malloc': events 5-6
                          |
                          |    7 | {
                          |      | ^
                          |      | |
                          |      | (5) entering 'wrapped_malloc'
                          |    8 |   return malloc (size);
                          |      |          ~~~~~~~~~~~~~
                          |      |          |
                          |      |          (6) calling 'malloc'
                          |
            <-------------+
            |
         'test': event 7
            |
            |  138 |   free_boxed_int (obj);
            |      |   ^~~~~~~~~~~~~~~~~~~~
            |      |   |
            |      |   (7) calling 'free_boxed_int'
            |
        (etc)

• -fdiagnostics-show-path-depths — this one hands you extra info when printing control-flow paths tied to a diagnostic. Depth check ✨

If this option is provided, the stack depth gets printed for each run of events within -fdiagnostics-path-format=inline-events. And if you pair it with -fdiagnostics-path-format=separate-events, then the stack depth and the function declaration get appended when printing each event.

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. Devs only, iykyk 💅

• -fno-show-column — do not print column numbers in diagnostics. This may be necessary if the diagnostics are being scanned by a program that doesn't understand column numbers, such as dejagnu. Column-blind tools exist and we respect them.
• -fdiagnostics-column-unit=UNIT — picks the units for the column number. This affects traditional diagnostics (when -fno-show-column isn't in play), and also JSON format diagnostics if those are requested.

The default UNIT, display, counts the number of display columns occupied by each character. This can be bigger than the number of bytes needed to encode the character — think tab characters — or 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 needs two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding needs four bytes. Unicode math, but make it cute 🫶

• -fdiagnostics-column-unit=UNIT — wait, this one's already been vibing, but its bestie is here: setting UNIT to byte makes the column number the raw byte count in all the cases, which is exactly what GCC did traditionally before version 11.1.0. Old-school behavior, no cap. 🫶
• -fdiagnostics-column-origin=ORIGIN — picks where column numbers start, bestie. Like, which column gets to be column one? The default 1 is traditional GCC behavior and also what the GNU style guide says. Some utilities lowkey do better with an origin of 0 though — any non-negative value is fair game. ✨
• -fdiagnostics-escape-format=FORMAT — okay so when GCC prints the pertinent source lines for a diagnostic, it normally just tries to print the source bytes directly. But some diagnostics are about encoding drama in the source file, like malformed UTF-8, or Unicode normalization issues. Those diagnostics get flagged so GCC will escape bytes that aren't printable ASCII when it prints their pertinent source lines. 💅

This option controls how those bytes get escaped.

The default FORMAT, unicode, displays Unicode characters that aren't printable ASCII in the form <U+XXXX>, and bytes that don't correspond to a Unicode character validly-encoded in UTF-8 get shown 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. Iconic, honestly. 🎀

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

The none value suppresses the printing of such diagrams. The ascii value will ensure that such diagrams are pure ASCII ("ASCII art"). The unicode value will allow for conservative use of unicode drawing characters (such as box-drawing characters). The emoji value further adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character).

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

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

The default is text.

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

The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical, apart from where the JSON is emitted to — with the former, the JSON is emitted to stderr, whereas with json-file it is written to source.gcc.json.

The emitted JSON consists of a top-level JSON array containing JSON objects representing the diagnostics.

Diagnostics can have child diagnostics. For example, this error and note:

         misleading-indentation.c:15:3: warning: this 'if' clause does not
           guard... [-Wmisleading-indentation]
            15 |   if (flag)
               |   ^~
         misleading-indentation.c:17:5: note: ...this statement, but the latter
           is misleadingly indented as if it were guarded by the 'if'
            17 |     y = 2;
               |     ^

might be printed in JSON form (after formatting) like this:

         [
             {
                 "kind": "warning",
                 "locations": [
                     {
                         "caret": {
                             "display-column": 3,
                             "byte-column": 3,
                             "column": 3,
                             "file": "misleading-indentation.c",
                             "line": 15
                         },
                         "finish": {
                             "display-column": 4,
                             "byte-column": 4,
                             "column": 4,
                             "file": "misleading-indentation.c",
                             "line": 15
                         }
                     }
                 ],
                 "message": "this \u2018if\u2019 clause does not guard...",
                 "option": "-Wmisleading-indentation",
                 "option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
                 "children": [
                     {
                         "kind": "note",
                         "locations": [
                             {
                                 "caret": {
                                     "display-column": 5,
                                     "byte-column": 5,
                                     "column": 5,
                                     "file": "misleading-indentation.c",
                                     "line": 17
                                 }
                             }
                         ],
                         "escape-source": false,
                         "message": "...this statement, but the latter is ..."
                     }
                 ]
                 "escape-source": false,
                 "column-origin": 1,
             }
         ]

where the "note" is a child of the "warning". 🦋

A diagnostic has a "kind". If this is "warning", then there's an "option" key describing the command-line option controlling the warning.

A diagnostic can contain zero or more locations. Each location has an optional "label" string and up to three positions within it: a "caret" position and optional "start" and "finish" positions. A position is described by a "file" name, a "line" number, and three numbers indicating a column position:

• "display-column" counts display columns, accounting for tabs and multibyte characters.
• "byte-column" counts raw bytes.
• "column" is equal to one of the previous two, as dictated by the -fdiagnostics-column-unit option.

All three columns are relative to the origin specified by -fdiagnostics-column-origin, which is typically equal to 1 but may be set, for instance, to 0 for compatibility with other utilities that number columns from 0. The column origin is recorded in the JSON output in the "column-origin" tag. In the remaining examples below, the extra column number outputs have been omitted for brevity.

For example, this error:

         bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
            'struct s'} and 'T' {aka 'struct t'})
            64 |   return callee_4a () + callee_4b ();
               |          ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
               |          |              |
               |          |              T {aka struct t}
               |          S {aka struct s}

has three locations. Its primary location is at the + token at column 23. It has two secondary locations, describing the left and right-hand sides of the expression, which have labels. It might be printed in JSON form as:

             {
                 "children": [],
                 "kind": "error",
                 "locations": [
                     {
                         "caret": {
                             "column": 23, "file": "bad-binary-ops.c", "line": 64
                         }
                     },
                     {
                         "caret": {
                             "column": 10, "file": "bad-binary-ops.c", "line": 64
                         },
                         "finish": {
                             "column": 21, "file": "bad-binary-ops.c", "line": 64
                         },
                         "label": "S {aka struct s}"
                     },
                     {
                         "caret": {
                             "column": 25, "file": "bad-binary-ops.c", "line": 64
                         },
                         "finish": {
                             "column": 36, "file": "bad-binary-ops.c", "line": 64
                         },
                         "label": "T {aka struct t}"
                     }
                 ],
                 "escape-source": false,
                 "message": "invalid operands to binary + ..."
             }

If a diagnostic contains fix-it hints, it has a "fixits" array, consisting of half-open intervals, similar to the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it hint:

         demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
           mean 'color'?
             8 |   return ptr->colour;
               |               ^~~~~~
               |               color

might be printed in JSON form as:

                       {
                           "children": [],
                           "fixits": [
                               {
                                   "next": {
                                       "column": 21,
                                       "file": "demo.c",
                                       "line": 8
                                   },
                                   "start": {
                                       "column": 15,
                                       "file": "demo.c",
                                       "line": 8
                                   },
                                   "string": "color"
                               }
                           ],
                           "kind": "error",
                           "locations": [
                               {
                                   "caret": {
                                       "column": 15,
                                       "file": "demo.c",
                                       "line": 8
                                   },
                                   "finish": {
                                       "column": 20,
                                       "file": "demo.c",
                                       "line": 8
                                   }
                               }
                           ],
                           "escape-source": false,
                           "message": "\u2018struct s\u2019 has no member named ..."
                       }

okay so where the fix-it hint suggests swapping out the text from "start" up to but NOT including "next" with the value of "string" 💅. Deletions? they show up as an empty value for "string" 🫥. Insertions? that's when "start" equals "next" ✨.

If the diagnostic has a path of control-flow events attached to it, it's got a "path" array of objects, each one representing an event 🦋. Every event object has a "description" string, a "location" object, plus a "function" string and a "depth" number for representing interprocedural paths 👑. The "function" is the current function at that event, and the "depth" is the stack depth relative to some baseline: the higher it is, the more frames are chillin' in the stack 🥞.

For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its path:

                       "path": [
                           {
                               "depth": 0,
                               "description": "when 'PyList_New' fails, returning NULL",
                               "function": "test",
                               "location": {
                                   "column": 10,
                                   "file": "test.c",
                                   "line": 25
                               }
                           },
                           {
                               "depth": 0,
                               "description": "when 'i < count'",
                               "function": "test",
                               "location": {
                                   "column": 3,
                                   "file": "test.c",
                                   "line": 27
                               }
                           },
                           {
                               "depth": 0,
                               "description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
                               "function": "test",
                               "location": {
                                   "column": 5,
                                   "file": "test.c",
                                   "line": 29
                               }
                           }
                       ]

Diagnostics also have a boolean attribute "escape-source", which hints at whether non-ASCII bytes should be escaped when printing the pertinent lines of source code ("true" for diagnostics involving source encoding issues) 🌸.

• -fno-diagnostics-json-formatting — so by default, whenever JSON gets emitted for diagnostics (via -fdiagnostics-format=sarif-stderr, -fdiagnostics-format=sarif-file, -fdiagnostics-format=json, -fdiagnostics-format=json-stderr, -fdiagnostics-format=json-file), GCC will sprinkle in newlines and indentation to visually emphasize the hierarchical structure of the JSON 🎀. Use -fno-diagnostics-json-formatting to suppress that whitespace. It must be passed before the option it is to affect. This is intended for compatibility with tools that don't expect the output to contain newlines, like the stuff emitted by older GCC releases 🙈.

---

✨ Options to Request or Suppress Warnings ✨

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

The following language-independent options don't enable specific warnings, they just control the kinds of diagnostics GCC produces 💖:

• -fsyntax-only — check the code for syntax errors, but like... don't do anything beyond that. that's it, that's the whole vibe 🫶.
• -fmax-errors=n — caps the maximum number of error messages at n, and once you hit that, GCC just bails out instead of trying to keep processing the source code 💀. If n is 0 (the default), there's no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option 👑.
• -w — inhibit ALL warning messages. like, every single one. gone. silenced. shhh 🤫.
• -Werror — make all warnings into errors 😈.
• -Werror= — make the specified warning into an error. You append the specifier for the warning; for example -Werror=switch turns the warnings controlled by -Wswitch into errors ✨. This switch also takes a negative form, so you can negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect 🙌. The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.) Also note: specifying -Werror=foo automatically implies -Wfoo 🔥. However, -Wno-error=foo does not imply anything 🫥.
• -Wfatal-errors — this one makes the compiler abort compilation on the first error that occurs, rather than trying to keep going and printing further error messages 🛑.

You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations 🌈. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example -Wno-implicit 💅. This manual lists only one of the two forms, whichever is not the default 👑. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer; 🔍

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

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

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

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

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

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

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

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based 🎀. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.) 🫠

-pedantic-errors

okay so bestie, this one is the ✨strict✨ sibling of -Wpedantic. It throws an error whenever the base standard (peep -Wpedantic) says a diagnostic is required — including some cases where there's undefined behavior at compile-time, plus some other cases that don't even stop you from compiling programs that are valid according to the standard. 🫠

Heads up: this is not the same thing as -Werror=pedantic. The latter is honestly lowkey not useful, because it only turns the diagnostics controlled by -Wpedantic into errors, whereas this option also messes with required diagnostics that are always enabled or that are controlled by options other than -Wpedantic. 💅

So if you want those required diagnostics that are warnings by default to become errors, but you don't also wanna turn on the -Wpedantic diagnostics, you can do:

-pedantic-errors -Wno-pedantic

or, if you want them enabled but only as warnings:

-pedantic-errors -Wno-error=pedantic

Some required diagnostics are errors by default but can be downgraded to warnings using -fpermissive or their own specific warning option, like -Wno-error=narrowing. 🎀

And some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but are also made into errors by -pedantic-errors. For instance, bestie, this whole squad right here:

• -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 era. This one takes some required diagnostics about nonconformant code and drops them from errors down to warnings. So yeah, 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)

And the tea: -fpermissive is the default for historic C language modes — -std=c89, -std=gnu89, -std=c90, -std=gnu90. 🫶

---

-Wall

This turns on all the warnings about constructions that some users consider questionable, and that are easy to avoid (or easy to modify to prevent the warning) — even in conjunction with macros. 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:

• -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

Notably, bestie: 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 wanna check for; others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually. 💀

---

-Wextra

This one enables some extra warning flags that aren't enabled by -Wall. 🦋 (Fun fact: this option used to be called -W. The older name is still supported, but the newer name is more descriptive.)

• -Wabsolute-value (only for C/ObjC)
• -Walloc-size
• -Wcalloc-transposed-args
• -Wcast-function-type
• -Wclobbered
• -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-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
• -Wunused-parameter (only with -Wunused or -Wall)
• -Wunused-but-set-parameter (only with -Wunused or -Wall)

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

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

---

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

Warn about code affected by ABI changes. That includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target. 😭

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

-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 is not 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 is generated is compatible. (iconic chaotic energy, honestly.)

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 does not allow for overloading of functions taking vectors of different sizes.

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

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

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

• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing "va_arg" to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than "int".

Also, 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.

These issues were corrected in -fabi-version=6.

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

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

• When it was mangling a function type that had function-cv-qualifiers, the un-qualified function type was being treated as a substitution candidate and that was wrong. Fixed in -fabi-version=8, which is the default for GCC 5.1. 💅
  • decltype(nullptr) lowkey had an alignment of 1, which meant unaligned accesses. But bestie, note that this did not affect the ABI of a function with a nullptr_t parameter, because 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 — like ia32 calling conventions on a function type (stdcall, regparm, etc.) — were not affecting the mangled name, which led to name collisions when function pointers were used as template arguments. Not the vibe. Fixed in -fabi-version=10, the default for GCC 6.1. 🫶

This option also turns on warnings about psABI-related changes. The known psABI changes at this point include:

• For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. Prior to GCC 4.4, that was not the case. For example:

union U {
  long double ld;
  int i;
};

union U is now always passed in memory. Periodt.

✨ -Wno-changes-meaning (C++ and Objective-C++ only) ✨

C++ requires that unqualified uses of a name within a class have the same meaning in the complete scope of the class, so declaring the name after you use it is ill-formed:

struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaning

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

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

🌸 -Wchar-subscripts 🌸

Warns if an array subscript has type char. This is a common cause of error, since programmers often forget that this type is signed on some machines. This warning is enabled by -Wall. 😭

💀 -Wno-coverage-mismatch 💀

Warns if feedback profiles do not match when you're using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC can't use the profile feedback information. By default this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning, or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as 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 and GCC gives up coverage. Coverage is 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-invalid-line-number 🦋

Warns in case a function ends earlier than it begins, due to an invalid linenum macros. 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 be used to disable the warning, or -Wno-error=coverage-invalid-line-number can be used to disable the error. 💅

🍓 -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) 🍓

Suppresses warning messages emitted by #warning directives. No cap.

🔥 -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) 🔥

Gives a warning when a value of type float is implicitly promoted to double. CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation. 💸

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

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

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

⭐ -Wduplicate-decl-specifier (C and Objective-C only) ⭐

Warns if a declaration has duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall.

👑 -Wformat / -Wformat=n 👑

• -Wformat
• -Wformat=n

Checks calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

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

-Wformat=1 / -Wformat

Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. 💖

-Wformat=2

Enables -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k.

---

🦋 -Wno-format-contains-nul 🦋

If -Wformat is specified, do not warn about format strings that contain NUL bytes.

💜 -Wno-format-extra-args 💜

If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored.

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

💀 -Wformat-overflow / -Wformat-overflow=level 💀

• -Wformat-overflow
• -Wformat-overflow=level

Warns about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive can't be determined at compile-time, it's estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. 🙈

-Wformat-overflow / -Wformat-overflow=1

Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. 😬

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

-Wformat-overflow=2

Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long. 🔥

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

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

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}

---

-Wno-format-zero-length

If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. yes bestie, empty is a valid vibe here 💅

-Wformat-nonliteral

If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list. 👀

-Wformat-security

If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.) 🚨

-Wformat-signedness

If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. signed/unsigned drama, we love to see it

-Wformat-truncation

-Wformat-truncation=level

Warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic -Wformat-overflow. 😭

• -Wformat-truncation
• -Wformat-truncation=1 — Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation. lowkey chill about it
• -Wformat-truncation=2 — Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude. highkey thorough

-Wformat-y2k

If -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. y2k but make it a compiler warning 🎀

-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 detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. bestie the precision is giving "it depends"

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

Warn if the compiler does not elide the copy from a local variable to the return value of a function in a context where it is allowed by [class.copy.elision]. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler cannot elide copies from both v1 and v2, so it elides neither. 💀

std::vector<int> f()
{
  std::vector<int> v1, v2;
  // ...
  if (cond) return v1;
  else return v2; // warning: not eliding copy
}

-Winfinite-recursion

Warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions. -Winfinite-recursion is included in -Wall. 🔁

Compare with -Wanalyzer-infinite-recursion which provides a similar diagnostic, but is implemented in a different way (as part of -fanalyzer). same energy, different plumbing

-Winit-self (C, C++, Objective-C and Objective-C++ only)

Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

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

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

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

-Wno-implicit-int (C and Objective-C only)

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

This warning is upgraded to an error by -pedantic-errors. pedantic-errors said "no 💅"

-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 did not enable an option from its set (for which see -fhardened). For instance, using -fhardened and -fstack-protector at the same time on the command line causes -Whardened to warn because -fstack-protector-strong will not be enabled by -fhardened. 😭

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. shortcut queen

-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;
  }

This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.

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

Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur:

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

C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies 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. anything goes, iconic
• -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:

So the regex list for -Wimplicit-fallthrough=3 — the one you'll meet basically every day — is this whole vibe, bestie:

*<"-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]*)?">

And -Wimplicit-fallthrough=4 case sensitively matches one of these:

*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">

And then there's -Wimplicit-fallthrough=5, which is the strictest of them all: it doesn't recognize any comments as fallthrough comments, only attributes disable the warning. No comments, no exceptions, periodt. 💅

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.

• -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — controls whether the warnings triggered by the "warn_if_not_aligned" attribute get issued. These warnings are enabled by default. 🎀
• -Wignored-qualifiers (C and C++ only) — warns you if the return type of a function has a type qualifier like "const". For ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or "void". And heads up: ISO C prohibits qualified "void" return types on function definitions, so those return types always receive a warning even without this option. This warning is also enabled by -Wextra. ✨
• -Wno-ignored-attributes (C and C++ only) — this one controls warnings when an attribute is ignored. It's different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. This warning is enabled by default. 🦋
• -Wmain — warns if the type of "main" is 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. Also: this warning is 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".  */

In the case of mixed tabs and spaces, 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 such as the following example:

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

Also, the warning is not issued after a "#line" directive, since this typically indicates autogenerated code, and no assumptions can be made 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.. (yes bestie that double period is in the original, we don't fix it, we respect it 💀)

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. This option 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*);

__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 — warns about unsafe multiple statement macros that appear to be guarded by a clause such as "if", "else", "for", "switch", or "while", in which only the first statement is actually guarded after the macro is expanded.

For example:

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

will increment "y" unconditionally, not just when "c" holds. The can usually be fixed by wrapping the macro in a do-while loop:

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

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

• -Wparentheses — warns if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.

Also warns if a comparison like "x<=y<=z" appears; this is equivalent to "(x<=y ? 1 : 0) <= z", which is a different interpretation from that of ordinary mathematical notation.

Also warns for dangerous uses of the GNU extension to "?:" with omitted middle operand. 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.

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) — bestie this one snitches on you when a value gets moved to itself using std::move. And like… such a std::move typically does absolutely nothing, so it's giving pointless 💀

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

This warning is enabled by -Wall.

• -Wsequence-point — warns about code that might have undefined semantics because it's breaking the sequence point rules from the C and C++ standards. 😭

So here's the tea: the C and C++ standards define the order in which expressions get evaluated in terms of sequence points, which are basically a partial ordering between the execution of parts of the program — the stuff that runs before the sequence point, and the stuff that runs after it. These pop up after the evaluation of a full expression (one that isn't part of a bigger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after its arguments and the expression denoting the called function get evaluated), and in certain other places. Other than what the sequence point rules say, the order of evaluation of subexpressions of an expression is not specified. All these rules only describe a partial order, not a total order — like, if two functions are called within one expression with no sequence point between them, the order they get called in is not specified. However, the standards committee 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 say: "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. 🙈

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's been found pretty effective at detecting this sort of problem in programs.

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's some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <https://gcc.gnu.org/readings.html>.

This warning is enabled by -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. 🫶
• -Wreturn-mismatch — warns about return statements without an expression in functions that don't 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.

This warning is specific to C and enabled by default. In C99 and later language dialects, it's 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. 😭

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 left shift a negative value. 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 outside the enumeration range also provoke warnings when this option is used (even if there's a default label). This warning is enabled by -Wall.
• -Wswitch-default — warns whenever a switch statement doesn't have a default case.
• -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 outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there's a default label. 💅
• -Wno-switch-bool — don't warn when a switch statement has an index of boolean type and the case values are outside the range of a boolean type. It's possible to suppress this warning by casting the controlling expression to a type other than bool. For example:

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

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

• -Wno-switch-outside-range — this option controls warnings when a switch case has a value that's outside of its respective type range. This warning is enabled by default for C and C++ programs.
• -Wno-switch-unreachable — don't warn when a switch statement contains statements between the controlling expression and the first case label, which will never be executed. For example:

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

-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:

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

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

• -Wsync-nand (C and C++ only) — warns when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4. 😭
• -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init can't initialize the automatic variable. A common situation is an automatic variable that's declared between the controlling expression and the first case label of a switch statement.
• -Wunused-but-set-parameter — warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).

To suppress this warning use the unused attribute.

This warning is also enabled by -Wunused together with -Wextra.

• -Wunused-but-set-variable — warns whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall.

To suppress this warning use the unused attribute.

This warning is also enabled by -Wunused, which is enabled by -Wall.

• -Wunused-function — warns whenever a static function is declared but not defined, or a non-inline static function is unused. This warning is enabled by -Wall.
• -Wunused-label — warns whenever a label is declared but not used. This warning is enabled by -Wall.

To suppress this warning use the unused attribute.

• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warns when a typedef locally defined in a function isn't used. This warning is enabled by -Wall.
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration. This option is not enabled by -Wunused unless -Wextra is also specified.

To suppress this warning use the unused attribute.

• -Wno-unused-result — don't 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 — bestie, this one yells at you whenever a local or static variable is just... sitting there, unused, existing only for its declaration and doing absolutely nothing else with its life. It also quietly turns on -Wunused-const-variable=1 for C, but not for C++. Enabled by -Wall.

Wanna shut it up? Slap the "unused" attribute on that variable and move on. 💅

• -Wunused-const-variable / -Wunused-const-variable=n — warns when a constant static variable is out here being unused aside from its declaration. Same energy as above.

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 you included. This level is enabled by either -Wunused-variable or -Wunused for C, but not for C++. Also, a heads-up: in C this declares variable storage, but in C++ this is not an error since const variables take the place of "#define"s.
• -Wunused-const-variable=2 — this level also warns for unused constant static variables in headers (excluding system headers, thank god). It's equivalent to the short form -Wunused-const-variable. And you must explicitly request this level in both C and C++, because fr fr it might be hard to clean up all headers included. 😭

• -Wunused-value — warns whenever a statement computes a result that is explicitly not used. To suppress 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. Example time: an expression like "x[i,j]" causes a warning, while "x[(void)i,j]" does not. No cap. Enabled by -Wall.
• -Wunused — the whole squad of the above -Wunused options combined, except those documented as needing to be specified explicitly. 👑

Wanna get a warning about an unused function parameter? You gotta either specify -Wextra -Wunused (and yes, -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter.

Also important: -Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++. Periodt.

• -Wuse-after-free (C, Objective-C, C++ and Objective-C++ only) / -Wuse-after-free=n — this one warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. It's enabled at all optimization levels but might yield different results with optimization than without. 🌸
  • -Wuse-after-free=1 — at level 1, the warning only tries to diagnose 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 plus uses in arithmetic and relational expressions. Even though it's undefined, 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, it also diagnoses conditional uses of pointers made indeterminate by a deallocation call. And just like at level 2 (yes, it says 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, it 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're diagnosed at a separate level to help gradually transition legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:

        void adjust_pointers (int**, int);

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

To dodge the warning at this level, store offsets into allocated memory instead of pointers. That approach obviates needing to adjust the stored pointers after reallocation. 🧚

• -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. Lowkey iconic. This warning does not occur when a class object is converted to a non-reference type, since 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. 💀

Plus, 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 instead.)

Wanna warn about code that uses the uninitialized value of the variable in its own initializer? That's -Winit-self's whole job.

These warnings happen 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.

Also note: 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 printed. Sneaky.

In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about "b" being uninitialized in this 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's initialized, but there exist some other paths where the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths aren't executed at run time. 🦋

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

These warnings are only possible in optimizing compilation, because otherwise GCC doesn't keep track of the state of variables.

They're made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here's one example of how that can happen:

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

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

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

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

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warns when a "#pragma" directive is encountered that GCC doesn't understand. And if this command-line option is used, warnings are even issued for unknown pragmas in system header files. That's not the case if the warnings are only enabled by the -Wall command-line option. 🌈
• -Wno-pragmas — do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas.
• -Wno-prio-ctor-dtor — okay so bestie, this one says: don't warn when a priority from 0 to 100 gets used for a constructor or destructor. Because the constructor and destructor attributes let you hand a priority to that constructor/destructor to control the order it runs in — either before "main" gets called or after it returns. 💅 But here's the tea: those priority values must be greater than 100, because the compiler reserves the 0--100 range for the implementation itself. So if you're using 0-100, that's a you problem, and GCC is like "not me warning about this."
• -Wstrict-aliasing — this one only does anything when -fstrict-aliasing is active, iykyk. It warns about code that might break the strict aliasing rules the compiler is using for optimization. Now, it does not catch every single case, but it does try to catch the more common pitfalls. It's included in -Wall. And it's equivalent to -Wstrict-aliasing=3. periodt. ✨
• -Wstrict-aliasing=n — again, 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 (so fewer false positives, slay). Higher levels also = more effort, kinda like how -O works. And remember, -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 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 one's only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. So this warning depends on the optimization level, bestie. 🫶

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

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

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

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

-Wstring-compare is enabled by -Wextra.

• -Wno-stringop-overflow
• -Wstringop-overflow
• -Wstringop-overflow=type — warn for calls to string manipulation functions such as memcpy and strcpy that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for functions that operate on character arrays but not for raw memory functions like memcpy which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object or at most SIZE_MAX / 2 bytes. The option produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. 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
}

Option -Wstringop-overflow=2 is enabled by default. 👑

• -Wstringop-overflow
• -Wstringop-overflow=1 — the -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine 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 determine 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 determine 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 determine 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 iconic example, this next call to strncpy copies into d literally just the characters before that terminating NUL — and then doesn't append the NUL at the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake (we've all been there bestie 😭), so the call gets diagnosed. If you're not expecting a NUL-terminated result, just call memcpy instead and dodge the warning entirely.

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

And in this little guy, the call to strncpy uses the size of the destination buffer as the bound. If the source string's length is equal to or greater than that size, the copy won't be NUL-terminated. So yeah, also diagnosed. To avoid it, pass 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);
  ...
}

But say you want a character array that stores a sequence of bytes with no terminating NUL — that's valid, no cap. You can annotate that array with attribute nonstring to skip this warning. Heads up though: those arrays are not suitable arguments to functions that expect NUL-terminated strings. To catch accidental misuse of them, GCC issues warnings unless it can prove the use is safe. 🛡️

---

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

Warns about improper usages of flexible array members, based on the level of the strict_flex_array (level) attribute attached to the trailing array field of a structure if it's available — otherwise based on the level of the option -fstrict-flex-arrays=level. Note that -Wstrict-flex-arrays is only effective when level is greater than 0, fr fr.

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

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

---

🌈 -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]returns_nonnull| 🌈

Warns for cases where adding an attribute might be beneficial. The attributes currently supported are listed below, bestie:

• -Wsuggest-attribute=pure
• -Wsuggest-attribute=const
• -Wsuggest-attribute=noreturn
• -Wmissing-noreturn
• -Wsuggest-attribute=malloc
• -Wsuggest-attribute=returns_nonnull
• -Wno-suggest-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 (for pure and const) if it can't prove the function returns normally. A function returns normally if it doesn't contain an infinite loop and doesn't return abnormally by throwing, calling abort, or trapping. This analysis requires the option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve how accurate the analysis is. 📈

• -Wsuggest-attribute=format
• -Wmissing-format-attribute

Warn about function pointers that might be candidates for format attributes. Lowkey note: these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes 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 should also have a format attribute to avoid the warning.

GCC also warns about function definitions that might be candidates for format attributes. Again, only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but that might not always be the case, and some functions where format attributes would be appropriate may not get detected at all. 🙈

• -Wsuggest-attribute=cold

Warn about functions that might be candidates for the cold attribute. This is based on static detection and generally only warns about functions which always lead to a call to another cold function — think 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 calls 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 the sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning, no cap.

The first argument to calloc is documented to be the number of elements in the array, while the second argument is the size of each element — so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If having sizeof in the earlier argument and not the latter is intentional, you can suppress the warning by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4), or just use sizeof in the later argument as well. Slay. ✨

---

🦋 -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 this option can be disabled either by specifying a 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. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger.

---

🔥 -Walloca 🔥

This option warns on all uses of alloca in the source. All of them. Every single one. 👀

---

🌈 -Walloca-larger-than=byte-size 🌈

This option warns on calls to alloca with an integer argument whose value is either zero, or that isn't bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. Arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range. 😭

For example, a bounded case of alloca could be:

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

In the above example, passing -Walloca-larger-than=1000 would not issue a warning because the call to alloca is known to be at most 1000 bytes. But if -Walloca-larger-than=500 were passed, the compiler would emit a warning. Slippery slope fr.

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

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.

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

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

In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. 💀

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 🌟

Disable -Walloca-larger-than= warnings. This option is equivalent to -Walloca-larger-than=SIZE_MAX or larger.

---

💅 -Warith-conversion 💅

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

void f (char c, int i)
{
  c = c + i; // warns with B<-Wconversion>
  c = c + 1; // only warns with B<-Warith-conversion>
}

---

⭐ -Warray-bounds ⭐

⭐ -Warray-bounds=n ⭐

Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It's more effective when -ftree-vrp is active (the default for -O2 and above), but a subset of instances are issued even without optimization. 🫶

So like, bestie, by default the trailing array of a struct is treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it's declared as any of these:

• a flexible array member per the C99 standard onwards — that's []
• a GCC zero-length array extension — that's [0]
• a one-element array — that's [1]

And because of that, out-of-bounds subscripts or offsets into zero-length arrays or one-element arrays just… don't get warned about by default. Silent. Sneaky. Iconic but dangerous. 💅

You can add -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treats the trailing array of a structure as a flexible array member:

• when level<=1, no change to the default behavior.
• when level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays;
• when level=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays.
• -Warray-bounds=1 — this is the default warning level of -Warray-bounds and it's enabled by -Wall. The higher levels are not, and you have to explicitly request them. No auto-upgrade, bestie.
• -Warray-bounds=2 — this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. Heads up tho: this level may give a larger number of false positives, and it is deactivated by default. 🫠

---

-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. Freebie warning, you're welcome. ✨

---

-Warray-parameter / -Warray-parameter=n 📏

Warns about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds.

So here's the tea: 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. Caught in 4K.

        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. And the -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments. VLAs stay messy, we been knew.

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. Bugs. In MY code? It's more likely than you think. 😭
• -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. That's the whole roster, no subs. 👑

-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. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous — for instance, it may seem that a piece of code is not commented out, whereas it in fact is. That's literally a jumpscare. 💀

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. Order matters, bestie, keep track. 🎀

---

-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. Always false? Girl, that's not a condition, that's a vibe. 🙈

---

-Wbool-operation 🔁

Warns about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.)

This warning is enabled by -Wall. ✨

---

-Wduplicated-branches 🪞

Warns when an if-else has identical branches. This warning detects cases like

        if (p != NULL)
          return 0;
        else
          return 0;

It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators:

          int i = x ? *p : *p;

Same branch twice? That's not a choice, that's a bit. 💅

---

-Wduplicated-cond 🔂

Warns about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

        if (p->q != NULL) { ... }
        else if (p->q != NULL) { ... }

Bestie you already asked that. It's the same question. 😭

---

-Wframe-address 🪜

Warns when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -Wall. Crash the program?? Not the crashout arc. 💀

---

-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. You're allowed to be messy on purpose, bestie.

---

-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. Pedantic mode does NOT play. Periodt. 🎀

---

-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-length-bounds 0️⃣

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. Undefined behavior, bestie. We do not stan.

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
                   }

And bestie, just so you know: -Wzero-length-bounds is enabled by -Warray-bounds. 💖

• -Wno-div-by-zero — Don't warn about integer division by zero when it's known at compile time. Floating-point division by zero? That's not warned about, because it can be a totally legit way to get infinities and NaNs. ✨
• -Wsystem-headers — Print warning messages for stuff found in system header files. Normally warnings from system headers are suppressed, because the assumption is they usually don't indicate real problems and would only make compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. But heads up: using -Wall together with this option does not warn about unknown pragmas in system headers—for that, -Wunknown-pragmas must also be used. 💅
• -Wtautological-compare — Warn if a self-comparison always evaluates to true or false. This warning catches various mistakes like:

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

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

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

which will always be false. This warning is enabled by -Wall. 👑

• -Wtrampolines — Warn 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's made up of data only and thus requires no special treatment. But for most targets, it's made up of code and thus requires the stack to be made executable in order for the program to work properly. 😭
• -Wfloat-equal — Warn if floating-point values are used in equality comparisons. The idea behind this is that sometimes it's convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you're doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check 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) — Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. Here's the whole vibe check:
  • 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. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
• -Wtraditional-conversion (C and Objective-C only) — Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument except when the same as the default promotion. 🔥
• -Wdeclaration-after-statement (C and Objective-C only) — Warn when a declaration is found after a statement in a block. This construct, 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 is 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 — bestie, this one clocks your function frames 👀 if a frame gets bigger than byte-size, it warns you. BUT ⚠️ the math it does to figure out the stack frame size is approximate and not conservative — so the real requirements might be somewhat greater than byte-size even if you don't get a warning. And plot twist: any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when deciding whether to warn you. -Wframe-larger-than=PTRDIFF_MAX is enabled by default. You can turn these warnings off either by specifying a byte-size of SIZE_MAX or more, or with -Wno-frame-larger-than.
• -Wno-frame-larger-than — turns off the -Wframe-larger-than= warnings. It's the exact same energy as -Wframe-larger-than=SIZE_MAX or larger. no cap 💅
• -Wfree-nonheap-object — warns when you try to deallocate something that either wasn't allocated on the heap, or you're using a pointer that wasn't returned from a prior call to the corresponding allocation function. Like, for example: since stpcpy returns a pointer to the terminating nul character and not to the beginning of the object, that free call down there gets diagnosed:

void f (char *p)
{
  p = stpcpy (p, "abc");
  // ...
  free (p);   // warning
}

-Wfree-nonheap-object is included in -Wall. 🫶

• -Wstack-usage=byte-size — warns if a function's stack usage might exceed byte-size. The computation here is conservative (opposite vibe from the frame one, iykyk). Any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when determining whether to warn you.

The message matches 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. You can disable these warnings either by specifying a byte-size of SIZE_MAX or more, or with -Wno-stack-usage.

• -Wno-stack-usage — disables the -Wstack-usage= warnings. Equivalent to -Wstack-usage=SIZE_MAX or larger. ✨
• -Wunsafe-loop-optimizations — warns if the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. With -funsafe-loop-optimizations, it warns if the compiler does make such assumptions. the way that... 💀
• -Wno-pedantic-ms-format (MinGW targets only) — when you use this together with -Wformat and -pedantic without GNU extensions, it 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 — warns 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++, it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.

Heads up bestie: this warning is upgraded to an error by -pedantic-errors. 😭

• -Wno-pointer-compare — does not warn if a pointer is compared with a zero character constant. That usually means the pointer was meant to be dereferenced. For example:

const char *p = foo ();
if (p == '\0')
  return 42;

Note that the code above is invalid in C++11. 🚨

This warning is enabled by default.

• -Wno-tsan — disables warnings about unsupported features in ThreadSanitizer.

ThreadSanitizer does not support std::atomic_thread_fence and can report false positives. 🙈

• -Wtype-limits — warns if a comparison is always true or always false due to the limited range of the data type, but does 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) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, calling abs(3.14) triggers the warning because the appropriate function to call to compute the absolute value of a double argument is fabs. The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast, and it's also enabled by -Wextra.
• -Wcomment
• -Wcomments — warns 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 — warns 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 is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.

• -Wundef — warns if an undefined identifier is evaluated in an #if directive. Such identifiers are replaced with zero.
• -Wexpansion-to-defined — warns whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.
• -Wunused-macros — warns about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note 📝: if a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — does not warn whenever an #else or an #endif are followed by text. This sometimes happens in older programs with code of the form

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

The second and third FOO should be in comments. This warning is on by default.

• -Wbad-function-cast (C and Objective-C only) — warns when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type. 💀
• -Wc90-c99-compat (C and Objective-C only) — warns about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable length arrays, long long type, 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 present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _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, warn about 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__. The name -Wc11-c2x-compat is deprecated. 👑

When not compiling in C23 mode, these warnings are upgraded to errors by -pedantic-errors.

• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit conversion from void * to a pointer to non-void type.
• -Wc++11-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.
• -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is 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. This warning is 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. This warning is enabled by -Wall.
• -Wno-c++11-extensions (C++ and Objective-C++ only) — do not warn about C++11 constructs in code being compiled using 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 using 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 using 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) — bestie, this one says don't yell at me about C++20 constructs when I'm compiling with an older C++ standard. And lowkey even if you don't pass it, some C++20 constructs only get diagnosed when you're using -Wpedantic anyway. So it's giving "chill, I know what I'm doing" 💅
• -Wno-c++23-extensions (C++ and Objective-C++ only) — same energy, but for C++23 constructs in code being compiled with an older C++ standard. And fr fr, even without this option, some C++23 constructs are only diagnosed if -Wpedantic is in the mix. ✨
• -Wno-c++26-extensions (C++ and Objective-C++ only) — and the trilogy concludes: no warnings about C++26 constructs when you're compiling against an older C++ standard. Again, some C++26 constructs only get diagnosed under -Wpedantic, option or no option. 🦋
• -Wcast-qual — okay so this one goes off whenever a pointer gets cast in a way that strips a type qualifier off the target type. Like, if a const char * gets cast to a plain old char *? Yeah, that's a warning, bestie. 😭

It also warns when a cast introduces a type qualifier in an unsafe way. Example: casting char ** to const char ** is unsafe, and here's literally why:

/* 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 — warn whenever a pointer cast makes the required alignment of the target go ⬆️ up. Classic case: a char * getting cast to an int * on machines where ints can only be accessed at two- or four-byte boundaries. Alignment is not a vibe, it's a rule. 🙈
• -Wcast-align=strict — the exact same warning (pointer cast raises required alignment of the target, e.g. char * → int *) but regardless of the target machine. No "well, this machine is fine" passes. Strict is strict, periodt. 👑
• -Wcast-function-type — warn when a function pointer gets cast to an incompatible function pointer. The deets: 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. Likewise type qualifiers are ignored. 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. And heads up — this warning is enabled by -Wextra. 🔥
• -Wcast-user-defined — warn when a cast to reference type doesn't involve a user-defined conversion that the programmer might have expected to be called. Like:

struct A { operator const int&(); } a;
auto r = (int&)a; // warning

This warning is enabled by default. So it's just there, watching. 👀

• -Wwrite-strings — okay so in C, this gives string constants the type const char[length], which means copying the address of one into a non-const char * pointer produces a warning. These warnings help you find code at compile time that can try to write into a string constant — but only if you've been super careful about using const in declarations and prototypes. Otherwise it's just a nuisance, and that's literally why -Wall doesn't request these warnings. 💀

In C++, it warns about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.

Also: this warning is upgraded to an error by -pedantic-errors in C++11 mode or later. Not a drill. 🚨

• -Wclobbered — warn about variables that might get changed by longjmp or vfork. Also enabled by -Wextra. Those variables really said "I'm in danger" 😭
• -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. Snitchy little front end, honestly. 🙈

• -Wcompare-distinct-pointer-types (C and Objective-C only) — warn if pointers of distinct types are compared without a cast. Enabled by default. Don't compare apples to int *, bestie. 🍓
• -Wconversion — warn for implicit conversions that may alter a value. This includes:
  • conversions between real and integer, like abs (x) when x is double
  • conversions between signed and unsigned, like unsigned ui = -1
  • conversions to smaller types, like sqrtf (M_PI)

It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value isn't changed by the conversion — like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.

For C++, it also warns for confusing overload resolution for user-defined conversions, and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.

And one more: warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion. 🌸

• -Wdangling-else — warn about constructions where there might be confusion about which if statement an else branch belongs to. Here's an example of such a case:

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

In C/C++, every else branch belongs to the innermost possible if statement — which in this example is if (b). That is often not what the programmer expected, as the indentation in the above example illustrates. When there's potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there's no way the else can belong to the enclosing if. The resulting code looks like this:

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

This warning is enabled by -Wparentheses. 🎀

• -Wdangling-pointer
• -Wdangling-pointer=n — warn about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. This includes local variables declared in nested blocks, compound literals and other unnamed temporary objects. On top of that, it warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels, but it may yield different results with optimization than without. 👑
  • -Wdangling-pointer=1 — at level 1, the warning diagnoses only unconditional uses of dangling pointers.
  • -Wdangling-pointer=2 — at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of dangling pointers.

The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 have the same effect of disabling the warnings. And -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 of its scope:

char f (char c1, char c2, char c3)
{
  char *p;
  {
    p = (char[]) { c1, c2, c3 };
  }
  // warning: using dangling pointer 'p' to an unnamed temporary
  return *p;
}

In the following function, the store of the address of the local variable x in the escaped pointer *p triggers the warning at level 1:

void g (int **p)
{
  int x = 7;
  // warning: storing the address of local variable 'x' in '*p'
  *p = &x;
}

In this example, the array a is out of scope when the pointer s is used. Since the code that sets s is conditional, the warning triggers at level 2:

extern void frob (const char *);
void h (char *s)
{
  if (!s)
    {
      char a[12] = "tmpname";
      s = a;
    }
  // warning: dangling pointer 's' to 'a' may be used
  frob (s);
}

• -Wdate-time — warn when the macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered, because they might prevent bit-wise-identical reproducible compilations. Not the reproducible-build girlies' fav 😭
• -Wempty-body — warn if an empty body occurs in an if, else or do while statement. Also enabled by -Wextra. An empty body said absolutely nothing and that was the problem 💀
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif. Live your truth, leave your little notes there. 🫶
• -Wenum-compare — warn about a comparison between values of different enumerated types. In C++, enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C, this warning is enabled by -Wall. Comparing two different enums is giving "these are not the same thing bestie" 💅
• -Wenum-conversion — warn when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by -Wextra in C. 🌈
• -Wenum-int-mismatch (C and Objective-C only) — warn 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. However, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues. 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) — okay bestie, so this one yells at you when a goto or a switch jumps forward across a variable's initialization, or jumps backward to a label after that variable already got initialized. It only cares about variables that get initialized at the moment they're declared, bestie, so if you declared it then assigned it later, this warning is not the one. Also it's literally only a thing for C and Objective-C — in C++ this kinda branch is just an error, no warning, no mercy, periodt. 💅

This one is bundled up inside -Wc++-compat, and you can turn it off with -Wno-jump-misses-init. slay.

• -Wsign-compare — warns you when a comparison between a signed value and an unsigned value could give you a wrong answer, specifically when that signed value gets converted to unsigned and just. ruins everything. In C++, -Wall turns this on too. In C, -Wextra is the one that turns it on. 🌸
• -Wsign-conversion — warns about implicit conversions that might flip the sign of an integer value, like assigning a signed integer expression into an unsigned integer variable. An explicit cast silences it, no cap. In C, this option also gets enabled by -Wconversion.
• -Wflex-array-member-not-at-end (C and C++ only) — warns when a struct that has a C99 flexible array member as its last field is itself not sitting at the end of another struct. It's giving "this layout is not what you think it is." For example, it warns about:

struct flex  { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };

😭 iconic problem honestly.

• -Wfloat-conversion — warns about implicit conversions that reduce the precision of a real value. That includes real → integer, and higher-precision real → lower-precision real. Also gets enabled by -Wconversion. 🦋
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order. simple as that bestie.
• -Wsizeof-array-div — warns about dividing two sizeof operators when the first one is applied to an array and the divisor isn't equal to the size of the array's element. Because in that case the computation is NOT gonna give you the number of elements in the array, which is like, almost definitely what you wanted. It warns about stuff 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 the pointer size by the element size — which is literally the classic way to compute array size but does NOT work out when you're holding a pointer. It warns about e.g. sizeof (ptr) / sizeof (ptr[0]) when ptr is not an array but a pointer. Enabled by -Wall. 🔥
• -Wsizeof-pointer-memaccess — warns for suspicious length parameters handed to certain string and memory built-in functions when the argument uses sizeof. Like it triggers on memset (ptr, 0, sizeof (ptr)); when ptr is a pointer, not an array — and it even suggests a possible fix. Also flags stuff like memcpy (&foo, ptr, sizeof (&foo));.

It ALSO warns about calls to bounded string copy functions like strncat or strncpy that pass a sizeof expression of the source array as the bound. Look at this — the strncat call passes the size of the source string as the bound, which is almost certainly a mistake, so the call gets diagnosed:

void make_file (const char *name)
{
  char path[PATH_MAX];
  strncpy (path, name, sizeof path - 1);
  strncat (path, ".text", sizeof ".text");
  ...
}

This option is enabled by -Wall. ✨

• -Wno-sizeof-array-argument — do not warn when the sizeof operator is applied to a parameter that's declared as an array in a function definition. This warning is enabled by default for C and C++ programs. 🌈
• -Wmemset-elt-size — warns for suspicious calls to the memset built-in 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. Translation: you forgot to multiply by the element size, bestie. 💀 Enabled by -Wall.
• -Wmemset-transposed-args — warns for suspicious memset calls where the second argument isn't zero and the third argument is zero. Like memset (buf, sizeof buf, 0) gets diagnosed, because you clearly meant memset (buf, 0, sizeof buf) and just. swapped them. 🙈

Important detail though: the diagnostic only fires if that third argument is a literal zero. If it's an expression that folds to zero, or a cast of zero to some type, then it's way less likely the args got transposed, so no warning. Enabled by -Wall.

• -Waddress — warns about suspicious uses of address expressions. 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 ();
}

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 mean a programmer goofed: the address of most functions and objects necessarily evaluates to true (the exception is weak symbols), so using one in a conditional might mean you dropped parentheses in a function call or dropped a dereference in an array expression. The object-pointer subset of the warning can be silenced by casting the pointer operand to an integer type like intptr_t or uintptr_t. Comparisons against string literals give you unspecified behavior and aren't portable, and honestly they just scream "I meant to call strcmp." The warning is suppressed if the suspicious expression came from macro expansion. 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 where a bit-wise operator was probably expected. It also warns when both operands of a logical operator are literally the same:

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

bestie. that's the same thing twice. 💅

• -Wlogical-not-parentheses — warns about a logical not used on the left-hand-side operand of a comparison. It does NOT warn if the right operand is considered a boolean expression. Its whole purpose is catching suspicious code like:

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

You can suppress the warning by wrapping the LHS in parentheses:

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

Enabled by -Wall. 🌸

• -Waggregate-return — warns if any functions that return structures or unions get defined or called. (And in languages where you can return an array, that also gets a warning.) 🦋
• -Wno-aggressive-loop-optimizations — warns if, in a loop with a constant number of iterations, the compiler detects undefined behavior in some statement during one or more of those iterations. 😭
• -Wno-attributes — do not warn if an unexpected __attribute__ is used, like unrecognized attributes, function attributes slapped onto variables, etc. This does NOT stop errors for incorrect use of supported attributes, so don't get it twisted.

Warnings about ill-formed uses of standard attributes get upgraded to errors by -pedantic-errors.

Also — using -Wno-attributes=, you can suppress warnings about unknown scoped attributes (in C++11 and C23). For example, -Wno-attributes=vendor::attr disables the warning about this declaration:

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

And you can disable warnings about all attributes in a namespace with -Wno-attributes=vendor::, which prevents warnings about both of these:

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

Note that -Wno-attributes= does not imply -Wno-attributes. periodt. 💜

• -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype is called with arguments whose promoted types don't match what the function expects. When -Wextra is specified, it ALSO warns when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by default. To dodge the warning, include the appropriate header so the prototypes of built-in functions come into scope. ✨

For example, the memset call below gets diagnosed because the function expects a value of type size_t as its argument but the type of 32 is int. With -Wextra, the declaration of the function gets diagnosed too:

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__. 🌈
• -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if it's preceded by a declaration that specifies the argument types.) 👑
• -Wold-style-declaration (C and Objective-C only) — warns you when you're doing obsolescent stuff in a declaration, per the C Standard. Like bestie, if your storage-class specifiers such as static are not the very first thing in a declaration, you're getting a warning. Also switched on by -Wextra. 👑
• -Wold-style-definition (C and Objective-C only) — screams at you if you used an old-style function definition. It warns even if there's a previous prototype chilling there already. Lowkey important: a definition using () is not counted as old-style in C23 mode, because in that case it's equivalent to (void) — but for older standards it totally is old-style. 🙈
• -Wmissing-parameter-type (C and Objective-C only) — a function parameter got declared with no 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 has a parameter name with no type. Heads up though: those declarations don't give you a function prototype and they block 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. That error can be downgraded to a warning with -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 with no previous prototype declaration. It fires even if the definition itself provides a prototype. Use this to sniff out global functions that don't have a matching prototype declaration in a header file. Not valid for C++, bestie — because in C++ all function declarations provide prototypes, and a non-matching declaration just declares an overload rather than conflicting with an earlier declaration. For C++ missing declarations, use -Wmissing-declarations.
• -Wmissing-variable-declarations (C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use it to catch global variables that don't have a matching extern declaration in a header file. 🎀
• -Wmissing-declarations — warns if a global function is defined without a previous declaration, even when the definition itself gives you a prototype. Use this to find global functions that aren't declared in header files. In C, no warnings for functions with previous non-prototype declarations — use -Wmissing-prototypes for those. In C++, no warnings for function templates, inline functions, or functions in anonymous namespaces. ✨
• -Wmissing-field-initializers — warns when a structure's initializer is missing some fields. Example: this code triggers it because x.h is implicitly zero:

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

In C this option does not warn about designated initializers, so this modification doesn't trigger anything:

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

And in C it also doesn't warn about the universal zero initializer { 0 }:

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

Same energy in C++: this option does not warn about the empty { } initializer, like:

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

This warning is included in -Wextra. Want the other -Wextra warnings without this one? Use -Wextra -Wno-missing-field-initializers. 💅

• -Wno-missing-requires — by default, the compiler warns about a concept-id showing up as a C++20 simple-requirement:

bool satisfied = requires { C<T> };

Here satisfied will be true if C<T> is a valid expression, which it is for all T. Presumably you meant to write:

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

so 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 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 you can get false positives. To silence it, wrap the expression in parentheses. For example, this next bit is treated as a template, even where m and N are integers:

void NotATemplate (my_class t)
{
  int N = 5;

  bool test = t.m < N > (0); // Treated as a template.
  test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}

This warning can be disabled with -Wno-missing-template-keyword.

• -Wno-multichar — do not warn if a multicharacter constant ('FOOF') is used. Usually these are a typo in your code, because they have implementation-defined values and should not be used in portable code. 💀
• -Wnormalized=[none|id|nfc|nfkc] — in ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes, when characters outside the basic ASCII set are used, you can end up with two different character sequences that look identical. To avoid confusion, the ISO 10646 standard lays out normalization rules that, when applied, make two look-alike sequences turn into the same sequence. GCC can warn you if you're using identifiers that haven't been normalized — this option controls that warning. 🌈

There are four levels of warning GCC supports. The default is -Wnormalized=nfc, which warns about any identifier not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It's equivalent to -Wnormalized.

Unfortunately, some characters allowed in identifiers by ISO C and ISO C++ — when turned into NFC — are not allowed in identifiers. Like, there's genuinely 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. The hope is that future versions of the standards involved will fix 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. Only do this if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 😭

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially after formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular n that's been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all of these into a standard form too, and GCC warns if your code isn't in NFKC when you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier containing the letter O because it might be confused with the digit 0 — so it's not the default, but it can be useful as a local coding convention if the programming environment can't be fixed to display these characters distinctly.

• -Wno-attribute-warning — do not warn about usage of functions declared with the warning attribute. By default this warning is enabled. -Wno-attribute-warning can be used to disable the warning, or -Wno-error=attribute-warning can be used to disable the error when compiled with the -Werror flag. 🫶
• -Wno-deprecated — do not warn about usage of deprecated features.
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated via 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 — warns about potentially suboptimal choices related to OpenACC parallelism.
• -Wno-openmp — warns about suspicious OpenMP code.
• -Wopenmp-simd — warns if the vectorizer cost model overrides the OpenMP simd directive you set. The -fsimd-cost-model=unlimited option can be used to relax the cost model. ⭐
• -Woverride-init (C and Objective-C only) — warns if an initialized field without side effects gets overridden when you're using designated initializers.

This warning is included in -Wextra. Want the other -Wextra warnings without this one? Use -Wextra -Wno-override-init.

• -Wno-override-init-side-effects (C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.
• -Wpacked — warns if a structure is given the packed attribute, but that packed attribute has zero effect on the layout or size of the structure. Such structures may end up mis-aligned for basically no benefit. For instance, here the variable f.x in struct bar is misaligned even though struct bar doesn't itself have the packed attribute:

struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};

• -Wnopacked-bitfield-compat — the 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This got fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC lets you know when the offset of such a field has changed in GCC 4.4. For example there's no longer a 4-bit padding between field a and b in this structure:

struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

---

This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warns you when a struct field that has explicitly specified alignment sitting inside a packed struct or union ends up misaligned. Like, bestie, you asked for aligned(8) and then packed the whole thing, so of course it's crooked. For example, a warning gets issued on struct S, something 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 — warns if padding gets baked into a structure, whether it's there to align one element or to align the whole structure. And honestly? Sometimes when this happens you can rearrange the fields of the structure to shrink the padding and make the whole thing smaller. So it's giving helpful, not just judgy. 💅
• -Wredundant-decls — warns if literally anything is declared more than once in the same scope. Even in the cases where declaring it twice is valid and changes absolutely nothing. Redundant? yes. Illegal? no. Still gonna tell you. 🙈
• -Wrestrict — warns when an object referenced by a restrict-qualified parameter (or, in C++, a __restrict-qualified parameter) gets aliased by another argument, or when copies between such objects overlap. So here's the vibe: the call to the strcpy function below is trying to truncate the string by replacing its initial characters with the last four. But because the call writes the terminating NUL into a[4], the copies overlap and the call gets diagnosed. 😭

void foo (void)
{
  char a[] = "abcd1234";
  strcpy (a, a + 4);
  ...
}

The -Wrestrict option catches some instances of simple overlap even with no optimization at all, but it works best at -O2 and above. It is included in -Wall.

• -Wnested-externs (C and Objective-C only) — warns if an extern declaration shows up inside a function. Why is it in there. Get it out. 💀
• -Winline — warns if a function that's declared as inline cannot actually be inlined. Even with this option on, the compiler does not warn about failures to inline functions declared in system headers. Not our problem, not our drama.

The compiler uses a whole variety of heuristics to decide whether or not to inline a function. 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 seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. Iconic chaotic behaviour, honestly.

• -Winterference-size — warns about use of C++17 std::hardware_destructive_interference_size without specifying its value with --param destructive-interference-size. Also warns about questionable values for that option. 🚩

This variable is intended to be used for controlling class layout, to 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.

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, sometimes to a range if 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 the rule. 💖

If ABI stability is important, such as if the use is in a header for a library, you should probably not use the hardware interference size variables at all. Alternatively, you can force a particular value with --param.

If you are 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.

• -Wint-in-bool-context — warns for suspicious use of integer values where boolean values are expected. Like conditional expressions (?:) using non-boolean integer constants in boolean context, like if (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, like for (a = 0; 1 << a; a++);. Same energy 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. 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. Found it, can't use it. It's giving ghosted. 🌸
• -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 turned into error with -pedantic-errors.
• -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. Not a drill, fr fr.

• -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.
• -Wvector-operation-performance — warns if a vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for performance tuning. 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; and as a single scalar, which means the vector fits into a scalar type. Three flavours, all valid, some faster. ✨
• -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 little optimizer. 🙈

-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.

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 like, the vibe is "I said volatile so it's safe" and the compiler is like "no 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 language, different lore. iykyk.

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 complexity budget. 💀
• -Wpointer-sign (C and Objective-C only) — warns when you pass a pointer argument or do an assignment where the signedness doesn't match. ngl this one is C/Objective-C exclusive, bestie. It's implied by -Wall and by -Wpedantic, and you can turn it off with -Wno-pointer-sign.

Heads up: -pedantic-errors upgrades this warning to a full-on error. 💀

• -Wstack-protector — only does anything when -fstack-protector is active. When it is, it warns about functions that aren't protected against stack smashing. 🛡️
• -Woverlength-strings — warns about string constants that are longer than the "minimum maximum" length the C standard specifies. Modern compilers generally let you go way past the standard's minimum limit, but if your program needs to be very portable you should not be out here using longer strings. ✨

The limit applies after string constant concatenation, and it does not count the trailing NUL. In C90 the limit was 509 characters; in C99 it got raised to 4095. C++98 doesn't specify a normative minimum maximum, so we do not diagnose overlength strings in C++. Periodt.

This option is implied by -Wpedantic, and you can disable it with -Wno-overlength-strings.

• -Wunsuffixed-float-constants (C and Objective-C only) — issues a warning for any floating constant that has no suffix. When you use it together with -Wsystem-headers it also warns about such constants inside system header files. This is genuinely useful when you're prepping code to use with the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99. 🍓
• -Wno-lto-type-mismatch — during link-time optimization, don't warn about type mismatches in global declarations coming from different compilation units. Requires -flto to be enabled. It's enabled by default.
• -Wno-designated-init (C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that's been marked with the "designated_init" attribute.

---

🔮 Options That Control Static Analysis 🔮

• -fanalyzer — turns on a static analysis of program flow that hunts for "interesting" interprocedural paths through your code, and issues warnings for the problems it finds along them. 🔍

This analysis is much more expensive than other GCC warnings. Like, dramatically.

In technical terms: it performs coverage-guided symbolic execution of the code being compiled. 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. No cap.

In this release, the analyzer is only suitable for use on C code.

Enabling this option effectively enables the following warnings:

-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-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

This option is only available if GCC was configured with analyzer support enabled. 🙈

• -Wanalyzer-symbol-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to try to track the state of memory, but sufficiently complicated code can defeat them.

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. The -Wanalyzer-symbol-too-complex option warns you when this 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 sufficiently complicated code can defeat them too.

By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it hits an internal limit. The -Wanalyzer-too-complex option warns you when this happens. 💅

• -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 whose size 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 suggests the pointer could have been NULL. Cases like that suggest the NULL check is either redundant, or it needs to be moved to before the pointer is dereferenced. 🫠

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

This diagnostic is unlikely to be reported when any level of optimization is enabled, because GCC's optimization logic will typically consider such NULL checks redundant and optimize them away before the analyzer ever "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 gets written to an output file (like, writing a password into 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 (like 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 the other way round.

This diagnostic 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 hasn't 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 — okay bestie, this one needs -fanalyzer to even turn on, and it's the analyzer that enables it, so if you wanna shut it up you use -Wno-analyzer-fd-type-mismatch. 💅

This diagnostic is the one that clocks paths through your code where you're doing an operation on the wrong type of file descriptor. Like, it will warn when you're out here trying to do socket operations on a file descriptor you got from open, or when you're attempting a stream socket operation on a datagram socket. Wrong vibe entirely, and the analyzer WILL call it out. 😭

• -Wno-analyzer-fd-use-after-close — requires -fanalyzer, which enables it; hit it with -Wno-analyzer-fd-use-after-close to disable.

This one warns for paths through code where a read or write gets called on a closed file descriptor. Rude. 🙈

It also warns for paths where a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) gets called with a closed file descriptor at referenced argument N.

• -Wno-analyzer-fd-use-without-check — requires -fanalyzer, which enables it; -Wno-analyzer-fd-use-without-check disables it.

This diagnostic warns for paths through code where a file descriptor is used without being checked for validity. No vibe check, no mercy. ✨

It also warns for paths 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 — requires -fanalyzer, which enables it; -Wno-analyzer-file-leak turns it off.

This one warns for paths through the code where a <stdio.h> FILE * stream object is leaked. It's giving "forgot to close the door on your way out" energy. 🚪

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 — requires -fanalyzer, which enables it; -Wno-analyzer-free-of-non-heap disables it.

This diagnostic warns for paths through the code where free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). Ma'am, that is NOT yours to free. 💀

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; -Wno-analyzer-imprecise-fp-arithmetic disables it.

This diagnostic warns for paths through the code where floating-point arithmetic is used in locations where precise computation is needed. At the moment, it only warns on use of floating-point operands inside the calculation of an allocation size. Math girlies stay winning, but only with integers here. 🎀

• -Wno-analyzer-infinite-loop — requires -fanalyzer, which enables it; -Wno-analyzer-infinite-loop disables it.

This diagnostic warns for paths through the code which appear to lead to an infinite loop. We've all been there emotionally. 🌀

Specifically, the analyzer will issue this warning when it "sees" a loop in which:

• no externally-visible work could be being done within the loop
• there is no way to escape from the loop
• the analyzer is sufficiently confident about the program state throughout the loop to know that the above are true

One way for this warning to be emitted is when there is an execution path through a loop for which taking the path on one iteration implies that the same path will be taken on all subsequent iterations.

For example, consider:

          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 for the above case because if opcode ever matches none of the cases, the switch will follow the implicit default case, making the body of the loop be a no-op with cpu_state.pc unchanged, and thus using the same value of opcode on all subseqent iterations, leading to an 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; -Wno-analyzer-infinite-recursion disables it.

This diagnostic warns for paths through the code which appear to lead to infinite recursion. It's the function calling itself forever and ever, no thoughts head empty. 🔁

Specifically, when the analyzer "sees" a recursive call, it will compare the state of memory at the entry to the new frame with that at the entry to the previous frame of that function on the stack. The warning is issued if nothing in memory appears to be changing; any changes observed to parameters or globals are assumed to lead to termination of the recursion and thus suppress the warning.

This diagnostic is likely to miss cases of infinite recursion that are convered to iteration by the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic.

Compare with -Winfinite-recursion, which provides a similar diagnostic, but is implemented in a 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; -Wno-analyzer-jump-through-null disables it.

This diagnostic warns for paths through the code where a NULL function pointer is called. There is nothing there. Nobody is home. Do not call them. 😭

• -Wno-analyzer-malloc-leak — requires -fanalyzer, which enables it; -Wno-analyzer-malloc-leak disables it.

This diagnostic warns for paths through the code where a pointer allocated via an allocator is leaked: either malloc, or a function marked with attribute malloc. The memory is just out there, unbothered, unclaimed, unserfed. 🌸

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; -Wno-analyzer-mismatching-deallocation disables it.

This diagnostic warns for paths through the code where the wrong deallocation function is called on a pointer value, based on which function was used to allocate the pointer value. The diagnostic will warn about mismatches between free, scalar delete and vector delete[], and those marked as allocator/deallocator pairs using attribute malloc. It's giving "you cannot return that, it's from a different store." 🛍️

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; -Wno-analyzer-out-of-bounds disables it.

This diagnostic warns for paths through the code where a buffer is definitely read or written out-of-bounds. The diagnostic applies for cases where the analyzer is able to determine a constant offset and for accesses past the end of a buffer, also a constant capacity. Further, the diagnostic does limited checking for accesses past the end when the offset as well as the capacity is symbolic. Boundaries exist for a reason, bestie. 🚧

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

For cases where the analyzer is able, it will emit a text art diagram visualizing the spatial relationship between the memory region that the analyzer predicts would be accessed, versus the range of memory that is valid to access: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data involved. This diagram can be suppressed using -fdiagnostics-text-art-charset=none. A whole little diagram, she's an artist, she's an icon. 🎨

• -Wno-analyzer-overlapping-buffers — requires -fanalyzer, which enables it; -Wno-analyzer-overlapping-buffers disables it.

This diagnostic warns for paths through the code where overlapping buffers are passed to an API for which the behavior on such buffers is undefined. Sharing is not always caring. 😬

Specifically, the diagnostic occurs on calls to the following functions

• "memcpy"
• "strcat"
• "strcpy"

for cases where the buffers are known to overlap.

• -Wno-analyzer-possible-null-argument — requires -fanalyzer, which enables it; -Wno-analyzer-possible-null-argument disables it.

This diagnostic warns for paths through the code where a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. The function said no NULLs and you said "well, maybe." ✋

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; -Wno-analyzer-possible-null-dereference disables it.

This diagnostic warns for paths through the code where a possibly-NULL value is dereferenced. Possibly. Maybe. Potentially. And the analyzer is watching. 👀

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; -Wno-analyzer-null-argument disables it.

This diagnostic warns for paths through the code where a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. Not "possibly", bestie. Known. Certain. Absolutely. 💀

See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).

• -Wno-analyzer-null-dereference — requires -fanalyzer, which enables it; -Wno-analyzer-null-dereference disables it.

This diagnostic warns for paths through the code where a value known to be NULL is dereferenced. There is nothing at that address. You are knocking on a door that does not exist. 🚪💀

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; -Wno-analyzer-putenv-of-auto-var disables it.

This diagnostic warns for paths through the code in which a call to putenv is passed a pointer to an automatic variable or an on-stack buffer. Bestie, that buffer is literally about to evaporate 😭

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 — this one needs -fanalyzer, which is what turns it on in the first place; use -Wno-analyzer-shift-count-negative to turn it back off.

It warns about paths through the code where somebody tries to shift by a negative count. It's the analyzer-flavored cousin of the -Wshift-count-negative diagnostic that lives in the C/C++ front ends — except this one is built on analyzing interprocedural paths instead of just parsing the syntax tree. Real talk though: the analyzer doesn't prioritize hunting these paths down, so you're more likely to get false negatives here than with the other warnings. 🫠

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

Warns about paths where a shift is attempted with a count that's greater than or equal to the precision of the operand's type. Same deal as -Wshift-count-overflow in the C/C++ front ends, but again, it's built on interprocedural path analysis rather than merely parsing the syntax tree. And yeah — the analyzer doesn't prioritize detecting these either, so false negatives are more likely compared to the other warnings. 🙈

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

Warns about paths where longjmp gets called to rewind to a jmp_buf that relates to a setjmp call in a function that has already returned. 💀

Here's the tea: when setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid the moment the function containing the setjmp call returns. Trying to rewind to it via longjmp means you're referencing a stack frame that does not exist anymore, and that's very likely gonna crash (or worse). Not cute. Not the vibe.

• -Wno-analyzer-tainted-allocation-size — needs -fanalyzer which enables it; use -Wno-analyzer-tainted-allocation-size to disable it.

Warns about paths where a value that could be under an attacker's control gets used as the size of an allocation without being sanitized. Which means an attacker could inject an excessively large allocation and potentially cause a denial of service attack. Sabotage, basically.

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

• -Wno-analyzer-tainted-assertion — needs -fanalyzer which enables it; use -Wno-analyzer-tainted-assertion to disable it.

Warns about paths where a value that could be under an attacker's control is used as part of a condition without being sanitized first, and that condition guards a call to a function marked with attribute noreturn (like the function __builtin_unreachable). Functions like that typically mean the program is terminating abnormally — assertion failure handlers, that kind of thing. Example time:

assert (some_tainted_value < SOME_LIMIT);

In cases like this:

• 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.

Also important, bestie: when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer even 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. Heads up though: these functions are sometimes removed by the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic. 🎀

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

This warning can also report problematic constructions like

switch (some_tainted_value) {
case 0:
  /* [...etc; various valid cases omitted...] */
  break;

default:
  __builtin_unreachable (); /* BUG: attacker can trigger this  */
}

even though, strictly speaking, the above isn't an assertion failure at all. Plot twist.

• -Wno-analyzer-tainted-array-index — needs -fanalyzer which enables it; use -Wno-analyzer-tainted-array-index to disable it.

Warns about paths where a value that could be under an attacker's control gets used as the index of an array access without being sanitized — so an attacker could inject an out-of-bounds access. 👑

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

• -Wno-analyzer-tainted-divisor — needs -fanalyzer which enables it; use -Wno-analyzer-tainted-divisor to disable it.

Warns about paths 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, so an attacker could inject a division-by-zero. Math crimes!!

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

• -Wno-analyzer-tainted-offset — needs -fanalyzer which enables it; use -Wno-analyzer-tainted-offset to disable it.

Warns about paths where a value that could be under an attacker's control gets used as a pointer offset without being sanitized, so an attacker could inject an out-of-bounds access. 💅

See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).

• -Wno-analyzer-tainted-size — needs -fanalyzer which enables it; use -Wno-analyzer-tainted-size to disable it.

Warns about paths where a value that could be under an attacker's control gets used as the size of an operation such as memset without being sanitized, so an attacker could inject an out-of-bounds access. Same energy, different function.

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

• -Wno-analyzer-undefined-behavior-strtok — needs -fanalyzer, which enables it; use -Wno-analyzer-undefined-behavior-strtok to disable it.

Warns about paths 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. That's it. That's the whole thing. Don't do it.

• -Wno-analyzer-unsafe-call-within-signal-handler — needs -fanalyzer, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to disable it.

Warns about paths where a function that's known to be async-signal-unsafe (like fprintf) gets called from a signal handler. 🚩

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

• -Wno-analyzer-use-after-free — needs -fanalyzer, which enables it; use -Wno-analyzer-use-after-free to disable it.

Warns about paths where a pointer is used after a deallocator was called on it — either free, or a deallocator referenced by attribute malloc. You freed it, bestie. It's gone. Let it go.

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

• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — needs -fanalyzer, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.

Warns about paths where a pointer is dereferenced that points to a variable sitting in a stale stack frame. 💀

• -Wno-analyzer-va-arg-type-mismatch — needs -fanalyzer, which enables it; use -Wno-analyzer-va-arg-type-mismatch to disable it.

Warns about interprocedural paths where the analyzer catches an attempt to use va_arg to extract a value that was passed to a variadic call — but using a type that does not match the type of the expression that was passed to the call. Type mismatch girlies, this one's for you.

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

• -Wno-analyzer-va-list-exhausted — needs -fanalyzer, which enables it; use -Wno-analyzer-va-list-exhausted to disable it.

Warns about interprocedural paths where the analyzer catches an attempt to use va_arg to access the next value passed to a variadic call, except all the values in the va_list have already been consumed. 🍽️ Nothing left on the plate, babe.

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

• -Wno-analyzer-va-list-leak — needs -fanalyzer, which enables it; use -Wno-analyzer-va-list-leak to disable it.

Warns about interprocedural paths where the analyzer catches that va_start or va_copy was called on a va_list without a corresponding call to va_end. 🫠

• -Wno-analyzer-va-list-use-after-va-end — needs -fanalyzer, which enables it; use -Wno-analyzer-va-list-use-after-va-end to disable it.

Warns about interprocedural paths where the analyzer catches an attempt to use a va_list after va_end has been called on it. va_list. 💅

• -Wno-analyzer-write-to-const — needs -fanalyzer, which enables it; use -Wno-analyzer-write-to-const to disable it.

...and yeah, this diagnostic? it warns about paths through the code where the analyzer spots an attempt to write through a pointer to a const object. but — and this is the tea — the analyzer does not prioritize detecting these paths, so false negatives are more likely here than with its other warnings. so like, don't get comfy just bc it stayed quiet bestie 💀

---

• -Wno-analyzer-write-to-string-literal — this warning needs -fanalyzer to even be on (that's what enables it); use -Wno-analyzer-write-to-string-literal to turn it back off.

it warns about paths through the code where the analyzer catches an attempt to write through a pointer to a string literal. and again, the analyzer does not prioritize finding these paths, so false negatives are more likely relative to other warnings. 🙈

• -Wno-analyzer-use-of-uninitialized-value — requires -fanalyzer, which is what enables it; use -Wno-analyzer-use-of-uninitialized-value to disable it.

this one warns about paths through the code where an uninitialized value gets used. ngl that's just asking for chaos 🫠

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

---

🧠 what the analyzer just knows (hardcoded knowledge)

the analyzer has hardcoded knowledge about the behavior of these 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"

of these 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"

of these functions for working with "<stdio.h>" streams:

• The built-in functions "__builtin_fprintf", "__builtin_fprintf_unlocked", "__builtin_fputc", "__builtin_fputc_unlocked", "__builtin_fputs", "__builtin_fputs_unlocked", "__builtin_fwrite", "__builtin_fwrite_unlocked", "__builtin_printf", "__builtin_printf_unlocked", "__builtin_putc", "__builtin_putchar", "__builtin_putchar_unlocked", "__builtin_putc_unlocked", "__builtin_puts", "__builtin_puts_unlocked", "__builtin_vfprintf", and "__builtin_vprintf"
• "fopen"
• "fclose"
• "ferror"
• "fgets"
• "fgets_unlocked"
• "fileno"
• "fread"
• "getc"
• "getchar"
• "fprintf"
• "printf"
• "fwrite"

and of these functions:

• The built-in functions "__builtin_expect", "__builtin_expect_with_probability", "__builtin_strchr", "__builtin_strcpy", "__builtin_strcpy_chk", "__builtin_strlen", "__builtin_va_copy", and "__builtin_va_start"
• The GNU extensions "error" and "error_at_line"
• "getpass"
• "longjmp"
• "putenv"
• "setjmp"
• "siglongjmp"
• "signal"
• "sigsetjmp"
• "strcat"
• "strchr"
• "strlen"

plus, various functions with an "__analyzer_" prefix have special meaning to the analyzer — those are described in the GCC Internals manual. iykyk 📖✨

---

🎛️ the params that actually matter here

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

---

⚙️ options that control the analyzer

• -fanalyzer-call-summaries — simplifies interprocedural analysis by computing the effect of certain calls instead of exploring every single path through the function from callsite to each possible return. it's giving "shortcut, but make it smart" 💅

if it's enabled, call summaries are only used for functions that have 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 checker called name, and enables it. one checker, no entourage. 👑
• -fanalyzer-debug-text-art-headings — this one is for analyzer developers. if it's enabled, the analyzer adds extra annotations to any diagrams it generates. 🎨
• -fno-analyzer-feasibility — this option is intended for analyzer developers.

by default the analyzer verifies that there's a feasible control flow path for every diagnostic it emits — meaning the conditions that hold aren't mutually exclusive. diagnostics where no feasible path can be found get rejected. you can suppress that filtering 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, every single statement gets its own edge. so demure, so mindful ✨

• -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's literally counting the clones 💀
• -fanalyzer-show-events-in-system-headers — by default the analyzer emits simplified diagnostic paths by hiding events fully located within a system header. with -fanalyzer-show-events-in-system-headers, those events are no longer suppressed. nothing stays hidden bestie 🙈
• -fno-analyzer-state-merge — this option is intended for analyzer developers.

by default the analyzer tries to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". with -fno-analyzer-state-merge that merging can be suppressed, for debugging state-handling issues.

• -fno-analyzer-state-purge — this option is intended for analyzer developers.

by default the analyzer tries to simplify analysis by purging aspects of state at a program point that look like they're no longer 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 stops exploring an execution path after hitting certain diagnostics, so it doesn't potentially issue a cascade of follow-up diagnostics.

the diagnostics that terminate analysis along a path are:

• -Wanalyzer-null-argument
• -Wanalyzer-null-dereference
• -Wanalyzer-use-after-free
• -Wanalyzer-use-of-pointer-in-stale-stack-frame
• -Wanalyzer-use-of-uninitialized-value

with -fno-analyzer-suppress-followups the analyzer keeps exploring those paths even after such diagnostics — which can be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior.

• -fanalyzer-transitivity — enables transitivity of constraints within the analyzer. transitive queen 👑
• -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 already happened, which leads to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code. 😭

by default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls.

with -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled, which may help when debugging issues in the analyzer.

• -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. yapping mode: activated 🗣️
• -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 get emitted for analyzer diagnostics.

the level can be one of:

• 0 — at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. for example, for a double-"free" diagnostic, both calls to "free" will be shown.
• 1 — as per the previous level, but also shows events for the entry to each function.
• 2 — as per the previous level, but also shows 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.

• 3 — as per the previous level, but shows all control flow events, not just the significant ones.
• 4 — this level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.

• -fdump-analyzer — dumps internal details about what the analyzer is doing to file.analyzer.txt. -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. 🌐
• -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 text version of the "exploded graph" into one whole file per node, named file.eg-id.txt. and bestie… that's a LOT of files. like, so many. brace yourself. 💀
• -fdump-analyzer-exploded-paths — dumps a text version of the "exploded path" for each diagnostic into file.idx.kind.epath.txt.
• -fdump-analyzer-feasibility — spills the analyzer's internal tea about its search for feasible paths 🫖 into filenames shaped like file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt, all formatted so you can peek at them in GraphViz.
• -fdump-analyzer-infinite-loop — dumps internal deets about the analyzer's hunt for infinite loops 🌀, GraphViz-viewable, into filenames like file.*.infinite-loop.dot.
• -fdump-analyzer-json — dumps a compressed JSON version of the analyzer's internals into file.analyzer.json.gz. ngl the exact format is subject to change, so don't get too attached. 🙈
• -fdump-analyzer-state-purge — same energy as -fdump-analyzer-supergraph, dumps a GraphViz-ready representation of the "supergraph", but it annotates the graph with what state gets purged at each node. lands in file.state-purge.dot.
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" (GraphViz-friendly ✨) into file.supergraph.dot and file.supergraph-eg.dot. these show every control flow graph in the program, plus the interprocedural edges for calls and returns. the second dump has annotations showing nodes in the "exploded graph" and the diagnostics attached to them.
• -fdump-analyzer-untracked — emits custom warnings with internal details, literally just for analyzer developers. it's giving insider info only. 👑

---

🌸 Options for Debugging Your Program 🌸

okay so, to tell GCC to emit extra info for a debugger, in like almost all cases you just add -g to your other options. periodt. some debug formats can co-exist (think DWARF and CTF 💞) when you explicitly enable each one by adding its own command line option to your other options.

GCC lets you use -g with -O. the shortcuts optimized code takes can occasionally be surprising: some variables you declared may not exist at all 😭; flow of control may briefly move somewhere 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 got moved out of loops. nevertheless, it IS possible to debug optimized output. so it's actually reasonable to use the optimizer for programs that might have bugs. slay.

if you're not using some other optimization option, consider using -Og with -g. fr fr — with no -O option at all, some compiler passes that collect info useful for debugging don't run at all, so -Og may give you 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 info that only GDB can use. this extra info 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 using the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. iconic.
• -gdwarf, -gdwarf-version — produces debugging information in DWARF format (if that is supported). version can 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 they apply to all currently-supported versions of DWARF.

• -gbtf — requests 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 debug formats are enabled explicitly via their respective command line options. 🦋
• -gctf, -gctflevel — requests CTF debug information and uses level to specify how much CTF debug info 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 debug formats are enabled explicitly via their respective command line options.

• Level 0 produces no CTF debug information at all. so -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. so -g0 negates -g.
  • Level 1 produces minimal information, enough for making backtraces in parts of the program 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. periodt.

-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 aren't 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. ⚠️ only use this with debuggers that can't handle the way GCC normally emits debugging info for classes, because using this option increases the size of debugging information by as much as a factor of two. that's a big yikes.
• -fno-merge-debug-strings — tells the linker to not merge together strings in the debugging information that are identical in different object files. merging isn't supported by all assemblers or linkers. merging decreases the size of the debug info 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 replace a build-time path with an install-time path in the debug info. it can also 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 — runs the 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 info). 💖

it's enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.

• -fvar-tracking-assignments — annotates assignments to user variables early in the compilation and attempts to carry the annotations over throughout the compilation all the way to the end, trying to improve debug info while optimizing. using -gdwarf-4 along with it is recommended. ✨

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 info as possible into a separate output file with the extension .dwo. this option lets the build system 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, -gdwarf32 selects the 32-bit DWARF format and -gdwarf64 selects the 64-bit DWARF format. the default is target specific, on most targets it's -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 — adds description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs.
• -gpubnames — generates DWARF .debug_pubnames and .debug_pubtypes sections. 🌈
• -ggnu-pubnames — bestie this one makes .debug_pubnames and .debug_pubtypes sections in a format that can be converted into a GDB index ✨. It's only useful with a linker that can produce GDB index version 7, so like, only then. Periodt.
• -fdebug-types-section — when you're using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of being part of .debug_info 🎀. It's more efficient to put them in a separate comdat section since the linker can then remove duplicates. But — and this is the tea — not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types produces larger instead of smaller debugging information. Not me getting betrayed by an optimization 💀.
• -grecord-gcc-switches, -gno-record-gcc-switches — this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information 🫶. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file.
• -gstrict-dwarf — disallow using extensions of later DWARF standard version than selected with -gdwarf-version 🙈. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed.
• -gno-strict-dwarf — allow using extensions of later DWARF standard version than selected with -gdwarf-version. Slay, freedom ✨.
• -gas-loc-support — this tells the compiler that the assembler supports .loc directives 💅. It may then use them for the assembler to generate DWARF2+ line number tables.

This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. Iconic behavior honestly.

This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives.

• -gno-as-loc-support — force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. Okay, control freak, we respect it 💜.
• -gas-locview-support — inform the compiler that the assembler supports "view" assignment and reset assertion checking in .loc directives 👑

This option will be enabled by default if, at GCC configure time, the assembler was found to support them.

• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
• -gcolumn-info, -gno-column-info — emit location column information into DWARF debugging information, rather than just file and line 🌸. This option is enabled by default.
• -gstatement-frontiers, -gno-statement-frontiers — this option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table 🦋. This is 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 — augment variable location lists with progressive view numbers implied from the line number table ✨. This enables debug information consumers to 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 will cause the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that are not aware of these augmentations, but they won't derive any benefit from them either. It's giving "works on your machine either way, bestie" 🫶.

This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location views 😭. (Silently! The audacity.)

There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it 💀. So like, do not casually reach for this one.

• -ginternal-reset-location-views, -gno-internal-reset-location-views — attempt to determine location views that can be omitted from location view lists 🙈. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists. The GNU assembler will flag any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function.
• -ginline-points, -gno-inline-points — generate extended debug information 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 information. 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 is only enabled by default if location views are enabled.
• -gz[=type] — produce compressed debug sections in DWARF format, if that is supported 🎀. If type is not 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 does not support them, -gz is silently ignored when producing object files 👀.
• -femit-struct-debug-baseonly — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined 🦋.

This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger 😭. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-reduced — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header 🌸.

This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-detailed[=spec-list] — specify the struct-like types for which the compiler generates debug information 👑. The intent is to reduce duplicate struct debug information 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 specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain 😭. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.

The third word specifies the source files for those structs for which the compiler should emit debug information. 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 information 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, fr fr 🧚.

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 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 is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space 💀.

---

✨ Options That Control Optimization ✨

These options control various sorts of optimizations 🔥.

okay so bestie, real talk about what the compiler is actually doing to your code, because this is the part where it stops being a translator and starts being a ✨whole entire stylist✨ for your binary.

When you pass no optimization option at all, the compiler's whole goal is just to keep compilation cheap and make debugging do the thing you expect. Statements stay independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable, or change the program counter to any other statement in the function, and get exactly the results you expect from the source code. It's giving "what you wrote is what you get" 🫶.

Flip on optimization flags though and it's a whole different vibe — the compiler starts attempting to improve performance and/or code size, but the price is compilation time and possibly the ability to debug the program. 😭 tradeoffs, always.

It does all this based on the knowledge it has of the program. And here's a cute little trick: compiling multiple files at once to a single output file mode lets the compiler use information gained from all of the files when compiling each of them. it's giving group project but everyone actually participates 💅.

Not all optimizations are controlled directly by a flag. Only 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. Same energy with -Og: it suppresses many optimization passes too.

And 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. Wanna know the exact set for real? Invoke GCC with -Q --help=optimizers and it'll spill everything. iykyk 🔥

---

💖 the -O levels 💖

• -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 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 -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 don't 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-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. 👀

• -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. no thoughts, just vibes 🍓
• -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. smol but mighty 🧚

• -Ofast — Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens. It turns off -fsemantic-interposition. living dangerously, love that for you 💀
• -Og — Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It'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.

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 👑

---

✨ the -f flags section ✨

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. easy math bestie 🎀

The following options control specific optimizations. They're either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired.

• -fno-defer-pop — For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once. bulk pop, iconic 🌈
• -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.

• -ffp-contract=style — -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.

The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast otherwise.

• -fomit-frame-pointer — Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. free real estate 💅

On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't 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-sibling-calls — Optimize sibling and tail recursive calls.

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

• -foptimize-strlen — Optimize various standard C string functions (e.g. strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives.

Enabled at levels -O2, -O3.

• -finline-stringops[=fn] — Expand memory and string operations (for now, only memset) inline, even when the length is variable or big enough as to require looping. This is most useful along with -ffreestanding and -fno-builtin.

In some circumstances, it enables the compiler to generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches. This option is disabled by default.

• -fno-inline — Do not expand any functions inline apart from those marked with the always_inline attribute. This is the default when not optimizing.

Single functions can be exempted from inlining by marking them with the noinline attribute. not me, not today 🙈

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

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

and that's the energy for this chunk bestie, more coming so don't go anywhere 💜

• -findirect-inlining — this one inlines the indirect calls that turn out to be known at compile time, all thanks to inlining that already happened earlier 💫. Lowkey it does nothing unless inlining itself is switched on by -finline-functions or -finline-small-functions. no cap.

It's enabled at levels -O2, -O3, -Os.

• -finline-functions — bestie, this considers every single function for inlining, even the ones that never said inline in their whole life 🫶. The compiler heuristically decides which functions are actually worth integrating like that.

If all calls to a given function get integrated, and the function is declared static, then normally it just... doesn't get output as assembler code in its own right. bye girl 💅.

Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.

• -finline-functions-called-once — consider all static functions that get called exactly once for inlining into their caller, even if they're not marked inline ✨. If a call to a given function gets 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 — inline functions marked by always_inline, plus functions whose body seems smaller than the function call overhead, early — before -fprofile-generate instrumentation and the real inlining pass even happen 🌸. Doing it this way makes profiling significantly cheaper and usually makes inlining faster on programs with big long chains of nested wrapper functions. it's giving efficiency.

Enabled by default.

• -fipa-sra — perform interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference with parameters passed by value 🧚. so much cleanup, obsessed.

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

• -finline-limit=n — by default GCC limits the size of functions that can be inlined, and this flag gives you coarse control over that limit 👑. n is the size of functions that can be inlined, measured in number of pseudo instructions.

Inlining is actually controlled by a bunch of parameters, which you can specify individually with --param name=value. The -finline-limit=n option sets some of these parameters like this:

• 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 of -finline-limit that results in default behavior. like, none. it's giving "you can't go home again" 😭.

Note: "pseudo instruction" here means an abstract measurement of a 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 don't come for me if the number vibes differently next version 💀.

• -fno-keep-inline-dllexport — this is a more fine-grained version of -fkeep-inline-functions, and it applies only to functions declared using the dllexport attribute or declspec. specific queen 🎀.
• -fkeep-inline-functions — in C, emit static functions that are declared inline into the object file, even if the function got inlined into all of its callers 🫶. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emit any and all inline functions into the object file.
• -fkeep-static-functions — emit static functions into the object file, even if the function is never used. keeping it for the plot ✨.
• -fkeep-static-consts — emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

GCC enables this option by default. If you want to force the compiler to check whether a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

• -fmerge-constants — attempt to merge identical constants (string constants and floating-point constants) across compilation units 💖.

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

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

• -fmerge-all-constants — attempt to merge identical constants and identical variables 🔥.

This option implies -fmerge-constants. On top of -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 😬. it's giving "breaking the rules, knowingly".

• -fmodulo-sched — perform swing modulo scheduling immediately before the first scheduling pass 🌈. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. slay.
• -fmodulo-sched-allow-regmoves — perform more aggressive SMS-based modulo scheduling, with register moves allowed 💅. By setting this flag, certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled.
• -fno-branch-count-reg — disable the optimization pass that scans for opportunities to use decrement and branch instructions on a count register, instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result 🙈. This option is only meaningful on architectures that support such instructions — which include x86, PowerPC, IA-64 and S/390. Note that the -fno-branch-count-reg option doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes. she's just declining to add more, not cleaning up everyone else's mess 😭.

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. iykyk.

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.

• -fthread-jumps — perform optimizations that check whether 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. main character redirect energy.

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

• -fsplit-wide-types — when using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently 🎀. This normally generates better code for those types, but may make debugging more difficult 😬.

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

• -fsplit-wide-types-early — fully split wide types early, instead of very late 💫. This option has no effect unless -fsplit-wide-types is turned on.

This is the default on some targets.

• -fcse-follow-jumps — in common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path 🌈. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false.

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

• -fcse-skip-blocks — this is similar to -fcse-follow-jumps, but it 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-run common subexpression elimination after loop optimizations are performed 🔁. she's doing it again, and honestly? iconic.

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

• -fgcse — perform a global common subexpression elimination pass 💅. This pass also performs global constant and copy propagation.

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.

• -fgcse-lm — when -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves 👑. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. chef's kiss.

Enabled by default when -fgcse is enabled.

• -fgcse-sm — when -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination 🧚. This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop.

Not enabled at any optimization level. so you have to ask for it specifically, bestie.

• -fgcse-las — when -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies) ✨.

Not enabled at any optimization level.

• -fgcse-after-reload — when -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload 💫. The purpose of this pass is to clean up redundant spilling. tidying up the mess fr fr.

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

• -faggressive-loop-optimizations — this option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop 🔥. This assumes that loop code does not invoke undefined behavior — by, for example, causing signed integer overflows or out-of-bound array accesses 😭. The bounds for the number of iterations of a loop are used to guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
• -funconstrained-commons — bestie, heads up: this tells the compiler that variables declared in common blocks (like, e.g., Fortran ones) might get overridden later with longer trailing arrays. Which means it blocks certain optimizations that would otherwise depend on knowing the array bounds. So like, it's a "we can't be sure, so don't get cocky" flag. 💅
• -fcrossjumping — does the cross-jumping transformation, which unifies equivalent code and saves code size. Now, does the result actually run faster? Honestly, maybe, maybe not — it may or may not perform better than without cross-jumping. It's giving "smaller but who knows" energy. 🦋
  • Enabled at levels -O2, -O3, -Os.
• -fauto-inc-dec — combines increments or decrements of addresses with memory accesses. But wait: this pass is always skipped on architectures that don't have instructions to support it. No instruction, no vibe.
  • 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. This includes use of conditional moves, min, max, set flags and abs instructions, plus some tricks doable by standard arithmetics. The use of conditional execution on chips where it's available is controlled by -fif-conversion2.
  • 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. Same fam, different flavour.
  • 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 — which literally means two copies of the function. 😭 With this option, the base and complete variants get changed into thunks that call a common implementation. Iconic, honestly.
  • Enabled by -Os. 💖
• -fdelete-null-pointer-checks — assumes that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This option enables simple constant folding optimizations at all optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so if a pointer is checked after it's already been dereferenced, it cannot be null. 🕵️‍♀️

Bestie, WARNING: in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior. 🚨

This option is enabled by default on most targets. On Nios II ELF, it defaults to off. 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. This happens 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 the analysis of the type inheritance graph, it determines for a given call the set of likely targets. If the set is small — preferably of size 1 — it changes the call into a conditional deciding between direct and indirect calls. These 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 "we tried, we un-tried" energy and I respect it. 💅
• -fdevirtualize-at-ltrans — streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode. This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default. (trade-off alert 🫶)
• -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive. Yes, that's the actual name, they said the quiet part out loud.
  • Enabled at levels -O2, -O3, -Os.
• -free — attempts to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. ✨
  • 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 when the object is destroyed. Normally dead store elimination takes advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization. 🫠

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, you can 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. This is helpful for fast processors with small or moderate size register sets. 🧚
• -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 is not implemented for all architectures, but for those targets that do support it, it is the default because it generates better code. 👑
• -fira-region=region — use specified regions for the integrated register allocator. The region argument should be one of the following:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).
  • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
• -fira-hoist-pressure — uses IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down. 🐢
  • This option is enabled at level -Os for all targets.
• -fira-loop-pressure — uses IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down.
  • This option is 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 a separate stack slot, and as a result function stack frames are larger. 💀
• -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger. 💀
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it is profitable. 💰
  • 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 due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating-point instruction is required. ✨
  • Enabled at levels -O2, -O3.
• -fschedule-insns2 — similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.
  • Enabled at levels -O2, -O3, -Os.
• -fno-sched-interblock — disables instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. 🙈
• -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. This only makes sense when scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage of this option can improve the generated code and decrease its size by preventing register pressure increase above the number of available hard registers and subsequent spills in register allocation. 🎀
• -fsched-spec-load — allows speculative motion of some load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions. This only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. And yeah bestie, it's literally called "dangerous" — the name is not a vibe, it's a warning. 💅
• -fsched-stalled-insns, -fsched-stalled-insns=n — okay so this one sets how many insns (if any, bestie) get to be moved prematurely out of the queue of stalled insns and into the ready list during the second scheduling pass. 💅
  • -fno-sched-stalled-insns means literally zero insns get moved prematurely.
  • -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely.
  • -fsched-stalled-insns with no value at all is the same thing as -fsched-stalled-insns=1.
• -fsched-stalled-insns-dep, -fsched-stalled-insns-dep=n — this one 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. 🌀
  • Heads up: this only does anything during the second scheduling pass, and only if -fsched-stalled-insns is in play.
  • -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0.
  • -fsched-stalled-insns-dep without a value is equivalent to -fsched-stalled-insns-dep=1.
• -fsched2-use-superblocks — when you're scheduling after register allocation, this uses superblock scheduling, which lets motion happen across basic block boundaries and that gives you faster schedules. ✨
  • > Bestie, fair warning: this option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. 👀
  • It only makes sense when scheduling happens after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
• -fsched-group-heuristic — turns on the group heuristic in the scheduler. It favors the instruction that belongs to a schedule group. On by default whenever scheduling is on, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🎀
• -fsched-critical-path-heuristic — turns on the critical-path heuristic. It favors instructions on the critical path. On by default whenever scheduling is on, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🔥
• -fsched-spec-insn-heuristic — turns on the speculative instruction heuristic. It favors speculative instructions with greater dependency weakness. On by default whenever scheduling is on, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 💫
• -fsched-rank-heuristic — turns on the rank heuristic. It favors the instruction belonging to a basic block with greater size or frequency. On by default whenever scheduling is on, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 👑
• -fsched-last-insn-heuristic — turns on the last-instruction heuristic. It favors the instruction that's less dependent on the last instruction scheduled. On by default whenever scheduling is on, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🌸
• -fsched-dep-count-heuristic — turns on the dependent-count heuristic. It favors the instruction that has more instructions depending on it. On by default whenever scheduling is on, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🦋
• -freschedule-modulo-scheduled-loops — so modulo scheduling gets performed before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule, and this option is how you 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 it runs instead of the second scheduler pass. ⭐
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling. Ngl, 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, this also pipelines outer loops. Same deal: no effect unless -fsel-sched-pipelining is turned on. 💜
• -fsemantic-interposition — some object formats, like ELF, allow interposing of symbols by the dynamic linker. What that means is that for symbols exported from the DSO, the compiler can't perform interprocedural propagation, inlining and other optimizations, because it has to anticipate that the function or variable in question may change. 😭
  • While this feature is useful — for example, to rewrite memory allocation functions by a debugging implementation — it is 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.
  • The 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 them, instead of 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 it. ✂️
• -fcaller-saves — enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. That allocation only happens when it seems to result in better code. 💾
  • This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
  • Enabled at levels -O2, -O3, -Os.
• -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them. Enabled by default at -O1 and higher. 🧩
• -fipa-ra — use caller save registers for allocation if those registers aren't used by any called function. In that case it's not necessary to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. 🔍
  • Enabled at levels -O2, -O3, -Os, however the option is disabled if the generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly — which happens on targets that don't expose prologues and epilogues in RTL.
• -fconserve-stack — attempt to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400. 📉
• -ftree-reassoc — perform reassociation on trees. This flag is enabled by default at -O1 and higher. 🌳
• -fcode-hoisting — perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well. This flag is enabled by default at -O2 and higher. 🚀
• -ftree-pre — perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3. 🍓
• -ftree-partial-pre — make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3. 💥
• -ftree-forwprop — perform forward propagation on trees. This flag is enabled by default at -O1 and higher. ➡️
• -ftree-fre — perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O1 and higher. 🧠
• -ftree-phiprop — perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O1 and higher. 🎈
• -fhoist-adjacent-loads — speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. This flag is enabled by default at -O2 and higher. 🎪
• -ftree-copy-prop — perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at -O1 and higher. 📋
• -fipa-pure-const — discover which functions are pure or constant. Enabled by default at -O1 and higher. 💎
• -fipa-reference — discover which static variables do not escape the compilation unit. Enabled by default at -O1 and higher. 🔒
• -fipa-reference-addressable — discover read-only, write-only and non-addressable static variables. Enabled by default at -O1 and higher. 🔎
• -fipa-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. > Heads up bestie: this option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level. 💀
• -fipa-profile — perform interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also, functions executed once — such as cold, noreturn, static constructors or destructors — are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O1 and higher. ❄️
• -fipa-modref — perform interprocedural mod/ref analysis. This optimization analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary. This flag is enabled by default at -O1 and higher. 🌟
• -fipa-cp — interprocedural constant propagation, bestie 💖 It peeks at the whole program to figure out when the values you're passing into functions are actually constants, and then optimizes around that knowledge. If your app is out here passing constants into functions, this can be a substantial glow-up for performance. Enabled by default at -O2, -Os and -O3, and it's also switched on by -fprofile-use and -fauto-profile. ✨
• -fipa-cp-clone — function cloning that makes interprocedural constant propagation even stronger 💅 When it's on, IPA-CP goes ahead and clones a function when an externally visible function can be called with constant arguments. Heads up though: because this can create multiple copies of functions, it may significantly increase code size (see --param ipa-cp-unit-growth=value). Enabled by default at -O3, and also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, does interprocedural bitwise constant propagation 🎀 Enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled. No -fipa-cp, no bit-cp, periodt.
• -fipa-vrp — when enabled, does interprocedural propagation of value ranges 🌸 Enabled by default at -O2. And yeah, it requires that -fipa-cp is enabled too.
• -fipa-icf — Identical Code Folding for functions and read-only variables 🦋 It reduces code size, and it may disturb unwind stacks by replacing a function with an equivalent one that has a different name. (Yeah, dramatic, we know.) The optimization works more effectively with link-time optimization enabled.

Even though 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 found only by Gold. Both ate, just differently 💅

Enabled by default at -O2 and -Os.

• -flive-patching=level — controls GCC's optimizations so the output is suitable for live-patching 👑

Okay so here's the tea: if the compiler's optimization uses a function's body, or info extracted from its body, to optimize or change another function, that other function is called an impacted function of the former. And if a function gets patched, its impacted functions should be patched too. Fr fr.

The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.

Usually, the more IPA optimizations 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 enables 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 enables inlining of static functions. As a result, when patching a static function, all its callers are impacted and so need to be patched as well.

In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp

When -flive-patching is specified without any value, the default value is inline-clone.

This flag is disabled by default.

Note that -flive-patching is not supported with link-time optimization (-flto). Like, don't even try it bestie 🙈

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer 💀 It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. 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 due to a null value 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 erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by -O2 in the future.
• -ftree-sink — performs forward store motion on trees 🌈 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 is 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 in order to simplify the definitions 🎀 For example, this pass strips sign operations if the sign of a value never matters. 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 to initializations from a scalar array 🦋 Enabled by default at -O2 and higher.
• -ftree-tail-merge — looks for identical code sequences, and when it finds them, replaces one with a jump to the other 👑 This optimization is known as tail merging or cross jumping. 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-dce — performs dead code elimination (DCE) on trees ✨ 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 💖 Enabled by default at -O2 and higher if -Os is not also specified.
• -ffinite-loops — assumes that a loop with an exit will eventually take the exit and not loop indefinitely 🌀 This allows the compiler to remove loops that otherwise have no side-effects, and it's not considering eventual endless looping as such a side-effect.

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

• -ftree-dominator-opts — performs a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal 💅 This also performs jump threading (to reduce jumps to jumps). Enabled by default at -O1 and higher.
• -ftree-dse — performs dead store elimination (DSE) on trees 🌈 A dead store is a store into a memory location that's later overwritten by another store without any intervening loads. In that case, the earlier store can be deleted. Enabled by default at -O1 and higher.
• -ftree-ch — performs loop header copying on trees 🎀 This is beneficial since it increases the effectiveness of code motion optimizations. It also saves one jump. Enabled by default at -O1 and higher. It is not enabled for -Os, since it usually increases code size.
• -ftree-loop-optimize — performs loop optimizations on trees 🌸 Enabled by default at -O1 and higher.
• -ftree-loop-linear, -floop-strip-mine, -floop-block — perform 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.
• -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.
• -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. This option is experimental 💀 (so, like, tread carefully bestie).
• -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's profitable to parallelize the loops. Not me checking the vibe and just sending 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 💜 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 — bestie, this one tries to turn conditional jumps in the innermost loops into branch-less equivalents ✨. The whole goal? Strip control-flow out of those innermost loops so the vectorization pass has an easier time handling them 💅. It's on by default whenever vectorization is enabled, periodt.
• -ftree-loop-distribution — does loop distribution 🌀. This flag can level up cache performance on big loop bodies and lets further loop optimizations happen, like parallelization or vectorization. For example, this loop

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

gets transformed into

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

This flag is enabled by default at -O3. It's also enabled by -fprofile-use and -fauto-profile 🔥.

• -ftree-loop-distribute-patterns — loop distribution, but make it library-call coded 👑. It distributes 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. it's giving optimized 💖

• -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 — think function calls, operations that expand to nontrivial sequences of insns. With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion.
• -ftree-loop-ivcanon — creates a canonical counter for the number of iterations in loops where figuring out the iteration count needs complicated analysis 😭. Later optimizations then may determine the number easily. Useful especially in connection with unrolling, iykyk 💅.
• -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, then uses of the final value get replaced by such a computation — provided it's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher.
• -fivopts — performs induction variable optimizations on trees: strength reduction, induction variable merging and induction variable elimination 🌈.
• -ftree-parallelize-loops=n — parallelize loops, bestie!! 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. Heads up: 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 — merging of narrow stores to consecutive memory addresses 🎀. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores, which reduces the number of instructions. Enabled by default at -O2 and higher as well as -Os.
• -ftree-ter — performs temporary expression replacement during the SSA->normal phase 🌀. Single use/single def temporaries 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, which results in better RTL generation. 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. Enabled by default at -O1 and higher.
• -ftree-vectorize — performs vectorization on trees ✨. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if they're not explicitly specified.
• -ftree-loop-vectorize — performs loop vectorization on trees 🔥. 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 — initialize automatic variables with either a pattern or with zeroes 🛡️, which increases the security and predictability of a program by preventing uninitialized memory disclosure and use. Okay but listen: GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized. -Wuninitialized and -Wanalyzer-use-of-uninitialized-value will still report warning messages on such automatic variables, and the compiler will perform optimization as if the variable were uninitialized 😤. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the first case of a switch statement. Use -Wtrivial-auto-var-init to report all such cases.

The three values of choice are:

• uninitialized — doesn't initialize any automatic variables. This is C and C++'s default 💅.
• pattern — initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump and without being values that programmers can rely on for useful program semantics. The current value is a byte-repeatable pattern with byte "0xFE". The values used for pattern initialization might be changed in the future 👀.
• zero — initialize automatic variables with zeroes.

The default is uninitialized.

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. This means the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception is that (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 — alter 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 only allows vectorization if the vector code would entirely replace the scalar code that is being vectorized. For example, if each iteration of a vectorized loop would only be able to handle exactly four iterations of the scalar loop, the very-cheap model would only allow vectorization if the scalar iteration count is known to be a multiple of four.

The default cost model depends on other optimization flags and is either dynamic or cheap 🦋.

• -fsimd-cost-model=model — bestie, this changes the cost model used when GCC vectorizes loops that are marked with the OpenMP simd directive. ✨ The model argument needs to be one of unlimited, dynamic, cheap. Every one of those values means the same thing it means over in -fvect-cost-model, and if you don't say anything here, whatever cost model you set with -fvect-cost-model is the one that gets used. 💅
• -ftree-vrp — this does Value Range Propagation on trees. 🌈 It's giving constant propagation's cooler cousin: instead of propagating values, it propagates ranges of values. That lets the optimizers yeet unnecessary range checks, like array bound checks and null pointer checks. It's on by default at -O2 and higher. 🫶 One lil caveat: null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled. 👑
• -fsplit-paths — splits the paths that lead to loop backedges. 🦋 This can level up dead code elimination and common subexpression elimination. Enabled by default at -O3 and above. ✨
• -fsplit-ivs-in-unroller — this one lets GCC express the values of induction variables in later iterations of an unrolled loop using the value from the first iteration. 🔥 It breaks up those long dependency chains, which makes the scheduling passes way more efficient. slay.

A combo of -fweb and CSE is often enough to get the same effect. However, that's not reliable when the loop body is more complicated than a single basic block. 😭 And it doesn't work at all on some architectures because of restrictions in the CSE pass.

This optimization is enabled by default. 💖

• -fvariable-expansion-in-unroller — with this one, the compiler makes multiple copies of some local variables while unrolling a loop, which can result in superior code. 👑

This optimization is enabled by default for PowerPC targets, but disabled by default everywhere else. 🎀

• -fpartial-inlining — inline parts of functions. 🌸 Heads up though: this only has any effect when inlining itself is turned on, via 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) that were already performed in previous iterations of loops. 🦋

This option is enabled at level -O3. It's also enabled by -fprofile-use and -fauto-profile. ✨

• -fprefetch-loop-arrays — if the target machine supports it, this generates instructions to prefetch memory so loops that access large arrays perform better. 🔥

Fair warning, bestie: this option may generate better or worse code. 😭 Results are highly dependent on the structure of the loops within your source code.

Disabled at level -Os. 🙈

• -fno-printf-return-value — do not substitute constants for the known return value of formatted output functions like sprintf, snprintf, vsprintf, and vsnprintf (but not printf of fprintf). 🎀 This transformation lets GCC optimize or even eliminate branches based on the known return value of those functions when they're 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 its 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're implemented in the compiler; some targets use one, some use the other, and a few use both.

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

• -fno-guess-branch-probability — don't guess branch probabilities using heuristics. 🚫

GCC uses heuristics to guess branch probabilities when profiling feedback (-fprofile-arcs) hasn't given it any. 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 the effects of __builtin_expect are easier to understand.

It's also possible to specify the expected probability of the expression with the __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 so there are fewer taken branches and better 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 the 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 — on top of reordering basic blocks in the compiled function to reduce the number of taken branches, this partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance. 💅

This optimization is automatically turned off when exception handling or unwind tables are in play (on targets using setjump/longjump or a target-specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that doesn't support named sections. 🚫 When -fsplit-stack is used, this option isn't enabled by default (to avoid linker errors), but it may be enabled explicitly (if you're using a working linker).

Enabled for x86 at levels -O2, -O3, -Os. ✨

• -freorder-functions — reorder functions in the object file to improve code locality. 🌈 This is implemented by using special subsections .text.hot for the most frequently executed functions and .text.unlikely for the unlikely-to-execute ones. The reordering is done by the linker, so the object file format must support named sections and the linker must place them in a reasonable way. 👑

This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes. 😭

Enabled at levels -O2, -O3, -Os. 💖

• -fstrict-aliasing — 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 live 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, bestie:

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 the rules of -fstrict-aliasing get applied across function boundaries. ✨ Note that if multiple functions get inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary. 🦋

The -fipa-strict-aliasing option is enabled by default and is effective only in combination with -fstrict-aliasing. 👑

• -falign-functions
• -falign-functions=n
• -falign-functions=n:m
• -falign-functions=n:m:n2
• -falign-functions=n:m:n2:m2 — 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, and -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less. 🔥

That second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. 🎀 If m2 is not specified, it defaults to n2. ✨

Some assemblers lowkey only support this flag when n is a power of two — and in that case, bestie, it just gets rounded up. 💅

-fno-align-functions and -falign-functions=1 are literally equivalent, and they mean functions are not aligned. Periodt.

If n isn't specified or it's zero, use a machine-dependent default. The maximum allowed n option value is 65536. No cap on the vibes though. ✨

Enabled at levels -O2, -O3.

• -flimit-function-alignment — if this is on, 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. It's giving "align, but know your limits" energy. 🎀
• -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. It's giving "no one gets left behind" fr fr. 💖
• -fno-allocation-dce — do not remove unused C++ allocations in dead code elimination. Not me keeping the dead allocations around... 🙈
• -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.

⚠️ Okay so heads up bestie: this one is literally about data races. The manpage says it may introduce new ones. Read the whole thing before you turn it on. 💀

• -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. Main character exception, obviously. 👑

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. ✨

To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example:

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

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. It's giving "two files, one vibe". 🦋

Another (simpler) way to enable link-time optimization is:

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

The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.

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

To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions. 💅

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

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing.

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

Okay so if you're at link time and you don't pass an optimization level like -O, GCC just goes "bestie, I'll use whatever the highest -O level was that got used when compiling the object files" ✨. But here's the tea: passing an optimization level only at link time and not at compile time is basically pointless, for two reasons. One, compiling without optimization suppresses the compiler passes that gather the info you'd need to actually optimize well at link time. Two, some early optimization passes can only run at compile time — never at link time. So like, it's giving nothing. 💀

There are a few code generation flags GCC preserves when it's generating bytecodes, because they're needed during the final link. Currently these options and their settings get taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm, plus all the -m target flags.

The following options — -fPIC, -fpic, -fpie and -fPIE — get combined according to this scheme, no cap:

-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 have to match across every compilation unit, periodt. If you try to override that at link time with a conflicting value, the override just gets ignored. This includes options like -freg-struct-return and -fpcc-struct-return.

Other options — like -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing — get 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. These ones you can override at link time. 🫶

Diagnostic options like -Wstringop-overflow also get passed through to the link stage, and their setting matches the compile-step's setting at function granularity. Heads up: this only actually matters for diagnostics emitted during optimization. And also — code transforms like inlining can make warnings turn on or off for regions of code that don't match the compile-time setting. 🙈

When you need to pass options to the assembler via -Wa or -Xassembler, make sure you either compile those translation units with -fno-lto, or consistently use the same assembler options on all of them. Or alternatively, you can just specify the assembler options at LTO link time. 💅

To get 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, then the link enables debug info generation too. But fancy debug info settings like the dwarf level -gdwarf-5 have to be explicitly repeated on the linker command line — and mixing different settings across different translation units is discouraged, so like, don't. 😭

If LTO runs into objects with C linkage declared with incompatible types in separate translation units that are being linked together (which is undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may get issued. The behavior is still undefined at run time though — the diagnostic is not a fix, bestie. Similar diagnostics may be raised for other languages too.

---

Another iconic feature of LTO: you can apply interprocedural optimizations on files written in different languages. Obsessed, honestly:

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 pull in the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when you're mixing languages in LTO mode, use the same link command options you'd use when mixing languages in a regular (non-LTO) compilation. 🌈

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — you can 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 make sure the library actually 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 so they become part of the aggregated GIMPLE image that gets 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. 🎀

Link-time optimizations do not require the whole program to be present to operate. If the program doesn't need any symbols exported, you can combine -flto and -fwhole-program to let the interprocedural optimizers use more aggressive assumptions, which may lead to improved optimization opportunities. You do not need -fwhole-program when the linker plugin is active (see -fuse-linker-plugin).

The current implementation of LTO makes no attempt to generate bytecode that's portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. It's giving exclusive. 💀

Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. Just so you know. 😭

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. Slay.

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 it's available, or otherwise fall back to autodetecting the number of CPU threads present in your system.

-flto-partition=alg 👑

Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1, to specify a partitioning mirroring the original source files, or balanced, to specify partitioning into equally sized chunks (whenever possible), or max, to create a new partition for every symbol where possible. Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as a workaround for various code ordering issues, the max partitioning is intended for internal testing only — don't be out here using it in prod, bestie. 🙈 The value one specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.

-flto-compression-level=n 🗜️

This option specifies the level of compression used for intermediate language written to LTO object files, and it's only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values outside this range get clamped to either the minimum or maximum of the supported values. If the option isn't given, a default balanced compression setting is used.

-fuse-linker-plugin 🔌

Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.

This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). 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. This 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 — so after register allocation and post-register allocation instruction splitting, GCC goes looking for arithmetic instructions that compute processor flags the same way a comparison operation would, based on that arithmetic. If it can, it ✨eliminates✨ the explicit comparison operation. This pass only applies to certain targets that can't 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 into memory loads/stores. Enabled at levels -O2, -O3.
• -fcprop-registers — after register allocation and post-register allocation instruction splitting, run a copy-propagation pass to try to reduce scheduling dependencies and, every once in a while, straight up eliminate the copy. Enabled at levels -O1, -O2, -O3, -Os.
• -fprofile-correction — bestie. Profiles collected from an instrumented binary for multi-threaded programs can be inconsistent because some counter updates just get missed, it's giving race condition energy 💀. When you specify this option, GCC uses heuristics to correct or smooth out those inconsistencies. By default, GCC emits an error message when it detects an inconsistent profile. This option is enabled by -fauto-profile.
• -fprofile-partial-training — with -fprofile-use, every portion of a program that didn't get executed during the train run is optimized aggressively for size rather than speed. And sometimes it's just not practical to train all the possible hot paths in a program (say the program has functions specific to a given hardware and your training didn't cover all the hardware configurations the program actually runs on 🙈). With -fprofile-partial-training, profile feedback gets ignored for all functions not executed during the train run, which leads them to be optimized as if they were compiled without profile feedback at all. This leads to better performance when the train run isn't representative — but also leads to significantly bigger code. Tradeoffs, babes.
• -fprofile-use, -fprofile-use=path — enable 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. That's the whole prerequisite, no skipping 👑

By default, GCC emits an error message if the feedback profiles don't match the source code. You can turn that error into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code 😭. Also, by default, GCC emits a warning message if the feedback profiles don't exist at all (see -Wmissing-profile).

If path is specified, GCC looks at path to find the profile feedback data files. See -fprofile-dir.

• -fauto-profile, -fauto-profile=path — enable 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 it's 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 info, 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 into a format GCC can actually use. You also have to 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. And heads up — all of them must be specifically enabled. No freebies here, you gotta ask nicely.

• -ffloat-store — don't store floating-point variables in registers, and inhibit other options that might change whether a floating-point value gets taken from a register or from memory.

This option prevents undesirable excess precision on machines like the 68000, where the floating registers (of the 68881) keep more precision than a double is supposed to have. Same situation on the x86 architecture, fr fr. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for those programs — after modifying them to store all pertinent intermediate computations into variables. Yes, that part's on you.

• -fexcess-precision=style — this option lets you control excess precision even further on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types.

By default, -fexcess-precision=fast is in effect; this means 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.

Heads up bestie: -fexcess-precision=standard is not implemented for languages other than C or C++. On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. 💀

• -ffast-math — sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.

This option causes the preprocessor macro __FAST_MATH__ to be defined. Iconic.

This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications. Your call, your vibe 🦋.

• -fno-math-errno — don't set errno after calling math functions that are executed with a single instruction, e.g. sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while keeping IEEE arithmetic compatibility.

This option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets errno. So there's literally no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default there. Slay, Darwin.

• -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 include libraries or startup files that change the default FPU control word or other similar optimizations.

This option is not turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications. 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 series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52). It may also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

• -freciprocal-math — allow the reciprocal of a value to be used instead of dividing by the value, if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value.

The default is -fno-reciprocal-math.

• -ffinite-math-only — okay so this one lets GCC do FP math optimizations that assume your arguments and your results are never NaNs and never ±Infs. The catch? No -O option turns this on, because it can straight up produce incorrect output for programs that need math functions to follow IEEE or ISO rules/specifications to the letter. If your program doesn't need those guarantees though? Faster code, bestie. ✨

The default is -fno-finite-math-only.

• -fno-signed-zeros — this one allows optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic is very much a "we care about this" situation: it specifies distinct behavior for +0.0 and -0.0, which then forbids simplifying expressions like x+0.0 or 0.0*x (yes, even with -ffinite-math-only). Turning this on implies the sign of a zero result isn't significant.

The default is -fsigned-zeros.

• -fno-trapping-math — compile code assuming floating-point operations cannot generate user-visible traps. Which traps, you ask? 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.

This option should never be turned on by any -O option, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

The default is -ftrapping-math.

Future versions of GCC may 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 — disables transformations and optimizations that assume default floating-point rounding behavior. And what's "default", exactly? Round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. Specify this for programs that change the FP rounding mode dynamically, or that might be executed with a non-default rounding mode. It disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

Heads up babes, this option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. 🙈 Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -ftrapping-math to specify the default state for FENV_ACCESS.

• -fsignaling-nans — compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this disables optimizations that may change the number of exceptions visible with signaling NaNs. It also implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined. 🎀

The default is -fno-signaling-nans.

Also experimental, bestie — it does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

• -fno-fp-int-builtin-inexact — do not allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to 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 these functions to do so. ✨

The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect.

Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the "inexact" exception may be raised if the library implementation does not follow TS 18661. 😭

• -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. That's it, that's the whole vibe.
• -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is 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 is 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 is 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. 😳 Yeah. Read that twice. Proceed with caution, gorgeous.

• -fbranch-probabilities — so after running a program compiled with -fprofile-arcs, you can compile it a second time</u> using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The info in that data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. See details about the file naming in -fprofile-arcs.

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. 🔥

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

• -fprofile-values — if combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.

With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.

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

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order. Sorting, but make it a glow-up. ✨

Enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator. iykyk.

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

• -frename-registers — attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register". 😭

Enabled by default with -funroll-loops.

• -fschedule-fusion — performs a target dependent pass over the instruction stream to schedule instructions of same type together, because the target machine can execute them more efficiently if they are adjacent to each other in the instruction flow. Besties who sit together, slay together. 🫶

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

• -ftracer — perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function, allowing other optimizations to do a better job. Teamwork makes the dream work. 🌈

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

• -funroll-loops — unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster. No promises, babe.

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

• -funroll-all-loops — unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. 😬 -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops for which there is enough information that they do not 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). Peeling, but for loops, not for your skincare routine. 🍓

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

• -fmove-loop-invariants — enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og.
• -fmove-loop-stores — enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration. Note: for this option to have an effect, -ftree-loop-im has to be enabled as well. Enabled at level -O1 and higher, except for -Og.
• -fsplit-loops — split 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. Divide and conquer, literally. 💅

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

• -funswitch-loops — move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition). Basically yeeting the decision out of the loop so it stops asking every single time. 👑

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

• -fversion-loops-for-strides — bestie, if a loop is striding through an array with a variable stride, this makes a whole second version of the loop that just assumes the stride is 1. Iconic behaviour, honestly. Like, watch this:

for (int i = 0; i < n; ++i)
  x[i * stride] = ...;

and then it 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 lets the compiler vectorize way better assuming contiguous accesses. This flag is enabled by default at -O3. It's also enabled by -fprofile-use and -fauto-profile.

• -ffunction-sections
• -fdata-sections

Put each function or data item into its own section in the output file, if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. (Two flags, one slay.)

Use these on systems where the linker can do 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 though.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.

• -fstdarg-opt — optimizes the prologue of variadic argument functions with respect to usage of those arguments. Little prologue glow-up 💅
• -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. Security girl summer 🔥

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 — okay so in some places, GCC uses various constants to control how much optimization gets done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the --param option.

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. (fr fr, don't get attached)

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:

predictable-branch-outcome

When branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable.

max-rtl-if-conversion-insns

RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.

max-rtl-if-conversion-predictable-cost

RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internal seq_cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH_COST target macro.

max-crossjump-edges

The maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size.

min-crossjump-insns

The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched.

max-grow-copy-bb-insns

The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction.

max-goto-duplication-insns

The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored.

max-delay-slot-insn-search

The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.

max-delay-slot-live-search

When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.

max-gcse-memory

The approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.

max-gcse-insertion-ratio

If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.

max-pending-list-length

The maximum number of pending dependencies scheduling allows before flushing the current state and starting over. Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources.

max-modulo-backtrack-attempts

The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time.

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 maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++).

max-inline-insns-auto

When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto).

max-inline-insns-small

This is bound applied to calls which are considered relevant with -finline-small-functions.

max-inline-insns-size

This is bound applied to calls which 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) that gets applied to inline-insns-single, inline-insns-single-O2 and inline-insns-auto when the inline heuristics are hinting that inlining is very profitable (like, it'll unlock later optimizations, bestie). 💅
• uninlined-thunk-insns, uninlined-thunk-time — literally the same energy as --param uninlined-function-insns and --param uninlined-function-time, except it's applied to function thunks. 🫶
• inline-min-speedup — when the estimated performance improvement of caller + callee runtime goes over this threshold (in percent), the function can be inlined no matter what, ignoring the limit on --param max-inline-insns-single and --param max-inline-insns-auto. it just gets to do it. iconic.
• large-function-insns — the limit that says what counts as a really large function. for functions bigger than this limit after inlining, inlining is constrained by --param large-function-growth. this parameter is mainly there to avoid absolutely unhinged compilation times caused by the non-linear algorithms the back end uses. 😭
• large-function-growth — specifies the maximal growth of a large function caused by inlining, in percents. for example, a parameter value of 100 limits large function growth to 2.0 times the original size. no cap.
• large-unit-insns — the limit that says what counts as a large translation unit. growth caused by inlining of units bigger than this limit is limited by --param inline-unit-growth. for small units this might be too tight. for example, think of a unit made of function A that's inline and B that just calls A three times. if B is small relative to A, the growth of the unit is 300% and yet that inlining is very sane. but for very large units made of small inlineable functions, the overall unit growth limit is needed to avoid exponential explosion of code size. so for smaller units, the size gets bumped up to --param large-unit-insns before --param inline-unit-growth is applied. periodt.
• lazy-modules — maximum number of concurrently open C++ module files when lazy loading.
• inline-unit-growth — specifies the 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 the 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 the IPA-CP pass considers large.
• large-stack-frame — the limit that says what counts as a large stack frame. while inlining, the algorithm is trying to not grow past this limit too much.
• large-stack-frame-growth — specifies the 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 maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by doing recursive inlining.

--param max-inline-insns-recursive applies to functions declared inline. for functions not declared inline, recursive inlining only happens when -finline-functions (which is included in -O3) is enabled; --param max-inline-insns-recursive-auto applies instead. slay.

• max-inline-recursive-depth, max-inline-recursive-depth-auto — specifies the maximum recursion depth used for recursive inlining.

--param max-inline-recursive-depth applies to functions declared inline. for functions not declared inline, recursive inlining only happens when -finline-functions (which is included in -O3) is enabled; --param max-inline-recursive-depth-auto applies instead.

• min-inline-recursive-probability — recursive inlining is only profitable for functions that have deep recursion on average, and it can actually hurt functions with little recursion depth by increasing the prologue size or the complexity of the function body for the 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 to only those call expressions whose probability exceeds the given threshold (in percents).

• early-inlining-insns — specify the growth the early inliner is allowed to make. in effect it increases the amount of inlining for code that has a large abstraction penalty.
• max-early-inliner-iterations — the limit on iterations of the early inliner. this basically bounds the number of nested indirect calls the early inliner can resolve. deeper chains are still handled by late inlining.
• comdat-sharing-probability — the probability (in percent) that a C++ inline function with comdat visibility is shared across multiple compilation units.
• modref-max-bases, modref-max-refs, modref-max-accesses — specifies the maximal number of base pointers, references and accesses stored for a single function by mod/ref analysis.
• modref-max-tests — specifies the maxmal number of tests the alias oracle can perform to disambiguate memory locations using the mod/ref information. this parameter ought to be bigger than --param modref-max-bases and --param modref-max-refs.
• modref-max-depth — specifies the maximum depth of the DFS walk used by modref escape analysis. setting it to 0 disables the analysis completely. 💀
• modref-max-escape-points — specifies the maximum number of escape points tracked by modref per SSA-name.
• modref-max-adjustments — specifies the maximum number the access range is enlarged during modref dataflow analysis.
• profile-func-internal-id — a parameter to control whether to use the function internal id in profile database lookup. if the value is 0, the compiler uses an id that's based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc.
• 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 specified by this option to allow vectorization.
• gcse-cost-distance-ratio — scaling factor in the calculation of the maximum distance an expression can be moved by GCSE optimizations. this is currently supported only in the code hoisting pass. the bigger the ratio, the more aggressive code hoisting is with simple expressions, i.e. the expressions that have cost less than gcse-unrestricted-cost. specifying 0 disables hoisting of simple expressions.
• gcse-unrestricted-cost — cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. this is currently supported only in the code hoisting pass. the lesser the cost, the more aggressive code hoisting is. specifying 0 allows all expressions to travel unrestricted distances. ✨
• max-hoist-depth — the depth of search in the dominator tree for expressions to hoist. this is used to avoid quadratic behavior in the hoisting algorithm. the value 0 doesn't limit the search, but may slow down compilation of huge functions.
• max-tail-merge-comparisons — the maximum amount of similar bbs to compare a bb with. this is used to avoid quadratic behavior in tree tail merging.
• max-tail-merge-iterations — the maximum amount of iterations of the pass over the function. this is 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 in the attempt 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 in the attemt to 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 is unrolled, this parameter also determines how many times the loop code is unrolled.
• max-average-unrolled-insns — the maximum number of instructions, biased by the 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 minimum threshold for probability of semi-invariant condition statement to trigger loop split.

---

iv-consider-all-candidates-bound

Bound on number of candidates for induction variables, below which 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 avoid 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

Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer. 🐌

---

scev-max-expr-complexity

Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer.

---

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 (such as 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 vectorizer. Value -1 means no limit.

---

max-iterations-to-track

The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.

---

hot-bb-count-fraction

The denominator n of fraction 1/n of the maximal execution count of a basic block in the entire program that a basic block needs to at least have in order to be considered hot. The default is 10000, which means that a basic block is considered hot if its execution count is greater than 1/10000 of the maximal execution count. 0 means that it is never considered hot. Used in non-LTO mode. 🔥

---

hot-bb-count-ws-permille

The number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program to which the execution count of a basic block must be part of in order to be considered hot. The default is 990, which means that 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 that it is never considered hot. Used in LTO mode.

---

hot-bb-frequency-fraction

The denominator n of fraction 1/n of the execution frequency of the entry block of a function that a basic block of this function needs to at least have in order to be considered hot. The default is 1000, which means that a basic block is considered hot in a function if it is executed more frequently than 1/1000 of the frequency of the entry block of the function. 0 means that it is never considered hot.

---

unlikely-bb-count-fraction

The denominator n of fraction 1/n of the number of profiled runs of the entire program below which the execution count of a basic block must be in order for the basic block to be considered unlikely executed. The default is 20, which means that a basic block is considered unlikely executed if it is executed in fewer than 1/20, or 5%, of the runs of the program. 0 means that it is always considered unlikely executed. 😬

---

max-predicted-iterations

The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one. 🥶

---

builtin-expect-probability

Control the probability of the expression having the specified value. This parameter takes a percentage (i.e. 0 ... 100) as input.

---

builtin-string-cmp-inline-length

The maximum length of a constant string for a builtin string cmp call eligible for inlining.

---

align-threshold

Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block.

---

align-loop-iterations

A loop expected to iterate at least the selected number of iterations is aligned.

---

tracer-dynamic-coverage, tracer-dynamic-coverage-feedback

This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.

The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value. 🎢

---

tracer-max-code-growth

Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth.

---

tracer-min-branch-ratio

Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).

---

tracer-min-branch-probability, tracer-min-branch-probability-feedback

Stop forward growth if the best edge has probability lower than this threshold.

Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-probability-feedback is used for compilation with profile feedback and tracer-min-branch-probability compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

---

stack-clash-protection-guard-size

Specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 🛡️

Bestie, that last bit is the important bit. Bigger than the OS guard = vulnerable. Do not do it.

---

stack-clash-protection-probe-interval

Stack clash protection involves probing stack space as it is allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 💀

---

max-cse-path-length

The maximum number of basic blocks on path that CSE considers.

---

max-cse-insns

The maximum 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 garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation. 🚮

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging. 🐢

---

ggc-min-heapsize

Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and has no effect on code generation.

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC is not able to 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 maximum number of instruction reload should look backward for equivalent register. Increasing values mean more aggressive optimization, making the compilation time increase with probably slightly better performance. ⚡

• max-cselib-memory-locations — the max number of memory locations cselib is allowed to keep track of, bestie. Crank it up and the compiler gets more aggressive with optimizations, so compile time goes 📈 but performance probably gets a lil better. Trade-offs, you know? 💅
• max-sched-ready-insns — how many instructions that are ready to be issued the scheduler will actually consider at any one moment during the first scheduling pass. Bigger number = more thorough searching, which means longer compile time for probably barely any benefit. Not the glow-up you're hoping for. 🙈
• max-sched-region-blocks — the max number of blocks in a region that get considered for interblock scheduling.
• max-pipeline-region-blocks — the max number of blocks in a region considered for pipelining in the selective scheduler.
• max-sched-region-insns — the max number of insns in a region that get considered for interblock scheduling.
• max-pipeline-region-insns — the max number of insns in a region 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. Set it to 0 and region extensions are fully disabled, periodt.
• max-sched-insn-conflict-delay — the max conflict delay for an insn to even be considered for speculative motion.
• sched-spec-prob-cutoff — the minimal probability of speculation success (in percents), so that speculative insns actually get scheduled.
• sched-state-edge-prob-cutoff — the minimum probability an edge has to have for the scheduler to save its state across it.
• sched-mem-true-dep-cost — the 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 the depth of search for available instructions. 👀
• selsched-max-sched-times — the max number of times an instruction gets scheduled during selective scheduling. This is the limit on how many iterations the instruction may be pipelined through.
• selsched-insns-to-rename — the max number of best instructions in the ready list that get considered for renaming in the selective scheduler.
• sms-min-sc — the minimum value of stage count that the 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. That's it, that's the whole vibe. ✨
• integer-share-limit — small integer constants can share a data structure, which cuts the compiler's memory usage and speeds it up 🏃‍♀️💨. This sets the maximum value of a shared integer constant.
• ssp-buffer-size — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protector is used.
• min-size-for-stack-sharing — the minimum size of variables that take part in stack slot sharing when not optimizing.
• max-jump-thread-duplication-stmts — the 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 hunting for jump threading opportunities. When arriving at a block, incoming edges only get 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. Math girlie behavior fr fr. 🧮
• max-fields-for-field-sensitive — the maximum number of fields in a structure treated in a field sensitive manner during pointer analysis.
• prefetch-latency — an estimate of the average number of instructions executed before a prefetch finishes. The distance prefetched ahead is proportional to this constant. Bumping this number up may also lead to less streams being prefetched (peep simultaneous-prefetches).
• simultaneous-prefetches — the maximum number of prefetches that can run at the same time.
• l1-cache-line-size — the size of a cache line in the L1 data cache, in bytes.
• l1-cache-size — the size of the L1 data cache, in kilobytes.
• l2-cache-size — the size of the L2 data cache, in kilobytes.
• prefetch-dynamic-strides — whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. In some cases this may be beneficial, though the fact that the stride is non-constant may make it hard to predict when there's a clear benefit to issuing these hints.

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

• prefetch-minimum-stride — the minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than this threshold, prefetch hints will not be issued.

This setting is useful for processors that have hardware prefetchers, since there may be conflicts between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum stride they can handle, that value should be used here to improve the use of software prefetchers.

A value of -1 means we don't have a threshold and therefore prefetch hints can be issued for any constant stride.

This setting is only useful for strides that are known and constant.

• destructive-interference-size, constructive-interference-size — these are the values for the C++17 variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. The destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects; the constructive interference size is the maximum recommended size of contiguous memory accessed together. Typically both will be the size of an L1 cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size will be the small end of the range and the destructive size will be the large end.

The destructive interference size is intended to be used for layout, and thus has ABI impact. 🚨 The default value is not expected to be stable, and on some targets 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 it is used in that context, 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 to be interchanged.
• loop-interchange-stride-ratio — the minimum ratio between the stride of two loops for interchange to be profitable.
• min-insn-to-prefetch-ratio — the minimum ratio between the number of instructions and the number of prefetches 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 to enable prefetching in a loop.
• use-canonical-types — whether the compiler should use the "canonical" type system. This should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. But — plot twist — if bugs in the canonical type system are causing compilation failures, set this value to 0 to disable canonical types. 🩹
• switch-conversion-max-branch-ratio — switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch.
• max-partial-antic-length — the maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the enhanced partial redundancy elimination optimization can run away, consuming all of the memory available on the host machine 😭. This parameter sets a limit on the length of the sets that are computed, which prevents that runaway behavior. Setting a value of 0 for this parameter allows an unlimited set length (so like, don't, unless you're feeling chaotic).
• rpo-vn-max-loop-depth — the maximum loop depth that is value-numbered optimistically. When the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically and the remaining ones not.
• sccvn-max-alias-queries-per-access — the maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is 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 — okay so IRA has this whole sophisticated algorithm for squishing the conflict table down, but bestie, for truly huge functions that table can STILL eat absurd amounts of memory 😭. So if a function's conflict table could be bigger than the size in MB you give this parameter, the register allocator just nopes out and switches to a faster, simpler, lower-quality algorithm that doesn't need to build a pseudo-register conflict table at all. Tradeoff queen 💅
• ira-loop-reserved-regs — IRA can be used to get a more accurate read on register pressure in loops, which feeds into decisions about moving loop invariants (peep -O3). This parameter says how many available registers get reserved for other purposes. Default is the best value found from numerous experiments — scientists were sciencing 🧪
• ira-consider-dup-in-all-alts — Makes IRA care heavily about matching constraints (that's a duplicated operand number, iykyk) across all available alternatives for a preferred register class. Set it to zero and IRA only respects the matching constraint when it shows up in the only available alternative with an appropriate register class. Otherwise, IRA checks all available alternatives for the preferred register class even if it already found a choice with an appropriate register class, and respects the qualified matching constraint it found. Obsessed with thoroughness fr 💖
• ira-simple-lra-insn-threshold — Approximates the function's insn count in 1K units that triggers simple local RA.
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values it already reloaded into registers in subsequent insns, and this little glow-up is called inheritance ✨. EBB is the region where it does this. This parameter sets the minimal fall-through edge probability, in percentage, needed to add a BB to the inheritance EBB in LRA. The default was picked from numerous runs of SPEC2000 on x86-64, no cap.
• loop-invariant-max-bbs-in-loop — Loop invariant motion can get very expensive — both in compile time and in compile-time memory — when loops are huge. Any loop with more basic blocks than this parameter simply won't get loop invariant motion optimization done on it. Sorry bestie, too big 🫶
• loop-max-datarefs-for-datadeps — Building data dependencies is expensive for very large loops, so this parameter caps how many data references inside loops get considered for data dependence analysis. Those large loops just don't get handled by the optimizations that use loop data dependencies.
• max-vartrack-size — Sets the maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If you blow past this limit with variable tracking at assignments enabled, the analysis for that function gets retried without it, after all debug insns are removed from the function. And if you blow past it even without debug insns? Var tracking analysis is completely disabled for that function 💀. Setting this parameter to zero makes it unlimited.
• max-vartrack-expr-depth — Sets the maximum number of recursion levels when trying to map variable names or debug temporaries to value expressions. It's literally trading compilation time for more complete debug info. Set it too low and value expressions that ARE available and could be represented in debug info might just end up unused 😭; set it higher and the compiler may 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 a complexity explosion at inlining or when expanding to RTL. If a function has more of these gimple stmts than the limit you set, those stmts get dropped from the inlined copy of a function, and from its RTL expansion.
• min-nondebug-insn-uid — Uses uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may still get (non-overlapping) uids above it if that reserved range gets exhausted 👑
• ipa-sra-deref-prob-threshold — IPA-SRA only replaces a pointer known not to be NULL with one or more new parameters when the probability (in percent, relative to function entry) of it being dereferenced is higher than this parameter.
• ipa-sra-ptr-growth-factor — IPA-SRA only replaces a pointer to an aggregate with one or more new parameters when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
• ipa-sra-ptrwrap-growth-factor — The additional maximum allowed growth of the total size of new parameters that ipa-sra replaces a pointer to an aggregate with, if it points to a local variable that the caller only writes to and passes as an argument to other functions.
• ipa-sra-max-replacements — The maximum pieces of an aggregate that IPA-SRA tracks. As a consequence, it's also the maximum number of replacements of a formal parameter.
• sra-max-scalarization-size-Ospeed, sra-max-scalarization-size-Osize — The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of an aggregate considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize), respectively. Two moods, two numbers 💅
• sra-max-propagations — The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, to facilitate copy propagation.
• tm-max-aggregate-size — When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables get saved with the logging functions as opposed to save/restore code sequence pairs. This option only applies when 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 lifts that bound entirely 🦋. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP.
• hardcfr-max-blocks — Disables -fharden-control-flow-redundancy for functions with a larger number of blocks than the value you specify. Zero removes any limit.
• 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 value you specify.
• loop-block-tile-size — Loop blocking or 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. The strip length can be changed using the loop-block-tile-size parameter.
• ipa-jump-function-lookups — Specifies the number of statements visited during jump function offset discovery.
• ipa-cp-value-list-size — IPA-CP tries to track all possible values and types passed to a function's parameter so it can propagate them and do devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function.
• ipa-cp-eval-threshold — IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities whose scores exceed ipa-cp-eval-threshold.
• ipa-cp-max-recursive-depth — Maximum depth of recursive cloning for a self-recursive function.
• ipa-cp-min-recursive-probability — Recursive cloning happens only when the probability of the call being executed exceeds the parameter.
• ipa-cp-profile-count-base — When using the -fprofile-use option, IPA-CP will consider the measured execution count of a call graph edge at this percentage position in their histogram as the basis for its heuristics calculation.
• ipa-cp-recursive-freq-factor — The number of times interprocedural copy propagation expects recursive functions to call themselves.
• ipa-cp-recursion-penalty — The percentage penalty recursive functions receive when they're evaluated for cloning.
• ipa-cp-single-call-penalty — The percentage penalty that functions containing a single call to another function receive when they're evaluated for cloning.
• ipa-max-agg-items — IPA-CP is also capable of propagating a number of scalar values passed in an aggregate. ipa-max-agg-items controls the maximum number of such values per one parameter.
• ipa-cp-loop-hint-bonus — When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of that candidate. Main character bonus energy fr 🔥
• ipa-max-loop-predicates — The maximum number of different predicates IPA will use to describe when loops in a function have known properties.
• ipa-max-aa-steps — During its analysis of function bodies, IPA-CP employs alias analysis to track values pointed to by function parameters. So it doesn't spend forever analyzing huge functions, it gives up and considers all memory clobbered after examining ipa-max-aa-steps statements that modify memory. Respect the timeout bestie ⏳
• ipa-max-switch-predicate-bounds — The maximal number of boundary endpoints of case ranges of a switch statement. For a switch that exceeds this limit, IPA-CP will not construct a cloning cost predicate — which is what's used to estimate cloning benefit — for the default case of the switch statement.

okay bestie we're still deep in the --param vault, so let's keep scrolling 💅✨

• ipa-max-param-expr-ops — so IPA-CP will look at a conditional statement that references some function parameter to estimate the benefit of cloning it for a certain constant value. BUT if the number of operations in a parameter expression goes over ipa-max-param-expr-ops, that expression gets labeled as a complicated one and IPA analysis just... won't touch it. 🙈
• lto-partitions — sets the number of partitions you want produced during WHOPR compilation. Important vibe check: that number should be bigger than the number of CPUs you're compiling with.
• lto-min-partition — the size of the minimal partition for WHOPR, measured in estimated instructions. It exists so tiny little programs don't get shredded into a zillion partitions and waste everyone's time. 💀
• lto-max-partition — size of the max partition for WHOPR (again, in estimated instructions), giving an upper bound on how big any single partition can get. Meant to be used only with balanced partitioning, bestie. 🎀
• lto-max-streaming-parallelism — the max number of parallel processes used for LTO streaming. that's it, that's the whole thing. 👑
• cxx-max-namespaces-for-diagnostic-help — the max number of namespaces GCC will consult for suggestions when C++ name lookup fails for an identifier. it's giving helpful friend energy 🫶
• sink-frequency-threshold — the maximum relative execution frequency (in percents) of the target block compared to a statement's original block, which is what allows a statement to be sunk. Bigger numbers = more aggressive sinking. And 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 either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled.
• case-values-threshold — the smallest number of different values for which a jump-table beats a tree of conditional branches. If the value is 0, GCC uses the default for the machine. ✨
• jump-table-max-growth-ratio-for-size — the max code size growth ratio (in percent) when expanding into a jump table. This one is used when you're optimizing for size.
• jump-table-max-growth-ratio-for-speed — same deal, max code size growth ratio (in percent) when expanding into a jump table, except this parameter is the one used when optimizing for speed. 🏎️
• tree-reassoc-width — sets the maximum number of instructions executed in parallel in a reassociated tree. If it has a non zero value, this parameter overrides the target dependent heuristics used by default.
• sched-pressure-algorithm — pick between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely one to stop 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 leans way more heavily on having a regular register file and accurate register pressure classes. For more details, go peek at haifa-sched.cc in the GCC sources. 👀

The default choice depends on the target.

• max-slsr-cand-scan — the maximum number of existing candidates considered when seeking a basis for a new straight-line strength reduction candidate.
• asan-globals — turns on buffer overflow detection for global objects. This kind of protection is enabled by default if you're using the -fsanitize=address option. To turn global objects protection off, use --param asan-globals=0.
• asan-stack — turns on buffer overflow detection for stack objects. Enabled by default when using -fsanitize=address. To disable stack protection use the --param asan-stack=0 option.
• asan-instrument-reads — turns on buffer overflow detection for memory reads. Enabled by default when using -fsanitize=address. To disable memory reads protection, use --param asan-instrument-reads=0.
• asan-instrument-writes — turns on buffer overflow detection for memory writes. Enabled by default when using -fsanitize=address. To disable memory writes protection, use the --param asan-instrument-writes=0 option.
• asan-memintrin — turns on detection for built-in functions. Enabled by default when using -fsanitize=address. To disable built-in functions protection, use --param asan-memintrin=0.
• asan-use-after-return — turns on detection of use-after-return. Enabled by default when using the -fsanitize=address option. To disable it, use --param asan-use-after-return=0.

Heads up bestie: by default the check is disabled at run time. To enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS. 🚨

• asan-instrumentation-with-call-threshold — if the number of memory accesses in a function being instrumented is greater than or equal to this number, GCC uses callbacks instead of inline checks. E.g. to disable inline code, use --param asan-instrumentation-with-call-threshold=0.
• asan-kernel-mem-intrinsic-prefix — if nonzero, prefix calls to "memcpy", "memset" and "memmove" with __asan_ or __hwasan_ for -fsanitize=kernel-address or -fsanitize=kernel-hwaddress, respectively. 🌈
• hwasan-instrument-stack — enables hwasan instrumentation of statically sized stack-allocated variables. This kind of instrumentation is enabled by default when using -fsanitize=hwaddress and disabled by default when using -fsanitize=kernel-hwaddress. To disable stack instrumentation, use --param hwasan-instrument-stack=0, and to enable it use --param hwasan-instrument-stack=1.
• hwasan-random-frame-tag — when using stack instrumentation, decides tags for stack variables with a deterministic sequence that begins at a random tag for each frame. With this parameter unset, tags get chosen using that same sequence but starting from 1. It's enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress. To disable it, use --param hwasan-random-frame-tag=0. 🎲
• hwasan-instrument-allocas — enables hwasan instrumentation of dynamically sized stack-allocated variables. Enabled by default when using -fsanitize=hwaddress and disabled by default when using -fsanitize=kernel-hwaddress. To disable instrumentation of such variables, use --param hwasan-instrument-allocas=0, and to enable it use --param hwasan-instrument-allocas=1.
• hwasan-instrument-reads — enables hwasan checks on memory reads. Instrumentation of reads is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking memory reads, use --param hwasan-instrument-reads=0.
• hwasan-instrument-writes — enables hwasan checks on memory writes. Instrumentation of writes is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking memory writes, use --param hwasan-instrument-writes=0.
• hwasan-instrument-mem-intrinsics — enables hwasan instrumentation of builtin functions. Instrumentation of these builtin functions is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable instrumentation of builtin functions, use --param hwasan-instrument-mem-intrinsics=0.
• use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller than or equal to this number, GCC directly poisons (or unpoisons) 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 — the chunk size of the omp schedule for loops parallelized by parloops.
• parloops-schedule — the schedule type of the 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. Slight plot twist: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two. 🌸
• max-ssa-name-query-depth — the maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion = following one use-def chain link. 🔗
• max-speculative-devirt-maydefs — the maximum number of may-defs analyzed when looking for a must-def that specifies the dynamic type of an object which invokes a virtual call we might 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 count as 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 — the maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that's considered unpredictable. ⭐
• max-variable-expansions-in-unroller — so if -fvariable-expansion-in-unroller is switched on, this caps how many times a single variable is allowed to get expanded while loop unrolling is happening. 💫
• partial-inlining-entry-probability — the maximum probability of the entry BB of a split region (in percent, relative to the entry BB of the function) that makes partial inlining go ahead. yes it's as specific as it sounds.
• max-tracked-strlens — the most strings the strlen optimization pass will bother tracking string lengths for.
• gcse-after-reload-partial-fraction — the threshold ratio for doing partial redundancy elimination after reload. ✨
• gcse-after-reload-critical-fraction — the threshold ratio of critical edges execution count that permits redundancy elimination after reload.
• max-loop-header-insns — the max number of insns in a loop header that the copy loop headers pass is allowed to duplicate.
• vect-epilogues-nomask — turns on loop epilogue vectorization using a smaller vector size.
• vect-partial-vector-usage — controls when the loop vectorizer thinks about using partial vector loads and stores instead of just giving up and falling back to scalar code. here's the tea: 🫖
  • 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.

lowkey important: this parameter only does anything on targets that support partial vector loads and stores. no cap.

• vect-inner-loop-cost-factor — the maximum factor the loop vectorizer applies to the cost of statements in an inner loop, relative to the loop being vectorized. the factor it actually applies is the max of the estimated number of iterations of the inner loop and this parameter. default is 50. 🌸
• vect-induction-float — enables loop vectorization of floating point inductions.
• vrp-sparse-threshold — max number of basic blocks before VRP switches to using a sparse bitmap cache.
• vrp-switch-limit — max number of outgoing edges in a switch before VRP refuses to process it. periodt.
• vrp-vector-threshold — max 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 it's non-zero, reassociation considers the benefit of parallelizing the FMA's multiplication part and its 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 that the swing modulo scheduler considers.
• sms-dfa-history — the number of cycles the swing modulo scheduler looks at when checking conflicts using DFA.
• graphite-allow-codegen-errors — whether codegen errors should be ICEs when -fchecking is on. 😭
• sms-max-ii-factor — a factor for tuning the upper bound that the swing modulo scheduler uses when scheduling a loop.
• lra-max-considered-reload-pseudos — the max number of reload pseudos considered while spilling a non-reload pseudo.
• max-pow-sqrt-depth — maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant. ✨
• max-dse-active-local-stores — maximum number of active local stores in RTL dead store elimination.
• asan-instrument-allocas — enables asan allocas/VLAs protection.
• max-iterations-computation-cost — a bound on the cost of an expression used to compute the number of iterations.
• max-isl-operations — maximum number of isl operations; 0 means unlimited.
• graphite-max-arrays-per-scop — maximum number of arrays per scop.
• max-vartrack-reverse-op-size — max size of loc list for which reverse ops should be added.
• fsm-scale-path-stmts — scale factor to apply to the number of statements in a threading path crossing a loop backedge, when comparing against --param=max-jump-thread-duplication-stmts.
• uninit-control-dep-attempts — maximum number of nested calls to search for control dependencies during uninitialized variable analysis.
• uninit-max-chain-len — maximum number of predicates anded for each predicate ored in the normalized predicate chain.
• uninit-max-num-chains — maximum number of predicates ored in the normalized predicate chain.
• sched-autopref-queue-depth — hardware autoprefetcher scheduler model control flag. it's the number of lookahead cycles the model looks into; at ' ' only enable instruction sorting heuristic. (yes, that's literally a space in there, we don't make the rules 💅)
• loop-versioning-max-inner-insns — the maximum 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 maximum number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy — and it discounts any instructions in inner loops that directly benefit from versioning.
• ssa-name-def-chain-limit — the maximum number of SSA_NAME assignments to follow when determining a property of a variable, like its value. this limits the number of iterations or recursive calls GCC performs when optimizing certain statements, or when determining their validity before issuing diagnostics.
• store-merging-max-size — maximum size of a single store merging region, in bytes.
• hash-table-verification-limit — the number of elements for which hash table verification is done, for each searched element.
• max-find-base-term-values — maximum number of VALUEs handled during a single find_base_term call.
• analyzer-max-enodes-per-program-point — the maximum number of exploded nodes per program point inside the analyzer, before it terminates analysis of that point. 💀
• analyzer-max-constraints — the maximum number of constraints per state.
• analyzer-min-snodes-for-call-summary — the minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites.
• analyzer-max-enodes-for-full-dump — the maximum 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 maximum number of times a callsite can appear in a call stack within the analyzer, before it terminates analysis of a call that would recurse deeper.
• analyzer-max-svalue-depth — the maximum depth of a symbolic value, before it approximates the value as unknown.
• analyzer-max-infeasible-edges — the maximum number of infeasible edges to reject before declaring a diagnostic as infeasible.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block that gets considered hot. this parameter is used only in GIMPLE FE.
• analyzer-bb-explosion-factor — the maximum 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 to ellipsize string literals in analyzer text art diagrams. 🎀
• analyzer-text-art-ideal-canvas-width — the ideal width in characters of text art diagrams generated by the analyzer.
• analyzer-text-art-string-ellipsis-head-len — the number of literal bytes to show at the head of a string literal in text art when ellipsizing it.
• analyzer-text-art-string-ellipsis-tail-len — the number of literal bytes to show at the tail of a string literal in text art when ellipsizing it.
• ranger-logical-depth — maximum depth of logical expression evaluation the ranger will look through when evaluating outgoing edge ranges.
• ranger-recompute-depth — maximum depth of instruction chains to consider for recomputation in the outgoing range calculator.
• relation-block-limit — maximum number of relations the oracle will register in a basic block.
• min-pagesize — minimum page size for warning purposes.
• openacc-kernels — specify the mode of OpenACC kernels constructs handling. with --param=openacc-kernels=decompose, OpenACC kernels constructs get decomposed into parts — a sequence of compute constructs, each then handled individually. heads up bestie, this is work in progress 🚧. with --param=openacc-kernels=parloops, OpenACC kernels constructs are handled by the parloops pass, en bloc. this is the current default.
• openacc-privatization — 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.

---

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 — how many Newton iterations we do when calculating the reciprocal for double type. When division approximation is turned on, the precision of division is proportional to this param. Default is 2. 💅
• aarch64-autovec-preference — forces an ISA selection strategy for auto-vectorization, bestie. Takes values 0 through 4, inclusive:
  • 0 — use the default heuristics 🧚
  • 1 — use only Advanced SIMD for auto-vectorization
  • 2 — use only SVE for auto-vectorization
  • 3 — use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal
  • 4 — use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal

The default value is 0. ✨

• aarch64-ldp-policy — fine-grained policy for load pairs, like the little details matter (they do 💖). With --param=aarch64-ldp-policy=default, use the policy of the tuning structure — this is the current default. With --param=aarch64-ldp-policy=always, emit ldp regardless of alignment. With --param=aarch64-ldp-policy=never, do not emit ldp. With --param=aarch64-ldp-policy=aligned, emit ldp only if the source pointer is aligned to at least double the alignment of the type.
• aarch64-stp-policy — fine-grained policy for store pairs. With --param=aarch64-stp-policy=default, use the policy of the tuning structure — current default, slay. 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 how many alias checks the AArch64 load/store pair fusion pass performs when it's trying to form an ldp/stp. Higher values make the pass more aggressive at re-ordering loads over stores, but the tradeoff is increased compile time. 👑
• aarch64-ldp-writeback — param controlling which writeback opportunities we try to handle in the AArch64 load/store pair fusion pass. A value of zero disables writeback handling. One means we try to form pairs involving one or more existing individual writeback accesses where possible. A value of two means we also try to opportunistically form writeback opportunities by folding in trailing destructive updates of the base register used by a pair. 🌀
• aarch64-loop-vect-issue-rate-niters — the tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If this parameter is set to n, GCC will not use this heuristic for loops that are known to execute in fewer than n Advanced SIMD iterations.
• aarch64-vect-unroll-limit — the vectorizer uses available tuning information to figure out whether it'd be beneficial to unroll the main vectorized loop, and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four. 🎀

---

The following choices of name are available on GCN targets:

• gcn-preferred-vectorization-factor — preferred vectorization factor: default, 32, 64.

The following choices of name are available on i386 and x86_64 targets:

• x86-stlf-window-ninsns — instructions number above which STFL stall penalty can be compensated.
• x86-stv-max-visits — the maximum number of use and def visits when discovering a STV chain before the discovery is aborted.

---

✨ Program Instrumentation Options ✨

GCC supports a bunch of command-line options that control adding run-time instrumentation to the code it normally generates. Like, one purpose of instrumentation is collecting profiling statistics for finding program hot spots, code coverage analysis, or profile-guided optimizations. Another whole class of program instrumentation is adding run-time checking to catch programming errors like invalid pointer dereferences or out-of-bounds array accesses, plus deliberately hostile attacks such as stack smashing or C++ vtable hijacking. 😤 And there's also a general hook you can use to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. It's giving swiss army knife, fr fr. 🔥

• -p
• -pg — generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. Bestie, don't forget the link step, I'm begging 🙈

You can use the function attribute "no_instrument_function" to suppress profiling of individual functions when compiling with these options.

• -fprofile-arcs — add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable. 💖

When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).

Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. "gcc a.c b.c -o binary" would generate binary-a.gcda and binary-b.gcda files.

• -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". ✨
• --coverage — this option is used to compile and link code instrumented for coverage analysis. It's a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details. 👑
  1. 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. 🦋
  2. 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.
  3. Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
  4. 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.

5. For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
6. 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. 🧚

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.

• -ftest-coverage — produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. 💅
• -fprofile-abs-path — automatically convert relative source file names to absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories. 🌈
• -fprofile-dir=path — tells GCC which directory to go look in for the profile data files, bestie. It's the path. 💖 Heads up though: this one only touches the profile data that -fprofile-generate, -ftest-coverage and -fprofile-arcs make, and the data that -fprofile-use, -fbranch-probabilities and their little friend-options consume. Both absolute and relative paths are totally fine here. Default vibe? GCC just uses the current directory as path, so the profile data file ends up chillin' right next to the object file. And to stop file names from beefing with each other: if the object file name isn't an absolute path, GCC mangles the absolute path of the sourcename.gcda file and uses that as the name of the .gcda file. For the full tea on file naming, peep -fprofile-arcs. And there's a similar one, -fprofile-note 👀.

So when you run an executable in a massive parallel environment, it's recommended to save profiles to different folders. You can do that with these variables in path that get exported at run-time:

• %p — process ID. 🆔
• %q{VAR} — value of environment variable VAR.

• -fprofile-generate
• -fprofile-generate=path — this turns on the options that are usually used for instrumenting an application so it produces profile info that's useful for later recompilation with profile-feedback-based optimization. ✨ Important: you gotta use -fprofile-generate both when compiling and when linking your program. Both. Fr fr.

These are the options it enables: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.

If path is given, GCC looks at that path to find the profile feedback data files. See -fprofile-dir.

And to actually optimize your program based on all that collected profile info? Use -fprofile-use. 🫶

• -fprofile-info-section
• -fprofile-info-section=name — registers the profile information in the section you name instead of using a constructor/destructor. The section name is name if you specify it; otherwise it defaults to ".gcov_info". For every translation unit, a pointer to the profile information generated by -fprofile-arcs gets placed in the specified section. Heads up: this option disables the profile information registration through a constructor, and it disables the profile information processing through a destructor.

This one is not meant for hosted environments like GNU/Linux. It's for freestanding environments (think embedded systems 🧚) with limited resources that don't support constructors/destructors or the C library file I/O.

The linker could collect the input sections into one continuous memory block and define start and end symbols. Here's a GNU linker script example that defines a linker output section:

.gcov_info      :
{
  PROVIDE (__gcov_info_start = .);
  KEEP (*(.gcov_info))
  PROVIDE (__gcov_info_end = .);
}

And the program could dump the profiling information registered in this linker set, like, 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 can 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 the .gcno file into that path location. But careful: if you combine this option with multiple source files, the .gcno file will get overwritten. 😭 One file, one note, bestie.
• -fprofile-prefix-path=path — you can use this alongside profile-generate=profile_dir and profile-use=profile_dir to tell GCC where the base directory of the built source tree is. By default, profile_dir holds files with mangled absolute paths of all the object files in the built project. That's not it when the directory you used to build the instrumented binary is different from the directory you used to build the binary optimized with profile feedback — because then the profile data won't be found during the optimized build. 🙈 In those setups, -fprofile-prefix-path=path with path pointing at the base directory of the build strips off the irrelevant part of the path and keeps all file names relative to the main build directory. Clean. ✨
• -fprofile-prefix-map=old=new — when compiling files that live in directory old, record profiling information (with --coverage) describing them as if they lived in directory new instead. See also -ffile-prefix-map and -fcanon-prefix-map. 👑
• -fprofile-update=method — changes 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 great for single-threaded applications; the second one prevents profile corruption by emitting thread-safe code. 🧵

Warning: when an application doesn't properly join all threads (or creates a detached thread), a profile file can still get corrupted.

Using prefer-atomic gets transformed either into atomic (when the target supports it) or into single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is on the command line; otherwise the default method is single.

If atomic is selected, the profile information is updated using atomic operations on a best-effort basis. Ideally, it's updated through atomic operations in hardware. But if the target platform doesn't support the required atomic operations in hardware, yet libatomic is available, then the profile info gets updated through calls to libatomic. And if the target platform supports neither the required atomic hardware ops nor libatomic? Then the profile information is not atomically updated and a warning is issued. In that case the profiling info you get 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. And if a signal interrupts those two operations updating a counter, the profiling information may end up in an inconsistent state. 😬

• -fprofile-filter-files=regex — instruments 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 — instruments 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 located in the /usr/ folder. 🚫

• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — controls the level of reproducibility of the profile gathered by -fprofile-generate. This makes it possible to rebuild a program with the same outcome, which is useful for, like, distribution packages. 📦

With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it isn't multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of a program run but also before the fork function is invoked.

Note that it's pretty common for execution counts of some part of programs to depend on, for example, the length of temporary file names or memory space randomization (which can affect hash-table collision rate). Such non-reproducible parts of programs may be annotated by the no_instrument_function function attribute. And gcov-dump with -l can be used to dump the gathered data and verify that it's indeed reproducible. 🔍

With -fprofile-reproducible=parallel-runs the collected profile stays reproducible regardless of the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of the instrumented program in parallel (like with make -j). This reduces the quality of the gathered data, in particular of indirect call profiling. 🌈

• -fsanitize=address — bestie, this is AddressSanitizer 💖 a fast little memory error detector. Memory access instructions get instrumented so it can catch out-of-bounds and use-after-free bugs. Turning this on also turns on -fsanitize-address-use-after-scope. Full deets at <https://github.com/google/sanitizers/wiki/AddressSanitizer>. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. Set it to "help=1" and the available options are shown at startup of the instrumented program. List of supported options lives at <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>. This option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. And heads up: 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.

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). 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 — AddressSanitizer but make it Linux kernel 🧚✨. See <https://github.com/google/kernel-sanitizers> for more details.
• -fsanitize=hwaddress — Hardware-assisted AddressSanitizer! 🦋 It uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with a low memory overhead. 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://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html> for more details. 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. This one cannot be combined with -fsanitize=thread or -fsanitize=address, and is currently only available on AArch64.
• -fsanitize=kernel-hwaddress — Hardware-assisted AddressSanitizer for compilation of the Linux kernel 🌈. Similar to -fsanitize=kernel-address but using an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences necessary for compiling the Linux kernel. Those differences are 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 👀. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. It 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 ➖. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. It 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 — Enable 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.

Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).

• -fsanitize=leak — Enable 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 — Enable 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 🚫0️⃣.
  • -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 — okay so this is the strict girly version of array bounds instrumentation 💅. Most out-of-bounds accesses get caught, including flexible array member-like arrays. But bestie, listen: initializers of variables with static storage are not instrumented. Not them. They're exempt. 🙈
• -fsanitize=alignment — this one checks pointer alignment when pointers get dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object. Alignment vibe check, basically. ✨
• -fsanitize=object-size — instruments memory references using the __builtin_dynamic_object_size function. Various out-of-bounds pointer accesses get detected. It's giving caught in 4K. 👑
• -fsanitize=float-divide-by-zero — detects floating-point division by zero. But here's the tea: unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. It's not a bug, it's a lifestyle. 🦋
• -fsanitize=float-cast-overflow — enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. And again, unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also it does not work well with FE_INVALID exceptions enabled — so like, don't. 😭
• -fsanitize=nonnull-attribute — instruments calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute. Passing null where you promised not to? Absolutely not, we don't do that here. 💀
• -fsanitize=returns-nonnull-attribute — instruments return statements in functions marked with the returns_nonnull function attribute, to detect returning null values from such functions. You said non-null, bestie. You said it. 🫶
• -fsanitize=bool — instruments loads from bool. If a value other than 0/1 is loaded, a run-time error is issued. Bool said binary only, and it meant it. ⭐
• -fsanitize=enum — instruments loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued. Stay in your lane, enum. 🌸
• -fsanitize=vptr — this is the C++ one 😌. It instruments C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. Like, are you actually who you say you are? Main character verification. 💜
• -fsanitize=pointer-overflow — instruments pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. Math with pointers going off the rails = caught. 🔥
• -fsanitize=builtin — instruments arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. So like… don't pass 0 there. Seriously. 🙈

---

Heads up bestie: sanitizers tend to increase the rate of false positive warnings, most notably those around -Wmaybe-uninitialized. We recommend against combining -Werror and [the use of] sanitizers. The drama is not worth it. 😭

While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages. Fyi. 💅

---

• -fno-sanitize=all — this disables all previously enabled sanitizers. And no, -fsanitize=all is not allowed, because some sanitizers cannot be used together. They're not friends. They don't vibe. Periodt. 💀
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. Science girlie behavior, we love it. 🧚
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections. si may contain wildcards. Wildcards!! ✨
• -fsanitize-recover[=opts] — okay this one is long, sit down. -fsanitize-recover= controls error recovery mode for sanitizers mentioned in the comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code.

Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.

Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable.

Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:

undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

• -fsanitize-address-use-after-scope — enables sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to none. Snitching on your own locals, but make it safe. 🎀
• -fsanitize-trap[=opts] — the -fsanitize-trap= option instructs the compiler to report for sanitizers mentioned in the comma-separated list of opts undefined behavior using __builtin_trap rather than a libubsan library routine. If this option is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer, __builtin_trap will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover=.

The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. No library? No problem. 💅

Currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. -fsanitize-trap=all can be also specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs libubsan support, -fsanitize-trap=vptr is not allowed. 🚫

• -fsanitize-undefined-trap-on-error — this option is the deprecated equivalent of -fsanitize-trap=all. She's retired, but she still shows up sometimes. 🫶
• -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 with both operands variable, or __sanitizer_cov_trace_const_cmp1, __sanitizer_cov_trace_const_cmp2, __sanitizer_cov_trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. Look at all those little trace functions, they're iconic honestly. 👑
• -fcf-protection=[full|branch|return|none|check] — enables code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. This is intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Security girlie, we stan. 🔥

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 none turns off instrumentation.

To override -fcf-protection, -fcf-protection=none needs to be added and then with -fcf-protection=xxx. Yes, in that order. Don't fight it. 🙈

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 macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for the return.

You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. Opting out, but make it explicit. 💜

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 with -fharden-conditional-branches to cover all conditionals. Double-checking the double-check, highkey paranoid but honestly? Respect. 🦋
• -fharden-conditional-branches — bestie, for every single non-vectorized conditional branch that makes it through gimple optimizations, GCC goes and emits extra code that computes the reversed condition and verifies it, then calls __builtin_trap if the result is unexpected. Use it together with -fharden-compares to cover all the conditionals. 💅
• -fharden-control-flow-redundancy — this one emits extra code that sets booleans when you enter basic blocks, then verifies and traps at function exits whenever those booleans don't form an execution path that's compatible with the control flow graph. It's giving security-guard energy fr fr. 🛡️

Verification goes down 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. The tuning options --param hardcfr-max-blocks and --param hardcfr-max-inline-blocks are there for you. ✨

Tail call optimization happens too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends — plus any calls marked early enough as potential tail calls — would also get verification issued before the call. But ngl these possibilities are merely theoretical, because those conditions can only be met when you're using custom compiler plugins. 🙈

• -fhardcfr-skip-leaf — turns off -fharden-control-flow-redundancy in leaf functions. Simple as that. 🍃
• -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is active, this checks the recorded execution path against the control flow graph at exception escape points, as if the function body were wrapped in a cleanup handler that performed the check and reraised. This option is enabled by default, so use -fno-hardcfr-check-exceptions if you wanna turn it off. 🎀
• -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, this checks 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 block tail-call optimization — whether or not that call ends up 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 or disable it (using its negated form) explicitly, regardless of the optimizations. Iconic behaviour tbh. 💖

• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, this checks the recorded execution path against the control flow graph before noreturn calls. Either all of them (always), or those that aren't expected to return control to the caller through an exception (no-xthrow, which is the default), or 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 make checking happen more than once, if the exception gets caught in the caller — whether by a handler or a cleanup. When -fhardcfr-check-exceptions is also enabled, the compiler avoids associating a noreturn call with the implicitly-added cleanup handler, since that would be redundant with the check done before the call. But other handlers or cleanups in the function, if they get activated, will modify the recorded execution path and check it again when another checkpoint is hit. And the checkpoint may even be another noreturn call, so checking can end up performed multiple times. 😭

Various optimizers may cause calls to be marked as noreturn and/or nothrow, even without the corresponding attributes, which may affect where checks get placed before calls, and whether implicit cleanup handlers get added for them. This unpredictability, plus the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, is why no-xthrow is the default for this option: it excludes from the noreturn treatment only internal functions used to (re)raise exceptions, which aren't affected by these optimizations. 💀

• -fhardened — enables a set of flags for C and C++ that level up the security of the generated code without touching its ABI. The precise flags enabled may change between major releases of GCC, but currently they are:

-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE  -pie  -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)

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. 👀

This option is intended to be used in production builds, not merely in debug builds. It said what it said. 💅

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. ✨

• -fstack-protector — emits extra code to check for buffer overflows, like stack smashing attacks. It does this 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 count — optimized-away variables or variables allocated in registers don't. 🔥
• -fstack-protector-all — like -fstack-protector, except all functions are protected. No skipping, periodt. 💖
• -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. Again, only variables 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. VIP only, iykyk. 🎀
• -fstack-check — generates code to verify that you don't 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 it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems when there's only one stack. 🧚

Note that this switch does not actually cause checking to be done — the operating system or the language runtime has to do that. The switch causes generation of code to ensure that they see the stack being extended. 👑

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, and specific means use the best checking method, which 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 these drawbacks:

1. Modified allocation strategy for large objects: they're always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking isn't 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's not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. 🌈

• -fstack-clash-protection — generates code to prevent stack clash style attacks. When this option is enabled, the compiler only allocates 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 don't 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. ✨

• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit — generates code to ensure that the stack doesn't 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 — generates 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. 🦋

---

okay so picture this: code that was built with -fsplit-stack calls code that was not built with -fsplit-stack, and suddenly there might not be much stack space left for that second chunk to actually run in. 😭 rude.

If compiling every single thing — library code included — with -fsplit-stack is just not on the table, then the linker can come through and fix up those calls so the code compiled without -fsplit-stack always has a big stack. 🫶 Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. periodt.

---

-fstrub=disable

Turn stack scrubbing all the way off, ignoring any "strub" attributes you may have sprinkled around. See

-fstrub=strict

Functions default to "strub" mode "disabled", and this applies the restriction strictly: 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"). no exceptions bestie 💅

-fstrub=relaxed

Restores the default stack scrub ("strub") setting, meaning "strub" is only enabled as required by "strub" attributes attached to function and data types. "Relaxed" means strub contexts are only prevented from calling functions explicitly associated with "strub" mode "disabled". This option is only useful to override other -fstrub=* options that come before it in the command line. 🎀

-fstrub=at-calls

Enables "at-calls" "strub" mode where it's viable. Mainly a testing thing. It exercises the "strub" machinery in scenarios strictly local to a translation unit. Heads up: this "strub" mode modifies function interfaces, so any function that's visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. Go read the "strub" attribute documentation for the details on viability and eligibility requirements. 👑

-fstrub=internal

Enables "internal" "strub" mode where it's viable. Also mainly for testing. This one is intended to thoroughly exercise the parts of the "strub" machinery that implement the less efficient, but interface-preserving "strub" mode. Functions that wouldn't be affected by this option are, like, quite uncommon. 🌸

-fstrub=all

Enables some "strub" mode where it's viable. When both strub modes are viable, "at-calls" is preferred. -fdump-ipa-strubm adds function attributes that tell you which mode got selected for each function. Primary use of this option is testing, to thoroughly exercise the "strub" machinery. 💖

-fvtable-verify=[std|preinit|none]

This one is only available when compiling C++ code. It turns on (or off, if you're using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer the call is made through is actually valid for the type of the object — and that it hasn't been corrupted or overwritten. If an invalid vtable pointer gets detected at run time, an error is reported and execution of the program is halted immediately. 💀

This option causes run-time data structures to be built at program startup, and those are what verifies the vtable pointers. The options std and preinit control the timing of when those data structures get built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std makes the data structures get built after shared libraries have been loaded and initialized. -fvtable-verify=preinit makes them get built before shared libraries have been loaded and initialized.

If this option shows up multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. the way that priority order just exists 🧚

-fvtv-debug

When used alongside -fvtable-verify=std or -fvtable-verify=preinit, this causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also makes the compiler log information about which vtable pointers it finds for each class. That info gets written to a file named vtv_set_ptr_data.log, in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. 🍓

Note: this feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one first, bestie. 🙈

-fvtv-counts

This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts, and logs this information for each compilation unit. The compiler writes this information 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 information to vtv_class_set_sizes.log in the same directory. ✨

Note: this feature appends data to the log files. To get fresh log files, be sure to delete any existing ones. 🫶

-finstrument-functions

Generates instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, these profiling functions get called with the address of the current function and its call site. (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.) 😭

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which you can look up exactly in the symbol table. 🔍

This instrumentation also happens for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. That means addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (That's normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) 💅

A function may be given the attribute no_instrument_function, in which case this instrumentation is not done. You can use this for, like, the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot 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 — i.e. the first profiling function is called after the first entry into the instrumented function, and the second profiling function is called before the exit corresponding to this first entry. 🎀

The definition of "once" for the purposes of this option is a little vague, because the implementation is not protected against data races. As a result, the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread — but the calls are always paired, that is to say, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function. 🦋

-finstrument-functions-exclude-file-list=file,file,...

Sets the list of functions that are excluded from instrumentation (peep the description of -finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it 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 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,...

Similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name that gets matched is its user-visible name, like vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, it counts as a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. ✨

-fpatchable-function-entry=N[,M]

Generates N NOPs right at the beginning of each function, with the function entry point before the 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 that you can use to patch in whatever instrumentation you want at run time — provided that the code segment is writable. 👀 The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.

For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary. 💖

Also heads up bestie: the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over the command-line option -fpatchable-function-entry=N,M. So you can use the attribute to grow the area or yeet it entirely for one single function. And if N=0? No pad location gets recorded, periodt.

The NOP instructions get inserted at — and maybe before, it depends on M — the function entry address, and yes, that's 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. 💅

The max value for both 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.

---

✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which gets run on every C source file before the actual compilation happens. 🧚

If you use the -E option, literally nothing happens except preprocessing. Some of these options only make sense alongside -E, because they make the preprocessor output unsuitable for actual compilation. Not me warning you twice. 💀

On top of the options listed right here, there are a bunch of options controlling search paths for include files, documented over in Directory Options. And options controlling preprocessor diagnostics live in Warning Options.

• -D name — Predefine name as a macro, with definition 1.
• -D name=definition — The contents of definition are tokenized and processed as if they showed up during translation phase three in a #define directive. In particular, the definition gets truncated by embedded newline characters. 🙈

If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters like spaces that mean something in shell syntax.

Wanna define a function-like macro on the command line? Write its argument list with surrounding parentheses before the equals sign (if there is one). Parentheses are meaningful to most shells, so quote the option, bestie. With sh and csh, -D'name(args...)=definition' works. 🫶

-D and -U options are processed in the order they're given on the command line. All -imacros file and -include file options are processed after all the -D and -U options. Order matters, iykyk.

• -U name — Cancel any previous definition of name, whether it was built in or provided with a -D option. Gone. Deleted. 💅
• -include file — Process file as if #include "file" appeared as the very first line of the primary source file. BUT — and this is the twist — the first directory searched for file is the preprocessor's working directory, not 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 as normal.

If multiple -include options are given, the files are included in the order they appear on the command line.

• -imacros file — Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines stay defined though. This lets you scoop up all the macros from a header without also processing its declarations. Iconic, honestly.

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. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.
• -M — Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files — including those coming from -include or -imacros command-line options. 🎀

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with the object file suffix, and with any leading directory parts removed. If there are many included files, then the rule gets split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, like -dM. To avoid mixing that debug output with the dependency rules, you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

• -MM — Like -M but do not mention header files found in system header directories, nor header files included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output.

• -MF file — When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given, the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

If file is -, then the dependencies are written to stdout. ✨

• -MG — In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, since a missing header file renders that useless.

This feature is used in automatic updating of makefiles.

• -Mno-modules — Disable dependency generation for compiled module interfaces.
• -MP — This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around the errors make gives you if you remove header files without updating the Makefile to match. 😭

This is typical output:

test.o: test.c test.h

test.h:

• -MT target — Change the target of the rule emitted by dependency generation. By default, CPP takes the name of the main input file, deletes any directory components and any file suffix like .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? You can specify them as a single argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give:

$(objpfx)foo.o: foo.c

• -MQ target — Same as -MT, but it quotes any characters which are special to Make. So -MQ '$(objpfx)foo.o' gives:

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ.

• -MD — -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d; otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file.

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Sneaky and efficient, we love her. 💅

• -MMD — Like -MD except mention only user header files, not system header files.
• -fpreprocessed — Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. 🧚

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. Those are the extensions GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — When preprocessing, handle directives, but do not expand macros.

The option's behavior depends on the -E and -fpreprocessed options.

With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. In addition, the -dD option is implicitly enabled.

With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only.

With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only.

• -fdollars-in-identifiers — Accept $ in identifiers.
• -fextended-identifiers — Accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. ✨
• -fno-canonical-system-headers — When preprocessing, do not shorten system header paths with canonicalization.
• -fmax-include-depth=depth — Set the maximum depth of the nested #include. The default is 200.
• -ftabstop=width — Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8.
• -ftrack-macro-expansion[=level] — okay so this one tracks where all the tokens go when macros expand, and bestie, that's lowkey a whole vibe 💫. It lets the compiler spill the tea about the current macro-expansion stack when a compilation error happens inside a macro expansion. Downside? It makes the preprocessor AND the compiler eat more memory 😭. The level param picks how precise the tracking is, so you can dial down memory if you need to:
  • Value 0 of level — de-activates this option, periodt.
  • Value 1 — tracks token locations in a degraded mode for minimal memory overhead. In this mode every token that comes out of expanding an argument of a function-like macro gets the same location. It's giving "close enough" energy.
  • Value 2 — tracks token locations completely. This value is the most memory hungry, fr fr.
  • When the option is given with no argument, the default parameter value is 2.

Heads up: -ftrack-macro-expansion=2 is activated by default. Yes queen, it's already on 💅.

• -fmacro-prefix-map=old=new — when you're preprocessing files that live in directory old, this expands the __FILE__ and __BASE_FILE__ macros as if those files actually lived in directory new instead. You can use this to flip an absolute path into a relative one by using . for new, which can give you more reproducible builds that are location independent ✨. This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map and -fcanon-prefix-map — they're all in the same friend group 🫶.
• -fexec-charset=charset — sets the execution character set, which is what's 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 one matches the width of wchar_t and the big-endian or little-endian byte order being used for code generation. Same as -fexec-charset, charset can be any encoding supported by the system's iconv library routine; however, you're gonna have problems with encodings that don't fit exactly in wchar_t. Not a slay, sorry bestie 💀.
• -finput-charset=charset — sets the input character set, used to translate from the character set of the input file over to the source character set GCC uses. If the locale doesn't specify it, or GCC can't get this info 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 — she's the main character here 👑. charset can be any encoding supported by the system's iconv library routine.
• -fpch-deps — when you're using precompiled headers, this flag makes the dependency-output flags also list the files from the precompiled header's dependencies. If you don't specify it, only the precompiled header gets listed and not the files that were used to create it, because those files aren't consulted when a precompiled header is used. Makes sense, iconic behavior 🌈.
• -fpch-preprocess — this option lets you use 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, plus 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 gets 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 — turns on generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this 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 that can be inhibited with the negated form -fno-working-directory. And if the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever. Bye bestie 🙈.
• -A predicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred over the older form -A predicate(answer), which is still supported, because it doesn't use shell special characters. New form ate, no cap 💅.
• -A -predicate=answer — cancel an assertion with the predicate predicate and answer answer.
• -C — do not discard comments. All comments get passed through to the output file, except for comments in processed directives, which are deleted along with the directive. Ngl, be prepared for side effects when you use -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.

On top of 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. Slay for the linters 🦋.

• -P — inhibit generation of linemarkers in the output from the preprocessor. This might be useful when you're running the preprocessor on something that isn't C code, and it's gonna be sent to a program which might get confused by the linemarkers. Protect the innocent, periodt 💖.
• -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 the details, iykyk.

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. Don't come for her with anything else 💀.

• -trigraphs — support 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. Pick your lane ✨.

• -remap — enable special code to work around file systems which only permit very short file names, such as MS-DOS. Retro problem, retro fix 🌸.
• -H — print 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're found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! . Vibe check on your headers, always 👑.
• -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 — that's a hard no, bestie 😭.
  • -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's predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. Plot twist 🌀.

• -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. Minimalist queen 💅.
• -dI — output #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. It's giving "I'm only here for the debug girlies" 🧚.
• -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's 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. Live dangerously if you want, but she warned you 🙈.
• -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. Two-step, but make it fashion 💜.
• -no-integrated-cpp — bestie, by default GCC just does the preprocessing inside the whole tokenizing-and-parsing situation, all integrated, one big vibe. Pass this and it does preprocessing as its own separate pass before compilation instead. Concretely: the right language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) gets invoked twice — once for preprocessing only, and once to actually compile the preprocessed input. Lowkey useful with -B or -wrapper if you wanna slot in an alternate preprocessor or do extra processing on the source between normal preprocessing and compilation. 💅
• -flarge-source-files — tells GCC to expect large source files, at the cost of slower compilation and higher memory usage. Here's the tea: GCC normally tracks both column numbers and line numbers inside source files, and prints both in diagnostics. But once it's chewed through a certain number of source lines, it stops tracking column numbers and only tracks line numbers. Which means diagnostics for later lines come with no column numbers, and options like -Wmisleading-indentation just stop working at that point — though the compiler does print a note when that happens, so at least it tells you. Passing -flarge-source-files significantly increases the number of source lines GCC can handle before it gives up on columns. 🦋

---

✨ Passing Options to the Assembler ✨

Yes bestie, you can pass options straight through to the assembler. Obsessed.

• -Wa,option — passes option as an option to the assembler. If option contains commas, it gets split into multiple options at the commas. (So comma = new option, got it.)
• -Xassembler option — passes option as an option to the assembler. Use this one to supply system-specific assembler options that GCC doesn't even recognize. 🙈

Heads up: if you wanna pass an option that takes an argument, you have to use -Xassembler twice — once for the option and once for the argument. Fr fr.

---

💖 Options for Linking 💖

These ones only come into play when the compiler links object files into an executable output file. If there's no link step happening, they're meaningless. No link, no vibe check. 😭

• object-file-name — a file name that doesn't end in a special recognized suffix is considered to name an object file or library. (Object files get distinguished from libraries by the linker based on the file contents, iykyk.) If linking is done, these object files are used as input to the linker.
• -c
• -S
• -E — if any of these three is used, then the linker is not run, and object file names should not be used as arguments. Periodt.
• -flinker-output=type — this one controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin. But for debugging the compiler, or if you want incremental linking with a non-LTO object file, manually controlling type can be useful. Let's break down the types:
  • 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 allows building shared libraries without position-independent code on architectures where that's possible, i.e. on x86.
  • If type is pie, code generation produces an -fpie executable. This results in similar optimizations as exec, except that -fpie is not disabled if specified at compilation time.
  • If type is rel, the compiler assumes incremental linking is being done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. Also, if -ffat-lto-objects is specified, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. And at link time, the result of incremental linking also loads faster than a static library — assuming the majority of objects in the library are actually 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 are linked together, the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of the benefits of whole program optimizations are lost.

During the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of the object files in the 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 — searches the library named library when linking. (The second form, with the library as a separate argument, is only there for POSIX compliance and is not recommended, so maybe don't.)

The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details — the general description below applies to the GNU linker.

The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.

It makes a difference where in the command you write this option — the linker searches and processes libraries and object files in the order they are 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. 💀 Order matters, bestie.

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
• -nostartfiles — don't use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — don't use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries — 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. These entry points should be supplied through some other mechanism when this option is specified. 🫶

• -nolibc — don't use the C library or system libraries tightly coupled with it when linking. It 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, plus system libraries that normally go with it and become meaningless when absence of a C library is assumed — for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there really is no C library available.
• -nostdlib — don't use the standard system startup files or libraries when linking. No startup files, and only the libraries you specify are passed to the linker. Options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

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++ — don't implicitly link with standard C++ libraries.
• -e entry
• --entry= entry — specifies 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 — bestie, this links you up with the POSIX threads library 💅. It's supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Heads up tho: on some targets this option also sets flags for the preprocessor, so you gotta use it consistently for both compilation and linking, no cap.
• -r — produces a relocatable object as output. Also known as partial linking ✨.
• -rdynamic — passes 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 the used ones 👑. You need this for some uses of dlopen or to allow obtaining backtraces from within a program.
• -s — removes 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 — produces 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 in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc 🫶.

Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do 💖.

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.

• -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 ✨. 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 — binds references to global symbols when building a shared object. Warns about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option 👀.
• -T script — uses 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 — passes 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 — passes 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 — pretends 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.

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🎀 Options for Directory Search 🎀

These options specify directories to search for header files, for libraries and for parts of the compiler 🦋:

• -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.

• okay so lowkey the messiest part of -I- — it also blocks the directory of the current file from being the first search directory for #include "file". And bestie… there is no way to override this effect of -I-. none. zero. it's giving final boss. periodt. 💅
• -iprefix prefix — sets prefix as the prefix for the -iwithprefix options that come after it. if that prefix is a whole directory, pls include the trailing / or it'll be weird. 🎀
• -iwithprefix dir / -iwithprefixbefore dir — append dir to whatever prefix you set earlier with -iprefix, and drop the resulting directory onto the include search path. The difference: -iwithprefixbefore slots it in the same place -I would, and -iwithprefix slots it where -idirafter would. two vibes, one prefix, iconic. ✨
• -isysroot dir — basically --sysroot but only for header files (…except on Darwin targets, where it does headers and libraries, main character behaviour tbh). Go peek at the --sysroot option for the deets. 🦋
• -imultilib dir — use dir as a subdirectory of the directory that holds the target-specific C++ headers. yes it's niche, yes it's important. 💜
• -nostdinc — do NOT search the standard system directories for header files. The only places searched are the directories you explicitly gave with -I, -iquote, -isystem, and/or -idirafter (plus the directory of the current file, if that's appropriate). you're on your own now bestie. 😭
• -nostdinc++ — skip the C++-specific standard directories for headers, but still search all the other standard directories. (This one gets used when you're building the C++ library itself.) 🧚
• -iplugindir= dir — sets the directory to search for plugins that get passed by -fplugin=name instead of -fplugin=path/name.so. and like… this is not for the user — it's only passed by the driver. hands off, it's a driver thing. 🙈
• -L dir — add directory dir to the list of directories searched for -l. 📁
• -B prefix — tells the compiler where to find the executables, libraries, include files, and data files of the compiler itself. it's the compiler's own lil treasure map 🗺️

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.

For each subprogram to be run, the compiler driver first tries the -B prefix, if there is one. If that name isn't found, or if -B wasn't given at all, the driver tries two standard prefixes: /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those lands on a file name that's actually found, then the unmodified program name gets searched for using the directories in your PATH environment variable. so like, plan A, plan B, plan C, and then PATH. ✨

The compiler checks whether the path from -B refers to a directory, and if necessary it adds a directory separator character at the end of the path. tidying up the lil path, as one does. 💅

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates those options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates those options into -isystem options for the preprocessor. In that case, the compiler appends include to the prefix. so sneaky, so efficient. 🎀

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. brutal but true. 😭

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. 🌸

And 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 gets replaced by [dir/]include. This is to help with boot-strapping the compiler. a kludge!! a real one!! documented and everything!! 👑

• -no-canonical-prefixes — do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. leave the path exactly as 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. same energy, new root. 🌈

If you use both this option and the -isysroot option, then --sysroot applies to libraries, but -isysroot applies to header files. split duties, no drama. 💅

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. so check your linker before you get your hopes up. 💀

• --no-sysroot-suffix — for some targets, a suffix gets added to the root directory specified with --sysroot, depending on the other options used, so 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. no suffix, no problem. ✨

---

✨ Options for Code Generation Conventions ✨

these machine-independent options control the interface conventions used in code generation. 🎀

Most of them have both a positive and a negative form — 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. it's giving choose-your-own-adventure but for flags. 💜

• -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 the 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 the 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 legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, and 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, and 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. last one standing wins, fr fr. 🔥
• -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. wrapping around, cute and chaotic. 🌀
• -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. pointers get the same wrap treatment. 🎀
• -fstrict-overflow — this option implies -fno-wrapv -fno-wrapv-pointer, and when negated implies -fwrapv -fwrapv-pointer. package deal, no substitutions. ✨
• -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 don't 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're compiling older C++ programs that don't use exception handling. bigger data, same speed, choose wisely bestie. 🦋
• -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. so like… traps can throw, signal handlers can't. important distinction!! 💅
• -fdelete-dead-exceptions — okay so bestie, this one says: if some instructions might throw exceptions but they literally don't do anything else for the program's execution, we can just yeet them in optimization. Important caveat though: this does not affect calls to functions, except the ones rocking the pure or const attributes. It's on by default for the Ada and C++ compilers, since the language specs say that's allowed. And the optimization passes that actually remove dead exceptions get turned on independently at different optimization levels, so like, they're doing their own thing. 💅
• -funwind-tables — kinda like -fexceptions, except it only generates whatever static data is needed and does not touch the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling turns it on for you. So just... let them handle it, bestie. 🫶
• -fasynchronous-unwind-tables — generates an unwind table in DWARF format, if the target machine supports it. The table is exact at every single instruction boundary, which means it can be used for stack unwinding from asynchronous events — think debuggers or garbage collectors doing their little surprise appearances. ✨
• -fno-gnu-unique — okay so on systems with a 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 stay unique even when RTLD_LOCAL is in the picture. This is necessary to avoid problems where a library used by two different RTLD_LOCAL plugins depends on a definition in one of them, and therefore disagrees with the other one about the binding of the symbol. 😭 But — and here's the plot twist — this causes dlclose to be ignored for affected DSOs. So if your program relies on reinitialization of a DSO via dlclose and dlopen, you can use -fno-gnu-unique. There's your escape hatch. 🚪
• -fpcc-struct-return — returns short struct and union values in memory like the longer ones, instead of in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers — particularly the Portable C Compiler (pcc).

The precise convention for returning structures in memory depends on the target configuration macros.

Short structures and unions 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. This is more efficient for small structures 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 is 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. Slay. 👑

⚠️ 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 needs for the declared range of possible values. Specifically, the enum type is equivalent to the smallest integer type that has enough room. Tiny enum, tiny footprint. 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.

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. 💀

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. 🏛️

• -fno-ident — ignore the #ident directive. That's it. That's the whole vibe. 🙈
• -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. Leave it alone bestie, it's not for you. 🎀
• -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). iykyk ✨

-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. Don't get attached, bestie. 💅

• -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. This switch is 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 (the dynamic loader is not part of GCC; it is 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.)

Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent.

When this flag is set, the macros __pic__ and __PIC__ are defined to 1. ✨

• -fPIC — if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC.

Position-independent code requires special support, and therefore works only on certain machines.

And when this flag is set, the macros __pic__ and __PIC__ are defined to 2. 💫

---

• -fpie, -fPIE — okay so these are the cousins of -fpic and -fPIC, except the position-independent code they generate can only be linked into executables. No shared-lib slay for these ones, bestie. Usually you reach for them when you're compiling code that's gonna get linked using the -pie GCC option. Both of them define the macros __pie__ and __PIE__ — the macros hold 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, the callee address gets loaded at call sites from the GOT and then we branch to it. It's giving efficiency: PLT stubs get eliminated and GOT loads get exposed to optimizations. On architectures like 32-bit x86, where PLT stubs expect the GOT pointer in a specific register, this hands the compiler way more register allocation freedom. ⚠️ Lazy binding requires use of the PLT, so 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 that would be more efficient than other code generation strategies. This one's your girl when you're working alongside -fpic or -fPIC to build code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT at all, and then this option isn't needed. 🫶
• -fno-bit-tests — do not use bit tests for switch statements, even where that would be more efficient than other code generation strategies. Periodt.
• -ffixed-reg — treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer, or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice.
• -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. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

A different sort of disaster results from the use of this flag for a register in which function values may be returned. 😭 This flag does not have a negative form, because it specifies a three-way choice.

• -fpack-struct[=n] — without a value specified, pack all structure members together without holes. When a value is specified (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. 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. 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 generate 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. 🌸

ngl bestie this is the part where GCC lowkey starts talking to itself — the compiler-nerd dev options. buckle up 🎀

Ok so quick heads-up: -fvisibility does affect C++ vague linkage entities. which means — for example — an exception class that's getting thrown between DSOs has to be explicitly marked with default visibility so the type_info nodes get unified between the DSOs. no unified type_info, no vibe. 💅

There's a whole overview of these techniques + their benefits + how to use them at <https://gcc.gnu.org/wiki/Visibility>. go read it, it's giving academic but in a cute way.

---

✨ -fstrict-volatile-bitfields ✨

Use this one if accesses to volatile bit-fields (or other structure fields, though ngl the compiler usually honors those types anyway) should be a single access of the width of the field's type, aligned to a natural alignment if that's possible.

Example time, bestie: targets with memory-mapped peripheral registers might literally 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, like, maybe a more efficient 32-bit access. 📏

If this option is disabled, the compiler just uses the most efficient instruction. in the previous example that might be a 32-bit load instruction — even though that touches bytes that don't contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. messy but fast, yk? 💀

In some cases, like when the packed attribute is applied to a structure field, it might not be possible to access the field with a single read or write that's correctly aligned for the target machine. when that happens, GCC falls back to generating multiple accesses rather than code that'll fault or truncate the result at run time. 🫶

Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. so it's recommended to define all bits of the field's type as bit-field members. periodt.

The default value of this option is determined by the application binary interface for the target processor. (iykyk — it's an ABI thing.)

---

✨ -fsync-libcalls ✨

This option controls whether any out-of-line instance of the __sync family of functions may be used to implement the C++11 __atomic family of functions. 🦋

The default value is enabled, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. slay, libatomic.

---

💖 GCC Developer Options 💖

This section describes command-line options that are primarily of interest to GCC developers — the compiler girlies who write the compiler, not just use it. This includes options to support compiler testing and investigation of compiler bugs and compile-time performance problems.

Specifically it covers:

• options that produce debug dumps at various points in the compilation
• options that print statistics like memory use and execution time
• options that print info about GCC's configuration, like where it searches for libraries

Real talk: you should rarely need any of these for ordinary compilation and linking tasks. these are the backstage-pass options. 🎟️

---

dump files, explained (the =filename thing) 🧚

Many developer options that make GCC dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. 📤

If =filename is omitted, a default dump file name gets constructed by concatenating:

1. the base dump file name,
2. a pass number,
3. a phase letter, and
4. a pass name.

The base dump file name is the name of the output file produced by the compiler if it was explicitly specified and not an executable; otherwise it's the source file name.

The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they're executed earlier. messy, I know. 🙈

The phase letter is one of:

• i — inter-procedural analysis
• l — language-specific
• r — RTL
• t — tree

The files are created in the directory of the output file. cute and tidy. 🎀

---

✨ -fcallgraph-info ✨

✨ -fcallgraph-info=MARKERS ✨

Makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. 📊

It can be decorated with additional, per-node and/or per-edge information if you additionally specify a list of comma-separated markers. When the su marker is specified, the callgraph is decorated with stack usage information — it's equivalent to -fstack-usage. When the da marker is specified, the callgraph is decorated with information about dynamically allocated objects. 🌸

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. ngl that's a lil chaotic but that's the deal.

---

✨ -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. heads up.

Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings, bestie:

• -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-sibling — Dump after sibling call optimizations.
• -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.
• -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-unshare — Dump after all rtl has been unshared.
• -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. 💀 literally just vibes and nothing else.
• -da, -fdump-rtl-all — Produce all the dumps listed above. the whole menu, chef's kiss. 👑

---

ok that's a whole lotta dumps and we're still going — more coming, don't go anywhere bestie 🫶

• -dA — slaps annotations on the assembler output with all the miscellaneous debugging tea ☕✨
• -dD — dumps every macro definition at the end of preprocessing, on top of the normal output. like, everything, bestie 💅
• -dH — produces a core dump whenever an error occurs. yes fr, on purpose 🙈
• -dp — annotates the assembler output with a comment saying which pattern and alternative got used, plus prints the length and cost of every instruction 🔍
• -dP — dumps the RTL in the assembler output as a comment before each instruction. also flips on -dp annotation while it's at it ✨
• -dx — just generates RTL for a function instead of actually compiling it. usually chills with -fdump-rtl-expand 🫶

---

• -fdump-debug — dumps the debugging info that got generated during the debug generation phase 🐛
• -fdump-earlydebug — dumps the debugging info that got generated during the early debug generation phase 🌅
• -fdump-noaddr — when it's doing debugging dumps, suppresses address output. this makes it way more feasible to run diff on debugging dumps for compiler invocations that use different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. iconic for comparing stuff 👑
• -freport-bug — collects and dumps debug information into a temporary file if an internal compiler error (ICE) happens. so if it crashes, you've got receipts 💀
• -fdump-unnumbered — when doing debugging dumps, suppresses instruction numbers and address output. makes running diff on debugging dumps for compiler invocations with different options way more feasible — especially with and without -g 🌈
• -fdump-unnumbered-links — when doing debugging dumps (peep the -d option up above), suppresses instruction numbers for the links to the previous and next instructions in a sequence 🔗
• -fdump-ipa-switch
• -fdump-ipa-switch-options — controls the dumping at various stages of inter-procedural analysis language tree to a file 📂 the file name gets 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 dumps you can get are:
  • all — enables all inter-procedural analysis dumps. all of them. the whole buffet 🍓
  • cgraph — dumps info about call-graph optimization, unused function removal, and inlining decisions.
  • inline — dump after function inlining.
  • strubm — dump after selecting "strub" modes, and recording the selections as function attributes.
  • strub — dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking.

on top of that, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, and they default to -optimized ✨

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. snitching on both sides fr 👀

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 info about the creation of call graph node clones and the removals of call graph nodes during inter-procedural optimizations and transformations 🧬 its main intended use is so that tools that create live-patches can figure out the set of functions that need to be live-patched to completely replace a particular function (see -flive-patching). the file name gets 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.

• -fdump-lang — dumps language-specific information 🌍 the file name is made by appending .lang to the source file name.
• -fdump-lang-all
• -fdump-lang-switch
• -fdump-lang-switch-options
• -fdump-lang-switch-options=filename — controls the dumping of language-specific information. the options and filename portions behave exactly as described in the -fdump-tree option. the switch values it accepts are:
  • all — enable all language-specific dumps.
  • 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.
  • raw — dump the raw internal tree data. this option is applicable to C++ only.
• -fdump-passes — prints the list of optimization passes that are turned on and off by the current command-line options, right there on stderr 👀
• -fdump-statistics-option — enables and controls dumping of pass statistics into a separate file 📊 the file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit, while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled.
• -fdump-tree-all
• -fdump-tree-switch
• -fdump-tree-switch-options
• -fdump-tree-switch-options=filename — controls the dumping at various stages of processing the intermediate language tree to a file 🌳 if the -options form is used, options is a list of - separated options which control the details of the dump. not all options are applicable to all dumps — the ones that aren't meaningful just get ignored, no hard feelings 💅 the following options are available:
  • address — print the address of each node. usually this is not meaningful since it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment.
  • asmname — if "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". its primary use is ease of use working backward from mangled names in the assembly file.
  • slim — when dumping front-end intermediate representations, inhibits dumping of members of a scope or body of a function merely because that scope has been reached. only 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.
  • 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 includes 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 📈
  • 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 — turn on all the options, except raw, slim, verbose and lineno. not those four, bestie, they're on their own vibe 💅
• optall — turn on all the optimization options, i.e. optimized, missed, and note. so like, the whole optimization glow-up ✨

So to figure out what tree dumps are even available, or to hunt down the dump for a pass you're obsessed with, just follow these steps, okay? 👑

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. so tree-vrp1 and tree-vrp2 are literally the same pass running at different times, the number is how you tell them apart 🫶
2. To enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below. 💖
3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter t for tree passes (and the letter r for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another. so yeah, 038 today might be something else tomorrow, do not hardcode that number, i am begging 😭

---

• -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. hyphen-separated, bestie, not comma, not underscore 💅

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. last one wins, classic 🎀

The following options control which kinds of messages should be emitted:

• optimized — print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized. 🦋
• missed — print information about missed optimizations. Individual passes control which information to include in the output. the "aw girl you almost had it" report 😭
• note — print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
• all — print detailed optimization information. This includes optimized, missed, and note. the 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. GCC devs only, no cap 🙈

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 upgrade package, includes everything 👑

If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".

If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. one filename only, the rest get ghosted 💀

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. so if you compile a bunch of TUs, the file gets clobbered each time, pls use stderr if you want them all together 🫶

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. order? irrelevant, she doesn't care 💅

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities. only the first one survives, the second one is literally not invited 😭

---

• -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. EXPERIMENTAL. the format can change on you, do not build your whole toolchain on this without knowing that 💀

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, the whole family tree of inlining, iconic
• 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. nesting, like a little matryoshka of compiler tea 🎀

---

• -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 0 < 1 < 2 < 3 < 5+, each level adds more, and yeah the manpage literally says "it also output", we keep the typos around here 🫶

---

• -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.

DEBUGGING ONLY, bestie!! if you're just a normal compiler user, use the regular options, these are for the GCC devs poking at the internals 🔧

• -fdisable-ipa-pass — disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fdisable-rtl-pass, -fdisable-rtl-pass=range-list — disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes. so 5:9 means 5 through 9, both ends included, and if you just want one function you can write the single number, no colon needed ✨
• -fdisable-tree-pass, -fdisable-tree-pass=range-list — disable tree pass pass. See -fdisable-rtl for the description of option arguments.
• -fenable-ipa-pass — enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
• -fenable-rtl-pass, -fenable-rtl-pass=range-list — enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
• -fenable-tree-pass, -fenable-tree-pass=range-list — enable tree pass pass. See -fdisable-rtl for the description of option arguments.

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 the compiler's internal consistency checking. What it defaults to depends on how your compiler was built (its configuration), okay? And -fchecking=2 goes even further and turns on extra internal consistency checking that might actually affect code generation. 👀
• -frandom-seed=string — this hands GCC a seed to use instead of random numbers when it's making certain symbol names that have to be different in every single compiled file. It also drops unique stamps into coverage data files and the object files that produce them. Bestie, the reason you'd even want this: use -frandom-seed to get reproducibly identical object files. Slay. ✨

The string can be either a number (decimal, octal or hex) or an arbitrary string — and in that case it gets converted to a number by computing CRC32. Wild, I know.

Lowkey important: the string should be different for every file you compile. No reusing, no sharing. 💅

• -save-temps — stores the usual "temporary" intermediate files permanently; they get named as auxiliary output files, as specified/described under -dumpbase and -dumpdir.

When you use it together with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that has the same extension as an intermediate file. Wanna get that corresponding intermediate file anyway? Rename the source file before using -save-temps. Easy. 🎀

• -save-temps=cwd — equivalent to -save-temps -dumpdir ./.
• -save-temps=obj — equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file you named after the -o option, including any directory separators. And if you didn't use -o at all? Then -save-temps=obj just behaves like -save-temps=cwd. ✨
• -time[=file] — reports the CPU time taken by each subprocess in the compilation sequence. For C source files, that's the compiler proper and assembler (plus the linker, if linking is happening).

Without naming an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time" — time spent executing the program itself. The second number is "system time" — time spent executing operating system routines on behalf of the program. Both numbers are in seconds, no cap. ⏱️

With an output file named, the output is appended to that file and looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

So now the "user time" and "system time" get moved before the program name, and the options passed to the program are shown too — so later you can tell what file was being compiled, and with which options. Iconic for debugging. 👑

• -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the name of the dump file is determined by appending .gkd to the dump base name, see -dumpbase.
• -fcompare-debug[=opts] — if no error happens during compilation, it 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. Detective energy fr fr. 🕵️

If the equal sign is omitted, the default -gtoggle is 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 it's used for opts — otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation — preventing even GCC_COMPARE_DEBUG from taking effect.

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). Want just a warning instead of a hard reject? Setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do it. 💅

• -fcompare-debug-second — this option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and it omits other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first. 🎀

When this option is passed to the compiler driver, it causes the first compilation to be skipped — which makes it useful for little other than debugging the compiler proper. Not me saying that, the manual says it. 🙈

• -gtoggle — turns off generation of debug info if leaving out this option generates it, or turns it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times you give it. It's mainly intended to be used with -fcompare-debug.
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. Cute symmetry, honestly. ✨
• -Q — makes the compiler print out each function name as it's 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 SARIF output. Heads up though: the precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. 🫶

• -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. And if the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10. Math-y but make it fashion. 💅
• -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 those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time.

Like "TFLAGS", this allows the target libraries to be built for portable baseline environments, while the compiler defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use to build their own programs if the defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. 💅

Unlike "TFLAGS", the use of specs enables different flags to be selected for different multilibs. The way to accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.

This option is discarded by the driver once it's done processing driver self spec.

It'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 consistency of the (estimated) profile and 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. auxname is generated from the name of the output file, if 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 that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes. 🦋

The qualifier "dynamic" means the function is out here doing the most with the stack — like, besides the static allocation we already talked about, it also makes stack adjustments inside the body of the function, for example to push/pop arguments around function calls. 💅 If the qualifier "bounded" is also present, then the amount of those adjustments is bounded at compile time, and the second field is an upper bound of the total amount of stack used by the function. If "bounded" is not there, then the amount of those adjustments is not bounded at compile time, and the second field only represents the bounded part. (so like, a partial answer, not the whole vibe check.)

---

• -fstats — Emit statistics about front-end processing at the end of the compilation. ✨ This option is supported only by the C++ front end, and ngl 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 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 second, third, fourth, tenth and eleventh invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations. 🧮
• -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. 🛑 With this option, GCC does not compile or link anything; it just prints the file name. Periodt.
• -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 -, without spaces between multiple switches. This is supposed to ease shell processing. 🐚
• -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, the trailing slash is not optional, bestie.)

• -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 allow you to 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.

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 SysV-like data model where int, long int and pointers are 32 bits, and lp64 for SysV-like data model where int is 32 bits, but long int and pointers are 64 bits. 💾

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. 🚨 (do not mix, bestie, it's giving chaos.)

• -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. ✨
• -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. 💀 (pick one, you cannot have both.)
• -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. ⚠️

• -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. 🩹
• -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. 🔗
• -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. 🎯
• -mlow-precision-sqrt, -mno-low-precision-sqrt — Enable or disable the 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 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 — turn the division approximation on or off. ⚠️ heads up bestie: this literally does nothing unless you also pass -ffast-math or -funsafe-math-optimizations. And when you do turn it on, it trades away precision — division results get crunched down to about 16 bits for single precision and 32 bits for double precision. It's giving "fast but blurry" 💅
• -mtrack-speculation, -mno-track-speculation — turn generation of extra code on/off that tracks speculative execution through conditional branches. That tracking state then gets used by the compiler when it expands calls to __builtin_speculation_safe_copy, so it can spit out a more efficient code sequence. 🕵️✨
• -moutline-atomics, -mno-outline-atomics — turn calls to out-of-line helpers on/off, the ones that implement atomic operations. These helpers figure out at runtime whether the LSE instructions from ARMv8.1-A are available; if they're not, they fall back to the load/store-exclusive instructions that live in the base ARMv8.0 ISA. 🔄

This option only even applies when you're compiling for the base ARMv8.0 instruction set. If you're on a later revision — say -march=armv8.1-a or -march=armv8-a+lse — the ARMv8.1-Atomics instructions get used directly. Same story with -mcpu= when the cpu you picked supports the lse feature. And fun fact: this option is on by default. 👑

• -march=name — name the target architecture, and optionally tack on one or more feature modifiers. The shape is -march=arch{+[no]feature}*.

Here's the rundown of what arch values you're allowed to use and which features they turn on by default (no cap, this is the whole list 🫶):

• 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
• armv8.4-a — Architecture: Armv8.4-A — Includes by default: armv8.3-a, +flagm, +fp16fml, +dotprod
• armv8.5-a — Architecture: Armv8.5-A — Includes by default: armv8.4-a, +sb, +ssbs, +predres
• 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, +ls64
• 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, +ls64
• 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
• armv8-r — Architecture: Armv8-R — Includes by default: armv8-r

The value native is available on native AArch64 GNU/Linux and makes the compiler just grab whatever architecture the host system has. If the compiler can't recognize the host's architecture, this option does absolutely nothing. 🙈

As for what feature values are allowed — those are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. And if you specify conflicting feature modifiers? The right-most feature wins. Periodt.

GCC uses name to decide what kind of instructions it's allowed to emit when it generates assembly code. And if you specify -march without also giving -mtune or -mcpu, the code gets tuned to perform well across a whole range of target processors that implement that target architecture. 🌈

• -mtune=name — name the target processor GCC should tune the performance of the code for. The permissible values are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-n3, neoverse-v1, neoverse-v2, neoverse-v3, neoverse-v3ae, grace, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, hip12, 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-x1, cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-x925, cortex-a510, cortex-a520, cortex-a710, cortex-a715, cortex-a720, cortex-a725, ampere1, ampere1a, ampere1b, cobalt-100 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 mean GCC should tune for a big.LITTLE system. 🏙️🌆

The value neoverse-512tvb means 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 — and I'm quoting the vibe here — 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 one specific core like Neoverse V1, but more specific than the default tuning described below. 🎯

Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. If the compiler can't recognize the host system's processor, this option does nothing.

And where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

⚠️ One thing: this option cannot be suffixed by feature modifiers. Not even one. Sorry bestie. 💅

• -mcpu=name — name the target processor, optionally suffixed with one or more feature modifiers. The shape is -mcpu=cpu{+[no]feature}*, and the permissible values for cpu are the exact same ones available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Conflicting feature modifiers? Right-most one wins, every time. ✨

GCC uses name to figure out what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor to tune for performance (as if by -mtune). And if you use this option alongside -march or -mtune, those options take precedence over the appropriate part of this one. Teamwork, but with a hierarchy. 👑

-mcpu=neoverse-512tvb is special — it doesn't refer to one specific core, it refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless it's overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. And unless it's overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb. 🦋

• -moverride=string — override tuning decisions that the back-end made in response to a -mtune= switch. 🚨 Real talk: the syntax, semantics, and accepted values for string here are not guaranteed to be consistent across releases.

This option is only intended to be useful when you're developing GCC. It is not for you, casual user. It is for the compiler devs. 🙈

• -mverbose-cost-dump — enable verbose cost model dumping in the debug dump files. This option exists for debugging the compiler. 🔍
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — turn PC-relative literal loads on/off. With this on, literal pools get accessed using a single instruction and are emitted after each function. That limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny. 📏
• -msign-return-address=scope — pick the function scope that return address signing applies to. The permissible values are none, which disables return address signing entirely, non-leaf, which enables pointer signing for functions that are not leaf functions, and all, which enables pointer signing for all functions. Default value is none. ⚠️ Heads up: this option has been deprecated by -mbranch-protection.
• -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti — pick which 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 extends the signing to include leaf functions too. The optional argument b-key signs the functions with the B-key instead of the A-key.
  • bti turns on the branch target identification mechanism.
• -mharden-sls=opts — enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of these options:

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Plus, -mharden-sls=all enables all SLS hardening, while -mharden-sls=none disables all SLS hardening. 🔥

• -mearly-ra=scope — decide 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, like SME2's strided LD1 and ST1. 🧚

The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass.

-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise. ✨

• -mearly-ldp-fusion — enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by default at -O and above. 👑
• -mlate-ldp-fusion — enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by default at -O and above. 💅
• -msve-vector-bits=bits — specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled. 🌸

GCC's got two whole flavors of SVE code generation, bestie: vector-length agnostic output that works with literally any size of vector register, and vector-length specific output that lets GCC make assumptions about the vector length when that's actually useful for optimization reasons. ✨

So the possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. All other values generate 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. Like, fr fr, don't build your whole life on 256 okay? 😭

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. 💅

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🌈 -march and -mcpu Feature Modifiers 🌈

Feature modifiers used with -march and -mcpu can be any of the following, and their inverses nofeature (so like, crc has nocrc, you get the vibe 😌):

• 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.
• 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. This option is enabled by default for -march=armv8.7-a.
• 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.
• pauth — Enable the Pointer Authentication Extension.
• cssc — Enable the Common Short Sequence Compression instructions.
• sme — Enable the Scalable Matrix Extension.
• sme-i16i64 — Enable the FEAT_SME_I16I64 extension to SME.
• sme-f64f64 — Enable the FEAT_SME_F64F64 extension to SME.

+@item sme2 Enable the Scalable Matrix Extension 2.  This also enables SME instructions.

• 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.
• rcpc3 — Enable the RCpc3 (Release Consistency) extension.

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. 🦋 It's giving chain reaction, iykyk.

---

🧚 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 — Preferentially allocate registers that allow short instruction generation. This can result in increased instruction count, so this may either reduce or increase overall code size. 🙈
• *-mbranch-cost=num* — Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
• -mcmove — Enable the generation of conditional moves.
• *-mnops=num* — Emit num NOPs before every other generated instruction.
• -mno-soft-cmpsf — For single-precision floating-point comparisons, emit an fsub instruction and test the flags. This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons. 😭

So like, speed vs. correctness, pick your fighter.

• -mstack-offset=num — Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range sp+0...sp+7 can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Note also that this option *changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num* option. 💅
• -mno-round-nearest — Make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
• -mlong-calls — If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
• -mshort-calls — If not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl instructions, so use these instructions for direct calls. The default is -mlong-calls.
• -msmall16 — Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect.
• *-mfp-mode=mode* — Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. 🌸

mode can be set to one the following values:

• caller — honestly the chillest mode 🫶 Any mode at function entry is valid, and it gets retained or restored when the function returns, and also when it calls other functions. It's giving "universal adapter" energy: perfect for compiling libraries or other compilation units you might wanna drop into different programs with different prevailing FPU modes. And the convenience of being able to use a single object file outweighs the size and speed overhead of any extra mode switching that might be needed, compared with what you'd need with a more specific choice of prevailing FPU mode.
• truncate — this is the mode used for floating-point calculations with truncating rounding mode, i.e. round towards zero. That includes conversion from floating point to integer. no cap 💅
• 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 — change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as 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. 👀 Heads up though: 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 — split 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 said slay, reality said no.
• -m1reg-reg — specify 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 that no register is used for this purpose. The default is -m1reg-none.

✨ AMD GCN Options ✨

These options are defined specifically for the AMD GCN port. 🌈

• -march=gpu, -mtune=gpu — set architecture type or tuning for gpu. Supported values for gpu are:
  • fiji — compile for GCN3 Fiji devices (gfx803). Support deprecated; availablility depends on how GCC has been configured, see --with-arch and --with-multilib-list. 🙈
  • gfx900 — compile for GCN5 Vega 10 devices (gfx900).
  • gfx906 — compile for GCN5 Vega 20 devices (gfx906).
  • gfx908 — compile for CDNA1 Instinct MI100 series devices (gfx908).
  • gfx90a — compile for CDNA2 Instinct MI200 series devices (gfx90a).
  • gfx90c — compile for GCN5 Vega 7 devices (gfx90c).
  • gfx1030 — compile for RDNA2 gfx1030 devices (GFX10 series).
  • gfx1036 — compile for RDNA2 gfx1036 devices (GFX10 series).
  • gfx1100 — compile for RDNA3 gfx1100 devices (GFX11 series).
  • gfx1103 — compile for RDNA3 gfx1103 devices (GFX11 series).
• -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 — specify how many bytes of stack space will be requested for each GPU thread (wave-front). ⚠️ Beware that there may be many threads and limited memory available. The size of the stack allocation may also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise.
• -mxnack=on, -mxnack=off, -mxnack=any — compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise.

✨ ARC Options ✨

The following options control the architecture variant for which code is being compiled: 🦋

• -mbarrel-shifter — generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture.
• -mcpu=cpu — set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:
  • arc600 — compile for ARC600. Aliases: -mA6, -mARC600.
  • arc601 — compile for ARC601. Alias: -mARC601.
  • arc700 — compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
  • arcem — compile for ARC EM.
  • archs — compile for ARC HS.
  • em — compile for ARC EM CPU with no hardware extensions.
  • em4 — compile for ARC EM4 CPU.
  • em4_dmips — compile for ARC EM4 DMIPS CPU.
  • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — compile for ARC HS34 CPU.
  • hs38 — compile for ARC HS38 CPU.
  • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on.
  • hs4x — compile for ARC HS4x CPU.
  • hs4xd — compile for ARC HS4xD CPU.
  • hs4x_rel31 — compile for ARC HS4x CPU release 3.10a.
  • arc600_norm — compile for ARC 600 CPU with "norm" instructions enabled.
  • arc600_mul32x16 — compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc600_mul64 — compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
  • arc601_norm — compile for ARC 601 CPU with "norm" instructions enabled.
  • arc601_mul32x16 — compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc601_mul64 — compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
  • nps400 — compile for ARC 700 on NPS400 chip.
  • em_mini — compile for ARC EM minimalist configuration featuring reduced register set.
• -mdpfp, -mdpfp-compact — generate double-precision FPX instructions, tuned for the compact implementation.
• -mdpfp-fast — generate double-precision FPX instructions, tuned for the fast implementation.
• -mno-dpfp-lrsr — disable "lr" and "sr" instructions from using FPX extension aux registers.
• -mea — generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. Only valid for -mcpu=ARC700.
• -mno-mpy — do not generate "mpy"-family instructions for ARC700. 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 — this option is ignored; it is provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can be overridden by FPX options; -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
• -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 option is on by default for ARC HS.
• -mll64 — enable double load/store operations for ARC HS cores.
• -mtp-regno=regno — specify 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 numeric value for multo. wlh1 is the default value. The recognized values are:
  • 0, none — no multiplier available.
  • 1, w — 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw" and "mpyuw".
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 4, wlh3 — two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 5, wlh4 — one 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".

6. wlh5 — one 32x4 multiplier, blocking, sequential. It also unlocks these instructions: 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. ✨

Quick heads up bestie: this option is only available for ARCv2 cores. 🫶

• -mfpu=fpu — turns on support for specific floating-point hardware extensions for ARCv2 cores. Here's everything fpu can be:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. Single-precision comes along for the ride too. Not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. Single-precision floating-point is also enabled. This one is only available for ARC EM.
  • fpuda_div — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. 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. 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 option includes option fpus_div. 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. This option includes option fpus_fma. Not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions.
  • 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 — names the general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is written 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 — sets how many registers get replicated into the second register bank on entry to a fast interrupt. Fast interrupts = interrupts with the highest priority level P0. These interrupts save only the PC and STATUS32 registers, so no memory transactions happen during the interrupt entry and exit sequences. Use this option when you're using fast interrupts in an ARC V2 family processor. Permitted values are 4, 8, 16, and 32.
• -mlpc-width=width — sets 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 does not attempt 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 specify, the compiler and run-time library might keep using the loop mechanism for various needs. This option defines 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 — enables use of "bi" or "bih" instructions to implement jump tables.

---

📦 The following options are passed through to the assembler, and 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 — passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol "__Xxy".

---

✍️ The following options control how the assembly code is annotated

• -misize — annotate assembler instructions with estimated addresses.
• -mannotate-align — does literally nothing. Preserved for backward compatibility. 🙈

---

🔗 The following options are passed through to the linker

• -marclinux — passed through to the linker, to specify use of the "arclinux" emulation. This option is enabled 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. This option is enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling is requested.

---

🧠 The following options control the semantics of generated code

• -mlong-calls — generate calls as register indirect calls, which gives you access to the full 32-bit address range.
• -mmedium-calls — don't use less than a 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, so you get the 25-bit range instead of the 21-bit range you'd have with conditional branch-and-link. This is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations.
• -mno-sdata — don't generate sdata references. This is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets.
• -mvolatile-cache — use ordinarily cached memory accesses for volatile references. This is the default.
• -mno-volatile-cache — enable cache bypass for volatile references.

---

🎛️ The following options fine tune code generation

• -malign-call — does nothing. Preserved for backward compatibility.
• -mauto-modify-reg — enable the use of pre/post modify with register displacement.
• -mbbit-peephole — does nothing. Preserved for backward compatibility.
• -mno-brcc — disables a target-specific pass in arc_reorg to generate compare-and-branch ("brcc") instructions. It has no effect on generation of these instructions driven by the combiner pass.
• -mcase-vector-pcrel — use PC-relative switch case tables to enable case table shortening. This is the default for -Os.
• -mcompact-casesi — enable compact "casesi" pattern. This is the default for -Os, and only available for ARCv1 cores. This option is deprecated.
• -mno-cond-exec — disable the ARCompact-specific pass that generates conditional execution instructions.

Okay so why does this pass even exist? Because of delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass for generating conditional execution is often lacking — so the ARC port has kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is why there's an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead.

• -mearly-cbranchsi — enable pre-reload use of the "cbranchsi" pattern.
• -mexpand-adddi — expand "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. This option is deprecated.
• -mindexed-loads — enable the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case. 😭
• -mlra — enable Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e. -mno-lra).
• -mlra-priority-none — don't indicate any priority for target registers.
• -mlra-priority-compact — tells the compiler the target registers r0..r3 / r12..r15 are high priority 💅
• -mlra-priority-noncompact — opposite vibe, bestie: it lowers the priority of those same target registers r0..r3 / r12..r15 🫶
• -mmillicode — okay so when you're optimizing for size (that's -Os), prologues and epilogues that gotta save or restore a whole pile of registers often get shortened by calling a special function in libgcc. That's called a millicode call ✨ but these calls can cause performance issues and/or linking issues if you're linking in a nonstandard way, so this option exists to turn millicode call generation on or off 🔥
• -mcode-density-frame — lets the compiler emit enter and leave instructions. Heads up tho: these instructions are only valid for CPUs with the code-density feature ⚠️
• -mmixed-code — literally does nothing 🙈 kept around purely for backward compatibility.
• -mq-class — this one is deprecated, no cap. Enables q instruction alternatives. It's the default for -Os.
• -mRcq — does nothing 💀 preserved for backward compatibility.
• -mRcw — also does nothing 💀 preserved for backward compatibility.
• -msize-level=level — fine-tune size optimization re: instruction lengths and alignment 🎀 the recognized values for level are:
  • 0 — no size optimization. This level is deprecated and treated like 1.
  • 1 — short instructions are used opportunistically.
  • 2 — in addition, alignment of loops and of code after barriers are dropped.
  • 3 — in addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, when this isn't 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 following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release 🚨:

• -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 💖

• -mabi=name — generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt ✨
• -mapcs-frame — generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even if it's not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer alongside this option means stack frames won't be generated for leaf functions. The default is -mno-apcs-frame. Heads up: this option is deprecated ⚠️
• -mapcs — this is a synonym for -mapcs-frame and is deprecated 🙈
• -mthumb-interwork — generate code that supports calling between the ARM and Thumb instruction sets 🦋 without this option, on pre-v5 architectures, the two instruction sets can't be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code gets generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless.
• -mno-sched-prolog — stops the reordering of instructions in the function prologue, and stops the merging of those instructions with the instructions in the function's body. Which means all functions start with a recognizable set of instructions (or actually one of a choice from a small set of different function prologues), and that info can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog 💅
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
  • Specifying soft makes GCC generate output containing library calls for floating-point operations.
  • softfp allows generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.
  • hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not link-compatible 🚨 you must compile your entire program with the same ABI, and link with a compatible set of libraries.

• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This prevents the compiler from using floating-point and Advanced SIMD registers, but it will not impose any restrictions on the assembler 🎀
• -mlittle-endian — generate code for a processor running in little-endian mode. This is the default for all standard configurations ✨
• -mbig-endian — generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.
• -mbe8, -mbe32 — when linking a big-endian image, select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default depends on the selected target architecture: for ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM ⚠️
• -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. This option can be used in conjunction 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, armv9-a, iwmmxt and iwmmxt2.

Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: armv4.

Many of the architectures support extensions ✨ these can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions it depends on. For example, the +crypto extension will always enable the +simd extension. The exception to the additive construction is for extensions prefixed with +no...: these disable the specified option and any other extensions that may depend on the presence of that extension.

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 💅

Most extension names are generically named, but have an effect that depends on the architecture to which they're applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a 🦋

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.

🦋 the fp/simd extension tea (cont.)

• +vfpv3 — the VFPv3 floating-point instructions, with 32 double-precision registers. bestie the registers really said double it up 💅
• +vfpv3-d16-fp16 — VFPv3 floating-point instructions, 16 double-precision registers, plus the half-precision floating-point conversion operations. combo meal ✨
• +vfpv3-fp16 — VFPv3 floating-point instructions, 32 double-precision registers, and the half-precision floating-point conversion operations.
• +vfpv4-d16 — VFPv4 floating-point instructions, with 16 double-precision registers.
• +vfpv4 — VFPv4 floating-point instructions, with 32 double-precision registers. upgraded era fr fr 🔥
• +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. iconic duo tbh 👑
• +nosimd — Disable the Advanced SIMD instructions (does not disable floating point). not me taking away the neon but leaving the fp like it's nothing 😭
• +nofp — Disable the floating-point and Advanced SIMD instructions. full shutdown energy 💀

---

armv7ve

the extended version of the ARMv7-A architecture with support for virtualization. it's giving main character architecture 🫶

• +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. so many aliases, so little time 🌸
• +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. that's a triple takeaway, ngl 😳
• +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

heads up bestie, these two share the exact same list of extensions 💖

• +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. brain instructions?? ok genius behavior 🧠✨

---

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 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.
• +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.

---

armv8.5-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 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 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 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.
• +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. ok so you can turn integer division OFF again, wild but slay 🙈

---

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 💀

Okay so -march=native makes the compiler auto-detect the architecture of the build computer. Ngl, at present 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. It's giving "tried my best, no notes" 🙈.

---

✨ -mtune=name ✨

This option says: hey, here's the name of the target ARM processor GCC should tune the performance of the code for. For some ARM implementations you genuinely get better performance by using this. Permissible names are:

arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110,
strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t,
arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s,
arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s,
arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53,
cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76,
cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710,
ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52,
cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4,
cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52, cortex-m55,
cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1,
marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2,
ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1

Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. Permissible names are:

cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55

-mtune=generic-arch says GCC should tune the performance for a blend of processors within architecture arch. The whole point is to generate code that runs well on the current most popular processors — balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go, fr fr 🦋.

-mtune permits the same extension options as -mcpu, but bestie, the extension options do not affect the tuning of the generated code. Noted 📝.

-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect 💀.

---

✨ -mcpu=name[+extension...] ✨

This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune). But the way that ✨precedence✨ works: where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. Got it? 🫶

Many of the supported CPUs implement optional architectural extensions. Where this is so, the architectural extensions are normally 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. For floating-point and 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. Iconic, honestly.

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 and cortex-m55 🎀
• +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 🙈.

---

✨ -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. She's doing the deciding for you, bestie 💅.

Okay so if your selected floating-point hardware includes the NEON extension (like -mfpu=neon), bestie, listen — floating-point ops are not generated by GCC's auto-vectorization pass unless you also throw in -funsafe-math-optimizations. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic (specifically, denormal values get treated as zero 🫠), so using NEON instructions may lead to a loss of precision. It's giving "fast but a lil imprecise", and that's the whole vibe.

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. So flexible 💅.

---

• -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 just… not defined. Iconic "if you don't set it, it doesn't exist" energy ✨.
• -mstructure-size-boundary=n — The sizes of all structures and unions get rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains — for the COFF targeted toolchain the default is 8. A value of 64 is only allowed if the underlying ABI supports it, no exceptions, that's the rule 🎀.

Specifying a larger number can produce faster, more efficient code, but it can also increase the size of your program. And here's the tea ☕: 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. So like, pick one and commit, bestie.

This option is deprecated. 🚫

• -mabort-on-noreturn — Generate a call to the function abort at the end of a noreturn function. It gets executed if the function tries to return. So it's like a little security guard at the end of the function going "excuse me?? you're not supposed to be here" 💀.
• -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: 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. Rules upon rules, it's giving bureaucracy but make it compiler 🌸.

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. Not me pointing out the fine print 🙈.

• -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 the runtime does the homework, not the compiler ✨.
• -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 the 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. Autopilot vibes 🦋.
• -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. Sneaky little chain reaction, lowkey iconic 💜.
• -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). Forensic detective work, honestly 👑

• -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. Per-function glow-ups, we love to see it 💅.

• -mflip-thumb — Switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of mixed Thumb/ARM code generation, and is not intended for ordinary use in compiling code. So like, it's a lab experiment, not a lifestyle 🌈.
• -mtpcs-frame — Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
• -mtpcs-leaf-frame — Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame. Yes bestie, that's what it says, I'm not gonna "fix" it for you, copy it exactly 💖.
• -mcallee-super-interworking — Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default. Sorry, rules are rules 🎀.
• -mcaller-super-interworking — Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default. Same disclaimer, different flag, iykyk.
• -mtp=name — Specify the access model for the thread local storage pointer. The model soft generates calls to __aeabi_read_tp. Other accepted models are tpidrurw, tpidruro and tpidrprw which fetch the thread pointer from the corresponding system register directly (supported from the arm6k architecture and later). These system registers are accessed through the CP15 co-processor interface and the argument cp15 is also accepted as a convenience alias of tpidruro. The argument auto uses the best available method for the selected processor. The default setting is auto. Let it cook ✨.
• -mtls-dialect=dialect — Specify the dialect to use for accessing thread local storage. Two dialects are supported --- gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. Upgrade tax, unfortunately 💀.
• -mword-relocations — Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. Pick a side, bestie, you can't have both 🫶.
• -mfix-cortex-m3-ldrd — Some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified. Protecting you from the chaos, no cap 🛡️.
• -mfix-cortex-a57-aes-1742098
• -mno-fix-cortex-a57-aes-1742098
• -mfix-cortex-a72-aes-1655431
• -mno-fix-cortex-a72-aes-1655431 — Enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. This option is enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified. Yes those numbers are erratum IDs and yes they go in the flag name, we don't make the rules 👑
• -munaligned-access
• -mno-unaligned-access — Enables (or disables) reading and writing of 16- and 32-bit values from addresses that are not 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 is not enabled then words in packed data structures are accessed a byte at a time. Slow and steady, but correct 🐢.

The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. Receipts in the object file, we love documentation 💖.

• -mneon-for-64bits — This option is deprecated and has no effect. It's giving ghost flag 👻, it's literally just vibes and nothing else.
• -mslow-flash-data — Assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default. It conflicts with -mword-relocations. Again: choose one, queen 🌸.
• -masm-syntax-unified — Assume inline assembler is using unified asm syntax. The default is currently off which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. Future you might have different defaults, so heads up 🦋.
• -mrestrict-it — Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode. Structured, tidy, disciplined — the main character of instruction blocks ✨.
• -mprint-tune-info — drops CPU tuning info into your assembler file as a comment ✨ This one is strictly for regression-testing the compiler itself, not for anyone actually out here compiling real code, okay? It's off by default 💅
• -mverbose-cost-dump — turns on verbose cost model dumping in the debug dump files 🫶 It exists purely so compiler devs can debug the compiler. That's it. That's the whole job. Not for you, bestie, unless you're literally fixing GCC 😭
• -mpure-code — says NO to constant data living in code sections 🚫 On top of that, when you're compiling for the ELF object format it gives every text section the ELF processor-specific section attribute SHF_ARM_PURECODE 👑 Heads up tho: this option is only available when generating non-pic code for M-profile targets. Periodt.
• -mcmse — generates secure code exactly as the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification" says 🌸 You can find that doc at <https://developer.arm.com/documentation/ecm0359818/latest/>. Go read it, it's giving very serious academic energy 📚
• -mfix-cmse-cve-2021-35465 — mitigates a potential security issue with the VLLDM instruction on some M-profile devices when you're using CMSE (CVE-2021-365465) 😳 This one is enabled by default whenever the option -mcpu= is used with cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85 or star-mc1. Wanna turn the mitigation off? Use -mno-fix-cmse-cve-2021-35465 🙈
• *-mstack-protector-guard=guard* — and — *-mstack-protector-guard-offset=offset* — generates stack protection code with the canary sitting at guard 🐤 The supported locations are global for a global canary, or tls for a canary you can reach via the TLS register. Lowkey important: the option -mstack-protector-guard-offset= is meant to be used with -fstack-protector-guard=tls and not for use in user-land code 🚫
• -mfdpic — and its opposite — -mno-fdpic — picks 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 you didn't hand it any of the PIC/PIE-related options. On other targets? It only enables the FDPIC-specific code generation features, and you gotta explicitly provide the PIC/PIE-related options yourself as needed, no cap 🤷‍♀️

Also note: static linking is not supported here, 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 honestly 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]] — turns on branch protection features (armv8.1-m.main only, bestie, don't get cute) 🛡️
  • 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 is to sign all functions that save the return address to memory.
  • leaf — when return address signing is enabled, also sign leaf functions even if they do not 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 will remain backwards-compatible with earlier versions of the architecture, but the additional security can be enabled at run time on processors that support the PACBTI extension ✨

Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti 👀

Any setting other than none is supported only on armv8-m.main or later. And the default? Generate code without branch protection or return address signing. Iconic, unbothered, moisturized 💅

---

🌈 AVR Options 🌈

These options are defined for AVR implementations, and they're honestly a whole vibe:

• *-mmcu=mcu* — specify the AVR instruction set architecture (ISA) or device type 💖 The default for this option is avr2.

Here's the thing tho, bestie — the following AVR devices and ISAs are supported. Note: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs (<https://gcc.gnu.org/wiki/avr-gcc#spec-files>) file specs-mcu 📦 Only the latter is provided by the compiler according to the supported *mcu*s below. The rest is supported by AVR-LibC (<https://www.nongnu.org/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</p> with up to 64 KiB of combined program memory and RAM, and where program memory is visible in the RAM address space. 💖 mcu = a whole lil army:

"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",
"avr16la14", "avr16la20", "avr16la28", "avr16la32", "avr32da28",
"avr32da28s", "avr32da32", "avr32da32s", "avr32da48",
"avr32da48s", "avr32db28", "avr32db32", "avr32db48",
"avr32dd14", "avr32dd20", "avr32dd28", "avr32dd32", "avr32du14",
"avr32du20", "avr32du28", "avr32du32", "avr32ea28", "avr32ea32",
"avr32ea48", "avr32eb14", "avr32eb20", "avr32eb28", "avr32eb32",
"avr32la14", "avr32la20", "avr32la28", "avr32la32", "avr32sd20",
"avr32sd28", "avr32sd32".

• "avrxmega4" — "XMEGA"</p> girlies with more than 64 KiB and up to 128 KiB of program memory. mcu = the roster:

"atxmega64a3", "atxmega64a3u",
"atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3",
"atxmega64d3", "atxmega64d4", "avr128da28", "avr128da28s",
"avr128da32", "avr128da32s", "avr128da48", "avr128da48s",
"avr128da64", "avr128da64s", "avr128db28", "avr128db32",
"avr128db48", "avr128db64".

• "avrxmega5" — "XMEGA"</p> devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu = just these two:

"atxmega64a1", "atxmega64a1u".

• "avrxmega6" — "XMEGA"</p> devices with more than 128 KiB of program memory. mcu = the big ones:

"atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3",
"atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3",
"atxmega256a3b", "atxmega256a3bu", "atxmega256a3u",
"atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".

• "avrxmega7" — "XMEGA"</p> devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = three icons:

"atxmega128a1", "atxmega128a1u",
"atxmega128a4u".

• "avrtiny" — "Reduced Tiny"</p> Tiny core devices, and the way that they only have 16 general purpose registers and 512 B up to 4 KiB of program memory. literally so smol 🥺 mcu = the tiny squad:

"attiny4", "attiny5", "attiny9", "attiny10", "attiny102",
"attiny104", "attiny20", "attiny40".

• "avr1" — this ISA is implemented by the minimal AVR core and it's supported for assembler only. mcu = the classics:

"attiny11", "attiny12", "attiny15",
"attiny28", "at90s1200".

---

• -mabsdata — bestie, assume that all data in static storage can be accessed by LDS / STS instructions. 💅 This option only does anything on reduced Tiny devices like ATtiny40. See also the "absdata" AVR Variable Attributes,variable attribute. 👑
• -maccumulate-args — accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call.

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack — like calls to printf-like functions. 🍓

• *-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. ✨
• -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.
• *-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. iykyk 💅
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the AVR Function Attributes,,"no_gccisr" function attribute. This feature is activated per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).
• -mint8 — assume int to be an 8-bit integer. This affects the sizes of all the types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes.

🚨 okay so heads up bestie: this option does not conform to the C standards. But it results in smaller code size, so. 🤷♀️

• -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-interrupts — generated code is not compatible with hardware interrupts. Code size is smaller.
• -mrelax — try to replace CALL resp. JMP instruction by the shorter RCALL resp. RJMP instruction if 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.

Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code. 🙈

Relaxing must be turned on if linker stubs are needed, see the section on "EIND" and linker stubs below. 👇

• -mrodata-in-ram
• -mno-rodata-in-ram — locate the .rodata sections for read-only data in RAM resp. in program memory. For most devices, there is no choice and this option acts rather like an assertion. 💀

Since v14 and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default, provided the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM.

• -mstrict-X — use address register X in a way proposed by the hardware. This means that X is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the X register may be used in the same way as Y or Z — which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as

        adiw r26, const   ; X += const
        ld   <Rn>, X        ; <Rn> = *X
        sbiw r26, const   ; X -= const

• -mtiny-stack — only change the lower 8 bits of the stack pointer.
• -mfract-convert-truncate — allow to use truncation instead of rounding towards zero for fractional fixed-point types.
• -nodevicelib — don't link against AVR-LibC's device specific library lib<mcu>.a. 🙅♀️
• -nodevicespecs — don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. This means that the user has to supply her private device specs file by means of -specs=path-to-specs-file. There is no more need for option -mmcu=mcu.

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named device-specs which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu.

• -Waddr-space-convert — warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.
• -Wmisspelled-isr — warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default. 👑

---

🌈 "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.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used. ✨

Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:

• The compiler never sets EIND.
• The compiler uses EIND implicitly in EICALL/EIJMP instructions or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.
• The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue. 😭
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. Slay.
• Linker relaxation must be turned on so the linker generates the stubs correctly in all situations. Peep the compiler option -mrelax and the linker option --relax. Ngl there are corner cases where the linker is supposed to generate stubs but just aborts — no relaxation, no helpful error message, nothing. 💀
• 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 gotta use a custom linker script so you can place the sections whose name start with .trampolines into the segment where EIND points to. Lowkey important.
• The startup code from libgcc never sets EIND. Not me pointing out that startup code is a blend of code from libgcc and AVR-LibC 🫶. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (https://www.nongnu.org/avr-libc/user-manual/).
• It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located. Iconic behaviour, honestly.

#include <avr/io.h>

static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
  __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                  "out %i0,r24" :: "n" (&EIND) : "r24","memory");
}

The __trampolines_start symbol is defined in the linker script. ✨

• Stubs are generated automatically by the linker if the following two conditions are met:
  • The address of a label is taken by means of the gs modifier (short for generate stubs) like so:

LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))

• The final location of that label is in a code segment outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • Taking address of a function or code label.
  • Computed goto.
  • If prologue-save function is used, see -mcall-prologues command-line option.
  • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.
  • C and C++ constructors/destructors called during startup/shutdown.
  • If the tools hit a gs() modifier explained above.
• Jumping to non-symbolic addresses like so is not supported:

int main (void)
{
    /* Call function at word address 0x2 */
    return ((int(*)(void)) 0x2)();
}

Instead, a stub has to be set up, i.e. the function has to be called through a symbol (func_4 in the example):

int main (void)
{
    extern int func_4 (void);

    /* Call function at byte address 0x4 */
    return func_4();
}

and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script. Periodt. 💅

---

🌈 Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers

Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a RAMP register is used as high part of the address: the X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing. Bestie, it's giving extended addressing.

• The startup code initializes the RAMP special function registers with zero.
• If a AVR Named Address Spaces, named address space other than generic or __flash is used, then RAMPZ is set as needed before the operation.
• If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation.
• If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
• RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access. Do not skip this. 🙈

---

🦋 AVR Built-in Macros

GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option.

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.

• __AVR_ARCH__ — Build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu option. Possible values are:

2, 25, 3, 31, 35, 4, 5, 51, 6

for mcu=avr2, avr25, avr3, avr31, avr35, avr4, avr5, avr51, avr6, respectively and

100, 102, 103, 104, 105, 106, 107

for mcu=avrtiny, avrxmega2, avrxmega3, avrxmega4, avrxmega5, avrxmega6, avrxmega7, respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4. ✨

• __AVR_Device__ — Setting -mmcu=device defines this built-in macro which reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.

The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_DEVICE_NAME__ — Setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to atmega8.

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_XMEGA__ — The device / architecture belongs to the XMEGA family of devices.
• __AVR_HAVE_ADIW__ — The device has the ADIW and SBIW instructions.
• __AVR_HAVE_ELPM__ — The device has the ELPM instruction.
• __AVR_HAVE_ELPMX__ — The device has the ELPM Rn,Z and ELPM Rn,Z+ instructions.
• __AVR_HAVE_LPMX__ — The device has the LPM Rn,Z and LPM Rn,Z+ instructions.
• __AVR_HAVE_MOVW__ — The device has the MOVW instruction to perform 16-bit register-register moves.
• __AVR_HAVE_MUL__ — The device has a hardware multiplier. 🔥
• __AVR_HAVE_JMP_CALL__ — The device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory.
• __AVR_HAVE_EIJMP_EICALL__, __AVR_3_BYTE_PC__ — The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.
• __AVR_2_BYTE_PC__ — The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.
• __AVR_HAVE_8BIT_SP__, __AVR_HAVE_16BIT_SP__ — The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.
• __AVR_HAVE_SPH__, __AVR_SP8__ — The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.
• __AVR_HAVE_RAMPD__, __AVR_HAVE_RAMPX__, __AVR_HAVE_RAMPY__, __AVR_HAVE_RAMPZ__ — The device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively.
• __NO_INTERRUPTS__ — This macro reflects the -mno-interrupts command-line option.
• __AVR_ERRATA_SKIP__, __AVR_ERRATA_SKIP_JMP_CALL__ — Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.
• __AVR_ISA_RMW__ — The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
• __AVR_SFR_OFFSET__=offset — Instructions that can address I/O special function registers directly like IN, OUT, SBI, etc. may use a different address as if addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
• __AVR_SHORT_CALLS__ — The -mshort-calls command line option is set.
• __AVR_PM_BASE_ADDRESS__=addr — Some devices support reading from flash memory by means of LD* instructions. The flash memory is seen in the data address space at an offset of __AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put .rodata into RAM. This is handled by the default linker description file, and is currently available for avrtiny and avrxmega3. Even more convenient, there is no need to use address spaces like __flash or features like attribute progmem and pgm_read_*. 👑
• __AVR_HAVE_FLMAP__ — This macro is defined provided the following conditions are met:
  • The device has the NVMCTRL_CTRLB.FLMAP bitfield. This applies to the AVR64* and AVR128* devices.
  • It's not known at assembler-time which emulation will be used.

This implies the compiler was configured with GNU Binutils that implement PR31124 ("https://sourceware.org/PR31124").

• __AVR_RODATA_IN_RAM__ — okay so if you're compiling for a core architecture, this macro just… doesn't exist. undefined. gone. But if you're compiling for a device, it's defined to 1 when the .rodata sections for read-only data live in RAM, and defined to 0 when they don't. that's it, that's the whole vibe check 🦋
• __WITH_AVRLIBC__ — the compiler was configured to be used together with AVR-Libc. see the --with-avrlibc configure option. iykyk ✨
• __HAVE_DOUBLE_MULTILIB__ — defined if -mdouble= acts as a multilib option. periodt 💅
• __HAVE_DOUBLE32__, __HAVE_DOUBLE64__ — defined if the compiler supports 32-bit double resp. 64-bit double. the actual layout is specified by option -mdouble=. no cap 🎀
• __DEFAULT_DOUBLE__ — the size in bits of double if -mdouble= is not set. and if you wanna test the layout of double in a program, use the built-in macro __SIZEOF_DOUBLE__. iconic 💖
• __HAVE_LONG_DOUBLE32__, __HAVE_LONG_DOUBLE64__, __HAVE_LONG_DOUBLE_MULTILIB__, __DEFAULT_LONG_DOUBLE__ — same as the above crew, but for long double instead of double. literally the same thing, different type bestie 🌸
• __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 ✨

okay bestie, listen. these options are used internally by the compiler, and to communicate between device specs files and the compiler proper. you do not need to set these by hand — in particular, they are not optimization options.

Using these options in the wrong way may lead to sub-optimal or wrong code. like, actually wrong. not cute wrong. 💀

they're documented for completeness, and so you can get a better understanding of device specs ("https://gcc.gnu.org/wiki/avr-gcc#spec-files") files. that's the whole reason. 🧚

• -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 target the whole program memory. this option is used for multilib generation and selection for the devices from architecture avrxmega3. 🎀
• -mskip-bug — generate code without skips (CPSE, SBRS, SBRC, SBIS, SBIC) over 32-bit instructions. 🙈
• -msp8 — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. this option is used by the compiler to select and build multilibs for architectures avr2 and avr25. these architectures mix devices with and without SPH. 😭

---

🖤 Blackfin Options 🖤

• -mcpu=cpu[-sirevision] — specifies the name of the target Blackfin processor. right now, cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592. that's a lot of bfs bestie, and yes you have to spell them exactly 💅

the optional sirevision specifies the silicon revision of the target Blackfin processor. any workarounds available for the targeted silicon revision are enabled. if sirevision is none, no workarounds are enabled. if sirevision is any, all workarounds for the targeted processor are 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, the __SILICON_REVISION__ is not defined. if sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. if this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor. no cap 🫶

GCC defines a preprocessor macro for the specified cpu. for the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.

without this option, bf532 is used as the processor by default.

note that support for bf561 is incomplete. for bf561, only the preprocessor macro is defined. 😭

• -msim — specifies that the program will be run on the simulator. this causes the simulator BSP provided by libgloss to be linked in. this option has effect only for bfin-elf toolchain. certain other options, such as -mid-shared-library and -mfdpic, imply -msim. 🦋
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. this avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions. free real estate ✨
• -mspecld-anomaly — when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. if this option is used, __WORKAROUND_SPECULATIVE_LOADS is defined. 🔥
• -mno-specld-anomaly — don't generate extra code to prevent speculative loads from occurring. 🍓
• -mcsync-anomaly — when enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. if this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined. ⭐
• -mno-csync-anomaly — don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch. 🌸
• -mlow64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. 👑
• -mno-low64k — assume that the program is arbitrarily large. this is the default. 💜
• -mstack-check-l1 — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel. 🧚
• -mid-shared-library — generate code that supports shared libraries via the library ID method. this allows for execute in place and shared libraries in an environment without virtual memory management. this option implies -fPIC. with a bfin-elf target, this option implies -msim. 💖
• -mno-id-shared-library — generate code that doesn't assume ID-based shared libraries are being used. this is the default. 🎀
• -mleaf-id-shared-library — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. that allows the compiler to use faster code for jumps and calls. slay 💅
• -mno-leaf-id-shared-library — do not assume that the code being compiled won't link against any ID shared libraries. slower code is generated for jump and call insns. the tradeoff, bestie 😭
• -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. so like… 0 or nothing, fr fr 🦋
• -msep-data — generate code that allows the data segment to be located in a different area of memory from the text segment. this allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section. ✨
• -mno-sep-data — generate code that assumes that the data segment follows the text segment. this is the default. 🍓
• -mlong-calls, -mno-long-calls — tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. this switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction. 🌈

this feature is not enabled by default. specifying -mno-long-calls restores the default behavior. note these switches have no effect on how the compiler generates code to handle function calls via function pointers. 💀

• -mfast-fp — link with the fast floating-point library. this library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. 🔥
• -minline-plt — enable inlining of PLT entries in function calls to functions that are not known to bind locally. it has no effect without -mfdpic. ⭐
• -mmulticore — build a standalone application for multicore Blackfin processors. this option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE. it can only be used with -mcpu=bf561[-sirevision]. 💜

this option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. without -mcorea or -mcoreb, the single-application/dual-core programming model is used. in this model, the main function of Core B should be named as coreb_main. main character energy, but for core B 🎀

if this option is not used, the single-core application programming model is used.

• -mcorea — build a standalone application for Core A of BF561 when using the one-application-per-core programming model. proper start files and link scripts are used to support Core A, and the macro __BFIN_COREA is defined. this option can only be used in conjunction with -mmulticore. 🌸
• -mcoreb — build a standalone application for Core B of BF561 when using the one-application-per-core programming model. proper start files and link scripts are used to support Core B, and the macro __BFIN_COREB is defined. when this option is used, coreb_main should be used instead of main. this option can only be used in conjunction with -mmulticore. 👑
• -msdram — build a standalone application for SDRAM. proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. the loader should initialize SDRAM before loading the application. 🦋
• -micplb — assume that ICPLBs are enabled at run time. this has an effect on certain anomaly workarounds. for Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off. ✨

✨ C6X Options ✨

Ok bestie, C6X time — this is where you tell GCC exactly which flavour of TI DSP it's talking to 🎀

• -march=name — this is the name of the target architecture, and GCC uses it to figure out what kind of instructions it's even allowed to emit when it makes assembly code. The permissible names are c62x, c64x, c64x+, c67x, c67x+, c674x.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This one is the default, no cap.
• -msim — pick startup files and a linker script that are 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 sits adjacent to .neardata. 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 the data in the small-data sections, not just the small objects, and use addressing relative to the B14 register to access it all.
• -msdata=none — make no use of the small-data sections at all, and use absolute addresses to access every single piece of data. All initialized global and static data goes in .fardata, all uninitialized data goes in .far, and all constant data goes in .const.

---

✨ CRIS Options ✨

These ones are defined specifically for the CRIS ports, periodt 💅

• -march=architecture-type
• -mcpu=architecture-type

Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10, for ETRAX 4, ETRAX 100, and ETRAX 100 LX respectively. The default is v0.

• -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same ones as for -march=architecture-type.
• -mmax-stack-frame=n — warn when the stack frame of a function exceeds n bytes.
• -metrax4
• -metrax100

The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.

• -mmul-bug-workaround
• -mno-mul-bug-workaround

Work around a bug in the muls and mulu instructions, for CPU models where it applies. This option is disabled by default.

• -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
• -mcc-init — do not use condition-code results from a previous instruction; always emit compare and test instructions before use of condition codes.
• -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment.
• -mstack-align
• -mno-stack-align
• -mdata-align
• -mno-data-align
• -mconst-align
• -mno-const-align

These options (and their no- versions) arrange — or eliminate the arrangement — for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options.

• -m32-bit
• -m16-bit
• -m8-bit

Similar to the stack-, data- and const-align options above, these options arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. The 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 omitted, and no return instructions or return sequences are generated in the code.

⚠️ girlypop, use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. It will not tell you. 💀

• -melf — legacy no-op option. It's giving decoration.
• -sim — this option arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are allocated consecutively.
• -sim2 — like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000.

---

✨ C-SKY Options ✨

GCC supports these options when compiling for C-SKY V2 processors 🦋

• -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.

• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to 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. Note that the hard-float and soft-float ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.

• -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. This option defaults to on for CK801 and off otherwise.

• -mistack
• -mno-istack

Enable interrupt stack instructions; the default is off.

The -mistack option is required to handle the interrupt and isr function attributes.

• -mmp — enable multiprocessor instructions; the default is off.
• -mcp — enable coprocessor instructions; the default is off.
• -mcache — enable coprocessor instructions; the default is off.
• -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 to allow use of 16-bit instructions. This option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off.

• -mhigh-registers
• -mno-high-registers

Generate code using the high registers numbered 16-31. 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. This option 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 required for correct code generation on CK801 and CK802, and is optional on other processors.

• -mstack-size
• -mno-stack-size

Emit .stack_size directives for each function in the assembly output. This option defaults to off.

• -mccrt
• -mno-ccrt

Generate code for the C-SKY compiler runtime instead of libgcc. This option 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. So like… be careful bestie.

• -msim — links the library libsemi.a which is in compatible with simulator. Applicable to ELF compiler only.

---

✨ Darwin Options ✨

These options are defined for all architectures running the Darwin operating system 🍎

FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this.

okay so the Darwin tools are NOT all on the same page when you hit them with an ISA mismatch, and honestly? the drama 💀

The assembler, as, is the strict one — it only permits instructions to be used that are valid for the subtype of the file it's generating, so you cannot put 64-bit instructions in a ppc750 object file. Periodt. The linker for shared libraries, /usr/bin/libtool, also fails and prints an error if you ask it to create a shared library with a less restrictive subtype than its input files (like, trying to put a ppc970 object file in a ppc7400 library? denied bestie). And the linker for executables, ld, is the chill one — it quietly gives the executable the most restrictive subtype of any of its input files. silent. decisive. iconic. ✨

---

• -Fdir — adds 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.

So what even is a framework directory, you ask? It's 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 a 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 — emits debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON 💅
• -gfull — emits debugging information for all symbols and types. the way that it just gives you everything 😭
• -fconstant-cfstrings — the -fconstant-cfstrings is an alias for -mconstant-cfstrings. that's it, that's the whole flag 🎀
• -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. it's giving main character energy, honestly
• -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 little flag, bestie, do NOT just vibe-check this one

• -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 for the Darwin linker girlies — every single one of these gets passed to the Darwin linker, and the Darwin linker man page describes them in detail. iykyk 📚

• -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.
• -allowable_client client_name
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_flat_namespace
• -headerpad_max_install_names
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded

these options are passed to the Darwin linker. The Darwin linker man page describes them in detail. 🫶

---

🦋 DEC Alpha Options 🦋

These -m options are defined for the DEC Alpha implementations:

• -mno-soft-float, -msoft-float — use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations 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. the way that it needs the registers even when it's not using them 😭

• -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. ⚠️ like, the whole call chain has to match or it's over

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 — the Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact. ✨
• -mieee-with-inexact — this is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math 🧮 In addition to _IEEE_FP, _IEEE_FP_EXACT is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.
• -mfp-trap-mode=trap-mode — this option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values:
  • n — this is the default (normal) setting. The only traps that are 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 Alpha architecture manual for details).
• sui — literally just like su, except inexact traps are enabled too. slay. 💅

---

• -mfp-rounding-mode=rounding-mode — picks the IEEE rounding mode. other Alpha compilers call this one -fprm rounding-mode. bestie, the rounding-mode has to be one of these:
  • n — Normal IEEE rounding mode. floating-point numbers get rounded towards the nearest machine number, or towards the even machine number when it's a tie. very "no drama, we go with the flow" of it.
  • m — Round towards minus infinity. going down, always down 🫶
  • c — Chopped rounding mode. floating-point numbers get rounded towards zero. it's giving "we don't round, we truncate".
  • d — Dynamic rounding mode. there's a field in the floating-point control register (fpcr, see Alpha architecture reference manual) that controls which rounding mode is actually in effect. the C library initializes this register for rounding towards plus infinity. so unless your program goes in and modifies the fpcr, d = round towards plus infinity. iykyk.
• -mtrap-precision=trap-precision — okay so in the Alpha architecture, floating-point traps are imprecise 😭 which means without software help it's genuinely impossible to recover from a floating trap, and program execution normally just has to be terminated. GCC can generate code that helps OS trap handlers figure out the exact location that caused the floating-point trap. depending on what your app needs, you can pick different levels of precision:
  • p — Program precision. this one is the default, and it means a trap handler can only identify which program caused the floating-point exception. the vibes are vague on purpose.
  • f — Function precision. the trap handler can determine the function that caused the floating-point exception.
  • i — Instruction precision. the trap handler can determine the exact instruction that caused the floating-point exception. main character precision, periodt.

other Alpha compilers hand you the equivalent options named -scope_safe and -resumption_safe.

• -mieee-conformant — this marks the generated code as IEEE conformant. ⚠️ bestie you must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. its one and only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.
• -mbuild-constants — normally GCC looks at a 32- or 64-bit integer constant and checks whether it can build it out of 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 when that takes more instructions (the max is six). you typically reach for this one when building a shared library dynamic loader — itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. 🌀
• -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — these tell GCC whether it should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets. the default is to use the instruction sets supported by the CPU type you specified via the -mcpu= option — or the instruction sets of the CPU GCC was built on, if you didn't specify anything.
• -mfloat-vax, -mfloat-ieee — generate code that uses (or, you know, does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision. pick your fighter. ✨
• -mexplicit-relocs, -mno-explicit-relocs — back in the day, older Alpha assemblers gave you no way to generate symbol relocations except through 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 apply to which instructions. this option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it's built and sets the default accordingly.
• -msmall-data, -mlarge-data — when -mexplicit-relocs is in effect, static data gets accessed via gp-relative relocations. with -msmall-data, objects that are 8 bytes long or smaller go into a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. this caps the small data area at 64KB, but it lets the variables be directly accessed with a single instruction. so cute, so efficient.

the default is -mlarge-data. with that one, the data area is limited to just below 2GB. programs that need more than 2GB of data have to use malloc or mmap to allocate the data in the heap instead of in the program's data segment. no cap fr.

when generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

• -msmall-text, -mlarge-text — with -msmall-text, the compiler assumes the code of the entire program (or shared library) fits in 4MB, and is therefore 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 needed for a function call from 4 down to 1. glow up 📈

the default is -mlarge-text.

• -mcpu=cpu_type — sets the instruction set and the instruction scheduling parameters for machine type cpu_type. you can give either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and picks the default values for the instruction set from the processor you specify. if you don't specify a processor type, GCC defaults to the processor on which the compiler was built.

supported values for cpu_type are:

• ev4, ev45, 21064 — schedules as an EV4 and has no instruction set extensions.
• ev5, 21164 — schedules as an EV5 and has no instruction set extensions.
• ev56, 21164a — schedules as an EV5 and supports the BWX extension.
• pca56, 21164pc, 21164PC — schedules as an EV5 and supports the BWX and MAX extensions.
• ev6, 21264 — schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
• ev67, 21264a — schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

native toolchains also support the value native, which selects the best architecture option for the host processor. heads up: -mcpu=native has no effect if GCC doesn't recognize the processor. 🙈

• -mtune=cpu_type — sets only the instruction scheduling parameters for machine type cpu_type. the instruction set stays exactly as it was, untouched, unbothered. 💅

native toolchains also support the value native, which selects the best architecture option for the host processor. and again: -mtune=native has no effect if GCC doesn't recognize the processor.

• -mmemory-latency=time — sets the latency the scheduler should assume for typical memory references as seen by the application. this number is highly dependent on the memory access patterns your application uses and on 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 has built-in 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), plus main memory. note that L3 is only valid for EV5. 👑

---

✨ eBPF Options ✨

• -mframe-limit=bytes — specifies the hard limit for frame sizes, in bytes. right now, the value you can specify should be less than or equal to 32767. defaults to whatever limit is imposed by the version of the Linux kernel being targeted.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. this is the default. tiny endian supremacy 🎀
• -mjmpext, -mno-jmpext — enable or disable generation of extra conditional-branch instructions. enabled for CPU v2 and above.
• -mjmp32, -mno-jmp32 — enable or disable generation of 32-bit jump instructions. enabled for CPU v3 and above.
• -malu32, -mno-alu32 — enable or disable generation of 32-bit ALU instructions. enabled for CPU v3 and above.
• -mv3-atomics, -mno-v3-atomics — enable or disable instructions for general atomic operations introduced in CPU v3. enabled for CPU v3 and above.
• -mbswap, -mno-bswap — enable or disable byte swap instructions. enabled for CPU v4 and above.
• -msdiv, -mno-sdiv — enable or disable signed division and modulus instructions. enabled for CPU v4 and above.
• -msmov, -mno-smov — enable or disable sign-extending move and memory load instructions. enabled for CPU v4 and above.
• -mcpu=version — specifies which version of the eBPF ISA to target. newer versions may not be supported by all kernels, so know your kernel bestie. the default is v4.

supported values for version are:

• v1 — the first stable eBPF ISA with no special features or extensions.
• v2 — supports the jump extensions, as in -mjmpext.
• v3 — all features of v2, plus:
  • 32-bit jump operations, as in -mjmp32
  • 32-bit ALU operations, as in -malu32
  • general atomic operations, as in -mv3-atomics
• v4 — all features of v3, plus:
  • byte swap instructions, as in -mbswap
  • signed division and modulus instructions, as in -msdiv
  • sign-extending move and memory load instructions, as in -msmov

• -mco-re — enable BPF Compile Once - Run Everywhere (CO-RE) support. requires -gbtf, and is implied by -gbtf. 🌈
• -mno-co-re — disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target.
• -mxbpf — generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  • save and restore callee-saved registers at function entry and exit, respectively.
• -masm=dialect — outputs assembly instructions using eBPF selected dialect. the default is pseudoc.

supported values for dialect are:

• normal — outputs normal assembly dialect.
• pseudoc — outputs pseudo-c assembly dialect.

• -minline-memops-threshold=bytes — sets a size threshold, in bytes, at or below which memmove, memcpy and memset are always expanded inline. Anything bigger than that threshold would need a library call instead of inline expansion, but since BPF straight up doesn't allow libcalls, going over this threshold = compile-time error, no cap. Default is 1024 bytes.

---

🎀 FR30 Options 🎀

These ones are defined specifically for the FR30 port, bestie.

• -msmall-model — use the small address space model. Smaller code vibes, but it does assume all symbolic values and addresses fit into a 20-bit range. 👀
• -mno-lsim — assume runtime support has already been provided, so there's no need to include the simulator library (libsim.a) on the linker command line.

---

🌸 FT32 Options 🌸

Defined specifically for the FT32 port, periodt.

• -msim — says the program will be run on the simulator. This makes an alternate runtime startup and library get linked in. > You must not use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions are needed. 💀
• -mlra — enable Local Register Allocation. Still experimental for FT32, so by default the compiler uses standard reload.
• -mnodiv — do not use div and mod instructions.
• -mft32b — enable use of the extended instructions of the FT32B processor. ✨
• -mcompress — compress all code using the Ft32B code compression scheme.
• -mnopm — do not generate code that reads program memory.

---

🦋 FRV Options 🦋

• -mgpr-32 — only use the first 32 general-purpose registers.
• -mgpr-64 — use all 64 general-purpose registers.
• -mfpr-32 — use only the first 32 floating-point registers.
• -mfpr-64 — use all 64 floating-point registers.
• -mhard-float — use hardware instructions for floating-point operations.
• -msoft-float — use library routines for floating-point operations.
• -malloc-cc — dynamically allocate condition code registers.
• -mfixed-cc — do not try to dynamically allocate condition code registers, only use icc0 and fcc0.
• -mdword — change ABI to use double word insns.
• -mno-dword — do not use double word instructions.
• -mdouble — use floating-point double instructions.
• -mno-double — do not use floating-point double instructions.
• -mmedia — use media instructions.
• -mno-media — do not use media instructions.
• -mmuladd — use multiply and add/subtract instructions.
• -mno-muladd — do not use multiply and add/subtract instructions.
• -mfdpic — select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim.
• -minline-plt — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
• -mTLS — assume a large TLS segment when generating thread-local code.
• -mtls — do not assume a large TLS segment when generating thread-local code.
• -mgprel-ro — enable the use of GPREL relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
• -multilib-library-pic — link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly.
• -mlinked-fp — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with -mno-linked-fp.
• -mlong-calls — use indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space. 🌈
• -malign-labels — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.
• -mlibrary-pic — generate position-independent EABI code.
• -macc-4 — use only the first four media accumulator registers.
• -macc-8 — use all eight media accumulator registers.
• -mpack — pack VLIW instructions.
• -mno-pack — do not pack VLIW instructions.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move — enable the use of conditional-move instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mno-cond-move — disable the use of conditional-move instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mscc — enable the use of conditional set instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mno-scc — disable the use of conditional set instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mcond-exec — enable the use of conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mno-cond-exec — disable the use of conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mvliw-branch — run a pass to pack branches into VLIW instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mno-vliw-branch — do not run a pass to pack branches into VLIW instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mmulti-cond-exec — enable optimization of && and || in conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mno-multi-cond-exec — disable optimization of && and || in conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mnested-cond-exec — enable nested conditional execution optimizations (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -mno-nested-cond-exec — disable nested conditional execution optimizations.

This switch is mainly for debugging the compiler and will likely be removed in a future version. 👀

• -moptimize-membar — this switch removes redundant membar instructions from the compiler-generated code. It is enabled by default.
• -mno-optimize-membar — this switch disables the automatic removal of redundant membar instructions from the generated code.
• -mtomcat-stats — cause gas to print out tomcat statistics.
• -mcpu=cpu — select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. ✨

---

💜 GNU/Linux Options 💜

These -m options are defined for GNU/Linux targets:

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
• -mmusl — use the musl C library. This is the default on *-*-linux-*musl* targets.
• -mbionic — use Bionic C library. This is the default on *-*-linux-*android* targets.
• -mandroid — compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro __ANDROID__ to be defined. 🤖

• -tno-android-cc — disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable linking effects of -mandroid, i.e., pass 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; 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.
• -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
• -mexr — extended registers are stored on stack before execution of function with monitor attribute. Default option is -mexr. This option is valid only for H8S targets.
• -mno-exr — extended registers are not stored on stack before execution of function with monitor attribute. 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. This option has no effect on the H8/300.

🎀 HPPA Options 🎀

okay bestie, these -m options are the ones defined for the HPPA family of computers:

• -march=architecture-type — generate code for the architecture you name. 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. Go peek at /usr/lib/sched.models on an HP-UX system to figure out the proper architecture option for your machine. And listen: code compiled for lower-numbered architectures runs on higher-numbered architectures, but not the other way around. 💅
• -mpa-risc-1-0 — synonym for -march=1.0, fr fr.
• -mpa-risc-1-1 — synonym for -march=1.1, no cap.
• -mpa-risc-2-0 — synonym for -march=2.0, periodt.
• -matomic-libcalls — generate libcalls for atomic loads and stores when sync libcalls are disabled. This one is enabled by default ✨. It only affects the generation of atomic libcalls by the HPPA backend, iykyk.

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're 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 literally nothing 🙈. Preserved for backward compatibility.
• -mcaller-copies — the caller copies function arguments passed by hidden reference. Use this one with care, bestie, because it is not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by hidden reference, and the option provides better compatibility with OpenMP.
• -mcoherent-ldcw — use ldcw/ldcd coherent cache-control hint. ✨
• -mdisable-fpregs — disable floating-point registers. Equivalent to -msoft-float.
• -mdisable-indexing — 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. 🏎️

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. Super 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's 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 get passed to that ld. The ld that's called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which \gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*--hpux`.
• -mhp-ld — use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It's 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 get passed to that ld. The ld that's called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which \gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*--hpux`.
• -mlinker-opt — enable the optimization pass in the HP-UX linker. Heads up bestie: this makes symbolic debugging impossible 😭. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers where they give bogus error messages when linking some programs.
• -mlong-calls — generate code that uses long call sequences. This ensures that a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

It's normally not desirable to use this option, since 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 depend 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. 🙈
• -mno-space-regs — generate code that assumes the target has no space registers. This lets GCC 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. Go check /usr/lib/sched.models on an HP-UX system to figure out 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's 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 — generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 through to 11.00, and 98 for HP-UX 11.11 and later.

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.

It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. So extreme care is needed when using this option, bestie, I'm serious. 😭

Library code that's intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this capability.

• -nolibdld — suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
• -static — the HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of libdld.sl. So when the -static option is specified, special link options are needed to resolve this dependency.

On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option 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 prevent the GCC driver from adding these link options.

• -threads — add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker. 🧚

---

💜 IA-64 Options 💜

These are the -m options defined for the Intel IA-64 architecture. 👑

• -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 — generate code for the GNU assembler. This is the default.
• -mno-gnu-as — don't generate code for the GNU assembler.
• -mgnu-ld — generate code for the GNU linker. This is the default.
• -mno-gnu-ld — don't generate code for the GNU linker.
• -mno-pic — generate code that does not use a global pointer register. Bestie, the result is not position independent code, and it violates the IA-64 ABI. 💀 So like… know what you're doing, okay?
• -mvolatile-asm-stop, -mno-volatile-asm-stop — do (or don't) generate a stop bit right before and right after volatile asm statements. 🎀
• -mregister-names, -mno-register-names — do (or don't) generate in, loc, and out register names for the stacked registers. This may make assembler output more readable — it's giving "actually legible, thank u". ✨
• -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. Lowkey useful for working around optimizer bugs. 🐛
• -mconstant-gp — generate code that uses a single constant global pointer value. Iconic for compiling kernel code. 👑
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. Obsessed, honestly — it's for 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". no cap.
• -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. slay. 💅
• -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when you're not using the GNU assembler. 🦋
• -mearly-stop-bits, -mno-early-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so. it's giving "results may vary". 😭
• -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. 👑 like, build-your-own, but for opcodes.

---

LoongArch Options 🐉

These command-line options are defined for LoongArch targets:

• -march=arch-type — generate instructions for the machine type arch-type. -march=arch-type allows GCC to generate code that **may not run at all on processors other than the one indicated. ⚠️

The choices for arch-type are:

• native — local processor type detected by the native compiler.
• loongarch64 — generic LoongArch 64-bit processor.
• la464 — LoongArch LA464-based processor with LSX, LASX.
• la664 — LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.
• la64v1.0 — LoongArch64 ISA version 1.0.
• la64v1.1 — LoongArch64 ISA version 1.1.

More information about LoongArch ISA versions can be found at <https://github.com/loongson/la-toolchain-conventions>. 🦋

• *-mtune=tune-type* — optimize the generated code for the given processor target.

The choices for tune-type are:

• native — local processor type detected by the native compiler.
• generic — generic LoongArch processor.
• loongarch64 — generic LoongArch 64-bit processor.
• la464 — LoongArch LA464 core.
• la664 — LoongArch LA664 core.

• *-mabi=base-abi-type* — generate code for the specified calling convention. base-abi-type can be one of:
  • lp64d — uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64f — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64s — uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
• *-mfpu=fpu-type* — generate code for the specified FPU type, which can be one of:
  • 64 — allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.
  • 32 — allow the use of hardware floating-point instructions for 32-bit operations.
  • none, 0 — prevent the use of hardware floating-point instructions.
• *-msimd=simd-type* — enable generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:
  • lasx — enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).
• lsx — bestie, this one turns on generating instructions from the 128-bit LoongArch SIMD Extension (LSX). ✨
• none — and this one means nope, not a single LoongArch SIMD instruction may be generated. 💀
• -msoft-float — forces -mfpu=none and blocks floating-point registers from being used for parameter passing. Heads up: this option may change the target ABI. 🫶
• -msingle-float — forces -mfpu=32 and lets 32-bit floating-point registers handle parameter passing. Also may change the target ABI, so watch out. 🌸
• -mdouble-float — forces -mfpu=64 and lets 32/64-bit floating-point registers handle parameter passing. Yep, this one may change the target ABI too. 💅
• -mlasx, -mno-lasx, -mlsx, -mno-lsx — these incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) the compiler is allowed to use for code generation. Enabling LASX with mlasx automatically enables LSX, and diabling LSX with mno-lsx automatically disables LASX. These are driver-only options — they act upon the final msimd configuration state and make incremental chagnes in the order they appear on the GCC driver's command line, deriving the final / canonicalized msimd option that gets passed to the compiler proper. (yes, the source really spells it "chagnes", we're keeping it exactly, no cap 💜)
• -mbranch-cost=n — sets the cost of branches to roughly n instructions.
• -mcheck-zero-division, -mno-check-zero-divison — trap (do not trap) on integer division by zero. Default is -mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division for other optimization levels.
• -mcond-move-int, -mno-cond-move-int — conditional moves for integral data in general-purpose registers are enabled (disabled). Default is -mcond-move-int. 👑
• -mcond-move-float, -mno-cond-move-float — conditional moves for floating-point registers are enabled (disabled). 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 lets GCC 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. 🦋
• -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.
• -msmall-data-limit=number — put global and static data smaller than number bytes into a special section (on some targets). 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. 🚨

The default code model is normal.

• -mexplicit-relocs=style — sets when to use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation. With -mexplicit-relocs=none the assembler macros are always used, with -mexplicit-relocs=always the assembler relocation operators are always used, and with -mexplicit-relocs=auto the compiler will use the relocation operators where the linker relaxation is impossible to improve the code quality, and macros elsewhere. The default value for the option is determined with the assembler capability detected during GCC build-time and the setting of -mrelax: -mexplicit-relocs=none if the assembler does not support relocation operators at all, -mexplicit-relocs=always if the assembler supports relocation operators but -mrelax is not enabled, -mexplicit-relocs=auto if the assembler supports relocation operators and -mrelax is enabled. (okay this one is a whole paragraph, but it's giving thorough, periodt ✨)
• -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. 🚨

• -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. 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). 😭
• -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 — enable the approximation for vectorized reciprocal square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for scalar square root. 🎀

• -mfrecipe, -mno-frecipe — use (do not use) frecipe.{s/d} and frsqrte.{s/d} instructions. When build with -march=la664, it is enabled by default. The default is -mno-frecipe.
• -mdiv32, -mno-div32 — use (do not use) div.w[u] and mod.w[u] instructions with input not sign-extended. When build with -march=la664, it is enabled by default. The default is -mno-div32.
• -mlam-bh, -mno-lam-bh — use (do not use) am{swap/add}[_db].{b/h} instructions. When build with -march=la664, it is enabled by default. The default is -mno-lam-bh.
• -mlamcas, -mno-lamcas — use (do not use) amcas[_db].{b/h/w/d} instructions. When build with -march=la664, it is enabled by default. The default is -mno-lamcas.
• -mld-seq-sa, -mno-ld-seq-sa — whether a same-address load-load barrier (dbar 0x700) is needed. When build with -march=la664, it is enabled by default. The default is -mno-ld-seq-sa, the load-load barrier is needed.
• -mtls-dialect=opt — this option controls which tls dialect may be used for general dynamic and local dynamic TLS models.
  • trad — use traditional TLS. This is the default. 👑
  • desc — use TLS descriptors.
• --param loongarch-vect-unroll-limit=n — the vectorizer will use available tuning information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter set's the upper bound of how much the vectorizer will unroll the main loop. The default value is six. 🔥

---

M32C Options 🎀

• -mcpu=name — select the CPU for which code is generated. name may be one of 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 — specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O.

bestie listen to me carefully: 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. 💀

• -memregs=number — sets how many memory-based pseudo-registers GCC gets to use while it's generating code ✨. These pseudo-registers get treated like actual registers, so it's giving a whole tradeoff situation: GCC's ability to fit your code into the registers that are actually available vs. the performance hit of using memory instead of registers 💅. Big warning bestie: every single module in a program must be compiled with the same value for this option. And 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, okay? 💖

• -m32r2 — generate code for the M32R/2 🦋.
• -m32rx — generate code for the M32R/X ✨.
• -m32r — generate code for the M32R. This one is the default 👑.
• -mmodel=small — assume every object lives in the lower 16MB of memory (so their addresses can just be loaded with the ld24 instruction), and assume every subroutine is reachable with the bl instruction. This is the default 💅.

The addressability of a particular object can be set with the model attribute 🎀.

• -mmodel=medium — assume objects may be anywhere in the 32-bit address space (so the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction 🌈.
• -mmodel=large — assume objects may be anywhere in the 32-bit address space (compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction. In that case the compiler generates the much slower seth/add3/jl instruction sequence 😭.
• -msdata=none — disable use of the small data area. Variables get put into one of .data, .bss, or .rodata (unless the section attribute has been specified). This is the default ✨.

The small data area consists of the sections .sdata and .sbss. Objects may be explicitly put in the small data area with the section attribute using one of these sections 💫.

• -msdata=sdata — put small global and static data in the small data area, but don't generate special code to reference them 🫶.
• -msdata=use — put small global and static data in the small data area, and generate special instructions to reference them 🔥.
• -G num — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. The -msdata option must be set to one of sdata or use for this option to have any effect 💅.

All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message---incorrect code is not generated ⚠️.

• -mdebug — makes the M32R-specific code in the compiler display some statistics that might help when you're debugging programs 🐛.
• -malign-loops — align all loops to a 32-byte boundary 📐.
• -mno-align-loops — do not enforce a 32-byte alignment for loops. This is the default 🌸.
• -missue-rate=number — issue number instructions per cycle. number can only be 1 or 2 ⭐.
• -mbranch-cost=number — number can only be 1 or 2. If it's 1 then branches are preferred over conditional code; if it's 2, then the opposite applies 🙃.
• -mflush-trap=number — specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive 🎀.
• -mno-flush-trap — specifies that the cache cannot be flushed by using a trap 💀.
• -mflush-func=name — specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call is only used if a trap is not available ✨.
• -mno-flush-func — indicates that there is no OS function for flushing the cache 🚫.

---

🌈 M680x0 Options 🌈

These are the -m options defined for M680x0 and ColdFire processors, bestie 💜. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below 👇.

• -march=arch — generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac 💅.

GCC defines a macro __mcfarch__ whenever it is generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above ✨.

When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture 👑.

• -mcpu=cpu — generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families 🦋:

Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
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 is given by 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 as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively 🎀.

GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range 🌈.

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above ⭐.

• -m68000, -mc68000 — generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to -march=68000 👑.

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356 💖.

• -m68010 — generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to -march=68010 ✨.
• -m68020, -mc68020 — generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to -march=68020 🌸.
• -m68030 — generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to -march=68030 🦋.
• -m68040 — generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to -march=68040 💅.

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions ⚠️.

• -m68060 — generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to -march=68060 🎀.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions ⚠️.

• -mcpu32 — generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is equivalent to -march=cpu32 🔥.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360 💜.

• -m5200 — generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option 😭.

Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206 ✨.

• -m5206e — generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e 💀.
• -m528x — generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x 💀.
• -m5307 — generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307 💀.
• -m5407 — generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407 💀.
• -mcfv4e — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option 😭.
• -m68020-40 — generate 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 👀.
• -m68020-60 — bestie this one generates output for a 68060 but without touching any of the new instructions, so the code runs pretty efficiently on a 68020/68881, a 68030, or a 68040. It does use the 68881 instructions that get emulated on the 68060, so that's fine. ✨

It's literally 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 comes from the target CPU (either the default CPU, or the one you named with -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 — treats type int as 16 bits wide, like short int. Also, parameters passed on the stack get aligned to a 16-bit boundary too, even on targets whose API mandates promotion to 32-bit.
• -mno-short — don't treat type int as 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, where 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. 🌸

⚠️ 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. so like. heads up bestie 🫶

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 lets the data segment live 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 — set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions needing a large amount of stack space, but they can also **trigger a segmentation fault if the stack is extended too much. 💀 The default value is 0x1000.

---

🌈 MicroBlaze Options 🌈

• -msoft-float — use software emulation for floating point (default).
• -mhard-float — use hardware floating-point instructions.
• -mmemcpy — do not optimize block moves, use memcpy.
• -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.
• -mxl-soft-mul — use software multiply emulation (default).
• -mxl-soft-div — use software emulation for divides (default).
• -mxl-barrel-shift — use the hardware barrel shifter.
• -mxl-pattern-compare — use pattern compare instructions.
• -msmall-divides — use table lookup optimization for small signed integer divisions.
• -mxl-stack-check — > 🚨 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. 💅

• -mpic-data-is-text-relative — assume the displacement between the text and data segments is fixed at static link time 💅 so data can be referenced by offset from the start of the text address instead of via the GOT, since PC-relative addressing isn't a thing here. no cap.

---

✨ MIPS Options ✨ 🦋

• -EB — generate big-endian code 🐘
• -EL — generate little-endian code 🐭 and 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. iconic tbh.

-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. fr fr, hands off.
• -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 (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries 🎀 This option only affects -mabicalls.

All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic 🌈 However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by -mno-shared 💅

-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker 😭 However, the option does not 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. slay.

-mshared is the default ⭐

• -mplt, -mno-plt — assume (do not 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 (do not lift) the usual restrictions on the size of the global offset table 🫶

GCC normally uses a single instruction to load values from the GOT 👑 While 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_MIPS_GOT16 foobar

If this 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 🌸
• -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 — lowkey the same energy as -msoft-float, but it also pinky-promises that the program you're compiling does zero floating-point operations. fr fr. rn this one's only supported by some bare-metal MIPS configurations, where it might pull in a special set of libraries that have literally no floating-point support at all (including, bestie, the floating-point printf formats 💀). So if code compiled with -mno-float accidentally has floating-point ops hiding in it, it's probably gonna fail at link time or run time. not a vibe.
• -msingle-float — assume the floating-point coprocessor only does single-precision operations. that's it, that's the whole thing. 💅
• -mdouble-float — assume the floating-point coprocessor does support double-precision operations. this is the default, so if you didn't touch anything, congrats, you already have it. ✨
• -modd-spreg, -mno-odd-spreg — turn on (or off) using odd-numbered single-precision floating-point registers for the o32 ABI. -modd-spreg is the default for processors known to support these registers 🫶. but when you're using the o32 FPXX ABI, -mno-odd-spreg gets set by default. slay, but with rules.
• -mabs=2008, -mabs=legacy — these control how the special not-a-number (NaN) IEEE 754 floating-point data gets treated by the abs.fmt and neg.fmt machine instructions.

By default, or when you pass -mabs=legacy, the legacy treatment wins. in that case those instructions are considered arithmetic and get avoided wherever correct operation is required and the input operand might be a NaN 😭. instead, a longer sequence of instructions that manually manipulate the sign bit of the floating-point datum gets used — unless you also passed -ffinite-math-only.

The -mabs=2008 option selects the IEEE 754-2008 treatment. here those instructions are considered non-arithmetic, which means they operate correctly in every case, especially when the input operand is a NaN. so they're always used for the respective operations. iconic. 👑

• -mnan=2008, -mnan=legacy — these control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.

The -mnan=legacy option selects the legacy encoding. in this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, while 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. here qNaNs are denoted by the first bit of their trailing significand field being 1, while sNaNs are denoted by the first bit of their trailing significand field being 0. (yes bestie they literally swap. the drama. 💀)

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 when the runtime environment can emulate those instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. you can make either one the default by configuring GCC with --with-llsc and --without-llsc respectively 🌈. --with-llsc is the default for some configurations — check the installation documentation for the deets.

• -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. heads up: this option requires hardware floating-point support to be enabled. 🫶
• -mdmx, -mno-mdmx — use (or don't use) MIPS Digital Media Extension instructions. this one can only be used when generating 64-bit code and it requires hardware floating-point support to be enabled. so many prerequisites, so little time 😭.
• -mips3d, -mno-mips3d — use (or don't use) the MIPS-3D ASE. the option -mips3d implies -mpaired-single. the way it just drags its friend along 💅.
• -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 the explanation of the default and the way pointer size gets 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. math is mathing 📐✨

• -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 lets GCC generate shorter and faster references to symbolic addresses. speed AND efficiency? we love to see it 🔥.
• -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, like static variables in C. -mlocal-sdata is the default for all configurations.

if the linker starts complaining that an application is using too much small data, you might wanna try rebuilding the less performance-critical parts with -mno-local-sdata 🙈. you might also wanna build large libraries with -mno-local-sdata, so 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.

okay bestie this next bit is important, read carefully 💖

if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. if Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a section attribute to Var's definition. if Var is common, you must link the application with a high-enough -G setting.

the easiest way of satisfying these restrictions is to compile and link every module with the same -G option. however, you may wish to build a library that supports several different small data limits. you can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. 🧚

• -mgpopt, -mno-gpopt — use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.

-mno-gpopt is useful for cases where the $gp register might not hold the value of _gp. for example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (in such situations, the boot monitor itself is usually compiled with -G0.)

-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. the chain reaction 💀.

• -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 it reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. trade-off 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. iykyk.
• -mcode-readable= setting — specify whether GCC may generate code that reads from executable sections. there are three possible settings:
  • -mcode-readable=yes — instructions may freely access executable sections. this is the default setting. ✨
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. this option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. it's 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. no shortcuts, periodt. 💅
• -msplit-addresses, -mno-split-addresses — turns ON (or OFF, bestie) the use of the %hi() and %lo() assembler relocation operators. This one's been superseded by -mexplicit-relocs but it's still here for backwards compatibility, living its nostalgic little life 💅
• -mexplicit-relocs=none, -mexplicit-relocs=base, -mexplicit-relocs=pcrel, -mexplicit-relocs, -mno-explicit-relocs — these control whether explicit relocs (like %gp_rel, bestie) get used or not. The default value depends on which version of GAS was around when GCC itself got built.

The "base" explicit-relocs support landed in GAS in 2001. The "pcrel" explicit-relocs support landed in GAS in 2014, and it's the one that supports %pcrel_hi and %pcrel_lo.

• -mcheck-zero-division, -mno-check-zero-division — trap (or don't trap) on integer division by zero. Periodt.

The default is -mcheck-zero-division.

• -mdivide-traps, -mdivide-breaks — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps gives you smaller code, but it's only supported on MIPS II and later. Also — and this is the annoying part 😭 — some versions of the Linux kernel have a bug that stops trap from generating the proper signal (SIGFPE). Use -mdivide-traps to allow conditional traps on architectures that support them, and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division.

• -mload-store-pairs, -mno-load-store-pairs — enable (disable) an optimization that pairs consecutive load or store instructions so load/store bonding can happen. It's enabled by default, but it only does anything when the selected architecture is known to support bonding. iykyk 🦋
• -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 by trap & emulate. So -mstrict-align may be needed by kernel. The options -munaligned-access and -mno-unaligned-access are obsoleted, and only hang around for backward compatibility.
• -mmemcpy, -mno-memcpy — force (or don't force) the use of memcpy for non-trivial block moves. The default is -mno-memcpy, which lets GCC inline most constant-sized copies. slay 💖
• -mlong-calls, -mno-long-calls — disable (don't disable) use of the jal instruction. Calling functions using jal is more efficient, but it requires the caller and callee to be in the same 256 megabyte segment. So like... it's a vibe tradeoff.

This option has no effect on abicalls code. The default is -mno-long-calls.

• -mmad, -mno-mad — enable (disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA.
• -mimadd, -mno-imadd — enable (disable) use of the madd and msub integer instructions. The default is -mimadd on architectures that support madd and msub, except for the 74k architecture, where it was found to generate slower code. not the 74k catching strays 😭
• -mfused-madd, -mno-fused-madd — enable (disable) use of the floating-point multiply-accumulate instructions, when they're available. The default is -mfused-madd.

On the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors, the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

• -nocpp — tell the MIPS assembler to NOT run its preprocessor over user assembler files (the ones with a .s suffix) when assembling them. leave those files alone bestie 🙈
• -mfix-24k, -mno-fix-24k — work around the 24K E48 errata (that's lost data on stores during refill, iconic behavior in the worst way 💀). The workarounds are implemented by the assembler rather than by GCC.
• -mfix-r4000, -mno-fix-r4000 — work around certain R4000 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
  • A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
  • An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
• -mfix-r4400, -mno-fix-r4400 — work around certain R4400 CPU errata:
  • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
• -mfix-r10000, -mno-fix-r10000 — work around certain R10000 errata:
  • ll/sc sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 😳

This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.

• -mfix-r5900, -mno-fix-r5900 — do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a nop instruction there instead. The short loop bug, under certain conditions, causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.
• -mfix-rm7000, -mno-fix-rm7000 — work around the RM7000 dmult/dmultu errata. The workarounds are implemented by the assembler rather than by GCC.
• -mfix-vr4120, -mno-fix-vr4120 — work around certain VR4120 errata:
  • dmultu does not always produce the correct result.
  • div and ddiv do not always produce the correct result if one of the operands is negative.

The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are only provided by the mips64vr*-elf configurations.

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.

• -mfix-vr4130 — work around the VR4130 mflo/mfhi errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead. iconic workaround energy 👑
• -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. 💀

This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. 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 a full description, 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 ensure 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 — specifies the function to call to flush the I and D caches, or to not call any such function at all. If it is called, the function must take the same arguments as the common _flush_func, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly it's either _flush_func or __cpu_flush.
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune setting. fr fr, heuristics are like that 🌸
• -mbranch-likely, -mno-branch-likely — turns Branch Likely instructions on or off, no matter what the selected architecture's default says 💅. Normally, Branch Likely instructions may get generated if your architecture supports them. The one exception: MIPS32 and MIPS64 architectures, plus any processor implementing them — for those, Branch Likely instructions do not get generated by default, because MIPS32 and MIPS64 straight up deprecate using them. Rude of them but okay.
• -mcompact-branches=never, -mcompact-branches=optimal, -mcompact-branches=always — these three control which form of branches get generated. Default is -mcompact-branches=optimal ✨.
  • -mcompact-branches=never — compact branch instructions will literally never be generated, periodt.
  • -mcompact-branches=always — a compact branch instruction gets generated if one is available for MIPS Release 6 onwards. If a compact branch instruction isn't available (or you're pre-R6), a delay slot form of the branch gets used instead.
  • If you use it for MIPS16/microMIPS targets, it's just ignored right now. The behaviour for MIPS16/microMIPS may change in the future, since they do have some compact branch instructions. 👀
  • -mcompact-branches=optimal — causes 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 gets chosen if one is available.
• -mfp-exceptions, -mno-fp-exceptions — says whether FP exceptions are enabled. This affects how FP instructions get scheduled for some processors. Default: FP exceptions are enabled 🫶.
  • For example, 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 (or disable) generation of synci instructions on architectures that support it. The synci instructions (if enabled) get 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 😭. So like, be careful bestie.
• -mrelax-pic-calls, -mno-relax-pic-calls — try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call 🔥.
  • -mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler.
• -mmcount-ra-address, -mno-mcount-ra-address — emit (or don't 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.
  • Default is -mno-mcount-ra-address.
• -mframe-header-opt, -mno-frame-header-opt — enable (or 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's unused 🎀.
  • This optimization is off by default at all optimization levels.
• -mlxc1-sxc1, -mno-lxc1-sxc1 — when applicable, enable (or disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default ✨.
• -mmadd4, -mno-madd4 — when applicable, enable (or 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, bestie:

• -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 (or don't 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 (or don't 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 (or don't use) the probable-branch instructions, when static branch prediction indicates a probable branch.
• -mbase-addresses, -mno-base-addresses — generate (or don't 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 (or don't 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 — do not generate code using features specific to the AM33 processor. This is the default.
• -mam33-2 — generate code using features specific to the AM33/2.0 processor.
• -mam34 — generate code using features specific to the AM34 processor.
• -mtune=cpu-type — use the timing characteristics of the indicated CPU type when scheduling instructions. This does **not change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
• -mreturn-pointer-on-d0 — when generating a function that returns a pointer, return the pointer in both a0 and d0. Otherwise the pointer is returned only in a0, and attempts to call such functions without a prototype result in errors 👀. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it.
• -mno-crt0 — do not link in the C run-time initialization object file.
• -mrelax — indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.
  • > This option makes symbolic debugging impossible 💀. Not a vibe if you were planning to debug, fr fr.
• -mliw — allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __LIW__.
• -mno-liw — do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro __NO_LIW__.
• -msetlb — allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __SETLB__.
• -mno-setlb — do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro __NO_SETLB__.

---

🌈 Moxie Options 🌈

• -meb — generate big-endian code. This is the default for moxie-*-* configurations.
• -mel — generate little-endian code.
• -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.

---

💖 MSP430 Options 💖

These options are defined for the MSP430:

• -masm-hex — force assembly output to always use hex constants. Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes ✨ (aesthetic purposes, iconic).
• -mmcu= — select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with __. This in turn is used by the msp430.h header file to select an MCU-specific supplementary header file.

The option also sets which ISA to use. If the MCU name is one that's only known to support the 430 ISA, then that's the one that gets picked — otherwise it's the 430X ISA. And you can also just pass the generic MCU name msp430 to select the 430 ISA. Same energy: the generic msp430x MCU name selects the 430X ISA. 💅

On top of that, an MCU-specific linker script gets added to the linker command line. The script's name is just the MCU name with .ld stuck on the end. So like, 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 support for the different MCUs is hard-coded into GCC. But bestie, plot twist: an external devices.csv file can be used to extend device support beyond the ones that are hard-coded. ✨

GCC searches for the devices.csv file using the following methods in this precedence order — where the first method takes precedence over the second, which takes precedence over the third. 👑

• Include path specified with -I and -L — devices.csv will be 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 value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path of the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is found, this directory will also be registered as an include path and linker library path. So header files and linker scripts in this directory can be used without manually specifying -I and -L on the command line. 🫶
• The msp430-elf{,bare}/include/devices directory — finally, GCC will examine msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if you created it and copied devices.csv there, then the MCU data will be read. As above, this directory will also be 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 — this enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default. 🦋
• -mcpu= — specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is deprecated, so like… use the -mmcu= option to select the ISA instead. ⚠️
• -msim — link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the -mmcu= option.
• -mlarge — use large-model addressing (20-bit pointers, 20-bit "size_t").
• -msmall — use small-model addressing (16-bit pointers, 16-bit "size_t").
• -mrelax — this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that can't be done 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. You can also give a value of auto, which 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. But when compiling at -O3 or higher, the hardware multiplier is invoked inline. That makes for bigger, but faster code. 🚀

The hardware multiply routines disable interrupts while running and restore the previous interrupt state when they finish. So they're safe to use inside interrupt handlers as well as in normal code. 🫶

• -minrt — enable the use of a minimum runtime environment — no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.
• -mtiny-printf — enable reduced code size "printf" and "puts" library functions. The tiny implementations of these functions are not reentrant, so they must be used with caution in multi-threaded applications. 💀

Support for streams has been removed and the string to be printed will always be sent to stdout via the "write" syscall. The string is not buffered before it is sent to write.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.

• -mmax-inline-shift= — this option takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an mspabi helper function is used instead. The default value is 4. 🎀

This only affects cases where a shift by multiple positions can't be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead.

• -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= — this option passes on a request to the assembler to enable the fixes for the named silicon errata.
• -msilicon-errata-warn= — this option 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 isn't 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 — do not generate v3 push25/pop25 instructions.
• -m16-bit — generate 16-bit instructions.
• -mno-16-bit — do not generate 16-bit instructions.
• -misr-vector-size=num — specify the size of each interrupt vector, which must be 4 or 16.
• -mcache-block-size=num — specify the size of each cache block, which must be a power of 2 between 4 and 512.
• -march=arch — specify the name of the target architecture.
• -mcmodel=code-model — set the code model to one of:
  • small — all the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.
  • medium — the data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. The text segment should still be within 16MB addressing space.
  • large — all the text and data segments can be within 4GB addressing space.
• -mctor-dtor — enable constructor/destructor feature.
• -mrelax — guide linker to relax instructions.

---

🌸 Nios II Options 🌸

These are the options defined for the Altera Nios II processor:

• -G num — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. 🎀
• -mgpopt=option, -mgpopt, -mno-gpopt — generate (do not generate) GP-relative accesses. The following option names are recognized:
  • none — do not generate GP-relative accesses.
  • local — generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. Also use GP-relative addressing for objects that have been explicitly placed in a small data section via a "section" attribute.
  • global — as for local, but also generate GP-relative accesses for small data objects that are external, weak, or common. If you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same -G setting. ⚠️
  • data — generate GP-relative accesses for all data objects in the program. If you use this option, the entire data and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer. 💀
• all — generate GP-relative addresses for function pointers and data pointers. If you pick this one, lowkey the entire text, data, and BSS segments of your program have to fit inside 64K of memory, and you must use an appropriate linker script to allocate them within the addressable range of the global pointer. No pressure bestie 💅

-mgpopt is literally the same as -mgpopt=local, and -mno-gpopt is the same as -mgpopt=none.

The default is -mgpopt, except when -fpic or -fPIC is specified to generate position-independent code. Heads up though: the Nios II ABI does not permit GP-relative accesses from shared libraries. That's just the rules, periodt.

You may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections. In this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section. It's giving "the address space said no" 😭

---

• -mgprel-sec=regexp — this option specifies additional section names that can be accessed via GP-relative addressing. It is most useful in conjunction with "section" attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression. Obsessed with the precision tbh ✨

This option does not affect the behavior of the -G option, and the specified sections are in addition to the standard ".sdata" and ".sbss" small-data sections that are recognized by -mgpopt.

• -mr0rel-sec=regexp — this option specifies names of sections that can be accessed via a 16-bit offset from "r0"; that is, in the low 32K or high 32K of the 32-bit address space. It is most useful in conjunction with "section" attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression. 🎀

In contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory.

• -mel
• -meb — generate little-endian (default) or big-endian (experimental) code, respectively. Pick your endianness, bestie 🌸
• -march=arch — this specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: r1, r2. 👑

The preprocessor macro __nios2_arch__ is available to programs, with value 1 or 2, indicating the targeted ISA level.

• -mbypass-cache
• -mno-bypass-cache — force all load and store instructions to always bypass cache by using I/O variants of the instructions. The default is not to bypass the cache. She's skipping the line entirely 💀
• -mno-cache-volatile
• -mcache-volatile — volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.
• -mno-fast-sw-div
• -mfast-sw-div — do not use table-based fast divide for small numbers. The default is to use the fast divide at -O3 and above. Fast math stays winning 🔥
• -mno-hw-mul
• -mhw-mul
• -mno-hw-mulx
• -mhw-mulx
• -mno-hw-div
• -mhw-div — enable or disable emitting "mul", "mulx" and "div" family of instructions by the compiler. The default is to emit "mul" and not emit "div" and "mulx".
• -mbmx
• -mno-bmx
• -mcdx
• -mno-cdx — enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. Enabling these instructions also requires -march=r2. Since these instructions are optional extensions to the R2 architecture, the default is not to emit them. Optional extras, we respect it 💅
• -mcustom-insn=N
• -mno-custom-insn — each -mcustom-insn=N option enables use of a custom instruction with encoding N when generating code that uses insn. For example, -mcustom-fadds=253 generates custom instruction 253 for single-precision floating-point add operations instead of the default behavior of using a library call. Custom hardware era ✨

The following values of insn are supported. Except as otherwise noted, floating-point operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions.

Single-precision floating point:

• fadds, fsubs, fdivs, fmuls — binary arithmetic operations.
• fnegs — unary negation.
• fabss — unary absolute value.
• fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes — comparison operations.
• fmins, fmaxs — floating-point minimum and maximum. These instructions are only generated if -ffinite-math-only is specified.
• fsqrts — unary square root operation.
• fcoss, fsins, ftans, fatans, fexps, flogs — floating-point trigonometric and exponential functions. These instructions are only generated if -funsafe-math-optimizations is also specified.

Double-precision floating point:

• faddd, fsubd, fdivd, fmuld — binary arithmetic operations.
• fnegd — unary negation.
• fabsd — unary absolute value.
• fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned — comparison operations.
• fmind, fmaxd — double-precision minimum and maximum. These instructions are only generated if -ffinite-math-only is specified.
• fsqrtd — unary square root operation.
• fcosd, fsind, ftand, fatand, fexpd, flogd — double-precision trigonometric and exponential functions. These instructions are only generated if -funsafe-math-optimizations is also specified.

Conversions:

• fextsd — conversion from single precision to double precision.
• ftruncds — conversion from double precision to single precision.
• fixsi, fixsu, fixdi, fixdu — conversion from floating point to signed or unsigned integer types, with truncation towards zero.
• round — conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. This corresponds to the __builtin_lroundf function when -fno-math-errno is used.
• floatis, floatus, floatid, floatud — conversion from signed or unsigned integer types to floating-point types.

In addition, all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions. Custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. The other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used. It's giving very precise register choreography 🦋

• fwrx — write src1 into the least significant half of X and src2 into the most significant half of X.
• fwry — write src1 into Y.
• frdxhi, frdxlo — read the most or least (respectively) significant half of X and store it in dest.
• frdy — read the value of Y and store it into dest.

Note that you can gain more local control over generation of Nios II custom instructions by using the target("custom-insn=N") and target("no-custom-insn") function attributes or pragmas. Local control, we love to see it 🫶

• -mcustom-fpu-cfg=name — this option enables a predefined, named set of custom instruction encodings (see -mcustom-insn above). Currently, the following sets are defined:
  • -mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant
  • -mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant
  • -mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243 -mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246 -mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant
  • -mcustom-fpu-cfg=fph2 is equivalent to: -mcustom-fabss=224 -mcustom-fnegs=225 -mcustom-fcmpnes=226 -mcustom-fcmpeqs=227 -mcustom-fcmpges=228 -mcustom-fcmpgts=229 -mcustom-fcmples=230 -mcustom-fcmplts=231 -mcustom-fmaxs=232 -mcustom-fmins=233 -mcustom-round=248 -mcustom-fixsi=249 -mcustom-floatis=250 -mcustom-fsqrts=251 -mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255

Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line. The individual one always wins, no cap 👑

Note that you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg=name") function attribute or pragma.

The name fph2 is an abbreviation for Nios II Floating Point Hardware 2 Component. Please note that the custom instructions enabled by -mcustom-fmins=233 and -mcustom-fmaxs=234 are only generated if -ffinite-math-only is specified. The custom instruction enabled by -mcustom-round=248 is only generated if -fno-math-errno is specified. In contrast to the other configurations, -fsingle-precision-constant is not set. Little exceptions, gotta respect them 😭

---

These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

• -mhal — link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with -msys-crt0= to specify the location of the alternate startup code provided by the HAL BSP.
• -msmallc — link with a limited version of the C library, -lsmallc, rather than Newlib.
• -msys-crt0=startfile — startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with -mhal. 🌈
• -msys-lib=systemlib — systemlib is the name of the library that hands over the low-level system calls the C library needs, like read and write. Bestie, you'd normally reach for this when you're linking against a library some HAL BSP gave you. 🫶

---

🎮 Nvidia PTX Options

These ones are for Nvidia PTX, periodt:

• -m64 — lowkey ignored, but kept around for backward compatibility vibes. Only the 64-bit ABI is supported, so like... there's nothing else to pick. 💀
• -march=architecture-string — generate code for the PTX ISA target architecture you name (e.g. sm_35). The valid architecture strings are sm_30, sm_35, sm_53, sm_70, sm_75 and sm_80. What you get by default depends on how the compiler was configured, see --with-arch.

This option sets the value of the preprocessor macro __PTX_SM__; for instance, for sm_35 it has the value 350. ✨

• -misa=architecture-string — literally just an alias of -march=. Same energy, different name. 🦋
• -march-map=architecture-string — picks the closest available -march= value that is not more capable. For instance, for -march-map=sm_50 select -march=sm_35, and for -march-map=sm_53 select -march=sm_53.
• -mptx=version-string — generate code for the PTX ISA version you name (e.g. 7.0). Valid version strings include 3.1, 6.0, 6.3, and 7.0. The default PTX ISA version is 6.0, unless a higher version is required for the PTX ISA target architecture you specified via -march=. 👑

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.

• -mmainkernel — link in code for a __main kernel. This is for stand-alone execution instead of offloading execution.
• -moptimize — apply partitioned execution optimizations. This is the default whenever any level of optimization is selected. Slay. 🔥
• -msoft-stack — generate code that does not use .local memory 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 the underlying storage, it makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is what's 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 — switch 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 the current lane index to compute the master lane index. Main character energy fr fr. 💅
• -mgomp — generate code for use in OpenMP offloading: enables the -msoft-stack and -muniform-simt options, and selects the corresponding multilib variant. 🎀

---

🌿 OpenRISC Options

These ones are defined for OpenRISC:

• -mboard=name — configure a board specific runtime. This will be 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. 💀
• -msoft-div, -mhard-div — select software or hardware divide (l.div, l.divu) instructions. The default is hardware divide.
• -msoft-mul, -mhard-mul — select software or hardware multiply (l.mul, l.muli) instructions. The default is hardware multiply.
• -msoft-float, -mhard-float — select software or hardware for floating point operations. The default is software.
• -mdouble-float — when -mhard-float is selected, enables generation of double-precision floating point instructions. By default, functions from libgcc are used to perform double-precision floating point operations. ✨
• -munordered-float — when -mhard-float is selected, enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default, functions from libgcc are used to perform unordered floating point compare and set flag operations.
• -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.) 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. 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. Implies -msoft-float -mno-split.
• -mint16, -mno-int32 — use 16-bit int. This is the default.
• -mint32, -mno-int16 — use 32-bit int.
• -msplit — target has split instruction and data space. 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. 🧚

---

⚡ 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. 🫶

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.

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.

• -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.

Optional Return Value Pointer

Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer.

The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti. 😭

Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.

---

🌊 RISC-V Options

These command-line options are defined for RISC-V targets:

• -mbranch-cost=n — set the cost of branches to roughly n instructions.
• -mplt, -mno-plt — when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is -mplt. 💜
• -mabi=ABI-string — this one's the ✨calling convention✨ setting, and it covers both integers and floats. ABI-string has two parts: the size of integer types, and the registers used for floating-point types. So for example, -march=rv64ifd -mabi=lp64d means long and pointers are 64-bit (which implicitly defines int to be 32-bit, no cap), and floating-point values up to 64 bits wide get passed in F registers. 💅

Now contrast that with -march=rv64ifd -mabi=lp64f — this still lets the compiler generate code that uses the F and D extensions, but it only allows floating-point values up to 32 bits long to be passed in registers. And then -march=rv64ifd -mabi=lp64? In that one, no floating-point arguments will be passed in registers. literally none. 💀

The default for this argument is system dependent, so if you want a specific calling convention, bestie, you 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 — so, heads up. 🫶

• -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. easy. ✨
• -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 — tells GCC which 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 the given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case, okay? Examples include rv64i, rv32g, rv32e, and rv32imaf. Also, there's a special value help (-march=help) that's 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 — and bestie, get comfy, there's a whole ✨menu✨:

• 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 — supported version 1.0 — Vector NIST Algorithm Suite with GCM extension, zvkng will expand to zvkn and zvkg.
• zvks — supported version 1.0 — Vector ShangMi algorithm suite extension, zvks will expand to zvksed, zvksh, zvkb and zvkt.
• zvksc — supported version 1.0 — Vector ShangMi algorithm suite with carryless multiplication extension, zvksc will expand to zvks and zvbc.
• zvksg — supported version 1.0 — Vector ShangMi algorithm suite with GCM extension, zvksg will expand to zvks and zvkg.
• zvkt — supported version 1.0 — Vector data independent execution latency extension.
• zfh — supported version 1.0 — Half-precision floating-point extension.
• zfhmin — supported version 1.0 — Minimal half-precision floating-point extension.
• zvfh — supported version 1.0 — Vector half-precision floating-point extension.
• zvfhmin — supported version 1.0 — Vector minimal half-precision floating-point extension.
• zvfbfmin — supported version 1.0 — Vector BF16 converts extension.
• zfa — supported version 1.0 — Additional floating-point extension.
• zmmul — supported version 1.0 — Integer multiplication extension.
• zca — supported version 1.0 — Integer compressed instruction extension.
• zcf — supported version 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 when -march= isn't given? it just borrows whatever you set in -mcpu. easy 💫

and if neither -march nor -mcpu= is specified?? the default is system dependent bestie 😭 so if you actually want specific architecture extensions, pls just say so explicitly. don't leave it to the vibes.

---

• -mcpu=processor-string — use the architecture of, and optimize the output for, the processor you name via its particular CPU name. the allowed values 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.

heads up: -mcpu does not override -march or -mtune. it's giving "i'm just here to help" energy 🙈

• -mtune=processor-string — optimize the output for the processor you name, given by microarchitecture or by particular CPU name. allowed values: rocket, sifive-3-series, sifive-5-series, sifive-7-series, thead-c906, size, sifive-p400-series, sifive-p600-series, and literally every valid option for -mcpu=.

if -mtune= isn't specified, it takes the setting from -mcpu, and if both are missing the default is rocket. ⭐

the size choice is not meant for end-users, ok? it's what gets used when -Os is specified. it overrides the instruction cost info that -mtune= provides, but it does not override the pipeline info. it's there to shrink code size while still keeping good performance. 💅

• -mpreferred-stack-boundary=num — tries to keep the stack boundary aligned to a 2 raised to num byte boundary. if you don't pass -mpreferred-stack-boundary, the default is 4 (that's 16 bytes, aka 128-bits).

⚠️ warning: if you use this switch you must build all modules with the same value, libraries included. yes, that means the system libraries and startup modules too. no cap, don't mix and match.

• -msmall-data-limit=n — puts 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. fast > smol here, periodt.
• -mmovcc
• -mno-movcc — do, or don't, produce branchless conditional-move code sequences even on targets that don't have specific instructions for conditional operations. if it's enabled, sequences of ALU operations get produced using base integer ISA instructions where that's profitable. 🧚
• -minline-atomics
• -mno-inline-atomics — do, or don't, use smaller but slower subword atomic emulation code that calls libatomic functions. the default is fast inline subword atomics that don't need libatomic.
• -minline-strlen
• -mno-inline-strlen — do, or don't, attempt to inline strlen calls if possible. inlining only happens 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 don't, attempt to inline strcmp calls if possible. inlining only happens 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 don't, attempt to inline strncmp calls if possible. inlining only happens 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. 💫

• -mshorten-memrefs
• -mno-shorten-memrefs — do, or don't, try to make more use of compressed load/store instructions by swapping a load/store of base register + large offset for a new load/store of new base + small offset. if that new base gets stored in a compressed register, then the new load/store can be compressed. currently it only targets 32-bit integer load/stores. 🎀
• -mstrict-align
• -mno-strict-align — don't, or do, generate unaligned memory accesses. the default depends on whether the processor you're optimizing for supports fast unaligned access or not. vibe check on the hardware, basically.
• -mcmodel=medlow — generate code for the medium-low code model. the program and its statically defined symbols must lie inside a single 2 GiB address range and must sit between absolute addresses -2 GiB and +2 GiB. programs can be statically or dynamically linked. this is the 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 the medium-any code model generates is position-independent, but it's not guaranteed to function correctly when linked into position-independent executables or libraries. noted, bestie. 😭

• -mexplicit-relocs
• -mno-exlicit-relocs — use, or don't use, assembler relocation operators when dealing with symbolic addresses. the alternative is using assembler macros instead, which may limit optimization. (yes that spelling is exactly what it says, don't come for me 💀)
• -mrelax
• -mno-relax — take advantage of linker relaxations to cut down the number of instructions needed to materialize symbol addresses, or don't. the default is to take advantage of linker relaxations. slay. ✨
• -mriscv-attribute
• -mno-riscv-attribute — emit (or don't emit) the RISC-V attribute to record extra info into ELF objects. this feature needs at least binutils 2.32. 🦋
• -mcsr-check
• -mno-csr-check — enables or disables the CSR checking.
• -malign-data=type — controls how GCC aligns variables and constants of array, structure, or union types. supported values for type are xlen, which uses x register width as the alignment value, and natural, which uses natural alignment. xlen is the default. 💜
• -mbig-endian — generate big-endian code. this is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-* target.
• -mlittle-endian — generate little-endian code. this is the default when GCC is configured for a riscv64-*-* or riscv32-*-* target — but not a riscv64be-*-* or riscv32be-*-* target. read it twice, it's fine. 🙈
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. supported locations are global for a global canary, or tls for a per-thread canary in the TLS block.

with that tls choice, the -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset options additionally say which register to use as the base register for reading the canary, and from what offset off that base register. there's no default register or offset, because this is entirely for use inside the Linux kernel. yep. 👑

• -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 one is the default. ⭐

---

✨ RL78 Options ✨

• -msim — links in additional target libraries so things can actually operate inside a simulator. 🌈
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — bestie, this is where you tell GCC what kind of hardware multiply-and-divide support you want it to lean on. The simplest one is none, which just uses software for both multiplication and division — and yes, that's the default. The g13 value is for the hardware multiply/divide peripheral that lives on the RL78/G13 (S2 core) targets. The g14 value selects the multiplication and division instructions that the RL78/G14 (S3 core) parts support. And then the value rl78 is an alias for g14, while the value mg10 is an alias for none.

Also, a C preprocessor macro gets defined based on whatever you set here. The possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__.

• -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — this one says which RL78 core you're targeting. Default is the G14 core, which is also known as an S3 core, or honestly just "RL78". The G13, aka S2 core, has no multiply or divide instructions — it uses a hardware peripheral for those operations instead. And the G10, aka S1 core, doesn't have register banks at all, so it uses a different calling convention. 💅

Now here's the plot twist: if you set this option, it also picks the type of hardware multiply support to use — unless you override it with an explicit -mmul=none on the command line. So specifying -mcpu=g13 enables the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables hardware multiplications entirely.

Note: even though the RL78/G14 core is the default target, actually writing -mcpu=g14 or -mcpu=rl78 on the command line does change how the toolchain behaves, because it also turns on G14 hardware multiply support. If you don't pass those options, software multiplication routines get used even though the code targets the RL78 core. Why? Backwards compatibility with older toolchains that didn't have hardware multiply and divide support. It's giving legacy energy. 🫶

And yep, a C preprocessor macro gets defined here too, based on the setting. Possible values: __RL78_G10__, __RL78_G13__ or __RL78_G14__.

• -mg10, -mg13, -mg14, -mrl78 — these are just aliases for the matching -mcpu= option. They exist purely for backwards compatibility. ✨
• -mallregs — lets the compiler use all of the available registers. By default, registers r24..r31 are reserved for use in interrupt handlers. Flip this on and those registers become fair game in ordinary functions too.
• -m64bit-doubles, -m32bit-doubles — makes the double data type either 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 — says that interrupt handler functions should preserve the MDUC registers. You only need this if normal code might actually use the MDUC registers — like, say, because it does multiplication and division operations. The default is to ignore the MDUC registers, because that makes the interrupt handlers faster. Heads up though: you need to pass the target option -mg13 for this to work, since the feature only exists on the G13 target (S2 core). Also, the MDUC registers only get 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, bestie:

• -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 to specify which instructions are available on the processor you're using. The default value of these options is determined when GCC is configured. Specifying -mcpu=cpu_type overrides the specification of these options. And fr, we recommend you use -mcpu=cpu_type instead of the options listed above. 👑

So: specifying -mpowerpc-gpopt lets GCC use the optional PowerPC architecture instructions in the General Purpose group, floating-point square root included. Specifying -mpowerpc-gfxopt lets GCC use the optional PowerPC architecture instructions in the Graphics group, floating-point select included.

The -mmfcrf option lets GCC 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 lets GCC 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 lets GCC generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. The -mfprnd option lets GCC 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 lets GCC generate the compare-bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. And the -mhard-dfp option lets GCC generate the decimal floating-point instructions implemented on some POWER processors.

The -mpowerpc64 option lets GCC generate the additional 64-bit instructions found in the full PowerPC64 architecture, and treats GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.

• -mcpu=cpu_type — sets the architecture type, register usage, and instruction scheduling parameters for the 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 the cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. Heads up: -mcpu=native has no effect if GCC doesn't 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. Lowkey dramatic but true.

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 want 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 doesn't 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 — sets the instruction scheduling parameters for machine type cpu_type, but does 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 you specify both, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune.
• -mcmodel=small — generate PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux.
• -mcmodel=large — generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space.
• -maltivec, -mno-altivec — generate code that uses (or doesn't use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

When -maltivec is used, the element order for AltiVec intrinsics such as vec_splat, vec_extract, and vec_insert matches array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. ✨

• -mvrsave, -mno-vrsave — generate VRSAVE instructions when generating AltiVec code.
• -msecure-plt — generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is 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. This is a PowerPC 32-bit SYSV ABI option.
• -misel, -mno-isel — this switch enables or disables the generation of ISEL instructions.
• -mvsx, -mno-vsx — generate code that uses (or doesn't use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set. 🦋
• -mcrypto, -mno-crypto — turns on (or off) the built-in functions that give you direct access to the cryptographic instructions that got added back in version 2.07 of the PowerPC ISA. 🔐 slay.
• -mhtm, -mno-htm — enables (disables) the built-in functions that let you reach the Hardware Transactional Memory (HTM) instructions directly, also from version 2.07 of the PowerPC ISA. 🧠✨
• -mpower8-fusion, -mno-power8-fusion — makes GCC generate code that keeps (or doesn't keep) some integer operations adjacent so the instructions can be fused together on power8 and later processors. 🫶
• -mquad-memory, -mno-quad-memory — generate code that uses (or doesn't use) the non-atomic quad word memory instructions. Heads up bestie, -mquad-memory requires 64-bit mode. 💅
• -mquad-memory-atomic, -mno-quad-memory-atomic — same energy but for the atomic quad word memory instructions. And yeah, -mquad-memory-atomic also requires 64-bit mode. no cap.
• -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. 🫶
  • 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 don't specify to generate ISA 3.0 instructions or you're 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 four mess with the generation of the TOC (Table Of Contents), which gets created for every executable file. 📚

-mfull-toc is the 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. But only 16,384 entries are available in the TOC. 😭

If you get a linker error message saying you've overflowed the available TOC space, you can reduce the TOC space used with -mno-fp-in-toc and -mno-sum-in-toc. -mno-fp-in-toc stops GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code that calculates the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these. Each one causes GCC to produce very slightly slower and larger code, at the expense of conserving TOC space.

If you still run out of TOC space even with both of those on, specify -mminimal-toc instead. This one makes GCC create only one TOC entry for every file. When you specify it, GCC produces code that is slower and larger but uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 🐌

• -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 the AIX-compatible ABI. It passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. It does not assume that the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double. And it uses 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). Links an application written to use message passing with special startup code so the application can 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 -mpe and -pthread are incompatible. 💀
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, -malign-natural overrides the ABI-defined alignment of larger types, like floating-point doubles, on their natural size-based boundary. -malign-power tells 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 (or does use) the floating-point register set. Software floating-point emulation is provided if you use -msoft-float, and pass the option to GCC when linking. 💅
• -mmultiple, -mno-multiple — generate code that uses (or doesn't use) the load multiple word instructions and the store multiple word instructions. These are generated by default on POWER systems, and not generated on PowerPC systems.

Bestie, 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 (or doesn't 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's a small window between the time the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. ⚠️

• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (or not avoid) the use of indexed load or store instructions. These 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 — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These 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. 🌀
• -mmulhw, -mno-mulhw — generate code that uses (or doesn't use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors. 🧮
• -mdlmzb, -mno-dlmzb — generate code that uses (or doesn't use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors. 🔍
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (or do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field. 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. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. 🎀
• -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, do not (or 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, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary. 🦋
• -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a .fixup section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. 🫶
• -mno-toc, -mtoc — on System V.4 and embedded PowerPC systems, do not (-mno-toc) / do (-mtoc) assume that register 2 holds a pointer to a global area pointing at the addresses your program uses. tiny register, big responsibilities ✨
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. -mlittle-endian is literally the same thing as -mlittle, no cap. 🦋
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. And -mbig-endian is the same as -mbig. same energy, different spelling 💅
• -mdynamic-no-pic — on Darwin / macOS systems, compile code so it's not relocatable, but its external references are relocatable. The resulting code is giving "applications only" — it is not suitable for shared libraries. 🚫📚
• -msingle-pic-base — treat the register used for PIC addressing as read-only, instead of loading it fresh in the prologue of every single function. The runtime system is the one responsible for initializing this register with an appropriate value before execution begins. not your job bestie, that's the runtime's job 🫶
• -mprioritize-restricted-insns=priority — this one controls the priority assigned to dispatch-slot restricted instructions during the second scheduling pass. The priority argument takes the value 0, 1, or 2, to assign no, highest, or second-highest priority (respectively) to dispatch-slot restricted instructions. ✨
• -msched-costly-dep=dependence_type — controls which dependences the target treats as costly during instruction scheduling. The dependence_type argument takes one of these values:
  • no — no dependence is costly. chill, nothing's expensive here
  • all — all dependences are costly. everything is expensive, we're all suffering 😭
  • 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 whose latency is greater than or equal to number is costly.
• -minsert-sched-nops=scheme — controls which NOP insertion scheme gets used during the second scheduling pass. The scheme argument takes one of these values:
  • no — don't insert NOPs. we simply do not ✋
  • 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 into 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 into 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. 👑
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options. two for one, slay
• -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. 🐧
• -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 — pick the type of traceback table. Valid values for traceback_type are full, part, and no. full, part, or nope 💅
• -maix-struct-return — return all structures in memory (as specified by the AIX ABI). every struct, straight to memory
• -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 away from IEEE extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled. > ⚠️ warning incoming, listen to it bestie.
• -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 away from IBM extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled. > ⚠️ same deal, do not ignore the warning fr fr.
• -mabi=elfv1 — change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways. 💀 > like, catastrophic icon behavior. don't.
• -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 has to 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. one bit, so much drama 🎀
• -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 the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
• -mads — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
• -myellowknife — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a. yellowknife the name is sending me 💛
• -mvxworks — on System V.4 and embedded PowerPC systems, specify that you're 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 (-meabi) / do not (-mno-eabi) 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. -meabi is on by default if you configured GCC using one of the powerpc*-*-eabi* options. 🫶
• -msdata=eabi — 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. -msdata=eabi is incompatible with the -mrelocatable option. -msdata=eabi also sets the -memb option. > ⚠️ incompatible means incompatible, don't do it.
• -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. -msdata=sysv is incompatible with the -mrelocatable option. > ⚠️ same warning, same no-no.
• -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. pick a lane, it picks for you ✨
• -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.
• -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 (like 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 (like calls to memcmp or structure compares) less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare is disabled. The default value is target-specific. ⭐
• -mblock-compare-inline-loop-limit=num — okay so this one says: go ahead and do an inline expansion using loop code for every block compare that's less than or equal to num bytes but bigger than the limit for the non-loop inline block compare expansion. If the block length isn't constant, at most num bytes get compared before memcmp is called in to handle the rest of the block. Default value? Target-specific, bestie. It's giving "ask your target". 🎀
• -mstring-compare-inline-limit=num — compare at most num string bytes with inline code. If the difference or the end of the string isn't found by the end of that inline compare, a call to strcmp or strncmp takes care of the rest of the comparison. Default is 64 bytes. ✨
• -G num — on embedded PowerPC systems, this drops global and static items that are 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 also gets passed to the linker. And listen — all modules should be compiled with the same -G num value. Consistency is the vibe. 💅
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or don't) emit register names in the assembly language output using symbolic forms. Pick your fighter. 🦋
• -mlongcall, -mno-longcall — by default this assumes that all calls are far away, so a longer and more expensive calling sequence is required. Which, yeah, is needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't be that far away. You can override this setting with the shortcall function attribute, or with #pragma longcall(0).

Some linkers can detect out-of-range calls and generate glue code on the fly. On those systems, long calls are unnecessary and they generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. The plan is to add this feature to the GNU linker for 32-bit PowerPC systems too.

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). Those 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. 🧚

On Mach-O (Darwin) systems, this option directs the compiler to emit the glue for every direct call, and the Darwin linker decides whether to use it or discard it.

In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. Future tea, iykyk. 🍓

• -mpltseq, -mno-pltseq — implement (or don't implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if that support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with --enable-secureplt. Heads up: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together. 🫶
• -mtls-markers, -mno-tls-markers — mark (or don't mark) calls to __tls_get_addr with a relocation specifying the function argument. That relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. Slay. ✨
• -mrecip, -mno-recip — this option enables the use of reciprocal estimate and reciprocal square root estimate instructions with extra Newton-Raphson steps to increase precision, instead of doing a divide or a square root and divide for floating-point arguments. You should use -ffast-math when you're using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Ngl, note this: 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. So like, fast but slightly messy. 💀
• -mrecip=opt — this controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, and any of them may be preceded by a ! to invert 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 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 are the ones handling the double-precision reciprocal square root calculations. Selective queen behavior. 👑

• -mrecip-precision, -mno-recip-precision — assume (or don't 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 don't provide an estimate that converges after three steps. No cap. 🌸
• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is mass, which says to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to acosd2, acosf4, acoshd2, acoshf4, asind2, asinf4, asinhd2, asinhf4, atan2d2, atan2f4, atand2, atanf4, atanhd2, atanhf4, cbrtd2, cbrtf4, cosd2, cosf4, coshd2, coshf4, erfcd2, erfcf4, erfd2, erff4, exp2d2, exp2f4, expd2, expf4, expm1d2, expm1f4, hypotd2, hypotf4, lgammad2, lgammaf4, log10d2, log10f4, log1pd2, log1pf4, log2d2, log2f4, logd2, logf4, powd2, powf4, sind2, sinf4, sinhd2, sinhf4, sqrtd2, sqrtf4, tand2, tanf4, tanhd2, and tanhf4 when generating code for power7. Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled. And the MASS libraries must be specified at link time. Don't forget that part bestie, fr fr. 🔥
• -mfriz, -mno-friz — generate (or don't generate) the friz instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. The friz instruction does not return the same value if the floating-point number is too large to fit in an integer. 🙈
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (or don't generate) code to load up the static chain register (r11) when calling through a pointer on AIX and 64-bit Linux systems, where a function pointer points to a 3-word descriptor giving the function address, the TOC value to be loaded in register r2, and the static chain value to be loaded in register r11. The -mpointers-to-nested-functions is on by default. And important: you cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions. So like, careful. 💜
• -msave-toc-indirect, -mno-save-toc-indirect — generate (or don't generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on 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. The -mno-save-toc-indirect option is the default. 🎀
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that's compatible with functions compiled with older versions of GCC.

The -mno-compat-align-parm option is the default.

• -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 with GNU libc version 2.4 or later).

With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. And -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. ✨

• -mpcrel, -mno-pcrel — generate (or don't generate) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. Gotta have both, no skipping. 💖
• -mprefixed, -mno-prefixed — bestie, this one's giving address mode energy 💫 generate (or, with -mno-prefixed, don't generate) addressing modes that use prefixed load and store instructions. Fair warning tho: -mprefixed is not gonna work unless you've got -mcpu=power10 (or later) turned on. No power10, no prefixing, periodt.
• -mmma, -mno-mma — generate (or don't) the MMA instructions 🧮. Same deal as above: the -mma option requires that -mcpu=power10 (or later) is enabled. It's giving "you need the vip pass to get in."
• -mrop-protect, -mno-rop-protect — generate (or don't generate) ROP protection instructions, but only when the target processor actually supports them 🙈. Heads up tho: right now this option disables the shrink-wrap optimization, which is -fshrink-wrap. So like, you're trading one thing for another, iykyk.
• -mprivileged, -mno-privileged — generate (or don't) code that's gonna run in privileged state 👑. Main character energy, literally.
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generate (or don't) unaligned vsx loads and stores, specifically for the inline expansion of memcpy and memmove 🌈.
• --param rs6000-vect-unroll-limit= — okay so the vectorizer checks in with target info to figure out whether unrolling the main vectorized loop would actually be worth it, and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. Default value? Four. Slay ✨.

---

🌸 RX Options 🌸

These command-line options are the ones defined for RX targets, bestie:

• -m64bit-doubles, -m32bit-doubles — makes the double data type either 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. And no, that's not a typo — RX floating-point hardware only works on 32-bit values, which is literally why the default is -m32bit-doubles. It's giving "the hardware said no 💀."
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) 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, tho, so the FPU hardware isn't used for doubles if you're using the -m64bit-doubles option. RIP to your 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 😭. Not me being honest about it.

• *-mcpu=name* — picks the type of RX CPU you're targeting. Right now 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 tea.

The RX200 series doesn't have a hardware floating-point unit, so -nofpu is enabled by default when you select this type. She's just built different (derogatory).

• -mbig-endian-data, -mlittle-endian-data — store data (but not code, bestie, keep your eyes on the fine print 👀) 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 the area doesn't overflow 🫶. Also, when the small data area is used, one of the RX's registers (usually r13) gets reserved for pointing to this area, so it's no longer available for the compiler to use. That could result in slower and/or larger code if variables get pushed onto the stack instead of being held in this register. It's giving "there are no free lunches."

Note: common variables (aka variables that haven't been initialized) and constants are not placed into the small data area, since they're assigned to other sections in the output executable.

The default value is zero, which disables this feature. And 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 actually 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 that's 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. Fussy little syntax, honestly.
• *-mmax-constant-size=N* — specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Even tho 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 tries 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, doesn't reserve any registers. Zero, none, nada ✨.
• -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 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, which is a benefit 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 totally possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line. Iconic, honestly.

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, since the RX only supports one such interrupt 🚨.
• -mallow-string-insns, -mno-allow-string-insns — enables or disables use of the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL SWHILE and also the RMPA instruction. These instructions may prefetch data, which is not safe to do if you're accessing an I/O register ⚠️. (See section 12.2.7 of the RX62N Group User's Manual for more information.)

The default is to allow these instructions, but it's not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use can't be disabled automatically. Instead it relies on the programmer to use the -mno-allow-string-insns option if their program accesses I/O space. You gotta do it yourself, bestie.

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. This option can be used when code size exceeds the range of BSR instructions 📦. Note that -mno-jsr does not mean "don't use JSR" — instead it means that any type of branch may be used. Read that twice, it trips people up.

Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the interrupt function attribute. This attribute indicates a function intended to process fast interrupts. GCC makes sure it only uses the registers r10, r11, r12 and/or r13, and only provided that the normal use of the corresponding registers has been restricted via the -ffixed-reg or -mint-register command-line options. So many conditions, I know, but that's the vibe 💜.

---

👑 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 (or don't use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform 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 (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform 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 — bestie, this one decides whether the address of the caller's frame gets stored as a backchain pointer into the callee's stack frame (or not, if you pick the no version). Why would you want a backchain? So that debugging tools that do not understand DWARF call frame information can still figure out what's going on. When -mno-packed-stack is in effect, that backchain pointer lives at the bottom of the stack frame. When -mpacked-stack is in effect, the backchain gets placed into the topmost word of the 96/160 byte register save area. 💅

Generally speaking, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain — cute, right? — however, if you actually want to use the backchain for debugging purposes, you usually need the whole binary built with -mbackchain. Also heads up: the combo of -mbackchain, -mpacked-stack and -mhard-float is not supported. And if you're building a linux kernel, use -msoft-float.

The default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (or don't use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose, and unused fields still eat up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area, and unused space gets reused for other purposes — which means more efficient use of the available stack space, slay. But when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.

As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code compiled with -mpacked-stack. Also, 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 use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or don't 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 doesn't have this limitation.
• -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 making 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. In addition to 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 don't 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 don't print) additional debug information when compiling. The default is to not print debug information.
• *-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14, 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 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, ngl.
• -mstack-guard=stack-guard — *-mstack-size=stack-size* — if these options are provided, the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows *downward). If the stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation. The given values have to be exact powers of 2, and *stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient, the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. The stack-guard option can only be used in conjunction with stack-size. 👑
• *-mhotpatch=pre-halfwords*,post-halfwords — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000).

If both arguments are zero, hotpatching is disabled.

This option can be overridden for individual functions with the "hotpatch" attribute.

---

✨ SH Options ✨

These -m options are defined for the SH implementations, bestie:

• -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 — codegen for the SH4-100, but locked in so that no double-precision floating-point ops ever happen. singles only bestie 💅
• -m4-200 — make code for the SH4-200. that's it, that's the whole vibe ✨
• -m4-200-nofpu — SH4-200 code, arranged so the floating-point unit does not get used at all. fp who? never heard of her 💀
• -m4-200-single — SH4-200 code that assumes the FPU is in single-precision mode by default. manifesting single precision 🎀
• -m4-200-single-only — SH4-200, but arranged so that no double-precision floating-point operations are used. no doubles allowed in this house 🚫
• -m4-300 — generate code for the SH4-300 🦋
• -m4-300-nofpu — SH4-300 code generated so the floating-point unit is not used. leaving the FPU on read 🙈
• -m4-300-single — SH4-300 code, but in such a way that no double-precision floating-point operations are used.
• -m4-300-single-only — same deal for the SH4-300: absolutely zero double-precision floating-point operations, periodt 👑
• -m4-340 — code for the SH4-340. and just so you know: no MMU, no FPU. bare minimum queen 💖
• -m4-500 — code for the SH4-500 (again, no FPU). this one also passes -isa=sh4-nofpu to the assembler, so the assembler knows what's up ✨
• -m4a-nofpu — code for the SH4al-dsp, or for a SH4a, generated in such a way that the floating-point unit is not used.
• -m4a-single-only — SH4a code, arranged so that no double-precision floating-point operations are used.
• -m4a-single — SH4a code assuming the floating-point unit is in single-precision mode by default 💅
• -m4a — generate code for the SH4a. short, sweet, iconic 🎀
• -m4al — literally the same as -m4a-nofpu, except it implicitly passes -dsp to the assembler. heads up tho: GCC doesn't generate any DSP instructions at the moment. so, yk. it's giving promise 💀
• -mb — compile code for the processor in big-endian mode.
• -ml — compile code for the processor in little-endian mode. (yes bestie, -mb and -ml, that's the whole naming scheme, we love a minimal queen 🦋)
• -mdalign — align doubles at 64-bit boundaries. ⚠️ but fr fr pay attention: this changes the calling conventions, which means some functions from the standard C library will not work unless you recompile it first with -mdalign. no cap, recompile first or suffer 😭
• -mrelax — shorten some address references at link time when possible. it does this by using the linker option -relax. tiny code, big slay ✨
• -mbigtable — use 32-bit offsets in switch tables. the default is to use 16-bit offsets, so this is the ✨upgrade✨ option.
• -mbitops — enable the use of bit manipulation instructions on the SH2A 🔥
• -mfmovd — enable the use of the instruction fmovd. and bestie, go check -mdalign for the alignment constraints, they're connected 👀
• -mrenesas — comply with the calling conventions defined by Renesas 💜
• -mno-renesas — comply with the calling conventions defined for GCC before the Renesas conventions existed. this is the default for all targets of the SH toolchain, so if you didn't touch anything, congrats, you're already here 🎀
• -mnomacsave — mark the MAC register as call-clobbered, even if -mrenesas is given. yes even then. it's not listening 🫶
• -mieee — control the IEEE compliance of floating-point comparisons. this affects how it handles cases where the result of a comparison is unordered.
• -mno-ieee — the other half of that pair. 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. and both implicit settings can be overridden by explicitly specifying either -mieee or -mno-ieee. so you always get the final say 💅
• -minline-ic_invalidate — inline code to invalidate instruction cache entries after setting up nested function trampolines. okay so, the deets: this option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) doesn't allow the use of the icbi instruction. if the selected code generation option doesn't allow icbi and -musermode is not in effect, then the inlined code manipulates the instruction cache address array directly with an associative write. ⚠️ and that not only requires privileged mode at run time, it also fails if the cache line had been mapped via the TLB and has become unmapped. messy, be careful 😭
• -misize — dump instruction size and location in the assembly code. lil receipt for every instruction 🧾✨
• -mpadstruct — this option is deprecated. it pads structures to multiple of 4 bytes, which is incompatible with the SH ABI. old and problematic, we don't stan 🙈
• -matomic-model=model — sets the model of atomic operations, plus additional parameters, as a comma separated list. for details on the atomic built-in functions see __atomic Builtins. the supported models and parameters are:
  • 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. these generated sequences require extra support from the interrupt/exception handling code of the system, and they're only suitable for SH3* and SH4* single-core systems. this 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. it's a variation of the gUSA sequences that can also be used on SH1* and SH2* targets. same deal: needs additional support from the interrupt/exception handling code of the system, and only suitable for single-core systems. when you use 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. the good news: 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. and for this model, additional support from the interrupt/exception handling code of the system is not required 🔥
  • 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 value you give must be an integer multiple of four and in the range 0-1020. math check bestie, 0-1020 and divisible by 4 📐✨
  • strict — this parameter prevents mixed usage of multiple atomic models, even if they're compatible, and makes the compiler generate atomic sequences of the specified model only. one model only, monogamy 💅
• -mtas — generate the tas.b opcode for __atomic_test_and_set. ⚠️ but notice: depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by the tas.b instruction. and on multi-core SH4A processors the tas.b instruction must be used with caution since it can result in data corruption for certain cache configurations. no cap, be careful with this one 😭
• -mprefergot — when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. GOT > PLT, apparently ✨
• -musermode — don't 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. this is the default when the target is sh*-*-linux*. and if the target is SH1* or SH2*, -musermode has no effect, since there is no user mode. nothing to leave, bestie 🦋
• -mno-usermode — the flip side: allow the compiler to generate privileged mode code. pick your fighter 💅
• -multcost=number — set the cost to assume for a multiply insn. multiply girlies, set your price 🎀
• -mdiv=strategy — set the division strategy to be 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 perform the operation. division by zero calculates an unspecified result and does not trap. this is the default except for SH4, SH2A and SHcompact.
  • call-fp — calls a library function that performs the operation in double precision floating point. division by zero causes a floating-point exception. this is the default for SHcompact with FPU. specifying this for targets that do not have a double precision FPU defaults to call-div1.
  • call-table — calls a library function that uses a lookup table for small divisors and the div1 instruction with case distinction for larger divisors. division by zero calculates an unspecified result and does not trap. this is the default for SH4. specifying this for targets that do not have dynamic shift instructions defaults to call-div1.

when a division strategy has not been specified, the default strategy is selected based on the current target. for SH2A the default strategy is to use the divs and divu instructions instead of library function calls. so many defaults, so little time 💜

• -maccumulate-outgoing-args — reserve space once for outgoing arguments in the function prologue rather than around each call. generally beneficial for performance and size ✨ it's also needed for unwinding, to avoid changing the stack frame around conditional code. lowkey a two-for-one 🎀
• -mdivsi3_libfunc=name — set the name of the library function used for 32-bit signed division to name. heads up: 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. renaming the girl doesn't change her register requirements 💅
• -mfixed-range=register-range — bestie, this tells the compiler to treat that register range as fixed registers, aka ones the register allocator is simply not allowed to touch 💅. Super handy when you're compiling kernel code. You write a register range as two registers separated by a dash, and you can stack multiple ranges by separating them with a comma.
• -mbranch-cost=num — pretend num is the cost of a branch instruction. Bigger numbers = the compiler tries harder to generate branch-free code where it can. If you don't specify it, the value gets picked based on the processor type you're compiling for ✨.
• -mzdcbranch
• -mno-zdcbranch — assume (or don't assume, respectively) that the zero displacement conditional branch instructions bt and bf are fast 🔥. With -mzdcbranch, the compiler prefers zero displacement branch code sequences. It's enabled by default when generating code for SH4 and SH4A, and you can explicitly turn it off with -mno-zdcbranch.
• -mcbranch-force-delay-slot — forces delay slots to be used for conditional branches, which means if it can't find a suitable instruction, it just stuffs the delay slot with a nop 💀. By default this option is disabled. You'd enable it to work around hardware bugs, like the ones found in the original SH7055.
• -mfused-madd
• -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These get generated by default if hardware floating point is in play. Heads up though: the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd maps to -ffp-contract=off 🫶.
• -mfsca
• -mno-fsca — allow or disallow the compiler from emitting the fsca instruction for sine and cosine approximations 🌸. -mfsca must be used together with -funsafe-math-optimizations. It's enabled by default when generating code for SH4A. Using -mno-fsca disables those sine and cosine approximations even if -funsafe-math-optimizations is in effect.
• -mfsrra
• -mno-fsrra — allow or disallow the compiler from emitting the fsrra instruction for reciprocal square root approximations 🌈. -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It's 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 give you faster code on the SH4 processor.
• -mfdpic — generate code using the FDPIC ABI ✨.

---

🌞 Solaris 2 Options 🌞

These -m options are supported on Solaris 2, bestie:

• -mclear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler 🧹. You only need this when object files use ISA extensions that aren't 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. With this option you can link position-dependent code into a shared object.

-mimpure-text suppresses the relocations remain against allocatable but non-writable sections linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes 😭. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

These switches are supported in addition to the above on Solaris 2:

• -pthreads — this is a synonym for -pthread 💖.

---

✨ SPARC Options ✨

These -m options are supported on the SPARC:

• -mno-app-regs
• -mapp-regs — specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications 🎀. Like global register 1, each global register 2 through 4 is then treated as an allocable register that gets 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 get 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 can't be done directly in cross-compilation — you have to 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 though 🫶.

-msoft-float changes the calling convention in the output file, so it's 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 for this to work.

• -mhard-quad-float — generate output containing quad-word (long double) floating-point instructions 🌟.
• -msoft-quad-float — generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions 🙈. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.

• -mno-unaligned-doubles
• -munaligned-doubles — assume that doubles have 8-byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they're 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 assumes that structures should have 8-byte alignment 🔥. This enables the use of pairs of ldd and std instructions for copies in structure assignment, instead of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI 💀. So it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI.
• -mstd-struct-return
• -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI 🔍.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode.

• -mlra
• -mno-lra — enable Local Register Allocation ✨. This has been the default for SPARC since GCC 7, so you need to pass -mno-lra to get the old Reload.
• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type 👑. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor 🌈. -mcpu=native has no effect if GCC doesn't recognize the processor.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here's a list of each supported architecture and their supported implementations 💖:

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3, leon5
• sparclite — f930, f934, sparclite86x
• sparclet — tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture 🌸. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7 ✨. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 🧚

With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip. ✨

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. 👑

• With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips.
• With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips.
• With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips.
• With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips.
• With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips.
• With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips.
• With -mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips.
• With -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips. 💖

---

• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that the option -mcpu=cpu_type does. Like, scheduling vibes only, bestie.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used. 🦋

• -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. 💅
• -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 -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, -mvis3, -mvis2 and -mvis. 😭 so many flags, but we love her
• -mcbcond, -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is -mcbcond when targeting a CPU that supports such instructions, such as Niagara-4 and later.
• -mfmaf, -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is -mfmaf when targeting a CPU that supports such instructions, such as Niagara-3 and later.
• -mfsmuld, -mno-fsmuld — with -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. The default is -mfsmuld when targeting a CPU supporting the architecture versions V8 or V9 with FPU except -mcpu=leon.
• -mpopc, -mno-popc — with -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. The default is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and later.
• -msubxc, -mno-subxc — with -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is -msubxc when targeting a CPU that supports such an instruction, such as Niagara-7 and later. 🌸
• -mfix-at697f — enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
• -mfix-ut699 — enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
• -mfix-ut700 — enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — enable the documented workaround for the back-to-back store errata of the GR712RC processor. 🫶

These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:

• -m32, -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
• -mcmodel=which — set the code model to one of:
  • medlow — the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
  • medmid — the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • medany — the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • embmedany — the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). 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 — set the memory model in force on the processor to one of:
  • 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 that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. 💀 (the -2047 is not a typo, bestie, she's just like that)

---

✨ Options for System V ✨

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

• -G — create a shared object. It is recommended that -symbolic or -shared be used instead. 👀
• -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.

---

🌈 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 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 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. tiny code era 🎀
• -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 — bestie, this one shoves all your static and global variables that are n bytes or smaller into the first 32 kilobytes of memory. tiny vars, prime real estate 💖
• -mv850 — tells GCC the target processor is the V850. that's it, that's the whole vibe ✨
• -mv850e3v5 — target processor = V850E3V5. when you use this one, the preprocessor constant __v850e3v5__ gets defined 🎀
• -mv850e2v4 — target processor = V850E3V5. yeah. it says V850E3V5. this is an alias for the -mv850e3v5 option, no cap 💅
• -mv850e2v3 — target processor = V850E2V3, and the preprocessor constant __v850e2v3__ is defined when you use it 🦋
• -mv850e2 — target processor = V850E2, and yes, the preprocessor constant __v850e2__ is defined 🌸
• -mv850e1 — target processor = V850E1. this one's generous: it defines both __v850e1__ and __v850e__ 💖
• -mv850es — target processor = V850ES. an alias for -mv850e1, same energy 🫶
• -mv850e — target processor = V850E, and the preprocessor constant __v850e__ gets defined ✨

okay so here's the tea: if none of -mv850, -mv850e, -mv850e1, -mv850e2, -mv850e2v3 or -mv850e3v5 are defined, then a default target processor is chosen and the relevant __v850*__ preprocessor constant gets defined. and the preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target. iconic consistency 👑

• -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 😤

it's enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. and if "CALLT" instructions are being generated, then the C preprocessor symbol __V850_CALLT__ is defined 🎀

• -mrelax, -mno-relax — passes on (or does not pass on) the -mrelax command-line option to the assembler. just relaying the message fr fr 📨
• -mlong-jumps, -mno-long-jumps — disables (or re-enables) the generation of PC-relative jump instructions 🌈
• -msoft-float, -mhard-float — disables (or re-enables) the generation of hardware floating point instructions. this one only matters 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. 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 — generates 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 🔒

---

🌟 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 🫶
• -mg — output code for G-format floating-point numbers instead of D-format 🔥
• -mlra, -mno-lra — enable Local Register Allocation. this is still experimental for the VAX, so by default the compiler uses standard reload 👑

---

🧚 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 this option. this causes libraries libc.a and libsim.a to be linked ✨
• -mfpu, -mhard-float — generate code containing floating-point instructions. this is the default 💖
• -mno-fpu, -msoft-float — generate code containing library calls for floating-point.

okay listen up, this one's important: -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 😭

• -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 — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would 🌈
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on the access to general registers. this is the default 👑
• -muser-mode — generate code for the user mode, where the 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 🚫

---

💜 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 🫡
• -mdebug-main=prefix — flag the first routine whose name starts with prefix as the main routine for the debugger 🔍
• -mmalloc64 — default to 64-bit memory allocation routines 💾
• -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, and no for supporting only 32 bit pointers. the later option disables "pragma pointer_size" 🎯

---

🌸 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__ 🔥
• -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 ⚠️
• -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 🤝
• -Xbind-lazy — enable lazy binding of function calls. this option is equivalent to -Wl,-z,now and is defined for compatibility with Diab 💤
• -Xbind-now — disable lazy binding of function calls. this option is the default and is defined for compatibility with Diab ⚡

---

👑 x86 Options 👑 (main character section, obviously)

these -m options are defined for the x86 family of computers:

• -march=cpu-type — generate instructions for the machine type cpu-type. and here's the difference, bestie: 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 🖥️
• x86-64 — a generic CPU with 64-bit extensions ✨

• x86-64-v2, x86-64-v3, x86-64-v4 — these cpu-type picks hand you the matching micro-architecture level straight out of the x86-64 psABI. On ABIs that are not the x86-64 psABI, they still select the same CPU features that the x86-64 psABI documents for that particular micro-architecture level.

okay so heads up bestie: these cpu-type values have no matching -mtune setting of their own, so using -march with them just turns on generic tuning. Want something more specific? Flip on specific tuning with the -mtune=other-cpu-type option and an appropriate other-cpu-type value. 💅

• i386 — the OG Intel i386 CPU.
• i486 — Intel i486 CPU. (fr, no scheduling is implemented for this chip. 💀)
• 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.
• i686 — when you use it with -march, the Pentium Pro instruction set is what gets used, so the code runs on all i686 family chips. When you use it with -mtune, it means the same thing 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; the low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. This is the one Centrino notebooks used. 🎀
• 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, 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 and PREFETCHW instruction set support.
• skylake — Intel Skylake 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 and SGX instruction set support.
• skylake-avx512 — Intel Skylake Server 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, 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.
• 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.
• 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 and SM4 instruction set support.
• sapphirerapids, emeraldrapids — Intel's Sapphire Rapids / Emerald Rapids server chips, 64-bit extensions and honestly the longest feature list you've ever seen in your life 💀 she's giving maximalist:

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, same energy as above but she went and added two more at the end ✨:

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, and she said and also one more time 🫶 (yes, AMX-COMPLEX gets her own spot at the end):

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.

• bonnell, atom — Intel Bonnell, the lil low-power baby 🍓 short list, cozy:

Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

• silvermont, slm — Intel Silvermont, step two of the tiny-chip glow-up ✨:

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, now with crypto and XSAVE-adjacent stuff unlocked 🔥:

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 — Goldmont Plus, same as Goldmont but she picked up PTWRITE, RDPID and SGX along the way 💅:

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, the plus-plus version, now with GFNI-SSE in the mix 🌸:

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, the efficiency-core girlies, and she is stacked 💀:

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, and ngl her list is character for character the same as sierraforest 😭 iconic twins, no notes:

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, and she showed up with SHA512, SM3, SM4, USER_MSR and PREFETCHI like a whole new personality 👑:

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.

• knl — Intel Knights Landing. Bestie, notice that MOVBE shows up twice in this list — that's not a typo on my part, that's literally how it's written, and I am not touching it 🫶:

Intel Knights Landing 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, AVX512PF, AVX512ER, AVX512F, AVX512CD and PREFETCHWT1 instruction set support.

• knm — Intel Knights Mill, the KNL glow-up with AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ bolted on 🔥:

Intel Knights Mill 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, AVX512PF, AVX512ER, AVX512F, AVX512CD and PREFETCHWT1, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support.

• k6 — AMD K6, and her whole personality is MMX, periodt:

AMD K6 CPU with MMX instruction set support.

• k6-2, k6-3 — improved AMD K6 versions, now with MMX and 3DNow! 💅:

Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.

• athlon, athlon-tbird — AMD Athlon, riding in with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions 🌈:

AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.

• athlon-4, athlon-xp, athlon-mp — improved AMD Athlon, and now she has the full SSE instruction set, no crumbs:

Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.

• k8, opteron, athlon64, athlon-fx — AMD K8-core processors, covering the AMD Opteron, Athlon 64, and Athlon 64 FX chips. This one supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and the 64-bit instruction set extensions ✨:

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.)

• k8-sse3, opteron-sse3, athlon64-sse3 — improved AMD K8 cores, now with SSE3 in the building 🎀:

Improved versions of AMD K8 cores with SSE3 instruction set support.

• amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64. She supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions 🦋:

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 — AMD Family 15h cores with x86-64. 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 💜:

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. 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 🌸:

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, and she picked up FSGSBASE since last time 🫶:

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. Now with BMI2, AVX2 and MOVBE joining the party 🔥:

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. 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 💖:

AMD Family 17h core based CPUs with x86-64 instruction set support.  (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)

• znver2 — AMD Family 17h core based CPUs with x86-64. Adds CLWB, RDPID and WBNOINVD to the znver1 set ✨:

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. Now also supersets PKU, VPCLMULQDQ and VAES 💅:

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, and she went full AVX-512 on us 😭 the whole alphabet of AVX512 things plus GFNI:

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.)

• 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.
• 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. 🙈
• 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.
• geode — AMD Geode embedded processor with MMX and 3DNow! instruction set support.

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🎀 -mtune=cpu-type

Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. Bestie, hear me out — 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. So like, 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. It's giving "dress for the job you want but you can still show up to the old job." 💅

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. Not me warning you that your "reproducible" build is on a timer 😭

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. Periodt.

• 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.

There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common 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. Same energy, different chip 💀

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🧚 -mcpu=cpu-type

A deprecated synonym for -mtune. She's retired, she's iconic, she's still answering the group chat. 👑

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🌸 -mfpmath=unit

Generate floating-point arithmetic for selected unit unit. The choices for unit are:

• 387 — Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description. 🫶

This is the default choice for non-Darwin x86-32 targets.

• sse — Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too. 🌈

For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default.

The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but 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 — Attempt 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 the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance. 💀

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💜 -masm=dialect

Output assembly instructions using selected dialect. 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. (RIP to Darwin intel stans, iykyk.)

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🎀 -mieee-fp

🎀 -mno-ieee-fp

Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. It's giving "yes she handles the edge case" energy ✨

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🦋 -m80387

🦋 -mhard-float

Generate output containing 80387 instructions for floating point.

---

🦋 -mno-80387

🦋 -msoft-float

Generate output containing library calls for floating point.

⚠️ Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler 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. No cap, GCC is not gonna hand you the libraries — that's on you, bestie. 💅

the tail of that last one, bestie: on machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may still be emitted even if -msoft-float is used. yeah. rude but true. 💀

• -mno-fp-ret-in-387 — do not use the FPU registers for function return values.

okay so the usual calling convention is that functions hand back float and double values in an FPU register, even when there's literally no FPU present. the whole idea is that the operating system is supposed to emulate an FPU for you. cute, right? 🎀

-mno-fp-ret-in-387 says nope to all that and makes those values come back in ordinary CPU registers instead.

• -mno-fancy-math-387 — some 387 emulators simply do not support the sin, cos and sqrt instructions for the 387. specify this option to avoid generating those instructions. 🧚

heads up though: this option gets overridden when -march indicates that the target CPU always has an FPU, because then the instruction doesn't need emulation at all. also, these instructions are not generated unless you also use the -funsafe-math-optimizations switch. iykyk.

• -malign-double, -mno-align-double — these control whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary. aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium, at the expense of more memory. tradeoffs, bestie, always tradeoffs.

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. but in arrays or structures conforming to the ABI, that's just 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 — and this is important — that neither of these options enables any extra precision over the x87 standard of 80 bits for a long double. periodt.

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. hence they are not binary-compatible with code compiled without that switch. 😭

• -mlong-double-64, -mlong-double-80, -mlong-double-128 — these switches control the size of the long double type too. a size of 64 bits makes the long double type equivalent to the double type — that's the default for 32-bit Bionic C library. a size of 128 bits makes the long double type equivalent to the __float128 type — that's 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, as well as modifying the function calling convention for functions taking long double. hence they are not binary-compatible with code compiled without that switch. 🙈

• -malign-data=type — control how GCC aligns variables. the supported values for type are: compat, which uses an increased alignment value compatible with GCC 4.8 and earlier; abi, which uses the alignment value as specified by the psABI; and cacheline, which uses an increased alignment value to match the cache line size. compat is the default. ✨
• -mlarge-data-threshold=threshold — when -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold are placed in large data sections. the default is 65535.
• -mrtd — use 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 queen.

you can specify that an individual function is called with this calling sequence using the function attribute stdcall. you can also override the -mrtd option by using the function attribute cdecl.

Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 💀

also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. and in addition, seriously incorrect code results if you call a function with too many arguments. (normally, extra arguments are harmlessly ignored, but not here bestie.)

• -mregparm=num — control how many registers are used to pass integer arguments. by default, no registers are used to pass arguments, and at most 3 registers can be used. you can control this behavior for a specific function by using the function attribute regparm.

Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. this includes the system libraries and startup modules. 🌸

• -msseregparm — use 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. 🔥

• -mvect8-ret-in-mem — return 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. 🎀
• -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 are 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. 👑

setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. but note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. 😭 that's a real one.

• -mdaz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. SSE and AVX instructions, including scalar and vector instructions, could benefit from enabling the FTZ and DAZ flags when -mdaz-ftz is specified. don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is specified — -mdaz-ftz will set FTZ/DAZ flags even with -shared. 🦋
• -mstackrealign — realign the stack at entry. on the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. this supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. see also the attribute force_align_arg_pointer, applicable to individual functions. ✨
• -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. if -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).

Warning: when generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to an 8 byte boundary. since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in a controlled environment where stack space is an important limitation. this option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with a misaligned stack. in this case, SSE instructions may lead to misaligned memory access traps. in addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. you must build all modules with -mpreferred-stack-boundary=3, including any libraries. this includes the system libraries and startup modules. 💀

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. if -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

on Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. on Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. 🌈

to ensure proper alignment of these values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. further, every function must be generated such that it keeps the stack aligned. thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. it is recommended that libraries that use callbacks always use the default setting. 🫶

okay so this extra alignment? yeah, it does consume extra stack space, and it generally increases code size too. 💅 Code that's sensitive to stack space usage — think embedded systems and operating system kernels, the girlies who count every single byte — may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. periodt.

---

Now bestie, buckle up, because this next bit is a whole alphabet of CPU instruction sets. 🎀

-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-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
-mavx5124fmaps
-mavx512vnni
-mavxvnni
-mavx5124vnniw
-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

These switches enable the use of instructions in the MMX, SSE, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, 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, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, 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 or CLDEMOTE extended instruction sets. 😌 Every single one of them has a matching -mno- option to turn that instruction use off again. slay.

These extensions are also available as built-in functions — go peek at x86 Built-in Functions for the deets on which functions these switches enable and disable. 🦋

Wanna generate SSE/SSE2 instructions automatically from floating-point code (instead of 387 instructions)? That's -mfpmath=sse, bestie.

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 their default settings. You can then feed those names 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 is not preceded with ^, otherwise it's turned off. Heads up though: -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. 😭

for real, that one is devs-only, don't come for me.

• -mno-default — instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features.
• -mcld — instructs GCC to emit a cld instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of cld instructions can be suppressed with the -mno-cld compiler option in this case. ✨
• -mvzeroupper — instructs GCC to emit a vzeroupper instruction before a transfer of control flow out of the function, to minimize the AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics. 💜
• -mprefer-avx128 — instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
• -mprefer-vector-width=opt — instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform.
• -mpartial-vector-fp-math — enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless -fno-trapping-math is specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when -fno-trapping-math is in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using -mdaz-ftz to set denormals-are-zero (DAZ) flag in the MXCSR register. 🌸

This option is enabled by default.

• -mmove-max=bits — instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are 128, 256 and 512.
• -mstore-max=bits — instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits. The valid bits are 128, 256 and 512.
• none — No extra limitations applied to GCC other than defined by the selected platform.
• 128 — Prefer 128-bit vector width for instructions.
• 256 — Prefer 256-bit vector width for instructions.
• 512 — Prefer 512-bit vector width for instructions.
• -mnoreturn-no-callee-saved-registers — optimizes functions with noreturn attribute or _Noreturn specifier by not saving in the function prologue callee-saved registers which are used in the function (except for the BP register). This option can interfere with debugging of the caller of the noreturn function or any function further up in the call stack, so it is not enabled by default. 🫶
• -mcx16 — enables GCC to generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement __sync Builtins. However, for __atomic Builtins operating on 128-bit integers, a library call is always used. 💅
• -msahf — enables generation of SAHF instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize 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). 🌈
• -mcrc32 — enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction.
• -mmwait — enables built-in functions __builtin_ia32_monitor, and __builtin_ia32_mwait to generate the monitor and mwait machine instructions.
• -mrecip — enables use of RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Note 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). 😭

Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip.

Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. 💖

• -mrecip=opt — bestie, this one decides which reciprocal estimate instructions GCC is allowed to pull up. opt is a comma-separated list of options, and any of them can get a ! in front to flip it to the opposite. The menu:
  • all — turn on every estimate instruction. all of them. the whole buffet 💅
  • default — just the default set, which is literally the same thing as plain -mrecip.
  • none — shut them all off, which is exactly -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 like, for example, -mrecip=all,!sqrt turns on every reciprocal approximation except square root. iconic.

• -mveclibabi=type — tells GCC which ABI type to use when it vectorizes intrinsics using an external library. The supported values for type are svml (for the Intel short vector math library) and acml (for the AMD math core library). Heads up though: to actually use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML- or ACML-ABI-compatible library has to be specified at link time. no shortcuts bestie.

Right now GCC emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for the corresponding function type when -mveclibabi=svml is used, and __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used. yeah it's a lot. she's thorough. 💜

• -mabi=name — generates code for the calling convention you name. The permissible values are sysv (the ABI used on GNU/Linux and other systems) and ms (the Microsoft ABI). Default behavior: it uses the Microsoft ABI when targeting Microsoft Windows, and the SysV ABI on every other system. You can control this for specific functions by using the function attributes ms_abi and sysv_abi. cute, right?
• -mforce-indirect-call — forces all calls to functions to be indirect. This is useful when you're using Intel Processor Trace, because it generates more precise timing information for function calls. ⭐
• -mmanual-endbr — inserts the ENDBR instruction at function entry only via the cf_check function attribute. This is useful when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry.
• -mcet-switch — so by default, CET instrumentation is turned off on switch statements that use a jump table, and indirect branch track is disabled. Since jump tables are stored in read-only memory, this does not cause a direct loss of hardening. But — and this is the tea — if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET. This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets being reachable via any indirect jumps.
• -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 these registers is emitted inline, which gives you pretty lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to do these saves and restores, 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 — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default.
• -mno-push-args — the off switch. 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 gets 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. Also this switch implies -mno-push-args.
• -mthreads — supports 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 — enable bit-field layout compatible with the native Microsoft Windows compiler.
• -mno-ms-bitfields — disable it. 🙈

If packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Especially when you're moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be necessary to access either format.

This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.

The Microsoft structure layout algorithm is pretty simple, except for the bit-field packing. The padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.

MSVC interprets zero-length bit-fields in the following ways:

1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.

For example:

struct
 {
   unsigned long bf_1 : 12;
   unsigned long : 0;
   unsigned long bf_2 : 12;
 } t1;

The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1's size would be 4 bytes.

2. If a zero-length bit-field is inserted after a bit-field, foo, and the alignment of the zero-length bit-field is greater than the member that follows it, bar, bar is aligned as the type of the zero-length bit-field.

For example:

struct
 {
   char foo : 4;
   short : 0;
   char bar;
 } t2;

struct
 {
   char foo : 4;
   short : 0;
   double bar;
 } t3;

For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.

Taking this into account, it's important to note the following:

1. If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as a whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:

struct
 {
   char foo : 6;
   long : 0;
 } t4;

Here, t4 takes up 4 bytes.

3. Zero-length bit-fields following non-bit-field members are ignored:

struct
 {
   char foo;
   long : 0;
   char bar;
 } t5;

Here, t5 takes up 2 bytes.

• -mno-align-stringops — do not align the destination of inlined string operations. This switch reduces code size and improves performance in the case where the destination is already aligned, but GCC doesn't know about it. 😭 sneaky.
• -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This option enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. 🦋
• -minline-stringops-dynamically — for string ops where we literally don't know the size ahead of time, this does a run-time check: inline code for the small blocks, and a library call for the big ones. best of both worlds bestie ✨
• -mstringop-strategy=alg — overrides the internal decision heuristic for which algorithm gets used when inlining string operations. the allowed values for alg are, and yes we're listing all of them:
  • rep_byte, rep_4byte, rep_8byte — expand using the i386 rep prefix of the specified size. slay 💅
  • byte_loop, loop, unrolled_loop — expand into an inline loop.
  • libcall — always use a library call. no thoughts, just vibes, just the library.
• -mmemcpy-strategy=strategy — overrides the internal decision heuristic for whether __builtin_memcpy should be inlined, and which inline algorithm to use when the expected size of the copy 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 that inline algorithm alg is allowed for. for the last triplet, max_size must be -1. the max_size of the triplets in the list must be specified in increasing order (chronological order bestie, we love a sorted list 📈). the minimal byte size for alg is 0 for the first triplet, and max_size + 1 of the preceding range.
• -mmemset-strategy=strategy — same energy as -mmemcpy-strategy=, except it's the one controlling __builtin_memset expansion. twins fr fr 🫶
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. this avoids the instructions to save, set up, and restore frame pointers, and makes an extra register available in leaf functions. note that the option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. so, iconic for speed, a lil cursed for debugging 😭
• -mtls-direct-seg-refs / -mno-tls-direct-seg-refs — controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

for systems that use the GNU C Library, the default is on.

• -msse2avx / -mno-sse2avx — specify that the assembler should encode SSE instructions with VEX prefix. the option -mavx turns this on by default.
• -mfentry / -mno-fentry — if profiling is active (-pg), put the profiling counter call before the prologue. note: on x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg. 🙈
• -mrecord-mcount / -mno-record-mcount — if profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. this is useful for automatically patching and out calls.
• -mnop-mcount / -mno-nop-mcount — if profiling is active (-pg), generate the calls to the profiling functions as NOPs. this is useful when they should be patched in later dynamically. this is likely only useful together with -mrecord-mcount.
• -minstrument-return=type — instrument function exit in -pg -mfentry instrumented functions with a call to the specified function. this only instruments true returns ending with ret, but 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. periodt.
• -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 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 don't have those issues, regardless of the RAX register value. so if you're on anything modern you're chillin 💜

• -m8bit-idiv / -mno-8bit-idiv — on some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. this option generates a run-time check. if both 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. tiny numbers, tiny math, big speedup ✨
• -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 — generate stack protection code using a canary at guard. supported locations are global for global canary, or tls for per-thread canary in the TLS block (the default). this option has effect only when -fstack-protector or -fstack-protector-all is specified.

with the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as the base register for reading the canary, and from what offset from that base register. the default for those is as specified in the relevant ABI. 🎀

• -mgeneral-regs-only — generate code that uses only the general-purpose registers. this prevents the compiler from using floating-point, vector, mask and bound registers. minimalism era 💅
• -mrelax-cmpxchg-loop — when emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins lacking a native instruction, optimize 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. so considerate of the CPU's energy tbh 🔋
• -mindirect-branch=choice — convert 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.

note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model. 💀

note that -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 — convert 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.

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 — force indirect call and jump via register. no detours, straight through the register 🏃‍♀️
• -mharden-sls=choice — generate 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. go big or go home bestie 👑
• -mindirect-branch-cs-prefix — add 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, to allow them to be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time. very specific, very iconic.
• -mapx-inline-asm-use-gpr32 — for inline asm support with APX, by default the EGPR feature was disabled to prevent potential illegal instruction with EGPR occurring. to invoke egpr usage in inline asm, use the new compiler option -mapx-inline-asm-use-gpr32 and the user should ensure the instruction supports EGPR. not me telling you to double check your own work 🙈
• -mevex512 / -mno-evex512 — enables/disables 512-bit vector. 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 now we're in the 64-bit girlies section 🌈

• -m32, -m64, -mx32, -m16, -miamcu — 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.

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.

the -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.

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.

the -miamcu option generates code which conforms to Intel MCU psABI. it requires the -m32 option to be turned on. no -m32, no -miamcu, that's the rule 📏

• -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 is 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. (and yes it's called a red zone, it's giving danger but it's actually fine 🚨)
• -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. small but mighty 💖
• -mcmodel=kernel — okay so this one's for kernel code, bestie. The kernel lives in the negative 2 GB of the address space, and this model has to be used for Linux kernel code. No exceptions, it's giving very specific energy. 👑
• -mcmodel=medium — the medium model! Your program gets linked in the lower 2 GB of the address space, and small symbols get placed there too. But symbols bigger than -mlarge-data-threshold? Those get yeeted into large data or BSS sections and can live above 2GB. Programs can be statically or dynamically linked, so flexible queen behavior fr fr. ✨
• -mcmodel=large — the large model, the main character who trusts nobody. This one makes no assumptions about addresses and sizes of sections. Live your truth. 🌈
• -maddress-mode=long — generates code for long address mode. Only supported for 64-bit and x32 environments, and it's the default address mode for 64-bit environments. Slay.
• -maddress-mode=short — generates code for short address mode. Only supported for 32-bit and x32 environments, and it's the default address mode for 32-bit and x32 environments. Matching set, we love to see it. 💅
• -mneeded, -mno-needed — emit the GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target, which tells everyone the micro-architecture ISA level required to actually execute the binary. It's like a little name tag for your executable. 🎀
• -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. Not a vibe. Be careful. 💀

• -munroll-only-small-loops — controls conservative small loop unrolling. It is default enabled by O2, and unrolls loop with less than 4 insns by 1 time. Explicit -f[no-]unroll-[all-]loops would disable this flag to avoid any unintended unrolling behavior that user does not want. Respecting your boundaries, honestly. 🫶
• -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. Sneaky little metadata pockets, iconic.

---

🪟 x86 Windows 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 it's enabled by default on those targets. Default behavior, we stan.
• -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. It's giving "don't ship this to prod," bestie. 🙈
• -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. Dynamic queen. 💫
• -mnop-fun-dllimport — available for Cygwin and MinGW targets. It specifies that the "dllimport" attribute should be ignored. Not me pretending that attribute doesn't exist.
• -mthreads — available for MinGW targets. It specifies that MinGW-specific thread support is to be used. Threads but make it MinGW. 🧵
• -municode — available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. Unicode rights, as it should be. 🌍
• -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 does not influence the choice of runtime library/startup code. Macros only, she's just here for the preprocessor drama.
• -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. Windows with actual windows, lowkey meta. 🪟✨
• -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. No API, no flag, no problem.
• -fwritable-relocated-rdata — available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the ".data" section. This is necessary for older runtimes not supporting modification of ".rdata" sections for pseudo-relocation. Grandmacore runtimes get accommodations too. 🎀
• -mpe-aligned-commons — available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature. Auto-detected and auto-enabled, she's self-sufficient.

See also under x86 Options for standard options.

---

🌩️ Xstormy16 Options 🌩️

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. Simulator mode, we go virtual. 🎮

---

🦋 Xtensa Options 🦋

These options are supported for Xtensa targets:

• -mconst16, -mno-const16 — enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is currently not a standard option from Tensilica. When enabled, "CONST16" instructions are always used in place of the standard "L32R" instructions. The use of "CONST16" is enabled by default only if the "L32R" instruction is not available. Conditional defaults, we love a plot twist.
• -mfused-madd, -mno-fused-madd — enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. Precision girlies, this one's for you. 🔬
• -mserialize-volatile, -mno-serialize-volatile — when this option is enabled, GCC inserts "MEMW" instructions before "volatile" memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the "MEMW" instructions. Ordering, but make it enforced. 💅
• -mforce-no-pic — for targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. Kernel privileges only, sorry bestie. 🚫
• -mtext-section-literals, -mno-text-section-literals — these options control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. This may be necessary for large assembly files. Literals for each function are placed right before that function. Keeping your constants close, emotionally available literals. 🫂
• -mauto-litpools, -mno-auto-litpools — these options control the treatment of literal pools. The default is -mno-auto-litpools, which places literals in a separate section in the output file unless -mtext-section-literals is used. With -mauto-litpools the literals are interspersed in the text section by the assembler. Compiler does not produce explicit ".literal" directives and loads literals into registers with "MOVI" instructions instead of "L32R" to let the assembler do relaxation and place literals as necessary. This option allows the assembler to create several literal pools per function and assemble very big functions, which may not be possible with -mtext-section-literals. Big function energy, the assembler handles it. 🔥
• -mtarget-align, -mno-target-align — when this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. The assembler attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like "LOOP", which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. "LOOP" does what it wants, always has. 💁‍♀️
• -mlongcalls, -mno-longcalls — okay so when this is on, GCC tells the assembler "bestie, pls translate direct calls into indirect ones" — but only if it can't figure out that the target of a direct call is inside the range the call instruction allows. This translation typically 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. The default is -mno-longcalls. Use this in programs where the call target could potentially be out of range. ✨

Heads up: this option is implemented in the assembler, NOT the compiler — so the assembly code GCC generates still shows direct call instructions. Look at the disassembled object code to see the actual instructions. Also note the assembler uses an indirect call for every cross-file call, not just the ones that really are out of range. no cap 😭

• -mabi=name — generate code for the specified ABI. The permissible values are: call0, windowed. The default ABI is chosen by the Xtensa core configuration. 🎀
• -mabi=call0 — with this on, function parameters get passed in registers a2 through a7, registers a12 through a15 are caller-saved, and register a15 may be used as a frame pointer. When this version of the ABI is enabled, the C preprocessor symbol __XTENSA_CALL0_ABI__ gets defined. slay 💅
• -mabi=windowed — with this on, function parameters get passed in registers a10 through a15, and the called function rotates the register window by 8 registers on entry so its arguments are found in registers a2 through a7. Register a7 may be used as a frame pointer. The register window is rotated 8 registers back upon return. When this version of the ABI is enabled, the C preprocessor symbol __XTENSA_WINDOWED_ABI__ gets 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 the literal pool using L32R or synthesizing the constant from a small one with a couple of arithmetic instructions. The default value is 0. ⭐
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary, as described in the architecture specification. The default is -mno-strict-align for cores that support both unaligned loads and stores in hardware, and -mstrict-align for all other cores. 🌸

---

🧚 zSeries Options 🧚

These are listed under

🌈 ENVIRONMENT 🌈

okay bestie, this part is the whole vibe check for how GCC behaves before it even looks at your flags 💅. There's a bunch of environment variables that change how GCC operates — some of them work by handing it directories or prefixes to search when it's hunting for files, and some just... set the whole compilation environment differently. ✨

And before you get any ideas: you can also tell GCC where to search using options like -B, -I and -L. Those take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. So the priority chain is literally options > env vars > whatever GCC was built with. no cap. 👑

---

💖 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL

These environment variables control the way that GCC uses localization information, which allows GCC to 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. 🦋

The LC_CTYPE environment variable 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 giving "please don't chop my string in the wrong place" 😭

The LC_MESSAGES environment variable specifies the language to use in diagnostic messages. (yes, your compiler errors can speak French. iconic.)

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. 🇬🇧

---

🎀 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. so it's the group chat between compilation stages fr fr 💬

---

✨ 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. (nearly, bestie, nearly — go read the option docs.) 🙈

---

👑 GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script.

Other prefixes specified with -B take precedence over this prefix.

This prefix is also used for finding files such as crt0.o that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. 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. 🌸

---

🦋 COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX.

---

💜 LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). 🍓

---

🌟 LANG (yes, again — she does double duty)

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. 🧚

---

🔥 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.

---

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

💖 DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too. slay, one env var doing the job of three flags. ✨

🌸 SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted. 🙈

🦋 SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become reproducible. (reproducible builds girlies, this one's for you 🫶)

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of "date +%s" on GNU/Linux and other systems that support the %s extension in the "date" command.

The value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process.

---

💀 BUGS 💀

For instructions on reporting bugs, see <http://bugzilla.redhat.com/bugzilla>. go tell them everything bestie, they wanna know. 🐛

---

📝 FOOTNOTES 📝

1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. -shared suppresses the addition of startup code to alter the floating-point environment as done with -ffast-math, -Ofast or -funsafe-math-optimizations on some targets. 😭 subtle defects are the WORST kind, do not skip the flags.

---

👀 SEE ALSO 👀

gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb. the whole friend group, honestly. 🫶

---

🌟 AUTHOR 🌟

See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. (yes, an entire crowd of people. main character energy, collectively. 👑)

---

🎀 COPYRIGHT 🎀

Copyright (c) 1988-2024 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.

---

okay that's the WHOLE page bestie, we made it to the end 🥳 this one shipped as gcc-14, dated 2026-07-24, and it's the GCC(1) man page itself. go forth, compile something iconic, and remember: read the docs, respect the precedence order, and hydrate. 💖✨🎀🦋🌸💅🫶