GCC(1)

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

okay bestie, deep breath, because you just opened the man page for GCC(1) and she is long (129 parts long, we're on part 1, buckle up 🎀). It's the manual for gcc, the GNU project C and C++ compiler, it lives in section 1, and I'm gonna walk you through every single crumb of it. no cap, nothing gets skipped. 💖

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🌸 NAME 🌸

gcc — GNU project C and C++ compiler 👑

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🦋 SYNOPSIS 🦋

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

Only the most useful options are listed right there — the rest of the whole entire stash is further down, keep scrolling. Also: g++ accepts mostly the same options as gcc. ✨

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💜 DESCRIPTION 💜

So when you invoke GCC, it normally does four things in a row: preprocessing, compilation, assembly and linking. The "overall options" let you stop that whole process at an intermediate stage, like hitting pause mid-glow-up. For example, the -c option says do not run the linker — so then your output is just object files that the assembler dropped. 💅

Other options get passed on to one or more stages of processing. Some of them control the preprocessor, others control the compiler itself. And yet others control the assembler and linker — most of those aren't documented here, because honestly you rarely need to use any of them. not me gatekeeping, it's just true 😌

Most of the command-line options you can use with GCC are useful for C programs. When an option is only useful with another language (usually C++), the explanation says so explicitly. And if the description of a particular option doesn't mention a source language at all? Then you can use that option with all supported languages. blessed. 🌈

The usual way to run GCC is to run the executable called gcc, or machine-gcc when you're cross-compiling, or machine-gcc-version to run one specific version of GCC. And when you're compiling C++ programs, you should invoke GCC as g++ instead. periodt. 🫶

The gcc program accepts options and file names as operands. Many options have multi-letter names, and therefore multiple single-letter options may not be grouped — -dv is very different from -d -v. like, they are not the same girl, do not treat them the same girl 😭

You can mix options and other arguments. For the most part, the order you use doesn't matter. But 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 lowkey order is the main character sometimes 👑

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. (yes the man page really squishes that dash blob together like that, iykyk 💀) Most of these have both a positive and a negative form — the negative form of -ffoo is -fno-foo. This manual documents only one of those two forms: whichever one is not the default. 🙈

Some options take one or more arguments, typically separated from the option name either by a space or by the equals sign (=). Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers, and 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. Those kinds of arguments are designated by byte-size in the following text. For the full listing and explanation of the binary and decimal byte size prefixes, refer to the NIST, IEC, and other relevant national and international standards. 📚✨

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💅 OPTIONS 💅

🌟 Option Summary 🌟

Here's the whole summary of all the options, grouped by type. The explanations live in the following sections. (We're only at part 1, so the explanations are coming, bestie — this is the trailer, not the movie 🎬)

🍓 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]

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and that's the option summary, every single one of them, no crumbs left behind 💖 more parts coming, so don't go anywhere — we're literally just getting started 🎀✨

💅 Warning Options 💅

okay bestie, deep breath. this is the warning-flag roster and it is long. like, unnervingly long. your girl is gonna hold your hand through all of it because every single one of these matters and i am not about to let you skim 🫶

here's the whole lineup, no skips, no cuts:

-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

see? iconic. exhausting, but iconic. ✨

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🔍 Static Analyzer Options 🔍

these ones are the analyzer girlies — they poke at your code on a whole other level. anyway here's the full cast, every last one, fr fr:

-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

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🍓 C and Objective-C-only Warning Options 🍓

okay these next ones are exclusive — they only show up when you're doing C or Objective-C, so if you're a C++ girlie they're just not your business. iykyk 💜

-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

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🦋 Debugging Options 🦋

and now... the debugging era. debugging girlies, this is your moment. these control the debug info situation. here's every single one:

-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

and that's the roster, bestie — every name exactly as it lives, nothing dropped, nothing invented 💖 more incoming, so stay tuned...

⚡ Optimization Options ⚡

okay bestie, hydration check 🍓 because this right here is the biggest list on the entire page — the optimization flags, aka the whole reason people fight in the group chat about which -O level is the moment. Every single one is below, in the exact order the original has them, nothing skipped, nothing merged, no cap. 💅

the alignment girlies go first 🎀

• -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

still with me? good, because the -fno- girlies are about to walk in and they have opinions 😭

• -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

the tree-pass squad has entered the building 🌳✨

• -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, bestie. iconic. 👑

---

🧪 Program Instrumentation Options 🧪

these are the "make the compiler tell on the program" flags — profiling, coverage, sanitizing, hardening, all of it. they also get their own -O-sized moment. sit pretty. 💜

• -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]

instrumented, hardened, profiled, protected — she contains multitudes 🫶

---

🌸 Preprocessor Options 🌸

the before-the-compiler-actually-does-anything era. macros, includes, charsets, and the little -M family that everybody googles every single time (iykyk).

• -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 icons only, we love efficiency ✨

• -Wa,option
• -Xassembler option

---

🔗 Linker Options 🔗

the "please actually connect my object files, bestie" zone. libraries, entry points, static vs shared, and the -Wl, passthrough everybody abuses. 💀

• 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 to go looking for headers, libraries, and the rest of the treasure map. note the many -i-flavored girlies here — they are not a vibe, they are a whole alphabet. 🧚

• -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 actual "what does the machine code end up looking like" department. positioning, unwinding, struct returns, visibility, and one flag (-ftrapv) that is going to ruin somebody's afternoon in a documented way. 🙈

• -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 🧑‍💻

the deep-lore section. dumps, dumps of dumps, statistics, reports, memory reports, timing reports, and the -print- icons that tell you where GCC hides its stuff. this is the group chat's weirdest and most beloved corner. 💖

• -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]

and that's a wrap on the option index for this chunk, bestie — the rest of the page keeps going, so don't wander off 🌈

🛠️ Machine-Dependent Options 🛠️

okay bestie, breathe. this is the bit where GCC shows off that she speaks literally every chip's language, and she does it with a whole separate menu for each family 💅

every architecture down here gets its own pile of -m... flags — plus a few chaotic curveballs that don't even start with -m, and we respect their audacity. i'm listing every single one, in the order gcc writes them, because absolutely none of these are skippable. not me gatekeeping a target flag, never 🫶

---

✨ AArch64 Options ✨

the AArch64 menu, top to bottom, no edits, no skipping:

• -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

that's the whole squad, no notes 💖

---

🧚 Adapteva Epiphany Options 🧚

okay so this one is giving very specific, very niche, very her. the entire lineup:

• -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 🌈

short, sweet, three items, ate and left no crumbs:

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

---

🏹 ARC Options 🏹

this list is long long, bestie. buckle up 🫠

• -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 🎀 (one single capital G and honestly? iconic)
• -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

yes bestie, -mRcq and -mRcw really do have capital letters in them. case-sensitive queen 💅

---

💪 ARM Options 💪

another massive one. these are in the exact order gcc puts them:

• -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]]

that last one has a space in the middle of it, and i'm copying it character for character because that's the assignment 🫡

---

🌸 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 ⭐

five items, quick and cute:

• -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 🍎

bestie these ones don't even start with -m and i'm obsessed with the lawlessness 💅 here they all are:

• -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

not me pointing out that -sectobjectsymbols, -segs_read_only_addr and -segs_read_write_addr each show up twice in this list 😭 i'm not touching them. they're staying exactly as written. both of them.

---

👑 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 💫

two. that's it. that's the whole section. she's lowkey mysterious:

• -msmall-model
• -mno-lsim

---

🌟 FT32 Options 🌟

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

---

💖 FRV Options 💖

big list incoming, stay with me 🫶

• -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

-mTLS and -mtls sitting right next to each other, differing only in vibes (and a capital letter). case sensitivity is the moment 💀

---

🐧 GNU/Linux Options 🐧

the little libc-and-android corner of the world:

• -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

-static and -threads never even got the -m memo and i love that for them 💅

---

🚀 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

the -msched- family really said "we're all showing up together or not at all" and honestly, respect the unity 🌈

---

🍓 LM32 Options 🍓

last one for this part, bestie, and she's short and sweet:

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

---

okay so that's a lot of machines and we are not even close to done 💀 but every single flag above is exactly where gcc put it, in the exact order, with the exact spelling. no cap. keep scrolling bestie, there's more alphabet to get through 💖

🦋 LoongArch Options 🦋

okay bestie, this is the LoongArch buffet and literally every single one of these is on the menu, no skipping:

• -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 💅

M32R/D said "here's my whole personality" and we're listening:

• -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 🌈

short, sweet, iconic:

• -mcpu=cpu
• -msim
• -memregs=number

---

✨ M680x0 Options ✨

the 680x0 lineup is giving vintage, and yes it goes on forever, that's the vibe:

• -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 🎀

bestie the MCore flags walked in and owned the room:

• -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 🫶

MicroBlaze really said "I contain multitudes" and listed them all:

• -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 👑

okay so MIPS is THAT girl and this list is long, but we do not skim, we do not scroll past, we read every single one because every single one matters. no cap. 💖

• -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

ate, no crumbs left. periodt. 💅

---

⭐ MMIX Options ⭐

MMIX came through with the whole set:

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

---

🌸 MN10300 Options 🌸

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

---

🧚 Moxie Options 🧚

tiny but mighty, obsessed:

• -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 💀

• -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 🦋

• -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 💖

bestie, look over there: See RS/6000 and PowerPC Options. they're right below, we're not skipping, promise. ✨

---

🎀 PRU Options 🎀

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

---

👑 RISC-V Options 👑

RISC-V pulled up looking immaculate and brought every single flag with it:

• -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 bestie deep breath, this is the BIG one, the main character list, and we are reading every single flag because that's just who we are:

• -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

we did it. we survived. iconic behavior tbh. 💅✨

So continuing from where we left off, bestie, here are more options for specific architectures. Buckle up! 🎢

---

RX Options 🦋

If you're on RX, here are all the options you can vibe with:

• -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

That's a whole lot of options, but you got this! 💅

---

S/390 and zSeries Options 👑

Mainframe icons, rise up! Here's what you can use:

• -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

So many choices, but you're the main character here. 🌟

---

SH Options 🌸

SH gang, let's go! Here are your options:

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

That's a whole mood. 🎀

---

Solaris 2 Options ☀️

Solaris besties, here's your list:

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

Short and sweet! 💖

---

SPARC Options 🔥

SPARC squad, assemble! Here are all the options:

• -mcpu=cpu-type
• -mtune=cpu-type
• -mcmodel=code-model
• -mmemory-model=mem-model
• -m32
• -m64
• -mapp-regs
• -mno-app-regs
• -mfaster-structs
• -mno-faster-structs
• -mflat
• -mno-flat
• -mfpu
• -mno-fpu
• -mhard-float
• -msoft-float
• -mhard-quad-float
• -msoft-quad-float
• -mstack-bias
• -mno-stack-bias
• -mstd-struct-return
• -mno-std-struct-return
• -munaligned-doubles
• -mno-unaligned-doubles
• -muser-mode
• -mno-user-mode
• -mv8plus
• -mno-v8plus
• -mvis
• -mno-vis
• -mvis2
• -mno-vis2
• -mvis3
• -mno-vis3
• -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

So many options, so little time. 😭

---

System V Options 🧚

System V, keeping it classic:

• -Qy
• -Qn
• -YP,paths
• -Ym,dir

Iconic. 💜

---

V850 Options 🌟

V850 fam, here's your menu:

• -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

Slay. 💅

---

VAX Options 🦕

VAX, the retro queen:

• -mg
• -mgnu
• -munix
• -mlra

Vintage vibes. 🎀

---

Visium Options 🦋

Visium options, coming right up:

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

So chic. ✨

---

VMS Options 🌿

VMS besties:

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

Got it? 💚

---

VxWorks Options 🚀

VxWorks, let's go:

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

Periodt. 💀

---

x86 Options 👑

Okay, x86 bestie, this is the big one. Here's the whole list, no skips:

• -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, that's a lot. You're a legend for reading all that. 👑

---

x86 Windows Options 🪟

Windows x86 besties:

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

Stay cozy. 💖

---

Xstormy16 Options ⛈️

Just one option, keep it simple:

• -msim

---

Xtensa Options 🦋

Xtensa crew:

• -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

You're doing amazing, sweetie. ✨

---

zSeries Options 👑

See S/390 and zSeries Options. No cap.

---

Options Controlling the Kind of Output 🌈

Okay so like, compilation can happen in up to four stages, bestie: preprocessing, compilation proper, assembly, and linking. Always in that order, no cap. GCC can preprocess and compile several files into either several assembler input files or one assembler input file. Then each assembler input file makes an object file, and linking combines all the object files (the new ones and the ones you gave as input) into an executable file. It's giving very organized, honestly.

For any input file, the file name suffix tells you what kind of compilation is done, so pay attention:

• file.c — C source code that must be preprocessed. Like, it needs the whole preprocessing step, no skipping.
• file.i — C source code that should not be preprocessed. So leave it alone, bestie.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. Oh, and heads up: you must link with the libobjc library to make an Objective-C program work. Don't forget! 😘
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program work. Also, .M is a literal capital M, so don't play. 💅
• 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 (that's the default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch). So versatile!
• 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. And .C refers to a literal capital C. Got it? 👑
• file.mm, file.M — Objective-C++ source code that must be preprocessed. (Yeah, these appear again, different context, keep up!)
• file.mii — Objective-C++ source code that should not be preprocessed. (Also appears again, it's fine.)
• 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). Old school!
• 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. Gopher vibes! 🦫
• 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. Fancy!
• 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. So if it's not on the list, it's an object file, periodt.

You can specify the input language explicitly with the -x option:

• -x language — okay bestie, this is you telling gcc exactly what language the next input files are in, instead of letting it guess from the filename suffix like it usually does. It applies to all the input files that follow, right up until the next -x shows up. The possible values for language are:

        c  c-header  cpp-output
        c++  c++-header  c++-system-header c++-user-header c++-cpp-output
        objective-c  objective-c-header  objective-c-cpp-output  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 — shuts the language specification off completely, so every file after this is handled by its filename suffix again (literally as if -x was never used at all). the way that it just resets the whole vibe. ✨

So if you only want some of the compilation stages, you use -x (or the filename suffixes) to tell gcc where to start, and one of -c, -S, or -E to say where gcc should stop. Heads up though — some combos are lowkey a whole nothing-burger. For example, -x cpp-output -E tells gcc to do literally nothing. 💀

• -c — compile or assemble the source files, but do not link. The linking stage just… doesn't happen, periodt. The final output is one object file per 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 — the ones that don't need compiling or assembling — are just ignored. not their moment, they get skipped. 🙈

• -S — stop right after the compilation-proper stage; do not assemble. The output is an assembler code file for every non-assembler input file you 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 need compiling are ignored.

• -E — stop after the preprocessing stage; do not run the compiler proper. The output is preprocessed source code, and it goes straight to standard output.

Input files that don't need preprocessing are ignored.

• -o file — put the primary output in file. This works for whatever kind of output you're making — an executable file, an object file, an assembler file, or preprocessed C code. one flag, all the fits. 💅

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

Now listen — even though -o names only the primary output, it also affects the naming of auxiliary and dump outputs too. (See the examples below.) Unless it gets overridden, both auxiliary outputs and dump outputs land in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with the suffix 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're 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 all of 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 one sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when they're preserved, aren't regarded as primary outputs — they count 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.

If this option isn't there, 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) get the input name suffix, and aux output files (those requested by other non-dump options, e.g. -save-temps, -gsplit-dwarf, -fcallgraph-info) get it without. ✨

Similar suffix differentiation of dump and aux outputs can be attained for an 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. that's a full takeover, no cap. 👑

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 output file (not a dump one, bestie, aux only), drop the trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase — that is, the suffix of the input file when -dumpbase isn't present on the command line, or when 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 doesn't 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 meant 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 does. In case multiple such options are given, the last one prevails:

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

…and that outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* is out here overriding the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*. Periodt. 💅

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

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

outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:

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

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

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

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

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

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

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

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

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

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

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

---

Okay bestie, now let's get into the actual options themselves. 💖

• -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. 👑
• -### — 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. 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. 📖
• --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.

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

--help=target,undocumented

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

--help=warnings,^joined,^undocumented

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

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

--help=target,optimizers

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

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

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

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

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

-Q -O2 --help=optimizers

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

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled

• --version — Display the version number and copyrights of the invoked GCC. 📜
• -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 .... 🎁

• -ffile-prefix-map=old=new — okay so when you're compiling files that live in directory old, this makes the compiler write down every reference to them like they actually lived in directory new instead 🫶. Specifying this one is literally the same as specifying all the individual -f*-prefix-map options. It's the move for making reproducible builds that don't care where you built them. Fair warning tho: directories referenced by directives are not affected by these options. Also go peek at -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map. ✨
• -fcanon-prefix-map — so for the -f*-prefix-map options, the comparison of the old prefix against the filename that would normally be referenced in the compilation result is normally done by textual comparison of the prefixes — or, on case insensitive filesystems, by ignoring character case, and on DOS based filesystems by treating slashes and backslashes as equal. This flag makes those comparisons happen on the canonicalized paths of old and the referenced filename instead 💅
• -fplugin=name.so — load the plugin code sitting in file name.so, assumed to be a shared object that the compiler will dlopen. The base name of the shared object file is what identifies the plugin for argument parsing purposes (see -fplugin-arg-name-key=value right below bestie). Every plugin should define the callback functions laid out in the Plugins API 👑
• -fplugin-arg-name-key=value — define an argument called key with a value of value for the plugin called name. That's it, that's the whole thing 💖
• -fdump-ada-spec[-slim] — for C and C++ source and include files, generate the corresponding Ada specs. Iconic.
• -fada-spec-parent=unit — teaming up with -fdump-ada-spec[-slim] above, this generates the Ada specs as child units of parent unit 🌸
• -fdump-go-spec=file — for input files in any language, generate the corresponding Go declarations into file. This spits out Go "const", "type", "var", and "func" declarations, which lowkey can be a really useful way to start writing a Go interface to code that was written in some other language 🦋
• @file — read command-line options from file. The options read get inserted in place of the original @file option. And if file doesn't exist, or can't be read, then the option is treated literally — and not removed. No cap.

Options inside file are separated by whitespace. Want a whitespace character inside an option? Surround the entire option in either single or double quotes 💅 Any character at all (backslash included) can be included by prefixing the character you want with a backslash. And the file itself may contain even more @file options — any of those get processed recursively. It's turtles all the way down 🐢

---

Compiling C++ Programs

C++ source files conventionally use one of these suffixes: .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx. C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc. 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 you'd call it for compiling C programs (usually with the name gcc).

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

When you compile C++ programs, you may specify many of the same command-line options 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. So much freedom, we love to see it 💖

---

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's 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) — things like the "asm" and "typeof" keywords, and predefined macros such as "unix" and "vax" that identify the type of system you're on. It also enables the undesirable and rarely used ISO trigraph feature 🙈 And for the C compiler, it disables recognition of C++ style // comments as well as the "inline" keyword.

The alternate keywords "__asm__", "__extension__", "__inline__" and "__typeof__" keep working despite -ansi. You would not want to use them in an ISO C program, of course, but it's useful to put them in header files that might get 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.

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. Noted 📝

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

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

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

• c90
• c89
• iso9899:1990 — support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
• iso9899:199409 — ISO C90 as modified in amendment 1.
• c99
• c9x
• iso9899:1999
• iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/c99status.html> for more info. The names c9x and iso9899:199x are deprecated ⚠️
• c11
• c1x
• iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. Substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
• c17
• c18
• iso9899:2017
• iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is the same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of "__STDC_VERSION__", and so is supported to the same extent as C11. 💜
• c23
• c2x
• iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (expected to be published in 2024). 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. 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 — 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. 💅
• gnu++2b, gnu++23 — GNU dialect of -std=c++2b. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 🦋
• c++2c, c++26 — the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 🎀
• gnu++2c, gnu++26 — GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 🌸
• -aux-info filename — output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C. Besides declarations, the file indicates, 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. 🫶
• -fno-asm — do not recognize "asm", "inline" or "typeof" as a keyword, so that 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. 👑
• -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. 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++. 😭
• -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at "main". The most obvious example is an OS kernel. This is equivalent to -fno-hosted. 🧚
• -fgimple — enable parsing of function definitions marked with "__GIMPLE". This is an experimental feature that allows unit testing of GIMPLE passes. 🔥
• -fgnu-tm — when the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. For more information on GCC's support for transactional memory, the original manpage just trails off here—no extra link, no cap. Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions). 💀
• -fgnu89-inline — the option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode. Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions. The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode. The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions. 💜
• -fhosted — assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which "main" has a return type of "int". Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding. 🌈
• -flax-vector-conversions — allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code. 🙈
• -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. 🍓

• -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". ⭐
• -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. 💖
• -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. ✨
• -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. 👑
• -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. 💅
• -fopenmp-target-simd-clone
• -fopenmp-target-simd-clone=device-type

Okay so bestie 🫶 normally GCC makes SIMD clones for functions marked with the declare simd directive. But with this option on, it also makes clones for functions marked with the OpenMP declare target directive, and those clones are vectorization-friendly. The device-type can be none, host, nohost, or any — literally matching the keywords for the device_type clause of the declare target directive, and clones get generated for the intersection of devices you specified. -fopenmp-target-simd-clone alone is the same 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 it's off by default. 💅

• -fpermitted-flt-eval-methods=style

So ISO/IEC TS 18661-3 defines new permissible values for FLT_EVAL_METHOD, and those values say that ops 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 the ones allowed under C99/C11, and C99/C11 doesn't even specify the meaning of other positive values of FLT_EVAL_METHOD. Which means: code that conforms to C11 might not have been written expecting these new values to show up. 🙈

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

style is either c11 or ts-18661-3, whichever fits.

Default when you're in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. Default in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3. ✨

The -fdeps-* options exist to extract structured dependency info for a source. That means figuring out what resources other source files provide that you'll need to compile this source, and what resources this source provides. Then you can add required dependencies between compilation rules of dependent sources based on their contents instead of having to reflect all that info inside the build tools. it's giving smart build systems 🌈

• -fdeps-file=file

Where to write that structured dependency info. 🎀

• -fdeps-format=format

The format to use for structured dependency info. p1689r5 is the only supported format right now. Heads up tho: when this argument is specified, the output of -MF gets stripped of some information (namely C++ modules) so it doesn't use extended makefile syntax that most tools don't understand. 👑

• -fdeps-target=file

Basically -MT but for structured dependency info. It indicates the target which will ultimately need any required resources and provide any resources extracted from the source that might be required by other sources. 🦋

• -fplan9-extensions

Accepts 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. Only supported for C, not C++.

• -fsigned-bitfields
• -funsigned-bitfields
• -fno-signed-bitfields
• -fno-unsigned-bitfields

These control whether a bit-field is signed or unsigned when the declaration doesn't say signed or unsigned. By default such a bit-field is signed, because that's consistent — the basic integer types like int are signed types, bestie. 💜

• -fsigned-char

Lets the type char be signed, like signed char.

Ngl this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Same energy the other way: -fno-signed-char is equivalent to -funsigned-char. 💅

• -funsigned-char

Lets the type char be unsigned, like unsigned char.

Every kind of machine has a default for what char should be — 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) lets you make such a program work with the opposite default. 🫶

The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. no cap.

• -fstrict-flex-arrays (C and C++ only)
• -fstrict-flex-arrays=level (C and C++ only)

Controls when to treat the trailing array of a structure as a flexible array member, for the purpose of accessing elements of such an array. The value of level controls how strict it gets.

-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 that case a trailing array is treated as a flexible array member only when it's declared as one 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 one specific trailing array field of a structure by using the variable attribute strict_flex_array.

Also the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. (Yes, one has a dash and one has an underscore. iconic behavior, honestly.)

• -fsso-struct=endianness

Sets the default scalar storage order of structures and unions to the specified endianness. Accepted values are big-endian, little-endian and native for the native endianness of the target (that one's the default). Not supported for C++.

Warning: the -fsso-struct switch makes GCC generate code that is not binary compatible with code generated without it, if the specified endianness is not the native endianness of the target. 😭 be careful bestie

---

🌈 Options Controlling C++ Dialect 🌈

This section describes the command-line options that are only meaningful for C++ programs. 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. ✨

Some options for compiling C programs, like -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. Which means... the ABI you get with version 0 will change in different versions of G++ as ABI bugs get fixed. 🙈 sorry bestie, 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, declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.

Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. 💀 oops

Version 13, which first appeared in G++ 8.2, fixes that accidental change in version 12.

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. 🦋

• -fabi-version=0 — bestie, this is the default, and it quietly maps to 13 (aka GCC 8.2 compatibility vibes ✨). But if you go and explicitly pick some other ABI version, this thing flips to defaulting to 0 instead. And if your whole goal is staying friends with GCC versions 3.2 through 4.9? You want -fabi-compat-version=2. Slay.

Also, the lore 🫶: if you never pass -fabi-version but you do pass -Wabi=n, then that n version is the one used for the compatibility aliases. And if you pass this option alongside -Wabi (with no version attached, just vibes), then the version from this option is what gets used for the warning. iykyk.

• -fno-access-control — turns off all the access checking. Like, all of it. This one's mainly for sneaking around bugs in the access control code itself. Very "let me in, it's not a phase" energy. 🚪
• -faligned-new — enables support for C++17 new for types that need more alignment than void* ::operator new(std::size_t) hands out. You can drop a numeric argument like -faligned-new=32 to say how much alignment (in bytes) that function provides — but fr fr, barely anyone needs to override the default of alignof(std::max_align_t). 😌

Heads up: this is on by default for -std=c++17. Periodt.

• -fchar8_t, -fno-char8_t — turns on support for char8_t, exactly as it was adopted for C++20. That's a whole glow-up: a brand-new char8_t fundamental type, changed types for UTF-8 string and character literals, new signatures for user-defined literals, the associated standard library updates, plus the shiny new __cpp_char8_t and __cpp_lib_char8_t feature test macros. 💅

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

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

and it introduces new signatures for user-defined literals:

int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;

Bestie, listen — the change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. Like literally, the code below is well-formed under ISO C++11 but becomes ill-formed the moment you say -fchar8_t. 💀

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 — checks that the pointer coming back from operator new is non-null before it tries to mess with the storage it allocated. Normally you don't need this at all, because the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler always checks the return value anyway, no flag required. In literally every other case — when operator new has a non-empty exception specification — running out of memory is signalled by throwing std::bad_alloc. See also new (nothrow). 🧠
• -fconcepts, -fconcepts-ts — enables 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 switched on with -fconcepts-ts.

Sad news: -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. RIP to a real one. 😭

• -fconstexpr-depth=n — sets the maximum nested evaluation depth for C++11 constexpr functions to n. You need some limit or endless recursion during constant expression evaluation would just keep going forever. The minimum specified by the standard is 512.
• -fconstexpr-cache-depth=n — sets the maximum level of nested evaluation depth for C++11 constexpr functions that'll be cached, to n. It's a heuristic trading compilation speed (when the cache dodges repeated calculations) against memory consumption (when the cache grows absolutely huge from highly recursive evaluations). The default is 8. Very few users are likely to want to touch it, but if your code does heavy constexpr calculations you might want to experiment and find whichever value works best for you. 🔬
• -fconstexpr-fp-except — Annex F of the C standard says IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers historically ignored that advice and treated 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 the other rules that say a particular operation is undefined. Life-changing fr fr.

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

• -fconstexpr-loop-limit=n — sets the maximum number of iterations for a loop in C++14 constexpr functions to n. You need a limit to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).
• -fconstexpr-ops-limit=n — sets the maximum number of operations during a single constexpr evaluation. Even when a single loop's iteration count is capped by the limit above, you can still get in trouble: if there are several nested loops and each has many iterations but still fewer than that limit, or if inside some loop — or even outside of a loop — way too many expressions need evaluating, the resulting constexpr evaluation might take, like, forever. The default is 33554432 (1<<25). 😮‍💨
• -fcontracts — enables experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This whole thing is intended mostly for those interested in experimentation towards refining the feature to get it into shape for a future C++ standard. It's giving "beta test the future". 🧪

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. And bestie, these sets cannot be used together. Pick your lane. 🚫

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

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

• "ignore" — this contract has no effect. 🫥
• "assume" — this contract is treated like C++23 [[assume]].
• "check_never_continue", "never", "abort" — this contract is checked. If it fails, the violation handler is called. If the handler returns, std::terminate is called. 💀
• "check_maybe_continue", "maybe" — this contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally. 😌

• -fcoroutines — enables support for the C++ coroutines extension (experimental). Experimental girlie behavior. 🌈
• -fdiagnostics-all-candidates — permits the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected. The compiler is being thorough. We love that for us. 📝
• -fno-elide-constructors — the C++ standard allows an implementation to omit creating a temporary that's only used to initialize another object of the same type. Specifying this option disables that optimization and forces G++ to call the copy constructor in all cases. This option also makes G++ call trivial member functions which would otherwise be expanded inline.

In C++17 the compiler is required to omit these temporaries — but this option still affects trivial member functions. So it's not fully retired. 🎀

• -fno-enforce-eh-specs — don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but it may be useful for reducing code size in production builds, much like defining NDEBUG.

Careful, bestie: 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. Do not play with this one. 🚨

---

• -fextern-tls-init, -fno-extern-tls-init — ok so the C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization 🌀. To make that work, every use of such a variable has to go through a lil wrapper function that does whatever initialization is needed. When the use and the definition are in the same translation unit, that overhead can be optimized right out the window ✨ — but when the use is in a different TU, the overhead is genuinely significant, even when the variable doesn't actually need dynamic initialization 😭. So: if the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or because a use of the variable in the defining TU will execute before any uses in another TU), they can dodge all that overhead with -fno-extern-tls-init 💅.

On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init 👑

• -ffold-simple-inlines, -fno-fold-simple-inlines — lets the C++ frontend fold calls to std::move, std::forward, std::addressof and std::as_const 🫶. And unlike inlining, this means no debug information gets generated for those calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active. periodt 💖
• -fno-gnu-keywords — don't recognize typeof as a keyword, so your code can use that word as an identifier 🙈. You can use the keyword __typeof__ instead. Heads up: 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, which is 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 💅. So the call to id(t) is in an immediate context, meaning it doesn't have to produce a constant — that's literally the mechanism that allows consteval function composition 🧚. 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 — make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function 🌸. You can use this option in C++14 mode or later. It can result in initialization changing from dynamic to static, plus other optimizations. ate 🔥
• -fno-implicit-templates — never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations 👑. And bestie, if you use this option you must take care to structure your code so it includes all the necessary explicit instantiations, otherwise you'll get undefined symbols at link time 😭.
• -fno-implicit-inline-templates — don't emit code for implicit instantiations of inline templates either 🙅. The default is to handle inlines differently, so that compiles with and without optimization need the same set of explicit instantiations.
• -fno-implement-inlines — to save space, don't emit out-of-line copies of inline functions controlled by #pragma implementation 🎀. Warning though: this causes linker errors if these functions aren't 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's not currently implicitly enabled just by selecting that standard version 👀.
• -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 from traditional C++ behavior, for good reasons. However, it may result in difficulty during code porting. This option makes such function definitions implicitly inline. It does, however, generate an ABI incompatibility, so you must use it everywhere or nowhere 💀. (Such definitions outside of a named module remain implicitly inline, regardless.)
• -fno-module-lazy — disable lazy module importing and module mapper creation 🙈.
• -fmodule-mapper=[hostname]:port[?ident], -fmodule-mapper=|program[?ident] args..., -fmodule-mapper==socket[?ident], -fmodule-mapper=<>[inout][?ident], -fmodule-mapper=<in>out[?ident], -fmodule-mapper=file[?ident] — an oracle to query for module name to filename mappings 🔮. If unspecified, the CXX_MODULE_MAPPER environment variable is used, and if that is unset, an in-process default is provided ✨.
• -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file 💅.
• -fms-extensions — disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax 👑
• -fnew-inheriting-ctors — enable the P0136 adjustment to the semantics of C++11 constructor inheritance 🌟. This is part of C++17 but is 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 aren't 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 💖. 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 😳.
• -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords 🎀.
• -fno-optional-diags — disable diagnostics that the standard says a compiler does not need to issue 🌸. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
• -fno-pretty-templates — when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so it's clear which template is involved 👑. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors makes it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them 🙈.
• -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 don't require run-time type information, i.e. casts to void * or to unambiguous base classes.

Mixing code compiled with -frtti with code compiled with -fno-rtti may not work 😭. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti.

• -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.
• -ftemplate-depth=n — sets the max instantiation depth for template classes to n. bestie why do we need a limit? because otherwise endless recursion during template class instantiation would just... never stop 💀 so there's a cap. ANSI/ISO C++ conforming programs must not rely on a max depth greater than 17 (this got changed to 1024 in C++11). the default value is 900, because in some situations the compiler can run out of stack space before it even hits 1024. iconic self-preservation honestly.
• -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 can use this option to reduce code size slightly in code that doesn't need to be thread-safe. lowkey a little dangerous, use with intention 🫶
• -fuse-cxa-atexit — register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. this option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit.
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. this causes std::uncaught_exception to be incorrect, but it's necessary if the runtime routine isn't available. no cap, it's a tradeoff.
• -fvisibility-inlines-hidden — this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

the effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. literally the glow-up your link times were waiting for ✨

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.

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

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

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

---

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

• -Wabi-tag (C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. see C++ Attributes for more information about ABI tags.
• -Wcomma-subscript (C++ and Objective-C++ only) — warn about uses of a comma expression within a subscripting expression. this usage was deprecated in C++20 and is going to be removed in C++23. however, a comma expression wrapped in "( )" is not deprecated. example:

void f(int *a, int b, int c) {
    a[b,c];     // deprecated in C++20, invalid in C++23
    a[(b,c)];   // OK
}

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

enabled by default with -std=c++20 unless -Wno-deprecated, and 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. bestie your class is giving locked-room mystery 🔒
• -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. 😭

Okay but the warning is like, chill about std::span-ish classes. We're talking classes shaped like this:

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

Those count as "std::span-like" bestie — meaning it's a non-union class that's got a pointer data member and a trivial destructor. slay 💅

And if you wanna turn this warning OFF? Just use the gnu::no_dangling attribute. iconic.

This warning is enabled by -Wall. no cap.

---

• -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — Warns you when delete is used to destroy an instance of a class that has virtual functions and a non-virtual destructor. Like, it is unsafe to delete an instance of a derived class through a pointer to a base class if the base class doesn't have a virtual destructor. 🙈 This warning is enabled by -Wall.
• -Wdeprecated-copy (C++ and Objective-C++ only) — Warns 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. And with -Wdeprecated-copy-dtor, it also deprecates if the class has a user-provided destructor. 💀
• -Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) — Kills the warning about when the usual arithmetic conversions get applied to operands where one is of enumeration type and the other is of a different enumeration type. That conversion got deprecated in C++20, fr fr. For example:

enum E1 { e };
enum E2 { f };
int k = f - e;

-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. And in pre-C++20 dialects, you can turn this warning on with -Wenum-conversion. 🫶

• -Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only) — Kills the warning for when the usual arithmetic conversions hit operands where one is of enumeration type and the other is of a floating-point type. Also deprecated in C++20. Example time:

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, -Wenum-conversion can enable this one too. ✨

• -Wno-elaborated-enum-base — For C++11 and above, warns if an (invalid) additional enum-base is used in an elaborated-type-specifier. Basically: if an enum with a given underlying type and no enumerator list shows up in a declaration that isn't just a standalone declaration of the enum. Enabled by default. And this warning gets upgraded to an error with -pedantic-errors. 😤
• -Wno-init-list-lifetime (C++ and Objective-C++ only) — Don't warn about uses of std::initializer_list that are probably gonna leave you with dangling pointers. Because the underlying array for an initializer_list is handled like a normal C++ temporary object, it's so easy to accidentally keep a pointer to the array past the end of the array's lifetime. For example:
  • If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. 💀
  • If a new-expression creates an initializer_list, the array only lives until the end of the enclosing full-expression, so the initializer_list in the heap has a dangling pointer. no ma'am.
  • 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. watch:

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

• When a list constructor stores the begin pointer from the initializer_list argument, that 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 — Warns when a function never produces a constant expression. In C++20 and earlier, for every constexpr function and function template, there has to 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 you can totally have a function or function template marked constexpr for which no invocation satisfies the requirements of a core constant expression. 🙃

This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, you can turn on -Winvalid-constexpr, in which case it'll be an ordinary warning. Like:

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

• -Winvalid-imported-macros — Verifies that all imported macro definitions are valid at the end of compilation. Not enabled by default, because it needs extra processing to determine. It can be useful when you're preparing sets of header-units to make sure macros stay consistent. 🎀
• -Wno-literal-suffix (C++ and Objective-C++ only) — Don't warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens, to keep 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 that 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 the standard requires. 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 doesn't change the meaning of well-formed code — narrowing conversions are still considered ill-formed in SFINAE contexts. 🤓

With -Wnarrowing in C++98, it warns when a narrowing conversion prohibited by C++11 occurs within { }, like:

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

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

• -Wnoexcept (C++ and Objective-C++ only) — Warns when a noexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception. 💜
• -Wnoexcept-type (C++ and Objective-C++ only) — Warns if that cute 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. 👑

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>. the way it just changed on us 😭

• -Wclass-memaccess (C++ and Objective-C++ only) — Warns when the destination of a call to a raw memory function like memset or memcpy is an object of class type, and when writing into 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. periodt 💅

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

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

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — Warns when a class has virtual functions and an accessible non-virtual destructor, either in itself or in an accessible polymorphic base class. In that case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or the base class. This warning is automatically enabled if -Weffc++ is specified. But lowkey, the -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred, because it warns about the unsafe cases without false positives. 🫶
• -Wregister (C++ and Objective-C++ only) — Warns on uses of the register storage class specifier, except when it's part of the GNU Explicit Register Variables extension. Using the register keyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. 💀
• -Wreorder (C++ and Objective-C++ only) — bestie, this one drags you when the order you wrote your member initializers in is not the order they actually get executed in. Look at this:

struct A {
  int i;
  int j;
  A(): j (0), i (1) { }
};

The compiler is like "okay so I'm gonna rearrange those member initializers for i and j so they match the declaration order of the members" — and then it emits a warning telling you it did that. Enabled by -Wall. 💅

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this warning clocks when a call to std::move is blocking copy elision. 😭 The classic scenario where copy elision can happen: you're returning from a function with a class return type, the expression you're returning is the name of a non-volatile automatic object, it's not a function parameter, and it has the same type as the function's return type. Like so:

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

But in this example, that std::move call prevents the copy elision. Rude of it, honestly. Enabled by -Wall.

• -Wno-redundant-move (C++ and Objective-C++ only) — this one warns about redundant calls to std::move, meaning the move operation would've happened anyway even without you writing std::move. That happens because the compiler is forced to treat the object as if it were an rvalue in certain situations, like returning a local variable, where copy elision isn't applicable. Consider:

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

Here, the std::move call is redundant. fr fr. And because G++ implements Core Issue 1579, here's another example:

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

In this one, copy elision isn't applicable because the type of the expression being returned and the function return type differ — yet G++ treats the return value as if it were designated by an rvalue. Sneaky. Enabled by -Wextra.

• -Wrange-loop-construct (C++ and Objective-C++ only) — warns when a C++ range-based for-loop is making an unnecessary copy. ✨ This can happen when the range declaration isn't 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. (Under 64 bytes? Live your life. 🦋)

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

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

In the example above, every single iteration of the loop creates and destroys a temporary value of type double, and the reference const double & gets bound to it. Every loop. That's a lot of tiny funerals. 🙈

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 any ambiguity. For example:

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

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

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

iykyk — sometimes the tag is load-bearing and the compiler knows it. 👑

• -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. Why does this matter? If a type A depends on a type B with no linkage or internal linkage, then 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 shows up in a single translation unit, the best way to silence the warning is to give A internal linkage too, by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. Heads up: -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 here to warn about the lack of a virtual destructor in accessible non-polymorphic base classes too. 👀

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

• -Wno-exceptions (C++ and Objective-C++ only) — disables the warning for when an exception handler is shadowed by another handler, which is a way of pointing out that your exception handlers are in the wrong order. 🎀
• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about using an uncasted NULL as a sentinel. When you're compiling with GCC only, this is a valid sentinel, because NULL is defined as __null. And even though it's a null pointer constant rather than a null pointer, it's guaranteed to be the same size as a pointer. But this use is not portable across different compilers, bestie. Portability matters. 🌈
• -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings for when non-template friend functions are declared inside a template. In very old versions of GCC, from before the ISO standard was implemented, declarations like 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 those compatibility problems, and it 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 inside 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. ✨
• -Woverloaded-virtual, -Woverloaded-virtual=n (C++ and Objective-C++ only) — 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 so it un-hides 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 into a plain pointer. 🫶
• -Wsign-promo (C++ and Objective-C++ only) — warns when overload resolution picks 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 so, so. 🤷
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates entirely, and this may be used to enforce that rule. The warning is inactive inside a system header file, like the STL, so you can still use the STL. You may also instantiate or specialize templates. (So: no new templates, but all your faves still work. 💅)
• -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 that 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
}

okay so the related option -Wmismatched-dealloc literally diagnoses mismatches involving allocation and deallocation functions other than operator new and operator delete. and -Wmismatched-new-delete is included in -Wall. periodt. 💅

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

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

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

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

• -Wmultiple-inheritance — (C++ and Objective-C++ only) warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, 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 define classes that indirectly use multiple inheritance. 🦋
• -Wvirtual-inheritance — warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, 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 define classes that indirectly use virtual inheritance. 🌸
• -Wno-virtual-move-assign — suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. this is dangerous because if the virtual base is reachable along more than one path, it is 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 — warn when a namespace definition is opened. Some coding rules disallow namespaces, 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 use using directives and qualified names. 🎀
• -Wno-template-id-cdtor — (C++ and Objective-C++ only) disable the warning about the use of 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) disable the warning about a throw-expression that will immediately result in a call to terminate. 🙈
• -Wno-vexing-parse — (C++ and Objective-C++ only) warn 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 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. 🌈
• -Wvolatile — (C++ and Objective-C++ only) warn 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) warn when a literal 0 is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11. ⭐
• -Waligned-new — warn about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall.

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

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

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

This warning is enabled by default.

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

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

• -Wplacement-new=2 — At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. 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) warn 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) warn for conditionally-supported (C++11 [intro.defs]) constructs. 🧚
• -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) warn about redundant semicolons after in-class function definitions. 💅
• -Wno-global-module — (C++ and Objective-C++ only) disable 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 — suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. 🌸
• -Wno-invalid-offsetof — (C++ and Objective-C++ only) suppress 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) warn 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 — ok so this one snitches on any type with virtual methods that would honestly be so much better off if you just slapped the C++11 final specifier on it, or — if you can swing it — dropped it into an anonymous namespace instead. Why do we care? Because once a type is final, GCC can go absolutely feral devirtualizing those polymorphic calls, and the code just flies. Bonus tea: this warning hits way harder with link-time optimization, where GCC actually has the full class-hierarchy graph and knows who's related to who. 💅
• -Wsuggest-final-methods — same energy, but for virtual methods. It'll tell you when a method would be better off marked final, or when its whole type should be final or tucked into an anonymous namespace. Also way more effective with link-time optimization for the same reason (fuller class hierarchy graph = clearer picture). Pro tip from the manpage itself: handle the -Wsuggest-final-types suggestions first, then rebuild with your new annotations. One thing at a time, bestie. 🎀
• -Wsuggest-override — warns you about overriding virtual functions that forgot to say the override keyword. Like, girl, you are an override, just label yourself. ✨
• -Wno-conversion-null (C++ and Objective-C++ only) — turns off the warning for conversions between NULL and non-pointer types. Heads up: -Wconversion-null is enabled by default, so this is the off-switch. 🔥

---

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

Quick note: this manual does not describe the Objective-C and Objective-C++ languages themselves. We're just doing the compiler-flag tour, bestie.

Ok so this whole section is about the command-line options that only mean anything for Objective-C and Objective-C++ programs. That said, you can absolutely still use most of the language-independent GNU compiler options alongside them. Like, say you wanna compile a file some_class.m — you'd do:

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

See how -fgnu-runtime in there is an option that's only for Objective-C and Objective-C++ programs? The other two can hang out with literally any language GCC supports. 🦋

And since Objective-C is an extension of C, your Objective-C compilations can also use options specific to the C front-end (think: -Wtraditional). Matching energy: Objective-C++ compilations get to use C++-specific options (like -Wabi). It's giving "you get the best of both worlds". 💖

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

• -fconstant-string-class=class-name — sets class-name as the class to instantiate for every literal string you write with the "@"..."" syntax. The default class name is "NXConstantString" if you're on the GNU runtime, and "NSConstantString" if you're on the NeXT runtime (more on both of those below). Darwin / macOS girlies listen up: if the -fconstant-cfstrings option is also present, it overrides your -fconstant-string-class setting and makes those "@"..."" literals get laid out as constant CoreFoundation strings instead. Also, heads up: -fconstant-cfstrings is just an alias for the target-specific -mconstant-cfstrings equivalent. ✨
• -fgnu-runtime — generates object code compatible with the standard GNU Objective-C runtime. This is the default on most types of systems. 🫶
• -fnext-runtime — generates output compatible with the NeXT runtime. This is the default on NeXT-based systems, including Darwin / macOS. And the macro "__NEXT_RUNTIME__" gets predefined if and only if this option is used — no sneaky extra definitions. 💅
• -fno-nil-receivers — tells GCC to assume that every single Objective-C message dispatch ("[receiver message:arg]") in this translation unit makes sure the receiver isn't nil. What's the payoff? The runtime can use more efficient entry points. ⚠️ But note: this option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. Don't try it anywhere else.
• -fobjc-abi-version=n — picks version n of the Objective-C ABI for whichever runtime you selected. Currently this is only supported for the NeXT runtime, so here's the lore on that:
  • Version 0 — the traditional (32-bit) ABI, with no support for properties and other Objective-C 2.0 additions.
  • Version 1 — the traditional (32-bit) ABI, but with support for properties and other Objective-C 2.0 additions.
  • Version 2 — the modern (64-bit) ABI.

If you don't specify anything, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. 📚

• -fobjc-call-cxx-cdtors — okay this one is iconic, stay with me. For each Objective-C class, GCC checks whether any of its instance variables is a C++ object with a non-trivial default constructor. If yes, it synthesizes a special - (id) .cxx_construct instance method that runs those non-trivial default constructors on any such instance variables, in order, and then returns self. Similarly, it checks if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesizes a special - (void) .cxx_destruct method that runs all those default destructors, in reverse order. 🌀

Important detail: those - (id) .cxx_construct and - (void) .cxx_destruct methods only operate on instance variables declared in the current Objective-C class — not the ones inherited from superclasses. It's the Objective-C runtime's job to invoke all such methods across an object's inheritance hierarchy. The - (id) .cxx_construct methods get invoked by the runtime right after a new object instance is allocated; the - (void) .cxx_destruct methods get invoked right before the runtime deallocates an object instance. 😭

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later actually supports invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods.

• -fobjc-direct-dispatch — allows fast jumps to the message dispatcher. On Darwin, this happens via the comm page. ⚡
• -fobjc-exceptions — enables syntactic support for structured exception handling in Objective-C, similar to what C++ offers. You literally need this option to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. Available with both the GNU and NeXT runtimes — 🚨 except it is not available with the NeXT runtime on Mac OS X 10.2 and earlier.
• -fobjc-gc — turns on garbage collection (GC) in Objective-C and Objective-C++ programs. Only available with the NeXT runtime. The GNU runtime has a different garbage collection implementation that doesn't need special compiler flags, so don't come at it with this one. 🗑️✨
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, this checks for a nil receiver in method invocations before doing the actual method call. This is the default, and you can turn it off with -fno-objc-nilcheck. 📌 Caveat time: class methods and super calls are never checked for nil this way, no matter what you set this flag to. And currently this flag does absolutely nothing when the GNU runtime is used, or with an older version of the NeXT runtime ABI.
• -fobjc-std=objc1 — makes you conform to the language syntax of Objective-C 1.0, aka the language GCC 4.0 recognized. This only affects the Objective-C additions to C/C++; it does not affect conformance to C/C++ standards — that's controlled by the separate C/C++ dialect option flags. When used with the Objective-C or Objective-C++ compiler, any Objective-C syntax GCC 4.0 wouldn't recognize gets rejected. Useful if you need to be sure your Objective-C code can compile with older versions of GCC. 🕰️
• -freplace-objc-classes — emits a special marker telling ld(1) not to statically link in the resulting object file, and instead letting dyld(1) load it in at run time. This pairs with the Fix-and-Continue debugging mode, where the object file in question can be recompiled and dynamically reloaded during program execution — no restarting the program itself needed. 😮 Currently, Fix-and-Continue functionality is only available with the NeXT runtime on Mac OS X 10.3 and later.
• -fzero-link — so when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the class name 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 that behavior and causes calls to objc_getClass("...") to be retained. Why would you want that? It's useful in Zero-Link debugging mode, since it lets individual class implementations be modified during program execution. 🐛 And FYI: the GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options.
• -fno-local-ivars — by default, instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. Cute, but 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 those variable shadowing issues. Problem solved, slay.
• -fivar-visibility=[public|protected|private|package] — sets the default instance variable visibility to whichever option you pick, so that instance variables declared outside the scope of any access modifier directives just default to that visibility. 🎚️
• -gen-decls — dumps interface declarations for all classes seen in the source file into a file named sourcename.decl. Very "here's everything, in writing, bestie". 📝
• -Wassign-intercept (Objective-C and Objective-C++ only) — warns you whenever an Objective-C assignment is being intercepted by the garbage collector. 👀
• -Wno-property-assign-default (Objective-C and Objective-C++ only) — don't warn if a property for an Objective-C object has no assign semantics specified. Silence is golden. 🤫
• -Wno-protocol (Objective-C and Objective-C++ only) — okay so the deal here: if a class is declared to implement a protocol, a warning gets issued for every method in the protocol that the class doesn't implement. And the default behavior is to warn for every method not explicitly implemented in the class — even if a method implementation is inherited from the superclass. 😤 But if you use the -Wno-protocol option, then methods inherited from the superclass count as implemented, and no warning gets issued for them. Much chiller.
• -Wobjc-root-class (Objective-C and Objective-C++ only) — warns if a class interface lacks a superclass. Most classes will inherit from something like "NSObject" (or "Object"), so a missing superclass is usually a red flag. 🚩 When you're declaring classes that are meant to be root classes, you can suppress the warning by marking their interfaces with __attribute__((objc_root_class)). Permission granted. 👑
• -Wselector (Objective-C and Objective-C++ girlies only) — warns you if the compiler stumbles on multiple methods with different types but the same selector while it's compiling. 💅 The actual vibe check happens on the method list at the final stage of compilation. On top of that, it also checks every selector that shows up in a @selector(...) expression, if a matching method for that selector got found during compilation. ⚠️ But here's the catch, bestie: because these checks only scan the method table at the very end, you get no warnings at all if the final stage never happens — like if an error killed the compile early, or if you're using -fsyntax-only. 😭
• -Wstrict-selector-match (Objective-C and Objective-C++ only, again) — warns when you're trying to send a message with a selector to a receiver of type id or Class, and there are multiple methods with differing argument and/or return types for that selector. When this flag is off (which is the default, lowkey), the compiler skips those warnings if all the differences it found are only in types that share the same size and alignment. 🧚
• -Wundeclared-selector (Objective-C and Objective-C++ squad) — warns you if a @selector(...) expression refers to a selector that was never declared. 🫶 A selector counts as undeclared when no method with that name has been declared before the @selector(...) expression — not explicitly in an @interface or @protocol declaration, and not implicitly in an @implementation section. This option is the impatient icon it is: it runs its checks the moment a @selector(...) expression shows up, whereas -Wselector waits until the final stage of compilation. ✨ Bonus: it also enforces that whole "methods and selectors must be declared before being used" coding-style convention. Periodt.
• -print-objc-runtime-info — spits out a C header describing the largest structure that gets passed by value, if there even is one. 👑

---

🌈 Options to Control Diagnostic Messages Formatting 🌈

Back in the day, diagnostic messages were formatted with zero regard for the output device's actual vibe (its width, etc. 💀). The options below let you take the wheel on the formatting algorithm for diagnostics — like how many characters per line, and how often source location info gets reported. Just know that some language front ends might straight-up ignore these options. 🙈

• -fmessage-length=n — tries to format error messages so they fit on lines of roughly n characters. If n is zero, no line-wrapping happens at all — every error message lives on one single line. That's the default for all front ends. ✨ Heads up: this option also changes how the #error and #warning pre-processor directives display, plus the deprecated function/type/variable attribute. It does not, however, touch the pragma GCC warning and pragma GCC error pragmas. 🙂‍↔️
• -fdiagnostics-plain-output — asks for diagnostic output to look as plain as humanly possible. Super useful when you're running dejagnu or other utilities that need to parse diagnostic output and really want it to stay stable over time. 🎀 Right now, -fdiagnostics-plain-output is exactly equivalent to these:

-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 in the future, if GCC changes the default look of its diagnostics, whatever option disables the new behavior will get added to this list too. 💖

• -fdiagnostics-show-location=once — only means anything in line-wrapping mode. Tells the diagnostic reporter to emit source location info once: if a message is too long to fit on one physical line and has to wrap, the source location won't get re-emitted as a prefix over and over on the continuation lines. This is the default behavior. ⭐
• -fdiagnostics-show-location=every-line — also only meaningful in line-wrapping mode. Tells the reporter to emit the same source location info (as a prefix) on every physical line that comes out of breaking a message that was too long for one line. 🔥
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — turns color on for diagnostics. WHEN is never, always, or auto. The default depends on how the compiler was configured — it can be any of those WHEN options, or never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when standard error is a terminal, and not when running inside an emacs shell. 🦋 Also: -fdiagnostics-color and -fno-diagnostics-color are just aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.

The colors themselves are defined by the GCC_COLORS environment variable. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands get interpreted by your terminal or terminal emulator (peep your text terminal's documentation for the permitted values and what they mean as character attributes 👀). These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate:

• 1 for bold
• 4 for underline
• 5 for blink
• 7 for inverse
• 39 for default foreground color
• 30 to 37 for foreground colors
• 90 to 97 for 16-color mode foreground colors
• 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors
• 49 for default background color
• 40 to 47 for background colors
• 100 to 107 for 16-color mode background colors
• 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors

The default GCC_COLORS is

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

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. 🙅‍♀️ The supported capabilities are:

• `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=, like the identifiers of individual events and lines showing interprocedural calls and returns.
• `range1=` — SGR substring for the first additional range.
• `range2=` — SGR substring for the second additional range.
• `locus=` — SGR substring for location information, file:line or file:line:column etc.
• `quote=` — SGR substring for information printed inside 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 inside diagnostics. Like, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, it can embed a URL for that option's docs. 🔗

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

The default depends on how the compiler was 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 the default is affected by environment variables. Under that configuration, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the compiler's environment, or never if neither is. 🌸

But even with -fdiagnostics-urls=always, the behavior still depends on those environment variables: if GCC_URLS is set to empty or no, no URLs get embedded in diagnostics. If it's set to st, URLs use ST escape sequences. If it's set to bel (the default), URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS isn't set, TERM_URLS is used 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. 🔔

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

• -fno-diagnostics-show-option — by default, each diagnostic emitted includes text pointing at the command-line option that directly controls it (if the diagnostic machinery knows such an option). Passing the -fno-diagnostics-show-option flag suppresses that behavior. 🎀

okay bestie, buckle up, we are still in the diagnostics flags and this is genuinely the ✨glow-up✨ corner of gcc — all the flags here are about making compiler errors look pretty, readable, or machine-munchable. every single one of them still matters. no skipping. 💅

---

• -fno-diagnostics-show-caret

so by default, every diagnostic gcc tosses at you prints the original source line AND a little caret ^ pointing at the exact column. this flag suppresses all that. note that the source line gets truncated to n characters if you passed -fmessage-length=n. and when the output is going to a terminal, the width is capped at whatever COLUMNS (the environment variable, yes) says — or, if COLUMNS isn't set, at the terminal width. 🌸

• -fno-diagnostics-show-labels

by default, when gcc is printing source code (that's via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent info — like the types of expressions, bestie:

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

this option kills the labels. in the example up there, that means the vertical bars and the "char *" and "long int" text just… don't show up. 🙈

• -fno-diagnostics-show-cwe

diagnostic messages can optionally carry an associated CWE identifier — that's https://cwe.mitre.org/index.html for the uninitiated, iykyk. gcc itself only provides that metadata for some of the -fanalyzer diagnostics. gcc plugins may also ship diagnostics with that metadata. by default, if the info is there, it gets printed with the diagnostic. this option suppresses printing of that metadata. periodt.

• -fno-diagnostics-show-rules

diagnostic messages can optionally have rules attached — like from a coding standard, or a spec. gcc itself does not do this for any of its diagnostics (she's not that girl), but plugins absolutely may. by default, if the info is present, it prints with the diagnostic. this option suppresses printing of that metadata. 💜

• -fno-diagnostics-show-line-numbers

by default, when printing source code (via -fdiagnostics-show-caret again), a left margin gets printed showing line numbers. this option suppresses that left margin. the numbers? gone. the vibe? cleaner. ✨

• -fdiagnostics-minimum-margin-width=width

this one controls the minimum width of that left margin printed by -fdiagnostics-show-line-numbers. it defaults to 6. width is, obviously, a width.

• -fdiagnostics-parseable-fixits

this emits fix-it hints in a machine-parseable format, suitable for consumption by IDEs. for each fix-it, a line gets printed after the relevant diagnostic, and that line starts with the string "fix-it:". like so:

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

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

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

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

an empty replacement string 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 the given position.

• -fdiagnostics-generate-patch

this prints fix-it hints to stderr in unified diff format, after any diagnostics are printed. for example:

--- test.c
+++ test.c
@ -42,5 +42,5 @

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

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

• -fdiagnostics-show-template-tree

in the C++ frontend, when printing diagnostics that show 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 get highlighted with color ("double" and "float" in this case). iconic honestly. 🎀

• -fno-elide-type

by default, when the C++ frontend prints diagnostics showing mismatching template types, the common parts of the types are printed as "[...]" to simplify the error message. for example:

  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

specifying the -fno-elide-type flag suppresses that behavior. and heads up — this flag also affects the output of the -fdiagnostics-show-template-tree flag. 🦋

• -fdiagnostics-path-format=KIND

this 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, and inline-events is the default.

none means to not print diagnostic paths at all. 💀

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

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

inline-events means to print the events "inline" within the source code. this view 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 be printed as:

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

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

for example:

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

• -fdiagnostics-show-path-depths

this option hands you extra info when printing control-flow paths that are associated with a diagnostic.

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

heads up though — this is intended for use by gcc developers and plugin developers when debugging diagnostics that report interprocedural control flow. iykyk, it's giving insider tooling. 🫶

• -fno-show-column

do not print column numbers in diagnostics. this may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu. 💀

• -fdiagnostics-column-unit=UNIT

this selects the units for the column number. it affects traditional diagnostics (in the absence of -fno-show-column), as well as JSON format diagnostics if those were requested.

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

Setting UNIT to byte changes the column number to the raw byte count in all cases — that's literally what GCC was doing before version 11.1.0, so it's giving "vintage, but make it bytes" ✨

• -fdiagnostics-column-origin=ORIGIN — picks where the column numbering starts, i.e. which column gets to be column number one. Default is 1, which is both traditional GCC behavior and what the GNU style guide says, so if you want to be normal, leave it 🎀. Some utilities do better with an origin of 0, and honestly bestie, any non-negative value is fair game — you can pass whatever you want.
• -fdiagnostics-escape-format=FORMAT — okay so when GCC shows you the relevant source lines for a diagnostic, it normally just prints the source bytes straight up, raw, no filter 💅. BUT some diagnostics are about encoding drama in the source file — malformed UTF-8, Unicode normalization being messy, that kind of thing. Those particular diagnostics get flagged, and when GCC prints their source lines, it escapes any bytes that aren't printable ASCII.

This option is the one that controls how those bytes get escaped, bestie. Choose your own adventure.

The default FORMAT, unicode, shows Unicode characters that aren't printable ASCII like <U+XXXX>, and bytes that don't correspond to a Unicode character validly encoded in UTF-8 get shown as hexadecimal like <XX>.

For example: a source line with the string before, then the Unicode character U+03C0 ("GREEK SMALL LETTER PI", UTF-8 encoding 0xCF 0x80), then the byte 0xBF (a stray UTF-8 trailing byte), then the string after — for a diagnostic like that it prints:

         before<U+03C0><BF>after

Set FORMAT to bytes and it'll show all non-printable-ASCII bytes as <XX>, which means you see the actual underlying encoding of non-ASCII Unicode characters. Same example, bytes mode, you get:

         before<CF><80><BF>after

• -fdiagnostics-text-art-charset=CHARSET — okay some diagnostics come with "text art" diagrams: little visualizations made out of text, meant to be looked at in a monospaced font. They're so cute actually 🎨

This option picks which characters get used to print those diagrams — if any get printed at all. CHARSET can be none, ascii, unicode, or emoji.

• none — no diagrams. Cancelled. 🚫
• ascii — keeps them pure ASCII, so: "ASCII art", periodt.
• unicode — allows conservative use of unicode drawing characters (box-drawing characters and friends, you know the vibe).
• emoji — goes further and allows emoji in the output, like emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to pick the emoji variant of that character. iconic honestly 🌈

Default is emoji — UNLESS the environment variable LANG is set to C, in which case the default is ascii. iykyk.

• -fdiagnostics-format=FORMAT — swaps out the whole printing format for diagnostics. FORMAT is text, sarif-stderr, sarif-file, json, json-stderr, or json-file.

Default is text, obviously, like a normal person.

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

json is literally just a synonym for json-stderr. And json-stderr and json-file are identical to each other, apart from where the JSON goes — the former puts the JSON on stderr, while json-file writes it to source.gcc.json.

The emitted JSON is a top-level JSON array, and inside it are JSON objects representing the diagnostics. So like, the whole thing is one big array energy 💖

Diagnostics can have child diagnostics, which is so family-coded of them. 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 get 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". Family tree, fr fr.

So a diagnostic has a "kind". If that kind is "warning", then there's an "option" key describing the command-line option that controls the warning. Very demure, very mindful of it.

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, plus optional "start" and "finish" positions. A position is described by a "file" name, a "line" number, and three numbers that indicate a column position:

• "display-column" — counts display columns, so it accounts for tabs and multibyte characters.
• "byte-column" — counts raw bytes.
• "column" — equals one of the previous two, whichever the -fdiagnostics-column-unit option says.

All three columns are relative to the origin specified by -fdiagnostics-column-origin, which is typically 1 but can be set to something else — 0, for instance, for compatibility with other utilities that number columns from 0. The column origin gets recorded in the JSON output in the "column-origin" tag. And in the rest of the examples below, those extra column number outputs have been left out, for brevity. (ngl, appreciated 🙏)

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, bestie. 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, and those have labels. It might get 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 made 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 get 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 ..."
                       }

So basically that fixits thing? it's the glow-up suggestion 🎀 the fix-it hint is telling you to swap out the text from start up to but not including next, and drop string's value in its place. Deletions are just an empty string — nothing there, gone, bye 💨 — and insertions happen when start equals next. that's it, that's the whole trick.

If the diagnostic has a whole lil control-flow journey attached to it, it gets a "path" array of objects, one per event. Each event object has a "description" string, a "location" object, plus a "function" string and a "depth" number — those two are for representing interprocedural paths. The "function" is the current function at that event, and "depth" is the stack depth relative to some baseline: higher number = more frames are chillin' in the stack.

For example, that intraprocedural example shown over at -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 carry a boolean attribute called "escape-source", and it's lowkey hinting at whether non-ASCII bytes should be escaped when printing the relevant lines of source code. It's "true" for diagnostics that involve source encoding issues. 🦋

• -fno-diagnostics-json-formatting — okay so normally, when 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 adds newlines and indentation to visually emphasize the hierarchical structure of the JSON. cute, right? This flag suppresses all that whitespace. But heads up: it must be passed before the option it is to affect. It's meant for compatibility with tools that do not expect the output to contain newlines — like the output from older GCC releases. 💅

---

🚨 Options to Request or Suppress Warnings 🚨

Warnings are diagnostic messages reporting constructions that are not inherently erroneous but that are risky or suggest there may have been an error. So like… not a crime, but sus. 🙈

The following language-independent options don't enable specific warnings — they control the kinds of diagnostics GCC produces.

• -fsyntax-only — check the code for syntax errors, and then just… do nothing beyond that. queen of doing the bare minimum 💅
• -fmax-errors=n — caps the maximum number of error messages at n, and once you hit it, GCC bails out instead of trying to keep grinding through the source code. If n is 0 (the default), there's no limit on error messages. And if -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option. rules are rules ✨
• -w — inhibit all warning messages. all of them. gone.
• -Werror — make all warnings into errors. no mercy, bestie 😭
• -Werror= — make the specified warning into an error. you append the specifier for a warning; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch also takes a negative form, used to negate -Werror for specific warnings — for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect.

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

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

• -Wfatal-errors — this option makes the compiler abort compilation on the first error that occurred, instead of trying to keep going and printing further error messages. rip to the rest of your errors, they never got their moment 💀

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 warnings off; 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, like -Wall and -Wextra, turn on other options, like -Wunused, which may turn on further options, like -Wunused-value. it's chain reactions all the way down 🦋 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. iykyk 💅

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. no shortcuts for you, bestie 🎀

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.) that's just math, fr fr ✨

• -pedantic-errors — this one's giving strict mode, but make it error. Whenever the base standard (peep -Wpedantic) says "okay bestie you gotta say something here," this makes it an actual error. And yes, that covers some cases where there's undefined behavior at compile-time, plus a bunch of other cases that lowkey don't even stop valid standard programs from compiling. 💅

Heads up though: this is not the same as -Werror=pedantic. That latter one is basically not useful — it only turns the diagnostics controlled by -Wpedantic into errors, whereas this option also hits the required diagnostics that are always on or controlled by totally different options. Not the same vibe at all.

So if you want those required diagnostics (the ones that are warnings by default) to become errors, but you do not wanna light up the -Wpedantic diagnostics too? Just do -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic if you want them enabled but only as warnings). ✨

Some required diagnostics are errors by default already, but you can downgrade them to warnings with -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 -pedantic-errors makes those errors too. For instance: 💀

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

• -fpermissive — the chill pill 🧚. This downgrades some required diagnostics about nonconformant code from errors to warnings. So yeah, -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 fun fact: -fpermissive is the default for historic C language modes — -std=c89, -std=gnu89, -std=c90, -std=gnu90. Throwback era behavior fr fr. 👑

• -Wall — okay this is the one everyone types. It turns on all the warnings about constructions some users consider questionable, and that are easy to avoid (or tweak so the warning stops), even when macros are in the mix. It also enables some language-specific warnings described in the C++ Dialect Options and the Objective-C / 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

Real talk though: some warning flags are not implied by -Wall. Some of them warn about constructions users generally don't consider questionable but that occasionally you might wanna check for. Others warn about stuff that's necessary or hard to avoid in some cases, and there's no simple way to tweak the code to make the warning go away. Some get turned on by -Wextra, but a whole lot of them you gotta enable individually. No shortcuts bestie. 🙈

• -Wextra — this enables some extra warning flags that -Wall does not turn on. (And yes, this used to be called -W. The older name still works, but the newer one is way more descriptive. Glow-up complete. ✨)
  • -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 these 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) — this warns you about code affected by ABI changes. That includes code that might not be compatible with the vendor-neutral C++ ABI, plus the psABI for whatever particular target you're on. 😬

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. But -Wabi warns about more stuff if you select an older ABI version (with -fabi-version=n).

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

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

Ngl, an effort has been made to warn about all such cases, but there are probably some cases that don't get warned about even though G++ is generating incompatible code. And there may also be cases where warnings get emitted even though the code that's generated is compatible. So don't come for me. 😭

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

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 mangles a function type that has function-cv-qualifiers, the un-qualified function type was getting treated as a substitution candidate, which was wrong. Fixed in -fabi-version=8, which is the default for GCC 5.1. ✨
• decltype(nullptr) had an alignment of 1, which is giving unaligned accesses 😭. Heads up tho: this did not affect the ABI of a function with a nullptr_t parameter, because parameters have a minimum alignment. 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.) — did not affect the mangled name, which caused name collisions when function pointers were used as template arguments 💀. Fixed in -fabi-version=10, the default for GCC 6.1.

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

• 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. 🧚

---

🌸 Some more options (yes there are so many) 🌸

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

C++ requires that unqualified uses of a name within a class have the same meaning across 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. Or, alternately, you can reduce the error case to a warning with -Wno-error=changes-meaning or -fpermissive. 🙈

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

---

✨ -Wchar-subscripts ✨

Warns if an array subscript has type char. This is a super common cause of error, because 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 changes 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.

Bestie, listen: disabling the error for this warning can result in poorly optimized code, and it's useful only in the case of very minor changes like bug fixes to an existing code-base. Completely disabling the warning is not recommended 🫶

---

✨ -Wno-coverage-too-many-conditions ✨

Warns if -fcondition-coverage is used and an expression has too many terms, and GCC gives up on coverage. Coverage gets given up when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default.

---

✨ -Wno-coverage-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.

---

✨ -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 a machine like that, doing computations using double values is much more expensive, because of the overhead required for software emulation 😭

It is 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. 💀

---

✨ -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 ✨

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, iykyk). 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, so like... it's fine, bestie.

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. 💅

---

✨ -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 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. 💀

✨ -Wformat-overflow ✨

✨ -Wformat-overflow=1 ✨

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

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. ✨

At level 2, that call from the example gets flagged again, but this time the tea is different: with a equal to a 32-bit INT_MIN, that first %i directive is gonna write some of its digits waaay past the end of your destination buffer. 😭 To make the call safe no matter what those two variables are doing, you gotta bump the destination buffer up to at least 34 bytes. GCC is nice enough to drop the minimum buffer size in an informational note right after the warning, so read the note bestie. 💖

Another way to fix it (instead of just making the buffer bigger) is to constrain the range of the formatted values. The maximum length of string arguments can be bounded by specifying the precision in the format directive. And when numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, picking an appropriate length modifier for the format specifier will shrink 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 brings the maximum required buffer size down to 24 bytes. 💅

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

• -Wno-format-zero-length — if -Wformat is specified, do not warn about zero-length formats. The C standard says zero-length formats are allowed, so like… they're legal, okay? ✨
• -Wformat-nonliteral — if -Wformat is specified, also warn when the format string is not a string literal and therefore 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. Right now, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, like 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 the 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. the way that it just clocks the mismatch 💀
• -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 can't be figured out at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic as -Wformat-overflow.
  • -Wformat-truncation
  • -Wformat-truncation=1 — Level 1 of -Wformat-truncation, enabled by -Wformat, employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
  • -Wformat-truncation=2 — Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
• -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. it's giving Y2K era anxiety 🎀
• -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. 🫶
• -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).

• -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.

• -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.
• -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 a 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.
• -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:

• -Wimplicit-fallthrough=4 — this one matches, case sensitively, exactly one of these regexes 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]*)?">

• -Wimplicit-fallthrough=5 — nope, absolutely not, this level does not recognize any comment as a fallthrough comment. The only thing that silences the warning is attributes. Periodt. 💀

The comment has 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. ✨

---

🌈 a whole bunch more -W flags 🌈

• -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — this one controls whether warnings triggered by the warn_if_not_aligned attribute even get issued. These warnings are on by default, so they're just kinda vibing there unless you turn them off. ✨
• -Wignored-qualifiers (C and C++ only) — warns you when a function's return type has a type qualifier like const. For ISO C that qualifier does literally nothing, because the value a function hands back isn't an lvalue. For C++, the warning only fires for scalar types or void. And heads up: ISO C straight-up prohibits qualified void return types on function definitions, so those return types always get a warning whether you asked for this option or not. Also enabled by -Wextra. 🫶
• -Wno-ignored-attributes (C and C++ only) — controls warnings for when an attribute gets ignored. It's different from -Wattributes because it yells whenever the compiler decides to drop an attribute — not when the attribute is unknown or used in the wrong place or whatever. Enabled by default. 🎀
• -Wmain — warns you if the type of main is suspicious 👀. main should be a function with external linkage, returning int, and taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and gets enabled by either -Wall or -Wpedantic.

It also gets upgraded to an error by -pedantic-errors. so like, be careful. 😭

• -Wmisleading-indentation (C and C++ only) — warns when your indentation is lying about the block structure. Specifically, it fires for if, else, while, and for clauses that have a guarded statement with no braces, followed by an unguarded statement sitting at the same indentation. It's giving chaos. 💅

In this example the call to bar is misleadingly indented as if it were guarded by the if conditional:

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

When you've got mixed tabs and spaces, the warning uses the -ftabstop= option to figure out if the statements line up (and that defaults to 8).

The warning doesn't fire for code that involves multiline preprocessor logic, like this example:

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

It also doesn't fire after a #line directive, because that usually means autogenerated code, and you can't make any assumptions about the layout of the file that directive points at.

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 could hurt the correctness or efficiency of the generated code. For example, it fires for declarations of aliases that use attributes specifying less restrictive requirements than their targets. Usually that's 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 it considers include: alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.

In C++, the warning fires 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. The attributes deprecated, error, and warning suppress the warning..

You can use the copy attribute to slap the same set of attributes on a declaration as another declaration has, without listing them all out yourself. That attribute works on declarations of functions, variables, or types.

-Wmissing-attributes is enabled by -Wall.

For example, since the declaration of the primary function template below uses both attribute malloc and alloc_size, the declaration of the explicit specialization gets diagnosed for missing one of the attributes:

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

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

• -Wmissing-braces — warns if an aggregate or union initializer isn't fully bracketed. In the example below the initializer for a isn't fully bracketed, but b's is:

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

Enabled by -Wall. 💖

• -Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only) — warns if a user-supplied include directory straight up doesn't exist. 👑 This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it's partially enabled by default — it only warns for -I and -J.
• -Wno-missing-profile — controls warnings when feedback profiles are missing while you're using the -fprofile-use option. This one catches the cases where a new function or a new file gets added between compiling with -fprofile-generate and compiling with -fprofile-use, without regenerating the profiles. In those cases the profile feedback data files don't contain any profile feedback info for the newly added function or file. Also, if profile count data (.gcda) files get removed, GCC can't use any profile feedback info at all. In all those cases, warnings get issued to let you know 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 that is not recommended and should only be done when non-existent profile data is justified. fr fr.
• -Wmismatched-dealloc — warns for calls to deallocation functions with pointer arguments that came back 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 using attribute malloc. Unless it's 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 example below, 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 look like they're guarded by a clause like if, else, for, switch, or while, when actually only the first statement is guarded after the macro expands. Sneaky!! 💀

For example:

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

will increment y unconditionally, not just when c holds. You can usually fix this 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, like when there's an assignment in a spot where a truth value is expected, or when operators are nested in ways that people often get confused about. 😵‍💫

It also warns if a comparison like x<=y<=z shows up; that's equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from the one ordinary mathematical notation gives you.

It also warns for dangerous uses of the GNU extension to ?: with an omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Programmers often 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 point to someone trying to do 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) — ngl bestie, this one calls you out when you move a value into itself with std::move. And like… that std::move usually does absolutely nothing. 💀

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 out here violating the sequence point rules from the C and C++ standards. 🚨

Okay so the C and C++ standards define the order expressions get evaluated in terms of sequence points, which are basically a partial ordering between the execution of parts of the program: the bits that run before the sequence point, and the bits that run after it. These pop up after the evaluation of a full expression (one that isn't part of a bigger expression), after evaluating the first operand of a &&, ||, ? : or , (comma) operator, before a function gets 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 subexpressions get evaluated in is not specified. And all these rules only describe a partial order, not a total one — like, if two functions get called inside one expression with no sequence point between them, the order they're called in is not specified. That said, the standards committee ruled that function calls do not overlap. ✨

It's also not specified when, between sequence points, modifications to object values actually take effect. Programs whose behavior depends on that 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 has been found fairly 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 is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at <https://gcc.gnu.org/readings.html>.

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

• -Wno-return-local-addr — do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. 🙈
• -Wreturn-mismatch — warns about return statements with no 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, that 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 is treated as an error. It can be downgraded to a warning using -fpermissive (along with other warnings), or for just this warning, with -Wno-error=return-mismatch.

• -Wreturn-type — warns whenever a function is defined with a return type that defaults to int (unless -Wimplicit-int is active, which takes precedence). Also warns if execution may reach the end of the function body, or if the function doesn't contain any return statement at all. 🫠

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

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

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 is a default label). This warning is enabled by -Wall. 🔀
• -Wswitch-default — warns whenever a switch statement doesn't have a default case. periodt. 💅
• -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 is a default label. 👑
• -Wno-switch-bool — do not warn when a switch statement has an index of boolean type and the case values are outside the range of a boolean type. It's possible to suppress this warning by casting the controlling expression to a type other than bool. For example:

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

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

• -Wno-switch-outside-range — this option controls warnings when a switch case has a value that's outside of its respective type range. This warning is enabled by default for C and C++ programs. 🎯
• -Wno-switch-unreachable — do not warn when a switch statement contains statements between the controlling expression and the first case label, which will never be executed. 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 is not used. This warning is enabled by -Wall. 🦋
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration. This option is not enabled by -Wunused unless -Wextra is also specified.

To suppress this warning use the unused attribute.

• -Wno-unused-result — do not warn if a caller of a function marked with attribute warn_unused_result 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, aside from its declaration. It also implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall.

Wanna shut it up? Use the "unused" attribute. 💅

• -Wunused-const-variable
• -Wunused-const-variable=n — warn whenever a constant static variable is unused aside from its declaration.

To suppress this warning, use the "unused" attribute.

• -Wunused-const-variable=1 — warn about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.

-Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of "#define"s. no cap.

• -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (excluding system headers). It is equivalent to the short form -Wunused-const-variable. This level must be explicitly requested in both C and C++ because it might be hard to clean up all headers included. 😭

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

This warning is enabled by -Wall.

• -Wunused — all the above -Wunused options combined ✨, except those documented as needing to be specified explicitly.

In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter.

-Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++. 🎀

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

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

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

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

void adjust_pointers (int**, int);

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

To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation. 💡 iconic problem-solving honestly

• -Wuse-after-free=2 is included in -Wall.

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

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

• -Wuninitialized — warn if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warn if a non-static reference or non-static "const" member appears in a class without constructors.

In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.) fr fr

If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.

These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared "volatile". Because these warnings depend on optimization, the exact variables or elements for which there are warnings depend on the precise optimization options and version of GCC used.

Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. 🫶

In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about "b" being uninitialized in the following snippet:

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

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

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

-Winvalid-memory-model is enabled by default.

• -Wmaybe-uninitialized — for an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time.

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

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

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

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

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

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

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

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option. 👑
• -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 — bestie, this one shuts off the warning you'd get if you assign a priority from 0 to 100 to a constructor or destructor. Real talk: the constructor and destructor attributes let you give a priority to that constructor/destructor so you can control the order it runs in — before main gets called, or after it returns. But the priority values must be greater than 100, because the compiler is out here reserving priority values between 0 and 100 for the implementation itself. Respect the reservation, okay? 💅
• -Wstrict-aliasing — this one only does anything when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules your compiler is leaning on for optimization. Heads up though: it does not catch all cases, it just tries to catch the more common pitfalls. It's included in -Wall, and it's equivalent to -Wstrict-aliasing=3. ✨

---

-Wstrict-aliasing=n levels, unlocked 🔓

Again, only active when -fstrict-aliasing is active — same deal as above, warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels = higher accuracy (fewer false positives). But 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 your code, since it has very few false negatives. But babes, it has many false positives. It warns for all pointer conversions between possibly incompatible types, even if they're never dereferenced. Runs in the front end only.
• Level 2 — aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
• Level 3 (default for -Wstrict-aliasing) — should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types. 👑

---

-Wstrict-overflow and -Wstrict-overflow=n 🌈

Only active when signed overflow is undefined, bestie. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. And like — 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. It's giving dependency. 😌

An optimization that assumes signed overflow doesn't occur is perfectly safe if the values of the variables involved are such that overflow never actually occurs. Which is why this warning can easily give a false positive — a warning about code that isn't even a problem. To help focus on important issues, several warning levels are defined. Also, 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 — warns 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 warns about other cases where a comparison is simplified to a constant. 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. And -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — also warns about other cases where a comparison is simplified. For example: "x + 1 > 1" is simplified to "x > 0".
• -Wstrict-overflow=4 — also warns about other simplifications not covered by the above cases. For example: "(x * 10) / 5" is simplified to "x * 2".
• -Wstrict-overflow=5 — also warns 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 — warns 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. Like, for example, the call to "strcmp" below is diagnosed because its result is necessarily non-zero no matter what's in 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. 🌸

---

-Wstringop-overflow and friends 🍓

Options: -Wno-stringop-overflow, -Wstringop-overflow, -Wstringop-overflow=type

These 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. (Yes, one greater. Not me double-checking that. 💀)

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 it 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. Not it catching that. 🙈

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 — 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's 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 — 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 — 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 — 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 — warns 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);
}

Like, say you're calling strncpy and it copies to d just the characters that come before the terminating NUL — and then it does not append the NUL at the end. 💀 Bestie, assuming the result of strncpy is automatically a NUL-terminated string is honestly one of the most common mistakes ever, so yeah, the call gets diagnosed. If you already know the result isn't supposed to be NUL-terminated and you want the warning to chill, just call memcpy instead.

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

Next example, next drama 🎀 — here the call to strncpy passes the size of the destination buffer as the bound. So if the length of the source string is equal to or greater than that size? The copy will not be NUL-terminated. Which means... diagnosed again, sorry. To dodge the warning, you'd specify sizeof buf - 1 as the bound and then set the last element of the buffer to NUL.

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

Now, sometimes you actually want a character array to just hold a sequence of bytes with no terminating NUL at all 💅. In that case you can annotate the array with the nonstring attribute and the warning goes away. But — and this is the important part — arrays like that are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuses of those arrays, 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, and it's all about the level, bestie. The level comes from the strict_flex_array (level) attribute attached to the trailing array field of a structure, if that's available — otherwise it falls back to whatever the -fstrict-flex-arrays=level option says. And psst: -Wstrict-flex-arrays only actually does anything when level is greater than 0.

• Level 1: warnings get 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.
• Level 2: on top of level 1, extra warnings fire for a trailing one-element array reference of a structure, if the array is referenced as a flexible array member.
• Level 3: on top of level 2, even more warnings — this time for a trailing zero-length array reference of a structure, if the array is referenced as a flexible array member.

This one hits different when -ftree-vrp is active (which is the default for -O2 and above), but some warnings may still get 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. 👑

• -Wsuggest-attribute=pure
• -Wsuggest-attribute=const
• -Wsuggest-attribute=noreturn
• -Wmissing-noreturn
• -Wsuggest-attribute=malloc
• -Wsuggest-attribute=returns_nonnull
• -Wno-suggest-attribute=returns_nonnull

Warns about functions that might be candidates for the attributes pure, const, noreturn, malloc, or returns_nonnull. Note the tea here: the compiler only warns for functions that are visible in other compilation units — or, for the pure and const cases, if it cannot prove that the function returns normally. A function counts as returning normally if it doesn't contain an infinite loop and doesn't return abnormally by throwing, calling abort, or trapping. This whole analysis needs option -fipa-pure-const, which is enabled by default at -O and higher. And higher optimization levels make the analysis more accurate. 🔥

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

Warns about function pointers that might be candidates for format attributes. And bestie, note the ✨vibe✨: these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing, or return statements should have a corresponding format attribute in the resulting type. Meaning the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function (respectively) should also carry a format attribute, otherwise you get 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 that would be appropriate for format attributes may not get detected. 🤷‍♀️

• -Wsuggest-attribute=cold

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

---

💖 -Walloc-size 💖

Warns about calls to allocation functions decorated with the 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 aligned_alloc, alloca, calloc, malloc, and realloc. No cap, that's the list. 🎀

🌸 -Walloc-zero 🌸

Warns about calls to allocation functions decorated with the attribute alloc_size that specify zero bytes — again including the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. And here's why you should care: because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc it's been deprecated), relying on it may result in subtle portability bugs and should be avoided. Fr fr, avoid it. 🫶

🦋 -Wcalloc-transposed-args 🦋

Warns about calls to allocation functions decorated with attribute alloc_size that take two arguments, where sizeof is used as the earlier size argument and not as the later one. This is a coding style warning, okay, it's giving aesthetic. The first argument to calloc is documented to be the number of elements in the array, while the second is the size of each element — so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If the 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 by using sizeof in the later argument as well. 💅

⭐ -Walloc-size-larger-than=byte-size ⭐

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

💜 -Wno-alloc-size-larger-than 💜

Disables -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. Easy peasy. ✨

---

🎀 -Walloca 🎀

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

🔥 -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 limiting its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. And arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range. Iconic, honestly.

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 example above, 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. Got it? 💅

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

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, which causes a larger than expected argument to be implicitly cast into the alloca call. Sneaky. 🙈

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

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

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

💜 -Wno-alloca-larger-than 💜

Disables -Walloca-larger-than= warnings. The 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. Obsessed with the chaos, ngl.

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 🌸

Warns about out-of-bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It hits harder when -ftree-vrp is active (the default for -O2 and above), but a subset of instances get issued even without optimization. 💖

okay so lowkey the flexible-array-member lore is so specific and I need you to lock in bestie 💖

By default, that trailing array sitting at the end 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 three things:

• a flexible array member per the C99 standard onwards ([]) ✨
• a GCC zero-length array extension ([0])
• a one-element array ([1])

And because of that, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned about by default. No cap.

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

• when level<=1, no change to the default behavior 🙈
• when level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays
• when level=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays

-Warray-bounds=1

This is the default warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested. She doesn't come free, 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 though: this warning level may give a larger number of false positives and is deactivated by default. It's giving 🚩 but go off I guess.

---

🌸 -Warray-compare 🌸

Warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:

int arr1[5];
int arr2[5];
bool same = arr1 == arr2;

-Warray-compare is enabled by -Wall. ✨

---

💅 -Warray-parameter / -Warray-parameter=n 💅

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

If the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the "T[static N]" form.

For example, the warning triggers for the second declaration of "f" because the first one with the keyword "static" specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to "f".

void f (int[static 4]);
void f (int[]);           // warning (inconsistent array form)

void g (void)
{
  int *p = (int *)malloc (1 * sizeof (int));
  f (p);                  // warning (array too small)
  ...
}

At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.

void g (int*);
void g (int[]);     // no warning
void g (int[8]);    // warning (inconsistent array bound)

-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments.

The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form -Wno-array-parameter is equivalent to -Warray-parameter=0. 🫶

---

🦋 -Wattribute-alias=n / -Wno-attribute-alias 🦋

Warn about declarations using the "alias" and similar attributes whose target is incompatible with the type of the alias.

-Wattribute-alias=1

The default warning level of the -Wattribute-alias option diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs. 👀

-Wattribute-alias=2

At this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.

Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice". Periodt.

-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0.

---

🌈 -Wbidi-chars=[none|unpaired|any|ucn] 🌈

Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 😭

There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.

By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified. 💜

---

🍓 -Wbool-compare 🍓

Warn about boolean expression compared with an integer value different from "true"/"false". For instance, the following comparison is always false:

int n = 5;
...
if ((n > 1) == 2) { ... }

This warning is enabled by -Wall. ✨

---

🧚 -Wbool-operation 🧚

Warn 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 👑

Warn 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;

---

💖 -Wduplicated-cond 💖

Warn 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) { ... }

---

🔥 -Wframe-address 🔥

Warn 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. Bestie, do not play with this one.

---

🎀 -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. She's just minding her own business fr fr.

---

⭐ -Wno-discarded-array-qualifiers (C and Objective-C only) ⭐

Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a "const int (*)[]" variable is passed to a function that takes a "int (*)[]" parameter. This option can be used to suppress such a warning.

---

🦋 -Wno-incompatible-pointer-types (C and Objective-C only) 🦋

Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.

By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=incompatible-pointer-types.

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

---

💜 -Wno-int-conversion (C and Objective-C only) 💜

Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used.

By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=int-conversion.

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

---

🌸 -Wzero-length-bounds 🌸

Warn about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are undefined. 🫥

For example, the first two stores in function "bad" are diagnosed because the array elements overlap the subsequent members "b" and "c". The third store is diagnosed by -Warray-bounds because it is beyond the bounds of the enclosing object.

struct X { int a[0]; int b, c; };
struct X x;

                   void bad (void)
                   {
                     x.a[0] = 0;   // -Wzero-length-bounds
                     x.a[1] = 1;   // -Wzero-length-bounds
                     x.a[2] = 2;   // -Warray-bounds
                   }

okay so the tea here: -Wzero-length-bounds is switched on by -Warray-bounds. they're a package deal bestie 💅

---

• -Wno-div-by-zero — bestie, do not warn when you divide an integer by zero at compile time. but floating-point division by zero? that's not warned about, because lowkey it can be a totally legit way to get yourself infinities and NaNs ✨
• -Wsystem-headers — prints out warning messages for stuff found in system header files. normally those warnings are suppressed, because the assumption is they usually don't mean there's a real problem and they'd just make the compiler output harder to read (it's giving wall-of-text 💀). using this command-line option tells GCC to emit warnings from system headers as if they happened in your own user code. but heads up: using -Wall together with this option does not warn about unknown pragmas in system headers — for that you gotta also use -Wunknown-pragmas. iykyk 🫶
• -Wtautological-compare — yells at you if a self-comparison always evaluates to true or false. it catches all kinds of oopses like:

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

it also warns about bitwise comparisons that always come out true or false, for instance:

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

which will always be false. bestie. it literally cannot be true 😭

this warning is enabled by -Wall.

• -Wtrampolines — warns about trampolines that get generated for pointers to nested functions. so what even is a trampoline? it's a small piece of data or code created at run time on the stack when you take the address of a nested function, and it's used to call that nested function indirectly. for some targets it's made up of data only, so it needs no special treatment 🌸. but for most targets it's made up of code, which means the stack has to be made executable for the program to work properly. that's the whole vibe check 👑
• -Wfloat-equal — warns if floating-point values are used in equality comparisons. ngl this one's important so lemme explain the reasoning: sometimes it's convenient (for you, the programmer) to think of floating-point values as approximations to infinitely precise real numbers. if that's the situation you're in, then you need to compute — either by analyzing the code or by some other means — the maximum or likely maximum error that the computation introduces, and allow for it when you do comparisons (and when you produce output too, but that's a whole different problem, not our business rn). specifically: instead of testing for equality, you should check whether the two values have ranges that overlap, and you do that with the relational operators. so yeah, equality comparisons are probably mistaken. periodt 💅
• -Wtraditional (C and Objective-C only) — warns about certain constructs that behave differently in traditional C versus ISO C. it also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs you should really be avoiding. the rundown:
  • Macro parameters that show up inside string literals in the macro body. in traditional C, macro replacement takes place inside string literals — in ISO C it does not. different behaviour, bestie.
  • In traditional C, some preprocessor directives straight up didn't exist. traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. so -Wtraditional warns about directives that traditional C understands but ignores because the # isn't the first character on the line. it also suggests you hide directives like #pragma that traditional C doesn't understand by indenting them. and some traditional implementations don't recognize #elif at all, so this option suggests avoiding it entirely 🙈.
  • 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, just so you know.) note that these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. using these macros in user code would normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases. iconic behaviour honestly ✨
  • 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. some traditional C compilers don't even accept this construct.
  • 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. no cap.
  • 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. real talk. this is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. this warning is intentionally not issued for prototype declarations or variadic functions, because these ISO C features appear in your code when you're using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. this warning is also bypassed for nested functions, because that feature was already a GCC extension and thus isn't relevant to traditional C compatibility. 🦋
• -Wtraditional-conversion (C and Objective-C only) — warns if a prototype causes a type conversion that's different from what would happen to the same argument if there were no prototype at all. this includes conversions of fixed point to floating and vice versa, plus conversions changing the width or signedness of a fixed-point argument — except when it's the same as the default promotion. 💜
• -Wdeclaration-after-statement (C and Objective-C only) — warns when a declaration shows up after a statement in a block. this construct, which you'll know from C++, was introduced with ISO C99 and is by default allowed in GCC. ISO C90 does not support it 🫠.

this warning is upgraded to an error by -pedantic-errors. so like, don't play.

• -Wshadow — warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) — or whenever a built-in function gets 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 all the instances of local shadowing too. which means -Wno-shadow=local and -Wno-shadow=compatible-local get ignored when -Wshadow is in use 🙃. 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. that's it, that's the flag 💅
• -Wshadow=global — warns for any shadowing. same as -Wshadow.
• -Wshadow=local — warns when a local variable shadows another local variable or parameter.
• -Wshadow=compatible-local — warns when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. in C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. the creation of this flag (on top of -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it's most likely intentional — not a bug or a typo — as shown in this example:

for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
  for (int i = 0; i < N; ++i)
  {
    ...
  }
  ...
}

since those two variables i in the example have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. and because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch it and emit an error or warning anyway. using this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing ✨. note that this also means shadowing const char *i with char *i does not emit a warning. sneaky.

this warning is also enabled by -Wshadow=local.

• *-Wlarger-than=byte-size* — warns whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is enabled by default. the warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-larger-than.

it also warns for calls to bounded functions like memchr or strnlen that specify a bound greater than the largest possible object, which by default is PTRDIFF_MAX bytes. these warnings can only be disabled by -Wno-larger-than — that's the only way out, bestie 🫶

• -Wno-larger-than — disables -Wlarger-than= warnings. the option is equivalent to -Wlarger-than=SIZE_MAX or larger. 💀
• -Wframe-larger-than=byte-size — ok so this one yells at you when a function's stack frame is bigger than byte-size. BUT bestie listen, the math it does to figure out the frame size is approximate and not conservative, so the real requirement could be lowkey bigger than byte-size even when it stays silent. Also, anything you grabbed with alloca, variable-length arrays, or related constructs is not counted by the compiler when it decides whether to warn. -Wframe-larger-than=PTRDIFF_MAX is on by default. You can kill these warnings either by handing it a byte-size of SIZE_MAX or more, or by using -Wno-frame-larger-than. 💅
• -Wno-frame-larger-than — turns off the -Wframe-larger-than= warnings. It's exactly the same as -Wframe-larger-than=SIZE_MAX or bigger. ✨
• -Wfree-nonheap-object — screams at you when you try to deallocate something that was either not heap-allocated at all, or you're freeing it through a pointer that did not come back from a prior call to the matching allocation function. Like, classic example: stpcpy returns a pointer to the terminating nul character, not the start of the object, so the free down here gets called out:

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

It's part of -Wall, so if you've got -Wall you already have it. 🎀

• -Wstack-usage=byte-size — warns when a function's stack usage might exceed byte-size. Unlike the frame one, the computation here is conservative, and any space from alloca, variable-length arrays, or related constructs is included when the compiler decides whether to warn.

The message matches the output of -fstack-usage. Here's the tea:

• If the stack usage is fully static but goes over the amount you specified, you get:

                  warning: stack usage is 1120 bytes

• If the stack usage is (partly) dynamic but bounded, you get:

                  warning: stack usage might be 1648 bytes

• If the stack usage is (partly) dynamic and not bounded, you get:

                  warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is on by default. You can disable these warnings either by specifying 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 when the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And with -funsafe-loop-optimizations, it warns when the compiler does make such assumptions. 😭
• -Wno-pedantic-ms-format (MinGW targets only) — combine this with -Wformat and -pedantic and no GNU extensions, and it disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I 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 gives those types a size of 1, purely for convenience when you're doing calculations with void * pointers and pointers to functions. In C++, it also warns when an arithmetic operation involves NULL. -Wpedantic enables this one too.

Heads up: -pedantic-errors upgrades this warning to an actual error. 💀

• -Wno-pointer-compare — stops warning when a pointer is compared against a zero character constant. That pattern usually means the pointer was meant to be dereferenced. Like:

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

Note bestie, 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 when a comparison is always true or always false because of the limited range of the data type, but it does not warn for constant expressions. Example: warn if an unsigned variable gets compared against zero with < or >=. -Wextra enables this one too. 🔥
• -Wabsolute-value (C and Objective-C only) — warns when you call a standard function that computes the absolute value of an argument when a more appropriate standard function exists. Classic: calling abs(3.14) triggers it, because the right call for the absolute value of a double argument is fabs. It also fires when the argument in a call to such a function has an unsigned type. You can silence it with an explicit type cast, and -Wextra enables it too. 👑
• -Wcomment
• -Wcomments — warns whenever a comment-start sequence /* shows up inside a /* comment, or whenever a backslash-newline appears inside a // comment. -Wall enables this. 💜
• -Wtrigraphs — warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs inside comments are not warned about, except the ones that would form escaped newlines.

-Wall implies this option. If you didn't pass -Wall, it's still enabled unless trigraphs are enabled. If you want trigraph conversion without the warnings but still want the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. 🌸

• -Wundef — warns if an undefined identifier gets evaluated inside an #if directive. Those identifiers get replaced with zero. ⭐
• -Wexpansion-to-defined — warns whenever defined shows up in the expansion of a macro (including when the macro is expanded by an #if directive). That usage is not portable. -Wpedantic and -Wextra enable this one too. 🧚
• -Wunused-macros — warns about macros defined in the main file that never get used. A macro counts as used if it's expanded or tested for existence at least once. The preprocessor also warns if the macro hasn't been used by the time it gets 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 inside skipped conditional blocks, the preprocessor still reports it as unused. To dodge the warning in that case, you could improve the scope of the macro's definition — like moving it into the first skipped block. Or you could just give it a dummy use, something like:

        #if defined the_macro_causing_the_warning
        #endif

• -Wno-endif-labels — don't warn when an #else or #endif is followed by text. Older programs do this sometimes, with code shaped like:

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

The second and third FOO are supposed to be in comments. This warning is on by default. 🎀

• -Wbad-function-cast (C and Objective-C only) — warns when a function call gets cast to a non-matching type. Example: warn if a call to a function returning an integer type gets cast to a pointer type. 😭
• -Wc90-c99-compat (C and Objective-C only) — warns about features that aren't in ISO C90 but are in ISO C99. Like: variable length arrays, the long long type, the bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled inside the expression that follows __extension__. ✨
• -Wc99-c11-compat (C and Objective-C only) — warns about features that aren't in ISO C99 but are in ISO C11. Like: anonymous structures and unions, the _Atomic type qualifier, the _Thread_local storage-class specifier, the _Alignas specifier, the Alignof operator, the _Generic keyword, and so on. Also independent of the standards mode, and 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 that aren't in ISO C11 but are in ISO C23. Like: omitting the string in _Static_assert, using [[]] syntax for attributes, using decimal floating-point types, and so on. Independent of the standards mode, and warnings are disabled in the expression after __extension__. FYI the name -Wc11-c2x-compat is deprecated. 💀

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

• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that live outside the common subset of ISO C and ISO C++, e.g. a request for implicit conversion from void * to a pointer to a non-void type. 🍓
• -Wc++11-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers in ISO C++ 1998 that became keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall. 🔥
• -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by -Wall. 💅
• -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by -Wall. 🌈
• -Wc++20-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by -Wall. ⭐
• -Wno-c++11-extensions (C++ and Objective-C++ only) — don't warn about C++11 constructs in code you're compiling with an older C++ standard. Even without this option, some C++11 constructs only get diagnosed if you use -Wpedantic. 🙈
• -Wno-c++14-extensions (C++ and Objective-C++ only) — don't warn about C++14 constructs in code being compiled with an older C++ standard. Even without this option, some C++14 constructs only get diagnosed if -Wpedantic is used. 💜
• -Wno-c++17-extensions (C++ and Objective-C++ only) — don't warn about C++17 constructs in code being compiled with an older C++ standard. Even without this option, some C++17 constructs only get diagnosed if -Wpedantic is used. 🌸
• -Wno-c++20-extensions (C++ and Objective-C++ only) — okay bestie, if you're compiling with an older C++ standard and you do not want to hear about C++20 constructs, this is your girl. She says: no warnings for C++20 stuff. But lowkey, even without this option, some C++20 constructs will only be diagnosed if you use -Wpedantic. So like, -Wpedantic is the strict queen who catches everything. 💅
• -Wno-c++23-extensions (C++ and Objective-C++ only) — same energy as above but for C++23 constructs in your older-standard code. No warnings unless you bring in -Wpedantic, because some C++23 things only get diagnosed there. Periodt. ✨
• -Wno-c++26-extensions (C++ and Objective-C++ only) — yes, C++26 is already a thing, and this flag says "don't warn me about C++26 constructs when I'm using an older standard." But again, even without this option, some C++26 constructs will only be diagnosed if you use -Wpedantic. The future is now, bestie. 🦋
• -Wcast-qual — warns whenever you cast a pointer in a way that removes a type qualifier from the target type. For example, if you cast a "const char *" to an ordinary "char *", GCC will be like "bestie, why are you trying to write to read-only memory?!" 😭

It also warns when you make a cast that introduces a type qualifier in an unsafe way. For example, casting "char **" to "const char **" is unsafe, as in this example:

/* p is char ** value.  */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK.  */
*q = "string";
/* Now char** pointer points to read-only memory.  */
**p = 'b';

See? That's a whole mess. Don't do it. 🚫

• -Wcast-align — warns whenever you cast a pointer such that the required alignment of the target is increased. Like, if you cast a "char *" to an "int *" on machines where integers can only be accessed at two- or four-byte boundaries. The compiler is basically like "bestie, that pointer is not aligned for that type, this is giving undefined behavior energy." 💀
• -Wcast-align=strict — same as above but regardless of the target machine. So even if your machine is chill about unaligned access, GCC will still warn you if you cast a "char *" to an "int *". Strict queen behavior. 👑
• -Wcast-function-type — warns when a function pointer is cast to an incompatible function pointer. Here's the tea: in a cast involving function types with a variable argument list, only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, like "int" vs. "long" on ILP32 targets. Likewise type qualifiers are ignored. The function type "void (*) (void)" is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types, this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by -Wextra. So if you're doing sketchy function pointer casts, this one has your back. 🔥
• -Wcast-user-defined — warns when a cast to reference type does not involve a user-defined conversion that the programmer might expect to be called. Like this:

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

This warning is enabled by default. So if you're casting to a reference and you think a conversion operator is gonna run but it doesn't, GCC is like "bestie, no, that's not what's happening." 🙈

• -Wwrite-strings — okay so when compiling C, this gives string constants the type "const char[length]" so that copying the address of one into a non-"const" "char *" pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about using "const" in declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings. 💅

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

This warning is upgraded to an error by -pedantic-errors in C++11 mode or later. So yeah, it's serious. 😤

• -Wclobbered — warns for variables that might be changed by "longjmp" or "vfork". This warning is also enabled by -Wextra. So if you're doing wild jumps and forks, GCC is like "bestie, that variable might get clobbered, watch out." 🫶
• -Wno-complain-wrong-lang — by default, language front ends complain when a command-line option is valid, but not applicable to that front end. This may be disabled with -Wno-complain-wrong-lang, which is mostly useful when invoking a single compiler driver for multiple source files written in different languages, for example:

$ g++ -fno-rtti a.cc b.f90

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. The latter front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which may be disabled with -Wno-complain-wrong-lang. So if you're mixing languages in one command, this is your "stop complaining, I know what I'm doing" flag. 😌

• -Wcompare-distinct-pointer-types (C and Objective-C only) — warns if pointers of distinct types are compared without a cast. This warning is enabled by default. So if you're comparing an "int *" to a "char *" without a cast, GCC is like "bestie, those are not the same thing, are you sure?" 🤨
• -Wconversion — warns 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"; and conversions to smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like "abs ((int) x)" and "ui = (unsigned) -1", or if the value is not 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++, also warn 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.

Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion. So yeah, lots of layers here. Stay woke. 👀

• -Wdangling-else — warns about constructions where there may be confusion to which "if" statement an "else" branch belongs. Here is 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)". This is often not what the programmer expected, as illustrated in the above example by indentation the programmer chose. When there is the 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 is 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. So if your indentation is lying to you, GCC will call it out. 💅

• -Wdangling-pointer
• -Wdangling-pointer=n — warns 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. In addition, warn about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but 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. -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);
}

So basically, if you're using a pointer to something that's already gone, GCC is like "bestie, that's a ghost, stop it." 👻

• -Wdate-time — warns when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are encountered as they might prevent bit-wise-identical reproducible compilations. So if you're trying to get reproducible builds, these macros are the enemy. GCC is like "bestie, that timestamp is gonna mess up your reproducibility, just so you know." ⏰
• -Wempty-body — warns if an empty body occurs in an "if", "else" or "do while" statement. This warning is also enabled by -Wextra. So if you accidentally write if (x); with a semicolon and no body, GCC is like "bestie, did you mean to do nothing here? Because that's what you did." 💀
• -Wno-endif-labels — do not warn about stray tokens after "#else" and "#endif". So if you have like #endif FOO and you don't want GCC to complain about the FOO being there, this is your flag. It's like "bestie, I know it's extra, leave me alone." 🙈
• -Wenum-compare — warns 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. So if you're comparing apples to oranges (or enum A to enum B), GCC is like "bestie, those are not the same type, why are you comparing them?" 🤨
• -Wenum-conversion — warns when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by -Wextra in C. So if you're assigning one enum to another enum type without a cast, GCC is like "bestie, that's a conversion you might not have meant." 🎀
• -Wenum-int-mismatch (C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. For example:

enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);

In C, an enumerated type is compatible with "char", a signed integer type, or an unsigned integer type. 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. So if you're declaring the same function with different return types in different places, GCC is like "bestie, pick a type and stick with it, this is giving portability problems." 🌍

• -Wjump-misses-init (C, Objective-C only) — bestie, this one's watching your jumps 👀 It yells at you if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after that variable has already been initialized. It's picky tho — it only cares about variables that get initialized right when they're declared. And fr fr it only works for C and Objective-C; in C++ this kind of branch is literally an error anyway, no warning needed. This one is included in -Wc++-compat, and you can shut it up with the -Wno-jump-misses-init option.
• -Wsign-compare — warns you when a comparison between signed and unsigned values could give you a wrong answer because the signed value gets converted to unsigned 😭 In C++, this warning is also enabled by -Wall. In C, it's also enabled by -Wextra.
• -Wsign-conversion — warns about implicit conversions that might flip the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning, no cap. In C, this option is also enabled by -Wconversion.
• -Wflex-array-member-not-at-end (C and C++ only) — warns when a structure that contains a C99 flexible array member as its last field is not sitting at the end of another structure. Like, it'll call out stuff like this:

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

• -Wfloat-conversion — warns about implicit conversions that reduce the precision of a real value 💀 That covers conversions from real to integer, and from a higher precision real to a lower precision real value. This option is also enabled by -Wconversion.
• -Wno-scalar-storage-order — tells the compiler not to warn on suspicious constructs involving reverse scalar storage order. Chill vibes only ✨
• -Wsizeof-array-div — warns about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element 🫠 Because in that case the computation will not give you the number of elements in the array, which is lowkey what you probably meant. It warns about stuff like this:

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

This warning is 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 the normal way to compute array size but will not work out correctly with pointers, bestie 😭 It warns about e.g. sizeof (ptr) / sizeof (ptr[0]) if ptr is not an array, but a pointer. This warning is enabled by -Wall.
• -Wsizeof-pointer-memaccess — warns for suspicious length parameters passed to certain string and memory built-in functions when the argument uses sizeof. It triggers for example for memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer — and it suggests a possible fix 🫶 Or about memcpy (&foo, ptr, sizeof (&foo));.

-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. For example, in the following function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake and so the call is diagnosed. 💅

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

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

• -Wno-sizeof-array-argument — tells the compiler not to warn when the sizeof operator is applied to a parameter that is 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 function, if 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 😳 Which means the user has omitted a multiplication by the element size. This warning is enabled by -Wall.
• -Wmemset-transposed-args — warns for suspicious calls to the memset built-in function where the second argument is not zero and the third argument is zero 🫣 For example, the call memset (buf, sizeof buf, 0) is diagnosed because memset (buf, 0, sizeof buf) was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it's an expression that gets folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is 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, such as in:

        void f (void);
        void g (void)
        {
          if (!f)   // warning: expression evaluates to false
            abort ();
        }

comparisons of a pointer to a string literal, such as in:

        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, such as in:

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

Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception are weak symbols), so using them in a conditional might indicate missing parentheses in a function call or a missing dereference in an array expression. The subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as intptr_t or uintptr_t. Comparisons against string literals result in unspecified behavior and are not portable, and suggest the intent was to call strcmp. The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress warning is enabled by -Wall.

• -Wno-address-of-packed-member — do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value 🙈 This is enabled by default.
• -Wlogical-op — warns about suspicious uses of logical operators in expressions. That includes using logical operators in contexts where a bit-wise operator is likely to be expected. It also warns when the operands of a logical operator are the same:

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

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

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

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

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

This warning is enabled by -Wall.

• -Waggregate-return — warns if any functions that return structures or unions are defined or called 👑 (In languages where you can return an array, this also elicits a warning.)
• -Wno-aggressive-loop-optimizations — 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 applied to variables, etc. This does not stop errors for incorrect use of supported attributes. ⚠️

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

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

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

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

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

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

• -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature or as a non-function 🫠 Or when a built-in function declared with a type that does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When -Wextra is specified, it also warns when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by default. To dodge the warning, include the appropriate header to bring the prototypes of built-in functions into scope.

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

        extern void* memset ();
        void f (void *d)
        {
          memset (d, '\0', 32);
        }

• -Wno-builtin-macro-redefined — do not warn if certain built-in macros are redefined 🙅‍♀️ This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.
• -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 about obsolescent usages in a declaration, per the C Standard. Like, for example, it'll yell if storage-class specifiers such as static aren't the first things in a declaration. This warning is also enabled by -Wextra.
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition gets used. And bestie, it warns even if there's a previous prototype. A definition using () is not considered an old-style definition in C23 mode, because in that case it's equivalent to (void) — but it is an old-style definition for older standards.
• -Wmissing-parameter-type (C and Objective-C only) — a function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }

This warning is also enabled by -Wextra.

• -Wno-declaration-missing-parameter-type (C and Objective-C only) — do not warn if a function declaration contains a parameter name without a type. Such function declarations don't provide a function prototype and prevent most type checking in function calls.

This warning is enabled by default. In C99 and later dialects of C, it's treated as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=declaration-missing-parameter-type.

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

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning fires even if the definition itself provides a prototype. Use this option to detect global functions that don't have a matching prototype declaration in a header file. This option is not valid for C++ because all function declarations provide prototypes, and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.
• -Wmissing-variable-declarations (C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use this option to detect global variables that don't have a matching extern declaration in a header file.
• -Wmissing-declarations — warns if a global function is defined without a previous declaration. And it does so even if the definition itself provides a prototype. Use this option to detect global functions that aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations — use -Wmissing-prototypes to detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces.
• -Wmissing-field-initializers — warns if a structure's initializer has some fields missing. For example, the following code causes such a warning, because x.h is implicitly zero:

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

In C this option does not warn about designated initializers, so the following modification does not trigger a warning:

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

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

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

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

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

This warning is included in -Wextra. To get 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 appearing as a C++20 simple-requirement:

bool satisfied = requires { C<T> };

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

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

so satisfied is only true if concept C is satisfied for type T.

This warning can be disabled with -Wno-missing-requires.

• -Wno-missing-template-keyword — the member access tokens ., -> and :: must be followed by the template keyword if the parent object is dependent and the member being named is a template.

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

In rare cases it's possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:

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

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

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

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

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

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It's hoped that future versions of the standards involved will correct this, which is why this option is not the default.

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 😭

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

• -Wno-attribute-warning — do not warn about usage of functions declared with the warning attribute. By default, this warning is enabled. -Wno-attribute-warning can be used to disable the warning, or -Wno-error=attribute-warning can be used to disable the error when compiled with the -Werror flag.
• -Wno-deprecated — do not warn about usage of deprecated features.
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute.
• -Wno-overflow — do not warn about compile-time overflow in constant expressions.
• -Wno-odr — warn about One Definition Rule violations during link-time optimization. Enabled by default.
• -Wopenacc-parallelism — warn about potentially suboptimal choices related to OpenACC parallelism.
• -Wno-openmp — warn about suspicious OpenMP code.
• -Wopenmp-simd — warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The -fsimd-cost-model=unlimited option can be used to relax the cost model.
• -Woverride-init (C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers.

This warning is included in -Wextra. To get the other -Wextra warnings without this one, use -Wextra -Wno-override-init.

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

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

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

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

• This warning is on by default, bestie. Use -Wno-packed-bitfield-compat if you wanna shut it up.
• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — screams at you if a struct field with explicitly specified alignment inside a packed struct or union ends up misaligned. Like, a warning gets issued on struct S, something like warning: alignment 1 of 'struct S' is less than 8, in this code right here:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

This warning is enabled by -Wall.

• -Wpadded — yells if padding sneaks into a structure, whether it's there to align an element or to align the whole entire struct. Sometimes when this happens you can literally juggle the fields around to shrink the padding and make the struct smaller. ✨
• -Wredundant-decls — flags anything that gets declared more than once in the same scope, even in the cases where the multiple declaration is totally valid and changes absolutely nothing. like girl we heard you the first time 💅
• -Wrestrict — warns when an object referenced by a restrict-qualified parameter (or in C++, a __restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the strcpy function below tries 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 going on, but it works best at -O2 and above. It's included in -Wall.

• -Wnested-externs (C and Objective-C only) — warns if an extern declaration shows up inside a function. not the vibe, bestie 😭
• -Winline — warns if a function 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.

The compiler uses a whole bunch of heuristics to figure out whether or not to inline a function. Like, it considers the size of the function being inlined and the amount of inlining that's already happened in the current function. So yes, seemingly insignificant changes in the source program can make the warnings from -Winline appear or disappear. chaotic behavior 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 meant 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. main character spacing, honestly 👑

By default, --param destructive-interference-size and --param constructive-interference-size are set based on the current -mtune option, typically to the L1 cache line size for the particular target CPU, 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).

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

If you're confident that your use of the variable doesn't 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, no matter 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. so close yet so far 😭
• -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 an 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 long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.

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

• -Wvariadic-macros — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
• -Wno-varargs — don't warn upon questionable usage of the macros used to handle variable arguments like va_start. These warnings are enabled by default.
• -Wvector-operation-performance — warns if a vector operation 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 whole flavors, pick your fave 🍓
• -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.

-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. 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 looks like the code meant exponentiation. Specifically, the warning happens 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 strokes, different operators 💀

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 mean there's anything wrong with your code; it merely means GCC's optimizers couldn't handle the code effectively. Often the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. it's not a you problem, it's a "this is gonna take forever" problem 🫶
• -Wpointer-sign (C and Objective-C only, bestie 🫶) — warns when you pass a pointer as an argument or assign a pointer with a different signedness. Only C and Objective-C get this one, okay? 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 — this one 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 says. Modern compilers are like yeah whatever, we'll take strings way longer than the standard's minimum limit, but if you want your program to be very portable, pls don't use longer strings. 🌈

The limit kicks in 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 don't diagnose overlength strings in C++ at all — not our problem, iykyk. 💅

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

• -Wunsuffixed-float-constants (C and Objective-C only) — warns for any floating constant with no suffix. If you pair it with -Wsystem-headers it'll also warn about such constants inside system header files. 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 — okay this one is the main character. 🎀 It enables a static analysis of program flow that hunts for "interesting" interprocedural paths through your code and issues warnings for problems it finds on them.

Fair warning: this analysis is much more expensive than the other GCC warnings. It is not a casual little check, bestie.

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 is a bug-finding tool, not a tool for proving program correctness. 📌

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. No analyzer support, no vibes. 😭

• -Wanalyzer-symbol-too-complex — if -fanalyzer is on, the analyzer uses various heuristics to try to track the state of memory, but sufficiently complicated code can absolutely 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 that happens. 🫶

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

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

• -Wno-analyzer-allocation-size — this warning requires -fanalyzer, which enables it; to disable it, use -Wno-analyzer-allocation-size.

This diagnostic warns for paths through the code where a pointer to a buffer is assigned to point at a buffer with a size that isn't 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 gets checked for NULL after it's already been dereferenced — which lowkey suggests the pointer could have been NULL all along. 😭 Such cases mean either the NULL check is redundant, or it needs to be moved to before the pointer is dereferenced.

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

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 even "sees" them. So: disable optimization if you're trying to trigger this diagnostic. fr fr.

• -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. Rude.

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 to a log file. 🙈 Girl. No.

See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).

• -Wanalyzer-exposure-through-uninit-copy — this warning requires both -fanalyzer and the use of a plugin to specify a function that copies across a "trust boundary". Use -Wno-analyzer-exposure-through-uninit-copy to disable it.

This diagnostic warns for "infoleaks" — paths through the code where uninitialized values are copied across a security boundary (think code inside an OS kernel that copies a partially-initialized struct on the stack to user space 💀).

See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).

• -Wno-analyzer-fd-access-mode-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-access-mode-mismatch to disable it.

This diagnostic warns for paths through code where a read on a write-only file descriptor is attempted, or vice versa. ✋

It also warns for code paths where a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N, or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.

• -Wno-analyzer-fd-double-close — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-double-close to disable it.

This diagnostic warns for paths through code where a file descriptor can be closed more than once.

See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).

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

This diagnostic warns for paths through code where an open file descriptor is leaked. 💧

See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).

• -Wno-analyzer-fd-phase-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-phase-mismatch to disable it.

This diagnostic warns for paths through code where an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it'll warn on attempts to call accept on a stream socket that hasn't yet had listen successfully called on it. Timing is everything, bestie. ⏳

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 so this warning needs -fanalyzer to exist (that's what turns it on, bestie), and you turn it off with -Wno-analyzer-fd-type-mismatch.

This one's whole thing is catching the paths through your code where an operation happens on the wrong type of file descriptor. Like, it'll yell at you for trying socket operations on an fd you got from open, or for trying a stream socket operation on a datagram socket. Wrong descriptor, wrong energy, we don't do that 🙅‍♀️.

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

This diagnostic clocks paths through the code where a read or write gets called on a file descriptor that's already been closed. The audacity 😭.

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

• -Wno-analyzer-fd-use-without-check — needs -fanalyzer, which enables it; use -Wno-analyzer-fd-use-without-check to disable it.

This one warns for paths through the code where a file descriptor gets used without ever being checked for validity. no vibe check, no service 🫠.

And same deal — it also warns for paths where a function with the fd_arg (N), fd_arg_read (N) or fd_arg_write (N) attribute is called with a file descriptor at referenced argument N that never got its validity checked.

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

This diagnostic warns for paths through the code where a <stdio.h> FILE * stream object just... leaks. Gone. Never released. A whole grief situation 💀.

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 — needs -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.

This one warns for paths through the code where free gets called on a non-heap pointer — like an on-stack buffer, or a global. Not its pointer bestie, put it down 😳.

See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).

• -Wno-analyzer-imprecise-fp-arithmetic — needs -fanalyzer, which enables it; use -Wno-analyzer-imprecise-fp-arithmetic to disable it.

This diagnostic warns for paths through the code where floating-point arithmetic is used in places that genuinely need precise computation. As of right now, it only warns on floating-point operands used inside the calculation of an allocation size. Tiny scope, big feelings.

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

This diagnostic warns for paths through the code that look like they lead to an infinite loop. Forever. No exit. Not the cute kind of forever 🌀.

Specifically, the analyzer fires this warning when it "sees" a loop where:

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

One way this warning shows up is when there's an execution path through a loop where taking that path on one iteration means the exact same path gets taken on every iteration after that.

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 about the above because if opcode ever matches none of the cases, the switch follows the implicit default case, which makes the loop body a total no-op with cpu_state.pc sitting there unchanged — so the same opcode value hits on all subsequent iterations and it just loops forever and ever amen.

See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).

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

This diagnostic warns for paths through the code that appear to lead to infinite recursion. It's giving "I called myself and I never stopped" 🔁.

Specifically, when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame against the state at the entry to the previous frame of that same function on the stack. The warning drops if nothing in memory seems to be changing. Any changes it spots to parameters or globals are assumed to lead to termination of the recursion, which shuts the warning up.

Heads up: this diagnostic is likely to miss cases of infinite recursion that the optimizer turns into iteration before the analyzer even "sees" them. So optimization should be disabled if you're trying to trigger this one.

Compare it with -Winfinite-recursion, which gives a similar diagnostic but is implemented in a whole different way. Same vibe, different girl 💅.

See CWE-674: Uncontrolled Recursion (https://cwe.mitre.org/data/definitions/674.html).

• -Wno-analyzer-jump-through-null — needs -fanalyzer, which enables it; use -Wno-analyzer-jump-through-null to disable it.

This diagnostic warns for paths through the code where a NULL function pointer gets called. Jumping into the void, literally 🕳️.

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

This one warns for paths through the code where a pointer allocated via an allocator is leaked — either malloc, or a function marked with the malloc attribute. You allocated it and never said goodbye 😭.

See CWE-401: Missing Release of Memory after Effective Lifetime (https://cwe.mitre.org/data/definitions/401.html).

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

This diagnostic warns for paths through the code where the wrong deallocation function gets called on a pointer value, based on which function was used to allocate that pointer value in the first place. It'll warn about mismatches between free, scalar delete and vector delete[], plus those marked as allocator/deallocator pairs using the malloc attribute. You can't just free anything with anything bestie, that's not how this works 🙈.

See CWE-762: Mismatched Memory Management Routines (https://cwe.mitre.org/data/definitions/762.html).

• -Wno-analyzer-out-of-bounds — needs -fanalyzer, which enables it; use -Wno-analyzer-out-of-bounds to disable it.

This diagnostic warns for paths through the code where a buffer is definitely read or written out-of-bounds. It applies to cases where the analyzer can work out a constant offset, and to accesses past the end of a buffer (which also has a constant capacity). It also does limited checking for accesses past the end when both the offset and the capacity are symbolic.

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

When the analyzer can, it straight up emits a text art diagram visualizing the spatial relationship between the memory region it predicts would be accessed and the range of memory that's valid to access — whether they overlap, are touching, are close or far apart; which one comes before or after in memory; the relative sizes involved; the direction of the access (read vs write); and in some cases, the values of the data involved. It's honestly iconic of it. You can suppress this diagram with -fdiagnostics-text-art-charset=none.

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

This diagnostic warns for paths through the code where overlapping buffers get passed to an API whose behavior on such buffers is undefined.

Specifically, it fires on calls to these functions:

• "memcpy"
• "strcat"
• "strcpy"

— for cases where the buffers are known to overlap.

• -Wno-analyzer-possible-null-argument — needs -fanalyzer, which enables it; use -Wno-analyzer-possible-null-argument to disable it.

This one warns for paths through the code where a possibly-NULL value gets passed to a function argument marked with __attribute__((nonnull)), which means it requires a non-NULL value. Rude of you, honestly.

See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).

• -Wno-analyzer-possible-null-dereference — needs -fanalyzer, which enables it; use -Wno-analyzer-possible-null-dereference to disable it.

This diagnostic warns for paths through the code where a possibly-NULL value gets dereferenced. Maybe fine, maybe a crash, we don't gamble here 💀.

See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).

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

This diagnostic warns for paths through the code where a value that's known to be NULL gets passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. Known NULL. Absolutely cooked 🔥.

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

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

This one warns for paths through the code where a value that's known to be NULL gets dereferenced. No cap, that's a bad time.

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

• -Wno-analyzer-putenv-of-auto-var — needs -fanalyzer, which enables it; use -Wno-analyzer-putenv-of-auto-var to disable 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.

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 — heads up bestie, this warning needs -fanalyzer, which is the thing that turns it on in the first place; use -Wno-analyzer-shift-count-negative if you wanna turn it back off 🎀

It flags the paths through your code where a shift is attempted with a negative count. Think of it as the cousin of the -Wshift-count-negative diagnostic that lives in the C/C++ front ends — except this one is built by analyzing interprocedural paths instead of just parsing the syntax tree. ✨ Small catch though: the analyzer does not prioritize detection of such paths, so false negatives are more likely here relative to other warnings. 🙈

• -Wno-analyzer-shift-count-overflow — this one also requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-overflow to disable it 💖

It warns about paths through the code where a shift is attempted with a count greater than or equal to the precision of the operand's type. Same deal as above: analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but implemented based on analyzing interprocedural paths rather than merely parsing the syntax tree. And again, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. 😭

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

This diagnostic warns for paths through the code in which longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has returned.

So like: 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 would reference a stack frame that no longer exists, and likely lead to a crash (or worse). 💀

• -Wno-analyzer-tainted-allocation-size — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-allocation-size to disable it 👑

It warns for paths through the code in which a value that could be under an attacker's control is used as the size of an allocation without being sanitized — so an attacker could inject an excessively large allocation and potentially cause a denial of service attack. 🫶

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

• -Wno-analyzer-tainted-assertion — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-assertion to disable it 🌸

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as part of a condition without being first sanitized, and that condition guards a call to a function marked with attribute noreturn (such as the function __builtin_unreachable). Those functions typically indicate abnormal termination of the program, like for assertion failure handlers. Example incoming:

        assert (some_tainted_value < SOME_LIMIT);

In such cases:

• when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure
• when assertion-checking is disabled, such as by defining NDEBUG, an attacker could inject data that subverts the process, since it presumably violates a precondition that is being assumed by the code.

Note that when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer has a chance to "see" them, so this diagnostic can only generate warnings on builds in which assertion-checking is enabled. ✨

For the purpose of this warning, any function marked with attribute noreturn is considered as a possible assertion failure handler, including __builtin_unreachable. Note that 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).

The warning can also report problematic constructions such as:

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

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

despite the above not being an assertion failure, strictly speaking. 😭

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

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the index of an array access without being sanitized, so that 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 — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-divisor to disable it 🧚

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the divisor in a division or modulus operation without being sanitized, so that an attacker could inject a division-by-zero.

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

• -Wno-analyzer-tainted-offset — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-offset to disable it 🌈

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as a pointer offset without being sanitized, so that 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 — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-size to disable it ⭐

This diagnostic warns for paths through the code in which a value that could be under an attacker's control is used as the size of an operation such as memset without being sanitized, so that an attacker could inject an out-of-bounds access. 💅

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

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

This diagnostic warns for paths through the code in which 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. 💀

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

This diagnostic warns for paths through the code in which a function known to be async-signal-unsafe (such as fprintf) is 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 — requires -fanalyzer, which enables it; use -Wno-analyzer-use-after-free to disable it 😭

This diagnostic warns for paths through the code in which a pointer is used after a deallocator is called on it: either free, or a deallocator referenced by attribute malloc.

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

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

This diagnostic warns for paths through the code in which a pointer is dereferenced that points to a variable in a stale stack frame. Yeah. That old chestnut. 🙈

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

This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use va_arg to extract a value passed to a variadic call, but uses a type that does not match that of the expression passed to the call.

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

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

This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use va_arg to access the next value passed to a variadic call, but all of the values in the va_list have already been consumed. Rip. 🫶

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

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

This diagnostic warns for interprocedural paths through the code for which the analyzer detects that va_start or va_copy has been called on a va_list without a corresponding call to va_end.

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

This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use a va_list after va_end has been called on it. va_list. (yes, that's how the page ends the sentence, we don't question it 💀)

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

...so like, this diagnostic warns you 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 finding these paths, so false negatives are more likely here than with other warnings. 🙈

---

• -Wno-analyzer-write-to-string-literal — okay so this warning needs -fanalyzer (which is what turns it on in the first place); use -Wno-analyzer-write-to-string-literal to turn it off again. It warns about paths through the code where the analyzer catches an attempt to write through a pointer to a string literal. Same disclaimer though bestie: the analyzer does not prioritize detecting these paths, so false negatives are more likely compared to other warnings. 😭
• -Wno-analyzer-use-of-uninitialized-value — this one also needs -fanalyzer to enable it; use -Wno-analyzer-use-of-uninitialized-value to disable it. It warns about paths through the code where an uninitialized value gets used. 👀

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

---

🧠 What the analyzer already knows (its hardcoded lore)

The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:

• alloca
• the built-in functions __builtin_alloc, __builtin_alloc_with_align, __builtin_calloc, __builtin_free, __builtin_malloc, __builtin_memcpy, __builtin_memcpy_chk, __builtin_memset, __builtin_memset_chk, __builtin_realloc, __builtin_stack_restore, and __builtin_stack_save
• calloc
• free
• malloc
• memset
• operator delete
• operator delete []
• operator new
• operator new []
• realloc
• strdup
• strndup

of the following functions for working with file descriptors:

• open
• close
• creat
• dup, dup2 and dup3
• isatty
• pipe, and pipe2
• read
• write
• socket, bind, listen, accept, and connect

of the following functions for working with <stdio.h> streams:

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

and of the following functions:

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

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

---

🎛️ Params that control the exploration

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

The following options control the analyzer. buckle up bestie, there's a lot 💅

• -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. If it's enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value). 🎀
• -fanalyzer-checker=name — restricts the analyzer so it runs just the named checker, and enables that checker. one checker, main character energy only 👑
• -fanalyzer-debug-text-art-headings — this option is intended for analyzer developers. if 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 each diagnostic it emits: that the conditions that hold are not mutually exclusive. Diagnostics for which no feasible path can be found get rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code. 🕵️
• -fanalyzer-fine-grained — this option is intended for analyzer developers. Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information. By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. ✂️
• -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. 🗂️
• -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 such events are no longer suppressed. 👁️
• -fno-analyzer-state-merge — this option is intended for analyzer developers. By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues. 🔀
• -fno-analyzer-state-purge — this option is intended for analyzer developers. By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant — e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis. With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues. 🧹
• -fno-analyzer-suppress-followups — this option is intended for analyzer developers. By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics. The diagnostics that terminate analysis along a path are:
  • -Wanalyzer-null-argument
  • -Wanalyzer-null-dereference
  • -Wanalyzer-use-after-free
  • -Wanalyzer-use-of-pointer-in-stale-stack-frame
  • -Wanalyzer-use-of-uninitialized-value

With -fno-analyzer-suppress-followups the analyzer will continue to explore such paths even after such diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. 💀

• -fanalyzer-transitivity — this option enables transitivity of constraints within the analyzer. 🔗
• -fno-analyzer-undo-inlining — this option is intended for analyzer developers. -fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code. By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls. With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled, which may be of help when debugging issues in the analyzer. 🪄
• -fanalyzer-verbose-edges — this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. 📝
• -fanalyzer-verbose-state-changes — this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths. 🔍
• -fanalyzer-verbosity=level — this option controls the complexity of the control flow paths that are emitted for analyzer diagnostics. The level can be one of:
  1. 0 — At this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-free diagnostic, both calls to free will be shown.
  2. 1 — As per the previous level, but also show events for the entry to each function.
  3. 2 — As per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. "true path taken" at a conditional). This level is the default. ⭐
  4. 3 — As per the previous level, but show all control flow events, not just significant ones.
  5. 4 — This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.
• -fdump-analyzer — dumps internal details about what the analyzer is doing to file.analyzer.txt. -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. 🌈
• -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 — drops a text version of the "exploded graph" into one dump file per node, named file.eg-id.txt. Heads up bestie: this is usually a lot of files. Like, a lot a lot. 💀
• -fdump-analyzer-exploded-paths — dumps a text version of the "exploded path" for every single diagnostic, into files called file.idx.kind.epath.txt. Receipts for each path, periodt. 🦋
• -fdump-analyzer-feasibility — spills the internal deets about the analyzer's hunt for feasible paths. Written so you can literally open it in GraphViz, with filenames shaped like file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt. ✨
• -fdump-analyzer-infinite-loop — dumps the internal deets about the analyzer's search for infinite loops, formatted for GraphViz viewing, into filenames like file.*.infinite-loop.dot. It's giving very-much-not-ending. 🔁
• -fdump-analyzer-json — dumps a compressed JSON representation of the analyzer's internals to file.analyzer.json.gz. Fair warning: the precise format is subject to change. So don't you dare build anything load-bearing on it. 🙈
• -fdump-analyzer-state-purge — same energy as -fdump-analyzer-supergraph, dumps a representation of the "supergraph" that's GraphViz-friendly, but it annotates the graph with info about what state will be purged at each node. Written to file.state-purge.dot. 👑
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" for GraphViz viewing into file.supergraph.dot and into file.supergraph-eg.dot. These show all the control flow graphs in the program, with interprocedural edges for calls and returns. The second dump has annotations showing nodes in the "exploded graph" plus the diagnostics tied to them. 🌈
• -fdump-analyzer-untracked — emits custom warnings with internal details, aimed at analyzer developers specifically. If that's not you, this is not your moment, bestie. 🧚

---

🌸 Options for Debugging Your Program 🌸

okay so, if you want GCC to spit out extra info for a debugger to chew on? In literally almost every case you just add -g to your other options and you're done. Some debug formats can co-exist (like DWARF chilling alongside CTF) — but only when you explicitly turn each one on by adding its respective command line option to your pile of options. 💅

GCC does let you use -g together with -O. But the shortcuts optimized code takes may occasionally be surprising, no cap:

• some variables you declared may not exist at all
• flow of control may briefly move where you didn't expect it
• some statements may not be executed because they compute constant results or their values are already at hand
• some statements may execute in different places because they have been moved out of loops

Nevertheless, it is possible to debug optimized output. Which is exactly what makes it reasonable to use the optimizer for programs that might have bugs. 🫶

If you're not using some other optimization option, consider using -Og together with -g. Here's why: with no -O option at all, some compiler passes that collect information useful for debugging don't run at all — so -Og may give you a better debugging experience. ✨

---

• -g — produce debugging information in the operating system's native format (that's stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information. On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; that 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 — produce 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. GDB girlies, this one's for you. 💖
• -gdwarf / -gdwarf-version — produce debugging information in DWARF format (if that's supported). version can be either 2, 3, 4 or 5; the default version for most targets is 5 — with the exceptions 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. And for historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. iykyk. 🦋
• -gbtf — request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options. Two formats, one slay. ✨
• -gctf / -gctflevel — request CTF debug information and use 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 of the debug formats are enabled explicitly via their respective command line options. And now the levels, because the levels are the whole plot:
  • Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf. 💀
  • Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.
  • Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.
• -gvms — produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems. Retro era, we respect it. 👑
• -gcodeview — produce debugging information in CodeView debug format (if that is supported). This is the format used by Microsoft Visual C++ on Windows. 🪟
• -glevel / -ggdblevel / -gvmslevel — request debugging information and also use level to specify how much information. The default level is 2.
  • Level 0 produces no debug information at all. Thus, -g0 negates -g.
  • Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
  • Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.

If you use multiple -g options, with or without level numbers, the last such option is the one that is effective. Last one wins, always. 💅

Also important: -gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF.

• -fno-eliminate-unused-debug-symbols — by default, no debug information is produced for symbols that are not actually used. Use this option if you want debug information for all symbols. FOMO but for symbols. 🙈
• -femit-class-debug-always — instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes — because using this option increases the size of debugging information by as much as a factor of two. You have been warned, bestie. 🎀
• -fno-merge-debug-strings — direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default. 📎
• -fdebug-prefix-map=old=new — when compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using . for new. This can give more reproducible builds, which are location independent — but it may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map and -fcanon-prefix-map. 🌈
• -fvar-tracking — run 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 information). It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it. ✨
• -fvar-tracking-assignments — annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of -gdwarf-4 is recommended along with it. It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled. 🧚
• -gsplit-dwarf — if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension .dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading .dwo files. 🔥
• -gdwarf32 / -gdwarf64 — if DWARF debugging information is enabled, -gdwarf32 selects the 32-bit DWARF format and -gdwarf64 selects the 64-bit DWARF format. The default is target specific — on most targets it is -gdwarf32 though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet. Size vs. support, the eternal struggle. 💀
• -gdescribe-dies — add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. ✏️
• -gpubnames — generate DWARF .debug_pubnames and .debug_pubtypes sections. 📖
• -ggnu-pubnames — emits .debug_pubnames and .debug_pubtypes sections in a format you can convert into a GDB index. Only useful if your linker can spit out GDB index version 7, so if it can't, this is giving nothing bestie. 💅
• -fdebug-types-section — when you're on DWARF Version 4 or higher, type DIEs can live in their own lil .debug_types section instead of being crammed into .debug_info. Putting them in a separate comdat section is lowkey more efficient because the linker can yeet the duplicates. But — not all DWARF consumers support .debug_types yet, and on some objects it literally makes your debugging info bigger instead of smaller. 🙈
• -grecord-gcc-switches, -gno-record-gcc-switches — this one appends the command-line options you used to invoke the compiler that might affect code generation onto the DW_AT_producer attribute in your DWARF debugging info. The options get concatenated, spaces separating them from each other and from the compiler version. ✨ It's enabled by default ✨. Also peep -frecord-gcc-switches if you want another way of stuffing compiler options into the object file.
• -gstrict-dwarf — no using extensions from a later DWARF standard version than whatever you picked with -gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed, so this is the strict one. 💀
• -gno-strict-dwarf — okay, you can use extensions of later DWARF standard versions than selected with -gdwarf-version. Live a little. 🦋
• -gas-loc-support — tells the compiler that your assembler supports .loc directives, so it can use them to have the assembler generate DWARF2+ line number tables. This is generally desirable, because assembler-generated line-number tables are a lot more compact than the ones the compiler can make itself. 🎀 It gets enabled by default if, at GCC configure time, the assembler was found to support such directives.
• -gno-as-loc-support — forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are gonna be generated. No delegating. 🫶
• -gas-locview-support — informs the compiler that the assembler supports "view" assignment and reset assertion checking in .loc directives. Enabled by default if, at GCC configure time, the assembler was found to support them. ✨
• -gno-as-locview-support — forces GCC to assign view numbers internally, if -gvariable-location-views is explicitly requested. 🛠
• -gcolumn-info, -gno-column-info — emit location column info into DWARF debugging information, not just file and line. Enabled by default, bestie. 👑
• -gstatement-frontiers, -gno-statement-frontiers — makes GCC create markers in its internal representation at the beginning of statements, and keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. Enabled by default when compiling with optimization (-Os, -O1, -O2, ...), and outputting DWARF 2 debug information at the normal level. 🌸
• -gvariable-location-views, -gvariable-location-views=incompat5, -gno-variable-location-views — augments variable location lists with progressive view numbers implied from the line number table. This lets debug info consumers inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this 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 info consumers that aren't aware of these augmentations — but they won't derive any benefit from them either. 😭

Enabled by default when outputting DWARF 2 debug information at the normal level, as long as there's assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support isn't available, this may still be enabled, but it'll force GCC to output internal line number tables, and if -ginternal-reset-location-views isn't enabled, that will most certainly lead to silently mismatching location views. 💀

There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, enabled with -gvariable-location-views=incompat5. This option may be removed in the future, and is only provided as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they'd be rendered unable to decode location lists using it. 🙈

• -ginternal-reset-location-views, -gno-internal-reset-location-views — attempts to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that doesn't support location view lists. The GNU assembler will flag any such error as a view number mismatch. Only enabled on ports that define a reliable estimation function. 🎀
• -ginline-points, -gno-inline-points — generates extended debug info for inlined functions. Location view tracking markers get inserted at inlined entry points, so address and view numbers can be computed and output in debug info. This can be enabled independently of location views, in which case the view numbers won't be output — but it can only be enabled along with statement frontiers, and it's only enabled by default if location views are enabled. 💫
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported. If type isn't given, the default type depends on the capabilities of the assembler and linker used. type may be one of none (don't compress debug sections) or zlib (use zlib compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler doesn't support them, -gz is silently ignored when producing object files. 🤫
• -femit-struct-debug-baseonly — emit debug info for struct-like types only when the base name of the compilation source file matches the base name of the file in which the struct is defined. This substantially reduces the size of debugging 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 info for struct-like types only when the base name of the compilation source file matches the base name of the file in which the type is defined, unless the struct is a template or defined in a system header. Significantly reduces the size of debugging information, with some potential loss in type info 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] — specifies the struct-like types for which the compiler generates debug info. The intent is to reduce duplicate struct debug info between different object files within the same program. This is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serve for most needs. ✨

A specification has the syntax:

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

The optional first word limits the spec to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it's the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

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

The third word specifies the source files for those structs for which the compiler should emit debug info. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug info is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application. 🔬

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

This option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as a compiler generated .eh_frame section instead of using GAS .cfi_* directives. 🛠
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it's useful to have GCC emit debugging info for all types declared in a compilation unit, regardless of whether or not they're actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that isn't actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. 🗑️

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🌟 Options That Control Optimization 🌟

These options control various sorts of optimizations. 👑

okay so here's the ✨tea✨ on optimization, bestie: if you don't turn on a single optimization option, the compiler's whole entire goal is just to reduce the cost of compilation and to make debugging produce the expected results. 🔍

Every statement stays 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. no surprises, no cap. 🫶

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. it's giving trade-offs, always with the trade-offs 😭

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. group-project energy fr fr 🫶

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. iykyk.

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. 💅

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. very demure, very "go check for yourself" 👑

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-O, -O1 — Optimize

Optimizing compilation takes somewhat more time, and a lot more memory for a large function. it's hungry.

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 do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code. ate. 🔥

-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. bestie, that one is important, don't skip it. 🙈

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2, in Ubuntu 24.04 and later versions, -D_FORTIFY_SOURCE=3, is set by default, and is activated when -O is set to 2 or higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.

NOTE: In Debian 13 and Ubuntu 24.04 and later versions, -D_TIME_BITS=64 together with -D_FILE_OFFSET_BITS=64 is set by default on the 32bit architectures armel, armhf, hppa, m68k, mips, mipsel, powerpc and sh4.

-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. the "i am not doing all that" setting, and honestly? valid. 💅

-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. 🔥

-Og — Optimize debugging experience

-Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0. the main character of the debug-and-chill arc 🦋

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, always. 💅

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. so convenient, we love a predictable naming scheme ✨

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

-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. stacking them like a little to-do list 🍓

-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 cannot be omitted.

Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.

Enabled by default at -O1 and higher.

-foptimize-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. glow-up for your string functions 💅

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 — bestie, this one says do NOT expand any functions inline, except the ones wearing the always_inline attribute like it's a VIP wristband 💅. It's the default when you're not optimizing at all. Got one lil function you want left alone? Slap the noinline attribute on it and it's exempt, periodt.
• -finline-small-functions — the compiler goes "hmm, this function's body is smaller than the code a call to it would take, so let's just squish it into the caller" (net result: whole program gets smaller 🌸). It's a heuristic call — GCC decides which functions are simple enough to be worth integrating. Applies to all functions, even ones never declared inline, no cap.

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

• -findirect-inlining — indirect calls that got figured out as known at compile time thanks to previous inlining? Yeah, inline those too 🦋. It only does literally anything when inlining itself is switched on by -finline-functions or -finline-small-functions.

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

• -finline-functions — consider all the functions for inlining, even the ones that never said inline out loud. Compiler heuristically picks which ones are worth it ✨.

If every single call to a given function gets integrated, and that function is declared static, then normally it isn't output as assembler code in its own right — it's just absorbed, iconic.

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

• -finline-functions-called-once — every static function that's called exactly once gets considered for inlining into its caller, even if it never got marked inline 👑. If a call to a given function is integrated, that 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 the always_inline-marked functions, plus any function whose body seems smaller than the function call overhead, early — before the -fprofile-generate instrumentation and the real inlining pass even happen. Doing this makes profiling significantly cheaper and usually makes inlining faster on programs with big chains of nested wrapper functions 🧚.

Enabled by default.

• -fipa-sra — interprocedural scalar replacement of aggregates, removal of unused parameters, and swapping out parameters passed by reference for parameters passed by value 🔥.

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

• -finline-limit=n — by default GCC caps how big a function can be and still get inlined. This flag is your coarse-volume knob for that limit: n is the size of functions that can be inlined, measured in pseudo instructions.

Real inlining control actually lives in a bunch of parameters, which you can set one at a time with --param name=value. What this option does is set some of them like so:

• max-inline-insns-single — is set to n/2.
• max-inline-insns-auto — is set to n/2.

Scroll down for docs on the individual parameters controlling inlining and their defaults 👇.

Note: there may be no value of -finline-limit that gives you default behavior. It's giving "the road back home is gone" 😭.

Note: "pseudo instruction" here is an abstract measurement of a function's size. It is in no way a count of assembly instructions, and its exact meaning might change from one release to another. Do not build your personality on it 💀.

• -fno-keep-inline-dllexport — a more fine-grained version of -fkeep-inline-functions, applying only to functions declared using the dllexport attribute or declspec 🎀.
• -fkeep-inline-functions — in C, emit static functions that are declared inline into the object file, even if the function has been inlined into every single one of its callers. Heads up: 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, boom 💖.
• -fkeep-static-functions — emit static functions into the object file even if the function is never used at all. Just vibes, hanging out in there 🌈.
• -fkeep-static-consts — emit variables declared static const when optimization isn't turned on, even if nothing ever references them.

GCC enables this option by default ✨. If you want to force the compiler to check whether a variable is referenced, regardless of whether optimization is 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 is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit that behavior.

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

• -fmerge-all-constants — attempt to merge identical constants and identical variables. It implies -fmerge-constants automatically.

On top of what -fmerge-constants does, this one also considers e.g. even constant initialized arrays, or initialized constant variables with integral or floating-point types 🍓.

Okay so major warning: languages like C or C++ require each variable — including multiple instances of the same variable in recursive calls — to have distinct locations. Using this option results in non-conforming behavior. Not the moment to be cute 💅.

• -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 🌀.
• -fmodulo-sched-allow-regmoves — perform more aggressive SMS-based modulo scheduling, with register moves allowed. Setting this flag deletes certain anti-dependences edges, which triggers the generation of reg-moves based on the life-range analysis 🔥. Only effective with -fmodulo-sched enabled — otherwise it's just standing there.
• -fno-branch-count-reg — disables the optimization pass that hunts for chances 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 on the result 🎯. Only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.

Note: -fno-branch-count-reg does not remove the decrement and branch instructions from the generated instruction stream that other optimization passes introduced. It only stops this one from adding more, bestie 😭.

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 gives you less efficient code, but some strange hacks that alter the assembler output may get confused by the optimizations performed when this option is not used.

The default is -ffunction-cse.

• -fno-zero-initialized-in-bss — if the target supports a BSS section, GCC by default drops variables initialized to zero into BSS, which can save space in the resulting code 👑.

This option turns that behavior off, because some programs explicitly rely on variables going to the data section — e.g. so 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 one, is found. If so, the first branch gets 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 🧠.

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

• -fsplit-wide-types — when you're using a type that occupies multiple registers, like long long on a 32-bit system, split those registers apart and allocate them independently 🌸. This normally generates better code for those types, but it may make debugging more difficult. Trade-offs, babe.

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

• -fsplit-wide-types-early — fully split wide types early, instead of very late. 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 isn't 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 — 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 have been performed 🔁.

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

• -fgcse — perform a global common subexpression elimination pass. This pass also does global constant and copy propagation 👑.

Note: when compiling a program that uses 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 lets a loop containing a load/store sequence 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 runs after global common subexpression elimination 🌈. This pass attempts to move stores out of loops. When used together 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.

• -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.

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• -fgcse-after-reload — okay so when you turn this on, a whole redundant load elimination pass happens after reload, and its entire job is to clean up any messy redundant spilling. It's giving cleanup crew energy. 🧹 Enabled by -O3, -fprofile-use and -fauto-profile.
• -faggressive-loop-optimizations — this tells the loop optimizer to use language constraints to figure out how many times a loop will run (the bounds). It's assuming your loop code isn't out here causing undefined behavior like signed integer overflows or out-of-bound array accesses. 🙈 Those bounds then guide loop unrolling, peeling, and loop exit test optimizations. Enabled by default, bestie.
• -funconstrained-commons — this one tells the compiler that variables declared in common blocks (like in Fortran) might later be overridden with longer trailing arrays. So it prevents certain optimizations that would otherwise depend on knowing the array bounds. Lowkey a safety net for that specific case. 🛡️
• -fcrossjumping — perform cross-jumping transformation, which unifies equivalent code and saves code size. The resulting code may or may not perform better — it's a vibe check, not a guarantee. 😅 Enabled at levels -O2, -O3, -Os.
• -fauto-inc-dec — combine increments or decrements of addresses with memory accesses. It's always skipped on architectures that don't have instructions to support this. Enabled by default at -O1 and higher on architectures that do support it. If you got the hardware, it's already on. ✨
• -fdce — perform dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher. It's literally just deleting the code that does nothing. 💀
• -fdse — perform dead store elimination (DSE) on RTL. Enabled by default at -O1 and higher. Bye bye useless stores. 👋
• -fif-conversion — attempt to transform conditional jumps into branch-less equivalents. This includes using 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. No branches, no problem. 💅
• -fif-conversion2 — use conditional execution (where available) to transform conditional jumps into branch-less equivalents. Enabled at levels -O1, -O2, -O3, -Os, but not with -Og. It's the sequel, and it's still slaying. 🔥
• -fdeclone-ctor-dtor — okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, meaning two copies of the function. With this option, the base and complete variants become thunks that call a common implementation. It's giving deduplication. Enabled by -Os.
• -fdelete-null-pointer-checks — assume that programs cannot safely dereference null pointers, and that no code or data element lives 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. Those analyses 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 can't be null.

⚠️ But bestie, 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 — attempt 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. Making virtual calls direct? That's a glow-up. ✨
• -fdevirtualize-speculatively — attempt to convert calls to virtual functions into speculative direct calls. Based on the analysis of the type inheritance graph, it figures out for a given call the set of likely targets. If that set is small, preferably of size 1, it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, like inlining. And if they seem useless after further optimization, they get converted back into their original form. It's a whole vibe. 🦋
• -fdevirtualize-at-ltrans — stream 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. It's giving data-hungry. 🍓
• -fexpensive-optimizations — perform a number of minor optimizations that are relatively expensive. Enabled at levels -O2, -O3, -Os. Expensive but worth it, like your skincare routine. 💅
• -free — attempt 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. Cleaning up redundant extensions, periodt. 🧼
• -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 object's lifetime are dead once the object is destroyed. Normally dead store elimination takes advantage of this. But if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable that optimization.

To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor, 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. So many options, bestie. 😵‍💫

• -flive-range-shrinkage — attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets. Shrinking that range, slaying that pressure. 💅
• -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. CB stays winning. 👑
• -fira-region=region — use specified regions for the integrated register allocator. The region argument should be one of these:
  • 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.

Pick your fighter. 🎀

• -fira-hoist-pressure — use 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. Enabled at level -Os for all targets. Smaller code, slower compile — a trade-off, bestie. ⚖️
• -fira-loop-pressure — use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually generates faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down. Enabled at level -O3 for some targets. If you got the registers, it's a whole mood. 🌈
• -fno-ira-share-save-slots — disable 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. No sharing, bigger frames. 🙅‍♀️
• -fno-ira-share-spill-slots — disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard register gets a separate stack slot, and as a result function stack frames are larger. Same energy, different registers. 💀
• -flra-remat — enable 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. Recalculate instead of reload — it's giving efficiency. ✨
• -fdelayed-branch — if supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions. Enabled at levels -O1, -O2, -O3, -Os, but not at -Og. Delayed branch? More like delayed slay. 💅
• -fschedule-insns — if supported for the target machine, attempt 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. Scheduling those instructions so nothing stalls. 🔥
• -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. It's the sequel, and it's still booked and busy. 📚
• -fno-sched-interblock — disable instruction scheduling across basic blocks, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. No inter-block scheduling, bestie. 🛑
• -fno-sched-spec — disable 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. No speculative moves, no surprises. 🙈
• -fsched-pressure — turn on register pressure sensitive insn scheduling before register allocation. Bestie, this only makes sense when scheduling before register allocation is even a thing, i.e. with -fschedule-insns or at -O2 or higher. Using it can make the generated code better and shrink it, by stopping register pressure from rising above the number of available hard registers and the resulting spills during register allocation. 🩸
• -fsched-spec-load — allow speculative motion of some load instructions. Again, only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. ✨
• -fsched-spec-load-dangerous — allow speculative motion of more load instructions. Yeah, dangerous, the name is doing the work for us 💀. Same rule: only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-stalled-insns, *-fsched-stalled-insns=n*** — this one defines how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass. -fno-sched-stalled-insnsmeans no insns get moved prematurely,-fsched-stalled-insns=0means there is *no limit* on how many queued insns can be moved prematurely. And-fsched-stalled-insnswith no value? Equivalent to-fsched-stalled-insns=1`. cute 🌸
• -fsched-stalled-insns-dep, *-fsched-stalled-insns-dep=n*** — defines how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This only hits during the second scheduling pass, and only if -fsched-stalled-insnsis being used.-fno-sched-stalled-insns-depis equivalent to-fsched-stalled-insns-dep=0. And -fsched-stalled-insns-depwithout a value is equivalent to-fsched-stalled-insns-dep=1`. 🦋
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling. This allows motion across basic block boundaries, which gives you faster schedules. ⚡ Heads up though: this option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. Not all heroes are stable, ok? 😭

And it only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

• -fsched-group-heuristic — enable the group heuristic in the scheduler. It favors the instruction that belongs to a schedule group. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 👑
• -fsched-critical-path-heuristic — enable the critical-path heuristic in the scheduler. It favors instructions on the critical path. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. main character instructions only 💅
• -fsched-spec-insn-heuristic — enable the speculative instruction heuristic in the scheduler. This one favors speculative instructions with greater dependency weakness. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-rank-heuristic — enable the rank heuristic in the scheduler. It favors the instruction belonging to a basic block with greater size or frequency. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. big block energy 🫶
• -fsched-last-insn-heuristic — enable the last-instruction heuristic in the scheduler. It favors the instruction that is less dependent on the last instruction scheduled. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-dep-count-heuristic — enable the dependent-count heuristic in the scheduler. It favors the instruction that has more instructions depending on it. Enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. popular instruction behavior fr fr 🌈
• -freschedule-modulo-scheduled-loops — modulo scheduling happens before traditional scheduling. So if a loop is modulo scheduled, later scheduling passes might change its schedule. Use this option to control that behavior. 🎀
• -fselective-scheduling — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass. 🔥
• -fselective-scheduling2 — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass.
• -fsel-sched-pipelining — enable software pipelining of innermost loops during selective scheduling. This option does literally nothing unless one of -fselective-scheduling or -fselective-scheduling2 is turned on. girl needs her prerequisites 💜
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops. Also does nothing unless -fsel-sched-pipelining is turned on. see above bestie 🙈
• -fsemantic-interposition — okay so some object formats, like ELF, let the dynamic linker interpose symbols. Which means: for symbols exported from the DSO, the compiler cannot do interprocedural propagation, inlining and other optimizations, because it has to assume the function or variable in question might change. This feature is genuinely useful — for example to rewrite memory allocation functions with a debugging implementation — but it's expensive in terms of code quality. With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Same deal for variables: if interposition happens for variables, the constructor of the variable will be the same. Heads up though — this flag has no effect for functions explicitly declared inline (where interposition changing semantics is never allowed) and for symbols explicitly declared weak. 🤝
• -fshrink-wrap — emit function prologues only before the parts of the function that actually need it, rather than dumping them at the top of the function like a lil intro nobody asked for. This flag is enabled by default at -O and higher. ✂️
• -fshrink-wrap-separate — shrink-wrap the separate parts of the prologue and epilogue separately, so those parts only get executed when needed. This option is on by default, but it has no effect unless -fshrink-wrap is also turned on and the target supports this. 🧚
• -fcaller-saves — enable allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. This allocation only happens when it seems like it'll result in better code.

This option is always enabled by default on certain machines, usually the ones with no call-preserved registers to use instead.

Enabled at levels -O2, -O3, -Os. 🎯

• -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them. Enabled by default at -O1 and higher. matchmaking but for stack ops 💞
• -fipa-ra — use caller save registers for allocation if those registers aren't used by any called function. In that case there's no need to save and restore them around calls. But this is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. 😤

Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be instrumented for profiling (-p, or -pg) or if callee's register usage cannot be known exactly (this happens on targets that do not 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. slow but skinny, we love her 🌸 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. go off 💥
• -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. bye bye redundant copies 🫶
• -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. ⚠️ 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. like, any of them. not one. 💀
• -fipa-profile — bestie, this does interprocedural profile propagation ✨ Functions that only ever get called from cold functions get marked as cold themselves. It also sniffs out functions that get executed exactly once — think "cold", "noreturn", static constructors, static destructors, all of those. Then cold functions, plus the loop-less bits of those once-and-done functions, get optimized for size. Enabled by default at -O1 and higher.
• -fipa-modref — interprocedural mod/ref analysis, no cap 💅 This one analyzes the side effects of functions — like which memory locations get modified or referenced — and that lets GCC optimize way better across the function-call boundary. Enabled by default at -O1 and higher.
• -fipa-cp — interprocedural constant propagation, and it ATE 👑 It analyzes your whole program to figure out when values passed to functions are actually constants, then optimizes accordingly. If your app passes constants around a lot, this can substantially increase performance. Enabled by default at -O2, -Os and -O3. Also enabled by -fprofile-use and -fauto-profile.
• -fipa-cp-clone — makes interprocedural constant propagation stronger by doing function cloning 🦋 When it's on, IPC does function cloning whenever 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 — check --param ipa-cp-unit-growth=value. Enabled by default at -O3. Also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, this does interprocedural bitwise constant propagation 💖 Enabled by default at -O2, and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled.
• -fipa-vrp — when enabled, this propagates value ranges interprocedurally 🌈 Enabled by default at -O2. It requires that -fipa-cp is enabled.
• -fipa-icf — Identical Code Folding, for functions and read-only variables ✨ This optimization shrinks code size, but lowkey it may disturb unwind stacks, because it replaces a function with an equivalent one that has a different name. Works more effectively with link-time optimization enabled.

Okay so even though the vibe is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels, so the optimizations are not the same — there are equivalences that only GCC finds and equivalences that only Gold finds. Both iconic, both different.

Enabled by default at -O2 and -Os.

• -flive-patching=level — this controls GCC's optimizations to produce output that's suitable for live-patching 🩹

Here's the deal: if the compiler's optimization uses a function's body — or info extracted from its body — to optimize/change another function, that other one is called an impacted function of the former. So if a function gets patched, its impacted functions should get patched too.

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

Usually, the more IPA optimizations you enable, the larger the number of impacted functions per function. To control how many impacted functions there are — and to make computing that list of impacted functions easier — IPA optimizations can be partially enabled at two different levels.

The level argument should be one of these:

• inline-clone — only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. Result? When you patch a function, all its callers and its clones' callers are impacted, so they need patching too.

-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. Result? When you patch a static function, all its callers are impacted and need patching too.

On top of all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables these 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.

Not supported with link-time optimization (-flto). Just so you know 💀

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior because of dereferencing a null pointer 😭 It isolates those paths from the main control flow and turns the statement with the erroneous/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 — same energy, but the trigger is a null value being used in a way that a "returns_nonnull" or "nonnull" attribute forbids 🚫 Isolates those paths from the main control flow and turns that statement into a trap. This is not currently enabled, but it may be enabled by -O2 in the future.
• -ftree-sink — performs forward store motion on trees ✨ 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. 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 🦋 Only operates on local scalar variables, and it's enabled by default at -O1 and higher.
• -fssa-backprop — propagates info about uses of a value up the definition chain, so the definitions can get simplified 🌸 For example, this pass strips sign operations if the sign of a value never actually matters. Enabled by default at -O1 and higher.
• -fssa-phiopt — does pattern matching on SSA PHI nodes to optimize conditional code ✨ Enabled by default at -O1 and higher, except for -Og.
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array 💫 Enabled by default at -O2 and higher.
• -ftree-tail-merge — looks for identical code sequences, and when it finds one, it 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 the max-tail-merge-comparisons parameter and the 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 — but only if -Os is not also specified.
• -ffinite-loops — assumes that a loop with an exit will eventually take that exit and not loop forever 🌀 This lets the compiler remove loops that otherwise have no side-effects, since it's not considering eventual endless looping as a side effect.

Enabled by default at -O2 for C++ with -std=c++11 or higher.

• -ftree-dominator-opts — does a variety of simple scalar cleanups based on a dominator tree traversal: constant/copy propagation, redundancy elimination, range propagation and expression simplification ✨ It 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 with no 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 because 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.
• -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.
• -ftree-coalesce-vars — okay so while the compiler is transforming your program out of the SSA representation, this makes it try to reduce copying by coalescing versions of different user-defined variables, not just the compiler's own temporaries. 💅

big warning bestie: this "may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments." So like, adorable for speed, kinda cursed for debugging. 🫶

In the negated form, this flag prevents SSA coalescing of user variables. It's enabled by default if optimization is enabled, and ngl it does very little otherwise. 🙈

• -ftree-loop-if-convert — tries to transform conditional jumps in the innermost loops into branch-less equivalents. The intent (her intent, she's intentional ✨) is to remove control-flow from the innermost loops so the vectorization pass has an easier time handling those loops. Enabled by default if vectorization is enabled. 🎀
• -ftree-loop-distribution — performs loop distribution. This flag can improve cache performance on big loop bodies, and it lets further loop optimizations — think parallelization or vectorization — actually happen. Like, it ate and left no crumbs. For example, the loop:

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

is transformed to:

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

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🔥

• -ftree-loop-distribute-patterns — performs loop distribution of patterns that can be code generated with calls to a library. 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. Obsessed with the efficiency honestly. 💖

• -floop-interchange — performs loop interchange outside of graphite. This flag can improve cache performance on loop nest and lets further loop optimizations, like vectorization, 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. Enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. Main character optimization tbh. 👑
• -ftree-loop-im — performs loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level (function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion. 🌸
• -ftree-loop-ivcanon — creates a canonical counter for number of iterations in loops for which determining the number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling. Bestie this is the setup arc for the unrolling moment fr fr. 🦋
• -ftree-scev-cprop — performs final value replacement. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, it replaces uses of the final value by such a computation — provided it is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher. ✨
• -fivopts — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. Three for one, we love to see it. 💖
• -ftree-parallelize-loops=n — parallelizes loops, i.e., splits their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. 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. Enabled by default at -O1 and higher, except for -Og. 🎀
• -ftree-sra — performs scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. Enabled by default at -O1 and higher, except for -Og. 🧚
• -fstore-merging — performs merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores, to reduce the number of instructions. Enabled by default at -O2 and higher as well as -Os. The way it just combines them… iconic. ⭐
• -ftree-ter — performs temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on, resulting in better RTL generation. 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 not explicitly specified. 💜
• -ftree-loop-vectorize — performs loop vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile. ✨
• -ftree-slp-vectorize — performs basic block vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile. 🌸
• -ftrivial-auto-var-init=choice — initializes automatic variables with either a pattern or with zeroes, to increase the security and predictability of a program by preventing uninitialized memory disclosure and use. 🛡️

Okay but real talk, keep reading: 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 — and this is the 💀 part — 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 initializes 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 byte-repeatable pattern with byte "0xFE". The values used for pattern initialization might be changed in the future.
• zero initializes 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 — bestie this sets the cost model for vectorization, aka whether GCC thinks turning your loop into vector code is even worth it 💅 the model you pass has to be one of unlimited, dynamic, cheap, or very-cheap.

With unlimited, the vectorized code-path is just assumed to be profitable, no questions asked ✨ with dynamic, a runtime check guards the vectorized code-path so it's only enabled for iteration counts that will likely execute faster than running the original scalar loop 🦋

The cheap model kills vectorization of loops where doing so would be cost prohibitive — like, say, because of required runtime checks for data dependence or alignment — but otherwise it's equal to dynamic. And very-cheap is the pickiest of all: it only allows vectorization if the vector code would entirely replace the scalar code being vectorized. For example, if each iteration of a vectorized loop would only be able to handle exactly four iterations of the scalar loop, very-cheap would only allow vectorization if the scalar iteration count is known to be a multiple of four 👑

The default cost model depends on your other optimization flags and is either dynamic or cheap.

• -fsimd-cost-model=model — same vibe but for vectorization of loops marked with the OpenMP simd directive 🎀 the model argument should be one of unlimited, dynamic, cheap. All values of model mean the same thing as described in -fvect-cost-model, and by default a cost model defined with -fvect-cost-model is used.
• -ftree-vrp — Perform Value Range Propagation on trees 🌈 it's kinda like the constant propagation pass, except instead of pushing values, it pushes ranges of values. This lets the optimizers nuke unnecessary range checks like array bound checks and null pointer checks 💀 no cap, it's enabled by default at -O2 and higher. Quick heads up tho — null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled.
• -fsplit-paths — Split paths leading to loop backedges ✨ this can improve dead code elimination and common subexpression elimination. Enabled by default at -O3 and above, periodt.
• -fsplit-ivs-in-unroller — Enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration 🔥 this breaks long dependency chains, which is giving… better efficiency for the scheduling passes, naturally.

A combination of -fweb and CSE is often sufficient to get the same effect. However, that's not reliable when the loop body is more complicated than a single basic block 🙈 it also 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 option the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code 👑 This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
• -fpartial-inlining — Inline parts of functions 💅 this option only does anything when inlining itself is turned on by the -finline-functions or -finline-small-functions options. Enabled at levels -O2, -O3, -Os.
• -fpredictive-commoning — Perform predictive commoning optimization, aka reusing computations (especially memory loads and stores) 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 supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays 🦋 real talk: this option may generate better or worse code — results are highly dependent on the structure of loops within your source code. Disabled at level -Os.
• -fno-printf-return-value — Do not substitute constants for known return value of formatted output functions such as sprintf, snprintf, vsprintf, and vsnprintf (but not printf or fprintf) 😭 this transformation lets GCC optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible.

For example, when -fprintf-return-value is in effect, both the branch and the body of the if statement (but not the call to snprint) can be optimized away when i is a 32-bit or smaller integer because the return value is guaranteed to be at most 8 🎀

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  ...

The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above 💜 it works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default.

• -fno-peephole — Disable any machine-specific peephole optimizations 💅
• -fno-peephole2 — also disables machine-specific peephole optimizations, and 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 — Do not guess branch probabilities using heuristics, bestie 🤚 GCC uses heuristics to guess branch probabilities if they're not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account 🌈

The interactions between the heuristics and __builtin_expect can be complex, and in some cases it may be useful to disable the heuristics so the effects of __builtin_expect are easier to understand. It's also possible to specify 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 to reduce the number of taken branches and improve code locality 🎀 enabled at levels -O1, -O2, -O3, -Os.
• -freorder-blocks-algorithm=algorithm — Use the specified algorithm for basic block reordering 👑 the algorithm argument can be:
  • simple — does not increase code size (except sometimes due to secondary effects like alignment).
  • stc — the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.

The default is simple at levels -O1, -Os, and stc at levels -O2, -O3 ✨

• -freorder-blocks-and-partition — 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 💅

Heads up bestie, this optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that does not support named sections 😭 when -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but 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 most frequently executed functions and .text.unlikely for unlikely executed functions. Reordering is done by the linker, so the object file format must support named sections and the linker must place them in a reasonable way.

This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes 💀 enabled at levels -O2, -O3, -Os.

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

Pay special attention to code like this, 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 rules of -fstrict-aliasing are 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 — okay so this one lines up the start of your functions all pretty 🌸 — it aligns them to the next power-of-two that's greater than or equal to n, skipping up to m-1 bytes while it's at it. The whole point is that at least the first m bytes of the function can get fetched by the CPU without ever crossing an n-byte alignment boundary. It's a code-performance glow-up, and ngl alignment is ignored for functions considered cold 🥶. If you need alignment on literally every function, use -fmin-function-alignment instead.

If m isn't specified, it defaults to n. 💅

Examples, bestie, watch: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if that can be done by skipping 23 bytes or less, and -falign-functions=32:7 aligns to the next 32-byte boundary only if that can be done by skipping 6 bytes or less. ✨

The second pair of n2:m2 values unlocks a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if that can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2.

Some assemblers only support this flag when n is a power of two; in that case, it gets rounded up. 🫶

Also: -fno-align-functions and -falign-functions=1 are equivalent and mean functions are not aligned, periodt.

If n isn't specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536. 🔥

Enabled at levels -O2, -O3.

• -flimit-function-alignment — turn this on and the compiler tries to avoid unnecessarily overaligning functions, like it's being considerate 😌. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function.
• -falign-labels, -falign-labels=n, -falign-labels=n:m, -falign-labels=n:m:n2, -falign-labels=n:m:n2:m2 — aligns all branch targets to a power-of-two boundary. 🎯

The parameters work just like the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean 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 isn't specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -falign-loops, -falign-loops=n, -falign-loops=n:m, -falign-loops=n:m:n2, -falign-loops=n:m:n2:m2 — aligns loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. 🌀 It's a code-performance optimization, 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 loops are not aligned. The maximum allowed n option value is 65536.

If n isn't specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3.

• -falign-jumps, -falign-jumps=n, -falign-jumps=n:m, -falign-jumps=n:m:n2, -falign-jumps=n:m:n2:m2 — aligns branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping 🦘. In this case, no dummy operations need be executed. This is a code-performance optimization, 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 isn't 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 — sets the minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike -falign-functions this alignment is applied to all functions (even those considered cold 🧊). The alignment is also not affected by -flimit-function-alignment.
• -fno-allocation-dce — do not remove unused C++ allocations in dead code elimination. 🗑️
• -fallow-store-data-races — this lets the compiler perform optimizations that may introduce new data races on stores, without proving that the variable can't be concurrently accessed by other threads 😭. It does not affect optimization of local data. It's safe to use this option if it's known that global data will not be accessed by multiple threads.

Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context ⚠️. Note that on some processors, if-conversions may be required in order to enable vectorization.

Enabled at level -Ofast.

• -funit-at-a-time — this option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors. 🫠

Enabled by default.

• -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it's 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 assigns each web an 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 — assumes that the current compilation unit represents the whole program being compiled 🌍. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers.

With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful if no linker plugin is used or during incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).

• -flto[=n] — okay bestie this one is the 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's 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. 🔥

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.

---

Okay bestie, this is the LTO lore drop and it is long, but it's giving "you will pass your build" energy so let's lock in 🔒✨

To make whole program optimization actually hit, you need some whole program assumptions. The compiler has to know which functions and variables can be reached by libraries and runtime stuff outside the link-time optimized unit. When the linker supports it, the linker plugin (peep -fuse-linker-plugin) hands that info to the compiler — used symbols, externally visible symbols, the whole vibe. But when there's no linker plugin available? You gotta use -fwhole-program yourself so the compiler is allowed to make those assumptions, which means more aggressive optimization decisions. Slay, but only if you mean it.

When a file gets compiled with -flto without -fuse-linker-plugin, the generated object file is bigger than a regular object file, because it's carrying GIMPLE bytecodes and the usual final code (that's the -ffat-lto-objects situation). Which means: object files with LTO info can still be linked as normal object files, and if -fno-lto gets passed to the linker, zero interprocedural optimizations happen. Lowkey important: when -fno-fat-lto-objects is on, the compile stage is faster, but bestie you cannot do a regular non-LTO link on them. 😭

When you're producing the final binary, GCC only applies link-time optimizations to the files that actually contain bytecode. So you can absolutely mix and match object files and libraries with GIMPLE bytecodes and final object code — GCC figures out on its own which files get optimized in LTO mode and which ones just get linked with no further processing. Main character behavior honestly.

Generally, options you specify at link time override the ones from compile time, though in some cases GCC tries to infer link-time options from the settings used to compile the input files. So it's not always a straight override, it's giving "it depends" 🙈

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

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

There are some code generation flags that GCC preserves when generating bytecodes, because they're needed during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm, and all the -m target flags.

The following options — -fPIC, -fpic, -fpie, -fPIE — get combined based on this scheme:

-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpie

Certain ABI-changing flags must match in all compilation units, and trying to override this at link time with a conflicting value just gets ignored. That includes options like -freg-struct-return and -fpcc-struct-return. No arguing with the ABI, bestie, it's not the vibes, it's the rules 💅

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. You can override them at link time though, so you're not stuck 🫶

Diagnostic options such as -Wstringop-overflow get passed through to the link stage, and their setting matches that of the compile-step at function granularity. Note that this only matters for diagnostics emitted during optimization. Also note: code transforms such as inlining can lead to warnings being enabled or disabled for regions of code that aren't consistent with the setting at compile time. Sneaky.

When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto, or consistently use the same assembler options on all translation units. Alternatively, you can also specify assembler options at LTO link time. Pick a lane, babe 🎀

To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation too. But any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. Don't do it. Seriously. 🙅

If LTO runs into objects with C linkage declared with incompatible types in separate translation units being linked together (that's undefined behavior according to ISO C99 6.2.7, so, yikes), a non-fatal diagnostic may be issued. The behavior is still undefined at run time, the diagnostic is just a heads-up, not a fix. Similar diagnostics may be raised for other languages too.

Another feature of LTO: you can apply interprocedural optimizations on files written in different languages. Obsessed.

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 the final link is done with g++ so you get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Consistency is the whole game ✨

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it's possible to extract and use them in an LTO link if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; and 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 pulls the needed GIMPLE files out of libfoo.a and passes them on to the running GCC, so they become part of the aggregated GIMPLE image that gets optimized. It's giving teamwork 🌈

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 get extracted and linked as usual — but they do not participate in the LTO optimization process. To make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Best of both worlds, no cap.

Link-time optimizations do not require the whole program to be present to operate. But if the program doesn't require any symbols to be 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 don't 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. No time travel, unfortunately 💀

Also: link-time optimization doesn't 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. 🚀

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 autodetection of the number of CPU threads present in your system. Automatic. Effortless. Iconic. 👑

• -flto-partition=alg — specifies 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. 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 are 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.

---

Still going, bestie — the LTO saga continues… 🦋

• the flag from the part right before this one 💫 — this option enables the extraction of object files with GIMPLE bytecode straight out of library archives. And why is that a slay? Because it 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). The resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. For the description of the effect of this flag and how to use it, go look at -flto.

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 — okay so fat LTO objects are object files that contain both the intermediate language and the object code. That makes them usable for both LTO linking and normal linking. Heads up bestie: this option is effective only when compiling with -flto, and it's 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 plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).

The default is -fno-fat-lto-objects on targets with linker plugin support. ✨

• -fcompare-elim — after register allocation and post-register allocation instruction splitting, this identifies arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, it eliminates the explicit comparison operation. 🧹

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.

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

• -ffold-mem-offsets, -fno-fold-mem-offsets — tries to eliminate add instructions by folding them in memory loads/stores. That's it, that's the whole vibe.

Enabled at levels -O2, -O3.

NOTE: On Debian/Ubuntu this is disabled by default on m68k-linux-gnu. 💀

• -fcprop-registers — after register allocation and post-register allocation instruction splitting, this performs a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Tiny little glow-up for your instruction scheduling 🌸

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

• -fprofile-correction — so here's the tea: profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. 😭

This option is enabled by -fauto-profile.

• -fprofile-partial-training — with -fprofile-use all portions of programs not executed during train run are optimized aggressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, program may contain functions specific for a given hardware and training may not cover all hardware configurations program is run on.) With -fprofile-partial-training profile feedback will be ignored for all functions not executed during the train run, leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when train run is not representative but also leads to significantly bigger code. 🫶
• -fprofile-use, -fprofile-use=path — enables profile feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions

Before you can use this option, you must first generate profiling information. No shortcuts, bestie. 👑

By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile).

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.

• -fauto-profile, -fauto-profile=path — enables sampling-based feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correction

path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.

Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see https://perf.wiki.kernel.org/.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo.

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

---

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. ✨

• -ffloat-store — do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory. 🧊

This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. 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 such programs, after modifying them to store all pertinent intermediate computations into variables.

• -fexcess-precision=style — this one lets you have further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C or C++, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C or C++ if a strict conformance option such as -std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used. If -fexcess-precision=16 is specified, constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision. 🔬

-fexcess-precision=standard is not implemented for languages other than C or C++. 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. 🎀

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 do not require the guarantees of these specifications. 🔥

• -fno-math-errno — do not 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 maintaining 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 do not require the guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets errno. There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default. 🍓

• -funsafe-math-optimizations — bestie, this one lets GCC do floating-point math optimizations that (a) just assume the arguments and results are valid and (b) might straight up violate IEEE or ANSI standards. And when you use it at link time? It might pull in libraries or startup files that change the default FPU control word, or do other similar optimizations. It's giving chaotic but fast. 💅

No -O option ever turns this on, because it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. That said, it may yield faster code for programs that don't need those guarantees. It enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

• -fassociative-math — allows re-association of operands in a series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing the computation result. NOTE: the re-ordering may change the sign of zero, ignore NaNs, and inhibit or create underflow or overflow (so it literally cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52). It may also reorder floating-point comparisons, so no using it when ordered comparisons are required. This option needs both -fno-signed-zeros and -fno-trapping-math to be in effect. Also, 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 that unlocks optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Heads up though: this loses precision and increases the number of flops operating on the value. 😭

The default is -fno-reciprocal-math.

• -ffinite-math-only — allow optimizations for floating-point arithmetic that assume arguments and results are not NaNs or +-Infs.

This 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. But it may yield faster code for programs that don't require those guarantees.

The default is -fno-finite-math-only.

• -fno-signed-zeros — allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant. ✨

The default is -fsigned-zeros.

• -fno-trapping-math — compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting it may allow faster code if you rely on "non-stop" IEEE arithmetic, for example.

bestie this option should never be turned on by any -O option, because 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. That default is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. You want this option for programs that change the FP rounding mode dynamically, or that may 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.

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 option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

It causes the preprocessor macro __SUPPORT_SNAN__ to be defined. 💖

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

• -fno-fp-int-builtin-inexact — 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.
• -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there's no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but it's enabled by -ffast-math. 🔥

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages.

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there's no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case.

The default is -fno-cx-fortran-rules.

---

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

• -fbranch-probabilities — after running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to improve optimizations based on 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 this 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're 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.

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.

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're adjacent to each other in the instruction flow.

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. 🎀

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

• -funroll-loops — okay bestie so this one unrolls loops when the compiler can figure out the iteration count at compile time, or when it figures it out on entry to the loop 🌀 -funroll-loops also implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling, which is literally the whole removal of loops that only have a small constant number of iterations. Heads up though: this option makes your code bigger, and it may or may not make it run faster — no cap, it's a gamble ✨

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

• -funroll-all-loops — no thoughts, just vibes: unroll all the loops, even the ones where the number of iterations is totally 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 when there's enough info that they don't roll much (info coming from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations) 🍓

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

• -fmove-loop-invariants — turns on the loop invariant motion pass in the RTL loop optimizer ✨ Enabled at level -O1 and higher, except for -Og.
• -fmove-loop-stores — turns on 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 that for this option to actually do anything, -ftree-loop-im has to be enabled as well. Enabled at level -O1 and higher, except for -Og.
• -fsplit-loops — splits a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other 🦋

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

• -funswitch-loops — moves branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to the result of the condition) 💅

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

• -fversion-loops-for-strides — if a loop iterates over an array with a variable stride, this creates another version of the loop that assumes the stride is always one 👀 For example:

for (int i = 0; i < n; ++i)
  x[i * stride] = ...;

becomes:

if (stride == 1)
  for (int i = 0; i < n; ++i)
    x[i] = ...;
else
  for (int i = 0; i < n; ++i)
    x[i * stride] = ...;

This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses 😌 This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -ffunction-sections
• -fdata-sections — these two 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.

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space ✨ Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping) 🫶

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats 👀

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower 😭 These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.

• -fstdarg-opt — optimizes the prologue of variadic argument functions with respect to usage of those arguments 🌸
• -fsection-anchors — tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects 🦋 This transformation can help to 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 out the call-used registers at function return to increase program security, either by mitigating Return-Oriented Programming (ROP) attacks or by preventing information leakage through registers 🔥

The possible values of choice are the same as for the zero_call_used_regs attribute. The default is skip.

You can control this behavior for a specific function by using the function attribute zero_call_used_regs.

• --param name=value — 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 💀

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 a branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable ✨

max-rtl-if-conversion-insns

RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions 👑 This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.

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 biggest number of pending dependencies the scheduler is allowed to hold before it says "okay bestie, flushing this" and starts fresh. Big functions with barely any branches or calls can build up huge lists and those lists just eat your memory and resources for literally no reason. 🙈
• max-modulo-backtrack-attempts — how many times the scheduler is allowed to back it up and try again when it's modulo scheduling a loop. Bigger values can make your compile time explode exponentially. like, exponentially exponentially. 💀
• max-inline-functions-called-once-loop-depth — the max loop depth of a call that inline heuristics will even look at, for the heuristics that try to inline every single function called exactly once.
• max-inline-functions-called-once-insns — the max estimated size of the functions that get produced while inlining functions-called-once. basically a size cap, estimate edition.
• max-inline-insns-single — okay so several parameters control the tree inliner in GCC, and this number sets the maximum number of instructions (counted in GCC's internal representation, iykyk) in a single function that the tree inliner will even consider for inlining. heads up tho: this only affects functions declared inline and methods implemented inside a class declaration (that's the C++ girlies ✨).
• max-inline-insns-auto — when you use -finline-functions (which is included in -O3 btw), a whole bunch of functions that the compiler would normally not even glance at for inlining get investigated. For those functions a different, more restrictive limit than the one for functions declared inline can be applied — that's --param max-inline-insns-auto.
• max-inline-insns-small — the bound applied to calls that are considered relevant with -finline-small-functions.
• max-inline-insns-size — the bound applied to calls that are optimized for size. A lil growth can be desirable here so you can anticipate the optimization opportunities that inlining exposes. (yes the original spells it "oppurtunities", no I will not be fixing that, it's her little thing 💅)
• uninlined-function-insns — the number of instructions the inliner accounts for function overhead, like the function prologue and epilogue.
• uninlined-function-time — extra time the inliner accounts for function overhead, like the time needed to actually execute the function prologue and epilogue.
• inline-heuristics-hint-percent — the scale (in percents, obvs) applied to inline-insns-single, inline-insns-single-O2, and inline-insns-auto when the inline heuristics hint that inlining is very profitable (meaning it'll enable later optimizations). slay.
• uninlined-thunk-insns, uninlined-thunk-time — same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks instead. thunk rights, basically. 🫶
• inline-min-speedup — when the estimated performance improvement of caller + callee runtime goes over this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. no cap, it just goes.
• large-function-insns — the limit that defines what counts as a really large function. For functions bigger than this limit after inlining, inlining gets constrained by --param large-function-growth. This parameter is mostly here to avoid extreme compilation time caused by the non-linear algorithms the back end uses.
• large-function-growth — the maximal growth of a large function caused by inlining, in percents. Example: parameter value 100 limits large function growth to 2.0 times the original size.
• large-unit-insns — the limit that defines a large translation unit. Growth caused by inlining in units bigger than this limit gets bounded by --param inline-unit-growth. For small units this can be way too tight. Example: picture a unit made of function A which is inline, and B which 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, you need that overall unit growth limit to avoid an 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. 🧚
• lazy-modules — the max number of concurrently open C++ module files when lazy loading.
• inline-unit-growth — the maximal overall growth of the compilation unit caused by inlining. Example: parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or via profile feedback) are not counted into the unit size, they get a pass. 🥶
• ipa-cp-unit-growth — the maximal overall growth of the compilation unit caused by interprocedural constant propagation. Example: parameter value 10 limits unit growth to 1.1 times the original size.
• ipa-cp-large-unit-insns — the size of a translation unit that the IPA-CP pass considers large.
• large-stack-frame — the limit that defines a large stack frame. While inlining, the algorithm tries not to blow past this limit too much.
• large-stack-frame-growth — the maximal growth of large stack frames caused by inlining, in percents. Example: parameter value 1000 limits large stack frame growth to 11 times the original size. yes, eleven. 🔥
• max-inline-insns-recursive, max-inline-insns-recursive-auto — the max number of instructions an out-of-line copy of a self-recursive inline function is allowed to grow into by doing recursive inlining.

--param max-inline-insns-recursive applies to functions declared inline. For functions that are not declared inline, recursive inlining only happens when -finline-functions (which is included in -O3) is enabled, and then --param max-inline-insns-recursive-auto applies instead.

• max-inline-recursive-depth, max-inline-recursive-depth-auto — the maximum recursion depth used for recursive inlining.

--param max-inline-recursive-depth applies to functions declared inline. For functions that are not declared inline, recursive inlining happens only when -finline-functions (included in -O3) is enabled, and --param max-inline-recursive-depth-auto applies instead. same energy, different auto. 💫

• min-inline-recursive-probability — recursive inlining is only profitable for functions that have deep recursion on average, and it can genuinely hurt a function with only a little recursion depth, by growing the prologue size or making the function body more complex for the other optimizers to deal with. 😭

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 — specifies 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 iteration limit 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, so don't stress. 🫶
• comdat-sharing-probability — the probability (in percent) that a C++ inline function with comdat visibility gets shared across multiple compilation units.
• modref-max-bases, modref-max-refs, modref-max-accesses — 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. btw this parameter ought to be bigger than --param modref-max-bases and --param modref-max-refs. (and yes, "maxmal" — she committed to that spelling, we respect it 🙈)
• modref-max-depth — specifies the maximum depth of the DFS walk used by modref escape analysis. Setting it to 0 disables the analysis completely. gone. poof. ✨
• modref-max-escape-points — specifies the maximum number of escape points that modref tracks per SSA-name.
• modref-max-adjustments — specifies the maximum number of times the access range gets enlarged during modref dataflow analysis.
• profile-func-internal-id — a parameter controlling whether to use the function internal id for profile database lookup. If the value is 0, the compiler uses an id based on the function assembler name and filename, which makes old profile data more tolerant to source changes like function reordering and so on.
• min-vect-loop-bound — the minimum number of iterations under which loops do not get vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option for vectorization to be allowed. so basically, hit the number or no glow-up for you. 💅
• gcse-cost-distance-ratio — the scaling factor used when calculating the maximum distance an expression can be moved by GCSE optimizations. Currently this is only supported in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting gets with simple expressions, i.e. the expressions whose cost is less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions entirely.
• gcse-unrestricted-cost — the cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations stop constraining the distance an expression is allowed to travel. Currently only supported in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 lets all expressions travel unrestricted distances. 🌈
• max-hoist-depth — the depth of search in the dominator tree for expressions to hoist. This exists to avoid quadratic behavior in the hoisting algorithm. A value of 0 doesn't limit the search, but it may slow down compilation of huge functions. ⚠️
• max-tail-merge-comparisons — the maximum number of similar bbs (basic blocks, bestie) to compare a bb with. This is used to avoid quadratic behavior in tree tail merging.
• max-tail-merge-iterations — how many times the pass gets to run over the function, max. basically it's the lil budget cap so tree tail merging doesn't eat your whole afternoon compiling 🌸
• store-merging-allow-unaligned — lets the store merging pass sneak in unaligned stores, but only if that's legal. if it's not legal, it's not happening bestie ✨
• max-stores-to-merge — the most stores the store merging pass will even attempt to squish together into wider stores.
• max-store-chains-to-track — how many store chains it keeps an eye on at once while trying to merge them into wider stores. multitasking queen 👑
• max-stores-to-track — the max number of stores tracked at the same time in the attempt to merge them into wider stores in the store merging pass. (yes it's a lot of tracking, it's giving surveillance 💅)
• max-unrolled-insns — the most instructions a loop can have and still be allowed to get unrolled. and if it does get unrolled, this same param decides how many times the loop code gets unrolled. two jobs, one icon 🎀
• max-average-unrolled-insns — same energy as above, except the instruction count is biased by the probabilities of their execution. and yeah, if the loop gets unrolled, this one also sets how many times the loop code gets unrolled.
• max-unroll-times — the max number of unrollings of one single loop. one loop, so many unrolls 🌀
• max-peeled-insns — the most instructions a loop can have and still be eligible for peeling. and if it's peeled, this parameter also decides how many times the loop code gets peeled.
• max-peel-times — max number of peelings of a single loop.
• max-peel-branches — the max number of branches allowed on the hot path that goes through the peeled sequence 🔥
• max-completely-peeled-insns — the max number of insns in a completely peeled loop.
• max-completely-peel-times — the max number of loop iterations a loop can have and still be considered suitable for complete peeling.
• max-completely-peel-loop-nest-depth — deepest loop nest depth that's still suitable for complete peeling 🪆
• max-unswitch-insns — max number of insns of an unswitched loop.
• max-unswitch-depth — the deepest loop nest depth that can get unswitched.
• lim-expensive — the minimum cost threshold for an expression to count as "expensive" in loop invariant motion. gotta be pricey to qualify 💸
• min-loop-cond-split-prob — when FDO profile info is available, this sets the minimum probability threshold for a semi-invariant condition statement to actually trigger a loop split.
• iv-consider-all-candidates-bound — a bound on how many induction variable candidates there are. below this number, every candidate gets considered for each use in induction variable optimizations. go above it and only the most relevant ones get considered, because we are NOT doing quadratic time complexity today 🙂‍↔️
• iv-max-considered-uses — the induction variable optimizations straight up give up on loops that have more induction variable uses than this.
• iv-always-prune-cand-set-bound — if the candidate set has fewer members than this value, always try to remove unnecessary ivs from the set whenever a new one gets added. spring cleaning era 🧹
• avg-loop-niter — the average number of iterations of a loop.
• dse-max-object-size — the maximum size, in bytes, of objects tracked bytewise by dead store elimination. bigger values = potentially bigger compilation times. say it with me: bigger 📈
• dse-max-alias-queries-per-store — the max number of queries into the alias oracle per store. larger values mean larger compilation times, but might also mean more dead stores get removed. tradeoffs, babe ✨
• scev-max-expr-size — the bound on the size of expressions used in the scalar evolutions analyzer. big expressions make the analyzer slow, and nobody wants that 😭
• scev-max-expr-complexity — the bound on how complex expressions can get in the scalar evolutions analyzer. complex expressions = slow analyzer. same vibe, different axis.
• max-tree-if-conversion-phi-args — the max number of arguments in a PHI that TREE if conversion supports — unless the loop is marked with the simd pragma, in which case this limit doesn't apply 💫
• vect-max-layout-candidates — the max number of possible vector layouts (think permutations) to consider when optimizing code that's about to be vectorized.
• vect-max-version-for-alignment-checks — the max number of run-time checks allowed when the vectorizer does loop versioning for alignment.
• vect-max-version-for-alias-checks — the max number of run-time checks allowed when the vectorizer does loop versioning for alias.
• vect-max-peeling-for-alignment — max number of loop peels used to improve access alignment for the vectorizer. value -1 means no limit. go off, unlimited 💅
• max-iterations-to-track — the max number of loop iterations that the brute-force algorithm for analyzing a loop's iteration count will even try to evaluate.
• hot-bb-count-fraction — here's the vibe: n is the denominator of the fraction 1/n of the maximal execution count of a basic block in the whole program, and a basic block needs at least that much to count as hot. the default is 10000, meaning a block is hot if its execution count is greater than 1/10000 of the maximal execution count. a value of 0 means it is never considered hot. this one's used in non-LTO mode.
• hot-bb-count-ws-permille — the number of most-executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program that a basic block's execution count must be part of to count as hot. default is 990, so a block is hot if its execution count lands in the upper 990 permilles — that's 99.0% — of the profiled execution of the whole program. 0 means never hot. used in LTO mode.
• hot-bb-frequency-fraction — n is the denominator of fraction 1/n of the execution frequency of a function's entry block, and a basic block in that function needs at least that much to be considered hot. default 1000 → a block counts as hot in a function if it runs more frequently than 1/1000 of the frequency of the function's entry block. 0 means never hot.
• unlikely-bb-count-fraction — n is the denominator of fraction 1/n of the number of profiled runs of the entire program; a basic block's execution count has to be below that for the block to be considered unlikely executed. default is 20, so a block is unlikely executed if it runs in fewer than 1/20 — that's 5% — of the program's runs. 0 means it's always considered unlikely executed 👀
• max-predicted-iterations — the max number of loop iterations we predict statically. super useful when a function has one loop with a known bound and another with an unknown bound: the known iteration count gets predicted correctly, while the unknown one averages out to roughly 10 — which means the loop with no bounds ends up looking artificially cold next to the other one 🥶
• builtin-expect-probability — controls the probability of the expression having the specified value. takes a percentage, so 0 ... 100, as input.
• builtin-string-cmp-inline-length — the max length of a constant string for a builtin string cmp call to be eligible for inlining.
• align-threshold — picks the fraction of the maximal frequency of executions of a basic block in a function, and that's what gets used to align the basic block 📐
• align-loop-iterations — a loop that's expected to iterate at least this selected number of iterations gets aligned.
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value caps superblock formation once the given percentage of executed instructions has been covered. it keeps unnecessary code size expansion in check 🛑

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

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

just like tracer-dynamic-coverage, there are two of these. tracer-min-branch-probability-feedback is for compiling with profile feedback, tracer-min-branch-probability is for compiling without. and the feedback one needs to be more conservative (higher) for the tracer to actually be effective.

• stack-clash-protection-guard-size — sets the size of the operating system provided stack guard, expressed as 2 raised to num bytes. higher values may cut down the number of explicit probes, but a value larger than the OS-provided guard leaves your code vulnerable to stack clash style attacks.

⚠️ bestie that last bit is the real tea: bigger than the OS guard = stack clash style attacks can get in. do not do that. 💀

• stack-clash-protection-probe-interval — stack clash protection probes stack space as it gets allocated, and 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 OS-provided guard will leave code vulnerable to stack clash style attacks.

same warning, same drama: past the OS-provided guard and you're wide open to stack clash style attacks 🔥

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• max-cse-path-length — the max number of basic blocks on a path that CSE will consider. like, how far down the hallway is it allowed to peek before it gives up 🚪✨
• max-cse-insns — the max number of instructions CSE processes before it flushes. everyone needs a lil limit, bestie 🙈
• max-cse-insns… no wait, we did that one. moving on fr fr ⭐

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• ggc-min-expand — okay so GCC manages its own memory allocation with a garbage collector, and this parameter sets the minimum percentage by which the garbage collector's heap is allowed to expand between collections. tuning this may improve compilation speed, and 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" here is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC isn't able to calculate RAM on a particular platform, the lower bound of 30% is what you get.

Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. That is extremely slow 💀 but it can be useful for debugging.

• ggc-min-heapsize — the minimum size of the garbage collector's heap before it even starts bothering to collect garbage. the first collection happens after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. again, tuning this may improve compilation speed, and it has no effect on code generation 🫶

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to make sure RLIMIT_DATA or RLIMIT_AS aren't exceeded, with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC isn't able to calculate RAM on a particular platform, the lower bound is used.

Setting this parameter very large effectively disables garbage collection. And setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. yeah. same energy as above 😭

• max-reload-search-insns — the max number of instructions reload should look backward for an equivalent register. bigger values = more aggressive optimization, so compilation time goes up, probably for slightly better performance 📈
• max-cselib-memory-locations — the max number of memory locations cselib should take into account. bigger values = more aggressive optimization, compilation time goes up, probably slightly better performance ✨
• max-sched-ready-insns — the max number of instructions that are ready to be issued that the scheduler should consider at any given time during the first scheduling pass. bigger values = more thorough searches, so compilation time goes up with probably little benefit 🙃
• max-sched-region-blocks — the max number of blocks in a region 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 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. a value of 0 disables region extensions 🚫
• max-sched-insn-conflict-delay — the max conflict delay for an insn to be considered for speculative motion 🏃‍♀️
• sched-spec-prob-cutoff — the minimal probability of speculation success (in percents), so that speculative insns get scheduled 🎯
• sched-state-edge-prob-cutoff — the minimum probability an edge must have for the scheduler to save its state across it 🔗
• sched-mem-true-dep-cost — 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 a depth of search for available instructions 👀
• selsched-max-sched-times — the max number of times an instruction is scheduled during selective scheduling. this is the limit on the number of 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 swing modulo scheduler generates 🎡
• max-last-value-rtl — the max size, measured as the 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 🧬
• integer-share-limit — small integer constants can share a data structure, which reduces 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 🛡️

The default before Ubuntu 10.10 was "8". Currently it is "4", to increase the number of functions protected by the stack protector.

• min-size-for-stack-sharing — the minimum size of variables taking part in stack slot sharing when not optimizing 📦
• max-jump-thread-duplication-stmts — the max number of statements allowed in a block that needs to be duplicated when threading jumps ✂️
• max-jump-thread-paths — the max number of paths to consider when searching for jump threading opportunities. when arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges does not exhaust the specified max number of paths to consider 🛤️
• max-fields-for-field-sensitive — the max number of fields in a structure that get 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 may also lead to fewer streams being prefetched (see simultaneous-prefetches) 🌊
• simultaneous-prefetches — the max 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, because there may be conflicts between the hardware prefetchers and the software prefetchers. if the hardware prefetchers have a maximum stride they can handle, that's the value to use here, to improve the use of software prefetchers.

A value of -1 means we don't have a threshold, and therefore prefetch hints can be issued for any constant stride. and this setting is only useful for strides that are known and constant, periodt 💜

• destructive-interference-size, constructive-interference-size — the values for the C++17 variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. the destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects 👯 the constructive interference size is the maximum recommended size of contiguous memory accessed together 🫂 typically both are the size of an L1 cache line for the target, in bytes. for a generic target covering a range of L1 cache line sizes, typically the constructive interference size is the small end of the range and the destructive size is the large end.

The destructive interference size is intended to be used for layout, and thus has ABI impact 😱 its default value is not expected to be stable, and on some targets it varies with -mtune, so using this variable in a context where ABI stability is important — such as 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 max 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 — okay so this one is literally about whether the compiler should use the “canonical” type system. bestie, it should ALWAYS be 1 — that uses a way more efficient internal mechanism for comparing types in C++ and Objective-C++. but if bugs in the canonical type system are making compilation fail? set it to 0 to disable canonical types. periodt. 💅
• switch-conversion-max-branch-ratio — Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. it’s giving “don’t make arrays too big” energy. ✨
• max-partial-antic-length — Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. so like, for some sorts of source code the enhanced partial redundancy elimination optimization can literally 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 the runaway behavior. setting a value of 0 for this parameter allows an unlimited set length. yikes, but you do you. 💀
• rpo-vn-max-loop-depth — 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. slay. 🌈
• sccvn-max-alias-queries-per-access — 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. iykyk. 🔥
• 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. so like, it’s a vibe check for loops. 🦋
• ira-max-conflict-table-size — Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require excessive amounts of memory for huge functions 😭. if the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table. it’s giving “we tried, but we’re gonna take the easy way out”. 🙈
• ira-loop-reserved-regs — IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3). the number of available registers reserved for some other purposes is given by this parameter. default of the parameter is the best found from numerous experiments. iconic. 👑
• ira-consider-dup-in-all-alts — Make IRA to consider matching constraint (duplicated operand number) heavily in all available alternatives for preferred register class. if it is set as zero, it means IRA only respects the matching constraint when it’s in the only available alternative with an appropriate register class. otherwise, it means IRA will check all available alternatives for preferred register class even if it has found some choice with an appropriate register class and respect the found qualified matching constraint. no cap. 💯
• ira-simple-lra-insn-threshold — Approximate function insn number in 1K units triggering simple local RA. simple as that. ⭐
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values reloaded in registers in subsequent insns. this optimization is called inheritance. EBB is used as a region to do this optimization. the parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. the default value was chosen from numerous runs of SPEC2000 on x86-64. vintage. 🎀
• loop-invariant-max-bbs-in-loop — Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops. loops with more basic blocks than this parameter won’t have loop invariant motion optimization performed on them. it’s a boundary, bestie. 🫶
• loop-max-datarefs-for-datadeps — Building data dependencies is expensive for very large loops. this parameter limits the number of data references in loops that are considered for data dependence analysis. these large loops are no handled by the optimizations using loop data dependencies. (yes, the original says “no handled” — we’re keeping that exactly, character for character 😭). 💀
• max-vartrack-size — Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. if this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. if the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. setting the parameter to zero makes it unlimited. so like, no cap on the hash table. 💖
• max-vartrack-expr-depth — Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. this trades compilation time for more complete debug information. if this is set too low, value expressions that are available and could be represented in debug information may end up not being used 😭. setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. it’s a tradeoff, bestie. ⚖️
• max-debug-marker-count — Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL. if a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion. boom, dropped. 🔥
• min-nondebug-insn-uid — Use 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 get (non-overlapping) uids above it if the reserved range is exhausted. so like, it’s a VIP section for debug insns. 🎟️
• ipa-sra-deref-prob-threshold — IPA-SRA replaces a pointer which is known not be NULL with one or more new parameters only when the probability (in percent, relative to function entry) of it being dereferenced is higher than this parameter. so it’s a threshold for deref vibes. ✨
• ipa-sra-ptr-growth-factor — IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. math, but make it compiler. 🧮
• ipa-sra-ptrwrap-growth-factor — Additional maximum allowed growth of 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 it as an argument to other functions. extra allowance, periodt. 💅
• ipa-sra-max-replacements — Maximum pieces of an aggregate that IPA-SRA tracks. as a consequence, it is also the maximum number of replacements of a formal parameter. so it’s a two-for-one limit. 🍓
• 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 aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. so like, one for speed, one for size. 🦋
• sra-max-propagations — The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to facilitate copy propagation. keep it tracked. 👑
• tm-max-aggregate-size — When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. this option only applies when using -fgnu-tm. transaction vibes. 💸
• graphite-max-nb-scop-params — To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. a value of zero can be used to lift the bound. a variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP. so like, parameters are just mysterious variables. 🔮
• hardcfr-max-blocks — Disable -fharden-control-flow-redundancy for functions with a larger number of blocks than the specified value. zero removes any limit. no blocks, no problem? actually more blocks, no hardening. 💀
• hardcfr-max-inline-blocks — Force -fharden-control-flow-redundancy to use out-of-line checking for functions with a larger number of basic blocks than the specified value. out-of-line, out of mind. 🙈
• 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. tile size, slay. ✨
• ipa-jump-function-lookups — Specifies number of statements visited during jump function offset discovery. short and sweet. 🍬
• ipa-cp-value-list-size — IPA-CP attempts to track all possible values and types passed to a function’s parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. so it’s a list limit. 📋
• ipa-cp-eval-threshold — IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold. score threshold, bestie. 🎯
• ipa-cp-max-recursive-depth — Maximum depth of recursive cloning for self-recursive function. recursion, but make it clone. 🌀
• ipa-cp-min-recursive-probability — Recursive cloning only when the probability of call being executed exceeds the parameter. so like, don’t clone if it’s unlikely. 🎲
• ipa-cp-profile-count-base — bestie, when you're rolling with -fprofile-use, IPA-CP looks at the measured execution count of a call graph edge at this percentage position in the histogram and uses that as the basis for its heuristics math. ✨
• ipa-cp-recursive-freq-factor — how many times interprocedural copy propagation expects recursive functions to call themselves. yes, they call themselves, it's giving main character energy 💅
• ipa-cp-recursion-penalty — the percentage penalty recursive functions eat when they're being evaluated for cloning. not me being penalized for being recursive 😭
• ipa-cp-single-call-penalty — the percentage penalty that functions containing just one single call to another function get hit with when they're evaluated for cloning. so rude, but okay 🫶
• ipa-max-agg-items — IPA-CP can also propagate a bunch of scalar values that were passed inside an aggregate, and ipa-max-agg-items controls the max number of those values per one single parameter. no cap, except this cap 🌸
• ipa-cp-loop-hint-bonus — when IPA-CP figures out that a cloning candidate would make the number of iterations of a loop known, it drops a bonus of ipa-cp-loop-hint-bonus onto that candidate's profitability score. little treat, well deserved 🍓
• ipa-max-loop-predicates — the max number of different predicates IPA will use to describe when loops in a function have known properties.
• ipa-max-aa-steps — while analyzing function bodies, IPA-CP uses alias analysis to track the values that function parameters point to. but it's not about to sit there forever analyzing enormous functions, so after it examines ipa-max-aa-steps statements that modify memory, it gives up and considers all memory clobbered. 💀
• ipa-max-switch-predicate-bounds — the maximal number of boundary endpoints of case ranges of a switch statement. for a switch that goes over this limit, IPA-CP will not construct a cloning cost predicate — that's the thing it uses to estimate cloning benefit — for the default case of the switch. 🙈
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter to estimate the benefit of cloning on a certain constant value. but if the number of operations in a parameter expression goes over ipa-max-param-expr-ops, that expression gets treated as a complicated one and it does not get handled by IPA analysis. it's giving "I don't do complicated" 💅
• lto-partitions — specify the desired number of partitions produced during WHOPR compilation. heads up: the number of partitions should exceed the number of CPUs used for compilation. 👑
• lto-min-partition — the size of the minimal partition for WHOPR, in estimated instructions. this exists to prevent the expense of splitting really small programs into way too many partitions.
• lto-max-partition — the size of the max partition for WHOPR, in estimated instructions, to give an upper bound for the individual size of a partition. meant to be used only with balanced partitioning, okurrr.
• lto-max-streaming-parallelism — the maximal number of parallel processes used for LTO streaming. maximum parallelism, as she deserves 🔥
• cxx-max-namespaces-for-diagnostic-help — the max number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. it's giving "did you mean…?" energy ✨
• sink-frequency-threshold — the max relative execution frequency (in percents) of the target block relative to a statement's original block, which allows statement sinking of a statement. larger numbers = more aggressive statement sinking. psst: a small positive adjustment gets applied for statements with memory operands, since those are even more profitable to sink. 🎀
• max-stores-to-sink — the max number of conditional store pairs that can be sunk. set it to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. lowkey important, don't skip it 💜
• case-values-threshold — the smallest number of different values for which it's best to use a jump-table instead of a tree of conditional branches. if the value is 0, use the default for the machine.
• jump-table-max-growth-ratio-for-size — the max code size growth ratio when expanding into a jump table, in percent. this parameter gets used when optimizing for size. 🌸
• jump-table-max-growth-ratio-for-speed — the max code size growth ratio when expanding into a jump table, in percent. this parameter gets used when optimizing for speed. 🔥
• tree-reassoc-width — set the max number of instructions executed in parallel in a reassociated tree. this parameter overrides the target-dependent heuristics used by default if it has a non-zero value.
• sched-pressure-algorithm — choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely one to prevent instructions from being reordered. Algorithm 2 was designed as a compromise between the relatively conservative approach of Algorithm 1 and the rather aggressive approach of the default scheduler. it relies more heavily on having a regular register file and accurate register pressure classes. see haifa-sched.cc in the GCC sources for more details, bestie. the default choice depends on the target. 🦋
• max-slsr-cand-scan — set the max number of existing candidates that get considered when seeking a basis for a new straight-line strength reduction candidate.
• asan-globals — enable buffer overflow detection for global objects. this kind of protection is on by default if you're using the -fsanitize=address option. to disable global objects protection, use --param asan-globals=0. 🛡️
• asan-stack — enable buffer overflow detection for stack objects. this kind of protection is on by default when using -fsanitize=address. to disable stack protection, use the --param asan-stack=0 option.
• asan-instrument-reads — enable buffer overflow detection for memory reads. this kind of protection is on by default when using -fsanitize=address. to disable memory reads protection, use --param asan-instrument-reads=0.
• asan-instrument-writes — enable buffer overflow detection for memory writes. this kind of protection is on by default when using -fsanitize=address. to disable memory writes protection, use the --param asan-instrument-writes=0 option.
• asan-memintrin — enable detection for built-in functions. this kind of protection is on by default when using -fsanitize=address. to disable built-in functions protection, use --param asan-memintrin=0.
• asan-use-after-return — enable detection of use-after-return. this kind of protection is on by default when using the -fsanitize=address option. to disable it, use --param asan-use-after-return=0.

Note: by default the check is disabled at run time. to enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS. ✨

• asan-instrumentation-with-call-threshold — if the number of memory accesses in the function being instrumented is greater than or equal to this number, use callbacks instead of inline checks. e.g. to disable inline code, use --param asan-instrumentation-with-call-threshold=0.
• asan-kernel-mem-intrinsic-prefix — if nonzero, prefix calls to "memcpy", "memset" and "memmove" with __asan_ or __hwasan_ for -fsanitize=kernel-address or -fsanitize=kernel-hwaddress, respectively.
• hwasan-instrument-stack — enable hwasan instrumentation of statically sized stack-allocated variables. this kind of instrumentation is on by default when using -fsanitize=hwaddress and off 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, decide tags for stack variables using a deterministic sequence beginning at a random tag for each frame. with this parameter unset, tags are chosen using the same sequence but beginning from 1. this is enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress. to disable it, use --param hwasan-random-frame-tag=0. 🎲
• hwasan-instrument-allocas — enable hwasan instrumentation of dynamically sized stack-allocated variables. this kind of instrumentation is on by default when using -fsanitize=hwaddress and off 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 — enable 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 — enable 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 — enable 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, directly poison (or unpoison) shadow memory instead of using run-time callbacks. 🧚
• tsan-distinguish-volatile — emit special instrumentation for accesses to volatiles. ✨
• tsan-instrument-func-entry-exit — emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit(). 🎀
• max-fsm-thread-path-insns — the absolute max number of instructions we're allowed to copy when duplicating blocks along a finite state automaton jump thread path. no cap on the ambition, cap on the copies 💅
• threader-debug — threader-debug=[none|all] turns on verbose dumping of the threader solver.
• parloops-chunk-size — chunk size of the omp schedule for loops that parloops parallelizes.
• parloops-schedule — the schedule type of the omp schedule for loops that parloops parallelizes: static, dynamic, guided, auto, or runtime.
• parloops-min-per-thread — the minimum number of iterations per thread for an innermost parallelized loop before the parallelized variant is preferred over the single threaded one. note bestie: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.
• max-ssa-name-query-depth — max depth of recursion when we're querying properties of SSA names in stuff like fold routines. one level of recursion = following one use-def chain link. slay but don't go infinite 🌀
• max-speculative-devirt-maydefs — the max number of may-defs we'll analyze while hunting for a must-def that specifies the dynamic type of an object invoking a virtual call we might be able to devirtualize speculatively.
• ranger-debug — says what type of debug output gets issued for ranges.
• unroll-jam-min-percent — the minimum percentage of memory references that have to be optimized away for the unroll-and-jam transformation to count as profitable. gotta actually save work to be worth it, fr.
• unroll-jam-max-unroll — the max number of times the outer loop should be unrolled by the unroll-and-jam transformation.
• max-rtl-if-conversion-unpredictable-cost — the max permissible cost for the sequence the RTL if-conversion pass would generate for a branch that's considered unpredictable.
• max-variable-expansions-in-unroller — if -fvariable-expansion-in-unroller is used, this is the max number of times one individual variable gets expanded during loop unrolling.
• partial-inlining-entry-probability — the max probability of the entry BB of a split region (as a percent relative to the entry BB of the function) for partial inlining to happen.
• max-tracked-strlens — the max number of strings whose lengths the strlen optimization pass will track.
• 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 performing redundancy elimination after reload.
• max-loop-header-insns — the max number of insns in a loop header that the copy loop headers pass will duplicate.
• vect-epilogues-nomask — enables loop epilogue vectorization using a smaller vector size. 🦋
• vect-partial-vector-usage — controls when the loop vectorizer considers using partial vector loads and stores instead of just falling back to scalar code. 0 = the vectorizer never uses partial vector loads and stores, periodt. 1 = allows partial vector loads and stores if vectorization removes the need for the code to iterate. 2 = allows partial vector loads and stores in all loops. this one only does anything on targets that support partial vector loads and stores, iykyk.
• vect-inner-loop-cost-factor — the max factor the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized. the factor actually applied is the max of the estimated number of iterations of the inner loop and this parameter. the default value 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 is like "nope, not processing that."
• vrp-vector-threshold — max number of basic blocks for VRP to use a basic cache vector.
• avoid-fma-max-bits — max number of bits for which we avoid creating FMAs.
• fully-pipelined-fma — whether the target fully pipelines FMA instructions. if non-zero, reassociation considers the benefit of parallelizing the multiplication part and the addition part of an FMA, assuming FMUL and FMA use the same units — 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 considers when checking conflicts using DFA.
• graphite-allow-codegen-errors — whether codegen errors should be ICEs when -fchecking. spicy choice, handle with care 🔥
• sms-max-ii-factor — a factor for tuning the upper bound 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 — max depth of sqrt chains to use when synthesizing exponentiation by a real constant.
• max-dse-active-local-stores — max 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 — max number of isl operations; 0 means unlimited. go off, no ceiling 🌈
• graphite-max-arrays-per-scop — max 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 — max number of nested calls to search for control dependencies during uninitialized variable analysis.
• uninit-max-chain-len — max number of predicates anded for each predicate ored in the normalized predicate chain.
• uninit-max-num-chains — max number of predicates ored in the normalized predicate chain.
• sched-autopref-queue-depth — hardware autoprefetcher scheduler model control flag. it's the number of lookahead cycles the model looks into; at ' ' it only enables the instruction sorting heuristic.
• loop-versioning-max-inner-insns — the max number of instructions an inner loop can have before the loop versioning pass decides it's too big to copy.
• loop-versioning-max-outer-insns — the max number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy — and heads up, this discounts any instructions in inner loops that directly benefit from versioning.
• ssa-name-def-chain-limit — the max number of SSA_NAME assignments to follow when determining a property of a variable, like its value. this caps how many iterations or recursive calls GCC does when optimizing certain statements, or when determining their validity before issuing diagnostics.
• store-merging-max-size — max 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 — max number of VALUEs handled during a single find_base_term call.
• analyzer-max-enodes-per-program-point — the max number of exploded nodes per program point within the analyzer before it terminates analysis of that point. 💀
• analyzer-max-constraints — the max number of constraints per state.
• analyzer-min-snodes-for-call-summary — the minimum number of supernodes within a function for the analyzer to consider summarizing its effects at call sites.
• analyzer-max-enodes-for-full-dump — the max depth of exploded nodes that should show up in a dot dump before switching to a less verbose format.
• analyzer-max-recursion-depth — the max number of times a callsite can appear in a call stack within the analyzer before it terminates analysis of a call that would recurse deeper.
• analyzer-max-svalue-depth — the max depth of a symbolic value before the value gets approximated as unknown.
• analyzer-max-infeasible-edges — the max number of infeasible edges to reject before declaring a diagnostic as infeasible.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block that counts as hot. this parameter is used only in GIMPLE FE.
• analyzer-bb-explosion-factor — the max number of 'after supernode' exploded nodes within the analyzer per supernode before terminating analysis. 💥
• analyzer-text-art-string-ellipsis-threshold — the number of bytes at which to ellipsize string literals in analyzer text art diagrams.
• analyzer-text-art-ideal-canvas-width — the ideal width, in characters, of text art diagrams generated by the analyzer.
• analyzer-text-art-string-ellipsis-head-len — the number of literal bytes to show at the head of a string literal in text art when ellipsizing it.
• analyzer-text-art-string-ellipsis-tail-len — the number of literal bytes to show at the tail of a string literal in text art when ellipsizing it.
• ranger-logical-depth — max depth of logical expression evaluation the ranger will look through when evaluating outgoing edge ranges.
• ranger-recompute-depth — max depth of instruction chains to consider for recomputation in the outgoing range calculator.
• relation-block-limit — max number of relations the oracle will register in a basic block.
• min-pagesize — the minimum page size, but like, for warning purposes only. She's just here to warn, bestie. 👀
• openacc-kernels — picks how OpenACC kernels constructs get handled. With --param=openacc-kernels=decompose, those kernels constructs get broken into parts — a whole sequence of compute constructs — and each one gets handled on its own. Heads up though: this is work in progress, so don't come for me if it's messy. With --param=openacc-kernels=parloops, the kernels constructs get handed to the parloops pass, en bloc. That one's the current default. ✨
• openacc-privatization — decides whether -fopt-info-omp-note and the applicable -fdump-tree-*-details options spit out OpenACC privatization diagnostics. --param=openacc-privatization=quiet → no diagnostics, and yep, that's the current default. --param=openacc-privatization=noisy → okay, diagnose away. 📢

The following choices of name are available on AArch64 targets:

• aarch64-vect-compare-costs — when vectorizing, this one goes "what if we tried several different approaches and let the cost model pick the cheapest?" and honestly? main character energy. What that includes:
  • trying both SVE and Advanced SIMD, whenever SVE is actually available.
  • trying to use 64-bit Advanced SIMD vectors for the teeniest data elements instead of forcing 128-bit vectors on literally everything.
  • trying to use "unpacked" SVE vectors for smaller elements — that means storing smaller elements in bigger containers and poking at elements with extending loads and truncating stores. 💅
• aarch64-float-recp-precision — the number of Newton iterations used to calculate the reciprocal for float type. The precision of division scales with this param whenever division approximation is enabled. Default value is 1.
• aarch64-double-recp-precision — same deal but for double type: the number of Newton iterations for calculating the reciprocal. Precision of division is proportional to this param when division approximation is on. Default value is 2.
• aarch64-autovec-preference — forces an ISA selection strategy for auto-vectorization. Takes values from 0 to 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, but prefer Advanced SIMD when the costs come out equal.
  • 4 — use both Advanced SIMD and SVE, but prefer SVE when the costs come out equal.

The default value is 0.

• aarch64-ldp-policy — fine-grained policy for load pairs. --param=aarch64-ldp-policy=default → just use the policy of the tuning structure, and that's the current default. --param=aarch64-ldp-policy=always → emit ldp regardless of alignment, no thoughts. --param=aarch64-ldp-policy=never → do not emit ldp, periodt. --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 — the exact same energy but for store pairs. --param=aarch64-stp-policy=default → policy of the tuning structure, current default. --param=aarch64-stp-policy=always → emit stp regardless of alignment. --param=aarch64-stp-policy=never → no stp for you. --param=aarch64-stp-policy=aligned → emit stp only when the source pointer is aligned to at least double the alignment of the type. 🫶
• aarch64-ldp-alias-check-limit — a limit on how many alias checks the AArch64 load/store pair fusion pass is allowed to do when it's trying to form an ldp/stp. Higher values = the pass gets more aggressive about re-ordering loads over stores, and the trade-off is increased compile time. 😭
• aarch64-ldp-writeback — controls which writeback opportunities we try to handle in the AArch64 load/store pair fusion pass. 0 disables writeback handling entirely. 1 means we try to form pairs involving one or more existing individual writeback accesses, where possible. 2 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 weigh both latencies and issue rates when deciding whether a loop should be vectorized with SVE, vectorized with Advanced SIMD, or not vectorized at all. Set this parameter to n and GCC will skip that heuristic for loops that are known to execute in fewer than n Advanced SIMD iterations. 🧚
• aarch64-vect-unroll-limit — the vectorizer uses whatever tuning info it has to figure out whether unrolling the main vectorized loop would be beneficial, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll that main loop. Default value is 4. 🌸

The following choices of name are available on GCN targets:

• gcn-preferred-vectorization-factor — preferred vectorization factor: default, 32, 64. That's it, that's the list. ✨

The following choices of name are available on i386 and x86_64 targets:

• x86-stlf-window-ninsns — the instruction count above which the STFL stall penalty can be compensated for.
• x86-stv-max-visits — the maximum number of use and def visits allowed while discovering a STV chain before the discovery gets aborted. 🛑

---

🎀 Program Instrumentation Options 🎀

GCC supports a whole bunch of command-line options that control adding run-time instrumentation to the code it normally generates. Like, for example, one purpose of instrumentation is to collect profiling statistics you can use for finding program hot spots, doing code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to catch programming errors — invalid pointer dereferences, out-of-bounds array accesses — and also deliberately hostile attacks like stack smashing or C++ vtable hijacking (rude). 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. Versatile queen. 👑

• -p
• -pg — generate extra code that writes 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. No skipping steps, bestie. You can use the function attribute no_instrument_function to suppress profiling of individual functions while compiling with these options. 🙈
• -fprofile-arcs — adds code so that program flow arcs get 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. Iconic, honestly. When the compiled program exits, it saves this data to a file called auxname.gcda for each source file. That data can 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 it's explicitly specified and it is not the final executable; otherwise it's the basename of the source file. Either way, any suffix gets removed — e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o.

Heads up: if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. Like, gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. 💅

• -fcondition-coverage — adds code so that program conditions get instrumented. During execution the program records which terms in a conditional actually contribute to a decision — which you can use to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. You can read the result with gcov --conditions. 🔍
• --coverage — this option compiles and links code instrumented for coverage analysis. It's a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). Check the docs for those options for more deets. The whole routine goes:
  • Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, also use the additional -ftest-coverage option. You do not need to profile every source file in a program, so breathe.
  • Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This lets gcov find the correct sources in projects where compilations happen with different working directories.
  • Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
  • Run the program on a representative workload to generate the arc profile information. You can repeat this any number of times. You can even run concurrent instances of your program, and — provided 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. Also, an object file can be recompiled multiple times and the corresponding .gcda file merges, as long as the source file and the compiler options are unchanged. ✨
  • For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
  • For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Go read the gcov documentation for further information. 📖

okay bestie, so with -fprofile-arcs, for each function of your program GCC literally builds a whole program flow graph, then finds a spanning tree for that graph. Only the arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count how many times those arcs get executed. And when an arc is the only exit or only entrance to a block, the instrumentation code can just chill in the block; otherwise, a whole new basic block has to be created just to hold the instrumentation code. it's giving graph theory realness 🫶.

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. slay.

• -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. no cap, this one saves you from path chaos.
• -fprofile-dir=path — Set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. See details about the file naming in -fprofile-arcs. See similar option -fprofile-note.

When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:

• %p — process ID.
• %q{VAR} — value of environment variable VAR

• -fprofile-generate, -fprofile-generate=path — Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program. fr fr, don't skip the link step.

The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.

To optimize the program based on the collected profile information, use -fprofile-use.

• -fprofile-info-section, -fprofile-info-section=name — Register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to ".gcov_info". A pointer to the profile information generated by -fprofile-arcs is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O.

The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:

                     .gcov_info      :
                     {
                       PROVIDE (__gcov_info_start = .);
                       KEEP (*(.gcov_info))
                       PROVIDE (__gcov_info_end = .);
                     }

The program could dump the profiling information registered in this linker set for example like this:

                   #include <gcov.h>
                   #include <stdio.h>
                   #include <stdlib.h>

                   extern const struct gcov_info *const __gcov_info_start[];
                   extern const struct gcov_info *const __gcov_info_end[];

                   static void
                   dump (const void *d, unsigned n, void *arg)
                   {
                     const unsigned char *c = d;

                     for (unsigned i = 0; i < n; ++i)
                       printf ("%02x", c[i]);
                   }

                   static void
                   filename (const char *f, void *arg)
                   {
                     __gcov_filename_to_gcfn (f, dump, arg );
                   }

                   static void *
                   allocate (unsigned length, void *arg)
                   {
                     return malloc (length);
                   }

                   static void
                   dump_gcov_info (void)
                   {
                     const struct gcov_info *const *info = __gcov_info_start;
                     const struct gcov_info *const *end = __gcov_info_end;

                     /* Obfuscate variable to prevent compiler optimizations.  */
                     __asm__ ("" : "+r" (info));

                     while (info != end)
                     {
                       void *arg = NULL;
                       __gcov_info_to_gcda (*info, filename, dump, allocate, arg);
                       putchar ('\n');
                       ++info;
                     }
                   }

                   int
                   main (void)
                   {
                     dump_gcov_info ();
                     return 0;
                   }

The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the "__gcov_filename_to_gcfn" and "__gcov_info_to_gcda" functions and merge the profile information into .gcda files on the host filesystem. iykyk.

• -fprofile-note=path — If path is specified, GCC saves .gcno file into path location. If you combine the option with multiple source files, the .gcno file will be overwritten. 💀 watch out for that overwrite bestie.
• -fprofile-prefix-path=path — This option can be used in combination with profile-generate=profile_dir and profile-use=profile_dir to inform GCC where is the base directory of built source tree. By default profile_dir will contain files with mangled absolute paths of all object files in the built project. This is not desirable when directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback because the profile data will not be found during the optimized build. In such setups -fprofile-prefix-path=path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory. it's giving path therapy ✨.
• -fprofile-prefix-map=old=new — When compiling files residing in directory old, record profiling information (with --coverage) describing them as if the files resided in directory new instead. See also -ffile-prefix-map and -fcanon-prefix-map.
• -fprofile-update=method — Alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code.

⚠️ Warning: When an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted. 😭

Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the default method is single.

If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, libatomic is available, then the profile information is updated through calls to libatomic. If the target platform neither supports the required atomic operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications.

For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state. highkey chaotic but that's what the manpage says.

• -fprofile-filter-files=regex — Instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons).

For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'.

• -fprofile-exclude-files=regex — Instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons).

For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files that are located in the /usr/ folder.

• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — Control level of reproducibility of profile gathered by "-fprofile-generate". This makes it possible to rebuild program with same outcome which is useful, for example, for distribution packages.

With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before "fork" function is invoked. 🎀

Still on the profile-reproducible tea, bestie, because it gets spicy:

One thing to know: it's genuinely super common that execution counts in some chunk of a program depend on stuff like the length of temp file names or memory space randomization (which can mess with hash-table collision rate). That non-reproducible slice of your program can be annotated with the "no_instrument_function" function attribute. And gcov-dump with -l is your bestie here — use it to dump the gathered data and verify it's actually, truly reproducible. ✨

With -fprofile-reproducible=parallel-runs, the collected profile stays reproducible no matter what order the data gets streamed into the gcda files. That setting means you can run multiple instances of your instrumented program in parallel (like with "make -j"). Downside though — it reduces the quality of the gathered data, especially the indirect call profiling. 😭

• -fsanitize=address — Enables AddressSanitizer, a fast little memory error detector. Memory access instructions get instrumented so it can catch out-of-bounds and use-after-free bugs. This option also enables -fsanitize-address-use-after-scope. More deets at https://github.com/google/sanitizers/wiki/AddressSanitizer. You can influence the run-time behavior with the ASAN_OPTIONS environment variable — set it to "help=1" and the available options get shown at startup of the instrumented program. Full list of supported options at https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags. Heads up: this option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. And ngl, the only targets -fsanitize=hwaddress is currently supported on are x86-64 (only with "-mlam=u48" or "-mlam=u57" options) and AArch64 — in both those cases only in ABIs with 64-bit pointers.

For more accurate stack traces, you can reach for options like -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls — this one is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). And since multiple runs of the program may hand you backtraces with different addresses thanks to ASLR (Address Space Layout Randomization), turning ASLR off may be desirable. On Linux, you do it like this:

setarch `uname -m` -R ./prog

• -fsanitize=kernel-address — Enables AddressSanitizer for the Linux kernel. More details at https://github.com/google/kernel-sanitizers. 🐧
• -fsanitize=hwaddress — Enables Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with low memory overhead. Memory access instructions get instrumented to catch out-of-bounds and use-after-free bugs. Enables -fsanitize-address-use-after-scope too. More at https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html. You can influence run-time behavior with the HWASAN_OPTIONS environment variable — set it to "help=1" and the available options show up at startup of the instrumented program. This option cannot be combined with -fsanitize=thread or -fsanitize=address, and it's currently only available on AArch64.
• -fsanitize=kernel-hwaddress — Enables 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 exist to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.

Note: this option has different defaults to -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default, but 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 — Instrument 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 — Instrument 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). The way it works: a function's return address gets saved to a separately allocated shadow call stack in the function prologue, then restored from the shadow call stack in the function epilogue. Instrumentation only happens 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 user space programs, runtime support is not currently provided in libc and libgcc — so if you wanna use this feature in user space, you need to provide your own support for the runtime. And 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". Which generally means any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18 — otherwise functions compiled without -ffixed-x18 might clobber "x18" and therefore corrupt the shadow stack pointer. Yikes.

Also, because there's no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions.

More details at https://clang.llvm.org/docs/ShadowCallStack.html.

• -fsanitize=thread — Enables ThreadSanitizer, a fast data race detector. Memory access instructions get instrumented to detect data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more. You can influence run-time behavior with the TSAN_OPTIONS environment variable; supported options list lives at https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags. This option cannot be combined with -fsanitize=address or -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 — Enables LeakSanitizer, a memory leak detector. This option only matters for linking of executables. The executable gets linked against a library that overrides "malloc" and other allocator functions. More at https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer. You can influence run-time behavior with the LSAN_OPTIONS environment variable. This option cannot be combined with -fsanitize=thread.
• -fsanitize=undefined — Enables UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations get instrumented so undefined behavior can be detected at runtime. More at https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html. You can influence run-time behavior with the UBSAN_OPTIONS environment variable. Current suboptions are:
  • -fsanitize=shift — Enables checking that the result of a shift operation isn't undefined. Note that what exactly counts as undefined differs slightly between C and C++, and also between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent.
  • -fsanitize=shift-exponent — Enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.
  • -fsanitize=shift-base — If the second argument of a shift operation is within range, check that the result of the shift operation isn't undefined. Note that what exactly counts as 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. Iconic, honestly.
  • -fsanitize=unreachable — With this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message call instead. When you actually reach the "__builtin_unreachable" call, the behavior is undefined.
  • -fsanitize=vla-bound — Instructs the compiler to check that the size of a variable length array is positive.
  • -fsanitize=null — Enables pointer checking. Specifically, an application built with this 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 to by a NULL pointer.
  • -fsanitize=return — 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 — okay so this one turns on signed integer overflow checking 💥 We watch that your +, *, and both unary and binary - don't overflow when you're doing signed arithmetic. It also catches INT_MIN / -1 signed division, bestie, which is notorious. But heads up: integer promotion rules gotta be taken into account, so something like this is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — this is giving array bounds instrumented ✨ All the various out-of-bounds accesses get detected. But listen: flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented — the ONE exception being flexible array member-like arrays where -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. iykyk.
• -fsanitize=bounds-strict — the strict version, no cap 💅 Most out-of-bounds accesses get detected, including flexible array member-like arrays. Still, initializers of variables with static storage are not instrumented. We're thorough, not psychic.
• -fsanitize=alignment — checks the alignment of pointers when they 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 🎯
• -fsanitize=object-size — this instruments memory references using the __builtin_dynamic_object_size function. All the various out-of-bounds pointer accesses get detected 🔍
• -fsanitize=float-divide-by-zero — detects floating-point division by zero 🚨 And like, unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can legitimately be how you obtain infinities and NaNs. So it's valid, it's just chaotic.
• -fsanitize=float-cast-overflow — enables floating-point type to integer conversion checking. We check that the result of the conversion doesn't overflow. Same deal: unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also, this option does not work well with FE_INVALID exceptions enabled. 🫠
• -fsanitize=nonnull-attribute — instruments calls and checks whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute 🙅‍♀️
• -fsanitize=returns-nonnull-attribute — instruments return statements in functions marked with the returns_nonnull function attribute, to detect returning of null values from such functions. Not me returning null from a function that swore it wouldn't 💀
• -fsanitize=bool — instruments loads from bool. If a value other than 0/1 is loaded, a run-time error gets issued ⚠️
• -fsanitize=enum — instruments loads from an enum type. If a value outside the range of values for the enum type gets loaded, boom, run-time error 🌈
• -fsanitize=vptr — 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 👑
• -fsanitize=pointer-overflow — instruments pointer arithmetic. If the pointer arithmetic overflows, a run-time error is issued 📍
• -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. fr fr, don't do that 🙈

Okay real talk: 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. Just don't do it bestie, it's not worth the drama 😭

And like, 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.

---

• -fno-sanitize=all — disables all previously enabled sanitizers 🧹 And heads up: -fsanitize=all is not allowed, since some sanitizers cannot be used together. They beef, basically.
• -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 🧪
• -fsanitize-sections=s1,s2,... — sanitizes global variables in selected user-defined sections. Your si may contain wildcards ✨
• -fsanitize-recover[=opts] — okay so -fsanitize-recover= controls error recovery mode for the sanitizers mentioned in the comma-separated list of opts. Enabling this for a sanitizer component makes it attempt to keep running the program as if no error happened. Which means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported 👀 The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and the 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 are 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 also be enabled on the runtime library side, otherwise the failures are still fatal 💀 The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable.

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 🎀
• -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 🌱

Currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. -fsanitize-trap=all can also be specified, which enables it for 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, and -fsanitize-trap=vptr is not allowed 🙅‍♀️

• -fsanitize-undefined-trap-on-error — this option is the deprecated equivalent of -fsanitize-trap=all. Old news, but still here 🫶
• -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block 🎯
• -fsanitize-coverage=trace-cmp — enables dataflow guided fuzzing code instrumentation. Inserts calls 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 🌈
• -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. It's intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Very main character security energy 💅

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 ✨

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.

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer.

NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found.

---

• -fharden-compares — bestie, for every logical test that survives gimple optimizations and is not the condition in a conditional branch (so like, conditions tested for conditional moves, or stored in boolean variables), it emits extra code to compute and verify the reversed condition, and to call __builtin_trap if the results don't match up 😭. Use it with -fharden-conditional-branches to cover all conditionals. no cap.
• -fharden-conditional-branches — for every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call __builtin_trap if the result is unexpected. Use with -fharden-compares to cover all conditionals. the two of them together? iconic. 👑
• -fharden-control-flow-redundancy — emit extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans don't form an execution path that's compatible with the control flow graph. it's giving maximum paranoia and honestly? valid. 💅

Verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls). Tuning options --param hardcfr-max-blocksand--param hardcfr-max-inline-blocks` are available.

Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls would also have verification issued before the call — but these possibilities are merely theoretical, since these conditions can only be met when using custom compiler plugins. lowkey wild.

• -fhardcfr-skip-leaf — disable -fharden-control-flow-redundancy in leaf functions. just leaves them alone, as they deserve. 🍓
• -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. This option is enabled by default; use -fno-hardcfr-check-exceptions to disable it.
• -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before any function call immediately followed by a return of its result, if any, so as to not prevent tail-call optimization — whether or not it's ultimately optimized to a tail call.

This option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but it can be enabled (or disabled, using its negated form) explicitly, regardless of the optimizations.

• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before noreturn calls. which ones tho?? pick your fighter:
  • always — all of them, every single one 🫶
  • no-xthrow — only those that aren't expected to return control to the caller through an exception (this is the default)
  • nothrow — only those that may not return control to the caller through an exception either
  • never — none of them, we're off duty

Checking before a noreturn function that may return control to the caller through an exception may cause checking to be performed more than once, if the exception is caught in the caller, whether by a handler or a cleanup. When -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a noreturn call with the implicitly-added cleanup handler, since it would be redundant with the check performed before the call — but other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit. The checkpoint may even be another noreturn call, so checking may end up performed multiple times. 💀

Various optimizers may cause calls to be marked as noreturn and/or nothrow, even in the absence of the corresponding attributes, which may affect the placement of checks before calls, as well as the addition of implicit cleanup handlers for them. This unpredictability, and the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, have made no-xthrow the default setting for this option: it excludes from the noreturn treatment only internal functions used to (re)raise exceptions, that are not affected by these optimizations.

• -fhardened — enable a set of flags for C and C++ that improve the security of the generated code without affecting its ABI. glow-up but make it safe ✨. The precise flags enabled may change between major releases of GCC, but are currently:

-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. bestie this is for the real world, not the sandbox 🎀

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. the way it respects your choices? obsessed.

• -fstack-protector — emit extra code to check for buffer overflows, like stack smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits 😭. Only variables that are actually allocated on the stack are considered — optimized away variables or variables allocated in registers don't count. periodt.
• -fstack-protector-all — like -fstack-protector except that all functions are protected. all of them. no exceptions. 💅
• -fstack-protector-strong — like -fstack-protector but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count.

NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found.

• -fstack-protector-explicit — like -fstack-protector but only protects those functions which have the stack_protect attribute. only the ones who asked for it 🫶
• -fstack-check — generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there's only one stack.

Note that this switch does not actually cause checking to be done; the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code. ⚠️
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. it's giving slow 😭

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. fr fr, different tool different job.

• -fstack-clash-protection — generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system. it's giving no shortcuts 🔥

Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.

NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++, ObjC, ObjC++, unless -fno-stack-clash-protection is found.

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• -fstack-limit-register=reg — -fstack-limit-symbol=sym — -fno-stack-limit — okay so this trio makes the compiler spit out code that keeps the stack from growing past a certain value 💅 and that value is either whatever's chillin' in a register (reg) or the address of a symbol (sym). Ask for a bigger stack than that? A signal gets raised at run time, periodt. And good news bestie: for most targets the signal fires before the stack actually overruns the boundary, so you can catch it without doing any special little precautions 🎀

Like, say the stack starts at absolute address 0x80000000 and grows downwards — you'd use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Ngl, note that this may only work with the GNU linker 🙈

And you can locally override stack limit checking by using the "no_stack_limit" function attribute. iconic.

• -fsplit-stack — this one generates code that automatically splits the stack before it overflows 😤 so the resulting program has a discontiguous stack which can only overflow if the program literally cannot allocate any more memory. It's giving most useful when running threaded programs, because you no longer have to sit there calculating a good stack size for each thread. Currently this is only implemented for the x86 targets running GNU/Linux, fyi 🦋

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run 😭 If compiling all the code — library code included — with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later ✨

• -fstrub=disable — disables stack scrubbing entirely, ignoring any "strub" attributes. See
• -fstrub=strict — functions default to "strub" mode "disabled", and it strictly enforces the restriction that only functions associated with "strub"-"callable" modes ("at-calls", "callable" and "always_inline" "internal") are "callable" by functions with "strub"-enabled modes ("at-calls" and "internal"). no cap.
• -fstrub=relaxed — restores the default stack scrub ("strub") setting, namely, "strub" is only enabled as required by "strub" attributes associated with function and data types. "Relaxed" means that strub contexts are only prevented from calling functions explicitly associated with "strub" mode "disabled". Heads up: this option is only useful to override other -fstrub=* options that precede it in the command line 👑
• -fstrub=at-calls — enables "at-calls" "strub" mode where viable. The primary use of this option is for testing 🧪 It exercises the "strub" machinery in scenarios strictly local to a translation unit. This "strub" mode modifies function interfaces, so any function that is visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. See the "strub" attribute documentation for details on viability and eligibility requirements 📖
• -fstrub=internal — enables "internal" "strub" mode where viable. The primary use of this option is for testing. This one is intended to exercise thoroughly the parts of the "strub" machinery that implement the less efficient, but interface-preserving "strub" mode. Functions that would not be affected by this option are quite uncommon 👀
• -fstrub=all — enables some "strub" mode where viable. When both strub modes are viable, "at-calls" is preferred 🥇 -fdump-ipa-strubm adds function attributes that tell which mode was selected for each function. The primary use of this option is for testing, to exercise thoroughly the "strub" machinery. slay.
• -fvtable-verify=[std|preinit|none] — bestie 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 single virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten 🔥 If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. dramatic but necessary.

This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches "main". Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized ⏰

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.

• -fvtv-debug — when used in conjunction with -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 causes the compiler to log information about which vtable pointers it finds for each class. That info gets written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined, or the current working directory otherwise 📝

Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one first, okay?

• -fvtv-counts — this is a debugging flag 🐛 When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, it causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this 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, the following profiling functions get called with the address of the current function and its call site. (On some platforms, "__builtin_return_address" does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise 🙈)

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table 🔍

This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size 😬 If you use "extern inline" in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies 💅)

A function may be given the attribute "no_instrument_function", in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (maybe signal handlers, if the profiling routines generate output or allocate memory 👀).

• -finstrument-functions-once — similar to -finstrument-functions, but the profiling functions are called only once per instrumented function, i.e. the first profiling function is called after the first entry into the instrumented function and the second profiling function is called before the exit corresponding to this first entry 🎯

The definition of "once" for the purpose of this option is a little vague because the implementation is not protected against data races 😭 As a result, the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread — but the calls are always paired, that is to say, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function. fr fr.

• -finstrument-functions-exclude-file-list=file,file,... — sets the list of functions that are excluded from instrumentation (peep the description of -finstrument-functions above 💖). If the file that contains a function definition matches with one of file, then that function is not instrumented. And the match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match 🔍

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. ate.

If, for some reason, you want to include the letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option) 🌸

• -finstrument-functions-exclude-function-list=sym,sym,... — okay so this is literally the sibling of -finstrument-functions-exclude-file-list, except instead of files you're listing function names that get yeeted out of instrumentation 🫶. The name you match on is the user-visible one, like vector<int> blah(const vector<int> &), not the cursed internal mangled name (you know, the _Z4blahRSt6vectorIiSaIiEE situation 💀). And the matching? It's straight-up substring-coded: if your sym parameter shows up anywhere inside the function name, that's a match, no cap. One more thing bestie: for C99 and C++ extended identifiers, you have to hand over the function name in UTF-8 — do not use universal character names. 🙈
• -fpatchable-function-entry=N[,M] — this one generates N NOPs smack at the beginning of every function, and the function entry point sits before the Mth NOP. Skip the M and it defaults to 0, so the function entry points right at the address of the very first NOP ✨. Those NOP instructions are reserving extra space you can patch any instrumentation you want into at run time — but only provided the code segment is writable, so no sneaky stuff otherwise. How much space you get is controllable indirectly, through the number of NOPs; the NOP instruction used is whatever the internal GCC back-end interface gen_nop emits. This whole behavior is target-specific and might also depend on the architecture variant and/or other compilation options — it's giving "depends on the vibes of your target" 😭.

For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary. 🎀

Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over the command-line option -fpatchable-function-entry=N,M. That's your little override button: use it to increase the area size, or to remove it completely on a single function. And if N=0, no pad location is recorded. periodt.

The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address. 👑

The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14. 🌸

---

🧚 Options Controlling the Preprocessor 🧚

These options are the ones running the whole C preprocessor show — it gets run on every C source file before the actual compilation even starts. main character energy fr fr 💅

If you use the -E option, literally nothing happens except preprocessing. And some of these options only make sense together with -E, because they make the preprocessor output unsuitable for actual compilation — like, that output is not going to compile, bestie, that's the whole bit. 😭

On top of everything listed here, there are a number of options controlling search paths for include files, documented over in Directory Options. And options controlling preprocessor diagnostics live in Warning Options. iykyk 🦋

---

• -D name — Predefine name as a macro, with definition 1. Simple. Iconic. ✨
• -D name=definition — The contents of definition get tokenized and processed as if they showed up during translation phase three in a #define directive. In particular — heads up — the definition is truncated by embedded newline characters. 💀

If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax. (Yes, the shell is fighting you, it always is. 🫠)

Want to define a function-like macro on the command line? Write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so quote the option. With sh and csh, -D'name(args...)=definition' works. 🎀

Also important: -D and -U options are processed in the order they're given on the command line. And all -imacros file and -include file options are processed after all -D and -U options. Order girlies, order matters. 👑

• -U name — Cancel any previous definition of name, whether it was built in or you handed it over with a -D option. Undefined. Gone. Not me erasing it. 🧹
• -include file — Process file as if #include "file" appeared as the first line of the primary source file. But here's the plot 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. 🦋

Give multiple -include options and the files get included in the order they appear on the command line. Periodt.

• -imacros file — Exactly like -include, except any output produced by scanning file is thrown away. But the macros it defines stay defined, so slay — this lets you scoop up all the macros from a header without also processing its declarations. Efficient queen. 💅

All files specified by -imacros are processed before all files specified by -include.

• -undef — Do not predefine any system-specific or GCC-specific macros. The standard predefined macros stay defined though, so not a total blank slate. 🌸
• -pthread — Define the additional macros required for using the POSIX threads library. Use this option consistently for both compilation and linking, bestie — do not be flaky about it. 😤 This one 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 spits out 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 you specify it 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 a lot of included files, the rule gets split into several lines using \-newline. And the rule has no commands. 🤷

This option does not suppress the preprocessor's debug output, such as -dM. So 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 it does 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 shows up in -MM dependency output. Sneaky.

• -MF file — When used with -M or -MM, this 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. And if file is -, then the dependencies are written to stdout. ✨

• -MG — In conjunction with an option such as -M that requests dependency generation, -MG assumes that 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. Also, -MG suppresses preprocessed output, since a missing header file renders that useless. 🫠

This feature is used in automatic updating of makefiles. iykyk 💅

• -Mno-modules — Disable dependency generation for compiled module interfaces. That's it, that's the whole thing. 🙈
• -MP — This option tells CPP to add a phony target for each dependency other than the main file, making each one depend on nothing. Those dummy rules are the workaround for the errors make throws at you when you remove header files without updating the Makefile to match. 💀

Typical output, bestie:

test.o: test.c test.h

test.h:

• -MT target — Change the target of the rule emitted by dependency generation. By default, CPP takes the name of the main input file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. That result is the target.

An -MT option sets the target to be exactly the string you specify. Want multiple targets? Either hand them over 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 figures out 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. Two birds, one compile. 🦋

• -MMD — Like -MD except it mentions only user header files, not system header files. System girlies stay private. 💅
• -fpreprocessed — Tell 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. But 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 gets limited to just handling directives like #define, #ifdef, and #error, bestie. Other preprocessor stuff — macro expansion, trigraph conversion — literally does not happen. And plot twist: -dD gets implicitly enabled. 💅

With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros like __LINE__ that are contextually dependent are handled normally, so it's all good. This enables compilation of files previously preprocessed with -E -fdirectives-only.

With both -E and -fpreprocessed? The rules for -fpreprocessed take precedence, periodt. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only.

---

• -fdollars-in-identifiers — accept $ in identifiers. yes literally, dollar signs, in your identifiers 👑
• -fextended-identifiers — accept universal character names and extended characters in identifiers. This one's 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. leave those paths long, fr fr.
• -fmax-include-depth=depth — sets the maximum depth of the nested #include. The default is 200. 🌀 (don't go nesting past that, bestie)
• -ftabstop=width — sets 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] — tracks locations of tokens across macro expansions. This lets the compiler emit diagnostics about the current macro expansion stack when a compilation error happens inside a macro expansion. Heads up though: using this makes the preprocessor and the compiler consume more memory. 😭

The level parameter chooses the precision of token location tracking, so you can dial down the memory consumption if you need to:

• Value 0 of level de-activates this option.
• Value 1 tracks token locations in a degraded mode for the sake of minimal memory overhead. In this mode, all tokens resulting from the expansion of an argument of a function-like macro have the same location.
• Value 2 tracks token locations completely. This value is the most memory hungry. 🍓

When this option is given no argument, the default parameter value is 2.

Note that -ftrack-macro-expansion=2 is activated by default. no cap.

• -fmacro-prefix-map=old=new — when preprocessing files residing in directory old, expand the __FILE__ and __BASE_FILE__ macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using . for new, which can result in more reproducible builds that are location independent. 🗺️ This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map and -fcanon-prefix-map.
• -fexec-charset=charset — sets the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's iconv library routine. 💜
• -fwide-exec-charset=charset — sets the wide execution character set, used for wide string and character constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the width of wchar_t and the big-endian or little-endian byte order being used for code generation. As with -fexec-charset, charset can be any encoding supported by the system's iconv library routine — however, you will have problems with encodings that do not fit exactly in wchar_t. 💀
• -finput-charset=charset — sets the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify it, or GCC cannot 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. charset can be any encoding supported by the system's iconv library routine. 🎀
• -fpch-deps — when using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header is listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used. 🙈
• -fpch-preprocess — this option allows use of a precompiled header together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename" in the output to mark the place where the precompiled header was found, 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 is switched on by -save-temps.

You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.

• -fworking-directory — enables generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever. 🧚
• -A predicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. 💫
• -A -predicate=answer — cancel an assertion with the predicate predicate and answer answer. (note the -predicate, bestie — that leading dash is doing the work 🫶)
• -C — do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive. 🙈

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. 😭

• -CC — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

• -P — inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers. ✨
• -traditional, -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details.

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly. 👑

• -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.

• -remap — enable special code to work around file systems which only permit very short file names, such as MS-DOS. 💾
• -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 are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...!. 🌸
• -dletters — says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. 💀
  • -dM — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. 🌟

• -dD — like -dM except that it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
• -dN — like -dD, but emit only the macro names, not their expansions. 🎀
• -dI — output #include directives in addition to the result of preprocessing.

• -dU — it's giving -dD but picky bestie 💅 Only macros that actually get expanded, or whose definedness gets tested in preprocessor directives, come out. The output is delayed until the macro is used or tested, and #undef directives get printed too for macros that were tested but undefined at the time.
• -fdebug-cpp — only useful if you're literally debugging GCC itself, no cap. When used from CPP or with -E, it dumps debugging info about location maps. Every single token in the output gets preceded by the dump of the map its location belongs to. When used from GCC without -E, this option has no effect at all 🙈
• -Wp,option — you can use -Wp,option to bypass the compiler driver and yeet option directly through to the preprocessor. If option contains commas, it gets split into multiple options at the commas. But (and this is the important part) many options are modified, translated, or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses that whole phase 😭

The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. Like, seriously, it's giving "f around and find out" energy.

• -Xpreprocessor option — passes option as an option to the preprocessor. Use this to supply system-specific preprocessor options that GCC doesn't recognize 🫶

If you want to pass an option that takes an argument, you have to use -Xpreprocessor twice — once for the option and once for the argument. Two trips, one result, iconic.

• -no-integrated-cpp — performs preprocessing as a separate pass before compilation ✨ By default, GCC does preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only, and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation 🌸
• -flarge-source-files — adjusts GCC to expect large source files, at the expense of slower compilation and higher memory usage. Tradesies, fr.

Specifically: GCC normally tracks both column numbers and line numbers within source files, and it normally prints both of these numbers in diagnostics. However, once it has processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers 📉 This means diagnostics for later lines don't include column numbers. It also means options like -Wmisleading-indentation cease to work at that point, although the compiler prints a note if this happens. Passing -flarge-source-files significantly increases the number of source lines that GCC can process before it stops tracking columns 🔥

---

🧰 Passing Options to the Assembler 🧰

You can pass options to the assembler. It's a whole thing, bestie.

• -Wa,option — pass option as an option to the assembler. If option contains commas, it gets split into multiple options at the commas.
• -Xassembler option — pass option as an option to the assembler 🫶 Use this to supply system-specific assembler options that GCC does not recognize.

If you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option and once for the argument. Same dance as before 💃

---

🔗 Options for Linking 🔗

These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler isn't doing a link step. Like, literally nothing happens. Periodt.

• object-file-name — a file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents, not the vibes.) If linking is done, these object files are used as input to the linker 🎀
• -c
• -S
• -E — if any of these options is used, then the linker is not run, and object file names should not be used as arguments. Do not do it. Just don't 💀
• -flinker-output=type — this option controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin. For debugging the compiler, and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually 🧚

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 you to build shared libraries without position-independent code on architectures where this is 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 that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, 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. At link time the result of incremental linking also loads faster than a static library, assuming that the majority of objects in the library are 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 an incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole program optimization, it is recommended to link such objects into a static library instead. Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects 🌈

• -fuse-ld=bfd — use the bfd linker instead of the default linker.
• -fuse-ld=gold — use the gold linker instead of the default linker.
• -fuse-ld=lld — use the LLVM lld linker instead of the default linker.
• -fuse-ld=mold — use the Modern Linker (mold) instead of the default linker ✨
• -llibrary
• -l library — search the library named library when linking. (The second alternative, with the library as a separate argument, is only for POSIX compliance and is not recommended 🚫)

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. Thus, 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 truly matters, bestie.

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program 🎀
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.

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 😭 (yes, really — that's on you now)

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. Still links with the startup files, libgcc, or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++, unless options preventing their inclusion are used as well. This typically removes -lc from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed — for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there is indeed no C library available 🫶
• -nostdlib — do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages 🌸

okay so real talk, bestie: in most cases you literally need libgcc.a even when you're trying to ghost all the other standard libraries. in other words, when you pass -nostdlib or -nodefaultlibs, you should usually also pass -lgcc. why? because that's what makes sure you have no unresolved references to internal GCC library subroutines. (a lil example of one of those internal subroutines? "__main" — it's the one that makes sure your C++ constructors actually get called. iconic of it. 👑)

• -nostdlib++ — do not implicitly link with the standard C++ libraries. that's it, that's the whole thing. 💅
• -e entry, --entry=entry — say that the program entry point is entry. plot twist: that entry argument gets interpreted by the linker, and the GNU linker will accept either a symbol name or an address. dealer's choice ✨
• -pie — produce a dynamically linked position independent executable, on targets that support it. 🫶 but for predictable results, you've also gotta specify the exact same set of options you used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. no cap.
• -no-pie — don't produce a dynamically linked position independent executable. simple as. 🦋
• -static-pie — produce a static position independent executable, on targets that support it. a static PIE is basically a static executable's cousin, except it can be loaded at any address without a dynamic linker. and yeah, same rule as before: for predictable results you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option.
• -pthread — link with the POSIX threads library. this one'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 it should be used consistently for both compilation and linking. consistency is the vibe check. 🌸
• -r — produce a relocatable object as output. this is also known as partial linking. cute, right?
• -rdynamic — passes the flag -export-dynamic to the ELF linker, on targets that support it. this instructs the linker to add all symbols to the dynamic symbol table, not only the used ones. you need this for some uses of dlopen, or if you wanna allow obtaining backtraces from within a program. 👑
• -s — remove all symbol table and relocation information from the executable. ✂️ bye bestie
• -static — on systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. on other systems? it literally has no effect. 🙈
• -shared — produce a shared object which can then be linked with other objects to form an executable. not all systems support this option! and 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 the static version, respectively. if no shared version of libgcc was built when the compiler was configured, these options have no effect. 💀

there are several situations where an application should use the shared libgcc instead of the static version. the most common one is when the application wishes to throw and catch exceptions across different shared libraries. in that case, every one 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. periodt.

if, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. here's the tea: 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. slay.

however — and this is the part that matters — 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, so 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's normally fine, ngl. however, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. the -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically. 💜
• -symbolic — bind references to global symbols when building a shared object. it warns about any unresolved references (unless that's overridden by the link editor option -Xlinker -z -Xlinker defs). only a few systems support this option, so yeah, rare queen behavior. 👑
• -T script — use script as the linker script. this option is supported by most systems using the GNU linker. on some targets, like bare-board targets without an operating system, the -T option may be required when linking, to avoid references to undefined symbols.
• -Xlinker option — pass option as an option to the linker. you can use this to supply system-specific linker options that GCC does not recognize. 🌈

if you want to pass an option that takes a separate argument, you must use -Xlinker twice — once for the option and once for the argument. for example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. it does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. 😭 learn from my mistakes besties

when using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. for example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. other linkers may not support this syntax for command-line options.

• -Wl,option — pass option as an option to the linker. if option contains commas, it is split into multiple options at the commas. you can use this syntax to pass an argument to the option. for example, -Wl,-Map,output.map passes -Map output.map to the linker. and when using the GNU linker, you can also get the same effect with -Wl,-Map=output.map. 🫶

NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use -Wl,-z,norelro.

• -u symbol — pretend the symbol symbol is undefined, to force linking of library modules to define it. you can use -u multiple times with different symbols to force loading of additional library modules. 🧚
• -z keyword — -z is passed directly on to the linker along with the keyword keyword. see the section in the documentation of your linker for permitted values and their meanings.

---

📂 Options for Directory Search 🎀

these options specify directories to search for header files, for libraries, and for parts of the compiler:

• -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.

okay so you can use -I to override a system header file and swap in your own version, because these directories get searched before the standard system header file directories. but bestie listen 👑 do not use this option to add directories that contain vendor-supplied system header files — use -isystem for that. periodt.

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. ✨

And here's the sneaky bit: if a standard system include directory, or a directory specified with -isystem, is also specified with -I, then the -I option is ignored. 😭 Like, the directory is still searched, but as a system directory at its normal position in the system include chain. Why? 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. that's the whole vibe 💅

• -I- — Split the include path. 🚨 This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers requested with "#include "file""; they are not searched for "#include <file>". If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for "#include "file"". There is no way to override this effect of -I-. 💀

• -iprefix prefix — Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. 🎀
• -iwithprefix dir, -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would. 🦋
• -isysroot dir — This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information.
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers. 🌸
• -nostdinc — Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched.
• -nostdinc++ — Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.)
• -iplugindir=dir — Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. 🙈
• -Ldir — Add directory dir to the list of directories to be searched for -l. 💜
• -Bprefix — This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. 👑

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 any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.

The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means. 😭

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. 🧚

• -no-canonical-prefixes — Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix.
• --sysroot=dir — Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. 🌈

If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not.

• --no-sysroot-suffix — For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix.

---

🎀 Options for Code Generation Conventions 🎀

These machine-independent options control the interface conventions used in code generation. 🔥

Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it. iykyk 💅

• -fstack-reuse=reuse-level — This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. 🚨

For example,

int *p;
{
  int local1;

  p = &local1;
  local1 = 10;
  ....
}
{
   int local2;
   local2 = 20;
   ...
}

if (*p == 10)  // out of scope use of local1
  {

  }

Another example:

struct A
{
    A(int k) : i(k), j(k) { }
    int i;
    int j;
};

A *ap;

void foo(const A& ar)
{
   ap = &ar;
}

void bar()
{
   foo(A(10)); // temp object's lifetime ends when foo returns

   {
     A a(20);
     ....
   }
   ap->i+= 10;  // ap references out of scope temp whose space
                // is reused with a. What is the value of ap->i?
}

The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. 💀

• -ftrapv — This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. 🔥
• -fwrapv — This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. ⭐
• -fwrapv-pointer — bestie this one tells the compiler to just assume that when pointer arithmetic overflows on addition or subtraction, it wraps around in twos-complement representation 🔄✨. It turns off some optimizations that lowkey assume pointer overflow is invalid. So like, if your code does the sketchy thing, this says "yeah bestie I got you, wrap it". 🎀
• -fstrict-overflow — this option implies -fno-wrapv -fno-wrapv-pointer, and when it gets negated it implies -fwrapv -fwrapv-pointer. That's the whole vibe check. 💅
• -fexceptions — turns on exception handling, periodt. It generates the extra code needed to propagate exceptions ✨. For some targets, this means GCC starts generating frame unwind information for all the functions, which can produce significant data size overhead — although ngl 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 don't normally require it 🙂. BUT — you might need to enable it when you're compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable it if you're compiling older C++ programs that don't use exception handling. Options, bestie, you have them. 💜
• -fnon-call-exceptions — generates code that allows trapping instructions to throw exceptions 👀. Heads up though: this requires platform-specific runtime support that honestly does not exist everywhere. Also it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as SIGALRM 🚫. This enables -fexceptions.
• -fdelete-dead-exceptions — considers that instructions which may throw exceptions but don't otherwise contribute to the execution of the program can just be optimized away ✂️. This does not affect calls to functions except those with the pure or const attributes. It's enabled by default for the Ada and C++ compilers, as permitted by the language specifications. And just so you know, optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels. Layers, okay? 🧚
• -funwind-tables — similar to -fexceptions, except it just generates any needed static data and does not affect the generated code in any other way. You normally do not need to enable this; instead, a language processor that needs this handling enables it on your behalf 🫶.
• -fasynchronous-unwind-tables — generates an unwind table in DWARF format, if supported by the target machine ✨. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as a debugger or a garbage collector). Accurate! Iconic! 🎀
• -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 are unique even in the presence of RTLD_LOCAL 🌈. This is necessary to avoid problems with a library used by two different RTLD_LOCAL plugins depending on a definition in one of them, and therefore disagreeing with the other one about the binding of the symbol. Drama! But it 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.
• -fpcc-struct-return — returns short struct and union values in memory like the longer ones, rather than 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 😌 queen behavior.

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 📏✨. 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 — grandpa code gets to live another day, we love that for it 👑

• -fno-ident — ignore the #ident directive. That's it, that's the whole thing 🎀.
• -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. Like genuinely, don't touch it 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). Niche! But slay.

-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

The added comments include:

• information on the compiler version and command-line options,
• the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
• hints on which high-level expressions correspond to the various assembly instruction operands.

For example, given this C source file:

int test (int n)
{
  int i;
  int total = 0;

  for (i = 0; i < n; i++)
    total += i * i;

  return total;
}

compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -

gcc -S test.c -fverbose-asm -Os -o -

gives output similar to this:

        .file   "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
  [...snip...]
# options passed:
  [...snip...]

        .text
        .globl  test
        .type   test, @function
test:
.LFB0:
        .cfi_startproc
# test.c:4:   int total = 0;
        xorl    %eax, %eax      # <retval>
# test.c:6:   for (i = 0; i < n; i++)
        xorl    %edx, %edx      # i
.L2:
# test.c:6:   for (i = 0; i < n; i++)
        cmpl    %edi, %edx      # n, i
        jge     .L5     #,
# test.c:7:     total += i * i;
        movl    %edx, %ecx      # i, tmp92
        imull   %edx, %ecx      # i, tmp92
# test.c:6:   for (i = 0; i < n; i++)
        incl    %edx    # i
# test.c:7:     total += i * i;
        addl    %ecx, %eax      # tmp92, <retval>
        jmp     .L2     #
.L5:
# test.c:10: }
        ret
        .cfi_endproc
.LFE0:
        .size   test, .-test
        .ident  "GCC: (GNU) 7.0.0 20160809 (experimental)"
        .section        .note.GNU-stack,"",@progbits

The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change 🦋. So don't go parsing them, bestie, they're just vibes.

• -frecord-gcc-switches — okay so this is the "receipts" switch 💅 it makes the exact command line you used to invoke the compiler get recorded into the object file you're creating. Cute, right? But listen — it's only implemented on some targets, and the exact format of the recording depends on the target and the binary file format. Usually it shows up as a section full of ASCII text. It's lowkey cousins with -fverbose-asm, except that one only records info in the assembler output file as comments, so it never even reaches the object file. Also peep -grecord-gcc-switches if you want another way to stash compiler options into the object file 👑
• -fpic — generate position-independent code (PIC) suitable for use in a shared library, if it's supported for the target machine. Bestie, this kind of code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves those GOT entries when the program starts — and heads up, the dynamic loader is not part of GCC, it's part of the operating system 🫶

If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker saying -fpic does not work 😭 in that case, recompile with -fPIC instead. The maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit, main character energy fr fr.

Position-independent code requires special support, so it only works on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. And code generated for the IBM RS/6000 is always position-independent, iconic.

When this flag is set, the macros __pic__ and __PIC__ are defined to 1 ✨

• -fPIC — if it's 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, so again: only works on certain machines.

When this flag is set, the macros __pic__ and __PIC__ are defined to 2 💖

• -fpie
• -fPIE — these are basically the little siblings of -fpic and -fPIC, but the generated position-independent code can only be linked into executables. Usually you use these to compile code that'll be linked using the -pie GCC option.

-fpie and -fPIE both define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE.

• -fno-plt — do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations 🌈 On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time.

Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions.

In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.

• -fno-jump-tables — don't use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT and this option isn't needed. Periodt.
• -fno-bit-tests — don't use bit tests for switch statements even where it would be more efficient than other code generation strategies. That's the whole vibe 🎀
• -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 don't 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 it for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

This flag does not have a negative form, because it specifies a three-way choice.

• -fcall-saved-reg — treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it 🫶

💀 It is an error to use this flag with the frame pointer or stack pointer. And 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] — with no value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).

⚠️ Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. On top of that, 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, iykyk.

⚠️ 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, ngl.

• -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 don't need them — like, for example, HP-PA or IA-64 — do nothing 💅

A trampoline is a small piece of code that's created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly.

-fno-trampolines is enabled by default on a language-by-language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.

⚠️ Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care.

For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.

• -ftrampoline-impl=[stack|heap] — by default, trampolines are generated on stack. However, certain platforms (such as the Apple M1) do not permit an executable stack 🫠 Compiling with -ftrampoline-impl=heap generates calls to __gcc_nested_func_ptr_created and __gcc_nested_func_ptr_deleted in order to allocate and deallocate trampoline space on the executable heap. These functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux.

🚨 PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if you setjmp, instantiate nested functions, and then longjmp back to a state prior to having allocated those nested functions. Like, that's a whole trauma, bestie.

• -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.

---

Okay bestie, so like, 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/>). But 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. It works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads. Making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility. 💅

Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. 🫶

extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with no modifications. However, this means that calls to extern functions with no explicit visibility use the PLT, so it is more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.

Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs.

An overview of these techniques, their benefits and how to use them is at <https://gcc.gnu.org/wiki/Visibility>.

• -fstrict-volatile-bitfields — This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.

If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated.

In some cases, such as when the packed attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time.

Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. It is therefore recommended to define all bits of the field's type as bit-field members.

The default value of this option is determined by the application binary interface for the target processor.

• -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 of this option is enabled, thus the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.

🌈 GCC Developer Options 🌈

This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks.

Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error.

If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is 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 are executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file.

• -fcallgraph-info, -fcallgraph-info=MARKERS — Makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When the su marker is specified, the callgraph is decorated with stack usage information; it is 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.

• -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.

Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings:

• -fdump-rtl-alignments — Dump after branch alignments have been computed.
• -fdump-rtl-asmcons — Dump after fixing rtl statements that have unsatisfied in/out constraints.
• -fdump-rtl-auto_inc_dec — Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
• -fdump-rtl-barriers — Dump after cleaning up the barrier instructions.
• -fdump-rtl-bbpart — Dump after partitioning hot and cold basic blocks.
• -fdump-rtl-bbro — Dump after block reordering.
• -fdump-rtl-btl1, -fdump-rtl-btl2 — -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
• -fdump-rtl-bypass — Dump after jump bypassing and control flow optimizations.
• -fdump-rtl-combine — Dump after the RTL instruction combination pass.
• -fdump-rtl-compgotos — Dump after duplicating the computed gotos.
• -fdump-rtl-ce1, -fdump-rtl-ce2, -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
• -fdump-rtl-cprop_hardreg — Dump after hard register copy propagation.
• -fdump-rtl-csa — Dump after combining stack adjustments.
• -fdump-rtl-cse1, -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
• -fdump-rtl-dce — Dump after the standalone dead code elimination passes.
• -fdump-rtl-dbr — Dump after delayed branch scheduling.
• -fdump-rtl-dce1, -fdump-rtl-dce2 — -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
• -fdump-rtl-eh — Dump after finalization of EH handling code.
• -fdump-rtl-eh_ranges — Dump after conversion of EH handling range regions.
• -fdump-rtl-expand — Dump after RTL generation.
• -fdump-rtl-fwprop1, -fdump-rtl-fwprop2 — -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
• -fdump-rtl-init-regs — Dump after the initialization of the registers.
• -fdump-rtl-initvals — Dump after the computation of the initial value sets.
• -fdump-rtl-into_cfglayout — Dump after converting to cfglayout mode.
• -fdump-rtl-ira — Dump after iterated register allocation.
• -fdump-rtl-jump — Dump after the second jump optimization.
• -fdump-rtl-loop2 — -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
• -fdump-rtl-mach — Dump after performing the machine dependent reorganization pass, if that pass exists.
• -fdump-rtl-mode_sw — dumps everything right after it yanks out the redundant mode switches 💅
• -fdump-rtl-rnreg — dumps after register renumbering, aka the glow-up where the register numbers get reshuffled ✨
• -fdump-rtl-outof_cfglayout — dumps after converting out of cfglayout mode, bestie, it's leaving the cfg layout era 🦋
• -fdump-rtl-peephole2 — dumps after the peephole pass (yes that's the actual name, and yes it's iconic for no reason 😭)
• -fdump-rtl-postreload — dumps after the post-reload optimizations do their lil cleanup 🎀
• -fdump-rtl-pro_and_epilogue — dumps after the function prologues and epilogues get generated 👑
• -fdump-rtl-sched1, -fdump-rtl-sched2 — these two enable dumping after the basic block scheduling passes, periodt 🔥
• -fdump-rtl-ree — dumps after sign/zero extension elimination 🌸
• -fdump-rtl-seqabstr — dumps after common sequence discovery ✨
• -fdump-rtl-shorten — dumps after shortening branches ✂️
• -fdump-rtl-sibling — dumps 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. five. rounds. 💀
• -fdump-rtl-sms — dumps after modulo scheduling. heads up bestie, this pass only runs on some architectures 🧚
• -fdump-rtl-stack — dumps after converting from GCC's "flat register file" registers to the x87's stack-like registers. and this pass only runs on x86 variants 🎀
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — these two enable dumping after the two subreg expansion passes 🌈
• -fdump-rtl-unshare — dumps after all rtl has been unshared 💜
• -fdump-rtl-vartrack — dumps after variable tracking 🔍
• -fdump-rtl-vregs — dumps after converting virtual registers to hard registers ✨
• -fdump-rtl-web — dumps 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 — ngl these dumps are defined but they always produce empty files. it's giving placeholder energy 💀
• -da, -fdump-rtl-all — produce all the dumps listed above. all of them. every single one 👑
• -dA — annotate the assembler output with miscellaneous debugging information 💅
• -dD — dump all macro definitions, at the end of preprocessing, in addition to normal output ✨
• -dH — produce a core dump whenever an error occurs 🌸
• -dp — annotate the assembler output with a comment indicating which pattern and alternative is used. the length and cost of each instruction also get printed 🎀
• -dP — dump the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation ✨
• -dx — just generate RTL for a function instead of compiling it. usually used with -fdump-rtl-expand 🦋
• -fdump-debug — dump debugging information generated during the debug generation phase 🔍
• -fdump-earlydebug — dump debugging information generated during the early debug generation phase 🌸
• -fdump-noaddr — when doing debugging dumps, suppress address output. this makes it way more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. lowkey genius 🧚
• -freport-bug — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs 🧊
• -fdump-unnumbered — when doing debugging dumps, suppress instruction numbers and address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different options, especially with and without -g 💅
• -fdump-unnumbered-links — when doing debugging dumps (see the -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence 🔗
• -fdump-ipa-switch, -fdump-ipa-switch-options — control the dumping at various stages of inter-procedural analysis language tree to a file. the file name is made by appending a switch-specific suffix to the source file name, and the file gets created in the same directory as the output file 🌈

the following dumps are possible:

• all — enables all inter-procedural analysis dumps 👑
• cgraph — dumps info about call-graph optimization, unused function removal, and inlining decisions 🕸️
• inline — dump after function inlining ✨
• strubm — dump after selecting "strub" modes, and recording the selections as function attributes 🎀
• strub — dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking 💧

additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized 💖

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined 😭

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis 🫶

• -fdump-ipa-clones — create a dump file containing info about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use is so that tools that create live-patches can determine the set of functions that need to be live-patched to completely replace a particular function (see -flive-patching). the file name is made 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 🧬

when it's a call graph clone creation, the individual fields are:

1. the 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 it's 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 it's 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.

when it's a call graph clone removal, the individual fields are:

1. the 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 it's 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 — dump language-specific information. the file name is made by appending .lang to the source file name 🗣️
• -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — control the dumping of language-specific information. the options and filename portions behave as described in the -fdump-tree option. the following switch values are accepted:
  • 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 — print on stderr the list of optimization passes that are turned on and off by the current command-line options 🎀
• -fdump-statistics-option — enable and control dumping of pass statistics in a separate file. the file name is made 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 — control the dumping at various stages of processing the intermediate language tree to a file. if the -options form is used, options is a list of - separated options which control the details of the dump. not all options are applicable to all dumps — the ones that aren't meaningful just get ignored, no biggie 🙈 the following options are available:
  • address — print the address of each node. usually this isn't meaningful since it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment 🔍
  • asmname — if "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". its primary use is ease of use working backward from mangled names in the assembly file 💅
• slim — okay so when you're dumping front-end intermediate representations, this one says bestie, do NOT dump members of a scope or the body of a function just because you reached that scope?? Like, only dump those items when they're directly reachable by some other path. 💅

And when you're dumping pretty-printed trees, this option also stops the bodies of control structures from getting dumped.

And when you're dumping RTL, it prints the RTL in slim (condensed) form instead of the default LISP-like representation. it's giving minimalist era ✨

• raw — prints a raw representation of the tree. by default trees get pretty-printed into a C-like representation, so this is the un-glammed version 🫶
• details — turns on more detailed dumps (not every dump option honors this btw 💀). also includes info from the optimization passes.
• stats — enables dumping various statistics about the pass (again, not honored by every dump option, no cap).
• blocks — enables 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. Every function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot. obsessed 😻

This option currently only works for RTL dumps, and the RTL is always dumped in slim form.

• vops — enables showing virtual operands for every statement.
• lineno — enables showing line numbers for statements.
• uid — enables showing the unique ID ("DECL_UID") for each variable.
• verbose — enables showing the tree dump for each statement.
• eh — enables showing the EH region number holding each statement.
• scev — enables showing scalar evolution analysis details.
• optimized — enables showing optimization information (only available in certain passes).
• missed — enables showing missed optimization information (only available in certain passes). the missed ones are the ones that keep us up at night fr 😭
• note — enables other detailed optimization information (only available in certain passes).
• all — turns on all options, except raw, slim, verbose and lineno.
• optall — turns on all optimization options, i.e. optimized, missed, and note.

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so to figure out what tree dumps are available, or to find the dump for a pass you're actually interested in, follow these steps bestie 👇

1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. For example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass. 🔢
2. To enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter t for tree passes (and the letter r for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another. 💀

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💖 -fopt-info 💖

-fopt-info
-fopt-info-options
-fopt-info-options=filename

Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations. periodt ✨

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. (order matters bestie 🫶)

The following options control which kinds of messages should be emitted:

• optimized — print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized. slay 💅
• missed — print information about missed optimizations. Individual passes control which information to include in the output.
• note — print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
• all — print detailed optimization information. This includes optimized, missed, and note.

The following option controls the dump verbosity:

• internals — by default, only "high-level" messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. iykyk 👑

One or more of the following option keywords can be used to describe a group of optimizations:

• ipa — enable dumps from all interprocedural optimizations.
• loop — enable dumps from all loop optimizations.
• inline — enable dumps from all inlining optimizations.
• omp — enable dumps from all OMP (Offloading and Multi Processing) optimizations.
• vec — enable dumps from all vectorization optimizations.
• optall — enable dumps from all optimizations. This is a superset of the optimization groups listed above.

If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".

If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. 🙈

Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead. so don't come crying when your file only has the last one in it 💀

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter. 🌈

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities. the way that second file simply does not exist 😭

---

💜 -fsave-optimization-record 💜

Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info.

This option is experimental and the format of the data within the compressed JSON file is subject to change. 🚨

It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message:

• the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
• the function name of the code being optimized, where applicable,
• the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
• objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
• the GCC pass that emitted the message, and
• the location in GCC's own code from which the message was emitted

Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. 🧚

---

🌸 -fsched-verbose=n 🌸

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.

For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also 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. the numbers just keep going bestie 🔥

---

🦋 -fenable-kind-pass / -fdisable-kind-pass=range-list 🦋

-fenable-kind-pass
-fdisable-kind-pass=range-list

This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead. 👑 (so if you're just a normal person compiling your code, respectfully, this is not for you 🫶)

• -fdisable-ipa-pass — bestie, this one says "nope, skip this IPA pass entirely." pass is the pass name, obvi. And here's the tea: if that same pass gets statically invoked in the compiler more than once, you gotta slap a sequential number on the pass name, counting up from 1. 🚫
• -fdisable-rtl-pass
• -fdisable-rtl-pass=range-list — same vibe but for an RTL pass: disables pass. pass is the pass name, and yep, multiple static invocations means appending a sequential number starting from 1. Now range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair split by a colon, and it's inclusive on both ends. If the range is trivial you can just simplify it to a single number. If the function's call graph node's uid lands inside one of your specified ranges, the pass gets disabled for that function. Where do you find the uid?? It's shown in the function header of a dump file, and bestie you can dump the pass names with -fdump-passes. ✨
• -fdisable-tree-pass
• -fdisable-tree-pass=range-list — disables the tree pass pass. For what all the option arguments mean, go peek at -fdisable-rtl. 🌳
• -fenable-ipa-pass — turn IPA pass pass ON. pass is the pass name. Same deal as before: statically invoked multiple times? append a sequential number starting at 1. ✅
• -fenable-rtl-pass
• -fenable-rtl-pass=range-list — enables RTL pass pass. Go see -fdisable-rtl for the argument description and examples. 🔥
• -fenable-tree-pass
• -fenable-tree-pass=range-list — enables tree pass pass. Again, -fdisable-rtl has the description of the option arguments. 💅

Here are some examples showing uses of these options. Copy them exactly, no improvising:

# disable ccp1 for all functions
   -fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
   -fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
   -fdisable-rtl-gcse2=foo,foo2
# disable early inlining
   -fdisable-tree-einline
# disable ipa inlining
   -fdisable-ipa-inline
# enable tree full unroll
   -fenable-tree-unroll

---

• -fchecking
• -fchecking=n — turns on internal consistency checking. The default? Depends on your compiler configuration, no cap. And -fchecking=2 enables further internal consistency checking that might affect code generation. 👀
• -frandom-seed=string — this option hands GCC a seed to use in place of random numbers when it's generating certain symbol names that have to be different in every compiled file. It also gets used to place unique stamps in coverage data files and the object files that produce them. Use -frandom-seed if you want reproducibly identical object files. 🎲

The string can either be a number (decimal, octal or hex) or an arbitrary string — and in that case it gets converted to a number by computing CRC32.

Warning bestie: the string should be different for every file you compile. Every. Single. One. 😭

• -save-temps — stores the usual "temporary" intermediate files permanently instead of yeeting them. 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. And you can still get the corresponding intermediate file by renaming the source file before you use -save-temps.

• -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 specified after the -o option, directory separators and all included. And if you don't use -o?? Then -save-temps=obj 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 done). ⏱️

Without specifying 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", aka 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, bestie.

With an output file specified, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

The "user time" and the "system time" get moved before the program name, and the options passed to the program are displayed, so later you can tell what file was being compiled and with which options. Iconic, honestly. 👑

• -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the dump file name is determined by appending .gkd to the dump base name, see -dumpbase. 📝
• -fcompare-debug[=opts] — if no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to that second compilation. Dump the final internal representation in both compilations, and print an error if they differ. 👯

If the equal sign is omitted, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. 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). To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. 🐛

• -fcompare-debug-second — this one gets implicitly passed to the compiler for that second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files are renamed to contain the .gk additional extension during the second compilation, so they don't overwrite the ones from 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. That's it. That's the whole gig. 💀

• -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. Mainly intended to be used with -fcompare-debug. 🔁
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. Matching energy fr fr. 🎀
• -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. ⏳

And ok this is important: if SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr, then the -ftime-report information is instead emitted in JSON form as part of SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. 🙈

• -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 than or equal to 10, the dump output is sent to stderr using the same format as n minus 10. 📊
• -flto-report — prints a report with internal details on the workings of the link-time optimizer. Heads up: 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. 💖

okay so here's the thing about TFLAGS versus specs, bestie: TFLAGS is one-size-fits-all, but specs? specs let you pick different flags for different multilibs ✨. The glow-up move is to build with make TFLAGS=-fmultiflags, but only after you've configured with --with-specs=%{fmultiflags:...}. That's the whole recipe, no cap.

Also — this option gets discarded by the driver once it's finished processing the driver self spec. Bye bestie 💅.

And it's lowkey iconic for double-checking that TFLAGS are actually being used to build all the target libraries. How? Configure a non-bootstrap compiler with --with-specs='%{!fmultiflags:%emissing TFLAGS}' and then build the compiler and the target libraries. Vibe check accomplished 🫶.

---

• -fprofile-report — makes the compiler spill some stats about how consistent the (estimated) profile is, and what each individual pass actually did. Receipts, basically 📊.
• -fstack-usage — makes the compiler output stack usage info for the program, one function at a time. The dump filename = auxname with .su glued on the end. And what even is auxname, you ask? It's generated from the name of the output file — if you explicitly specified one and it's not an executable — otherwise it's the basename of the source file. Cute.

Every entry is made up of three fields:

1. The name of the function.
2. A number of bytes.
3. One or more qualifiers: static, dynamic, bounded.

The qualifier static means the function handles the stack statically: a fixed number of bytes get allocated for the frame when the function enters, and released when it exits; no other stack adjustments happen inside the function. The second field is that fixed number of bytes. Simple. Clean. Iconic.

The qualifier dynamic means the function handles the stack dynamically: on top of the static allocation described above, stack adjustments also happen in the body of the function — like pushing/popping arguments around function calls, you know the vibe. 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 on the total stack the function uses. But if it's not present, then those adjustments are not bounded at compile time, and the second field only represents the bounded part. F in the chat for your stack analysis 😭.

• -fstats — emits statistics about front-end processing at the end of the compilation. Heads up though: this one is only supported by the C++ front end, and the info is generally only useful to the G++ development team. iykyk 🌈.
• -fdbg-cnt-list — prints the name and the counter upper bound for all the debug counters. The whole roster, bestie 👑.
• *-fdbg-cnt=counter-value-list* — sets the internal debug counter's 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 counter's name plus a list of closed intervals. The lower_bound is optional and gets zero-initialized if you don't set it. For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for the second, third, fourth, tenth and eleventh invocation. And for dbg_cnt(tail_call), true is returned for the first 10 invocations. Math but make it slay ✨.
• *-print-file-name=library* — prints the full absolute name of the library file library that would be used when linking — and does nothing else. With this option GCC does not compile or link anything; it just prints the file name. Just the name, periodt 💅.
• -print-multi-directory — prints the directory name corresponding to the multilib selected by any other switches hanging out in the command line. That directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib — prints the mapping from multilib directory names to the compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, with no spaces between multiple switches. This is supposed to ease shell processing. Fancy 🎀.
• -print-multi-os-directory — prints the path to the OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries live in the lib subdirectory and no multilibs are used, this is usually just .. If OS libraries live in libsuffix sibling directories, it prints e.g. ../lib64, ../lib or ../lib32. Or, if OS libraries live in lib/subdir subdirectories, it prints e.g. amd64, sparcv9 or ev6.
• -print-multiarch — prints the path to the OS libraries for the selected multiarch, relative to some lib subdirectory.
• *-print-prog-name=program* — like -print-file-name, but it searches for a program such as cpp. Same energy, different target 🦋.
• -print-libgcc-file-name — same as -print-file-name=libgcc.a.

Super useful when you're using -nostdlib or -nodefaultlibs but you do still want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

• -print-search-dirs — prints the name of the configured installation directory plus a list of program and library directories gcc searches — and does nothing else. Go off, directory reveal 💅.

This one's the hero when gcc spits out the error message installation problem, cannot exec cpp0: No such file or directory. To fix that vibe, you either gotta 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 /. It will absolutely notice 🙈.

• -print-sysroot — prints the target sysroot directory that's used during compilation. This is the target sysroot specified either at configure time or with the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. Silence. Ghosted 🌸.
• -print-sysroot-headers-suffix — prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler isn't configured with such a suffix — and does nothing else.
• -dumpmachine — prints the compiler's target machine (for example, i686-pc-linux-gnu) — and does nothing else. Machine reveal ✨.
• -dumpversion — prints the compiler version (for example, 3.0, 6.3.0 or 7) — and does nothing else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler was 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). Surprise format, honestly 🎁.
• -dumpfullversion — prints the full compiler version — and does nothing else. The output is always three numbers separated by dots: major, minor and patchlevel version. No mystery here, bestie.
• -dumpspecs — prints the compiler's built-in specs — and does nothing else. (This is used when GCC itself is being built. Very meta of it 👑.)

🎀 Machine-Dependent Options 🎀

Every target machine GCC supports can have its own options — like, ones that let you compile for a particular processor variant or ABI, or control optimizations that are specific to that machine. By convention, the names of machine-specific options start with -m. That's your tell 💅.

Some configurations of the compiler also support extra target-specific options, usually so they play nice with other compilers on the same platform. Networking but for compilers 🫶.

🌸 AArch64 Options 🌸

These options are defined for AArch64 implementations:

• *-mabi=name* — generates code for the specified data model. The permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a SysV-like data model where int is 32 bits but long int and pointers are 64 bits.

The default depends on the specific target configuration. And heads up, bestie: 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. No mixing, fr fr 😤.

• -mbig-endian — generates big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generates code that uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers, but it will not impose any restrictions on the assembler. Compiler, you're on your own.
• -mlittle-endian — generates little-endian code. This is the default when GCC is configured for an aarch64-*-* target, but not an aarch64_be-*-* target.
• -mcmodel=tiny — generates code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
• -mcmodel=small — generates code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. And this one is the default code model 👑.
• -mcmodel=large — generates code for the large code model. It makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. And the drama: -mcmodel=large is incompatible with -mabi=ilp32, -fpic and -fPIC. Pick a lane 😭.
• *-mtp=name* — specifies the system register to use as a thread pointer. The valid values are tpidr_el0, tpidrro_el0, tpidr_el1, tpidr_el2, tpidr_el3. For backwards compatibility, the aliases el0, el1, el2, el3 are also accepted. The default setting is tpidr_el0. It's recommended to compile all code that's meant to interoperate with the same value of this option, so you don't end up accessing a different thread pointer from the wrong exception level. Stay consistent, bestie 🦋.
• -mstrict-align
• -mno-strict-align Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. Your call 🎀.
• -momit-leaf-frame-pointer
• -mno-omit-leaf-frame-pointer Omit or keep the frame pointer in leaf functions. The former behavior (omitting it) is the default 💅.
• *-mstack-protector-guard=guard*
• *-mstack-protector-guard-reg=reg*
• *-mstack-protector-guard-offset=offset* Generate stack protection code using a canary at guard. Supported locations are global for a global canary, or sysreg for a canary in an appropriate system register.

With that second choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset go further and specify which system register to use as the base register for reading the canary, and from what offset from that base register. And ngl: there is no default register or offset, because this is entirely for use within the Linux kernel. Bring your own values, babes 🫶.

• -mtls-dialect=desc — uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. And plot twist, this is the default. Slay ✨.

---

• -mtls-dialect=traditional — tells the compiler to use traditional TLS as the thread-local storage mechanism whenever it's doing dynamic accesses of TLS variables. old school but it works 💅
• -mtls-size=size — sets the bit size of immediate TLS offsets. the valid values are 12, 24, 32, 48. heads up bestie: this one needs binutils 2.26 or newer, no cap.
• -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769 — turn the workaround for the ARM Cortex-A53 erratum number 835769 on or off. what does the workaround even do? it inserts a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. little nap between the math, iconic 🎀
• -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. important detail: this erratum workaround gets made at link time, so this option only passes the corresponding flag along to the linker. it's giving middleman energy.
• -mlow-precision-recip-sqrt, -mno-low-precision-recip-sqrt — enable or disable the reciprocal square root approximation. this option only does anything if -ffast-math or -funsafe-math-optimizations is used as well. enabling it drops the precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. fast and loose, lowkey.
• -mlow-precision-sqrt, -mno-low-precision-sqrt — enable or disable the square root approximation. same deal: only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. enabling it reduces the precision of square root results to about 16 bits for single precision and to 32 bits for double precision. also, if you enable this one it implies -mlow-precision-recip-sqrt — it's a package deal fr fr.
• -mlow-precision-div, -mno-low-precision-div — enable or disable the division approximation. yep, you guessed it: only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. enabling it reduces the precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation, -mno-track-speculation — enable or disable the generation of extra code that tracks speculative execution through conditional branches. the tracking state can then be used by the compiler when it's expanding calls to __builtin_speculation_safe_copy, which lets it generate a more efficient code sequence. the compiler is doing detective work 🔍
• -moutline-atomics, -mno-outline-atomics — enable or disable calls to out-of-line helpers that implement atomic operations. at runtime these helpers figure out whether the LSE instructions from ARMv8.1-A can be used, and if they can't, they fall back to the load/store-exclusive instructions that are present in the base ARMv8.0 ISA. flexible queen ✨

this option is only applicable when compiling for the base ARMv8.0 instruction set. if you're using a later revision, like -march=armv8.1-a or -march=armv8-a+lse, then the ARMv8.1-Atomics instructions get used directly. same story when you use -mcpu= and the selected cpu supports the lse feature. this option is on by default.

• -march=name — specify the name of the target architecture, and optionally one or more feature modifiers. the form 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:

arch value : Architecture : Includes by default
armv8-a : Armv8-A : +fp, +simd
armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
armv8.2-a : Armv8.2-A : armv8.1-a
armv8.3-a : Armv8.3-A : armv8.2-a, +pauth
armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
armv8.7-a : Armv8.7-A : armv8.6-a, +ls64
armv8.8-a : Armv8.8-a : armv8.7-a, +mops
armv8.9-a : Armv8.9-a : armv8.8-a
armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
armv9.1-a : Armv9.1-A : armv9-a, +bf16, +i8mm
armv9.2-a : Armv9.2-A : armv9.1-a, +ls64
armv9.3-a : Armv9.3-A : armv9.2-a, +mops
armv9.4-a : Armv9.4-A : armv9.3-a
armv8-r : Armv8-R : armv8-r

the value native is available on native AArch64 GNU/Linux and makes the compiler just pick the architecture of the host system. and if the compiler can't recognize the architecture of the host system, this option has no effect. it tried its best 🥺

the permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. where conflicting feature modifiers are specified, the right-most feature wins — last one standing, periodt.

GCC uses name to decide what kind of instructions it can emit when generating assembly code. and if -march is specified without either -mtune or -mcpu also being specified, the code gets tuned to perform well across a range of target processors implementing the target architecture. versatile 💖

• -mtune=name — specify the name of the target processor that GCC should tune the performance of the code for.

the permissible values for this option are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, 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. that's a lot of names bestie, I counted them so you don't have to 😭

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. two brains, one chip, main character energy 👑

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, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). this is more general than tuning for a specific core like Neoverse V1, but more specific than the default tuning described below. niche but make it flexible ✨

also, on native AArch64 GNU/Linux systems the value native tunes performance to the host system. this option has no effect if the compiler can't recognize the processor of the host system.

where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

🚫 this option cannot be suffixed by feature modifiers. don't try it.

• -mcpu=name — specify the name of the target processor, optionally suffixed by one or more feature modifiers. the form is -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as the ones available for -mtune. the permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. where conflicting feature modifiers are specified, the right-most feature is used. same rule as before 💅

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). where this option is used together with -march or -mtune, those options take precedence over the appropriate part of this option. they're the boss, not -mcpu.

-mcpu=neoverse-512tvb is special: it doesn't refer to a 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. it's giving "I contain multitudes" 🌈

• -moverride=string — override tuning decisions made by the back-end in response to a -mtune= switch. heads up: the syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases. so like. don't build your whole personality on it.

this option is only intended to be useful when developing GCC. it's a dev tool, not a lifestyle.

• -mverbose-cost-dump — turn on verbose cost model dumping in the debug dump files. this option exists for debugging the compiler. iykyk, compiler girlies only 🔥
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — enable or disable PC-relative literal loads. with this on, literal pools get accessed using a single instruction and emitted after each function. catch: this limits the maximum size of functions to 1MB. it's enabled by default for -mcmodel=tiny.
• -msign-return-address=scope — pick the function scope that return address signing gets applied to. the permissible values are:
  • none — disables return address signing
  • non-leaf — enables pointer signing for functions that are not leaf functions
  • all — enables pointer signing for all functions

the default value is none.

⚠️ this option has been deprecated by -mbranch-protection. it's out here living its retired life 💀

• -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti — select the branch protection features to use.
  • none is the default and turns off all types of branch protection.
  • standard turns on all types of branch protection features. if a feature has additional tuning options, then standard sets it to its standard level. standard issue slay ✨
  • pac-ret[+leaf] turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. the optional argument leaf can be used to extend the signing to include leaf functions. the optional argument b-key can be used to sign the functions with the B-key instead of the A-key. very choose-your-own-adventure 🎀
  • bti turns on the branch target identification mechanism. protected and cute 🛡️
• -mharden-sls=opts — bestie, this turns on compiler hardening against straight line speculation (SLS) 💅. And opts here is a comma-separated list of these:
  • retbr
  • blr

Plus, <hotpink>bonus values</hotpink>: -mharden-sls=all enables all the SLS hardening, and -mharden-sls=none disables all of it. That's the whole menu 🍓.

---

• -mearly-ra=scope — this one decides when to run an early register allocation pass. That pass happens before instruction scheduling, and it goes hunting for a spill-free allocation of floating-point and vector code 🦋. It also tries to use strided multi-register instructions, like SME2's strided LD1 and ST1. Iconic.

The possible values of scope are:

• all — runs the pass on all functions
• strided — runs the pass on functions that have access to strided multi-register instructions
• none — disables the pass entirely, we're closed 😭

Defaults, no cap: -mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise.

• -mearly-ldp-fusion — enables 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 — enables the copy of that same AArch64 load/store pair fusion pass, except this one runs after register allocation. Also enabled by default at -O and above 💫
• -msve-vector-bits=bits — sets the number of bits in an SVE vector register. Heads up tho: this option only does anything when SVE is enabled 🙈

GCC supports two forms of SVE code generation:

• "vector-length agnostic" output, which works with any size of vector register
• "vector-length specific" output, which lets GCC make assumptions about the vector length when that's useful for optimization reasons

The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. 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.

Ngl this part matters: 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. Protect your peace, use scalable 💀

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code 👑

---

🌈 -march and -mcpu Feature Modifiers 🌈

Feature modifiers used with -march and -mcpu can be any of the following, and also their inverses, written as nofeature 🧚

• crc — enable CRC extension. 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. On by default for all possible values for options -march and -mcpu.
• simd — enable Advanced SIMD instructions. This also enables floating-point instructions. On by default for all possible values for options -march and -mcpu.
• sve — enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
• lse — enable Large System Extension instructions. On by default for -march=armv8.1-a.
• rdma — enable Round Double Multiply Accumulate instructions. On by default for -march=armv8.1-a.
• fp16 — enable FP16 extension. This also enables floating-point instructions.
• fp16fml — enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. Enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported 🚫
• rcpc — enable the RCpc extension. This enables the use of the LDAPR instructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension.
• 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 there 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. 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. 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. 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. Enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• f32mm — enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• f64mm — enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• bf16 — enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. Enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• ls64 — enable the 64-byte atomic load and store instructions for accelerators. Enabled by default for -march=armv8.7-a.
• mops — enable the instructions to accelerate memory operations like memcpy, memmove, memset. Enabled by default for -march=armv8.8-a
• flagm — enable the Flag Manipulation instructions Extension.
• 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. (Also: in the original text there's a stray +@item sme2 Enable the Scalable Matrix Extension 2. This also enables SME instructions. line glued onto the end of this entry — the sme2 bit is listed properly right below 🫶)
• 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 ⭐

---

🎀 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. Ngl this can result in an increased instruction count, so it may either reduce or increase overall code size. It's giving chaotic neutral 😭
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. Heads up: 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. Yes, really 🍓
• -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 it can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero ⚠️

The default is -msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.

• -mstack-offset=num — 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.

Bestie, listen: values other than 8 or 16 are untested and unlikely to work. Also, 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. Periodt 💅

• -mno-round-nearest — tells the scheduler to assume the rounding mode is set to truncating. the default? -mround-nearest, bestie 🌸
• -mlong-calls — unless some attribute says otherwise, assume every call might be way out past the offset range of the b / bl instructions, so load the function address into a register before doing what would otherwise be a direct call. this is the default 💅
• -mshort-calls — unless an attribute says otherwise, assume all direct calls fit inside the range of the b / bl instructions, so just use those instructions for direct calls. the default here is -mlong-calls, so like, don't get it twisted 🙈
• -msmall16 — assume addresses can be loaded as 16-bit unsigned values. note that this does not apply to function addresses, because those are under -mlong-calls semantics 🎀
• -mfp-mode=mode — sets the prevailing mode of the floating-point unit. this decides what FP mode is provided and expected at function call and return time. and ooh, if you make this mode match what you mostly need at the start of a function, your programs get smaller and faster because you skip all the unnecessary mode-switching ✨

mode can be one of these:

• caller — any mode at function entry is valid, and it's retained or restored when the function returns, and when it calls other functions. this one's great for compiling libraries or other compilation units you might wanna drop into different programs that have different prevailing FPU modes. the convenience of reusing a single object file outweighs the size and speed overhead of any extra mode switching that might be needed, compared to what a more specific prevailing FPU mode would cost you 💖
• truncate — the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. that includes conversion from floating point to integer.
• round-nearest — the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
• int — the mode used to do 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 — switches the preferred SIMD mode to SImode. the default is -mvect-double, which uses DImode as the preferred SIMD mode 👑
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. default is 8. also heads up: this is an ABI change, even though a lot of library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of the relevant types 😭
• -msplit-vecmove-early — splits vector moves into single word moves before reload. in theory this can give better register allocation, but so far the reverse seems to generally be the case 💀
• -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 (both of which specify using that register as a fixed register), and none, which means no register is used for this purpose. the default is -m1reg-none 🍓

---

🌟 AMD GCN Options 🌟

these ones are defined specifically for the AMD GCN port, and they're lowkey iconic 💜

• -march=gpu
• -mtune=gpu — set the architecture type or the tuning for gpu. supported values for gpu are:
  • fiji — compile for GCN3 Fiji devices (gfx803). support is deprecated; availability depends on how GCC was 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: there may be many threads and only limited memory available 😭 the size of the stack allocation can also impact run-time performance. the default is 32KB when using OpenACC or OpenMP, and 1MB otherwise 🦋
• -mxnack=on
• -mxnack=off
• -mxnack=any — compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. some devices always require XNACK and some let the user configure XNACK. the compiled code must match the device mode. the default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise 💅

---

🦋 ARC Options 🦋

the following options control which architecture variant your code is being compiled for:

• -mbarrel-shifter — generate instructions supported by the barrel shifter. this is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — force calling a function using the jli_s instruction. this option is valid only for the ARCv2 architecture 🔥
• -mcpu=cpu — set architecture type, register usage, and instruction scheduling parameters for cpu. there are also shortcut alias options available for backward compatibility and convenience. supported values for cpu are:
  • arc600 — compile for ARC600. aliases: -mA6, -mARC600.
  • arc601 — compile for ARC601. alias: -mARC601.
  • arc700 — compile for ARC700. aliases: -mA7, -mARC700. this is the default when configured with --with-cpu=arc700.
  • arcem — compile for ARC EM.
  • archs — compile for ARC HS.
  • em — compile for ARC EM CPU with no hardware extensions.
  • em4 — compile for ARC EM4 CPU.
  • em4_dmips — compile for ARC EM4 DMIPS CPU.
  • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — compile for ARC HS34 CPU.
  • hs38 — compile for 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 a 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's 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 — 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 the thread pointer register number 🧚
• -mmpy-option=multo — bestie, this one compiles your ARCv2 code with a specific multiplier design option. You can hand it multo as either a string or a numeric value, whichever your heart desires. wlh1 is the default. Here's the whole squad of recognized values:
  • 0, none — no multiplier available. not a single one. we're multiplying with vibes only, which is to say we are not multiplying at all. 💀
  • 1, w — 16x16 multiplier, fully pipelined. This enables the following instructions: mpyw and mpyuw.
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). Additionally enabled: mpy, mpyu, mpym, mpymu, and mpy_s. (this is the default, so if you say nothing you get this one 🫶)
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). Also additionally enabling: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 4, wlh3 — two 16x16 multipliers, blocking, sequential. Also additionally enabling: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 5, wlh4 — one 16x16 multiplier, blocking, sequential. Also additionally enabling: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 6, wlh5 — one 32x4 multiplier, blocking, sequential. Also additionally enabling: 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. ✨

Heads up bestie: this option is only available for ARCv2 cores. Not a v2? Not for you. 💅

• -mfpu=fpu — turns on support for specific floating-point hardware extensions on ARCv2 cores. The supported values for fpu are, and I will be listing every single one because we don't do half-measures:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. Single-precision is also switched on. Not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. Single-precision is also switched on. This one is only available for ARC EM.
  • fpuda_div — double-precision floating-point hardware extensions via double-precision assist instructions. Single-precision, square-root, and divide extensions are also enabled. Only available for ARC EM.
  • fpuda_fma — double-precision floating-point hardware extensions via double-precision assist instructions. Single-precision plus fused multiply and add hardware extensions also enabled. Only for ARC EM.
  • fpuda_all — double-precision floating-point hardware extensions via double-precision assist instructions. All single-precision floating-point hardware extensions also enabled. Only 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 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 fpus_fma. Not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions. all of them. the whole collection. 👑
  • fpud_all — enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM.
• -mirq-ctrl-saved=register-range, blink, lp_count — like, this tells the compiler which general-purpose registers the processor auto-saves/restores on interrupt entry and exit (it does it by itself, no cap 🫶). register-range is written as two registers with a dash between them. The range always starts with r0, and the upper limit is the fp register. blink and lp_count are optional. Only valid for ARC EM and ARC HS cores.
• -mrgf-banked-regs=number — says how many registers get replicated into the second register bank when a fast interrupt hits. "Fast interrupts" = interrupts at the top priority level, P0. These save only the PC and STATUS32 registers so nobody's doing memory transactions during the entry and exit sequences — efficient queen behavior. Use it when you're running fast interrupts on an ARC V2 family processor. Permitted values: 4, 8, 16, and 32.
• -mlpc-width=width — sets the width of the lp_count register. Valid width values are 8, 16, 20, 24, 28 and 32 bits. Default width is locked in at 32 bits. If your width is less than 32, the compiler won't try turning your loops into the zero-delay loop mechanism unless it knows lp_count can actually hold the loop-counter value you need. And depending on the width you picked, the compiler and the run-time library might still use the loop mechanism for various needs. This option defines the macro __ARC_LPC_WIDTH__ with the value of width. 🎀
• -mrf16 — tells the compiler to generate code for a 16-entry register file. This 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. iykyk, we're handing these straight to the assembler bestie:

• -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 __Xdvbf. ⚠️ Deprecated.
• -mlock — passed down to the assembler to enable the locked load/store conditional extension. Also sets __Xlock.
• -mmac-d16 — passed down to the assembler. Also sets __Xxmac_d16. ⚠️ Deprecated.
• -mmac-24 — passed down to the assembler. Also sets __Xxmac_24. ⚠️ Deprecated.
• -mrtsc — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets __Xrtsc. ⚠️ Deprecated.
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. Also sets __Xswape.
• -mtelephony — passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets __Xtelephony. ⚠️ Deprecated.
• -mxy — passed down to the assembler to enable the XY memory extension. Also sets __Xxy.

---

The following options control how the assembly code is annotated, aka the little notes we scribble in the margins ✏️:

• -misize — annotate assembler instructions with estimated addresses.
• -mannotate-align — does nothing. Preserved for backward compatibility. (it's giving "I'm just here so I don't get fined" 🙈)

The following options are passed straight through to the linker:

• -marclinux — passed through to the linker to specify use of the arclinux emulation. 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. 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 the generated code. the big behavior stuff:

• -mlong-calls — generate calls as register indirect calls, which gives you access to the full 32-bit address range. 🌈
• -mmedium-calls — don't use anything 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 the 25-bit range can be used instead of the 21-bit range you'd get with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. Default num is 4 for any ARC configuration, or 8 when we've got 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, the little tweaks that make it chef's kiss 💅:

• -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 that generates compare-and-branch (brcc) instructions. It has no effect on generation of these instructions driven by the combiner pass. So, like, half the story only.
• -mcase-vector-pcrel — use PC-relative switch case tables so case tables can be shortened. This is the default for -Os.
• -mcompact-casesi — enable the compact casesi pattern. 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.

Delay slot scheduling being the absolute chaos gremlin it is, plus operand numbers, literal sizes, instruction lengths and conditional execution support all being in their little gang together, the target-independent pass that generates conditional execution is often not it. So the ARC port kept a special pass on life support that goes hunting for more conditional-execution opportunities after register allocation, branch shortening, and delay slot scheduling have all finished their thing. This pass generally, but not always, makes performance and code size better, and the price tag is extra compilation time — which is literally why there's an option to turn it off. And bestie, if call instructions are blowing past their allowable offset range because they got conditionalized, you should consider using -mmedium-calls instead. 💅

---

• -mearly-cbranchsi — turns on the pre-reload use of the cbranchsi pattern. slay.
• -mexpand-adddi — expands adddi3 and subdi3 at RTL generation time into add.f, adc and friends. ngl this option is deprecated, so 💀.
• -mindexed-loads — enables indexed loads. ngl this can be a lil problematic, because some optimizers then go and assume indexed stores exist too, and they simply do not. the audacity honestly.
• -mlra — enables Local Register Allocation. this is still experimental for ARC, so by default the compiler uses standard reload (i.e. -mno-lra). don't say we didn't warn you 🫶
• -mlra-priority-none — don't indicate any priority for target registers. none of them. no favorites here.
• -mlra-priority-compact — indicate target register priority for r0..r3 / r12..r15.
• -mlra-priority-noncompact — reduce target register priority for r0..r3 / r12..r15. sorry besties.
• -mmillicode — when optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of registers often get shortened by using a call to a special function in libgcc; this is called a millicode call. but these calls can cause performance issues and/or linking issues when you link in a nonstandard way 💀, which is exactly why this option exists — to turn millicode call generation on or off.
• -mcode-density-frame — lets the compiler emit enter and leave instructions. these are only valid for CPUs with the code-density feature, so don't go wild with them elsewhere 👑
• -mmixed-code — does nothing. preserved for backward compatibility. iconic behavior honestly.
• -mq-class — this option is deprecated. enables q instruction alternatives. this is the default for -Os.
• -mRcq — does nothing. preserved for backward compatibility.
• -mRcw — does nothing. preserved for backward compatibility.
• -msize-level=level — fine-tune size optimization with regards to instruction lengths and alignment. the recognized values for level are:
  • 0 — No size optimization. this level is deprecated and treated like 1.
  • 1 — Short instructions are used opportunistically.
  • 2 — in addition, alignment of loops and of code after barriers are dropped.
  • 3 — in addition, optional data alignment is dropped, and the option Os is enabled.

this defaults to 3 when -Os is in effect. otherwise, the behavior when this is not set is equivalent to level 1.

• -mtune=cpu — set instruction scheduling parameters for cpu, overriding any 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 — cost to assume for a multiply instruction, with 4 being equal to a normal instruction.
• -munalign-prob-threshold=probability — does nothing. preserved for backward compatibility.

The 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 — values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively.
• -multcost=num — Replaced by -mmultcost.

---

🦋 ARM Options 🦋

These -m options are defined for the ARM port, bestie, get comfy:

• -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 is compliant with the ARM Procedure Call Standard for all functions, even if this isn't strictly necessary for correct execution of the code. specifying -fomit-frame-pointer with this option causes the stack frames not to be generated for leaf functions. the default is -mno-apcs-frame. This option is deprecated.
• -mapcs — this is a synonym for -mapcs-frame and is deprecated.
• -mthumb-interwork — 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 reliably be used inside one program 😬. the default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. in AAPCS configurations this option is meaningless.
• -mno-sched-prolog — Prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. this means all functions start with a recognizable set of instructions (or in fact one of a choice from a small set of different function prologues), and this 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 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. no mixing and matching, periodt.

• -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 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...] — 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 upon which it depends. for example, the +crypto extension will always enable the +simd extension. the exception to the additive construction is for extensions that are prefixed with +no...: these extensions 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. the way that order matters here is so real.

most extension names are generically named, but have an effect that depends on the architecture it's applied to. 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. versatile queen ✨

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, bestie. and +vfpv2 is literally an alias for this extension, periodt. 💅
  • +nofp — disables the floating-point instructions. no cap. 🚫
• armv7 — the common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures, iykyk.
  • +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. and guess what? +vfpv3-d16 can be used as an alias for this extension, slay. note that floating-point is not supported by the base ARMv7-M architecture, but it is compatible with both the ARMv7-A and ARMv7-R architectures. so don't try it on M, it's not the vibe. 😭
  • +nofp — disables the floating-point instructions. bye. ✨
• armv7-a
  • +mp — the multiprocessing extension. 🔥
  • +sec — the security extension. 🔒
  • +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. +vfpv3-d16 is an alias for this extension, periodt.
  • +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. iconic. ✨
  • +vfpv3 — the VFPv3 floating-point instructions, with 32 double-precision registers.
  • +vfpv3-d16-fp16 — the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations. 🎀
  • +vfpv3-fp16 — the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv4-d16 — the VFPv4 floating-point instructions, with 16 double-precision registers.
  • +vfpv4 — the VFPv4 floating-point instructions, with 32 double-precision registers.
  • +neon-fp16 — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +neon-vfpv4 — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
  • +nosimd — disables the Advanced SIMD instructions (it does not disable floating point, just fyi). 🙅
  • +nofp — disables the floating-point and Advanced SIMD instructions.
• armv7ve — the extended version of the ARMv7-A architecture with support for virtualization. so extra. 💁
  • +fp — the VFPv4 floating-point instructions, with 16 double-precision registers. +vfpv4-d16 can be used as an alias for this extension.
  • +simd — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. +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. +neon-vfpv3 can be used as an alias for this extension.
  • +neon-fp16 — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +nosimd — disables the Advanced SIMD instructions (it does not disable floating point, just so you know).
  • +nofp — disables the floating-point and Advanced SIMD instructions.
• armv8-a
  • +crc — the Cyclic Redundancy Check (CRC) instructions. 🧐
  • +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
  • +crypto — the cryptographic instructions. 🔐
  • +nocrypto — disables the cryptographic instructions.
  • +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction. 🛡️
  • +predres — Execution and Data Prediction Restriction Instructions.
• armv8.1-a
  • +simd — the ARMv8.1-A Advanced SIMD and floating-point instructions.
  • +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions.
  • +nocrypto — disables the cryptographic instructions.
  • +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
• armv8.2-a
• armv8.3-a
  • +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions.
  • +fp16fml — the half-precision floating-point fmla extension. this also enables the half-precision floating-point extension and 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 — disables the cryptographic extension.
  • +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
  • +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
• armv8.4-a
  • +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
  • +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions 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 — disables the cryptographic extension.
  • +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
  • +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
• armv8.5-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 — disables the cryptographic extension.
  • +nofp — disables 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 — disables the cryptographic extension.
  • +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. This also turns on Advanced SIMD and floating-point instructions, so it's giving multitasker energy. 💅
  • +bf16 — Brain half-precision floating-point instructions. Also enables Advanced SIMD and floating-point instructions. two-for-one, obsessed. ✨
• armv7-r
  • +fp.sp — the single-precision VFPv3 floating-point instructions. And bestie, +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 one. ✨
  • +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations. look at her go 🦋
  • +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers plus the half-precision floating-point conversion operations.
  • +nofp — disable the floating-point extension. bye bestie 👋
  • +idiv — the ARM-state integer division instructions.
  • +noidiv — disable the ARM-state integer division extension.
• armv7e-m
  • +fp — the single-precision VFPv4 floating-point instructions.
  • +fpv5 — the single-precision FPv5 floating-point instructions.
  • +fp.dp — the single- and double-precision FPv5 floating-point instructions.
  • +nofp — disable the floating-point extensions.
• armv8.1-m.main
  • +dsp — the DSP instructions.
  • +mve — the M-Profile Vector Extension (MVE) integer instructions.
  • +mve.fp — the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.
  • +fp — the single-precision floating-point instructions.
  • +fp.dp — the single- and double-precision floating-point instructions.
  • +nofp — disable the floating-point extension.
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7. pick your fighter 💅
  • +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. (yes it's the same deal as armv8.1-m.main, they're twinning 💖)
• armv8-r
  • +crc — the Cyclic Redundancy Check (CRC) instructions. checksums slay 🔥
  • +fp.sp — the single-precision FPv5 floating-point instructions.
  • +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
  • +crypto — the cryptographic instructions. so secretive 🤫
  • +nocrypto — disable the cryptographic instructions.
  • +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions. absolutely nothing left, it's giving minimalism 😭

Heads up bestie: -march=native makes the compiler auto-detect the architecture of the build computer. Right now this feature is only supported on GNU/Linux, and not all architectures are recognized. And if the auto-detect is unsuccessful, the option has no effect at all. 🙈

---

• -mtune=name — okay so this one tells GCC the name of the target ARM processor it should tune the performance of the code for. For some ARM implementations you can get better performance by using this option. no cap. 🫶

The 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.

ALSO, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. The permissible names for that 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 aim? generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go. it's giving adaptable queen 🦋

-mtune permits the same extension options as -mcpu, but lowkey: the extension options don't affect the tuning of the generated code.

-mtune=native makes the compiler auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. same energy as before fr fr 😭

---

• -mcpu=name[+extension...] — this specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if you'd said it with -march) and the ARM processor type to tune for performance (as if you'd said it with -mtune). 👑

Where this option is used together with -march or -mtune, those options take precedence over the appropriate part of this option. they said "my rules now" 💅

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.

For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions respectively.

Permissible names for this option are the same as those for -mtune. (scroll back up bestie, it's the whole list ✨)

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. And it 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. that's a long list and it's all getting switched off 💀
• +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. SIMD only, bestie, the floating-point stays 😌
• +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. 🔐✨

• -mcpu=generic-arch — also totally allowed, and it lowkey just means -march=arch -mtune=generic-arch. Peep -mtune if you want the deets. 💫
• -mcpu=native — makes the compiler sniff out the CPU of the build machine all by itself. Heads up tho: rn this only works on GNU/Linux, and not every architecture gets recognized. If the auto-detect flops, the option just does literally nothing. 🙈

---

So the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers, and it supports these extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. They all mean the exact same thing as the extensions to -march=armv7-a. iykyk 💅

---

• -mfpu=name — tells the compiler what floating-point hardware (or hardware emulation, no judgment ✨) is chillin' on the target. The names you're allowed to use 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. Also bestie, neon is just an alias for neon-vfpv3, and vfp is an alias for vfpv2. periodt.

auto is the default and it's kinda special — it makes the compiler pick the floating-point and Advanced SIMD instructions based on whatever -mcpu and -march are set to. slay 🌸

🚨 If the floating-point hardware you picked includes the NEON extension (like with -mfpu=neon), real talk: floating-point operations will not be generated by GCC's auto-vectorization pass unless you also specify -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 can legit cause a loss of precision. Not the vibe you want. 😭

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. cute, right? 🎀

• -mfp16-format=name — specifies the format of the "__fp16" half-precision floating-point type. The names you can use are none, ieee, and alternative; the default is none, and in that case the "__fp16" type just... isn't defined. that's it, that's the tea 🦋
• -mstructure-size-boundary=n — the sizes of all structures and unions get rounded up to a multiple of however many bits you set with this option. Allowed values are 8, 32 and 64. The default value differs per toolchain — for the COFF targeted toolchain the default is 8. A value of 64 is only allowed if the underlying ABI supports it. 💅

Setting a bigger number can give you faster, more efficient code, but it can also bloat your program's size. And ngl, different values are potentially incompatible — code compiled with one value can't necessarily expect to work with code or libraries compiled with another value, if they're swapping info through structures or unions. so pick carefully bestie 🙏

This option is deprecated. 💀

• -mabort-on-noreturn — generates a call to the function "abort" at the end of a "noreturn" function. It fires off if the function tries to return. no cap.
• -mlong-calls
• -mno-long-calls — tells the compiler to do function calls by first loading the address of the function into a register, then doing a subroutine call on that register. This switch is literally needed if the target function lives outside the 64-megabyte addressing range of the offset-based version of subroutine call instruction. 📏

Even if this switch is enabled, not every single function call turns into a long call. The heuristic is: static functions, functions that have the "short_call" attribute, functions inside the scope of a "#pragma no_long_calls" directive, and functions whose definitions have already been compiled within the current compilation unit — none of those get turned into long calls. The exceptions to this rule are: weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions within the scope of a "#pragma long_calls" directive — those always become long calls. 👑

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, and so does putting your function calls inside the scope of a "#pragma long_calls_off" directive. Also worth noting — these switches have zero effect on how the compiler generates code to handle function calls via function pointers. 💫

• -msingle-pic-base — treats the register used for PIC addressing as read-only, instead of loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. 🎀
• -mpic-register=reg — specifies which register to use for PIC addressing. For the standard PIC base case, the default is any suitable register determined by the compiler. For the single PIC base case, the default is R9 if the target is EABI based or stack-checking is enabled — otherwise the default is R10. 💅
• -mpic-data-is-text-relative — assumes the displacement between the text and data segments is fixed at static link time. This lets you use PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable -msingle-pic-base by default. slay 🌈
• -mpoke-function-name — writes the name of each function into the text section, directly preceding the function prologue. The generated code looks something like this:

             t0
                 .ascii "arm_poke_function_name", 0
                 .align
             t1
                 .word 0xff000000 + (t1 - t0)
             arm_poke_function_name
                 mov     ip, sp
                 stmfd   sp!, {fp, ip, lr, pc}
                 sub     fp, ip, #4

When 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 there's a function name embedded immediately preceding this location, and it has length "((pc[-3]) & 0xff000000)". iconic detective work tbh 🔍

• -mthumb
• -marm — picks 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 that default can be changed by configuring GCC with the --with-mode=state configure option. ✨

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. 💜

• -mflip-thumb — switches ARM/Thumb modes on alternating functions. This option is literally just for regression testing of mixed Thumb/ARM code generation, and is not intended for ordinary use in compiling code. like. don't. 🙈
• -mtpcs-frame — generates a stack frame that's compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that doesn't call any other functions, bestie.) The default is -mno-tpcs-frame. 🍓
• -mtpcs-leaf-frame — generates a stack frame that's compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that calls no other functions.) The default is -mno-apcs-leaf-frame. 🌸
• -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 lets these functions be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default there. 💅
• -mcaller-super-interworking — allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code was compiled for interworking or not. There's 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 there. giving "same caveat, different day" energy 😭
• -mtp=name — specifies 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. main character energy fr fr 👑
• -mtls-dialect=dialect — specifies 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 it does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. ✨
• -mword-relocations — only generates 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. ⚠️
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used. This option avoids generating those instructions. It's enabled by default when -mcpu=cortex-m3 is specified. 💀
• -mfix-cortex-a57-aes-1742098
• -mno-fix-cortex-a57-aes-1742098
• -mfix-cortex-a72-aes-1655431
• -mno-fix-cortex-a72-aes-1655431 — enable (or disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. This option is enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified. 🔥
• -munaligned-access, -mno-unaligned-access — turns reading and writing of 16- and 32-bit values on or off when they live at addresses that aren't 16- or 32-bit aligned. By default unaligned access is off for all pre-ARMv6, all ARMv6-M and ARMv8-M Baseline architectures, and on for every other architecture. If it's not enabled, then words inside packed data structures get accessed a byte at a time (slower, but hey, aligned).

The ARM attribute "Tag_CPU_unaligned_access" gets set in the generated object file to either true or false depending on how you set this option. And if unaligned access is enabled, the preprocessor symbol "__ARM_FEATURE_UNALIGNED" gets defined too. ✨

• -mneon-for-64bits — bestie this one is deprecated and does literally nothing. 💀 No effect. Zero. Nada.
• -mslow-flash-data — assumes that loading data from flash is slower than fetching an instruction, so literal loads get minimized for better performance. It's only supported when compiling for ARMv7 M-profile, and it's off by default. Heads up: it conflicts with -mword-relocations.
• -masm-syntax-unified — tells GCC to assume the inline assembler is using unified asm syntax. Default right now is off, which means divided syntax. This one has no impact on Thumb2, but ngl that may change in future releases of GCC. Also, divided syntax should be considered deprecated.
• -mrestrict-it — restricts generation of IT blocks so they conform to ARMv8-A's rules: an IT block can only hold a single 16-bit instruction, and only from a select set of instructions. This one is on by default for ARMv8-A Thumb mode.
• -mprint-tune-info — prints CPU tuning info as a comment in the assembler file. This is only for regression testing of the compiler and genuinely not intended for ordinary use when compiling code. Disabled by default.
• -mverbose-cost-dump — enables verbose cost model dumping in the debug dump files. This one's for debugging the compiler itself, iykyk. 🔍
• -mpure-code — says no to constant data being placed in code sections. Also, when compiling for ELF object format, it gives all text sections the ELF processor-specific section attribute "SHF_ARM_PURECODE". Only available when you're generating non-pic code for M-profile targets.
• -mcmse — generates secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which you can find at <https://developer.arm.com/documentation/ecm0359818/latest/>. 🛡️
• -mfix-cmse-cve-2021-35465 — mitigates a potential security issue with the "VLLDM" instruction in some M-profile devices when using CMSE (CVE-2021-365465). This option is enabled by default when the option -mcpu= is used with "cortex-m33", "cortex-m35p", "cortex-m52", "cortex-m55", "cortex-m85" or "star-mc1". If you want the mitigation gone, -mno-fix-cmse-cve-2021-35465 turns it off.
• -mstack-protector-guard=guard, -mstack-protector-guard-offset=offset — generates stack protection code using a canary at guard. The supported locations are global for a global canary, or tls for a canary you can reach through the TLS register. The option -mstack-protector-guard-offset= is for use with -fstack-protector-guard=tls and not for use in user-land code.
• -mfdpic, -mno-fdpic — selects the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for "arm-*-uclinuxfdpiceabi" targets, this option is on by default and implies -fPIE if none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and you, the user, should explicitly provide the PIC/PIE-related options as needed.

Note that static linking is not supported, because it would still involve the dynamic linker when the program self-relocates. If that behavior is acceptable to you, use -static and -Wl,-dynamic-linker options.

The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using the same ("arm-*-uclinuxfdpiceabi") toolchain as the one used to build the userland programs.

• -mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]] — enables branch protection features (armv8.1-m.main only). Breaking it down, bestie:
  • none — generate code without branch protection or return address signing.
  • standard[+leaf] — generate code with all branch protection features enabled at their standard level.
  • pac-ret[+leaf] — generate code with return address signing set to its standard level, which means signing all functions that save the return address to memory.
  • leaf — when return address signing is enabled, also sign leaf functions even if they don't write the return address to memory.
  • +bti — add landing-pad instructions at the permitted targets of indirect branch instructions.

If the +pacbti architecture extension is not enabled, then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code stays backwards-compatible with earlier versions of the architecture, but you can enable that extra security at run time on processors that support the PACBTI extension. 🌈

Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.

Any setting other than none is supported only on armv8-m.main or later.

The default is to generate code without branch protection or return address signing.

---

🌸 AVR Options 🌸

These options are defined for AVR implementations:

• -mmcu=mcu — specify the AVR instruction set architecture (ISA) or device type. The default for this option is "avr2".

The following AVR devices and ISAs are supported. Note: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs (<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 — the "XMEGA" girlies with more than 8 KiB and up to 64 KiB of program memory 💅 here's the roster, write it down bestie:

mcu = "atxmega8e5", "atxmega16a4", "atxmega16a4u",
"atxmega16c4", "atxmega16d4", "atxmega16e5", "atxmega32a4",
"atxmega32a4u", "atxmega32c3", "atxmega32c4", "atxmega32d3",
"atxmega32d4", "atxmega32e5", "avr64da28", "avr64da28s",
"avr64da32", "avr64da32s", "avr64da48", "avr64da48s",
"avr64da64", "avr64da64s", "avr64db28", "avr64db32",
"avr64db48", "avr64db64", "avr64dd14", "avr64dd20", "avr64dd28",
"avr64dd32", "avr64du28", "avr64du32", "avr64ea28", "avr64ea32",
"avr64ea48", "avr64sd28", "avr64sd32", "avr64sd48".

• avrxmega3 — "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space (iconic honestly). the cast list:

mcu = "attiny202", "attiny204", "attiny212", "attiny214",
"attiny402", "attiny404", "attiny406", "attiny412", "attiny414",
"attiny416", "attiny416auto", "attiny417", "attiny424",
"attiny426", "attiny427", "attiny804", "attiny806", "attiny807",
"attiny814", "attiny816", "attiny817", "attiny824", "attiny826",
"attiny827", "attiny1604", "attiny1606", "attiny1607",
"attiny1614", "attiny1616", "attiny1617", "attiny1624",
"attiny1626", "attiny1627", "attiny3214", "attiny3216",
"attiny3217", "attiny3224", "attiny3226", "attiny3227",
"atmega808", "atmega809", "atmega1608", "atmega1609",
"atmega3208", "atmega3209", "atmega4808", "atmega4809",
"avr16dd14", "avr16dd20", "avr16dd28", "avr16dd32", "avr16du14",
"avr16du20", "avr16du28", "avr16du32", "avr16ea28", "avr16ea32",
"avr16ea48", "avr16eb14", "avr16eb20", "avr16eb28", "avr16eb32",
"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" devices with more than 64 KiB and up to 128 KiB of program memory. the roster:

mcu = "atxmega64a3", "atxmega64a3u",
"atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3",
"atxmega64d3", "atxmega64d4", "avr128da28", "avr128da28s",
"avr128da32", "avr128da32s", "avr128da48", "avr128da48s",
"avr128da64", "avr128da64s", "avr128db28", "avr128db32",
"avr128db48", "avr128db64".

• avrxmega5 — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. short list, big energy:

mcu = "atxmega64a1", "atxmega64a1u".

• avrxmega6 — "XMEGA" devices with more than 128 KiB of program memory. look at this lineup:

mcu = "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3",
"atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3",
"atxmega256a3b", "atxmega256a3bu", "atxmega256a3u",
"atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".

• avrxmega7 — "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. the whole roster is three names, and they ate:

mcu = "atxmega128a1", "atxmega128a1u",
"atxmega128a4u".

• avrtiny — "Reduced Tiny" Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory. tiny but mighty 🫶

mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny102",
"attiny104", "attiny20", "attiny40".

• avr1 — this ISA is implemented by the minimal AVR core and is supported for assembler only. like, assembler. that's it.

mcu = "attiny11", "attiny12", "attiny15",
"attiny28", "at90s1200".

---

• -mabsdata — assume that all data in static storage can be reached by LDS / STS instructions. heads up: this only does anything on reduced Tiny devices, like the ATtiny40. see also the "absdata" AVR Variable Attributes,variable attribute 💖
• -maccumulate-args — stack up the outgoing function arguments and grab/release the stack space they need once, in the function's prologue/epilogue. without this option, outgoing arguments get pushed before calling a function and popped afterwards.

and popping them after the call can be expensive on AVR, so accumulating the stack space can get you smaller executables, because the args don't have to be pulled off the stack after a function call.

it can shrink code for functions that make several calls to functions that take their arguments on the stack — think 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 get expanded as calls to the appropriate subroutines. code size is smaller. periodt ✨
• -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 a specific value for bits is even allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the exact same deal applies to the default values of these options.
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction that GNU Binutils supports. if this option is on, you can still switch the feature off for individual ISRs with the AVR Function Attributes,,"no_gccisr" function attribute. it's turned on per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils supports PR21683 (https://sourceware.org/PR21683).
• -mint8 — assume int is an 8-bit integer. this messes with the sizes of every type: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and a long long is 4 bytes. please note: this option does not conform to the C standards — but it does give you smaller code size. your call bestie 🙈
• -mmain-is-OS_task — don't save registers in main. the effect is the same as slapping the AVR Function Attributes,,"OS_task" attribute onto main. it activates per default if optimization is on.
• -mno-interrupts — the generated code is not compatible with hardware interrupts. code size is smaller. trade-offs, we love them 💀
• -mrelax — try to swap a CALL / JMP instruction for the shorter RCALL / RJMP one if that's applicable. setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

jump relaxing happens in the linker, because jump offsets aren't known until the code is actually located. so the assembler code the compiler spits out is identical, but the instructions in the executable may differ from the ones 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 — put the .rodata sections for read-only data in RAM, or in program memory, respectively. for most devices there's genuinely no choice, and this option acts more like an assertion than anything else.

since v14 and for the AVR64* and AVR128* devices, .rodata lives in flash memory per default — as long as the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is there. and in that case, -mrodata-in-ram takes you back to the old layout with .rodata in RAM.

• -mstrict-X — use the address register X the way the hardware actually proposes. meaning X is only used in indirect, post-increment or pre-decrement addressing. no cap.

without this option, X may be used the same way as Y or Z, and that gets emulated with extra instructions. for example, loading a value with X+const addressing where a small non-negative const < 64 goes into 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 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 handing the sub-processes — compiler proper, assembler and linker — the appropriate command line options. that means you have to supply your own private device specs file via -specs=path-to-specs-file. and there's no more need for the option -mmcu=mcu.

this option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point at a directory containing 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 the __vector prefix. enabled by default ✨

---

🌈 "EIND" and Devices with More Than 128 Ki Bytes of Flash 🌈

okay so, pointers in the implementation are 16 bits wide. the address of a function or label is represented as a word address, so indirect jumps and calls can target any code address in the range of 64 Ki words.

and in order to facilitate indirect jumps on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND that serves as the most significant part of the target address whenever the EICALL or EIJMP instructions are used 💜

Okay so indirect jumps and calls on these lil devices get handled by the compiler like this, and bestie there are some limits, so pay attention 💅

• The compiler never sets EIND.
• The compiler uses EIND implicitly inside EICALL/EIJMP instructions, or it might read EIND directly to emulate an indirect call/jump via a RET instruction. iykyk 🫶
• The compiler assumes EIND never changes during the startup code or during the application. Like, specifically: EIND is not saved/restored in function or interrupt service routine prologue/epilogue. So don't come for it. 🙈
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines ✨ So the indirect call/jump jumps to such a stub, and the stub contains a direct jump to the desired address.
• Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 😭 Not a vibe.
• The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (https://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. Slay 🎀

#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. 💖

---

🌈 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 the 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. Iconic behavior, honestly 👑

• 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. Periodt. 💅

---

✨ 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.

🌸 okay that's the whole macro menu bestie, each one is basically a lil yes/no vibe check for your device 🌸

• "__AVR_PM_BASE_ADDRESS__=addr" — okay so some devices can read straight out of flash memory using LD* instructions 💫 and that flash shows up in the data address space at an offset of __AVR_PM_BASE_ADDRESS__. If this macro isn't defined, bestie, that feature just isn't there. If it is defined, the address space is linear and you do not need to shove .rodata into RAM. The default linker description file handles all of this, and rn it's available for avrtiny and avrxmega3. And the even more convenient part? No need to mess with address spaces like __flash or features like attribute progmem and pgm_read_*. ✨
• "__AVR_HAVE_FLMAP__" — this macro gets defined provided the following conditions are met:
  • The device has the NVMCTRL_CTRLB.FLMAP bitfield. This applies to the AVR64* and AVR128* devices. 🎀
  • It's not known at assembler-time which emulation will be used.

This implies the compiler was configured with GNU Binutils that implement PR31124 (https://sourceware.org/PR31124).

• "__AVR_RODATA_IN_RAM__" — this macro is undefined when the code is compiled for a core architecture.

When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data is located in RAM; and defined to 0, otherwise.

• "__WITH_AVRLIBC__" — the compiler is configured to be used together with AVR-Libc 💜. See the --with-avrlibc configure option.
• "__HAVE_DOUBLE_MULTILIB__" — defined if -mdouble= acts as a multilib option.
• "__HAVE_DOUBLE32__" / "__HAVE_DOUBLE64__" — defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=. 🦋
• "__DEFAULT_DOUBLE__" — the size in bits of double if -mdouble= is not set. To test the layout of double in a program, use the built-in macro __SIZEOF_DOUBLE__. (iykyk 😌)
• "__HAVE_LONG_DOUBLE32__" / "__HAVE_LONG_DOUBLE64__" / "__HAVE_LONG_DOUBLE_MULTILIB__" / "__DEFAULT_LONG_DOUBLE__" — same as above, but for long double instead of double. 💅
• "__WITH_DOUBLE_COMPARISON__" — reflects the --with-double-comparison={tristate|bool|libf7} configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to 2 or 3.
• "__WITH_LIBF7_LIBGCC__" / "__WITH_LIBF7_MATH__" / "__WITH_LIBF7_MATH_SYMBOLS__" — reflects the --with-libf7={libgcc|math|math-symbols} configure option (https://gcc.gnu.org/install/configure.html#avr).

---

🎀 AVR Internal Options 🎀

So these next ones are used internally by the compiler, and also to communicate between device specs files and the compiler proper. You genuinely do not need to set these by hand, and especially not — they are not optimization options, bestie. 👑 Using these options the wrong way may lead to sub-optimal or wrong code, and we do not want that energy. They're documented for completeness, and in order to get a better understanding of device specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) files.

• -mn-flash=num — assume that the flash memory has a size of num times 64 KiB. This determines which __flashN address spaces are available. 🔥
• -mflmap — the device has the FLMAP bit field located in special function register NVMCTRL_CTRLB.
• -mrmw — assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT. ⭐
• -mshort-calls — 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. Currently, cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.

The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for 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.

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. 🫶
• -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. 💅
• -mno-leaf-id-shared-library — do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns.
• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option.
• -msep-data — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section. 🌈
• -mno-sep-data — generate code that assumes that the data segment follows the text segment. This is the default.
• -mlong-calls / -mno-long-calls — tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. 👑

• -mfast-fp — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance.
• -minline-plt — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic.
• -mmulticore — build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision]. 🔥

This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main function of Core B should be named as coreb_main.

If this option is not used, the single-core application programming model is used.

• -mcorea — build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with -mmulticore. 💖

Okay so we are STILL in the machine-dependent options zone and honestly the BFIN section is almost done, hang in there bestie 💖

• -mcoreb — builds a standalone application for Core B of BF561 when you're using the one-application-per-core programming model. Proper start files and link scripts get used to support Core B, and the macro "__BFIN_COREB" gets defined. When you use this one, "coreb_main" should be used instead of "main". No cap. And heads up: this option can only be used in conjunction with -mmulticore. 🫶
• -msdram — builds a standalone application for SDRAM. Proper start files and link scripts get used to put the application into SDRAM, and the macro "__BFIN_SDRAM" is defined. The loader should initialize SDRAM before loading the application, iykyk.
• -micplb — assumes that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.

---

✨ C6X Options ✨

• -march=name — this specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default. 🍓
• -msim — choose startup files and linker script suitable for the simulator.
• -msdata=default — puts small global and static data in the .neardata section, which is pointed to by register B14. Puts small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Puts small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const.
• -msdata=all — puts all data, not just small objects, into the sections reserved for small data, and uses addressing relative to the B14 register to access them.
• -msdata=none — makes no use of the sections reserved for small data, and uses absolute addresses to access all data. Puts all initialized global and static data in the .fardata section, and all uninitialized data in the .far section. Puts all constant data into the .const section.

---

👑 CRIS Options 👑

These options are defined specifically for the CRIS ports, bestie.

• -march=architecture-type
• -mcpu=architecture-type

Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0. 💅

• -mtune=architecture-type — tunes everything applicable about the generated code to architecture-type, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type.
• -mmax-stack-frame=n — warns 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

Works around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is disabled by default. 💀

• -mpdebug — enables 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 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 (the no- options) arrange (eliminate arrangements) 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 which set up the stack frame are omitted and no return instructions or return sequences are generated in the code.

⚠️ 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. Like, fr fr, go look at it with your own eyes. 😭

• -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 passes 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, periodt.

• -march=arch — specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810.
• -mcpu=cpu — specify the C-SKY target processor. Valid values for cpu are: ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t, ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht, ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1, ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1, ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1, ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft, ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft, ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and ck810ftv. 🔥 (yes that's the whole list, we do not skip a single one in this house)
• -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. Do not mix them, bestie, it will not slay.

• -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, enables generation of double-precision float instructions. This is the default except when compiling for CK803.

• -mfdivdu
• -mno-fdivdu

When -mhard-float is in effect, enables 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. (okay yes, this description is literally identical to -mcp's, that's not me copy-pasting by accident, that's genuinely what the page says 💀 iconic behavior honestly)
• -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. Big yikes energy, proceed with caution. 😭

---

• -msim — links in libsemi.a, which plays nice with the simulator. ELF compiler only, bestie — don't come crying to me if you try this elsewhere. 💅

---

✨ Darwin Options ✨

Okay so these are the flags for every architecture that runs the Darwin operating system. 🍎 Two different worlds here, and you need to know which one you're in:

• FSF GCC on Darwin does not make "fat" object files. It makes one object file, for the single architecture GCC was built to target. Periodt.
• Apple's GCC on Darwin does make "fat" files when you throw multiple -arch options at it — it does that by running the compiler or linker a bunch of times and gluing the results together with lipo.

The subtype of the file it makes (think ppc7400, ppc970, i686) is decided by whatever flags tell it which ISA GCC is targeting — stuff like -mcpu or -march. Want to override that? -force_cpusubtype_ALL has entered the chat. 👑

And the Darwin tools? They all react differently to an ISA mismatch, which is honestly giving chaos:

• The assembler, as, only lets you use instructions that are valid for the subtype of the file it's generating — so no sneaking 64-bit instructions into a ppc750 object file. 🙅‍♀️
• The linker for shared libraries, /usr/bin/libtool, fails and prints an error if you ask it to build a shared library with a less restrictive subtype than its input files (like trying to shove a ppc970 object file into a ppc7400 library).
• The linker for executables, ld, just quietly gives the executable the most restrictive subtype out of all its input files. silent but deadly 💀

---

• -Fdir — adds the framework directory dir to the head of the list of directories searched for header files. These get interleaved with whatever you specified via -I, and everything is scanned left-to-right. 📚

Okay so a framework directory is a directory that has frameworks inside it. A framework is a directory that has a Headers and/or PrivateHeaders directory sitting directly inside it, and whose name ends in .framework. The name of the framework is just the name of that directory minus the .framework part. Headers that belong to the framework live in one of those two directories, with Headers getting searched first. 🌸

A subframework is a framework directory living inside a framework's Frameworks directory. Includes of subframework headers can only show up in a header of a framework that contains that subframework, or in a sibling subframework header. Two subframeworks are siblings if they sit in the same framework. A subframework should never have the same name as a framework — you get a warning if you violate that. And as of right now, a subframework cannot have subframeworks; maybe one day the mechanism gets extended to support it, but not today. 🙈

The standard frameworks live in /System/Library/Frameworks and /Library/Frameworks. An example include looks like:

#include <Framework/header.h>

where Framework is the name of the framework and header.h is found in the PrivateHeaders or Headers directory.

• -iframeworkdir — same energy as -F, except the directory gets treated as a system directory. The main glow-up difference between -iframework and -F is that with -iframework the compiler does not warn you about constructs inside header files it found via dir. 🫶 Heads up though: this one's only valid for the C family of languages.
• -gused — emits debugging info for symbols that are used. For the stabs debugging format, this turns on -feliminate-unused-debug-symbols. And it's ON by default, bestie. ✨
• -gfull — emits debugging info for all symbols and types. Everything. The whole closet. 👑
• -fconstant-cfstrings — this one is just an alias for -mconstant-cfstrings. Same girl, different name. 💅
• -mconstant-cfstrings — when the NeXT runtime is in use (which is the default on these systems), this overrides any -fconstant-string-class setting and makes @"..." literals get laid out as constant CoreFoundation strings. 🎀
• -mmacosx-version-min=version — sets version as the earliest version of MacOS X this executable will run on. Typical supported values include 12, 10.12, and 10.5.8.

Now if the compiler was built to use the system's headers by default, then this option defaults to whatever system version the compiler itself is running on. Otherwise, the default is to make choices compatible with as many systems and code bases as possible. 🌈

• -mkernel — turns on kernel development mode. This option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti wherever applicable. It also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets. 🛠️
• -mone-byte-bool — overrides the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when you're compiling for Darwin/PowerPC and 1 when you're compiling for Darwin/x86 — so this option does literally nothing on x86.

⚠️ WARNING: The -mone-byte-bool switch makes GCC generate code that is not binary compatible with code generated without that switch. You may have to recompile every other module in the program, including the system libraries. Only use this switch to conform to a non-default data model. Don't say I didn't warn you. 💀

• -mfix-and-continue, -ffix-and-continue, -findirect-data — these generate code suitable for fast turnaround development, like letting GDB dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility. 🔄
• -all_load — loads all members of static archive libraries. See man ld(1) for more info. 📖
• -arch_errors_fatal — makes errors about files having the wrong architecture fatal. 🔥
• -bind_at_load — marks the output file so the dynamic linker binds all undefined references when the file is loaded or launched.
• -bundle — produces a Mach-o bundle format file. See man ld(1) for more info. 🎁
• -bundle_loader executable — specifies the executable that will load the build output file being linked. See man ld(1) for more info.
• -dynamiclib — when you pass this, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command. 🏗️
• -force_cpusubtype_ALL — makes GCC's output file have the ALL subtype instead of one controlled by the -mcpu or -march option. 👑
• -nodefaultrpaths — don't add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath gets added by default unless the user adds -nodefaultrpaths to the link line. And heads up: run paths are needed (and therefore enforced) to build on macOS version 10.11 or later. 🛣️

These next ones? All besties that get passed straight to the Darwin linker — the Darwin linker man page describes them in detail, so go read it if you want the lore:

• -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
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded

---

💜 DEC Alpha Options 💜

These -m options are defined for the DEC Alpha implementations, babes:

• -mno-soft-float, -msoft-float — use (or do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to carry out floating-point operations. Unless those routines get replaced by routines that emulate the floating-point operations, or get compiled in a way that calls such emulation routines, these routines issue floating-point operations. 🧮 So if you're compiling for an Alpha with no floating-point operations, you must make sure the library is built so it doesn't call them.

Also note: Alpha implementations without floating-point operations are still required to have floating-point registers. Wild, I know. 💀

• -mfp-reg, -mno-fp-regs — generate code that uses (or doesn't use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set isn't used, floating-point operands get passed in integer registers as if they were integers, and floating-point results get passed in $0 instead of $f0. That's a non-standard calling sequence, so any function with a floating-point argument or return value that gets called by code compiled with -mno-fp-regs must also be compiled with that option. No exceptions bestie. 🙅‍♀️

A typical use of this option is building a kernel that doesn't use any floating-point registers — and therefore doesn't need to save and restore them. 🌸

• -mieee — the Alpha architecture implements floating-point hardware optimized for maximum performance. It's mostly compliant with the IEEE floating-point standard. But for full compliance, you need software assistance. ✨ This option generates fully IEEE-compliant code, except that the inexact-flag is not maintained (see below). If this option is on, the preprocessor macro _IEEE_FP gets defined during compilation. The resulting code is less efficient, but it can correctly support denormalized numbers and exceptional IEEE values like not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.

DEBIAN SPECIFIC: This option is on by default for alpha-linux-gnu, unless -ffinite-math-only (which is part of the -ffast-math set) is specified — because the software functions in the GNU libc math libraries generate denormalized numbers, NaNs, and infs, and all of those will cause a program to SIGFPE when it tries to use the results without -mieee. 😭 So like, don't skip it.

• -mieee-with-inexact — bestie, this one's like -mieee but turned ALL the way up 💅 like, the generated code also keeps the IEEE inexact-flag maintained. Flip this on and your generated code implements fully-compliant IEEE math. On top of _IEEE_FP, you also get _IEEE_FP_EXACT defined as a preprocessor macro. Heads up though: on some Alpha implementations the resulting code can run significantly slower than the default code, so since there's basically very little code out there that even depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this one -ieee_with_inexact. iykyk 🫶
• *-mfp-trap-mode=trap-mode* — this option controls which floating-point-related traps get enabled. Other Alpha compilers call this -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 — on top of the traps enabled by n, underflow traps are enabled too.
  • su — like u, but the instructions are marked to be safe for software completion (see the Alpha architecture manual for the details, bestie 📖).
  • sui — like su, but inexact traps are enabled as well.
• *-mfp-rounding-mode=rounding-mode* — selects the IEEE rounding mode. Other Alpha compilers call this -fprm rounding-mode. The rounding-mode can be one of:
  • n — normal IEEE rounding mode ✨ floating-point numbers get rounded towards the nearest machine number, or towards the even machine number in case of a tie.
  • m — round towards minus infinity.
  • c — chopped rounding mode 🪓 floating-point numbers get rounded towards zero.
  • d — dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls the rounding mode actually in effect. The C library initializes this register for rounding towards plus infinity. So, unless your program modifies the fpcr, d corresponds to round towards plus infinity.
• *-mtrap-precision=trap-precision* — so in the Alpha architecture, floating-point traps are imprecise 😭 which means without software assistance it is literally impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that assists operating system trap handlers in figuring out the exact location that caused a floating-point trap. Depending on what your app needs, you can pick different levels of precision:
  • p — program precision. This option is the default, and it means a trap handler can only identify which program caused a floating-point exception.
  • f — function precision. The trap handler can determine the function that caused a floating-point exception.
  • i — instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception. 👑

Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

• -mieee-conformant — this option marks the generated code as IEEE conformant. But bestie, listen: you must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.

Not me being dramatic but seriously, don't skip the pairing requirement on that one 💀

• -mbuild-constants — normally GCC looks at a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time.

Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six 🫣).

You typically use this option to build a shared library dynamic loader. Itself a shared library, it must relocate itself in memory before it can find the variables and constants in its own data segment.

• -mbwx
• -mno-bwx
• -mcix
• -mno-cix
• -mfix
• -mno-fix
• -mmax
• -mno-max — indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via the -mcpu= option, or that of the CPU on which GCC was built if none is specified. ✨
• -mfloat-vax
• -mfloat-ieee — generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.
• -mexplicit-relocs
• -mno-explicit-relocs — older Alpha assemblers gave you no way to generate symbol relocations except via assembler macros, and using those macros doesn't allow optimal instruction scheduling 😤 GNU binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it's built and sets the default accordingly.
• -msmall-data
• -mlarge-data — when -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller get placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but it lets the variables be directly accessed via a single instruction 💅

The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

• -msmall-text
• -mlarge-text — when -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1. 🔥

The default is -mlarge-text.

• *-mcpu=cpu_type* — set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you don't specify a processor type, GCC defaults to the processor on which the compiler was built.

Supported values for cpu_type are

• ev4
• ev45
• 21064 — schedules as an EV4 and has no instruction set extensions.
• ev5
• 21164 — schedules as an EV5 and has no instruction set extensions.
• ev56
• 21164a — schedules as an EV5 and supports the BWX extension.
• pca56
• 21164pc
• 21164PC — schedules as an EV5 and supports the BWX and MAX extensions.
• ev6
• 21264 — schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
• ev67
• 21264a — schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions. 👑

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

• *-mtune=cpu_type* — set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed. 🎀

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.

• *-mmemory-latency=time* — sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

Valid options for time are

• number — a decimal number representing clock cycles.
• L1
• L2
• L3
• main — the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

---

🧚 eBPF Options 🧚

• *-mframe-limit=bytes* — this specifies the hard limit for frame sizes, in bytes. Currently, the value that can be specified should be less than or equal to 32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted. 🍓
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default. ✨
• -mjmpext
• -mno-jmpext — 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* — this specifies which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The default is v4.

Supported values for version are:

• v1 — the first stable eBPF ISA with no special features or extensions.
• v2 — supports the jump extensions, as in -mjmpext.
• v3 — all features of v2, plus:

• - 32-bit jump operations, as in -mjmp32
  • - 32-bit ALU operations, as in -malu32
  • - general atomic operations, as in -mv3-atomics
• v4 — bestie, v4 is giving everything v3 has 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 — turns on BPF Compile Once - Run Everywhere (CO-RE) support 💫. It requires -gbtf and is implied by it too, so like, they're besties fr fr.
• -mno-co-re — turns BPF Compile Once - Run Everywhere (CO-RE) support off. Ngl, CO-RE support is on by default when you're generating BTF debug info for the BPF target. Periodt.
• -mxbpf — generates code for an expanded version of BPF that relaxes some of the restrictions the BPF architecture imposes:
  • - Save and restore callee-saved registers at function entry and exit, respectively.
• -masm=dialect — outputs assembly instructions using the eBPF-selected dialect 🦋. Default is pseudoc.

The supported values for dialect are, slay:

• 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 shall always be expanded inline ✨. Ops dealing with sizes bigger than this threshold would have to be implemented with a library call instead of being expanded inline — but since BPF doesn't allow libcalls, going over this threshold means you get a compile-time error. 💀 Default is 1024 bytes.

---

🌸 FR30 Options 🌸

These options are defined specifically for the FR30 port.

• -msmall-model — uses the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. Lowkey a big assumption, just sayin.
• -mno-lsim — assumes runtime support has already been provided, so there's no need to put the simulator library (libsim.a) on the linker command line.

---

🧚 FT32 Options 🧚

These options are defined specifically for the FT32 port.

• -msim — says the program is gonna run on the simulator. This causes an alternate runtime startup and library to be 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 — enables Local Register Allocation. It's still experimental for FT32, so by default the compiler uses standard reload.
• -mnodiv — do not use div and mod instructions.
• -mft32b — enables 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 — selects the FDPIC ABI, which uses function descriptors to represent pointers to functions 👑. With no PIC/PIE-related options present, 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 — enables inlining of PLT entries in function calls to functions that aren't known to bind locally. It has no effect without -mfdpic. It's enabled by default if you're optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option like -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 — enables the use of GPREL relocations in the FDPIC ABI for data that's 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 needing a GOT entry for the referenced symbol, so it's more likely to be a win. If it isn't, -mno-gprel-ro can be used to disable it. (The way that it's conditional on your flags… iconic.)
• -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. Ngl 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 lets the functions 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 to generate code for. The 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 the Android platform. This is the default on *-*-linux-*android* targets. 📱

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. And finally, this option causes the preprocessor macro __ANDROID__ to be defined. Main character energy, honestly.

• -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 💖

okay bestie, these -m options are the ones defined for the H8/300 implementations — tiny embedded vibes, big personality ✨

• -mrelax — shortens some address references at link time when it can; it uses the linker option -relax to do it.
• -mh — generates code for the H8/300H. 💅
• -ms — generates code for the H8S.
• -mn — generates code for the H8S and H8/300H in normal mode. 🫶 heads up: you must use this switch either with -mh or with -ms, no solo runs.
• -ms2600 — generates code for the H8S/2600. this one must be used with -ms.
• -mexr — extended registers get stored on the stack before execution of a function with the monitor attribute. the default option is -mexr. this option is valid only for H8S targets.
• -mno-exr — extended registers are not stored on the stack before execution of a function with the monitor attribute. the default option is -mno-exr. (yes bestie, they both say default, we don't make the rules, we just report them 💀) this option is valid only for H8S targets.
• -mint32 — makes int data 32 bits by default.
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. the default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries; -malign-300 causes them to be aligned on 2-byte boundaries instead. this option has no effect on the H8/300. 🙈

---

🌈 HPPA Options 🌈

and now for the HPPA family of computers — these -m options are the whole lineup:

• -march=architecture-type — generate code for the specified architecture. the choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. pop over to /usr/lib/sched.models on an HP-UX system to figure out the proper architecture option for your machine. 📋 code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around. it's giving one-way street energy.
• -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0 — synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively. same slay, shorter name 💅
• -matomic-libcalls — generate libcalls for atomic loads and stores when sync libcalls are disabled. this option is enabled by default. it only affects the generation of atomic libcalls by the HPPA backend. both the sync and libatomic libcall implementations use locking, so as a result, processor stores are not atomic with respect to other atomic operations. processor loads up to DImode are atomic with respect to other atomic operations, provided they are implemented as a single access. the PA-RISC architecture does not support any atomic operations in hardware except for the ldcw instruction — so all atomic support is implemented using sync and atomic libcalls. sync libcall support lives in libgcc.a. atomic libcall support lives in libatomic. this option generates __atomic_exchange calls for atomic stores, and it also provides special handling for atomic DImode accesses on 32-bit targets. 🧚
• -mbig-switch — does nothing. preserved for backward compatibility. iconic nap behavior tbh 😴
• -mcaller-copies — the caller copies function arguments passed by hidden reference. use this option 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. worth it for the OpenMP girlies 🫶
• -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 does faster indirect calls. 🔥 but warning: this option does not work in the presence of shared libraries or nested functions. periodt.
• -mfixed-range=register-range — generate code treating the given register range as fixed registers. a fixed register is one the register allocator cannot use. this is useful when compiling kernel code. a register range is specified as two registers separated by a dash, and you can specify multiple register ranges 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 is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. you can print the linker GCC uses with 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 is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. you can print the linker GCC uses with 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. ngl, note this makes symbolic debugging impossible. it also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs. 💀
• -mlong-calls — generate code that uses long call sequences. this ensures a call is always able to reach linker generated stubs. the default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used. the limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes. distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker. it is normally not desirable to use this option as it degrades performance — but 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 allows GCC to generate faster indirect calls and use unscaled index address modes. such code is suitable for level 0 PA systems and kernels. 👑
• -mordered — assume memory references are ordered and barriers are not needed.
• -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems.
• -mschedule=cpu-type — schedule code according to the constraints for the machine type cpu-type. the choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. the default scheduling is 8000. ✨
• -msio — generate the predefine _SIO for server IO. the default is -mwsio. this generates the predefines __hp9000s700, __hp9000s700__ and _WSIO, for workstation IO. these options are available under HP-UX and HI-UX.
• -msoft-float — generate output containing library calls for floating point. ⚠️ warning: the requisite libraries are not available for all HPPA targets. normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation — you must make your own arrangements to provide suitable library functions for cross-compilation. -msoft-float changes the calling convention in the output file, therefore it is only useful if you compile all of a program with this option. in particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. no half measures bestie 💅
• -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. thus, extreme care is needed in using this option. 🚨 library code that is 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 — so here's the ✨situation✨ bestie: the HP-UX implementation of setlocale in libc has a dependency on libdld.sl. And there is no 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 the linker 👑

---

🦅 IA-64 Options 🦅

These are the -m options defined for the Intel IA-64 architecture.

• -mbig-endian — generate code for a big-endian target. This is the default for HP-UX.
• -mlittle-endian — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
• -mgnu-as — 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. The result is not position independent code, and it violates the IA-64 ABI 🚨.
• -mvolatile-asm-stop — generate a stop bit immediately before and after volatile asm statements.
• -mno-volatile-asm-stop — don't generate a stop bit immediately before and after volatile asm statements.
• -mregister-names — generate in, loc, and out register names for the stacked registers. This may make assembler output more readable 🌸.
• -mno-register-names — don't generate in, loc, and out register names for the stacked registers.
• -mno-sdata — disable optimizations that use the small data section. This may be useful for working around optimizer bugs 🐛.
• -msdata — enable optimizations that use the small data section. This may be useful for working around optimizer bugs.
• -mconstant-gp — generate code that uses a single constant global pointer value. This is useful when compiling kernel code 👑.
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code.
• -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm.
• -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm.
• -mno-inline-float-divide — do not generate inline code for divides of floating-point values.
• -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm.
• -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm.
• -mno-inline-int-divide — do not generate inline code for divides of integer values.
• -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm.
• -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm.
• -mno-inline-sqrt — do not generate inline code for sqrt.
• -mfused-madd — generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions.
• -mno-fused-madd — don't generate code that uses the fused multiply/add or multiply/subtract instructions.
• -mno-dwarf2-asm — don't generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.
• -mdwarf2-asm — do generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler.
• -mearly-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but it does not always do so 😭.
• -mno-early-stop-bits — don't allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit.
• -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 — generate code for a 32-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. These are HP-UX specific flags.
• -mlp64 — generate code for a 64-bit environment. 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 — disable 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-br-data-spec — enable 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 — enable 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-ar-data-spec — disable data speculative scheduling after reload. The default setting is enabled.
• -mno-sched-control-spec — disable 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-control-spec — enable 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 — enable 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.
• -mno-sched-br-in-data-spec — disable speculative scheduling of the instructions that are dependent on the data speculative loads before reload.
• -msched-ar-in-data-spec — enable 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.
• -mno-sched-ar-in-data-spec — disable speculative scheduling of the instructions that are dependent on the data speculative loads after reload.
• -msched-in-control-spec — enable 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-in-control-spec — disable speculative scheduling of the instructions that are dependent on the control speculative loads.
• -mno-sched-prefer-non-data-spec-insns — disable this: 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.
• -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 — disable this: 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.
• -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 — disable this: 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-count-spec-in-critical-path — if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.
• -msched-spec-ldc — use a simple data speculation check. This option is on by default.
• -msched-control-spec-ldc — use a simple check for control speculation. This option is on by default.
• -msched-stop-bits-after-every-cycle — place a stop bit after every cycle when scheduling. This option is on by default.
• -msched-fp-mem-deps-zero-cost — assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.
• -msel-sched-dont-check-control-spec — generate checks for control speculation in selective scheduling. This flag is disabled by default.
• -msched-max-memory-insns=max-insns — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1.
• -msched-max-memory-insns-hard-limit — makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.

---

🐄 LM32 Options 🐄

These -m options are defined for the LatticeMico32 architecture:

• -mbarrel-shift-enabled — enable barrel-shift instructions.
• -mdivide-enabled — enable divide and modulus instructions.
• -mmultiply-enabled — enable multiply instructions.
• -msign-extend-enabled — enable sign extend instructions.
• -muser-enabled — enable user-defined instructions.

---

🐉 LoongArch Options 🐉

These command-line options are defined for LoongArch targets:

• -march=arch-type — 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 — 64-bit general-purpose registers + 32/64-bit floating-point registers for passing params around. The data model is LP64: int is 32 bits, but long int and pointers are a whole 64 bits. 💅
• lp64f — 64-bit general-purpose registers + 32-bit floating-point registers for parameter passing. Data model is still LP64 (int = 32 bits, long int and pointers = 64 bits).
• lp64s — 64-bit general-purpose registers and no floating-point registers at all for parameter passing. Yep, LP64 again: int 32 bits, long int and pointers 64 bits.
• -mfpu=fpu-type — generate code for the FPU type you name, which can be any of these:
  • 64 — allow hardware floating-point instructions for both 32-bit and 64-bit operations. 🌸
  • 32 — allow hardware floating-point instructions for 32-bit operations only.
  • none, 0 — nope, zero hardware floating-point instructions, bestie. Prevent them entirely. 🙈
• -msimd=simd-type — turns on generating LoongArch SIMD instructions, for vectorization and via builtin functions. Pick one value:
  • lasx — enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).
  • lsx — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
  • none — no LoongArch SIMD instruction may be generated, periodt.
• -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, so 32-bit floating-point registers are fair game for parameter passing. Also potentially an ABI shaker: this option may change the target ABI.
• -mdouble-float — forces -mfpu=64, letting 32/64-bit floating-point registers handle parameter passing. And yes, this one may change the target ABI too.
• -mlasx, -mno-lasx, -mlsx, -mno-lsx — these nudge the scope of the SIMD extensions (none / LSX / LASX) the compiler is allowed to use for code generation, incrementally. Catch this: enabling LASX with -mlasx automatically enables LSX, and disabling LSX with -mno-lsx automatically disables LASX. These are driver-only options, and they act on the final -msimd configuration state, making incremental changes in the exact order they show up on the GCC driver's command line — which is how the final / canonicalized -msimd option that gets passed to the compiler proper is derived. 🔥
• -mbranch-cost=n — set the cost of branches to roughly n instructions.
• -mcheck-zero-division, -mno-check-zero-divison — trap (or don't trap) on integer division by zero. Default is -mcheck-zero-division when you're at -O0 or -Og, and -mno-check-zero-division at every other optimization level.
• -mcond-move-int, -mno-cond-move-int — conditional moves for integral data in general-purpose registers, enabled (or disabled). Default is -mcond-move-int.
• -mcond-move-float, -mno-cond-move-float — conditional moves for floating-point registers, enabled (or disabled). Default is -mcond-move-float.
• -mmemcpy, -mno-memcpy — force (or don't force) the use of memcpy for non-trivial block moves. Default is -mno-memcpy, which lets GCC inline most constant-sized copies. Setting the optimization level to -Os also forces memcpy, but -mno-memcpy may override that behavior if you explicitly specify it — regardless of the order these options sit 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. Default is -mno-strict-align.
• -msmall-data-limit=number — shove global and static data smaller than number bytes into a special section (on some targets, anyway). Default value is 0.
• -mmax-inline-memcpy-size=n — inline all block moves (like calls to memcpy or structure copies) that are less than or equal to n bytes. Default value of n is 1024. 🫶
• -mcmodel=code-model — set the code model to one of these:
  • tiny-static — (Not implemented yet)
  • tiny — (Not implemented yet)
  • normal — the text segment must live within 128MB addressing space, and the data segment must live within 2GB addressing space.
  • medium — both the text segment and the data segment must be within 2GB addressing space.
  • large — (Not implemented yet)
  • extreme — this mode puts no limit on the size of the code segment or the data segment. But ⚠️ -mcmodel=extreme is incompatible with -fplt and/or -mexplicit-relocs=none.

The default code model is normal. ✨

• -mexplicit-relocs=style — pick when assembler relocation operators get used for symbolic addresses. The alternative is assembler macros, which can limit instruction scheduling but allow linker relaxation. So: with -mexplicit-relocs=none the assembler macros are always used; with -mexplicit-relocs=always the assembler relocation operators are always used; with -mexplicit-relocs=auto the compiler uses relocation operators wherever linker relaxation is impossible (better code quality) and macros everywhere else. The default value is decided by the assembler capability detected at GCC build-time plus the setting of -mrelax: -mexplicit-relocs=none if the assembler doesn't support relocation operators at all, -mexplicit-relocs=always if it supports them but -mrelax isn't enabled, and -mexplicit-relocs=auto if it supports them and -mrelax is enabled. 🧠
• -mexplicit-relocs — an alias of -mexplicit-relocs=always, kept around for backward compatibility.
• -mno-explicit-relocs — an alias of -mexplicit-relocs=none, also just for backward compatibility.
• -mdirect-extern-access, -mno-direct-extern-access — do not use, or use, GOT to access external symbols. Default is -mno-direct-extern-access: GOT is used for external symbols with default visibility, but not for other external symbols.

With -mdirect-extern-access, GOT isn't used and all external symbols are PC-relatively addressed. It's only suitable for environments where no dynamic link happens — firmwares, OS kernels, executables linked with -static or -static-pie. And 💀 -mdirect-extern-access is not compatible with -fPIC or -fpic.

• -mrelax, -mno-relax — take (or don't take) advantage of linker relaxations. If -mpass-mrelax-to-as is enabled, this option also gets handed off to the assembler. The default is determined at GCC build-time by detecting the corresponding assembler support: -mrelax if the assembler supports both the -mrelax option and conditional branch relaxation (and it's required, or the .align directives and conditional branch instructions in the assembly code GCC outputs may get rejected by the assembler because of a relocation overflow), otherwise -mno-relax. 😤
• -mpass-mrelax-to-as, -mno-pass-mrelax-to-as — pass (or don't pass) the -mrelax or -mno-relax option along to the assembler. The default is figured out at GCC build-time by detecting the corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the -mrelax option, otherwise -mno-pass-mrelax-to-as. This one is mostly for debugging, or for interop with assemblers other than the build-time one. 🔧
• -mrecip — this enables the reciprocal estimate and reciprocal square root estimate instructions, with extra Newton-Raphson steps for precision, instead of doing a divide (or square root and divide) for floating-point arguments. These instructions only get generated when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Off by default. Before you use this, make sure your target CPU actually 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 can drop by up to 2 ulp — as in, the inverse of 1.0 equals 0.99999994. 😭
• -mrecip=opt — controls which reciprocal estimate instructions are allowed. opt is a comma-separated list of options, and any of them may be preceded by ! to invert it:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the approximation for scalar division.
  • vec-div — enable the approximation for vectorized division.
  • sqrt — enable the approximation for scalar square root.
  • vec-sqrt — enable the approximation for vectorized square root.
  • 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 (or don't use) frecipe.{s/d} and frsqrte.{s/d} instructions. When built with -march=la664, it's enabled by default. The default is -mno-frecipe.
• -mdiv32, -mno-div32 — use (or don't use) div.w[u] and mod.w[u] instructions with input that isn't sign-extended. When built with -march=la664, it's enabled by default. The default is -mno-div32.
• -mlam-bh, -mno-lam-bh — use (or don't use) am{swap/add}[_db].{b/h} instructions. When built with -march=la664, it's enabled by default. The default is -mno-lam-bh.
• -mlamcas, -mno-lamcas — use (or don't use) amcas[_db].{b/h/w/d} instructions. When built with -march=la664, it's enabled by default. The default is -mno-lamcas.
• -mld-seq-sa, -mno-ld-seq-sa — whether a same-address load-load barrier (dbar 0x700) is needed. When built with -march=la664, it's enabled by default. The default is -mno-ld-seq-sa, meaning the load-load barrier is needed. 🌈
• -mtls-dialect=opt — 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. that's it, that's the whole thing 💅

---

• --param loongarch-vect-unroll-limit=n — okay so the vectorizer is gonna look at all the tuning info it's got and decide whether unrolling the main vectorized loop is even worth it, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll that main loop. Default is six. 🌀

---

🌸 M32C Options 🌸

• -mcpu=name — picks the CPU you're generating code for. name can be r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series. 🎯
• -msim — says the program is gonna run on the simulator. Which means an alternate runtime library gets linked in, one that supports stuff like file I/O. ⚠️ You must not use this option when you're generating programs that'll run on real hardware — for those you have to provide your own runtime library for whatever I/O functions you need. no cap.
• -memregs=number — tells GCC how many memory-based pseudo-registers to use during code generation. These pseudo-registers act like real registers, so there's a tradeoff: GCC's ability to fit the code into the available registers vs. the performance penalty of hitting memory instead of registers. 🧠 Note that all modules 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:

• -m32r2 — Generate code for the M32R/2. ✨
• -m32rx — Generate code for the M32R/X. ✨
• -m32r — Generate code for the M32R. This is the default. 👑
• -mmodel=small — Assume all objects live in the lower 16MB of memory (so their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

The addressability of a particular object can be set with the model attribute.

• -mmodel=medium — Assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.
• -mmodel=large — Assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler generates the much slower seth/add3/jl instruction sequence). 🐌
• -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 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 in debugging programs. 🔍
• -malign-loops — Align all loops to a 32-byte boundary.
• -mno-align-loops — Do not enforce a 32-byte alignment for loops. This is the default.
• -missue-rate=number — Issue number instructions per cycle. number can only be 1 or 2.
• -mbranch-cost=number — number can only be 1 or 2. If it's 1, then branches are preferred over conditional code; if it's 2, then the opposite applies.
• -mflush-trap=number — Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15 inclusive.
• -mno-flush-trap — Specifies that the cache cannot be flushed by using a trap.
• -mflush-func=name — Specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but a function call is only used if a trap is not available.
• -mno-flush-func — Indicates that there is no OS function for flushing the cache.

---

💜 M680x0 Options 💜

These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below. 🛠️

• -march=arch — Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and isac.

GCC defines a macro __mcfarch__ whenever it is generating code for a ColdFire target. The arch in this macro is one of the -march arguments given above.

When used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture.

• -mcpu=cpu — Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
  • Family 51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
  • Family 5206 : 5202 5204 5206
  • Family 5206e : 5206e
  • Family 5208 : 5207 5208
  • Family 5211a : 5210a 5211a
  • Family 5213 : 5211 5212 5213
  • Family 5216 : 5214 5216
  • Family 52235 : 52230 52231 52232 52233 52234 52235
  • Family 5225 : 5224 5225
  • Family 52259 : 52252 52254 52255 52256 52258 52259
  • Family 5235 : 5232 5233 5234 5235 523x
  • Family 5249 : 5249
  • Family 5250 : 5250
  • Family 5271 : 5270 5271
  • Family 5272 : 5272
  • Family 5275 : 5274 5275
  • Family 5282 : 5280 5281 5282 528x
  • Family 53017 : 53011 53012 53013 53014 53015 53016 53017
  • Family 5307 : 5307
  • Family 5329 : 5327 5328 5329 532x
  • Family 5373 : 5372 5373 537x
  • Family 5407 : 5407
  • Family 5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 🚫

GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family 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 — makes output for a member of the ColdFire 528X fam 💫 heads up tho, this one's deprecated now — use the equivalent -mcpu=528x instead.
• -m5307 — output for a ColdFire 5307 CPU. Also deprecated bestie, the glow-up is -mcpu=5307.
• -m5407 — output for a ColdFire 5407 CPU. Yep you guessed it: deprecated, pls use -mcpu=5407.
• -mcfv4e — output for a ColdFire V4e family CPU (like 547x/548x) 🌀 and yes it pulls in hardware floating-point instructions. Equivalent to -mcpu=547x, and also deprecated in favor of that one.
• -m68020-40 — output for a 68040, but without touching any of the new instructions. The tea ☕: this gives you code that runs ✨relatively efficiently✨ on a 68020/68881, a 68030, or a 68040. The generated code does use the 68881 instructions, which get emulated on the 68040.

Equivalent to -march=68020 -mtune=68020-40.

• -m68020-60 — output for a 68060, again with none of the new instructions. Same deal: code that runs relatively efficiently on a 68020/68881, a 68030, or a 68040. The generated code does use the 68881 instructions, which are emulated on the 68060.

Equivalent to -march=68020 -mtune=68020-60.

• -mhard-float, -m68881 — generate floating-point instructions 💅 this is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro "__HAVE_68881__" on M680x0 targets and "__mcffpu__" on ColdFire targets.
• -msoft-float — do not generate floating-point instructions; use library calls instead. Default for 68000, 68010, and 68832 targets, and 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". Bonus: parameters passed on the stack also get aligned to a 16-bit boundary, even on targets whose API mandates promotion to 32-bit.
• -mno-short — do not treat type "int" as 16 bits wide. This is the default 🫶
• -mnobitfield, -mno-bitfield — don't 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're using a configuration designed for a 68020 💖
• -mrtd — use a different function-calling convention where functions taking a fixed number of arguments return with the "rtd" instruction, which pops their arguments while returning. Saves one instruction in the caller since there's no need to pop the arguments there. Slay, but—

🚨 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 on top of that, 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 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.

• -mpcrel — 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 with no virtual memory management. 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 ✨ allows for execute-in-place and shared libraries in an environment with no virtual memory management. 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 just 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 isn't needed; -fPIC suffices 💅

GCC normally uses a single instruction to load values from the GOT. While that's relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger makes the linker 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, bestie 💜

• -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 speed up 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 the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. Bestie, this one rocks the startup file crt1.o and sets the start address of the program to 0x800.
  • bootstrap — for applications that get loaded by a bootloader. This model uses the startup file crt2.o, which does not contain a processor reset vector handler. Perfect when you want control on a processor reset to go to the bootloader instead of your app. slay.
  • novectors — for applications that genuinely don't need any of the MicroBlaze vectors. Lowkey useful for apps running inside a monitoring application. This model uses crt3.o as its startup file.

Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model. 👑 (deprecated, so like… move on bestie)

• -mpic-data-is-text-relative — assume that the displacement between the text and data segments is fixed at static link time. This lets data be referenced by offset from the start of the text address instead of from the GOT, since PC-relative addressing is not supported. ✨

---

💖 MIPS Options 💖

• -EB — generate big-endian code.
• -EL — generate little-endian code. This is the default for mips*el-*-* configurations.
• *-march=arch* — generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor.

The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6.

The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e, loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp.

The special value from-abi picks 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 *n*f1_1.

GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000.

Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and 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. it's giving ✨main character processor✨

-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. 🧪 like a science experiment, not your vibe
• -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. 🫶 an oldie but a goodie
• -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.

-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. 😭 worth it tho

okay so a little linker tea before we get into the rest of these MIPS flags: some linkers can create multiple GOTs. If yours is that girl, you should only reach for -mxgot when a single object file accesses more than 64k's worth of GOT entries. Lowkey almost nobody does. 🙈

Also — heads up bestie — these options have no effect unless GCC is generating position independent code. Periodt.

---

• -mgp32 — assume general-purpose registers are 32 bits wide.
• -mgp64 — assume general-purpose registers are 64 bits wide.
• -mfp32 — assume floating-point registers are 32 bits wide.
• -mfp64 — assume floating-point registers are 64 bits wide.
• -mfpxx — do not assume the width of floating-point registers at all. mystery era. 🦋
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead. ✨
• -mno-float — equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations.

fr fr be careful with this one — it's presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats). If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. 💀

• -msingle-float — assume the floating-point coprocessor only supports single-precision operations.
• -mdouble-float — assume the floating-point coprocessor supports double-precision operations. This is the default. 👑
• -modd-spreg, -mno-odd-spreg — enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, -mno-odd-spreg is set by default.
• -mabs=2008, -mabs=legacy — these two control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the "abs.fmt" and "neg.fmt" machine instructions.

By default, or when -mabs=legacy is used, the legacy treatment is selected. In that case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead — unless the -ffinite-math-only option has also been specified.

The -mabs=2008 option selects the IEEE 754-2008 treatment. In that case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations. slay 💅

• -mnan=2008, -mnan=legacy — these control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.

The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.

The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0. (yes they literally just swapped, iconic behavior 🎀)

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

• -mllsc, -mno-llsc — use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details. 📖

• -mdsp, -mno-dsp — use (do not use) revision 1 of the MIPS DSP ASE.

This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.

• -mdspr2, -mno-dspr2 — use (do not use) revision 2 of the MIPS DSP ASE.

This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2.

• -msmartmips, -mno-smartmips — use (do not use) the MIPS SmartMIPS ASE.
• -mpaired-single, -mno-paired-single — use (do not use) paired-single floating-point instructions.

this option requires hardware floating-point support to be enabled. no hardware, no vibes. 🔥

• -mdmx, -mno-mdmx — use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.
• -mips3d, -mno-mips3d — use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single. ✨
• -mmicromips, -mno-micromips — generate (do not generate) microMIPS code.

MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and "nomicromips" attributes.

• -mmt, -mno-mt — use (do not use) MT Multithreading instructions.
• -mmcu, -mno-mcu — use (do not use) the MIPS MCU ASE instructions.
• -meva, -mno-eva — use (do not use) the MIPS Enhanced Virtual Addressing instructions.
• -mvirt, -mno-virt — use (do not use) the MIPS Virtualization (VZ) instructions.
• -mxpa, -mno-xpa — use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
• -mcrc, -mno-crc — use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
• -mginv, -mno-ginv — use (do not use) the MIPS Global INValidate (GINV) instructions.
• -mloongson-mmi, -mno-loongson-mmi — use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
• -mloongson-ext, -mno-loongson-ext — use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
• -mloongson-ext2, -mno-loongson-ext2 — use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
• -mlong64 — force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined.
• -mlong32 — force "long", "int", and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller.

• -msym32, -mno-sym32 — assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses. ⚡
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.

The default -G option depends on the configuration.

• -mlocal-sdata, -mno-local-sdata — extend (do not extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the default for all configurations.

If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. 🫶

• -mextern-sdata, -mno-extern-sdata — assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations.

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 (do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.)

-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.

• -membedded-data, -mno-embedded-data — allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. 🌸
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data.
• -mcode-readable= setting — specify whether GCC may generate code that reads from executable sections. There are three possible settings:
• -mcode-readable=yes — okay so instructions can just, like, freely access executable sections. No restrictions, no vibes check. This is the default setting ✨
• -mcode-readable=pcrel — so MIPS16 PC-relative load instructions can touch executable sections, but every other instruction has to stay out of there 💅 This one's your girl on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It's also a slay on processors that can be configured with a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM 🎀
• -mcode-readable=no — nope, instructions must NOT access executable sections, periodt. Useful on targets configured with a dual instruction/data SRAM interface that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM 🫶

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• -msplit-addresses / -mno-split-addresses — turn on (or off) the use of the %hi() and %lo() assembler relocation operators. Ngl this one got superseded by -mexplicit-relocs, it's just still hanging around for backwards compatibility 💀
• -mexplicit-relocs=none, -mexplicit-relocs=base, -mexplicit-relocs=pcrel, -mexplicit-relocs / -mno-explicit-relocs — these control whether explicit relocs (like %gp_rel) get used or not. The default depends on which version of GAS was around when GCC itself got built 👑

So: 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 😭

Default is -mcheck-zero-division, bestie. Don't say I didn't warn you.

• -mdivide-traps / -mdivide-breaks — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Traps give you smaller code, but they're only supported on MIPS II and later. Also, 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 breaks instead.

The default is usually -mdivide-traps, but that can be overridden at configure time with --with-divide=breaks. And if you wanna completely turn off divide-by-zero checks, that's -mno-check-zero-division, iykyk.

• -mload-store-pairs / -mno-load-store-pairs — enable (or disable) an optimization that pairs consecutive load or store instructions so load/store bonding can happen 🎀 It's enabled by default, but fr fr it only actually does anything when the selected architecture is known to support bonding.
• -mstrict-align, -mno-strict-align, -munaligned-access, -mno-unaligned-access — disable (or enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, either by hardware or by trap&emulate. So -mstrict-align may be needed by kernel 😭 Heads up: -munaligned-access and -mno-unaligned-access are obsoleted — they're only there for backward-compatible vibes.
• -mmemcpy / -mno-memcpy — force (or don't force) the use of memcpy for non-trivial block moves. Default is -mno-memcpy, which lets GCC inline most constant-sized copies ✨
• -mlong-calls / -mno-long-calls — disable (or don't disable) use of the jal instruction. Calling functions with jal is more efficient, but it requires the caller and callee to live in the same 256 megabyte segment 👑

This option has zero effect on abicalls code. Default is -mno-long-calls.

• -mmad / -mno-mad — enable (or disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA 💅
• -mimadd / -mno-imadd — enable (or disable) use of the madd and msub integer instructions. Default is -mimadd on architectures that support madd and msub — except for the 74k architecture, where it was found to generate slower code 🙈
• -mfused-madd / -mno-fused-madd — enable (or disable) use of the floating-point multiply-accumulate instructions, when they're available. 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 😳 That 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 it's assembling them 🧚
• -mfix-24k / -mno-fix-24k — work around the 24K E48 errata (that's lost data on stores during refill, in case you were wondering 💀). The workarounds are implemented by the assembler, not 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 💀💀

You can only use this option 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 — don't even try 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. Always schedule a nop instruction there instead 🫶 The short loop bug, under certain conditions, causes loops to execute only once or twice, because of a hardware bug in the R5900 chip. And yep, 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 — those errata are handled by the assembler, not by GCC itself.

• -mfix-vr4130 — work around the VR4130 mflo/mfhi errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead ✨
• -mfix-sb1 / -mno-fix-sb1 — work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and "F2" floating-point errata.) 🎀

---

• -mr10k-cache-barrier=setting — this one decides whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. Buckle up, bestie, this needs lore 💅

Like many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts those instructions if the predicted outcome was 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. 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 that 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. No pressure or anything 🙈

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 — ok so this one picks which function gets called to flush the I-cache and D-cache. If it's called, that function has to take the same args as the usual _flush_func bestie: the address of the memory range being flushed, the size of that range, and the number 3 (which means flush both caches, periodt). The default is whatever target GCC was configured for, but usually it's either _flush_func or __cpu_flush 💅
• -mno-flush-func — the vibe-flip version: don't call any such function at all. No flush, no function, no problem (well, maybe a problem, but that's between you and your caches).
• -mbranch-cost=num — sets the cost of branches to roughly num "simple" instructions. Heads up though, this cost is only a heuristic and it is not guaranteed to produce consistent results across releases 😭. A zero cost redundantly selects the default, which is based on the -mtune setting. It's giving "vibes-based scheduling", ngl.
• -mbranch-likely — turn on use of Branch Likely instructions, regardless of what the default is for the architecture you picked.
• -mno-branch-likely — turn them off, same deal, regardless of the arch default. By default Branch Likely instructions may be generated if the selected architecture supports them. ✨ The exception: MIPS32 and MIPS64 architectures, and the processors that implement them. For those, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. So if you were hoping for them there... no 💀
• -mcompact-branches=never — this option controls which form of branches get generated, and this particular value ensures that compact branch instructions will never be generated. None. Zero. Absolutely not.
• -mcompact-branches=optimal — the other form of the same control knob, and this one is the default. It 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 is chosen if one is available. iykyk 🌸
• -mcompact-branches=always — the third flavor. Ensures a compact branch instruction will be generated if one is available for MIPS Release 6 onwards. If a compact branch instruction is not available (or you're pre-R6), a delay slot form of the branch gets used instead. Also, if it's used for MIPS16/microMIPS targets, it will be just ignored for now — the behaviour for MIPS16/microMIPS may change in future, since they do have some compact branch instructions. So watch this space 👀
• -mfp-exceptions — specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.
• -mno-fp-exceptions — the off-switch for FP exceptions. Real talk, this changes scheduling: on the SB-1, for instance, if FP exceptions are disabled and we're emitting 64-bit code, we can use both FP pipes. Otherwise we can only use one FP pipe. Two pipes vs one pipe is a whole different girl 💅
• -mvr4130-align — the VR4130 pipeline is two-way superscalar, but it can only issue two instructions together if the first one is 8-byte aligned. Turn this on and GCC aligns pairs of instructions that it thinks should execute in parallel. It only has an effect when optimizing for the VR4130, and it normally makes code faster but also bigger (the classic tradeoff, we've all been there). It is enabled by default at optimization level -O3 🔥
• -mno-vr4130-align — disables that alignment behaviour. Your code stays smaller, but good luck with your parallel instruction pairs bestie.
• -msynci — enables generation of synci instructions on architectures that support them. Those synci instructions (when enabled) get generated when __builtin___clear_cache is compiled.
• -mno-synci — disables that generation. Note: 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 — tries to turn PIC calls that are normally dispatched via register $25 into direct calls. Only possible if the linker can resolve the destination at link time and the destination is within range for a direct call. This 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.
• -mno-relax-pic-calls — turns that off in the compiler... except, plot twist: with -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone, without any help from the compiler. Teamwork makes the dream work 🫶
• -mmcount-ra-address — 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 these:
  • Returning the new address in register $31.
  • Storing the new address in *ra-address, if ra-address is nonnull.
• -mno-mcount-ra-address — don't emit that code. And this one is the default.
• -mframe-header-opt — enables 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 suppresses the allocation of the frame header if it can be determined that it's unused. Free stack space, we love to see it 💖
• -mno-frame-header-opt — disables it. Heads up: this optimization is off by default at all optimization levels. All of them. Even the big ones.
• -mlxc1-sxc1 — when applicable, enables generation of the lwxc1, swxc1, ldxc1, and sdxc1 instructions. Enabled by default ✨
• -mno-lxc1-sxc1 — when applicable, disables generation of those same instructions.
• -mmadd4 — when applicable, enables generation of the 4-operand madd.s, madd.d and related instructions. Enabled by default 👑
• -mno-madd4 — when applicable, disables generation of those 4-operand bad boys.

---

✨ MMIX Options ✨

These options are defined for the MMIX:

• -mlibfuncs — specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. All of them. Every size. Registers only 💅
• -mno-libfuncs — don't specify that.
• -mepsilon — generate floating-point comparison instructions that compare with respect to the rE epsilon register.
• -mno-epsilon — don't generate those comparisons.
• -mabi=mmixware — generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up.
• -mabi=gnu — the other ABI option 💫 this one uses global registers $231 and up, as opposed to the mmixware ABI above.
• -mzero-extend — when reading data from memory in sizes shorter than 64 bits, use zero-extending load instructions by default, rather than sign-extending ones.
• -mno-zero-extend — do not use zero-extending load instructions by default for those short reads; sign-extending ones it is.
• -mknuthdiv — makes the result of a division that yields a remainder have the same sign as the divisor. Fr fr, this one's the interesting case 🌸
• -mno-knuthdiv — the default, where the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, and the latter (this default one) is the one that's almost exclusively used.
• -mtoplevel-symbols — prepend a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
• -mno-toplevel-symbols — do not prepend that :.
• -melf — generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
• -mbranch-predict — use the probable-branch instructions when static branch prediction indicates a probable branch.
• -mno-branch-predict — don't use them.
• -mbase-addresses — 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. Which means a program that uses lots of static data may require -mno-base-addresses 😭
• -mno-base-addresses — do not generate code that uses base addresses.
• -msingle-exit — force generated code to have a single exit point in each function.
• -mno-single-exit — do not force that.

---

✨ 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 (and honestly, dodging bugs is a whole mood).
• -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.

⚠️ Real talk: this option makes symbolic debugging impossible. So like, choose your battles 💀

• -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. Big-endian besties rise up 👑
• -mel — generate little-endian code. 🦋
• -mmul.x — ok bestie, this one says generate mul.x and umul.x instructions. It's the default for moxiebox-*-* configurations. slay ✨
• -mno-crt0 — do NOT link in the C run-time initialization object file. that's it. 🚫

💖 MSP430 Options 💖

okay so these cuties are defined for the MSP430: 🦋

• -masm-hex — forces your assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. it's giving hex realness 💅
• -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 the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA.

In addition an MCU-specific linker script is added to the linker command line. The script's name is the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol "__XXX__" and cause the linker to search for a script called xxx.ld.

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond those that have been hard-coded.

GCC searches for the devices.csv file using the following methods in the given precedence order, where the first method takes precedence over the second which takes precedence over the third.

• Include path specified with -I and -L — devices.csv 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 — Define the value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is found, this directory will also be registered as an include path, and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying -I and -L on the command line. 💖
• The msp430-elf{,bare}/include/devices directory — Finally, GCC will examine msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if the user has created it and copied devices.csv there, then the MCU data 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 — enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default. 👑
• -mcpu= — specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is deprecated. The -mmcu= option should be used to select the ISA. ⚠️ deprecated bestie, don't get attached.
• -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 cannot be done until the final link. 🧚
• mhwmult= — describes the type of hardware multiply supported by the target. Accepted values are:
  • none — no hardware multiply. 🙅
  • 16bit — the original 16-bit-only multiply supported by early MCUs.
  • 32bit — the 16/32-bit multiply supported by later MCUs.
  • f5series — the 16/32-bit multiply supported by F5-series MCUs.
  • auto — can also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified or if the MCU name is not recognized then no hardware multiply support is assumed. "auto" is the default setting. 💖

Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code.

The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. 🔥

• -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. tiny but mighty ✨
• -mtiny-printf — enable reduced code size printf and puts library functions. The tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications.

Support for streams has been removed and the string to be printed will always be sent to stdout via the write syscall. The string is not buffered before it is sent to write.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io. 🍓

• -mmax-inline-shift= — this option takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an mspabi helper function is used instead. The default value is 4.

This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead. math girlie moment 💅

• -mcode-region=, -mdata-region= — these options tell the compiler where to place functions and data that do not have one of the "lower", "upper", "either" or "section" attributes. Possible values are "lower", "upper", "either" or "any". The first three behave like the corresponding attribute. The fourth possible value - "any" - is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (.text, .data, etc) to the memory regions. 🗺️
• -msilicon-errata= — this option passes on a request to 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 is not found or there are problem 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 be still 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 — literally the local vibe but louder 💅 it also generates GP-relative accesses for small data objects that are external, weak, or common. But bestie, listen: use this and you must make sure every single part of your program (libraries included, no exceptions) is compiled with the same -G setting. Same setting or it's not giving what you think it's giving.
• data — generate GP-relative accesses for all data objects in the program, periodt. The catch? The entire data and BSS segments of your program have to fit in 64K of memory, and you must use an appropriate linker script to allocate them within the addressable range of the global pointer. 64K. That's it. That's the budget. 😭
• all — GP-relative addresses for function pointers as well as data pointers. Now the entire text, data, and BSS segments of your program must fit in 64K of memory, and again you must use an appropriate linker script to allocate them within the addressable range of the global pointer. Iconic ambition, tiny address space ✨

-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.

The default is -mgpopt except when -fpic or -fPIC is specified to generate position-independent code. Note that the Nios II ABI does not permit GP-relative accesses from shared libraries. So yeah — shared libraries, not the move here, no cap.

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. Basically the offset is short and it can't reach 😭

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✨ -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. Fancy, fr fr.

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. No defaults, no conventions, you're on your own bestie 🫶

---

Endianness o'clock 🌈

• -mel, -meb — generate little-endian (default) or big-endian (experimental) code, respectively. Note the experimental on the big-endian one. She's trying her best 🧚

Architecture era 🦋

• -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.

Cache behaviour, lowkey 👀

• -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.
• -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.

Divide era 💀

• -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.

The multiply/divide hardware menu 🎀

• -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".

BMX and CDX ✨

• -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.

---

👑 Custom instructions: the whole saga 👑

• -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. Glow up your FPU, bestie 🌸

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 choreography but make it registers 💅

• 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.

---

🎀 -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. Order does not matter here, the specific one wins, periodt.

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. Yeah, fph2 is the odd one out and we love her for it 🌈

---

These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

• -mhal — links with HAL BSP, bestie ✨. It suppresses linking with the GCC-provided C runtime startup and termination code, and it's basically always used alongside -msys-crt0= so you can point at where the HAL BSP's alternate startup code lives.
• -msmallc — links with a limited version of the C library, -lsmallc, instead of Newlib. Smaller vibes only 🫶.
• *-msys-crt0=startfile* — startfile is the file name of the startfile (crt0) to use when linking. Heads up: this one is only useful together with -mhal, no cap.
• *-msys-lib=systemlib* — systemlib is the library name of the library that provides the low-level system calls the C library needs, like "read" and "write". Typically you use this to link with a library that a HAL BSP hands you.

---

👑 Nvidia PTX Options 👑

These options are defined for Nvidia PTX:

• -m64 — literally ignored, but preserved for backward compatibility 💀. Only the 64-bit ABI is supported, so there's nothing else to pick anyway.
• *-march=architecture-string* — generates code for the specified PTX ISA target architecture (e.g. sm_35). Valid architecture strings are sm_30, sm_35, sm_53, sm_70, sm_75 and sm_80. The default depends on how the compiler was configured, so peek at --with-arch.

This option sets the value of the preprocessor macro "__PTX_SM__" — for instance, for sm_35 it has the value 350. slay ✨

• *-misa=architecture-string* — alias of -march=. Same energy, different name 💅.
• *-march-map=architecture-string* — picks the closest available -march= value that is not more capable. So for -march-map=sm_50 you get -march=sm_35, and for -march-map=sm_53 you get -march=sm_53.
• *-mptx=version-string* — generates code for the specified PTX ISA version (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 — links in code for a __main kernel. This is for stand-alone execution instead of offloading execution, iykyk.
• -moptimize — applies partitioned execution optimizations. This is the default whenever any level of optimization is selected 👑.
• -msoft-stack — generates code that does not use ".local" memory directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This unlocks 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 OpenMP offloading uses, but the option is exposed on its own for the purpose of testing the compiler 🧪. If you want code that's actually suitable for linking into programs using OpenMP offloading, use -mgomp.
• -muniform-simt — switches to the code generation variant that lets all threads in each warp execute, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff the 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. The shared memory array "int __nvptx_uni[]" stores all-zeros or all-ones bitmasks for each warp, indicating the 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. iconic, honestly 🔥
• -mgomp — generates code for use in OpenMP offloading: it enables the -msoft-stack and -muniform-simt options, and selects the corresponding multilib variant.

---

🌸 OpenRISC Options 🌸

These options are defined for OpenRISC:

• *-mboard=name* — configures a board-specific runtime. This gets passed to the linker for newlib board library linking. The default is "or1ksim".
• -mnewlib — this option is ignored; it's for compatibility purposes only 🙈. It used to select linker and preprocessor options for use with newlib.
• -msoft-div, -mhard-div — selects software or hardware divide ("l.div", "l.divu") instructions. The default is hardware divide.
• -msoft-mul, -mhard-mul — selects software or hardware multiply ("l.mul", "l.muli") instructions. The default is hardware multiply.
• -msoft-float, -mhard-float — selects software or hardware for floating point operations. The default is software.
• -mdouble-float — when -mhard-float is selected, this enables generation of double-precision floating point instructions. By default, functions from libgcc are used to perform double-precision floating point operations.
• -munordered-float — when -mhard-float is selected, this enables generation of unordered floating point compare and set flag ("lf.sfun*") instructions. By default, functions from libgcc are used to perform unordered floating point compare and set flag operations.
• -mcmov — enables generation of conditional move ("l.cmov") instructions. By default the equivalent gets generated using set and branch.
• -mror — enables generation of rotate right ("l.ror") instructions. By default, functions from libgcc are used to perform rotate right operations.
• -mrori — enables generation of rotate right with immediate ("l.rori") instructions. By default, functions from libgcc are used to perform rotate right with immediate operations.
• -msext — enables generation of sign extension ("l.ext*") instructions. By default memory loads are used to perform sign extension.
• -msfimm — enables generation of compare and set flag with immediate ("l.sf*i") instructions. By default extra instructions get generated to store the immediate to a register first.
• -mshftimm — enables generation of shift with immediate ("l.srai", "l.srli", "l.slli") instructions. By default extra instructions get generated to store the immediate to a register first.
• -mcmodel=small — generates OpenRISC code for the small model: the GOT is limited to 64k. This is the default model 💖.
• -mcmodel=large — generates 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 — uses hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported, just so you know 😭.) Implies -m45.
• -msoft-float — do not use hardware floating point.
• -mac0 — returns floating-point results in ac0 (fr0 in Unix assembler syntax).
• -mno-ac0 — returns floating-point results in memory. This is the default.
• -m40 — generates code for a PDP-11/40. Implies -msoft-float -mno-split.
• -m45 — generates code for a PDP-11/45. This is the default.
• -m10 — generates 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 — links with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but some standard C runtime features get 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, fr fr.

The minimal startup code would not pass "argc" and "argv" arguments to "main", so the latter must be declared as "int main (void)". This is already the norm for most firmware projects, so lowkey you're fine.

• *-mmcu=mcu* — specifies 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 — makes GCC pass the --no-relax command-line option to the linker instead of the --relax option.
• -mloop — allows (or does not allow) GCC to use the LOOP instruction.
• *-mabi=variant* — specifies 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* — sets the cost of branches to roughly n instructions.
• -mplt, -mno-plt — when you're generating PIC code, this is the vibe check on whether PLTs are allowed or not. If you're not doing PIC? It literally ignores you, no notes. Default is -mplt. 💅
• -mabi=ABI-string — sets the integer and floating-point calling convention, bestie. Your <italic>ABI-string</italic> is two things stitched together: how big integer types are, and which registers floating-point types get to ride in. So for example, -march=rv64ifd -mabi=lp64d means long and pointers are 64-bit (which implicitly makes int 32-bit, iykyk), and floating-point values up to 64 bits wide get passed in F registers. ✨

Now contrast that with -march=rv64ifd -mabi=lp64f, which still lets the compiler generate code using the F and D extensions, but only lets floating-point values up to 32 bits long be passed in registers. Or -march=rv64ifd -mabi=lp64, where literally no floating-point arguments get passed in registers at all.

The default here is system dependent — so if you want a specific calling convention, say it explicitly, don't be shy. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. 😌

Some calling conventions are genuinely 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. Like, the math isn't mathing. 💀 There's 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 don't get attached.

• -mfdiv, -mno-fdiv — do or don't use hardware floating-point divide and square root instructions. This needs the F or D extensions for floating-point registers. Default is to use them if the specified architecture has these instructions. 🎀
• -mdiv, -mno-div — do or don't use hardware instructions for integer division. This one requires the M extension. Default is to use them if the specified architecture has these instructions.
• -misa-spec=ISA-spec-string — pick which version of the RISC-V Unprivileged (formerly User-Level) ISA specification you want your code to conform to. 🌸 Your 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, no shouting. Examples include rv64i, rv32g, rv32e, and rv32imaf. Also, a special value help (-march=help) is accepted to list all supported extensions. 🦋

The syntax of the ISA string is defined as follows:

• "The string must start with rv32 or rv64, followed by" i, e, or g, referred to as the base ISA.
• "The subsequent part of the string is a list of extension names. Extension" names can be categorized as multi-letter (e.g. zba) and single-letter (e.g. v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores.
• "An underscore can appear anywhere after the base ISA. It has no specific" effect but is used to improve readability and can act as a separator.
• "Extension names may include an optional version number, following the" syntax <major>p<minor> or <major>, (e.g. m2p1 or m2).

Supported extension are listed below: 💖

• i — supported version: 2.0, 2.1 — Base integer extension.
• e — supported version: 2.0 — Reduced base integer extension.
• g — supported version: - — General-purpose computing base extension, g will expand to i, m, a, f, d, zicsr and zifencei.
• m — supported version: 2.0 — Integer multiplication and division extension.
• a — supported version: 2.0, 2.1 — Atomic extension.
• f — supported version: 2.0, 2.2 — Single-precision floating-point extension.
• d — supported version: 2.0, 2.2 — Double-precision floating-point extension.
• c — supported version: 2.0 — Compressed extension.
• h — supported version: 1.0 — Hypervisor extension.
• v — supported version: 1.0 — Vector extension.
• zicsr — supported version: 2.0 — Control and status register access extension.
• zifencei — supported version: 2.0 — Instruction-fetch fence extension.
• zicond — supported version: 1.0 — Integer conditional operations extension.
• za64rs — supported version: 1.0 — Reservation set size of 64 bytes.
• za128rs — supported version: 1.0 — Reservation set size of 128 bytes.
• zawrs — supported version: 1.0 — Wait-on-reservation-set extension.
• zba — supported version: 1.0 — Address calculation extension.
• zbb — supported version: 1.0 — Basic bit manipulation extension.
• zbc — supported version: 1.0 — Carry-less multiplication extension.
• zbs — supported version: 1.0 — Single-bit operation extension.
• zfinx — supported version: 1.0 — Single-precision floating-point in integer registers extension.
• zdinx — supported version: 1.0 — Double-precision floating-point in integer registers extension.
• zhinx — supported version: 1.0 — Half-precision floating-point in integer registers extension.
• zhinxmin — supported version: 1.0 — Minimal half-precision floating-point in integer registers extension.
• zbkb — supported version: 1.0 — Cryptography bit-manipulation extension.
• zbkc — supported version: 1.0 — Cryptography carry-less multiply extension.
• zbkx — supported version: 1.0 — Cryptography crossbar permutation extension.
• zkne — supported version: 1.0 — AES Encryption extension.
• zknd — supported version: 1.0 — AES Decryption extension.
• zknh — supported version: 1.0 — Hash function extension.
• zkr — supported version: 1.0 — Entropy source extension.
• zksed — supported version: 1.0 — SM4 block cipher extension.
• zksh — supported version: 1.0 — SM3 hash function extension.
• zkt — supported version: 1.0 — Data independent execution latency extension.
• zk — supported version: 1.0 — Standard scalar cryptography extension.
• zkn — supported version: 1.0 — NIST algorithm suite extension.
• zks — supported version: 1.0 — ShangMi algorithm suite extension.
• zihintntl — supported version: 1.0 — Non-temporal locality hints extension.
• zihintpause — supported version: 1.0 — Pause hint extension.
• zicboz — supported version: 1.0 — Cache-block zero extension.
• zicbom — supported version: 1.0 — Cache-block management extension.
• zicbop — supported version: 1.0 — Cache-block prefetch extension.
• zic64b — supported version: 1.0 — Cache block size isf 64 bytes.
• ziccamoa — supported version: 1.0 — Main memory supports all atomics in A.
• ziccif — supported version: 1.0 — Main memory supports instruction fetch with atomicity requirement.
• zicclsm — supported version: 1.0 — Main memory supports misaligned loads/stores.
• ziccrse — supported version: 1.0 — Main memory supports forward progress on LR/SC sequences.
• zicntr — supported version: 2.0 — Standard extension for base counters and timers.
• zihpm — supported version: 2.0 — Standard extension for hardware performance counters.
• ztso — supported version: 1.0 — Total store ordering extension.
• zve32x — supported version: 1.0 — Vector extensions for embedded processors.
• zve32f — supported version: 1.0 — Vector extensions for embedded processors.
• zve64x — supported version: 1.0 — Vector extensions for embedded processors.
• zve64f — supported version: 1.0 — Vector extensions for embedded processors.
• zve64d — supported version: 1.0 — Vector extensions for embedded processors.
• zvl32b — supported version: 1.0 — Minimum vector length standard extensions
• zvl64b — supported version: 1.0 — Minimum vector length standard extensions
• zvl128b — supported version: 1.0 — Minimum vector length standard extensions
• zvl256b — supported version: 1.0 — Minimum vector length standard extensions
• zvl512b — supported version: 1.0 — Minimum vector length standard extensions
• zvl1024b — supported version: 1.0 — Minimum vector length standard extensions
• zvl2048b — supported version: 1.0 — Minimum vector length standard extensions
• zvl4096b — supported version: 1.0 — Minimum vector length standard extensions
• zvbb — supported version: 1.0 — Vector basic bit-manipulation extension.
• zvbc — supported version: 1.0 — Vector carryless multiplication extension.
• zvkb — supported version: 1.0 — Vector cryptography bit-manipulation extension.
• zvkg — supported version: 1.0 — Vector GCM/GMAC extension.
• zvkned — supported version: 1.0 — Vector AES block cipher extension.
• zvknha — supported version: 1.0 — Vector SHA-2 secure hash extension.
• zvknhb — supported version: 1.0 — Vector SHA-2 secure hash extension.
• zvksed — supported version: 1.0 — Vector SM4 Block Cipher extension.
• zvksh — supported version: 1.0 — Vector SM3 Secure Hash extension.
• zvkn — supported version: 1.0 — Vector NIST Algorithm Suite extension, zvkn will expand to zvkned, zvknhb, zvkb and zvkt.
• zvknc — supported version: 1.0 — Vector NIST Algorithm Suite with carryless multiply extension, zvknc will expand to zvkn and zvbc.
• zvkng — 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 (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

okaay so when you don't pass -march=, GCC just kinda borrows the setting from -mcpu — bestie behaviour fr fr 💅

And if you leave out both -march and -mcpu=, the default is system dependent, so like, if you actually want a specific set of architecture extensions you should just say so out loud and specify one explicitly. No vibes-based guessing, periodt ✨

---

• -mcpu=processor-string — use the architecture of the given processor and optimize the output for it, all based on the particular CPU name you hand over. The permissible 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 bestie: -mcpu does not override -march or -mtune. It simply does not have that power, iykyk 😭

• -mtune=processor-string — optimize the output for the given processor, specified by microarchitecture or by a particular CPU name. The permissible values are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, thead-c906, size, sifive-p400-series, sifive-p600-series, and literally all valid options for -mcpu= 🌈

When -mtune= isn't specified, it uses the setting from -mcpu, and if neither of them is specified the default is rocket 🚀

The size choice is not meant for end-users, okay? This one gets used when -Os is specified. It overrides the instruction cost info that -mtune= gives you, but it does not override the pipeline info. The whole point is to shrink your code size while still keeping good performance. It's giving "small but mighty" 💀✨

• -mpreferred-stack-boundary=num — try to keep the stack boundary aligned to a 2 raised to num byte boundary. If you don't specify -mpreferred-stack-boundary, the default is 4 (which is 16 bytes, aka 128-bits) 🎀

⚠️ Warning: if you use this switch, then you have to build all modules with the same value, libraries included. And yes, that means the system libraries and startup modules too. No mixing, no exceptions, it will Not go well ✋

• -msmall-data-limit=n — put global and static data smaller than n bytes into a special section (on some targets) 💖
• -msave-restore, -mno-save-restore — do, or don't, use smaller-but-slower prologue and epilogue code that uses library function calls. The default is to use the fast inline prologues and epilogues ⚡
• -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 you enable it, sequences of ALU operations get produced using base integer ISA instructions where profitable 👑
• -minline-atomics, -mno-inline-atomics — do, or don't, use smaller-but-slower subword atomic emulation code that uses libatomic function calls. The default is to use the fast inline subword atomics that don't need libatomic 🧚
• -minline-strlen, -mno-inline-strlen — do, or don't, attempt to inline strlen calls if it's possible. Inlining only happens if the string is properly aligned and the 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 it's possible. Inlining only happens if the strings are properly aligned and the 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 it's possible. Inlining only happens if the strings are properly aligned and the 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, attempt to make more use of compressed load/store instructions by replacing a load/store of 'base register + large offset' with a new load/store of 'new base + small offset'. And get this: if the new base ends up stored in a compressed register, then the new load/store can itself be compressed. Currently this only targets 32-bit integer load/stores, so don't get your hopes up beyond that 🔥
• -mstrict-align, -mno-strict-align — don't, or do, generate unaligned memory accesses. The default is set depending on whether the processor we're optimizing for supports fast unaligned access or not 👀
• -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model ✨
• -mcmodel=medany — generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked. 🎀

The code generated by the medium-any code model is position-independent, but it is not guaranteed to function correctly when linked into position-independent executables or libraries. So like… keep that in mind bestie 😭

• -mexplicit-relocs, -mno-exlicit-relocs — use, or don't use, assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization 💜
• -mrelax, -mno-relax — take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to take advantage of linker relaxations 🫶
• -mriscv-attribute, -mno-riscv-attribute — emit (or do not emit) the RISC-V attribute to record extra information into ELF objects. Heads up: this feature requires at least binutils 2.32 ⭐
• -mcsr-check, -mno-csr-check — enables or disables the CSR checking 👑
• -malign-data=type — control how GCC aligns variables and constants of array, structure, or union types. The 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-*-* but not a riscv64be-*-* or riscv32be-*-* target 🌸
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. The supported locations are global for a global canary, or tls for a per-thread canary in the TLS block 🌱

With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally specify which register to use as the base register for reading the canary, and from what offset from that base register. There is no default register or offset, because this is entirely for use within the Linux kernel 🐧

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables 🎀
• -mtls-dialect=trad — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default ✨

---

🌸 RL78 Options 🌸

• -msim — links in additional target libraries so you can support operation within a simulator 🧚
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — bestie this one picks what kind of hardware multiply/divide support you're using 💅

So like, the most basic one is none, which means software handles both multiplication and division. And yeah that's the default, no cap. The g13 value is for the hardware multiply/divide peripheral that lives on the RL78/G13 (S2 core) targets. The g14 value picks the multiply and divide instructions that the RL78/G14 (S3 core) parts support. And for the aliases: rl78 is literally just another name for g14, and g10 is an alias for none. 🫶

Oh also bestie — 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 just "RL78". The G13 (S2) core doesn't have multiply or divide instructions — instead it uses a hardware peripheral for those operations. And the G10 (S1) core doesn't have register banks at all, so it uses a different calling convention.

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, periodt.

Ok now this part matters: even though RL78/G14 is the default target, actually putting -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 specify these on the command line, then 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. iykyk 💀

And just like the last one, a C preprocessor macro gets defined based on this setting. Possible values: __RL78_G10__, __RL78_G13__ or __RL78_G14__ 🌸

• -mg10, -mg13, -mg14, -mrl78 — these are aliases for the corresponding -mcpu= option, kept around for backwards compatibility 🧚
• -mallregs — lets the compiler use all the available registers 👑

By default, registers r24..r31 are reserved for interrupt handlers. With this option on, those registers can be used in ordinary functions too. slay.

• -m64bit-doubles, -m32bit-doubles — makes the double data type 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 🎯

This is only necessary if normal code might use the MDUC registers — like, for example, because it's doing multiplication and division operations. The default is to ignore the MDUC registers since that makes interrupt handlers faster. The target option -mg13 needs to be passed for this to even work, because this feature is only available on the G13 target (S2 core). And the MDUC registers will only actually be saved if the interrupt handler does a multiplication or division operation, or if it calls another function. no cap.

---

IBM RS/6000 and PowerPC Options 🦋

These -m options are defined for the IBM RS/6000 and PowerPC, bestie, get comfy:

• -mpowerpc-gpopt, -mno-powerpc-gpopt
• -mpowerpc-gfxopt, -mno-powerpc-gfxopt
• -mpowerpc64, -mno-powerpc64
• -mmfcrf, -mno-mfcrf
• -mpopcntb, -mno-popcntb
• -mpopcntd, -mno-popcntd
• -mfprnd, -mno-fprnd
• -mcmpb, -mno-cmpb
• -mhard-dfp, -mno-hard-dfp

You use these options to say which instructions are available on the processor you're using. The default value of these options is decided when GCC gets configured. Specifying -mcpu=cpu_type overrides the specification of these options. And real talk — we recommend you use the -mcpu=cpu_type option instead of the options listed above 💅

Specifying -mpowerpc-gpopt lets GCC use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt lets GCC use the optional PowerPC architecture instructions in the Graphics group, including floating-point select. ✨

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. The -mhard-dfp option lets GCC generate the decimal floating-point instructions implemented on some POWER processors. 😭 (that's a lot of architectures bestie)

The -mpowerpc64 option lets GCC generate the additional 64-bit instructions found in the full PowerPC64 architecture, and treat 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 machine type cpu_type 🎀

Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, power10, power11, powerpc, powerpc64, powerpc64le, rs64, and native. (yes that's a whole vibe check of CPU names, drink some water)

-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor of the system doing the compilation. Heads up though: -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. 💀

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. So like, if you wanna set an individual option to a particular value, you can 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 can 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 like -mcpu=cpu_type does 🧚

The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune.

• -mcmodel=small — generates PowerPC64 code for the small model: the TOC is limited to 64k 🌸
• -mcmodel=medium — generates 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 — generates 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 (does not 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 match 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 — generates code that lets ld and ld.so build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option. 🌈
• -mbss-plt — generates 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. (writable and executable, bestie, just letting that sit there) 🙈
• -misel, -mno-isel — this switch enables or disables the generation of ISEL instructions 💅
• -mvsx, -mno-vsx — generate code that uses (does not 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, your call 💀) the built-in functions that let you go straight at the cryptographic instructions that showed up in version 2.07 of the PowerPC ISA. Direct access, no middleman. ✨
• -mhtm, -mno-htm — same vibe, different instruction family: enables (disables) the built-ins that give you direct access to the Hardware Transactional Memory (HTM) instructions, also added in version 2.07 of the PowerPC ISA.
• -mpower8-fusion, -mno-power8-fusion — generate code that keeps (or does not keep 🙈) some integer operations sitting right next to each other so the instructions can fuse together on power8 and later processors. Adjacency slay. 🔥
• -mquad-memory, -mno-quad-memory — generate code that uses (or doesn't use) the non-atomic quad word memory instructions. ⚠️ Heads up: -mquad-memory requires use of 64-bit mode. No 64-bit, no quad word, periodt.
• -mquad-memory-atomic, -mno-quad-memory-atomic — generate code that uses (or doesn't use) the atomic quad word memory instructions. Same rule applies bestie: -mquad-memory-atomic requires use of 64-bit mode. 💅
• -mfloat128, -mno-float128 — enable/disable the __float128 keyword for IEEE 128-bit floating point, and pick whether that support rides on software emulation or on actual hardware instructions.
  • The VSX instruction set (-mvsx) must be enabled if you want any IEEE 128-bit floating point support at all.
  • IEEE 128-bit floating point is only supported on Linux. That's the whole list.
  • Default tea ☕: -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.
  • If you're using the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support also switches on generation of ISA 3.0 IEEE 128-bit floating point instructions. 🎀
  • Otherwise — meaning you didn't ask for ISA 3.0 instructions, or you're targeting a 32-bit big endian system — IEEE 128-bit floating point gets done with software emulation.
• -mfloat128-hardware, -mno-float128-hardware — enable/disable using ISA 3.0 hardware instructions to support the __float128 data type.
  • Default: -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 (GNU/Linux counts, hi 🫶).
  • The 32-bit environment sets int, long and pointer to 32 bits, and generates code that runs on any PowerPC variant. Versatility icon.
  • The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, same as -mpowerpc64.
• -mfull-toc, -mno-fp-in-toc, -mno-sum-in-toc, -mminimal-toc — the TOC (Table Of Contents) squad. These modify generation of the TOC, which gets created for every executable file.
  • -mfull-toc is selected by default. In that case GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also drops floating-point constants into the TOC. The catch: only 16,384 entries are available in the TOC. 🫠
  • If you get a linker error saying you've overflowed the available TOC space, you can shrink the amount of 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.
    • -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 makes GCC produce very slightly slower and larger code, in exchange for conserving TOC space.
  • Still out of space even with both of those? Say less — specify -mminimal-toc instead. This makes GCC create only one TOC entry for every file. When you use it, GCC produces code that's slower and larger, but it uses an extremely little amount of TOC space. You may wish to use this option only on files that contain less frequently-executed code. Bestie advice: don't put it everywhere. 🙈
• -maix64, -maix32 — enable the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. -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 you're using an AIX-compatible ABI. Concretely, this means:
  • Pass floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack, in addition to argument FPRs.
  • Do not assume the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double.
  • Use XL symbol names for long double support routines.

So here's the lore: the AIX calling convention got extended but was not initially documented, all to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that don't fit in the RSA from the stack when a subroutine is compiled without optimization. And because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default — it's only necessary when you're calling subroutines compiled by IBM XL compilers without optimization.

• -mpe — support the IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code so the application can actually run.
  • The system must have PE installed in the standard location /usr/lpp/ppe.poe/, or the specs file must be overridden with the -specs= option to point at the appropriate directory location.
  • The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible. Can't have both, sorry bestie 😭.
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, -malign-natural overrides the ABI-defined alignment of larger types — floating-point doubles and friends — putting them on their natural size-based boundary. -malign-power tells GCC to follow the ABI-specified alignment rules instead. 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 there. That's just the rules, ngl.
• -msoft-float, -mhard-float — generate code that does not use (uses) the floating-point register set. If you go with -msoft-float, software floating-point emulation is provided — and you have to pass the option to GCC when linking too. Do not forget the link step, I'm begging 🫶.
• -mmultiple, -mno-multiple — generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions.
  • These instructions are generated by default on POWER systems, and not generated on PowerPC systems.
  • ⚠️ Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode.
  • The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. Special little guys. 🎀
• -mupdate, -mno-update — generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default.
  • ⚠️ If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored. That means code that walks the stack frame across interrupts or signals may get corrupted data. Lowkey terrifying, be so careful. 💀
• -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 (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used.
  • Big update bestie: 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 (does not 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 (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors too. 🌸
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.
  • Example, because seeing it helps: 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, that structure is aligned to a 1-byte boundary and is 1 byte in size. Tiny. Cute. Slay. 🦋
• -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, do not (do) assume that unaligned memory references are handled by the system. That's the whole thing, no cap.
• -mrelocatable, -mno-relocatable — generate code that allows (does not allow) a static executable to be relocated to a different address at run time.
  • A simple embedded PowerPC system loader should relocate the entire contents of .got2 and the 4-byte locations listed in the .fixup section — that's 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 it 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. Mix and match, bestie. 🫶
• -mno-toc, -mtoc — PowerPC glow-up, System V.4 and embedded edition ✨ the -mno-toc version says bestie NO, do not assume register 2 is holding a pointer to a global area pointing at the addresses used in your program. -mtoc says actually yes, assume that. Choose your fighter 💅
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode 🦋 and yeah, -mlittle-endian is literally the exact same option as -mlittle, they're twins, no cap 👯
• -mbig, -mbig-endian — same energy but make it big-endian 🌸 compile for the processor in big-endian mode on System V.4 and embedded PowerPC systems. And -mbig-endian is the same as -mbig — again, twins fr fr 🫶
• -mdynamic-no-pic — Darwin / macOS girlies listen up 🍓 compile code so it is not relocatable, but its external references are relocatable. The code that falls out is perfect for applications, but not for shared libraries. So like, know what you're making before you commit 😭
• -msingle-pic-base — treat the register used for PIC addressing as read-only instead of loading it in the prologue for each and every function 🙈. The runtime system is the one responsible for initializing that register with an appropriate value before execution begins — that's not on GCC, that's on your runtime, bestie 💀
• -mprioritize-restricted-insns=priority — this one controls the priority assigned to dispatch-slot restricted instructions during the second scheduling pass ✨. The argument priority takes the value 0, 1, or 2, which assign no, highest, or second-highest priority (respectively) to dispatch-slot restricted instructions. Pick a number, any number 🎀
• -msched-costly-dep=dependence_type — controls which dependences the target considers costly during instruction scheduling 💸. The argument dependence_type takes one of these values:
  • no — no dependence is costly. chill. free. unbothered 💅
  • all — every single dependence is costly. all of them. the whole group chat 😭
  • 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 argument scheme takes one of these values:
  • no — don't insert NOPs. periodt.
  • pad — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping 🎀
  • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn into a new group, according to the estimated processor grouping ✨
  • number — insert NOPs to force costly dependent insns into separate groups, and insert number NOPs to force an insn to a new group 🔢
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement 📜. This is the default unless you configured GCC using powerpc-*-eabiaix.
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi at once. two for one, iconic 🫶
• -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi. matching set 💜
• -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 your traceback table type bestie 🎀 valid values for traceback_type are full, part, and no.
• -maix-struct-return — return all structures in memory, exactly as specified by the AIX ABI 📦
• -msvr4-struct-return — return structures smaller than 8 bytes in registers, as specified by the SVR4 ABI ✨
• -mabi=abi-type — extend the current ABI with a particular extension, or remove such an 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. Heads up though: this is not likely to work if your system defaults to using IEEE extended-precision long double 😭. If you change the long double type from IEEE extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option. Also requires -mlong-double-128 to be enabled. read the fine print bestie 💅
• -mabi=ieeelongdouble — change the current ABI to use IEEE extended-precision long double. Same warning energy: not likely to work if your system defaults to using IBM extended-precision long double 😭. If you change the long double type from IBM extended-precision, the compiler will issue a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be enabled. no skipping this one 🫶
• -mabi=elfv1 — change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. But listen: overriding the default ABI requires special system support and is likely to fail in spectacular ways 💀💥 iconic behaviour, terrible idea. like, spectacular.
• -mabi=elfv2 — change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. And same as above: overriding the default ABI requires special system support and is likely to fail in spectacular ways 💀 don't say I didn't warn you, fr fr.
• -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 that bit. neat, right? 💅
• -msim — on embedded PowerPC systems, assume 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 my beloved)
• -mvxworks — on System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
• -memb — on embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate that eabi extended relocations are used 🏁
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, do (or do not) adhere to the Embedded Applications Binary Interface (EABI) — which is a set of modifications to the System V.4 specifications 📚. Selecting -meabi means the stack is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Selecting -mno-eabi means the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and the -msdata option only uses "r13" to point to a single small data area. And -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. ⚠️ the -msdata=eabi option is incompatible with the -mrelocatable option. Also, -msdata=eabi sets the -memb option. two birds 💅
• -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. ⚠️ the -msdata=sysv option is incompatible with the -mrelocatable option.
• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi; otherwise compile code the same as -msdata=sysv 🦋. the "dealer's choice" of small data options.
• -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. But do not use register "r13" to address small data, okay? 🙅. 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) that are 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. no universal answer bestie, it depends 💅
• -mblock-compare-inline-limit=num — generate non-looping inline code for all block compares (like calls to "memcmp" or structure compares) that are less than or equal to num bytes 🔍. If num is 0, all inline expansion (both non-loop and loop) of block compare is disabled 🚫. The default value is target-specific.
• -mblock-compare-inline-loop-limit=num — bestie, this one makes GCC inline-expand via loop code every block compare that's <= num bytes, as long as it's bigger than the limit for the non-loop inline block compare expansion ✨. If the block length isn't constant? Then it compares at most num bytes before calling memcmp to handle the rest of the block. Default is target-specific, iykyk.
• -mstring-compare-inline-limit=num — compare at most num string bytes using inline code 💅. If the difference or the end of string doesn't show up by the end of that inline compare, a call to strcmp or strncmp takes care of the rest. Default is 64 bytes, periodt.
• -G num — on embedded PowerPC systems, this puts global and static items that are <= num bytes into the small data or BSS sections instead of the normal data or BSS section 🌸. Default num is 8. The -G num switch also gets passed to the linker, so all modules should be compiled with the same -G num value — no mixing, okay? 🙈
• -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 ✨.
• -mlongcall, -mno-longcall — by default, assume all calls are far away so a longer and more expensive calling sequence is required 🔥. This is needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't be that far away. You can override this 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 generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It's planned 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 (peep -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 gets 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 🔮.

• -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 are 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 a function call with its argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. Slay.
• -mrecip, -mno-recip — this enables the reciprocal estimate and reciprocal square root estimate instructions with extra Newton-Raphson steps to boost precision, instead of doing a divide (or square root and divide) for floating-point arguments 🌈. You should use -ffast-math when using -mrecip — or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math. Note that while the throughput of this sequence is generally higher than the non-reciprocal instruction, the precision can drop by up to 2 ulp (so like, the inverse of 1.0 equals 0.99999994) for reciprocal square roots 😭.
• -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 handle the double-precision reciprocal square root calculations 🔥.

• -mrecip-precision, -mno-recip-precision — assume (or don't assume) that the reciprocal estimate instructions give higher-precision estimates than the PowerPC ABI mandates 🎀. Picking -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions aren't generated by default on low-precision machines, since they don't provide an estimate that converges after three steps.
• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library ✨. The only type supported right now is mass, which says to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics via 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 💖.
• -mfriz, -mno-friz — generate (or don't generate) the friz instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point 🌸. Careful though: the friz instruction does not return the same value if the floating-point number is too large to fit in an integer 😬.
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — 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, TOC value to be loaded in register r2, and static chain value to be loaded in register r11 💅. The -mpointers-to-nested-functions is on by default. But bestie — 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 💀.
• -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 (that's 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 further 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, no cap.
• -mprefixed, -mno-prefixed — bestie, this one controls whether we generate addressing modes that use prefixed load and store instructions (or don't). Heads up tho, using -mprefixed requires that -mcpu=power10 (or something later, we love an upgrade path 💅) is enabled.
• -mmma, -mno-mma — do we generate the MMA instructions or not, that's the question 🎀. And same deal as above: the -mma option requires -mcpu=power10 (or later) to be enabled. No power10? No MMA. Periodt.
• -mrop-protect, -mno-rop-protect — turns ROP protection instructions on (or off) whenever the target processor actually supports them. But wait, there's a catch: right now, this option disables the shrink-wrap optimization (-fshrink-wrap). It's giving trade-off energy fr fr.
• -mprivileged, -mno-privileged — generate (or don't generate) code that runs in privileged state. Iconic, elite, above your pay grade 💅.
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generate (or don't generate) unaligned vsx loads and stores for inline expansion of memcpy and memmove. The way that memcpy stays memcpy-ing ✨.
• --param rs6000-vect-unroll-limit= — 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 param sets the upper bound of how much the vectorizer will unroll the main loop. Default value is four. Not five. Four. 🍓

---

✨ RX Options ✨

These command-line options are defined for RX targets, bestie. RX gang rise up 🦋:

• -m64bit-doubles, -m32bit-doubles — make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. Default is -m32bit-doubles. Why? Because 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 what it said" 💀.
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. Default is enabled for the RX600 series and disabled for the RX200 series.

Floating-point instructions are only generated for 32-bit floating-point values, though, so the FPU hardware is not used for doubles if you're out here using -m64bit-doubles. Sad but true 😭.

Note: if the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. Why? Because the RX FPU instructions are themselves unsafe. No pretending otherwise, bestie, that's just canon.

• -mcpu=name — picks which type of RX CPU we're targeting. Currently three types are supported: the generic RX600 and RX200 series hardware, plus the specific RX610 CPU. 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 has no hardware floating-point unit, so -nofpu is enabled by default when this type is selected. Makes sense, she doesn't have the hardware, she can't serve.

• -mbig-endian-data, -mlittle-endian-data — store data (but not code, mind you) in big-endian format. Default is -mlittle-endian-data, i.e. storing data in the little-endian format. Code stays out of this drama entirely, she's unbothered 💅.
• -msmall-data-limit=N — specifies the max 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) is reserved for pointing to this area, so it is no longer available for use by the compiler. Which could legit result in slower and/or larger code if variables get pushed onto the stack instead of just chillin' in this register. Not me sacrificing a register for the greater good 😭.

Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, since they get assigned to other sections in the output executable.

The default value is zero, which disables this feature. Note: this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature actually benefits their program. See the description of the -mpid option for how the actual register to hold the small data area pointer is chosen. iykyk 🔥

• -msim, -mno-sim — use the simulator runtime. Default is to use the libgloss board-specific runtime. Pick your fighter 🌈.
• -mas100-syntax, -mno-as100-syntax — when generating assembler output, use a syntax compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions, so it is not generated by default. The restrictions said "not today" 🙈.
• -mmax-constant-size=N — specifies the maximum size, in bytes, of a constant that can be used as an operand in an RX instruction. Even though the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value equates to a longer instruction. So in some circumstances it can be beneficial to restrict the size of constants used in instructions. Constants that are too big get placed into a constant pool instead and referenced via register indirection.

The value N can be between 0 and 4. A value of 0 (the default) or 4 means constants of any size are allowed. Big constant energy, we love to see it 👑

• -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. She's an opt-in queen 🎀.
• -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, reserves no registers. Register countdown, let's goooo ✨
• -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, e.g. because it performs 64-bit multiplications. Default is to ignore the accumulator, because that makes the interrupt handlers faster. Speed over safety, but make it a choice 💅.
• -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 to embedded applications with tight memory constraints. Data that can be modified is not affected by this option. Constantly constant, she doesn't move, she's just found differently 🦋.

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 price of flexibility, bestie 😭.

The actual register chosen to hold the constant data base address depends on whether -msmall-data-limit and/or -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 is possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line. Register allocation: a whole storyline 🌸.

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. Default is to issue a warning for each extra fast interrupt handler found, because the RX only supports one such interrupt. Only one seat at the table, sorry 🫶.
• -mallow-string-insns, -mno-allow-string-insns — enables or disables the 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 info.)

The default is to allow these instructions, but it is 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 cannot be disabled automatically. Instead it relies on the programmer to use -mno-allow-string-insns if their program accesses I/O space. GCC out here trusting you to know your own I/O, which is lowkey terrifying but okay 💀.

When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__. Preprocessor symbols, we love a define moment ✨.

• -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 "do not use JSR" — it instead means that any type of branch may be used. Read that twice, bestie, it's tricky 🙈.

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 ensures that 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. Teamwork, but make it registers 💖

---

✨ S/390 and zSeries Options ✨

These are the -m options defined for the S/390 and zSeries architecture. Big iron besties, this one's for you 🏛️:

• -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. Hardware girlies stay winning 👑.
• -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. Decimals, but make them fast 🍓.
• -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. If you wanted long double to feel special, well, sometimes she's just double with a different name 😭✨
• -mbackchain, -mno-backchain — so this one decides whether we store (or don't store 🙅‍♀️) the address of the caller's frame as a backchain pointer inside the callee's stack frame. A backchain might be needed so that debugging tools that don't 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. However — and this is the tea ☕ — using the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. And if you want to build a linux kernel, use -msoft-float.

The default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (or don't use) the packed stack layout.

With -mno-packed-stack, the compiler uses all fields of the 96/160 byte register save area only for their default purpose, and any unused fields still take up stack space. With -mpacked-stack, register save slots get densely packed at the top of the register save area, and the unused space is reused for other purposes — which is a way more efficient use of the available stack space. ✨

That said, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.

As long as the stack frame backchain isn't being used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Heads up though: some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging. That kind of code is not call-compatible with code compiled with -mpacked-stack. Also, like, again: the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported, and to build a linux kernel you want -msoft-float.

The default is to not use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or don't generate) code that uses the bras instruction to do subroutine calls. This only works reliably if the total executable size doesn't exceed 64k. The default is to use the basr instruction instead, which doesn't have that limitation. 🤏
• -m64, -m31 — 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 is what lets GCC, in particular, generate 64-bit instructions. For the s390 targets the default is -m31, while the s390x targets default to -m64.
• -mzarch, -mesa — when -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. 🚫

When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.

• -mhtm, -mno-htm — the -mhtm option enables a set of builtins that make use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. -mhtm is enabled by default when using -march=zEC12. 🔥
• -mvx, -mno-vx — when -mvx is specified, generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation.

This option changes the ABI for some vector type values when it comes to alignment and calling conventions. And if vector type values are being used in an ABI-relevant context, a GAS .gnu_attribute command gets added to mark the resulting binary with the ABI used. -mvx is enabled by default when using -march=z13. 🦋

• -mzvector, -mno-zvector — the -mzvector option enables vector language extensions and builtins using instructions available with the vector extension facility introduced with the IBM z13 machine generation. It adds support for vector to be used as a keyword to define vector type variables and arguments. But vector is only available when GNU extensions are enabled — it will not be expanded when you're requesting strict standard compliance, e.g. with -std=c99. 😤

On top of the GCC low-level builtins, -mzvector enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To actually make use of those builtins, you need to include the header file vecintrin.h. -mzvector is disabled by default.

• -mmvcle, -mno-mvcle — generate (or don't generate) code that uses the mvcle instruction to perform block moves. When -mno-mvcle is specified, use a mvc loop instead. That's the default unless you're optimizing for size.
• -mdebug, -mno-debug — print (or don't print) additional debug information while compiling. The default is to not print debug info.
• *-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. The 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 the cpu type can be used to select the best architecture option for the host processor. -march=native has no effect if GCC doesn't recognize the processor.

• *-mtune=cpu-type* — tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as the one for -march. The default is the value used for -march.
• -mtpf-trace, -mno-tpf-trace — generate code that adds (or doesn't add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS.
• -mtpf-trace-skip, -mno-tpf-trace-skip — generate code that changes (or doesn't change) the default branch targets enabled by -mtpf-trace to point to specialized trace routines that provide the ability of selectively skipping function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace. 💀
• -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. 🍓
• *-mwarn-framesize=framesize* — emit a warning if the current function exceeds the given frame size. Because this is a compile-time check, it doesn't have to be a real problem when the program runs. It's intended to sniff out functions that most probably cause a stack overflow, and it's useful in an environment with a 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, the stack on S/390 grows *downward 📉). If the *stack-guard option is omitted, the smallest power of 2 that's larger than the frame size of the compiled function is chosen.

These options are meant to help debug stack overflow problems. The additionally emitted code causes only a little overhead, so it 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. To be efficient, the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. And 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 function 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:

• -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 isn't used.
• -m2a-single-only — generate code for the SH2a-FPU in such a way that no double-precision floating-point operations are used.
• -m2a-single — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
• -m2a — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
• -m3 — generate code for the SH3.
• -m3e — generate code for the SH3e.
• -m4-nofpu — generate code for the SH4 without a floating-point unit.
• -m4-single-only — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
• -m4-single — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
• -m4 — generate code for the SH4.
• -m4-100 — generate code for SH4-100.
• -m4-100-nofpu — generate code for SH4-100 in such a way that the floating-point unit isn't used.
• -m4-100-single — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — generate code for the SH4-100 but like... never touch double-precision floats, bestie. Not once. 💅
• -m4-200 — generate code for the SH4-200. slay ✨
• -m4-200-nofpu — generate code for the SH4-200 in such a way that the FPU is just... not invited to the party 🚫
• -m4-200-single — generate code for the SH4-200, assuming the floating-point unit is chilling in single-precision mode by default 🌸
• -m4-200-single-only — generate code for the SH4-200 where no double-precision floating-point operations are used, periodt 🎀
• -m4-300 — generate code for the SH4-300 🦋
• -m4-300-nofpu — generate code for the SH4-300 in such a way that the floating-point unit is not used 🚫
• -m4-300-single — generate code for the SH4-300 in such a way that no double-precision floating-point operations are used 🎀
• -m4-300-single-only — generate code for the SH4-300 in such a way that no double-precision floating-point operations are used 🎀
• -m4-340 — generate code for the SH4-340 (no MMU, no FPU, just vibes) 💀
• -m4-500 — generate code for the SH4-500 (no FPU). It passes -isa=sh4-nofpu to the assembler 👑
• -m4a-nofpu — generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used 🚫
• -m4a-single-only — generate code for the SH4a, in such a way that no double-precision floating-point operations are used 🎀
• -m4a-single — generate code for the SH4a assuming the floating-point unit is in single-precision mode by default ✨
• -m4a — generate code for the SH4a 💖
• -m4al — literally the same as -m4a-nofpu, except it implicitly passes -dsp to the assembler. Also ngl, GCC doesn't generate any DSP instructions at the moment 🙈
• -mb — compile code for the processor in big-endian mode 🌈
• -ml — compile code for the processor in little-endian mode 🌈
• -mdalign — align doubles at 64-bit boundaries. ok but heads up bestie, this changes the calling conventions, so some functions from the standard C library will not work unless you recompile it first with -mdalign. the way that's a whole ordeal 😭
• -mrelax — shorten some address references at link time when possible; uses the linker option -relax ✨
• -mbigtable — use 32-bit offsets in switch tables. the default is to use 16-bit offsets 🎀
• -mbitops — enable the use of bit manipulation instructions on SH2A 👑
• -mfmovd — enable the use of the instruction fmovd. check -mdalign for alignment constraints 👀
• -mrenesas — comply with the calling conventions defined by Renesas 💅
• -mno-renesas — comply with the calling conventions defined for GCC before the Renesas conventions even existed. this option is the default for all targets of the SH toolchain ⭐
• -mnomacsave — mark the MAC register as call-clobbered, even if -mrenesas is given 💀
• -mieee
• -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects how we handle cases where the result of a comparison is unordered. by default -mieee is implicitly enabled ✨ If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which gives you faster floating-point greater-equal and less-equal comparisons 🔥 The implicit settings can be overridden by specifying either -mieee or -mno-ieee 🎀
• -minline-ic_invalidate — inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi instruction 👀 If the selected code generation option does not allow the use of the icbi instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. And bestie... this 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 😭
• -misize — dump instruction size and location in the assembly code 📝
• -mpadstruct — this option is deprecated. it pads structures to multiple of 4 bytes, which is incompatible with the SH ABI 🚩
• -matomic-model=model — sets the model of atomic operations and additional parameters as a comma separated list. for details on the atomic built-in functions see __atomic Builtins. the following models and parameters are supported:
  • none — disable compiler generated atomic sequences and emit library calls for atomic operations. this is the default if the target is not sh*-*-linux* 💅
  • soft-gusa — generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. the generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems 🌸 This option is enabled by default when the target is sh*-*-linux* and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions movli.l and movco.l to create more efficient code, unless strict is specified ✨
  • soft-tcb — generate software atomic sequences that use a variable in the thread control block. this is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets 🦋 The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well ⚠️
  • soft-imask — generate software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111. this model works only when the program runs in privileged mode and is only suitable for single-core systems 🔒 Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is sh*-*-linux* and SH1* or SH2* 🎀
  • hard-llcs — generate hardware atomic sequences using the movli.l and movco.l instructions only. this is only available on SH4A and is suitable for multi-core systems 👑 Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model 🌈
  • gbr-offset= — this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the soft-tcb model has been selected. For other models this parameter is ignored 🙈 The specified value must be an integer multiple of four and in the range 0-1020 🔢
  • strict — this parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the compiler generate atomic sequences of the specified model only 🚫
• -mtas — generate the tas.b opcode for __atomic_test_and_set. Notice that 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 😭 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 💀
• -mprefergot — when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table ✨
• -musermode
• -mno-usermode — don't allow (allow) the compiler to generate privileged mode code 🔒 Specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code would not work in user mode. -musermode is the default when the target is sh*-*-linux*. If the target is SH1* or SH2*, -musermode has no effect, since there is no user mode 🤷‍♀️
• -multcost=number — set the cost to assume for a multiply insn 🔢
• -mdiv=strategy — set the division strategy to be used for integer division operations. strategy can be one of:
  • call-div1 — calls a library function that uses the single-step division instruction div1 to perform the operation. Division by zero calculates an unspecified result and does not trap 🙈 This is the default except for SH4, SH2A and SHcompact.
  • call-fp — calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception 💀 This is the default for SHcompact with FPU. Specifying this for targets that do not have a double precision FPU defaults to call-div1 🎀
  • call-table — calls a library function that uses a lookup table for small divisors and the div1 instruction with case distinction for larger divisors 📊 Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that do not have dynamic shift instructions defaults to call-div1 🦋

When a division strategy has not been specified, the default strategy is selected based on the current target 👀 For SH2A the default strategy is to use the divs and divu instructions instead of library function calls ✨

• -maccumulate-outgoing-args — reserve space once for outgoing arguments in the function prologue rather than around each call 💖 Generally beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around conditional code 🎀
• -mdivsi3_libfunc=name — set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present 👑
• -mfixed-range=register-range — bestie, this one tells the compiler to ✨lock✨ the given register range and treat it as fixed registers. What's a fixed register? It's one the register allocator simply cannot touch, periodt. Super useful when you're compiling kernel code 💅. A register range is written as two registers with a dash in between, like regA-regB. Wanna do more than one? Separate the ranges with a comma, easy peasy 🌸.
• -mbranch-cost=num — this one's just vibes-and-math: assume num is the cost of a branch instruction. The higher the number, the harder the compiler tries to generate branch-free code wherever it possibly can 🦋. And if you don't specify anything? No worries, the value just gets picked based on whatever processor type you're compiling for 💖.
• -mzdcbranch
• -mno-zdcbranch — assume (or do not assume, respectfully 🙈) that the zero displacement conditional branch instructions bt and bf are fast. If you pass -mzdcbranch, the compiler starts preferring zero displacement branch code sequences 👑. This is enabled by default when generating code for SH4 and SH4A, but you can explicitly turn it off by specifying -mno-zdcbranch. It's giving "on by default but consent matters" energy fr fr 😭.
• -mcbranch-force-delay-slot — this one is forceful, lowkey. It forces the usage of delay slots for conditional branches, and if a suitable instruction can't be found 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. Not me offering a bug workaround as a love language 🫶.
• -mfused-madd
• -mno-fused-madd — generate code that uses (or does not use, your call bestie) the floating-point multiply and accumulate instructions 🧚. These instructions get generated by default if hardware floating point is being used. Plot twist though: 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. They said "we're rebranding" and honestly, iconic ✨.
• -mfsca
• -mno-fsca — allow (or disallow) the compiler to emit the fsca instruction for sine and cosine approximations 🌈. Heads up: -mfsca must be used in combination with -funsafe-math-optimizations. It's enabled by default when generating code for SH4A. And using -mno-fsca disables those sine and cosine approximations even if -funsafe-math-optimizations is in effect — no take-backsies, it means it 💅.
• -mfsrra
• -mno-fsrra — allow (or disallow) the compiler to emit the fsrra instruction for reciprocal square root approximations 🍓. Important: -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only — it needs both, it's high maintenance and that's fine 😌. 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 both in effect.
• -mpretend-cmove — this one is literally in the name bestie, we're pretending 😭. It makes the compiler prefer zero-displacement conditional branches for conditional move instruction patterns. Result? Potentially faster code on the SH4 processor 🔥.
• -mfdpic — generate code using the FDPIC ABI. Short, sweet, slays 💜.

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☀️ 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 actually need this when object files use ISA extensions that aren't supported by the current machine, but they 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 😳.

Now for the real talk: -mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. However (and this is a big however, bestie 💀), the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. So instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. It's giving "the easy way is a trap" energy fr fr, do it the right way 🫶.

These switches are supported in addition to the above on Solaris 2:

• -pthreads — this is just a synonym for -pthread. Same vibes, different name, we love a nickname moment 💖.

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🌟 SPARC Options 🌟

These -m options are supported on the SPARC, babes:

• -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 🔥. Just like the 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, no cap.

But if you want to be fully SVR4 ABI-compliant — at the cost of some performance loss 😭 — specify -mno-app-regs. And bestie, listen: you should compile libraries and system software with this option. It's the correct-slash-respectful choice 👑.

• -mflat
• -mno-flat — with -mflat, the compiler does not generate save/restore instructions and instead 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 they 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, the main character mode, the default behavior we know and love.

• -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, bestie ⚠️: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler get used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support though, so there's that 🫶.

Also: -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 in order for this to work. All or nothing, that's the rule.

• -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. And because of that trap handler overhead, it's much slower than calling the ABI library routines. So that's exactly why -msoft-quad-float is the default. The default is the default for a reason, periodt 💀.

• -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 😭. Trade-offs, bestie, trade-offs.

• -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. Sounds great, right? But 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. You've been warned, be honest with yourself 💅.
• -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. And heads up: this option has no effect in 64-bit mode 🤷.

• -mlra
• -mno-lra — enable Local Register Allocation 👑. This is the default for SPARC since GCC 7, so -mno-lra needs to be passed to get the old Reload. We love a modern default but respect the nostalgic girlies who switch back 💖.
• -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. Yes, that's a lot of names, it's giving roster drop 🌈.

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor 👑. But -mcpu=native has no effect if GCC does not recognize the processor. Ngl, sometimes it just doesn't know her 🙈.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. Those are v7, v8, sparclite, sparclet, v9.

Here's the list of each supported architecture and their supported implementations, bestie:

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3, leon5
• sparclite
  • f930, f934, sparclite86x
• sparclet
  • tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

By default (unless 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. Throwback energy 🎀.

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. Vintage icon behavior, we love to see it ✨.

• -mcpu=sparclite — bestie, this is the SPARClite variant of the SPARC architecture. It adds in the integer multiply, the integer divide step and the scan ("ffs") instructions — all of which exist in SPARClite but not in SPARC-V7. Add -mcpu=f930 and the compiler also optimizes for the Fujitsu MB86930 chip, which is the original SPARClite, no FPU at all. Add -mcpu=f934 and it also optimizes for the Fujitsu MB86934 chip, which is the more recent SPARClite, and yes, this one has the FPU. 💅
• -mcpu=sparclet — now we're doing the SPARClet variant of the SPARC architecture. This brings the integer multiply, the multiply/accumulate, the integer divide step and the scan ("ffs") instructions, none of which live in SPARC-V7. With -mcpu=tsc701 the compiler additionally optimizes for the TEMIC SPARClet chip. ✨
• -mcpu=v9 — this generates code for the V9 variant of the SPARC architecture. It adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers, and conditional move instructions. Then:
  • -mcpu=ultrasparc → the compiler additionally optimizes for the Sun UltraSPARC I/II/IIi chips.
  • -mcpu=ultrasparc3 → additionally optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips.
  • -mcpu=niagara → additionally optimizes for Sun UltraSPARC T1 chips.
  • -mcpu=niagara2 → additionally optimizes for Sun UltraSPARC T2 chips.
  • -mcpu=niagara3 → additionally optimizes for Sun UltraSPARC T3 chips.
  • -mcpu=niagara4 → additionally optimizes for Sun UltraSPARC T4 chips.
  • -mcpu=niagara7 → additionally optimizes for Oracle SPARC M7 chips.
  • -mcpu=m8 → additionally optimizes for Oracle M8 chips.
• -mtune=cpu_type — this one sets the instruction scheduling parameters for machine type cpu_type, but it does not set the instruction set or register set that -mcpu=cpu_type does. Same values work here as for -mcpu=cpu_type, but the only useful ones are those that select a particular CPU implementation, and those are: cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. And 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? 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, like 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, like 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, like 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, like m8 and later. Setting -mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis. 🌈
• -mcbcond, -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is -mcbcond when targeting a CPU that supports such instructions, like 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, like 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, like 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, like Niagara-7 and later. 🔥
• -mfix-at697f — enables the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
• -mfix-ut699 — enables the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
• -mfix-ut700 — enables the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — enables the documented workaround for the back-to-back store errata of the GR712RC processor. 💜

---

And now for the plot twist, bestie: 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.

✨ 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.
• -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 — any static or global variable that's n bytes or smaller gets dropped into the first 32 kilobytes of memory. cozy lil spot for the small ones ✨
• -mv850 — tells the compiler the target processor is the V850.
• -mv850e3v5 — target processor is the V850E3V5 💅. Using this option defines the preprocessor constant __v850e3v5__.
• -mv850e2v4 — the manpage says: specify that the target processor is the V850E3V5. It's literally just an alias for -mv850e3v5. (yes bestie, it says V850E3V5, we don't make the rules, we just report them 🫶)
• -mv850e2v3 — target processor is the V850E2V3. Using this option defines the preprocessor constant __v850e2v3__.
• -mv850e2 — target processor is the V850E2. Using this option defines the preprocessor constant __v850e2__.
• -mv850e1 — target processor is the V850E1. Using this option defines the preprocessor constants __v850e1__ and __v850e__.
• -mv850es — target processor is the V850ES. This is an alias for -mv850e1. two names, one vibe 🦋
• -mv850e — target processor is the V850E. Using this option defines the preprocessor constant __v850e__.

If none of -mv850, -mv850e, -mv850e1, -mv850e2, -mv850e2v3 or -mv850e3v5 are defined, then a default target processor gets chosen and the relevant __v850*__ preprocessor constant is defined. (the manpage says "are defined" here, not "are used" — iconic wording, we keep it fr fr)

The preprocessor constants __v850 and __v851__ are always defined, no matter which processor variant you're targeting. periodt 💖

---

• -mdisable-callt
• -mno-disable-callt — this one 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 (peep -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. the way it just tells you through a macro 😌

• -mrelax
• -mno-relax — pass on (or don't pass on) the -mrelax command-line option down to the assembler.
• -mlong-jumps
• -mno-long-jumps — disable (or re-enable) generation of PC-relative jump instructions.
• -msoft-float
• -mhard-float — disable (or re-enable) generation of hardware floating point instructions. This option only matters when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated, the C preprocessor symbol __FPU_OK__ is defined; otherwise the symbol __NO_FPU__ is defined. one or the other, no in-between ✨
• -mloop — enables use of the e3v5 LOOP instruction. Heads up: this is not enabled by default when the e3v5 architecture is selected, because its use is still experimental. 👀
• -mrh850-abi
• -mghs — enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI, here are the rules:
  • 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 rules are:
  • 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 being aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect, the C preprocessor symbol __V850_8BYTE_ALIGN__ is defined.
• -mbig-switch — generate code suitable for big switch tables. ⚠️ Only use this option if the assembler/linker complain about out-of-range branches within a switch table. like, only reach for it when someone's yelling at you.
• -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 can't 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 that does 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 that does 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.

⚠️ Note that -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. all of it or none of it, bestie 😭

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5, kept around 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 -mcpu=cpu_type would. tune only, no full glow-up 💅
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on 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. That last 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're 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. (yes, the "lazy" one equals now, iconic chaos, we report it faithfully 🫶)
• -Xbind-now — disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.

---

🔥 x86 Options 🔥

These -m options are defined for the x86 family of computers. main character architecture fr fr 👑

• -march=cpu-type — generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. 💀 Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

• native — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
• x86-64 — a generic CPU with 64-bit extensions.

• x86-64-v2, x86-64-v3, x86-64-v4 — bestie these three pick the matching micro-architecture level straight out of the x86-64 psABI. And lowkey if you're on an ABI that is not the x86-64 psABI? They still select the exact same CPU features that the x86-64 psABI documents for that particular micro-architecture level. Consistent queen behavior. ✨

Okay but here's the tea 💅 — these cpu-type values have no corresponding -mtune setting of their own. So if you use -march with them, you get generic tuning. Want actual specific tuning? You gotta say -mtune=other-cpu-type with an appropriate other-cpu-type value yourself. No cap.

---

• i386 — the original Intel i386 CPU. The blueprint, the blueprint. 🌸
• i486 — Intel i486 CPU. Heads up though: no scheduling is implemented for this chip, so don't expect any choreography. 💀
• i586, pentium — Intel Pentium CPU, and it does not have MMX support. Not a single MMX to its name.
• lakemont — Intel Lakemont MCU, which is based on the Intel Pentium CPU. Small but mighty 🦋
• pentium-mmx — Intel Pentium MMX CPU, built on the Pentium core but with MMX instruction set support added. Glow up. ✨
• pentiumpro — Intel Pentium Pro CPU. Iconic.
• i686 — ooh this one's sneaky, pay attention 👀 When you use it with -march, the Pentium Pro instruction set gets used, so your code runs on all i686 family chips. But when you use it with -mtune, it means the exact same thing as generic. Same name, different personality, main character energy tbh.
• pentium2 — Intel Pentium II CPU, based on the Pentium Pro core with MMX and FXSR instruction set support. 🎀
• pentium3, pentium3m — Intel Pentium III CPU, based on the Pentium Pro core with MMX, FXSR and SSE instruction set support.
• pentium-m — Intel Pentium M: it's the low-power version of the Intel Pentium III CPU, with MMX, SSE, SSE2 and FXSR instruction set support. Famously used by Centrino notebooks. Little laptop queen 💻💜
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support. The hot one from back in the day 🔥
• prescott — improved version of the Intel Pentium 4 CPU, with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
• nocona — also an improved version of the Intel Pentium 4 CPU, but this one's got 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support. 64-bit era unlocked 🌈
• core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support. The people's champion 👑
• 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. Look at that list, it's giving comprehensive 💅
• 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. AVX has entered the chat 😭
• 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. AVX2 ate and left no crumbs 🍓
• 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. (Yes, MOVBE is listed twice, that's just what the page says, not me being messy 🫶)
• 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. She's stacked 💅
• 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. AVX512 said hi bestie ✨
• 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. The acronyms are acronying 🧚
• 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. Client side stay winning 💅
• 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. Server edition said hold my ✨
• 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. (The page really does jam AVX512CD PKU together with no comma there, and honestly? Mood. Leave it be 😭)
• 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. Four names, one icon, we love a shared spotlight 🫶
• 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. She got upgrades and she's thriving 🌈
• 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. SHA512, SM3 AND SM4?? the way she just keeps going 💅
• 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. Panther said almost the same as Lunar but watch me 💅✨
• sapphirerapids, emeraldrapids — Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions, and literally the whole snack platter: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16 and AVX512BF16 instruction set support. 💅
• graniterapids — Intel Granite Rapids CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16 and PREFETCHI instruction set support. ✨
• graniterapids-d — Intel Granite Rapids D CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI and AMX-COMPLEX instruction set support. 🔥

no cap, notice that AVX512FP16 here has no dash but AVX512-FP16 above does — that's not me being sloppy bestie, that's literally how it's spelled. copy it character for character or the compiler will side-eye you. 💀

• bonnell, atom — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. humble but valid 🎀
• silvermont, slm — Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support. 🌸
• goldmont — Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT and FSGSBASE instruction set support. 🌼
• goldmont-plus — Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID and SGX instruction set support. ✨
• tremont — Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support. 🦋
• sierraforest — Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support. 💚
• grandridge — Intel Grand Ridge CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support. 🔥
• clearwaterforest — Intel Clearwater Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI instruction set support. 🌊
• knl — 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 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.

bestie, knl and knm are written on the same line as their description in the original, so if you're diffing against it, that's why they look a lil different. iykyk 💅

---

🎀 the AMD section, aka the plot twist 🎀

• k6 — AMD K6 CPU with MMX instruction set support. 💜
• k6-2, k6-3 — Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support. ✨
• athlon, athlon-tbird — AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
• athlon-4, athlon-xp, athlon-mp — Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support. 🌈
• k8, opteron, athlon64, athlon-fx — Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.)
• k8-sse3, opteron-sse3, athlon64-sse3 — Improved versions of AMD K8 cores with SSE3 instruction set support.
• amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
• bdver1 — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver2 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver3 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver4 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• znver1 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.) 👑
• znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.) ✨
• znver3 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.) 💜
• znver4 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.) 🌟

and the list keeps going, bestie — do not close the tab, we are nowhere near done here 🔥

• 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. 💜

---

✨ -mtune=cpu-type ✨

okay bestie, this one is the vibes dial, not the features dial 🎚️ It tunes to cpu-type everything applicable about the generated code — except for the ABI and the set of available instructions. So like, when you pick a specific cpu-type, it schedules things appropriately for that particular chip, but the compiler does not generate any code that cannot run on the default machine type unless you also use a -march=cpu-type option. fr fr, the code still runs everywhere default could run. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. 💖

The choices for cpu-type are the same as for -march. plus -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.

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 🫶

• intel — Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option. 💙

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released.

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. ✨

---

✨ -mcpu=cpu-type ✨

⚠️ deprecated alert ⚠️ bestie this is literally just a synonym for -mtune. That's it. That's the whole option. 💀

---

✨ -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. no cap, that's a real warning. 🫠

---

✨ -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. 🦋

---

✨ -mieee-fp / -mno-ieee-fp ✨

• -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. 🌸

---

✨ -m80387 / -mhard-float ✨

• -m80387, -mhard-float — Generate output containing 80387 instructions for floating point. 🔥

---

✨ -mno-80387 / -msoft-float ✨

• -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. bestie do not skip this one, it will bite you 😭

On machines where a function hands back floating-point results in the 80387 register stack, some floating-point opcodes may still get emitted even if you used -msoft-float. Wild, I know, but that's the deal. 💀

• -mno-fp-ret-in-387 — bestie, do not use the FPU registers for return values of functions.

Normally the calling convention says functions return values of type float and double in an FPU register, even when there's literally no FPU anywhere. The whole idea is that the operating system is supposed to emulate an FPU for you.

The -mno-fp-ret-in-387 option 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. So you pass this option to avoid generating them. 🚫

Heads up: this option gets overridden when -march says the target CPU always has an FPU, because then the instruction doesn't need emulation anyway. And plot twist — these instructions aren't generated at all unless you also use the -funsafe-math-optimizations switch.

• -malign-double
• -mno-align-double — these two 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 makes code run somewhat faster on a Pentium, at the cost of using more memory. Trade-offs, always trade-offs. 💅

On x86-64, -malign-double is on by default.

Warning: if you use the -malign-double switch, structures containing those types get aligned differently than the published application binary interface specs for x86-32, and they 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 says the size should 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 inside arrays or structures that conform 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. Little stuffing, big consequences. 🎀

In the x86-64 compiler, -m128bit-long-double is the default choice, because its ABI specifies that long double is aligned on a 16-byte boundary.

Notice that neither of these options gives you any extra precision beyond the x87 standard of 80 bits for a long double. Facts only. ✨

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, and it also modifies the function calling convention for functions taking long double. So they are not binary-compatible with code compiled without that switch. 🙈

• -mlong-double-64
• -mlong-double-80
• -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes long double equivalent to double. That's the default for 32-bit Bionic C library. A size of 128 bits makes long double equivalent to __float128. 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, and it also modifies the function calling convention for functions taking long double. So they are not binary-compatible with code compiled without that switch. 💀

• -malign-data=type — controls how GCC aligns variables. Supported values for type are: compat, which uses the 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 the increased alignment value that matches the cache line size. compat is the default. 🦋
• -mlarge-data-threshold=threshold — when -mcmodel=medium or -mcmodel=large is specified, data objects bigger than threshold get placed in large data sections. The default is 65535.
• -mrtd — uses a different function-calling convention, one where functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. The glow-up here is that it saves one instruction in the caller, since there's no need to pop the arguments there. 🌈

You can say that an individual function is called with this calling sequence by using the function attribute stdcall. And 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. Nope, not happening. 🚫

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code gets generated for calls to those functions.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored, but not here bestie. 😭)

• -mregparm=num — controls how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for one 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. Everyone on the same page or it falls apart. 💅

• -msseregparm — uses SSE register passing conventions for float and double arguments and return values. You can control this behavior for one specific function via 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 — returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks, to match the ABI of the Sun Studio compilers up to version 12. Only use this option if you need to stay compatible with existing code produced by those previous compiler versions or older versions of GCC. Nostalgia is not always free. 🎀
• -mpc32
• -mpc64
• -mpc80 — set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations get rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision); and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer unless they set the FPU control word explicitly.

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs up 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 those 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. 😭

• -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. Rude of it, honestly. 💀
• -mstackrealign — realigns 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 code that keeps 4-byte stack alignment with modern code that keeps 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, which you can apply to individual functions. ✨
• -mpreferred-stack-boundary=num — attempts to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary isn't specified, the default is 4 (16 bytes or 128 bits).

Warning: when generating code for 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 requires 16 byte stack alignment, this is ABI incompatible and is 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 — assumes the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary isn't specified, the one specified by -mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double), or they suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it isn't 16-byte aligned. 😤

To make sure these values are properly aligned on the stack, the stack boundary 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. So calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🫶

okay so real talk bestie — that extra alignment? it's not free. It consumes extra stack space and generally makes your code bigger too. So if your code is sensitive to how much stack it's using (embedded systems, OS kernels, that whole crowd 😭), you might wanna dial the preferred alignment down to -mpreferred-stack-boundary=2. do it for your own peace of mind 💅

---

✨ the instruction-set girlies ✨

here's the whole squad 👑 every single one of these turns on a family of instructions, and every single one has a -mno- twin that turns it right back off:

• -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 let GCC use instructions from 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. and yes, each and every one of them has a matching -mno- option to switch the instructions back off. 🌈

✨ also!! these extensions are available as built-in functions too — go peek at x86 Built-in Functions for the tea on which functions each switch enables and disables.

wanna generate SSE/SSE2 instructions automatically from floating-point code (instead of 387 instructions)? that's -mfpmath=sse, bestie 💖

and here's a fun one: GCC depresses SSEx instructions when -mavx is used. Instead it generates brand new AVX instructions, or the AVX equivalent for all SSEx instructions, whenever they're needed. glow-up fr.

these options let GCC use those extended instructions in the code it generates, even without -mfpmath=sse. 🫶 if your app does run-time CPU detection, you must compile separate files for each supported architecture with the appropriate flags — and especially, the file holding the CPU detection code should be compiled without these options. no cap.

---

• -mdump-tune-features — tells GCC to dump the names of the x86 performance tuning features and their default settings. Those names are exactly what you feed into -mtune-ctrl=feature-list. it's giving ✨receipts✨
• -mtune-ctrl=feature-list — fine grain control over x86 code generation features, babes. feature-list is a comma separated list of feature names. see also -mdump-tune-features. When specified, the feature is turned on if it's not preceded with ^, otherwise it's turned off.

🚨 okay important: -mtune-ctrl=feature-list is meant for GCC developers. using it may lead to code paths that aren't covered by testing, and can potentially cause compiler ICEs or runtime errors. proceed with caution, queen 💀

• -mno-default — turns off all tunable features. see also -mtune-ctrl=feature-list and -mdump-tune-features.
• -mcld — emits a "cld" instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to pick between autoincrement or autodecrement mode. The ABI says the DF flag is cleared on function entry, but some operating systems violate that spec and don't clear the DF flag in their exception dispatchers 😭. The exception handler can get invoked with the DF flag set, which leads to the wrong direction mode when string instructions get used. This option can be on by default on 32-bit x86 targets if you configure GCC with the --enable-cld configure option. In that case you can suppress generation of "cld" instructions with the -mno-cld compiler option.
• -mvzeroupper — emits a "vzeroupper" instruction before a transfer of control flow out of the function, which minimizes the AVX to SSE transition penalty and also removes unnecessary "zeroupper" intrinsics. efficient queen 💅
• -mprefer-avx128 — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.
• -mprefer-vector-width=opt — tells GCC to use opt-bit vector width in instructions instead of the default on the selected platform.
• -mpartial-vector-fp-math — lets GCC generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements live in the low part of the 128-bit SSE register. Unless -fno-trapping-math is specified, the compiler guarantees correct behavior by sanitizing all input operands so they have zeroes in the unused upper part of the vector register. ⚠️ but heads up: if you use built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, which can cause possible performance issues when -fno-trapping-math is in effect. You can mitigate those issues by manually sanitizing the upper part of the partial vector argument register, or by using -mdaz-ftz to set the denormals-are-zero (DAZ) flag in the MXCSR register.

This option is enabled by default. 🍓

• -mmove-max=bits — sets the maximum number of bits that can be moved from memory to memory efficiently to bits. The valid bits are 128, 256 and 512.
• -mstore-max=bits — sets the maximum number of bits that 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 the "noreturn" attribute or "_Noreturn" specifier by not saving, in the function prologue, the callee-saved registers that are used in the function (except for the "BP" register). 🚨 this one 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. know that before you turn it on.
• -mcx16 — lets GCC generate "CMPXCHG16B" instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. iconic for atomic updates of data structures that are bigger than one machine word 🔥. The compiler uses this instruction to implement __sync Builtins. But 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, before the Pentium 4 G1 step landed in December 2005, lacked the "LAHF" and "SAHF" instructions that AMD64 supports. Those two are load and store instructions, respectively, for certain status flags. In 64-bit mode, the "SAHF" instruction is used to optimize the "fmod", "drem", and "remainder" built-in functions — see Other Builtins for the details.
• -mmovbe — enables use of the "movbe" instruction to optimize byte swapping of four and eight byte entities. lil glow-up for your bytes ✨
• -mshstk — enables the shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). 🎀
• -mcrc32 — enables the built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di so they generate the "crc32" machine instruction.
• -mmwait — enables the built-in functions __builtin_ia32_monitor, and __builtin_ia32_mwait so they generate the "monitor" and "mwait" machine instructions.
• -mrecip — enables use of the "RCPSS" and "RSQRTSS" instructions (and their vectorized variants "RCPPS" and "RSQRTPS") with an extra Newton-Raphson step to bump up precision, instead of "DIVSS" and "SQRTSS" (and their vectorized variants), for single-precision floating-point arguments. These instructions only get generated 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 drop by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).

Also note: GCC already implements 1.0f/sqrtf(x) in terms of "RSQRTSS" (or "RSQRTPS") with -ffast-math (or that above option combination), and doesn't need -mrecip.

And also also note 📢: GCC already emits the above sequence with an additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) with -ffast-math (or that above option combination), and doesn't need -mrecip. so like, she's got this already, bestie 💜

• -mrecip=opt — bestie, this one is your remote control for which reciprocal estimate instructions GCC is allowed to bust out. opt is a comma-separated lil list of options, and any of them can get a ! in front to flip it to the opposite 💅
  • all — turn on every single estimate instruction, go feral, enable them all ✨
  • default — the default ones only, literally the same thing as -mrecip 🎀
  • none — nope, none of them, equivalent to -mno-recip 🙈
  • div — enable the approximation for scalar division.
  • vec-div — enable the approximation for vectorized division.
  • sqrt — enable the approximation for scalar square root.
  • vec-sqrt — enable the approximation for vectorized square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. iconic behaviour honestly 🌸

• -mveclibabi=type — says which ABI type to use when vectorizing intrinsics with 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. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. no skipping steps bestie 🫶

GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for 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. that whole guest list, yes ma'am 💖

• -mabi=name — generates code for the calling convention you name. The permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. per-function vibes, we love to see it 💅
• -mforce-indirect-call — forces all calls to functions to be indirect. This is useful when using Intel Processor Trace, where it generates more precise timing information for function calls. indirect era 🔥
• -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. hands-on, manual, artisanal ✨
• -mcet-switch — okay 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 result in a direct loss of hardening. But if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET 😭 This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets reachable via any indirect jumps. hardening unlocked 🔥
• -mcall-ms2sysv-xlogues — so here's the tea: due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, which gives you fairly lengthy prologues and epilogues 😮‍💨 Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to do those saves and restores, which reduces function size at the cost of a few extra instructions. smaller functions, little tradeoff, we love a bargain 🎀
• -mtls-dialect=type — generates 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 cannot be satisfied on all systems. highkey risky on some systems, so know your target bestie 🦋
• -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. sometimes less is more, periodt 💅
• -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size 😭 This switch implies -mno-push-args. faster but chonkier, pick your fighter ✨
• -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. compile and link, both, don't be half-in 🙈
• -mms-bitfields, -mno-ms-bitfields — enable/disable bit-field layout that's compatible with the native Microsoft Windows compiler.

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. Particularly when 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. cross-compiler handshakes are always a whole thing 🫶

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 fairly simple, with the exception of 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. one lil line, double the size, no thank you 💀

2. If a zero-length bit-field is inserted after a bit-field, foo, and the alignment of the zero-length bit-field is greater than the member that follows it, bar, bar is aligned as the type of the zero-length bit-field.

For example:

struct
 {
   char foo : 4;
   short : 0;
   char bar;
 } t2;

struct
 {
   char foo : 4;
   short : 0;
   double bar;
 } t3;

For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.

Taking this into account, it is important to note the following:

1. If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as a whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:

struct
 {
   char foo : 6;
   long : 0;
 } t4;

Here, t4 takes up 4 bytes.

3. Zero-length bit-fields following non-bit-field members are ignored:

struct
 {
   char foo;
   long : 0;
   char bar;
 } t5;

Here, t5 takes up 2 bytes.

okay that was a whole lecture but you needed it bestie, layout math is not beating the allegations 🌸

• -mno-align-stringops — do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. GCC being oblivious is so real 😭
• -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. more inlining, more code, more speed for short stuff — slay 🔥

---

• -minline-stringops-dynamically — bestie, for string operations where nobody knows the size at compile time, this does a lil run-time check ✨: inline code for the smol blocks, and a library call for the big chonky blocks. Eat.
• -mstringop-strategy=alg — override GCC's internal decision heuristic and tell it exactly which algorithm to use when inlining string operations. The allowed values for alg are, no cap:
  • rep_byte, rep_4byte, rep_8byte — expand using the i386 "rep" prefix of the specified size.
  • byte_loop, loop, unrolled_loop — expand into an inline loop. 🌀
  • libcall — always use a library call, every single time, periodt.
• -mmemcpy-strategy=strategy — overrides the internal decision heuristic that decides whether "__builtin_memcpy" should get inlined, and which inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets 💅. alg is specified in -mstringop-strategy, and max_size says the max byte size for which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet, and "max_size + 1" of the preceding range. It's giving very precise, and honestly? Iconic.
• -mmemset-strategy=strategy — literally the same situation as -mmemcpy-strategy= above, except this one controls "__builtin_memset" expansion instead. 🫶
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids all the instructions to save, set up, and restore frame pointers, and it frees up an extra register in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder 😭. So like, cute for speed, sad 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 even 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 (don't instrument at all), call (generate a call to __return__), or nop5 (generate a 5 byte nop). Slay.
• -mrecord-return -mno-record-return — generate a __return_loc section pointing to all return instrumentation code. 📍
• -mfentry-name=name — set the name of the __fentry__ symbol called at function entry for -pg -mfentry functions.
• -mfentry-section=name — set the name of the section used to record -mrecord-mcount calls (default __mcount_loc). ✨
• -mskip-rax-setup -mno-skip-rax-setup — when generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up the RAX register when there are no variable arguments passed in vector registers.

Warning: since the RAX register is used to avoid unnecessarily saving vector registers on the stack when passing variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location. 😭 GCC 4.4 or newer don't have those issues, regardless of the RAX register value. So like, if you're on 4.4+ you're fine bestie.

• -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 the range of 0 to 255, an 8-bit unsigned integer divide is used instead of a 32-bit/64-bit integer divide. ⚡
• -mavx256-split-unaligned-load -mavx256-split-unaligned-store — split 32-byte AVX unaligned load and store. Clean, simple, iconic. 🎀
• -mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset — generate stack protection code using the canary at guard. Supported locations are global for the global canary, or tls for a per-thread canary in the TLS block (that one's the default). This option only has effect when -fstack-protector or -fstack-protector-all is specified. 💅

With the latter choice (tls), 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. No frills, just vibes. 🙈
• -mrelax-cmpxchg-loop — when emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins that lack 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. 🔋
• -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. Periodt.
• -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. 🛡️
• -mindirect-branch-cs-prefix — add 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, allowing them to be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time. Lowkey genius.
• -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. 🙏
• -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. 🌈

• -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 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.

• -mno-red-zone — do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it's a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. 🔴
• -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. 👑🌸
• -mcmodel=kernel — bestie we're going into the kernel. This makes code for the kernel code model, where the kernel lives in the negative 2 GB of the address space 🖤. This model has to be used for Linux kernel code, periodt.
• -mcmodel=medium — code for the medium model: the program gets linked in the lower 2 GB of the address space, and small symbols hang out there too. But symbols bigger than -mlarge-data-threshold get yeeted into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked, no cap.
• -mcmodel=large — the large model, the one that makes no assumptions about addresses and sizes of sections. It's giving unbothered. ✨
• -maddress-mode=long — long address mode code. Only supported for 64-bit and x32 environments, and it's literally the default address mode for 64-bit environments.
• -maddress-mode=short — short address mode code. Only supported for 32-bit and x32 environments, and yep, it's the default address mode for those.
• -mneeded, -mno-needed — emits the GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target, so the binary can announce which micro-architecture ISA level it needs to actually run 👑
• -mno-direct-extern-access — okay so: without -fpic or -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, and no I'm not being dramatic: shared libraries compiled with -mno-direct-extern-access and an executable compiled with -mdirect-extern-access may not be binary compatible if protected symbols are used in shared libraries and executable. 😭

• -munroll-only-small-loops — controls conservative small loop unrolling. It's default enabled by O2, and unrolls loops with less than 4 insns by 1 time. Explicit -f[no-]unroll-[all-]loops would disable this flag, so you avoid any unintended unrolling behavior that you did not ask for 🙈
• -mlam=choice — LAM (linear-address masking) lets special bits in the pointer 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.

---

🪟 x86 Windows Options 🪟

These additional options are available for Microsoft Windows targets, and they're so real:

• -mconsole — says a console application is to be generated, by telling 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 🌈
• -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 that CRT library, and links with it. The 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 isn't specified, then the default MinGW import library msvcrt is used for linking and no other adjustment for 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 🧪. This option is available for MinGW targets.
• -mdll — available for Cygwin and MinGW targets. It says a DLL — a dynamic link library, bestie — is to be generated, which enables the selection of the required runtime startup object and entry point 🎀
• -mnop-fun-dllimport — available for Cygwin and MinGW targets. It says the dllimport attribute should just be ignored, like we ignore our exes ✨
• -mthreads — available for MinGW targets. It says MinGW-specific thread support is to be used.
• -municode — available for MinGW-w64 targets. It causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code 🌸
• -mwin32 — available for Cygwin and MinGW targets. It says the typical Microsoft Windows predefined macros are to be set in the pre-processor, but it does not influence the choice of runtime library/startup code. Boundaries, we have them 💅
• -mwindows — available for Cygwin and MinGW targets. It says a GUI application is to be generated by telling the linker to set the PE header subsystem type appropriately 👑
• -fno-set-stack-executable — available for MinGW targets. It says 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, because there the User32 API — which is what sets executable privileges — isn't available.
• -fwritable-relocated-rdata — available for MinGW and Cygwin targets. It says relocated-data in a read-only section is put into the .data section. This is necessary for older runtimes that don't support modification of .rdata sections for pseudo-relocation 🦋
• -mpe-aligned-commons — available for Cygwin and MinGW targets. It says the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It's enabled by default if GCC detects that the target assembler found during configuration supports the feature 🙌

See also under x86 Options for standard options.

---

⚡ Xstormy16 Options ⚡

These options are defined for Xstormy16:

• -msim — chooses startup files and linker script suitable for the simulator. Slay, honestly.

---

🌟 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.
• -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, which produces results with more bits of precision than the IEEE standard specifies 😳. 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.
• -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.
• -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. Main character kernel energy, fr fr.
• -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 🫶
• -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. The 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 💪
• -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. Aligned and unbothered 💅
• -mlongcalls, -mno-longcalls — turn this on and GCC tells the assembler to translate direct calls into indirect calls, unless it can figure out that the target of that direct call sits in the range the call instruction can actually reach. This translation typically happens for calls to functions in other source files. Concretely: the assembler turns a direct CALL instruction into an L32R followed by a CALLX instruction. Default is -mno-longcalls. Reach for this one in programs where the call target can potentially be out of range. 💅 Heads up though — it's implemented in the assembler, not the compiler, so the assembly code GCC hands you still shows direct call instructions. If you wanna see the actual instructions, go look at the disassembled object code. Also note: the assembler uses an indirect call for every cross-file call, not just the ones that genuinely are out of range. Ngl, that's a little extra.
• -mabi=name — generate code for the ABI you name. Permissible values are: call0, windowed. The default ABI gets chosen by your Xtensa core configuration. ✨
• -mabi=call0 — with this ABI, 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. And when this version of the ABI is enabled, the C preprocessor symbol __XTENSA_CALL0_ABI__ is defined. 🎀
• -mabi=windowed — with this one, function parameters get passed in registers a10 through a15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registers a2 through a7. Register a7 may be used as a frame pointer. The register window gets rotated 8 registers back on return. And when this version of the ABI is enabled, the C preprocessor symbol __XTENSA_WINDOWED_ABI__ is defined. 🦋
• -mextra-l32r-costs=n — specify an extra cost, in clock cycles, for instruction RAM/ROM access on L32R instructions. When optimizing for speed, this affects whether a constant gets loaded from the literal pool using L32R, or gets synthesized from a small one with a couple of arithmetic instructions. Default value is 0. ⭐
• -mstrict-align, -mno-strict-align — avoid (-mstrict-align) or allow (-mno-strict-align) 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. 🌸

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👑 zSeries Options 👑

These are listed under

okay bestie that sentence literally just ends there, no cap — the actual list of options picks up right after this. Not me leaving you on a cliffhanger 🙈

🌸 ENVIRONMENT 🌸

okay so this whole section is about the env vars that change the way GCC does its lil thing 💖 Some of them work by handing over directories or prefixes to use when hunting for various kinds of files. Others just, like, describe other aspects of the compilation environment.

Real talk though: you can also tell GCC where to search using options like -B, -I and -L. Those take precedence over places specified with environment variables — and those in turn take precedence over places specified by the configuration of GCC. So it's a whole ranking system, main character hierarchy fr. 👑

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✨ LANG, LC_CTYPE, LC_MESSAGES, LC_ALL

These ones control how GCC uses localization info, which is what lets GCC vibe with different national conventions. GCC peeks at the locale categories LC_CTYPE and LC_MESSAGES — but only if it's been configured to do so. Those categories can be set to any value your installation supports. A typical value is en_GB.UTF-8, which is English in the United Kingdom encoded in UTF-8, no cap.

• LC_CTYPE — specifies character classification. GCC uses it to figure out the character boundaries in a string. This is needed for some multibyte encodings that contain quote and escape characters which would otherwise get interpreted as a string end or an escape. lowkey important!!
• LC_MESSAGES — specifies the language to use in diagnostic messages.

If LC_ALL is set, it overrides the value of LC_CTYPE and LC_MESSAGES. Otherwise, LC_CTYPE and LC_MESSAGES just default to whatever LANG is set to. And if none of these variables are set, GCC defaults to traditional C English behavior. periodt. 🎀

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🦋 TMPDIR

If TMPDIR is set, it tells you which directory to use for temporary files. GCC uses temp files to hold the output of one stage of compilation that's gonna be used as input to the next stage — for example, the output of the preprocessor, which is the input to the compiler proper. it's giving assembly line energy 🏭✨

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💅 GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of that option for more details.

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👑 GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash gets 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. your call bestie 💁

• 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. that's so niche of it honestly 🌈

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🍓 COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX.

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🧚 LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). order matters bb 💖

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🌸 LANG

(okay yes it's back, it does two jobs, we love a multitasker)

This variable is used to pass locale information to the compiler. One way that info gets used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

• C-JIS — Recognize JIS characters.
• C-SJIS — Recognize SJIS characters.
• C-EUCJP — Recognize EUCJP characters.

If LANG is not defined, or if it has some other value, then the compiler uses "mblen" and "mbtowc" as defined by the default locale to recognize and translate multibyte characters.

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💜 GCC_EXTRA_DIAGNOSTIC_OUTPUT

If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable.

• fixits-v1 — Emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
• fixits-v2 — As "fixits-v1", but columns are expressed as display columns, as per -fdiagnostics-column-unit=display.

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Some additional environment variables affect the behavior of the preprocessor. yes we're still going, she has range 💫

🫶 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. That 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. sneaky but iconic 😌

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🔥 DEPENDENCIES_OUTPUT

If this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. System header files are ignored in the dependency output.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too. two-for-one deal, ate 💅

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⭐ SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.

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🌈 SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible. reproducible builds girlies rise up 🧚✨

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.

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🐛 BUGS 🐛

For instructions on reporting bugs, see <file:///usr/share/doc/gcc-14/README.Bugs>. go snitch, they actually want you to 🫶

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📌 FOOTNOTES 📌

1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. -shared suppresses the addition of startup code to alter the floating-point environment as done with -ffast-math, -Ofast or -funsafe-math-optimizations on some targets.

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👀 SEE ALSO 👀

gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb. the whole friend group fr fr 💖

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💜 AUTHOR 💜

See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC.

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📜 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.

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and that's a wrap on the whole thing bestie 😭💖 this lil manual is gcc-14, dated 2026-09-02, and it lives at GCC(1). you made it to the end, which is honestly main character energy — go compile something cute 🎀✨