GCC(1)

💅 gcc(1) — GNU project C and C++ compiler 👑

bestie. sit down. hydrate. we're reading the gcc manual page together and honestly it's a whole journey 💖

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✨ NAME ✨

gcc — the GNU project C and C++ compiler, aka the icon, the legend, the thing that turns your beautiful little text file into a binary that segfaults at 3am 😭

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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...

okay so like — this is only the most useful options, not the whole menu. The full buffet is down below. And <hotpink>g++ accepts mostly the same options as gcc</hotpink>, so if you're a C++ girlie you're not left out 🫶

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🦋 DESCRIPTION 🦋

When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" let you stop this whole process at an intermediate stage — like hitting pause mid-routine. For example, the -c option says do not run the linker. So the output is just object files, freshly output by the assembler ✨

Other options get passed on to one or more stages of processing. Some options control the preprocessor, others control the compiler itself. And then there are yet other options that control the assembler and the linker — most of those are not documented here, because ngl you rarely need to use any of them 💀

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, no games. And if the description for a particular option does not mention a source language, you can use that option with all supported languages. periodt 💅

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

The gcc program accepts options and file names as operands. Many options have multi-letter names, therefore multiple single-letter options may not be grouped — -dv is very different from -d -v. not the same vibe at all, do not confuse them 🙈

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

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

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

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🔥 OPTIONS 🔥

🌸 Option Summary 🌸

Here is a summary of all the options, grouped by type. Explanations are in the following sections. Buckle up bestie, this is the master list 👑

⭐ 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  -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 --assemble  --compile  --dumpbase dumpbase
--dumpbase-ext auxdropsuf  --dumpdir dumppfx --language=language
--output=file  --pass-exit-codes --pipe  --preprocess  --verbose

⭐ C Language Options

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

⭐ C++ Language Options

--compile-std-module -fabi-compat-version=n  -fabi-version=n
-fno-access-control  -faligned-new=[n]
-fno-assume-sane-operators-new-delete -fchar8_t  -fcheck-new
-fconcepts  -fconcepts-diagnostics-depth=n -fconstexpr-depth=n
-fconstexpr-cache-depth=n -fconstexpr-loop-limit=n
-fconstexpr-ops-limit=n -fcontracts
-fcontract-evaluation-semantic=[ignore|observe|enforce|quick_enforce]
-fcontracts-conservative-ipa  -fcontract-checks-outlined
-fcontract-disable-optimized-checks
-fcontracts-client-check=[none|pre|all]
-fcontracts-definition-check=[on|off] -fcoroutines
-fdiagnostics-all-candidates -fno-elide-constructors
-fno-enforce-eh-specs -fext-numeric-literals -fno-gnu-keywords
-fno-immediate-escalation -fno-implement-inlines
-fimplicit-constexpr -fno-implicit-inline-templates
-fno-implicit-templates -fmodule-header[=kind]
-fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-only -fmodules
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins  -fnothrow-opt  -fno-operator-names
-fno-optional-diags -fno-pretty-templates  -frange-for-ext-temps
-freflection -fno-rtti  -fsized-deallocation -fstrict-enums
-fstrong-eval-order[=kind] -ftemplate-backtrace-limit=n
-ftemplate-depth=n -fno-threadsafe-statics  -fuse-cxa-atexit
-fno-use-cxa-get-exception-ptr -fno-weak  -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabbreviated-auto-in-template-arg
-Wabi-tag  -Waligned-new[=kind] -Wcatch-value  -Wcatch-value=n
-Wno-class-conversion  -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy  -Wdangling-reference
-Wno-defaulted-function-deleted -Wno-delete-incomplete
-Wdelete-non-virtual-dtor  -Wno-deprecated-array-compare
-Wdeprecated-copy  -Wdeprecated-copy-dtor
-Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion
-Wno-deprecated-literal-operator
-Wdeprecated-variadic-comma-omission -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions
-Wno-expose-global-module-tu-local  -Wno-external-tu-local
-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
-Wno-sfinae-incomplete -Wstrict-null-sentinel
-Wno-subobject-linkage  -Wtemplates -Wno-non-c-typedef-for-linkage
-Wno-non-template-friend  -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions  -Wself-move  -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types  -Wsuggest-override
-Wno-template-body -Wno-template-id-cdtor  -Wtemplate-names-tu-local
-Wno-terminate  -Wno-vexing-parse  -Wvirtual-inheritance
-Wno-virtual-move-assign  -Wvolatile

⭐ Objective-C and Objective-C++ Language Options

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

⭐ OpenMP and OpenACC Options

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

⭐ Diagnostic Message Formatting Options

-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file]
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
-fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-event-links
-fno-diagnostics-show-labels  -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe -fno-diagnostics-show-rules
-fno-diagnostics-show-highlight-colors -fno-diagnostics-show-nesting
-fno-diagnostics-show-nesting-locations
-fdiagnostics-show-nesting-levels
-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]
-fdiagnostics-show-context[=depth]

🚨 Warning Options 🚨

okay so this is the section EVERYONE actually opens the man page for, and tbh it's a whole buffet 😭 there are so many of these that the list itself needs a snack. quick heads-up bestie: the -Wno- entries below are their own separate flags listed exactly as they appear, so don't come for me about the "no", iykyk 💅

here's the full roster, no skips, no cuts, periodt:

• -fsyntax-only — just check the syntax, that's the whole assignment ✨
• -fmax-errors=n — cap the error spam at n, we love a limit 🎀
• -Wpedantic — the "well, actually" of warning flags
• -pedantic-errors — pedantic but it means it
• -fpermissive — the chill pill flag
• -w — lowercase, one letter, absolute silence 🔇
• -Wextra — the extras, the bonus tracks
• -Wall — the icon, the main character, the one everybody uses 👑
• -Wabi=n — ABI vibes, version n
• -Waddress — address-related callouts
• -Wno-address-of-packed-member — the off-switch for that one
• -Waggregate-return — aggregate returns get the side-eye
• -Walloc-size — allocation size, keeping it on brand
• -Walloc-size-larger-than=byte-size — threshold time, in bytes
• -Walloc-zero — zero-size allocation, noted
• -Walloca — alloca gets its own flag, obvi
• -Walloca-larger-than=byte-size — same but with a byte limit
• -Wauto-profile — auto-profile era 💫
• -Wno-aggressive-loop-optimizations — turning off the aggressive loop stuff
• -Warith-conversion — arithmetic conversions, under the microscope
• -Warray-bounds — out-of-bounds array behaviour gets flagged
• -Warray-bounds=n — same, but pick your level n
• -Warray-compare — comparing arrays, spotted
• -Warray-parameter — array parameters, noticed
• -Warray-parameter=n — array parameters but with a level
• -Wno-attributes — attributes, but make it off
• -Wattribute-alias=n — attribute aliasing, level n
• -Wno-attribute-alias — the off version
• -Wno-attribute-warning — off again, respectfully
• -Wbidi-chars=[none|unpaired|any|ucn] — bidi character check, with four modes: none, unpaired, any, ucn 🦋
• -Wbool-compare — bools getting compared
• -Wbool-operation — bools getting operated on
• -Wno-builtin-declaration-mismatch — mismatched builtin decls, silenced
• -Wno-builtin-macro-redefined — redefined builtin macros, silenced
• -Wc90-c99-compat — C90 vs C99 compatibility tea ☕
• -Wc99-c11-compat — C99 vs C11
• -Wc11-c23-compat — C11 vs C23
• -Wc23-c2y-compat — C23 vs C2y, the new era
• -Wc++-compat — C vs C++, the classic crossover episode
• -Wc++11-compat — C++11 compatibility
• -Wc++14-compat — C++14 compatibility
• -Wc++17-compat — C++17 compatibility
• -Wc++20-compat — C++20 compatibility
• -Wc++26-compat — C++26 compatibility, forward-thinking queen 🌈
• -Wno-c++11-extensions — extensions off
• -Wno-c++14-extensions — extensions off
• -Wno-c++17-extensions — extensions off
• -Wno-c++20-extensions — extensions off
• -Wno-c++23-extensions — extensions off (yes there are five of these, I don't make the rules)
• -Wcalloc-transposed-args — transposed calloc args
• -Wcannot-profile — literally "cannot profile", a mood
• -Wcast-align — casting and alignment
• -Wcast-align=strict — same but strict mode 🔥
• -Wcast-function-type — function pointer casts
• -Wcast-qual — casts that drop qualifiers
• -Wchar-subscripts — char subscripts, underrated
• -Wclobbered — clobbered variables, yes ma'am
• -Wcomment — comment-related concerns
• -Wcompare-distinct-pointer-types — comparing pointers of different types
• -Wno-complain-wrong-lang — "don't complain about the wrong language"
• -Wconversion — conversions, all up in your business
• -Wno-coverage-mismatch — coverage mismatch, off
• -Wno-cpp — preprocessor warnings, off
• -Wdangling-else — the dangling else classic 😭
• -Wdangling-pointer — dangling pointers
• -Wdangling-pointer=n — dangling pointers, level n
• -Wdate-time — __DATE__/__TIME__-ish energy
• -Wno-deprecated — deprecated stuff, off
• -Wno-deprecated-declarations — deprecated declarations, off
• -Wno-designated-init — designated init, off
• -Wno-deprecated-openmp — deprecated OpenMP, off
• -Wdisabled-optimization — disabled optimizations get reported
• -Wno-discarded-array-qualifiers — off
• -Wno-discarded-qualifiers — also off
• -Wno-div-by-zero — div by zero, conspicuously off 👀
• -Wdouble-promotion — double promotion, promoted to a warning
• -Wduplicated-branches — duplicated branches
• -Wduplicated-cond — duplicated conditions
• -Wempty-body — empty bodies
• -Wno-endif-labels — #endif labels, off
• -Wenum-compare — comparing enums
• -Wenum-conversion — converting enums
• -Wenum-int-mismatch — enum/int mismatch
• -Werror — every warning becomes an error, no chill, iconic 💀
• -Werror=* — same but targeted at specific ones
• -Wexpansion-to-defined — macro expansion into defined
• -Wfatal-errors — first error and we're DONE
• -Wflex-array-member-not-at-end — flex array member not at the end
• -Wfloat-conversion — float conversions
• -Wfloat-equal — comparing floats with ==, the eternal discourse
• -Wformat — the printf family check we all know 🫶
• -Wformat=2 — the deluxe version
• -Wno-format-contains-nul — off
• -Wno-format-diag — off
• -Wno-format-extra-args — off
• -Wformat-nonliteral — non-literal format strings
• -Wformat-overflow=n — format overflow, level n
• -Wformat-security — format security, bestie pls
• -Wformat-signedness — signedness in formats
• -Wformat-truncation=n — truncation, level n
• -Wformat-y2k — yes, y2k. it survived the vibes 🎀
• -Wframe-address — frame addresses
• -Wframe-larger-than=byte-size — frame size limit in bytes
• -Wno-free-nonheap-object — off
• -Wheader-guard — header guards
• -Wno-if-not-aligned — off
• -Wno-ignored-attributes — off
• -Wignored-qualifiers — ignored qualifiers, noticed
• -Wno-incompatible-pointer-types — off
• -Whardened — hardened mode 🛡️
• -Wimplicit — implicit stuff
• -Wimplicit-fallthrough — the switch fallthrough of it all
• -Wimplicit-fallthrough=n — fallthrough, level n
• -Wno-implicit-function-declaration — off
• -Wno-implicit-int — off
• -Winfinite-recursion — infinite recursion, gonna be a no from me
• -Winit-self — self-initialization
• -Winline — inlining
• -Wno-int-conversion — off
• -Wint-in-bool-context — int where a bool should be
• -Wno-int-to-pointer-cast — off
• -Wno-invalid-memory-model — off
• -Winvalid-pch — invalid precompiled headers
• -Winvalid-utf8 — invalid UTF-8, rude
• -Wno-unicode — off
• -Wjump-misses-init — jump skipping initialization
• -Wkeyword-macro — keyword macros
• -Wlarger-than=byte-size — bigger than byte-size
• -Wleading-whitespace=kind — leading whitespace, kind kind
• -Wlogical-not-parentheses — ! and parens drama
• -Wlogical-op — logical operators
• -Wlong-long — long long, the traditional one
• -Wno-lto-type-mismatch — off
• -Wmain — the main function gets checked, main character energy indeed 👑
• -Wmaybe-uninitialized — maybe uninitialized, the suspense 😭
• -Wmemset-elt-size — memset element size
• -Wmemset-transposed-args — transposed memset args
• -Wmisleading-indentation — indentation that lies to you
• -Wmissing-attributes — missing attributes
• -Wmissing-braces — missing braces
• -Wmissing-field-initializers — missing field initializers
• -Wmissing-format-attribute — missing format attribute
• -Wmissing-include-dirs — include dirs that aren't there
• -Wmissing-noreturn — missing noreturn
• -Wmusttail-local-addr — musttail + local address
• -Wmaybe-musttail-local-addr — the "maybe" version, indecisive queen 🦋
• -Wno-missing-profile — off
• -Wno-multichar — off
• -Wmultistatement-macros — multi-statement macros
• -Wnonnull — nonnull args getting null
• -Wnonnull-compare — nonnull comparisons
• -Wnormalized=[none|id|nfc|nfkc] — normalization modes: none, id, nfc, nfkc ✨
• -Wnull-dereference — null dereference, the classic
• -Wno-odr — ODR, off
• -Wopenacc-parallelism — OpenACC parallelism
• -Wopenmp — OpenMP
• -Wopenmp-simd — OpenMP SIMD
• -Wno-overflow — off
• -Woverlength-strings — strings that are too long
• -Wno-override-init-side-effects — off
• -Wpacked — packed structs
• -Wno-packed-bitfield-compat — off
• -Wpacked-not-aligned — packed and not aligned
• -Wpadded — padding 🎀
• -Wparentheses — parentheses, use them bestie
• -Wno-pedantic-ms-format — off
• -Wpointer-arith — pointer arithmetic
• -Wno-pointer-compare — off
• -Wno-pointer-to-int-cast — off
• -Wno-pragmas — off
• -Wno-pragma-once-outside-header — off
• -Wno-prio-ctor-dtor — off
• -Wno-psabi — off
• -Wredundant-decls — redundant declarations, we see you
• -Wrestrict — restrict violations
• -Wno-return-local-addr — off
• -Wreturn-type — return types
• -Wno-scalar-storage-order — off
• -Wsequence-point — sequence points
• -Wshadow — shadowing, the naming crime 🕵️
• -Wshadow=global — global shadowing
• -Wshadow=local — local shadowing
• -Wshadow=compatible-local — compatible-local shadowing
• -Wno-shadow-ivar — off
• -Wno-shift-count-negative — off
• -Wno-shift-count-overflow — off
• -Wshift-negative-value — shifting a negative value
• -Wno-shift-overflow — off
• -Wshift-overflow=n — shift overflow, level n
• -Wsign-compare — signed/unsigned comparison, the villain of every codebase 💀
• -Wsign-conversion — sign conversions
• -Wno-sizeof-array-argument — off
• -Wsizeof-array-div — sizeof array division
• -Wsizeof-pointer-div — sizeof pointer division
• -Wsizeof-pointer-memaccess — sizeof pointer memaccess
• -Wstack-protector — stack protector
• -Wstack-usage=byte-size — stack usage in bytes
• -Wstrict-aliasing — strict aliasing
• -Wstrict-aliasing=n — strict aliasing, level n
• -Wstrict-overflow — strict overflow
• -Wstrict-overflow=n — strict overflow, level n
• -Wstring-compare — string comparison
• -Wno-stringop-overflow — off
• -Wno-stringop-overread — off
• -Wno-stringop-truncation — off
• -Wstrict-flex-arrays — strict flex arrays
• -Wsuggest-attribute=attribute-name — suggests an attribute, name it in attribute-name
• -Wswitch — switches
• -Wno-switch-bool — off
• -Wswitch-default — missing default
• -Wswitch-enum — switch over enums
• -Wno-switch-outside-range — off
• -Wno-switch-unreachable — off
• -Wsync-nand — the __sync_nand situation
• -Wsystem-headers — warnings from system headers too, brave
• -Wtautological-compare — comparing something to itself
• -Wtrailing-whitespace — trailing whitespace, the lint of lints
• -Wtrailing-whitespace=kind — trailing whitespace, kind kind
• -Wtrampolines — trampolines 🎪
• -Wtrigraphs — trigraphs, ancient lore
• -Wtrivial-auto-var-init — trivial auto var init
• -Wno-tsan — ThreadSanitizer warnings, off
• -Wtype-limits — type limits
• -Wundef — undefined identifiers in #if
• -Wuninitialized — uninitialized vars, bestie pls initialize
• -Wunknown-pragmas — pragmas nobody recognizes
• -Wunsuffixed-float-constants — float constants with no suffix
• -Wunterminated-string-initialization — unterminated string initialization
• -Wunused — the whole unused family, the umbrella
• -Wunused-but-set-parameter — set but never used, parameter edition
• -Wunused-but-set-parameter=n — same, level n
• -Wunused-but-set-variable — set but never used, variable edition
• -Wunused-but-set-variable=n — same, level n
• -Wunused-const-variable — unused const variable
• -Wunused-const-variable=n — same, level n
• -Wunused-function — unused functions
• -Wunused-label — unused labels
• -Wunused-local-typedefs — unused local typedefs
• -Wunused-macros — unused macros
• -Wunused-parameter — unused parameters
• -Wno-unused-result — off
• -Wunused-value — unused values
• -Wunused-variable — unused variables
• -Wuse-after-free — use after free, the horror 💀
• -Wuse-after-free=n — same, level n
• -Wuseless-cast — casts that do literally nothing
• -Wno-varargs — off
• -Wvariadic-macros — variadic macros
• -Wvector-operation-performance — vector operation performance
• -Wvla — variable length arrays
• -Wvla-larger-than=byte-size — VLA bigger than byte-size
• -Wno-vla-larger-than — off
• -Wvolatile-register-var — volatile register variables
• -Wwrite-strings — write strings
• -Wno-xor-used-as-pow — off
• -Wzero-as-null-pointer-constant — zero as a null pointer constant, we've all done it
• -Wzero-length-bounds — zero-length bounds
• --all-warnings — long-form, all of them
• --extra-warnings — long-form, the extras
• --no-warnings — long-form, none of them
• --pedantic — long-form pedantic
• --pedantic-errors — long-form pedantic-errors

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🔍 Static Analyzer Options 🔍

okay so this is the analyzer era, the "let me actually look at your code before you run it" moment 💅 it's a whole ecosystem down here, with dumps and checkers and verbosity levels. no cap, it's a lot.

• -fanalyzer — the whole analyzer, the big one, the moment 🔥
• -fanalyzer-assume-nothrow — assume nothing throws
• -fanalyzer-call-summaries — call summaries
• -fanalyzer-checker=name — pick a specific checker by name
• -fno-analyzer-feasibility — feasibility, off
• -fanalyzer-show-events-in-system-headers — show events happening in system headers
• -fno-analyzer-state-merge — state merging, off
• -fno-analyzer-state-purge — state purging, off
• -fno-analyzer-suppress-followups — followups not suppressed
• -fanalyzer-transitivity — transitivity
• -fno-analyzer-undo-inlining — inlining undo, off
• -fanalyzer-verbose-edges — verbose edges
• -fanalyzer-verbose-state-changes — verbose state changes
• -fanalyzer-verbosity=level — verbosity, level level
• -fdump-analyzer — dump it all 🗑️
• -fdump-analyzer-callgraph — dump the callgraph
• -fdump-analyzer-exploded-graph — exploded graph, dramatic name, love it
• -fdump-analyzer-exploded-nodes — exploded nodes
• -fdump-analyzer-exploded-nodes-2 — exploded nodes, the sequel
• -fdump-analyzer-exploded-nodes-3 — exploded nodes, threequel
• -fdump-analyzer-exploded-paths — exploded paths
• -fdump-analyzer-feasibility — dump feasibility
• -fdump-analyzer-infinite-loop — dump the infinite loop
• -fdump-analyzer-json — dump it as JSON 🎀
• -fdump-analyzer-state-purge — dump state purge
• -fdump-analyzer-stderr — dump to stderr
• -fdump-analyzer-supergraph — supergraph 💫
• -fdump-analyzer-untracked — dump untracked
• -Wno-analyzer-double-fclose — off
• -Wno-analyzer-double-free — off
• -Wno-analyzer-exposure-through-output-file — off
• -Wno-analyzer-exposure-through-uninit-copy — off
• -Wno-analyzer-fd-access-mode-mismatch — off
• -Wno-analyzer-fd-double-close — off
• -Wno-analyzer-fd-leak — off
• -Wno-analyzer-fd-phase-mismatch — off
• -Wno-analyzer-fd-type-mismatch — off
• -Wno-analyzer-fd-use-after-close — off
• -Wno-analyzer-fd-use-without-check — off
• -Wno-analyzer-file-leak — off
• -Wno-analyzer-free-of-non-heap — off
• -Wno-analyzer-imprecise-fp-arithmetic — off
• -Wno-analyzer-infinite-loop — off
• -Wno-analyzer-infinite-recursion — off
• -Wno-analyzer-jump-through-null — off
• -Wno-analyzer-malloc-leak — off
• -Wno-analyzer-mismatching-deallocation — off
• -Wno-analyzer-null-argument — off
• -Wno-analyzer-null-dereference — off
• -Wno-analyzer-out-of-bounds — off
• -Wno-analyzer-overlapping-buffers — off
• -Wno-analyzer-possible-null-argument — off
• -Wno-analyzer-possible-null-dereference — off
• -Wno-analyzer-putenv-of-auto-var — off
• -Wno-analyzer-shift-count-negative — off
• -Wno-analyzer-shift-count-overflow — off
• -Wno-analyzer-stale-setjmp-buffer — off
• -Wno-analyzer-tainted-allocation-size — off
• -Wno-analyzer-tainted-assertion — off
• -Wno-analyzer-tainted-array-index — off
• -Wno-analyzer-tainted-divisor — off
• -Wno-analyzer-tainted-offset — off
• -Wno-analyzer-tainted-size — off
• -Wno-analyzer-throw-of-unexpected-type — off
• -Wanalyzer-symbol-too-complex — symbol too complex, relatable 😭
• -Wanalyzer-too-complex — analyzer too complex, also relatable
• -Wno-analyzer-undefined-behavior-ptrdiff — off
• -Wno-analyzer-undefined-behavior-strtok — off
• -Wno-analyzer-unsafe-call-within-signal-handler — off
• -Wno-analyzer-use-after-free — off
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — off
• -Wno-analyzer-use-of-uninitialized-value — off
• -Wno-analyzer-va-arg-type-mismatch — off
• -Wno-analyzer-va-list-exhausted — off
• -Wno-analyzer-va-list-leak — off
• -Wno-analyzer-va-list-use-after-va-end — off
• -Wno-analyzer-write-to-const — off
• -Wno-analyzer-write-to-string-literal — off

---

🌸 C and Objective-C-only Warning Options 🌸

these ones are exclusively for the C and Objective-C girlies — if you're writing C++ they're just not your lane, and that's okay 🫶

• -Wbad-function-cast — bad function casts
• -Wdeprecated-non-prototype — deprecated non-prototype style
• -Wfree-labels — free labels
• -Wmissing-declarations — missing declarations
• -Wmissing-parameter-name — parameter with no name
• -Wmissing-parameter-type — parameter with no type
• -Wdeclaration-missing-parameter-type — declaration missing a parameter type
• -Wmissing-prototypes — missing prototypes
• -Wmissing-variable-declarations — missing variable declarations
• -Wmultiple-parameter-fwd-decl-lists — multiple parameter forward decl lists
• -Wnested-externs — nested externs
• -Wold-style-declaration — old-style declarations, vintage
• -Wold-style-definition — old-style definitions, also vintage
• -Wstrict-prototypes — strict prototypes
• -Wtraditional — traditional mode, ancient times 🏺
• -Wtraditional-conversion — traditional conversions
• -Wdeclaration-after-statement — declarations after statements
• -Wpointer-sign — pointer signedness

---

🐛 Debugging Options 🐛

okay debug girlies rise, this is your section — -g and all its little friends, plus the DWARF lore and the dumpy bits. ngl there is SO much down here but we're getting through it together 💜

• -g — the one, the only, generate debug info 🎀
• -glevel — -g but with a level
• -gdwarf — DWARF format
• -gdwarf-version — DWARF, but pick a version
• -gbtf — BTF
• -gctf — CTF
• -gctflevel — CTF with a level
• -gno-prune-btf — don't prune BTF
• -ggdb — the GDB-flavoured version 🫶
• -gno-record-gcc-switches — don't record the gcc switches
• -gstrict-dwarf — strict DWARF
• -gas-loc-support — asm loc support
• -gas-locview-support — asm locview support
• -gcodeview — CodeView output
• -gcolumn-info — column info
• -gdwarf32 — DWARF32
• -gdwarf64 — DWARF64
• -gno-statement-frontiers — no statement frontiers
• -gno-variable-location-views — no variable location views
• -gvariable-location-views=incompat5 — variable location views, incompat5 (yes, exactly like that)
• -ginternal-reset-location-views — internal reset location views
• -ginline-points — inline points
• -gvms — VMS format
• -gz[=type] — compress the debug sections, optionally as type ✨
• -gsplit-dwarf — split DWARF files
• -gdescribe-dies — describe DIEs
• -fdebug-prefix-map=old=new — remap old to new in debug info
• -fdebug-types-section — a debug types section
• -fno-eliminate-unused-debug-types — don't eliminate unused debug types
• -femit-struct-debug-baseonly — emit struct debug info, base only
• -femit-struct-debug-reduced — emit struct debug info, reduced
• -femit-struct-debug-detailed[=spec-list] — emit struct debug info, detailed, with an optional spec-list
• -fno-eliminate-unused-debug-symbols — don't eliminate unused debug symbols
• -femit-class-debug-always — always emit class debug info
• -fno-merge-debug-strings — don't merge debug strings
• -fno-dwarf2-cfi-asm — no DWARF2 CFI asm
• -fvar-tracking — variable tracking
• -fvar-tracking-assignments — variable tracking assignments
• -fvar-tracking-uninit — variable tracking, uninit edition
• --debug — long-form debug, the glamorous alias 💅

okay so we're still in OPTIONS and now we're hitting the optimization ones, which are literally the main character energy of gcc. no cap, there are so many flags here that it's giving "i have a spreadsheet for my coffee order." here's every single one, bestie, do not lose focus:

✨ Optimization Options ✨

this is the biggest vibe check in the whole page. it's giving "i have 300 settings and i'm not afraid to use them." every flag is listed exactly, no skips, no edits:

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

and that's the optimization section, periodt. now let's talk instrumentation.

🎀 Program Instrumentation Options 🎀

this is the section where you tell gcc to spy on your program for science. it's giving "i'm not nosy, i'm just collecting data." every flag, unchanged:

• -p
• -pg
• -fprofile-arcs
• -coverage
• -ftest-coverage
• -fcondition-coverage
• -fpath-coverage
• -fprofile
• -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
• -fcf-protection
• -fcf-protection=[full|branch|return|none|check]
• -fharden-compares
• -fharden-conditional-branches
• -fhardened
• -fharden-control-flow-redundancy
• -fhardcfr-skip-leaf
• -fhardcfr-check-exceptions
• -fhardcfr-check-returning-calls
• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
• -fstack-protector
• -fstack-protector-all
• -fstack-protector-strong
• -fstack-protector-explicit
• -fstack-check
• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit
• -fsplit-stack
• -fstrub=disable
• -fstrub=strict
• -fstrub=relaxed
• -fstrub=all
• -fstrub=at-calls
• -fstrub=internal
• -fvtable-verify=[std|preinit|none]
• -fvtv-counts
• -fvtv-debug
• -finstrument-functions
• -finstrument-functions-once
• -finstrument-functions-exclude-function-list=sym,sym,...
• -finstrument-functions-exclude-file-list=file,file,...
• -fprofile-prefix-map=old=new
• -fpatchable-function-entry=N[,M]
• --coverage
• --profile

instrumentation? slayed. moving on.

🦋 Preprocessor Options 🦋

this is where the preprocessor lives, bestie. it's the part that runs before your code even knows what hit it. all the flags, exactly:

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

preprocessor done. now the little assembler corner.

🌸 Assembler Options 🌸

just three, bestie, but they matter:

• -Wa,option
• -Xassembler option
• --for-assembler=option

💜 Linker Options 💜

this is the linker section and it's highkey iconic. every single option, no cap:

• object-file-name
• -flink-libatomic
• -fuse-ld=linker
• -llibrary
• -nostartfiles
• -nodefaultlibs
• -nolibc
• -nostdlib
• -nostdlib++
• -e entry
• -pie
• -pthread
• -r
• -rdynamic
• -s
• -static
• -static-pie
• -static-libgcc
• -static-libstdc++
• -static-libasan
• -static-libhwasan
• -static-liblsan
• -static-libtsan
• -static-libubsan
• -shared
• -shared-libgcc
• -symbolic
• -T script
• -Wl,option
• -Xlinker option
• -u symbol
• -Tbss=addr
• -Tdata=addr
• -Ttext=addr
• -N
• -n
• -t
• -Z
• -z keyword
• --entry=entry
• --for-linker=option
• --force-link=symbol
• --no-standard-library
• --pie
• --static
• --static-pie
• --symbolic

🧚 Directory Options 🧚

directory paths and include stuff, all here:

• -Bprefix
• -Idir
• -I-
• -idirafter dir
• -imacros file
• -imultilib dir
• -imultiarch dir
• -iplugindir=dir
• -iprefix file
• -iquote dir
• -isysroot dir
• -isystem dir
• -iwithprefix dir
• -iwithprefixbefore dir
• -Ldir
• -no-canonical-prefixes
• --no-sysroot-suffix
• -nostdinc
• -nostdinc++
• --embed-dir=dir
• --embed-directory=dir
• --include-barrier
• --include-directory=dir
• --include-directory-after=dir
• --include-prefix=prefix
• --include-with-prefix=prefix
• --include-with-prefix-after=prefix
• --include-with-prefix-before=prefix
• --no-canonical-prefixes
• --no-standard-includes
• --prefix=prefix
• --sysroot=dir

(yes, --no-canonical-prefixes shows up twice in the original, so it shows up twice here too. consistency, bestie.)

🔥 Code Generation Options 🔥

last little group in this part, and it's giving "i control how the code actually gets made." all the flags:

• -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
• -fwrapv-pointer
• -fvisibility=[default|internal|hidden|protected]
• -fstrict-volatile-bitfields
• -fsync-libcalls
• -fzero-init-padding-bits=value
• -Qy
• -Qn

okay that's the code gen options done. next part is still coming, don't go anywhere. 💅

🛠️ Developer Options 🛠️

okay bestie, this is the compiler’s backstage-pass zone: dev-only knobs, dumps, reports, and “what is GCC even doing” switches. No cap, every single one is here 💅

• -dletters — the dev dump switch where letters does the lil’ dump thing ✨
• -dumpspecs — dumps the specs, literally what it says 🎀
• -dumpmachine — prints the target machine 🖥️
• -dumpversion — prints the version, short and sweet 💖
• -dumpfullversion — prints the full version, no truncation energy 🌸
• -fcallgraph-info[=su,da] — callgraph info, optionally with su,da 🦋
• -fchecking — turns on checking vibes 🔍
• -fchecking=n — checking at level n, bestie ✨
• -fdbg-cnt-list — lists the debug counter names 💅
• -fdbg-cnt=counter-value-list — sets debug counters from counter-value-list 🎀
• -fdisable-ipa-pass_name — disables the IPA pass called pass_name 🙈
• -fdisable-rtl-pass_name — disables the RTL pass called pass_name 🔥
• -fdisable-rtl-pass-name=range-list — disables that RTL pass for range-list 💀
• -fdisable-tree-pass_name — disables the tree pass called pass_name 🌿
• -fdisable-tree-pass-name=range-list — disables that tree pass for range-list 🍓
• -fdump-debug — dumps debug info 🐛
• -fdump-earlydebug — dumps early debug info, before the glow-up ✨
• -fdump-noaddr — dumps without addresses 🙅
• -fdump-unnumbered — dumps without numbers 🔢
• -fdump-unnumbered-links — dumps unnumbered links 🔗
• -fdump-final-insns[=file] — dumps final insns, optionally into file 📄
• -fdump-internal-locations — dumps internal locations 📍
• -fdump-ipa-all — dumps all the IPA stuff 🌈
• -fdump-ipa-cgraph — dumps the IPA callgraph 🕸️
• -fdump-ipa-inline — dumps IPA inlining info 🫶
• -fdump-lang-all — dumps all the language-specific stuff 🗣️
• -fdump-lang-switch — language dump switch, iykyk 🔀
• -fdump-lang-switch-options — language dump switch options 🎛️
• -fdump-lang-switch-options=filename — same but writes to filename 📝
• -fdump-passes — dumps the passes 👑
• -fdump-rtl-pass — dumps the RTL pass, the classic 💅
• -fdump-rtl-pass=filename — dumps the RTL pass into filename 📁
• -fdump-statistics — dumps statistics, numbers girlie 📊
• -fdump-tree-all — dumps all the tree passes 🌳
• -fdump-tree-switch — tree dump switch 🔀
• -fdump-tree-switch-options — tree dump switch options 🎛️
• -fdump-tree-switch-options=filename — tree dump switch options into filename 📄
• -fcompare-debug[=opts] — compares debug output, optionally with opts 🐞
• -fcompare-debug-second — the second compare-debug pass, bestie ✌️
• -fenable-kind-pass — enables a kind of pass by name ✅
• -fenable-kind-pass=range-list — enables that kind of pass for range-list 🎯
• -fira-verbose=n — IRA verbosity level n 🗣️
• -flto-report — LTO report time 📈
• -flto-report-wpa — LTO WPA report time 📊
• -fmem-report-wpa — WPA memory report 🧠
• -fmem-report — memory report, the receipts 💾
• -fpre-ipa-mem-report — pre-IPA memory report 🧮
• -fpost-ipa-mem-report — post-IPA memory report 🧮
• -fopt-info — optimization info, the tea ☕
• -fopt-info-options[=file] — opt-info options, optionally into file 📁
• -fmultiflags — multiflags mode, periodt 💅
• -fprofile-report — profile report 📋
• -frandom-seed=string — sets the random seed from string 🎲
• -fsched-verbose=n — scheduler verbosity level n 🗓️
• -fsel-sched-verbose — selective scheduler verbosity 🗣️
• -fsel-sched-dump-cfg — dumps the selective scheduler CFG 🕸️
• -fsel-sched-pipelining-verbose — selective scheduler pipelining verbosity 🚰
• -fstats — stats mode 📊
• -fstack-usage — stack usage info 🥞
• -ftime-report — time report ⏱️
• -ftime-report-details — time report with details 🔍
• -fvar-tracking-assignments-toggle — toggles var-tracking assignments 🔁
• -gtoggle — toggles debug info generation 🐛
• -print-autofdo-gcov-version — prints the AutoFDO gcov version 🎀
• -print-file-name=library — prints the full path of library 📚
• -print-libgcc-file-name — prints the libgcc file name 💖
• -print-multi-directory — prints the multi directory 📂
• -print-multi-lib — prints multi-lib info 🧚
• -print-multi-os-directory — prints the multi OS directory 🗂️
• -print-multiarch — prints the multiarch 🌍
• -print-prog-name=program — prints the name of program 🏷️
• -print-search-dirs — prints search dirs 🔎
• -Q — the compiler’s “what are you doing” switch 💅
• -print-sysroot — prints the sysroot 🌱
• -print-sysroot-headers-suffix — prints the sysroot headers suffix 🏷️
• -save-temps — saves temp files, no deleting the receipts 💾
• -save-temps=cwd — saves temps in the current working directory 📍
• -save-temps=obj — saves temps by the object output 📦
• -specs=file — uses file as the specs file 📜
• -time[=file] — times stuff, optionally into file ⏱️
• --dump=letters — long-form dump switch with letters ✨
• --print-autofdo-gcov-version — long-form AutoFDO gcov version print 🎀
• --print-file-name=library — long-form print of library’s path 📚
• --print-libgcc-file-file-name — long-form libgcc file name print, exactly as written bestie 💖
• --print-multi-directory — long-form multi directory print 📂
• --print-multi-lib — long-form multi-lib print 🧚
• --print-multi-os-directory — long-form multi OS directory print 🗂️
• --print-multiarch — long-form multiarch print 🌍
• --print-prog-name=program — long-form print of program’s name 🏷️
• --print-search-dirs — long-form search-dirs print 🔎
• --print-sysroot — long-form sysroot print 🌱
• --print-sysroot-headers-suffix — long-form sysroot headers suffix print 🏷️
• --save-temps — long-form save-temps 💾
• --specs=file — long-form specs file 📜
• --param name=value — sets parameter name to value 🎛️

---

🌍 Target-Specific Options 🌍

these are the per-architecture menus, bestie — same compiler, different little machine-specific flavors 🍓

🌸 AArch64 Options (AArch64 Options) 🌸

• -mabi=name — ABI is name 🏷️
• -mbig-endian — big-endian mode 🐘
• -mlittle-endian — little-endian mode 🐜
• -menable-sysreg-checking — enables sysreg checking 🔍
• -mgeneral-regs-only — general registers only, no extras 💅
• -mcmodel=tiny — code model tiny 🐣
• -mcmodel=small — code model small 🐥
• -mcmodel=large — code model large 🦅
• -mstrict-align — strict alignment vibes 📐
• -momit-leaf-frame-pointer — omits the leaf frame pointer 🍃
• -mtls-dialect=desc — TLS dialect desc 🧵
• -mtls-dialect=traditional — TLS dialect traditional 🧶
• -mtls-size=size — TLS size is size 📏
• -mtp=name — thread pointer style name 📍
• -mfix-cortex-a53-835769 — the Cortex-A53 835769 fix 🛠️
• -mfix-cortex-a53-843419 — the Cortex-A53 843419 fix 🛠️
• -mlow-precision-recip-sqrt — low-precision reciprocal sqrt 🧮
• -mlow-precision-sqrt — low-precision sqrt 🧮
• -mlow-precision-div — low-precision divide ➗
• -mmax-vectorization — max vectorization mode 🚀
• -mautovec-preference=name — autovec preference name 🎛️
• -mpc-relative-literal-loads — PC-relative literal loads 📦
• -msign-return-address=scope — return-address signing scope scope ✍️
• -mbranch-protection=features — branch protection features 🛡️
• -mharden-sls=opts — SLS hardening with opts 🧱
• -march=name — architecture is name 🏛️
• -mcpu=name — CPU is name 🧠
• -mtune=name — tune for name 🎶
• -moverride=string — override string string 🎛️
• -mstack-protector-guard=guard — stack protector guard is guard 🛡️
• -mstack-protector-guard-reg=sysreg — guard register is sysreg 📍
• -mstack-protector-guard-offset=offset — guard offset is offset 📏
• -mtrack-speculation — tracks speculation 👀
• -moutline-atomics — outline atomics mode 🧵
• -mearly-ra — early RA mode 🌅
• -mearly-ldp-fusion — early LDP fusion 🧬
• -mlate-ldp-fusion — late LDP fusion 🧬
• -mnarrow-gp-writes — narrow GP writes ✍️
• -msve-vector-bits=bits — SVE vector bits is bits 🧮

🌼 Adapteva Epiphany Options (Adapteva Epiphany Options) 🌼

• -mhalf-reg-file — half register file mode 🧺
• -mprefer-short-insn-regs — prefers short insn regs 🩳
• -mbranch-cost=num — branch cost is num 🌿
• -mcmove — cmove mode 🔀
• -mnops=num — nops is num 😴
• -msoft-cmpsf — soft cmpsf mode 🧸
• -msplit-lohi — split lo/hi 🔪
• -mpost-inc — post increment mode ➕
• -mpost-modify — post modify mode ✏️
• -mstack-offset=num — stack offset is num 📏
• -mround-nearest — round nearest 🎯
• -mlong-calls — long calls mode 📞
• -mshort-calls — short calls mode 📞
• -msmall16 — small16 mode 🐜
• -mfp-mode=mode — FP mode is mode 🧮
• -mmay-round-for-trunc — may round for trunc 🌀
• -mfp-iarith — FP iarith mode ➗
• -mvect-double — vector double mode 🧬
• -max-vect-align=num — max vector alignment is num 📐
• -msplit-vecmove-early — split vecmove early 🌅
• -m1reg-reg — 1reg-reg mode 1️⃣

🔥 AMD GCN Options (AMD GCN Options) 🔥

• -march=gpu — architecture is gpu 🏛️
• -mtune=gpu — tune for gpu 🎶
• -mgang-private-size=bytes — gang-private size is bytes 📦
• -msram-ecc=[on|off|any] — SRAM ECC is on, off, or any 🛡️
• -mxnack=[on|off|any] — XNACK is on, off, or any 🧵
• -Wopenacc-dims — OpenACC dims warning 🌈

🧚 ARC Options (ARC Options) 🧚

• -mbarrel-shifter — barrel shifter mode 🌀
• -mjli-always — JLI always mode 💅
• -mcpu=cpu — CPU is cpu 🧠
• -mA6 — A6 mode 🅰️6️⃣
• -mARC600 — ARC600 mode 🅰️
• -mA7 — A7 mode 🅰️7️⃣
• -mARC700 — ARC700 mode 🅰️
• -mdpfp — DPFP mode 🧮
• -mdpfp-compact — DPFP compact 🧳
• -mdpfp-fast — DPFP fast ⚡
• -mno-dpfp-lrsr — no DPFP LRSR 🚫
• -mea — EA mode 🌱
• -mmul32x16 — 32x16 multiply ✖️
• -mmul64 — 64-bit multiply ✖️
• -matomic — atomic mode ⚛️
• -mnorm — norm mode 🧼
• -mspfp — SPFP mode 🧮
• -mspfp-compact — SPFP compact 🧳
• -mspfp-fast — SPFP fast ⚡
• -msimd — SIMD mode 🧬
• -msoft-float — soft float mode 🧸
• -mswap — swap mode 🔄
• -mlock — lock mode 🔒
• -mswape — swape mode 🔄
• -mxy — XY mode 🧭
• -misize — isize mode 📏
• -marclinux — ARC Linux mode 🐧
• -marclinux_prof — ARC Linux prof mode 📈
• -mlong-calls — long calls mode 📞
• -mmedium-calls — medium calls mode 📞
• -msdata — sdata mode 📦
• -mirq-ctrl-saved — IRQ ctrl saved 🚨
• -mrgf-banked-regs — RGF banked regs 🏦
• -mlpc-width=width — LPC width is width 📏
• -G num — G is num 🅶
• -mvolatile-cache — volatile cache mode 🌪️
• -mtp-regno=regno — TP regno is regno 📍
• -mbitops — bitops mode 🧮
• -mcmem — cmem mode 🧠
• -munaligned-access — unaligned access mode 📐
• -mauto-modify-reg — auto modify reg 🔁
• -mno-brcc — no brcc 🚫
• -mcase-vector-pcrel — case vector PC-relative 🧭
• -mno-cond-exec — no cond exec 🚫
• -mearly-cbranchsi — early cbranchsi 🌅
• -mindexed-loads — indexed loads 📥
• -mlra-priority-none — LRA priority none 🙅
• -mlra-priority-compact — LRA priority compact 🧳
• -mlra-priority-noncompact — LRA priority noncompact 🧳
• -mmillicode — millicode mode 🧬
• -msize-level=level — size level is level 📏
• -mtune=cpu — tune for cpu 🎶
• -mmultcost=num — mult cost is num ✖️
• -mcode-density-frame — code density frame 🧱
• -mmpy-option=multo — mpy option is multo ✖️
• -mdiv-rem — div/rem mode ➗
• -mcode-density — code density mode 🧱
• -mll64 — LL64 mode 🧬
• -mfpu=fpu — FPU is fpu 🧮
• -mrf16 — RF16 mode 🧺
• -mbranch-index — branch index mode 🌿

🦋 ARM Options (ARM Options) 🦋

• -mapcs-frame — APCS frame mode 🖼️
• -mapcs — APCS mode 🧵
• -mabi=name — ABI is name 🏷️
• -mgeneral-regs-only — general regs only 💅
• -mno-sched-prolog — no scheduled prolog 🚫
• -mlittle-endian — little-endian mode 🐜
• -mbig-endian — big-endian mode 🐘
• -mbe8 — BE8 mode 🧭
• -mbe32 — BE32 mode 🧭
• -mfloat-abi=name — float ABI is name 🧮
• -mfp16-format=name — FP16 format is name 🎀
• -mthumb-interwork — Thumb interwork mode 🔀
• -mcpu=name — CPU is name 🧠
• -march=name — architecture is name 🏛️
• -mfpu=name — FPU is name 🧮
• -mtune=name — tune for name 🎶
• -mstructure-size-boundary=n — structure size boundary is n 📐
• -mabort-on-noreturn — abort on noreturn 🚨
• -mlong-calls — long calls mode 📞
• -msingle-pic-base — single PIC base 1️⃣
• -mpic-register=reg — PIC register is reg 📍
• -mpic-data-is-text-relative — PIC data is text-relative 🧭
• -mnop-fun-dllimport — no fun dllimport 🚫
• -mpoke-function-name — poke function name 📌
• -mthumb — Thumb mode 👍
• -marm — ARM mode 💪
• -mtpcs-frame — TPCS frame mode 🖼️
• -mtpcs-leaf-frame — TPCS leaf frame mode 🍃
• -mcaller-super-interworking — caller super-interworking 📞
• -mcallee-super-interworking — callee super-interworking 📞
• -mtp=name — thread pointer style name 📍
• -mtls-dialect=dialect — TLS dialect is dialect 🧵
• -mword-relocations — word relocations mode 📦
• -mfix-cortex-m3-ldrd — Cortex-M3 LDRD fix 🛠️
• -mfix-cortex-a57-aes-1742098 — Cortex-A57 AES 1742098 fix 🛠️
• -mfix-cortex-a72-aes-1655431 — Cortex-A72 AES 1655431 fix 🛠️
• -munaligned-access — unaligned access mode 📐
• -mslow-flash-data — slow flash data mode 🐢
• -masm-syntax-unified — unified asm syntax 🧵
• -mrestrict-it — restrict IT mode 🚧
• -mpure-code — pure code mode 🧼
• -mcmse — CMSE mode 🛡️
• -mfix-cmse-cve-2021-35465 — CMSE CVE-2021-35465 fix 🛠️
• -mstack-protector-guard=guard — stack protector guard is guard 🛡️
• -mstack-protector-guard-offset=offset — guard offset is offset 📏
• -mfdpic — FDPIC mode 🧭
• -mbranch-protection=features — branch protection features 🛡️
• -mvectorize-with-neon-quad — vectorize with NEON quad 🍀
• -mvectorize-with-neon-double — vectorize with NEON double 🍀

🍓 AVR Options (AVR Options) 🍓

• -mmcu=mcu — MCU is mcu 🧠
• -mabsdata — absdata mode 📦
• -maccumulate-args — accumulate args mode 🧺
• -mcvt — CVT mode 🔄
• -mbranch-cost=cost — branch cost is cost 🌿
• -mfuse-add=level — fuse add level level ➕
• -mfuse-move=level — fuse move level level 🚚
• -mfuse-move2 — fuse move 2 mode 🚚
• -mcall-prologues — call prologues mode 📞
• -mgas-isr-prologues — gas ISR prologues mode 🧵
• -mint8 — int8 mode 8️⃣
• -mflmap — flmap mode 🗺️
• -mdouble=bits — double is bits 🧮
• -mlong-double=bits — long double is bits 🧮
• -mno-call-main — no call main 🚫
• -mn_flash=size — n_flash is size 📏
• -mfract-convert-truncate — fract convert truncate mode 🌀
• -mno-interrupts — no interrupts 🚫
• -mmain-is-OS_task — main is OS_task mode 🧵
• -mrelax — relax mode 😌
• -mpmem-wrap-around — pmem wrap around mode 🌀
• -mrmw — RMW mode 🔁
• -mstrict-X — strict X mode 📐
• -mtiny-stack — tiny stack mode 🐜
• -mrodata-in-ram — rodata in RAM mode 🧠
• -msplit-bit-shift — split bit shift mode 🔪
• -msplit-ldst — split ldst mode 🔪
• -mshort-calls — short calls mode 📞
• -mskip-bug — skip bug mode 🐛
• -muse-nonzero-bits — use nonzero bits mode 1️⃣
• -nodevicelib — no devicelib 🚫
• -nodevicespecs — no devicespecs 🚫
• -Waddr-space-convert — address-space conversion warning ⚠️
• -Wmisspelled-isr — misspelled ISR warning ⚠️

🖤 Blackfin Options (Blackfin Options) 🖤

• -mcpu=cpu[-sirevision] — CPU is cpu with optional [-sirevision], exactly like that 🧠
• -msim — simulator mode 🎮
• -momit-leaf-frame-pointer — omits leaf frame pointer 🍃
• -mspecld-anomaly — specld anomaly mode 🐛
• -mcsync-anomaly — csync anomaly mode 🐛
• -mlow-64k — low 64K mode 🔽
• -mstack-check-l1 — L1 stack check mode 🥞
• -mid-shared-library — ID shared library mode 📚
• -mleaf-id-shared-library — leaf ID shared library mode 🍃
• -mshared-library-id=n — shared library ID is n 🆔
• -msep-data — separate data mode 🗂️
• -mlong-calls — long calls mode 📞
• -mfast-fp — fast FP mode ⚡
• -minline-plt — inline PLT mode 📥
• -mmulticore — multicore mode 🧠
• -mcorea — CoreA mode 🅰️
• -mcoreb — CoreB mode 🅱️
• -msdram — SDRAM mode 🧠
• -micplb — ICPLB mode 📚

🧵 C6X Options (C6X Options) 🧵

• -mbig-endian — big-endian mode 🐘
• -mlittle-endian — little-endian mode 🐜
• -march=cpu — architecture is cpu 🏛️
• -msim — simulator mode 🎮
• -msdata=sdata-type — sdata type is sdata-type 📦
• -mdsbt — DSBT mode 🧭
• -mlong-calls — long calls mode 📞

🌿 CRIS Options (CRIS Options) 🌿

• -mcpu=cpu — CPU is cpu 🧠
• -march=cpu — architecture is cpu 🏛️
• -mtune=cpu — tune for cpu 🎶
• -mmax-stackframe=n — max stackframe is n 🥞
• -metrax4 — ETRAX 4 mode 4️⃣
• -metrax100 — ETRAX 100 mode 💯
• -mpdebug — pdebug mode 🐛
• -mcc-init — CC init mode 🧵
• -mno-side-effects — no side effects 🚫
• -mstack-align — stack align mode 📐
• -mdata-align — data align mode 📐
• -mconst-align — const align mode 📐
• -m32-bit — 32-bit mode 3️⃣2️⃣
• -m16-bit — 16-bit mode 1️⃣6️⃣
• -m8-bit — 8-bit mode 8️⃣
• -mno-prologue-epilogue — no prologue/epilogue 🚫
• -mbest-lib-options — best lib options mode 🏆
• -moverride-best-lib-options — override best lib options mode 🎛️
• -mtrap-using-break8 — trap using break8 mode 🚨
• -mtrap-unaligned-atomic — trap unaligned atomic mode 🚨
• -munaligned-atomic-may-use-library — unaligned atomic may use library mode 📚
• -sim — sim mode 🎮
• -sim2 — sim2 mode 🎮
• -mmul-bug-workaround — multiply bug workaround 🐛

🧊 C-SKY Options (C-SKY Options) 🧊

• -march=arch — architecture is arch 🏛️
• -mcpu=cpu — CPU is cpu 🧠
• -mbig-endian — big-endian mode 🐘
• -mlittle-endian — little-endian mode 🐜
• -mfpu=fpu — FPU is fpu 🧮
• -mdouble-float — double float mode 🧮
• -mfdivdu — fdivdu mode ➗
• -mfloat-abi=name — float ABI is name 🧮
• -melrw — elrw mode 🔁
• -mistack — istack mode 🥞
• -mmp — MP mode 🧠
• -mcp — CP mode 🧵
• -mcache — cache mode 🗄️
• -msecurity — security mode 🛡️
• -mtrust — trust mode 🤝
• -mdsp — DSP mode 🎛️
• -medsp — EDSP mode 🎛️
• -mvdsp — VDSP mode 🎛️
• -mdiv — div mode ➗
• -msmart — smart mode 🧠
• -mhigh-registers — high registers mode ⬆️
• -manchor — anchor mode ⚓
• -mpushpop — push/pop mode 📥
• -mmultiple-stld — multiple st/ld mode 📚
• -mconstpool — constpool mode 🏊
• -mstack-size — stack size mode 🥞
• -mccrt — CCRT mode 🧵
• -mbranch-cost=n — branch cost is n 🌿
• -msched-prolog — scheduled prolog mode 🗓️
• -msim — simulator mode 🎮

🐧 Cygwin and MinGW Options (Cygwin and MinGW Options) 🐧

• -mconsole — console mode 🖥️
• -mcrtdll=library — CRT DLL is library 📚
• -mdll — DLL mode 📦
• -mnop-fun-dllimport — no fun dllimport 🚫
• -mthreads — threads mode 🧵
• -municode — Unicode mode 🌈
• -mwin32 — Win32 mode 🪟
• -mwindows — Windows mode 🪟
• -fno-set-stack-executable — no set stack executable 🚫
• -fwritable-relocated-rdata — writable relocated rdata mode ✍️
• -mpe-aligned-commons — PE aligned commons mode 📐
• -muse-libstdc-wrappers — use libstdc++ wrappers mode 🎁

🍎 Darwin Options (Darwin Options) 🍎

• -all_load — all_load mode 📥
• -allowable_client — allowable client mode 👤
• -arch name — architecture is name 🏛️
• -arch_errors_fatal — arch errors fatal mode 🚨
• -asm_macosx_version_min=version — asm macOS version min is version 🍎
• -bind_at_load — bind at load mode 🔗
• -bundle — bundle mode 🎁
• -bundle_loader — bundle loader mode 🚚
• -client_name — client name mode 👤
• -compatibility_version — compatibility version mode 🧾
• -current_version — current version mode 🧾
• -dead_strip — dead strip mode ☠️
• -dependency-file — dependency file mode 📄
• -dylib_file — dylib file mode 📚
• -dylinker — dylinker mode 🔗
• -dylinker_install_name — dylinker install name mode 🏷️
• -dynamic — dynamic mode 🌀
• -dynamiclib — dynamiclib mode 📚
• -exported_symbols_list — exported symbols list mode 📋
• -fapple-kext — Apple kext mode 🍎
• -fconstant-cfstrings — constant CFStrings mode 🧵
• -ffix-and-continue — fix and continue mode 🛠️
• -filelist — filelist mode 📋
• -findirect-data — findirect data mode 🧭
• -flat_namespace — flat namespace mode 🧺
• -force_cpusubtype_ALL — force CPU subtype ALL mode 💥
• -force_flat_namespace — force flat namespace mode 🧺
• -framework name — framework is name 🧱
• -gfull — gfull mode 🐛
• -gused — gused mode 🐛
• -headerpad_max_install_names — headerpad max install names mode 📏
• -iframework — iframework mode 🧱
• -image_base — image base mode 🖼️
• -init symbol-name — init symbol is symbol-name 🚀
• -install_name — install name mode 🏷️
• -keep_private_externs — keep private externs mode 🔒
• -matt-stubs — Matt stubs mode 👑
• -mconstant-cfstrings — constant CFStrings mode 🧵
• -mdynamic-no-pic — dynamic no-PIC mode 🌀
• -mfix-and-continue — fix and continue mode 🛠️
• -mkernel — kernel mode 🧠
• -mmacosx-version-min=version — macOS version min is version 🍎
• -mone-byte-bool — one-byte bool mode 1️⃣
• -msymbol-stubs — symbol stubs mode 🧵
• -mtarget-linker[=]version — target linker [=]version mode 🔗
• -nodefaultexport — no default export 🚫
• -nodefaultrpaths — no default rpaths 🚫
• -pagezero_size — pagezero size mode 📏
• -preload — preload mode 📥
• -read_only_relocs — read-only relocs mode 🔒
• -sectalign — sectalign mode 📐
• -sectcreate — sectcreate mode 🧱
• -seg_addr_table — segment address table mode 📋
• -seg1addr — seg1addr mode 📍
• -segaddr — segaddr mode 📍
• -segprot — segprot mode 🛡️
• -segs_read_only_addr — segs read-only addr mode 🔒
• -segs_read_write_addr — segs read-write addr mode ✍️
• -sub_library — sub_library mode 📚
• -sub_umbrella — sub_umbrella mode ☂️
• -twolevel_namespace — two-level namespace mode 2️⃣
• -twolevel_namespace_hints — two-level namespace hints mode 2️⃣
• -umbrella — umbrella mode ☂️
• -undefined — undefined mode ❓
• -unexported_symbols_list — unexported symbols list mode 📋
• -weak_framework name — weak framework is name 🧱
• -weak_reference_mismatches — weak reference mismatches mode 🧵
• -whatsloaded — whatsloaded mode ❓
• -whyload — whyload mode ❓
• -F — -F mode 🅵
• -ObjC — Objective-C mode 🍎
• -ObjC++ — Objective-C++ mode 🍎
• -Wnonportable-cfstrings — nonportable CFStrings warning ⚠️

💜 DEC Alpha Options (DEC Alpha Options) 💜

• -mno-fp-regs — no FP regs 🚫
• -msoft-float — soft float mode 🧸
• -mieee — IEEE mode 🧮
• -mieee-with-inexact — IEEE with inexact mode 🧮
• -mieee-conformant — IEEE conformant mode 🧮
• -mfp-trap-mode=mode — FP trap mode is mode 🚨
• -mfp-rounding-mode=mode — FP rounding mode is mode 🎯
• -mtrap-precision=mode — trap precision is mode 🎯
• -mbuild-constants — build constants mode 🧱
• -mcpu=cpu-type — CPU is cpu-type 🧠
• -mtune=cpu-type — tune for cpu-type 🎶
• -mbwx — BWX mode 🧮
• -mmax — MAX mode 💯
• -mfix — FIX mode 🛠️
• -mcix — CIX mode 🧮
• -msafe-bwa — safe BWA mode 🛡️
• -msafe-partial — safe partial mode 🛡️
• -mfloat-vax — VAX float mode 🧮
• -mfloat-ieee — IEEE float mode 🧮
• -mexplicit-relocs — explicit relocs mode 🧭
• -msmall-data — small data mode 🐜
• -mlarge-data — large data mode 🐘
• -msmall-text — small text mode 🐜
• -mlarge-text — large text mode 🐘
• -mmemory-latency=time — memory latency is time ⏱️
• -mtls-kernel — TLS kernel mode 🧵
• -mtls-size=bitsize — TLS size is bitsize 📏
• -mlong-double-128 — long double 128 mode 1️⃣2️⃣8️⃣
• -mlong-double-64 — long double 64 mode 6️⃣4️⃣

🧬 eBPF Options (eBPF Options) 🧬

• -mbig-endian — big-endian mode 🐘
• -mlittle-endian — little-endian mode 🐜
• -mframe-limit=bytes — frame limit is bytes 📏
• -mxbpf — xBPF mode 🧬
• -mco-re — CO-RE mode 🧭
• -mjmpext — jump extension mode 🦘
• -mjmp32 — JMP32 mode 3️⃣2️⃣
• -malu32 — ALU32 mode 3️⃣2️⃣
• -mv3-atomics — v3 atomics mode ⚛️
• -mbswap — BSWAP mode 🔄
• -msdiv — SDIV mode ➗
• -msmov — SMOV mode 🚚
• -mcpu=version — CPU is version 🧠
• -masm=dialect — asm dialect is dialect 🧵
• -minline-memops-threshold=bytes — inline memops threshold is bytes 📏
• -Wco-re — CO-RE warning ⚠️

🌱 FR30 Options (FR30 Options) 🌱

• -msmall-model — small model mode 🐜
• -mno-lsim — no lsim mode 🚫

🧚 FRV Options (FRV Options) 🧚

• -mgpr-32 — GPR 32 mode 3️⃣2️⃣
• -mgpr-64 — GPR 64 mode 6️⃣4️⃣
• -mfpr-32 — FPR 32 mode 3️⃣2️⃣
• -mfpr-64 — FPR 64 mode 6️⃣4️⃣
• -mhard-float — hard float mode 🧮
• -msoft-float — soft float mode 🧸
• -malloc-cc — alloc CC mode 🧵
• -mfixed-cc — fixed CC mode 📌
• -mdword — dword mode 📦
• -mdouble — double mode 🧮
• -mmedia — media mode 🎬
• -mmuladd — muladd mode ✖️
• -mfdpic — FDPIC mode 🧭
• -minline-plt — inline PLT mode 📥
• -mgprel-ro — GPREL RO mode 🧭
• -multilib-library-pic — multilib library PIC mode 📚
• -mlinked-fp — linked FP mode 🔗
• -mlong-calls — long calls mode 📞
• -malign-labels — align labels mode 📐
• -mlibrary-pic — library PIC mode 📚
• -macc-4 — ACC 4 mode 4️⃣
• -macc-8 — ACC 8 mode 8️⃣
• -mpack — pack mode 📦
• -mno-eflags — no eflags 🚫
• -mno-cond-move — no cond move 🚫
• -mno-optimize-membar — no optimize membar 🚫
• -mno-scc — no SCC 🚫
• -mno-cond-exec — no cond exec 🚫
• -mno-vliw-branch — no VLIW branch 🚫
• -mno-multi-cond-exec — no multi cond exec 🚫
• -mno-nested-cond-exec — no nested cond exec 🚫
• -mtomcat-stats — Tomcat stats mode 🐱
• -mTLS — TLS mode 🧵
• -mtls — tls mode 🧵
• -mcpu=cpu — CPU is cpu 🧠

🧸 FT32 Options (FT32 Options) 🧸

• -msim — simulator mode 🎮
• -mnodiv — no div mode 🚫
• -mft32b — FT32B mode 🧸
• -mcompress — compress mode 🗜️
• -mnopm — nopm mode 😴

🐧 GNU/Linux Options (GNU/Linux Options) 🐧

• -mglibc — glibc mode 🐧
• -muclibc — uClibc mode 🐧
• -mmusl — musl mode 🐧
• -mbionic — Bionic mode 🐧
• -mandroid — Android mode 🤖
• -tno-android-cc — no Android CC mode 🚫
• -tno-android-ld — no Android LD mode 🚫

🧵 H8/300 Options (H8/300 Options) 🧵

• -mrelax — relax mode 😌
• -mh — H mode 🅷
• -ms — S mode 🆂
• -mn — N mode 🅽
• -msx — SX mode 🆂
• -ms2600 — S2600 mode 2️⃣6️⃣0️⃣0️⃣
• -mquickcall — quickcall mode 📞
• -mslowbyte — slowbyte mode 🐢
• -mexr — EXR mode 🧮
• -mint32 — int32 mode 3️⃣2️⃣
• -malign-300 — align 300 mode 📐

🌸 HPPA Options

okay bestie, HPPA time — here's the whole flag buffet, every single one, no skips:

• -march=architecture-type
• -mno-atomic-libcalls
• -mcaller-copies
• -mdisable-fpregs
• -mdisable-indexing
• -mordered
• -mfast-indirect-calls
• -mgas
• -mgnu-ld
• -mhp-ld
• -mfixed-range=register-range
• -mcoherent-ldcw
• -mlinker-opt
• -mlong-calls
• -mlong-load-store
• -mno-space-regs
• -msoft-float
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0
• -mportable-runtime
• -mschedule=cpu-type
• -msoft-mult
• -msio
• -mwsio
• -munix=unix-std
• -nolibdld
• -static
• -threads

---

✨ IA-64 Options

Itanium said "I'm gonna have SO many flags" and honestly? She delivered. Here's the list 💅

• -mbig-endian
• -mlittle-endian
• -mgnu-as
• -mgnu-ld
• -mno-pic
• -mvolatile-asm-stop
• -mregister-names
• -msdata
• -mconstant-gp
• -mauto-pic
• -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-stop-bits-after-every-cycle
• -msched-count-spec-in-critical-path
• -msel-sched-dont-check-control-spec
• -msched-fp-mem-deps-zero-cost
• -msched-max-memory-insns-hard-limit
• -msched-max-memory-insns=max-insns

---

🎀 LM32 Options

smol architecture, tidy little flag list, we love a minimal queen 🦋

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

---

🌈 LoongArch Options

okay so LoongArch has range — march, mtune, mabi, mfpu, msind… no wait, -msimd, and a whole crew after it:

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

---

💖 LynxOS Options

three flags, maximum drama, minimum scrolling:

• -mshared
• -mthreads
• -mlegacy-threads

---

🍓 M32C Options

• -mcpu=cpu
• -msim
• -memregs=number

---

🔥 M32R/D Options

the M32R/D menu, served fresh, nothing left off the plate:

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

---

💜 M680x0 Options

the 68k family reunion 😭 so many cousins. All of them, in order:

• -march=arch
• -mcpu=cpu
• -mtune=tune
• -m68000
• -m68020
• -m68020-40
• -m68020-60
• -m68030
• -m68040
• -m68060
• -m68302
• -m68332
• -m68851
• -mcpu32
• -mfidoa
• -m5200
• -m5206e
• -m528x
• -m5307
• -m5407
• -mcfv4e
• -mbitfield
• -mc68000
• -mc68020
• -mrtd
• -mdiv
• -mshort
• -mhard-float
• -m68881
• -msoft-float
• -mpcrel
• -malign-int
• -mstrict-align
• -msep-data
• -mshared-library-id=n
• -mid-shared-library
• -mxgot
• -mxtls
• -mlong-jump-table-offsets

---

🧚 MCore Options

• -mhardlit
• -mdiv
• -mrelax-immediates
• -mwide-bitfields
• -m4byte-functions
• -mcallgraph-data
• -mslow-bytes
• -mno-lsim
• -mlittle-endian
• -mbig-endian
• -m210
• -m340
• -mstack-increment

---

👑 MicroBlaze Options

not her being a whole soft-core icon with this many flags:

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

---

⭐ MIPS Options

okay this is THE big one, bestie. Like, MIPS said "let me have every option ever" and then had more. Buckle up, we're going the whole way down, nothing summarized, nothing skipped 🫶

• -EL
• -EB
• -mel
• -meb
• -march=arch
• -mtune=arch
• -mips1
• -mips2
• -mips3
• -mips4
• -mips32
• -mips32r2
• -mips32r3
• -mips32r5
• -mips32r6
• -mips64
• -mips64r2
• -mips64r3
• -mips64r5
• -mips64r6
• -mips16
• -mmips16e2
• -mflip-mips16
• -minterlink-compressed
• -minterlink-mips16
• -mabi=abi
• -mabicalls
• -mshared
• -mplt
• -mxgot
• -mgp32
• -mgp64
• -mfp32
• -mfpxx
• -mfp64
• -mhard-float
• -msoft-float
• -mno-float
• -msingle-float
• -mdouble-float
• -modd-spreg
• -mabs=mode
• -mnan=encoding
• -mdsp
• -mdspr2
• -mmcu
• -meva
• -mvirt
• -mxpa
• -mcrc
• -mginv
• -mmicromips
• -mmsa
• -mloongson-mmi
• -mloongson-ext
• -mloongson-ext2
• -mfpu=fpu-type
• -msmartmips
• -mpaired-single
• -mdmx
• -mips3d
• -mmt
• -mllsc
• -mlong64
• -mlong32
• -msym32
• -Gnum
• -mno-local-sdata
• -mno-extern-sdata
• -mno-gopt
• -membedded-data
• -muninit-const-in-rodata
• -mcode-readable=setting
• -mno-data-in-code
• -mcode-xonly
• -msplit-addresses
• -mexplicit-relocs
• -mexplicit-relocs=release
• -mno-check-zero-division
• -mdivide-traps
• -mdivide-breaks
• -mno-load-store-pairs
• -mstrict-align
• -mno-unaligned-access
• -mmemcpy
• -mlong-calls
• -mmad
• -mimadd
• -mno-fused-madd
• -nocpp
• -mfix-24k
• -mfix-r4000
• -mfix-r4400
• -mfix-r5900
• -mfix-r10000
• -mfix-rm7000
• -mfix-vr4120
• -mfix-vr4130
• -mfix-sb1
• -mfix4300
• -mr10k-cache-barrier=setting
• -mflush-func=func
• -mno-flush-func
• -mbranch-cost=num
• -mbranch-likely
• -mcompact-branches=policy
• -mno-fp-exceptions
• -mvr4130-align
• -msynci
• -mno-lxc1-sxc1
• -mno-madd4
• -mno-relax-pic-calls
• -mmcount-ra-address
• -mframe-header-opt

phew. MIPS ate and left no crumbs 💀

---

🌸 MMIX Options

Knuth's baby, and she comes with receipts:

• -mlibfuncs
• -mepsilon
• -mabi=gnu
• -mabi=mmixware
• -mzero-extend
• -mknuthdiv
• -mtoplevel-symbols
• -melf
• -mbranch-predict
• -mbase-addresses
• -msingle-exit
• -mset-data-start=address
• -mset-program-start=address
• -mno-set-program-start

---

🎀 MN10300 Options

• -mmult-bug
• -mno-mult-bug
• -mam33
• -mam33-2
• -mam34
• -mtune=cpu-type
• -mno-return-pointer-on-d0
• -mno-crt0
• -mrelax
• -mno-liw
• -mno-setlb

---

🦋 Moxie Options

tiny list, big main character energy:

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

🍓 MSP430 Options

the microcontroller girlies are gonna live for this one:

• -msim
• -masm-hex
• -mmcu=name
• -mlarge
• -msmall
• -mrelax
• -mwarn-mcu
• -mwarn-devices-csv
• -mcode-region=where
• -mdata-region=where
• -muse-lower-region-prefix
• -msilicon-errata=name[,name...]
• -msilicon-errata-warn=name[,name...]
• -mhwmult=type
• -minrt
• -mtiny-printf
• -mmax-inline-shift=n

note bestie, those errata flags literally take name[,name...] lists — keep the commas, they're load-bearing 🫶

---

💅 NDS32 Options

• -mbig-endian
• -mlittle-endian
• -EB
• -EL
• -mabi=name
• -mfloat-abi=name
• -mreduced-regs
• -mfull-regs
• -malways-align
• -malign-functions
• -mfp-as-gp
• -mcmov
• -mhw-abs
• -mext-perf
• -mext-perf2
• -mext-string
• -mext-dsp
• -mext-fpu-fma
• -mext-fpu-sp
• -mext-fpu-dp
• -mv3push
• -m16-bit
• -mvh
• -misr-vector-size=num
• -misr-secure=num
• -mcache-block-size=num
• -march=arch
• -mcpu=cpu
• -mconfig-fpu=num
• -mconfig-mul=type
• -mconfig-register-ports=kind
• -mcmodel=code-model
• -mctor-dtor
• -mrelax
• -mrelax-hint
• -msched-prolog-epilog
• -mno-ret-in-naked-func
• -malways-save-lp
• -munaligned-access
• -minline-asm-r15

---

🔥 Nvidia PTX Options

GPU-flavoured flags, no cap:

• -m64
• -march=arch
• -misa=arch
• -march-map=arch
• -mptx=version
• -mmainkernel
• -moptimize
• -msoft-stack
• -msoft-stack-reserve-local=size
• -muniform-simt
• -mgomp

---

🌈 OpenRISC Options

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

---

💜 PDP-11 Options

vintage icon behaviour, she's literally from the 70s and still showing up ✨

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

---

🧚 Picolibc Options

these are the long-option girlies, all four of them:

• --oslib=library
• --crt0=[none|minimal|hosted|semihost]
• --printf=[d|f|l|i|m]
• --scanf=[d|f|l|i|m]

---

👑 PowerPC Options

see RS/6000 and PowerPC Options. that's it, that's the whole entry, she's pointing you at her other section 💅

---

🎀 PRU Options

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

---

🦋 RISC-V Options

the newest cool girl on the block and she has OPINIONS. full list, in order, exactly as written:

• -mbranch-cost=N-instruction
• -mabi=ABI-string
• -mfdiv
• -mdiv
• -mno-fence-tso
• -misa-spec=ISA-spec-string
• -march=[ISA|Profile|Profile_ISA|processor-string]
• -mcpu=processor-string
• -mtune=processor-string
• -mpreferred-stack-boundary=num
• -msmall-data-limit=N-bytes
• -msave-restore
• -mno-shorten-memrefs
• -mstrict-align
• -mscalar-strict-align
• -mno-vector-strict-align
• -mcmodel=medlow
• -mcmodel=medany
• -mcmodel=large
• -mexplicit-relocs
• -mrelax
• -mriscv-attribute
• -malign-data=type
• -mbig-endian
• -mlittle-endian
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mcsr-check
• -momit-leaf-frame-pointer
• -mmovcc
• -mno-inline-atomics
• -mno-inline-strlen
• -mno-inline-strcmp
• -mno-inline-strncmp
• -mstringop-strategy=strategy
• -mtls-dialect=desc
• -mtls-dialect=trad
• -mrvv-vector-bits=value
• -mrvv-max-lmul=value
• -madjust-lmul-cost
• -mmax-vectorization
• -mno-autovec-segment

---

⭐ RL78 Options

last one in this batch and she's bringing the microcontroller energy:

• -msim
• -mallregs
• -mrelax
• -mes0
• -mmul=none
• -mmul=g13
• -mmul=g14
• -mmul=rl78
• -mcpu=g10
• -mcpu=g13
• -mcpu=g14
• -mcpu=rl78
• -mg10
• -mg13
• -mg14
• -mrl78
• -msave-mduc-in-interrupts

🌈 RS/6000 and PowerPC Options (RS/6000 and PowerPC Options)

bestie there are like. SO many of these. buckle up 💅

-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mprofile-kernel
-mpowerpc64 -maltivec -mpowerpc-gpopt -mpowerpc-gfxopt -mmfcrf
-mpopcntb -mpopcntd -mfprnd -mcmpb -mhard-dfp -mfull-toc -mminimal-toc
-mno-fp-in-toc -mno-sum-in-toc -maix64 -maix32 -m64 -m32 -mxl-compat
-mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple
-mupdate -mavoid-indexed-addresses -mfused-madd -mbit-align -mbit-word
-mstrict-align -mrelocatable -mrelocatable-lib -mlittle -mlittle-endian
-mbig -mbig-endian -mdynamic-no-pic -msingle-pic-base
-mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi -mcall-freebsd
-mcall-linux -mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-msplit-patch-nops -mregnames -mlongcall -mpltseq -mtls-markers
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mblock-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel -mvsx -mvrsave -mmulhw -mdlmzb
-mprototype -msim -mmvme -mads -myellowknife -memb -meabi -msdata
-msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip -mrecip=opt
-mno-recip -mrecip-precision -mveclibabi=type -mfriz
-mpointers-to-nested-functions -msave-toc-indirect -mpower8-fusion
-mcrypto -mhtm -mquad-memory -mquad-memory-atomic -mcompat-align-parm
-mfloat128 -mfloat128-hardware -mgnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mprefixed -mpcrel -mmma
-mdense-math -mrop-protect -mprivileged -mno-splat-word-constant
-mno-splat-float-constant -mno-ieee128-constant -mno-warn-altivec-long

like, -mno-splat-word-constant?? the name alone is giving chaos goblin 😭 anyway. slay, moving on ✨

---

🦋 RX Options (RX Options)

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

short, sweet, to the point. we love a low-maintenance king 👑💖

---

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

mainframe besties stand UP 🫶

-mtune=cpu-type -march=cpu-type -mhard-float -msoft-float -mhard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain -mpacked-stack -msmall-exec
-mmvcle -m64 -m31 -mdebug -mesa -mzarch -mhtm -mvx -mzvector -mtpf-trace
-mtpf-trace-skip -mmain -mfused-madd -mno-fused-madd
-mwarn-framesize=framesize -mwarn-dynamicstack -mstack-size=stack-size
-mno-stack-size -mstack-guard=stack-guard -mno-stack-guard
-mstack-protector-guard=guard -mstack-protector-guard-record
-mhotpatch=pre-halfwords,post-halfwords -mno-pic-data-is-text-relative
-mindirect-branch=choice -mindirect-branch-jump=choice
-mindirect-branch-call=choice -mfunction-return=choice
-mfunction-return-mem=choice -mfunction-return-reg=choice
-mindirect-branch-table -mfentry -mrecord-mcount -mnop-mcount
-mpreserve-args -munaliged-symbols

ok so -munaliged-symbols is literally spelled that way in the source, like with the typo and everything. not me, not a typo on my end — copy it exactly or it won't work fr fr 🙈

---

🌸 SH Options (SH Options)

this list is a whole mood board of CPU generations 💅

-m1 -m2 -m2e -m2a -m2a-nofpu -m2a-single -m2a-single-only -m3 -m3e -m4
-m4-nofpu -m4-single -m4-single-only -m4-100 -m4-100-nofpu -m4-100-single
-m4-100-single-only -m4-200 -m4-200-nofpu -m4-200-single
-m4-200-single-only -m4-300 -m4-300-nofpu -m4-300-single
-m4-300-single-only -m4-340 -m4-400 -m4-500 -m4a -m4al -m4a-nofpu
-m4a-single -m4a-single-only -mb -ml -mdalign -mrelax -mbigtable -mbitops
-mfmovd -mrenesas -mnomacsave -mieee -misize -minline-ic_invalidate
-mprefergot -musermode -multcost=number -mdiv=strategy
-mdivsi3_libfunc=name -mfixed-range=register-range
-maccumulate-outgoing-args -matomic-model=atomic-model
-mbranch-cost=num -mzdcbranch -mcbranch-force-delay-slot -mfsca -mfsrra
-mpretend-cmove -mfdpic -mtas -mlra

---

🎀 Solaris 2 Options (Solaris 2 Options)

• -mclear-hwcap — turn on the hwcap clearing 💫
• -mno-clear-hwcap — turn it back off, icon behavior
• -mimpure-text — impure text mode, spooky 🍓
• -mno-impure-text — pure text again, we're cleansed
• -gsctf — the -gsctf flag, no notes 🔥

---

⭐ SPARC Options (SPARC Options)

-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mmemory-model=mem-model -m32 -m64 -mptr32 -mptr64 -mapp-regs
-mfaster-structs -mflat -mfpu -mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mstack-bias -mstd-struct-return -munaligned-doubles
-muser-mode -mv8plus -mvis -mvis2 -mvis3 -mvis3b -mvis4 -mvis4b -mcbcond
-mfmaf -mfsmuld -mpopc -msubxc -mfix-at697f -mfix-ut699 -mfix-ut700
-mfix-gr712rc

the -mvis → -mvis4b glow-up is honestly character development 🦋✨

---

💀 System V Options (System V Options)

• -YP,paths — specify the paths for the -YP option, comma-separated bestie
• -Ym,dir — point -Ym at a dir

only two options and yet somehow still mysterious. respect 😌

---

🧚 V850 Options (V850 Options)

-mlong-calls -mep -mprolog-function -mspace -mtda=n -msda=n -mzda=n
-mv850 -mv850e3v5 -m850e2v4 -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e
-mdisable-callt -mrelax -mlong-jumps -msoft-float -mhard-float -mloop
-mrh850-abi -mghs -mgcc-abi -m8byte-align -mbig-switch -mapp-regs
-msmall-sld -mno-strict-align -mjump-tables-in-data-section

(tiny detail that matters: yes, -m850e2v4 really doesn't have the v prefix while its neighbors do. copy it exactly, i'm begging 😭)

---

🍓 VAX Options (VAX Options)

• -munix — unix vibes
• -mgnu — gnu vibes
• -md — -md!
• -md-float — -md but float
• -mg — -mg!
• -mg-float — -mg but float
• -mlra — lra mode
• -mvaxc-alignment — vaxc alignment
• -mqmath — qmath mode

greatest hits of short flag names honestly 👑

---

💖 Visium Options (Visium Options)

• -mdebug — debug mode
• -msim — simulator mode
• -mfpu — fpu, on
• -mhard-float — hard float
• -msoft-float — soft float
• -mcpu=cpu-type — pick your cpu-type
• -mtune=cpu-type — tune for that cpu-type
• -msv-mode — supervisor mode
• -muser-mode — user mode

---

🫶 VMS Options (VMS Options)

• -mvms-return-codes — VMS-style return codes 💫
• -mdebug-main=prefix — set the debug main prefix
• -mmalloc64 — 64-bit malloc energy
• -mpointer-size=size — pointers are size big, no cap

---

🔥 VxWorks Options (VxWorks Options)

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

look at -Bstatic and -Bdynamic living their best linker lives 💅

---

✨ x86 Options (x86 Options)

okay this one is the main character of the entire manpage. deep breath. here we go 😭💖

-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
-mieee-fp -mrtd -malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mmwait
-mrecip -mrecip=opt -mvzeroupper -mstv -mprefer-avx128
-mprefer-vector-width=opt -mpartial-vector-fp-math -mmove-max=bits
-mstore-max=bits -mnoreturn-no-callee-saved-registers -mmmx -msse -msse2
-msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f
-mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi
-msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig
-mwbnoinvd -mptwrite -mclflushopt -mclwb -mxsavec -mxsaves -msse4a
-m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt
-mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero
-mpku -mgfni -mvaes -mwaitpkg -mshstk -mmanual-endbr -mcet-switch
-mforce-indirect-call -mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq
-mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx512vnni
-mprfchw -mrdpid -mrdseed -msgx -mavx512vp2intersect -mserialize
-mtsxldtrk -mamx-tile -mamx-int8 -mamx-bf16 -muintr -mhreset -mavxvnni
-mamx-fp8 -mavx512fp16 -mavxifma -mavxvnniint8 -mavxneconvert
-mcmpccxadd -mamx-fp16 -mprefetchi -mraoint -mamx-complex -mavxvnniint16
-msm3 -msha512 -msm4 -mapxf -musermsr -mavx10.1 -mavx10.2 -mamx-avx512
-mamx-tf32 -mmovrs -mamx-movrs -mavx512bmm -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 -mstack-arg-probe -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 -mrecord-return
-mfentry-name=name -mfentry-section=name -mskip-rax-setup
-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 -mtls-dialect=type -mrelax-cmpxchg-loop
-mindirect-branch=choice -mfunction-return=choice
-mindirect-branch-register -mharden-sls=choice
-mindirect-branch-cs-prefix -mapx-inline-asm-use-gpr32 -mgather
-mscatter -mneeded -mno-direct-extern-access -munroll-only-small-loops
-mdispatch-scheduler -mlam=choice

that's it, that's the whole vibe. i'm winded. drink some water bestie 🍓💧

---

🐚 x86 Windows Options

See Cygwin and MinGW Options. that's literally all it says. go look over there 👀

---

🌟 Xstormy16 Options (Xstormy16 Options)

• -msim — simulator go brrr ✨

one (1) option. minimalism. we stan a low-effort icon 🎀

---

🌸 Xtensa Options (Xtensa Options)

• -mconst16 — const16 mode 💫
• -mforce-no-pic — force no-pic
• -mno-serialize-volatile — don't serialize volatile
• -mtext-section-literals — literals in the text section
• -mauto-litpools — auto litpools
• -mno-target-align — no target align
• -mlongcalls — long calls
• -mabi=abi-type — pick your abi-type
• -mextra-l32r-costs=cycles — extra l32r costs, measured in cycles
• -mstrict-align — strict alignment, behave urself 😤

---

💜 zSeries Options

See S/390 and zSeries Options. yes bestie it's literally the same section, scroll up 💅

---

👑 Options Controlling the Kind of Output

okay so this is the part where gcc actually explains itself. iconic of it honestly.

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking — always in that order, no skipping, no shuffling, that's the lineup periodt 🎀

GCC is capable of preprocessing and compiling several files, either:

• into several assembler input files, or
• into one assembler input file.

then each assembler input file produces an object file, and linking combines all the object files — the ones that were newly compiled and the ones you passed in as input — into a single executable file. one big happy family 🫶

---

For any given input file, the file name suffix determines what kind of compilation is done. the suffix is literally the vibe check 🔥

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed. (already preprocessed, don't come for it)
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. ⚠️ note that you must link with the libobjc library to make an Objective-C program work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. ⚠️ note that you must link with the libobjc library to make an Objective-C++ program work. also note that .M refers to a literal capital M — yes the case matters bestie, i said what i said 💀
• 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 a C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch). two completely different destinies for one little .h file, main character energy fr fr ✨

and that's the suffix lore. next part's gonna keep going, obviously, because this manpage is a journey and we're only on part 6 of 130 😭💖

ok so like, the compiler sniffs the file name and goes "ah yes, I know her" 💅 here's the whole little vibe-check table of suffixes:

• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. ⚠️ note that in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C (it's case-sensitive bestie, don't play with her).
• file.mm, file.M — Objective-C++ source code that must be preprocessed 🦋
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec. ✨
• file.f, file.for, file.ftn, file.fi — Fixed form Fortran source code that should not be preprocessed.
• file.F, file.FOR, file.fpp, file.FPP, file.FTN — Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.f90, file.f95, file.f03, file.f08, file.fii — Free form Fortran source code that should not be preprocessed 🌸
• file.F90, file.F95, file.F03, file.F08 — Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.cob, file.COB, file.cbl, file.CBL — COBOL source code 👑
• file.go — Go source code.
• file.d — D source code. 🎀
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.
• file.s — Assembler code.
• file.S, file.sx — Assembler code that must be preprocessed.
• other — An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way. lowkey the catch-all bin 🫶

---

Or you can just tell her the language yourself with the -x option, no guessing required:

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

c  c-header  cpp-output
c++  c++-header  c++-system-header c++-user-header c++-cpp-output
c++-system-module
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
cobol
d
f77  f77-cpp-input  f95  f95-cpp-input
go
modula-2  modula-2-cpp-output
rust
algol68
lto

ngl this is important: -x does not imply a particular language standard. Like, -x f77 may also require -std=legacy for some older source codes. it's giving "I picked the language, not the era" 😭

• -x none — Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as if -x has not been used at all). reset era 💀

---

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

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

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

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

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

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

Input files that don't require compilation are ignored.

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

Input files that don't require preprocessing are ignored.

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

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

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

gcc -c foo.c ...

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

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

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

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

A linker output will instead prefix aux and dump outputs:

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

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

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

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

in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*. 🎀 she noticed the match, she's being cute about it

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, --dumpbase dumpbase — This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs:

gcc -save-temps -S foo.c

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

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

preprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o. see how the ✨plot twist✨ changed everything

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

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

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

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

creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir. she said "no ❤️" to all of them

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.*. no collisions, we love a considerate queen 👑

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. 🦋 and that's the naming lore, iykyk

• -dumpbase-ext auxdropsuf, --dumpbase-ext auxdropsuf — okay so when GCC is forming the name of an auxiliary output file (but not a dump file, bestie 💅), it drops the trailing auxdropsuf from dumpbase before appending any suffixes. If you don't specify it, this option defaults to the suffix of a default dumpbase — i.e. the suffix of the input file when -dumpbase is not present on the command line, or when dumpbase is combined with dumppfx. 🎀

Example:

gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...

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

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

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

This 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, --dumpdir dumppfx — when GCC is forming the name of an auxiliary or dump output file, it uses dumppfx as a prefix. 🦋

gcc -dumpdir pfx- -c foo.c ...

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

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

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

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

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

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

This outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides 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.*. 🎀

When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus an 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 ...

This 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. iconic LTO behavior 💜

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

• -v, --verbose — prints (on standard error output) the commands executed to run the stages of compilation. It also prints 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 — prints (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified, then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. 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 — prints (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. 🎯
• --help={class|[^]qualifier}[,...] — prints (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. 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 shows up on the command line before the --help= option, then the descriptive text that --help= shows you gets switched up. Instead of describing the options it's displaying, it just tells you whether each one is enabled, disabled, or set to some specific value (assuming the compiler actually knows that at the point where the --help= option gets used).

Here's 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's totally possible to find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Or, alternatively, you can discover which binary optimizations are enabled by -O3 by using:

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

• --version — Displays the version number and copyrights of the invoked GCC. 💅
• -pass-exit-codes, --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, --pipe — Uses pipes rather than temporary files for communication between the various stages of compilation. Ngl this fails to work on some systems where the assembler can't read from a pipe, but the GNU assembler has no trouble at all. 🫶
• -wrapper — Invokes 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 — When compiling files residing in directory old, records any references to them in the result of the compilation as if the files resided in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. Directories referenced by directives are not affected by these options. See also -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.
• -fcanon-prefix-map — For the -f*-prefix-map options, normally comparison of old prefix against the filename that would be normally referenced in the result of the compilation is done using textual comparison of the prefixes, or ignoring character case for case insensitive filesystems and considering slashes and backslashes as equal on DOS based filesystems. The -fcanon-prefix-map causes such comparisons to be done on canonicalized paths of old and the referenced filename. ✨
• -fplugin=name.so — Loads the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (see -fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins API.
• -fplugin-arg-name-key=value — Defines an argument called key with a value of value for the plugin called name.
• -fdump-ada-spec[-slim] — For C and C++ source and include files, generates corresponding Ada specs. 🎀
• -fada-spec-parent=unit — In conjunction with -fdump-ada-spec[-slim] above, generates Ada specs as child units of parent unit.
• -fdump-go-spec=file — For input files in any language, generates corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written in some other language.
• @file — Reads command-line options from file. The options read are inserted in place of the original @file option. If file doesn't exist, or can't be read, then the option gets treated literally, and not removed. 👑

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

---

🌸 Compiling C++ Programs 🌸

C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; preprocessed C++ files use the suffix .ii; and C++20 module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm. 💖

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

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

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

---

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

The compiler can accept several base standards, 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 don't have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. 😭 The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.

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

• c90, c89, iso9899:1990 — Supports all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
• iso9899:199409 — ISO C90 as modified in amendment 1.
• c99, c9x, iso9899:1999, iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/projects/c-status.html> for more information. The names c9x and iso9899:199x are deprecated. 💀
• c11, c1x, iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
• c17, c18, iso9899:2017, iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of "__STDC_VERSION__", and so is supported to the same extent as C11.
• c23, c2x, iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated.
• c2y — The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 🙈
• gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features).
• gnu99, gnu9x — GNU dialect of ISO C99. The name gnu9x is deprecated.
• gnu11, gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated.
• gnu17, gnu18 — GNU dialect of ISO C17.
• gnu23, gnu2x — GNU dialect of ISO C23. This is the default for C code. The name gnu2x is deprecated. ⭐
• gnu2y — The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The name gnu2x is deprecated. 🙃
• c++98, c++03 — The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
• gnu++98, gnu++03 — GNU dialect of -std=c++98.
• c++11, c++0x — The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
• gnu++11, gnu++0x — GNU dialect of -std=c++11. The name gnu++0x is deprecated. 🎀

• c++14, c++1y — the 2014 ISO C++ standard plus amendments. Heads up: the name c++1y is deprecated.
• gnu++14, gnu++1y — GNU dialect of -std=c++14. Also: the name gnu++1y is deprecated.
• c++17, c++1z — the 2017 ISO C++ standard plus amendments. The name c++1z is deprecated, bestie.
• gnu++17, gnu++1z — GNU dialect of -std=c++17. The name gnu++1z is deprecated.
• c++20, c++2a — the 2020 ISO C++ standard plus amendments. C++20 modules support is still experimental and needs to be enabled with the -fmodules option. The name c++2a is deprecated.
• gnu++20, gnu++2a — GNU dialect of -std=c++20. This is the default for C++ code, periodt. C++20 modules support is still experimental and needs to be enabled with -fmodules. The name gnu++2a is deprecated.
• c++23, c++2b — the 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases 😭. The name c++2b is deprecated.
• gnu++23, gnu++2b — GNU dialect of -std=c++23. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2b is deprecated.
• c++2c, c++26 — the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases 💀.
• gnu++2c, gnu++26 — GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

---

• -aux-info filename — writes prototyped declarations to the given filename for all functions declared and/or defined in a translation unit, including the ones chilling in header files. It's silently ignored in any language other than C, no cap.

Besides declarations, the file tells you — in comments — the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). And for function definitions, you also get a K&R-style list of arguments followed by their declarations, inside comments, after the declaration 🎀.

• -fno-asm — do not recognize asm, inline or typeof as a keyword, so your code can use those 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. iykyk 💅

• -fno-builtin, -fno-builtin-function — don't recognize built-in functions that don't 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. Kind of a niche flex but okay 🔥.
• -fgnu-tm — when the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.

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

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).

• -fgnu89-inline — the option -fgnu89-inline tells GCC to use the traditional GNU semantics for inline functions when in C99 mode.

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

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

The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. Handy, fr fr ✨.

• -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. Like, don't 💀.
• -fms-extensions — accept some non-standard constructs used in Microsoft header files.

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

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

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

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

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

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

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

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

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

• -fdeps-file= file — where to write structured dependency information.
• -fdeps-format= format — the format to use for structured dependency information. p1689r5 is the only supported format right now. Note that when this argument is specified, the output of -MF is stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools.
• -fdeps-target= file — analogous to -MT but for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources.
• -fplan9-extensions — accept some non-standard constructs used in Plan 9 code.

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

• -fsigned-bitfields, -funsigned-bitfields, -fno-signed-bitfields, -fno-unsigned-bitfields — these four besties decide whether a bit-field is signed or unsigned when the declaration is out here being indecisive and doesn't say signed or unsigned at all 😌. Default? Signed, because that's the consistent vibe — basic integer types like int are signed types, so it just matches. Periodt.
• -fsigned-char, -funsigned-char — okay so -fsigned-char lets the type char be signed, literally just like signed char, and -funsigned-char lets char be unsigned, like unsigned char 💅.

Note that -fsigned-char is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, -fno-signed-char is equivalent to -funsigned-char. Sneaky little mirror situation, iykyk 🪞.

Every target GCC supports has a default for what char should be, and it's typically spelled out in the processor-specific ABI document for that target. So it's either like unsigned char by default or like signed char by default — no in-between, no deciding for yourself.

Ideally a portable program should always use signed char or unsigned char when it actually depends on the signedness of an object. But like… so many programs were written to use plain char and expect it signed, or expect it unsigned, depending on the machines they were written for 😭. These options let you make such a program work with the opposite default. Iconic rescue behavior.

And char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two. It's giving main character with a borrowed personality 🎀.

• -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 the elements of such an array. The value of level controls the level of strictness 🦋.

-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest — a trailing array is treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards. Maximum standards-conformity energy ✨.

The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members. Chaos mode, honestly 💀.

There are two more levels in between 0 and 3, provided to support older code that uses the GCC zero-length array extension ([0]) or one-element array as flexible array members ([1]). When level is 1, the trailing array is treated as a flexible array member when it is declared as either [], [0], or [1]. When level is 2, the trailing array is treated as a flexible array member when it is declared as either [], or [0].

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

Heads up bestie: the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. (Yes, that one's written with underscores, not a typo on my part — copy it exactly 💖)

• -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 (the default). Note: this option is not supported for C++ 🚫.

⚠️ Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. Like, actually not compatible. Do not play with this one casually fr fr 🙈

---

✨ 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, such as -std, are also relevant for C++ programs.

Here is a list of options that are only for compiling C++ programs 👇:

• --compile-std-module — build the compiled module interfaces for the <bits/stdc++.h> header unit and the std and std.compat modules before compiling any source files explicitly specified on the command line. This is intended to be useful for building simple programs that use import std; with a single command 🔥. One command, whole module situation, slay.
• -fabi-version=n — use version n of the C++ ABI. The default is version 0.

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. It's giving "moving target but make it spec-compliant" 💀.

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, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.

Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).

Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.

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

Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. Cleanup era ✨.

Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.

Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.

Version 16, which first appeared in G++ 11, changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names.

Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.

Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.

Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags, handling of cv-qualified [[no_unique_address]] members, and adds mangling of C++20 constraints on function templates.

Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.

Version 21, which first appeared in G++ 16, fixes unnecessary captures in noexcept lambdas (c++/119764), layout of a base class with all explicitly defaulted constructors (c++/120012), and mangling of class and array objects with implicitly zero-initialized non-trailing subobjects (c++/121231).

See also -Wabi 👀.

• -fabi-compat-version=n — on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name 🎀. This switch specifies which ABI version to use for the alias.

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

If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning. The way these two negotiate is honestly kind of iconic 💅.

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

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

• -fno-assume-sane-operators-new — okay bestie, this one is about the C++ standard letting you replace the global new, new[], delete and delete[] operators, even though a lot of C++ programs don’t replace them and just use the implementation-provided version. The C++ standard also allows omitting those calls if they’re made from new or delete expressions, and by extension the same is assumed if __builtin_operator_new or __builtin_operator_delete functions are used. This option gives you control over some optimizations around calls to those operators. With -fassume-sane-operators-new-delete, GCC may assume that calls to the replaceable global operators from new or delete expressions, or from __builtin_operator_new or __builtin_operator_delete calls, don’t read or modify any global variables or variables whose address could escape to the operators — that’s global state, except for errno for the new and new[] operators. This allows most optimizations across those calls and is something the implementation-provided operators satisfy unless malloc implementation details are observable in the code or unless malloc hooks are used, but it might not be satisfied if a program replaces those operators. This behavior is enabled by default. With -fno-assume-sane-operators-new-delete, GCC must assume all these calls — whether from new or delete expressions or called directly — may read and write global state unless proven otherwise, e.g. when GCC compiles their implementation. Use this option if those operators are or may be replaced and code needs to expect such behavior. ✨
• -fchar8_t, -fno-char8_t — enables support for char8_t as adopted for C++20. This includes adding a new char8_t fundamental type, changing the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros. 💅

This option 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;

The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when -fchar8_t is specified:

const char *cp = u8"xx";// error: invalid conversion from
                        //        `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx");     // error: invalid conversion from
                        //        `const char8_t*' to `const char*'
std::string s{u8"xx"};  // error: no matching function for call to
                        //        `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s;            // error: conversion from
                        //        `basic_string<char8_t>' to non-scalar
                        //        type `basic_string<char>' requested

• -fcheck-new — checks that the pointer returned by operator new is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that operator new only returns 0 if it is declared throw(), in which case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also new (nothrow). 🫶
• -fconcepts — 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, could additionally be enabled by -fconcepts-ts. The option -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15; users are expected to convert their code to C++20 concepts.

Heads up bestie: -fconcepts-ts is gone in GCC 15. Not a drill — convert to C++20 concepts. 😭

• -fconcepts-diagnostics-depth=n — sets the maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is 1. 🎀
• -fconstexpr-depth=n — sets the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512. 🔥
• -fconstexpr-cache-depth=n — sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is a heuristic that trades off compilation speed — when the cache avoids repeated calculations — against memory consumption — when the cache grows very large from highly recursive evaluations. The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. 🦋
• -fconstexpr-fp-except — Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined. 🌸

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

• -fconstexpr-loop-limit=n — sets the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18). 👑
• -fconstexpr-ops-limit=n — sets the maximum number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop — or even outside of a loop — too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25). 🌈
• -fcontracts — enables support for the C++ Contracts feature, as specified in the C++26 working draft (N5003). ⭐

On violation of an enforced or observed contract — see -fcontract-evaluation-semantic below — a violation handler is called; the standard library provides a default handler that emits information about the contract that failed.

Users can replace the default violation handler by defining:

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

• -fcontract-evaluation-semantic=semantic — sets the semantic with which contracts will be evaluated to semantic. 💜

Available values for the evaluation mode are:

• '"ignored"' — the contract checks will be elided, with no checking added at either compile or runtime.
• '"observed"' — the contract checks are performed at runtime and, if one fails, a handler is called that allows actions such as logging or emitting run-time warnings. When the handler returns, execution of the code will continue. At compile-time the checks are performed for constexpr evaluations, and in this case a diagnostic is emitted if the check fails.
• '"enforce"' — the contract checks are performed and the handler is called if one fails. When the handler returns, execution of the code is terminated preventing it from continuing past the failed case. At compile-time the checks are applied to constexpr and, if one fails, the program is ill-formed; an error will be emitted.
• '"quick_enforce"' — the contract checks are performed at runtime and, if one fails, execution is terminated immediately without calling any handler. At compile-time there is no difference in behaviour between this option and the enforce case, since no handler is invoked at compile time.

• -fcontracts-conservative-ipa — prevents inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. This allows for the case that contract evaluation conditions are permitted to differ between TUs, which means such actions would be potentially incorrect. 🧚
• -fcontract-checks-outlined — by default, contract checks for "pre" and "post" condition checks are expanded in-line into function bodies. This option causes a small function to be created instead — containing the checks — that is called in the relevant position. This permits different criteria to be applied to the compilation of the checking and the remainder of the function body. 🍓
• -fcontract-disable-optimized-checks — when "pre" and "post" condition checks are outlined using -fcontract-checks-outlined, this option disables the optimisation steps for those functions which can avoid unwanted elision of checking steps. 💀
• -fcontracts-client-check=mode — enables insertion of checks at call sites, a.k.a. caller-side checking. 🎀

The value of mode determines which contract checks are made:

• '"none"' — no caller/client side checking (default)
• '"pre"' — preconditions are checked at the caller/client side
• '"all"' — both pre and post-conditions are checked at the caller/client side

• -fcontracts-definition-check=mode — introduces the ability to disable callee/definition-side checks, which are otherwise enabled by default when the contracts feature is active. 🙈

The variable mode has the values '"on"' and '"off"'.

• -fcoroutines — turns on support for the C++ coroutines extension. With -std=c++20 and above, coroutines are literally part of the language standard, so -fcoroutines defaults to on. ✨
• -fdiagnostics-all-candidates — lets the C++ front end note all candidates during an overload resolution failure, including when a deleted function is selected. More receipts for your template chaos, bestie. 💅
• -fdump-lang-, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — control the dumping of C++-specific information. The options and filename portions behave as described in the -fdump-tree option. The following switch values are accepted:
  • all — enable all of the below. 👑
  • class — dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified.
  • 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.
  • raw — dump the raw internal tree data. no cap, just vibes 💀
  • tinst — dump the sequence of template instantiations, indented to show the depth of recursion. The lineno option adds the source location where the instantiation was triggered, and the details option also dumps pre-instantiation substitutions such as those performed during template argument deduction. Lines in the .tinst dump start with I for an instantiation, S for another substitution, and R[IS] for the reopened context of a deferred instantiation. 🧚
• -fno-elide-constructors — the C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline. In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions. 🫶
• -fno-enforce-eh-specs — don’t generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. ⚠️
• -fextern-tls-init, -fno-extern-tls-init — the C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn’t actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option. On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init. 🌈
• -ffold-simple-inlines, -fno-fold-simple-inlines — permit the C++ frontend to fold calls to std::move, std::forward, std::addressof, std::to_underlying and std::as_const. In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active. 🎀
• -fno-gnu-keywords — do not recognize typeof as a keyword, so that code can use this word as an identifier. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc. ✨
• -fno-immediate-escalation — do not enable immediate function escalation whereby certain functions can be promoted to consteval, as specified in P2564R3. For example:

consteval int id(int i) { return i; }

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

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

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

• -fimplicit-constexpr — make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations. 👑
• -fno-implicit-templates — never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. 💀
• -fno-implicit-inline-templates — don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations. 🌸
• -fno-implement-inlines — to save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called. ⚠️
• -fmodules, -fno-modules — enable support for C++20 modules. The -fno-modules is usually not needed, as that is the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. 🦋
• -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file to create an importable header unit. ✨
• -fmodule-implicit-inline — member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.) 💅
• -fno-module-lazy — disable lazy module importing and module mapper creation. 🙈
• -fmodule-mapper=[hostname]:port[?ident], -fmodule-mapper=|program[?ident] args..., -fmodule-mapper==socket[?ident], -fmodule-mapper=<>[inout][?ident], -fmodule-mapper=<in>out[?ident], -fmodule-mapper=file[?ident] — an oracle to query for module name to filename mappings. If unspecified the CXX_MODULE_MAPPER environment variable is used, and if that is unset, an in-process default is provided. 🧚
• -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file. 🎀
• -fms-extensions — disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax. 😌
• -fnew-inheriting-ctors — enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified. 👑
• -fnew-ttp-matching — enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for -std=c++17. 🔥
• -fno-nonansi-builtins — disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions. 💜
• -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 — okay so when an error message points at a specialization of a function template, the compiler normally shows off by printing the whole signature, then the template arguments, then any typedefs or typenames chillin' in the signature (like void f(T) [with T = int] instead of just void f(int)) so you can actually tell which template is being messy. And when the error is about a class template specialization, it leaves out any template arguments that match the defaults for that template. But if either of those flexes makes the error harder to read instead of easier? This flag turns both off. You're welcome 💅
• -frange-for-ext-temps — bestie this one extends the lifetime of C++ range-based for temporaries ✨ With -std=c++23 and up, that's literally just the language standard now, so temporaries live until the end of the loop by default, no cap. This option lets you turn that behavior on in earlier versions of the standard too. So iconic.
• -freflection — enables experimental C++26 Reflection 🪞 (yes, experimental, so like… handle with care, it's giving beta energy).
• -fno-rtti — stops the compiler from generating that info about every class with virtual functions that the C++ run-time type identification features (dynamic_cast and typeid) need. If you don't use those parts of the language, this flag saves you some space 🎀 Note that exception handling uses the same information, but G++ generates it as needed. Also: the dynamic_cast operator still works for casts that don't need run-time type info, i.e. casts to void * or to unambiguous base classes.

Real talk: mixing code compiled with -frtti and code compiled with -fno-rtti may just… not work 😭 Like, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. Be careful, bestie.

• -fsized-deallocation — enables the built-in global declarations

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

as introduced in C++14 🔥 Super useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. It's enabled by default under -std=c++14 and above. Oh and -Wsized-deallocation warns you about places that might want to add a definition 👀

• -fstrict-enums — lets the compiler optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that fits 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. Also, this option has no effect for an enumeration type with a fixed underlying type.
• -fstrong-eval-order, -fstrong-eval-order=kind — evaluates member access, array subscripting, and shift expressions left-to-right, and evaluates assignment right-to-left, exactly as adopted for C++17 🫶 -fstrong-eval-order is the same thing as -fstrong-eval-order=all, and it's enabled by default with -std=c++17 or later.
  • -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and it's the default for C++ dialects before C++17.
  • -fstrong-eval-order=none is equivalent to -fno-strong-eval-order.
• -ftemplate-backtrace-limit=n — caps the maximum number of template instantiation notes for a single warning or error at n. Default is 10. No more scrolling for a thousand years 💀
• -ftemplate-depth=n — sets the maximum instantiation depth for template classes to n. You need a limit on template instantiation depth to detect endless recursions during template class instantiation (iykyk). ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). Default value is 900, because the compiler can run out of stack space before hitting 1024 in some situations 😭
• -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. Use this if you wanna shrink your code size slightly and your code genuinely doesn't need to be thread-safe 🧚
• -fuse-cxa-atexit — registers destructors for objects with static storage duration with the __cxa_atexit function instead of the atexit function. This one's required for fully standards-compliant handling of static destructors, but it only works if your C library supports __cxa_atexit 💜
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This makes std::uncaught_exception incorrect 😬 but it's necessary if the runtime routine isn't available.
• -fvisibility-inlines-hidden — this switch declares that you, the user, are not out here comparing pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. Periodt.

The effect: GCC may effectively mark inline methods with __attribute__ ((visibility ("hidden"))) so they don't show up in a DSO's export table and don't need a PLT indirection when used inside the DSO. Turning this on can have a dramatic effect on load and link times of a DSO 🔥 because it massively reduces the size of the dynamic export table when the library makes heavy use of templates.

Heads up though: this switch isn't quite the same as marking the methods hidden directly, because it doesn't affect static variables local to the function, and it doesn't make the compiler deduce that the function is defined in only one shared object.

You can mark a method as having visibility explicitly to cancel the effect of the switch for that method — like, if you genuinely do wanna compare pointers to one particular inline method, mark it as having default visibility. And marking the enclosing class with explicit visibility? Does nothing, sorry 🫠

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

• -fvisibility-ms-compat — this flag tries to use visibility settings to make GCC's C++ linkage model compatible with Microsoft Visual Studio's. The changes it makes 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're permitted when this option is not used.

In new code it's better to use -fvisibility=hidden and export the classes you actually want externally visible 🎀 Unfortunately, code can rely on the Visual Studio behavior, maybe accidentally.

Among the consequences of these changes: static data members of the same type with the same name but defined in different shared objects are different, so changing one doesn't change the other 😱 and pointers to function members defined in different shared objects may not compare equal. And when this flag is given, defining types with the same name differently is a violation of the ODR. So like… don't.

• -fno-weak — don't use weak symbol support, even if the linker provides it. By default G++ uses weak symbols when they're available. This option exists only for testing and should not be used by end-users; it results in inferior code and has no benefits 💀 It may be removed in a future release of G++. So basically: not for you, bestie.
• -fext-numeric-literals (C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off, those suffixes are treated as C++11 user-defined literal numeric suffixes. It's on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. It's off by default for ISO C++11 onwards (-std=c++11, ...).
• -nostdinc++ — don't search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library 🛠️)
• -flang-info-include-translate, -flang-info-include-translate-not, -flang-info-include-translate=header — inform you about include translation events ✨ The first notes accepted include translations, the second notes declined include translations, and the header form informs you about include translations relating to that specific header. If header is of the form user or <system>, it gets resolved to a specific user or system header using the include path 👑
• -flang-info-module-cmi, -flang-info-module-cmi=module — inform you about Compiled Module Interface pathnames. The first notes all read CMI pathnames. The module form notes reading a specific module's CMI. And 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 lets you specify 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) — warns when a type with an ABI tag is used in a context that doesn't have that ABI tag. See C++ Attributes for more info about ABI tags 🏷️
• -Wno-abbreviated-auto-in-template-arg — disables the error for an auto placeholder type used within a template argument list to declare a C++20 abbreviated function template, e.g.

void f(S<auto>);

this bit was proposed in the Concepts TS but did not make it into C++20, bestie — in the standard, a placeholder in a parameter declaration has to show up as a decl-specifier. The error can also be softened down to a warning with -fpermissive or -Wno-error=abbreviated-auto-in-template-arg.

-Wcomma-subscript (C++ and Objective-C++ only) 💅

Warns you when a comma expression shows up inside a subscripting expression. That usage got deprecated in C++20 and is on its way out in C++23. But heads up: a comma expression wrapped in ( ) is not deprecated. Example:

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

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

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

This warning gets upgraded to an error by -pedantic-errors in C++23 mode or later. 😭

-Wctad-maybe-unsupported (C++ and Objective-C++ only) 🦋

Warns when you're doing class template argument deduction (CTAD) on a type that has no explicitly written deduction guides. It's basically flagging the cases where CTAD only succeeded because the compiler synthesized the implicit deduction guides — which might not be what the programmer actually intended, iykyk. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. You can suppress this warning 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) 🎀

Warns 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 warns if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. It's giving "sealed box, no keys" energy.

-Wdangling-reference (C++ and Objective-C++ only) 💀

Warns when a reference is bound to a temporary whose lifetime has ended. Like:

int n = 1;
const int& r = std::max(n - 1, n + 1); // r is dangling

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

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

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

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

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

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

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

-Wdangling-reference also warns about code like

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

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

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

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

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

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

This warning is enabled by -Wextra.

-Wdelete-non-virtual-dtor (C++ and Objective-C++ only) 🔥

Warns when delete is used to destroy an instance of a class that has virtual functions and a non-virtual destructor. Deleting an instance of a derived class through a pointer to a base class is unsafe if the base class does not have a virtual destructor — that's a whole undefined-behavior jump scare. 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.

-Wdeprecated-copy-dtor (C++ and Objective-C++ only) 🌸

Similar to -Wdeprecated-copy, but it also deprecates if the class has a user-provided destructor.

-Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) 🧚

Disables the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:

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

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

-Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only) 🌈

Disables the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:

enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;

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

-Wdeprecated-literal-operator (C++ and Objective-C++ only) ⭐

Warns that the declaration of a user-defined literal operator with a space before the suffix is deprecated. This warning is enabled by default in C++23, or with explicit -Wdeprecated.

string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred

-Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only) 💜

Warns that omitting a comma before the varargs ... at the end of a function parameter list is deprecated. This warning is enabled by default in C++26, or with explicit -Wdeprecated.

void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated

-Wno-elaborated-enum-base 🫶

For C++11 and above, warns if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with -pedantic-errors.

-Wno-init-list-lifetime (C++ and Objective-C++ only) 🎀

Stops warning about uses of std::initializer_list that are likely to result in dangling pointers. Since the underlying array for an initializer_list is handled like a normal C++ temporary object, it is so easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example:

• If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
• If a new-expression creates an initializer_list, the array only lives until the end of the enclosing full-expression, so the initializer_list in the heap has a dangling pointer.
• When an initializer_list variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the initializer_list variable has a dangling pointer.

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

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

-Winvalid-constexpr 🙈

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

This one's enabled as a pedantic warning by default in C++20 and earlier, bestie. In C++23, -Winvalid-constexpr can be turned on, and then it becomes an ordinary warning instead. Like so:

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 the imported macro definitions are actually valid at the end of compilation. Ngl this one is not enabled by default, because it takes extra processing to figure out. But it's genuinely useful when you're putting together sets of header-units, since it makes sure your macros stay consistent. 🎀
• -Wno-literal-suffix (C++ and Objective-C++ only) — tells GCC to chill and not warn when a string or character literal is followed by a ud-suffix that doesn't start with an underscore. As a conforming extension, GCC treats those suffixes as separate preprocessing tokens, all so it stays backwards compatible with code that uses formatting macros from <inttypes.h>. Example time:

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

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

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

This option also controls the warnings you get when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore — because those identifiers are reserved for future standardization, iykyk.

These warnings are enabled by default.

• -Wno-narrowing (C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions get diagnosed by default, as the standard demands. A narrowing conversion from a constant produces an error, and one from a non-constant produces a warning — but -Wno-narrowing suppresses the diagnostic entirely. Heads up though: this doesn't change the meaning of well-formed code, and narrowing conversions are still ill-formed in SFINAE contexts. 🙈

With -Wnarrowing in C++98, you get a warning when a narrowing conversion that C++11 prohibits happens inside { }, 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 it's calling a function that doesn't have a non-throwing exception specification (i.e. throw() or noexcept) but the compiler knows that function never actually throws. The audacity, honestly. 😭
• -Wnoexcept-type (C++ and Objective-C++ only) — warns if that C++17 feature that makes noexcept part of a function type changes the mangled name of a symbol compared to C++14. Enabled by -Wabi and -Wc++17-compat. Here's the 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 more you know 🌈

• -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a raw memory function call like memset or memcpy is an object of class type, and when writing into that object might bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of objects like that can violate invariants that member functions of the class are out here maintaining, bestie. 💀

For example, the memset call below is undefined behaviour because it modifies a non-trivial class object — so it gets diagnosed. The safe way to initialize or clear the storage of objects like this is to use the appropriate constructor or assignment operator, if one's available.

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

The -Wclass-memaccess option is enabled by -Wall. And if you explicitly cast the pointer to the class object to void * or to a type that the raw memory function can safely access, the warning goes away. 🫶

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor, either on 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 the -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred, because it warns about the unsafe cases without the false positives. Periodt.
• -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, so like... it's giving legacy. 👑 Enabled by default with -std=c++17.
• -Wreorder (C++ and Objective-C++ only) — warns when the order of member initializers in your code doesn't match the order they actually have to be executed in. Example:

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

The compiler rearranges the member initializers for i and j to match the declaration order of the members, and emits a warning about it. This warning is enabled by -Wall. ✨

• -Wno-pessimizing-move (C++ and Objective-C++ only) — warns when a call to std::move is actively preventing copy elision. 😭 The typical scenario where copy elision can happen is when you return from a function with a class return type, the expression being returned 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.

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

But in this example, that std::move call is literally blocking copy elision. Not the flex you thought it was. 💀

This warning is enabled by -Wall.

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

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

Here, the std::move call is redundant. 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's return type differ — yet G++ treats the return value as if it were designated by an rvalue. Sneaky. 🦋

This warning is 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. Small victories. 🎀

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

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

In the example above, on every single iteration of the loop a temporary value of type double is created and destroyed, and the reference const double & gets bound to it. Every. Single. Time. 💀

This warning is enabled by -Wall.

• -Wredundant-tags (C++ and Objective-C++ only) — warns about redundant class-key and enum-key in references to class types and enumerated types, in contexts where the key can be dropped without causing ambiguity. For example:

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

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

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

See, context is everything. ✨

• -Wno-subobject-linkage (C++ and Objective-C++ only) — don't 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. Because if a type A depends on a type B with no linkage or internal linkage, defining A in multiple translation units would be an ODR violation — the meaning of B is different in each translation unit, bestie. 😭

If A only shows up in a single translation unit, the best way to silence the warning is to give it internal linkage too, by sticking it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default. 🫶

• -Weffc++ (C++ and Objective-C++ only) — warns about violations of these style guidelines from Scott Meyers' Effective C++ series of books:
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have operator= return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload &&, ||, or ,.

Basically the canon, fr fr. 📚

Okay bestie, the previous option is still going — it's got one more thing to tell you:

This flag also switches on -Wnon-virtual-dtor, which is, like, one of the actually-good C++ recommendations too. But the check gets extended: it'll also warn you when an accessible non-polymorphic base class is missing a virtual destructor. Extra credit energy 💅

When you pick this option, lowkey be aware: the standard library headers do not obey all of these guidelines. So use grep -v to filter out those warnings. Don't come for me, come for the STL 🫶

---

• -Wno-exceptions (C++ and Objective-C++ only) — turns OFF the warning for when an exception handler gets shadowed by another handler. Which, honestly? That warning is a little snitch that can point out a wrong ordering of your exception handlers. Silence it at your own risk 🔥
• -Wsfinae-incomplete (C++ and Objective-C++ only) — warns you about a class that turns out to be incomplete, or a function with auto return type whose return type hasn't been deduced yet, in a context where that causes substitution failure instead of an error — and then the class or function gets defined later in the translation unit. Why is this a problem? Because template instantiations or concept checks could give you different results depending on whether they first happen before or after that definition. Messy 😭

This warning is enabled by default, no cap. And -Wsfinae-incomplete=2 adds an extra warning right at the point of substitution failure, so you can actually track down the problems the default mode flagged. Level-up behavior ⭐

• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns when you use an uncasted "NULL" as a sentinel. If you're compiling with GCC and only GCC, this is a totally valid sentinel, because "NULL" is defined to "__null". And yeah, it's a null pointer constant rather than a null pointer, but it's guaranteed to be the same size as a pointer. The catch? This use is not portable across different compilers. So it works until it doesn't 💀
• -Wno-non-c-typedef-for-linkage (C++ and Objective-C++ only) — disables the pedwarn for unnamed classes that have a typedef name for linkage purposes but contain C++-specific members, base classes, default member initializers or lambda expressions — including those on nested member classes. Here's the vibe:

typedef struct {
  int a; // non-static data members are ok
  struct T { int b; }; // member classes too
  enum E { E1, E2, E3 }; // member enumerations as well
  int c = 42; // default member initializers are not ok
  struct U : A { int c; }; // classes with base classes are not ok
  typedef int V; // typedef is not ok
  using W = int; // using declaration is not ok
  decltype([](){}) x; // lambda expressions not ok
} S;

In all of these cases, the tag name S should be added after the struct keyword. Periodt 🎀

• -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings for when non-template friend functions are declared inside a template. Back in very old versions of GCC, like, before the ISO standard was implemented, declarations such as friend int foo(int) — where the name of the friend is an unqualified-id — could be interpreted as a particular specialization of a template function. The warning exists to diagnose those compatibility problems, and it's enabled by default. Ancient history, but the warning stayed 👑
• -Wold-style-cast (C++ and Objective-C++ only) — warns you if an old-style (C-style) cast to a non-void type shows up inside a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are way less vulnerable to unintended effects and so much easier to search for. Glow-up, honestly ✨
• -Woverloaded-virtual (C++ and Objective-C++ only)
• -Woverloaded-virtual=n — warns when a function declaration hides virtual functions from a base class. Example time:

struct A {
  virtual void f();
};

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

Here the A class version of f is hidden in B, and code like this:

B* b;
b->f();

just fails to compile. Sad! 😭

In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function — like, add using A::f; to B. Easy fix, chef's kiss 🎀

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. Reasonable!

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

• -Wno-pmf-conversions (C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function into a plain pointer. Bye-bye, warning 🙈
• -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. So, yeah, it is what it is 😤
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates entirely, and this is how you'd enforce that rule. The warning is inactive inside a system header file (like the STL), so you can still use the STL. And you can also instantiate or specialize templates — that's fine, bestie 🦋
• -Wmismatched-new-delete (C++ and Objective-C++ only) — warns about mismatches between calls to operator new or operator delete and the corresponding call to the allocation or deallocation function. That 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 operator delete isn't a suitable deallocator — plus calls to other deallocation functions with pointers returned from operator new where the deallocation function isn't suitable. It's giving "wrong tool for the job" 🔥

For example, the delete expression in the function below gets diagnosed because it doesn't match the array form of the new expression the pointer argument came from. Same deal — the call to free is also diagnosed:

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

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

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

And just so you know: -Wmismatched-new-delete is included in -Wall ⭐

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

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

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

Now, it is not wrong to declare a class with the class-key struct, like the example above shows. The -Wmismatched-tags option is just here to help you achieve a consistent style of class declarations. And in code meant to be portable to Windows-based compilers, this warning helps prevent unresolved references caused by the difference in mangling of symbols declared with different class-keys. You can use the option on its own or in conjunction with -Wredundant-tags 💜

• -Wmultiple-inheritance (C++ and Objective-C++ only) — warns when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this is how you enforce that rule. Inactive inside a system header file (like the STL), so you can still use the STL. And you can also define classes that indirectly use multiple inheritance — that's fine 🌸
• -Wvirtual-inheritance — warns 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. Same energy as above, honestly 🧚
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. And bestie, this is dangerous: if the virtual base is reachable along more than one path, it gets moved multiple times, which can mean both objects end up in the moved-from state 😭 If the move assignment operator is written to avoid moving from a moved-from object, then yeah, this warning can be disabled. Fair.
• -Wnamespaces — warns 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. So like, no new namespaces, but you can still use them 👑
• -Wno-template-body (C++ and Objective-C++ only) — disables diagnosing errors when parsing a template, and instead issues an error only upon instantiation of the template. This flag can also be used to downgrade such errors into warnings with Wno-error= or -fpermissive 🫶
• -Wno-template-id-cdtor (C++ and Objective-C++ only) — disables the warning about using simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. 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's also enabled by -Wc++20-compat. Slay 🌈

• -Wtemplate-names-tu-local — warns you when a template body hides an exposure of a translation-unit-local entity 💅 in most cases, referring to a translation-unit-local entity (like an internal linkage declaration) inside an entity that gets emitted into a module's CMI is straight-up an error. But — and this is the plot twist — inside the initializer of a variable, or in the body of a non-inline function, it's not an exposure, so no error gets emitted, no drama.

Why do we care? Because this can make variable or function templates accidentally become unusable if they reference such an entity, since other translation units that import the template will never be able to instantiate it 😭 this warning tries to catch those cases before they ruin your day. The presence of an explicit instantiation silences the warning. Iconic behaviour, honestly.

This flag is enabled by -Wextra.

• -Wno-expose-global-module-tu-local — okay so an exposure of a translation-unit-local entity from a module interface is invalid, because it may cause ODR violations and manifest in link errors or other unexpected behaviour 💀 however (and this is the messy part), lots of existing libraries declare TU-local entities in their interface, and avoiding exposures of these entities may be difficult in some cases.

As an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment ✨ this warning indicates when such an invalid exposure has occurred, and can be silenced using diagnostic pragmas either at the site of the exposure, or at the point of declaration of the internal declaration. This also applies to export using declarations naming such entities.

When combined with -Wtemplate-names-tu-local, GCC will also warn about non-exposure references to TU-local entities in template bodies 🫶 such templates can still be instantiated in other TUs, but the above risks regarding exposures of translation-unit-local entities apply.

This warning is enabled by default, and is upgraded to an error by -pedantic-errors.

• -Wno-external-tu-local — warns when you're naming a TU-local entity outside of the translation unit it was declared in 🙈 such declarations will be ignored during name lookup. This can occur when performing ADL from a template declared in the same TU as the internal function:

export module M;
template <typename T> void foo(T t) {
  bar(t);
}
struct S {} s;
static void bar(S) {}  // internal linkage

// instantiating foo(s) from outside this TU can see ::bar,
// but naming it there is ill-formed.

You can work around this by making bar attached to the global module, using extern "C++" 💖

This warning is enabled by default, and is upgraded to an error by -pedantic-errors.

• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate 😭 like, why are we even throwing bestie.
• -Wno-vexing-parse (C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity 👑 this warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:

void f(double a) {
  int i();        // extern int i (void);
  int n(int(a));  // extern int n (int);
}

Another example:

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

The warning will suggest options how to deal with such an ambiguity — e.g., it can suggest removing the parentheses or using braces instead 💅 honestly a little life coach.

This warning is enabled by default.

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

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

• -Waligned-new
• -Waligned-new=[none|global|all] — warns about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter 🦋 this option is enabled by -Wall.

Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. Slay, more coverage 💖

• -Wno-placement-new
• -Wplacement-new=n — warns about placement new expressions with undefined behavior, like 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) — warns about catch handlers that do not catch via reference 💅 with -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference. -Wcatch-value is enabled by -Wall.
• -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs 🌸 like, "supported… conditionally" bestie, pick a lane.
• -Wno-defaulted-function-deleted (C++ and Objective-C++ only) — warns when an explicitly defaulted function is deleted by the compiler 🫶 that can occur when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default.
• -Wno-delete-incomplete (C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime 💀 this warning is enabled by default.
• -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolons ✨ there are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):

struct S {
  void foo () {};
};

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

struct S {
};
;

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

struct S {
  int a;
  ;
};

• -Wno-global-module (C++ and Objective-C++ only) — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives 🙈 like girl, that's the whole point of the fragment.
• -Wno-inaccessible-base (C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity 👑 the warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the -Wextra option.

struct A { int a; };

struct B : A { };

struct C : B, A { };

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

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

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

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

without a definition of the corresponding sized deallocation function

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

...or the other way around, whatever 💅. Enabled by -Wextra along with -fsized-deallocation.

• -Wsuggest-final-types — this one warns you about types that have virtual methods but would be so much better if you'd just slap the C++11 final specifier on them, or (if you can swing it) declare them in an anonymous namespace. Why? Because then GCC can go full main-character mode and devirtualize those polymorphic calls way more aggressively 🔥. Heads up: this warning hits different with link-time optimization, since that's when the class hierarchy graph info is actually complete. iykyk.
• -Wsuggest-final-methods — same energy but for virtual methods instead of types. It warns about virtual methods that would be higher quality if the method got the C++11 final specifier, or if its type were declared in an anonymous namespace or with final 🎀. Again, way more effective with link-time optimization, because that's when the class hierarchy graph info is complete. Pro tip from the manpage itself: consider the -Wsuggest-final-types suggestions first, then rebuild with your new annotations. Order matters bestie ✨.
• -Wsuggest-override — warns you about overriding virtual functions that aren't marked with the override keyword. That's it. That's the whole vibe.
• -Wno-conversion-null (C++ and Objective-C++ only) — do NOT warn about conversions between NULL and non-pointer types. Because -Wconversion-null is on by default, this is how you shut it up 🙈.

---

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

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.

Okay so this whole section is about the command-line options that are only meaningful for Objective-C and Objective-C++ programs. But like — you can still use most of the language-independent GNU compiler options too, obviously. For example, you might compile a file some_class.m like this:

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

In that example, -fgnu-runtime is an option that's meant only for Objective-C and Objective-C++ programs — but the other options? Those work with literally any language GCC supports, slay 💖.

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

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

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string you write with the @"..." syntax 🎀. The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below). On Darwin / macOS platforms, the -fconstant-cfstrings option — if it's also present — overrides the -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings. And fyi, -fconstant-cfstrings is just an alias for the target-specific -mconstant-cfstrings equivalent.
• -fgnu-runtime — generate object code that's compatible with the standard GNU Objective-C runtime. This is the default for most types of systems.
• -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin / macOS. Also, the macro __NEXT_RUNTIME__ gets predefined if (and only if) this option is used 👑
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This lets the runtime use more efficient entry points ✨. Caveat: this option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
• -fobjc-abi-version=n — use version n of the Objective-C ABI for the selected runtime. Currently this option is only supported for the NeXT runtime. In that case:
  • Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions.
  • Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions.
  • Version 2 is the modern (64-bit) ABI.

If 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 — for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then returns self 🫶. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods generated this way 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 immediately after a new object instance is allocated; the - (void) .cxx_destruct methods get invoked immediately before the runtime deallocates an object instance.

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

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what C++ offers. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized 🔥. It's available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that doesn't require special compiler flags.
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil this way no matter what this flag is set to 🎀. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards — those are controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that isn't recognized by GCC 4.0 gets rejected. Super useful if you need to make sure your Objective-C code can compile with older versions of GCC ✨.
• -freplace-objc-classes — emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution — no need to restart the program itself 😭👑. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
• -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained 🦋. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options.
• -fno-local-ivars — by default, instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name 🙈. Specifying the -fno-local-ivars flag disables this behavior, thus avoiding variable shadowing issues.
• -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option, so that instance variables declared outside the scope of any access modifier directives default to that visibility.
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
• -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector.
• -Wno-property-assign-default (Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified.
• -Wno-protocol (Objective-C and Objective-C++ only) — if a class is declared to implement a protocol, a warning gets issued for every method in the protocol that the class doesn't implement. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass 💅. But if you use the -Wno-protocol option, then methods inherited from the superclass count as implemented, and no warning is issued for them.
• -Wobjc-root-class (Objective-C and Objective-C++ only) — this one's looking out for your root classes 👑. It hollers if a class interface has no superclass at all. Like, most classes are gonna inherit from NSObject (or Object), bestie, that's the whole vibe. But if you're deliberately declaring a class that's meant to be a root class? Just mark its interface with __attribute__((objc_root_class)) and the warning gets suppressed, easy. 💅
• -Wselector (Objective-C and Objective-C++ only) — warns you when multiple methods of different types for the same selector show up during compilation 🫠. The check runs on that list of methods in the final stage of compilation. On top of that, it also checks each selector that appears in a @selector(...) expression, plus that a corresponding method for that selector actually got found during compilation. ⚠️ But here's the catch, and it's kind of a big one: because these checks scan the method table only at the end of compilation, you won't get these warnings at all if the final stage never gets reached — like if an error is found during compilation, or if you're using -fsyntax-only. So no cap, if it dies early, you get nothing. 😭
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warns if multiple methods with differing argument and/or return types are found for a given selector when you're trying to send a message with that selector to a receiver of type id or Class. When this flag is off (which, btw, is the default behavior ✨), the compiler just omits those warnings if any differences it found are confined to types that share the same size and alignment. So same-size-same-alignment drama = not its problem.
• -Wundeclared-selector (Objective-C and Objective-C++ only) — warns when a @selector(...) expression is referring to a selector that was never declared 🙈. A selector counts as undeclared if no method with that name has been declared before the @selector(...) expression — whether that's explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. And the timing? This option always does its checks the instant a @selector(...) expression shows up, while -Wselector only does its checks in the final stage of compilation. Different energy, honestly. It also enforces that coding style convention that methods and selectors must be declared before being used — so it's lowkey your lint bestie too. 💖
• -print-objc-runtime-info — generates a C header describing the largest structure that is passed by value, if any. That's it, that's the whole thing. 🎀

---

🌈 Options Controlling OpenMP and OpenACC 🌈

So, GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option ✨. Same deal for OpenACC — those extensions are supported in all three languages with -fopenacc. Parallelism is literally the main character here. 🧚

• -foffload=disable
• -foffload=default
• -foffload=target-list — these tell GCC which OpenMP and OpenACC offload targets it should generate code for 🎯. The default behavior — which is exactly what -foffload=default gives you — 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 get specified in GCC's internal target-triplet format. Pro tip: 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 🫶. Wanna send options that apply only to a specific target or targets? Use the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option. Iconic symmetry, we love to see it.

Typical command lines look like:

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

• -fopenacc — enables handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran, plus !$acc, c$acc and *$acc in fixed-form Fortran. 🌀 Heads up: this option implies -pthread, and thus is only supported on targets that have support for -pthread. No pthread, no party.
• -fopenacc-dim=geom — sets the default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of :-separated sizes, in this order: gang, worker, and vector 🎯. You can omit a size to use a target-specific default value. Mwah, flexible king.
• -fopenmp — enables handling of OpenMP directives #pragma omp, [[omp::decl(...)]], [[omp::directive(...)]], and [[omp::sequence(...)]] in C/C++ 🔥. In Fortran, it enables !$omp and the conditional compilation sentinel !$. And in fixed source form Fortran, those sentinels can also start with c or *.

Also: this option implies -pthread, so again — only supported on targets that have support for -pthread. And -fopenmp implies -fopenmp-simd too. It's giving "and I brought my friend." 💜

• -fopenmp-simd — enables handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directive, and of combined or composite directives with simd as 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 *. All the other OpenMP directives get ignored. And unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause. Kind of a whole situation, but okay. 💅
• -fopenmp-target-simd-clone
• -fopenmp-target-simd-clone=device-type — so beyond generating SIMD clones for functions marked with the declare simd directive, GCC also generates clones for functions marked with the OpenMP declare target directive that are suitable for vectorization when this option is in effect 🌟. The device-type may be one of none, host, nohost, and any — which correspond to keywords for the device_type clause of the declare target directive, and clones are generated for the intersection of devices specified. -fopenmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=any, and -fno-openmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=none.

At -O2 and higher (but not -Os or -Og), this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it's disabled by default. Okay so the defaults switch up on you depending on optimization level — clock that. ⭐

---

🎀 Options to Control Diagnostic Messages Formatting 🎀

Traditionally, diagnostic messages were formatted irrespective of the output device's aspect (e.g. its width, ...). Rude, honestly 😤. You can use the options described below to control the formatting algorithm for diagnostic messages — like how many characters per line, and how often source location information should be reported. 🌸

Heads up bestie: some language front ends may not honor these options. So like... vibe check your front end first.

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

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

• -fdiagnostics-plain-output — requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time 🫶. -fdiagnostics-plain-output is currently equivalent to the following options: -fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers -fdiagnostics-color=never -fdiagnostics-urls=never -fdiagnostics-path-format=separate-events -fdiagnostics-text-art-charset=none -fno-diagnostics-show-event-links -fno-diagnostics-show-nesting. In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. So this list might grow on you, no cap. 👑
• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once — that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines 💜. This is the default behavior.
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line 🔁. Every line gets the prefix. Obsessed with consistency.
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — use color in diagnostics 🌈. WHEN is never, always, or auto. The default depends on how the compiler has been configured — it can be any of the above WHEN options, or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.

The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) 🖍️

These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors. ✨

The default GCC_COLORS is

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

so like, decoding that: 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, periodt. The supported capabilities are literally these bestie:

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

---

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

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

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

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

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

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

• -fno-diagnostics-show-option — by default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option flag suppresses that behavior, no cap.
• -fno-diagnostics-show-caret — by default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width 📐.
• -fno-diagnostics-show-labels — by default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:

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

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

• -fno-diagnostics-show-event-links — by default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, such as the line connecting events 1 and 2 in:

                     3 |   if (p)
                       |       ^
                       |       |
                       |       (1) following `false' branch (when `p' is NULL)... ->-+
                       |                                                             |
                       |                                                             |
                       |+------------------------------------------------------------+
                     4 ||    return 0;
                     5 ||  return *p;
                       ||         ~
                       ||         |
                       |+-------->(2) ...to here
                       |          (3) dereference of NULL `p'

This option suppresses the printing of such connector lines 🧚.

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

For example, in

                 demo.c: In function `test_bad_format_string_args':
                 ../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
                    25 |   printf("hello %i", msg);
                       |                 ~^   ~~~
                       |                  |   |
                       |                  int const char *
                       |                 %s

• the %i and int in the message and the int in the quoted source are colored using highlight-a (bold green by default), and
• the const char * in the message and in the quoted source are both colored using highlight-b (bold blue by default).

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

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

• -fno-diagnostics-show-line-numbers — by default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin.
• -fdiagnostics-minimum-margin-width=width — this option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6 📏.
• -fdiagnostics-show-context[=depth], -fno-diagnostics-show-context — with this option, the compiler might print the interesting control flow chain that guards the basic block of the statement which has the warning. depth is the maximum depth of the control flow chain. Currently, the list of the impacted warning options includes: -Warray-bounds, -Wstringop-overflow, -Wstringop-overread, -Wstringop-truncation. and -Wrestrict. More warning options might be added to this list in future releases. The forms -fdiagnostics-show-context and -fno-diagnostics-show-context are aliases for -fdiagnostics-show-context=1 and -fdiagnostics-show-context=0, respectively 🌀.
• -fdiagnostics-parseable-fixits — emits fix-it hints in a machine-parseable format, suitable for consumption by IDEs 🤖. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string fix-it:. For example:

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

okay so back to the fix-it deets because these matter bestie 💅

The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the above example, 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 "\"", non-printable characters as octal (e.g. vertical tab as "\013").

An empty replacement string indicates that the given range is to be removed. An empty range (e.g. "45:3-45:3") indicates that the string is to be inserted at the given position. ✨ iconic little edge cases tbh

---

• -fdiagnostics-generate-patch — Print fix-it hints to stderr in unified diff format, after any diagnostics are printed. Like, it literally hands you the patch 👑 For example:

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

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

The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). no cap, it's the same color rules 🎀

• -fdiagnostics-show-template-tree — In the C++ frontend, when printing diagnostics showing mismatching template types, such as:

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

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

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

The parts that differ are highlighted with color ("double" and "float" in this case). literally a whole tree just to drag your types 😭🌳

• -fno-elide-type — By default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as "[...]" to simplify the error message. For example:

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

Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. so it's giving "show me EVERYTHING, no shorthand" 💅

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

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

none means to not print diagnostic paths.

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

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

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

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

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

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

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 provides additional information when printing control-flow paths associated with a diagnostic. 🔍

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

This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. yeah it's a devs-only lil treat 🧚

• -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. fr fr sometimes the columns just confuse the poor script 😭
• -fdiagnostics-column-unit=UNIT — Select the units for the column number. This affects traditional diagnostics (in the absence of -fno-show-column).

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

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

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

This option controls how such bytes should be escaped.

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

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

before<U+03C0><BF>after

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

before<CF><80><BF>after

• -fdiagnostics-text-art-charset=CHARSET — Some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a monospaced font. 🎨

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

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

The default is emoji, except when the environment variable LANG is set to C, in which case the default is ascii. honestly the emoji default is so main character of GCC 💅

• -fno-diagnostics-show-nesting — Okay so some GCC diagnostics have an internal tree-like structure of nested sub-diagnostics, like when C++ templates are getting instantiated and everything gets dramatic. By default GCC uses indentation and bullet points in its text output to show the nesting structure of these diagnostics, moves location information to separate lines to make the structure clearer, and eliminates redundant repeated information. Selecting -fno-diagnostics-show-nesting suppresses this indentation, reformatting, and elision, restoring an older “look” for the diagnostics. 🫶
• -fno-diagnostics-show-nesting-locations — When fdiagnostics-show-nesting is enabled, file names and line- and column- numbers are displayed on separate lines from the messages. This location information can be disabled altogether with -fno-diagnostics-show-nesting-locations. This option exists for use by GCC developers, for writing DejaGnu test cases. 🙈
• -fdiagnostics-show-nesting-levels — When fdiagnostics-show-nesting is enabled, use fdiagnostics-show-nesting-levels to also display numbers showing the depth of the nesting. This option exists for use by GCC developers for debugging nested diagnostics, but it may be of use to plugin authors. 🔥
• -fdiagnostics-format=FORMAT — Select a different format for printing diagnostics. FORMAT is text, sarif-stderr, or sarif-file.

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

The default is text.

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

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

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

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

etc.

Schemes, keys, or values with a name prefixed experimental may change or be removed without notice. Keys can be per-scheme, or related to GCC as a whole.

SCHEME can be

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

Supported keys for the text scheme are:

• color=[yes|no] — Override colorization settings from -fdiagnostics-color for this text output.
• show-nesting=[yes|no] — Enable a mode that emphasizes hierarchical relationships within diagnostics messages, as per -fdiagnostics-show-nesting. Defaults to yes.
• show-nesting-locations=[yes|no] — If show-nesting=yes, then by default locations are shown; set this key to no to disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.
• show-nesting-levels=[yes|no] — This is a debugging option for use with show-nesting=yes. Set this key to yes to print explicit nesting levels in the output. This exists for use by GCC developers.

• sarif — Emit diagnostics to a file in SARIF format.

Supported keys for the sarif scheme are:

• file=FILENAME — Specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults to source.sarif.
• serialization=[json] — Specify the serialization format to use when writing out the SARIF. Currently this can only be json, but it is present as an extension point for experimenting with other serializations.
• version=[2.1|2.2-prerelease] — Specify the version of SARIF to use for the output. If not specified, defaults to 2.1. 2.2-prerelease uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release.

There is also this key intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:

• state-graphs=[yes|no] — This is a debugging feature and defaults to no. If state-graphs=yes, then attempt to capture detailed state information from -fanalyzer in the generated SARIF.

• experimental-html — Emit diagnostics to a file in HTML format. This scheme is experimental, and may go away in future GCC releases. The keys and details of the output are also subject to change.

Supported keys for the experimental-html scheme are:

• css=[yes|no] — Add an embedded <style> to the generated HTML. Defaults to yes.
• file=FILENAME — Specify the filename to write the HTML output to, potentially with a leading absolute or relative path. If not specified, it defaults to source.html.
• javascript=[yes|no] — Add an embedded <script> to the generated HTML providing a barebones UI for viewing results. Defaults to yes.

There are also these keys intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:

• show-state-diagrams=[yes|no] — This is a debugging feature and defaults to no. If show-state-diagrams=yes, then attempt to use dot to generate SVG diagrams in the generated HTML, visualizing the state at each event in a diagnostic path. These are visible by pressing j and k to single-step forward and backward through events. Enabling this option will slow down HTML generation.
• show-graph-dot-src=[yes|no] — This is a debugging feature and defaults to no. If show-graph-dot-src=yes then if show-state-diagrams=yes, the generated state diagrams will also show the .dot source input to GraphViz used for the diagram.
• show-graph-sarif=[yes|no] — This is a debugging feature and defaults to no. If show-graph-sarif=yes then if show-state-diagrams=yes, the generated state diagrams will also show a SARIF representation of the state.

As well as scheme-specific keys, the following GCC-related key is usable on sinks of any scheme:

• cfgs=[yes|no] — If cfgs=yes for a sink, then GCC will attempt to send information to that sink about the control flow graphs for the functions it is compiling. Text sinks ignore the information. SARIF sinks will add the graphs within theRun.graphs. HTML sinks will generate SVG displaying the graphs. The precise form of the information is subject to change without notice.

For example,

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

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

In EBNF:

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

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

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

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

<key> = ? string without a '=' ? ;
<value> = ? string without a ',' ? ;

• -fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC — This works in a similar way to -fdiagnostics-add-output= except that instead of adding an additional “output sink” for diagnostics, it replaces all existing output sinks, such as from -fdiagnostics-format=, -fdiagnostics-add-output=, or a prior call to -fdiagnostics-set-output=. 💅
• -fno-diagnostics-json-formatting — By default, when JSON is emitted for diagnostics (via -fdiagnostics-format=sarif-stderr or -fdiagnostics-format=sarif-file), GCC will add newlines and indentation to visually emphasize the hierarchical structure of the JSON.

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

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

✨ Options to Request or Suppress Warnings ✨

Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. Basically: not always a full-on crime, but the vibe check says something might be off. 🚨

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

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

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

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

• -Wfatal-errors — bestie, this one is the "i'm done" button 😭 the second the compiler hits its first error it just aborts the whole compilation instead of trying to keep going and printing a million more error messages. no cap, sometimes you just need it to stop talking.

---

So here's the tea on warnings ☕ you can request a ton of specific warnings with options that start with -W — like -Wunused-variable to get warned about variables you declared and then never used (we've all been there 🙈). And every single one of these specific warning options also has a negative form starting with -Wno- that turns it off — so like, -Wno-unused-variable. The way this manual works is it only lists one of the two forms, whichever one is not the default. iykyk.

For more language-specific options you also wanna peek at C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. And plot twist: you can get even more warnings by enabling the static analyzer ✨

Some options — looking at you, -Wall and -Wextra — turn on other options, like -Wunused, which then turns on further options, like -Wunused-variable. It's giving cascade 🦋. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, no matter where they sit in the command line. For options of the same specificity, the last one wins. And options turned on or off via pragmas take effect as if they were tacked onto the very end of the command line. periodt.

When you ask for an unrecognized warning option (e.g. -Wunknown-warning), GCC gives an error saying it doesn't recognize it. But the -Wno- form is lowkey different: no diagnostic gets produced for -Wno-unknown-warning unless other diagnostics are already being produced. That's on purpose, so you can use new -Wno- options with old compilers — but if something goes wrong, the compiler will warn you that an unrecognized option is hanging around.

Real talk: the effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is way more effective with link-time optimization. Some other warnings might not get issued at all unless optimization is on. And while optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it can also cause false positives 💀 so, vibe check your output.

✨ -Wpedantic, -pedantic, --pedantic ✨

Okay so this one is the strict-ISO-C-and-C++ hall monitor 📏 it issues all the warnings demanded by strict ISO C and ISO C++, diagnoses all programs that use forbidden extensions, and drags some other programs that don't follow ISO C and ISO C++ too. It follows whichever version of the ISO C or C++ standard you specified with any -std option.

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few need -ansi or a -std option specifying the standard version). But without this option, certain GNU extensions and traditional C/C++ features are supported as well — and with this option, they get diagnosed (or straight-up rejected with -pedantic-errors). 🔥

Some things -Wpedantic does not do:

• It doesn't cause warning messages for use of the alternate keywords whose names begin and end with __. That 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__.
• But like… only system header files should use these escape routes. Application programs should avoid them, bestie, pls 🫶

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

And when the standard specified with -std represents a GNU extended dialect of C, like gnu90 or gnu99, there's a corresponding base standard — the version of ISO C that the GNU extended dialect is based on. Warnings from -Wpedantic are given where they're required by the base standard. (And it wouldn't make sense for such warnings to be given only for features not in the specified GNU C dialect, because by definition the GNU dialects of C include all the features the compiler supports with the given option — there'd be literally nothing to warn about 💀)

🌸 -pedantic-errors, --pedantic-errors 🌸

Whenever the base standard (see -Wpedantic) requires a diagnostic, this gives you an error — in some cases where there's undefined behavior at compile-time, and in some other cases that don't prevent compilation of programs that are valid according to the standard.

Not me needing to be super clear: this is not equivalent to -Werror=pedantic. That latter option is unlikely to be useful, since it only makes errors out of the diagnostics controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic. 😤

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

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

Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but are also made errors by -pedantic-errors. For instance:

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

🦋 -fpermissive 🦋

This one is the chill pill 💊 it downgrades some required diagnostics about nonconformant code from errors to warnings. So using -fpermissive lets some nonconforming code compile. Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:

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

Fun fact 🎀 -fpermissive is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90).

👑 -Wall, --all-warnings 👑

This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning) — even in conjunction with macros. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

-Wall turns on the following warning flags:

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

Heads up bestie ⚠️ some warning flags are not implied by -Wall. Some of them warn about constructions users generally don't consider questionable, but which occasionally you might wish to check for. Others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually. iykyk 😌

💜 -Wextra, --extra-warnings 💜

This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W. The older name is still supported, but the newer name is more descriptive. glow-up ✨)

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

And like, -Wextra also prints warning messages for the following cases, bestie:

• 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)

Okay so this one warns you about code affected by ABI changes — yes, the scary binary-compatibility stuff 💅. This includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target. The latter warnings are also controlled separately by -Wpsabi, which is implied by -Wabi.

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

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

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

ngl, although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code 😭. There may also be cases where warnings are emitted even though the code that is generated is compatible. It's giving "we tried our best."

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

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

• A template with a non-type template parameter of reference type was mangled incorrectly:

extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

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

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

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

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

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

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

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

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

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

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

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

• decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a nullptr_t parameter, as parameters have a minimum alignment.

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

• Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.

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

---

🦋 -Wpsabi (C, Objective-C, C++ and Objective-C++ only)

-Wpsabi enables warnings about processor-specific ABI changes, such as changes in alignment requirements or how function arguments are passed. On several targets, including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390, these details have changed between different versions of GCC and/or different versions of the C or C++ language standards in ways that affect binary compatibility of compiled code. With -Wpsabi, GCC can detect potentially incompatible usages and warn you about them. processor-specific drama, detected ✨

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

---

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

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

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

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

Both diagnostics are also suppressed by -fms-extensions.

---

🌸 -Wchar-subscripts

Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on some machines 😭. @xref{Characters implementation}, and char type signedness.

This warning is enabled by -Wall. When enabled, the warning is given regardless of whether char is unsigned by default on the target, and it is also not affected by the -fsigned-char or -funsigned-char options. it does not care about your char opinions, bestie

---

💜 -Wno-coverage-mismatch

Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended.

🫶 real talk: don't just fully silence this one, it's there for a reason.

---

⭐ -Wno-coverage-too-many-conditions

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

---

🌈 -Wno-coverage-too-many-paths

Warn if -fpath-coverage is used and a function has too many paths and GCC gives up coverage. Giving up is controlled by -fpath-coverage-limit. This warning is enabled by default.

---

🍓 -Wno-coverage-invalid-line-number

Warn 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)

Suppress warning messages emitted by #warning directives. bye bye little warnings ✨

---

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

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

It 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. the literal betrayed you, fr fr

---

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

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

• -Wformat, -Wformat=n — okay so this is the format-string vibe check 💅 It checks calls to printf, scanf, and friends to make sure the arguments you pass actually have types that fit the format string, and that the conversions in the format string make sense. This covers standard functions plus others specified by format attributes, across the printf, scanf, strftime, and strfmon families — and yes, strfmon is an X/Open extension, not in the C standard — plus other target-specific families. Which functions get checked without format attributes being specified depends on the selected standard version, and those checks of functions without the attribute are disabled by -ffreestanding or -fno-builtin.

The formats are checked against the features supported by GNU libc version 2.2. That includes all ISO C90 and C99 features, plus features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library’s limitations. But if you use -Wpedantic with -Wformat, you get warnings about format features not in the selected standard version — except not for strfmon formats, because those aren’t in any version of the C standard. iykyk ✨

• -Wformat=1, -Wformat — the classic level-one format check 🎀 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 these options: -Wno-format-contains-nul, -Wno-format-diag, -Wno-format-extra-args, and -Wno-format-zero-length. And yes, -Wformat is enabled by -Wall.
• -Wformat=2 — level two, extra format checks, main character energy 👑 Enable -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. simple as that, bestie 🫶
• -Wno-format-diag — If -Wformat is specified, do not warn about format strings that are unsuitable for GCC diagnostics. not every string is built for the diagnostics spotlight 🙈
• -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. iconic, they literally don’t matter 💀

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. periodt, the spec said so 🌈

• -Wformat-overflow, -Wformat-overflow=level — buffer-overflow warning time, but make it formatted I/O 🔥 Warn 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. ngl, compiler warnings have trust issues too 😭
  • -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. so close, yet so buffer-y 🦋

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. level 2 is not playing around 💅

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

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

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

• -Wno-format-zero-length — If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. empty format string? valid. slay. 🎀
• -Wformat-nonliteral — If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list. if I can’t read it, I can’t check it 👀
• -Wformat-security — If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. this is the “bestie, do NOT trust that string” warning 🚨 (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)
• -Wformat-signedness — If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed, and vice versa. signed vs unsigned, the eternal group-chat drama 😭
• -Wformat-truncation, -Wformat-truncation=level — truncation warnings, because sometimes your output just gets cut off mid-sentence ✂️ Warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time, it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic as -Wformat-overflow. same energy, different function family 🦋
  • -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. if you’re not even checking the return value, bestie… 🙈
  • -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. level 2 said “and what if the input is huge?” 🔥
• -Wformat-y2k — If -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. year 2000 called, it wants its ambiguity back 💀
• -Wnonnull — Warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute. if it says non-null, please do not send null 🫶

-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. the “why are you checking for null when you already promised non-null” warning 😭

-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. undefined behavior is not the vibe, no cap 🚨
• -Wno-musttail-local-addr — Do not warn about passing a pointer (or in C++, a reference) to a local variable or label to argument of a musttail call. Those variables go out of scope before the tail call instruction. they’re literally gone before the call, so chill 🧚
• -Wmaybe-musttail-local-addr — Warn when address of a local variable can escape to a musttail call, unless it goes out of scope already before the musttail call. vibe check: does this local address escape? 👑

int foo (int *);

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

This warning is enabled by -Wextra. 💜

---

🌸 -Wnrvo (C++ and Objective-C++ only)

Warns you when the compiler doesn't elide the copy from a local variable into a function's return value — but only in the contexts where [class.copy.elision] would have allowed it. Bestie, this elision has a whole government name: the Named Return Value Optimization. 🎀

So like, look at this cutie:

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

The compiler literally cannot elide copies from both v1 and v2 here, so it elides neither. Tragic, but fair. 😭

---

🔁 -Winfinite-recursion

Warns you about infinitely recursive calls — the "oops I called myself forever" situation. 💀 The warning is effective at all optimization levels, but ngl it requires optimization in order to detect infinite recursion in calls <italic>between two or more functions*. So like, mutual recursion shenanigans? Yeah, that needs the optimizer's help.

-Winfinite-recursion is included in -Wall. 👑

Compare it with -Wanalyzer-infinite-recursion, which gives you a similar diagnostic but is implemented in a completely different way (as part of -fanalyzer). Same vibe, different machinery. ✨

---

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

Warns about uninitialized variables that are initialized with themselves. Yes bestie, int i = i; is exactly as cursed as it looks. 💅

Heads up: this option can only be used together with -Wuninitialized. No solo runs. 🙈

For example, GCC only warns about i being uninitialized in this snippet 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 one controls warnings for when a declaration doesn't specify a type at all. Rude, honestly.

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)

Same energy, different crime: this controls warnings when a function is used before being declared. Bestie, at least say hi first?? 😭

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)

The combo meal 🍓 — same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.

---

🛡️ -Whardened

Warns you when -fhardened did not enable an option from its own set (go read -fhardened for the lineup, iykyk). So 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. The set said no and you overrode it, basically. 💅

This warning is enabled by default, and it 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. Shortcuts, slay.

---

🕳️ -Wimplicit-fallthrough=n

Warns you when a switch case falls through into the next one. Like this lil guy:

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

This warning does not fire when the last statement of a case cannot fall through — e.g. when there's a return statement, or a call to a function declared with the noreturn attribute. Goodbye is goodbye. 👋

-Wimplicit-fallthrough= also takes control flow statements, such as ifs, into account, 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 and C23 provide 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's 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:
  • "-fallthrough"
  • "@fallthrough@"
  • "lint -fallthrough[ \t]*"
  • "[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?"
  • "[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?"
  • "[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
• -Wimplicit-fallthrough=4 — case sensitively matches one of the following regular expressions:
  • "-fallthrough"
  • "@fallthrough@"
  • "lint -fallthrough[ \t]*"
  • "[ \t]*FALLTHR(OUGH|U)[ \t]*"
• -Wimplicit-fallthrough=5 — doesn't recognize any comments as fallthrough comments, only attributes disable the warning.

The comment needs to be followed after optional whitespace and other comments by case or default keywords, or by a user label that precedes some case or default label. 💅

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

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra. 🦋

---

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

Controls whether warnings triggered by the warn_if_not_aligned attribute should be issued. These warnings are enabled by default. 🎀

---

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

Warns when the return type of a function has a type qualifier such as const. Bestie, for ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. 👑

ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option. Periodt.

This warning is also enabled by -Wextra. ✨

---

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

This option controls warnings when an attribute is ignored. It's different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute — not that the attribute is either unknown, used in a wrong place, etc. Subtle but iconic distinction. 💅

This warning is enabled by default. 🌸

---

👑 -Wmain

Warns when the type of main is suspicious. Like, main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. That's the whole dress code, bestie. 🎀

This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.

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

---

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

Warns you when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the same indentation. It's giving "looks fine, is a lie." 💀

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

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

In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8). 📏

The warning is not issued for code involving multiline preprocessor logic such as the following example.

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

Okay bestie, this warning is not issued after a #line directive, since that typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references. 💅

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

• -Wmissing-attributes — Warns when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice. ✨

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

You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types. Common Attributes.

-Wmissing-attributes is enabled by -Wall.

For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes. 😭

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

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

• -Wmissing-braces — Warns if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed. 🎀

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

This warning is enabled by -Wall.

• -Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only) — Warns if a user-supplied include directory does not exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for -I and -J, only. 🌸
• -Wno-missing-profile — This option controls warnings if feedback profiles are missing when using the -fprofile-use option. This option diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that a profile generation step is due. Ignoring the warning can result in poorly optimized code. -Wno-missing-profile can be used to disable the warning, but this is not recommended and should be done only when non-existent profile data is justified. 💀
• -Wmismatched-dealloc — Warns for calls to deallocation functions with pointer arguments returned from allocation functions for which the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by use of attribute malloc. Unless disabled by the -fno-builtin option the standard functions calloc, malloc, realloc, and free, as well as the corresponding forms of C++ operator new and operator delete are implicitly associated as matching allocators and deallocators. In the following example mydealloc is the deallocator for pointers returned from myalloc. 🦋

void mydealloc (void*);

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

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

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

Option -Wmismatched-dealloc is included in -Wall. 🔥

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

For example:

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

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

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

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

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

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

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

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

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

This warning is enabled by -Wall. 💖

• -Wno-self-move (C++ and Objective-C++ only) — This warning warns when a value is moved to itself with std::move. Such a std::move typically has no effect. 🫶

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

This warning is enabled by -Wall.

• -Wsequence-point — Warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. 🚨

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

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

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 — bestie, this one's about return statements that don't match the function's vibe 😤 It warns about return statements with no expression in functions that don't return void. Also flags a return with an expression inside a function whose return type is void — unless the expression's type is also void. As a GNU extension, that second case is accepted with no warning at all unless -Wpedantic is on. Ngl, tryna use the return value of a non-void function (other than main, obvs) that just kinda falls off the end by hitting the closing curly brace? That's undefined behavior, periodt. This warning is C-specific and enabled by default ✨ In C99 and later dialects it's treated as an error. You can downgrade it to a warning with -fpermissive (along with other warnings), or just this one alone 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, 'cause that one takes precedence 👑). Also warns if execution might reach the end of the function body, or if the function has no return statement at all. Same undefined-behavior caveat as above: tryna use the return value of a non-void function other than main that flows off the end and hits the closing curly brace = undefined 🫣 And unlike in C, in C++ flowing off the end of a non-void function other than main is undefined behavior even when the value isn't used. This one's enabled by default in C++ and by -Wall otherwise. 🌈
• -Wno-shift-count-negative — controls warnings when a shift count is negative. Enabled by default. ✨
• -Wno-shift-count-overflow — controls warnings when a shift count is greater than or equal to the bit width of the type. Enabled by default. ✨
• -Wshift-negative-value — warns if you left shift a negative value 🔥 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. Let's break it down:
  • -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 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 in C++20 mode (and newer) since signed left shifts always wrap. 🌀
  • -Wshift-overflow=2 — this 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 is missing a case for one or more of the named codes of that enumeration. (A default label prevents this warning, iykyk.) case labels that don't correspond to enumerators also provoke warnings with this option, unless the enumeration is marked with the flag_enum attribute. Enabled by -Wall. ✨
• -Wswitch-default — warns whenever a switch statement has no default case. 🎀
• -Wswitch-enum — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more named codes of that enumeration. case labels that don't correspond to enumerators also provoke warnings here too, unless the enumeration is marked with the flag_enum attribute. The only difference from -Wswitch is that this one warns about an omitted enumeration code even if there's a default label. 👑
• -Wno-switch-bool — don't warn when a switch statement has an index of boolean type and the case values are outside the range of a boolean type. You can also 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 — controls warnings when a switch case has a value outside its respective type range. Enabled by default for C and C++ programs. ✨
• -Wno-switch-unreachable — don't warn when a switch statement contains statements between the controlling expression and the first case label, which will never be executed. 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 the __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4, so like... heads up 🫣
• -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init can't initialize the automatic variable. A common situation is an automatic variable declared between the controlling expression and the first case label of a switch statement. 😭
• -Wunused-but-set-parameter — same as -Wunused-but-set-parameter=3, and -Wno-unused-but-set-parameter is the same as -Wunused-but-set-parameter=0. 🎀
• -Wunused-but-set-parameter=n — warns whenever a function parameter is assigned to but otherwise unused (aside from its declaration). To suppress this warning, use the unused attribute. -Wunused-but-set-parameter=0 disables the warning. With =1, all uses except initialization and left-hand side of assignment which isn't further used disable the warning. With =2, additionally uses of the parameter in ++ and -- operators don't count as uses. And finally with =3, additionally uses in parm @= rhs outside of rhs don't count as uses. See the -Wunused-but-set-variable=n option for examples. 👀

This -Wunused-but-set-parameter=3 warning is also enabled by -Wunused together with -Wextra. 🔥

• -Wunused-but-set-variable — same as -Wunused-but-set-variable=3, and -Wno-unused-but-set-variable is the same as -Wunused-but-set-variable=0. 💜
• -Wunused-but-set-variable=n — warns whenever a local variable is assigned to but otherwise unused (aside from its declaration). This -Wunused-but-set-variable=3 warning is enabled by -Wall. To suppress it, use the unused attribute. =0 disables the warning. With =1, all uses except initialization and left-hand side of assignment which isn't further used disable the warning. With =2, additionally uses of the variable in ++ and -- operators don't count as uses. And finally with =3, additionally uses in parm @= rhs outside of rhs don't count as uses. This -Wunused-but-set-variable=3 warning is also enabled by -Wunused, which is enabled by -Wall. 🎀

void foo (void)
{
  int a = 1; // B<-Wunused-variable> warning
  int b = 0; // Warning for <n> >= 1
  b = 1; b = 2;
  int c = 0; // Warning for <n> >= 2
  ++c; c--; --c; c++;
  int d = 0; // Warning for <n> >= 3
  d += 4;
  int e = 0; // No warning, cast to void
  (void) e;
  int f = 0; // No warning, f used
  int g = f = 5;
  (void) g;
  int h = 0; // No warning, preincrement result used
  int i = ++h;
  (void) i;
  int j = 0; // No warning, postdecrement result used
  int k = j--;
  (void) k;
  int l = 0; // No warning, l used
  int m = l |= 2;
  (void) m;
}

• -Wunused-function — warns whenever a static function is declared but not defined, or a non-inline static function is unused. Enabled by -Wall. ✨
• -Wunused-label — warns whenever a label is declared but not used. Enabled by -Wall. To suppress, use the unused attribute. 🏷️
• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warns when a typedef locally defined in a function isn't used. 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, use the unused attribute. 🫶
• -Wno-unused-result — don't warn if a caller of a function marked with the warn_unused_result attribute doesn't use its return value. The default is -Wunused-result. 👑
• -Wunused-variable — warns whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. Enabled by -Wall. To suppress, use the unused attribute. 🌸
• -Wunused-const-variable, -Wunused-const-variable=n — warns whenever a constant static variable is unused aside from its declaration. To suppress, use the unused attribute. 🎀
  • -Wunused-const-variable=1 — warns about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included. -Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ this isn't an error since const variables take the place of #defines. 🦋
• -Wunused-const-variable=2 — this level also warns you about unused constant static variables hiding in headers (system headers stay excluded, bestie, they're untouchable 👑). It's literally the same as the short form -Wunused-const-variable. You have to ask for this level explicitly in both C and C++ because ngl, cleaning up every header you include can be a whole situation 😭
• -Wunused-value — warns whenever a statement computes a result that's explicitly not used. Wanna silence it? Just cast that unused expression to "void", iconic move. This covers 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 the ones documented as needing to be specified explicitly. So it's giving "greatest hits album" but not the deluxe tracks 💅

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. No free rides, bestie.

-Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++. C++ girlies gotta opt in themselves 💀

---

🦋 -Wuse-after-free (C, Objective-C, C++ and Objective-C++ only)

• -Wuse-after-free
• -Wuse-after-free=n

Warns you about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. Translation: you freed it, so let it go 🙈 The warning is enabled at all optimization levels but may yield different results with optimization than without. -Wuse-after-free is equivalent to -Wuse-after-free=1. ✨

• -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. Lowkey the gentle intro 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. Big brain energy fr fr 🧠✨

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

---

• -Wuseless-cast (C, Objective-C, C++ and Objective-C++ only) — warns when an expression is cast to its own type. Like, why are you casting yourself to yourself, main character? 💅 This warning does not occur when a class object is converted to a non-reference type as that is a way to create a temporary:

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

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

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

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

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. So like, your mileage literally varies by build, no cap.

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. GCC said "that whole plotline? cut it" 🎬💀

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. "maybe" is doing so much heavy lifting here bestie 😭

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. Cute little "she doesn't even go here" annotation 💅

These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables. On the other hand, -Wmaybe-uninitialized is known not to warn in many situations (false negatives) due to optimizations taking advantage of undefinedness of uninitialized uses like constant propagation. So it's giving flaky, honestly 🌸

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. Just tell the compiler what's up, bestie 📢

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. Trust issues: mutual 🫠

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

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

• -Wunknown-pragmas — warns 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 — like, it's not scared of system headers, periodt. 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. Pragmas get a pass today 🎀
• -Wno-pragma-once-outside-header — do not warn when "#pragma once" is used in a file that is not a header file, such as a main file. She's allowed to be there, actually 💅
• -Wno-prio-ctor-dtor — do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution before "main" is called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between 0--100 for the implementation. So 0–100 is the compiler's lane, stay out of it 🚫👑
• -Wstrict-aliasing — only does anything when -fstrict-aliasing is active, bestie. It warns you about code that might break the strict aliasing rules the compiler is using to optimize. Heads up: it does not catch every case, it just tries to catch the more common pitfalls. It's included in -Wall, and it's literally the same thing as -Wstrict-aliasing=3.
• -Wstrict-aliasing=n — same deal, only active when -fstrict-aliasing is active 💅 warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels = higher accuracy (fewer false positives). Higher levels also = more effort, kinda like how -O works. And yeah, -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.

Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code, because it has very few false negatives. However, it has many false positives. It warns for all pointer conversions between possibly incompatible types, even if never dereferenced. Runs in the front end only.

Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address is taken. Warns about incomplete types. Runs in the front end only.

Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: *(int*)&some_float. If optimization is enabled, it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types. ✨

---

• -Wstrict-overflow
• -Wstrict-overflow=n — this one's only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur 🙈 Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. So like… this warning depends on the optimization level, iykyk.

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

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

---

• -Wstring-compare — 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. For example, the call to strcmp below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array a:

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

-Wstring-compare is enabled by -Wextra. 🎀

---

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

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

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

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

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

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

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

void f (int[static 4]);

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

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

• -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 is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value.
• -Wstringop-overflow=2 — 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, same energy for the next strncpy situation: this call copies to d only the characters that come before the terminating NUL, and then doesn't append the NUL at the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake, so the call gets diagnosed. If you're not expecting a NUL-terminated result and wanna dodge the warning, just call memcpy instead. 🫶

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

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

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

Now, if you've got a character array that's genuinely meant to store a sequence of bytes with no terminating NUL, you can annotate that array with attribute nonstring to avoid this warning. Cute, right? BUT — and this is the important part — such arrays are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuse 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, according to the level of the strict_flex_array (level) attribute attached to the trailing array field of a structure — if that attribute's available — otherwise according to the level of the option -fstrict-flex-arrays=level. Heads up: -Wstrict-flex-arrays is only effective 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 — in addition to level=1, extra warnings get issued for a trailing one-element array reference of a structure, if the array is referenced as a flexible array member.
• level=3 — in addition to level=2, extra warnings get issued for a trailing zero-length array reference of a structure, if the array is referenced as a flexible array member.

This option is more effective when -ftree-vrp is active (that's the default for -O2 and above), but some warnings may be diagnosed even without optimization. ✨

-Wsuggest-attribute=attribute-name 🦋

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

• -Wsuggest-attribute=pure, -Wsuggest-attribute=const, -Wsuggest-attribute=noreturn, -Wmissing-noreturn, -Wsuggest-attribute=malloc, -Wsuggest-attribute=returns_nonnull — warn about functions that might be candidates for attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units, or (in the case of pure and const) if it can't prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort or trapping. This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis. 🔥
• -Wsuggest-attribute=format, -Wmissing-format-attribute — warn about function pointers that might be candidates for format attributes. NGL, these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function respectively should also have a format attribute to avoid the warning.

GCC also warns about function definitions that might be candidates for format attributes. Again — only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but that might not always be the case, and some functions for which format attributes are appropriate may not be detected. It's giving "vibes-based heuristic" ngl. 🙈

• -Wsuggest-attribute=cold — warn about functions that might be candidates for cold attribute. This one's based on static detection and generally only warns about functions which always lead to a call to another cold function, like wrappers of C++ throw or fatal error reporting functions leading to abort. 💀

---

-Walloc-size 🎀

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

-Walloc-zero 🌈

Warns about calls to allocation functions decorated with attribute alloc_size that specify zero bytes — including those to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc.

Ok so listen: because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc has been deprecated), relying on it may result in subtle portability bugs and should be avoided. Periodt. ⭐

-Wcalloc-transposed-args 🍓

Warns about calls to allocation functions decorated with attribute alloc_size with two arguments, which use sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning.

Here's the deal: the first argument to calloc is documented to be number of elements in array, while the second argument is size of each element, so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If having sizeof in the earlier argument and not the latter is intentional, the warning can be suppressed by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4) or just 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 the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than.

-Wno-alloc-size-larger-than 💜

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

-Walloca 🔥

This option warns on all uses of alloca in the source. Like, all of them. No cap.

-Wauto-profile 🦋

Output warnings about auto-profile inconsistencies.

-Wcannot-profile ✨

Warn when profiling instrumentation was requested, but could not be applied to a certain function. 😭

-Walloca-larger-than=byte-size 👑

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

For example, a bounded case of alloca could be:

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

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

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

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

If -Walloca-larger-than=500 were passed, the above would trigger a warning — but this time because of the lack of bounds checking.

Also, note that even seemingly correct code involving signed integers could cause a warning. Rude, I know:

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

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

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

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

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

-Wno-alloca-larger-than 🌸

Disable -Walloca-larger-than= warnings. 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.

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 — bestie, this one yells at you when you index outside an array or take an offset that's out of bounds 💅. It's switched on by -Wall. It hits way harder when -ftree-vrp is active (which is the default at -O2 and above), but a subset of these still get reported even with zero optimization. Ngl, it's giving "the compiler sees your crimes" energy.

By default, the trailing array of a structure gets treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it's declared as any of: a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or a one-element array ([1]). The tea here: because of that, out-of-bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default 🙈.

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 it's enabled by -Wall; the higher levels are not included, so you have to explicitly request them, mwah 💋.
• -Warray-bounds=2 — this level also warns about the intermediate results of pointer arithmetic that may yield out-of-bounds values. Heads up though: this level may give a larger number of false positives, and it's deactivated by default. It's giving "paranoid bestie" and honestly? valid.

• -Wunterminated-string-initialization (C and Objective-C only) — warns about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has the "nonstring" attribute. Like so, bestie:

        char arr[3] = "foo"; /* Warning.  */
        char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning.  */

This warning is enabled by -Wextra. And if -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the "nonstring" warning, because in C++ such initializations are an error 😭. C++ said "absolutely not" fr fr.

• -Warray-compare — warns about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20, so like… it's giving legacy behavior. Example time:

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

-Warray-compare is enabled by -Wall. Slay, it caught that for you ✨.

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

Okay so here's the lore 🧚: 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". Inconsistent much? 💀

        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 — they're basically matching outfits, no cap 👑

        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. Also: the short form of the option -Warray-parameter is equivalent to -Warray-parameter=2, and the negative form -Wno-array-parameter is equivalent to -Warray-parameter=0. cute little symmetry, we love to see it 🎀

• -Wattribute-alias=n, -Wno-attribute-alias — warns about declarations using the "alias" and similar attributes whose target is incompatible with the type of the alias. It's giving "you set them up and they don't match" 😭
  • -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. Red flag behavior fr.
  • -Wattribute-alias=2 — at this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.

Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice". That's the whole guest list, bestie 🫶

-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. Your call, queen 👑

• -Wbidi-chars=[none|unpaired|any|ucn] — warns about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Bestie this one is genuinely scary 😭 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. Straight-up gaslighting code 💀

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. Little conditional logic moment, iykyk 🌸

• -Wbool-compare — warns about boolean expression compared with an integer value different from "true"/"false". For instance, the following comparison is always false, bestie, like… whoever wrote this was not having a good day:

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

This warning is enabled by -Wall. Iconic catch tbh 🎀

• -Wbool-operation — warns about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense — and honestly? why would you even 😭 (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. It's giving "why are you like this" energy fr fr.

• -Wduplicated-branches — warns when an if-else has identical branches. This warning detects cases like:

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

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

          int i = x ? *p : *p;

Like bestie… both roads lead to *p. Pick a lane 💅

• -Wduplicated-cond — warns about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

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

The second one is never gonna hit, queen. Deceased 💀

• -Wframe-address — warns when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program 😭 The warning is included in -Wall. Do not do this, bestie, I'm begging 🫶
• -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. Sometimes you just gotta drop the qualifier and move on, no questions asked 🙈
• -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. Same vibe, different array 🦋
• -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. So like… -pedantic-errors said "no fun allowed" 💀

• -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. Implicit vs explicit, bestie — the compiler cares about the difference even if your situationship doesn't 🌈

okay so by default, in C99 and later dialects of C, GCC treats this whole issue as an error. Rude but fair. You can downgrade that error into a lil warning with -fpermissive (which does the same for certain other errors too, bestie, it's a package deal), or — if you only want this one softened — with -Wno-error=int-conversion. And heads up: -pedantic-errors upgrades this warning right back into an error. 💅

• -Wzero-as-null-pointer-constant — screams when a literal 0 gets used as a null pointer constant. Just say nullptr babe. ✨
• -Wzero-length-bounds — warns you 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 accessing them is undefined — like, undefined undefined. 💀

For example, the first two stores in function bad get diagnosed because the array elements overlap the subsequent members b and c. The third store gets diagnosed by -Warray-bounds instead, because it's beyond the bounds of the enclosing object:

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

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

Option -Wzero-length-bounds is enabled by -Warray-bounds. 🫶

• -Wno-div-by-zero — do not warn about compile-time integer division by zero. And floating-point division by zero isn't warned about either, because it can legitimately be how you obtain infinities and NaNs. so true, let her cook 🍓
• -Wsystem-headers — prints warning messages for constructs found in system header files. Normally warnings from system headers are suppressed, on the assumption that they usually don't indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. Chaos mode: enabled. But note — using -Wall together with this option does not warn about unknown pragmas in system headers; for that, -Wunknown-pragmas must also be used. iykyk 😭
• -Wtautological-compare — warns if a self-comparison always evaluates to true or false. It catches all those classic oopsie mistakes like:

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

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

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

will always be false. not the bitwise delusion 😭

This warning is enabled by -Wall. 👑

• -Wtrailing-whitespace
• -Wtrailing-whitespace=kind — warn about trailing whitespace at the end of lines, including inside of comments, but excluding trailing whitespace in raw string literals. -Wtrailing-whitespace is equivalent to -Wtrailing-whitespace=blanks and warns just about trailing space and horizontal tab characters. -Wtrailing-whitespace=any warns about those or trailing form feed or vertical tab characters. -Wno-trailing-whitespace or -Wtrailing-whitespace=none disables the warning, which is the default. This is a coding style warning. 🎀
• -Wleading-whitespace=kind — warns about style issues in leading whitespace, but not about the amount of indentation. Some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of -ftabstop=n) followed by zero or fewer than n spaces. No warning is emitted on lines which contain solely whitespace (although -Wtrailing-whitespace= warning might be emitted), and no warnings are emitted inside of raw string literals. Warnings are also emitted for leading whitespace inside of multi-line comments. -Wleading-whitespace=spaces warns about leading whitespace other than spaces, for projects which want to indent just by spaces. -Wleading-whitespace=tabs warns about leading whitespace other than horizontal tabs, for projects which want to indent just by horizontal tabs. -Wleading-whitespace=blanks warns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or about n or more consecutive spaces in leading whitespace (where n is argument of -ftabstop=n, 8 by default). -Wleading-whitespace=none disables the warning, which is the default. This is a coding style warning. 🌸
• -Wtrampolines — warns about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets it's made up of data only and thus requires no special treatment. But for most targets it's made up of code, and thus requires the stack to be made executable in order for the program to work properly. executable stack, bestie, that's a whole vibe and not the good kind 💀
• -Wfloat-equal — warns if floating-point values are used in equality comparisons.

The idea behind this is that sometimes it's convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you're doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap — and that's done with the relational operators, so equality comparisons are probably mistaken. fr fr, floats are liars 🧚

• -Wtraditional (C and Objective-C only) — warns about certain constructs that behave differently in traditional C and ISO C. Also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. The full lore:
  • Macro parameters that appear within string literals in the macro body. In traditional C, macro replacement takes place within string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
  • Usage of ISO string concatenation is detected.
  • Initialization of automatic aggregates.
  • Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
  • Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case.
  • Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
• -Wtraditional-conversion (C and Objective-C only) — warns if a prototype causes a type conversion that's different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument — except when it's the same as the default promotion. ✨
• -Wdeclaration-after-statement (C and Objective-C only) — warns when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. old-school C said no 💅

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

• -Wshadow — warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If this warning is enabled, it includes also all instances of local shadowing — which means -Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global. 🦋
• -Wno-shadow-ivar (Objective-C only) — bestie, do not warn when a local variable shadows an instance variable inside an Objective-C method. Let it live. 💅
• -Wshadow=global — warn for any shadowing at all, periodt. Same as -Wshadow. 👑
• -Wshadow=local — ok so this one warns when a local variable shadows another local variable or parameter. Just the local-on-local drama, nothing else.
• -Wshadow=compatible-local — warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, "type compatible" here literally means the shadowing variable's type can be converted to the shadowed variable's type. ✨

The whole reason this flag exists (on top of -Wshadow=local) is the idea that when a local shadows another one of incompatible type, it's most likely intentional, not a bug or a typo — like, the way that this example shows:

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

Since the two variable "i" in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. Highkey iconic behaviour. And because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. ngl there's a catch tho: this also means that shadowing const char *i by char *i does not emit a warning. 🙈

This warning is also enabled by -Wshadow=local.

• -Wlarger-than=byte-size — warn whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-larger-than. 💀

Also warn for calls to bounded functions such as memchr or strnlen that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. These warnings can only be disabled by -Wno-larger-than.

• -Wno-larger-than — disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger. 🫶
• -Wframe-larger-than=byte-size — warn if the size of a function frame exceeds byte-size. Heads up bestie: the computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you do not get a warning. Plus, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. 😭

-Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-frame-larger-than.

• -Wno-frame-larger-than — disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or larger.
• -Wfree-nonheap-object — warn when attempting to deallocate an object that was either not allocated on the heap, or by using a pointer that was not returned from a prior call to the corresponding allocation function. Like, for example, because the call to stpcpy returns a pointer to the terminating nul character and not to the beginning of the object, the call to free below is diagnosed. caught in 4k fr fr 📸

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

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

• -Wstack-usage=byte-size — warn if the stack usage of a function might exceed byte-size. Unlike the frame one, the computation done to determine the stack usage here is conservative. And any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning. 🌸

The message is in keeping with the output of -fstack-usage:

• If the stack usage is fully static but exceeds the specified amount, it's:

warning: stack usage is 1120 bytes

• If the stack usage is (partly) dynamic but bounded, it's:

warning: stack usage might be 1648 bytes

• If the stack usage is (partly) dynamic and not bounded, it's:

warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or by -Wno-stack-usage.

• -Wno-stack-usage — disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger. 💜
• -Wunsafe-loop-optimizations — warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. With -funsafe-loop-optimizations warn if the compiler makes such assumptions. lowkey unhinged but ok 🌀
• -Wno-pedantic-ms-format (MinGW targets only) — when used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime. 🪟
• -Wpointer-arith — warn about anything that depends on the "size of" a function type or of void. GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions. In C++, warn also when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic. 👑

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

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

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

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

This warning is enabled by default.

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

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

• -Wtype-limits — warn if a comparison is always true or always false due to the limited range of the data type, but do not warn for constant expressions. For example, warn if an unsigned variable is compared against zero with < or >=. This warning is also enabled by -Wextra. 🌈
• -Wabsolute-value (C and Objective-C only) — warn for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, calling abs(3.14) triggers the warning because the appropriate function to call to compute the absolute value of a double argument is fabs. The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast and it is also enabled by -Wextra. 🍓
• -Wcomment, -Wcomments — warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. This warning is enabled by -Wall. 💬
• -Wtrigraphs — warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines. 🔺

This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs.

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

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

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

#if defined the_macro_causing_the_warning
#endif

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

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

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

• -Wbad-function-cast (C and Objective-C only) — warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type. bestie that's not the vibe 🚫
• -Wc90-c99-compat (C and Objective-C only) — warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable length arrays, long long type, bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. 🕰️
• -Wc99-c11-compat (C and Objective-C only) — warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous structures and unions, _Atomic type qualifier, _Thread_local storage-class specifier, _Alignas specifier, Alignof operator, _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. ⏭️
• -Wc11-c23-compat (C and Objective-C only) — -Wc11-c2x-compat (C and Objective-C only) — okay so this one warns you about stuff that isn't in ISO C11 but is in ISO C23. Like: leaving the string out of _Static_assert, using [[]] attribute syntax, decimal floating-point types, that whole vibe. It's independent of the standards mode you're in, bestie. Also, warnings get switched off inside the expression right after __extension__. Heads up: the name -Wc11-c2x-compat is deprecated.

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

• -Wc23-c2y-compat (C and Objective-C only) — -Wc23-c2y-compat (C and Objective-C only) — warns about features that aren't in ISO C23 but are in ISO C2Y. Example: _Generic selecting with a type name instead of an expression. Also independent of standards mode, and warnings are disabled in the expression following __extension__.

Same deal: when not compiling in C2Y mode, -pedantic-errors bumps these to errors.

yes, that option is listed twice in the original. iconic of it honestly. 💅

• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that fall outside the common subset of ISO C and ISO C++, like asking for an 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 that were fine in ISO C++ 1998 but are keywords in ISO C++ 2011. This one 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.
• -Wc++26-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2023 and the upcoming ISO C++ 2026. Enabled by -Wall.
• -Wno-c++11-extensions (C++ and Objective-C++ only) — don't warn about C++11 constructs when you're compiling with an older C++ standard. Real talk though: 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 compiled with an older C++ standard. Same caveat: some C++14 constructs only get diagnosed with -Wpedantic.
• -Wno-c++17-extensions (C++ and Objective-C++ only) — don't warn about C++17 constructs under an older C++ standard. (And yeah — some of them only show up with -Wpedantic.)
• -Wno-c++20-extensions (C++ and Objective-C++ only) — don't warn about C++20 constructs under an older C++ standard. Some C++20 constructs still only get diagnosed with -Wpedantic.
• -Wno-c++23-extensions (C++ and Objective-C++ only) — don't warn about C++23 constructs under an older C++ standard. Yep, you guessed it: some only get diagnosed with -Wpedantic.
• -Wno-c++26-extensions (C++ and Objective-C++ only) — don't warn about C++26 constructs when compiling with an older C++ standard. And again, some C++26 constructs only get diagnosed if -Wpedantic is in play.

---

• -Wcast-qual — warns whenever a pointer gets cast in a way that strips a type qualifier off the target type. Example: casting a const char * to a plain old char *.

It also warns when a cast introduces a type qualifier in an unsafe way. Like casting char ** to const char ** — that's unsafe, bestie, and here's the whole crime scene:

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

• -Wcast-align — warns whenever a pointer is cast such that the required alignment of the target gets increased. For example, warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.

Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * regardless of the target machine.

• -Wcast-function-type — warns when a function pointer gets cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of the initial arguments that are actually provided get considered. Any parameter of pointer-type matches any other pointer-type. Benign differences in integral types are ignored — like int vs. long on ILP32 targets. Type qualifiers are ignored too. The function type void (*) (void) is special and matches everything, which you can use to silence this warning. And in a cast involving pointer-to-member types, this warns whenever the cast changes the pointer-to-member type. Enabled by -Wextra.
• -Wcast-user-defined — warns when a cast to reference type doesn't involve a user-defined conversion that the programmer might have expected to be called. 👀

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

This warning is enabled by default.

• -Wwrite-strings — in 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 catch, at compile time, code that might try to write into a string constant — but only if you've been super careful about using const in declarations and prototypes. Otherwise it's just a nuisance. That's literally why -Wall doesn't request these warnings.

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

And it gets upgraded to an error by -pedantic-errors in C++11 mode or later. 🔥

• -Wclobbered — warns about variables that might get changed by longjmp or vfork. Also enabled by -Wextra.
• -Wno-complain-wrong-lang — so by default, language front ends complain when a command-line option is valid but not applicable to that front end. You can turn that off with -Wno-complain-wrong-lang, which is mostly useful when you're invoking a single compiler driver on multiple source files written in different languages. Like:

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

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. That latter front end diagnoses: f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran — which you can disable with -Wno-complain-wrong-lang.

It can also disable warnings like cc1plus: note: CTF debug info requested, but not supported for 'GNU C++20' frontend — that one gets produced by -gctf or -gsctf for unsupported languages.

• -Wcompare-distinct-pointer-types (C and Objective-C only) — warns if pointers of distinct types get compared without a cast. Enabled by default. 🫶
• -Wconversion — warns about implicit conversions that might alter a value. That 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). It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or when the value isn't changed by the conversion, like in abs (2.0). Warnings about conversions between signed and unsigned integers can be turned off with -Wno-sign-conversion.

For C++, it also warns about confusing overload resolution for user-defined conversions, and conversions that never use a type conversion operator: conversions to void, to the same type, to a base class, or to a reference to them. In C++, warnings about signed/unsigned conversions are off by default unless you explicitly enable -Wsign-conversion.

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

• -Wdangling-else — warns about constructions where it's ambiguous which if statement an else branch belongs to. Here's the classic:

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

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

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

This warning is enabled by -Wparentheses.

• -Wdangling-pointer — -Wdangling-pointer=n — warns about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. That includes local variables declared in nested blocks, compound literals, and other unnamed temporary objects. It also warns about storing the address of such objects in escaped pointers. Enabled at all optimization levels, but it may give different results with optimization than without. 👑
  • -Wdangling-pointer=1 — at level 1, the warning diagnoses only unconditional uses of dangling pointers.
• -Wdangling-pointer=2 — bestie, level 2 is level 1 plus it also calls out conditional uses of dangling pointers, not just the unconditional ones 💅

So like, the short form -Wdangling-pointer is literally the same thing as -Wdangling-pointer=2, and -Wno-dangling-pointer and -Wdangling-pointer=0 both do the same "turn it all off" thing. Oh and -Wdangling-pointer=2 is included in -Wall ✨

This next example triggers the warning at level 1 — the address of the unnamed temporary is unconditionally referenced outside of its scope, which is a big nope:

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;
}

And in this function, storing the address of the local variable x in the escaped pointer *p triggers the warning at level 1:

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

In this last one, the array a is out of scope when the pointer s is used 😭 and since the code that sets s is conditional, the warning triggers at level 2:

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

• -Wdate-time — yells at you when the macros __TIME__, __DATE__ or __TIMESTAMP__ show up, because they might ruin your bitwise-identical reproducible compilations 🕰️ like, no cap, reproducibility is the vibe.
• -Wempty-body — warns if there's an empty body in an if, else or do while statement. This one is also enabled by -Wextra 🫶
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif. simple as that 💅
• -Wenum-compare — warns about comparisons between values of different enumerated types. In C++ enumerated type mismatches in conditional expressions get diagnosed too, and the warning is on by default. In C, this warning is enabled by -Wall ✨
• -Wenum-conversion — warns when a value of enumerated type is implicitly converted to a different enumerated type. In C this one is enabled by -Wextra 🎀
• -Wenum-int-mismatch (C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. For example:

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

In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. But since the choice of the underlying type of an enumerated type is implementation-defined, these mismatches may cause portability issues 😬 In C++ such mismatches are literally an error. In C, this warning is enabled by -Wall and -Wc++-compat 💖

• -Wjump-misses-init (C, Objective-C only) — warns if a goto statement or a switch statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. It only warns about variables that are initialized when they're declared, iykyk. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case 🙈

-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option 🌸

• -Wsign-compare — warns when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned 😭 In C++, this warning is also enabled by -Wall. In C, it's also enabled by -Wextra.
• -Wsign-conversion — warns for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning, slay 💅 In C, this option is enabled also by -Wconversion.
• -Wflex-array-member-not-at-end (C and C++ only) — warns when a structure containing a C99 flexible array member as the last field is not at the end of another structure 🦋 This warning warns e.g. about:

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

• -Wfloat-conversion — warns for implicit conversions that reduce the precision of a real value. This includes conversions from real to integer, and from higher precision real to lower precision real values 🔥 This option is also enabled by -Wconversion.
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order 🙅‍♀️
• -Wsizeof-array-div — warns about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element 😱 In such a case, the computation will not yield the number of elements in the array, which is probably what the user intended. This warning warns e.g. about:

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 usual way to compute the array size but won't work out correctly with pointers 💀 This warning warns e.g. about 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 to certain string and memory built-in functions if the argument uses sizeof. It triggers for example for memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and 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 — do not 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 😬 This indicates that 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 is folded to zero, or a cast of zero to some type, it's 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 their use 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. The -Waddress warning is enabled by -Wall 🔥

• -Wno-address-of-packed-member — do not warn when the address of a packed member of a struct or union is taken, which usually results in an unaligned pointer value 😖 This is enabled by default.
• -Wlogical-op — warns about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bitwise 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 purpose is to detect suspicious code like the following:

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

It's genuinely possible to shush that warning by wrapping the LHS in parentheses:

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

This warning is enabled by -Wall.

• -Waggregate-return — warns you if any function that returns a structure or union gets defined or called. (And in languages where returning an array is a thing, that also sets it off. 💅)
• -Wno-aggressive-loop-optimizations — turns off the warning you'd get when the compiler spots undefined behavior in a loop with a constant number of iterations. Like, the compiler is being a lil detective 👀 and you're telling it to chill. -Waggressive-loop-optimizations is enabled by default.
• -Wno-attributes — silences the warning for an unexpected "__attribute__" — think unrecognized attributes, function attributes slapped onto variables, that whole situation. Heads up though: this does not stop errors for incorrect use of supported attributes. 🫶

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

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

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

And you can kill warnings for all attributes in a namespace with -Wno-attributes=vendor::, which stops the warning for both of these:

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

Real talk: -Wno-attributes= does not imply -Wno-attributes. They're different things bestie, don't mix them up. 💀

• -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype is called with arguments whose promoted types don't match what the function expects. And 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 on by default. To dodge the warning, include the appropriate header so the prototypes of built-in functions are in scope. 📚

Example time — the call to "memset" below gets 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 gets diagnosed too:

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

• -Wno-builtin-macro-redefined — no warning if certain built-in macros get redefined. This suppresses warnings for redefinition of "__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__". 🎀
• -Wkeyword-macro — warns if a keyword is defined as a macro or undefined. For C++, identifiers with special meaning or standard attribute identifiers get diagnosed as well. This warning is enabled by default for C++26 if "-Wpedantic" is on, and it emits a pedwarn in that case. ⭐
• -Wfree-labels (C and Objective-C only) — warns if a label is applied to a non-statement, or if it shows up at the end of a compound statement. C23 and later dialects of C allow those labels, and they're available as a GCC extension in all the other dialects. 🦋

This warning is also enabled by -Wc11-c23-compat. And it gets turned into an error if you're building for a C version before C23 with -pedantic-errors.

• -Wheader-guard — warns if a valid preprocessor header multiple inclusion guard has a "#define" directive right after the "#ifndef" or "#if !defined" directive for the multiple inclusion guard, and that "#define" defines a different macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding "#ifndef" directive at the end of the header, and the "#define" directive defines an object-like macro with an empty definition. In that case, it's often just a misspelled guard name — either in the "#ifndef" / "#if !defined" directive or in the subsequent "#define" directive. 😭 This warning is enabled by -Wall.
• -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is totally fine, no warning, if it's preceded by a declaration that specifies the argument types. 🫶)
• -Wold-style-declaration (C and Objective-C only) — warns about obsolescent usages in a declaration, per the C Standard. For example, it warns if storage-class specifiers like "static" aren't the first things in a declaration. This warning is also enabled by -Wextra.
• -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. You get a warning even if there's a previous prototype. A definition using () is not considered an old-style definition in C23 mode, because it's equivalent to (void) in that case — but it is considered an old-style definition for older standards. 💅
• -Wmultiple-parameter-fwd-decl-lists (C and Objective-C only) — warns if more than one list of forward declarations of parameters shows up in a function prototype. This warning is also enabled by -Wextra. 🌸
• -Wdeprecated-non-prototype (C and Objective-C only) — warns if a function declared with an empty parameter list () is called with one or more arguments, or if a function definition with one or more parameters is encountered after such a declaration. Both of these are errors in C23 and later dialects of C. ⚠️

This warning is also enabled by -Wc11-c23-compat.

• -Wmissing-parameter-name (C and Objective-C only) — warns if a function definition omits a parameter name, specifying only its type. You can use this to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects. ✨

This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if you're building for a C version before C23 with -pedantic-errors.

• -Wmissing-parameter-type (C and Objective-C only) — a function parameter 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) — no warning if a function declaration contains a parameter name without a type. Those declarations don't provide a function prototype, and they block most type checking in function calls. 🙈

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

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

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

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

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

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

In C this option also doesn't warn about the universal zero initializer { 0 }:

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

Same energy in C++ — this option doesn't warn about the empty { } initializer, e.g.:

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

This warning is included in -Wextra. Wanna 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 that 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. Like so:

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

In rare cases it's possible to get false positives. To silence those, 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 — bestie, do NOT warn when a multicharacter constant like 'FOOF' shows up. Like… usually these are lowkey a typo in the user's code 💀 because they have implementation-defined values and honestly should not be used in portable code, periodt.
• -Wnormalized=[none|id|nfc|nfkc] — okay so this is the whole "do these two identifiers actually look the same" situation. In ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes when you use characters outside the basic ASCII set, you can get two different character sequences that look identical 👀. 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's not 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 is hoped that future versions of the standards involved will correct this, which is why this option is not the default.

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you'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 is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0, and so 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. iykyk 💅

• -Wno-attribute-warning — don't warn about usage of functions declared with the "warning" attribute. By default, this warning is enabled. You can use -Wno-attribute-warning to disable the warning, or -Wno-error=attribute-warning to disable the error when compiled with the -Werror flag. pick your fighter 🫶
• -Wno-deprecated — don't warn about usage of deprecated features.

In C++, explicitly specifying -Wdeprecated also enables warnings about some features that are deprecated in later language standards, specifically -Wcomma-subscript, -Wvolatile, -Wdeprecated-enum-float-conversion, -Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and -Wdeprecated-variadic-comma-omission. that's a whole list, we love an overachiever ✨

• -Wno-deprecated-declarations — don't warn about uses of functions, variables, or types marked as deprecated by using the "deprecated" attribute.
• -Wno-deprecated-openmp — don't warn about deprecated OpenMP code.
• -Wno-overflow — don't warn about compile-time overflow in constant expressions.
• -Wno-odr — warn about One Definition Rule violations during link-time optimization. Enabled by default. (yes the name is confusing, no we don't make the rules 🤷‍♀️)
• -Wopenacc-parallelism — warn about potentially suboptimal choices related to OpenACC parallelism. not me judging your parallelism choices 💀
• -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. slay, tune that cost model 🔥
• -Woverride-init (C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers.

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

• -Wno-override-init-side-effects (C and Objective-C only) — don't 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 — bestie, storytime 📖: the 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This was fixed in GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there's no longer a 4-bit padding between field a and b in this structure:

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

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

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued on struct S, like, warning: alignment 1 of 'struct S' is less than 8, in this code:

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

This warning is enabled by -Wall.

• -Wpadded — warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens it's possible to rearrange the fields of the structure to reduce the padding and so make the structure smaller. glow-up your structs fr 🎀
• -Wredundant-decls — warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid and changes nothing. like bestie we GET it, you already said that 💅
• -Wrestrict — warn when an object referenced by a "restrict"-qualified parameter (or, in C++, a "__restrict"-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to the "strcpy" function below attempts to truncate the string by replacing its initial characters with the last four. However, because the call writes the terminating NUL into a[4], the copies overlap and the call is diagnosed. 😭

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

The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It's included in -Wall.

• -Wnested-externs (C and Objective-C only) — warn if an "extern" declaration is encountered within a function. like why is extern in here, it's giving lost 🧚
• -Winline — warn if a function that's declared as inline cannot be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers.

The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the compiler takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. chaotic behavior ngl 💀

• -Winterference-size, -Wno-interference-size — warn about questionable uses of the C++17 "constexpr" variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size.

These variables are intended to be used for controlling class layout, to avoid false sharing in concurrent code:

struct independent_fields {
  alignas(std::hardware_destructive_interference_size)
    std::atomic<int> one;
  alignas(std::hardware_destructive_interference_size)
    std::atomic<int> two;
};

Here one and two are intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line.

By default, these variables are given values 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). 👑 consistency is key

If ABI stability is important, such as if the use is in a header for a library, you should probably not use the hardware interference size variables at all. 👈 important!!

These warnings are enabled by default. If you're confident that your use of these variables 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 — warn 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++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall. sir that is not a boolean 😭
• -Wno-int-to-pointer-cast — suppress 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) — bestie, this one shuts up the warning you get when you cast a pointer to an integer type of a different size 🫶 like it's not judging you, it's just not gonna nag.
• -Winvalid-pch — warns you when there's a precompiled header sitting in the search path that it straight up cannot use 😭 imagine finding a snack in your bag and it's expired. tragic. this tells you.
• -Winvalid-utf8 — yells if it finds an invalid UTF-8 character. this one's lowkey on by default for C++23 if you're using -finput-charset=UTF-8, and gets upgraded to an actual error by -pedantic-errors 💀 so like, it stops being a vibe check and becomes a whole thing.
• -Wno-unicode — don't diagnose invalid forms of delimited or named escape sequences that get treated as separate tokens. Wunicode is enabled by default ✨ so this is the "actually, leave me alone about it" switch.
• -Wlong-long — warns if the long long type shows up. it's enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. to inhibit the warning messages, use -Wno-long-long 🙈

heads up: this warning gets upgraded to an error by -pedantic-errors. so dramatic.

• -Wvariadic-macros — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. enabled by either -Wpedantic or -Wtraditional. to inhibit the warning messages, use -Wno-variadic-macros 💅
• -Wno-varargs — do not warn upon questionable usage of the macros used to handle variable arguments like va_start. these warnings are enabled by default, so this is you telling it "i know what i'm doing" (iconic, possibly delusional, but iconic).
• -Wvector-operation-performance — warns if a vector operation is not implemented via the SIMD capabilities of the architecture. mainly useful for performance tuning 🔥 vector operation can be implemented:
  • piecewise — the scalar operation is performed on every vector element
  • in parallel — the vector operation is implemented using scalars of wider type, which normally is more performance efficient
  • as a single scalar — the vector fits into a scalar type
• -Wvla — warns if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array 🦋

yup, upgraded to an error by -pedantic-errors. it's giving consequences.

• -Wvla-larger-than=byte-size — if you use this option, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. it's similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays 💜

ngl, 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. it's 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 appears the code meant exponentiation. specifically, the warning occurs when the left-hand side is the decimal constant 2 or 10 and the right-hand side is also a decimal constant 👀

in C and C++, ^ means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. so yeah, easy mix-up, no shame.

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

• -Wdisabled-optimization — warns if a requested optimization pass is disabled. this warning does not generally indicate that there is anything wrong with your code 🫶 it merely indicates that GCC's optimizers are unable to handle the code effectively. often the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time 😮‍💨
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. this option is only supported for C and Objective-C. it's implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign 🌸

upgraded to an error by -pedantic-errors, bestie. watch out.

• -Wstack-protector — this option is only active when -fstack-protector is active 🛡️ it warns about functions that are not protected against stack smashing. not me being the one function without a shield…
• -Woverlength-strings — warns about string constants that are longer than the "minimum maximum" length specified in the C standard 📏 modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings ✨

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

this option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.

• -Wunsuffixed-float-constants (C and Objective-C only) — issues a warning for any floating constant that does not have a suffix. when used together with -Wsystem-headers it warns about such constants in system header files 🍓 this can be useful when preparing code to use with the FLOAT_CONST_DECIMAL64 pragma from the decimal floating-point extension to C99.
• -Wno-lto-type-mismatch — during the link-time optimization, do not warn about type mismatches in global declarations from different compilation units. requires -flto to be enabled. enabled by default 🧚
• -Wno-designated-init (C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with the designated_init attribute 🙈

---

🕵️ Options That Control Static Analysis 🕵️

• -fanalyzer — okay this one is a whole main character ✨ it enables a static analysis of program flow which looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them 👑

real talk: this analysis is much more expensive than other GCC warnings. like, highkey expensive.

in technical terms, it performs coverage-guided symbolic execution of the code being compiled. it is neither sound nor complete — so yes it can have false positives and false negatives 💀 it's a bug-finding tool, rather than a tool for proving program correctness. very much "i have a hunch" energy, not "i have a proof" energy.

the analyzer is only suitable for use on C code in this release 🫶

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-throw-of-unexpected-type
-Wanalyzer-undefined-behavior-ptrdiff
-Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal

important: this option is only available if GCC was configured with analyzer support enabled 🎀

• -Wanalyzer-symbol-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to attempt to track the state of memory, but these can be defeated by sufficiently complicated code 😭

By default, the analysis silently stops tracking values of expressions if they exceed an internal limit, and falls back to an imprecise representation for such expressions. The -Wanalyzer-symbol-too-complex option warns if this occurs.

• -Wanalyzer-too-complex — okay so if -fanalyzer is on, the analyzer pulls out various heuristics to try to explore the control flow and data flow in the program, but bestie, sufficiently complicated code can just defeat those heuristics, no cap.

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

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

This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is not a multiple of sizeof (*pointer). It's giving wrong-math energy fr.

See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html).

• -Wno-analyzer-deref-before-check — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-deref-before-check to disable it.

This diagnostic warns for paths through the code in which a pointer is checked for NULL after it has already been dereferenced, suggesting that the pointer could have been NULL. Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced. Like bestie, the order of operations is not optional here 😭

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

This diagnostic is unlikely to be reported when any level of optimization is enabled, as GCC's optimization logic will typically consider such checks for NULL as being redundant, and optimize them away before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic. 💀

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

This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. Closing it twice? Not the slay you think it is 🎀

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

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

This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either free, or a deallocator referenced by attribute malloc. Double-free is not main character energy, it's iconic in the bad way 🙈

See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html).

• -Wno-analyzer-exposure-through-output-file — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). Bestie. Bestie. Not the password in the log file 🔥

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

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

This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). Uninit data crossing a trust boundary is giving leak era and we are not here for it 💀

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

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

This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa. Reading a write-only fd? The audacity 😭

This diagnostic also warns for code paths in a which a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.

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

This diagnostic warns for paths through code in which a file descriptor can be closed more than once. Once is plenty, queen 🎀

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

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

This diagnostic warns for paths through code in which an open file descriptor is leaked. Hold onto your friends, not your fds 🫶

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

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

This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it will warn on attempts to call accept on a stream socket that has not yet had listen successfully called on it. Skipping a step? In this economy? ✨

See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).

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

This diagnostic warns for paths through code in which an operation is attempted on the wrong type of file descriptor. For example, it will warn on attempts to use socket operations on a file descriptor obtained via open, or when attempting to use a stream socket operation on a datagram socket. Wrong vibe, wrong fd, wrong everything 🌸

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

This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor. It's closed, bestie, let it go 😭

This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a closed file descriptor at referenced argument N.

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

This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity. Do your vibe check first, fr fr 👑

This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor, at referenced argument N, without being checked for validity.

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

This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked.

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

• -Wno-analyzer-free-of-non-heap — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.

This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). You cannot free the stack, that's literally not its era 🍓

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

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

This diagnostic warns for paths through the code in which floating-point arithmetic is used in locations where precise computation is needed. Floating point where you need exact math is giving chaos 💀 This diagnostic only warns on use of floating-point operands inside the calculation of an allocation size at the moment.

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

This diagnostics warns for paths through the code which appear to lead to an infinite loop. We love a loop, we do not love a loop that never ends 🌀

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

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

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

For example, consider:

          while (1)
            {
              char opcode = *cpu_state.pc;
              switch (opcode)
               {
               case OPCODE_FOO:
                 handle_opcode_foo (&cpu_state);
                 break;
               case OPCODE_BAR:
                 handle_opcode_bar (&cpu_state);
                 break;
               }
            }

The analyzer will complain for the above case because if opcode ever matches none of the cases, the switch will follow the implicit default case, making the body of the loop be a "no-op" with cpu_state.pc unchanged, and thus using the same value of opcode on all subseqent iterations, leading to an infinite loop. It literally just keeps going forever, no notes 💅

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

• -Wno-analyzer-infinite-recursion — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-infinite-recursion to disable it.
• this diagnostic is out here warning u about paths through the code that look like they lead to infinite recursion 🔁😵

ok so like, specifically: when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame with the state at the entry to the previous frame of that function on the stack. warning gets issued if nothing in memory appears to be changing — any changes observed to parameters or globals are assumed to lead to termination of the recursion, and thus suppress the warning. so basically if u're actually mutating stuff, it chills out 🧘‍♀️

heads up bestie: this diagnostic is likely to miss cases of infinite recursion that are converted to iteration by the optimizer before the analyzer "sees" them. hence optimization should be disabled when attempting to trigger this diagnostic 💀

compare with -Winfinite-recursion, which provides a similar diagnostic but is implemented in a different way (sisters, not twins ✨).

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

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

this diagnostic warns for paths through the code in which a "NULL" function pointer is called 🫠 like, u called nothing, babe. that's not a function.

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

this diagnostic warns for paths through the code in which a pointer allocated via an allocator is leaked: either "malloc", or a function marked with attribute "malloc" 🗑️✨ memory said "i'll be back" and then never came back.

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

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

this diagnostic warns for paths through the code in which the wrong deallocation function is called on a pointer value, based on which function was used to allocate that pointer value. so ur code is out here buying shoes from one store and trying to return them to another 🥿➡️👟 the diagnostic will warn about mismatches between "free", scalar "delete" and vector "delete[]", and those marked as allocator/deallocator pairs using attribute "malloc".

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

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

this diagnostic warns for paths through the code in which a buffer is definitely read or written out-of-bounds 🚧😭 the diagnostic applies for cases where the analyzer is able to determine a constant offset, and for accesses past the end of a buffer (also a constant capacity). further, the diagnostic does limited checking for accesses past the end when the offset as well as the capacity is symbolic.

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

and for cases where the analyzer is able, it will emit a text art diagram visualizing the spatial relationship between the memory region that the analyzer predicts would be accessed, versus the range of memory that is valid to access 🌈🖼️ so like: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data involved. iconic honestly. this diagram can be suppressed using -fdiagnostics-text-art-charset=none.

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

this diagnostic warns for paths through the code in which overlapping buffers are passed to an API for which the behavior on such buffers is undefined 😬

specifically, the diagnostic occurs on calls to the following functions

• "memcpy"
• "strcat"
• "strcpy"

for cases where the buffers are known to overlap. two buffers walked into the same room and the vibes were undefined 💀

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

this diagnostic warns for paths through the code in which a possibly-NULL value is passed to a function argument marked with "__attribute__((nonnull))" as requiring a non-NULL value 🙈 "this seat is taken" and u sat down anyway.

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

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

this diagnostic warns for paths through the code in which a possibly-NULL value is dereferenced 🫠

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

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

this diagnostic warns for paths through the code in which a value known to be NULL is passed to a function argument marked with "__attribute__((nonnull))" as requiring a non-NULL value 😭 no guessing here, the analyzer knows.

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

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

this diagnostic warns for paths through the code in which a value known to be NULL is dereferenced 💅

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

• -Wno-analyzer-putenv-of-auto-var — this warning requires -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 🥴 bestie that stack frame is leaving, it cannot be holding ur env var, it's giving "texting someone who already changed their number".

see POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument (https://wiki.sei.cmu.edu/confluence/x/6NYxBQ).

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

this diagnostic warns for paths through the code in which a shift is attempted with a negative count 😵‍💫 shifting by negative amounts is not a vibe. it is analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. however, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings 👀 so like... don't rely on it catching everything, mmkay?

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

this diagnostic warns for paths through the code in which a shift is attempted with a count greater than or equal to the precision of the operand's type 🌀 it is analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. however (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 — this warning 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 🕰️💀 like rewinding to a location that literally does not exist anymore.

when "setjmp" is called on a "jmp_buf" to record a rewind location, it records the stack frame. the stack frame becomes invalid when the function containing the "setjmp" call returns. attempting to rewind to it via "longjmp" would reference a stack frame that no longer exists, and likely lead to a crash (or worse) 😱 yeah. or worse. that's the whole warning.

• -Wno-analyzer-tainted-allocation-size — this warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-allocation-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 allocation without being sanitized 🎯 so that an attacker could inject an excessively large allocation and potentially cause a denial of service attack. not the kind of "big energy" we want, ngl.

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

• -Wno-analyzer-tainted-assertion — this warning 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") 🚨 such functions typically indicate abnormal termination of the program, such as for assertion failure handlers. for example:

        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 📝 important bestie!!

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 ⚠️ (same energy as before, the optimizer keeps eating the evidence)

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  */
}

...even though, ngl, strictly speaking that's not an assertion failure. 🙈

• -Wno-analyzer-tainted-array-index — okay so this one needs -fanalyzer to even exist (that's what turns it on), and you use -Wno-analyzer-tainted-array-index to turn it off 💅. What it flags: any path through the code where a value that could be under an attacker's control gets used as an array index without being sanitized first — which means an attacker could inject an out-of-bounds access. Iconic of them, honestly. See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
• -Wno-analyzer-tainted-divisor — requires -fanalyzer (that's the enabler bestie); -Wno-analyzer-tainted-divisor disables it. It warns on paths where a possibly-attacker-controlled value is used as the divisor in a division or modulus operation without being sanitized, so an attacker could inject a division-by-zero. 🫠 See CWE-369: Divide By Zero (https://cwe.mitre.org/data/definitions/369.html).
• -Wno-analyzer-tainted-offset — requires -fanalyzer, which enables it; -Wno-analyzer-tainted-offset turns it off. Warns for paths where a value that could be under an attacker's control is used as a pointer offset without being sanitized, so an attacker could inject an out-of-bounds access. ✨ See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).
• -Wno-analyzer-tainted-size — requires -fanalyzer, which enables it; -Wno-analyzer-tainted-size disables it. Warns for paths where a value that could be under an attacker's control is used as the size of an operation such as memset without being sanitized, so an attacker could inject an out-of-bounds access. 💀 See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).
• -Wno-analyzer-throw-of-unexpected-type — requires -fanalyzer which enables it; -Wno-analyzer-throw-of-unexpected-type disables it. Quick lore drop: dynamic exception specifications are only available in C++14 and earlier 👑. This diagnostic warns for paths through the code in which a an exception is thrown from a function with a dynamic exception specification where the exception does not comply with the specification.
• -Wno-analyzer-undefined-behavior-ptrdiff — requires -fanalyzer, which enables it; -Wno-analyzer-undefined-behavior-ptrdiff disables it. Warns for paths in which a pointer subtraction happens where the pointers refer to different chunks of memory. That code is literally relying on undefined behavior, bestie — pointer subtraction is only defined when both pointers point within (or just after) the same array. 🚨 See CWE-469: Use of Pointer Subtraction to Determine Size (https://cwe.mitre.org/data/definitions/469.html).
• -Wno-analyzer-undefined-behavior-strtok — requires -fanalyzer, which enables it; -Wno-analyzer-undefined-behavior-strtok disables it. Warns for paths in which a call is made to strtok with undefined behavior. Specifically (and this is the part people always miss 😭): 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; -Wno-analyzer-unsafe-call-within-signal-handler disables it. Warns for paths in which a function known to be async-signal-unsafe (such as fprintf) is called from a signal handler. No cap, do not do this. 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; -Wno-analyzer-use-after-free disables it. Warns for paths 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; -Wno-analyzer-use-of-pointer-in-stale-stack-frame disables it. Warns for paths in which a pointer is dereferenced that points to a variable in a stale stack frame. 🫠
• -Wno-analyzer-va-arg-type-mismatch — requires -fanalyzer, which enables it; -Wno-analyzer-va-arg-type-mismatch disables it. Warns for interprocedural paths through the code where the analyzer catches an attempt to use va_arg to extract a value passed to a variadic call, but with a type that does not match the type 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; -Wno-analyzer-va-list-exhausted disables it. Warns for interprocedural paths where the analyzer catches 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. empty. gone. nothing left 🍓 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; -Wno-analyzer-va-list-leak disables it. Warns for interprocedural paths where 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; -Wno-analyzer-va-list-use-after-va-end disables it. Warns for interprocedural paths where the analyzer detects an attempt to use a va_list after va_end has been called on it. va_list.
• -Wno-analyzer-write-to-const — requires -fanalyzer, which enables it; -Wno-analyzer-write-to-const disables it. Warns for paths where the analyzer detects an attempt to write through a pointer to a const object. BUT ⚠️ — the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.
• -Wno-analyzer-write-to-string-literal — requires -fanalyzer, which enables it; -Wno-analyzer-write-to-string-literal disables it. Warns for paths where the analyzer detects an attempt to write through a pointer to a string literal. Same disclaimer as above bestie: the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. 💅
• -Wno-analyzer-use-of-uninitialized-value — requires -fanalyzer, which enables it; -Wno-analyzer-use-of-uninitialized-value disables it. Warns for paths in which an uninitialized value is used. 😭 See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).

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

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

...and of 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

...and 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

Also ✨ — various functions with an __analyzer_ prefix have special meaning to the analyzer, and those are described in the GCC Internals manual. Various internal parameters, settable via --param, are used to control the exploration; those are also documented in the GCC Internals manual. iykyk 🦋

The following options control the analyzer.

• -fanalyzer-assume-nothrow — okay so by default, if -fexceptions is enabled, the analyzer assumes that a call to any function without the "nothrow" attribute could throw an exception 🫶. That's genuinely helpful for catching execution paths that leak because exceptions bypass your clean-up code, but ngl it can also give you false positives when you're using headers where the author just never added the "nothrow" attribute 🙈. If -fanalyzer-assume-nothrow is enabled, the analyzer assumes external functions do not throw exceptions. Lowkey useful for reducing "noise" from the analyzer when you enable -fexceptions on C code — but it could hide true issues if an exception could be raised by something your C code calls. So like, use with intention 💅
• -fanalyzer-call-summaries — simplifies interprocedural analysis by computing the effect of certain calls instead of exploring, like, every single path through the function from callsite to each possible return ✨. If enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated. Fancy and efficient, we love to see it.
• -fanalyzer-checker=name — restricts the analyzer to run just the named checker, and enables it. One checker, main character energy, no distractions 👑
• -fanalyzer-debug-text-art-headings — this one is for analyzer developers 🛠️. If enabled, the analyzer adds extra annotations to any diagrams it generates.
• -fno-analyzer-feasibility — this option is 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 are rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code 🐛
• -fanalyzer-fine-grained — does nothing. Preserved for backward compatibility. Literally just vibing there 💀
• -fanalyzer-show-duplicate-count — analyzer-developer option: 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. Show me everything, bestie.
• -fno-analyzer-simplify-supergraph — analyzer developers only 🛠️. By default, the analyzer performs various simplifications to the program supergraph before analyzing it. With -fno-analyzer-simplify-supergraph, this simplification can be suppressed, for debugging issues with it.
• -fno-analyzer-state-merge — for the analyzer devs. 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 — analyzer developers, this one's yours. 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 — analyzer developers again 🛠️. By default the analyzer stops exploring an execution path after encountering certain diagnostics, so it doesn't issue a whole cascade of follow-up diagnostics. The diagnostics that terminate analysis along a path are:
  • -Wanalyzer-null-argument
  • -Wanalyzer-null-dereference
  • -Wanalyzer-use-after-free
  • -Wanalyzer-use-of-pointer-in-stale-stack-frame
  • -Wanalyzer-use-of-uninitialized-value

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

• -fanalyzer-transitivity — this option enables transitivity of constraints within the analyzer ✨
• -fno-analyzer-undo-inlining — 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 — for analyzer developers. Enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths 🌀
• -fanalyzer-verbose-state-changes — analyzer devs: enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths 🔍
• -fanalyzer-verbosity=level — this one controls the complexity of the control flow paths that are emitted for analyzer diagnostics. The level can be one of:
  • 0 — at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-"free" diagnostic, both calls to "free" will be shown.
  • 1 — as per the previous level, but also show events for the entry to each function.
  • 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 ⭐
  • 3 — as per the previous level, but show all control flow events, not just significant ones.
  • 4 — 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 📄. Note that -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 — dumps a textual representation of the "exploded graph" to one dump file per node, to file.eg-id.txt. This is typically a large number of dump files 😭
• -fdump-analyzer-exploded-paths — dumps a textual representation of the "exploded path" for each diagnostic to file.idx.kind.epath.txt 🛤️
• -fdump-analyzer-feasibility — dumps internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the form file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt ✨
• -fdump-analyzer-infinite-loop — dumps internal details about the analyzer's search for infinite loops. The details are written in a form suitable for viewing with GraphViz to filenames of the form file.*.infinite-loop.dot 🔁
• -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change ⚠️
• -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dumps a representation of the "supergraph" suitable for viewing with GraphViz, but annotates the graph with information on what state will be purged at each node. The graph is written to file.state-purge.dot 🧹
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" suitable for viewing with GraphViz to file.supergraph.index.kind.dot. These show all of the control flow graphs in the program, at various stages of the analysis. The precise set of dumps and what they show is subject to change ⚠️
• -fdump-analyzer-untracked — emits custom warnings with internal details intended for analyzer developers 🛠️

🐛 Options for Debugging Your Program 🐛

To tell GCC to emit extra information for use by a debugger, in almost all cases you only need to add -g to your other options 🫶. Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options.

GCC allows you to use -g with -O ✨. The shortcuts taken by optimized code may occasionally be surprising, so like, brace yourself: some variables you declared may not exist at all 😭; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless, it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs. Slay.

If you are not using some other optimization option, consider using -Og with -g 💡. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so -Og may result in a better debugging experience.

• -g, --debug — produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information 💖

On most systems that use stabs format, -g also turns on some extra debugging info that literally only GDB can use 🫶 — it makes debugging work better in GDB but probably makes other debuggers crash or flat-out refuse to read the program 💀. If you want to control for certain whether that extra info gets generated, use -gvms (peep below).

• -ggdb — produces debugging info specifically for GDB, meaning it uses the most expressive format available (DWARF, stabs, or the native format if neither of those is supported), including GDB extensions if at all possible. GDB gets the deluxe treatment ✨
• -gdwarf, -gdwarf-version — produces debugging info in DWARF format (if that's supported). The value of version may 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. iykyk 🌈

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables. Yeah, it's giving "complicated" 😭

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher. Check your GDB's vibe before you commit 👑

GCC no longer supports DWARF Version 1 — it's substantially different from Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but they apply to all currently-supported versions of DWARF. The names are just stuck in the past, bestie 💅

• -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 object file, no cap 🦋
• -gprune-btf, -gno-prune-btf — prunes BTF information before emission. When pruning, only type information for types used by global variables and file-scope functions will be emitted. If you're compiling for the BPF target with BPF CO-RE enabled, type information will also be emitted for types used in BPF CO-RE relocations. In addition, struct and union types which are only referred to via pointers from members of other struct or union types shall be pruned and replaced with BTF_KIND_FWD, as though those types were only present in the input as forward declarations.

This option substantially reduces the size of produced BTF information, but at significant loss in the amount of detailed type information 😭 It's primarily useful when compiling for the BPF target, to minimize the size of the resulting object, and to eliminate BTF information which is not immediately relevant to the BPF program loading process.

This option is enabled by default for the BPF target when generating BTF information. Default behavior, iconic 🎀

• -gctf, -gctflevel — request CTF debug information and use level to specify how much CTF debug information should be produced. If -gctf is specified without a value for level, the default level of CTF debug information is 2.

CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.

Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf. Poof, gone 💨

Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.

Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.

• -gvms — produces debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems. Retro vibes only 🧚
• -gcodeview — produces debugging information in CodeView debug format (if that is supported). This is the format used by Microsoft Visual C++ on Windows. Windows besties, this one's for you 💜
• -glevel, -ggdblevel, -gvmslevel — request debugging information and also use level to specify how much information. The default level is 2. Okay so levels, let's go:

Level 0 produces no debug information at all. Thus, -g0 negates -g. Nothing. Nada. 🙈

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 2 is the normal setting to produce debuggable code. The debug output includes all the information from level 1, plus information about local variables and typedefs to allow stepping through code and examining its state. For C++ code, additional information is also emitted to assist in debugging namespace, class, and template functions. This is the main character setting ⭐

Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3. Maximum lore 🔥

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

Heads up: -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. No sneaky shortcuts here 😌

• -fno-eliminate-unused-debug-symbols — by default, no debug information is produced for symbols that aren't actually used. Use this option if you want debug information for all symbols. Every last one 🫶
• -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. Bloat era, honestly 😭
• -fno-merge-debug-strings — directs 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. Speed vs. size, the eternal struggle 🦋
• -fdebug-prefix-map=old=new — when compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using . for new. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map and -fcanon-prefix-map. Path glow-up ✨
• -fvar-tracking — runs the variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).

It's enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it. Free upgrade, we love to see 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. So much effort for nothing 😭 By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.

• -fvar-tracking-uninit, -fno-var-tracking-uninit — perform variable tracking and also mark uninitialized variables in the debug information. This flag is enabled by default by -fvar-tracking; it also implies -fvar-tracking. To do variable tracking without marking uninitialized variables, use -fvar-tracking -fno-var-tracking-uninit. Mix and match, bestie 💅
• -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. Split it, don't link it 🎀
• -gdwarf32, -gdwarf64 — if DWARF debugging information is enabled, the -gdwarf32 selects the 32-bit DWARF format and the -gdwarf64 selects the 64-bit DWARF format. The default is target specific — on most targets it's -gdwarf32, though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet. Pick your fighter 🌈
• -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. Giving the nameless a little something 🫶
• -gpubnames — generate DWARF .debug_pubnames and .debug_pubtypes sections.
• -ggnu-pubnames — generate .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7. Niche but iconic ✨
• -gno-pubnames — don't generate DWARF .debug_pubnames and .debug_pubtypes sections. The anti-option, we respect it 🙈
• -fdebug-types-section — when using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of making them part of the .debug_info section. It's more efficient to put them in a separate comdat section since the linker can then remove duplicates. But not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types produces larger instead of smaller debugging information. Not me getting bigger after optimizing 😭
• -grecord-gcc-switches, -gno-record-gcc-switches — this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file. Receipts, in the debug info 💅
• -gstrict-dwarf — no later-DWARF-version shenanigans for you 🚫. This bans 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 — this option says "not today."
• -gno-strict-dwarf — the chill one 💅. Allows using extensions of later DWARF standard version than the one selected with -gdwarf-version.
• -gas-loc-support — telling the compiler "bestie, the assembler gets .loc directives" 📍. It may then use them so the assembler generates DWARF2+ line number tables. This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. Enabled by default if, at GCC configure time, the assembler is found to support such directives.
• -gno-as-loc-support — force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. DIY era 🛠️.
• -gas-locview-support — inform 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 is found to support them.
• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
• -gcolumn-info, -gno-column-info — emit location column information into DWARF debugging information, rather than just file and line. Enabled by default ✨.
• -gstatement-frontiers, -gno-statement-frontiers — causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. Enabled by default when compiling with optimization (-Os, -O1, -O2, ...), and outputting DWARF 2 debug information at the normal level 🚩.
• -gvariable-location-views, -gvariable-location-views=incompat5, -gno-variable-location-views — augment variable location lists with progressive view numbers implied from the line number table. This enables debug information consumers to inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point 🔍. If the assembler lacks support for view numbers in line number tables, this causes the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that are not aware of these augmentations — but they won't derive any benefit from them either.

Enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it forces GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that most certainly leads 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, and you can switch it on 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 information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it 💀.

• -ginternal-reset-location-views, -gno-internal-reset-location-views — attempt to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate instruction length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists 🙈. The GNU assembler flags any such error as a "view number mismatch". Only enabled on ports that define a reliable estimation function.
• -ginline-points, -gno-inline-points — generate extended debug information for inlined functions. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won't be output, but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled 🎀.
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported 🗜️. If type is not given, the default type depends on the capabilities of the assembler and linker used. type may be one of none (don't compress debug sections), zlib (use zlib compression in ELF gABI format), or zstd (use zstd compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler does not support them, -gz is silently ignored when producing object files 🤫.
• -femit-struct-debug-baseonly — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of the file in which the struct is defined. This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger 😬. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control. This option works only with DWARF debug output.
• -femit-struct-debug-reduced — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of the file in which the type is defined, unless the struct is a template or defined in a system header. This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control. This option works only with DWARF debug output.
• -femit-struct-debug-detailed[=spec-list] — specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program 🧠.

This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.

A specification has the syntax:

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

The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

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

The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application 🧪.

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

This option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as compiler generated .eh_frame section instead of using GAS .cfi_* directives.
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it's useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared) 🪄. More often, however, this results in a significant amount of wasted space.

---

✨ Options That Control Optimization ✨

These options control various sorts of optimizations, bestie — this is where the compiler stops being a literal translator and starts having opinions 🔥.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code 🎯. It's giving "no surprises" energy, and honestly? Iconic.

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 ⚖️. Trade-offs, always 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 👑 — main character move, honestly.

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section 📋.

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes 💤.

Real talk before we get into the levels: 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. Like, your build might be built different 💅. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level — icon behaviour, honestly.

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• -O, -O1, --optimize — optimizes, bestie. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

-O is the recommended optimization level for large machine-generated code as a sensible balance between time taken to compile and memory use: higher optimization levels perform optimizations with greater algorithmic complexity than at -O.

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants
-fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time

• -O2 — optimize even more, like it's not even trying to be chill about it. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code.

-O2 turns on all optimization flags specified by -O1. It also turns on the following optimization flags:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdep-fusion
-fdevirtualize  -fdevirtualize-speculatively
-fexpensive-optimizations -ffinite-loops -fgcse  -fgcse-lm
-fhoist-adjacent-loads -finline-functions -finline-small-functions
-findirect-inlining -fipa-bit-cp  -fipa-cp  -fipa-icf -fipa-ra
-fipa-sra  -fipa-vrp -fisolate-erroneous-paths-dereference
-flra-remat -foptimize-crc -foptimize-sibling-calls
-foptimize-strlen -fpartial-inlining -fpeephole2
-freorder-blocks-algorithm=stc -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop -fschedule-insns
-fschedule-insns2 -fsched-interblock  -fsched-spec
-fspeculatively-call-stored-functions -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

🫶 heads up, bestie: please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. Not me forgetting to tell you, that's literally where it lives.

• -O3 — optimize yet more, the main character of optimization levels 🔥. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags:

-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides

• -O0 — reduce compilation time and make debugging produce the expected results. This is the default 🙈.

At -O0, GCC completely disables most optimization passes; they are not run even if you explicitly enable them on the command line, or are listed by -Q --help=optimizers as being enabled by default. Many optimizations performed by GCC depend on code analysis or canonicalization passes that are enabled by -O, and it would not be useful to run individual optimization passes in isolation. So no, turning them on manually does not make them a thing. Periodt.

• -Os — optimize for size 🎀. -Os enables all -O2 optimizations except those that often increase code size:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. Small but mighty vibes, iykyk.

• -Ofast — disregard strict standards compliance, we're going feral. -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.

💀 fr fr though: "not valid for all standard-compliant programs" is not a vibe, it's a legit caveat. Standards compliance is out the window at this level.

• -Og — optimize while keeping in mind debugging experience 🦋. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable blend of optimization, fast compilation and debugging experience especially for code with a high abstraction penalty. In contrast to -O0, this enables -fvar-tracking-assignments and -fvar-tracking which handle debug information in the prologue and epilogue of functions better than -O0.

Like -O0, -Og completely skips a number of optimization passes so that individual options controlling them have no effect. Otherwise -Og enables all -O1 optimization flags except for those known to greatly 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, no take-backs 👑

---

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. Math, but make it compiler flags 🧮✨.

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.
• -fforward-propagate — perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.

• -favoid-store-forwarding, -fno-avoid-store-forwarding — many CPUs will stall for many cycles when a load partially depends on previous smaller stores 😭. This pass tries to detect such cases and avoid the penalty by changing the order of the load and store and then fixing up the loaded value.

Disabled by default.

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

• -ffp-int-builtin-inexact — 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 -fno-fp-int-builtin-inexact, disallowing the exception to be raised, unless C17 or an earlier 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.

• -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, lowkey 💅.

On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.

Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions.

Enabled by default at -O1 and higher.

• -foptimize-crc — detect loops calculating CRC (performing polynomial long division) and replace them with a faster implementation 🔥. Detect 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and bit-reversed cases. If the target supports a CRC instruction and the polynomial used in the source code matches the polynomial used in the CRC instruction, generate that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, generate CRC using it; otherwise generate table-based CRC.

Enabled by default at -O2 and higher. Slay.

• -foptimize-sibling-calls — optimize sibling and tail recursive calls. That's it, that's the flag 🫶.
• -foptimize-strlen — rewrites all those classic C string functions — strlen, strchr, strcpy, and even their _FORTIFY_SOURCE twins — into snappier equivalents. Same job, faster fit. 💅

On at -O2 and -O3.

• -finline-atomics, -fno-inline-atomics — inlines __atomic operations whenever a lock-free instruction sequence is available. Like, if the hardware can do it without a lock, GCC just smooshes it right in. This one is on by default, periodt.
• -finline-stringops[=fn] — expands memory and string ops inline — for now only memset — even when the length is variable or so big it'd need a loop. Chef's kiss combo with -ffreestanding and -fno-builtin. ✨

Sometimes this lets GCC generate code that flexes known alignment and length multipliers, but even then it can be less efficient than a tuned runtime implementation, and it can also balloon your code size so much that a slower-but-shared implementation ends up running faster purely because it plays nicer with code caches. Off by default. 🙈

• -fno-inline — no inline expansion for anybody, except the functions tagged with the always_inline attribute. This is the default when you're not optimizing.

Want to rescue a single function from inlining? Tag it noinline and it's exempt.

• -finline-small-functions — pulls functions into their callers when the body is smaller than the function-call code would be — so the whole program gets smaller. GCC heuristically picks which functions are simple enough to be worth it. Heads up: this applies to all functions, even the ones you never declared inline.

On at -O2, -O3, -Os.

• -findirect-inlining — inlines the indirect calls too, the ones that previous inlining revealed to be known at compile time. No effect unless inlining itself is already on via -finline-functions or -finline-small-functions.

On at -O2, -O3, -Os.

• -finline-functions — considers every function for inlining, declared inline or not; GCC heuristically decides which ones are worth integrating.

If literally every call to a function gets integrated and that function is declared static, it normally doesn't get output as assembler code on its own — it's fully absorbed. 🫶

On at -O2, -O3, -Os. Also switched on by -fprofile-use and -fauto-profile.

• -finline-functions-called-once — considers all static functions that are called exactly once for inlining into their caller, even without the inline mark. If the call gets integrated, the function isn't emitted as assembler code separately.

On at -O1, -O2, -O3 and -Os — but not -Og.

• -fearly-inlining — inlines always_inline functions and functions whose body looks smaller than the call overhead early, before -fprofile-generate instrumentation and the real inlining pass happen. This makes profiling way cheaper and usually makes inlining faster on programs with big chains of nested wrapper functions.

On by default.

• -fipa-sra — interprocedural scalar replacement of aggregates: removes unused parameters and swaps parameters passed by reference for parameters passed by value. Glow-up for your call signatures. 💖

On at -O2, -O3 and -Os.

• -finline-limit=n — GCC caps how big an inlinable function can be by default; this flag gives you coarse control over that cap. n is the size of functions that can be inlined, counted in pseudo instructions.

Inlining is actually controlled by a bunch of internal parameters, documented in the GCC Internals manual, and honestly you shouldn't normally set those directly.

Note: there may be no value of -finline-limit that gives you the default behavior. Yeah. It's like that.

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 can change from one release to the next. Do not come for it, it's doing its best. 😭

• -fno-keep-inline-dllexport — a more fine-grained version of -fkeep-inline-functions, applying only to functions declared with the dllexport attribute or declspec.
• -fkeep-inline-functions — in C, emits static functions declared inline into the object file even if the function got inlined into all of its callers. Doesn't touch functions using the extern inline extension in GNU C90. In C++, emits any and all inline functions into the object file.
• -fkeep-static-functions — emits static functions into the object file even if they're never used. Keeping them for the plot. 👑
• -fkeep-static-consts — emits variables declared static const when optimization is off, even if nothing references them.

GCC enables this by default. Want to force the compiler to actually check whether a variable is referenced, optimization or not? Use -fno-keep-static-consts.

• -fmerge-constants — tries to merge identical constants (string constants and floating-point constants) across compilation units.

This is the default for optimized compilation when the assembler and linker support it. Use -fno-merge-constants to block the behavior.

On at -O1, -O2, -O3, -Os.

• -fmerge-all-constants — tries to merge identical constants and identical variables.

Implies -fmerge-constants. On top of that, it also considers things like constant-initialized arrays, or initialized constant variables with integral or floating-point types. But listen — languages like C or C++ require every variable, including multiple instances of the same variable in recursive calls, to have distinct locations. So this option gives you non-conforming behavior. It's giving rulebreaker. 💀

• -fmodulo-sched — does swing modulo scheduling right before the first scheduling pass. This pass stares at innermost loops and reorders their instructions by overlapping different iterations.
• -fmodulo-sched-allow-regmoves — more aggressive SMS-based modulo scheduling, with register moves allowed. Setting this flag deletes certain anti-dependence edges, which triggers reg-move generation based on life-range analysis. Only effective with -fmodulo-sched enabled.
• -fno-branch-count-reg — disables the optimization pass that hunts for chances to use decrement and branch instructions on a count register instead of the whole decrement-a-register / compare-against-zero / branch-on-result sequence. Only meaningful on architectures that support such instructions: x86, PowerPC, IA-64 and S/390 are on the list. Also note that -fno-branch-count-reg does not strip the decrement and branch instructions other optimization passes already introduced into the generated instruction stream.

The default is -fbranch-count-reg at -O1 and higher, except for -Og.

• -fno-function-cse — doesn't put function addresses in registers; every instruction that calls a constant function has to spell out the function's address explicitly.

Less efficient code, but some strange hacks that alter assembler output can get confused by the optimizations that happen when this option is off.

The default is -ffunction-cse

• -ffuse-ops-with-volatile-access — allows limited optimization of operations with volatile memory access when doing so doesn't change the semantics outlined in

The default is -ffuse-ops-with-volatile-access

No cap, that sentence just ends there in the source. That's the whole sentence. I'm not making it up, bestie. 🫠

• -fno-zero-initialized-in-bss — if the target has a BSS section, GCC by default drops variables initialized to zero into BSS. Saves space in the resulting code. ✨

This option turns that off, because some programs explicitly rely on their variables landing in the data section — like, so the resulting executable can find the beginning of that section and/or make assumptions based on it.

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

• -fthread-jumps — optimizes by checking whether a jump branches to a spot where another comparison subsumed by the first one is sitting. If so, the first branch gets redirected to either the destination of the second branch or a point immediately after it — depends on whether the condition is known true or false.

On at -O1, -O2, -O3, -Os.

• -fsplit-wide-types — when a type occupies multiple registers — think long long on a 32-bit system — splits those registers apart and allocates them independently. Usually better code for those types, but it can make debugging harder. 🙃

On at -O1, -O2, -O3, -Os.

• -fsplit-wide-types-early — fully splits wide types early instead of very late. Does nothing unless -fsplit-wide-types is on.

This is the default on some targets.

• -fcse-follow-jumps — in common subexpression elimination (CSE), scans through jump instructions when the jump's target isn't reached by any other path. Like, when CSE bumps into an if statement with an else clause, it follows the jump when the condition tested is false.

On at -O2, -O3, -Os.

• -fcse-skip-blocks — similar to -fcse-follow-jumps, but it makes CSE follow jumps that conditionally skip over blocks. When CSE hits a simple if with no else clause, -fcse-skip-blocks makes it follow the jump around the body of the if.

On at -O2, -O3, -Os.

• -frerun-cse-after-loop — re-runs common subexpression elimination after the loop optimizations have done their thing.

On at -O2, -O3, -Os.

• -fgcse — runs a global common subexpression elimination pass. This pass also does global constant and copy propagation. 🌈

Note: when you're compiling a program that uses computed gotos (a GCC extension), you may get better run-time performance by disabling the global common subexpression elimination pass — just add -fno-gcse to the command line.

On at -O2, -O3, -Os.

• -fgcse-lm — okay so when this one is on, global common subexpression elimination gets a little ✨adventurous✨ and tries to move loads that are only killed by stores into themselves. Basically it takes a loop with a load/store sequence and goes "nah bestie, let's make it a load outside the loop and a copy/store inside the loop" 🌀

Enabled by default when -fgcse is enabled. iykyk 💅

• -fgcse-sm — turn this on and a store motion pass runs after global common subexpression elimination. This pass is the one that's like "stores, babes, let's get you out of the loop" 🚪 When you pair it with -fgcse-lm, loops containing a load/store sequence can become a load before the loop and a store after the loop. Obsessed tbh.

Not enabled at any optimization level. 😔

• -fgcse-las — with this enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location — and it does both partial and full redundancies 💀

Not enabled at any optimization level.

• -fgcse-after-reload — when this is enabled, a redundant load elimination pass is performed after reload. The whole purpose? Cleaning up redundant spilling 🧹✨

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

• -faggressive-loop-optimizations — this tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. Translation: it assumes your loop code doesn't invoke undefined behavior, like causing signed integer overflows or out-of-bound array accesses 😬 The bounds it derives are then used to guide loop unrolling and peeling and loop exit test optimizations.

This option is enabled by default.

Bestie, note the assumption. If your code is out here causing signed integer overflows or reading out of bounds, that's undefined behavior territory and this option is not going to be your friend 🙈

• -funconstrained-commons — this one tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. Which prevents certain optimizations that depend on knowing the array bounds 🫶
• -fcrossjumping — perform cross-jumping transformation ✨ It unifies equivalent code and saves code size. The resulting code may or may not perform better than without cross-jumping — ngl, no promises on speed, it's a size thing 💁‍♀️

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

• -fauto-inc-dec — combines increments or decrements of addresses with memory accesses 🔥 This pass is always skipped on architectures that don't have instructions to support this, so if your chip can't, it just sits there like "not me, not my problem" 💅 Enabled by default at -O1 and higher on architectures that support this.
• -fdce — perform dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher. Rude and efficient, we love her 🗑️
• -fdse — perform dead store elimination (DSE) on RTL. Enabled by default at -O1 and higher. The store-side sibling of -fdce, living her best life 🦋
• -fif-conversion — attempt to transform conditional jumps into branch-less equivalents 🎀 This includes use of conditional moves, min, max, set flags and abs instructions, and 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. Periodt.

• -fif-conversion2 — use conditional execution (where available) to transform conditional jumps into branch-less equivalents ✨ Same vibe, the sequel.

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

• -fdeclone-ctor-dtor — ok so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards 😭 For a hierarchy with virtual bases, the base and complete variants are clones, which means two copies of the function. With this option, the base and complete variants are changed to be thunks that call a common implementation — deduplicated, iconic, no cap 👑

Enabled by -Os.

• -fdelete-null-pointer-checks — assume that programs cannot safely dereference null pointers, and that no code or data element resides at address zero 🚫 This option enables simple constant folding optimizations at all optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer is checked after it has already been dereferenced, it cannot be null.

Note however that 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. Fr fr, check your environment 🙏

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

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

• -fdevirtualize — attempt to convert calls to virtual functions to direct calls 🎯 This is done both within a procedure and interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
• -fdevirtualize-speculatively — attempt to convert calls to virtual functions to speculative direct calls 🔮 Based on the analysis of the type inheritance graph, it determines for a given call the set of likely targets. If the set is small, preferably of size 1, it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, such as inlining. And when they seem useless after further optimization, they're converted back into original form — she said "jk, never mind" 💀
• -fdevirtualize-at-ltrans — streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode 📡 This option enables more devirtualization but significantly increases the size of streamed data. For this reason it is disabled by default. Big brain but big data, pick your struggle 🫠
• -fexpensive-optimizations — perform a number of minor optimizations that are relatively expensive 💸 Yeah, "minor" and "expensive" in the same sentence, she contains multitudes.

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

• -fext-dce, -fno-ext-dce — perform dead code elimination on zero and sign extensions, with special dataflow analysis ✨ Cleanup for your extension instructions, with the fancy analysis and everything 🧠
• -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. Like, it literally does it for you, so the redundant ones can go 🫶

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

• -fno-lifetime-dse — in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed 💀 Normally dead store elimination takes advantage of this; if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization.

To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor, you can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. And -flifetime-dse=0 is equivalent to -fno-lifetime-dse. The whole little family, all accounted for 💅

• -flive-range-shrinkage — attempt to decrease register pressure through register live range shrinkage 🩹 Helpful for fast processors with small or moderate size register sets. Squish, bestie, squish.
• -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. Slay.
• -fira-region=region — use specified regions for the integrated register allocator. The region argument should be one of the following:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set 🌸
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...) 👑
  • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0 🎀
• -fira-hoist-pressure — 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. Trade-off era.

This option is enabled at level -Os for all targets.

• -fira-loop-pressure — use IRA to evaluate register pressure in loops for decisions to move loop invariants 🔁 This option usually results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it can slow the compiler down. Fancy but pricey 💅

This option is enabled at level -O3 for some targets.

• -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, no shrinkage — bigger frames, that's the deal.
• -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 slots 😭
• -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. Recalculate > reload, when the math works out.

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

• -fdelayed-branch — if supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions 🌈 If your target doesn't support it, she simply doesn't show up.

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

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• -fschedule-insns — ok so if the target machine supports it, GCC will try to reorder your instructions so you don't get those sad little execution stalls where the data you need just isn't there yet 💀. It's a lifesaver for machines with slow floating-point or memory-load instructions, because it lets other instructions go out and party until the load or FP result is actually needed. Conventionally it's on at -O2 and -O3, but ngl a ton of targets override that: on x86 it's disabled at all levels, and on AArch64 it's only enabled at -O3. Target-dependent behavior, bestie, that's just how it is ✨
• -fschedule-insns2 — literally the same vibe as -fschedule-insns, but it asks for an additional pass of instruction scheduling after register allocation has happened 🎀. Especially useful on machines with a relatively small number of registers, and where memory load instructions take more than one cycle. Enabled at -O2, -O3, -Os.
• -fno-sched-interblock — turns off instruction scheduling across basic blocks. That scheduling is normally enabled when you're scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher 🔥
• -fno-sched-spec — disables speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. No speculative vibes today 🙈
• -fsched-pressure — enables register-pressure-sensitive insn scheduling before register allocation 💅. It only makes sense when scheduling before register allocation is on, i.e. with -fschedule-insns or at -O2 or higher. Using it can genuinely improve the generated code and shrink it, by preventing register pressure from rising above the number of available hard registers and the subsequent spills in register allocation. Iconic, honestly.
• -fsched-spec-load — allows speculative motion of some load instructions 🦋. Only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — allows speculative motion of more load instructions 😭 (yes, the flag is literally named dangerous, it's giving main character energy). Only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-stalled-insns, -fsched-stalled-insns=n — defines how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass ⭐. -fno-sched-stalled-insns means no insns are moved prematurely, and -fsched-stalled-insns=0 means there's no limit on how many queued insns can be moved prematurely. -fsched-stalled-insns with no value is equivalent to -fsched-stalled-insns=1.
• -fsched-stalled-insns-dep, -fsched-stalled-insns-dep=n — defines how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns 🧚. This only does anything during the second scheduling pass, and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0, and -fsched-stalled-insns-dep with no value is equivalent to -fsched-stalled-insns-dep=1.
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling 💜. This 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 😳. And it only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.

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• -fsched-group-heuristic — enables the group heuristic in the scheduler, which favors the instruction that belongs to a schedule group 🌸. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-critical-path-heuristic — enables the critical-path heuristic, which favors instructions on the critical path 👑. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-spec-insn-heuristic — enables the speculative instruction heuristic, which favors speculative instructions with greater dependency weakness ✨. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-rank-heuristic — enables the rank heuristic, which favors the instruction belonging to a basic block with greater size or frequency 🔥. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-last-insn-heuristic — enables the last-instruction heuristic, which favors the instruction that's less dependent on the last instruction scheduled 🎀. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.
• -fsched-dep-count-heuristic — enables the dependent-count heuristic, which favors the instruction that has more instructions depending on it 💖. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher.

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• -fspeculatively-call-stored-functions — this one is lowkey wild 🦋. It attempts to convert indirect calls of function pointers into pointers loaded from a structure field, if all visible stores to that field store just a single candidate. When it does that, it turns the call into a conditional deciding between the direct call and the original indirect one. These speculative calls often unlock more optimizations, like inlining. And if they seem useless after further optimization, they get converted back into their original form. No harm done 💅
• -freschedule-modulo-scheduled-loops — modulo scheduling happens before traditional scheduling. So if a loop is modulo scheduled, later scheduling passes may change its schedule 😭. Use this option to control that behavior.
• -fselective-scheduling — schedule instructions using the selective scheduling algorithm, which runs instead of the first scheduler pass ⭐
• -fselective-scheduling2 — schedule instructions using the selective scheduling algorithm, which runs instead of the second scheduler pass ⭐⭐
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling 🌈. This has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
• -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops 💫. This has no effect unless -fsel-sched-pipelining is turned on.

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• -fsemantic-interposition — some object formats, like ELF, let the dynamic linker interpose symbols 🙈. Basically that means for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining, or other optimizations, because the function or variable in question might change on it. While that feature is useful — say, for rewriting memory allocation functions with a debugging implementation — it's expensive when it comes to code quality 💀. With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where interposition is never allowed to change semantics) and for symbols explicitly declared weak.
• -fshrink-wrap — emits function prologues only before the parts of the function that actually need them, instead of at the top of the function 🎀. Enabled by default at -O and higher.
• -fshrink-wrap-separate — shrink-wraps separate parts of the prologue and epilogue separately, so those parts are only executed when needed ✨. On by default, but it has no effect unless -fshrink-wrap is also turned on and the target supports this.
• -fcaller-saves — enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls 💅. That allocation only happens when it seems to result in better code. This option is always enabled by default on certain machines — usually the ones that have no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os.
• -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them 🔥. Enabled by default at -O1 and higher.
• -fipa-ra — uses caller-save registers for allocation if those registers aren't used by any called function 🙌. In that case you don't need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. Enabled at levels -O2, -O3, -Os, but it's disabled if the generated code will be instrumented for profiling (-p, or -pg), or if the callee's register usage can't be known exactly — which happens on targets that don't expose prologues and epilogues in RTL.
• -fconserve-stack — attempt to minimize stack usage 🍓. The compiler tries to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.

---

• -ftree-reassoc — perform reassociation on trees 🌳. Enabled by default at -O1 and higher.
• -fcode-hoisting — perform code hoisting 🎈. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. It's especially useful as a code size optimization, but it often helps for code speed as well. Enabled by default at -O2 and higher.
• -ftree-pre — perform partial redundancy elimination (PRE) on trees ✨. Enabled by default at -O2 and -O3.
• -ftree-partial-pre — makes partial redundancy elimination (PRE) more aggressive 😤. Enabled by default at -O3.
• -ftree-forwprop — does forward propagation on trees ✨ this flag is enabled by default at -O1 and higher.
• -ftree-fre — does 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 — does hoisting of loads from conditional pointers on trees 🎀 this pass is enabled by default at -O1 and higher.
• -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction 🦋 this flag is enabled by default at -O2 and higher.
• -ftree-copy-prop — does copy propagation on trees 💖 this pass eliminates unnecessary copy operations. This flag is enabled by default at -O1 and higher.
• -fipa-pure-const — discovers which functions are pure or constant 🌸 enabled by default at -O1 and higher.
• -fipa-reference — discovers which static variables do not escape the compilation unit 🌈 enabled by default at -O1 and higher.
• -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables 👑 enabled by default at -O1 and higher.
• -fipa-reorder-for-locality — groups call chains close together in the binary layout to improve code locality and minimize jump distances between frequently called functions 🔥 unlike -freorder-functions this pass considers the call chains between functions and groups them together, rather than grouping all hot/normal/cold/never-executed functions into separate sections. Unlike -fprofile-reorder-functions it aims to improve code locality throughout the runtime of the program rather than focusing on program startup. This option is incompatible with an explicit -flto-partition= option since it enforces a custom partitioning scheme. If using this option it is recommended to also use profile feedback, but this option is not enabled by default otherwise.
• -fipa-stack-alignment — reduces stack alignment on call sites if possible 💜 enabled by default.
• -fipa-pta — does interprocedural pointer analysis and interprocedural modification and reference analysis 🫶 this option can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at any optimization level.
• -fipa-profile — does interprocedural profile propagation 💫 the functions called only from cold functions are marked as cold. Also functions executed once (such as "cold", "noreturn", static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at -O1 and higher.
• -fipa-modref — does interprocedural mod/ref analysis 🧚 this optimization analyzes the side effects of functions (memory locations that are modified or referenced) and enables better optimization across the function call boundary. This flag is enabled by default at -O1 and higher.
• -fipa-cp — does interprocedural constant propagation ⭐ this optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. This optimization can substantially increase performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os and -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -fipa-cp-clone — does function cloning to make interprocedural constant propagation stronger 💥 when enabled, interprocedural constant propagation performs function cloning when externally visible function can be called with constant arguments. Because this optimization can create multiple copies of functions, it may significantly increase code size. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
• -fipa-bit-cp — when enabled, does interprocedural bitwise constant propagation 🎀 this flag is enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled.
• -fipa-vrp — when enabled, does interprocedural propagation of value ranges 🌸 this flag is enabled by default at -O2. It requires that -fipa-cp is enabled.
• -fipa-icf-functions, -fipa-icf-variables, -fipa-icf — does Identical Code Folding for functions (-fipa-icf-functions), read-only variables (-fipa-icf-variables), or both (-fipa-icf) 🦋 the optimization reduces code size and may disturb unwind stacks by replacing a function by an equivalent one with a different name. The optimization works more effectively with link-time optimization enabled.

Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not same — there are equivalences that are found only by GCC and equivalences found only by Gold.

-fipa-icf is enabled by default at -O2 and -Os.

• -flate-combine-instructions — enables two instruction combination passes that run relatively late in the compilation process ✨ one of the passes runs before register allocation and the other after register allocation. The main aim of the passes is to substitute definitions into all uses.

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

• -flive-patching=level — controls GCC's optimizations to produce output suitable for live-patching 💅 if the compiler's optimization uses a function's body or information extracted from its body to optimize/change another function, the latter is called an impacted function of the former. If a function is patched, its impacted functions should be patched too.

The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when inlining a function into its caller, cloning a function and changing its caller to call this new clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.

Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In order to control the number of impacted functions and more easily compute the list of impacted function, IPA optimizations can be partially enabled at two different levels.

The level argument should be one of the following:

• inline-clone — only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining 🫶 as a result, when patching a function, all its callers and its clones' callers are impacted, therefore need to be patched as well.

-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref

• inline-only-static — only enables inlining of static functions 🌈 as a result, when patching a static function, all its callers are impacted and so need to be patched as well.

In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp

When -flive-patching is specified without any value, the default value is inline-clone.

This flag is disabled by default.

Note that -flive-patching is not supported with link-time optimization (-flto). 😭

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer (with -fdelete-null-pointer-checks enabled) or a division by zero 💀 it isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. This flag is enabled by default at -O2 and higher.
• -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a returns_nonnull or nonnull attribute 🎀 it isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by -O2 in the future.
• -ftree-sink — does forward store motion on trees 🌸 this flag is enabled by default at -O1 and higher.
• -ftree-bit-ccp — does sparse conditional bit constant propagation on trees and propagates pointer alignment information 💖 this pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. It requires that -ftree-ccp is enabled.
• -ftree-ccp — does sparse conditional constant propagation (CCP) on trees ⭐ this pass only operates on local scalar variables and is enabled by default at -O1 and higher.
• -fssa-backprop — propagates information about uses of a value up the definition chain in order to simplify the definitions 🌈 for example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default at -O1 and higher.
• -fssa-phiopt — does pattern matching on SSA PHI nodes to optimize conditional code 👑 this pass is enabled by default at -O1 and higher, except for -Og.
• -ftree-switch-conversion — does conversion of simple initializations in a switch to initializations from a scalar array ✨ this flag is enabled by default at -O2 and higher.
• -ftree-tail-merge — looks for identical code sequences 🔥 when found, replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.
• -ftree-cselim — does conditional store elimination on trees 🌳 like it's cleaning out a closet. Enabled by default at -O1 and higher, but only on targets that actually have conditional move instructions. iykyk 💅
• -ftree-dce — dead code elimination (DCE) on trees 🪦 bestie, if it's dead we're literally not keeping it. On by default at -O1 and higher.
• -ftree-builtin-call-dce — conditional dead code elimination (DCE), but specifically for calls to built-in functions that might set errno yet are otherwise free of side effects 🫶. On by default at -O2 and higher — unless -Os is also specified, in which case nope 🙈.
• -ffinite-loops — tells the compiler to assume a loop with an exit will eventually take that exit and not loop forever ⏳. That means it can straight-up remove loops that otherwise have no side-effects, because it's not counting "eventual endless looping" as a side effect. Enabled by default at -O2 for C++ with -std=c++11 or higher. main character assumption energy fr 💀
• -ftree-dominator-opts — a whole lil spa day of simple scalar cleanups based on a dominator tree traversal 🌸: constant/copy propagation, redundancy elimination, range propagation and expression simplification. It also does jump threading (to reduce jumps to jumps 🦘). Enabled by default at -O1 and higher.
• -ftree-dse — dead store elimination (DSE) on trees 🗑️. A dead store is when you store into a memory location and then it gets overwritten by another store with no loads in between — so the earlier store can just be deleted, she's irrelevant. Enabled by default at -O1 and higher.
• -ftree-ch — loop header copying on trees 👑. It's a slay because it increases the effectiveness of code motion optimizations, and it also saves one jump. Enabled by default at -O1 and higher. Not enabled for -Os, since it usually increases code size (the tradeoff was not worth it, ngl).
• -ftree-loop-optimize — loop optimizations on trees 🔁. Enabled by default at -O1 and higher.
• -ftree-loop-linear, -floop-strip-mine, -floop-block — loop nest optimizations ✨. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. So like, check that first bestie 🫶
• -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 🌀. It's a generic loop nest optimizer based on the Pluto optimization algorithms, and it calculates a loop structure optimized for data-locality and parallelism. This option is experimental ⚠️ like bestie it works but she's still in beta.
• -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 actually profitable to parallelize the loops. no vibe check, just vibes 😭
• -ftree-coalesce-vars — while transforming the program out of the SSA representation, it attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries 🧚. Heads up though: this may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments 💀. In the negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise (she's basically chilling).
• -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops into branch-less equivalents 🎀. The intent is to remove control-flow from the innermost loops so the vectorization pass has an easier time handling these loops. Enabled by default if vectorization is enabled.
• -ftree-loop-distribution — performs loop distribution 🌈. This can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. Example time, bestie — this 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 ✨ the way that loop just became a memset? iconic.

• -floop-interchange — performs loop interchange outside of graphite 🦋. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. So like, this loop:

        for (int i = 0; i < N; i++)
          for (int j = 0; j < N; j++)
            for (int k = 0; k < N; k++)
              c[i][j] = c[i][j] + a[i][k]*b[k][j];

is transformed to:

        for (int i = 0; i < N; i++)
          for (int k = 0; k < N; k++)
            for (int j = 0; j < N; j++)
              c[i][j] = c[i][j] + a[i][k]*b[k][j];

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.
• -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 where figuring out the number of iterations requires complicated analysis 😵‍💫. Later optimizations then may determine the number easily. Useful especially in connection with unrolling 🎀
• -ftree-scev-cprop — performs final value replacement 🔄. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, it replaces uses of the final value by such a computation — provided it is sufficiently cheap 💸. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher.
• -fivopts — performs induction variable optimizations on trees 🌀 (strength reduction, induction variable merging and induction variable elimination). Enabled by default at -O1 and higher.
• -ftree-parallelize-loops, -ftree-parallelize-loops=n — parallelizes loops, i.e., splits their iteration space to run in multiple threads 🧵🔥. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread ⚠️

When a positive value n is specified, the number of threads is fixed at compile time and cannot be changed after compilation. The compiler generates #pragma omp parallel num_threads(n).

When used without =n (i.e., -ftree-parallelize-loops), the number of threads is determined at program execution time via the OMP_NUM_THREADS environment variable 🌱. If OMP_NUM_THREADS is not set, the OpenMP runtime automatically detects the number of available processors and uses that value. This enables creating binaries that adapt to different hardware configurations without recompilation — lowkey the whole point, and we love her for it 💖

• -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 — performing 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. efficiency queen 👑
• -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. go off bestie 💅
• -ftree-slp-vectorize — bestie, this does basic block vectorization on trees ✨ It's on by default at -O2, and also gets switched on by -ftree-vectorize, -fprofile-use, and -fauto-profile. Free speed, no cap.
• -ftrivial-auto-var-init=choice — okay so this one is a whole thing, so buckle up 🎀 This initializes automatic variables or temporary objects with either a pattern or with zeroes, to increase the security and predictability of a program by preventing uninitialized memory disclosure and use.

Here's the tea though: 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 or temporary objects, and the compiler will perform optimization as if the variable were uninitialized 👀 With this option, GCC will also initialize any padding of automatic variables or temporary objects that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables whose initialization is bypassed through "switch" or "goto" statement. Use -Wtrivial-auto-var-init to report all such cases 💅

The three values of choice are:

1. uninitialized — doesn't initialize any automatic variables.
2. pattern — initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump and without being values that programmers can rely on for useful program semantics. The current value is byte-repeatable pattern with byte "0xFE". The values used for pattern initialization might be changed in the future.
3. zero — initialize automatic variables with zeroes.

The default is uninitialized except for C++26, in which case if -ftrivial-auto-var-init= is not specified at all automatic variables or temporary objects are zero initialized, but zero initialization of padding bits does not happen 🦋

Lowkey important note: 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" standard attribute or the C++26 "[[indeterminate]]".

• -fvect-cost-model=model — alter the cost model used for vectorization, bestie. The model argument should be one of unlimited, dynamic, cheap or very-cheap ⭐
  • With the unlimited model the vectorized code-path is assumed to be profitable.
  • With the dynamic model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop.
  • The cheap model disables vectorization of loops where doing so would be cost prohibitive, for example due to required runtime checks for data dependence or alignment, but otherwise is equal to the dynamic model.
  • The very-cheap model disables vectorization of loops when any runtime check for data dependence or alignment is required; it also disables vectorization of epilogue loops but otherwise is equal to the cheap model.

The default cost model depends on other optimization flags and is either dynamic or cheap.

• -fsimd-cost-model=model — same energy but specifically for vectorization of loops marked with the OpenMP simd directive 🌈 The model argument should be one of unlimited, dynamic, cheap. All values of model have the same meaning as described in -fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.
• -ftree-vrp — perform Value Range Propagation on trees 🔥 This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled, iykyk.
• -fsplit-paths — split paths leading to loop backedges ✂️ This can improve dead code elimination and common subexpression elimination. This is enabled by default at -O3 and above.
• -fsplit-ivs-in-unroller — enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes 💖

A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass 😭

This optimization is enabled by default.

• -fvariable-expansion-in-unroller — with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code 👑

This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.

• -fpartial-inlining — inline parts of functions, not the whole thing, we're being selective ✨ This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

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

• -fpredictive-commoning — perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops 🌀

This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -fprefetch-loop-arrays — if supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays 🧚

This option may generate better or worse code; results are highly dependent on the structure of loops within the source code. So like, yolo, but responsibly.

Disabled at level -Os.

• -fno-printf-return-value — do not substitute constants for known return value of formatted output functions such as sprintf, snprintf, vsprintf, and vsnprintf (but not printf of fprintf) 🙈 This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when -fprintf-return-value is in effect, both the branch and the body of the if statement (but not the call to snprint) can be optimized away when i is a 32-bit or smaller integer because the return value is guaranteed to be at most 8:

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  ...

The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default 💜

• -fno-peephole, -fno-peephole2 — disable any machine-specific peephole optimizations 🎀 The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.

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

• -fno-guess-branch-probability — do not guess branch probabilities using heuristics, bestie, we're keeping it honest 💀

GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand 🌸

It is also possible to specify expected probability of the expression with __builtin_expect_with_probability built-in function.

The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

• -freorder-blocks — reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality 🔥

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

• -freorder-blocks-algorithm=algorithm — use the specified algorithm for basic block reordering, pick your fighter 💅 The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.

The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.

• -freorder-blocks-and-partition — in addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance 💖

That whole optimization just dips out automatically when exception handling or unwind tables are around (like, on targets that use setjump/longjump or some target-specific scheme), and it also bails for linkonce sections, for functions with a user-defined section attribute, and on any architecture that doesn't support named sections 💅. Oh and when -fsplit-stack is in play this option is not enabled by default (to dodge linker errors, bestie), but you can totally enable it explicitly if your linker is actually working 🫶.

Enabled for x86 at levels -O2, -O3, -Os ✨.

• -freorder-functions — reorders functions in the object file so the code locality gets better 🎀. Unlike -fipa-reorder-for-locality, this one prioritises grouping all functions within a category (hot/normal/cold/never-executed) together. It does this using the special subsections .text.hot for the most frequently executed functions and .text.unlikely for the ones that (probably) never run. The reordering is done by the linker, so the object file format must support named sections and the linker must place them reasonably. Ngl, this option isn't effective unless you either give it profile feedback (peep -fprofile-arcs for the deets) or manually annotate functions with the hot or cold attributes 😌.

Enabled at levels -O2, -O3, -Os ⭐.

• -fstrict-aliasing — lets the compiler assume the strictest aliasing rules that apply to the language being compiled 👑. For C (and C++), this activates optimizations based on the type of the expressions. Specifically: you can't access an object of one type through an expression of a different type, unless the types are compatible types, differ only in signedness or qualifiers, or the expression has a character type. Accessing scalar objects via a corresponding vector type is also allowed ✨.

For example, an unsigned int can alias an int, but it can not alias a void* or a double. A character type may alias any other type 💜.

Pay special attention to code like this, okay? 👀

union a_union {
  int i;
  double d;
};

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

Reading from a different union member than the one you most recently wrote to (it's called "type-punning", iykyk) is super common. Even with -fstrict-aliasing, type-punning is allowed in C provided the memory is accessed through the union type 🫶. In ISO C++, type-punning through a union type is undefined behavior, but GCC supports it as an extension. So the code above works as expected. However, this next one might not 😭:

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

Similarly, accessing by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior — even if the cast uses a union type, like:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

🚨 The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os. Watch yourself!

• -fipa-strict-aliasing — controls whether the rules of -fstrict-aliasing get applied across function boundaries 🦋. Small note: if multiple functions get inlined into a single function, those memory accesses are no longer considered to be crossing a function boundary 💅.

The -fipa-strict-aliasing option is enabled by default and is effective only in combination with -fstrict-aliasing ✨.

• -falign-functions
• -falign-functions=n
• -falign-functions=n:m
• -falign-functions=n:m:n2
• -falign-functions=n:m:n2:m2 — aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes 🎀. This makes sure at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. It's an optimization of code performance, and alignment is ignored for functions considered cold. If you need alignment for all functions, use -fmin-function-alignment instead 😌.

If m isn't specified, it defaults to n.

Examples, because these hit different: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if it can do so by skipping 23 bytes or less, and -falign-functions=32:7 aligns to the next 32-byte boundary only if it can do so by skipping 6 bytes or less ✨.

The second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if 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 🫶.

-fno-align-functions and -falign-functions=1 are equivalent and mean functions are not aligned, periodt 💅.

If n isn't specified or is zero, it uses a machine-dependent default. The maximum allowed n option value is 65536 👑.

Enabled at levels -O2, -O3 ⭐.

• -flimit-function-alignment — with this one on, the compiler tries to avoid unnecessarily overaligning functions 🎀. It attempts to tell the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. Clever, honestly.
• -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 of this option are analogous to 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, it uses 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 get executed a lot, this makes up for any execution of those dummy padding instructions. It's an optimization of code performance, and alignment is ignored for loops considered cold 🥶.

If -falign-labels is greater than this value, then its value is used instead.

Parameters are analogous to -falign-functions. -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, it uses 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, but only for branch targets that can only be reached by jumping 🦘. In this case, no dummy operations need to be executed. It's an optimization of code performance, and alignment is ignored for jumps considered cold 🥶.

If -falign-labels is greater than this value, then its value is used instead.

Parameters are analogous to -falign-functions. -fno-align-jumps and -falign-jumps=1 are equivalent and mean loops are not aligned 💅.

If n isn't specified or is zero, it uses a machine-dependent default. The maximum allowed n option value is 65536 🫶.

Enabled at levels -O2, -O3 ⭐.

• -fmin-function-alignment — specifies the minimal alignment of functions to the next power-of-two greater than or equal to n 🎀. Unlike -falign-functions, this alignment also gets applied to all functions (even the cold ones 🥶). And the alignment is also not affected by -flimit-function-alignment 👑.
• -fno-allocation-dce — don't remove unused C++ allocations (using operator new and operator delete) during dead code elimination 🙈.

See also -fmalloc-dce 👀.

• -fallow-store-data-races — allows the compiler to do optimizations that may introduce new data races on stores, without proving that the variable can't be concurrently accessed by other threads 🔥. Does not affect optimization of local data. It's safe to use this option if you know global data won't be accessed by multiple threads 🫶.

Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value already sitting in memory to be re-written with that same value 💅. That kind of re-writing is safe single-threaded 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 — don't reorder top-level functions, variables, and asm statements 🙅. Output them in the same order 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 it 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 — okay so this one constructs webs (as in the register-allocation kind, not the spider kind 🕸️) and gives each web its very own pseudo register. Why do we care? Because it lets the register allocation pass work directly on pseudos, and it also lowkey strengthens a bunch of other optimization passes — CSE, the loop optimizer, and the trivial dead code remover all eat this up. 💅 The catch? It can make debugging straight-up impossible, because variables no longer stay in a "home register". So like, iconic for shipping, cursed for gdb girlies. 😭

Fun fact: it's enabled by default with -funroll-loops. ✨

• -fwhole-program — this is the compiler equivalent of "no thoughts, just vibes, this file IS the whole program". 💀 It assumes the current compilation unit represents the entire program being compiled. So every public function and variable — except main and the ones merged by the attribute externally_visible — becomes a static function, and they get optimized way more aggressively by the interprocedural optimizers. Slay, but be careful, it's giving main character syndrome for real.

With -flto this option has limited use. In most cases the precise list of symbols used or exported from the binary is known from the resolution info passed to the link-time optimizer by the linker plugin. It's still useful if no linker plugin is used, or during the incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).

• -flto[=n] — the big one, bestie. 🔥 This runs the standard link-time optimizer. When you invoke it with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. Then when the object files get 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. It's literally giving "we are one" energy. 🫶

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

So 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. And because both foo.o and bar.o are merged into a single image, all the interprocedural analyses and optimizations in GCC work across the two files as if they were a single one. That means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. Crossover episode, we love to see it. 🎀

Another (simpler) way to enable link-time optimization is:

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

The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation, and optimizes them as usual to produce myprog.

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

To make whole-program optimization effective, it is necessary to make certain assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions — which leads to more aggressive optimization decisions. 👑

When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster, but you cannot perform a regular, non-LTO link on them. Pick your fighter. ⚔️

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. She's organized like that. 💅

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

If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time. So like, do it at both stages or don't bother, fr fr. 🙈

There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm, and all the -m target flags.

The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:

B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. Like, the compiler is not taking feedback on this one, bestie. 🚫

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically, -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time.

Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code not consistent with the setting at compile time. Slightly chaotic, but that's the vibe check. 🌸

When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time.

To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. (Read: don't do it, it's giving messy. 💀)

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages. So like, it'll warn you, but the chaos is still on you. 😭

Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Multicultural queen behavior. 🌈

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. It's giving teamwork, it's giving collaboration. 🫶

If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Best of both worlds, exactly like we deserve. ✨

Okay so bestie, link-time optimizations do not require the whole program to be present to operate. If the program doesn’t require any symbols to be exported, it is possible to combine -flto and -fwhole-program to let the interprocedural optimizers use more aggressive assumptions, which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when the linker plugin is active (see -fuse-linker-plugin). 💅

The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. 😭

Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. Noted, bestie. 🫶

If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. 🛠️

You can also specify -flto=jobserver to use GNU make’s job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make’s job server.

Use -flto=auto to use GNU make’s job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. ✨

• -flto-partition=alg — Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a partitioning mirroring the original source files, or balanced to specify partitioning into equally sized chunks (whenever possible), or max to create a new partition for every symbol where possible, or cache to balance chunk sizes while keeping related symbols together for better caching in incremental LTO. 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-incremental=path — Enable incremental LTO, with its cache in the given existing directory. Can significantly shorten edit-compile cycles with LTO. When used with LTO (-flto), the output of translation units inside LTO is cached. Cached translation units are likely to be encountered again when recompiling with small code changes, leading to recompile time reduction. Multiple GCC instances can use the same cache in parallel. 🎀
• -flto-incremental-cache-size=n — Specifies number of cache entries in incremental LTO after which to prune old entries. This is a soft limit; temporarily there may be more entries. 🍓
• -flto-compression-level=n — This option specifies the level of compression used for intermediate language written to LTO object files, and is 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 is not given, a default balanced compression setting is used.
• -flto-toplevel-asm-heuristics — Enables heuristics to find symbols used in top-level basic "asm". This will restrict link-time optimizations that could cause renaming or deletion of such symbols, which would result in missing symbol errors by linker. This flag is intended for projects that have not converted to using top-level extended "asm", which specify the usage directly without any false positives. The heuristics are simple and do not parse the assembly. The heuristics scan through top-level assembly for all possible identifiers; if an identifier is found among declared symbols, the symbol will be marked to restrict link-time optimizations. Static symbols disable more optimizations. Identifiers followed by ':' disable more optimizations as well, because they might be a locally defined symbol in assembly, even when the declaration is marked extern. 💀
• -fuse-linker-plugin — Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer. This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it. This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold). 🔥
• -ffat-lto-objects — Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time. -fno-fat-lto-objects improves compilation time over plain LTO, but 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, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. Enabled at levels -O1, -O2, -O3, -Os.
• -ffold-mem-offsets, -fno-fold-mem-offsets — Try to eliminate add instructions by folding them in memory loads/stores. Enabled at levels -O2, -O3.
• -fcprop-registers — After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Enabled at levels -O1, -O2, -O3, -Os.
• -fprofile-correction — Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. This option is enabled by -fauto-profile.
• -fprofile-partial-training — With -fprofile-use, all portions of programs not executed during training runs 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, it may contain functions specific to a given hardware and training may not cover all hardware configurations the program later runs on.) With -fprofile-partial-training, profile feedback is ignored for all functions not executed during the training runs, causing them to be optimized as if they were compiled without profile feedback. This leads to better performance when the training is not representative at the cost of significantly bigger code. 🌸
• -fprofile-use, -fprofile-use=path — Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions

Before you can use this option, you must first generate profiling information. By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile). If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. 🦋

• -fauto-profile, -fauto-profile=path — Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correction

Okay so picking right back up with the AutoFDO tea ☕✨: path is the name of a file containing AutoFDO profile information. If you leave it out, it defaults to fbdata.afdo in the current directory. 💅

To actually produce an AutoFDO profile data file, you gotta run your program with the perf utility on a supported GNU/Linux target system. For more info, see <https://perfwiki.github.io/main/>. 🐧🔍

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data into a format GCC can use. You also have to supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>. 🛠️✨

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

• -fauto-profile-inlining — when auto-profile is available, inline all relevant functions which was inlined in the tran run before reading the profile feedback. This improves context sensitivity of the profile. Enabled by default. 🔁💖

---

The following options control compiler behavior regarding floating-point arithmetic 🧮✨. These options trade off between speed and correctness. All must be specifically enabled — no sneaky defaults doing the work for you, bestie.

• -fexcess-precision=style — this option lets you control excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard, and where floating-point types get interchanged. An example of such a target is x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format. For most programs, the excess precision is harmless, but some programs may rely on the requirements of the C or C++ language standards for handling IEEE values. 😮‍💨📐

By default, -fexcess-precision=fast is in effect; this means 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. 😭💀

• -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 has generally been subsumed by -fexcess-precision=standard, which is more general. If you do use -ffloat-store, you may need to modify your program to explicitly store intermediate computations in temporary variables since -ffloat-store handles rounding to IEEE format only on assignments and not casts as -fexcess-precision=standard does. 🧠💾
• -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 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. 😌⚡

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 — allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. 😳⚠️

This option is not turned on by any -O option 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. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math. 🔥🧨

The default is -fno-unsafe-math-optimizations. 🛑✨

• -fassociative-math — allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52). May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect. 🔄⚠️

The default is -fno-associative-math. 🛑✨

• -freciprocal-math — allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 🔁💔

The default is -fno-reciprocal-math. 🛑✨

• -ffinite-math-only — allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs. 🚫♾️

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. 😬⚡

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. 🥶0️⃣

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 this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example. 🪤🚫

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. 😬⚡

The default is -ftrapping-math. 🛑✨

Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -frounding-math to specify the default state for FENV_ACCESS. 🔮📜

• -frounding-math — disable transformations and optimizations that assume default floating-point rounding behavior (round-to-nearest). This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe 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. 🚨🧮

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

The default is -fno-signaling-nans. 🛑✨

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. 🧪💀

• -fsingle-precision-constant — bestie, this makes floating-point constants be treated as single precision instead of quietly getting upgraded to double-precision constants. no sneaky silent promotion 💅
• -fcx-limited-range — when it's on, you're telling the compiler: "nah, we don't need a range reduction step when doing complex division." also it skips checking whether the result of a complex multiplication or division is "NaN + I*NaN" with a rescue attempt for that situation. it's turned on by -ffast-math. ✨

this option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. but also — important — the option applies to all languages, not just C. 🌈

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. range reduction is done as part of complex division, but there's still no checking whether the result of a complex multiplication or division is "NaN + I*NaN" with an attempt to rescue it in that case. 🫶
• *-fcx-method=method* — complex multiplication and division follow whatever method you state. the method argument should be one of limited-range, fortran or stdc. 👑

the default is to honor language specific constraints, which means fortran for Fortran and stdc otherwise. iykyk

---

okay so the next batch of options? they control optimizations that may improve performance, but none of the -O options enable them. and heads up, this section includes experimental options that may produce broken code 💀

• -fbranch-probabilities — after you've run a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities to get better optimizations based on how many times each branch was actually taken. when a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. the info in that data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. file naming details live over in -fprofile-arcs. 🍓

with -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. those 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. slay.

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

• -fprofile-values — if you combine it with -fprofile-arcs, it adds code so that some data about values of expressions in the program gets 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, which collects the first time of execution of a function and orders those functions in ascending order. the goal? optimize program startup through more efficient loading of text segments. literally main character energy for your startup time. ⭐

enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this tells the compiler to add code to gather info 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 — attempts to avoid false dependencies in scheduled code by using registers left over after register allocation. this optimization most benefits processors with lots of registers. but depending on the debug info format your target adopted, it can make debugging impossible, since variables no longer stay in a "home register". 😭

enabled by default with -funroll-loops.

• -fschedule-fusion — does a target dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently when they're adjacent to each other in the instruction flow. 🔥

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

• -fdep-fusion — detects macro-op fusible pairs made of single-use instructions and their uses, and places those pairs together in the instruction stream to increase fusion opportunities in hardware. this pass runs once before register allocation, and another time before register renaming. 💜

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

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

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

• -funroll-loops — unrolls loops whose number of iterations can be determined at compile time or upon entry to the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. it also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). this option makes code larger, and may or may not make it run faster. 🙈

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

• -funroll-all-loops — unrolls all loops, even if their number of iterations is uncertain when the loop is entered. this usually makes programs run more slowly 😬. -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops for which there's enough information that they don't roll much (from profile feedback or static analysis). it also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). 🎀

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

• -fmalloc-dce — controls whether "malloc" (and its variants such as "calloc" or "strdup") can be optimized away, provided its return value is only used as a parameter of a "free" call or compared with "NULL". if -fmalloc-dce=1 is used, only calls to "free" are allowed, while with -fmalloc-dce=2 comparisons with "NULL" pointer are also considered safe to remove. 💅

the default is -fmalloc-dce=2. see also -fallocation-dce.

• -fmove-loop-invariants — enables the loop invariant motion pass in the RTL loop optimizer. enabled at level -O1 and higher, except for -Og. 🦋
• -fmove-loop-stores — enables the loop store motion pass in the GIMPLE loop optimizer. this moves invariant stores to after the end of the loop, in exchange for carrying the stored value in a register across the iteration. note: for this option to have an effect, -ftree-loop-im has to be enabled as well. enabled at level -O1 and higher, except for -Og. 🌸
• -fsplit-loops — splits a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other. 🔪

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

• -funswitch-loops — moves branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to the result of the condition). 👑

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

• -fversion-loops-for-strides — if a loop iterates over an array with a variable stride, it 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's 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.

okay bestie, listen: only use these options when there are significant benefits from doing so. when you specify them, the assembler and linker create larger object and executable files and are also slower. these options affect code generation — they prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. an example of such an optimization is relaxing calls to short call instructions. 😭

• -fstdarg-opt — optimizes the prologue of variadic argument functions with respect to usage of those arguments. 🫶
• -fsection-anchors — tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. this transformation can help reduce the number of GOT entries and GOT accesses on some targets. 💜

for example, the implementation of the following function "foo":

static int a, b, c;
int foo (void) { return a + b + c; }

usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. the effect is similar to the following pseudocode (which isn't valid C):

int foo (void)
{
  register int *xr = &x;
  return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}

not all targets support this option. 🙈

• -fzero-call-used-regs=choice — zeroes out call-used registers when a function returns, which boosts program security by either mitigating Return-Oriented Programming (ROP) attacks or stopping info from leaking through registers. The possible values of choice are the same as for the zero_call_used_regs attribute, and the default is skip. You can control this behavior for a specific function by using the function attribute zero_call_used_regs. 🛡️✨

✨ Program Instrumentation Options ✨

GCC supports a bunch of command-line options that control adding run-time instrumentation to the code it normally generates. Like, one purpose of instrumentation is collecting profiling statistics for finding program hot spots, code coverage analysis, or profile-guided optimizations. Another whole class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, plus deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook that can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. 💅🦋

• -p, --profile, -fprofile, -pg — generate extra code to write profile information suitable for the analysis program prof (for -p, --profile, and -fprofile) 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. You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. 📊
• -fprofile-arcs — adds code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable. When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o). Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. 🎀
• -fcondition-coverage — adds code so that program conditions are instrumented. During execution the program records what terms in a conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read with gcov --conditions. 🧚
• -fpath-coverage — adds code so that the paths taken are tracked. During execution the program records the prime paths taken. The number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up instrumentation if the approximate number of paths exceeds the limit controlled by -fpath-coverage-limit. The result can be read with gcov --prime-paths --prime-paths-lines --prime-paths-source, 🌈
• -fpath-coverage-limit=limit — the threshold at which point -fpath-coverage gives up on instrumenting a function. This limit is approximate and conservative, as GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and gives up if the threshold is reached before finding all the paths. This option is not for fine grained control over which functions to instrument — rather it is intended to limit the effect of path explosion and keep compile times reasonable. The default is 250000. 💀
• --coverage, -coverage — this option is used to compile and link code instrumented for coverage analysis. The options -coverage and --coverage are equivalent; both are a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details. 👑
  • Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
  • Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
  • Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
  • Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting. Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged.
  • For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
  • For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information.

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. 🌸

With -fcondition-coverage, for each conditional in your program GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression. 🫶

With -fpath-coverage, GCC finds and enumerates and records the taken prime paths of each function, unless the number of paths would exceed the limit controlled by -fpath-coverage-limit. If the limit is exceeded the function is not instrumented as if -fpath-coverage was not used. A prime path is the longest sequence of unique blocks, except possibly the first and last, which is not a subpath of any other path. ⭐

• -ftest-coverage — produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. 🎀
• -fprofile-abs-path — automatically convert relative source file names to absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories. 🔥
• -fprofile-dir=path — set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. See details about the file naming in -fprofile-arcs. See similar option -fprofile-note. When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:
  • %p — process ID.
  • %q{VAR} — value of environment variable VAR. 💜
• -fprofile-generate, -fprofile-generate=path — enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program. The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp. If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. To optimize the program based on the collected profile information, use -fprofile-use. 🦋
• -fprofile-info-section, -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. 💖

Okay so picture this: the linker could collect the input sections in one continuous memory block and define start and end symbols. 💅 A GNU linker script example that defines a linker output section is literally this:

.gcov_info      :
{
  PROVIDE (__gcov_info_start = .);
  KEEP (*(.gcov_info))
  PROVIDE (__gcov_info_end = .);
}

And a program could dump the profiling information registered in this linker set, like, for example, this whole thing:

#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;
}

Then 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. 🫶

---

Now, the flags. Buckle up, bestie — this batch is giving profile-and-sanitizer main character energy. ✨

• -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. 😭
• -fprofile-prefix-path=path — this one can be used in combination with profile-generate=profile_dir and profile-use=profile_dir to inform GCC where the base directory of the built source tree is.
  • By default, profile_dir will contain files with mangled absolute paths of all object files in the built project.
  • That is not desirable when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback, because the profile data will not be found during the optimized build.
  • In such setups, -fprofile-prefix-path=path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory.
• -fprofile-prefix-map=old=new — when compiling files residing in directory old, record profiling information (with --coverage) describing them as if the files resided in directory new instead. See also -ffile-prefix-map and -fcanon-prefix-map.
• -fprofile-update=method — alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code.

⚠️ Warning: when an application does not properly join all threads (or creates a detached thread), a profile file can still be corrupted.

• Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the default method is single.
• If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, libatomic is available, then the profile information is updated through calls to libatomic. If the target platform neither supports the required atomic operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications.
• For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state.

• -fprofile-filter-files=regex — instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons).
  • For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'.
• -fprofile-exclude-files=regex — instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons).
  • For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files that are located in the /usr/ folder.
• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — control level of reproducibility of profile gathered by -fprofile-generate. This makes it possible to rebuild program with same outcome which is useful, for example, for distribution packages. The values are multithreaded, parallel-runs, and serial.
  • With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before fork function is invoked.
  • Note that it is quite common that execution counts of some part of programs depends, for example, on length of temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-reproducible part of programs may be annotated by no_instrument_function function attribute. gcov-dump with -l can be used to dump gathered data and verify that they are indeed reproducible.
  • With -fprofile-reproducible=parallel-runs collected profile stays reproducible regardless the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of instrumented program in parallel (such as with make -j). This reduces quality of gathered data, in particular of indirect call profiling.
• -fsanitize=address — enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://github.com/google/sanitizers/wiki/AddressSanitizer for more details. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. When set to help=1, the available options are shown at startup of the instrumented program. See https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags for a list of supported options. The option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. Note that the only targets -fsanitize=hwaddress is currently supported on are x86-64 (only with -mlam=u48 or -mlam=u57 options) and AArch64, in both cases only in ABIs with 64-bit pointers. Similarly, -fsanitize=memtag-stack is currently only supported on AArch64 ABIs with 64-bit pointers.
  • When compiling with -fsanitize=address, you should also use -g to produce more meaningful output. To get more accurate stack traces, it is possible to use options such as -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls; this option is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). Using -fno-omit-frame-pointer also improves stack traces. Since multiple runs of the program may yield backtraces with different addresses due to ASLR (Address Space Layout Randomization), it may be desirable to turn ASLR off. On Linux, this can be achieved with setarch `uname -m` -R ./prog.
• -fsanitize=kernel-address — enable AddressSanitizer for Linux kernel. See https://github.com/google/kernel-sanitizers for more details.
• -fsanitize=hwaddress — enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to allow the detection of memory errors with a low memory overhead. Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html for more details. The run-time behavior can be influenced using the HWASAN_OPTIONS environment variable. When set to help=1, the available options are shown at startup of the instrumented program. The option cannot be combined with -fsanitize=thread or -fsanitize=address, and is currently only available on AArch64. 🦋
• -fsanitize=kernel-hwaddress — turns on Hardware-assisted AddressSanitizer so you can compile the Linux kernel with it 💅 it's like -fsanitize=kernel-address but with a whole different instrumentation method, and like -fsanitize=hwaddress but with the tweaks you need for kernel compilation. those tweaks exist so we (a) avoid hwasan library initialization calls and (b) account for the stack pointer having a different value in its top byte 🫶

Note: this option has different defaults than -fsanitize=hwaddress. instrumenting the stack and alloca calls are not on by default, and using a random frame tag is not implemented for kernel instrumentation. no cap.

• -fsanitize=memtag-stack — uses Memory Tagging Extension instructions instead of instrumentation so you can catch memory errors ✨ it's similar to HWASAN in that it's also a probabilistic method, bestie. heads up: this one is only available on those AArch64 architectures that support Memory Tagging Extensions 🌸
• -fsanitize=pointer-compare — instruments comparison operations (<, <=, >, >=) that have pointer operands 👀 you must combine it with either -fsanitize=kernel-address or -fsanitize=address, and 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 the invalid operation only when both pointers are non-null 🎀
• -fsanitize=pointer-subtract — instruments subtraction with pointer operands 🔥 same rules, bestie: it must be combined with either -fsanitize=kernel-address or -fsanitize=address, and it cannot be combined with -fsanitize=thread. Note: by default the check is disabled at run time — enable it by adding "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. using "detect_invalid_pointer_pairs=1" detects the invalid operation only when both pointers are non-null 💖
• -fsanitize=shadow-call-stack — enables ShadowCallStack, a security enhancement mechanism that protects programs against return address overwrites (think stack buffer overflows) 👑 the way it works: it saves a function's return address to a separately allocated shadow call stack in the function prologue, then restores the return address from the shadow call stack in the function epilogue. instrumentation only happens in functions that actually need to save the return address to the stack ✨

currently it only supports the aarch64 platform, and it's specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. for user space programs, runtime support is not currently provided in libc and libgcc — so if you wanna use this in user space you have to provide your own runtime support. and yes, it should be noted that this may cause the ABI rules to be broken 😭

on aarch64, the instrumentation makes use of the platform register "x18". this generally means any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18 — otherwise functions compiled without -ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer 💀

also, because there's no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. use -fno-exceptions to turn off exceptions 🙈

see <https://clang.llvm.org/docs/ShadowCallStack.html> for more details 🦋

• -fsanitize=thread — enables ThreadSanitizer, a fast data race detector 🔥 memory access instructions are instrumented to detect data race bugs. see <https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. the run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options. this option cannot be combined with -fsanitize=address, -fsanitize=leak.

when compiling with -fsanitize=thread, you should also use -g to produce more meaningful output, bestie 💅

note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions) 🌸

• -fsanitize=leak — enables LeakSanitizer, a memory leak detector 🫶 this option only matters for linking of executables. the executable is linked against a library that overrides "malloc" and other allocator functions. see <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. the run-time behavior can be influenced using the LSAN_OPTIONS environment variable. the option cannot be combined with -fsanitize=thread.
• -fsanitize=undefined — enables UndefinedBehaviorSanitizer, a fast undefined behavior detector ✨ various computations are instrumented to detect undefined behavior at runtime. see <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. the run-time behavior can be influenced using the UBSAN_OPTIONS environment variable. current suboptions are:
  • -fsanitize=shift — this enables checking that the result of a shift operation is not undefined 💜 note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. this option has two suboptions: -fsanitize=shift-base and -fsanitize=shift-exponent.
  • -fsanitize=shift-exponent — this enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument 🌈
  • -fsanitize=shift-base — if the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. 🎀
  • -fsanitize=integer-divide-by-zero — detect integer division by zero. iconic behavior honestly 💅
  • -fsanitize=unreachable — with this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message call instead. when reaching the "__builtin_unreachable" call, the behavior is undefined 💀
  • -fsanitize=vla-bound — this instructs the compiler to check that the size of a variable length array is positive 🌸
  • -fsanitize=null — this enables pointer checking. particularly, 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 by a NULL pointer 😭
  • -fsanitize=return — this enables return statement checking. programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. this option works in C++ only 🫶
  • -fsanitize=signed-integer-overflow — this enables signed integer overflow checking 🔥 we check that the result of "+", "*", and both unary and binary "-" does not overflow in the signed arithmetics. this also detects "INT_MIN / -1" signed division. note, integer promotion rules must be taken into account. that is, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — this enables instrumentation of array bounds. various out of bounds accesses are detected ✨ flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented — with the exception of flexible array member-like arrays for which "-fstrict-flex-arrays" or "-fstrict-flex-arrays=" options or "strict_flex_array" attributes say they shouldn't be treated like flexible array member-like arrays 👑
• -fsanitize=bounds-strict — this enables strict instrumentation of array bounds 💅 most out of bounds accesses are detected, including flexible array member-like arrays. initializers of variables with static storage are not instrumented.
• -fsanitize=alignment — this enables checking of alignment of pointers when they are 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 enables instrumentation of memory references using the "__builtin_dynamic_object_size" function. various out of bounds pointer accesses are detected 🦋
• -fsanitize=float-divide-by-zero — detect floating-point division by zero. unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs 🌈
• -fsanitize=float-cast-overflow — this enables floating-point type to integer conversion checking. we check that the result of the conversion does not overflow. unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. this option does not work well with "FE_INVALID" exceptions enabled 🙈
• -fsanitize=nonnull-attribute — this enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the "nonnull" function attribute ⭐
• -fsanitize=returns-nonnull-attribute — this enables instrumentation of return statements in functions marked with "returns_nonnull" function attribute, to detect returning of null values from such functions 💜
• -fsanitize=bool — this enables instrumentation of loads from bool. if a value other than 0/1 is loaded, a run-time error is issued 🔥
• -fsanitize=enum — this enables instrumentation of loads from an enum type. if a value outside the range of values for the enum type is loaded, a run-time error is issued 🎀

• -fsanitize=vptr — bestie this one instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, all so it can verify the referenced object has the correct dynamic type ✨
• -fsanitize=pointer-overflow — this instruments pointer arithmetics, and if the pointer arithmetics overflows, a run-time error gets issued 🫶
• -fsanitize=builtin — this one instruments the arguments to selected builtin functions, and if an invalid value gets passed to such arguments, a run-time error is issued. Case in point: passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option 💅

okay important vibe check: 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. no cap, just don't 🙈

Also, 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 — this disables all previously enabled sanitizers. And ngl, -fsanitize=all is not allowed, because some sanitizers cannot be used together 💀
• -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,... — sanitize global variables in selected user-defined sections. Heads up: si may contain wildcards ⭐
• -fsanitize-recover[=opts] — so -fsanitize-recover= controls error recovery mode for the sanitizers mentioned in the comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened 😭

Which means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. 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 is also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it 👑

Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while 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 🔥

real talk: syntax without an explicit opts parameter is deprecated. It's equivalent to specifying an opts list of: undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

• -fsanitize-address-use-after-scope — enables sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to none 🦋
• -fsanitize-trap[=opts] — this option instructs the compiler to report for the 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 be also specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs libubsan support, and -fsanitize-trap=vptr is not allowed 🙅

• -fsanitize-undefined-trap-on-error — this option is the deprecated equivalent of -fsanitize-trap=all 🎀
• -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation. It inserts a call to __sanitizer_cov_trace_pc into every basic block 🔥
• -fsanitize-coverage=trace-cmp — enables dataflow guided fuzzing code instrumentation. It inserts a call to __sanitizer_cov_trace_cmp1, __sanitizer_cov_trace_cmp2, __sanitizer_cov_trace_cmp4 or __sanitizer_cov_trace_cmp8 for integral comparison with both operands variable, or __sanitizer_cov_trace_const_cmp1, __sanitizer_cov_trace_const_cmp2, __sanitizer_cov_trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, plus __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], -fcf-protection — this enables code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. It prevents diverting the flow of control to an unexpected target, and it's intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP) 💅

The -fcf-protection= keywords are interpreted as follows:

• The value branch tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions.
• The value return implements checking of validity at the point of returning from a function.
• The value full is an alias for specifying both branch and return.
• The value check is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different -fcf-protection values. The value check is ignored at the compile time.
• The value none turns off instrumentation.

-fcf-protection is an alias for -fcf-protection=full. To override a previous -fcf-protection option on the command line, add -fcf-protection=none and then -fcf-protection=kind ⭐

The macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for 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.

• -fharden-compares — for every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call __builtin_trap if the results do not match. Use with -fharden-conditional-branches to cover all conditionals 🔥
• -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 💜
• -fharden-control-flow-redundancy — emit extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans do not form an execution path that is compatible with the control flow graph 🦋

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.

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 — though these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins 🙈

• -fhardcfr-skip-leaf — disable -fharden-control-flow-redundancy in leaf functions 🍓
• -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 is 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] — okay so when -fharden-control-flow-redundancy is doing its thing, this decides whether we check the recorded execution path against the control flow graph before noreturn calls. Your picks are: always (check, like, all of them 💅), no-xthrow (only the ones that aren't expected to hand control back to the caller via an exception — this is the default, bestie), nothrow (the ones that may not return through an exception either), or never (check none of them, we're chilling).

And here's the tea 🫖: checking before a noreturn function that may return control to the caller through an exception can make checking happen more than once if the exception gets caught in the caller — whether by a handler or a cleanup. And when -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a noreturn call with the implicitly-added cleanup handler, since that'd be redundant with the check done before the call. But other handlers or cleanups in the function, if they get activated, will modify the recorded execution path and check it again when another checkpoint is hit. The checkpoint may even be another noreturn call, so yeah, checking may end up performed multiple times 😭.

Also, various optimizers may cause calls to be marked as noreturn and/or nothrow even when the corresponding attributes aren't there, which can affect where checks get placed before calls and the addition of implicit cleanup handlers for them. This unpredictability, plus the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, is why no-xthrow is the default for this option: it excludes from noreturn treatment only the internal functions used to (re)raise exceptions, which aren't affected by these optimizations. Slay, sensible defaults ✨.

• -fhardened — turns on a whole little squad of flags for C and C++ that level up the security of the generated code without affecting its ABI 🛡️. The exact flags may change between major releases of GCC, but right now they're:

-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE  -pie  -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)

You can see the whole list of what -fhardened enables with the --help=hardened option 👀.

If your system glibc is older than 2.35, it uses -D_FORTIFY_SOURCE=2 instead.

This one's meant for production builds, not just debug builds — like, wear your cute outfit to the party, not just to try it on 💃.

Currently -fhardened is only supported on GNU/Linux targets.

And here's the polite part: -fhardened only enables a particular option if it wasn't already specified anywhere on the command line. So like, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. It respects your choices, icon behaviour 🎀.

• -fstack-protector — emits extra code to check for buffer overflows, like stack smashing attacks 🥊. It does this by adding a guard variable to functions with vulnerable objects — that includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards get initialized when a function is entered and then checked when it exits. If a guard check fails, an error message is printed and the program exits. Only variables actually allocated on the stack count; optimized-away variables or variables allocated in registers don't count, sorry bestie 💅.
• -fstack-protector-all — like -fstack-protector except all functions are protected. Everyone gets a bodyguard, no cap 👑
• -fstack-protector-strong — like -fstack-protector but it includes extra functions in the protection squad — those that have local array definitions, or that have references to local frame addresses 🫶. Again, only variables actually allocated on the stack are considered; optimized-away variables or ones in registers don't count.
• -fstack-protector-explicit — like -fstack-protector but it only protects those functions which have the stack_protect attribute. Exclusive club, invite only ✨
• -fstack-check — generates code to verify you don't go beyond the boundary of the stack 🧐. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems when there's only one stack.

Important caveat, bestie: this switch does not actually cause checking to be done — the operating system or the language runtime has to do that. The switch causes generation of code to make sure 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 needs no specific target support in the compiler, but it comes with these drawbacks 😬:

1. Modified allocation strategy for large objects: they're always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it's topped by a particular function, stack checking isn't reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. 💀

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It's not generally sufficient to protect against stack-clash attacks though — to protect against those you want -fstack-clash-protection 🌈.

• -fstack-clash-protection — generates code to prevent stack clash style attacks 🛡️. When this is enabled, the compiler will only allocate one page of stack space at a time, and each page is accessed immediately after allocation. So it prevents allocations from jumping over any stack guard page the operating system provides. Periodt.

Most targets don't fully support stack clash protection. But on those targets, -fstack-clash-protection will protect dynamic stack allocations. It may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.

• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit — generate code to ensure the stack doesn't grow beyond a certain value, either the value of a register or the address of a symbol 🚧. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so you can catch the signal without taking special precautions.

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker, iykyk 🔗.

You can locally override stack limit checking by using the no_stack_limit function attribute.

• -fsplit-stack — generates code to automatically split the stack before it overflows 🧚. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is super useful when running threaded programs, because it's no longer necessary to calculate a good stack size for each thread. Currently this is only implemented for the x86 targets running GNU/Linux.

Heads up: when code compiled with -fsplit-stack calls code compiled without it, there may not be much stack space available for the latter code to run 😬. If compiling all code — including library code — with -fsplit-stack isn't 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. We pretend strub isn't even a thing 🙈.
• -fstrub=strict — functions default to strub mode disabled, and it strictly applies 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). Rules are rules, bestie 🎀.
• -fstrub=relaxed — restores the default stack scrub (strub) setting, namely, strub is only enabled as required by strub attributes associated with function and data types. relaxed means strub contexts are only prevented from calling functions explicitly associated with strub mode disabled. This option is only useful to override other -fstrub=* options that come before it in the command line — it's the "undo" button of strub 💅.
• -fstrub=at-calls — enables at-calls strub mode where viable. The main 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's visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. See the strub attribute documentation for the details on viability and eligibility requirements, iykyk 📖.
• -fstrub=internal — enables internal strub mode where viable. Again, mostly for testing 🧪. This option is intended to thoroughly exercise the parts of the strub machinery that implement the less efficient but interface-preserving strub mode. Functions that wouldn't be affected by this option are, like, super 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 you which mode was selected for each function. The primary use of this option is for testing, to exercise the strub machinery thoroughly. Obsessed with a flag that just goes "pick the best one" 💖.
• -fvtable-verify=[std|preinit|none] — C++ only, bestie! It turns on (or off, if you say -fvtable-verify=none) the security feature that, at run time, for every single virtual call, checks that the vtable pointer the call goes through is actually valid for the type of the object — and hasn't been corrupted or overwritten. If it catches an invalid vtable pointer at run time, it reports an error and immediately halts execution of the program. 💀

This option makes run-time data structures get built at program startup, and those are the things used to verify the vtable pointers. The std and preinit values control when those data structures get built. In both cases they're built before execution reaches main. With -fvtable-verify=std, the data structures are built after shared libraries have been loaded and initialized. With -fvtable-verify=preinit, they're built before shared libraries have been loaded and initialized. ⏱️

If this option shows up multiple times on the command line with different values, none takes highest priority over both std and preinit, and preinit takes priority over std.

• -fvtv-debug — when you use this alongside -fvtable-verify=std or -fvtable-verify=preinit, it makes debug versions of the runtime functions for the vtable verification feature get called. It also makes the compiler log info about which vtable pointers it finds for each class. That info goes into a file named vtv_set_ptr_data.log, in the directory named by the environment variable VTV_LOGS_DIR if that's defined — otherwise the current working directory. 🗂️

Note: this feature appends data to the log file. If you want a fresh log file, make sure you delete any existing one first!

• -fvtv-counts — debugging flag, bestie. Used with -fvtable-verify=std or -fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts how many calls to certain run-time library functions it inserts, and logs all of that for each compilation unit. The compiler writes this info to a file named vtv_count_data.log, in the directory named by VTV_LOGS_DIR if that's defined — otherwise the current working directory. It also counts the size of the vtable pointer sets for each class and writes that to vtv_class_set_sizes.log in the same directory. 📊

Note: this one appends to the log files too. To get fresh ones, delete any existing ones first!

• -finstrument-functions — generates instrumentation calls for entry and exit to functions. Just after a function is entered and just before it exits, these profiling functions get called with the address of the current function and its call site. (On some platforms, __builtin_return_address doesn't work beyond the current function, so the call site info might not be available to the profiling functions otherwise. 😭)

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function — and that can be looked up exactly in the symbol table. 🔍

This instrumentation also happens for functions that get expanded inline inside other functions. The profiling calls show where, conceptually, the inline function is entered and exited. Which means addressable versions of those functions have to be available. If literally all your uses of a function get expanded inline, this could mean extra code size. And if you use extern inline in your C code, an addressable version of those functions must be provided. (That's normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies. 🍀)

A function can be given the attribute no_instrument_function, and then this instrumentation just doesn't happen for it. You'd use that for, like, the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions can't safely be called (maybe signal handlers, if the profiling routines generate output or allocate memory). 🙅‍♀️

• -finstrument-functions-once — similar to -finstrument-functions, except the profiling functions get called only once per instrumented function. So the first profiling function is called after the first entry into the instrumented function, and the second profiling function is called before the exit that corresponds to this first entry.

Okay so the definition of "once" here is a little vague, because the implementation is not protected against data races. So the implementation only guarantees the profiling functions are called at least once per process and at most once per thread — but the calls are always paired. Meaning: if a thread calls the first function, it will call the second function, unless it never reaches the exit of the instrumented function. 🤷‍♀️

• -finstrument-functions-exclude-file-list=file,file,... — sets the list of functions excluded from instrumentation (peep the description of -finstrument-functions). If the file that contains a function definition matches one of file, then that function just isn't instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it counts as a match. 🎯

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option). 🙈

• -finstrument-functions-exclude-function-list=sym,sym,... — similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names excluded from instrumentation. The function name that gets matched is its user-visible name, like vector<int> blah(const vector<int> &) — not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, it's considered a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. ✨
• -fpatchable-function-entry=N[,M] — generates N NOPs right at the beginning of each function, with the function entry point placed before the Mth NOP. If M is omitted, it defaults to 0, so the function entry points to the address just at the first NOP. Those NOP instructions reserve extra space, and that space can be used to patch in any instrumentation you want at run time — provided the code segment is writable. 👀 The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.

For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary. 📦

Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over the command-line option -fpatchable-function-entry=N,M. You can use this to increase the area size or to remove it completely on a single function. If N=0, no pad location is recorded.

The NOP instructions are inserted at---and maybe before, depending on M---the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. See the -msplit-patch-nops option to change this.

The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14 when not using -msplit-patch-nops. 🔢

✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which gets run on each C source file before the actual compilation happens. 🧚

If you use the -E option, nothing is done except preprocessing. Some of these options only make sense together with -E, because they cause the preprocessor output to be unsuitable for actual compilation. 💅

In addition to the options listed here, there are a number of options to control search paths for include files, documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options.

• -D name
• --define-macro=name
• --define-macro name

Predefines name as a macro, with definition 1. ✅

• -D name=definition
• --define-macro=name=definition
• --define-macro name=definition

The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, 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. 🐚

If you 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 you should quote the option. With sh and csh, -D'name(args...)=definition' works.

-D and -U options are processed in the order they're given on the command line. All -imacros file and -include file options are processed after all -D and -U options. 🫶

• -U name
• --undefine-macro=name
• --undefine-macro name

Cancels any previous definition of name, whether it was built in or provided with a -D option. 🚫

• -include file, **--include=*file***, **--include*file*** — okay so this one's iconic: it processes *file* as if#include "file"was literally the first line of your 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 rest of the#include "..."search chain like normal, no cap. And if you stack multiple-include` options? They're included in the order they appear on the command line, periodt. 💅
• -imacros file, **--imacros=*file***, **--imacros*file*** — exactly like-include, except any output produced by scanning *file* gets **thrown away** 🗑️. Macros it defines stay defined, though. So it's giving "I just want the macros from this header, not its whole declaration situation" — very main character energy. Heads up: all files specified by -imacrosare processed **before** all files specified by-include`. ✨
• -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined, so like, the basics stay, everything else gets the ✂️.
• -pthread — defines additional macros required for using the POSIX threads library 🧵. Use it consistently for both compilation and linking, bestie, or you're gonna have a bad time. Supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Slay.
• -M, --dependencies — instead of outputting the result of preprocessing, it outputs a rule suitable for make describing the dependencies of the main source file 🫶. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files — including those coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with the object file suffix and with any leading directory parts removed. If there are many included files, the rule gets split into several lines using \-newline. The rule has no commands, fr fr.

This option does not suppress the preprocessor's debug output, such as -dM 😭. To avoid mixing that debug output with the dependency rules, 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. Not me just casually implying -E 💀.

• -MM, --user-dependencies — 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 🙈. Which implies the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output. iykyk.
• -MF file — when used with -M or -MM, specifies a file to write the dependencies to 🌸. If no -MF switch is given, the preprocessor sends the rules to the same place it would send preprocessed output. When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. And if file is -, then the dependencies are written to stdout. Cute.
• -MG, --print-missing-file-dependencies — in conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error ✨. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, since a missing header file renders this useless. This feature is used in automatic updating of makefiles 👑
• -Mno-modules — disable dependency generation for compiled module interfaces. Simple, iconic, does exactly what it says on the tin. 🎀
• -MP — instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing 🦋. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. The way this saves you from your own chaos… 😭

This is typical output:

    test.o: test.c test.h

    test.h:

• -MT target — changes 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? Pass them as a single argument to -MT, or use multiple -MT options. Easy.

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. Iconic behavior honestly.

• -MD, --write-dependencies — -MD is equivalent to -M -MF file, except that -E is not implied 🙅. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d; otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file. The vibes shift depending on context, okurrr.

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process ✨

• -MMD, --write-user-dependencies — like -MD except mention only user header files, not system header files 🍓
• -fpreprocessed — indicates to the preprocessor that the input file has already been preprocessed 🌸. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so you can pass a file preprocessed with -C to the compiler without problems 🫶. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps. Noted, bestie 📝

• -fdirectives-only — when preprocessing, handle directives, but don't expand macros 🎀. The option's behavior depends on the -E and -fpreprocessed options:
  • With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. In addition, the -dD option is implicitly enabled. ✨
  • With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only. 💖
  • With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only. 🦋
• -fdollars-in-identifiers — accept $ in identifiers 💅. That's it. That's the tweet.
• -fextended-identifiers — accept universal character names and extended characters in identifiers 🌈. This option is enabled by default for C99 (and later C standard versions) and C++. So mostly you don't gotta touch it, but now you know 👑
• -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization 🙈
• -fmax-include-depth=depth — sets the maximum depth of the nested #include. The default is 200. Guardrails, bestie 🚧
• -fsearch-include-path[=kind] — look for input files on the #include path, not just the current directory 🔥. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly:

g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc

kind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the #include <...> search path. Choose your fighter 💅

• -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 occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory, so, tradeoffs 😭.

The level parameter chooses the level of precision of token location tracking, thus decreasing memory consumption if necessary:

1. Value 0 of level de-activates this option.
2. 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.
3. 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. So like, you're already in it, bestie 💜

• -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. Go off, encoding icon 👑
• -fwide-exec-charset=charset — sets the wide execution character set, bestie, the one used for wide string and character constants 💅 the default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE — whichever one matches the width of wchar_t plus the big-endian or little-endian byte order your code generation is using. Same energy as -fexec-charset: charset can be any encoding your system's iconv library routine supports — but heads up, you will have problems with encodings that don't fit exactly in wchar_t. like, it's giving mismatched vibes 😭
• -finput-charset=charset — sets the input character set, the one used to translate from the character set of the input file to the source character set GCC uses. The default is UTF-8 ✨ charset can be any encoding supported by the system's iconv library routine.

If the input character set is UTF-8, warnings about ill-formed code unit sequences get issued if -Winvalid-utf8 is enabled. Otherwise, no diagnostics at all when the input character set matches the execution character set. But if they're different? ill-formed code unit sequences result in an error during transcoding to the execution character set 💀 no cap.

• -fpch-deps — when you're using precompiled headers, this flag makes the dependency-output flags also list the files from the precompiled header's dependencies 🫶 if you don't specify it, only the precompiled header gets listed, not the files that were used to create it — because those files aren't consulted when a precompiled header is used. periodt.
• -fpch-preprocess — this option lets you use a precompiled header together with -E. It sticks a special #pragma, #pragma GCC pch_preprocess "filename", into the output to mark where the precompiled header was found, plus its filename 🎀 when -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It gets switched on by -save-temps.

Do not write this #pragma in your own code, bestie 🙈 but it is safe to edit the filename if the PCH file lives somewhere else. 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 🌸 with this on, 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. It's implicitly enabled if debugging information is enabled — but you can inhibit that with the negated form -fno-working-directory. And if -P is present in the command line, this option has no effect, since no #line directives are emitted whatsoever. iconic behavior ngl.
• -C, --comments — do not discard comments 💖 all comments get passed through to the output file, except for comments in processed directives, which get deleted along with the directive.

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, --comments-in-macros — do not discard comments, including during macro expansion 🔥 this is like -C, except that comments contained within macros also get passed through to the output file where the macro is expanded.

On top of all the side effects of -C, 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. not me being protected from myself.

The -CC option is generally used to support lint comments.

• -P, --no-line-commands — inhibits generation of linemarkers in the output from the preprocessor 🦋 might be useful when you're running the preprocessor on something that isn't C code, and it'll be sent to a program that might get confused by the linemarkers. fair honestly.
• -traditional, --traditional, -traditional-cpp, --traditional-cpp — tries to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors 🌈 see the GNU CPP manual for the 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, --trigraphs — support ISO C trigraphs ✨ these are three-character sequences, all starting with ??, that ISO C defines to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline. the way that works is so unserious.

The nine trigraphs and their replacements are:

        Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
        Replacement:      [    ]    {    }    #    \    ^    |    ~

By default, GCC ignores trigraphs — but in standard-conforming modes it converts them. See the -std and -ansi options 👀

• -remap — enables special code to work around file systems that only permit very short file names, such as MS-DOS 💀 retro.
• -H, --trace-includes — 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 get printed too, even if they're found to be invalid — an invalid precompiled header file is printed with ...x and a valid one with ...!. iykyk.
• -dletters, --dump=letters, --dump letters — 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. And if you specify letters whose behavior conflicts? the result is undefined 😭
  • -dM, --dump=M — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros 🌸 this gives you a way of finding out what's predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

        touch foo.h; cpp -dM foo.h

shows all the predefined macros ✨

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach 👀 plot twist.

• -dD, --dump=D — 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, --dump=N — like -dD, but emit only the macro names, not their expansions 🫶 minimal queen.
• -dI, --dump=I — output #include directives in addition to the result of preprocessing.
• -dU, --dump=U — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, get output 💅 the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time. so thorough.

• -fdebug-cpp — this option is only useful for debugging GCC 🙈 when used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.

When used from GCC without -E, this option has no effect. literally nothing. 💀

• -Wp,option — you can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor ✨ if option contains commas, it gets split into multiple options at the commas. However — and this is the important part, bestie — many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase 😳 the preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead. fr fr, just let the driver cook.
• -Xpreprocessor option — pass option as an option to the preprocessor 🌸 you can 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 must use -Xpreprocessor twice — once for the option and once for the argument 🎀

• -no-integrated-cpp, --no-integrated-cpp — perform preprocessing as a separate pass before compilation 💜 by default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If you provide this option, 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 together with the -B or -wrapper options to specify an alternate preprocessor, or to perform additional processing of the program source between normal preprocessing and compilation 🔥

---

🧚 Passing Options to the Assembler 🧚

You can pass options to the assembler. yes bestie, the assembler gets love too 💖

• -Wa,option — pass option as an option to the assembler. If option contains commas, it's split into multiple options at the commas ✨
• -Xassembler option, --for-assembler=option, --for-assembler option — pass option as an option to the assembler 🌸 you can use this to supply system-specific assembler options that GCC doesn't recognize.

If you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option and once for the argument 💅

---

🦋 Options for Linking 🦋

These options come into play when the compiler links object files into an executable output file ⭐ and they're meaningless if the compiler isn't doing a link step. like, don't even bother, they simply won't eat 💀

• object-file-name — so if a file name does not end in one of the special recognized suffixes, it lowkey gets treated as an object file or a library, periodt. (The linker tells object files apart from libraries by looking at the file contents, iykyk.) If linking actually happens, these object files get fed straight into the linker as input. 🫶
• -c, -S, -E — if you use any of these three, the linker does not run, and ngl you should not be passing object file names as arguments either. 💅
• -flink-libatomic — turns on linking of libatomic if it's supported by the target, and bestie it's enabled by default. If you wanna be explicit about not linking it, use the negative form -fno-link-libatomic. ✨
• -flinker-output=type — okay so this one controls code generation of the link-time optimizer. By default, the linker output is automatically determined by the linker plugin. But for debugging the compiler, and if you want incremental linking with a non-LTO object file, it can be useful to control the type manually. 🎀
  • exec — code generation produces a static binary. In this case -fpic and -fpie are both disabled.
  • dyn — code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This lets you build shared libraries without position-independent code on architectures where that's possible, i.e. on x86. 💀
  • pie — code generation produces an -fpie executable. This gives you similar optimizations as exec, except that -fpie is not disabled if you specified it at compilation time.
  • rel — the compiler assumes incremental linking is being done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. On top of that, 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 the majority of objects in the library are used. 🌸
  • nolto-rel — finally, this 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. GNU Binutils 2.44 or later is needed 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, link such objects into a static library instead. 🧚

• -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.
• -fuse-ld=wild — use the Wild linker (wild) 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 — fr fr, don't do it.)

GCC passes the -l option directly to the linker, so definitely check your linker documentation for the exact details. The general description below applies to the GNU linker, bestie. 🫶

The linker searches a standard list of directories for the library. Those directories include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used. ✨

It makes a difference where in the command you write this option — the linker searches and processes libraries and object files in the order they are specified. So foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded. 😭

• -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. 🎀

Heads up: the compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc, so when this option is specified, those entry points should be supplied through some other mechanism. 💀

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. It still links with the startup files, libgcc, or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++ — unless options preventing their inclusion are used as well. This typically removes -lc from the link command line, 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, --no-standard-libraries — 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. 🌈

Same warning as above, bestie: the compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. 👑

In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called.) 🔥

• -nostdlib++ — do not implicitly link with standard C++ libraries. simple as that 💅
• -e entry, --entry=entry, --entry entry — specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. ⭐
• -pie, --pie — produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. 🎀
• -no-pie — don't produce a dynamically linked position independent executable. that's it, that's the vibe 🙈
• -static-pie, --static-pie — produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. 🌸
• -pthread — link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. 🧵✨
• -r — produce a relocatable object as output. This is also known as partial linking. 💜
• -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program. 🔥
• -s — remove all symbol table and relocation information from the executable. bye bye debug info 👋💀
• -static, --static — on systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect. 🌈
• -shared, --shared — produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1] 🎀
• -shared-libgcc, -static-libgcc — on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect. 😭

There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc. 🫶

So like, that's literally why the G++ driver just automatically adds -shared-libgcc whenever you build a shared library or a main executable — because C++ programs typically use exceptions, so it's the right thing to do. 💅

But bestie, here's the plot twist: if you use the GCC driver to create shared libraries instead, you might find they're not always linked with the shared libgcc. 🙈 If GCC finds, at its configuration time, that you've got a non-GNU linker or a GNU linker that doesn't 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. And that's actually iconic of it, because it lets exceptions propagate through such shared libraries without incurring relocation costs at library load time. ✨

However — and this is the important part, no cap — if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc. Don't skip that step, ok? 🫶

• -static-libasan — when -fsanitize=address is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and -static 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-libhwasan — when -fsanitize=hwaddress is used to link a program, the GCC driver automatically links against libhwasan. If libhwasan is available as a shared library, and -static is not used, then this links against the shared version of libhwasan. The -static-libhwasan option directs the GCC driver to link libhwasan statically, without necessarily linking other libraries statically. 🦋
• -static-liblsan — when -fsanitize=leak is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and -static 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-libtsan — when -fsanitize=thread is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and -static 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-libubsan — when -fsanitize=undefined is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and -static 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 -static is not used, then this links against the shared version of libstdc++. That's normally fine, like genuinely. 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, --symbolic — bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option, so vibe check your platform first. 🌸
• -T script — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols. 😭
• -Xlinker option — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize. 👑

If you want to pass an option that takes a separate argument, you must use -Xlinker twice — once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. 💀

When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options, ngl. 🙈

• -Wl,option, --for-linker=option, --for-linker option — pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map. ✨
• -u symbol, --force-link=symbol, --force-link 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. 🫶
• -Tbss=addr, -Tdata=addr, -Ttext=addr, -N, -n, -t, -Z, -z keyword — these options are passed through to the linker without interpretation by GCC. Refer to your linker documentation for the meanings of these options. Not GCC's problem, bestie — it's just the messenger. 🧚

---

🔍 Options for Directory Search 🔍

These options specify directories to search for header files, for libraries and for parts of the compiler. Like, they're the map to your filesystem, and honestly? Iconic. 🗺️

• -I dir, -iquote dir, -isystem dir, -idirafter dir, --include-directory-after=dir, --include-directory-after dir, --include-directory=dir, --include-directory dir — add the directory dir to the list of directories to be searched for header files during preprocessing. --include-directory is an alias for -I, while --include-directory-after is an alias for -idirafter. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot. 💖

Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives. 🎀

You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:

1. For the quote form of the include directive, the directory of the current file is searched first.
2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However — big however — you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that. 🙅‍♀️

The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories. ✨

If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. 🔥

• -I-, --include-barrier — 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-. 😭 Like, zero. None. It's giving "no escape". 💀

• -iprefix prefix, --include-prefix=prefix, --include-prefix 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, --include-with-prefix=prefix, --include-with-prefix prefix, --include-with-prefix-after=prefix, --include-with-prefix-after prefix, --include-with-prefix-before=prefix, --include-with-prefix-before prefix — 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. 🦋

--include-with-prefix and --include-with-prefix-after are both aliases for -iwithprefix, while --include-with-prefix-before is an alias for -iwithprefixbefore. Slay, aliases. 💅

• -isysroot dir — okay bestie, this is basically --sysroot but it only applies to header files. Except on Darwin targets, where it applies to both header files and libraries. Peep the --sysroot option for the full tea ☕.
• -imultilib dir — use dir as a subdirectory of the directory that contains target-specific C++ headers. Slay, very target-specific 💅.
• -imultiarch dir — use dir as a subdirectory of the directory that contains architecture-specific C++ headers. Architecture girlies rise 🌸.
• -nostdinc, --no-standard-includes — 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. No cap, that’s the whole vibe check 🔥.
• -nostdinc++ — do not search for header files in the C++-specific standard directories, but do still search the other standard directories. This one is used when building the C++ library, iykyk 🎀.
• --embed-dir=dir, --embed-directory=dir, --embed-directory dir — append dir directory to the list of searched directories for the "#embed" preprocessing directive or "__has_embed" macro. There are no default directories for "#embed", periodt 💀.

If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot. Cute little substitution moment ✨.

• -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. So bestie, don’t touch unless you’re the driver 💅.
• -Ldir, --library-directory=dir, --library-directory dir — add directory dir to the list of directories to be searched for -l. Linker girlies, this one’s for you 🫶.
• -Bprefix, --prefix=prefix, --prefix prefix — this option specifies where to find the executables, libraries, include files, and data files of the compiler itself. Main character energy for the whole toolchain 👑.

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. Very “I know where everything lives” of it.

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. PATH girlies know 😭.

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. Little detail, but she matters 🎀.

-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. Translating options like a multilingual icon 💅.

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. So yeah, sometimes she’s just not invited to the link party 🙈.

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. Environment variable cameo 🦋.

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. Kludge? More like vintage hack couture ✨.

• -no-canonical-prefixes, --no-canonical-prefixes — do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. Leave those paths exactly as they are, bestie 💜.
• --sysroot=dir, --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. It’s giving “new root, same compiler” 🌈.

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. They’re splitting duties, your honor 💅.

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. So check your linker’s vibe first, ngl 😭.

• --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. No extra suffix, no extra drama 🎀.

---

✨ Options for Code Generation Conventions ✨

These machine-independent options control the interface conventions used in code generation. Interface conventions? We love a structured queen 👑.

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. It’s giving “choose your own flag adventure” 💅.

• -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. Stack reuse? She’s recycling but make it compiler 💀.

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. Bestie, this is the part where old code can get absolutely got 😭.

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. So yeah, it’s a real “respect the old code’s boundaries” situation 🫶.

• -ftrapv — this option generates traps for signed overflow on addition, subtraction, multiplication operations. Traps! We love a dramatic entrance 💀.

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. Override wars, but make it compiler flags ⚔️.

• -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. Same override tea, different flag ☕.
• -fwrapv-pointer — this option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid. Pointer girlies get their own wrap-around moment 💅.
• -fstrict-overflow — this option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer. Strict overflow said “I contain multitudes” ✨.
• -fexceptions — enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution. Bigger data? Maybe. Slower execution? Not this one, bestie 🎀.

If you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. However, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. You may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. C and C++ interop? We love a crossover episode 🌈.

• -fnon-call-exceptions — generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM". This enables -fexceptions. So it’s exceptions with a very specific guest list, no random signal-handler crashers allowed 🙈.
• -fdelete-dead-exceptions — okay so this one's like, "instructions that might throw but honestly aren't contributing to the program's actual execution? bestie, they can be optimized away." 🗑️✨ It does not affect calls to functions, except the ones with the pure or const attributes. This is enabled by default for the Ada and C++ compilers — as permitted by the language specs, no cap. And heads up: the optimization passes that actually cause dead exceptions to be removed are enabled independently at different optimization levels. iykyk 💅
• -funwind-tables — lowkey similar to -fexceptions, except it only generates any needed static data and does not touch the generated code in any other way. You normally don't need to turn this on yourself — instead, a language processor that needs this handling enables it on your behalf. 🫶
• -fasynchronous-unwind-tables — generates the unwind table in DWARF format, if it's supported by the target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (like a debugger or a garbage collector). 🧚✨
• -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 definitions of template static data members and static local variables in inline functions stay unique even when RTLD_LOCAL is in the mix. This is necessary to avoid problems with a library used by two different RTLD_LOCAL plugins depending on a definition in one of them — and therefore disagreeing with the other one about the binding of the symbol. BUT 💀 this causes dlclose to be ignored for affected DSOs. 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 the glow-up is that it allows 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. fr fr.

"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. For small structures this is more efficient than -fpcc-struct-return. 💖

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there 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. 👑

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. 🫶

• -fno-ident, -Qy, -Qn — -fno-ident suppresses emission of .ident assembler directives and causes the #ident preprocessor directive to be ignored. -Qy and -Qn are obsolete synonyms for -fident and -fno-ident, respectively. 💅
• -finhibit-size-directive — don't output a .size assembler directive, or anything else that would cause trouble if the function is split in the middle and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else. 🚫
• -fverbose-asm — puts extra commentary info in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). 🕵️‍♀️

-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

The added comments include:

• information on the compiler version and command-line options,
• the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
• hints on which high-level expressions correspond to the various assembly instruction operands.

For example, given this C source file:

        int test (int n)
        {
          int i;
          int total = 0;

          for (i = 0; i < n; i++)
            total += i * i;

          return total;
        }

compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -

        gcc -S test.c -fverbose-asm -Os -o -

gives output similar to this:

                .file   "test.c"
        # GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
          [...snip...]
        # options passed:
          [...snip...]

                .text
                .globl  test
                .type   test, @function
        test:
        .LFB0:
                .cfi_startproc
        # test.c:4:   int total = 0;
                xorl    %eax, %eax      # <retval>
        # test.c:6:   for (i = 0; i < n; i++)
                xorl    %edx, %edx      # i
        .L2:
        # test.c:6:   for (i = 0; i < n; i++)
                cmpl    %edi, %edx      # n, i
                jge     .L5     #,
        # test.c:7:     total += i * i;
                movl    %edx, %ecx      # i, tmp92
                imull   %edx, %ecx      # i, tmp92
        # test.c:6:   for (i = 0; i < n; i++)
                incl    %edx    # i
        # test.c:7:     total += i * i;
                addl    %ecx, %eax      # tmp92, <retval>
                jmp     .L2     #
        .L5:
        # test.c:10: }
                ret
                .cfi_endproc
        .LFE0:
                .size   test, .-test
                .ident  "GCC: (GNU) 7.0.0 20160809 (experimental)"
                .section        .note.GNU-stack,"",@progbits

The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change. 🌀

• -frecord-gcc-switches — this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on some targets and the exact format of the recording is target- and binary-file-format-dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file. 🎀
• -fpic — generates position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it's part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.) 🔥

Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. 🌈

When this flag is set, the macros __pic__ and __PIC__ are defined to 1. ✨

• -fPIC — okay bestie, if your target machine supports it, this emits position-independent code that’s suitable for dynamic linking and avoids any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and SPARC. Position-independent code requires special support, and therefore works only on certain machines. When this flag is set, the macros __pic__ and __PIC__ are defined to 2. 🫶
• -fpie, -fPIE — these are similar to -fpic and -fPIC, but the generated position-independent code can be only linked into executables. Usually these options are used to compile code that will be linked using the -pie GCC option. Both -fpie and -fPIE define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE. ✨
• -fno-plt — do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time. Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions. In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead. 🔥
• -fno-jump-tables — do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed. 💅
• -fno-bit-tests — do not use bit tests for switch statements even where it would be more efficient than other code generation strategies. 💀
• -ffixed-reg — treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file. This flag does not have a negative form, because it specifies a three-way choice. 🎀
• -fcall-used-reg — treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg. It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine’s execution model produces disastrous results. This flag does not have a negative form, because it specifies a three-way choice. 😭
• -fcall-saved-reg — treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it. It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine’s execution model produces disastrous results. A different sort of disaster results from the use of this flag for a register in which function values may be returned. This flag does not have a negative form, because it specifies a three-way choice. 🙈
• -fpack-struct[=n] — without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface. 🫶

• -fleading-underscore — this option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code.

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch. 🌸

• -ftls-model=model — alter the thread-local storage model to be used. The model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line. The default without -fpic is initial-exec; with -fpic the default is global-dynamic. ⭐
• -ftrampolines — for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that do not need them, like for example HP-PA or IA-64, do nothing. A trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly. -fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada. Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care. For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions. 🧚
• -ftrampoline-impl=[stack|heap] — by default, trampolines are generated on stack. However, certain platforms (such as the Apple M1) do not permit an executable stack. Compiling with -ftrampoline-impl=heap generate calls to __gcc_nested_func_ptr_created and __gcc_nested_func_ptr_deleted in order to allocate and deallocate trampoline space on the executable heap. These functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux. PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if you setjmp, instantiate nested functions, and then longjmp back to a state prior to having allocated those nested functions. 💜
• -fvisibility=[default|internal|hidden|protected] — set the default ELF image symbol visibility to the specified option---all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public. A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by How To Write Shared Libraries by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/)---however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads---making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, "operator new" and "operator delete" must always be of default visibility. Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. 🌈

Heads up bestie: "extern" declarations do not get touched by -fvisibility, so like, a ton of code can be recompiled with -fvisibility=hidden and literally nothing needs changing 💅. But — and this is the plot twist — that means calls to "extern" functions with no explicit visibility go through the PLT, so it's way more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which "extern" declarations should be treated as hidden. iykyk.

Also note 📌: -fvisibility does affect C++ vague linkage entities. Translation: if you've got an exception class getting thrown between DSOs, it must be explicitly marked with default visibility so the type_info nodes get unified between the DSOs. No cap, that one bites people.

There's a whole overview of these techniques, their benefits and how to use them at <https://gcc.gnu.org/wiki/Visibility> — go read it, it's giving study guide ✨.

---

🧊 -fstrict-volatile-bitfields

Use this one when accesses to volatile bit-fields (or other structure fields too, though ngl 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. Classic example: targets with memory-mapped peripheral registers might need all such accesses to be 16 bits wide. With this flag you can declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, like, a maybe more efficient 32-bit access. Slay.

If this option is disabled, the compiler just uses the most efficient instruction. In that same example, that might be a 32-bit load instruction — even though that accesses bytes that don't contain any portion of the bit-field, or memory-mapped registers that have nothing to do with the one being updated 😭.

Sometimes — like 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's correctly aligned for the target machine. In that case GCC falls back to generating multiple accesses rather than code that'll fault or truncate the result at run time. We love a graceful fallback 🫶.

Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended to define all bits of the field's type as bit-field members. fr fr, do that.

The default value of this option is determined by the application binary interface for the target processor.

---

🔄 -fsync-libcalls

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 is enabled, so the only actually useful form is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. Cute, right? 🎀

---

🫧 -fzero-init-padding-bits=value

Guarantees zero initialization of padding bits in automatic variable initializers. Some languages already promise zeroed padding bits in certain cases — e.g. C23 when you use empty initializers ("{}"), or C++ when you use zero-initialization, or C guaranteeing that fields not specified in an initializer have their padding bits zero initialized. This option lets you change when padding bits in initializers are guaranteed to be zero initialized ✨.

The default is -fzero-init-padding-bits=standard, which makes no further guarantees than the corresponding standard. Like so:

struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.

• -fzero-init-padding-bits=unions — guarantees zero initialization of padding bits in unions on top of what the standards guarantee, if the initializer of a union is empty (then all bits of the union are zero initialized) or if the initialized member of the union is smaller than the size of the union (in that case it guarantees padding bits outside the initialized member are zero initialized). This was literally the GCC behavior before GCC 15, and in the example above it guarantees zero initialization of the last sizeof (unsigned long long) - 1 bytes in the union 💖.
• -fzero-init-padding-bits=all — guarantees additionally zero initialization of padding bits of other aggregates, so the padding in between b.a and b.b (if any) and tail padding in the structure (if any). go off 👑

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🛠️ GCC Developer Options ✨

This section describes command-line options that are primarily of interest to GCC developers — like, the people building the compiler, not you casually compiling your homework (no shade, ily). It includes options to support compiler testing and investigation of compiler bugs and compile-time performance problems. So: options that produce debug dumps at various points in the compilation; that print statistics like memory use and execution time; and that print information about GCC's configuration, like where it searches for libraries. You should rarely need any of these for ordinary compilation and linking tasks 🎀.

Many developer options that make GCC dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. iconic.

If =filename is omitted, a default dump file name gets constructed by concatenating the base dump file name, a pass number, a phase letter, and a pass name. Breaking it down for you bestie:

• The base dump file name is the name of the output file produced by the compiler if explicitly specified and not an executable; otherwise it's the source file name.
• The pass number is determined by the order passes are registered with the compiler's pass manager. Generally that's the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late get numbered higher than the pass named final, even if they're executed earlier. messy but ok 💀.
• 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. periodt 🌸.

---

🌳 -fcallgraph-info, -fcallgraph-info=MARKERS

Makes the compiler output callgraph information for the program, on a per-object-file basis. The info is generated in the common VCG format ✨. It can be decorated with extra per-node and/or per-edge info if you also specify a comma-separated list of markers:

• su — the callgraph gets decorated with stack usage information; it's equivalent to -fstack-usage.
• da — the callgraph gets decorated with information about dynamically allocated objects.

When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files. multitasking queen 🫶.

---

📝 -dletters, --dump=letters, --dump letters, -fdump-rtl-pass, -fdump-rtl-pass-options, -fdump-rtl-pass-options=filename

Says to make debugging dumps during compilation at the times specified by letters (when using -d) or by pass (when using -fdump-rtl). This is what you use for debugging the RTL-based passes of the compiler. no cap, this is deep-in-the-compiler territory 🔥.

Some -dletters switches have different meaning when -E is used for preprocessing 👀.

The -options form allows greater control over the details of the dump. See -fdump-tree.

Here are actual instances of command-line options following these patterns, and what they mean:

• -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 only runs 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 — these two 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 — these three enable dumping after the three if conversion passes. trio slay 💅.
• -fdump-rtl-cprop_hardreg — dump after hard register copy propagation.
• -fdump-rtl-csa — dump after combining stack adjustments.
• -fdump-rtl-cse1, -fdump-rtl-cse2 — these two 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 — these two 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 — these two enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — these two 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 — enables dumping after the rtl loop optimization passes. 🔁
• -fdump-rtl-mach — dumps the tea after the machine-dependent reorganization pass runs… but only if that pass even exists on your target. If it doesn't, well, nothing to spill 🫖
  • -fdump-rtl-mode_sw — dumps right after it clears out the redundant mode switches. So much cleaner, honestly ✨
  • -fdump-rtl-rnreg — dumps after register renumbering. Fresh numbers, fresh me 💅
  • -fdump-rtl-outof_cfglayout — dumps after it converts out of cfglayout mode. Bye cfglayout, you were iconic but it's time 🦋
  • -fdump-rtl-peephole2 — dumps after the peephole pass. Squint through the tiny hole, bestie 👀
  • -fdump-rtl-postreload — dumps after the post-reload optimizations. Post-reload glow-up 💖
  • -fdump-rtl-pro_and_epilogue — dumps after generating the function prologues and epilogues. The hello and the bye-bye of every function 🎀
  • -fdump-rtl-sched1, -fdump-rtl-sched2 — these two turn on dumping after the basic block scheduling passes. Scheduling queen behavior, no cap 👑
  • -fdump-rtl-ree — dumps after sign/zero extension elimination. Redundant extensions? Not on my watch 🚫
  • -fdump-rtl-seqabstr — dumps after common sequence discovery. Finding the twins in your code fr fr 👯
  • -fdump-rtl-shorten — dumps after shortening branches. Short branches, short attention span, we love it 🌸
  • -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — these bad boys enable dumping after five whole rounds of instruction splitting. Five!! That's a whole era 😭
  • -fdump-rtl-sms — dumps after modulo scheduling. Heads up though: this pass only runs on some architectures, so don't come for me if your dump folder is empty 💀
  • -fdump-rtl-stack — dumps after converting from GCC's "flat register file" registers over to the x87's stack-like registers. This pass is only run on x86 variants, iykyk 🖥️
  • -fdump-rtl-subreg1, -fdump-rtl-subreg2 — these enable dumping after the two subreg expansion passes. Two passes, two dumps, two moods 🫶
  • -fdump-rtl-vartrack — dumps after variable tracking. Tracking your faves like a stan account 📲
  • -fdump-rtl-vregs — dumps after converting virtual registers to hard registers. Glow-up from virtual to real, we love the character development 🦋
  • -fdump-rtl-web — dumps after live range splitting. Split that range, periodt ✂️
  • -fdump-rtl-regclass, -fdump-rtl-subregs_of_mode_init, -fdump-rtl-subregs_of_mode_finish, -fdump-rtl-dfinit, -fdump-rtl-dfinish — okay so these dumps are *defined*… but they always produce empty files. Literally giving nothing. Empty. Void. Blank. Not a single crumb 😭
  • -da, --dump=a, -fdump-rtl-all — produces all the dumps listed above. All of them. Every single one. Maximum chaos, maximum data 🔥
  • -dA, --dump=A — annotates the assembler output with miscellaneous debugging info. A little behind-the-scenes commentary for your asm 💅
  • -dD, --dump=D — dumps all macro definitions at the end of preprocessing, in addition to the normal output. All your #defines, exposed 🫣
  • -dH, --dump=H — produces a core dump whenever an error occurs. Every error gets a souvenir 💀
  • -dp, --dump=p — annotates the assembler output with a comment showing which pattern and alternative is used. The length and cost of each instruction get printed too. Receipts for every instruction 🧾
  • -dP, --dump=P — dumps the RTL into the assembler output as a comment before each instruction. Also turns on -dp annotation, so you get the pattern comments too for free 🎁
  • -dx, --dump=x — just generates RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand. RTL only, no commit 🔥

---

• -fdump-debug — dumps the debugging info generated during the debug generation phase. Debugging your debug, very meta ✨
• -fdump-earlydebug — dumps the debugging info generated during the early debug generation phase. Early bird debug energy 🐦
• -fdump-noaddr — when it's doing debugging dumps, this suppresses the address output. Why would you want that? So you can actually run diff on dumps from compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. Addresses change, everything else doesn't — diff girlies rise 🫶
• -freport-bug — collects and dumps debug info into a temporary file if an internal compiler error (ICE) occurs. Your crash gets a whole scrapbook 📔
• -fdump-unnumbered — when doing debugging dumps, suppresses instruction numbers and address output. Again with the diff-ability: this makes it more feasible to diff dumps for compiler invocations with different options — especially with and without -g. Consistent dumps >>> everything 💖
• -fdump-unnumbered-links — when doing debugging dumps (see the -d option above), it suppresses instruction numbers for the links to the previous and next instructions in a sequence. The links stay, the numbers dip 🦋
• -fdump-internal-locations — dumps the detailed tea on GCC's internal representation of source code locations. Where did this line come from?? Now you know 📍
• -fdump-ipa-switch, -fdump-ipa-switch-options — these control the dumping at various stages of inter-procedural analysis language tree, written out to a file. The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. Possible dumps:
  • all — enables all the inter-procedural analysis dumps. Everything, all at once, iconic behavior 👑
  • cgraph — dumps info about call-graph optimization, unused function removal, and inlining decisions. Basically the group chat receipts of your call graph 📱
  • inline — dumps after function inlining. In we go 🏃‍♀️
  • strubm — dumps after selecting "strub" modes, and recording those selections as function attributes. Receipts noted 📝
  • strub — dumps the "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking. Scrub that stack 🧽

Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, and they default to -optimized.

For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. Both lists, bestie. The tea is complete 🍵

By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

• -fdump-ipa-clones — creates a dump file full of info about the creation of call graph node clones and the removals of call graph nodes during inter-procedural optimizations and transformations. Its main intended use: tools that create live-patches can figure out the set of functions that need to be live-patched to completely replace a particular function (see -flive-patching). The file name is generated by appending 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 lives on its own separate line, containing semicolon-separated fields. So organized of it 🎀

In the case of call graph clone creation, the individual fields are:

1. String "Callgraph clone".
2. Name of the function being cloned as it is presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases. (Read that again, it matters 😭)
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. Same deal — unique only within a compilation unit or within whole-program analysis, but likely different in the two phases.
9. The file name of the source file where the source code location of the new clone points to.
10. The line to which the source code location of the new clone points to.
11. The column to which the source code location of the new clone points to.
12. A string that determines the reason for cloning.

In the case of call graph clone removal, the individual fields are:

1. String "Callgraph removal".
2. Name of the function being removed as it would be presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. Yep — unique only within a compilation unit or within whole-program analysis, likely different in the two phases. Every time, bestie 💀
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.

• -fdump-lang — dumps language-specific information. The file name is made by appending .lang to the source file name. Slap a .lang on it and call it a day 💅
• -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — these control the dumping of language-specific information. The options and filename portions behave exactly as described in the -fdump-tree option. -fdump-tree-all enables all language-specific dumps; the other options vary with the language. For instance, see

(yeah, the original page trails off right there — I'm not making up the rest for you, bestie 🙈)

• -fdump-passes — prints on stderr the list of optimization passes that are turned on and off by the current command-line options. A full roster of who's active and who got benched 📋
• -fdump-statistics-option — enables and controls dumping of pass statistics into a separate file. The file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. If the -option form is used, -stats causes counters to be summed over the whole compilation unit, while -details dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled. Numbers girlies, this is your moment 📊
• -fdump-tree-all, -fdump-tree-switch, -fdump-tree-switch-options, -fdump-tree-switch-options=filename — these control the dumping at various stages of processing the intermediate language tree out to a file. If the -options form is used, options is a list of --separated options that control the details of the dump. Heads up: not all options are applicable to all dumps — the ones that aren't meaningful are just ignored, no drama 🌈 Available options:
  • address — prints the address of each node. Usually this is not meaningful, since it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment. Cute for debugging, useless otherwise 💅
  • 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 working backward from mangled names in the assembly file. Follow the mangled breadcrumbs 🍞
• slim — When dumping front-end intermediate representations, this says don’t dump members of a scope or the body of a function just because that scope has been reached. Only dump those items when they’re directly reachable by some other path.

When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.

• raw — Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
• details — Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.
• stats — Enable dumping various statistics about the pass (not honored by every dump option).
• blocks — Enable showing basic block boundaries (disabled in raw dumps).
• graph — For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

RTL is always dumped in slim form.

• vops — Enable showing virtual operands for every statement.
• lineno — Enable showing line numbers for statements.
• uid — Enable showing the unique ID (DECL_UID) for each variable.
• verbose — Enable showing the tree dump for each statement.
• eh — Enable showing the EH region number holding each statement.
• scev — Enable showing scalar evolution analysis details.
• optimized — Enable showing optimization information (only available in certain passes).
• missed — Enable showing missed optimization information (only available in certain passes).
• note — Enable other detailed optimization information (only available in certain passes).
• folding — Enable dumping information about match-and-simplify (match.pd) patterns, when they are applied.
• all — Turn on all options, except raw, slim, verbose and lineno.
• optall — Turn on all optimization options, i.e., optimized, missed, and note.

To determine what tree dumps are available or find the dump for a pass of interest follow the steps below.

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.

🌟 The -fopt-info family: -fopt-info, -fopt-info-options, -fopt-info-options=filename

Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations.

The options can be divided into three groups:

1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.

The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line.

The following options control which kinds of messages should be emitted:

• optimized — Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.
• missed — Print information about missed optimizations. Individual passes control which information to include in the output.
• note — Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.
• all — Print detailed optimization information. This includes optimized, missed, and note.

The following option controls the dump verbosity:

• internals — By default, only “high-level” messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers.

One or more of the following option keywords can be used to describe a group of optimizations:

• ipa — Enable dumps from all interprocedural optimizations.
• loop — Enable dumps from all loop optimizations.
• inline — Enable dumps from all inlining optimizations.
• omp — Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
• vec — Enable dumps from all vectorization optimizations.
• optall — Enable dumps from all optimizations. This is a superset of the optimization groups listed above.

If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as “internals”.

If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.

Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead.

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter.

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced which contains dumps from the vectorizer about missed opportunities.

🌈 -fsave-optimization-record

Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info.

This option is experimental and the format of the data within the compressed JSON file is subject to change.

It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message:

• the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
• the function name of the code being optimized, where applicable,
• the “inlining chain” for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
• objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
• the GCC pass that emitted the message, and
• the location in GCC’s own code from which the message was emitted

Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes.

🎀 -fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.

For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info.

• -fenable-kind-pass, -fdisable-kind-pass=range-list — This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.
• -fdisable-ipa-pass — bestie, this one just turns off an IPA pass. pass is literally the pass name, okay? And if that same pass gets statically invoked more than once by the compiler, you gotta tack on a sequential number starting from 1, or it won't know which one you mean 💅
• -fdisable-rtl-pass
• -fdisable-rtl-pass=range-list — kills the RTL pass pass. Same deal: pass is the pass name, and if it's statically invoked multiple times in the compiler, append a sequential number starting from 1. Now range-list is where it gets spicy — it's a comma-separated list of function ranges or assembler names 🌸 Each range is a number pair separated by a colon, and the range is inclusive on both ends. If the range is trivial, you can simplify the number pair down to a single number, iconic. So if the function's call graph node's uid falls inside one of the ranges you specified, the pass is disabled for that function. The uid shows up in the function header of a dump file, and you can dump the pass names with the -fdump-passes option, iykyk 🫶
• -fdisable-tree-pass
• -fdisable-tree-pass=range-list — disables the tree pass pass. See -fdisable-rtl for the description of the option arguments, because it's literally the same energy ✨
• -fenable-ipa-pass — the opposite bestie, this enables IPA pass pass. pass is the pass name, and if the same pass is statically invoked multiple times in the compiler, append a sequential number starting from 1. Slay.
• -fenable-rtl-pass
• -fenable-rtl-pass=range-list — enables RTL pass pass. See -fdisable-rtl for the option argument description and examples, no cap 🎀
• -fenable-tree-pass
• -fenable-tree-pass=range-list — enables tree pass pass. See -fdisable-rtl for the description of the option arguments, same as before, we stay consistent 💖

Here's some examples showing uses of these options, because a vibe check is worth a thousand words:

# 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 — enables internal consistency checking 🔥 The default depends on the compiler configuration, so like, it's giving "ask your compiler". And -fchecking=2 enables further internal consistency checking that might affect code generation. Highkey more checking, more vibes 💀
• -frandom-seed=string — okay so this option gives GCC a seed 🌱 and GCC uses it in place of random numbers when generating certain symbol names that have to be different in every compiled file. It's also used to place unique stamps in coverage data files and the object files that produce them. So you can use -frandom-seed to produce reproducibly identical object files, periodt 🎀

The string can either be a number (decimal, octal or hex) or an arbitrary string — and in that case it's converted to a number by computing CRC32, cute.

Pro tip from the manpage itself: the string should be different for every file you compile. Don't be lazy about it bestie 😭

• -save-temps
• --save-temps — stores the usual "temporary" intermediate files permanently 💾 Names them as auxiliary output files, as specified described under -dumpbase and -dumpdir. So your lil temp files become main characters.

When used in combination with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file with the same extension as an intermediate file 👑 The corresponding intermediate file may be obtained by renaming the source file before using -save-temps.

• -save-temps=cwd — equivalent to -save-temps -dumpdir ./. — literally just that, no notes 💅
• -save-temps=obj — equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file specified after the -o option, including any directory separators. And if -o isn't used? Then the -save-temps=obj switch behaves like -save-temps=cwd, easy 🌸
• -specs=file
• --specs=file
• --specs file — processes 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. 🧚 You can specify more than one -specs=file on the command line, and they're processed in order, from left to right. Left to right bestie, order matters.
• -time[=file] — reports the CPU time taken by each subprocess in the compilation sequence ⏱️ For C source files, this is the compiler proper and assembler (plus the linker if linking is done).

Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time", that is, time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds — iconic.

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options. Lowkey forensic, we love receipts 🕵️‍♀️✨

• -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the name of the dump file is determined by appending .gkd to the dump base name, see -dumpbase 💖
• -fcompare-debug[=opts] — if no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation 👀 Dumps the final internal representation in both compilations, and prints an error if they differ. Like a spot-the-difference but for compiler internals.

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. Sneaky but valid 💅

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). And to get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. Pick your chaos 🌈

• -fcompare-debug-second — this option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect 🎀 Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first. Ngl, very considerate.

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 💀 So like, don't reach for it casually bestie.

• -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 doesn't matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it's given. This is mainly intended to be used with -fcompare-debug, iykyk.
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g 🦋 Toggle-ception.
• -Q — when used on the command line prior to --help=, -Q acts as a modifier to the help output ✨

Otherwise, this option makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. Chatty king 👑

• -ftime-report — makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes ⏱️🔥

If SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr then the -ftime-report information is instead emitted in JSON form as part of SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. So yeah, close but not 1:1, don't @ me 💀

• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass 🎀 Detailed queen.
• -fira-verbose=n — controls the verbosity of the dump file for the integrated register allocator 👑 The default value is 5. And if the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10. Math but make it compiler energy.
• -flto-report — prints a report with internal details on the workings of the link-time optimizer 🌈 The contents of this report vary from version to version, so like, don't expect stability bestie. It's meant to be useful to GCC developers when processing object files in LTO mode (via -flto). And guess what — it's disabled by default 🙈
• -flto-report-wpa — like -flto-report, but only prints for the WPA phase of link-time optimization. Same vibe, narrower scope 💅
• -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 🌸 Timing is everything.
• -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 🔥 Big main-character energy for build configs.

---

So, like, just like TFLAGS, this lets the target libraries be built for chill portable baseline environments while the compiler itself defaults to more demanding ones. That's actually iconic because users can easily override the defaults the compiler is configured to use when building their own programs if those defaults aren't ideal for their target environment—whereas rebuilding the runtime libraries is usually not as easy or desirable. 🙈

Unlike TFLAGS, using specs means different flags can be selected for different multilibs. The way to do it is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}. ✨

This option is discarded by the driver once it's done processing driver self spec.

It's also useful for checking that TFLAGS are being used to build all target libraries by configuring a non-bootstrap compiler with --with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the compiler and target libraries.

• -fprofile-report — Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes. Lowkey useful for vibe-checking your profile data. 📊
• -fstack-usage — Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
  • The name of the function.
  • A number of bytes.
  • One or more qualifiers: static, dynamic, bounded.

The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.

• -fstats — Emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. It's giving "dev-only lore" fr. 🧚
• -fdbg-cnt-list — Print the name and the counter upper bound for all debug counters.
• -fdbg-cnt=counter-value-list — Set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...] tuples which sets the name of the counter and list of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for second, third, fourth, tenth and eleventh invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations.
• -print-autofdo-gcov-version, --print-autofdo-gcov-version — Print the current version of GCOV being used by the AutoFDO infrastructure.
• -print-file-name=library, --print-file-name=library, --print-file-name library — Print the full absolute name of the library file library that would be used when linking—and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name. Just a little path reveal moment. 🫶
• -print-multi-directory, --print-multi-directory — Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
• -print-multi-lib, --print-multi-lib — Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches. This is supposed to ease shell processing.
• -print-multi-os-directory, --print-multi-os-directory — Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
• -print-multiarch, --print-multiarch — Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
• -print-prog-name=program, --print-prog-name=program, --print-prog-name program — Like -print-file-name, but searches for a program such as cpp.
• -print-libgcc-file-name, --print-libgcc-file-name — Same as -print-file-name=libgcc.a.

This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

• -print-search-dirs, --print-search-dirs — Print the name of the configured installation directory and a list of program and library directories gcc searches—and don't do anything else.

This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /. 😭

• -print-sysroot, --print-sysroot — Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.
• -print-sysroot-headers-suffix, --print-sysroot-headers-suffix — Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix—and don't do anything else.
• -dumpmachine — Print the compiler's target machine (for example, i686-pc-linux-gnu)—and don't do anything else.
• -dumpversion — Print the compiler version (for example, 3.0, 6.3.0 or 7)—and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version).
• -dumpfullversion — Print the full compiler version—and don't do anything else. The output is always three numbers separated by dots, major, minor and patchlevel version.
• -dumpspecs — Print the compiler's built-in specs—and don't do anything else. (This is used when GCC itself is being built.)
• --param name=value, --param=name=value — GCC by convention uses parameters that can be specified on the command line instead of hard-wired constants to represent arbitrary compiler limits or heuristics. Many parameters are related to optimization; for example, GCC does not inline functions that contain more than a certain number of instructions. The static analyzer similarly uses parameters to limit complexity, link-time optimization uses parameters to control partitioning, and so on. Other parameters control aspects of GCC that are completely internal, such as its memory allocation and garbage collection strategy. Still others control target-specific behavior.

The --param option provides a uniform interface for specifying values for these compiler parameters. However, 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. You should not depend on parameter settings for correct compilation of your program. They are exposed via the command line for the convenience of developers in debugging compilation problems or, in some cases, to provide workarounds for compiler bugs.

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🎯 Target-Specific Options

Each target machine supported by GCC can have its own options—for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. Similarly, GCC also has options that are specific to particular operating systems or runtime environments on the target.

By convention, the names of machine-specific options start with -m. Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.

🦋 AArch64 Options

These options are defined for AArch64 implementations:

• -mabi=name — Generate code for the specified data model. Permissible values are ilp32 for SysV-like data model where int, long int and pointers are 32 bits, and lp64 for SysV-like data model where int is 32 bits, but long int and pointers are 64 bits.

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.

⚠️ Heads up bestie: the ilp32 model is deprecated. 💀

• -mbig-endian — Generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mlittle-endian — Generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
• -menable-sysreg-checking — Generates an error message if an attempt is made to access a system register which is not available on the target architecture.
• -mgeneral-regs-only — bestie, this one tells GCC to only use the general-purpose registers, periodt. It stops the compiler from touching floating-point and Advanced SIMD registers. Does it put any restrictions on the assembler? Nope, none at all. 💅
• -mcmodel=tiny — generates code for the tiny code model. The program and its statically defined symbols have to live within 1MB of each other. Programs can be statically or dynamically linked, so flexible king behavior. ✨
• -mcmodel=small — the small code model: the program and its statically defined symbols must be within 4GB of each other. Statically or dynamically linked, both fine. Also? This is the default code model. Main character energy fr. 👑
• -mcmodel=large — the large code model, which makes zero assumptions about addresses and sizes of sections. Catch: programs can be statically linked only. Also -mcmodel=large is incompatible with -mabi=ilp32, -fpic and -fPIC — so don't try to mix them, it's giving chaos. 💀
• -mtp=name — picks which system register to use as the thread pointer. 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. Default is tpidr_el0. And honestly? It's recommended to compile all code intended 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. Rookie mistake avoidance, we love to see it. 🫶
• -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. That's it, that's the vibe check. ⭐
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior (-momit-leaf-frame-pointer) is the default. 🌸
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or sysreg for a canary in an appropriate system register.

With the latter choice (sysreg), the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset — this is entirely for use within the Linux kernel. iykyk. 🧚

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. And guess what, this is the default. 💖
• -mtls-dialect=traditional — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. Classic flavor. 🎀
• -mtls-size=size — specify the bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. Heads up: this option requires binutils 2.26 or newer. 🙈
• -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769 — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. What does that actually do? It involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround. Very "we'll handle it, but only if it's relevant" of it. 🔥
• -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. Big difference here: this erratum workaround is made at link time, and this will only pass the corresponding flag to the linker. This flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround. ✨
• -mlow-precision-recip-sqrt, -mno-low-precision-recip-sqrt — enable or disable the reciprocal square root approximation. Important: this option only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. Speed over precision, the tradeoff era. ⚡
• -mlow-precision-sqrt, -mno-low-precision-sqrt — enable or disable the square root approximation. Again, only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. And if enabled, it implies -mlow-precision-recip-sqrt — package deal, bestie. 🦋
• -mlow-precision-div, -mno-low-precision-div — enable or disable the division approximation. You already know the rule: only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision. 🍓
• -mtrack-speculation, -mno-track-speculation — enable or disable generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to __builtin_speculation_safe_value to permit a more efficient code sequence to be generated. Lowkey iconic compiler cooperation. 💜
• -moutline-atomics, -mno-outline-atomics — enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.

This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. Same energy when using -mcpu= when the selected cpu supports the lse feature. Oh and also — this option is on by default. 👑

• -mmax-vectorization, -mno-max-vectorization — enable or disable an override to the vectorizer cost model making vectorization always appear profitable. This option can be combined with -mautovec-preference allowing precise control over which ISA will be used for auto-vectorization. And no, unlike -fno-vect-cost-model or -fvect-cost-model=unlimited, this option does not turn off cost comparison between different vector modes. Details matter, periodt. 🌈
• -mautovec-preference=name — force an ISA selection strategy for auto-vectorization. The possible values of name are:
  • default — use the default heuristics.
  • asimd-only — use only Advanced SIMD for auto-vectorization.
  • sve-only — use only SVE for auto-vectorization.
  • prefer-asimd — use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.
  • prefer-sve — use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.

For best performance it is highly recommended to use -mcpu or -mtune instead. This parameter should only be used for code exploration. Like, exploring, not shipping. 🧚

• -march=name — specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The list below summarizes the permissible values for arch and the features that they enable 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, +fcma, +jscvt
• armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod, +rcpc2
• armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres, +frintts, +flagm2
• armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
• armv8.7-a : Armv8.7-A : armv8.6-a, +wfxt, +xs
• 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, +wfxt, +xs
• armv9.3-a : Armv9.3-A : armv9.2-a, +mops
• armv9.4-a : Armv9.4-A : armv9.3-a, +sve2p1
• armv9.5-a : Armv9.5-A : armv9.4-a, cpa, +faminmax, +lut
• armv8-r : Armv8-R : armv8-r

The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system. When -march=native is given and no other -mcpu or -mtune is given then GCC will pick the host CPU as the CPU to tune for as well as select the architecture features from. That is, -march=native is treated as -mcpu=native. ✨

The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers, -march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. (Right-most wins — like who spoke last in the group chat. 💅)

GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture. Team-player behavior. 🫶

• -mtune=name — specify the name of the target processor for which GCC should tune the performance of the code. Permissible values for this option are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2, grace, neoverse-v3, neoverse-v3ae, armagicpu, neoverse-n3, olympus, rigel, cortex-a725, cortex-x925, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, 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-r82ae, cortex-x1, cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a520ae, cortex-a710, cortex-a715, cortex-a720, cortex-a720ae, ampere1, ampere1a, ampere1b, ampere1c, cobalt-100, apple-m1, apple-m2, apple-m3, apple-m4, apple-m5, c1-nano, c1-pro, c1-premium c1-ultra and native. That's a whole lot of names but every single one counts, no skips. 🎀

Okay bestie, continuing the -mtune= saga: those values — cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 — are GCC’s way of saying “tune me for a big.LITTLE system.” 💅✨

The value neoverse-512tvb says GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. Translation: GCC is tuning for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and can execute an equivalent number of SVE arithmetic instructions per cycle — that’s 2 for 256-bit SVE, 4 for 128-bit SVE. It’s more general than tuning for a specific core like Neoverse V1, but more specific than the default tuning described below. 🦋

Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. This option has no effect if the compiler can’t recognize the processor of the host system. No cap.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

And heads up, bestie: this option cannot be suffixed by feature modifiers. No +feature glow-up here. 🙈

• -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 cpu values are the same as those available for -mtune. The permissible feature values 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. Fr fr.

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.

• -moverride=string — override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases. This option is only intended to be useful when developing GCC. 🛠️
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — enable or disable PC-relative literal loads. With this option, literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny. ✨
• -msign-return-address=scope — select the function scope on which return address signing will be applied. The permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none. This option has been deprecated by -mbranch-protection. ⚠️
• -mbranch-protection=features — select the branch protection features to use. features can have one of these forms:
  • none is the default and turns off all types of branch protection.
  • standard turns on all types of branch protection features. If a feature has additional tuning options, then standard sets it to its standard level.
  • pac-ret turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions practically always do this) using the A-key.
  • pac-ret+leaf extends the pac-ret signing to include leaf functions.
  • pac-ret+b-key or pac-ret+leaf+b-key can be used to sign the functions with the B-key instead of the A-key.
  • bti turns on branch target identification mechanism.
  • gcs turns on guarded control stack compatible code generation.
• -mharden-sls=opts — enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of the following options:
  • retbr
  • blr

In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening. 💀

• -mearly-ra=scope — determine when to enable an early register allocation pass. This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, such as SME2's strided LD1 and ST1.

The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass.

-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise.

• -mearly-ldp-fusion, -mno-early-ldp-fusion — enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by default at -O and above. 🎀
• -mlate-ldp-fusion, -mno-late-ldp-fusion — enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by default at -O and above. 🎀
• -mnarrow-gp-writes — enable conversion of 64-bit general purpose register writes to equivalent 32-bit operations when the upper 32 bits are known to be zero. This pass can be controlled with -mnarrow-gp-writes and is active at -O2 and above, but not enabled by default, except for -mcpu=olympus. 👑
• -msve-vector-bits=bits — specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.

GCC supports two forms of SVE code generation: “vector-length agnostic” output that works with any size of vector register and “vector-length specific” output that allows GCC to make assumptions about the vector length when it is 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. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths.

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. 🌈

-march and -mcpu Feature Modifiers 🧚

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:

• crc — enable CRC extension. This is on by default for -march=armv8.1-a.
• crypto — enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
• fp — enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• simd — enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• sve — enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
• lse — enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
• rdma — enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
• fp16 — enable FP16 extension. This also enables floating-point instructions.
• fp16fml — enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• rcpc — enable the RCpc extension. This enables the use of the LDAPR instructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension.
• dotprod — enable the Dot Product extension. This also enables Advanced SIMD instructions.
• aes — enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
• sha2 — enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
• sha3 — enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• sm4 — enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• profile — enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation.
• rng — enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.
• memtag — enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not supported.
• sb — enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• ssbs — enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a. 💜
• predres — turns on the Armv8-a Execution and Data Prediction Restriction instructions ✨. Real talk though: this one is assembler-only, it just lets the assembler accept the extension and it does not affect code generation. It's on by default for -march=armv8.5-a 💅.
• sve2 — enables the Armv8-a Scalable Vector Extension 2, and bestie, it also brings SVE instructions along for the ride 🦋.
• sve2-bitperm — enables SVE2 bitperm instructions. Also enables SVE2 instructions, obviously 💫.
• sve2-sm4 — enables SVE2 sm4 instructions, plus SVE2 instructions 🙈.
• sve2-aes — enables SVE2 aes instructions, plus SVE2 instructions 🎀.
• sve2-sha3 — enables SVE2 sha3 instructions, plus SVE2 instructions 💜.
• sve2p1 — enables SVE2.1 instructions. Also enables SVE2 instructions 🌸.
• tme — enables the Transactional Memory Extension 🔥.
• i8mm — enables 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions 🫶. It's enabled by default for -march=armv8.6-a. Heads up: using this with architectures prior to Armv8.2-A is not supported 💀.
• f32mm — enables 32-bit Floating point Matrix Multiply instructions, and it also enables SVE instructions ✨. Again: use with architectures prior to Armv8.2-A is not supported 😭.
• f64mm — enables 64-bit Floating point Matrix Multiply instructions, also enabling SVE instructions. Same warning, same energy: prior to Armv8.2-A is not supported 💀.
• bf16 — enables brain half-precision floating-point instructions, plus Advanced SIMD and floating-point instructions 👑. Enabled by default for -march=armv8.6-a. And yes, prior to Armv8.2-A is not supported 🚫.
• ls64 — enables the 64-byte atomic load and store instructions for accelerators ⚡.
• mops — enables the instructions that accelerate memory operations like memcpy, memmove, memset 🧚. This one's enabled by default for -march=armv8.8-a.
• flagm — enables the Flag Manipulation instructions Extension 🚩.
• flagm2 — enables the FlagM2 flag conversion instructions 🌈.
• pauth — enables the Pointer Authentication Extension 🔐.
• cssc — enables the Common Short Sequence Compression instructions 🗜️.
• cmpbr — enables the shorter compare and branch instructions, cbb, cbh and cb ✂️.
• sme — enables the Scalable Matrix Extension 💠.
• sme-i16i64 — enables the FEAT_SME_I16I64 extension to SME. Also enables SME instructions 🫶.
• sme-f64f64 — enables the FEAT_SME_F64F64 extension to SME. Also enables SME instructions 💜.
• sme2 — enables the Scalable Matrix Extension 2, and yes, it also enables SME instructions 🌟.
• sme-f8f16 — enables the FEAT_SME_F8F16 extension to SME. This also enables SME2 and FP8 instructions 🔥.
• sme-f8f32 — enables the FEAT_SME_F8F32 extension to SME. Also enables SME2 and FP8 instructions ⚡.
• sme-b16b16 — enables the FEAT_SME_B16B16 extension to SME. Also enables SME2 and SVE_B16B16 instructions 🧠.
• sme-f16f16 — enables the FEAT_SME_F16F16 extension to SME. Also enables SME2 instructions ✨.
• sme2p1 — enables the Scalable Matrix Extension version 2.1. Also enables SME2 instructions 💫.
• fcma — enables the complex number SIMD extensions 🌀.
• jscvt — enables the fjcvtzs JavaScript conversion instruction 🟨 (iykyk).
• frintts — enables floating-point round to integral value instructions 🔢.
• wfxt — enables wfet and wfit instructions 😴.
• xs — enables the XS memory attribute extension 🧬.
• lse128 — enables the LSE128 128-bit atomic instructions extension. Also enables LSE instructions 💥.
• d128 — enables support for 128-bit system register read/write instructions. This also enables the LSE128 extension 🎯.
• gcs — enables support for Armv9.4-a Guarded Control Stack extension 🛡️.
• the — enables support for Armv8.9-a/9.4-a translation hardening extension 🔒.
• rcpc2 — enables the RCpc2 extension 🔁.
• rcpc3 — enables the RCpc3 (Release Consistency) extension 🌀.
• fp8 — enables the fp8 (8-bit floating point) extension 🎈.
• fp8fma — enables the fp8 (8-bit floating point) multiply accumulate extension ✖️.
• ssve-fp8fma — enables the fp8 (8-bit floating point) multiply accumulate extension in streaming mode 🌊.
• fp8dot4 — enables the fp8 (8-bit floating point) to single-precision 4-way dot product extension 🎯.
• ssve-fp8dot4 — enables the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode 🌊.
• fp8dot2 — enables the fp8 (8-bit floating point) to half-precision 2-way dot product extension ✌️.
• ssve-fp8dot2 — enables the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode 🌊.
• faminmax — enables the Floating Point Absolute Maximum/Minimum extension 📈.
• lut — enables the Lookup Table extension 📖.
• sme-lutv2 — enables the SME Lookup Table v2 (LUTv2) extension 📚.
• cpa — enables the Checked Pointer Arithmetic instructions ✅.
• sve-b16b16 — enables the SVE non-widening brain floating-point (bf16) extension 🧠. Lowkey important: this only has an effect when sve2 or sme2 are also enabled 👀.
• sve-bfscale — enables the SVE_BFSCALE extension ⚖️.
• poe2 — enables the Permission Overlays Extension 2 🔑.
• tev — enables the TIndex Exception-like Vector Extension 🧵.
• tlbid — enables the TLBI Domains Extension 🌐.
• gcie — enables the GICv5 (Generic Interrupt Controller) CPU Interface Extension 🎛️.
• lscp — enables the load acquire and store release pair extension 🔄.
• mops-go — enables tag only variants of MOPS instructions. This also enables the instructions to accelerate memory operations and Armv8.5-a Memory Tagging Extensions 🏷️.
• sve2p3 — enables SVE2.3. This also enables SVE2.2 instructions 💫.
• sme2p3 — enables SME2.3. This also enables SME2.2 instructions ✨.
• f16f32dot — enables Armv9.7-a f16f32dot instructions. Also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions 🔥.
• sve-b16mm — enables the SVE B16MM Extension. Also enables SVE instructions, Advanced SIMD and floating-point instructions 💜.
• mtetc — enables Data cache tag block operations. Also enables Armv8.5-a Memory Tagging Extensions 🏷️.
• f16f32mm — enables Armv9.7-a f16f32mm instructions. Also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions 🎀.
• f16mm — enables f16mm instructions. Also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions 🌸.

okay so this next bit is like the ✨implication lore✨ and you gotta keep it straight: 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, -mno-half-reg-file — don't allocate any register in the range r32...r63 🙅. That's what lets your code run on hardware variants that lack these registers 💅.
• -mprefer-short-insn-regs, -mno-prefer-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 — vibe check your binary, bestie 🤷.
• -mbranch-cost=*num* — set the cost of branches to roughly num "simple" instructions 🔢. This cost is only a heuristic and is not guaranteed to produce consistent results across releases 😬.
• -mcmove, -mno-cmove — enable the generation of conditional moves 🎛️.
• -mnops=*num* — emit num NOPs before every other generated instruction 😴 (yes, every other one, iconic behaviour).
• -mno-soft-cmpsf, -msoft-cmpsf — for single-precision floating-point comparisons, emit an fsub instruction and test the flags 🚩. That's 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. Values other than 8 or 16 are untested and unlikely to work 🙈. Also note that this option changes the ABI — compiling a program with a different stack offset than the libraries have been compiled with generally does not work 🚫. It can be useful if you want to evaluate whether a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=*num* option 👑.
• -mno-round-nearest, -mround-nearest — -mno-round-nearest makes the scheduler assume that the rounding mode has been set to truncating ✂️. The default is -mround-nearest 🎯.
• -mlong-calls — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing a (otherwise direct) call 📞. This is the default 💅.
• -mshort-calls — if not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl instructions, so use these instructions for direct calls ✨.

The default is -mlong-calls. Note that -mlong-calls is equivalent to -mno-short-calls, and similarly -mno-long-calls is equivalent to -mshort-calls 🔁.

• -msmall16, -mno-small16 — assume addresses can be loaded as 16-bit unsigned values 🔢. Heads up: this does not apply to function addresses for which -mlong-calls semantics are in effect 🙅.
• -mfp-mode=*mode* — set the prevailing mode of the floating-point unit 🎛️. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches ⚡.

mode can be set to one the following values: 🌀

• caller — any mode at function entry is valid, and it gets retained or restored when the function returns, and also when it calls other functions. This is THE mode for compiling libraries or other compilation units you might wanna drop into different programs with different prevailing FPU modes. The convenience of using one single object file outweighs the size and speed overhead of any extra mode switching that might be needed, compared to what you'd need with a more specific choice of prevailing FPU mode. 💖
• 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 perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller 🎀

---

• -mmay-round-for-trunc, -mno-may-round-for-trunc — this option lets floating point to integer truncation be replaced with rounding to save mode switching. It's disabled by default. lowkey a trade, bestie 🙈
• -mfp-iarith, -mno-fp-iarith — this option enables use of the floating-point unit for integer add and subtract. Disabled by default.
• -msplit-lohi, -mno-split-lohi — codegen tweak for splitting of 32-bit loads.
• -mpost-inc, -mno-post-inc — codegen tweak for generation of post-increment addresses.
• -mpost-modify, -mno-post-modify — codegen tweak for generation of post-modify addresses.

The defaults are msplit-lohi, -mpost-inc, and -mpost-modify. ✨

• -mno-vect-double — change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as the preferred SIMD mode.
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8.

⚠️ heads up bestie, this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types. like, don't just flip it for funsies 💅

• -msplit-vecmove-early, -mno-split-vecmove-early — split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case 😭
• -m1reg-reg — specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means no register is used for this purpose. The default is -m1reg-none. 👑

---

🌈 AMD GCN Options 🌈

These options are defined specifically for the AMD GCN port. fr fr, this is a whole dedicated lil world. 🫶

• -march=gpu
• -mtune=gpu

Set architecture type or tuning for gpu. Supported values for gpu are:

• gfx900 — compile for GCN5 Vega 10 devices (gfx900).
• gfx902 — compile for GCN5 Vega gfx902 devices. (Experimental)
• gfx904 — compile for GCN5 Vega gfx904 devices. (Experimental)
• gfx906 — compile for GCN5 Vega 20 devices (gfx906).
• gfx908 — compile for CDNA1 Instinct MI100 series devices (gfx908).
• gfx909 — compile for GCN5 Vega gfx909 devices. (Experimental)
• gfx90a — compile for CDNA2 Instinct MI200 series devices (gfx90a).
• gfx90c — compile for GCN5 Vega 7 devices (gfx90c).
• gfx942 — compile for CDNA3 Instinct MI300 series devices (gfx942). (Experimental)
• gfx950 — compile for the CDNA3 gfx950 devices. (Experimental)
• gfx9-generic — compile generic code for Vega devices, executable on the following subset of GFX9 devices: gfx900, gfx902, gfx904, gfx906, gfx909 and gfx90c.
• gfx9-4-generic — compile generic code for CDNA3 devices, executable on the following subset of GFX9 devices: gfx942 and gfx950. (Experimental)
• gfx1030 — compile for RDNA2 gfx1030 devices (GFX10 series).
• gfx1031 — compile for RDNA2 gfx1031 devices (GFX10 series). (Experimental)
• gfx1032 — compile for RDNA2 gfx1032 devices (GFX10 series). (Experimental)
• gfx1033 — compile for RDNA2 gfx1033 devices (GFX10 series). (Experimental)
• gfx1034 — compile for RDNA2 gfx1034 devices (GFX10 series). (Experimental)
• gfx1035 — compile for RDNA2 gfx1035 devices (GFX10 series). (Experimental)
• gfx1036 — compile for RDNA2 gfx1036 devices (GFX10 series).
• gfx10-3-generic — compile generic code for GFX10-3 devices, executable on gfx1030, gfx1031, gfx1032, gfx1033, gfx1034, gfx1035, and gfx1036.
• gfx1100 — compile for RDNA3 gfx1100 devices (GFX11 series).
• gfx1101 — compile for RDNA3 gfx1101 devices (GFX11 series). (Experimental)
• gfx1102 — compile for RDNA3 gfx1102 devices (GFX11 series). (Experimental)
• gfx1103 — compile for RDNA3 gfx1103 devices (GFX11 series).
• gfx1150 — compile for RDNA3 gfx1150 devices (GFX11 series). (Experimental)
• gfx1151 — compile for RDNA3 gfx1151 devices (GFX11 series). (Experimental)
• gfx1152 — compile for RDNA3 gfx1152 devices (GFX11 series). (Experimental)
• gfx1153 — compile for RDNA3 gfx1153 devices (GFX11 series). (Experimental)
• gfx11-generic — compile generic code for GFX11 devices, executable on gfx1100, gfx1101, gfx1102, gfx1103, gfx1150, gfx1151, gfx1152, and gfx1153.

---

• -mgang-private-size=bytes — set the amount of local data-share (LDS) memory to reserve for gang-private variables. The default is 512. 🍓
• -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. 🔥

• -mxnack=on
• -mxnack=off
• -mxnack=any

Compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise. 😭 careful with this one, the mode matching is not a vibe suggestion

• -Wopenacc-dims
• -Wno-openacc-dims

Control warnings about invalid OpenACC dimensions. 💜

---

🦋 ARC Options 🦋

The following options control the architecture variant for which code is being compiled:

• -mbarrel-shifter — generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture. ⭐
• -mcpu=cpu — set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:
  • arc600 — compile for ARC600. Aliases: -mA6, -mARC600.
  • arc601 — compile for ARC601. Alias: -mARC601.
  • arc700 — compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
  • arcem — compile for ARC EM.
  • archs — compile for ARC HS.
  • em — compile for ARC EM CPU with no hardware extensions.
  • em4 — compile for ARC EM4 CPU.
  • em4_dmips — compile for ARC EM4 DMIPS CPU.
  • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — compile for ARC HS34 CPU.
  • hs38 — compile for ARC HS38 CPU.
  • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on.
  • hs4x — compile for ARC HS4x CPU.
  • hs4xd — compile for ARC HS4xD CPU.
  • hs4x_rel31 — compile for ARC HS4x CPU release 3.10a.
  • arc600_norm — compile for ARC 600 CPU with "norm" instructions enabled.
  • arc600_mul32x16 — compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc600_mul64 — compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
  • arc601_norm — compile for ARC 601 CPU with "norm" instructions enabled.
  • arc601_mul32x16 — compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc601_mul64 — compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
  • nps400 — compile for ARC 700 on NPS400 chip.
  • em_mini — compile for ARC EM minimalist configuration featuring reduced register set.
• -mdpfp
• -mdpfp-compact

Generate double-precision FPX instructions, tuned for the compact implementation. ✨

• -mdpfp-fast — generate double-precision FPX instructions, tuned for the fast implementation.
• -mno-dpfp-lrsr
• -mdpfp-lrsr

Control whether "lr" and "sr" instructions use FPX extension aux registers. This is enabled by default. 🎀

• -mea — generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. Only valid for -mcpu=ARC700.
• -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 — turns on generation of ARC SIMD instructions through target-specific builtins ✨ only valid if you're rocking -mcpu=ARC700, bestie, no exceptions.
• -msoft-float — okay so this one is literally ignored 🙈 it's just sitting here for compatibility purposes only. Software floating-point code gets emitted by default, and that default can be overridden by the FPX options: -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision. She's a legacy queen, respect her.
• -mswap — generate swap instructions 💅
• -matomic — enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Heads up though: not available for ARC 6xx or ARC EM cores 😭
• -mdiv-rem — enables the div and rem instructions for ARCv2 cores. Slay.
• -mcode-density, -mno-code-density — enables code density instructions for ARC EM. This option is on by default for ARC HS 🫶
• -mll64 — enables double load/store operations for ARC HS cores.
• -mtp-regno=regno — specifies the thread pointer register number. that's it, that's the whole vibe.
• -mbitops — enables use of NPS400 bit operations ✨
• -mcmem — enables use of the NPS400 xld/xst extension.
• -mmpy-option=multo — compiles ARCv2 code with a multiplier design option. You can pass multo as either a string or a numeric value, and wlh1 is the default. The recognized values are, and I'm quoting the whole menu because you deserve the full tea:
  • 0, none — no multiplier available. we die like real ones 💀
  • 1, w — 16x16 multiplier, fully pipelined. Enables these instructions: mpyw and mpyuw.
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 4, wlh3 — two 16x16 multipliers, blocking, sequential. Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 5, wlh4 — one 16x16 multiplier, blocking, sequential. Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 6, wlh5 — one 32x4 multiplier, blocking, sequential. Additionally enables: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 7, plus_dmpy — ARC HS SIMD support.
  • 8, plus_macd — ARC HS SIMD support.
  • 9, plus_qmacw — ARC HS SIMD support.

This option is only available for ARCv2 cores, periodt.

• -mfpu=fpu — enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. This option is only available for ARC EM.
  • fpuda_div — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM.
  • fpuda_fma — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM.
  • fpuda_all — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision floating-point hardware extensions are also enabled. This option is only available for ARC EM.
  • fpus_div — enables support for single-precision floating-point, square-root and divide hardware extensions.
  • fpud_div — enables support for double-precision floating-point, square-root and divide hardware extensions. This option includes option fpus_div. Not available for ARC EM.
  • fpus_fma — enables support for single-precision floating-point and fused multiply and add hardware extensions.
  • fpud_fma — enables support for double-precision floating-point and fused multiply and add hardware extensions. This option includes option fpus_fma. Not available for ARC EM.
  • fpus_all — enables support for all single-precision floating-point hardware extensions.
  • fpud_all — enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM.

and yes, that's twelve of them, I counted, we don't skip options in this house 🎀

• -mirq-ctrl-saved=register-range, blink, lp_count — specifies general-purpose registers that the processor automatically saves/restores on interrupt entry and exit. register-range is given as two registers separated by a dash, it always starts with r0, and the upper limit is the fp register. blink and lp_count are optional. Only valid for ARC EM and ARC HS cores 💖
• -mrgf-banked-regs=number — specifies the number of registers replicated in the second register bank on entry to a fast interrupt. Quick vocab check: fast interrupts are the interrupts with the highest priority level P0, and they save only the PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. Use this when you're using fast interrupts in an ARC V2 family processor. Permitted values are 4, 8, 16, and 32 ⭐
• -mlpc-width=width — specifies the width of the lp_count register. Valid values for width are 8, 16, 20, 24, 28 and 32 bits, and the default width is fixed to 32 bits. If the width is less than 32, the compiler does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it's known that the lp_count register can hold the required loop-counter value 😤 depending on the width specified, the compiler and run-time library might continue to use the loop mechanism for various needs. This option defines the macro __ARC_LPC_WIDTH__ with the value of width.
• -mrf16 — tells the compiler to generate code for a 16-entry register file. This option defines the __ARC_RF16__ preprocessor macro ✨
• -mbranch-index — enables use of bi or bih instructions to implement jump tables 🔥

---

🧚 the assembler pass-throughs

The following options are passed through to the assembler, and they also define preprocessor macro symbols. two jobs, one flag, main character energy:

• -mlock — passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol __Xlock.
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol __Xswape.
• -mxy — passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol __Xxy.

🎀 the annotation corner

The following options control how the assembly code is annotated:

• -misize — annotate assembler instructions with estimated addresses.

🌸 linker pass-throughs

The following options are passed through to the linker:

• -marclinux, -mno-arclinux — passed through to the linker to specify use of the arclinux emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested 🫶
• -marclinux_prof, -mno-arclinux_prof — passed through to the linker to specify use of the arclinux_prof emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested.

💜 code semantics, aka the big vibes

The following options control the semantics of generated code:

• -mlong-calls — generates calls as register indirect calls, which gives you access to the full 32-bit address range. iconic reach 💅
• -mmedium-calls, -mno-medium-calls — don't use less than a 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed so you can use the 25-bit range instead of the 21-bit range with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -G num — puts definitions of externally-visible data in a small data section if that data is no bigger than num bytes. Default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations ✨
• -mno-sdata — don't generate sdata references. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -mvolatile-cache, -mno-volatile-cache — control how volatile references are accessed. The default is -mvolatile-cache, which uses ordinary cached memory accesses for volatile references. Use -mno-volatile-cache to enable cache bypass for volatile references 🦋

🧁 fine-tuning time

The following options fine tune code generation:

• -mauto-modify-reg — enables the use of pre/post modify with register displacement.
• -mno-brcc, -mbrcc — controls a target-specific pass in arc_reorg that generates compare-and-branch (brcc) instructions, which is enabled by default. Real talk: it has no effect on generation of these instructions driven by the combiner pass.
• -mcase-vector-pcrel — uses PC-relative switch case tables to enable case table shortening. This is the default for -Os.
• -mno-cond-exec — disables the ARCompact-specific pass that generates conditional execution instructions.

Okay so back to that ARC conditional-execution pass, because she’s a whole situation: due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass to generate conditional execution is often lowkey lacking. That’s why the ARC port has kept a special pass around — it tries to find more conditional-execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass generally, but not always, improves performance and code size, at the cost of extra compilation time, which is exactly why there’s an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, bestie, you should consider using -mmedium-calls instead. 💅

• -mearly-cbranchsi — Enables pre-reload use of the "cbranchsi" pattern. ✨
• -mindexed-loads — Enables the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case. So like… be aware, okay? 🙈
• -mlra-priority-none — Don’t indicate any priority for target registers. Chill vibes, no ranking.
• -mlra-priority-compact — Indicate target register priority for r0..r3 / r12..r15. 🎀
• -mlra-priority-noncompact — Reduce target register priority for r0..r3 / r12..r15.
• -mmillicode — When optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of registers are often shortened by using call to a special function in libgcc; this is referred to as a millicode call. As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn on or off millicode call generation. Big-brain size hack, but mind the link tea.
• -mcode-density-frame — This option enable the compiler to emit "enter" and "leave" instructions. These instructions are only valid for CPUs with code-density feature. ⭐
• -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 enable usage of "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. 🔥

🌸 ARM Options 🌸

These -m options are defined for the ARM port:

• -mabi=name — Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs and aapcs-linux. 👑
• -mapcs-frame — Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is not 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. Same energy, still old.
• -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 cannot be reliably used inside one program. The default is -mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS configurations this option is meaningless. 🎀
• -msched-prolog, -mno-sched-prolog — Allow or prevent the reordering of instructions in the function prologue, or the merging of those instruction with the instructions in the function’s body. With -mno-sched-prolog, this means that 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 information 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, bestie — link compatibility is not a vibe.

• -mgeneral-regs-only — Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers but will not impose any restrictions on the assembler. 🫶
• -mlittle-endian — Generate code for a processor running in little-endian mode. This is the default for all standard configurations. Little-endian girlies stay winning.
• -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 is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. 💀
• -march=name[+extension...] — This specifies the name of the target ARM architecture. GCC uses this name to determine 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, 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. Love a cancel-out moment.

Most extension names are generically named, but have an effect that is dependent upon the architecture to which it is applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.

The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions.

armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk

• +fp — The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this extension.
• +nofp — Disable the floating-point instructions.

armv7

The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.

• +fp — The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.
• +nofp — Disable the floating-point instructions.

armv7-a

• +mp — The multiprocessing extension.
• +sec — The security extension.
• +fp — The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
• +simd — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can be used as aliases for this extension.
• +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 — turns OFF the Advanced SIMD instructions (and no, it does not disable floating point, bestie — those are separate, don't get it twisted) 🙈
• +nofp — disables the floating-point and Advanced SIMD instructions, like, both, gone, bye 💀
• armv7ve — the extended version of the ARMv7-A architecture with support for virtualization 👑
  • +fp — the VFPv4 floating-point instructions, with 16 double-precision registers. Also, +vfpv4-d16 can be used as an alias for this extension 🫶
  • +simd — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. And +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. And +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 (does not disable floating point, ok?) 🙈
  • +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. Lowkey 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 — enables the Dot Product extension. This also enables Advanced SIMD instructions 🎯
  • +nocrypto — disables the cryptographic extension 🚫
  • +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions 💀
  • +sb — Speculation Barrier Instruction 🛡️
  • +predres — Execution and Data Prediction Restriction Instructions 🌈
  • +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions 🧮
  • +bf16 — "Brain" half-precision floating-point instructions (that's the manpage's spelling, not mine, iykyk). 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 enables Advanced SIMD and floating-point instructions 🧮
  • +bf16 — "Brain" half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions 🧠
• armv7-r 🎀
  • +fp.sp — the single-precision VFPv3 floating-point instructions. The extension +vfpv3xd can be used as an alias for this extension 🫶
  • +fp — the VFPv3 floating-point instructions with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension ✨
  • +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations 🌸
  • +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations 🦋
  • +nofp — disables the floating-point extension 💀
  • +idiv — the ARM-state integer division instructions ➗
  • +noidiv — disables 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 — disables 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, bestie 💖
  • +nofp — nope, no floating-point extension for us 🙅‍♀️ disabled.
  • +cdecp0, +cdecp1, ... , +cdecp7 — turns on the Custom Datapath Extension (CDE) on the coprocessors you pick, according to the numbers you give in the options, in the range 0 to 7 ✨
  • +pacbti — enables the Pointer Authentication and Branch Target Identification Extension 🎀

armv8-m.main 🌸

• +dsp — the DSP instructions 🎛️
• +nodsp — disables the DSP extension 💅
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- and double-precision floating-point instructions 💖
• +nofp — disables the floating-point extension.
• +cdecp0, +cdecp1, ... , +cdecp7 — enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 ✨

armv8-r 🦋

• +crc — the Cyclic Redundancy Check (CRC) instructions 🔍
• +fp.sp — the single-precision FPv5 floating-point instructions.
• +simd — the ARMv8-A Advanced SIMD and floating-point instructions 🌈
• +crypto — the cryptographic instructions 🔐
• +nocrypto — disables the cryptographic instructions.
• +nofp — disables the floating-point, Advanced SIMD and cryptographic instructions. all of them. gone. 💀

---

-march=native makes the compiler auto-detect the architecture of the build computer 🕵️‍♀️. Heads up though: rn this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect — so like, it just silently does nothing, no cap 🙈.

-march=unset makes the compiler ignore any -march=... options that appear earlier on the command line and behave as if the option was never passed 🫶. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications. lowkey a lifesaver for your build logs ✨

---

• -mtune=name — specifies the name of the target ARM processor for which GCC should tune the performance of the code 🎯. For some ARM implementations better performance can be obtained by using this option. 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. that's the whole guest list, bestie, we're having a party 👑🎉

Also, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. Permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55. big brain + little brain energy fr fr 🧠✨

-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch. The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go. it's giving "everybody's invited but nobody's the main character" 💅

-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code 🫶

-mtune=native causes the compiler to auto-detect the CPU of the build computer 🕵️‍♀️. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.

• -mcpu=name[+extension...] — specifies the name of the target ARM processor 👑. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. so like, this one's the vibe-setter, but the specialists can overrule it 🤷‍♀️

Many of the supported CPUs implement optional architectural extensions. Where this is so the architectural extensions are normally enabled by default ✨. If implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. For floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions 🫶

For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD or both the SIMD and floating-point instructions respectively 💅

Permissible names for this option are the same as those for -mtune 👯‍♀️

The following extension options are common to the listed CPUs:

• +nodsp — disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Also disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85 🎛️
• +nopacbti — disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
• +nomve — disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85 🚫
• +nomve.fp — disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 on cortex-m52, cortex-m55, cortex-m85 and star-mc1 ✨
• +nofp — disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4, cortex-m7, cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Disables the floating-point and SIMD instructions on generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and cortex-a55 🫶
• +nofp.dp — disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.
• +nosimd — disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9 💅
• +crypto — enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55 🔐

Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a 🌈

-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more information 🫶

-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect 🙈

-mcpu=unset causes the compiler to ignore any -mcpu=... options that appear earlier on the command line and behave as if the option was never passed ✨. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications.

• -mfpu=name — specifies what floating-point hardware (or hardware emulation) is available on the target 🎀. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2 🦋

The setting auto is the default and is special ⭐. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march — like, it reads the room and matches the fit 💅

If the selected floating-point hardware includes the NEON extension (like when you pass -mfpu=neon), bestie, pls note: GCC's auto-vectorization pass will not generate floating-point operations unless -funsafe-math-optimizations is also specified. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic (denormal values get treated as zero, specifically), so using NEON instructions may lead to a loss of precision. 😭

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. ✨

• -mfp16-format=name — Specify the format of the __fp16 half-precision floating-point type. Permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined. 🫶
• -mstructure-size-boundary=n — The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it. 🌸

Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.

🚨 This option is deprecated.

• -mabort-on-noreturn — Generate a call to the function abort at the end of a noreturn function. It is executed if the function tries to return. 💀
• -mlong-calls, -mno-long-calls — Tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction. 🔥

Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers.

• -msingle-pic-base — Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. 🧚
• -mpic-register=reg — Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-checking is enabled, otherwise the default is R10. 💅
• -mpic-data-is-text-relative — Assume that the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable -msingle-pic-base by default. 🌈
• -mpoke-function-name — Write the name of each function into the text section, directly preceding the function prologue. The generated code is similar to this:

                        t0
                            .ascii "arm_poke_function_name", 0
                            .align
                        t1
                            .word 0xff000000 + (t1 - t0)
                        arm_poke_function_name
                            mov     ip, sp
                            stmfd   sp!, {fp, ip, lr, pc}
                            sub     fp, ip, #4

When performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000). 🎀

• -mthumb, -marm — Select between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the --with-mode=state configure option. 🦋

You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas.

• -mtpcs-frame — Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame. 🌸
• -mtpcs-leaf-frame — Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame. 🫶
• -mcallee-super-interworking — Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default. ✨
• -mcaller-super-interworking — Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There is a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default. 💜
• -mtp=name — Specify the access model for the thread local storage pointer. The model soft generates calls to __aeabi_read_tp. Other accepted models are tpidrurw, tpidruro and tpidrprw which fetch the thread pointer from the corresponding system register directly (supported from the arm6k architecture and later). These system registers are accessed through the CP15 co-processor interface and the argument cp15 is also accepted as a convenience alias of tpidruro. The argument auto uses the best available method for the selected processor. The default setting is auto. 🔥
• -mtls-dialect=dialect — Specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. 🌈
• -mword-relocations — Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. 💅
• -mfix-cortex-m3-ldrd — Some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when -mcpu=cortex-m3 is specified. 😭
• -mfix-cortex-a57-aes-1742098, -mno-fix-cortex-a57-aes-1742098, -mfix-cortex-a72-aes-1655431, -mno-fix-cortex-a72-aes-1655431 — Enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. This option is enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified. 👑
• -munaligned-access, -mno-unaligned-access — Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures are accessed a byte at a time. 🦋

The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined.

• -mslow-flash-data — Assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and off by default. It conflicts with -mword-relocations. 🐢
• -masm-syntax-unified — Assume inline assembler is using unified asm syntax. The default is currently off which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. 💀
• -mrestrict-it — Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode. 🎀
• -mpure-code — Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format give all text sections the ELF processor-specific section attribute SHF_ARM_PURECODE. This option is only available when generating non-pic code for M-profile targets. ⭐
• -mcmse — generates secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which lives at <https://developer.arm.com/documentation/ecm0359818/latest/>. 💅
• -mfix-cmse-cve-2021-35465 — shields you from a potential security issue with the VLLDM instruction in some M-profile devices when you're using CMSE (CVE-2021-365465). It's on by default whenever -mcpu= is used with cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85 or star-mc1. Wanna turn the mitigation off? That's -mno-fix-cmse-cve-2021-35465, bestie.
• -mstack-protector-guard=guard</bold> and -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a canary you reach via the TLS register. Heads up 🙈: -mstack-protector-guard-offset= is for use with -fstack-protector-guard=tls and not for use in user-land code.
• -mfdpic, -mno-fdpic — picks the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for arm-*-uclinuxfdpiceabi targets, this one is on by default and implies -fPIE if none of the PIC/PIE-related options is provided. On other targets it only switches on the FDPIC-specific code generation features, and you gotta explicitly hand over the PIC/PIE-related options yourself 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 honestly required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs. No cap. 🫶

• -mbranch-protection=features</bold> — enables branch protection features (armv8.1-m.main only, just so you know ✨).

features can have one of these forms:

• none — generates code without branch protection or return address signing.
• standard — generates code with all branch protection features enabled at their standard level.
• pac-ret — generates code with return address signing set to its standard level, which is to sign all functions that save the return address to memory.
• pac-ret+leaf — extends the pac-ret signing to include leaf functions even if they do not write the return address to memory.
• bti — adds landing-pad instructions at the permitted targets of indirect branch instructions.

If support for the +pacbti architecture extension is not enabled (like, via -march=), 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 turn the extra security on at run time on processors that support the PACBTI extension. 🌈

Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.

Any setting other than none is supported only on armv8-m.main or later.

The default is to generate code without branch protection or return address signing. 💀

• -mvectorize-with-neon-quad, -mvectorize-with-neon-double — control whether vectorization uses NEON quad-word or double-word registers. The default is -mvectorize-with-neon-quad — slay. 👑

---

🌸 AVR Options 🌸

These options are defined for AVR implementations, bestie — yes, the tiny chip energy is real:

• -mmcu=mcu</bold> — specify the AVR instruction set architecture (ISA) or device type. The default for this option is avr2. 🎀

Here are all the AVR devices and ISAs that are supported. Note: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs (<https://gcc.gnu.org/wiki/avr-gcc#spec-files>) file specs-mcu. Only the latter is provided by the compiler according to the supported *mcu*s below — the rest is supported by AVR-LibC (<https://github.com/avrdudes/avr-libc/>), or by means of atpack (<https://gcc.gnu.org/wiki/avr-gcc#atpack>) files from the hardware manufacturer. 🦋

• avr2 — "Classic" devices with up to 8 KiB of program memory.
  • mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.
• avr25 — "Classic" devices with up to 8 KiB of program memory and with the MOVW instruction.
  • mcu = attiny13, attiny13a, attiny24, attiny24a, attiny25, attiny261, attiny261a, attiny2313, attiny2313a, attiny43u, attiny44, attiny44a, attiny45, attiny48, attiny441, attiny461, attiny461a, attiny4313, attiny84, attiny84a, attiny85, attiny87, attiny88, attiny828, attiny841, attiny861, attiny861a, ata5272, ata6616c, at86rf401.
• avr3 — "Classic" devices with 16 KiB up to 64 KiB of program memory.
  • mcu = at76c711, at43usb355.
• avr31 — "Classic" devices with 128 KiB of program memory.
  • mcu = atmega103, at43usb320.
• avr35 — "Classic" devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction.
  • mcu = attiny167, attiny1634, atmega8u2, atmega16u2, atmega32u2, ata5505, ata6617c, ata664251, at90usb82, at90usb162.
• avr4 — "Enhanced" devices with up to 8 KiB of program memory.
  • mcu = atmega48, atmega48a, atmega48p, atmega48pa, atmega48pb, atmega8, atmega8a, atmega8hva, atmega88, atmega88a, atmega88p, atmega88pa, atmega88pb, atmega8515, atmega8535, ata5795, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.
• avr5 — "Enhanced" devices with 16 KiB up to 64 KiB of program memory.
  • mcu = atmega16, atmega16a, atmega16hva, atmega16hva2, atmega16hvb, atmega16hvbrevb, atmega16m1, atmega16u4, atmega161, atmega162, atmega163, atmega164a, atmega164p, atmega164pa, atmega165, atmega165a, atmega165p, atmega165pa, atmega168, atmega168a, atmega168p, atmega168pa, atmega168pb, atmega169, atmega169a, atmega169p, atmega169pa, atmega32, atmega32a, atmega32c1, atmega32hvb, atmega32hvbrevb, atmega32m1, atmega32u4, atmega32u6, atmega323, atmega324a, atmega324p, atmega324pa, atmega324pb, atmega325, atmega325a, atmega325p, atmega325pa, atmega328, atmega328p, atmega328pb, atmega329, atmega329a, atmega329p, atmega329pa, atmega3250, atmega3250a, atmega3250p, atmega3250pa, atmega3290, atmega3290a, atmega3290p, atmega3290pa, atmega406, atmega64, atmega64a, atmega64c1, atmega64hve, atmega64hve2, atmega64m1, atmega64rfr2, atmega640, atmega644, atmega644a, atmega644p, atmega644pa, atmega644rfr2, atmega645, atmega645a, atmega645p, atmega649, atmega649a, atmega649p, atmega6450, atmega6450a, atmega6450p, atmega6490, atmega6490a, atmega6490p, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5787, ata5835, ata5700m322, ata5702m322, at90pwm161, at90pwm216, at90pwm316, at90can32, at90can64, at90scr100, at90usb646, at90usb647, at94k, m3000.
• avr51 — "Enhanced" devices with 128 KiB of program memory.
  • mcu = atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.
• avr6 — "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory.
  • mcu = atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.
• avrxmega2 — "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory.
  • mcu = atxmega8e5, atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5, avr64da28, avr64da28s, avr64da32, avr64da32s, avr64da48, avr64da48s, avr64da64, avr64da64s, avr64db28, avr64db32, avr64db48, avr64db64, avr64dd14, avr64dd20, avr64dd28, avr64dd32, avr64du28, avr64du32, avr64ea28, avr64ea32, avr64ea48, avr64sd28, avr64sd32, avr64sd48.

avrxmega3 🦋

"XMEGA" cuties with up to 64 KiB of combined program memory and RAM, and where program memory is visible in the RAM address space. The mcu = roster is long, bestie, so buckle up:

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" girlies with more than 64 KiB and up to 128 KiB of program memory. mcu = atxmega64a3, atxmega64a3u, atxmega64a4u, atxmega64b1, atxmega64b3, atxmega64c3, atxmega64d3, atxmega64d4, avr128da28, avr128da28s, avr128da32, avr128da32s, avr128da48, avr128da48s, avr128da64, avr128da64s, avr128db28, avr128db32, avr128db48, avr128db64.

avrxmega5 💜

"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory plus more than 64 KiB of RAM. She's got range. mcu = atxmega64a1, atxmega64a1u.

avrxmega6 ⭐

"XMEGA" besties with more than 128 KiB of program memory. mcu = atxmega128a3, atxmega128a3u, atxmega128b1, atxmega128b3, atxmega128c3, atxmega128d3, atxmega128d4, atxmega192a3, atxmega192a3u, atxmega192c3, atxmega192d3, atxmega256a3, atxmega256a3b, atxmega256a3bu, atxmega256a3u, atxmega256c3, atxmega256d3, atxmega384c3, atxmega384d3.

avrxmega7 👑

"XMEGA" icons with more than 128 KiB of program memory and more than 64 KiB of RAM. Main character energy fr fr. mcu = atxmega128a1, atxmega128a1u, atxmega128a4u.

avrtiny 🧚

The "Reduced Tiny" Tiny core girlies — only 16 general purpose registers and 512 B up to 4 KiB of program memory. Small 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 only, no cap. mcu = attiny11, attiny12, attiny15, attiny28, at90s1200.

---

• -mabsdata — Assume that all data in static storage can be accessed by LDS / STS instructions. Heads up: this option only does anything on reduced Tiny devices like the ATtiny40. See also the absdata AVR variable attribute 🫶.
• -mcvt — Use a compact vector table ✨. Some devices support a CVT with only four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ, 3=Prio0 IRQs. This option links startup code from crtmcu-cvt.o instead of the usual crtmcu.o. Besides giving you a compact vector table, the startup code sets bit CPUINT_CTRLA.CPUINT_CVT, which enables the CVT on the device.

If you don't want the startup code setting CPUINT_CTRLA.CPUINT_CVT, you can satisfy the symbol __init_cvt so that the respective code is no longer pulled in from libmcu.a. For example, you can link with -Wl,--defsym,__init_cvt=0.

The CVT startup code is available since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1010).

• -mdouble= bits
• -mlong-double= bits — Set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether or not a specific value for bits is allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the 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 supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs via the no_gccisr AVR function attribute. This feature is activated per default if optimization is on (but not with -Og, see Optimize Options), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683) 🔥.
• -mint8 — Assume int to be an 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Please note that this option does not conform to the C standards — but it does result in smaller code size, so. Your call 🙈.
• -mmain-is-OS_task — Do not save registers in main. The effect is the same as attaching the OS_task AVR attribute to main. It is activated per default if optimization is on.
• -mno-call-main — Don't run main by means of

XCALL  main
XJMP   exit

Instead, put main in section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so that no call is required. By setting this option, the user asserts that main will not return.

This option can be used for devices with very limited resources in order to save a few bytes of code and stack space. It will work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions, there will be no performance gain 💀.

• -mno-interrupts — Generated code is not compatible with hardware interrupts. Code size is smaller. (Yes, that's the trade. Yes, it's real.)
• -mrelax — Try to replace the CALL resp. JMP instruction with the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

Jump relaxing is performed by the linker because jump offsets aren't known before the code is located. So the assembler code generated by the compiler is the same, but the instructions in the executable may differ from the instructions in the assembler code 👀.

Relaxing must be turned on if linker stubs are needed — see the section on EIND and linker stubs below.

• -mpmem-wrap-around — Enable program counter wrap-around in linker relaxation.
• -mrodata-in-ram
• -mno-rodata-in-ram — Locate the .rodata sections for read-only data in RAM resp. in program memory. For most devices there is no choice here and this option acts rather like an assertion.

Since v14, and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default — provided the required GNU Binutils support (PR31124, https://sourceware.org/PR31124) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM 💅.

• -mtiny-stack — Only change the lower 8 bits of the stack pointer.
• -mfract-convert-truncate — Allow using truncation instead of rounding towards zero for fractional fixed-point types.
• -nodevicelib — Don't link against AVR-LibC's device specific library libmcu.a.

Heads up bestie 🚨 — notice that since AVR-LibC v2.3, that library contains code that is essential for the correct functioning of a program. In particular, it contains parts of the startup code like: __init_sp (https://github.com/avrdudes/avr-libc/issues/1011) to initialize the stack pointer with the symbol __stack; __init_cvt (https://github.com/avrdudes/avr-libc/issues/1010) to set up the hardware to use a compact vector table with -mcvt; __call_main (https://github.com/avrdudes/avr-libc/issues/1012) to call main and exit; and __do_flmap_init (https://github.com/avrdudes/avr-libc/issues/931) to set up FLMAP according to the symbol __flmap.

• -nodevicespecs — Don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. This means you have to supply your own private device specs file by means of -specs=path-to-specs-file. There is no more need for the option -mmcu=mcu 🫶.

This option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point to a directory containing a folder named device-specs, which 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 ⭐.

---

🔥 AVR Optimization Options 🔥

The following options are pure optimization options. Quick reality check though: -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from above are only almost optimization options, since there are rare occasions where their different code generation actually matters 👀.

• -maccumulate-args — Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in the function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards. See also the -fdefer-pop optimization option.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, because arguments need not be removed from the stack after such a function call 💖.

• -mbranch-cost=cost — sets how expensive a conditional branch is to cost. Reasonable values are small, non-negative integers, and the default is 0. Like, literally just 0.
• -mcall-prologues — function prologues and epilogues get expanded as calls to the appropriate subroutines instead of being inlined. Code size goes down 📉 (that's the whole point).
• -mfuse-add, -mno-fuse-add, -mfuse-add=level — optimizes indirect memory accesses on reduced Tiny devices ✨. By default it uses level=1 for optimizations -Og and -O1, and level=2 for the higher optimizations. Valid values for level are 0, 1 and 2. That's it, that's the list 🎀
• -mfuse-move, -mno-fuse-move, -mfuse-move=level — runs a post-reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations 🔥. The default uses level=3 for optimization -O1, and level=23 for higher optimizations. Valid values for level are in the range 0 ... 23, which is a 3:2:2:2 mixed radix value — each digit controls some aspect of the optimization. iykyk 💅
• -mfuse-move2 — runs a post-combine optimization pass that tries to fuse move instructions. Lowkey a whole other pass, don't confuse it with the one above 🫶
• -mstrict-X — uses address register X the way the hardware actually proposes. Meaning X is only used in indirect, post-increment or pre-decrement addressing. Periodt.

Without this option, the X register may be used in the same way as Y or Z — which then gets emulated by additional instructions 😭. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as:

        adiw r26, const   ; X += const
        ld   <Rn>, X        ; <Rn> = *X
        sbiw r26, const   ; X -= const

• -msplit-bit-shift — splits multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset ✨. This optimization is turned on per default for -O2 and higher, including -Os but excluding -Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by -mfuse-move. Wild, I know.
• -msplit-ldst — splits multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for -O2 and higher 🌸
• -muse-nonzero-bits — enables optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for -O2 and higher 💜

---

🦋 "EIND" and Devices with More Than 128 Ki Bytes of Flash 🦋

Okay bestie, listen up, this part is a whole vibe check on its own.

Pointers in the implementation are 16 bits wide. The address of a function or label is represented as a word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND that serves as the most significant part of the target address when EICALL or EIJMP instructions are used 👑

Indirect jumps and calls on these devices are handled as follows by the compiler, and are subject to some limitations:

• The compiler never sets EIND. Like, not once. Never.
• The compiler uses EIND implicitly in EICALL/EIJMP instructions, or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.
• The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue. So don't be out here changing it mid-run, okay? 🙈
• 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 💀 you've been warned.
• The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/) 🧚
• It is legitimate for user-specific startup code to set up EIND early — for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located. Iconic behaviour, honestly:

#include <avr/io.h>

static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
  __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                  "out %i0,r24" :: "n" (&EIND) : "r24","memory");
}

The __trampolines_start symbol is defined in the linker script ⭐

• Stubs are generated automatically by the linker if the following two conditions are met:
  • The address of a label is taken by means of the gs modifier (short for generate stubs) like so:

LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))

• The final location of that label is in a code segment outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • Taking address of a function or code label.
  • Computed goto.
  • If prologue-save function is used, see -mcall-prologues command-line option.
  • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.
  • C and C++ constructors/destructors called during startup/shutdown.
  • If the tools hit a gs() modifier explained above.
• Jumping to non-symbolic addresses like so is not supported 🙅‍♀️:

int main (void)
{
    /* Call function at word address 0x2 */
    return ((int(*)(void)) 0x2)();
}

Instead, a stub has to be set up — i.e. the function has to be called through a symbol (func_4 in the example):

int main (void)
{
    extern int func_4 (void);

    /* Call function at byte address 0x4 */
    return func_4();
}

and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, func_4 can be defined in the linker script ✨

---

🎀 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.

• The startup code initializes the RAMP special function registers with zero.
• If an AVR Named Address Spaces — a 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. No cap, that's mandatory.

---

⭐ 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. So cute, honestly 🌸

The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase 💅

If device is not a device but only a core architecture like avr51, this macro is not defined.

__AVR_DEVICE_NAME__

Setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to atmega8 👑

If device is not a device but only a core architecture like avr51, this macro is not defined.

__AVR_CVT__

The code is being compiled with option -mcvt to use a compact vector table 🔥

• "__AVR_XMEGA__" — the device / architecture is in the XMEGA family of devices. 💅
• "__AVR_HAVE_ADIW__" — the device has the "ADIW" and "SBIW" instructions.
• "__AVR_HAVE_ELPM__" — the device has the "ELPM" instruction.
• "__AVR_HAVE_ELPMX__" — the device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
• "__AVR_HAVE_LPMX__" — the device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
• "__AVR_HAVE_MOVW__" — the device has the "MOVW" instruction to perform 16-bit register-register moves.
• "__AVR_HAVE_MUL__" — the device has a hardware multiplier. 🔥
• "__AVR_HAVE_JMP_CALL__" — the device has the "JMP" and "CALL" instructions. This is the case for devices with more than 8 KiB of program memory.
• "__AVR_HAVE_EIJMP_EICALL__", "__AVR_3_BYTE_PC__" — the device has the "EIJMP" and "EICALL" instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.
• "__AVR_2_BYTE_PC__" — the program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.
• "__AVR_HAVE_8BIT_SP__", "__AVR_HAVE_16BIT_SP__" — the stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.
• "__AVR_HAVE_SPH__", "__AVR_SP8__" — the device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.
• "__AVR_HAVE_RAMPD__", "__AVR_HAVE_RAMPX__", "__AVR_HAVE_RAMPY__", "__AVR_HAVE_RAMPZ__" — the device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively.
• "__NO_INTERRUPTS__" — this macro reflects the -mno-interrupts command-line option.
• "__AVR_ERRATA_SKIP__", "__AVR_ERRATA_SKIP_JMP_CALL__" — some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is only defined if "__AVR_HAVE_JMP_CALL__" is also set.
• "__AVR_ISA_RMW__" — the device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
• "__AVR_SFR_OFFSET__=offset" — instructions that can address I/O special function registers directly like "IN", "OUT", "SBI", etc. may use a different address as if addressed by an instruction to access RAM like "LD" or "STS". This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
• "__AVR_SHORT_CALLS__" — the -mshort-calls command line option is set.
• "__AVR_PM_BASE_ADDRESS__=addr" — some devices support reading from flash memory by means of "LD*" instructions. The flash memory is seen in the data address space at an offset of "__AVR_PM_BASE_ADDRESS__". If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put ".rodata" into RAM. This is handled by the default linker description file, and is currently available for "avrtiny" and "avrxmega3". Even more convenient, there is no need to use address spaces like "__flash" or features like attribute "progmem" and "pgm_read_*". 🦋
• "__AVR_HAVE_FLMAP__" — this macro is defined provided the following conditions are met:
  • The device has the "NVMCTRL_CTRLB.FLMAP" bitfield. This applies to the AVR64* and AVR128* devices.
  • It's not known at assembler-time which emulation will be used.

This implies the compiler was configured with GNU Binutils that implement PR31124 ("https://sourceware.org/PR31124").

• "__AVR_RODATA_IN_RAM__" — this macro is undefined when the code is compiled for a core architecture.

When the code is compiled for a device, the macro is defined to 1 when the ".rodata" sections for read-only data is located in RAM; and defined to 0, otherwise.

• "__WITH_AVRLIBC__" — the compiler is configured to be used together with AVR-LibC. See the --with-avrlibc configure option.
• "__HAVE_SIGNAL_N__" — the compiler supports the signal(num) and interrupt(num) AVR Attributes,,function attributes with an argument num that specifies the number of the interrupt service routine.
• "__HAVE_DOUBLE_MULTILIB__" — defined if -mdouble= acts as a multilib option.
• "__HAVE_DOUBLE32__", "__HAVE_DOUBLE64__" — defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=.
• "__DEFAULT_DOUBLE__" — the size in bits of "double" if -mdouble= is not set. To test the layout of "double" in a program, use the built-in macro "__SIZEOF_DOUBLE__".
• "__HAVE_LONG_DOUBLE32__", "__HAVE_LONG_DOUBLE64__", "__HAVE_LONG_DOUBLE_MULTILIB__", "__DEFAULT_LONG_DOUBLE__" — same as above, but for "long double" instead of "double".
• "__WITH_DOUBLE_COMPARISON__" — reflects the "--with-double-comparison={tristate|bool|libf7}" configure option ("https://gcc.gnu.org/install/configure.html#avr") and is defined to 2 or 3.
• "__WITH_LIBF7_LIBGCC__", "__WITH_LIBF7_MATH__", "__WITH_LIBF7_MATH_SYMBOLS__" — reflects the "--with-libf7={libgcc|math|math-symbols}" configure option ("https://gcc.gnu.org/install/configure.html#avr").

🧚 AVR Internal Options

Okay so these next ones are internal, bestie: the compiler uses them, and they also communicate between device specs files and the compiler proper. You don't need to set these options by hand, in particular they are not optimization options.

⚠️ Using these options in the wrong way may lead to sub-optimal or wrong code.

They are documented for completeness, and in order to get a better understanding of device specs ("https://gcc.gnu.org/wiki/avr-gcc#spec-files") files.

• -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, -mno-specld-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.
• -mcsync-anomaly, -mno-csync-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.
• -mlow64k, -mno-low64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. The default behavior is to assume that the program is arbitrarily large (-mno-low64k).
• -mstack-check-l1 — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
• -mid-shared-library, -mno-id-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. The default is -mno-id-shared-library, to generate code that doesn't assume ID-based shared libraries are being used.
• -mleaf-id-shared-library, -mno-leaf-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. 🌈

...and honestly the default being -mno-leaf-id-shared-library is kind of a whole thing, because under that default the compiler makes no assumption that the code it's compiling won't get linked against any ID shared libraries 😭 — and the price you pay for that safety is that slower code gets generated for jump and call insns. not glamorous, but there it is 💅

• -mshared-library-id=n — tells the compiler the identification number of the ID-based shared library it's building. setting the value to 0 gets you more compact code ✨; any other value forces that number to be allocated to the current library, but it's genuinely no more space- or time-efficient than just not passing this option at all. so like... 0 or nothing, bestie 🎀
• -msep-data, -mno-sep-data — generates code that lets the data segment live in a completely different area of memory than the text segment 🦋. why do you care? because it allows execute in place in an environment with no virtual memory management, by eliminating relocations against the text section. the default is -mno-sep-data, which tells GCC to generate code that assumes the data segment follows the text segment. periodt 💜
• -mlong-calls, -mno-long-calls — makes the compiler do function calls by first loading the address of the function into a register, then doing a subroutine call on that register 🔥. you need this switch if the target function sits outside the 24-bit addressing range of the offset-based version of the subroutine call instruction. it is not enabled by default, and -mno-long-calls restores the default behavior. ⚠️ heads up though: these switches have no effect on how the compiler generates code to handle function calls made via function pointers. iykyk.
• -mfast-fp — links against the fast floating-point library 🧚. this library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), all in the interest of performance. yes, that is the trade, and yes, it is iconic of it.
• -minline-plt — enables inlining of PLT entries in function calls to functions that aren't known to bind locally. 🫶 important: it has no effect without -mfdpic.
• -mmulticore — builds a standalone application for multicore Blackfin processors 👑. this option makes proper start files and link scripts that support multicore get used, and it defines the macro "__BFIN_MULTICORE". it can only be used with -mcpu=bf561[-sirevision].

it can also be used together with -mcorea or -mcoreb, which selects the one-application-per-core programming model. without -mcorea or -mcoreb, you get the single-application/dual-core programming model — and in that model, the main function of Core B should be named "coreb_main" 💀.

if you don't use this option at all, the single-core application programming model is what you get.

• -mcorea — builds a standalone application for Core A of BF561 when you're using the one-application-per-core programming model 🌸. proper start files and link scripts get used to support Core A, and the macro "__BFIN_COREA" is defined. this option can only be used in conjunction with -mmulticore.
• -mcoreb — builds a standalone application for Core B of BF561 when you're on the one-application-per-core programming model 🎀. proper start files and link scripts get used to support Core B, and the macro "__BFIN_COREB" is defined. when you use this one, "coreb_main" should be used instead of "main". and yeah — 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. your loader should initialize SDRAM before loading the application. that's on you, not on GCC 🙈
• -micplb — assumes 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 — specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it's allowed to emit when generating assembly code 🦋. permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. this is the default ✨.
• -msim — choose startup files and linker script suitable for the simulator 🧚.
• -msdata=default — put small global and static data in the ".neardata" section, which is pointed to by register "B14" 🫶. put small uninitialized global and static data in the ".bss" section, which sits adjacent to the ".neardata" section. put small read-only data into the ".rodata" section. the corresponding sections used for large pieces of data are ".fardata", ".far" and ".const".
• -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the "B14" register to access them 👑
• -msdata=none — make no use of the sections reserved for small data, and use absolute addresses to access all data 💀. put all initialized global and static data in the ".fardata" section, and all uninitialized data in the ".far" section. put all constant data into the ".const" section.
• -mdsbt — compile for the DSBT shared library ABI 🌈. this option is required to compile with -fpic or -fPIC, and it implies -fpic.
• -mlong-calls — avoid generating PC-relative calls; use indirection instead 🔥.

---

🎀 CRIS Options 🎀

these options are defined specifically for the CRIS ports, bestie 💅

• -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture 🌸. the choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. default is v0.
• -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions ✨. the choices for architecture-type are the same as for -march=architecture-type.
• -mmax-stackframe=n — warn when the stack frame of a function exceeds n bytes 🫶.
• -metrax4, -metrax100 — these two are just synonyms for -march=v3 and -march=v8 respectively. cute little nicknames 💜
• -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the "muls" and "mulu" instructions, for the CPU models where it applies 😭. this option is disabled by default.
• -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code. this option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file 💀.
• -mcc-init — do not use condition-code results from a previous instruction; always emit compare and test instructions before condition codes get used 🔥.
• -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- versions) arrange (or 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 arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned ✨. the default is 32-bit alignment.
• -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame get 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, it will not tell you. you just have to look. scary but okay.
• -mbest-lib-options, -moverride-best-lib-options — -mbest-lib-options selects the most feature-enabling options allowed by other options 👑. this option has no no- form, but -moverride-best-lib-options disables it regardless of the relative order of the two options on the command line.
• -mtrap-using-break8 — emit traps as break 8 🎀. this is the default for CRIS v3 and up. if it's disabled, calls to "abort" are used instead.
• -mtrap-unaligned-atomic — emit checks that cause break 8 instructions to execute when atomic builtins get applied to misaligned memory 💥
• -munaligned-atomic-may-use-library — handle atomic builtins that may be applied to unaligned data by calling library functions 🧚. this option overrides -mtrap-unaligned-atomic.
• -sim — 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 it 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 🦋

• -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. like yes there are a LOT of them, that's the list, i don't make the rules 😭
• -mbig-endian, -mlittle-endian — 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.

⚠️ bestie, note this: 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.

• -mdouble-float, -mno-double-float — When -mhard-float is in effect, this enables generation of double-precision float instructions. It's the default except when compiling for CK803.
• -mfdivdu, -mno-fdivdu — When -mhard-float is in effect, this enables generation of frecipd, fsqrtd, and fdivd instructions. It's 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, -mno-mp — Enable multiprocessor instructions; the default is off.
• -mcp, -mno-cp — Enable coprocessor instructions; the default is off.
• -mcache, -mno-cache — Enable coprocessor instructions; the default is off.
• -msecurity, -mno-security — Enable C-SKY security instructions; the default is off.
• -mtrust, -mno-trust — Enable C-SKY trust instructions; the default is off.
• -mdsp, -mno-dsp, -medsp, -mno-edsp, -mvdsp, -mno-vdsp — 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, -mno-multiple-stld — 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.
• -msim, -mno-sim — Links the library libsemi.a which is in compatible with simulator. Applicable to ELF compiler only.

---

💻 Cygwin and MinGW Options 💻

These additional options are available for Microsoft Windows targets:

• -mconsole — This option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets.
• -mcrtdll=library — Preprocess, compile or link with specified C RunTime DLL library. This option adjusts predefined macros __CRTDLL__, __MSVCRT__, _UCRT and __MSVCRT_VERSION__ for the specified CRT library, chooses the start file for the CRT library, and links with the CRT library. Recognized CRT library names for proprocessor 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 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 — This option is available for Cygwin and MinGW targets. It specifies that a DLL---a dynamic link library---is to be generated, enabling the selection of the required runtime startup object and entry point.
• -mnop-fun-dllimport — This option is available for Cygwin and MinGW targets. It specifies that the dllimport attribute should be ignored.
• -mthreads — Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data.
• -municode — This option is available for MinGW-w64 targets. It causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code.
• -mwin32 — This option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.
• -mwindows — This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately.
• -fno-set-stack-executable — This option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available.
• -fwritable-relocated-rdata — This option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the .data section. This is necessary for older runtimes not supporting modification of .rdata sections for pseudo-relocation.
• -mpe-aligned-commons — This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature.
• -muse-libstdc-wrappers — Use Cygwin DLL wrappers to support C++ operators new and delete.

See also under x86 Options for standard options.

---

🍎 Darwin Options 🍎

These options are defined for all architectures running the Darwin operating system.

FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.

The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this.

The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files.

• -Fdir — Add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by -I options and are scanned in a left-to-right order.

okay so bestie, a framework directory is literally a directory with frameworks in it. A framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework is the name of this directory excluding the .framework. Headers associated with the framework are found in one of those two directories, with Headers being searched first. A subframework is a framework directory that is in a framework's Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like "#include <Framework/header.h>", where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory.

• -iframeworkdir — Like -F except the directory is a treated as a system directory. The main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.
• -arch name — Generate output for architecture name. As described above, GCC generates output for the architecture it was configured for, using its usual options to select subarchitecture variants. The -arch option is accepted for compatibility, but an error is diagnosed if name is inconsistent with GCC's own idea of the target architecture.
• -dependency-file filename — Alias for the preprocessor option -MF filename.
• -fapple-kext — Generate code for Darwin loadable kernel extensions.
• -gused — Emit debugging information for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON.
• -gfull — Emit debugging information for all symbols and types.
• -matt-stubs, -mno-att-stubs — Enable AT&T-style PIC stubs. This is the default when supported by the target architecture (currently x86 only).
• -mconstant-cfstrings, -fconstant-cfstrings — When the NeXT runtime is being used (the default on these systems), override any -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings. -fconstant-cfstrings is an alias for -mconstant-cfstrings.
• -mdynamic-no-pic — Generate code suitable for executables (not shared libraries). This option is incompatible with -fpic, -fPIC, -fpie, or -fPIE.
• -mfix-and-continue, -ffix-and-continue, -findirect-data — Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
• -mkernel — Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
• -mmacosx-version-min= version, -asm_macosx_version_min= version — The -mmacosx-version-min option specifies the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include 12, 10.12, and 10.5.8.

If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible.

-asm_macosx_version_min=version is similar, but the GCC driver passes its version information only to the assembler.

• -mone-byte-bool — Override the defaults for "bool" so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.

Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model.

• -msymbol-stubs, -mno-symbol-stubs — Force generation of external symbol indirection stubs for PIC references. By default, this option is enabled automatically if the target linker version (-mtarget-linker) is old enough to require them.
• -mtarget-linker= version, -mtarget-linker version — Specify the target ld64 version, overriding any version specified in the GCC configuration. Newer linker versions support improved code generation in some cases, for example for PIC code.
• -ObjC — Equivalent to -x objective-c; specifies that the input is is Objective-C source code.
• -ObjC++ — Equivalent to -x objective-c++; specifies that the input is is Objective-C++ source code.
• -Wnonportable-cfstrings, -Wno-nonportable-cfstrings — Warn if constant CoreFoundation string objects contain non-portable characters. This warning is enabled by default.
• -all_load — Loads all members of static archive libraries. See man ld(1) for more information.
• -arch_errors_fatal — Cause the errors having to do with files that have the wrong architecture to be fatal.
• -bind_at_load — Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
• -bundle — Produce a Mach-o bundle format file. See man ld(1) for more information.
• -bundle_loader executable — This option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
• -dynamiclib — When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command.
• -force_cpusubtype_ALL — This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
• -nodefaultrpaths — Do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds -nodefaultrpaths to the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later.
• -nodefaultexport — Do not add default symbol exports to modules or dynamic libraries.
• These options are passed to the Darwin linker:

-allowable_client  client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dylib_file
-dylinker
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-framework
-headerpad_max_install_names
-image_base
-init symbol-name
-install_name
-keep_private_externs
-pagezero_size
-preload
-read_only_relocs
-sectalign
-sectcreate
-seg_addr_table
-seg1addr
-segaddr
-segprot
-segs_read_only_addr
-segs_read_write_addr
-sub_library
-sub_umbrella
-twolevel_namespace
-twolevel_namespace_hints
-umbrella
-undefined
-unexported_symbols_list
-weak_framework
-weak_reference_mismatches
-whatsloaded
-whyload

These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.

🐘 DEC Alpha Options 🐘

These -m options are defined for the DEC Alpha implementations:

• -mno-soft-float, -msoft-float — Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

Note that Alpha implementations without floating-point operations are required to have floating-point registers.

• -mfp-regs, -mno-fp-regs — Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option.

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

• -mieee — The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.
• -mieee-with-inexact — This is like -mieee, except the generated code also maintains the IEEE inexact-flag. Turning this on makes the generated code implement fully-compliant IEEE math. On top of _IEEE_FP, it also defines _IEEE_FP_EXACT as a preprocessor macro. On some Alpha implementations, the resulting code may execute significantly slower than the default code. Since very little code actually depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.
• -mfp-trap-mode=trap-mode — This controls which floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values:
  • n — This is the default (normal) setting. The only traps enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
  • u — In addition to the traps enabled by n, underflow traps are enabled as well.
  • su — Like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
  • sui — Like su, but inexact traps are enabled as well.
• -mfp-rounding-mode=rounding-mode — Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:
  • n — Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
  • m — Round towards minus infinity.
  • c — Chopped rounding mode. Floating-point numbers are rounded towards zero.
  • d — Dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus infinity.
• -mtrap-precision=trap-precision — In the Alpha architecture, floating-point traps are imprecise. That means without software assistance it is impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected:
  • p — Program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
  • f — Function precision. The trap handler can determine the function that caused a floating-point exception.
  • i — Instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.

Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

• -mieee-conformant — This option marks the generated code as IEEE conformant.

🚨 Bestie, you must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.

• -mbuild-constants — Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, 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 -mcpu= option or that of the CPU on which GCC was built if none is specified.
• -msafe-bwa, -mno-safe-bwa — Indicate whether in the absence of the optional BWX instruction set GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses.
• -msafe-partial, -mno-safe-partial — Indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses, including piecemeal accesses to unaligned data as well as block accesses to leading and trailing parts of aggregate types or other objects in memory that do not respectively start and end on an aligned 64-bit data boundary.
• -mfloat-vax, -mfloat-ieee — Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.
• -mexplicit-relocs, -mno-explicit-relocs — Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these macros does not allow optimal instruction scheduling. GNU Binutils as of version 2.12 supports a new syntax that allows the compiler to explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
• -msmall-data, -mlarge-data — When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller are placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but allows the variables to be directly accessed via a single instruction.

The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.

• -msmall-text, -mlarge-text — When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1.

The default is -mlarge-text.

• -mcpu=cpu_type — Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you do not specify a processor type, GCC defaults to the processor on which the compiler was built.

Supported values for cpu_type are:

• ev4, ev45, 21064 — Schedules as an EV4 and has no instruction set extensions.
• ev5, 21164 — Schedules as an EV5 and has no instruction set extensions.
• ev56, 21164a — Schedules as an EV5 and supports the BWX extension.
• pca56, 21164pc, 21164PC — Schedules as an EV5 and supports the BWX and MAX extensions.
• ev6, 21264 — Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
• ev67, 21264a — Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

• -mtune=cpu_type — Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed.

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.

• -mmemory-latency=time — Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

Valid options for time are:

• number — A decimal number representing clock cycles.
• L1, L2, L3, main — The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

• -mtls-kernel — Emit rdval instead of rduniq for thread pointer.
• -mtls-size=bitsize — Specify bit size of immediate TLS offsets. Valid values for bitsize are 16, 32, and 64; it defaults to 32.
• -mlong-double-128, -mlong-double-64 — Specify the size of the long double type. Note that -mlong-double-128 is incompatible with VAX floating point.

---

🛸 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.

Okay so the eBPF CPU-feature toggles keep rolling, bestie — here’s the next batch, no cap: ✨

• -malu32, -mno-alu32 — turns generation of 32-bit ALU instructions on or off. It’s enabled for CPU v3 and above. 💅
• -mv3-atomics, -mno-v3-atomics — toggles instructions for general atomic operations introduced in CPU v3. Enabled for CPU v3 and above. 🦋
• -mbswap, -mno-bswap — toggles byte swap instructions. Enabled for CPU v4 and above. 🌸
• -msdiv, -mno-sdiv — toggles signed division and modulus instructions. Enabled for CPU v4 and above. 🎀
• -msmov, -mno-smov — toggles sign-extending move and memory load instructions. Enabled for CPU v4 and above. 💖
• -mcpu=version — picks which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The default is v4. 👑

Supported values for version are:

• v1 — the first stable eBPF ISA, with no special features or extensions.
• v2 — supports the jump extensions, as in -mjmpext.
• v3 — all features of v2, plus:
  • 32-bit jump operations, as in -mjmp32
  • 32-bit ALU operations, as in -malu32
  • general atomic operations, as in -mv3-atomics
• v4 — all features of v3, plus:
  • byte swap instructions, as in -mbswap
  • signed division and modulus instructions, as in -msdiv
  • sign-extending move and memory load instructions, as in -msmov

• -mco-re, -mno-co-re — toggles BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target (-gbtf). 🫶
• -mxbpf — generates code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  • Save and restore callee-saved registers at function entry and exit, respectively. 💜
• -masm=dialect — outputs assembly instructions using the eBPF selected dialect. The default is pseudoc. 🎀

Supported values for dialect are:

• normal — outputs normal assembly dialect.
• pseudoc — outputs pseudo-c assembly dialect.

• -minline-memops-threshold=bytes — sets a size threshold in bytes at or below which memmove, memcpy and memset shall always be expanded inline. Operations dealing with sizes larger than this threshold would have to be implemented using a library call instead of being expanded inline, but since BPF doesn’t allow libcalls, exceeding this threshold results in a compile-time error. The default is 1024 bytes. 💀
• -Wco-re — enables warnings for scenarios where -mco-re is enabled, but it may not be possible to generate CO-RE compatible code. 🦋

---

🌸 FR30 Options 🌸

These options are defined specifically for the FR30 port. Short and sweet, no cap. 🧚

• -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. ✨
• -mno-lsim — assumes that runtime support has been provided, so there’s no need to include the simulator library (libsim.a) on the linker command line. 💜

---

🔥 FRV Options 🔥

FRV options are a whole buffet, lowkey iconic. Every single one is below, periodt. 💅

• -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. Slay. ⚡
• -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, -mno-dword — control whether the ABI uses double-word instructions. 🌈
• -mdouble, -mno-double — enable or disable use of floating-point double instructions. 💖
• -mmedia, -mno-media — enable or disable use of media instructions. 🎀
• -mmuladd, -mno-muladd — enable or disable use of multiply and add/subtract instructions. 🔥
• -mfdpic — selects the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim. 👑
• -minline-plt — enables inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It’s enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line. 🎀
• -mTLS — assumes a large TLS segment when generating thread-local code. 🦋
• -mtls — does 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 is known to be in read-only sections. It’s enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it’s more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it. 💜
• -multilib-library-pic — link with the (library, not FD) pic libraries. It’s implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly. 😭
• -mlinked-fp — follows 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 — uses indirect addressing to call functions outside the current compilation unit. This allows the functions to be placed anywhere within the 32-bit address space. 🌈
• -malign-labels — tries 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 — generates position-independent EABI code. ✨
• -macc-4 — use only the first four media accumulator registers. 🍓
• -macc-8 — use all eight media accumulator registers. 🍓
• -mpack, -mno-pack — enable or disable packing VLIW instructions. 🎀
• -mno-eflags — do not mark ABI switches in e_flags. 🦋
• -mcond-move, -mno-cond-move — enable or disable the use of conditional-move instructions; it is enabled by default. This switch is mainly for debugging the compiler and will likely be removed in a future version. ⚠️
• -mscc, -mno-scc — enable or disable the use of conditional set instructions; it is enabled by default. This switch is mainly for debugging the compiler and will likely be removed in a future version. ⚠️
• -mcond-exec, -mno-cond-exec — enable or disable the use of conditional execution; it is enabled by default. This switch is mainly for debugging the compiler and will likely be removed in a future version. ⚠️
• -mvliw-branch, -mno-vliw-branch — enable or disable an optimization pass to pack branches into VLIW instructions; it is enabled by default. This switch is mainly for debugging the compiler and will likely be removed in a future version. ⚠️
• -mmulti-cond-exec, -mno-multi-cond-exec — enable or disable optimization of && and || in conditional execution; it is enabled by default. This switch is mainly for debugging the compiler and will likely be removed in a future version. ⚠️
• -mnested-cond-exec, -mno-nested-cond-exec — enable or disable nested conditional execution optimizations; it is enabled by default. This switch is mainly for debugging the compiler and will likely be removed in a future version. ⚠️
• -moptimize-membar, -mno-optimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by default. 🔥
• -mtomcat-stats — causes gas to print out tomcat statistics. 🍓
• -mcpu=cpu — selects the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. 👑

---

🦋 FT32 Options 🦋

FT32 time! Tiny but mighty, and yes, there’s a real warning here. 🦋

• -msim — specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed. 🚨
• -mnodiv — do not use div and mod instructions. 🦋
• -mft32b — enable use of the extended instructions of the FT32B processor. 🎀
• -mcompress — compress all code using the Ft32B code compression scheme. 🌸
• -mnopm — do not generate code that reads program memory. 💜

---

🐧 GNU/Linux Options 🐧

Now we’re in GNU/Linux territory, bestie — library-picking era, iykyk. 🐧

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, -mno-android — 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. Finally, this option causes the preprocessor macro __ANDROID__ to be defined. ✨

This option can be disabled completely with -mno-android. 💖

• -tno-android-cc — disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. 🎀
• -tno-android-ld — disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker. 🌈

✨ H8/300 Options ✨

These -m options are the ones defined for the H8/300 implementations, bestie — tiny list, big vibes 💅

• -mrelax — shortens some address references at link time when it can, and it does that by using the linker option -relax.
• -mh — generates code for the H8/300H 🎀
• -ms — generates code for the H8S.
• -mn — generates code for the H8S and H8/300H in the normal mode. Heads up though: this switch must be used with either -mh or -ms, no solo runs 🙈
• -msx — generates H8SX code.
• -ms2600 — generates code for the H8S/2600. This switch must be used with -ms.
• -mquickcall — uses registers for argument passing. Efficient little icon 💫
• -mslowbyte — treats access to byte-sized memory as slow.
• -mexr, -mno-exr — stores extended registers on the stack before execution of functions with the "monitor" attribute. The default is -mexr, and this option is valid only for H8S targets.
• -mint32 — makes "int" data 32 bits by default.
• -malign-300 — on the H8/300H and H8S, uses 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, and -malign-300 causes them to be aligned on 2-byte boundaries instead. This option has no effect on the H8/300.

---

🌸 HPPA Options 🌸

These -m options are defined for the HPPA family of computers — buckle up, there's a lot 🫶

• -march=architecture-type — generates 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. Peep /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, iykyk.
• -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0 — synonyms for -march=1.0, -march=1.1, and -march=2.0, respectively.
• -matomic-libcalls, -mno-atomic-libcalls — generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default, and it only affects the generation of atomic libcalls by the HPPA backend.

Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations, provided they are implemented as a single access.

The PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction. So literally all atomic support is implemented using sync and atomic libcalls — sync libcall support lives in libgcc.a, and 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. Multi-tasking queen 🔥

• -mcaller-copies — the caller copies function arguments passed by hidden reference. Use this one with care, bestie, because it's not compatible with the default 32-bit runtime 😭 That said, only aggregates larger than eight bytes are passed by hidden reference, and the option gives you better compatibility with OpenMP.
• -mcoherent-ldcw — use the ldcw/ldcd coherent cache-control hint.
• -mdisable-fpregs — disables floating-point registers. Equivalent to -msoft-float.
• -mdisable-indexing — stops the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH 🙈
• -mfast-indirect-calls — generates code that assumes calls never cross space boundaries, which lets GCC emit code that performs faster indirect calls ⚡

Fr fr, this option does not work in the presence of shared libraries or nested functions. Noted.

• -mfixed-range=register-range — generates code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use, which is super 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 — enables the use of assembler directives only GAS understands.
• -mgnu-ld — uses options specific to GNU ld. This passes -shared to ld when building a shared library, and it's the default when GCC is configured, explicitly or implicitly, with the GNU linker. It does not affect which ld gets called — it only changes what parameters are passed to that ld. Which ld gets called is decided by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mhp-ld — uses 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. Same deal as above: it does not affect which ld is called, only what parameters get passed to it. The ld that's called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld. Only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
• -mlinker-opt — enables the optimization pass in the HP-UX linker. Just know that this makes symbolic debugging impossible 💀
• -mlong-calls — generates code that uses long call sequences, which guarantees 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 (whichever applies) 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. Sibling calls are always limited at 240,000 bytes 🧚

Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

It's normally not desirable to use this option since it degrades performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.

The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. On systems that support long absolute calls, and long PIC symbol-difference or PC-relative calls, the impact should be relatively small. But an indirect call is used on 32-bit ELF systems in PIC code, and that one is quite long 😭

• -mlong-load-store — generates 3-instruction load and store sequences, as sometimes required by the HP-UX 10 linker. Equivalent to the +k option to the HP compilers.
• -mno-space-regs — generates code that assumes the target has no space registers. This lets GCC generate faster indirect calls and use unscaled index address modes. Such code is suitable for level 0 PA systems and kernels 👑
• -mordered — assumes memory references are ordered and barriers are not needed.
• -mportable-runtime — uses the portable calling conventions proposed by HP for ELF systems.
• -mschedule=cpu-type — schedules code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. Peep /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000.
• -msio, -mwsio — -msio generates the predefine "_SIO" for server IO. The default is -mwsio, which generates the predefines "__hp9000s700", "__hp9000s700__" and "_WSIO" for workstation IO. These options are available under HP-UX and HI-UX.
• -msoft-float — generates 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 can't be done directly in cross-compilation — you have to make your own arrangements to provide suitable library functions for cross-compilation. 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 for this to work 🍓

• -msoft-mult — uses software integer multiplication. This disables the use of the "xmpyu" instruction.
• -munix=unix-std — generates compiler predefines and selects 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, and 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 though 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", plus the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", plus the startfile unix98.o.

Important to note, bestie: this option changes the interfaces for various library routines, and it also affects the operational behavior of the C library. So extreme care is needed when using this option 💀

Library code 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 — suppresses the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later.
• -static — the HP-UX C library implementation of "setlocale" has a dependency on libdld.sl, and there isn't an archive version of libdld.sl. So when the -static option is specified, special link options are needed to resolve this dependency 🎀

okay so about that HP-UX situation 💅 — on HP-UX 10 and later, the GCC driver straight up adds the necessary options to link with libdld.sl whenever you pass -static. and like... plot twist, that makes the resulting binary dynamic 🫠 yes bestie, -static doing a little chaos. meanwhile on the 64-bit port the linkers are already generating dynamic binaries by default in any case, so it's giving nothing new. and if that's not the vibe for you? the -nolibdld option is right there to stop the GCC driver from adding these link options. slay.

• -threads — adds support for multithreading with the dce thread library under HP-UX. it sets flags for both the preprocessor and the linker, so both sides are in on it 🧵✨

---

💙 IA-64 Options 💙

these are the -m options defined for the Intel IA-64 architecture. buckle up 🎀

• -mbig-endian — generate code for a big-endian target. this is the default for HP-UX 🔥
• -mlittle-endian — generate code for a little-endian target. this is the default for AIX5 and GNU/Linux 💫
• -mgnu-as, -mno-gnu-as — generate (or don't) code for the GNU assembler. this is the default 🛠️
• -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker. this is the default 🔗
• -mno-pic — generate code that does not use a global pointer register. heads up: the result is not position independent code, and it violates the IA-64 ABI 😭 like, actually rule-breaking, bestie
• -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don't) a stop bit immediately before and after volatile asm statements ⏸️
• -mregister-names, -mno-register-names — generate (or don't) in, loc, and out register names for the stacked registers. this may make assembler output more readable 👀✨
• -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. this may be useful for working around optimizer bugs 🐛
• -mconstant-gp — generate code that uses a single constant global pointer value. useful when compiling kernel code 👑
• -mauto-pic — generate code that is self-relocatable. this implies -mconstant-gp. useful when compiling firmware code 🌱
• -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm ⚡
• -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm 🚀
• -mno-inline-float-divide — do not generate inline code for divides of floating-point values 🙅‍♀️
• -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm ⚡
• -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm 🚀
• -mno-inline-int-divide — do not generate inline code for divides of integer values 🙅‍♀️
• -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm ⚡
• -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm 🚀
• -mno-inline-sqrt — do not generate inline code for sqrt 🚫
• -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF line number debugging info. this may be useful when not using the GNU assembler 🐞
• -mearly-stop-bits, -mno-early-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. this can improve instruction scheduling — but it does not always do so 🙃 ngl, no promises
• -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 🎛️ (yes the original says "Valid values are itanium..." with that stray comma, we're keeping the energy)
• -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. the 32-bit environment sets int, long and pointer to 32 bits. the 64-bit environment sets int to 32 bits and long and pointer to 64 bits. these are HP-UX specific flags 💜
• -mno-sched-br-data-spec, -msched-br-data-spec — (dis/en)able data speculative scheduling before reload. this results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). the default setting is disabled 💀
• -msched-ar-data-spec, -mno-sched-ar-data-spec — (en/dis)able data speculative scheduling after reload. this results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). the default setting is enabled ✨
• -mno-sched-control-spec, -msched-control-spec — (dis/en)able control speculative scheduling. this feature is available only during region scheduling (i.e. before reload). this results in generation of the ld.s instructions and the corresponding check instructions chk.s. the default setting is disabled 🚧
• -msched-br-in-data-spec, -mno-sched-br-in-data-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. this is effective only with -msched-br-data-spec enabled. the default setting is enabled 🌟
• -msched-ar-in-data-spec, -mno-sched-ar-in-data-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. this is effective only with -msched-ar-data-spec enabled. the default setting is enabled 🌟
• -msched-in-control-spec, -mno-sched-in-control-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. this is effective only with -msched-control-spec enabled. the default setting is enabled 🌟
• -mno-sched-count-spec-in-critical-path, -msched-count-spec-in-critical-path — if enabled, speculative dependencies are considered during computation of the instructions priorities. this makes the use of the speculation a bit more conservative. the default setting is disabled 🧐
• -msched-spec-ldc, -mno-sched-spec-ldc — use a simple data speculation check. this option is on by default ✅
• -msched-control-spec-ldc, -mno-sched-control-spec-ldc — use a simple check for control speculation. this option is on by default ✅
• -msched-stop-bits-after-every-cycle, -mno-sched-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, -mno-sched-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, -mno-sel-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, -mno-sched-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 😬 fr fr, that's the deal

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 📜
• la32v1.0 — LoongArch32 ISA version 1.0 📜
• la32rv1.0 — LoongArch32 Reduced ISA version 1.0 📜

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 🔥
• loongarch32 — generic LoongArch 32-bit processor 🧱

• *-mabi=base-abi-type* — generate code for the specified calling convention. base-abi-type can be one of:
  • lp64d — uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. data model is LP64, where int is 32 bits, while long int and pointers are 64 bits 💖
  • lp64f — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. data model is LP64, where int is 32 bits, while long int and pointers are 64 bits 💖
  • lp64s — uses 64-bit general purpose registers and no floating-point registers for parameter passing. data model is LP64, where int is 32 bits, while long int and pointers are 64 bits 💖
• *-mfpu=fpu-type* — generate code for the specified FPU type, which can be one of:
• 64 — lets the hardware floating-point instructions flex for both 32-bit and 64-bit operations 💖
• 32 — hardware floating-point instructions, but only for 32-bit operations
• none / 0 — nope, zero, nada: blocks any use of hardware floating-point instructions 🙅‍♀️

-msimd=simd-type 🦋

turns on generating LoongArch SIMD instructions — both for vectorization and via builtin functions. the value is one of these cuties:

• lasx — enables generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX)
• lsx — enables generating instructions from the 128-bit LoongArch SIMD Extension (LSX)
• none — literally no LoongArch SIMD instruction may be generated, not even one 💀

the floaty ABI ones 🎀

• -msoft-float — forces -mfpu=none and straight-up prevents floating-point registers from being used for parameter passing. heads up bestie: this option may change the target ABI
• -msingle-float — forces -mfpu=32 and allows 32-bit floating-point registers for parameter passing. again: this option may change the target ABI
• -mdouble-float — forces -mfpu=64 and allows 32/64-bit floating-point registers for parameter passing. and yep, this option may change the target ABI

-mlasx / -mno-lasx / -mlsx / -mno-lsx ✨

these four incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) the compiler is allowed to use for code generation. the way they chain is so satisfying: enabling LASX with -mlasx automatically enables LSX, and disabling LSX with -mno-lsx automatically disables LASX 🫶

these are driver-only options — they act upon the final -msimd configuration state and make incremental changes in the order they appear on the GCC driver's command line, deriving the final / canonicalized -msimd option that gets passed to the compiler proper. order matters, iykyk 💅

-mbranch-cost=n 🔥

sets the cost of branches to roughly n instructions.

-maddr-reg-reg-cost=n 🌸

sets the cost of ADDRESS_REG_REG to the value calculated by n.

-mcheck-zero-division / -mno-check-zero-divison 💀

traps (or does not trap) on integer division by zero. the default is -mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division for other optimization levels.

fr fr note the spelling on that second one — it's -mno-check-zero-divison in the manual, missing an "i". that's just how it's written, don't come for me 🙈

-mbreak-code=code 👑

emits a break code instruction for irrecoverable traps from __builtin_trap or ones inserted by the compiler (for example an erroneous path isolated with -fisolate-erroneous-paths-dereference) — or an amswap.w $r0, $r1, $r0 instruction, which will cause the hardware to trigger an Instruction Not-defined Exception if code is negative or greater than 32767.

the default is -1, meaning: use the amswap.w instruction.

-mcond-move-int / -mno-cond-move-int 🌈

conditional moves for integral data in general-purpose registers are enabled (disabled). default? -mcond-move-int, slay.

-mcond-move-float / -mno-cond-move-float 🦋

conditional moves for floating-point registers are enabled (disabled). default is -mcond-move-float.

-mmemcpy / -mno-memcpy ⭐

forces (or doesn't force) the use of memcpy for non-trivial block moves. the default is -mno-memcpy, which lets GCC inline most constant-sized copies.

setting the optimization level to -Os also forces the use of memcpy, but -mno-memcpy may override this behavior if explicitly specified — regardless of the order these options appear on the command line. chaotic but valid 💅

-mstrict-align / -mno-strict-align 🎀

avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. the default is -mno-strict-align.

-G num 🍓

puts global and static data smaller than num bytes into a small data section. the default value is 0.

-mmax-inline-memcpy-size=n ✨

inlines all block moves (like calls to memcpy or structure copies) less than or equal to n bytes. the default value of n is 1024.

-mcmodel=code-model 👑

sets the code model to one of:

• tiny-static — (Not implemented yet)
• tiny — (Not implemented yet)
• normal — the text segment must be within 128MB addressing space. the data segment must be within 2GB addressing space.
• medium — the text segment and data segment must be within 2GB addressing space. this is the default code model unless GCC has been configured with --with-cmodel= specifying a different default code model.
• large — (Not implemented yet)
• extreme — this mode does not limit the size of the code segment and data segment. the -mcmodel=extreme option is incompatible with -fplt and/or -mexplicit-relocs=none.

lowkey obsessed with how half these models are just "(Not implemented yet)" 😭 iconic honestly

-mexplicit-relocs=style / -mexplicit-relocs / -mno-explicit-relocs 🧚

sets when to use assembler relocation operators when dealing with symbolic addresses. the alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation.

• with -mexplicit-relocs=none, the assembler macros are always used
• with -mexplicit-relocs=always, the assembler relocation operators are always used
• with -mexplicit-relocs=auto, the compiler uses the relocation operators where linker relaxation is impossible (to improve code quality), and macros elsewhere

the default value for the option is determined by the assembler capability detected during GCC build-time plus the setting of -mrelax:

• -mexplicit-relocs=none if the assembler does not support relocation operators at all
• -mexplicit-relocs=always if the assembler supports relocation operators but -mrelax is not enabled
• -mexplicit-relocs=auto if the assembler supports relocation operators and -mrelax is enabled

for backward compatibility: -mexplicit-relocs is equivalent to -mexplicit-relocs=always, while -mno-explicit-relocs is equivalent to -mexplicit-relocs=none. 🫶

-mdirect-extern-access / -mno-direct-extern-access 💜

controls use of the GOT to access external symbols. the default is -mno-direct-extern-access: the GOT is used for external symbols with default visibility, but not used for other external symbols.

with -mdirect-extern-access, the GOT is not used and all external symbols are PC-relatively addressed. it's only suitable for environments where no dynamic link is performed — like firmwares, OS kernels, executables linked with -static or -static-pie.

-mdirect-extern-access is not compatible with -fPIC or -fpic. bestie, do not mix these, I'm begging 😭

-mrelax / -mno-relax 🌟

takes (or doesn't take) advantage of linker relaxations. if -mpass-mrelax-to-as is enabled, this option is also passed to the assembler.

the default is determined during GCC build-time by detecting corresponding assembler support:

• -mrelax if the assembler supports both the -mrelax option and the conditional branch relaxation — and that second part is required, otherwise the .align directives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow 💀
• -mno-relax otherwise

-mpass-mrelax-to-as / -mno-pass-mrelax-to-as 🌸

passes (or does not pass) the -mrelax or -mno-relax option to the assembler. the default is determined during GCC build-time by detecting corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the -mrelax option, -mno-pass-mrelax-to-as otherwise.

this one is mostly useful for debugging, or for interoperation with assemblers different from the build-time one.

-mrecip 🔥

enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision, instead of doing a divide or square root and divide for floating-point arguments.

these instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. this option is off by default.

before you can use this option, you must be sure the target CPU supports the frecipe and frsqrte instructions. 👑

and note: while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). so like… faster but slightly feral 🫠

-mrecip=opt 🎀

controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:

• all — enable all estimate instructions
• default — enable the default instructions, equivalent to -mrecip
• none — disable all estimate instructions, equivalent to -mno-recip
• div — enable the approximation for scalar division
• vec-div — enable the approximation for vectorized division
• sqrt — enable the approximation for scalar square root
• vec-sqrt — enable the approximation for vectorized square root
• rsqrt — enable the approximation for scalar reciprocal square root
• 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. see how clean that is ✨

-mfrecipe / -mno-frecipe 🦋

use (do not use) frecipe.{s/d} and frsqrte.{s/d} instructions. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-frecipe.

-mdiv32 / -mno-div32 ⭐

use (do not use) div.w[u] and mod.w[u] instructions with input not sign-extended. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-div32.

-mlam-bh / -mno-lam-bh 🍓

use (do not use) am{swap/add}[_db].{b/h} instructions. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-lam-bh.

-mlamcas / -mno-lamcas 💜

use (do not use) amcas[_db].{b/h/w/d} instructions. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-lamcas.

-mld-seq-sa / -mno-ld-seq-sa 🌈

whether a same-address load-load barrier (dbar 0x700) is needed. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-ld-seq-sa, and the load-load barrier is needed.

-mscq / -mno-scq 👑

use (do not use) the 16-byte conditional store instruction sc.q. the default is -mscq if the machine type specified with -march= supports this instruction, -mno-scq otherwise.

• -mtls-dialect=opt — bestie, this one controls which TLS dialect may be used for general dynamic and local dynamic TLS models. The opt argument can be one of:
  • trad — use traditional TLS. This is the default. 💅
  • desc — use TLS descriptors. ✨
• -mannotate-tablejump, -mno-annotate-tablejump — this creates an annotation section .discard.tablejump_annotate to correlate the jirl instruction and the jump table when a jump table is used to optimize the switch statement. Some external tools, for example objtool of the Linux kernel building system, need the annotation to analyze the control flow. The default is -mno-annotate-tablejump. 🔥

🎀 LynxOS Options 🎀

These options are available for LynxOS targets.

• -mshared — use shared libraries. The default is to link with static libraries. 🫶
• -mthreads — support multi-threading. 💖
• -mlegacy-threads — support legacy multi-threading. This option is incompatible with -mthreads. 💀

🍓 M32C Options 🍓

• -mcpu=name — select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series. 👑
• -msim — specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed. 🚨
• -memregs=number — specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there is a tradeoff between GCC's ability to fit the code into available registers, and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries. 😭

🦋 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 that their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

The addressability of a particular object can be set with the model attribute. ✨

• -mmodel=medium — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction. 🦋
• -mmodel=large — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler generates the much slower seth/add3/jl instruction sequence). 💀
• -msdata=none — disable use of the small data area. Variables are 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 do not 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, -mno-align-loops — align all loops to a 32-byte boundary. This option is disabled by 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 is 1 then branches are preferred over conditional code, if it is 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 : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 🚨

GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above. 👑

• -mtune=tune — tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e. 🦋

You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively. 💖

GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range. 🌸

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above. ⭐

• -m68000, -mc68000 — generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to -march=68000.

Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. 🎀

• -m68010 — generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to -march=68010. ✨
• -m68020, -mc68020 — generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to -march=68020. 💜
• -m68030 — generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to -march=68030. 🌈
• -m68040 — generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to -march=68040.

This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions. 😭

• -m68060 — generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to -march=68060.

This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions. 🔥

• -m68302 — generate code for a 68302. 💅
• -m68332 — generate code for a 68332. 🫶
• -m68851 — bestie, this was an external MMU for the 68020 processor. GCC accepts this option but does not use it 😭. It’s giving “we have your order but the kitchen is closed.”
• -mcpu32 — generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It’s 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. ✨
• -mfidoa — generate code for a Fido A. That’s the whole vibe. 🐕
• -m5200 — generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It’s equivalent to -mcpu=5206, and is now deprecated in favor of that option. Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206. 💅
• -m5206e — generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e.
• -m528x — generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
• -m5407 — generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.
• -m68020-40 — generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040. The option is equivalent to -march=68020 -mtune=68020-40. 🎀
• -m68020-60 — generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060. The option is equivalent to -march=68020 -mtune=68020-60.
• -mhard-float, -m68881 — generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets. 💖
• -msoft-float — do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It is also the default for ColdFire devices that have no FPU.
• -mdiv, -mno-div — generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e. GCC defines the macro __mcfhwdiv__ when this option is enabled. ⚡
• -mshort, -mno-short, -mnoshort — consider type int to be 16 bits wide, like short int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit. This option is disabled by default.
• -mbitfield, -mno-bitfield, -mnobitfield — control use of the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield; the -m68020 option implies -mbitfield.
• -mrtd, -mno-rtd, -mnortd — control use of a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there.

⚠️ This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.) 😭

The rtd instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the 68000 or 5200. The default is -mno-rtd.

• -malign-int, -mno-align-int — control whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.

⚠️ Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. 💀

• -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, -mno-sep-data — with -msep-data, generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC. This option is disabled by default; GCC generates code that assumes that the data segment follows the text segment.
• -mid-shared-library, -mno-id-shared-library — if enabled, 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. This option is disabled by default. 🌈
• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.
• -mxgot, -mno-xgot — when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; -fPIC suffices.

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol. 💅

Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do. These options have no effect unless GCC is generating position-independent code.

• -mxtls — support TLS segment larger than 64K. The considerations for using this option are similar to those for -mxgot above; the default 16-bit offset addressing for TLS is more efficient, but this may be inadequate for some programs.
• -mlong-jump-table-offsets — use 32-bit offsets in switch tables. The default is to use 16-bit offsets.

---

🌸 MCore Options 🌸

These are the -m options defined for the Motorola M*Core processors.

• -mhardlit, -mno-hardlit — inline constants into the code stream if it can be done in two instructions or less. 🎀
• -mdiv, -mno-div — use the divide instruction. (Enabled by default).
• -mrelax-immediates, -mno-relax-immediates — 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- or big-endian target, respectively. The default is big-endian. Little-endian code is supported only with -m340.
• -m210 — generate code for the 210 processor.
• -m340 — generate code for the 340 processor.
• -mno-lsim — assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.
• -mstack-increment=size — set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need 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.
• -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, -mno-xl-soft-mul — use software multiply emulation. It's on by default, bestie, so you're already slaying it 💅
• -mxl-soft-div, -mno-xl-soft-div — use software emulation for divides. Also enabled by 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. smol divisions, big brain energy 🍓
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections 🔥
• -mxl-multiply-high — use multiply high instructions for the high part of a 32x32 multiply ✖️
• -mxl-float-convert — use hardware floating-point conversion instructions 🦋
• -mxl-float-sqrt — use the hardware floating-point square root instruction 🌸
• -mbig-endian — generate code for a big-endian target 👑
• -mlittle-endian — generate code for a little-endian target 🎀
• -mxl-reorder — use reorder instructions (swap and byte reversed load/store) 🔄
• -mxl-mode-app-model — pick your application model app-model. Valid models are, and honestly the range:
  • 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. This uses startup file crt1.o and sets the start address of the program to 0x800.
  • bootstrap — for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
  • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.
• -mxl-prefetch — enable insertion of prefetch (wic) instructions at call sites ⚡
• -mpic-data-is-text-relative — assume that the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from start of text address instead of GOT, since PC-relative addressing is not supported 🫶

---

🐚 MIPS Options

Ok so this is the MIPS section and it is a lot, but we're getting through it together, no cap 💜

• -EB, -meb — generate big-endian code.
• -EL, -mel — 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

...plus allegrex. And the special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs) 🌈

The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor 🙈

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r ✨

Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1 🧚

GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 sets _MIPS_ARCH to "r2000" and defines the macro _MIPS_ARCH_R2000 👑

Note that the _MIPS_ARCH macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given 💖

• -mtune=arch — optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march.

When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family 🎀

-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, which work in the same way as the -march ones described above.

• -mips1 — equivalent to -march=mips1.
• -mips2 — equivalent to -march=mips2.
• -mips3 — equivalent to -march=mips3.
• -mips4 — equivalent to -march=mips4.
• -mips32 — equivalent to -march=mips32.
• -mips32r3 — equivalent to -march=mips32r3.
• -mips32r5 — equivalent to -march=mips32r5.
• -mips32r6 — equivalent to -march=mips32r6.
• -mips64 — equivalent to -march=mips64.
• -mips64r2 — equivalent to -march=mips64r2.
• -mips64r3 — equivalent to -march=mips64r3.
• -mips64r5 — equivalent to -march=mips64r5.
• -mips64r6 — equivalent to -march=mips64r6.
• -mips16, -mno-mips16 — generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE 🌸

MIPS16 code generation can also be controlled on a per-function basis by means of mips16 and nomips16 attributes.

• -mmips16e2, -mno-mips16e2 — use (do not use) the MIPS16e2 ASE. This option modifies the behavior of the -mips16 option such that it targets the MIPS16e2 ASE ⭐
• -mflip-mips16, -mflip-mips16 — yes, the page really does list it twice, we're just as confused as you are bestie 😭 — generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code ⚠️
• -minterlink-compressed, -mno-interlink-compressed — require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa 🔗

For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.

• -minterlink-mips16, -mno-interlink-mips16 — aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility 🫶
• -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — generate code for the given ABI.

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead 💅

For information about the O64 ABI, see https://gcc.gnu.org/projects/mipso64-abi.html.

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors 🔥

The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved 👑

Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes 🦋

• -mabicalls, -mno-abicalls — generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems 🎀
• -mshared, -mno-shared — generate (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 (or don't assume, bestie, pick a lane) that the static and dynamic linkers support PLTs and copy relocations. This one only does anything for -mno-shared + -mabicalls. And for the n64 ABI it's literally a no-op without -msym32. 🙈

You can make -mplt the default by configuring GCC with --with-mips-plt. Otherwise the default is -mno-plt. Periodt.

• -mxgot, -mno-xgot — lift (or don't lift) the usual restrictions on how big the global offset table can get.

So normally GCC loads values from the GOT with a single instruction. Cute and efficient, sure, but it only works while the GOT is under about 64k. Anything bigger and the linker comes for you with an error like:

relocation truncated to fit: R_MIPS_GOT16 foobar

If that happens, recompile with -mxgot. It handles very large GOTs, but the code is less efficient too — fetching a global symbol now takes three instructions instead of one. 😭

Ngl, some linkers can create multiple GOTs. If yours does, you should only need -mxgot when a single object file touches more than 64k's worth of GOT entries. Very few do.

Also these options do nothing unless GCC is generating position independent code. Fr fr.

• -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 — don't assume the width of floating-point registers at all. Mysterious. Iconic. 💅
• -mhard-float — use floating-point coprocessor instructions.
• -msoft-float — don't use floating-point coprocessor instructions. Do your floating-point math via library calls instead.
• -mno-float — same energy as -msoft-float, but additionally it asserts that the program you're compiling performs zero floating-point operations. Right now this is only supported by some bare-metal MIPS configurations, where it may pick a special set of libraries with no floating-point support at all (including, for example, the floating-point printf formats).

Bestie, if code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. 💀 Don't do that.

• -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. That's the default for processors known to support these registers. When you're on the o32 FPXX ABI, though, -mno-odd-spreg is set by default.
• -mabs=2008, -mabs=legacy — these control how the special not-a-number (NaN) IEEE 754 floating-point data gets treated by the abs.fmt and neg.fmt machine instructions.

By default, or when -mabs=legacy is used, you get the legacy treatment. In that case these instructions count as arithmetic and get avoided wherever correct operation is required and the input operand might be a NaN. Instead it uses a longer sequence of instructions that manually manipulates the sign bit of the floating-point datum — unless you also passed -ffinite-math-only.

The -mabs=2008 option selects the IEEE 754-2008 treatment. Now these instructions count as non-arithmetic, so they operate correctly in all cases, including specifically when the input operand is a NaN. So they're 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.

-mnan=legacy selects the legacy encoding: quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, while signaling NaNs (sNaNs) are denoted by that first bit being 1.

-mnan=2008 selects the IEEE 754-2008 encoding: now qNaNs are denoted by the first bit of their trailing significand field being 1, while sNaNs are denoted by that first bit being 0. Literally flipped, no cap.

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

• -mllsc, -mno-llsc — use (or don't use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either one the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations — check the installation documentation for the details 🎀

• -mdsp, -mno-dsp — use (or don't use) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro __mips_dsp. It also defines __mips_dsp_rev to 1.
• -mdspr2, -mno-dspr2 — use (or don't use) revision 2 of the MIPS DSP ASE. This one defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines __mips_dsp_rev to 2.
• -msmartmips, -mno-smartmips — use (or don't use) the MIPS SmartMIPS ASE.
• -mpaired-single, -mno-paired-single — use (or don't use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled 🫶
• -mdmx, -mno-mdmx — use (or don't use) MIPS Digital Media Extension instructions. You can only use this one when generating 64-bit code, and it requires hardware floating-point support to be enabled.
• -mips3d, -mno-mips3d — use (or don't use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single. Package deal 💅
• -mmicromips, -mno-micromips — generate (or don't generate) microMIPS code.

MicroMIPS code generation can also be controlled on a per-function basis by means of micromips and nomicromips attributes. Fancy.

• -mmt, -mno-mt — use (or don't use) MT Multithreading instructions.
• -mmcu, -mno-mcu — use (or don't use) the MIPS MCU ASE instructions.
• -meva, -mno-eva — use (or don't use) the MIPS Enhanced Virtual Addressing instructions.
• -mvirt, -mno-virt — use (or don't use) the MIPS Virtualization (VZ) instructions.
• -mxpa, -mno-xpa — use (or don't use) the MIPS eXtended Physical Address (XPA) instructions.
• -mcrc, -mno-crc — use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions.
• -mginv, -mno-ginv — use (or don't use) the MIPS Global INValidate (GINV) instructions.
• -mmsa, -mno-msa — use (or don't use) the MIPS MSA extension instructions.
• -mloongson-mmi, -mno-loongson-mmi — use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
• -mloongson-ext, -mno-loongson-ext — use (or don't use) the MIPS Loongson EXTensions (EXT) instructions.
• -mloongson-ext2, -mno-loongson-ext2 — use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
• -mlong64 — force long types to be 64 bits wide. See -mlong32 for an explanation of the default and how pointer size gets determined 👀
• -mlong32 — force long, int, and pointer types to be 32 bits wide.

The default size of ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, and so does the 64-bit EABI; the others use 32-bit longs. Pointers are the same size as longs, or the same size as integer registers — whichever is smaller. The way that works is lowkey cursed but okay 🦋

• -msym32, -mno-sym32 — assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it lets GCC generate shorter and faster references to symbolic addresses ✨
• -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. Then GCC can generate more efficient accesses to the data; see -mgpopt for details.

The default -G option depends on the configuration.

• -mlocal-sdata, -mno-local-sdata — extend (or don't extend) the -G behavior to local data too, like static variables in C. -mlocal-sdata is the default for all configurations 💖

If the linker 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 also might want to build large libraries with -mno-local-sdata, so the libraries leave more room for the main program. Be considerate 🫶

• -mextern-sdata, -mno-extern-sdata — assume (or don't assume) that externally-defined data is in a small data section if that data's size is within the -G limit. -mextern-sdata is the default for all configurations.

Okay so this one's got rules, bestie, pay attention: if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that's no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a section attribute to Var's definition. And if Var is common, you must link the application with a high-enough -G setting. No cap, those are the rules 😤

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. But you may wish to build a library that supports several different small data limits — you can do that 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. Main character library energy 👑

• -mgpopt, -mno-gpopt — use (or don't use) GP-relative accesses for symbols known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations ⭐
• -mno-gpopt — okay bestie, this one is for when the $gp register might not hold the value of _gp. Like, imagine the code is part of a library that might get used inside a boot monitor — programs that call boot monitor routines pass an unknown value in $gp. (And in those situations, the boot monitor itself is usually compiled with -G0, iykyk.)

Lowkey important: -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. Periodt.

• -membedded-data, -mno-embedded-data — allocates variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. It's giving slightly slower code than the default, but it reduces the amount of RAM required when executing — so it may be preferred for some embedded systems. 🌸
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — puts uninitialized const variables in the read-only data section. Heads up though: this option is only meaningful in conjunction with -membedded-data. 💅
• *-mcode-readable=setting* — specifies whether GCC may generate code that reads from executable sections. There are three possible settings, and they're all a whole vibe:
  • -mcode-readable=yes — instructions may freely access executable sections. This is the default setting. ✨
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This one is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It's also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
  • -mcode-readable=no — instructions must not access executable sections. Can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.

And on SDE targets, -mcode-data-in-code is available as a traditional alias for -mcode-readable=no, and -mcode-xonly for -mcode-readable=pcrel. iykyk 🫶

• -msplit-addresses, -mno-split-addresses — enable (disable) use of the %hi() and %lo() assembler relocation operators. Ngl, this option has been superseded by -mexplicit-relocs but is retained for backwards compatibility. 💀
• -mexplicit-relocs=none, -mexplicit-relocs=base, -mexplicit-relocs=pcrel, -mexplicit-relocs, -mno-explicit-relocs — these options control whether explicit relocs (such as %gp_rel) are used. The default value depends on the version of GAS when GCC itself was built, so it's giving "it depends" energy.

The base explicit-relocs support was introduced into GAS in 2001. The pcrel explicit-relocs support was introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo. History lesson slay ✨

• -mcheck-zero-division, -mno-check-zero-division — trap (do not trap) on integer division by zero. The default is -mcheck-zero-division. 💅
• -mdivide-traps, -mdivide-breaks — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but it's only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal (SIGFPE) 😭. Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. And divide-by-zero checks can be completely disabled using -mno-check-zero-division.

• -mload-store-pairs, -mno-load-store-pairs — enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. Enabled by default, but only takes effect when the selected architecture is known to support bonding. Lowkey conditional, bestie.
• -mstrict-align, -mno-strict-align, -munaligned-access, -mno-unaligned-access — disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, either by hardware or by trapping and emulation. In the latter case -mstrict-align may be needed by the operating system kernel. Also, -munaligned-access and -mno-unaligned-access are obsolete and only provided for backward compatibility. Dead flags walking 💀
• -mmemcpy, -mno-memcpy — force (do not force) the use of memcpy for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies. Slay.
• -mlong-calls, -mno-long-calls — disable (do not disable) use of the jal instruction. Calling functions using jal is more efficient but requires the caller and callee to be in the same 256 megabyte segment. So yeah, tradeoffs.

This option has no effect on abicalls code. The default is -mno-long-calls.

• -mmad, -mno-mad — enable (disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA. 🎀
• -mimadd, -mno-imadd — enable (disable) use of the madd and msub integer instructions. The default is -mimadd on architectures that support madd and msub, except for the 74k architecture, where it was found to generate slower code. Not the 74k catching strays 😭
• -mfused-madd, -mno-fused-madd — enable (disable) use of the floating-point multiply-accumulate instructions, when they're available. The default is -mfused-madd. ✨

Careful though: on the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

• -nocpp — tells the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling them. 💅
• -mfix-24k, -mno-fix-24k — works around the 24K E48 errata (that's lost data on stores during refill, bestie, a whole situation). The workarounds are implemented by the assembler rather than by GCC.
• -mfix-r4000, -mno-fix-r4000 — works 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.

Three whole bugs. Pray for the R4000 🙈

• -mfix-r4400, -mno-fix-r4400 — works 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 — works around certain R10000 errata:
  • ll/sc sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 💀

This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.

• -mfix-r5900, -mno-fix-r5900 — does not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedules a nop instruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.
• -mfix-rm7000, -mno-fix-rm7000 — works around the RM7000 dmult/dmultu errata. The workarounds are implemented by the assembler rather than by GCC. 💅
• -mfix-vr4120, -mno-fix-vr4120 — works around certain VR4120 errata:
  • dmultu does not always produce the correct result.
  • div and ddiv do not always produce the correct result if one of the operands is negative.

The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are only provided by the mips64vr*-elf configurations. 🫶

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.

• -mfix-vr4130, -mno-fix-vr4130 — works around the VR4130 mflo/mfhi errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead. Iconic workaround behaviour ✨
• -mfix-sb1, -mno-fix-sb1 — works around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 F1 and F2 floating-point errata.) 🎀
• -mfix4300, -mno-fix4300 — works around a bug in early VR4300 silicon that causes multiplies with certain operands to corrupt immediately following multiplies. Absolutely cursed, but okay 🔥
• *-mr10k-cache-barrier=setting* — specifies whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. Buckle up, this one's a saga:

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 😭

This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description, including other potential problems. (Like, read the manual bestie, fr fr.)

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:

okay so back in MIPS land, the little safety bubble covers exactly these three things, no cap:

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.

and it is the kernel's responsibility to make sure speculative accesses to those regions are actually safe, bestie. like, that's the whole vibe check.

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. periodt.

the option has three forms:

• -mr10k-cache-barrier=load-store — insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=store — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=none — disable the insertion of cache barriers. this is the default setting. 💅
• -mflush-func=func, -mno-flush-func — specifies the function to call to flush the I and D caches, or to not call any such function. if called, the function must take the same arguments as the common _flush_func, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). the default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush. 🧚
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. this cost is only a heuristic and is not guaranteed to produce consistent results across releases. a zero cost redundantly selects the default, which is based on the -mtune setting. lowkey important: it's a heuristic, not a promise.
• -mbranch-likely, -mno-branch-likely — enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. by default, Branch Likely instructions may be generated if they are supported by the selected architecture. an exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures specifically deprecate their use. 🫶
• -mcompact-branches=never, -mcompact-branches=optimal, -mcompact-branches=always — these options control which form of branches are generated. the default is -mcompact-branches=optimal.

the -mcompact-branches=never option ensures that compact branch instructions are never generated. 🚫

the -mcompact-branches=always option ensures that a compact branch instruction is generated if available for MIPS Release 6 onwards. if a compact branch instruction is not available (or pre-R6), a delay slot form of the branch is used instead.

if it is used for MIPS16/microMIPS targets, it is just ignored now. the behavior for MIPS16/microMIPS may change in future, since they do have some compact branch instructions. iykyk.

the -mcompact-branches=optimal option 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. ✨

• -mfp-exceptions, -mno-fp-exceptions — specifies whether FP exceptions are enabled. this affects how FP instructions are scheduled for some processors. the default is that FP exceptions are enabled.

for instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes. otherwise, we can only use one FP pipe. 🔥

• -mvr4130-align, -mno-vr4130-align — the VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. when this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel.

this option only has an effect when optimizing for the VR4130. it normally makes code faster, but at the expense of making it bigger. it is enabled by default at optimization level -O3. bigger but faster, classic tradeoff.

• -msynci, -mno-synci — enable (disable) generation of synci instructions on architectures that support it. the synci instructions (if enabled) are generated when __builtin___clear_cache is compiled.

this option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.

when compiling code for single processor systems, it is generally safe to use synci. however, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. ⚠️ like, do not ignore that one.

• -mrelax-pic-calls, -mno-relax-pic-calls — try to turn PIC calls that are normally dispatched via register $25 into direct calls. this is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call.

-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. with -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler. 🎀

• -mmcount-ra-address, -mno-mcount-ra-address — emit (do not emit) code that allows _mcount to modify the calling function's return address. when enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12. _mcount can then modify the return address by doing both of the following:
  • returning the new address in register $31.
  • storing the new address in *ra-address, if ra-address is nonnull.

the default is -mno-mcount-ra-address. 💖

• -mframe-header-opt, -mno-frame-header-opt — enable (disable) frame header optimization in the o32 ABI. when using the o32 ABI, calling functions allocates 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 is unused.

this optimization is off by default at all optimization levels. 🙈

• -mlxc1-sxc1, -mno-lxc1-sxc1 — when applicable, enable (disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. enabled by default. 💅
• -mmadd4, -mno-madd4 — when applicable, enable (disable) the generation of 4-operand madd.s, madd.d and related instructions. enabled by default. 👑

---

🦋 MMIX Options 🦋

these options are defined for the MMIX:

• -mlibfuncs, -mno-libfuncs — specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
• -mepsilon, -mno-epsilon — generate floating-point comparison instructions that compare with respect to the rE epsilon register.
• -mabi=mmixware, -mabi=gnu — generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up. 🌀
• -mzero-extend, -mno-zero-extend — when reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones.
• -mknuthdiv, -mno-knuthdiv — make the result of a division yielding a remainder have the same sign as the divisor. with the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. both methods are arithmetically valid, the latter being almost exclusively used. math girlie moment.
• -mtoplevel-symbols, -mno-toplevel-symbols — prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
• -melf — generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator. 💫
• -mbranch-predict, -mno-branch-predict — use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
• -mbase-addresses, -mno-base-addresses — generate (do not generate) code that uses base addresses. using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. the register is used for one or more base address requests within the range 0 to 255 from the value held in the register. this generally leads to short and fast code, but the number of different data items that can be addressed is limited. this means that a program that uses lots of static data may require -mno-base-addresses. 🍓
• -msingle-exit, -mno-single-exit — force (do not force) generated code to have a single exit point in each function.
• -mset-data-start=address — when linking, set the start of the data section to address.
• -mset-program-start=address, -mno-set-program-start — when linking, set the program start address to address. it defaults to 0x100 unless -mno-set-program-start is used to suppress this option entirely. ⭐

---

🌸 MN10300 Options 🌸

these -m options are defined for Matsushita MN10300 architectures:

• -mmult-bug, -mno-mult-bug — when enabled, generate code to avoid bugs in the multiply instructions for the MN10300 processors. this is the default.
• -mam33, -mno-am33 — generate code using features specific to the AM33 processor. the default is -mno-am33.
• -mam33-2, -mno-am33-2 — generate code using features specific to the AM33/2.0 processor. the default is -mno-am33-2.
• -mam34, -mno-am34 — generate code using features specific to the AM34 processor. the default is -mno-am34.
• -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, -mno-return-pointer-on-d0 — okay so when the compiler makes a function that hands back a pointer, this makes it return that pointer in both a0 and d0 💅. Without it, the pointer only comes back in a0, and then if you try to call one of those functions without a prototype you get errors, no cap 😭. Heads up: this one is on by default, so reach for -mno-return-pointer-on-d0 if you wanna turn it off.
• -mno-crt0 — don't link in the C run-time initialization object file, bestie. That's it, that's the whole vibe.
• -mrelax — tells the linker "go off, do a relaxation optimization pass" so branches, calls and absolute memory addresses get shortened ✨. Fr fr this only does anything when you use it on the command line for the final link step. Also, big caveat: this option makes symbolic debugging impossible 💀. Choose wisely.
• -mliw, -mno-liw — lets the compiler generate Long Instruction Word instructions if your target is the AM33 or later. On by default 🫶. -mliw defines the preprocessor macro __LIW__, and -mno-liw defines the preprocessor macro __NO_LIW__.
• -msetlb, -mno-setlb — lets the compiler generate the SETLB and Lcc instructions if the target is AM33 or later. Also enabled by default ✨. -msetlb defines __SETLB__, -mno-setlb defines __NO_SETLB__.

---

🦋 Moxie Options 🦋

• -meb — generate big-endian code. This is the default for moxie-*-* configurations 👑
• -mel — generate little-endian code 🌸
• -mmul.x, -mno-mul.x — generate mul.x and umul.x instructions. This one's enabled by default for moxiebox-*-* configurations 💖
• -mno-crt0 — do not link in the C run-time initialization object file 🎀

---

🌈 MSP430 Options 🌈

These options are defined for the MSP430, iykyk:

• -masm-hex — forces assembly output to always use hex constants 🔥. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. (Yes, aesthetics. We love a functional-but-pretty moment.)
• -mmcu=name — picks the MCU you're targeting 🎯. This gets used to create a C preprocessor symbol built from the MCU name, converted to upper case and pre- and post-fixed with __. Then msp430.h uses that to pick an MCU-specific supplementary header file.

The option also sets which ISA to use. If the MCU name is one that's known to only support the 430 ISA, then that's the one selected — otherwise the 430X ISA gets 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 💅

On top of all that, an MCU-specific linker script gets added to the linker command line. The script's name is the name of the MCU with .ld appended. So specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and causes the linker to search for a script called xxx.ld.

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond those that have been hard-coded ✨

GCC searches for the devices.csv file using the following methods in the given precedence order, where the first method takes precedence over the second, which takes precedence over the third:

Include path specified with -I and -L

devices.csv is searched for in each of the directories specified by include paths and linker library search paths 👑

Path specified by the environment variable MSP430_GCC_INCLUDE_DIR

Define the value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is found, this directory is also registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying -I and -L on the command line 🫶

The msp430-elf{,bare}/include/devices directory

Finally, GCC examines msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation 🙈, but if you've created it and copied devices.csv there, then the MCU data is read. Same as above, this directory is also registered as an include path and linker library path.

If none of the above search methods find devices.csv, then the hard-coded MCU data is used.

• -mwarn-mcu, -mno-warn-mcu — enables or disables warnings about conflicts between the MCU name 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.
• -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=type — describes the type of hardware multiply supported by the target. Accepted values for type 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

A value of 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 isn't recognized, then no hardware multiply support is assumed. "auto" is the default setting 👑

Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher however, the hardware multiplier is invoked inline — 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 without support for static initializers or constructors. It's intended for memory-constrained devices 🥺. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.
• -mtiny-printf — enable reduced code size printf and puts library functions. The tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications ⚠️

Support for streams has been removed, and the string to be printed is always 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=n — takes an integer n between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an mspabi helper function is used instead. The default value is 4 ✨

This only affects cases where a shift by multiple positions can't be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead 🧮

• -mcode-region=where, -mdata-region=where — these tell the compiler where to place functions and data that don't have one of the "lower", "upper", "either" or "section" attributes. Possible values for where 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 💅
• -muse-lower-region-prefix — add the .lower prefix to section names when compiling with -mcode-region=lower or -mdata-region=lower. Disabled by default 🎀
• -msilicon-errata=name[,name...] — passes on a request to the assembler to enable the fixes for the named silicon errata. Refer to the assembler documentation for details 🙈
• -msilicon-errata-warn=name[,name...] — passes on a request to the assembler to enable warning messages when a named silicon errata might need to be applied. Refer to the assembler documentation for details 📖
• -mwarn-devices-csv, -mno-warn-devices-csv — warn if devices.csv is not found or there are problems parsing it (default: on) ⭐

---

🧚 NDS32 Options 🧚

These options are defined for NDS32 implementations:

• -mbig-endian, -EB — generate code in big-endian mode 👑
• -mlittle-endian, -EL — generate code in little-endian mode 🌸
• -mabi=2, -mfloat-abi=soft — use the soft floating-point ABI 🫶
• -mabi=2fp+, -mfloat-abi=hard — use the hard floating-point ABI 🔥
• -mreduced-regs — use reduced-set registers for register allocation 💅
• -mfull-regs — use full-set registers for register allocation ✨
• -malways-align — always align function entry, jump targets, and return address 🎯
• -malign-functions — align function entry to 4 bytes 📏
• -mfp-as-gp, -mno-fp-as-gp — enable/disable fp-as-gp optimization 🦋
• -mcmov, -mno-cmov — enable/disable generation of conditional move instructions 💜
• -mhw-abs, -mno-hw-abs — enable/disable generation of hardware abs instructions 🍓
• -mext-perf, -mno-ext-perf — enable/disable generation of performance extension instructions ⭐
• -mext-perf2, -mno-ext-perf2 — enable/disable generation of performance extension 2 instructions 🌈
• -mext-string, -mno-ext-string — enable/disable generation of string extension instructions 🎀
• -mext-dsp, -mno-ext-dsp — enable/disable generation of DSP extension instructions 💖
• -mext-fpu-fma, -mno-ext-fpu-fma — enable/disable generation of floating-point multiply-accumulation instructions 🧮

Okay bestie, we're still deep in the Andes machine options — no new heading, just vibes and flags 🫶 Let's keep going.

• -mext-fpu-sp, -mno-ext-fpu-sp — turn on/off generating single-precision floating-point instructions. Precision, but make it single 💅
• -mext-fpu-dp, -mno-ext-fpu-dp — turn on/off generating double-precision floating-point instructions. Double the precision, double the slay ✨
• -mv3push, -mno-v3push — turn on/off generating the v3 push25/pop25 instructions. Push it, pop it 🎀
• -m16-bit, -mno-16-bit — turn on/off generating 16-bit instructions. Small but mighty, fr fr.
• -mvh, -mno-vh — turn on/off Virtual Hosting support. It's giving main-character hosting energy 👑
• -misr-vector-size=num — set the size of each interrupt vector. Heads up: it **must be 4 or 16, no in-between, no negotiation 😭
• *-misr-secure=num* — set the security level of c-isr for the whole file. One level to rule them all 🔒
• -mcache-block-size=num — set the size of each cache block. Rules: **must be a power of 2, and it has to sit between 4 and 512 inclusive. Math is mathing, no cap ✨
• *-march=arch* — pick the target architecture. arch can be one of v2, v3, v3j, v3m, v3f, or v3s. Choose your fighter 🌈
• *-mcpu=cpu* — pick the CPU to compile for. cpu may be one of n6, n650, n7, n705, n8, n801, sn8, sn801, s8, s801, e8, e801, n820, s830, e830, n9, n903, n903a, n968, n968a, n10, n1033, n1033a, n1033-fpu, n1033-spu, n1068, n1068a, n1068-fpu, n1068a-fpu, d10, d1088, d1088-fpu, d1088-spu, n15, d15, n15s, d15s, n15f, d15f, n12, n1213, n1233, n1233-fpu, n1233-spu, n13, n1337, n1337-fpu, n1337-spu, or simple. That's the whole roster, bestie — it's giving a very long guest list 💀
• *-mconfig-fpu=num* — set an FPU configuration value from 0 to 7. Okay so 0--3 is as the FPU spec says, and 4--7 correspond to 0--3. Same energy, different number 🌀
• *-mconfig-mul=type* — set the configuration for the multiply instruction. type can be one of fast, fast1 (equivalent to fast), fast2, or slow. The default is fast1. Fast girlie by default, obviously 💅
• *-mconfig-register-ports=kind* — set the numbers of read/write ports for n9/n10 cores. The value should be 3r2w or 2r1w. Ports, but make it a whole personality 🎀
• *-mcmodel=code-model* — set the code model to one of these three:
  • small — all the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space. Cozy little neighborhood 🏡
  • medium — the data segment must be within 512KB, while the read-only data segment can be within 4GB addressing space. The text segment should still be within 16MB addressing space. Mid-size, mid-vibes ✨
  • large — all the text and data segments can be within 4GB addressing space. Big energy, no cap 🔥

The default is large on GNU/Linux targets and medium on other ELF targets. Noted, bestie 📝

• -mctor-dtor, -mno-ctor-dtor — turn on/off the constructor/destructor feature. Beginnings and endings, iconic 🦋
• -mrelax, -mno-relax — turn on/off the linker option to relax instructions. Relax, it's just instructions 💆‍♀️
• -mrelax-hint, -mno-relax-hint — turn on/off insertion of hints for the linker to do relaxation. Little notes to your linker bestie 🫶
• -msched-prolog-epilog, -mno-sched-prolog-epilog — turn on/off scheduling of a function's prologue and epilogue sequence. Bookends matter, periodt 📚
• -mret-in-naked-func, -mno-ret-in-naked-func — turn on/off generating return instructions in naked functions. This option is enabled by default. Naked functions are already out here exposed 😭
• -malways-save-lp, -mno-always-save-lp — turn on/off always saving $lp on the stack. Always save your faves 💖
• -munaligned-access, -mno-unaligned-access — turn on/off unaligned word and halfword accesses to packed data. It's giving chaos but make it structured 🙈
• -minline-asm-r15, -mno-inline-asm-r15 — allow/disallow use of r15 for inline asm. Permission slip, signed or denied 🎟️

---

🌸 Nvidia PTX Options 🌸

These options are defined for Nvidia PTX — GPU bestie territory 👑

• -m64 — ignored, but preserved for backward compatibility. Only 64-bit ABI is supported. It's just here for the memories 🫶
• *-march=architecture-string* — generate code for the specified PTX ISA target architecture. Valid architecture strings are sm_30, sm_35, sm_37, sm_50, sm_52, sm_53, sm_61, sm_70, sm_75, sm_80, and sm_89. The default depends on how the compiler has been configured — see --with-arch.

This option sets the value of the preprocessor macro __PTX_SM__; for instance, for sm_35, it has the value 350. Numbers, but make it iconic ✨

• *-misa=architecture-string* — alias of -march=. Same thing, different name, iykyk 💅
• -march-map=architecture-string — select the closest available -march= value that is **not more capable. For instance, for -march-map=sm_50 select -march=sm_37, and for -march-map=sm_53 select -march=sm_53. It's giving "bestie, we're matching you to the nearest vibe" 🎯
• -mptx=version-string — generate code for the specified PTX ISA version. Valid version strings are 3.1, 4.0, 4.1, 4.2, 5.0, 6.0, 6.3, 7.0, 7.3, and 7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target architecture, see -march=, but **at least 6.3, or 7.3 for -march=sm_52 and higher.

This option sets the values of the preprocessor macros __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__; for instance, for 3.1 the macros have the values 3 and 1, respectively. Major and minor, the duo we deserved 🫶

• -mmainkernel — link in code for a __main kernel. This is for stand-alone instead of offloading execution. Main-character kernel energy, literally 👑
• -moptimize, -mno-optimize — turn on/off partitioned execution optimizations. This option is enabled by default when any level of optimization is selected. Optimization stays on unless you say otherwise 💅
• -msoft-stack, -mno-soft-stack — okay so this one has lore, buckle up:

For -mno-soft-stack (the default, unless -mgomp has been specified), use PTX "native" stacks — that is, generate code that uses .local memory or PTX "alloca" directly for stack storage. Unless -mptx=7.3 or higher and -march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with "alloca" are not supported. Read that again, bestie 😭

For -msoft-stack (implied by -mgomp), generate code that does not use .local memory or PTX "alloca" directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler — to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp. Testing-only, don't ship it by accident 🫶

• *-msoft-stack-reserve-local=size* — specify the size of .local memory used for the stack when the exact amount is not known. It defaults to 128. When in doubt, 128 ✨
• -muniform-simt, -mno-uniform-simt — turn on/off a code generation variant that allows execution of all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array int __nvptx_uni[] stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position tid.y with current lane index to compute the master lane index. Yeah, it's a lot — but it all matters, don't skip it 💀
• -mgomp, -mno-gomp — turn on/off generation of code for use in OpenMP offloading. -mgomp enables -msoft-stack and -muniform-simt options, and selects a corresponding multilib variant. One flag, three jobs, main character energy 👑

---

🧚 OpenRISC Options 🧚

These options are defined for OpenRISC — open source, open heart 🫶

• *-mboard=name* — configure a board specific runtime. This is passed to the linker for newlib board library linking. The default is or1ksim. Board meeting, but make it linker 💅
• -msoft-div, -mhard-div — select software or hardware divide (l.div, l.divu) instructions. This default is hardware divide. Dividing, but make it hardware 🔥
• -msoft-mul, -mhard-mul — select software or hardware multiply (l.mul, l.muli) instructions. This default is hardware multiply. Multiplication stays fast ✨
• -msoft-float, -mhard-float — select software or hardware for floating point operations. The default is software. Floating point, but the soft way 🎀
• -mdouble-float — when -mhard-float is selected, enables generation of double-precision floating point instructions. By default functions from libgcc are used to perform double-precision floating point operations. Double the float, double the fun 💖
• -munordered-float — when -mhard-float is selected, enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default functions from libgcc are used to perform unordered floating point compare and set flag operations. Unordered but unbothered 🌈
• -mcmov — enable generation of conditional move (l.cmov) instructions. By default the equivalents are generated using set and branch. Conditional move said "I'll do it myself" 💅
• -mror — enable generation of rotate right (l.ror) instructions. By default functions from libgcc are used to perform rotate right operations. Rotate right, rotate slay 🔄
• -mrori — enable generation of rotate right with immediate (l.rori) instructions. By default functions from libgcc are used to perform rotate right with immediate operations. Immediate, like your reaction ⚡
• -msext — enable generation of sign extension (l.ext*) instructions. By default memory loads are used to perform sign extension. Extend the sign, extend the vibe ✨
• -msfimm — enable generation of compare and set flag with immediate (l.sf*i) instructions. By default extra instructions are generated to store the immediate to a register first. No extra steps if you don't have to 🎀
• -mshftimm — enable generation of shift with immediate (l.srai, l.srli, l.slli) instructions. By default extra instructions are generated to store the immediate to a register first. Shifting into the immediate, no middleman 🦋
• -mcmodel=small — generate OpenRISC code for the small model: the GOT is limited to 64k and function call jumps are limited to 64M offsets. This is the default model. Small but mighty, fr fr 🏡
• -mcmodel=large — generate OpenRISC code for the large model: the GOT may grow up to 4G in size and function call jumps can target the full 4G address space. Big model energy, go big or go home 🌍

---

💀 PDP-11 Options 💀

These options are defined for the PDP-11 — vintage bestie, respect the elders 👑

• -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies -m45. Hardware it is, unless stated otherwise ✨
• -msoft-float — do not use hardware floating point. Soft float era 🎀
• -mac0, -mno-ac0 — with -mac0, return floating-point results in ac0 (fr0 in Unix assembler syntax). The default, -mno-ac0, is to return floating-point results in memory. Pick your return address, bestie 🫶
• -m40 — generate code for a PDP-11/40. Implies -msoft-float -mno-split. The 40 said soft float, no split, and it meant it 💅
• -m45 — generate code for a PDP-11/45. This is the default. The 45 is the moment 👑
• -m10 — generate code for a PDP-11/10. Implies -msoft-float -mno-split. The 10 keeps it soft and unsplit, iconic 🔥
• -mint16, -mno-int32 — use 16-bit int 💖 this is the default, bestie, so if you do nothing you already got it.
• -mint32, -mno-int16 — use 32-bit int. big int energy ✨
• -msplit — tells GCC the target has split instruction and data space. it implies -m45. iykyk 🫶
• -munix-asm — use Unix assembler syntax.
• -mdec-asm — use DEC assembler syntax.
• -mgnu-asm — use GNU assembler syntax. this one's the default, she's the main character 👑
• -mlra — use the new LRA register allocator. by default the old "reload" allocator is the one getting the job, so this is you upgrading her 💅

---

✨ Picolibc Options ✨

these bad girls control compilation and linking when you're using Picolibc 🎀

• --oslib=library — search for the library literally named library after the C library, so symbols the C library left undefined can get defined by this one instead. the C library, libgcc and this library all get placed between --start-group and --end-group flags so each of them can refer to symbols in the others 🫶. and for many targets, Picolibc hands you a semihost variant (you ask for it with --oslib=semihost) which provides enough basic OS functionality to support console and file I/O when it's run in an emulator or when you're using an in-circuit debugger. iconic.
• --crt0=[none|minimal|hosted|semihost] — swaps the default crt0.o name out for crt0-variant.o. here's the lineup 🌈:
  • none gives you no startup code at all, which means you supply your own — you're on your own, babe, but in a cool way.
  • minimal performs basic memory setup but does not invoke any constructors.
  • when no -crt0 option is provided, the default initialization code adds calls to all constructors.
  • hosted adds a call to exit when main returns.
  • semihost accesses a command-line parameter supplied via the semihosting interface and splits that into arguments at whitespace boundaries, passing the resulting array of strings to main in argc and argv. on some targets — including AArch64, ARC, ARM, LoongArch, M680x0, RISC-V, SH, and x86 — semihost also traps hardware exceptions and prints information to the console 🖨️. heads up though: --crt0=semihost depends upon APIs provided by --oslib=semihost.
• --printf=[d|f|l|i|m] — picks your printf variant. Picolibc gives you five different printf variants, and they offer decreasing levels of functionality along with decreasing code size (trade-off queen 💀):
  • d is the default level, offering full C17 and POSIX.1-2024 conformance.
  • f gives the same feature set, but supports float values instead of double, and those get passed using the printf_float macro.
  • l elides all floating-point and POSIX positional parameter support.
  • i limits integers to those no larger than long.
  • m removes support for most formatting options, including width and precision. the formats and arguments are still parsed correctly, but the output just does not respect those parameters 🙈.
• --scanf=[d|f|l|i|m] — same deal but for scanf. five variants, decreasing functionality, decreasing code size:
  • d is the default level, offering full C17 and POSIX.1-2024 conformance.
  • f removes support for double values.
  • l elides all floating point support.
  • i limits integers to those no larger than long.
  • m removes support for %[ conversion specifiers.

---

✨ PowerPC Options ✨

these are listed under — that's literally what it says, no cap, they're listed under 🦋

---

✨ PRU Options ✨

these command-line options are defined for the PRU target, okurrr:

• -minrt — link 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: the compiler could still generate code with static initializers and constructors. it is up to the programmer to ensure that the source program does not use those features. (yes, that's on you, bestie.)

the minimal startup code does not pass argc and argv arguments to main, so main must be declared as int main (void). this is already the norm for most firmware projects, so like, chill.

• -mmcu=mcu — specify the PRU hardware variant to use. a correspondingly-named spec file gets loaded, which passes the memory region sizes to the linker and defines 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. go get it ✨

• -mno-relax — makes GCC pass the --no-relax command-line option to the linker instead of the --relax option.
• -mloop, -mno-loop — allow (or do not allow) GCC to use the LOOP instruction.
• -mmul, -mno-mul — allow (or do not allow) GCC to use the PRU multiplier unit.
• -mfillzero, -mno-fillzero — allow (or do not allow) GCC to use the FILL and ZERO instructions.
• -mabi=variant — specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI, while -mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions. the differences, bestie, are these 👇:

🎀 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.

🎀 Size Of Struct Containing Bit-fields 🎀

TI ABI mandates that struct size is determined by the bit-field type, if it contains any. on the other hand, GNU allocates the smallest amount of bytes which would fit the bit-field.

for example, TI ABI reserves 4 bytes for this struct, whereas GNU reserves a single byte:

struct S { int i:1; };

🎀 Access Size For Volatile Bit-fields 🎀

TI ABI mandates that volatile bit-fields are accessed using their type. in contrast, GNU ABI uses the smallest integer type fitting the bit-field.

for example, TI ABI requires a single load of 4 bytes for the following bit-field. GNU generates a load of 1 byte:

struct S { volatile int i:1; };

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, periodt 🌸:

• -mbranch-cost=n — set the cost of branches to roughly n instructions.
• -mabi=ABI-string — specify the integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. for example -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers ✨. contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments are passed in registers.

the default for this argument is system dependent; if you want a specific calling convention you should specify one explicitly, bestie, don't leave it to fate 🙈. the valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide 💀. there are also the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the rv64e. those ABIs are not well-specified at present, and are subject to change — so like, don't get too attached.

• -mfdiv, -mno-fdiv — do or don't use hardware floating-point divide and square root instructions. this requires the F or D extensions for floating-point registers. the default is to use them if the specified architecture has these instructions 🫶.
• -mfence-tso, -mno-fence-tso — do or don't use the fence.tso instruction, which is unimplemented on some processors (including those from T-Head). if the fence.tso instruction is not available then a stronger fence is used instead. she improvises 💅
• -mdiv, -mno-div — do or don't use hardware instructions for integer division. this requires the M extension. the default is to use them if the specified architecture has these instructions.
• -misa-spec=ISA-spec-string — specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification that generated code should conform to. the possibilities for ISA-spec-string are:
  • 2.2 — produce code conforming to version 2.2.
  • 20190608 — produce code conforming to version 20190608.
  • 20191213 — produce code conforming to version 20191213.

the default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version ⭐.

• -march=[ISA|Profile|Profile_ISA|processor-string] — generate code for the given RISC-V ISA or profile or a combination of them (e.g. rv64im rvi20u64 rvi20u64_zbb). the names of ISAs and profiles must be lower case, no shouting allowed 🎀. examples include rv64i, rv32g, rv32e, rv32imaf, rva22u64 and rva23u64. to combine a named profile with optional RISC-V ISA extensions, give the profile first and then append the extension name(s) using an underscore as a delimiter (e.g. rvi20u64_zca_zcb rva23u64_zacas). additionally, a special value help (-march=help) is accepted to list all supported extensions. cute, right?

-march=unset causes the compiler to ignore any -march=... options that appear earlier on the command line, behaving as if the option was never passed. this is useful for ensuring that the architecture is taken from the -mcpu option, and an error results if no -mcpu option is given when -march=unset is used ⚠️.

okay so the syntax of the ISA string is defined as follows:

• The string has to start with rv32 or rv64, then i, e, or g — that bit is the base ISA 💅
• After that comes a list of extension names. These come in two flavours: multi-letter (like zba) and single-letter (like v). Single-letter extensions can be stacked up back-to-back, but multi-letter ones must be separated by underscores. no cap 🫶
• An underscore can show up anywhere after the base ISA. It doesn't actually do anything mechanically — it's purely a readability thing and can double as a separator 💖
• Extension names can carry an optional version number, written as <major>p<minor> or <major>, e.g. m2p1 or m2 ✨

---

Supported extensions are listed below — yes, all of them, bestie. Buckle up 🎀

• g — version - — General-purpose computing base extension; g expands to i, m, a, f, d, zicsr and zifencei.
• e — version 2.0 — Reduced base integer extension
• i — versions 2.0, 2.1 — Base integer extension
• m — version 2.0 — Integer multiplication and division extension
• a — versions 2.0, 2.1 — Atomic extension
• f — versions 2.0, 2.2 — Single-precision floating-point extension
• d — versions 2.0, 2.2 — Double-precision floating-point extension
• c — version 2.0 — Compressed extension
• b — version 1.0 — Standard extension for bit manipulation functions
• v — version 1.0 — Vector extension
• h — version 1.0 — Hypervisor extension
• zic64b — version 1.0 — Cache block size is 64 bytes
• zicbom — version 1.0 — Cache-block management extension
• zicbop — version 1.0 — Cache-block prefetch extension
• zicboz — version 1.0 — Cache-block zero extension
• ziccamoa — version 1.0 — Main memory supports all atomics in A
• ziccif — version 1.0 — Main memory supports instruction fetch with atomicity requirement
• zicclsm — version 1.0 — Main memory supports misaligned loads/stores
• ziccrse — version 1.0 — Main memory supports forward progress on LR/SC sequences
• zicfilp — version 1.0 — Control-flow integrity landing pad extension
• zicfiss — version 1.0 — Control-flow integrity shadow stack extension
• zicntr — version 2.0 — Standard extension for base counters and timers
• zicond — version 1.0 — Integer conditional operations extension
• zicsr — version 2.0 — Control and status register access extension
• zifencei — version 2.0 — Instruction-fetch fence extension
• zihintntl — version 1.0 — Non-temporal locality hints extension
• zihintpause — version 2.0 — Pause hint extension
• zihpm — version 2.0 — Standard extension for hardware performance counters
• zimop — version 1.0 — May-be-operations extension
• zilsd — version 1.0 — Load/store pair instructions extension
• zmmul — version 1.0 — Integer multiplication extension
• za128rs — version 1.0 — Reservation set size of 128 bytes
• za64rs — version 1.0 — Reservation set size of 64 bytes
• zaamo — version 1.0 — Atomic memory operations extension
• zabha — version 1.0 — Byte and halfword atomic memory operations extension
• zacas — version 1.0 — Atomic compare-and-swap instructions extension
• zalrsc — version 1.0 — Load-reserved/store-conditional subset of the A extension
• zawrs — version 1.0 — Wait-on-reservation-set extension
• zama16b — version 1.0 — Misaligned loads, stores, and AMOs that are fully contained within a naturally-aligned 16-byte boundary are atomic
• zfa — version 1.0 — Additional floating-point extension
• zfbfmin — version 1.0 — Minimal BF16 support extension
• zfh — version 1.0 — Half-precision floating-point extension
• zfhmin — version 1.0 — Minimal half-precision floating-point extension
• zfinx — version 1.0 — Single-precision floating-point in integer registers extension
• zdinx — version 1.0 — Double-precision floating-point in integer registers extension
• zca — version 1.0 — Integer compressed instruction extension
• zcb — version 1.0 — Simple compressed instruction extension
• zcd — version 1.0 — Compressed double-precision floating point loads and stores extension
• zce — version 1.0 — Compressed instruction extensions for embedded processors
• zcf — version 1.0 — Compressed single-precision floating point loads and stores extension
• zcmop — version 1.0 — Compressed may-be-operations extension
• zcmp — version 1.0 — Compressed push pop extension
• zcmt — version 1.0 — Table jump instruction extension
• zclsd — version 1.0 — Compressed load/store pair instructions extension
• zba — version 1.0 — Address calculation extension
• zbb — version 1.0 — Basic bit manipulation extension
• zbc — version 1.0 — Carry-less multiplication extension
• zbkb — version 1.0 — Cryptography bit-manipulation extension
• zbkc — version 1.0 — Cryptography carry-less multiply extension
• zbkx — version 1.0 — Cryptography crossbar permutation extension
• zbs — version 1.0 — Single-bit operation extension
• zk — version 1.0 — Standard scalar cryptography extension
• zkn — version 1.0 — NIST algorithm suite extension
• zknd — version 1.0 — AES Decryption extension
• zkne — version 1.0 — AES Encryption extension
• zknh — version 1.0 — Hash function extension
• zkr — version 1.0 — Entropy source extension
• zks — version 1.0 — ShangMi algorithm suite extension
• zksed — version 1.0 — SM4 block cipher extension
• zksh — version 1.0 — SM3 hash function extension
• zkt — version 1.0 — Data independent execution latency extension
• ztso — version 1.0 — Total store ordering extension
• zvbb — version 1.0 — Vector basic bit-manipulation extension
• zvbc — version 1.0 — Vector carry-less multiplication extension
• zve32f — version 1.0 — Vector extensions for embedded processors
• zve32x — version 1.0 — Vector extensions for embedded processors
• zve64d — version 1.0 — Vector extensions for embedded processors
• zve64f — version 1.0 — Vector extensions for embedded processors
• zve64x — version 1.0 — Vector extensions for embedded processors
• zvfbfmin — version 1.0 — Vector BF16 converts extension
• zvfbfwma — version 1.0 — Vector BF16 widening multiply/add extension
• zvfh — version 1.0 — Vector half-precision floating-point extension
• zvfhmin — version 1.0 — Vector minimal half-precision floating-point extension
• zvkb — version 1.0 — Vector cryptography bit-manipulation extension
• zvkg — version 1.0 — Vector GCM/GMAC extension
• zvkn — version 1.0 — Vector NIST Algorithm Suite extension, zvkn will expand to
• zvknc — version 1.0 — Vector NIST Algorithm Suite with carry-less multiply extension, zvknc
• zvkned — version 1.0 — Vector AES block cipher extension
• zvkng — version 1.0 — Vector NIST Algorithm Suite with GCM extension, zvkng will expand
• zvknha — version 1.0 — Vector SHA-2 secure hash extension
• zvknhb — version 1.0 — Vector SHA-2 secure hash extension
• zvks — version 1.0 — Vector ShangMi algorithm suite extension, zvks will expand
• zvksc — version 1.0 — Vector ShangMi algorithm suite with carry-less multiplication extension,
• zvksed — version 1.0 — Vector SM4 block cipher extension
• zvksg — version 1.0 — Vector ShangMi algorithm suite with GCM extension
• zvksh — version 1.0 — Vector SM3 secure hash extension
• zvkt — version 1.0 — Vector data independent execution latency extension

---

okay the vector-length girlies are next, and ngl the wording flip-flops between plural and singular — I'm keeping it exactly as written because that's the vibe the source committed to 💀:

• zvl1024b — version 1.0 — Minimum vector length standard extensions
• zvl128b — version 1.0 — Minimum vector length standard extensions
• zvl16384b — version 1.0 — Minimum vector length standard extension
• zvl2048b — version 1.0 — Minimum vector length standard extensions
• zvl256b — version 1.0 — Minimum vector length standard extensions
• zvl32768b — version 1.0 — Minimum vector length standard extension
• zvl32b — version 1.0 — Minimum vector length standard extensions
• zvl4096b — version 1.0 — Minimum vector length standard extensions
• zvl512b — version 1.0 — Minimum vector length standard extensions
• zvl64b — version 1.0 — Minimum vector length standard extensions
• zvl65536b — version 1.0 — Minimum vector length standard extension
• zvl8192b — version 1.0 — Minimum vector length standard extension

---

and the rest of the alphabet soup ✨:

• zhinx — version 1.0 — Half-precision floating-point in integer registers extension
• zhinxmin — version 1.0 — Minimal half-precision floating-point in integer registers extension
• sdtrig — version 1.0 — Debug triggers extension
• sha — version 1.0 — The augmented hypervisor extension
• shcounterenw — version 1.0 — Support writeable enables for any supported counter
• shgatpa — version 1.0 — SvNNx4 mode supported for all modes supported by satp
• shlcofideleg — version 1.0 — Delegating LCOFI interrupts to VS-mode
• shtvala — version 1.0 — The htval register provides all needed values
• shvstvala — version 1.0 — The vstval register provides all needed values
• shvstvecd — version 1.0 — The vstvec register supports direct mode
• shvsatpa — version 1.0 — The vsatp register supports all modes supported by satp
• smaia — version 1.0 — Advanced interrupt architecture extension
• smcntrpmf — version 1.0 — Cycle and instret privilege mode filtering
• smcsrind — version 1.0 — Machine-level indirect CSR access

that's the whole roster, bestie — every last one of them, nothing skipped 💖

• smepmp — 1.0 — PMP enhancements for memory access and execution prevention on machine mode
• smmpm — 1.0 — Supervisor-mode pointer masking extension
• smnpm — 1.0 — Supervisor-mode pointer masking extension
• smrnmi — 1.0 — Resumable non-maskable interrupts
• smstateen — 1.0 — State enable extension
• smdbltrp — 1.0 — Double trap extensions
• ssaia — 1.0 — Advanced interrupt architecture extension for supervisor mode
• ssccptr — 1.0 — Main memory supports page table reads
• sscofpmf — 1.0 — Count overflow and filtering extension
• sscounterenw — 1.0 — Support writeable enables for any supported counter
• sscsrind — 1.0 — Supervisor-mode indirect CSR access
• ssnpm — 1.0 — Supervisor-mode pointer masking extension
• sspm — 1.0 — Supervisor-mode pointer masking extension
• ssstateen — 1.0 — Supervisor-mode state-enable extension
• sstc — 1.0 — Supervisor-mode timer interrupts extension
• sstvala — 1.0 — Stval provides all needed values
• sstvecd — 1.0 — Stvec supports direct mode
• ssstrict — 1.0 — Unimplemented reserved encodings raise illegal instruction exceptions and no non-conforming extensions are present
• ssdbltrp — 1.0 — Double trap extensions
• ssu64xl — 1.0 — UXLEN=64 must be supported
• supm — 1.0 — User-mode pointer masking 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
• svvptc — 1.0 — Extension for obviating memory-management instructions after marking PTEs valid
• svadu — 1.0 — Hardware updating of A/D bits extension
• svade — 1.0 — Cause exception when hardware updating of A/D bits is disabled
• svbare — 1.0 — Satp mode bare is supported
• xcvalu — 1.0 — Core-V miscellaneous ALU extension
• xcvbi — 1.0 — Core-V immediate branch extension
• xcvelw — 1.0 — Core-V event load word extension
• xcvmac — 1.0 — Core-V multiply-accumulate extension
• xcvsimd — 1.0 — Core-V SIMD extension
• xsfcease — 1.0 — SiFive CEASE instruction extension
• xsfvcp — 1.0 — SiFive VCIX vector coprocessor extension
• xsfvfnrclipxfqf — 1.0 — SiFive FP32-to-int8 ranged clip instructions
• xsfvqmaccdod — 1.0 — SiFive int8 matrix multiplication extension
• xsfvqmaccqoq — 1.0 — SiFive int8 matrix multiplication 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
• xtheadvector — 1.0 — T-head vector extension
• xventanacondops — 1.0 — Ventana integer conditional operations extension
• xmipscmov — 1.0 — Mips conditional move extension
• xmipscbop — 1.0 — Mips prefetch extension
• xandesperf — 5.0 — Andes performace extension
• xandesbfhcvt — 5.0 — Andes bfloat16 conversion extension
• xandesvbfhcvt — 5.0 — Andes vector bfloat16 conversion extension
• xandesvsintload — 5.0 — Andes vector INT4 load extension
• xandesvpackfph — 5.0 — Andes vector packed FP16 extension
• xandesvdot — 5.0 — Andes vector dot product extension
• xsmtvdot — 1.0 — SpacemiT vector dot product extension

Okay bestie, breathe — that was the whole extension buffet 💅 and honestly it ate.

When -march= is not specified, GCC uses the setting from -mcpu. Slay, it just inherits, no cap.

If both -march and -mcpu= are not specified, the default for this argument is system dependent; so like, if you want a specific architecture extension, you should specify one explicitly. Don't just vibe and hope, okay? 🫶

When the RISC-V specifications define an extension as depending on other extensions, GCC implicitly adds the dependent extensions to the enabled extension set if they weren't added explicitly. Like it literally brings the whole friend group along 🎀

💖 Core Name 💖

• -mcpu=processor-string — Use architecture of and optimize the output for the given processor, specified by particular CPU name. Here's the guest list, all permissible values for this option:

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-p450,
sifive-p670,
thead-c906,
xt-c908,
xt-c908v,
xt-c910,
xt-c910v2,
xt-c920,
xt-c920v2,
tt-ascalon-x,
tt-ascalon-d8,
xiangshan-nanhu,
xiangshan-kunminghu,
mips-p8700,
andes-n22,
andes-n25,
andes-a25,
andes-nx25,
andes-ax25,
andes-a27,
andes-ax27,
andes-n225,
andes-d23,
andes-n45,
andes-nx45,
andes-a45,
andes-ax45,
spacemit-x60.

Heads up bestie ✨ Note that -mcpu does not override -march or -mtune. It's not that girl, it does not have that power.

🌸 Tune Name 🌸

• -mtune=processor-string — Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are:

generic,
rocket,
sifive-3-series,
sifive-5-series,
sifive-7-series,
sifive-p400-series,
sifive-p600-series,
tt-ascalon-x,
tt-ascalon-d8,
thead-c906,
xt-c908,
xt-c908v,
xt-c910,
xt-c910v2,
xt-c920,
xt-c920v2,
xiangshan-nanhu,
xiangshan-kunminghu,
spacemit-x60,
generic-ooo,
size,
mips-p8700,
andes-25-series,
andes-23-series,
andes-45-series,

…and all valid options for -mcpu=. Yes, every single one, the whole roster 💀

When -mtune= is not specified, GCC uses the setting from -mcpu. The default is generic if neither is specified.

The size choice is not intended for use by end-users 🚫 — this is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but does not override the pipeline info. It helps reduce code size while still giving good performance. So like, don't touch it, it's doing its job 🙈

---

• -mpreferred-stack-boundary=num — Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128-bits).

⚠️ Warning: If you use this switch, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. Not a vibe if you mix them, fr fr 😭

• -msmall-data-limit=n — Put global and static data smaller than n bytes into a special section (on some targets).
• -msave-restore, -mno-save-restore — Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues. (Fast > small, that's the default energy 💅)
• -mmovcc, -mno-movcc — Do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable.
• -minline-atomics, -mno-inline-atomics — Do or don't use smaller but slower subword atomic emulation code that uses libatomic function calls. The default is to use fast inline subword atomics that do not require libatomic — iconic, no library needed 🎀
• -minline-strlen, -mno-inline-strlen — Do or do not attempt to inline "strlen" calls if possible. Inlining can only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to inline "strlen" calls.
• -minline-strcmp, -mno-inline-strcmp — Do or do not attempt to inline "strcmp" calls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inline "strcmp" calls.
• -minline-strncmp, -mno-inline-strncmp — Do or do not attempt to inline "strncmp" calls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inline "strncmp" calls. (Yes bestie, all three of them get the same treatment, they're a package deal 🫶)
• -mstringop-strategy=strategy — Specify a particular strategy for inlining string and memory operations. strategy may be one of auto, libcall, scalar, or vector.
• -mshorten-memrefs, -mno-shorten-memrefs — Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of 'base register + large offset' with a new load/store of 'new base + small offset'. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only. 🦋
• -mstrict-align, -mno-strict-align — bestie, this is the "do we do unaligned memory accesses or not" switch. -mno-strict-align = yes we do generate them, -mstrict-align = no we do not. The default just depends on whether the processor we're optimizing for supports fast unaligned access or not. Slay, adaptive defaults 💅
• -mscalar-strict-align, -mno-scalar-strict-align — literally the exact same vibe as above: do not or do generate unaligned memory accesses, with the default set depending on whether the processor we're optimizing for supports fast unaligned access or not. It's an alias for -mstrict-align, so don't stress about picking one 🫶
• -mvector-strict-align, -mno-vector-strict-align — same energy but for vector memory accesses: do not or do generate unaligned vector memory accesses. The default is set to off unless the processor we're optimizing for explicitly supports element-misaligned vector memory access. Iconic conditional behaviour ✨
• -mmax-vectorization, -mno-max-vectorization — enable or disable an override to the vectorizer cost model that makes vectorization always appear profitable. And no, it's not the same as -fno-vect-cost-model or -fvect-cost-model=unlimited — unlike those, this option does not turn off the cost comparison between different vector modes. Important distinction, iykyk 💀
• -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model unless GCC has been configured with --with-cmodel= specifying a different default code model. Cozy little address box 🎀
• -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… read that twice, bestie 😭

• -mcmodel=large — generate code for a large code model, which has no restrictions on size or placement of symbols. Free-range symbols, no fence 🌈
• -mexplicit-relocs, -mno-explicit-relocs — use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. Pick your fighter 💅
• -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 — work smarter not harder, periodt ✨
• -mriscv-attribute, -mno-riscv-attribute — emit (do not emit) 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. Simple as that, no cap ✅
• -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. Little optimization moment 🍓
• -malign-data=type — controls how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen, which uses x register width as the alignment value, and natural, which uses natural alignment. xlen is the default. Alignment lore 👑
• -mbig-endian — generate big-endian code. This is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-* target.

Support for RISC-V big-endian is experimental. The ABI is not yet stable and could change in incompatible ways in future releases. Like, don't build your whole empire on it 💀

• -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. The chill default vibe 🦋
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or tls for per-thread canary in the TLS block.

With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset, as this is entirely for use within the Linux kernel. Kernel-only behaviour, bestie, don't touch 🙈

• -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. Classic for a reason 🎀
• -mrvv-vector-bits=value — specify how the number of bits for an RVV vector register, as taken from the -march= option, is interpreted. The value parameter is specified as a string keyword and may be one of scalable or zvl. The default is scalable, which tells GCC to interpret the number as a minimum, while zvl tells GCC to use exactly the number of bits specified. Math but make it fashion ✨
• -mrvv-max-lmul=value — this option allows explicit control over the maximum length multiplier (LMUL) used when generating code for the RISC-V Vector Extensions (RVV). The value parameter is specified as a string keyword and may be one of m1, m2, m4, m8, or dynamic. The default is m1 for compatibility with existing hardware that does not support the other options. Backwards compat queen 👑
• -madjust-lmul-cost, -mno-adjust-lmul-cost — this option adjusts the cost model used to schedule vector instructions to multiply the latency of instructions by the RVV length multiplier, LMUL. It is disabled by default. Opt-in only, bestie 🫶
• -mautovec-segment, -mno-autovec-segment — enable or disable generation of vector segment load/store instructions. This option is enabled by default. Free feature, we love to see it 💖

---

🌸 RL78 Options 🌸

• -msim — links in additional target libraries to support operation within a simulator. Simulator support, cute 💅
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — specifies the type of hardware multiplication and division support to be used. Let's break it down:
  • The simplest is none, which uses software for both multiplication and division. This is the default.
  • The g13 value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets.
  • The g14 value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts.
  • The value rl78 is an alias for g14 and the value mg10 is an alias for none.

In addition, a C preprocessor macro is defined, based upon the setting of this option. Possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__. Macro lore unlocked 🎀

• -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions — instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention.

If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit -mmul=none option on the command line. Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware multiplications altogether.

Note: although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain, since it also enables G14 hardware multiply support. If these options are not specified on the command line, then software multiplication routines are used even though the code targets the RL78 core. This is for backward compatibility with older toolchains which did not have hardware multiply and divide support. Bestie, the default is doing less than you think 😭

In addition, a C preprocessor macro is defined, based upon the setting of this option. Possible values are: __RL78_G10__, __RL78_G13__ or __RL78_G14__.

• -mg10, -mg13, -mg14, -mrl78 — these are aliases for the corresponding -mcpu= option. They are provided for backwards compatibility. Short and sweet 💅
• -mallregs — allow the compiler to use all of the available registers. By default, registers r24..r31 are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well. All the registers, no holding back 🔥
• -mrelax, -mno-relax — enable/disable assembler and linker relaxation. This is enabled by default at -Os. Optimize for size, get relaxations free ✨
• -mes0 — assume ES is zero throughout program execution, and use it to address read-only data. Manifesting ES = 0 for the whole run 🧚
• -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster.

The target option -mg13 needs to be passed for this to work, as this feature is only available on the G13 target (S2 core). The MDUC registers are only saved if the interrupt handler performs a multiplication or division operation or it calls another function. Conditional saving, very demure 💀

---

👑 IBM RS/6000 and PowerPC Options 👑

These -m options are defined for the IBM RS/6000 and PowerPC:

• -mcpu=cpu_type — set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, power10, power11, future, powerpc, powerpc64, powerpc64le, rs64, and native. That's the whole roster, no skips 🫶

-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. -mcpu=native has no effect if GCC does not recognize the processor. Main character energy, but only if GCC recognizes you 💅

The other options each pick one specific processor, and here's the tea: code generated under those options runs best on that processor — and honestly might not run at all on any other one. 💀

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 -mdense-math -mrop-protect

Okay so the exact set of options any given CPU flips on varies between compiler versions — it all depends on what setting seems to produce the optimal code for that CPU. It does not necessarily reflect what the actual hardware is capable of. Iconic but slightly unhinged, we love her.

If you want to set an individual option to a particular value, just 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. 🙈

In other cases, we recommend you use the -mcpu=cpu_type option rather than the options that control generation of specific instructions. 💅

---

• -mtune=cpu_type — Sets the instruction scheduling parameters for machine type cpu_type, but doesn't set the architecture type or register usage the way -mcpu=cpu_type does. Same values for cpu_type here as for -mcpu. If you specify both, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune. Teamwork, bestie.
• -mpowerpc64, -mno-powerpc64 — -mpowerpc64 lets GCC generate those extra 64-bit instructions found in the full PowerPC64 architecture, and treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
• -mcmodel=small — Generate PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — Generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux. ✨
• -mcmodel=large — Generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space. big model energy
• -mprofile-kernel — Available on PowerPC64 GNU/Linux targets. When used with -pg, it causes calls to mcount to be inserted before the function prologue instead of after it. 🦋
• -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 matches array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. endianness is literally the main character here ngl

• -mpowerpc-gpopt, -mno-powerpc-gpopt — Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying
• -mpowerpc-gfxopt, -mno-powerpc-gfxopt — -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.
• -mmfcrf, -mno-mfcrf — The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture. 🍓
• -mpopcntb, -mno-popcntb — The -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture.
• -mpopcntd, -mno-popcntd — The -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture. 🔥
• -mfprnd, -mno-fprnd — The -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture.
• -mcmpb, -mno-cmpb — The -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture. ✨
• -mhard-dfp, -mno-hard-dfp — The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors.
• -mvrsave, -mno-vrsave — Generate VRSAVE instructions when generating AltiVec code. 💜
• -msecure-plt — Generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option. secure queen behavior
• -mbss-plt — Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.

👀 Heads up: that writable-and-executable combo is exactly what it says on the tin — -mbss-plt is the one that needs .plt and .got to be both writable and executable, unlike -msecure-plt. Pick accordingly, bestie.

• -msplit-patch-nops — When adding NOPs for a patchable area via the -fpatchable-function-entry option, emit the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point. This makes the sequence of NOPs not consecutive when a global entry point is generated. Without this option the NOPs are emitted directly before and after the local entry point, making them consecutive but moving the global and local entry point further apart. If only a single entry point is generated, this option has no effect. 🧚
• -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 — Enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA. 🔐
• -mhtm, -mno-htm — Enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA.
• -mpower8-fusion, -mno-power8-fusion — Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors. 😭 (yes, the manpage really says "does not keeps" and honestly? relatable)
• -mquad-memory, -mno-quad-memory — Generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory option requires use of 64-bit mode.
• -mquad-memory-atomic, -mno-quad-memory-atomic — Generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic option requires use of 64-bit mode. 💅
• -mfloat128, -mno-float128 — Enable/disable the __float128 keyword for IEEE 128-bit floating point, and use either software emulation for IEEE 128-bit floating point or hardware instructions.

The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point is only supported on Linux. 🐧

The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.

If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support also enables the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you don't specify generation of ISA 3.0 instructions or you're targeting a 32-bit big-endian system, IEEE 128-bit floating point is handled with software emulation. emulation era, we make it work

• -mfloat128-hardware, -mno-float128-hardware — Enable/disable using ISA 3.0 hardware instructions to support the __float128 data type.

The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems. ✨

• -m32, -m64 — Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64. 🎀
• -mfull-toc, -mno-fp-in-toc, -mno-sum-in-toc, -mminimal-toc — Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. 😭

If you receive a linker error message saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space. tradeoffs, tradeoffs, always with the tradeoffs

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 🫶

• -maix64, -maix32 — okay so this is the 64-bit AIX era switch 💅. -maix64 turns on the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and all the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32, no cap.
• -mxl-compat, -mno-xl-compat — this one makes your code cosplay IBM XL compiler semantics more closely when you're using the AIX-compatible ABI. It passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. It also says: do not assume that the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double. And it uses XL symbol names for long double support routines. The lore: the AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. iykyk ✨
• -mpe — support for the IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code so the app can actually run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. Also: PE does not support threads, so -mpe and -pthread are incompatible. not those two together, 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, such as floating-point doubles, on their natural size-based boundary. -malign-power tells GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI. On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported. respect the ABI, periodt 🎀
• -msoft-float, -mhard-float — code that does not use (uses) the floating-point register set. Software floating-point emulation is provided if you use -msoft-float, and pass the option to GCC when linking. soft float is doing a lot of emotional labor 😭
• -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. serious caveat, no playing around ⚠️
• -mupdate, -mno-update — generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default. If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. corrupted data is not the vibe 💀
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise. Power6 said “not today” 🔥
• -mfused-madd, -mno-fused-madd — generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. the mapping is giving “old name, new era” 🦋
• -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. specific chip energy 🎀
• -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. string-search slay ✨
• -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. For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. the size glow-up is real 🫶
• -mbit-word, -mno-bit-word — allow bit-fields to cross word boundaries. boundary crossing, but make it bit-fields 🌸
• -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. trust issues with alignment, honestly 🦋
• -mrelocatable, -mno-relocatable — generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section, 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. all objects or it does not count, bestie ⭐
• -mrelocatable-lib, -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a .fixup section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. mix-and-match friendly 💅
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle. little-endian girlies rise 🌈
• -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig. big-endian main character energy 👑
• -mdynamic-no-pic — on Darwin / macOS systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. apps yes, shared libraries no 🙈
• -msingle-pic-base — treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. runtime said “I got this” ✨
• -mprioritize-restricted-insns=priority — this option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. scheduling drama, but prioritized 💅
• -msched-costly-dep=dependence_type — this option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:
  • no — no dependence is costly.
  • all — all dependences are costly.
  • true_store_to_load — a true dependence from store to load is costly.
  • store_to_load — any dependence from store to load is costly.
  • number — any dependence for which the latency is greater than or equal to number is costly.
• -minsert-sched-nops=scheme — this option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:
  • no — don't insert NOPs.
  • pad — pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
  • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
  • number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix. March 1995 said “I'm the standard now” 🎀
• -mcall-sysv-eabi, -mcall-eabi — specify both -mcall-sysv and -meabi options. two options, one bullet, iconic ✨
• -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi options. same energy, no eabi 💅
• -mcall-aixdesc — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system. AIX mode activated 🦋
• -mcall-linux — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system. Linux-based GNU system, periodt 🐧
• -mcall-freebsd — on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system. FreeBSD representation 😌
• -mcall-netbsd — on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system. NetBSD said hi 🌸
• -mcall-openbsd — on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system. OpenBSD in the chat 🫶
• -mtraceback=traceback_type — select the type of traceback table. Valid values for traceback_type are full, part, and no. traceback table vibe check 💅
• -maix-struct-return — return all structures in memory (as specified by the AIX ABI). memory return, no shortcuts ✨
• -msvr4-struct-return — return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI). small structs get the register treatment 🎀
• -mabi=abi-type — okay so this one lets you extend the current ABI with a specific extension, or yeet one right off 🫶. The valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and (for AIX girlies specifically) vec-extabi and vec-default.
• -mabi=ibmlongdouble — switches the current ABI over to IBM extended-precision long double 💅. Heads up though: this is not likely to work if your system defaults to IEEE extended-precision long double. And if you change the long double type away from IEEE extended-precision, the compiler will throw a warning at you unless you pass -Wno-psabi. Also requires -mlong-double-128 to be enabled, no exceptions ✨.
• -mabi=ieeelongdouble — same energy but the other direction: switches the current ABI to use IEEE extended-precision long double 🦋. Same deal — not likely to work if your system defaults to IBM extended-precision long double, and if you change the long double type away from IBM extended-precision the compiler complains unless you use -Wno-psabi. Also needs -mlong-double-128 enabled. Symmetry, we love to see it 💖.
• -mabi=elfv1 — flips the current ABI to the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux 👑. Overriding the default ABI requires special system support and is, and I quote, likely to fail in spectacular ways 💀 no cap, that's the actual warning, please take it seriously.
• -mabi=elfv2 — flips the current ABI to the ELFv2 ABI, which is the default for little-endian PowerPC 64-bit Linux 🌈. Same caveat: overriding the default ABI needs special system support and is likely to fail in spectacular ways 😭 iconic warning tbh.
• -mgnu-attribute, -mno-gnu-attribute — emit (or don't emit) .gnu_attribute assembly directives that set tag/value pairs in a .gnu.attributes section, which specify ABI variations in function parameters or return values 🎀.
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, assume (or don't assume) that all calls to variable argument functions are properly prototyped 🌸. When you don't assume it, the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR), so it can 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. Efficient queen behavior 💅.
• -msim — on embedded PowerPC systems, assumes 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, iykyk.
• -mmvme — on embedded PowerPC systems, assumes the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a ✨.
• -mads — on embedded PowerPC systems, assumes the startup module is called crt0.o and the standard C libraries are libads.a and libc.a 🍓.
• -myellowknife — on embedded PowerPC systems, assumes the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a 💛 (yes the flag is literally named yellowknife, main character energy).
• -mvxworks — on System V.4 and embedded PowerPC systems, says "bestie I'm compiling for a VxWorks system" 🦋.
• -memb — on embedded PowerPC systems, sets 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 whole set of modifications to the System V.4 specs 👑. Picking -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.

Picking -mno-eabi means:

• the stack is aligned to a 16-byte boundary,
• no EABI initialization function is called from main,
• and -msdata only uses r13 to point to a single small data area.

The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options 💖.

• -msdata=eabi — on System V.4 and embedded PowerPC systems, puts small initialized const global and static data in the .sdata2 section, which register r2 points to 🌸. Puts small initialized non-const global and static data in the .sdata section, pointed to by register r13. Puts small uninitialized global and static data in the .sbss section, which sits right next to .sdata. Fair warning bestie: -msdata=eabi is incompatible with the -mrelocatable option 💀, and it also sets the -memb option.
• -msdata=sysv — on System V.4 and embedded PowerPC systems, puts small global and static data in the .sdata section, pointed to by register r13 ✨. Puts small uninitialized global and static data in .sbss, adjacent to .sdata. Also incompatible with -mrelocatable, so don't try it 🙈.
• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems: if -meabi is used, compile code the same as -msdata=eabi; otherwise compile it the same as -msdata=sysv 🫶. Vibes-based delegation, honestly iconic.
• -msdata=data — on System V.4 and embedded PowerPC systems, puts small global data in the .sdata section, and small uninitialized global data in the .sbss section 🎀. But — and this is the plot twist — do not use register r13 to address small data here. This is the default behavior unless other -msdata options are used 👀.
• -msdata=none, -mno-sdata — on embedded PowerPC systems, puts all initialized global and static data in the .data section, and all uninitialized data in the .bss section 💅. Simple, clean, no small-data drama.
• -mreadonly-in-sdata — also puts read-only objects in the .sdata section 🌈. This is the default, so it's already doing this unless you tell it otherwise.
• -mblock-move-inline-limit=num — inlines all block moves (stuff 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, so check your target's vibe.
• -mblock-compare-inline-limit=num — generates 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 (non-loop and loop) of block compare is disabled 🚫. Default value is target-specific.
• -mblock-compare-inline-loop-limit=num — generates an inline expansion using loop code for all block compares less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion 🌀. If the block length isn't constant, at most num bytes get compared before memcmp is called to handle the remainder of the block. Default is target-specific.
• -mstring-compare-inline-limit=num — compares at most num string bytes using inline code 🎀. If the difference or the end of the string isn't found by the end of that inline compare, a call to strcmp or strncmp takes care of the rest of the comparison. The default is 64 bytes, periodt 💖.
• -G num — on embedded PowerPC systems, puts global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section 🌸. By default, num is 8. The -G num switch also gets passed to the linker, and — important — all modules should be compiled with the same -G num value or things get messy 😭.
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms 🦋. Your asm, your aesthetic.
• -mlongcall, -mno-longcall — by default, assumes all calls are far away, so a longer and more expensive calling sequence is required 🔥. This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't possibly be that far away. This setting can be overridden by the shortcall function attribute, or by #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 just generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's planned to add this feature to the GNU linker for 32-bit PowerPC systems too 🌈.

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq) ✨. PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls 🙅.

On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code) 🏝️. The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island gets appended to the body of the calling function, and it computes the full 32-bit address of the callee and jumps to it 💀 (in a cool way).

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 👀. Just so you're not surprised later.

• -mpltseq, -mno-pltseq — implement (or don't implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries 🌟. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if the support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with --enable-secureplt 💅. Warning bestie: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together 😭.
• -mtls-markers, -mno-tls-markers — mark (or don't mark) calls to __tls_get_addr with a relocation specifying the function argument 🎀. The relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better ✨. Teamwork, honestly.
• -mrecip, -mno-recip — bestie, this one lets GCC use reciprocal estimate and reciprocal square root estimate instructions with extra Newton-Raphson steps to bump precision, instead of doing a divide or square root and divide for floating-point arguments. You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Note that while the throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for reciprocal square roots. 💅
• -mrecip=opt — this controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the reciprocal approximation instructions for both single and double precision.
  • divf — enable the single-precision reciprocal approximation instructions.
  • divd — enable the double-precision reciprocal approximation instructions.
  • rsqrt — enable the reciprocal square root approximation instructions for both single and double precision.
  • rsqrtf — enable the single-precision reciprocal square root approximation instructions.
  • rsqrtd — enable the double-precision reciprocal square root approximation instructions.

So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations. ✨

• -mrecip-precision, -mno-recip-precision — assume (or, with -mno-recip-precision, do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they do not provide an estimate that converges after three steps. 🫶
• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is mass, which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to acosd2, acosf4, acoshd2, acoshf4, asind2, asinf4, asinhd2, asinhf4, atan2d2, atan2f4, atand2, atanf4, atanhd2, atanhf4, cbrtd2, cbrtf4, cosd2, cosf4, coshd2, coshf4, erfcd2, erfcf4, erfd2, erff4, exp2d2, exp2f4, expd2, expf4, expm1d2, expm1f4, hypotd2, hypotf4, lgammad2, lgammaf4, log10d2, log10f4, log1pd2, log1pf4, log2d2, log2f4, logd2, logf4, powd2, powf4, sind2, sinf4, sinhd2, sinhf4, sqrtd2, sqrtf4, tand2, tanf4, tanhd2, and tanhf4 when generating code for power7. Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled. The MASS libraries must be specified at link time. 💖
• -mfriz, -mno-friz — generate (do not generate) the friz instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. 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 (do not 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. 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 (do not 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 is not saved in the prologue, it is 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 (do not 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 is 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 — generate stack protection code using canary at guard. Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default with GNU libc version 2.4 or later).

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. 🎀

• -mpcrel, -mno-pcrel — generate (do not generate) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. ⭐
• -mprefixed, -mno-prefixed — generate (do not generate) addressing modes using prefixed load and store instructions. The -mprefixed option requires that the option -mcpu=power10 (or later) is enabled. 🦋
• -mmma, -mno-mma — generate (do not generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or later) is enabled. 💀
• -mdense-math, -mno-dense-math — generate (do not generate) Dense Math Facility (DMF) instructions. The -mdense-math option requires that the option -mcpu=future (or later) is enabled. 🌈
• -mrop-protect, -mno-rop-protect — generate (do not generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). 🧚
• -mprivileged, -mno-privileged — generate (do not generate) code that runs in privileged state. 💜
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generate (do not generate) unaligned vsx loads and stores for inline expansion of memcpy and memmove. 🍓
• -msplat-word-constant, -mno-splat-word-constant — generate (do not generate) code that uses the XXSPLTIW instruction. This option is enabled by default. ✨
• -msplat-float-constant, -mno-splat-float-constant — generate (do not generate) code that uses the XXSPLTIDP instruction. This option is enabled by default. 💖
• -mieee128-constant, -mno-ieee128-constant — generate (do not generate) code that uses the LXVKQ instruction. This option is enabled by default. 🫶
• -mwarn-altivec-long, -mno-warn-altivec-long — enable or disable warnings about deprecated vector long ... Altivec type usage. This option is enabled by default. 🌸

RX Options 🎀

These command-line options are defined for RX targets:

• -m64bit-doubles, -m32bit-doubles — make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. Note RX floating-point hardware only works on 32-bit values, which is why the default is -m32bit-doubles. 🦋
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.

Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used.

Note: if the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is because the RX FPU instructions are themselves unsafe. 😭

• -mcpu=name — selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600 and RX200 series hardware and the specific RX610 CPU. The default is RX600.

The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction.

The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by default when this type is selected. 🔥

• -mbig-endian-data, -mlittle-endian-data — store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the little-endian format. 💅
• -msmall-data-limit=N — specifies the maximum size in bytes of global and static variables that can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow. Also when the small data area is used one of the RX's registers (usually r13) is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register.

Note, common variables (variables that have not been initialized) and constants are not placed into the small data area as they are assigned to other sections in the output executable. 🌟

The default value is zero, which turns this whole feature off. And ngl, it's not on by default even at higher optimization levels (-O2 and friends), because reserving a register can genuinely hurt you. Bestie, this is a "you experiment and find out if it helps your program" situation, no cap. Peep the -mpid description below for how the actual register that holds the small data area pointer gets picked. 💅

• -msim, -mno-sim — use the simulator runtime. Default is the libgloss board-specific runtime, iykyk.
• -mas100-syntax, -mno-as100-syntax — when you're generating assembler output, use a syntax that plays nice with Renesas's AS100 assembler. GAS can handle this syntax too, but it comes with some restrictions, which is why it's not what you get by default.
• -mmax-constant-size=N — sets the max size, in bytes, of a constant that can be used as an operand in an RX instruction. So here's the tea: the RX instruction set does let you use constants up to 4 bytes long inside instructions, but a longer value means a longer instruction. So sometimes it's lowkey beneficial to cap how big your in-instruction constants get. Constants that are too big get yeeted into a constant pool and referenced via register indirection instead.

The value N can be between 0 and 4. A value of 0 (the default) or 4 means constants of any size are allowed. 🫶

• -mrelax — enable linker relaxation. That's the process where the linker tries to shrink your program by finding shorter versions of various instructions. Disabled by default.
• -mint-register=N — says how many registers to reserve for fast interrupt handler functions. N can be between 0 and 4. A value of 1 means r13 is reserved exclusively for 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 — that's the default — reserves nothing at all. ✨
• -msave-acc-in-interrupts — says interrupt handler functions should preserve the accumulator register. You only need this if normal code might actually use the accumulator, like when it does 64-bit multiplications. Default is to ignore the accumulator, because that makes the interrupt handlers faster. 🏃‍♀️💨
• -mpid, -mno-pid — enables generating position-independent data. When it's on, any access to constant data happens via an offset from a base address held in a register. That means the location of constant data can be figured out at run time without the executable needing relocation — huge win for embedded apps with tight memory constraints. Data that can be modified is not affected by this option, so don't worry about that.

Heads up bestie: this feature reserves a register — usually r13 — for the constant data base address. That can make your code slower and/or bigger, especially in complicated functions. 😭

Which register actually gets picked for the constant data base address depends on whether -msmall-data-limit and/or -mint-register are enabled on the command line. Starting at r13 and going downwards, registers get allocated first to satisfy -mint-register, then -mpid, and finally -msmall-data-limit. So yes, it's genuinely possible for the small data area register to be r8 if you pass both -mint-register=4 and -mpid. 👑

By default this is off. You can get the default back with -mno-pid.

• -mno-warn-multiple-fast-interrupts, -mwarn-multiple-fast-interrupts — stops GCC from throwing a warning when it finds more than one fast interrupt handler while compiling a file. The default is to warn for each extra fast interrupt handler it finds, because the RX only supports one such interrupt. fr fr, only one.
• -mallow-string-insns, -mno-allow-string-insns — turns on or off the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL, SWHILE and also the RMPA instruction. These instructions may prefetch data, which is not safe 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 GCC literally cannot reliably detect every circumstance where a string instruction might touch an I/O register, so it can't disable them automatically. Instead it's on the programmer — that's you, bestie — to use -mno-allow-string-insns if the program accesses I/O space. 🙈

When these instructions are enabled, GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__; otherwise it defines __RX_DISALLOW_STRING_INSNS__.

• -mjsr, -mno-jsr — use only (or not only) JSR instructions to access functions. This comes in clutch when your code size exceeds the range of BSR instructions. Careful though: -mno-jsr does not mean "don't use JSR" — it means any type of branch may be used. 💀
• -mlra — use the new LRA register allocator. By default the old reload allocator is what you get.

🎀 Note: the generic GCC command-line option -ffixed-reg has special significance for the RX port when it's used with the interrupt function attribute. That attribute marks a function as one intended to process fast interrupts. GCC makes sure it only uses the registers r10, r11, r12 and/or r13 — and only provided that the normal use of the corresponding registers has been restricted via the -ffixed-reg or -mint-register command-line options.

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🦋 S/390 and zSeries Options 🦋

These are the -m options defined for the S/390 and zSeries architecture. Big iron energy, we love to see it. 🏗️

• -mhard-float, -msoft-float — use (or don't use) the hardware floating-point instructions and registers for floating-point operations. With -msoft-float, functions in libgcc.a handle the floating-point math. With -mhard-float, the compiler generates IEEE floating-point instructions. -mhard-float is the default. ✨
• -mhard-dfp, -mno-hard-dfp — use (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations. With -mno-hard-dfp, functions in libgcc.a do the decimal-floating-point work. With -mhard-dfp, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. 🌟
• -mlong-double-64, -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes long double equivalent to double. That's the default.
• -mbackchain, -mno-backchain — store (or don't store) the address of the caller's frame as a backchain pointer into the callee's stack frame. You might need a backchain so that debugging tools which don't understand DWARF call frame information can still do their thing. When -mno-packed-stack is in effect, the backchain pointer goes at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.

Generally speaking, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain — but using the backchain for debugging usually requires the whole binary to be built with -mbackchain. Also note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. If you're building a linux kernel, use -msoft-float.

The default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (or don't use) the packed stack layout. With -mno-packed-stack, the compiler uses all the fields of the 96/160 byte register save area only for their default purpose — and 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 unused space gets reused for other stuff, which means more efficient use of the available stack space. 🎀 However, when -mbackchain is also in effect, the topmost word of the save area is always used for the backchain, and the return address register is always saved two words below the backchain.

As long as the stack frame backchain isn't used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. But watch out: some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging. That kind of code is not call-compatible with code compiled with -mpacked-stack. Also note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported — to build a linux kernel, use -msoft-float.

The default is to not use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or don't generate) code 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 — with -m31, generate code compliant with the GNU/Linux for S/390 ABI. With -m64, generate code compliant with the GNU/Linux for zSeries ABI. This is what lets GCC in particular generate 64-bit instructions. For s390 targets the default is -m31, while s390x targets default to -m64.

⚠️ Note: -m31 is deprecated and support for it will be removed. Pour one out. 😭

• -mzarch, -mesa — with -mzarch, generate code using the instructions available on z/Architecture. With -mesa, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant with the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant with the GNU/Linux for zSeries ABI, the default is -mzarch. 🌈
• -mhtm, -mno-htm — the -mhtm girlie turns on a whole set of builtins that use the instructions from the transactional execution facility that came with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. And -mhtm is on by default when you're using -march=zEC12, periodt. 💅
• -mvx, -mno-vx — when you say -mvx, you get code using the instructions from the vector extension facility introduced with the IBM z13 machine generation. Heads up tho: this option changes the ABI for some vector type values when it comes to alignment and calling conventions. If vector type values show up in an ABI-relevant context, a GAS .gnu_attribute command gets added to mark the resulting binary with the ABI it's using. -mvx is enabled by default when using -march=z13. 🦋
• -mzvector, -mno-zvector — -mzvector enables vector language extensions and builtins that use instructions from the vector extension facility introduced with the IBM z13 machine generation. It also adds support for vector as a keyword so you can define vector type variables and arguments. Lowkey important: vector is only available when GNU extensions are enabled — it does not expand when you're requesting strict standard compliance, like with -std=c99. On top of the GCC low-level builtins, -mzvector also enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To use those builtins you gotta include the header file vecintrin.h. -mzvector is disabled by default, no cap. ✨
• -mmvcle, -mno-mvcle — generate (or don't generate) code that uses the "mvcle" instruction to do block moves. Say -mno-mvcle and it uses an "mvc" loop instead. That's the default unless you're optimizing for size. 🎀
• -mdebug, -mno-debug — print (or don't print) extra debug info while compiling. Default is to not print debug info. Simple as that. 🫶
• -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, z17/arch15, and native.

The default is -march=z900.

And if you pass native as the cpu type, it picks the best architecture option for the host processor. But real talk: -march=native has no effect if GCC doesn't recognize the processor. 😭

• -mtune=cpu-type — tune everything applicable about the generated code to cpu-type, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for -march. The default is whatever value you used for -march. 👑
• -mtpf-trace, -mno-tpf-trace — generate code that adds (or does not add) TPF OS specific branches to trace routines in the operating system. This option is off by default, even when you're compiling for the TPF OS. 🙈
• -mtpf-trace-skip, -mno-tpf-trace-skip — generate code that changes (or does not change) the default branch targets enabled by -mtpf-trace so they point to specialized trace routines that can selectively skip function trace entries for the TPF OS. This one is off by default too, even when you're compiling for the TPF OS and specifying -mtpf-trace. 💀
• -mmain — for the TPF OS target, use this when linking to add the startup code and other linker options so you produce a main object. 🔥
• -mfused-madd, -mno-fused-madd — generate code that uses (or does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. 🌈
• -mwarn-framesize=framesize — emit a warning if the current function exceeds the given frame size. Since this is a compile-time check, it doesn't have to be an actual problem when the program runs — it's meant to flag functions that most probably cause a stack overflow. Super useful for environments with a limited stack size, e.g. the Linux kernel.

A value of zero disables this warning, and that's the default. ⭐

• -mwarn-dynamicstack, -mno-warn-dynamicstack — emit a warning if the function calls "alloca" or uses dynamically-sized arrays. Generally a bad idea when you've got a limited stack size. 😬
• -mstack-guard=stack-guard, -mstack-size=stack-size, -mno-stack-guard, -mno-stack-size — if these are enabled, the S/390 back end emits extra 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, bestie). If you omit the stack-guard option, the smallest power of 2 larger than the frame size of the compiled function gets 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 much performance degradation. 💖

The values you give have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. And to be efficient, the extra code assumes the stack starts at an address aligned to the value given by stack-size.

The -mstack-guard= option can only be used together with -mstack-size=. You can override these options on the command line with -mno-stack-guard and -mno-stack-size, respectively.

• -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 gets prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes get appended, using the largest NOP-like instructions the architecture allows (maximum 1000000).

If both arguments are zero, hotpatching is disabled.

You can override this per-function with the "hotpatch" attribute. 🧚

• -mpic-data-is-text-relative, -mno-pic-data-is-text-relative — when compiling for S/390 with -fpic or -fPIC, GCC normally emits code using relative addressing between code and data. But for hotpatching you might need to introduce new ".text" parts while using the existing ".data" and ".bss" sections. In that case, use -mno-pic-data-is-text-relative to force addressing through the GOT instead. 💜
• -mstack-protector-guard=guard, -mstack-protector-guard-record — 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).

Option -mstack-protector-guard-record results in generating a section "__stack_protector_loc" containing pointers to all the instructions that load the address of the global guard. So it only has an effect together with -mstack-protector-guard=global. The intended use is for the Linux kernel. 🍓

• -mindirect-branch=choice, -mindirect-branch-jump=choice, -mindirect-branch-call=choice, -mfunction-return=choice, -mfunction-return-mem=choice, -mfunction-return-reg=choice — this whole squad of options handles security vulnerabilities related to speculative execution and indirect branch prediction by letting you replace indirect branches and function returns with direct branches to a thunk.

Passing a choice of keep causes generation of the default code. thunk triggers generation of out-of-line thunks and replaces the formerly indirect branch with a direct branch to the thunk. thunk-extern does the branch replacement like thunk but does not emit the thunks. thunk-inline is only available with -mindirect-branch-jump — and you should prefer it over thunk in user-space applications so you get correct stack unwinding and exception handling. 🌸

-mindirect-branch sets the value of -mindirect-branch-jump and -mindirect-branch-call. -mfunction-return sets the value of -mfunction-return-reg and -mfunction-return-mem.

All of these can also be set per-function using function attributes. ✨

• -mindirect-branch-table, -mno-indirect-branch-table — useful together with the -mindirect-branch and -mfunction-return options. When enabled, it generates tables pointing to the branch locations that those options patched. The tables are emitted in sections named ".s390_indirect_jump", ".s390_indirect_call", ".s390_return_reg", and ".s390_return_mem". Each section is an array of 32-bit elements, and each entry holds the offset from the entry to the patched location. 👑
• -mfentry, -mno-fentry — when used together with the -pg option, emit profiling code using the "__fentry__" hook provided by GLIBC 2.29 or later. This is only available for 64-bit code. If this option isn't enabled, profiling uses the less efficient "_mcount" hook instead. 💅
• -mrecord-mcount, -mno-record-mcount — when enabled, generate a "__mcount_loc" section with the locations of all generated profiling calls to "_mcount" or "__fentry__". 🔥
• mnop-mcount, mno-nop-mcount — instead of generating calls to "_mcount" or "__fentry__" with -pg, insert a sequence of no-op instructions of the same length at the locations where the calls would have gone. This can be used together with the -mrecord-mcount option to patch the call sequences into the object file without recompiling it. 🎀
• -mpreserve-args, -mno-preserve-args — when enabled, save all argument registers to the stack and generate corresponding CFI information. Handy for applications that want to implement their own stack unwinding and need access to function arguments. 🫶
• -munaligned-symbols, -mno-unaligned-symbols — assume that external symbols with a natural alignment of 1 are potentially unaligned. By default, all symbols without explicit alignment are assumed to be aligned on a 2-byte boundary, as mandated by the IBM Z ABI. 😌

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🌈 SH Options 🌈

These -m options are defined for the SH implementations, bestie:

• -m1 — generate code for the SH1. ⭐
• -m2 — generate code for the SH2. ⭐
• -m2e — generate code for the SH2e. ⭐

More SH family realness, bestie — this whole chunk is literally "okay but which SH chip are we even talking about," and the answer is: so many. Buckle in. 🦋

• -m2a-nofpu — code for the SH2a with no FPU at all, or for a SH2a-FPU where you're making very sure the floating-point unit doesn't get used. ✨
• -m2a-single-only — code for the SH2a-FPU, set up so that no double-precision floating-point operations happen. Ever. 🚫
• -m2a-single — code for the SH2a-FPU, assuming the FPU is chilling in single-precision mode by default.
• -m2a — code for the SH2a-FPU, assuming the FPU is in double-precision mode by default. Fancy. 👑
• -m3 — code for the SH3.
• -m3e — code for the SH3e.
• -m4-nofpu — code for the SH4 with no floating-point unit.
• -m4-single-only — code for the SH4 with an FPU that only does single-precision arithmetic. That's its whole range. 💅
• -m4-single — code for the SH4, assuming the FPU is in single-precision mode by default.
• -m4 — code for the SH4. Simple. Iconic.
• -m4-100 — code for SH4-100.
• -m4-100-nofpu — code for SH4-100 so that the floating-point unit is not used.
• -m4-100-single — code for SH4-100, assuming the FPU is in single-precision mode by default.
• -m4-100-single-only — code for SH4-100 so that no double-precision floating-point operations are used.
• -m4-200 — code for SH4-200.
• -m4-200-nofpu — code for SH4-200 so that the floating-point unit is not used.
• -m4-200-single — code for SH4-200, assuming the FPU is in single-precision mode by default.
• -m4-200-single-only — code for SH4-200 so that no double-precision floating-point operations are used.
• -m4-300 — code for SH4-300.
• -m4-300-nofpu — code for SH4-300 so that the floating-point unit is not used.
• -m4-300-single — code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-300-single-only — code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-340 — code for SH4-340 (no MMU, no FPU). Bare bones bestie. 💀
• -m4-400 — code for SH4-400 (no MMU, no FPU).
• -m4-500 — code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
• -m4a-nofpu — code for the SH4al-dsp, or for a SH4a so that the floating-point unit is not used.
• -m4a-single-only — code for the SH4a so that no double-precision floating-point operations are used.
• -m4a-single — code for the SH4a, assuming the FPU is in single-precision mode by default.
• -m4a — code for the SH4a.
• -m4al — same as -m4a-nofpu, except it implicitly passes -dsp to the assembler. Heads up though: 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. Big caveat energy: this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign. 😭 So don't just sprinkle this one around.
• -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, -mno-renesas — comply with the calling conventions defined by Renesas. The default for all targets of the SH toolchain is -mno-renesas, which uses the calling conventions defined for GCC before the Renesas conventions were available.
• -mnomacsave — mark the MAC register as call-clobbered, even if -mrenesas is given.
• -mieee, -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects how cases where the result of a comparison is unordered get handled. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either -mieee or -mno-ieee.
• -minline-ic_invalidate, -mno-inline-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 that's not just spicy, it requires privileged mode at run time, and 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.
• -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. Real talk though: depending on the particular hardware and software configuration, this can degrade overall performance due to the operand cache line flushes that are implied by the tas.b instruction. And on multi-core SH4A processors the tas.b instruction must be used with caution, since it can result in data corruption for certain cache configurations. 💀 Not the vibe.
• -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 instruction.
• -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 — okay so this one 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, -mno-accumulate-outgoing-args — reserve space once for outgoing arguments in the function prologue rather than around each call. This is generally beneficial for performance and size, and also needed for unwinding to avoid changing the stack frame around conditional code. -maccumulate-outgoing-args is enabled the default.
• -mdivsi3_libfunc=name — set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present.
• -mfixed-range=register-range — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma.
• -mbranch-cost=num — assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If not specified the value is selected depending on the processor type that is being compiled for.
• -mzdcbranch, -mno-zdcbranch — assume (do not assume) that zero displacement conditional branch instructions bt and bf are fast. If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying -mno-zdcbranch.
• -mcbranch-force-delay-slot — force the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if a suitable instruction cannot be found. By default this option is disabled. It can be enabled to work around hardware bugs as found in the original SH7055.
• -mfsca, -mno-fsca — allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations. The option -mfsca must be used in combination with -funsafe-math-optimizations. It is enabled by default when generating code for SH4A. Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect.
• -mfsrra, -mno-fsrra — allow or disallow the compiler to emit the fsrra instruction for reciprocal square root approximations. The option -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It is enabled by default when generating code for SH4A. Using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
• -mpretend-cmove, -mno-pretend-cmove — prefer or don't prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor.
• -mfdpic, -mno-fdpic — generate code using the FDPIC ABI.
• -mlra, -mno-lra — use the new LRA register allocator. By default, the old reload allocator is used.

☀️ Solaris 2 Options ☀️

These options are supported on Solaris 2:

• -mclear-hwcap, -mno-clear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them.
• -mimpure-text, -mno-impure-text — -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.

-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.

• -gsctf — generate Solaris CTF. Needs to be used both for compilation and linking. See ctf(7) for more information. This is only supported since Solaris 11.4 SRU 84 where the necessary toolchain support was added.

🌞 SPARC Options 🌞

These -m options are supported on the SPARC:

• -mno-app-regs, -mapp-regs — specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like the global register 1, each global register 2 through 4 is then treated as an allocable register that is clobbered by function calls. This is the default.

To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option.

• -mflat, -mno-flat — with -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.

With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode.

• -mfpu, -mhard-float — generate output containing floating-point instructions. This is the default.
• -mno-fpu, -msoft-float — generate output containing library calls for floating point. Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work.

• -mhard-quad-float — generate output containing quad-word (long double) floating-point instructions.
• -msoft-quad-float — generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.

As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.

• -mno-unaligned-doubles, -munaligned-doubles — assume that doubles have 8-byte alignment. This is the default.

With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code.

• -muser-mode, -mno-user-mode — do not generate code that can only run in supervisor mode. This is relevant only for the casa instruction emitted for the LEON3 processor. This is the default.
• -mfaster-structs, -mno-faster-structs — with -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of ldd and std instructions for copies in structure assignment, in place of twice as many ld and st pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI.
• -mstd-struct-return, -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode.

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here is a list of each supported architecture and their supported implementations.

• 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 you went and configured it otherwise), GCC spits out code for the V7 variant of the SPARC architecture. Slap -mcpu=cypress on there and the compiler also optimizes for the Cypress CY7C602 chip — the one living inside the SPARCStation/SPARCServer 3xx series. It's also a whole vibe for the older SPARCStation 1, 2, IPX, etc. 💅

With -mcpu=v8, GCC generates code for the V8 variant. The only glow-up from V7 code? The compiler now emits the integer multiply and integer divide instructions that exist in SPARC-V8 but literally do not exist in SPARC-V7. And with -mcpu=supersparc, it additionally optimizes for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series. ✨

With -mcpu=sparclite, GCC generates code for the SPARClite variant. This adds the integer multiply, integer divide step and scan ("ffs") instructions that exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes for the Fujitsu MB86930 chip — the original SPARClite, with no FPU, bless. With -mcpu=f934, it additionally optimizes for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 🎀

With -mcpu=sparclet, GCC generates code for the SPARClet variant. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions that exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes for the TEMIC SPARClet chip. 🦋

With -mcpu=v9, GCC generates code for the V9 variant. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. Then the optimization era begins, bestie:

• -mcpu=ultrasparc — 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

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• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that -mcpu=cpu_type does. No cap, those are two different things. 👑

The same values you'd use for -mcpu=cpu_type work for -mtune=cpu_type, but the only useful ones are those that select a particular CPU implementation. Those are: cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used. 🌈

• -mv8plus, -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors. 💖
• -mvis, -mno-vis — with -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis. ✨
• -mvis2, -mno-vis2 — with -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis. 🎀
• -mvis3, -mno-vis3 — with -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis. 🦋
• -mvis3b, -mno-vis3b — with -mvis3b, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011. The default is -mvis3b when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis3b also sets -mvis3, -mvis2 and -mvis. 🌸
• -mvis4, -mno-vis4 — with -mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis4 when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis4 also sets -mvis3b, -mvis3, -mvis2 and -mvis. 💅
• -mvis4b, -mno-vis4b — with -mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. The default is -mvis4b when targeting a cpu that supports such instructions, such as m8 and later. Setting -mvis4b also sets -mvis4, -mvis3b, -mvis3, -mvis2 and -mvis. 👑
• -mcbcond, -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is -mcbcond when targeting a CPU that supports such instructions, such as Niagara-4 and later. 🫶
• -mfmaf, -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is -mfmaf when targeting a CPU that supports such instructions, such as Niagara-3 and later. 🔥
• -mfsmuld, -mno-fsmuld — with -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. The default is -mfsmuld when targeting a CPU supporting the architecture versions V8 or V9 with FPU — except -mcpu=leon. 🙈
• -mpopc, -mno-popc — with -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. The default is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and later. ⭐
• -msubxc, -mno-subxc — with -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is -msubxc when targeting a CPU that supports such an instruction, such as Niagara-7 and later. 💜
• -mfix-at697f — enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor). 🩹
• -mfix-ut699 — enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
• -mfix-ut700 — enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — enable the documented workaround for the back-to-back store errata of the GR712RC processor.

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These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments, bestie — yes, there's more, hydrate before continuing: 💧

• -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. 📏
• -mptr32, -mptr64 — use 32- or 64-bit pointers, respectively. Unlike the -m32 and -m64 options, this affects only the pointer size and not the ABI. Subtle. Iconic. 🎀
• -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. (Yes, that's a "please use the other thing" note. Noted. 🙈)
• -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 — so these two are the "do not / do" pair for this one specific optimization 🎀: whether to optimize basic blocks that use the same index pointer 4 or more times to copy the pointer into the "ep" register and then use the shorter "sld" and "sst" instructions. And -mep is on by default if you optimize, iykyk ✨
• -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 🫶. Heads up bestie: those external functions are slower, but they use less code space if more than one function saves the same number of registers. -mprolog-function is on by default if you optimize 💅
• -mspace — try to make the code as small as possible 🐜✨. Right now that just means it turns on -mep and -mprolog-function. Little bit iconic, little bit tiny.
• -mtda=n — put static or global variables whose size is n bytes or less into the tiny data area that register "ep" points to 🎀. That tiny data area holds up to 256 bytes total (128 bytes for byte references) — it's giving studio apartment, ngl.
• -msda=n — put static or global variables whose size is n bytes or less into the small data area that register "gp" points to 🌸. This one can hold up to 64 kilobytes — so like, the tiny data area's older sibling with a slightly bigger place.
• -mzda=n — put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory 🦋.
• -mv850 — specify that the target processor is the V850 👑
• -mv850e3v5 — specify that the target processor is the V850E3V5 ✨. The preprocessor constant "__v850e3v5__" gets defined if you use this option.
• -mv850e2v4 — specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option 💖 (yes, it says V850E3V5 — we're just reading it, bestie 😭)
• -mv850e2v3 — specify that the target processor is the V850E2V3 ✨. The preprocessor constant "__v850e2v3__" gets defined if this option is used.
• -mv850e2 — specify that the target processor is the V850E2 ✨. Preprocessor constant "__v850e2__" gets defined if you use it.
• -mv850e1 — specify that the target processor is the V850E1 ✨. The preprocessor constants "__v850e1__" and "__v850e__" are defined if this option is used.
• -mv850es — specify that the target processor is the V850ES. This is an alias for the -mv850e1 option 🫶
• -mv850e — specify that the target processor is the V850E ✨. The preprocessor constant "__v850e__" gets defined if this option is used.

If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined, then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined 🎀

And the preprocessor constants "__v850" and "__v851__" are always defined, no matter which processor variant is the target ⭐ — those two are the ride-or-dies fr fr.

• -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 (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If "CALLT" instructions are being generated, then the C preprocessor symbol "__V850_CALLT__" gets defined ✨

• -mrelax, -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler 🦋
• -mlong-jumps, -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions 💫
• -msoft-float, -mhard-float — disable (or re-enable) generation of hardware floating point instructions 🌈. Important: this option is only significant when the target architecture is V850E2V3 or higher ⚠️. If hardware floating point instructions are being generated, then the C preprocessor symbol "__FPU_OK__" is defined; otherwise the symbol "__NO_FPU__" is defined.
• -mloop — enables use of the e3v5 LOOP instruction 🔁. It is not enabled by default when the e3v5 architecture is selected, because its use is still experimental 🙈 — so like, proceed with caution bestie.
• -mrh850-abi, -mghs — enables support for the RH850 version of the V850 ABI. This is the default 👑. With this version of the ABI, these rules apply:
  • Integer sized structures and unions are returned via a memory pointer rather than a register 🎀
  • Large structures and unions (more than 8 bytes in size) are passed by value
  • Functions are aligned to 16-bit boundaries
  • The -m8byte-align command-line option is supported ✨
  • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported 💀

When this version of the ABI is enabled, the C preprocessor symbol "__V850_RH850_ABI__" is defined 💖

• -mgcc-abi — enables support for the old GCC version of the V850 ABI 🫶. With this version, these rules apply:
  • Integer sized structures and unions are returned in register "r10" ⭐
  • Large structures and unions (more than 8 bytes in size) are passed by reference
  • Functions are aligned to 32-bit boundaries, unless optimizing for size
  • The -m8byte-align command-line option is not supported 🙈
  • The -mdisable-callt command-line option is supported but not enabled by default

When this version of the ABI is enabled, the C preprocessor symbol "__V850_GCC_ABI__" is defined ✨

• -m8byte-align, -mno-8byte-align — enables support for "double" and "long long" types to be aligned on 8-byte boundaries 🌸. The default is to restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect, the C preprocessor symbol "__V850_8BYTE_ALIGN__" is defined 💫
• -mbig-switch, -mno-big-switch — generate code suitable for big switch tables 🔥. Use this option only if the assembler/linker complain about out of range branches within a switch table ⚠️ — it's giving "only for emergencies, bestie".
• -mapp-regs, -mno-app-regs — -mapp-regs causes r2 and r5 to be used in the code generated by the compiler. This setting is the default 👑. -mno-app-regs causes r2 and r5 to be treated as fixed registers 💅
• -msmall-sld, -mno-small-sld — enable or disable the use of the short load instructions 🎀
• -mno-strict-align — do not enforce strict data alignment 🦋
• -mjump-tables-in-data-section, -mno-jump-tables-in-data-section — enable or disable placement of jump tables for switch statements in the ".data" section rather than the ".code" section ✨. The default is -mno-jump-tables-in-data-section.

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🌈 VAX Options 🌈

These -m options are defined for the VAX, bestie — retro icon behavior tbh 💜:

• -munix — do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot handle across long ranges 🙈
• -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used ✨
• -md, -md-float — use the D_floating data format for double-precision floating-point numbers instead of G_floating 💫
• -mg, -mg-float — use the G_floating data format for double-precision floating-point numbers instead of D_floating 💫
• -mlra, -mno-lra — enable Local Register Allocation 🎀. This is still experimental for the VAX, so by default the compiler uses standard reload ⚠️
• -mvaxc-alignment — use VAXC conventions for alignment of structure members 🌸
• -mqmath, -mno-qmath — enable or disable new instruction patterns for 64-bit integer addition and subtraction 🔥

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🦋 Visium Options 🦋

• -mdebug — use this option when linking programs that perform file I/O and are destined to run on an MCM target 🎀. It causes the libraries libc.a and libdebug.a to be linked. You should run the program on the target under the control of the GDB remote debugging stub 👑
• -msim — use this option when linking programs that perform file I/O and are destined to run on the simulator ✨. It causes the 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 🌸

⚠️ Bestie listen: -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 💀

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type 🎀. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5, present for backward compatibility 🫶

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture ✨

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions 💅

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that -mcpu=cpu_type would 🌈
• -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 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 🙈

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💜 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" ⚠️

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⭐ 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 👑 — so go check those if you need the hardware-specific tea, bestie 🍓

• -mrtp — ok so GCC can make code for both VxWorks kernels and real time processes (RTPs), and this option flips you from the first vibe to the second 💅 it also defines the preprocessor macro __RTP__, no cap.
• -msmp — picks SMP runtimes for linking ✨ heads up though: it's not available on architectures other than PowerPC, and it's also not available on VxWorks version 7 or later, because in those the selection is part of the VxWorks build configuration and the library paths are the same either way 🫶
• -mvthreads — links against the VxWorks AE vThread runtime environment 🎀
• -non-static — links an RTP executable against shared libraries instead of static ones 🌸 the options -static and -shared can also be used for RTPs, and -static is the default (iykyk).
• -Bstatic, -Bdynamic — these get passed straight down to the linker, bestie 💫 they exist for compatibility with Diab.
• -Xbind-lazy — enables lazy binding of function calls 🔥 this one is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
• -Xbind-now — disables lazy binding of function calls 💀 this is the default, and yes, it's also defined for compatibility with Diab.

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🦋 x86 Options 🦋

This section is literally the whole -m catalogue for the x86 family of computers, so buckle up 🎢

The following group of options lets you compile for one specific processor 👑

• -march=cpu-type — generates instructions for the machine type cpu-type. And the difference matters, so listen: unlike -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type lets GCC generate code that may not run at all on processors other than the one indicated 😭 also, specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

• native — picks 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, oops 🙈); 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, MMX, SSE, SSE2, and FXSR instruction set support 💜
• x86-64-v2, x86-64-v3, x86-64-v4 — these cpu-type choices select the corresponding micro-architecture level from the x86-64 psABI ⭐ on ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for that particular micro-architecture level.

Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values enables generic tuning 🎀 specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value.

• i386 — Original Intel i386 CPU.
• i486 — Intel i486 CPU (no scheduling is implemented for this chip, so don't come for me).
• i586, pentium — Intel Pentium CPU with no MMX support.
• lakemont — Intel Lakemont MCU, based on Intel Pentium CPU.
• pentium-mmx — Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
• pentiumpro — Intel Pentium Pro CPU with no MMX support.
• i686 — when used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips 🌈 when used with -mtune, it has the same meaning as generic.
• pentium2 — Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.
• pentium3, pentium3m — Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.
• pentium-m — Intel Pentium M; a low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support 🫶 used by Centrino notebooks, iconic era fr.
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.
• prescott — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
• nocona — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, CX16 and FXSR instruction set support.
• core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support ✨
• nehalem, corei7 — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.
• westmere — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.
• sandybridge, corei7-avx — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support 💖
• ivybridge, core-avx-i — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
• haswell, core-avx2 — Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support 🔥
• broadwell — Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support.
• skylake — Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support 🦋
• skylake-avx512 — Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.
• cascadelake — Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support 💜
• cannonlake — Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support.
• cooperlake — Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.
• icelake-client — Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support 🍓
• icelake-server — Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support.
• tigerlake — Intel Tiger Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support 👑
• 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, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support. ✨
• arrowlake — Intel Arrow Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT and CMPCCXADD instruction set support. 🦋
• arrowlake-s, lunarlake — Intel Arrow Lake S/Lunar Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3 and SM4 instruction set support. 💖
• pantherlake, wildcatlake — Intel Panther Lake/Wildcat Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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. 🐆
• novalake — Intel Nova Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, PREFETCHI, APX_F, AVX10.1, AVX10.2 and MOVRS instruction set support. ⭐
• sapphirerapids, emeraldrapids — Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16 and AVX512BF16 instruction set support. 💎
• graniterapids — Intel Granite Rapids CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16 and PREFETCHI instruction set support. 🪨
• graniterapids-d — Intel Granite Rapids D CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI and AMX-COMPLEX instruction set support. 🔥
• diamondrapids — Intel Diamond Rapids CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI, AMX-COMPLEX, AVX10.1-512, AVX-IFMA, AVX-NE-CONVERT, AVX-VNNI-INT16, AVX-VNNI-INT8, CMPccXADD, SHA512, SM3, SM4, AVX10.2-512, APX_F, AMX-AVX512, AMX-FP8, MOVRS and AMX-MOVRS instruction set support. 💎💅
• bonnell, atom — Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3 and CX16 instruction set support. 🍓
• silvermont, slm — Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support. 🌸
• goldmont — Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT and FSGSBASE instruction set support. ✨
• goldmont-plus — Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID and SGX instruction set support. 🌟
• tremont — Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support. 🎀
• sierraforest — Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, 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, CX16, 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, CX16, 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. 💧

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okay and now we're entering the AMD portion of the menu, bestie — the classic lineup 💜

• 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 CX16, 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. 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. ok AMD era 💅
• bdver2 — AMD Family 15h core based CPUs with x86-64 instruction set support. 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. 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. 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. the "4" is not a vibe, it's a whole upgrade 🫶
• znver1 — AMD Family 17h core based CPUs with x86-64 instruction set support. Supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions. Zen has entered the chat ✨
• znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. 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. 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. 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. the AVX512 wall of text, iconic 😭
• znver5 — AMD Family 1ah core based CPUs with x86-64 instruction set support. 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.
• znver6 — AMD Family 1ah core based CPUs with x86-64 instruction set support. 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, AVXVNNIINT8, AVXIFMA, AVX512FP16, AVXNECONVERT, AVX512BMM and 64-bit instruction set extensions. yes the list keeps growing, she ate and left no crumbs 🔥
• btver1 — CPUs based on AMD Family 14h cores with x86-64 instruction set support. 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. (note: this one says "includes" not "supersets", keep that in mind bestie, the wording is doing its own thing 💀)
• 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.) yes every single one of these says no scheduling, that's not me being lazy, that's literally what the page says 🫶
• 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.

⚠️ ok that last bit is important bestie: the silicon can do AVX and F16C, but -march=lujiazui does not turn them on, and it's for performance reasons. not a bug, on purpose, don't @ me 💅

• yongfeng — ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support. ✨
• shijidadao — ZHAOXIN shijidadao CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support. (yes it's the exact same list as yongfeng, no I'm not gaslighting you, that's how the page is written 🍓)
• geode — AMD Geode embedded processor with MMX and 3DNow! instruction set support. embedded king 👑
• c86-4g-m4 — HYGON c86-4g-m4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support.
• c86-4g-m6 — HYGON c86-4g-m6 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support.
• c86-4g-m7 — HYGON c86-4g-m7 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, GFNI, AVX512VNNI, VAES, AVX512BITALG, AVX512VPOPCNTDQ, AVX512VP2INTERSECT, AVXVNNI, VPCLMULQDQ, WBNOINVD instruction set support. m7 said "let me add the whole AVX512 alphabet" 😭🔥
• c86-4g-m8 — HYGON c86-4g-m8 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, GFNI, AVX512VNNI, VAES, AVX512BITALG, AVX512VPOPCNTDQ, AVX512VP2INTERSECT, AVXVNNI, VPCLMULQDQ, WBNOINVD instruction set support. (same energy as m7, and yeah, same list too — the page just lists them separately, so I'm listing them separately 🌸)
• -mtune=cpu-type — tunes everything applicable about the generated code for cpu-type... except the ABI and the set of available instructions, bestie. Picking a specific cpu-type schedules things appropriately for that particular chip, but the compiler does not generate any code that can't run on the default machine type unless you also use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. 💅

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

• generic — produces code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But if you do not know exactly what CPU users of your application will have, then you should use this option. 🫶

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. (it's giving "living document", fr fr)

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 🧚

• intel — produces code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Diamond Rapids and Clearwater Forest, 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.

---

The following options allow more detailed control over which instruction set extensions are targeted by GCC. Each has a corresponding -mno-option to disable use of these instructions. 🎀

These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches.

These options enable GCC to use these extended instructions in generated code. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. ⚠️

To control whether 387 or SSE/AVX instructions are generated automatically for floating-point arithmetic, see -mfpmath=, below. 👀

• -mmmx — support MMX built-in functions.
• -msse — support MMX and SSE built-in functions and code generation.
• -msse2 — support MMX, SSE and SSE2 built-in functions and code generation.
• -msse3 — support MMX, SSE, SSE2 and SSE3 built-in functions and code generation.
• -mssse3 — support MMX, SSE, SSE2, SSE3 and SSSE3 built-in functions and code generation.
• -msse4.1 — support MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 built-in functions and code generation.
• -msse4.2 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.
• -msse4 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.

Note that -msse4 enables both SSE4.1 and SSE4.2 support, while -mno-sse4 turns off those features; neither form of the option affects SSE4A support, controlled separately by -msse4a. 💀

• -msse4a — support MMX, SSE, SSE2, SSE3 and SSE4A built-in functions and code generation.
• -mavx — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and AVX built-in functions and code generation.
• -mavx2 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 built-in functions and code generation.
• -mavx512f — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVX512F built-in functions and code generation.
• -mavx512cd — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512CD built-in functions and code generation.
• -mavx512vl — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VL built-in functions and code generation.
• -mavx512bw — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BW built-in functions and code generation.
• -mavx512dq — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512DQ built-in functions and code generation.
• -mavx512ifma — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512IFMA built-in functions and code generation.
• -mavx512vbmi — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI built-in functions and code generation.
• -mavx512vpopcntdq — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VPOPCNTDQ built-in functions and code generation.
• -mavx512vp2intersect — support AVX512VP2INTERSECT built-in functions and code generation.
• -mavx512vnni — support AVX512VNNI built-in functions and code generation.
• -mavx512vbmi2 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI2 built-in functions and code generation.
• -mavx512bf16 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BF16 built-in functions and code generation.
• -mavx512fp16 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512-FP16 built-in functions and code generation.
• -mavx512bitalg — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BITALG built-in functions and code generation.
• -mavx512bmm — support AVX512BMM built-in functions and code generation.
• -mavxvnni — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXVNNI built-in functions and code generation.
• -mavxifma — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXIFMA built-in functions and code generation.
• -mavxvnniint8 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT8 built-in functions and code generation.
• -mavxneconvert — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXNECONVERT build-in functions and code generation.
• -mavxvnniint16 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT16 built-in functions and code generation.
• -mavx10.1 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVX10.1 built-in functions and code generation.
• -mavx10.2 — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1 and AVX10.2 built-in functions and code generation.
• -msha — support SHA1 and SHA256 built-in functions and code generation.
• -maes — support AES built-in functions and code generation.
• -mpclmul — support PCLMUL built-in functions and code generation.
• -mclflushopt — support CLFLUSHOPT instructions.
• -mclwb — support CLWB instruction.
• -mfsgsbase — support FSGSBASE built-in functions and code generation.
• -mptwrite — support PTWRITE built-in functions and code generation.
• -mrdrnd — support RDRND built-in functions and code generation.
• -mf16c — support F16C built-in functions and code generation.
• -mfma — support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and FMA built-in functions and code generation.
• -mfma4 — support FMA4 built-in functions and code generation.
• -mpconfig — support PCONFIG built-in functions and code generation.
• -mwbnoinvd — support WBNOINVD built-in functions and code generation.
• -mprfchw — support PREFETCHW instruction.
• -mrdpid — support RDPID built-in functions and code generation.
• -mrdseed — support RDSEED instruction.
• -msgx — support SGX built-in functions and code generation.
• -mxop — support XOP built-in functions and code generation.
• -mlwp — support LWP built-in functions and code generation.
• -m3dnow — support 3DNow! built-in functions. (retro era, honestly iconic)
• -m3dnowa — support Athlon 3Dnow! built-in functions.
• -mpopcnt — support code generation of popcnt instruction.
• -mabm — support code generation of Advanced Bit Manipulation (ABM) instructions.
• -madx — support flag-preserving add-carry instructions.
• -mbmi — support BMI built-in functions and code generation.
• -mbmi2 — support BMI2 built-in functions and code generation.
• -mlzcnt — support LZCNT built-in function and code generation.
• -mfxsr — support FXSAVE and FXRSTOR instructions.
• -mxsave — support XSAVE and XRSTOR instructions.
• -mxsaveopt — support XSAVEOPT instruction.
• -mxsavec — support XSAVEC instructions.
• -mxsaves — support XSAVES and XRSTORS instructions.
• -mrtm — support RTM built-in functions and code generation.
• -mhle — support Hardware Lock Elision prefixes.
• -mtbm — support TBM built-in functions and code generation.
• -mmwaitx — support MWAITX and MONITORX built-in functions and code generation.
• -mclzero — support CLZERO built-in functions and code generation.
• -mpku — support PKU built-in functions and code generation.
• -mgfni — support GFNI built-in functions and code generation.
• -mvaes — turns on VAES built-in functions and code generation 💖
• -mwaitpkg — turns on WAITPKG built-in functions and code generation ✨
• -mvpclmulqdq — turns on VPCLMULQDQ built-in functions and code generation
• -mmovdiri — turns on MOVDIRI built-in functions and code generation
• -mmovdir64b — turns on MOVDIR64B built-in functions and code generation
• -menqcmd — turns on ENQCMD built-in functions and code generation
• -muintr — turns on UINTR built-in functions and code generation
• -mtsxldtrk — turns on TSXLDTRK built-in functions and code generation
• -mcldemote — turns on CLDEMOTE built-in functions and code generation
• -mserialize — turns on SERIALIZE built-in functions and code generation 🌸
• -mamx-tile — turns on AMX-TILE built-in functions and code generation
• -mamx-int8 — turns on AMX-INT8 built-in functions and code generation
• -mamx-bf16 — turns on AMX-BF16 built-in functions and code generation
• -mhreset — turns on HRESET built-in functions and code generation
• -mkl — turns on KL built-in functions and code generation
• -mwidekl — turns on WIDEKL built-in functions and code generation
• -mcmpccxadd — turns on MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and CMPCCXADD build-in functions and code generation. (yes, "build-in", the manpage said it like that, we're not touching it 🫶)
• -mamx-fp16 — turns on AMX-FP16 built-in functions and code generation
• -mprefetchi — turns on PREFETCHI built-in functions and code generation
• -mraoint — turns on RAOINT built-in functions and code generation
• -mamx-complex — turns on AMX-COMPLEX built-in functions and code generation
• -msm3 — turns on MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM3 built-in functions and code generation
• -msm4 — turns on MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM4 built-in functions and code generation
• -msha512 — turns on MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SHA512 built-in functions and code generation
• -mapxf — turns on code generation for APX features, including EGPR, PUSH2POP2, NDD, PPX, NF, CCMP and ZU 🔥
• -musermsr — turns on USER_MSR built-in functions and code generation
• -mamx-avx512 — turns on MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1, AVX10.2 and AMX-AVX512 built-in functions and code generation
• -mamx-tf32 — turns on AMX-TF32 built-in functions and code generation
• -mamx-fp8 — turns on AMX-FP8 built-in functions and code generation
• -mmovrs — turns on MOVRS built-in functions and code generation
• -mamx-movrs — turns on AMX-MOVRS built-in functions and code generation

---

So like, these additional options are available for the x86 processor family, bestie 💅

• -mfpmath=unit — generates floating-point arithmetic for whichever unit you pick. The choices for unit are, and I quote, the whole menu:
  • 387 — use the standard 387 floating-point coprocessor that's on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere, iconic. The temporary results get computed in 80-bit precision instead of the precision the type actually specifies, so you get slightly different results compared to most other chips 🙈. Peep -ffloat-store for a 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 on the AMD side by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, so 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 actually effective. For the x86-64 compiler, these extensions are enabled by default, no cap.

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.

GCC depresses SSE instructions when -mavx (or another option enabling AVX extensions) is used. Instead, it generates new AVX instructions or AVX equivalents for all SSE instructions when needed.

• 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 though, because it is still experimental — the GCC register allocator does not model separate functional units well, resulting in unstable performance 💀

• -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 — generate output containing 80387 instructions for floating point.
• -mno-80387, -msoft-float — generate output containing library calls for floating point.

Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation 😬

On machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used.

• -mno-fp-ret-in-387 — do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types float and double in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU.

The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.

• -mno-fancy-math-387 — some 387 emulators do not support the sin, cos and sqrt instructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the -funsafe-math-optimizations switch 🌸
• -malign-double, -mno-align-double — control whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary. Aligning double variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory.

On x86-64, -malign-double is enabled by default.

Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch 💀

• -m96bit-long-double, -m128bit-long-double — these switches control the size of long double type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

Modern architectures (Pentium and newer) prefer long double to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero ✨

In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on 16-byte boundary.

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double.

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch 😭

• -mlong-double-64, -mlong-double-80, -mlong-double-128 — these switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library.

Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch 🫶

• -malign-data=type — control how GCC aligns variables. Supported values for type are: compat uses increased alignment value compatible with GCC 4.8 and earlier, abi uses alignment value as specified by the psABI, and cacheline uses increased alignment value to match the cache line size. compat is the default 💖
• -mlarge-data-threshold=threshold — when -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold are placed in large data sections. The default is 65535.
• -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there 👑

You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.

⚠️ Warning: this calling convention is incompatible with the one normally used on Unix, so you literally cannot use it if you need to call libraries compiled with the Unix compiler. ngl that's a whole vibe killer 😭

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. like, the compiler will just Guess Wrong and it's over 💀

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored. not here bestie 🫶)

• -mregparm=num — controls how many registers get used to pass integer arguments. By default, zero registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm. 🎀

⚠️ Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. no mixing and matching, that's the rule ✨

• -msseregparm — use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm. 🦋

⚠️ Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. same deal as above, everyone in the group chat has to agree 💅

• -mvect8-ret-in-mem — return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. like it's giving legacy-support-only, don't reach for it casually 🙈
• -mpc32, -mpc64, -mpc80 — set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. 😤

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. 💀 the way "catastrophic cancellation" is a real technical term and not just how my math homework feels

• -mdaz-ftz, -mno-daz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. The -Ofast, -ffast-math, or -funsafe-math-optimizations options normally link in startup code that sets these flags — except when building a shared library (-shared). You can use the -mdaz-ftz and -mno-daz-ftz options to explicitly enable or disable setting these flags, regardless of other options passed to GCC. 🔥 explicit is the moment
• -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. 🧚
• -mstack-arg-probe — emit stack probing code in the function prologue. lil' safety check, slay ✨
• -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits). 📐

⚠️ Warning: when generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is 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 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. 😭 like please read this one twice fr fr

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used. 🌸

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🫶

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. small stack energy 👑

• -mdump-tune-features — this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in -mtune-ctrl=feature-list. it's literally the receipts 💅
• *-mtune-ctrl=feature-list* — this option is used to do fine-grain control of x86 code generation features. feature-list is a comma-separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with ^; otherwise, it is turned off. -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. 🔥 okay so this one is devs-only, no cap — you're off the tested path and that's a whole risk
• -mno-default — this option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features. everything off, periodt 💀
• -mcld — this option instructs GCC to emit a cld instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of cld instructions can be suppressed with the -mno-cld compiler option in this case. 😤 some OSes really said "spec? never met her"
• -mvzeroupper — this option instructs GCC to emit a vzeroupper instruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessary zeroupper intrinsics. ✨ smooth transitions only
• -mstv, -mno-stv — enable/disable Scalar to Vectorization pass transforming 64-bit integer computation into vector ones. This optimization is restricted to -O2 and higher. 🦋
• -mprefer-avx128 — this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. small and mighty 💖
• *-mprefer-vector-width=opt* — this option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform. 👑
• -mpartial-vector-fp-math, -mno-partial-vector-fp-math — this option enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless -fno-trapping-math is specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when -fno-trapping-math is in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using -mdaz-ftz to set denormals-are-zero (DAZ) flag in the MXCSR register. 😭 the "leaking into the upper part" imagery is giving contamination horror movie

This option is enabled by default. 🎀

• *-mmove-max=bits* — this option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are 128, 256 and 512. 🌸
• *-mstore-max=bits* — this option instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits. The valid bits are 128, 256 and 512. 🫶

And here's the lil' value rundown that shows up in this stretch:

• 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 — okay so this one optimizes functions that have the noreturn attribute or the _Noreturn specifier by not saving callee-saved registers in the function prologue that are used in the function (except the BP register). It can interfere with debugging the caller of the noreturn function or any function further up in the call stack, so it is not enabled by default. 💅
• -mcx16 — lets GCC generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement __sync Builtins. However, for __atomic Builtins operating on 128-bit integers, a library call is always used. ✨
• -msahf — enables generation of SAHF instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the LAHF and SAHF instructions which are supported by AMD64. These are load and store instructions, respectively, for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize fmod, drem, and remainder built-in functions; see Other Builtins for details. 🌸
• -mmovbe — enables use of the movbe instruction to optimize byte swapping of four and eight byte entities. 🦋
• -mshstk — enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). 🔥
• -mcrc32 — enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction. ⭐
• -mmwait — enables built-in functions __builtin_ia32_monitor, and __builtin_ia32_mwait to generate the monitor and mwait machine instructions. 💜
• -mrecip — this one enables use of RCPSS and RSQRTSS instructions (and their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to increase precision instead of DIVSS and SQRTSS (and their vectorized variants) for single-precision floating-point arguments.

These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). 😭

Note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the above option combination), and doesn't need -mrecip.

Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip.

• -mrecip=opt — controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — enable all estimate instructions.
  • default — enable the default instructions, equivalent to -mrecip.
  • none — disable all estimate instructions, equivalent to -mno-recip.
  • div — enable the approximation for scalar division.
  • vec-div — enable the approximation for vectorized division.
  • sqrt — enable the approximation for scalar square root.
  • vec-sqrt — enable the approximation for vectorized square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. 🎀

• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library, aocl for the math library (LibM) from AMD Optimizing CPU Libraries (AOCL) and acml for the end-of-life AMD core math library (to which AOCL-LibM is the successor). To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time.

GCC currently emits calls to these for the corresponding function type when -mveclibabi=svml is used:

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
vmlsAcos4

For the corresponding function type when -mveclibabi=aocl is used, it emits calls to:

amd_vrs4_acosf
amd_vrs16_acosf
amd_vrd8_asin
amd_vrs4_asinf
amd_vrs8_asinf
amd_vrs16_asinf
amd_vrd2_atan
amd_vrd8_atan
amd_vrs4_atanf
amd_vrs8_atanf
amd_vrs16_atanf
amd_vrd2_cos
amd_vrd4_cos
amd_vrd8_cos
amd_vrs4_cosf
amd_vrs8_cosf
amd_vrs16_cosf
amd_vrs4_coshf
amd_vrs8_coshf
amd_vrd2_erf
amd_vrd4_erf
amd_vrd8_erf
amd_vrs4_erff
amd_vrs8_erff
amd_vrs16_erff
amd_vrd2_exp
amd_vrd4_exp
amd_vrd8_exp
amd_vrs4_expf
amd_vrs8_expf
amd_vrs16_expf
amd_vrd2_exp10
amd_vrs4_exp10f
amd_vrd2_exp2
amd_vrd4_exp2
amd_vrd8_exp2
amd_vrs4_exp2f
amd_vrs8_exp2f
amd_vrs16_exp2f
amd_vrs4_expm1f
amd_vrd2_log
amd_vrd4_log
amd_vrd8_log
amd_vrs4_logf
amd_vrs8_logf
amd_vrs16_logf
amd_vrd2_log10
amd_vrs4_log10f
amd_vrs8_log10f
amd_vrs16_log10f
amd_vrd2_log1p
amd_vrs4_log1pf
amd_vrd2_log2
amd_vrd4_log2
amd_vrd8_log2
amd_vrs4_log2f
amd_vrs8_log2f
amd_vrs16_log2f
amd_vrd2_pow
amd_vrd4_pow
amd_vrd8_pow
amd_vrs4_powf
amd_vrs8_powf
amd_vrs16_powf
amd_vrd2_sin
amd_vrd4_sin
amd_vrd8_sin
amd_vrs4_sinf
amd_vrs8_sinf
amd_vrs16_sinf
amd_vrd2_tan
amd_vrd4_tan
amd_vrd8_tan
amd_vrs16_tanf
amd_vrs4_tanhf
amd_vrs8_tanhf
amd_vrs16_tanhf

And for the corresponding function type when -mveclibabi=acml is used, it emits calls to:

__vrd2_sin
__vrd2_cos
__vrd2_exp
__vrd2_log
__vrd2_log2
__vrd2_log10
__vrs4_sinf
__vrs4_cosf
__vrs4_expf
__vrs4_logf
__vrs4_log2f
__vrs4_log10f
__vrs4_powf

• -mabi=name — generate code for the specified calling convention. 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. 🫶
• -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. 🌈
• -mforce-indirect-call — force 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. 🔥
• -mmanual-endbr — insert ENDBR instruction at function entry only via the cf_check function attribute. This is useful when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry. ✨
• -mcet-switch — 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. 👑
• -mcall-ms2sysv-xlogues — due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, resulting in fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions. 💜
• -mtls-dialect=type — generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all systems. 🦋
• -mpush-args, -mno-push-args — use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. ⭐
• -maccumulate-outgoing-args — if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args. 😭
• -mms-bitfields, -mno-ms-bitfields — enable/disable bit-field layout 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. 💅

okay so this behavior is enabled by default for Microsoft Windows targets, bestie 💖. This behavior can also be controlled locally by use of variable or type attributes. For more information, see the x86 Attributes section. ✨

The Microsoft structure layout algorithm is lowkey fairly simple, except the bit-field packing is giving chaos 😭. 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, and this is where it gets spicy 🌶️:

1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.

For example:

struct
 {
   unsigned long bf_1 : 12;
   unsigned long : 0;
   unsigned long bf_2 : 12;
 } t1;

The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1's size would be 4 bytes.

2. If a zero-length bit-field is inserted after a bit-field, foo, and the alignment of the zero-length bit-field is greater than the member that follows it, bar, bar is aligned as the type of the zero-length bit-field.

For example:

struct
 {
   char foo : 4;
   short : 0;
   char bar;
 } t2;

struct
 {
   char foo : 4;
   short : 0;
   double bar;
 } t3;

For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.

Taking this into account, it 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.

---

• -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. 🫶
• -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. ✨
• -minline-stringops-dynamically — For string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. 💅
• -mstringop-strategy=alg — Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
  • rep_byte, rep_4byte, rep_8byte — Expand using i386 rep prefix of the specified size.
  • byte_loop, loop, unrolled_loop — Expand into an inline loop.
  • libcall — Always use a library call.
• -mmemcpy-strategy=strategy — Override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. 🔥
• -mmemset-strategy=strategy — This option is similar to -mmemcpy-strategy= except that it controls the __builtin_memset expansion. 🎀
• -momit-leaf-frame-pointer — Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -momit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. 😭
• -mtls-direct-seg-refs, -mno-tls-direct-seg-refs — Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on. 🌸

• -msse2avx, -mno-sse2avx — Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default. ⚡
• -mfentry, -mno-fentry — If profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg. 💀
• -mrecord-mcount, -mno-record-mcount — If profiling is active (-pg), generate a section that contains pointers to each profiling call; this is useful for automatically patching the calls. You can use the mfentry-section= option to set the name of the section; it defaults to __mcount_loc. 🦋
• -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 — With -pg -mfentry, instrument function exits according to type, which may be one of none to not instrument, call to generate a call to __return__, or nop5 to generate a 5-byte nop sequence. This option only instruments true returns ending with a ret, not sibling calls via a jump. 🌈
• -mrecord-return, -mno-record-return — Generate a __return_loc section pointing to all return instrumentation code. ⭐
• -mfentry-name=name — Set name of __fentry__ symbol called at function entry for -pg -mfentry functions. 💜
• -mfentry-section=name — Set name of section to record -mrecord-mcount calls. The default is __mcount_loc. 🧚
• -mskip-rax-setup, -mno-skip-rax-setup — When generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up RAX register when there are no variable arguments passed in vector registers. 🙈
• -m8bit-idiv, -mno-8bit-idiv — On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. 🍓
• -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — Split 32-byte AVX unaligned load and store. 💫
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset, -mstack-protector-guard-symbol=symbol — Generate stack protection code using canary at guard. Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default). This option has effect only when -fstack-protector or -fstack-protector-all is specified.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as 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.

-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. 🌟

• -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. 💖
• -mrelax-cmpxchg-loop — when GCC is emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins that don’t have a native instruction, this optimizes for the highly contended case by issuing an atomic load before the CMPXCHG instruction, and using the PAUSE instruction to save CPU power when restarting the loop. 💅
• -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. lowkey direct, ngl.
• -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. This allows them to potentially be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run time. ⭐
• -mapx-inline-asm-use-gpr32 — by default, EGPR usage is disabled in inline asm constraints as GCC cannot be aware of whether the asm instructions support GP32 or not. If your inline asm can handle EGPR, use -mapx-inline-asm-use-gpr32. 🧚
• -mgather — enable vectorization for gather instructions. 🌀
• -mscatter — enable vectorization for scatter instructions. 🌈

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. 💖

These next ones generate code for a 16-bit, 32-bit, or 64-bit environment:

• -m32 — generate code for a 32-bit environment: sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode. 🍓
• -m64 — generate code for a 64-bit environment: sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options. 🦋
• -mx32 — generate code for an x32 environment: sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture. 🌸
• -m16 — the same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode. 🎀
• -miamcu — the -miamcu option generates code that 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 is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. 🚫
• -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 — generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code. 🐧
• -mcmodel=medium — generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked. 🫶
• -mcmodel=large — generate code for the large model. This model makes no assumptions about addresses and sizes of sections. 🌌
• -maddress-mode=long — generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments. 📏
• -maddress-mode=short — generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments. 📐
• -mneeded, -mno-needed — emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. 🏷️
• -mno-direct-extern-access — 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: shared libraries compiled with -mno-direct-extern-access and executable compiled with -mdirect-extern-access may not be binary compatible if protected symbols are used in shared libraries and executable. 💀

• -munroll-only-small-loops, -mno-unroll-only-small-loops — controls conservative small loop unrolling. It is enabled by default with -O2, and unrolls loops with less than 4 instructions by 1 time. This gives better utilization of the instruction decoding pipeline on modern processors. You can disable this with -mno-unroll-only-small-loops, and it is also disabled if the more general options -funroll-loops or -funroll-all-loops are either enabled or explicitly disabled. 🔁
• -mdispatch-scheduler — enable instruction scheduling. This is only supported on bdver1, bdver2, bdver3, bdver4, and znver1 processors and additionally requires -fschedule-insns -fsched-pressure. 🗓️
• -mlam=choice — LAM(linear-address masking) allows special bits in the pointer to be used for metadata. The default is none. With u48, pointer bits in positions 62:48 can be used for metadata; With u57, pointer bits in positions 62:57 can be used for metadata. 🧠

🪟 x86 Windows Options

🦋 Xstormy16 Options

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. 🎮

🌸 Xtensa Options

These options are supported for Xtensa targets:

• -mconst16, -mno-const16 — enable or disable use of CONST16 instructions for loading constant values. 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. 🧩
• -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, -mno-force-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. 🐧
• -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. 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 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. 🎯
• -mlongcalls, -mno-longcalls — okay so when this one's turned on, GCC tells the assembler to turn direct calls into indirect calls unless it can prove the target of a direct call is inside the range the call instruction can actually reach 🎯. This translation typically happens for calls to functions in other source files. Like, specifically: the assembler rewrites a direct CALL instruction into an L32R followed by a CALLX instruction. The default is -mno-longcalls. Use this in programs where the call target could potentially be out of range. Heads up though — this option lives in the assembler, not the compiler 🙈, so the assembly code GCC spits out still shows direct call instructions… you gotta look at the disassembled object code to see the actual instructions. And also, the assembler uses an indirect call for every cross-file call, not just the ones that are genuinely out of range. So it's giving "when in doubt, indirect" energy 💅
• -mabi=name — generates code for the specified ABI ✨. Permissible values are: call0, windowed. The default ABI is chosen by the Xtensa core configuration, iykyk.
• -mabi=call0 — with this one enabled, function parameters get passed in registers a2 through a7, registers a12 through a15 are caller-saved, and register a15 may be used as a frame pointer 💖. When this version of the ABI is enabled, the C preprocessor symbol __XTENSA_CALL0_ABI__ is defined.
• -mabi=windowed — with this one enabled, function parameters are passed in registers a10 through a15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registers a2 through a7 🌀. Register a7 may be used as a frame pointer. The register window gets rotated 8 registers back upon return. When this version of the ABI is enabled, the C preprocessor symbol __XTENSA_WINDOWED_ABI__ is defined.
• -mextra-l32r-costs=n — specifies an extra cost of instruction RAM/ROM access for L32R instructions, measured in clock cycles ⏱️. When you're optimizing for speed, this affects whether a constant gets loaded from the literal pool using L32R, or synthesized from a small one with a couple of arithmetic instructions. The default value is 0.
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary, as described in the architecture specification 🧚. The default is -mno-strict-align for cores that support both unaligned loads and stores in hardware, and -mstrict-align for all other cores. Choose wisely bestie 💀

---

🦋 zSeries Options 🦋

These are listed under

…and that's literally it, that's the whole thing 😭 the page just drops you there mid-sentence, main character cliffhanger energy, fr fr. More is coming in the next part, don't panic 🫶

🌸 ENVIRONMENT 🌸

Okay bestie, this section is literally the ✨vibes layer✨ of GCC — the environment variables that change how it behaves. Some of them just point at directories or prefixes to search when looking for files. Others tweak whole other aspects of your compilation environment. Slay.

Heads up though 💅 — you can also tell GCC where to look with options like -B, -I and -L, and those take precedence over anything you set with environment variables. Env vars in turn beat whatever GCC's configuration says. That's the hierarchy, periodt.

---

🎀 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL

These four control how GCC uses localization info, so it can vibe with different national conventions for diagnostic and informational output. GCC looks at the locale categories LC_CTYPE and LC_MESSAGES if it's been configured to do that. Those categories can be set to literally any value your installation supports — a very typical one is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8. 🇬🇧✨

If LC_ALL is set, it overrides the value of LC_CTYPE and LC_MESSAGES. If it isn't, then LC_CTYPE and LC_MESSAGES default to the value of LANG. And if none of these variables are set at all? GCC falls back to traditional C English behavior. iconic honestly.

Lowkey important 🙈: these environment variables do not affect the encodings of input files and strings in output files produced by GCC — those are controlled by the -finput-charset and -fexec-charset options respectively. Different thing entirely, okurr.

🗂️ TMPDIR

If TMPDIR is set, it says which directory to use for temporary files. GCC uses temp files to hold the output of one stage of compilation that's gonna be the input to the next stage — for example, the output of the preprocessor, which is the input to the compiler proper. Passing notes between stages, we love to see it. 📝

🩺 GCC_DIAGNOSTICS_LOG

If GCC_DIAGNOSTICS_LOG is set, extra information about the diagnostics subsystem gets emitted. Set it to an empty value and that info goes to stderr; otherwise GCC will attempt to open that file and write the information there.

The precise content and format of that info is subject to change — it's intended for GCC developers, not end-users. So like, not your business unless you're built different. 💀

🆚 GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. Peep that option's documentation for the full details, bestie.

🧬 GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with a subprogram's name, but you can specify a prefix ending with a slash if you wish. Your call, main character.

If GCC_EXEC_PREFIX isn't set, GCC tries to figure out an appropriate prefix based on the pathname it was invoked with.

If GCC can't find the subprogram using the specified prefix, it tries looking in the usual places instead. Fallback queen. 👑

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix of 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 like crt0.o that get used for linking.

And in addition — this is the quirky part — the prefix is used in an unusual way when finding directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. So with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. And 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. Wilder than my group chat 😭

🧭 COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries those directories when searching for subprograms, if it can't find the subprograms using GCC_EXEC_PREFIX.

📚 LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it can't 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. No cutting the line, -L. 💅

💬 GCC_EXTRA_DIAGNOSTIC_OUTPUT

If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, extra text gets emitted to stderr when fix-it hints are emitted. And just so you know: -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable. Options always win, that's the rule.

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

Same as fixits-v1, but columns are expressed as display columns, as per -fdiagnostics-column-unit=display.

🔌 EXPERIMENTAL_SARIF_SOCKET

If EXPERIMENTAL_SARIF_SOCKET is set in the environment, the compiler will attempt to connect to a UNIX domain stream socket with that name, and send an "OnSarifResult" JSON-RPC 2.0 notification to it for each diagnostic that occurs, where the value of the notification is a SARIF "result" object. Fancy networking behaviour, we stan. 🦋

The compiler will fail immediately if EXPERIMENTAL_SARIF_SOCKET is set and it cannot connect to it. No chill, no retry.

⚠️ This feature is experimental and subject to change or removal without notice. Do not build your whole personality on it, bestie.

---

Some additional environment variables affect the behavior of the preprocessor. Yes, there's more. Buckle up. 🎀

🍓 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's a semicolon, and for almost all other targets it's 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 gets 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 tells 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. See how that works? lil empty string doing so much 💖

📦 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. Not invited to the party, sorry.

The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.

In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too. Combo move! 🔥

☀️ SUNPRO_DEPENDENCIES

This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. However, the dependence on the main input file is omitted.

⏰ SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible. Reproducible builds are so iconic, no cap.

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. Like, don't just make one up, bestie. 🫶

---

💀 BUGS 💀

For instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>. Go off, file that report, the devs literally want to hear it.

---

📎 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. Also — and this is the spicy part — -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. Okay? Okay. 🧚

---

👀 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. 🌈

---

👑 AUTHOR 👑

See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. The contributors are the main characters here, honestly.

---

📜 COPYRIGHT 📜

Copyright (c) 1988-2026 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.

(a) The FSF's Front-Cover Text is:

     A GNU Manual

(b) The FSF's Back-Cover Text is:

     You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development.

---

Okay that's the whole page, bestie — you made it to the very end, which is genuinely a flex. 🫶 This lil masterpiece is gcc-16.2.0, dated 2026-08-07, and it's giving documentation royalty. Now go compile something and slay. 💅✨🎀