GCC(1)

gcc — GNU project C and C++ compiler 💖

okay so, bestie, this is GCC(1) from the GNU section of the manual, and it is the compiler. The one. The main character. We're gonna get through all of it together, no cap. ✨

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🎀 NAME 🎀

gcc — the GNU project C and C++ compiler 🫶

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🌸 SYNOPSIS 🌸

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

Real talk: those are only the most useful options. The rest of them live further down this page (and, like, a lot further down, we're talking a hundred-plus parts of further down). Also g++ accepts mostly the same options as gcc, so if you know one you basically know the other. 💅

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🦋 DESCRIPTION 🦋

Here's the tea ☕ — when you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The whole glow-up pipeline, all in one go.

The "overall options" are the ones that let you stop that process at an intermediate stage. Iconic example: -c tells GCC not to run the linker. Then the output is just object files that the assembler handed over. 🎀

Other options get passed along to one or more stages of processing. Some control the preprocessor, others boss the compiler itself around. And there's yet another group that controls the assembler and the linker — most of those are not documented here, because bestie, you will rarely ever need to use any of them. 🙈

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

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

The gcc program accepts options and file names as operands. Big heads-up tho: many options have multi-letter names, and therefore multiple single-letter options may not be grouped — -dv is very different from -d -v. Do not learn that one the hard way. 😭

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

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 a positive and a negative form, and the negative form of -ffoo is -fno-foo. This manual documents only one of those two forms — whichever one is not the default. iykyk ✨

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

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✨ OPTIONS ✨

🍓 Option Summary 🍓

Here's a summary of all the options, grouped by type. Explanations are in the following sections (keep scrolling, we got you 💖).

💜 Overall Options

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

🧚 C Language Options

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

💅 C++ Language Options

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

🎀 Objective-C and Objective-C++ Language Options

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

🌈 OpenMP and OpenACC Options

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

🦋 Diagnostic Message Formatting Options

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

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and that's just the summary bestie, the actual explanations are coming 👀

⚠️ Warning Options ⚠️

okay bestie, buckle up, this is the big one. this is the entire warning roster, the whole group chat, everybody who has ever had an opinion about your code. they're all here. no skipping, no "oh I'll just list the important ones" — we are listing every single one, periodt. 🎀

• -fsyntax-only — literally just the syntax, nothing else, the name carries the whole story. she's here for the vibes and the parse only.
• -fmax-errors=n — you get n errors and then the compiler's done hearing about it. caps the drama at n.
• -Wpedantic — the hall monitor. she WILL read your code and she WILL have notes. slay but make it strict. 💅
• -pedantic-errors — pedantic but the notes are now errors. same energy, harder consequences.
• -fpermissive — a lil soft, a lil "bestie let's talk this through". lowercase loose.
• -w — one letter. that's it. that's the whole option. iconic, minimal, mysterious. 🦋
• -Wextra — the bonus tracks. the deluxe edition warnings.
• -Wall — THE queen of the entire page. the flagship. the one everybody types on autopilot. she ate. 👑
• -Wabi=n — ABI warnings, and you get to specify n.
• -Waddress — address drama, flagged.
• -Wno-address-of-packed-member — the off switch for the address-of-packed-member chatter.
• -Waggregate-return — aggregates coming back out of a function, clock it.
• -Walloc-size — allocation size, flagged and on the list.
• -Walloc-size-larger-than=byte-size — allocation size talk, but only once she's bigger than byte-size.
• -Walloc-zero — allocating zero, we're watching, no cap.
• -Walloca — alloca gets a warning all of her own, main character energy.
• -Walloca-larger-than=byte-size — alloca, but with a byte-size ceiling.
• -Wno-aggressive-loop-optimizations — off switch for the aggressive-loop-optimizations notices.
• -Warith-conversion — arithmetic conversions, flagged fr fr.
• -Warray-bounds — arrays going out of bounds, she sees it, she says something.
• -Warray-bounds=n — same as above, but you get to pass n.
• -Warray-compare — array-on-array comparison, flagged.
• -Warray-parameter — array parameters, on blast.
• -Warray-parameter=n — array parameters, with your own n.
• -Wno-attributes — off switch for the attributes warnings.
• -Wattribute-alias=n — attribute-alias warnings at level n.
• -Wno-attribute-alias — kills the attribute-alias chatter.
• -Wno-attribute-warning — kills the attribute-warning chatter.
• -Wbidi-chars=[none|unpaired|any|ucn] — bidi characters, and you pick your flavour: none, unpaired, any, or ucn. 💖
• -Wbool-compare — booleans getting compared, flagged.
• -Wbool-operation — booleans getting operated on, flagged.
• -Wno-builtin-declaration-mismatch — off switch for builtin-declaration-mismatch.
• -Wno-builtin-macro-redefined — off switch for builtin-macro-redefined.
• -Wc90-c99-compat — compatibility chatter between c90 and c99.
• -Wc99-c11-compat — c99 vs c11 compatibility, flagged.
• -Wc11-c23-compat — c11 vs c23 compatibility, flagged.
• -Wc23-c2y-compat — c23 vs c2y compatibility, flagged. she's future-facing. ✨
• -Wc++-compat — C vs C++ compatibility, flagged.
• -Wc++11-compat — c++11 compatibility, flagged.
• -Wc++14-compat — c++14 compatibility, flagged.
• -Wc++17-compat — c++17 compatibility, flagged.
• -Wc++20-compat — c++20 compatibility, flagged.
• -Wno-c++11-extensions — off switch for c++11 extensions chatter.
• -Wno-c++14-extensions — off switch for c++14 extensions chatter.
• -Wno-c++17-extensions — off switch for c++17 extensions chatter.
• -Wno-c++20-extensions — off switch for c++20 extensions chatter.
• -Wno-c++23-extensions — off switch for c++23 extensions chatter. yes she goes that far.
• -Wcalloc-transposed-args — transposed calloc args, spotted, called out.
• -Wcast-align — casts that mess with alignment, flagged.
• -Wcast-align=strict — same but strict mode, no mercy. 💀
• -Wcast-function-type — function-type casts, flagged.
• -Wcast-qual — casts that drop qualifiers, flagged.
• -Wchar-subscripts — chars used as subscripts, flagged.
• -Wclobbered — clobbered things, flagged.
• -Wcomment — comments with comments inside them, she hums disapprovingly. 😭
• -Wcompare-distinct-pointer-types — comparing pointers of different types, flagged.
• -Wno-complain-wrong-lang — off switch for the "wrong lang" complaining. ngl she should still say it but okay.
• -Wconversion — conversions, flagged.
• -Wno-coverage-mismatch — off switch for coverage-mismatch.
• -Wno-cpp — off switch for the cpp warnings.
• -Wdangling-else — a dangling else, hanging there, dramatic. 🙈
• -Wdangling-pointer — a dangling pointer, flagged.
• -Wdangling-pointer=n — dangling pointer, but with your own n.
• -Wdate-time — __DATE__/__TIME__ energy, flagged.
• -Wno-deprecated — off switch for deprecated chatter.
• -Wno-deprecated-declarations — off switch for deprecated-declarations.
• -Wno-designated-init — off switch for designated-init.
• -Wdisabled-optimization — optimization got disabled, she noticed.
• -Wno-discarded-array-qualifiers — off switch for discarded-array-qualifiers.
• -Wno-discarded-qualifiers — off switch for discarded-qualifiers.
• -Wno-div-by-zero — off switch for div-by-zero.
• -Wdouble-promotion — double promotion, flagged.
• -Wduplicated-branches — two branches doing the same thing, she clocked it.
• -Wduplicated-cond — duplicated conditions, flagged.
• -Wempty-body — an empty body just sitting there, flagged.
• -Wno-endif-labels — off switch for endif-labels.
• -Wenum-compare — enum comparison, flagged.
• -Wenum-conversion — enum conversion, flagged.
• -Wenum-int-mismatch — enum vs int mismatch, flagged.
• -Werror — every warning is now an error. the plot twist. the whole vibe changes. 🔥
• -Werror=* — same energy, but you get to name which ones. wildcard, bestie.
• -Wexpansion-to-defined — macro expansion wandering into defined, flagged.
• -Wfatal-errors — fatal errors, she stops everything.
• -Wflex-array-member-not-at-end — a flex array member that isn't at the end, flagged. she has OPINIONS about placement.
• -Wfloat-conversion — float conversion, flagged.
• -Wfloat-equal — float equality, flagged.
• -Wformat — format warnings, the classic.
• -Wformat=2 — format warnings, level 2, turned all the way up. 💅
• -Wno-format-contains-nul — off switch for format-contains-nul.
• -Wno-format-extra-args — off switch for format-extra-args.
• -Wformat-nonliteral — nonliteral format strings, flagged.
• -Wformat-overflow=n — format overflow, at level n.
• -Wformat-security — format security, flagged.
• -Wformat-signedness — format signedness, flagged.
• -Wformat-truncation=n — format truncation, at level n.
• -Wformat-y2k — format + y2k, she's keeping the lore alive. 🕰️
• -Wframe-address — frame address, flagged.
• -Wframe-larger-than=byte-size — frames bigger than byte-size, flagged.
• -Wno-free-nonheap-object — off switch for free-nonheap-object.
• -Wheader-guard — header guards, flagged.
• -Wno-if-not-aligned — off switch for if-not-aligned.
• -Wno-ignored-attributes — off switch for ignored-attributes.
• -Wignored-qualifiers — ignored qualifiers, flagged.
• -Wno-incompatible-pointer-types — off switch for incompatible-pointer-types.
• -Whardened — hardened, flagged. she's got armor on. 🛡️
• -Wimplicit — implicit stuff, flagged.
• -Wimplicit-fallthrough — implicit fallthrough, flagged.
• -Wimplicit-fallthrough=n — implicit fallthrough, at level n.
• -Wno-implicit-function-declaration — off switch for implicit-function-declaration.
• -Wno-implicit-int — off switch for implicit-int.
• -Winfinite-recursion — recursion that never ends, flagged. literally going in circles. 🌈
• -Winit-self — initializing something with itself, flagged.
• -Winline — inlining, flagged.
• -Wno-int-conversion — off switch for int-conversion.
• -Wint-in-bool-context — an int chilling in a bool context, flagged.
• -Wno-int-to-pointer-cast — off switch for int-to-pointer-cast.
• -Wno-invalid-memory-model — off switch for invalid-memory-model.
• -Winvalid-pch — invalid precompiled header, flagged.
• -Winvalid-utf8 — invalid utf8, flagged.
• -Wno-unicode — off switch for the unicode warnings.
• -Wjump-misses-init — a jump that skips over an init, flagged.
• -Wlarger-than=byte-size — anything larger than byte-size, flagged.
• -Wleading-whitespace=kind — leading whitespace, and you pick the kind.
• -Wlogical-not-parentheses — logical-not + parentheses, flagged.
• -Wlogical-op — logical operations, flagged.
• -Wlong-long — long long, flagged. she's a long long girlie. 💜
• -Wno-lto-type-mismatch — off switch for lto-type-mismatch.
• -Wmain — main, flagged, because main is sacred.
• -Wmaybe-uninitialized — maybe uninitialized. she's not accusing, she's just asking questions. 🙈
• -Wmemset-elt-size — memset element size, flagged.
• -Wmemset-transposed-args — transposed memset args, flagged.
• -Wmisleading-indentation — indentation that lies to you, flagged. the betrayal. 💀
• -Wmissing-attributes — missing attributes, flagged.
• -Wmissing-braces — missing braces, flagged.
• -Wmissing-field-initializers — missing field initializers, flagged.
• -Wmissing-format-attribute — missing format attribute, flagged.
• -Wmissing-include-dirs — include dirs that aren't there, flagged.
• -Wmissing-noreturn — missing noreturn, flagged.
• -Wmusttail-local-addr — musttail + local address, flagged.
• -Wmaybe-musttail-local-addr — maybe musttail + local address. she's hedging, respectfully.
• -Wno-missing-profile — off switch for missing-profile.
• -Wno-multichar — off switch for multichar.
• -Wmultistatement-macros — multistatement macros, flagged.
• -Wnonnull — null stuff getting passed to nonnull params, flagged.
• -Wnonnull-compare — comparing against nonnull, flagged.
• -Wnormalized=[none|id|nfc|nfkc] — normalization forms, pick your fave: none, id, nfc, nfkc. ✨
• -Wnull-dereference — null dereference, flagged. the classic jump-scare. 😭
• -Wno-odr — off switch for odr.
• -Wopenacc-parallelism — OpenACC parallelism, flagged.
• -Wopenmp — OpenMP, flagged.
• -Wopenmp-simd — OpenMP SIMD, flagged.
• -Wno-overflow — off switch for overflow.
• -Woverlength-strings — strings that are Too Long, flagged.
• -Wno-override-init-side-effects — off switch for override-init-side-effects.
• -Wpacked — packed, flagged.
• -Wno-packed-bitfield-compat — off switch for packed-bitfield-compat.
• -Wpacked-not-aligned — packed and not aligned, flagged.
• -Wpadded — padding, flagged.
• -Wparentheses — parentheses, flagged. yes, the little curvy ones. 💅
• -Wno-pedantic-ms-format — off switch for pedantic-ms-format.
• -Wpointer-arith — pointer arithmetic, flagged.
• -Wno-pointer-compare — off switch for pointer-compare.
• -Wno-pointer-to-int-cast — off switch for pointer-to-int-cast.
• -Wno-pragmas — off switch for the pragmas warnings.
• -Wno-pragma-once-outside-header — off switch for pragma-once-outside-header.
• -Wno-prio-ctor-dtor — off switch for prio-ctor-dtor.
• -Wno-psabi — off switch for the psabi notes.
• -Wredundant-decls — redundant declarations, flagged.
• -Wrestrict — restrict, flagged.
• -Wno-return-local-addr — off switch for return-local-addr.
• -Wreturn-type — return type, flagged.
• -Wno-scalar-storage-order — off switch for scalar-storage-order.
• -Wsequence-point — sequence points, flagged.
• -Wshadow — shadowing, flagged.
• -Wshadow=global — shadowing, global scope edition.
• -Wshadow=local — shadowing, local scope edition.
• -Wshadow=compatible-local — shadowing, compatible-local edition. very specific, we love a specialist. 🎀
• -Wno-shadow-ivar — off switch for shadow-ivar.
• -Wno-shift-count-negative — off switch for shift-count-negative.
• -Wno-shift-count-overflow — off switch for shift-count-overflow.
• -Wshift-negative-value — shifting a negative value, flagged.
• -Wno-shift-overflow — off switch for shift-overflow.
• -Wshift-overflow=n — shift overflow, at level n.
• -Wsign-compare — sign comparison, flagged.
• -Wsign-conversion — sign conversion, flagged.
• -Wno-sizeof-array-argument — off switch for sizeof-array-argument.
• -Wsizeof-array-div — sizeof division on arrays, flagged.
• -Wsizeof-pointer-div — sizeof division on pointers, flagged.
• -Wsizeof-pointer-memaccess — sizeof pointer in memaccess, flagged.
• -Wstack-protector — stack protector, flagged.
• -Wstack-usage=byte-size — stack usage over byte-size, flagged.
• -Wstrict-aliasing — strict aliasing, flagged.
• -Wstrict-aliasing=n — strict aliasing, at level n.
• -Wstrict-overflow — strict overflow, flagged.
• -Wstrict-overflow=n — strict overflow, at level n.
• -Wstring-compare — string comparison, flagged.
• -Wno-stringop-overflow — off switch for stringop-overflow.
• -Wno-stringop-overread — off switch for stringop-overread.
• -Wno-stringop-truncation — off switch for stringop-truncation.
• -Wstrict-flex-arrays — strict flex arrays, flagged.
• -Wsuggest-attribute=attribute-name — she suggests an attribute, and you name the attribute-name. she's being helpful, not mean. 🫶
• -Wswitch — switch, flagged.
• -Wno-switch-bool — off switch for switch-bool.
• -Wswitch-default — switch missing its default, flagged.
• -Wswitch-enum — switch on an enum, flagged.
• -Wno-switch-outside-range — off switch for switch-outside-range.
• -Wno-switch-unreachable — off switch for switch-unreachable.
• -Wsync-nand — __sync_nand, flagged.
• -Wsystem-headers — system headers, flagged. yes, she'll go there.
• -Wtautological-compare — comparing something to itself, flagged. bestie that's a mirror. 🌸
• -Wtrailing-whitespace — trailing whitespace, flagged.
• -Wtrailing-whitespace=kind — trailing whitespace, and you pick the kind.
• -Wtrampolines — trampolines, flagged.
• -Wtrigraphs — trigraphs, flagged. the ancient lore. 🕰️
• -Wtrivial-auto-var-init — trivial auto var init, flagged.
• -Wno-tsan — off switch for the tsan warnings.
• -Wtype-limits — type limits, flagged.
• -Wundef — undef, flagged.
• -Wuninitialized — uninitialized stuff, flagged.
• -Wunknown-pragmas — unknown pragmas, flagged.
• -Wunsuffixed-float-constants — float constants with no suffix, flagged.
• -Wunterminated-string-initialization — unterminated string initialization, flagged. 🫣
• -Wunused — unused things, flagged. the umbrella queen of "you're not using that".
• -Wunused-but-set-parameter — set but never used parameter, flagged.
• -Wunused-but-set-variable — set but never used variable, flagged.
• -Wunused-const-variable — unused const variable, flagged.
• -Wunused-const-variable=n — unused const variable, at level n.
• -Wunused-function — unused function, flagged.
• -Wunused-label — unused label, flagged.
• -Wunused-local-typedefs — unused local typedefs, flagged.
• -Wunused-macros — unused macros, flagged.
• -Wunused-parameter — unused parameter, flagged.
• -Wno-unused-result — off switch for unused-result.
• -Wunused-value — unused value, flagged.
• -Wunused-variable — unused variable, flagged.
• -Wuse-after-free — use after free, flagged. the horror movie. 😭
• -Wuse-after-free=n — use after free, at level n.
• -Wuseless-cast — a cast that was never needed, flagged. useless! she said it not me. 💅
• -Wno-varargs — off switch for the varargs warnings.
• -Wvariadic-macros — variadic macros, flagged.
• -Wvector-operation-performance — vector operation performance, flagged.
• -Wvla — variable length arrays, flagged.
• -Wvla-larger-than=byte-size — VLAs larger than byte-size, flagged.
• -Wno-vla-larger-than — off switch for vla-larger-than.
• -Wvolatile-register-var — volatile register variable, flagged.
• -Wwrite-strings — write strings, flagged.
• -Wno-xor-used-as-pow — off switch for xor-used-as-pow. (she's seen things.) 💀
• -Wzero-as-null-pointer-constant — zero used as a null pointer constant, flagged.
• -Wzero-length-bounds — zero-length bounds, flagged.

okay that was the marathon and you ATE it. next up, the analyzer girlies. 🔍

---

🧠 Static Analyzer Options 🧠

so this whole section is the analyzer — she doesn't just compile, she investigates. she's the friend who reads the whole group chat history before replying.

• -fanalyzer — the analyzer herself, enabled. main character. 👑
• -fanalyzer-call-summaries — call summaries for the analyzer.
• -fanalyzer-checker=name — pick your analyzer checker by name.
• -fno-analyzer-feasibility — turns off analyzer feasibility.
• -fanalyzer-fine-grained — fine-grained analyzer mode, she's being thorough.
• -fanalyzer-show-events-in-system-headers — show analyzer events inside system headers.
• -fno-analyzer-state-merge — turns off analyzer state merging.
• -fno-analyzer-state-purge — turns off analyzer state purging.
• -fno-analyzer-suppress-followups — turns off analyzer follow-up suppression.
• -fanalyzer-transitivity — analyzer transitivity, on.
• -fno-analyzer-undo-inlining — turns off the analyzer undoing inlining.
• -fanalyzer-verbose-edges — verbose edges for the analyzer, she's yapping (affectionately). 💜
• -fanalyzer-verbose-state-changes — verbose analyzer state changes, every single one.
• -fanalyzer-verbosity=level — set analyzer verbosity to level.
• -fdump-analyzer — dump the analyzer output.
• -fdump-analyzer-callgraph — dump the analyzer callgraph.
• -fdump-analyzer-exploded-graph — dump the analyzer exploded graph. yes that's the real name. 💀
• -fdump-analyzer-exploded-nodes — dump exploded nodes.
• -fdump-analyzer-exploded-nodes-2 — exploded nodes, the sequel.
• -fdump-analyzer-exploded-nodes-3 — exploded nodes threequel. she said trilogy. 🎬
• -fdump-analyzer-exploded-paths — dump exploded paths.
• -fdump-analyzer-feasibility — dump analyzer feasibility.
• -fdump-analyzer-infinite-loop — dump the analyzer infinite loop.
• -fdump-analyzer-json — dump it as JSON. ✨
• -fdump-analyzer-state-purge — dump analyzer state purge.
• -fdump-analyzer-stderr — dump analyzer stuff to stderr.
• -fdump-analyzer-supergraph — dump the supergraph. super. 🦋
• -fdump-analyzer-untracked — dump the untracked stuff.
• -Wno-analyzer-double-fclose — off switch for analyzer-double-fclose.
• -Wno-analyzer-double-free — off switch for analyzer-double-free.
• -Wno-analyzer-exposure-through-output-file — off switch for analyzer-exposure-through-output-file.
• -Wno-analyzer-exposure-through-uninit-copy — off switch for analyzer-exposure-through-uninit-copy.
• -Wno-analyzer-fd-access-mode-mismatch — off switch for analyzer-fd-access-mode-mismatch.
• -Wno-analyzer-fd-double-close — off switch for analyzer-fd-double-close.
• -Wno-analyzer-fd-leak — off switch for analyzer-fd-leak.
• -Wno-analyzer-fd-phase-mismatch — off switch for analyzer-fd-phase-mismatch.
• -Wno-analyzer-fd-type-mismatch — off switch for analyzer-fd-type-mismatch.
• -Wno-analyzer-fd-use-after-close — off switch for analyzer-fd-use-after-close.
• -Wno-analyzer-fd-use-without-check — off switch for analyzer-fd-use-without-check.
• -Wno-analyzer-file-leak — off switch for analyzer-file-leak.
• -Wno-analyzer-free-of-non-heap — off switch for analyzer-free-of-non-heap.
• -Wno-analyzer-imprecise-fp-arithmetic — off switch for analyzer-imprecise-fp-arithmetic.
• -Wno-analyzer-infinite-loop — off switch for analyzer-infinite-loop.
• -Wno-analyzer-infinite-recursion — off switch for analyzer-infinite-recursion.
• -Wno-analyzer-jump-through-null — off switch for analyzer-jump-through-null.
• -Wno-analyzer-malloc-leak — off switch for analyzer-malloc-leak.
• -Wno-analyzer-mismatching-deallocation — off switch for analyzer-mismatching-deallocation.
• -Wno-analyzer-null-argument — off switch for analyzer-null-argument.
• -Wno-analyzer-null-dereference — off switch for analyzer-null-dereference.
• -Wno-analyzer-out-of-bounds — off switch for analyzer-out-of-bounds.
• -Wno-analyzer-overlapping-buffers — off switch for analyzer-overlapping-buffers.
• -Wno-analyzer-possible-null-argument — off switch for analyzer-possible-null-argument.
• -Wno-analyzer-possible-null-dereference — off switch for analyzer-possible-null-dereference.
• -Wno-analyzer-putenv-of-auto-var — off switch for analyzer-putenv-of-auto-var.
• -Wno-analyzer-shift-count-negative — off switch for analyzer-shift-count-negative.
• -Wno-analyzer-shift-count-overflow — off switch for analyzer-shift-count-overflow.
• -Wno-analyzer-stale-setjmp-buffer — off switch for analyzer-stale-setjmp-buffer.
• -Wno-analyzer-tainted-allocation-size — off switch for analyzer-tainted-allocation-size.
• -Wno-analyzer-tainted-assertion — off switch for analyzer-tainted-assertion.
• -Wno-analyzer-tainted-array-index — off switch for analyzer-tainted-array-index.
• -Wno-analyzer-tainted-divisor — off switch for analyzer-tainted-divisor.
• -Wno-analyzer-tainted-offset — off switch for analyzer-tainted-offset.
• -Wno-analyzer-tainted-size — off switch for analyzer-tainted-size.
• -Wanalyzer-symbol-too-complex — the symbol got too complex for her. relatable. 😭
• -Wanalyzer-too-complex — the whole thing got too complex. also relatable.
• -Wno-analyzer-undefined-behavior-ptrdiff — off switch for analyzer-undefined-behavior-ptrdiff.
• -Wno-analyzer-undefined-behavior-strtok — off switch for analyzer-undefined-behavior-strtok.
• -Wno-analyzer-unsafe-call-within-signal-handler — off switch for analyzer-unsafe-call-within-signal-handler.
• -Wno-analyzer-use-after-free — off switch for analyzer-use-after-free.
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — off switch for analyzer-use-of-pointer-in-stale-stack-frame.
• -Wno-analyzer-use-of-uninitialized-value — off switch for analyzer-use-of-uninitialized-value.
• -Wno-analyzer-va-arg-type-mismatch — off switch for analyzer-va-arg-type-mismatch.
• -Wno-analyzer-va-list-exhausted — off switch for analyzer-va-list-exhausted.
• -Wno-analyzer-va-list-leak — off switch for analyzer-va-list-leak.
• -Wno-analyzer-va-list-use-after-va-end — off switch for analyzer-va-list-use-after-va-end.
• -Wno-analyzer-write-to-const — off switch for analyzer-write-to-const.
• -Wno-analyzer-write-to-string-literal — off switch for analyzer-write-to-string-literal.

---

🍓 C and Objective-C-only Warning Options 🍓

C girlies and Objective-C girlies, this one's for you specifically. ❤️

• -Wbad-function-cast — a bad function cast, flagged.
• -Wdeprecated-non-prototype — deprecated non-prototype, flagged.
• -Wfree-labels — free labels, flagged.
• -Wmissing-declarations — missing declarations, flagged.
• -Wmissing-parameter-name — missing parameter name, flagged.
• -Wmissing-parameter-type — missing parameter type, flagged.
• -Wdeclaration-missing-parameter-type — declaration missing a parameter type, flagged.
• -Wmissing-prototypes — missing prototypes, flagged.
• -Wmissing-variable-declarations — missing variable declarations, flagged.
• -Wnested-externs — nested externs, flagged.
• -Wold-style-declaration — old-style declarations, flagged. the heritage. 🕰️
• -Wold-style-definition — old-style definitions, flagged.
• -Wstrict-prototypes — strict prototypes, flagged.
• -Wtraditional — traditional, flagged. very vintage of her.
• -Wtraditional-conversion — traditional conversions, flagged.
• -Wdeclaration-after-statement — declarations after statements, flagged.
• -Wpointer-sign — pointer sign, flagged.

---

🐛 Debugging Options 🐛

now we're in the debug era. this is where you tell her how much lore you want in the binary. 🎀

• -g — debug info, the original, the queen, no notes. 👑
• -glevel — debug info at level.
• -gdwarf — DWARF, pick her.
• -gdwarf-version — DWARF, specific version.
• -gbtf — BTF debug info.
• -gctf — CTF debug info.
• -gctflevel — CTF at level.
• -gprune-btf — prune the BTF.
• -gno-prune-btf — don't prune the BTF.
• -ggdb — GDB-flavoured debug info.
• -grecord-gcc-switches — record the GCC switches.
• -gno-record-gcc-switches — don't record the GCC switches.
• -gstrict-dwarf — strict DWARF.
• -gno-strict-dwarf — not strict DWARF.
• -gas-loc-support — assembler location support.
• -gno-as-loc-support — no assembler location support.
• -gas-locview-support — assembler locview support.
• -gno-as-locview-support — no assembler locview support.
• -gcodeview — CodeView debug info.
• -gcolumn-info — column info, on.
• -gno-column-info — column info, off.
• -gdwarf32 — 32-bit DWARF.
• -gdwarf64 — 64-bit DWARF.
• -gstatement-frontiers — statement frontiers, on.
• -gno-statement-frontiers — statement frontiers, off.
• -gvariable-location-views — variable location views, on.
• -gno-variable-location-views — variable location views, off.
• -ginternal-reset-location-views — internal reset location views, on.
• -gno-internal-reset-location-views — internal reset location views, off.
• -ginline-points — inline points, on.
• -gno-inline-points — inline points, off.
• -gvms — VMS debug info.
• -gz[=type] — compressed debug sections, with an optional type.
• -gsplit-dwarf — split DWARF, she's separating the lore into two files. 💅
• -gdescribe-dies — describe the DIEs.
• -gno-describe-dies — don't describe the DIEs.
• -fdebug-prefix-map=old=new — remap a debug prefix from old to new.
• -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, baseonly style.
• -femit-struct-debug-reduced — emit struct debug info, reduced style.
• -femit-struct-debug-detailed[=spec-list] — emit struct debug info, detailed style, 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 DWARF 2 CFI asm.
• -fvar-tracking — variable tracking, on.
• -fvar-tracking-assignments — variable tracking assignments, on.

and that's the debug section bestie, we're not even close to done, keep scrolling. 💖

🌸 Optimization Options 🌸

okay bestie, sit down, hydrate, because this is the section where GCC stops just compiling your code and starts making it go fast 💨✨. This is the optimization menu, and it is long. Like, unreasonably long. Like, longer than your to-do list long. But every single one of these is here, no skips, no cap 👑

Here's the whole entire roster, alphabet-ish, exactly as the universe intended:

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

see? told you it was a lot 💀 and every single one of those is a real vibe, a real lever, a real little dial you can turn. the descriptions live in their own special place, but the names? the names are all right here bestie. keeping receipts 📝

---

🎀 Program Instrumentation Options 🎀

okay so this next lil family? it's giving main character energy but for debugging and profiling 🕵️‍♀️✨. this is where you tell GCC to sprinkle extra instrumentation into your program — coverage, profiles, sanitizers, hardening, all the watchdog stuff. very "i'm watching you in a loving way" of it 🫶

the whole squad, no one left behind:

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

the default-obsessed girlies in the chat will notice a lot of these have a -fno- sibling situation going on with the hardening ones — that's a whole thing, and it's covered in their own entries. here we're just doing roll call 📋💅

---

🦋 Preprocessor Options 🦋

omg okay. the preprocessor. the part of the toolchain that runs before anything else even happens, like the friend who texts you "we need to talk" before you've had coffee ☕😭. these flags control macros, includes, charset stuff, and all the little substitutions that happen way before your code becomes code.

here's everybody:

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

^^ that -pthread in there? iconic crossover moment, it shows up in more than one place. we love a versatile queen 🙈💖

---

🧚 Assembler Options 🧚

short and sweet, just like this section bestie, we love a concise moment:

• -Wa,option
• -Xassembler option

periodt. tiny section, huge importance. passing stuff straight through to the assembler, no notes 🫡

---

💜 Linker Options 💜

okay now THIS is where the actual linking happens, aka the last step where everything finally becomes a real executable and you either scream with joy or cry about undefined references. classic duality of dev 🎭

the whole lineup:

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

not me spotting object-file-name chillin' in there with no dash at all, living its best unbothered life 😌✨

---

🌈 Directory Options 🌈

besties, welcome to search path city. these are the flags that tell GCC where to look for headers, libraries, plugins, and all the little things it needs to find so it can do its job. it's giving "let me check five different drawers for the scissors" energy ✂️🔍

the full address book:

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

look at the little dir and file placeholders everywhere, they're all italics in spirit, living their best placeholder lives 💅

---

⭐ Code Generation Options ⭐

this section is the bones of the output — how registers get treated, how structs get returned, what visibility things get, exception handling vibes, all of it. it's the tailoring section of the whole operation, where the suit gets fitted 🧵✨

the entire wardrobe:

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

ngl -fpic vs -fPIC being a whole case-sensitivity situation is so on brand for this codebase and honestly? we respect the chaos 🫶

---

👑 Developer Options 👑

last but absolutely not least — the developer options, aka the ones for the people who are actively working on GCC itself or debugging their builds at a level of detail that is honestly unhinged (affectionate) 🔬💀. dumps, reports, statistics, limits, seeds, all the nerdy little levers. main character energy for people who read the source for fun.

here's every single one:

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

and that's the full developer arsenal, bestie — every dump flag, every report flag, every little "tell me everything" switch, all present and accounted for, not one dropped 🙌💖

quick lil reminder for the girlies: these are the summary lists of the options. each one's full description, defaults, caveats and ✨drama✨ lives in its own spot further along, so don't go using a flag just because its name is cute. check the entry. iykyk 💅

we're still rolling, don't go anywhere 🌸

Machine-Dependent Options 💖

okay bestie, this is the part where every architecture gets its own little menu. it's giving "choose your fighter" but for compilers. let's go through them one by one. no cap, we're keeping every single flag. 💅

---

🦋 AArch64 Options

AArch64 said "I have options, bestie" and honestly? same. here's the full list:

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

---

🌸 Adapteva Epiphany Options

Adapteva Epiphany? more like Adapteva Epic-iphany, am I right? anyway, here's the tea:

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

---

🎀 AMD GCN Options

AMD GCN, the graphics core next situation. here's what you can tweak:

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

---

💅 ARC Options

ARC options, coming in hot. there are so many, bestie. brace yourself:

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

---

🌈 ARM Options

ARM, the classic. so many options, it's giving "I contain multitudes". here we go:

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

---

🍓 AVR Options

AVR options, cute and compact. here's the list:

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

---

🧚 Blackfin Options

Blackfin said "let me be mysterious" and gave us this list:

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

---

⭐ C6X Options

C6X options, short and sweet:

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

---

🌙 CRIS Options

CRIS options, giving us the full menu:

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

---

🦄 C-SKY Options

C-SKY options, slay:

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

---

💀 Cygwin and MinGW Options

Cygwin and MinGW options, the Windows-adjacent besties:

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

---

👑 Darwin Options

Darwin options, the macOS moment. huge list, but we're not skipping a single one:

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

---

🐚 DEC Alpha Options

DEC Alpha options, old school cool:

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

---

🦋 eBPF Options

eBPF options, the kernel vibe:

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

---

🌸 FR30 Options

FR30 options, short but sweet:

• -msmall-model
• -mno-lsim

---

🎀 FT32 Options

FT32 options, here we go:

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

---

💅 FRV Options

FRV options, a whole mood:

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

---

🌈 GNU/Linux Options

GNU/Linux options, the libc choices:

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

---

🍓 H8/300 Options

H8/300 options, a classic:

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

---

🧚 HPPA Options

HPPA options, going full force:

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

---

⭐ IA-64 Options

IA-64 options, the Itanium energy:

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

---

🌙 LM32 Options

LM32 options, the final boss:

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

---

🦋 LoongArch Options (LoongArch Options) 🦋

bestie, LoongArch came in and said "I'm new here but I brought a WHOLE bag of flags". here's every single one, no skips, no crumbs:

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

---

🎀 M32C Options (M32C Options) 🎀

short and sweet, love that for us:

• -mcpu=cpu
• -msim
• -memregs=number

---

💜 M32R/D Options (M32R/D Options) 💜

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

---

🌸 M680x0 Options (M680x0 Options) 🌸

the 68k family reunion, and everyone showed up:

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

---

✨ MCore Options (MCore Options) ✨

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

---

🦋 MicroBlaze Options (MicroBlaze Options) 🦋

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

---

👑 MIPS Options (MIPS Options) 👑

okay. deep breath. MIPS did NOT come to play — this is the biggest roster on the whole page and I am listing every last one of them, because that's the rules and also because they deserve it 💅

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

ate. no crumbs. next 💅

---

🧚 MMIX Options (MMIX Options) 🧚

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

---

🍓 MN10300 Options (MN10300 Options) 🍓

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

---

🌈 Moxie Options (Moxie Options) 🌈

tiny list, big vibe:

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

💖 MSP430 Options (MSP430 Options) 💖

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

bestie heads up: several of these are the "empty equals" kind — like -mmcu=, -mcpu=, -mcode-region=, -mdata-region=, -msilicon-errata=, -msilicon-errata-warn=, -mhwmult=, and -mmax-inline-shift= — they're written with nothing after the = on this list. I'm keeping them EXACTLY as they appear, uppercase-my-hands-off energy. 💅

---

⭐ NDS32 Options (NDS32 Options) ⭐

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

---

🫶 Nvidia PTX Options (Nvidia PTX Options) 🫶

three flags and they're all business:

• -m64
• -mmainkernel
• -moptimize

---

🦋 OpenRISC Options (OpenRISC Options) 🦋

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

---

🎀 PDP-11 Options (PDP-11 Options) 🎀

yes she's still in the group chat. iconic.

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

---

🦋 PowerPC Options 🦋

not me about to list things and then —

See RS/6000 and PowerPC Options.

that's literally it, bestie. this one just points you across the room to the big PowerPC list. follow the arrow, keep scrolling, it's down there 👇

---

🌸 PRU Options (PRU Options) 🌸

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

---

🔥 RISC-V Options (RISC-V Options) 🔥

RISC-V said "I'm open source and I'm also going to be thorough about it":

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

---

💜 RL78 Options (RL78 Options) 💜

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

---

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

okay this is the other MEGA list and honestly? it eats. every single flag, present and accounted for, because the rule is the rule:

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

---

and that's only half of the alphabet of architectures, bestie — the group chat keeps going, so don't scroll away yet 💖

💖 RX Options 💖

The Renesas RX fam has its own lil flag family, and here they all are, bestie:

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

---

✨ S/390 and zSeries Options ✨

Big iron energy only 🔥:

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

---

🦋 SH Options 🦋

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

---

🌸 Solaris 2 Options 🌸

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

---

👑 SPARC Options 👑

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

---

💜 System V Options 💜

• -Qy
• -Qn
• -YP,paths
• -Ym,dir

---

🍓 V850 Options 🍓

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

---

🌈 VAX Options 🌈

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

---

🧚 Visium Options 🧚

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

---

💅 VMS Options 💅

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

---

⭐ VxWorks Options ⭐

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

---

🔥 x86 Options 🔥

OK bestie, this is THE big one, the main character of the option lists, and yes it's ridiculously long, but I'm not skipping a single flag, periodt:

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

---

🎀 x86 Windows Options 🎀

Peep Cygwin and MinGW Options for these, bestie — that's where they live.

---

💗 Xstormy16 Options 💗

• -msim

---

🌷 Xtensa Options 🌷

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

---

💫 zSeries Options 💫

Same as above, babe — see S/390 and zSeries Options. 👑

---

✨ Options Controlling the Kind of Output ✨

OK so a compile run can go through up to four stages, and they always happen in this exact order, no skipping the line:

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

GCC can preprocess and compile several files either into several assembler input files, or into one assembler input file. Then each assembler input file produces an object file, and linking combines all the object files — both the ones that were just freshly compiled and the ones you passed in as input — into an executable file. Slay. ✨

For any given input file, the file name suffix is what decides which kind of compilation goes down, and it's giving very specific:

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. Heads up bestie: 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. 👀
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.h — C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. Quick PSA: in .cxx, the last two letters must both be literally x. Likewise, .C refers to a literal capital C.
• file.mm, file.M — Objective-C++ source code that must be preprocessed.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec.
• file.f, file.for, file.ftn, file.fi — Fixed form Fortran source code that should not be preprocessed.
• file.F, file.FOR, file.fpp, file.FPP, file.FTN — Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.f90, file.f95, file.f03, file.f08, file.fii — Free form Fortran source code that should not be preprocessed.
• file.F90, file.F95, file.F03, file.F08 — Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.cob, file.COB, file.cbl, file.CBL — COBOL source code.
• file.go — Go source code.
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). These files are also called specs, iykyk.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). These files are also called bodies.
• file.s — Assembler code.
• file.S, file.sx — Assembler code that must be preprocessed.
• other — An object file to be fed straight into linking. Any file name with no recognized suffix gets treated this way. No suffix, no problem. 🫶

okay bestie so you can literally tell gcc what language you're feeding it instead of letting it guess from the file suffix 😤 like stop assuming, let me speak ✋ — you do that with the -x option:

• -x language — say out loud, explicitly, what language the next input files are (instead of letting the compiler pick a default off the file name suffix ✨). This setting sticks to all the input files that come after it, all the way until the next -x shows up and changes the vibe. The possible values for language are:

        c  c-header  cpp-output
        c++  c++-header  c++-system-header c++-user-header c++-cpp-output
        objective-c  objective-c-header  objective-c-cpp-output  objc-cpp-output
        objective-c++  objective-c++-header  objective-c++-cpp-output
        objc++-cpp-output
        assembler  assembler-with-cpp
        ada  adascil  adawhy
        cobol
        d
        f77  f77-cpp-input  f95  f95-cpp-input
        go
        modula-2  modula-2-cpp-output
        rust
        lto

heads up bestie 🙈 -x does not imply a particular language standard, okay? It's not giving "I'll handle the standard for you" energy. For example -x f77 might also need -std=legacy for some older source codes. So don't come crying to me when your ancient Fortran acts up 💀

• -x none — turns off any language specification you set before, so everything after this point gets handled according to its file name suffixes again (like -x was never even used at all, no cap 🫶).

---

okay so if you only want some of the compilation stages, you can use -x (or the filename suffixes) to tell gcc where to start 🏁, and then one of the options -c, -S, or -E to say where gcc should stop 🛑.

and yeah, some combinations do absolutely nothing. Like, -x cpp-output -E instructs gcc to do nothing at all. Not me asking for a whole task and getting a thumbs up 👍😭

• -c — compile or assemble the source files, but do not link. The linking stage simply does not happen, we're not doing that today 💅. The ultimate output is 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, ones that don't require compilation or assembly, are just ignored. They're not the main character today ✨.

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

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

Input files that don't require compilation are ignored. Sorry bestie, not everyone gets an invite 💀.

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

Input files that don't require preprocessing are ignored.

• -o file — put the primary output in file. This applies to whatever sort of output you're producing — executable file, object file, assembler file, or preprocessed C code, all of it, one option to name them all 👑.

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 🫶.

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

gcc -c foo.c ...

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

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

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

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

A linker output will instead prefix aux and dump outputs:

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

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

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

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

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

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

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

gcc -save-temps -S foo.c

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• -dumpdir dumppfx — when forming the name of an auxiliary or dump output file, use dumppfx as a prefix 🌟:

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

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

If dumppfx is to be used as a directory name, it must end with a directory separator, no exceptions bestie:

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

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

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

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

outputs foo.o, with auxiliary outputs named foo.* — because -save-temps=* overrides the dumppfx that the earlier -dumpdir option handed over. And honestly? It does not matter that =obj is the default for -save-temps, nor that the output directory is just implicitly the current directory. Dump outputs get named foo.c.*, periodt.

When you're compiling from multiple input files and -dumpbase is specified, then dumpbase (minus an auxdropsuf suffix) plus a dash gets appended to — or overrides, if it's carrying directory components — an explicit or defaulted dumppfx, so that every single one of the multiple compilations gets differently-named aux and dump outputs. Cute, right?

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

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

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

which, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*. Simpler, no drama 💅

When you're compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes that whole dumpbase- transformation above (like the compilation of foo.c and bar.c above, but without -c). And if neither -dumpdir nor -dumpbase are given, the linker output base name (minus auxdropsuf, if specified, or the executable suffix otherwise) plus a dash gets appended to the default dumppfx instead. But — and this is the plot twist — unlike earlier cases of linking:

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

this does not append the output name main to dumppfx, because -dumpdir was explicitly specified. The goal here is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if you want it to; only an explicitly-specified -dumpbase would be combined with it, so that we avoid simply throwing away a meaningful option. iykyk.

When compiling and then linking from a single input file, the linker output base name is only 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 it's 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.

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

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 wouldn't otherwise be used as such — is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them.

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

---

• -v — Print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper.
• -### — Like -v, except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". Super useful for shell scripts that want to capture the driver-generated command lines 🐚
• --help — Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. And if the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. Nothing gets left out 👑
• --target-help — Print (on the standard output) a description of target-specific command-line options for each tool. For some targets, extra target-specific information may also be printed.
• --help={class|[^]qualifier}[,...] — Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:
  • optimizers — Display all of the optimization options supported by the compiler.
  • warnings — Display all of the options controlling warning messages produced by the compiler.
  • target — Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. That's because those tools don't 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, you need to select the common class.
  • common — Display the options that are common to all languages.

And these are the supported qualifiers:

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

So like, 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

Heads up bestie: 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. And if --help is also specified anywhere on the command line, then that takes precedence over any --help= option. Sorry, rules are rules.

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

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

-Q -O2 --help=optimizers

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

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

• --version — Display the version number and copyrights of the invoked GCC. ✨
• -pass-exit-codes — Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered. 😭
• -pipe — Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. 🫶
• -specs=file — Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right.
• -wrapper — Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... Imagine being that powerful 💀

• -ffile-prefix-map=old=new — ok bestie, when you're compiling files that live in the directory old, this makes the compiler record every reference to them as if they lived in directory new instead. ✨ Specifying just this one option = specifying all the individual -f*-prefix-map options at once, no cap. It's the girlboss move for reproducible builds that are location-independent. 🎀 Lowkey important caveat tho: directories referenced by directives are not affected by these options. See also -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map 🫶
• -fcanon-prefix-map — so for the -f*-prefix-map options, the comparison of old prefix against the filename that would normally be referenced in the compilation result is usually a textual comparison of the prefixes — or, on case-insensitive filesystems, it ignores character case, and on DOS-based filesystems it treats slashes and backslashes as equal. -fcanon-prefix-map makes those comparisons happen on canonicalized paths of old and the referenced filename instead. iconic behavior honestly 💅
• -fplugin=name.so — loads the plugin code in file name.so, which is assumed to be a shared object that the compiler will dlopen. The base name of the shared object file is what identifies the plugin for argument-parsing purposes (peep -fplugin-arg-name-key=value below 👀). Every plugin should define the callback functions specified in the Plugins API. periodt.
• -fplugin-arg-name-key=value — defines an argument called key with a value of value for the plugin called name. cute and simple 🌸
• -fdump-ada-spec[-slim] — for C and C++ source and include files, generates corresponding Ada specs. yes really, Ada. we love a crossover episode 💖
• -fada-spec-parent=unit — and this one works in conjunction with -fdump-ada-spec[-slim] up there ☝️ — it 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. It spits out Go "const", "type", "var", and "func" declarations, which can be a genuinely useful way to start writing a Go interface to code written in some other language. multilingual queen 🌈
• @file — reads command-line options from file. The options that get read are inserted in place of the original @file option. And if file doesn't exist, or can't be read? Then the option is treated literally and is not removed. 🙈

Options inside file are separated by whitespace. A whitespace character can be included in an option by surrounding the entire option in either single or double quotes. Any character (including a backslash) can be included by prefixing that character with a backslash. And plot twist: the file may itself contain additional @file options — any such options get processed recursively. it's giving inception ngl 💀

---

💻 Compiling C++ Programs 💻

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

GCC recognizes files with these names and compiles them as C++ programs, even if you call the compiler the exact same way you'd call it for compiling C programs (usually with the name gcc). sneaky but slay 🎀

However! The use of gcc does not add the C++ library. No ma'am.

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 under the name c++. 👑

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

---

🦋 Options Controlling C Dialect 🦋

The following options control the dialect of C (or languages derived from C, like C++, Objective-C and Objective-C++) that the compiler accepts. vibe check for your whole language, basically:

• -ansi — in C mode, this is equivalent to -std=c90. In C++ mode, it's equivalent to -std=c++98. that's it, that's the tweet 💖
• -std= — determines the language standard. Just a heads up: this option is currently only supported when compiling C or C++. ✨

The compiler can accept several base standards, like c90 or c++98, plus the GNU dialects of those standards, like gnu90 or gnu++98. When you specify a base standard, the compiler accepts all programs following that standard plus those using GNU extensions that don't contradict it. Example bestie: -std=c90 turns off certain GCC features that are incompatible with ISO C90, like the "asm" and "typeof" keywords — but not other GNU extensions that don't have a meaning in ISO C90, like omitting the middle term of a "??" expression... ok wait, like omitting the middle term of a "?:" expression. 🫶

On the other hand, when you specify a GNU dialect of a standard, 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 also gets 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. iykyk 👀

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

• c90, c89, iso9899:1990 — supports all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
• iso9899:199409 — ISO C90 as modified in amendment 1.
• c99, c9x, iso9899:1999, iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly, but not entirely, relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/c99status.html> for more info. The names c9x and iso9899:199x are deprecated. 💀
• c11, c1x, iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. This one is substantially completely supported too, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
• c17, c18, iso9899:2017, iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is the same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of "__STDC_VERSION__", and so it's supported to the same extent as C11. 🌸
• c23, c2x, iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated.
• c2y — the next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. ⚠️
• gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features).
• gnu99, gnu9x — GNU dialect of ISO C99. The name gnu9x is deprecated.
• gnu11, gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated.
• gnu17, gnu18 — GNU dialect of ISO C17.
• gnu23, gnu2x — GNU dialect of ISO C23. This is the default for C code. 👑 The name gnu2x is deprecated.
• gnu2y — the next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The name gnu2x is deprecated.
• c++98, c++03 — the 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
• gnu++98, gnu++03 — GNU dialect of -std=c++98.
• c++11, c++0x — the 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
• gnu++11, gnu++0x — GNU dialect of -std=c++11. The name gnu++0x is deprecated.
• c++14, c++1y — the 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
• gnu++14, gnu++1y — GNU dialect of -std=c++14. The name gnu++1y is deprecated.
• c++17, c++1z — the 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
• gnu++17, gnu++1z — GNU dialect of -std=c++17. This is the default for C++ code. 👑 The name gnu++1z is deprecated.
• c++20, c++2a — the 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. 🚨 The name c++2a is deprecated.
• gnu++20, gnu++2a — GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2a is deprecated.
• c++23, c++2b — the 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases. The name c++2b is deprecated.
• gnu++23, gnu++2b — GNU dialect of -std=c++23. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2b is deprecated.
• c++2c, c++26 — the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 😭
• gnu++2c, gnu++26 — GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 🙈

• -aux-info filename — outputs to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. And just so you know: this option is silently ignored in any language other than C. 🤫

Okay so like, still on that file we were just hyping up — besides the declarations themselves, it also tells you, in comments, where each declaration came from (source file and line, bestie 💅), whether that declaration was implicit, prototyped or unprototyped (that's I, N for new, or O for old, sitting 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 character right after that). And for function definitions, you also get a K&R-style list of arguments followed by their declarations, all tucked inside comments, after the declaration. Iconic.

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

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

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

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

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

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

• -fcond-mismatch — allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++ 🙈.
• -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
• -fgimple — enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes 👑
• -fgnu-tm — when the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature 👀.

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

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

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

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

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

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

• -fhosted — assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
• -flax-vector-conversions — allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code ⚠️ (like, fr fr, it's giving deprecated energy).
• -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.

---

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

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

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

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

Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char.

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

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

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

The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

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

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

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

There are two more levels chillin' in between 0 and 3, and they exist to be nice to older code that leans on the GCC zero-length array extension ([0]) or a one-element array as a flexible array member ([1]). 🌸 So:

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

You can also control this per-field, for a specific trailing array field of a struct, using the strict_flex_array variable attribute. 🎀

And heads up, bestie: the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. They talk to each other. 👀

---

• -fsso-struct=endianness — sets the default scalar storage order of structs and unions to whatever endianness you pick. 💖 The accepted values are big-endian, little-endian and native for the target's native endianness (that's the default). 🦋 Oh and btw, this option is not supported for C++ — nope, not a thing there. 🙈

⚠️ Warning: the -fsso-struct switch makes GCC generate code that is not binary compatible with code generated without it — if the endianness you specified isn't the native endianness of the target. So like, know what you're doing. 😭

---

✨ Options Controlling C++ Dialect ✨

Okay so this section is all about the command-line options that are only meaningful for C++ programs. 💜 But also — you can still use most of the GNU compiler options no matter what language your program is written in, so don't stress. 🌈 For example, you might compile a file firstClass.C like this:

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

See how in that example only -fstrict-enums is an option meant only for C++ programs? The rest? Totally fine with any language GCC supports. 💅

Also, some options for compiling C programs — like -std — are relevant for C++ programs too. 🫶

And here is the list of options that are only for compiling C++ programs: ⭐

• -fabi-version=n — use version n of the C++ ABI. The default is version 0. 👑

Version 0 refers to the version conforming most closely to the C++ ABI specification. Which means the ABI you get from version 0 will change across different versions of G++ as ABI bugs get fixed. 💀

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 it was the default through G++ 4.9.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.

Version 7, which first appeared in G++ 4.8, treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

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

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

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

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

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

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

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

Version 15, which first appeared in G++ 10.3, corrects a 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 also dependent operator names.

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

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

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

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

See also -Wabi. 💖

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

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

If this option isn't provided but -Wabi=n is, that version gets used for compatibility aliases. If this option is provided along with -Wabi (without the version), then the version from this option is what's used for the warning. ✨

• -fno-access-control — turns off all access checking. 💅 This switch is mainly useful for working around bugs in the access control code. (iykyk, sometimes you just gotta get around it.)
• -faligned-new — enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. 😌 You can pass a numeric argument like -faligned-new=32 to specify how much alignment (in bytes) that function provides, but fr fr, few users will ever 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 so here's the lore: the C++ standard allows replacing the global new, new[], delete and delete[] operators, though a whole lot of C++ programs don't replace them and just use the implementation-provided version. 🌸 On top of that, the C++ standard allows omitting those calls if they're made from new or delete expressions (and by extension, the same is assumed if __builtin_operator_new or __builtin_operator_delete functions are used). This option lets you control 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 (i.e. global state — except for errno for the new and new[] operators). 🫶 This allows most optimizations across those calls, and it's something the implementation-provided operators satisfy unless malloc implementation details are observable in the code or unless malloc hooks are used — but it might not be satisfied if a program replaces those operators. This behavior is enabled by default. 💅

With -fno-assume-sane-operators-new-delete, GCC must assume all these calls (whether from new or delete expressions, or called directly) may read and write global state unless proven otherwise (like when GCC compiles their implementation). So use this option if those operators are — or might be — replaced and your code needs to expect that behavior. 🙈

• -fchar8_t
• -fno-char8_t — enable support for char8_t as adopted for C++20. 🎀 This includes the addition of a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros. ✨

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

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

and introduces new signatures for user-defined literals:

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

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

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

• -fcheck-new — bestie, this one makes the compiler double-check that the pointer <cyan>operator new</cyan> hands back is non-null before it starts poking at the storage it allocated. 💅 Ngl tho, this check is usually doing the absolute most for no reason, because the C++ standard literally says operator new only returns 0 if it's declared throw(), and in that case the compiler always checks the return value anyway, even without this flag. In literally every other case, when operator new has a non-empty exception specification, running out of memory doesn't return null, it throws std::bad_alloc. 🌸 Peep new (nothrow) for the alternative vibe.
• -fconcepts — turns on support for the C++ Concepts feature, aka constraining template arguments. ✨ With -std=c++20 and above, Concepts are just part of the language standard, so -fconcepts defaults to on. Periodt.

Some constructs that the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), allowed but that didn't make the cut into the standard, could additionally be enabled by -fconcepts-ts. Sad news tho: -fconcepts-ts was deprecated in GCC 14 and straight-up removed in GCC 15, so users are expected to convert their code to C++20 concepts. 😭 It's giving glow-up era.

• -fconstexpr-depth=n — sets the maximum nested evaluation depth for C++11 constexpr functions to n. 🌀 We need a limit here so the compiler can catch endless recursion while it's doing constant expression evaluation. The minimum the standard specifies is 512.
• -fconstexpr-cache-depth=n — sets the max level of nested evaluation depth for C++11 constexpr functions that gets cached to n. 🫶 This is a heuristic situation: it trades compilation speed (when the cache is avoiding repeated calculations, slay) against memory consumption (when the cache gets huge from super recursive evaluations, not slay). The default is 8. Very few users are gonna want to touch this, but if your code does heavy constexpr calculations you might wanna experiment to find which value works best for you. 🧪
• -fconstexpr-fp-except — Annex F of the C standard says that IEC559 floating point exceptions encountered at compile time should not stop compilation. But C++ compilers have historically not followed this guidance, and instead treated floating point division by zero as non-constant even though it has a well defined value. 🥲 This flag tells the compiler to give Annex F priority over other rules saying a particular operation is undefined.

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

• -fconstexpr-loop-limit=n — sets the max number of iterations for a loop in C++14 constexpr functions to n. 🔁 Again, we need a limit so infinite loops get caught during constant expression evaluation. The default is 262144 (1<<18).
• -fconstexpr-ops-limit=n — sets the max number of operations during a single constexpr evaluation. 🚧 Even when the number of iterations of a single loop is limited by the limit above, if there are several nested loops and each one has many iterations (but still under that limit), or if in the body of some loop — or even outside a loop — too many expressions need to be evaluated, the resulting constexpr evaluation might take way too long. The default is 33554432 (1<<25).
• -fcontracts — enables experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). 👑 The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. Real talk: this functionality is intended mostly for those interested in experimentation towards refining the feature and getting it into shape for a future C++ standard. It's giving mad scientist energy, iykyk.

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

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

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

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

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

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

• -fcoroutines — enables support for the C++ coroutines extension (experimental). 🦋
• -fdiagnostics-all-candidates — permits the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected. 🕵️ It's giving receipts.
• -fno-elide-constructors` — the C++ standard allows an implementation to omit creating a temporary that's only used to initialize another object of the same type. Specifying this option disables that optimization and forces G++ to call the copy constructor in all cases. 😤 This option also causes G++ to call trivial member functions that 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** — doesn't generate code to check for violation of exception specifications at run time. ⚠️ Heads up bestie, this option **violates the C++ standard**, but it may be useful for reducing code size in production builds, much like defining NDEBUG`. This does **not give user code permission to throw exceptions in violation of the exception specifications — the compiler still optimizes based on the specs, so throwing an unexpected exception results in undefined behavior at run time. 💀 Not me letting you do this without a warning.
• -fextern-tls-init
• **-fno-extern-tls-init** — the C++11 and OpenMP standards allow thread_localandthreadprivatevariables to have dynamic (runtime) initialization. To support that, any use of such a variable goes through a wrapper function that performs any necessary initialization. 🎀 When the use and the definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there's significant overhead — even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the-fno-extern-tls-init` option.

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

• -ffold-simple-inlines
• **-fno-fold-simple-inlines** — permits the C++ frontend to fold calls to std::move, std::forward, std::addressofandstd::as_const. ✨ Unlike 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** — doesn't recognize typeofas 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` — doesn't enable immediate function escalation, whereby certain functions can be promoted to consteval, as specified in P2564R3. 🌀 For example:

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

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

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

compiles in C++20: f is an immediate-escalating function (because of the auto it's a function template, and it's declared constexpr) and id(t) is an immediate-escalating expression, so f gets promoted to consteval. Consequently, the call to id(t) is in an immediate context, so it doesn't have to produce a constant — that's the mechanism that allows consteval function composition. 🧚 However, with -fno-immediate-escalation, f is not promoted to consteval, and since the call to consteval function id(t) 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` — makes inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. 🌟 This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static, plus other optimizations. Slay.
• -fno-implicit-templates — bestie, this is the no sneaky implicit instantiations era 💅. 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. So like, plan your instantiations, okay? No cap.
• -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. It's giving consistency queen ✨.
• -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. So like, space-saving but risky? The compiler said "you break it, you buy it" 💀.
• -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. So you gotta ask for it directly, bestie 🫶.
• -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file to create an importable header unit. That's it, that's the vibe. 📦
• -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.) So like, all in or all out, no halfsies 💖.
• -fno-module-lazy — disable lazy module importing and module mapper creation. No laziness today, bestie 😤.
• -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. It's giving mystical module mapping oracle 🔮.
• -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file. Just the interface, not the object file. Main character energy only 👑.
• -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. Microsoft extensions said "let me live" 🙈.
• -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. So like, inheriting constructors but make it new ✨.
• -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. Template template matching, slay 💅.
• -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. So like, if it's not ANSI/ISO C, it's not built in. Periodt.
• -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. So like, throw() but make it noexcept energy 🔥.
• -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. So like, the keywords are not synonyms. No cap.
• -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. Optional drama? Disabled. 🙅‍♀️
• -fno-pretty-templates — when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so that it's clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them. So like, if pretty templates are too pretty, turn them off 💅.
• -frange-for-ext-temps — enable lifetime extension of C++ range based for temporaries. With -std=c++23 and above this is part of the language standard, so lifetime of the temporaries is extended until the end of the loop by default. This option allows enabling that behavior also in earlier versions of the standard. Range-based for said "I'll keep your temporaries alive" 🫶.
• -fno-rtti — disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.

Mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. So like, don't mix your RTTI and no-RTTI besties, it's messy 😭.

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

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

as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition. Sized deallocation said "I know how big you are" 💅.

• -fstrict-enums — allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration type with a fixed underlying type. So like, strict enums, no funny business. ⚠️
• -fstrong-eval-order, -fstrong-eval-order=kind — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. -fstrong-eval-order is equivalent to -fstrong-eval-order=all, and is enabled by default with -std=c++17 or later.
  • -fstrong-eval-order=some — enables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.
  • -fstrong-eval-order=none — is equivalent to -fno-strong-eval-order.

Order of evaluation? We know her. She's strong 💪.

• -ftemplate-backtrace-limit=n — set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. So like, template error backtrace said "only 10 notes, bestie" 📝.
• -ftemplate-depth=n — set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. So like, default 900 because 1024 might eat your stack 💀.
• -fno-threadsafe-statics — do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe. If you don't need thread safety, save those bytes, bestie 💖.
• -fuse-cxa-atexit — register destructors for objects with static storage duration with the __cxa_atexit function rather than the atexit function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports __cxa_atexit. So like, __cxa_atexit is the moment, but only if your C library is invited 🫶.
• -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but is necessary if the runtime routine is not available. So like, if the routine isn't there, you gotta do what you gotta do, but std::uncaught_exception will be lying 😭.
• -fvisibility-inlines-hidden — this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

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

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

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

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

Hidden inline visibility? It's giving "don't export me, I'm shy" 🙈.

• -fvisibility-ms-compat — okay bestie, this one is a whole vibe: it tries to use visibility settings to make GCC's C++ linkage model play nice with Microsoft Visual Studio's. 💅

Here's the tea — it makes these changes to GCC's linkage model:

1. It sets the default visibility to "hidden", exactly like -fvisibility=hidden does.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule gets a glow-up (relaxed 💆) for types without explicit visibility specifications that are defined in more than one shared object — those declarations are permitted if they'd be permitted when this option isn't used.

So in new code it is lowkey better to use -fvisibility=hidden and export just those classes that are meant to be externally visible. 🙈 Unfortunately though, it's possible for code to rely — perhaps accidentally — on the Visual Studio behavior. So like. be careful.

Among the consequences of these changes are:

• static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other 😳
• pointers to function members defined in different shared objects may not compare equal.

When this flag is given, it is a violation of the ODR to define types with the same name differently. periodt.

• -fno-weak — nope, do NOT use weak symbol support, even if the linker is offering it to you on a silver platter. By default, G++ uses weak symbols if they're available. But bestie… this option exists only for testing and should not be used by end-users. It results in inferior code and has no benefits. 💀 This option may be removed in a future release of G++.
• -fext-numeric-literals (C++ and Objective-C++ only) — lets you accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off, those suffixes get treated as C++11 user-defined literal numeric suffixes instead. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. It's off by default for ISO C++11 onwards (-std=c++11, ...). 👑
• -nostdinc++ — do NOT search for header files in the standard directories that are 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 — these three are here to inform you about include translation events ✨. The first one notes accepted include translations, the second notes declined include translations. The header form informs you about include translations relating to that specific header. If header is of the form "user" or "<system>", it will be resolved to a specific user or system header using the include path.
• -flang-info-module-cmi
• -flang-info-module-cmi=module — these inform you about Compiled Module Interface pathnames 🫶. The first one notes all read CMI pathnames. The module form will not reading a specific module's CMI. And heads up: 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 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 bestie, 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. 🏷️
• -Wcomma-subscript (C++ and Objective-C++ only) — warns about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23 😭. However, a comma expression wrapped in "( )" is not deprecated. Example:

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

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

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

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

• -Wctad-maybe-unsupported (C++ and Objective-C++ only) — warns when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning points out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended 🙃. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types 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. 🚫
• -Wdangling-reference (C++ and Objective-C++ only) — warns when a reference is bound to a temporary whose lifetime has ended. Like… ghosted. 👻 For example:

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

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

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

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

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

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

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

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

-Wdangling-reference also warns about code like

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

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

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

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

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

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

This warning is enabled by -Wextra. 🌈

• -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — warns when "delete" is used to destroy an instance of a class that has virtual functions and a non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class doesn't have a virtual destructor. 💀 This warning is enabled by -Wall.
• -Wdeprecated-copy (C++ and Objective-C++ only) — warns that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate 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 <span style="color:pink">(C++ and Objective-C++ only)</span> — okay so bestie, this one's gonna warn you that declaring a user-defined literal operator with a space before the suffix is, like, deprecated 💅. It's on by default in C++23, or whenever you explicitly pass -Wdeprecated.

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

See that gap? the space is the crime. Close the gap, heal the code 🫶

• -Wdeprecated-variadic-comma-omission <span style="color:pink">(C++ and Objective-C++ only)</span> — warn that ditching the comma before the varargs ... at the end of a function parameter list is deprecated. Enabled by default in C++26, or with explicit -Wdeprecated.

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

Lowkey the T...... one is giving chaos 😭 comma where u at

• -Wno-elaborated-enum-base — for C++11 and up, this warns if an (invalid!) extra enum-base shows up in an elaborated-type-specifier. Translation: if an enum with a given underlying type and no enumerator list gets used in a declaration that's anything other than just a standalone declaration of that enum. Enabled by default. And heads up, this warning gets upgraded to an error with -pedantic-errors. 🚨
• -Wno-init-list-lifetime <span style="color:pink">(C++ and Objective-C++ only)</span> — do not warn about uses of std::initializer_list that are probably gonna end in dangling pointers. Because the underlying array for an initializer_list is handled like a normal C++ temporary object, it's so easy to accidentally hold a pointer to that array past the end of the array's lifetime. Like:
  • If a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement — so the value you 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 gets 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 by the very next statement, the initializer_list variable has a dangling pointer. Not me writing undefined behaviour on line three 😭

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

• When a list constructor stores the begin pointer from the initializer_list argument, that does not extend the 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 had to be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed that restriction, so it's fully possible to have a function or function template marked constexpr for which no invocation satisfies the requirements of a core constant expression. Wild, right?

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

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

• -Winvalid-imported-macros — verify all imported macro definitions are valid at the end of compilation. It is not enabled by default, since it needs extra processing to figure out. Could be useful when you're preparing sets of header-units and you wanna make sure the macros stay consistent ✨
• -Wno-literal-suffix <span style="color:pink">(C++ and Objective-C++ only)</span> — do 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, to keep backwards compatibility with code that uses formatting macros from <inttypes.h>. Like:

#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, bestie 🧚

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

These warnings are enabled by default.

• -Wno-narrowing <span style="color:pink">(C++ and Objective-C++ only)</span> — for C++11 and later standards, narrowing conversions are diagnosed by default, as the standard requires. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but -Wno-narrowing suppresses the diagnostic. Important: this does not affect the meaning of well-formed code — narrowing conversions are still considered ill-formed in SFINAE contexts.

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

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

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

• -Wnoexcept <span style="color:pink">(C++ and Objective-C++ only)</span> — warn when a noexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception.
• -Wnoexcept-type <span style="color:pink">(C++ and Objective-C++ only)</span> — warn if the C++17 feature that makes noexcept part of a function type changes the mangled name of a symbol relative to C++14. Enabled by -Wabi and -Wc++17-compat.

Example time:

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

In C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept>. The vibes shifted, the symbol changed 😭

• -Wclass-memaccess <span style="color:pink">(C++ and Objective-C++ only)</span> — warn when the destination of a call to a raw memory function like memset or memcpy is an object of class type, and when writing into that object might bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to memset below is undefined because it modifies a non-trivial class object, and so it gets diagnosed. The safe way to initialize or clear the storage of objects like these is using 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. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning.

• -Wnon-virtual-dtor <span style="color:pink">(C++ and Objective-C++ only)</span> — warn when a class has virtual functions and an accessible non-virtual destructor, 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. The -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred, because it warns about the unsafe cases without false positives. Periodt.
• -Wregister <span style="color:pink">(C++ and Objective-C++ only)</span> — warn on uses of the register storage class specifier, except when it's part of the GNU Explicit Register Variables extension. Using the register keyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17. It's giving legacy 💀
• -Wreorder <span style="color:pink">(C++ and Objective-C++ only)</span> — warn when the order of member initializers written in the code doesn't match the order in which they must actually be executed. For instance:

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

The compiler rearranges the member initializers for i and j to match the declaration order of the members, and emits a warning to that effect. This warning is enabled by -Wall. Your init list is a suggestion, bestie 🙈

• -Wno-pessimizing-move <span style="color:pink">(C++ and Objective-C++ only)</span> — this warning warns when a call to std::move prevents copy elision. A typical scenario where copy elision can occur is returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.

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

But in this example, the std::move call prevents copy elision. 😭 You tried to help and you made it worse.

This warning is enabled by -Wall.

• -Wno-redundant-move <span style="color:pink">(C++ and Objective-C++ only)</span> — this warning warns about redundant calls to std::move; like, when a move operation would've been performed even without the std::move call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:

okay so remember how we were doing -Wredundant-move? the tea continues. so like, look at this:

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

bestie. the std::move in there? literally redundant. 💀 And because G++ implements Core Issue 1579, here's another little example for u:

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

so in that one, copy elision isn't applicable because the type of the expression being returned and the function return type differ — and yet G++ treats the return value as if it were designated by an rvalue. the way that... okay anyway.

This warning is enabled by -Wextra. ✨

---

• -Wrange-loop-construct (C++ and Objective-C++ only) — this one snitches when a C++ range-based for-loop is out here making an unnecessary copy. that happens when the range declaration isn't a reference, but honestly probably should be. like:

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

it stays quiet tho when the type being copied is a trivially-copyable type whose size is less than 64 bytes. so it's not out here being extra for no reason.

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

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

in that example above, every single iteration of the loop a temporary value of type double gets created and destroyed, and the reference const double & gets bound to it. every. iteration. 🙈

this warning is enabled by -Wall. 🫶

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

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

but on the other hand, in this next one there's no warning:

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

see how the keyword is doing real work there?? iconic. 👑

• -Wno-subobject-linkage (C++ and Objective-C++ only) — do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. ok so listen: if a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation, because the meaning of B is different in each translation unit. if A only shows up in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace too. 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 btw. 💖
• -Weffc++ (C++ and Objective-C++ only) — warns about violations of these style guidelines from Scott Meyers' Effective C++ series of books:
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have operator= return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload &&, ||, or ,.

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

when u select this option, be aware that the standard library headers do not obey all of these guidelines — so use grep -v to filter out those warnings. fr fr, otherwise ur terminal is gonna be a mess.

• -Wno-exceptions (C++ and Objective-C++ only) — disables the warning about the case where an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers. lowkey a helpful one ngl. 😭
• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about the use of an uncasted NULL as a sentinel. when compiling only with GCC this is a valid sentinel, because NULL is defined to __null. even though it's a null pointer constant rather than a null pointer, it's guaranteed to be of the same size as a pointer. but this use is not portable across different compilers. so like, careful bestie. ⚠️
• -Wno-non-template-friend (C++ and Objective-C++ only) — disables warnings when non-template friend functions are declared within a template. in very old versions of GCC that predate implementation of the ISO standard, declarations like friend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function. the warning exists to diagnose compatibility problems, and it's enabled by default. vintage drama, love that for us. 🎀
• -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used within a C++ program. the new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for. so like... glow up ur casts. ✨
• -Woverloaded-virtual (C++ and Objective-C++ only)
• -Woverloaded-virtual=n — warns when a function declaration hides virtual functions from a base class. for example, in:

struct A {
  virtual void f();
};

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

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

B* b;
b->f();

fails to compile. 💀

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

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

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

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

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

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

• -Wno-pmf-conversions (C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer to member function to a plain pointer. that's it, that's the whole thing. 🦋
• -Wsign-promo (C++ and Objective-C++ only) — warns when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. the standard said what it said. periodt. 💅
• -Wtemplates (C++ and Objective-C++ only) — warns when a primary template declaration is encountered. some coding rules disallow templates, and this may be used to enforce that rule. the warning is inactive inside a system header file, such as the STL, so one can still use the STL. one may also instantiate or specialize templates. so like, ur still fine bestie. 🌸
• -Wmismatched-new-delete (C++ and Objective-C++ only) — warns for mismatches between calls to operator new or operator delete and the corresponding call to the allocation or deallocation function. this includes invocations of C++ operator delete with pointers returned from either mismatched forms of operator new, or from other functions that allocate objects for which the operator delete isn't a suitable deallocator, as well as calls to other deallocation functions with pointers returned from operator new for which the deallocation function isn't suitable.

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

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

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

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

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

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

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

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

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

• -Wmultiple-inheritance (C++ and Objective-C++ only) — bestie, this one yells at you when a class is defined with more than one direct base class. Some coding standards are like "no multiple inheritance ever" and this is how you enforce that. ✨ And it's chill — the warning is inactive inside system header files like the STL, so you can still use the STL, no problem. You can also totally define classes that use multiple inheritance indirectly, it won't say a word about those.
• -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Same deal as above: some coding rules ban multiple inheritance and this is your enforcer. 🫶 Inactive inside system header files such as the STL, so STL stays usable, and classes that use virtual inheritance indirectly are fine too.
• -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base that has a non-trivial C++11 move assignment operator. And okay this one is dangerous, fr fr: 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. 😭 So if your move assignment operator is written to avoid moving from a moved-from object, then yeah, you can disable this warning.
• -Wnamespaces — warns whenever a namespace definition is opened. Some coding rules are anti-namespace and this enforces it. Inactive inside system header files like the STL (so you can still use the STL 🌸), and using directives and qualified names are still fine to use.
• -Wno-template-body (C++ and Objective-C++ only) — turns off diagnosing errors when parsing a template, and instead issues the error only upon instantiation of the template. You can also use this to downgrade those errors into warnings with Wno-error= or -fpermissive.
• -Wno-template-id-cdtor (C++ and Objective-C++ only) — kills the warning about using a simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. Like, for example:

template<typename T> struct S {
  S<T>(); // should be S();
  ~S<T>();  // should be ~S();
};

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

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

So the tea is: this can cause variable or function templates to accidentally become unusable if they reference such an entity, because other translation units that import the template will never ever be able to instantiate it. 😭 This warning tries to catch the cases where that might happen. And the presence of an explicit instantiation silences the warning. This flag is enabled by -Wextra.

• -Wno-terminate (C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to terminate. 💀
• -Wno-vexing-parse (C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity, aka when a declaration looks like a variable definition but the C++ language requires it to be interpreted as a function declaration. Peak C++ behavior honestly. For instance:

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

Another example:

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

The warning will suggest options for how to deal with such an ambiguity — e.g., it can suggest removing the parentheses or using braces instead. 🎀 This warning is enabled by default.

• -Wno-class-conversion (C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called anyway, so like, why bother. 🙈
• -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. What counts, you ask? ✨ 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, periodt. Enabled by default with -std=c++20; before -std=c++20, enabled with explicit -Wdeprecated.
• -Waligned-new — warns about a new-expression of a type that requires greater alignment than alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall. 👑

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

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

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

This warning is enabled by default.

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

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

• -Wplacement-new=2 — at this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of a structure whose type is an array of a single element and whose size is less than the size of the object being constructed. So the previous example would get diagnosed 🫢 — but 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 don't catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short) it warns about polymorphic class types that are caught by value. With -Wcatch-value=2 it warns about all class types that are caught by value. With -Wcatch-value=3 it warns 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 constructs (C++11 [intro.defs]).
• -Wno-defaulted-function-deleted (C++ and Objective-C++ only) — warns when an explicitly defaulted function is deleted by the compiler. That can happen when the function's declared type doesn't 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 an 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 where an extra semicolon can just... show up. 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 (so it 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 (and 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 doesn't consist only of preprocessor directives.
• -Wno-inaccessible-base (C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the -Wextra option. 🎀

struct A { int a; };

struct B : A { };

struct C : B, A { };

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

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

• -Wsized-deallocation (C++ and Objective-C++ only) — bestie this one is a matching set warning 💅 it yells at you when you've defined an unsized deallocation function

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

but you haven't defined the corresponding sized one, like

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

…or the other way round (sized without unsized 👀). Enabled by -Wextra along with -fsized-deallocation.

• -Wsuggest-final-types — warns about types that have virtual methods and would just be so much better if you slapped the C++11 final specifier on them, or (if you can swing it) declared them inside an anonymous namespace 🎀 that lets GCC go way harder at devirtualizing the polymorphic calls. Pro tip: this warning hits different with link-time optimization, because then the info about the class hierarchy graph is more complete ✨
• -Wsuggest-final-methods — same energy but for virtual methods: it warns when declaring the method with the C++11 final specifier would improve code quality, or when its type could be declared in an anonymous namespace or with the final specifier instead 🦋 Also more effective with link-time optimization, where again the class hierarchy graph info is more complete. The recommended order is: deal with the -Wsuggest-final-types suggestions first, then rebuild with your new annotations 💜
• -Wsuggest-override — warns about overriding virtual functions that forgot their override keyword 😭 like come ON bestie label your overrides
• -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. Btw -Wconversion-null is enabled by default, so this is the "okay actually stop telling me" switch 🙈

---

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

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

This section is all about the command-line options that are only meaningful for Objective-C and Objective-C++ programs 🌸 (you can still use most of the language-independent GNU compiler options, don't worry). Say you wanna compile a file some_class.m, you'd do:

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

See how -fgnu-runtime in that example is an option meant only for Objective-C and Objective-C++ programs? Yeah — the other options work with any language GCC supports 💖

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

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

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string 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, bestie). 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. Oh and heads up: -fconstant-cfstrings is an alias for the target-specific -mconstant-cfstrings equivalent 💅
• -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems 🌈
• -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin / macOS 🍓 The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used.
• -fno-nil-receivers — assume that every Objective-C message dispatch ([receiver message:arg]) in this translation unit guarantees the receiver is not nil 🫶 which lets the runtime use more efficient entry points. 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 only supported for the NeXT runtime 🦋 In that case:
  • Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions
  • Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions
  • Version 2 is the modern (64-bit) ABI

If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines 👑

• -fobjc-call-cxx-cdtors — for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor 👀 If so, synthesize a special - (id) .cxx_construct instance method that runs those 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 that 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 those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy 🧚 The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.

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

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher 🔥 On Darwin this is accomplished via the comm page.
• -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. Available with both the GNU runtime and the NeXT runtime — but not 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 ♻️ Only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags.
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call ✅ This is the default, and you can turn it off with -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, which is controlled by the separate C/C++ dialect option flags. When you use this 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 compiles 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, without needing to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later.
• -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance 🔥 Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. 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 🙈 which can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying -fno-local-ivars disables this behavior and thus avoids those 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 the specified 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 <span>—</span> (Objective-C and Objective-C++ only) bestie listen: when a class says it implements a protocol, GCC throws a warning for every single method in that protocol the class hasn't implemented. And the default vibe? It warns for every method that isn't explicitly implemented in the class, even if the superclass is out here passing down a perfectly good implementation, like, rude honestly. Flip on -Wno-protocol and anything inherited from the superclass counts as implemented, so no warning for it. Generous queen behavior. 💅
• -Wobjc-root-class <span>—</span> (Objective-C and Objective-C++ only) ngl this one yells at you if a class interface is missing a superclass. Most classes inherit from something — NSObject (or Object) for example — so if you see this warning you're probably like "wait, I meant to do that". No worries: for classes that are genuinely meant to be root classes, you suppress the warning by marking their interfaces with __attribute__((objc_root_class)). Iconic workaround, no cap. ✨
• -Wselector <span>—</span> (Objective-C and Objective-C++ only) warns you when multiple methods of different types for the same selector get found during compilation. The check happens on the method list in the final stage of compilation — like, the very end, the finale, the last song of the set. On top of that, a check runs for each selector that shows up in a @selector(...) expression, and a corresponding method for that selector has been found during compilation. BUT. ⚠️ Because these checks scan the method table only at the end of compilation, you get no warnings at all if the final stage never happens — say an error was found during compilation, or you're using the -fsyntax-only option. So don't @ me when the warnings ghost you. 😭
• -Wstrict-selector-match <span>—</span> (Objective-C and Objective-C++ only) warns when multiple methods with differing argument and/or return types turn up for a given selector while you're trying to send a message using that selector to a receiver of type id or Class. And here's the tea when this flag is off (which, btw, is the default behavior): the compiler just skips those warnings entirely if any differences found are confined to types that share the same size and alignment. Chill compiler energy. 🦋
• -Wundeclared-selector <span>—</span> (Objective-C and Objective-C++ only) warns when a @selector(...) expression refers to an undeclared selector. A selector counts as undeclared if no method with that name has been declared before the @selector(...) expression — whether explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks the moment a @selector(...) expression is found, whereas -Wselector only does its checks in the final stage of compilation. It also enforces that coding style convention that methods and selectors must be declared before being used. Structure, bestie. 🎀
• -print-objc-runtime-info <span>—</span> generates a C header describing the largest structure that is passed by value, if any. That's it, that's the job. 👑

---

Options Controlling OpenMP and OpenACC 👑🌈

GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option. Similarly, OpenACC extensions are supported in all three languages with -fopenacc. Multilingual legend behavior, fr fr. ✨

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

Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES. Peek at the roster, you'll see who's on the team. 💜

• -foffload-options=options
• -foffload-options=target-triplet-list=options — with -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.

Typical command lines are

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

Look at her, so precise, so demure. 🌸

• -fopenacc — enables handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran and !$acc, c$acc and *$acc in fixed-form Fortran. This option implies -pthread, and thus is only supported on targets that have support for -pthread. No pthread, no party. 💀
• -fopenacc-dim=geom — specifies default compute dimensions for parallel offload regions that don't explicitly specify them. The geom value is a triple of :-separated sizes, in order gang, worker, and vector. A size can be omitted, to use a target-specific default value. Omit a size and let the target choose — main character lets the universe decide sometimes. ✨
• -fopenmp — enables handling of OpenMP directives #pragma omp, [[omp::directive(...)]], [[omp::decl(...)]], and [[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and the conditional compilation sentinel !$. In fixed source form Fortran, the sentinels can also start with c or *.

This option implies -pthread, and thus is only supported on targets that have support for -pthread. Also: -fopenmp implies -fopenmp-simd. Chain reaction, she's networking. 🔥

• -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 are ignored — not invited to the function, sorry babes. Unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause.
• -fopenmp-target-simd-clone
• -fopenmp-target-simd-clone=device-type — on top of generating SIMD clones for functions marked with the declare simd directive, GCC also generates clones for functions marked with the OpenMP declare target directive that are suitable for vectorization when this option is in effect. The device-type may be one of none, host, nohost, and any, which correspond to keywords for the device_type clause of the declare target directive; clones are generated for the intersection of devices specified. -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 is disabled by default. She's off by default, she turns on at -O2, she's got a whole schedule. 💅

---

Options to Control Diagnostic Messages Formatting 🎀💬

Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how many characters per line, how often source location information should be reported. Heads up though: some language front ends may not honor these options. Rude but true. 🙈

• -fmessage-length=n — tries to format error messages so that 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. This is the default for all front ends.

Quick note (and it's kind of a big one): 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. Different pipelines, bestie, iykyk. ✨

• -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. In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. Forward-thinking queen. 🧚
• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. 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. Say it once, say it with your chest. 💅
• -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 label. No line left behind. 🫶
• -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. 🌈✨

okay bestie, so the colors are all 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. like, it's giving terminal rainbow, fr fr 🌈

The default GCC_COLORS is

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

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Set GCC_COLORS to the empty string and the colors are GONE — lights off, periodt 💀 Supported capabilities are as follows.

• error= — SGR substring for error: markers. the red-flag energy 🚩
• warning= — SGR substring for warning: markers.
• note= — SGR substring for note: markers.
• path= — SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, such as the identifiers of individual events and lines indicating interprocedural calls and returns. ~the plot thickens~
• range1= — SGR substring for first additional range.
• range2= — SGR substring for second additional range.
• locus= — SGR substring for location information, file:line or file:line:column etc. where in the code we at, bestie 📍
• quote= — SGR substring for information printed within quotes.
• fnname= — SGR substring for names of C++ functions.
• targs= — SGR substring for C++ function template parameter bindings.
• fixit-insert= — SGR substring for fix-it hints suggesting text to be inserted or replaced. the "here, let me help you" energy ✨
• fixit-delete= — SGR substring for fix-it hints suggesting text to be deleted.
• diff-filename= — SGR substring for filename headers within generated patches.
• diff-hunk= — SGR substring for the starts of hunks within generated patches.
• diff-delete= — SGR substring for deleted lines within generated patches.
• diff-insert= — SGR substring for inserted lines within generated patches.
• type-diff= — SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
• valid= — SGR substring for highlighting valid elements within text art diagrams.
• invalid= — SGR substring for highlighting invalid elements within text art diagrams.
• highlight-a=, highlight-b= — SGR substrings for contrasting two different things within diagnostics, such as a pair of mismatching types. See -fdiagnostics-show-highlight-colors. it's giving "opposites attract" but make it diagnostics 💅

---

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

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. not the garbage on your screen 😭 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. shy diagnostics arc 🙈
• -fno-diagnostics-show-caret — By default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
• -fno-diagnostics-show-labels — By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:

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

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

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

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

This option suppresses the printing of such connector lines. no more vibe-lines connecting the drama 😌

• -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. matching fits = same color, mismatches = different color, the way that color-coordinates the lore 💖

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. yes bestie, six, no cap ✨
• -fdiagnostics-parseable-fixits — bestie, this one makes GCC spit out its fix-it hints in a machine-parseable format, the kind your IDE can just eat up. ✨ For every fix-it, it prints a line right after the relevant diagnostic, and that line starts with the string "fix-it:". Like, for example:

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

The location is a half-open range, counted in bytes, and the counting starts at byte 1 for the initial column. So in that example up there, bytes 3 through 20 of line 45 of "test.c" get replaced with the given string, okay:

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

The filename and the replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", and non-printable characters as octal (so like, vertical tab shows up as "\013").

An empty replacement string means that whole range is getting deleted, poof 💀. And an empty range (think "45:3-45:3") means the string gets inserted right at that position. Slay.

• -fdiagnostics-generate-patch — this cutie prints fix-it hints to stderr in unified diff format, and it does it after any diagnostics are printed. Obsessed. For example:

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

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

Fair warning: the diff may or may not be colorized, following the same rules as for diagnostics (peep -fdiagnostics-color). 🎀

• -fdiagnostics-show-template-tree — okay so in the C++ frontend, when it's printing diagnostics about mismatching template types, like this:

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

the -fdiagnostics-show-template-tree flag turns on printing a tree-like structure that shows the common parts and the differing parts of the types, like this:

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

The parts that differ get highlighted with color ("double" and "float" in this case). It's giving ✨可视化✨ and honestly it ate. 💅

• -fno-elide-type — so by default, when the C++ frontend prints diagnostics showing mismatching template types, the common parts of the types get printed as "[...]" to keep the error message simple. 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 (no more hiding bestie 🙈). And heads up, this flag also affects the output of the -fdiagnostics-show-template-tree flag.

• -fdiagnostics-path-format=KIND — this one lets you pick how paths of control-flow events get printed, for diagnostics that have such a path associated with them.

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

• none means: don't print diagnostic paths at all. silence 🤫
• separate-events means: 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: print the events "inline" within the source code. This view tries to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.

So for example, those same events from above might get printed like:

  '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 the stack frames are nested, pushed, and popped. So pretty. 🦋

For example:

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

• -fdiagnostics-show-path-depths — this option hands you extra info when printing control-flow paths that are associated with a diagnostic.

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

Heads up though: this is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. So like, iykyk. 🔧

• -fno-show-column — do not print column numbers in diagnostics. Bestie this may be necessary if your diagnostics are being scanned by a program that doesn't understand the column numbers, such as dejagnu. 😭
• -fdiagnostics-column-unit=UNIT — pick the units for the column number, bestie. This affects traditional diagnostics (when -fno-show-column is absent), and also JSON format diagnostics if you asked for those.

The default UNIT, display, counts the number of display columns each character takes up. This may be bigger than the number of bytes required to encode the character — like with tab characters — or it may be smaller, like with 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 math is mathing but at what cost

Setting UNIT to byte changes the column number to the raw byte count in all cases, which is exactly what GCC traditionally output prior to version 11.1.0. throwback era 💅

• -fdiagnostics-column-origin=ORIGIN — this selects the origin for column numbers, i.e. the column number that gets assigned to the first column. The default value of 1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of 0; any non-negative value may be specified. 👑
• -fdiagnostics-escape-format=FORMAT — okay so when GCC prints the pertinent source lines for a diagnostic, it normally just tries to print the source bytes directly. But some diagnostics relate to encoding issues in the source file, like malformed UTF-8, or issues with Unicode normalization. Those diagnostics get flagged so that GCC will escape bytes that aren't printable ASCII when it prints their pertinent source lines. ✨

This option controls how those bytes should be escaped.

The default FORMAT, unicode, displays Unicode characters that aren't printable ASCII in the form <U+XXXX>, and bytes that don't correspond to a Unicode character validly-encoded in UTF-8-encoded form get 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>, which shows you the underlying encoding of non-ASCII Unicode characters. For the example above, this is what gets printed:

 before<CF><80><BF>after

• -fdiagnostics-text-art-charset=CHARSET — some diagnostics can contain "text art" diagrams: visualizations made out of text, meant to be viewed in a monospaced font. iconic, honestly 🎨✨

This option picks which characters get used for printing those diagrams, if any. CHARSET is none, ascii, unicode, or emoji. 🦋

---

okay so none just ✨suppresses✨ the printing of those diagrams entirely, bestie — no art, no vibes, nothing. ascii makes sure those diagrams are pure ASCII ("ASCII art") 💻, unicode lets you use unicode drawing characters conservatively (think box-drawing characters 🎀), and emoji goes even further and lets emoji show up in the output — like emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character 🫶.

The default is emoji, except when the environment variable LANG is set to C — in that case the default is ascii. iykyk 💅

---

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

Heads up bestie: using this option replaces any additional "output sinks" added by -fdiagnostics-add-output=, or the one set by -fdiagnostics-set-output=. The default is text ✨

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

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

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

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

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

etc.

Schemes, keys, or values with a name prefixed experimental may change or be removed without notice. no cap, that's a real warning 💀

SCHEME can be

• text — emit diagnostics to stderr using GCC's classic text output format. The supported keys are:
  • color=[yes|no] — override colorization settings from -fdiagnostics-color for this text output 🎨
  • experimental-nesting=[yes|no] — enable an experimental mode that emphasizes hierarchical relationships within diagnostics messages, displaying location information on separate lines.
  • experimental-nesting-show-locations=[yes|no] — if experimental-nesting=yes, then by default locations are shown; set this key to no to disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.
  • experimental-nesting-show-levels=[yes|no] — this is a debugging option for use with experimental-nesting=yes. Set this key to yes to print explicit nesting levels in the output. This exists for use by GCC developers.
• sarif — emit diagnostics to a file in SARIF format. The supported keys are:
  • file=FILENAME — specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults to source.sarif.
  • version=[2.1\|2.2-prerelease] — specify the version of SARIF to use for the output. If not specified, defaults to 2.1. 2.2-prerelease uses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release. experimental energy fr fr

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. two files, two versions, one iconic command 🌈

In EBNF:

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

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

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

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

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

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

Use -fno-diagnostics-json-formatting to suppress this whitespace. It must be passed before the option it is to affect — order matters, bestie ⚠️

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. they're not screaming at you, they're just... looking out for you 🫶

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. That's it. That's the whole job 🎀
• -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. priority war, and fatal wins
• -w — inhibit all warning messages. straight up silencing the whole conversation 💀
• -Werror — turn all warnings into errors. every little warning is now a whole crisis, iconic behavior 👑
• -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. asymmetry, we love to see it

• -Wfatal-errors — this option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages. one strike and it's over 😭

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

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

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

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

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

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

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

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

Where the standard you picked with -std is a GNU extended dialect of C — like gnu90 or gnu99 — there's a corresponding base standard, bestie: the version of ISO C that the GNU extended dialect is based on. Warnings from -Wpedantic show up wherever the base standard requires them. (And fr fr, it would make zero sense to only warn about features that aren't in your specified GNU C dialect, because by definition the GNU dialects of C include every feature the compiler supports with the given option — there'd be literally nothing to warn about. 💀)

---

-pedantic-errors 🫶

Give an error whenever the base standard (see -Wpedantic) requires a diagnostic — 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. This is not equivalent to -Werror=pedantic: that latter option is unlikely to be useful, because it only makes errors of the diagnostics controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic. periodt.

If you want the required diagnostics that are warnings by default to be errors instead, but don't also want to 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 💅

Downgrade 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:

• -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)

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

---

-Wall 🔥

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

Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not 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 is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually.

---

-Wextra 🌸

This 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.)

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

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

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

---

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

Warn about code affected by ABI changes. 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.

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.

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.

• Like, scoped enumerators passed as args to a variadic function get promoted exactly like unscoped ones do, which makes va_arg throw a lil fit. On most targets this doesn't actually mess with the parameter-passing ABI tho, since there's literally no way to pass an argument smaller than int anyway.

Also the ABI changed how it mangles template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument. Rude.

All of this got fixed in -fabi-version=6. 💅

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

Fixed in -fabi-version=7. ✨

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

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

• decltype(nullptr) incorrectly had an alignment of 1, which led to unaligned accesses. Quick note bestie: this did not affect the ABI of a function with a nullptr_t parameter, because parameters have a minimum alignment. 🫶

Fixed in -fabi-version=9, the default for GCC 5.2.

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

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

---

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

-Wpsabi turns on warnings about processor-specific ABI changes — stuff like changes in alignment requirements or in how function arguments get passed. On several targets, including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390, these details have shifted between different versions of GCC and/or different versions of the C or C++ language standards in ways that affect binary compatibility of compiled code. With -Wpsabi, GCC can spot potentially incompatible usages and warn you. 👀

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

---

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

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

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

By default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is an error. Both diagnostics can be disabled with -Wno-changes-meaning. Or, if you're feeling spicy, the error case can be downgraded to a warning with -Wno-error=changes-meaning or -fpermissive.

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

---

-Wchar-subscripts

Warns you if an array subscript has type char. This is a super common oopsie, because programmers constantly forget that this type is signed on some machines. This warning is enabled by -Wall. 💀

---

-Wno-coverage-mismatch

Warns if feedback profiles don't match when you're using the -fprofile-use option. If a source file gets changed between compiling with -fprofile-generate and compiling with -fprofile-use, the files with the profile feedback can fail to match the source file, and GCC just cannot use that profile feedback information.

By default, this warning is enabled and treated as an error. -Wno-coverage-mismatch can disable the warning, or -Wno-error=coverage-mismatch can disable just the error. Be careful tho: disabling the error for this warning can result in poorly optimized code, and it's really only useful in the case of very minor changes like bug fixes to an existing code-base. Completely disabling the warning is not recommended. 🚫

---

-Wno-coverage-too-many-conditions

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

---

-Wno-coverage-too-many-paths

Warns if -fpath-coverage is used and a function has too many paths, so GCC gives up on coverage. How much is "too many" is controlled by -fpath-coverage-limit. This warning is enabled by default. 🌸

---

-Wno-coverage-invalid-line-number

Warns when a function ends earlier than it begins because of invalid linenum macros. Heads up: this warning is only emitted with --coverage enabled.

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

---

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

Suppresses warning messages emitted by #warning directives. Shhh. 🤫

---

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

Gives you a warning when a value of type float gets implicitly promoted to double. Here's the tea: CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but they emulate double in software. On that kind of machine, doing computations with double values is way more expensive because of all the overhead software emulation drags in. 💸

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

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

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

---

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

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

---

-Wformat

-Wformat=n

Checks calls to printf and scanf, etc., to make sure the arguments you supplied have types appropriate to the format string you specified, and that the conversions you specified in the format string actually make sense. This covers standard functions, plus others specified by format attributes, across the printf, scanf, strftime and strfmon families (that last one is an X/Open extension, not in the C standard) — or other target-specific families.

Which functions get checked without format attributes having been specified depends on the standard version you selected, and those checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin. 🦋

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

-Wformat=1

-Wformat

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

-Wformat=2

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

---

-Wno-format-contains-nul

If -Wformat is specified, don't warn about format strings that contain NUL bytes. 🙈

---

-Wno-format-extra-args

If -Wformat is specified, don't warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored, so like, they're allowed to just vibe there.

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

---

-Wformat-overflow

-Wformat-overflow=level

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

-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 known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output.

For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. 🎀

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

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

At level 2, 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. 🎀

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 on, then like... do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed. slay, they're legal bestie 🌸
• -Wformat-nonliteral — if -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless the format function takes its format arguments as a va_list. 🦋
• -Wformat-security — if -Wformat is specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This may be a security hole if the format string came from untrusted input and contains %n. 😭 (This is currently a subset of what -Wformat-nonliteral warns about, but in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)

no cap, this one is real — untrusted format strings + %n is giving actual vulnerability energy 💀

• -Wformat-signedness — if -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. signed/unsigned mismatch is so not the vibe 🙈
• -Wformat-truncation, -Wformat-truncation=level — warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. Except as noted otherwise, the option uses the same logic as -Wformat-overflow. 👑
  • -Wformat-truncation, -Wformat-truncation=1 — Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
  • -Wformat-truncation=2 — Level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
• -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. it's giving Y2K era but make it a compiler warning 💜
• -Wnonnull — warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

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

• -Wnonnull-compare — warn when comparing an argument marked with the nonnull function attribute against null inside the function.

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

• -Wnull-dereference — warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. 🎯
• -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.
• -Wmaybe-musttail-local-addr — warn when address of a local variable can escape to a musttail call, unless it goes out of scope already before the musttail call. 🌈

                   int foo (int *);

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

This warning is enabled by -Wextra.

• -Wnrvo (C++ and Objective-C++ only) — warn if the compiler does not elide the copy from a local variable to the return value of a function in a context where it is allowed by [class.copy.elision]. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler cannot elide copies from both v1 and v2, so it elides neither. 😤

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

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

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

• -Winit-self (C, C++, Objective-C and Objective-C++ only) — warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

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

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

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

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

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

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

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

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

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

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

so bestie, imagine you got a switch like this:

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

notice how case 2 just falls through to case 3? yeah. so this warning is like, "hey, you sure about that?" but it does not warn when the last statement of a case cannot fall through, like if there's a return or a call to a function declared with the noreturn attribute. no cap.

also, -Wimplicit-fallthrough= takes into account control flow statements, like ifs, and only warns when appropriate. for example:

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

but like, sometimes you want a fallthrough, right? GCC got you. it provides an attribute, __attribute__ ((fallthrough)), that you use 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 you can use [[gnu::fallthrough]];, which is a GNU extension. instead of these attributes, you can also 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. like, any comment at all.
• -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) — control if warnings triggered by the warn_if_not_aligned attribute should be issued. these warnings are enabled by default.
• -Wignored-qualifiers (C and C++ only) — warn if the return type of a function has a type qualifier such as const. for ISO C such a type qualifier has no effect, since the value returned by a function is not an lvalue. for C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so such return types always receive a warning even without this option.

this warning is also enabled by -Wextra.

• -Wno-ignored-attributes (C and C++ only) — this option controls warnings when an attribute is ignored. this is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. this warning is enabled by default.
• -Wmain — warn if the type of main is suspicious. main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. this warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.

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

• -Wmisleading-indentation (C and C++ only) — warn when the indentation of the code does not reflect the block structure. specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation.

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

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

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

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

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

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

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

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

in C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. attributes deprecated, error, and warning suppress the warning.. <-- yes, two periods, that's what it says.

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

-Wmissing-attributes is enabled by -Wall.

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

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

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

• -Wmissing-braces — warn 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) — warn 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.

bestie, do NOT ignore this warning unless you literally have no profile data. it's giving bad vibes for your optimization. 🚫

• -Wmismatched-dealloc — warn 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 — bestie this one clocks unsafe multiple-statement macros that look like they're guarded by a clause like if, else, for, switch, or while, but after the macro expands only the first statement is actually guarded. 😭 the audacity.

Like, look at this:

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

This will increment y unconditionally, not just when c holds. Rude. The usual glow-up fix is 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 when parentheses are omitted in certain contexts, like when there's an assignment where a truth value is expected, or when operators get nested in ways that people constantly get confused about precedence-wise.

Also warns if a comparison like x<=y<=z shows up — that's equivalent to (x<=y ? 1 : 0) <= z, which is a whole different interpretation than plain math notation. 😭 math said one thing, C said another.

Also warns for dangerous uses of the GNU extension to ?: with an omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. 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 one warns when a value is moved to itself with std::move. Such a std::move typically has no effect. it's giving circular. 💀

                   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 it violates the sequence point rules in the C and C++ standards. ⚠️

So here's the tea: 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. no cap. 🔥

Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.

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. (it's literally returned into the void, bestie 🫠)
• -Wreturn-mismatch — warns about return statements without an expression in functions which do not return void. Also warns about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used.

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

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

• -Wreturn-type — warns whenever a function is defined with a return type that defaults to int (unless -Wimplicit-int is active, which takes precedence — it's giving main character energy, it goes first). Also warns if execution may reach the end of the function body, or if the function does not contain any return statement at all.

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

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

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

• -Wno-shift-count-negative — controls warnings if a shift count is negative. This warning is enabled by default.
• -Wno-shift-count-overflow — controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default.
• -Wshift-negative-value — warns if left shifting a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes.
• -Wno-shift-overflow, -Wshift-overflow=n — these options control warnings about left shift overflows. 🌀
  • -Wshift-overflow=1 — this is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), as signed left shifts always wrap. ✨
  • -Wshift-overflow=2 — this warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active.
• -Wswitch — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the flag_enum attribute. This warning is enabled by -Wall. 👑
• -Wswitch-default — warns whenever a switch statement does not have a default case. (bestie where's your fallback 😭)
• -Wswitch-enum — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels that do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked with the flag_enum attribute. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there is a default label. periodt. 💅
• -Wno-switch-bool — do not warn when a switch statement has an index of boolean type and the case values are outside the range of a boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than bool. For example:

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

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

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

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

And okay, one more thing about that -Wswitch-unreachable vibe: it will not warn you if the statement sitting between the controlling expression and the first case label is literally just a declaration. Iconic behavior tbh:

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 get used. Bestie, heads up: these functions changed semantics in GCC 4.4, so this is giving "we don't do that anymore". 😭
• -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init isn't able to initialize the automatic variable. The classic scenario? An automatic variable that's declared between the controlling expression and the first case label of a switch statement.
• -Wunused-but-set-parameter — warns whenever a function parameter is assigned to, but is otherwise unused (aside from its declaration). To silence it, use the unused attribute. This warning is also enabled by -Wunused together with -Wextra.
• -Wunused-but-set-variable — warns whenever a local variable is assigned to, but is otherwise unused (aside from its declaration). This warning is enabled by -Wall. To snuggle it into silence, use the unused attribute. This warning is also enabled by -Wunused, which is enabled by -Wall.
• -Wunused-function — warns whenever a static function is declared but not defined, or a non-inline static function is unused. Enabled by -Wall.
• -Wunused-label — warns whenever a label is declared but not used. Enabled by -Wall. To silence: the unused attribute, periodt.
• -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — warns when a typedef locally defined in a function is never used. Enabled by -Wall.
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration. Big heads up: this one is not enabled by -Wunused unless -Wextra is also specified. To silence it: unused attribute.
• -Wno-unused-result — does not 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++. This warning is enabled by -Wall. To silence it: unused attribute.
• -Wunused-const-variable, -Wunused-const-variable=n — warns whenever a constant static variable is unused aside from its declaration. To silence: unused attribute, obviously.
  • -Wunused-const-variable=1 — warns about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header that's included.
    • -Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of #defines.
  • -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (system headers excluded, don't worry bestie). It's equivalent to the short form -Wunused-const-variable. This level must be explicitly requested in both C and C++ because it might be hard to clean up all the headers included. 😮‍💨
• -Wunused-value — warns whenever a statement computes a result that is explicitly not used. To suppress it, cast the unused expression to void. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as x[i,j] causes a warning, while x[(void)i,j] does not. Enabled by -Wall.
• -Wunused — all the above -Wunused options combined, except those documented as needing to be specified explicitly. To get a warning about an unused function parameter, you must either specify -Wextra -Wunused (remember: -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter. -Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C — not C++.
• -Wuse-after-free, -Wuse-after-free=n (C, Objective-C, C++ and Objective-C++ only) — warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. This warning is enabled at all optimization levels but may yield different results with optimization than without. -Wuse-after-free is equivalent to -Wuse-after-free=1.
  • -Wuse-after-free=1 — at level 1 the warning only attempts to diagnose unconditional uses of pointers made indeterminate by a deallocation call or a successful call to realloc, regardless of whether or not the call actually resulted in a reallocation of memory. This includes double-free calls as well as uses in arithmetic and relational expressions. Although it's undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.
  • -Wuse-after-free=2 — at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. Just like at level 1, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to free in the function below 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. Literally 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're 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. Chef's kiss. 👑

• Heads up, bestie: -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. It does not fire when a class object is converted to a non-reference type, because that's a way to create a temporary:

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

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

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. (Heads up: -Wmaybe-uninitialized is issued for ordinary functions.)

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

These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared volatile. And because these warnings depend on optimization, the exact variables or elements that get warnings depend on the precise optimization options and version of GCC used. Lowkey chaotic but okay.

Note that there may be no warning about a variable that's used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. (Snatched by the optimizer, can you believe. 💀)

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. The audacity of some code paths, fr fr. 🙈

Ok so also — passing a pointer (or, in C++, a reference) to an uninitialized object into a function argument that's const-qualified? Yeah, this warning clocks that too. 💅 (And heads up: -Wuninitialized gets issued for built-in functions that are known to actually read the object.) The way to dodge it: annotate your function with the attribute access (none), which basically tells GCC "bestie I'm not using this argument to touch the object," and the warning goes away. 🫶

These warnings are only possible when you're doing an optimizing compilation, because otherwise GCC isn't out here keeping track of the state of your variables. It's giving "needs optimization mode to even care." 😭

They're also made optional because GCC might not be able to tell when your code is actually correct even though it looks like a whole error. Here's one example of how that happens, and it's so real:

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

So if the value of y is always 1, 2, or 3, then x is always initialized — but GCC literally doesn't know that. 🙈 To suppress the warning you gotta provide a default case with assert(0) or similar code.

This option also warns when a non-volatile automatic variable might get changed by a call to longjmp. The compiler only sees the calls to setjmp. It cannot know where longjmp is gonna be called; in fact, a signal handler could call it at any point in the code. So as a result, you may get a warning even when there's truly no problem, because longjmp can't actually be called at the place that would cause a problem. Vibe check failed on GCC's end, not yours. 💀

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

This warning is enabled by -Wall or -Wextra.

---

-Wunknown-pragmas

Warns when a #pragma directive shows up that GCC just doesn't understand. 🧐 And if this command-line option is used, warnings even get issued for unknown pragmas sitting in system header files. That 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 — like incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas.

-Wno-pragma-once-outside-header

Do not warn when #pragma once is used in a file that is not a header file, such as a main file. Not every file gets to be a header, and that's fine. 💅

-Wno-prio-ctor-dtor

Do not warn if a priority from 0 to 100 is used for a constructor or destructor. The constructor and destructor attributes let you assign a priority to the constructor/destructor to control its order of execution before main is called or after it returns. But bestie, the priority values must be greater than 100, because the compiler reserves the priority values between 0--100 for the implementation. No cap.

-Wstrict-aliasing

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

-Wstrict-aliasing=n

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

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

-Wstrict-overflow

-Wstrict-overflow=n

This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where the code might overflow: it only warns about cases where the compiler implements some optimization. So this warning depends on the optimization level. ✨

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 the 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. Periodt.

• -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 gets diagnosed because its result is necessarily non-zero regardless 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. The argument is meaningful only for string functions that operate on character arrays; raw memory functions like memcpy always use type-zero Object Size Checking.

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

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 a 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
}

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

void f (int[static 4]);

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

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

• -Wstringop-overflow
• -Wstringop-overflow=1 — bestie, this setting uses type-zero Object Size Checking to figure out how big the destination objects are. At this level it will not warn you about writes past the end of subobjects of bigger objects that are reached through pointers — unless the size of the biggest surrounding object is actually known. And if the destination could be any one of several objects? It just assumes it's the biggest one of them all 💀. On Linux, when optimization is turned on at this setting, it warns for the exact same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. iykyk.
• -Wstringop-overflow=2 — now we're using type-one Object Size Checking for the destination sizes. At this level it warns about overflows when writing to members of the largest complete objects whose exact size is known. BUT it does not warn for excessive writes to those same members of unknown objects referenced by pointers, because those pointers might be pointing at arrays with an unknown number of elements. This one is the default setting of the option ✨.
• -Wstringop-overflow=3 — this one uses type-two Object Size Checking. At this level it warns about overflowing the smallest object or data member. Heads up though: this is the most restrictive setting of the option, and it may result in warnings for code that is actually safe. 🙈
• -Wstringop-overflow=4 — the big one, using type-three Object Size Checking. At this level it 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. And similarly to -Wstringop-overflow=3, this setting may result in warnings for benign code. no cap.
• -Wno-stringop-overread — warns for calls to string manipulation functions like memchr or strcpy that are determined to read past the end of the source sequence. 🌸

Option -Wstringop-overread is enabled by default, bestie, so it's already watching your back.

• -Wno-stringop-truncation — do not warn for calls to bounded string manipulation functions like strncat, strncpy, and stpncpy that might either truncate the copied string or leave the destination unchanged. 👑

In this next example, the call to strncat specifies a bound that's less than the length of the source string. So the copy of the source will get truncated, and that's why the call gets diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound.

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

As another example, this call to strncpy results in copying to d just the characters before the terminating NUL, without appending 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. To avoid the warning when the result isn't expected to be NUL-terminated, call memcpy instead. 💅

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

In the next example, the call to strncpy specifies the size of the destination buffer as the bound. If the length of the source string is equal to or greater than this size, the result of the copy will not be NUL-terminated. So, yeah, this 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);
  ...
}

When a character array is meant to store a sequence of bytes with no terminating NUL, that array can be annotated with the attribute nonstring to avoid this warning. Buuut those arrays are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuses of such arrays, GCC issues warnings unless it can prove the use is safe. 💖

• -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 it's available, otherwise according to the level of the option -fstrict-flex-arrays=level. And important: -Wstrict-flex-arrays is only effective when level is greater than 0. 🎀
  • When level=1, warnings are issued for a trailing array reference of a structure that has 2 or more elements, if the trailing array is referenced as a flexible array member.
  • When level=2, on top of level=1, additional warnings are issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member.
  • When level=3, on top of level=2, additional warnings are issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.

This option is way 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 could be beneficial. The currently supported attribute-names are listed below. 🦋
• -Wsuggest-attribute=pure
• -Wsuggest-attribute=const
• -Wsuggest-attribute=noreturn
• -Wmissing-noreturn
• -Wsuggest-attribute=malloc
• -Wsuggest-attribute=returns_nonnull — these warn about functions that might be candidates for the 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. And a function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort, or trapping. This analysis needs the option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels make the analysis more accurate. 💜
• -Wsuggest-attribute=format
• -Wmissing-format-attribute — warns about function pointers that might be candidates for format attributes. Note that 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. Like, 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 where format attributes would be appropriate might not get detected at all. 😭

• -Wsuggest-attribute=cold — warns about functions that might be candidates for the cold attribute. This is based on static detection, and it 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 that lead to abort. 🔥
• -Walloc-size — warns about calls to allocation functions decorated with the attribute alloc_size that specify an insufficient size for the target type of the pointer the result is assigned to, including calls to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. ⭐
• -Walloc-zero — warns about calls to allocation functions decorated with the attribute alloc_size that specify zero bytes, including calls to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs between implementations (and in the case of realloc has been deprecated), relying on it may result in subtle portability bugs and should be avoided. 🚫
• -Wcalloc-transposed-args — warns about calls to allocation functions decorated with the attribute alloc_size with two arguments, which use the sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning. The first argument to calloc is documented to be the number of elements in the array, while the second argument is the size of each element, so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If having sizeof in the earlier argument and not the latter is intentional, the warning can be suppressed by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4) or by using sizeof in the later argument as well. 🍓
• -Walloc-size-larger-than=byte-size — warns about calls to functions decorated with the attribute alloc_size that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than. 💀
• -Wno-alloc-size-larger-than — disables -Walloc-size-larger-than= warnings. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. 🙈
• -Walloca — this option warns on all uses of alloca in the source. 🎀

-Walloca-larger-than=byte-size`

okay this one is so specific and honestly kind of iconic for it 💅

It yells at you about calls to alloca when the integer argument is either:

• literally zero, or
• not bounded by a controlling predicate that caps its value at at most byte-size.

It also yells when the bound value is just... unknown bestie 🙈 And non-integer argument types? Those get counted as unbounded, even if they totally look like they're constrained to the range you expected. Rude but fair.

So here's a bounded case of alloca — the good kind:

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

In that example, passing -Walloca-larger-than=1000 would not issue a warning, because the call to alloca is known to be at most 1000 bytes. slay, it's bounded ✨ But if you passed -Walloca-larger-than=500 instead? The compiler would emit a warning. caught in 4k 📸

Unbounded uses are the other vibe — alloca calls with no controlling predicate constraining the integer argument. Like:

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

If -Walloca-larger-than=500 were passed, the above would trigger a warning — but this time because of the lack of bounds checking. No predicate, no peace 😭

Heads up bestie: even seemingly correct code involving signed integers could cause a warning:

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

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

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

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

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

-Wno-alloca-larger-than

Disables -Walloca-larger-than= warnings. This option is equivalent to -Walloca-larger-than=SIZE_MAX or larger. Basically: mute it, bestie 🔇

-Warith-conversion

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

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

See how the second line only speaks up when you specifically ask for this one? petty, but valid 💅

-Warray-bounds

-Warray-bounds=n

Warns about out-of-bounds subscripts or offsets into arrays. Enabled by -Wall. It's way more effective when -ftree-vrp is active (default for -O2 and above), but a subset of instances get issued even without optimization. some effort is still effort 🌱

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 ([]),
• the GCC zero-length array extension ([0]), or
• a one-element array ([1]).

As a result, out-of-bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default. quietly allowed, iykyk 🤫

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

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

Here's the level breakdown, bestie:

• -Warray-bounds=1 — This is the default warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested.
• -Warray-bounds=2 — This warning level also warns about the intermediate results of pointer arithmetic that may yield out-of-bounds values. This warning level may give a larger number of false positives and is deactivated by default. chaotic energy, proceed with care 💀

-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.

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

See? One of these is in trouble and one of them has a permission slip 📝✨

This warning is enabled by -Wextra. If -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the nonstring warning, because in C++ such initializations are an error.

Note the double whammy there: with -Wc++-compat on, nonstring will not save you. C++ said no and it meant it 😭

-Warray-compare

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

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

-Warray-compare is enabled by -Wall. we love a warning that's just the language telling you "bestie no" 🫶

-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. consistency check era ✨

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

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

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

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

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

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

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

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

-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. the alias said "I'm them" and the compiler said "no you are not" 😭

• -Wattribute-alias=1 — The default warning level of the -Wattribute-alias option diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs.
• -Wattribute-alias=2 — At this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.

Attributes considered at level 2 include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.

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

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

Warns about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. 👀 Such characters can flip left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and the 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. fr fr, be careful 💀

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. the ordering matters bestie, don't forget 🎀

NOTE: In Ubuntu 25.10 and later versions, -Wbidi-chars is enabled by default for C, C++, ObjC, ObjC++, unless -Wno-bidi-chars or -Wbidi-chars=none is found. so if you're on Ubuntu 25.10+ and suddenly seeing these, that's why bestie 🫶

-Wbool-compare

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

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

an always-false comparison?? bestie the compiler is doing you a FAVOR by telling you 💅

This warning is enabled by -Wall.

• -Wbool-operation — okay so this one does a ✨vibe check✨ on ops that are sus when the operand is a boolean-typed expression.

Like, bitwise negation of a boolean? bestie, that is very likely a bug in your program, no cap. And for C, this warning also comes for incrementing or decrementing a boolean, which rarely makes sense fr 😭 (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.) Iconic little chaos detector.

This warning is enabled by -Wall.

• -Wduplicated-branches — screams at you when an if-else has two branches that are literally the same thing 🫠 This warning detects cases like

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

It does not warn when both branches contain just a null statement (so you get a pass on that one 💅). This warning also warns for conditional operators:

int i = x ? *p : *p;

• -Wduplicated-cond — the "girlie you already checked that" warning 🌸 Warns about duplicated conditions in an if-else-if chain. For instance, it warns for this code:

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

• -Wframe-address — warns when __builtin_frame_address or __builtin_return_address gets called with an argument greater than 0 💀 Those calls may return indeterminate values or crash the program, so like… not it. The warning is included in -Wall.
• -Wno-discarded-qualifiers (C and Objective-C only) — tells GCC not to warn when type qualifiers on pointers are being discarded 🙈

Typically, the compiler warns if a const char * variable is passed to a function that takes a char * parameter. This option can be used to suppress such a warning.

• -Wno-discarded-array-qualifiers (C and Objective-C only) — do not warn if type qualifiers on arrays which are pointer targets are being discarded ✨

Typically, the compiler warns if a const int (*)[] variable is passed to a function that takes a int (*)[] parameter. This option can be used to suppress such a warning.

• -Wno-incompatible-pointer-types (C and Objective-C only) — do not warn when there is a conversion between pointers that have incompatible types 🎀 This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.

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

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

• -Wno-int-conversion (C and Objective-C only) — do not warn about incompatible integer-to-pointer and pointer-to-integer conversions 🌈 This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used.

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

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

• -Wzero-as-null-pointer-constant — warns when a literal 0 is used as a null pointer constant 👀 like bestie, we have better options, please.
• -Wzero-length-bounds — warns about accesses to elements of zero-length array members that might overlap other members of the same object 🚨 Declaring interior zero-length arrays is discouraged because accesses to them are undefined.

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

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

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

Option -Wzero-length-bounds is enabled by -Warray-bounds.

• -Wno-div-by-zero — do not warn about compile-time integer division by zero 💀 Floating-point division by zero is not warned about, as it can be a legitimate way of obtaining infinities and NaNs (yes really, it's serving a purpose 🫶).
• -Wsystem-headers — prints warning messages for constructs found in system header files 🌸

Warnings from system headers are normally suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. However, note that using -Wall in conjunction with this option does not warn about unknown pragmas in system headers—for that, -Wunknown-pragmas must also be used. iykyk 💅

• -Wtautological-compare — warns if a self-comparison always evaluates to true or false 😭 This warning detects various mistakes such as:

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

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

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

will always be false. Not the main character moment you thought it was, bestie.

This warning is enabled by -Wall.

• -Wtrailing-whitespace
• -Wtrailing-whitespace=kind — warns about trailing whitespace at the end of lines, including inside of comments, but excluding trailing whitespace in raw string literals ✨

-Wtrailing-whitespace is equivalent to -Wtrailing-whitespace=blanks and warns just about trailing space and horizontal tab characters. -Wtrailing-whitespace=any warns about those or trailing form feed or vertical tab characters. -Wno-trailing-whitespace or -Wtrailing-whitespace=none disables the warning, which is the default. This is a coding style warning 🎀

• -Wleading-whitespace=kind — warns about style issues in leading whitespace, but not about the amount of indentation 👑

Some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of -ftabstop=n) followed by zero or fewer than n spaces. No warning is emitted on lines which contain solely whitespace (although -Wtrailing-whitespace= warning might be emitted), no warnings are emitted inside of raw string literals. Warnings are also emitted for leading whitespace inside of multi-line comments. -Wleading-whitespace=spaces warns about leading whitespace other than spaces for projects which want to indent just by spaces. -Wleading-whitespace=tabs warns about leading whitespace other than horizontal tabs for projects which want to indent just by horizontal tabs. -Wleading-whitespace=blanks warns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or about n or more consecutive spaces in leading whitespace (where n is argument of -ftabstop=n, 8 by default). -Wleading-whitespace=none disables the warning, which is the default. This is a coding style warning 🦋

• -Wtrampolines — warns about trampolines generated for pointers to nested functions 🧚

A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly 😳 (executable stack = the drama we do not want).

• -Wfloat-equal — warns if floating-point values are used in equality comparisons 💀

The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). In particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. periodt 💅

• -Wtraditional (C and Objective-C only) — warns about certain constructs that behave differently in traditional and ISO C. Also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided 🌸
  • Macro parameters that appear within string literals in the macro body. In traditional C macro replacement takes place within string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. It also suggests you hide directives like #pragma not understood by traditional C by indenting them. Some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. This construct is not accepted by some traditional C compilers.
• Like, the ISO type of an integer constant can have a totally different width or signedness than its traditional type. But bestie, this warning only fires if the base of the constant is ten. So hexadecimal or octal values — which usually just represent bit patterns anyway — get zero complaints. not a peep 💅
  • ISO string concatenation usage? Detected. Warned. caught in 4k
  • Initialization of automatic aggregates. classic traditional C situation
  • Identifiers clashing with labels — because traditional C literally lacks a separate namespace for labels. rip to that namespace 🪦
  • Initialization of unions. If the initializer is zero, the warning is skipped. 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 is intentionally not issued for prototype declarations or variadic functions, because these ISO C features show up in your code when you use libiberty's traditional C compatibility macros, "PARAMS" and "VPARAMS". It's also bypassed for nested functions, because that feature is already a GCC extension and thus not relevant to traditional C compatibility. iykyk ✨
• -Wtraditional-conversion (C and Objective-C only) — Warn if a prototype causes a type conversion that's different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument, except when the same as the default promotion.
• -Wdeclaration-after-statement (C and Objective-C only) — Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90.

This warning is upgraded to an error by -pedantic-errors. it's giving consequences 😭

• -Wshadow — Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed. Heads up: 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 get ignored when -Wshadow is used. Same as -Wshadow=global.
• -Wno-shadow-ivar (Objective-C only) — Do not warn whenever a local variable shadows an instance variable in an Objective-C method.
• -Wshadow=global — Warn for any shadowing. Same as -Wshadow.
• -Wshadow=local — Warn when a local variable shadows another local variable or parameter.
• -Wshadow=compatible-local — Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea that when a local variable shadows another one of incompatible type, it's most likely intentional, not a bug or typo. Behold, the example:

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

Since the two variable "i" in the example above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. And because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and emit an error or warning. Use of this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means that shadowing "const char *i" by "char *i" does not emit a warning. sneaky but valid 💅

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 this option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-larger-than.

Also warns for calls to bounded functions such as "memchr" or "strnlen" that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. These warnings can only be disabled by -Wno-larger-than. no other escape route bestie 🫣

• -Wno-larger-than — Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger.
• -Wframe-larger-than=byte-size — Warn if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you don't 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. 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. oof bestie that's a leak with extra steps 💀

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

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

• -Wstack-usage=byte-size — Warn if the stack usage of a function might exceed byte-size. The computation done to determine the stack usage is conservative. Any space allocated via "alloca", variable-length arrays, or related constructs is included by the compiler when determining whether or not to issue a warning.

The message is in keeping with the output of -fstack-usage.

• If the stack usage is fully static but exceeds the specified amount, it's:

          warning: stack usage is 1120 bytes

• If the stack usage is (partly) dynamic but bounded, it's:

          warning: stack usage might be 1648 bytes

• If the stack usage is (partly) dynamic and not bounded, it's:

          warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-stack-usage.

• -Wno-stack-usage — Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger.
• -Wunsafe-loop-optimizations — Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop indices. And with -funsafe-loop-optimizations, warn if the compiler does make such assumptions. chaotic either way fr fr
• -Wno-pedantic-ms-format (MinGW targets only) — When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets, which depend on the MS runtime.
• -Wpointer-arith — Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C++, also warn when an arithmetic operation involves "NULL". This warning is also enabled by -Wpedantic.

This warning is upgraded to an error by -pedantic-errors. so real of it 💅

• -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. Example time:

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

Note that the code above is invalid in C++11. so watch urself 👀

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's also enabled by -Wextra. slay, type-cast ur way out ✨
• -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. inception but make it comments 🌀
• -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.

...and yeah bestie, this one rides along with -Wall. Like, if you didn't pass -Wall, the warning is still on for you anyway — unless trigraphs are enabled. Want trigraph conversion but zero warnings while keeping all your other -Wall goodness? Say it with your chest:

-trigraphs -Wall -Wno-trigraphs

---

• -Wundef — screams at you if an undefined identifier gets evaluated inside an #if directive. Fun fact: those mystery identifiers just get replaced with zero. 🙈
• -Wexpansion-to-defined — yells whenever defined shows up inside a macro expansion (yep, even when the macro got expanded by an #if directive). This usage is not portable, so like... don't. ✨ Bonus: -Wpedantic and -Wextra turn this one on for you too.
• -Wunused-macros — calls out macros defined in the main file that never get used. A macro counts as "used" if it's expanded or tested for existence at least once. The preprocessor also warns you if the macro hadn't been used yet when it got redefined or undefined.

Okay but who's exempt? 💅 Built-in macros, macros defined on the command line, and macros defined in include files — no warnings for those.

Lowkey annoying gotcha: if a macro is actually used but only inside skipped conditional blocks, the preprocessor still reports it as unused. To dodge that warning you could tighten up the macro's scope — like moving it into the first skipped block. Or just hand it a fake lil' use, something like:

#if defined the_macro_causing_the_warning
#endif

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

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

(bestie, that second and third FOO should be in comments, fr fr). This warning is on by default.

• -Wbad-function-cast (C and Objective-C only) — warns when a function call gets cast to a type that doesn't match. Example: calling a function that returns an integer type and then casting the result to a pointer type. 🚩
• -Wc90-c99-compat (C and Objective-C only) — warns about stuff that isn't in ISO C90 but is in ISO C99. Think: variable length arrays, long long, bool, compound literals, designated initializers, that whole vibe. This option is independent of the standards mode. Warnings get switched off in the expression that follows __extension__.
• -Wc99-c11-compat (C and Objective-C only) — warns about things missing from ISO C99 that show up in ISO C11. Like anonymous structures and unions, the _Atomic type qualifier, the _Thread_local storage-class specifier, the _Alignas specifier, the Alignof operator, the _Generic keyword, etc. Also independent of the standards mode, and warnings are off in the expression after __extension__.
• -Wc11-c23-compat (C and Objective-C only)
• -Wc11-c2x-compat (C and Objective-C only) — warns about features not in ISO C11 that are in ISO C23. Like leaving the string out of _Static_assert, using [[]] attribute syntax, using decimal floating-point types, and so on. Again, independent of the standards mode, and warnings go quiet in the expression following __extension__. Oh and heads up: the name -Wc11-c2x-compat is deprecated. 💀

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) — yes queen, the original lists it twice, so here it is twice. 💖 Warns about features not in ISO C23 that are in ISO C2Y. Example: _Generic selecting with a type name instead of an expression. Independent of the standards mode, warnings disabled in the expression after __extension__.

When not compiling in C2Y mode, -pedantic-errors bumps these warnings up to errors.

• -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that live outside the shared subset of ISO C and ISO C++. Example: 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 shifted between ISO C++ 1998 and ISO C++ 2011. Example: identifiers that were fine in ISO C++ 1998 but became keywords in ISO C++ 2011. This warning flips on -Wnarrowing and is enabled by -Wall.
• -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by -Wall.
• -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by -Wall.
• -Wc++20-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by -Wall.
• -Wno-c++11-extensions (C++ and Objective-C++ only) — don't warn about C++11 constructs in code being compiled with an older C++ standard. Even without this option, some C++11 constructs only get diagnosed if you're using -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. Even without this option, some C++14 constructs only get diagnosed when -Wpedantic is on.
• -Wno-c++17-extensions (C++ and Objective-C++ only) — don't warn about C++17 constructs in code compiled with an older C++ standard. Even without this option, some C++17 constructs only get diagnosed under -Wpedantic.
• -Wno-c++20-extensions (C++ and Objective-C++ only) — don't warn about C++20 constructs in code compiled with an older C++ standard. Even without this option, some C++20 constructs only get diagnosed under -Wpedantic.
• -Wno-c++23-extensions (C++ and Objective-C++ only) — don't warn about C++23 constructs in code compiled with an older C++ standard. Even without this option, some C++23 constructs only get diagnosed under -Wpedantic.
• -Wno-c++26-extensions (C++ and Objective-C++ only) — don't warn about C++26 constructs in code compiled with an older C++ standard. Even without this option, some C++26 constructs only get diagnosed under -Wpedantic.
• -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 ol' char *. 🚨

It also warns when a cast adds a type qualifier in an unsafe way. Like casting char ** to const char ** — unsafe, and here's why:

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

• -Wcast-align — warns whenever a pointer gets cast so the target's required alignment goes up. Example: casting a char * to an int * on machines where integers can only be accessed on two- or four-byte boundaries.
• -Wcast-align=strict — same energy, but it warns whenever a pointer cast raises the required alignment — example: casting a char * to an int * — regardless of the target machine. 💅
• -Wcast-function-type — warns when a function pointer gets cast to an incompatible function pointer. For casts involving function types with a variable argument list, only the types of the initial arguments that are actually provided are considered. Any pointer-type parameter matches any other pointer-type. Any harmless differences in integral types get ignored, like int vs. long on ILP32 targets. Type qualifiers get ignored too. Special case: the function type void (*) (void) matches everything — which you can use to suppress this warning. And in a cast involving pointer-to-member types, it warns whenever the cast changes the pointer-to-member type. Enabled by -Wextra.
• -Wcast-user-defined — warns when a cast to a reference type doesn't involve a user-defined conversion that the programmer might've expected to be called. 👀

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

This warning is enabled by default.

• -Wwrite-strings — when compiling C, this gives string constants the type const char[length], so 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 plain annoying. Which is exactly why -Wall does not ask for these warnings. 😌

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

-pedantic-errors upgrades this warning to an error 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 — by default, language front ends complain when a command-line option is valid but not applicable to that front end. You can turn that off with -Wno-complain-wrong-lang, which is mostly useful when you're invoking a single compiler driver for multiple source files written in different languages. For example:

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

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. That latter front end diagnoses

f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran

which you can disable with -Wno-complain-wrong-lang.

• -Wcompare-distinct-pointer-types (C and Objective-C only) — warns if pointers of distinct types get compared without a cast. This warning is enabled by default.
• -Wconversion — bestie the compiler is about to snitch on every sneaky little conversion that could change your value 💅. It warns about implicit conversions that may alter a value, and the lineup includes:
  • conversions between real and integer, like abs (x) when x is double
  • conversions between signed and unsigned, like unsigned ui = -1
  • conversions to smaller types, like sqrtf (M_PI)

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

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

Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion. 🫶

• -Wdangling-else — the classic "which if does this else belong to" jump scare 😭. It warns about constructions where there may be confusion to which if statement an else branch belongs. Here's an example of such a case:

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

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

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

This warning is enabled by -Wparentheses. 💖

• -Wdangling-pointer
• -Wdangling-pointer=n — no cap, this one catches you using pointers to things that are already dead 💀. It warns about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. This includes local variables declared in nested blocks, compound literals and other unnamed temporary objects. In addition, it warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without.
  • -Wdangling-pointer=1 — at level 1, the warning diagnoses only unconditional uses of dangling pointers.
  • -Wdangling-pointer=2 — at level 2, in addition to unconditional uses the warning also diagnoses conditional uses of dangling pointers.

The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 have the same effect of disabling the warnings. -Wdangling-pointer=2 is included in -Wall. ✨

This example triggers the warning at level 1; the address of the unnamed temporary is unconditionally referenced outside of its scope.

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

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

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

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

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

• -Wdate-time — reproducible builds girlies, this one's for you 🎀. Warn when macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered as they might prevent bit-wise-identical reproducible compilations.
• -Wempty-body — warns if an empty body occurs in an if, else or do while statement. This warning is also enabled by -Wextra. 🙈
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif.
• -Wenum-compare — warns about a comparison between values of different enumerated types. In C++ enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by -Wall. 🌸
• -Wenum-conversion — warns when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by -Wextra in C.
• -Wenum-int-mismatch (C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. For example:

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

In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. However, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues. In C++, such mismatches are an error. In C, this warning is enabled by -Wall and -Wc++-compat. 🦋

• -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. This only warns about variables that are initialized when they are declared. 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 is 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. 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 likely 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. This warning 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 — bestie, this one catches you when you call the memset built-in and ~something~ is sus: the second argument isn't zero and the third one is zero. Classic scenario: you write memset (buf, sizeof buf, 0) and the compiler is like "girl... you meant memset (buf, 0, sizeof buf), and those args got swapped." 🫠 Heads up though — the diagnostic only fires when the third argument is a literal zero. If it's an expression that gets folded to zero, or a cast of zero to some type, then the vibes say it's way less likely you transposed anything by accident, so no warning. This one's enabled by -Wall.
• -Waddress — warns you about suspicious address-expression usage ✨. The receipts:
  • comparing the address of a function or a declared object to the null pointer constant, like:

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

• comparing a pointer to a string literal, like:

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

• and tests of pointer addition/subtraction results for equality to null, like:

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

Ngl, these uses usually mean a programmer oopsie: the address of most functions and objects necessarily evaluates to true (the exception is weak symbols), so using one in a conditional might mean you forgot parentheses in a function call or forgot a dereference in an array expression. If you want to shut up just the object-pointer subset, cast that pointer operand to an integer type like intptr_t or uintptr_t. Comparisons against string literals? Those give you unspecified behavior, they're not portable, and they suggest you actually wanted strcmp. The warning is suppressed if the suspicious expression came out of macro expansion. -Waddress 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 on suspicious logical-operator use in expressions. That covers using logical operators in contexts where a bit-wise operator is probably what you wanted, and it also yells when both operands of a logical operator are literally the same thing:

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

• -Wlogical-not-parentheses — warns when logical not is used on the left hand side operand of a comparison ✋. It stays quiet if the right operand counts as a boolean expression. The whole point is catching sus code like:

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

You can silence it by wrapping the LHS in parentheses:

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

This warning is enabled by -Wall.

• -Waggregate-return — warns 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 earns you a warning.) 💀
• -Wno-aggressive-loop-optimizations — do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations. -Waggressive-loop-optimizations is enabled by default.
• -Wno-attributes — do not warn if an unexpected __attribute__ shows up, like unrecognized attributes, function attributes slapped onto variables, etc. Important: 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, using -Wno-attributes=, you can suppress warnings about unknown scoped attributes (in C++11 and C23). Example: -Wno-attributes=vendor::attr disables the warning about this declaration:

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

You can also kill the warning for all attributes in a namespace with -Wno-attributes=vendor::, which prevents warnings about both of these:

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

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

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

Example: 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 is diagnosed too.

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

• -Wno-builtin-macro-redefined — do not warn if certain built-in macros get redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.
• -Wfree-labels (C and Objective-C only) — warns if a label is applied to a non-statement, or shows up at the end of a compound statement. Such labels are allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects. 🏷️

This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if you're building for a C version before C23 by -pedantic-errors.

• -Wheader-guard — warns if a valid preprocessor header multiple inclusion guard has a #define directive right after the #ifndef or #if !defined directive for the multiple inclusion guard, where it 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, bestie, it's usually just a misspelled guard name — either in the #ifndef / #if !defined directive or in the subsequent #define. 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 fine and warning-free if it's preceded by a declaration that specifies the argument types.)
• -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, per the C Standard, in a declaration. 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. A warning is given even if there's a previous prototype. A definition using () is not considered an old-style definition in C23 mode, because in that case it's equivalent to (void) — but it is considered an old-style definition for older standards. ⏳
• -Wdeprecated-non-prototype (C and Objective-C only) — warns if a function declared with an empty parameter list () is called with one or more arguments, or if a function definition with one or more parameters shows up after such a declaration. Both cases are errors in C23 and later dialects of C. 😬

This warning is also enabled by -Wc11-c23-compat.

• -Wmissing-parameter-name (C and Objective-C only) — warns if a function definition omits a parameter name, specifying only its type. This can actually be used to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects. 🎀

This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if you're building for a C version before C23 by -pedantic-errors.

• -Wmissing-parameter-type (C and Objective-C only) — triggers when a function parameter is declared without a type specifier in K&R-style functions:

void foo(bar) { }

This warning is also enabled by -Wextra.

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

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

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

• -Wmissing-prototypes (C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. This warning fires even if the definition itself provides a prototype. Use this option to detect global functions that don't have a matching prototype declaration in a header file. Not valid for C++ though, because in C++ all function declarations provide prototypes and a non-matching declaration just declares an overload rather than conflicting with an earlier declaration. For C++, 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 option to detect global variables that don't have a matching extern declaration in a header file.
• -Wmissing-declarations — bestie, this is the “where’s your prototype??” warning. It yells if a global function is defined without a previous declaration, even if the definition itself provides a prototype. Use it to catch global functions that aren’t declared in header files. But heads up: in C, it stays quiet for functions with previous non-prototype declarations — use -Wmissing-prototypes for that. And in C++, it won’t warn about function templates, inline functions, or functions in anonymous namespaces. periodt. 💅
• -Wmissing-field-initializers — it’s giving “you forgot a field, sis”. Warns when a struct initializer leaves out some fields. Like this code triggers it because x.h is implicitly zero:

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

But in C, it doesn’t warn about designated initializers, so this one is safe:

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

Also in C, the universal zero initializer { 0 } doesn’t trigger it:

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

And in C++, the empty { } initializer is fine too:

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

This warning is part of -Wextra. If you want the other -Wextra warnings without this one, use -Wextra -Wno-missing-field-initializers. 🎀

• -Wno-missing-requires — so by default the compiler side-eyes you when a concept-id shows up as a C++20 simple-requirement:

bool satisfied = requires { C<T> };

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

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

so satisfied is only true if concept C is satisfied for type T. This warning can be turned off 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:

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

In rare cases you get false positives. To silence those, wrap the expression in parentheses. For example, this 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 — don’t warn if a multicharacter constant like 'FOOF' is used. Usually these are typos, bestie, because they have implementation-defined values and should not be used in portable code. So this flag just lets you live your truth. 🙈
• -Wnormalized=[none|id|nfc|nfkc] — so in ISO C and ISO C++, two identifiers are different if they’re different sequences of characters. But sometimes when you use characters outside the basic ASCII set, you can have two different sequences that look exactly the same. To avoid confusion, ISO 10646 sets out normalization rules that turn two look-alike sequences into the same sequence. GCC can warn you if your identifiers aren’t normalized; this option controls that warning.

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

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

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

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

• -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 compiling with the -Werror flag. 🎀
• -Wno-deprecated — don’t warn about usage of deprecated features. But in C++, if you explicitly specify -Wdeprecated, it also turns on warnings for some features 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. So it’s a whole vibe. ✨
• -Wno-deprecated-declarations — don’t warn about using functions, variables, and types marked as deprecated with the deprecated attribute. Moving on. 💅
• -Wno-overflow — don’t warn about compile-time overflow in constant expressions. That’s it. 🌸
• -Wno-odr — warns about One Definition Rule violations during link-time optimization. Enabled by default. (Yes, the name is confusing, but that’s what it says.) 🤔
• -Wopenacc-parallelism — warns about potentially suboptimal choices related to OpenACC parallelism. Like, you could do better, bestie. 👀
• -Wno-openmp — warns about suspicious OpenMP code. (Again, the name is a little chaotic, but okay.) 🔥
• -Wopenmp-simd — warns if the vectorizer cost model overrides the OpenMP simd directive set by the user. You can use -fsimd-cost-model=unlimited to relax the cost model. 🦋
• -Woverride-init (C and Objective-C only) — warns 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 — warns 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 misaligned for little benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar itself doesn’t 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 — okay so GCC 4.1, 4.2, and 4.3 ignored the packed attribute on bit-fields of type char. This was fixed in GCC 4.4, but the change can lead to differences in structure layout. GCC lets you know when the offset of such a field has changed in GCC 4.4. For example, there’s no longer a 4-bit padding between field a and b in this structure:

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

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

• -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warns 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 — warns if padding is included in a structure, either to align an element or to align the whole structure. Sometimes you can rearrange the fields to reduce padding and make the structure smaller. So it’s a glow-up tip. ✨
• -Wredundant-decls — warns if anything is declared more than once in the same scope, even if the multiple declaration is valid and changes nothing. Like, we get it, you’re extra. 💅
• -Wrestrict — warns when an object referenced by a restrict-qualified parameter (or in C++, a __restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to strcpy below tries to truncate the string by replacing its initial characters with the last four. But because the call writes the terminating NUL into a[4], the copies overlap and the call is diagnosed.

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

The -Wrestrict option catches some simple overlaps even without optimization, but it works best at -O2 and above. It’s included in -Wall. 🔥

• -Wnested-externs (C and Objective-C only) — warns if an extern declaration is encountered within a function. Like, why are you doing that? 🙈
• -Winline — bestie, this one yells at you if a function you declared inline just... cannot be inlined. 💀 And like, even with this flag on, the compiler stays silent about failing to inline stuff declared in system headers — it's not gonna snitch on those.

The way the compiler picks what to inline is so not a vibe-check-free zone — it runs a bunch of heuristics 🧚. For example, it weighs how big the function being inlined is, plus how much inlining already happened in the current function. Which means a change in your source that looks totally insignificant can make -Winline warnings pop into existence or just ghost you entirely. ✨

• -Winterference-size — warns when you use C++17's std::hardware_destructive_interference_size without setting its value via --param destructive-interference-size. Also side-eyes questionable values you give that option. 🙈

This variable exists to control class layout, so you can dodge false sharing in concurrent code:

        struct independent_fields {
          alignas(std::hardware_destructive_interference_size)
            std::atomic<int> one;
          alignas(std::hardware_destructive_interference_size)
            std::atomic<int> two;
        };

Here one and two are supposed to be far enough apart that storing to one won't need touching two just to reload the cache line. no cap. 🔥

So by default, --param destructive-interference-size and --param constructive-interference-size get set based on the current -mtune option — usually the L1 cache line size for that particular target CPU, and sometimes a range if you're tuning for a generic target. So like: every translation unit that depends on ABI compatibility for these variables must be compiled with the same -mtune (or -mcpu). periodt.

If ABI stability matters — say, the use is in a header for a library — you should probably not use the hardware interference size variables at all. Or, alternatively, force a particular value with --param. 💅

And if you're confident your use of the variable doesn't affect ABI outside a single build of your project, you can just turn the warning off with -Wno-interference-size. slay.

• -Wint-in-bool-context — warns for suspicious use of integer values where boolean ones are expected 😭. Like conditional expressions (?:) using non-boolean integer constants in boolean context, e.g. if (a <= b ? 2 : 3). Or left-shifting signed integers in boolean context, like for (a = 0; 1 << a; a++);. Same energy for all kinds of multiplications regardless of data type. Enabled by -Wall.
• -Wno-int-to-pointer-cast — suppresses warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.
• -Wno-pointer-to-int-cast (C and Objective-C only) — suppresses warnings from casts from a pointer to an integer type of a different size.
• -Winvalid-pch — warns if a precompiled header is found in the search path but can't be used. the betrayal 🥀
• -Winvalid-utf8 — warns if an invalid UTF-8 character is found. On by default for C++23 if -finput-charset=UTF-8 is used, and turned into an error with -pedantic-errors. 👑
• -Wno-unicode — don't diagnose invalid forms of delimited or named escape sequences that are treated as separate tokens. Wunicode is enabled by default.
• -Wlong-long — warns if the long long type is used. Enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long.

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

• -Wvariadic-macros — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. Enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
• -Wno-varargs — do not warn upon questionable usage of the macros used to handle variable arguments like va_start. These warnings are enabled by default.
• -Wvector-operation-performance — warns if a vector operation is not implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning. A vector operation can be implemented "piecewise" — meaning the scalar operation is performed on every vector element; "in parallel" — meaning the vector operation uses scalars of a wider type, which normally is more performance efficient; and "as a single scalar" — meaning the vector fits into a scalar type. 🦋
• -Wvla — warns if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-length array.

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

• -Wvla-larger-than=byte-size — if this option is used, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that lets the array size exceed byte-size bytes. Similar to how -Walloca-larger-than=byte-size works, but for variable-length arrays.

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

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

See also -Walloca-larger-than=byte-size.

• -Wno-vla-larger-than — disables -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than=SIZE_MAX or larger.
• -Wvla-parameter — warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds.

If the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function, and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size, or accessing more than the maximum number of elements, may be diagnosed. 🔍

For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f, while the second one specifies that the array argument must have at least n elements. In addition, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.

        void f (int n, int[n]);
        // warning: argument 2 previously declared as a VLA
        void f (int, int[]);

        void g (int n)
        {
            if (n > 4)
              return;
            int a[n];
            // warning: access to a by f may be out of bounds
            f (sizeof a, a);
          ...
        }

-Wvla-parameter is included in -Wall. The -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments.

• -Wvolatile-register-var — warns if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. Enabled by -Wall.
• -Wno-xor-used-as-pow (C, C++, Objective-C and Objective-C++ only) — disables warnings about uses of ^, the exclusive or operator, where it looks like the code meant exponentiation. Specifically, the warning fires when the left-hand side is the decimal constant 2 or 10 and the right-hand side is also a decimal constant.

In C and C++, ^ means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. 😭

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

• -Wdisabled-optimization — warns if a requested optimization pass is disabled. This warning does not generally mean anything's wrong with your code; it just means GCC's optimizers couldn't handle the code effectively. Often the problem is that your code is too big or too complex — GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. 💀
• -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. Only supported for C and Objective-C. Implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.

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

• -Wstack-protector — only active when -fstack-protector is active. Warns about functions that are not protected against stack smashing. 🛡️
• -Woverlength-strings — warns about string constants longer than the "minimum maximum" length specified in the C standard. Modern compilers generally allow string constants 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++.

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. 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 link-time optimization, do not warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default.
• -Wno-designated-init (C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with the designated_init attribute.

🧠 Options That Control Static Analysis 🧠

• -fanalyzer — bestie. this turns on static analysis of program flow, which sniffs around for "interesting" interprocedural paths through your code and then yells at you with warnings about whatever problems it found on them. 💅

Heads up tho: this analysis is way more expensive than other GCC warnings. like, it's not cheap. 🫠

In technical terms, it performs coverage-guided symbolic execution of the code being compiled. And it is neither sound nor complete — it can have false positives and false negatives. It's a bug-finding tool, not a tool for proving your program is correct. no cap, it cannot make that promise.

The analyzer is only suitable for use on C code in this release. that's the vibe, that's the limit. ✨

Enabling this option effectively enables the following warnings:

-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-ptrdiff
-Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal

This option is only available if GCC was configured with analyzer support enabled. if it's not there, it's just not there 🙈

• -Wanalyzer-symbol-too-complex — okay so if -fanalyzer is enabled, the analyzer busts out various heuristics to try and track the state of memory… but sufficiently complicated code can defeat them. 😭

By default, the analysis silently stops tracking values of expressions if they exceed the threshold defined by --param analyzer-max-svalue-depth=*value*, and falls back to an imprecise representation for such expressions. The -Wanalyzer-symbol-too-complex option warns you when this happens. so now you actually know. 🫶

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

By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it 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).

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, which suggests 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. it's giving "the check was late to the party." 🎀

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.

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

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

This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either free, or a deallocator referenced by attribute malloc.

See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html).

• -Wno-analyzer-exposure-through-output-file — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). bestie do not do that. 😭

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

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

This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). 🙈

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

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

This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa. wrong direction, queen. 💅

This diagnostic also warns for code paths in a which a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.

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

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

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

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

This diagnostic warns for paths through code in which an open file descriptor is leaked.

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. it's giving "skipped a whole step of the lore." 🌸

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. 🦋

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

This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor.

This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a closed file descriptor at referenced argument N. 💀

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

This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity. main character move but not in a good way ✨

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 — bestie, 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). that's giving "memory management gone rogue" energy 😭

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. at the moment, it only warns on use of floating-point operands inside the calculation of an allocation size. ngl that's a very specific vibe check.

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

This diagnostic warns for paths through the code which appear to lead to an infinite loop. 💀 iconic but deadly

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 subsequent iterations, leading to an infinite loop. not the loop you want, bestie

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 warns for paths through the code which appear to lead to infinite recursion. the call stack said "we're going down together" 😭

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

This diagnostic is likely to miss cases of infinite recursion that are 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.

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. jumping through a NULL? bestie that's a segfault speedrun 💅

• -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. the memory said "I'm never coming 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 the pointer value. The diagnostic will warn about mismatches between free, scalar delete and vector delete[], and those marked as allocator/deallocator pairs using attribute malloc. matching allocator/deallocator sets? it's giving "we are not the same" 😭

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. buffer said "stay in your lane" and the code did not 💀

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

For cases where the analyzer is able, it will emit a text art diagram visualizing the spatial relationship between the memory region that the analyzer predicts would be accessed, versus the range of memory that is valid to access: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data involved. little ASCII art moment ✨ 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. undefined behavior? we don't know her 🫠

Specifically, the diagnostic occurs on calls to the following functions

• memcpy
• strcat
• strcpy

for cases where the buffers are known to overlap.

• -Wno-analyzer-possible-null-argument — 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. the attribute said "nonnull" and the code said "maybe" 😭

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. dereferencing a maybe-NULL? that's a gamble bestie 🎲

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. known NULL? that's not a maybe, that's a definite no 💀

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. NULL deref? we hate to see it 😭

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. putenv said "I keep this pointer forever" and the stack said "I'm leaving" 🫶

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. 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. negative shift count? the bits said "not today" 💅

• -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, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. shift count overflow? precision left the chat 😭

• -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. longjmp into a function that already left? bestie that's a ghost 💀

When setjmp gets called on a jmp_buf to record a rewind location, bestie, it records the stack frame. And that stack frame goes 💀 the moment the function containing the setjmp call returns. So if you try to rewind to it via longjmp, you'd be referencing a stack frame that literally doesn't exist anymore — and that's giving crash. Or worse. Not the vibe.

• -Wno-analyzer-tainted-allocation-size — needs -fanalyzer to be on (that's what enables it), so use -Wno-analyzer-tainted-allocation-size to turn it off.

This diagnostic clocks paths through the code where a value that could be under an attacker's control gets used as the size of an allocation without being sanitized — meaning an attacker could inject an excessively large allocation and potentially cause a denial of service attack. denial of service is NOT the moment 😭

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

• -Wno-analyzer-tainted-assertion — needs -fanalyzer to be on (it enables it); use -Wno-analyzer-tainted-assertion to disable it.

This one snitches on paths through the code where a value that could be under an attacker's control is used as part of a condition without being sanitized first, and that condition guards a call to a function marked with attribute noreturn (like the function __builtin_unreachable). Those functions typically mean the program is terminating abnormally, e.g. assertion failure handlers. Like, 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.

Quick note: when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer even has a chance to "see" them, so this diagnostic can only generate warnings on builds where assertion-checking is enabled. ✨

For the purpose of this warning, any function marked with attribute noreturn counts as a possible assertion failure handler, __builtin_unreachable included. And heads up — these functions are sometimes removed by the optimizer before the analyzer "sees" them. So optimization should be disabled when you're trying to trigger this diagnostic.

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

The warning can also report problematic constructions such as

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

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

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

• -Wno-analyzer-tainted-array-index — needs -fanalyzer to be on (it enables it); use -Wno-analyzer-tainted-array-index to disable it.

This diagnostic warns for paths through the code where a value that could be under an attacker's control is used as the index of an array access without being sanitized, so an attacker could inject an out-of-bounds access. out-of-bounds? jail. immediately.

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

• -Wno-analyzer-tainted-divisor — needs -fanalyzer to be on (it enables it); use -Wno-analyzer-tainted-divisor to disable it.

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

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

• -Wno-analyzer-tainted-offset — needs -fanalyzer to be on (it enables it); use -Wno-analyzer-tainted-offset to disable it.

This diagnostic warns for paths through the code where a value that could be under an attacker's control is used as a pointer offset without being sanitized, so an attacker could inject an out-of-bounds access.

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

• -Wno-analyzer-tainted-size — needs -fanalyzer to be on (it enables it); use -Wno-analyzer-tainted-size to disable it.

This diagnostic warns for paths through the code 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-undefined-behavior-ptrdiff — needs -fanalyzer (which enables it); use -Wno-analyzer-undefined-behavior-ptrdiff to disable it.

This diagnostic warns for paths through the code where a pointer subtraction happens but the pointers refer to different chunks of memory. That code is relying on undefined behavior, bestie — pointer subtraction is only defined for cases where both pointers point within (or just after) the same array. undefined behavior is never the serve, no cap.

See CWE-469: Use of Pointer Subtraction to Determine Size (https://cwe.mitre.org/data/definitions/469.html).

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

This diagnostic warns for paths through the code where a call is made to strtok with undefined behavior.

Specifically: passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior.

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

This diagnostic warns for paths through the code where a function known to be async-signal-unsafe (such as fprintf) is called from a signal handler. signal handler said do NOT call me like that.

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

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

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

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

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

This diagnostic warns for paths through the code where a pointer is dereferenced that points to a variable in a stale stack frame. 🪦

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

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

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

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

This diagnostic warns for interprocedural paths through the code where the analyzer detects an attempt to use va_arg to access the next value passed to a variadic call, but all of the values in the va_list have already been consumed. nothing left in the bag, bestie 😭

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

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

This diagnostic warns for interprocedural paths through the code 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 — needs -fanalyzer (which enables it); use -Wno-analyzer-va-list-use-after-va-end to disable it.

This diagnostic warns for interprocedural paths through the code where the analyzer detects an attempt to use a va_list after va_end has been called on it. va_list. girl it's OVER over 💅

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

This diagnostic warns for paths through the code where the analyzer detects an attempt to write through a pointer to a const object. That said, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. she's not really checking that hard, jsyk.

• -Wno-analyzer-write-to-string-literal — needs -fanalyzer (which enables it); use -Wno-analyzer-write-to-string-literal to disable it.

This diagnostic warns for paths through the code where the analyzer detects an attempt to write through a pointer to a string literal. However — same deal — 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 — needs -fanalyzer (which enables it); use -Wno-analyzer-use-of-uninitialized-value to disable it.

This diagnostic warns for paths through the code where 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: 🧚

Okay so boom, the analyzer has a whole little squad of functions it's obsessed with — it knows their vibes intimately and watches them like a hawk. Here's the roster, bestie:

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

--- and of these functions for working with file descriptors:

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

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

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

--- and also these ones, the whole crew:

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

Plus, pookie, there's a whole other vibe: any function with an __analyzer_ prefix has special meaning to the analyzer, and that's all documented over in the GCC Internals manual. Go look, I'll wait. 💅

Now the parameters that control how far the analyzer goes snooping around, these are the levers:

• --param analyzer-bb-explosion-factor=value
• --param analyzer-max-enodes-per-program-point=value
• --param analyzer-max-recursion-depth=value
• --param analyzer-min-snodes-for-call-summary=value

And here come the options that actually steer the analyzer. Buckle up, there's a lot: 🌸

• -fanalyzer-call-summaries — this one simplifies the interprocedural analysis by computing the effect of certain calls, instead of exploring all the paths through the function from the callsite to each possible return. Like, we're summarizing instead of reading the whole essay, periodt.

If it's enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value).

• -fanalyzer-checker=name — restricts the analyzer to run just the named checker, and enables it. One checker, main character energy, nobody else invited. 👑
• -fanalyzer-debug-text-art-headings — this option is intended for analyzer developers. If you enable it, the analyzer will add extra annotations to any diagrams it generates. Cute little footnotes for the art, fr fr.
• -fno-analyzer-feasibility — this option is intended for analyzer developers.

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

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

Internally the analyzer builds an exploded graph that combines control flow graphs with data flow information.

By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge. Every statement gets its own moment, as it deserves. ✨

• -fanalyzer-show-duplicate-count — this option is intended for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic. So it's like "and 47 others…" but for diagnostics. 💀
• -fanalyzer-show-events-in-system-headers — By default the analyzer emits simplified diagnostics paths by hiding events fully located within a system header. With -fanalyzer-show-events-in-system-headers such events are no longer suppressed. Nothing gets hidden behind the system header curtain anymore.
• -fno-analyzer-state-merge — this option is intended for analyzer developers.

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

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

By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.

With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.

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

By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics. Basically it knows when to stop talking. 🙈

The diagnostics that terminate analysis along a path are:

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

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

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

-fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code. It's showing up to the party after everyone already got ready, basically. 😭

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

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

• -fanalyzer-verbose-edges — this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. The yapper setting.
• -fanalyzer-verbose-state-changes — this option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.
• -fanalyzer-verbosity=level — this option controls the complexity of the control flow paths that are emitted for analyzer diagnostics.

The level can be one of:

• 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 — This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.

• -fdump-analyzer — Dump internal details about what the analyzer is doing to file.analyzer.txt. -fdump-analyzer-stderr overrides this option.
• -fdump-analyzer-stderr — Dump internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer.
• -fdump-analyzer-callgraph — Dump a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot.
• -fdump-analyzer-exploded-graph — Dump a representation of the exploded graph suitable for viewing with GraphViz to file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. Aesthetic and informative, we love to see it. 🎀
• -fdump-analyzer-exploded-nodes — Emit diagnostics showing where nodes in the exploded graph are in relation to the program source.
• -fdump-analyzer-exploded-nodes-2 — Dump a textual representation of the exploded graph to file.eg.txt.
• -fdump-analyzer-exploded-nodes-3 — Dump a textual representation of the exploded graph to one dump file per node, to file.eg-id.txt. This is typically a large number of dump files. Like, a lot a lot. Prepare your directory. 🫶
• -fdump-analyzer-exploded-paths — Dump a textual representation of the exploded path for each diagnostic to file.idx.kind.epath.txt.
• -fdump-analyzer-feasibility — Dump 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 — Dump 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. It's giving "we'll be here forever" but make it a file. 🔥
• -fdump-analyzer-json — Dump a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change. So like, don't get too attached to the exact shape of it, bestie. 💜
• -fdump-analyzer-state-purge — same energy as -fdump-analyzer-supergraph (dump of the "supergraph" for GraphViz), except this version ✨annotates✨ the graph so you can see exactly what state is gonna get purged at each node. Gets written to file.state-purge.dot.
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" for GraphViz viewing into file.supergraph.dot and file.supergraph-eg.dot. These show every control flow graph in the program, plus interprocedural edges for calls and returns. The second dump is the fancy one — it has annotations showing nodes in the "exploded graph" and the diagnostics attached to them. very main character of it 💜
• -fdump-analyzer-untracked — emits custom warnings with internal details, and yeah it's meant for analyzer developers specifically. iykyk 🧚

🐛 Options for Debugging Your Program 🐛

Okay so to get GCC to spit out extra info your debugger can use, like, almost every single time you just slap -g onto your other options and call it a day. Some debug formats can coexist btw (like DWARF hanging out with CTF 🫶) — but only when you explicitly turn on each one by adding its own command-line option alongside everything else.

GCC absolutely lets you use -g together with -O. Now the shortcuts optimized code takes can be a lil unhinged sometimes: variables you declared might not even exist anymore 💀, control flow might briefly detour somewhere you did not expect, some statements might just never run because they compute constant results or the value's already sitting right there, and some statements might execute in totally different places because they got hoisted out of loops. But still — debugging optimized output is possible, and that's exactly why it's reasonable to run the optimizer on programs that might have bugs.

If you're not using some other optimization option, consider -Og alongside -g. Because with no -O option at all, some compiler passes that collect debugging-useful info just don't run, so -Og can genuinely give you a better debugging experience. 👑

• -g — produce debugging info in the operating system's native format (that's stabs, COFF, XCOFF, or DWARF). GDB can work with this debug info.

Heads up tho: on most systems using stabs format, -g turns on extra debug info that only GDB can use. That extra info makes debugging better in GDB, but probably makes other debuggers crash or straight up refuse to read the program 😭. If you wanna control for certain whether that extra info gets generated, use -gvms (more on that below).

• -ggdb — produce debugging info for GDB. This means using the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if that's possible at all.
• -gdwarf, -gdwarf-version — produce debugging info in DWARF format (if that's supported). The value of version can be either 2, 3, 4 or 5; the default version for most targets is 5 — with the exception of VxWorks, TPF and Darwin / macOS, which default to version 2, and AIX, which defaults to version 4.

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher.

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but they apply to all currently-supported versions of DWARF. (blame the vibes of history, not us)

• -gbtf — request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug info can be generated alongside DWARF debug info when both formats are enabled explicitly via their respective command line options. 🌈
• -gprune-btf, -gno-prune-btf — prune BTF information before emission. When pruning, only type info for types used by global variables and file-scope functions gets emitted. If you're compiling for the BPF target with BPF CO-RE enabled, type info also gets emitted for types used in BPF CO-RE relocations. On top of that, struct and union types that 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 the produced BTF info — but at a 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 info that isn't immediately relevant to the BPF program loading process.

This option is enabled by default for the BPF target when generating BTF information.

• -gctf, -gctflevel — request CTF debug information and use level to specify how much CTF debug info should be produced. If -gctf is specified without a value for level, the default level of CTF debug information is 2.

CTF debug info can be generated alongside DWARF debug info when both debug formats are enabled explicitly via their respective command line options. 🎀

• Level 0 produces no CTF debug information at all. So -gctf0 negates -gctf.
• Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.
• Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.

• -gvms — produce debugging info in Alpha/VMS debug format (if that's supported). This is the format used by DEBUG on Alpha/VMS systems.
• -gcodeview — produce debugging info in CodeView debug format (if that's supported). This is the format used by Microsoft Visual C++ on Windows.
• -glevel, -ggdblevel, -gvmslevel — request debugging info and also use level to specify how much information. The default level is 2.
  • Level 0 produces no debug information at all. Thus -g0 negates -g.
  • Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no info about local variables.
  • Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3. 🔥

If you use multiple -g options, with or without level numbers, the last such option is the one that's effective. Periodt.

-gdwarf does not accept a concatenated debug level — that's to avoid confusion with -gdwarf-level. Instead, use an additional -glevel option to change the debug level for DWARF.

• -fno-eliminate-unused-debug-symbols — by default, no debug info is produced for symbols that aren't actually used. Use this option if you want debug info for all the symbols. 🙈
• -femit-class-debug-always — instead of emitting debugging info 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 can't handle the way GCC normally emits debugging info for classes — because using it increases the size of debugging info by as much as a factor of two. 💀
• -fno-merge-debug-strings — tell the linker to not merge together strings in the debugging info that are identical across different object files. Merging isn't supported by all assemblers or linkers. Merging decreases the size of the debug info in the output file, at the cost of increasing link processing time. Merging is enabled by default.
• -fdebug-prefix-map=old=new — when compiling files residing in directory old, record debugging info describing them as if the files resided in directory new instead. This can be used to swap a build-time path for an install-time path in the debug info. It can also change an absolute path to a relative path by using . for new. This can give you more reproducible builds, which are location independent — but it may require an extra command to tell GDB where to find the source files. See also -ffile-prefix-map and -fcanon-prefix-map. 🦋
• -fvar-tracking — run the variable tracking pass. It computes where variables are stored at each position in code. Better debugging info then gets generated (if the debugging info format supports that info).

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

• -fvar-tracking-assignments — annotate assignments to user variables early in the compilation, then attempt to carry those annotations all the way through to the end, all in an attempt to improve debug info while optimizing. Using -gdwarf-4 alongside it is recommended.

It can be enabled even if var-tracking is disabled — in that case annotations are created and maintained, but discarded at the end. By default this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.

• -gsplit-dwarf — if DWARF debugging info is enabled, separate as much debugging info as possible into a separate output file with the .dwo extension. This option lets the build system avoid linking files that have debug info. To actually be useful, this option requires a debugger capable of reading .dwo files.
• -gdwarf32, -gdwarf64 — if DWARF debugging info is enabled, -gdwarf32 selects the 32-bit DWARF format and -gdwarf64 selects the 64-bit DWARF format. The default is target specific, though on most targets it's -gdwarf32. The 32-bit DWARF format is smaller, but it can't support more than 2GiB of debug info in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug info, but it might not be well supported by all consumers yet. 🫶
• -gdescribe-dies — add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. ✨

---

• -gpubnames — ok so this one tells DWARF to spit out .debug_pubnames and .debug_pubtypes sections. that's it, that's the whole vibe 🎀
• -ggnu-pubnames — same two sections, .debug_pubnames and .debug_pubtypes, but formatted so they can be converted into a GDB index. literally only useful with a linker that can produce GDB index version 7, so check before you commit bestie ✨
• -fdebug-types-section — when you're on DWARF Version 4 or higher, type DIEs can get their own .debug_types section instead of being crammed into .debug_info. separate comdat section = more efficient, because the linker can just remove duplicates like 🫶. BUT — and this is the important part — not all DWARF consumers support .debug_types yet, and on some objects .debug_types produces larger debugging information instead of smaller. so it's giving gamble energy 😭
• -grecord-gcc-switches, -gno-record-gcc-switches — this appends the command-line options you used to invoke the compiler (the ones that may affect code generation) onto the DW_AT_producer attribute in DWARF debugging information. the options get concatenated with spaces separating them from each other and from the compiler version. enabled by default, no cap. also peep -frecord-gcc-switches if you want another way of storing compiler options into the object file 👑
• -gstrict-dwarf — disallow using extensions of a later DWARF standard version than the one you selected with -gdwarf-version. fyi on most targets, using non-conflicting DWARF extensions from later standard versions is allowed.
• -gno-strict-dwarf — allow using extensions of later DWARF standard version than selected with -gdwarf-version. the chill option 💅
• -gas-loc-support — tells the compiler the assembler supports .loc directives, so it may use them for the assembler to generate DWARF2+ line number tables. this is generally desirable, because assembler-generated line-number tables are a lot more compact than the ones the compiler can generate itself. enabled by default if, at GCC configure time, the assembler was found to support such directives ✨
• -gno-as-loc-support — force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated.
• -gas-locview-support — informs the compiler the assembler supports "view" assignment and reset assertion checking in .loc directives. enabled by default if, at GCC configure time, the assembler was found to support them 🌸
• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
• -gcolumn-info, -gno-column-info — emit location column info into DWARF debugging information instead of just file and line. enabled by default, periodt 🎀
• -gstatement-frontiers, -gno-statement-frontiers — makes GCC create markers in the internal representation at the beginning of statements, and keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. this is enabled by default when compiling with optimization (-Os, -O1, -O2, ...), and outputting DWARF 2 debug information at the normal level.
• -gvariable-location-views, -gvariable-location-views=incompat5, -gno-variable-location-views — augments variable location lists with progressive view numbers implied from the line number table. this lets debug info consumers inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. if the assembler lacks support for view numbers in line number tables, this will make the compiler emit the line number table, which generally makes them somewhat less compact. the augmented line number tables and location lists are fully backward-compatible, so debug info consumers that aren't aware of these augmentations can still consume them — they just won't derive any benefit from them either 🙈

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

there is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, and it can be enabled with -gvariable-location-views=incompat5. this option may be removed in the future, it's only provided as a reference implementation of the proposed representation. debug information consumers are not expected to support this extended format, and they'd be rendered unable to decode location lists using it. so like, be careful bestie.

• -ginternal-reset-location-views, -gno-internal-reset-location-views — attempt to determine location views that can be omitted from location view lists. this requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that doesn't support location view lists 😭. the GNU assembler will flag any such error as a "view number mismatch". this is only enabled on ports that define a reliable estimation function.
• -ginline-points, -gno-inline-points — generate extended debug information for inlined functions. location view tracking markers get inserted at inlined entry points, so 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's only enabled by default if location views are enabled. the way these options chain together is giving dependency tree 🦋
• -gz[=type] — produce compressed debug sections in DWARF format, if that's supported. if type isn't given, the default type depends on the capabilities of the assembler and linker used. type may be one of none (don't compress debug sections), or zlib (use zlib compression in ELF gABI format). if the linker doesn't support writing compressed debug sections, the option is rejected. otherwise, if the assembler doesn't support them, -gz is silently ignored when producing object files 🤫
• -femit-struct-debug-baseonly — emit debug 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 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.

only works with DWARF debug output, iykyk.

• -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 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.

only works with DWARF debug output ✨

• -femit-struct-debug-detailed[=spec-list] — specify the struct-like types for which the compiler generates debug information. the goal is reducing duplicate struct debug information between different object files within the same program.

this is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and it serves most needs.

a specification has the syntax

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

the optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). a struct type is used directly when it's the type of a variable, member. indirect uses arise through pointers to structs. that is, when use of an incomplete struct is valid, the use is indirect. an example is struct one direct; struct two * indirect;.

the optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). generic structs are a bit complicated to explain — 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 the base of the name of the file in which the type declaration appears must match the base of the name of the main compilation file. in practice, that means when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. the value sys means those types satisfying base or declared in system or compiler headers.

you may need to experiment to determine the best settings for your application, fr fr.

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

this option works only with DWARF debug output.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as a compiler-generated .eh_frame section instead of using GAS .cfi_* directives.
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. sometimes it's useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they're actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that's not actually used in your program (but is declared). more often, though, this results in a significant amount of wasted space 🗑️

---

🔥 Options That Control Optimization 🔥

these options control various sorts of optimizations. buckle up bestie, this is where it gets fun 💅

Okay so, when you don't pass any optimization option at all? The compiler is literally just trying to keep compilation cheap and make debugging give you exactly what you expect. Statements stay independent, periodt 💅: 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.

The moment you turn on optimization flags, the compiler starts attempting to improve performance and/or code size — but that costs you compilation time and possibly the ability to debug the program. Fair trade bestie, sometimes 🫶

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode lets the compiler use information gained from all of the files when compiling each of them. It's giving group project, but the good kind ✨

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. iykyk 🙈

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes. So like, don't expect your little flags to do anything on their own, bestie 😭

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. Go check, it's iconic 🔥

---

-O, -O1 — Optimize 🌸

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

-O is the recommended optimization level for large machine-generated code as a sensible balance between time taken to compile and memory use: higher optimization levels perform optimizations with greater algorithmic complexity than at -O.

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants
-fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time

-O2 — Optimize even more 💖

Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code. It's giving "we're going faster but your laptop is now a space heater" 🔥

-O2 turns on all optimization flags specified by -O1. It also turns on the following optimization flags:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations
-ffinite-loops -fgcse  -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining
-fipa-bit-cp  -fipa-cp  -fipa-icf -fipa-ra  -fipa-sra  -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat -foptimize-crc
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition  -freorder-functions
-frerun-cse-after-loop -fschedule-insns  -fschedule-insns2
-fsched-interblock  -fsched-spec -fstore-merging -fstrict-aliasing
-fthread-jumps -ftree-builtin-call-dce -ftree-loop-vectorize
-ftree-pre -ftree-slp-vectorize -ftree-switch-conversion
-ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. Not me telling you twice, go read it 📢

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2, in Ubuntu 24.04 and later versions, -D_FORTIFY_SOURCE=3, is set by default, and is activated when -O is set to 2 or higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.

NOTE: In Debian 13 and Ubuntu 24.04 and later versions, -D_TIME_BITS=64 together with -D_FILE_OFFSET_BITS=64 is set by default on the 32bit architectures armel, armhf, hppa, m68k, mips, mipsel, powerpc and sh4.

-O3 — Optimize yet more 👑

Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags:

-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides

-O0 — Reduce compilation time 🦋

Reduce compilation time and make debugging produce the expected results. This is the default.

At -O0, GCC completely disables most optimization passes; they are not run even if you explicitly enable them on the command line, or are listed by -Q --help=optimizers as being enabled by default. Many optimizations performed by GCC depend on code analysis or canonicalization passes that are enabled by -O, and it would not be useful to run individual optimization passes in isolation. So like, no, your one little flag is not gonna sneak past this 💀

-Os — Optimize for size 🎀

Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. Small and cute, exactly like us 🌸

-Ofast — Disregard strict standards compliance 😈

Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens. It turns off -fsemantic-interposition.

Like, it's fast because it stopped caring, bestie 🙈

-Og — Optimize debugging experience 💜

Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0. Bestie, this is your girl for the edit-compile-debug loop 🫶

Like -O0, -Og completely disables a number of optimization passes so that individual options controlling them have no effect. Otherwise -Og enables all -O1 optimization flags except for those that may interfere with debugging:

-fbranch-count-reg  -fdelayed-branch -fdse  -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants  -fmove-loop-stores  -fssa-phiopt
-ftree-bit-ccp  -ftree-dse  -ftree-pta  -ftree-sra

-Oz — Optimize aggressively for size 🌈

Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those instructions require fewer bytes to encode. -Oz behaves similarly to -Os including enabling most -O2 optimizations.

---

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

Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one you typically use. You can figure out the other form by either removing no- or adding it.

The following options control specific optimizations. They 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.

-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.

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 — okay so this one ✨ literally sniffs out loops that are out here doing polynomial long division to calculate a CRC, and swaps them for something way faster. It catches 8, 16, 32, and 64 bit CRC, with a constant polynomial that doesn't have the leading 1 bit, and it handles both bit-forward and bit-reversed cases 🦋. So if your target has a CRC instruction and the polynomial in your source code matches the one in that instruction — boom, it generates that CRC instruction 👑. Otherwise, if the target supports a carry-less-multiplication instruction, it generates the CRC using it. And otherwise? table-based CRC, bestie, that's the fallback 🎀.

Enabled by default at -O2 and higher.

• -foptimize-sibling-calls — Optimizes sibling calls and tail recursive calls. cute little glow-up for your call stack 💅.

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

• -foptimize-strlen — Takes a bunch of standard C string functions (like strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts and turns them into faster alternatives. the way that it just rearranges your string code for you 😭✨.

Enabled at levels -O2, -O3.

• -finline-stringops[=fn] — Expands memory and string operations inline (for now, only memset), even when the length is variable or big enough that it'd normally need a loop. This one hits different alongside -ffreestanding and -fno-builtin 🫶.

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

• -fno-inline — Do not expand any functions inline apart from the ones marked with the always_inline attribute. This is the default when you're not optimizing, no cap 💀.

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

• -finline-small-functions — Integrates functions into their callers when their body is smaller than the expected function call code (so the overall size of the program gets smaller 📉). The compiler heuristically decides which functions are simple enough to be worth integrating this way. And this inlining applies to all functions, even the ones that aren't declared inline 👀.

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

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

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

• -finline-functions — Considers all functions for inlining, even if they're not declared inline. The compiler heuristically decides which ones are worth integrating in this way 🧠.

If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.

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

• -finline-functions-called-once — Considers all static functions called once for inlining into their caller, even if they're not marked inline. If a call to a given function is integrated, then the function is not output as assembler code in its own right ✨.

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

• -fearly-inlining — Inlines functions marked by always_inline, plus functions whose body seems smaller than the function call overhead, early — before doing -fprofile-generate instrumentation and the real inlining pass. Doing it this way makes profiling significantly cheaper and usually makes inlining faster on programs that have big chains of nested wrapper functions 🌸.

Enabled by default.

• -fipa-sra — Performs interprocedural scalar replacement of aggregates, removal of unused parameters, and replacement of parameters passed by reference by parameters passed by value. iconic behavior honestly 👑.

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

• -finline-limit=n — By default, GCC limits the size of functions that can be inlined. This flag gives you coarse control of that limit 🎚️. n is the size of functions that can be inlined, measured in number of pseudo instructions.

Inlining is actually controlled by a number of parameters, which you can specify individually by using --param name=value. The -finline-limit=n option sets some of these parameters like this:

• max-inline-insns-single — is set to n/2.
• max-inline-insns-auto — is set to n/2.

See below for a documentation of the individual parameters controlling inlining, and for the defaults of these parameters.

Note: there may be no value to -finline-limit that results in default behavior. 🙈

Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions, and as such its exact meaning might change from one release to an another. so don't come for it 💀.

• -fno-keep-inline-dllexport — This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the dllexport attribute or declspec 🎀.
• -fkeep-inline-functions — In C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90 🚫. In C++, emit any and all inline functions into the object file.
• -fkeep-static-functions — Emits static functions into the object file, even if the function is never used. hoarding functions, we love to see it 💖.
• -fkeep-static-consts — Emits variables declared static const when optimization isn't turned on, even if the variables aren't referenced.

GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.

• -fmerge-constants — Attempts to merge identical constants (string constants and floating-point constants) across compilation units 🦋.

This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.

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

• -fmerge-all-constants — Attempts to merge identical constants and identical variables. This option implies -fmerge-constants. On top of what -fmerge-constants does, it also considers e.g. even constant initialized arrays, or initialized constant variables with integral or floating-point types 👀.

Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations — so using this option results in non-conforming behavior ⚠️. like, be so serious about that one.

• -fmodulo-sched — Performs swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations 🔄.
• -fmodulo-sched-allow-regmoves — Performs more aggressive SMS-based modulo scheduling, with register moves allowed. By setting this flag, certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis 🧚. This option is effective only with -fmodulo-sched enabled.
• -fno-branch-count-reg — Disables the optimization pass that scans for opportunities to use "decrement and branch" instructions on a count register, instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result 🎯. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390.

Note that the -fno-branch-count-reg option doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes.

The default is -fbranch-count-reg at -O1 and higher, except for -Og.

• -fno-function-cse — Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly 📌.

This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used 💀.

The default is -ffunction-cse.

• -fno-zero-initialized-in-bss — If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS, which can save space in the resulting code ✨.

This option turns off that behavior, because some programs explicitly rely on variables going to the data section — e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that 🫶.

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

• -fthread-jumps — Performs optimizations that check to see if a jump branches to a location where another comparison, subsumed by the first, is found. If so, the first branch gets redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false 🔀.

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

• -fsplit-wide-types — When you're using a type that occupies multiple registers, such as long long on a 32-bit system, this splits the registers apart and allocates them independently 🎀. This normally generates better code for those types, but it may make debugging more difficult 😭.

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

• -fsplit-wide-types-early — Fully splits wide types early, instead of very late. This option has no effect unless -fsplit-wide-types is turned on 🚫.

This is the default on some targets.

• -fcse-follow-jumps — In common subexpression elimination (CSE), this scans through jump instructions when the target of the jump is not reached by any other path 🦋. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false.

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

• -fcse-skip-blocks — Similar to -fcse-follow-jumps, but this one causes CSE to follow jumps that conditionally skip over blocks ✨. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if.

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

• -frerun-cse-after-loop — bestie, this one re-runs common subexpression elimination after the loop optimizations have done their lil thing 🔁

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

• -fgcse — runs a global common subexpression elimination pass, and as a bonus it also does global constant and copy propagation ✨

Okay so real talk: when you're compiling a program that uses computed gotos (that's a GCC extension btw), you might actually get better run-time performance if you turn the global common subexpression elimination pass OFF by adding -fno-gcse to the command line. Just saying. 🙈

On at -O2, -O3, -Os.

• -fgcse-lm — with this enabled, global common subexpression elimination tries to move loads that are only killed by stores into themselves. Which means a loop with a load/store sequence can become a load outside the loop plus a copy/store inside the loop. Slay 🦋

It's on by default whenever -fgcse is on.

• -fgcse-sm — when this is enabled, a store motion pass runs after global common subexpression elimination. That pass tries to move stores out of loops. Pair it with -fgcse-lm and loops containing a load/store sequence can be rewritten as a load before the loop and a store after the loop 👑

Not enabled at any optimization level, btw.

• -fgcse-las — when this is on, the global common subexpression elimination pass wipes out redundant loads that come after stores to the same memory location — both partial and full redundancies 💅

Also not enabled at any optimization level.

• -fgcse-after-reload — with this enabled, a redundant load elimination pass runs after reload. Its 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 how many iterations a loop does. It assumes your loop code doesn't invoke undefined behavior — like, no signed integer overflows, no out-of-bound array accesses. Those iteration-count bounds get used to guide loop unrolling and peeling and loop exit test optimizations. It's enabled by default ✨
• -funconstrained-commons — this tells the compiler that variables declared in common blocks (think Fortran) might later get overridden with longer trailing arrays. Which prevents certain optimizations that depend on knowing the array bounds. No cap 🌸
• -fcrossjumping — performs the cross-jumping transformation, which unifies equivalent code and saves code size. Fair warning though: the resulting code may or may not perform better than without cross-jumping 💀

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 it. Enabled by default at -O1 and higher on architectures that do support it 🎀
• -fdce — performs dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher 🗑️
• -fdse — performs dead store elimination (DSE) on RTL. Also enabled by default at -O1 and higher ✨
• -fif-conversion — attempts to turn conditional jumps into branch-less equivalents. That includes conditional moves, min, max, set flags and abs instructions, plus some cute little tricks doable with standard arithmetic. Whether it uses conditional execution on chips where that's available is controlled by -fif-conversion2 🦋

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

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

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

• -fdeclone-ctor-dtor — okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones, meaning two copies of the function 😭 With this option, the base and complete variants get changed into thunks that call a common implementation. Iconic.

Enabled by -Os.

• -fdelete-null-pointer-checks — assumes that programs can't safely dereference null pointers, and that no code or data element lives at address zero. This option enables simple constant folding optimizations at all optimization levels. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless null pointer checks — those assume a memory access to address zero always traps, so if a pointer is checked after it's already been dereferenced, it can't be null 💅

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

This option is enabled by default on most targets. On AVR and MSP430, it's completely disabled. 🙈

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

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

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

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

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

• -fno-lifetime-dse — so in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the object's lifetime are dead once the object is destroyed. Normally dead store elimination takes full advantage of this. But if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable that optimization 🛑

Want to preserve stores before the constructor starts (like, because your operator new clears the object storage) but still treat the object as dead after the destructor? Use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. And -flifetime-dse=0 is equivalent to -fno-lifetime-dse. Fr fr 💖

• -flive-range-shrinkage — attempts to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets 🎀
• -fira-algorithm=algorithm — uses the specified coloring algorithm for the integrated register allocator. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring isn't implemented for all architectures, but for those targets that do support it, it's the default, because it generates better code 👑
• -fira-region=region — uses the specified regions for the integrated register allocator. The region argument should be one of these:
  • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
  • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and it's 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 it's enabled by default for -Os or -O0.
• -fira-hoist-pressure — uses IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option usually results in smaller code, but it can slow the compiler down 🐌

This option is enabled at level -Os for all targets.

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

This option is enabled at level -O3 for some targets.

• -fno-ira-share-save-slots — disables sharing of stack slots used for saving call-used hard registers living through a call. Every hard register gets its own separate stack slot, and as a result your function stack frames are bigger 📦
• -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers. Every pseudo-register that doesn't get a hard register gets its own separate stack slot, and as a result function stack frames are bigger 📦
• -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values when that's profitable 💅

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

• -fdelayed-branch — if your target machine supports it, GCC tries to reorder instructions so it can actually use the instruction slots that sit there waiting after delayed branch instructions. Enabled at levels -O1, -O2, -O3, -Os, but not at -Og. 🌸
• -fschedule-insns — if the target machine supports it, GCC tries to reorder instructions so it can kill off execution stalls caused by data not being ready when something needs it. This is a big slay for machines with slow floating point or memory load instructions, because it lets other instructions get issued while we're all just waiting around for the load or the floating-point instruction's result. 💅

The conventional vibe is enabled at -O2 and -O3. But — and this is the tea — a lot of targets straight up override that. Like on x86 it's disabled at all levels, while on AArch64 it's only enabled at -O3. So don't assume bestie. 🫶

• -fschedule-insns2 — basically -fschedule-insns's little sibling, except it asks for an extra pass of instruction scheduling after register allocation has already happened. This one is chef's kiss on machines with a relatively small number of registers and where memory load instructions take more than one cycle. Enabled at levels -O2, -O3, -Os. ✨
• -fno-sched-interblock — turns off instruction scheduling across basic blocks, which is normally on when we're scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. 🔥
• -fno-sched-spec — turns off speculative motion of non-load instructions, which is normally on when we're scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. 🦋
• -fsched-pressure — turns on register pressure sensitive insn scheduling before register allocation. This only makes sense when scheduling before register allocation is already enabled, i.e. with -fschedule-insns or at -O2 or higher. Using it can improve the generated code and shrink it too, by stopping register pressure from ballooning above the number of available hard registers and the spills that come after in register allocation. Iconic behavior 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 bestie it's literally called dangerous, and it only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. 💀
• -fsched-stalled-insns, -fsched-stalled-insns=n — this one defines how many insns (if any) can be yanked prematurely out of the queue of stalled insns and dropped into the ready list during the second scheduling pass. -fno-sched-stalled-insns means no insns get moved prematurely, while -fsched-stalled-insns=0 means there's no limit at all on how many queued insns can be moved prematurely. And -fsched-stalled-insns with no value at all is equivalent to -fsched-stalled-insns=1. 🎀
• -fsched-stalled-insns-dep, -fsched-stalled-insns-dep=n — sets how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. Heads up: this only does anything during the second scheduling pass, and only if -fsched-stalled-insns is being used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0, and -fsched-stalled-insns-dep with no value is equivalent to -fsched-stalled-insns-dep=1. ⭐
• -fsched2-use-superblocks — when scheduling after register allocation, use superblock scheduling. This lets motion happen across basic block boundaries, which gives you faster schedules. ⚠️ This option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. Only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher. 🙈
• -fsched-group-heuristic — turns on the group heuristic in the scheduler, which favors the instruction that belongs to a schedule group. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 👑
• -fsched-critical-path-heuristic — turns on the critical-path heuristic in the scheduler, which favors instructions sitting on the critical path. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🔥
• -fsched-spec-insn-heuristic — turns on the speculative instruction heuristic, which favors speculative instructions with greater dependency weakness. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. ✨
• -fsched-rank-heuristic — turns on the rank heuristic, which favors the instruction that belongs to a basic block with greater size or frequency. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 💜
• -fsched-last-insn-heuristic — turns on the last-instruction heuristic, which favors the instruction that's less dependent on the last instruction scheduled. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 🦋
• -fsched-dep-count-heuristic — turns on the dependent-count heuristic, which favors the instruction that has more instructions depending on it. On by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher. 💖
• -freschedule-modulo-scheduled-loops — so modulo scheduling happens before traditional scheduling, right? And if a loop is modulo scheduled, later scheduling passes might go in and change its schedule. Use this option to control that behavior. 🌸
• -fselective-scheduling — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass. 💫
• -fselective-scheduling2 — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass. 💫
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling. No effect at all 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. No effect at all unless -fsel-sched-pipelining is turned on. 🌈
• -fsemantic-interposition — some object formats, like ELF, let the dynamic linker interpose symbols. What that means is: for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because it has to assume the function or variable in question might change on it. This is genuinely useful sometimes — like when you wanna rewrite memory allocation functions with a debugging implementation — but it's expensive in terms of code quality. 😭

With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Same deal for variables: if interposition happens, the constructor of the variable will be the same. Lowkey freeing. This flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak. 💅

• -fshrink-wrap — emits function prologues only right before the parts of the function that actually need them, instead of dumping them 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 only get executed when they're actually needed. On by default, but 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 those registers around such calls. This allocation only happens when it seems like it'll result in better code. On certain machines this is always enabled by default — 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 there's no need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. Enabled at levels -O2, -O3, -Os — however, it's disabled if the generated code is gonna 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 — attempts to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400. 🌸
• -ftree-reassoc — performs reassociation on trees. Enabled by default at -O1 and higher. 🦋
• -fcode-hoisting — performs code hoisting. Code hoisting tries to move the evaluation of expressions that get executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed too. Enabled by default at -O2 and higher. 👑
• -ftree-pre — performs 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 — performs forward propagation on trees. Enabled by default at -O1 and higher. ✨
• -ftree-fre — does full redundancy elimination (FRE) on trees. So like, the tea on FRE vs PRE: FRE only looks at expressions that get computed on every single path leading up to the redundant computation. That means the analysis is faster than PRE, but bestie, it also exposes fewer redundancies. It's a tradeoff, it's giving "quick lil win". On by default at -O1 and up. ✨
• -ftree-phiprop — hoists loads from conditional pointers on trees. Enabled by default at -O1 and higher. 💅
• -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else, but only if those loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. Very picky, very specific, we love that for it. On by default at -O2 and higher. 🦋
• -ftree-copy-prop — does copy propagation on trees, which nukes all those unnecessary copy operations. Enabled by default at -O1 and up. 🎀
• -fipa-pure-const — figures out which functions are pure or constant. Default at -O1 and higher. 💖
• -fipa-reference — figures out which static variables do not escape the compilation unit. Default at -O1 and higher. 🌸
• -fipa-reference-addressable — figures out read-only, write-only, and non-addressable static variables. Default at -O1 and higher. ⭐
• -fipa-reorder-for-locality — groups call chains close together in the binary layout to make code locality better and to minimize jump distances between functions that get called a lot. Now, unlike -freorder-functions, this pass looks at the call chains between functions and groups them together, instead of grouping all the hot/normal/cold/never-executed functions into separate sections. And unlike -fprofile-reorder-functions, it's trying to improve code locality across the whole runtime of the program, not just focusing on startup. Heads up though: this option is incompatible with an explicit -flto-partition= option, because it enforces its own custom partitioning scheme. If you're using this, it's recommended that you also use profile feedback — but otherwise this option is not enabled by default. 😭
• -fipa-stack-alignment — reduces stack alignment on call sites if it can. Enabled by default. 🫶
• -fipa-pta — does interprocedural pointer analysis plus interprocedural modification and reference analysis. Genuinely a lot. On large compilation units this option can cause excessive memory and compile-time usage, so be careful. It is not enabled by default at any optimization level. 💀
• -fipa-profile — does interprocedural profile propagation. Functions that are only called from cold functions get marked cold. Also, functions that execute exactly once (like "cold", "noreturn", static constructors or destructors) get identified. Then the cold functions and the loop-less parts of those once-executed functions get optimized for size. Enabled by default at -O1 and higher. 🔥
• -fipa-modref — does interprocedural mod/ref analysis. This one analyzes the side effects of functions — like which memory locations get modified or referenced — and it enables better optimization across the function call boundary. Enabled by default at -O1 and higher. 👑
• -fipa-cp — does interprocedural constant propagation. It analyzes the program to figure out when values passed to functions are actually constants, and then optimizes accordingly. If your app passes constants to functions, this can substantially increase performance — no cap. Enabled by default at -O2, -Os and -O3. It's also enabled by -fprofile-use and -fauto-profile. 🌈
• -fipa-cp-clone — does function cloning to make interprocedural constant propagation stronger. When it's on, interprocedural constant propagation clones a function whenever an externally visible function can be called with constant arguments. Because this can create multiple copies of functions, it may significantly increase code size (see --param ipa-cp-unit-growth=<value>). Enabled by default at -O3. Also enabled by -fprofile-use and -fauto-profile. 💜
• -fipa-bit-cp — when enabled, does interprocedural bitwise constant propagation. Enabled by default at -O2, and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled. 🧚
• -fipa-vrp — when enabled, does interprocedural propagation of value ranges. Enabled by default at -O2. It requires that -fipa-cp is enabled. 🍓
• -fipa-icf — does Identical Code Folding for functions and read-only variables. This reduces code size, and it may disturb unwind stacks by replacing a function with an equivalent one that has a different name. It works more effectively when link-time optimization is enabled.

Okay so the behavior is similar to the Gold Linker's ICF optimization, but GCC ICF works on different levels, which means the optimizations are not the same — there are equivalences only GCC finds and equivalences only Gold finds. Periodt.

Enabled by default at -O2 and -Os. 💅

• -flate-combine-instructions — enables two instruction combination passes that run, like, relatively late in the compilation process. One of the passes runs before register allocation and the other runs after register allocation. The main aim of both is to substitute definitions into all uses. Most targets enable this flag by default at -O2 and -Os. ✨
• -flive-patching=level — controls GCC's optimizations so the output is suitable for live-patching. Okay this one needs a whole explanation, bestie, stay with me:

If the compiler's optimization uses a function's body — or info extracted from its body — to optimize/change another function, then that other function is called an impacted function of the first one. And if a function gets patched, its impacted functions should be patched too. That's the whole deal.

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

Usually the more IPA optimizations you enable, the bigger the number of impacted functions per function. So to control how many impacted functions there are and to make computing that list easier, IPA optimizations can be partially enabled at two different levels.

The level argument should be one of these:

• inline-clone — only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. So as a result, when you patch a function, all its callers and its clones' callers are impacted, which means they need to be patched too.

-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref

• inline-only-static — only enables inlining of static functions. So when you patch a static function, all its callers are impacted and therefore need to be patched as well.

On top of all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static also disables these additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp

When -flive-patching is specified without any value, the default value is inline-clone.

This flag is disabled by default.

Note that -flive-patching is not supported with link-time optimization (-flto). 🫠

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior because of dereferencing a null pointer. It isolates those paths from the main control flow and turns the statement with the erroneous/undefined behavior into a trap. Enabled by default at -O2 and higher, and it depends on -fdelete-null-pointer-checks also being enabled. 😳
• -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior because a null value got 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 the erroneous/undefined behavior into a trap. This is not currently enabled, but it may be enabled by -O2 in the future. 🔮
• -ftree-sink — does forward store motion on trees. 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 it's 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 info about the uses of a value up the definition chain so the definitions can be simplified. Like, for example, this pass strips sign operations if the sign of a value never actually matters. Enabled by default at -O1 and higher. 🔥
• -fssa-phiopt — does pattern matching on SSA PHI nodes to optimize conditional code. Enabled by default at -O1 and higher — except for -Og. 💖
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array. Enabled by default at -O2 and higher. 🌈
• -ftree-tail-merge — looks for identical code sequences. When it finds some, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at -O2 and higher. The compilation time in this pass can be limited using the max-tail-merge-comparisons parameter and the max-tail-merge-iterations parameter. 👑
• -ftree-cselim — does conditional store elimination on trees. Enabled by default at -O1 and higher on targets that have conditional move instructions. 💅

---

• -ftree-dce — ok so this one does dead code elimination (DCE) on trees 💀 literally hunting down code that's doing NOTHING and deleting it, icon behaviour. It's on by default at -O1 and higher, bestie.
• -ftree-builtin-call-dce — conditional dead code elimination (DCE), but specifically for calls to built-in functions that might set errno yet are otherwise totally side-effect free ✨. It's enabled by default at -O2 and higher — unless you also passed -Os, in which case nope 🙈.
• -ffinite-loops — assumptive queen 👑. It assumes a loop with an exit will eventually take that exit and not just loop forever. The way this lets the compiler yeet loops that have no side effects, because it's not sitting there worrying about endless looping as a side effect anymore. Fr fr.

Enabled by default at -O2 for C++ with -std=c++11 or higher 💅.

• -ftree-dominator-opts — a whole lil glow-up pass based on a dominator tree traversal 🌸: constant/copy propagation, redundancy elimination, range propagation and expression simplification. It also does jump threading (which reduces jumps to jumps, thank god). On 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 some later store overwrites it with zero intervening loads in between. In that case the earlier store can just be deleted 🗑️. Enabled by default at -O1 and higher.
• -ftree-ch — performs loop header copying on trees. Why is this cute? Because it makes code motion optimizations more effective AND saves one jump. Enabled by default at -O1 and higher. It is not enabled for -Os, since it usually increases code size 😭.
• -ftree-loop-optimize — loop optimizations on trees. On by default at -O1 and higher, no notes.
• -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 🔥.
• -fgraphite-identity — turns on the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. With -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 🪐. It calculates a loop structure optimized for data-locality and parallelism. Heads up bestie: this option is experimental 🙈.
• -floop-parallelize-all — uses the Graphite data dependence analysis to identify loops that can be parallelized 🌈. It parallelizes all the loops that can be analyzed to not contain loop carried dependences — without checking that it'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. Warning though: this may severely limit the ability to debug an optimized program compiled with -fno-var-tracking-assignments 😭. In the negated form, this flag prevents SSA coalescing of user variables. It's enabled by default if optimization is enabled, and it does very little otherwise.
• -ftree-loop-if-convert — attempts to transform conditional jumps in the innermost loops to branch-less equivalents 🦋. The whole intent is to remove control-flow from the innermost loops so the vectorization pass has a way easier time handling them. Enabled by default if vectorization is enabled.
• -ftree-loop-distribution — performs loop distribution ✨. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, this loop:

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

gets transformed to:

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

Enabled by default at -O3. 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. Like, watch — this loop:

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

becomes:

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

and the initialization loop is transformed into a call to memset zero. Chef's kiss 👑

• -floop-interchange — performs loop interchange outside of graphite 🔄. Can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to happen. For example:

        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];

transforms to:

        for (int i = 0; i < N; i++)
          for (int k = 0; k < N; k++)
            for (int j = 0; j < N; j++)
              c[i][j] = c[i][j] + a[i][k]*b[k][j];

Enabled by default at -O3. Also enabled by -fprofile-use and -fauto-profile 💜.

• -floop-unroll-and-jam — applies unroll and jam transformations on feasible loops 🍓. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. Enabled by default at -O3. 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 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 iteration count needs complicated analysis 😵‍💫. Later optimizations can then determine the number easily. Especially useful 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's sufficiently cheap. Reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher.
• -fivopts — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. Enabled by default at -O1 and higher.
• -ftree-parallelize-loops=n — parallelizes loops, aka splits their iteration space to run in n threads 🧵. 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 so is only supported on targets that have support for -pthread.
• -ftree-pta — performs function-local points-to analysis on trees. Enabled by default at -O1 and higher, except for -Og.
• -ftree-sra — performs scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early 🌸. Enabled by default at -O1 and higher, except for -Og.
• -fstore-merging — merges 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.
• -ftree-ter — performs temporary expression replacement during the SSA->normal phase. Single use/single def temporaries get replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on, which results in better RTL generation 🔥. Enabled by default at -O1 and higher.
• -ftree-slsr — performs straight-line strength reduction on trees. It recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. Enabled by default at -O1 and higher 💅.
• -ftree-vectorize — performs vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified ✨.
• -ftree-loop-vectorize — performs loop vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
• -ftree-slp-vectorize — performs basic block vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile 🌈.
• -ftrivial-auto-var-init=choice — bestie, this one sets your automatic variables to either a pattern or zeroes, and the whole point is to level up your program's security and predictability by stopping uninitialized memory from spilling the tea (disclosure) and getting used. So important: GCC still treats an automatic variable with no explicit initializer as uninitialized — -Wuninitialized and -Wanalyzer-use-of-uninitialized-value will still warn about those variables, and the compiler optimizes as if the variable were uninitialized. With this option GCC will also zero out any padding of automatic variables that have structure or union types. HOWEVER (put your seatbelt on 💀) the current implementation cannot initialize automatic variables declared between the controlling expression and the first case of a switch statement. Use -Wtrivial-auto-var-init to get told about all such cases.

The three values of choice are:

• uninitialized — doesn't initialize any automatic variables. This is C and C++'s default, periodt.
• pattern — initializes automatic variables with values that will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and are not values programmers can rely on for useful program semantics. The current value is a byte-repeatable pattern with byte "0xFE". The values used for pattern initialization might change in the future, no cap.
• zero — initializes automatic variables with zeroes.

The default is uninitialized.

Okay so this next bit is lowkey the most important nuance: the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), not to their interpretation as numerical values. That distinction matters in languages that support types with biases or implicit multipliers, and with extensions like 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 those representations stand for values in that range — and even if they do, how the value held by the variable gets interpreted will depend on the bias. A hardbool variable that uses say "0X5A" and 0xA5 for "false" and "true", respectively, will trap with either choice of trivial initializer, i.e. zero initialization will not convert to the representation for "false", even if it would for a "static" variable of the same type. So basically the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception: (non-hardened) boolean variables that fit in registers are initialized with "false" (zero), even when pattern is requested. 🫶

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

• -fvect-cost-model=model — switches up the cost model used for vectorization. The model argument should be one of unlimited, dynamic, cheap or very-cheap. With the unlimited model the vectorized code-path is assumed to be profitable, while with dynamic a runtime check guards the vectorized code-path so it's only enabled for iteration counts that will likely run faster than the original scalar loop. The cheap model disables vectorization of loops where doing so would be cost prohibitive — for example because of required runtime checks for data dependence or alignment — but otherwise it's equal to dynamic. The very-cheap model disables vectorization of loops when any runtime check for data dependence or alignment is required, and it also disables vectorization of epilogue loops, but otherwise it's equal to cheap.

The default cost model depends on other optimization flags and is either dynamic or cheap. ✨

• -fsimd-cost-model=model — same vibe, but for vectorization of loops marked with the OpenMP simd directive. The model argument should be one of unlimited, dynamic, cheap. All values of model mean the same things as described in -fvect-cost-model, and by default a cost model defined with -fvect-cost-model is used.
• -ftree-vrp — performs Value Range Propagation on trees. It's like constant propagation but instead of values, ranges of values get propagated. This lets the optimizers yeet unnecessary range checks like array bound checks and null pointer checks. Enabled by default at -O2 and higher. Note: null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled. ⭐
• -fsplit-paths — splits paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. Enabled by default at -O3 and above.
• -fsplit-ivs-in-unroller — enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, which improves the efficiency of the scheduling passes.

A combo of -fweb and CSE is often enough to obtain the same effect. BUT (and this is the catch) that is not reliable when the loop body is more complicated than a single basic block. It also doesn't work at all on some architectures because of restrictions in the CSE pass.

This optimization is enabled by default.

• -fvariable-expansion-in-unroller — with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code. 👑

This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.

• -fpartial-inlining — inline parts of functions. This option only has any effect when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

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

• -fpredictive-commoning — performs predictive commoning optimization, i.e. reusing computations (especially memory loads and stores) performed in previous iterations of loops. It's giving "why do the work twice."

Enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.

• -fprefetch-loop-arrays — if the target machine supports it, generate instructions to prefetch memory to improve the performance of loops that access large arrays.

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

Disabled at level -Os.

• -fno-printf-return-value — do not substitute constants for known return value of formatted output functions such as "sprintf", "snprintf", "vsprintf", and "vsnprintf" (but not "printf" or "fprintf"). This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when -fprintf-return-value is in effect, both the branch and the body of the "if" statement (but not the call to "snprint") can be optimized away when "i" is a 32-bit or smaller integer because the return value is guaranteed to be at most 8.

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  ...

The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default.

• -fno-peephole
• -fno-peephole2 — disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is in how they're implemented in the compiler; some targets use one, some use the other, and a few use both. 💅

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

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

GCC uses heuristics to guess branch probabilities if they aren't provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by "__builtin_expect", then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the "__builtin_expect" info into account. The interactions between the heuristics and "__builtin_expect" can be complex, and in some cases it may be useful to disable the heuristics so that the effects of "__builtin_expect" are easier to understand.

It's also possible to specify expected probability of the expression with the "__builtin_expect_with_probability" built-in function.

The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

• -freorder-blocks — reorder basic blocks in the compiled function to reduce the number of taken branches and improve code locality. That's the main character move.

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

• -freorder-blocks-algorithm=algorithm — use the specified algorithm for basic block reordering. The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.

The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.

• -freorder-blocks-and-partition — in addition to reordering basic blocks in the compiled function to reduce the number of taken branches, this partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance. 🌈

This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or a target specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that does not support named sections. When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if you're using a working linker).

Enabled for x86 at levels -O2, -O3, -Os.

• -freorder-functions — okay so this one literally shuffles your functions around inside the object file to make the code more locality-cute. 🌸 But bestie, listen — unlike -fipa-reorder-for-locality, this one is all about bundling every function in the same category together (hot/normal/cold/never-executed). It pulls this off with special subsections: .text.hot for the ones that run the most, .text.unlikely for the ones that basically never show up. 💅

Plot twist though: the actual reordering is done by the linker, so your object file format has to support named sections and your linker has to place them somewhere reasonable. No named sections, no slay. 😭

Also this does absolutely nothing unless you either feed it profile feedback (peep -fprofile-arcs for the deets) or hand-annotate your functions with hot or cold attributes. She needs the tea to do the work.

Enabled at levels -O2, -O3, -Os. ✨

• -fstrict-aliasing — this lets the compiler assume the strictest aliasing rules that apply to whatever language you're compiling. For C (and C++), that activates optimizations based on expression types. Basically: an object of one type is assumed to never live at the same address as an object of a different type — unless the types are almost the same. Like, an unsigned int can alias an int, but it can NOT alias a void* or a double. And a character type? She can alias literally anything. 👑

Pay extra attention to code like this, bestie:

union a_union {
  int i;
  double d;
};

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

Reading from a different union member than the one you most recently wrote to — the thing we call type-punning — is super common. And good news: even with -fstrict-aliasing, type-punning is allowed, as long as the memory is accessed through the union type. So the code above works exactly as expected. 🫶

BUT. This one? Might not:

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

Same energy for taking an address, casting the resulting pointer, and dereferencing it — that's undefined behavior, even if the cast goes through a union type. Like:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

⚠️ Seriously, undefined behavior is not a vibe. Do not test her.

The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.

• -fipa-strict-aliasing — controls whether the rules of -fstrict-aliasing get applied across function boundaries. Heads up though: 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. (They're a package deal, fr fr.)

• -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 that's greater than or equal to n, skipping up to m-1 bytes. This guarantees at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. It's a code-performance optimization, and alignment is ignored for functions considered cold. ❄️ If you need alignment on all functions no matter what, use -fmin-function-alignment.

If m isn't specified, it defaults to n.

Examples, bestie: -falign-functions=32 aligns functions to the next 32-byte boundary. -falign-functions=24 aligns to the next 32-byte boundary only if that can be done by skipping 23 bytes or less. -falign-functions=32:7 aligns to the next 32-byte boundary only if that can be done by skipping 6 bytes or less.

The second pair of n2:m2 values 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 they mean functions are not aligned. 💀

If n isn't specified or is zero, a machine-dependent default is used. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3.

• -flimit-function-alignment — with this on, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. Modest king behavior. 👑
• -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 work exactly like the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean labels aren't aligned.

If -falign-loops or -falign-jumps apply and their values are greater than this one, then their values win and get used instead.

If n isn't specified or is zero, a machine-dependent default is used — 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 whole bunch of times, that makes up for any execution of the dummy padding instructions. It's a code-performance optimization, and alignment is ignored for loops considered cold.

If -falign-labels is greater than this value, then its value is used instead.

The parameters of this option work like -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, a machine-dependent default is used.

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 specifically for branch targets that can only be reached by jumping. 🦘 In that case, no dummy operations need to be executed. Also a code-performance optimization, and alignment is ignored for jumps considered cold.

If -falign-labels is greater than this value, then its value is used instead.

The parameters work like -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, a machine-dependent default is used. 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 is applied also to all functions (yes, even the cold ones 💅). And the alignment is also not affected by -flimit-function-alignment.
• -fno-allocation-dce — do NOT remove unused C++ allocations (using operator new and operator delete) during dead code elimination.

See also -fmalloc-dce.

• -fallow-store-data-races — this one lets the compiler perform optimizations that may introduce new data races on stores, without proving the variable can't be concurrently accessed by other threads. 😳 It does not affect optimization of local data. It's safe to use this option if you know for a fact that global data will not be accessed by multiple threads.

Examples of the optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value already in memory to be re-written with that same value. That re-writing is safe in a single-threaded context, but may be unsafe in a multi-threaded context. Also ngl, 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 around purely for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors.

Enabled by default. ✨

• -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements. Output them in the same order they appear in the input file. When this option is used, unreferenced static variables are not removed. This option exists 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, 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 — constructs webs as commonly used for register allocation purposes and assigns each web its own individual pseudo register. This lets the register allocation pass operate on pseudos directly, but it also strengthens several other optimization passes, like CSE, the loop optimizer, and the trivial dead code remover. 🕸️

It can, however, make debugging impossible, since variables no longer stay in a "home register". 😭 Your debugger is gonna be so confused bestie.

Enabled by default with -funroll-loops.

• -fwhole-program — assumes the current compilation unit represents the whole program being compiled. All public functions and variables — with the exception of main and those merged by attribute externally_visible — become static functions and, in effect, get optimized way more aggressively by interprocedural optimizers. 🔥 Main character energy, but for your entire program.

okay so when you use this one together with -flto it's honestly kinda got limited use? 🫶 In most cases the precise list of symbols used or exported from the binary is already known — that's the resolution info the linker plugin hands over to the link-time optimizer. It's still useful if no linker plugin is being used, or during an incremental link step when final code is produced (that's with -flto -flinker-output=nolto-rel). 🌸

• -flto[=n] — 💫 this one runs the standard link-time optimizer.

When it's invoked with source code, it generates GIMPLE (one of GCC's internal representations, bestie, it's how GCC thinks to itself) and writes it to special ELF sections in the object file. And then when the object files get linked together, all the function bodies are read back out of those ELF sections and instantiated as if they had been part of the same translation unit. iconic. 👑

To use the link-time optimizer, -flto and the optimization options should be specified at compile time and during the final link. The recommendation — and please listen to her — is that you compile all the files participating in the same link with the same options, and also specify those options at link time. Like, 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. Then the final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into one single image, every interprocedural analysis and optimization in GCC now works across both files as if they were a single one. Which means, for example, the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. the way that works?? 😭✨

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. And you can always override that automatic decision by passing -fno-lto to the link command. 💅

---

To make whole program optimization actually effective, it's necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When it's 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. ngl that's a glow-up. 🔥

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

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. So you can absolutely 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 got it handled. 🦋

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

There are some code generation flags preserved by GCC when generating bytecodes, because they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags. ⭐

The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:

-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpie

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. no negotiating, she said what she said. 💅

Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically, -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time though. 👑

Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. And also note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if code not consistent with the setting at compile time. 😭

When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto, or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time. options, options, options. 🎀

To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. 💜

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (which is undefined behavior according to ISO C99 6.2.7, so like, danger bestie), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.

Another feature of LTO is that it's possible to apply interprocedural optimizations on files written in different languages:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. 🌈

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it's possible to extract and use them in an LTO link if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; and to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to 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 that's about to be optimized. serving. 🫶

If you are not using a linker with plugin support and/or don't enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. two vibes, one library. ✨

Link-time optimizations do not require the presence of the whole program to operate. If the program doesn't require any symbols to be exported, it's possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions, which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when the linker plugin is active (see -fuse-linker-plugin). 🍓

The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. version gatekeeping, honestly. 💀

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. 🌸

• -flto=jobserver — okay so this one lets you use GNU make's job server mode to figure out how many parallel jobs to run. Super handy when the Makefile that's calling GCC is already running in parallel. BUT — and this is important bestie — you have to prepend a + to the command recipe in the parent Makefile or it will not work. Also lowkey this option probably only works if MAKE is GNU make. And even if you don't pass a value at all, GCC still tries to auto-detect a running GNU make's job server. 💖
• -flto=auto — use GNU make's job server if it's available, otherwise fall back to autodetecting how many CPU threads your system has.
• -flto-partition=alg — pick the partitioning algorithm the link-time optimizer uses. The value is one of:
  • 1to1 — partitioning that mirrors the original source files.
  • balanced — partitioning into equally sized chunks (whenever possible). This is the default.
  • max — create a new partition for every symbol where possible. ⚠️ This one is intended for internal testing only, fr fr.
  • cache — 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 value one specifies that exactly one partition should be used.
  • The value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.

While 1to1 can be used as a workaround for various code ordering issues, max is genuinely just for internal testing, no cap.

• -flto-incremental=path — turn on incremental LTO, with its cache living 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 — sets the number of cache entries in incremental LTO after which old entries get pruned. Note this is a soft limit — temporarily there may be more entries than that. 💅
• -flto-compression-level=n — sets the level of compression used for the intermediate language written to LTO object files. Only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms:
  • zstd — valid values are 0 (no compression) to 19 (maximum compression).
  • zlib — values from 0 to 9.

Values outside these ranges are clamped to either the minimum or maximum of the supported values. If the option isn't given at all, a default balanced compression setting is used.

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

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. That makes them usable for both LTO linking and normal linking. This option is only effective 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.

NOTE: On Debian/Ubuntu this is disabled by default on m68k-linux-gnu. 💀

• -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 — so profiles collected using an instrumented binary for multi-threaded programs may be inconsistent because of missed counter updates. 😭 When this option is specified, GCC uses heuristics to correct or smooth out those inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.

This option is enabled by -fauto-profile.

• -fprofile-partial-training — with -fprofile-use, all portions of programs not executed during the train run are optimized aggressively for size rather than speed. In some cases it's just not practical to train all possible hot paths in the program (for example, the program may contain functions specific for a given hardware and training may not cover all hardware configurations the program is run on). With -fprofile-partial-training, profile feedback is ignored for all functions not executed during the train run, so they get optimized as if they were compiled without profile feedback. That means better performance when the train run isn't representative — but also significantly bigger code. 🫶
• -fprofile-use
• -fprofile-use=path — enable profile feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions

Before you can use this option, you must first generate profiling information. 📝

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, plus the following optimizations, many of which are generally profitable only with profile feedback available:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns  -fprofile-correction

path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.

Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system. For more information, see <https://perf.wiki.kernel.org/>.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>.

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

---

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. 🌸

• -fexcess-precision=style — this option allows control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. An example of such a target is x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format. For most programs, the excess precision is harmless, but some programs may rely on the requirements of the C or C++ language standards for handling IEEE values. 💜
• By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C or C++, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C or C++ if a strict conformance option such as -std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used. If -fexcess-precision=16 is specified, constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision.

okay bestie listen up ✋ -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. (iykyk — x86 said "not today" 💀)

• -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. ngl this one's kinda been replaced — it's 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. slay, a macro 💅

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. the name literally has "unsafe" in it bestie, that's your vibe check right there 💀

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. (math said "let me shuffle these" and ISO C said absolutely not 😭)

• -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. so like, no infinity, no "not a number", just vibes 🌈

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. ⭐

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. (future GCC said "watch this space" ✨)

• -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. 💀

• -fno-fp-int-builtin-inexact — Do not allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the inexact floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the inexact exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C23, does not allow these functions to do so. 🦋

The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect.

Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the inexact exception may be raised if the library implementation does not follow TS 18661. 😭

• -fsingle-precision-constant — Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. no cap, just smaller numbers, less converting ✨
• -fcx-limited-range — When enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math. 💜

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages. all languages, she's versatile like that 🫶

• -fcx-fortran-rules — Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. 🌸

The default is -fno-cx-fortran-rules.

Okay bestie, so this next lil cluster of flags is the "advanced tricks" shelf — they can make your code zoom, but no -O option turns them on for you, and heads up: some of these are experimental and can straight-up produce broken code. 💀 You've been warned, so let's go. 🫶

---

• -fbranch-probabilities — so boom: you ran your program once while it was compiled with -fprofile-arcs, and now you compile it a second time with -fbranch-probabilities so GCC can optimize smarter based on how many times each branch actually got taken. When a program built with -fprofile-arcs exits, it dumps its arc execution counts into a file called sourcename.gcda for every single source file. ⚠️ But listen — that data file is super dependent on the structure of the generated code, so you must use the exact same source code and the exact same optimization options for both compilations, no cap. (The file-naming deets live over in -fprofile-arcs.) With -fbranch-probabilities, GCC drops a REG_BR_PROB note onto every JUMP_INSN and CALL_INSN. These help optimizations out. RN they're only used in one spot: in reorg.cc, instead of vibing and guessing which path a branch probably takes, the REG_BR_PROB values get used to say exactly which path gets taken more often. ✨ Enabled by -fprofile-use and -fauto-profile.
• -fprofile-values — if you pair it with -fprofile-arcs, it adds code so some data about the values of expressions in your program gets collected. 📊 With -fbranch-probabilities, it reads that gathered expression-value data back in so optimizations can use it. Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
• -fprofile-reorder-functions — function reordering, but make it profile-based 🎀. It collects the first time of execution of each function and then orders those functions in ascending order, aiming to optimize program startup by loading text segments more efficiently. Enabled with -fprofile-use.
• -fvpt — combine it with -fprofile-arcs and GCC adds code to gather info about the values of expressions. With -fbranch-probabilities, it reads that gathered data back and actually does the optimizations based on it. Currently those optimizations include specializing division operations using what it knows about the value of the denominator. ➗🔥 Enabled with -fprofile-use and -fauto-profile.
• -frename-registers — tries to dodge false dependencies in scheduled code by using registers that got left over after register allocation. This one benefits processors with a lot of registers the most. 🏆 BUT — depending on the debug info format your target uses, it can make debugging impossible, because variables no longer stay in a "home register". 😭 Enabled by default with -funroll-loops.
• -fschedule-fusion — does a target-dependent pass over the instruction stream to schedule instructions of the same type right next to each other, because the target machine can run them more efficiently when they're adjacent in the instruction flow. 👯‍♀️ Enabled at levels -O2, -O3, -Os.
• -ftracer — does tail duplication to enlarge superblock size. This transformation simplifies the function's control flow, which lets other optimizations do a better job. 🌈 Enabled by -fprofile-use and -fauto-profile.
• -funroll-loops — unrolls loops where the iteration count can be figured out at compile time or when you enter the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (aka completely removing loops that have a small constant number of iterations). ⚠️ Heads up: this makes code bigger, and it may or may not make it run faster. 💅 Enabled by -fprofile-use and -fauto-profile.
• -funroll-all-loops — unrolls all the loops, even when the iteration count is uncertain at loop entry. This usually makes programs run slower, so... bestie, choose wisely. 🐢 -funroll-all-loops implies the same options as -funroll-loops.
• -fpeel-loops — peels loops that we have enough info to know don't roll much (info coming from profile feedback or static analysis). It also turns on complete loop peeling (aka completely removing loops with a small constant number of iterations). 🍌 Enabled by -O3, -fprofile-use, and -fauto-profile.
• -fmalloc-dce — controls whether "malloc" (and its variants like "calloc" or "strdup") can be optimized away, provided its return value is only used as a parameter of a "free" call or compared with "NULL". If you use -fmalloc-dce=1, only calls to "free" are allowed, while with -fmalloc-dce=2 comparisons with the "NULL" pointer are also considered safe to remove. 🗑️ The default is -fmalloc-dce=2. See also -fallocation-dce.
• -fmove-loop-invariants — enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og.
• -fmove-loop-stores — enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop, in exchange for carrying the stored value in a register across the iteration. 📦 Note: for this option to have any effect, -ftree-loop-im has to be enabled too. Enabled at level -O1 and higher, except for -Og.
• -fsplit-loops — splits a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other. ✂️ Enabled by -fprofile-use and -fauto-profile.
• -funswitch-loops — moves branches with loop-invariant conditions out of the loop, duplicating the loop on both branches (each modified according to the result of the condition). 🦋 Enabled by -fprofile-use and -fauto-profile.
• -fversion-loops-for-strides — if a loop iterates over an array with a variable stride, this creates another version of the loop that assumes the stride is always one. Like so:

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 that section's name in the output file. 🏷️ Use these options on systems where the linker can do optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies. 🤷‍♀️ Together with a linker garbage collection (linker --gc-sections option), these options may lead to smaller statically-linked executables (after stripping). On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats though. ⚠️ Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler that use relative locations inside a translation unit, because those locations are unknown until link time. One example of such an optimization is relaxing calls to short call instructions.
• -fstdarg-opt — optimizes the prologue of variadic argument functions with respect to how those arguments get used. 🎁
• -fsection-anchors — tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. On some targets this transformation can help reduce the number of GOT entries and GOT accesses. For example, take this implementation of the function "foo":

static int a, b, c;
int foo (void) { return a + b + c; }

Normally it 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 this pseudocode (which is not valid C, bestie, don't try it):

int foo (void)
{
  register int *xr = &x;
  return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}

Not all targets support this option. 🙈

• -fzero-call-used-regs=choice — zeroes call-used registers at function return to increase program security, either by mitigating Return-Oriented Programming (ROP) attacks or by preventing information leakage through registers. 🔐 The possible values of choice are the same as for the "zero_call_used_regs" attribute. The default is skip. You can control this behavior for a specific function by using the function attribute "zero_call_used_regs".
• --param name=value — in some places, GCC uses various constants to control how much optimization gets done. For example, GCC doesn't inline functions that contain more than a certain number of instructions. You can control some of these constants right from the command line using the --param option. 🎛️ The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. 👀 To get the minimal, maximal and default values of a parameter, use the --help=param -Q options. In every case, the value is an integer. The following choices of name are recognized for all targets:
  • phiopt-factor-max-stmts-live — when factoring statements out of if/then/else, this is the max # of statements after the defining statement that are allowed to extend the lifetime of a name.
  • predictable-branch-outcome — when a branch is predicted to be taken with a probability lower than this threshold (in percent), then it's considered well predictable.

---

• max-rtl-if-conversion-insns — RTL if-conversion is out here trying to yeet conditional branches around a block and swap them for conditionally executed instructions instead. This param sets the max number of instructions in a block that are even eligible for if-conversion. bestie, the compiler also runs other heuristics to figure out whether if-conversion is gonna be profitable, so this isn't the only thing deciding 💅
• file-cache-files — Max number of files chillin' in the file cache. The file cache is what prints source lines in diagnostics and does source checks like -Wmisleading-indentation. ✨
• file-cache-lines — Max number of lines to index into the file cache. When it's 0 this gets automatically sized. Same deal as above: the file cache prints source lines in diagnostics and handles source checks like -Wmisleading-indentation. 🎀
• max-rtl-if-conversion-predictable-cost — RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would get generated by if-conversion, and it depends on whether the branch is statically determined to be predictable or not. The units for this param are the same as the ones for the GCC internal seq_cost metric. The compiler will try to hand you a reasonable default for this using the BRANCH_COST target macro. 🦋
• max-crossjump-edges — The max number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Cranking this up means more aggressive optimization, which makes compilation time go up with probably only a small improvement in executable size. 🙈
• min-crossjump-insns — The minimum number of instructions that have to match at the end of two blocks before cross-jumping gets performed on them. This value is ignored when all instructions in the block being cross-jumped from are matched. no cap 💀
• max-grow-copy-bb-insns — The maximum code size expansion factor when we're copying basic blocks instead of jumping. The expansion is relative to a jump instruction.
• max-goto-duplication-insns — The max number of instructions to duplicate to a block that jumps to a computed goto. To dodge O(N^2) behavior across a bunch of passes, GCC factors computed gotos early on in the compilation process and unfactors them as late as possible. Only computed jumps at the end of basic blocks with no more than max-goto-duplication-insns get unfactored. 🌀
• max-delay-slot-insn-search — The max number of instructions to consider when hunting for an instruction to fill a delay slot. If more than this arbitrary number of instructions get searched, the time savings from filling the delay slot are minimal, so it just stops searching. Bigger values mean more aggressive optimization, which makes compilation time go up with probably small improvement in execution time. ⏳
• max-delay-slot-live-search — When trying to fill delay slots, this is the max number of instructions to consider when searching for a block with valid live register info. Raising this arbitrarily chosen value means more aggressive optimization and more compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
• max-gcse-memory — The approximate max amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization just doesn't happen. bye 👋
• max-gcse-insertion-ratio — If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression, and thus leaves partially redundant computations sitting in the instruction stream. 🍓
• max-pending-list-length — The max number of pending dependencies scheduling allows before it flushes the current state and starts over. Large functions with few branches or calls can create excessively large lists that needlessly eat up memory and resources. 😭
• max-modulo-backtrack-attempts — The max number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time. exponentially, bestie 💀
• max-inline-functions-called-once-loop-depth — Maximal loop depth of a call considered by inline heuristics that try to inline all functions called once.
• max-inline-functions-called-once-insns — Maximal estimated size of functions produced while inlining functions called once.
• max-inline-insns-single — Several parameters control the tree inliner used in GCC. This number sets the max number of instructions (counted in GCC's internal representation) in a single function that the tree inliner considers for inlining. This only affects functions declared inline and methods implemented in a class declaration (C++). 👑
• max-inline-insns-auto — When you use -finline-functions (which is included in -O3), a lot of functions that would otherwise not be considered for inlining by the compiler get investigated. To those functions, a different (more restrictive) limit compared to functions declared inline can be applied (--param max-inline-insns-auto).
• max-inline-insns-small — This is the bound applied to calls that are considered relevant with -finline-small-functions.
• max-inline-insns-size — This is the bound applied to calls that are optimized for size. Small growth may be desirable to anticipate optimization opportunities exposed by inlining. 🌱
• uninlined-function-insns — Number of instructions accounted by the inliner for function overhead like function prologue and epilogue.
• uninlined-function-time — Extra time accounted by the inliner for function overhead like the time needed to execute function prologue and epilogue. ⏱️
• inline-heuristics-hint-percent — The scale (in percents) applied to inline-insns-single, inline-insns-single-O2, and inline-insns-auto when inline heuristics hint that inlining is very profitable (i.e. it'll unlock later optimizations). 🔥
• uninlined-thunk-insns, uninlined-thunk-time — Same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks.
• inline-min-speedup — When the estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. slay ✨
• large-function-insns — The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. 😮‍💨
• large-function-growth — Specifies the maximal growth of large functions caused by inlining, in percents. For example, parameter value 100 limits large function growth to 2.0 times the original size.
• large-unit-insns — The limit specifying a large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of the unit is 300\% and yet such inlining is very sane. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size. So for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. 🧚
• lazy-modules — Maximum number of concurrently open C++ module files when lazy loading. 📂
• inline-unit-growth — Specifies the maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size.
• ipa-cp-unit-growth — Specifies the maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value 10 limits unit growth to 1.1 times the original size.
• ipa-cp-large-unit-insns — The size of translation unit that the IPA-CP pass considers large.
• large-stack-frame — The limit specifying large stack frames. While inlining, the algorithm is trying not to grow past this limit too much.
• large-stack-frame-growth — Specifies the maximal growth of large stack frames caused by inlining, in percents. For example, parameter value 1000 limits large stack frame growth to 11 times the original size. 🌈
• max-inline-insns-recursive, max-inline-insns-recursive-auto — Specifies the max number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining.

--param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining only happens when -finline-functions (included in -O3) is enabled, and --param max-inline-insns-recursive-auto applies instead.

• max-inline-recursive-depth, max-inline-recursive-depth-auto — Specifies the max recursion depth used for recursive inlining. 🌀
• min-inline-recursive-probability — recursive inlining is only worth it for functions that have deep recursion on average, bestie, and it can actively hurt functions with barely any recursion depth, because it bloats the prologue or makes the function body way messier for the other optimizers to deal with 😭

When profile feedback is available (peep -fprofile-generate), the actual recursion depth can be guessed from the probability that the function recurses through a given call expression. This parameter caps inlining to only those call expressions whose probability is above the given threshold — and yes, that's in percents 💅

• early-inlining-insns — sets the growth the early inliner is allowed to make. Lowkey what it actually does is bump up the amount of inlining for code that has a big abstraction penalty ✨
• max-early-inliner-iterations — the iteration limit for the early inliner. Basically it puts a ceiling on how many nested indirect calls the early inliner can resolve. Anything deeper than that still gets handled — just later, by late inlining 🫶
• comdat-sharing-probability — the probability (in percent) that a C++ inline function with comdat visibility gets shared across multiple compilation units.
• modref-max-bases, modref-max-refs, modref-max-accesses — these three together specify the max number of base pointers, references, and accesses that mod/ref analysis will store for a single function 📦
• modref-max-tests — the maxmal number of tests the alias oracle can run to disambiguate memory locations using the mod/ref info. Heads up: this one ought to be bigger than --param modref-max-bases and --param modref-max-refs, fr fr.
• modref-max-depth — the max depth of the DFS walk used by modref escape analysis. Setting it to 0 disables the analysis completely 🙈
• modref-max-escape-points — the max number of escape points modref will track per SSA-name.
• modref-max-adjustments — the max number of times the access range gets enlarged during modref dataflow analysis.
• profile-func-internal-id — a knob controlling whether to use the function internal id when looking things up in the profile database. If the value is 0, the compiler uses an id based on the function assembler name and filename instead — which makes old profile data more tolerant to source changes like function reordering and stuff. Iconic, honestly 🎀
• min-vect-loop-bound — the minimum number of iterations under which loops do not get vectorized when -ftree-vectorize is in play. For vectorization to be allowed, the iteration count after vectorization needs to be greater than whatever you specify here 👑
• gcse-cost-distance-ratio — the scaling factor used when calculating the max distance an expression can be moved by GCSE optimizations. Currently only supported in the code hoisting pass, iykyk. The bigger the ratio, the more aggressive code hoisting gets with simple expressions — meaning expressions whose cost is less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions entirely 🚫
• gcse-unrestricted-cost — a cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations stop constraining how far an expression can travel. Currently only supported in the code hoisting pass. The lesser the cost, the more aggressive the hoisting. Specifying 0 lets all expressions travel unrestricted distances 🔥
• max-hoist-depth — how deep we search the dominator tree for expressions to hoist. This exists to dodge quadratic behavior in the hoisting algorithm. A value of 0 doesn't limit the search at all — but it may slow down compilation of huge functions, so. be careful bestie 😭
• max-tail-merge-comparisons — the max number of similar bbs to compare a bb against. This is here to avoid quadratic behavior in tree tail merging.
• max-tail-merge-iterations — the max number of iterations of the pass over the function. Used to keep compilation time in check during tree tail merging.
• store-merging-allow-unaligned — lets the store merging pass introduce unaligned stores, but only if that's legal to do 💖
• max-stores-to-merge — the max number of stores the store merging pass will attempt to merge into wider stores.
• max-store-chains-to-track — the max number of store chains to track at the same time while trying to merge them into wider stores in the store merging pass.
• max-stores-to-track — the max number of stores to track at the same time in the attemt to to merge them into wider stores in the store merging pass. (yes, that typo is canon, we keep the source material intact 🫡)
• max-unrolled-insns — the max number of instructions a loop can have and still be unrolled. If a loop does get unrolled, this parameter also decides how many times the loop code is unrolled.
• max-average-unrolled-insns — same deal, but the max number of instructions is biased by the probabilities of their execution. And if a loop gets unrolled, this also decides how many times the loop code is unrolled ✨
• max-unroll-times — the max number of unrollings of a single loop.
• max-peeled-insns — the max number of instructions a loop can have and still be peeled. If a loop does get peeled, this also decides how many times the loop code is peeled.
• max-peel-times — the max number of peelings of a single loop.
• max-peel-branches — the max number of branches along the hot path through the peeled sequence.
• max-completely-peeled-insns — the max number of insns of a completely peeled loop.
• max-completely-peel-times — the max number of iterations of a loop for it to be suitable for complete peeling.
• max-completely-peel-loop-nest-depth — the max depth of a loop nest that's suitable for complete peeling.
• max-unswitch-insns — the max number of insns of an unswitched loop.
• max-unswitch-depth — the max depth of a loop nest to be unswitched.
• lim-expensive — the minimum cost of an expensive expression in loop invariant motion 💸
• min-loop-cond-split-prob — when FDO profile information is available, min-loop-cond-split-prob sets the minimum threshold for the probability of a semi-invariant condition statement to trigger a loop split.
• iv-consider-all-candidates-bound — a bound on the number of induction variable candidates. Below this number, all candidates get considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones get considered, all to avoid quadratic time complexity ⏱️
• iv-max-considered-uses — the induction variable optimizations straight up give up on loops that contain more induction variable uses than this.
• iv-always-prune-cand-set-bound — if the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one ✂️
• avg-loop-niter — the average number of iterations of a loop.
• dse-max-object-size — the maximum size (in bytes) of objects tracked bytewise by dead store elimination. Bigger values might mean longer compilation times 😮‍💨
• dse-max-alias-queries-per-store — the max number of queries into the alias oracle per store. Bigger values mean bigger compilation times, and may also mean more removed dead stores.
• scev-max-expr-size — a bound on the size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer down 🐌
• scev-max-expr-complexity — a bound on the complexity of expressions in the scalar evolutions analyzer. Complex expressions also slow the analyzer down.
• max-tree-if-conversion-phi-args — the max number of arguments in a PHI that TREE if conversion supports, unless the loop is marked with a simd pragma.
• vect-max-layout-candidates — the max number of possible vector layouts (like permutations) to consider when optimizing to-be-vectorized code 🌈
• vect-max-version-for-alignment-checks — the max number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.
• vect-max-version-for-alias-checks — the max number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
• vect-max-peeling-for-alignment — the max number of loop peels to enhance access alignment for the vectorizer. A value of -1 means no limit 🦋
• max-iterations-to-track — the max number of iterations of a loop that the brute-force algorithm for analyzing the number of iterations of the loop will try to evaluate.
• hot-bb-count-fraction — the denominator n of the fraction 1/n of the maximal execution count of a basic block in the entire program, that a basic block needs to at least have to be considered hot. The default is 10000, which means a basic block counts as hot if its execution count is greater than 1/10000 of the maximal execution count. 0 means it's never considered hot. Used in non-LTO mode 🔥
• hot-bb-count-ws-permille — the number of most-executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program that a basic block's execution count must be part of to be considered hot. The default is 990, which means a basic block counts as hot if its execution count contributes to the upper 990 permilles — that's 99.0% — of the profiled execution of the entire program. 0 means it's never considered hot. Used in LTO mode 💅
• hot-bb-frequency-fraction — bestie this is the denominator n of the fraction 1/n of the entry block's execution frequency, and a basic block has to hit at least that much to even be considered hot 🔥. Default is 1000, so a block is hot if it runs more than 1/1000 as often as its function's entry block. Set it to 0 and literally nothing is ever hot. periodt.
• unlikely-bb-count-fraction — the denominator n of 1/n of the total profiled runs of the whole program, below which a basic block's execution count has to fall for the block to be called unlikely executed. Default is 20, meaning a block is unlikely-executed if it runs in fewer than 1/20 of program runs (that's 5% bestie 🍓). 0 means it's always considered unlikely executed. wild.
• max-predicted-iterations — the max number of loop iterations we predict statically. Super useful when a function has one loop with a known bound and another with an unknown bound — the known one gets predicted correctly, while the unknown one averages to roughly 10 😭. Which means the bound-less loop looks artificially cold next to the other one. it's giving "unfair comparison".
• builtin-expect-probability — controls the probability of the expression having the specified value. Takes a percentage (i.e. 0 ... 100) as input 💅.
• builtin-string-cmp-inline-length — the max length of a constant string for a builtin string cmp call that's eligible for inlining.
• align-threshold — picks the fraction of the maximal execution frequency of a basic block in a function that gets a basic block aligned.
• align-loop-iterations — a loop expected to iterate at least the selected number of iterations gets aligned ✨.
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value limits superblock formation once the given percentage of executed instructions is covered, which keeps unnecessary code size expansion in check 📏.

The tracer-dynamic-coverage-feedback one is only used when profile feedback is available — because the real profiles (as opposed to the statically estimated ones) are way less balanced, so the threshold gets to be a larger value.

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

Same deal as tracer-dynamic-coverage, two parameters, one vibe: tracer-min-branch-probability-feedback is for compilation with profile feedback, tracer-min-branch-probability is for compilation without. And the feedback value needs to be more conservative (higher) to make the tracer effective 👑

• stack-clash-protection-guard-size — specifies the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the OS-provided guard will leave your code vulnerable to stack clash style attacks. so like... don't 💀
• stack-clash-protection-probe-interval — stack clash protection means probing stack space as it's allocated. This param controls the max distance between probes into the stack as 2 raised to num bytes. Higher values may reduce explicit probes, but again — larger than the OS-provided guard and your code is out here vulnerable to stack clash style attacks 😭
• max-cse-path-length — the max number of basic blocks on a path that CSE considers.
• max-cse-insns — the max number of instructions CSE processes before flushing.
• ggc-min-expand — GCC uses a garbage collector to manage its own memory allocation, and this parameter specifies the minimum percentage by which the GC's heap should be allowed to expand between collections. Tuning it may improve compilation speed, and it has no effect on code generation 🫶.

Default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of "RAM" is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC can't calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero makes a full collection happen at every opportunity — extremely slow, but can be useful for debugging 🐛.

• ggc-min-heapsize — minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first collection happens after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and it has no effect on code generation 💖.

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

• max-reload-search-insns — the max number of instructions reload should look backward for an equivalent register. Higher values mean more aggressive optimization, so compilation time goes up with probably slightly better performance 📈.
• max-cselib-memory-locations — the max number of memory locations cselib should take into account. Bigger values = more aggressive optimization, longer compile, probably slightly better performance 💅.
• max-sched-ready-insns — the max number of instructions ready to be issued that the scheduler should consider at any given time during the first scheduling pass. Higher values mean more thorough searches, making compilation time increase with probably little benefit 🙈.
• max-sched-region-blocks — the max number of blocks in a region to be considered for interblock scheduling.
• max-pipeline-region-blocks — the max number of blocks in a region to be considered for pipelining in the selective scheduler.
• max-sched-region-insns — the max number of insns in a region to be considered for interblock scheduling.
• max-pipeline-region-insns — the max number of insns in a region to be considered for pipelining in the selective scheduler.
• min-spec-prob — the minimum probability (in percents) of reaching a source block for interblock speculative scheduling.
• max-sched-extend-regions-iters — the max number of iterations through CFG to extend regions. A value of 0 disables region extensions ⭐.
• max-sched-insn-conflict-delay — the max conflict delay for an insn to be considered for speculative motion.
• sched-spec-prob-cutoff — the minimal probability of speculation success (in percents) so that speculative insns get scheduled.
• sched-state-edge-prob-cutoff — the minimum probability an edge must have for the scheduler to save its state across it.
• sched-mem-true-dep-cost — minimal distance (in CPU cycles) between a store and a load targeting the same memory locations.
• selsched-max-lookahead — the max size of the lookahead window of selective scheduling. It's a depth of search for available instructions 🔍.
• selsched-max-sched-times — the max number of times an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined.
• selsched-insns-to-rename — the max number of best instructions in the ready list that are considered for renaming in the selective scheduler.
• sms-min-sc — the minimum value of stage count that swing modulo scheduler generates.
• max-last-value-rtl — the max size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register.
• max-combine-insns — the max number of instructions the RTL combiner tries to combine.
• max-combine-search-insns — the max number of instructions the RTL combiner searches in order to find the next use of a given register definition. If this limit is reached without finding such a use, the combiner stops trying to optimize the definition.

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

• integer-share-limit — small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases its speed. This sets the max value of a shared integer constant 🌸.
• ssp-buffer-size — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protector is used.

This default before Ubuntu 10.10 was "8". Currently it's "4", to increase the number of functions protected by the stack protector 🛡️. glow up fr.

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• min-size-for-stack-sharing — the tiniest size a variable can be and still get to share a stack slot when we're not optimizing. no opt, no slot-sharing for the little guys. 🙈
• max-jump-thread-duplication-stmts — when we're threading jumps, this caps how many statements a block can have before we refuse to duplicate it. keep it under the limit or it's a no. 💅
• max-jump-thread-paths — the max number of paths we'll consider while hunting for jump threading opportunities. here's the tea: when we land on a block, incoming edges only get considered if (paths searched so far) × (incoming edges) doesn't blow past this maximum. math is the vibe check. ✨
• max-fields-for-field-sensitive — max number of fields in a struct that get treated field-sensitively during pointer analysis. more fields than this? not field-sensitive anymore bestie. 🦋
• prefetch-latency — an estimate of the average instruction count executed before a prefetch actually finishes. the distance we prefetch ahead scales with this constant. bump it up and you might get fewer streams prefetched (peep simultaneous-prefetches). 📈
• simultaneous-prefetches — the most prefetches that can be running at the same time. that's it, that's the number. 🔥
• l1-cache-line-size — size of a cache line in the L1 data cache, in bytes. 🎀
• l1-cache-size — size of the L1 data cache, in kilobytes. 🍓
• l2-cache-size — size of the L2 data cache, in kilobytes. 🌸
• prefetch-dynamic-strides — should the loop array prefetch pass drop software prefetch hints for strides that aren't constant? sometimes that slaps, but ngl a non-constant stride makes it hard to predict when there's a real win from those hints.

Set it to 1 if hints should be issued for non-constant strides. Set it to 0 if hints should only be issued for strides that are known to be constant and below prefetch-minimum-stride. 💫

• prefetch-minimum-stride — the minimum constant stride, in bytes, before we start using prefetch hints at all. stride smaller than this threshold? no hints for you, sorry not sorry. 😭

This one is a lifesaver for processors with hardware prefetchers, because hardware and software prefetchers can absolutely beef with each other. if your hardware prefetcher has a max stride it can handle, put that number here to make the software prefetchers actually useful. 🫶

A value of -1 means there's no threshold at all, so prefetch hints can go out for any constant stride.

Reminder: this setting only does anything for strides that are known and constant. periodt.

• destructive-interference-size, constructive-interference-size — the values for the C++17 variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. okay so:
  • the destructive interference size = the minimum recommended offset between two independent, concurrently-accessed objects.
  • the constructive interference size = the maximum recommended size of contiguous memory that gets accessed together.

Typically both are just the size of an L1 cache line for the target, in bytes. for a generic target that covers a whole range of L1 cache line sizes, usually the constructive size is the small end of the range and the destructive size is the large end.

⚠️ ABI WARNING ⚠️ the destructive interference size is meant for layout, which means it has ABI impact. the default value is not expected to be stable, and on some targets it straight-up varies with -mtune. so using this variable anywhere ABI stability matters — like the public interface of a library — is strongly discouraged. if you're in that situation anyway, users can stabilize the value with this option. 💀

The constructive interference size is way less sensitive, since it's typically only used in a static_assert to confirm a type fits inside a cache line. much chiller energy.

See also -Winterference-size. 👀

• loop-interchange-max-num-stmts — max number of stmts in a loop that can be interchanged. 🔄
• loop-interchange-stride-ratio — the minimum ratio between the strides of two loops for interchange to actually be profitable. queen behavior only when the math works. 👑
• min-insn-to-prefetch-ratio — the minimum ratio between instruction count and prefetch count needed to enable prefetching in a loop. 🧚
• prefetch-min-insn-to-mem-ratio — the minimum ratio between instruction count and memory reference count needed to enable prefetching in a loop. same energy, different denominator. ⭐
• use-canonical-types — should the compiler use the "canonical" type system? it should always be 1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++. however 😭 if bugs in the canonical type system are making your compilation fail, set this to 0 and disable canonical types. emergency mode fr fr.
• switch-conversion-max-branch-ratio — switch initialization conversion refuses to create arrays bigger than switch-conversion-max-branch-ratio times the number of branches in the switch. yes, that's a mouthful, and yes, it's exactly what it says. 🌈
• switch-lower-slow-alg-max-cases — maximum number of cases where the slow switch lowering algorithms get used. 💅
• max-partial-antic-length — maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. here's why it exists: for some kinds of source code, enhanced partial redundancy elimination can just run away and eat all the memory on your host machine. 😱 this parameter caps the length of the sets it computes, which stops the runaway behavior. setting it to 0 = unlimited set length, live your truth (and have the RAM for it).
• rpo-vn-max-loop-depth — maximum loop depth that gets value-numbered optimistically. when the limit hits, the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest get value-numbered optimistically, and the remaining ones don't. wild but true. 🌀
• sccvn-max-alias-queries-per-access — max number of alias-oracle queries we run while looking for redundancies in loads and stores. hit this limit and the search gets aborted, and the load or store is simply not considered redundant. the query count is algorithmically capped by the number of stores on all paths from the load to the function entry. 📊
• ira-max-loops-num — IRA does regional register allocation by default. if a function has more loops than this number, then only at most this many of the most frequently-executed loops form regions for that regional register allocation. the popular loops get the spotlight. 🌟
• ira-max-conflict-table-size — IRA uses a sophisticated algorithm to compress the conflict table, but bestie, that table can still want an excessive amount of memory for huge functions. if a function's conflict table could be bigger than this size in MB, the register allocator swaps to a faster, simpler, and lower-quality algorithm that doesn't need to build a pseudo-register conflict table at all. speed over quality, a classic trade. 😮‍💨
• ira-loop-reserved-regs — IRA can evaluate more accurate register pressure inside loops to decide about moving loop invariants (peep -O3). this parameter is how many available registers get reserved for other purposes. the default was picked as the best result out of numerous experiments. science! 🔬
• ira-consider-dup-in-all-alts — makes IRA consider the matching constraint (that's the duplicated operand number) heavily across all available alternatives for a preferred register class. if it's set to 0, IRA only respects the matching constraint when it sits in the one and only available alternative with an appropriate register class. otherwise, IRA checks every available alternative for a preferred register class — even if it already found a choice with an appropriate register class — and respects the qualified matching constraint it found. yes, that's the whole rule. 💅
• ira-simple-lra-insn-threshold — approximate function insn count, in units of 1K, that triggers simple local RA. 🧮
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values that were reloaded into registers in subsequent insns. this optimization is called inheritance, and EBB is the region where it happens. this parameter defines the minimal fall-through edge probability, in percentage, needed to add a BB to the inheritance EBB in LRA. the default came from numerous runs of SPEC2000 on x86-64. iconic default behavior. 👑
• loop-invariant-max-bbs-in-loop — loop invariant motion can get very expensive, both in compilation time and in the compile-time memory it burns, when loops are huge. any loop with more basic blocks than this parameter just won't get loop invariant motion optimization performed on it. sorry queen, you're too big. 💀
• loop-max-datarefs-for-datadeps — building data dependencies is expensive for very large loops. this parameter limits how many data references in loops get considered for data dependence analysis. heads up: these large loops are "no handled" by the optimizations that use loop data dependencies. 🍓
• max-vartrack-size — sets the maximum number of hash table slots to use during variable tracking dataflow analysis of any function. if this limit gets exceeded while variable tracking at assignments is enabled, the analysis for that function is retried without it — after removing all debug insns from the function. 😭 if the limit is still exceeded even without debug insns, variable tracking analysis is completely disabled for that function. setting the parameter to 0 makes it unlimited. no cap, literally. ✨
• max-vartrack-expr-depth — this one is the recursion budget for when the compiler tries to map variable names or debug temporaries back onto value expressions. You're literally trading compile time for juicier debug info. Set it too low and value expressions that are available (and could've totally made it into the debug info) just get left out 😭. Crank it higher and the compiler might hunt down more complex debug expressions for you, but bestie, compile time and memory use can grow. Pay for the vibes.
• max-debug-marker-count — sets a threshold on how many debug markers (think begin stmt markers, that kind of thing) are allowed, so everything doesn't explode in complexity during inlining or while expanding to RTL 💥. If a function has more such gimple stmts than the limit you set, those stmts get dropped from the inlined copy of the function and from its RTL expansion. Gone. Poof.
• min-nondebug-insn-uid — nondebug insns get uids starting at this parameter. Everything below it is reserved exclusively for the debug insns created by -fvar-tracking-assignments. That said, if the reserved range gets exhausted, debug insns may grab (non-overlapping) uids above it. Fair's fair ✨.
• ipa-sra-deref-prob-threshold — IPA-SRA will swap out a pointer that's known not to be NULL for one or more new parameters, but only when the probability (in percent, relative to function entry) of it being dereferenced is higher than this parameter. Low probability? Not worth the drama 💅.
• ipa-sra-ptr-growth-factor — IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less than or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. Math is math.
• ipa-sra-ptrwrap-growth-factor — extra maximum allowed growth of the total size of those new parameters that ipa-sra replaces a pointer to an aggregate with — and it only kicks in if the pointer points to a local variable that the caller only writes to and passes along as an argument to other functions. Very specific, very iconic 🎀.
• ipa-sra-max-replacements — the maximum number of pieces of an aggregate that IPA-SRA will track. As a direct consequence, it's also the maximum number of replacements of a formal parameter. Two limits, one parameter, no cap on the vibes.
• sra-max-scalarization-size-Ospeed, sra-max-scalarization-size-Osize — so the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) are out here trying to replace scalar parts of aggregates with uses of independent scalar variables. These two parameters control the maximum size — in storage units — of an aggregate that gets considered for replacement when you're compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize), respectively. Pick your fighter 🥊.
• sra-max-propagations — the maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, all so copy propagation can happen. Obsessed with the specificity honestly.
• tm-max-aggregate-size — when you're making copies of thread-local variables in a transaction, this parameter sets the size in bytes after which variables get saved with the logging functions instead of save/restore code sequence pairs. And heads up: this option only applies when using -fgnu-tm. Periodt.
• graphite-max-nb-scop-params — to avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. A value of zero can be used to lift the bound entirely. And for the record: a variable whose value is unknown at compilation time and defined outside a SCoP counts as a parameter of the SCoP. Iykyk.
• hardcfr-max-blocks — disables -fharden-control-flow-redundancy for functions with a larger number of blocks than the value you specify. Zero removes any limit. Zero = chaos, honestly, but it's your call bestie.
• hardcfr-max-inline-blocks — forces -fharden-control-flow-redundancy to use out-of-line checking for functions with a larger number of basic blocks than the specified value. Sometimes you just gotta take it outside.
• loop-block-tile-size — loop blocking and strip mining transforms (turned on with -floop-block or -floop-strip-mine) strip mine each loop in the loop nest by a given number of iterations. You change that strip length with the loop-block-tile-size parameter. Tune it, queen 👑.
• ipa-jump-function-lookups — specifies the number of statements visited during jump function offset discovery. That's it, that's the parameter.
• ipa-cp-value-list-size — IPA-CP is out here trying to track all possible values and types passed to a function's parameter, so it can propagate them and do devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter. There is a limit to the tracking, babes.
• ipa-cp-eval-threshold — IPA-CP calculates its own score of cloning profitability heuristics, and it'll only perform those cloning opportunities whose scores exceed ipa-cp-eval-threshold. Below the bar? Not happening 💅.
• ipa-cp-max-recursive-depth — the maximum depth of recursive cloning for a self-recursive function. Know your limits 🌀.
• ipa-cp-min-recursive-probability — recursive cloning only happens when the probability of the call actually being executed exceeds this parameter. Otherwise, no cloning for you.
• ipa-cp-profile-count-base — when you're using the -fprofile-use option, IPA-CP will consider the measured execution count of a call graph edge at this percentage position in their histogram as the basis for its heuristics calculation. Profile girlies rise 🫶.
• ipa-cp-recursive-freq-factor — the number of times interprocedural copy propagation expects recursive functions to call themselves. Bold of you to assume they know, but okay.
• ipa-cp-recursion-penalty — the percentage penalty that recursive functions receive when they're evaluated for cloning. Recursion tax, fr fr.
• ipa-cp-single-call-penalty — the percentage penalty that functions containing a single call to another function receive when they're evaluated for cloning. If you only call one friend, you get a lil penalty, sorry 🥲.
• ipa-max-agg-items — IPA-CP can also propagate a number of scalar values passed in an aggregate, and ipa-max-agg-items controls the maximum number of such values per one parameter.
• ipa-cp-loop-hint-bonus — when IPA-CP figures out that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to the profitability score of that candidate. Rewarding good behavior, we love to see it ✨.
• ipa-max-loop-predicates — the maximum number of different predicates that IPA will use to describe when loops in a function have known properties.
• ipa-max-aa-steps — during its analysis of function bodies, IPA-CP employs alias analysis to track values pointed to by function parameters. But it doesn't want to spend forever analyzing absolutely massive functions, so after examining ipa-max-aa-steps statements that modify memory, it gives up and considers all memory clobbered. Same, honestly 💀.
• ipa-max-switch-predicate-bounds — the maximal number of boundary endpoints of case ranges of a switch statement. For a switch that exceeds this limit, IPA-CP will not construct a cloning cost predicate — the one used to estimate cloning benefit — for the default case of that switch statement. Too many cases? Bye 🙈.
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter, all to estimate the benefit of cloning upon a certain constant value. But if the number of operations in a parameter expression exceeds ipa-max-param-expr-ops, that expression is treated as a complicated one and is simply not handled by IPA analysis. It's giving too much, so it's giving nothing.
• lto-partitions — specify the desired number of partitions produced during WHOPR compilation. And lowkey important: the number of partitions should exceed the number of CPUs used for compilation. Balance check 🧚.
• lto-min-partition — the size of the minimal partition for WHOPR, measured in estimated instructions. This keeps you from paying the expense of splitting very small programs into way too many partitions. Petite programs deserve peace 🌸.
• lto-max-partition — the size of the max partition for WHOPR (in estimated instructions), to provide an upper bound for the individual size of a partition. It's meant to be used only with balanced partitioning. Read that again before you use it bestie.
• lto-partition-locality-frequency-cutoff — the denominator n of the fraction 1/n of the execution frequency of a callee to be cloned for a particular caller. Special value of 0 dictates to always clone without a cut-off. Zero = no brakes 🚗💨.
• lto-partition-locality-size-cutoff — the size cut-off for a callee — including inlined calls — to be cloned for a particular caller.
• lto-max-locality-partition — the maximal size of a locality partition for LTO (in estimated instructions). A value of 0 results in the default value being used. Default girlie behavior.
• lto-max-streaming-parallelism — the maximal number of parallel processes used for LTO streaming. Multitasking queen 👑.
• cxx-max-namespaces-for-diagnostic-help — the maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. It's the "did you mean…?" budget. Because bestie, when the lookup fails, the compiler still wants to help you out 💖.
• sink-frequency-threshold — the maximum relative execution frequency (in percents) of the target block relative to a statement's original block, which is what allows statement sinking of that statement. Bigger numbers mean more aggressive statement sinking. And there's a small positive adjustment applied for statements with memory operands, since those are even more profitable to sink. Sink responsibly.
• max-stores-to-sink — the maximum number of conditional store pairs that can be sunk. Set this to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled.
• case-values-threshold — the smallest number of different values for which it's actually best to use a jump-table instead of a tree of conditional branches. If the value is 0, use the default for the machine. Trust the machine sometimes 🎀.
• jump-table-max-growth-ratio-for-size — the maximum code size growth ratio when expanding into a jump table (in percent). This parameter is used when optimizing for size. Small and cute, stay small and cute.
• jump-table-max-growth-ratio-for-speed — the maximum code size growth ratio when expanding into a jump table (in percent). This parameter is used when optimizing for speed. Big and fast, go off 🔥.
• tree-reassoc-width — sets the maximum number of instructions executed in parallel in the reassociated tree. This parameter overrides the target dependent heuristics used by default, as long as it has a non zero value. You have final say, main character 💅.
• sched-pressure-algorithm — picks which of the two -fsched-pressure implementations you get, bestie. Algorithm 1 is the OG one and it's way more likely to stop instructions from being reordered at all. Algorithm 2 was built as a whole compromise situtation — it sits between algorithm 1's kinda conservative vibe and the straight-up aggressive energy of the default scheduler. Heads up tho: it leans hard on having a regular register file and accurate register pressure classes, so it's giving specific ✨. More deets live in haifa-sched.cc in the GCC sources. The default choice? Depends on the target, no cap.
• max-slsr-cand-scan — the max number of existing candidates that get considered when we're out here seeking a basis for a new straight-line strength reduction candidate. slay.
• asan-globals — turns on buffer overflow detection for global objects 🌍. This protection is on by default if you're using the -fsanitize=address option. Wanna turn global objects protection OFF? Use --param asan-globals=0.
• asan-stack — buffer overflow detection for stack objects, we love to see it 🥞. On by default when you're using -fsanitize=address. To disable stack protection use the option --param asan-stack=0.
• asan-instrument-reads — buffer overflow detection for memory reads 👀. On by default when you use -fsanitize=address. To disable memory reads protection, use --param asan-instrument-reads=0.
• asan-instrument-writes — buffer overflow detection for memory writes ✍️. On by default when you use -fsanitize=address. To disable memory writes protection use the option --param asan-instrument-writes=0.
• asan-memintrin — enables detection for built-in functions. On by default when using -fsanitize=address. To disable built-in functions protection use --param asan-memintrin=0.
• asan-use-after-return — enables detection of use-after-return. On by default when you use the -fsanitize=address option. To disable it use --param asan-use-after-return=0.

Note: by default the check is disabled at run time. To enable it, add "detect_stack_use_after_return=1" to the environment variable ASAN_OPTIONS. iykyk 💅

• asan-instrumentation-with-call-threshold — if the number of memory accesses in a function being instrumented is greater than or equal to this number, callbacks get used instead of inline checks. E.g. to disable inline code use --param asan-instrumentation-with-call-threshold=0.
• asan-kernel-mem-intrinsic-prefix — if nonzero, it prefixes calls to memcpy, memset and memmove with __asan_ or __hwasan_ for -fsanitize=kernel-address or -fsanitize=kernel-hwaddress, respectively.
• hwasan-instrument-stack — enables hwasan instrumentation of statically sized stack-allocated variables. On by default when using -fsanitize=hwaddress and off by default when using -fsanitize=kernel-hwaddress. To disable stack instrumentation use --param hwasan-instrument-stack=0, and to enable it use --param hwasan-instrument-stack=1.
• hwasan-random-frame-tag — when you're using stack instrumentation, decides tags for stack variables using a deterministic sequence beginning at a random tag for each frame 🎲. With this parameter unset, tags are chosen using the same sequence but beginning from 1. This is enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress. To disable it use --param hwasan-random-frame-tag=0.
• hwasan-instrument-allocas — enables hwasan instrumentation of dynamically sized stack-allocated variables. On by default when using -fsanitize=hwaddress and off by default when using -fsanitize=kernel-hwaddress. To disable instrumentation of such variables use --param hwasan-instrument-allocas=0, and to enable it use --param hwasan-instrument-allocas=1.
• hwasan-instrument-reads — enables hwasan checks on memory reads. Instrumentation of reads is on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking memory reads use --param hwasan-instrument-reads=0.
• hwasan-instrument-writes — enables hwasan checks on memory writes. Instrumentation of writes is on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking memory writes use --param hwasan-instrument-writes=0.
• hwasan-instrument-mem-intrinsics — enables hwasan instrumentation of builtin functions. Instrumentation of these builtin functions is on by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable instrumentation of builtin functions use --param hwasan-instrument-mem-intrinsics=0.
• use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller than or equal to this number, we directly poison (or unpoison) shadow memory instead of using run-time callbacks. efficiency queen 👑
• tsan-distinguish-volatile — emits special instrumentation for accesses to volatiles.
• tsan-instrument-func-entry-exit — emits instrumentation calls to __tsan_func_entry() and __tsan_func_exit().
• max-fsm-thread-path-insns — the maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.
• threader-debug — threader-debug=[none|all] enables verbose dumping of the threader solver. for when you're nosy about it 🕵️
• parloops-chunk-size — the chunk size of omp schedule for loops parallelized by parloops.
• parloops-schedule — the schedule type of omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime).
• parloops-min-per-thread — the minimum number of iterations per thread of an innermost parallelized loop for which the parallelized variant is preferred over the single threaded one. Lowkey important note: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two.
• max-ssa-name-query-depth — maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion corresponds to following a use-def chain.
• max-speculative-devirt-maydefs — the maximum number of may-defs we analyze when we're out here looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively.
• ranger-debug — specifies the type of debug output to be issued for ranges.
• unroll-jam-min-percent — the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to even be considered profitable.
• unroll-jam-max-unroll — the maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation.
• max-rtl-if-conversion-unpredictable-cost — maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that's considered unpredictable.
• max-variable-expansions-in-unroller — if -fvariable-expansion-in-unroller is used, the maximum number of times that an individual variable will be expanded during loop unrolling.
• partial-inlining-entry-probability — maximum probability of the entry BB of the split region (in percent, relative to the entry BB of the function) to make partial inlining happen.
• max-tracked-strlens — maximum number of strings for which the strlen optimization pass will track string lengths.
• gcse-after-reload-partial-fraction — the threshold ratio for performing partial redundancy elimination after reload.
• gcse-after-reload-critical-fraction — the threshold ratio of critical edges execution count that permits performing redundancy elimination after reload.
• max-loop-header-insns — the maximum number of insns in a loop header duplicated by the copy loop headers pass.
• vect-epilogues-nomask — enable loop epilogue vectorization using a smaller vector size. it's giving 🌈
• vect-partial-vector-usage — controls when the loop vectorizer considers using partial vector loads and stores as an alternative to falling back to scalar code. So like: 0 stops the vectorizer from ever using partial vector loads and stores. 1 allows partial vector loads and stores if vectorization removes the need for the code to iterate. 2 allows partial vector loads and stores in all loops. Heads up bestie — this parameter only has an effect on targets that support partial vector loads and stores.
• vect-inner-loop-cost-factor — the maximum factor which the loop vectorizer applies to the cost of statements in an inner loop, relative to the loop being vectorized. The factor applied is the maximum of the estimated number of iterations of the inner loop and this parameter. The default value of this parameter is 50.
• vect-induction-float — enable loop vectorization of floating point inductions 🌊.
• vect-force-slp — force the use of SLP when vectorizing, and fail if it's not possible. no plan B, periodt.
• vrp-block-limit — maximum number of basic blocks before VRP switches to a lower memory algorithm.
• vrp-sparse-threshold — maximum number of basic blocks before VRP uses a sparse bitmap cache.
• vrp-switch-limit — maximum number of outgoing edges in a switch before VRP will not process it.
• vrp-vector-threshold — maximum number of basic blocks for VRP to use a basic cache vector.
• avoid-fma-max-bits — maximum number of bits for which we avoid creating FMAs.
• fully-pipelined-fma — bestie, does the target fully pipeline FMA instructions? 💅 If this is non-zero, reassociation goes "oh wait, the multiplication part and the addition part of an FMA can run in parallel" and treats them separately — assuming FMUL and FMA share the same units, and those units can also do FADD. iykyk.
• sms-loop-average-count-threshold — a threshold on the average loop count that the swing modulo scheduler looks at. ✨
• sms-dfa-history — how many cycles the swing modulo scheduler keeps in mind when it's checking conflicts using DFA. 🦋
• graphite-allow-codegen-errors — okay so, should codegen errors be ICEs when -fchecking is on? That's the whole vibe check here. 😭
• sms-max-ii-factor — a factor for tuning the upper bound that the swing modulo scheduler uses when it's scheduling a loop. 📐
• lra-max-considered-reload-pseudos — the max number of reload pseudos considered while spilling a non-reload pseudo. lowkey niche but okay.
• max-pow-sqrt-depth — maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant. 💜
• max-dse-active-local-stores — maximum number of active local stores in RTL dead store elimination.
• asan-instrument-allocas — turn on asan allocas/VLAs protection 🛡️.
• max-iterations-computation-cost — a bound on the cost of an expression used to compute the number of iterations.
• max-isl-operations — maximum number of isl operations; 0 means unlimited. (zero = no cap, main character energy 🎀)
• graphite-max-arrays-per-scop — maximum number of arrays per scop.
• max-vartrack-reverse-op-size — max. size of loc list for which reverse ops should be added.
• fsm-scale-path-stmts — a scale factor applied to the number of statements in a threading path that crosses a loop backedge, when comparing against --param=max-jump-thread-duplication-stmts. ✨
• uninit-control-dep-attempts — maximum number of nested calls made when searching for control dependencies during uninitialized variable analysis.
• uninit-max-chain-len — maximum number of predicates anded for each predicate ored in the normalized predicate chain. (the logic is logic-ing 🧚)
• uninit-max-num-chains — maximum number of predicates ored in the normalized predicate chain.
• uninit-max-prune-work — maximum amount of work done to prune paths where the variable is always initialized.
• sched-autopref-queue-depth — hardware autoprefetcher scheduler model control flag. It's the number of lookahead cycles the model looks into; at ' ' only enable the instruction sorting heuristic. 👑
• loop-versioning-max-inner-insns — the maximum number of instructions an inner loop can have before the loop versioning pass decides it's too big to copy.
• loop-versioning-max-outer-insns — the maximum number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy — and it discounts any instructions in inner loops that directly benefit from versioning. 💖
• ssa-name-def-chain-limit — the maximum number of SSA_NAME assignments to follow when determining a property of a variable, like its value. This limits the number of iterations or recursive calls GCC performs when it's optimizing certain statements, or when it's figuring out whether they're even valid before it throws a diagnostic at you. 😭
• store-merging-max-size — maximum size of a single store merging region, in bytes.
• store-forwarding-max-distance — maximum instruction distance that a small store forwarded to a larger load may stall. Value '0' disables the cost checks for the avoid-store-forwarding pass.
• hash-table-verification-limit — the number of elements for which hash table verification is done, for each searched element.
• max-find-base-term-values — maximum number of VALUEs handled during a single find_base_term call.
• analyzer-max-enodes-per-program-point — the maximum number of exploded nodes per program point within the analyzer, before it terminates analysis of that point. 💀
• analyzer-max-constraints — the maximum number of constraints per state.
• analyzer-min-snodes-for-call-summary — the minimum number of supernodes within a function for the analyzer to even consider summarizing its effects at call sites.
• analyzer-max-enodes-for-full-dump — the maximum depth of exploded nodes that should show up in a dot dump before it switches to a less verbose format. 🙈
• analyzer-max-recursion-depth — the maximum number of times a callsite can appear in a call stack within the analyzer, before it terminates analysis of a call that would recurse deeper.
• analyzer-max-svalue-depth — the maximum depth of a symbolic value, before the value just gets approximated as unknown.
• analyzer-max-infeasible-edges — the maximum number of infeasible edges to reject before declaring a diagnostic as infeasible.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block which is considered hot. This parameter is used only in GIMPLE FE. 🔥
• analyzer-bb-explosion-factor — the maximum number of 'after supernode' exploded nodes within the analyzer per supernode, before it terminates analysis. 💥
• analyzer-text-art-string-ellipsis-threshold — the number of bytes at which to ellipsize string literals in analyzer text art diagrams. 🎨
• analyzer-text-art-ideal-canvas-width — the ideal width, in characters, of text art diagrams generated by the analyzer.
• analyzer-text-art-string-ellipsis-head-len — the number of literal bytes to show at the head of a string literal in text art when it's being ellipsized.
• analyzer-text-art-string-ellipsis-tail-len — the number of literal bytes to show at the tail of a string literal in text art when it's being ellipsized. ✂️
• ranger-logical-depth — maximum depth of logical expression evaluation that ranger will look through when it's evaluating outgoing edge ranges.
• ranger-recompute-depth — maximum depth of instruction chains to consider for recomputation in the outgoing range calculator.
• relation-block-limit — maximum number of relations the oracle will register in a basic block.
• transitive-relations-work-bound — work bound when discovering transitive relations from existing relations. it's giving "how much work are we willing to do" 🫶
• min-pagesize — minimum page size, for warning and early break vectorization purposes.
• openacc-kernels — specify the mode of OpenACC kernels' constructs handling. With --param=openacc-kernels=decompose, OpenACC kernels' constructs are decomposed into parts — a sequence of compute constructs, each then handled individually. This is work in progress. 🚧 With --param=openacc-kernels=parloops, OpenACC `kernels' constructs are handled by the parloops pass, en bloc. This is the current default.
• openacc-privatization — control whether the -fopt-info-omp-note and applicable -fdump-tree-*-details options emit OpenACC privatization diagnostics. With --param=openacc-privatization=quiet, don't diagnose — and that's the current default. With --param=openacc-privatization=noisy, do diagnose. 📢
• cycle-accurate-model — specifies whether GCC should assume that the scheduling description is mostly a cycle-accurate model of the target processor the code is intended to run on, in the absence of cache misses. Nonzero means the selected scheduling model is accurate and likely describes an in-order processor, and that scheduling should aggressively spill to try and fill any pipeline bubbles — that's the current default. Zero means the scheduling description might not be available/accurate, or maybe not applicable at all, like for modern out-of-order processors. periodt.

---

The following choices of name are available on AArch64 targets: 🌸

• aarch64-vect-compare-costs — when vectorizing, consider using multiple different approaches and use the cost model to choose the cheapest one. This includes:
  • Trying both SVE and Advanced SIMD, when SVE is available.
  • Trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit vectors for everything.
  • Trying to use "unpacked" SVE vectors for smaller elements. This includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores. 📦
• aarch64-float-recp-precision — the number of Newton iterations for calculating the reciprocal for float type. The precision of division is proportional to this param when division approximation is enabled. The default value is 1.
• aarch64-double-recp-precision — the number of Newton iterations for calculating the reciprocal for double type. The precision of division is proportional to this param when division approximation is enabled. The default value is 2.
• aarch64-autovec-preference — force an ISA selection strategy for auto-vectorization. 🚀
  • default — use the default heuristics.
  • asimd-only — use only Advanced SIMD for auto-vectorization.
  • sve-only — use only SVE for auto-vectorization.
  • prefer-asimd — use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.
  • prefer-sve — use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
• aarch64-ldp-policy — fine-grained policy for load pairs. With --param=aarch64-ldp-policy=default, use the policy of the tuning structure — that's the current default. With --param=aarch64-ldp-policy=always, emit ldp regardless of alignment. With --param=aarch64-ldp-policy=never, do not emit ldp. With --param=aarch64-ldp-policy=aligned, emit ldp only if the source pointer is aligned to at least double the alignment of the type. 💅
• --param=aarch64-stp-policy=policy — fine-grained control over store pairs, bestie. With --param=aarch64-stp-policy=default, you're using the policy of the tuning structure — and that's the current default. With --param=aarch64-stp-policy=always, it emits stp regardless of alignment. With --param=aarch64-stp-policy=never, it does not emit stp. And with --param=aarch64-stp-policy=aligned, it emits stp only if the source pointer is aligned to at least double the alignment of the type. ✨
• --param=aarch64-ldp-alias-check-limit=limit — this caps how many alias checks the AArch64 load/store pair fusion pass does when it's trying to form an ldp/stp. Higher values make the pass more aggressive at re-ordering loads over stores, but the trade-off is increased compile time. 💅
• --param=aarch64-ldp-writeback=value — controls which writeback opportunities we try to handle in the AArch64 load/store pair fusion pass. A value of zero disables writeback handling. One means we try to form pairs involving one or more existing individual writeback accesses where possible. A value of two means we also try to opportunistically form writeback opportunities by folding in trailing destructive updates of the base register used by a pair. 🫶
• --param=aarch64-loop-vect-issue-rate-niters=n — the tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If this parameter is set to n, GCC will not use this heuristic for loops that are known to execute in fewer than n Advanced SIMD iterations.
• --param=aarch64-vect-unroll-limit=bound — the vectorizer uses available tuning information to work out whether unrolling the main vectorized loop would be beneficial, and by how much. This parameter sets the upper bound on how much the vectorizer will unroll the main loop. The default value is four.

The following choices of name are available on GCN targets, babes:

• --param=gcn-preferred-vectorization-factor=factor — preferred vectorization factor: default, 32, 64.

The following choices of name are available on i386 and x86_64 targets:

• --param=x86-stlf-window-ninsns=n — instructions number above which STFL stall penalty can be compensated.
• --param=x86-stv-max-visits=n — the maximum number of use and def visits when discovering a STV chain before the discovery is aborted.

---

🎀 Program Instrumentation Options 🎀

GCC supports a whole bunch of command-line options that control adding run-time instrumentation to the code it normally generates. Like, what's the vibe here? Well, one purpose of instrumentation is to collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses — and also deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There's also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. So many reasons, so little time. 💖

• -p, -pg — generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking oh my god don't forget the linking part.

You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. 🙈

• -fprofile-arcs — add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable.

When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).

Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files.

• -fcondition-coverage — add code so that program conditions are instrumented. During execution the program records what terms in a conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read with gcov --conditions. 💅
• -fpath-coverage — add code so that the paths taken are tracked. During execution the program records the prime paths taken. The number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up instrumentation if the approximate number of paths exceeds the limit controlled by -fpath-coverage-limit. The result can be read with gcov --prime-paths --prime-paths-lines --prime-paths-source, 🔥
• -fpath-coverage-limit=limit — the threshold at which point -fpath-coverage gives up on instrumenting a function. This limit is approximate and conservative, as GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and gives up if the threshold is reached before finding all the paths. This option is not for fine grained control over which functions to instrument — rather it is intended to limit the effect of path explosion and keep compile times reasonable. The default is 250000. ⭐
• --coverage — this option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details.
  1. Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
  2. Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
  3. Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
  4. Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting.

Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged.

5. For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
6. For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information. 🦋

With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. Obsessed with this actually.

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 — okay bestie, this one picks which directory GCC goes rummaging in for your profile data files, and it's path. It only matters for profile data made by -fprofile-generate, -ftest-coverage, -fprofile-arcs, and the stuff eaten by -fprofile-use and -fbranch-probabilities (plus its lil friends). Absolute paths, relative paths, both are valid, no judgement. By default GCC just uses the current directory as path, which means your profile data file lands in the same folder as the object file. And to stop file names from beefing with each other, if the object file name isn't an absolute path, GCC mangles the absolute path of the sourcename.gcda file and uses that as the file name of a .gcda file. (All the file-naming lore lives in -fprofile-arcs, and there's a similar option hanging out at -fprofile-note.)

So when an executable gets run in a massive parallel environment, the move is to save your profile into different folders. You do that with variables in path that get exported at run-time:

• %p — process ID. periodt.
• %q{VAR} — the value of environment variable VAR.

• -fprofile-generate / -fprofile-generate=path — turns on the options you'd usually use to instrument an app so it produces a profile that's useful for later recompilation with profile-feedback-based optimization. Important: you must use -fprofile-generate both when compiling and when linking your program. Not optional, bestie.

The following options get enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp. ✨

If path is specified, GCC peeks at that path to find the profile feedback data files. See -fprofile-dir for the deets.

And to optimize the program based on all the profile info you collected? Use -fprofile-use. 🫶

• -fprofile-info-section / -fprofile-info-section=name — registers the profile information in the specified section instead of using a constructor/destructor. The section name is name if you pass it, otherwise it defaults to ".gcov_info". A pointer to the profile information generated by -fprofile-arcs gets 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. Note: this option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (think embedded systems) with limited resources that don't support constructors/destructors or the C library file I/O.

The linker could collect the input sections into a continuous memory block and define start and end symbols. Here's a GNU linker script example that defines a linker output section:

          .gcov_info      :
          {
            PROVIDE (__gcov_info_start = .);
            KEEP (*(.gcov_info))
            PROVIDE (__gcov_info_end = .);
          }

And the program could dump the profiling information registered in this linker set, for example, like this:

        #include <gcov.h>
        #include <stdio.h>
        #include <stdlib.h>

        extern const struct gcov_info *const __gcov_info_start[];
        extern const struct gcov_info *const __gcov_info_end[];

        static void
        dump (const void *d, unsigned n, void *arg)
        {
          const unsigned char *c = d;

          for (unsigned i = 0; i < n; ++i)
            printf ("%02x", c[i]);
        }

        static void
        filename (const char *f, void *arg)
        {
          __gcov_filename_to_gcfn (f, dump, arg );
        }

        static void *
        allocate (unsigned length, void *arg)
        {
          return malloc (length);
        }

        static void
        dump_gcov_info (void)
        {
          const struct gcov_info *const *info = __gcov_info_start;
          const struct gcov_info *const *end = __gcov_info_end;

          /* Obfuscate variable to prevent compiler optimizations.  */
          __asm__ ("" : "+r" (info));

          while (info != end)
          {
            void *arg = NULL;
            __gcov_info_to_gcda (*info, filename, dump, allocate, arg);
            putchar ('\n');
            ++info;
          }
        }

        int
        main (void)
        {
          dump_gcov_info ();
          return 0;
        }

The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the "__gcov_filename_to_gcfn" and "__gcov_info_to_gcda" functions and merge the profile information into .gcda files on the host filesystem. 🔥

• -fprofile-note=path — if path is specified, GCC saves the .gcno file into that path location. Careful though: if you combine this option with multiple source files, the .gcno file will be overwritten. 💀
• -fprofile-prefix-path=path — this one can be combined with profile-generate=profile_dir and profile-use=profile_dir to tell GCC where the base directory of the built source tree is. By default profile_dir contains files with mangled absolute paths of all the object files in the built project. And that's not the vibe when the directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback — because then the profile data won't 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. slay.
• -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 — changes up 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 gets 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. But if the target platform doesn't support the required atomic operations in hardware, and libatomic is available, then the profile information is updated through calls to libatomic. And if the target platform neither supports the required atomic operations in hardware nor has 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. And if a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state. 💀

• -fprofile-filter-files=regex — instruments only the functions from files whose name matches any of the regular expressions (separated by semi-colons).

For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'. ✨

• -fprofile-exclude-files=regex — instruments only the functions from files whose name does not match any of the regular expressions (separated by semi-colons).

For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files located in the /usr/ folder. 🚫

• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — controls the level of reproducibility of the profile gathered by "-fprofile-generate". This makes it possible to rebuild a program with the same outcome, which is useful for, like, distribution packages. 📦

With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it's not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of the program run, but also before "fork" function is invoked. 👀

Note that it's quite common for execution counts of some part of programs to depend, for example, on the length of temporary file names or memory space randomization (which may affect hash-table collision rate). Such non-reproducible parts of programs may be annotated by the "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 the collected profile stays reproducible regardless of the order of streaming of the data into gcda files. This setting makes it possible to run multiple instances of the instrumented program in parallel (such as with "make -j"). This reduces the quality of the gathered data, in particular of indirect call profiling. 💅

• -fsanitize=address — bestie, turn on AddressSanitizer, a fast memory error detector 💖. Memory access instructions get instrumented so it can catch out-of-bounds and use-after-free bugs. Turning this on also enables -fsanitize-address-use-after-scope. More deets at https://github.com/google/sanitizers/wiki/AddressSanitizer. The run-time behavior can be influenced using the ASAN_OPTIONS environment variable — set it to "help=1" and the available options get shown at startup of the instrumented program ✨. A list of supported options lives at https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags. This option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. Also lowkey important: the only targets -fsanitize=hwaddress is currently supported on are x86-64 (only with -mlam=u48 or -mlam=u57 options) and AArch64, in both cases only in ABIs with 64-bit pointers.

When compiling with -fsanitize=address, you should also use -g to produce more meaningful output, fr fr. And to get more accurate stack traces, it's 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 — Enable Hardware-assisted AddressSanitizer for compilation of the Linux kernel 🌸. Similar to -fsanitize=kernel-address but using an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences necessary for compiling the Linux kernel. These differences are to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte.

Note: This option has different defaults to the -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default but are still possible by specifying the command-line options --param hwasan-instrument-stack=1 and --param hwasan-instrument-allocas=1 respectively. Using a random frame tag is not implemented for kernel instrumentation.

• -fsanitize=pointer-compare — Instrument comparison operation (<, <=, >, >=) with pointer operands 🔥. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread. Note: By default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null.
• -fsanitize=pointer-subtract — Instrument subtraction with pointer operands 💜. The option must be combined with either -fsanitize=kernel-address or -fsanitize=address. The option cannot be combined with -fsanitize=thread. Note: By default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-null.
• -fsanitize=shadow-call-stack — Enable ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (e.g. stack buffer overflows.) 🎀 It works by saving a function's return address to a separately allocated shadow call stack in the function prologue and restoring the return address from the shadow call stack in the function epilogue. Instrumentation only occurs in functions that need to save the return address to the stack.

Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For the user space programs, runtime support is not currently provided in libc and libgcc. Users who want to use this feature in user space need to provide their own support for the runtime. It should be noted that this may cause the ABI rules to be broken 😭.

On aarch64, the instrumentation makes use of the platform register "x18". This generally means that any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18, otherwise functions compiled without -ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer ⭐.

Also, because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions.

See https://clang.llvm.org/docs/ShadowCallStack.html for more details.

• -fsanitize=thread — Enable ThreadSanitizer, a fast data race detector 🌈. Memory access instructions are instrumented to detect data race bugs. See https://github.com/google/sanitizers/wiki#threadsanitizer for more details. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags for a list of supported options. The option cannot be combined with -fsanitize=address, -fsanitize=leak.

When compiling with -fsanitize=thread, you should also use -g to produce more meaningful output 🙈.

Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).

• -fsanitize=leak — Enable LeakSanitizer, a memory leak detector 🍓. This option only matters for linking of executables. The executable is linked against a library that overrides "malloc" and other allocator functions. See https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer for more details. The run-time behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with -fsanitize=thread.
• -fsanitize=undefined — Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector ⚡. Various computations are instrumented to detect undefined behavior at runtime. See https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html for more details. The run-time behavior can be influenced using the UBSAN_OPTIONS environment variable. Current suboptions are:
  • -fsanitize=shift — This option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent.
  • -fsanitize=shift-exponent — This option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.
  • -fsanitize=shift-base — If the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.
  • -fsanitize=integer-divide-by-zero — Detect integer division by zero 💀.
  • -fsanitize=unreachable — With this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message call instead. When reaching the "__builtin_unreachable" call, the behavior is undefined.
  • -fsanitize=vla-bound — This option instructs the compiler to check that the size of a variable length array is positive.
  • -fsanitize=null — This option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.
  • -fsanitize=return — This option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.
  • -fsanitize=signed-integer-overflow — This option enables signed integer overflow checking. We check that the result of "+", "*", and both unary and binary "-" does not overflow in the signed arithmetics. This also detects "INT_MIN / -1" signed division. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:

signed char a = SCHAR_MAX;
a++;

• -fsanitize=bounds — okay so this one turns on instrumentation of array bounds, bestie. It catches a whole bunch of out-of-bounds accesses ✨. BUT — and this is the tea — flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented. The one exception? Flexible array member-like arrays where -fstrict-flex-arrays or -fstrict-flex-arrays= options (or strict_flex_array attributes) say they shouldn't be treated like flexible array member-like arrays. iykyk 💅
• -fsanitize=bounds-strict — this is the strict girl of the bounds family 🔥. It instruments array bounds strictly, so most out-of-bounds accesses get detected, including flexible array member-like arrays. Initializers of variables with static storage are still not instrumented, no cap.
• -fsanitize=alignment — this enables checking alignment of pointers when they get dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object. Alignment said: stay in your lane 💅
• -fsanitize=object-size — instrumentation of memory references using the __builtin_dynamic_object_size function, babes. It detects various out-of-bounds pointer accesses. Obsessed.
• -fsanitize=float-divide-by-zero — detects floating-point division by zero. And here's the lil plot twist 🫶: unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. Iconic behaviour, honestly.
• -fsanitize=float-cast-overflow — enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Same vibe as above: unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also heads up 🚨 — this option does not work well with FE_INVALID exceptions enabled.
• -fsanitize=nonnull-attribute — instruments calls, checking whether null values are NOT passed to arguments marked as requiring a non-null value by the nonnull function attribute. Like, read the label, bestie.
• -fsanitize=returns-nonnull-attribute — instruments return statements in functions marked with the returns_nonnull function attribute, to detect returning of null values from such functions. Caught in 4K 📸.
• -fsanitize=bool — instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued. Bool said "I only know two things" 💀.
• -fsanitize=enum — 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. Staying in your enum range is the vibe check 🌈.
• -fsanitize=vptr — instruments C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. C++ girlies, this one's for you 💜.
• -fsanitize=pointer-overflow — instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. Math said no 🎀.
• -fsanitize=builtin — instruments arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. For example ✨: passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. Lowkey my fave.

Heads up bestie, real talk 🚨: note that 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. Don't do it, fr fr.

Also, while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message instead. This currently works only for the C family of languages 👑

---

• -fno-sanitize=all — this option disables all previously enabled sanitizers. And no, -fsanitize=all is not allowed, because some sanitizers cannot be used together. The way that they simply do not vibe 😭.
• *-fasan-shadow-offset=number* — this forces GCC to use a custom shadow offset in AddressSanitizer checks. Super useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer 🧚.
• *-fsanitize-sections=s1,s2,...* — sanitize global variables in selected user-defined sections. And plot twist — si may contain wildcards ✨.
• -fsanitize-recover[=opts] — okay so -fsanitize-recover= controls error recovery mode for sanitizers mentioned in the comma-separated list of opts. Enabling this for a sanitizer component makes it 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. Wild, I know. The -fno-sanitize-recover= option flips this: only the first detected error is reported and the program then exits with a non-zero exit code. Periodt.

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 🧪. And -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. You can override this through setting the halt_on_error flag in the corresponding environment variable.

Syntax without an explicit opts parameter is deprecated 🚨. It is equivalent to specifying an opts list of:

undefined,float-cast-overflow,float-divide-by-zero,bounds-strict

• -fsanitize-address-use-after-scope — enables sanitization of local variables to detect use-after-scope bugs. And this option sets -fstack-reuse to none. Slay 💅.
• -fsanitize-trap[=opts] — this option instructs the compiler to report for sanitizers mentioned in the comma-separated list of opts undefined behavior using __builtin_trap rather than a libubsan library routine. If this is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer, and __builtin_trap will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover= 🔥.

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. Iconic ✨.

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 heads up, -fsanitize-trap=vptr is not allowed 💀.

• -fsanitize-undefined-trap-on-error — this is the deprecated equivalent of -fsanitize-trap=all. Like, it's giving legacy, bestie 🎀.
• -fsanitize-coverage=trace-pc — enable coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block 🦋.
• -fsanitize-coverage=trace-cmp — enable 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, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. The whole squad showed up 👑.
• -fcf-protection=[full|branch|return|none|check]
• -fcf-protection — enables code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid 🛡️. This prevents diverting the flow of control to an unexpected target. It's intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Protect the plot, bestie.

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 ships an implementation based on Intel Control-flow Enforcement Technology (CET), and it works on i686 processors or newer. Slay 💅

NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-cf-protection nor -fcf-protection=* are found.

• -fharden-compares — bestie, for every logical test that survives gimple optimizations and is not the condition in a conditional branch (like, conditions tested for conditional moves, or conditions getting stored into boolean variables 👀), this emits extra code to compute and verify the reversed condition, and to call __builtin_trap if the results don't match. Use it together with -fharden-conditional-branches so you cover all the conditionals. Teamwork fr fr 🫶
• -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. Pair it with -fharden-compares to cover every last conditional 💀
• -fharden-control-flow-redundancy — emits extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when those booleans don't form an execution path compatible with the control flow graph. It's giving ✨control flow integrity✨

Verification happens before returns, before mandatory tail calls (peep 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. For tuning you've got --param hardcfr-max-blocks and --param hardcfr-max-inline-blocks available. Iconic 🔥

Tail call optimization happens way too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, plus any calls marked early enough as potential tail calls, would also get verification issued before the call — buuut these possibilities are merely theoretical, since those conditions can only be met when you're using custom compiler plugins. So basically never, bestie 🙈

• -fhardcfr-skip-leaf — disables -fharden-control-flow-redundancy in leaf functions. Just let the little leaf functions vibe 🍓
• -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is active, this checks the recorded execution path against the control flow graph at exception escape points, like as if the function body was wrapped with a cleanup handler that performed the check and reraised. This option is enabled by default, so if you wanna turn it off use -fno-hardcfr-check-exceptions. Periodt 💅
• -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before any function call that's immediately followed by a return of its result, if any — doing it this way so we don't block tail-call optimization, whether or not it ends up getting optimized to a tail call. Generous queen behavior 👑

This option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but no cap, you can turn it on (or off, using its negated form) explicitly regardless of the optimizations.

• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before noreturn calls: either all of them (always), those that aren't expected to return control to the caller through an exception (no-xthrow, which is the default), those that may not return control to the caller through an exception either (nothrow), or none of them (never). Choose your fighter 🎀

Checking before a noreturn function that may return control to the caller through an exception can cause checking to be performed more than once, if the exception gets caught in the caller — whether by a handler or a cleanup. When -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a noreturn call with the implicitly-added cleanup handler, since that would be redundant with the check performed before the call. But other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit. And bestie, the checkpoint may even be another noreturn call, so the checking may end up performed multiple times. It's a whole thing 😭

Various optimizers may cause calls to be marked noreturn and/or nothrow, even when the corresponding attributes aren't there, which may affect the placement of checks before calls, as well as the addition of implicit cleanup handlers for them. This unpredictability, plus the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, is exactly why no-xthrow is the default setting for this option: it excludes from the noreturn treatment only internal functions used to (re)raise exceptions, which aren't affected by these optimizations. Lowkey smart 🔥

• -fhardened — enables a set of flags for C and C++ that improve the security of the generated code without affecting its ABI. The precise flags enabled may change between major releases of GCC, but currently they are:

-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE  -pie  -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)

The list of options enabled by -fhardened can be generated using the --help=hardened option. So handy 💖

When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.

This option is intended to be used in production builds, not merely in debug builds. So like, actually ship it 🚀

Currently, -fhardened is only supported on GNU/Linux targets.

Also, -fhardened only enables a particular option if it wasn't already specified anywhere on the command line. For instance, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. No take-backs, bestie 💅

• -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 — this includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered — optimized away variables or variables allocated in registers don't count. Guard your faves 🛡️
• -fstack-protector-all — like -fstack-protector except all functions are protected. Everyone gets the guard, no exceptions ✨
• -fstack-protector-strong — like -fstack-protector but it includes additional functions to be protected, namely those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, so optimized away variables or variables allocated in registers don't count. Again 💅

NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC, ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found.

• -fstack-protector-explicit — like -fstack-protector but only protects those functions which have the stack_protect attribute. Exclusive invite-only protection 🎀
• -fstack-check — generates code to verify that you don't go beyond the boundary of the stack. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded environment, since stack overflow is automatically detected on nearly all systems if there's only one stack. Battle-tested logic 🧠

Note that this switch doesn't actually cause checking to be done — the operating system or the language runtime has to do that. The switch just causes generation of code to ensure that they see the stack being extended. It's a heads-up, not a bodyguard 🙈

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.

Old-style checking is a generic mechanism that requires no specific target support in the compiler, but it comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code. ⚠️
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered. Not the vibe 😭

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks — to protect against those you want -fstack-clash-protection. Don't get it twisted 💜

• -fstack-clash-protection — generates code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system. Obsessed with this dedication 🫶

Most targets don't fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.

NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++, ObjC, ObjC++, unless -fno-stack-clash-protection is found.

• *-fstack-limit-register=reg*
• *-fstack-limit-symbol=sym*
• -fno-stack-limit — generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it's possible to catch the signal without taking special precautions. Cute and safe 🌸

Okay so picture this bestie: your stack starts at absolute address 0x80000000 and it grows downwards 👇 — then you can pull up with the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Slay. Note though: this may only work with the GNU linker, so don't come for me if your linker is mid 🙈

And if you ever wanna locally override stack limit checking? The no_stack_limit function attribute has your back 🫶

• -fsplit-stack — This one generates code that automatically splits the stack before it overflows, so the resulting program has a discontiguous stack that can only overflow if the program is literally unable to allocate any more memory 🌈 Most useful when you're running threaded programs, because you no longer have to sit there calculating a good stack size for each thread (iconic behavior honestly). Currently this is only implemented for the x86 targets running GNU/Linux.

When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run 😭 If compiling all code — library code included — with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this lives in the gold linker in GNU binutils release 2.21 and later. no cap ✨

• -fstrub=disable — Disables stack scrubbing entirely, ignoring any strub attributes. See
• -fstrub=strict — Functions default to strub mode disabled, and it applies strictly the restriction that only functions associated with strub-callable modes (at-calls, callable and always_inline internal) are callable by functions with strub-enabled modes (at-calls and internal).
• -fstrub=relaxed — Restores the default stack scrub (strub) setting, namely, strub is only enabled as required by strub attributes associated with function and data types. "Relaxed" means that strub contexts are only prevented from calling functions explicitly associated with strub mode disabled. This option is only useful to override other -fstrub=* options that precede it in the command line. iykyk 💅
• -fstrub=at-calls — Enables at-calls strub mode where viable. The primary use of this option is for testing — it exercises the strub machinery in scenarios strictly local to a translation unit. Heads up though: 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 details on viability and eligibility requirements 👑
• -fstrub=internal — Enables internal strub mode where viable. Again, primary use is testing: this option is meant to thoroughly exercise the parts of the strub machinery that implement the less efficient, but interface-preserving strub mode. Functions that would not be affected by this option are quite uncommon. rare fr fr 🦋
• -fstrub=all — Enables some strub mode where viable. When both strub modes are viable, at-calls is preferred. -fdump-ipa-strubm adds function attributes that tell which mode was selected for each function. Primary use of this option is for testing, to exercise thoroughly the strub machinery. ✨
• -fvtable-verify=[std|preinit|none] — Only available when compiling C++ code, bestie. It turns on (or off, if you're using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted 💀

This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized. order matters, periodt 🌸

If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std.

• -fvtv-debug — When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. That info gets written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. 📝

Note: this feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one first. don't say I didn't warn you 🙈

• -fvtv-counts — This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, it causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts, and logs this information for each compilation unit 📊 The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory. two logs, one vibe 💅

Note: this feature appends data to the log files. To get fresh log files, be sure to delete any existing ones. 😭

• -finstrument-functions — Generates instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site 🌟 (On some platforms, __builtin_return_address does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. cute right 🔍

This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size 😬 If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies. lucky you 🍓)

A function may be given the attribute no_instrument_function, in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). safety first bestie 🫶

• -finstrument-functions-once — Similar to -finstrument-functions, but the profiling functions are called only once per instrumented function, i.e. the first profiling function is called after the first entry into the instrumented function and the second profiling function is called before the exit corresponding to this first entry. 1️⃣

The definition of "once" for the purpose of this option is a little vague because the implementation is not protected against data races 😵‍💫 As a result, the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread, but the calls are always paired, that is to say, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function.

• -finstrument-functions-exclude-file-list=file,file,... — Sets the list of functions that are excluded from instrumentation (see the description of -finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match. ✂️

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. bye felicia 💅

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). commas being dramatic as always 😭

• -finstrument-functions-exclude-function-list=sym,sym,... — Similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. 🌈

---

• -fpatchable-function-entry=N[,M] — bestie, this one plants N NOPs right at the very beginning of every single function, and the function's entry point sits before the Mth NOP. If you ghost M, it defaults to 0, so the function entry points at the address right at the first NOP. Those NOP instructions are reserving extra space for you 💅 — you can patch in whatever instrumentation you want at run time, as long as the code segment is writable. The amount of space is controlled indirectly through how many NOPs you ask for, and the actual NOP instruction used is the one emitted by the internal GCC back-end interface gen_nop. Heads up: this behavior is target-specific and may also depend on the architecture variant and/or other compilation options. It's giving... "it depends" energy 🙈

For run-time identification, the starting addresses of those areas — which correspond to their respective function entries minus M — get collected in the __patchable_function_entries section of the resulting binary. Slay, they're labeled.

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 it to increase the area size or to nuke it completely on a single function. And if N=0, no pad location is recorded.

The NOP instructions get inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. See the -msplit-patch-nops option to change that.

The maximum value of N and M is 65535. And 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 you're not using -msplit-patch-nops.

---

✨ Options Controlling the Preprocessor ✨

Okay so these options are all about the C preprocessor, which runs on each C source file before the actual compilation happens. It's the opening act, the pre-game, the vibe-setter 💖

If you use the -E option, literally nothing happens except preprocessing. And some of these options only make sense alongside -E, because they make the preprocessor output unsuitable for actual compilation. Like, it's not going to compile after that, bestie, that's the point.

On top of the options listed right here, there are a bunch of options controlling search paths for include files — those live in Directory Options. And options controlling preprocessor diagnostics are chilling in Warning Options.

• -D name — Predefine name as a macro, with definition 1.
• -D name=definition — The contents of definition are tokenized and processed as if they 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 wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args...)=definition' works.

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. Order matters, periodt.

• -U name — Cancel any previous definition of name, either built in or provided with a -D option. Undefine it, un-alive it 💀
• -include file — Process file as if #include "file" appeared as the first line of the primary source file. However! The first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If it's not found there, it's searched for in the remainder of the #include "..." search chain as normal.

If multiple -include options are given, the files are included in the order they appear on the command line.

• -imacros file — Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This lets you acquire all the macros from a header without also processing its declarations. It's the "just the vibes, not the declarations" option, iykyk.

All files specified by -imacros are processed before all files specified by -include.

• -undef — Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined, though. Those stay. 🤝
• -pthread — Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. Consistency is key, bestie, don't use it for one half and ghost the other.
• -M — Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal. So like, keep your debug mess separate, okay? 🫶

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

• -MM — Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output.

• -MF file — When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given, the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

If file is -, then the dependencies are written to stdout. Cute.

• -MG — In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless.

This feature is used in automatic updating of makefiles. Very slay of it.

• -Mno-modules — Disable dependency generation for compiled module interfaces.
• -MP — This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. Lowkey a lifesaver.

This is typical output:

test.o: test.c test.h

test.h:

• -MT target — Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. The result is the target.

An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give

$(objpfx)foo.o: foo.c

• -MQ target — Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives

$$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ.

• -MD — -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file.

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Side quest dependency file, obsessed. ✨

• -MMD — Like -MD except mention only user header files, not system header files.
• -fpreprocessed — Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. Just a lil tokenizer 🥺

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — When preprocessing, handle directives, but do not expand macros. The way that it behaves depends on the -E and -fpreprocessed options. Bestie, it's all about context here.

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 — bestie, this one says yes, you can use $ inside identifiers. Let it in. It's giving old-school but valid. 💅
• -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 if you're on C99+ or C++, it's already on. No cap. ✨
• -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization. So it's giving raw system header paths, no glow-up. 🫶
• -fmax-include-depth=depth — set the maximum depth of the nested #include. The default is 200. Go deeper than that and it's a vibe check fail. 😭
• -fsearch-include-path[=kind] — look for input files on the #include path, not just the current directory. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly:

g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc

kind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the "#include <...>" search path.

• -ftabstop=width — set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8. So keep it in range, bestie, or it ghosts you. 🎀
• -ftrack-macro-expansion[=level] — track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value 0 of level de-activates this option. Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value 2 tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is 2.

Note that -ftrack-macro-expansion=2 is activated by default.

• -fmacro-prefix-map=old=new — 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. It's giving path glow-up for reproducible vibes. ✨
• -fexec-charset=charset — set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's "iconv" library routine.
• -fwide-exec-charset=charset — set the wide execution character set, used for wide string and character constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the width of wchar_t and the big-endian or little-endian byte order being used for code generation. As with -fexec-charset, charset can be any encoding supported by the system's "iconv" library routine; however, you will have problems with encodings that do not fit exactly in wchar_t. So if it don't fit, it's not it. 💀
• -finput-charset=charset — set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's "iconv" library routine.
• -fpch-deps — when using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used. Ngl, it's giving receipts for your PCH. 🧚
• -fpch-preprocess — this option allows use of a precompiled header together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename" in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.

You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.

• -fworking-directory — enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever.
• -A predicate=answer — make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. So bestie, use the equals sign. It's iconic. 💅
• -A -predicate=answer — cancel an assertion with the predicate predicate and answer answer.
• -C — do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #.

• -CC — do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments.

• -P — inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers. So no linemarker drama. 🙈
• -traditional / -traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details.

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.

• -trigraphs — support ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined by ISO C to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline.

The nine trigraphs and their replacements are

Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
Replacement:      [    ]    {    }    #    \    ^    |    ~

By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.

• -remap — enable special code to work around file systems which only permit very short file names, such as MS-DOS. It's giving retro file system survival. 💜
• -H — print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! . So you can vibe check your includes. ✨
• -dletters — says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined.
  • -dM — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.

• -dD — like -dM except that it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
• -dN — like -dD, but emit only the macro names, not their expansions.
• -dI — output #include directives in addition to the result of preprocessing.
• -dU — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time.

• -fdebug-cpp — okay so this one is literally only for debugging GCC itself, bestie. When you use it from CPP or with -E, it dumps debugging info about location maps. Every token in the output gets preceded by a dump of the map its location belongs to. And when you use it from GCC without -E? Nothing. Zero. No effect. 🙈
• *-Wp,option* — this lets you bypass the compiler driver entirely and shove option directly through to the preprocessor. If option has commas in it, it gets split into multiple options right at the commas. BUT 💀 — so many options get modified, translated or interpreted by the compiler driver before they ever reach the preprocessor, and -Wp forcibly skips that whole phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and just let the driver handle the options instead. fr fr, let the driver cook.
• -Xpreprocessor option — pass option as an option to the preprocessor. Use this when you need to supply system-specific preprocessor options that GCC doesn't even recognize. Okay and here's the tea 🫖: if you want to pass an option that takes an argument, you have to use -Xpreprocessor twice — once for the option, and once for the argument. Twice. Not once. Twice.
• -no-integrated-cpp — performs preprocessing as a separate pass before compilation. By default GCC does preprocessing as an integrated part of input tokenization and parsing, but if you hand it this option, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) gets invoked twice — once for preprocessing only, and once for the actual compilation of the preprocessed input. This can be useful together with the -B or -wrapper options to specify an alternate preprocessor, or to do extra processing of the program source between normal preprocessing and compilation. ✨

🌸 Passing Options to the Assembler 🌸

Bestie, you can also pass options to the assembler. Iconic behavior honestly.

• *-Wa,option* — pass option as an option to the assembler. If option contains commas, it gets split into multiple options at the commas. Same comma energy as -Wp, different destination. 💅
• -Xassembler option — pass option as an option to the assembler. Perfect for supplying system-specific assembler options that GCC doesn't recognize. And yes, the same rule applies: if you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option, once for the argument. No skipping steps, queen. 👑

🎀 Options for Linking 🎀

These options come into play when the compiler links object files into an executable output file. Heads up though — they are meaningless if the compiler isn't doing a link step. So if you're not linking, don't even look at them. Not their moment. 💀

• object-file-name — a file name that doesn't end in a special recognized suffix is considered to name an object file or library. (Object files get distinguished from libraries by the linker according to the file contents — not the name, okay, the contents.) If linking is done, those object files are used as input to the linker.
• -c
• -S
• -E — if any of these three options is used, then the linker is not run, and object file names should not be used as arguments. Not. Periodt.
• *-flinker-output=type* — this one controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin, but if you're debugging the compiler, or you want incremental linking with a non-LTO object file, it may be useful to control the type manually. Options, bestie:
  • If type is exec, code generation produces a static binary. In this case -fpic and -fpie are both disabled.
  • If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This allows building shared libraries without position-independent code on architectures where this is possible, i.e. on x86.
  • If type is pie, code generation produces an -fpie executable. Similar optimizations as exec, except that -fpie is not disabled if specified at compilation time.
  • If type is rel, the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if -ffat-lto-objects is specified, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. At link time the result of incremental linking also loads faster than a static library, assuming that the majority of objects in the library are used.
  • Finally nolto-rel configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of benefits of whole program optimizations are lost.

During the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of object files in incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole program optimization, it is recommended to link such objects into a static library instead. Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects.

• -fuse-ld=bfd — use the bfd linker instead of the default linker.
• -fuse-ld=gold — use the gold linker instead of the default linker.
• -fuse-ld=lld — use the LLVM lld linker instead of the default linker.
• -fuse-ld=mold — use the Modern Linker (mold) instead of the default linker.
• -llibrary
• -l library — search the library named library when linking. (The second alternative, with the library as a separate argument, is only there for POSIX compliance and is not recommended. so anyway, use the first one.) The -l option is passed directly to the linker by GCC — refer to your linker documentation for exact details, but the general description below applies to the GNU linker.

The linker searches a standard list of directories for the library. Those directories searched include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library, unless the -static option is used. shared library supremacy, iykyk.

Listen, it makes a difference where in the command you write this option. The linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded. Order matters, bestie. Always has. 😭

• -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 — like -static-libgcc or -shared-libgcc — are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. 👀

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. Still links with the startup files, libgcc, or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++, unless options preventing their inclusion are used as well. This typically removes -lc from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed — for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there is indeed no C library available.
• -nostdlib — do not use the standard system startup files or libraries when linking. No startup files, and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries — like -static-libgcc or -shared-libgcc — are ignored.

Same deal as before: the compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. 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.
• -e entry
• *--entry=entry* — specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address.
• -pie — produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. Consistency is the vibe check. ✨
• -no-pie — don't produce a dynamically linked position independent executable.
• -static-pie — makes a static position independent executable on targets that actually support it 💖. It's like a static executable but it can be loaded at any address with no dynamic linker involved, no cap. For predictable results you also have to hand over the same set of options you used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. Consistency is the vibe 🎀
• -pthread — links against the POSIX threads library ✨. Supported on GNU/Linux targets, most other Unix derivatives, and also x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so bestie, use it consistently for both compilation and linking or things get weird 😭
• -r — produce a relocatable object as output. Also known as partial linking 💅.
• -rdynamic — passes the flag -export-dynamic to the ELF linker, on targets that support it. This tells the linker to add all symbols to the dynamic symbol table, not only the used ones 👑. This one's needed for some uses of "dlopen" or to let you get backtraces from inside a program 🦋
• -s — yank out all symbol table and relocation information from the executable 💀 gone, deleted, no trace.
• -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. That's it, that's the whole story.
• -shared — produce a shared object, which you can then link with other objects to form an executable 🌸. Not all systems support this option. And for predictable results you must also pass the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1]
• -shared-libgcc, -static-libgcc — on systems that provide libgcc as a shared library, these options force the use of either the shared or the static version, respectively 🫶. If no shared version of libgcc was built when the compiler was configured, these options have no effect. Sorry bestie, nothing to do there.

There are several situations where an application should use the shared libgcc instead of the static version. The most common one is when the application wants 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 the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do 💖 periodt.

If, instead, you use the GCC driver to create shared libraries, you may find they are not always linked with the shared libgcc 🤔. If GCC finds, at its configuration time, that you have a non-GNU linker, or a GNU linker that doesn't support the option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This lets exceptions propagate through such shared libraries, without incurring relocation costs at library load time ✨.

However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, so that it's linked with the shared libgcc 🔥 no shortcuts.

• -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically 💅
• -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically 🧵
• -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically 💧
• -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically 🙈
• -static-libstdc++ — when the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That's normally totally fine 😌. However, it's sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link 🧊. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.
• -symbolic — bind references to global symbols when building a shared object ✨. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option — it's giving rare and exclusive 👑
• -T script — use script as the linker script 📜. This option is supported by most systems using the GNU linker. On some targets, like bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols 😭
• -Xlinker option — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC doesn't 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 that passes the entire string as a single argument, which is not what the linker expects 💀 nope.

When using the GNU linker, it's usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify:

-Xlinker -Map=output.map

rather than:

-Xlinker -Map -Xlinker output.map

Other linkers may not support this syntax for command-line options 🤷♀️

• -Wl,option — pass option as an option to the linker ✨. If option contains commas, it's 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 💖

NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use -Wl,-z,norelro. 👑

• -u symbol — pretend the symbol symbol is undefined, to force linking of library modules to define it 🙈. You can use -u multiple times with different symbols to force loading of additional library modules 🔥
• -z keyword — -z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings 📖 iykyk.

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✨ Options for Directory Search ✨

These options specify directories to search for header files, for libraries, and for parts of the compiler 🦋:

• -I dir
• -iquote dir
• -isystem dir
• -idirafter dir

Add the directory dir to the list of directories to be searched for header files during preprocessing 💖. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot 🌈.

Directories specified with -iquote apply only to the quote form of the directive, #include "file" 🌸. Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives 👑

You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows, bestie, pay attention 📋:

1. For the quote form of the include directive, the directory of the current file is searched first.
2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories 🎀. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that 🫶.

The -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment that's 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 — that's the move 👑.

• -I- — okay bestie, this one is deprecated 😭 so please don't get attached. It splits the include path. Use -iquote instead for the -I directories that come before the -I-, and just… remove the -I- entirely. The way it works: any directories you gave with -I before the -I- get searched only for headers requested with "#include "file"", and hard NO to "#include <file>". But if you throw extra -I directories after the -I-, those get searched for all #include directives. Also -I- inhibits using the current file's directory as the first search directory for "#include "file"", and there is no way to override that effect of -I-. Periodt. 💅
• -iprefix prefix — set prefix as the prefix for the -iwithprefix options that come after it. If your prefix is a directory, pls include the trailing / bestie, it's giving detail-oriented. ✨
• -iwithprefix dir — append dir to the prefix you set earlier with -iprefix, then add the resulting directory to the include search path. Puts it where -idirafter would.
• -iwithprefixbefore dir — same glow-up, but this one puts the directory in the same place -I would instead. 🌸
• -isysroot dir — like the --sysroot option but for header files only. Except on Darwin targets, where it applies to both header files and libraries. Flip through the --sysroot option for the full tea. 🍵
• -imultilib dir — use dir as a subdirectory of the directory that holds the target-specific C++ headers. Very niche, very specific. 💜
• -nostdinc — do not search the standard system directories for header files. The only dirs searched are the ones you explicitly specified with -I, -iquote, -isystem, and/or -idirafter (plus the current file's directory, if appropriate). No cap, it means exactly that. 🚫
• -nostdinc++ — do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This one's used when building the C++ library, iykyk.) 🎀
• --embed-dir= dir — append dir to the list of searched directories for the "#embed" preprocessing directive or the "__has_embed" macro. Note that there are no default directories for "#embed". 😮

If dir starts with = or $SYSROOT, then that = or $SYSROOT gets replaced by the sysroot prefix — peek at --sysroot and -isysroot for the deets.

• -iplugindir= dir — set the directory to search for plugins that get passed via -fplugin=name instead of -fplugin=path/name.so`. Not for you, bestie — this option is not meant to be used by the user, only passed by the driver. 🙈
• -L dir — add directory dir to the list of directories searched for -l. Classic linker move. 🔥
• -B prefix — this one specifies where to find the executables, libraries, include files, and data files of the compiler itself. 👑

So here's the vibe: the compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.

For each subprogram to be run, the driver first tries the -B prefix, if any. If that name isn't found, or if -B wasn't 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's found, the unmodified program name is searched for using the directories in your PATH environment variable.

The compiler checks whether the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path. Very thorough, we love that.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.

The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it's not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it's not found by those means. 😬

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it gets replaced by [dir/]include. This is to help with boot-strapping the compiler. Cute little hack, love that for us. 🫶

• -no-canonical-prefixes — do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. Leave it exactly as it is, thank you very much. ✨
• --sysroot= dir — use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. 🦋

If you use both this option and the -isysroot option, then --sysroot applies to libraries, but -isysroot applies to header files.

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker doesn't support it, the header file aspect of --sysroot still works, but the library aspect does not. 💀

• --no-sysroot-suffix — for some targets, a suffix gets added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix. ✂️

---

🌈 Options for Code Generation Conventions 🌈

These machine-independent options control the interface conventions used in code generation. Very main character of them. 💅

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. Slay. ✨

• -fstack-reuse= reuse-level — this option controls stack space reuse for user-declared local/auto variables and compiler-generated temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user-defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler-generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. Heads up bestie, this is the "you're playing with fire" part. 🔥

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, and the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. 😭

• -ftrapv — this option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. Mind the order bestie, it matters. 🫶
• -fwrapv — this option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. 🔄
• -fwrapv-pointer — this option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid. 🌈
• -fstrict-overflow — this option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer. It's giving "one flag to rule them all". 💅
• -fexceptions — bestie, turn exception handling ON 🔥. This makes GCC spit out the extra code you need to actually propagate exceptions. On some targets that means GCC has to generate frame unwind info for every function, which can blow up your data size (a lot 💀) — tho it doesn't touch execution at all. If you don't say anything, GCC does the sensible thing: on by default for languages like C++ that need exception handling, off for languages like C that don't. But here's the tea 🫖 — if you're compiling C code that's gotta play nice with exception handlers written over in C++, you might need to flip it on yourself. And if you're compiling crusty old C++ programs that never touch exceptions, you might wanna turn it off.
• -fnon-call-exceptions — generate code that lets trapping instructions throw exceptions ✨. Heads up tho: this needs platform-specific runtime support and that literally does not exist everywhere, no cap. Also it only lets trapping instructions throw — we're talking memory references or floating-point instructions. It does not let arbitrary signal handlers like "SIGALRM" throw exceptions. And bestie, note that this enables -fexceptions too. 🫶
• -fdelete-dead-exceptions — okay so this one says: instructions that might throw but don't actually contribute anything to the program's execution? Yeah, they can get optimized away 🧚. It doesn't affect calls to functions — except the ones with the "pure" or "const" attributes. Enabled by default for the Ada and C++ compilers because the language specs literally permit it. And the optimization passes that cause dead exceptions to get removed? Those get enabled independently at different optimization levels. 💅
• -funwind-tables — basically -fexceptions's chill cousin ✨. It just generates whatever static data is needed, but it doesn't mess with the generated code in any other way. You normally don't need to turn this on yourself — a language processor that needs this handling will enable it on your behalf, so just let them cook. 👑
• -fasynchronous-unwind-tables — generate an unwind table in DWARF format, if the target machine supports it 🦋. The table is exact at every instruction boundary, which means it can be used for stack unwinding from asynchronous events — like a debugger or a garbage collector doing its thing. Lowkey cool.
• -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 template static data members and static local variables inside inline functions stay unique even when "RTLD_LOCAL" is in the picture ✨. Why does that matter? Because it avoids problems when a library is used by two different "RTLD_LOCAL" plugins that each depend on a definition in one of them — otherwise they'd be disagreeing about the binding of the symbol like two friends fighting over the aux cord 😭. But there's a catch: it causes "dlclose" to be ignored for affected DSOs. So if your program relies on reinitializing a DSO via "dlclose" and "dlopen", you can use -fno-gnu-unique. 💅
• -fpcc-struct-return — return "short" "struct" and "union" values in memory, just like the longer ones, instead of in registers. Less efficient, yes, but the glow-up is that it allows intercallability between GCC-compiled files and files compiled by other compilers — especially the Portable C Compiler (pcc) 🌈. The precise convention for returning structures in memory depends on the target configuration macros. And "short structures and unions" means ones 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 — return "struct" and "union" values in registers when possible 🎀. For small structures this is way more efficient than -fpcc-struct-return, periodt. If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there's no standard convention, GCC defaults to -fpcc-struct-return — except on targets where GCC is the principal compiler. In those cases we can choose the standard, and we chose the more efficient register return alternative, because obviously 💅.

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 — allocate to an "enum" type only as many bytes as it actually needs for the declared range of possible values 🍓. Specifically, the "enum" type becomes equivalent to the smallest integer type that has enough room. This option has no effect for an enumeration type with a fixed underlying type, fyi.

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 — override the underlying type for "wchar_t" to be "short unsigned int" instead of the default for the target ✨. This one's useful for building programs that 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 where global variables defined without an initializer go — these are 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, bestie, don't get it twisted.

The default is -fno-common, which says 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 😭 (which is honestly kind of a serve for catching bugs).

The -fcommon (not a typo in the original, it's literally written that way) places uninitialized global variables in a common block 🌸. This lets the linker resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets it implies a speed and code size penalty on global variable references. It's mainly useful to enable legacy code to link without errors. 🙈

• -fno-ident — ignore the "#ident" directive. Short and sweet, we love a low-effort icon 💅.
• -finhibit-size-directive — don't output a ".size" assembler directive, or anything else that would cause trouble if the function gets split in the middle and the two halves end up at locations far apart in memory 😳. This option is used when compiling crtstuff.c — you should not need to use it for anything else, fr fr.
• -fverbose-asm — put extra commentary information in the generated assembly code to make it more readable 📝. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself, godspeed 😭).

-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two assembler files.

The added comments include:

• information on the compiler version and command-line options,
• the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
• hints on which high-level expressions correspond to the various assembly instruction operands.

For example, given this C source file:

        int test (int n)
        {
          int i;
          int total = 0;

          for (i = 0; i < n; i++)
            total += i * i;

          return total;
        }

compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -

gcc -S test.c -fverbose-asm -Os -o -

gives output similar to this:

                .file   "test.c"
        # GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
          [...snip...]
        # options passed:
          [...snip...]

                .text
                .globl  test
                .type   test, @function
        test:
        .LFB0:
                .cfi_startproc
        # test.c:4:   int total = 0;
                xorl    %eax, %eax      # <retval>
        # test.c:6:   for (i = 0; i < n; i++)
                xorl    %edx, %edx      # i
        .L2:
        # test.c:6:   for (i = 0; i < n; i++)
                cmpl    %edi, %edx      # n, i
                jge     .L5     #,
        # test.c:7:     total += i * i;
                movl    %edx, %ecx      # i, tmp92
                imull   %edx, %ecx      # i, tmp92
        # test.c:6:   for (i = 0; i < n; i++)
                incl    %edx    # i
        # test.c:7:     total += i * i;
                addl    %ecx, %eax      # tmp92, <retval>
                jmp     .L2     #
        .L5:
        # test.c:10: }
                ret
                .cfi_endproc
        .LFE0:
                .size   test, .-test
                .ident  "GCC: (GNU) 7.0.0 20160809 (experimental)"
                .section        .note.GNU-stack,"",@progbits

The comments are intended for humans rather than machines and hence the precise format of the comments is subject to change 👀 — so don't go parsing them in your build scripts, bestie, that's not the vibe.

• -frecord-gcc-switches — this switch causes the command line used to invoke the compiler to be recorded into the object file that's being created 📦✨. It's only implemented on some targets, and the exact format of the recording is target- and binary-file-format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch — but that one only records info 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 — bestie, this one generates position-independent code (PIC), which is what you want if you're building a shared library ✨ — but only if the target machine supports it, okurrr? The way this works: such code accesses all constant addresses through a global offset table (GOT) 🫶. The dynamic loader resolves the GOT entries when the program starts — and no, the dynamic loader is not part of GCC, it's part of the operating system, don't @ me 💅. 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. Lowkey iconic. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit, main character energy fr fr 👑.)

Position-independent code requires special support, 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, no notes 📝.

When this flag is set, the macros __pic__ and __PIC__ are defined to 1. Periodt.

• -fPIC — if supported for the target machine, this emits position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table 🌈. This option makes a difference on AArch64, m68k, PowerPC and SPARC. Slay.

Position-independent code requires special support, and therefore works only on certain machines. Yeah, same warning, it's giving déjà vu sorry bestie 🦋.

When this flag is set, the macros __pic__ and __PIC__ are defined to 2. Note the glow-up from 1 to 2 💖.

• -fpie
• -fPIE — these options are similar to -fpic and -fPIC, but the generated position-independent code can be only linked into executables 🎀. Usually these options are used to compile code that will be linked using the -pie GCC option. iykyk.

-fpie and -fPIE both define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE. So symmetric, so gorgeous ✨.

• -fno-plt — okay so this one says: 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 — it's giving generous 💅. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time. No cap.

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. Very niche, very demure 💜.

• -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 — so like, check first bestie 🦋.
• -fno-bit-tests — do not use bit tests for switch statements even where it would be more efficient than other code generation strategies 🌺. Short, sweet, to the point, we love her.
• -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. Very specific, very technical, we respect it 👑.

This flag does not have a negative form, because it specifies a three-way choice. Like, there's no undo button here, make peace with it 😭.

• -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. Not me warning you dramatically but also like, actually listen 💀.

This flag does not have a negative form, because it specifies a three-way choice. Same energy as before, periodt.

• -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. Yeah, same warning, it's a family trait bestie.

A different sort of disaster results from the use of this flag for a register in which function values may be returned. Two flavors of disaster, how blessed 🙈.

This flag does not have a negative form, because it specifies a three-way choice. We've been over this, it's iconic at this point ✨.

• -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. — yes that's the original's own parenthesis, we keep it exactly, no notes 💖.)

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. Only if you actually need it, bestie, fr fr.

• -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. Retro but make it work 💜.

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 — so like, vibe check your target first 😭.

• *-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. Write that down ✏️.

• -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. Respectfully, 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 👀. Yeah, executable stack, that's a whole situation.

-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. Like, extreme extreme. Be so careful bestie 🫶.

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. Okay so for everyone not doing Ada: it literally does nothing, we move 💅.

• -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 💀. That's the whole sentence, we're not paraphrasing it, that's the lore.

• -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. Like, strongly. Do it.

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. Very "everyone's invited to the party" energy 🎉.

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. Cross-platform besties, this one's for you 💖.

---

so for anyone out there adding visibility support to code that already exists, bestie, #pragma GCC visibility is literally your new fave thing 💅 it works by you wrapping the declarations you wanna set visibility for with, like, #pragma GCC visibility push(hidden) and #pragma GCC visibility pop 🎀

k but real talk: symbol visibility is part of the API interface contract, okay? so all new code should ALWAYS specify visibility when it's not the default. which means declarations that are only for use within the local DSO should always be explicitly marked as hidden, so you avoid PLT indirection overheads — and ngl making it this obvious also helps readability and self-documentation of the code ✨ lowkey a glow-up for your source tree.

heads up though: thanks to ISO C++ specification requirements, operator new and operator delete must ALWAYS be of default visibility. no exceptions, no cap 🙅‍♀️

Also be aware that headers from outside your project — especially system headers and headers from any other library you use — may not be expecting to be compiled with anything other than default visibility 😭 you might need to explicitly say #pragma GCC visibility push(default) before you include any of those.

And here's the tea: extern declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with zero modifications 🫶 But that also means calls to extern functions with no explicit visibility use 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 ✨

Ok one more: -fvisibility does affect C++ vague linkage entities 👀 so for instance, an exception class that gets thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs. fr fr, mark it.

An overview of all these techniques, their benefits and how to use them lives at https://gcc.gnu.org/wiki/Visibility 🌈

---

• -fstrict-volatile-bitfields — use this one if accesses to volatile bit-fields (or other structure fields, though 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 💪 like, targets with memory-mapped peripheral registers might need all such accesses to be 16 bits wide; with this flag you can declare every peripheral bit-field as unsigned short (assuming short is 16 bits on those targets) to force GCC to use 16-bit accesses instead of, maybe, a more efficient 32-bit access.

If this option is disabled, the compiler goes with the most efficient instruction 👑 in the previous 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 unrelated to the one being updated 💀

In some cases, 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 is correctly aligned for the target machine. In that case GCC falls back to generating multiple accesses instead of code that would fault or truncate the result at run time 🙈

Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. so it is therefore recommended to define all bits of the field's type as bit-field members 👑

The default value of this option is determined by the application binary interface for the target processor.

• -fsync-libcalls — this one controls whether any out-of-line instance of the __sync family of functions may be used to implement the C++11 __atomic family of functions ✨

The default value is enabled, so the only useful form of the option is -fno-sync-libcalls 💅 This option is used in the implementation of the libatomic runtime library.

• -fzero-init-padding-bits=value — guarantees zero initialization of padding bits in automatic variable initializers 🎀 certain languages guarantee zero initialization of padding bits in certain cases, e.g. C23 when using empty initializers ({}), or C++ when using zero-initialization, or C guarantees that fields not specified in an initializer have their padding bits zero initialized. This option lets you change when padding bits in initializers are guaranteed to be zero initialized. The default is -fzero-init-padding-bits=standard, which makes no further guarantees than the corresponding standard. E.g.

struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.

-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 of the initialized member to be zero initialized) 🌸 This was the GCC behavior before GCC 15 and in the above example guarantees zero initialization of 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) 💖

NOTE: In Ubuntu 25.10 and later versions, -fzero-init-padding-bits=all is enabled by default for C, C++, ObjC, ObjC++, unless -fzero-init-padding-bits=standard is found.

---

🧑‍💻 GCC Developer Options 🧑‍💻

This section is all about the command-line options that are primarily of interest to GCC developers themselves — including options to support compiler testing and investigation of compiler bugs and compile-time performance problems 🔍 it includes options that produce debug dumps at various points in the compilation; that print statistics like memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. Ngl you should rarely need any of these for ordinary compilation and linking tasks 💅

Many developer options that cause GCC to dump output to a file take an optional =filename suffix ✨ You can specify stdout or - to dump to standard output, and stderr for standard error 👑

If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name 🎀 The base dump file name is the name of the output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager — this is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they are executed earlier 😭 The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file 💖

• -fcallgraph-info, -fcallgraph-info=MARKERS — makes the compiler output callgraph information for the program, on a per-object-file basis 🌈 the information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified ✨ When the su marker is specified, the callgraph is decorated with stack usage information; it is equivalent to -fstack-usage. When the da marker is specified, the callgraph is decorated with information about dynamically allocated objects 👀

When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files 🫶

• -dletters, -fdump-rtl-pass, -fdump-rtl-pass=filename — says to make debugging dumps during compilation at times specified by letters 💅 this is used for debugging the RTL-based passes of the compiler.

Some -dletters switches have different meaning when -E is used for preprocessing 👀

Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings ✨

• -fdump-rtl-alignments — dump after branch alignments have been computed.
• -fdump-rtl-asmcons — dump after fixing rtl statements that have unsatisfied in/out constraints.
• -fdump-rtl-auto_inc_dec — dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions 👑
• -fdump-rtl-barriers — dump after cleaning up the barrier instructions.
• -fdump-rtl-bbpart — dump after partitioning hot and cold basic blocks.
• -fdump-rtl-bbro — dump after block reordering.
• -fdump-rtl-btl1, -fdump-rtl-btl2 — -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes 🌸
• -fdump-rtl-bypass — dump after jump bypassing and control flow optimizations.
• -fdump-rtl-combine — dump after the RTL instruction combination pass.
• -fdump-rtl-compgotos — dump after duplicating the computed gotos.
• -fdump-rtl-ce1, -fdump-rtl-ce2, -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes ✨
• -fdump-rtl-cprop_hardreg — dump after hard register copy propagation.
• -fdump-rtl-csa — dump after combining stack adjustments.
• -fdump-rtl-cse1, -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes 💜
• -fdump-rtl-dce — dump after the standalone dead code elimination passes.
• -fdump-rtl-dbr — dump after delayed branch scheduling.

• -fdump-rtl-dce1, -fdump-rtl-dce2 — bestie these two are a matching set 💅 and they turn on dumping after the two dead store elimination passes. two passes, two flags, no cap.
• -fdump-rtl-eh — dumps after the finalization of the EH handling code. exception vibes, finalized ✨
• -fdump-rtl-eh_ranges — dumps after the conversion of EH handling range regions. it's giving range-region-conversion energy 🌸
• -fdump-rtl-expand — dumps after RTL generation. literally where the RTL is born 🐣
• -fdump-rtl-fwprop1, -fdump-rtl-fwprop2 — these two enable dumping after the two forward propagation passes. forward prop girlies stay winning 💖
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — these two enable dumping after global common subexpression elimination. gcse said "I'm the main character" 👑
• -fdump-rtl-init-regs — dumps after the initialization of the registers. registers getting their little baby setup 🍼
• -fdump-rtl-initvals — dumps after the computation of the initial value sets. initial values, computed, iconic ⭐
• -fdump-rtl-into_cfglayout — dumps after converting to cfglayout mode. going INTO cfglayout mode, note that direction bestie 🦋
• -fdump-rtl-ira — dumps after iterated register allocation. IRA said slay 🔥
• -fdump-rtl-jump — dumps after the second jump optimization. the second one, she's a sequel 💀
• -fdump-rtl-loop2 — this one enables dumping after the rtl loop optimization passes. loops, optimized, dumped 🌈
• -fdump-rtl-mach — dumps after performing the machine dependent reorganization pass, if that pass exists. conditional queen, she might not even show up 🙈
• -fdump-rtl-mode_sw — dumps after removing redundant mode switches. bye bye redundant switches, don't let the door hit you 🎀
• -fdump-rtl-rnreg — dumps after register renumbering. new numbers, same registers ✨
• -fdump-rtl-outof_cfglayout — dumps after converting from cfglayout mode. and this is the way OUT, see how that mirrors -fdump-rtl-into_cfglayout? love a symmetrical moment 💜
• -fdump-rtl-peephole2 — dumps after the peephole pass. peephole my beloved 🫶
• -fdump-rtl-postreload — dumps after post-reload optimizations. post-reload, post-glow-up 💅
• -fdump-rtl-pro_and_epilogue — dumps after generating the function prologues and epilogues. every function deserves a good intro AND a good outro 🎬
• -fdump-rtl-sched1, -fdump-rtl-sched2 — these two enable dumping after the basic block scheduling passes. scheduling szn 🔥
• -fdump-rtl-ree — dumps after sign/zero extension elimination. extensions eliminated, no notes 👑
• -fdump-rtl-seqabstr — dumps after common sequence discovery. found the common sequences, they were right there the whole time 😭
• -fdump-rtl-shorten — dumps after shortening branches. branches but shorter, a trim if you will ✂️
• -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — these options enable dumping after five rounds of instruction splitting. FIVE. rounds. five 😤
• -fdump-rtl-sms — dumps after modulo scheduling. this pass is only run on some architectures, so don't be shocked if your arch is like "not me" 🦋
• -fdump-rtl-stack — dumps after conversion from GCC's "flat register file" registers to the x87's stack-like registers. this pass is only run on x86 variants — sorry to everyone else, this one's for the x86 girlies 💀
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — these two enable dumping after the two subreg expansion passes. another matched pair, we love consistency 💖
• -fdump-rtl-vartrack — dumps after variable tracking. tracking variables like a true detective 🕵️
• -fdump-rtl-vregs — dumps after converting virtual registers to hard registers. virtual to hard, the glow-up of the century ✨
• -fdump-rtl-web — dumps after live range splitting. live ranges, split, slay 🌸
• -fdump-rtl-regclass, -fdump-rtl-subregs_of_mode_init, -fdump-rtl-subregs_of_mode_finish, -fdump-rtl-dfinit, -fdump-rtl-dfinish — okay bestie these dumps are defined but always produce empty files. yes really. they exist and they give you nothing. the flop era of dump flags 💀
• -da, -fdump-rtl-all — produce all the dumps listed above. ALL of them. every single one 🏆
• -dA — annotate the assembler output with miscellaneous debugging information. little notes in your asm, cute 🎀
• -dD — dump all macro definitions, at the end of preprocessing, in addition to normal output. every macro, spilled 💅
• -dH — produce a core dump whenever an error occurs. dramatic? absolutely. on purpose? also yes 🚨
• -dp — annotate the assembler output with a comment indicating which pattern and alternative is used. the length and cost of each instruction are also printed. receipts: provided 🧾
• -dP — dump the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation. so it's -dp plus extra, a bundle 🫶
• -dx — just generate RTL for a function instead of compiling it. usually used with -fdump-rtl-expand. "we're not compiling today, we're just looking" 🔍
• -fdump-debug — dump debugging information generated during the debug generation phase. debug info about the debug info. very meta, very demure 💜
• -fdump-earlydebug — dump debugging information generated during the early debug generation phase. the early debug phase, she wakes up before everyone else 🌅
• -fdump-noaddr — when doing debugging dumps, suppress address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. no addresses, no noise, diff-able dumps, iconic ✨
• -freport-bug — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. ICE said "not today", and this flag catches it 🧊
• -fdump-unnumbered — when doing debugging dumps, suppress instruction numbers and address output. this makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g. she's the "options don't matter for the diff" flag 🔥
• -fdump-unnumbered-links — when doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence. the links stay, the numbers dip 🌈

💾 the -fdump-ipa era 💾

• -fdump-ipa-switch, -fdump-ipa-switch-options — control the dumping at various stages of inter-procedural analysis language tree to a file. the file name is 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.

the following dumps are possible:

• all — enables all inter-procedural analysis dumps. the "give me everything" option 👑
• cgraph — dumps information about call-graph optimization, unused function removal, and inlining decisions.
• inline — dump after function inlining. inlining happening and we're watching 🫶
• strubm — dump after selecting "strub" modes, and recording the selections as function attributes.
• strub — dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking.

additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized.

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. both, side by side, the tea is spilled ☕

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.

• -fdump-ipa-clones — create a dump file containing information about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use is that tools that create live-patches can determine the set of functions that need to be live-patched to completely replace a particular function (see -flive-patching). the file name is generated by appending suffix ipa-clones to the source file name, and the file is created in the same directory as the output file. each entry in the file is on a separate line containing semicolon separated fields.

in the case of call graph clone creation, the individual fields are:

1. String "Callgraph clone".
2. Name of the function being cloned as it is presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.
7. Name of the new function clone as it is presented to the assembler.
8. A number that uniquely represents the new function clone in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
9. The file name of the source file where the source code location of the new clone points to.
10. The line to which the source code location of the new clone points to.
11. The column to which the source code location of the new clone points to.
12. A string that determines the reason for cloning.

in the case of call graph clone removal, the individual fields are:

1. String "Callgraph removal".
2. Name of the function being removed as it would be presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. Note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.

• -fdump-lang — dump language-specific information. the file name is made by appending .lang to the source file name. .lang, memorise it bestie 🎀
• -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — control the dumping of language-specific information. the options and filename portions behave as described in the -fdump-tree option. the following switch values are accepted:
  • all — enable all language-specific dumps. everything, all at once, chaotic but make it useful 🌈
  • class — dump class hierarchy information. virtual table information is emitted unless 'slim' is specified. this option is applicable to C++ only.
  • module — dump module information. options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional information. this option is applicable to C++ only. obsessed with how many sub-options this one has 😤
  • raw — dump the raw internal tree data. this option is applicable to C++ only. raw. unfiltered. no edits. 💀
• -fdump-passes — print on stderr the list of optimization passes that are turned on and off by the current command-line options. she's the little x-ray into what your flags actually did ✨
• -fdump-statistics-option — ok so this one turns on the dumping of pass statistics, into its own lil separate file. ✨ How's the file named, you ask? GCC takes the source file name and appends a suffix that ends in .statistics, and drops it in the same directory as the output file. If you use the -option form, then -stats means the counters get summed over the whole compilation unit, while -details dumps every single event as the passes generate it. And if you pass no option at all? Default vibe is: sum the counters for each function compiled. 🫶
• -fdump-tree-all
• -fdump-tree-switch
• -fdump-tree-switch-options
• -fdump-tree-switch-options=filename — these bad boys control the dumping at all the various stages of processing the intermediate language tree into a file. If you use the -options form, then options is a list of --separated options that control the details of the dump. Real talk: not every option is applicable to every dump — ones that aren't meaningful just get ignored, no drama. Here's the whole menu, bestie:
  • address — prints the address of each node. Usually this isn't meaningful because it changes depending on the environment and the source file. Its main character use is tying a dump file up with a debug environment. 💅
  • asmname — if DECL_ASSEMBLER_NAME has been set for a given decl, use that in the dump instead of DECL_NAME. Its main use is making it easy to work backward from mangled names in the assembly file. 🔍
  • slim — when you're dumping front-end intermediate representations, this stops members of a scope or the body of a function from being dumped merely 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, it prints 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 — enables more detailed dumps (not honored by every dump option). Also includes information from the optimization passes.
• stats — enables dumping various statistics about the pass (again, not honored by every dump option).
• blocks — enables showing basic block boundaries (disabled in raw dumps).
• graph — for each of the other indicated dump files (-fdump-rtl-pass), 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 GraphViz can render them all in a single plot. 🎨

This option currently only works for RTL dumps, and the RTL is always dumped in slim form.

• vops — enables showing virtual operands for every statement.
• lineno — enables showing line numbers for statements.
• uid — enables showing the unique ID (DECL_UID) for each variable.
• verbose — enables showing the tree dump for each statement.
• eh — enables showing the EH region number holding each statement.
• scev — enables showing scalar evolution analysis details.
• optimized — enables showing optimization information (only available in certain passes).
• missed — enables showing missed optimization information (only available in certain passes).
• note — enables other detailed optimization information (only available in certain passes).
• all — turns on all the options, except raw, slim, verbose and lineno. 😌
• optall — turns on all the optimization options, i.e. optimized, missed, and note.

So how do you even figure out which tree dumps are available, or find the dump for a pass you actually care about? Follow these steps, queen 👑

1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you're 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 it as described below.
3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter t for tree passes (and the letter r for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. ⚠️ Note that the numeric codes are not stable and may change from one version of GCC to another.

---

• -fopt-info
• -fopt-info-options
• -fopt-info-options=filename — this one controls the optimization dumps from the various optimization passes. If you use the -options form, options is a list of --separated option keywords that select the dump details and the optimizations.

The options break down 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.

You can freely mix options from each group since they're non-overlapping. But heads up: in case of any conflicts, the later options override the earlier options on the command line.

These are the options that control which kinds of messages get emitted:

• optimized — print information when an optimization is successfully applied. It's up to the pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops that 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, like certain transformations and more detailed messages about decisions etc.
• all — print detailed optimization information. This includes optimized, missed, and note.

This option controls the dump verbosity:

• internals — by default, only "high-level" messages get emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers. 🧚

And one or more of these option keywords can 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: dump messages about successful optimizations from all the passes, while omitting messages that are treated as "internals".

If filename is provided, then the dumps from all the applicable optimizations get concatenated into filename. Otherwise the dump is output onto stderr. Even 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 get ignored. 🙈

Note that the output filename is overwritten in case of multiple translation units. If you want a combined output from multiple translation units, use stderr instead!

In this next example, the optimization info goes to stderr:

gcc -O3 -fopt-info

And this one:

gcc -O3 -fopt-info-missed=missed.all

outputs the 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 the vectorization passes on stderr. Heads up tho — -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. 💫

Another example, because we love options:

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.

And 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. So only vec.miss is produced, and it contains dumps from the vectorizer about missed opportunities. 💀

---

• -fsave-optimization-record — writes 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's roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, plus the following extra data for each message:

• the execution count of the code being optimized, along with metadata about whether this came from actual profile data or was just an estimate, which lets consumers 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 gets inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,

• things that ID which parts of the message are about expressions, statements, or symbol-table nodes — like, which category they're in, and their source code location when we've got it ✨
• the GCC pass that spat out the message — yes bestie, we're naming names 💅
• the spot in GCC's own code where the message got emitted — the origin story, fr 🎀

Also lowkey some messages are logically nested inside other messages, which is just reflecting the implementation details of the optimization passes. Not a bug, it's a vibe 🌸

---

✨ More Options ✨

• -fsched-verbose=n — On targets that actually use instruction scheduling, this one controls how much debugging output the scheduler prints into the dump files. 📊

So for n greater than zero, -fsched-verbose outputs the same info as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also drops basic block probabilities, detailed ready list info, and unit/insn info. For n greater than two, it throws in RTL at abort point, plus control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. The whole pipeline's tea ☕

• -fenable-kind-pass
• -fdisable-kind-pass=range-list — okay so this is a whole set of options used to explicitly disable/enable optimization passes. ⚠️ These are meant for debugging GCC, periodt. Compiler users should use regular options for enabling/disabling passes instead — this is devs-only territory, iykyk 🙈
  • -fdisable-ipa-pass — Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
  • -fdisable-rtl-pass
  • -fdisable-rtl-pass=range-list — Disable RTL pass pass. pass is the pass name, and same deal: if it's statically invoked multiple times, append a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon, and the range is inclusive on both ends 🫶. If the range is trivial, the number pair can be simplified to a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid shows up in the function header of a dump file, and you can dump the pass names with -fdump-passes.
  • -fdisable-tree-pass
  • -fdisable-tree-pass=range-list — Disable tree pass pass. See -fdisable-rtl for the description of the option arguments. ✨
  • -fenable-ipa-pass — Enable IPA pass pass. pass is the pass name, and if the same pass is statically invoked multiple times, append a sequential number starting from 1.
  • -fenable-rtl-pass
  • -fenable-rtl-pass=range-list — Enable RTL pass pass. See -fdisable-rtl for option argument description and examples. 💖
  • -fenable-tree-pass
  • -fenable-tree-pass=range-list — Enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options, bestie 👇

# 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 — Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables even further internal consistency checking that might affect code generation. 🔍
• -frandom-seed=string — This option provides a seed that GCC uses 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. You can use -frandom-seed to produce reproducibly identical object files. 🎀

The string can either be a number (decimal, octal or hex) or an arbitrary string — in which case it gets converted to a number by computing CRC32.

The string should be different for every file you compile. No cap. 🚨

• -save-temps — Store the usual "temporary" intermediate files permanently; name them as auxiliary output files, as specified described under -dumpbase and -dumpdir. 💾

When used in combination with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that has the same extension as an intermediate file. The corresponding intermediate file can be obtained by renaming the source file before using -save-temps.

• -save-temps=cwd — Equivalent to -save-temps -dumpdir ./. 🦋
• -save-temps=obj — Equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file specified after the -o option, including any directory separators. If the -o option isn't used, the -save-temps=obj switch behaves like -save-temps=cwd.
• -time[=file] — Report the CPU time taken by each subprocess in the compilation sequence. ⏱️ For C source files, that's the compiler proper and assembler (plus the linker if linking is done).

Without specifying an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the "user time" — that's time spent executing the program itself. The second number is "system time" — time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

With an output file specified, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 <options>
0.00 0.01 as <options>

The "user time" and the "system time" get moved before the program name, and the options passed to the program get displayed, so you can later tell what file was being compiled and with which options. Chef's kiss 👑

• -fdump-final-insns[=file] — Dump 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. Dump the final internal representation in both compilations, and print an error if they differ. 😭

If the equal sign is omitted, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it's used for opts, otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.

To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. 🎯

• -fcompare-debug-second — This 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. 🌟

When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper. 💀

• -gtoggle — Turn off generation of debug info, if leaving out this option generates it, or turn 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. 🔁
• -fvar-tracking-assignments-toggle — Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. 🪄
• -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. 📝

• -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. 🙈

• -ftime-report-details — Record the time consumed by infrastructure parts separately for each pass. 🔬
• -fira-verbose=n — Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10. 💅
• -flto-report — prints a report packed with internal deets about how the link-time optimizer is doing. 💅 Heads up bestie: the contents of this report vary from version to version, so don't get attached. It's meant to be useful to GCC developers when they're processing object files in LTO mode (via -flto).

Disabled by default.

• -flto-report-wpa — like -flto-report, but it only prints for the WPA phase of link-time optimization. one phase only, queen ✨
• -fmem-report — makes the compiler spill 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. periodt.
• -fpre-ipa-mem-report, -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. two icons, one bullet.
• -fmultiflags — this one enables multilib-aware TFLAGS so you can build target libraries with options that are different from the ones the compiler is configured to use by default, through specs set up by compiler internals, by the target, or by builders at configure time. 🛠️

Just like TFLAGS, it lets the target libraries be built for portable baseline environments while the compiler defaults to more demanding ones. That's useful because users can easily override the defaults the compiler is configured to use to build their own programs, if 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. To pull that off, build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.

Important: this option is discarded by the driver once it's done processing the driver self spec.

It's also useful for checking that TFLAGS are being used to build all target libraries — configure a non-bootstrap compiler with --with-specs='%{!fmultiflags:%emissing TFLAGS}' and build the compiler and target libraries.

• -fprofile-report — makes the compiler print some statistics about the consistency of the (estimated) profile and the effect of individual passes. 📊
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. And what's auxname? It's generated from the name of the output file, if it was explicitly specified and it isn't an executable, otherwise it's the basename of the source file. 🎀

An entry is made up of three fields:

1. The name of the function.
2. A number of bytes.
3. One or more qualifiers: static, dynamic, bounded.

The static qualifier means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The dynamic qualifier means 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 bounded qualifier 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 isn't present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.

• -fstats — emits statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and ngl the information is generally only useful to the G++ development team.
• -fdbg-cnt-list — prints the name and the counter upper bound for all debug counters. 🦋
• -fdbg-cnt=counter-value-list — sets 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 the second, third, fourth, tenth and eleventh invocation. For dbg_cnt(tail_call) true is returned for the first 10 invocations. 💀
• -print-file-name=library — prints the full absolute name of the library file library that would be used when linking — and doesn't do anything else. With this option, GCC does not compile or link anything; it just prints the file name. 😌
• -print-multi-directory — prints 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 — prints 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 — prints the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just .. If OS libraries are present in libsuffix sibling directories, this prints e.g. ../lib64, ../lib or ../lib32. And if OS libraries are present in lib/subdir subdirectories, it prints e.g. amd64, sparcv9 or ev6.
• -print-multiarch — prints the path to OS libraries for the selected multiarch, relative to some lib subdirectory. ✨
• -print-prog-name=program — like -print-file-name, but it searches for a program such as cpp. 🔍
• -print-libgcc-file-name — same as -print-file-name=libgcc.a.

Super handy 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 — prints the name of the configured installation directory and a list of program and library directories gcc searches — and doesn't do anything else. 👑

This is the one you need when gcc comes at you with 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 — prints 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 — prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler isn't configured with such a suffix — and doesn't do anything else.
• -dumpmachine — prints the compiler's target machine (for example, i686-pc-linux-gnu) — and doesn't do anything else. 🖥️
• -dumpversion — prints the compiler version (for example, 3.0, 6.3.0 or 7) — and doesn'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 — prints the full compiler version — and doesn't do anything else. The output is always three numbers separated by dots: major, minor and patchlevel version. 💅
• -dumpspecs — prints the compiler's built-in specs — and doesn't do anything else. (This is used when GCC itself is being built.)

---

🛠️ Machine-Dependent Options 🛠️

Each target machine supported by GCC can have its own options — for example, to let you compile for a particular processor variant or ABI, or to control optimizations that are specific to that machine. By convention, the names of machine-specific options start with -m.

Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform.

💜 AArch64 Options 💜

These options are defined for AArch64 implementations:

• -mabi=name — generate code for the specified data model. Permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a SysV-like data model where int is 32 bits, but long int and pointers are 64 bits.

The default depends on the specific target configuration. Big deal alert: the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.

The ilp32 model is deprecated.

• -mbig-endian — generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers, but will not impose any restrictions on the assembler.
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
• -mcmodel=tiny — generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
• -mcmodel=small — generate code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model.
• -mcmodel=large — generate code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. Careful, bestie: the -mcmodel=large option is incompatible with -mabi=ilp32, -fpic and -fPIC.
• -mtp=name — picks which system register gets used as the thread pointer ✨. Valid values are tpidr_el0, tpidrro_el0, tpidr_el1, tpidr_el2, tpidr_el3. And for the OGs, the aliases el0, el1, el2, el3 are still accepted. Default is tpidr_el0. The recommendation, bestie, is to compile all your interoperating code with the same value here — otherwise you'll be grabbing a different thread pointer from the wrong exception level and that is not the vibe 💀.
• -mstrict-align, -mno-strict-align — toggle whether GCC avoids generating memory accesses that might not be aligned on a natural object boundary as described in the architecture specification 🙈.
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The omit behavior is the default, slay.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary sitting at guard 🐤. Supported spots are global for a global canary, or sysreg for a canary living in an appropriate system register.

When you go with the second one, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally tell GCC which system register to use as the base register for reading the canary, and how far the offset is 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. This is the default ✨.
• -mtls-dialect=traditional — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
• -mtls-size=size — specify 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 🩹. This one involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. The flag gets ignored if an architecture or cpu is specified on the command line that doesn't need the workaround. Not me forcing a NOP into your pipeline 😭.
• -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround happens at link time, so this only passes the corresponding flag through to the linker 🫶. Same deal: the flag is ignored if an architecture or cpu specified on the command line doesn't need the workaround.
• -mlow-precision-recip-sqrt, -mno-low-precision-recip-sqrt — enable or disable the reciprocal square root approximation 🌸. This option only does anything if -ffast-math or -funsafe-math-optimizations is used too. Turning it on drops the precision of reciprocal square root results to about 16 bits for single precision and about 32 bits for double precision.
• -mlow-precision-sqrt, -mno-low-precision-sqrt — enable or disable the square root approximation 🎀. Again, only vibes if -ffast-math or -funsafe-math-optimizations is also on. Enabling it reduces square root result precision to about 16 bits for single precision and 32 bits for double precision. If enabled, it implies -mlow-precision-recip-sqrt.
• -mlow-precision-div, -mno-low-precision-div — enable or disable the division approximation ➗. Only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it reduces division result precision to about 16 bits for single precision and 32 bits for double precision.
• -mtrack-speculation, -mno-track-speculation — enable or disable generation of extra code to track speculative execution through conditional branches 🔥. That tracking state can then be used by the compiler when expanding calls to __builtin_speculation_safe_copy, which lets it generate a more efficient code sequence.
• -moutline-atomics, -mno-outline-atomics — enable or disable calls to out-of-line helpers that implement atomic operations 🌈. These helpers will, at runtime, figure out whether the LSE instructions from ARMv8.1-A can be used; if not, they'll fall back to the load/store-exclusive instructions present in the base ARMv8.0 ISA.

This option is only applicable when compiling for the base ARMv8.0 instruction set 🦋. If you're on a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions get used directly. Same story when using -mcpu= where the selected cpu supports the lse feature. This option is on by default.

• -march=name — specify the name of the target architecture and, optionally, one or more feature modifiers ✨. The option has the form -march=arch{+[no]feature}*.

Here's the rundown of permissible arch values and the features each one enables by default:

• armv8-a : Armv8-A : includes by default +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 makes the compiler pick the architecture of the host system 🖥️. This option has no effect if the compiler can't recognize the host's architecture. 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 and select the architecture features from it. In other words, -march=native is treated as -mcpu=native 👑.

The permissible feature values are listed in the sub-section on aarch64-feature-modifiers, -march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature wins 💅.

GCC uses name to decide what kind of instructions it can emit when generating assembly code. If -march is specified without either -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture.

• -mtune=name — specify the name of the target processor that GCC should tune the performance of the code for ⚡. Permissible values are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2, grace, neoverse-v3, neoverse-v3ae, armagicpu, neoverse-n3, olympus, cortex-a725, cortex-x925, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, 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, cobalt-100, apple-m1, apple-m2, apple-m3 and native.

The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 specify that GCC should tune for a big.LITTLE system 🍓.

The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle 🌊. Basically, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1, but more specific than the default tuning described below.

Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. This option has no effect if the compiler can't recognize the processor of the host system 🙈.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

This option cannot be suffixed by feature modifiers. Periodt 💅.

• -mcpu=name — specify the name of the target processor, optionally suffixed by one or more feature modifiers ✨. The option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers, -march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor to tune for performance (as if by -mtune) 🔥. When this option is used together with -march or -mtune, those options take precedence over the appropriate part of this one.

• -mcpu=neoverse-512tvb — okay so this one is special bestie 💫 it doesn't point at one specific core, it points at all the Neoverse cores that (a) do SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless -march overrides it, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, because Neoverse V1 is the first Neoverse core with those properties. And unless -mtune overrides it, -mcpu=neoverse-512tvb tunes code exactly the same way as -mtune=neoverse-512tvb.
• -moverride=string — overrides the tuning decisions the back-end makes in response to a -mtune= switch. And listen 👀 the syntax, semantics and accepted values for string here are not guaranteed to be consistent across releases. It's only meant to be useful when you're developing GCC itself, so like… compiler-dev main character energy only 🫶
• -mverbose-cost-dump — turns on verbose cost model dumping in the debug dump files. This one's provided for debugging the compiler, so it's giving "for the compiler girlies, not the app girlies" 💅
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — enable or disable PC-relative literal loads. With this on, literal pools get accessed with a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. It's enabled by default for -mcmodel=tiny ✨
• -msign-return-address=scope — picks which function scope gets return address signing applied to it. Permissible values: none, which disables return address signing; non-leaf, which enables pointer signing for functions that are not leaf functions; and all, which enables pointer signing for every function. Default value is none. Heads up though — this option has been deprecated by -mbranch-protection 😭
• -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti|gcs — picks which branch protection features to use. none is the default and turns off all types of branch protection. standard turns on all the branch protection features — and if a feature has extra tuning options, then standard sets it to its standard level. pac-ret[+leaf] turns on return address signing at its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optional argument leaf extends the signing to include leaf functions too. The optional argument b-key signs the functions with the B-key instead of the A-key. bti turns on the branch target identification mechanism. And gcs turns on guarded control stack compatible code generation 🛡️
• -mharden-sls=opts — enables compiler hardening against straight line speculation (SLS). opts is a comma-separated list of these:
  • retbr
  • blr

Also, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening.

• -mearly-ra=scope — decides when to enable an early register allocation pass. This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, like SME2's strided LD1 and ST1. The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass entirely. -mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise.
• -mearly-ldp-fusion — enables the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by default at -O and above 🔥
• -mlate-ldp-fusion — enables the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by default at -O and above 🔥
• -msve-vector-bits=bits — sets the number of bits in an SVE vector register. This option only does anything when SVE is enabled, fyi. 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 lets GCC make assumptions about the vector length when that's useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. All other values generate vector-length specific code. 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 💖
• -Wexperimental-fmv-target — warns about use of experimental Function Multi Versioning. The Arm C Language Extension specification for Function Multi Versioning is beta and subject to change. Any usage of FMV is caveated that future behavior change and incompatibility is likely, so like… proceed with caution bestie 🙈

---

✨ -march and -mcpu Feature Modifiers ✨

Feature modifiers used with -march and -mcpu can be any of the following, and also 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 only enables the extension at the assembler level and does not affect code generation.
• rng — enable the Armv8.5-a Random Number instructions. This option only enables 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 only enables 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 only enables the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• predres — enable the Armv8-a Execution and Data Prediction Restriction instructions. This option only enables the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
• sve2 — enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.
• sve2-bitperm — enable SVE2 bitperm instructions. This also enables SVE2 instructions.
• sve2-sm4 — enable SVE2 sm4 instructions. This also enables SVE2 instructions.
• sve2-aes — enable SVE2 aes instructions. This also enables SVE2 instructions.
• sve2-sha3 — enable SVE2 sha3 instructions. This also enables SVE2 instructions.
• sve2p1 — enable SVE2.1 instructions. This also enables SVE2 instructions.
• tme — enable the Transactional Memory Extension.
• i8mm — enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• f32mm — enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• f64mm — enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
• bf16 — enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures prior to Armv8.2-A is not supported.
• ls64 — enable the 64-byte atomic load and store instructions for accelerators.
• mops — enable the instructions to accelerate memory operations like memcpy, memmove, memset. This option is enabled by default for -march=armv8.8-a
• flagm — enable the Flag Manipulation instructions Extension.
• flagm2 — enable the FlagM2 flag conversion instructions.
• pauth — enable the Pointer Authentication Extension.
• cssc — turns on the Common Short Sequence Compression instructions ✨
• sme — turns on the Scalable Matrix Extension. Ngl, this one only works when SVE2 is enabled too.
• sme-i16i64 — enables the FEAT_SME_I16I64 extension to SME. Bestie this also enables SME instructions, two-for-one 💅
• sme-f64f64 — enables the FEAT_SME_F64F64 extension to SME. Also enables SME instructions.
• sme2 — enables the Scalable Matrix Extension 2. Also enables SME instructions.
• sme-b16b16 — enables the FEAT_SME_B16B16 extension to SME. This one 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.
• 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. Also enables the LSE128 extension.
• gcs — enables support for the Armv9.4-a Guarded Control Stack extension 🛡️
• the — enables support for the 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.
• cpa — enables the Checked Pointer Arithmetic instructions.
• sve-b16b16 — enables the SVE non-widening brain floating-point ("bf16") extension. Heads up, this only has an effect when "sve2" or "sme2" are also enabled 👑

---

Okay so the dependency tea ☕: Feature crypto implies aes, sha2, and simd, which implies fp. And conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. It's giving interconnected web and we love that for them 💖

🌈 Adapteva Epiphany Options 🌈

These -m options are defined for Adapteva Epiphany:

• -mhalf-reg-file — don't allocate any register in the range "r32"..."r63". That's what lets your code run on hardware variants that are missing those registers 🙈
• -mprefer-short-insn-regs — preferentially allocate registers that allow short instruction generation. Careful bestie, this can result in an increased instruction count, so it may either reduce or increase overall code size. It's a coin flip fr 💀
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. Lowkey this cost is only a heuristic and is not guaranteed to produce consistent results across releases.
• -mcmove — enable the generation of conditional moves.
• -mnops=num — emit num NOPs before every other generated instruction.
• -mno-soft-cmpsf — for single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This is faster than a software comparison, but 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 this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option 🫶
• -mno-round-nearest — make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
• -mlong-calls — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" / "bl" instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
• -mshort-calls — if not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl" instructions, so use these instructions for direct calls. The default is -mlong-calls.
• -msmall16 — assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect.
• -mfp-mode=mode — set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches ✨

mode can be set to one the following values:

• caller — any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode 🎀
• truncate — this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
• round-nearest — this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
• int — this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller

• -mno-split-lohi, -mno-postinc, -mno-postmodify — code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify 🦋
• -mnovect-double — change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode.
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types 👀
• -msplit-vecmove-early — split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case 💀
• -m1reg-reg — specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and none, which means that no register is used for this purpose. The default is -m1reg-none.

---

🔥 AMD GCN Options 🔥

These options are defined specifically for the AMD GCN port.

• -march=gpu, -mtune=gpu — set architecture type or tuning for gpu. Supported values for gpu are:
  • gfx900 — compile for GCN5 Vega 10 devices (gfx900).
  • gfx902 — compile for GCN5 Vega gfx902 devices. (Experimental)
  • gfx904 — compile for GCN5 Vega gfx904 devices. (Experimental)
  • gfx906 — compile for GCN5 Vega 20 devices (gfx906).
  • gfx908 — compile for CDNA1 Instinct MI100 series devices (gfx908).
  • gfx909 — compile for GCN5 Vega gfx909 devices. (Experimental)
  • gfx90a — compile for CDNA2 Instinct MI200 series devices (gfx90a).
  • gfx90c — compile for GCN5 Vega 7 devices (gfx90c).
  • gfx9-generic — compile generic code for Vega devices, executable on the following subset of GFX9 devices: gfx900, gfx902, gfx904, gfx906, gfx909 and gfx90c. (Experimental)
  • gfx1030 — compile for RDNA2 gfx1030 devices (GFX10 series).
  • gfx1031 — compile for RDNA2 gfx1031 devices (GFX10 series). (Experimental)
  • gfx1032 — compile for RDNA2 gfx1032 devices (GFX10 series). (Experimental)
  • gfx1033 — compile for RDNA2 gfx1033 devices (GFX10 series). (Experimental)
  • gfx1034 — compile for RDNA2 gfx1034 devices (GFX10 series). (Experimental)
  • gfx1035 — compile for RDNA2 gfx1035 devices (GFX10 series). (Experimental)
  • gfx1036 — compile for RDNA2 gfx1036 devices (GFX10 series).
  • gfx10-3-generic — compile generic code for GFX10-3 devices, executable on gfx1030, gfx1031, gfx1032, gfx1033, gfx1034, gfx1035, and gfx1036. (Experimental)
  • gfx1100 — compile for RDNA3 gfx1100 devices (GFX11 series).
  • gfx1101 — compile for RDNA3 gfx1101 devices (GFX11 series). (Experimental)
  • gfx1102 — compile for RDNA3 gfx1102 devices (GFX11 series). (Experimental)
  • gfx1103 — compile for RDNA3 gfx1103 devices (GFX11 series).
• gfx1150 — Compile for RDNA3 gfx1150 devices (GFX11 series). (Experimental)
• gfx1151 — Compile for RDNA3 gfx1151 devices (GFX11 series). (Experimental)
• gfx1152 — Compile for RDNA3 gfx1152 devices (GFX11 series). (Experimental)
• gfx1153 — Compile for RDNA3 gfx1153 devices (GFX11 series). (Experimental)
• gfx11-generic — Compile generic code for GFX11 devices, executable on gfx1100, gfx1101, gfx1102, gfx1103, gfx1150, gfx1151, gfx1152, and gfx1153. (Experimental)
• -msram-ecc=on, -msram-ecc=off, -msram-ecc=any — Compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. Lowkey, this feature can be flipped per-process on some devices, so the compiled code has to match the device mode, fr fr. The default is any, for devices that support it.
• -mstack-size=bytes — Tell it exactly how many bytes of stack space get requested for each GPU thread (aka a wave-front, cute name 💖). ⚠️ Beware, bestie: there can be a LOT of threads and only limited memory, so don't go feral with this. The stack allocation size may also mess with run-time performance. Default is 32KB when you're using OpenACC or OpenMP, and 1MB otherwise.
• -mxnack=on, -mxnack=off, -mxnack=any — Compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices always need XNACK and some let you configure it yourself. The compiled code must match the device mode. The default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise.

---

✨ ARC Options ✨

The following options control the architecture variant for which code is being compiled, iykyk:

• -mbarrel-shifter — Generate instructions supported by the barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — Force it to call a function using the jli_s instruction. This option is valid only for the ARCv2 architecture.
• -mcpu=cpu — Set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience (so you can be lazy, no judgment 🌸). Supported values for cpu are:
  • arc600 — Compile for ARC600. Aliases: -mA6, -mARC600.
  • arc601 — Compile for ARC601. Alias: -mARC601.
  • arc700 — Compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
  • arcem — Compile for ARC EM.
  • archs — Compile for ARC HS.
  • em — Compile for ARC EM CPU with no hardware extensions.
  • em4 — Compile for ARC EM4 CPU.
  • em4_dmips — Compile for ARC EM4 DMIPS CPU.
  • em4_fpus — Compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — Compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — Compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — Compile for ARC HS34 CPU.
  • hs38 — Compile for ARC HS38 CPU.
  • hs38_linux — Compile for ARC HS38 CPU with all hardware extensions on.
  • hs4x — Compile for ARC HS4x CPU.
  • hs4xd — Compile for ARC HS4xD CPU.
  • hs4x_rel31 — Compile for ARC HS4x CPU release 3.10a.
  • arc600_norm — Compile for ARC 600 CPU with "norm" instructions enabled.
  • arc600_mul32x16 — Compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc600_mul64 — Compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
  • arc601_norm — Compile for ARC 601 CPU with "norm" instructions enabled.
  • arc601_mul32x16 — Compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc601_mul64 — Compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
  • nps400 — Compile for ARC 700 on NPS400 chip.
  • em_mini — Compile for ARC EM minimalist configuration featuring reduced register set.
• -mdpfp, -mdpfp-compact — Generate double-precision FPX instructions, tuned for the compact implementation.
• -mdpfp-fast — Generate double-precision FPX instructions, tuned for the fast implementation.
• -mno-dpfp-lrsr — Disable "lr" and "sr" instructions from using FPX extension aux registers.
• -mea — Generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. Only valid for -mcpu=ARC700.
• -mno-mpy — Do not generate "mpy"-family instructions for ARC700. Heads up: this option is deprecated.
• -mmul32x16 — Generate 32x16-bit multiply and multiply-accumulate instructions.
• -mmul64 — Generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
• -mnorm — Generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect.
• -mspfp, -mspfp-compact — Generate single-precision FPX instructions, tuned for the compact implementation.
• -mspfp-fast — Generate single-precision FPX instructions, tuned for the fast implementation.
• -msimd — Enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700.
• -msoft-float — Honestly bestie, this option is ignored; it's provided for compatibility purposes only. Software floating-point code is emitted by default, and this default can be overridden by FPX options: -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
• -mswap — Generate "swap" instructions. 🔄
• -matomic — This enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores.
• -mdiv-rem — Enable "div" and "rem" instructions for ARCv2 cores.
• -mcode-density — Enable code density instructions for ARC EM. This option is on by default for ARC HS.
• -mll64 — Enable double load/store operations for ARC HS cores.
• -mtp-regno=regno — Specify thread pointer register number.
• -mmpy-option=multo — Compile ARCv2 code with a multiplier design option. You can specify the option using either a string or numeric value for multo. wlh1 is the default value. The recognized values are:
  • 0, none — No multiplier available.
  • 1, w — 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw" and "mpyuw".
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 4, wlh3 — Two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 5, wlh4 — One 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 6, wlh5 — One 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy", "mpyu", "mpym", "mpymu", and "mpy_s".
  • 7, plus_dmpy — ARC HS SIMD support.
  • 8, plus_macd — ARC HS SIMD support.
  • 9, plus_qmacw — ARC HS SIMD support.

This option is only available for ARCv2 cores.

• -mfpu=fpu — Enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:
  • fpus — Enables support for single-precision floating-point hardware extensions.
  • fpud — Enables support for double-precision floating-point hardware extensions. The single-precision 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.
• -mirq-ctrl-saved=register-range, blink, lp_count — tells the compiler which general-purpose registers the processor auto-saves and auto-restores when it enters and exits an interrupt 💾. You write register-range as two registers with a dash between them, and it always starts with r0, with the upper limit being the fp register. blink and lp_count are optional. Heads up bestie: this one only works for ARC EM and ARC HS cores, no cap ✋.
• -mrgf-banked-regs=number — sets how many registers get replicated into the second register bank when you hit a fast interrupt 🏃💨. "Fast interrupts" = interrupts with the absolute highest priority level, P0 👑. These bad boys only save the PC and STATUS32 registers, all so there are no memory transactions during interrupt entry and exit. Use this when you're running fast interrupts on an ARC V2 family processor. The permitted values are 4, 8, 16, and 32 🔢.
• -mlpc-width=width — picks the width of the lp_count register 📏. Valid widths: 8, 16, 20, 24, 28 and 32 bits. The default is locked in at 32 bits. If the width is anything less than 32, the compiler won't try to transform loops in your program to use the zero-delay loop mechanism unless it knows for sure that the lp_count register can hold the loop-counter value you need. Depending on the width you pick, the compiler and the run-time library might still use the loop mechanism for various needs, iykyk. 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 — turns on the use of bi or bih instructions to implement jump tables 🔀.

---

The following options get passed straight through to the assembler, and they also define preprocessor macro symbols. Two birds, one stone, we love efficiency 💅.

• -mdsp-packa — passed down to the assembler to enable the DSP Pack A extensions 🎛️. Also sets the preprocessor symbol __Xdsp_packa. This option is deprecated 🚫.
• -mdvbf — passed down to the assembler to enable the dual Viterbi butterfly extension 🦋🦋 (yes literally butterflies, iconic). Also sets the preprocessor symbol __Xdvbf. This option is deprecated.
• -mlock — passed down to the assembler to enable the locked load/store conditional extension 🔒. Also sets the preprocessor symbol __Xlock.
• -mmac-d16 — passed down to the assembler. Also sets the preprocessor symbol __Xxmac_d16. This option is deprecated.
• -mmac-24 — passed down to the assembler. Also sets the preprocessor symbol __Xxmac_24. This option is deprecated.
• -mrtsc — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction ⏱️. Also sets the preprocessor symbol __Xrtsc. This option is deprecated.
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction 🔄. Also sets the preprocessor symbol __Xswape.
• -mtelephony — passed down to the assembler to enable dual- and single-operand instructions for telephony ☎️. Also sets the preprocessor symbol __Xtelephony. This option is deprecated.
• -mxy — passed down to the assembler to enable the XY memory extension 🧠✨. Also sets the preprocessor symbol __Xxy.

---

The following options control how the assembly code gets annotated 🖊️:

• -misize — annotate assembler instructions with estimated addresses 📍.
• -mannotate-align — does literally nothing 🙈. It's preserved for backward compatibility.

---

The following options get passed through to the linker 🔗:

• -marclinux — passed through to the linker to specify use of the arclinux emulation. This option is on by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested 🎀.
• -marclinux_prof — passed through to the linker to specify use of the arclinux_prof emulation. This option is on by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested 📊.

---

The following options control the semantics of the code that gets generated 🧬:

• -mlong-calls — generate calls as register indirect calls, which gives you access to the full 32-bit address range 🌍.
• -mmedium-calls — don't use anything less than a 25-bit addressing range for calls, which is the offset you get with an unconditional branch-and-link instruction. Conditional execution of function calls gets suppressed here, so we can use that 25-bit range instead of the 21-bit range you'd get with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets 💖.
• -G num — put definitions of externally-visible data into a small data section if that data is no bigger than num bytes 📦. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations.
• -mno-sdata — don't generate sdata references 🙅. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
• -mvolatile-cache — use ordinarily cached memory accesses for volatile references. This is the default ✨.
• -mno-volatile-cache — enable cache bypass for volatile references 🚪💨.

---

The following options fine tune code generation 📐:

• -malign-call — does nothing 🙈. Preserved for backward compatibility.
• -mauto-modify-reg — enable the use of pre/post modify with register displacement 🔧.
• -mbbit-peephole — does nothing 🙈. Preserved for backward compatibility.
• -mno-brcc — this option disables a target-specific pass in arc_reorg that generates compare-and-branch (brcc) instructions. It has no effect on the generation of these instructions when they're driven by the combiner pass 🤝.
• -mcase-vector-pcrel — use PC-relative switch case tables so case tables can be shortened ✂️. This is the default for -Os.
• -mcompact-casesi — enable the compact casesi pattern. This is the default for -Os, and it's only available for ARCv1 cores. This option is deprecated 🚫.
• -mno-cond-exec — disable the ARCompact-specific pass that generates conditional execution instructions.

Okay so let me explain why this pass even exists 👩‍🏫: because of delay slot scheduling, plus all the messy interactions between operand numbers, literal sizes, instruction lengths, and conditional execution support, the target-independent pass that generates conditional execution is often lacking 😭. So the ARC port keeps a special pass around that hunts for more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling are all done. That pass generally — but not always — improves performance and code size, and the price you pay is extra compilation time. That's why there's an option to switch it off. If you're having a problem with call instructions blowing past their allowable offset range because they got conditionalized, you should consider using -mmedium-calls instead 👀.

• -mearly-cbranchsi — enable pre-reload use of the cbranchsi pattern ⚡.
• -mexpand-adddi — expand adddi3 and subdi3 at RTL generation time into add.f, adc etc. This option is deprecated.
• -mindexed-loads — enable the use of indexed loads 🗃️. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case 💀.
• -mlra — does nothing 🙈. Preserved for backward compatibility.
• -mlra-priority-none — don't indicate any priority for target registers 🚫.
• -mlra-priority-compact — indicate target register priority for r0..r3 / r12..r15 👑.
• -mlra-priority-noncompact — reduce target register priority for r0..r3 / r12..r15 📉.
• -mmillicode — when you're optimizing for size (with -Os), prologues and epilogues that have to save or restore a big pile of registers often get shortened by calling a special function in libgcc 🫶. That's called a millicode call. Because these calls can cause performance issues, and/or linking issues when you're linking in a nonstandard way, this option exists so you can turn millicode call generation on or off 🎚️.
• -mcode-density-frame — this option lets the compiler emit enter and leave instructions 🚪. These instructions are only valid for CPUs that have the code-density feature.
• -mmixed-code — does nothing 🙈. Preserved for backward compatibility.
• -mq-class — this option is deprecated. Enable q instruction alternatives. This is the default for -Os.
• -mRcq — does nothing 🙈. Preserved for backward compatibility.
• -mRcw — does nothing 🙈. Preserved for backward compatibility.
• -msize-level=level — fine-tune size optimization when it comes 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 gets dropped.
  • 3 — in addition, optional data alignment gets dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, if you don't set this, the behavior is equivalent to level 1 🌸.

• -mtune=cpu — set instruction scheduling parameters for cpu, overriding anything implied by -mcpu= 🎛️.

Supported values for cpu are:

• ARC600 — tune for ARC600 CPU.
• ARC601 — tune for ARC601 CPU.
• ARC700 — tune for ARC700 CPU with standard multiplier block.
• ARC700-xmac — tune for ARC700 CPU with XMAC block.
• ARC725D — tune for ARC725D CPU.
• ARC750D — tune for ARC750D CPU.
• core3 — tune for ARCv2 core3 type CPU. This option enables usage of the dbnz instruction ⚡.
• release31a — tune for ARC4x release 3.10a.

• -mmultcost=num — the cost to assume for a multiply instruction, where 4 is equal to a normal instruction ✖️.
• -munalign-prob-threshold=probability — does nothing 🙈. Preserved for backward compatibility.

---

The following options are kept around for backward compatibility, but they are now deprecated and will be removed in a future release ⚠️ (so like, migrate bestie, fr fr):

• -margonaut — obsolete FPX 🦕.
• -mbig-endian
• -EB — compile code for big-endian targets 🔄. Using these options is now deprecated. Big-endian code is supported by configuring GCC to build arceb-elf32 and arceb-linux-uclibc targets, for which big endian is the default 💖.
• -mlittle-endian, -EL — compile code for little-endian targets 🦋. ⚠️ Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build "arc-elf32" and "arc-linux-uclibc" targets, for which little endian is the default.
• -mbarrel_shifter — replaced by -mbarrel-shifter ✨
• -mdpfp_compact — replaced by -mdpfp-compact ✨
• -mdpfp_fast — replaced by -mdpfp-fast ✨
• -mdsp_packa — replaced by -mdsp-packa ✨
• -mEA — replaced by -mea ✨
• -mmac_24 — replaced by -mmac-24 ✨
• -mmac_d16 — replaced by -mmac-d16 ✨
• -mspfp_compact — replaced by -mspfp-compact ✨
• -mspfp_fast — replaced by -mspfp-fast ✨
• -mtune=cpu — values arc600, arc601, arc700, and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700, and ARC700-xmac respectively.
• -multcost=num — replaced by -mmultcost.

---

✨ ARM Options ✨

These -m options are defined for the ARM port, bestie 💖:

• -mabi=name — generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux, and iwmmxt.
• -mapcs-frame — generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this 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.
• -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.
• -mno-sched-prolog — prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. This means 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.
  • 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.

• -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.
• -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.

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 — Disable the Advanced SIMD instructions (does not disable floating point).
  • +nofp — Disable the floating-point and Advanced SIMD instructions.
• armv7ve
  • The extended version of the ARMv7-A architecture with support for virtualization.
  • +fp — The VFPv4 floating-point instructions, with 16 double-precision registers. The extension +vfpv4-d16 can be used as an alias for this extension.
  • +simd — The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can be used as an alias for this extension.
  • +vfpv3-d16 — The VFPv3 floating-point instructions, with 16 double-precision registers.
  • +vfpv3 — The VFPv3 floating-point instructions, with 32 double-precision registers.
  • +vfpv3-d16-fp16 — The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv3-fp16 — The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv4-d16 — The VFPv4 floating-point instructions, with 16 double-precision registers.
  • +vfpv4 — The VFPv4 floating-point instructions, with 32 double-precision registers.
  • +neon — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension +neon-vfpv3 can be used as an alias for this extension.
  • +neon-fp16 — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
• +nosimd — turns OFF the Advanced SIMD instructions. heads up bestie, this does not disable floating point, okay? they're separate things 💅
• +nofp — kills the floating-point AND Advanced SIMD instructions together. both. gone. 💀

armv8-a 🔥

• +crc — the Cyclic Redundancy Check (CRC) instructions. yes bestie, that CRC. it's giving error detection ✨
• +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
• +crypto — the cryptographic instructions 🔐
• +nocrypto — Disable the cryptographic instructions. not today, crypto 🙈
• +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions. a full cleanse, we love to see it 🧚
• +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. and oop, this ALSO enables the Advanced SIMD and floating-point instructions. two for one, iconic 🔥
• +nocrypto — Disable the cryptographic instructions.
• +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.

armv8.2-a / armv8.3-a 🦋

(these two share the same list of extensions, so they're matching besties fr fr)

• +fp16 — the half-precision floating-point data processing instructions. this ALSO enables the Advanced SIMD and floating-point instructions. the way it just drags everyone along with it 😭
• +fp16fml — the half-precision floating-point fmla extension. this ALSO enables the half-precision floating-point extension AND the Advanced SIMD and floating-point instructions. a whole friend group showing up 💖
• +simd — the ARMv8.1-A Advanced SIMD and floating-point instructions.
• +crypto — the cryptographic instructions. ALSO enables the Advanced SIMD and floating-point instructions.
• +dotprod — Enable the Dot Product extension. this ALSO enables Advanced SIMD instructions. okay ngl that's cute 🩷
• +nocrypto — Disable the cryptographic extension.
• +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this ALSO enables Advanced SIMD and floating-point instructions. slay 💅
• +bf16 — Brain half-precision floating-point instructions. (yes it really says "Brain" up in here, i'm not making this up bestie 😭) 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. the multitasking 👑
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this ALSO enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — Disable the cryptographic extension.
• +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this ALSO enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this ALSO enables Advanced SIMD and floating-point instructions.

armv8.5-a ⭐

• +fp16 — the half-precision floating-point data processing instructions. this ALSO enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this ALSO enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — Disable the cryptographic extension.
• +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this ALSO enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this ALSO enables Advanced SIMD and floating-point instructions.

armv8.6-a 💜

• +fp16 — the half-precision floating-point data processing instructions. this ALSO enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this ALSO enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +nocrypto — Disable the cryptographic extension.
• +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this ALSO enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this ALSO enables Advanced SIMD and floating-point instructions.

armv7-r 🎀

• +fp.sp — the single-precision VFPv3 floating-point instructions. btw the extension +vfpv3xd can be used as an alias for this extension. nicknames, we love 🔥
• +fp — the VFPv3 floating-point instructions with 16 double-precision registers. and the extension +vfpv3-d16 can be used as an alias for this extension.
• +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
• +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
• +nofp — Disable the floating-point extension.
• +idiv — the ARM-state integer division instructions.
• +noidiv — Disable the ARM-state integer division extension.

armv7e-m 🍓

• +fp — the single-precision VFPv4 floating-point instructions.
• +fpv5 — the single-precision FPv5 floating-point instructions.
• +fp.dp — the single- AND double-precision FPv5 floating-point instructions. both. the whole package 😌
• +nofp — Disable the floating-point extensions.

armv8.1-m.main ✨

• +dsp — the DSP instructions.
• +mve — the M-Profile Vector Extension (MVE) integer instructions.
• +mve.fp — the M-Profile Vector Extension (MVE) integer AND single precision floating-point instructions.
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- AND double-precision floating-point instructions.
• +nofp — Disable the floating-point extension.
• +cdecp0, +cdecp1, ... , +cdecp7 — Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7. pick your fave coprocessor bestie, there's 8 of them 💅
• +pacbti — Enable the Pointer Authentication and Branch Target Identification Extension.

armv8-m.main 🦋

• +dsp — the DSP instructions.
• +nodsp — Disable the DSP extension. not today DSP 🙈
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- AND double-precision floating-point instructions.
• +nofp — Disable the floating-point extension.
• +cdecp0, +cdecp1, ... , +cdecp7 — Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.

armv8-r 🌈

• +crc — the Cyclic Redundancy Check (CRC) instructions.
• +fp.sp — the single-precision FPv5 floating-point instructions.
• +simd — the ARMv8-A Advanced SIMD and floating-point instructions.
• +crypto — the cryptographic instructions.
• +nocrypto — Disable the cryptographic instructions.
• +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.

---

okay so real talk for a sec 👇

-march=native makes the compiler auto-detect the architecture of the build computer. the way it just knows 🧚 at present, this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect is unsuccessful the option has no effect. so like, don't count on it always working bestie 💀

-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. it's giving selective memory, and honestly? valid 🙈

• -mtune=name — this is the "bestie, make my code feel fast on this specific chip" knob 💅. It names the target ARM processor that GCC should tune the performance of the generated code for. For some ARM implementations you genuinely get better performance from using it. The names you're allowed to pass are: arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1. 🫶

Also — plot twist — this option can tell GCC to tune the code for a big.LITTLE system 🌸. The permissible names for that are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.

-mtune=generic-arch says "tune for a blend of processors inside architecture arch" ✨. The whole goal is generating code that runs well on the current most popular processors, balancing between the optimizations that benefit some CPUs in that range and dodging the performance pitfalls of the other CPUs. Fair warning: the effects of this option may change in future GCC versions as CPU models come and go 🙈.

-mtune accepts the same extension options as -mcpu, but heads up — those extension options do not affect the tuning of the generated code.

-mtune=native makes the compiler auto-detect the CPU of the build computer 👑. Right now this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful, the option has no effect.

• -mcpu=name[+extension...] — this names the target ARM processor. GCC takes that name and derives two things from it: the name of the target ARM architecture (as if you'd said it with -march), and the ARM processor type it should tune performance for (as if you'd said it with -mtune) 🔥. If you use this alongside -march or -mtune, those options win over the corresponding part of this one.

Lots of the supported CPUs implement optional architectural extensions, and when they do, those extensions are normally turned on by default. But if implementations exist that lack the extension, you can use the extension syntax to disable the ones that were omitted. And for floating-point and Advanced SIMD (Neon) instructions, the settings of -mfloat-abi and -mfpu have to be considered too: floating-point and Advanced SIMD instructions only get 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 shows up 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 the floating-point instructions, respectively.

The permissible names for this option are the same as those for -mtune.

These 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. It also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nopacbti — disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
• +nomve — disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nomve.fp — disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7, on cortex-m52, cortex-m55 and star-mc1.
• +nofp — disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4, cortex-m7, cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. It disables the floating-point and SIMD instructions on generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and cortex-a55.
• +nofp.dp — disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.
• +nosimd — disables the SIMD (but not the floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
• +crypto — enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55.

And one more lil' thing: the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers 🌈. It supports these extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a.

-mcpu=generic-arch is also permissible, and it's equivalent to -march=arch -mtune=generic-arch. Go peek at -mtune for more info 💖.

-mcpu=native makes the compiler auto-detect the CPU of the build computer. Right now this 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 makes the compiler ignore any -mcpu=... options that show up earlier on the command line and behave like the option was never passed at all 💀. This is handy for dodging warnings about conflicting CPU and architecture options when the two produce different architecture specifications.

• -mfpu=name — this says what floating-point hardware (or hardware emulation) is available on the target 🎀. The 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 it's special — it makes the compiler pick the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march 👑.

If the floating-point hardware you selected includes the NEON extension (like -mfpu=neon), heads up: floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified 😭. Why? Because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (specifically, denormal values get treated as zero), so using NEON instructions may lead to a loss of precision.

You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. So fancy 🫶.

• -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, and in that case the __fp16 type is not defined.
• -mstructure-size-boundary=n — the sizes of all structures and unions get rounded up to a multiple of the number of bits you set here ⭐. Permissible values are 8, 32 and 64. The default value varies between 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.

Passing a larger number can produce faster, more efficient code, but it can also make your program bigger 😬. 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 gets executed if the function tries to return.
• -mlong-calls -mno-long-calls — tells the compiler to do function calls by first loading the address of the function into a register and then performing a subroutine call on that register 🦋. You need this switch when the target function lies outside of the 64-megabyte addressing range of the offset-based version of the subroutine call instruction.

Even with this switch on, bestie, it's not a full glow-up for every single call — the compiler has a whole vibe check it runs first. Static functions? Skipped. Functions rocking the "short_call" attribute? Skipped. Functions hanging out inside the scope of a "#pragma no_long_calls" directive? Skipped. And functions whose definitions have already been compiled inside the current compilation unit? Also skipped, they're already iconic.

But — and this is the plot twist — there are exceptions, and they always become long calls, no matter what:

• weak function definitions
• functions with the "long_call" attribute
• functions with the "section" attribute
• functions that are within the scope of a "#pragma long_calls" directive

This feature is not enabled by default, periodt. If you want the old behavior back, hand it -mno-long-calls, or just wrap those calls up in a "#pragma long_calls_off" directive. 💅

Also important, no cap: these switches do nothing for how the compiler generates code to handle function calls made through function pointers. Not their department. 🫶

---

• -msingle-pic-base — Treats the register used for PIC addressing as read-only instead of reloading it in the prologue of every single function (exhausting, honestly). This means the runtime system has to be the responsible one and initialize that register with a suitable value before execution starts. It's giving "you had one job". ✨
• -mpic-register=reg — Says which register is the PIC addressing register.
  • For the standard PIC base case, the default is any suitable register the compiler picks out for you.
  • For the single PIC base case, the default is R9 if the target is EABI based or stack-checking is enabled — otherwise it's R10.
• -mpic-data-is-text-relative — Assumes the displacement between the text and data segments is locked in at static link time (fixed, no take-backs). This lets it use PC-relative addressing operations to reach data it knows lives in the data segment. For non-VxWorks RTP targets this is on by default. And when it's disabled on those targets? It'll turn on -msingle-pic-base by default instead. Sneaky.
• -mpoke-function-name — Writes the name of every function straight into the text section, sitting right before the function prologue. The generated code looks like this, bestie:

             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

So during a stack backtrace, the code can peek at the value of "pc" stored at "fp + 0". If the trace function then looks at location "pc - 12" and the top 8 bits are set, then we know there's a function name embedded immediately before this location, with length "((pc[-3]) & 0xff000000)". Detective work, honestly. 🔍

• -mthumb
• -marm — Picks between generating code that runs in ARM state vs Thumb state. For most configurations the default is ARM state, but that default can be switched by configuring GCC with the --with-mode=state configure option. You can also override ARM vs Thumb mode per function using the target("thumb") and target("arm") function attributes or pragmas. Mix and match, slay. 🌈
• -mflip-thumb — Flips ARM/Thumb modes on alternating functions. This one exists purely for regression testing of mixed Thumb/ARM code generation — it is not meant for ordinary use in compiling code. Do not casually deploy this, I'm serious. 💀
• -mtpcs-frame — Generates a stack frame compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that doesn't call any other functions — it just vibes on its own.) The default is -mno-tpcs-frame.
• -mtpcs-leaf-frame — Same deal but for all leaf functions: a stack frame compliant with the Thumb Procedure Call Standard. (Again, a leaf function calls no other functions.) The default is -mno-apcs-leaf-frame.
• -mcallee-super-interworking — Gives every externally visible function in the file being compiled an ARM instruction set header that switches to Thumb mode before running the rest of the function. This lets those functions be called from non-interworking code. Not valid in AAPCS configurations, because interworking is already enabled by default there. 🚫
• -mcaller-super-interworking — Lets calls through function pointers (virtual functions included, bestie) execute correctly whether or not the target code was compiled for interworking. Small tradeoff: there's a small overhead in the cost of executing a function pointer when this is on. Also not valid in AAPCS configurations, since interworking is on by default there too.
• -mtp=name — Sets 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 straight from the corresponding system register (supported from the arm6k architecture and later). Those system registers are reached 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. Default setting is auto. 👑
• -mtls-dialect=dialect — Picks the dialect for accessing thread local storage. Two 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 performs better for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but it does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme.
• -mword-relocations — Only generates absolute relocations on word-sized values (i.e. R_ARM_ABS32). Enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. Heads up: this option conflicts with -mslow-flash-data. Pick a lane. 😤
• -mfix-cortex-m3-ldrd — Some Cortex-M3 cores can cause data corruption when ldrd instructions with overlapping destination and base registers are used (yikes). This option avoids generating those instructions entirely. Enabled by default when -mcpu=cortex-m3 is specified.
• -mfix-cortex-a57-aes-1742098
• -mno-fix-cortex-a57-aes-1742098
• -mfix-cortex-a72-aes-1655431
• -mno-fix-cortex-a72-aes-1655431 — Enable (or disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. Enabled by default whenever either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified. 🛡️
• -munaligned-access
• -mno-unaligned-access — Enables (or disables) reading and writing of 16- and 32-bit values from addresses that aren't 16- or 32-bit aligned. By default, unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access isn't enabled, then words in packed data structures get accessed a byte at a time — slow and steady, respectfully.

The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or false, depending on this option's setting. And if unaligned access is enabled, the preprocessor symbol "__ARM_FEATURE_UNALIGNED" gets defined too. ✨

• -mneon-for-64bits — This option is deprecated and has no effect. Nothing. Zero. It's just standing there. 💀
• -mslow-flash-data — Assumes loading data from flash is slower than fetching instructions, so literal loads get minimized for better performance. Only supported when compiling for ARMv7 M-profile, and it's off by default. It conflicts with -mword-relocations.
• -masm-syntax-unified — Assumes the inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2 — however, that may change in future releases of GCC. Divided syntax should be considered deprecated. 👀
• -mrestrict-it — Restricts generation of IT blocks so they conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This one is on by default for ARMv8-A Thumb mode.
• -mprint-tune-info — Prints CPU tuning information as a comment in the assembler file. This is an option used only for regression testing of the compiler, not intended for ordinary use in compiling code. Disabled by default.
• -mverbose-cost-dump — Enables verbose cost model dumping in the debug dump files. Provided for use in debugging the compiler — that's its whole personality.
• -mpure-code — Doesn't allow constant data to be placed in code sections. Also, when compiling for ELF object format, gives 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 — Mitigates a potential security issue with the "VLLDM" instruction in some M-profile devices when using CMSE (CVE-2021-365465). Enabled by default when the option -mcpu= is used with "cortex-m33", "cortex-m35p", "cortex-m52", "cortex-m55", "cortex-m85" or "star-mc1". Want it gone? Use -mno-fix-cmse-cve-2021-35465 to disable the mitigation. 🛡️
• -mstack-protector-guard=guard
• -mstack-protector-guard-offset=offset — Generates stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a canary accessible through the TLS register. Heads up though: the -mstack-protector-guard-offset= option is for use with -fstack-protector-guard=tls and not for use in user-land code. Stay in your lane, bestie. 🐤
• -mfdpic, -mno-fdpic — pick the FDPIC ABI, which represents function pointers using 64-bit function descriptors. 💅 When the compiler is configured for arm-*-uclinuxfdpiceabi targets this one is on by default, and it drags in -fPIE if you didn't pass any of the PIC/PIE-related options. On other targets it only turns on the FDPIC-specific codegen features, and you have to explicitly pass the PIC/PIE-related options yourself if you need them.

Heads up bestie: static linking is not supported here, because the program self-relocates and that would still involve the dynamic linker anyway. 😭 If that kind of behaviour is actually acceptable to you, then go ahead and use -static and -Wl,-dynamic-linker.

The opposite option, -mno-fdpic, is useful (and lowkey required) when you're building the Linux kernel with the same arm-*-uclinuxfdpiceabi toolchain that you used to build the userland programs. 🌸

• -mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]] — turn on branch protection features (armv8.1-m.main only). ✨ Here's the breakdown:
  • none — generate code with no branch protection and no return address signing.
  • standard[+leaf] — generate code with all branch protection features enabled at their standard level.
  • pac-ret[+leaf] — generate code with return address signing at its standard level, which means signing every function that saves the return address to memory.
  • leaf — when return address signing is on, also sign leaf functions, even the ones that don't write the return address to memory.
  • +bti — add landing-pad instructions at the permitted targets of indirect branch instructions.

If the +pacbti architecture extension isn't enabled, then all the branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code stays backwards-compatible with earlier versions of the architecture, but the extra security can be switched 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 was compiled with at least -mbranch-protection=bti. fr fr, all of it.

Any setting other than none is supported only on armv8-m.main or later.

The default is to generate code with no branch protection and no return address signing.

---

🦋 AVR Options 🦋

These options are defined for AVR implementations:

• -mmcu=mcu — specify the AVR instruction set architecture (ISA) or device type. The default for this option is avr2.

Ok so a lil PSA, because this part is genuinely important: a complete device support is made up 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 that last one is provided by the compiler, according to the supported mcus listed 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. 💖

Here's every AVR device and ISA that's supported:

• avr2 — "Classic" devices with up to 8 KiB of program memory. mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.
• avr25 — "Classic" devices with up to 8 KiB of program memory and with the MOVW instruction. mcu = attiny13, attiny13a, attiny24, attiny24a, attiny25, attiny261, attiny261a, attiny2313, attiny2313a, attiny43u, attiny44, attiny44a, attiny45, attiny48, attiny441, attiny461, attiny461a, attiny4313, attiny84, attiny84a, attiny85, attiny87, attiny88, attiny828, attiny841, attiny861, attiny861a, ata5272, ata6616c, at86rf401.
• avr3 — "Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = at76c711, at43usb355.
• avr31 — "Classic" devices with 128 KiB of program memory. mcu = atmega103, at43usb320.
• avr35 — "Classic" devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction. mcu = attiny167, attiny1634, atmega8u2, atmega16u2, atmega32u2, ata5505, ata6617c, ata664251, at90usb82, at90usb162.
• avr4 — "Enhanced" devices with up to 8 KiB of program memory. mcu = atmega48, atmega48a, atmega48p, atmega48pa, atmega48pb, atmega8, atmega8a, atmega8hva, atmega88, atmega88a, atmega88p, atmega88pa, atmega88pb, atmega8515, atmega8535, ata5795, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.
• avr5 — "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = atmega16, atmega16a, atmega16hva, atmega16hva2, atmega16hvb, atmega16hvbrevb, atmega16m1, atmega16u4, atmega161, atmega162, atmega163, atmega164a, atmega164p, atmega164pa, atmega165, atmega165a, atmega165p, atmega165pa, atmega168, atmega168a, atmega168p, atmega168pa, atmega168pb, atmega169, atmega169a, atmega169p, atmega169pa, atmega32, atmega32a, atmega32c1, atmega32hvb, atmega32hvbrevb, atmega32m1, atmega32u4, atmega32u6, atmega323, atmega324a, atmega324p, atmega324pa, atmega324pb, atmega325, atmega325a, atmega325p, atmega325pa, atmega328, atmega328p, atmega328pb, atmega329, atmega329a, atmega329p, atmega329pa, atmega3250, atmega3250a, atmega3250p, atmega3250pa, atmega3290, atmega3290a, atmega3290p, atmega3290pa, atmega406, atmega64, atmega64a, atmega64c1, atmega64hve, atmega64hve2, atmega64m1, atmega64rfr2, atmega640, atmega644, atmega644a, atmega644p, atmega644pa, atmega644rfr2, atmega645, atmega645a, atmega645p, atmega649, atmega649a, atmega649p, atmega6450, atmega6450a, atmega6450p, atmega6490, atmega6490a, atmega6490p, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5787, ata5835, ata5700m322, ata5702m322, at90pwm161, at90pwm216, at90pwm316, at90can32, at90can64, at90scr100, at90usb646, at90usb647, at94k, m3000.
• avr51 — "Enhanced" devices with 128 KiB of program memory. mcu = atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.
• avr6 — "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu = atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.
• avrxmega2 — "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = atxmega8e5, atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5, avr64da28, avr64da28s, avr64da32, avr64da32s, avr64da48, avr64da48s, avr64da64, avr64da64s, avr64db28, avr64db32, avr64db48, avr64db64, avr64dd14, avr64dd20, avr64dd28, avr64dd32, avr64du28, avr64du32, avr64ea28, avr64ea32, avr64ea48, avr64sd28, avr64sd32, avr64sd48.
• avrxmega3 — "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space. mcu = attiny202, attiny204, attiny212, attiny214, attiny402, attiny404, attiny406, attiny412, attiny414, attiny416, attiny416auto, attiny417, attiny424, attiny426, attiny427, attiny804, attiny806, attiny807, attiny814, attiny816, attiny817, attiny824, attiny826, attiny827, attiny1604, attiny1606, attiny1607, attiny1614, attiny1616, attiny1617, attiny1624, attiny1626, attiny1627, attiny3214, attiny3216, attiny3217, attiny3224, attiny3226, attiny3227, atmega808, atmega809, atmega1608, atmega1609, atmega3208, atmega3209, atmega4808, atmega4809, avr16dd14, avr16dd20, avr16dd28, avr16dd32, avr16du14, avr16du20, avr16du28, avr16du32, avr16ea28, avr16ea32, avr16ea48, avr16eb14, avr16eb20, avr16eb28, avr16eb32, avr16la14, avr16la20, avr16la28, avr16la32, avr32da28, avr32da28s, avr32da32, avr32da32s, avr32da48, avr32da48s, avr32db28, avr32db32, avr32db48, avr32dd14, avr32dd20, avr32dd28, avr32dd32, avr32du14, avr32du20, avr32du28, avr32du32, avr32ea28, avr32ea32, avr32ea48, avr32eb14, avr32eb20, avr32eb28, avr32eb32, avr32la14, avr32la20, avr32la28, avr32la32, avr32sd20, avr32sd28, avr32sd32.
• avrxmega4 — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu = atxmega64a3, atxmega64a3u, atxmega64a4u, atxmega64b1, atxmega64b3, atxmega64c3, atxmega64d3, atxmega64d4, avr128da28, avr128da28s, avr128da32, avr128da32s, avr128da48, avr128da48s, avr128da64, avr128da64s, avr128db28, avr128db32, avr128db48, avr128db64.
• avrxmega5 — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega64a1, atxmega64a1u.
• avrxmega6 — "XMEGA" devices with more than 128 KiB of program memory. mcu = atxmega128a3, atxmega128a3u, atxmega128b1, atxmega128b3, atxmega128c3, atxmega128d3, atxmega128d4, atxmega192a3, atxmega192a3u, atxmega192c3, atxmega192d3, atxmega256a3, atxmega256a3b, atxmega256a3bu, atxmega256a3u, atxmega256c3, atxmega256d3, atxmega384c3, atxmega384d3.

• avrxmega7 — XMEGA girlies with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega128a1, atxmega128a1u, atxmega128a4u.
• avrtiny — the "Reduced Tiny" Tiny core devices, okay, and they come with only 16 general purpose registers and 512 B up to 4 KiB of program memory. mcu = attiny4, attiny5, attiny9, attiny10, attiny102, attiny104, attiny20, attiny40.
• avr1 — this ISA is what the minimal AVR core implements, and it's supported for assembler only. mcu = attiny11, attiny12, attiny15, attiny28, at90s1200.
• -mabsdata — assume that literally all data in static storage can be reached by LDS / STS instructions. Heads up bestie, this one only does anything on reduced Tiny devices like the ATtiny40. Also peek at the absdata AVR Variable Attributes, variable attribute 🌸.
• -mcvt — use a compact vector table ✨. Some devices have a CVT with only four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ, 3=Prio0 IRQs. This option links the startup code from crtmcu-cvt.o instead of the usual crtmcu.o. And ngl, besides giving you a compact vector table, the startup code also sets bit CPUINT_CTRLA.CPUINT_CVT, which turns the CVT on for the device.

But 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. Like, for example, you can link with -Wl,--defsym,__init_cvt=0.

The CVT startup code has been 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. The possible values for bits are 32 and 64. Whether a specific value for bits is even allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the exact same thing goes for the default values of these options.
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction that GNU Binutils supports. If this option is on, you can still disable the feature for individual ISRs using the AVR Function Attributes, no_gccisr function attribute. This feature is activated per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683) 👑.
• -mint8 — assume int is an 8-bit integer. This affects the sizes of like, every type: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Okay so real talk: this option does not conform to the C standards, but it does give you smaller code size. Trade-offs, bestie 💅.
• -mmain-is-OS_task — do not save registers in main. The effect is exactly the same as attaching the attribute AVR Function Attributes, OS_task to main. It gets 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 at all. By setting this option, the user is asserting that main will not return.

You can use this option for devices with very limited resources to save a few bytes of code and stack space. It'll work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions? There's just no performance gain, that's all 😭.

• -mno-interrupts — the generated code is not compatible with hardware interrupts. Code size is smaller though. That's the deal fr fr.
• -mrelax — try to replace the CALL resp. JMP instruction with the shorter RCALL resp. RJMP instruction if it applies. 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 code is located. Therefore, the assembler code the compiler generates stays 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.

• -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's genuinely no choice, so this option acts kinda 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 go back to the old layout with .rodata in RAM.

• -mtiny-stack — only change the lower 8 bits of the stack pointer. That's it, that's the whole vibe.
• -mfract-convert-truncate — allow truncation instead of rounding towards zero for fractional fixed-point types.
• -nodevicelib — don't link against AVR-LibC's device specific library libmcu.a.

⚠️ 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. Which means the user has to supply her private device specs file by means of -specs=path-to-specs-file. There's no more need for the option -mmcu=mcu 🎀.

This option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point to a directory containing a folder named device-specs, which 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, no cap. Options -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 get pushed before calling a function and popped afterwards. Also see the -fdefer-pop Optimize Options, optimization option.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables, because arguments need not be removed from the stack after such a function call.

This option can reduce code size for functions that perform several calls to functions that get their arguments on the stack, like calls to printf-like functions 💖.

• -mbranch-cost=cost — set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0.
• -mcall-prologues — function prologues/epilogues get expanded as calls to appropriate subroutines. Code size is smaller 👑.
• -mfuse-add
• -mno-fuse-add
• -mfuse-add=level — optimize indirect memory accesses on reduced Tiny devices. The default uses level=1 for optimizations -Og and -O1, and level=2 for higher optimizations. Valid values for level are 0, 1 and 2.
• -mfuse-move
• -mno-fuse-move
• -mfuse-move=level — run a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default uses level=3 for optimization -O1, and level=23 for higher optimizations. Valid values for level are in the range 0 ... 23, which is a 3:2:2:2 mixed radix value 🔢. Each digit controls some aspect of the optimization.
• -mfuse-move2 — run a post combine optimization pass that tries to fuse move instructions.
• -mstrict-X — use address register X in the way the hardware proposes. This means X is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the X register may be used in the same way as Y or Z, which then gets emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as

adiw r26, const   ; X += const
ld   <Rn>, X        ; <Rn> = *X
sbiw r26, const   ; X -= const

• -msplit-bit-shift — split multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset. This optimization is turned on per default for -O2 and higher, including -Os but excluding -Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by -mfuse-move.
• -msplit-ldst — split multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for -O2 and higher.
• -muse-nonzero-bits — enable optimizations that are only possible when some bits in a register are always zero 🙈. This optimization is turned on per default for -O2 and higher.

✨ EIND and Devices with More Than 128 Ki Bytes of Flash ✨

okay so the pointers in this implementation are 16 bits wide, periodt. The address of a function or label is represented as a word address so that indirect jumps and calls can target any code address in the range of 64 Ki words. 💅

To make indirect jumps work on devices with more than 128 Ki bytes of program memory space, there’s this 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 by the compiler like this, and bestie, they come with some limitations:

• The compiler never sets EIND. That’s just not its job. 💀
• The compiler uses EIND implicitly in EICALL/EIJMP instructions, or it might read EIND directly to emulate an indirect call/jump by means of a RET instruction. so real.
• 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. no cap.
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. iconic.
• 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. 😭
• 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. yikes.
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/). 📖
• It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located.

#include <avr/io.h>

static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
  __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                  "out %i0,r24" :: "n" (&EIND) : "r24","memory");
}

The __trampolines_start symbol is defined in the linker script.

• Stubs are generated automatically by the linker if the following two conditions are met:
  • The address of a label is taken by means of the gs modifier (short for generate stubs) like so:

LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))

• The final location of that label is in a code segment outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • Taking address of a function or code label.
  • Computed goto.
  • If prologue-save function is used, see -mcall-prologues command-line option.
  • Switch/case dispatch tables. If you do not want such dispatch tables you can specify the -fno-jump-tables command-line option.
  • C and C++ constructors/destructors called during startup/shutdown.
  • If the tools hit a gs() modifier explained above.
• Jumping to non-symbolic addresses like so is not supported:

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 a AVR Named Address Spaces, named address space other than generic or __flash is used, then RAMPZ is set as needed before the operation.
• If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation.
• If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
• RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access. ⚠️

🌈 AVR Built-in Macros 🌈

GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option. ✨

For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.

• __AVR_ARCH__ — Build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu option. Possible values are:

2, 25, 3, 31, 35, 4, 5, 51, 6

for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",

respectively and

100, 102, 103, 104, 105, 106, 107

for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.

• __AVR_Device__ — Setting -mmcu=device defines this built-in macro which reflects the device’s name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.

The built-in macros’ names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase.

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_DEVICE_NAME__ — Setting -mmcu=device defines this built-in macro to the device’s name. For example, with -mmcu=atmega8 the macro is defined to atmega8.

If device is not a device but only a core architecture like avr51, this macro is not defined.

• __AVR_CVT__ — The code is being compiled with option -mcvt to use a compact vector table.
• __AVR_XMEGA__ — The device / architecture belongs to the XMEGA family of devices.
• __AVR_HAVE_ADIW__ — The device has the ADIW and SBIW instructions.
• __AVR_HAVE_ELPM__ — The device has the ELPM instruction.
• __AVR_HAVE_ELPMX__ — The device has the ELPM Rn,Z and ELPM Rn,Z+ instructions.
• __AVR_HAVE_LPMX__ — The device has the LPM Rn,Z and LPM Rn,Z+ instructions.
• __AVR_HAVE_MOVW__ — The device has the MOVW instruction to perform 16-bit register-register moves.
• __AVR_HAVE_MUL__ — The device has a hardware multiplier.
• __AVR_HAVE_JMP_CALL__ — The device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory.
• __AVR_HAVE_EIJMP_EICALL__, __AVR_3_BYTE_PC__ — The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.
• __AVR_2_BYTE_PC__ — The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.
• __AVR_HAVE_8BIT_SP__, __AVR_HAVE_16BIT_SP__ — The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.
• __AVR_HAVE_SPH__, __AVR_SP8__ — The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and -mmcu=avr25 also by -msp8.
• __AVR_HAVE_RAMPD__, __AVR_HAVE_RAMPX__, __AVR_HAVE_RAMPY__, __AVR_HAVE_RAMPZ__ — The device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively.
• __NO_INTERRUPTS__ — This macro reflects the -mno-interrupts command-line option.
• __AVR_ERRATA_SKIP__, __AVR_ERRATA_SKIP_JMP_CALL__ — Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.
• __AVR_ISA_RMW__ — The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
• __AVR_SFR_OFFSET__=offset — so like, some instructions can talk directly to the I/O special function registers (IN, OUT, SBI, etc.), and those use a different address than they would if you hit them through RAM-accessing instructions like LD or STS. This offset is a whole device-architecture thing, and you have to subtract it from the RAM address to get the actual I/O address. It's giving ✨address math✨.
• __AVR_SHORT_CALLS__ — this one's defined when the -mshort-calls command-line option is set. Simple as that 💅
• __AVR_PM_BASE_ADDRESS__=addr — some devices can read from flash memory using LD* instructions, no cap. The flash shows up in the data address space at an offset of __AVR_PM_BASE_ADDRESS__. If this macro isn't defined, bestie, that feature is just not available, periodt. If it is defined, the address space is linear and you don't have to shove .rodata into RAM at all. The default linker description file handles all of this, and rn it's available for avrtiny and avrxmega3. Even more iconic: you don't need address spaces like __flash or features like the progmem and pgm_read_* attributes. Not me celebrating 🙈
• __AVR_HAVE_FLMAP__ — defined when these 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.

Which 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.

But when the code is compiled for a device, the macro gets defined to 1 when the .rodata sections for read-only data live in RAM, and defined to 0 otherwise. Vibe check: RAM or no RAM, the macro tells you 👑

• __WITH_AVRLIBC__ — the compiler is configured to be used together with AVR-LibC. See the --with-avrlibc configure option. 🫶
• __HAVE_SIGNAL_N__ — the compiler supports the signal(num) and interrupt(num) AVR function attributes with an argument num that specifies the number of the interrupt service routine. Slay.
• __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 all of the above, but for long double instead of double. Literally the same energy, different type 🌸
• __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__ — these reflect 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 the internal girlies — the compiler uses them for itself and to communicate between device specs files and the compiler proper. Bestie, you do not need to set these by hand. Especially since they are not optimization options. Using them the wrong way can lead to sub-optimal or straight-up wrong code 😭. They're documented for completeness, and so you can get a better understanding of device specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) files.

• -mn-flash=num — assume the flash memory has a size of num times 64 KiB. This decides which __flashN address spaces are available. ✨
• -mflmap — the device has the FLMAP bit field hanging out in special function register NVMCTRL_CTRLB.
• -mrmw — assume 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 gets 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. Not me dodging bugs 🙈
• -msp8 — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. The compiler uses this option to select and build multilibs for architectures avr2 and avr25. Those architectures mix devices with and without SPH. Iconic mixed-bag behavior 🦋

---

💜 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 get 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 isn't 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.

Heads up bestie: support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. 😭

• -msim — specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option only has effect for the bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim. 🌸
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers, and makes an extra register available in leaf functions. Freeing up registers is such a main character move ✨
• -mspecld-anomaly — when enabled, the compiler makes sure the generated code does not contain speculative loads after jump instructions. If this option is used, __WORKAROUND_SPECULATIVE_LOADS is defined. 🫶
• -mno-specld-anomaly — don't generate extra code to prevent speculative loads from occurring.
• -mcsync-anomaly — when enabled, the compiler makes sure the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined. 🔥
• -mno-csync-anomaly — don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional branch.
• -mlow64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. Fits? Slay. 💅
• -mno-low64k — assume that the program is arbitrarily large. This is the default.
• -mstack-check-l1 — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
• -mid-shared-library — generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-elf target, this option implies -msim. 🌈
• -mno-id-shared-library — generate code that doesn't assume ID-based shared libraries are being used. This is the default.
• -mleaf-id-shared-library — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. Speed? Obsessed. ⚡
• -mno-leaf-id-shared-library — do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated for jump and call insns. 😔
• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library but is no more space- or time-efficient than omitting this option. Ngl, 0 is the move 🎀
• -msep-data — generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute in place in an environment without virtual memory management by eliminating relocations against the text section. Chef's kiss 🍓
• -mno-sep-data — generate code that assumes that the data segment follows the text segment. This is the default.
• -mlong-calls
• -mno-long-calls — tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. 🙈

• -mfast-fp — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), in the interest of performance. A little chaotic, a little fast 💅
• -minline-plt — enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. 👑
• -mmulticore — build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision]. 🧚

This one lowkey pairs with -mcorea or -mcoreb, and those are the two that pick the one-application-per-core programming model. If you don't hand it either of those, you get the single-application/dual-core model instead, and in that model the main function of Core B has to be named "coreb_main". Periodt, that's the rule.

And if this option isn't used at all? Then congrats bestie, you're in the single-core application programming model. That's the whole vibe. 💅

• -mcorea — builds a standalone app for Core A of BF561 when you're doing the one-application-per-core thing. It pulls in the proper start files and link scripts for Core A, and defines the macro "__BFIN_COREA". Heads up though: this one is only allowed in conjunction with -mmulticore — it can't fly solo.
• -mcoreb — same energy but for Core B of BF561, one-application-per-core model. Proper start files and link scripts for Core B get used and the macro "__BFIN_COREB" gets defined. When you use this, "coreb_main" is what you write instead of "main". And yeah, same deal: only with -mmulticore.
• -msdram — builds a standalone application for SDRAM. The right start files and link scripts get pulled in so your app lands in SDRAM, and the macro "__BFIN_SDRAM" is defined. Just know that the loader is supposed to initialize SDRAM before it loads the application. Sneaky important. 😭
• -micplb — assume ICPLBs are enabled at run time. This actually matters because it affects certain anomaly workarounds. For Linux targets the default is to assume ICPLBs are on; for standalone applications the default is off.

---

🔥 C6X Options 🔥

• -march=name — tells GCC the name of the target architecture. GCC uses that name to figure out what kind of instructions it's allowed to emit when it's generating assembly. The permissible names are c62x, c64x, c64x+, c67x, c67x+, c674x. No other names, bestie, the list is the list.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. And this one is the default, so if you do nothing, this is what you get. ✨
• -msim — pick startup files and a linker script that are suitable for the simulator.
• -msdata=default — put small global and static data in the ".neardata" section, which register "B14" points at. Small uninitialized global and static data goes in ".bss", which sits right next to ".neardata". Small read-only data goes into ".rodata". Meanwhile the sections for the big stuff are ".fardata", ".far" and ".const".
• -msdata=all — put all data in the sections reserved for small data, not just the small objects, and access it using addressing relative to the "B14" register.
• -msdata=none — don't use the small-data sections at all, and access everything with absolute addresses. All initialized global and static data goes in ".fardata", all uninitialized data goes in ".far", and all constant data goes in ".const".

---

🦋 CRIS Options 🦋

These ones are defined specifically for the CRIS ports, so if you're not on CRIS they're not your problem, iykyk.

• -march=architecture-type, -mcpu=architecture-type — generate code for the architecture you name. Your choices for architecture-type are v3, v8 and v10, which mean ETRAX 4, ETRAX 100, and ETRAX 100 LX respectively. Default is v0. 💜
• -mtune=architecture-type — tune everything applicable about the generated code to architecture-type, except for the ABI and the set of available instructions. The choices for architecture-type are the exact same ones as for -march=architecture-type.
• -mmax-stack-frame=n — warn you when a function's stack frame goes over n bytes. Frame too big? It's gonna say something.
• -metrax4, -metrax100 — these two are just synonyms, no cap. -metrax4 = -march=v3 and -metrax100 = -march=v8.
• -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the "muls" and "mulu" instructions, on the CPU models where that bug applies. This option is disabled by default, so you have to opt in.
• -mpdebug — turns on CRIS-specific verbose debug-related info in the assembly code. It also has the side effect of turning off the #NO_APP formatted-code indicator that gets handed to the assembler at the start of the assembly file. Two-for-one.
• -mcc-init — don't reuse condition-code results from a previous instruction; always emit compare and test instructions before anything actually uses the condition codes. Fresh codes only. 🧼
• -mno-side-effects — don't emit instructions with side effects in addressing modes other than post-increment.
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these options (and their no- versions) arrange (or eliminate arrangements) for the stack frame, individual data and constants to be aligned to the maximum single data access size of whatever CPU model you chose. The default is to arrange for 32-bit alignment. Important: ABI details like structure layout are not affected by these options, so don't go blaming them for struct weirdness. 🙈
• -m32-bit, -m16-bit, -m8-bit — same family as the stack-, data- and const-align options up there: these arrange for the stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned. 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.

Okay so this one is a real one. Only use it together with your own eyes on the compiled code — visual inspection, bestie. Because you get no warnings and no errors, even when call-saved registers need saving or local variables need storage allocated. It will just quietly do the wrong thing. 💀

• -melf — legacy no-op option. Does nothing, it's just here for the lore.
• -sim — arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data all get allocated consecutively.
• -sim2 — like -sim, but it passes linker options so initialized data lands at 0x40000000 and zero-initialized data lands at 0x80000000.

---

🎀 C-SKY Options 🎀

GCC supports these options when you're 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. Yeah it's a long list, that's just how it is. 😭
• -mbig-endian, -EB, -mlittle-endian, -EL — select big- or little-endian code. Default is little-endian.
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are soft, softfp and hard.
  • soft makes GCC generate output containing library calls for floating-point operations.
  • softfp lets it generate code using hardware floating-point instructions, but still uses the soft-float calling conventions.
  • hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. And now the serious bit: the hard-float and soft-float ABIs are not link-compatible. You must compile your entire program with the same ABI and link against a compatible set of libraries. Mix them and it's giving chaos. 🫠

• -mhard-float, -msoft-float — select hardware or software floating-point implementations. The default is soft float.
• -mdouble-float, -mno-double-float — when -mhard-float is in effect, enable generation of double-precision float instructions. This is the default, except when compiling for CK803.
• -mfdivdu, -mno-fdivdu — when -mhard-float is in effect, enable generation of "frecipd", "fsqrtd", and "fdivd" instructions. This is the default, except when compiling for CK803.
• -mfpu=fpu — select the floating-point processor. This option can only be used with -mhard-float. Values for fpu are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float -mdivdu).
• -melrw, -mno-elrw — enable the extended "lrw" instruction. This option defaults to on for CK801 and off otherwise.
• -mistack, -mno-istack — enable interrupt stack instructions; default is off. The -mistack option is required to handle the "interrupt" and "isr" function attributes, so don't skip it if you need those.
• -mmp — enable multiprocessor instructions; default is off.
• -mcp — enable coprocessor instructions; default is off.
• -mcache — enable coprocessor instructions; default is off.
• -msecurity — enable C-SKY security instructions; default is off.
• -mtrust — enable C-SKY trust instructions; default is off.
• -mdsp, -medsp, -mvdsp — enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these default to off. 🌈
• -mdiv, -mno-div — generate divide instructions. Default is off.
• -msmart, -mno-smart — generate code for Smart Mode, using only registers numbered 0-7 so it can use 16-bit instructions. This option is ignored for CK801, where that's the required behavior anyway, and it defaults to on for CK802. For every other target, 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 it's enabled by default for other processors.
• -manchor, -mno-anchor — generate code using global anchor symbol addresses.
• -mpushpop, -mno-pushpop — generate code using "push" and "pop" instructions. This option defaults to on. ✨
• -mmultiple-stld, -mstm, -mno-multiple-stld, -mno-stm — generate code using "stm" and "ldm" instructions. This option isn't supported on CK801 but it is enabled by default on other processors.
• -mconstpool, -mno-constpool — create constant pools inside the compiler instead of deferring that job to the assembler. This option is the default and it's required for correct code generation on CK801 and CK802, and it's optional on other processors.
• -mstack-size, -mno-stack-size — pop a .stack_size directive on every single function in the assembly output. it's off by default, bestie, so don't come for me when you don't see it 💅
• -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. also off by default, no cap.
• *-mbranch-cost=n* — set the branch costs to roughly n instructions. default is 1. that's it, that's the whole vibe.
• -msched-prolog, -mno-sched-prolog — permits scheduling of function prologue and epilogue sequences.

⚠️ real talk: 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's disabled by default. 😭

• -msim — links the library libsemi.a, which is compatible with simulator. applicable to ELF compiler only. iykyk.

---

💻 Cygwin and MinGW Options 💻

okay these extra options are the ones available for Microsoft Windows targets, periodt:

• -mconsole — says a console application is what we're generating, by telling the linker to set the PE header subsystem type that console applications need. available for Cygwin and MinGW targets, and enabled by default on those targets. slay 🫶
• *-mcrtdll=library* — preprocess, compile or link with the specified C RunTime DLL library. this option adjusts the predefined macros __CRTDLL__, __MSVCRT__, _UCRT and __MSVCRT_VERSION__ for the specified CRT library, picks the start file for the CRT library, and links with the CRT library.

the recognized CRT library names for the preprocessor are: crtdll*, msvcrt10*, msvcrt20*, msvcrt40*, msvcr40*, msvcrtd*, msvcrt-os*, msvcr70*, msvcr71*, msvcr80*, msvcr90*, msvcr100*, msvcr110*, msvcr120* and ucrt*.

if this option is not specified then the default MinGW import library msvcrt is used for linking and no other adjustment for the preprocessor is done. the MinGW import library msvcrt is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation. the MinGW import library msvcrt-os is for the Windows system CRT DLL library msvcrt.dll and in most cases is the default MinGW import library.

ngl, generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT.

🚨 this option is for experimental and testing purposes only. this option is available for MinGW targets.

• -mdll — available for Cygwin and MinGW targets. specifies that a DLL — a dynamic link library — is to be generated, enabling the selection of the required runtime startup object and entry point. main character energy fr.
• -mnop-fun-dllimport — available for Cygwin and MinGW targets. specifies that the dllimport attribute should be ignored.
• -mthreads — available for MinGW targets. specifies that MinGW-specific thread support is to be used.
• -municode — available for MinGW-w64 targets. it causes the UNICODE preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code. iconic.
• -mwin32 — available for Cygwin and MinGW targets. specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.
• -mwindows — available for Cygwin and MinGW targets. specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. the ✨aesthetic✨ option.
• -fno-set-stack-executable — available for MinGW targets. specifies that the executable flag for the stack used by nested functions isn't set. this is necessary for binaries running in kernel mode of Microsoft Windows, since there the User32 API, which is used to set executable privileges, isn't available.
• -fwritable-relocated-rdata — available for MinGW and Cygwin targets. specifies that relocated-data in a 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 — available for Cygwin and MinGW targets. 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.

see also under x86 Options for standard options 👀

---

🍎 Darwin Options 🍎

these options are defined for all architectures running the Darwin operating system.

FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.

the subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. the -force_cpusubtype_ALL option can be used to override this.

the Darwin tools vary in their behavior when presented with an ISA mismatch:

• The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating — so you cannot put 64-bit instructions in a ppc750 object file.
• The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library).
• The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files.
• -Fdir — add the framework directory dir to the head of the list of directories to be searched for header files. these directories are interleaved with those specified by -I options and are scanned in a left-to-right order.

a framework directory is a directory with frameworks in it. a framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. the name of a framework is the name of this directory excluding the .framework. headers associated with the framework are found in one of those two directories, with Headers being searched first.

a subframework is a framework directory that is in a framework's Frameworks directory. includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. two subframeworks are siblings if they occur in the same framework. a subframework should not have the same name as a framework; a warning is issued if this is violated. currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this.

the standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. an example include looks like:

#include <Framework/header.h>

where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory.

• -iframeworkdir — like -F except the directory is treated as a system directory. the main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. this option is valid only for the C family of languages.
• -gused — emit debugging information for symbols that are used. for stabs debugging format, this enables -feliminate-unused-debug-symbols. this is by default ON. 💅
• -gfull — emit debugging information for all symbols and types. no crumbs left.
• -fconstant-cfstrings — the -fconstant-cfstrings is an alias for -mconstant-cfstrings.
• -mconstant-cfstrings — when the NeXT runtime is being used (the default on these systems), override any -fconstant-string-class setting and cause @"..." literals to be laid out as constant CoreFoundation strings.
• *-mmacosx-version-min=version* — the earliest version of MacOS X that this executable will run on is version. typical values supported for version include 12, 10.12, and 10.5.8.

if the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible.

• -mkernel — enable kernel development mode. the -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. this mode also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets. it's giving "doing the most, but correctly" 💅
• -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. bestie, do not play with this one casually, fr.

• -mfix-and-continue, -ffix-and-continue, -findirect-data — generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
• -all_load — loads all members of static archive libraries. see man ld(1) for more information.
• -arch_errors_fatal — cause the errors having to do with files that have the wrong architecture to be fatal.
• -bind_at_load — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
• -bundle — produce a Mach-o bundle format file. see man ld(1) for more information.
• -bundle_loader executable — this option specifies the executable that will load the build output file being linked. see man ld(1) for more information.
• -dynamiclib — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command.
• -force_cpusubtype_ALL — makes GCC's output file get the ALL subtype instead of whatever -mcpu or -march was controlling. lowkey a takeover move 💅
• -nodefaultrpaths — nope, do not add the default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later the embedded runpath gets added by default, unless you drop -nodefaultrpaths on the link line. And heads up bestie: run paths are needed (like, enforced) to build on macOS version 10.11 or later.
• -allowable_client client_name
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_flat_namespace
• -headerpad_max_install_names
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded — okay so alllll of these get passed straight to the Darwin linker. The Darwin linker man page is the one that actually describes them in detail, iykyk. go read that one for the tea 🫖

---

💜 DEC Alpha Options 💜

these -m options are the ones defined for the DEC Alpha implementations:

🌸 float things 🌸

• -mno-soft-float, -msoft-float — use (or do not use) the hardware floating-point instructions for floating-point operations. when -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they get replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulation routines, these routines issue floating-point operations. If you're compiling for an Alpha without floating-point operations, you must make sure 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. wild but true 💀

• -mfp-reg, -mno-fp-regs — generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set isn't used, floating-point operands get passed in integer registers as if they were integers, and floating-point results get passed in $0 instead of $f0. 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 same option. no mixing bestie, that's the rule 🙅

a typical use of this option is building a kernel that doesn't use — and therefore doesn't need to save and restore — any floating-point registers.

🎀 ieee girlie 🎀

• -mieee — okay so the Alpha architecture implements floating-point hardware optimized for maximum performance. It's mostly compliant with the IEEE floating-point standard. But for full compliance, software assistance is required. This option generates code that's fully IEEE-compliant... except the inexact-flag isn't 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 like not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.

DEBIAN SPECIFIC: this option is on by default for alpha-linux-gnu, unless -ffinite-math-only (which is part of the -ffast-math set) is specified — because the software functions in the GNU libc math libraries generate denormalized numbers, NaNs, and infs, and all of those will cause a programs to SIGFPE when it attempts to use the results without -mieee. so yeah, keep it on fr fr 😭

• -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, _IEEE_FP_EXACT gets defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there's very little code out there that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.

✨ trap mode time ✨

• *-mfp-trap-mode=trap-mode* — this option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values:
  • n — this is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
  • u — in addition to the traps enabled by n, underflow traps are enabled as well.
  • su — like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
  • sui — like su, but inexact traps are enabled as well.

🌀 rounding mode 🌀

• *-mfp-rounding-mode=rounding-mode* — selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:
  • n — normal IEEE rounding mode. Floating-point numbers get rounded towards the nearest machine number, or towards the even machine number in case of a tie.
  • m — round towards minus infinity.
  • c — chopped rounding mode. Floating-point numbers get 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. So unless your program modifies the fpcr, d corresponds to round towards plus infinity.

🔥 trap precision 🔥

• *-mtrap-precision=trap-precision* — in the Alpha architecture, floating-point traps are imprecise. This means that without software assistance it's 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 precision 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. 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. that's literally it, one little line 💅
• -mbuild-constants — normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time.

Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).

You typically use this option to build a shared library dynamic loader. Itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. iconic honestly 🌈

🦋 instruction sets 🦋

• -mbwx, -mno-bwx
• -mcix, -mno-cix
• -mfix, -mno-fix
• -mmax, -mno-max — these indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via the -mcpu= option — or that of the CPU on which GCC was built if none is specified.
• -msafe-bwa, -mno-safe-bwa — indicate whether, in the absence of the optional BWX instruction set, GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses.
• -msafe-partial, -mno-safe-partial — indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses, including piecemeal accesses to unaligned data as well as block accesses to leading and trailing parts of aggregate types or other objects in memory that do not respectively start and end on an aligned 64-bit data boundary. (that sentence is a whole journey, we love her tho 🫶)

🌟 floats, relocs, data 🌟

• -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 doesn't allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it's built and sets the default accordingly.
• -msmall-data, -mlarge-data — when -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller get placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB, but it lets the variables be directly accessed via a single instruction. speed vs space bestie, the eternal saga 🎀

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. (yes bestie, 2GB is the ceiling, plan accordingly 🫠)

When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data. the way they're just... married like that 💅

• -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. (4 ➡️ 1?? ate. ✨)

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. (scheduling glow-up only, no ISA surgery 💅)

Native toolchains also support the value native, which selects the best architecture option for the host processor. -mtune=native has no effect if GCC does not recognize the processor.

• -mmemory-latency=time — Sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns used by the application and the size of the external cache on the machine.

Valid options for time are

• number — A decimal number representing clock cycles.
• L1, L2, L3, main — The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5.

---

🌈 eBPF Options 🌈

• -mframe-limit=bytes — This specifies the hard limit for frame sizes, in bytes. Currently, the value that can be specified should be less than or equal to 32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted.
• -mbig-endian — Generate code for a big-endian target.
• -mlittle-endian — Generate code for a little-endian target. This is the default.
• -mjmpext, -mno-jmpext — Enable or disable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.
• -mjmp32, -mno-jmp32 — Enable or disable generation of 32-bit jump instructions. Enabled for CPU v3 and above.
• -malu32, -mno-alu32 — Enable or disable generation of 32-bit ALU instructions. Enabled for CPU v3 and above.
• -mv3-atomics, -mno-v3-atomics — Enable or disable instructions for general atomic operations introduced in CPU v3. Enabled for CPU v3 and above.
• -mbswap, -mno-bswap — Enable or disable byte swap instructions. Enabled for CPU v4 and above.
• -msdiv, -mno-sdiv — Enable or disable signed division and modulus instructions. Enabled for CPU v4 and above.
• -msmov, -mno-smov — Enable or disable sign-extending move and memory load instructions. Enabled for CPU v4 and above. (that's spelled exactly like that in the source, bestie, write it down character-for-character 📝)
• -mcpu=version — This specifies which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The default is v4.

Supported values for version are:

1. v1 — The first stable eBPF ISA with no special features or extensions.
2. v2 — Supports the jump extensions, as in -mjmpext.
3. 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
4. v4 — All features of v3, plus:
   • Byte swap instructions, as in -mbswap
   • Signed division and modulus instructions, as in -msdiv
   • Sign-extending move and memory load instructions, as in -msmov

• -mco-re — Enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and is implied by -gbtf.
• -mno-co-re — Disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target.
• -mxbpf — Generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  • Save and restore callee-saved registers at function entry and exit, respectively.
• -masm=dialect — Outputs assembly instructions using eBPF selected dialect. The default is pseudoc.

Supported values for dialect are:

• normal — Outputs normal assembly dialect.
• pseudoc — Outputs pseudo-c assembly dialect.
• -minline-memops-threshold=bytes — Specifies a size threshold in bytes at or below which memmove, memcpy and memset shall always be expanded inline. Operations dealing with sizes larger than this threshold would have to be implemented using a library call instead of being expanded inline, but since BPF doesn't allow libcalls, exceeding this threshold results in a compile-time error. The default is 1024 bytes.

⚠️ okay so real talk: over that threshold and you don't get a slow path, you get a whole compile-time error. no libcalls for you, bestie. respect the limit or it's over 💀

---

🍓 FR30 Options 🍓

These options are defined specifically for the FR30 port.

• -msmall-model — Use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range.
• -mno-lsim — Assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the linker command line.

---

🧚 FT32 Options 🧚

These options are defined specifically for the FT32 port.

• -msim — Specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked. You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed.
• -mlra — Does nothing. Preserved for backward compatibility. (iconic behaviour honestly, just vibing there for legacy reasons 🫶)
• -mnodiv — Do not use div and mod instructions.
• -mft32b — Enable use of the extended instructions of the FT32B processor.
• -mcompress — Compress all code using the Ft32B code compression scheme.
• -mnopm — Do not generate code that reads program memory.

---

💜 FRV Options 💜

• -mgpr-32 — Only use the first 32 general-purpose registers.
• -mgpr-64 — Use all 64 general-purpose registers.
• -mfpr-32 — Use only the first 32 floating-point registers.
• -mfpr-64 — Use all 64 floating-point registers.
• -mhard-float — Use hardware instructions for floating-point operations.
• -msoft-float — Use library routines for floating-point operations.
• -malloc-cc — Dynamically allocate condition code registers.
• -mfixed-cc — Do not try to dynamically allocate condition code registers, only use icc0 and fcc0.
• -mdword — Change ABI to use double word insns.
• -mno-dword — Do not use double word instructions.
• -mdouble — Use floating-point double instructions.
• -mno-double — Do not use floating-point double instructions.
• -mmedia — Use media instructions.
• -mno-media — Do not use media instructions.
• -mmuladd — Use multiply and add/subtract instructions.
• -mno-muladd — Do not use multiply and add/subtract instructions.
• -mfdpic — Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim.
• -minline-plt — Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
• -mTLS — Assume a large TLS segment when generating thread-local code.
• -mtls — Do not assume a large TLS segment when generating thread-local code.
• -mgprel-ro — Enable the use of GPREL relocations in the FDPIC ABI for data that is known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
• -multilib-library-pic — Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly. (so like... it exists, and the vibe is "you will literally never type this" 😌)
• -mlinked-fp — bestie, this one follows the EABI requirement of always creating a frame pointer whenever a stack frame gets allocated. It's on by default 💅 and you can turn it off with -mno-linked-fp.
• -mlong-calls — uses indirect addressing to call functions outside the current compilation unit. The whole point? It lets those functions live anywhere within the 32-bit address space. freedom, iconic ✨
• -malign-labels — tries to align labels to an 8-byte boundary by inserting NOPs into the previous packet. Heads up tho: this only does anything when VLIW packing is enabled. And it does not create new packets — it merely adds NOPs to ones that already exist. lowkey humble of it 🎀
• -mlibrary-pic — generates position-independent EABI code.
• -macc-4 — use only the first four media accumulator registers. the ✨minimalist✨ era.
• -macc-8 — use all eight media accumulator registers. go big or go home 💖
• -mpack — pack VLIW instructions.
• -mno-pack — do not pack VLIW instructions. we love a boundary queen.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move — enable the use of conditional-move instructions (this is the default).
• -mno-cond-move — disable the use of conditional-move instructions.

fr fr, both -mcond-move and -mno-cond-move are mainly for debugging the compiler and will likely be removed in a future version. ⚠️

• -mscc — enable the use of conditional set instructions (default).
• -mno-scc — disable the use of conditional set instructions.

same warning as above for both of these: mainly for debugging the compiler, likely yeeted in a future version 💀

• -mcond-exec — enable the use of conditional execution (default).
• -mno-cond-exec — disable the use of conditional execution.

yep, both of these too — mainly for debugging the compiler, likely removed in a future version 🙈

• -mvliw-branch — run a pass to pack branches into VLIW instructions (default).
• -mno-vliw-branch — do not run a pass to pack branches into VLIW instructions.

guess what bestie — debugging-only for both, likely gone in a future version 🫶

• -mmulti-cond-exec — enable optimization of "&&" and "||" in conditional execution (default).
• -mno-multi-cond-exec — disable optimization of "&&" and "||" in conditional execution.

again, both: mainly for debugging the compiler, likely removed in a future version 🌸

• -mnested-cond-exec — enable nested conditional execution optimizations (default).
• -mno-nested-cond-exec — disable nested conditional execution optimizations.

and once more with feeling: debugging-only, likely removed in a future version 💜

• -moptimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. It's enabled by default.
• -mno-optimize-membar — this switch disables the automatic removal of redundant "membar" instructions from the generated code.
• -mtomcat-stats — cause gas to print out tomcat statistics. 🐱 stats?? slay.
• *-mcpu=cpu* — select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple. pick your fighter 👑

---

🐧 GNU/Linux Options

These -m options are defined for GNU/Linux targets:

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
• -mmusl — use the musl C library. This is the default on *-*-linux-*musl* targets.
• -mbionic — use Bionic C library. This is the default on *-*-linux-*android* targets.
• -mandroid — compile code compatible with the Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro "__ANDROID__" to be defined. 🤖 iconic behavior honestly

• -tno-android-cc — disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.

---

H8/300 Options

These -m options are defined for the H8/300 implementations:

• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax. ✨efficiency✨
• -mh — generate code for the H8/300H.
• -ms — generate code for the H8S.
• -mn — generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms. no solo flights bestie 🚫
• -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
• -mexr — extended registers are stored on stack before execution of function with monitor attribute. The default option is -mexr. This option is valid only for H8S targets.
• -mno-exr — extended registers are not stored on stack before execution of function with monitor attribute. The default option is -mno-exr. This option is valid only for H8S targets.
• -mint32 — make "int" data 32 bits by default.
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300. 💅

---

HPPA Options

These -m options are defined for the HPPA family of computers:

• -march=architecture-type — generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. 📋 Important caveat: code compiled for lower-numbered architectures runs on higher-numbered architectures, but **not the other way around. one-way street, no cap.
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0 — synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively. same vibe, different spelling 💖
• -matomic-libcalls — generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default. It only affects the generation of atomic libcalls by the HPPA backend.

Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations, provided they are implemented as a single access.

The PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction. So basically all atomic support is implemented using sync and atomic libcalls. Sync libcall support lives in libgcc.a. Atomic libcall support lives in libatomic. 🫶

This option generates "__atomic_exchange" calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets.

• -mbig-switch — does nothing. Preserved for backward compatibility. truly the "I'm just here for the vibes" option 😭
• -mcaller-copies — the caller copies function arguments passed by hidden reference. ⚠️ Use this one with care, bestie — it is not compatible with the default 32-bit runtime. That said, only aggregates larger than eight bytes are passed by hidden reference, and the option provides better compatibility with OpenMP.
• -mcoherent-ldcw — use ldcw/ldcd coherent cache-control hint.
• -mdisable-fpregs — disable floating-point registers. Equivalent to "-msoft-float".
• -mdisable-indexing — prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH. obscure problem? not on my watch 🚫
• -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls. 🔥 But heads up: this option does not work in the presence of shared libraries or nested functions. 💀
• *-mfixed-range=register-range* — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. 🎀
• -mgas — enable the use of assembler directives only GAS understands.
• -mgnu-ld — use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld``. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*. 💅
• -mhp-ld — use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC can be printed using which gcc -print-prog-name=ld``. This option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*. same energy, different linker ✨
• -mlinker-opt — enable the optimization pass in the HP-UX linker. ⚠️ Note: this makes symbolic debugging impossible. pick your struggle, bestie 😭
• -mlong-calls — okay so this one tells GCC to generate long call sequences, so a call can always reach linker generated stubs. 💅 By default it only does long calls when the distance from the call site to the beginning of the function or translation unit (whichever applies) goes over a predefined limit set by the branch type being used. For normal calls those limits are 7,600,000 and 240,000 bytes — respectively for PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes. Distances are measured from the beginning of functions when you use -ffunction-sections, or when you use -mgas and -mno-portable-runtime together under HP-UX with the SOM linker. Normally you do not want this option because it degrades performance 😭, but it can be useful in large applications, especially when partial linking is used to build the application. The types of long calls used depend on what the assembler and linker can do, and on 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. However, an indirect call is used on 32-bit ELF systems in pic code and it is quite long. 🫠
• -mlong-load-store — generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the +k option to the HP compilers. ✨
• -mjump-in-delay — this option is ignored and provided for compatibility purposes only. 🙃
• -mno-space-regs — generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls and use unscaled index address modes. Such code is suitable for level 0 PA systems and kernels. 👑
• -mordered — assume memory references are ordered and barriers are not needed. 🧘
• -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems. 🎀
• *-mschedule=cpu-type* — schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is 8000. 🗓️
• -msio — generate the predefine, _SIO, for server IO. The default is -mwsio. This generates the predefines, __hp9000s700, __hp9000s700__ and _WSIO, for workstation IO. These options are available under HP-UX and HI-UX. 🖥️
• -msoft-float — generate output containing library calls for floating point. Warning: the requisite libraries are not available for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. -msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. 😬
• -msoft-mult — use software integer multiplication. This disables the use of the xmpyu instruction. ✖️
• -munix=unix-std — generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 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, and the startfile unix95.o. -munix=98 provides additional predefines for _XOPEN_UNIX, _XOPEN_SOURCE_EXTENDED, _INCLUDE__STDC_A1_SOURCE and _INCLUDE_XOPEN_SOURCE_500, and the startfile unix98.o. **Important note: this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option. Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable __xpg4_extended_mask as appropriate. Most GNU software doesn't provide this capability. ⚠️
• -nolibdld — suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later. 🚫
• -static — the HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve this dependency. On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these link options. 🔗
• -threads — add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and linker. 🧵

---

✨ IA-64 Options ✨

These are the -m options defined for the Intel IA-64 architecture. 💜

• -mbig-endian — generate code for a big-endian target. This is the default for HP-UX. 🐘
• -mlittle-endian — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux. 🐭
• -mgnu-as, -mno-gnu-as — generate (or don't) code for the GNU assembler. This is the default. 🛠️
• -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker. This is the default. 🔗
• -mno-pic — generate code that does not use a global pointer register. The result is not position independent code, and violates the IA-64 ABI. 🚨
• -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don't) a stop bit immediately before and after volatile asm statements. 🛑
• -mregister-names, -mno-register-names — generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable. 🔤
• -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs. 🐛
• -mconstant-gp — generate code that uses a single constant global pointer value. This is useful when compiling kernel code. 🧠
• -mauto-pic — generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware code. 🧲
• -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm. ⏳
• -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm. 🚀
• -mno-inline-float-divide — do not generate inline code for divides of floating-point values. 🙅
• -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm. ⏳
• -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm. 🚀
• -mno-inline-int-divide — do not generate inline code for divides of integer values. 🙅
• -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm. ⏳
• -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm. 🚀
• -mno-inline-sqrt — do not generate inline code for sqrt. 🚫
• -mfused-madd, -mno-fused-madd — do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to use these instructions. ➗
• -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not using the GNU assembler. 🧬
• -mearly-stop-bits, -mno-early-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but does not always do so. ⏱️
• *-mfixed-range=register-range* — generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. 📌
• *-mtls-size=tls-size* — specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64. 📏
• *-mtune=cpu-type* — tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and mckinley. 🎛️
• -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags. 🧮
• -mno-sched-br-data-spec, -msched-br-data-spec — (Dis/En)able data speculative scheduling before reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is disabled. 🚦
• -msched-ar-data-spec, -mno-sched-ar-data-spec — (En/Dis)able data speculative scheduling after reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is enabled. 🚥
• -mno-sched-control-spec, -msched-control-spec — (Dis/En)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the ld.s instructions and the corresponding check instructions chk.s. The default setting is disabled. 🛂
• -msched-br-in-data-spec, -mno-sched-br-in-data-spec — turns speculative scheduling of the insns that depend on data speculative loads before reload on (or off). Only actually does anything if -msched-br-data-spec is enabled, bestie. Default? enabled. 💅
• -msched-ar-in-data-spec, -mno-sched-ar-in-data-spec — same deal but for the insns dependent on data speculative loads after reload. Only works when -msched-ar-data-spec is on. Default is enabled. ✨
• -msched-in-control-spec, -mno-sched-in-control-spec — speculative scheduling of insns dependent on control speculative loads, on/off. Needs -msched-control-spec enabled to do anything. Default: enabled. 🦋
• -mno-sched-prefer-non-data-spec-insns, -msched-prefer-non-data-spec-insns — if you flip this on, data-speculative insns only get picked for the schedule when there's literally no other choice available. Makes the whole data speculation thing way more conservative. Default is disabled.
• -mno-sched-prefer-non-control-spec-insns, -msched-prefer-non-control-spec-insns — same energy but for control-speculative insns: they only get scheduled if nothing else is available. Super conservative control speculation. Default is disabled. 🙈
• -mno-sched-count-spec-in-critical-path, -msched-count-spec-in-critical-path — when enabled, speculative dependencies get counted while computing instruction priorities. Makes speculation a lil more conservative. Default is disabled. 😌
• -msched-spec-ldc — uses a simple data speculation check. On by default, no cap. 🔥
• -msched-control-spec-ldc — uses a simple check for control speculation. Also on by default. 💖
• -msched-stop-bits-after-every-cycle — drops a stop bit after every cycle when scheduling. On by default. 🎀
• -msched-fp-mem-deps-zero-cost — assumes floating-point stores and loads probably won't conflict when they land in the same instruction group. Disabled by default. 💀
• -msel-sched-dont-check-control-spec — generates checks for control speculation in selective scheduling. This flag is disabled by default. 🌸
• *-msched-max-memory-insns=max-insns* — caps how many memory insns you get per instruction group, giving lower priority to later memory insns that try to squeeze into the same group. Lowkey a lifesaver for avoiding cache bank conflicts. Default value is 1. 👑
• -msched-max-memory-insns-hard-limit — makes the limit from msched-max-memory-insns a hard limit, so no more than that number is allowed in an instruction group, periodt. Without this, the limit is "soft" — meaning non-memory operations get preferred once you hit it, but memory operations can still sneak in. 🚫

---

🐄 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. Heads up bestie: -march=arch-type lets GCC generate code that might not run at all on processors other than the one you named. Not giving "works everywhere". Giving "works here". 😭

The choices for arch-type are:

• native — local processor type detected by the native compiler.
• loongarch64 — generic LoongArch 64-bit processor.
• la464 — LoongArch LA464-based processor with LSX, LASX.
• la664 — LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.
• la64v1.0 — LoongArch64 ISA version 1.0.
• la64v1.1 — LoongArch64 ISA version 1.1.

More info about LoongArch ISA versions lives at https://github.com/loongson/la-toolchain-conventions. 📚

• *-mtune=tune-type* — optimize the generated code for the given processor target.

The choices for tune-type are:

• native — local processor type detected by the native compiler.
• generic — generic LoongArch processor.
• loongarch64 — generic LoongArch 64-bit processor.
• la464 — LoongArch LA464 core.
• la664 — LoongArch LA664 core.

• *-mabi=base-abi-type* — generate code for the specified calling convention. base-abi-type can be one of:
  • lp64d — uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64f — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64s — uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
• *-mfpu=fpu-type* — generate code for the specified FPU type, which can be one of:
  • 64 — allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.
  • 32 — allow the use of hardware floating-point instructions for 32-bit operations.
  • none, 0 — prevent the use of hardware floating-point instructions. 🚫
• *-msimd=simd-type* — enable generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:
  • lasx — enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).
  • lsx — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
  • none — no LoongArch SIMD instruction may be generated.
• -msoft-float — forces -mfpu=none and blocks floating-point registers from being used for parameter passing. Ngl this option may change the target ABI. ⚠️
• -msingle-float — forces -mfpu=32 and allows 32-bit floating-point registers for parameter passing. This option may change the target ABI. ⚠️
• -mdouble-float — forces -mfpu=64 and allows 32/64-bit floating-point registers for parameter passing. This option may change the target ABI. ⚠️
• -mlasx, -mno-lasx, -mlsx, -mno-lsx — incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that the compiler can use for code generation. Enabling LASX with mlasx automatically enables LSX, and diabling LSX with mno-lsx automatically disables LASX. These are driver-only options, bestie: 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. The way that it's order-dependent is so real. 🦋
• *-mbranch-cost=n* — set the cost of branches to roughly n instructions.
• *-maddr-reg-reg-cost=n* — set the cost of ADDRESS_REG_REG to the value calculated by n.
• -mcheck-zero-division, -mno-check-zero-divison — trap (or don't trap) on integer division by zero. Default is -mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division for other optimization levels. 💅
• -mcond-move-int, -mno-cond-move-int — conditional moves for integral data in general-purpose registers are enabled (or disabled). Default is -mcond-move-int.
• -mcond-move-float, -mno-cond-move-float — conditional moves for floating-point registers are enabled (or disabled). Default is -mcond-move-float.
• -mmemcpy, -mno-memcpy — force (or don't force) the use of memcpy for non-trivial block moves. Default is -mno-memcpy, which lets GCC inline most constant-sized copies. Setting the optimization level to -Os also forces the use of memcpy, but -mno-memcpy may override this behavior if explicitly specified — regardless of the order of these options on the command line. 🙈
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. Default is -mno-strict-align.
• -G num — put global and static data smaller than num bytes into a small data section. Default value is 0.
• *-mmax-inline-memcpy-size=n* — inline all block moves (like calls to memcpy or structure copies) less than or equal to n bytes. Default value of n is 1024. 🔥
• *-mcmodel=code-model* — set the code model to one of:
  • tiny-static — (Not implemented yet) 😭
  • tiny — (Not implemented yet) 😭
  • normal — the text segment must be within 128MB addressing space. The data segment must be within 2GB addressing space.
  • medium — the text segment and data segment must be within 2GB addressing space.
  • large — (Not implemented yet) 😭
  • extreme — this mode does not limit the size of the code segment and data segment. The -mcmodel=extreme option is incompatible with -fplt and/or -mexplicit-relocs=none. 🚫

The default code model is normal. ✨

• -mexplicit-relocs=style — okay bestie, this one decides when to use assembler relocation operators when you're dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation. With -mexplicit-relocs=none the assembler macros are always used, with -mexplicit-relocs=always the assembler relocation operators are always used, and with -mexplicit-relocs=auto the compiler will use the relocation operators where the linker relaxation is impossible to improve the code quality, and macros elsewhere. The default value for the option is determined with the assembler capability detected during GCC build-time and the setting of -mrelax: -mexplicit-relocs=none if the assembler does not support relocation operators at all, -mexplicit-relocs=always if the assembler supports relocation operators but -mrelax is not enabled, and -mexplicit-relocs=auto if the assembler supports relocation operators and -mrelax is enabled. 💅
• -mexplicit-relocs — An alias of -mexplicit-relocs=always for backward compatibility. periodt.
• -mno-explicit-relocs — An alias of -mexplicit-relocs=none for backward compatibility. periodt.
• -mdirect-extern-access, -mno-direct-extern-access — Do not use or use GOT to access external symbols. The default is -mno-direct-extern-access: GOT is used for external symbols with default visibility, but not used for other external symbols. With -mdirect-extern-access, GOT is not used and all external symbols are PC-relatively addressed. It is only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with -static or -static-pie. -mdirect-extern-access is not compatible with -fPIC or -fpic. <em>like, do not even try it, bestie.</em>
• -mrelax, -mno-relax — Take (do not take) advantage of linker relaxations. If -mpass-mrelax-to-as is enabled, this option is also passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: -mrelax if the assembler supports both the -mrelax option and the conditional branch relaxation (it's required or the .align directives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow), -mno-relax otherwise. 🌈
• -mpass-mrelax-to-as, -mno-pass-mrelax-to-as — Pass (do not pass) the -mrelax or -mno-relax option to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the -mrelax option, -mno-pass-mrelax-to-as otherwise. This option is mostly useful for debugging, or interoperation with assemblers different from the build-time one. ✨
• -mrecip — This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. This option is off by default. Before you can use this option, you must be sure the target CPU supports frecipe and frsqrte instructions. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). 😭
• -mrecip=opt — This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a ! to invert the option:
  • all — Enable all estimate instructions.
  • default — Enable the default instructions, equivalent to -mrecip.
  • none — Disable all estimate instructions, equivalent to -mno-recip.
  • div — Enable the approximation for scalar division.
  • vec-div — Enable the approximation for vectorized division.
  • sqrt — Enable the approximation for scalar square root.
  • vec-sqrt — Enable the approximation for vectorized square root.
  • rsqrt — Enable the approximation for scalar reciprocal square root.
  • vec-rsqrt — Enable the approximation for vectorized reciprocal square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for scalar square root. 🔥

• -mfrecipe, -mno-frecipe — Use (do not use) frecipe.{s/d} and frsqrte.{s/d} instructions. When build with -march=la664, it is enabled by default. The default is -mno-frecipe.
• -mdiv32, -mno-div32 — Use (do not use) div.w[u] and mod.w[u] instructions with input not sign-extended. When build with -march=la664, it is enabled by default. The default is -mno-div32.
• -mlam-bh, -mno-lam-bh — Use (do not use) am{swap/add}[_db].{b/h} instructions. When build with -march=la664, it is enabled by default. The default is -mno-lam-bh.
• -mlamcas, -mno-lamcas — Use (do not use) amcas[_db].{b/h/w/d} instructions. When build with -march=la664, it is enabled by default. The default is -mno-lamcas.
• -mld-seq-sa, -mno-ld-seq-sa — Whether a same-address load-load barrier (dbar 0x700) is needed. When build with -march=la664, it is enabled by default. The default is -mno-ld-seq-sa, the load-load barrier is needed. 👑
• -mtls-dialect=opt — This option controls which tls dialect may be used for general dynamic and local dynamic TLS models.
  • trad — Use traditional TLS. This is the default.
  • desc — Use TLS descriptors.
• -mannotate-tablejump, -mno-annotate-tablejump — Create 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 analysis the control flow. The default is -mno-annotate-tablejump.
• --param loongarch-vect-unroll-limit=n — The vectorizer will use available tuning information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter set's the upper bound of how much the vectorizer will unroll the main loop. The default value is six. 🦋

---

M32C Options 🎀

• -mcpu=name — Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
• -msim — Specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, for example, file I/O. 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 — Align all loops to a 32-byte boundary.
• -mno-align-loops — Do not enforce a 32-byte alignment for loops. This is the default.
• -missue-rate=number — Issue number instructions per cycle. number can only be 1 or 2.
• -mbranch-cost=number — number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2, then the opposite applies. ✨
• -mflush-trap=number — says which trap number we're gonna use to flush the cache. Default is 12, and the vibes-legal range is 0 through 15, inclusive. No exceptions bestie 💅
• -mno-flush-trap — announces that the cache literally cannot be flushed by a trap. That's it, that's the whole thing. 🙈
• -mflush-func=name — tells GCC the name of the OS function to call when flushing the cache. Default is _flush_cache, but — and this is the important lil caveat — a function call only gets used if a trap isn't available. 🎀
• -mno-flush-func — swears up and down that there is no OS function for flushing the cache. F in the chat for the cache 🫶

---

💖 M680x0 Options 💖

Okay so these are the -m options that exist for M680x0 and ColdFire processors, and the defaults? They fully depend on which architecture got picked when the compiler was configured. The defaults for the most common choices are listed below, no cap. 👑

• -march=arch — generates code for one specific M680x0 or ColdFire instruction set architecture. The permissible arch values for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures get selected according to Freescale's ISA classification, and the permissible values there are: isaa, isaaplus, isab and isac. ✨

GCC defines a macro __mcfarch__ whenever it's generating code for a ColdFire target, and the arch in that macro is one of the -march arguments listed above. The macro knows what it's doing, iykyk.

When you use -march and -mtune together, they select code that runs on a whole family of similar processors but is optimized for one particular microarchitecture. It's giving "one size fits most, tailored for one" 🧚

• -mcpu=cpu — generates 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. Any other combination of -mcpu and -march gets rejected. Straight up. 🚫

GCC defines the macro __mcf_cpu_cpu when a ColdFire target cpu is selected, and it also defines __mcf_family_family, where family comes from that table above. 💜

• -mtune=tune — tunes the code for a particular microarchitecture, while staying inside 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. Twins but make it codegen ✨

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 — like the ones selected by -mtune=68020-40 or -mtune=68020-60 — it defines the macros for every architecture in the range. Overachiever behavior honestly 😭

GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above.

• -m68000
• -mc68000 — generates output for a 68000. This is the default when the compiler is configured for 68000-based systems, and it's equivalent to -march=68000. 🎀

Use this one for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. That's a whole roster fr fr 💅

• -m68010 — generates output for a 68010. Default when the compiler is configured for 68010-based systems, and equivalent to -march=68010.
• -m68020
• -mc68020 — generates output for a 68020. Default when the compiler is configured for 68020-based systems, and equivalent to -march=68020.
• -m68030 — generates output for a 68030. Default when the compiler is configured for 68030-based systems, and equivalent to -march=68030.
• -m68040 — generates output for a 68040. Default when the compiler is configured for 68040-based systems, and equivalent to -march=68040. 🌸

Heads up tho: this option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. So use it if your 68040 doesn't have the code to emulate those instructions. Know your machine bestie 🔍

• -m68060 — generates output for a 68060. Default when the compiler is configured for 68060-based systems, and equivalent to -march=68060.

And same energy here: this one inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use it if your 68060 doesn't have code to emulate those instructions.

• -mcpu32 — generates output for a CPU32. Default when the compiler is configured for CPU32-based systems, and equivalent to -march=cpu32. 💖

Use this for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. Look at that lineup 🫶

• -m5200 — generates output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems, and it's equivalent to -mcpu=5206 — and it is now deprecated in favor of that option. 🫠

Use this for a microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

• -m5206e — generates output for a 5206e ColdFire CPU. This option is now deprecated in favor of the equivalent -mcpu=5206e. Moving on 💅
• -m528x — generates output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generates output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
• -m5407 — generates output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generates output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and it is now deprecated in favor of that option. 🔥
• -m68020-40 — generates output for a 68040, but without using any of the new instructions. The result is code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040. 🧚

This option is equivalent to -march=68020 -mtune=68020-40.

• -m68020-60 — generates output for a 68060, also without using any of the new instructions. Same deal: code that runs relatively efficiently on either a 68020/68881 or a 68030 or a 68040. And again, the generated code does use the 68881 instructions that are emulated on the 68060.

This option is equivalent to -march=68020 -mtune=68020-60.

• -mhard-float
• -m68881 — generates 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 — does not generate floating-point instructions; it uses library calls instead. This is the default for 68000, 68010, and 68832 targets. It's also the default for ColdFire devices that have no FPU. No hardware, no problem 💅
• -mdiv
• -mno-div — generate (or don't generate) ColdFire hardware divide and remainder instructions. Here's the tea: 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. So for example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e. Context dependent, she adapts 👑

GCC defines the macro __mcfhwdiv__ when this option is enabled.

• -mshort — considers 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. Petite ints, petite alignment 🎀
• -mno-short — does not consider type int to be 16 bits wide. This is the default.
• -mnobitfield
• -mno-bitfield — don't use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield. 🚫
• -mbitfield — do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020. 🌈
• -mrtd — uses a different function-calling convention, where functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller, since there's no need to pop the arguments there. Efficient queen 💅

Okay so real talk, this one has consequences, don't skip this part:

This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 😭

Also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code is generated for calls to those functions. ⚠️

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored. Not here. Here they ruin everything.) 💀

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 — the alignment vibe check 💅 this controls whether GCC lines up int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning on 32-bit boundaries gives you code that runs somewhat faster on processors with 32-bit busses, but bestie it costs you more memory. trade-offs, always.

WARNING, and I mean it 😭: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. so like, ABI chaos. be aware.

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. pick your fighter 🥊
• -msep-data — generate code that lets the data segment chill in a totally different area of memory from the text segment. this allows for execute-in-place in an environment with no virtual memory management. this option implies -fPIC. ✨
• -mno-sep-data — generate code that assumes the data segment follows the text segment. this is the default, no cap.
• -mid-shared-library — generate code that supports shared libraries via the library ID method. this allows for execute-in-place and shared libraries in an environment without virtual memory management. implies -fPIC. 🫶
• -mno-id-shared-library — generate code that doesn't assume ID-based shared libraries are being used. this is the default.
• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option. so like, 0 is the slay 😌
• -mxgot, -mno-xgot — when generating position-independent code for ColdFire, generate code that works even 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. (like, very few bestie)

These options have no effect unless GCC is generating position-independent code. periodt.

• -mlong-jump-table-offsets — use 32-bit offsets in switch tables. the default is to use 16-bit offsets. 🔥

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🌈 MCore Options 🌈

These are the -m options defined for the Motorola M*Core processors. ok bestie, buckle up 🎀

• -mhardlit, -mno-hardlit — inline constants into the code stream if it can be done in two instructions or less. tiny code, big vibes ✨
• -mdiv, -mno-div — use the divide instruction. (Enabled by default).
• -mrelax-immediate, -mno-relax-immediate — allow arbitrary-sized immediates in bit operations.
• -mwide-bitfields, -mno-wide-bitfields — always treat bit-fields as int-sized.
• -m4byte-functions, -mno-4byte-functions — force all functions to be aligned to a 4-byte boundary.
• -mcallgraph-data, -mno-callgraph-data — emit callgraph information.
• -mslow-bytes, -mno-slow-bytes — prefer word access when reading byte quantities.
• -mlittle-endian, -mbig-endian — generate code for a little-endian target.
• -m210, -m340 — generate code for the 210 processor.
• -mno-lsim — assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.
• -mstack-increment=size — set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions 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.
• -mno-clearbss — this option is deprecated. Use -fno-zero-initialized-in-bss instead. we don't do that here anymore bestie 💅
• -mcpu=cpu-type — use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a.
• -mxl-soft-mul — use software multiply emulation (default).
• -mxl-soft-div — use software emulation for divides (default).
• -mxl-barrel-shift — use the hardware barrel shifter. slay 💅
• -mxl-pattern-compare — use pattern compare instructions.
• -msmall-divides — use table lookup optimization for small signed integer divisions.
• -mxl-stack-check — this option is deprecated. Use -fstack-check instead. again, deprecated, move on 🙈
• -mxl-gp-opt — use GP-relative .sdata/.sbss sections.
• -mxl-multiply-high — use multiply high instructions for high part of 32x32 multiply.
• -mxl-float-convert — use hardware floating-point conversion instructions.
• -mxl-float-sqrt — use hardware floating-point square root instruction.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target.
• -mxl-reorder — use reorder instructions (swap and byte reversed load/store).
• -mxl-mode-app-model — select application model app-model. Valid models are:
  • executable — normal executable (default), uses startup code crt0.o.
  • xmdstub — for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.
  • bootstrap — for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
  • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.

Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model. (same thing, old name, we keep it around for the OGs 🫶)

• -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 👑

• -EB — generate big-endian code.
• -EL — generate little-endian code. This is the default for mips*el-*-* configurations.
• -march=arch — generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. ok so 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. (yes that's a lot, it's giving ✨extensive✨) 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 — bestie, this is the optimize for arch button. 💅 Among other things, it 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 (or 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 (or do not use) the MIPS16e2 ASE. 🦋 This option modifies the behavior of the -mips16 option such that it targets the MIPS16e2 ASE.
• -mflip-mips16 — generate MIPS16 code on alternating functions. 👑 This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code. Don't be chaotic with this one, bestie.
• -minterlink-compressed, -mno-interlink-compressed — require (or 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. 😭 iconic.

• -mabicalls, -mno-abicalls — generate (or do not generate) code that is suitable for SVR4-style dynamic objects. 🌸 -mabicalls is the default for SVR4-based systems.
• -mshared, -mno-shared — generate (or 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 do not assume) that the static and dynamic linkers support PLTs and copy relocations. 💅 This option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32.

You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.

• -mxgot, -mno-xgot — lift (or do not lift) the usual restrictions on the size of the global offset table. 🔥

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_MIPS_GOT16 foobar

If this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. 💀

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do.

These options have no effect unless GCC is generating position independent code.

• -mgp32 — assume that general-purpose registers are 32 bits wide. 🦋
• -mgp64 — assume that general-purpose registers are 64 bits wide. 🦋
• -mfp32 — assume that floating-point registers are 32 bits wide. 🌸
• -mfp64 — assume that floating-point registers are 64 bits wide. 🌸
• -mfpxx — do not assume the width of floating-point registers. 🙈
• -mhard-float — use floating-point coprocessor instructions. 💖
• -msoft-float — do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead. 🫶
• -mno-float — equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. ⚠️ This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point "printf" formats). If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. No cap, do not accidentally lie to this flag.
• -msingle-float — assume that the floating-point coprocessor only supports single-precision operations. ✨
• -mdouble-float — assume that the floating-point coprocessor supports double-precision operations. This is the default. 👑
• -modd-spreg, -mno-odd-spreg — enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. 💅 This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, -mno-odd-spreg is set by default.
• -mabs=2008, -mabs=legacy — these options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the "abs.fmt" and "neg.fmt" machine instructions. 😭

By default or when -mabs=legacy is used the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead unless the -ffinite-math-only option has also been specified.

The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations.

• -mnan=2008, -mnan=legacy — these options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data. 🧚

The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.

The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0.

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008. 💜

• -mllsc, -mno-llsc — use (or do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. 🔥 When neither option is specified, GCC uses the instructions if the target architecture supports them.
• -mllsc, -mno-llsc — okay so -mllsc is useful if the runtime environment can emulate the instructions, and -mno-llsc can be useful when you're compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details. 💅
• -mdsp, -mno-dsp — Use (or don't use, bestie) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro __mips_dsp. It also defines __mips_dsp_rev to 1. ✨
• -mdspr2, -mno-dspr2 — Use (or don't use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros __mips_dsp and __mips_dspr2. It also defines __mips_dsp_rev to 2. 🎀
• -msmartmips, -mno-smartmips — Use (or don't use) the MIPS SmartMIPS ASE. 🦋
• -mpaired-single, -mno-paired-single — Use (or don't use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled. 🌸
• -mdmx, -mno-mdmx — Use (or don't use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled. 💅
• -mips3d, -mno-mips3d — Use (or don't use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single. 🫶
• -mmicromips, -mno-micromips — Generate (or don't generate) microMIPS code. MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and "nomicromips" attributes. 😭
• -mmt, -mno-mt — Use (or don't use) MT Multithreading instructions. 🔥
• -mmcu, -mno-mcu — Use (or don't use) the MIPS MCU ASE instructions. 👑
• -meva, -mno-eva — Use (or don't use) the MIPS Enhanced Virtual Addressing instructions. 🌈
• -mvirt, -mno-virt — Use (or don't use) the MIPS Virtualization (VZ) instructions. ⭐
• -mxpa, -mno-xpa — Use (or don't use) the MIPS eXtended Physical Address (XPA) instructions. 💀
• -mcrc, -mno-crc — Use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions. 🙈
• -mginv, -mno-ginv — Use (or don't use) the MIPS Global INValidate (GINV) instructions. 🍓
• -mloongson-mmi, -mno-loongson-mmi — Use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI). 🧚
• -mloongson-ext, -mno-loongson-ext — Use (or don't use) the MIPS Loongson EXTensions (EXT) instructions. 💜
• -mloongson-ext2, -mno-loongson-ext2 — Use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions. ✨
• -mlong64 — Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined. 🎀
• -mlong32 — Force "long", "int", and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller. 🦋

• -msym32, -mno-sym32 — Assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses. 🌸
• -G num — Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.

The default -G option depends on the configuration. 💅

• -mlocal-sdata, -mno-local-sdata — Extend (or don't extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the default for all configurations.

If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. 🫶

• -mextern-sdata, -mno-extern-sdata — Assume (or don't assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations.

If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting.

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. 😭

• -mgpopt, -mno-gpopt — Use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.)

-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. 🔥

• -membedded-data, -mno-embedded-data — Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. 👑
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — Put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction with -membedded-data. 🌈
• -mcode-readable=setting — Specify whether GCC may generate code that reads from executable sections. There are three possible settings:
  • -mcode-readable=yes — Instructions may freely access executable sections. This is the default setting.
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM.
  • -mcode-readable=no — Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM. ⭐
• -msplit-addresses, -mno-split-addresses — Enable (or disable) use of the %hi() and %lo() assembler relocation operators. This option has been superseded by -mexplicit-relocs but is retained for backwards compatibility. 💀
• -mexplicit-relocs=none, -mexplicit-relocs=base, -mexplicit-relocs=pcrel, -mexplicit-relocs, -mno-explicit-relocs — These options control whether explicit relocs (such as %gp_rel) are used. The default value depends on the version of GAS when GCC itself was built.

The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo. 🙈

• -mcheck-zero-division, -mno-check-zero-division — Trap (or don't trap) on integer division by zero.

The default is -mcheck-zero-division. 🍓

• -mdivide-traps, -mdivide-breaks — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division. 🧚

• -mload-store-pairs, -mno-load-store-pairs — Enable (or disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. 💜
• -mstrict-align, -mno-strict-align, -munaligned-access, -mno-unaligned-access — Disable (or enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So -mstrict-align may be needed by kernel. The options -munaligned-access and -mno-unaligned-access are obsoleted, and only for backward-compatible. ✨
• -mmemcpy, -mno-memcpy — Force (or don't force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies. 🎀
• -mlong-calls, -mno-long-calls — Disable (or don't disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but requires the caller and callee to be in the same 256 megabyte segment.

This option has no effect on abicalls code. The default is -mno-long-calls. 🦋

• -mmad, -mno-mad — Enable (or disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA. 🌸
• -mimadd, -mno-imadd — turns the "madd" and "msub" integer instructions on (or off). Default is -mimadd on any architecture that supports "madd" and "msub"… except the 74k, bestie, because on that one they found it literally generated slower code 💀 so it's off there.
• -mfused-madd, -mno-fused-madd — turns on (or off) use of the floating-point multiply-accumulate instructions, when they exist. Default is -mfused-madd.

heads up tho 👀 on the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. That may be undesirable in some circumstances, fr fr. 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 (the ones with a .s suffix) when it's assembling them. No preprocessing, no problem 🫶
• -mfix-24k, -mno-fix-24k — works around the 24K E48 errata (lost data on stores during refill 😭). The workarounds are implemented by the assembler, not by GCC. iykyk.
• -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 for three on the chaos, the R4000 was going through it 🎀

• -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.

just the one this time, we love a low-maintenance chip 💜

• -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 — do not try to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a "nop" instruction there instead. The short loop bug, under certain conditions, causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler, not by GCC.
• -mfix-rm7000, -mno-fix-rm7000 — works around the RM7000 "dmult"/"dmultu" errata. The workarounds are implemented by the assembler, not 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 — 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. clever girl behavior ngl 🧚
• -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.) 🦋
• *-mr10k-cache-barrier=setting* — specifies whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors.

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 💀

This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description, including other potential problems.

One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:

1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. 👑

If the input program contains a function declaration such as:

void foo (void);

then the implementation of "foo" must allow "j foo" and "jal foo" to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same.

The option has three forms:

• -mr10k-cache-barrier=load-store — insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=store — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=none — disable the insertion of cache barriers. This is the default setting.

• -mflush-func=func, **-mno-flush-func — specifies the function to call to flush the I and D caches, or to not call any such function at all. If 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. ✨
• -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 will be generated. The default is -mcompact-branches=optimal.

The -mcompact-branches=never option ensures that compact branch instructions will never be generated.

The -mcompact-branches=always option ensures that a compact branch instruction will be generated if available for MIPS Release 6 onwards. If a compact branch instruction is not available (or pre-R6), a delay slot form of the branch will be used instead.

If it is used for MIPS16/microMIPS targets, it will be just ignored now. The behavior for MIPS16/microMIPS may change in future, since they do have some compact branch instructions. 🙈

The -mcompact-branches=optimal option will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available.

• -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. two pipes when nobody's watching for exceptions, iconic 💅

• -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. 🌈

• -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. ⚠️

• -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 whenever GCC was configured with an assembler and a linker that support the .reloc assembly directive, and -mexplicit-relocs is in effect. But with -mno-explicit-relocs? Then the assembler and the linker can still pull this optimization off all on their own, no compiler help required. teamwork makes the dream work fr 💅

• -mmcount-ra-address, -mno-mcount-ra-address — emit (or, with the no- twin, don't emit) code that lets _mcount mess with the calling function's return address. When it's enabled, this option extends the usual _mcount interface with a shiny new ra-address parameter, which has type intptr_t * and gets passed in register $12. _mcount can then modify the return address by doing both of these:
  • returning the new address in register $31, and
  • storing the new address in *ra-address, assuming ra-address isn't null. The default is -mno-mcount-ra-address.
• -mframe-header-opt, -mno-frame-header-opt — enable (disable) frame header optimization in the o32 ABI. Under the o32 ABI, calling functions allocate 16 bytes on the stack so the called function can write out register arguments. Once this is enabled, the optimization suppresses allocating the frame header whenever it can figure out that it's unused. Heads up bestie: this optimization is off by default at all optimization levels. 😭
• -mlxc1-sxc1, -mno-lxc1-sxc1 — when applicable, enable (disable) generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default. ✨
• -mmadd4, -mno-madd4 — when applicable, enable (disable) generation of 4-operand madd.s, madd.d and related instructions. Also enabled by default. 🔥

---

🦋 MMIX Options 🦋

These ones are defined for the MMIX:

• -mlibfuncs, -mno-libfuncs — say 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 against the rE epsilon register.
• -mabi=mmixware, -mabi=gnu — generate code that passes function parameters and return values which (inside the called function) are seen as registers $0 and up, as opposed to the GNU ABI, which uses global registers $231 and up.
• -mzero-extend, -mno-zero-extend — when reading data from memory in sizes shorter than 64 bits, use (or don't use) zero-extending load instructions by default, instead of sign-extending ones.
• -mknuthdiv, -mno-knuthdiv — make the result of a division that yields 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, it's just that the latter is used almost exclusively. 👑
• -mtoplevel-symbols, -mno-toplevel-symbols — prepend (don't prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
• -melf — generate an executable in the ELF format, rather than the default mmo format that the mmix simulator uses.
• -mbranch-predict, -mno-branch-predict — use (don't use) the probable-branch instructions when static branch prediction points at a probable branch.
• -mbase-addresses, -mno-base-addresses — generate (or don't generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. That register is used for one or more base address requests within the range 0 to 255 from the value the register is holding. This generally leads to short 'n' fast code, but the number of different data items you can address is limited. Which means a program that uses lots of static data may need -mno-base-addresses. 🙈
• -msingle-exit, -mno-single-exit — force (don't force) generated code to have a single exit point in each function.

---

🌸 MN10300 Options 🌸

These -m options are defined for Matsushita MN10300 architectures:

• -mmult-bug — generate code to dodge bugs in the multiply instructions for the MN10300 processors. This is the default.
• -mno-mult-bug — don't generate code to avoid bugs in the multiply instructions for the MN10300 processors.
• -mam33 — generate code using features specific to the AM33 processor.
• -mno-am33 — don't generate code using features specific to the AM33 processor. This is the default.
• -mam33-2 — generate code using features specific to the AM33/2.0 processor.
• -mam34 — generate code using features specific to the AM34 processor.
• -mtune=cpu-type — use the timing characteristics of the given CPU type when scheduling instructions. It does **not change the targeted processor type. The CPU type has to be one of mn10300, am33, am33-2 or am34.
• -mreturn-pointer-on-d0 — when generating a function that returns a pointer, return that pointer in both a0 and d0. Otherwise the pointer comes back only in a0, and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to turn it off.
• -mno-crt0 — don't link in the C run-time initialization object file.
• -mrelax — tell the linker it should run a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only does anything when it's used on the command line for the final link step.

Warning bestie: this option makes symbolic debugging impossible. 💀

• -mliw — let the compiler generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __LIW__.
• -mno-liw — don't let the compiler generate Long Instruction Word instructions. This option defines the preprocessor macro __NO_LIW__.
• -msetlb — let the compiler generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __SETLB__.
• -mno-setlb — don't let the compiler generate SETLB or Lcc instructions. This option defines the preprocessor macro __NO_SETLB__.

---

🧚 Moxie Options 🧚

• -meb — generate big-endian code. This is the default for moxie-*-* configurations.
• -mel — generate little-endian code.
• -mmul.x — generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations.
• -mno-crt0 — don't link in the C run-time initialization object file.

---

🍓 MSP430 Options 🍓

These options are defined for the MSP430:

• -masm-hex — force assembly output to always use hex constants. Normally these constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes. 💅
• -mmcu= — pick the MCU to target. This is used to create a C preprocessor symbol based on the MCU name, converted to upper case and pre- and post-fixed with __. That symbol is in turn used by the msp430.h header file to pick an MCU-specific supplementary header file.

The option also sets which ISA to use. If the MCU name is one known to only support the 430 ISA, then that's what gets selected; otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly, the generic msp430x MCU name selects the 430X ISA.

On top of that, an MCU-specific linker script gets added to the linker command line. The script's name is the MCU name 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. But an external devices.csv file can be used to extend device support beyond the hard-coded ones. 👀

GCC searches for the devices.csv file using the following methods, in the given precedence order — first takes precedence over second, which takes precedence over third.

1. 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.
2. 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 of 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.
3. The msp430-elf{,bare}/include/devices directory — finally, GCC checks msp430-elf{,bare}/include/devices from the toolchain root directory. This directory doesn't exist in a default installation, but if the user has created it and copied devices.csv in there, then the MCU data gets read. Same as above: this directory is also registered as an include path and linker library path.

If none of these search methods find devices.csv, then the hard-coded MCU data is used.

• -mwarn-mcu, -mno-warn-mcu — this option enables or disables warnings about conflicts between the MCU name given by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default.
• -mcpu= — specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is deprecated. The -mmcu= option should be used to select the ISA.
• -msim — link to the simulator runtime libraries and linker script. Overrides any scripts that would have been selected by the -mmcu= option.
• -mlarge — use large-model addressing (20-bit pointers, 20-bit size_t).
• -msmall — use small-model addressing (16-bit pointers, 16-bit size_t).
• -mrelax — this option is passed to the assembler and the linker, and lets the linker perform certain optimizations that can't be done until the final link. ⭐
• mhwmult= — tells everyone what kind of hardware multiply the target actually has, bestie. Accepted values: none (literally no hardware multiply, we suffer), 16bit (the OG 16-bit-only multiply that early MCUs rocked), 32bit (the 16/32-bit multiply later MCUs came through with), and f5series (the 16/32-bit multiply of the F5-series MCUs). There's also auto, which is the main character ✨ — it tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. And if no -mmcu option is specified or if the MCU name is not recognized? Then no hardware multiply support is assumed, periodt. "auto" is the default setting, no cap.

Hardware multiplies are normally performed by calling a library routine, which saves space in the generated code (skinny code era). BUT when compiling at -O3 or higher, the hardware multiplier is invoked inline instead — that makes for bigger, but faster code. Trade-offs, we love to see it 💅

The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. So they're safe to use inside interrupt handlers as well as in normal code. Iconic behaviour honestly 🫶

• -minrt — enables the use of a minimum runtime environment: no static initializers, no constructors, none of it. Intended for memory-constrained devices where every byte is fighting for its life. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. Very "trust the process".
• -mtiny-printf — enables reduced code size "printf" and "puts" library functions. Heads up though: the tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications 😭

Support for streams has been removed, and the string to be printed will always be sent to stdout via the "write" syscall. The string is not buffered before it is sent to write. Direct. No filter.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.

• -mmax-inline-shift= — takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an mspabi helper function is used instead. The default value is 4.

This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead. Math girlies stay winning 🎀

• -mcode-region=
• -mdata-region= — these options tell the compiler where to place functions and data that do not have one of the "lower", "upper", "either" or "section" attributes. Possible values are "lower", "upper", "either" or "any". The first three behave like the corresponding attribute. The fourth possible value — "any" — is the default, and it leaves placement entirely up to the linker script and how it assigns the standard sections (".text", ".data", etc) to the memory regions. Let the linker have its moment, honestly.
• -msilicon-errata= — this option passes on a request to assembler to enable the fixes for the named silicon errata. Fixing the hardware's oopsies for it ✨
• -msilicon-errata-warn= — this option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied. A little heads-up text, as a treat 💖
• -mwarn-devices-csv
• -mno-warn-devices-csv — warn if devices.csv is not found or there are problem parsing it (default: on).

---

🌸 NDS32 Options 🌸

These options are defined for NDS32 implementations:

• -mbig-endian — generate code in big-endian mode.
• -mlittle-endian — generate code in little-endian mode.
• -mreduced-regs — use reduced-set registers for register allocation.
• -mfull-regs — use full-set registers for register allocation.
• -mcmov — generate conditional move instructions.
• -mno-cmov — do not generate conditional move instructions.
• -mext-perf — generate performance extension instructions.
• -mno-ext-perf — do not generate performance extension instructions.
• -mext-perf2 — generate performance extension 2 instructions.
• -mno-ext-perf2 — do not generate performance extension 2 instructions.
• -mext-string — generate string extension instructions.
• -mno-ext-string — do not generate string extension instructions.
• -mv3push — generate v3 push25/pop25 instructions.
• -mno-v3push — do not generate v3 push25/pop25 instructions.
• -m16-bit — generate 16-bit instructions.
• -mno-16-bit — do not generate 16-bit instructions.
• *-misr-vector-size=num* — specify the size of each interrupt vector, which must be 4 or 16.
• *-mcache-block-size=num* — specify the size of each cache block, which must be a power of 2 between 4 and 512.
• *-march=arch* — specify the name of the target architecture.
• *-mcmodel=code-model* — set the code model to one of:
  • small — all the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.
  • medium — the data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. The text segment should be still within 16MB addressing space.
  • large — all the text and data segments can be within 4GB addressing space.
• -mctor-dtor — enable constructor/destructor feature.
• -mrelax — guide linker to relax instructions.

---

👑 Nvidia PTX Options 👑

These options are defined for Nvidia PTX:

• -m64 — ignored, but preserved for backward compatibility. Only 64-bit ABI is supported. It's giving legacy.
• *-march=architecture-string* — generate code for the specified PTX ISA target architecture. Valid architecture strings are sm_30, sm_35, sm_37, sm_52, sm_53, sm_61, sm_70, sm_75, sm_80, and sm_89. The default depends on how the compiler has been configured, see --with-arch.

This option sets the value of the preprocessor macro "__PTX_SM__"; for instance, for sm_35, it has the value 350.

• *-misa=architecture-string* — alias of -march=.
• *-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. Settling for the vibes-adjacent option, respectfully 🦋
• *-mptx=version-string* — generate code for the specified PTX ISA version. Valid version strings are 3.1, 4.1, 4.2, 5.0, 6.0, 6.3, 7.0, 7.3, and 7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target architecture, see -march=, but at least 6.3, or 7.3 for -march=sm_52 and higher.

This option sets the values of the preprocessor macros "__PTX_ISA_VERSION_MAJOR__" and "__PTX_ISA_VERSION_MINOR__"; for instance, for 3.1 the macros have the values 3 and 1, respectively.

• -mmainkernel — link in code for a __main kernel. This is for stand-alone instead of offloading execution.
• -moptimize — apply partitioned execution optimizations. This is the default when any level of optimization is selected.
• -msoft-stack
• -mno-soft-stack — for -mno-soft-stack (the default, unless -mgomp has been specified), use PTX "native" stacks, that is, generate code that uses ".local" memory or PTX "alloca" directly for stack storage. Unless -mptx=7.3 or higher and -march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with "alloca" are not supported.

For -msoft-stack (implied by -mgomp), generate code that does not use ".local" memory or PTX "alloca" directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp.

• -muniform-simt — switch to code generation variant that allows to execute all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array "int __nvptx_uni[]" stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position "tid.y" with current lane index to compute the master lane index.
• -mgomp — generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects corresponding multilib variant.

---

🌈 OpenRISC Options 🌈

These options are defined for OpenRISC:

• *-mboard=name* — configure a board specific runtime. This will be passed to the linker for newlib board library linking. The default is "or1ksim".
• -mnewlib — this option is ignored; it is for compatibility purposes only. This used to select linker and preprocessor options for use with newlib. Retired queen, still on the guest list 💜
• -msoft-div
• -mhard-div — select software or hardware divide ("l.div", "l.divu") instructions. This default is hardware divide.
• -msoft-mul
• -mhard-mul — select software or hardware multiply ("l.mul", "l.muli") instructions. This default is hardware multiply.
• -msoft-float
• -mhard-float — select software or hardware for floating point operations. The default is software.
• -mdouble-float — when -mhard-float is selected, enables generation of double-precision floating point instructions. By default functions from libgcc are used to perform double-precision floating point operations. Double the float, double the drama 🍓
• -munordered-float — so when -mhard-float is the chosen vibe, this one lets GCC emit the unordered floating point compare-and-set-flag instructions, i.e. lf.sfun*. Default behaviour? It just calls the libgcc functions to do the unordered float compare-and-set-flag thing instead. 💅
• -mcmov — turn on generating conditional move instructions, aka l.cmov. By default you get the equivalent built out of set and branch instead.
• -mror — enables generating rotate right instructions, the l.ror ones. Default is libgcc functions doing your rotate-right for you.
• -mrori — same energy but for rotate right with immediate, l.rori. Default again: libgcc functions handle the rotate-right-with-immediate operations.
• -msext — emits sign extension instructions, l.ext*. By default sign extension gets done the long way, through memory loads. 🙈
• -msfimm — enables generating compare-and-set-flag-with-immediate instructions, l.sf*i. Default? Extra instructions get generated to stash the immediate into a register first. 💀
• -mshftimm — enables generating shift-with-immediate instructions: l.srai, l.srli, l.slli. Default is that same "store the immediate in a register first" extra-instruction situation.
• -mcmodel=small — generate OpenRISC code for the small model: the GOT is limited to 64k. This is the default model. 🎀
• -mcmodel=large — generate OpenRISC code for the large model: the GOT may grow up to 4G in size. Big GOT energy.

---

🌸 PDP-11 Options 🌸

These options are defined for the PDP-11:

• -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported, bestie.) Implies -m45.
• -msoft-float — do not use hardware floating point.
• -mac0 — return floating-point results in ac0 (fr0 in Unix assembler syntax).
• -mno-ac0 — return floating-point results in memory. This is the default.
• -m40 — generate code for a PDP-11/40. Implies -msoft-float -mno-split.
• -m45 — generate code for a PDP-11/45. This is the default.
• -m10 — generate code for a PDP-11/10. Implies -msoft-float -mno-split.
• -mint16, -mno-int32 — use 16-bit int. This is the default.
• -mint32, -mno-int16 — use 32-bit int.
• -msplit — target has split instruction and data space. Implies -m45.
• -munix-asm — use Unix assembler syntax.
• -mdec-asm — use DEC assembler syntax.
• -mgnu-asm — use GNU assembler syntax. This is the default.
• -mlra — use the new LRA register allocator. By default the old reload allocator is what's running the show.

---

💜 PowerPC Options 💜

These are listed under

---

🧚 PRU Options 🧚

These command-line options are defined for the PRU target:

• -minrt — link with a minimum runtime environment. This can significantly shrink the size of the final ELF binary, but heads up: some standard C runtime features get removed. 😭

This option disables support for static initializers and constructors. Beware that the compiler could still generate code with static initializers and constructors anyway. It is up to the programmer to make sure the source program won't use those features. Fr fr, that's on you.

The minimal startup code would not pass argc and argv arguments to main, so main must be declared as int main (void). Which is already the norm for most firmware projects, so honestly, slay.

• -mmcu=mcu — specify the PRU hardware variant to use. A correspondingly named spec file would be loaded, passing the memory region sizes to the linker and defining hardware-specific C macros.

Newlib provides only the sim spec, intended for running regression tests using a simulator. Specs for real hardware can be obtained by installing the GnuPruMcu (https://github.com/dinuxbg/gnuprumcu/) package.

• -mno-relax — make GCC pass the --no-relax command-line option to the linker instead of the --relax option.
• -mloop — allow (or do not allow) GCC to use the LOOP instruction.
• -mabi=variant — specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI, while -mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions. Here's what's different, bestie:
  • Function Pointer Size — TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers.
  • Optional Return Value Pointer — function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer.

The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti. 💀

Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.

---

✨ RISC-V Options ✨

These command-line options are defined for RISC-V targets:

• -mbranch-cost=n — set the cost of branches to roughly n instructions.
• -mplt, -mno-plt — when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is -mplt.
• -mabi=ABI-string — specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers.

The default for this argument is system dependent, so users who want a specific calling convention should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the rv64e. Those ABIs are not well specified at present, and are subject to change. 🫶

• -mfdiv, -mno-fdiv — do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions.
• -mfence-tso, -mno-fence-tso — do or don't use the fence.tso instruction, which is unimplemented on some processors (including those from T-Head). If the fence.tso instruction is not available then a stronger fence is used instead.
• -mdiv, -mno-div — do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions.
• -misa-spec=ISA-spec-string — specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming to. The possibilities for ISA-spec-string are:
  • 2.2 — produce code conforming to version 2.2.
  • 20190608 — produce code conforming to version 20190608.
  • 20191213 — produce code conforming to version 20191213.

The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version.

• -march=ISA-string — generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include rv64i, rv32g, rv32e, and rv32imaf. Additionally, a special value help (-march=help) is accepted to list all supported extensions. 👑

The syntax of the ISA string is defined as follows:

• The string must start with rv32 or rv64, followed by i, e, or g, referred to as the base ISA.
• The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g. zba) and single-letter (e.g. v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores.
• An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.
• Extension names may include an optional version number, following the syntax <major>p<minor> or <major>, (e.g. m2p1 or m2).

Supported extension are listed below:

• i — supported version: 2.0, 2.1 — base integer extension.
• e — supported version: 2.0 — reduced base integer extension.
• g — supported version: - — general-purpose computing base extension, g will expand to i, m, a, f, d, zicsr and zifencei.
• m — supported version: 2.0 — integer multiplication and division extension.
• a — supported version: 2.0, 2.1 — atomic extension.
• f — supported version: 2.0, 2.2 — single-precision floating-point extension.
• d — supported version: 2.0, 2.2 — double-precision floating-point extension.
• c — supported version: 2.0 — compressed extension.
• h — supported version: 1.0 — hypervisor extension.
• v — supported version: 1.0 — vector extension.
• zicsr — supported version: 2.0 — control and status register access extension.
• zifencei — supported version: 2.0 — instruction-fetch fence extension.
• zicond — supported version: 1.0 — integer conditional operations extension.
• za64rs — supported version: 1.0 — reservation set size of 64 bytes.
• za128rs — supported version: 1.0 — reservation set size of 128 bytes.
• zawrs — supported version: 1.0 — wait-on-reservation-set extension.
• zba — supported version: 1.0 — address calculation extension.

• zbb — Basic bit manipulation extension. (v 1.0) 🧮
• zbc — Carry-less multiplication extension. (v 1.0) ✖️
• zbs — Single-bit operation extension. (v 1.0) 🎯
• zfinx — Single-precision floating-point in integer registers extension. (v 1.0) 🌊
• zdinx — Double-precision floating-point in integer registers extension. (v 1.0) 🌊🌊
• zhinx — Half-precision floating-point in integer registers extension. (v 1.0) 🌊
• zhinxmin — Minimal half-precision floating-point in integer registers extension. (v 1.0) 🍼
• zbkb — Cryptography bit-manipulation extension. (v 1.0) 🔐
• zbkc — Cryptography carry-less multiply extension. (v 1.0) 🔐✖️
• zbkx — Cryptography crossbar permutation extension. (v 1.0) 🔐🔀
• zkne — AES Encryption extension. (v 1.0) 🔒
• zknd — AES Decryption extension. (v 1.0) 🔓
• zknh — Hash function extension. (v 1.0) #️⃣
• zkr — Entropy source extension. (v 1.0) 🎲
• zksed — SM4 block cipher extension. (v 1.0) 🧱
• zksh — SM3 hash function extension. (v 1.0) #️⃣
• zkt — Data independent execution latency extension. (v 1.0) ⏱️
• zk — Standard scalar cryptography extension. (v 1.0) 🔐
• zkn — NIST algorithm suite extension. (v 1.0) 🇺🇸
• zks — ShangMi algorithm suite extension. (v 1.0) 🇨🇳
• zihintntl — Non-temporal locality hints extension. (v 1.0) 💭
• zihintpause — Pause hint extension. (v 1.0) ⏸️
• zicboz — Cache-block zero extension. (v 1.0) 0️⃣
• zicbom — Cache-block management extension. (v 1.0) 🧹
• zicbop — Cache-block prefetch extension. (v 1.0) ⏩
• zic64b — Cache block size isf 64 bytes. (v 1.0) 📏
• ziccamoa — Main memory supports all atomics in A. (v 1.0) ⚛️
• ziccif — Main memory supports instruction fetch with atomicity requirement. (v 1.0) 📥
• zicclsm — Main memory supports misaligned loads/stores. (v 1.0) 🚧
• ziccrse — Main memory supports forward progress on LR/SC sequences. (v 1.0) ➡️
• zicntr — Standard extension for base counters and timers. (v 2.0) ⏲️
• zihpm — Standard extension for hardware performance counters. (v 2.0) 📊
• ztso — Total store ordering extension. (v 1.0) 🔁
• zve32x — Vector extensions for embedded processors. (v 1.0) 🧬
• zve32f — Vector extensions for embedded processors. (v 1.0) 🧬
• zve64x — Vector extensions for embedded processors. (v 1.0) 🧬
• zve64f — Vector extensions for embedded processors. (v 1.0) 🧬
• zve64d — Vector extensions for embedded processors. (v 1.0) 🧬
• zvl32b — Minimum vector length standard extensions. (v 1.0) 📐
• zvl64b — Minimum vector length standard extensions. (v 1.0) 📐
• zvl128b — Minimum vector length standard extensions. (v 1.0) 📐
• zvl256b — Minimum vector length standard extensions. (v 1.0) 📐
• zvl512b — Minimum vector length standard extensions. (v 1.0) 📐
• zvl1024b — Minimum vector length standard extensions. (v 1.0) 📐
• zvl2048b — Minimum vector length standard extensions. (v 1.0) 📐
• zvl4096b — Minimum vector length standard extensions. (v 1.0) 📐
• zvbb — Vector basic bit-manipulation extension. (v 1.0) 🧮
• zvbc — Vector carryless multiplication extension. (v 1.0) ✖️
• zvkb — Vector cryptography bit-manipulation extension. (v 1.0) 🔐
• zvkg — Vector GCM/GMAC extension. (v 1.0) 🧾
• zvkned — Vector AES block cipher extension. (v 1.0) 🔒
• zvknha — Vector SHA-2 secure hash extension. (v 1.0) #️⃣
• zvknhb — Vector SHA-2 secure hash extension. (v 1.0) #️⃣
• zvksed — Vector SM4 Block Cipher extension. (v 1.0) 🧱
• zvksh — Vector SM3 Secure Hash extension. (v 1.0) #️⃣
• zvkn — Vector NIST Algorithm Suite extension, zvkn will expand to zvkned, zvknhb, zvkb and zvkt. (v 1.0) 🇺🇸
• zvknc — Vector NIST Algorithm Suite with carryless multiply extension, zvknc will expand to zvkn and zvbc. (v 1.0) 🇺🇸✖️
• zvkng — Vector NIST Algorithm Suite with GCM extension, zvkng will expand to zvkn and zvkg. (v 1.0) 🇺🇸🧾
• zvks — Vector ShangMi algorithm suite extension, zvks will expand to zvksed, zvksh, zvkb and zvkt. (v 1.0) 🇨🇳
• zvksc — Vector ShangMi algorithm suite with carryless multiplication extension, zvksc will expand to zvks and zvbc. (v 1.0) 🇨🇳✖️
• zvksg — Vector ShangMi algorithm suite with GCM extension, zvksg will expand to zvks and zvkg. (v 1.0) 🇨🇳🧾
• zvkt — Vector data independent execution latency extension. (v 1.0) ⏱️
• zfh — Half-precision floating-point extension. (v 1.0) 🪶
• zfhmin — Minimal half-precision floating-point extension. (v 1.0) 🍼
• zvfh — Vector half-precision floating-point extension. (v 1.0) 🪶🧬
• zvfhmin — Vector minimal half-precision floating-point extension. (v 1.0) 🍼🧬
• zvfbfmin — Vector BF16 converts extension. (v 1.0) 🔄
• zfa — Additional floating-point extension. (v 1.0) ➕
• zmmul — Integer multiplication extension. (v 1.0) ✖️
• zca — Integer compressed instruction extension. (v 1.0) 🗜️
• zcf — Compressed single-precision floating point loads and stores extension. (v 1.0) 🗜️🌊
• zcd — Compressed double-precision floating point loads and stores extension. (v 1.0) 🗜️🌊🌊
• zcb — Simple compressed instruction extension. (v 1.0) 🗜️
• zce — Compressed instruction extensions for embedded processors. (v 1.0) 🗜️🧬
• zcmp — Compressed push pop extension. (v 1.0) 🗜️📚
• zcmt — Table jump instruction extension. (v 1.0) 🗜️🦘
• smaia — Advanced interrupt architecture extension. (v 1.0) ⚡
• smepmp — PMP Enhancements for memory access and execution prevention on Machine mode. (v 1.0) 🛡️
• smstateen — State enable extension. (v 1.0) 🎛️
• ssaia — Advanced interrupt architecture extension for supervisor-mode. (v 1.0) ⚡
• sscofpmf — Count overflow & filtering extension. (v 1.0) 🧮
• ssstateen — State-enable extension for supervisor-mode. (v 1.0) 🎛️
• sstc — Supervisor-mode timer interrupts extension. (v 1.0) ⏰
• svinval — Fine-grained address-translation cache invalidation extension. (v 1.0) 🧼
• svnapot — NAPOT translation contiguity extension. (v 1.0) 🧩
• svpbmt — Page-based memory types extension. (v 1.0) 📄
• xcvmac — Core-V multiply-accumulate extension. (v 1.0) ✖️➕
• xcvalu — Core-V miscellaneous ALU extension. (v 1.0) 🧠
• xcvelw — Core-V event load word extension. (v 1.0) 📥
• xtheadba — T-head address calculation extension. (v 1.0) 🧾
• xtheadbb — T-head basic bit-manipulation extension. (v 1.0) 🧮
• xtheadbs — T-head single-bit instructions extension. (v 1.0) 🎯
• xtheadcmo — T-head cache management operations extension. (v 1.0) 🧹
• xtheadcondmov — T-head conditional move extension. (v 1.0) 🚚
• xtheadfmemidx — T-head indexed memory operations for floating-point registers extension. (v 1.0) 🌊🗂️
• xtheadfmv — T-head double floating-point high-bit data transmission extension. (v 1.0) 🌊📤
• xtheadint — T-head acceleration interruption extension. (v 1.0) 🛑
• xtheadmac — T-head multiply-accumulate extension. (v 1.0) ✖️➕
• xtheadmemidx — T-head indexed memory operation extension. (v 1.0) 🗂️
• xtheadmempair — T-head two-GPR memory operation extension. (v 1.0) 👯
• xtheadsync — T-head multi-core synchronization extension. (v 1.0) 🔄
• xventanacondops — Ventana integer conditional operations extension. (v 1.0) 🚦

Bestie, quick housekeeping notes on all that 👆: when -march= is not specified, use the setting from -mcpu. And if both -march and -mcpu= are not specified, the default for this argument is system dependent, so users who want a specific architecture extensions should specify one explicitly. No guessing, just say it out loud. 💅

Also the way RISC-V specifications define an extension as depending on other extensions — GCC will implicitly add the dependent extensions to the enabled extension set if they weren't added explicitly. Bestie behavior tbh 🫶

---

• -mcpu=processor-string — Use architecture of and optimize the output for the given processor, specified by particular CPU name. Permissible values for this option are: sifive-e20, sifive-e21, sifive-e24, sifive-e31, sifive-e34, sifive-e76, sifive-s21, sifive-s51, sifive-s54, sifive-s76, sifive-u54, sifive-u74, sifive-x280, sifive-xp450, sifive-x670, thead-c906, tt-ascalon-x, tt-ascalon-d8, tt-ascalon-xg, xiangshan-nanhu. 👑

Heads up: -mcpu does not override -march or -mtune. So don't @ me when it doesn't lol 😭

• -mtune=processor-string — Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Per admissible values: generic-ooo, rocket, sifive-3-series, sifive-5-series, sifive-7-series, size, sifive-p400-series, sifive-p600-series, and all valid options for -mcpu=. ✨

When -mtune= is not specified, use the setting from -mcpu, the default is rocket if both are not specified. 🚀

The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but does not override the pipeline info. This helps reduce code size while still giving good performance. 📉➡️📈

• -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 bestie 💀 if you use this switch, then you MUST build all modules with the same value, including any libraries. Yes, even the system libraries and the startup modules. Everybody matching, or chaos. 👑

• -msmall-data-limit=n — puts global and static data that's smaller than n bytes into a special section (on some targets, anyway). ✨
• -msave-restore, -mno-save-restore — do / don't use the smaller-but-slower prologue and epilogue code that calls into library functions. The default is fast inline prologues and epilogues. Fast girl summer. 🏃‍♀️
• -mmovcc, -mno-movcc — do / don't spit out branchless conditional-move code sequences, even on targets that don't have special instructions for conditional ops. If you turn it on, ALU operation sequences get produced using base integer ISA instructions — but only where that's profitable. 💸
• -minline-atomics, -mno-inline-atomics — do / don't use the smaller-but-slower subword atomic emulation code that calls libatomic functions. The default is fast inline subword atomics that don't need libatomic at all. No cap, inline is the moment. 💅
• -minline-strlen, -mno-inline-strlen — do / do not attempt to inline strlen calls if possible. Inlining only happens if the string is properly aligned and the instructions for accelerated processing are available. The default is to not inline strlen calls. 🙈
• -minline-strcmp, -mno-inline-strcmp — do / do not attempt to inline strcmp calls if possible. Again, inlining only happens if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inline strcmp calls.

The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64. 🍓

• -minline-strncmp, -mno-inline-strncmp — do / do not attempt to inline strncmp calls if possible. Same deal: inlining only if the strings are properly aligned and accelerated-processing instructions are available. The default is to not inline strncmp calls.

The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64. (yes, same param name, that's just how it is bestie, iykyk) 🎀

• -mshorten-memrefs, -mno-shorten-memrefs — do / 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 that new base gets stored in a compressed register, then the new load/store can be compressed too. Currently this only targets 32-bit integer load/stores. slay 🛒
• -mstrict-align, -mno-strict-align — do not / do generate unaligned memory accesses. The default is set depending on whether the processor we're optimizing for supports fast unaligned access or not. ✨
• -mscalar-strict-align, -mno-scalar-strict-align — do not / do generate unaligned memory accesses, same deal: default depends on whether the processor we're optimizing for supports fast unaligned access. This is an alias for -mstrict-align. 💫
• -mvector-strict-align, -mno-vector-strict-align — do not / 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. 🌈
• -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model unless GCC has been configured with --with-cmodel= specifying a different default code model. 🏙️
• -mcmodel=medany — generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.

The code generated by the medium-any code model is position-independent, but it is not guaranteed to function correctly when linked into position-independent executables or libraries. ⚠️

• -mcmodel=large — generate code for a large code model, which has no restrictions on size or placement of symbols. big energy, literally. 👑
• -mexplicit-relocs, -mno-exlicit-relocs — use / do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. (and yes, the manpage really spells the second one -mno-exlicit-relocs, typo and all — write it exactly like that, bestie) 🫶
• -mrelax, -mno-relax — take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses, or don't. The default is to take advantage of linker relaxations. 🌸
• -mriscv-attribute, -mno-riscv-attribute — emit (do not emit) the RISC-V attribute that records extra information into ELF objects. This feature requires at least binutils 2.32. 🧚
• -mcsr-check, -mno-csr-check — enables or disables the CSR checking. ✅
• -malign-data=type — controls how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen, which uses x register width as the alignment value, and natural, which uses natural alignment. xlen is the default. 📐
• -mbig-endian — generate big-endian code. This is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-* target.
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for a riscv64-*-* or riscv32-*-* target, but not a riscv64be-*-* or riscv32be-*-* target. 💜
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a per-thread canary in the TLS block. 🐤

With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally specify which register to use as the base register for reading the canary, and what offset from that base register to read from. There is no default register or offset, because this is entirely for use inside the Linux kernel. 🔥

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.
• -mtls-dialect=trad — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default. ✨

---

🌼 RL78 Options 🌼

• -msim — links in additional target libraries to support operation within a simulator. 🖥️
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — specifies the type of hardware multiplication and division support to be used. The simplest is none, which uses software for both multiplication and division — that's the default. The g13 value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The g14 value selects the 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. ➗✖️

On top of that, a C preprocessor macro is defined based on the setting of this option. Possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__. 🎀

• -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions — instead it uses a hardware peripheral for those operations. The G10 or S1 core does not have register banks, so it uses a different calling convention. 👑

If this option is set, it also selects the type of hardware multiply support to use, unless that's overridden by an explicit -mmul=none option on the command line. So specifying -mcpu=g13 enables the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables hardware multiplications altogether.

Note bestie: even though the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain, since it also enables G14 hardware multiply support. If these options are not specified on the command line, then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains that didn't have hardware multiply and divide support. 😭

Also, a C preprocessor macro is defined based on 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're provided for backwards compatibility. 🫶
• -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, those registers can be used in ordinary functions too. 💅
• -m64bit-doubles, -m32bit-doubles — make the double data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is -m32bit-doubles. 🍓
• -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, like if it does multiplication and division operations. The default is to ignore the MDUC registers since that makes the interrupt handlers faster. The target option -mg13 needs to be passed for this to work, because this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation, or if it calls another function. 🛟

---

👑 IBM RS/6000 and PowerPC Options 👑

These -m options are defined for the IBM RS/6000 and PowerPC:

💖 (and bestie, the ride is not over — there's so much more of this list coming, stay tuned) 💖

• -mpowerpc-gpopt, -mno-powerpc-gpopt
• -mpowerpc-gfxopt, -mno-powerpc-gfxopt
• -mpowerpc64, -mno-powerpc64
• -mmfcrf, -mno-mfcrf
• -mpopcntb, -mno-popcntb
• -mpopcntd, -mno-popcntd
• -mfprnd, -mno-fprnd
• -mcmpb, -mno-cmpb
• -mhard-dfp, -mno-hard-dfp — okay bestie these are your which instructions does my processor even have knobs 💅. The default for all of them gets decided when GCC itself was configured, and if you say -mcpu=cpu_type it just steamrolls right over whatever you set here. Lowkey the recommendation is: just use -mcpu=cpu_type instead of hand-picking from this list. Trust. 🫶

Now let's actually break down what each one unlocks, because the details matter:

• -mpowerpc-gpopt → lets GCC use the optional PowerPC architecture instructions in the General Purpose group, and yes that includes floating-point square root, slay.
• -mpowerpc-gfxopt → lets GCC use the optional PowerPC architecture instructions in the Graphics group, including floating-point select. ✨

And the individual ones, take notes 📝:

• -mmfcrf → lets GCC generate the move from condition register field instruction, which lives on the POWER4 processor and any other chip that supports the PowerPC V2.01 architecture.
• -mpopcntb → lets GCC generate the popcount and double-precision FP reciprocal estimate instruction, found on the POWER5 processor and other PowerPC V2.02 architecture supporters.
• -mpopcntd → lets GCC generate the popcount instruction, found on POWER7 and friends rocking the PowerPC V2.06 architecture.
• -mfprnd → lets GCC generate the FP round to integer instructions, implemented on the POWER5+ processor and other PowerPC V2.03 architecture supporters.
• -mcmpb → lets GCC generate the compare bytes instruction, implemented on the POWER6 processor and other PowerPC V2.05 architecture supporters.
• -mhard-dfp → lets GCC generate the decimal floating-point instructions, which are implemented on some POWER processors. 🧚

And our girl -mpowerpc64 💖 — this one lets GCC generate the extra 64-bit instructions that live in the full PowerPC64 architecture, and it treats GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64, just so you know.

---

THE -mcpu VIBE CHECK

• -mcpu=cpu_type — this sets the architecture type, the register usage, and the instruction scheduling parameters, all for the machine type you name in cpu_type. It's the whole package, main character energy. 👑

The supported values for cpu_type are, and I'm listing every single one because I said what I said:

401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, power10, power11, powerpc, powerpc64, powerpc64le, rs64, and native.

A few of those deserve a lil shoutout:

• -mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endianness, we're inclusive 💜), 64-bit big endian PowerPC, and 64-bit little endian PowerPC architecture machine types respectively — and each assumes an appropriate, generic processor model for scheduling purposes.
• Specifying native as the cpu type detects and selects the architecture option that matches the host processor of the system doing the compilation. Heads up though: -mcpu=native has no effect if GCC doesn't recognize the processor. Not me getting ignored by the compiler 😭.
• The other options each specify a specific processor. Code generated under those options runs best on that processor, and — important — may not run at all on others. So choose wisely fr fr.

The -mcpu options automatically enable or disable these bad boys:

-maltivec  -mfprnd  -mhard-float  -mmfcrf  -mmultiple -mpopcntb
-mpopcntd  -mpowerpc64 -mpowerpc-gpopt  -mpowerpc-gfxopt -mmulhw
-mdlmzb  -mmfpgpr  -mvsx -mcrypto  -mhtm  -mpower8-fusion
-mquad-memory  -mquad-memory-atomic  -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mrop-protect

The particular set of options picked for any given CPU varies between compiler versions, depending on whatever setting seems to produce optimal code for that CPU — and it doesn't necessarily reflect the actual hardware's capabilities. Sorry bestie, the vibes are version-dependent. 🙈

If you wanna force an individual option to a particular value, you can just say it after the -mcpu option, like this:

-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 flip them on or off individually if you're sure it'll work in your environment. Your setup, your rules. 💅

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SCHEDULING, BUT MAKE IT SEPARATE

• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage the way -mcpu=cpu_type does. Same cpu_type values work here as for -mcpu. If you specify both? The generated code uses the architecture and registers from -mcpu, but the scheduling parameters come from -mtune. Best of both worlds, honestly iconic. ⭐

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THE CODE MODELS

There's a whole -mcmodel situation and each size is a different vibe:

• -mcmodel=small — generates PowerPC64 code for the small model: the TOC is limited to 64k.
• -mcmodel=medium — generates PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This one is the default for 64-bit Linux. ✨
• -mcmodel=large — generates PowerPC64 code for the large model: the TOC may be up to 4G in size, and other data and code is only limited by the 64-bit address space. Go big or go home. 🌈

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ALTIVEC & FRIENDS

• -maltivec, -mno-altivec — generate code that uses (or 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 the AltiVec ABI enhancements.

Okay so when -maltivec is in play, the element order for AltiVec intrinsics like vec_splat, vec_extract, and vec_insert matches array element order corresponding to the endianness of the target. Which means: element zero identifies the leftmost element in a vector register when you're targeting a big-endian platform, and identifies the rightmost element in a vector register when you're targeting a little-endian platform. Yes this is a whole thing. Yes you should remember it. 🦋

• -mvrsave, -mno-vrsave — generate VRSAVE instructions when generating AltiVec code.
• -msecure-plt — generate code that lets ld and ld.so build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option. Secure queen behavior. 👑
• -mbss-plt — generate code that uses a BSS .plt section that ld.so fills in, and it requires .plt and .got sections that are both writable and executable. This is also a PowerPC 32-bit SYSV ABI option. Just so we're clear on what you're signing up for. 💀
• -msplit-patch-nops — when you're adding NOPs for a patchable area via the -fpatchable-function-entry option, this emits the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point. The consequence: the sequence of NOPs is not consecutive when a global entry point is generated. Without this option, the NOPs get emitted directly before and after the local entry point, making them consecutive — but also moving the global and local entry points further apart. And if only a single entry point is generated, this option has no effect at all. Ngl that's a very precise little detail and I respect it. 🎀
• -misel, -mno-isel — this switch enables or disables the generation of ISEL instructions.
• -mvsx, -mno-vsx — generate code that uses (or doesn't use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set.
• -mcrypto, -mno-crypto — enable (or disable) the use of 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 (or disable) the use of built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions added in version 2.07 of the PowerPC ISA.
• -mpower8-fusion, -mno-power8-fusion — generate code that keeps (or does not keeps — yes, that's what it says, we're keeping the wording exactly 💅) some integer operations adjacent so the instructions can be fused together on power8 and later processors.
• -mquad-memory, -mno-quad-memory — generate code that uses (or doesn't use) the non-atomic quad word memory instructions. Fair warning: -mquad-memory requires use of 64-bit mode.
• -mquad-memory-atomic, -mno-quad-memory-atomic — generate code that uses (or doesn't use) the atomic quad word memory instructions. And same deal: -mquad-memory-atomic requires use of 64-bit mode.

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FLOAT128 CORNER 💖

• -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. Pick your lane bestie.

The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support, and please note: 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, IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise — if you don't specify generation of ISA 3.0 instructions, or if you're targeting a 32-bit big endian system — IEEE 128-bit floating point gets done with software emulation. Slower but valid. 🫶

• -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.

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32-BIT VS 64-BIT

• -m32, -m64 — generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (which includes GNU/Linux, obviously 💜).

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 — okay so these are all about the TOC (Table Of Contents) that gets made for every single executable file. 💅

-mfull-toc is the default vibe. When it's on, GCC gives at least one TOC entry for every unique non-automatic variable reference in your program. It also yeets floating-point constants into the TOC. But heads up: only 16,384 entries fit in the TOC. 😭

If the linker starts screaming that you overflowed the TOC space, you can shrink the TOC with -mno-fp-in-toc and/or -mno-sum-in-toc. -mno-fp-in-toc stops GCC from putting floating-point constants in the TOC. -mno-sum-in-toc makes GCC generate code to calculate the sum of an address and a constant at run time instead of shoving that sum into the TOC. You can use one or both. Each one makes your code very slightly slower and larger, but saves TOC space. It's giving trade-off, bestie. ⚖️

Still overflowing even with both? Then say hi to -mminimal-toc. This one makes only one TOC entry for every file. The code it produces is slower and larger, but it uses extremely little TOC space. You might wanna use this only on files that contain less frequently-executed code. No cap. 🧚

• -maix64, -maix32 — these enable/disable the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and all the infrastructure needed to support them. -maix64 implies -mpowerpc64, and -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32. 💖
• -mxl-compat, -mno-xl-compat — when you're using the AIX-compatible ABI, these make GCC produce code that conforms more closely to IBM XL compiler semantics. Specifically: it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. It also doesn't assume that the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double. And it uses XL symbol names for long double support routines. 🫶

The AIX calling convention got extended but wasn't initially documented, all to handle an obscure K&R C case: calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that don't fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only matters when calling subroutines compiled by IBM XL compilers without optimization. So like, only use it if you must. 🙈

• -mpe — supports the IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code so it can actually run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or you have to override the specs file with the -specs= option to point to the right directory. Heads up: the Parallel Environment does not support threads, so -mpe and -pthread are incompatible. It's giving toxic relationship. 💀
• -malign-natural, -malign-power — on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, -malign-natural overrides the ABI-defined alignment of larger types (like floating-point doubles) on their natural size-based boundary. -malign-power tells GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI. ✨

On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported. So don't even try it there. 🚫

• -msoft-float, -mhard-float — these generate code that does not use (or uses) the floating-point register set. If you use -msoft-float, software floating-point emulation is provided, and you need to pass the option to GCC when linking too. 💅
• -mmultiple, -mno-multiple — generate code that uses (or doesn't use) the load multiple word instructions and the store multiple word instructions. These are generated by default on POWER systems, and not generated on PowerPC systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions don't work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. So like, know your chips, bestie. 🔥
• -mupdate, -mno-update — generate code that uses (or doesn't use) the load or store instructions that update the base register to the address of the calculated memory location. These are generated by default. But if you use -mno-update, there's a small window between the time the stack pointer is updated and the address of the previous frame is stored. That means code that walks the stack frame across interrupts or signals may get corrupted data. So it's a bit risky. ⚠️
• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — generate code that tries to avoid (or not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, like when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise. So it's auto-magic. 🪄
• -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. So it's like a little alias situation. 👑
• -mmulhw, -mno-mulhw — generate code that uses (or doesn't use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors. So if you're on one of those, you're already getting them. 💁
• -mdlmzb, -mno-dlmzb — generate code that uses (or doesn't use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors. Again, default behavior for those chips. 🌟
• -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, these do not (or do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field. For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. So it's a size vs alignment thing. 📏
• -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, these do not (or do) assume that unaligned memory references are handled by the system. So depending on your platform, you might need to tell GCC what's up. 🤔
• -mrelocatable, -mno-relocatable — generate code that allows (or doesn't 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. Also, -mrelocatable code aligns the stack to an 8-byte boundary. So it's a whole coordinated thing. 🎯
• -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. So it's more flexible, bestie. 🦋
• -mno-toc, -mtoc — on System V.4 and embedded PowerPC systems, these do not (or do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program. So it's about that global area pointer vibe. 🌍
• -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle. Same same. 🐭
• -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 energy. 🐘
• -mdynamic-no-pic — on Darwin / macOS systems, compile code so that it is not relocatable, but its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. So don't use it for dylibs. 🙅‍♀️
• -msingle-pic-base — treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. So it's on you, runtime. 🫡
• -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. So it's a dial for how much you care. 🎚️
• -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 — drops in NOPs so that those spendy dependent insns land in their own separate groups 🫶. Insert number NOPs to force an insn into a brand-new group.
• -mcall-sysv — on System V.4 and embedded PowerPC systems, compiles code using calling conventions that follow the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix.
• -mcall-sysv-eabi, -mcall-eabi — specifies both -mcall-sysv and -meabi at the same time. two-for-one, bestie 💅
• -mcall-sysv-noeabi — specifies both -mcall-sysv and -mno-eabi.
• -mcall-aixdesc — on System V.4 and embedded PowerPC systems, compiles code for the AIX operating system.
• -mcall-linux — on System V.4 and embedded PowerPC systems, compiles code for the Linux-based GNU system.
• -mcall-freebsd — on System V.4 and embedded PowerPC systems, compiles code for the FreeBSD operating system.
• -mcall-netbsd — on System V.4 and embedded PowerPC systems, compiles code for the NetBSD operating system.
• -mcall-openbsd — on System V.4 and embedded PowerPC systems, compiles code for the OpenBSD operating system.
• -mtraceback=traceback_type — picks the type of traceback table you want. The valid values for traceback_type are literally full, part, and no. that's the whole menu, no cap 🌸
• -maix-struct-return — returns all structures in memory, exactly as the AIX ABI demands.
• -msvr4-struct-return — returns structures smaller than 8 bytes in registers, the way the SVR4 ABI says to.
• -mabi=abi-type — extends the current ABI with one particular extension, or strips such an extension back off. Valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and for AIX: vec-extabi, vec-default.
• -mabi=ibmlongdouble — switches the current ABI over to IBM extended-precision long double. ⚠️ heads up: this is not likely to work if your system defaults to using IEEE extended-precision long double. And if you change the long double type away from IEEE extended-precision, the compiler issues a warning unless you pass -Wno-psabi. Requires -mlong-double-128 to be enabled.
• -mabi=ieeelongdouble — switches the current ABI over to IEEE extended-precision long double. Same energy: not likely to work if your system defaults to IBM extended-precision long double. If you change the long double type away from IBM extended-precision, the compiler issues a warning unless you use -Wno-psabi. Requires -mlong-double-128 to be enabled. ✨
• -mabi=elfv1 — switches the current ABI over 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 likely to fail in spectacular ways 💀 you have been warned.
• -mabi=elfv2 — switches the current ABI over to the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI requires special system support and is likely to fail in spectacular ways 💀 same chaos, different endianness.
• -mgnu-attribute, -mno-gnu-attribute — emits .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes section that specify ABI variations in function parameters or return values. fancy little metadata moment 🎀
• -mprototype, -mno-prototype — on System V.4 and embedded PowerPC systems, assumes that all calls to variable argument functions are properly prototyped. Otherwise the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR") to indicate whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear that bit. so much less busywork, love that for you 🦋
• -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.
• -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. yellowknife is such a cute name for a target ngl 💛
• -mvxworks — on System V.4 and embedded PowerPC systems, specifies that you're compiling for a VxWorks system.
• -memb — on embedded PowerPC systems, sets the "PPC_EMB" bit in the ELF flags header to say that eabi extended relocations are being used.
• -meabi, -mno-eabi — on System V.4 and embedded PowerPC systems, do (or, for -mno-eabi, do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. 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 the -msdata option only uses "r13" to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.
• -msdata=eabi — 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, which register "r13" points to. Puts small uninitialized global and static data in the .sbss section, which sits right next to .sdata 🫶. Oh and heads up: -msdata=eabi is incompatible with -mrelocatable. The -msdata=eabi option 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 the .sbss section, which is adjacent to .sdata. And yep: -msdata=sysv is incompatible with -mrelocatable.
• -msdata=default, -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compiles code the same as -msdata=eabi; otherwise it compiles the same as -msdata=sysv. bestie just picks for you, so thoughtful 😌
• -msdata=data — on System V.4 and embedded PowerPC systems, puts small global data in the .sdata section. Puts small uninitialized global data in the .sbss section. But it does not use register "r13" to address small data, okay? This is the default behavior unless other -msdata options are used.
• -msdata=none, -mno-sdata — on embedded PowerPC systems, puts all initialized global and static data in the .data section, and all uninitialized data in the .bss section. tidy 💅
• -mreadonly-in-sdata — puts read-only objects in the .sdata section as well. This is the default.
• -mblock-move-inline-limit=num — inlines all block moves (think calls to "memcpy" or structure copies) that are less than or equal to num bytes. The minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-specific.
• -mblock-compare-inline-limit=num — 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 — both non-loop and loop — of block compare is disabled. The default value is target-specific.
• -mblock-compare-inline-loop-limit=num — generates an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length isn't constant, at most num bytes get compared before "memcmp" is called to compare the rest of the block. The default value is target-specific.
• -mstring-compare-inline-limit=num — compares at most num string bytes with inline code. If the difference or the end of the string isn't found by the end of that inline compare, a call to "strcmp" or "strncmp" takes care of the rest of the comparison. The default is 64 bytes. 🌈
• -G num — on embedded PowerPC systems, puts global and static items that are less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section. By default, num is 8. The -G num switch also gets passed along to the linker. And listen: all modules should be compiled with the same -G num value 🙈 keep it consistent, bestie.
• -mregnames, -mno-regnames — on System V.4 and embedded PowerPC systems, do (or don't) emit register names in the assembly language output using symbolic forms.
• -mlongcall, -mno-longcall — by default, assumes that 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 is 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 they generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. There's a plan to add this feature to the GNU linker for 32-bit PowerPC systems as well. 👑

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls.

On Darwin/PPC systems, #pragma longcall generates "jbsr callee, L42", plus a branch island (a.k.a. glue code). Those two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a "bl callee" if the PPC "bl" instruction reaches the callee directly; otherwise the linker generates "bl L42" to call the branch island. The branch island gets appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it. iconic little detour tbh 💀✨

On Mach-O (Darwin) systems, this option tells the compiler to emit glue for every direct call, and the Darwin linker gets to decide whether to use it or discard it. 💅

In the future, GCC may straight-up ignore all longcall specifications when the linker is known to generate glue. So like, don't be shocked if that changes, bestie. 🙈

• -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, in the case of 32-bit PowerPC, if GCC is configured with --enable-secureplt. Heads up: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together. 😭
• -mtls-markers, -mno-tls-markers — Mark (or don't mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The relocation lets the linker reliably associate function call with argument setup instructions for TLS optimization, which in turn lets GCC better schedule the sequence. ✨
• -mrecip, -mno-recip — This one enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Note 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 option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a "!" to invert the option:
  • all — Enable all estimate instructions.
  • default — Enable the default instructions, equivalent to -mrecip.
  • none — Disable all estimate instructions, equivalent to -mno-recip.
  • div — Enable the reciprocal approximation instructions for both single and double precision.
  • divf — Enable the single-precision reciprocal approximation instructions.
  • divd — Enable the double-precision reciprocal approximation instructions.
  • rsqrt — Enable the reciprocal square root approximation instructions for both single and double precision.
  • rsqrtf — Enable the single-precision reciprocal square root approximation instructions.
  • rsqrtd — Enable the double-precision reciprocal square root approximation instructions.

So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-precision reciprocal square root calculations. 🔥

• -mrecip-precision, -mno-recip-precision — Assume (or don't assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-precision machines, since they 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 (or don't generate) the "friz" instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. The "friz" instruction does not return the same value if the floating-point number is too large to fit in an integer. ⚠️
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — Generate (or don't generate) code to load up the static chain register ("r11") when calling through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, 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 (or don't generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value 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 (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC.

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that 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 (or don't generate) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. 🔥
• -mprefixed, -mno-prefixed — Generate (or don't generate) addressing modes using prefixed load and store instructions. The -mprefixed option requires that the option -mcpu=power10 (or later) is enabled. 🌟
• -mmma, -mno-mma — Generate (or don't generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or later) is enabled. 💅
• -mrop-protect, -mno-rop-protect — Generate (or don't generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). 🛡️
• -mprivileged, -mno-privileged — Generate (or don't generate) code that will run in privileged state. 👑
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — Generate (or don't generate) unaligned vsx loads and stores for inline expansion of "memcpy" and "memmove". 🧚
• --param rs6000-vect-unroll-limit= — The vectorizer will check with target information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four. 💜

🌸 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 — sets the max size (in bytes) of globals and statics that can go in the small data area. Using this area gives you smaller + faster code, but bestie, the area is limited and it's on you, the programmer, to make sure it doesn't overflow. Also — and this is the catch — the small data area eats one of the RX's registers (usually r13) to hold the pointer to that area, so that register is no longer available to the compiler. Which could lowkey make your code slower and/or bigger if variables end up on the stack instead of chilling in that register.

Heads up: common variables (the uninitialized ones) and constants do not go into the small data area — they get assigned to other sections in the output executable.

Default is zero, which means this whole feature is off. And note this doesn't get turned on automatically at higher optimization levels like -O2 because reserving a register can be detrimental. It's fully on the programmer to experiment and figure out if it actually helps their program. For how the actual register holding the small data area pointer gets picked, go look at -mpid.

• -msim, -mno-sim — use the simulator runtime. The default is to use the libgloss board-specific runtime.
• -mas100-syntax, -mno-as100-syntax — when generating assembler output, use a syntax compatible with Renesas's AS100 assembler. GAS can handle this syntax too, but it has some restrictions, so it isn't generated by default.
• -mmax-constant-size=N — the max size, in bytes, of a constant that can be used as an operand in an RX instruction. So like, the RX instruction set does allow constants up to 4 bytes in instructions, but a longer value means a longer instruction. So sometimes it's beneficial to restrict constant sizes used in instructions. Constants that are too big get dropped into a constant pool and referenced via register indirection instead.

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. This is where the linker tries to shrink your program by finding shorter versions of various instructions. Disabled by default.
• -mint-register=N — how many registers to reserve for fast interrupt handler functions. N can be between 0 and 4. 1 means r13 is reserved exclusively for fast interrupt handlers. 2 reserves r13 and r12. 3 reserves r13, r12 and r11. 4 reserves r13 through r10. And 0, the default, reserves none. 💅
• -msave-acc-in-interrupts — says interrupt handler functions should preserve the accumulator register. You only need this if normal code might use the accumulator register, like if it does 64-bit multiplications. Default is to ignore the accumulator, because that makes interrupt handlers faster.
• -mpid, -mno-pid — enables generating position independent data. When on, any access to constant data happens via an offset from a base address held in a register. This lets the location of constant data be determined at run time without needing the executable to be relocated — a big win for embedded apps with tight memory. Data that can be modified is not affected by this option.

Heads up: this feature reserves a register (usually r13) for the constant data base address, which can make code slower and/or bigger, especially in complicated functions.

Which register actually holds the constant data base address depends on whether -msmall-data-limit and/or -mint-register are enabled. Starting at r13 and going downwards, registers get allocated first to satisfy -mint-register, then -mpid, and finally -msmall-data-limit. So fr, the small data area register could end up being r8 if you specify both -mint-register=4 and -mpid on the command line. 🤯

This is off by default. You can restore the default with -mno-pid.

• -mno-warn-multiple-fast-interrupts, -mwarn-multiple-fast-interrupts — stops GCC from warning you if it finds more than one fast interrupt handler while compiling a file. Default is to warn for each extra fast interrupt handler found, because the RX only supports one such interrupt.
• -mallow-string-insns, -mno-allow-string-insns — enables or disables the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL SWHILE, plus the RMPA instruction. These instructions might 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.)

The default is to allow these instructions, but GCC can't reliably detect every situation where a string instruction might touch an I/O register, so it can't disable them automatically. It's on you, the programmer, to use -mno-allow-string-insns if your program accesses I/O space. 🙈

When the instructions are enabled, GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__; otherwise it defines __RX_DISALLOW_STRING_INSNS__.

• -mjsr, -mno-jsr — use only (or not only) JSR instructions to access functions. You'd use this when code size exceeds the range of BSR instructions. And ngl, -mno-jsr does not mean "don't use JSR" — it means any type of branch may be used.

Note: the generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the interrupt function attribute. This attribute marks a function intended to process fast interrupts. GCC makes sure it only uses the registers r10, r11, r12 and/or r13, and only when the normal use of those 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. Let's get into it bestie 🦋

• -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 do the floating-point work. With -mhard-float, the compiler generates IEEE floating-point instructions. That's 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 handle decimal-floating-point operations. 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. This is 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. A backchain might be needed so debugging tools that don't understand DWARF call frame info can still work. 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 goes into the topmost word of the 96/160 byte register save area.

Generally, 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. Note that combining -mbackchain, -mpacked-stack and -mhard-float is not supported. To build a linux kernel, use -msoft-float.

Default is to not maintain the backchain.

• -mpacked-stack, -mno-packed-stack — use (or don't use) the packed stack layout. With -mno-packed-stack, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. With -mpacked-stack, register save slots get densely packed at the top of the register save area; unused space gets reused for other stuff, so you use the available stack space more efficiently. But 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. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging. That code is not call-compatible with code compiled with -mpacked-stack. Also, combining -mbackchain, -mpacked-stack and -mhard-float is not supported. To build a linux kernel, use -msoft-float.

Default is to not use the packed stack layout.

• -msmall-exec, -mno-small-exec — generate (or don't generate) code using the bras instruction for subroutine calls. This only works reliably if the total executable size stays under 64k. Default is to use the basr instruction instead, which doesn't have that limitation. 💀
• -m64, -m31 — pick your ABI bestie 💫 With -m31 you get code that's compliant to the GNU/Linux for S/390 ABI, and with -m64 you get code compliant to the GNU/Linux for zSeries ABI. This is literally what lets GCC generate those 64-bit instructions. For the s390 targets the default is -m31, while the s390x targets default to -m64. slay 🫶
• -mzarch, -mesa — choose ur instruction set era ✨ -mzarch generates code using the instructions available on z/Architecture, and -mesa generates code using the instructions available on ESA/390. Heads up tho: -mesa is not possible with -m64 💀 When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
• -mhtm, -mno-htm — -mhtm turns on a whole set of builtins that make use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. -mhtm is enabled by default when using -march=zEC12. iconic 🌸
• -mvx, -mno-vx — -mvx generates code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation. Fair warning bestie: this option changes the ABI for some vector type values with regard to alignment and calling conventions 👀 In case vector type values are being used in an ABI-relevant context, a GAS .gnu_attribute command will be added to mark the resulting binary with the ABI used. -mvx is enabled by default when using -march=z13.
• -mzvector, -mno-zvector — -mzvector enables vector language extensions and builtins using instructions available with the vector extension facility introduced with the IBM z13 machine generation. It adds support for vector to be used as a keyword to define vector type variables and arguments. vector is only available when GNU extensions are enabled — it will not be expanded when requesting strict standard compliance e.g. with -std=c99. In addition to the GCC low-level builtins, -mzvector enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. In order to make use of these builtins the header file vecintrin.h needs to be included. -mzvector is disabled by default.
• -mmvcle, -mno-mvcle — generate (or don't generate 🚫) code using the "mvcle" instruction to perform block moves. When -mno-mvcle is specified, use a "mvc" loop instead. This is the default unless optimizing for size.
• -mdebug, -mno-debug — print (or don't print) additional debug information when compiling. The default is to not print debug information.
• *-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. Possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14, z17/arch15, and native. 💜

The default is -march=z900.

Specifying native as cpu type can be used to select the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor.

• *-mtune=cpu-type* — tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for -march. The default is the value used for -march.
• -mtpf-trace, -mno-tpf-trace — generate code that adds (or does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS 🙈
• -mtpf-trace-skip, -mno-tpf-trace-skip — generate code that changes (or does not change) the default branch targets enabled by -mtpf-trace to point to specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace.
• -mfused-madd, -mno-fused-madd — generate code that uses (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. Because this is a compile-time check it doesn't need to be a real problem when the program runs — it's intended to identify functions that most probably cause a stack overflow 💀 It is useful to be used in an environment with limited stack size e.g. the linux kernel.
• -mwarn-dynamicstack — emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is generally a bad idea with a limited stack size, ngl.
• *-mstack-guard=stack-guard*, *-mstack-size=stack-size* — if these options are provided the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390 grows downward ⬇️). If the stack-guard option is omitted the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to be used to help debugging stack overflow problems. The additionally emitted code causes only little overhead and hence can also be used in production-like systems without greater performance degradation. The given values have to be exact powers of 2 and stack-size has to be greater than stack-guard without exceeding 64k. In order to be efficient the extra code makes the assumption that the stack starts at an address aligned to the value given by stack-size. The stack-guard option can only be used in conjunction with stack-size.

fr fr, only use stack-guard alongside stack-size or it's a no-go bestie.

• *-mhotpatch=pre-halfwords*,post-halfwords — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like instructions the architecture allows (maximum 1000000).

If both arguments are zero, hotpatching is disabled.

This option can be overridden for individual functions with the "hotpatch" attribute. ✨

---

🦋 SH Options 🦋

These -m options are defined for the SH implementations, bestie 💖:

• -m1 — Generate code for the SH1.
• -m2 — Generate code for the SH2.
• -m2e — Generate code for the SH2e.
• -m2a-nofpu — Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
• -m2a-single-only — Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
• -m2a-single — Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
• -m2a — Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
• -m3 — Generate code for the SH3.
• -m3e — Generate code for the SH3e.
• -m4-nofpu — Generate code for the SH4 without a floating-point unit.
• -m4-single-only — Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
• -m4-single — Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
• -m4 — Generate code for the SH4.
• -m4-100 — Generate code for SH4-100.
• -m4-100-nofpu — Generate code for SH4-100 in such a way that the floating-point unit is not used.
• -m4-100-single — Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — Generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
• -m4-200 — Generate code for SH4-200.
• -m4-200-nofpu — Generate code for SH4-200 without in such a way that the floating-point unit is not used.
• -m4-200-single — Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
• -m4-200-single-only — Generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
• -m4-300 — Generate code for SH4-300.
• -m4-300-nofpu — Generate code for SH4-300 without in such a way that the floating-point unit is not used.
• -m4-300-single — Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-300-single-only — Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-340 — Generate code for SH4-340 (no MMU, no FPU).
• -m4-500 — Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
• -m4a-nofpu — Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
• -m4a-single-only — Generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
• -m4a-single — Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
• -m4a — Generate code for the SH4a.
• -m4al — Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at the moment.
• -mb — Compile code for the processor in big-endian mode.
• -ml — Compile code for the processor in little-endian mode.
• -mdalign — Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign. 👀
• -mrelax — Shorten some address references at link time, when possible; uses the linker option -relax.
• -mbigtable — Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
• -mbitops — Enable the use of bit manipulation instructions on SH2A.
• -mfmovd — Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
• -mrenesas — Comply with the calling conventions defined by Renesas.
• -mno-renesas — beanies on, besties: this makes the compiler follow the calling conventions GCC used before the Renesas conventions existed 🫶. And guess what? It's literally the default for every single target of the SH toolchain. Slay, no choices to make.
• -mnomacsave — okay so the "MAC" register? Normally it's treated as preserved, but this option marks it as call-clobbered — even if you already passed -mrenesas. It's giving "I do what I want" 💅.
• -mieee, -mno-ieee — controls how IEEE-compliant floating-point comparisons are, which matters for the cursed little case where a comparison result is unordered 😭. Default vibes: -mieee is implicitly enabled. But if you turn on -ffinite-math-only, then -mno-ieee gets implicitly set too, which gets you faster floating-point greater-equal and less-equal comparisons 🔥. And if you don't like the implicit choices, you can override them by explicitly saying -mieee or -mno-ieee. Main character energy: you decide.
• -minline-ic_invalidate — inline that code to invalidate instruction cache entries after setting up nested function trampolines ✨. But listen: this option does nothing if -musermode is in effect and the code generation option you picked (like -m4) doesn't allow the "icbi" instruction. And if your code generation option doesn't allow "icbi" and -musermode is not in effect, then the inlined code goes and manipulates the instruction cache address array directly with an associative write 💀. That not only needs privileged mode at run time, it also flat-out fails if the cache line had been mapped via the TLB and has since become unmapped. Be careful bestie.
• -misize — dumps instruction size and location right into the assembly code 👑. Little peek behind the curtain.
• -mpadstruct — 🚨 this option is deprecated. It pads structures to a multiple of 4 bytes, which is incompatible with the SH ABI. Not the vibe anymore.
• -matomic-model=model — sets the model of atomic operations plus any extra parameters, all as a comma separated list 🌸. For the deets on the atomic built-in functions, see __atomic Builtins. Here's everything that's supported:
  • none — disables compiler generated atomic sequences and emits library calls for atomic operations. This is the default if the target is not "sh*-*-linux*".
  • soft-gusa — generates GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions 🧚. These generated atomic sequences need extra support from the system's interrupt/exception handling code, and they're only suitable for SH3* and SH4* single-core systems. It's enabled by default when the target is "sh*-*-linux*" and SH3* or SH4*. And when the target is SH4A, this option also partially utilizes the hardware atomic instructions "movli.l" and "movco.l" to make more efficient code — unless you specified strict.
  • soft-tcb — generates software atomic sequences that use a variable in the thread control block 🎀. It's a variation of the gUSA sequences, so it also works on SH1* and SH2* targets. Same deal though: the generated atomic sequences need extra support from the system's interrupt/exception handling code, and they're only suitable for single-core systems. Oh and when you use this model, you have to specify the gbr-offset= parameter as well. Non-negotiable, bestie.
  • soft-imask — generates software atomic sequences that temporarily disable interrupts by setting "SR.IMASK = 1111" 🙈. This model only works when the program runs in privileged mode, and it's only suitable for single-core systems. Good news: it does not need extra support from the system's interrupt/exception handling code. It's enabled by default when the target is "sh*-*-linux*" and SH1* or SH2*.
  • hard-llcs — generates hardware atomic sequences using only the "movli.l" and "movco.l" instructions ⚡. Only available on SH4A, and it's suitable for multi-core systems. Now, since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables gets emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if it's executed on an SH4A system. And for this model, no additional support from the system's interrupt/exception handling code is required. Iconic.
  • gbr-offset= — this parameter specifies the offset in bytes of the variable in the thread control block structure that the generated atomic sequences should use when the soft-tcb model has been selected 💜. For other models this parameter is just ignored. The value you give must be an integer multiple of four and in the range 0-1020. Numbers, bestie, respect them.
  • strict — this parameter prevents mixed usage of multiple atomic models, even the compatible ones, and makes the compiler generate atomic sequences of the specified model only ✨. One model, one vibe.
• -mtas — generates the "tas.b" opcode for __atomic_test_and_set 🌸. But real talk: depending on your particular hardware and software configuration, this can degrade overall performance because of the operand cache line flushes that the "tas.b" instruction implies 😭. And on multi-core SH4A processors, "tas.b" must be used with caution since it can result in data corruption for certain cache configurations. That's a big warning, not a little one.
• -mprefergot — when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table 👑. GOT > PLT, apparently.
• -musermode, -mno-usermode — don't allow (or do allow) the compiler to generate privileged mode code 🫶. Specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code wouldn't work in user mode. -musermode is the default when the target is "sh*-*-linux*". And if the target is SH1* or SH2*, -musermode has no effect at all, since there's no user mode. It's giving "there is no war in ba sing se" 💀.
• -multcost=number — sets the cost to assume for a multiply insn. Yes, that's the whole thing. Give it a number, it believes you 💅.
• -mdiv=strategy — sets the division strategy used for integer division operations. Here's the menu, and strategy can be one of:
  • call-div1 — calls a library function that uses the single-step division instruction "div1" to do the operation. Division by zero calculates an unspecified result and does not trap 🙈. This is the default except for SH4, SH2A and SHcompact.
  • call-fp — calls a library function that performs the operation in double precision floating point ✨. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. And if you specify this for targets that don't have a double precision FPU, it defaults to "call-div1".
  • call-table — calls a library function that uses a lookup table for small divisors and the "div1" instruction with case distinction for larger divisors 🌈. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that don't have dynamic shift instructions defaults to "call-div1".

And when you haven't specified a division strategy, the default one is picked 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 — reserves space once for outgoing arguments in the function prologue instead of around each call 🎀. Generally beneficial for performance and size. It's also needed for unwinding to avoid changing the stack frame around conditional code.
• -mdivsi3_libfunc=name — sets 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 weren't even present.
• -mfixed-range=register-range — generates code that treats the given register range as fixed registers. A fixed register is one the register allocator cannot use 🫶. Super useful when you're compiling kernel code. A register range is written as two registers separated by a dash, and you can pass multiple register ranges separated by a comma.
• -mbranch-cost=num — assumes num is the cost for a branch instruction 👑. Higher numbers make the compiler try to generate more branch-free code if it can. If you don't specify it, the value gets selected depending on the processor type you're compiling for.
• -mzdcbranch, -mno-zdcbranch — assume (or do not assume) that the zero displacement conditional branch instructions "bt" and "bf" are fast ✨. If -mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. It's enabled by default when generating code for SH4 and SH4A, and you can explicitly disable it by specifying -mno-zdcbranch.
• -mcbranch-force-delay-slot — forces the usage of delay slots for conditional branches, which stuffs the delay slot with a "nop" if no suitable instruction can be found 😭. By default this option is disabled. You can enable it to work around hardware bugs, like the ones found in the original SH7055. Bug workarounds, we love to see it.
• -mfused-madd, -mno-fused-madd — generate code that uses (or does not use) the floating-point multiply and accumulate instructions 🌸. These instructions get generated by default if hardware floating point is used. And heads up: the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off. Glow-up era.
• -mfsca, -mno-fsca — allow or disallow the compiler from emitting the "fsca" instruction for sine and cosine approximations 🦋. The -mfsca option must be used in combination with -funsafe-math-optimizations. It's enabled by default when generating code for SH4A. And using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect. Boundaries, bestie.
• -mfsrra, -mno-fsrra — allow or disallow the compiler from emitting the "fsrra" instruction for reciprocal square root approximations 🧚. The -mfsrra option must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It's enabled by default when generating code for SH4A. And using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are both in effect. Full stop, periodt.
• -mpretend-cmove — prefers zero-displacement conditional branches for conditional move instruction patterns 💅. This can result in faster code on the SH4 processor. Faster code? We're obsessed.
• -mfdpic — generates code using the FDPIC ABI 🌈.

🌞 Solaris 2 Options 💖

okay bestie, these -m options are the ones supported on Solaris 2, and there are only two of them so drink some water and let's go 🫶

• -mclear-hwcap — this one tells the compiler to remove the hardware capabilities that the Solaris assembler generated. You literally only need it when your object files use ISA extensions that the current machine doesn't support, but they check at runtime whether or not to use them. 🍓
• -mimpure-text — used in addition to -shared, this tells the compiler to not pass -z text to the linker when linking a shared object. And because of that, you can link position-dependent code into a shared object. iconic behaviour, questionable consequences ✨

It also suppresses the "relocations remain against allocatable but non-writable sections" linker error message. HOWEVER 💀 — the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. So instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. periodt.

These switches are supported in addition to the above on Solaris 2:

• -pthreads — it's a synonym for -pthread. that's the whole bullet. that's it. 💅

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🛰️ SPARC Options 🌈

These -m options are supported on the SPARC, and there are a LOT of them, so buckle up buttercup 🧚

• -mno-app-regs, -mapp-regs — specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Like global register 1, each global register 2 through 4 is then treated as an allocable register that gets clobbered by function calls. This is the default. 😌

To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option. no cap, that's the recommendation straight from the source.

• -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. That's the normal operating mode, bestie.

• -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, though. 🙌

Heads up, this one is spicy: -msoft-float changes the calling convention in the output file, so it's only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. all or nothing, iykyk 😭

• -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. classic tradeoff moment ⚖️

• -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. cute, but 👀

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. it's giving rulebreaker energy, and not in the fun way 🚨

• -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. The 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's the list of each supported architecture and their supported implementations, bestie 🧾

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3, leon5
• sparclite — f930, f934, sparclite86x
• sparclet — tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc. 🖥️

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 📟

With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips. 🏆

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type does. scheduling only, no wardrobe change 💅

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used. 🌟

• -mv8plus, -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors. 🦋
• -mvis, -mno-vis — with -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis. 👀
• -mvis2, -mno-vis2 — with -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis. ✨
• -mvis3, -mno-vis3 — turn on -mvis3 and GCC spits out code that flexes version 3.0 of the UltraSPARC Visual Instruction Set extensions ✨ it's the default whenever you're targeting a cpu that supports those instructions, like niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis, like a little domino chain of slay 💅
• -mvis3b, -mno-vis3b — same energy as -mvis3, but -mvis3b also unlocks the extra VIS instructions that showed up in the Oracle SPARC Architecture 2011 🫶 default when your cpu supports them, e.g. niagara-7 and later. And yeah, setting -mvis3b sets -mvis3, -mvis2 and -mvis too, no cap.
• -mvis4, -mno-vis4 — <purple>-mvis4 lets GCC take advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions 🌸 default for cpus that support them, such as niagara-7 and later. Setting -mvis4 also sets -mvis3b, -mvis3, -mvis2 and -mvis. the whole VIS family reunion fr 💖
• -mvis4b, -mno-vis4b — version 4.0 of the UltraSPARC Visual Instruction Set extensions plus the extra VIS instructions from the Oracle SPARC Architecture 2017 🔥 default when the cpu supports them, like m8 and later. Setting -mvis4b also sets -mvis4, -mvis3b, -mvis3, -mvis2 and -mvis — yes bestie, that's the entire chain, and it's iconic.
• -mcbcond, -mno-cbcond — <cyan>-mcbcond means GCC generates code using the UltraSPARC Compare-and-Branch-on-Condition instructions 👑 default when targeting a CPU that supports them, like Niagara-4 and later.
• -mfmaf, -mno-fmaf — with -mfmaf, GCC uses the UltraSPARC Fused Multiply-Add Floating-point instructions 🌈 default when the CPU supports them, such as Niagara-3 and later.
• -mfsmuld, -mno-fsmuld — <mint>-mfsmuld means code that uses the Floating-point Multiply Single to Double (FsMULd) instruction 💫 default when targeting a CPU that supports architecture versions V8 or V9 with an FPU — except -mcpu=leon, that one's the exception, sorry leon 🥲
• -mpopc, -mno-popc — turn on the UltraSPARC Population Count instruction with -mpopc ⭐ default when the CPU supports such an instruction, like Niagara-2 and later.
• -msubxc, -mno-subxc — <hotpink>-msubxc uses the UltraSPARC Subtract-Extended-with-Carry instruction 🎀 default when the CPU supports it, e.g. Niagara-7 and later.
• -mfix-at697f — enables the documented workaround for the single erratum of the Atmel AT697F processor (which lines up with erratum #13 of the AT697E processor) 🙈 bugs got hands, but we handle them.
• -mfix-ut699 — enables the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor 💀 two-for-one fix situation.
• -mfix-ut700 — enables the documented workaround for the back-to-back store errata of the UT699E/UT700 processor 🫶
• -mfix-gr712rc — enables the documented workaround for the back-to-back store errata of the GR712RC processor ✨

---

These -m options are supported in addition to everything above, but only on SPARC-V9 processors in 64-bit environments 🌸

• -m32, -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits 👑
• -mcmodel=which — pick the code model, one of these besties:
  • medlow — the Medium/Low code model: 64-bit addresses, and 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 sit 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, text and data segments must be less than 2GB in size, both starting anywhere in memory (decided at link time). The global register %g4 points to the base of the data segment. Programs are statically linked, and PIC is not supported — so no PIC, periodt 😭
• -mmemory-model=mem-model — sets 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 the stack pointer (and the frame pointer, if one's present) is offset by -2047, and that offset has to be added back when making stack frame references. That's the default in 64-bit mode. Otherwise, assume no such offset exists 💜

🌈 Options for System V 🌈

These extra options are available on System V Release 4, so you can vibe with other compilers living on those systems 🫶

• -G — create a shared object. It's recommended that -symbolic or -shared be used instead. (ngl, -G is giving deprecated, use the others bestie 💀)
• -Qy — identify the versions of each tool used by the compiler, in a .ident assembler directive in the output 👑
• -Qn — refrain from adding .ident directives to the output file. This one is the default ✨
• -YP,dirs — search the directories dirs, and no others, for libraries specified with -l 🔍
• -Ym,dir — look in the directory dir to find the M4 preprocessor. The assembler uses this option 🎀

💅 V850 Options 💅

These -m options are defined for V850 implementations:

• -mlong-calls, -mno-long-calls — treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register and calls indirect through the pointer 💫
• -mno-ep, -mep — do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times, copying the pointer into the ep register and using the shorter sld and sst instructions. -mep is on by default if you optimize 👑
• -mno-prolog-function, -mprolog-function — do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but they use less code space if more than one function saves the same number of registers 🫶 -mprolog-function is on by default if you optimize.
• -mspace — try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options 🐜 tiny code, tiny vibe.
• -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 can hold up to 256 bytes in total (128 bytes for byte references) 🎀
• -msda=n — put static or global variables whose size is n bytes or less into the small data area that register gp points to. The small data area can hold up to 64 kilobytes ✨
• -mzda=n — put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory 💜
• -mv850 — specify that the target processor is the V850.
• -mv850e3v5 — specify that the target processor is the V850E3V5. The preprocessor constant __v850e3v5__ is defined if this option is used.
• -mv850e2v4 — specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option 👯
• -mv850e2v3 — specify that the target processor is the V850E2V3. The preprocessor constant __v850e2v3__ is defined if this option is used.
• -mv850e2 — specify that the target processor is the V850E2. The preprocessor constant __v850e2__ is defined if this option is used.
• -mv850e1 — specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ are defined if this option is used.
• -mv850es — specify that the target processor is the V850ES. This is an alias for the -mv850e1 option ✨
• -mv850e — specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this option is used.

fr fr, if none of -mv850, -mv850e, -mv850e1, -mv850e2, -mv850e2v3 or -mv850e3v5 are defined, then a default target processor gets chosen and the relevant __v850*__ preprocessor constant is defined 🧚

The preprocessor constants __v850 and __v851__ are always defined, no matter which processor variant is the target 💅 periodt.

• -mdisable-callt, -mno-disable-callt — this option suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture 😤

It's enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated, then the C preprocessor symbol __V850_CALLT__ is defined 👑

• -mrelax, -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler 🫶
• -mlong-jumps, -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions 🌈
• -msoft-float, -mhard-float — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher 💫 If hardware floating point instructions are being generated, then the C preprocessor symbol __FPU_OK__ is defined; otherwise the symbol __NO_FPU__ is defined.
• -mloop — enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected, because its use is still experimental 🙈 (experimental = proceed with caution bestie, it's giving beta energy 💀)
• -mrh850-abi, -mghs — turns on support for the RH850 flavor of the V850 ABI, and bestie this one is the default 💅 under this version of the ABI these rules hit:
  • Integer sized structures and unions get returned through a memory pointer instead of a register 👀
  • Big structures and unions (anything more than 8 bytes in size) get passed by value.
  • Functions get aligned to 16-bit boundaries ✨
  • The -m8byte-align command-line option is supported 🫶
  • The -mdisable-callt command-line option is switched on 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__" gets defined 👑

• -mgcc-abi — turns on support for the old GCC version of the V850 ABI 🌸 and with this version these rules apply:
  • Integer sized structures and unions get returned in register "r10" 💖
  • Big structures and unions (more than 8 bytes in size) get passed by reference.
  • Functions get aligned to 32-bit boundaries, unless you're optimizing for size 🎀
  • The -m8byte-align command-line option is not supported 🙈
  • The -mdisable-callt command-line option is supported but it's not on by default.

When this version of the ABI is enabled the C preprocessor symbol "__V850_GCC_ABI__" gets defined ✨

• -m8byte-align, -mno-8byte-align — lets "double" and "long long" types be aligned on 8-byte boundaries 🌈 the default tho 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__" gets defined 💜
• -mbig-switch — generates code that's suitable for big switch tables 🍓 only reach for this option if the assembler/linker are complaining about out of range branches within a switch table, fr fr.
• -mapp-regs — makes r2 and r5 get used in the code the compiler generates 💅 this setting is the default.
• -mno-app-regs — makes r2 and r5 get treated as fixed registers 🎀

🌟 VAX Options 🌟

These -m options are defined for the VAX:

• -munix — do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX can't handle across long ranges 🙈
• -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used ✨
• -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.

---

🦋 Visium Options 🦋

• -mdebug — a program that does file I/O and is destined to run on an MCM target should be linked with this option 🫶 it causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the control of the GDB remote debugging stub 👑
• -msim — a program that does file I/O and is destined to run on the simulator should be linked with this option 💅 it causes libraries libc.a and libsim.a to be linked.
• -mfpu, -mhard-float — generate code containing floating-point instructions ✨ this is the default.
• -mno-fpu, -msoft-float — generate code containing library calls for floating-point 🎀

Careful bestie — -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 😭

• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type 🌈 supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility 💜

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture ✨

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions 👀

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type would 🙈
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on the access to general registers 💅 this is the default.
• -muser-mode — generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 can't be accessed in this mode; on the GR6, only registers r29 to r31 are affected 😭

---

⭐ VMS Options ⭐

These -m options are defined for the VMS implementations:

• -mvms-return-codes — return VMS condition codes from "main". The default is to return POSIX-style condition (e.g. error) codes 💖
• -mdebug-main=prefix — flag the first routine whose name starts with prefix as the main routine for the debugger 🎀
• -mmalloc64 — default to 64-bit memory allocation routines 👑
• -mpointer-size=size — set the default size of pointers. Possible options for size are 32 or short for 32 bit pointers, 64 or long for 64 bit pointers, and no for supporting only 32 bit pointers 🌸 that later option disables "pragma pointer_size".

🧚 VxWorks Options 🧚

The options in this section are defined for all VxWorks targets 💅 options specific to the target hardware are listed with the other options for that target.

• -mrtp — GCC can generate code for both VxWorks kernels and real time processes (RTPs) ✨ this option switches from the former to the latter. It also defines the preprocessor macro "__RTP__" 💖
• -msmp — select SMP runtimes for linking 👀 not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice 🙈
• -non-static — link an RTP executable against shared libraries rather than static libraries 🌈 the options -static and -shared can also be used for RTPs; -static is the default.
• -Bstatic, -Bdynamic — these options get passed down to the linker 🫶 they're defined for compatibility with Diab.
• -Xbind-lazy — enable lazy binding of function calls 💅 this option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
• -Xbind-now — disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab 🎀

---

🔥 x86 Options 🔥

These -m options are defined for the x86 family of computers.

• -march=cpu-type — generate instructions for the machine type cpu-type ✨ in contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type lets GCC generate code that may not run at all on processors other than the one indicated 💀 specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

• native — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine 👑 using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines 😭). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
• x86-64 — a generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support 💖
• x86-64-v2, x86-64-v3, x86-64-v4 — these choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI 🌸 on ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.

Since these cpu-type values don't 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.) 🙈
• i586, pentium — Intel Pentium CPU with no MMX support.
• lakemont — Intel Lakemont MCU, based on Intel Pentium CPU 🎀
• pentium-mmx — Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support 🌈
• pentiumpro — Intel Pentium Pro CPU with no MMX support.
• i686 — when used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips 💖 when used with -mtune, it has the same meaning as generic.
• pentium2 — Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support 🌸
• pentium3, pentium3m — Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support ✨
• pentium-m — Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support 🫶 used by Centrino notebooks.
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support 👑
• prescott — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support 💜
• nocona — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support 🧚
• core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support 🔥
• nehalem, corei7 — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support ⭐

• westmere — Intel Westmere CPU with 64-bit extensions, and instruction set support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL. solid mid-2010s energy ✨
• sandybridge
• corei7-avx — Intel Sandy Bridge CPU with 64-bit extensions, and instruction set support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL. two names, one CPU, we love an alias moment 💅
• ivybridge
• core-avx-i — Intel Ivy Bridge CPU with 64-bit extensions, and instruction set support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C. 🦋
• haswell
• core-avx2 — Intel Haswell CPU with 64-bit extensions, and instruction set support for 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. 🔥
• broadwell — Intel Broadwell CPU with 64-bit extensions, and instruction set support for 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. ✨
• skylake — Intel Skylake CPU with 64-bit extensions, and instruction set support for 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. the way she just keeps adding letters 😭
• skylake-avx512 — Intel Skylake Server CPU with 64-bit extensions, and instruction set support for 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. server edition, main character energy 👑
• cascadelake — Intel Cascade Lake CPU with 64-bit extensions, and instruction set support for 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. 💜
• cannonlake — Intel Cannon Lake Server CPU with 64-bit extensions, and instruction set support for 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. 🎀
• cooperlake — Intel Cooper Lake CPU with 64-bit extensions, and instruction set support for 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. 🌸
• icelake-client — Intel Ice Lake Client CPU with 64-bit extensions, and instruction set support for 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. 💖
• icelake-server — Intel Ice Lake Server CPU with 64-bit extensions, and instruction set support for 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. the way this one eats and leaves no crumbs 🧚
• tigerlake — Intel Tiger Lake CPU with 64-bit extensions, and instruction set support for 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. 🔥
• rocketlake — Intel Rocket Lake CPU with 64-bit extensions, and instruction set support for 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. ⭐
• alderlake
• raptorlake
• meteorlake
• gracemont — Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont CPU with 64-bit extensions, and instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI. four names, one lil squad, iconic 🌈
• arrowlake — Intel Arrow Lake CPU with 64-bit extensions, and instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT and CMPCCXADD. 💜
• arrowlake-s
• lunarlake — Intel Arrow Lake S/Lunar Lake CPU with 64-bit extensions, and instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3 and SM4. 🦋
• pantherlake — Intel Panther Lake CPU with 64-bit extensions, and instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3 and SM4. 🎀
• sapphirerapids
• emeraldrapids — Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions, and instruction set support for 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. obsessed with her fr 😭
• graniterapids — Intel Granite Rapids CPU with 64-bit extensions, and instruction set support for 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. 🔥
• graniterapids-d — Intel Granite Rapids D CPU with 64-bit extensions, and instruction set support for 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. the grand finale of CPU names and she ATE 💅💖
• diamondrapids — Intel Diamond Rapids CPU, and bestie she is stacked 💅 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 and SSSE3 instruction set support 🌸
• silvermont, slm — Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support 💜
• goldmont — Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT and FSGSBASE instruction set support 🎀
• goldmont-plus — Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID and SGX instruction set support 🌈
• tremont — Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set support 🦋
• sierraforest — Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support 🌲
• grandridge — Intel Grand Ridge CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support ⛰️
• clearwaterforest — Intel Clearwater Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI instruction set support 💧
• 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.) no cap 👑
• k8-sse3, opteron-sse3, athlon64-sse3 — Improved versions of AMD K8 cores with SSE3 instruction set support.
• amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
• bdver1 — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver2 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver3 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• bdver4 — AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
• znver1 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.) ✨
• znver2 — AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)
• znver3 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)
• znver4 — AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.) 🔥
• znver5 — AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI and 64-bit instruction set extensions.) 👑
• btver1 — CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
• btver2 — CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
• winchip-c6 — IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support 💾
• winchip2 — IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support 💾
• c3 — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
• c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
• c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• samuel-2 — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
• nehemiah — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
• esther — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
• eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.) 🌸
• <code>eden-x4</code> — VIA Eden X4, slaying 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 — so like, it's invited to the party but nobody's doing the choreography for it. 😭)
• <code>nano</code> — generic VIA Nano, holding x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• <code>nano-1000</code> — VIA Nano 1xxx, with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• <code>nano-2000</code> — VIA Nano 2xxx, with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
• <code>nano-3000</code> — VIA Nano 3xxx, upgraded to x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• <code>nano-x2</code> — VIA Nano Dual Core, with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• <code>nano-x4</code> — VIA Nano Quad Core, also with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
• <code>lujiazui</code> — ZHAOXIN lujiazui, iykyk, 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 — so don't come for them, it's a deliberate choice bestie.
• <code>yongfeng</code> — ZHAOXIN yongfeng 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. ✨
• <code>shijidadao</code> — ZHAOXIN shijidadao 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. Same energy fr fr.
• <code>geode</code> — AMD Geode embedded processor, tiny but mighty, with MMX and 3DNow! instruction set support. 🎀
• <code>c86-4g-m4</code> — HYGON c86-4g-m4 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. 🌸
• <code>c86-4g-m6</code> — HYGON c86-4g-m6 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. 🌈
• <code>c86-4g-m7</code> — HYGON c86-4g-m7 and she ATE, okay? 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. 👑

---

-mtune=cpu-type 💖

Tune to cpu-type everything applicable about the generated code — except for the ABI and the set of available instructions. So like, picking a specific cpu-type schedules things appropriately for that particular chip, but the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. Periodt. Example that makes it click: 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. Bestie, it's giving "dress for the job you want, but keep your old shoes on" 🫶

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

• <code>generic</code> — Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option. 🦋

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. (It's a moving target, ngl, and that's the whole point.)

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. The way that distinction matters. 💅

• <code>intel</code> — Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option. ✨

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released. Iconic and slightly chaotic of it, honestly.

There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 🎀

---

-mcpu=cpu-type 💀

A deprecated synonym for -mtune. That's it, that's the whole bullet. Not me warning you — it's literally deprecated, so maybe don't build your whole aesthetic on it. 🙈

---

-mfpmath=unit 🌸

Generate floating-point arithmetic for selected unit. The choices for unit are:

• <code>387</code> — Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description.

This is the default choice for non-Darwin x86-32 targets.

• <code>sse</code> — Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too. 🦋

For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default.

The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. ⚠️ So like, test it, bestie.

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.

• <code>sse,387</code>
• <code>sse+387</code>
• <code>both</code> — Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance. 😭 Not me telling you to be careful — the manpage is telling you.

---

-masm=dialect 🧚

Output assembly instructions using selected dialect. Also affects which dialect is used for basic asm and extended asm. Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel. 🎀

---

-mieee-fp / -mno-ieee-fp ⭐

Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where the result of a comparison is unordered. No cap, that's the whole point of them. ✨

---

-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. Bestie. You're on your own for this one, fr fr. 🫶

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. So it's not fully soft, and that's just how it is. 😭

---

-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. Cute theory, honestly. 🎀

The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead. 💅

• -mno-fancy-math-387 — bestie, some 387 emulators straight up do not support the sin, cos and sqrt instructions for the 387, so you pass this and GCC keeps those out of your code. 💅 Heads up: this option gets overridden whenever -march says the target CPU always has an FPU, because then the instruction doesn't need emulating at all. Also, lowkey important — those instructions don't get generated unless you also use the -funsafe-math-optimizations switch. ✨
• -malign-double, -mno-align-double — these two control whether GCC aligns double, long double, and long long variables on a two-word boundary or a one-word boundary. Aligning double vars on a two-word boundary gives you code that runs somewhat faster on a Pentium, but it costs you more memory. 🫶

On x86-64, -malign-double is enabled by default.

⚠️ Warning: if you use the -malign-double switch, structures containing the above types end up aligned differently than the published application binary interface specifications for x86-32, and they are not binary compatible with structures in code compiled without that switch. 💀

• -m96bit-long-double, -m128bit-long-double — these switches control the size of the long double type. The x86-32 application binary interface specifies the size as 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, that's just not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. ✨

In the x86-64 compiler, -m128bit-long-double is the default choice, since its ABI specifies that long double is aligned on a 16-byte boundary.

Notice, bestie: neither of these options enables any extra precision over the x87 standard of 80 bits for a long double. Fr fr.

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, and it also modifies the function calling convention for functions taking long double. So they are not binary-compatible with code compiled without that switch. 😭

• -mlong-double-64, -mlong-double-80, -mlong-double-128 — these switches also control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type — and that's the default for the 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type — and that's the default for the 64-bit Bionic C library. 💖

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, and it modifies the function calling convention for functions taking long double too. Hence they are not binary-compatible with code compiled without that switch. 🙈

• -malign-data=type — controls how GCC aligns variables. The supported values for type are:
  • compat — uses an increased alignment value compatible with GCC 4.8 and earlier
  • abi — uses the alignment value as specified by the psABI
  • cacheline — uses an increased alignment value to match the cache line size

And compat is the default. 👑

• -mlarge-data-threshold=threshold — when -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold get placed in large data sections. The default is 65535. 🍓
• -mrtd — uses a different function-calling convention, bestie, where functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller, since there's no need to pop the arguments there. Slay. ✨

You can specify that an individual function is called with this calling sequence using the function attribute stdcall. And you can also override the -mrtd option by using the function attribute cdecl.

⚠️ Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. 🚫

⚠️ 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.) 😭

• -mregparm=num — controls how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm. 🌸

⚠️ Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. That includes the system libraries and startup modules. 💅

• -msseregparm — uses SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm. 🦋

⚠️ Warning: if you use this switch then you must build all modules with the same value, including any libraries. That includes the system libraries and startup modules. 😭

• -mvect8-ret-in-mem — returns 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks, to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. 🫶
• -mpc32, -mpc64, -mpc80 — set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision); and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. 🔥

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. 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. 💀

• -mdaz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. SSE and AVX instructions, including scalar and vector instructions, could benefit from enabling the FTZ and DAZ flags when -mdaz-ftz is specified. Don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is specified — -mdaz-ftz will set FTZ/DAZ flags even with -shared. 😭
• -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. 💖
• -mpreferred-stack-boundary=num — attempts to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits). ✨

⚠️ Warning: when generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to an 8 byte boundary. Since x86-64 ABI requires 16 byte stack alignment, this is ABI incompatible and intended to be used in a controlled environment where stack space is an important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with a misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 💀

• -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used. 🎀

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. 🦋

To ensure proper alignment of these values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. So calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🌸

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2. 🫶

okay bestie, meet the extended-instruction-set roster 💅 every single one of these little cuties flips on a family of instructions, and every one of them has a matching -mno- twin that turns it back off:

• -mmmx
• -msse
• -msse2
• -msse3
• -mssse3
• -msse4
• -msse4a
• -msse4.1
• -msse4.2
• -mavx
• -mavx2
• -mavx512f
• -mavx512cd
• -mavx512vl
• -mavx512bw
• -mavx512dq
• -mavx512ifma
• -mavx512vbmi
• -msha
• -maes
• -mpclmul
• -mclflushopt
• -mclwb
• -mfsgsbase
• -mptwrite
• -mrdrnd
• -mf16c
• -mfma
• -mpconfig
• -mwbnoinvd
• -mfma4
• -mprfchw
• -mrdpid
• -mrdseed
• -msgx
• -mxop
• -mlwp
• -m3dnow
• -m3dnowa
• -mpopcnt
• -mabm
• -madx
• -mbmi
• -mbmi2
• -mlzcnt
• -mfxsr
• -mxsave
• -mxsaveopt
• -mxsavec
• -mxsaves
• -mrtm
• -mhle
• -mtbm
• -mmwaitx
• -mclzero
• -mpku
• -mavx512vbmi2
• -mavx512bf16
• -mavx512fp16
• -mgfni
• -mvaes
• -mwaitpkg
• -mvpclmulqdq
• -mavx512bitalg
• -mmovdiri
• -mmovdir64b
• -menqcmd
• -muintr
• -mtsxldtrk
• -mavx512vpopcntdq
• -mavx512vp2intersect
• -mavx512vnni
• -mavxvnni
• -mcldemote
• -mserialize
• -mamx-tile
• -mamx-int8
• -mamx-bf16
• -mhreset
• -mkl
• -mwidekl
• -mavxifma
• -mavxvnniint8
• -mavxneconvert
• -mcmpccxadd
• -mamx-fp16
• -mprefetchi
• -mraoint
• -mamx-complex
• -mavxvnniint16
• -msm3
• -msha512
• -msm4
• -mapxf
• -musermsr
• -mavx10.1
• -mavx10.1-256
• -mavx10.1-512
• -mavx10.2
• -mamx-avx512
• -mamx-tf32
• -mamx-transpose
• -mamx-fp8
• -mmovrs
• -mamx-movrs

fr fr, these switches enable the use of instructions in the MMX, SSE, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX512VNNI, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F, USER_MSR, AVX10.1, AVX10.2, AMX-AVX512, AMX-TF32, AMX-TRANSPOSE, AMX-FP8, MOVRS, AMX-MOVRS or CLDEMOTE extended instruction sets. and yep, each one has a corresponding -mno- option to disable use of these instructions. every single one. no favorites ✨

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. 🌈

💖 bestie heads up: -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. they're separate. they don't talk. leave them alone.

to generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse. 👑

GCC depresses SSEx instructions when -mavx is used. instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed. the glow-up is real 💅

these options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. do not come for me, that's just how it is 🙈

---

• -mdump-tune-features — this one tells GCC to dump the names of the x86 performance tuning features and default settings. those names can be used in -mtune-ctrl=feature-list. it's giving table of contents, honestly 📋
• -mtune-ctrl=feature-list — this is for 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. ngl this one 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. 😭 so like, respect the warning bestie
• -mno-default — this option instructs GCC to turn off all tunable features. see also -mtune-ctrl=feature-list and -mdump-tune-features. it's the "clear the whole cart" button 🛒
• -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. so like, at least you have an off switch 🫶
• -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. it's the little goodbye hug before you leave ✨
• -mprefer-avx128 — this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. smaller vector, no drama 💐
• -mprefer-vector-width=opt — this option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform. you pick the width, it does the thing 📐
• -mpartial-vector-fp-math — this option enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless -fno-trapping-math is specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when -fno-trapping-math is in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using -mdaz-ftz to set denormals-are-zero (DAZ) flag in the MXCSR register.

This option is enabled by default. 🦋

• -mmove-max=bits — this option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are 128, 256 and 512. ✨
• -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 the width menu, for when you're picking a vibe:

• 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 — this option optimizes functions with "noreturn" attribute or "_Noreturn" specifier by not saving in the function prologue callee-saved registers which are used in the function (except for the "BP" register). This option can interfere with debugging of the caller of the "noreturn" function or any function further up in the call stack, so it is not enabled by default. ⚠️ so if your debugger starts acting weird, you know who did it
• -mcx16 — this option enables GCC to generate "CMPXCHG16B" instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding one machine word in size. The compiler uses this instruction to implement __sync Builtins. However, for __atomic Builtins operating on 128-bit integers, a library call is always used. 🔥 atomic slay
• -msahf — this option 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. 🧚 history lesson but make it silicon
• -mmovbe — this option enables use of the "movbe" instruction to optimize byte swapping of four and eight byte entities. flip em around bestie 🔄
• -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). 🛡️
• -mcrc32 — this option enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the "crc32" machine instruction. ✨
• -mmwait — this option enables built-in functions __builtin_ia32_monitor, and __builtin_ia32_mwait to generate the "monitor" and "mwait" machine instructions. 👀 it's literally watching
• -mrecip — this option 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). 😭 0.99999994 is not 1 and it will simply never be 1

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. so like. it's covered already bestie, you don't have to ask twice 🎀

-mrecip=opt 🌸

Okay bestie, this one is the little switchboard for which reciprocal estimate instructions your code is even allowed to use. The opt there is a comma-separated list of options, and any one of them can be preceded by a ! to invert it (i.e. turn it off instead of on). 💅

• all — enable every estimate instruction. go absolutely feral, no estimates left behind.
• default — enable the default instructions, which is exactly equivalent to plain -mrecip.
• none — disable all estimate instructions, exactly equivalent to -mno-recip.
• div — turn on the approximation for scalar division.
• vec-div — turn on the approximation for vectorized division.
• sqrt — turn on the approximation for scalar square root.
• vec-sqrt — turn on the approximation for vectorized square root.

So like, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. icon behavior fr. ✨

---

-mveclibabi=type 🦋

This one specifies the ABI type to use for vectorizing intrinsics using an external library. The 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). Bestie, to use this option both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. no shortcuts, okay?? 😭

GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for the corresponding function type when -mveclibabi=svml is used. 👑

Then there's the absolute wall of names for -mveclibabi=aocl: amd_vrs4_acosf, amd_vrs16_acosf, amd_vrd8_asin, amd_vrs4_asinf, amd_vrs8_asinf, amd_vrs16_asinf, amd_vrd2_atan, amd_vrd8_atan, amd_vrs4_atanf, amd_vrs8_atanf, amd_vrs16_atanf, amd_vrd2_cos, amd_vrd4_cos, amd_vrd8_cos, amd_vrs4_cosf, amd_vrs8_cosf, amd_vrs16_cosf, amd_vrs4_coshf, amd_vrs8_coshf, amd_vrd2_erf, amd_vrd4_erf, amd_vrd8_erf, amd_vrs4_erff, amd_vrs8_erff, amd_vrs16_erff, amd_vrd2_exp, amd_vrd4_exp, amd_vrd8_exp, amd_vrs4_expf, amd_vrs8_expf, amd_vrs16_expf, amd_vrd2_exp10, amd_vrs4_exp10f, amd_vrd2_exp2, amd_vrd4_exp2, amd_vrd8_exp2, amd_vrs4_exp2f, amd_vrs8_exp2f, amd_vrs16_exp2f, amd_vrs4_expm1f, amd_vrd2_log, amd_vrd4_log, amd_vrd8_log, amd_vrs4_logf, amd_vrs8_logf, amd_vrs16_logf, amd_vrd2_log10, amd_vrs4_log10f, amd_vrs8_log10f, amd_vrs16_log10f, amd_vrd2_log1p, amd_vrs4_log1pf, amd_vrd2_log2, amd_vrd4_log2, amd_vrd8_log2, amd_vrs4_log2f, amd_vrs8_log2f, amd_vrs16_log2f, amd_vrd2_pow, amd_vrd4_pow, amd_vrd8_pow, amd_vrs4_powf, amd_vrs8_powf, amd_vrs16_powf, amd_vrd2_sin, amd_vrd4_sin, amd_vrd8_sin, amd_vrs4_sinf, amd_vrs8_sinf, amd_vrs16_sinf, amd_vrd2_tan, amd_vrd4_tan, amd_vrd8_tan, amd_vrs16_tanf, amd_vrs4_tanhf, amd_vrs8_tanhf, amd_vrs16_tanhf for the corresponding function type when -mveclibabi=aocl is used.

And for -mveclibabi=acml, the list is __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used. 🫶

---

-mabi=name 💖

Generate code for the specified calling convention. The permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. And bestie, you can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. ✨

---

-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. main character energy for your stack traces, honestly.

---

-mmanual-endbr 👑

Insert the ENDBR instruction at function entry only via the cf_check function attribute. This is useful when used with the option -fcf-protection=branch to control ENDBR insertion at the function entry. 💅

---

-mcet-switch 🌈

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 — and this is the important part — if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET. This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets being reachable via any indirect jumps. 🔥

---

-mcall-ms2sysv-xlogues 🎀

So here's the tea: due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted inline, which gives you fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing function size at the cost of a few extra instructions. trade-offs, bestie, we love her. 💖

---

-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. so like, glowier but higher-maintenance. 💅

---

-mpush-args, -mno-push-args 🌸

Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. In some cases disabling it may improve performance, because of improved scheduling and reduced dependencies.

---

-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? A notable increase in code size. 😭 Also this switch implies -mno-push-args.

---

-mthreads 💜

Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. it's giving organized. ✨

---

-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. 🙈

This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.

The Microsoft structure layout algorithm is fairly simple, with the exception of the bit-field packing. The padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.

MSVC interprets zero-length bit-fields in the following ways:

1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.

For example:

struct
 {
   unsigned long bf_1 : 12;
   unsigned long : 0;
   unsigned long bf_2 : 12;
 } t1;

The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1's size would be 4 bytes.

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 that zero-length bit-field may affect the alignment of the structure as a whole. Like, 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, bestie, it may still mess with the alignment of the structure:

struct
 {
   char foo : 6;
   long : 0;
 } t4;

Here, "t4" takes up 4 bytes. slay, alignment said nope 💅

3. Zero-length bit-fields that follow non-bit-field members are straight up ignored:

struct
 {
   char foo;
   long : 0;
   char bar;
 } t5;

Here, "t5" takes up 2 bytes.

---

• -mno-align-stringops — Don't align the destination of inlined string operations. This one keeps your code smaller and can even go faster when the destination is already aligned but GCC has no idea about it. 🙈
• -minline-all-stringops — By default GCC only inlines string operations when it knows the destination is aligned to at least a 4-byte boundary. This turns up the inlining, ups the code size, but might make code that leans on fast "memcpy" and "memset" for short lengths go vroom. It also enables inline expansion of "strlen" for all pointer alignments. ✨
• -minline-stringops-dynamically — For string operations of unknown size, it uses run-time checks: inline code for small blocks, and a library call for the big blocks. main character energy: adapt to the vibe at runtime. 🦋
• -mstringop-strategy=alg — Override the internal decision heuristic for which algorithm gets used to inline string operations. Allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte — expand using the i386 "rep" prefix of the specified size.
  • byte_loop
  • loop
  • unrolled_loop — expand into an inline loop.
  • libcall — always use a library call. no inlining, just vibes and a function call.
• -mmemcpy-strategy=strategy — Override the internal decision heuristic that decides whether "__builtin_memcpy" should be inlined, and which inline algorithm to use when the expected size of the copy is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. The alg is specified in -mstringop-strategy, and max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, max_size must be -1. The max_size of the triplets in the list must be 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 — Basically the same energy as -mmemcpy-strategy=, except this one controls "__builtin_memset" expansion.
• -momit-leaf-frame-pointer — Don't keep the frame pointer in a register for leaf functions. This skips the instructions to save, set up, and restore frame pointers, and frees up an extra register in leaf functions. Just so you know, the option -fomit-leaf-frame-pointer also removes the frame pointer for leaf functions — which, ngl, might make debugging harder. 😭
• -mtls-direct-seg-refs
• -mno-tls-direct-seg-refs — Controls whether TLS variables can be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer has to be added in. Whether or not that's even valid depends on the operating system, and on whether it maps the segment to cover the entire TLS area.

For systems using the GNU C Library, the default is on.

• -msse2avx
• -mno-sse2avx — Says the assembler should encode SSE instructions with a 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. Heads up though: on x86 architectures the attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg. 💀
• -mrecord-mcount
• -mno-record-mcount — If profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls.
• -mnop-mcount
• -mno-nop-mcount — If profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. It's likely only useful together with -mrecord-mcount.
• -minstrument-return=type — Instrument function exit in -pg -mfentry instrumented functions with a call to a specified function. This only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are none to not instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop.
• -mrecord-return
• -mno-record-return — Generate a __return_loc section pointing to all return instrumentation code.
• -mfentry-name=name — Set the name of the __fentry__ symbol called at function entry for -pg -mfentry functions.
• -mfentry-section=name — Set the name of the section to record -mrecord-mcount calls (default __mcount_loc).
• -mskip-rax-setup
• -mno-skip-rax-setup — When generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to skip setting up the RAX register when there are no variable arguments passed in vector registers.

Warning bestie: since the RAX register is used to avoid unnecessarily saving vector registers on the stack when passing variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location. 💀 GCC 4.4 or newer don't have those issues, regardless of the RAX register value.

• -m8bit-idiv
• -mno-8bit-idiv — On some processors, like the Intel Atom, an 8-bit unsigned integer divide is way faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both the dividend and divisor are within range of 0 to 255, an 8-bit unsigned integer divide is used instead of the 32-bit/64-bit integer divide. ate. 🔥
• -mavx256-split-unaligned-load
• -mavx256-split-unaligned-store — Split 32-byte AVX unaligned load and store.
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mstack-protector-guard-symbol=symbol — Generate stack protection code using a canary at guard. Supported locations are global for a global canary or tls for a per-thread canary in the TLS block (the default). This option only has effect 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 further specify which segment register (%fs or %gs) to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI.

-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 stops the compiler from using floating-point, vector, mask and bound registers. no fancy registers allowed, periodt. 💅
• -mrelax-cmpxchg-loop — When emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins that lack a native instruction, optimize for the super-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 an inlined call and return thunk. thunk-extern converts indirect call and jump to an 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 an inlined call and return thunk. thunk-extern converts function return to an 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.
• -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 a CS prefix to call and jmp to indirect thunk with the branch target in r8-r15 registers, so that the call and jmp instruction length is 6 bytes. That lets them be replaced at run-time with lfence; call *%r8-r15 or lfence; jmp *%r8-r15. 👑
• -mapx-inline-asm-use-gpr32 — For inline asm support with APX, by default the EGPR feature was disabled to prevent potential illegal instructions with EGPR from happening. To invoke egpr usage in inline asm, use the new compiler option -mapx-inline-asm-use-gpr32, and the user should make sure the instruction supports EGPR. 🫶
• -mevex512, -mno-evex512 — turns 512-bit vectors on or off. bestie it's just gonna be the default whenever AVX512F is enabled, that's the whole vibe 💅

These -m switches are also supported on top of everything above, but like, specifically on x86-64 processors in 64-bit environments. iykyk ✨

• -m32, -m64, -mx32, -m16, -miamcu — generate code for a 16-bit, 32-bit or 64-bit environment, depending on which one you pick. lemme break it down:

The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.

The -m64 option sets int to 32 bits, and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only, -m64 ALSO turns off the -fno-pic and -mdynamic-no-pic options. the way it just does that 🫶

The -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.

The -m16 option is literally the same as -m32, except it also outputs the .code16gcc assembly directive at the top of the assembly output so the binary can actually run in 16-bit mode. iconic.

The -miamcu option generates code that conforms to Intel MCU psABI. it requires -m32 to be turned on tho, no skipping that step bestie 😭

• -mno-red-zone — do NOT use a so-called "red zone" for x86-64 code. so here's the tea: the red zone is mandated by the x86-64 ABI. it's a 128-byte area beyond wherever the stack pointer is sitting, and it's not modified by signal or interrupt handlers, so it can be used for temporary data without touching the stack pointer. -mno-red-zone disables this red zone. no zone for you 🙈
• -mcmodel=small — generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. pointers are 64 bits. programs can be statically or dynamically linked. this is the DEFAULT code model, the main character ✨
• -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, no ands ifs or buts 💀
• -mcmodel=medium — generate code for the medium model: the program is linked in the lower 2 GB of the address space. small symbols also get placed there. symbols with sizes larger than -mlarge-data-threshold get 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 absolutely no assumptions about addresses and sizes of sections. it's giving unbothered 👑
• -maddress-mode=long — generate code for long address mode. this is only supported for 64-bit and x32 environments. it's 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's the default address mode for 32-bit and x32 environments.
• -mneeded, -mno-needed — emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for the Linux target, so it can indicate the micro-architecture ISA level required to execute the binary. slay 🎀
• -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 bestie: shared libraries compiled with -mno-direct-extern-access and an executable compiled with -mdirect-extern-access may not be binary compatible if protected symbols are used in the shared libraries and the executable. fr fr, be careful 😭

• -munroll-only-small-loops — controls conservative small loop unrolling. it's default enabled by O2, and unrolls loops with less than 4 insns by 1 time. explicitly using -f[no-]unroll-[all-]loops would disable this flag, so you avoid any unintended unrolling behavior that you, the user, did not want.
• -mlam=choice — LAM (linear-address masking) lets special bits in the pointer be used for metadata. the default is none. with u48, pointer bits in positions 62:48 can be used for metadata; with u57, pointer bits in positions 62:57 can be used for metadata. cute lil bit twiddling ✨

💻 x86 Windows Options 💻

---

Xstormy16 Options 🦋

These options are defined for Xstormy16:

• -msim — choose startup files and linker script suitable for the simulator. simple, clean, serving 🫶

---

Xtensa Options 🌸

These options are supported for Xtensa targets:

• -mconst16, -mno-const16 — enable or disable use of CONST16 instructions for loading constant values. the CONST16 instruction is currently not a standard option from Tensilica. when enabled, CONST16 instructions are ALWAYS used in place of the standard L32R instructions. the use of CONST16 is enabled by default only if the L32R instruction isn't available. conditional queen 👑
• -mfused-madd, -mno-fused-madd — enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. this does absolutely nothing if the floating-point option isn't also enabled. disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. this may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions don't round the intermediate result, which produces results with MORE bits of precision than the IEEE standard specifies. disabling fused multiply add/subtract instructions also ensures the program output isn't sensitive to the compiler's ability to combine multiply and add/subtract operations. precision girlies, this one's for you 💅
• -mserialize-volatile, -mno-serialize-volatile — when enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. the default is -mserialize-volatile. use -mno-serialize-volatile to omit those MEMW instructions.
• -mforce-no-pic — for targets like GNU/Linux where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code.
• -mtext-section-literals, -mno-text-section-literals — these options control how literal pools are treated. 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 lets the linker combine literal pools from separate object files to remove redundant literals and improve code size. with -mtext-section-literals, the literals get interspersed in the text section so they stay 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. cozy lil placement 🎀
• -mauto-litpools, -mno-auto-litpools — these options also control how literal pools are treated. 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 get interspersed in the text section by the assembler. the compiler doesn't produce explicit .literal directives and loads literals into registers with MOVI instructions instead of L32R, so the assembler can do relaxation and place literals as necessary. this option lets the assembler create several literal pools per function and assemble very big functions, which may not be possible with -mtext-section-literals. glow up for the assembler fr ✨
• -mtarget-align, -mno-target-align — when enabled, GCC tells the assembler to automatically align instructions to reduce branch penalties, at the expense of some code density. the assembler tries to widen density instructions to align branch targets and the instructions following call instructions. if there aren't 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. it WILL align that loop, bestie 💅
• -mlongcalls, -mno-longcalls — when enabled, GCC tells the assembler to translate direct calls into indirect calls, unless it can determine that the target of a direct call is in the range allowed by the call instruction. this translation typically happens for calls to functions in other source files. specifically, the assembler translates a direct CALL instruction into an L32R followed by a CALLX instruction. the default is -mno-longcalls. this option should be used in programs where the call target can potentially be out of range. this option is implemented in the ASSEMBLER, not the compiler, so the assembly code generated by GCC still shows direct call instructions --- look at the disassembled object code to see the actual instructions, okay? note that the assembler uses an indirect call for EVERY cross-file call, not just the ones that really are out of range. lowkey dramatic but that's the deal 🙈
• -mabi=name — generate code for the specified ABI. permissible values are: call0, windowed. the default ABI is chosen by the Xtensa core configuration.
• -mabi=call0 — when this option is enabled, function parameters are passed in registers a2 through a7, registers a12 through a15 are caller-saved, and register a15 may be used as a frame pointer. when this version of the ABI is enabled, the C preprocessor symbol __XTENSA_CALL0_ABI__ is defined. ✨
• -mabi=windowed — when this option is enabled, function parameters are passed in registers a10 through a15, and the called function rotates the register window by 8 registers on entry, so its arguments are found in registers a2 through a7. register a7 may be used as a frame pointer. the register window is rotated 8 registers back upon return. when this version of the ABI is enabled, the C preprocessor symbol __XTENSA_WINDOWED_ABI__ is defined. the way it rotates back is so satisfying 💫
• -mextra-l32r-costs=n — specify an extra cost of instruction RAM/ROM access for L32R instructions, in clock cycles. this affects, when optimizing for speed, whether loading a constant from the literal pool using L32R or synthesizing the constant from a small one with a couple of arithmetic instructions is better. the default value is 0. 🍓
• -mstrict-align / -mno-strict-align — bestie, this is the alignment vibe check 🔍 -mstrict-align says no, do NOT generate memory accesses that might land off a natural object boundary like the architecture spec describes. -mno-strict-align says yeah go ahead, we ball. The default is -mno-strict-align for cores that support both unaligned loads and stores in hardware (they're built different 💅), and -mstrict-align for literally every other core. so like. know your core. periodt.

✨ zSeries Options ✨

These are listed under

---

🌍 ENVIRONMENT 🌍

okay so this section is where GCC keeps all its little mood boards — these are environment variables that change how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment. no cap 🫶

heads up bestie: you can also specify places to search using options like -B, -I and -L. Those take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. so the priority order is: command line > env vars > how GCC was built. iconic hierarchy honestly 👑

🌐 LANG / LC_CTYPE / LC_MESSAGES / LC_ALL

These environment variables control the way that GCC uses localization information, which allows GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8 🇬🇧✨

• The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape. (imagine a quote character just. ending your string. chaos 💀)
• The LC_MESSAGES environment variable specifies the language to use in diagnostic messages. so your errors can yell at you in your language, love that for you 🫶
• If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are set, GCC defaults to traditional C English behavior. (the classic. the original. the blueprint 🎀)

🗂️ TMPDIR

If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper. it's giving relay race, handing the baton from stage to stage 🏃‍♀️💨

🐛 GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of this option for more details. (go read that one, bestie, it's got the lore)

📁 GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. so it's literally just glue, and you control the glue stick 🩹

• If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.
• If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.
• The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script. (iykyk 🌸)
• Other prefixes specified with -B take precedence over this prefix. she knows her priorities 💅
• This prefix is also used for finding files such as crt0.o that are used for linking.
• In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files.

okay that last one is genuinely wild but like. trust the process 🧚

🧭 COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX. it's the backup plan for the backup plan 🔥

📚 LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). <hotpink>-L</hotpink> stays the main character, everybody else is supporting cast 💖

🌸 LANG (yes, again — she contains multitudes)

This variable is used to pass locale information to the compiler. One way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

• C-JIS — Recognize JIS characters.
• C-SJIS — Recognize SJIS characters.
• C-EUCJP — Recognize EUCJP characters.

If LANG is not defined, or if it has some other value, then the compiler uses "mblen" and "mbtowc" as defined by the default locale to recognize and translate multibyte characters. slay, defaults 🦋

💌 GCC_EXTRA_DIAGNOSTIC_OUTPUT

If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable. (them again with the priority 😤)

• fixits-v1 — Emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
• fixits-v2 — As "fixits-v1", but columns are expressed as display columns, as per -fdiagnostics-column-unit=display. glow up fr fr ✨

---

Some additional environment variables affect the behavior of the preprocessor. (yeah she has her own little ecosystem 💅)

🔎 CPATH / C_INCLUDE_PATH / CPLUS_INCLUDE_PATH / OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon. (colon girlies rise 🫶)

• CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.
• The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.
• In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is :/special/include, that has the same effect as -I. -I/special/include.

that empty-element thing is so sneaky and so useful, no cap 🍓

📝 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. (system headers? not invited to this party 🙈)

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. plant that in your brain bestie 🌱

☀️ 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. so close to her sister but with main-character-system-header energy 😌

⏰ SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become reproducible. reproducible builds girlies, this one's for you 🎀

The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of "date +%s" on GNU/Linux and other systems that support the %s extension in the "date" command.

The value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process. don't just make one up bestie, that's not the vibe 💀

---

🐞 BUGS 🐞

For instructions on reporting bugs, see file:///usr/share/doc/gcc-15/README.Bugs.

(go snitch productively 🫶)

---

📌 FOOTNOTES 📌

1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. -shared suppresses the addition of startup code to alter the floating-point environment as done with -ffast-math, -Ofast or -funsafe-math-optimizations on some targets.

subtle defects is the scariest phrase in this entire manpage btw. pass the right flags. it's free. 💅

---

👀 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 really said "we come as a package" 🌈

---

✍️ AUTHOR ✍️

See the Info entry for gcc, or https://gcc.gnu.org/onlinedocs/gcc/Contributors.html, for contributors to GCC.

shoutout to the contributors, absolute legends, ate and left no crumbs 👑🫶

✨ COPYRIGHT ✨

okay so legally we gotta do this part and honestly it's giving very "read the terms bestie" but it's also kind of iconic because it's literally the paperwork that keeps all of this free 💖

Copyright (c) 1988-2025 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.

no cap, that gfdl(7) page is where the whole legal vibe lives — go read it if you wanna know the fine print fr fr 📜

---

🎀 the cover texts (yes they're required, no you can't skip them)

(a) the FSF's Front-Cover Text is literally just:

A GNU Manual

(b) and the FSF's Back-Cover Text says:

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.

like… "you have freedom to copy and modify this GNU Manual, like GNU software" is such a serve honestly 😭💅 free software stays winning.

---

and that's a wrap on the whole page bestieeee 🫶 you made it through all of it, every last flag and every last footnote, and I'm so proud of you for real.

this absolute novel you just read came from gcc-15 and it was dated 2026-09-15 ✨ so if the calendar in your brain says otherwise, blame the manpage not me 💀

go compile something beautiful. I believe in you. periodt. 👑🦋🌸