GCC(1)

📦
this version · 3 versions
, in ubuntu stonking

okay bestie, pull up a chair and grab a snack 🍓, because this is the manual page for gcc — GNU project C and C++ compiler — straight outta the GNU manual, section 1. She's a big one. We're doing this page in 132 parts and this is part 1, so hydrate and let's GO. 💅

✨ NAME ✨

gcc — GNU project C and C++ compiler.

That's it. That's the name section. Short queen. 👑

🦋 SYNOPSIS 🦋

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

So real talk: that list up there is literally just the most useful options, not the whole thing. The rest of them live down below (and there are so many). Also, plot twist: g++ accepts mostly the same options as gcc, so if you're a C++ girlie you are not left out. 🫶

💅 DESCRIPTION 💅

Okay so when you invoke GCC, it normally does preprocessing, compilation, assembly and linking — the whole glow-up pipeline. The "overall options" let you stop that process at an intermediate stage. For example, -c says don't run the linker, and then the output is just object files output by the assembler. Iconic, efficient, no notes. ✨

Other options get passed on to one or more stages of processing. Some of them control the preprocessor, others control the compiler itself. And yet other options control the assembler and the linker — but most of those are not documented here, because you rarely ever need to touch them. So don't stress about the ones you never see. 🙈

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

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 a specific version of GCC. And when you compile C++ programs, you should invoke GCC as g++ instead. Periodt. 💖

The gcc program accepts options and file names as operands. Many options have multi-letter names, so heads up: multiple single-letter options may not be grouped. -dv is very different from -d -v. Do not mix those up, bestie, I am begging. 😭

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. So sometimes the vibe is alphabetical and sometimes it is strictly chronological. 👑

Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on. Most of these have both positive and negative forms, and the negative form of -ffoo is -fno-foo. This manual documents only one of these two forms — whichever one is not the default. So if you go looking for the other one and can't find it, that's why. 💀

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 — things like kB and KiB for kilobyte and kibibyte respectively, MB and MiB for megabyte and mebibyte, GB and GiB for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. For the full listing and explanation of the binary and decimal byte size prefixes, refer to the NIST, IEC, and other relevant national and international standards. 📏

🔥 OPTIONS 🔥

Option Summary

Here's a summary of all the options, grouped by type. The actual explanations are in the following sections — this part is just the roll call. 🌸

Overall Options

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

C Language Options

  • -ansi
  • --ansi
  • -std=standard
  • -aux-info filename
  • -fno-asm
  • -fno-builtin
  • -fno-builtin-function
  • -fcond-mismatch
  • -fdeps-file=file
  • -fdeps-format=format
  • -fdeps-target=file
  • -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
  • -fsso-struct=endianness
  • -fstrict-flex-arrays[=n]

C++ Language Options

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

Objective-C and Objective-C++ Language Options

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

OpenMP and OpenACC Options

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

Diagnostic Message Formatting Options

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

And that's the summary, bestie — every single option name, no skips, no "and so on" hand-waving. 🌟 Next up we get into what all of these actually do, so keep scrolling, there's so much more coming. 🦋

✨ Warning Options ✨

okay bestie, buckle up, because this is the list. every single flag in here is about warnings — the little "hey girl, maybe look at this" notes your compiler leaves you. some of them start with -W (the classic), some with -f, some with plain old --. some are on, some are off, some are situational. and yes, there are SO many. we're listing every last one, no skipping, no "etc." 🫶

-fsyntax-only  -fmax-errors=n  -Wpedantic -pedantic-errors
-fpermissive -w  -Wextra  -Wall  -Wabi=n -Waddress

here's the full roster, in the exact order the manpage serves it 💅

  • -fsyntax-only
  • -fmax-errors=n
  • -Wpedantic
  • -pedantic-errors
  • -fpermissive
  • -w
  • -Wextra
  • -Wall
  • -Wabi=n
  • -Waddress
  • -Wno-address-of-packed-member
  • -Waggregate-return
  • -Walloc-size
  • -Walloc-size-larger-than=byte-size
  • -Walloc-zero
  • -Walloca
  • -Walloca-larger-than=byte-size
  • -Wauto-profile
  • -Wno-aggressive-loop-optimizations
  • -Warith-conversion
  • -Warray-bounds
  • -Warray-bounds=n
  • -Warray-compare
  • -Warray-parameter
  • -Warray-parameter=n
  • -Wno-attributes
  • -Wattribute-alias=n
  • -Wno-attribute-alias
  • -Wno-attribute-warning
  • -Wbidi-chars=[none|unpaired|any|ucn]
  • -Wbool-compare
  • -Wbool-operation
  • -Wno-builtin-declaration-mismatch
  • -Wno-builtin-macro-redefined
  • -Wc90-c99-compat
  • -Wc99-c11-compat
  • -Wc11-c23-compat
  • -Wc23-c2y-compat
  • -Wc++-compat
  • -Wc++11-compat
  • -Wc++14-compat
  • -Wc++17-compat
  • -Wc++20-compat
  • -Wc++26-compat
  • -Wno-c++11-extensions
  • -Wno-c++14-extensions
  • -Wno-c++17-extensions
  • -Wno-c++20-extensions
  • -Wno-c++23-extensions
  • -Wcalloc-transposed-args
  • -Wcannot-profile
  • -Wcast-align
  • -Wcast-align=strict
  • -Wcast-function-type
  • -Wcast-qual
  • -Wchar-subscripts
  • -Wclobbered
  • -Wcomment
  • -Wcompare-distinct-pointer-types
  • -Wno-complain-wrong-lang
  • -Wconstant-logical-operand
  • -Wconversion
  • -Wno-coverage-mismatch
  • -Wno-cpp
  • -Wdangling-else
  • -Wdangling-pointer
  • -Wdangling-pointer=n
  • -Wdate-time
  • -Wno-deprecated
  • -Wno-deprecated-declarations
  • -Wno-designated-init
  • -Wno-deprecated-openmp
  • -Wdisabled-optimization
  • -Wno-discarded-array-qualifiers
  • -Wno-discarded-qualifiers
  • -Wno-div-by-zero
  • -Wdouble-promotion
  • -Wduplicated-branches
  • -Wduplicated-cond
  • -Wempty-body
  • -Wno-endif-labels
  • -Wenum-compare
  • -Wenum-conversion
  • -Wenum-int-mismatch
  • -Werror
  • -Werror=*
  • -Wexpansion-to-defined
  • -Wfatal-errors
  • -Wflex-array-member-not-at-end
  • -Wfloat-conversion
  • -Wfloat-equal
  • -Wformat
  • -Wformat=2
  • -Wno-format-contains-nul
  • -Wno-format-diag
  • -Wno-format-extra-args
  • -Wformat-nonliteral
  • -Wformat-overflow=n
  • -Wformat-security
  • -Wformat-signedness
  • -Wformat-truncation=n
  • -Wformat-y2k
  • -Wframe-address
  • -Wframe-larger-than=byte-size
  • -Wno-free-nonheap-object
  • -Wheader-guard
  • -Wno-if-not-aligned
  • -Wno-ignored-attributes
  • -Wignored-qualifiers
  • -Wno-incompatible-pointer-types
  • -Whardened
  • -Wimplicit
  • -Wimplicit-fallthrough
  • -Wimplicit-fallthrough=n
  • -Wno-implicit-function-declaration
  • -Wno-implicit-int
  • -Winfinite-recursion
  • -Winit-self
  • -Winline
  • -Wno-int-conversion
  • -Wint-in-bool-context
  • -Wno-int-to-pointer-cast
  • -Wno-invalid-memory-model
  • -Winvalid-pch
  • -Winvalid-utf8
  • -Wno-unicode
  • -Wjump-misses-init
  • -Wkeyword-macro
  • -Wlarger-than=byte-size
  • -Wleading-whitespace=kind
  • -Wlogical-not-parentheses
  • -Wlogical-op
  • -Wlong-long
  • -Wno-lto-type-mismatch
  • -Wmain
  • -Wmaybe-uninitialized
  • -Wmemset-elt-size
  • -Wmemset-transposed-args
  • -Wmisleading-indentation
  • -Wmissing-attributes
  • -Wmissing-braces
  • -Wmissing-field-initializers
  • -Wmissing-format-attribute
  • -Wmissing-include-dirs
  • -Wmissing-noreturn
  • -Wmusttail-local-addr
  • -Wmaybe-musttail-local-addr
  • -Wno-missing-profile
  • -Wno-multichar
  • -Wmultistatement-macros
  • -Wnonnull
  • -Wnonnull-compare
  • -Wnormalized=[none|id|nfc|nfkc]
  • -Wnull-dereference
  • -Wno-odr
  • -Wopenacc-parallelism
  • -Wopenmp
  • -Wopenmp-simd
  • -Wno-overflow
  • -Woverlength-strings
  • -Wno-override-init-side-effects
  • -Wpacked
  • -Wno-packed-bitfield-compat
  • -Wpacked-not-aligned
  • -Wpadded
  • -Wparentheses
  • -Wno-pedantic-ms-format
  • -Wpointer-arith
  • -Wno-pointer-compare
  • -Wno-pointer-to-int-cast
  • -Wno-pragmas
  • -Wno-pragma-once-outside-header
  • -Wno-prio-ctor-dtor
  • -Wno-psabi
  • -Wredundant-decls
  • -Wrestrict
  • -Wno-return-local-addr
  • -Wreturn-type
  • -Wno-scalar-storage-order
  • -Wsequence-point
  • -Wshadow
  • -Wshadow=global
  • -Wshadow=local
  • -Wshadow=compatible-local
  • -Wno-shadow-ivar
  • -Wno-shift-count-negative
  • -Wno-shift-count-overflow
  • -Wshift-negative-value
  • -Wno-shift-overflow
  • -Wshift-overflow=n
  • -Wsign-compare
  • -Wsign-conversion
  • -Wno-sizeof-array-argument
  • -Wsizeof-array-div
  • -Wsizeof-pointer-div
  • -Wsizeof-pointer-memaccess
  • -Wstack-protector
  • -Wstack-usage=byte-size
  • -Wstrict-aliasing
  • -Wstrict-aliasing=n
  • -Wstring-compare
  • -Wno-stringop-overflow
  • -Wno-stringop-overread
  • -Wno-stringop-truncation
  • -Wstrict-flex-arrays
  • -Wsuggest-attribute=attribute-name
  • -Wswitch
  • -Wno-switch-bool
  • -Wswitch-default
  • -Wswitch-enum
  • -Wno-switch-outside-range
  • -Wno-switch-unreachable
  • -Wsync-nand
  • -Wsystem-headers
  • -Wtautological-compare
  • -Wtrailing-whitespace
  • -Wtrailing-whitespace=kind
  • -Wtrampolines
  • -Wtrigraphs
  • -Wtrivial-auto-var-init
  • -Wno-tsan
  • -Wtype-limits
  • -Wundef
  • -Wuninitialized
  • -Wunknown-pragmas
  • -Wunsuffixed-float-constants
  • -Wunterminated-string-initialization
  • -Wunused
  • -Wunused-but-set-parameter
  • -Wunused-but-set-parameter=n
  • -Wunused-but-set-variable
  • -Wunused-but-set-variable=n
  • -Wunused-const-variable
  • -Wunused-const-variable=n
  • -Wunused-function
  • -Wunused-label
  • -Wunused-local-typedefs
  • -Wunused-macros
  • -Wunused-parameter
  • -Wno-unused-result
  • -Wunused-value
  • -Wunused-variable
  • -Wuse-after-free
  • -Wuse-after-free=n
  • -Wuseless-cast
  • -Wno-varargs
  • -Wvariadic-macros
  • -Wvector-operation-performance
  • -Wvla
  • -Wvla-larger-than=byte-size
  • -Wno-vla-larger-than
  • -Wvolatile-register-var
  • -Wwrite-strings
  • -Wno-xor-used-as-pow
  • -Wzero-as-null-pointer-constant
  • -Wzero-length-bounds
  • -Wzero-init-padding-bits=value
  • --all-warnings
  • --extra-warnings
  • --no-warnings
  • --pedantic
  • --pedantic-errors

🔍 Static Analyzer Options 🔍

okay so this next bit? it's the deep-scan crew 🌈 these flags turn on GCC's static analyzer and then fine-tune literally everything about how it thinks, what it dumps out, and which of its little warnings it's allowed to yell about. flags starting with -Wno-analyzer-... are analyzer warnings that come switched OFF by default, and the two without a no- (-Wanalyzer-symbol-too-complex and -Wanalyzer-too-complex) are the ones that come switched on. iykyk.

  • -fanalyzer
  • -fanalyzer-assume-nothrow
  • -fanalyzer-call-summaries
  • -fanalyzer-checker=name
  • -fno-analyzer-feasibility
  • -fanalyzer-show-events-in-system-headers
  • -fno-analyzer-state-merge
  • -fno-analyzer-state-purge
  • -fno-analyzer-suppress-followups
  • -fanalyzer-transitivity
  • -fno-analyzer-undo-inlining
  • -fanalyzer-verbose-edges
  • -fanalyzer-verbose-state-changes
  • -fanalyzer-verbosity=level
  • -fdump-analyzer
  • -fdump-analyzer-callgraph
  • -fdump-analyzer-exploded-graph
  • -fdump-analyzer-exploded-nodes
  • -fdump-analyzer-exploded-nodes-2
  • -fdump-analyzer-exploded-nodes-3
  • -fdump-analyzer-exploded-paths
  • -fdump-analyzer-feasibility
  • -fdump-analyzer-infinite-loop
  • -fdump-analyzer-json
  • -fdump-analyzer-state-purge
  • -fdump-analyzer-stderr
  • -fdump-analyzer-supergraph
  • -fdump-analyzer-untracked
  • -Wno-analyzer-double-fclose
  • -Wno-analyzer-double-free
  • -Wno-analyzer-exposure-through-output-file
  • -Wno-analyzer-exposure-through-uninit-copy
  • -Wno-analyzer-fd-access-mode-mismatch
  • -Wno-analyzer-fd-double-close
  • -Wno-analyzer-fd-leak
  • -Wno-analyzer-fd-phase-mismatch
  • -Wno-analyzer-fd-type-mismatch
  • -Wno-analyzer-fd-use-after-close
  • -Wno-analyzer-fd-use-without-check
  • -Wno-analyzer-file-leak
  • -Wno-analyzer-free-of-non-heap
  • -Wno-analyzer-imprecise-fp-arithmetic
  • -Wno-analyzer-infinite-loop
  • -Wno-analyzer-infinite-recursion
  • -Wno-analyzer-jump-through-null
  • -Wno-analyzer-malloc-leak
  • -Wno-analyzer-mismatching-deallocation
  • -Wno-analyzer-mkostemp-redundant-flags
  • -Wno-analyzer-mktemp-missing-placeholder
  • -Wno-analyzer-mktemp-of-string-literal
  • -Wno-analyzer-null-argument
  • -Wno-analyzer-null-dereference
  • -Wno-analyzer-out-of-bounds
  • -Wno-analyzer-overlapping-buffers
  • -Wno-analyzer-possible-null-argument
  • -Wno-analyzer-possible-null-dereference
  • -Wno-analyzer-putenv-of-auto-var
  • -Wno-analyzer-shift-count-negative
  • -Wno-analyzer-shift-count-overflow
  • -Wno-analyzer-stale-setjmp-buffer
  • -Wno-analyzer-tainted-allocation-size
  • -Wno-analyzer-tainted-assertion
  • -Wno-analyzer-tainted-array-index
  • -Wno-analyzer-tainted-divisor
  • -Wno-analyzer-tainted-offset
  • -Wno-analyzer-tainted-size
  • -Wno-analyzer-throw-of-unexpected-type
  • -Wanalyzer-symbol-too-complex
  • -Wanalyzer-too-complex
  • -Wno-analyzer-undefined-behavior-ptrdiff
  • -Wno-analyzer-undefined-behavior-strtok
  • -Wno-analyzer-unsafe-call-within-signal-handler
  • -Wno-analyzer-use-after-free
  • -Wno-analyzer-use-of-pointer-in-stale-stack-frame
  • -Wno-analyzer-use-of-uninitialized-value
  • -Wno-analyzer-va-arg-type-mismatch
  • -Wno-analyzer-va-list-exhausted
  • -Wno-analyzer-va-list-leak
  • -Wno-analyzer-va-list-use-after-va-end
  • -Wno-analyzer-write-to-const
  • -Wno-analyzer-write-to-string-literal

🍎 C and Objective-C-only Warning Options 🌈

these ones are C and Objective-C girlies only — C++ people, you can scroll past, this isn't your moment 🙈 but for the C besties these are real and they matter, so here they all are:

  • -Wbad-function-cast
  • -Wdeprecated-non-prototype
  • -Wfree-labels
  • -Wmissing-declarations
  • -Wmissing-parameter-name
  • -Wmissing-parameter-type
  • -Wdeclaration-missing-parameter-type
  • -Wmissing-prototypes
  • -Wmissing-variable-declarations
  • -Wmultiple-parameter-fwd-decl-lists
  • -Wnested-externs
  • -Wold-style-declaration
  • -Wold-style-definition
  • -Wstrict-prototypes
  • -Wtraditional
  • -Wtraditional-conversion
  • -Wdeclaration-after-statement
  • -Wpointer-sign

🐛 Debugging Options 🐛

debugging era 💀✨ this is where you tell GCC how much debug info to bake into your binary, which format to use, and how it should peek at your variables while you're stepping through. so many -g variants. so many. and then a whole pile of -f friends riding along at the end. every one, coming right up:

  • -g
  • -glevel
  • -gdwarf
  • -gdwarf-version
  • -gbtf
  • -gctf
  • -gctflevel
  • -gno-prune-btf
  • -ggdb
  • -gno-record-gcc-switches
  • -gstrict-dwarf
  • -gas-loc-support
  • -gas-locview-support
  • -gcodeview
  • -gcolumn-info
  • -gdwarf32
  • -gdwarf64
  • -gno-statement-frontiers
  • -gno-variable-location-views
  • -gvariable-location-views=incompat5
  • -ginternal-reset-location-views
  • -ginline-points
  • -gvms
  • -gz[=type]
  • -gsplit-dwarf
  • -gdescribe-dies
  • -fdebug-prefix-map=old=new
  • -fdebug-types-section
  • -fno-eliminate-unused-debug-types
  • -femit-struct-debug-baseonly
  • -femit-struct-debug-reduced
  • -femit-struct-debug-detailed[=spec-list]
  • -fno-eliminate-unused-debug-symbols
  • -femit-class-debug-always
  • -fno-merge-debug-strings
  • -fno-dwarf2-cfi-asm
  • -fvar-tracking
  • -fvar-tracking-assignments
  • -fvar-tracking-uninit
  • --debug

okay so this is the big one bestie — the optimization buffet 🍓. Literally hundreds of flags. Are we listing every single one? Yes. We do not gatekeep here 💅. And they're in the order they came in, so if the alphabetization feels chaotic, that's not me, that's the vibe. Also, this part of the page is just the roster — the actual gossip about what each one does comes later, so don't yell at me for not explaining them yet 😭

🔥 OPTIMIZATION OPTIONS 🔥 🔥

  • -faggressive-loop-optimizations — loops but make it aggressive 😤
  • -falign-functions[=n[:m:[n2[:m2]]]] — alignment saga, part one. Look at those nested brackets, they're giving nesting-doll 🪆
  • -falign-jumps[=n[:m:[n2[:m2]]]] — same bracket situation, different target
  • -falign-labels[=n[:m:[n2[:m2]]]] — labels getting the alignment treatment
  • -falign-loops[=n[:m:[n2[:m2]]]] — and loops too. Four for four 💅
  • -fmin-function-alignment=[n] — note the [n], it's got the brackets
  • -fno-allocation-dce — the "no" version, your honour 🙈
  • -fallow-store-data-races — the name is literally right there
  • -fassociative-math — math, but make it associate
  • -fauto-profile — auto profile, main character energy
  • -fauto-profile[=path] — same but you can hand it a path
  • -fauto-profile-inlining — the inlining chapter of the auto-profile-verse
  • -fauto-inc-dec — short and snappy, love that for it
  • -fbranch-probabilities — probabilities, but for branches 🌿
  • -fcaller-saves — the caller is saving, we love a saver 🫶
  • -fcombine-stack-adjustments — combining things, as one does
  • -fconserve-stack — conserving the stack, very eco of it 🌱
  • -ffold-mem-offsets — folding those offsets like laundry 🧺
  • -fcompare-elim — eliminating compares, slay
  • -fcprop-registers — registers getting propagated, iconic
  • -fcrossjumping — CROSSJUMPING. say it with your chest 🔥
  • -fcse-follow-jumps — following jumps, nosy behaviour (affectionate)
  • -fcse-skip-blocks — skipping blocks, avoidant queen
  • -fcx-fortran-rules — Fortran rules in the house
  • -fcx-limited-range — limited range, mood
  • -fcx-method — just "method", no notes
  • -fdata-sections — data sections, plural, we love options
  • -fdce — three letters, zero explanation, mysterious 💀
  • -fdelayed-branch — the branch is late, it's on its way, it texted
  • -fdelete-null-pointer-checks — deleting checks, bold of it
  • -fdep-fusion — fusion! like a dance move 💃
  • -fdevirtualize — devirtualizing, very demure
  • -fdevirtualize-speculatively — same but with speculation 👀
  • -fdevirtualize-at-ltrans — and this one's specifically at ltrans
  • -fdse — another three-letter icon, we stan
  • -fearly-inlining — inlining but early, it's punctual ⏰
  • -fexcess-precision=style — takes a style, fancy
  • -fexpensive-optimizations — expensive, bestie. luxury optimizations 💸
  • -fext-dce — the "ext" version
  • -ffast-math — FAST MATH. zoom zoom ⚡
  • -ffat-lto-objects — fat objects, no body shaming here 🫶
  • -ffinite-loops — finite loops, they know when to stop
  • -ffinite-math-only — math only when finite, strict bestie
  • -ffloat-store — storing floats, where else would they go
  • -fforward-propagate — forwards, always forwards ➡️
  • -ffp-contract=style — another one that wants a style
  • -ffp-int-builtin-inexact — okay the name is the whole thing
  • -ffunction-sections — functions getting sections too
  • -ffuse-ops-with-volatile-access — a whole sentence in a flag, respect
  • -fgcse — four letters of mystery
  • -fgcse-after-reload — and it happens after the reload
  • -fgcse-las — the -las edition
  • -fgcse-lm — the -lm edition
  • -fgraphite-identity — graphite!! giving art student 🎨
  • -fgcse-sm — the -sm edition, that's all three now
  • -fhoist-adjacent-loads — hoisting, adjacent, loads. three words, one flag
  • -fif-conversion — if conversion, number one
  • -fif-conversion2 — if conversion, the sequel 🎬
  • -findirect-inlining — inlining, but indirect
  • -finline-atomics — inlining the atomics, tiny units ⚛️
  • -finline-functions — the classic. the blueprint
  • -finline-functions-called-once — specifically the ones called once, exclusive ✨
  • -finline-limit=n — takes an n, put a number in it
  • -finline-small-functions — small functions only, they're baby 🥺
  • -finline-stringops[=fn] — optional fn in brackets, cute
  • -fipa-modref — ippa! say it how it's spelled
  • -fipa-cp — the -cp one
  • -fipa-cp-clone — the -cp-clone one, it multiplied 🪞
  • -fipa-bit-cp — now with bits
  • -fipa-vrp — -vrp, four letters
  • -fipa-pta — -pta, also four letters
  • -fipa-profile — ippa does profiles now
  • -fipa-pure-const — pure const, sounds like a fragrance
  • -fipa-reference — ippa references (icago)
  • -fipa-reference-addressable — the reference one but addressable now
  • -fipa-reorder-for-locality — reordering for locality, community-minded
  • -fipa-sra — -sra, three letters again
  • -fipa-stack-alignment — ippa and the stack, aligned
  • -fipa-icf — -icf, mystery acronym
  • -fipa-icf-functions — the functions branch of it
  • -fipa-icf-variables — and the variables branch
  • -fira-algorithm=algorithm — takes an algorithm, name the algorithm bestie
  • -flate-combine-instructions — combine them, but late this time
  • -flifetime-dse — lifetime dse, very poetic actually
  • -flive-patching=level — takes a level
  • -fira-region=region — takes a region, the flag is just naming itself
  • -fira-hoist-pressure — hoisting AND pressure, stressful but iconic
  • -fira-loop-pressure — loop pressure, relatable honestly 😮‍💨
  • -fno-ira-share-save-slots — the no version, with a hyphenated hyphen situation
  • -fno-ira-share-spill-slots — the sibling of the above, spill this time
  • -fisolate-erroneous-paths-dereference — isolating erroneous paths, the dereference edition
  • -fisolate-erroneous-paths-attribute — same family, attribute edition
  • -fivopts — -ivopts, short king 👑
  • -fkeep-inline-functions — keeping the inline functions, we don't throw things away here
  • -fkeep-static-functions — keeping the static ones too
  • -fkeep-static-consts — and the consts. hoarder behaviour (affectionate)
  • -flimit-function-alignment — limits the alignment, boundaries babe
  • -flive-range-shrinkage — shrinkage but make it live range
  • -floop-block — loop block, four syllables of business
  • -floop-interchange — swapping them loops 🔄
  • -floop-strip-mine — strip mine, sounds like Minecraft
  • -floop-unroll-and-jam — unroll AND jam?? it's a whole breakfast 🥐
  • -floop-nest-optimize — optimizing the nest, cozy
  • -floop-parallelize-all — ALL of them, parallel, no exceptions 💅
  • -flra-remat — -lra-remat, say that five times fast
  • -flto — the famous three letters
  • -flto-compression-level=n — compression, with a numeric level
  • -flto-toplevel-asm-heuristics — heuristics at the top level, very main character
  • -flto-partition=alg — takes an alg
  • -flto-incremental=path — incremental, and it wants a path
  • -flto-incremental-cache-size=n — cache size, with a number
  • -fmalloc-dce — malloc dce, done
  • -fmerge-all-constants — merging ALL the constants, thorough 💅
  • -fmerge-constants — regular merge, the restrained version
  • -fmodulo-sched — modulo scheduling, math girlies rise
  • -fmodulo-sched-allow-regmoves — and it allows regmoves, generous
  • -fmove-loop-invariants — move them, they're in the way
  • -fmove-loop-stores — stores this time
  • -fno-branch-count-reg — the no version again
  • -fno-defer-pop — do not defer the pop 🚫
  • -fno-function-cse — no cse for functions
  • -fno-guess-branch-probability — no guessing. certainty only ✋
  • -fno-inline — no inlining, respect
  • -fno-math-errno — no errno for math, it's on its own
  • -fno-peephole — no peephole 😳
  • -fno-peephole2 — no peephole, the sequel
  • -fno-printf-return-value — don't do the printf return value thing
  • -fno-sched-interblock — no interblock scheduling
  • -fno-sched-spec — no spec scheduling
  • -fno-signed-zeros — zeros stay unsigned ig
  • -fno-toplevel-reorder — no reordering at the top level, leave it where it is 🚫
  • -fno-trapping-math — no trapping the math, free her
  • -fno-zero-initialized-in-bss — a whole mouthful, the no version
  • -fomit-frame-pointer — omitting the frame pointer, iconic and slightly scary
  • -foptimize-crc — optimizing CRC, nerds rise up 🧮
  • -foptimize-sibling-calls — sibling calls, they're family 🫶
  • -fpartial-inlining — partially inlining, it's doing its best
  • -fpeel-loops — peeling loops like an orange 🍊
  • -fpredictive-commoning — predictive commoning, she sees the future 🔮
  • -fprefetch-loop-arrays — prefetching, ahead of schedule ⏰
  • -fprofile-correction — correcting the profile, glow up
  • -fprofile-use — use the profile
  • -fprofile-use=path — use the profile AT this path
  • -fprofile-partial-training — partial training arc 📈
  • -fprofile-values — profile values, plural
  • -fprofile-reorder-functions — profile-reorder the functions, double duty
  • -freciprocal-math — reciprocal math, mutual math 💞
  • -free — just "free". iconic. no notes 🕊️
  • -frename-registers — renaming the registers, they needed a rebrand 💅
  • -freorder-blocks — reorder the blocks
  • -freorder-blocks-algorithm=algorithm — and it takes an algorithm
  • -freorder-blocks-and-partition — blocks AND partition, multitasking queen
  • -freorder-functions — functions get reordered too, equal treatment
  • -frerun-cse-after-loop — re-run cse after the loop, encore 🎤
  • -freschedule-modulo-scheduled-loops — rescheduling the modulo-scheduled loops, recursive name energy
  • -frounding-math — rounding math, it goes around 🔄
  • -fsave-optimization-record — saving the record, scrapbook behaviour 📸
  • -fsched2-use-superblocks — note the 2, it's the sequel
  • -fsched-pressure — pressure again, drink water 💧
  • -fsched-spec-load — spec load
  • -fsched-spec-load-dangerous — DANGEROUS. it's in the name, bestie ⚠️
  • -fsched-stalled-insns-dep[=n] — optional n, stalled
  • -fsched-stalled-insns[=n] — same family, no -dep
  • -fsched-group-heuristic — heuristic number one: group
  • -fsched-critical-path-heuristic — heuristic: critical path, very important
  • -fsched-spec-insn-heuristic — heuristic: spec insn
  • -fsched-rank-heuristic — heuristic: rank
  • -fsched-last-insn-heuristic — heuristic: last insn
  • -fsched-dep-count-heuristic — heuristic: dep count. that's the whole set 🫡
  • -fschedule-fusion — schedule fusion, two things becoming one
  • -fschedule-insns — schedule insns, the original
  • -fschedule-insns2 — number two 🎬
  • -fsection-anchors — anchors for sections, very nautical ⚓
  • -fselective-scheduling — selective scheduling, picky (valid)
  • -fselective-scheduling2 — and its sequel
  • -fsel-sched-pipelining — pipelining, but selected
  • -fsel-sched-pipelining-outer-loops — outer loops this time
  • -fsemantic-interposition — semantic interposition, fancy words
  • -fshrink-wrap — shrink wrap 📦
  • -fshrink-wrap-separate — shrink wrap, but separate
  • -fsignaling-nans — signaling NANs 📢
  • -fsingle-precision-constant — one precision only, monogamous
  • -fsplit-ivs-in-unroller — splitting ivs in the unroller, very specific
  • -fsplit-loops — split them loops ✂️
  • -fspeculatively-call-stored-functions — a whole sentence again
  • -fsplit-wide-types — wide types getting split
  • -fsplit-wide-types-early — the same, but early
  • -fssa-backprop — backprop but ssa
  • -fssa-phiopt — phiopt, love a portmanteau
  • -fstdarg-opt — stdarg, optimized
  • -fstore-merging — merging stores, retail therapy
  • -fstrict-aliasing — strict aliasing, she has boundaries ✋
  • -fipa-strict-aliasing — and the ippa version of her
  • -fthread-jumps — thread jumps 🧵
  • -ftracer — the tracer, detective era 🔍
  • -ftree-bit-ccp — tree bit ccp, four words
  • -ftree-builtin-call-dce — tree builtin call dce, five words
  • -ftree-ccp — tree ccp
  • -ftree-ch — tree ch, two letters! minimalist
  • -ftree-coalesce-vars — coalescing vars, they're merging 🫂
  • -ftree-copy-prop — copy prop, short for propagate
  • -ftree-cselim — cselim, one word, one vibe
  • -ftree-dce — tree dce
  • -ftree-dominator-opts — dominator opts, powerful
  • -ftree-dse — tree dse
  • -ftree-forwprop — forwprop, forwarding again
  • -ftree-fre — tree fre
  • -fcode-hoisting — note there's no tree- here, hoisting code
  • -ftree-loop-im — tree loop im
  • -ftree-phiprop — phiprop, say it
  • -ftree-loop-distribution — distribution time 📦
  • -ftree-loop-distribute-patterns — patterns this time
  • -ftree-loop-ivcanon — ivcanon, canon
  • -ftree-loop-linear — linear loops, straight lines only
  • -ftree-loop-optimize — the classic loop one
  • -ftree-loop-vectorize — vectorize the loops
  • -ftree-parallelize-loops[=n] — parallelize them, with an optional n
  • -ftree-pre — tree pre
  • -ftree-partial-pre — partial version
  • -ftree-pta — tree pta
  • -ftree-reassoc — reassoc, it's been through things
  • -ftree-scev-cprop — scev cprop
  • -ftree-sink — sink 🌊
  • -ftree-slsr — tree slsr
  • -ftree-sra — tree sra
  • -ftree-switch-conversion — converting the switch 🔀
  • -ftree-tail-merge — merging tails, cute
  • -ftree-ter — tree ter, three-letter icon
  • -ftree-vectorize — vectorize 🌈
  • -ftree-vrp — tree vrp
  • -ftrivial-auto-var-init — trivial auto var init, mouthful but valid
  • -funconstrained-commons — unconstrained commons, free spirits
  • -funit-at-a-time — one unit at a time, no rush 🧘
  • -funroll-all-loops — ALL of them, unfurled
  • -funroll-loops — just the normal unrolling
  • -funsafe-math-optimizations — UNSAFE. it says it. right there ⚠️
  • -funswitch-loops — unswitch them
  • -fipa-ra — ippa ra
  • -fvariable-expansion-in-unroller — variable expansion, in the unroller, peak specificity
  • -fvect-cost-model — the cost model, budget girlie
  • -fvpt — vpt, three letters
  • -fweb — web 🕸️
  • -fwhole-program — the WHOLE program, go big
  • -fwpa — wpa, three letters
  • -fuse-linker-plugin — use the linker plugin 🔌
  • -fzero-call-used-regs — zero the call used regs, tidy
  • -O — the one-letter legend 👑
  • -O0 — the one with a zero
  • -O1 — the one with a one
  • -O2 — the one with a two
  • -O3 — the one with a three
  • -Os — the one with an s
  • -Ofast — the one that's fast, it's in the name
  • -Og — the one with a g
  • -Oz — the one with a z
  • --optimize — the long-form spelling, for when hyphens aren't your thing

🎀 PROGRAM INSTRUMENTATION OPTIONS 🎀

okay so this is the ✨watch what my program is doing✨ section — all the flags that put little cameras and tripwires everywhere 📸. Surgical. Obsessive. Iconic.

  • -p — single letter, big impact
  • -pg — two letters, even bigger
  • -fprofile-arcs — profile arcs, curved profiling
  • -coverage — coverage, one hyphen
  • -ftest-coverage — test coverage, the "test" edition
  • -fcondition-coverage — coverage for conditions specifically
  • -fpath-coverage — and coverage for paths
  • -fprofile — the plain profile
  • -fprofile-abs-path — the abs path version, absolute 💅
  • -fprofile-dir=path — point it at a path
  • -fprofile-generate — generate mode
  • -fprofile-generate=path — generate mode, into this path
  • -fprofile-info-section — info section, mysterious
  • -fprofile-info-section=name — same but you name it
  • -fprofile-note=path — a note, at a path 📝
  • -fprofile-prefix-path=path — prefix path, so many paths
  • -fprofile-update=method — takes a method
  • -fprofile-filter-files=regex — filter files by regex, picky and valid
  • -fprofile-exclude-files=regex — exclude files by regex, the other side of the coin
  • -fprofile-reproducible=[multithreaded|parallel-runs|serial] — three choices in brackets: multithreaded, parallel-runs, serial 🎀
  • -fsanitize=style — sanitize, takes a style
  • -fsanitize-recover — recover mode
  • -fsanitize-recover=style — recover, in a particular style
  • -fsanitize-trap — trap mode 🪤
  • -fsanitize-trap=style — trap, in a style
  • -fasan-shadow-offset=number — asan shadow offset, with a number
  • -fsanitize-sections=s1,s2,... — comma-separated sections, listed out
  • -fsanitize-undefined-trap-on-error — trap on error, drama 💀
  • -fcf-protection — cf protection, the plain version
  • -fcf-protection=[full|branch|return|none|check] — five options in brackets: full, branch, return, none, check
  • -fharden-compares — hardening the compares, tough love
  • -fharden-conditional-branches — and the conditional branches
  • -fhardened — fully hardened, she's been through it 🛡️
  • -fharden-control-flow-redundancy — hardening the redundancy, extra
  • -fhardcfr-skip-leaf — hardcfr, skip the leaf 🍃
  • -fhardcfr-check-exceptions — and check the exceptions
  • -fhardcfr-check-returning-calls — and the returning calls
  • -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — four choices: always, no-xthrow, nothrow, never
  • -fstack-protector — the classic stack protector 🛡️
  • -fstack-protector-all — ALL of them protected
  • -fstack-protector-strong — the STRONG one 💪
  • -fstack-protector-explicit — the explicit one, says what it means
  • -fstack-check — check that stack
  • -fstack-limit-register=reg — limit register, with a reg
  • -fstack-limit-symbol=sym — limit symbol, with a sym
  • -fno-stack-limit — no limit, no boundaries
  • -fsplit-stack — split the stack
  • -fstrub=disable — first of SIX strub modes: disable
  • -fstrub=strict — strub: strict
  • -fstrub=relaxed — strub: relaxed
  • -fstrub=all — strub: all
  • -fstrub=at-calls — strub: at-calls
  • -fstrub=internal — strub: internal. that's the whole set 🫡
  • -fvtable-verify=[std|preinit|none] — three choices: std, preinit, none
  • -fvtv-counts — vtv counts, counting things 🔢
  • -fvtv-debug — vtv debug, debugging things
  • -finstrument-functions — instrumenting the functions, orchestra era 🎻
  • -finstrument-functions-once — but just once, exclusive
  • -finstrument-functions-exclude-function-list=sym,sym,... — exclude a comma list of symbols
  • -finstrument-functions-exclude-file-list=file,file,... — exclude a comma list of files
  • -fprofile-prefix-map=old=new — map old to new, glow-up in flag form
  • -fpatchable-function-entry=N[,M] — N with an optional comma M, brackets and all
  • --coverage — the double-dash coverage
  • --profile — the double-dash profile

✨ PREPROCESSOR OPTIONS ✨

the preprocessor era 💖 everything here is about the stuff that happens before your code even gets to be real code. the opening act. the soundcheck. and bestie, there are a LOT of them.

  • -C — capital C, iconic single letter
  • -CC — double C, twice the letter
  • -Dmacro[=defn] — define a macro, with the optional [=defn] right there
  • -dD — lowercase d, capital D
  • -dI — d and I
  • -dM — d and M
  • -dN — d and N
  • -dU — d and U, that's the whole d-set
  • -fdebug-cpp — debug the cpp, detective vibes 🔍
  • -fdirectives-only — directives only, nothing else, boundaries
  • -fdollars-in-identifiers — dollars allowed in identifiers 💵
  • -fexec-charset=charset — exec charset, takes a charset
  • -fextended-identifiers — extended identifiers, they got longer
  • -finput-charset=charset — input charset, another charset
  • -fmacro-prefix-map=old=new — map old to new again, this time for macros
  • -fmax-include-depth=depth — max include depth, with a depth
  • -fno-canonical-system-headers — no canonical system headers, rogue
  • -fpch-deps — pch deps, four letters
  • -fpch-preprocess — pch preprocess, preprocess the pch
  • -fpreprocessed — it's already preprocessed, don't touch it
  • -ftabstop=width — tabstop, with a width
  • -ftrack-macro-expansion — tracking macro expansion, following the trail
  • -fwide-exec-charset=charset — wide exec charset, wide edition
  • -fworking-directory — the working directory, where the magic happens
  • -H — capital H, one letter
  • -imacros file — imacros, then a file
  • -include file — include, then a file
  • -M — capital M
  • -MD — M and D
  • -MF — M and F
  • -MG — M and G
  • -MM — double M
  • -MMD — double M plus D
  • -MP — M and P
  • -MQ — M and Q
  • -MT — M and T, and that's the whole M-family 🫶
  • -Mno-modules — M no modules, the rebel of the group
  • -no-integrated-cpp — no integrated cpp 🚫
  • -P — capital P, mysterious
  • -pthread — pthread, familiar face
  • -remap — remap, she changed her whole look 💅
  • -traditional — traditional, old school
  • -traditional-cpp — traditional cpp, old school but for cpp
  • -trigraphs — trigraphs 👁️👄👁️
  • -Umacro — undefine that macro, bye bestie 👋
  • -undef — undef, do it
  • -Wp,option — Wp with a comma, then an option
  • -Xpreprocessor option — Xpreprocessor, then an option
  • --comments — double-dash comments
  • --comments-in-macros — comments, but inside the macros
  • --define-macro=macro[=defn] — the long-form define macro, with the optional [=defn]
  • --dependencies — dependencies, plural, we love a list
  • --dump=letters — dump, with letters
  • --imacros=file — imacros, equals version
  • --include=file — include, equals version
  • --no-integrated-cpp — the long spelling of no integrated cpp
  • --no-line-commands — no line commands
  • --print-missing-file-dependencies — print the missing file dependencies, helpful bestie 🫶
  • --traditional — long-form traditional
  • --traditional-cpp — long-form traditional cpp
  • --trigraphs — long-form trigraphs
  • --trace-includes — trace the includes, follow the breadcrumbs
  • --undefine-macro=macro — undefine macro, equals version
  • --user-dependencies — user dependencies, the user edition
  • --write-dependencies — write dependencies out
  • --write-user-dependencies — write the USER dependencies out

🦋 ASSEMBLER OPTIONS 🦋

three flags. tiny section. tiny but mighty, the moment of peace before the linker section 💅

  • -Wa,option — Wa, comma, then an option
  • -Xassembler option — Xassembler, then an option
  • --for-assembler=option — for-assembler, equals, then an option

👑 LINKER OPTIONS 👑

okay so this is where all the pieces actually get stitched together 🧵. and yes, lots of these are just single letters, because the linker is confident like that.

  • object-file-name — not even a dash, just hand it your object file, it's a placeholder for a name
  • -flink-libatomic — link libatomic, the f prefix showed up
  • -fuse-ld=linker — use this linker
  • -llibrary — the classic lowercase L library flag 💖
  • -nostartfiles — no start files, we're skipping the intro
  • -nodefaultlibs — no default libs, custom only
  • -nolibc — no libc, bold
  • -nostdlib — no standard library, fully indie
  • -nostdlib++ — no standard library, but with a ++ this time
  • -e entry — e, then an entry
  • -pie — pie 🥧 (position independent, but also. pie.)
  • -pthread — pthread again, hello again bestie
  • -r — lowercase r
  • -rdynamic — r plus dynamic
  • -s — lowercase s
  • -static — static, she's not going anywhere
  • -static-pie — static AND pie, both, at once 🥧
  • -static-libgcc — static libgcc
  • -static-libstdc++ — static libstdc++
  • -static-libasan — static libasan
  • -static-libhwasan — static libhwasan
  • -static-liblsan — static liblsan
  • -static-libtsan — static libtsan
  • -static-libubsan — static libubsan, and that's all the sanitizer libs 🫡
  • -shared — shared, sharing is caring
  • -shared-libgcc — shared libgcc, the opposite of the static one
  • -symbolic — symbolic, symbolic of what, we don't know
  • -T script — T, then a script
  • -Wl,option — Wl, comma, then an option
  • -Xlinker option — Xlinker, then an option
  • -u symbol — u, then a symbol
  • -Tbss=addr — Tbss, equals, then an addr
  • -Tdata=addr — Tdata, equals, then an addr
  • -Ttext=addr — Ttext, equals, then an addr. that's all three T's
  • -N — capital N
  • -n — lowercase n, the plot twist
  • -t — lowercase t
  • -Z — capital Z
  • -z keyword — lowercase z, then a keyword
  • --entry=entry — the long-form entry
  • --for-linker=option — for-linker, equals, then an option
  • --force-link=symbol — force link this symbol, forceful 💪
  • --no-standard-library — the long spelling of no standard library
  • --pie — the long spelling of pie 🥧
  • --static — the long spelling of static
  • --static-pie — the long spelling of static pie
  • --symbolic — the long spelling of symbolic

🌈 DIRECTORY OPTIONS 🌈

where do we LOOK for things 🗺️. that's the whole vibe here. paths, prefixes, sysroots, the whole filing cabinet.

  • -Bprefix — capital B, then a prefix
  • -Idir — capital I, then a dir
  • -I- — capital I, then a hyphen. that's the whole thing. iconic in its own way
  • -idirafter dir — idirafter, then a dir
  • -imacros file — imacros again, from the preprocessor section, haunting this one too
  • -imultilib dir — imultilib, then a dir
  • -imultiarch dir — imultiarch, then a dir
  • -iplugindir=dir — plugindir, equals, then a dir
  • -iprefix file — iprefix, then a file
  • -iquote dir — iquote, then a dir
  • -isysroot dir — isysroot, then a dir
  • -isystem dir — isystem, then a dir
  • -iwithprefix dir — iwithprefix, then a dir
  • -iwithprefixbefore dir — iwithprefixbefore, then a dir. say that one out loud
  • -Ldir — capital L, then a dir
  • -no-canonical-prefixes — no canonical prefixes, again (she gets around)
  • --no-sysroot-suffix — no sysroot suffix
  • -nostdinc — no standard includes 🚫
  • -nostdinc++ — no standard includes, ++ edition
  • --embed-dir=dir — embed dir, equals, then a dir
  • --embed-directory=dir — embed directory, the long spelling of the same energy
  • --include-barrier — include barrier, a lil wall 🧱
  • --include-directory=dir — include directory, with a dir
  • --include-directory-after=dir — include directory after, so after what, we'll find out
  • --include-prefix=prefix — include prefix, with a prefix
  • --include-with-prefix=prefix — include WITH prefix, the "with" one
  • --include-with-prefix-after=prefix — with prefix, after
  • --include-with-prefix-before=prefix — with prefix, before. that's the whole before/after saga
  • --no-canonical-prefixes — yes, it's listed a second time. it's here. twice. periodt
  • --no-standard-includes — the long spelling of no standard includes
  • --prefix=prefix — prefix, equals, prefix. poetry
  • --sysroot=dir — sysroot, equals, then a dir

💜 CODE GENERATION OPTIONS 💜

the "what does the actual machine code look like" corner of the universe 🛠️. big section, big energy. let's go.

  • -fcall-saved-reg — call saved reg
  • -fcall-used-reg — call used reg
  • -ffixed-reg — fixed reg, don't move her
  • -fexceptions — exceptions ✨
  • -fnon-call-exceptions — non-call exceptions, the plot twist
  • -fdelete-dead-exceptions — deleting the dead exceptions, grim and efficient 💀
  • -funwind-tables — unwind tables, relaxing 🧘
  • -fasynchronous-unwind-tables — asynchronous unwind tables, multitasking
  • -fno-gnu-unique — no gnu unique
  • -finhibit-size-directive — inhibit the size directive, repressed
  • -fcommon — common, just common
  • -fno-ident — no ident
  • -fpcc-struct-return — pcc struct return, a blast from the past
  • -fpic — pic, lowercase, chill
  • -fPIC — PIC, uppercase, LOUD 📢
  • -fpie — pie again 🥧
  • -fPIE — PIE, uppercase, also LOUD
  • -fno-plt — no plt
  • -fno-jump-tables — no jump tables, the tables have been escorted out
  • -fno-bit-tests — no bit tests
  • -frecord-gcc-switches — record the gcc switches, keep receipts 📝
  • -freg-struct-return — reg struct return, the reg edition
  • -fshort-enums — short enums, they're petite
  • -fshort-wchar — short wchar, also petite
  • -fverbose-asm — verbose asm, chatty assembly 🗣️
  • -fpack-struct[=n] — pack the struct, with the optional [=n]
  • -fleading-underscore — leading underscore, it goes first
  • -ftls-model=model — tls model, with a model
  • -fstack-reuse=reuse_level — stack reuse, with a reuse_level. the variable is literally named after the flag
  • -ftrampolines — trampolines 🤸
  • -ftrampoline-impl=[stack|heap] — two choices for the trampoline: stack or heap
  • -ftrapv — trapv, one word
  • -fwrapv — wrapv, the sibling
  • -fwrapv-pointer — and then pointer showed up too
  • -fvisibility=[default|internal|hidden|protected] — four choices: default, internal, hidden, protected
  • -fstrict-volatile-bitfields — strict volatile bitfields, strict about it
  • -fsync-libcalls — sync the libcalls 🤝
  • -fzero-init-padding-bits=value — zero init padding bits, with a value
  • -Qy — capital Q, lowercase y
  • -Qn — capital Q, lowercase n. and that's the end of the list ✨

and there's more coming, bestie — don't go anywhere 💖

💅 Developer Options 💅

Okay bestie, this next batch is the compiler-nerd zone. These are the flags the people who build GCC reach for when they're deep in the sauce — dumping, checking, reporting, profiling, and just generally staring at internals. No cap, they're iconic for exactly the kind of person who reads assembly for fun. Here's every single one, verbatim, because we do not skip flags in this house 🫶

  • -dletters — letters is a literal placeholder here, fill it in with actual letters, iykyk.
  • -dumpspecs — dumping the specs. Yes really, that's the whole name.
  • -dumpmachine — the machine gets dumped. Periodt.
  • -dumpversion — version-dumping behaviour, verbatim name.
  • -dumpfullversion — the full version one. Emphasis on full ✨
  • -fcallgraph-info[=su,da] — note the optional bit, and the values are exactly su,da.
  • -fchecking — checking mode: on.
  • -fchecking=n — same vibe, but you hand it an n.
  • -fdbg-cnt-list — the list version, loves a good enumerate.
  • -fdbg-cnt=counter-value-list — takes a counter-value-list, keep the name exact.
  • -fdisable-ipa-pass_name — takes a <italic>pass_name</italic>, IPA flavour.
  • -fdisable-rtl-pass_name — same shape, RTL flavour.
  • -fdisable-rtl-pass-name=range-list — the equals form, and it wants a range-list.
  • -fdisable-tree-pass_name — tree flavour, <italic>pass_name</italic> goes here.
  • -fdisable-tree-pass-name=range-list — tree flavour with the range-list on the end.
  • -fdump-debug — debug gets dumped, bestie.
  • -fdump-earlydebug — the early debug dump. Punctual queen 🎀
  • -fdump-noaddr — dump, no addresses.
  • -fdump-unnumbered — dump, unnumbered.
  • -fdump-unnumbered-links — unnumbered and the links too.
  • -fdump-final-insns[=file] — optional =file, keep those brackets.
  • -fdump-internal-locations — the internal locations one 💅
  • -fdump-ipa-all — IPA, all of it.
  • -fdump-ipa-cgraph — IPA cgraph specifically.
  • -fdump-ipa-inline — IPA inline specifically.
  • -fdump-lang-all — language dump, the whole menu.
  • -fdump-lang-switch — lang switch, no argument.
  • -fdump-lang-switch-options — lang switch options, still no argument.
  • -fdump-lang-switch-options=filename — now with a <italic>filename</italic>.
  • -fdump-passes — passes get dumped. Iconic.
  • -fdump-rtl-pass — RTL pass, bare version.
  • -fdump-rtl-pass=filename — RTL pass plus your <italic>filename</italic>.
  • -fdump-statistics — statistics, dumped. Numbers girlies rise 🌸
  • -fdump-tree-all — tree dump, everything, go off.
  • -fdump-tree-switch — tree switch, bare.
  • -fdump-tree-switch-options — tree switch options, bare.
  • -fdump-tree-switch-options=filename — same but with a <italic>filename</italic>.
  • -fcompare-debug[=opts] — optional =opts in brackets, exactly as written.
  • -fcompare-debug-second — the second one. Second, bestie, don't mix them up 💀
  • -fenable-kind-pass — note it's <italic>kind</italic> and <italic>pass</italic> squished together.
  • -fenable-kind-pass=range-list — same, with range-list attached.
  • -fira-verbose=n — wants an n.
  • -flto-report — LTO report, incoming.
  • -flto-report-wpa — LTO report, whole-program analysis flavour.
  • -fmem-report-wpa — mem report, WPA flavour.
  • -fmem-report — mem report, plain.
  • -fpre-ipa-mem-report — pre-IPA mem report, right on time.
  • -fpost-ipa-mem-report — post-IPA mem report, fashionably late.
  • -fopt-info — the classic, no argument.
  • -fopt-info-options[=file] — options flavour, optional =file in brackets, don't lose them.
  • -fmultiflags — multi flags. Plural. Respect it ✨
  • -fprofile-report — profile report, slay.
  • -frandom-seed=string — takes a <italic>string</italic> as the seed.
  • -fsched-verbose=n — scheduler verbose with a level n.
  • -fsel-sched-verbose — selective scheduler verbose, bare.
  • -fsel-sched-dump-cfg — dumps CFG, name says it.
  • -fsel-sched-pipelining-verbose — pipelining verbose. Long iconique name 👑
  • -fstats — stats. Short, sweet, dramatic.
  • -fstack-usage — stack usage, tracked.
  • -ftime-report — where did the compile time go? This one.
  • -ftime-report-details — the details version for the extra-obsessed 🫶
  • -fvar-tracking-assignments-toggle — a toggle, and it's a whole mouthful.
  • -gtoggle — g toggle, little guy.
  • -print-autofdo-gcov-version — prints that autofdo gcov version, exactly.
  • -print-file-name=library — hand it a <italic>library</italic>.
  • -print-libgcc-file-name — libgcc file name, single dash version.
  • -print-multi-directory — multi directory, printed.
  • -print-multi-lib — multi lib, printed.
  • -print-multi-os-directory — the multi OS directory, printed.
  • -print-multiarch — multiarch, printed. One word, no dashes, careful 🎀
  • -print-prog-name=program — takes a <italic>program</italic>.
  • -print-search-dirs — search dirs, printed. Plural, as is.
  • -Q — capital Q, one character, main character energy 💅
  • -print-sysroot — sysroot, printed.
  • -print-sysroot-headers-suffix — sysroot headers suffix, printed.
  • -save-temps — keep those temps, bestie.
  • -save-temps=cwd — temps into the cwd.
  • -save-temps=obj — temps into the obj spot.
  • -specs=file — takes a <italic>file</italic> of specs.
  • -time[=file] — lowercase -time, optional =file in brackets.
  • --dump=letters — double dash, and <italic>letters</italic> again.
  • --print-autofdo-gcov-version — double-dash twin of the one above.
  • --print-file-name=library — double-dash, same library placeholder.
  • --print-libgcc-file-file-name — yes, it really says file-file, we do not "fix" it 🙈
  • --print-multi-directory — double-dash multi directory.
  • --print-multi-lib — double-dash multi lib.
  • --print-multi-os-directory — double-dash multi OS directory.
  • --print-multiarch — double-dash multiarch.
  • --print-prog-name=program — double-dash, takes a <italic>program</italic>.
  • --print-search-dirs — double-dash search dirs.
  • --print-sysroot — double-dash sysroot.
  • --print-sysroot-headers-suffix — double-dash sysroot headers suffix.
  • --save-temps — double-dash temps saving.
  • --specs=file — double-dash specs file.
  • --param name=value — yes, there's a space between the flag and name=value, keep it exactly like that ⭐

🎯 Target-Specific Options 🎯

Now we enter the architecture wing of the museum, and it is huge. Every chip family gets its own little shrine of -m flags. Full list, every single one, no picking and choosing 💖

AArch64 Options (AArch64 Options) 🧚

  • -mabi=name — takes a <italic>name</italic>.
  • -mbig-endian — big endian mode.
  • -mlittle-endian — little endian mode.
  • -menable-sysreg-checking — enables sysreg checking.
  • -mgeneral-regs-only — general regs only, nothing else invited.
  • -mcmodel=tiny — code model: tiny.
  • -mcmodel=small — code model: small.
  • -mcmodel=large — code model: large.
  • -mstrict-align — strict alignment. Uptight, but valid 💅
  • -momit-leaf-frame-pointer — omits that leaf frame pointer.
  • -mtls-dialect=desc — TLS dialect desc.
  • -mtls-dialect=traditional — TLS dialect traditional.
  • -mtls-size=size — takes a <italic>size</italic>.
  • -mtp=name — takes a <italic>name</italic>.
  • -mfix-cortex-a53-835769 — keep that number exact: 835769.
  • -mfix-cortex-a53-843419 — and this one: 843419.
  • -mlow-precision-recip-sqrt — low precision reciprocal square root.
  • -mlow-precision-sqrt — low precision square root.
  • -mlow-precision-div — low precision division.
  • -mmax-vectorization — max vectorization, go big ✨
  • -mautovec-preference=name — takes a <italic>name</italic>.
  • -mpc-relative-literal-loads — PC-relative literal loads.
  • -msign-return-address=scope — takes a <italic>scope</italic>.
  • -mbranch-protection=features — takes <italic>features</italic>.
  • -mharden-sls=opts — takes <italic>opts</italic>.
  • -march=name — takes a <italic>name</italic>.
  • -mcpu=name — takes a <italic>name</italic>.
  • -mtune=name — takes a <italic>name</italic>.
  • -moverride=string — takes a <italic>string</italic>.
  • -mstack-protector-guard=guard — takes a <italic>guard</italic>.
  • -mstack-protector-guard-reg=sysreg — takes a <italic>sysreg</italic>.
  • -mstack-protector-guard-offset=offset — takes an <italic>offset</italic>.
  • -mtrack-speculation — tracks speculation.
  • -moutline-atomics — outlines atomics.
  • -mearly-ra — early register allocation.
  • -mearly-ldp-fusion — early LDP fusion.
  • -mlate-ldp-fusion — late LDP fusion. (Yes, there's an early AND a late one, they're a duo 🫶)
  • -mnarrow-gp-writes — narrow GP writes.
  • -msve-vector-bits=bits — takes <italic>bits</italic>.

Adapteva Epiphany Options (Adapteva Epiphany Options) 🌈

  • -mhalf-reg-file — half register file.
  • -mprefer-short-insn-regs — prefers short instruction regs.
  • -mbranch-cost=num — takes a <italic>num</italic>.
  • -mcmove — cmove, four letters of power.
  • -mnops=num — takes a <italic>num</italic>.
  • -msoft-cmpsf — soft cmpsf.
  • -msplit-lohi — split lo/hi.
  • -mpost-inc — post increment.
  • -mpost-modify — post modify.
  • -mstack-offset=num — takes a <italic>num</italic>.
  • -mround-nearest — rounds to nearest.
  • -mlong-calls — long calls.
  • -mshort-calls — short calls. They're rivals, obviously 💀
  • -msmall16 — small16.
  • -mfp-mode=mode — takes a <italic>mode</italic>.
  • -mmay-round-for-trunc — may round for truncation.
  • -mfp-iarith — floating point integer arithmetic.
  • -mvect-double — vector double.
  • -max-vect-align=num — takes a <italic>num</italic>.
  • -msplit-vecmove-early — split vecmove, early.
  • -m1reg-reg — yes, -m1reg-reg, exactly that 🙈

AMD GCN Options (AMD GCN Options) 🔥

  • -march=gpu — takes a <italic>gpu</italic>.
  • -mtune=gpu — takes a <italic>gpu</italic> too.
  • -mgang-private-size=bytes — takes <italic>bytes</italic>.
  • -msram-ecc=[on|off|any] — one of on, off, or any, keep the pipe characters.
  • -mxnack=[on|off|any] — same trio: on, off, any.
  • -Wopenacc-dims — a warning flag, not a -m one, watch the capital W.

ARC Options (ARC Options) 🌸

  • -mbarrel-shifter — barrel shifter.
  • -mjli-always — JL, just-in-time style.
  • -mcpu=cpu — takes a <italic>cpu</italic>.
  • -mA6 — capital A, then 6.
  • -mARC600 — capital ARC, then 600.
  • -mA7 — capital A, then 7.
  • -mARC700 — capital ARC, then 700.
  • -mdpfp — double precision FP.
  • -mdpfp-compact — the compact one.
  • -mdpfp-fast — the fast one.
  • -mno-dpfp-lrsr — the no- version of dpfp lrsr.
  • -mea — EA.
  • -mmul32x16 — 32x16, lowercase x, no spaces.
  • -mmul64 — mul64.
  • -matomic — atomic.
  • -mnorm — norm.
  • -mspfp — single precision FP.
  • -mspfp-compact — spfp, compact style.
  • -mspfp-fast — spfp, fast style.
  • -msimd — SIMD.
  • -msoft-float — soft float.
  • -mswap — swap.
  • -mlock — lock.
  • -mswape — swape. Yes, swap and swape are both here, they're different 💅
  • -mxy — XY.
  • -misize — instruction size.
  • -marclinux — ARC Linux, all lowercase in one word.
  • -marclinux_prof — and this one has an underscore in it, keep it.
  • -mlong-calls — long calls.
  • -mmedium-calls — medium calls. Small, medium, large energy.
  • -msdata — small data.
  • -mirq-ctrl-saved — IRQ ctrl saved.
  • -mrgf-banked-regs — RGF banked regs.
  • -mlpc-width=width — takes a <italic>width</italic>.
  • -G num — capital G, then a <italic>num</italic>. Not a -m flag, note the space.
  • -mvolatile-cache — volatile cache.
  • -mtp-regno=regno — takes a <italic>regno</italic>.
  • -mbitops — bit ops.
  • -mcmem — cmem.
  • -munaligned-access — unaligned access.
  • -mauto-modify-reg — auto modify register.
  • -mno-brcc — no BRCC.
  • -mcase-vector-pcrel — case vector, PC-relative.
  • -mno-cond-exec — no conditional execution.
  • -mearly-cbranchsi — early cbranchsi.
  • -mindexed-loads — indexed loads.
  • -mlra-priority-none — LRA priority: none.
  • -mlra-priority-compact — LRA priority: compact.
  • -mlra-priority-noncompact — LRA priority: noncompact.
  • -mmillicode — millicode.
  • -msize-level=level — takes a <italic>level</italic>.
  • -mtune=cpu — takes a <italic>cpu</italic>.
  • -mmultcost=num — takes a <italic>num</italic>.
  • -mcode-density-frame — code density frame.
  • -mmpy-option=multo — takes multo, and yes that's the spelling, don't come for me 😭
  • -mdiv-rem — div rem.
  • -mcode-density — code density.
  • -mll64 — ll64.
  • -mfpu=fpu — takes an <italic>fpu</italic>.
  • -mrf16 — RF16.
  • -mbranch-index — branch index.

ARM Options (ARM Options) 👑

  • -mapcs-frame — APCS frame.
  • -mapcs — APCS, bare.
  • -mabi=name — takes a <italic>name</italic>.
  • -mgeneral-regs-only — general regs only.
  • -mno-sched-prolog — no scheduled prologue.
  • -mlittle-endian — little endian.
  • -mbig-endian — big endian.
  • -mbe8 — BE8.
  • -mbe32 — BE32.
  • -mfloat-abi=name — takes a <italic>name</italic>.
  • -mfp16-format=name — takes a <italic>name</italic>.
  • -mthumb-interwork — Thumb interwork.
  • -mcpu=name — takes a <italic>name</italic>.
  • -march=name — takes a <italic>name</italic>.
  • -mfpu=name — takes a <italic>name</italic>.
  • -mtune=name — takes a <italic>name</italic>.
  • -mstructure-size-boundary=n — takes an n.
  • -mabort-on-noreturn — aborts on no-return.
  • -mlong-calls — long calls.
  • -msingle-pic-base — single PIC base.
  • -mpic-register=reg — takes a <italic>reg</italic>.
  • -mpic-data-is-text-relative — PIC data is text relative.
  • -mnop-fun-dllimport — nop fun dllimport.
  • -mpoke-function-name — pokes the function name.
  • -mthumb — Thumb mode.
  • -marm — ARM mode. (The eternal argument, we don't take sides 💅)
  • -mtpcs-frame — TPCS frame.
  • -mtpcs-leaf-frame — TPCS leaf frame.
  • -mcaller-super-interworking — caller super interworking.
  • -mcallee-super-interworking — callee super interworking.
  • -mtp=name — takes a <italic>name</italic>.
  • -mtls-dialect=dialect — takes a <italic>dialect</italic>.
  • -mword-relocations — word relocations.
  • -mfix-cortex-m3-ldrd — the M3 LDRD fix.
  • -mfix-cortex-a57-aes-1742098 — that number is 1742098, character for character.
  • -mfix-cortex-a72-aes-1655431 — and this one is 1655431.
  • -munaligned-access — unaligned access.
  • -mslow-flash-data — slow flash data.
  • -masm-syntax-unified — unified asm syntax.
  • -mrestrict-it — restrict IT.
  • -mpure-code — pure code.
  • -mcmse — CMSE.
  • -mfix-cmse-cve-2021-35465 — CVE number 2021-35465, keep every digit.
  • -mstack-protector-guard=guard — takes a <italic>guard</italic>.
  • -mstack-protector-guard-offset=offset — takes an <italic>offset</italic>.
  • -mfdpic — FDPIC.
  • -mbranch-protection=features — takes <italic>features</italic>.
  • -mvectorize-with-neon-quad — vectorize with NEON quad.
  • -mvectorize-with-neon-double — vectorize with NEON double.

AVR Options (AVR Options) 🍓

  • -mmcu=mcu — takes an <italic>mcu</italic>.
  • -mabsdata — absolute data.
  • -maccumulate-args — accumulates args.
  • -masm-len-notes — asm length notes.
  • -mbranch-cost=cost — takes a <italic>cost</italic>.
  • -mcvt — CVT.
  • -mdemote-switch — demote switch.
  • -mfuse-add=level — takes a <italic>level</italic>.
  • -mfuse-ifelse — fuse if-else.
  • -mfuse-move=level — takes a <italic>level</italic>.
  • -mfuse-move2 — fuse move, part 2. The sequel 🔥
  • -mcall-prologues — call prologues.
  • -mgas-isr-prologues — GAS ISR prologues.
  • -mint8 — int8.
  • -mflmap — FLMAP.
  • -mdouble=bits — takes <italic>bits</italic>.
  • -mlong-double=bits — takes <italic>bits</italic>.
  • -mno-call-main — no call main.
  • -mn_flash=size — underscore in there: mn_flash, then takes a <italic>size</italic>.
  • -mfract-convert-truncate — fract convert truncate.
  • -mno-interrupts — no interrupts.
  • -mmain-is-OS_task — main is OS_task, mixed case and an underscore, exactly so.
  • -mrelax — relax, bestie.
  • -mpmem-wrap-around — pmem wrap around.
  • -mrmw — RMW.
  • -mstrict-addr-space-subsets — strict address space subsets.
  • -mstrict-X — capital X at the end, don't lowercase it.
  • -mtiny-stack — tiny stack, tiny vibe.
  • -mrodata-in-ram — rodata in RAM.
  • -msplit-bit-shift — split bit shift.
  • -msplit-ldst — split load/store.
  • -mshort-calls — short calls.
  • -mskip-bug — skip bug. We love a workaround 💅
  • -muse-nonzero-bits — use nonzero bits.
  • -nodevicelib — no device lib.
  • -nodevicespecs — no device specs.
  • -Wasm-len-notes — capital W, this one's a warning.
  • -Waddr-space-convert — capital W again.
  • -Wmisspelled-isr — capital W, and yes, "misspelled". The spelling stays.

Blackfin Options (Blackfin Options) 💜

  • -mcpu=cpu[-sirevision] — takes a <italic>cpu</italic> and optionally the [-sirevision] bit, brackets included.
  • -msim — sim mode.
  • -momit-leaf-frame-pointer — omit leaf frame pointer.
  • -mspecld-anomaly — specld anomaly.
  • -mcsync-anomaly — csync anomaly.
  • -mlow-64k — low 64k.
  • -mstack-check-l1 — stack check L1.
  • -mid-shared-library — ID shared library.
  • -mleaf-id-shared-library — leaf ID shared library.
  • -mshared-library-id=n — takes an n.
  • -msep-data — separate data.
  • -mlong-calls — long calls.
  • -mfast-fp — fast floating point.
  • -minline-plt — inline PLT.
  • -mmulticore — multicore.
  • -mcorea — core A.
  • -mcoreb — core B. Siblings 💞
  • -msdram — SDRAM.
  • -micplb — ICPLB.

C6X Options (C6X Options) ⭐

  • -mbig-endian — big endian.
  • -mlittle-endian — little endian.
  • -march=cpu — takes a <italic>cpu</italic>.
  • -msim — sim mode.
  • -msdata=sdata-type — takes an <italic>sdata-type</italic>.
  • -mdsbt — DSBT.
  • -mlong-calls — long calls.

CRIS Options (CRIS Options) 🎀

  • -mcpu=cpu — takes a <italic>cpu</italic>.
  • -march=cpu — takes a <italic>cpu</italic>.
  • -mtune=cpu — takes a <italic>cpu</italic>.
  • -mmax-stackframe=n — takes an n.
  • -metrax4 — ETRAX 4.
  • -metrax100 — ETRAX 100.
  • -mpdebug — P debug.
  • -mcc-init — CC init.
  • -mno-side-effects — no side effects.
  • -mstack-align — stack align.
  • -mdata-align — data align.
  • -mconst-align — const align.
  • -m32-bit — 32-bit, dash and all.
  • -m16-bit — 16-bit.
  • -m8-bit — 8-bit.
  • -mno-prologue-epilogue — no prologue/epilogue, though it's spelled without the slash in the flag.
  • -mbest-lib-options — best lib options.
  • -moverride-best-lib-options — override best lib options.
  • -mtrap-using-break8 — trap using break8.
  • -mtrap-unaligned-atomic — trap unaligned atomic.
  • -munaligned-atomic-may-use-library — unaligned atomic may use library. Long queen 👑
  • -sim — note: -sim, no m.
  • -sim2 — the sequel, also no m.
  • -mmul-bug-workaround — mul bug workaround.

C-SKY Options (C-SKY Options) 🦋

  • -march=arch — takes an <italic>arch</italic>.
  • -mcpu=cpu — takes a <italic>cpu</italic>.
  • -mbig-endian — big endian.
  • -mlittle-endian — little endian.
  • -mfpu=fpu — takes an <italic>fpu</italic>.
  • -mdouble-float — double float.
  • -mfdivdu — fdivdu.
  • -mfloat-abi=name — takes a <italic>name</italic>.
  • -melrw — ELRW.
  • -mistack — iStack.
  • -mmp — MP.
  • -mcp — CP.
  • -mcache — cache.
  • -msecurity — security.
  • -mtrust — trust.
  • -mdsp — DSP.
  • -medsp — enhanced DSP.
  • -mvdsp — vector DSP.
  • -mdiv — div.
  • -msmart — smart.
  • -mhigh-registers — high registers.
  • -manchor — anchor.
  • -mpushpop — push-pop.
  • -mmultiple-stld — multiple store/load.
  • -mconstpool — const pool.
  • -mstack-size — stack size.
  • -mccrt — CCRT.
  • -mbranch-cost=n — takes an n.
  • -msched-prolog — scheduled prologue.
  • -msim — sim mode.

Cygwin and MinGW Options (Cygwin and MinGW Options) 🌸

  • -mconsole — console.
  • -mcrtdll=library — takes a <italic>library</italic>.
  • -mdll — DLL.
  • -mnop-fun-dllimport — nop fun dllimport.
  • -mthreads — threads.
  • -municode — unicode.
  • -mwin32 — win32.
  • -mwindows — windows.
  • -fno-set-stack-executable — an -f flag, not -m, heads up.
  • -fwritable-relocated-rdata — also -f, writable relocated rdata.
  • -mpe-aligned-commons — PE aligned commons.
  • -muse-libstdc-wrappers — use libstdc wrappers.

Darwin Options (Darwin Options) 🌈

  • -all_load — loads all, single dash, two words with an underscore.
  • -allowable_client — allowable client.
  • -arch name — takes a <italic>name</italic> with a space.
  • -arch_errors_fatal — arch errors, fatal.
  • -asm_macosx_version_min=version — takes a <italic>version</italic>.
  • -bind_at_load — bind at load.
  • -bundle — bundle.
  • -bundle_loader — bundle loader.
  • -client_name — client name.
  • -compatibility_version — compatibility version.
  • -current_version — current version.
  • -dead_strip — dead strip.
  • -dependency-file — dependency file.
  • -dylib_file — dylib file.
  • -dylinker — dylinker.
  • -dylinker_install_name — dylinker install name.
  • -dynamic — dynamic.
  • -dynamiclib — dynamic lib.
  • -exported_symbols_list — exported symbols list.
  • -fapple-kext — Apple kext.
  • -fconstant-cfstrings — constant CF strings.
  • -ffix-and-continue — fix and continue.
  • -filelist — file list.
  • -findirect-data — indirect data.
  • -flat_namespace — flat namespace.
  • -force_cpusubtype_ALL — capital ALL at the end, keep it.
  • -force_flat_namespace — force flat namespace.
  • -framework name — takes a <italic>name</italic> with a space.
  • -gfull — full debug info.
  • -gused — used debug info.
  • -headerpad_max_install_names — header pad, max install names.
  • -iframework — i-framework.
  • -image_base — image base.
  • -init symbol-name — takes a <italic>symbol-name</italic> after a space.
  • -install_name — install name.
  • -keep_private_externs — keep private externs.
  • -matt-stubs — Matt stubs. Shoutout Matt 💅
  • -mconstant-cfstrings — the -m version of the cfstrings one.
  • -mdynamic-no-pic — dynamic, no PIC.
  • -mfix-and-continue — the -m version of fix and continue.
  • -mkernel — kernel mode.
  • -mmacosx-version-min=version — takes a <italic>version</italic>.
  • -mone-byte-bool — one byte bool.
  • -msymbol-stubs — symbol stubs.
  • -mtarget-linker[=]version — that's [=] in the middle, then a <italic>version</italic>. Copy it exactly.
  • -nodefaultexport — no default export, one word.
  • -nodefaultrpaths — no default rpaths, one word.
  • -pagezero_size — page zero size.
  • -preload — preload.
  • -read_only_relocs — read only relocs.
  • -sectalign — section align.
  • -sectcreate — section create.
  • -seg_addr_table — segment address table.
  • -seg1addr — segment 1 address, no underscore between seg and 1.
  • -segaddr — segment address.
  • -segprot — segment protection.
  • -segs_read_only_addr — segments read only address, plural segs.
  • -segs_read_write_addr — segments read write address.
  • -sub_library — sub library.
  • -sub_umbrella — sub umbrella.
  • -twolevel_namespace — two level namespace.
  • -twolevel_namespace_hints — two level namespace hints.
  • -umbrella — umbrella.
  • -undefined — undefined.
  • -unexported_symbols_list — unexported symbols list.
  • -weak_framework name — takes a <italic>name</italic> after a space.
  • -weak_reference_mismatches — weak reference mismatches.
  • -whatsloaded — what's loaded, no apostrophe.
  • -whyload — why load.
  • -F — capital F, on its own.
  • -ObjC — capital O, capital C.
  • -ObjC++ — with the plus plus, obviously.
  • -Wnonportable-cfstrings — capital W, this one warns.

DEC Alpha Options (DEC Alpha Options) 💖

  • -mno-fp-regs — no FP registers.
  • -msoft-float — soft float.
  • -mieee — IEEE.
  • -mieee-with-inexact — IEEE, with inexact.
  • -mieee-conformant — IEEE, conformant.
  • -mfp-trap-mode=mode — takes a <italic>mode</italic>.
  • -mfp-rounding-mode=mode — takes a <italic>mode</italic>.
  • -mtrap-precision=mode — takes a <italic>mode</italic>.
  • -mbuild-constants — build constants.
  • -mcpu=cpu-type — takes a <italic>cpu-type</italic>.
  • -mtune=cpu-type — takes a <italic>cpu-type</italic>.
  • -mbwx — BWX.
  • -mmax — MAX.
  • -mfix — FIX.
  • -mcix — CIX.
  • -msafe-bwa — safe BWA.
  • -msafe-partial — safe partial.
  • -mfloat-vax — float VAX.
  • -mfloat-ieee — float IEEE.
  • -mexplicit-relocs — explicit relocations.
  • -msmall-data — small data.
  • -mlarge-data — large data.
  • -msmall-text — small text.
  • -mlarge-text — large text.
  • -mmemory-latency=time — takes a <italic>time</italic>.
  • -mtls-kernel — TLS kernel.
  • -mtls-size=bitsize — takes a <italic>bitsize</italic>.
  • -mlong-double-128 — long double, 128.
  • -mlong-double-64 — long double, 64.

eBPF Options (eBPF Options) 🔥

  • -mbig-endian — big endian.
  • -mlittle-endian — little endian.
  • -mframe-limit=bytes — takes <italic>bytes</italic>.
  • -mxbpf — xBPF.
  • -mco-re — CO-RE, with that dash in the middle.
  • -mjmpext — jump extension.
  • -mjmp32 — jump 32.
  • -malu32 — ALU 32.
  • -mv3-atomics — v3 atomics.
  • -mbswap — byte swap.
  • -msdiv — signed div.
  • -msmov — signed mov.
  • -mcpu=version — takes a <italic>version</italic>.
  • -masm=dialect — takes a <italic>dialect</italic>.
  • -minline-memops-threshold=bytes — takes <italic>bytes</italic>.
  • -Wco-re — capital W, warning flag.

FR30 Options (FR30 Options) 🧚

  • -msmall-model — small model.
  • -mno-lsim — no lsim.

FRV Options (FRV Options) 🌟

  • -mgpr-32 — GPR 32.
  • -mgpr-64 — GPR 64.
  • -mfpr-32 — FPR 32.
  • -mfpr-64 — FPR 64.
  • -mhard-float — hard float.
  • -msoft-float — soft float.
  • -malloc-cc — allocate CC.
  • -mfixed-cc — fixed CC.
  • -mdword — double word.
  • -mdouble — double.
  • -mmedia — media.
  • -mmuladd — multiply-add.
  • -mfdpic — FDPIC.
  • -minline-plt — inline PLT.
  • -mgprel-ro — GP-relative, read-only.
  • -multilib-library-pic — multilib library PIC.
  • -mlinked-fp — linked frame pointer.
  • -mlong-calls — long calls.
  • -malign-labels — align labels.
  • -mlibrary-pic — library PIC.
  • -macc-4 — 4 accumulators.
  • -macc-8 — 8 accumulators.
  • -mpack — pack.
  • -mno-eflags — no eflags.
  • -mno-cond-move — no conditional move.
  • -mno-optimize-membar — no optimize memory barrier.
  • -mno-scc — no SCC.
  • -mno-cond-exec — no conditional execution.
  • -mno-vliw-branch — no VLIW branch.
  • -mno-multi-cond-exec — no multi conditional execution.
  • -mno-nested-cond-exec — no nested conditional execution.
  • -mtomcat-stats — Tomcat stats. Cute name, keep it.
  • -mTLS — capital TLS.
  • -mtls — lowercase tls. Both exist, they are not the same, do not swap them 💀
  • -mcpu=cpu — takes a <italic>cpu</italic>.

FT32 Options (FT32 Options) 🍓

  • -msim — sim mode.
  • -mnodiv — no div.
  • -mft32b — FT32B.
  • -mcompress — compress.
  • -mnopm — no PM.

GNU/Linux Options (GNU/Linux Options) 🐧

  • -mglibc — glibc.
  • -muclibc — uClibc.
  • -mmusl — musl.
  • -mbionic — bionic.
  • -mandroid — Android mode.
  • -tno-android-cc — note the t start, not m.
  • -tno-android-ld — same deal, t start.

H8/300 Options (H8/300 Options) 🎀

  • -mrelax — relax.
  • -mh — H.
  • -ms — S.
  • -mn — N.
  • -msx — SX.
  • -ms2600 — S2600.
  • -mquickcall — quick call.
  • -mslowbyte — slow byte.
  • -mexr — EXR.
  • -mint32 — int 32.
  • -malign-300 — align 300.

And that's the whole target wing, bestie — every architecture, every flag, nothing dropped. The tour is not over though, so don't go anywhere 💖

🎀 HPPA Options (HPPA Options)

the HP Precision Architecture section, bestie — enjoyed, obsessed, here's the full list 💖

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

✨ IA-64 Options (IA-64 Options)

Itanium girlies, this one's for you 🦋

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

💜 LM32 Options (LM32 Options)

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

🌸 LoongArch Options (LoongArch Options)

not the little dragon architecture serving lewks 🔥

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

🧚 LynxOS Options (LynxOS Options)

  • -mshared
  • -mthreads
  • -mlegacy-threads

🍓 M32R/D Options (M32R/D Options)

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

⭐ M680x0 Options (M680x0 Options)

legacy models ate, no cap 💅

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

🌈 MCore Options (MCore Options)

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

🫶 MicroBlaze Options (MicroBlaze Options)

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

👑 MIPS Options (MIPS Options)

okay this one is a LOT, bestie. stay with me, we're getting through all of them 💀

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

proud of you for making it through that one 🫶 periodt.

💫 MMIX Options (MMIX Options)

Knuth girlies rise 🙈

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

🎀 MN10300 Options (MN10300 Options)

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

🦋 Moxie Options (Moxie Options)

cute name, cute flags 💖

  • -meb
  • -mel
  • -mmul.x
  • -mno-crt0

✨ MSP430 Options (MSP430 Options)

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

💜 NDS32 Options (NDS32 Options)

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

🌸 Nvidia PTX Options (Nvidia PTX Options)

GPU girlie moment 💅✨

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

🧚 OpenRISC Options (OpenRISC Options)

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

🍓 PDP-11 Options (PDP-11 Options)

the retro baddie of architectures 💀 iconic fr

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

⭐ Picolibc Options (Picolibc Options)

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

🫶 PowerPC Options

See RS/6000 and PowerPC Options.

🌈 PRU Options (PRU Options)

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

👑 RISC-V Options (RISC-V Options)

the main character of modern ISAs, we love to see it 🔥

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

🌸 RL78 Options (RL78 Options)

last one in this batch, you're doing amazing sweetie 🥹

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

👑 RS/6000 and PowerPC Options 👑

bestie, this is the IBM RS/6000 + PowerPC family of -m flags. There are so many. Character-for-character, no cap:

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

✨ RX Options ✨

Renames only, no invented lore, here's the exact list 🫶

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

🌈 S/390 and zSeries Options 🌈

mainframes said "we're the moment" and honestly? iconic:

-mtune=cpu-type  -march=cpu-type -mhard-float  -msoft-float
-mhard-dfp -mlong-double-64  -mlong-double-128 -mbackchain
-mpacked-stack -msmall-exec  -mmvcle -m64  -mdebug  -mzarch -mhtm
-mvx  -mzvector -mtpf-trace  -mtpf-trace-skip  -mmain -mfused-madd
-mno-fused-madd -mwarn-framesize=framesize  -mwarn-dynamicstack
-mstack-size=stack-size  -mno-stack-size -mstack-guard=stack-guard
-mno-stack-guard -mstack-protector-guard=guard
-mstack-protector-guard-record
-mhotpatch=pre-halfwords,post-halfwords
-mno-pic-data-is-text-relative -mindirect-branch=choice
-mindirect-branch-jump=choice  -mindirect-branch-call=choice
-mfunction-return=choice -mfunction-return-mem=choice
-mfunction-return-reg=choice -mindirect-branch-table -mfentry
-mrecord-mcount  -mnop-mcount -mpreserve-args  -munaliged-symbols
Not me pointing out that -munaliged-symbols is spelled exactly like that. That's the page's spelling, so that's our spelling. 💅

🦋 SH Options 🦋

SuperH time, babes:

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

🌸 Solaris 2 Options 🌸

short + sweet, the mini-arc of option groups:

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

🔥 SPARC Options 🔥

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

💜 System V Options 💜

only two. they understood the assignment:

-YP,paths  -Ym,dir

🎀 V850 Options 🎀

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

🌟 VAX Options 🌟

-munix  -mgnu  -md  -md-float  -mg
-mg-float  -mlra -mvaxc-alignment  -mqmath

🧚 Visium Options 🧚

-mdebug  -msim  -mfpu  -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  -mvthreads
-non-static  -Bstatic  -Bdynamic -Xbind-lazy  -Xbind-now

💅 x86 Options 💅

okay this one is the main character. it's a whole semester of material. every single one, exactly as printed, no shortcuts bestie:

-mtune=cpu-type  -march=cpu-type
-mtune-ctrl=feature-list  -mdump-tune-features  -mno-default
-mfpmath=unit -masm=dialect  -mno-fancy-math-387 -mno-fp-ret-in-387
-m80387  -mhard-float  -msoft-float  -mieee-fp -mrtd  -malign-double
-mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld
-mcx16  -msahf  -mmovbe  -mcrc32  -mmwait -mrecip  -mrecip=opt
-mvzeroupper  -mstv  -mprefer-avx128  -mprefer-vector-width=opt
-mpartial-vector-fp-math -mmove-max=bits  -mstore-max=bits
-mnoreturn-no-callee-saved-registers -mmmx  -msse  -msse2  -msse3
-mssse3  -msse4.1  -msse4.2  -msse4  -mavx -mavx2  -mavx512f
-mavx512cd  -mavx512vl -mavx512bw  -mavx512dq  -mavx512ifma
-mavx512vbmi  -msha  -maes -mpclmul  -mfsgsbase  -mrdrnd  -mf16c
-mfma  -mpconfig  -mwbnoinvd -mptwrite  -mclflushopt  -mclwb
-mxsavec  -mxsaves -msse4a  -m3dnow  -m3dnowa  -mpopcnt  -mabm
-mbmi  -mtbm  -mfma4  -mxop -madx  -mlzcnt  -mbmi2  -mfxsr  -mxsave
-mxsaveopt  -mrtm  -mhle  -mlwp -mmwaitx  -mclzero  -mpku  -mgfni
-mvaes  -mwaitpkg -mshstk  -mmanual-endbr  -mcet-switch
-mforce-indirect-call -mavx512vbmi2  -mavx512bf16  -menqcmd
-mvpclmulqdq  -mavx512bitalg  -mmovdiri  -mmovdir64b
-mavx512vpopcntdq -mavx512vnni  -mprfchw  -mrdpid -mrdseed  -msgx
-mavx512vp2intersect  -mserialize  -mtsxldtrk -mamx-tile  -mamx-int8
-mamx-bf16  -muintr  -mhreset -mavxvnni  -mamx-fp8  -mavx512fp16
-mavxifma  -mavxvnniint8 -mavxneconvert  -mcmpccxadd  -mamx-fp16
-mprefetchi  -mraoint -mamx-complex  -mavxvnniint16  -msm3  -msha512
-msm4  -mapxf -musermsr  -mavx10.1  -mavx10.2  -mamx-avx512  -mmovrs
-mamx-movrs -mavx512bmm  -mavx10v2aux  -macev1 -mcldemote
-mms-bitfields  -mno-align-stringops  -minline-all-stringops
-minline-stringops-dynamically  -mstringop-strategy=alg -mkl
-mwidekl -mmemcpy-strategy=strategy  -mmemset-strategy=strategy
-mpush-args  -maccumulate-outgoing-args  -m128bit-long-double
-m96bit-long-double  -mlong-double-64  -mlong-double-80
-mlong-double-128 -mregparm=num  -msseregparm -mveclibabi=type
-mvect8-ret-in-mem -mpc32  -mpc64  -mpc80  -mdaz-ftz  -mstackrealign
-mstack-arg-probe -momit-leaf-frame-pointer  -mno-red-zone
-mno-tls-direct-seg-refs -mcmodel=code-model  -mabi=name
-maddress-mode=mode -m32  -m64  -mx32  -m16  -miamcu
-mlarge-data-threshold=num -msse2avx  -mfentry  -mrecord-mcount
-mnop-mcount  -m8bit-idiv -minstrument-return=type  -mrecord-return
-mfentry-name=name  -mfentry-section=name -mskip-rax-setup
-mavx256-split-unaligned-load  -mavx256-split-unaligned-store
-malign-data=type  -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol -mgeneral-regs-only
-mcall-ms2sysv-xlogues  -mtls-dialect=type -m128bit-atomic
-mrelax-cmpxchg-loop -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register
-mharden-sls=choice -mindirect-branch-cs-prefix
-mapx-inline-asm-use-gpr32 -mgather  -mscatter -mneeded
-mno-direct-extern-access -munroll-only-small-loops
-mdispatch-scheduler  -mlam=choice

x86 Windows Options — See Cygwin and MinGW Options.

🌈 Xstormy16 Options 🌈

one flag. one vibe. periodt:

-msim

🫶 Xtensa Options 🫶

-mconst16  -mforce-no-pic
-mno-serialize-volatile -mtext-section-literals  -mauto-litpools
-mno-target-align -mlongcalls  -mabi=abi-type
-mextra-l32r-costs=cycles  -mstrict-align  -mforce-l32

zSeries Options — See S/390 and zSeries Options.

✨ Options Controlling the Kind of Output ✨

okay bestie, now we get to the actual what is even happening section, and it's lowkey the most important one so far, so lock in 🔒

Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking — and it's always in that order, no switching it up, that's the canon.

GCC is capable of preprocessing and compiling several files either into several assembler input files, or into one assembler input file. Then each assembler input file produces an object file, and linking combines all the object files — both the newly compiled ones and the ones you handed it as input — into a single executable file. it's giving ✨assembly line✨ but for your code.

💖 the filename suffix tells GCC what to do 💖

For any given input file, the file name suffix determines what kind of compilation is done. here's the whole menu, no skips:

  • 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 queen: 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. you must link with the libobjc library to make an Objective-C++ program work. and yes, .M refers to a literal capital M — casing matters, don't @ me. 🙈
  • 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. 🌸 Heads up bestie: in .cxx, the last two letters have to both be a literal x. And .C means a literal capital C, okay?? Case matters, she's sensitive.
  • 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 that gets turned into a precompiled header or an 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 containing 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. ✨
  • 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. Like, any file name with no recognized suffix is treated this way. 🫶

You can also just tell her the language, out loud, with the -x option:

  • -x language, --language=language, --language language — Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next -x option. Possible values for language are:
c  c-header  cpp-output
c++  c++-header  c++-system-header c++-user-header c++-cpp-output
c++-system-module
objective-c  objective-c-header  objective-c-cpp-output  objc-cpp-output
objective-c++  objective-c++-header  objective-c++-cpp-output
objc++-cpp-output
assembler  assembler-with-cpp
ada  adascil  adawhy
cobol
d
f77  f77-cpp-input  f95  f95-cpp-input
go
modula-2  modula-2-cpp-output
rust
algol68
lto
🚨 Real talk: -x does not imply a particular language standard. So like, -x f77 might also need -std=legacy for some older source codes. Don't @ me, that's just how it is.
  • -x none — Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as if -x had not been used at all). 💀 Goodbye language, we barely knew ye.

So here's the vibe: if you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of -c, -S, or -E to say where gcc should stop. Just know that some combos (for example, -x cpp-output -E) instruct gcc to do absolutely nothing at all. Iconic behaviour honestly, she said "I'm not doing that" and she meant it. 🙈

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

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

Unrecognized input files that don't require compilation or assembly are just ignored. 🎀

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

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

Input files that don't require compilation are ignored.

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

Input files that don't require preprocessing are ignored.

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

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

Okay 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, --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
  • --dumpbase-ext auxdropsuf

okay bestie so when gcc is out here naming an auxiliary output file (and no, not a dump — more on that in a sec), it takes dumpbase and drops any trailing auxdropsuf off the end before it slaps any suffixes on. 💅

If you don't specify it? It just defaults to the suffix of a default dumpbase, which lowkey means the suffix of the input file when -dumpbase isn't chillin' on the command line, or dumpbase combined with dumppfx. 🦋

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

this one ✨ 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.*. no cap. 🫶

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

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

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

  • -dumpdir dumppfx
  • --dumpdir dumppfx

when forming the name of an auxiliary or dump output file, use dumppfx as a prefix, bestie:

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

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

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

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

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

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

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

outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*. the way that just quietly happens is so real 😭

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

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

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

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

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

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

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

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

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

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

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

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

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

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

  • -v
  • --verbose

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. basically receipts for everything that's running, obsessed. 🔥

  • -###

Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines. 🎀

  • --help

Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified then --help is also passed on to the various processes invoked by gcc, so that they can display the command-line options they accept. If the -Wextra option has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. 👑

  • --target-help

Print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. 🌸

  • --help={class|[^]qualifier}[,...]

Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. These are the supported classes:

  • optimizers — Display all of the optimization options supported by the compiler. ✨
  • warnings — Display all of the options controlling warning messages produced by the compiler. 😭
  • target — Display target-specific options. Unlike the --target-help option however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax. 💀
  • params — Display the values recognized by the --param option. 🦋
  • language — Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to select common class. 🌈
  • common — Display the options that are common to all languages. 🫶

These are the supported qualifiers:

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

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

--help=target,undocumented

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

--help=warnings,^joined,^undocumented

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

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

--help=target,optimizers

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

Okay bestie, so if you drop the -Q option on the command line before the --help= option, the cute descriptive text that --help= shows you gets ✨swapped✨. Instead of telling you what each option does, it tells you whether the option is enabled, disabled, or set to a specific value — but only if the compiler actually knows that at the point where --help= shows up. iykyk.

Here's a truncated little example from the ARM port of gcc, look at her:

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

And the way that the output depends on all the command-line options that came before it?? Obsessed. So for example you can find out which optimizations are enabled at -O2 like this:

-Q -O2 --help=optimizers

Or, alternatively, you can sleuth out which binary optimizations -O3 turns on with this little duo:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
  • --version — Displays the version number and the copyrights of the GCC you just invoked. iconic, honestly.
  • -pass-exit-codes, --pass-exit-codes — So normally the gcc program exits with code 1 if any phase of the compiler comes back with a non-success return code. But if you say -pass-exit-codes, the gcc program returns the numerically highest error produced by any phase that returned an error indication instead. Also fyi: the C, C++, and Fortran front ends return 4 if they hit an internal compiler error. 😭
  • -pipe, --pipe — Use pipes instead of temporary files for the chit-chat between the various stages of compilation. Heads up tho: this fails to work on some systems where the assembler can't read from a pipe — but the GNU assembler has no trouble at all, she's fine. 💅
  • -wrapper — Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters get passed as a comma separated list, like so:
gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, so the invocation of cc1 is gdb --args cc1 ....

  • -ffile-prefix-map=old=new — When you're compiling files that live in directory old, record any references to them in the result of the compilation as if those files lived in directory new instead. Specifying this option is equivalent to specifying all the individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent. Directories referenced by directives are not affected by these options. See also -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.
  • -fcanon-prefix-map — Okay so for the -f*-prefix-map options, normally the comparison of old prefix against the filename that would be normally referenced in the result of the compilation is done using textual comparison of the prefixes — or ignoring character case for case insensitive filesystems, and treating slashes and backslashes as equal on DOS based filesystems. -fcanon-prefix-map makes those comparisons happen on canonicalized paths of old and the referenced filename instead. 🦋
  • -fplugin=name.so — Loads the plugin code in file name.so, assumed to be a shared object that the compiler will dlopen. The base name of the shared object file is used to identify the plugin for the purposes of argument parsing (see -fplugin-arg-name-key=value below, bestie). Each plugin should define the callback functions specified in the Plugins API.
  • -fplugin-arg-name-key=value — Define an argument called key with a value of value for the plugin called name. Simple as that. ✨
  • -fdump-ada-spec[-slim] — For C and C++ source and include files, this generates corresponding Ada specs.
  • -fada-spec-parent=unit — In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
  • -fdump-go-spec=file — For input files in any language, generate corresponding Go declarations in file. This generates Go "const", "type", "var", and "func" declarations, which is literally such a useful way to start writing a Go interface to code written in some other language. 🫶
  • @file — Read command-line options from file. The options that get read are inserted in place of the original @file option. If file doesn't exist, or can't be read, then the option is treated literally, and not removed. No cap.

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

🌸 Compiling C++ Programs 🌸

C++ source files conventionally use one of 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. so many suffixes, she contains multitudes.

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). 👑

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

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

💜 Options Controlling C Dialect 💜

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

  • -ansi, --ansi — In C mode, this is equivalent to -std=c90. In C++ mode, it's equivalent to -std=c++98.
  • -std= — Determines the language standard. This option is currently only supported when compiling C or C++. you must give a value, no skipping.

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 a base standard is specified, the compiler accepts all programs that follow that standard, plus those using GNU extensions that don't contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords — but not other GNU extensions that have no meaning in ISO C90, like omitting the middle term of a "?:" expression.

On the other hand, when a GNU dialect of a standard is specified, all the 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 get rejected. 😭

The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example, -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.

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

  • c90, c89, iso9899:1990 — Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
  • iso9899:199409 — ISO C90 as modified in amendment 1.
  • c99, c9x, iso9899:1999, iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/projects/c-status.html> for more information. The names c9x and iso9899:199x are deprecated.
  • c11, c1x, iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
  • c17, c18, iso9899:2017, iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is the same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of "__STDC_VERSION__", and so is supported to the same extent as C11.
  • c23, c2x, iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated.
  • c2y — The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 👀
  • gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features).
  • gnu99, gnu9x — GNU dialect of ISO C99. The name gnu9x is deprecated.
  • gnu11, gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated.
  • gnu17, gnu18 — GNU dialect of ISO C17.
  • gnu23, gnu2x — GNU dialect of ISO C23. This is the default for C code. The name gnu2x is deprecated.
  • gnu2y — The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The name gnu2x is deprecated. (yes bestie, the manpage really does say gnu2x is deprecated here — we do not ask questions, we copy faithfully 💀)
  • 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, bestie. Heads up tho: the name c++1y is deprecated. 🫠
  • gnu++14, gnu++1y — the GNU dialect of -std=c++14. And yeah, gnu++1y is deprecated too, so maybe stop typing it, iykyk. 💅
  • c++17, c++1z — the 2017 ISO C++ standard plus amendments. c++1z is deprecated.
  • gnu++17, gnu++1z — GNU dialect of -std=c++17. gnu++1z is deprecated.
  • c++20, c++2a — the 2020 ISO C++ standard plus amendments. Okay so C++20 modules support is still experimental, and you have to turn it on with the -fmodules option. The name c++2a is deprecated.
  • gnu++20, gnu++2a — GNU dialect of -std=c++20. ✨ This is the default for C++ code, so if you're just vibing with C++ and not saying anything, this is what you get. C++20 modules support is still experimental and needs -fmodules to be enabled. And the name gnu++2a is deprecated.
  • c++23, c++2b — the 2023 ISO C++ standard plus amendments (published in 2024, fashionably late 💅). Support is experimental, and it could change in ways that are incompatible in future releases. The name c++2b is deprecated.
  • gnu++23, gnu++2b — GNU dialect of -std=c++23. Same warning bestie: support is experimental and could change incompatibly in future releases. The name gnu++2b is deprecated.
  • c++26, c++2c — the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and it will almost certainly change in incompatible ways in future releases. Do not build your whole personality on it. 😭
  • gnu++26, gnu++2c — GNU dialect of -std=c++26. Also highly experimental, also almost certainly going to change incompatibly in future releases.
  • c++29, c++2d — the upcoming revision of the ISO C++ standard, planned for 2029. Support is highly experimental and will almost certainly change in incompatible ways in future releases.
  • gnu++29, gnu++2d — GNU dialect of -std=c++29. Highly experimental squad, and yeah, it will almost certainly break in incompatible ways in future releases. 🫶
  • -aux-info filename — okay so this one writes, into the filename you give it, prototyped declarations for all the functions declared and/or defined in a translation unit — and yes, that includes the ones hiding in header files. 🐛 This option gets silently ignored in any language that isn't C.

Besides the declarations themselves, the file also tells you, in comments, where each declaration came from (source file and line), plus whether the declaration was implicit, prototyped or unprototyped — that's I, N for new, or O for old, respectively, sitting in the first character after the line number and the colon. Then the next character says whether it came from a declaration or a definition — that's C or F, respectively. And when it's a function definition, you also get a K&R-style list of arguments followed by their declarations, tucked inside comments, after the declaration. It's giving archaeology dig 🔍.

  • -fno-asm — do not recognize asm, inline or typeof as keywords, so that your code is free to use those words as identifiers instead. You can use the keywords __asm__, __inline__ and __typeof__ in their place. In C, -ansi implies -fno-asm.

In C++ tho, inline is a standard keyword and this switch does not touch it. You probably want -fno-gnu-keywords 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, because 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. The plot twist nobody asked for 😭.

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

GCC normally generates special code to handle certain built-in functions way more efficiently; for instance, calls to alloca may become single instructions that 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 can't set a breakpoint on those calls, and you can't change the behavior of the functions by linking with a different library either. Also, when a function is recognized as a built-in, GCC may use info about that function to warn you about problems with calls to it, 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 is disabled. And function must not begin with __builtin_. If you name a function that isn't built-in in this version of GCC, this option is just ignored. There is no corresponding -fbuiltin-function option; if you want to enable built-in functions selectively when you're 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++.
  • -fdeps-file=file , -fdeps-format=format , -fdeps-target=file — the -fdeps-* options are all about extracting structured dependency information for a source file. This involves figuring out what resources provided by other source files are required to compile the source, and what resources are provided by the source. You can use that info to add required dependencies between compilation rules of dependent sources based on their contents, instead of needing that info to be reflected within the build tools as well. Basically it's the build-system glow-up 🦋.

-fdeps-file=file specifies where to write the structured dependency information.

-fdeps-format=format specifies the format to use for structured dependency information.

p1689r5 ("https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1689r5.html") 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 that most tools don't understand.

-fdeps-target=file is analogous to -MT, but for structured dependency information. This indicates the target of the build rule for the source file that is currently being processed for dynamic dependencies, and it's used to associate the dependency information with that target in the output. Normally this is the name of the .o file generated during compilation.

  • -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one where the standard library may not exist at all, 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 you specify -fgnu-tm, 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. And please note that not all architectures are supported for this feature.

For more information on GCC's support for transactional memory, (bestie the sentence literally just stops there in the original, I'm not editing the manpage 😭)

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

  • -fgnu89-inline — this option 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 actually in effect for inline functions. Vibe check your inline semantics, lowkey important 💜.

  • -fhosted — assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one where the entire standard library is available, and where 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, do not. Periodt.
  • -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 you're in a standards-compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. And the default when you're in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3. That's the whole vibe bestie ✨

  • -fplan9-extensions — okay so this one lets you accept some non-standard constructs that show up in Plan 9 code 💅 Here's what it does, all three things: it enables -fms-extensions, it permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and it permits referring to anonymous fields declared using a typedef. Heads up though — this is only supported for C, not C++. Sorry C++ girlies 😭
  • -fsigned-bitfields, -funsigned-bitfields, -fno-signed-bitfields, -fno-unsigned-bitfields — these four control whether a bit-field is signed or unsigned when the declaration doesn't use either signed or unsigned. And by default, such a bit-field is signed, because that's the consistent choice: the basic integer types like int are signed types. periodt 👑
  • -fsigned-char, -funsigned-char — -fsigned-char lets the type char be signed, like signed char, while -funsigned-char lets the type char be unsigned, like unsigned char.

Not me needing to spell out the equivalences, but here we go: -fsigned-char is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. And likewise, -fno-signed-char is equivalent to -funsigned-char. ikykyk 🔥

Every target GCC supports has a default for what char should be, and it's typically specified in the processor-specific ABI document for that target. So it's either like unsigned char by default or like signed char by default — depends on the target, bestie.

Ideally, a portable program should always use signed char or unsigned char whenever it depends on the signedness of an object. But like… so many programs out there were written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for 🙈 These options let you make such a program work with the opposite default. slay.

And one more thing that's lowkey important: 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. 💀

  • -fsso-struct=endianness — sets the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian and native for the native endianness of the target (which is the default). This option is not supported for C++ 🚫
⚠️ Warning bestie, for real: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it, if the specified endianness is not the native endianness of the target. That's a whole mess you don't want.
  • -fstrict-flex-arrays (C and C++ only), -fstrict-flex-arrays=level (C and C++ only) — these 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 one — a trailing array is treated as a flexible array member only when it's declared as a flexible array member per the 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. no thoughts, head empty, everything is flexible 🦋

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

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

Also, the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. (Yes, one has an underscore in the middle and one has dashes — that's just how it is, don't @ me 😭)

✨ Options Controlling C++ Dialect ✨

This section describes the command-line options that are only meaningful for C++ programs. But bestie, you can also use most of the GNU compiler options regardless of what language your program is in — they don't gatekeep 🌈

For example, you might compile a file firstClass.C like this:

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

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

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

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

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

Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed. (she's evolving, she's growing 💅)

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

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

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

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

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

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

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

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

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

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

Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. (not her finest moment 😭)

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

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

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

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

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

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

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

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

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

See also -Wabi. 👀

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

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

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

  • -fno-access-control — turns off all access checking 😳 This switch is mainly useful for working around bugs in the access control code. Like, it's a workaround, not a lifestyle, okay?
  • -faligned-new, -faligned-new=alignment — enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument such as -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of alignof(std::max_align_t). 🫶

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

  • -fno-assume-sane-operators-new — okay so the C++ standard is like, "bestie, you can replace the global new, new[], delete and delete[] operators," but honestly? A ton of C++ programs just don't, and they vibe with the implementation-provided version instead. On top of that, the standard also lets those calls get omitted entirely when they come from new or delete expressions (and yeah, the same assumption applies if __builtin_operator_new or __builtin_operator_delete functions are being used). This option is your little control panel over the optimizations hovering around those operator calls.

With -fassume-sane-operators-new-delete, GCC is allowed to assume that calls to the replaceable global operators — from new or delete expressions, or from __builtin_operator_new / __builtin_operator_delete calls — do not read or modify any global variables (or variables whose address could escape to the operators). We're talking global state here, except for errno in the new and new[] operator cases. That unlocks most optimizations across those calls, and it's genuinely something the implementation-provided operators satisfy — unless malloc implementation details are observable in your code, or unless malloc hooks are being used. But it might not be satisfied if a program replaces those operators. This behavior is on by default ✨.

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

  • -fchar8_t, -fno-char8_t — turns on support for char8_t as adopted for C++20 💅. This is a whole glow-up: a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, the associated standard library updates, and brand-new __cpp_char8_t and __cpp_lib_char8_t feature test macros.

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

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

and it introduces new signatures for user-defined literals:

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

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

const char *cp = u8"xx";// error: invalid conversion from
                        //        `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx");     // error: invalid conversion from
                        //        `const char8_t*' to `const char*'
std::string s{u8"xx"};  // error: no matching function for call to
                        //        `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s;            // error: conversion from
                        //        `basic_string<char8_t>' to non-scalar
                        //        type `basic_string<char>' requested
  • -fcheck-new — double-checks that the pointer coming back from operator new is non-null before anyone tries to modify the storage that got allocated 👀. Normally this check is totally unnecessary, because the C++ standard says operator new only returns 0 if it's declared throw(), and in that case the compiler always checks the return value anyway — no option required. In literally every other case, when operator new has a non-empty exception specification, running out of memory gets signalled by throwing std::bad_alloc. See also new (nothrow). 💀
  • -fconcepts — enables support for the C++ Concepts feature for constraining template arguments 🎀. With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on.

Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn't make it into the standard, could additionally be enabled by -fconcepts-ts. But heads up: the option -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15; users are expected to convert their code to C++20 concepts. No cap, it's gone 😭.

  • -fconcepts-diagnostics-depth=n — sets the maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is 1.
  • -fconstexpr-depth=n — sets the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed so we can actually detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.
  • -fconstexpr-cache-depth=n — sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. It's a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows super huge from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might wanna experiment to find which value works best for you 🧚.
  • -fconstexpr-fp-except — Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically ignored this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined.
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except
  • -fconstexpr-loop-limit=n — sets the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).
  • -fconstexpr-ops-limit=n — sets the maximum number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the above limit, things can still spiral: if there are several nested loops and each of them has many iterations (but still smaller than the above limit), or if in the body of some loop — or even outside of 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 support for the C++ Contracts feature, as specified in the C++26 working draft (N5003) 📜.

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

Users can replace the default violation handler by defining

void
handle_contract_violation (const std::contracts::contract_violation&);
  • -fcontract-evaluation-semantic=semantic — sets the semantic with which contracts will be evaluated to semantic.

Available values for the evaluation mode are:

  • "ignored" — the contract checks will be elided, with no checking added at either compile or runtime. 🚫
  • "observed" — the contract checks are performed (at runtime) and, if one fails, a handler is called that allows actions such as logging or emitting run-time warnings. When the handler returns, execution of the code will continue. At compile-time the checks are performed for constexpr evaluations — in this case a diagnostic is emitted if the check fails. 👀
  • "enforce" — the contract checks are performed and the handler is called if one fails. When the handler returns, execution of the code is terminated, preventing it from continuing past the failed case. At compile-time the checks are applied to constexpr and, if one fails, the program is ill-formed; an error will be emitted. 💥
  • "quick_enforce" — the contract checks are performed (at runtime) and, if one fails, execution is terminated immediately (without calling any handler). At compile-time there is no difference in behaviour between this option and the "enforce" case, since no handler is invoked at compile time. 🔥
  • -fcontracts-conservative-ipa — this prevents inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. This allows for the case that contract evaluation conditions are permitted to differ between TUs, which means that such actions would be potentially incorrect. 🛑
  • -fcontract-checks-outlined — by default, contract checks for pre and post condition checks are expanded in-line into function bodies. This option causes a small function to be created instead (containing the checks) that is called in the relevant position. This permits different criteria to be applied to the compilation of the checking and the remainder of the function body. ✨
  • -fcontract-disable-optimized-checks — when pre and post condition checks are outlined (using -fcontract-checks-outlined), this option disables the optimisation steps for those functions, which can avoid unwanted elision of checking steps. 🙈
  • -fcontracts-client-check=mode — this option enables insertion of checks at call sites (caller-side checking).

The value of mode determines which contract checks are made:

  • "none" — no caller/client side checking (default)
  • "pre" — preconditions are checked at the caller/client side.
  • "all" — both pre and post-conditions are checked at the caller/client side.
  • -fcontracts-definition-check=mode — this option introduces the ability to disable callee/definition-side checks, which are otherwise enabled by default when the contracts feature is active.

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

  • -fcoroutines — turns on support for the C++ coroutines extension ✨. with -std=c++20 and up, coroutines are literally part of the language standard, so -fcoroutines defaults to on. slay 💅
  • -fdiagnostics-all-candidates — lets the C++ front end spill all the candidates during an overload resolution failure, even when a deleted function got picked. the way it just tells you everything 😭
  • -fdump-lang-, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — control the dumping of C++-specific info 🎀. the options and filename parts behave exactly as described in the -fdump-tree option. accepted switch values, bestie:
    • all — enable all of the below. icon behavior 👑
    • class — dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified.
    • module — dump module information. Options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional information.
    • raw — dump the raw internal tree data.
    • tinst — dump the sequence of template instantiations, indented to show the depth of recursion. The lineno option adds the source location where the instantiation was triggered, and the details option also dumps pre-instantiation substitutions such as those performed during template argument deduction.

Lines in the .tinst dump start with I for an instantiation, S for another substitution, and R[IS] for the reopened context of a deferred instantiation. not me memorizing the whole legend 🫶

  • -fno-elide-constructors — the C++ standard allows an implementation to omit creating a temporary that's only used to initialize another object of the same type 🦋. specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline.

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

  • -fno-enforce-eh-specs — don't generate code to check for violation of exception specifications at run time 🙈. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. bestie that's a real warning 💀
  • -fextern-tls-init, -fno-extern-tls-init — the C++11 and OpenMP standards allow thread_local and threadprivate variables to have dynamic (runtime) initialization ✨. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the -fno-extern-tls-init option.

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

  • -ffold-simple-inlines, -fno-fold-simple-inlines — permit the C++ frontend to fold calls to std::move, std::forward, std::addressof, std::to_underlying and std::as_const 🌸. In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active.
  • -fno-gnu-keywords — do not recognize typeof as a keyword, so that code can use this word as an identifier 👑. You can use the keyword __typeof__ instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
  • -fno-immediate-escalation — do not enable immediate function escalation whereby certain functions can be promoted to consteval, as specified in P2564R3. For example:
consteval int id(int i) { return i; }

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

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

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

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

  • -fimplicit-constexpr — make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function ✨. This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations. lowkey free real estate 💅
  • -fno-implicit-templates — never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations 🎀. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. no cap, this one bites.
  • -fno-implicit-inline-templates — don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.
  • -fno-implement-inlines — to save space, do not emit out-of-line copies of inline functions controlled by #pragma implementation. This causes linker errors if these functions are not inlined everywhere they are called 💀
  • -fmodules, -fno-modules — enable support for C++20 modules 🌈. The -fno-modules is usually not needed, as that is the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. rude but okay.
  • -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file to create an importable header unit 🦋.
  • -fmodule-implicit-inline — member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere 🙈 (Such definitions outside of a named module remain implicitly inline, regardless.)
  • -fno-module-lazy — disable lazy module importing and module mapper creation.
  • -fmodule-mapper=[hostname]:port[?ident], -fmodule-mapper=|program[?ident] args..., -fmodule-mapper==socket[?ident], -fmodule-mapper=<>[inout][?ident], -fmodule-mapper=<in>out[?ident], -fmodule-mapper=file[?ident] — an oracle to query for module name to filename mappings ✨. If unspecified the CXX_MODULE_MAPPER environment variable is used, and if that is unset, an in-process default is provided.
  • -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file.
  • -fms-extensions — disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax 👑
  • -fnew-inheriting-ctors — enable the P0136 adjustment to the semantics of C++11 constructor inheritance 🌸. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified.
  • -fnew-ttp-matching — enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters 💜. This flag is enabled by default for -std=c++17.
  • -fno-nonansi-builtins — disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions.
  • -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification to reduce or eliminate the text size overhead relative to a function with no exception specification 🔥. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected. periodt.
  • -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. the way the alphabet soup just goes away 😭
  • -fno-optional-diags — disable diagnostics that the standard says a compiler does not need to issue 💅. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.
  • -fno-pretty-templates — okay so when an error message is dragging a specialization of a function template, the compiler is normally extra and prints the whole template signature, then the template arguments, then any typedefs or typenames hanging out in that signature (like void f(T) [with T = int] instead of just void f(int)) so you actually know which template is being messy. And when the error is about a specialization of a class template, it skips any template arguments that match that template's default template arguments. But bestie, if either of these behaviors is making the error message harder to read instead of easier? -fno-pretty-templates turns both of them off. 💅
  • -frange-for-ext-temps — turns on lifetime extension for C++ range based for temporaries. With -std=c++23 and above this is literally part of the language standard, so the temporaries' lifetime already gets extended until the end of the loop by default. This option lets you have that same behavior in earlier versions of the standard too. ✨
  • -freflection — enables experimental C++26 Reflection. 🪞✨ (it's giving experimental, so you know the vibe)
  • -fno-rtti — stops the generation of info about every class with virtual functions that the C++ run-time type identification features need — that's dynamic_cast and typeid. If you're not using those parts of the language, you can save some space with this flag. Heads up tho: exception handling uses the same information, but G++ generates it as needed. And the dynamic_cast operator can still be used for casts that don't require run-time type information, i.e. casts to void * or to unambiguous base classes. 🫶

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

  • -fsized-deallocation — enables the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

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

  • -fstrict-enums — lets the compiler optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that 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. Also this option has no effect for an enumeration type with a fixed underlying type.
  • -fstrong-eval-order
  • -fstrong-eval-order=kind — evaluates member access, array subscripting, and shift expressions in left-to-right order, and evaluates assignment in right-to-left order, as adopted for C++17. -fstrong-eval-order is equivalent to -fstrong-eval-order=all, and it's enabled by default with -std=c++17 or later. 👑

-fstrong-eval-order=some enables just the ordering of member access and shift expressions, and it's the default for C++ dialects before C++17.

-fstrong-eval-order=none is equivalent to -fno-strong-eval-order.

  • -ftemplate-backtrace-limit=n — sets the max number of template instantiation notes for a single warning or error to n. The default value is 10. 🎀
  • -ftemplate-depth=n — sets the maximum instantiation depth for template classes to n. You genuinely need a limit on template instantiation depth to detect endless recursions while instantiating template classes. 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, because the compiler can run out of stack space before even hitting 1024 in some situations. 😭
  • -fno-threadsafe-statics — don't emit the extra code that uses the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe. ✨
  • -fuse-cxa-atexit — registers destructors for objects with static storage duration with the __cxa_atexit function instead of the atexit function. This option is required for fully standards-compliant handling of static destructors, but it only works if your C library supports __cxa_atexit. 🫶
  • -fno-use-cxa-get-exception-ptr — don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect 💀, but it's necessary if the runtime routine isn't available.
  • -fvisibility-inlines-hidden — this switch declares that the user isn't attempting to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they don't show up in the export table of a DSO and don't 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 because it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥

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

You can 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, since their linkage might otherwise cross a shared library boundary.

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

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

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

In new code it's better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it's possible for code to rely, maybe accidentally, on the Visual Studio behavior.

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

  • -fno-weak — don't use weak symbol support, even if the linker provides it. By default, G++ uses weak symbols if they're available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. It may get removed in a future release of G++. 🙈
  • -fext-numeric-literals (C++ and Objective-C++ only) — accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off, those suffixes are treated as C++11 user-defined literal numeric suffixes. It's on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. It's off by default for ISO C++11 onwards (-std=c++11, ...). 🌈
  • -nostdinc++ — don't search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) 🧚
  • -flang-info-include-translate
  • -flang-info-include-translate-not
  • -flang-info-include-translate=header — inform of include translation events. The first one notes accepted include translations, the second one notes declined include translations. The header form informs of include translations relating to that specific header. If header is of the form "user" or <system>, it gets resolved to a specific user or system header using the include path. ✨
  • -flang-info-module-cmi
  • -flang-info-module-cmi=module — inform of Compiled Module Interface pathnames. The first notes all read CMI pathnames. The module form notes reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in <> or ""). 💜
  • -stdlib=libstdc++,libc++ — when G++ is configured to support this option, it lets you specify alternate C++ runtime libraries. Two options are available: libstdc++ (the default, native C++ runtime for G++) and libc++, which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit -lstdc++ or -lc++ respectively, when a C++ runtime is required for linking. 🌸

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

  • -Wabi-tag (C++ and Objective-C++ only) — warns when a type with an ABI tag is used in a context that doesn't have that ABI tag. See C++ Attributes for more info about ABI tags. 🎀
  • -Wno-abbreviated-auto-in-template-arg — disables the error for an auto placeholder type used within a template argument list to declare a C++20 abbreviated function template, e.g.
void f(S<auto>);

Oops, that one was so close to making it into C++20 — this feature was proposed in the Concepts TS, but the standard said nah, not adopted. In the real standard, a placeholder in a parameter declaration has to show up as a decl-specifier. And hey, if you're getting this as an error and you're not about it, you can downgrade it to a warning with -fpermissive or -Wno-error=abbreviated-auto-in-template-arg. Your call, bestie. ✨

  • -Wcomma-subscript (C++ and Objective-C++ only) — warns you when you use a comma expression inside a subscripting expression. That usage got deprecated in C++20 and is on the chopping block for removal in C++23. BUT — and this is the plot twist — a comma expression wrapped in "( )" is not deprecated. Look:
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 totally valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and it supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that isn't valid in C++23 but used to be valid-but-deprecated in C++20, with a pedantic warning that you can turn off with -Wno-comma-subscript.

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

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

  • -Wctad-maybe-unsupported (C++ and Objective-C++ only) — warns you when you do class template argument deduction (CTAD) on a type with no explicitly written deduction guides. Basically it's calling out the cases where CTAD only worked because the compiler synthesized the implicit deduction guides, which might not be what the programmer actually intended. Some style guides only allow CTAD on types that specifically "opt-in" — like, types that were designed to support CTAD. You can hush this warning with this exact 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 looks completely unusable because all the constructors or destructors in it are private, and it has neither friends nor public static member functions. It 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. The class is giving "sealed shut, no keys, no vibes" 💀
  • -Wdangling-reference (C++ and Objective-C++ only) — warns when a reference gets bound to a temporary whose lifetime has already ended. Example time:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is dangling

So in that example above, two temporaries get created — one for each argument — and a reference to one of the temporaries is returned. The problem? Both temporaries get 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 get checked too, but only their object argument:

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

Certain functions are actually safe in this regard — for example "std::use_facet": they take and return a reference, but they don't return one of their arguments, which is exactly the kind of thing that can fool the warning. You can exclude such functions 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 does not 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.
  • -Wdeprecated-copy-dtor (C++ and Objective-C++ only) — similar to -Wdeprecated-copy, but it also deprecates if the class has a user-provided destructor.
  • -Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only) — disables the warning about the case when the usual arithmetic conversions get 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 get applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;

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

  • -Wdeprecated-literal-operator (C++ and Objective-C++ only) — warns that declaring a user-defined literal operator with a space before the suffix is deprecated. This warning is enabled by default in C++23, or with explicit -Wdeprecated.
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred
  • -Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only) — warns that omitting a comma before the varargs "..." at the end of a function parameter list is deprecated. This warning is enabled by default in C++26, or with explicit -Wdeprecated.
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated
  • -Wno-elaborated-enum-base — for C++11 and above, warns if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with a given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with -pedantic-errors. ⭐
  • -Wno-init-list-lifetime (C++ and Objective-C++ only) — do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers. Because the underlying array for an "initializer_list" is handled like a normal C++ temporary object, it's so easy to accidentally keep a pointer to the array past the end of the array's lifetime. For example:
    • If a function returns a temporary "initializer_list", or a local "initializer_list" variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
    • If a new-expression creates an "initializer_list", the array only lives until the end of the enclosing full-expression, so the "initializer_list" in the heap has a dangling pointer.
    • When an "initializer_list" variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the "initializer_list" variable has a dangling pointer.
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
  • When a list constructor stores the "begin" pointer from the "initializer_list" argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary "initializer_list", the pointer is left dangling by the end of the variable declaration statement.
  • -Winvalid-constexpr — warns when a function never produces a constant expression. In C++20 and earlier, for every "constexpr" function and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's possible to have a function or a function template marked "constexpr" for which no invocation actually satisfies the requirements of a core constant expression. 🙈
If you want to turn this one off, -Wno-invalid-constexpr is a thing.

Okay bestie, quick recap so you're not lost: this warning is a pedantic warning by default in C++20 and earlier. In C++23 you can turn on -Winvalid-constexpr and then it becomes a regular, ordinary warning. Like, it graduates. 💅 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);
}

✨ moar warnings (we're literally not done) ✨

  • -Winvalid-imported-macros — bestie this one checks that every imported macro definition is still valid at the end of compilation. It's not on by default, because figuring that out needs extra processing (it's giving extra credit homework energy 😭). Where it slays is when you're preparing sets of header-units and you want to make sure your macros stay consistent. Consistent macros = no drama. ✨
  • -Wno-invalid-specialization (C++ and Objective-C++ only) — this one says: don't error on explicit specializations of templates that are marked with the "no_specializations" attribute. This diagnostic is enabled by default, and you can shut it up with either -Wno-invalid-specialization or #pragma GCC ignored "-Winvalid-specializations". Periodt. 💅
  • -Wno-literal-suffix (C++ and Objective-C++ only) — okay so normally GCC would warn you when a string or character literal is followed by a ud-suffix that doesn't begin with an underscore. This option turns that warning off. Why does GCC even tolerate it? As a conforming extension, GCC treats those suffixes as separate preprocessing tokens, all for backwards compatibility with code that uses formatting macros from <inttypes.h>. The vibe check:
#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. No cap. 🦋

This option also controls the warnings you get when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore — because literal suffix identifiers that don't start with an underscore are reserved for future standardization (so, off-limits, reserved for the future, not yours 🙈).

These warnings are enabled by default.

  • -Wno-narrowing (C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions get diagnosed by default, because the standard said so. 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. 🫶
Heads up, bestie: this does not change the meaning of well-formed code. Narrowing conversions are still considered ill-formed in SFINAE contexts.

With -Wnarrowing in C++98, you'll get a warning when a narrowing conversion that C++11 prohibits happens inside { }, like:

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

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

  • -Wnoexcept (C++ and Objective-C++ only) — warns you when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. throw() or "noexcept") — but that the compiler knows will never throw an exception. It's giving "you said you'd never do this to me but you didn't put it in writing". 😭
  • -Wnoexcept-type (C++ and Objective-C++ only) — warns if the C++17 feature that makes "noexcept" part of a function type changes the mangled name of a symbol compared to C++14. Enabled by -Wabi and -Wc++17-compat. Iconic example moment:
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>". Same girl, different name. 💀

  • -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function like "memset" or "memcpy" is an object of class type, and 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, and we do not do that. For example, the call to "memset" below is undefined because it modifies a non-trivial class object — so it gets diagnosed. The safe way to initialize or clear the storage of objects like this is to use the appropriate constructor or assignment operator, if one is available. 🧚

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

The -Wclass-memaccess option is enabled by -Wall. And if you explicitly cast the pointer to the class object to "void *" — or to a type that can be safely accessed by the raw memory function — the warning is suppressed. Slay.

  • -Wnon-virtual-dtor (C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor, either itself or in an accessible polymorphic base class. In that case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or the base class. This warning is automatically enabled if -Weffc++ is specified. But bestie — -Wdelete-non-virtual-dtor (enabled by -Wall) is the one you actually want, because it warns about the unsafe cases without false positives.
  • -Wregister (C++ and Objective-C++ only) — warns on uses of the "register" storage class specifier, except when it's part of the GNU Explicit Register Variables extension. FYI the "register" keyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with -std=c++17.
  • -Wreorder (C++ and Objective-C++ only) — warns when the order of member initializers in your code doesn't match the order they actually have to be executed in. Exhibit A:
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. The compiler said "I'll fix it but I'm telling everyone". 💅 This warning is enabled by -Wall.

  • -Wno-pessimizing-move (C++ and Objective-C++ only) — this one warns when a call to "std::move" prevents copy elision. A typical scenario where copy elision can happen is when you return inside a function with a class return type, the expression being returned is the name of a non-volatile automatic object, it's not a function parameter, and it has the same type as the function return type:
struct T {
...
};
T fn()
{
  T t;
  ...
  return std::move (t);
}

But in this example, that "std::move" call is blocking copy elision. Rude. 😤 This warning is enabled by -Wall.

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

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

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

In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ — yet G++ treats the return value as if it were designated by an rvalue. Wild. 🌈 This warning is enabled by -Wextra.

  • -Wrange-loop-construct (C++ and Objective-C++ only) — warns when a C++ range-based for-loop is creating an unnecessary copy. That can happen when the range declaration isn't a reference but probably should be. Like:
struct S { char arr[128]; };
void fn () {
  S arr[5];
  for (const auto x : arr) { ... }
}

It does not warn when the type being copied is a trivially copy constructible type whose size is less than 64 bytes. (Small enough, she's fine, let her live. 🍓)

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

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

In the example above, every single iteration of the loop creates and destroys a temporary value of type "double", and the reference const double & is bound to it. Every. Loop. 😭 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 the contexts where the key can be eliminated without causing ambiguity. Example:
struct foo;
struct foo *p;   // warn that keyword struct can be eliminated

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

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

See? Necessary. The keyword is the main character here. 👑

  • -Wno-subobject-linkage (C++ and Objective-C++ only) — do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. Here's the tea: if a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation, because the meaning of B is different in each translation unit. 🙈 If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
  • -Weffc++ (C++ and Objective-C++ only) — warns about violations of the following style guidelines from Scott Meyers' Effective C++ series of books (yes, the sacred texts 📚):
    • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors. Like, use the member initializer list, bestie, don't do the assign-in-the-body thing — it's giving extra work for zero reason. 💅
  • Have operator= return a reference to *this. Chainable assignments only, periodt.
  • Don't try to return a reference when you must return an object. If a fresh object has to be born, let it be born — returning a ref to a local is giving dangling pointer energy and we do NOT stan. 😭
  • Distinguish between prefix and postfix forms of increment and decrement operators. They are not the same girl and they should not have the same signature.
  • Never overload &&, ||, or ,. Fr fr, just don't. The short-circuit vibe is sacred. ✨

This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too. 👑

When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. Like, the STL is not going to pass this vibe check and that's not your fault, so pipe through grep -v and move on. 🫶

  • -Wno-exceptions (C++ and Objective-C++ only) — turns off the warning for when an exception handler gets shadowed by another handler, iykyk. That shadowing thing is usually pointing out that your exception handlers are in the wrong order, so honestly the warning is doing you a favor.
  • -Wsfinae-incomplete (C++ and Objective-C++ only) — warns about a class that's found to be incomplete, or a function with auto return type that hasn't had its type deduced yet, in a context where that causes substitution failure rather than an error, and then that class or function gets defined later in the translation unit. Why is this problematic, you ask? Because template instantiations or concept checks could have totally different results depending on whether they first happen before or after that definition. 💀 Inconsistent results = cursed.

This warning is enabled by default. And -Wsfinae-incomplete=2 adds a warning at the point of substitution failure, which makes it way easier to track down the problems the default mode flagged.

  • -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about using an uncasted NULL as a sentinel. When you're compiling only with GCC this is a valid sentinel, because NULL is defined to __null. Even though it's a null pointer constant and not a null pointer, it's guaranteed to be the same size as a pointer. But — and this is the tea — that use is not portable across different compilers. 🌸
  • -Wno-non-c-typedef-for-linkage (C++ and Objective-C++ only) — disables the pedwarn for unnamed classes that have a typedef name for linkage purposes and contain C++-specific members, base classes, default member initializers or lambda expressions, including those on nested member classes. Like this:
typedef struct {
  int a; // non-static data members are ok
  struct T { int b; }; // member classes too
  enum E { E1, E2, E3 }; // member enumerations as well
  int c = 42; // default member initializers are not ok
  struct U : A { int c; }; // classes with base classes are not ok
  typedef int V; // typedef is not ok
  using W = int; // using declaration is not ok
  decltype([](){}) x; // lambda expressions not ok
} S;

In all these cases, the tag name S should be added after the struct keyword. Noted. 📝

  • -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.
  • -Wold-style-cast (C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (dynamic_cast, static_cast, reinterpret_cast, and const_cast) are less vulnerable to unintended effects and much easier to search for. Glow-up your casts, bestie. ✨
  • -Woverloaded-virtual (C++ and Objective-C++ only) / -Woverloaded-virtual=n — warns when a function declaration hides virtual functions from a base class. For example, in:
struct A {
  virtual void f();
};

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

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

B* b;
b->f();

fails to compile. 😭

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

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

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

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

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

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

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

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

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

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

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) — warns when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance.
  • -Wvirtual-inheritance — warns when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance.
  • -Wno-virtual-move-assign — suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous 🫣 because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.
  • -Wnamespaces — warns when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names.
  • -Wno-template-body (C++ and Objective-C++ only) — disables diagnosing errors when parsing a template, and instead issues an error only upon instantiation of the template. This flag can also be used to downgrade such errors into warnings with Wno-error= or -fpermissive.
  • -Wno-template-id-cdtor (C++ and Objective-C++ only) — disables the warning about the use of simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. For example:
template<typename T> struct S {
  S<T>(); // should be S();
  ~S<T>();  // should be ~S();
};
  • -Wtemplate-id-cdtor — heads up bestie, this one's on by default once you're on -std=c++20, and it also rides along when you turn on -Wc++20-compat.
  • -Wtemplate-names-tu-local — this one yells at you when a template body hides an exposure of a translation-unit-local entity. Okay so context: usually, if you refer to a TU-local entity (think: something with internal linkage) inside a thing that gets emitted into a module's CMI, that's an error. BUT — and this is the little loophole — inside a variable's initializer, or inside the body of a non-inline function, that's not an exposure, so no error, no problem, we move on. 💅

The catch (there's always a catch): this can make your variable or function templates accidentally become unusable, because other TUs that import the template will never be able to instantiate it. This warning exists to sniff out those cases before they ruin your whole day. Also, if there's an explicit instantiation hanging around, that silences the warning. 🫶

Enabled by -Wextra.

  • -Wno-expose-global-module-tu-local — exposing a translation-unit-local entity from a module interface is invalid, and like, for good reason: it can cause ODR violations and then manifest as link errors or other chaotic unexpected behaviour. 😭 That said, a lot of existing libraries declare TU-local entities in their interface, and dodging those exposures is genuinely hard sometimes.

So as an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment. This warning tells you when one of those invalid exposures happened, and you can shush it with diagnostic pragmas — either at the site of the exposure, or at the point of declaration of the internal declaration. This also applies to export using declarations naming such entities.

When you combine it with -Wtemplate-names-tu-local, GCC will also warn about non-exposure references to TU-local entities in template bodies. Those templates can still be instantiated in other TUs, but all the risky risks above re: exposures of translation-unit-local entities still apply, so, y'know. Be careful.

This warning is enabled by default, and -pedantic-errors upgrades it to an error. 💀

  • -Wno-external-tu-local — warns when you name a TU-local entity outside of the translation unit it was declared in. Those declarations get straight-up ignored during name lookup, which, rude. This can happen when you do ADL from a template declared in the same TU as the internal function:
export module M;
template <typename T> void foo(T t) {
  bar(t);
}
struct S {} s;
static void bar(S) {}  // internal linkage

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

Workaround? Get bar attached to the global module using extern "C++". ✨

Enabled by default, and -pedantic-errors bumps it up to an error.

  • -Wno-terminate (C++ and Objective-C++ only) — turns off the warning about a throw-expression that's about to immediately result in a call to terminate. Bye warning, hello chaos. 🙈
  • -Wno-vexing-parse (C++ and Objective-C++ only) — squashes the warning about the most vexing parse syntactic ambiguity. This is when something looks like a variable definition but the C++ language requires it to be read as a function declaration instead. Classic. For instance:
void f(double a) {
  int i();        // extern int i (void);
  int n(int(a));  // extern int n (int);
}

Another one:

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 how to deal with the ambiguity — like, it might tell you to drop the parentheses or switch to braces. Iconic of it honestly.

Enabled by default. 💖

  • -Wno-class-conversion (C++ and Objective-C++ only) — don't warn when a conversion function converts an object to the same type, to a base class of that type, or to void. Because ngl, such a conversion function will never ever be called. Never. 😴
  • -Wvolatile (C++ and Objective-C++ only) — warns about deprecated uses of the volatile qualifier. That includes: postfix and prefix ++ and -- expressions on volatile-qualified types, simple assignments where the left operand is a volatile-qualified non-class type (used 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. All of this was deprecated back in C++20. 🫠

Enabled by default with -std=c++20. Before -std=c++20? You need explicit -Wdeprecated to get it.

  • -Waligned-new
  • -Waligned-new=[none|global|all] — warns about a new-expression of a type that needs greater alignment than alignof(std::max_align_t) but uses an allocation function with no explicit alignment parameter. Enabled by -Wall.

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

  • -Wno-placement-new
  • -Wplacement-new=n — warns about placement new expressions with undefined behavior, like constructing an object in a buffer that's smaller than the object's type. For example, this placement new expression below gets diagnosed because it's trying to build an array of 64 integers inside a buffer that's 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 does not fire for some strictly undefined constructs that GCC allows as extensions, for legacy-code-compatibility reasons. Like, the following new expression is not diagnosed at this level even though it has undefined behavior per 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, on top of diagnosing everything level 1 does, a diagnostic also gets 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 be diagnosed, but the following construct uses the flexible member array extension to dodge 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 just -Wcatch-value for short) it warns about polymorphic class types caught by value. With -Wcatch-value=2 it warns about all class types caught by value. With -Wcatch-value=3 it warns about all types that are not caught by reference. -Wcatch-value is enabled by -Wall. 🎀
  • -Wconditionally-supported (C++ and Objective-C++ only) — warns for conditionally-supported constructs (that's C++11 [intro.defs], iykyk). 👑
  • -Wno-defaulted-function-deleted (C++ and Objective-C++ only) — warns when an explicitly defaulted function gets deleted by the compiler. That can happen when the function's declared type doesn't match the type of the function that would've been implicitly declared. This warning is enabled by default. 🔥
  • -Wno-delete-incomplete (C++ and Objective-C++ only) — don't warn when you delete a pointer to an incomplete type, which may cause undefined behavior at runtime. Enabled by default. (The warning is, I mean. Not the UB. The UB is on you bestie. 💀)
  • -Wextra-semi (C++, Objective-C++ only) — warns about redundant semicolons. There are several contexts where an extra semicolon can 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's a pedwarn in C++98):

struct S {
};
;

And yet another is an extra semicolon in class definitions, also allowed since C++11 (so, yep, pedwarn in C++98):

struct S {
  int a;
  ;
};
  • -Wno-global-module (C++ and Objective-C++ only) — turns off 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 instead.
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. This warning is on by default because the ellipsis is not inherited. Periodt.
  • -Wno-invalid-offsetof (C++ and Objective-C++ only) — suppresses warnings from applying the offsetof macro to a non-POD type. Per the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations though, offsetof typically gives meaningful results. So this flag is for the girlies who know they're writing nonportable code and have deliberately chosen to ignore the warning about it. Respect honestly. 🫡

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 SO specific and I love that for it. It yells at you when you define an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

but you don't define the matching sized deallocation function

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

or the other way around, sized-but-not-unsized. Either direction, it's giving "hello?? where's your other half??" 💀 Enabled by -Wextra alongside -fsized-deallocation.

  • -Wsuggest-final-types — warns you about types that have virtual methods, where your code quality would be chef's kiss improved if you declared the type with the C++11 final specifier, or — if you can swing it — declared it in an anonymous namespace. Why do we care?? Because it lets GCC devirtualize those polymorphic calls way more aggressively. ✨ Pro tip from the page itself: this warning hits harder with link-time optimization, where the info about the class hierarchy graph is more complete. (i.e. it actually knows what it's talking about)
  • -Wsuggest-final-methods — same energy but for virtual methods instead of types. Warns about methods where life would be better if the method were declared with the C++11 final specifier, or if its type were declared in an anonymous namespace or with the final specifier. Also more effective with link-time optimization, for the same "the class hierarchy graph info is more complete" reason. 💅 The page literally recommends: first go look at what -Wsuggest-final-types is telling you, and then rebuild with your new annotations. One at a time, queen.
  • -Wsuggest-override — warns about overriding virtual functions that aren't marked with the override keyword. That's it. That's the whole vibe. slay with your annotations 🌸
  • -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. Just so you know: -Wconversion-null is enabled by default, so this is you turning that off.

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

NOTE: this manual does not describe the Objective-C and Objective-C++ languages themselves. We're just doing flags here, bestie.

okay so 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 with them too!! For example, you might compile a file some_class.m like this:

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

In that example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; all the other options there you can use with any language GCC supports. 💖

Also important: since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (like -Wtraditional). Same deal on the flip side — Objective-C++ compilations may use C++-specific options (like -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 specify with the @"..." syntax. Default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (more on that below). Heads up: on Darwin / macOS platforms, the -fconstant-cfstrings option, **if also present, overrides the -fconstant-string-class setting and causes @"..." literals to be laid out as constant CoreFoundation strings. Also also — -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 all Objective-C message dispatches ([receiver message:arg]) in this translation unit make suuure that the receiver is not nil. That lets the runtime use more efficient entry points. ⚠️ This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
  • *-fobjc-abi-version=n* — use version n of the Objective-C ABI for the selected runtime. Currently this option is only supported for the NeXT runtime. In that case:
    • Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions.
    • Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions.
    • Version 2 is the modern (64-bit) ABI.

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

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

The - (id) .cxx_construct and - (void) .cxx_destruct methods generated this way only operate on instance variables declared in the current Objective-C class — not 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 get invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods get invoked immediately before the runtime deallocates an object instance.

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

  • -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. iconic ✨
  • -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 available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier). 🙈
  • -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. ⚠️ Only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that doesn't require special compiler flags.
  • -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default, and you can turn it off with -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way, no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. 🫶
  • -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 gets rejected. This is useful if you need to make sure your Objective-C code can be compiled 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. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. 👑
  • -fno-local-ivars — by default, instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. 😭 Specifying the -fno-local-ivars flag disables this behavior, thus avoiding variable shadowing issues.
  • -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option, so that instance variables declared outside the scope of any access modifier directives default to that 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. lowkey polite of it fr fr
  • -Wno-protocol — Objective-C and Objective-C++ only, bestie 💅</p> So if a class is declared to implement a protocol, GCC will warn you about every method in that protocol the class doesn't implement. The default behavior (fr fr) is to warn for every method that isn't explicitly implemented in the class, even if a method implementation is straight up inherited from the superclass. But if you pass -Wno-protocol, then methods inherited from the superclass count as implemented, and no warning gets issued for them. Slay, inheritance is real. 🫶
  • -Wobjc-root-class — Objective-C and Objective-C++ only</p> Warns you if a class interface is missing a superclass (it's giving orphan energy 😭). Most classes will inherit from NSObject or Object, for example. When you're declaring classes that are meant to be root classes, you can shut the warning up by marking their interfaces with __attribute__((objc_root_class)). Iconic workaround behavior. ✨
  • -Wselector — Objective-C and Objective-C++ only</p> Warns if multiple methods of different types for the same selector show up during compilation. The check runs on the list of methods in the final stage of compilation. On top of that, a check runs for each selector appearing in a @selector(...) expression when a corresponding method for that selector has been found during compilation. And here's the tea: because these checks scan the method table only at the end of compilation, those warnings never appear if the final stage of compilation never gets reached — like if an error pops up during compilation, or if you're using -fsyntax-only. 💀
  • -Wstrict-selector-match — Objective-C and Objective-C++ only</p> Warns if multiple methods with differing argument and/or return types are found for a given selector when you're trying to send a message using that selector to a receiver of type id or Class. When this flag is off (which is literally the default behavior), the compiler just omits those warnings if any differences found are confined to types that share the same size and alignment. Lowkey respectful of your time. 🌸
  • -Wundeclared-selector — Objective-C and Objective-C++ only</p> Warns if a @selector(...) expression referring to an undeclared selector is found. A selector counts as undeclared if no method with that name has been declared before the @selector(...) expression — either explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always performs its checks the moment a @selector(...) expression is found, whereas -Wselector only performs its checks in the final stage of compilation. This also enforces the coding style convention that methods and selectors must be declared before being used. Declare before you use, bestie, it's the law. 👑
  • -print-objc-runtime-info — Generates a C header describing the largest structure that is passed by value, if any. That's it, that's the whole 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. Three languages, one compiler, infinite chaos. 🦋

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

Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES. Go look, it's right there. 🔍

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

Typical command lines are

-foffload-options='-fno-math-errno -ffinite-math-only' \
        -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906
  • -fopenacc — Enables handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran, plus !$acc, c$acc and *$acc in fixed-form Fortran. This option implies -pthread, and thus is only supported on targets that have support for -pthread. No pthreads, 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. Skipping a size = trusting the target. ✨
  • -fopenmp — Enables handling of OpenMP directives #pragma omp, [[omp::decl(...)]], [[omp::directive(...)]], and [[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and the conditional compilation sentinel !$. 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. -fopenmp implies -fopenmp-simd. Chain reactions, we love to see it. 🔥

  • -fopenmp-simd — Enables handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directive, and of combined or composite directives with simd as a constituent, with #pragma omp, [[omp::directive(...)]], [[omp::sequence(...)]] and [[omp::decl(...)]] in C/C++ and !$omp in Fortran. It additionally enables the conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the sentinels can also start with c or *.

Other OpenMP directives are ignored. Unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause. Noted, no cap. 📝

  • -fopenmp-ompt
  • -fopenmp-ompt=level — Controls how much detail GCC reports to an OMPT tool — that's a tool that registers callbacks with the OpenMP runtime to observe events such as when a parallel region or worksharing construct starts and ends. The level argument is one of minimal, basic, or extended. -fopenmp-ompt without an argument is equivalent to -fopenmp-ompt=basic. OMPT support itself is always present with -fopenmp; this option only changes how much is reported and at what runtime cost. Requires -fopenmp.
    • minimal — The level used when -fopenmp-ompt is not specified. Some OpenMP constructs are reported to an OMPT tool as a single combined "encountered" event, so a tool cannot tell how long the construct's region actually ran. No extra runtime calls are generated beyond what the construct's semantics already require. Minimal effort, minimal overhead, honestly valid. 🙈
    • basic — For the single, masked, and master constructs, and for distribute and worksharing-loop (for/do) constructs that use static scheduling, GCC instead reports a separate "begin" event when the region is entered and an "end" event when it's left. This lets an OMPT tool measure the time actually spent in those regions, at the cost of one extra runtime call per region. Pay one call, get real timings. 💖
    • extended — In addition to what basic reports, report a "dispatch" event each time a new chunk of loop iterations begins within a statically scheduled distribute or worksharing-loop construct (a chunk is the contiguous run of iterations handed to one thread or team at a time). This lets an OMPT tool see how work is actually split across threads, but it adds a runtime call for every chunk, which can be measurably slower for loops with many small chunks. ⚠️ Maximum tea, maximum cost.
There is no -fno-openmp-ompt: the lowest available level is -fopenmp-ompt=minimal, because OMPT support cannot be disabled outright once -fopenmp is in effect. Can't turn it off, bestie, only turn it down. 😭
  • -fopenmp-target-simd-clone
  • -fopenmp-target-simd-clone=device-type — In addition to 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's disabled by default. So it sorta just shows up when the optimization level is feeling generous. 🌟

🩷 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 — like how many characters per line, and how often source location information should be reported. Note that some language front ends may not honor these options. Rude of them, but okay. 🙃

  • -fmessage-length=n — Try to format error messages so that they fit on lines of about n characters. If n is zero, then no line-wrapping is done at all; each error message appears on a single line. This is the default for all front ends.
Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not however affect the pragma GCC warning and pragma GCC error pragmas. Special little exceptions, iykyk. ✨
  • -fdiagnostics-plain-output — This option requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time. -fdiagnostics-plain-output is currently equivalent to the following options: -fno-diagnostics-show-caret -fno-diagnostics-show-line-numbers -fdiagnostics-color=never -fdiagnostics-urls=never -fdiagnostics-path-format=separate-events -fdiagnostics-text-art-charset=none -fno-diagnostics-show-event-links -fno-diagnostics-show-nesting

In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list. Forward-thinking queen. 💅

  • -fdiagnostics-show-location=once — only means anything when you're in line-wrapping mode, bestie. It tells the diagnostic reporter to spill the source location info once — so if a message is too long to squeeze onto one physical line and has to get wrapped, the source location does not get re-emitted as a prefix over and over and over on every continuation line. Which is, like, the default behavior. Periodt. 💅
  • -fdiagnostics-show-location=every-line — again, only meaningful in line-wrapping mode. This one tells the reporter to emit that same source location info (as a prefix) on every physical line that comes out of breaking up a message that's too long to fit on a single line. So yeah, it's giving repetitive. 🌸
  • -fdiagnostics-color[=WHEN]
  • -fno-diagnostics-color — use color in diagnostics! 🎨✨ WHEN is never, always, or auto. The default depends on how the compiler was configured when it was built — 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 it's not executing inside an emacs shell. And btw, the forms -fdiagnostics-color and -fno-diagnostics-color are literally just aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. No cap. 🫶

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

The default GCC_COLORS is

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

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors entirely. The supported capabilities are as follows, bestie 👇

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

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

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

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

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

At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs — or at least had bugs in some versions that are still in use — where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with -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, that is). Specifying the -fno-diagnostics-show-option flag suppresses that behavior. Poof, gone. 🎀
  • -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, like 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, like the line connecting events 1 and 2 in:
            3 |   if (p)
              |       ^
              |       |
              |       (1) following `false' branch (when `p' is NULL)... ->-+
              |                                                             |
              |                                                             |
              |+------------------------------------------------------------+
            4 ||    return 0;
            5 ||  return *p;
              ||         ~
              ||         |
              |+-------->(2) ...to here
              |          (3) dereference of NULL `p'

This option suppresses the printing of such connector lines. ✂️

  • -fno-diagnostics-show-cwe — diagnostic messages can optionally have an associated CWE (https://cwe.mitre.org/index.html) identifier. GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata. 🕵️♀️
  • -fno-diagnostics-show-rules — diagnostic messages can optionally have rules associated with them, like 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

So the compiler draws that bad-format-string situation like this:

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

And inside that little masterpiece:

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

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

This additional colorization is on 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 — okay so normally, whenever the compiler prints source code (via -fdiagnostics-show-caret), it also prints a left margin showing line numbers. This option suppresses that left margin. bye margin 🫶
  • -fdiagnostics-minimum-margin-width=width — this one controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. 📏
  • -fdiagnostics-show-context[=depth], -fno-diagnostics-show-context — with this option, the compiler might print the interesting control flow chain that guards the basic block of the statement that has the warning. depth is the maximum depth of the control flow chain.

Currently, the list of the impacted warning options includes: -Warray-bounds, -Wstringop-overflow, -Wstringop-overread, -Wstringop-truncation, and -Wrestrict. More warning options might be added to this list in future releases. 👀

The forms -fdiagnostics-show-context and -fno-diagnostics-show-context are aliases for -fdiagnostics-show-context=1 and -fdiagnostics-show-context=0, respectively.

  • -fdiagnostics-parseable-fixits — emits fix-it hints in a machine-parseable format, perfect for IDEs to gobble up. For each fix-it, a line gets printed after the relevant diagnostic, starting with the string "fix-it:". Like, for example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

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

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

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

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

  • -fdiagnostics-generate-patch — prints fix-it hints to stderr in unified diff format, after any diagnostics are printed. Example time:
--- test.c
+++ test.c
@ -42,5 +42,5 @

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

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

  • -fdiagnostics-show-template-tree — in the C++ frontend, when printing diagnostics that show 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 enables printing a tree-like structure showing the common and differing parts of the types, like this:

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

The parts that differ are highlighted with color ("double" and "float" in this case). 🌈

  • -fno-elide-type — by default, when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as "[...]" to simplify the error message. For example:
  could not convert 'std::map<int, std::vector<double> >()'
    from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. 🚫✨

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

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

none means: do not print diagnostic paths. nope, nothing, we're done here 🙈

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 attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.

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

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

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

For example:

  'test': events 1-2
    |
    |  133 | {
    |      | ^
    |      | |
    |      | (1) entering 'test'
    |  134 |   boxed_int *obj = make_boxed_int (i);
    |      |                    ~~~~~~~~~~~~~~~~~~
    |      |                    |
    |      |                    (2) calling 'make_boxed_int'
    |
    +--> 'make_boxed_int': events 3-4
           |
           |  120 | {
           |      | ^
           |      | |
           |      | (3) entering 'make_boxed_int'
           |  121 |   boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
           |      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |      |                                    |
           |      |                                    (4) calling 'wrapped_malloc'
           |
           +--> 'wrapped_malloc': events 5-6
                  |
                  |    7 | {
                  |      | ^
                  |      | |
                  |      | (5) entering 'wrapped_malloc'
                  |    8 |   return malloc (size);
                  |      |          ~~~~~~~~~~~~~
                  |      |          |
                  |      |          (6) calling 'malloc'
                  |
    <-------------+
    |
 'test': event 7
    |
    |  138 |   free_boxed_int (obj);
    |      |   ^~~~~~~~~~~~~~~~~~~~
    |      |   |
    |      |   (7) calling 'free_boxed_int'
    |
(etc)
  • -fdiagnostics-show-path-depths — this option provides additional information when printing control-flow paths associated with a diagnostic.

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

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

  • -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu.
  • -fdiagnostics-column-unit=UNIT — selects the units for the column number. This affects traditional diagnostics (in the absence of -fno-show-column).

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

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

  • -fdiagnostics-column-origin=ORIGIN — okay so this picks the origin for column numbers, aka the column number that the very first column gets. Default is 1, which is what GCC has always done and what the GNU style guide says, so if it ain't broke 💅. But some utilities vibe better with an origin of 0, and honestly any non-negative value is allowed, so go off bestie ✨
  • -fdiagnostics-escape-format=FORMAT — so when GCC prints the relevant source lines for a diagnostic, it normally just tries to print the source bytes directly, raw, no filter. But some diagnostics are about encoding drama in the source file — malformed UTF-8, Unicode normalization issues, that kind of thing 😭 — and those get flagged so GCC will escape any bytes that aren't printable ASCII when it prints the pertinent source lines.

This option controls how those bytes get escaped 🎀

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

Example time, because yes: 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

Set FORMAT to bytes and it displays all non-printable-ASCII bytes in the form <XX>, so you see the underlying encoding of non-ASCII Unicode characters. For the example above, you get:

         before<CF><80><BF>after
  • -fdiagnostics-text-art-charset=CHARSET — some diagnostics contain "text art" diagrams: visualizations made of text, meant to be looked at in a monospaced font 🎨✨

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

  • none suppresses printing of such diagrams entirely, they're gone, deleted, not invited 🙈
  • ascii makes sure such diagrams are pure ASCII ("ASCII art") 💀
  • unicode allows for conservative use of unicode drawing characters (box-drawing characters and friends) 🦋
  • emoji further adds the possibility of emoji in the output (like emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character) 👑

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

  • -fno-diagnostics-show-nesting — some GCC diagnostics have an internal tree-like structure of nested sub-diagnostics, like when C++ templates are doing template things 🌳

By default GCC uses indentation and bullet points in its text output to show the nesting structure of these diagnostics, moves location information to separate lines so the structure is clearer, and eliminates redundant repeated information. very demure, very mindful

Selecting -fno-diagnostics-show-nesting suppresses this indentation, reformatting, and elision, restoring an older `look" for the diagnostics. Yes. Retro.

  • -fno-diagnostics-show-nesting-locations — when fdiagnostics-show-nesting is enabled, file names and line- and column-numbers are displayed on separate lines from the messages. This location information can be disabled altogether with -fno-diagnostics-show-nesting-locations. This option exists for use by GCC developers, for writing DejaGnu test cases 👩‍💻
  • -fdiagnostics-show-nesting-levels — when fdiagnostics-show-nesting is enabled, use fdiagnostics-show-nesting-levels to also display numbers showing the depth of the nesting. This option exists for use by GCC developers for debugging nested diagnostics, but may be of use to plugin authors 🫶
  • -fdiagnostics-format=FORMAT — picks a different format for printing diagnostics. FORMAT is text, sarif-stderr or sarif-file.

Using this option replaces any additional "output sinks" added by -fdiagnostics-add-output=, or that set by -fdiagnostics-set-output=. it overrides, okay, it does not stack 💅

The default is text.

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

  • -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC — adds 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. So maybe don't build your whole life on them, bestie 😭 Keys can be per-scheme, or related to GCC as a whole.

SCHEME can be:

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

Supported keys for the "text" scheme are:

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

Supported keys for the "sarif" scheme are:

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

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

  • state-graphs=[yes|no] — this is a debugging feature and defaults to "no". If "state-graphs=yes", then attempt to capture detailed state information from -fanalyzer in the generated SARIF.
  • experimental-html — emit diagnostics to a file in HTML format. This scheme is experimental, and may go away in future GCC releases. The keys and details of the output are also subject to change. 🫠

Supported keys for the "experimental-html" scheme are:

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

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

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

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

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

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 sinks, two files, no notes ✨

In EBNF:

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

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

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

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

         <key> = ? string without a '=' ? ;
         <value> = ? string without a ',' ? ;
  • -fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC — okay so this is like -fdiagnostics-add-output= but with a twist 💅 instead of just adding one more output sink for diagnostics, it straight up yeets all the existing output sinks first. So like, anything that came from -fdiagnostics-format=, or -fdiagnostics-add-output=, or an earlier -fdiagnostics-set-output= call? Gone. Replaced. Bye bestie 🫶
  • -fno-diagnostics-json-formatting — so by default, when GCC spits out JSON for diagnostics (think -fdiagnostics-format=sarif-stderr or -fdiagnostics-format=sarif-file), it's gonna add newlines and indentation so the hierarchical structure of the JSON visually pops off ✨ it's giving organized, it's giving readable.

But if you hit it with -fno-diagnostics-json-formatting, all that whitespace gets suppressed, no cap.

⚠️ Heads up: you gotta pass it before the option it's supposed to affect. Order matters, bestie.

This one's lowkey for compatibility with tools that don't expect newlines in the output — like the stuff older GCC releases used to emit. Vibes only 🌸

Options to Request or Suppress Warnings 🚨📢

Okay so warnings? They're diagnostic messages that report constructions that aren't inherently wrong but are risky, or they're basically hinting that maybe there was an error somewhere. Not an error, but like... a situationship with errors 💀

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

  • -fsyntax-only — checks the code for syntax errors, and that's it. Nothing beyond that, periodt. Just syntax vibes.
  • -fmax-errors=n — caps the max number of error messages at n, and once you hit that? GCC bails instead of trying to keep going through the source code. If n is 0 (which is the default), there's no limit at all on error messages. Oh and if -Wfatal-errors is also specified, -Wfatal-errors takes precedence over this option. It wins the argument 🙈
  • -w, --no-warnings — inhibits all warning messages. shut them all down.
  • -Werror — turns all warnings into errors. yup, all of them. glow-up from warning to error 🔥
  • -Werror= — turns the specified warning into an error. You append the specifier for the warning; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. It also has a negative form, used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect. flexible queen 💅

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 the option in the warning message can be disabled using the -fno-diagnostics-show-option flag, if the receipts are too much.)

📌 Note: specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. no cap.
  • -Wfatal-errors — this option makes the compiler abort compilation on the first error that occurs, instead of trying to keep going and printing further error messages. it's giving "we're done here" 😭

You can request a whole bunch of specific warnings with options that start with -W, like -Wunused-variable to request warnings on declarations of variables that are never used. Every one of these specific warning options also has a negative form starting with -Wno- to turn the warnings off; for example, -Wno-unused-variable. This manual only lists one of the two forms — whichever one is not the default. For more language-specific options, also go look at C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer;

Some options, like -Wall and -Wextra, turn on other options, like -Wunused, which may turn on even further options, like -Wunused-variable. The combined effect of positive and negative forms is that more specific options have priority over less specific ones — independently of where they sit in the command line. For options of the same specificity, the last one takes effect. And options enabled or disabled via pragmas take effect as if they appeared at the end of the command line. hierarchy is real, bestie 👑

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC gives an error stating that the option is not recognized. But — plot twist — 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 lets you use new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. sneaky but useful 🧚

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. And 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, --pedantic — issues all the warnings demanded by strict ISO C and ISO C++; diagnoses all programs that use forbidden extensions, and some other programs that don't follow ISO C and ISO C++. This follows the version of the ISO C or C++ standard specified by whatever -std option you used. ✨

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few need -ansi or a -std option specifying the version of the standard). But without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option? They get diagnosed (or rejected outright with -pedantic-errors). strict mode activated 😤

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows __extension__. However — only system header files should use these escape routes; application programs should avoid them. no cheating in your own code, bestie 🙈

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

Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there's a corresponding base standard — the version of ISO C that the GNU extended dialect is based on. Warnings from -Wpedantic are given where they're required by the base standard. (It doesn't make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there'd be nothing to warn about. so like, moot point fr fr)

  • -pedantic-errors, --pedantic-errors — gives 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, since it only makes errors of the diagnostics that are controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic. it does more, it's the main character here 👑

If you want the required diagnostics that are warnings by default to be errors instead, but you 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). choices, we love choices 🎀

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 — downgrades some required diagnostics about nonconformant code from errors to warnings. So like, using -fpermissive lets some nonconforming code actually compile. Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag. generosity icon 🌈
    • -Wabbreviated-auto-in-template-arg (C++ and Objective-C++ only)
    • -Wdeclaration-missing-parameter-type (C and Objective-C only)
    • -Wimplicit-function-declaration (C and Objective-C only)
    • -Wimplicit-int (C and Objective-C only)
    • -Wincompatible-pointer-types (C and Objective-C only)
    • -Wint-conversion (C and Objective-C only)
    • -Wnarrowing (C++ and Objective-C++ only)
    • -Wreturn-mismatch (C and Objective-C only)
    • -Wtemplate-body (C++ and Objective-C++ only)

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

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

...and the roster keeps going, bestie, because -Wall is a whole lifestyle 💅 here's the rest of the crew it drags in:

  • -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
  • -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

okay pause, important note bestie 🫶 not every warning flag is implied by -Wall. Some of them are out here warning 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. the way that some of them just have to be enabled by hand… 😭

✨ -Wextra ✨

✨ --extra-warnings ✨

This one turns on some extra warning flags that are not enabled by -Wall. Fun little glow-up: this option used to be called -W. The older name is still supported, but the newer name is more descriptive. iconic rebrand honestly 💖

The besties it brings along:

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

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. two warnings, one flag, we love a multitasker ✨

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. so like… it's doing its best, okay 🥺

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

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

  • A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

🌈 -Wpsabi 🌈

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

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

-Wpsabi is enabled by default, and is also implied by -Wabi. no flag needed, she's already here ✨

🦋 -Wno-changes-meaning 🦋

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

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

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

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

Both diagnostics are also suppressed by -fms-extensions.

🍓 -Wchar-subscripts 🍓

Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that this type is signed on some machines. @xref{Characters implementation}, and "char" type signedness. yes babe, signedness WILL get you, this is not a drill 😭

This warning is enabled by -Wall. When enabled, the warning is given regardless of whether "char" is unsigned by default on the target, and it is also not affected by the -fsigned-char or -funsigned-char options. she does not care about your defaults, she answers to no one 🫶

  • -Wno-coverage-mismatch — okay so this one's all about yelling at you when your feedback profiles don't match while you're using -fprofile-use. Like, if a source file gets changed between compiling with -fprofile-generate and then compiling with -fprofile-use, the files holding the profile feedback can straight up fail to match the source file, and then GCC cannot use the profile feedback information. 💀 By default this warning is enabled and it's treated as an error. So if you want it gone, -Wno-coverage-mismatch disables the warning, or -Wno-error=coverage-mismatch disables the error. But listen, bestie, disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. And completely disabling the warning is not recommended. 🙅‍♀️
  • -Wno-coverage-too-many-conditions — warns you if -fcondition-coverage is used and an expression has wayyy too many terms, so GCC just gives up on coverage. Coverage is given up when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default. 💅
  • -Wno-coverage-too-many-paths — warns you if -fpath-coverage is used and a function has too many paths, so GCC gives up on coverage. The giving-up part is controlled by -fpath-coverage-limit. Also enabled by default. ✨
  • -Wno-coverage-invalid-line-number — warns you in case a function ends earlier than it begins because of invalid linenum macros. The warning is emitted only with --coverage enabled. By default this warning is enabled and treated as an error. Use -Wno-coverage-invalid-line-number to disable the warning, or -Wno-error=coverage-invalid-line-number to disable the error. 🫶
  • -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) — suppresses warning messages emitted by #warning directives. That's it, that's the whole vibe. 🤫
  • -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. Why care? Because CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but they emulate double in software. On a machine like that, doing computations using double values is much more expensive because of the overhead required for software emulation. 😭

And it is so easy to accidentally do computations with double, because floating-point literals are implicitly of type double. Like, 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 you if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall. 👑
  • -Wformat
  • -Wformat=n — bestie this one is a whole icon, it 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 specified in the format string actually make sense. 🧚

This includes standard functions, and others specified by format attributes, in the printf, scanf, strftime and strfmon families (and strfmon 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 selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.

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

  • -Wformat=1
  • -Wformat — Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-diag, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. ✨
  • -Wformat=2 — Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. Let's goooo 🔥
  • -Wno-format-contains-nul — if -Wformat is specified, do not warn about format strings that contain NUL bytes. 💅
  • -Wno-format-diag — if -Wformat is specified, do not warn about format strings that are unsuitable for GCC diagnostics. 🤷‍♀️
  • -Wno-format-extra-args — if -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored anyway, so like, whatever. 🍓

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

  • -Wformat-overflow
  • -Wformat-overflow=level — warns you about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. 😱 When the exact number of bytes written by a format directive cannot be determined at compile-time, it is estimated based on heuristics that depend on the level argument and on optimization. And while enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. 💀
    • -Wformat-overflow
    • -Wformat-overflow=1 — Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length are assumed to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. 🫣
void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}
  • -Wformat-overflow=2 — Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long.

At level 2, 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 specified, do not warn about zero-length formats. The C standard specifies that zero-length formats are allowed, so it's lowkey fine. 💜
  • -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. Can't check what you can't see, bestie. 👑
  • -Wformat-security — ok so if -Wformat is on, this one ALSO flags uses of format functions that could be a whole security situation 💀. rn it warns about calls to printf and scanf functions where the format string isn't a string literal and there are no format arguments, like printf (foo);. and why do we care?? bestie, if that format string came from untrusted input and has a %n in it, that's literally a security hole. (right now it's a subset of what -Wformat-nonliteral warns about, but bestie, future warnings may get added to -Wformat-security that aren't included in -Wformat-nonliteral.)
  • -Wformat-signedness — if -Wformat is specified, also warn when the format string needs an unsigned argument and you handed it a signed one, or the other way around. sign mismatch = not the vibe 🙈
  • -Wformat-truncation, -Wformat-truncation=level — warns about calls to formatted input/output functions like snprintf and vsnprintf that might end up truncating the output ✂️. 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. turning on optimization will in most cases make the warning more accurate, but it may also hand you false positives (the duality of it all 😭). except as noted otherwise, this option uses the same logic as -Wformat-overflow.
    • -Wformat-truncation, -Wformat-truncation=1 — level 1, which is what -Wformat turns on, is the conservative one: it only warns about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
    • -Wformat-truncation=2 — level 2 also warns about calls to bounded functions whose return value IS used and that might result in truncation given an argument of sufficient length or magnitude.
  • -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may give you only a two-digit year. y2k called, it wants its whole bug back 📞✨
  • -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. you can turn it off with the -Wno-nonnull option.

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

-Wnonnull-compare is included in -Wall. you can turn it off with the -Wno-nonnull-compare option.

  • -Wnull-dereference — warn if the compiler detects paths that trigger erroneous or undefined behavior because of dereferencing a null pointer 😵. this option is only active when -fdelete-null-pointer-checks is active, which optimizations enable in most targets. how precise the warnings are depends on the optimization options you used.
  • -Wno-musttail-local-addr — do NOT warn about passing a pointer (or in C++, a reference) to a local variable or label to an argument of a musttail call. those variables go out of scope before the tail call instruction.
  • -Wmaybe-musttail-local-addr — warn when the address of a local variable can escape to a musttail call, unless it already goes out of scope 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 [class.copy.elision] allows it. this elision is commonly known as the Named Return Value Optimization. for instance, in the example below the compiler can't elide copies from both v1 and v2, so it elides neither 🫠
std::vector<int> f()
{
  std::vector<int> v1, v2;
  // ...
  if (cond) return v1;
  else return v2; // warning: not eliding copy
}
  • -Winfinite-recursion — warn about infinitely recursive calls 🔁. the warning is effective at all optimization levels, BUT it 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 😭. heads up, 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 doesn't specify a type. this warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. the error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-int.

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

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

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

  • -Wimplicit (C and Objective-C only) — same as -Wimplicit-int and -Wimplicit-function-declaration. this warning is enabled by -Wall.
  • -Whardened — warn when -fhardened didn't enable an option from its set 👀 (for that set, 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. like, example:
switch (cond)
  {
  case 1:
    a = 1;
    break;
  case 2:
    a = 2;
  case 3:
    a = 3;
    break;
  }

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

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

since there are occasions where a switch case fall through is actually desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that's meant to be used along with a null statement to suppress this warning that would normally happen anyway:

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. and instead of these attributes, it's also possible to add a fallthrough comment to silence the warning ✨. the whole body of the C or C++ style comment should match the given regular expressions listed below. the option argument n specifies what kind of comments are accepted:

  • -Wimplicit-fallthrough=0 disables the warning altogether.
  • -Wimplicit-fallthrough=1 matches ".*" regular expression, any comment is used as fallthrough comment.
  • -Wimplicit-fallthrough=2 case insensitively matches ".*falls?[ \t-]*thr(ough|u).*" regular expression.
  • -Wimplicit-fallthrough=3 case sensitively matches one of the following regular expressions:

…and level 3 (the case-insensitive one, bestie) matches any one of these regular expressions, no cap:

*<"-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 — this one is picky about CASE, okay? It sensitively matches one of these regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
  • -Wimplicit-fallthrough=5 — doesn't recognize any comments as fallthrough comments. The only thing that disables the warning here is attributes. periodt. 💅

The comment needs to be followed — after optional whitespace and other comments — by case or default keywords, or by a user label that precedes some case or default label. So like, this vibe:

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

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

  • -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — controls whether warnings triggered by the warn_if_not_aligned attribute get issued. These warnings are on by default, fyi.
  • -Wignored-qualifiers (C and C++ only) — warns if a function's return type has a type qualifier like const. For ISO C that qualifier literally does nothing, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. And ISO C prohibits qualified void return types on function definitions, so those return types always get a warning even without this option. 😤

This warning is also enabled by -Wextra.

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

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

  • -Wmisleading-indentation (C and C++ only) — warns when the indentation of the code doesn't reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the same indentation. The audacity of this formatting, honestly. 💀

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

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

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

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

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

-Wmissing-attributes is enabled by -Wall.

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

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

template <>
void* __attribute__ ((malloc))   // missing alloc_size
allocate<void> (size_t);
  • -Wmissing-braces — warns if an aggregate or union initializer isn't fully bracketed. In the following example, the initializer for a is not fully bracketed, but the one for b is. Balance, bestie 🎀
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

  • -Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only) — warns if a user-supplied include directory doesn't exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for -I and -J, only.
  • -Wno-missing-profile — this option controls warnings if feedback profiles are missing when using the -fprofile-use option. It diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files don't 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.
fr fr, don't silence this one unless you actually know what you're doing 🙈
  • -Wmismatched-dealloc — warns for calls to deallocation functions with pointer arguments returned from allocation functions for which the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by use of attribute malloc. Unless disabled by the -fno-builtin option, the standard functions calloc, malloc, realloc, and free, as well as the corresponding forms of C++ operator new and operator delete, are implicitly associated as matching allocators and deallocators. In the following example mydealloc is the deallocator for pointers returned from myalloc:
void mydealloc (void*);

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

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

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

Option -Wmismatched-dealloc is included in -Wall.

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

For example:

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

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

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

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

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

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

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

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

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

This warning is enabled by -Wall.

-Wno-self-move (C++ and Objective-C++ only)

Bestie. Have you ever heard of moving a thing into itself? Because GCC has, and it wants to talk. This one warns you when a value gets moved to itself with std::move. And like… a std::move onto the same variable typically has no effect at all, so it's giving "why are you like this" energy. 💀

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

Slay, this warning is enabled by -Wall. ✨

-Wsequence-point

Okay so this is the big one — the ✨sequence points✨ lore. This warns about code that might have undefined semantics because it violates the sequence point rules in the C and C++ standards. 🫣

So here's the situation: the C and C++ standards define the order expressions get evaluated in terms of sequence points, which are basically a partial ordering between chunks of execution — the stuff that runs before the sequence point, and the stuff that runs after it. These happen:

  • after the evaluation of a full expression (a.k.a. one that isn't part of a bigger expression)
  • after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator
  • before a function is called (but after the evaluation of its arguments and the expression denoting the called function)
  • and in certain other places, iykyk

Other than what the sequence point rules say, the order of evaluation of subexpressions is not specified. And literally all these rules describe only a partial order, not a total order — because, like, if two functions get called in one expression with no sequence point between them, the order they're called in just isn't specified. But — plot twist — the standards committee ruled that function calls do not overlap. 🎀

It is also not specified when, between sequence points, modifications to the values of objects actually take effect. Programs whose behavior depends on that have undefined behavior. The C and C++ standards say it like this:

"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 your program breaks these rules, the results on any particular implementation are entirely unpredictable. Not chaotic-cute. Just chaotic. 😭

Examples of code with undefined behavior: a = a++;, a[n] = b[n++] and a[i++] = i;. Some messier cases don't get diagnosed by this option, and it may occasionally give a false positive — but in general it's been found pretty effective at catching 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 those examples above are no longer undefined. But this option will still warn about them, to help you avoid writing code that's undefined in C and earlier revisions of C++. (Protective bestie behavior, honestly. 💅)

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

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

-Wno-return-local-addr

Do not warn about returning a pointer (or, in C++, a reference) to a variable that goes out of scope after the function returns. That's the whole vibe. 🙈

-Wreturn-mismatch

Warns about return statements without an expression in functions that 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, that latter case is accepted without a warning unless -Wpedantic is used.

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

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

-Wreturn-type

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

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

Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used. That's the difference bestie, and it's a big one. 💀

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

-Wno-shift-count-negative

Controls warnings if a shift count is negative. This warning is enabled by default. 🌀

-Wno-shift-count-overflow

Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default. 🌈

-Wshift-negative-value

Warns if you left shift a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes. 👑

-Wno-shift-overflow, -Wshift-overflow=n

These options control warnings about left shift overflows. Two levels, two vibes:

  • -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. Extra spicy. 🌶️

-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 don't 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. Like, where's your safety net bestie. 🕸️

-Wswitch-enum

Warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels that don't 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. So it's the stricter, more thorough sibling. 💅

-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:
  ...
  }

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

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

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

-Wsync-nand (C and C++ only)

Warns when __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4, so like… watch out. ⚠️

-Wtrivial-auto-var-init

Warns when -ftrivial-auto-var-init cannot initialize the automatic variable. A common situation is an automatic variable that's declared between the controlling expression and the first case label of a switch statement. 🫠

-Wunused-but-set-parameter

-Wunused-but-set-parameter is the same as -Wunused-but-set-parameter=3 and -Wno-unused-but-set-parameter is the same as -Wunused-but-set-parameter=0. Easy math. 🧮

-Wunused-but-set-parameter=n

Warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).

To suppress this warning use the unused attribute.

Levels, bestie, levels:

  • -Wunused-but-set-parameter=0 disables the warning.
  • With -Wunused-but-set-parameter=1 all uses except initialization and left hand side of assignment which is not further used disable the warning.
  • With -Wunused-but-set-parameter=2 additionally uses of parameter in ++ and -- operators don't count as uses.
  • And finally with -Wunused-but-set-parameter=3 additionally uses in parm @= rhs outside of rhs don't count as uses.

See -Wunused-but-set-variable=n option for examples.

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

-Wunused-but-set-variable

-Wunused-but-set-variable is the same as -Wunused-but-set-variable=3 and -Wno-unused-but-set-variable is the same as -Wunused-but-set-variable=0. 🦋

-Wunused-but-set-variable=n

Warns whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This -Wunused-but-set-variable=3 warning is enabled by -Wall.

To suppress this warning use the unused attribute. ✨

okay so bestie, let's talk about the -Wunused-but-set-variable levels. 💅

-Wunused-but-set-variable=0 turns the warning off completely. -Wunused-but-set-variable=1 means all uses except initialization and the left hand side of an assignment which is not further used disable the warning. With -Wunused-but-set-variable=2, additionally uses of the variable in "++" and "--" operators don't count as uses. And finally, with -Wunused-but-set-variable=3, additionally uses in parm "@=" rhs outside of rhs don't count as uses. (Yes, that's exactly what it says. 💀)

This -Wunused-but-set-variable=3 warning is also enabled by -Wunused, which is enabled by -Wall. 👑

void foo (void)
{
  int a = 1; // B<-Wunused-variable> warning
  int b = 0; // Warning for <n> >= 1
  b = 1; b = 2;
  int c = 0; // Warning for <n> >= 2
  ++c; c--; --c; c++;
  int d = 0; // Warning for <n> >= 3
  d += 4;
  int e = 0; // No warning, cast to void
  (void) e;
  int f = 0; // No warning, f used
  int g = f = 5;
  (void) g;
  int h = 0; // No warning, preincrement result used
  int i = ++h;
  (void) i;
  int j = 0; // No warning, postdecrement result used
  int k = j--;
  (void) k;
  int l = 0; // No warning, l used
  int m = l |= 2;
  (void) m;
}
  • -Wunused-function — Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by -Wall. 🫶
  • -Wunused-label — Warn whenever a label is declared but not used. This warning is enabled by -Wall. To suppress this warning, use the "unused" attribute. ✨
  • -Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only) — Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall. 🌸
  • -Wunused-parameter — Warn whenever a function parameter is unused aside from its declaration. This option is not enabled by "-Wunused" unless "-Wextra" is also specified. To suppress this warning, use the "unused" attribute. 💖
  • -Wno-unused-result — Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return value. The default is -Wunused-result. 🙈
  • -Wunused-variable — Warn 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 suppress this warning, use the "unused" attribute. 💅
  • -Wunused-const-variable
  • -Wunused-const-variable=n — Warn whenever a constant static variable is unused aside from its declaration. To suppress this warning, use the "unused" attribute.
    • -Wunused-const-variable=1 — Warn about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included. -Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C, this declares variable storage, but in C++, this is not an error since const variables take the place of "#define"s. 🍓
    • -Wunused-const-variable=2 — This warning level also warns for unused constant static variables in headers (excluding system headers). It is equivalent to the short form -Wunused-const-variable. This level must be explicitly requested in both C and C++ because it might be hard to clean up all headers included. 🧚
  • -Wunused-value — Warn whenever a statement computes a result that is explicitly not used. To suppress this warning, cast the unused expression to "void". This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as "x[i,j]" causes a warning, while "x[(void)i,j]" does not. This warning is enabled by -Wall. 🌈
  • -Wunused — All the above -Wunused options combined, except those documented as needing to be specified explicitly. In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter. -Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++. 💜
  • -Wuse-after-free (C, Objective-C, C++ and Objective-C++ only)
  • -Wuse-after-free=n — Warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without. -Wuse-after-free is equivalent to -Wuse-after-free=1.
    • -Wuse-after-free=1 — At level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call to "realloc", regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-"free" calls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level. 🔥
    • -Wuse-after-free=2 — At level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to "free" in the following function is diagnosed at this level:
struct A { int refcount; void *data; };

void release (struct A *p)
{
  int refcount = --p->refcount;
  free (p);
  if (refcount == 0)
    free (p->data);   // warning: p may be used after free
}
  • -Wuse-after-free=3 — At level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined but equality tests sometimes appear after calls to "realloc" as an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:
void adjust_pointers (int**, int);

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

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

  • -Wuse-after-free=2 is included in -Wall. 💀
  • -Wuseless-cast (C, Objective-C, C++ and Objective-C++ only) — Warn when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type as that is a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
  g (S(arg)); // make arg prvalue so that it can bind to S&&
}
  • -Wuninitialized — Warn if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warn if a non-static reference or non-static "const" member appears in a class without constructors. In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.) If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option. These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared "volatile". Because these warnings depend on optimization, the exact variables or elements for which there are warnings depend on the precise optimization options and version of GCC used. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about "b" being uninitialized in the following snippet:
struct A {
  int a;
  int b;
  A() : a(b) { }
};
  • -Wno-invalid-memory-model — This option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the "memory_order" enumeration. For example, since the "__atomic_store" and "__atomic_store_n" built-ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is diagnosed:
void store (int *i)
{
  __atomic_store_n (i, 0, memory_order_consume);
}

-Winvalid-memory-model is enabled by default. ⭐

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

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

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

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

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

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

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

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

This warning is enabled by -Wall or -Wextra.

  • -Wunknown-pragmas — Warn when a #pragma directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option. 💅
  • -Wno-pragmas — Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas. 🎀
  • -Wno-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. 🦋
  • -Wno-prio-ctor-dtor — Do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution before main is called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between 0--100 for the implementation. 👑
  • -Wstrict-aliasing — This option warns about code that might break the strict aliasing rules that the compiler uses for optimization. The type-based checking done in the front end is performed even when -fstrict-aliasing is not active, provided -Wstrict-aliasing is requested explicitly; it is not performed for -fno-strict-aliasing -Wall alone. When optimization is enabled, the warning also runs in the back end, where it handles multiple-statement cases using flow-sensitive points-to information. The warning does not catch all cases, but does attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3. 🔥
  • -Wstrict-aliasing=n — This option warns about code that might break the strict aliasing rules that the compiler uses for optimization. The type-based checking done in the front end is performed even when -fstrict-aliasing is not active, provided -Wstrict-aliasing is requested explicitly. When optimization is enabled, the warning also runs in the back end, where it handles multiple-statement cases using flow-sensitive points-to information. 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, as 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 — Does nothing. Preserved for backward compatibility. 😴
    • -Wstrict-overflow=1 — Does nothing. Preserved for backward compatibility.
    • -Wstrict-overflow=2 — Does nothing. Preserved for backward compatibility.
    • -Wstrict-overflow=3 — Does nothing. Preserved for backward compatibility.
    • -Wstrict-overflow=4 — Does nothing. Preserved for backward compatibility.
    • -Wstrict-overflow=5 — Does nothing. Preserved for backward compatibility.
  • -Wstring-compare — Warn for calls to strcmp and strncmp whose result is determined to be either zero or non-zero in tests for such equality owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case of strncmp). Such calls could be mistakes. For example, the call to strcmp below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array a. 🕵️‍♀️
extern char a[4];
void f (char *d)
{
  strcpy (d, "string");
  ...
  if (0 == strcmp (a, d))   // cannot be true
    puts ("a and d are the same");
}

-Wstring-compare is enabled by -Wextra.

  • -Wno-stringop-overflow
  • -Wstringop-overflow
  • -Wstringop-overflow=type — Warn for code that can be statically determined to cause buffer overflows or memory overruns, such as 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.

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

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

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

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

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

void f (int[static 4]);

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

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

  • -Wstringop-overflow
  • -Wstringop-overflow=1 — The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the _FORTIFY_SOURCE macro is defined to a non-zero value. 🐧
  • -Wstringop-overflow=2 — okay so this level uses type-one Object Size Checking to clock how big the destination objects actually are. At this setting it yells at you about overflows when you write to members of the largest complete objects whose exact size is known. But bestie, it will not warn about excessive writes to those same members of unknown objects that pointers reference — because those pointers might be pointing to arrays with an unknown number of elements, and we're not about to guess. 💅 This is literally the default setting of the option, so if you don't pick a level, this is the one you're getting.
    • -Wstringop-overflow=3 — this one rolls with type-two Object Size Checking to figure out destination object sizes. Here 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 throw warnings at you for code that's actually safe. 😭 Not it being dramatic for no reason.
    • -Wstringop-overflow=4 — and this level runs type-three Object Size Checking to determine destination object sizes. 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 just like -Wstringop-overflow=3, this setting may result in warnings for benign code. 🙈
  • -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. 🚨

Lowkey important: option -Wstringop-overread is enabled by default.

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

So picture this: in the example below, the call to strncat specifies a bound that's less than the length of the source string. Which means the copy of the source gets truncated, and so the call gets diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound. (yeah, exactly like that, we're keeping it)

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

And as another example, this next call to strncpy copies to d just the characters preceding the terminating NUL, without appending the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake, and so the call is diagnosed. If you're not expecting the result to be NUL-terminated, call memcpy instead to avoid the warning. 💖

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

And one more, bestie, because this one gets people: here 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, the call is also diagnosed. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL. 🎀

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

Now, in situations where a character array is intended to store a sequence of bytes with no terminating NUL, you can annotate that array with attribute nonstring to avoid this warning. 🌸 BUT — and this is a big but — such arrays are not suitable arguments to functions that expect NUL-terminated strings. To help detect accidental misuses of such arrays, GCC issues warnings unless it can prove that the use is safe. 🫶

  • -Wstrict-flex-arrays (C and C++ only) — 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. If it's not available, it goes by the level of the option -fstrict-flex-arrays=level instead. Also: -Wstrict-flex-arrays is effective only when level is greater than 0. 💅
    • When level=1, warnings are issued for a trailing array reference of a structure that have 2 or more elements, if the trailing array is referenced as a flexible array member.
    • When level=2, in addition to level=1, additional warnings are issued for a trailing one-element array reference of a structure, if the array is referenced as a flexible array member.
    • When level=3, in addition to level=2, additional warnings are issued for a trailing zero-length array reference of a structure, if the array is referenced as a flexible array member.

This option is more effective when -ftree-vrp is active (which is 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 may be beneficial. The attribute-names currently supported 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 attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units, or (in the case of pure and const) if it can't prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop, or doesn't return abnormally by throwing, calling abort, or trapping. 🔥 This analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. And higher optimization levels improve the accuracy of the analysis, so yeah, more -O = better vibes.

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

These warn about function pointers that might be candidates for format attributes. But ngl, these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of the parameter variable, or the return type of the containing function 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 this might not always be the case, and some functions for which format attributes are appropriate may not be detected. 😭 It's giving "best guess energy."

  • -Wsuggest-attribute=cold — warns about functions that might be candidates for the cold attribute. This one is based on static detection, and generally only warns about functions which always leads to a call to another cold function — such as wrappers of C++ throw, or fatal error reporting functions leading to abort. 💀
  • -Walloc-size — warns about calls to allocation functions decorated with attribute alloc_size that specify insufficient size for the target type of the pointer the result is assigned to. This includes 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 attribute alloc_size that specify zero bytes, including calls to the built-in forms of aligned_alloc, alloca, calloc, malloc, and realloc. Because the behavior of these functions when called with a zero size differs among implementations (and in the case of realloc it 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 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, you can suppress the warning by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4), or by using sizeof in the later argument as well. 🎀
  • *-Walloc-size-larger-than=byte-size* — warns about calls to functions decorated with attribute alloc_size that attempt to allocate objects larger than the specified number of bytes, or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than. ⭐
  • -Wno-alloc-size-larger-than — disables -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. 🌈
  • -Walloca — warns on all uses of alloca in the source. Periodt. 💅
  • -Wauto-profile — outputs warnings about auto-profile inconsistencies. 🤔
  • -Wcannot-profile — warns when profiling instrumentation was requested, but could not be applied to a certain function. 😭
  • *-Walloca-larger-than=byte-size* — warns on calls to alloca with an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. And bestie, arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range. 🫠

For example, a bounded case of alloca could be:

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

So like, in that example up there 👆 passing -Walloca-larger-than=1000 would NOT throw a warning, bestie, because the call to alloca is known to be at most 1000 bytes. But the second you pass -Walloca-larger-than=500 instead? Yeah, the compiler goes off and emits a warning. Periodt. 💅

Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. Like, none. Zero. For example:

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

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

And btw, even code that looks correct and involves signed integers could still 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. The math said no, the cast said yes. 😭

This option also warns when alloca is used in a loop. (bestie why is it in a loop)

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

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

  • -Wno-alloca-larger-than — Disable -Walloca-larger-than= warnings. The option is equivalent to -Walloca-larger-than=SIZE_MAX or larger. Basically "I said what I said, turn it off." ✨
  • -Warith-conversion — Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion.
void f (char c, int i)
{
  c = c + i; // warns with -Wconversion
  c = c + 1; // only warns with -Warith-conversion
}
  • -Warray-bounds
  • -Warray-bounds=n — Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It is more effective when -ftree-vrp is active (the default for -O2 and above) but a subset of instances are issued even without optimization. 🦋

By default, the trailing array of a structure will be treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it is declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or an 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. iykyk.

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

  • when level<=1, no change to the default behavior.
  • when level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays;
  • when level=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays.
  • -Warray-bounds=1 — This is the default warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested. Ask nicely. 🎀
  • -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. It's giving "chaos but make it optional." 💀
  • -Wunterminated-string-initialization (C and Objective-C only) — Warn about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has the nonstring attribute. For example:
char arr[3] = "foo"; /* Warning.  */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning.  */

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

  • -Warray-compare — Warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;

-Warray-compare is enabled by -Wall.

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

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

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

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

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

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

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

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

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

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

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

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

  • -Wbidi-chars=[none|unpaired|any|ucn] — Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 😭

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

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

  • -Wbool-compare — Warn about boolean expression compared with an integer value different from true/false. For instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }

This warning is enabled by -Wall.

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

This warning is enabled by -Wall.

  • -Wduplicated-branches — Warn when an if-else has identical branches. This warning detects cases like
if (p != NULL)
  return 0;
else
  return 0;

…and like, it also does not warn when both branches contain just a null statement. no drama where there's no drama 💅

This warning also warn for conditional operators:

int i = x ? *p : *p;
  • -Wduplicated-cond — bestie, this one clocks duplicated conditions in an if-else-if chain. like, it will literally warn for this:
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. 😭 such calls may return indeterminate values or crash the program. fr fr. this warning is included in -Wall.
  • -Wno-discarded-qualifiers (C and Objective-C only) — don't warn if type qualifiers on pointers are being discarded. typically the compiler warns if a const char * variable is passed to a function that takes a char * parameter. this option can be used to suppress such a warning. 💅
  • -Wno-discarded-array-qualifiers (C and Objective-C only) — don't 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) — don't 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) — don't 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. just say NULL bestie, it's giving clarity 🫶
  • -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 — don't 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. so like, chill on the floats 🧚
  • -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 — heads up — using -Wall in conjunction with this option does not warn about unknown pragmas in system headers, for that, -Wunknown-pragmas must also be used. 😌
  • -Wtautological-compare — warns if a self-comparison always evaluates to true or false. this warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }

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

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

will always be false. not me comparing a value to itself and expecting a plot twist 😭

this warning is enabled by -Wall. 🔥

  • -Wtrailing-whitespace
  • -Wtrailing-whitespace=kind — warns about trailing whitespace at the end of lines, including inside of comments, but excluding trailing whitespace in raw string literals. -Wtrailing-whitespace is equivalent to -Wtrailing-whitespace=blanks and warns just about trailing space and horizontal tab characters. -Wtrailing-whitespace=any warns about those or trailing form feed or vertical tab characters. -Wno-trailing-whitespace or -Wtrailing-whitespace=none disables the warning, which is the default. this is a coding style warning. ✨
  • -Wleading-whitespace=kind — warns about style issues in leading whitespace, but not about the amount of indentation. some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of -ftabstop=n) followed by zero or fewer than n spaces. no warning is emitted on lines which contain solely whitespace (although -Wtrailing-whitespace= warning might be emitted), no warnings are emitted inside of raw string literals. warnings are also emitted for leading whitespace inside of multi-line comments. -Wleading-whitespace=spaces warns about leading whitespace other than spaces for projects which want to indent just by spaces. -Wleading-whitespace=tabs warns about leading whitespace other than horizontal tabs for projects which want to indent just by horizontal tabs. -Wleading-whitespace=blanks warns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or about n or more consecutive spaces in leading whitespace (where n is argument of -ftabstop=n, 8 by default). -Wleading-whitespace=none disables the warning, which is the default. this is a coding style warning. 🎀
  • -Wtrampolines — warns about trampolines generated for pointers to nested functions. a trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. for some targets, it is made up of data only and thus requires no special treatment. but, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly. 👀
  • -Wfloat-equal — warns if floating-point values are used in equality comparisons.

the idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. if you are doing this, then you need to compute (by analyzing the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow for it when performing comparisons (and when producing output, but that's a different problem). in particular, instead of testing for equality, you should check to see whether the two values have ranges that overlap; and this is done with the relational operators, so equality comparisons are probably mistaken. bestie, floats are not your friends like that 💔

  • -Wtraditional (C and Objective-C only) — warns about certain constructs that behave differently in traditional and ISO C. also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided.
    • macro parameters that appear within string literals in the macro body. in traditional C macro replacement takes place within string literals, but in ISO C it does not. 🙈
    • in traditional C, some preprocessor directives did not exist. traditional preprocessors only considered a line to be a directive if the # appeared in column 1 on the line. therefore -Wtraditional warns about directives that traditional C understands but ignores because the # does not appear as the first character on the line. it also suggests you hide directives like #pragma not understood by traditional C by indenting them. some traditional implementations do not recognize #elif, so this option suggests avoiding it altogether.
    • a function-like macro that appears without arguments.
    • the unary plus operator.
    • the U integer constant suffix, or the F or L floating-point constant suffixes. (traditional C does support the L suffix on integer constants.) note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases. 🧚
    • a function declared external in one block and then used after the end of the block.
    • a switch statement has an operand of type long.
    • a non-static function declaration follows a static one. this construct is not accepted by some traditional C compilers.
    • the ISO type of an integer constant has a different width or signedness from its traditional type. this warning is only issued if the base of the constant is ten. i.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
    • usage of ISO string concatenation is detected.
    • initialization of automatic aggregates.
    • identifier conflicts with labels. traditional C lacks a separate namespace for labels.
    • initialization of unions. if the initializer is zero, the warning is omitted. this is done under the assumption that the zero initializer in user code appears conditioned on e.g. __STDC__ to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case. 💜
  • Conversions by prototypes between fixed/floating-point values and vice versa. Like, the absence of these prototypes when you're compiling with traditional C causes serious problems. No cap. This is only a subset of the possible conversion warnings — for the whole set use -Wtraditional-conversion.
  • Use of ISO C style function definitions. Okay so this warning is intentionally not issued for prototype declarations or variadic functions, bestie, because those ISO C features show up in your code when you're using libiberty's traditional C compatibility macros, PARAMS and VPARAMS. It's also bypassed for nested functions because that feature is already a GCC extension so it's not relevant to traditional C compatibility at all. 🫶

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

Warns if a prototype causes a type conversion that's different from what would happen to that same argument if there were no prototype. This includes fixed point → floating and floating → fixed conversions, plus conversions that change the width or signedness of a fixed-point argument — except when it's the same as the default promotion.

✨ -Wdeclaration-after-statement (C and Objective-C only) ✨

Warns when a declaration shows up after a statement in a block. This construct is a C++ thing, it got introduced with ISO C99, and it's allowed by default in GCC. It is not supported by ISO C90.

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

✨ -Wshadow ✨

Warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C) — or whenever a built-in function is shadowed. 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 also includes all instances of local shadowing. Which means -Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global.

✨ -Wno-shadow-ivar (Objective-C only) ✨

Don't warn whenever a local variable shadows an instance variable in an Objective-C method. 💅

✨ -Wshadow=global ✨

Warns for any shadowing. Same as -Wshadow.

✨ -Wshadow=local ✨

Warns when a local variable shadows another local variable or parameter.

✨ -Wshadow=compatible-local ✨

Warns when a local variable shadows another local variable or parameter whose type is compatible with the shadowing variable's type. In C++, type compatibility here means the type of the shadowing variable can be converted to the type of the shadowed variable. The whole idea behind this flag (on top of -Wshadow=local) is that when a local variable shadows another one of incompatible type, it's most likely intentional, not a bug or typo — like in this iconic example:

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

Since the two i variables up there have incompatible types, enabling only -Wshadow=compatible-local doesn't 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 throw an error or warning anyway. Using this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this also means shadowing const char *i by char *i does not emit a warning.

This warning is also enabled by -Wshadow=local.

✨ -Wlarger-than=byte-size ✨

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

Also warns for calls to bounded functions like memchr or strnlen that specify a bound greater than the largest possible object — which is PTRDIFF_MAX bytes by default. Those warnings can only be disabled by -Wno-larger-than.

✨ -Wno-larger-than ✨

Disables -Wlarger-than= warnings. This option is equivalent to -Wlarger-than=SIZE_MAX or larger.

✨ -Wframe-larger-than=byte-size ✨

Warns if the size of a function frame exceeds byte-size. The computation done to figure out the stack frame size is approximate and not conservative — like, the actual requirements may be somewhat greater than byte-size even if you get no warning at all. On top of that, any space allocated via alloca, variable-length arrays, or related constructs is not included by the compiler when it decides whether or not to issue a warning. -Wframe-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-frame-larger-than.

✨ -Wno-frame-larger-than ✨

Disables -Wframe-larger-than= warnings. This option is equivalent to -Wframe-larger-than=SIZE_MAX or larger.

✨ -Wfree-nonheap-object ✨

Warns when you attempt to deallocate an object that was either not allocated on the heap, or that you're deallocating using a pointer that wasn't 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 gets diagnosed. 😭

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

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

✨ -Wstack-usage=byte-size ✨

Warns if the stack usage of a function might exceed byte-size. The computation done to figure out the stack usage is conservative. Any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when it decides whether or not to issue a warning.

The message matches the output of -fstack-usage.

  • If the stack usage is fully static but exceeds the specified amount, it's:
          warning: stack usage is 1120 bytes
  • If the stack usage is (partly) dynamic but bounded, it's:
          warning: stack usage might be 1648 bytes
  • If the stack usage is (partly) dynamic and not bounded, it's:
          warning: stack usage might be unbounded

-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-stack-usage.

✨ -Wno-stack-usage ✨

Disables -Wstack-usage= warnings. This option is equivalent to -Wstack-usage=SIZE_MAX or larger.

✨ -Wunsafe-loop-optimizations ✨

Warns if the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. With -funsafe-loop-optimizations, it warns if the compiler makes such assumptions. 👑

✨ -Wno-pedantic-ms-format (MinGW targets only) ✨

When used together 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 — those depend on the MS runtime.

✨ -Wpointer-arith ✨

Warns about anything that depends on the "size of" a function type or of void. GNU C assigns these types a size of 1, for convenience in calculations with void * pointers and pointers to functions. In C++, it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.

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

✨ -Wno-pointer-compare ✨

Don't warn if a pointer is compared with a zero character constant. Usually that means the pointer was meant to be dereferenced, like:

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

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

This warning is enabled by default.

✨ -Wno-tsan ✨

Disables warnings about unsupported features in ThreadSanitizer.

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

✨ -Wtype-limits ✨

Warns if a comparison is always true or always false due to the limited range of the data type — but it does not warn for constant expressions. For example, it warns if an unsigned variable is compared against zero with < or >=. This warning is also enabled by -Wextra.

✨ -Wabsolute-value (C and Objective-C only) ✨

Warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, calling abs(3.14) triggers the warning because the right function to call to compute the absolute value of a double argument is fabs. The option also triggers warnings when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an explicit type cast, and it's also enabled by -Wextra.

✨ -Wcomment / -Wcomments ✨

  • -Wcomment, -Wcomments — Warns whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. This warning is enabled by -Wall.

✨ -Wtrigraphs ✨

Warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about — except those that would form escaped newlines.

This option is implied by -Wall. If -Wall isn't given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. 🌸

✨ -Wundef ✨

Warns if an undefined identifier is evaluated in an #if directive. Such identifiers get replaced with zero.

✨ -Wexpansion-to-defined ✨

Warns whenever defined is encountered in the expansion of a macro — including the case where the macro is expanded by an #if directive. Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra. ✨

  • -Wunused-macros — bestie, this one snitches on macros you defined in the main file and then just… never used. 🙈 A macro counts as "used" the second it gets expanded or tested for existence even once. The preprocessor will ALSO yell at you if you haven't used the macro by the time you redefine it or #undef it. Iconic behavior, honestly.

Heads up though: built-in macros, macros defined on the command line, and macros defined in include files are all exempt — no warnings for those. ✨

💀 the ONE 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 the scope of the macro's definition — like moving it into the first skipped block, for example. Or you could just feed it a dummy use, like this:
#if defined the_macro_causing_the_warning
#endif
  • -Wno-endif-labels — turns OFF the warning you get whenever an #else or an #endif is followed by text. This is giving "older programs with code like this" energy:
#if FOO
...
#else FOO
...
#endif FOO

That second and third "FOO" are supposed to be in comments, bestie. 😭 This warning is on by default.

  • -Wbad-function-cast (C and Objective-C only) — yells at you when a function call gets cast to a type that doesn't match. Like, if you call a function that returns an integer type and then cast the result to a pointer type? Yeah, warned. 🚩
  • -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, the long long type, the bool type, compound literals, designated initializers, and so on and so forth. This option does not care about the standards mode you're in. Also, the warnings get disabled inside the expression that follows __extension__. 💅
  • -Wc99-c11-compat (C and Objective-C only) — same vibe, next level: warns about stuff not in ISO C99 but present in ISO C11. We're talking anonymous structures and unions, the _Atomic type qualifier, the _Thread_local storage-class specifier, the _Alignas specifier, the Alignof operator, the _Generic keyword, etc. Independent of standards mode, and again, warnings are disabled in the expression following __extension__. ✨
  • -Wc11-c23-compat (C and Objective-C only)
  • -Wc11-c2x-compat (C and Objective-C only) — warns about things that aren't in ISO C11 but are in ISO C23. For example: omitting the string in _Static_assert, using [[]] syntax for attributes, using decimal floating-point types, you get it. 🎀 Independent of standards mode. Warnings disabled in the expression after __extension__. Also, heads up: the name -Wc11-c2x-compat is deprecated. 💀

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

  • -Wc23-c2y-compat (C and Objective-C only)
  • -Wc23-c2y-compat (C and Objective-C only) — (yes, it's listed twice in the original, bestie, I don't make the rules 💅) warns about features not in ISO C23 but present in ISO C2Y. Example: _Generic selecting with a type name instead of an expression. Independent of standards mode, and warnings are disabled in the expression that follows __extension__.

When you're not compiling in C2Y mode, -pedantic-errors upgrades these warnings to errors. 🔥

  • -Wc++-compat (C and Objective-C only) — warns about ISO C constructs that live outside the common subset of ISO C and ISO C++, like requesting an implicit conversion from void * to a pointer to a non-void type. Lowkey a vibe check between the two languages. 🦋
  • -Wc++11-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning shifted between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers that were fine in ISO C++ 1998 but became keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall. ✨
  • -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. Enabled by -Wall. 👑
  • -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. Enabled by -Wall. 🌸
  • -Wc++20-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by -Wall. 💖
  • -Wc++26-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2023 and the upcoming ISO C++ 2026. Enabled by -Wall. ⭐
  • -Wno-c++11-extensions (C++ and Objective-C++ only) — stops warning about C++11 constructs when you're compiling with an older C++ standard. But ngl, even without this option, some C++11 constructs only get diagnosed if -Wpedantic is in play. 🫶
  • -Wno-c++14-extensions (C++ and Objective-C++ only) — stops warning about C++14 constructs in code compiled with an older C++ standard. Same deal: some C++14 constructs only get diagnosed with -Wpedantic. 🫶
  • -Wno-c++17-extensions (C++ and Objective-C++ only) — stops warning about C++17 constructs under an older C++ standard. And yeah, some C++17 constructs only show up with -Wpedantic. 🫶
  • -Wno-c++20-extensions (C++ and Objective-C++ only) — stops warning about C++20 constructs under an older C++ standard. Same energy: some C++20 constructs only get diagnosed via -Wpedantic. 🫶
  • -Wno-c++23-extensions (C++ and Objective-C++ only) — stops warning about C++23 constructs under an older C++ standard. And yep, some C++23 constructs only get diagnosed if -Wpedantic is used. 🫶
  • -Wno-c++26-extensions (C++ and Objective-C++ only) — stops warning about C++26 constructs under an older C++ standard. Some C++26 constructs? Only diagnosed with -Wpedantic. 🫶
  • -Wno-c++29-extensions (C++ and Objective-C++ only) — stops warning about C++29 constructs under an older C++ standard. And, once again, some C++29 constructs only get diagnosed when -Wpedantic is used. 🫶
  • -Wcast-qual — warns whenever a pointer gets cast in a way that strips a type qualifier off the target type. For example, warn if a const char * is cast to an ordinary char *. 🚩

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

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

Not the vibes. 💀

  • -Wcast-align — warns whenever a pointer is cast such that the required alignment of the target is increased. Example: warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries.

Warn whenever a pointer is cast such that the required alignment of the target is increased. Example: warn if a char * is cast to an int * regardless of the target machine. (Yes bestie, both of those are in the source, and both are staying. 💅)

  • -Wcast-function-type — warns when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list, only the types of the initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types get ignored, like int vs. long on ILP32 targets. Type qualifiers? Also ignored. 🦋 The function type void (*) (void) is special and matches everything, which means you can use it to suppress this warning. And in a cast involving pointer to member types, this warning fires whenever the type cast changes the pointer to member type. Enabled by -Wextra.
  • -Wcast-user-defined — warns when a cast to reference type doesn't involve a user-defined conversion that the programmer might expect to be called. 👀
struct A { operator const int&(); } a;
auto r = (int&)a; // warning

This warning is on 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 a nuisance. That's literally why -Wall doesn't ask for these warnings. 😭

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

In C++11 mode or later, this warning gets upgraded to an error by -pedantic-errors. 🚨

  • -Wclobbered — warns for variables that might get changed by longjmp or vfork. Also enabled by -Wextra. 🙈
  • -Wno-complain-wrong-lang — okay so by default, language front ends complain when a command-line option is valid but not applicable to that front end. You can disable that 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, like:
$ g++ -fno-rtti a.cc b.f90

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. That latter front end diagnoses:

f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran

…which may be disabled with -Wno-complain-wrong-lang. 🎀

This option can also be used to disable warnings like:

cc1plus: note: CTF debug info requested, but not supported for 'GNU C++20' frontend

…which get produced by -gctf or -gsctf for unsupported languages. 💅

  • -Wcompare-distinct-pointer-types (C and Objective-C only) — warns if pointers of distinct types get compared without a cast. This warning is on by default. ⭐
  • -Wconversion — bestie, this one yells at you for implicit conversions that might mess with your value 💅. That includes conversions between real and integer types (like abs (x) when x is double), conversions between signed and unsigned (like unsigned ui = -1), and conversions into smaller types (like sqrtf (M_PI)). But it's chill about explicit casts like abs ((int) x) and ui = (unsigned) -1, and it also stays quiet if the value literally doesn't change through the conversion, like in abs (2.0). Oh and those warnings about signed↔unsigned integer conversions? You can turn them off with -Wno-sign-conversion ✨.

For C++, it ALSO warns about confusing overload resolution for user-defined conversions, plus conversions that never even use a type conversion operator: conversions to void, the same type, a base class, or a reference to them. And heads up — warnings about conversions between signed and unsigned integers are disabled by default in C++ unless you explicitly enable -Wsign-conversion.

One more thing: warnings about conversion from arithmetic on a small type back to that type? Those only show up with -Warith-conversion 💀.

  • -Wdangling-else — okay so this warns about constructions where you could get confused about which if statement an else branch belongs to. Here's the example, and yes it's exactly as cursed as it looks:
{
  if (a)
    if (b)
      foo ();
  else
    bar ();
}

In C/C++, every else branch attaches to the innermost possible if statement — which in this example is if (b). That's usually not what the programmer was picturing, as the indentation up there so tragically illustrates 😭. When there's potential for this confusion, GCC throws a warning if you specify this flag. To kill the warning, add explicit braces around the innermost if statement so there's no way the else can belong to the enclosing if. Like this:

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

This warning is enabled by -Wparentheses. 🫶

  • -Wdangling-pointer
  • -Wdangling-pointer=n — warns about using pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended 👻. This includes local variables declared in nested blocks, compound literals, and other unnamed temporary objects. On top of that, it warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels, but word to the wise: it may yield different results with optimization than without 🔥.
    • -Wdangling-pointer=1 — at level 1, the warning only diagnoses unconditional uses of dangling pointers.
    • -Wdangling-pointer=2 — at level 2, on top of unconditional uses, it 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 both do the same thing: disable the warnings entirely. And -Wdangling-pointer=2 is included in -Wall ✨.

This example triggers the warning at level 1, because 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 into the escaped pointer *p triggers the warning at level 1:

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

And 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 — warns when the macros __TIME__, __DATE__ or __TIMESTAMP__ show up, because they might prevent bitwise-identical reproducible compilations ⏰. Reproducibility queen behavior, honestly.
  • -Wempty-body — warns if an empty body shows up in an if, else or do while statement. This warning is also enabled by -Wextra 🍓.
  • -Wno-endif-labels — don't warn about stray tokens sitting after #else and #endif.
  • -Wenum-compare — warns about comparisons 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, it's enabled by -Wall 💜.
  • -Wenum-conversion — warns when a value of an enumerated type gets implicitly converted to a different enumerated type. In C, this warning is enabled by -Wextra.
  • -Wenum-int-mismatch (C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. Example time:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);

In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. But since the choice of the underlying type of an enumerated type is implementation-defined, mismatches like this 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. It only warns about variables that are initialized when they're declared, fyi. And 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. You can turn it off 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 gets converted to unsigned 😭. In C++, this warning is also enabled by -Wall. In C, it's also enabled by -Wextra.
  • -Wsign-conversion — warns for implicit conversions that may change the sign of an integer value, like assigning a signed integer expression to an unsigned integer variable. An explicit cast silences the warning 💅. 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. So like, it warns about this:
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. That covers 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 — don't 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 🤨. Because in that case, the computation won't give you the number of elements in the array, which is probably what everyone wanted. It warns about stuff like:
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 simply will not work out correctly with pointers, bestie 💀. It warns about things like sizeof (ptr) / sizeof (ptr[0]) if ptr is not an array, but a pointer. This warning is enabled by -Wall.
  • -Wsizeof-pointer-memaccess — warns for suspicious length parameters passed to certain string and memory built-in functions when the argument uses sizeof. It triggers for example on memset (ptr, 0, sizeof (ptr)); if ptr is not an array, but a pointer, and it even suggests a possible fix 🎀. Also for 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. 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, so the call gets diagnosed (as it SHOULD 😤):

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 — don't 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: specifically when the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory 😱. That right there indicates the user forgot to multiply by the element size. This warning is enabled by -Wall.
  • -Wmemset-transposed-args — bestie, this one's a lil snitch for sus memset calls 💅 it yells when the second arg isn't zero and the third arg is zero. Like, memset (buf, sizeof buf, 0) gets called out because you obviously meant memset (buf, 0, sizeof buf) and just... flipped them 🙈. BUT — and this is the important part — the diagnostic only fires if that third argument is a literal zero. If it's an expression that folds down to zero, or a cast of zero to some type, it's wayyy less likely the args were accidentally swapped, so no warning, okay? Enabled by -Wall. ✨
  • -Waddress — side-eyes suspicious uses of address expressions 👀 this includes comparing the address of a function or a declared object to the null pointer constant, like:
void f (void);
void g (void)
{
  if (!f)   // warning: expression evaluates to false
    abort ();
}

comparisons of a pointer to a string literal, like:

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

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

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

Uses like these pretty much always mean a programmer oopsie: the address of most functions and objects necessarily evaluates to true (the exception is weak symbols 😭 so keep that in mind), so using one in a conditional might mean you forgot parentheses in a function call or forgot to dereference in an array expression. The object-pointer subset of this warning can be hushed by casting the pointer operand to an integer type such as intptr_t or uintptr_t. And heads up — comparisons against string literals result in unspecified behavior and are not portable, and they suggest you meant to call strcmp instead. The warning is suppressed if the suspicious expression is the result 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 gives you an unaligned pointer value. This is enabled by default. 🌸
  • -Wlogical-op — sniffs out suspicious uses of logical operators in expressions. That means logical operators showing up in contexts where a bitwise operator is probably what you wanted, plus it also warns when the operands of a logical operator are literally the same:
extern int a;
if (a < 0 && a < 0) { ... }

like girl why 😭

  • -Wconstant-logical-operand — catches another flavor of sus logical-operator vibes: when neither operand is boolean, and one of the operands is (or folds into) a constant other than 0 or 1, or an enumerator with value 1 if the enumeral type contains enumerators with values other than 0 or 1. In that case the warning will suggest using the corresponding bitwise operator instead. Example:
extern int a;
if (a && 64) { ... }

And if the warning is a false positive, you can clarify the code by using e.g. a && (64 != 0) instead. 💅

  • -Wlogical-not-parentheses — warns about a logical not used on the left hand side operand of a comparison. It does not warn if the right operand is considered to be a boolean expression. Its whole purpose is detecting suspicious code like this:
int a;
...
if (!a > 1) { ... }

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

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

This warning is enabled by -Wall. ✨

  • -Waggregate-return — warns if any functions that return structures or unions are defined or called. (And in languages where you can return an array, that also elicits a warning.) 👑
  • -Wno-aggressive-loop-optimizations — do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations. -Waggressive-loop-optimizations is enabled by default. 🦋
  • -Wno-attributes — do not warn if an unexpected __attribute__ is used, like unrecognized attributes, function attributes applied to variables, etc. Note this does not stop errors for incorrect use of supported attributes. Warnings about ill-formed uses of standard attributes are upgraded to errors by -pedantic-errors. 💀

Also — and this is a cute lil bonus — using -Wno-attributes=, it's possible to suppress warnings about unknown scoped attributes (in C++11 and C23). For example, -Wno-attributes=vendor::attr disables warning about the following declaration:

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

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

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

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

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

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

extern void* memset ();
void f (void *d)
{
  memset (d, '\0', 32);
}
  • -Wno-builtin-macro-redefined — do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__. ✨
  • -Wkeyword-macro — warns if a keyword is defined as a macro or undefined. For C++, identifiers with special meaning or standard attribute identifiers are diagnosed as well. This warning is enabled by default for C++26 if -Wpedantic and emits a pedwarn in that case. 👑
  • -Wfree-labels (C and Objective-C only) — warns if a label is applied to a non-statement, or occurs at the end of a compound statement. Such labels are allowed by C23 and later dialects of C, and they're 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 when a valid preprocessor header multiple inclusion guard has a #define directive right after the #ifndef or #if !defined directive for the multiple inclusion guard, which defines a different macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding #ifndef directive at the end of the header, and the #define directive defines an object-like macro with empty definition. In that case, it often is just a misspelled guard name, either in the #ifndef or #if !defined directive or in the subsequent #define directive. This warning is enabled by -Wall. 💅
  • -Wstrict-prototypes (C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if it's preceded by a declaration that specifies the argument types.) 🌸
  • -Wold-style-declaration (C and Objective-C only) — warns for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class specifiers like static are not the first things in a declaration. This warning is also enabled by -Wextra. 🦋
  • -Wold-style-definition (C and Objective-C only) — warns if an old-style function definition is used. A warning is given even if there's a previous prototype. A definition using () is not considered an old-style definition in C23 mode, because it is equivalent to (void) in that case — but it is considered an old-style definition for older standards. 🙈
  • -Wmultiple-parameter-fwd-decl-lists (C and Objective-C only) — warns if more than one list of forward declarations of parameters appears in a function prototype. This warning is also enabled by -Wextra. ✨
  • -Wdeprecated-non-prototype (C and Objective-C only) — warns if a function declared with an empty parameter list () is called with one or more arguments, or if a function definition with one or more parameters is encountered after such a declaration. Both of those cases are errors in C23 and later dialects of C. This warning is also enabled by -Wc11-c23-compat. 💀
  • -Wmissing-parameter-name (C and Objective-C only) — warns if a function definition omits a parameter name, specifying only its type. This can be used to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects. This warning is also enabled by -Wc11-c23-compat. It gets turned into an error if building for a C version before C23 by -pedantic-errors. 😭
  • -Wmissing-parameter-type (C and Objective-C only) — fires 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. Heads up though: such function declarations do not provide a function prototype and prevent most type checking in function calls 👀. This warning is enabled by default. In C99 and later dialects of C, it's treated as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=declaration-missing-parameter-type. This warning is upgraded to an error by -pedantic-errors. 💅
  • -Wmissing-prototypes (C and Objective-C only, bestie, soz C++ girlies) — warns you when a global function gets defined without a prototype declaration showing up first. And yes, it still yells at you even if the definition itself hands you a prototype. The whole point of this one is catching global functions that don't have a matching prototype declaration chilling in a header file somewhere. Not valid for C++ though, because in C++ every function declaration provides a prototype, and a non-matching declaration is just an overload, not a beef with an earlier declaration. Want to catch missing declarations in C++? Use -Wmissing-declarations.
  • -Wmissing-variable-declarations (C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use it to hunt down global variables that don't have a matching extern declaration in a header file.
  • -Wmissing-declarations — warns if a global function is defined without a previous declaration. Yes, even if the definition itself provides a prototype. Use it to detect global functions that are not declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations — use -Wmissing-prototypes for that missing-prototype detective work. And in C++, no warnings get issued for function templates, or for inline functions, or for functions in anonymous namespaces. She's picky like that. 💅
  • -Wmissing-field-initializers — warns when a structure's initializer is missing some fields. Like, this code below triggers the warning because x.h is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };

In C, this option does not warn about designated initializers, so this lil modification is totally fine:

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

Also in C, it does not warn about the universal zero initializer { 0 }:

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

Same energy in C++ — it does not warn about the empty { } initializer:

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

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

  • -Wno-missing-requires — so by default the compiler warns about a concept-id showing up as a C++20 simple-requirement:
bool satisfied = requires { C<T> };

Here satisfied is true if C<T> is a valid expression, which, bestie, it is for all T. The user presumably meant to write:

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

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

  • -Wno-missing-template-keyword — the member access tokens ., -> and :: have to be followed by the template keyword when the parent object is dependent and the member you're naming 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 can get false positives 😭. To silence those, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:

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

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

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

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

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

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

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

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

  • -Wno-attribute-warning — do not warn about usage of functions declared with the "warning" attribute. By default this warning is enabled. -Wno-attribute-warning can be used to disable the warning, or -Wno-error=attribute-warning can be used to disable the error when compiled with the -Werror flag.
  • -Wno-deprecated — do not warn about usage of deprecated features.

In C++, explicitly specifying -Wdeprecated also enables warnings about some features that are deprecated in later language standards, specifically -Wcomma-subscript, -Wvolatile, -Wdeprecated-enum-float-conversion, -Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and -Wdeprecated-variadic-comma-omission.

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

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

  • -Wno-override-init-side-effects (C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.
  • -Wpacked — warns when a structure is given the packed attribute, but that packed attribute has zero effect on the layout or size of the structure. Such structures may be misaligned for basically no benefit 🙈. For instance, in this code, the variable "f.x" in "struct bar" is misaligned even though "struct bar" doesn't itself have the packed attribute:
struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};
  • -Wnopacked-bitfield-compat — the 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This was fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC lets you know when the offset of such a field has changed in GCC 4.4. For example, there's no longer a 4-bit padding between field "a" and "b" in this structure:
struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

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

  • -Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only) — warn if a structure field with explicitly specified alignment inside a packed struct or union is misaligned. For example, a warning gets issued on "struct S", like, warning: alignment 1 of 'struct S' is less than 8, in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

This warning is enabled by -Wall.

  • -Wpadded — warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes, when this happens, you can rearrange the fields of the structure to reduce the padding and make the whole structure smaller. ✨
  • -Wredundant-decls — warn if anything is declared more than once in the same scope, even in cases where the multiple declaration is valid and changes literally nothing.
  • -Wrestrict — okay so bestie, this one is giving “your pointers are literally overlapping and it’s not cute” energy 💅. It 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. Like, for example, the call to the strcpy function below tries to truncate the string by replacing its initial characters with the last four. But because the call writes the terminating NUL into a[4], the copies overlap and the call is diagnosed. So messy 😭.
void foo (void)
{
  char a[] = "abcd1234";
  strcpy (a, a + 4);
  ...
}

The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2 and above. It is included in -Wall. So turn it on, bestie, it’s free real estate ✨.

  • -Wnested-externs (C and Objective-C only) — warns if an extern declaration is encountered within a function. Like, why is that extern in the function? It’s giving claustrophobic 🫣. Just don’t.
  • -Winline — warns if a function that is declared as inline cannot be inlined. But even with this option, the compiler does not warn about failures to inline functions declared in system headers. The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, it takes into account the size of the function being inlined and the amount of inlining that has already been done in the current function. Therefore, seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear. So lowkey unpredictable, highkey useful 🔥.
  • -Winterference-size, -Wno-interference-size — warns about questionable uses of the C++17 constexpr variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. These variables are intended to be used for controlling class layout, to avoid false sharing in concurrent code. Like this:
struct independent_fields {
  alignas(std::hardware_destructive_interference_size)
    std::atomic<int> one;
  alignas(std::hardware_destructive_interference_size)
    std::atomic<int> two;
};

Here one and two are intended to be far enough apart that stores to one won’t require accesses to the other to reload the cache line. By default, these variables are given values based on the current -mtune option, typically to the L1 cache line size for the particular target CPU, sometimes to a range if tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu). If ABI stability is important, such as if the use is in a header for a library, you should probably not use the hardware interference size variables at all. These warnings are enabled by default. If you are confident that your use of these variables does not affect ABI outside a single build of your project, you can turn off the warning with -Wno-interference-size. So yeah, know your vibe 🧚.

  • -Wint-in-bool-context — warns for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:) using non-boolean integer constants in boolean context, like if (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, like for (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by -Wall. So it’s like, bestie, why are you using an int as a bool? That’s not the move 💀.
  • -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. So if you’re doing some sketchy casting, this will quiet the alarm 🙈.
  • -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. Same energy, different direction 🌀.
  • -Winvalid-pch — warns if a precompiled header is found in the search path but cannot be used. Like, you had one job, PCH. One job 😭.
  • -Winvalid-utf8 — warns if an invalid UTF-8 character is found. This warning is on by default for C++23 if -finput-charset=UTF-8 is used and turned into error with -pedantic-errors. So it’s giving strict UTF-8 vibes 🌈.
  • -Wno-unicode — don’t diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens. Wunicode is enabled by default. So if you’re not about that unicode life, you can turn it off 💜.
  • -Wlong-long — warns if long long type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit the warning messages, use -Wno-long-long. This warning is upgraded to an error by -pedantic-errors. So long long is the problematic fave 👑.
  • -Wvariadic-macros — warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros. So macros with ... are on thin ice in old standards 🧊.
  • -Wno-varargs — do not warn upon questionable usage of the macros used to handle variable arguments like va_start. These warnings are enabled by default. So if you’re doing varargs shenanigans, this makes it chill 😌.
  • -Wvector-operation-performance — warns if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented piecewise, which means that the scalar operation is performed on every vector element; in parallel, which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and as a single scalar, which means that vector fits into a scalar type. So it’s like, are you even using SIMD, bestie? ⚡
  • -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. So VLAs are giving risky 🎀.
  • *-Wvla-larger-than=byte-size* — if this option is used, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. This is similar to how -Walloca-larger-than=byte-size works, but with variable-length arrays. Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. -Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above). See also -Walloca-larger-than=byte-size. So if your VLA is huge, you’ll hear about it 📢.
  • -Wno-vla-larger-than — disable -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than=SIZE_MAX or larger. So basically turning it off completely 🚫.
  • -Wvla-parameter — warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds. If the first function declaration uses the 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. So it’s like, keep your VLA parameters consistent, bestie 🦋.

  • -Wvolatile-register-var — warns if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by -Wall. So declaring a register variable volatile is giving confused 🌸.
  • -Wno-xor-used-as-pow (C, C++, Objective-C and Objective-C++ only) — disable warnings about uses of ^, the exclusive or operator, where it appears the code meant exponentiation. Specifically, the warning occurs when the left-hand side is the decimal constant 2 or 10 and the right-hand side is also a decimal constant. In C and C++, ^ means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. This warning can be silenced by converting one of the operands to hexadecimal as well as by compiling with -Wno-xor-used-as-pow. So if you’re using 2 ^ 3 thinking it’s 8, that’s a no from GCC 🙅‍♀️.
  • -Wdisabled-optimization — warns if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC’s optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. So it’s like, “bestie, I tried, but your code is doing too much” 😮‍💨.
  • -Wpointer-sign (C and Objective-C only) — warns for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign. This warning is upgraded to an error by -pedantic-errors. So signed vs unsigned pointers? Not the vibe 🌈.
  • -Wstack-protector — this option is only active when -fstack-protector is active. It warns about functions that are not protected against stack smashing. So if you’re not using stack protector, you won’t get these warnings. Security check, bestie 🔒.
  • -Woverlength-strings — warns about string constants that are longer than the “minimum maximum” length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard’s minimum limit, but very portable programs should avoid using longer strings. The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++. So if your string is extra long, it might be a problem for portability 📏.

This option is implied by -Wpedantic, and you can turn it off with -Wno-overlength-strings. Like, it comes free with the pedantic package, bestie. 🎀

  • -Wunsuffixed-float-constants (C and Objective-C only) — yells at any floating constant that forgot its suffix. And if you pair it with -Wsystem-headers, it will also snitch on those constants hiding inside system header files. Genuinely useful when you're prepping code to use the "FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99. 💅
  • -Wno-lto-type-mismatch — during link-time optimization, don't warn about type mismatches in global declarations coming from different compilation units. Needs -flto to actually be on. And heads up: it's enabled by default. 👑
  • -Wno-designated-init (C and Objective-C only) — shuts up the warnings you get when a positional initializer is used to initialize a structure that's been marked with the "designated_init" attribute.
  • -Wzero-init-padding-bits=value — warns about automatic variable initializers that might not zero-initialize padding bits.

Some languages promise zero initialization of padding bits in certain cases. The "-fzero-init-padding-bits=unions" and "-fzero-init-padding-bits=all" options hand you extra guarantees that padding bits get zero-initialized in other circumstances too.

This warning exists mainly so you can hunt down source code that might need to be edited, or else recompiled with "-fzero-init-padding-bits=unions" or "-fzero-init-padding-bits=all", in order to keep the behavior it had when it was compiled by versions of GCC older than 15.1. You can also use it to find code that might end up with uninitialized struct padding unless GCC is invoked with "-fzero-init-padding-bits=all" — yes, even though older GCC versions wouldn't have guaranteed initialization of struct padding either. 🙈

Okay so real talk: it does not follow that all source code producing warnings about uninitialized padding bits has undefined behavior. Usually the values of padding bits have zero effect on how a program actually executes. Don't panic. 🫶

The two options interact like this:

  • -Wzero-init-padding-bits=standard → X against f:standard, X against f:unions, X against f:all
  • -Wzero-init-padding-bits=unions → U against f:standard, X against f:unions, X against f:all
  • -Wzero-init-padding-bits=all → A against f:standard, S against f:unions, X against f:all

And the letter decoder ring:

  • X — No warnings about padding bits.
  • U — Warnings about padding bits of unions.
  • S — Warnings about padding bits of structs.
  • A — Warnings about padding bits of structs and unions.

Here's what union initialization looks like for ISO C23 with "-fzero-init-padding-bits=standard" and "-Wzero-init-padding-bits=unions":

          long long q;
          unsigned char r;
          union U { unsigned char a; long long b; };
          struct F { long long a; union U b; };

          // Padding bits are explicitly initialized
          union U h = {};

          // Padding bits might not be initialized
          union U i = { r };

          // Padding bits might not be initialized
          union U j = { .a = r };

          // Union is expected to have no padding bits
          union U k = { .b = q };

          // Padding bits (of union) are explicitly initialized
          struct F l = { q, {}, .b.a = r };

          // Padding bits (of union) might not be initialized
          struct F m = { q, {}, .b = {.a = r} };

So in the declaration of "m", an empty initializer that would normally guarantee zero initialization of the union's padding bits — like, the storage allocated for member "b" that isn't part of member "a" — gets overridden by an initializer listed after it. That's an anti-pattern, bestie. Not the vibe. 💀

Whether a given initializer triggers a warning can be predicted for simple cases, but in general it's target-dependent, because the layout of storage is target-dependent. Also: the level of optimization, the size, and the initial value of the object being initialized all affect whether warnings show up. ✨

✨ Options That Control Static Analysis ✨

  • -fanalyzer — this turns on a static analysis of program flow that goes looking for "interesting" interprocedural paths through the code, and drops warnings for problems it finds along the way.

Heads up though: this analysis is much more expensive than other GCC warnings. 😭

In technical terms, it performs coverage-guided symbolic execution of the code being compiled. It is neither sound nor complete: it can have false positives AND false negatives. It's a bug-finding tool, not a tool for proving your program is correct. Periodt.

The analyzer is only suitable for use on C code in this release.

Enabling this option effectively enables the following warnings — here's the whole starter pack:

-Wanalyzer-allocation-size                  -Wanalyzer-deref-before-check
-Wanalyzer-div-by-zero                      -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-mkostemp-redundant-flags         -Wanalyzer-mktemp-missing-placeholder
-Wanalyzer-mktemp-of-string-literal         -Wanalyzer-null-argument
-Wanalyzer-null-dereference                 -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers              -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference        -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative             -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer              -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index              -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor                  -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size                     -Wanalyzer-throw-of-unexpected-type
-Wanalyzer-undefined-behavior-ptrdiff       -Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch             -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak                     -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const                   -Wanalyzer-write-to-string-literal
This option is only available if GCC was configured with analyzer support enabled. No analyzer support, no vibe check. 🧚
  • -Wanalyzer-symbol-too-complex — if -fanalyzer is enabled, the analyzer leans on various heuristics to try and track the state of memory, but sufficiently complicated code can absolutely defeat those heuristics. By default, the analysis silently stops tracking the values of expressions once they blow past an internal limit, and falls back to an imprecise representation for those expressions. This option warns you when that happens. 👀
  • -Wanalyzer-too-complex — again, if -fanalyzer is enabled, it uses heuristics to try to explore the control flow and data flow in the program, and again, sufficiently complicated code can beat them. By default the analysis just silently stops if the code is too complicated for the analyzer to fully explore and it hits an internal limit. This option warns you when that happens.
  • -Wno-analyzer-allocation-size — this warning requires -fanalyzer, which enables it; to disable it, use -Wno-analyzer-allocation-size.

This diagnostic warns for paths through the code where a pointer to a buffer is assigned to point at a buffer whose size is not a multiple of "sizeof (*pointer)".

See CWE-131: Incorrect Calculation of Buffer Size ("https://cwe.mitre.org/data/definitions/131.html"). 🍓

  • -Wno-analyzer-div-by-zero — this warning requires -fanalyzer, which enables it; to disable it, use -Wno-analyzer-div-by-zero.

This diagnostic warns for paths through the code that attempt integer division by zero. It's analogous to -Wdiv-by-zero, but it's implemented in a different way. Same energy, different mechanism. 💅

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

  • -Wno-analyzer-deref-before-check — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-deref-before-check to disable it.

This diagnostic warns for paths through the code where a pointer gets checked for "NULL" after it's already been dereferenced — which strongly suggests the pointer could have been NULL the whole time. 😭 Cases like that mean the NULL check is either redundant, or it needs to be moved to before the pointer is dereferenced.

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

Ngl this one is unlikely to fire when any level of optimization is enabled, because GCC's optimization logic typically considers those NULL checks redundant and optimizes them away before the analyzer ever "sees" them. So if you're trying to trigger this diagnostic: disable optimization. 🌸
  • -Wno-analyzer-double-fclose — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it.

This diagnostic warns for paths through the code where "fclose" can be called on a "FILE *" more than once.

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

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

This diagnostic warns for paths through the code where a deallocator can be called on a pointer more than once — either "free", or a deallocator referenced by attribute "malloc".

See CWE-415: Double Free ("https://cwe.mitre.org/data/definitions/415.html"). 💀

  • -Wno-analyzer-exposure-through-output-file — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns for paths through the code where a security-sensitive value gets written to an output file — like, the classic "oops I wrote a password into the log file" situation. Do not do this. 🚨

…and the lil receipt for the one before it: it points you at See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html). bestie look it up, it's a whole thing 💀

  • -Wanalyzer-exposure-through-uninit-copy — heads up, this warning needs BOTH -fanalyzer AND a plugin to specify a function that copies across a "trust boundary" 🌈. Wanna turn it off? -Wno-analyzer-exposure-through-uninit-copy. periodt.

So what's it actually catching? "infoleaks" ✨ — code paths where uninitialized values get copied across a security boundary. The classic example they give: code inside an OS kernel copying a partially-initialized struct on the stack into user space. like. no ma'am. 🙈

Also: 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 — needs -fanalyzer, which is what turns it on in the first place; using -Wno-analyzer-fd-access-mode-mismatch shuts it back off 💅.

This one snitches on code paths where a "read" gets attempted on a write-only file descriptor, or the other way around (a write on a read-only fd). iconic behavior, terrible code.

It also flags paths where a function with attribute fd_arg_read (N) gets called with a file descriptor that was opened with O_WRONLY at referenced argument N 💀 — or a function with attribute fd_arg_write (N) gets called with a file descriptor opened with O_RDONLY at referenced argument N.

  • -Wno-analyzer-fd-double-close — needs -fanalyzer, which enables it; hit it with -Wno-analyzer-fd-double-close to disable 🔥.

Catches paths where a file descriptor can get closed more than once. girl. once is enough 🫶

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

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

Catches paths where an open file descriptor just… gets leaked. left on read, as it were 💔

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 — needs -fanalyzer, which enables it; -Wno-analyzer-fd-phase-mismatch disables it 🎀.

Flags paths where an operation gets attempted in the wrong phase of a file descriptor's lifetime. The example they give is chef's kiss: it'll warn on attempts to call accept on a stream socket that hasn't had listen successfully called on it yet. like, bestie, you have to listen first 😭

See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).

  • -Wno-analyzer-fd-type-mismatch — needs -fanalyzer, which enables it; -Wno-analyzer-fd-type-mismatch disables it ✨.

Warns on paths where an operation is attempted on the wrong type of file descriptor. For example it'll yell if you try to use socket operations on a file descriptor you got via open, or if you try to use a stream socket operation on a datagram socket. wrong vibe, wrong fd 🚫

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

Catches paths where a read or write gets called on a closed file descriptor. the audacity 🫠

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

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

Catches paths where a file descriptor gets used without ever being checked for validity. like walking into a room without knocking. rude.

Also flags paths where a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor, at referenced argument N, without it being checked for validity.

  • -Wno-analyzer-file-leak — needs -fanalyzer, which enables it; -Wno-analyzer-file-leak disables it 🦋.

Flags paths through the code where a <stdio.h> FILE * stream object is leaked. that stream wanted to be closed, bestie 😭

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

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

Catches paths where free gets called on a non-heap pointer — think an on-stack buffer, or a global. absolutely not 💀

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

  • -Wno-analyzer-imprecise-fp-arithmetic — needs -fanalyzer, which enables it; -Wno-analyzer-imprecise-fp-arithmetic disables it ⭐.

Flags paths where floating-point arithmetic is used in places where you actually need precise computation 🎀. as of right now, it only warns on floating-point operands being used inside the calculation of an allocation size. so. scope is scoped.

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

Flags paths through the code that appear to lead into an infinite loop. and it's specific about what that means — the analyzer fires this warning when it "sees" a loop where:

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

One way this warning gets emitted: an execution path through a loop where taking the path on one iteration implies the exact same path gets taken on every subsequent iteration. cursed repetition 💀

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 complains about the above because if opcode ever matches none of the cases, the switch follows the implicit default case, which makes the body of the loop a "no-op" with cpu_state.pc left unchanged — so the same value of opcode gets used on all subsequent iterations, and boom, infinite loop. trapped forever, no thoughts head empty 🌀

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

  • -Wno-analyzer-infinite-recursion — needs -fanalyzer, which enables it; -Wno-analyzer-infinite-recursion disables it 💅.

Flags paths through the code that appear to lead to infinite recursion. so how does it even know? here's the tea: when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame with the state at the entry to the previous frame of that function on the stack. The warning fires if nothing in memory appears to be changing. any changes it observes to parameters or globals are assumed to eventually lead to the recursion terminating, so those shut the warning up 🔇

lowkey important: this diagnostic is likely to miss cases of infinite recursion that get converted into iteration by the optimizer before the analyzer "sees" them. so if you're trying to trigger this diagnostic, optimization should be disabled. no cap.

Compare it with -Winfinite-recursion, which gives you 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 — needs -fanalyzer, which enables it; -Wno-analyzer-jump-through-null disables it 🌈.

Catches paths through the code where a NULL function pointer gets called. literally calling nothing. the void replied 🙈

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

Flags paths where a pointer allocated via an allocator gets leaked — and by allocator they mean either malloc, or a function marked with attribute malloc. it was supposed to be freed, bestie 😭

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

  • -Wno-analyzer-mismatching-deallocation — needs -fanalyzer, which enables it; -Wno-analyzer-mismatching-deallocation disables it 🎀.

Flags paths where the wrong deallocation function gets called on a pointer value, judged by which function was used to allocate that pointer value in the first place. it'll warn about mismatches between free, scalar delete and vector delete[], plus anything marked as allocator/deallocator pairs using attribute malloc. you can't just mix and match, this isn't a build-your-own-allocator bar 🍓

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

  • -Wno-analyzer-mkostemp-redundant-flags — needs -fanalyzer, which enables it; -Wno-analyzer-mkostemp-redundant-flags disables it 🔥.

Flags paths where mkostemp or mkostemps is called with flags that include O_RDWR, O_CREAT, or O_EXCL. These flags are already implied by the function, so specifying them is unnecessary — and it produces errors on some systems 💀. extra for no reason, periodt.

  • -Wno-analyzer-mktemp-missing-placeholder — needs -fanalyzer, which enables it; -Wno-analyzer-mktemp-missing-placeholder disables it ✨.

Flags paths where a function in the mktemp family — that's mkstemp, mkostemp, mkstemps, mkostemps, mkdtemp, mktemp — gets called with a template string that doesn't contain the placeholder XXXXXX at the expected position. the placeholder is not optional, bestie, it's the whole point 👑

  • -Wno-analyzer-mktemp-of-string-literal — needs -fanalyzer, which enables it; -Wno-analyzer-mktemp-of-string-literal disables it 🦋.

This diagnostic warns for paths through the code in which a function in the mktemp family — that's mkstemp, mkostemp, mkstemps, mkostemps, mkdtemp, mktemp — gets called on a string literal. 💀 And bestie, that's a whole problem, because these functions modify their argument in place, so passing them a string literal leads directly to undefined behavior.

See STR30-C. Do not attempt to modify string literals (https://wiki.sei.cmu.edu/confluence/x/VtYxBQ).

  • -Wno-analyzer-out-of-bounds — okay so this warning needs -fanalyzer, which is what turns it on in the first place; use -Wno-analyzer-out-of-bounds to switch it back off. 🫶

This diagnostic warns for paths through the code in which a buffer is definitely read or written out-of-bounds. ✨ It applies to cases where the analyzer can figure out a constant offset, and to accesses past the end of a buffer, also with a constant capacity. On top of that, it does some limited checking for accesses past the end when both the offset and the capacity are symbolic.

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

For cases where the analyzer is able, it will emit a text art diagram visualising the spatial relationship between the memory region that the analyzer predicts would be accessed, versus the range of memory that is actually valid to access: whether they overlap, are touching, are close or far apart; which one comes before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of the data involved. 🤯 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 get passed to an API whose behavior on such buffers is undefined.

Specifically, the diagnostic fires 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 gets passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.

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

  • -Wno-analyzer-possible-null-dereference — 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 gets dereferenced. 😭

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

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

This diagnostic warns for paths through the code in which a value known to be NULL gets passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.

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

  • -Wno-analyzer-null-dereference — 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 gets dereferenced.

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

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

This diagnostic warns for paths through the code in which a call to putenv gets passed a pointer to an automatic variable or an on-stack buffer. 🌸

See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument (https://wiki.sei.cmu.edu/confluence/x/6NYxBQ).

  • -Wno-analyzer-shift-count-negative — 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's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but it's implemented based on analyzing interprocedural paths, rather than just parsing the syntax tree. The catch, bestie: the analyzer does not prioritise detection of such paths, so false negatives are more likely relative to other warnings. 🙈

  • -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's analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but again it's implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. And same deal — the analyzer doesn't prioritise detection of such paths, so false negatives are more likely relative to other warnings. 💅

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

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

When setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid the moment the function containing the setjmp call returns. Attempting to rewind to it via longjmp would reference a stack frame that no longer exists, and likely lead to a crash (or worse). 💀

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

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

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

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

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

assert (some_tainted_value < SOME_LIMIT);

In such cases:

  • when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure
  • when assertion-checking is disabled, such as by defining NDEBUG, an attacker could inject data that subverts the process, since it presumably violates a precondition that is being assumed by the code.
Note that when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer even gets a chance to "see" them, so this diagnostic can only generate warnings on builds in which assertion-checking is enabled. 👑

For the purpose of this warning, any function marked with attribute noreturn is considered a possible assertion failure handler, including __builtin_unreachable. Note that these functions are sometimes removed by the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic.

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

The warning can also report problematic constructions such as:

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

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

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

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

This diagnostic warns for paths through the code in which a value that could be under an attacker's control gets used as the index of an array access without being sanitized, so that an attacker could inject an out-of-bounds access. 🎀

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

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

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

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

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

This diagnostic warns for paths through the code in which a value that could be under an attacker's control gets used as a pointer offset without being sanitized, so that an attacker could inject an out-of-bounds access.

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

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

okay bestie, so picture this 💀 — there's a path through your code where a value that an attacker could fully control gets used as the size of an operation like memset, and nobody ever sanitized it. that means an attacker could inject an out-of-bounds access. not the vibe, fr fr. 🚩

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

  • -Wno-analyzer-throw-of-unexpected-type — this one ✨needs✨ -fanalyzer, which is the flag that turns it on in the first place; you turn it back off with -Wno-analyzer-throw-of-unexpected-type. heads up though: dynamic exception specifications only exist in C++14 and earlier, so if you're on newer C++ this whole thing is ancient history 💅.

it warns about paths through the code where an exception gets thrown out of a function that has a dynamic exception specification, and that exception does not comply with the specification. like ma'am, that is not the guest list. 🙈

  • -Wno-analyzer-undefined-behavior-ptrdiff — enable it with -fanalyzer (that's what turns it on), disable it with -Wno-analyzer-undefined-behavior-ptrdiff.

this diagnostic warns about paths where a pointer subtraction happens but the two pointers refer to different chunks of memory. that code is relying on undefined behavior, periodt. pointer subtraction is only defined when both pointers point within the same array (or just after it). don't do it bestie, it's giving undefined. 💀

See CWE-469: Use of Pointer Subtraction to Determine Size (https://cwe.mitre.org/data/definitions/469.html).

  • -Wno-analyzer-undefined-behavior-strtok — enable with -fanalyzer, disable with -Wno-analyzer-undefined-behavior-strtok.

warns about paths where a call is made to strtok with undefined behavior. and specifically? passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior. so no. 🌸

  • -Wno-analyzer-unsafe-call-within-signal-handler — enable with -fanalyzer, disable with -Wno-analyzer-unsafe-call-within-signal-handler.

warns about paths where a function known to be async-signal-unsafe (think fprintf) gets called from inside a signal handler. bestie that is not the place for it 😭.

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

  • -Wno-analyzer-use-after-free — enable with -fanalyzer, disable with -Wno-analyzer-use-after-free.

warns about paths where a pointer is used after a deallocator was called on it: either free, or a deallocator referenced by attribute malloc. the classic. the iconique. the CWE-416. 💅

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

  • -Wno-analyzer-use-of-pointer-in-stale-stack-frame — enable with -fanalyzer, disable with -Wno-analyzer-use-of-pointer-in-stale-stack-frame.

warns about paths where a pointer gets dereferenced and it points at a variable sitting in a stale stack frame. that frame is so over, and it did not leave a forwarding address. 🫠

  • -Wno-analyzer-va-arg-type-mismatch — enable with -fanalyzer, disable with -Wno-analyzer-va-arg-type-mismatch.

warns about interprocedural paths where the analyzer catches an attempt to use va_arg to pull out a value that was passed to a variadic call — but using a type that doesn't match the type of the expression that was actually passed in. mismatched. not slaying. 🙃

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

  • -Wno-analyzer-va-list-exhausted — enable with -fanalyzer, disable with -Wno-analyzer-va-list-exhausted.

warns about interprocedural paths where the analyzer catches an attempt to use va_arg to grab the next value passed to a variadic call, except every value in the va_list has already been consumed. the list said "no more". respect the boundary. 🚫

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

  • -Wno-analyzer-va-list-leak — enable with -fanalyzer, disable with -Wno-analyzer-va-list-leak.

warns about interprocedural paths where the analyzer catches that va_start or va_copy was called on a va_list, and then there was no corresponding call to va_end. it's giving unfinished business. 🌸

  • -Wno-analyzer-va-list-use-after-va-end — enable with -fanalyzer, disable with -Wno-analyzer-va-list-use-after-va-end.

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

  • -Wno-analyzer-write-to-const — enable with -fanalyzer, disable with -Wno-analyzer-write-to-const.

warns about paths where the analyzer catches an attempt to write through a pointer to a const object. the audacity honestly 😭. BUT — real talk — the analyzer does not prioritize detecting these paths, so false negatives are more likely here relative to the other warnings. so don't get comfy just because it stayed quiet.

  • -Wno-analyzer-write-to-string-literal — enable with -fanalyzer, disable with -Wno-analyzer-write-to-string-literal.

warns about paths where the analyzer catches an attempt to write through a pointer to a string literal. again — important — the analyzer does not prioritize detection of these paths either, so false negatives are more likely relative to other warnings. 🙈

  • -Wno-analyzer-use-of-uninitialized-value — enable with -fanalyzer, disable with -Wno-analyzer-use-of-uninitialized-value.

warns about paths where an uninitialized value gets used. it wandered in from nowhere and started making decisions. no ma'am. 💀

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

okay so this analyzer? it has hardcoded knowledge about the behavior of these memory-management functions 👑:

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

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 of these functions 💜:

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

also, like, various functions with an __analyzer_ prefix have special meaning to the analyzer, and those are described in the GCC Internals manual ✨. various internal parameters, settable via --param, are used to control the exploration — those are documented in the GCC Internals manual too.

the following options control the analyzer, bestie. get comfy. 🎀

  • -fanalyzer-assume-nothrow — by default, if -fexceptions is enabled, the analyzer will assume that a call to any function without attribute nothrow could throw an exception. this can help detect execution paths that leak because exceptions bypass clean-up code, but it could also lead to false positives when you're using headers where the author never added the nothrow attribute. 😬

if -fanalyzer-assume-nothrow is enabled, then the analyzer will assume that external functions do not throw exceptions. this may be useful for reducing "noise" from the analyzer when you enable -fexceptions on C code — but it could hide true issues if an exception could actually be raised by something the C code calls. so like... choose wisely. 💅

  • -fanalyzer-call-summaries — simplifies interprocedural analysis by computing the effect of certain calls, instead of exploring all the paths through the function from callsite to each possible return. so much walking, we're skipping it 😌.

if enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated.

  • -fanalyzer-checker=name — restricts the analyzer to run just the named checker, and enables it. one checker. no entourage. 👑
  • -fanalyzer-debug-text-art-headings — this option is intended for analyzer developers 🛠️. if enabled, the analyzer will add extra annotations to any diagrams it generates. cute little labels for the devs.
  • -fno-analyzer-feasibility — this option is intended for analyzer developers.

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

  • -fanalyzer-fine-grained — does nothing. preserved for backward compatibility. 💀 iconic behavior honestly.
  • -fanalyzer-show-duplicate-count — okay so this one is for the analyzer devs specifically 🛠️. If a bunch of diagnostics got flagged as dupes of each other, it drops a lil note when it reports the best diagnostic, telling you how many extra diagnostics got swallowed by the dedup logic. Receipts, basically.
  • -fanalyzer-show-events-in-system-headers — by default the analyzer keeps it cute and emits simplified diagnostic paths, hiding events that live entirely inside a system header. Flip this on and those events stop being suppressed entirely. No more hiding 💅.
  • -fno-analyzer-simplify-supergraph — analyzer devs only 🛠️.

So by default the analyzer does a whole bunch of simplifications to the program supergraph before analyzing it. With -fno-analyzer-simplify-supergraph you can shut that simplification off, which is for debugging issues with the simplification itself.

  • -fno-analyzer-state-merge — analyzer devs only 🛠️.

By default the analyzer tries to simplify things by merging states that are sufficiently similar at each program point as it's building its "exploded graph". With -fno-analyzer-state-merge that merging gets suppressed, for debugging state-handling issues.

  • -fno-analyzer-state-purge — analyzer devs only 🛠️.

By default the analyzer tries to simplify analysis by purging aspects of state at a program point that look like they're not relevant anymore, e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.

With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.

  • -fno-analyzer-suppress-followups — analyzer devs only 🛠️.

By default the analyzer will stop exploring an execution path after it hits certain diagnostics, so it doesn't potentially issue a whole cascade of follow-up diagnostics.

The diagnostics that terminate analysis along a path are:

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

With -fno-analyzer-suppress-followups the analyzer will keep exploring those 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 one enables transitivity of constraints within the analyzer ✨.
  • -fno-analyzer-undo-inlining — analyzer devs only 🛠️.

-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, which leads to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code 😭.

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

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

  • -fanalyzer-verbose-edges — analyzer devs only 🛠️. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths.
  • -fanalyzer-verbose-state-changes — analyzer devs only 🛠️. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.
  • -fanalyzer-verbosity=level — this option controls the complexity of the control flow paths that get emitted for analyzer diagnostics.

The level can be one of:

  • 0 — at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-"free" diagnostic, both calls to "free" will be shown.
  • 1 — as per the previous level, but also 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 — dumps internal details about what the analyzer is doing to file.analyzer.txt. Note that -fdump-analyzer-stderr overrides this option.
  • -fdump-analyzer-stderr — dumps internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer.
  • -fdump-analyzer-callgraph — dumps a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot.
  • -fdump-analyzer-exploded-graph — dumps a representation of the "exploded graph" suitable for viewing with GraphViz to file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes. So aesthetic 🎨.
  • -fdump-analyzer-exploded-nodes — emits diagnostics showing where nodes in the "exploded graph" are in relation to the program source.
  • -fdump-analyzer-exploded-nodes-2 — dumps a textual representation of the "exploded graph" to file.eg.txt.
  • -fdump-analyzer-exploded-nodes-3 — dumps a textual representation of the "exploded graph" to one dump file per node, to file.eg-id.txt. This is typically a large number of dump files 😭.
  • -fdump-analyzer-exploded-paths — dumps a textual representation of the "exploded path" for each diagnostic to file.idx.kind.epath.txt.
  • -fdump-analyzer-feasibility — dumps internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the form file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt.
  • -fdump-analyzer-infinite-loop — dumps internal details about the analyzer's search for infinite loops. The details are written in a form suitable for viewing with GraphViz to filenames of the form file.*.infinite-loop.dot.
  • -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is subject to change, just so you know 🙈.
  • -fdump-analyzer-state-purge — as per -fdump-analyzer-supergraph, dump a representation of the "supergraph" suitable for viewing with GraphViz, but annotate the graph with information on what state will be purged at each node. The graph is written to file.state-purge.dot.
  • -fdump-analyzer-supergraph — dumps representations of the "supergraph" suitable for viewing with GraphViz to file.supergraph.index.kind.dot. These show all of the control flow graphs in the program, at various stages of the analysis. The precise set of dumps and what they show is subject to change.
  • -fdump-analyzer-untracked — emits custom warnings with internal details intended for analyzer developers.

🐛✨ Options for Debugging Your Program ✨🐛

So like, to tell GCC to emit extra information for use by a debugger, in almost all cases you literally just need to add -g to your other options. Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command-line option to your other options. Bestie behavior, honestly 🫶.

GCC allows you to use -g with -O. But fair warning: the shortcuts taken by optimized code may occasionally be surprising 😳 — some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.

If you are not using some other optimization option, consider using -Og with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. Trust 💜.

  • -g, --debug — produces debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program 💀. If you want to control for certain whether to generate the extra information, use -gvms (see below).

  • -ggdb — produces debugging information for use by GDB. This means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
  • -gdwarf, -gdwarf-version — produces debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4 or 5; the default version for most targets is 5 (with the exception of VxWorks, TPF and Darwin / macOS, which default to version 2, and AIX, which defaults to version 4).

Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher.

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. Naming is messy, we don't talk about it 🙈.

  • -gbtf — requests BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command-line options.
  • -gprune-btf, -gno-prune-btf — bestie, this one prunes your BTF info before it gets emitted. 💅

When pruning is on, only type info for types that are actually used by global variables and file-scope functions gets emitted. And 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 get pruned and replaced with BTF_KIND_FWD, just like those types were only ever in the input as forward declarations. SHRINK RAY ACTIVATED 🦋

This option substantially reduces the size of the BTF information produced — but it's giving major loss of detailed type information, ngl. It's mainly useful when you're compiling for the BPF target, to keep the resulting object tiny and to nuke BTF info that isn't immediately relevant to the BPF program loading process.

It's enabled by default for the BPF target when generating BTF information. automatic queen behavior 👑

  • -gctf, -gctflevel — ask for CTF debug info, and use level to say how much CTF debug info you want. If you say -gctf with no value for level, the default level of CTF debug info is 2.

You can generate CTF debug info right alongside DWARF debug info when both of the debug formats are enabled explicitly via their own respective command line options 🫶

  • Level 0 produces no CTF debug info at all. So -gctf0 is literally just negating -gctf. 💀
  • Level 1 produces CTF info for tracebacks only — callsite info included, but no type info.
  • Level 2 produces type info for entities (functions, data objects etc.) at file-scope or global-scope only.
  • -gvms — produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG on Alpha/VMS systems. vintage tech era ✨
  • -gcodeview — produce debugging information in CodeView debug format (if that is supported). This is the format Microsoft Visual C++ uses on Windows. 🪟
  • -glevel, -ggdblevel, -gvmslevel — request debugging information and use level to say how much info you want. The default level is 2. Periodt.
    • Level 0 produces no debug information at all. So -g0 negates -g. bye bye 👋
    • Level 1 produces minimal info — enough to make backtraces in parts of the program you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
    • Level 2 is the normal setting for producing debuggable code. The debug output has everything from level 1, plus info about local variables and typedefs so you can step through code and examine its state. For C++ code, extra information also gets emitted to help debug namespace, class, and template functions. the full experience 💖
    • Level 3 includes extra info, like all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3. macro girlies rise 🌸

If you use multiple -g options, with or without level numbers, the last such option is the one that's effective. Last one standing, fr fr.

Heads up bestie: -gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead, use an additional -glevel option to change the debug level for DWARF. Don't get it twisted 🙈
  • -fno-eliminate-unused-debug-symbols — by default, no debug information is produced for symbols that aren't actually used. Use this option if you want debug information for all symbols. nothing gets left out, we love a completionist 🫶
  • -femit-class-debug-always — instead of emitting debugging information for a C++ class in only one object file, emit it in all object files that use the class. This should only be used with debuggers that can't handle the way GCC normally emits debugging information for classes, because using this option increases the size of your debugging information by as much as a factor of two. 😭 So like, emergency use only, iykyk.
  • -fno-merge-debug-strings — tell the linker to not merge together strings in the debugging information that are identical in different object files. Ngl, merging isn't supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file, at the cost of increasing link processing time. Merging is enabled by default. ✨
  • -fdebug-prefix-map=old=new — when compiling files that live in directory old, record debugging information describing them as if the files lived in directory new instead. You can use this 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. That 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. path glow-up 🔥
  • -fvar-tracking — run the variable tracking pass. It computes where variables are stored at each position in the code, and then better debugging information gets generated (if the debugging information format supports this information). 🦋

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

  • -fvar-tracking-assignments — annotate assignments to user variables early in compilation and try to carry those annotations all the way over throughout the compilation to the end, in an attempt to improve debug information while optimizing. Using -gdwarf-4 along with it is recommended. ✨

It can be enabled even if var-tracking is disabled, and in that case the 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. little annotation besties 🎀

  • -fvar-tracking-uninit, -fno-var-tracking-uninit — do variable tracking and mark uninitialized variables in the debug information. This flag is enabled by default by -fvar-tracking; it also implies -fvar-tracking. If you wanna do variable tracking without marking uninitialized variables, use -fvar-tracking -fno-var-tracking-uninit. mix and match, queen 💅
  • -gsplit-dwarf — if DWARF debugging information is enabled, split as much debugging information as possible into a separate output file with the extension .dwo. This gives the build system a way to avoid linking files with debug information. To actually be useful though, this option requires a debugger capable of reading .dwo files. 👑
  • -gdwarf32, -gdwarf64 — if DWARF debugging information is enabled, -gdwarf32 picks the 32-bit DWARF format and -gdwarf64 picks the 64-bit DWARF format. The default is target specific — on most targets it's -gdwarf32 though.

The 32-bit DWARF format is smaller, but it can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information, and might not be well supported by all consumers yet. choose your fighter ⭐

  • -gdescribe-dies — add description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs. 🎀
  • -gpubnames — generate DWARF .debug_pubnames and .debug_pubtypes sections. 🌈
  • -ggnu-pubnames — generate the .debug_pubnames and .debug_pubtypes sections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7. 🔥
  • -gno-pubnames — don't generate the DWARF .debug_pubnames and .debug_pubtypes sections. nope, not today 🙈
  • -fdebug-types-section — when using DWARF Version 4 or higher, type DIEs can be put into their own .debug_types section instead of being part of the .debug_info section. It's more efficient to put them in a separate comdat section, since the linker can then remove duplicates. But — and this is the tea — not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types produces larger instead of smaller debugging information. plot twist 😭
  • -grecord-gcc-switches, -gno-record-gcc-switches — this switch makes the command-line options used to invoke the compiler that may affect code generation get appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also -frecord-gcc-switches for another way of storing compiler options into the object file. 💅
  • -gstrict-dwarf — disallow using extensions from a later DWARF standard version than the one selected with -gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed — this shuts that down. strict mode: activated 🎀
  • -gno-strict-dwarf — allow using extensions from a later DWARF standard version than the one selected with -gdwarf-version. okay we're being flexible 💜
  • -gas-loc-support — inform the compiler that the assembler supports .loc directives. It may then use them so the assembler generates DWARF2+ line number tables. 🦋

This is generally desirable, because assembler-generated line-number tables are a lot more compact than the ones the compiler can generate for itself. ✨

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

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

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

  • -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested. 🎀
  • -gcolumn-info, -gno-column-info — emit location column information into DWARF debugging information, instead of just file and line. This option is enabled by default. columns girlies stay winning 🌈
  • -gstatement-frontiers, -gno-statement-frontiers — okay so this one makes GCC drop little markers into its internal representation right at the beginning of statements, and then it keeps them ~roughly~ where they are all the way through compilation, using them to guide the output of "is_stmt" markers in the line number table. It's on by default when you're compiling with optimization (-Os, -O1, -O2, ...) and outputting DWARF 2 debug info at the normal level. 💅
  • -gvariable-location-views, -gvariable-location-views=incompat5, -gno-variable-location-views — this pimps your variable location lists out with progressive view numbers that are implied from the line number table. Why? So debug info consumers can peek at state at certain points of the program even if there are no instructions tied to the corresponding source locations at that point. slay.

If your assembler doesn't support view numbers in line number tables, this makes the compiler emit an internal line number table, which generally makes them a bit less compact. ngl. The augmented line number tables and location lists are fully backward-compatible, so debug info consumers that have no idea these augmentations exist can still consume them — they just won't get any of the benefits either. 🙈

It's enabled by default when outputting DWARF 2 debug info at the normal level, as long as there's assembler support, -fvar-tracking-assignments is enabled, and -gstrict-dwarf is not. When assembler support isn't available, it may still be enabled — but that forces GCC to output internal line number tables, and if -ginternal-reset-location-views isn't enabled, that most certainly leads to silently mismatching location views. 💀

There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, and you turn it on with -gvariable-location-views=incompat5. Heads up bestie: this option may be removed in the future, it's only there as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they'd be left unable to decode location lists using it. 😭

  • -ginternal-reset-location-views, -gno-internal-reset-location-views — tries to figure out which location views can be left out of location view lists. The catch? It needs the compiler to have very accurate instruction length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when you're using an assembler that doesn't support location view lists. The GNU assembler flags any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function. periodt.
  • -ginline-points, -gno-inline-points — generates extended debug info for inlined functions. Location view tracking markers get inserted at inlined entry points so that address and view numbers can be computed and output in debug info. You can enable this independently of location views — in which case the view numbers just won't be output — but it can only be enabled along with statement frontiers, and it's only on by default if location views are enabled. 🌸
  • -gz[=type] — produces compressed debug sections in DWARF format, if that's supported. If type isn't given, the default type depends on what your assembler and linker can do. type can be one of none (don't compress debug sections, tyvm), zlib (use zlib compression in ELF gABI format), or zstd (use zstd compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option gets rejected. Otherwise, if the assembler doesn't support them, -gz is silently ignored when producing object files. 🤫
  • -femit-struct-debug-baseonly — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the struct is defined.

This substantially shrinks your debugging info — but at a significant potential loss in type information to the debugger. Peep -femit-struct-debug-reduced for a less aggressive option, and -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output. 🦋

  • -femit-struct-debug-reduced — emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the type is defined — unless the struct is a template or is defined in a system header.

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

This option works only with DWARF debug output. 🎀

  • -femit-struct-debug-detailed[=spec-list] — lets you specify exactly which struct-like types the compiler generates debug info for. The whole intent here is to reduce duplicate struct debug info between different object files in the same program. It's the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and honestly it serves for most needs. 💅

A specification has the syntax:

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

The optional first word limits the spec to structs used directly (dir:) or used indirectly (ind:). A struct type is used directly when it's the type of a variable or member. Indirect uses come through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. Example incoming:

struct one direct; struct two * indirect;

The optional second word limits the spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are... a bit complicated to explain, ngl. 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 you want debug info emitted for. Values none and any have their normal meaning. The value base means the base of the name of the file where the type declaration appears must match the base of the name of the main compilation file. In practice: when compiling foo.c, debug info is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to figure out the best settings for your application. lowkey true.

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

This option works only with DWARF debug output. ✨

  • -fno-dwarf2-cfi-asm — emits DWARF unwind info as a compiler-generated ".eh_frame" section instead of using GAS ".cfi_*" directives.
  • -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it's useful to have GCC emit debugging info for all types declared in a compilation unit, regardless of whether they're actually used in that compilation unit — for example if, in the debugger, you want to cast a value to a type that isn't actually used in your program (but is declared). More often than not, though, this results in a significant amount of wasted space. 🫠

🚀 Options That Control Optimization 🚀

These options control various sorts of optimizations. the good stuff fr fr.

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. chef's kiss.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size — at the expense of compilation time and possibly the ability to debug the program. 😭 the tradeoff is real.

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. iconic behavior.

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 isn't set on the command line — even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. 👑

  • -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 — bestie, this one optimizes even more 💅 GCC runs basically every supported optimization that doesn't come with a space-versus-speed tradeoff. Compared to -O, this thing bumps up both your compilation time and the performance of the generated code. So yeah, you pay in wait-time, you get paid back in speed 🏎️

-O2 turns on every optimization flag that -O1 already specifies. And also it turns these on:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdep-fusion
-fdevirtualize  -fdevirtualize-speculatively
-fexpensive-optimizations -ffinite-loops -fgcse  -fgcse-lm
-fhoist-adjacent-loads -finline-functions -finline-small-functions
-findirect-inlining -fipa-bit-cp  -fipa-cp  -fipa-icf -fipa-ra
-fipa-sra  -fipa-vrp -fisolate-erroneous-paths-dereference
-flra-remat -foptimize-crc -foptimize-sibling-calls
-foptimize-strlen -fpartial-inlining -fpeephole2
-freorder-blocks-algorithm=stc -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop -fschedule-insns
-fschedule-insns2 -fsched-interblock  -fsched-spec
-fspeculatively-call-stored-functions -fstore-merging
-fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce
-ftree-loop-vectorize -ftree-pre -ftree-slp-vectorize
-ftree-switch-conversion  -ftree-tail-merge -ftree-vrp
-fvect-cost-model=very-cheap
ok so real talk tho 🫶 please go read the warning under -fgcse about invoking -O2 on programs that use computed gotos. It's giving "important", not "optional".
  • -O3 — optimize yet more, bestie, we keep climbing 📈 -O3 turns on everything -O2 specifies and also turns on these:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -ftree-loop-distribution -ftree-partial-pre
-funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides
  • -O0 — reduce compilation time and make debugging produce the expected results. This is the default. No cap, it's just the chill baseline ✨

At -O0, GCC completely disables most optimization passes — and like, 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. Why? Because tons of optimizations GCC does depend on code analysis or canonicalization passes that get enabled by -O, and running individual optimization passes in isolation just... wouldn't be useful. It's giving "you can't have the frosting without the cake" 🎂

  • -Os — optimize for size 🎀 -Os enables all -O2 optimizations except the ones 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, makes the compiler tune for code size instead of execution speed, and runs further optimizations that are literally designed to reduce code size. Tiny queen energy 🐜👑

  • -Ofast — disregard strict standards compliance. periodt. -Ofast enables all -O3 optimizations, and then 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 — plus -fno-protect-parens. And it turns off -fsemantic-interposition.

  • -Og — optimize while keeping the debugging experience in mind 🧚 -Og should be the optimization level of choice for the standard edit-compile-debug cycle. It's a reasonable blend of optimization, fast compilation, and debugging experience, especially for code with a high abstraction penalty. And unlike -O0, it enables -fvar-tracking-assignments and -fvar-tracking, which handle debug information in the prologue and epilogue of functions better than -O0 does. Main character debugging arc 💅

Like -O0, -Og completely skips a bunch of optimization passes, so individual options controlling them have no effect. Otherwise, -Og enables all -O1 optimization flags except the ones known to greatly interfere with debugging:

-fbranch-count-reg  -fdelayed-branch -fdse  -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants  -fmove-loop-stores  -fssa-phiopt
-ftree-bit-ccp  -ftree-dse  -ftree-pta  -ftree-sra
  • -Oz — optimize aggressively for size rather than speed 🌸 This may actually 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's effective. Last one wins, no exceptions 🫶

Options of the form -fflag specify machine-independent flags. Most flags have both a positive and a negative form; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one you typically use. You can figure out the other form by either removing no- or adding it. iykyk 💜

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. (yes, "fine-tuning" — she's fancy ✨)

  • -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; it lets the compiler let arguments accumulate on the stack for several function calls and pop them all at once. Efficiency slay 💅
  • -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 one 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. Not me rescuing the CPU from its own drama 💅

Disabled by default.

  • -ffp-contract=style — okay so, the styles: -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction, like forming fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard — 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), and -ffp-contract=fast otherwise.

  • -ffp-int-builtin-inexact — allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the "inexact" floating-point exception for noninteger arguments 🚨 ISO C99 and C11 allow these functions to raise the "inexact" exception, but ISO/IEC TS 18661-1:2014 — the C bindings to IEEE 754-2008, as integrated into ISO C23 — does not allow these functions to do so.

The default is -fno-fp-int-builtin-inexact, disallowing the exception to be raised, unless C17 or an earlier C standard is selected. Heads up: this option does nothing unless -ftrapping-math is in effect 🙈

And even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function, then the "inexact" exception may still be raised if the library implementation doesn't follow TS 18661. So like… no promises bestie 💀

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

On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't be omitted. Sorry babe 🫶

Also note: -fno-omit-frame-pointer does not guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions. It's giving "no take-backs guaranteed" 🌈

Enabled by default at -O1 and higher.

  • -foptimize-crc — detect loops calculating CRC (performing polynomial long division) and replace them with a faster implementation 🔥 It detects 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and bit-reversed cases. If the target supports a CRC instruction and the polynomial used in the source code matches the polynomial used in the CRC instruction, generate that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, generate CRC using it; otherwise generate table-based CRC. Three-tier glow-up plan ✨

Enabled by default at -O2 and higher.

  • -foptimize-sibling-calls — optimize sibling and tail recursive calls 💜

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

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

Enabled at levels -O2, -O3.

  • -finline-atomics, -fno-inline-atomics — inline __atomic operations when a lock-free instruction sequence is available. This optimization is enabled by default 🎀
  • -finline-stringops[=fn] — expand memory and string operations inline (for now, only memset), even when the length is variable or big enough as to require looping 🌸 This is most useful along with -ffreestanding and -fno-builtin.

...and yeah, sometimes it lets the compiler spit out code that flexes known alignment and length multipliers — but lowkey, even then it can be less efficient than a well-optimized runtime implementation 😭 and it can grow code size so much that a slower-but-shared implementation ends up running faster, purely because it plays nicer with code caches. And bestie? This option is disabled by default. Not the vibe you get for free.

  • -fno-inline — Do not expand any functions inline, periodt — except the ones marked with the always_inline attribute, they get a pass. This is the default when you're not optimizing at all.

And if you've got a single function that's acting brand new and you want it exempt from inlining? Just mark it with the noinline attribute. 🎀

  • -finline-small-functions — Squeezes functions into their callers when the function body is smaller than the expected function-call code — so the whole program just gets smaller overall. The compiler heuristically decides which functions are simple enough to be worth integrating like this. And the way it applies to all functions, even ones that were never declared inline? Iconic.

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

  • -findirect-inlining — Inlines indirect calls too, as long as they got discovered as compile-time known thanks to previous inlining. But heads up bestie: this only does anything when inlining itself is already turned on via -finline-functions or -finline-small-functions.

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

  • -finline-functions — Considers every function for inlining, even the ones that never said they were inline. The compiler does its little heuristic thing and decides which functions are actually worth integrating like this.

And the fun part: if every single call to a given function gets integrated, and that function is declared static, then normally it doesn't get output as assembler code in its own right at all. Gone. Poof. 💅

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

  • -finline-functions-called-once — Considers all static functions that are called exactly once for inlining into their caller, even if they're not marked inline. If a given call gets integrated, that function does not get output as assembler code in its own right.

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

  • -fearly-inlining — Inlines the functions marked always_inline plus any function whose body looks smaller than the function-call overhead — and it does this early, before the -fprofile-generate instrumentation and the real inlining pass even happen. Doing it early makes profiling significantly cheaper, and usually makes inlining faster on programs that have big chains of nested wrapper functions. Chain of wrappers? She's SEEN it. 🦋

Enabled by default.

  • -fipa-sra — Performs interprocedural scalar replacement of aggregates, removes unused parameters, and swaps parameters that were passed by reference for parameters passed by value. Cleanup queen behavior. 👑

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

  • *-finline-limit=n* — By default, GCC puts a limit on the size of functions that can be inlined, and this flag is your coarse little dial for that limit. Here n is the size of functions that can be inlined, measured in number of pseudo instructions.

Now, the tea: inlining is actually controlled by a whole bunch of internal parameters, and those are documented in the GCC Internals manual — you should not normally set them directly, bestie. Please. 🙈

Note: there may be no value of -finline-limit that results in default behavior. That's just how it is.

Note: "pseudo instruction," in this particular context, is an abstract measurement of a function's size. It is in no way a count of assembly instructions, and as such its exact meaning might change from one release to another. So don't get attached.

  • -fno-keep-inline-dllexport — This is a more fine-grained version of -fkeep-inline-functions, and it applies only to functions declared using the dllexport attribute or declspec.
  • -fkeep-inline-functions — In C, this emits static functions that are declared inline into the object file, even if the function got inlined into all of its callers. Note that this switch does not touch functions using the extern inline extension in GNU C90. And in C++, it emits any and all inline functions into the object file.
  • -fkeep-static-functions — Emits static functions into the object file, even if that function is never used. Never even called. Still showing up. Main character energy fr. ⭐
  • -fkeep-static-consts — Emits variables declared static const when optimization isn't turned on, even if those variables aren't referenced.

GCC enables this option by default. And if you want to force the compiler to actually check whether a variable is referenced, regardless of whether optimization is on or off? Use the -fno-keep-static-consts option.

  • -fmerge-constants — Attempts to merge identical constants — that's string constants and floating-point constants — across compilation units.

This one 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. Ambitious!

This option implies -fmerge-constants. And 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.

But okay, caution time bestie 🚨 — languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations. So using this option results in non-conforming behavior. It's giving "she's breaking the rules and she knows it."

  • -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. Loop glow-up 🌸
  • -fmodulo-sched-allow-regmoves — Performs more aggressive SMS-based modulo scheduling, with register moves allowed. Setting this flag deletes certain anti-dependence edges, which triggers generation of reg-moves based on the life-range analysis. Heads up: this option is effective only with -fmodulo-sched enabled.
  • -fno-branch-count-reg — Disables the optimization pass that scans for chances to use decrement and branch instructions on a count register, instead of instruction sequences that decrement a register, compare it against zero, and then branch based on the result.

This option is only meaningful on architectures that support such instructions — that includes x86, PowerPC, IA-64 and S/390.

And note: the -fno-branch-count-reg option does not remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes. So don't @ her about that.

The default is -fbranch-count-reg at -O1 and higher, except for -Og.

  • -fno-function-cse — Do not put function addresses in registers; instead, make each instruction that calls a constant function contain the function's address explicitly.

Yeah, this results in less efficient code. But some strange hacks that alter the assembler output may get confused by the optimizations performed when this option is not used — so it's a trade-off, iykyk.

The default is -ffunction-cse.

  • -ffuse-ops-with-volatile-access — Allows limited optimization of operations with volatile memory access when doing so does not change the semantics outlined in

The default is -ffuse-ops-with-volatile-access

  • -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. Cute and efficient ✨

This option turns that behavior off, 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 whether a jump branches to a location where another comparison is found, one that's subsumed by the first. If so, the first branch gets redirected — either to the destination of the second branch, or to a point immediately following it — depending on whether the condition is known to be true or false.

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

  • -fsplit-wide-types — When you're using a type that occupies multiple registers, like long long on a 32-bit system, this splits the registers apart and allocates them independently. Normally it generates better code for those types — but it may make debugging more difficult. Pick your fighter. 🫶

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. Sibling energy.

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 isn't reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false.

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

  • -fcse-skip-blocks — Similar to -fcse-follow-jumps, but it makes CSE follow jumps that conditionally skip over blocks. So when CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if.

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

  • -frerun-cse-after-loop — Re-runs common subexpression elimination after loop optimizations have been performed. Round two, bestie. 🔥

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

  • -fgcse — Performs a global common subexpression elimination pass. This pass also performs global constant and copy propagation. She does it all. 👑

Note: when you're compiling a program that uses computed gotos — a GCC extension — you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

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

  • -fgcse-lm — When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed into a load outside the loop, plus a copy/store within the loop. Loop glow-up part two 🧚

Enabled by default when -fgcse is enabled.

  • -fgcse-sm — When -fgcse-sm is enabled, a store motion pass runs after global common subexpression elimination. This pass attempts to move stores out of loops. And when it's used together with -fgcse-lm, loops containing a load/store sequence can be changed into a load before the loop and a store after the loop. That's the combo meal. 🍓

Not enabled at any optimization level. Like, zero. None of them. It's giving "off in every timeline." 💀

  • -fgcse-las — turn this on and the global common subexpression elimination pass goes full detective mode: it hunts down redundant loads that show up after stores to the same memory location. Both partial redundancies and full ones, bestie, we're not picking favourites. ✨ And no, no optimization level enables it — not enabled at any optimization level.
  • -fgcse-after-reload — flips on a redundant load elimination pass that runs after reload, and its whole little job description is cleaning up redundant spilling. That's it, that's the purpose. 🧹 Enabled by -O3, -fprofile-use and -fauto-profile.
  • -faggressive-loop-optimizations — this one tells the loop optimizer to use language constraints to derive bounds for how many iterations a loop runs, which is aggressive in exactly the way the name promises. ⚠️ Heads up though: it assumes your loop code doesn't invoke undefined behavior — like causing signed integer overflows or out-of-bound array accesses. So if your code is chaotic, that's on you. Those iteration-count bounds then guide loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
  • -funconstrained-commons — tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. Which blocks certain optimizations that depend on knowing the array bounds. Sorry to those optimizations, they had a good run. 🌸
  • -fcrossjumping — performs the cross-jumping transformation, which unifies equivalent code and saves code size. 🎀 Ngl though: the resulting code may or may not perform better than without cross-jumping. It's giving "smaller but who knows about faster." Enabled at levels -O2, -O3, -Os.
  • -fauto-inc-dec — combines increments or decrements of addresses with memory accesses, iykyk. This pass is always skipped on architectures that don't have instructions to support this. Enabled by default at -O1 and higher on architectures that support this. 👑
  • -fdce — performs dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher. bye bye dead code 🦋
  • -fdse — performs dead store elimination (DSE) on RTL. Also enabled by default at -O1 and higher. the stores that weren't doing anything have been politely escorted out 💅
  • -fif-conversion — attempts to transform conditional jumps into branch-less equivalents. Which includes use of conditional moves, min, max, set flags and abs instructions, plus some tricks that are doable by standard arithmetics. The use of conditional execution on chips where it's available is controlled by -fif-conversion2. Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.
  • -fif-conversion2 — uses conditional execution (where available) to transform conditional jumps into branch-less equivalents. Enabled at levels -O1, -O2, -O3, -Os, but not with -Og. Two flags, same energy, different toolbox. ✨
  • -fdeclone-ctor-dtor — okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones — which literally means two copies of the function. 😭 With this option, the base and complete variants get changed into thunks that call a common implementation. De-cloned. Iconic. Enabled by -Os.
  • -fdelete-null-pointer-checks — assumes that programs cannot safely dereference null pointers, and that no code or data element resides at address zero. This enables simple constant folding optimizations at all optimization levels, no cap. On top of that, other optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for null pointers — and those analyses assume a memory access to address zero always results in a trap, so if a pointer gets checked after it's already been dereferenced, it can't be null.
But bestie, listen: in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

This option is enabled by default on most targets. On AVR and MSP430, this option is completely disabled. Passes that use the dataflow information are enabled independently at different optimization levels.

  • -fdevirtualize — attempts to convert calls to virtual functions into direct calls. It happens both within a procedure and interprocedurally, as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os. 🔥
  • -fdevirtualize-speculatively — attempts to convert calls to virtual functions into speculative direct calls. Based on the analysis of the type inheritance graph, it figures out for a given call the set of likely targets. If that set is small — preferably of size 1 — it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, like inlining. And when they seem useless after further optimization, they get converted back into original form. a glow up and a glow down, we love range 🌈
  • -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. For this reason it is disabled by default. 💀
  • -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive. Yes, that's the whole plot. Enabled at levels -O2, -O3, -Os.
  • -fext-dce, -fno-ext-dce — performs dead code elimination on zero and sign extensions, with a special dataflow analysis. ✨
  • -free — attempts to remove redundant extension instructions. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os.
  • -fno-lifetime-dse — C++ lore time: the value of an object is only affected by changes within its lifetime. When the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination takes advantage of this. 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 (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor? Use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. And -flifetime-dse=0 is equivalent to -fno-lifetime-dse. Three settings, zero ambiguity. 💅

  • -flive-range-shrinkage — attempts to decrease register pressure through register live range shrinkage. 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 is not 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 the following:
    • all — use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.
    • mixed — use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O, -O2, ...).
    • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0.
  • -fira-hoist-pressure — uses IRA to evaluate register pressure in the code hoisting pass, so it can make decisions about hoisting 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, so it can decide whether to move loop invariants. This option 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. Each hard register gets its own separate stack slot, and as a result function stack frames are larger. 📏
  • -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers. Every pseudo-register that doesn't get a hard register gets its own separate stack slot, and as a result function stack frames are larger. Same vibe, different registers. ✨
  • -flra-remat — enables CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) those values if it is profitable. Enabled at levels -O2, -O3, -Os.
  • -fdelayed-branch — if it's supported for the target machine, it attempts to reorder instructions to exploit instruction slots available after delayed branch instructions. Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.
  • -fschedule-insns — if supported for the target machine, it attempts to reorder instructions to eliminate execution stalls caused by required data being unavailable. This helps machines that have slow floating point or memory load instructions by letting other instructions be issued until the result of the load or floating-point instruction is actually required. 🌀 Conventionally enabled at optimization levels -O2 and -O3. However, many targets override this behavior — for example, on x86 it's disabled at all levels, while on AArch64 it's enabled only at -O3.
  • -fschedule-insns2 — similar to -fschedule-insns, but it requests an additional pass of instruction scheduling after register allocation has been done. Especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle. Enabled at levels -O2, -O3, -Os. scheduling² 💖
  • -fno-sched-interblock — no scheduling across basic blocks bestie 💅 this one's usually on whenever you schedule before register allocation, like with -fschedule-insns or at -O2 or higher, so this flag is you saying "nvm, keep the blocks separate."
  • -fno-sched-spec — turns off speculative motion of non-load instructions 😌 which, same vibe as above, is normally enabled when scheduling happens before register allocation — i.e. with -fschedule-insns or at -O2 or higher.
  • -fsched-pressure — register pressure sensitive insn scheduling before register allocation ✨ this only makes sense when scheduling before register allocation is actually turned on, i.e. with -fschedule-insns or at -O2 or higher. Using it can improve the generated code and shrink its size by preventing register pressure from climbing above the number of available hard registers and the spills that come after in register allocation 🙈 iconically useful, ngl.
  • -fsched-spec-load — allows speculative motion of some load instructions 🦋 again, only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
  • -fsched-spec-load-dangerous — allows speculative motion of even more load instructions 💀 yeah the name is giving exactly what you think. 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* ** — sets how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list during the second scheduling pass 🌸 so: -fno-sched-stalled-insns means zero insns get moved prematurely, and -fsched-stalled-insns=0 means there's no limit on how many queued insns can be moved prematurely. And -fsched-stalled-insns with no value at all? That's equivalent to -fsched-stalled-insns=1. periodt.
  • -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 🧚 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, resulting in faster schedules. It's experimental though — not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm 😭. Also, this only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
  • -fsched-group-heuristic — enables the group heuristic in the scheduler 💜 it 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 — enables the critical-path heuristic in the scheduler 🎀 favors instructions 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 — enables the speculative instruction heuristic in the scheduler ✨ 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 — enables the rank heuristic in the scheduler 🌈 favors the instruction belonging 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 — enables the last-instruction heuristic in the scheduler 🫶 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 — enables the dependent-count heuristic in the scheduler 💖 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.
  • -fspeculatively-call-stored-functions — attempts to convert indirect calls of function pointers into pointers loaded from a structure field, but only if all visible stores to that field store just a single candidate 👀 when it does that, it turns the call into a conditional deciding between the direct call and the original indirect one. These speculative calls often enable more optimizations, like inlining, and when they seem useless after further optimization, they get converted back into their original form. cute little round trip, lowkey.
  • -freschedule-modulo-scheduled-loops — modulo scheduling happens before traditional scheduling 🔥 and if a loop is modulo scheduled, later scheduling passes may change its schedule. Use this option to control that behavior.
  • -fselective-scheduling — schedules instructions using the selective scheduling algorithm 🌸 selective scheduling runs instead of the first scheduler pass.
  • -fselective-scheduling2 — schedules 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 🦋 heads up: this option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
  • -fsel-sched-pipelining-outer-loops — when pipelining loops during selective scheduling, also pipeline outer loops 👑 again, no effect unless -fsel-sched-pipelining is turned on.
  • -fsemantic-interposition — some object formats, like ELF, allow interposing of symbols by the dynamic linker 😌 which means that for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining, and other optimizations, because the function or variable in question might change. This feature is useful, like for rewriting memory allocation functions with a debugging implementation, but it's expensive in terms of code quality 💀. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects), and similarly if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where it's never allowed for interposition to change semantics) and for symbols explicitly declared weak.
  • -fshrink-wrap — emits function prologues only before the parts of the function that actually need it, instead of dumping it at the top 🌸 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 needed ✨ this one's on by default, but has no effect unless -fshrink-wrap is also turned on and the target supports it.
  • -fcaller-saves — enables allocation of values to registers that get clobbered by function calls, by emitting extra instructions to save and restore the registers around those calls 💅 such allocation only happens when it seems to result in better code. This option is always enabled by default on certain machines, usually ones that have no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os.
  • -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them 🎀 enabled by default at -O1 and higher.
  • -fipa-ra — uses caller save registers for allocation if those registers aren't used by any called function 👀 in that case you don't need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. Enabled at levels -O2, -O3, -Os, but it's disabled if the generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly — which happens on targets that don't expose prologues and epilogues in RTL 😭.
  • -fconserve-stack — 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 🦋 it tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. This is especially useful as a code size optimization, but it often helps for code speed as well. 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 — performs full redundancy elimination (FRE) on trees 👑 the difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. Enabled by default at -O1 and higher.
  • -ftree-phiprop — performs hoisting of 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 the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction ✨ enabled by default at -O2 and higher.
  • -ftree-copy-prop — does copy propagation on trees. 💅 this pass yeets the unnecessary copy operations out of your life. enabled by default at -O1 and higher, so it's just doing its thing in the background bestie.
  • -fipa-pure-const — sniffs out which functions are pure or constant. enabled by default at -O1 and higher. ✨
  • -fipa-reference — finds the static variables that do not escape the compilation unit. enabled by default at -O1 and higher.
  • -fipa-reference-addressable — hunts down read-only, write-only, and non-addressable static variables. enabled by default at -O1 and higher. 🎀
  • -fipa-reorder-for-locality — groups call chains close together in the binary layout so code locality gets better and the jumps between frequently-called functions get shorter. 🦋 unlike -freorder-functions, this pass actually looks at the call chains between functions and groups those together, instead of dumping all hot/normal/cold/never-executed functions into separate sections. and unlike -fprofile-reorder-functions it's trying to make code locality better across the whole runtime of the program, not just flexing on program startup. this option is incompatible with an explicit -flto-partition= option, because it enforces its own custom partitioning scheme. if you're using this option it's recommended to also use profile feedback, but it's not enabled by default otherwise.
  • -fipa-stack-alignment — reduces stack alignment on call sites when it's possible. enabled by default. 💫
  • -fipa-pta — does interprocedural pointer analysis plus interprocedural modification and reference analysis. lowkey this one can eat excessive memory and compile-time on large compilation units. 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 get executed once (like cold, noreturn, static constructors or destructors) get identified. then cold functions and the loop-less parts of functions executed once get optimized for size. enabled by default at -O1 and higher. 👑
  • -fipa-modref — does interprocedural mod/ref analysis. this optimization looks at the side effects of functions (memory locations that get modified or referenced) and unlocks 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 the values passed to functions are constants, then optimizes accordingly. if your app passes constants to functions this can boost performance substantially fr fr. 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 functions 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 😬. 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-functions, -fipa-icf-variables, -fipa-icf — perform Identical Code Folding for functions (-fipa-icf-functions), read-only variables (-fipa-icf-variables), or both (-fipa-icf). this optimization reduces code size and 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.

even though the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels, so the optimizations are not the same — there are equivalences that only GCC finds and equivalences that only Gold finds. iykyk.

-fipa-icf is enabled by default at -O2 and -Os.

  • -flate-combine-instructions — enables two instruction combination passes that run pretty late in the compilation process. one of them runs before register allocation and the other runs after register allocation. the main goal of these passes is to substitute definitions into all uses. 💖

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

  • -flive-patching= level — controls GCC's optimizations so the output is suitable for live-patching. 🩹

if the compiler's optimization uses a function's body, or information pulled from its body, to optimize/change another function, that other one is called an impacted function of the first. so if a function gets patched, its impacted functions should be patched too.

the impacted functions get determined by the compiler's interprocedural optimizations. for example, a caller is impacted when a function gets inlined into its caller, or when a function gets cloned and its caller is changed to call this new clone, or when a function's pureness/constness info gets extracted to optimize its direct or indirect callers, etc.

usually, the more IPA optimizations you have enabled, the larger the number of impacted functions per function. to control how many impacted functions there are and to make it easier to compute the list of impacted functions, IPA optimizations can be partially enabled at two different levels.

the level argument should be one of these:

  • inline-clone — only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates, and partial inlining. as a result, when you patch a function, all its callers and its clones' callers are impacted, so they need patching as well.

-flive-patching=inline-clone disables these optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref

  • inline-only-static — only enable inlining of static functions. as a result, when you patch a static function, all its callers are impacted and so need patching as well.

on top of all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static disables these extra 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.

heads up bestie: -flive-patching is not supported with link-time optimization (-flto). 🙈
  • -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior from dereferencing a null pointer (with -fdelete-null-pointer-checks enabled) or from a division by zero. it isolates those paths away from the main control flow and turns the statement with erroneous or undefined behavior into a trap. ⚠️ enabled by default at -O2 and higher.
  • -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior because a null value is being used in a way that's forbidden by a returns_nonnull or nonnull attribute. it isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. this is not currently enabled, but may be enabled by -O2 in the future. 🔮
  • -ftree-sink — does forward store motion on trees. 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 works on local scalar variables and is enabled by default at -O1 and higher — except for -Og. it requires that -ftree-ccp is enabled. 🌸
  • -ftree-ccp — does sparse conditional constant propagation (CCP) on trees. this pass only works on local scalar variables and is enabled by default at -O1 and higher.
  • -fssa-backprop — propagates info about uses of a value up the definition chain so it can simplify the definitions. like, this pass strips sign operations when the sign of a value never even 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 them, 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 — does dead code elimination (DCE) on trees. enabled by default at -O1 and higher. 💀
  • -ftree-builtin-call-dce — does conditional dead code elimination (DCE) for calls to built-in functions that may set errno but are otherwise free of side effects. enabled by default at -O2 and higher, as long as -Os isn't also specified.
  • -ffinite-loops — assumes that a loop with an exit will eventually take that exit and won't loop forever. this lets the compiler remove loops that otherwise have no side-effects, because it doesn't count eventual endless looping as a side effect. 🔄

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

  • -ftree-dominator-opts — bestie this one does a whole lil spa day of simple scalar cleanups (that's constant/copy propagation, redundancy elimination, range propagation, and expression simplification) all based on a dominator tree traversal. It also does jump threading, which is just ✨removing jumps to jumps✨ so you're not hopping through pointless little middleman jumps. Enabled by default at -O1 and higher, no cap.
  • -ftree-dse — dead store elimination (DSE) on trees, periodt. So what's a dead store? It's when you store into a memory location, and then later some other store just overwrites it with no loads happening in between. That first store? Wasted. Deleted. Gone. 💅 On by default at -O1 and higher.
  • -ftree-ch — loop header copying on trees, and it's giving ✨beneficial✨ because it makes code motion optimizations way more effective. Bonus: it saves you one whole jump. Enabled by default at -O1 and higher, BUT it is not enabled for -Os, because it usually makes your code bigger and -Os said "not today, size queen."
  • -ftree-loop-optimize — loop optimizations on trees. That's it, that's the tweet. Enabled by default at -O1 and higher.
  • -ftree-loop-linear
  • -floop-strip-mine
  • -floop-block — all three of these do loop nest optimizations, and they're literally the same thing as -floop-nest-optimize. To actually use this code transformation tho, GCC has to be configured with --with-isl so the Graphite loop transformation infrastructure is enabled. No isl, no transform, sorry babe 🫶
  • -fgraphite-identity — turns on the identity transformation for graphite. For every SCoP we generate the polyhedral representation and then transform it right back to gimple. Using -fgraphite-identity you can vibe check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations also get performed by the code generator isl, like index splitting and dead code elimination in loops. It's giving "we went all the way around the block for the plot."
  • -floop-nest-optimize — enables the isl based loop nest optimizer. It's a generic loop nest optimizer based on the Pluto optimization algorithms, and it calculates a loop structure optimized for data-locality and parallelism. This option is experimental, so like… hold its hand 🥺
  • -floop-parallelize-all — uses the Graphite data dependence analysis to find loops that can be parallelized. Then it parallelizes all the loops that can be analyzed as not containing loop carried dependences — and it does NOT check whether it's actually profitable to parallelize them. So it's the "we're doing it anyway bestie" of optimizations 💀
  • -ftree-coalesce-vars — while it's transforming your program out of the SSA representation, it attempts to reduce copying by coalescing versions of different user-defined variables, not just compiler temporaries. Heads up: this may severely limit your ability to debug an optimized program compiled with -fno-var-tracking-assignments. In its negated form, this flag prevents SSA coalescing of user variables. Enabled by default if optimization is enabled, and honestly it does very little otherwise.
  • -ftree-loop-distribution — performs loop distribution. This flag can improve cache performance on big loop bodies and let further loop optimizations happen, like parallelization or vectorization. For example, the loop
        DO I = 1, N
          A(I) = B(I) + C
          D(I) = E(I) * F
        ENDDO

is transformed to

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

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

  • -ftree-loop-distribute-patterns — does loop distribution of patterns that can be code generated with calls to a library. Enabled by default at -O2 and higher, and by -fprofile-use and -fauto-profile.

This pass distributes the initialization loops and generates a call to memset zero. For example, the loop

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

is transformed to

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

and the initialization loop is transformed into a call to memset zero. memset said "I got this" 🫡

  • -floop-interchange — performs loop interchange outside of graphite. This can improve cache performance on loop nests and let further loop optimizations happen, like vectorization. For example, the loop
        for (int i = 0; i < N; i++)
          for (int j = 0; j < N; j++)
            for (int k = 0; k < N; k++)
              c[i][j] = c[i][j] + a[i][k]*b[k][j];

is transformed to

        for (int i = 0; i < N; i++)
          for (int k = 0; k < N; k++)
            for (int j = 0; j < N; j++)
              c[i][j] = c[i][j] + a[i][k]*b[k][j];

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.

  • -floop-unroll-and-jam — applies unroll and jam transformations on feasible loops. In a loop nest this unrolls the outer loop by some factor and fuses the resulting multiple inner loops. Enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
  • -ftree-loop-im — performs loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level — think function calls, and operations that expand to nontrivial sequences of insns. With -funswitch-loops it also moves operands of conditions that are invariant out of the loop, so we can use just trivial invariantness analysis in loop unswitching. The pass also includes store motion 🧚
  • -ftree-loop-ivcanon — creates a canonical counter for the number of iterations in loops where figuring out the iteration count requires complicated analysis. Later optimizations can then determine the number easily. Useful especially in connection with unrolling ✨
  • -ftree-scev-cprop — performs final value replacement. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, it replaces uses of the final value with such a computation — provided it's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher.
  • -fivopts — performs induction variable optimizations on trees: that's strength reduction, induction variable merging, and induction variable elimination. Enabled by default at -O1 and higher.
  • -ftree-parallelize-loops
  • -ftree-parallelize-loops=n — parallelizes loops, i.e. splits their iteration space so they run in multiple threads 🔥 This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive rather than constrained by stuff like memory bandwidth. This option implies -pthread, so it's only supported on targets that have support for -pthread.

When a positive value n is specified, the number of threads is fixed at compile time and cannot be changed after compilation. The compiler generates "#pragma omp parallel num_threads(n)".

When used without =n (i.e., -ftree-parallelize-loops), the number of threads is determined at program execution time via the OMP_NUM_THREADS environment variable. If OMP_NUM_THREADS is not set, the OpenMP runtime automatically detects the number of available processors and uses that value. This enables creating binaries that adapt to different hardware configurations without recompilation — fr fr, it's adaptive 💅

  • -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 so structures don't get committed 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 it 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 with less expensive calculations when possible. Enabled by default at -O1 and higher.
  • -ftree-vectorize — performs vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if they weren't explicitly specified.
  • -ftree-loop-vectorize — performs loop vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
  • -ftree-slp-vectorize — performs basic block vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile.
  • -ftrivial-auto-var-init=choice — initializes automatic variables or temporary objects with either a pattern or with zeroes, to bump up the security and predictability of a program by preventing uninitialized memory disclosure and use. GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized — -Wuninitialized and -Wanalyzer-use-of-uninitialized-value will still report warning messages on such automatic variables or temporary objects, and the compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also initialize to zeroes any padding of automatic variables or temporary objects that have structure or union types. However — the current implementation cannot initialize automatic variables whose initialization is bypassed through a "switch" or "goto" statement. Use -Wtrivial-auto-var-init to report all such cases.

The three values of choice are:

  • uninitialized doesn't initialize any automatic variables.
  • pattern — bestie, this one fills your automatic variables with values that are designed to turn logic bugs into crashes 💀 like, the whole point is that they're super easy to spot in a crash dump, and they're absolutely not values any programmer should ever rely on for useful program semantics. The current value is a byte-repeatable pattern with byte 0xFE. Ngl, the exact values used for pattern initialization might get changed in the future, so don't get attached. 🙈
    • zero — this one just initializes automatic variables with zeroes. Chill. 🎀

The default is uninitialized, except for C++26, in which case if -ftrivial-auto-var-init= is not specified at all, automatic variables or temporary objects are zero initialized — but zero initialization of padding bits does not happen. Periodt.

Okay so heads up: the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), not to their interpretation as numerical values. This distinction is lowkey important in languages that support types with biases or implicit multipliers, and with such extensions as hardbool.

For example ✨ a variable that uses 8 bits to represent (biased) quantities in the range 160..400 will be initialized with the bit patterns 0x00 or 0xFE, depending on choice — whether or not these representations stand for values in that range, and even if they do, the interpretation of the value held by the variable will depend on the bias. A hardbool variable that uses say 0x5A and 0xA5 for "false" and "true", respectively, will trap with either choice of trivial initializer. Like, zero initialization will not convert to the representation for "false", even if it would for a "static" variable of the same type. 😭 This means the initializer pattern doesn't generally depend on the type of the initialized variable.

One notable exception though, bestie: (non-hardened) boolean variables that fit in registers are initialized with "false" (zero), even when pattern is requested. 🫶

You can control this behavior for a specific variable by using the variable attribute "uninitialized" standard attribute or the C++26 "[[indeterminate]]".

  • -fvect-cost-model=model — this alters the cost model used for vectorization 👑. The model argument should be one of unlimited, dynamic, cheap or very-cheap. So here's the tea:
    • with the unlimited model, the vectorized code-path is assumed to be profitable 🔥
    • with the dynamic model, a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop
    • the cheap model disables vectorization of loops where doing so would be cost prohibitive — for example due to required runtime checks for data dependence or alignment — but otherwise is equal to the dynamic model
    • the very-cheap model disables vectorization of loops when any runtime check for data dependence or alignment is required, and it also disables vectorization of epilogue loops, but otherwise is equal to the cheap model

The default cost model depends on other optimization flags and is either dynamic or cheap. iykyk 💅

  • -fsimd-cost-model=model — alters the cost model used for vectorization of loops marked with the OpenMP simd directive 🌈. The model argument should be one of unlimited, dynamic, cheap. All values of model have the same meaning as described in -fvect-cost-model, and by default a cost model defined with -fvect-cost-model is used.
  • -ftree-vrp — performs Value Range Propagation on trees 🌳✨. This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This lets the optimizers remove unnecessary range checks like array bound checks and null pointer checks. It's enabled by default at -O2 and higher. Null pointer check elimination is only done if -fdelete-null-pointer-checks is enabled. Not me reminding you about that dependency 🙈
  • -fsplit-paths — okay bestie, this option is deprecated and does absolutely nothing 💀. It is accepted only for backward compatibility.
  • -fsplit-ivs-in-unroller — enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes. Slay. 🎀

A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass. 😭

This optimization is enabled by default.

  • -fvariable-expansion-in-unroller — with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code. 👑

This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.

  • -fpartial-inlining — inline parts of functions 🧩. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

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

  • -fpredictive-commoning — performs predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops 🔁✨.

This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.

  • -fprefetch-loop-arrays — if supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays 🏃‍♀️💨.

This option may generate better or worse code; results are highly dependent on the structure of loops within the source code. So like, vibe check your loops first, fr fr.

Disabled at level -Os.

  • -fno-printf-return-value — do not substitute constants for known return value of formatted output functions such as "sprintf", "snprintf", "vsprintf", and "vsnprintf" (but not "printf" of "fprintf"). This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible.

For example, when -fprintf-return-value is in effect, both the branch and the body of the "if" statement (but not the call to "snprint") can be optimized away when "i" is a 32-bit or smaller integer because the return value is guaranteed to be at most 8. 🌸

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  ...

The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default.

  • -fno-peephole
  • -fno-peephole2 — disable any machine-specific peephole optimizations 🔍. The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.

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

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

GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by "__builtin_expect", then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the "__builtin_expect" info into account. The interactions between the heuristics and "__builtin_expect" can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of "__builtin_expect" are easier to understand. No cap, it gets messy in there. 😵‍💫

It is also possible to specify expected probability of the expression with "__builtin_expect_with_probability" built-in function.

The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

  • -freorder-blocks — reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality 🧚.

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

  • -freorder-blocks-algorithm=algorithm — use the specified algorithm for basic block reordering. The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code. 💅

The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.

  • -freorder-blocks-and-partition — in addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance 🔥.
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections.

When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker). Careful with that one bestie 🫶

Enabled for x86 at levels -O2, -O3, -Os.

  • -freorder-functions — reorder functions in the object file in order to improve code locality. Unlike -fipa-reorder-for-locality this option prioritises grouping all functions within a category (hot/normal/cold/never-executed) together. This is implemented by using special subsections ".text.hot" for most frequently executed functions and ".text.unlikely" for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way. 🎀

This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with "hot" or "cold" attributes.

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

  • -fstrict-aliasing — bestie, this one lets the compiler assume the strictest aliasing rules that even apply to whatever language you're compiling fr. For C (and C++), it turns on optimizations based on the type of expressions. So like, accessing an object of one type through an expression of a different type? NOT allowed 🚫 unless the types are compatible types, differ only in signedness or qualifiers, or the expression has a character type. Accessing scalar objects via a corresponding vector type is also allowed btw ✨

For example, an unsigned int can alias an int, but it can not alias a void* or a double. A character type may alias any other type 💅

Pay extra special attention to code like this, no cap:

union a_union {
  int i;
  double d;
};

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

The practice of reading from a different union member than the one most recently written to (called "type-punning", iykyk) is common. Even with -fstrict-aliasing, type-punning is allowed in C, provided the memory is accessed through the union type. In ISO C++, type-punning through a union type is undefined behavior 😭, but GCC supports it as an extension. So, the code above works as expected. HOWEVER this code might not:

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

Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os. 📌
  • -fipa-strict-aliasing — this one controls whether the rules of -fstrict-aliasing get applied across function boundaries. Note that if multiple functions get inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary 👀
The -fipa-strict-aliasing option is enabled by default and is effective only in combination with -fstrict-aliasing. 💖
  • -falign-functions
  • -falign-functions=n
  • -falign-functions=n:m
  • -falign-functions=n:m:n2
  • -falign-functions=n:m:n2:m2 — align the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes 🎯. This ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. This is an optimization of code performance and alignment is ignored for functions considered cold 🥶. If alignment is required for all functions, use -fmin-function-alignment.

If m is not specified, it defaults to n.

Examples, because we love a demo: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less ✨

The second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2. Iconic behaviour tbh 💅

Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.

-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3. 🔥
  • -flimit-function-alignment — if this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function 🫶
  • -falign-labels
  • -falign-labels=n
  • -falign-labels=n:m
  • -falign-labels=n:m:n2
  • -falign-labels=n:m:n2:m2 — align all branch targets to a power-of-two boundary ⭐

Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead 👑.

If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3. 💜
  • -falign-loops
  • -falign-loops=n
  • -falign-loops=n:m
  • -falign-loops=n:m:n2
  • -falign-loops=n:m:n2:m2 — align loops to a power-of-two boundary 🔄. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. This is an optimization of code performance and alignment is ignored for loops considered cold 🥶

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1 are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.

If n is not specified or is zero, use a machine-dependent default.

Enabled at levels -O2, -O3. 🍓
  • -falign-jumps
  • -falign-jumps=n
  • -falign-jumps=n:m
  • -falign-jumps=n:m:n2
  • -falign-jumps=n:m:n2:m2 — align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by jumping 🦋. In this case, no dummy operations need be executed. This is an optimization of code performance and alignment is ignored for jumps considered cold 🥶

If -falign-labels is greater than this value, then its value is used instead.

Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.

If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3. 🌸
  • -fmin-function-alignment — specify minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike -falign-functions this alignment is applied also to all functions (even those considered cold 🥶). The alignment is also not affected by -flimit-function-alignment 💅
  • -fno-allocation-dce — do not remove unused C++ allocations (using operator new and operator delete) in dead code elimination 🗑️

See also -fmalloc-dce.

  • -fallow-store-data-races — allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the variable cannot be concurrently accessed by other threads 😳. Does not affect optimization of local data. It is safe to use this option if it is known that global data will not be accessed by multiple threads.

Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization 👀

Enabled at level -Ofast. ⚡
  • -funit-at-a-time — this option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors 🫠
Enabled by default. ✅
  • -fno-toplevel-reorder — do not reorder top-level functions, variables, and asm statements. Output them in the same order that they appear in the input file 📄. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible 💡

-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors.

  • -funreachable-traps — with this option, the compiler turns calls to __builtin_unreachable into traps 🪤, instead of using them for optimization. This also affects any such calls implicitly generated by the compiler.

This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but does not affect the values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one 👑

This option is enabled by default at -O0 and -Og. 💖
  • -fweb — constructs webs as commonly used for register allocation purposes and assigns each web an individual pseudo register 🕸️. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover 🔥. It can, however, make debugging impossible 💀, since variables no longer stay in a "home register".
Enabled by default with -funroll-loops. 🔁
  • -fwhole-program — assume that the current compilation unit represents the whole program being compiled 🌍. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers 💅

With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful if no linker plugin is used or during incremental link step when final code is produced (with -flto -flinker-output=nolto-rel) ✨

okay so -flto[=n] is literally the whole-program glow-up flag and I need you to be seated for this one 💖

This one turns on the standard link-time optimizer. When you compile source code with it, GCC generates GIMPLE (one of its internal representations, iconic behavior honestly) and tucks it into special ELF sections inside the object file. Then when the object files get linked together, all the function bodies get read back out of those ELF sections and instantiated as if they'd all been one big translation unit the whole time. the way that works?? unreal 🦋

To actually use the link-time optimizer, -flto and your optimization options need to be specified at compile time and during the final link. Bestie, the recommendation is: compile all the files that are participating in the same link with the same options, and also pass those same options at link time. Like so:

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

The first two invocations save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads that GIMPLE bytecode back from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. And because both are merged into one image, all the interprocedural analyses and optimizations in GCC work across the two files like they were a single one. Which means — for example — the inliner can inline functions in bar.o into functions in foo.o and vice-versa. cross-file inlining?? she ate 💅

There's also a simpler way to enable link-time optimization:

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

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

k but the important thing to keep in mind: to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs LTO 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 effective, you gotta make certain assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When the linker supports it, the linker plugin (see -fuse-linker-plugin) passes info to the compiler about used and externally visible symbols. When the linker plugin isn't available, -fwhole-program should be used to let the compiler make these assumptions — which leads to more aggressive optimization decisions 👑

When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file, because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means object files with LTO info can be linked as normal object files; and if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but — heads up 😭 — you cannot perform a regular, non-LTO link on them.

When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. So you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. it just knows 💅

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. Note that it's generally ineffective to specify an optimization level option only at link time and not at compile time — for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time. so like, do both okay?? 🙈

🎀 the flags that get preserved

There are some code generation flags preserved by GCC when generating bytecodes, since they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

The following options — -fPIC, -fpic, -fpie and -fPIE — are combined based on this scheme:

B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. no negotiating with her 💀

Other options — like -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing — are passed through to the link stage and merged conservatively for conflicting translation units. Specifically, -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time.

Diagnostic options such as -Wstringop-overflow are passed through to the link stage, and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. And note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code not consistent with the setting at compile time. it's giving slightly chaotic ngl 🌀

When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time. pick a lane bestie 🫶

To enable debug info generation you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. 🙅‍♀️

If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. But bestie — the behavior is still undefined at run time. Similar diagnostics may be raised for other languages. so like, don't do that 😭

Another feature of LTO? It's possible to apply interprocedural optimizations on files written in different languages:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. polyglot queen 🌈

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it's possible to extract and use them in an LTO link if you're using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure the library participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. teamwork makes the dream work ✨

If you're 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.

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. it's giving exclusivity but make it compiler versions 🙈

Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.

If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. parallelize and slay 👑

okay so you can also say -flto=jobserver and it'll ride GNU make's job server mode to figure out how many parallel jobs are happening ✨. This is the move when the Makefile that's calling GCC is already running in parallel — but bestie, you must prepend a + to the command recipe in the parent Makefile or it will not work 🫶. This option likely only works if MAKE is GNU make. And honestly? Even if you don't pass a value at all, GCC tries to automatically detect a running GNU make's job server 👑.

Then there's -flto=auto, which uses GNU make's job server if it's available, and otherwise falls back to autodetecting how many CPU threads your system has 🌸.

  • -flto-partition=alg — picks the partitioning algorithm the link-time optimizer uses. Your values are: 1to1 (partitioning that mirrors the original source files 💅), balanced (chunks of equal size, whenever that's even possible), max (a brand new partition for every symbol, where possible), or cache (balances chunk sizes while keeping related symbols together, which is better for caching in incremental LTO 🧚). Passing none disables partitioning and streaming completely. The default is balanced. Now: 1to1 can serve as a workaround for various code ordering issues, but max is intended for internal testing only 🔥. The value one says exactly one partition should be used, and none bypasses partitioning and runs the link-time optimization step directly from the WPA phase.
  • -flto-incremental=path — enables incremental LTO, with its cache living in the existing directory you give it. Can significantly shorten edit-compile cycles with LTO. When you use it with LTO (-flto), the output of translation units inside LTO gets cached, and cached translation units are likely to show up again when you recompile with small code changes — which means recompile time goes down 💖. Multiple GCC instances can use the same cache in parallel, no cap.
  • -flto-incremental-cache-size=n — sets the number of cache entries in incremental LTO after which old entries get pruned. It's a soft limit, so temporarily there may be more entries than that. It's giving "vibes, not a promise."
  • -flto-compression-level=n — the level of compression used for the intermediate language written to LTO object files. Only meaningful alongside LTO mode (-flto). GCC currently supports two LTO compression algorithms: for zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9. Values outside the range get clamped to either the minimum or the maximum of the supported values. If you don't give the option, a default balanced compression setting is used ✨.
  • -flto-toplevel-asm-heuristics — turns on heuristics that find symbols used in top-level basic "asm" 🌸. This restricts link-time optimizations that could cause renaming or deletion of such symbols, which would otherwise result in missing symbol errors by the linker. This flag is intended for projects that haven't converted to using top-level extended "asm", which specifies the usage directly without any false positives. Heads up: the heuristics are simple and do not parse the assembly — they scan through top-level assembly for all possible identifiers, and if an identifier is found among declared symbols, that symbol gets marked to restrict link-time optimizations. Static symbols disable more optimizations 💀. Identifiers followed by ':' disable more optimizations as well, because they might be a locally defined symbol in assembly, even when the declaration is marked 'extern'.
  • -fuse-linker-plugin — enables use of a linker plugin during link-time optimization. It 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, which 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 containing both the intermediate language and the object code, which makes them usable for both LTO linking and normal linking 💅. Only effective when compiling with -flto, and it's ignored at link time. Meanwhile -fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be LTO-aware. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non-fat LTO, makefiles need to be modified to use them 🫶. Note that modern binutils provide a plugin auto-load mechanism: installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib). The default is -fno-fat-lto-objects on targets with linker plugin support.
  • -fcompare-elim — after register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic, and 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 and -fno-fold-mem-offsets — try to eliminate add instructions by folding them into memory loads/stores 🎀. Enabled at levels -O2, -O3.
  • -fcprop-registers — after register allocation and post-register allocation instruction splitting, run a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy 💖. Enabled at levels -O1, -O2, -O3, -Os.
  • -fprofile-correction — profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates 😭. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. This option is enabled by -fauto-profile.
  • -fprofile-partial-training — with "-fprofile-use", all portions of programs not executed during training runs are optimized aggressively for size rather than speed. In some cases it's just not practical to train all possible hot paths in the program (for example, it may contain functions specific to a given hardware, and training may not cover all hardware configurations the program later runs on 🙈). With "-fprofile-partial-training", profile feedback is ignored for all functions not executed during the training runs, causing them to be optimized as if they were compiled without profile feedback. This leads to better performance when the training is not representative, at the cost of significantly bigger code 💀.
  • -fprofile-use, -fprofile-use=path — enable profile feedback-directed optimizations, 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://perfwiki.github.io/main/.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data 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
  • -fauto-profile-inlining — when auto-profile is available, inline all relevant functions which was inlined in the tran run before reading the profile feedback ✨. This improves context sensitivity of the profile. Enabled by default 🫶.

ok so bestie, buckle UP because we're about to enter the floating-point zone and this is where compilers get ✨opinionated✨. The next batch of options controls how the compiler behaves around floating-point arithmetic — and yes, every single one of them is a trade-off between speed and correctness. And ngl? All of them have to be specifically enabled. No freebies. Periodt. 💅

  • -fexcess-precision=style — this one lets you control excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types. Picture this: x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format. For most programs that extra precision is totally harmless, bestie, but some programs rely on the C or C++ language standards' requirements for handling IEEE values, so for them it matters. 🙈

By default, -fexcess-precision=fast is in effect — meaning operations may be carried out in a wider precision than the types specified in the source if that gets you faster code, and it's unpredictable when rounding to the source-code types actually happens. When compiling C or C++, if you specify -fexcess-precision=standard then excess precision follows the rules in ISO C99 or C++, and specifically 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++ when a strict conformance option like -std=c99 or -std=c++17 is used. And -ffast-math turns on -fexcess-precision=fast by default no matter what, even if you're using a strict conformance option. If you say -fexcess-precision=16, then constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision.

Heads up bestie: -fexcess-precision=standard is not implemented for languages other than C or C++. And on x86 it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified — in the former case IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. 😭
  • -ffloat-store — do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory. This option has generally been subsumed by -fexcess-precision=standard, which is more general (iconic of it honestly). If you do use -ffloat-store, you may need to modify your program to explicitly store intermediate computations in temporary variables, because -ffloat-store handles rounding to IEEE format only on assignments and not casts, the way -fexcess-precision=standard does. 🌸
  • -ffast-math — sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast. 🎀

This option causes the preprocessor macro __FAST_MATH__ to be defined.

This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications.

  • -fno-math-errno — don't set errno after calling math functions that are executed with a single instruction, e.g. sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while 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 don't require the guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets errno. So there's genuinely no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default there. 💖

  • -funsafe-math-optimizations — allow optimizations for floating-point arithmetic that (a) assume arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. 🦋

This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications. Enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

  • -fassociative-math — allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. ⚠️
NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52. May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required.

This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.

The default is -fno-associative-math.

  • -freciprocal-math — allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 😬

The default is -fno-reciprocal-math.

  • -ffinite-math-only — allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

This option is not turned on by any -O option besides -Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't 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.

  • -frounding-math — disable transformations and optimizations that assume default floating-point rounding behavior (round-to-nearest). This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes. 💜

The default is -fno-rounding-math.

This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode.

Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -ftrapping-math to specify the default state for FENV_ACCESS.

  • -fsignaling-nans — compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math. 🫶

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined.

The default is -fno-signaling-nans.

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.
  • -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. Simple. Clean. Slay. ✨
  • -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there's no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. The option is enabled by -ffast-math. 💅

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages.

  • -fcx-fortran-rules — complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there's no checking whether the result of a complex multiplication or division is NaN + I*NaN, with an attempt to rescue the situation in that case. 🌈
  • -fcx-method=method — complex multiplication and division follow the stated method. The method argument should be one of limited-range, fortran or stdc. 👑

The default is to honor language specific constraints, which means fortran for Fortran and stdc otherwise. 💅

Okay bestie, real talk — the next pile of flags is the "extra credit" section. These all control optimizations that can improve performance but are not switched on by any -O option. Oh, and heads up: this section also includes experimental options that may produce broken code. 😭 So like, vibe check before you yeet them into your build. ✨

  • -fbranch-probabilities — after you run a program compiled with -fprofile-arcs, you compile it a second time with -fbranch-probabilities so the compiler can level up its optimizations using how many times each branch actually got taken.

When a program compiled with -fprofile-arcs exits, it saves its arc execution counts into a file called sourcename.gcda for each source file. 🎀 The info in that data file is super dependent on the structure of the generated code, so listen: you must use the same source code and the same optimization options for both compilations. See the deets about file naming in -fprofile-arcs.

With -fbranch-probabilities, GCC sticks a REG_BR_PROB note on every JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently they're only used in one place: in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values exactly determine which path is taken more often. no cap 🔥

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

  • -fprofile-values — if you combine it with -fprofile-arcs, it adds code that gathers data about the values of expressions in your program. 👀

With -fbranch-probabilities, it reads back the data gathered from profiling expression values so optimizations can actually use it.

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

  • -fprofile-reorder-functions — function reordering based on profile instrumentation collects the first time of execution of a function and orders those functions in ascending order, aiming to optimize program startup through more efficient loading of text segments. it's giving "load fast, main character" 🦋

Enabled with -fprofile-use.

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

With -fbranch-probabilities, it reads that gathered data back and actually does the optimizations based on it. Currently the optimizations include specialization of division operations using knowledge about the value of the denominator. math girlies rise 🧚

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

  • -frename-registers — attempts to avoid false dependencies in scheduled code by using registers left over after register allocation. This optimization benefits processors with lots of registers the most. 💖 But — depending on the debug information format the target adopted, it can make debugging impossible, since variables no longer stay in a "home register". not the home register 💀

Enabled by default with -funroll-loops.

  • -fschedule-fusion — does a target dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently when they're adjacent to each other in the instruction flow. bestie behavior, honestly 🫶

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

  • -fdep-fusion — detects macro-op fusible pairs made of single-use instructions and their uses, and places those pairs together in the instruction stream to create more hardware fusion opportunities. This pass runs once before register allocation, and another time before register renaming. ✨

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

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

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

  • -funroll-loops — unrolls loops whose number of iterations can be determined at compile time or upon entry into the loop. -funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code larger, and may or may not make it run faster. 🔥

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

  • -funroll-all-loops — unrolls all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. 😭 -funroll-all-loops implies the same options as -funroll-loops.
  • -fpeel-loops — peels loops where there's enough information that they don't roll much (from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations).

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

  • -fmalloc-dce — controls whether "malloc" (and its variants such as "calloc" or "strdup") can be optimized away, provided its return value is only used as a parameter of a "free" call or compared with "NULL". If -fmalloc-dce=1 is used, only calls to "free" are allowed, while with -fmalloc-dce=2 comparisons with 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. Heads up: for this option to have an effect, -ftree-loop-im has to be enabled as well. Enabled at level -O1 and higher, except for -Og.
  • -fsplit-loops — splits a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other. ✂️

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

  • -funswitch-loops — moves branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to the result of the condition).

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

  • -fversion-loops-for-strides — if a loop iterates over an array with a variable stride, it creates another version of the loop that assumes the stride is always one. For example:
for (int i = 0; i < n; ++i)
  x[i * stride] = ...;

becomes:

if (stride == 1)
  for (int i = 0; i < n; ++i)
    x[i] = ...;
else
  for (int i = 0; i < n; ++i)
    x[i * stride] = ...;

This is especially 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. obsessed 😍

  • -ffunction-sections, -fdata-sections — place each function or data item into its own section in the output file, if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. 👑

With link-time optimization, an implicitly generated function section name is derived from the final assembler name, unless the compiler deliberately places related functions — such as a thunk and its target — in the same section. The final assembler name may include a compiler-generated suffix that distinguishes identically named internal-linkage items from different translation units.

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option), these options may lead to smaller statically-linked executables (after stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats. 🙈

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler that use relative locations inside a translation unit, since the locations are unknown until link time. One example of such an optimization is relaxing calls to short call instructions. so like, weigh it, bestie. 💅
  • -fstdarg-opt — optimizes the prologue of variadic argument functions with respect to usage of those arguments.
  • -fsection-anchors — tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help reduce the number of GOT entries and GOT accesses on some targets. 🎀

For example, the implementation of the following function "foo":

static int a, b, c;
int foo (void) { return a + b + c; }

usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):

int foo (void)
{
  register int *xr = &x;
  return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}

Not all targets support this option.

  • -fzero-call-used-regs=choice — zeroes call-used registers at function return to increase program security, either by mitigating Return-Oriented Programming (ROP) attacks or by preventing information leakage through registers. 🛡️✨

The possible values of choice are the same as for the "zero_call_used_regs" attribute. The default is skip.

You can control this behavior for a specific function by using the function attribute "zero_call_used_regs". main character security energy 💜

🎀 Program Instrumentation Options 💅

okay bestie so GCC has a whole lil wardrobe of command-line options that sprinkle run-time instrumentation onto the code it generates ✨. Like, one of the classic uses is collecting profiling statistics so you can find your program's hot spots 🔥, do code coverage analysis, or feed profile-guided optimizations. Another whole vibe of instrumentation is adding run-time checks that catch programming oopsies — invalid pointer dereferences, out-of-bounds array accesses — plus the deliberately hostile attacks like stack smashing or C++ vtable hijacking 💀. And there's also a general hook you can use to build other flavors of tracing or function-level instrumentation for debug/program analysis purposes, iykyk 🦋.

  • -p, --profile, -fprofile, -pg — bestie these four make GCC generate extra code that writes out profile info suitable for the analysis program prof (that's -p, --profile, and -fprofile) or gprof (that's -pg) 👑. You must use this option when you're compiling the source files you want data about, and you also have to use it when linking. fr fr, both times.

You can use the function attribute no_instrument_function to suppress profiling on individual functions when you're compiling with these options 🙈.

  • -fprofile-arcs — adds code so program flow arcs get instrumented 🌸. During execution the program records how many times each branch and call gets executed, and how many times it's taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors, and C++ constructors (and destructors) of classes 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 💾. That data can be used for profile-directed optimizations (-fbranch-probabilities) or for test coverage analysis (-ftest-coverage). Each object file's auxname comes from the name of the output file — if it's explicitly specified and it's not the final executable — otherwise it's the basename of the source file. Either way any suffix gets removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).

Heads up bestie: if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files 👀.
  • -fcondition-coverage — adds code so program conditions get instrumented ✨. During execution the program records which terms in a conditional actually contribute to a decision, which you can use to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read with gcov --conditions 🧚.
  • -fpath-coverage — adds code so the paths taken get tracked 🌈. During execution the program records the prime paths taken. The number of paths grows very fast with complexity, and to avoid the compile times absolutely exploding 💥, 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, because GCC uses a pessimistic heuristic that slightly overcounts the running number of paths, and it 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's meant to limit the effect of path explosion and keep compile times reasonable. The default is 250000.
  • --coverage, -coverage — this one compiles and links code instrumented for coverage analysis 🎀. The options -coverage and --coverage are equivalent; both are a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the docs for those options for more deets.
    • Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program 💅.
    • Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This lets gcov find the correct sources in projects where compilations happen with different working directories 🗂️.
    • Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
    • Run the program on a representative workload to generate the arc profile information. This can be repeated any number of times, no cap. You can run concurrent instances of your program, and as long as the file system supports locking, the data files will be correctly updated 🔒. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting 👏.

Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged.

  • For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities 🚀.
  • For test coverage analysis, use gcov to produce human-readable information from the .gcno and .gcda files. Refer to the gcov documentation for further info 📖.

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 those arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code.

With -fcondition-coverage, for each conditional in your program GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression 🎯.

With -fpath-coverage, GCC finds and enumerates and records the taken prime paths of each function, unless the number of paths would exceed the limit controlled by -fpath-coverage-limit. If the limit is exceeded the function is not instrumented, as if -fpath-coverage was not used at all 🙅. 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 — produces 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, jsyk bestie.
  • -fprofile-abs-path — automatically converts relative source file names to absolute path names in the .gcno files 🛤️. This lets gcov find the correct sources in projects where compilations occur with different working directories.
  • -fprofile-dir=path — sets the directory to search for the profile data files in, to path 📁. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, so the profile data file appears in the same directory as the object file. In order to prevent file name clashing, if the object file name is not an absolute path, GCC mangles the absolute path of the sourcename.gcda file and uses it as the file name of a .gcda file. See details about the file naming in -fprofile-arcs. See similar option -fprofile-note.

When an executable is run in a massive parallel environment, it's recommended to save profile to different folders 🌀. That can be done with variables in path that are exported during run-time:

  • %p — process ID.
  • %q{VAR} — value of environment variable VAR.
  • -fprofile-generate, -fprofile-generate=path — enables options usually used for instrumenting an application to produce a profile useful for later recompilation with profile feedback based optimization ✨. You must use -fprofile-generate both when compiling and when linking your program.

The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.

To optimize the program based on the collected profile information, use -fprofile-use 👑.

  • -fprofile-info-section, -fprofile-info-section=name — registers the profile information in the specified section instead of using a constructor/destructor 🎀. The section name is name if it's specified, otherwise the section name defaults to .gcov_info. A pointer to the profile information generated by -fprofile-arcs is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor, and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux 🚫. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O.

The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:

.gcov_info      :
{
  PROVIDE (__gcov_info_start = .);
  KEEP (*(.gcov_info))
  PROVIDE (__gcov_info_end = .);
}

The program could dump the profiling information registered in this linker set for example like this:

Here's the code in question, bestie — and yeah, it's a whole little program, I'm not even kidding 💅

#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;
}

And the merge-stream subcommand of gcov-tool? That's your girl for deserializing the data stream that the __gcov_filename_to_gcfn and __gcov_info_to_gcda functions generate — and then she merges the profile info into .gcda files on the host filesystem. slay. ✨

  • -fprofile-note=path — if you actually hand over a path, GCC writes the .gcno file right into that path location. But heads up bestie: if you combine this option with multiple source files, that .gcno file is gonna get overwritten. one file, one path, it's giving chaos otherwise 😭
  • -fprofile-prefix-path=path — okay so this one works with profile-generate=profile_dir and profile-use=profile_dir, and its whole job is telling GCC where the base directory of your built source tree is. By default, profile_dir ends up stuffed with files carrying mangled absolute paths of every object file in the built project. Which is annoying when the directory you used to build the instrumented binary is not the directory you used to build the binary that's optimized with profile feedback — because then the profile data just... isn't found during the optimized build. tragic. So in those setups you use -fprofile-prefix-path=path with path pointing at the base directory of the build, and it strips off the irrelevant part of the path and keeps all file names relative to the main build directory. chef's kiss 🫶
  • -fprofile-prefix-map=old=new — when you're compiling files that live in directory old, this records profiling information (with --coverage) describing them as if they lived in directory new instead. see also -ffile-prefix-map and -fcanon-prefix-map. ✨
  • -fprofile-update=method — changes up the update method for an application that's instrumented for profile-feedback-based optimization. method has to be one of single, atomic or prefer-atomic. The first one is the move for single-threaded applications, while the second one stops profile corruption by emitting thread-safe code.
Warning: when an application doesn't properly join all threads (or goes and creates a detached thread), a profile file can still get corrupted. 💀

Using prefer-atomic gets transformed into either atomic (when the target supports it) or single (otherwise). And the GCC driver automatically picks prefer-atomic whenever -pthread is on the command line — otherwise the default method is single.

If atomic is selected, then the profile information gets updated using atomic operations on a best-effort basis. Like, ideally it's updated through atomic operations in hardware. If the target platform doesn't support the required atomic operations in hardware but libatomic is available, then the profile info gets updated through calls to libatomic. And if the target platform supports neither the required hardware atomic operations nor libatomic, then the profile information is not atomically updated and a warning gets issued. In that case, the profiling info you end up with may be corrupt for multi-threaded applications. 🙈

And for performance reasons: if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance-critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations while they're updating a counter, then the profiling information may end up in an inconsistent state. noted. 📝

  • -fprofile-filter-files=regex — only instrument functions from files whose name matches any of the regular expressions (and those are 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'. iconic behaviour, honestly.

  • -fprofile-exclude-files=regex — the inverse vibe: only instrument functions from files whose name does not match any of the regular expressions (again, semi-colon separated).

For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files that are located in the /usr/ folder. bye felicia 👋

  • -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 stuff like distribution packages. 📦

With -fprofile-reproducible=serial, the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it isn't multi-threaded, and profile data streaming always happens in the same order. and side note: profile streaming happens at the end of the program run, but also before the fork function is invoked.

Also note that it's pretty common for execution counts of some parts of programs to depend on stuff like the length of temporary file names or memory space randomization (which can affect hash-table collision rate). That kind of non-reproducible part of a program can be annotated with the no_instrument_function function attribute. And gcov-dump with -l can be used to dump the gathered data and verify that it's actually reproducible. 💖

With -fprofile-reproducible=parallel-runs, the collected profile stays reproducible no matter what order the data gets streamed into the gcda files. This setting is what makes it possible to run multiple instances of the instrumented program in parallel (like with "make -j"). The trade-off? It reduces the quality of the gathered data — in particular of indirect call profiling. 😬

heads up: this option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. 💀 Also, the only targets -fsanitize=hwaddress is currently supported on are x86-64 (and only with the "-mlam=u48" or "-mlam=u57" options) and AArch64, in both cases only in ABIs with 64-bit pointers. Similarly, -fsanitize=memtag-stack is currently only supported on AArch64 ABIs with 64-bit pointers.

When you compile with -fsanitize=address, you should also use -g so the output is actually meaningful. And to get more accurate stack traces, you can use options like -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls — and this one 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. And since multiple runs of the program may produce backtraces with different addresses thanks to ASLR (Address Space Layout Randomization), you might wanna turn ASLR off — on Linux you can do that with setarch uname -m -R ./prog. 🌈

  • -fsanitize=kernel-address — turns on AddressSanitizer for the Linux kernel. see https://github.com/google/kernel-sanitizers for more. 🔥
  • -fsanitize=hwaddress — turns on Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with a low memory overhead. Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. This option enables -fsanitize-address-use-after-scope. Details at https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html. The run-time behaviour can be influenced with the HWASAN_OPTIONS environment variable — set it to "help=1" and the available options get shown at startup of the instrumented program.
and nope, this one cannot be combined with -fsanitize=thread or -fsanitize=address, and it's currently only available on AArch64. periodt. 👑
  • -fsanitize=kernel-hwaddress — turns on Hardware-assisted AddressSanitizer for compiling the Linux kernel. It's like -fsanitize=kernel-address but with an alternate instrumentation method, and like -fsanitize=hwaddress but with the instrumentation differences that are necessary to compile the Linux kernel. Those differences exist to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte. 🧚
Note: this option has different defaults than -fsanitize=hwaddress — instrumenting the stack and alloca calls is not on by default. And using a random frame tag isn't implemented for kernel instrumentation.
  • -fsanitize=memtag-stack — uses Memory Tagging Extension instructions instead of instrumentation so it can sniff out memory errors. Like HWASAN, it's a probabilistic method bestie, so it's not a perfect little angel. This one is only available on AArch64 architectures that support Memory Tagging Extensions. Periodt. 💅
  • -fsanitize=pointer-compare — instruments comparison operations (<, <=, >, >=) when they've got pointer operands. It must be combined with either -fsanitize=kernel-address or -fsanitize=address. It cannot be combined with -fsanitize=thread.
Heads up: by default the check is disabled at run time. To turn it on, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. If you use "detect_invalid_pointer_pairs=1" instead, it only flags an invalid operation when both pointers are non-null. 🫶
  • -fsanitize=pointer-subtract — instruments subtraction with pointer operands. Same rules, same bestie energy: it must be combined with either -fsanitize=kernel-address or -fsanitize=address, and it cannot be combined with -fsanitize=thread.
Also same as above: the check is disabled at run time by default. Enable it by adding "detect_invalid_pointer_pairs=2" to the ASAN_OPTIONS environment variable. "detect_invalid_pointer_pairs=1" only detects an invalid operation when both pointers are non-null. ✨
  • -fsanitize=shadow-call-stack — enables ShadowCallStack, a security glow-up that protects your programs against return address overwrites (hi, stack buffer overflows 👋). The way it works is genuinely iconic: it saves a function's return address to a separately allocated shadow call stack in the function prologue, and restores that return address from the shadow call stack in the function epilogue. Instrumentation only happens in functions that actually need to save the return address to the stack.

Right now it only supports the aarch64 platform. It's specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For user space programs, runtime support is not currently provided in libc and libgcc. If you wanna use this feature in user space, you gotta bring your own runtime support. Also it should be noted that this may cause the ABI rules to be broken. 😭

On aarch64, the instrumentation makes use of the platform register "x18". Which basically means: any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18, otherwise functions compiled without -ffixed-x18 might clobber "x18" and therefore corrupt the shadow stack pointer. Not the vibe.

Also, because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions. 🧚

More details live at https://clang.llvm.org/docs/ShadowCallStack.html.

When you compile with -fsanitize=thread, you should also use -g so you get more meaningful output. 🔥

Also note: sanitized atomic builtins cannot throw exceptions when they operate on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).

  • -fsanitize=leak — enables LeakSanitizer, a memory leak detector. This one only matters for linking of executables. The executable gets linked against a library that overrides malloc and other allocator functions. More at https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer. You can influence the run-time behavior using the LSAN_OPTIONS environment variable. This option cannot be combined with -fsanitize=thread.
  • -fsanitize=undefined — enables UndefinedBehaviorSanitizer</>, a fast undefined behavior detector. Various computations get instrumented so undefined behavior can be caught at runtime. More at https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html. You can influence the run-time behavior using the UBSAN_OPTIONS environment variable. Current suboptions are:
    • -fsanitize=shift — checks that the result of a shift operation is not undefined. Note that what exactly counts as undefined differs slightly between C and C++, and also between ISO C90 and C99, etc. This option has two suboptions: -fsanitize=shift-base and -fsanitize=shift-exponent.
    • -fsanitize=shift-exponent — checks 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, it checks that the result of the shift operation is not undefined. Again, what exactly counts as undefined differs slightly between C and C++, and between ISO C90 and C99, etc.
    • -fsanitize=integer-divide-by-zero — detects integer division by zero. 🚫
    • -fsanitize=unreachable — with this option, the compiler turns the __builtin_unreachable call into a diagnostics message call instead. When the __builtin_unreachable call is reached, the behavior is undefined. 💀
    • -fsanitize=vla-bound — tells the compiler to check that the size of a variable length array is positive.
    • -fsanitize=null — enables pointer checking. Specifically, an application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed to by a NULL pointer.
    • -fsanitize=return — enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.
    • -fsanitize=signed-integer-overflow — enables signed integer overflow checking. It checks that the result of +, *, and both unary and binary - does not overflow in the signed arithmetics. It 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 — enables instrumentation of array bounds. Various out of bounds accesses get detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented — with the exception of flexible array member-like arrays for which "-fstrict-flex-arrays" or "-fstrict-flex-arrays=" options or "strict_flex_array" attributes say they shouldn't be treated like flexible array member-like arrays.
  • -fsanitize=bounds-strict — enables strict instrumentation of array bounds. Most out of bounds accesses get detected, including flexible array member-like arrays. Initializers of variables with static storage are not instrumented.
  • -fsanitize=alignment — enables checking of alignment of pointers when they're dereferenced, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object.
  • -fsanitize=object-size — enables instrumentation of memory references using the __builtin_dynamic_object_size function. Various out of bounds pointer accesses get detected.
  • -fsanitize=float-divide-by-zero — detects floating-point division by zero. Unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because floating-point division by zero can legitimately be a way of obtaining infinities and NaNs. ✨
  • -fsanitize=float-cast-overflow — enables floating-point type to integer conversion checking. It checks that the result of the conversion doesn't overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also this option does not work well with "FE_INVALID" exceptions enabled.
  • -fsanitize=nonnull-attribute — enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the "nonnull" function attribute.
  • -fsanitize=returns-nonnull-attribute — enables instrumentation of return statements in functions marked with the "returns_nonnull" function attribute, to detect returning of null values from such functions.
  • -fsanitize=bool — enables instrumentation of loads from bool. If a value other than 0/1 gets loaded, a run-time error is issued.
  • -fsanitize=enum — enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type gets loaded, a run-time error is issued.
  • -fsanitize=vptr — enables instrumentation of C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. 👑
  • -fsanitize=pointer-overflow — enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued.
  • -fsanitize=builtin — enables instrumentation of 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. 🙈

Also bestie, heads up: sanitizers literally love to give you false-positive warnings, especially anything chilling around -Wmaybe-uninitialized. So we do not recommend combining -Werror with [the use of] sanitizers. It's giving chaos. 🙈

And the way that -ftrapv works? It causes traps for signed overflows to be emitted. -fsanitize=undefined instead gives you a diagnostic message. This currently works only for the C family of languages. 🫶

  • -fno-sanitize=all — this one yeets all previously enabled sanitizers. And no, -fsanitize=all is not allowed, because some sanitizers literally cannot be used together. Periodt. 💅
  • -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks. This is the move when you want to experiment with different shadow memory layouts in Kernel AddressSanitizer. ✨
  • -fsanitize-sections=s1,s2,... — sanitizes global variables in selected user-defined sections. And guess what, si may contain wildcards. 🦋
  • -fsanitize-recover[=opts] — okay so -fsanitize-recover= controls the error recovery mode for the sanitizers you name in that comma-separated list of opts. Enabling this for a sanitizer component makes it attempt to keep the program running as if no error happened. Which means, yes, multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported 😭. The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and then the program exits with a non-zero exit code.

Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted — the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.

Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable.

Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
  • -fsanitize-address-use-after-scope — enables sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to none. 👑
  • -fsanitize-trap[=opts] — the -fsanitize-trap= option instructs the compiler to report for sanitizers mentioned in the comma-separated list of opts undefined behavior using __builtin_trap rather than a libubsan library routine. If this is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer, __builtin_trap will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover=.

The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. ✨

Currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. -fsanitize-trap=all can be also specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs libubsan support, -fsanitize-trap=vptr is not allowed.

  • -fsanitize-undefined-trap-on-error — the deprecated equivalent of -fsanitize-trap=all. Old news, bestie, don't use it. 🙈
  • -fsanitize-coverage=trace-pc — enables coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block. 🔥
  • -fsanitize-coverage=trace-cmp — enables dataflow guided fuzzing code instrumentation. Inserts 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.
  • -fcf-protection=[full|branch|return|none|check]
  • -fcf-protection

Enable 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). Security slay. 🛡️

The -fcf-protection= keywords are interpreted as follows. 👇

The value branch tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions.

The value return implements checking of validity at the point of returning from a function.

The value full is an alias for specifying both branch and return.

The value check is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different -fcf-protection values. The value check is ignored at the compile time.

The value none turns off instrumentation.

-fcf-protection is an alias for -fcf-protection=full. To override a previous -fcf-protection option on the command line, add -fcf-protection=none and then -fcf-protection=kind.

The macro __CET__ is defined when -fcf-protection is used. The first bit of __CET__ is set to 1 for the value branch and the second bit of __CET__ is set to 1 for the return.

You can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. 🎀

Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer.

  • -fharden-compares — for every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emits extra code to compute and verify the reversed condition, and to call __builtin_trap if the results do not match. Use with -fharden-conditional-branches to cover all conditionals. 💀
  • -fharden-conditional-branches — for every non-vectorized conditional branch that survives gimple optimizations, emits extra code to compute and verify the reversed condition, and to call __builtin_trap if the result is unexpected. Use with -fharden-compares to cover all conditionals.
  • -fharden-control-flow-redundancy — emits extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans do not form an execution path that is compatible with the control flow graph.

Verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls).

Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls would also have verification issued before the call, but these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. 🧚

  • -fhardcfr-skip-leaf — disables -fharden-control-flow-redundancy in leaf functions.
  • -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is active, checks the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. This option is enabled by default; use -fno-hardcfr-check-exceptions to disable it. ✨
  • -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, checks the recorded execution path against the control flow graph before any function call immediately followed by a return of its result, if any, so as to not prevent tail-call optimization, whether or not it is ultimately optimized to a tail call.

This option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but it can be enabled (or disabled, using its negated form) explicitly, regardless of the optimizations.

  • -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, checks 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, the default), those that may not return control to the caller through an exception either (nothrow), or none of them (never). 🌈

Ngl bestie, this one's a whole saga. Checking before a noreturn function that might still hand control back to the caller through an exception can make checking happen more than once — like, if that 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, because that would be redundant with the check done before the call. But other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit. And the checkpoint could even be another noreturn call, so checking may end up performed multiple times. 💀

Various optimizers may cause calls to be marked as noreturn and/or nothrow, even when the corresponding attributes aren't there — which can affect where checks before calls get placed, and whether implicit cleanup handlers get added for them. This unpredictability, plus the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, is what made no-xthrow 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 those optimizations.

  • -fhardened — turns on a set of flags for C and C++ that boost the security of the generated code without touching its ABI. The exact flags enabled may change between major releases of GCC, but rn 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)

Bestie, you can literally generate the list of options that -fhardened turns on with the --help=hardened option. ✨

If your system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.

This option is meant for production builds, not just debug builds, ok?

Currently, -fhardened is only supported on GNU/Linux targets.

One thing: -fhardened only enables a particular option if it wasn't already specified anywhere on the command line. So like, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. The way it respects your choices?? Obsessed. 🫶

  • -fstack-protector — emits extra code to check for buffer overflows, like stack smashing attacks. It does this by adding a guard variable to functions with vulnerable objects — that includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards get initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack count here — optimized-away variables or variables allocated in registers don't. 💅
  • -fstack-protector-all — same vibe as -fstack-protector, except all functions are protected. 🛡️
  • -fstack-protector-strong — like -fstack-protector but it includes additional functions to be protected — those that have local array definitions, or that have references to local frame addresses. Same deal as before: only variables actually allocated on the stack are considered, so optimized-away variables or register-allocated variables don't count. ✨
  • -fstack-protector-explicit — like -fstack-protector but it only protects those functions which have the stack_protect attribute.
  • -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.
Heads up: this switch doesn't actually cause checking to be done — the operating system or the language runtime has to do that. The switch causes generation of code to ensure that they see the stack being extended. Don't get it twisted. 🙈

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 bestie, it comes with the following drawbacks 😭:

  1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code.
  2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
  3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered.

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code. But ngl, it's not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection. 👑

  • -fstack-clash-protection — generates code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. So it prevents allocations from jumping over any stack guard page provided by the operating system. Slay. 🎀

Most targets don't fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.

  • -fstack-limit-register=reg
  • -fstack-limit-symbol=sym
  • -fno-stack-limit — these generate code to ensure that the stack doesn't grow beyond a certain value, either the value of a register or the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it's possible to catch the signal without taking special precautions. 🌸

For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.

You can locally override stack limit checking by using the no_stack_limit function attribute.

  • -fsplit-stack — generates code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, since it's no longer necessary to calculate a good stack size to use for each thread. 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 — including library code — with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU Binutils release 2.21 and later. 💅

  • -fstrub=disable — disables stack scrubbing entirely, ignoring any strub attributes.
  • -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.
  • -fstrub=at-calls — enables at-calls strub mode where viable. The primary use of this option is for testing. It exercises the strub machinery in scenarios strictly local to a translation unit. This strub mode modifies function interfaces, so any function that is visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. See the strub attribute documentation for the deets on viability and eligibility requirements. 🦋
  • -fstrub=internal — enables internal strub mode where viable. The primary use of this option is for testing. This option is intended to exercise thoroughly the parts of the strub machinery that implement the less efficient, but interface-preserving strub mode. Functions that would not be affected by this option are quite uncommon.
  • -fstrub=all — enables some strub mode where viable. When both strub modes are viable, at-calls is preferred. -fdump-ipa-strubm adds function attributes that tell which mode was selected for each function. The primary use of this option is for testing, to exercise thoroughly the strub machinery. 🧚
  • -fvtable-verify=[std|preinit|none] — this option is only available when compiling C++ code. It turns on (or off, if you're using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and hasn't 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. Not me being dramatic but that's a full STOP. 🔥

The options std and preinit control when those structures get built. Either way they're built before execution reaches main. With -fvtable-verify=std the structures get built after shared libraries have been loaded and initialized. With -fvtable-verify=preinit they get built before shared libraries have been loaded and initialized. 👑

If this option shows up multiple times on the command line with different values, here's the drama: none takes highest priority over both std and preinit, and preinit takes priority over std. Periodt.

  • -fvtv-debug — when you use this alongside -fvtable-verify=std or -fvtable-verify=preinit, it makes the debug versions of the runtime functions for the vtable verification feature get called. This flag also makes the compiler log info about which vtable pointers it finds for each class. 📝 That info goes into a file named vtv_set_ptr_data.log, inside the directory named by the environment variable VTV_LOGS_DIR if that's defined — otherwise it lands in the current working directory.
Heads up bestie: this feature appends data to the log file. If you want a fresh log file, make sure you delete any existing one first. 💀
  • -fvtv-counts — this is a debugging flag. When used with -fvtable-verify=std or -fvtable-verify=preinit, it makes the compiler keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts, and logs this information for each compilation unit. 🔢 The compiler writes all that into a file named vtv_count_data.log, in the directory named by the environment variable VTV_LOGS_DIR if that's defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class and writes that to vtv_class_set_sizes.log in the same directory.
Same energy as above: this one appends to the log files too. Want fresh log files? Delete any existing ones. 🌸
  • -finstrument-functions — generates instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions get called with the address of the current function and its call site. (On some platforms, __builtin_return_address does not work beyond the current function, so the call site info may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. 🔍

This instrumentation also happens for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. Which means addressable versions of such functions must be available. 🫠 If all your uses of a function get expanded inline, this may mean an additional expansion of code size. And if you use extern inline in your C code, an addressable version of such functions must be provided. (That's normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)

A function can be given the attribute no_instrument_function, in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (maybe signal handlers, if the profiling routines generate output or allocate memory). 🚨

  • -finstrument-functions-once — similar to -finstrument-functions, but the profiling functions are called only once per instrumented function. Like, the first profiling function is called after the first entry into the instrumented function, and the second profiling function is called before the exit corresponding to this first entry.

The definition of "once" for this option is a little vague, because the implementation is not protected against data races. 😬 So 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, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function.

  • -finstrument-functions-exclude-file-list=file,file,... — sets the list of functions that are excluded from instrumentation (peep the description of -finstrument-functions). If the file that contains a function definition matches 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's considered a match. 👀

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.

If, for some reason, you want to include the letter , in one of sym, you escape it as \,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option). ✨

  • -finstrument-functions-exclude-function-list=sym,sym,... — same vibe as -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, like vector<int> blah(const vector<int> &), not the internal mangled name (e.g., _Z4blahRSt6vectorIiSaIiEE). The match is done on substrings: if the sym parameter is a substring of the function name, it's considered a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. 🦋
  • -fpatchable-function-entry=N[,M] — generates N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted, it defaults to 0, so the function entry points to the address just at the first NOP. 🎯

The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time — provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop. This behavior is target-specific and may also depend on the architecture variant and/or other compilation options.

For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the __patchable_function_entries section of the resulting binary.

Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over the command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If N=0, no pad location is recorded. 🎀

The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. See the -msplit-patch-nops option to change this.

The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14 when not using -msplit-patch-nops. 💅

🧚 Options Controlling the Preprocessor 🧚

These options control the C preprocessor, which gets run on each C source file before actual compilation. 🌈

If you use the -E option, nothing is done except preprocessing. Some of these options only make sense together with -E, because they cause the preprocessor output to be unsuitable for actual compilation.

In addition to the options listed right here, there are a bunch of options to control search paths for include files documented in Directory Options. And options to control preprocessor diagnostics are listed in Warning Options. 💖

  • -D name, --define-macro=name, --define-macro name — predefine name as a macro, with definition 1.
  • -D name=definition, --define-macro=name=definition, --define-macro name=definition — the contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition is truncated by embedded newline characters. 😳

If you're invoking the preprocessor from a shell or shell-like program, you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax.

If you 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. 🫶

  • -U name, --undefine-macro=name, --undefine-macro name — cancel any previous definition of name, whether it was built in or provided with a -D option. 🧹
  • -include file, --include=file, --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 gets searched for in the remainder of the #include "..." search chain as normal. 🌸

If you're stacking multiple -include options, they get included in the exact order they show up on the command line. Order matters, bestie, don't play with it. 💅

  • -imacros file / --imacros=file / --imacros file — literally the same as -include, except everything the scanner spits out while chewing through file gets yeeted into the void 🗑️. But the macros it defines? Still defined. Iconic. This means you can slurp up all the macros from a header without also dragging in its declarations. And here's the tea: every file you pass with -imacros gets processed before every file you pass with -include. Kinda obsessed with that ordering tbh.
  • -undef — do not predefine any system-specific or GCC-specific macros. The standard predefined ones stay put though, they're not going anywhere. 🫶
  • -pthread — defines the extra macros you need to use the POSIX threads library. Use this option consistently for both compilation and linking, bestie — don't be half-and-half about it. It's supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. ✨

🌸 Dependency Generation: the "-M" family 🌸

  • -M / --dependencies — instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor drops one make rule containing the object file name for that source file, a colon, and the names of all the included files — including the ones that came from -include or -imacros command-line options. 👑

Unless you specify it explicitly (with -MT or -MQ), the object file name is the source file's name with any suffix swapped for the object file suffix, and any leading directory parts deleted. If there are lots of included files, the rule gets split across several lines using \-newline. The rule has no commands.

Heads up ⚠️: this option does not suppress the preprocessor's debug output, like -dM. So if you don't want that debug noise mixing with your dependency rules, explicitly set the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output still goes to the regular output stream as normal either way.

Also, passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

  • -MM / --user-dependencies — like -M but do not mention header files found in system header directories, nor header files included (directly or indirectly) from such a header. This means whether you used angle brackets or double quotes in an #include directive doesn't by itself decide whether that header shows up in -MM dependency output. It's giving "user files only." 🦋
  • -MF file — when used with -M or -MM, specifies a file to write the dependencies to. If you don't pass an -MF switch, the preprocessor sends the rules to the same place it'd send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file. And if file is -, the dependencies get written to stdout. 😌

  • -MG / --print-missing-file-dependencies — together with an option like -M that requests 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 straight from the #include directive, no path prepended. -MG also suppresses preprocessed output, since a missing header file renders that useless anyway. This feature is used in automatic updating of makefiles. 🛠️
  • -Mno-modules — disable dependency generation for compiled module interfaces. Simple as that.
  • -MP — this option tells CPP to add a phony target for each dependency other than the main file, making each of them depend on nothing. These dummy rules work around the errors make throws at you if you delete header files without updating the Makefile to match. Classic chaotic move, and this is the fix. This is the typical output:
        test.o: test.c test.h

        test.h:
  • -MT target — change the target of the rule emitted by dependency generation. By default, CPP takes the name of the main input file, deletes any directory components and any file suffix like .c, and appends the platform's usual object suffix. That's your target.

An -MT option sets the target to be exactly the string you specify. Want multiple targets? Pass them as a single argument to -MT, or just use multiple -MT options. Both work. 💖

For example, -MT '$(objpfx)foo.o' might give you:

        $(objpfx)foo.o: foo.c
  • -MQ target — same as -MT, but it quotes any characters that are special to Make. So -MQ '$(objpfx)foo.o' gives you:
        $$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ. No cap.

  • -MD / --write-dependencies — -MD is equivalent to -M -MF file, except that -E is not implied. The driver figures out file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d; otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used together 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. Sneaky distinction, keep it straight bestie. 🙈

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Multitasking queen. 👑

  • -MMD / --write-user-dependencies — like -MD except it mentions only user header files, not system header files.
  • -fpreprocessed — tells the preprocessor that the input file is already preprocessed. This suppresses stuff like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments though, so you can pass a file preprocessed with -C to the compiler without any problems. In this mode, the integrated preprocessor is little more than a tokenizer for the front ends. 💀

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi — these are the extensions GCC uses for preprocessed files created by -save-temps.

  • -fdirectives-only — when preprocessing, handle directives, but do not expand macros. Its behavior depends on the -E and -fpreprocessed options. Let's break it down:

With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, like macro expansion and trigraph conversion, are not performed. On top of that, the -dD option is implicitly enabled.

With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros like __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". Full circle moment ✨

  • -fdollars-in-identifiers — accept $ in identifiers. Just letting the dollar signs in, periodt. 💅
  • -fextended-identifiers — accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. 🌈
  • -fno-canonical-system-headers — when preprocessing, do not shorten system header paths with canonicalization.
  • *-fmax-include-depth=depth* — set the maximum depth of the nested #include. The default is 200. Don't go infinite on me. 🌀
  • *-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. Pick your fighter. 🫶

  • -ftabstop=width — set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is **ignored. The default is 8. 🎀
  • -ftrack-macro-expansion[=level] — track locations of tokens across macro expansions. This lets the compiler emit diagnostics about the current macro expansion stack when a compilation error happens inside a macro expansion. Using this option makes the preprocessor and the compiler consume **more memory. 😭

The level parameter chooses the precision of token location tracking, so you can dial down memory consumption if you need to:

  • level 0 de-activates this option.
  • level 1 tracks token locations in a degraded mode for the sake of minimal memory overhead. In this mode, all tokens resulting from the expansion of an argument of a function-like macro have the same location.
  • level 2 tracks token locations completely. This value is the most memory hungry. 🍓

When this option is given no argument, the default parameter value is 2.

Note that -ftrack-macro-expansion=2 is activated by default. So you're already living this life, bestie.
  • *-fmacro-prefix-map=old*=new — when preprocessing files residing in directory old, expand the __FILE__ and __BASE_FILE__ macros as if the files resided in directory new instead. You can use this to change an absolute path into a relative path by using . for new, which can result in more reproducible builds that are location independent. Iconic for build reproducibility. 🌟 This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map and -fcanon-prefix-map.
  • *-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. Like with -fexec-charset, charset can be any encoding supported by the system's iconv library routine; **however, you will have problems with encodings that don't fit exactly in wchar_t. Consider yourself warned 💀
  • -finput-charset=charset — bestie this sets the input character set, i.e. how GCC translates from the charset of your input file over to the source character set it actually works in 💅 the default is UTF-8, and charset can be literally any encoding your system's iconv library routine supports ✨

If the input charset is UTF-8, you get warnings about ill-formed code unit sequences whenever -Winvalid-utf8 is on. Otherwise, no diagnostics get issued when the input character set matches the execution character set. But if the two are different, ill-formed code unit sequences become a full-on error during transcoding to the execution character set 😭

  • -fpch-deps — when you're using precompiled headers, this flag makes the dependency-output flags also list the files that the precompiled header depends on 🫶 if you don't specify it, only the precompiled header itself gets listed, not the files used to create it, because those files aren't consulted at all when a precompiled header is used. slay, that's just how it is.
  • -fpch-preprocess — this one lets you use a precompiled header together with -E ✨ it inserts a special #pragma, namely #pragma GCC pch_preprocess "filename", into the output, marking where the precompiled header was found and what its filename is. 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 really only suitable as input to GCC. It gets switched on by -save-temps.

You should not be writing this #pragma in your own code, bestie 🙈 but it IS safe to edit the filename if the PCH file lives somewhere else. The filename may be absolute, or it may be relative to GCC's current directory.

  • -fworking-directory — turns on generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing 🎀 when it's on, the preprocessor emits, right after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses that 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 whenever debugging information is enabled, but you can inhibit that with the negated form -fno-working-directory. And if -P is on the command line, this option has no effect at all, since no #line directives get emitted whatsoever 💀
  • -C, --comments — do not discard comments 💖 all comments get passed through to the output file, except for comments in processed directives, which get deleted along with the directive.
Be prepared for side effects here, bestie 😭 -C makes the preprocessor treat comments as tokens in their own right. So for example, comments appearing at the start of what would be a directive line end up turning that line into an ordinary source line, because the first token on the line is no longer a #.
  • -CC, --comments-in-macros — do not discard comments, including during macro expansion ✨ this is like -C, except that comments contained within macros also get passed through to the output file where the macro is expanded.

On top of all the side effects of -C, the -CC option causes all C++-style comments inside a macro to be converted into C-style comments. Why? To stop later use of that macro from inadvertently commenting out the rest of the source line 🙈

-CC is generally used to support lint comments. iconic.

  • -P, --no-line-commands — inhibit generation of linemarkers in the output from the preprocessor 🚫 super useful when you're running the preprocessor on something that isn't C code and you're sending it to a program that might get confused by the linemarkers.
  • -traditional, --traditional, -traditional-cpp, --traditional-cpp — try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors 🌸 see the GNU CPP manual for the details.
Heads up: GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when you invoke CPP explicitly.
  • -trigraphs, --trigraphs — support ISO C trigraphs ✨ these are three-character sequences, all starting with ??, that ISO C defines to stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline.

The nine trigraphs and their replacements are:

Trigraph:       ??(  ??)  ??<  ??>  ??=  ??/  ??'  ??!  ??-
Replacement:      [    ]    {    }    #    \    ^    |    ~

By default, GCC ignores trigraphs, but in standard-conforming modes it converts them 👑 see the -std and -ansi options.

  • -remap — enables special code to work around file systems that only permit very short file names, like MS-DOS 🎀
  • -H, --trace-includes — print the name of every header file used, on top of all the other normal activities 🦋 each name is indented to show how deep in the #include stack it sits. Precompiled header files get printed too, even if they turn out to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! ⭐
  • -dletters, --dump=letters, --dump letters — says to make debugging dumps during compilation as specified by letters 💅 the flags documented here are the ones relevant to the preprocessor. Other letters get interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. And if you specify letters whose behavior conflicts, the result is undefined 💀
    • -dM, --dump=M — instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros ✨ this gives you a way of finding out what's predefined in your version of the preprocessor. Assuming you have no file foo.h, the command:
touch foo.h; cpp -dM foo.h

shows all the predefined macros 👑

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.

  • -dD, --dump=D — like -dM except that it outputs both the #define directives and the result of preprocessing 🌸 both kinds of output go to the standard output file.
  • -dN, --dump=N — like -dD, but emit only the macro names, not their expansions 🎀
  • -dI, --dump=I — output #include directives in addition to the result of preprocessing 🫶
  • -dU, --dump=U — like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, get output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time 💖
  • -fdebug-cpp — this option is only useful for debugging GCC, ngl 🐛 when used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.

When used from GCC without -E, this option has no effect 🚫

  • -Wp,option — you can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor ✨ if option contains commas, it gets split into multiple options at the commas. However — and this is the important bit — many options are modified, translated, or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses that phase 😬 the preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid -Wp and let the driver handle the options instead. fr fr, listen to the manpage.
  • -Xpreprocessor option — pass option as an option to the preprocessor 💅 you can use this to supply system-specific preprocessor options that GCC doesn't recognize.

If you want to pass an option that takes an argument, you must use -Xpreprocessor twice — once for the option and once for the argument 🎀

  • -no-integrated-cpp, --no-integrated-cpp — perform preprocessing as a separate pass before compilation 🌸 by default, GCC does preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only, and once for the actual compilation of the preprocessed input. This option may be useful together with the -B or -wrapper options to specify an alternate preprocessor or to do additional processing of the program source between normal preprocessing and compilation ✨

🛠️ Passing Options to the Assembler 🛠️

You can pass options to the assembler, bestie 🫶 here's how:

  • -Wa,option — pass option as an option to the assembler ✨ if option contains commas, it gets split into multiple options at the commas.
  • -Xassembler option, --for-assembler=option, --for-assembler option — pass option as an option to the assembler 💅 you can use this to supply system-specific assembler options that GCC doesn't recognize.

If you want to pass an option that takes an argument, you must use -Xassembler twice — once for the option and once for the argument 🎀

🔗 Options for Linking 🔗

These options come into play when the compiler links object files into an executable output file 👑 they are meaningless if the compiler isn't doing a link step. periodt.

  • 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 are distinguished from libraries by the linker according to the file contents, iykyk). If linking is done, these object files get used as input to the linker.
  • -c, -S, -E — if any of these options is used, then the linker is not run, and object file names should not be used as arguments 🚫
  • -flink-libatomic — enable linking of libatomic if it's supported by the target, and it's enabled by default ✨ the negative form -fno-link-libatomic can be used to explicitly disable linking of libatomic 💅
  • -flinker-output=type — okay so this one ✨ literally controls how the link-time optimizer does its codegen. By default, the linker plugin just vibe-checks the output type all by itself and auto-decides. But if you're debugging the compiler, or you wanna do incremental linking with a non-LTO object file, you might lowkey want to set the type by hand. 👑
    • If type is exec, code generation produces a static binary. In this case -fpic and -fpie are both disabled. 💀
    • If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This lets you build shared libraries without position-independent code on architectures where that's actually possible, i.e. on x86. 🎀
    • If type is pie, code generation produces an -fpie executable. This gives you similar optimizations as exec except that -fpie is not disabled if it was 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 get merged, pre-optimized, and output to the resulting object file. On top of that, if -ffat-lto-objects is specified, binary code is produced for future non-LTO linking too. The object file produced by incremental linking is smaller than a static library produced from the same object files. And at link time the result of incremental linking also loads faster than a static library, assuming 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, iykyk), but most of the benefits of whole-program optimizations are lost. 😭

During the incremental link (by -r) the linker plugin defaults to rel. GNU Binutils 2.44 or later is needed to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of the object files in an incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole-program optimization, link such objects into a static library instead. 💅

  • -fuse-ld=bfd — use the bfd linker instead of the default linker.
  • -fuse-ld=gold — use the gold linker instead of the default linker.
  • -fuse-ld=lld — use the LLVM lld linker instead of the default linker.
  • -fuse-ld=mold — use the Modern Linker (mold) instead of the default linker.
  • -fuse-ld=wild — use the Wild linker (wild) instead of the default linker. 🌈
  • -llibrary, -l library — searches for the library named library when linking. (The second alternative, where the library is a separate argument, exists only for POSIX compliance and is not recommended, bestie. 🙈)

The -l option is passed directly to the linker by GCC. So go read your linker's documentation for the exact tea ☕ — the general description below applies to the GNU linker.

The linker searches a standard list of directories for the library. Those directories include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.

It makes a difference where in the command you write this option — the linker searches and processes libraries and object files in the order they are specified. 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. 😵‍💫

  • -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 still used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
  • -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify get passed to the linker, and options specifying linkage of the system libraries — like -static-libgcc or -shared-libgcc — are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. 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, --no-standard-libraries — do not use the standard system startup files or libraries when linking. No startup files, and only the libraries you specify get passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. 🍓

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.

In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures 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, --entry entry — specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address.
  • -pie, --pie — produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. ✨
  • -no-pie — don't produce a dynamically linked position independent executable.
  • -static-pie, --static-pie — produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option.
  • -pthread — link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. 🔥
  • -r — produce a relocatable object as output. This is also known as partial linking.
  • -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program. 💜
  • -s — remove all symbol table and relocation information from the executable.
  • -static, --static — on systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect.
  • -shared, --shared — produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1] 🌸
  • -shared-libgcc, -static-libgcc — on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.

There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.

Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. 💅

Okay bestie, we're picking up RIGHT where we left off, mid-gossip about shared libgcc, no biggie. 💅

If instead you're using the GCC driver itself to build shared libraries, heads up: they might not always end up linked against the shared libgcc. Here's the tea — if GCC figures out at its configuration time that you've got a non-GNU linker, or a GNU linker that doesn't support the option --eh-frame-hdr, then 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 libgcc, linking with the static libgcc by default instead. The upside? Exceptions can still propagate through those shared libraries, without paying relocation costs at library load time. ✨

However (and this is the plot twist 👀), if a library or a 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.

  • -static-libasan — so when -fsanitize=address is used to link a program, the GCC driver automatically links against libasan. If libasan exists as a shared library and you're not using -static, then yeah, it links against the shared version. This option tells the GCC driver to link libasan statically instead — without necessarily dragging every other library into static-land with it. 🎯
  • -static-libhwasan — same energy, different sanitizer. When -fsanitize=hwaddress links a program, the GCC driver automatically links against libhwasan. Shared library available + no -static? Then shared version it is. This option directs the GCC driver to link libhwasan statically, without necessarily linking other libraries statically.
  • -static-liblsan — you guessed it 💀 when -fsanitize=leak is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library and -static isn't used, it links the shared version. This option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically.
  • -static-libtsan — when -fsanitize=thread is used to link a program, the GCC driver automatically links against libtsan. Shared library available and -static not used? Shared version. This option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically.
  • -static-libubsan — when -fsanitize=undefined is used to link a program, the GCC driver automatically links against libubsan. Shared available, -static not used → shared version. This option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically.
  • -static-libstdc++ — okay so when the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library and -static isn't used, it links the shared version of libstdc++. That's normally fine, no notes. But sometimes it's genuinely useful to freeze the version of libstdc++ your program uses without going all the way to a fully static link. That's the moment this option shines: it directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically. 🧊
  • -symbolic, --symbolic — binds references to global symbols when building a shared object. Also warns you about any unresolved references (unless that's overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option, so vibe check your target first. 💫
  • -T script — use script as the linker script. This one's supported by most systems using the GNU linker. On some targets — like bare-board targets with no operating system at all — the -T option may be required when linking, just to avoid references to undefined symbols.
  • -Xlinker option — passes option as an option to the linker. Perfect for supplying system-specific linker options that GCC itself doesn't recognize. 🔧

Now listen closely, because this trips people up: if you want to pass an option that takes a separate argument, you have to use -Xlinker twice — once for the option, once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. Writing -Xlinker "-assert definitions" does not work, because that passes the whole string as a single argument, which is not what the linker expects. 😭

When you're using the GNU linker, it's usually way more convenient to pass arguments to linker options using the option=value syntax rather 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 though, so ymmv.

  • -Wl,option, --for-linker=option, --for-linker option — passes option as an option to the linker. If option contains commas, it gets split into multiple options at the commas. You can use this syntax to pass an argument to the option. Example time: -Wl,-Map,output.map passes -Map output.map to the linker. And when using the GNU linker, you can also get the same effect with -Wl,-Map=output.map. 🫶
  • -u symbol, --force-link=symbol, --force-link symbol — pretends the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules. Stack 'em, bestie. 📚
  • -Tbss=addr, -Tdata=addr, -Ttext=addr, -N, -n, -t, -Z, -z keyword — these all get passed straight through to the linker, no interpretation by GCC whatsoever. Go read your linker documentation for what these actually mean, because GCC is NOT gatekeeping that info for you. 🙈

These options specify directories to search for header files, for libraries, and for parts of the compiler. Very "where is everything" core. 🧭

  • -I dir, -iquote dir, -isystem dir, -idirafter dir, --include-directory-after=dir, --include-directory-after dir, --include-directory=dir, --include-directory dir — adds the directory dir to the list of directories searched for header files during preprocessing. --include-directory is an alias for -I, while --include-directory-after is an alias for -idirafter. And if dir begins with = or $SYSROOT, then that = or $SYSROOT gets replaced by the sysroot prefix — see --sysroot and -isysroot.

Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives. ✨

You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:

  1. For the quote form of the include directive, the directory of the current file is searched first.
  2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
  3. Directories specified with -I options are scanned in left-to-right order.
  4. Directories specified with -isystem options are scanned in left-to-right order.
  5. Standard system directories are scanned.
  6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files — use -isystem for that. 👑 (this is the correct way, no cap)

The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment applied to the standard system directories.

And here's a sneaky one: 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 instead.

  • -I-, --include-barrier — splits the include path. 🚧 This option has been deprecated. Please use -iquote instead for -I directories before the -I-, and remove the -I- option.

Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <file>. If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.

On top of that, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". And there is no way to override this effect of -I-. Nope. None. 💀

  • -iprefix prefix, --include-prefix=prefix, --include-prefix prefix — specifies prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /.
  • -iwithprefix dir, -iwithprefixbefore dir, --include-with-prefix=prefix, --include-with-prefix prefix, --include-with-prefix-after=prefix, --include-with-prefix-after prefix, --include-with-prefix-before=prefix, --include-with-prefix-before prefix — appends dir to the prefix specified previously with -iprefix, and adds the resulting directory to the include search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.

--include-with-prefix and --include-with-prefix-after are both aliases for -iwithprefix, while --include-with-prefix-before is an alias for -iwithprefixbefore. 🎀

  • -isysroot dir — this option is like the --sysroot option, but it applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. 🦋

And we're still going, don't you worry — next part's coming. 🌸

  • -imultilib dir — bestie, this makes dir a subdirectory of the directory that holds the target-specific C++ headers. that's it, that's the whole vibe. 🎀
  • -imultiarch dir — same energy but for architecture-specific C++ headers: dir becomes a subdirectory of the directory holding those. 🦋
  • -nostdinc, --no-standard-includes — okay so DO NOT search the standard system directories for header files. periodt. the only places that get searched are the ones you explicitly name with -I, -iquote, -isystem, and/or -idirafter (plus the directory of the current file, when that's appropriate). nothing else, no sneaky defaults. 💅
  • -nostdinc++ — skip the C++-specific standard directories for headers, but lowkey still search all the other standard directories. (this one's used when you're building the C++ library itself, iykyk.)
  • --embed-dir=dir, --embed-directory=dir, --embed-directory dir — appends the dir directory to the list of directories searched for the #embed preprocessing directive or the __has_embed macro. heads up: there are no default directories for #embed, so if you want any, you gotta say so. 🫶

if dir starts with = or $SYSROOT, that = or $SYSROOT gets swapped out for the sysroot prefix; see --sysroot and -isysroot.

  • -iplugindir=dir — sets the directory to search for plugins that get passed by -fplugin=name instead of -fplugin=path/name.so. ngl this option is not meant for you to use, it's only passed by the driver. hands off, bestie. 🙈
  • -Ldir, --library-directory=dir, --library-directory dir — adds directory dir to the list of directories searched for -l. 📚
  • -Bprefix, --prefix=prefix, --prefix prefix — this one specifies where to find the executables, libraries, include files, and data files of the compiler itself. like, the compiler's whole wardrobe. 👑

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 it's about to run, the compiler driver first tries the -B prefix, if there even is one. if that name isn't found, or if -B wasn't specified at all, the driver tries two standard prefixes: /usr/lib/gcc/ and /usr/local/lib/gcc/. if neither of those turns up a file name that's found, then the unmodified program name gets searched for using the directories specified in your PATH environment variable. the search continues, the plot thickens. 💀

the compiler checks whether the path -B gave it actually refers to a directory, and if necessary it adds a directory separator character at the end of the path. attention to detail, we love to see it.

-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 get tried, and that is all — no more. 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 is replaced by [dir/]include. this is to help with boot-strapping the compiler. yes it's a kludge, no we don't make the rules. ✨

  • -no-canonical-prefixes, --no-canonical-prefixes — do not expand any symbolic links, do not resolve references to /../ or /./, and do not make the path absolute when generating a relative prefix. leave the path exactly how you found it, no glow-up. 🌸
  • --sysroot=dir, --sysroot dir — use dir as the logical root directory for headers and libraries. for example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. it's giving relocation, it's giving "we moved and you didn't notice". 🌈

if you use both this option and the -isysroot option, then --sysroot applies to libraries, but -isysroot applies to header files. they split the job, no fighting.

the GNU linker (beginning with version 2.16) has the necessary support for this option. if your linker does not support it, the header file aspect of --sysroot still works, but the library aspect does not. so check your linker's version before you act surprised. 💅

  • --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. bye suffix, we hardly knew ye. 🦋

✨ Options for Code Generation Conventions ✨

these machine-independent options control the interface conventions used in code generation. the vibes, but for ABI.

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. you're smart, you got this. 🎀

  • -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 that extends local lifetime is likely to break with the stack reuse optimization. so like, be careful, this is genuinely the risky 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. sneaky but true, keep up bestie.

  • -fwrapv — this option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. this flag enables some optimizations and disables others.

the options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. same rule as above, it's literally the same energy mirrored. ✨

  • -fwrapv-pointer — this option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. this flag disables some optimizations which assume pointer overflow is invalid. pointers said "i contain multitudes" and the compiler respected it. 🦋
  • -fstrict-overflow — this option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer. it's a package deal, no substitutions. 💅
  • -fexceptions — enable exception handling. generates extra code needed to propagate exceptions. for some targets, this implies GCC generates frame unwind information for all functions, which can produce significant data size overhead, although it does not affect execution.

if you do not specify this option, GCC enables it by default for languages like C++ that normally require exception handling, and disables it for languages like C that do not normally require it. however, you may need to enable this option when compiling C code that needs to interoperate properly with exception handlers written in C++. you may also wish to disable this option if you are compiling older C++ programs that don't use exception handling. old code, new rules, choose wisely. 🎀

  • -fnon-call-exceptions — generate code that allows trapping instructions to throw exceptions. note that this requires platform-specific runtime support that does not exist everywhere. moreover, it only allows trapping instructions to throw exceptions, i.e. memory references or floating-point instructions. it does not allow exceptions to be thrown from arbitrary signal handlers such as "SIGALRM". this enables -fexceptions. 🔥
  • -fdelete-dead-exceptions — okay so this one is lowkey iconic: the compiler is allowed to consider that instructions which might throw exceptions but don't otherwise contribute to the execution of the program can just be optimized away. It does not affect calls to functions, except those with the "pure" or "const" attributes. This option is enabled by default for the Ada and C++ compilers, as permitted by the language specifications. And the optimization passes that cause dead exceptions to be removed are enabled independently at different optimization levels, so it's giving very modular energy. 💅
  • -funwind-tables — this is like -fexceptions's chill cousin: it just generates any needed static data, but does not affect the generated code in any other way. You normally do not need to enable this option; instead, a language processor that needs this handling enables it on your behalf. So like, not your job, bestie. 🫶
  • -fasynchronous-unwind-tables — generates an unwind table in DWARF format, if supported by the target machine. The table is exact at each instruction boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector). It's giving precise, main character energy. ✨
  • -fno-gnu-unique — on systems with recent GNU assembler and C library, the C++ compiler uses the "STB_GNU_UNIQUE" binding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence of "RTLD_LOCAL"; this is necessary to avoid problems with a library used by two different "RTLD_LOCAL" plugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causes "dlclose" to be ignored for affected DSOs; if your program relies on reinitialization of a DSO via "dlclose" and "dlopen", you can use -fno-gnu-unique. So like, it's a trade-off, no cap. 💀
  • -fpcc-struct-return — returns "short" "struct" and "union" values in memory like longer ones, rather than in registers. This convention is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files compiled with other compilers, particularly the Portable C Compiler (pcc). The precise convention for returning structures in memory depends on the target configuration macros. Short structures and unions are those whose size and alignment match that of some integer type. Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. So like, respect the ABI, bestie. 🎀
  • -freg-struct-return — returns "struct" and "union" values in registers when possible. This is more efficient for small structures than -fpcc-struct-return. If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative. 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. It's giving efficient but also chaotic. 😭
  • -fshort-enums — allocates to an "enum" type only as many bytes as it needs for the declared range of possible values. Specifically, the "enum" type is equivalent to the smallest integer type that has enough room. This option has no effect for an enumeration type with a fixed underlying type. Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Tiny enum energy, but be careful. 🦋
  • -fshort-wchar — overrides the underlying type for "wchar_t" to be "short unsigned int" instead of the default for the target. This option is useful for building programs to run under WINE. Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. WINE girlies know. 🍷
  • -fcommon — in C code, this option controls the placement of global variables defined without an initializer, known as tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with the "extern" keyword, which do not allocate storage. The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS section of the object file. This inhibits the merging of tentative definitions by the linker so you get a multiple-definition error if the same variable is accidentally defined in more than one compilation unit. The -fcommon places uninitialized global variables in a common block. This allows the linker to resolve all tentative definitions of the same variable in different compilation units to the same object, or to a non-tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size penalty on global variable references. It is mainly useful to enable legacy code to link without errors. So like, legacy code said slay, I guess. 💅
  • -fno-ident, -Qy, -Qn — -fno-ident suppresses emission of ".ident" assembler directives and causes the "#ident" preprocessor directive to be ignored. -Qy and -Qn are obsolete synonyms for -fident and -fno-ident, respectively. Obsolete but still here, iykyk. 🧚
  • -finhibit-size-directive — don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling crtstuff.c; you should not need to use it for anything else. So like, not for you, bestie. 🙈
  • -fverbose-asm — puts extra commentary information in the generated assembly code to make it more readable. This option is generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the compiler itself). -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 like, don't parse them, fr fr. 🤖

  • -frecord-gcc-switches — this switch causes the command line used to invoke the compiler to be recorded into the object file that is being created. This switch is only implemented on some targets and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to the -fverbose-asm switch, but that switch only records information in the assembler output file as comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing compiler options into the object file. So like, receipts but in the object file. 💅
  • -fpic — generates position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.) Position-independent code requires special support, and therefore works only on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent. When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1. So like, PIC is the vibe, but check your target. 🌈
  • -fPIC — bestie, this one emits position-independent code if the target machine supports it 💖 which means it's suitable for dynamic linking and it dodges any limit on the size of the global offset table. It only actually changes anything on AArch64, m68k, PowerPC and SPARC.

Position-independent code needs special support, so it only works on certain machines. Not universal, don't @ me.

When this flag is set, the macros __pic__ and __PIC__ are defined to 2. ✨

  • -fpie, -fPIE — these two are giving "-fpic and -fPIC but with a twist" 💅 The generated position-independent code can only be linked into executables. Usually you'd reach for these to compile code that's gonna be linked using the -pie GCC option.

-fpie and -fPIE both define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE. Iconic behavior actually.

  • -fno-plt — do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it 🦋

This gives you more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. On architectures like 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this hands the compiler way more register allocation freedom. Main character energy.

Lowkey important caveat though: 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.

  • -fno-jump-tables — do not use jump tables for switch statements, even where it would be more efficient than other code generation strategies 🎀

This is useful in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables don't require a GOT and this option isn't needed at all.

  • -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, efficient-avoiding.
  • -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.

Heads up bestie: this flag does not have a negative form, because it specifies a three-way choice. That's just how it is. 🧚
  • -fcall-used-reg — treat the register named reg as an allocable register that gets clobbered by function calls 💀 It may be allocated for temporaries or variables that don't live across a call. Functions compiled this way do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer. And using this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. Like, actually disastrous. Not cute.

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. And use of this flag for other registers with fixed pervasive roles in the machine's execution model produces disastrous results.

Also — a different sort of disaster results from using this flag for a register in which function values may be returned 😭 two flavors of chaos, obsessed.

This flag does not have a negative form, because it specifies a three-way choice.

  • -fpack-struct[=n] — without a value specified, pack all structure members together without holes ✨ When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment — that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch 🙅 Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.
  • -fleading-underscore — this option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file 💅 One use is to help link with legacy assembly code. Serving retro compatibility realness.
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. Also — not all targets provide complete support for this switch. So like, check 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. It's lowkey doing its own thing.

The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

  • -ftrampolines — for targets that normally need trampolines for nested functions, always generate them instead of using descriptors 🎀 Otherwise, for targets that don't need them — like for example HP-PA or IA-64 — do nothing.

A trampoline is a small piece of code created at run time on the stack when the address of a nested function is taken, and it's 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. Fun.

-fno-trampolines is enabled by default on a language-by-language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.

Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present 😭 This option must therefore be used on a program-wide basis and be manipulated with extreme care.

For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions. It's giving "we tried".

  • -ftrampoline-impl=[stack|heap] — by default, trampolines are generated on stack. However, certain platforms (such as the Apple M1) do not permit an executable stack 🚫

Compiling with -ftrampoline-impl=heap generates calls to __gcc_nested_func_ptr_created and __gcc_nested_func_ptr_deleted in order to allocate and deallocate trampoline space on the executable heap. These functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux.

PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if you setjmp, instantiate nested functions, and then longjmp back to a state prior to having allocated those nested functions 💀 like, the memory's just gonna vibe there. Be careful bestie.
  • -fvisibility=[default|internal|hidden|protected] — set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code ✨ Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. Like, genuinely, 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.

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by How To Write Shared Libraries by Ulrich Drepper (which can be found at <https://www.akkadia.org/drepper/>) — however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public 👑

This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects 🌈

For those adding visibility support to existing code, you may find #pragma GCC visibility of use. This works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop.

Bear in mind that symbol visibility should be viewed as part of the API interface contract and thus all new code should always specify visibility when it is not the default; i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code 🎀

Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility. No exceptions, that's just the rule.

Be aware that headers from outside your project, in particular system headers and headers from any other library you use, may not be expecting to be compiled with visibility other than the default 🙈 You may need to explicitly say #pragma GCC visibility push(default) before including any such headers.

"extern" declarations do not get touched by -fvisibility, which means a whole lotta code can be recompiled with -fvisibility=hidden with basically zero modifications. iconic 💅 But — and this is the plot twist 😭 — it also means that calls to "extern" functions with no explicit visibility go through the PLT, so honestly 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. tell it like it is bestie 🎀

Also lowkey important: -fvisibility does affect C++ vague linkage entities. So like, for instance, an exception class that is be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes get unified between the DSOs. no cap 🫶

There's a whole overview of these techniques, their benefits and how to use them over at https://gcc.gnu.org/wiki/Visibility ✨

  • -fstrict-volatile-bitfields — reach for this one if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as "unsigned short" (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access. 💪

If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 💀

In some cases, such as when the "packed" attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time. so it's giving messy but safe 🙈

Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. It is therefore recommended to define all bits of the field's type as bit-field members. fr fr, do that.

The default value of this option is determined by the application binary interface for the target processor.

  • -fsync-libcalls — controls whether any out-of-line instance of the "__sync" family of functions may be used to implement the C++11 "__atomic" family of functions. 🦋

The default value of this option 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. iykyk 💜

  • -fzero-init-padding-bits=value — guarantee 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 allows you to 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 an 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 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). the way it just fills in all the gaps 🥺

🛠️ GCC Developer Options 🛠️

okay so this section is the backstage pass 🎟️ — it describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks. like, for real, bestie, most of us will never touch these 💅

Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. 👑

If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they are executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file. the lore is deep 📚

  • -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. heads up 📢

  • -dletters, --dump=letters, --dump letters, -fdump-rtl-pass, -fdump-rtl-pass-options, -fdump-rtl-pass-options=filename — says to make debugging dumps during compilation at times specified by letters when using -d or by pass when using -fdump-rtl. This is used for debugging the RTL-based passes of the compiler. 🔍

Some -dletters switches have different meaning when -E is used for preprocessing. ⚠️

The -options form allows greater control over the details of the dump. See -fdump-tree.

Here are actual instances of command-line options following these patterns and their meanings: 👇

  • -fdump-rtl-alignments — Dump after branch alignments have been computed.
  • -fdump-rtl-asmcons — Dump after fixing rtl statements that have unsatisfied in/out constraints.
  • -fdump-rtl-auto_inc_dec — Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
  • -fdump-rtl-barriers — Dump after cleaning up the barrier instructions.
  • -fdump-rtl-bbpart — Dump after partitioning hot and cold basic blocks.
  • -fdump-rtl-bbro — Dump after block reordering.
  • -fdump-rtl-btl1, -fdump-rtl-btl2 — -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
  • -fdump-rtl-bypass — Dump after jump bypassing and control flow optimizations.
  • -fdump-rtl-combine — Dump after the RTL instruction combination pass.
  • -fdump-rtl-compgotos — Dump after duplicating the computed gotos.
  • -fdump-rtl-ce1, -fdump-rtl-ce2, -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
  • -fdump-rtl-cprop_hardreg — Dump after hard register copy propagation.
  • -fdump-rtl-csa — Dump after combining stack adjustments.
  • -fdump-rtl-cse1, -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
  • -fdump-rtl-dce — Dump after the standalone dead code elimination passes.
  • -fdump-rtl-dbr — Dump after delayed branch scheduling.
  • -fdump-rtl-dce1, -fdump-rtl-dce2 — -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
  • -fdump-rtl-eh — Dump after finalization of EH handling code.
  • -fdump-rtl-eh_ranges — Dump after conversion of EH handling range regions.
  • -fdump-rtl-expand — Dump after RTL generation.
  • -fdump-rtl-fwprop1, -fdump-rtl-fwprop2 — -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
  • -fdump-rtl-gcse1, -fdump-rtl-gcse2 — -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
  • -fdump-rtl-init-regs — Dump after the initialization of the registers.
  • -fdump-rtl-initvals — Dump after the computation of the initial value sets.
  • -fdump-rtl-into_cfglayout — Dump after converting to cfglayout mode.
  • -fdump-rtl-ira — Dump after iterated register allocation.
  • -fdump-rtl-jump — Dump after the second jump optimization.
  • -fdump-rtl-loop2 — -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
  • -fdump-rtl-mach — dumps right after the machine dependent reorganization pass does its thing, that is if that pass even exists for your target 🦋
  • -fdump-rtl-mode_sw — dumps after the redundant mode switches get yeeted ✨
  • -fdump-rtl-rnreg — dumps after register renumbering, bestie 💅
  • -fdump-rtl-outof_cfglayout — dumps after converting out of cfglayout mode 🔥
  • -fdump-rtl-peephole2 — dumps after the peephole pass does its lil peek-a-boo 👀
  • -fdump-rtl-postreload — dumps after the post-reload optimizations 🎀
  • -fdump-rtl-pro_and_epilogue — dumps after the function prologues and epilogues get generated 🌸
  • -fdump-rtl-sched1, -fdump-rtl-sched2 — these two enable dumping after the basic block scheduling passes 🫶
  • -fdump-rtl-ree — dumps after sign/zero extension elimination 👑
  • -fdump-rtl-seqabstr — dumps after common sequence discovery 🔍
  • -fdump-rtl-shorten — dumps after shortening branches ✂️
  • -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — these enable dumping after five rounds of instruction splitting, yes five, she's thorough like that 💀
  • -fdump-rtl-sms — dumps after modulo scheduling. heads up tho, this pass only runs on some architectures 🏗️
  • -fdump-rtl-stack — dumps after converting from GCC's "flat register file" registers to the x87's stack-like registers. also an architecture-gated one, this pass only runs on x86 variants 🧚
  • -fdump-rtl-subreg1, -fdump-rtl-subreg2 — these two enable dumping after the two subreg expansion passes 💜
  • -fdump-rtl-vartrack — dumps after variable tracking 🕵️
  • -fdump-rtl-vregs — dumps after converting virtual registers to hard registers 🔄
  • -fdump-rtl-web — dumps after live range splitting 🌈
  • -fdump-rtl-regclass, -fdump-rtl-subregs_of_mode_init, -fdump-rtl-subregs_of_mode_finish, -fdump-rtl-dfinit, -fdump-rtl-dfinish — okay so these dumps are literally defined but they always produce empty files. empty. nothing. void. no cap 🙈
  • -da, --dump=a, -fdump-rtl-all — produces all the dumps listed above. every single one. she's giving maximalism ✨
  • -dA, --dump=A — annotates the assembler output with miscellaneous debugging information 🎀
  • -dD, --dump=D — dumps all macro definitions at the end of preprocessing, on top of the normal output 🫶
  • -dH, --dump=H — produces a core dump whenever an error occurs. yeah. an actual core dump. dramatic queen behavior 💀
  • -dp, --dump=p — annotates the assembler output with a comment telling you which pattern and alternative is used. the length and cost of each instruction get printed too 🔥
  • -dP, --dump=P — dumps the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation 💅
  • -dx, --dump=x — just generates RTL for a function instead of compiling it. usually you'd pair this with -fdump-rtl-expand 👑
  • -fdump-debug — dumps debugging information generated during the debug generation phase 🐛
  • -fdump-earlydebug — dumps debugging information generated during the early debug generation phase 🌅
  • -fdump-noaddr — when doing debugging dumps, suppresses address output. this makes it way more feasible to run diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations 🔍
  • -freport-bug — collects and dumps debug information into a temporary file if an internal compiler error (ICE) occurs 🧊
  • -fdump-unnumbered — when doing debugging dumps, suppresses instruction numbers and address output. makes it way more feasible to diff debugging dumps for compiler invocations with different options, especially with and without -g ✨
  • -fdump-unnumbered-links — when doing debugging dumps (peep the -d option above), suppresses instruction numbers for the links to the previous and next instructions in a sequence 🔗
  • -fdump-internal-locations — dumps detailed information about GCC's internal representation of source code locations 📍
  • -fdump-ipa-switch, -fdump-ipa-switch-options — these control the dumping at various stages of inter-procedural analysis language tree to a file. the file name is generated by appending a switch specific suffix to the source file name, and the file gets created in the same directory as the output file. here are the possible dumps 💖
    • all — enables all inter-procedural analysis dumps 🌈
    • cgraph — dumps information about call-graph optimization, unused function removal, and inlining decisions 🕸️
    • inline — dumps after function inlining 📥
    • strubm — dumps after selecting "strub" modes, and recording the selections as function attributes 🧴
    • strub — dumps "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking 🧼

on top of that, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized 💫

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined 😭

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis 🔬

  • -fdump-ipa-clones — creates a dump file containing info about the creation of call graph node clones and the removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use is so that tools that create live-patches can determine the set of functions that need to be live-patched to completely replace a particular function (peep -flive-patching). the file name is generated by appending suffix "ipa-clones" to the source file name, and the file gets created in the same directory as the output file. each entry in the file sits on a separate line containing semicolon separated fields 📝

when it's 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.

and when it's call graph clone removal, the individual fields are:

  1. String "Callgraph removal".
  2. Name of the function being removed as it would be presented to the assembler.
  3. A number that uniquely represents the function being cloned in the call graph. note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
  4. The file name of the source file where the function is defined.
  5. The line on which the function definition is located.
  6. The column where the function definition is located.
  • -fdump-lang — dumps language-specific information. the file name is made by appending .lang to the source file name 🌐
  • -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — these control the dumping of language-specific information. the options and filename portions behave exactly as described in the -fdump-tree option. -fdump-tree-all enables all language-specific dumps; other options vary with the language. for instance, see 💜
  • -fdump-passes — prints on stderr the list of optimization passes that are turned on and off by the current command-line options 📋
  • -fdump-statistics-option — enables and controls dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in .statistics to the source file name, and the file gets created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled 📊
  • -fdump-tree-all, -fdump-tree-switch, -fdump-tree-switch-options, -fdump-tree-switch-options=filename — these control the dumping at various stages of processing the intermediate language tree to a file. if the -options form is used, options is a list of - separated options that control the details of the dump. not all options are applicable to all dumps; those that aren't meaningful get ignored, no drama 🙅

here are the available options:

  • address — prints the address of each node. usually this isn't meaningful since it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment 🗺️
  • asmname — if "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". its primary use is ease of use working backward from mangled names in the assembly file 🔤
  • slim — When GCC is dumping front-end intermediate representations, this says: don't dump members of a scope or the body of a function just because you happened to reach that scope, bestie. Only dump those items when they're directly reachable through some other path. When it's dumping pretty-printed trees, slim also stops it from dumping the bodies of control structures. And when it's dumping RTL, it prints the RTL in slim (condensed) form instead of the default LISP-like representation. 💅
  • raw — Prints a raw representation of the tree. By default trees get pretty-printed into a C-like representation, so this is the un-glammed-up version.
  • details — Turns on more detailed dumps. Slight catch: not every dump option honors this. It also includes info from the optimization passes.
  • stats — Dumps alllll the various statistics about the pass. Again, not honored by every dump option, so don't come for me if your dump is stats-less.
  • blocks — Shows basic block boundaries. It's disabled in raw dumps, fyi.
  • graph — For each of the other indicated dump files (-fdump-rtl-pass), dumps a representation of the control flow graph that's suitable for viewing with GraphViz, into file.passid.pass.dot. Every function in the file is pretty-printed as a subgraph, so GraphViz can render them all in one single plot. Also: RTL is always dumped in slim form with this one.
  • vops — Shows virtual operands for every statement. ✨
  • lineno — Shows line numbers for statements.
  • uid — Shows the unique ID (DECL_UID) for each variable.
  • verbose — Shows the tree dump for each statement.
  • eh — Shows the EH region number holding each statement.
  • scev — Shows scalar evolution analysis details. Not me knowing what that means but living for it 🦋
  • optimized — Shows optimization information (only available in certain passes).
  • missed — Shows missed optimization information (only available in certain passes).
  • note — Shows other detailed optimization information (only available in certain passes).
  • folding — Dumps information about match-and-simplify (match.pd) patterns, when they're applied.
  • all — Turns on all the options… except raw, slim, verbose and lineno. Those four are excluded, no cap.
  • optall — Turns on all the optimization options, i.e. optimized, missed, and note.

So like, how do you even figure out what tree dumps exist, or find the dump for the pass you actually care about? Follow these steps, bestie:

  1. Invoke GCC with -fdump-passes and go looking through the stderr output 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. Example: to enable the dump from the Early Value Range Propagation pass, invoke GCC with -fdump-tree-evrp. 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. So for example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. Heads up though: the numeric codes are not stable and may change from one version of GCC to another. 😭

💖 -fopt-info — the optimization gossip channel 💖

-fopt-info
-fopt-info-options
-fopt-info-options=filename

Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations.

The options can be divided into three groups:

  1. options describing what kinds of messages should be emitted,
  2. options describing the verbosity of the dump, and
  3. options describing which optimizations should be included.

The options from each group can be freely mixed, because they're non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line. Periodt.

The following options control which kinds of messages should be emitted:

  • optimized — Prints information when an optimization is successfully applied. It's up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.
  • missed — Prints information about missed optimizations. Individual passes control which information to include in the output.
  • note — Prints verbose information about optimizations, such as certain transformations, more detailed messages about decisions, etc.
  • all — Prints detailed optimization information. This includes optimized, missed, and note.

The following option controls the dump verbosity:

  • internals — By default only "high-level" messages are emitted. This option enables additional, more detailed messages, which are likely to only be of interest to GCC developers. (iykyk 🧚)

One or more of the following option keywords can be used to describe a group of optimizations:

  • ipa — Enables dumps from all interprocedural optimizations.
  • loop — Enables dumps from all loop optimizations.
  • inline — Enables dumps from all inlining optimizations.
  • omp — Enables dumps from all OMP (Offloading and Multi Processing) optimizations.
  • vec — Enables dumps from all vectorization optimizations.
  • optall — Enables 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, omitting messages that are treated as "internals".

If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. 💀

⚠️ Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead. Don't say I didn't warn you.

In the following example, the optimization info is output to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs a missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter. 🌈

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict, since only one output file is allowed. In this case only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is produced, which contains dumps from the vectorizer about missed opportunities.

🍓 -fsave-optimization-record

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. So don't build your whole personality on it, bestie.

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, with the following additional data for each message:

  • the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
  • the function name of the code being optimized, where applicable,
  • the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
  • objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
  • the GCC pass that emitted the message, and
  • the location in GCC's own code from which the message was emitted

Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. 🦋

🌸 -fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.

For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. The way the detail just keeps stacking… 👑

👑 -fenable-kind-pass / -fdisable-kind-pass=range-list

-fenable-kind-pass
-fdisable-kind-pass=range-list

This is a set of options used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead. So like, if you're not debugging GCC itself, this is not your lane, bestie. 💅

  • -fdisable-ipa-pass — bestie, this one just turns OFF an IPA pass. pass is the pass name, periodt. And if the same pass gets statically invoked in the compiler multiple times? You tack on a sequential number starting from 1. It's giving "which one, bestie, be specific" 💅
  • -fdisable-rtl-pass, -fdisable-rtl-pass=range-list — same deal but for an RTL pass. pass is the pass name (add that sequential number from 1 if it's statically invoked multiple times, same as above). And range-list is a comma-separated list of function ranges or assembler names ✨ Each range is a number pair separated by a colon, and it's inclusive on both ends. If the range is trivial you can simplify that number pair down to a single number. If the function's call graph node's uid falls inside one of the ranges you listed, the pass gets 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 a tree pass. Go read -fdisable-rtl for the description of the option arguments, fr fr 🦋
  • -fenable-ipa-pass — enable an IPA pass. pass is the pass name, and if it's statically invoked multiple times in the compiler, append a sequential number starting from 1. Same energy, opposite direction 🌈
  • -fenable-rtl-pass, -fenable-rtl-pass=range-list — enable an RTL pass. See -fdisable-rtl for the option argument description and examples 💖
  • -fenable-tree-pass, -fenable-tree-pass=range-list — enable a tree pass. See -fdisable-rtl for the description of option arguments ✨

Here are some examples showing uses of these options, and ngl they eat:

        # disable ccp1 for all functions
           -fdisable-tree-ccp1
        # disable complete unroll for function whose cgraph node uid is 1
           -fenable-tree-cunroll=1
        # disable gcse2 for functions at the following ranges [1,1],
        # [300,400], and [400,1000]
        # disable gcse2 for functions foo and foo2
           -fdisable-rtl-gcse2=foo,foo2
        # disable early inlining
           -fdisable-tree-einline
        # disable ipa inlining
           -fdisable-ipa-inline
        # enable tree full unroll
           -fenable-tree-unroll
  • -fchecking, -fchecking=n — turns ON internal consistency checking 🔍 The default depends on your compiler configuration. And -fchecking=2 enables further internal consistency checking — which, heads up, might affect code generation 👀
  • -frandom-seed=string — this one is the vibe-setter for reproducibility 🎀 It gives GCC a seed to use in place of random numbers when generating certain symbol names that have to be different in every compiled file. It's also used to place unique stamps in coverage data files and the object files that produce them. So you can use -frandom-seed to produce reproducibly identical object files. Iconic.

The string can be a number (decimal, octal, or hex) or an arbitrary string — which gets converted to a number by computing CRC32.

The string should be different for every file you compile. No copy-paste, bestie 😭
  • -save-temps, --save-temps — keeps the usual "temporary" intermediate files around permanently 💾 It names them as auxiliary output files, as specified described under -dumpbase and -dumpdir.

And when you use it together with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that has the same extension as an intermediate file 🫶 If you do want that intermediate file, just rename 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 you specified after the -o option, including any directory separators. And if you didn't use -o at all? Then -save-temps=obj just behaves like -save-temps=cwd. So chill 💅
  • -specs=file, --specs=file, --specs file — processes file after the compiler reads in the standard specs file, so you can override the defaults that the gcc driver program uses when figuring out what switches to pass to cc1, cc1plus, as, ld, etc. You can throw more than one -specs=file on the command line and they get processed in order, from left to right ✨
  • -time[=file] — reports the CPU time taken by each subprocess in the compilation sequence ⏱️ For C source files, that's the compiler proper and assembler (plus the linker if linking is done).

With no output file specified, the output looks like this:

        # cc1 0.12 0.01
        # as 0.00 0.01

First number on each line = "user time" — time spent executing the program itself. Second number = "system time" — time spent executing operating system routines on behalf of the program. Both in seconds, no cap 🕐

With an output file specified, the output gets appended to the named file and looks like this:

        0.12 0.01 cc1 <options>
        0.00 0.01 as <options>

Now the "user time" and "system time" move before the program name, and the options passed to the program get displayed — so later you can tell exactly what file was being compiled and with which options. Main character detective energy 🔍

  • -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file 📝 If the optional argument is omitted (or if file is .), the name of the dump file is determined by appending .gkd to the dump base name — see -dumpbase.
  • -fcompare-debug[=opts] — if no error occurs during compilation, it runs the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to that second compilation. Then it dumps the final internal representation in both compilations and prints an error if they differ 😱

If you omit the equal sign, the default -gtoggle is used.

The environment variable GCC_COMPARE_DEBUG, if defined, non-empty, and nonzero, implicitly enables -fcompare-debug 💖 And if GCC_COMPARE_DEBUG is defined to a string starting with a dash, then that'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 get renamed so they contain the .gk additional extension during the second compilation, so they don't overwrite the ones generated by the first 🎀

And when this option is passed to the compiler driver, it causes the first compilation to be skipped — which makes it useful for little other than debugging the compiler proper 💀

  • -gtoggle — turns off generation of debug info if leaving out this option generates it, or turns it on at level 2 otherwise 🔁 The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it's given. Mainly intended to be used with -fcompare-debug ✨
  • -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the exact 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 ⏱️

And if SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr, then the -ftime-report information is instead emitted in JSON form as part of SARIF output 🌈 Heads up though: the precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr — because it's written out at a slightly different place within the compiler 🙈

  • -ftime-report-details — records the time consumed by infrastructure parts separately for each pass 🔍
  • -fira-verbose=n — controls the verbosity of the dump file for the integrated register allocator. Default value is 5. And if the value n is greater than or equal to 10, the dump output is sent to stderr using the same format as n minus 10 💅
  • -flto-report — prints a report with internal details on the workings of the link-time optimizer 📋 The contents of this report vary from version to version. It's meant to be useful to GCC developers when processing object files in LTO mode (via -flto).

Disabled by default.

  • -flto-report-wpa — like -flto-report, but only print for the WPA phase of link-time optimization ✨
  • -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes 🧠
  • -fmem-report-wpa — makes the compiler print some statistics about permanent memory allocation for the WPA phase only 🌸
  • -fpre-ipa-mem-report, -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization 🎀
  • -fmultiflags — this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time 🔧

Like TFLAGS, this lets the target libraries get built for chill, portable baseline environments while the compiler itself defaults to the more demanding ones. And that's actually a serve, because users can just override the defaults the compiler was configured with when they build their own programs (if those defaults aren't it for their target environment), whereas rebuilding the runtime libraries is usually not as easy or as desirable. fr fr.

But unlike TFLAGS, using specs means different flags can be picked for different multilibs. The way you pull that off: build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.

This option gets yeeted by the driver once it's done processing the driver self spec. it did its job, it can go home now 💅

It's also lowkey useful for checking that TFLAGS are being used to build all target libraries — just 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 stats about how consistent the (estimated) profile is, plus the effect of individual passes. receipts ✨
  • -fstack-usage — makes the compiler spit out stack usage info for the program, per-function. The dump's filename is made by appending .su to the auxname. The auxname comes from the output file's name — but only if it's explicitly specified and it's not an executable — otherwise it's the basename of the source file. Each entry has four fields, separated by tab characters:
    • the name of the function, with its source location in front of it
    • the mangled name of the function
    • a number of bytes
    • one or more qualifiers: static, dynamic, bounded

The qualifier static means the function handles the stack statically: a fixed number of bytes gets allocated for the frame on function entry and released on function exit, and no other stack adjustments happen anywhere in the function. The second field is that fixed number of bytes.

The qualifier dynamic means the function handles the stack dynamically: on top of the static allocation described above, stack adjustments happen inside the function body — like pushing/popping arguments around function calls, for example. If the bounded qualifier is there too, the amount of those adjustments is bounded at compile time, and the second field is an upper bound on the total stack the function uses. If bounded is not there, the amount of adjustments is not bounded at compile time, and the second field only represents the bounded part. know the difference bestie, it matters 💖

  • -fstats — emit statistics about front-end processing at the end of compilation. This one is only supported by the C++ front end, and ngl the info is generally only useful to the G++ development team. iykyk 👑
  • -fdbg-cnt-list — print the name and the counter upper bound for every debug counter.
  • *-fdbg-cnt=counter-value-list* — set the internal debug counter's lower and upper bounds. 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 the list of closed intervals. The lower_bound is optional and gets zero-initialized if you don't set it. Example time: with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for the second, third, fourth, tenth and eleventh invocation. And for dbg_cnt(tail_call) it returns true for the first 10 invocations. math but make it debug 🧚
  • -print-autofdo-gcov-version, --print-autofdo-gcov-version — print the current version of GCOV being used by the AutoFDO infrastructure.
  • -print-file-name=library, *--print-file-name=library*, *--print-file-name *library — print the full absolute name of the library file library that would be used when linking — and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name. that's it. that's the whole job. ✨
  • -print-multi-directory, --print-multi-directory — print the directory name corresponding to the multilib that all the other switches on the command line selected. This directory is supposed to exist in GCC_EXEC_PREFIX.
  • -print-multi-lib, --print-multi-lib — print the mapping from multilib directory names to the compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, with no spaces between multiple switches. This is supposed to make shell processing easier. convenience queen 🎀
  • -print-multi-os-directory, --print-multi-os-directory — print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just .; if OS libraries are present in libsuffix sibling directories it prints e.g. ../lib64, ../lib or ../lib32; or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
  • -print-multiarch, --print-multiarch — print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
  • -print-prog-name=program, *--print-prog-name=program*, *--print-prog-name *program — like -print-file-name, but it searches for a program such as cpp. same energy, different target 💜
  • -print-libgcc-file-name, --print-libgcc-file-name — same as -print-file-name=libgcc.a.

This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
  • -print-search-dirs, --print-search-dirs — print the name of the configured installation directory and a list of the program and library directories gcc searches — and don't do anything else.

This one comes in clutch when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To fix that, 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 /. DO NOT forget the trailing slash, bestie 😭

  • -print-sysroot, --print-sysroot — print the target sysroot directory that's 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. silence. iconic. 💅
  • -print-sysroot-headers-suffix, --print-sysroot-headers-suffix — print the suffix added to the target sysroot when searching for headers, or give an error if the compiler isn't configured with such a suffix — and don't do anything else.
  • -dumpmachine — print the compiler's target machine (for example, i686-pc-linux-gnu) — and don't do anything else.
  • -dumpversion — print the compiler version (for example, 3.0, 6.3.0 or 7) — and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured, it can be just a single number (major version), two numbers separated by a dot (major and minor version), or three numbers separated by dots (major, minor and patchlevel version). sneaky variability, watch out 👀
  • -dumpfullversion — print the full compiler version — and don't do anything else. The output is always three numbers separated by dots: major, minor and patchlevel version. no surprises here, periodt ✨
  • -dumpspecs — print the compiler's built-in specs — and don't do anything else. (This is used when GCC itself is being built.) very meta of it 🦋
  • --param name=value, --param= name=value — GCC, by convention, uses parameters that you can specify on the command line instead of hard-wired constants to represent arbitrary compiler limits or heuristics. Many of these parameters are about optimization; for example, GCC does not inline functions that contain more than a certain number of instructions. The static analyzer likewise uses parameters to limit complexity, link-time optimization uses parameters to control partitioning, and so on. Other parameters control aspects of GCC that are completely internal, like its memory allocation and garbage collection strategy. And still others control target-specific behavior.

The --param option gives you one uniform interface for specifying values for these compiler parameters. However — the names of specific parameters, and the meaning of the values, are tied to the compiler's internals and are subject to change without notice in future releases. You should not depend on parameter settings for correct compilation of your program. They're exposed via the command line for the convenience of developers debugging compilation problems, or, in some cases, to provide workarounds for compiler bugs. do NOT build your whole life on these, bestie, that's a trap 💀

🌸 Target-Specific Options 🌸

Every target machine GCC supports can have its own options — for example, to let you compile for a particular processor variant or ABI, or to control optimizations specific to that machine. Same deal: GCC also has options that are specific to particular operating systems or runtime environments on the target.

By convention, the names of machine-specific options start with -m. Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform. friendly like that 🫶

🦋 AArch64 Options 🦋

These options are defined for AArch64 implementations: aarch64 girlies rise 💖

  • -mabi=name — generate code for the specified data model. The permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a SysV-like data model where int is 32 bits but long int and pointers are 64 bits.

The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. do not mix them, i am begging 😭

The ilp32 model is deprecated. deprecated. it's giving legacy energy 🫠

  • -mbig-endian — generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
  • -mlittle-endian — generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an aarch64_be-*-* target.
  • -menable-sysreg-checking — generates an error message if an attempt is made to access a system register that isn't available on the target architecture. caught in 4k 🔥
  • -mgeneral-regs-only — codegen that only touches the general-purpose registers, bestie. This stops the compiler from reaching for floating-point and Advanced SIMD registers — but heads up, it does not put any restrictions on the assembler. 🙈
  • -mcmodel=tiny — tiny code model energy ✨. The program and its statically defined symbols all have to live within 1MB of each other. Programs can be statically or dynamically linked.
  • -mcmodel=small — 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, so if you didn't ask for anything, this is what you got.
  • -mcmodel=large — large code model, no assumptions at all 💅. It makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. And fs fs, -mcmodel=large is incompatible with -mabi=ilp32, -fpic and -fPIC — don't try it, it's not gonna slay.
  • -mtp=name — pick which system register gets used as the thread pointer. Valid values are tpidr_el0, tpidrro_el0, tpidr_el1, tpidr_el2, tpidr_el3. And for backwards compatibility the aliases el0, el1, el2, el3 are also accepted (iconic of them tbh). The default setting is tpidr_el0. Highly recommended: compile all code that's meant to interoperate with the same value of this option, otherwise you'll be accessing a different thread pointer from the wrong exception level and that's a whole mess 😭.
  • -mstrict-align / -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary, exactly as described in the architecture specification.
  • -momit-leaf-frame-pointer / -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior (omitting it) is the default.
  • -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code with the canary sitting at guard. Supported locations are global for a global canary, or sysreg for a canary in an appropriate system register.

With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally specify which system register to use as the base register for reading the canary, and what offset from that base register to read it from. 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. The old-school way 🫶
  • -mtls-size=size — set 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 — turn the workaround for the ARM Cortex-A53 erratum number 835769 on or off. What does it do? It inserts a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag gets ignored if an architecture or CPU that doesn't need the workaround is specified on the command line. 💀
  • -mfix-cortex-a53-843419 / -mno-fix-cortex-a53-843419 — turn the workaround for the ARM Cortex-A53 erratum number 843419 on or off. This erratum workaround happens at link time, so this only passes the corresponding flag through to the linker. Same deal as above: ignored if an architecture or CPU that doesn't need the workaround is specified on the command line.
  • -mlow-precision-recip-sqrt / -mno-low-precision-recip-sqrt — enable or disable the reciprocal square root approximation. This only does anything if -ffast-math or -funsafe-math-optimizations is used too. Enabling it drops the precision of reciprocal square root results down 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 takes effect alongside -ffast-math or -funsafe-math-optimizations. Enabling it reduces square root precision to about 16 bits for single precision and about 32 bits for double precision. And the tea: if it's enabled, it implies -mlow-precision-recip-sqrt. 🫶
  • -mlow-precision-div / -mno-low-precision-div — enable or disable the division approximation. Same conditions, bestie: only matters if -ffast-math or -funsafe-math-optimizations is in play. Enabling reduces division result precision to about 16 bits for single precision and about 32 bits for double precision.
  • -mtrack-speculation / -mno-track-speculation — enable or disable generating extra code that tracks speculative execution through conditional branches. That tracking state can then be used by the compiler when it's expanding calls to __builtin_speculation_safe_value, 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. At runtime, these helpers figure out whether the LSE instructions from ARMv8.1-A can be used — and if not, they 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= and the selected CPU supports the lse feature. This option is on by default. 🔥

  • -mmax-vectorization / -mno-max-vectorization — enable or disable an override to the vectorizer cost model that makes vectorization always look profitable. You can pair this with -mautovec-preference for precise control over which ISA gets used for auto-vectorization. And unlike -fno-vect-cost-model or -fvect-cost-model=unlimited, this option does not turn off cost comparison between different vector modes. 💅
  • -mautovec-preference=name — force an ISA selection strategy for auto-vectorization. The possible values of name are:
    • default — use the default heuristics.
    • asimd-only — use only Advanced SIMD for auto-vectorization.
    • sve-only — use only SVE for auto-vectorization.
    • prefer-asimd — use both Advanced SIMD and SVE, but prefer Advanced SIMD when the costs are deemed equal.
    • prefer-sve — use both Advanced SIMD and SVE, but prefer SVE when the costs are deemed equal.

For best performance it's highly recommended to use -mcpu or -mtune instead. This parameter should only be used for code exploration, fr fr. 🧚

  • -march=name — name the target architecture, and optionally one or more feature modifiers. The form is -march=arch{+[no]feature}*.

Here's the rundown of the permissible arch values and which features they enable by default 🌈:

  • armv8-a — Architecture: Armv8-A — includes by default: +fp, +simd
  • armv8.1-a — Architecture: Armv8.1-A — includes by default: armv8-a, +crc, +lse, +rdma
  • armv8.2-a — Architecture: Armv8.2-A — includes by default: armv8.1-a
  • armv8.3-a — Architecture: Armv8.3-A — includes by default: armv8.2-a, +pauth, +fcma, +jscvt
  • armv8.4-a — Architecture: Armv8.4-A — includes by default: armv8.3-a, +flagm, +fp16fml, +dotprod, +rcpc2
  • armv8.5-a — Architecture: Armv8.5-A — includes by default: armv8.4-a, +sb, +ssbs, +predres, +frintts, +flagm2
  • armv8.6-a — Architecture: Armv8.6-A — includes by default: armv8.5-a, +bf16, +i8mm
  • armv8.7-a — Architecture: Armv8.7-A — includes by default: armv8.6-a, +wfxt, +xs
  • armv8.8-a — Architecture: Armv8.8-a — includes by default: armv8.7-a, +mops
  • armv8.9-a — Architecture: Armv8.9-a — includes by default: armv8.8-a
  • armv9-a — Architecture: Armv9-A — includes by default: armv8.5-a, +sve, +sve2
  • armv9.1-a — Architecture: Armv9.1-A — includes by default: armv9-a, +bf16, +i8mm
  • armv9.2-a — Architecture: Armv9.2-A — includes by default: armv9.1-a, +wfxt, +xs
  • armv9.3-a — Architecture: Armv9.3-A — includes by default: armv9.2-a, +mops
  • armv9.4-a — Architecture: Armv9.4-A — includes by default: armv9.3-a, +sve2p1
  • armv9.5-a — Architecture: Armv9.5-A — includes by default: armv9.4-a, cpa, +faminmax, +lut
  • armv8-r — Architecture: Armv8-R — includes by default: armv8-r

The value native is available on native AArch64 GNU/Linux and makes the compiler pick the architecture of the host system 🎀. This option has no effect if the compiler can't recognize the architecture of the host system. When -march=native is given and no other -mcpu or -mtune is given, then GCC will pick the host CPU as the CPU to tune for and select the architecture features from it. Basically: -march=native is treated as -mcpu=native. 👑

The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. That's the rule, no cap.

GCC uses name to figure out what kind of instructions it can emit when generating assembly code. And if -march is specified without either -mtune or -mcpu also being specified, the code gets tuned to perform well across a range of target processors implementing the target architecture. 💜

  • -mtune=name — name the target processor that GCC should tune the performance of the code for. The permissible values for this option are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2, grace, neoverse-v3, neoverse-v3ae, armagicpu, neoverse-n3, olympus, rigel, cortex-a725, cortex-x925, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, hip12, thunderxt83, thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82, cortex-r82ae, cortex-x1, cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a520ae, cortex-a710, cortex-a715, cortex-a720, cortex-a720ae, ampere1, ampere1a, ampere1b, ampere1c, cobalt-100, apple-m1, apple-m2, apple-m3, apple-m4, apple-m5, c1-nano, c1-pro, c1-premium c1-ultra and native. 🦋

For -mtune=, 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, and gb10 are all telling GCC to tune for a big.LITTLE system. 💅

Then there’s neoverse-512tvb. That one says GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, bestie, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below. 🌸

Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system. 😭

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

This option cannot be suffixed by feature modifiers. periodt.

-mcpu=name 👑

Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. 💅

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. 😤

-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.

-moverride=string 🧚

Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases.

This option is only intended to be useful when developing GCC. 🛠️

-mpc-relative-literal-loads, -mno-pc-relative-literal-loads 📦

Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny.

The -mpc-relative-literal-loads is deprecated. ⚠️

-msign-return-address=scope 🔏

Select the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none. This option has been deprecated by -mbranch-protection. 😭

-mbranch-protection=features 🛡️

Select the branch protection features to use. features can have one of the following forms:

  • none is the default and turns off all types of branch protection.
  • standard turns on all types of branch protection features. If a feature has additional tuning options, then standard sets it to its standard level.
  • pac-ret turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions practically always do this) using the A-key.
  • pac-ret+leaf extends the pac-ret signing to include leaf functions.
  • pac-ret+b-key or pac-ret+leaf+b-key can be used to sign the functions with the B-key instead of the A-key.
  • bti turns on branch target identification mechanism.
  • gcs turns on guarded control stack compatible code generation.

-mharden-sls=opts 🧱

Enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of the following options:

  • retbr
  • blr

In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening. 🔥

-mearly-ra=scope 🍓

Determine when to enable an early register allocation pass. This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, such as SME2's strided LD1 and ST1.

The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass.

-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise. 💅

-mearly-ldp-fusion, -mno-early-ldp-fusion 🧬

Enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by default at -O and above. ✨

-mlate-ldp-fusion, -mno-late-ldp-fusion 🦋

Enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by default at -O and above. 💜

-mnarrow-gp-writes 🪄

Enable conversion of 64-bit general purpose register writes to equivalent 32-bit operations when the upper 32 bits are known to be zero. This pass can be controlled with -mnarrow-gp-writes and is active at -O2 and above, but not enabled by default, except for -mcpu=olympus. 👑

-msve-vector-bits=bits 🌈

Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.

GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register and "vector-length specific" output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths.

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. 💖

🌟 -march and -mcpu Feature Modifiers 🌟

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:

  • crc — Enable CRC extension. This is on by default for -march=armv8.1-a.
  • crypto — Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
  • fp — Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
  • simd — Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
  • sve — Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
  • lse — Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
  • rdma — Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
  • fp16 — Enable FP16 extension. This also enables floating-point instructions.
  • fp16fml — Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported.
  • rcpc — Enable the RCpc extension. This enables the use of the LDAPR instructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension.
  • dotprod — Enable the Dot Product extension. This also enables Advanced SIMD instructions.
  • aes — Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
  • sha2 — Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
  • sha3 — Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
  • sm4 — Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported.
  • profile — Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation.
  • rng — Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation.
  • memtag — Enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not supported.
  • sb — Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
  • ssbs — Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
  • predres — turns on the Armv8-a Execution and Data Prediction Restriction instructions. Okay lowkey important: this one only enables the extension at the assembler level and does not affect code generation, so don't expect any ✨magic✨ out of the compiler. It's on by default for -march=armv8.5-a.
  • sve2 — turns on the Armv8-a Scalable Vector Extension 2 🦋. This also enables SVE instructions.
  • sve2-bitperm — turns on SVE2 bitperm instructions. This also enables SVE2 instructions.
  • sve2-sm4 — turns on SVE2 sm4 instructions. This also enables SVE2 instructions.
  • sve2-aes — turns on SVE2 aes instructions. This also enables SVE2 instructions.
  • sve2-sha3 — turns on SVE2 sha3 instructions. This also enables SVE2 instructions.
  • sve2p1 — turns on SVE2.1 instructions. This also enables SVE2 instructions.
  • sve2p2 — turns on SVE2.2 instructions. This also enables SVE2 and SVE2.1 instructions.
  • sve2p3 — turns on SVE2.3 instructions. This also enables SVE2 and SVE2.1 and SVE2.2 instructions.
  • i8mm — turns on 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. It's on by default for -march=armv8.6-a. Heads up bestie: using this with architectures prior to Armv8.2-A is not supported.
  • f32mm — turns on 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this with architectures prior to Armv8.2-A is not supported.
  • f64mm — turns on 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this with architectures prior to Armv8.2-A is not supported.
  • bf16 — turns on brain half-precision floating-point instructions 🧠. This also enables Advanced SIMD and floating-point instructions. On by default for -march=armv8.6-a. And again: use with architectures prior to Armv8.2-A is not supported.
  • ls64 — turns on the 64-byte atomic load and store instructions for accelerators.
  • mops — turns on the instructions that accelerate memory operations like "memcpy", "memmove", "memset". On by default for -march=armv8.8-a
  • flagm — turns on the Flag Manipulation instructions Extension.
  • flagm2 — turns on the FlagM2 flag conversion instructions.
  • pauth — turns on the Pointer Authentication Extension.
  • cssc — turns on the Common Short Sequence Compression instructions.
  • cmpbr — turns on the shorter compare and branch instructions, "cbb", "cbh" and "cb".
  • sme — turns on the Scalable Matrix Extension.
  • sme-i16i64 — turns on the FEAT_SME_I16I64 extension to SME. This also enables SME instructions.
  • sme-f64f64 — turns on the FEAT_SME_F64F64 extension to SME. This also enables SME instructions.
  • sme2 — turns on the Scalable Matrix Extension 2. This also enables SME instructions.
  • sme-f8f16 — turns on the FEAT_SME_F8F16 extension to SME. This also enables SME2 and FP8 instructions.
  • sme-f8f32 — turns on the FEAT_SME_F8F32 extension to SME. This also enables SME2 and FP8 instructions.
  • sme-b16b16 — turns on the FEAT_SME_B16B16 extension to SME. This also enables SME2 and SVE_B16B16 instructions.
  • sme-f16f16 — turns on the FEAT_SME_F16F16 extension to SME. This also enables SME2 instructions.
  • sme-mop4 — turns on the FEAT_SME_MOP4 extension to SME. This also enables SME2 instructions.
  • sme2p1 — turns on the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.
  • sme2p2 — turns on the Scalable Matrix Extension version 2.2. This also enables SME2 and SME2.1 instructions.
  • sme2p3 — turns on the Scalable Matrix Extension version 2.3. This also enables SME2, SME2.1 and SME2.2 instructions.
  • fcma — turns on the complex number SIMD extensions.
  • jscvt — turns on the "fjcvtzs" JavaScript conversion instruction.
  • frintts — turns on floating-point round to integral value instructions.
  • wfxt — turns on "wfet" and "wfit" instructions.
  • xs — turns on the XS memory attribute extension.
  • lse128 — turns on the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.
  • d128 — turns on support for 128-bit system register read/write instructions. This also enables the LSE128 extension.
  • gcs — turns on support for Armv9.4-a Guarded Control Stack extension.
  • the — turns on support for Armv8.9-a/9.4-a translation hardening extension.
  • rcpc2 — turns on the RCpc2 extension.
  • rcpc3 — turns on the RCpc3 (Release Consistency) extension.
  • fp8 — turns on the fp8 (8-bit floating point) extension.
  • fp8fma — turns on the fp8 (8-bit floating point) multiply accumulate extension.
  • ssve-fp8fma — turns on the fp8 (8-bit floating point) multiply accumulate extension in streaming mode.
  • fp8dot4 — turns on the fp8 (8-bit floating point) to single-precision 4-way dot product extension.
  • ssve-fp8dot4 — turns on the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.
  • fp8dot2 — turns on the fp8 (8-bit floating point) to half-precision 2-way dot product extension.
  • ssve-fp8dot2 — turns on the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.
  • f16f32dot — turns on the Half-precision to single-precision dot product extension.
  • faminmax — turns on the Floating Point Absolute Maximum/Minimum extension.
  • lut — turns on the Lookup Table extension.
  • sme-lutv2 — turns on the SME Lookup Table v2 (LUTv2) extension.
  • cpa — turns on the Checked Pointer Arithmetic instructions.
  • sve-b16b16 — turns on the SVE non-widening brain floating-point ("bf16") extension. And ngl, this one only has an effect when "sve2" or "sme2" are also enabled.
  • sve-bfscale — turns on the SVE_BFSCALE extension.
  • poe2 — turns on the Permission Overlays Extension 2.
  • tev — turns on the TIndex Exception-like Vector Extension.
  • tlbid — turns on the TLBI Domains Extension.
  • gcie — turns on the GICv5 (Generic Interrupt Controller) CPU Interface Extension.
  • lscp — turns on the load acquire and store release pair extension.
  • mops-go — turns on tag only variants of MOPS instructions. This also enables the instructions to accelerate memory operations and Armv8.5-a Memory Tagging Extensions.
  • sve2p3 — turns on SVE2.3. This also enables SVE2.2 instructions.
  • sme2p3 — turns on SME2.3. This also enables SME2.2 instructions.
  • f16f32dot — turns on Armv9.7-a f16f32dot instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.
  • sve-b16mm — turns on the SVE B16MM Extension. This also enables SVE instructions, Advanced SIMD and floating-point instructions.
  • mtetc — turns on Data cache tag block operations. This also enables Armv8.5-a Memory Tagging Extensions.
  • f16f32mm — turns on Armv9.7-a f16f32mm instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.
  • f16mm — turns on f16mm instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.

Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. 🫶 it's all connected bestie, the dependency chain is giving family tree energy.

🌈 Adapteva Epiphany Options 🌈

These -m options are defined for Adapteva Epiphany:

  • -mhalf-reg-file
  • -mno-half-reg-file — don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants that lack these registers. 🙈
  • -mprefer-short-insn-regs
  • -mno-prefer-short-insn-regs — preferentially allocate registers that allow short instruction generation. And okay listen: this can result in increased instruction count, so it may either reduce or increase overall code size. It's giving chaos, fr fr.
  • -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases.
  • -mcmove
  • -mno-cmove — enable the generation of conditional moves.
  • -mnops=num — emit num NOPs before every other generated instruction.
  • -mno-soft-cmpsf
  • -msoft-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 🙃. Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work. This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option.
  • -mno-round-nearest
  • -mround-nearest — -mno-round-nearest makes the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
  • -mlong-calls — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" / "bl" instructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.
  • -mshort-calls — if not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl" instructions, so use these instructions for direct calls.

The default is -mlong-calls. Note that -mlong-calls is equivalent to -mno-short-calls, and similarly -mno-long-calls is equivalent to -mshort-calls.

  • -msmall16
  • -mno-small16 — assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect.
  • -mfp-mode=mode — sets the prevailing mode of the floating-point unit, bestie. This is the FP mode that gets provided and expected at function call and return time. And like, the glow-up here? If you make this mode match whatever you predominantly need at function start, your programs get smaller and faster because you dodge all those unnecessary mode switches. ✨

mode can be set to one of these values:

  • caller — any mode at function entry is valid, and it's retained or restored when the function returns and when it calls other functions. This one's the move for compiling libraries or other compilation units you might want to drop into different programs with different prevailing FPU modes — the convenience of using a single object file outweighs the size and speed overhead from any extra mode switching that might be needed, compared with what a more specific choice of prevailing FPU mode would need. 🫶
  • truncate — this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer.
  • round-nearest — this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
  • int — this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate.

The default is -mfp-mode=caller. 💅

  • -mmay-round-for-trunc / -mno-may-round-for-trunc — this option lets floating point to integer truncation get replaced with rounding to save mode switching. It's disabled by default. 🙈
  • -mfp-iarith / -mno-fp-iarith — this one enables use of the floating-point unit for integer add and subtract. It's disabled by default.
  • -msplit-lohi, -mno-split-lohi, -mpost-inc, -mno-post-inc, -mpost-modify, -mno-post-modify — code generation tweaks that control, 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.
  • -mno-vect-double — changes 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. Not me warning you though — note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types. 😭
  • -msplit-vecmove-early / -mno-split-vecmove-early — splits vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. 💀
  • -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. It's giving very niche, very iconic. 👑

  • -march=gpu / -mtune=gpu — set architecture type or tuning for gpu. Supported values for gpu are:
    • gfx900 — compile for GCN5 Vega 10 devices (gfx900).
    • gfx902 — compile for GCN5 Vega gfx902 devices. (Experimental)
    • gfx904 — compile for GCN5 Vega gfx904 devices. (Experimental)
    • gfx906 — compile for GCN5 Vega 20 devices (gfx906).
    • gfx908 — compile for CDNA1 Instinct MI100 series devices (gfx908).
    • gfx909 — compile for GCN5 Vega gfx909 devices. (Experimental)
    • gfx90a — compile for CDNA2 Instinct MI200 series devices (gfx90a).
    • gfx90c — compile for GCN5 Vega 7 devices (gfx90c).
    • gfx942 — compile for CDNA3 Instinct MI300 series devices (gfx942). (Experimental)
    • gfx950 — compile for the CDNA3 gfx950 devices. (Experimental)
    • gfx9-generic — compile generic code for Vega devices, executable on the following subset of GFX9 devices: gfx900, gfx902, gfx904, gfx906, gfx909 and gfx90c.
    • gfx9-4-generic — compile generic code for CDNA3 devices, executable on the following subset of GFX9 devices: gfx942 and gfx950. (Experimental)
    • gfx1030 — compile for RDNA2 gfx1030 devices (GFX10 series).
    • gfx1031 — compile for RDNA2 gfx1031 devices (GFX10 series). (Experimental)
    • gfx1032 — compile for RDNA2 gfx1032 devices (GFX10 series). (Experimental)
    • gfx1033 — compile for RDNA2 gfx1033 devices (GFX10 series). (Experimental)
    • gfx1034 — compile for RDNA2 gfx1034 devices (GFX10 series). (Experimental)
    • gfx1035 — compile for RDNA2 gfx1035 devices (GFX10 series). (Experimental)
    • gfx1036 — compile for RDNA2 gfx1036 devices (GFX10 series).
    • gfx10-3-generic — compile generic code for GFX10-3 devices, executable on gfx1030, gfx1031, gfx1032, gfx1033, gfx1034, gfx1035, and gfx1036.
    • gfx1100 — compile for RDNA3 gfx1100 devices (GFX11 series).
    • gfx1101 — compile for RDNA3 gfx1101 devices (GFX11 series). (Experimental)
    • gfx1102 — compile for RDNA3 gfx1102 devices (GFX11 series). (Experimental)
    • gfx1103 — compile for RDNA3 gfx1103 devices (GFX11 series).
    • gfx1150 — compile for RDNA3 gfx1150 devices (GFX11 series). (Experimental)
    • gfx1151 — compile for RDNA3 gfx1151 devices (GFX11 series). (Experimental)
    • gfx1152 — compile for RDNA3 gfx1152 devices (GFX11 series). (Experimental)
    • gfx1153 — compile for RDNA3 gfx1153 devices (GFX11 series). (Experimental)
    • gfx11-generic — compile generic code for GFX11 devices, executable on gfx1100, gfx1101, gfx1102, gfx1103, gfx1150, gfx1151, gfx1152, and gfx1153.
  • -mgang-private-size=bytes — set the amount of local data-share (LDS) memory to reserve for gang-private variables. The default is 512.
  • -msram-ecc=on, -msram-ecc=off, -msram-ecc=any — compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature can be enabled per-process on some devices. The compiled code must match the device mode. The default is any, for devices that support it. ⭐
  • -mxnack=on, -mxnack=off, -mxnack=any — compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise.
  • -Wopenacc-dims / -Wno-openacc-dims — control warnings about invalid OpenACC dimensions.

🌸 ARC Options 🌸

The following options control the architecture variant for which code is being compiled:

  • -mbarrel-shifter — generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
  • -mjli-always — force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture.
  • -mcpu=cpu — set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:
    • arc600 — compile for ARC600. Aliases: -mA6, -mARC600.
    • arc601 — compile for ARC601. Alias: -mARC601.
    • arc700 — compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
    • arcem — compile for ARC EM.
    • archs — compile for ARC HS.
    • em — compile for ARC EM CPU with no hardware extensions.
    • em4 — compile for ARC EM4 CPU.
    • em4_dmips — compile for ARC EM4 DMIPS CPU.
    • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
    • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
    • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
    • hs34 — compile for ARC HS34 CPU.
    • hs38 — compile for ARC HS38 CPU.
    • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on.
    • hs4x — compile for ARC HS4x CPU.
    • hs4xd — compile for ARC HS4xD CPU.
    • hs4x_rel31 — compile for ARC HS4x CPU release 3.10a.
    • arc600_norm — compile for ARC 600 CPU with "norm" instructions enabled.
    • arc600_mul32x16 — compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
    • arc600_mul64 — compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
    • arc601_norm — compile for ARC 601 CPU with "norm" instructions enabled.
    • arc601_mul32x16 — compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
    • arc601_mul64 — compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
    • nps400 — compile for ARC 700 on NPS400 chip.
    • em_mini — compile for ARC EM minimalist configuration featuring reduced register set.
  • -mdpfp / -mdpfp-compact — generate double-precision FPX instructions, tuned for the compact implementation.
  • -mdpfp-fast — generate double-precision FPX instructions, tuned for the fast implementation.
  • -mno-dpfp-lrsr / -mdpfp-lrsr — control whether "lr" and "sr" instructions use FPX extension aux registers. This is enabled by default. 🎀
  • -mea — generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. Only valid for -mcpu=ARC700.
  • -mmul32x16 — generate 32x16-bit multiply and multiply-accumulate instructions.
  • -mmul64 — generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
  • -mnorm — okay so this one generates norm instructions ✨ and it's literally the default when -mcpu=ARC700 is in effect, bestie, so you probably already have it and didn't even know 💅
  • -mspfp, -mspfp-compact — both of these generate single-precision FPX instructions, tuned for the compact implementation 🎀
  • -mspfp-fast — same single-precision FPX instructions, but tuned for the fast implementation instead 🔥
  • -msimd — turns on generation of ARC SIMD instructions via target-specific builtins. Heads up though: it's only valid for -mcpu=ARC700 ⭐
  • -msoft-float — ngl this option is ignored, it's provided for compatibility purposes only 🙈 Software floating-point code is emitted by default, and that 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 — enables div and rem instructions for ARCv2 cores ✨
  • -mcode-density, -mno-code-density — enable code density instructions for ARC EM. This option is on by default for ARC HS 💅
  • -mll64 — enables double load/store operations for ARC HS cores 👑
  • *-mtp-regno=regno* — specify the thread pointer register number 🎀
  • -mbitops — enables use of NPS400 bit operations ⭐
  • -mcmem — enables use of the NPS400 xld/xst extension 🌸
  • *-mmpy-option=multo* — compile ARCv2 code with a multiplier design option. You can specify the option using either a string or a numeric value for multo. wlh1 is the default value. The recognized values are:
    • 0, none — no multiplier available 😭
    • 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.
And this one is only available for ARCv2 cores, so don't try it on anything else 💀
  • *-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. The supported values for fpu are, get comfy bestie, there's a lot:
    • 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 — specifies general-purposes registers that the processor automatically saves/restores on interrupt entry and exit. register-range is specified as two registers separated by a dash. The register range always starts with r0, the upper limit is fp register. blink and lp_count are optional. This option is only valid for ARC EM and ARC HS cores 🌈
  • *-mrgf-banked-regs=number* — specifies the number of registers replicated in second register bank on entry to fast interrupt. Fast interrupts are interrupts with the highest priority level P0. These interrupts save only PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. Use this option when you are using fast interrupts in an ARC V2 family processor. The permitted values are 4, 8, 16, and 32.
  • *-mlpc-width=width* — specify the width of the lp_count register. Valid values for width are 8, 16, 20, 24, 28 and 32 bits. The default width is fixed to 32 bits. If the width is less than 32, the compiler does not attempt to transform loops in your program to use the zero-delay loop mechanism unless it is known that the lp_count register can hold the required loop-counter value. Depending on the width specified, the compiler and run-time library might continue to use the loop mechanism for various needs. This option defines macro __ARC_LPC_WIDTH__ with the value of width ✨
  • -mrf16 — this option instructs the compiler to generate code for a 16-entry register file. This option defines the __ARC_RF16__ preprocessor macro 💅
  • -mbranch-index — enable use of bi or bih instructions to implement jump tables 🎀

The following options are passed through to the assembler, and also define preprocessor macro symbols, so they're doing double duty fr fr 👑

  • -mlock — passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol __Xlock.
  • -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol __Xswape.
  • -mxy — passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol __Xxy.

The following options control how the assembly code is annotated 🖊️

  • -misize — annotate assembler instructions with estimated addresses.

The following options are passed through to the linker 🔗

  • -marclinux, -mno-arclinux — passed through to the linker, to specify use of the arclinux emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is not requested.
  • -marclinux_prof, -mno-arclinux_prof — passed through to the linker, to specify use of the arclinux_prof emulation. This option is enabled by default in tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets when profiling is requested.

The following options control the semantics of generated code 🧠

  • -mlong-calls — generate calls as register indirect calls, thus providing access to the full 32-bit address range 🌈
  • -mmedium-calls, -mno-medium-calls — don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
  • -G num — put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations 🍓
  • -mno-sdata — do not generate sdata references. This is the default for tool chains built for arc-linux-uclibc and arceb-linux-uclibc targets.
  • -mvolatile-cache, -mno-volatile-cache — control how volatile references are accessed. The default is -mvolatile-cache, which uses ordinary cached memory accesses for volatile references. Use -mno-volatile-cache to enable cache bypass for volatile references ⚡

The following options fine tune code generation ✨

  • -mauto-modify-reg — enable the use of pre/post modify with register displacement.
  • -mno-brcc, -mbrcc — this option controls a target-specific pass in arc_reorg to generate compare-and-branch (brcc) instructions, which is enabled by default. It has no effect on generation of these instructions driven by the combiner pass.
  • -mcase-vector-pcrel — use PC-relative switch case tables to enable case table shortening. This is the default for -Os 💖
  • -mno-cond-exec — turn OFF the ARCompact-specific pass that generates conditional execution instructions.

Okay so here's the tea 🫖: 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 just… not it. So the ARC port keeps a special little pass around that hunts for extra conditional-execution opportunities after register allocation, branch shortening, and delay slot scheduling have already happened. This pass usually (but not always, bestie) makes performance and code size better — the price is extra compilation time, which is exactly why there's an option to switch it off. If you're having trouble with call instructions blowing past their allowable offset range because they got conditionalized, maybe consider -mmedium-calls instead. 💅

  • -mearly-cbranchsi — enable pre-reload use of the "cbranchsi" pattern.
  • -mindexed-loads — enable indexed loads. Lowkey a little risky ⚠️ because some optimizers then assume indexed stores exist too, and they do NOT.
  • -mlra-priority-none — don't give any priority to target registers.
  • -mlra-priority-compact — give target register priority for r0..r3 / r12..r15.
  • -mlra-priority-noncompact — reduce target register priority for r0..r3 / r12..r15.
  • -mmillicode — when optimizing for size (with -Os), prologues and epilogues that have to save or restore a large number of registers often get shortened by calling a special function in libgcc — that's what we call a millicode call ✨. But those calls can cause performance issues and/or linking issues when you link in a nonstandard way, so this option exists to turn millicode call generation on or off.
  • -mcode-density-frame — lets the compiler emit "enter" and "leave" instructions. Heads up though: those instructions are only valid for CPUs that have the code-density feature 🎀.
  • -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 it, the behavior is equivalent to level 1.

  • -mtune=cpu — set instruction scheduling parameters for cpu, overriding whatever -mcpu= implied.

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.

🌈 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 and aapcs-linux.
  • -mapcs-frame — generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even if it's not strictly necessary for the code to run correctly. If you also specify -fomit-frame-pointer with this option, stack frames won't be generated for leaf functions. The default is -mno-apcs-frame. This option is deprecated 🫠.
  • -mapcs — this is a synonym for -mapcs-frame and it 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, because specifying -mthumb-interwork makes slightly larger code. In AAPCS configurations this option is meaningless.
  • -msched-prolog, -mno-sched-prolog — allow or prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. With -mno-sched-prolog, this means all functions start with a recognizable set of instructions (or actually one of a choice from a small set of different function prologues), and that info can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
  • -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
    • soft makes GCC generate output that contains library calls for floating-point operations.
    • softfp allows generating code that uses hardware floating-point instructions, but still uses the soft-float calling conventions.
    • hard allows generating floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. Ngl, note that the hard-float and soft-float ABIs are not link-compatible 😭 — you have to compile your entire program with the same ABI, and link with a compatible set of libraries.

  • -mgeneral-regs-only — generate code that uses only the general-purpose registers. This stops the compiler from using floating-point and Advanced SIMD registers, but it won't 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. This option has no effect for little-endian images and gets ignored. The default depends on the selected target architecture: for ARMv6 and later architectures the default is BE8, and for older architectures the default is BE32. Also, BE32 format has been deprecated by ARM 💀.
  • -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. You can use this option alongside, or instead of, the -mcpu= option.

Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2.

Also, the following architectures, which don't support the Thumb execution state, are recognized but their support is deprecated: armv4.

Loads of these architectures support extensions 🌸. You add them by appending +extension to the architecture name. Extension options get processed in order and capabilities accumulate. An extension will also enable any necessary base extensions it depends on — for example, the +crypto extension always enables the +simd extension. The exception to this additive construction is for extensions prefixed with +no...: those disable the specified option and any other extensions that might depend on that extension being present.

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 their effect depends on the architecture you apply them to. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but it'll enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a, and the ARMv8-A variant for armv8-a.

The table below lists the supported extensions for each architecture. Architectures that aren't mentioned don't 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 it 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, plus the half-precision floating-point conversion operations. 🎀

+neon-vfpv4

The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.

+nosimd

Disables the Advanced SIMD instructions. 💅 Heads up tho — this does not disable floating point.

+nofp

Disables the floating-point and the Advanced SIMD instructions. Both. Gone. 🫶

armv7ve

The extended version of the ARMv7-A architecture, now with support for virtualization. 👑

  • +fp — The VFPv4 floating-point instructions, with 16 double-precision registers. Btw the extension +vfpv4-d16 can be used as an alias for this one.
  • +simd — The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can be used as an alias for this extension.
  • +vfpv3-d16 — The VFPv3 floating-point instructions, with 16 double-precision registers.
  • +vfpv3 — The VFPv3 floating-point instructions, with 32 double-precision registers.
  • +vfpv3-d16-fp16 — The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv3-fp16 — The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv4-d16 — The VFPv4 floating-point instructions, with 16 double-precision registers.
  • +vfpv4 — The VFPv4 floating-point instructions, with 32 double-precision registers.
  • +neon — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension +neon-vfpv3 can be used as an alias for this extension.
  • +neon-fp16 — The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +nosimd — Disable the Advanced SIMD instructions (does not disable floating point). 💅
  • +nofp — Disable the floating-point and Advanced SIMD instructions.

armv8-a

  • +crc — The Cyclic Redundancy Check (CRC) instructions.
  • +simd — The ARMv8-A Advanced SIMD and floating-point instructions.
  • +crypto — The cryptographic instructions. 🔐
  • +nocrypto — Disable the cryptographic instructions.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.

armv8.1-a

  • +simd — The ARMv8.1-A Advanced SIMD and floating-point instructions.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
  • +nocrypto — Disable the cryptographic instructions.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.

armv8.2-a and armv8.3-a

Okay so these two share the exact same extension list, bestie. Here it is 🦋

  • +fp16 — The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions.
  • +fp16fml — The half-precision floating-point fmla extension. This also enables the half-precision floating-point extension and Advanced SIMD and floating-point instructions.
  • +simd — The ARMv8.1-A Advanced SIMD and floating-point instructions.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
  • +dotprod — Enable the Dot Product extension. This also enables Advanced SIMD instructions.
  • +nocrypto — Disable the cryptographic extension.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
  • +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

armv8.4-a

  • +fp16 — The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
  • +simd — The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +nocrypto — Disable the cryptographic extension.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +sb — Speculation Barrier Instruction.
  • +predres — Execution and Data Prediction Restriction Instructions.
  • +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

armv8.5-a

  • +fp16 — The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
  • +simd — The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +nocrypto — Disable the cryptographic extension.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

armv8.6-a

  • +fp16 — The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.
  • +simd — The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +crypto — The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.
  • +nocrypto — Disable the cryptographic extension.
  • +nofp — Disable the floating-point, Advanced SIMD and cryptographic instructions.
  • +i8mm — 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions.
  • +bf16 — Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions.

armv7-r

  • +fp.sp — The single-precision VFPv3 floating-point instructions. The extension +vfpv3xd can be used as an alias for this extension.
  • +fp — The VFPv3 floating-point instructions with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
  • +vfpv3xd-d16-fp16 — The single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv3-d16-fp16 — The VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +nofp — Disable the floating-point extension.
  • +idiv — The ARM-state integer division instructions.
  • +noidiv — Disable the ARM-state integer division extension.

armv7e-m

  • +fp — The single-precision VFPv4 floating-point instructions.
  • +fpv5 — The single-precision FPv5 floating-point instructions.
  • +fp.dp — The single- and double-precision FPv5 floating-point instructions.
  • +nofp — Disable the floating-point extensions.

armv8.1-m.main

  • +dsp — The DSP instructions.
  • +mve — The M-Profile Vector Extension (MVE) integer instructions.
  • +mve.fp — The M-Profile Vector Extension (MVE) integer and single precision floating-point instructions. 🌈
  • +fp — the single-precision floating-point instructions ✨
  • +fp.dp — the single-and-double-precision floating-point instructions 💅
  • +nofp — nope, turn the floating-point extension all the way off 🙈
  • +cdecp0, +cdecp1, ... , +cdecp7 — turn on the Custom Datapath Extension (CDE) on whichever coprocessors you picked, chosen by the numbers in the options, range 0 to 7 🎀
  • +pacbti — turn on the Pointer Authentication and Branch Target Identification Extension 👑

armv8-m.main 🦋

  • +dsp — the DSP instructions 🌸
  • +nodsp — disable the DSP extension
  • +fp — the single-precision floating-point instructions ✨
  • +fp.dp — the single- and double-precision floating-point instructions
  • +nofp — disable the floating-point extension 🙈
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 🎀

armv8-r 🍓

  • +crc — the Cyclic Redundancy Check (CRC) instructions
  • +fp.sp — the single-precision FPv5 floating-point instructions
  • +simd — the ARMv8-A Advanced SIMD and floating-point instructions
  • +crypto — the cryptographic instructions 🔐
  • +nocrypto — disable the cryptographic instructions
  • +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions, all at once 💀
  • -march=native — makes the compiler auto-detect the architecture of the build computer 🕵️‍♀️. Right now bestie, this is only supported on GNU/Linux, and not every architecture gets recognized. If the auto-detect flops, the option just has no effect.
  • -march=unset — tells the compiler to ignore any -march=... options that show up earlier on the command line, and act like that option was never even passed 👻. Super useful for dodging those warnings about conflicting CPU and architecture options when the two produce different architecture specifications.

-mtune=name 💅

This option says the name of the target ARM processor GCC should tune the performance of the code for. For some ARM implementations you genuinely get better performance by using this option. So like, it's giving glow-up for your generated code ✨.

Permissible names are:

arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110,
strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t,
arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s,
arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s,
arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53,
cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76,
cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c,
cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8,
cortex-r52, cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3,
cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52,
cortex-m55, cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply,
cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1,
marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2,
ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, star-mc3,
xgene1.

Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system 🧚. Permissible names are those:

  • cortex-a15.cortex-a7
  • cortex-a17.cortex-a7
  • cortex-a57.cortex-a53
  • cortex-a72.cortex-a53
  • cortex-a72.cortex-a35
  • cortex-a73.cortex-a53
  • cortex-a75.cortex-a55
  • cortex-a76.cortex-a55

-mtune=generic-arch says GCC should tune the performance for a blend of processors within architecture arch 🌈. The whole goal is to generate code that runs well on the current most popular processors — balancing between optimizations that benefit some CPUs in the range while dodging performance pitfalls of the other CPUs in it. Heads up tho: the effects of this option may change in future GCC versions as CPU models come and go 👋.

-mtune permits the same extension options as -mcpu, but plot twist — the extension options do not affect the tuning of the generated code 💀.

-mtune=native causes the compiler to auto-detect the CPU of the build computer 🕵️‍♀️. At present this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.

-mcpu=name[+extension...] 🎀

This one specifies the name of the target ARM processor, bestie. GCC uses this name to derive the name of the target ARM architecture — as if you'd specified it with -march — and the ARM processor type to tune for performance, as if you'd specified it with -mtune ✨. Where this option is used together with -march or -mtune, those options take precedence over the appropriate part of this option. So they're the main characters here, periodt 👑.

Many of the supported CPUs implement optional architectural extensions. Where that's the case, the architectural extensions are normally enabled by default. If implementations that lack the extension exist, the extension syntax can be used to disable those extensions that have been omitted 🙈. For floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions. Yeah, it's a whole group chat of options deciding together 😭.

For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD 🍓. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions, respectively 💅.

Permissible names for this option are the same as those for -mtune — go look at that list above, it's exactly the same vibe 🔥.

The following extension options are common to the listed CPUs:

  • +nodsp — disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85 and star-mc3. Also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55, cortex-m85 and star-mc3 🙈
  • +nopacbti — disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52, cortex-m85 and star-mc3
  • +nomve — disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55, cortex-m85 and star-mc3
  • +nomve.fp — disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55, cortex-m85 and star-mc3
  • +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 on cortex-m52, cortex-m55, cortex-m85, star-mc1 and star-mc3 🎀
  • +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, cortex-m85 and star-mc3. 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-a53 and cortex-a75.cortex-a55 🔐

Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers 🌸. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a — iykyk 💅.

-mcpu=generic-arch is also permissible, and it's equivalent to -march=arch -mtune=generic-arch 👑. See -mtune for more information.

-mcpu=native causes the compiler to auto-detect the CPU of the build computer 🕵️‍♀️. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.

-mcpu=unset causes the compiler to ignore any -mcpu=... options that appear earlier on the command line and behave as if the option was never passed 👻. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications.

-mfpu=name 🦋

This one specifies what floating-point hardware (or hardware emulation, bestie) is available on the target 💖. Permissible names are:

auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd,
vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16,
neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8.

Quick lil heads-up: 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 causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.

So if the floating-point hardware you picked has the NEON extension in it (looking at you, -mfpu=neon), quick heads up: GCC's auto-vectorization pass is not gonna generate floating-point ops unless you also throw -funsafe-math-optimizations in there. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic — and the specific tea here is that denormal values just get treated as zero 🫠 — so using NEON instructions can absolutely lead to a loss of precision. It's giving "fast but slightly delulu."

You can also set the fpu name at the function level with target("fpu=") function attributes or pragmas, if you're feeling granular. 💅

  • -mfp16-format=name — Sets the format of the "__fp16" half-precision floating-point type. The names you're allowed to use are none, ieee, and alternative; default is none, and when it's none the "__fp16" type literally isn't defined at all. No type, no drama. 🙈
  • -mstructure-size-boundary=n — The sizes of every struct and union get rounded up to a multiple of however many bits this option sets. Permissible values: 8, 32 and 64. The default value varies across different toolchains. For the COFF targeted toolchain the default is 8. A value of 64 is only allowed if the underlying ABI supports it, bestie — otherwise it's a no.

Specifying a larger number can produce faster, more efficient code, but it can also blow up the size of your program. And different values are potentially incompatible with each other. Code compiled with one value can't necessarily expect to work with code or libraries compiled with another value if they exchange information using structures or unions. Mixing them is a trap.

This option is deprecated. Consider this your gentle warning. 📉
  • -mabort-on-noreturn — Generates a call to the function "abort" at the end of a "noreturn" function. It runs if the function tries to return. Like, excuse me? You said you'd never 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 if the target function lives outside the 64-megabyte addressing range of the offset-based version of the subroutine call instruction. It's the "your fave is out of range bestie" flag.

Even with the switch enabled, not all function calls become long calls. The heuristic is: static functions, functions that have the "short_call" attribute, functions inside the scope of a "#pragma no_long_calls" directive, and functions whose definitions have already been compiled within the current compilation unit — none of those become long calls. But here are the exceptions to that rule (yes, exceptions to the exceptions, iconic): weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions within the scope of a "#pragma long_calls" directive are always turned into long calls.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, and so does placing the function calls within the scope of a "#pragma long_calls_off" directive. And no cap, these switches have zero effect on how the compiler generates code to handle function calls via function pointers. Zero. 📌

  • -msingle-pic-base — Treat the register used for PIC addressing as read-only, instead of loading it in the prologue for each function. The runtime system is the one responsible for initializing this register with an appropriate value before execution begins. That's a runtime system problem, not a compiler problem. ✨
  • -mpic-register=reg — Specify the register used for PIC addressing. For the standard PIC base case, the default is any suitable register determined by the compiler. For the single PIC base case, the default is R9 if the target is EABI based or stack-checking is enabled, otherwise the default is R10. Pick your reg era. 🎀
  • -mpic-data-is-text-relative — Assumes the displacement between the text and data segments is fixed at static link time. This lets you use PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When it's disabled on those targets, it will enable -msingle-pic-base by default. Cascading defaults, we love to see it.
  • -mpoke-function-name — Writes the name of each function into the text section, directly preceding the function prologue. The generated code looks a lil something like this:
                      t0
                          .ascii "arm_poke_function_name", 0
                          .align
                      t1
                          .word 0xff000000 + (t1 - t0)
                      arm_poke_function_name
                          mov     ip, sp
                          stmfd   sp!, {fp, ip, lr, pc}
                          sub     fp, ip, #4

When doing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace function then looks at location "pc - 12" and the top 8 bits are set, then we know there's a function name embedded immediately preceding this location, and it has length "((pc[-3]) & 0xff000000)". Detective work fr. 🔍

  • -mthumb / -marm — Picks between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but that default can be changed by configuring GCC with the --with-mode=state configure option.

You can also override the ARM and Thumb mode for each function using the target("thumb") and target("arm") function attributes or pragmas. Per-function vibes. 💅

  • -mtpcs-frame — Generates a stack frame that's compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that doesn't call any other functions — it's just out here on its own, thriving.) The default is -mno-tpcs-frame.
  • -mtpcs-leaf-frame — Generates a stack frame that's compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that doesn't call any other functions.) The default is -mno-apcs-leaf-frame. Not a typo, that's what it says. 👀
  • -mcallee-super-interworking — Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This lets these functions be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default there. So don't even try it in AAPCS. 🙅‍♀️
  • -mcaller-super-interworking — Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code was compiled for interworking or not. There's a small overhead in the cost of executing a function pointer if this option is enabled. And same deal: this option is not valid in AAPCS configurations because interworking is enabled by default.
  • -mtp=name — Specify the access model for the thread local storage pointer. The model soft generates calls to "__aeabi_read_tp". Other accepted models are tpidrurw, tpidruro and tpidrprw, which fetch the thread pointer straight from the corresponding system register (supported from the arm6k architecture and later). These system registers are accessed through the CP15 co-processor interface, and the argument cp15 is also accepted as a convenience alias of tpidruro. The argument auto uses the best available method for the selected processor. Default setting is auto. Let it cook. ✨
  • -mtls-dialect=dialect — Specify the dialect to use for accessing thread local storage. Two dialects are supported — gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but 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. Glow up for your shared libs, but read the requirements first. 📋
  • -mword-relocations — Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. Pick a lane, bestie.
  • -mfix-cortex-m3-ldrd — Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used. This option avoids generating those instructions. It's enabled by default when -mcpu=cortex-m3 is specified. Because corruption is not the vibe. 💀
  • -mfix-cortex-a57-aes-1742098 / -mno-fix-cortex-a57-aes-1742098 / -mfix-cortex-a72-aes-1655431 / -mno-fix-cortex-a72-aes-1655431 — Enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. This option is enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified. Look at those erratum numbers, iconic. 🔢
  • -munaligned-access / -mno-unaligned-access — Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit aligned. By default, unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access isn't enabled, then words in packed data structures are accessed a byte at a time.

The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or false, depending on the setting of this option. If unaligned access is enabled, then the preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined. Two-for-one. 🎁

  • -mslow-flash-data — Assumes loading data from flash is slower than fetching instruction. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and it's off by default. It conflicts with -mword-relocations. Flash said "I'm tired." 😭
  • -masm-syntax-unified — Assumes inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. Future-facing tea, sip responsibly. ☕
  • -mrestrict-it — Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.
  • -mpure-code — Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute "SHF_ARM_PURECODE". This option is only available when generating non-pic code for M-profile targets. Exclusive club, only M-profile non-pic allowed. 👑
  • -mcmse — bestie, this one generates secure code exactly as laid out in the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which lives at <https://support.arm.com/documentation/ecm0359818/latest/>. periodt. 🛡️
  • -mfix-cmse-cve-2021-35465 — okay so there's a potential security oopsie 😭 with the VLLDM instruction on some M-profile devices when you're using CMSE, and this is the patch for it (CVE-2021-35465). It's on by default whenever you use the -mcpu= option with cortex-m33, cortex-m35p, cortex-m55, cortex-m85 or star-mc1. Wanna turn the mitigation off? Use -mno-fix-cmse-cve-2021-35465. slay 💅
  • -mstack-protector-guard=guard
  • -mstack-protector-guard-offset=offset — these two make stack protection code using a canary at guard. The supported locations are global (for a global canary) or tls (for a canary you reach through the TLS register). Heads up bestie, the -mstack-protector-guard-offset= option is meant for use with -fstack-protector-guard=tls and not for use in user-land code. 🫶
  • -mfdpic
  • -mno-fdpic — this picks the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for arm-*-uclinuxfdpiceabi targets, this option is on by default and implies -fPIE if none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and you should explicitly provide the PIC/PIE-related options as needed. no cap.
Note that static linking is not supported here, because it would still involve the dynamic linker when the program self-relocates. If that behavior is acceptable to you, use -static and -Wl,-dynamic-linker options. ✨

The opposite -mno-fdpic option is useful (and honestly required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs. 💀

  • -mbranch-protection=features — enables branch protection features (armv8.1-m.main only). features can take one of these forms: 🎀
    • none — generates code without branch protection or return address signing.
    • standard — generates code with all branch protection features enabled at their standard level.
    • pac-ret — generates code with return address signing set to its standard level, which is to sign all functions that save the return address to memory.
    • pac-ret+leaf — extends the pac-ret signing to include leaf functions even if they don't write the return address to memory.
    • bti — adds landing-pad instructions at the permitted targets of indirect branch instructions.

If support for the +pacbti architecture extension is not enabled (e.g. via -march=), then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code stays backwards-compatible with earlier versions of the architecture, but the extra security can be turned on at run time on processors that support the PACBTI extension. 🌈

Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.

Any setting other than none is supported only on armv8-m.main or later. 🙈

The default is to generate code without branch protection or return address signing.

  • -mvectorize-with-neon-quad
  • -mvectorize-with-neon-double — these control whether vectorization uses NEON quad-word or double-word registers. The default is -mvectorize-with-neon-quad. iconic. 👑

🌸 AVR Options 🌸

These options are defined for AVR implementations, bestie:

  • -mmcu=mcu — specify the AVR instruction set architecture (ISA) or device type. The default for this option is "avr2". 🦋

The following AVR devices and ISAs are supported. Note: a complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs file specs-mcu. Only the latter is provided by the compiler according to the supported mcus below. The rest is supported by AVR-LibC, or by means of atpack files from the hardware manufacturer. iykyk 📦

  • "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 babies 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 girlies 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 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 with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = "atxmega128a1", "atxmega128a1u", "atxmega128a4u". 👑
  • "avrtiny" — "Reduced Tiny" Tiny core devices, and she's tiny tiny: only 16 general purpose registers and 512 B up to 4 KiB of program memory. mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny102", "attiny104", "attiny20", "attiny40". 🎀
  • "avr1" — this ISA is implemented by the minimal AVR core and is supported for assembler only. mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200". 🫶
  • -mabsdata — assume that all data in static storage can be accessed by LDS / STS instructions. This one only does anything on reduced Tiny devices like ATtiny40. See also the absdata AVR Attributes variable attribute. 💅
  • -masm-len-notes, -mno-asm-len-notes — recognize (resp. ignore) [[len=spec]] notes placed in comments inside inline assembly code templates. They let you specify the exact Size of an asm construct. This option is on per default. ⭐
  • -mcvt — use a compact vector table. Some devices support a CVT with only four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ, 3=Prio0 IRQs. This option links startup code from crtmcu-cvt.o instead of the usual crtmcu.o. Besides giving you that compact vector table, the startup code sets bit CPUINT_CTRLA.CPUINT_CVT, which enables the CVT on the device. 🔥

If you don't want the startup code setting CPUINT_CTRLA.CPUINT_CVT, you can satisfy the symbol __init_cvt so the relevant code doesn't get pulled in from libmcu.a anymore. For example, you can link with -Wl,--defsym,__init_cvt=0. 💖

The CVT startup code is available since AVR-LibC v2.3. ✨

  • -mdouble=bits, -mlong-double=bits — set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether a specific value of bits is allowed depends on the --with-double= and --with-long-double= configure options, and the same goes for the default values of these options. 🌈
  • -mfract-convert-truncate — allow using truncation instead of rounding towards zero for fractional fixed-point types. 🍓
  • -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs using the no_gccisr AVR Attributes 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. 🌸
  • -mint8 — assume int to be an 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and a long long is 4 bytes. Just so you know — this option does not conform to the C standards, but it results in smaller code size. 😭
  • -mmain-is-OS_task — do not save registers in main. Same effect as attaching the OS_task AVR Attributes attribute to main. It is activated per default if optimization is on. 💜
  • -mno-call-main — don't run main by means of
XCALL  main
XJMP   exit

Instead, put main in section .init9 so that no call is required. By setting this option, the user asserts that main will not return. 🫶

This option can be used for devices with very limited resources in order to save a few bytes of code and stack space. It works as expected since AVR-LibC v2.3. With older versions, there will be no performance gain. 💀

  • -mno-interrupts — generated code is not compatible with hardware interrupts. Code size is smaller. 🙈
  • -mpmem-wrap-around — enable program counter wrap-around in linker relaxation. ⭐
  • -mrodata-in-ram, -mno-rodata-in-ram — locate the .rodata sections for read-only data in RAM resp. in program memory. For most devices there is no choice, and this option acts rather like an assertion. 👑

Since v14 and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default, provided the required GNU Binutils support (PR31124) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM. 💅

  • -mrelax — try to replace CALL resp. JMP instructions by the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line. 🔥

Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembly code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembly code. 🤯

Relaxing must be turned on if linker stubs are needed — see the section on EIND and linker stubs below. 🎀

  • -mstrict-addr-space-subsets — impose strict named address space subset relations. Without this option, every address space is considered to be a subset of all the other spaces, which allows you to write code like
char read_char (const char *addr, bool in_flash)
{
   return in_flash
     ? *(const __flash char*) addr  // Read from program memory.
     : *addr;                       // Read from RAM.
}

When the option is on, such code is rejected, since address space __flash is not a subset of the generic space, and hence the cast in the above code is prohibited. 💀

  • -mtiny-stack — only change the lower 8 bits of the stack pointer. 🦋
  • -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 to initialize the stack pointer with symbol __stack, __init_cvt to set up the hardware to use a compact vector table with -mcvt, __call_main to call main and exit, and __do_flmap_init to set up FLMAP according to symbol __flmap. 😭

  • -nodevicespecs — don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command-line options. This means the user has to supply her private device specs file by means of -specs=path-to-specs-file. There is no more need for option -mmcu=mcu. 💅

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named device-specs which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu. 🎀

  • -Waddr-space-convert — warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space. 👑
  • -Wasm-len-notes — warn about unrecognized [[len=spec]] Size of an asm / length annotations in inline assembly code templates. This Warning Options warning is on per default. Notice that in LTO mode, the code must be compiled with option -ffat-lto-objects in order for these diagnoses to appear. 🌟
  • -Wmisspelled-isr — warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default. ⭐

🌈 AVR Optimization Options 🌈

The following options are pure optimization options. 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 are pushed before calling a function and popped afterwards. See also the -fdefer-pop Optimize Options optimization option. 🫶

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call. 🔥

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack — like calls to printf-like functions. 💅

  • -mbranch-cost=cost — set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0. ⭐
  • -mcall-prologues — function prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller. 🌸
  • -mdemote-switch — without this option, the compiler will treat switch variables as 32-bit values at least. When this option is on, an AVR-specific pass will try to demote such switch variables to the widths as actually used in the program. This option is on per default when optimization is on. 👑
  • -mfuse-add, -mno-fuse-add, -mfuse-add=level — bestie this one optimizes indirect memory accesses on those lil 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-ifelse — this one optimizes compare-branch-compare-branch sequences down to compare-branch-branch, which typically shows up in if-else trees generated from dense switch statements 🌸. It's enabled per default if optimization is on, no cap.
  • -mfuse-move, -mno-fuse-move, -mfuse-move=level — runs a post-reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations ✨. The default uses level=3 for optimization -O1, and level=23 for higher optimizations. Valid values for level are in the range 0 ... 23 which is a 3:2:2:2 mixed radix value 🧚. Each digit controls some aspect of the optimization.
  • -mfuse-move2 — runs a post-combine optimization pass that tries to fuse move instructions 🔥.
  • -mstrict-X — use address register X in a way proposed by the hardware 💖. This means that X is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the X register may be used in the same way as Y or Z which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as

        adiw r26, const   ; X += const
        ld   <Rn>, X        ; <Rn> = *X
        sbiw r26, const   ; X -= const
  • -msplit-bit-shift — splits multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset 🌈. This optimization is turned on per default for -O2 and higher, including -Os but excluding -Oz. Splitting of shifts with a constant offset that is a multiple of 8 is controlled by -mfuse-move.
  • -msplit-ldst — splits multi-byte loads and stores into several byte loads and stores 👑. This optimization is turned on per default for -O2 and higher.
  • -muse-nonzero-bits — enables optimizations that are only possible when some bits in a register are always zero 💜. This optimization is turned on per default for -O2 and higher.

EIND and Devices with More Than 128 Ki Bytes of Flash 🦋

Okay so pointers in the implementation are 16 bits wide, periodt. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64 Ki words ✨.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used 💅.

Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:

  • The compiler never sets EIND 😭.
  • The compiler uses EIND implicitly in EICALL/EIJMP instructions or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.
  • The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue 🫶.
  • For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines 💀. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
  • Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations 🔥. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message 🙈.
  • The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to 💖.
  • The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual 🌸.
  • 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 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 assembly 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. fr fr.

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__ — bestie this chip literally has the LPM Rn,Z and LPM Rn,Z+ instructions ✨
  • __AVR_HAVE_MOVW__ — the chip has a MOVW instruction so it can do 16-bit register-register moves, no cap 🫶
  • __AVR_HAVE_MUL__ — it has a hardware multiplier 💅
  • __AVR_HAVE_JMP_CALL__ — the chip has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory 👑
  • __AVR_HAVE_EIJMP_EICALL__ and __AVR_3_BYTE_PC__ — the chip has the EIJMP and EICALL instructions, which happens for devices with more than 128 KiB of program memory. This also means 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__ and __AVR_HAVE_16BIT_SP__ — the stack pointer (SP) register gets treated as an 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack 🎀
  • __AVR_HAVE_SPH__ and __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__ and __AVR_ERRATA_SKIP_JMP_CALL__ — some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum 😭. The skip instructions are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.
  • __AVR_ISA_RMW__ — the device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT) 🔥
  • __AVR_SFR_OFFSET__=offset — instructions that can address I/O special function registers directly, like IN, OUT, SBI, etc., may use a different address than if they were addressed by an instruction accessing RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address. It's giving address-math homework, fr fr 💀
  • __AVR_SHORT_CALLS__ — the -mshort-calls command-line option is set.
  • __AVR_PM_BASE_ADDRESS__=addr — some devices support reading from flash memory by means of LD* instructions. The flash memory is seen in the data address space at an offset of __AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If it is defined, the address space is linear and there is no need to put .rodata into RAM 🦋. This is handled by the default linker description file, and is currently available for avrtiny and avrxmega3. Even more convenient: there's no need to use address spaces like __flash or features like attribute progmem and pgm_read_*. Iconic behaviour honestly.
  • __AVR_HAVE_FLMAP__ — this macro is defined provided the following conditions are met:
    • The device has the NVMCTRL_CTRLB.FLMAP bitfield. This applies to the AVR64* and AVR128* devices.
    • It's not known at assemble-time which emulation will be used.

This implies the compiler was configured with GNU Binutils that implement PR31124 👑

  • __AVR_RODATA_IN_RAM__ — this macro is undefined when the code is compiled for a core architecture.

When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data is located in RAM, and defined to 0 otherwise.

  • __WITH_AVRLIBC__ — the compiler is configured to be used together with AVR-LibC. See the --with-avrlibc configure option 💜
  • __HAVE_SIGNAL_N__ — the compiler supports the signal(num) and interrupt(num) AVR attributes, function attributes with an argument num that specifies the number of the interrupt service routine.
  • __HAVE_DOUBLE_MULTILIB__ — defined if -mdouble= acts as a multilib option.
  • __HAVE_DOUBLE32__ and __HAVE_DOUBLE64__ — defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=.
  • __DEFAULT_DOUBLE__ — the size in bits of double if -mdouble= is not set. To test the layout of double in a program, use the built-in macro __SIZEOF_DOUBLE__ ✨
  • __HAVE_LONG_DOUBLE32__, __HAVE_LONG_DOUBLE64__, __HAVE_LONG_DOUBLE_MULTILIB__, __DEFAULT_LONG_DOUBLE__ — same as above, but for long double instead of double 🫶
  • __WITH_DOUBLE_COMPARISON__ — reflects the --with-double-comparison={tristate|bool|libf7} configure option and is defined to 2 or 3.
  • __WITH_LIBF7_LIBGCC__, __WITH_LIBF7_MATH__, __WITH_LIBF7_MATH_SYMBOLS__ — reflects the --with-libf7={libgcc|math|math-symbols} configure option 🌸

🧚 AVR Internal Options 🧚

So these next ones are the compiler's own little group chat, okay? They're used internally by the compiler and to communicate between device specs files and the compiler proper. You do not need to set these options by hand, and in particular they are not optimization options 🚫. Using these options in the wrong way may lead to sub-optimal or wrong code 💀. They're documented for completeness, and in order to get a better understanding of device specs files.

  • -mn-flash=num — assume that the flash memory has a size of num times 64 KiB. This determines which __flashN address spaces are available.
  • -mflmap — the device has the FLMAP bit field located in special function register NVMCTRL_CTRLB.
  • -mrmw — assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT 🎀
  • -mshort-calls — assume that RJMP and RCALL can target the whole program memory. This option is used for multilib generation and selection for the devices from architecture avrxmega3.
  • -mskip-bug — generate code without skips (CPSE, SBRS, SBRC, SBIS, SBIC) over 32-bit instructions.
  • -msp8 — treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. This option is used by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH 👑

🔥 Blackfin Options 🔥

  • -mcpu=cpu[-sirevision] — specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.

The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the __SILICON_REVISION__ is not defined. If sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor.

GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.

Without this option, bf532 is used as the processor by default.

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 has effect only for bfin-elf toolchain. Certain other options, such as -mid-shared-library and -mfdpic, imply -msim.
  • -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and restore frame pointers and makes an extra register available in leaf functions ✨
  • -mspecld-anomaly, -mno-specld-anomaly — when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used, __WORKAROUND_SPECULATIVE_LOADS is defined.
  • -mcsync-anomaly, -mno-csync-anomaly — when enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If this option is used, __WORKAROUND_SPECULATIVE_SYNCS is defined.
  • -mlow64k, -mno-low64k — when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low 64k of memory. The default behavior is to assume that the program is arbitrarily large (-mno-low64k) 🫶
  • -mstack-check-l1 — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
  • -mid-shared-library, -mno-id-shared-library — generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-elf target, this option implies -msim. The default is -mno-id-shared-library, to generate code that doesn't assume ID-based shared libraries are being used 💅
  • -mleaf-id-shared-library, -mno-leaf-id-shared-library — generate code that supports shared libraries via the library ID method, but assumes that this library or executable won't link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls 🏃‍♀️

The default is -mno-leaf-id-shared-library, in which the no assumption is made 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 — bestie, this sets the ID number of the ID-based shared library you're compiling. Put 0 in there and you get more compact code ✨. Put any other number and it forces that number to be allocated to the current library — but it's no more space- or time-efficient than just leaving this option out entirely. iykyk.
  • -msep-data, -mno-sep-data — this one lets the data segment live in a totally different area of memory than the text segment 🫶. Why would you want that? Execute in place, babes — in an environment with no virtual memory management, because it eliminates relocations against the text section. The default is -mno-sep-data, which tells GCC to generate code assuming the data segment follows the text segment. no surprises, just adjacency.
  • -mlong-calls, -mno-long-calls — tells the compiler to do function calls by first loading the address of the function into a register, then doing a subroutine call on that register 💅. You need this switch if the target function lives outside the 24-bit addressing range of the offset-based version of the subroutine call instruction. This feature is not on by default. Saying -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. periodt.
  • -mfast-fp — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), all in the interest of performance 🔥. it's giving "speed over pedantry."
  • -minline-plt — enables inlining of PLT entries in function calls to functions that aren't known to bind locally 🎀. Heads up though: it has no effect without -mfdpic.
  • -mmulticore — build a standalone application for multicore Blackfin processors ✨. This option makes proper start files and link scripts supporting multicore get used, and it defines the macro "__BFIN_MULTICORE". It can only be used with -mcpu=bf561[-sirevision]. You can pair it with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used — and in that model, the main function of Core B should be named "coreb_main" 👑. If you don't use this option at all, the single-core application programming model is used.
  • -mcorea — build a standalone application for Core A of BF561 when you're doing the one-application-per-core programming model 💖. Proper start files and link scripts are used to support Core A, and the macro "__BFIN_COREA" is defined. This option can only be used in conjunction with -mmulticore. no solo runs, babe.
  • -mcoreb — build a standalone application for Core B of BF561 when you're doing the one-application-per-core programming model 🌸. Proper start files and link scripts support Core B, and the macro "__BFIN_COREB" is defined. When you use this option, "coreb_main" should be used instead of "main". And yep — this one can only be used in conjunction with -mmulticore too.
  • -msdram — build a standalone application for SDRAM 💜. Proper start files and link scripts are used to put the application into SDRAM, and the macro "__BFIN_SDRAM" is defined. Your loader should initialize SDRAM before loading the application — that part's on you, bestie.
  • -micplb — assume that ICPLBs are enabled at run time 🦋. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications, the default is off.

🧁 C6X Options 🧁

  • -march=name — specifies the name of the target architecture ✨. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. The permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
  • -mbig-endian — generate code for a big-endian target 💅.
  • -mlittle-endian — generate code for a little-endian target. This is the default. periodt.
  • -msim — choose startup files and linker script suitable for the simulator 🎀.
  • -msdata=default — put small global and static data in the ".neardata" section, which is pointed to by register "B14" 🫶. Put small uninitialized global and static data in the ".bss" section, which is adjacent to the ".neardata" section. Put small read-only data into the ".rodata" section. The corresponding sections used for large pieces of data are ".fardata", ".far" and ".const".
  • -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the "B14" register to access them 🔥.
  • -msdata=none — make no use of the sections reserved for small data at all, and use absolute addresses to access all data 💀. Put all initialized global and static data in the ".fardata" section, all uninitialized data in the ".far" section, and all constant data into the ".const" section.
  • -mdsbt — compile for the DSBT shared library ABI 🌈. This option is required to compile with -fpic or -fPIC, and it implies -fpic.
  • -mlong-calls — avoid generating PC-relative calls; use indirection instead 🦋.

💫 CRIS Options 💫

These options are defined specifically for the CRIS ports, bestie — a whole little world of its own.

  • -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture 👑. The choices for architecture-type are v3, v8 and v10 for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0.
  • -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions ✨. The choices for architecture-type are the same as for -march=architecture-type.
  • -mmax-stackframe=n — warn when the stack frame of a function exceeds n bytes 🎀.
  • -metrax4, -metrax100 — these two are synonyms for -march=v3 and -march=v8 respectively 💅.
  • -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the "muls" and "mulu" instructions for CPU models where it applies 🐛. This option is disabled by default.
  • -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code 🔍. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
  • -mcc-init — do not use condition-code results from a previous instruction; always emit compare and test instructions before use of condition codes ✅.
  • -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment 🙈.
  • -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these options (and their no- forms) arrange (or eliminate arrangements) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model 🌸. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options — important, bestie.
  • -m32-bit, -m16-bit, -m8-bit — similar to the stack-, data- and const-align options above, these arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned 💜. The default is 32-bit alignment.
  • -mno-prologue-epilogue, -mprologue-epilogue — with -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted, and no return instructions or return sequences are generated in the code 😭. > Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. fr fr, look at your output.
  • -mbest-lib-options, -moverride-best-lib-options — -mbest-lib-options selects the most feature-enabling options allowed by other options ✨. This option has no no- form, but -moverride-best-lib-options disables it regardless of the relative order of the two options on the command line. iconic behavior, honestly.
  • -mtrap-using-break8 — emit traps as break 8 🚨. This is the default for CRIS v3 and up. If disabled, calls to "abort" are used instead.
  • -mtrap-unaligned-atomic — emit checks causing break 8 instructions to execute when applying atomic builtins on misaligned memory ⚠️.
  • -munaligned-atomic-may-use-library — handle atomic builtins that may be applied to unaligned data by calling library functions 🫶. This option overrides -mtrap-unaligned-atomic.
  • -sim — this option arranges to link with input-output functions from a simulator library 🎀. Code, initialized data and zero-initialized data are allocated consecutively.
  • -sim2 — like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000 🌈.

🦋 C-SKY Options 🦋

GCC supports these options when compiling for C-SKY V2 processors 💖.

  • -march=arch — specify the C-SKY target architecture ✨. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810.
  • -mcpu=cpu — specify the C-SKY target processor 👑. Valid values for cpu are:
ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t,
ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht,
ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1,
ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1,
ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1,
ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft,
ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft,
ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and
ck810ftv.
  • -mbig-endian, -mlittle-endian — select big- or little-endian code 💅. The default is little-endian.
  • -mfloat-abi=name — specifies which floating-point ABI to use 🌸. Permissible values are: soft, softfp and hard.

Specifying soft causes GCC to generate output containing library calls for floating-point operations 🎀. softfp allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions 🔥.

The default depends on the specific target configuration. Heads up bestie: the hard-float and soft-float ABIs are not link-compatible 💀 — you have to compile your entire program with the same ABI, and link against a compatible set of libraries. No mixing, no matching, periodt.

  • -mdouble-float, -mno-double-float — when -mhard-float is in effect, this turns on generation of double-precision float instructions. It's the default, except when you're compiling for CK803.
  • -mfdivdu, -mno-fdivdu — when -mhard-float is in effect, this enables generation of "frecipd", "fsqrtd", and "fdivd" instructions. Default again, except when compiling for CK803.
  • -mfpu=fpu — pick your floating-point processor, bestie. This option can *only be used together with -mhard-float. The values for *fpu are fpv2_sf (which is equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float -mdivdu).
  • -melrw, -mno-elrw — enable the extended "lrw" instruction. Defaults to on for CK801 and off otherwise.
  • -mistack, -mno-istack — enable interrupt stack instructions; default is off.

The -mistack option is required to handle the "interrupt" and "isr" function attributes.

  • -mmp, -mno-mp — enable multiprocessor instructions; default is off.
  • -mcp, -mno-cp — enable coprocessor instructions; default is off.
  • -mcache, -mno-cache — enable coprocessor instructions; default is off.
  • -msecurity, -mno-security — enable C-SKY security instructions; default is off.
  • -mtrust, -mno-trust — enable C-SKY trust instructions; default is off.
  • -mdsp, -mno-dsp, -medsp, -mno-edsp, -mvdsp, -mno-vdsp — enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these default to off.
  • -mdiv, -mno-div — generate divide instructions. Default is off.
  • -msmart, -mno-smart — generate code for Smart Mode, using only registers numbered 0-7 so 16-bit instructions can be used. This option gets ignored for CK801 (where this is just the required behavior anyway), and it defaults to on for CK802. For other targets, the default is off.
  • -mhigh-registers, -mno-high-registers — generate code using the high registers numbered 16-31. 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. Defaults to on.
  • -mmultiple-stld, -mno-multiple-stld — generate code using "stm" and "ldm" instructions. Not supported on CK801, but enabled by default on other processors.
  • -mconstpool, -mno-constpool — create constant pools in the compiler instead of handing that job off to the assembler. This one is the default and is required for correct code generation on CK801 and CK802, and it's optional on other processors.
  • -mstack-size, -mno-stack-size — emit .stack_size directives for each function in the assembly output. Defaults to off.
  • -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. Defaults to off.
  • *-mbranch-cost=n* — set the branch costs to roughly n instructions. Default is 1.
  • -msched-prolog, -mno-sched-prolog — permit scheduling of function prologue and epilogue sequences. > Real talk: using this can give you code that is not compliant with the C-SKY V2 ABI prologue requirements, and it can't be debugged or backtraced 😭. It's disabled by default.
  • -msim, -mno-sim — links the library libsemi.a, which is compatible with the simulator. Only applicable to ELF compilers.

💻 Cygwin and MinGW Options 💻

These extra options are here for Microsoft Windows targets, bestie:

  • -mconsole — says a console application is getting generated, by telling the linker to set the PE header subsystem type that console apps need. Available for Cygwin and MinGW targets, and enabled by default on them.
  • *-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 you don't specify this option, then the default MinGW import library "msvcrt" gets used for linking and no other preprocessor adjustment happens. MinGW import library "msvcrt" is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation. MinGW import library "msvcrt-os" is for the Windows system CRT DLL library "msvcrt.dll" and in most cases is the default MinGW import library. Generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT. > This option is for experimental and testing purposes only 🙈. Available for MinGW targets.
  • -mdll — available for Cygwin and MinGW targets. It specifies that a DLL — a dynamic link library — is to be generated, which enables selection of the required runtime startup object and entry point.
  • -mnop-fun-dllimport — available for Cygwin and MinGW targets. Specifies that the "dllimport" attribute should be ignored.
  • -mthreads — support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data.
  • -municode — available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code.
  • -mwin32 — available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the preprocessor, but it does not influence the choice of runtime library/startup code.
  • -mwindows — available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated, by instructing the linker to set the PE header subsystem type appropriately.
  • -fno-set-stack-executable — available for MinGW targets. It says the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, since there the User32 API — which is what sets executable privileges — isn't available.
  • -fwritable-relocated-rdata — available for MinGW and Cygwin targets. It says relocated-data in the read-only section gets put into the ".data" section. Necessary for older runtimes that don't support modification of ".rdata" sections for pseudo-relocation.
  • -mpe-aligned-commons — available for Cygwin and MinGW targets. It says the GNU extension to the PE file format that permits correct alignment of COMMON variables should be used when generating code. Enabled by default if GCC detects that the target assembler it found during configuration supports the feature.
  • -muse-libstdc-wrappers — use Cygwin DLL wrappers to support the C++ operators "new" and "delete".

See also under x86 Options for standard options.

🍎 Darwin Options 🍎

These options are defined for all architectures running the Darwin operating system, bestie.

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 pulls that off 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 they get hit with an ISA mismatch. The assembler, as, only permits instructions that are valid for the subtype of the file it's 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 you ask it 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 searched for header files. These directories are interleaved with the ones specified by -I options and are scanned in a left-to-right order.

A framework directory is just a directory that has frameworks chilling inside it. A framework is a directory with a Headers and/or PrivateHeaders directory sitting directly inside it, and its name ends in .framework. The name of the framework = the name of that directory minus the .framework part, periodt. Headers for the framework live in one of those two dirs, and Headers gets searched first 💅. A subframework is a framework directory that lives inside a framework's Frameworks directory. Includes of subframework headers can only show up in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they're in the same framework. A subframework should not have the same name as a framework — you get a warning if you break this rule bestie. Currently a subframework can't have subframeworks; in the future, the mechanism may be extended to support this (not yet tho). The standard frameworks live in /System/Library/Frameworks and /Library/Frameworks. An example include looks like #include <Framework/header.h>, where Framework = the name of the framework and header.h lives in the PrivateHeaders or Headers directory ✨.

  • -iframeworkdir — literally -F but the directory gets treated as a system directory. The main glow-up difference between this -iframework and -F? With -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.
  • -arch name — generate output for architecture name. As described above, GCC generates output for the architecture it was configured for, using its usual options to select subarchitecture variants. The -arch option is accepted for compatibility, but an error is diagnosed if name is inconsistent with GCC's own idea of the target architecture 😭.
  • -dependency-file filename — alias for the preprocessor option -MF filename.
  • -fapple-kext — generate code for Darwin loadable kernel extensions.
  • -gused — emit debugging info for symbols that are used. For stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON 👑.
  • -gfull — emit debugging information for all symbols and types.
  • -matt-stubs
  • -mno-att-stubs — enable AT&T-style PIC stubs. This is the default when supported by the target architecture (currently x86 only).
  • -mconstant-cfstrings
  • -fconstant-cfstrings — when the NeXT runtime is being used (the default on these systems), override any -fconstant-string-class setting and cause @"..." literals to be laid out as constant CoreFoundation strings.

-fconstant-cfstrings is an alias for -mconstant-cfstrings.

  • -mdynamic-no-pic — generate code suitable for executables (not shared libraries). This option is incompatible with -fpic, -fPIC, -fpie, or -fPIE. Pick a lane bestie 🫶.
  • -mfix-and-continue
  • -ffix-and-continue
  • -findirect-data — generate code suitable for fast turnaround development, like letting GDB dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
  • -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 one flag to rule them all 💅.
  • -mmacosx-version-min=version
  • -asm_macosx_version_min=version — the -mmacosx-version-min option specifies the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include 12, 10.12, and 10.5.8.

If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible.

-asm_macosx_version_min=version is similar, but the GCC driver passes its version information only to the assembler.

  • -mone-byte-bool — override the defaults for bool so that sizeof(bool)==1. By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
⚠️ Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries 😭. Use this switch to conform to a non-default data model.
  • -msymbol-stubs
  • -mno-symbol-stubs — force generation of external symbol indirection stubs for PIC references. By default, this option is enabled automatically if the target linker version (-mtarget-linker) is old enough to require them.
  • -mtarget-linker=version
  • -mtarget-linker version — specify the target ld64 version, overriding any version specified in the GCC configuration. Newer linker versions support improved code generation in some cases, for example for PIC code.
  • -ObjC — equivalent to -x objective-c; specifies that the input is is Objective-C source code.
  • -ObjC++ — equivalent to -x objective-c++; specifies that the input is is Objective-C++ source code.
  • -Wnonportable-cfstrings
  • -Wno-nonportable-cfstrings — warn if constant CoreFoundation string objects contain non-portable characters. This warning is enabled by default.
  • -all_load — loads all members of static archive libraries. See man ld(1) for more information.
  • -arch_errors_fatal — cause the errors having to do with files that have the wrong architecture to be fatal 💀.
  • -bind_at_load — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
  • -bundle — produce a Mach-o bundle format file. See man ld(1) for more information.
  • -bundle_loader executable — this option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
  • -dynamiclib — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command.
  • -force_cpusubtype_ALL — this causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
  • -nodefaultrpaths — do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds -nodefaultrpaths to the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later.
  • -nodefaultexport — do not add default symbol exports to modules or dynamic libraries.
  • -allowable_client client_name
  • -client_name
  • -compatibility_version
  • -current_version
  • -dead_strip
  • -dylib_file
  • -dylinker
  • -dylinker_install_name
  • -dynamic
  • -exported_symbols_list
  • -filelist
  • -flat_namespace
  • -force_flat_namespace
  • -framework
  • -headerpad_max_install_names
  • -image_base
  • -init symbol-name
  • -install_name
  • -keep_private_externs
  • -pagezero_size
  • -preload
  • -read_only_relocs
  • -sectalign
  • -sectcreate
  • -seg_addr_table
  • -seg1addr
  • -segaddr
  • -segprot
  • -segs_read_only_addr
  • -segs_read_write_addr
  • -sub_library
  • -sub_umbrella
  • -twolevel_namespace
  • -twolevel_namespace_hints
  • -umbrella
  • -undefined
  • -unexported_symbols_list
  • -weak_framework
  • -weak_reference_mismatches
  • -whatsloaded
  • -whyload — these options are passed to the Darwin linker. The Darwin linker man page describes them in detail.

🦋 DEC Alpha Options 🦋

These -m options are defined for the DEC Alpha implementations:

  • -mno-soft-float
  • -msoft-float — use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.

Note that Alpha implementations without floating-point operations are required to have floating-point registers.

  • -mfp-regs
  • -mno-fp-regs — generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option 😤.

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

  • -mieee — the Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro _IEEE_FP is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity ✨. Other Alpha compilers call this option -ieee_with_no_inexact.
  • -mieee-with-inexact — bestie this is -mieee but the generated code ALSO keeps the IEEE inexact-flag alive 💅. Turning it on makes the generated code do fully-compliant IEEE math, no cap. On top of _IEEE_FP, it also defines _IEEE_FP_EXACT as a preprocessor macro. Heads up tho: on some Alpha implementations the resulting code may execute significantly slower than what you get by default. And since literally so little code depends on that inexact-flag, you should normally NOT specify this one. Other Alpha compilers call it -ieee_with_inexact.
  • *-mfp-trap-mode=trap-mode* — controls which floating-point related traps are enabled. Other Alpha compilers call this -fptm trap-mode. The trap mode can be one of four values:
    • n — the default (normal) setting 🌸. The only traps enabled are the ones that can't be disabled in software (e.g., division by zero trap).
    • u — in addition to everything n enables, underflow traps are on too.
    • su — like u, but the instructions get marked as safe for software completion (peep the Alpha architecture manual for the deets).
    • sui — like su, but inexact traps are enabled as well.
  • *-mfp-rounding-mode=rounding-mode* — picks the IEEE rounding mode. Other Alpha compilers call this -fprm rounding-mode. The rounding-mode can be one of:
    • n — normal IEEE rounding mode ✨. Floating-point numbers round towards the nearest machine number, or towards the even machine number when there's a tie.
    • m — round towards minus infinity.
    • c — chopped rounding mode. Floating-point numbers round towards zero.
    • d — dynamic rounding mode 🌀. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register, so unless your program modifies the fpcr — for example through fesetround — d corresponds to whatever rounding mode the C library selected.

The default is n, except that d is the default when -frounding-math is in effect, since only dynamic rounding can honor a rounding mode chosen at run time. This does not apply to -mfloat-vax, whose instructions have no dynamic rounding qualifier.

  • *-mtrap-precision=trap-precision* — okay so in the Alpha architecture, floating-point traps are imprecise 😭. That means 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 assists operating system trap handlers in figuring out the exact location that caused a floating-point trap. Depending on what an application needs, you can pick different levels of precision:
    • p — program precision. This one's the default, and it means a trap handler can only identify which program caused a floating-point exception.
    • f — function precision. The trap handler can determine the function that caused a floating-point exception.
    • i — instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception 🎯.

Other Alpha compilers have the equivalent options called -scope_safe and -resumption_safe.

  • -mieee-conformant — marks the generated code as IEEE conformant ✨. You must NOT use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui 🚨. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file.
  • -mbuild-constants — normally GCC looks at a 32- or 64-bit integer constant and checks whether 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 make GCC construct all integer constants using code, even if that takes more instructions (the max is six). You typically use this one 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. iykyk 🫶
  • -mbwx
  • -mno-bwx
  • -mcix
  • -mno-cix
  • -mfix
  • -mno-fix
  • -mmax
  • -mno-max — these say whether GCC should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets 👑. The default is to use whatever instruction sets the CPU type specified via -mcpu= supports, or the ones of the CPU GCC was built on if you didn't specify anything.
  • -msafe-bwa
  • -mno-safe-bwa — says 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 — says whether GCC should generate multi-thread and async-signal safe code for partial memory accesses 💫 — including piecemeal accesses to unaligned data, plus block accesses to leading and trailing parts of aggregate types or other objects in memory that don't respectively start and end on an aligned 64-bit data boundary.
  • -mfloat-vax
  • -mfloat-ieee — generate code that uses (or 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 had no way to generate symbol relocations except via assembler macros, and using those macros doesn't allow optimal instruction scheduling 😔. GNU Binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it is built and sets the default accordingly.
  • -msmall-data
  • -mlarge-data — when -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is used, objects 8 bytes long or smaller get placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register 💖. This limits the size of the small data area to 64KB, but it lets the variables be directly accessed via a single instruction. The default is -mlarge-data. With that option the data area is limited to just below 2GB. Programs that need more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment. When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
  • -msmall-text
  • -mlarge-text — when -msmall-text is used, the compiler assumes the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction ✨. When -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, and thus reduce the number of instructions required for a function call from 4 to 1. The default is -mlarge-text.
  • *-mcpu=cpu_type* — sets the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you don't specify a processor type, GCC defaults to the processor on which the compiler was built.

Supported values for cpu_type are

  • ev4
  • ev45
  • 21064 — schedules as an EV4 and has no instruction set extensions.
  • ev5
  • 21164 — schedules as an EV5 and has no instruction set extensions.
  • ev56
  • 21164a — schedules as an EV5 and supports the BWX extension.
  • pca56
  • 21164pc
  • 21164PC — schedules as an EV5 and supports the BWX and MAX extensions.
  • ev6
  • 21264 — schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
  • ev67
  • 21264a — schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.

Native toolchains also support the value native, which selects the best architecture option for the host processor 🌈. -mcpu=native has no effect if GCC does not recognize the processor.

  • -mtune=cpu_type — sets *only the instruction scheduling parameters for machine type *cpu_type. The instruction set is not changed, periodt. 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 the application uses and on the size of the external cache on the machine, fr fr.

Valid options for time are

  • number — a decimal number representing clock cycles.
  • L1
  • L2
  • L3
  • main — the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5 🫶.
  • -mtls-kernel — emit rdval instead of rduniq for thread pointer.
  • *-mtls-size=bitsize* — specify bit size of immediate TLS offsets. Valid values for bitsize are 16, 32, and 64; it defaults to 32.
  • -mlong-double-128
  • -mlong-double-64 — specify the size of the long double type. Note that -mlong-double-128 is incompatible with VAX floating point ⚠️.

✨ eBPF Options ✨

  • *-mframe-limit=bytes* — this specifies the hard limit for frame sizes, in bytes. Currently, the value you can specify 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 — turns generation of extra conditional-branch instructions on or off. It's on for CPU v2 and above.
  • -mjmp32, -mno-jmp32 — turns generation of 32-bit jump instructions on or off. On for CPU v3 and above.
  • -malu32, -mno-alu32 — turns generation of 32-bit ALU instructions on or off. On for CPU v3 and above.
  • -mv3-atomics, -mno-v3-atomics — turns instructions for general atomic operations (the ones introduced in CPU v3) on or off. On for CPU v3 and above.
  • -mbswap, -mno-bswap — turns byte swap instructions on or off. On for CPU v4 and above.
  • -msdiv, -mno-sdiv — turns signed division and modulus instructions on or off. On for CPU v4 and above.
  • -msmov, -mno-smov — turns sign-extending move and memory load instructions on or off. On for CPU v4 and above.
  • -mcpu=version — this is the one that picks which version of the eBPF ISA you're targeting. Fair warning bestie: newer versions might not be supported by every kernel out there. The default is v4.

Here's the menu of supported values for version:

  • v1 — the very first stable eBPF ISA, no special features, no extensions, just vibes. ✨
  • v2 — supports the jump extensions, same as -mjmpext.
  • v3 — everything from v2, plus:
    • 32-bit jump operations, as in -mjmp32
    • 32-bit ALU operations, as in -malu32
    • general atomic operations, as in -mv3-atomics
  • v4 — everything from v3, plus:
    • byte swap instructions, as in -mbswap
    • signed division and modulus instructions, as in -msdiv
    • sign-extending move and memory load instructions, as in -msmov
  • -mco-re, -mno-co-re — turns BPF Compile Once - Run Everywhere (CO-RE) support on or off. 💅 BPF CO-RE support is on by default when you're generating BTF debug information for the BPF target (-gbtf).
  • -mxbpf — generates code for an expanded version of BPF, one that relaxes some of the restrictions the BPF architecture puts on you:
    • save and restore callee-saved registers at function entry and exit, respectively.
  • -masm=dialect — spits out assembly instructions using the selected eBPF dialect. The default is pseudoc.

Supported values for dialect are:

  • normal — outputs the normal assembly dialect.
  • pseudoc — outputs the pseudo-c assembly dialect.
  • -minline-memops-threshold=bytes — sets the size threshold in bytes at or below which memmove, memcpy and memset always get expanded inline. Operations dealing with sizes bigger than this threshold would have to be implemented with a library call instead of being expanded inline — but since BPF doesn't allow libcalls, blowing past this threshold means a compile-time error. 😭 The default is 1024 bytes.
  • -Wco-re — enables warnings for the scenarios where -mco-re is enabled but it might not be possible to generate CO-RE compatible code.

🌸 FR30 Options 🌸

These options are defined specifically for the FR30 port.

  • -msmall-model — use the small address space model. It can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. 🙈
  • -mno-lsim — assumes that runtime support has already been provided, so there's no need to include the simulator library (libsim.a) on the linker command line.

🦋 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 — don't try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
  • -mdword, -mno-dword — controls whether the ABI uses double-word instructions.
  • -mdouble, -mno-double — enable or disable use of floating-point double instructions.
  • -mmedia, -mno-media — enable or disable use of media instructions.
  • -mmuladd, -mno-muladd — enable or disable use of multiply and add/subtract instructions.
  • -mfdpic — selects the FDPIC ABI, which uses function descriptors to represent pointers to functions. With no PIC/PIE-related options at all, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a bfin-elf target, this option implies -msim.
  • -minline-plt — enables inlining of PLT entries in function calls to functions that aren't known to bind locally. It has no effect without -mfdpic. It's enabled by default if you're optimizing for speed and compiling for shared libraries (i.e., -fPIC or -fpic), or when an optimization option 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 — enables the use of "GPREL" relocations in the FDPIC ABI for data that's known to be in read-only sections. It's enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it is not, -mno-gprel-ro can be used to disable it.
  • -multilib-library-pic — link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic without -mfdpic. You should never have to use it explicitly.
  • -mlinked-fp — follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with -mno-linked-fp.
  • -mlong-calls — use indirect addressing to call functions outside the current compilation unit. This lets the functions be placed anywhere within the 32-bit address space.
  • -malign-labels — try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only does anything when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.
  • -mlibrary-pic — generate position-independent EABI code.
  • -macc-4 — use only the first four media accumulator registers.
  • -macc-8 — use all eight media accumulator registers.
  • -mpack, -mno-pack — enable or disable packing VLIW instructions.
  • -mno-eflags — do not mark ABI switches in e_flags.
  • -mcond-move, -mno-cond-move — enable or disable the use of conditional-move instructions; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
  • -mscc, -mno-scc — enable or disable the use of conditional set instructions; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
  • -mcond-exec, -mno-cond-exec — enable or disable the use of conditional execution; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
  • -mvliw-branch, -mno-vliw-branch — enable or disable an optimization pass to pack branches into VLIW instructions; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
  • -mmulti-cond-exec, -mno-multi-cond-exec — enable or disable optimization of "&&" and "||" in conditional execution; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
  • -mnested-cond-exec, -mno-nested-cond-exec — enable or disable nested conditional execution optimizations; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
  • -moptimize-membar, -mno-optimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. It's enabled by default.
  • -mtomcat-stats — cause gas to print out tomcat statistics. 🐱
  • -mcpu=cpu — select the processor type to generate code for. Possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.

🎀 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.
  • -mnodiv — do not use div and mod instructions.
  • -mft32b — enable use of the extended instructions of the FT32B processor.
  • -mcompress — compress all code using the Ft32B code compression scheme.
  • -mnopm — do not generate code that reads program memory.

🐧 GNU/Linux Options 🐧

These -m options are defined for GNU/Linux targets:

  • -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
  • -muclibc — use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
  • -mmusl — use the musl C library. This is the default on *-*-linux-*musl* targets.
  • -mbionic — use Bionic C library. This is the default on *-*-linux-*android* targets.
  • -mandroid, -mno-android — compile code compatible with the Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro "__ANDROID__" to be defined.

This option can be disabled completely with -mno-android.

  • -tno-android-cc — turns off the compiling side of -mandroid, meaning it stops auto-enabling -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. slay, no surprise defaults 💅
  • -tno-android-ld — turns off the linking side of -mandroid, so it just hands your regular standard Linux linking options over to the linker instead 🫶

🦋 H8/300 Options 🦋

okay bestie, these -m options are the ones defined for the H8/300 implementations ✨

  • -mrelax — shortens some address references at link time when it can, and it does that by using the linker option -relax 💫
  • -mh — generates code for the H8/300H.
  • -ms — generates code for the H8S.
  • -mn — generates code for the H8S and H8/300H in normal mode. bestie, this switch has to be used together with either -mh or -ms, no solo runs 🙈
  • -msx — generates H8SX code.
  • -ms2600 — generates code for the H8S/2600. this switch must be used with -ms, periodt.
  • -mquickcall — uses registers for argument passing.
  • -mslowbyte — treats access to byte-sized memory as slow.
  • -mexr, -mno-exr — stores extended registers on the stack before executing functions with the "monitor" attribute. the default is -mexr. heads up, this option is valid only for H8S targets 👑
  • -mint32 — makes "int" data 32 bits by default.
  • -malign-300 — on the H8/300H and H8S, it uses the same alignment rules as the H8/300. the default over there is to align longs and floats on 4-byte boundaries, and -malign-300 makes them align on 2-byte boundaries instead. also, this option does nothing on the H8/300, so don't bother 🍓

🔥 HPPA Options 🔥

and these -m options are defined for the HPPA family of computers, iconic honestly 👑

  • -march=architecture-type — generates code for the architecture you name. your 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. on an HP-UX system, peek at /usr/lib/sched.models to figure out the right architecture option for your machine. code compiled for lower-numbered architectures runs on higher-numbered ones, but not the reverse — it's giving one-way street 💀
  • -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0 — synonyms for -march=1.0, -march=1.1, and -march=2.0, respectively.
  • -matomic-libcalls, -mno-atomic-libcalls — generates 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're implemented as a single access. fr fr, that's the fine print 😭

the PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction. so literally all atomic support is implemented using sync and atomic libcalls. sync libcall support lives in libgcc.a. 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 ✨

  • -mcaller-copies — the caller copies function arguments passed by hidden reference. use this one with care, bestie, because it is not compatible with the default 32-bit runtime 🙈 however, only aggregates larger than eight bytes get passed by hidden reference, and the option gives you better compatibility with OpenMP, so there's that 🌸
  • -mcoherent-ldcw — use the ldcw/ldcd coherent cache-control hint.
  • -mdisable-fpregs — disables floating-point registers. equivalent to -msoft-float.
  • -mdisable-indexing — stops the compiler from using indexing address modes. this dodges some pretty obscure problems when compiling MIG generated code under MACH 👀
  • -mfast-indirect-calls — generates code that assumes calls never cross space boundaries. this lets GCC emit code that does faster indirect calls 🔥
heads up: this option does not work in the presence of shared libraries or nested functions.
  • -mfixed-range=register-range — generates code treating the given register range as fixed registers. a fixed register is one the register allocator cannot use, which is handy when you're compiling kernel code. a register range is specified as two registers separated by a dash. you can specify multiple register ranges separated by commas 💅
  • -mgas — enables the use of assembler directives that only GAS understands.
  • -mgnu-ld — uses options specific to GNU ld. this passes -shared to ld when building a shared library. it's the default when GCC is configured, explicitly or implicitly, with the GNU linker. this option does not affect which ld gets called; it only changes what parameters get passed to that ld. which ld gets called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. you can print the linker GCC uses with which gcc -print-prog-name=ld``. also, this option is only available on the 64-bit HP-UX GCC, i.e. one configured with hppa*64*-*-hpux* ⭐
  • -mhp-ld — uses options specific to HP ld. this passes -b to ld when building a shared library, and passes +Accept TypeMismatch to ld on all links. it's the default when GCC is configured, explicitly or implicitly, with the HP linker. this option does not affect which ld gets called; it only changes what parameters get passed to that ld. which ld gets called is determined by the --with-ld configure option, GCC's program search path, and finally by the user's PATH. you can print the linker GCC uses with which gcc -print-prog-name=ld``. this option is only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux* 🌈
  • -mlinker-opt — enables the optimization pass in the HP-UX linker. note: this makes symbolic debugging impossible, so, uh, choose wisely 💀
  • -mlong-calls — generates code that uses long call sequences. this makes sure a call is always able to reach linker generated stubs. the default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit — as the case may be — exceeds a predefined limit set by the branch type being used. the limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. sibling calls are always limited at 240,000 bytes.

distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.

normally you don't want to use this option because it degrades performance. however, it can be useful in large applications, particularly 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. the impact on systems that support long absolute calls, and long PIC symbol-difference or PC-relative calls, should be relatively small. but an indirect call is used on 32-bit ELF systems in PIC code, and it is quite long 😭

  • -mlong-load-store — generates 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 👑
  • -mno-space-regs — generates code that assumes the target has no space registers. this lets GCC generate faster indirect calls and use unscaled index address modes.

such code is suitable for level 0 PA systems and kernels. slay ✨

  • -mordered — assumes memory references are ordered and barriers are not needed.
  • -mportable-runtime — uses the portable calling conventions proposed by HP for ELF systems.
  • -mschedule=cpu-type — schedules code according to the constraints for the machine type cpu-type. your choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000. on an HP-UX system, check /usr/lib/sched.models to figure out the proper scheduling option for your machine. the default scheduling is 8000 💫
  • -msio, -mwsio — -msio generates the predefine "_SIO" for server IO. the default is -mwsio, which generates the predefines "__hp9000s700", "__hp9000s700__" and "_WSIO" for workstation IO. these options are available under HP-UX and HI-UX. two for one, we love to see it 🫶
  • -msoft-float — generates output containing library calls for floating point.
warning bestie: 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 have to make your own arrangements to provide suitable library functions for cross-compilation 😭

-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. no half measures, periodt 💅

  • -msoft-mult — use software integer multiplication.

this disables the use of the "xmpyu" instruction.

  • -munix=unix-std — generates compiler predefines and selects a startfile for the specified UNIX standard. your choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. the default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 through to 11.00, and 98 for HP-UX 11.11 and later.

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", plus the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", plus the startfile unix98.o ⭐

it is important to note that this option changes the interfaces for various library routines. it also affects the operational behavior of the C library. thus, extreme care is needed in using this option. like, extreme extreme 🙈

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 — suppresses the generation of link options that search libdld.sl when the -static option is specified on HP-UX 10 and later.
  • -static — okay so HP-UX's C library version of setlocale is lowkey dependent on libdld.sl, and there is no archive version of libdld.sl 🫠. So when you pass -static, you need special link options to sort that dependency out.

On HP-UX 10 and later, the GCC driver just adds the necessary options to link with libdld.sl whenever -static is specified — but plot twist 👀: this causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic binaries by default in any case. If you want to stop the GCC driver from adding those link options, use -nolibdld.

  • -threads — adds support for multithreading with the dce thread library under HP-UX. This one sets flags for both the preprocessor and the linker ✨.

👑 IA-64 Options 👑

These are the -m options defined for the Intel IA-64 architecture 💜

  • -mbig-endian — generate code for a big-endian target. This is the default for HP-UX.
  • -mlittle-endian — generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
  • -mgnu-as, -mno-gnu-as — generate (or don't) code for the GNU assembler. This is the default ✨.
  • -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker. This is the default ✨.
  • -mno-pic — generate code that does not use a global pointer register. Heads up bestie: the result is not position independent code, and it violates the IA-64 ABI 🚨.
  • -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don't) a stop bit immediately before and after volatile asm statements.
  • -mregister-names, -mno-register-names — generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output more readable 👀.
  • -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section. This may be useful for working around optimizer bugs 🐛.
  • -mconstant-gp — generate code that uses a single constant global pointer value. Super useful when compiling kernel code 🧚.
  • -mauto-pic — generate code that is self-relocatable. It implies -mconstant-gp. Useful when compiling firmware code ✨.
  • -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm.
  • -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm.
  • -mno-inline-float-divide — do not generate inline code for divides of floating-point values.
  • -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm.
  • -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm.
  • -mno-inline-int-divide — do not generate inline code for divides of integer values.
  • -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm.
  • -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm.
  • -mno-inline-sqrt — do not generate inline code for sqrt.
  • -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when you're not using the GNU assembler.
  • -mearly-stop-bits, -mno-early-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit. This can improve instruction scheduling, but doesn't always do so 😭.
  • -mfixed-range=register-range — generate code treating the given register range as fixed registers. A fixed register is one the register allocator **cannot use. 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 the 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, iykyk 💅.
  • -mno-sched-br-data-spec, -msched-br-data-spec — (dis/en)able data speculative scheduling before reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is disabled.
  • -msched-ar-data-spec, -mno-sched-ar-data-spec — (en/dis)able data speculative scheduling after reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is enabled.
  • -mno-sched-control-spec, -msched-control-spec — (dis/en)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the ld.s instructions and the corresponding check instructions chk.s. The default setting is disabled.
  • -msched-br-in-data-spec, -mno-sched-br-in-data-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with -msched-br-data-spec enabled. The default setting is enabled.
  • -msched-ar-in-data-spec, -mno-sched-ar-in-data-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with -msched-ar-data-spec enabled. The default setting is enabled.
  • -msched-in-control-spec, -mno-sched-in-control-spec — (en/dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with -msched-control-spec enabled. The default setting is enabled.
  • -mno-sched-count-spec-in-critical-path, -msched-count-spec-in-critical-path — if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled.
  • -msched-spec-ldc, -mno-sched-spec-ldc — use a simple data speculation check. This option is on by default.
  • -msched-control-spec-ldc, -mno-sched-control-spec-ldc — use a simple check for control speculation. This option is on by default.
  • -msched-stop-bits-after-every-cycle, -mno-sched-stop-bits-after-every-cycle — place a stop bit after every cycle when scheduling. This option is on by default.
  • -msched-fp-mem-deps-zero-cost, -mno-sched-fp-mem-deps-zero-cost — assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.
  • -msel-sched-dont-check-control-spec, -mno-sel-sched-dont-check-control-spec — generate checks for control speculation in selective scheduling. This flag is disabled by default.
  • *-msched-max-memory-insns=max-insns* — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts ✨. The default value is 1.
  • -msched-max-memory-insns-hard-limit, -mno-sched-max-memory-insns-hard-limit — makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled.

🦋 LM32 Options 🦋

These -m options are defined for the LatticeMico32 architecture 💖:

  • -mbarrel-shift-enabled — enable barrel-shift instructions.
  • -mdivide-enabled — enable divide and modulus instructions.
  • -mmultiply-enabled — enable multiply instructions.
  • -msign-extend-enabled — enable sign extend instructions.
  • -muser-enabled — enable user-defined instructions.

🌈 LoongArch Options 🌈

These command-line options are defined for LoongArch targets 💜:

  • *-march=arch-type* — generate instructions for the machine type arch-type. -march=arch-type allows GCC to generate code that may not run at all on processors other than the one indicated 🚨.

The choices for arch-type are:

  • native — local processor type detected by the native compiler.
  • loongarch64 — generic LoongArch 64-bit processor.
  • la464 — LoongArch LA464-based processor with LSX, LASX.
  • la664 — LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.
  • la64v1.0 — LoongArch64 ISA version 1.0.
  • la64v1.1 — LoongArch64 ISA version 1.1.
  • la32v1.0 — LoongArch32 ISA version 1.0.
  • la32rv1.0 — LoongArch32 Reduced ISA version 1.0.

More information about LoongArch ISA versions can be found at <https://github.com/loongson/la-toolchain-conventions>.

  • *-mtune=tune-type* — optimize the generated code for the given processor target ✨.

The choices for tune-type are:

  • native — local processor type detected by the native compiler.
  • generic — generic LoongArch processor.
  • loongarch64 — generic LoongArch 64-bit processor.
  • la464 — LoongArch LA464 core.
  • la664 — LoongArch LA664 core.
  • loongarch32 — generic LoongArch 32-bit processor.
  • *-mabi=base-abi-type* — generate code for the specified calling convention. base-abi-type can be one of:
    • lp64d — uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
    • lp64f — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64s — 64-bit general purpose registers, and literally zero floating-point registers for passing parameters 🫶 the data model is LP64, where int is 32 bits while long int and pointers are 64 bits.
  • -mfpu=fpu-type — generate code for the FPU type you name, which can be one of:
    • 64 — allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.
    • 32 — allow the use of hardware floating-point instructions for 32-bit operations.
    • none, 0 — prevent the use of hardware floating-point instructions. no fpu for you bestie 💅
  • -msimd=simd-type — turn on 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). go big 🦋
    • lsx — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
    • none — no LoongArch SIMD instruction may be generated. nada. zip.
  • -msoft-float — forces -mfpu=none and prevents the use of floating-point registers for parameter passing. lowkey important: this option may change the target ABI 😭
  • -msingle-float — forces -mfpu=32 and allows the use of 32-bit floating-point registers for parameter passing. this one may change the target ABI too, so heads up ✨
  • -mdouble-float — forces -mfpu=64 and allows the use of 32/64-bit floating-point registers for parameter passing. you already know: it may change the target ABI 👑
  • -mlasx, -mno-lasx, -mlsx, -mno-lsx — these 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 disabling LSX with -mno-lsx automatically disables LASX. the way these work: they're driver-only options, so 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. it's giving ✨ sequential ✨
  • -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-division — trap (or do not trap) on integer division by zero 🚨 the default is -mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division for other optimization levels.
  • -mbreak-code=code — emit a "break" code instruction for irrecoverable traps from "__builtin_trap" or inserted by the compiler (for example an erroneous path isolated with -fisolate-erroneous-paths-dereference), or an "amswap.w $r0, $r1, $r0" instruction which will cause the hardware to trigger an Instruction Not-defined Exception if code is negative or greater than 32767. the default is -1, meaning to use the "amswap.w" instruction.
  • -mcond-move-int, -mno-cond-move-int — conditional moves for integral data in general-purpose registers are enabled (disabled). the default is -mcond-move-int.
  • -mcond-move-float, -mno-cond-move-float — conditional moves for floating-point registers are enabled (disabled). the default is -mcond-move-float 🌸
  • -mmemcpy, -mno-memcpy — force (do not force) the use of "memcpy" for non-trivial block moves. the default is -mno-memcpy, which lets GCC inline most constant-sized copies. setting the optimization level to -Os also forces the use of "memcpy", but -mno-memcpy may override this behavior if explicitly specified, regardless of the order these options on the command line. no cap 🫶
  • -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. the default is -mno-strict-align.
  • -G num — put global and static data smaller than num bytes into a small data section. the default value is 0.
  • -mmax-inline-memcpy-size=n — inline all block moves (such as calls to "memcpy" or structure copies) less than or equal to n bytes. the default value of n is 1024.
  • -mcmodel=code-model — set the code model to one of:
    • tiny-static (Not implemented yet)
    • tiny (Not implemented yet)
    • normal — the text segment must be within 128MB addressing space. the data segment must be within 2GB addressing space.
    • medium — the text segment and data segment must be within 2GB addressing space. this is the default code model unless GCC has been configured with --with-cmodel= specifying a different default code model.
    • large (Not implemented yet)
    • extreme — this mode does not limit the size of the code segment and data segment. heads up bestie: -mcmodel=extreme is incompatible with -fplt and/or -mexplicit-relocs=none 💀
  • -mexplicit-relocs=style, -mexplicit-relocs, -mno-explicit-relocs — set when to use assembler relocation operators when dealing with symbolic addresses. the alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation. so: 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 uses the relocation operators where 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 🧚

for backward compatibility, -mexplicit-relocs is equivalent to -mexplicit-relocs=always, while -mno-explicit-relocs is equivalent to -mexplicit-relocs=none.

  • -mdirect-extern-access, -mno-direct-extern-access — control use of the GOT to access external symbols. the default is -mno-direct-extern-access: the GOT is used for external symbols with default visibility, but not used for other external symbols.

with -mdirect-extern-access, the GOT is not used and all external symbols are PC-relatively addressed. it's only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with -static or -static-pie. also: -mdirect-extern-access is not compatible with -fPIC or -fpic 🚫

  • -mrelax, -mno-relax — take (do not take) advantage of linker relaxations. if -mpass-mrelax-to-as is enabled, this option is also passed to the assembler. the default is determined during GCC build-time by detecting corresponding assembler support: -mrelax if the assembler supports both the -mrelax option and the conditional branch relaxation (it's required, or the ".align" directives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow!), -mno-relax otherwise 😭
  • -mpass-mrelax-to-as, -mno-pass-mrelax-to-as — pass (do not pass) the -mrelax or -mno-relax option to the assembler. the default is determined during GCC build-time by detecting corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the -mrelax option, -mno-pass-mrelax-to-as otherwise. this option is mostly useful for debugging, or interoperation with assemblers different from the build-time one ✨
  • -mrecip — this option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision, instead of doing a divide or square root and divide for floating-point arguments. these instructions are generated only when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. this option is off by default. before you can use this option, you must be sure the target CPU supports the "frecipe" and "frsqrte" instructions. also ngl: 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 compiling with -march=la664, it is enabled by default. otherwise the default is -mno-frecipe.
  • -mdiv32, -mno-div32 — use (do not use) "div.w[u]" and "mod.w[u]" instructions with input not sign-extended. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-div32.
  • -mlam-bh, -mno-lam-bh — use (do not use) "am{swap/add}[_db].{b/h}" instructions. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-lam-bh 🌈
  • -mlamcas, -mno-lamcas — use (do not use) "amcas[_db].{b/h/w/d}" instructions. when compiling with -march=la664, it is enabled by default. otherwise the default is -mno-lamcas. periodt 💅
  • -mld-seq-sa
  • -mno-ld-seq-sa — do we need a same-address load-load barrier, aka "dbar 0x700"? 💅 when you're compiling with -march=la664 it's on by default. otherwise the default is -mno-ld-seq-sa and yeah bestie the load-load barrier is needed.
  • -mscq
  • -mno-scq — use (or don't use, your call 🫶) the 16-byte conditional store instruction "sc.q". the default? it's -mscq if the machine type you set with -march= supports this instruction, and -mno-scq if it doesn't. that's it, that's the rule ✨
  • -mtls-dialect=opt — this one is the boss of which TLS dialect gets used for general dynamic and local dynamic TLS models. the opt argument can be one of these two:
    • trad — use traditional TLS. this is the default. no thoughts, head empty, traditional 💖
    • desc — use TLS descriptors.
  • -mannotate-tablejump
  • -mno-annotate-tablejump — creates an annotation section .discard.tablejump_annotate so you can correlate the "jirl" instruction with the jump table whenever a jump table is being used to optimize a "switch" statement. some external tools — for example objtool from the Linux kernel building system — literally need that annotation to analyze the control flow. default is -mno-annotate-tablejump. 💅

🐱 LynxOS Options

these options are available for LynxOS targets, periodt.

  • -mshared — use shared libraries. the default is to link with static libraries instead (so if you want the shared life, you gotta ask for it 🎀).
  • -mthreads — support multi-threading. slay, threads! 🧵✨
  • -mlegacy-threads — support legacy multi-threading. and bestie, listen: this option is incompatible with -mthreads. pick a lane 💀

🍓 M32R/D Options

these -m options are defined for Renesas M32R/D architectures. let's go 🦋

  • -m32r2 — generate code for the M32R/2.
  • -m32rx — generate code for the M32R/X.
  • -m32r — generate code for the M32R. this is the default.
  • -mmodel=small — assume all objects live in the lower 16MB of memory (so their addresses can be loaded with the "ld24" instruction), and assume all subroutines are reachable with the "bl" instruction. this is the default.

the addressability of a particular object can be set with the "model" attribute, iykyk.

  • -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 (compiler generates "seth/add3" instructions to load their addresses), and assume subroutines may not be reachable with the "bl" instruction. so the compiler busts out the much slower "seth/add3/jl" instruction sequence 😭
  • -msdata=none — disable use of the small data area. variables get put into one of .data, .bss, or .rodata (unless the "section" attribute has been specified). this is the default.

btw 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. subtle, understated, lowkey ✨
  • -msdata=use — put small global and static data in the small data area, and generate special instructions to reference them. we're doing the whole thing now 💅
  • -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. and heads up: the -msdata option must be set to one of sdata or use for this option to have any effect at all.
⚠️ 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 — and good news, incorrect code is not generated. small mercies 🫶
  • -mdebug — makes the M32R-specific code in the compiler display some statistics that might help in debugging programs. little stats treat for the debug girlies 🌸
  • -malign-loops
  • -mno-align-loops — align all loops to a 32-byte boundary. this option is disabled by default.
  • -missue-rate=number — issue number instructions per cycle. and number can only be 1 or 2. that's the whole menu bestie 🎀
  • -mbranch-cost=number — number can only be 1 or 2. if it's 1 then branches are preferred over conditional code; if it's 2, then the opposite applies. pick your fighter 👑
  • -mflush-trap=number — specifies the trap number to use to flush the cache. the default is 12. valid numbers are between 0 and 15 inclusive.
  • -mno-flush-trap — specifies that the cache cannot be flushed by using a trap. no trap, no flush, no problem 💀
  • -mflush-func=name — specifies the name of the operating system function to call to flush the cache. the default is _flush_cache, but a function call is only used if a trap is not available.
  • -mno-flush-func — indicates that there is no OS function for flushing the cache. shrug, none exist 🤷

🔥 M680x0 Options

these are the -m options defined for M680x0 and ColdFire processors. the default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below. buckle up buttercup, there's a lot 🌈

  • -march=arch — generate code for a specific M680x0 or ColdFire instruction set architecture. the permissible values of arch for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are selected according to Freescale's ISA classification, and the permissible values are: isaa, isaaplus, isab and isac.

GCC defines a macro "__mcfarch__" whenever it is generating code for a ColdFire target. the arch in this macro is one of the -march arguments given above. 👑

when used together, -march and -mtune select code that runs on a family of similar processors but that is optimized for a particular microarchitecture. they're a duo fr fr 💖

  • -mcpu=cpu — generate code for a specific M680x0 or ColdFire processor. the M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332 and cpu32. the ColdFire cpus are given by the table below, which also classifies the CPUs into families (rendered as a list because apparently we're too cute for tables now ✨):
    • Family 51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
    • Family 5206 : 5202 5204 5206
    • Family 5206e : 5206e
    • Family 5208 : 5207 5208
    • Family 5211a : 5210a 5211a
    • Family 5213 : 5211 5212 5213
    • Family 5216 : 5214 5216
    • Family 52235 : 52230 52231 52232 52233 52234 52235
    • Family 5225 : 5224 5225
    • Family 52259 : 52252 52254 52255 52256 52258 52259
    • Family 5235 : 5232 5233 5234 5235 523x
    • Family 5249 : 5249
    • Family 5250 : 5250
    • Family 5271 : 5270 5271
    • Family 5272 : 5272
    • Family 5275 : 5274 5275
    • Family 5282 : 5280 5281 5282 528x
    • Family 53017 : 53011 53012 53013 53014 53015 53016 53017
    • Family 5307 : 5307
    • Family 5329 : 5327 5328 5329 532x
    • Family 5373 : 5372 5373 537x
    • Family 5407 : 5407
    • Family 5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485

-mcpu=cpu overrides -march=arch if arch is compatible with cpu. other combinations of -mcpu and -march are rejected. no mixing, no matching 💅

GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is selected. it also defines "__mcf_family_family", where the value of family is given by the table above. 🌸

  • -mtune=tune — tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. the M680x0 microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. the ColdFire microarchitectures are: cfv1, cfv2, cfv3, cfv4 and cfv4e.

you can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. these two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively. cute crossover moment 🦋

GCC defines the macros "__mcarch" and "__mcarch__" when tuning for 680x0 architecture arch. it also defines "mcarch" unless either -ansi or a non-GNU -std option is used. if GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range. ambitious queen 👑

GCC also defines the macro "__muarch__" when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above.

  • -m68000
  • -mc68000 — generate output for a 68000. this is the default when the compiler is configured for 68000-based systems. it is equivalent to -march=68000.

use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. so many numbers bestie 😭

  • -m68010 — generate output for a 68010. this is the default when the compiler is configured for 68010-based systems. it is equivalent to -march=68010.
  • -m68020
  • -mc68020 — generate output for a 68020. this is the default when the compiler is configured for 68020-based systems. it is equivalent to -march=68020.
  • -m68030 — generate output for a 68030. this is the default when the compiler is configured for 68030-based systems. it is equivalent to -march=68030.
  • -m68040 — generate output for a 68040. this is the default when the compiler is configured for 68040-based systems. it is equivalent to -march=68040.

this option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. use this option if your 68040 does not have code to emulate those instructions. ✨

  • -m68060 — generate output for a 68060. this is the default when the compiler is configured for 68060-based systems. it is equivalent to -march=68060.

this option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. use this option if your 68060 does not have code to emulate those instructions. 💅

  • -m68302 — generate code for a 68302.
  • -m68332 — generate code for a 68332.
  • -m68851 — the 68851 was an external MMU for the 68020 processor. GCC accepts this option, but does not use it. iconic legacy behavior honestly 💀
  • -mcpu32 — generate output for a CPU32. this is the default when the compiler is configured for CPU32-based systems. it is equivalent to -march=cpu32.

use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. 🎀

  • -mfidoa — generate code for a Fido A. 🐶
  • -m5200 — generate output for a 520X ColdFire CPU. this is the default when the compiler is configured for 520X-based systems. it is equivalent to -mcpu=5206, and is now deprecated in favor of that option. rip to the old way, we move on 🫶

okay so use this one for a microcontroller that's got a 5200 core — we're literally talking MCF5202, MCF5203, MCF5204 and MCF5206 💅

  • -m5206e — generates output for a 5206e ColdFire CPU. bestie it's deprecated now 🥲 use the equivalent -mcpu=5206e instead, no cap.
  • -m528x — generates output for a member of the ColdFire 528X family. also deprecated — the equivalent -mcpu=528x ate and left no crumbs, so use that.
  • -m5307 — generates output for a ColdFire 5307 CPU. yeah it's deprecated too, the equivalent -mcpu=5307 is the moment.
  • -m5407 — generates output for a ColdFire 5407 CPU. deprecated deprecation wave 🌊 — use the equivalent -mcpu=5407.
  • -mcfv4e — generates output for a ColdFire V4e family CPU (e.g. 547x/548x). this includes use of hardware floating-point instructions ✨. it's equivalent to -mcpu=547x, and it's now deprecated in favor of that option.
  • -m68020-40 — generates output for a 68040, but without using any of the new instructions. what you get 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, again 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. and 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. icon behavior honestly.
  • -msoft-float — nope, do not generate floating-point instructions; use library calls instead. this is the default for 68000, 68010, and 68832 targets. it's also the default for ColdFire devices that have no FPU.
  • -mdiv, -mno-div — generate (or don't generate) ColdFire hardware divide and remainder instructions. if -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. otherwise the default gets taken from the target CPU (either the default CPU, or the one you specified with -mcpu). for example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.
    • GCC defines the macro __mcfhwdiv__ when this option is enabled.
  • -mshort, -mno-short, -mnoshort — 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. this option is disabled by default 🫥.
  • -mbitfield, -mno-bitfield, -mnobitfield — controls use of the bit-field instructions. the -m68000, -mcpu32 and -m5200 options imply -mnobitfield; the -m68020 option implies -mbitfield. the way that these hints each other is lowkey iconic.
  • -mrtd, -mno-rtd, -mnortd — controls use of 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. slay, but:
    • this calling convention is incompatible with the one normally used on Unix, so you can't use it if you need to call libraries compiled with the Unix compiler.
    • also, you must provide function prototypes for all functions that take variable numbers of arguments (including printf); otherwise incorrect code 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 though 💀)
    • 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 — controls whether GCC aligns int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). aligning variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses, at the expense of more memory — trade-offs bestie 🫠.
Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than most published application binary interface specifications for the m68k. so like, be careful.
  • -mpcrel — use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. at present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
  • -mno-strict-align, -mstrict-align — do not (do) assume that unaligned memory references are handled by the system.
  • -msep-data, -mno-sep-data — with -msep-data, generates code that allows the data segment to be located in a different area of memory from the text segment 📍. this allows for execute-in-place in an environment without virtual memory management. this option implies -fPIC.
    • this option is disabled by default; GCC generates code that assumes the data segment follows the text segment.
  • -mid-shared-library, -mno-id-shared-library — if enabled, generates code that supports shared libraries via the library ID method. this allows for execute-in-place and shared libraries in an environment without virtual memory management. this option implies -fPIC.
    • this option is disabled by default.
  • -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.
  • -mxgot, -mno-xgot — when generating position-independent code for ColdFire, generates code that works if the GOT has more than 8192 entries. this code is larger and slower than code generated without this option 🐌. on M680x0 processors, this option isn't needed; -fPIC suffices.
    • GCC normally uses a single instruction to load values from the GOT. while 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. iykyk.
  • these options have no effect unless GCC is generating position-independent code.
  • -mxtls — support a TLS segment larger than 64K. the considerations for using this option are similar to those for -mxgot above; the default 16-bit offset addressing for TLS is more efficient, but this may be inadequate for some programs.
  • -mlong-jump-table-offsets — use 32-bit offsets in switch tables. the default is to use 16-bit offsets.

🎀 MCore Options 🎀

these are the -m options defined for the Motorola M*Core processors. okay bestie, we're in a whole new family now 💖

  • -mhardlit, -mno-hardlit — inlines constants into the code stream if it can be done in two instructions or less.
  • -mdiv, -mno-div — use the divide instruction. (enabled by default.)
  • -mrelax-immediates, -mno-relax-immediates — allow arbitrary-sized immediates in bit operations.
  • -mwide-bitfields, -mno-wide-bitfields — always treat bit-fields as int-sized.
  • -m4byte-functions, -mno-4byte-functions — force all functions to be aligned to a 4-byte boundary.
  • -mcallgraph-data, -mno-callgraph-data — emit callgraph information.
  • -mslow-bytes, -mno-slow-bytes — prefer word access when reading byte quantities.
  • -mlittle-endian, -mbig-endian — generate code for a little- or big-endian target, respectively. the default is big-endian. little-endian code is supported only with -m340.
  • -m210 — generate code for the 210 processor.
  • -m340 — generate code for the 340 processor.
  • -mno-lsim — assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.
  • -mstack-increment=size — set the maximum amount for a single stack increment operation. large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much 😭. the default value is 0x1000.

🦋 MicroBlaze Options 🦋

  • -msoft-float — use software emulation for floating point (default).
  • -mhard-float — use hardware floating-point instructions ✨.
  • -mmemcpy — do not optimize block moves, use memcpy.
  • -mcpu=cpu-type — use features of, and schedule code for, the given CPU. supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a.
  • -mxl-soft-mul, -mno-xl-soft-mul — use software multiply emulation. this is enabled by default.
  • -mxl-soft-div, -mno-xl-soft-div — use software emulation for divides. this is enabled by default.
  • -mxl-barrel-shift — use the hardware barrel shifter.
  • -mxl-pattern-compare — use pattern compare instructions.
  • -msmall-divides — use table lookup optimization for small signed integer divisions.
  • -mxl-gp-opt — use GP-relative .sdata/.sbss sections.
  • -mxl-multiply-high — use multiply high instructions for the high part of a 32x32 multiply.
  • -mxl-float-convert — use hardware floating-point conversion instructions.
  • -mxl-float-sqrt — use hardware floating-point square root instruction 🌸.
  • -mbig-endian — generate code for a big-endian target. bytes go one way 🌸
  • -mlittle-endian — generate code for a little-endian target. bytes go the other way, no drama 🦋
  • -mxl-reorder — use reorder instructions (that's swap and byte-reversed load/store, bestie) ✨
  • -mxl-mode-app-model — pick your application model, aka app-model. The valid models are:
    • executable — the normal executable, and yes it's the default, and it 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. It's the vibe for transferring control on a processor reset to the bootloader rather than the application. 🫶
    • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file. 💅
  • -mxl-prefetch — enable insertion of prefetch ("wic") instructions at call sites. so extra little wic moments, everywhere calls happen 🔥
  • -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, -meb — generate big-endian code. 🍓
  • -EL, -mel — generate little-endian code. This is the default for mips*el-*-* configurations. ✨
  • -march=arch — generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e, loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp. allegrex, The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).

The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor. 🤷

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r. ✨

Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1. 🧚

GCC defines two macros based on the value of this option. The first is "_MIPS_ARCH", which gives the name of target architecture, as a string. The second has the form "_MIPS_ARCH_foo", where foo is the capitalized value of "_MIPS_ARCH". For example, -march=r2000 sets "_MIPS_ARCH" to "r2000" and defines the macro "_MIPS_ARCH_R2000". 💖

Note that the "_MIPS_ARCH" macro uses the processor names given above. In other words, it has the full prefix and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). It names the default architecture when no -march option is given. 🌸

  • -mtune=arch — optimize for arch. Among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. The list of arch values is the same as for -march. 😌

When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family. 👑

-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the same way as the -march ones described above. 💅

  • -mips1 — equivalent to -march=mips1.
  • -mips2 — equivalent to -march=mips2.
  • -mips3 — equivalent to -march=mips3.
  • -mips4 — equivalent to -march=mips4.
  • -mips32 — equivalent to -march=mips32.
  • -mips32r3 — equivalent to -march=mips32r3.
  • -mips32r5 — equivalent to -march=mips32r5.
  • -mips32r6 — equivalent to -march=mips32r6.
  • -mips64 — equivalent to -march=mips64.
  • -mips64r2 — equivalent to -march=mips64r2.
  • -mips64r3 — equivalent to -march=mips64r3.
  • -mips64r5 — equivalent to -march=mips64r5.
  • -mips64r6 — equivalent to -march=mips64r6.
  • -mips16, -mno-mips16 — generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE. 🦋

MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16" attributes. 🎀

  • -mmips16e2, -mno-mips16e2 — use (do not use) the MIPS16e2 ASE. This option modifies the behavior of the -mips16 option such that it targets the MIPS16e2 ASE. 🫶
  • -mflip-mips16, -mflip-mips16 — 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. (and yeah, the page lists the flag twice — I'm not touching the source of truth, bestie 🙈)
  • -minterlink-compressed, -mno-interlink-compressed — require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa. 😭

For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed. 🎯

  • -minterlink-mips16, -mno-interlink-mips16 — aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility. 💜
  • -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — generate code for the given ABI. ✨

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead. 🧚

For information about the O64 ABI, see https://gcc.gnu.org/projects/mipso64-abi.html. 🌸

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the "mthc1" and "mfhc1" instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors. 🔥

The register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved. 💅

Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. 👑

  • -mabicalls, -mno-abicalls — generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems. 🍓
  • -mshared, -mno-shared — generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects -mabicalls. 🌈

All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by -mno-shared. ✨

-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. However, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. 🔥

-mshared is the default. 💜

  • -mplt, -mno-plt — assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32. 🫶

You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise. 🎀

  • -mxgot, -mno-xgot — lift (do not lift) the usual restrictions on the size of the global offset table. 😳

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_MIPS_GOT16 foobar

Okay so if you ever see that in your terminal, bestie, it's just the GOT being way too big for its little boots. 😭 The fix? Recompile your code with -mxgot. That 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. Slightly more work per symbol, but like… it runs. 💅

Ngl though, 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. So the odds you need this are lowkey tiny. 🦋

Heads up: these options have no effect unless GCC is generating position independent code. Fr fr. ✨
  • -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. Let them be mysterious, we love that for them. 💜
  • -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 — Same energy as -msoft-float, but it 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. 😬 So like, don't be messy with this one.
  • -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 (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.

-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details.

  • -mdsp, -mno-dsp — Use (do not use) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.
  • -mdspr2, -mno-dspr2 — Use (do not use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2.
  • -msmartmips, -mno-smartmips — Use (do not use) the MIPS SmartMIPS ASE.
  • -mpaired-single, -mno-paired-single — Use (do not use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled.
  • -mdmx, -mno-mdmx — Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.
  • -mips3d, -mno-mips3d — Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single. ✨ (it brings a friend along, we stan)
  • -mmicromips, -mno-micromips — Generate (do not generate) microMIPS code.

MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and "nomicromips" attributes.

  • -mmt, -mno-mt — Use (do not use) MT Multithreading instructions.
  • -mmcu, -mno-mcu — Use (do not use) the MIPS MCU ASE instructions.
  • -meva, -mno-eva — Use (do not use) the MIPS Enhanced Virtual Addressing instructions.
  • -mvirt, -mno-virt — Use (do not use) the MIPS Virtualization (VZ) instructions.
  • -mxpa, -mno-xpa — Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
  • -mcrc, -mno-crc — Use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
  • -mginv, -mno-ginv — Use (do not use) the MIPS Global INValidate (GINV) instructions.
  • -mmsa, -mno-msa — Use (do not use) the MIPS MSA extension instructions.
  • -mloongson-mmi, -mno-loongson-mmi — Use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
  • -mloongson-ext, -mno-loongson-ext — Use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
  • -mloongson-ext2, -mno-loongson-ext2 — Use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
  • -mlong64 — Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the pointer size is determined.
  • -mlong32 — Force "long", "int", and pointer types to be 32 bits wide.

The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller.

  • -msym32, -mno-sym32 — Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster references to symbolic addresses. 🏎️✨
  • -G num — Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see -mgpopt for details.

The default -G option depends on the configuration.

  • -mlocal-sdata, -mno-local-sdata — Extend (do not extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the default for all configurations.

If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program.

  • -mextern-sdata, -mno-extern-sdata — Assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations.

If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting.

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data.

  • -mgpopt, -mno-gpopt — Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.)

-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. 💅 (it drags them along, no choosing)

  • -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 — bestie, this one drops your uninitialized "const" variables into the read-only data section. ✨ But heads up: it's only meaningful when you've also got -membedded-data on. They're a package deal, fr.
  • -mcode-readable=setting — tells GCC whether it's allowed to generate code that reads from executable sections. Three vibes to pick from:
    • -mcode-readable=yes — instructions can just freely access executable sections, no thoughts, head empty. This is the default setting. 👑
    • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can reach executable sections, but every other instruction has to stay in its lane. Super useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. Also a slay on processors that can be configured with a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM. 💅
    • -mcode-readable=no — instructions are NOT allowed to touch executable sections, periodt. Handy on targets configured with a dual instruction/data SRAM interface that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.

On SDE targets, -mcode-data-in-code is available as the traditional alias for -mcode-readable=no, and -mcode-xonly stands in for -mcode-readable=pcrel. iykyk 😌

  • -msplit-addresses, -mno-split-addresses — enable (or disable) use of the %hi() and %lo() assembler relocation operators. ngl this one's been superseded by -mexplicit-relocs, but it's still hanging around for backwards compatibility. vintage, not cancelled. 🎀
  • -mexplicit-relocs=none, -mexplicit-relocs=base, -mexplicit-relocs=pcrel, -mexplicit-relocs, -mno-explicit-relocs — these control whether explicit relocs (like %gp_rel) get used at all. The default value depends on the version of GAS when GCC itself was built, so it's giving "it depends" energy 🙈

The "base" explicit-relocs support landed in GAS in 2001. The "pcrel" explicit-relocs support landed in GAS in 2014, and it 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 gives you smaller code, but it's only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that stops trap from generating the proper signal ("SIGFPE") — rude of them honestly. Use -mdivide-traps to allow conditional traps on architectures that support them, and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. And if you want zero divide-by-zero checks at all, you can just completely disable them with -mno-check-zero-division. living dangerously ✨

  • -mload-store-pairs, -mno-load-store-pairs — enable (disable) an optimization that pairs consecutive load or store instructions together to enable load/store bonding. Enabled by default, but lowkey it only takes effect when the selected architecture is actually known to support bonding. 🫶
  • -mstrict-align, -mno-strict-align, -munaligned-access, -mno-unaligned-access — disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, either by hardware or by trapping and emulation. In that trapping-and-emulation case, -mstrict-align may be needed by the operating system kernel. The options -munaligned-access and -mno-unaligned-access are obsolete and only provided for backward compatibility.
  • -mmemcpy, -mno-memcpy — force (or don't force) the use of memcpy for non-trivial block moves. The default is -mno-memcpy, which lets GCC inline most constant-sized copies. she loves to inline 💅
  • -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 — so like, same zip code, same vibes.

This option has no effect on abicalls code. The default is -mno-long-calls.

  • -mmad, -mno-mad — enable (disable) use of the mad, madu and mul instructions, as provided by the R4650 ISA. 🔥
  • -mimadd, -mno-imadd — enable (disable) use of the madd and msub integer instructions. The default is -mimadd on architectures that support madd and msub — except for the 74k architecture, where it was found to generate slower code. not the 74k catching strays 😭
  • -mfused-madd, -mno-fused-madd — enable (disable) use of the floating-point multiply-accumulate instructions, when they're available. The default is -mfused-madd.

On the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions.

  • -nocpp — tell the MIPS assembler to NOT run its preprocessor over user assembler files (the ones with a .s suffix) when assembling them. hands off 🙈
  • -mfix-24k, -mno-fix-24k — work around the 24K E48 errata (that's lost data on stores during refill, if you were wondering 💀). The workarounds are implemented by the assembler rather than by GCC.
  • -mfix-r4000, -mno-fix-r4000 — work around certain R4000 CPU errata:
    • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
    • A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
    • An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
  • -mfix-r4400, -mno-fix-r4400 — work around certain R4400 CPU errata:
    • A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
  • -mfix-r10000, -mno-fix-r10000 — work around certain R10000 errata:
    • ll/sc sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 😭

This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.

  • -mfix-r5900, -mno-fix-r5900 — do NOT attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a nop instruction there instead. The short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.
  • -mfix-rm7000, -mno-fix-rm7000 — work around the RM7000 dmult/dmultu errata. The workarounds are implemented by the assembler rather than by GCC.
  • -mfix-vr4120, -mno-fix-vr4120 — work around certain VR4120 errata:
    • dmultu does not always produce the correct result.
    • div and ddiv do not always produce the correct result if one of the operands is negative.

The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are only provided by the mips64vr*-elf configurations.

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.

  • -mfix-vr4130, -mno-fix-vr4130 — work around the VR4130 mflo/mfhi errata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids using mflo and mfhi if the VR4130 macc, macchi, dmacc and dmacchi instructions are available instead. iconic of it honestly ✨
  • -mfix-sb1, -mno-fix-sb1 — work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and "F2" floating-point errata.)
  • -mfix4300, -mno-fix4300 — work around a bug in early VR4300 silicon that causes multiplies with certain operands to corrupt immediately following multiplies. that's not a vibe, that's a crisis 💀
  • -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 — drops a cache barrier in before any load or store that might get speculatively executed, and that might have side effects even if it gets aborted. ✨
  • -mr10k-cache-barrier=store — same energy, but only before a store that might be speculatively executed and might have side effects even if it's aborted. 💅
  • -mr10k-cache-barrier=none — no cache barriers inserted at all, bestie. This is the default, so it's already the vibe unless you say otherwise. 🎀
  • -mflush-func=func, -mno-flush-func — tells GCC which function to call to flush the I and D caches, or tells it to not call any such function at all. If it is called, that function has to take the same arguments as the common _flush_func, which means: the address of the memory range whose cache is being flushed, the size of that memory range, and the number 3 (because that flushes both caches 🫶). The default depends on the target GCC was configured for, but it's commonly either _flush_func or __cpu_flush.
  • -mbranch-cost=num — sets the cost of branches to roughly num "simple" instructions. Heads up though: this cost is only a heuristic and it is not guaranteed to give you consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune setting. 😌
  • -mbranch-likely, -mno-branch-likely — turns Branch Likely instructions on or off, no matter what the default is for the architecture you picked. By default, Branch Likely instructions may be generated if the selected architecture supports them. 🌈 The exception is the MIPS32 and MIPS64 architectures and the processors that implement them — for those, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. periodt.
  • -mcompact-branches=never, -mcompact-branches=optimal, -mcompact-branches=always — these control which form of branches get generated. The default is -mcompact-branches=optimal. 🦋
    • -mcompact-branches=never makes it so compact branch instructions are never generated.
    • -mcompact-branches=always makes a compact branch instruction get generated if one is available for MIPS Release 6 onwards. If a compact branch instruction is not available (or it's pre-R6), a delay slot form of the branch is used instead.
    • If you use it for MIPS16/microMIPS targets, ngl, it's just ignored right now. The behavior for MIPS16/microMIPS may change in future, since they do have some compact branch instructions. 👀
    • -mcompact-branches=optimal uses a delay slot branch if one is available in the current ISA and the delay slot gets successfully filled. If the delay slot is not filled, a compact branch is chosen if one is available.
  • -mfp-exceptions, -mno-fp-exceptions — says whether FP exceptions are enabled or not. This affects how FP instructions get scheduled for some processors. The default is that FP exceptions are enabled. For instance, on the SB-1, if FP exceptions are disabled and we're emitting 64-bit code, we can use both FP pipes. Otherwise? Only one FP pipe, sorry bestie. 😭
  • -mvr4130-align, -mno-vr4130-align — the VR4130 pipeline is two-way superscalar, but it can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel. ⭐ This option only does anything when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It's enabled by default at optimization level -O3.
  • -msynci, -mno-synci — enables (or disables) generation of synci instructions on architectures that support them. The synci instructions (if enabled) get generated when __builtin___clear_cache is compiled. 🌸 This option defaults to -mno-synci, but that default can be overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it's generally safe to use synci. But on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. Be careful, okay? 💀
  • -mrelax-pic-calls, -mno-relax-pic-calls — tries to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time, and if the destination is within range for a direct call. 🔥 -mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive, and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be done by the assembler and the linker alone, no compiler help needed. 💪
  • -mmcount-ra-address, -mno-mcount-ra-address — emits (or doesn't emit) code that lets _mcount modify the calling function's return address. When it's enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12. Then _mcount can modify the return address by doing both of these:
    • Returning the new address in register $31.
    • Storing the new address in *ra-address, if ra-address is nonnull.

The default is -mno-mcount-ra-address.

  • -mframe-header-opt, -mno-frame-header-opt — enables (or disables) frame header optimization in the o32 ABI. When you're using the o32 ABI, calling functions allocates 16 bytes on the stack so the called function can write out register arguments. When this is enabled, the optimization suppresses the allocation of the frame header if it can be determined that it's unused. 🎀 This optimization is off by default at all optimization levels.
  • -mlxc1-sxc1, -mno-lxc1-sxc1 — when applicable, enables (or disables) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default. ✨
  • -mmadd4, -mno-madd4 — when applicable, enables (or disables) the generation of 4-operand madd.s, madd.d and related instructions. Enabled by default. 💖

🧚 MMIX Options

These options are defined for the MMIX, bestie:

  • -mlibfuncs, -mno-libfuncs — says that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
  • -mepsilon, -mno-epsilon — generates floating-point comparison instructions that compare with respect to the rE epsilon register.
  • -mabi=mmixware, -mabi=gnu — generates code that passes function parameters and return values that (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, rather than sign-extending ones.
  • -mknuthdiv, -mno-knuthdiv — makes the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, but the latter is almost exclusively used. 🫶
  • -mtoplevel-symbols, -mno-toplevel-symbols — prepends (or doesn't prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive.
  • -melf — generates an executable in the ELF format, rather than the default mmo format used by the mmix simulator. 💅
  • -mbranch-predict, -mno-branch-predict — uses (or doesn't use) the probable-branch instructions, when static branch prediction indicates a probable branch.
  • -mbase-addresses, -mno-base-addresses — generates (or doesn'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 held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. Which means a program that uses lots of static data may need -mno-base-addresses. 😭
  • -msingle-exit, -mno-single-exit — forces (or doesn't force) generated code to have a single exit point in each function.
  • -mset-data-start=address — when linking, sets the start of the data section to address.
  • -mset-program-start=address, -mno-set-program-start — when linking, sets the program start address to address. It defaults to 0x100 unless -mno-set-program-start is used to suppress this option entirely. ⭐

👑 MN10300 Options

These -m options are defined for Matsushita MN10300 architectures, bestie:

  • -mmult-bug, -mno-mult-bug — when enabled, generates code to dodge bugs in the multiply instructions for the MN10300 processors. This is the default. 🐛
  • -mam33, -mno-am33 — generates code using features specific to the AM33 processor. The default is -mno-am33.
  • -mam33-2, -mno-am33-2 — generates code using features specific to the AM33/2.0 processor. The default is -mno-am33-2.
  • -mam34, -mno-am34 — generates code using features specific to the AM34 processor. The default is -mno-am34.
  • -mtune=cpu-type — uses the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
  • -mreturn-pointer-on-d0, -mno-return-pointer-on-d0 — when generating a function that returns a pointer, return the pointer in both a0 and d0. Otherwise, the pointer is returned only in a0, and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it. 💅
  • -mno-crt0 — do not link in the C run-time initialization object file.
  • -mrelax — tells the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step. 🌸
Heads up: this option makes symbolic debugging impossible. 😭🔴
  • -mliw, -mno-liw — lets the compiler emit Long Instruction Word instructions when your target is AM33 or newer. It's on by default, bestie 💖. -mliw defines the preprocessor macro __LIW__; -mno-liw defines __NO_LIW__.
  • -msetlb, -mno-setlb — lets the compiler emit SETLB and Lcc instructions when the target is AM33 or newer. Also enabled by default ✨. -msetlb defines the macro __SETLB__; -mno-setlb defines __NO_SETLB__.

🎀 Moxie Options 🎀

  • -meb — generate big-endian code. This is the default for moxie-*-* configurations.
  • -mel — generate little-endian code.
  • -mmul.x, -mno-mul.x — generate mul.x and umul.x instructions. On by default for moxiebox-*-* configurations.
  • -mno-crt0 — do not link in the C run-time initialization object file.

🌸 MSP430 Options 🌸

These options are defined for the MSP430:

  • -masm-hex — forces assembly output to always use hex constants. Normally those constants are signed decimals, but this option exists for testsuite and/or aesthetic purposes 💅.
  • -mmcu=name — picks the MCU to target. It builds a C preprocessor symbol from the MCU name, converted to upper case and wrapped with __ on both sides. That symbol is then 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. The generic MCU name msp430 can also be used to select the 430 ISA, and 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 makes the linker 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 what's hard-coded 👑.

GCC searches for the devices.csv file using these methods in the given precedence order, where the first method takes precedence over the second, which takes precedence over the third.

  • Include path specified with -I and -L — devices.csv is searched for in each of the directories specified by include paths and linker library search paths.
  • Path specified by the environment variable MSP430_GCC_INCLUDE_DIR — set the value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is found, this directory is also registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying -I and -L on the command line.
  • The msp430-elf{,bare}/include/devices directory — finally, GCC examines msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if you've created it and copied devices.csv there, then the MCU data is read. As above, this directory is also registered as an include path and linker library path.

If none of the above search methods find devices.csv, then the hard-coded MCU data is used.

  • -mwarn-mcu, -mno-warn-mcu — enables or disables warnings about conflicts between the MCU name given by -mmcu and the ISA set by -mcpu and/or the hardware multiply support set by -mhwmult. It also toggles warnings about unrecognized MCU names. This option is on by default.
  • -msim — link to the simulator runtime libraries and linker script. It overrides any scripts that would otherwise be selected by the -mmcu= option.
  • -mlarge — use large-model addressing (20-bit pointers, 20-bit size_t).
  • -msmall — use small-model addressing (16-bit pointers, 16-bit size_t).
  • -mrelax — this option gets passed to the assembler and linker, and lets the linker perform certain optimizations that just can't happen until the final link 🔥.
  • mhwmult=type — describes the type of hardware multiply supported by the target. Accepted values for type are none for no hardware multiply, 16bit for the original 16-bit-only multiply supported by early MCUs, 32bit for the 16/32-bit multiply supported by later MCUs, and f5series for the 16/32-bit multiply supported by F5-series MCUs. A value of auto can also be given — it tells GCC to deduce the hardware multiply support from the MCU name provided by the -mmcu option. If no -mmcu option is specified, or if the MCU name isn't recognized, then no hardware multiply support is assumed. "auto" is the default setting.

Hardware multiplies are normally performed by calling a library routine, which saves space in the generated code. When compiling at -O3 or higher, though, the hardware multiplier is invoked inline — bigger code, but faster 🏎️.

The hardware multiply routines disable interrupts while they run and restore the previous interrupt state when they finish. That makes them safe to use inside interrupt handlers as well as in normal code.

  • -minrt — enables use of a minimum runtime environment, without support for static initializers or constructors. It's meant for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.
  • -mtiny-printf — enables reduced code size printf and puts library functions. Heads up though: the tiny implementations of these functions are not reentrant, so use them with caution in multi-threaded applications 😭.

Support for streams has been removed, and the string to be printed is always sent to stdout via the write syscall. The string is not buffered before it's sent to write.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.

  • -mmax-inline-shift=n — takes an integer n between 0 and 64 inclusive, and sets the maximum number of inline shift instructions that should be emitted to perform a shift operation by a constant amount. When that value would need to be exceeded, an mspabi helper function is used instead. The default value is 4.

This only affects cases where a shift by multiple positions can't be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value you pass for this option is divided by 2, and the resulting value is used instead.

  • -mcode-region=where, -mdata-region=where — these options tell the compiler where to place functions and data that don't have one of the "lower", "upper", "either" or "section" attributes. Possible values for where are lower, upper, either or any. The first three behave like the corresponding attribute. The fourth possible value, any, is the default — it leaves placement entirely up to the linker script and how it assigns the standard sections (.text, .data, etc) to the memory regions.
  • -muse-lower-region-prefix — adds the .lower prefix to section names when compiling with -mcode-region=lower or -mdata-region=lower. Disabled by default.
  • -msilicon-errata=name[,name...] — passes a request on to the assembler to enable the fixes for the named silicon errata. Refer to the assembler documentation for details.
  • -msilicon-errata-warn=name[,name...] — passes a request on to the assembler to enable warning messages when a named silicon errata might need to be applied. Refer to the assembler documentation for details.
  • -mwarn-devices-csv, -mno-warn-devices-csv — warn if devices.csv isn't found or there are problems parsing it (default: on).

🦋 NDS32 Options 🦋

These options are defined for NDS32 implementations:

  • -mbig-endian, -EB — generate code in big-endian mode.
  • -mlittle-endian, -EL — generate code in little-endian mode.
  • -mabi=2, -mfloat-abi=soft — use the soft floating-point ABI.
  • -mabi=2fp+, -mfloat-abi=hard — use the hard floating-point ABI.
  • -mreduced-regs — use reduced-set registers for register allocation.
  • -mfull-regs — use full-set registers for register allocation.
  • -malways-align — always align function entry, jump targets, and return address.
  • -malign-functions — align function entry to 4 bytes.
  • -mfp-as-gp, -mno-fp-as-gp — enable/disable fp-as-gp optimization.
  • -mcmov, -mno-cmov — enable/disable generation of conditional move instructions.
  • -mhw-abs, -mno-hw-abs — enable/disable generation of hardware abs instructions.
  • -mext-perf, -mno-ext-perf — enable/disable generation of performance extension instructions.
  • -mext-perf2, -mno-ext-perf2 — enable/disable generation of performance extension 2 instructions.
  • -mext-string, -mno-ext-string — enable/disable generation of string extension instructions.
  • -mext-dsp, -mno-ext-dsp — enable/disable generation of DSP extension instructions.
  • -mext-fpu-fma, -mno-ext-fpu-fma — enable/disable generation of floating-point multiply-accumulation instructions.
  • -mext-fpu-sp, -mno-ext-fpu-sp — enable/disable generation of single-precision floating-point instructions.
  • -mext-fpu-dp, -mno-ext-fpu-dp — enable/disable generation of double-precision floating-point instructions.
  • -mv3push, -mno-v3push — enable/disable generation of v3 push25/pop25 instructions.
  • -m16-bit, -mno-16-bit — enable/disable generation of 16-bit instructions.
  • -mvh, -mno-vh — enable/disable Virtual Hosting support.
  • -misr-vector-size=num — specify the size of each interrupt vector, which must be 4 or 16.
  • -misr-secure=num — specify the security level of c-isr for the whole file.
  • -mcache-block-size=num — okay so this sets the size of each cache block, and it **has to be a power of 2 sitting somewhere between 4 and 512. No vibes-based numbers allowed bestie 💅
  • *-march=arch* — pick your target architecture. arch can be v2, v3, v3j, v3m, v3f, or v3s. Choose wisely ✨
  • *-mcpu=cpu* — tell it which CPU to compile for. cpu may be any one of: n6, n650, n7, n705, n8, n801, sn8, sn801, s8, s801, e8, e801, n820, s830, e830, n9, n903, n903a, n968, n968a, n10, n1033, n1033a, n1033-fpu, n1033-spu, n1068, n1068a, n1068-fpu, n1068a-fpu, d10, d1088, d1088-fpu, d1088-spu, n15, d15, n15s, d15s, n15f, d15f, n12, n1213, n1233, n1233-fpu, n1233-spu, n13, n1337, n1337-fpu, n1337-spu, or simple. Yes it's a whole alphabet soup, it's giving Pokémon roster energy 😭
  • *-mconfig-fpu=num* — sets an FPU configuration value from 0 to 7. Here's the tea: 0–3 is as the FPU spec says, and 4–7 correspond to 0–3.
  • *-mconfig-mul=type* — configuration for the multiply instruction. type can be fast, fast1 (which is equivalent to fast), fast2, or slow. The default is fast1. 🏃
  • *-mconfig-register-ports=kind* — specifies the numbers of read/write ports for n9/n10 cores. The value should be 3r2w or 2r1w.
  • *-mcmodel=code-model* — sets the code model, and you get three flavors:

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.

The default is large on GNU/Linux targets and medium on other ELF targets. ✨

  • -mctor-dtor / -mno-ctor-dtor — enable/disable the constructor/destructor feature.
  • -mrelax / -mno-relax — enable/disable the linker option to relax instructions.
  • -mrelax-hint / -mno-relax-hint — enable/disable insertion of hints for the linker to do relaxation.
  • -msched-prolog-epilog / -mno-sched-prolog-epilog — enable/disable scheduling of a function's prologue and epilogue sequence.
  • -mret-in-naked-func / -mno-ret-in-naked-func — enable/disable generation of return instructions in naked functions. This option is enabled by default.
  • -malways-save-lp / -mno-always-save-lp — enable/disable always saving $lp on the stack.
  • -munaligned-access / -mno-unaligned-access — enable/disable unaligned word and halfword accesses to packed data.
  • -minline-asm-r15 / -mno-inline-asm-r15 — allow/disallow use of r15 for inline asm.

🎮 Nvidia PTX Options

These options are defined for Nvidia PTX, no cap:

  • -m64 — literally ignored, but preserved for backward compatibility. Only 64-bit ABI is supported, so there was no point in it anyway 💀
  • *-march=architecture-string* — generate code for the specified PTX ISA target architecture. Valid architecture strings are sm_30, sm_35, sm_37, sm_50, sm_52, sm_53, sm_61, sm_70, sm_75, sm_80, and sm_89. The default depends on how the compiler has been configured, see --with-arch.

This option sets the value of the preprocessor macro __PTX_SM__; for instance, for sm_35, it has the value 350.

  • *-misa=architecture-string* — alias of -march=. Same thing, different name, we love a nickname 🫶
  • *-march-map=architecture-string* — selects 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.
  • *-mptx=version-string* — generate code for the specified PTX ISA version. Valid version strings are 3.1, 4.0, 4.1, 4.2, 5.0, 6.0, 6.3, 7.0, 7.3, and 7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target architecture, see -march=, but at least 6.3, or 7.3 for -march=sm_52 and higher.

This option sets the values of the preprocessor macros __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__; for instance, for 3.1 the macros have the values 3 and 1, respectively. ✨

  • -mmainkernel — link in code for a __main kernel. This is for stand-alone instead of offloading execution.
  • -moptimize / -mno-optimize — enable/disable partitioned execution optimizations. This option is enabled by default when any level of optimization is selected.
  • -msoft-stack / -mno-soft-stack — okay this one's meaty so buckle up:

For -mno-soft-stack (the default, unless -mgomp has been specified), use PTX "native" stacks, that is, generate code that uses .local memory or PTX "alloca" directly for stack storage. Unless -mptx=7.3 or higher and -march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with "alloca" are not supported.

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. 🧚

  • *-msoft-stack-reserve-local=size* — specify the size of .local memory used for the stack when the exact amount is not known. It defaults to 128.
  • -muniform-simt / -mno-uniform-simt — enable/disable a code generation variant that allows execution of all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array int __nvptx_uni[] stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position tid.y with current lane index to compute the master lane index. 💅
  • -mgomp / -mno-gomp — enable/disable generation of code for use in OpenMP offloading. -mgomp enables -msoft-stack and -muniform-simt options, and selects a corresponding multilib variant. Two-for-one deal, obsessed 🔥

🌱 OpenRISC Options

These options are defined for OpenRISC:

  • *-mboard=name* — configure a board specific runtime. This is passed to the linker for newlib board library linking. The default is or1ksim.
  • -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.
  • -munordered-float — when -mhard-float is selected, enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default functions from libgcc are used to perform unordered floating point compare and set flag operations.
  • -mcmov — enable generation of conditional move (l.cmov) instructions. By default the equivalents are generated using set and branch.
  • -mror — enable generation of rotate right (l.ror) instructions. By default functions from libgcc are used to perform rotate right operations.
  • -mrori — enable generation of rotate right with immediate (l.rori) instructions. By default functions from libgcc are used to perform rotate right with immediate operations.
  • -msext — enable generation of sign extension (l.ext*) instructions. By default memory loads are used to perform sign extension.
  • -msfimm — enable generation of compare and set flag with immediate (l.sf*i) instructions. By default extra instructions are generated to store the immediate to a register first.
  • -mshftimm — enable generation of shift with immediate (l.srai, l.srli, l.slli) instructions. By default extra instructions are generated to store the immediate to a register first.
  • -mcmodel=small — generate OpenRISC code for the small model: The GOT is limited to 64k and function call jumps are limited to 64M offsets. This is the default model.
  • -mcmodel=large — generate OpenRISC code for the large model: The GOT may grow up to 4G in size and function call jumps can target the full 4G address space.

🕹️ PDP-11 Options

These options are defined for the PDP-11, yes we're going vintage bestie:

  • -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies -m45.
  • -msoft-float — do not use hardware floating point.
  • -mac0 / -mno-ac0 — with -mac0, return floating-point results in ac0 (fr0 in Unix assembler syntax). The default, -mno-ac0, is to return floating-point results in memory.
  • -m40 — generate code for a PDP-11/40. Implies -msoft-float -mno-split.
  • -m45 — generate code for a PDP-11/45. This is the default.
  • -m10 — generate code for a PDP-11/10. Implies -msoft-float -mno-split.
  • -mint16 / -mno-int32 — use 16-bit int. This is the default.
  • -mint32 / -mno-int16 — use 32-bit int.
  • -msplit — target has split instruction and data space. Implies -m45.
  • -munix-asm — use Unix assembler syntax.
  • -mdec-asm — use DEC assembler syntax.
  • -mgnu-asm — use GNU assembler syntax. This is the default. 👑
  • -mlra — use the new LRA register allocator. By default, the old reload allocator is used.

🧸 Picolibc Options

These options control compilation and linking when using Picolibc:

  • --oslib=library — bestie, this one goes hunting for the library called library after the C library, so any symbol the C library left undefined can get defined over here instead. The C library, libgcc, and this new library all get tucked between --start-group and --end-group flags, so each one can quietly reference symbols in the others 🫶. For a bunch of targets, Picolibc ships a semihost variant — you ask for it with --oslib=semihost — and that variant hands you just enough basic OS goodness to make console and file I/O work when you're running in an emulator or vibing with an in-circuit debugger 💅.
  • --crt0=[none|minimal|hosted|semihost] — swaps the default crt0.o name out for crt0-variant.o. Here's the tea on each variant:
    • none gives you literally zero startup code, so you get to supply your own ✨.
    • minimal does the basic memory setup but does not invoke any constructors.
    • when you don't pass a -crt0 option at all, the default init code adds calls to all constructors.
    • hosted adds a call to exit when main returns.
    • semihost grabs a command-line parameter through the semihosting interface and splits it into arguments at whitespace boundaries, then hands that resulting array of strings to main in argc and argv 🎀.

On some targets — including AArch64, ARC, ARM, LoongArch, M680x0, RISC-V, SH, and x86 — semihost also traps hardware exceptions and prints info to the console 😭. Heads up: --crt0=semihost depends on APIs provided by --oslib=semihost.

  • --printf=[d|f|l|i|m] — picks which printf variant you get. Picolibc serves up five of them, and the functionality drops as the code size drops (trade-offs, right?):
    • d is the default and it's giving full C17 and POSIX.1-2024 conformance 👑.
    • f has the same features but supports float values instead of double, and those get passed using the printf_float macro.
    • l elides all floating-point and POSIX positional parameter support.
    • i limits integers to stuff no larger than long.
    • m removes support for most formatting options including width and precision — the formats and arguments are still parsed correctly, but the output just does not respect those parameters 💀.
  • --scanf=[d|f|l|i|m] — same energy, but for scanf. Five variants, decreasing functionality, decreasing code size:
    • d is the default, full C17 and POSIX.1-2024 conformance ✨.
    • f removes support for double values.
    • l elides all floating point support.
    • i limits integers to those no larger than long.
    • m removes support for %[ conversion specifiers.

PowerPC Options

These are listed under

PRU Options

These command-line options are defined for the PRU target:

  • -minrt — link with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but some standard C runtime features get yeeted in the process 🙈.

It disables support for static initializers and constructors. Beware (fr fr): the compiler could still generate code with static initializers and constructors, and it is up to the programmer to make sure the source program doesn't use those features. No cap.

The minimal startup code does not pass argc and argv arguments to main, so main has to be declared as int main (void). Which is already the norm for most firmware projects, so slay 💅.

  • -mmcu=mcu — specifies the PRU hardware variant to use. A correspondingly-named spec file gets loaded, which passes the memory region sizes to the linker and defines hardware-specific C macros.

Newlib only provides the sim spec — that one is intended for running regression tests with a simulator. If you want specs for real hardware, you can obtain them by installing the GnuPruMcu package (https://github.com/dinuxbg/gnuprumcu/) 🦋.

  • -mno-relax — makes GCC pass the --no-relax command-line option to the linker instead of the --relax option.
  • -mloop, -mno-loop — allow (or do not allow) GCC to use the LOOP instruction.
  • -mmul, -mno-mul — allow (or do not allow) GCC to use the PRU multiplier unit.
  • -mfillzero, -mno-fillzero — allow (or do not allow) GCC to use the FILL and ZERO instructions.
  • -mabi=variant — specifies the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI, while -mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions. Here's the difference rundown, bestie:

Function Pointer Size

TI ABI says function (code) pointers are 16-bit, whereas GNU only supports 32-bit data and code pointers.

Optional Return Value Pointer

Function return values larger than 64 bits get 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 isn't using the returned value. GNU always passes and expects a valid return value pointer.

Size Of Struct Containing Bit-fields

TI ABI mandates that struct size is determined by the bit-field type, if it contains any. GNU, on the other hand, allocates the smallest amount of bytes that would fit the bit-field.

For example, TI ABI reserves 4 bytes for this struct, whereas GNU reserves a single byte:

struct S { int i:1; };

Access Size For Volatile Bit-fields

TI ABI mandates that volatile bit-fields are accessed using their type. In contrast, GNU ABI uses the smallest integer type fitting the bit-field.

For example, TI ABI requires a single load of 4 bytes for the following bit-field; GNU generates a load of 1 byte:

struct S { volatile int i:1; };
The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti. 😭

Relaxation is a GNU feature and, for safety reasons, is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions.

RISC-V Options

These command-line options are defined for RISC-V targets:

  • -mbranch-cost=n — set the cost of branches to roughly n instructions.
  • -mabi=ABI-string — specify the integer and floating-point calling convention. ABI-string has 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 (which implicitly defines 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 lets the compiler generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments are passed in registers at all.

The default for this argument is system dependent; if you want a specific calling convention you should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs — for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide 💀. There's also the ilp32e ABI, which can only be used with the rv32e architecture, and the lp64e ABI, which can only be used with 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 isn't 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 that generated code should conform to. The possibilities for ISA-spec-string are:
    • 2.2 — produce code conforming to version 2.2.
    • 20190608 — produce code conforming to version 20190608.
    • 20191213 — produce code conforming to version 20191213.

The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version.

  • -march=[ISA|Profile|Profile_ISA|processor-string] — generate code for the given RISC-V ISA or profile, or a combination of them (e.g. rv64im rvi20u64 rvi20u64_zbb). The names of ISAs and profiles must be lower case. Examples include rv64i, rv32g, rv32e, rv32imaf, rva22u64 and rva23u64. To combine a named profile with optional RISC-V ISA extensions, give the profile first and then append the extension name(s) using an underscore as a delimiter (e.g. rvi20u64_zca_zcb rva23u64_zacas). Additionally, a special value help (-march=help) is accepted to list all supported extensions ✨.

-march=unset causes the compiler to ignore any -march=... options that appear earlier on the command line, behaving as if the option was never passed. This is useful for ensuring that the architecture is taken from the -mcpu option, and an error results if no -mcpu option is given when -march=unset is used.

-march=native causes the compiler to work out the ISA of the machine it is running on. It is only available on a native GNU/Linux compiler; anywhere else the value is rejected like any other unknown ISA string 🙅. On a system whose cores are not all alike — such as one pairing fast and efficient cores — which of them the detected ISA describes is unspecified; write the ISA string out in full if it matters. If the running machine cannot be identified, the option is ignored, and the architecture is taken from -mcpu or from the configured default as usual.

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).

okay bestie, here comes the guest list 💅 Supported extensions are listed below — every single one, no skipping, we do not gatekeep in this house:

  • g — version: - — General-purpose computing base extension; g expands to i, m, a, f, d, zicsr and zifencei. the starter pack, fr fr
  • e — version: 2.0 — Reduced base integer extension. small but mighty
  • i — versions: 2.0, 2.1 — Base integer extension.
  • m — version: 2.0 — Integer multiplication and division extension.
  • a — versions: 2.0, 2.1 — Atomic extension.
  • f — versions: 2.0, 2.2 — Single-precision floating-point extension.
  • d — versions: 2.0, 2.2 — Double-precision floating-point extension.
  • c — version: 2.0 — Compressed extension.
  • b — version: 1.0 — Standard extension for bit manipulation functions.
  • v — version: 1.0 — Vector extension.
  • h — version: 1.0 — Hypervisor extension.
  • zic64b — version: 1.0 — Cache block size is 64 bytes.
  • zicbom — version: 1.0 — Cache-block management extension.
  • zicbop — version: 1.0 — Cache-block prefetch extension.
  • zicboz — version: 1.0 — Cache-block zero extension.
  • ziccamoa — version: 1.0 — Main memory supports all atomics in A.
  • ziccamoc — version: 1.0 — Main memory supports compare-and-swap atomics.
  • ziccid — version: 1.0 — Instruction/data coherence and consistency extension.
  • ziccif — version: 1.0 — Main memory supports instruction fetch with atomicity requirement.
  • zicclsm — version: 1.0 — Main memory supports misaligned loads/stores.
  • ziccrse — version: 1.0 — Main memory supports forward progress on LR/SC sequences.
  • zicfilp — version: 1.0 — Control-flow integrity landing pad extension.
  • zicfiss — version: 1.0 — Control-flow integrity shadow stack extension.
  • zicntr — version: 2.0 — Standard extension for base counters and timers.
  • zicond — version: 1.0 — Integer conditional operations extension.
  • zicsr — version: 2.0 — Control and status register access extension.
  • zifencei — version: 2.0 — Instruction-fetch fence extension.
  • zihintntl — version: 1.0 — Non-temporal locality hints extension.
  • zihintpause — version: 2.0 — Pause hint extension.
  • zihpm — version: 2.0 — Standard extension for hardware performance counters.
  • zimop — version: 1.0 — May-be-operations extension. may-be, bestie, no promises
  • zilsd — version: 1.0 — Load/store pair instructions extension.
  • zmmul — version: 1.0 — Integer multiplication extension.
  • za128rs — version: 1.0 — Reservation set size of 128 bytes.
  • za64rs — version: 1.0 — Reservation set size of 64 bytes.
  • zaamo — version: 1.0 — Atomic memory operations extension.
  • zabha — version: 1.0 — Byte and halfword atomic memory operations extension.
  • zacas — version: 1.0 — Atomic compare-and-swap instructions extension.
  • zalasr — version: 1.0 — Atomic load-acquire and store-release extension.
  • zalrsc — version: 1.0 — Load-reserved/store-conditional subset of the A extension.
  • zawrs — version: 1.0 — Wait-on-reservation-set extension.
  • zama16b — version: 1.0 — Misaligned loads, stores, and AMOs that are fully contained within a naturally-aligned 16-byte boundary are atomic.
  • zfa — version: 1.0 — Additional floating-point extension.
  • zfbfmin — version: 1.0 — Minimal BF16 support extension.
  • zfh — version: 1.0 — Half-precision floating-point extension.
  • zfhmin — version: 1.0 — Minimal half-precision floating-point extension.
  • zfinx — version: 1.0 — Single-precision floating-point in integer registers extension.
  • zdinx — version: 1.0 — Double-precision floating-point in integer registers extension.
  • zca — version: 1.0 — Integer compressed instruction extension.
  • zcb — version: 1.0 — Simple compressed instruction extension.
  • zcd — version: 1.0 — Compressed double-precision floating point loads and stores extension.
  • zce — version: 1.0 — Compressed instruction extensions for embedded processors.
  • zcf — version: 1.0 — Compressed single-precision floating point loads and stores extension.
  • zcmop — version: 1.0 — Compressed may-be-operations extension.
  • zcmp — version: 1.0 — Compressed push pop extension.
  • zcmt — version: 1.0 — Table jump instruction extension.
  • zclsd — version: 1.0 — Compressed load/store pair instructions extension.
  • zba — version: 1.0 — Address calculation extension.
  • zbb — version: 1.0 — Basic bit manipulation extension.
  • zbc — version: 1.0 — Carry-less multiplication extension.
  • zbkb — version: 1.0 — Cryptography bit-manipulation extension.
  • zbkc — version: 1.0 — Cryptography carry-less multiply extension.
  • zbkx — version: 1.0 — Cryptography crossbar permutation extension.
  • zbs — version: 1.0 — Single-bit operation extension.
  • zk — version: 1.0 — Standard scalar cryptography extension.
  • zkn — version: 1.0 — NIST algorithm suite extension.
  • zknd — version: 1.0 — AES Decryption extension.
  • zkne — version: 1.0 — AES Encryption extension.
  • zknh — version: 1.0 — Hash function extension.
  • zkr — version: 1.0 — Entropy source extension.
  • zks — version: 1.0 — ShangMi algorithm suite extension.
  • zksed — version: 1.0 — SM4 block cipher extension.
  • zksh — version: 1.0 — SM3 hash function extension.
  • zkt — version: 1.0 — Data independent execution latency extension.
  • ztso — version: 1.0 — Total store ordering extension.
  • zvbb — version: 1.0 — Vector basic bit-manipulation extension.
  • zvbc — version: 1.0 — Vector carry-less multiplication extension.
  • zve32f — version: 1.0 — Vector extensions for embedded processors.
  • zve32x — version: 1.0 — Vector extensions for embedded processors.
  • zve64d — version: 1.0 — Vector extensions for embedded processors.
  • zve64f — version: 1.0 — Vector extensions for embedded processors.
  • zve64x — version: 1.0 — Vector extensions for embedded processors.
  • zvfbfmin — version: 1.0 — Vector BF16 converts extension.
  • zvfofp8min — version: 0.2 — Vector FP8 minimum extension.
  • zvfbfwma — version: 1.0 — Vector BF16 widening multiply/add extension.
  • zvfh — version: 1.0 — Vector half-precision floating-point extension.
  • zvfhmin — version: 1.0 — Vector minimal half-precision floating-point extension.
  • zvkb — version: 1.0 — Vector cryptography bit-manipulation extension.
  • zvkg — version: 1.0 — Vector GCM/GMAC extension.
  • zvkn — version: 1.0 — Vector NIST Algorithm Suite extension, zvkn will expand to
  • zvknc — version: 1.0 — Vector NIST Algorithm Suite with carry-less multiply extension, zvknc
  • zvkned — version: 1.0 — Vector AES block cipher extension.
  • zvkng — version: 1.0 — Vector NIST Algorithm Suite with GCM extension, zvkng will expand
  • zvknha — version: 1.0 — Vector SHA-2 secure hash extension.
  • zvknhb — version: 1.0 — Vector SHA-2 secure hash extension.
  • zvks — version: 1.0 — Vector ShangMi algorithm suite extension, zvks will expand
  • zvksc — version: 1.0 — Vector ShangMi algorithm suite with carry-less multiplication extension,
  • zvksed — version: 1.0 — Vector SM4 block cipher extension.
  • zvksg — version: 1.0 — Vector ShangMi algorithm suite with GCM extension.
  • zvksh — version: 1.0 — Vector SM3 secure hash extension.
  • zvkt — version: 1.0 — Vector data independent execution latency extension.
  • zvl1024b — version: 1.0 — Minimum vector length standard extensions.
  • zvl128b — version: 1.0 — Minimum vector length standard extensions.
  • zvl16384b — version: 1.0 — Minimum vector length standard extension.
  • zvl2048b — version: 1.0 — Minimum vector length standard extensions.
  • zvl256b — version: 1.0 — Minimum vector length standard extensions.
  • zvl32768b — version: 1.0 — Minimum vector length standard extension.
  • zvl32b — version: 1.0 — Minimum vector length standard extensions.
  • zvl4096b — version: 1.0 — Minimum vector length standard extensions.
  • zvl512b — version: 1.0 — Minimum vector length standard extensions.
  • zvl64b — version: 1.0 — Minimum vector length standard extensions.
  • zvl65536b — version: 1.0 — Minimum vector length standard extension.
  • zvl8192b — version: 1.0 — Minimum vector length standard extension.
  • zhinx — version: 1.0 — Half-precision floating-point in integer registers extension.
  • zhinxmin — version: 1.0 — Minimal half-precision floating-point in integer registers extension.
  • zvabd — version: 0.7 — Vector absolute difference extension.
  • sdtrig — version: 1.0 — Debug triggers extension.
  • sha — version: 1.0 — The augmented hypervisor extension.
  • shcounterenw — version: 1.0 — Support writeable enables for any supported counter.
  • shgatpa — version: 1.0 — SvNNx4 mode supported for all modes supported by satp.
  • shlcofideleg — version: 1.0 — Delegating LCOFI interrupts to VS-mode.
  • shtvala — version: 1.0 — The htval register provides all needed values.
  • shvstvala — version: 1.0 — The vstval register provides all needed values.

and that's the whole roster, no crumbs left behind 🫶

  • shvstvecd — 1.0 — The vstvec register supports direct mode 💅
  • shvsatpa — 1.0 — The vsatp register supports all modes supported by satp
  • smaia — 1.0 — Advanced interrupt architecture extension
  • smcdeleg — 1.0 — Machine-mode counter delegation extension
  • smcntrpmf — 1.0 — Cycle and instret privilege mode filtering
  • smctr — 1.0 — Machine-mode control transfer records extension
  • smcsrind — 1.0 — Machine-level indirect CSR access
  • smepmp — 1.0 — PMP enhancements for memory access and execution prevention on machine mode
  • smmpm — 1.0 — Supervisor-mode pointer masking extension
  • smnpm — 1.0 — Supervisor-mode pointer masking extension
  • smrnmi — 1.0 — Resumable non-maskable interrupts
  • smstateen — 1.0 — State enable extension
  • smdbltrp — 1.0 — Double trap extensions
  • ssaia — 1.0 — Advanced interrupt architecture extension for supervisor mode
  • ssccfg — 1.0 — Supervisor-mode counter configuration extension
  • ssccptr — 1.0 — Main memory supports page table reads
  • sscofpmf — 1.0 — Count overflow and filtering extension
  • sscounterenw — 1.0 — Support writeable enables for any supported counter
  • ssctr — 1.0 — Supervisor-mode control transfer records extension
  • sscsrind — 1.0 — Supervisor-mode indirect CSR access
  • ssnpm — 1.0 — Supervisor-mode pointer masking extension
  • sspm — 1.0 — Supervisor-mode pointer masking extension
  • ssqosid — 1.0 — Supervisor-mode quality-of-service ID extension
  • ssstateen — 1.0 — Supervisor-mode state-enable extension
  • sstc — 1.0 — Supervisor-mode timer interrupts extension
  • sstvala — 1.0 — Stval provides all needed values
  • sstvecd — 1.0 — Stvec supports direct mode
  • ssstrict — 1.0 — Unimplemented reserved encodings raise illegal instruction exceptions and no non-conforming extensions are present
  • ssdbltrp — 1.0 — Double trap extensions
  • ssu64xl — 1.0 — UXLEN=64 must be supported
  • supm — 1.0 — User-mode pointer masking extension
  • svinval — 1.0 — Fine-grained address-translation cache invalidation extension 🌈
  • svnapot — 1.0 — NAPOT translation contiguity extension
  • svpbmt — 1.0 — Page-based memory types extension
  • svrsw60t59b — 1.0 — Page-table reserved-for-software bits 60-59 extension
  • svvptc — 1.0 — Extension for obviating memory-management instructions after marking PTEs valid
  • svadu — 1.0 — Hardware updating of A/D bits extension
  • svade — 1.0 — Cause exception when hardware updating of A/D bits is disabled
  • svbare — 1.0 — Satp mode bare is supported
  • xcvalu — 1.0 — Core-V miscellaneous ALU extension
  • xcvbi — 1.0 — Core-V immediate branch extension
  • xcvelw — 1.0 — Core-V event load word extension
  • xcvmac — 1.0 — Core-V multiply-accumulate extension
  • xcvsimd — 1.0 — Core-V SIMD extension
  • xsfcease — 1.0 — SiFive CEASE instruction extension
  • xsfvcp — 1.0 — SiFive VCIX vector coprocessor extension
  • xsfvfnrclipxfqf — 1.0 — SiFive FP32-to-int8 ranged clip instructions
  • xsfvqmaccdod — 1.0 — SiFive int8 matrix multiplication extension
  • xsfvqmaccqoq — 1.0 — SiFive int8 matrix multiplication extension
  • xtheadba — 1.0 — T-head address calculation extension
  • xtheadbb — 1.0 — T-head basic bit-manipulation extension
  • xtheadbs — 1.0 — T-head single-bit instructions extension
  • xtheadcmo — 1.0 — T-head cache management operations extension
  • xtheadcondmov — 1.0 — T-head conditional move extension
  • xtheadfmemidx — 1.0 — T-head indexed memory operations for floating-point registers extension
  • xtheadfmv — 1.0 — T-head double floating-point high-bit data transmission extension
  • xtheadint — 1.0 — T-head acceleration interruption extension
  • xtheadmac — 1.0 — T-head multiply-accumulate extension
  • xtheadmemidx — 1.0 — T-head indexed memory operation extension
  • xtheadmempair — 1.0 — T-head two-GPR memory operation extension
  • xtheadsync — 1.0 — T-head multi-core synchronization extension
  • xtheadvector — 1.0 — T-head vector extension
  • xventanacondops — 1.0 — Ventana integer conditional operations extension
  • xmipscmov — 1.0 — Mips conditional move extension
  • xmipscbop — 1.0 — Mips prefetch extension
  • xandesperf — 5.0 — Andes performance extension
  • xandesbfhcvt — 5.0 — Andes bfloat16 conversion extension
  • xandesvbfhcvt — 5.0 — Andes vector bfloat16 conversion extension
  • xandesvsintload — 5.0 — Andes vector INT4 load extension
  • xandesvpackfph — 5.0 — Andes vector packed FP16 extension
  • xandesvdot — 5.0 — Andes vector dot product extension
  • xsmtvdot — 1.0 — SpacemiT vector dot product extension
  • xsmtvdotii — 1.0 — SpacemiT vector dot product II extension

okay bestie, important little rules for the list above, no skipping 💖

When -march= is not specified, GCC uses the setting from -mcpu.

If both -march and -mcpu= are not specified, the default for this argument is system dependent; if you want a specific architecture extension, you should specify one explicitly. like, don't just vibe and hope.

When the RISC-V specifications define an extension as depending on other extensions, GCC implicitly adds the dependent extensions to the enabled extension set if they weren't added explicitly. it's giving "the compiler has your back" 🫶

Core Name 🎀

  • -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-p450,
    • sifive-p550,
    • sifive-p670,
    • sifive-p870-d,
    • thead-c906,
    • xt-c908,
    • xt-c908v,
    • xt-c910,
    • xt-c910v2,
    • xt-c920,
    • xt-c920v2,
    • xt-c9501fdvt,
    • tt-ascalon-x,
    • tt-ascalon-xg,
    • xiangshan-nanhu,
    • xiangshan-kunminghu,
    • mips-p8700,
    • andes-n22,
    • andes-n25,
    • andes-a25,
    • andes-nx25,
    • andes-ax25,
    • andes-a27,
    • andes-ax27,
    • andes-n225,
    • andes-d23,
    • andes-n45,
    • andes-nx45,
    • andes-a45,
    • andes-ax45,
    • spacemit-x60,
    • spacemit-x100,
    • spacemit-a100.
Note that -mcpu does not override -march or -mtune. read that again bestie, it does not. 🙈

Tune Name ⭐

  • -mtune=processor-string — Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are:
    • generic,
    • rocket,
    • sifive-3-series,
    • sifive-5-series,
    • sifive-7-series,
    • sifive-p400-series,
    • sifive-p600-series,
    • sifive-p550,
    • sifive-p870-d,
    • tt-ascalon-x,
    • tt-ascalon-xg,
    • thead-c906,
    • xt-c908,
    • xt-c908v,
    • xt-c910,
    • xt-c910v2,
    • xt-c920,
    • xt-c920v2,
    • xt-c9501fdvt,
    • xiangshan-nanhu,
    • xiangshan-kunminghu,
    • spacemit-x60,
    • spacemit-x100,
    • spacemit-a100,
    • arc-v-rhx-100-series,
    • generic-ooo,
    • size,
    • mips-p8700,
    • andes-25-series,
    • andes-23-series,
    • andes-45-series,
    • arc-v-rmx-100-series,
    • and all valid options for -mcpu=.

When -mtune= is not specified, GCC uses the setting from -mcpu. The default is generic if neither is specified.

-mtune=native and -mcpu=native identify the core the compiler is running on, subject to the same restrictions and the same caveat about unlike cores as -march=native above. -mcpu=native additionally selects the detected architecture. A core the compiler has no hardware identification for leaves the tuning at its default rather than causing an error. so chill, it won't yell at you 😌

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: If you use this switch, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. fr fr, mismatched values here is a whole mess, don't do it 💀
  • -msmall-data-limit=n — Put global and static data smaller than n bytes into a special section (on some targets).
  • -msave-restore
  • -mno-save-restore — Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is to use fast inline prologues and epilogues.
  • -mmovcc
  • -mno-movcc — Do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable.
  • -minline-atomics, -mno-inline-atomics — bestie, this one's a straight vibe check on your subword atomics 🔥 with it, GCC uses the fast inline subword atomics that don't need libatomic at all. Without it, you get the smaller-but-slower emulation code that literally calls into libatomic functions. The default? Fast inline, no libatomic required. Periodt. 💅
  • -minline-strlen, -mno-inline-strlen — do we try to inline strlen calls, or not? With it: attempt the inline, but ⚠️ only if the string is properly aligned and the instructions for accelerated processing are actually available. The default is to inline strlen calls. Slay. ✨
  • -minline-strcmp, -mno-inline-strcmp — same energy, but for strcmp 👑 Do or do not attempt to inline those calls. Again, inlining can only happen if the strings are properly aligned and accelerated-processing instructions are available. Default: inline them. 🫶
  • -minline-strncmp, -mno-inline-strncmp — and the trilogy closes with strncmp 💖 Do or do not attempt to inline them, only possible when the strings are properly aligned and the accelerated instructions exist. Default is to inline strncmp calls, no cap.
  • -mstringop-strategy=strategy — pick a specific strategy for inlining string and memory operations. strategy can be one of auto, libcall, scalar, or vector. That's it, that's the menu 🌈
  • -mshorten-memrefs, -mno-shorten-memrefs — do or do not try to squeeze more use out of compressed load/store instructions by swapping a load/store of 'base register + large offset' for a shiny new load/store of 'new base + small offset' ✨ and here's the trick: 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. iykyk.
  • -mstrict-align, -mno-strict-align — do not or do generate unaligned memory accesses 🫣 The default depends on whether the processor you're optimizing for supports fast unaligned access or not.
  • -mscalar-strict-align, -mno-scalar-strict-align — do not or do generate unaligned memory accesses, default set by whether your target processor supports fast unaligned access. PS: this is literally just an alias for -mstrict-align. 💀
  • -mvector-strict-align, -mno-vector-strict-align — do not or do generate unaligned vector memory accesses. The default is set to off unless the processor you're optimizing for explicitly supports element-misaligned vector memory access. 🦋
  • -mzilsd-word-align — lets Zilsd/Zclsd memory accesses be generated when they're known to be 4-byte aligned 🌸
  • -mzilsd-strict-align — requires Zilsd/Zclsd memory accesses to be naturally 8-byte aligned.
Heads up bestie: the precedence among -mzilsd-word-align, -mzilsd-strict-align, -mstrict-align, -mno-strict-align, -mscalar-strict-align, and -mno-scalar-strict-align is decided by the last one specified 👀 so order matters, okay?
  • -mmax-vectorization, -mno-max-vectorization — enable or disable an override to the vectorizer cost model that makes vectorization always look profitable 💅 and unlike -fno-vect-cost-model or -fvect-cost-model=unlimited, this option does not turn off the cost comparison between different vector modes. Different thing, same vibe.
  • -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 was configured with --with-cmodel= specifying a different default. 🎀
  • -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.
Okay so real talk 😭 the code generated by medium-any is position-independent, but it is not guaranteed to work correctly when linked into position-independent executables or libraries. Not "probably fine" — not guaranteed. Big difference, bestie.
  • -mcmodel=large — generate code for a large code model, which has no restrictions on size or placement of symbols. Free. Unbothered. 🌈
  • -mexplicit-relocs, -mno-explicit-relocs — use or don't use assembler relocation operators when dealing with symbolic addresses. The alternative is assembler macros instead, which may limit optimization. 👑
  • -mrelax, -mno-relax — take advantage of linker relaxations to cut down the number of instructions needed to materialize symbol addresses. Default is to take advantage of them. ✨
  • -mriscv-attribute, -mno-riscv-attribute — emit (or don't emit) the RISC-V attribute to record extra info into ELF objects. Heads up: this feature needs at least binutils 2.32 💖
  • -mcsr-check, -mno-csr-check — enables or disables the CSR checking. That's the whole thing 🫶
  • -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This dodges the instructions to save, set up, and restore frame pointers, and frees up an extra register in leaf functions. Little win, big slay. 💅
  • -malign-data=type — controls how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen, which uses the 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.
⚠️ Support for RISC-V big-endian is experimental. The ABI is not yet stable and could change in incompatible ways in future releases. You've been warned, bestie 😭
  • -mlittle-endian — generate little-endian code. This is the default when GCC is configured for a riscv64-*-* or riscv32-*-* but not a riscv64be-*-* or riscv32be-*-* target. 🌸
  • -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations are global for a global canary or tls for a per-thread canary in the TLS block. 💜

With the tls choice, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally specify which register to use as the base register for reading the canary, and from what offset from that base register. There is no default register or offset — this is entirely for use within the Linux kernel. 👑

  • -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. ✨
  • -mtls-dialect=trad — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default. 🫶
  • -munroll-only-small-loops, -mno-unroll-only-small-loops — controls conservative small loop unrolling. It's enabled by default at -O2 and above. When it's on, only loops whose RTL instruction count doesn't exceed a tune-specific threshold get unrolled, and the unroll factor is capped by a tune-specific parameter. The payoff: better utilization of the instruction decoding pipeline without blowing up bigger loops 🔥 You can disable it with -mno-unroll-only-small-loops; it also gets turned off when -funroll-loops or -funroll-all-loops is explicitly enabled, so that an explicit unrolling request applies to loops of any size. Make it make sense — the explicit ask wins. 💅
  • -mrvv-vector-bits=value — specify how the number of bits for an RVV vector register, as taken from the -march= option, is interpreted. The value parameter is given as a string keyword and may be one of scalable or zvl. The default is scalable, which tells GCC to interpret the number as a minimum, while zvl tells GCC to use exactly the number of bits specified. 🦋
  • -mrvv-max-lmul=value — this option gives you explicit control over the maximum length multiplier (LMUL) used when generating code for the RISC-V Vector Extensions (RVV). The value parameter is a string keyword and may be one of m1, m2, m4, m8, or dynamic. The default is m1 for compatibility with existing hardware that doesn't support the other options. 🌈
  • -madjust-lmul-cost, -mno-adjust-lmul-cost — this adjusts the cost model used to schedule vector instructions by multiplying the latency of instructions by the RVV length multiplier, LMUL. It's disabled by default. 🙈
  • -mautovec-segment, -mno-autovec-segment — enable or disable generation of vector segment load/store instructions. This option is enabled by default. Slay. 🔥

🌸 RL78 Options 🌸

  • -msim — links in additional target libraries to support operation within a simulator. Simulation vibes only. ✨
  • -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 — and 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 use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value rl78 is an alias for g14, and the value mg10 is an alias for none. 👑

In addition, a C preprocessor macro is defined based on the setting of this option. Possible values are: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__. 💖

  • -mcpu=g10
  • -mcpu=g13
  • -mcpu=g14
  • -mcpu=rl78

Specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions — instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention. 😭

If this option is set, it also selects the type of hardware multiply support to use, unless that's overridden by an explicit -mmul=none option on the command line. So specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables the use of hardware multiplications altogether. 🫶

Note, bestie: 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 aren't specified on the command line, then software multiplication routines are used even though the code targets the RL78 core. This is for backward compatibility with older toolchains that didn't have hardware multiply and divide support. Legacy behavior staying iconic, I guess 💅

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__". 🌸

  • -mg10, -mg13, -mg14, -mrl78 — bestie these are literally just aliases for the matching -mcpu= option 🫶 they exist purely for backwards compatibility, so old build scripts don't cry.
  • -mallregs — lets the compiler go feral and use all the available registers 👑 by default registers r24..r31 are reserved for interrupt handlers, but flip this on and ordinary functions get to borrow them too. generous queen behavior ✨
  • -mrelax, -mno-relax — turn assembler and linker relaxation on/off 🎀 it's on by default at -Os, no cap.
  • -mes0 — assume "ES" is zero for the entire program execution and use it to address read-only data 💅
  • -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — says interrupt handler functions should preserve the MDUC registers 🧚 this is only needed if normal code might touch MDUC registers, e.g. because it does multiplication and division operations. The default is to just ignore MDUC registers, because that makes the interrupt handlers faster (iconic, honestly). The target option -mg13 needs to be passed for this to work, since this feature is only available on the G13 target (S2 core). And heads up — the MDUC registers are only saved if the interrupt handler does a multiplication or division operation, or if it calls another function 😭

⚡ IBM RS/6000 and PowerPC Options ⚡

These -m options are defined for the IBM RS/6000 and PowerPC, bestie, and there are so many of them 🦋

  • -mcpu=cpu_type — sets the architecture type, register usage, and instruction scheduling parameters for machine type cpu_type 💖 supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, power10, power11, future, powerpc, powerpc64, powerpc64le, rs64, and native. 😤

-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.

Specifying native as CPU type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. -mcpu=native has no effect if GCC does not recognize the processor 🙈

The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others. lowkey dramatic but okay 💀

The -mcpu options automatically enable or disable the following options:

-maltivec  -mfprnd  -mhard-float  -mmfcrf  -mmultiple -mpopcntb
-mpopcntd  -mpowerpc64 -mpowerpc-gpopt  -mpowerpc-gfxopt -mmulhw
-mdlmzb  -mmfpgpr  -mvsx -mcrypto  -mhtm  -mpower8-fusion
-mquad-memory  -mquad-memory-atomic  -mfloat128 -mfloat128-hardware
-mprefixed  -mpcrel  -mmma -mdense-math -mrop-protect

The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities. If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like -mcpu=970 -mno-altivec. ✨

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present because AIX does not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment 💅

In other cases, we recommend you use the -mcpu=cpu_type option rather than the options that control generation of specific instructions. (do this, fr fr 🫶)

  • -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage, the way -mcpu=cpu_type does 😌 the same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune. cute lil division of labor 🌸
  • -mpowerpc64, -mno-powerpc64 — -mpowerpc64 lets GCC generate the additional 64-bit instructions found in the full PowerPC64 architecture and treat GPRs as 64-bit, doubleword quantities 👑 GCC defaults to -mno-powerpc64.
  • -mcmodel=small — generate PowerPC64 code for the small model: the TOC is limited to 64k 🎀
  • -mcmodel=medium — generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux ✨
  • -mcmodel=large — generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space 🌈
  • -mprofile-kernel — this option is available on PowerPC64 GNU/Linux targets 💜 when used with -pg, it causes calls to "mcount" to be inserted before the function prologue instead of after it. plot twist 😭
  • -maltivec, -mno-altivec — generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set 💅 you may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements.

When -maltivec is used, the element order for AltiVec intrinsics such as "vec_splat", "vec_extract", and "vec_insert" match array element order corresponding to the endianness of the target 🦋 that is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. iykyk ✨

  • -mpowerpc-gpopt, -mno-powerpc-gpopt — specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root 🌸 Specifying
  • -mpowerpc-gfxopt, -mno-powerpc-gfxopt — -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select 🎀
  • -mmfcrf, -mno-mfcrf — the -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the POWER4 processor and other processors that support the PowerPC V2.01 architecture 👑
  • -mpopcntb, -mno-popcntb — the -mpopcntb option allows GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5 processor and other processors that support the PowerPC V2.02 architecture 💖
  • -mpopcntd, -mno-popcntd — the -mpopcntd option allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture ✨
  • -mfprnd, -mno-fprnd — the -mfprnd option allows GCC to generate the FP round to integer instructions implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture 🌸
  • -mcmpb, -mno-cmpb — the -mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture 🧚
  • -mhard-dfp, -mno-hard-dfp — the -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors 💅
  • -mvrsave, -mno-vrsave — generate VRSAVE instructions when generating AltiVec code ⭐
  • -msecure-plt — generate code that allows ld and ld.so to build executables and shared libraries with non-executable ".plt" and ".got" sections 🛡️ this is a PowerPC 32-bit SYSV ABI option.
  • -mbss-plt — generate code that uses a BSS ".plt" section that ld.so fills in, and requires ".plt" and ".got" sections that are both writable and executable 😳 this is a PowerPC 32-bit SYSV ABI option.
  • -msplit-patch-nops — when adding NOPs for a patchable area via the -fpatchable-function-entry option, emit the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point 💜 this makes the sequence of NOPs not consecutive when a global entry point is generated. Without this option the NOPs are emitted directly before and after the local entry point, making them consecutive but moving global and local entry point further apart. If only a single entry point is generated this option has no effect. periodt 🫶
  • -misel, -mno-isel — this switch enables or disables the generation of ISEL instructions 🎀
  • -mvsx, -mno-vsx — generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set ✨
  • -mcrypto, -mno-crypto — enable (disable) the use of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA 🔐
  • -mhtm, -mno-htm — enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA 💖
  • -mpower8-fusion, -mno-power8-fusion — generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors 🔥 (yes, "does not keeps" — that's the original wording, bestie, we don't fix, we appreciate 😭 iconic.)
  • -mquad-memory, -mno-quad-memory — generate code that uses (does not use) the non-atomic quad word memory instructions 💅 the -mquad-memory option requires use of 64-bit mode.
  • -mquad-memory-atomic, -mno-quad-memory-atomic — generate code that uses (does not use) the atomic quad word memory instructions ⚛️ the -mquad-memory-atomic option requires use of 64-bit mode.
  • -mfloat128, -mno-float128 — enable/disable the __float128 keyword for IEEE 128-bit floating point, and use either software emulation for IEEE 128-bit floating point or hardware instructions ✨

The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support 👑 and the IEEE 128-bit floating point is only supported on Linux 🐧

  • -mfloat128 / -mno-float128 — turns the __float128 IEEE 128-bit floating point type on or off 💜 the default for -mfloat128 is on for PowerPC Linux systems that use the VSX instruction set, and off everywhere else.

And okay so if you're on a 64-bit system using the ISA 3.0 instruction set (-mcpu=power9), the IEEE 128-bit floating point support ALSO unlocks generation of ISA 3.0 IEEE 128-bit floating point instructions 🔥 Otherwise — like if you don't ask for ISA 3.0 instructions at all, or you're targeting a 32-bit big-endian system — IEEE 128-bit floating point gets handled with software emulation. slay, but slower 💅

  • -mfloat128-hardware / -mno-float128-hardware — enables/disables using ISA 3.0 hardware instructions to back the __float128 data type 🎀

Default for -mfloat128-hardware is on on PowerPC Linux systems using the ISA 3.0 instruction set, off on everything else. periodt.

  • -m32, -m64 — generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (GNU/Linux included, bestie 🫶). The 32-bit environment sets int, long and pointer to 32 bits and spits out code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits but long and pointer to 64 bits, and generates code for PowerPC64, same as -mpowerpc64. iconic behavior honestly 👑
  • -mfull-toc, -mno-fp-in-toc, -mno-sum-in-toc, -mminimal-toc — these four tweak how the TOC (Table Of Contents 😌) gets generated, and yeah a TOC is created for every executable file. -mfull-toc is the default. In that case GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program, and it also drops floating-point constants into the TOC. But — and this is the catch — only 16,384 entries fit in the TOC 😭

So if the linker yells at you that you overflowed the available TOC space, you can shrink your TOC usage with -mno-fp-in-toc and -mno-sum-in-toc. -mno-fp-in-toc stops GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code that calculates the sum of an address and a constant at run time instead of stuffing that sum into the TOC. You can use one or both 🌸 each one makes GCC produce very slightly slower and larger code, but hey, it conserves TOC space. trade-off queen.

Still running out of space even with both? Then specify -mminimal-toc instead 💀 this one makes GCC create only one TOC entry per file. You get code that's slower and larger but uses extremely little TOC space. Lowkey you only wanna use this on files holding less frequently-executed code, okay?

  • -maix64, -maix32 — enable the 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and all the infrastructure needed to support them 💖 specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32.
  • -mxl-compat / -mno-xl-compat — produce code that conforms more closely to IBM XL compiler semantics when you're using the AIX-compatible ABI ✨ it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack, in addition to argument FPRs. It does not assume that the most significant double in a 128-bit long double value is properly rounded when comparing values and converting to double. And it uses XL symbol names for long double support routines.

Story time, because this one's a whole saga: the AIX calling convention was extended but not initially documented 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. And since always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default — it's only necessary when calling subroutines compiled by IBM XL compilers without optimization. iykyk.

  • -mpe — supports the IBM RS/6000 SP Parallel Environment (PE) 🌈 it links an application written to use message passing with special startup code so the app can actually run. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or else the specs file has to be overridden with the -specs= option to point at the right directory. Oh and heads up — the Parallel Environment does not support threads, so -mpe and -pthread are incompatible 💀 can't have both, bestie, pick a lane.
  • -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) onto their natural size-based boundary 🦋 meanwhile -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 already the default, and -malign-power is not supported there. nope, not a thing 💅

  • -msoft-float, -mhard-float — generate code that doesn't use (or does use, respectively) the floating-point register set 🎀 software floating-point emulation is provided if you use -msoft-float, and you must pass the option to GCC when linking too. don't forget the linking part, that's the classic oopsie 😭
  • -mmultiple / -mno-multiple — generate code that uses (or doesn't use) the load multiple word instructions and the store multiple word instructions ✨ these get generated by default on POWER systems, and not generated on PowerPC systems. And listen — do not use -mmultiple on little-endian PowerPC systems, because those instructions don't work when the processor is in little-endian mode 💀 the exceptions are PPC740 and PPC750, which do permit these instructions in little-endian mode. lucky them 🍓
  • -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 moment the stack pointer is updated and the moment the address of the previous frame is stored — which means code that walks the stack frame across interrupts or signals may get corrupted data 😭 so, y'know, choose wisely.
  • -mavoid-indexed-addresses / -mno-avoid-indexed-addresses — generate code that tries to avoid (or not avoid) indexed load or store instructions 🌸 those 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.
  • -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. Also, plot twist: 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 ✨
  • -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 are generated by default when targeting those processors. automatic, no cap.
  • -mdlmzb / -mno-dlmzb — generate code that uses (or doesn't use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors 👑 this instruction is generated by default when targeting those processors.
  • -mno-bit-align, -mbit-align — on System V.4 and embedded PowerPC systems, do not (or do) force structures and unions containing bit-fields to be aligned to the base type of the bit-field 🌈

Example time, so it actually clicks: 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. Use -mno-bit-align and that same structure gets aligned to a 1-byte boundary and is 1 byte in size. wild, right? 🦋

  • -mbit-word / -mno-bit-word — allow bit-fields to cross word boundaries 💅 yes, that's the whole thing, bestie.
  • -mno-strict-align, -mstrict-align — on System V.4 and embedded PowerPC systems, do not (or do) assume that unaligned memory references are handled by the system ✨
  • -mrelocatable / -mno-relocatable — generate code that allows (or 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 the 4-byte locations listed in the ".fixup" section — a table of 32-bit addresses that this option generates. For this to actually work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. Also -mrelocatable code aligns the stack to an 8-byte boundary. all or nothing, fr fr.
  • -mrelocatable-lib / -mno-relocatable-lib — like -mrelocatable, -mrelocatable-lib generates a ".fixup" section so static executables can be relocated at run time, BUT -mrelocatable-lib doesn't use the smaller stack alignment of -mrelocatable 🫶 the best part: objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options. mix and match, we love flexibility 💖
  • -mlittle, -mlittle-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in little-endian mode 🌸 and -mlittle-endian is literally the same thing as -mlittle. two names, one vibe.
  • -mbig, -mbig-endian — on System V.4 and embedded PowerPC systems, compile code for the processor in big-endian mode ✨ -mbig-endian is the same as -mbig. same deal 💅
  • -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 for shared libraries. so don't try it for your .dylib, bestie, it's not gonna eat.
  • -msingle-pic-base — treat the register used for PIC addressing as read-only, instead of loading it in the prologue of each function 🌈 the runtime system is then responsible for initializing this register with an appropriate value before execution begins. that's on the runtime now, not you 😌
  • *-mprioritize-restricted-insns=priority* — this option controls the priority that gets assigned to dispatch-slot restricted instructions during the second scheduling pass 👑 the priority argument takes the value 0, 1, or 2, to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions. three choices, that's it, that's all ✨
  • -msched-costly-dep=dependence_type — bestie this one tells the target which dependences it should be pressed about (aka "costly") while it's doing instruction scheduling. So which dependence_type are we talking? take your pick:
    • no — literally none of them are costly. chill.
    • all — every single dependence is costly. no survivors.
    • true_store_to_load — a true dependence that goes from store → load is costly.
    • store_to_load — any dependence from store → load is costly.
    • number — any dependence whose latency is greater than or equal to number is costly.
  • -minsert-sched-nops=scheme — this picks which NOP insertion scheme gets used during the second scheduling pass 💅 the argument scheme takes one of these:
    • no — don't insert NOPs. periodt.
    • pad — pad with NOPs any dispatch group that has vacant issue slots, following the scheduler's grouping.
    • regroup_exact — insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn into a new group, per the estimated processor grouping.
    • number — insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
  • -mcall-sysv — on System V.4 and embedded PowerPC systems, compile code using calling conventions that stick to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix.
  • -mcall-sysv-eabi
  • -mcall-eabi — specify both -mcall-sysv and -meabi options. two for one, we love.
  • -mcall-sysv-noeabi — specify both -mcall-sysv and -mno-eabi options.
  • -mcall-aixdesc — on System V.4 and embedded PowerPC systems, compile code for the AIX operating system.
  • -mcall-linux — on System V.4 and embedded PowerPC systems, compile code for the Linux-based GNU system.
  • -mcall-freebsd — on System V.4 and embedded PowerPC systems, compile code for the FreeBSD operating system.
  • -mcall-netbsd — on System V.4 and embedded PowerPC systems, compile code for the NetBSD operating system.
  • -mcall-openbsd — on System V.4 and embedded PowerPC systems, compile code for the OpenBSD operating system.
  • -mtraceback=traceback_type — pick the type of traceback table. valid values for traceback_type are full, part, and no.
  • -maix-struct-return — return all structures in memory (as the AIX ABI specifies).
  • -msvr4-struct-return — return structures smaller than 8 bytes in registers (as the SVR4 ABI specifies).
  • -mabi=abi-type — extend the current ABI with a particular extension, or yeet such an extension. valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and for AIX: vec-extabi, vec-default.
  • -mabi=ibmlongdouble — change the current ABI to use IBM extended-precision long double. 🚨 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 throws a warning unless you use -Wno-psabi. requires -mlong-double-128 to be enabled.
  • -mabi=ieeelongdouble — change the current ABI to use IEEE extended-precision long double. 🚨 again, not likely to work if your system defaults to using IBM extended-precision long double. if you change the long double type away from IBM extended-precision, the compiler issues a warning unless you use -Wno-psabi. requires -mlong-double-128 to be enabled.
  • -mabi=elfv1 — change the current ABI to use the ELFv1 ABI. this is the default ABI for big-endian PowerPC 64-bit Linux. overriding the default ABI needs special system support and is likely to fail in spectacular ways. 💀 iconic honestly.
  • -mabi=elfv2 — change the current ABI to use the ELFv2 ABI. this is the default ABI for little-endian PowerPC 64-bit Linux. overriding the default ABI requires special system support and is likely to fail in spectacular ways.
  • -mgnu-attribute
  • -mno-gnu-attribute — emit .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes section that specify ABI variations in function parameters or return values.
  • -mprototype
  • -mno-prototype — on System V.4 and embedded PowerPC systems, assume that all calls to variable argument functions are properly prototyped (or don't assume that, respectively). otherwise the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR") to indicate whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. with -mprototype, only calls to prototyped variable argument functions set or clear the bit.
  • -msim — on embedded PowerPC systems, assume the startup module is called sim-crt0.o and the standard C libraries are libsim.a and libc.a. this is the default for powerpc-*-eabisim configurations.
  • -mmvme — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
  • -mads — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
  • -myellowknife — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a.
  • -mvxworks — on System V.4 and embedded PowerPC systems, specify that you're compiling for a VxWorks system.
  • -memb — on embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate that eabi extended relocations are used.
  • -meabi
  • -mno-eabi — on System V.4 and embedded PowerPC systems, do (or do not) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specs. selecting -meabi means the stack is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. selecting -mno-eabi means the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and the -msdata option only uses "r13" to point to a single small data area. the -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options.
  • -msdata=eabi — on System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the .sdata2 section, which is pointed to by register "r2". put small initialized non-"const" global and static data in the .sdata section, which is pointed to by register "r13". put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. ⚠️ the -msdata=eabi option is incompatible with the -mrelocatable option. the -msdata=eabi option also sets the -memb option.
  • -msdata=sysv — on System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, which is pointed to by register "r13". put small uninitialized global and static data in the .sbss section, which is adjacent to the .sdata section. ⚠️ the -msdata=sysv option is incompatible with the -mrelocatable option.
  • -msdata=default
  • -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise compile code the same as -msdata=sysv.
  • -msdata=data — on System V.4 and embedded PowerPC systems, put small global data in the .sdata section. put small uninitialized global data in the .sbss section. but do not use register "r13" to address small data. this is the default behavior unless other -msdata options are used.
  • -msdata=none
  • -mno-sdata — on embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section.
  • -mreadonly-in-sdata — put read-only objects in the .sdata section as well. this is the default. ✨
  • -mblock-move-inline-limit=num — inline all block moves (like calls to "memcpy" or structure copies) that are less than or equal to num bytes. the minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. the default value is target-specific.
  • -mblock-compare-inline-limit=num — generate non-looping inline code for all block compares (like calls to "memcmp" or structure compares) that are less than or equal to num bytes. if num is 0, all inline expansion (non-loop and loop) of block compare is disabled. the default value is target-specific.
  • -mblock-compare-inline-loop-limit=num — generate an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. if the block length isn't constant, at most num bytes are compared before "memcmp" is called to compare the rest of the block. the default value is target-specific.
  • -mstring-compare-inline-limit=num — compare at most num string bytes with inline code. if the difference or end of string isn't found at the end of the inline compare, a call to "strcmp" or "strncmp" handles the rest of the comparison. the default is 64 bytes.
  • -G num — on embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS section. by default, num is 8. the -G num switch is also passed to the linker. 📌 all modules should be compiled with the same -G num value.
  • -mregnames
  • -mno-regnames — on System V.4 and embedded PowerPC systems, do (or do not) emit register names in the assembly language output using symbolic forms.
  • -mlongcall
  • -mno-longcall — by default assume 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).

Linkers are getting ✨smart✨ bestie. Some of them can spot an out-of-range call and just generate glue code on the fly — like, on the fly?? iconic. On those systems, long calls are unnecessary and they just make your code slower, so skip 'em. As of this writing, the AIX linker can do it, and so can the GNU linker for PowerPC/64. It's planned to add this feature to the GNU linker for 32-bit PowerPC systems too, so stay tuned fr fr.

On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline PLT call sequence (peep -mpltseq). But PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline PLT calls — no cap, they just can't.

On Darwin/PPC systems, #pragma longcall generates jbsr callee, L42, plus a branch island (glue code). The two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address and generates a bl callee if the PPC bl instruction reaches the callee directly; otherwise, the linker generates bl L42 to call the branch island. The branch island is appended to the body of the calling function; it computes the full 32-bit address of the callee and jumps to it. Very "I'll get there eventually" energy ngl 💅

On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the Darwin linker decides whether to use or discard it. Linker keeps the receipts, compiler just provides.

In the future, GCC may ignore all longcall specifications when the linker is known to generate glue. So don't get too attached okay 🫶

  • -mpltseq, -mno-pltseq — Implement (or don't implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and are enabled by default if the support is detected when configuring GCC, and, in the case of 32-bit PowerPC, if GCC is configured with --enable-secureplt. Heads up bestie: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together. It's giving incompatible, periodt.
  • -mtls-markers, -mno-tls-markers — Mark (or don't mark) calls to __tls_get_addr with a relocation specifying the function argument. The relocation allows the linker to reliably associate function call with argument setup instructions for TLS optimization, which in turn allows GCC to better schedule the sequence. Scheduling slay 💖
  • -mrecip, -mno-recip — This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. You should use the -ffast-math option when using -mrecip (or at least -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Note 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. So, faster but slightly messier, pick your fighter ⚖️
  • -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. Cherry-picking, but make it compiler flags 🍒

  • -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. That's a whole lotta function names, but we keep every single one bestie 💜
  • -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. ⚠️ Too big = wrong answer, be careful.
  • -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. Like, that door closes fr.
  • -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. Legacy support, but make it cute 🎀

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.
  • -mdense-math, -mno-dense-math — Generate (or don't generate) Dense Math Facility (DMF) instructions. The -mdense-math option requires that the option -mcpu=future (or later) is enabled.
  • -mrop-protect, -mno-rop-protect — Generate (or don't generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). Trade-offs, bestie.
  • -mprivileged, -mno-privileged — Generate (or don't generate) code that runs 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.
  • -msplat-word-constant, -mno-splat-word-constant — Generate (or don't generate) code that uses the XXSPLTIW instruction. This option is enabled by default.
  • -msplat-float-constant, -mno-splat-float-constant — Generate (or don't generate) code that uses the XXSPLTIDP instruction. This option is enabled by default.
  • -mieee128-constant, -mno-ieee128-constant — Generate (or don't generate) code that uses the LXVKQ instruction. This option is enabled by default.
  • -mwarn-altivec-long, -mno-warn-altivec-long — Enable or disable warnings about deprecated vector long ... Altivec type usage. This option is enabled by default. Deprecated?? yeah, the warnings got your back ✨

🦋 RX Options 🦋

These command-line options are defined for RX targets:

  • -m64bit-doubles, -m32bit-doubles — bestie, this one just picks how thicc the double data type is: -m64bit-doubles makes it 64 bits, -m32bit-doubles makes it 32 bits. The default is -m32bit-doubles. And the reason for that default? The RX floating-point hardware only works on 32-bit values, periodt. So like, the default is literally just following the hardware's lead. 💅
  • -fpu, -nofpu — turns the use of RX floating-point hardware on (-fpu) or off (-nofpu). Default is enabled for the RX600 series, disabled for the RX200 series. But here's the tea: floating-point instructions are only generated for 32-bit floating-point values, so if you went and used -m64bit-doubles, the FPU hardware is not used for doubles. Fumbled, honestly.
⚠️ Note: if -fpu is enabled, then -funsafe-math-optimizations is also enabled automatically. Why? Because the RX FPU instructions are themselves unsafe. Not a vibe, just the truth. 😭
  • -mcpu=name — picks which type of RX CPU you're targeting. Right now three types are supported: the generic RX600 and RX200 series hardware, plus the specific RX610 CPU. Default is RX600.

The only difference between RX600 and RX610 is that the RX610 does not support the MVTIPL instruction. That's it, that's the whole difference. 🦋

The RX200 series does not have a hardware floating-point unit, so -nofpu is enabled by default when this type is selected. It's giving "no FPU, no problem."

  • -mbig-endian-data, -mlittle-endian-data — stores data (but not code) in big-endian format. Default is -mlittle-endian-data, i.e. store data in little-endian format. Choose your fighter. ✨
  • -msmall-data-limit=N — sets the maximum size in bytes of global and static variables that can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of the area is limited, and it is up to the programmer to make sure the area does not overflow. Also, when the small data area is used, one of the RX's registers (usually r13) is reserved for pointing to this area, so it is no longer available for use by the compiler. That could result in slower and/or larger code if variables get pushed onto the stack instead of being held in this register. Trade-offs, bestie. 🙈

Note, common variables (variables that have not been initialized) and constants are not placed into the small data area, since they are assigned to other sections in the output executable.

The default value is zero, which disables this feature. And note: this feature is not enabled by default with higher optimization levels (-O2 etc.) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen. 🔍

  • -msim, -mno-sim — use the simulator runtime. The default is to use the libgloss board-specific runtime. 🎀
  • -mas100-syntax, -mno-as100-syntax — when generating assembler output, use a syntax compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions, so it is not generated by default.
  • -mmax-constant-size=N — specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Now, the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, but a longer value equates to a longer instruction. So in some circumstances it can be beneficial to restrict the size of constants used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection. 🧚

The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed.

  • -mrelax — enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default. It's giving ✨optimization✨ but you have to ask for it.
  • -mint-register=N — specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means register r13 is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves r13 and r12. A value of 3 reserves r13, r12 and r11, and a value of 4 reserves r13 through r10. A value of 0, the default, does not reserve any registers. Clear as crystal, no cap. 💎
  • -msave-acc-in-interrupts — specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator, as this makes the interrupt handlers faster. Speed vs. safety, the eternal lore. 🔥
  • -mpid, -mno-pid — enables the generation of position-independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option. 🫶

Note, using this feature reserves a register, usually r13, for the constant data base address. This can result in slower and/or larger code, especially in complicated functions. 😭

The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register r13 and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line. Genius but chaotic.

By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.

  • -mno-warn-multiple-fast-interrupts, -mwarn-multiple-fast-interrupts — prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt. GCC is just looking out for you. 🥺
  • -mallow-string-insns, -mno-allow-string-insns — enables or disables the use of the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL, SWHILE and also the RMPA instruction. These instructions may prefetch data, which is not safe to do if accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more information.) 📖

The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. Instead it is reliant upon the programmer to use the -mno-allow-string-insns option if their program accesses I/O space. Your move, bestie.

When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__.

  • -mjsr, -mno-jsr — use only (or not only) JSR instructions to access functions. This option can be used when code size exceeds the range of BSR instructions. And heads up: -mno-jsr does not mean to not use JSR, but instead means that any type of branch may be used. Read that twice, bestie. 👀
  • -mlra — use the new LRA register allocator. By default, the old reload allocator is used. Glow-up for the register allocator, honestly. ✨
Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the interrupt function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers r10, r11, r12 and/or r13 and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options. 💜

S/390 and zSeries Options 👑

These are the -m options defined for the S/390 and zSeries architecture. Mainframe era, but make it cute. 🌸

  • -mhard-float, -msoft-float — use (or do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default. ⚡
  • -mhard-dfp, -mno-hard-dfp — use (or do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. Decimal floats are lowkey underrated. 🔢
  • -mlong-double-64, -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default. They're the same picture. 🖼️
  • -mbackchain, -mno-backchain — store (or do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area. 📚

In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float. 🌈

The default is to not maintain the backchain.

  • -mpacked-stack, -mno-packed-stack — okay so do we use the packed stack layout or don't we 🎀 When you say -mno-packed-stack, the compiler uses all the fields of the 96/160 byte register save area strictly for their default purpose — and the unused fields still eat up stack space, like they're just sitting there renting a room they never use 💀 But when you say -mpacked-stack, the register save slots get densely packed at the top of the register save area, and that unused space gets reused for other stuff, which means way more efficient use of the stack space you actually have. iconic BUT — and this is the catch bestie — when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. no exceptions 💅

As long as the stack frame backchain isn't used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. But heads up: some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes 😭 and that kind of code is not call-compatible with anything compiled with -mpacked-stack. Also, the combination of -mbackchain, -mpacked-stack and -mhard-float? Not supported. Not a vibe. If you wanna build a linux kernel, use -msoft-float 🫶

The default is to not use the packed stack layout.
  • -msmall-exec, -mno-small-exec — generate (or don't generate) code that uses the "bras" instruction to do subroutine calls ✨ This only works reliably if the total executable size does not exceed 64k, so like... it's giving "small and cute, do not push it" 🎀 The default is to use the "basr" instruction instead, which doesn't have that limitation at all. slay.
  • -m64 — okay so after the removal of -m31, the option -m64 literally has no counterpart anymore 🙈 For the sake of backwards compatibility it's still accepted, but it does not do anything anymore. So the generated code comprises 64-bit instructions and follows the GNU/Linux for zSeries ABI by default. it's just out here existing 💅
  • -mzarch — same story bestie: after the removal of -mesa, -mzarch has no counterpart anymore 😭 Again, backwards compatibility means it's still accepted but has no effect anymore. So the generated code comprises instructions available on z/Architecture by default. periodt.
  • -mhtm, -mno-htm — the -mhtm option enables a set of builtins that make use of the instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions 🔥 And -mhtm is enabled by default when you're using -march=zEC12. no cap.
  • -mvx, -mno-vx — when you specify -mvx, you generate code using the instructions available with the vector extension facility introduced with the IBM z13 machine generation ⭐ This option changes the ABI for some vector type values with regard to alignment and calling conventions, which is lowkey a big deal. In case vector type values are being used in an ABI-relevant context, a GAS .gnu_attribute command gets added to mark the resulting binary with the ABI used 💅 And -mvx is enabled by default when you're using -march=z13.
  • -mzvector, -mno-zvector — the -mzvector option enables vector language extensions and builtins using the instructions available with the vector extension facility introduced with the IBM z13 machine generation 🦋 This option adds support for vector to be used as a keyword to define vector type variables and arguments. BUT vector is only available when GNU extensions are enabled, bestie — it is not expanded when you're requesting strict standard compliance, e.g. with -std=c99 🙅 On top of the GCC low-level builtins, -mzvector enables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. And in order to make use of these builtins, you must include the header file vecintrin.h 📎 Also -mzvector is disabled by default. iykyk.
  • -mmvcle, -mno-mvcle — generate (or do not generate) code that uses the "mvcle" instruction to perform block moves ✨ When you specify -mno-mvcle, a "mvc" loop gets used instead. That's the default unless you're 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 the CPU type can be used to select the best architecture option for the host processor 💖 but -march=native has no effect if GCC does not recognize the processor. it's giving "we tried" 😭

  • -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 exact same as for -march, and the default is the value used for -march. cute, consistent, we love that 💅
  • -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. rude but okay 🙈
  • -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 that provide the ability to selectively skip function trace entries for the TPF OS ✨ This option is off by default too — even when compiling for the TPF OS and specifying -mtpf-trace 💀
  • -mmain — for the TPF OS target, use this option when linking to add the startup code and other linker options so you can produce a main object 👑 main character energy fr fr.
  • -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. easy.
  • -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 actually runs — it's just here to identify functions that most probably cause a stack overflow. Super useful for environments with limited stack size, e.g. the Linux kernel 🫶
A value of zero disables this warning; that is the default.
  • -mwarn-dynamicstack, -mno-warn-dynamicstack — emit a warning if the function calls alloca or uses dynamically-sized arrays 😭 and ngl, this is generally a bad idea when you've got a limited stack size. bestie please.
  • -mstack-guard=stack-guard, -mstack-size=stack-size, -mno-stack-guard, -mno-stack-size — if these options are enabled, 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 💥 (and remember, bestie, the stack on S/390 grows downward — yes really 🌸). If the stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function gets chosen.

These options are intended to be used to help debug stack overflow problems 🦋 and the additionally emitted code causes only little overhead, so they can also be used in production-like systems without greater performance degradation. love that for us.

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 -mstack-guard= option can only be used in conjunction with -mstack-size= 🙅 You can override these options on the command line with -mno-stack-guard and -mno-stack-size, respectively.

  • -mhotpatch=pre-halfwords,post-halfwords — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit 🔥 The function label 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 ✨

  • -mpic-data-is-text-relative, -mno-pic-data-is-text-relative — when compiling for S/390 with -fpic or -fPIC, normally GCC emits code using relative addressing between code and data 🎀 However, for hotpatching it might be required to introduce new .text parts while using the existing .data and .bss sections. In that case, use -mno-pic-data-is-text-relative to force addressing through the GOT instead 💅
  • -mstack-protector-guard=guard, -mstack-protector-guard-record — generate stack protection code using a canary at guard 🐤 Supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (that one's the default).

The option -mstack-protector-guard-record results in the generation of a section __stack_protector_loc containing pointers to all instructions which load the address of the global guard 👑 So this option only has an effect in conjunction with -mstack-protector-guard=global. The intended use is for the Linux kernel 🫶

  • -mindirect-branch=choice, -mindirect-branch-jump=choice, -mindirect-branch-call=choice, -mfunction-return=choice, -mfunction-return-mem=choice, -mfunction-return-reg=choice — this whole group of options addresses security vulnerabilities related to speculative execution and indirect branch prediction 🛡️ by enabling replacement of indirect branches and function returns with direct branches to a thunk. it's giving responsible queen behavior honestly 💅

Specifying a choice of keep causes generation of the default code. thunk triggers generation of out-of-line thunks and replaces the formerly indirect branch with a direct branch to the thunk. thunk-extern does the branch replacement just like thunk but does not emit the thunks. And thunk-inline is only available with -mindirect-branch-jump — it should be used in preference to thunk in user-space applications to support correct stack unwinding and exception handling ✨

-mindirect-branch sets the value of -mindirect-branch-jump and -mindirect-branch-call 🎀 and -mfunction-return sets the value of -mfunction-return-reg and -mfunction-return-mem. cute little inheritance situation.

All of these options can also be set on a per-function basis using function attributes 💖

  • -mindirect-branch-table, -mno-indirect-branch-table — okay so this one is the bestie of -mindirect-branch and -mfunction-return, they literally travel together 💖. Turn it on and it makes tables that point at all the branch locations that got patched by those two options. Those tables get emitted into sections named .s390_indirect_jump, .s390_indirect_call, .s390_return_reg, and .s390_return_mem. Each section is an array of 32-bit elements, and every entry holds the offset from the entry to the patched location. cute and precise, we love that for us ✨
  • -mfentry, -mno-fentry — when you pair this with -pg, it emits profiling code using the __fentry__ hook that GLIBC 2.29 or later provides. Only available for 64-bit code, bestie, no 32-bit crumbs here 🙈. If you don't enable this, profiling falls back to the less efficient _mcount hook instead. less efficient. we said what we said 💅
  • -mrecord-mcount, -mno-record-mcount — turn it on and it generates a __mcount_loc section listing the locations of all the generated profiling calls to _mcount or __fentry__. a little map of your profiling calls, iconic behavior 🎀
  • mnop-mcount, mno-nop-mcount — instead of generating calls to _mcount or __fentry__ with -pg, this inserts a sequence of no-op instructions of the same length right where those calls would have gone. You can run this alongside -mrecord-mcount to patch the call sequences into the object file without recompiling it. no recompile needed, we're just built different 🔥
  • -mpreserve-args, -mno-preserve-args — when it's on, all argument registers get saved to the stack and corresponding CFI information gets generated. Super useful if your app wants to implement its own stack unwinding and needs access to the function arguments. main character unwinding energy 👑
  • -munaligned-symbols, -mno-unaligned-symbols — assume that external symbols with a natural alignment of 1 are potentially unaligned. By default, every symbol without explicit alignment is assumed to be aligned on a 2-byte boundary, as the IBM Z ABI mandates. so it's literally the IBM Z ABI's rules, not mine 🌸

🦋 SH Options 🦋

These -m options are defined for the SH implementations:

  • -m1 — generate code for the SH1.
  • -m2 — generate code for the SH2.
  • -m2e — generate code for the SH2e.
  • -m2a-nofpu — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
  • -m2a-single-only — generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
  • -m2a-single — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
  • -m2a — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
  • -m3 — generate code for the SH3.
  • -m3e — generate code for the SH3e.
  • -m4-nofpu — generate code for the SH4 without a floating-point unit.
  • -m4-single-only — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
  • -m4-single — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
  • -m4 — generate code for the SH4.
  • -m4-100 — generate code for SH4-100.
  • -m4-100-nofpu — generate code for SH4-100 in such a way that the floating-point unit is not used.
  • -m4-100-single — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
  • -m4-100-single-only — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
  • -m4-200 — generate code for SH4-200.
  • -m4-200-nofpu — generate code for SH4-200 without in such a way that the floating-point unit is not used.
  • -m4-200-single — generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
  • -m4-200-single-only — generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
  • -m4-300 — generate code for SH4-300.
  • -m4-300-nofpu — generate code for SH4-300 without in such a way that the floating-point unit is not used.
  • -m4-300-single — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
  • -m4-300-single-only — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
  • -m4-340 — generate code for SH4-340 (no MMU, no FPU).
  • -m4-400 — generate code for SH4-400 (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. so like, the option is there, the instructions aren't, ok bestie we're manifesting 🦋
  • -mb — compile code for the processor in big-endian mode.
  • -ml — compile code for the processor in little-endian mode.
  • -mdalign — align doubles at 64-bit boundaries. Heads up though: this changes the calling conventions, so some functions from the standard C library do not work unless you recompile it first with -mdalign. like literally recompile the lib, no shortcuts 😭
  • -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
  • -mbigtable — use 32-bit offsets in switch tables. The default is to use 16-bit offsets.
  • -mbitops — enable the use of bit manipulation instructions on SH2A.
  • -mfmovd — enable the use of the instruction fmovd. Check -mdalign for alignment constraints.
  • -mrenesas, -mno-renesas — comply with the calling conventions defined by Renesas. The default for all targets of the SH toolchain is -mno-renesas, which uses the calling conventions defined for GCC before the Renesas conventions were available. vintage GCC conventions, we respect the elders 💜
  • -mnomacsave — mark the MAC register as call-clobbered, even if -mrenesas is given.
  • -mieee, -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either -mieee or -mno-ieee. you always get the final say, bestie 💅
  • -minline-ic_invalidate, -mno-inline-ic_invalidate — inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the use of the icbi instruction. If the selected code generation option does not allow the use of the icbi instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. And that's a problem because it not only requires privileged mode at run time, it also fails if the cache line had been mapped via the TLB and has become unmapped. so like, drama, but documented drama 🙈
  • -misize — dump instruction size and location in the assembly code.
  • *-matomic-model=model* — sets the model of atomic operations and additional parameters as a comma-separated list. For details on the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
    • none — disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not sh*-*-linux*.
    • soft-gusa — generate GNU/Linux-compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is sh*-*-linux* and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions movli.l and movco.l to create more efficient code, unless strict is specified.
    • soft-tcb — generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well.
    • soft-imask — generate software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111. This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is sh*-*-linux* and SH1* or SH2*.
    • hard-llcs — generate hardware atomic sequences using the movli.l and movco.l instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32-bit atomic variables, access to 8- or 16-bit variables is emulated with 32-bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model.
    • gbr-offset= — this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when the soft-tcb model has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.
  • strict — this one is the vibe killer for chaos: it stops you from mixing multiple atomic models, even when they'd technically play nice together, and forces the compiler to emit atomic sequences from only the model you picked. one model, one aesthetic, periodt 💅
  • -mtas — emits the "tas.b" opcode for "__atomic_test_and_set". ⚠️ heads up bestie: depending on your exact hardware + software combo, this can tank overall performance, because the "tas.b" instruction implies operand cache line flushes. And on multi-core SH4A processors you gotta use "tas.b" with caution, because it can straight-up cause data corruption on certain cache configurations. lowkey scary fr 😭
  • -mprefergot — when you're generating position-independent code, this makes function calls go through the Global Offset Table instead of the Procedure Linkage Table. GOT > PLT, we said what we said ✨
  • -musermode, -mno-usermode — don't allow (allow) the compiler to generate privileged mode code. specifying -musermode also implies -mno-inline-ic_invalidate if the inlined code wouldn't work in user mode. -musermode is the default when the target is "sh*-*-linux*". and if your target is SH1* or SH2*, -musermode does literally nothing, since there's no user mode to speak of 🙈
  • -multcost=number — sets the cost to assume for a multiply instruction. 💸
  • -mdiv=strategy — sets the division strategy used for integer division operations. strategy can be one of:
    • call-div1 — calls a library function that uses the single-step division instruction "div1" to do the thing. 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 does the operation in double-precision floating point. division by zero causes a floating-point exception. this is the default for SHcompact with FPU. 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 without dynamic shift instructions defaults to "call-div1".

and when you haven't specified a division strategy at all? the default gets picked based on the current target. for SH2A the default strategy is to use the "divs" and "divu" instructions instead of library function calls. slay 🫶

  • -maccumulate-outgoing-args, -mno-accumulate-outgoing-args — reserve space once for outgoing arguments in the function prologue, instead of around every single call. this is generally a glow-up for both performance and size, and it's also needed for unwinding so the stack frame doesn't shift around conditional code. -maccumulate-outgoing-args is enabled by default. 👑
  • -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 — the compiler still expects the exact same sets of input/output/clobbered registers as if this option weren't there at all. no cap.
  • -mfixed-range=register-range — generates code treating the given register range as fixed registers. a fixed register is one the register allocator cannot touch, which is genuinely useful when you're compiling kernel code. a register range is written as two registers separated by a dash, and you can specify multiple register ranges separated by commas.
  • -mbranch-cost=num — assume num is the cost for a branch instruction. higher numbers make the compiler try harder to generate branch-free code where possible. if you don't specify it, the value is selected depending on the processor type you're compiling for.
  • -mzdcbranch, -mno-zdcbranch — assume (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. this is enabled by default when generating code for SH4 and SH4A, and you can explicitly turn it off with -mno-zdcbranch.
  • -mcbranch-force-delay-slot — forces 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. 💀
  • -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. using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect.
  • -mfsrra, -mno-fsrra — allow or disallow the compiler from emitting the "fsrra" instruction for reciprocal square root approximations. -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. it's enabled by default when generating code for SH4A. using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
  • -mpretend-cmove, -mno-pretend-cmove — prefer or don't prefer zero-displacement conditional branches for conditional move instruction patterns. this can result in faster code on the SH4 processor. 🏎️
  • -mfdpic, -mno-fdpic — generate code using the FDPIC ABI.

🌞 Solaris 2 Options 🌞

these options are supported on Solaris 2, bestie:

  • -mclear-hwcap, -mno-clear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. this is only necessary when object files use ISA extensions that the current machine doesn't support, but check at runtime whether or not to use them. ✨
  • -mimpure-text, -mno-impure-text — -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object. with this option you can link position-dependent code into a shared object.

-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error message. however 😭 the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. do it the right way, i'm begging.

  • -gsctf — generates Solaris CTF. needs to be used both for compilation and linking. see ctf(7) for more info. this is only supported since Solaris 11.4 SRU 84, where the necessary toolchain support was added. 🎀

☀️ SPARC Options ☀️

these -m options are supported on the SPARC:

  • -mno-app-regs, -mapp-regs — specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. like global register 1, each global register 2 through 4 then gets treated as an allocable register that's clobbered by function calls. this is the default.

to be fully SVR4 ABI-compliant — at the cost of some performance loss 🥲 — specify -mno-app-regs. you should compile libraries and system software with this option.

  • -mflat, -mno-flat — with -mflat, the compiler doesn't generate save/restore instructions and uses a "flat" or single register window model. this model is compatible with the regular register window model. the local registers and the input registers (0--5) are still treated as "call-saved" registers and get saved on the stack as needed.

with -mno-flat (the default), the compiler generates save/restore instructions — except for leaf functions. this is the normal operating mode.

  • -mfpu, -mhard-float — generate output containing floating-point instructions. this is the default. 🔥
  • -mno-fpu, -msoft-float — generate output containing library calls for floating point. warning: the requisite libraries are not available for all SPARC targets. normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation — you have to make your own arrangements to provide suitable library functions for cross-compilation. the embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

-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. all of it or nah, bestie.

  • -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 — which is exactly why -msoft-quad-float is the default. 🙈

  • -mno-unaligned-doubles, -munaligned-doubles — assume that doubles have 8-byte alignment. this is the default.

with -munaligned-doubles, GCC assumes doubles have 8-byte alignment only if they're contained in another type, or if they have an absolute address. otherwise it assumes they have 4-byte alignment. specifying this option avoids some rare compatibility problems with code generated by other compilers. it's not the default because it results in a performance loss — especially for floating-point code. 💅

  • -muser-mode, -mno-user-mode — do not generate code that can only run in supervisor mode. this is relevant only for the "casa" instruction emitted for the LEON3 processor. this is the default. ✨
  • -mfaster-structs, -mno-faster-structs — so with -mfaster-structs the compiler lowkey just decides that structs should be 8-byte aligned 🫶. Why? So it can copy whole structs with pairs of ldd and std instructions instead of doing twice as many ld/st pairs. Faster? Yes bestie. Legal? ABSOLUTELY NOT 💀 — that changed alignment straight-up violates the SPARC ABI. So only use it on targets where the dev fully acknowledges their code is not in line with the ABI rules. It's giving "I know what I'm doing" energy, so mean it.
  • -mstd-struct-return, -mno-std-struct-return — with -mstd-struct-return, the compiler drops checking code into functions that return structs or unions, so it can spot size mismatches between the two sides of a function call, exactly as the 32-bit ABI says 🔍. The default is -mno-std-struct-return. Also, heads up: this option does literally nothing in 64-bit mode 🙈.
  • -mcpu=cpu_type — sets the instruction set, register set, and instruction scheduling params 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 picks the best architecture option for the host processor 👑. But -mcpu=native has no effect if GCC doesn't recognize the processor — it just shrugs.

Default instruction scheduling params get used for values that select an architecture and not an implementation. Those are v7, v8, sparclite, sparclet, v9.

Here's every supported architecture and its 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 for the Cypress CY7C602 chip, the one used in the SPARCStation/SPARCServer 3xx series 🌸. That's also appropriate for the older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions that exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series 💅.

With -mcpu=sparclite, GCC generates code for the SPARClite variant. This adds the integer multiply, integer divide step and scan (ffs) instructions that exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes for the Fujitsu MB86930 chip — the original SPARClite, with no FPU. With -mcpu=f934, it additionally optimizes for the Fujitsu MB86934 chip, the more recent SPARClite with FPU ✨.

With -mcpu=sparclet, GCC generates code for the SPARClet variant. This adds the integer multiply, multiply/accumulate, integer divide step and scan (ffs) instructions that exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes for the TEMIC SPARClet chip 🎀.

With -mcpu=v9, GCC generates code for the V9 variant. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers, and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, it additionally optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, it additionally optimizes for Sun UltraSPARC T1 chips. With -mcpu=niagara2, it additionally optimizes for Sun UltraSPARC T2 chips. With -mcpu=niagara3, it additionally optimizes for Sun UltraSPARC T3 chips. With -mcpu=niagara4, it additionally optimizes for Sun UltraSPARC T4 chips. With -mcpu=niagara7, it additionally optimizes for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes for Oracle M8 chips 🔥.

  • -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set like -mcpu=cpu_type does 🙅. Same values work as for -mcpu=cpu_type, but the only useful ones are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. And with native Solaris and GNU/Linux toolchains, native can also be used 💜.
  • -mv8plus, -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The glow-up from the V8 ABI? The global and out registers are treated as 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 🌈. 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 (they come as a bundle, bestie).
  • -mvis3, -mno-vis3 — with -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions 💫. The default is -mvis3 when targeting a CPU that supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis.
  • -mvis3b, -mno-vis3b — with -mvis3b, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011 🎀. The default is -mvis3b when targeting a CPU that supports such instructions, such as niagara-7 and later. Setting -mvis3b also sets -mvis3, -mvis2 and -mvis.
  • -mvis4, -mno-vis4 — with -mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions 🔥. The default is -mvis4 when targeting a CPU that supports such instructions, such as niagara-7 and later. Setting -mvis4 also sets -mvis3b, -mvis3, -mvis2 and -mvis.
  • -mvis4b, -mno-vis4b — with -mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017 ✨. The default is -mvis4b when targeting a CPU that supports such instructions, such as m8 and later. Setting -mvis4b also sets -mvis4, -mvis3b, -mvis3, -mvis2 and -mvis.
  • -mcbcond, -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions 💅. The default is -mcbcond when targeting a CPU that supports such instructions, such as Niagara-4 and later.
  • -mfmaf, -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions 🧮✨. The default is -mfmaf when targeting a CPU that supports such instructions, such as Niagara-3 and later.
  • -mfsmuld, -mno-fsmuld — with -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction 🌸. The default is -mfsmuld when targeting a CPU supporting the architecture versions V8 or V9 with FPU — except -mcpu=leon, which is not invited to this party 🙈.
  • -mpopc, -mno-popc — with -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction 🔢. The default is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and later.
  • -msubxc, -mno-subxc — with -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction ➖. The default is -msubxc when targeting a CPU that supports such an instruction, such as Niagara-7 and later.
  • -mfix-at697f — enables the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor) 🩹.
  • -mfix-ut699 — enables the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor 🩹.
  • -mfix-ut700 — enables the documented workaround for the back-to-back store errata of the UT699E/UT700 processor 🩹.
  • -mfix-gr712rc — enables the documented workaround for the back-to-back store errata of the GR712RC processor 🩹.

Okay so these -m options are supported in addition to everything above, on SPARC-V9 processors in 64-bit environments 👑:

  • -m32, -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits 📏.
  • -mptr32, -mptr64 — use 32- or 64-bit pointers, respectively. Unlike -m32 and -m64, this affects only the pointer size and not the ABI 🙅.
  • -mcmodel=which — set the code model to one of:
    • medlow — the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked 🫶.
    • medmid — the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size, and the data segment must be located within 2GB of the text segment 📐.
  • medany — the Medium/Anywhere code model bestie 💫: 64-bit addresses, programs can get linked anywhere in memory, but the text and data segments have to stay under 2GB each, and the data segment has to sit within 2GB of the text segment. No pressure or anything.
  • embmedany — Medium/Anywhere but make it embedded 🦋: 64-bit addresses, text and data segments both under 2GB, both starting literally anywhere in memory (the linker decides, main character behavior). The global register %g4 points to the base of the data segment. Programs are statically linked, and PIC is not supported — sorry, it's just not that girl.
  • -mmemory-model=mem-model — sets the memory model in force on the processor. Pick one of these 💅:
    • default — the default memory model for the processor and operating system. Lowkey the safe choice.
    • 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. Iconic documentation energy, fr fr 📖✨

  • -mstack-bias
  • -mno-stack-bias — with -mstack-bias, GCC assumes the stack pointer (and the frame pointer if there is one) is offset by -2047, which you have to add back whenever you make stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. Simple as that 🫶

✨ Options for System V ✨

These extra options exist on System V Release 4, for compatibility with other compilers on those systems (yes, it's giving legacy, but she serves 💜):

  • -G — create a shared object. It is recommended that -symbolic or -shared be used instead. Like, technically works, but there are cuter options, no cap.
  • -YP,dirs — search the directories dirs, and no others, for libraries specified with -l. It's giving exclusive guest list 🎀
  • -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, and there are so many of them. Buckle up bestie, we're going through all of them:

  • -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. It's not avoiding the call, it's just going the scenic route ✨
  • -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. The -mep option is on by default if you optimize.
  • -mno-prolog-function
  • -mprolog-function — do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. Those external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize.
  • -mspace — try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options. Tiny code, tiny code, tiny code 🧚
  • -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). It's a small space but she's doing her best.
  • -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. (Yes, it says V850E3V5. We are quoting, not judging 🙈)
  • -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. Two names, one queen 👑
  • -mv850e — specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this option is used.

If none of -mv850, -mv850e, -mv850e1, -mv850e2, -mv850e2v3 or -mv850e3v5 are defined, then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.

The preprocessor constants __v850 and __v851__ are always defined, no matter which processor variant is the target. Ride or die constants 💜

  • -mdisable-callt
  • -mno-disable-callt — this option suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated, then the C preprocessor symbol __V850_CALLT__ is defined.

  • -mrelax
  • -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler.
  • -mlong-jumps
  • -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions.
  • -msoft-float
  • -mhard-float — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated, then the C preprocessor symbol __FPU_OK__ is defined; otherwise the symbol __NO_FPU__ is defined.
  • -mloop — enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected, because its use is still experimental. Bestie is still in beta, we're being cautious 🫶
  • -mrh850-abi
  • -mghs — enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI, the following rules apply:
    • Integer sized structures and unions are returned via a memory pointer rather than a register.
    • Large structures and unions (more than 8 bytes in size) are passed by value.
    • Functions are aligned to 16-bit boundaries.
    • The -m8byte-align command-line option is supported.
    • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported.

When this version of the ABI is enabled, the C preprocessor symbol __V850_RH850_ABI__ is defined.

  • -mgcc-abi — enables support for the old GCC version of the V850 ABI. With this version of the ABI, the following rules apply:
    • Integer sized structures and unions are returned in register r10.
    • Large structures and unions (more than 8 bytes in size) are passed by reference.
    • Functions are aligned to 32-bit boundaries, unless optimizing for size.
    • The -m8byte-align command-line option is not supported.
    • The -mdisable-callt command-line option is supported but not enabled by default.

When this version of the ABI is enabled, the C preprocessor symbol __V850_GCC_ABI__ is defined.

  • -m8byte-align
  • -mno-8byte-align — enables support for double and long long types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4 bytes. When -m8byte-align is in effect, the C preprocessor symbol __V850_8BYTE_ALIGN__ is defined. Alignment glow-up ✨
  • -mbig-switch
  • -mno-big-switch — generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of range branches within a switch table. It's giving emergency backup plan.
  • -mapp-regs
  • -mno-app-regs — -mapp-regs causes r2 and r5 to be used in the code generated by the compiler. This setting is the default. -mno-app-regs causes r2 and r5 to be treated as fixed registers.
  • -msmall-sld
  • -mno-small-sld — enable or disable the use of the short load instructions.
  • -mno-strict-align — do not enforce strict data alignment. Live a little 💅
  • -mjump-tables-in-data-section
  • -mno-jump-tables-in-data-section — enable or disable placement of jump tables for switch statements in the .data section rather than the .code section. The default is -mno-jump-tables-in-data-section.

🌈 VAX Options 🌈

These -m options are defined for the VAX, and they're vintage in the cutest way:

  • -munix — do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX cannot handle across long ranges.
  • -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used. Trust fall with GNU 🫶
  • -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. Yes, she's the mirror image of the one above. Balance 💖
  • -mlra
  • -mno-lra — enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload. Proceed with caution, it's giving "we're still testing this".
  • -mvaxc-alignment — use VAXC conventions for alignment of structure members.
  • -mqmath
  • -mno-qmath — enable or disable new instruction patterns for 64-bit integer addition and subtraction.

🎀 Visium Options 🎀

  • -mdebug — use this option when linking programs that perform file I/O and are destined to run on an MCM target. It causes the libraries libc.a and libdebug.a to be linked. You should run the program on the target under the control of the GDB remote debugging stub. Debug era, let's go 🐛✨
  • -msim — use this option when linking programs that perform file I/O and are destined to run on the simulator. It causes the libraries libc.a and libsim.a to be linked. Simulation station, all aboard.
  • -mfpu
  • -mhard-float — generate code containing floating-point instructions. This is the default.
  • -mno-fpu
  • -msoft-float — generate code containing library calls for floating-point.
  • -msoft-float — okay so this one changes the calling convention in the output file, which is a big deal, bestie. That means it's only useful if you compile all of a program with this option. Like, all of it. In particular, you need to compile libgcc.a — the library that ships with GCC — with -msoft-float for this to actually work. No cap.
  • -mcpu=cpu_type — sets the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. The supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 that's just there for backward compatibility (it's giving legacy era 💅).

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions.

  • -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that -mcpu=cpu_type would. Scheduling vibes only, no architecture glow-up.
  • -msv-mode — generates code for the supervisor mode, where there are literally no restrictions on the access to general registers. This is the default, bestie, you're already living here.
  • -muser-mode — generates code for the user mode, where access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected. So like, hands off those registers, they're not for you 🙈.

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. Pick your era: retro VMS or POSIX, your call.
  • -mdebug-main=prefix — flags the first routine whose name starts with prefix as the main routine for the debugger. Main character energy, but for the debugger specifically ✨.
  • -mmalloc64 — defaults to 64-bit memory allocation routines. Bigger allocation, bigger vibes.
  • -mpointer-size=size — sets 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". So yeah, no is committing to the 32-bit life fr fr.

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 — iykyk, go look them up with their target bestie.

  • -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__. The way it just flips you from kernel girlie to RTP girlie 💅.
  • -msmp — selects SMP runtimes for linking. Not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice. So like, on VxWorks 7+ the choice is made for you, periodt.
  • -mvthreads — links against the VxWorks AE vThread runtime environment.
  • -non-static — links an RTP executable against shared libraries rather than static libraries. The options -static and -shared can also be used for RTPs; -static is the default.
  • -Bstatic
  • -Bdynamic — these options are passed down to the linker. They're defined for compatibility with Diab. Just relaying the message, bestie 📬.
  • -Xbind-lazy — enables lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
  • -Xbind-now — disables lazy binding of function calls. This option is the default and is defined for compatibility with Diab. So yeah, eager binding is where we start 🌈.

x86 Options 🔥

This section documents -m options available for the x86 family of computers. We love an architecture with a whole glow-up section 💖.

The following group of options allows compilation to target a specific processor.

  • -march=cpu-type — generates instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.
⚠️ ok warning bestie: -march can absolutely produce code that will not run at all on other processors. It's giving "this outfit was made for one specific body" 💀.

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 do not have a corresponding -mtune setting, using -march with these values enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value.

  • i386 — original Intel i386 CPU.
  • i486 — Intel i486 CPU. (No scheduling is implemented for this chip. She's just here, sitting pretty.)
  • i586, pentium — Intel Pentium CPU with no MMX support.
  • lakemont — Intel Lakemont MCU, based on Intel Pentium CPU.
  • pentium-mmx — Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
  • pentiumpro — Intel Pentium Pro CPU with no MMX support.
  • i686 — when used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as generic.
  • pentium2 — Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.
  • pentium3, pentium3m — Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.
  • pentium-m — Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. Used by Centrino notebooks. The it-girl of laptop chips 💅.
  • pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.
  • prescott — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
  • nocona — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, CX16 and FXSR instruction set support.
  • core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support.
  • nehalem, corei7 — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.
  • westmere — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.
  • sandybridge, corei7-avx — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.
  • ivybridge, core-avx-i — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
  • haswell, core-avx2 — Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support.
  • broadwell — Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support.
  • skylake — Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support.
  • skylake-avx512 — Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support.
  • cascadelake — Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support.
  • cannonlake — Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support.
  • cooperlake — Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support.
  • icelake-client — Intel Ice Lake Client CPU, and she's stacked 💅 64-bit extensions plus: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support.
  • icelake-server — Intel Ice Lake Server CPU, 64-bit extensions, and the roster is: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support.
  • tigerlake — Intel Tiger Lake CPU, 64-bit extensions, serving looks with: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support.
  • rocketlake — Intel Rocket Lake CPU, 64-bit extensions, and she said: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support.
  • alderlake, raptorlake, meteorlake, gracemont — Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont CPU, 64-bit extensions, and the whole girliepop lineup: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support.
  • arrowlake — Intel Arrow Lake CPU, 64-bit extensions, and OMG the list: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT and CMPCCXADD instruction set support.
  • arrowlake-s, lunarlake — Intel Arrow Lake S/Lunar Lake CPU, 64-bit extensions, and it's giving: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3 and SM4 instruction set support.
  • pantherlake, wildcatlake — Intel Panther Lake/Wildcat Lake CPU, 64-bit extensions, and bestie the flex is real: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3 and SM4 instruction set support.
  • novalake — Intel Nova Lake CPU, 64-bit extensions, and she ATE: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, PREFETCHI, APX_F, AVX10.1, AVX10.2 and MOVRS instruction set support.
  • sapphirerapids, emeraldrapids — Intel Sapphire Rapids/Emerald Rapids CPU, 64-bit extensions, and the lineup is: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16 and AVX512BF16 instruction set support.
  • graniterapids — Intel Granite Rapids CPU, 64-bit extensions, and no cap the list goes hard: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16 and PREFETCHI instruction set support.
  • graniterapids-d — Intel Granite Rapids D CPU, 64-bit extensions, and she's serving: MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI and AMX-COMPLEX instruction set support.
  • diamondrapids — Intel Diamond Rapids CPU, 64-bit extensions, and this one is the main character fr: 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, 64-bit extensions, and it's a small but mighty moment 🌸: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3 and CX16 instruction set support.
  • silvermont, slm — Intel Silvermont CPU, 64-bit extensions, and she's got: 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, 64-bit extensions, and the vibe is: 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, 64-bit extensions, and plus-sized for a reason: 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, 64-bit extensions, and she's lowkey stacked: 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, 64-bit extensions, and bestie it's a whole mood 🌈: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support.

okay bestie we are STILL deep in the -march=cpu-type list and honestly? it never ends 💀 but we're locked in, we're eating this alphabetically, no skips, no shortcuts. carrying on:

  • grandridge — Intel Grand Ridge CPU, 64-bit extensions plus MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR instruction set support. ✨
  • clearwaterforest — Intel Clearwater Forest CPU, 64-bit extensions plus MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI instruction set support.
  • k6 — AMD K6 CPU with MMX instruction set support.
  • k6-2, k6-3 — the glow-up versions of the AMD K6, with MMX and 3DNow! instruction set support. 💅
  • athlon, athlon-tbird — AMD Athlon CPU, bringin' 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 built on the AMD K8 core with x86-64 instruction set support, and that means the AMD Opteron, Athlon 64, and Athlon 64 FX processors too. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions.) ✨
  • k8-sse3, opteron-sse3, athlon64-sse3 — the improved AMD K8 cores, now with SSE3 instruction set support. slay.
  • amdfam10, barcelona — CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets CX16, MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
  • bdver1 — CPUs based on AMD Family 15h cores with x86-64 instruction set support. (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.)

okay now we're entering the znver era and bestie, this is the main character of the AMD list, I fear. 👑

  • 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.)
  • znver6 — 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, AVXVNNIINT8, AVXIFMA, AVX512FP16, AVXNECONVERT, AVX512BMM and 64-bit instruction set extensions.) 🔥

and then the btver girls, we love them, underrated fr:

  • 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.

okay switching families entirely, hi IDT and VIA besties 🫶 (heads up: a LOT of the VIA ones got no scheduling, and the manpage says it every single time, so I'm saying it every single time, that's the deal):

  • winchip-c6 — IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
  • winchip2 — IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
  • c3 — VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
  • c3-2 — VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
  • c7 — VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
  • samuel-2 — VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
  • nehemiah — VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)
  • esther — VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
  • eden-x2 — VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)
  • eden-x4 — VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)
  • nano — generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
  • nano-1000 — VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
  • nano-2000 — VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)
  • nano-3000 — VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
  • nano-x2 — VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
  • nano-x4 — VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)

and last but SO not least, the ZHAOXIN entries, and one of these has a little gossip note attached so pay attention bestie:

  • lujiazui — ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, FXSR, RDSEED instruction set support. And here's the tea: while the CPUs do support AVX and F16C, these aren't enabled by -march=lujiazui for performance reasons. so don't go looking for them, they're not coming.
  • yongfeng — ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support. 🌈

okay bestie that's this chunk of the cpu list and yes there's more coming, don't you dare close the tab 💜

  • shijidadao — ZHAOXIN shijidadao CPU, slaying with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support. 💅
  • geode — AMD Geode, the lil embedded era icon 🎀, rocking MMX and 3DNow! instruction set support.
  • c86-4g-m4 — HYGON c86-4g-m4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support. ✨
  • c86-4g-m6 — HYGON c86-4g-m6 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support. 🌸
  • c86-4g-m7 — HYGON c86-4g-m7 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, GFNI, AVX512VNNI, VAES, AVX512BITALG, AVX512VPOPCNTDQ, AVX512VP2INTERSECT, AVXVNNI, VPCLMULQDQ, WBNOINVD instruction set support. 👑
  • c86-4g-m8 — HYGON c86-4g-m8 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, GFNI, AVX512VNNI, VAES, AVX512BITALG, AVX512VPOPCNTDQ, AVX512VP2INTERSECT, AVXVNNI, VPCLMULQDQ, WBNOINVD instruction set support. 💖

✨ -mtune=cpu-type ✨

okay bestie this one is the vibe tuner 💅 — it tells GCC to tune everything applicable about the generated code to cpu-type, except for the ABI and the set of available instructions. So it's not changing what instructions exist, just how nicely the code is arranged for that chip.

The way that it works: while picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. fr fr, it's a scheduling glow-up, not an architecture switch. 🫶

For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. iconic, no cap.

The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:

generic

Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But, if you do not know exactly what CPU users of your application will have, then you should use this option. 🌈

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. ✨

There is no -march=generic option because -march indicates the instruction set the compiler can use, and there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 💜

intel

Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Diamond Rapids and Clearwater Forest, then you should use this option. 👑

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released. 🔥

There is no -march=intel option because -march indicates the instruction set the compiler can use, and there is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or, in this case, collection of processors) for which the code is optimized. 💖

The following options allow more detailed control over which instruction set extensions are targeted by GCC. Each has a corresponding -mno-option to disable use of these instructions. 🙈

These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches.

These options enable GCC to use these extended instructions in generated code. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular, the file containing the CPU detection code should be compiled without these options. ⚠️

To control whether 387 or SSE/AVX instructions are generated automatically for floating-point arithmetic, see -mfpmath=, below.

  • -mmmx — Support MMX built-in functions.
  • -msse — Support MMX and SSE built-in functions and code generation.
  • -msse2 — Support MMX, SSE and SSE2 built-in functions and code generation.
  • -msse3 — Support MMX, SSE, SSE2 and SSE3 built-in functions and code generation.
  • -mssse3 — Support MMX, SSE, SSE2, SSE3 and SSSE3 built-in functions and code generation.
  • -msse4.1 — Support MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 built-in functions and code generation.
  • -msse4.2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.
  • -msse4 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.
Note that -msse4 enables both SSE4.1 and SSE4.2 support, while -mno-sse4 turns off those features; neither form of the option affects SSE4A support, controlled separately by -msse4a. 💅
  • -msse4a — Support MMX, SSE, SSE2, SSE3 and SSE4A built-in functions and code generation.
  • -mavx — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and AVX built-in functions and code generation.
  • -mavx2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 built-in functions and code generation.
  • -mavx512f — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVX512F built-in functions and code generation.
  • -mavx512cd — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512CD built-in functions and code generation.
  • -mavx512vl — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VL built-in functions and code generation.
  • -mavx512bw — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BW built-in functions and code generation.
  • -mavx512dq — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512DQ built-in functions and code generation.
  • -mavx512ifma — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512IFMA built-in functions and code generation.
  • -mavx512vbmi — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI built-in functions and code generation.
  • -mavx512vpopcntdq — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VPOPCNTDQ built-in functions and code generation.
  • -mavx512vp2intersect — Support AVX512VP2INTERSECT built-in functions and code generation.
  • -mavx512vnni — Support AVX512VNNI built-in functions and code generation.
  • -mavx512vbmi2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI2 built-in functions and code generation.
  • -mavx512bf16 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BF16 built-in functions and code generation.
  • -mavx512fp16 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512-FP16 built-in functions and code generation.
  • -mavx512bitalg — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BITALG built-in functions and code generation.
  • -mavx512bmm — Support AVX512BMM built-in functions and code generation.
  • -mavxvnni — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXVNNI built-in functions and code generation.
  • -mavxifma — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXIFMA built-in functions and code generation.
  • -mavxvnniint8 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT8 built-in functions and code generation.
  • -mavxneconvert — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXNECONVERT build-in functions and code generation.
  • -mavxvnniint16 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT16 built-in functions and code generation.
  • -mavx10.1 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVX10.1 built-in functions and code generation.
  • -mavx10.2 — Support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1 and AVX10.2 built-in functions and code generation.
  • -msha — Support SHA1 and SHA256 built-in functions and code generation.
  • -maes — Support AES built-in functions and code generation.
  • -mpclmul — Support PCLMUL built-in functions and code generation.
  • -mclflushopt — Support CLFLUSHOPT instructions.
  • -mclwb — Support CLWB instruction.
  • -mfsgsbase — Support FSGSBASE built-in functions and code generation.
  • -mptwrite — turns on support for PTWRITE built-in functions and code generation. bestie said "write it" and gcc said "say less" ✍️
  • -mrdrnd — turns on RDRND built-in functions and code generation.
  • -mf16c — turns on F16C built-in functions and code generation.
  • -mfma — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and FMA built-in functions and code generation. it's giving whole-stack energy 💅
  • -mfma4 — support for FMA4 built-in functions and code generation.
  • -mpconfig — turns on PCONFIG built-in functions and code generation.
  • -mwbnoinvd — turns on WBNOINVD built-in functions and code generation.
  • -mprfchw — support for the PREFETCHW instruction.
  • -mrdpid — turns on RDPID built-in functions and code generation.
  • -mrdseed — support for the RDSEED instruction. seed girlie 🌱
  • -msgx — turns on SGX built-in functions and code generation.
  • -mxop — turns on XOP built-in functions and code generation.
  • -mlwp — turns on LWP built-in functions and code generation.
  • -m3dnow — support for 3DNow! built-in functions.
  • -m3dnowa — support for Athlon 3Dnow! built-in functions.
  • -mpopcnt — support for code generation of the popcnt instruction.
  • -mabm — support for code generation of Advanced Bit Manipulation (ABM) instructions.
  • -madx — support for flag-preserving add-carry instructions.
  • -mbmi — turns on BMI built-in functions and code generation.
  • -mbmi2 — turns on BMI2 built-in functions and code generation.
  • -mlzcnt — support for the LZCNT built-in function and code generation.
  • -mfxsr — support for the FXSAVE and FXRSTOR instructions.
  • -mxsave — support for the XSAVE and XRSTOR instructions.
  • -mxsaveopt — support for the XSAVEOPT instruction.
  • -mxsavec — support for the XSAVEC instructions.
  • -mxsaves — support for the XSAVES and XRSTORS instructions.
  • -mrtm — turns on RTM built-in functions and code generation.
  • -mhle — support for Hardware Lock Elision prefixes.
  • -mtbm — turns on TBM built-in functions and code generation.
  • -mmwaitx — turns on MWAITX and MONITORX built-in functions and code generation.
  • -mclzero — turns on CLZERO built-in functions and code generation.
  • -mpku — turns on PKU built-in functions and code generation.
  • -mgfni — turns on GFNI built-in functions and code generation.
  • -mvaes — turns on VAES built-in functions and code generation.
  • -mwaitpkg — turns on WAITPKG built-in functions and code generation.
  • -mvpclmulqdq — turns on VPCLMULQDQ built-in functions and code generation.
  • -mmovdiri — turns on MOVDIRI built-in functions and code generation.
  • -mmovdir64b — turns on MOVDIR64B built-in functions and code generation.
  • -menqcmd — turns on ENQCMD built-in functions and code generation.
  • -muintr — turns on UINTR built-in functions and code generation.
  • -mtsxldtrk — turns on TSXLDTRK built-in functions and code generation.
  • -mcldemote — turns on CLDEMOTE built-in functions and code generation.
  • -mserialize — turns on SERIALIZE built-in functions and code generation.
  • -mamx-tile — turns on AMX-TILE built-in functions and code generation.
  • -mamx-int8 — turns on AMX-INT8 built-in functions and code generation.
  • -mamx-bf16 — turns on AMX-BF16 built-in functions and code generation.
  • -mhreset — turns on HRESET built-in functions and code generation.
  • -mkl — turns on KL built-in functions and code generation.
  • -mwidekl — turns on WIDEKL built-in functions and code generation.
  • -mcmpccxadd — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and CMPCCXADD build-in functions and code generation.
  • -mamx-fp16 — turns on AMX-FP16 built-in functions and code generation.
  • -mprefetchi — turns on PREFETCHI built-in functions and code generation.
  • -mraoint — turns on RAOINT built-in functions and code generation.
  • -mamx-complex — turns on AMX-COMPLEX built-in functions and code generation.
  • -msm3 — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM3 built-in functions and code generation.
  • -msm4 — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM4 built-in functions and code generation.
  • -msha512 — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SHA512 built-in functions and code generation.
  • -mapxf — support for code generation for APX features, including EGPR, PUSH2POP2, NDD, PPX, NF, CCMP and ZU. that's a whole acronym mood board fr fr
  • -musermsr — turns on USER_MSR built-in functions and code generation.
  • -mamx-avx512 — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AMX-AVX512 built-in functions and code generation.
  • -mamx-fp8 — turns on AMX-FP8 built-in functions and code generation.
  • -mmovrs — turns on MOVRS built-in functions and code generation.
  • -mamx-movrs — turns on AMX-MOVRS built-in functions and code generation.
  • -mavx10v2aux — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1 and AVX10V2AUX built-in functions and code generation.
  • -macev1 — support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1 and ACEV1 built-in functions and code generation.

These additional options are available for the x86 processor family. 🖥️✨

  • -mfpmath=unit — generate floating-point arithmetic for the selected unit, aka unit. the choices for unit are, and yes we're going through all of them:
    • 387 — use the standard 387 floating-point coprocessor that's 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, which results in slightly different results compared to most of other chips. see -ffloat-store for a more detailed description, iykyk.

this is the default choice for non-Darwin x86-32 targets.

  • sse — use scalar floating-point instructions present in the SSE instruction set. this instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. the earlier version of the SSE instruction set supports only single-precision arithmetic, so 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 it may break some existing code that expects temporaries to be 80 bits. 😭

this is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.

GCC depresses SSE instructions when -mavx (or another option enabling AVX extensions) is used. Instead, it generates new AVX instructions or AVX equivalents for all SSE instructions when needed.
  • sse,387
  • sse+387
  • both — attempt to utilize both instruction sets at once. this effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. use this option with care, because it is still experimental — the GCC register allocator does not model separate functional units well, which results in unstable performance. it's giving chaos but make it floating point 💀
  • -mieee-fp
  • -mno-ieee-fp — control whether or not the compiler uses IEEE floating-point comparisons. these correctly handle the case where the result of a comparison is unordered.
  • -m80387
  • -mhard-float — generate output containing 80387 instructions for floating point.
  • -mno-80387
  • -msoft-float — generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. you must make your own arrangements to provide suitable library functions for cross-compilation. 🙈

on machines where a function returns floating-point results in the 80387 register stack, some floating-point opcodes may be emitted even if -msoft-float is used.

  • -mno-fp-ret-in-387 — do not use the FPU registers for return values of functions.

the usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there is no FPU. the idea is that the operating system should emulate an FPU.

the option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.

  • -mno-fancy-math-387 — some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. specify this option to avoid generating those instructions. this option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. these instructions are not generated unless you also use the -funsafe-math-optimizations switch.
  • -malign-double
  • -mno-align-double — control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a one-word boundary. aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory. 📐

on x86-64, -malign-double is enabled by default.

Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch.
  • -m96bit-long-double
  • -m128bit-long-double — these switches control the size of the "long double" type. the x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

okay so bestie, modern architectures (Pentium and newer) are lowkey obsessed with long double being aligned to an 8- or 16-byte boundary. But in arrays or structures conforming to the ABI, that's just not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. ✨

In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on a 16-byte boundary. 💅

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double. No cap. 🙈

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch. Periodt. 💀

  • -mlong-double-64, -mlong-double-80, -mlong-double-128 — these switches control the size of the long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the long double type equivalent to the __float128 type. This is the default for 64-bit Bionic C library. ⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing long double variables, as well as modifying the function calling convention for functions taking long double. Hence they are not binary-compatible with code compiled without that switch. 😭
  • -malign-data=type — control how GCC aligns variables. Supported values for type are: compat uses an increased alignment value compatible with GCC 4.8 and earlier, abi uses the alignment value as specified by the psABI, and cacheline uses an increased alignment value to match the cache line size. compat is the default. 🎀
  • -mlarge-data-threshold=threshold — when -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold are placed in large data sections. The default is 65535. 👑
  • -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning. This saves one instruction in the caller since there is no need to pop the arguments there. You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl. ⚠️ Warning: this calling convention is incompatible with the one normally used on Unix, so you 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 — control how many registers are used to pass integer arguments. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm. ⚠️ Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. 🚨
  • -msseregparm — use SSE register passing conventions for float and double arguments and return values. You can control this behavior for a specific function by using the function attribute sseregparm. ⚠️ Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. 🫶
  • -mvect8-ret-in-mem — return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. 🧚
  • -mpc32, -mpc64, -mpc80 — set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision); and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. 👀 Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. 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, -mno-daz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. The -Ofast, -ffast-math, or -funsafe-math-optimizations options normally link in startup code that sets these flags except when building a shared library (-shared). You can use the -mdaz-ftz and -mno-daz-ftz options to explicitly enable or disable setting these flags, regardless of other options passed to GCC. 🌈
  • -mstackrealign — realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. 💅
  • -mstack-arg-probe — emit stack probing code in the function prologue. 🛠️
  • -mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits). ⚠️ Warning: when generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to an 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 💀
  • -mincoming-stack-boundary=num — assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used. On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. To ensure proper alignment of these values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 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. 🎀
  • -mdump-tune-features — this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in -mtune-ctrl=feature-list. 📋
  • -mtune-ctrl=feature-list — this option is used to do fine-grain control of x86 code generation features. feature-list is a comma-separated list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not preceded with ^; otherwise, it is turned off. -mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. 🚨
  • -mno-default — this option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and -mdump-tune-features. ✨
  • -mcld — this tells GCC to drop a cld instruction right into the prologue of functions that use string instructions. 💅 And bestie, there's lore here: string instructions read the DF flag to decide between autoincrement and autodecrement mode. The ABI says DF is cleared on function entry, but some operating systems straight-up violate that spec and don't clear DF in their exception dispatchers. 😭 So your exception handler can get invoked with DF still set, and then string instructions go the wrong direction mode. Iconic of them, honestly. You can flip this on by default for 32-bit x86 targets by configuring GCC with the --enable-cld configure option. And if it's already the default and you want it gone, -mno-cld suppresses generation of cld instructions.
  • -mvzeroupper — this one makes GCC emit a vzeroupper instruction before a transfer of control flow out of the function, so you minimize the AVX to SSE transition penalty and it gets rid of unnecessary zeroupper intrinsics. double win, no cap 🫶
  • -mstv, -mno-stv — enable/disable the Scalar to Vectorization pass, which transforms 64-bit integer computation into vector ones. Heads up: this optimization is restricted to -O2 and higher. ✨
  • -mprefer-avx128 — tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. it's giving "smaller, but make it intentional" 💅
  • -mprefer-vector-width=opt — tells GCC to use opt-bit vector width in instructions instead of the default on the selected platform.
  • -mpartial-vector-fp-math, -mno-partial-vector-fp-math — this one lets GCC generate floating-point operations that might affect the set of floating-point status flags on partial vectors, i.e. where vector elements live in the low part of the 128-bit SSE register. Unless -fno-trapping-math is specified, the compiler guarantees correct behavior by sanitizing all input operands so they have zeroes in the unused upper part of the vector register. ⭐ But — and hear me out — if you use built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, and that can cause possible performance issues when -fno-trapping-math is in effect. 😬 You can mitigate these issues by manually sanitizing the upper part of the partial vector argument register, or by using -mdaz-ftz to set the denormals-are-zero (DAZ) flag in the MXCSR register.

This option is enabled by default. 🎀

  • -mmove-max=bits — instructs GCC to set the maximum number of bits that can be moved from memory to memory efficiently to bits. Valid bits are 128, 256 and 512.
  • -mstore-max=bits — instructs GCC to set the maximum number of bits that can be stored to memory efficiently to bits. Valid bits are 128, 256 and 512.
    • none — no extra limitations applied to GCC other than defined by the selected platform.
    • 128 — prefer 128-bit vector width for instructions.
    • 256 — prefer 256-bit vector width for instructions.
    • 512 — prefer 512-bit vector width for instructions.
  • -mnoreturn-no-callee-saved-registers — this one optimizes functions with the noreturn attribute or the _Noreturn specifier by not saving callee-saved registers in the function prologue — the ones that are used in the function, that is (except for the BP register). ⚠️ Warning time: 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. you've been warned, bestie 💀
  • -mcx16 — lets GCC generate CMPXCHG16B instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. Super useful for atomic updates of data structures that are bigger than one machine word. The compiler uses this instruction to implement __sync Builtins. 🔥
  • -msahf — enables generation of SAHF instructions in 64-bit code. Little history lesson: 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 that AMD64 supports. Those two are load and store instructions (respectively) for certain status flags. In 64-bit mode, the SAHF instruction is used to optimize the fmod, drem, and remainder built-in functions; see Other Builtins for details. 📚
  • -mmovbe — enables use of the movbe instruction to optimize byte swapping of four and eight byte entities. byte swap but make it fast 💫
  • -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). 🛡️
  • -mcrc32 — enables built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the crc32 machine instruction. four builtins, one instruction, we love a lineup 💅
  • -mmwait — enables built-in functions __builtin_ia32_monitor and __builtin_ia32_mwait to generate the monitor and mwait machine instructions. sleep tight, CPU 🌙
  • -mrecip — enables use of RCPSS and RSQRTSS instructions (plus their vectorized variants RCPPS and RSQRTPS) with an additional Newton-Raphson step to boost 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. so like... fast but slightly delulu 😭

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 an additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip. 🫶

  • -mrecip=opt — controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, and any of them may be preceded by a ! to invert the option. comma-separated AND invertible, the range??
    • all — enable all estimate instructions.
    • default — enable the default instructions, equivalent to -mrecip.
    • none — disable all estimate instructions, equivalent to -mno-recip.
    • div — enable the approximation for scalar division.
    • vec-div — enable the approximation for vectorized division.
    • sqrt — enable the approximation for scalar square root.
    • vec-sqrt — enable the approximation for vectorized square root.

So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root. 💅

  • -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library, aocl for the math library (LibM) from AMD Optimizing CPU Libraries (AOCL), and acml for the end-of-life AMD core math library (to which AOCL-LibM is the successor). To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. ⚠️ both, not one. BOTH.

GCC currently emits calls to vmldExp2, vmldLn2, vmldLog102, vmldPow2, vmldTanh2, vmldTan2, vmldAtan2, vmldAtanh2, vmldCbrt2, vmldSinh2, vmldSin2, vmldAsinh2, vmldAsin2, vmldCosh2, vmldCos2, vmldAcosh2, vmldAcos2, vmlsExp4, vmlsLn4, vmlsLog104, vmlsPow4, vmlsTanh4, vmlsTan4, vmlsAtan4, vmlsAtanh4, vmlsCbrt4, vmlsSinh4, vmlsSin4, vmlsAsinh4, vmlsAsin4, vmlsCosh4, vmlsCos4, vmlsAcosh4 and vmlsAcos4 for corresponding function type when -mveclibabi=svml is used, 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 __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. 📜 yes, that's the whole list, and yes, I typed every single one. you're welcome. 💖

  • -mabi=name — generates code for the specified calling convention. Permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes ms_abi and sysv_abi. 🤝
  • -masm=dialect — outputs assembly instructions using the selected dialect. It also affects which dialect is used for basic asm and extended asm. Supported choices (in dialect order) are att or intel. The default is att. 🚨 Darwin does not support intel.
  • -mforce-indirect-call — bestie, this one makes every single call to a function happen indirectly, no exceptions. Why would you want that? Because Intel Processor Trace eats it UP — it gives you way more precise timing info for function calls. It's giving ✨traceable✨.
  • -mmanual-endbr — this drops an ENDBR instruction at function entry only via the "cf_check" function attribute. Super handy when you're pairing it with -fcf-protection=branch so you can be the one in control of where ENDBR lands at the function entry. main character energy, honestly 💅.
  • -mcet-switch — okay so, by default CET instrumentation is turned off on switch statements that use a jump table, and indirect branch track is disabled. And like, that's fine-ish? Because jump tables live in read-only memory, so it's not a direct loss of hardening. BUT 👀 — if the jump table index is attacker-controlled, that indirect jump might not be constrained by CET. Yikes. This option turns CET instrumentation on to enable indirect branch track for switch statements with jump tables, which makes the jump targets reachable via any indirect jumps. Locked down 🔒.
  • -mcall-ms2sysv-xlogues — the 64-bit ABIs are doing the absolute most and don't agree, so any Microsoft ABI function that calls a System V ABI function has to treat RSI, RDI and XMM6-15 as clobbered. Default behaviour emits the save/restore code inline, which means long prologues and epilogues 😭. Use -mcall-ms2sysv-xlogues and you get prologues and epilogues that use stubs in the static portion of libgcc to do the saves and restores instead — smaller functions, at the cost of a few extra instructions. Trade-off queen 🎀.
  • -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 heads up bestie, it may add compile- and run-time requirements that **cannot be satisfied on all systems. Know your audience before you commit 💀.
  • -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 can level up performance, because of improved scheduling and reduced dependencies. Slay.
  • -maccumulate-outgoing-args — if it's enabled, the maximum amount of space needed for outgoing arguments gets computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary isn't equal to 2. The drawback? A notable increase in code size 📈. Also this switch implies -mno-push-args.
  • -mms-bitfields
  • -mno-ms-bitfields — enable/disable bit-field layout that's compatible with the native Microsoft Windows compiler.

If packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. Especially when you're moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), you may need to access either format.

This option is enabled by default for Microsoft Windows targets. You can also control this behaviour locally with variable or type attributes. For more info, see x86 Attributes.

The Microsoft structure layout algorithm is pretty simple, except for the bit-field packing part. 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
  1. 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.

  1. If a zero-length bit-field is inserted after a bit-field, foo, and the alignment of the zero-length bit-field is greater than the member that follows it, bar, bar is aligned as the type of the zero-length bit-field.

For example:

struct
 {
   char foo : 4;
   short : 0;
   char bar;
 } t2;

struct
 {
   char foo : 4;
   short : 0;
   double bar;
 } t3;

For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.

Taking this into account, it is important to note the following:

  1. If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as a whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short.
  2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:
struct
 {
   char foo : 6;
   long : 0;
 } t4;

Here, t4 takes up 4 bytes.

  1. Zero-length bit-fields following non-bit-field members are ignored:
struct
 {
   char foo;
   long : 0;
   char bar;
 } t5;

Here, t5 takes up 2 bytes.

  • -mno-align-stringops — do not align the destination of inlined string operations. This switch reduces code size and improves performance in the case where the destination is already aligned… but GCC doesn't know about it 🙈. It said "I can't tell" and you said "bestie I got this".
  • -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. We love a glow-up ✨.
  • -minline-stringops-dynamically — for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. It's giving versatility, it's giving range 💅.
  • *-mstringop-strategy=alg* — override the internal decision heuristic for the particular algorithm used for inlining string operations. The allowed values for alg are:
    • rep_byte
    • rep_4byte
    • rep_8byte

Expand using the i386 rep prefix of the specified size.

  • byte_loop
  • loop
  • unrolled_loop

Expand into an inline loop.

  • libcall

Always use a library call.

  • -mmemcpy-strategy=strategy — override the internal decision heuristic that decides if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, and max_size specifies the max byte size with which inline algorithm alg is allowed. For the *last triplet, the *max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and "max_size + 1" of the preceding range. Math girlies rise up 🧮.
  • *-mmemset-strategy=strategy* — this option is similar to -mmemcpy-strategy= except that it controls the __builtin_memset expansion. Same vibe, different builtin 🎀.
  • -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions needed to save, set up, and restore frame pointers, and makes an extra register available in leaf functions. The option -momit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder 😭. Just so you know, fr fr.
  • -mtls-direct-seg-refs
  • -mno-tls-direct-seg-refs — controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.

For systems that use the GNU C Library, the default is on.

  • -msse2avx
  • -mno-sse2avx — specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default. Fancy prefix, iconic behaviour ✨.
  • -mfentry
  • -mno-fentry — if profiling is active (-pg), put the profiling counter call before the prologue. Note: on x86 architectures the attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg. That's just how it is bestie, no cap 💀.
  • -mrecord-mcount, -mno-record-mcount — if profiling is on (-pg), this makes a whole section full of pointers, one to each profiling call, and it's literally built for auto-patching those calls later. You can rename that section with the mfentry-section= option — default name is __mcount_loc. ✨
  • -mnop-mcount, -mno-nop-mcount — with profiling active (-pg), the calls to the profiling functions get generated as NOPs instead. Iconic for when they're supposed to get patched in dynamically later. Ngl it's probably only useful alongside -mrecord-mcount. 💅
  • -minstrument-return=type — with -pg -mfentry, this instruments function exits according to type, which can be one of:
    • none — don't instrument at all, leave it alone 💤
    • call — generate a call to "__return__" 📞
    • nop5 — generate a 5-byte nop sequence, so it can be patched later

Heads up bestie: this only instruments true returns ending with a "ret", not sibling calls that happen via a jump. 🙈

  • -mrecord-return, -mno-record-return — generates a "__return_loc" section that points to all the return instrumentation code. she's organized 💖
  • -mfentry-name=name — sets the name of the "__fentry__" symbol that gets called at function entry for -pg -mfentry functions. 🎀
  • -mfentry-section=name — sets the name of the section used to record -mrecord-mcount calls. Default is __mcount_loc.
  • -mskip-rax-setup, -mno-skip-rax-setup — when you're generating code for x86-64 with SSE extensions disabled, -mskip-rax-setup lets you just... skip setting up the RAX register, as long as there are no variable arguments passed in vector registers. no RAX, no problem ✨
  • -m8bit-idiv, -mno-8bit-idiv — on some processors (hi Intel Atom 💅), 8-bit unsigned integer divide is way faster than 32-bit/64-bit integer divide. This option generates a run-time check, and if both the dividend and the divisor are in the 0 to 255 range, it does the 8-bit unsigned divide instead of the 32-bit/64-bit one. glow up 🔥
  • -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — split the 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 rocking a canary at guard. Supported locations:
    • global — for a global canary 🌍
    • tls — for a per-thread canary in the TLS block (this is the default, bestie) 🧵

This whole thing only does anything when -fstack-protector or -fstack-protector-all is specified. Pick tls and the extra options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset let you specify which segment register (%fs or %gs) to use as the base register for reading the canary, and what offset from that base register. Defaults for both are whatever the relevant ABI says, so don't come for me. 🙈

-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. 👑

  • -mgeneral-regs-only — generates code that uses only the general-purpose registers. This stops the compiler from touching floating-point, vector, mask and bound registers. general purpose only, as a treat 🎀
  • -m128bit-atomic — generates cmpxchg16b, 128-bit atomic vector load and store instructions for the x86-64 architecture. This is safe to use only on x86-64-v3 capable processors as well as AVX capable processors from Intel, AMD, Hygon and Zhaoxin, which guarantee that 128-bit aligned vector loads and stores are atomic. Fr fr, be careful. 🫶

It requires SSE2 and CMPXCHG16B. It implies -mcx16, which enables CMPXCHG16B. This option is enabled in 64-bit mode by default if the targeting processor supports it. For __atomic Builtins operating on 128-bit integers, this generates atomic instructions directly instead of calling a library function in the libatomic runtime library. slay 💅

Note that when SSE2 is disabled — i.e., by -mgeneral-regs-only — 128-bit atomic operations are unsupported, and libatomic shouldn't be used since it may use SSE2 or AVX instructions to implement 128-bit atomic operations. 💀
  • -mrelax-cmpxchg-loop — when emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins that are lacking a native instruction, this optimizes for the highly contended case by issuing an atomic load before the "CMPXCHG" instruction, and using the "PAUSE" instruction to save CPU power when restarting the loop. main character energy, saving watts ⭐
  • -mindirect-branch=choice — convert indirect call and jump with choice. The options:
    • keep — keeps indirect call and jump unmodified (this is the default)
    • thunk — converts indirect call and jump to call and return thunk 🔁
    • thunk-inline — converts indirect call and jump to inlined call and return thunk
    • thunk-extern — converts indirect call and jump to external call and return thunk, provided in a separate object file 📦

You can control this behavior for a specific function by using the function attribute "indirect_branch".

Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern, since the thunk function may not be reachable in the large code model. 😭
Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check. ✨
  • -mfunction-return=choice — convert function return with choice. The options:
    • keep — keeps function return unmodified (the default)
    • thunk — converts function return to call and return thunk
    • thunk-inline — converts function return to inlined call and return thunk
    • thunk-extern — converts function return to external call and return thunk, provided in a separate object file 📦

You can control this behavior for a specific function by using the function attribute "function_return".

Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch, since the external thunk can be made to enable control-flow check.
Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern, since the thunk function may not be reachable in the large code model. 💀
  • -mindirect-branch-register — force indirect call and jump via register. put it in a register, periodt 💅
  • -mharden-sls=choice — generate code to mitigate against straight line speculation (SLS) with choice. The options:
    • none — disables all SLS hardening (the default)
    • return — enables SLS hardening for function returns 🔙
    • indirect-jmp — enables SLS hardening for indirect jumps 🦘
    • all — enables all SLS hardening 🛡️
  • -mindirect-branch-cs-prefix — adds a CS prefix to call and jmp to indirect thunk when the branch target is in the r8-r15 registers, so that the call and jmp instruction length is 6 bytes. That way they can potentially be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run time. year-round patching 💖
  • -mapx-inline-asm-use-gpr32 — by default, EGPR usage is disabled in inline asm constraints, because GCC cannot be aware of whether the asm instructions support GP32 or not. If your inline asm can handle EGPR, use -mapx-inline-asm-use-gpr32. know your asm, bestie 🫶
  • -mgather, -mscatter — enable vectorization for gather and scatter instructions, respectively. 🧚

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. ✨

  • -m32, -m64, -mx32, -m16 — generate code for a 16-bit, 32-bit or 64-bit environment.
    • The -m32 option sets int, long, and pointer types to 32 bits, and generates code that runs in 32-bit mode.
    • The -m64 option sets int to 32 bits and long and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only, the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options. 🍎
    • The -mx32 option sets int, long, and pointer types to 32 bits, and generates code for the x86-64 architecture.
    • The -m16 option is the same as -m32, except for that it outputs the .code16gcc assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode. retro ✨
  • -miamcu — generates code that conforms to Intel MCU psABI. It requires the -m32 option to be turned on.
  • -mno-red-zone — do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it's a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers, and therefore can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red zone. 🚫🟥
  • -mcmodel=small — generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default code model. 👑
  • -mcmodel=kernel — generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model has to be used for Linux kernel code. 🐧
  • -mcmodel=medium — generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols are also placed there. Symbols with sizes larger than -mlarge-data-threshold 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 no assumptions about addresses and sizes of sections. free spirit vibes 🌈
  • -maddress-mode=long — generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.
  • -maddress-mode=short — generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.
  • -mneeded, -mno-needed — emit the 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. 🏷️
  • -mno-direct-extern-access — without -fpic or -fPIC, always use the GOT pointer to access external symbols. With -fpic or -fPIC, treat access to protected symbols as local symbols. The default is -mdirect-extern-access.
Warning: shared libraries compiled with -mno-direct-extern-access and an executable compiled with -mdirect-extern-access may not be binary compatible if protected symbols are used in shared libraries and executable. 💀 be so serious about this one, bestie 🫶
  • -munroll-only-small-loops, -mno-unroll-only-small-loops — this is the ✨conservative✨ tiny-loop unroll knob. It's on by default with -O2, and what it does is unroll loops that have fewer than 4 instructions exactly 1 time. Why? So modern processors get better utilization of the instruction decoding pipeline. Feels good, runs good 💅. You can turn it off with -mno-unroll-only-small-loops, and heads up bestie — it also gets disabled if the more general options -funroll-loops or -funroll-all-loops are either enabled or explicitly disabled. Either way, it's out 🫶.
  • -mdispatch-scheduler — turns on instruction scheduling. But it's not for everybody, no cap: this one is only supported on bdver1, bdver2, bdver3, bdver4, and znver1 processors, and on top of that it additionally requires -fschedule-insns and -fsched-pressure.
  • -mlam=choice — LAM stands for linear-address masking, and what it does is let special bits in the pointer be used for metadata. 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. Pick your flavor 💜.

🪟 x86 Windows Options

🦋 Xstormy16 Options

These options are defined for Xstormy16:

  • -msim — choose startup files and linker script suitable for the simulator. Sim time, babes 🧚.

🌸 Xtensa Options

These options are supported for Xtensa targets:

  • -mconst16, -mno-const16 — enable or disable use of CONST16 instructions for loading constant values. When enabled, CONST16 instructions are always used in place of the standard L32R instructions. The use of CONST16 is enabled by default only if the L32R instruction is not available. iykyk ☝️
  • -mserialize-volatile, -mno-serialize-volatile — when this option is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the MEMW instructions.
  • -mforce-no-pic, -mno-force-no-pic — for targets like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. yeah, kernel girlies get their own lane 🎀
  • -mtext-section-literals, -mno-text-section-literals — these control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. That's lowkey nice because it lets the literal pool 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 are interspersed in the text section to keep them as close as possible to their references. That may be necessary for large assembly files. Literals for each function are placed right before that function. Periodt.
  • -mauto-litpools, -mno-auto-litpools — again with the literal pools, bestie. The default is -mno-auto-litpools, which places literals in a separate section in the output file unless -mtext-section-literals is used. With -mauto-litpools the literals are interspersed in the text section by the assembler. The compiler does not produce explicit .literal directives and loads literals into registers with MOVI instructions instead of L32R, to let the assembler do relaxation and place literals as necessary. This option allows the assembler to create several literal pools per function and assemble very big functions, which may not be possible with -mtext-section-literals. Big function energy 👑
  • -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 attempts to widen density instructions to align branch targets and the instructions following call instructions. If there are not enough preceding safe density instructions to align a target, no widening is performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like LOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. That one's not up for debate 💅
  • -mlongcalls, -mno-longcalls — when enabled, GCC tells the assembler to translate direct calls to 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. Use this in programs where the call target can potentially be out of range. ⚠️ Warning, bestie: 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. And note that the assembler uses an indirect call for every cross-file call, not just those that really are out of range. No cap 😭
  • -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. slay 💖
  • -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 that 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.
  • -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 literal pool using L32R, or synthesizing the constant from a small one with a couple of arithmetic instructions. The default value is 0.
  • -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is -mno-strict-align for cores that support both unaligned loads and stores in hardware, and -mstrict-align for all other cores. Choose wisely ✨
  • -mforce-l32, -mno-force-l32 — when this option is enabled, GCC performs 1- or 2-byte loads in the generic address space (ie., default memory references) by bit-extracting the desired portion from the result of an aligned 4-byte load, instead of the instructions originally provided for those purposes. This option does not affect memory stores of such byte width, or the placement of those memory sections (see also __force_l32 address spaces described in Xtensa Named Address Spaces, and force_l32 attribute described in Xtensa Attributes). The default is -mno-force-l32.

💠 zSeries Options

These are listed under

ENVIRONMENT (aka the vibes your shell hands down to GCC) 💫

Okay bestie, so GCC is not just out here reading your mind — it also takes notes from your environment. This section is all about the environment variables that change how GCC does its thing. Some of them work by telling GCC which directories or prefixes to search when it's hunting for different kinds of files. Others just set the general compilation vibe.

Heads up, queen: you can ALSO tell GCC where to look with options like -B, -I and -L. Those take precedence over places specified using environment variables, and environment variables in turn take precedence over whatever GCC's configuration says. So options > env vars > config. Periodt.

LANG

LC_CTYPE

LC_MESSAGES

LC_ALL

These cuties control how GCC uses localization info — the stuff that lets GCC work with different national conventions for diagnostic and informational output. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it's been configured to do so. You can set these locale categories to any value your installation supports. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8. Slay.

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. And if none of these variables are set, GCC defaults to traditional C English behavior. So old-school. 🙈

These environment variables do not affect the encodings of input files and strings in output files produced by GCC. Those are controlled by the -finput-charset and -fexec-charset options, respectively. Don't get it twisted. 💅

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 assembly line, but make it temp files. ✨

GCC_DIAGNOSTICS_LOG

If GCC_DIAGNOSTICS_LOG is set, then extra info about the diagnostics subsystem gets emitted. If it's set to an empty value, then the info goes to stderr; otherwise, GCC will try to open that file and write the info there.

The precise content and format of this information is subject to change; it's intended for use by GCC developers, not end-users. So unless you're literally building GCC, this is not your main character moment. 🫶

GCC_COMPARE_DEBUG

Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the documentation of that option for more details. Lowkey a shortcut, highkey useful for debugging. 🐛

GCC_EXEC_PREFIX

If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish.

If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.

If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In many cases prefix is the value of "prefix" when you ran the configure script.

Other prefixes specified with -B take precedence over this prefix.

This prefix is also used for finding files such as crt0.o that are used for linking.

In addition, the prefix is used in an unusual way in finding the directories to search for header files. For each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. If a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. It's giving detective work, but for paths. 🔍

COMPILER_PATH

The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX. Bestie said "I'll check my other pockets." 👑

LIBRARY_PATH

The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). So -L is the VIP, LIBRARY_PATH is the plus-one. 💖

GCC_EXTRA_DIAGNOSTIC_OUTPUT

If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable.

  • fixits-v1 — Emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
  • fixits-v2 — As "fixits-v1", but columns are expressed as display columns, as per -fdiagnostics-column-unit=display.

EXPERIMENTAL_SARIF_SOCKET

If EXPERIMENTAL_SARIF_SOCKET is set in the environment, then the compiler will attempt to connect to a UNIX domain stream socket with that name, and send an "OnSarifResult" JSON-RPC 2.0 notification to it for each diagnostic that occurs, where the value of the notification is a SARIF "result" object.

The compiler will fail immediately if EXPERIMENTAL_SARIF_SOCKET is set and it cannot connect to it.

This feature is experimental and subject to change or removal without notice. So don't build your whole personality around it. 💀

COLLECT_GCC_OPTIONS

Set by the driver and read by collect2, lto-wrapper, and the LTO linker plugin to pass the driver's option list. If the list is too long (over 1 KiB by default), the driver writes it to a temporary file and sets this variable to @path instead. It's giving "too much text for a variable, let me put it in a file." 📝

Some additional environment variables affect the behavior of the preprocessor. The plot thickens. 🧚

CPATH

C_INCLUDE_PATH

CPLUS_INCLUDE_PATH

OBJC_INCLUDE_PATH

Each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.

The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.

In all these variables, an empty element instructs the compiler to search its current working directory. Empty elements can appear at the beginning or end of a path. For instance, if the value of CPATH is ":/special/include", that has the same effect as -I. -I/special/include. 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.

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. So it's like a combo meal, but for Make dependencies. 🍓

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. It's the rebel sibling. 🌈

SOURCE_DATE_EPOCH

If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become reproducible. Reproducible builds are iconic, actually. ✨

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. So like, don't just make it up, bestie. 🫶

BUGS (aka "not me breaking things" 😭)

For instructions on reporting bugs, see <https://gcc.gnu.org/bugs/>. They literally have a whole page for it. Go off. 🐛

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. So yeah, flags matter. Don't skip leg day. 💅

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 squad is here. 🫶

AUTHOR 🌟

See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. Give them their flowers. 💐

okay bestie, legal era 💅 this is the part where we do the rights bit and it's actually important, so let's not skim, mwah 🫶

Copyright (c) 1988-2026 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation — with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below, bestie, it's right there), and with the Back-Cover Texts being (b) (also below, don't worry). A copy of the license is included in the gfdl(7) man page. no cap, go read it if you want the full tea ☕

🎀 The Cover Texts (as promised) 🎀

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
software.  Copies published by the Free Software Foundation raise
funds for GNU development.

like, the whole vibe is: this manual is free, you can copy it and remix it, and if you're gonna print it then the front says A GNU Manual and the back says the little bit about freedom and funding GNU development. that's it, that's the whole legal slay 👑

friendly reminder: those two blocks above are verbatim — if you're reprinting this manual you literally copy them character for character, no ✨remixing✨ allowed on the cover text itself 🙈

and that's a wrap on the entire man page, bestie 😭 we did it, all 132 parts, main character energy from start to finish 💖

this beauty came out of gcc-17, dated 2026-09-20, living on the GCC(1) page 🦋

okay bye, go compile something iconic 🌸✨🎀

📦 where this page comes from