-march=, -mcpu=, and -mtune=

In GCC and Clang, there are three major options specifying the architecture and microarchitecture the generated code can run on. The general semantics are described below, but each target machine may assign different semantics.

  • -march=X: (execution domain) Generate code that can use instructions available in the architecture X
  • -mtune=X: (optimization domain) Optimize for the microarchitecture X, but does not change the ABI or make assumptions about available instructions
  • -mcpu=X: Specify both -march= and -mtune= but can be overridden by the two options. The supported values are generally the same as -mtune=. The architecture name is inferred from X

Below are some notes about GCC's implementation.

AArch32

See https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html.

AArch32 follows the general description.

-march=name[+extension...] may specify architecture extensions.

AArch64

See https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html.

AArch64 follows the general description.

See Compiler flags across architectures: -march, -mtune, and -mcpu.

gcc/config/aarch64/aarch64.cc:aarch64_validate_march verifies that -march= specifies a value defined in all_architectures. gcc/config/aarch64/aarch64.cc:aarch64_validate_mtune verifies that -mtune= specifies a value defined in all_cores.

The selected architecture is stored in gcc/config/aarch64/aarch64.opt:selected_arch (a TargetVariable). The selected tune microarchitecture is stored in gcc/config/aarch64/aarch64.opt:selected_tune (a TargetVariable).

MIPS

See https://gcc.gnu.org/onlinedocs/gcc/MIPS-Options.html.

-march= supports generic ISA names and processor names.

-mcpu= is not implemented in GCC.

PowerPC

See https://gcc.gnu.org/onlinedocs/gcc/RS_002f6000-and-PowerPC-Options.html.

-march= is not implemented in GCC. 

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% powerpc64le-linux-gnu-gcc -fsyntax-only -march=power10 a.c
powerpc64le-linux-gnu-gcc: error: unrecognized command-line option ‘-march=power10’; did you mean ‘-mcpu=power10’?

RISC-V

RISC-V follows the general description.

-mtune= must specify an element in gcc/config/riscv/riscv-cores.def. The default -mcpu= value is RISCV_TUNE_STRING_DEFAULT.

-mcpu= does not seem to infer -march= in GCC.

Sparc

-mcpu= must specify an element in gcc/config/sparc/sparc.md.

-march= is not allowed.

x86

See https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html.

-mcpu= has been deprecated since 2003-02. It is an alias for -mtune=.

-march= specifies a cpu-type. cpu-type is a microarchitecture name (e.g. skylake, znver3), a microarchitecture level (x86-64, x86-64-v2, x86-64-v3, x86-64-v4), or native. -march=native enables all instruction subsets supported by the local machine and recognized by the GCC version.

If -mtune= is not specified, use the -march= value (if a microarchitecture name) or generic. 

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% gcc -Q --help=target -march=skylake a.c |& grep --color 'march\|mtune\|mcpu'
  -march=                               skylake
  -mcpu=
  -mtune-ctrl=
  -mtune=                               skylake
  Known valid arguments for -march= option:
  Known valid arguments for -mtune= option:

-march= and -mtune= must specify an element in gcc/common/config/i386/i386-common.cc:processor_alias_table. -mtune= must specify an element without the PTA_NO_TUNE flag.

-mtune= decides features in gcc/config/i386/i386-options.cc:ix86_tune_features: 

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% gcc -c -mdump-tune-features a.c
List of x86 specific tuning parameter names:
schedule : on
partial_reg_dependency : on
sse_partial_reg_dependency : on
sse_split_regs : off
...
-mtune= decides instruction costs gcc/config/i386/i386-options.cc:processor_cost_table.

-mtune= defines some built-in macros __tune_* (see gcc/config/i386/i386-c.cc:ix86_target_macros_internal).

x86-64, x86-64-v2, x86-64-v3, x86-64-v4

The x86-64 psABI defines some microarchitecture levels. They are a very small set of choices suitable for a Linux distribution default with a neutral name (i.e. not tied to an AMD or Intel microarchitecture name). See New x86-64 micro-architecture levels for the original proposal on libc-alpha.

One major use case is glibc's hwcap support for x86-64.

I implemented x86-64-v2, x86-64-v3, and x86-64-v4 for Clang.

Recommendation

For the simplest case where only one option is desired, use -march= for x86 and -mcpu= for other targets.

When optimizing for the local machine, just use -march=native for x86 and -mcpu=native for other targets.

When the architecture and microarchitecture are both specified, i.e. when both the execution domain and the optimization domain need to be specified, specify -march= and -mtune=, and avoid -mcpu=. On PowerPC, specify both -mcpu= and -mtune=.

Clang

Clang driver parses -march=, -mcpu=, -mtune=, and pass -target-cpu, -tune-cpu, -target-feature to cc1.

-mtune= mostly affects MCSubtargetInfo::CPUSchedModel and target features.

For each target, -mcpu=native is implemented by llvm::sys::getHostCPUName.

--target=

Clang supports --target= to set the target triple (for cross compilation). The specified target should be built into Clang (see LLVM_TARGETS_TO_BUILD and LLVM_EXPERIMENTAL_TARGETS_TO_BUILD). See Compiler driver and cross compilation.

--target=aarch64-unknown-linux-gnu -mcpu=X can be used to cross compile for AArch64 and optimize for the microarchitecture X.

Function multi-versioning

GCC 4.4 added the function attribute target for x86. __attribute__((target(arch=k8))) is supported to override -march= for a function. GCC 4.8 extended the syntax with "default" which is then named function multi-versioning.

GCC 6 added the function attribute target_clones to support a new style of function multi-versioning for x86. target_clones supports multiple arch=X.

GNU assembler

-march=, -mcpu=, and -mtune= are optionally supported by various ports.

When -march= is defined, the assembler will report an error when assembling an instruction not supported by the specified architecture.

-mtune= is implemented for mips, ia64, visium, and x86. In the x86 port -mtune= just decides the desired NOP sequence. In the ia64 port -mtune= does some complex scheduling.

Clang does not support -Wa,-mtune=. I dropped a glibc use in i386: Remove -Wa,-mtune=i686.