Search code examples
gcccommand-linex86compiler-flagsmicro-architecture

What are my available march/mtune options?

Is there a way to get gcc to output the available -march=arch options? I'm getting build errors (tried -march=x86_64) and I don't know what my options are.

The compiler I'm using is a proprietary wrapper around gcc that doesn't seem to like -march=skylake. The flags should be the same so I assume whatever options I'd send to gcc to dump architectures would be the same for this wrapper.

I managed to cause gcc to error with a bogus parameter and it dumped a list, but I'm not seeing that now that I'm going through a wrapper.

How can I get gcc to tell me what it supports?

Solution

  • Use gcc --target-help

    -march=CPU[,+EXTENSION...]
                      generate code for CPU and EXTENSION, CPU is one of:
                       generic32, generic64, i386, i486, i586, i686,
                       pentium, pentiumpro, pentiumii, pentiumiii, pentium4,
                       prescott, nocona, core, core2, corei7, l1om, k1om,
                       iamcu, k6, k6_2, athlon, opteron, k8, amdfam10,
                       bdver1, bdver2, bdver3, bdver4, znver1, znver2,
                       btver1, btver2
...

    It's often not the general architecture like x86 or x86-64 but the specific microarchitectures. But there's x86-64 (not x86_64) for a generic x86 CPU with 64-bit extensions. The full list for each architecture can be found on GCC's -march manual. For x86:

    • -march=cpu-type

      Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type.

    ...

    https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html#index-march-13

    While the baseline version of -march is -march=x86-64, the baseline / default tune option is -mtune=generic. That aims to not be terrible anywhere, avoiding performance pitfalls even at the cost of extra instructions or code size.

    -march=native will pick the right arch and tune settings for the machine the compiler is running on, or tune=generic if the compiler doesn't recognize the specific model of CPU it's running on.

    (e.g. old gcc on a Skylake, will still enable -mavx2 -mpopcnt -mbmi2 and so on, but will set -mtune=generic instead of something closer to appropriate.)