CODEWITHSUNDEEP
linuxwindowsmacoscpujulia
Frames Catherine White
Frames Catherine White

Reputation: 28222

Dectecting CPU feature support (Eg sse2, fma4 etc)

I have some code that depends on CPU and OS support for various CPU features.

In particular I need to check for various SIMD instruction set support. Namely sse2, avx, avx2, fma4, and neon. (neon being the ARM SIMD feature. I'm less interested in that; given less ARM end-users.)

What I am doing right now is:

function cpu_flags()
    if is_linux()
        cpuinfo = readstring(`cat /proc/cpuinfo`);
        cpu_flag_string = match(r"flags\t\t: (.*)", cpuinfo).captures[1]
    elseif is_apple()
        sysinfo = readstring(`sysctl -a`);
        cpu_flag_string = match(r"machdep.cpu.features: (.*)", cpuinfo).captures[1]
    else
        @assert is_windows()
        warn("CPU Feature detection does not work on windows.")
        cpu_flag_string = ""
    end
    split(lowercase(cpu_flag_string))
end

This has two downsides:

  1. It doesn't work on windows
  2. I'm just not sure it is correct; it it? Or does it screw up, if for example the OS has a feature disabled, but physically the CPU supports it?

So my questions is:

  1. How can I make this work on windows.
  2. Is this correct, or even a OK way to go about getting this information?

This is part of a build script (with BinDeps.jl); so I need a solution that doesn't involve opening a GUI. And ideally one that doesn't add a 3rd party dependency. Extracting the information from GCC somehow would work, since I already require GCC to compile some shared libraries. (choosing which libraries, is what this code to detect the instruction set is for)

Upvotes: 6

Views: 946

Answers (1)

Matteo Italia
Matteo Italia

Reputation: 126907

I'm just not sure it is correct; it it? Or does it screw up, if for example the OS has a feature disabled, but physically the CPU supports it?

I don't think that the OS has any say in disabling vector instructions; I've seen the BIOS being able to disable stuff (in particular, the virtualization extensions), but in that case you won't find them even in /proc/cpuinfo - that's kind of its point :-) .

Extracting the information from GCC somehow would work, since I already require GCC to compile some shared libraries

If you always have gcc (MinGW on Windows) you can use __builtin_cpu_supports:

#include <stdio.h>

int main()
{
    if (__builtin_cpu_supports("mmx")) {
        printf("\nI got MMX !\n");
    } else
        printf("\nWhat ? MMX ? What is that ?\n");
    return (0);
}

and apparently this built-in functions work under mingw-w64 too.

AFAIK it uses the CPUID instruction to extract the relevant information (so it should reflect quite well the environment your code will run in).

(from https://stackoverflow.com/a/17759098/214671)

Upvotes: 2

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