[SOLVED] does gcc's __builtin_cpu_supports check for OS support?

Issue

GCC compiler provides a set of builtins to test some processor features, like availability of certain instruction sets. But, according to this thread we also may know certain cpu features may be not enabled by OS. So the question is: do __builtin_cpu_supports intrinsics also check if OS has enabled certain processor feature?

Solution

No.

I disabled AVX on my Skylake system by adding noxsave to the Linux kernel boot options. When I do cat /proc/cpuinfo AVX (and AVX2) no longer appear and when I run code with AVX instructions it crashes. This tells me that AVX has been disabled by the OS.

However, when I compile and run the following code

#include <stdio.h>

int main(void) {
  __builtin_cpu_init();
  printf("%d\n", __builtin_cpu_supports ("sse"));
  printf("%d\n", __builtin_cpu_supports ("avx"));
}

it returns 8 and 512. This means that __builtin_cpu_supports does not check to see if AVX was disabled by the OS.

Answered By - Z boson

Answer Checked By - Dawn Plyler (WPSolving Volunteer)