[Bug target/56068] -march=native creates Illegal instruction on KVM guests

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56068

--- Comment #13 from Uros Bizjak <ubizjak at gmail dot com> 2013-01-21 19:31:47 UTC ---
(In reply to comment #12)
> (In reply to comment #11)
> > I will try 4.6 to prove that. Off to build 4.6...
> 
> Thanks for checking.  See http://gcc.gnu.org/wiki/InstallingGCC for the
> foolproof way to do build it.
> 
> You could also try this with any GCC version on the KVM guest:
> 
> #include <stdio.h>
> #include "cpuid.h"
> 
> int main()
> {
>       unsigned bit_osxsave = bit_AVX >> 1;
> 
>       unsigned int eax, ebx, ecx, edx;
> 
>       __cpuid (1, eax, ebx, ecx, edx);
> 
>       printf("bit_OSXSAVE (%u) = %u\n", bit_osxsave, ecx & bit_osxsave);
>       printf("bit_AVX (%u) = %u\n", bit_AVX, ecx & bit_AVX);
> }
> 
> If that prints 0 for OSXSAVE and non-zero for AVX then it's the same problem I
> had.
> 
> Maybe GCC could work around it by checking both flags in the AVX detection
> logic.

Recent 4.6+ does. Please see driver-i386.c around line 470:

--snip--
  /* Get XCR_XFEATURE_ENABLED_MASK register with xgetbv.  */
#define XCR_XFEATURE_ENABLED_MASK    0x0
#define XSTATE_FP            0x1
#define XSTATE_SSE            0x2
#define XSTATE_YMM            0x4
  if (has_osxsave)
    asm (".byte 0x0f; .byte 0x01; .byte 0xd0"
     : "=a" (eax), "=d" (edx)
     : "c" (XCR_XFEATURE_ENABLED_MASK));

  /* Check if SSE and YMM states are supported.  */
  if (!has_osxsave
      || (eax & (XSTATE_SSE | XSTATE_YMM)) != (XSTATE_SSE | XSTATE_YMM))
    {
      has_avx = 0;
      has_fma = 0;
      has_fma4 = 0;
      has_xop = 0;
    }
--snip--