shithub: libvpx

Info  •  Files  •  Log  •  Branches

x86_simd_caps: check max cpuid before testing AVX2

structured extended feature flags require eax = 7; avoids incorrectly
detecting avx2 on some older processors that support avx.

from [1]:
INPUT EAX = 0: Returns CPUID’s Highest Value for Basic Processor
Information and the Vendor Identification String

[1] http://www.intel.com/content/www/us/en/processors/processor-identification-cpuid-instruction-note.html

Change-Id: I6b4735b5f7b7729a815e428fca767d1e5a10bcab

--- a/vpx_ports/x86.h

+++ b/vpx_ports/x86.h

@@ -152,7 +152,7 @@

 x86_simd_caps(void) {

   unsigned int flags = 0;

   unsigned int mask = ~0;

-  unsigned int reg_eax, reg_ebx, reg_ecx, reg_edx;

+  unsigned int max_cpuid_val, reg_eax, reg_ebx, reg_ecx, reg_edx;

   char *env;

   (void)reg_ebx;

@@ -168,9 +168,9 @@

     mask = strtol(env, NULL, 0);

   /* Ensure that the CPUID instruction supports extended features */

-  cpuid(0, 0, reg_eax, reg_ebx, reg_ecx, reg_edx);

+  cpuid(0, 0, max_cpuid_val, reg_ebx, reg_ecx, reg_edx);

-  if (reg_eax < 1)

+  if (max_cpuid_val < 1)

     return 0;

   /* Get the standard feature flags */

@@ -193,10 +193,12 @@

     if ((xgetbv() & 0x6) == 0x6) {

       flags |= HAS_AVX;

-      /* Get the leaf 7 feature flags. Needed to check for AVX2 support */

-      cpuid(7, 0, reg_eax, reg_ebx, reg_ecx, reg_edx);

+      if (max_cpuid_val >= 7) {

+        /* Get the leaf 7 feature flags. Needed to check for AVX2 support */

+        cpuid(7, 0, reg_eax, reg_ebx, reg_ecx, reg_edx);

-      if (reg_ebx & BIT(5)) flags |= HAS_AVX2;

+        if (reg_ebx & BIT(5)) flags |= HAS_AVX2;

+      }

     }

   }