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Fix AVX-512 capability detection

When Checking for AVX Support, only the CPU's Capabilities and YMM
Register support by the OS were queried. In case of AVX-512, that is
insufficient, and ZMM Register support by the OS needs querying,
otherwise the OS will raise an Illegal Operation Exception if the CPU
is capable of AVX-512 but the OS is not.

Change-Id: I3444b19156d5743841de96cecbdaac19cc3f2b3f

--- a/vpx_ports/x86.h

+++ b/vpx_ports/x86.h

@@ -202,6 +202,7 @@

   // bits 27 (OSXSAVE) & 28 (256-bit AVX)

   if ((reg_ecx & (BIT(27) | BIT(28))) == (BIT(27) | BIT(28))) {

+    // Check for OS-support of YMM state. Necessary for AVX and AVX2.

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

       flags |= HAS_AVX;

@@ -214,8 +215,10 @@

         // bits 16 (AVX-512F) & 17 (AVX-512DQ) & 28 (AVX-512CD) &

         // 30 (AVX-512BW) & 32 (AVX-512VL)

         if ((reg_ebx & (BIT(16) | BIT(17) | BIT(28) | BIT(30) | BIT(31))) ==

-            (BIT(16) | BIT(17) | BIT(28) | BIT(30) | BIT(31)))

-          flags |= HAS_AVX512;

+            (BIT(16) | BIT(17) | BIT(28) | BIT(30) | BIT(31))) {

+          // Check for OS-support of ZMM and YMM state. Necessary for AVX-512.

+          if ((xgetbv() & 0xe6) == 0xe6) flags |= HAS_AVX512;

+        }

       }

     }

   }