commit 8b8d0b5eacc08384664094285d559cc13b73467c
author Gustaf Ullberg <gustaf@webrtc.org> Fri Apr 29 06:54:29 2022
committer WebRTC LUCI CQ <webrtc-scoped@luci-project-accounts.iam.gserviceaccount.com> Fri Apr 29 08:41:52 2022
tree 5f6af824c5339c446f8b168004509d97e9827c61
parent 9c83d9d99ed1b43c2d7ea5c78d9675865a78f724

Check for BMI2 support before enabling AVX2 code paths

With MSVC, compiling with AVX2 support will also result in the
generation of BMI2 instructions. Some early Haswell CPUs reportedly support AVX2 but not BMI2. We have seen crashes (illegal instruction)
on these CPUs in our AVX2 code paths on MULX instructions (part of BMI2).

Including a check for BMI2 when checking for AVX2 support is
expected to solve the issue.

Please see the bug referenced below for more background on this issue.

Bug: chromium:1315519
Change-Id: I3a0a9838f1f632704ba505ecbb81a6f8b1889319
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/260323
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Henrik Andreasson <henrika@google.com>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#36701}

system_wrappers/source/cpu_features.cc

diff --git a/system_wrappers/source/cpu_features.cc b/system_wrappers/source/cpu_features.cc
index 23e0629..b676339 100644
--- a/system_wrappers/source/cpu_features.cc
+++ b/system_wrappers/source/cpu_features.cc
@@ -93,13 +93,14 @@
     //     a) AVX are supported by the CPU,
     //     b) XSAVE is supported by the CPU,
     //     c) XSAVE is enabled by the kernel.
-    // See http://software.intel.com/en-us/blogs/2011/04/14/is-avx-enabled
-    // AVX2 support needs (avx_support && (cpu_info7[1] & 0x00000020) != 0;).
-    return (cpu_info[2] & 0x10000000) != 0 &&
+    // Compiling with MSVC and /arch:AVX2 surprisingly generates BMI2
+    // instructions (see crbug.com/1315519).
+    return (cpu_info[2] & 0x10000000) != 0 /* AVX */ &&
            (cpu_info[2] & 0x04000000) != 0 /* XSAVE */ &&
            (cpu_info[2] & 0x08000000) != 0 /* OSXSAVE */ &&
            (xgetbv(0) & 0x00000006) == 6 /* XSAVE enabled by kernel */ &&
-           (cpu_info7[1] & 0x00000020) != 0;
+           (cpu_info7[1] & 0x00000020) != 0 /* AVX2 */ &&
+           (cpu_info7[1] & 0x00000100) != 0 /* BMI2 */;
   }
 #endif  // WEBRTC_ENABLE_AVX2
   return 0;