[go] internal/cpu: repair VNNI feature check

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David Chase (Gerrit)

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Dec 18, 2025, 1:59:34 PM (22 hours ago) Dec 18
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David Chase has uploaded the change for review

Commit message

internal/cpu: repair VNNI feature check

This is a pain to test.

It looks like processors that might lack this features
include Intel 11th generation and AMD Zen 4.  These might
or might not have bit 2 set in the 7th cpuid "leaf" (SM4)
which is what the incorrect test was checking; the bug
is triggered by ^VNNI & SM4.

The "Lion Cove" microarchitecture (Arrow Lake, Lunar Lake)
appears to trigger this problem, it's not clear if there are
others.  It was hard to verify this from online information.

Fixes #76881.
Change-Id: I21be6b4f47134d81e89799b0f06f89fcb6563264

Change diff

diff --git a/src/internal/cpu/cpu_x86.go b/src/internal/cpu/cpu_x86.go
index 4610ce8..711fb04 100644
--- a/src/internal/cpu/cpu_x86.go
+++ b/src/internal/cpu/cpu_x86.go
@@ -219,7 +219,7 @@
 	if eax7 >= 1 {
 		eax71, _, _, _ := cpuid(7, 1)
 		if X86.HasAVX {
-			X86.HasAVXVNNI = isSet(4, eax71)
+			X86.HasAVXVNNI = isSet(eax71, cpuid_AVXVNNI)
 		}
 	}

Change information

Files:
  • M src/internal/cpu/cpu_x86.go
Change size: XS
Delta: 1 file changed, 1 insertion(+), 1 deletion(-)
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David Chase (Gerrit)

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Keith Randall (Gerrit)

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Keith Randall . resolved

Looks like this may have been copied from x/sys/cpu:

https://cs.opensource.google/go/x/sys/+/refs/tags/v0.39.0:cpu/cpu_x86.go;l=154#:~:text=153-,154,-155

Definitely confusing that x/sys/cpu and internal/cpu have very different conventions.

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Keith Randall (Gerrit)

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    David Chase (Gerrit)

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    Cherry Mui (Gerrit)

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    Cherry Mui (Gerrit)

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    Looks like this may have been copied from x/sys/cpu:

    https://cs.opensource.google/go/x/sys/+/refs/tags/v0.39.0:cpu/cpu_x86.go;l=154#:~:text=153-,154,-155

    Definitely confusing that x/sys/cpu and internal/cpu have very different conventions.

    Cherry Mui

    I looked a bit into the history: x/sys/cpu and internal/cpu initially matched, until CL 94759 changed internal/cpu's x86 to match other architectures, which is a good thing itself, but diverged from x/sys. Maybe we want to update x/sys's x86 code as well...

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    David Chase (Gerrit)

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    Failure is some other flake, former failure was reproduced and corrected in the second patch set.

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      David Chase (Gerrit)

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      Dec 18, 2025, 9:37:14 PM (15 hours ago) Dec 18
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      David Chase submitted the change

      Change information

      Commit message:
      internal/cpu: repair VNNI feature check

      This is a pain to test.
      Also the original test was never executed, because it was wrong.


      It looks like processors that might lack this features
      include Intel 11th generation and AMD Zen 4.  These might
      or might not have bit 2 set in the 7th cpuid "leaf" (SM4)
      which is what the incorrect test was checking; the bug
      is triggered by ^VNNI & SM4.  Apparently the SM4 bit is
      not usually set, else we would have seen a test failure.


      The "Lion Cove" microarchitecture (Arrow Lake, Lunar Lake)
      appears to trigger this problem, it's not clear if there are
      others.  It was hard to verify this from online information.

      Fixes #76881.
      Change-Id: I21be6b4f47134d81e89799b0f06f89fcb6563264
      TryBot-Bypass: David Chase <drc...@google.com>
      Reviewed-by: Cherry Mui <cher...@google.com>
      Files:
      • M src/internal/cpu/cpu_x86.go
      • M src/simd/archsimd/internal/simd_test/simd_test.go
      Change size: S
      Delta: 2 files changed, 12 insertions(+), 8 deletions(-)
      Branch: refs/heads/master
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