[all-commits] [llvm/llvm-project] 56944e: [msan] Approximately handle AVX Galois Field Affin...

[Thurston Dang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 56944e606a1ba24e71fc89fad1eabb97e8240d65
      https://github.com/llvm/llvm-project/commit/56944e606a1ba24e71fc89fad1eabb97e8240d65
  Author: Thurston Dang <thurston at google.com>
  Date:   2025-07-30 (Wed, 30 Jul 2025)

  Changed paths:
    M llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
    M llvm/test/Instrumentation/MemorySanitizer/X86/avx512-gfni-intrinsics.ll

  Log Message:
  -----------
  [msan] Approximately handle AVX Galois Field Affine Transformation (#150794)

e.g.,
      <16 x i8> @llvm.x86.vgf2p8affineqb.128(<16 x i8>, <16 x i8>, i8)
      <32 x i8> @llvm.x86.vgf2p8affineqb.256(<32 x i8>, <32 x i8>, i8)
      <64 x i8> @llvm.x86.vgf2p8affineqb.512(<64 x i8>, <64 x i8>, i8)
       Out                                    A          x          b
where A and x are packed matrices, b is a vector, Out = A * x + b in
GF(2)

Multiplication in GF(2) is equivalent to bitwise AND. However, the
matrix computation also includes a parity calculation.

For the bitwise AND of bits V1 and V2, the exact shadow is:
Out_Shadow = (V1_Shadow & V2_Shadow) | (V1 & V2_Shadow) | (V1_Shadow &
V2)

We approximate the shadow of gf2p8affine using:
  Out_Shadow =   _mm512_gf2p8affine_epi64_epi8(x_Shadow, A_shadow, 0)
               | _mm512_gf2p8affine_epi64_epi8(x, A_shadow, 0)
               | _mm512_gf2p8affine_epi64_epi8(x_Shadow, A, 0)
               | _mm512_set1_epi8(b_Shadow)

This approximation has false negatives: if an intermediate dot-product
contains an even number of 1's, the parity is 0.

It has no false positives.

Updates the test from https://github.com/llvm/llvm-project/pull/149258



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