[all-commits] [llvm/llvm-project] 8ef171: [msan] Handle horizontal add/subtract intrinsic by...

[Thurston Dang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 8ef171ee831ff030e5aa81a74d68edc133d0cb4f
      https://github.com/llvm/llvm-project/commit/8ef171ee831ff030e5aa81a74d68edc133d0cb4f
  Author: Thurston Dang <thurston at google.com>
  Date:   2025-01-23 (Thu, 23 Jan 2025)

  Changed paths:
    M llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
    M llvm/test/Instrumentation/MemorySanitizer/X86/avx-intrinsics-x86.ll
    M llvm/test/Instrumentation/MemorySanitizer/X86/avx2-intrinsics-x86.ll
    M llvm/test/Instrumentation/MemorySanitizer/X86/mmx-intrinsics.ll
    M llvm/test/Instrumentation/MemorySanitizer/i386/avx-intrinsics-i386.ll
    M llvm/test/Instrumentation/MemorySanitizer/i386/avx2-intrinsics-i386.ll
    M llvm/test/Instrumentation/MemorySanitizer/i386/mmx-intrinsics.ll

  Log Message:
  -----------
  [msan] Handle horizontal add/subtract intrinsic by applying to shadow (#124159)

Horizontal add (hadd) and subtract (hsub) are currently heuristically
handled by `maybeHandleSimpleNomemIntrinsic()` (via
`handleUnknownIntrinsic()`), which computes the shadow by bitwise OR'ing
the two operands. This has false positives for hadd/hsub shadows. For
example, suppose the shadows for the two operands are 00000000 and
11111111 respectively. The expected shadow for the result is 00001111,
but `maybeHandleSimpleNomemIntrinsic` would compute it as 11111111.

This patch handles horizontal add using
`handleIntrinsicByApplyingToShadow` (from
https://github.com/llvm/llvm-project/pull/114490), which has no false
positives for hadd/hsub: if each pair of adjacent shadow values is zero
(fully initialized), the result will be zero (fully initialized). More
generally, it is precise for hadd/hsub if at least one of the two
adjacent shadow values in each pair is zero.

It does have some false negatives for hadd/hsub: if we add/subtract two
adjacent non-zero shadow values, some bits of the result may incorrectly
be zero. We consider this an acceptable tradeoff for performance. To
make shadow propagation precise, we want the equivalent of "horizontal
OR", but this is not available. Reducing horizontal OR to (permutation
plus bitwise OR) is left as an exercise for the reader.



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications