[all-commits] [llvm/llvm-project] 33a92a: [msan] Add off-by-default flag to fix false negati...

[Thurston Dang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 33a92af1b2260506356eb838125b356703bf02bb
      https://github.com/llvm/llvm-project/commit/33a92af1b2260506356eb838125b356703bf02bb
  Author: Thurston Dang <thurston at google.com>
  Date:   2025-06-20 (Fri, 20 Jun 2025)

  Changed paths:
    M llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
    M llvm/test/Instrumentation/MemorySanitizer/partial-poison.ll

  Log Message:
  -----------
  [msan] Add off-by-default flag to fix false negatives from partially undefined constant fixed-length vectors (#143837)

This patch adds an off-by-default flag which, when enabled via `-mllvm -msan-poison-undef-vectors=true`, fixes a false negative in MSan (partially-undefined constant fixed-length vectors). It is currently off by default since, by fixing the false positive, code/tests that previously passed MSan may start failing. The default will be changed in a future patch.

Prior to this patch, MSan computes that partially-undefined constant fixed-length vectors are fully initialized, which leads to false negatives; moreover, benign vector rewriting could theoretically flip MSan's shadow computation from initialized to uninitialized or vice-versa (*). `-msan-poison-undef-vectors=true` calculates the shadow precisely: for each element of the vector, the corresponding shadow is fully uninitialized if the element is undefined/poisoned, otherwise it is fully initialized.

Updates the test from https://github.com/llvm/llvm-project/pull/143823

(*) For example:
  ```
  %x = insertelement <2 x i64> <i64 0, i64 poison>, i64 42, i64 0
  %y = insertelement <2 x i64> <i64 poison, i64 poison>, i64 42, i64 0
  ```
%x and %y are equivalent but, prior to this patch, MSan incorrectly computes the shadow of %x as <0, 0> rather than <0, -1>.



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