llvm-project

33a92af1 - [msan] Add off-by-default flag to fix false negatives from partially undefined constant fixed-length vectors (#143837)

[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>.