llvm-project

0f35a701 - [SelectionDAG] Deal with POISON for INSERT_VECTOR_ELT/INSERT_SUBVECTOR (part 1)

[SelectionDAG] Deal with POISON for INSERT_VECTOR_ELT/INSERT_SUBVECTOR (part 1) As reported in #141034 SelectionDAG::getNode had some unexpected behaviors when trying to create vectors with UNDEF elements. Since we treat both UNDEF and POISON as undefined (when using isUndef()) we can't just fold away INSERT_VECTOR_ELT/INSERT_SUBVECTOR based on isUndef(), as that could make the resulting vector more poisonous. Same kind of bug existed in DAGCombiner::visitINSERT_SUBVECTOR. Here are some examples: This fold was done even if vec[idx] was POISON: INSERT_VECTOR_ELT vec, UNDEF, idx -> vec This fold was done even if any of vec[idx..idx+size] was POISON: INSERT_SUBVECTOR vec, UNDEF, idx -> vec This fold was done even if the elements not extracted from vec could be POISON: sub = EXTRACT_SUBVECTOR vec, idx INSERT_SUBVECTOR UNDEF, sub, idx -> vec With this patch we avoid such folds unless we can prove that the result isn't more poisonous when eliminating the insert. This patch in itself result in some regressions. Goal is to try to deal with those regressions in follow up commits. Fixes #141034