[all-commits] [llvm/llvm-project] d9b64b: [SelectionDAG] Deal with POISON for INSERT_VECTOR_...

[Björn Pettersson via All-commits]

  Branch: refs/heads/users/bjope/insertundef_2
  Home:   https://github.com/llvm/llvm-project
  Commit: d9b64b179dd33c7dd877900ff5f900dab930f436
      https://github.com/llvm/llvm-project/commit/d9b64b179dd33c7dd877900ff5f900dab930f436
  Author: Bjorn Pettersson <bjorn.a.pettersson at ericsson.com>
  Date:   2025-08-31 (Sun, 31 Aug 2025)

  Changed paths:
    M llvm/include/llvm/CodeGen/SelectionDAGNodes.h
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    M llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
    M llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
    M llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
    M llvm/test/CodeGen/AArch64/concat-vector-add-combine.ll
    M llvm/test/CodeGen/AArch64/sve-fixed-length-masked-gather.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vfw-web-simplification.ll
    M llvm/test/CodeGen/RISCV/rvv/fixed-vectors-vw-web-simplification.ll
    M llvm/test/CodeGen/X86/vector-shuffle-combining.ll
    M llvm/test/CodeGen/X86/zero_extend_vector_inreg_of_broadcast.ll

  Log Message:
  -----------
  [SelectionDAG] Deal with POISON for INSERT_VECTOR_ELT/INSERT_SUBVECTOR

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.

Fixes #141034



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