[all-commits] [llvm/llvm-project] f46a9c: [InstCombine] don't automatically drop poison-gene...

[RotateRight via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f46a9c8edd4421e875927d6808fea04090e28d37
      https://github.com/llvm/llvm-project/commit/f46a9c8edd4421e875927d6808fea04090e28d37
  Author: Sanjay Patel <spatel at rotateright.com>
  Date:   2021-12-13 (Mon, 13 Dec 2021)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
    M llvm/test/Transforms/InstCombine/X86/x86-muldq-inseltpoison.ll
    M llvm/test/Transforms/InstCombine/X86/x86-muldq.ll
    M llvm/test/Transforms/InstCombine/shuffle_select-inseltpoison.ll
    M llvm/test/Transforms/InstCombine/shuffle_select.ll
    M llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
    M llvm/test/Transforms/InstCombine/vec_demanded_elts.ll

  Log Message:
  -----------
  [InstCombine] don't automatically drop poison-generating flags in SimplifyVectorDemandedElts

I noticed this while reviewing the test diffs in D115460
(and so the diffs in that patch will be reduced if this one is applied first).

This is effectively a revert of 3436dc29239d ( https://reviews.llvm.org/rG3436dc29239d ) -
since that commit, we've made several enhancements, so the reasoning there is no longer
valid. Specifically, we added a poison value to IR, and we clarified the behavior of
undef/poison elements in a shuffle mask:
https://llvm.org/docs/LangRef.html#shufflevector-instruction

Alive2 seems to agree that the propagation of flags in the test diffs shown here are valid:
https://alive2.llvm.org/ce/z/UuY-jr
https://alive2.llvm.org/ce/z/GXoMD9
https://alive2.llvm.org/ce/z/nVCyVH

Differential Revision: https://reviews.llvm.org/D115526