[all-commits] [llvm/llvm-project] bf30c4: [X86] SimplifyDemandedVectorEltsForTargetNode - si...

[Simon Pilgrim via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: bf30c48419d8d0203088ef1d84599e7b3d685a4c
      https://github.com/llvm/llvm-project/commit/bf30c48419d8d0203088ef1d84599e7b3d685a4c
  Author: Simon Pilgrim <llvm-dev at redking.me.uk>
  Date:   2021-10-04 (Mon, 04 Oct 2021)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/test/CodeGen/X86/madd.ll
    M llvm/test/CodeGen/X86/shrink_vmul.ll

  Log Message:
  -----------
  [X86] SimplifyDemandedVectorEltsForTargetNode - simplify PMADDWD for known zero elements

Noticed while investigating the regressions in D110995 - if the RHS element is already zero, then we don't need the corresponding LHS element.

Technically we could also recheck RHS once we have LHS's known zeros, but I haven't seen any missed opportunities from that yet.