[all-commits] [llvm/llvm-project] 66998a: [X86][SSE] getFauxShuffle - ignore undemanded sour...

[Simon Pilgrim via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 66998ae59f4e334b285f735bb2d927976cdd323c
      https://github.com/llvm/llvm-project/commit/66998ae59f4e334b285f735bb2d927976cdd323c
  Author: Simon Pilgrim <llvm-dev at redking.me.uk>
  Date:   2020-07-25 (Sat, 25 Jul 2020)

  Changed paths:
    M llvm/lib/Target/X86/X86ISelLowering.cpp
    M llvm/test/CodeGen/X86/vector-reduce-mul.ll

  Log Message:
  -----------
  [X86][SSE] getFauxShuffle - ignore undemanded sources for PACKSS/PACKUS faux shuffles

If we don't care about an entire LHS/RHS of the PACK op, then can just treat it the same as undef (we don't care if it saturates) and is safe to treat as a shuffle.

This can happen if we attempt to decode as a faux shuffle before SimplifyDemandedVectorElts has been called on the PACK which should replace the source with UNDEF entirely.