[PATCH] D118635: [GlobalISel] Combine: (G_*MULO x, 0) -> 0 + no carry out

[Jessica Paquette via Phabricator via llvm-commits]

paquette created this revision.
paquette added reviewers: arsenm, foad, jroelofs.
Herald added subscribers: steven.zhang, hiraditya, rovka.
paquette requested review of this revision.
Herald added a subscriber: wdng.
Herald added a project: LLVM.

Similar to the following combine in `DAGCombiner::visitMULO`:

  // fold (mulo x, 0) -> 0 + no carry out
  if (isNullOrNullSplat(N1))
  return CombineTo(N, DAG.getConstant(0, DL, VT),
                      DAG.getConstant(0, DL, CarryVT));

This fixes some generally poor codegen for `*mulo`:

https://godbolt.org/z/eTxYsvz8f


https://reviews.llvm.org/D118635

Files:
  llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h
  llvm/include/llvm/Target/GlobalISel/Combine.td
  llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/prelegalizer-combiner-mulo-zero.mir

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