[PATCH] D55883: [TargetLowering] Fix propagation of undefs in zero extension ops (PR40091)

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: andreadb, dmgreen, spatel.
Herald added a subscriber: javed.absar.

As described on PR40091, we have several places where zext (and zext_vector_inreg) fold an undef input into an undef output. For zero extensions this incorrect as the output should guarantee to least have the new upper bits set to zero.

SimplifyDemandedVectorElts is the worst offender (and its the most likely to cause new undefs to appear) but DAGCombiner's tryToFoldExtendOfConstant has a similar issue.

Thanks to @dmgreen for catching this.


Repository:
  rL LLVM

https://reviews.llvm.org/D55883

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  lib/CodeGen/SelectionDAG/TargetLowering.cpp
  test/CodeGen/AArch64/pr40091.ll
  test/CodeGen/X86/fold-vector-sext-zext.ll

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