[PATCH] D93064: RegisterCoalescer: Prune undef subranges from copy pairs in loops

[Matt Arsenault via Phabricator via llvm-commits]

arsenm created this revision.
arsenm added reviewers: qcolombet, MatzeB, rampitec, foad, dstuttard, tpr.
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If we had a pair of copies inside a loop which introduced new liveness
to a subregister which was undef before the loop, we would have a
dummy phi-only segment remaining across the loop body. Later, this
false segment would confuse RenameIndependentSubregs causing it to
introduce IMPLICIT_DEFs with broken value numbering.

      

It seems always adding the lanes to ShrinkMask is OK, so any
conditions should be purely a compile time filter.


https://reviews.llvm.org/D93064

Files:
  llvm/lib/CodeGen/RegisterCoalescer.cpp
  llvm/test/CodeGen/AMDGPU/coalesce-identity-copies-undef-subregs.mir
  llvm/test/CodeGen/AMDGPU/loop-live-out-copy-undef-subrange.ll

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