[PATCH] D48102: Improve handling of COPY instructions with identical value numbers

[Krzysztof Parzyszek via Phabricator via llvm-commits]

kparzysz updated this revision to Diff 151982.
kparzysz added a comment.

Fix the "cannot join subranges" issue caused by redefining an undef value (more details in comments), roughly something like this:

  %0 <- undef
  %1 = COPY %0
  %0 = COPY %1  <- now %0 is "def", even though it's identical to what it was before (i.e. "undef").


Repository:
  rL LLVM

https://reviews.llvm.org/D48102

Files:
  lib/CodeGen/RegisterCoalescer.cpp
  test/CodeGen/AMDGPU/coalescer-extend-pruned-subrange.mir
  test/CodeGen/AMDGPU/coalescer-identical-values-undef.mir
  test/CodeGen/AMDGPU/coalescing-with-subregs-in-loop-bug.mir

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