[PATCH] D80077: [LiveVariables] Don't set undef reg PHI used as live for FromMBB

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope added a comment.

Maybe it could be considered as cheating a bit, but it seems like PHIElimination has some options that can help trigger the critical edge splitting without even having a loop.
So by adding some extra cmd line options I managed to create a reproducer that is quite similar to your description:

  # RUN: llc -mtriple=aarch64-linux-gnu -verify-machineinstrs -o /dev/null %s \
  # RUN:   -run-pass=livevars,phi-node-elimination,twoaddressinstruction \
  # RUN:   -no-phi-elim-live-out-early-exit=1 -phi-elim-split-all-critical-edges=1
  
  # Used to result in
  #
  #     *** Bad machine code: LiveVariables: Block should not be in AliveBlocks ***
  #
  # Just verify that we do not crash (or get verifier error).
  
  ---
  name: test
  tracksRegLiveness: true
  body: |
    bb.0:
      liveins: $nzcv, $wzr
      Bcc 8, %bb.2, implicit $nzcv
  
    bb.1:
      %x:gpr32 = COPY $wzr
  
    bb.2:
      %y:gpr32 = PHI %x:gpr32, %bb.1, undef %undef:gpr32, %bb.0
      $wzr = COPY %y:gpr32
  ...


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D80077/new/

https://reviews.llvm.org/D80077