[PATCH] D51848: [RegisterCoalescer] Fixup "Fixed inconsistent followCopyChain with subreg"

[Tim Renouf via Phabricator via llvm-commits]

tpr created this revision.
Herald added subscribers: llvm-commits, nhaehnle, jvesely, qcolombet, MatzeB.

This fixes up my earlier fix https://reviews.llvm.org/D49535 https://reviews.llvm.org/rL338070.

In followCopyChain, the comparison I added to check that two subranges
arrived back at the same def point was bogus. Fixed here.

The test fails with a "Couldn't join subrange" assert without this fix,
because followCopyChain gives the wrong answer and makes a coalesce of A
and B appear impossible where there are copies A->C->B and A is a subreg
def.

Change-Id: I4257fb93800d3834d2b1894359d778f3fdb43f40


Repository:
  rL LLVM

https://reviews.llvm.org/D51848

Files:
  lib/CodeGen/RegisterCoalescer.cpp
  test/CodeGen/AMDGPU/regcoal-followcopychain-bogus-subrange-comparison.mir

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