[PATCH] D48032: [RegisterCoalescer] Avoid erasing copy that breaks subranges

[Tim Renouf via Phabricator via llvm-commits]

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

I found a case where erasing a copy in coalescing causes a subrange to
be modified incorrectly.

It looks like it would be quite a large change to fix this properly,
such that subranges are updated correctly when a subreg is dead in and
live out of a copy being erased.

A possible alternative would be to recalculate subranges from scratch
for a register that is affected by this problem.

For now, I have just disabled that case of erasing a copy when subranges
are present.

Change-Id: Ib85a4e7e5458ec1b156cb84207c51cab63e92635


Repository:
  rL LLVM

https://reviews.llvm.org/D48032

Files:
  lib/CodeGen/RegisterCoalescer.cpp
  test/CodeGen/AMDGPU/coalescing-with-subregs-in-loop-bug.mir

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