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[RegisterCoalescer] Fixed inconsistent followCopyChain with subreg
Summary: The behavior of followCopyChain with a subreg depends on the order in which subranges appear in a live interval, which is bad. This commit fixes that, and allows the copy chain to continue only if all matching subranges that are not undefined take us to the same def. I don't have a test for this; the reproducer I had on my branch with various other local changes does not reproduce the problem on upstream llvm. Also that reproducer was an ll test; attempting to convert it to a mir test made the subranges appear in a different order and hid the problem. However I would argue that the old behavior was obviously wrong and needs fixing. Subscribers: MatzeB, qcolombet, llvm-commits Differential Revision: https://reviews.llvm.org/D49535 Change-Id: Iee7936ef305918f3b498ac432e2cf651ae5cc2df llvm-svn: 338070
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