[PATCH] D82258: [RegisterCoalescer] Fix IMPLICIT_DEF init removal for a register on joining

Wed Jul 15 16:42:45 PDT 2020

arsenm added a comment.

In D82258#2152354 <https://reviews.llvm.org/D82258#2152354>, @vpykhtin wrote:

> In D82258#2148331 <https://reviews.llvm.org/D82258#2148331>, @arsenm wrote:
>
> > It would help if 9d7bc0874cf20f44cd331c77f5a003b4c4b262bd had a test...
> >
> > I'm somewhat suspicious, since I have seen cases where the implicit def is needed in cases where the full register is never fully covered. However, it's possible this is still a leftover from before DetectDeadLanes was added
>
>
> Do you mean there might be a situation when some lane becomes uninitialized after RenameIndpendentSubregs? I checked RenameIndpendentSubregs pass - it has dedicated code in computeMainRangesFixFlags (line 310 ..) that creates IMPLICIT_DEF initialization for indpendent lanes.

The scheduler runs after RenameIndpendentSubregs, which makes it seem like anything can happen

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https://reviews.llvm.org/D82258