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

[Valery Pykhtin via Phabricator via llvm-commits]

vpykhtin added a comment.

In D82258#2230981 <https://reviews.llvm.org/D82258#2230981>, @qcolombet wrote:

> Honestly, I have no idea one way or another. I'm leaning toward @arsenm's theory that the implicit def may be needed in case the full register is never fully covered.
> But I would need to dig deeper into this.

I'm trying to imagine the situation when a subreg needs IMPLICIT_DEF when other not-intersecting subreg of the same whole register already contain some value. If RenameIndpendentSubregs would split the whole reg into separate registers the unitialized subreg will have its own IMPLICIT_DEF. Machine scheduler doens't touch IMPLICIT_DEFs but it erases undef flags before scheduling and reset it after.


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https://reviews.llvm.org/D82258