[PATCH] D155632: Preserve important metadata in JumpThreadingPass::unfoldSelectInstr

[Nikita Popov via Phabricator via llvm-commits]

nikic accepted this revision.
nikic added a comment.
This revision is now accepted and ready to land.

LGTM, but I do wonder whether we aren't working around a different issue here. JumpThreading generally does not perform threading across loop headers (it's behind a default-disabled option). As far as I understand, the select unfolding is just done to enable future threading. In this case, we unfold the select, but then can't thread it because that would require threading across a header.

We're left with what appear to me an unprofitable transform: We now have two loop latches, which is non-canonical. LoopSimplify will have to convert this back into a single latch.

So my thinking is that we should probably prevent select unfolding from happening here in the first place. I see that tryToUnfoldSelectInCurrBB() already guards against this, but tryToUnfoldSelec() does not.


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https://reviews.llvm.org/D155632