[PATCH] D101829: [SCEV] By more careful when traversing phis in isImpliedViaMerge.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D101829#2740317 <https://reviews.llvm.org/D101829#2740317>, @nikic wrote:

> What BasicAA does for this case is a CFG reachability check from the phi to the value. If it's reachable, you have some kind of cycle. I think what you're doing here is fine though, as this doesn't seem like a terribly important optimization to me, and the nature of SCEV makes those properlyDominates() checks likely to succeed for common cases (as non-SCEVUnknown expressions are effectively hoisted).

I think we should start out with a relatively simple/safe fix and go from there. The impact of the code seems relatively small, e.g. if unconditionally bail out in this case, ~2 benchmarks out of SPEC2000/SPEC2006/MultiSource have minor binary changes.


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