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[InstSimplify] Re-enable select ?, undef, X -> X transform when X is provably…

Authored by craig.topper on Jul 9 2020, 12:20 PM.

Description

[InstSimplify] Re-enable select ?, undef, X -> X transform when X is provably not poison

Follow up from the transform being removed in D83360. If X is probably not poison, then the transform is safe.

Still plan to remove or adjust the code from ConstantFolding after this.

Differential Revision: https://reviews.llvm.org/D83440

Event Timeline

A question about this: We got a couple of dozen MSAN warnings in the Google codebase from the revision that removed these -- I'm guessing that perhaps what happened was that the undef in question was a use-of-uninitialized-value, and this optimization was hiding the use so the MSAN checks didn't trigger. Is re-enabling this going to make those MSAN warnings go away again, re-hiding this undefined behavior?