[PATCH] D79294: [InstSimplify] Remove known bits constant folding

[Nikita Popov via Phabricator via llvm-commits]

nikic reopened this revision.
nikic added a subscriber: arsenm.
nikic added a comment.
This revision is now accepted and ready to land.

Had to revert this due to AMDGPU test failures. What happens there is that AMDGPU expands divisions in a custom CGP pass, then instructions get simplified as part of EarlyCSE run it schedules. The simplification in question is a `x ashr 31` sign bit extraction, which gets folded to zero.

Not quite sure what to do about this. I guess it's one of a) just accept the change b) add a known sign check in the AMDGPU div expansion code or c) make SimplifyAShr more aggressive (i.e. use known bits) if we're extracting the sign bit. I'm leaning towards b). cc @arsenm


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