[PATCH] D90382: [InstCombine] foldSelectRotate - generalize to foldSelectFunnelShift

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added a comment.

In D90382#2375905 <https://reviews.llvm.org/D90382#2375905>, @nikic wrote:

> @nlopes I think we should adjust the funnel shift definition to say that it blocks poison on one operand if the shift amount is zero. Basically the poison semantics should be "as if" the funnel shift were expanded, which does include an explicit select for the zero shift amount case.

The issue with having instructions block poison is that then you can't remove those instruction. Right now only `select` and `freeze` block poison. So I would try to stay away of making funnel shift block poison.

In D90382#2375906 <https://reviews.llvm.org/D90382#2375906>, @RKSimon wrote:

> Thanks @nlopes - I assume a suitably positioned isGuaranteedNotToBeUndefOrPoison guard should be enough to help with this?

Right. Or use freeze if the transformation is quite important for your benchmarks.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90382/new/

https://reviews.llvm.org/D90382