[PATCH] D93586: [InstCombine] use poison as placeholder for undemanded elems

[Sanjay Patel via Phabricator via llvm-commits]

spatel added reviewers: nlopes, regehr, RKSimon, zhengyangl, nikic, hfinkel.
spatel added a comment.

Thank you for working on this! Looking back at the bug comments (and adding reviewers based on those comments), this is a step towards killing undef that has been discussed for a long time now. :)

Besides changing IRBuilder and shufflevector's definition, I think we'll also need updates in the vectorizers, InstSimplify, and other places in InstCombine that use UndefValue for InsertElement and shuffle transforms.

> A conservative way is copying all tests having insertelement undef & replacing it with insertelement poison & run Alive2 on it, but it will create many tests and people won’t like it. :(

Do you have an estimate of how many tests are out there? If it's a temporary increase and not huge, I don't think there is much downside. But if we think the transition will take a long time, then maybe we don't want the duplication.


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