[PATCH] D84792: [InstSimplify/NewGVN] Add option to control the use of undef.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D84792#2179916 <https://reviews.llvm.org/D84792#2179916>, @efriedma wrote:

> Is the goal here that it's legal to replace the output value of SimplifyInstruction with the input value, in addition to the normal replacement of the input value with the output value?  Or do we specifically care about literal UndefValue constants somehow, as opposed to values which are undef?  If we had a PoisonValue constant, would we also need to exclude it?

I think we probably want to be able to replace in both directions as you mentioned. I should have been more specific. The immediate concern is undef constants, as they are most likely to introduce problems (e.g. if the the undef is interpreted as different value for different simplify calls). But we probably also want to limit distribution, as in @aqjune 's example in D84655 <https://reviews.llvm.org/D84655>. This is probably going to be much harder to audit for though I am afraid.


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