[PATCH] D124158: [Clang][Attr] Skip adding noundef attribute to arguments when function has convergent attribute

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

In D124158#3480566 <https://reviews.llvm.org/D124158#3480566>, @efriedma wrote:

> In D124158#3480384 <https://reviews.llvm.org/D124158#3480384>, @arsenm wrote:
>
>> I'm thinking noundef is a bit of red herring here. The real problem seems to be arising from the assume call which is inserted, which now introduces the assumption that the lane ID must be 0
>
> The optimizer is creating the llvm.assume call based on the violation of the noundef attribute.

I agree. As far as I can tell you have two options, both are specific to the shuffle functions:

1. Do not set noundef for calls to them as they allow undef values for all lanes we don't read the value.
2. Freeze the inputs unconditionally.

Convergent is unrelated to this.


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https://reviews.llvm.org/D124158