[PATCH] D57504: RFC: Prototype & Roadmap for vector predication in LLVM

[Andy Kaylor via Phabricator via llvm-commits]

andrew.w.kaylor added a comment.

In D57504#1861256 <https://reviews.llvm.org/D57504#1861256>, @simoll wrote:

> > Basically, I want to actively discourage front ends and optimizations from using the %evl argument in cases where it won't be optimal.
>
> TTI would tell front ends and optimizations that `%evl` is a no-go for your target. Is this enough discouragement?


In theory, yes. In practice, it will depend on how optimizations make use of that information. Your explanation of how the ExpandVectorPredicationPass will make this palatable to the backend worries me a little, because it essentially means that optimizations don't have to care that the target doesn't support this feature. They can generate IR that uses it and EVPP will smooth over it. Obviously, we could handle this on a case-by-case basis as it comes up. As you say, TTI will provide sufficient information for passes to make the decision.


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