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

[Renato Golin via Phabricator via llvm-commits]

rengolin added a comment.

In D57504#1724217 <https://reviews.llvm.org/D57504#1724217>, @cameron.mcinally wrote:

> Code explosion is the symptom, not the sickness. It's caused by using experimental intrinsics. Experimental intrinsics are a detriment to progress. They end up creating a ton more work and are designed to be inevitably replaced.


I think this is a big hammer argument for a nuanced topic.

We have used experimental intrinsics for a large number of disparate concepts, from exception handling to fuzzy vector extensions, and then after the semantics was defined and accepted, we baked the concepts into IR.

This is a proven track, and predication is a very similar example to past experiences, I see no contradiction here.

> IMHO, we'd be better off baking these new features into LLVM right from the start. These 3 topics are fairly significant features. It would be hard to argue that any one will go out of style in the foreseeable future...

The risk of getting it wrong and having to re-bake into IR is high. We've done that with exception handling before and it wasn't pretty.

Predication is already in native IR form, albeit complex and error prone. The nuances across targets are too many to have a simple implementation working for everyone, and having a concrete implementation of the idea in intrinsic form may help clear up the issues before we stick to anything.

It's quite possible, and I really hope, that only a few targets will actually implement them, and that will be enough, so intrinsics will be short lived. Meanwhile, previous IR patterns will still match, so nothing is lost.

Of course, as with any intrinsic, it's quite possible that it will "just work" and people will give up half-way through. But history has shown that more often than not, these group efforts finish with a reasonable implementation, better than what we had before.

cheers,
--renato


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