[PATCH] Implement pragma llvm.vectorizer.enable in LoopVectorizer

[Renato Golin]

On 29 November 2013 20:26, Arnold <aschwaighofer at apple.com> wrote:
> But at which width/factor? If the cost model "fails" how should the vectorizer select "the right" one? (This case is covered by pragma width/unroll)

As you said below, whatever the vectorizer thinks it's best. But
that's arguable, and I agree it's not a good alternative.


> I think #pragma vectorizer enable should just turn on vectorization with automatic selection of the VF/UF.
> You also would not need new meta data for this llvm.vectorizer.unroll (0) and llvm.vectorizer.width(0) already have this semantics.

Ok, I didn't know that.


> To implement Oz and O1 vectorization we would pass a flag to the loop vectorizer to only vectorize loops marked with the right metadata.

Yes, and then we'd enable the Loop/SLP vectorizers by default, but at
lower levels, only run with metadata.


> At Os we take these flags to mean vectorize with O3 heuristics.

I see. We'll need to document all of that, because it's getting
complicated for the user. But I agree with this change, too.

I'll discard this code review and start the global one.

Thaks!
--renato