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

[Andy Kaylor via Phabricator via llvm-commits]

andrew.w.kaylor added a subscriber: craig.topper.
andrew.w.kaylor added a comment.

In D57504#1851864 <https://reviews.llvm.org/D57504#1851864>, @cameron.mcinally wrote:

> > Btw, I guess that https://reviews.llvm.org/D71432 shows that op+select folding can be cleanly implemented in isel and that's also in line with my experiments for the VE target.
>
> This needs a caveat. Keeping the select glued to the operation takes some careful effort. Especially in the undef passthru case, there are a bunch of peeps that will incorrectly fold away the select. E.g. this transform from InstSimplify:
>
>   if (isa<UndefValue>(FalseVal))   // select ?, X, undef -> X
>     return TrueVal;
>   
>
> The VP intrinsics will certainly be immune to these, but if the plan is to eventually replace the VP select intrinsics with IR selects, then this problem will need to be solved. Just a heads up...


@hsaito and I had a discussion about this earlier today. I had the same concern, that optimizations after the vectorizer might do something to decouple the vp.select from the vp.{operation}, which could lead to the code generator not being able to create a masked operation with passthru on targets that support that and thus potentially invalidate the cost model assumptions that the vectorizer made when it generated the predicated operation. Hideki convinced me that the additional freedom from explicit dependencies gained by not having a passthru argument as part of the predicated operation was likely to be more beneficial than tight coupling. If we ever do find this to be a problem, we can do something to make the intervening optimizations less aggressive with this sort of pattern.

I also talked briefly with @craig.topper about the X86 codegen handling of this, and his off the cuff reaction was to think that we probably won't have any problem generating the desired passthru+masked instructions from separated vp.select operations.


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