[llvm-dev] [RFC] Vector Predication (original) (raw)

David Greene via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 31 11:03:48 PST 2019

Philip Reames <listmail at philipreames.com> writes:

Question 1 - Why do we need separate mask and lengths? Can't the length be easily folded into the mask operand?

e.g. newmask = (<4 x i1>)((i4)%y & (1 << %L -1)) and then pattern matched in the backend if needed

I'm a little concerned about how difficult it will be to maintain enough information throughout compilation to be able to match this on a machine with an explicit vector length value.

Question 2 - Have you explored using selects instead? What practical problems do you run into which make you believe explicit predication is required?

e.g. %sub = fsub <4 x float> %x, %y %result = select <4 x i1> %M, <4 x float> %sub, undef

That is semantically incorrect. According to IR semantics, the fsub is fully evaluated before the select comes along. It could trap for elements where %M is 0, whereas a masked intrinsic conveys the proper semantics of masking traps for masked-out elements. We need intrinsics and eventually (IMHO) fully first-class predication to make this work properly.

My context for these questions is that my experience recently w/o existing masked intrinsics shows us missing fairly basic optimizations, precisely because they weren't able to reuse all of the existing infrastructure. (I've been working on SimplifyDemandedVectorElts recently for exactly this reason.) My concern is that your EVL proposal will end up in the same state.

I think that's just the nature of the beast. We need IR-level support for masking and we have to teach LLVM about it.

                       -David

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