Tail-Loop Folding/Predication

I am looking for feedback to add support for a new loop pragma to Clang/LLVM.

With “#pragma tail_predicate” the idea would be to indicate that a loop

epilogue/tail can, or should be, folded into the main loop. I see two use

cases for this pragma.

First, this could be interesting for the vectorizer. It currently supports tail

folding by masking all loop instructions/blocks, but does this only when

optimising for size is enabled. This pragma could override the cost-model/opt-level.

Second use case would be the Armv8.1-M MVE vector extension, which supports

tail-predicated hardware loops. This version of hardware loops sets the vector

lanes to be masked, and is thus a nice optimisation that avoids generating a

tail loop when the number of elements processed is not a multiple of the vector

length.

For this use case, the tail predicate pragma could be good user experience

improvement, as it would for example allow this more compact form without

any predicated intrinsics:

#pragma tail_predicate

do {

VLD(…); // some vector load intrinsic

VST(…); // some vector store intrinsic

} while (N);

which can then be transformed and predication made explicit through data

dependencies like so:

do {

mask = vctp(N); // intrinsic that generates the mask of active lanes

VLD(…, mask);

VST(…, mask);

} while (N);

A vector loop in this form can easily be picked up the new hardware loop pass,

and the corresponding tail-predicated hardware loop can be generated. This is

only a small example, but we think for more complicated examples we think

the benefit could be substantial.

I have uploaded a patch for the initial Clang plumbing exercise here:

https://reviews.llvm.org/D64744

Cheers,
Sjoerd.

By “folded into the main loop”, do you actually mean replace the main loop? So, in effect, running the entire loop under predicate so there is only one loop body?

If so, I think that will be a useful pragma in general, but in my opinion, the name is not appropriate since it won’t have anything to do with the tail other than how this is accomplished at the moment. Is your thinking that the front end would generate the mask calculation, or are you just leveraging the exiting fold tail by masking and removing the original vectorized loop body?

I think the proper implementation should really be to generate the predicated instructions in the first place (I’d like to also see actual predicates on the instructions instead of selects, but that is another thread), so I think #pragma loop vectorize(enable) predicated(enable) (or something like that) seems a better choice. This would also allow you to disable loops run under predicate if the cost model in LLVM (or downstream) in the future thinks its best to generate this type of loop and performance numbers suggest otherwise.

Hi Scott,

Yes, I meant exactly that:

So, in effect, running the entire loop under predicate so there is only one loop body?

We see having one loop body as the easiest and also best way to detect and support these so called “tail-predicated hardware loops”. This solves the pass ordering problem of vectorizing and creating hardware loops and keeps the loop in an easier to analyse form.

…, or are you just leveraging the exiting fold tail by masking and removing the original vectorized loop body?

It looks like we can leverage the vectoriser as it already capable of folding the tail and predicating the instructions, but we just need to steer the decision making with e.g. this new pragma. For example, in LoopVectorizationLegality::canFoldTailByMasking() this decision making and transformation is happening.

Probably my suggestion for 1 generic pragma for both cases (the vectorizer, and tail-predicated hardware loops) was wrong. My reason however for suggesting 1 generic pragma was that I thought people would not be an awful less interested in an ARM MVE specific pragma; I thought I would increase my chances with a generic one, but that didn’t seem to work. :wink: I actually agree that a generic one like this:

#pragma tail_predicate

doesn’t seem a good fit for the vectorizer, which should indeed be something like this:

#pragma loop vectorize(enable) predicated(enable)

but this one doesn’t seem a great fit for my example, i.e. I see that it probably also work for my use case, but a vectorize pragma is probably not a great fit here. So that suggest 2 pragmas would be best.

It looks like people like the extra “predicated(enable)” hint as part of the vectorize pragma, so I will implement that first. This also allows to me to continue prototyping the actual transformation (rewriting unpredicated intrinsics to predicated ones), which might perhaps be a bit of an odd one, but I still think that could be convenient for users. I can always propose another pragma once I got more experience with my transformation.

Thanks,
Sjoerd.