[llvm-dev] Loop-vectorizer prototype for the EPI Project based on the RISC-V Vector Extension (Scalable vectors)

Thu Nov 5 00:59:56 PST 2020

Fold the epilog loop into the vector body.

  *   This is done by setting the vector length in each iteration. This induces a predicate/mask over all the vector instructions of the loop (any other predicates/masks in the vector body are needed for control flow).

That's what we do for Arm MVE using intrinsic get.active.lane.mask (*) which is emitted in the vectoriser. It generates a predicate that is used by the masked loads/stores. That's the current state of the art, long term that should indeed be using the VP intrinsics. Just wanted to point you at  get.active.lane.mask, because it would also be nice to get confirmation that this not only works for fixed vectors but also scalable vectors, which I think should be the case...

(*) https://llvm.org/docs/LangRef.html#llvm-get-active-lane-mask-intrinsics

Cheers,
Sjoerd.
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Vineet Kumar via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 05 November 2020 01:36
To: Renato Golin <rengolin at gmail.com>
Cc: LLVM Dev <llvm-dev at lists.llvm.org>; ROGER FERRER IBANEZ <roger.ferrer at bsc.es>; Arai, Masaki <arai.masaki at jp.fujitsu.com>
Subject: Re: [llvm-dev] Loop-vectorizer prototype for the EPI Project based on the RISC-V Vector Extension (Scalable vectors)

Hi Renato,

Thanks a lot for your comments!

(more inline.)

Thanks and Regards,

Vineet

On 2020-11-02 5:43 p.m., Renato Golin wrote:
Hi Vineet,

Thanks for sharing! I haven't looked at the code yet, just read the README file you have and it has already answered a lot of questions that I initially had. Some general comments...

I'm very happy to see that Simon's predication changes were useful to your work. It's a nice validation of their work and hopefully will help SVE, too.
Simon's vector predication ideas fit really nicely with our approach to predicated vectorization, specially the support for EVL parameter. We look forward to more discussions around it.

Your main approach to strip-mine + fuse tail loop is what I was going to propose for now. It matches well with the bite-sized approach VPlan has and could build on existing vector formats. For example, you always try to strip-mine (for scalable and non-scalable) and then only for scalable, you try to fuse the scalar loops, which would improve the solution and give RVV/SVVE an edge over the other extensions on the same hardware.
While our implemented approach with tail folding and predication is guided by the research interests of the EPI project, I agree that for a more general implementation your proposed approach for now makes more sense before moving on to better predication support and exploring other approaches.

There were also in the past proposals to vectorise the tail loop, which could be a similar step. For example, in case the main vector body is 8-way or 16-way, the tail loop would be 7-way or 15-way, which is horribly inefficient. The idea was to further vectorise the 7-way as 4+2+1 ways, same for 15. If those loops are then unrolled, you end up with a nice decaling down pattern. On scalable vectors, this becomes a noop.

There is a separate thread for vectorisation cost model [1] which talks about some of the challenges there, I think we need to include scalable vectors in consideration when thinking about it.
Agreed. It would be very useful to think about a scalable vectors aware cost-model right from the beginning now that there is effort already underway to integrate it into VPlan. There was also a discussion around it in the latest SVE/SVE2 sync-up meeting and I think almost everyone was in agreement.

The NEON vs RISCV register shadowing is interesting. It is true we mostly ignored 64-bit vectors in the vectoriser, but LLVM can still generate them with the (SLP) region vectoriser. IIRC, support for that kind of aliasing is not trivial (and why GCC's description of NEON registers sucked for so long), but the motivation of register pressure inside hot loops is indeed important. I'm adding Arai Masaki in CC as this is something he was working on.

Thanks for adding Arai! I will be happy to pick their brain on the the topic.

One specific place where we have to deal with it is when computing a feasible max VF. I am currently experimenting with an approach to have user specify (via a command line flag) a vector register width multiplier - a factor by which the operating vector register width would be the multiple of the minimum vector register width and then based on that, estimate the highest VF that won't spill registers (relies on TTI for information about the number of registers in relation to register width). This is definitely not a generic solution and probably not elegant either but personally it serves as a starting point to think about the broader issue.

Otherwise, I think working with the current folks on VPlan and scalable extensions will be a good way to upstreaming all the ideas you guys had in your work.
That's the plan!

Thanks!
--renato

[1] http://lists.llvm.org/pipermail/llvm-dev/2020-October/146236.html

On Mon, 2 Nov 2020 at 15:52, Vineet Kumar via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi all,

At the Barcelona Supercomputing Center, we have been working on an end-to-end vectorizer using scalable vectors for RISC-V Vector extension in context of the EPI Project<https://www.european-processor-initiative.eu/accelerator/>. We earlier shared a demo of our prototype implementation  (https://repo.hca.bsc.es/epic/z/9eYRIF, see below) with the folks involved with LLVM SVE/SVE2 development. Since there was an interest in looking at the source code during the discussions in the subsequent LLVM SVE/SVE2 sync-up meetings, we are also publishing a public copy of our repository.

It is available at https://repo.hca.bsc.es/gitlab/rferrer/llvm-epi and will sync with our ongoing development on a weekly basis. Note that this is very much a work in progress and the code in this repository is only for reference purpose. Please see the README<https://repo.hca.bsc.es/gitlab/rferrer/llvm-epi/-/blob/EPI/README.md> file in the repo for details on our approach, design decisions, and limitations.

We welcome any questions and feedback.

Thanks and Regards,
Vineet Kumar - vineet.kumar at bsc.es<mailto:vineet.kumar at bsc.es>
Barcelona Supercomputing Center - Centro Nacional de Supercomputación

On 2020-07-29 3:10 a.m., Vineet Kumar wrote:

Hi all,

Following up on the discussion in the last meeting about auto-
vectorization for RISC-V Vector extension (scalable vectors) at the
Barcelona Supercomputing Center, here are some additional details.

We have a working prototype for end-to-end compilation targeting the
RISC-V Vector extension. The auto-vectorizer supports two strategies to
generate LLVM IR using scalable vectors:

1) Generate a vector loop using VF (vscale x k) = whole vector register
width, followed by a scalar tail loop.

2) Generate only a vector loop with active vector length controlled by
the RISC-V `vsetvli` instruction and using Vector Predicated intrinsics
(https://reviews.llvm.org/D57504). (Of course, intrinsics come with
their own limitations but we feel it serves as a good proof of concept
for our use case.) We also extend the VPlan to generate VPInstructions
that are expanded using predicated intrinsics.

We also considered a third hybrid approach of having a vector loop with
VF = whole register width, followed by a vector tail loop using
predicated intrinsics. For now though, based on project requirements,
we favoured the second approach.

We have also taken care to not break any fixed-vector implementation.
All the scalable vector IR gen is guarded by conditions set by TTI.

For shuffles, the most used case is broadcast which is supported by the
current semantics of `shufflevector` instruction. For other cases like
reverse, concat, etc., we have defined our own intrinsics.

Current limitaitons:
The cost model for scalable vectors doesn't do much other than always
decideing to vectorize with VF based on TargetWidestType/SmallestType.
We also do not support interleaving yet.

Demo:
The current implementation is very much in alpha and eventually, once
it's more polished and thoroughly verified, we will put out patches on
Phabricator. Till then, we have set up a Compiler Explorer server
against our development branch to showcase the generated code.

You can see and experiment with the generated LLVM IR and VPlan for a
set of examples, with predicated vector loop (`-mprefer-predicate-over-
epilog`) at https://repo.hca.bsc.es/epic/z/JB4ZoJ
and with a scalar epilog (`-mno-prefer-predicate-over-epilog`) at
https://repo.hca.bsc.es/epic/z/0WoDGt.
Note that you can remove the `-emit-llvm` option to see the generated
RISC-V assembly.

We welcome any questions and feedback.

Thanks and Regards,
Vineet Kumar - vineet.kumar at bsc.es<mailto:vineet.kumar at bsc.es>
Barcelona Supercomputing Center - Centro Nacional de Supercomputación

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