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

[Renato Golin via llvm-dev]

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.

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.

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.

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.

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.

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> 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
> 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
> Barcelona Supercomputing Center - Centro Nacional de Supercomputación
>
>
>
>
>
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