[llvm-dev] Should we have a "GPU working group"?

Thu Nov 11 12:34:10 PST 2021

Many of these issues (like scheduling) also affect and impact other targets
too that aren't necessarily GPUs.  Definitely better and broader
infrastructure here would be useful.

Steve

On Thu, Nov 11, 2021, 10:22 AM Jason Eckhardt via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> +1 here as well, even if in lurk mode initially.
>
> Some additional topics of interest:
>
>    1. *Occupancy vs resource trade-offs (especially registers)*. Requires
>    closer coordination of scheduling + register allocation, in addition to
>    extensions in each.
>    2. *Divergence analysis/uniformity analysis*. Applications to sync
>    insertion, scalar vs vector register allocation, instruction selection, etc.
>    3. *Synchronization insertion*. Some GPUs have instructions to hint
>    where a good re-convergence point is.
>    4. *Global instruction scheduling*. Some GPU code is surprisingly
>    branchy. Numerous out-of-tree/proprietary global schedulers have been
>    written, we *still* don't have one upstream.
>    5. I*ncreasing overall back-end speed for JIT compilation*. GPU
>    back-ends are often situated within graphics drivers where they dynamically
>    compile shaders with absurdly stringent timing constraints. GlobalISel is a
>    step in the right direction, but more needs doing (particularly for small
>    shaders).
>
> As an example, #1 is handled (if at all) specially by each target. With
> the proliferation of GPUs (Teams Red, Green, Blue, and others), it would be
> useful for "all boats to float higher" with generic code (with
> target-specific hooks/overrides) supported by the wider community as
> opposed to redundant re-invention of the wheel (or no wheel at all, as in
> #4). Then specific targets can expend effort adding value for their
> particular needs.
>
> ------------------------------
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Lei Zhang
> via llvm-dev <llvm-dev at lists.llvm.org>
> *Sent:* Thursday, November 11, 2021 9:44 AM
> *To:* Valentin Churavy <v.churavy at gmail.com>
> *Cc:* Lieberman, Ron <Ron.Lieberman at amd.com>; justin.lebar at gmail.com <
> justin.lebar at gmail.com>; Chesterfield, Jonathan <
> Jonathan.Chesterfield at amd.com>; llvm-dev at lists.llvm.org <
> llvm-dev at lists.llvm.org>; tianshilei1992 at gmail.com <
> tianshilei1992 at gmail.com>; Rodgers, Gregory <Gregory.Rodgers at amd.com>;
> Huber, Joseph <huberjn at ornl.gov>; Narayanaswamy, Ravi <
> ravi.narayanaswamy at intel.com>
> *Subject:* Re: [llvm-dev] Should we have a "GPU working group"?
>
> *External email: Use caution opening links or attachments*
> +1. There are certainly efforts that need broad collaboration among
> various interest parties; they would benefit from more interactive
> discussions to have everybody on the same page. What immediately jumps into
> my mind include how we can push to have unified/better SPIR-V support in
> LLVM proper / cross LLVM/MLIR, etc. It's a big one that I'd assume will
> take a long time to unfold and will have back and forth. Having a place to
> discuss will actually make sure we put efforts towards it (or at least have
> plans and revisit periodically). There are certainly many other topics to
> discuss too, as said by Johannes. I think this is a great idea.
>
> Thanks,
> Lei
>
>
> On Wed, Nov 10, 2021 at 4:03 PM Valentin Churavy via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> I would certainly be interested in joining such an effort.
>
> -V
>
> On Wed, Nov 10, 2021 at 3:56 PM Johannes Doerfert via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hey,
>
> We nowadays have various "working groups" that meet regularly to discuss
> technical topics (incl. RISC-V, MLIR, OpenMP, Alias Analysis, ML, Flang,
> ...).
> It's a reasonably nice way for people to connect and present ideas, get
> feedback, etc. in an interactive environment.
> While the OpenMP meeting has a strong emphasis on offloading, we do not
> meet to discuss "generic GPU" topics as a community.
>
> This email is to determine if people would be interested in meeting once
> every N weeks (initially we could say 2) to coordinate efforts in this
> space, e.g., development of GPU-specific optimizations, organization of
> driver and backend code, dealing with issues like convergent functions, ...
>
> I CC'ed a bunch of people that might want to join such a meeting but
> certainly this is something everyone would be invited to and I hope
> people forward this to parties that might not read llvm-dev.
>
> Looking forward to hear what people think :)
>
> ~ Johannes
>
>
>
>
> --
> ───────────────────
> ∽ Johannes (he/his)
>
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