[cfe-dev] [Openmp-dev] [llvm-dev] RFC: Proposing an LLVM subproject for parallelism runtime and support libraries

[Chandler Carruth via cfe-dev]

On Fri, Apr 22, 2016 at 3:05 PM Mehdi Amini <mehdi.amini at apple.com> wrote:

>
> > On Apr 22, 2016, at 3:01 PM, Chandler Carruth <chandlerc at gmail.com>
> wrote:
> >
> > I feel like this thread got a bit stalled. I'd like to pick it up and
> try to suggest a path forward.
> >
> > I don't hear any real objections to the overall idea of having an LLVM
> subproject for parallelism runtimes and support libraries. I think we
> should get that created.
>
> I think it should be clarified if "parallelism runtimes and support
> libraries" are intended to expose user-level APIs or if these are intended
> to expose APIs for the compiler generated code (this may be part of your
> point about "writing up its charter, scope" but I also think it shouldn't
> be underestimated as a task so I called it out).
>

Absolutely. I think that needs to be clearly spelled out.

Personally, I'd like to see the subproject open to *both*. Here are some
libraries I would love to see (but don't necessarily have concrete plans
around):
- A nice vectorized math library
- Linear algebra libraries like BLAS implementations or such
- Highly tuned FFT or other domain specific libraries for GPUs. Essentially
the same is the vectorized math libraries but for GPUs and slightly higher
level.
- Stream executor
- Any generic components of the OpenMP libraries.

Clearly each of these would need to be discussed on a case by case basis,
but there seems to be a healthy mixture of both user-level APIs and
compiler-level APIs. I would suggest criteria for being here along the
lines of:

- Includes compiler-targeted APIs (maybe in addition to user-level APIs,
maybe even with overlap), or
- Leverages compiler details for its implementation (for example, using
vector extensions we know LLVM supports), or
- Wants to use compiler-specific packaging techniques or other integration
techniques (for example shipping as bitcode), or
- Helps support compiler or programming language functionality

The first three here seem clear cut to me. If any part of the library is
intended to be callable by the compiler, its a good fit. SE has such
interfaces. Vectorized math libraries do too, etc. If the implementation of
th elibrary really wants to use compiler internals like our vector math
extensions, again, I think it makes sense to keep it reasonably co-located
with the compiler.

The last seems a bit tricky, but I think its really important. Currently,
CUDA provides a pretty big programming surface, and having a well tuned
BLAS or FFT implementation for example that integrates with CUDA is pretty
important. Similarly in the future, we expect C++ to get lots of parallel
standard library interfaces, potentially even BLAS-looking ones and we
might want a good parallel BLAS implementation or other very fundamental
parallel library implementation to use when implementing it.

But at the same time, I think its really important to have a clear place
where any library here ties back into the compiler ecosystem and/or the
programming language ecosystem that are the core of LLVM.

Does this seem like its going in the right direction? (Jason can probably
take on the non-trivial task of writing this up more formally and make sure
it is clearly documented.)


> Otherwise you plan sounds good to me.
>
> --
> Mehdi
>
>
>
> >
> > I don't actually see any real objections to StreamExecutor being one of
> the runtimes. There are some interesting questions however:
> > - Is there common code in the OpenMP runtime that could be unified with
> this?
> > - Could OpenMP end up using SE or some common shared library between
> them as a basis for offloading?
> > - Would instead it make more sense to have the OpenMP offload library be
> a plugin for StreamExecutor?
> >
> > I don't know the answer to any of these really, but I also don't think
> that they should prevent us from making progress here. And I think if
> anything, they'll become easier to answer if we do.
> >
> > So my suggestion would be:
> > 1) Create the broader scoped LLVM subproject, including writing up its
> charter, scope, plans, etc.
> >
> > 2) Add stream executor to it
> >
> > 3) Initially, leave the OpenMP offloading stuff targeted at OpenMP.
> Then, as it evolves, consider moving it to be another runtime in the broad
> project if and when it makes sense.
> >
> > 4) As both OpenMP and SE evolve and are used some in the project,
> evaluate whether there is a common core that makes sense to extract. If so,
> do it and rebase them appropriately.
> >
> >
> > Does this make sense? Are there objections to moving forward here?
>
>
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