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

Sergey Ostanevich via cfe-dev cfe-dev at lists.llvm.org
Mon Apr 25 08:40:57 PDT 2016


Thank you for getting it up to ML top.

I believe we have to move broader than that you just mentioned. The natural
separation of the infrastructure into different parts can be across the
following lines:

- the parallel model of programming - these can be OpenMP, OpenACC,
CilkPlus, OpenCL, StreamExecutor, CUDA, C++ parallel extensions, etc.
- the offloading machinery to be used by any of those above and providing
unified interfaces across all targets to be supported
- the performance libraries collection that can be re-used in different
programming models and be targeting different host/targets planforms

I would like to touch the 2nd bullet, since I had most exerience with
it.There should be a single interface for all offloading players that are
willing to take part. Those are not limited to StreamExecutor and the
OpenMP already published in LLVM. There are number of solutions from Intel,
not saying of others, - it would be reasonable to become a platform for all
of them, and I got positive feedback on the idea within.
To name a few (don't take it as an ad):

 - Hetero Streams Library, https://01.org/hetero-streams-library
 - Beignet Project, https://01.org/beignet
 - Math Kernel Libraries, https://software.intel.com/en-us/intel-mkl
 - Intel Compiler, https://software.intel.com/en-us/intel-compilers

I believe we shouldn't make any difference between StreamExecutor and other
projects and to try to plug one into the other or vice versa. The better
would be to reuse the same ground level I/O machinery that will provide
efficiency to all of these and the newcomers. The machinery should have
some specific attributes, such as support of multitude of languages
currently employed by LLVM project and beyond. Also we have to take into
account different application of the compiler and infrastructure: there can
be server solutions where we are free to use full-featured C++ and there
can be embedded solutions, such as automotive, where customers are tend to
have as few runtime support as possible and like C the most.

Intel Compilers

On Sat, Apr 23, 2016 at 1:24 AM, Chandler Carruth via Openmp-dev <
openmp-dev at lists.llvm.org> wrote:

> 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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