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

Tue May 31 20:05:24 PDT 2016

----- Original Message -----

> From: "Andrey Bokhanko via Openmp-dev" <openmp-dev at lists.llvm.org>
> To: "Chandler Carruth" <chandlerc at gmail.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "Mehdi Amini"
> <mehdi.amini at apple.com>, "cfe-dev" <cfe-dev at lists.llvm.org>,
> openmp-dev at lists.llvm.org
> Sent: Wednesday, April 27, 2016 11:12:17 AM
> Subject: Re: [Openmp-dev] [llvm-dev] [cfe-dev] RFC: Proposing an LLVM
> subproject for parallelism runtime and support libraries

> Ahh, I just noticed that Chandler's proposal is to put SE only into
> this new project, and to keep libomptarget separately, in OpenMP
> project. I wonder why so? Why SE (a library serving only one PPM so
> far) is different from libomptarget (a library also serving only one
> PPM so far)?

> Are people have opinion on this?

I just clarified with Chandler that he proposes both to go into the new project. I agree that this is what we should do. 

-Hal 

> Yours,
> Andrey
> =====

> Software Engineer
> Intel Compiler Team

> On Tue, Apr 26, 2016 at 6:06 PM, Andrey Bokhanko <
> andreybokhanko at gmail.com > wrote:

> > Chandler,
> 

> > It seems that the general consensus is to save further discussions
> > for later and go ahead with your proposal. I can add my +1 to this
> > as well.
> 

> > Could you / Jason prepare a proposal on the new project, with all
> > the
> > usual questions covered? One interesting thing is where to put SE
> > and libomptarget in the project's tree. I would be happy to review
> > it from Intel / OpenMP side.
> 

> > Also, as Carlo noted, libomptarget is currently under review (a bit
> > stalled one, if I may say so). Do we expect SE to undergo through a
> > similar review -- piece by piece -- as well?
> 

> > Yours,
> 
> > Andrey
> 
> > ======
> 
> > Software Engineer
> 
> > Intel Compiler Team
> 

> > 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?
> > > 
> > 
> 

> > > _______________________________________________
> > 
> 
> > > Openmp-dev mailing list
> > 
> 
> > > Openmp-dev at lists.llvm.org
> > 
> 
> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/openmp-dev
> > 
> 

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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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