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

Mon Apr 25 11:25:47 PDT 2016

Hi

Just to clarify where is what to everybody, libomptarget is published as
three incremental patches, here:

http://reviews.llvm.org/D14031
http://reviews.llvm.org/D14253
http://reviews.llvm.org/D14254

This is related to an extension to the current openmp project. I am very
happy to discuss about having this as a separate project.
Right now libomptarget (design+implementation) is very specific to OpenMP,
meaning that its interfaces have been crafted by looking at the OpenMP
specifications. This does not mean that it cannot be improved/generalized
for wider use, but this should start from its design document (can be found
here: https://github.com/clang-omp/OffloadingDesign).

Thanks

-- Carlo

From:	Hal Finkel via Openmp-dev <openmp-dev at lists.llvm.org>
To:	C Bergström <cbergstrom at pathscale.com>
Cc:	llvm-dev <llvm-dev at lists.llvm.org>, cfe-dev
            <cfe-dev at lists.llvm.org>, openmp-dev at lists.llvm.org
Date:	04/25/2016 02:14 PM
Subject:	Re: [Openmp-dev] [cfe-dev] [llvm-dev] RFC: Proposing an LLVM
            subproject for parallelism runtime and support libraries
Sent by:	"Openmp-dev" <openmp-dev-bounces at lists.llvm.org>

----- Original Message -----
> From: "C Bergström 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>, "cfe-dev"
<cfe-dev at lists.llvm.org>, openmp-dev at lists.llvm.org
> Sent: Monday, April 25, 2016 12:51:51 PM
> Subject: Re: [Openmp-dev] [cfe-dev] [llvm-dev] RFC: Proposing an LLVM
subproject for parallelism runtime and support
> libraries
>
> Speaking from 1st hand experience -
>
> The off-the-cuff high level benefits
> 1) Better interopt of different programming models - For example what
> if someone wants to mix SE+OMP(4). Having the same runtime on the
> backend should make this a lot easier. (Maybe this particular example
> may never happen, but I hope you get what I'm talking about)
> 2) Less duplication of effort for common things.
> 3) Easier time for new programming models
>
> By ignoring the current technical issues it's just playing
> kick-the-can for someone less intimate with SE or OMP.
>
> I hate to pushback on SE for this general requirement, but it seems
> like the right time to do it. If not now - At which point does it
> make
> sense?
> -------------
> My real honest motivation here is for Google and Intel to join
> together on this. I believe SE will be much better long term if major
> stakeholders are aligned.

This is also my long-term desire, and having spoken offline with folks on
both sides, I think that everyone shares the desire to merge and
deduplicate where that makes technical sense. We'd all like LLVM's runtime
picture to truly be an infrastructure, not a patchwork of semi-related
overlapping things.

However, at this point we have two separate projects representing
significant independent planning and developer effort, user communities for
both, and a desire to have both in LLVM. It seems logical that there may be
reuse to be exploited between the two, but frankly, I don't even thing we
know exactly what the layering would be. What we need at this point are
concrete proposals, and patches, to merge functionality. Having these kinds
of discussions in the abstract is difficult, the code is too large, and its
design constraints are too complicated, for that to be productive. This
process could occur out-of-tree, but that's suboptimal. We want the full
engagement and relative transparency of the LLVM community, and our
community infrastructure, to facilitate this process. Trying to get these
issues worked out behind the scenes is unlikely to be successful, and while
I agree that the community has pre-commit leverage, the most effective way
to use that leverage is to make it clear to the parties involved that the
community expects cross-pollination to happen: by code integration, or just
by intellectual contribution.

Let's get both things committed. Committing them does not freeze the API,
or otherwise commit us to support for the current implementations. Once
that's done, we'll be in a much stronger, and better informed, position to
discuss further refactoring.

Thanks again,
Hal

>
> How many dsisjoint and fragmented programming models do "we" really
> need.. (I don't think SE falls into this category, but if we have a
> PHI and Intel backend.. why can't it be used)
>
>
>
> On Tue, Apr 26, 2016 at 1:38 AM, Chandler Carruth
> <chandlerc at gmail.com> wrote:
> > On Mon, Apr 25, 2016 at 12:14 PM C Bergström
> > <cbergstrom at pathscale.com>
> > wrote:
> >>
> >> I can't comment on all the things not directly used by llvm
> >> community,
> >> but I feel pretty strongly that
> >> 1) An independent project like liboffload should exist ; which
> >> 2) Projects like SE and OpenMP should both be using it ; and
> >> further
> >> 3) SE shouldn't just do their own thing because they haven't
> >> figured
> >> out how to make it work with other projects that already have some
> >> overlapping behaviour
> >> ---------
> >> If my points above are invalid - can someone clarify that SE and
> >> the
> >> "stuff" in OpenMP-llvm doesn't actually overlap in functionality.
> >
> >
> > It isn't that any of these points are invalid, it's just that I
> > don't think
> > we know how or if these projects have a useful overlap to extract
> > and keep
> > separate. That was the whole point of my email suggesting that we
> > shouldn't
> > try to force some hypothetical refactoring that we don't even know
> > will work
> > to happen. Several serious technical challenges have been raised
> > with doing
> > this refactoring, so its not just avoiding it for the sake of
> > avoiding it.
> >
> > Even if/when these issues are sorted out and it is feasible to
> > refactor
> > things to have a common layer, it still isn't clear whether the
> > overlap is
> > actually that useful. I think we're over analyzing and designing
> > this when
> > we don't even have the code in place to see if there is an
> > interesting
> > problem to solve here.
> _______________________________________________
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> Openmp-dev at lists.llvm.org
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>

--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory
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