[cfe-dev] RFC: clacc: translating OpenACC to OpenMP in clang

Joel E. Denny via cfe-dev cfe-dev at lists.llvm.org
Fri Dec 8 09:00:31 PST 2017


On Fri, Dec 8, 2017 at 11:32 AM, Jeff Hammond <jeff.science at gmail.com>
wrote:

>
>
> On Fri, Dec 8, 2017 at 7:51 AM, Joel E. Denny <jdenny.ornl at gmail.com>
> wrote:
> >
> > Hi Jeff, Hal,
> >
> > Thanks for your feedback.  My comments are inline below.
> >
> > On Tue, Dec 5, 2017 at 6:43 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> >>
> >> On 12/05/2017 05:11 PM, Jeff Hammond via cfe-dev wrote:
> >>
> >> All of the usage of OpenACC outside of benchmarks/research that I know
> about is done in Fortran.
> >
> > I agree that it's easier to find real apps that use OpenACC in Fortran
> than those that use OpenACC in C/C++.  However, the latter certainly
> exist.  For example:
>
> Two of the three examples you cite are primarily Fortran and using OpenACC
> exclusively in Fortran subroutines.
>

Are you saying that the occurrences of "pragma acc" in Nek5000 and NekCEM
are unused?


> > http://mrfil.github.io/PowerGrid/
>
> /tmp/PowerGrid$ git grep -il "pragma acc"
> PowerGrid/Gfft.hpp
> PowerGrid/Gnufft.hpp
> PowerGrid/ftCpu.hpp
> PowerGrid/gridding.hpp
> PowerGrid/griddingSupport.hpp
>
> From http://mrfil.github.io/PowerGrid/docs/Installation:
>
>
>
>
>
> *We have experience with PGC++ 15.7 from NVIDIA/The Portland Group as the
> version we have used most extensively. There is a free license available as
> part of the OpenACC Toolkit for academic users.GCC 6.1 has OpenACC support
> but has not yet been tested by the developers, we welcome reports of anyone
> trying to compile with it. We hope to support it alongside PGI compilers in
> the near future.For those lucky enough to have access to Cray
> supercomputers, the Cray compiler does support OpenACC, but we have not
> tried to build with it. Because the Cray compilers are not available on
> desktops, workstations, or non-Cray branded clusters, we have not dedicated
> resources to testing PowerGrid on it.*
>
> So these folks support OpenACC, but haven't bothered to try the GCC
> implementation in the 1+ year that it's been available.  How likely are
> they to use Clang's?
>

I cannot answer that. Perhaps they were waiting for GCC support to mature?

Thanks.

Joel


> > https://nek5000.mcs.anl.gov/ (look at the openacc branch in github)
>
> (on the openacc branch)
>
> /tmp/Nek5000$ git grep -il "\$acc "
> core/acc.f
> core/comm_mpi.f
> core/gmres.f
> core/hmholtz.f
> core/hsmg.f
> core/math.f
> core/navier1.f
> core/navier4.f
> core/plan4.f
> core/prepost.f
> core/subs2.f
>
> >
> > https://nekcem.mcs.anl.gov/
>
> (on master)
> /tmp/svn$ git grep -il "\$acc"
> branches/maxwell-experimental/src/cem_dg.F
> branches/maxwell-experimental/src/dssum2.F
> branches/maxwell-experimental/src/io.F
> branches/maxwell-experimental/src/mat1.F
> branches/maxwell-experimental/src/maxwell.F
> branches/maxwell-experimental/src/maxwell_acc.F
> branches/maxwell-experimental/src/mxm_acc.F
> branches/trunkQu/src/quantum_csr.F
> branches/trunkQu/src/quantum_setup.f
> branches/trunkQu/src/quantum_time.F
> trunk/examples/openacc_gpu=1/box.usr
> trunk/examples/openacc_gpu=8/box.usr
> trunk/src/acoustic.F
> trunk/src/cem_dg2.F
> trunk/src/complex.F
> trunk/src/drift1.F
> trunk/src/drift1_maud.F
> trunk/src/drive.F
> trunk/src/drive_maud.F
> trunk/src/dssum2.F
> trunk/src/hmholtz.F
> trunk/src/io.F
> trunk/src/mat1.F
> trunk/src/maxwell.F
> trunk/src/maxwell_acc.F
> trunk/src/mg_r2204.F
> trunk/src/mxm_acc.F
> trunk/src/poisson.F
> trunk/src/quantum2.F
> www/examples/libs/phpThumb/phpthumb.functions.php
> www/examples/phpthumb.functions.php
>
> >>   Can you provide a list of C/C++ applications using OpenACC today and
> estimate the number of users that will benefit from this feature?
> >>
> >>
> >> Such lists exist, although I don't know what can be shared (and Oak
> Ridge likely has better lists in this regard than I do).
> >
> > I'll look for a better list that I can share.
>
> That would be helpful.
>
> Best,
>
> Jeff
>
>
>
> >> I can tell you, from my own experience, that we're seeing an increase
> in development using OpenACC, in both C/C++ and Fortran, over the last
> couple of years (essentially because the compiler technology has improved
> to the point where that is now a potentially-productive choice).
> >
> >
> > Providing support in a production-quality, open-source compiler tool
> chain like LLVM will hopefully accelerate this trend.
> >
> > Joel
> >
> >>
> >> Also, we have a strong desire to enable tooling over code bases using
> OpenACC. Among many other things, at some point we'll likely want the
> option to automatically migrate much of this code to using OpenMP. Having
> an OpenACC-enabled Clang, with an implementation that maps to OpenMP, is an
> important step in that process.
> >>
> >>  -Hal
> >>
> >>
> >>
> >> Thanks,
> >>
> >> Jeff
> >>
> >> On Tue, Dec 5, 2017 at 11:06 AM, Joel E. Denny via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> >>>
> >>> Hi,
> >>>
> >>> We are working on a new project, clacc, that extends clang with
> OpenACC support.  Clacc's approach is to translate OpenACC (a descriptive
> language) to OpenMP (a prescriptive language) and thus to build on clang's
> existing OpenMP support.  While we plan to develop clacc to support our own
> research, an important goal is to contribute clacc as a production-quality
> component of upstream clang.
> >>>
> >>> We have begun implementing an early prototype of clacc.  Before we get
> too far into the implementation, we would like to get feedback from the
> LLVM community to help ensure our design would ultimately be acceptable for
> contribution.  For that purpose, below is an analysis of several high-level
> design alternatives we have considered and their various features.  We
> welcome any feedback.
> >>>
> >>> Thanks.
> >>>
> >>> Joel E. Denny
> >>> Future Technologies Group
> >>> Oak Ridge National Laboratory
> >>>
> >>>
> >>> Design Alternatives
> >>> -------------------
> >>>
> >>> We have considered three design alternatives for the clacc compiler:
> >>>
> >>> 1. acc src  --parser-->                     omp AST  --codegen-->
>  LLVM IR + omp rt calls
> >>> 2. acc src  --parser-->  acc AST                     --codegen-->
>  LLVM IR + omp rt calls
> >>> 3. acc src  --parser-->  acc AST  --ttx-->  omp AST  --codegen-->
>  LLVM IR + omp rt calls
> >>>
> >>> In the above diagram:
> >>>
> >>> * acc src = C source code containing acc constructs.
> >>> * acc AST = a clang AST in which acc constructs are represented by
> >>>   nodes with acc node types.  Of course, such node types do not
> >>>   already exist in clang's implementation.
> >>> * omp AST = a clang AST in which acc constructs have been lowered
> >>>   to omp constructs represented by nodes with omp node types.  Of
> >>>   course, such node types do already exist in clang's
> >>>   implementation.
> >>> * parser = the existing clang parser and semantic analyzer,
> >>>   extended to handle acc constructs.
> >>> * codegen = the existing clang backend that translates a clang AST
> >>>   to LLVM IR, extended if necessary (depending on which design is
> >>>   chosen) to perform codegen from acc nodes.
> >>> * ttx (tree transformer) = a new clang component that transforms
> >>>   acc to omp in clang ASTs.
> >>>
> >>> Design Features
> >>> ---------------
> >>>
> >>> There are several features to consider when choosing among the designs
> >>> in the previous section:
> >>>
> >>> 1. acc AST as an artifact -- Because they create acc AST nodes,
> >>>    designs 2 and 3 best facilitate the creation of additional acc
> >>>    source-level tools (such as pretty printers, analyzers, lint-like
> >>>    tools, and editor extensions).  Some of these tools, such as pretty
> >>>    printing, would be available immediately or as minor extensions of
> >>>    tools that already exist in clang's ecosystem.
> >>>
> >>> 2. omp AST/source as an artifact -- Because they create omp AST
> >>>    nodes, designs 1 and 3 best facilitate the use of source-level
> >>>    tools to help an application developer discover how clacc has
> >>>    mapped his acc to omp, possibly in order to debug a mapping
> >>>    specification he has supplied.  With design 2 instead, an
> >>>    application developer has to examine low-level LLVM IR + omp rt
> >>>    calls.  Moreover, with designs 1 and 3, permanently migrating an
> >>>    application's acc source to omp source can be automated.
> >>>
> >>> 3. omp AST for mapping implementation -- Designs 1 and 3 might
> >>>    also make it easier for the compiler developer to reason about and
> >>>    implement mappings from acc to omp.  That is, because acc and omp
> >>>    syntax is so similar, implementing the translation at the level of
> >>>    a syntactic representation is probably easier than translating to
> >>>    LLVM IR.
> >>>
> >>> 4. omp AST for codegen -- Designs 1 and 3 simplify the
> >>>    compiler implementation by enabling reuse of clang's existing omp
> >>>    support for codegen.  In contrast, design 2 requires at least some
> >>>    extensions to clang codegen to support acc nodes.
> >>>
> >>> 5. Full acc AST for mapping -- Designs 2 and 3 potentially
> >>>    enable the compiler to analyze the entire source (as opposed to
> >>>    just the acc construct currently being parsed) while choosing the
> >>>    mapping to omp.  It is not clear if this feature will prove useful,
> >>>    but it might enable more optimizations and compiler research
> >>>    opportunities.
> >>>
> >>> 6. No acc node classes -- Design 1 simplifies the compiler
> >>>    implementation by eliminating the need to implement many acc node
> >>>    classes.  While we have so far found that implementing these
> >>>    classes is mostly mechanical, it does take a non-trivial amount of
> >>>    time.
> >>>
> >>> 7. No omp mapping -- Design 2 does not require acc to be mapped to
> >>>    omp.  That is, it is conceivable that, for some acc constructs,
> >>>    there will prove to be no omp syntax to capture the semantics we
> >>>    wish to implement.  It is also conceivable that we might one day
> >>>    want to represent some acc constructs directly as extensions to
> >>>    LLVM IR, where some acc analyses or optimizations might be more
> >>>    feasible to implement.  This possibility dovetails with recent
> >>>    discussions in the LLVM community about developing LLVM IR
> >>>    extensions for various parallel programming models.
> >>>
> >>> Because of features 4 and 6, design 1 is likely the fastest design to
> >>> implement, at least at first while we focus on simple acc features and
> >>> simple mappings to omp.  However, we have so far found no advantage
> >>> that design 1 has but that design 3 does not have except for feature
> >>> 6, which we see as the least important of the above features in the
> >>> long term.
> >>>
> >>> The only advantage we have found that design 2 has but that design 3
> >>> does not have is feature 7.  It should be possible to choose design 3
> >>> as the default but, for certain acc constructs or scenarios where
> >>> feature 7 proves important (if any), incorporate design 2.  In other
> >>> words, if we decide not to map a particular acc construct to any omp
> >>> construct, ttx would leave it alone, and we would extend codegen to
> >>> handle it directly.
> >>>
> >>> Conclusions
> >>> -----------
> >>>
> >>> For the above reasons, and because design 3 offers the cleanest
> >>> separation of concerns, we have chosen design 3 with the possibility
> >>> of incorporating design 2 where it proves useful.
> >>>
> >>> Because of the immutability of clang's AST, the design of our proposed
> >>> ttx component requires careful consideration.  To shorten this initial
> >>> email, we have omitted those details for now, but we will be happy to
> >>> include them as the discussion progresses.
> >>>
> >>> _______________________________________________
> >>> cfe-dev mailing list
> >>> cfe-dev at lists.llvm.org
> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >>>
> >>
> >>
> >>
> >> --
> >> Jeff Hammond
> >> jeff.science at gmail.com
> >> http://jeffhammond.github.io/
> >>
> >>
> >> _______________________________________________
> >> cfe-dev mailing list
> >> cfe-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >>
> >>
> >> --
> >> Hal Finkel
> >> Lead, Compiler Technology and Programming Languages
> >> Leadership Computing Facility
> >> Argonne National Laboratory
> >
> >
>
>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>
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