[cfe-dev] [RFC] Proposal to contribute Intel’s implementation of C++17 parallel algorithms

Jeff Hammond via cfe-dev cfe-dev at lists.llvm.org
Fri Dec 8 13:55:49 PST 2017


On Fri, Dec 8, 2017 at 1:13 PM, Hal Finkel <hfinkel at anl.gov> wrote:

>
> On 12/07/2017 11:35 AM, Jeff Hammond wrote:
>
>
> On Wed, Dec 6, 2017 at 8:57 PM Hal Finkel <hfinkel at anl.gov> wrote:
>
>>
>> On 12/06/2017 10:23 PM, Jeff Hammond wrote:
>>
>>
>> On Wed, Dec 6, 2017 at 4:23 PM Hal Finkel <hfinkel at anl.gov> wrote:
>>
>>>
>>> On 12/04/2017 10:48 PM, Serge Preis via cfe-dev wrote:
>>>
>>> I agree that guarantees provided by ICC may be stronger than with other
>>> compilers, so yes, under OpenMP terms vectorization is permitted and cannot
>>> be assumed. However OpenMP clearly defines semantics of variables used
>>> within OpenMP region some being shared(scalar), some private(vector) and
>>> some being inductions. This goes far beyond typical compiler specific
>>> pragmas about dependencies and cost modelling and makes vectorization much
>>> simpler task with more predictable and robust results if properly
>>> implemented (admittedly, even ICC implementation is far from perfect). I
>>> hope Intel's efforts to standardize someting like this in core C++ will
>>> evntually come to fruition. Until then I as a regular application developer
>>> would appreciate OpenMP-simd based execution policy (hoping for good
>>> support for OpenMP SIMD in clang), but it shouldn't necessary be part of
>>> libc++. Since 'unordered' execution policy is currently not part of C++
>>> standard
>>>
>>>
>>> std::execution::par_unseq is part of C++17, and that essentially maps to
>>> '#pragma omp parallel for simd'.
>>>
>>>
>> Do you expect par/par_unseq to nest?
>>
>>
>> Yes.
>>
>>
>> Nesting omp-parallel is generally regarded as a Bad Idea.
>>
>>
>> Agreed. I suspect we'll want the mapping to be more like '#pragma omp
>> taskloop simd'.
>>
>>
> That won’t run in parallel unless in an omp-parallel-master region.
>
>
> Yes.
>
> That means OpenMP-based PSTL won’t be parallel unless the user knows to
> add back-end specific code about the PSTL.
>
>
> That obviously wouldn't be acceptable.
>
>
> What I’m trying to say is that OpenMP is a poor target for PSTL in its
> current form. Nested parallel regions is the only thing that works and it
> is likely to work poorly.
>
>
> I'm not sure that's true, but the technique may not be trivial. I believe
> that it is possible, however. For example, the mapping might be to
> something like:
>
> if (omp_in_parallel()) {
> #pragma omp taskloop simd
>   for (size_t i = 0; i < N; ++i)
>     F(X[i]);
> } else {
> #pragma omp parallel
>   {
> #pragma omp taskloop simd
>      for (size_t i = 0; i < N; ++i)
>        F(X[i]);
>   }
> }
>
> The fact that we'd need to use this kind of pattern is a bit unfortunate,
> but it can be easily abstracted into a template function, so it just
> becomes some implementation detail of the library.
>
>
You are right and that is probably the best way to do it with OpenMP.  I am
concerned about the absolute performance, based upon my observations of
omp-taskloop vs omp-for and tbb::parallel_for in the PRK project, but at
least it is sane from a semantic perspective.  Having motivating use cases
like PSTL should lead to improvements in OpenMP runtime performance w.r.t.
taskloop.

https://i.stack.imgur.com/MVd5j.png is a snapshot of the performance of PRK
stencil (https://github.com/ParRes/Kernels/tree/master/Cxx11), which shows
taskloop loses to TBB-based PSTL, OpenMP for, and tbb::parallel_for (pure
TBB beats TBB-based PSTL because I use tbb::blocked_range2d, which improves
cache utilization).  I think those results tuned taskloop grainsize as
well, so they may be an optimistic representation of taskloop in a general
usage.

I'll see if I can prototype this in RAJA or Intel PSTL.  It's not hard to
get results directly from the PRK tests, if the former attempts fail.

Best,

Jeff


> Thanks again,
> Hal
>
>
>
> Jeff
>
>
>>  -Hal
>>
>>
>>
>> Jeff
>>
>>
>>> I don't care much on how it will be implemneted in libc++ if it is. I
>>> just would like to ask Intel guys and community here to make implementation
>>> extensible in a sense that custom OpenMP-SIMD-based execution policy along
>>> with algorithms implementations (as specializations for the policy) can be
>>> used with the libc++ library. And I additionally would like to ask Intel
>>> guys to provide complete and compatible extension on github for developers
>>> like me to use.
>>>
>>>
>>> In the end, I think we want the following:
>>>
>>>  1. A design for libc++ that allows the thread-level parallelism to be
>>> implemented in terms of different underlying providers (i.e., OpenMP, GCD,
>>> Work Queues on Windows, whatever else).
>>>  2. To follow the same philosophy with respect to standards as we do
>>> everywhere else: Use standards where possible with compiler/system-specific
>>> extensions as necessary.
>>>
>>>  -Hal
>>>
>>>
>>>
>>> Regards,
>>> Serge.
>>>
>>>
>>>
>>> 04.12.2017, 12:07, "Jeff Hammond" <jeff.science at gmail.com>
>>> <jeff.science at gmail.com>:
>>>
>>> ICC implements a very aggressive interpretation of the OpenMP standard,
>>> and this interpretation is not shared by everyone in the OpenMP community.
>>> ICC is correct but other implementations may be far less aggressive, so
>>> _Pragma("omp simd") doesn't guarentee vectorization unless the compiler
>>> documentation says that is how it is implemented.  All the standard says
>>> that it means is that vectorization is _permitted_.
>>>
>>> Given that the practical meaning of _Pragma("omp simd") isn't guaranteed
>>> to be consistent across different implementations, I don't really know how
>>> to compare it to compiler-specific pragmas unless we define everything
>>> explicitly.
>>>
>>> In any case, my fundamental point remains: do not use OpenMP pragmas
>>> here, but instead use whatever the appropriate compiler-specific pragma is,
>>> or create a new one that meets the need.
>>>
>>> Best,
>>>
>>> Jeff
>>>
>>>
>>> On Sun, Dec 3, 2017 at 8:09 PM, Serge Preis <spreis at yandex-team.ru>
>>> wrote:
>>>
>>> Hello,
>>>
>>> _Pragma("omp simd") is semantically quite different from _Pragma("clang
>>> loop vectorize(assume_safety)"), _Pragma("GCC ivdep") and _Pragma("vector
>>> always"), so I am not sure all latter will work as expected in all cases.
>>> They definitely won't provide any vectorization guarantees which slightly
>>> defeat the purpose of using corresponding execution policy.
>>>
>>> I support the idea of having OpenMP orthogonal and definitely having
>>> -fopenmp enabled by default is not an option. Intel compiler has separate
>>> -qopenmp-simd option which doesn't affect performance outside explicitly
>>> marked loops, but even this is not enabled by default. I would say that
>>> there might exist multiple implementations of unordered policy, originally
>>> OpenMP SIMD based implementation may be more powerful and one based on
>>> other pragmas being default, but hinting about existence of faster option.
>>> Later on one may be brave enough to add some SIMD template library and
>>> implement default unordered policy using it (such implementation is
>>> possible even now using vector types, but it will be extremely complex if
>>> attempt to target all base data types, vector widths and target SIMD
>>> architectures clang supports. Even with the library this may be quite
>>> tedious).
>>>
>>> Without any standard way of expressing SIMD perallelism in pure C++ any
>>> implementer of SIMD execution policy is to rely on means avaialble for
>>> plaform/compiler and so it is not totaly unnatural to ask user to enable
>>> OpenMP SIMD for efficient support of corresponding execution policy.
>>>
>>> Reagrds,
>>> Serge Preis
>>>
>>> (Who once was part of Intel Compiler Vectorizer team and driven OpenMP
>>> SIMD efforts within icc and beyond, if anyone is keeping track of
>>> conflicts-of-interest)
>>>
>>>
>>> 04.12.2017, 08:46, "Jeff Hammond via cfe-dev" <cfe-dev at lists.llvm.org>:
>>>
>>> It would be nice to keep PSTL and OpenMP orthogonal, even if
>>> _Pragma("omp simd") does not require runtime support.  It should be trivial
>>> to use _Pragma("clang loop vectorize(assume_safety)") instead, by wrapping
>>> all of the different compiler vectorization pragmas in preprocessor logic.
>>> I similarly recommend _Pragma("GCC ivdep") for GCC and _Pragma("vector
>>> always") for ICC.  While this requires O(n_compilers) effort instead of
>>> O(1), but orthogonality is worth it.
>>>
>>> While OpenMP is vendor/compiler-agnostic, users should not be required
>>> to use -fopenmp or similar to enable vectorization from PSTL, nor should
>>> the compiler enable any OpenMP pragma by default.  I know of cases where
>>> merely using the -fopenmp flag alters code generation in a
>>> performance-visible manner, and enabling the OpenMP "simd" pragma by
>>> default may surprise some users, particularly if no other OpenMP pragmas
>>> are enabled by default.
>>>
>>> Best,
>>>
>>> Jeff
>>> (who works for Intel but not on any software products and has been a
>>> heavy user of Intel PSTL since it was released, if anyone is keeping track
>>> of conflicts-of-interest)
>>>
>>> On Wed, Nov 29, 2017 at 4:21 AM, Kukanov, Alexey via cfe-dev <
>>> cfe-dev at lists.llvm.org> wrote:
>>> >
>>> > Hello all,
>>> >
>>> > At Intel, we have developed an implementation of C++17 execution
>>> policies
>>> > for algorithms (often referred to as Parallel STL). We hope to
>>> contribute it
>>> > to libc++/LLVM, so would like to ask the community for comments on
>>> this.
>>> >
>>> > The code is already published at GitHub (https://github.com/intel/
>>> parallelstl).
>>> > It supports the C++17 standard execution policies (seq, par,
>>> par_unseq) as well as
>>> > the experimental unsequenced policy (unseq) for SIMD execution. At the
>>> moment,
>>> > about half of the C++17 standard algorithms that must support
>>> execution policies
>>> > are implemented; a few more will be ready soon, and the work continues.
>>> > The tests that we use are also available at GitHub; needless to say we
>>> will
>>> > contribute those as well.
>>> >
>>> > The implementation is not specific to Intel’s hardware. For
>>> thread-level parallelism
>>> > it uses TBB* (https://www.threadingbuildingblocks.org/) but abstracts
>>> it with
>>> > an internal API which can be implemented on top of other
>>> threading/parallel solutions –
>>> > so it is for the community to decide which ones to use. For SIMD
>>> parallelism
>>> > (unseq, par_unseq) we use #pragma omp simd directives; it is
>>> vendor-neutral and
>>> > does not require any OpenMP runtime support.
>>> >
>>> > The current implementation meets the spirit but not always the letter
>>> of
>>> > the standard, because it has to be separate from but also coexist with
>>> > implementations of standard C++ libraries. While preparing the
>>> contribution,
>>> > we will address inconsistencies, adjust the code to meet community
>>> standards,
>>> > and better integrate it into the standard library code.
>>> >
>>> > We are also proposing that our implementation is included into
>>> libstdc++/GCC.
>>> > Compatibility between the implementations seems useful as it can
>>> potentially
>>> > reduce the amount of work for everyone. We hope to keep the code
>>> mostly identical,
>>> > and would like to know if you think it’s too optimistic to expect.
>>> >
>>> > Obviously we plan to use appropriate open source licenses to meet the
>>> different
>>> > projects’ requirements.
>>> >
>>> > We expect to keep developing the code and will take the responsibility
>>> for
>>> > maintaining it (with community contributions, of course). If there are
>>> other
>>> > community efforts to implement parallel algorithms, we are willing to
>>> collaborate.
>>> >
>>> > We look forward to your feedback, both for the overall idea and – if
>>> supported –
>>> > for the next steps we should take.
>>> >
>>> > Regards,
>>> > - Alexey Kukanov
>>> >
>>> > * Note that TBB itself is highly portable (and ported by community to
>>> Power and ARM
>>> > architectures) and permissively licensed, so could be the base for the
>>> threading
>>> > infrastructure. But the Parallel STL implementation itself does not
>>> require TBB.
>>> >
>>> > _______________________________________________
>>> > 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
>>>
>>>
>>>
>>> --
>>> Jeff Hammond
>>> jeff.science at gmail.com
>>> http://jeffhammond.github.io/
>>>
>>>
>>>
>>> _______________________________________________
>>> cfe-dev mailing listcfe-dev at lists.llvm.orghttp://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/
>>
>>
>> --
>> 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/
>
>
> --
> 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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