[Llvm-bgq-discuss] Status of OpenMP support in BG/Q

[Jeff Hammond]

Do you mind if I send your C++11 tests to IBM?

Jeff

On Friday, February 7, 2014, Schlottke, Michael <
M.Schlottke at aia.rwth-aachen.de> wrote:

>   In general, ALCF finds that -O5 is not worthwhile and often worse
> than -O3.  The only exceptions of which we are aware are when the user
> has tuned the code to an extreme degree - e.g. manual unrolling,
> blocking for cache, vector intrinsics.
>
> Our experiences are similar. However, -O5 still has a slight (tiny,
> really) edge over -O3 for us (at the cost, of course, of 2+ hours
> compile&link times...), even though we do not use any fancy manual
> optimizations.
>
>   I would strongly encourage you to let IBM know that you're
> abandoning it due to their lack of C++11 support.  It is important for them
> to understand that their development model for XLC on Blue Gene harms users
> in significant ways.
>
>  At the beginning of the week, we had a BG/Q Porting & Tuning workshop at
> the Forschungszentrum Jülich. I talked to one of the IBM representatives
> and even he could not explain the lackluster support for C++ in the
> compiler development team.
>
>  A decade ago, we were forced to support many different compilers in
> Cactus, mostly because every supercomputer architecture came with its own
> compiler. These days, we support mostly GCC, Intel, and IBM's compilers,
> since the other compilers are too far behind, be it in terms of features,
> producing performant code, or not producing ICEs. Since Clang is so similar
> to GCC in its user interface, supporting it is almost trivial.
>
> We have made some good experiences with the current PGI (just released:
> 14.1) and Intel compilers. While the older PGI compilers still had quite a
> few bugs, the more recent ones do not cause any problems for us and
> actually found some (minor)  cases of bad programming that eluded gcc/clang.
>
>  The next infrastructure update of Cactus is very likely to use some
> C++11 features, either directly, or via threading or communication
> libraries that it uses.
>
>  If you're looking for a move to C++11 and wonder which C++11 features are
> supported on the XL machines... well, good luck with that. I wrote a small
> collection of tests that check if a compiler supports individual C++11
> features (https://github.com/sloede/cxx11tests) and I was surprised by
> how little is actually implemented in XL. For instance, the XL 12.1
> reference claims that valid 'constexpr' syntax is supported, which is not
> true for at least one example (constexpr float arrays). Although the tests
> are far from complete, they might give you a good overview of what's
> supported and what isn't.
>
>  Regards,
>
>  Michael
>
>  On Feb 6, 2014, at 19:33 , Erik Schnetter wrote:
>
>  A decade ago, we were forced to support many different compilers in
> Cactus, mostly because every supercomputer architecture came with its own
> compiler. These days, we support mostly GCC, Intel, and IBM's compilers,
> since the other compilers are too far behind, be it in terms of features,
> producing performant code, or not producing ICEs. Since Clang is so similar
> to GCC in its user interface, supporting it is almost trivial.
>
> Portability between different architectures is very important to us. IBM's
> compilers are currently causing the most trouble for us since they have
> weird restrictions and bugs, and Clang/LLVM is correcting errors and adding
> features at a much greater speed than xlC.
>
> The next infrastructure update of Cactus is very likely to use some C++11
> features, either directly, or via threading or communication libraries that
> it uses.
>
> -erik
>
> On Feb 6, 2014, at 12:24 , Jeff Hammond <jhammond at anl.gov<javascript:_e(%7B%7D,'cvml','jhammond at anl.gov');>>
> wrote:
>
> In general, ALCF finds that -O5 is not worthwhile and often worse than
>
> -O3.  The only exceptions of which we are aware are when the user has
>
> tuned the code to an extreme degree - e.g. manual unrolling, blocking
>
> for cache, vector intrinsics.
>
>
>  I would strongly encourage you to let IBM know that you're abandoning
>
> it due to their lack of C++11 support.  It is important for them to
>
> understand that their development model for XLC on Blue Gene harms
>
> users in significant ways.
>
>
>  Jeff
>
>
>

-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at anl.gov <jhammond at alcf.anl.gov> / jhammond at uchicago.edu / (630)
252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
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