OpenMP 4.0

Tim Prince n8tm@aol.com
Fri Aug 30 01:40:00 GMT 2013


On 8/29/2013 2:20 PM, Tobias Burnus wrote:
> José Luis García Pallero wrote:
>> I don't know if this is the correct place for this question, but I
>> haven't found any mailing list on the GOMP webpage.
>> The OpenMP 4.0 specifications were launched ten days ago. This new
>> standard includes several interesting characteristics as SIMD and
>> accelerators directives and error handling facilities. Is planned to
>> add this new version of OpenMP to libgomp and, then, to GCC 4.9?
>
> Well, it takes a while until features are implemented - and the 
> implementation work can only start after a specification/standard is 
> sufficiently finished to not change in a major way.
>
> Having said that, there is a GCC branch called gomp-4_0-branch (see
> http://gcc.gnu.org/svn.html), which is used for the on-going 
> implementation. I think SIMD already partially works (with C/C++, not 
> yet with Fortran).
>
> I believe that it is planed to support OpenMP 4 in GCC 4.9.
>
>
> Tim Prince wrote
>> OpenMP 4.0 simd facilities are related to Cilk(tm) Plus pragmas, for 
>> which there is a gcc branch on git (although I haven't figured out 
>> that stuff).
>
> As far as I gathered, Cilk+'s pragmas and OpenMP'pragmas are supposed 
> to be handled identically. (I think there were some differences but 
> they got resolved by changing Cilk+.) There are some Cilk+ branches, 
> which aim at consolidating the effort with OpenMP. Actually, some 
> Cilk+ patches has been submitted for inclusion - thus, expect more for 
> this. (The submitted patches do not include SIMD as far as I know. The 
> branches do support it.)

There are distinctions in Intel compilers between Cilk(tm) Plus and 
OpenMP 4.0.  For example, Cilk(tm) Plus expects use of simd firstprivate 
lastprivate where appropriate, while OpenMP 4.0 doesn't support those 
clauses, and depends on the compiler recognizing those cases of omp simd 
private.
Intel once talked of reconciling terminology (it seems unsatisfactory to 
market Fortran directives as Cilk(tm) Plus).
Intel takes Cilk(tm) Plus simd to require in-line simd instructions 
rather than automatic replacement by the special memset/memcpy library 
function calls, while the corresponding omp simd construct doesn't 
inhibit those automatic replacements.  I guess gfortran et al. aren't so 
likely to introduce these substitutions, so don't need a means to 
control them.
>
>
>> For example, I know of no one planning to implement user defined 
>> reduction. Some talk about proposing a specific standard on indexed 
>> min/max before deciding about user defined reductions.
>
> I think the gomp-4_0-branch already supports min/max since quite some 
> time. (For C/C++; Fortran supports it already since older OpenMP 
> specs.) Additionally, I believe that Jakub intents to implement 
> user-defined reductions (UDR) and that he has already done some prep 
> work on the branch. Ignoring "omp target", UDR seems to be the biggest 
> new feature.
C omp parallel reduction(min|max: ) was introduced in OpenMP 3.1 but I 
didn't find any tests for it in the gcc 4.9 testsuite. Corresponding omp 
simd reduction would not be so important for C++ if g++ could optimize 
min/max with maxp[sd]/minp[sd] as gfortran and icpc do.  No  omp max|min 
reductions are likely in the Intel icc/icpc 14.0 releases in a week or 
so, regardless of claims to support OpenMP 4.0.
>
> (Regarding "omp target" and other accelerator/GPU/hybrid-system 
> support: I think there is quite some interest to get it working with 
> GCC, however, it probably will take until 4.10 or longer.)
>
>
>> Among my ulterior motives for asking is my attempt to write a book 
>> centered on HPC development topics.
>
> That sounds interesting!
>
>
> Tobias
>
> PS: Regarding SIMD, in GCC 4.9 itself, some basic support has already 
> been merged a few days ago. However, it is not yet accessible from 
> user code (no front-end support) and I have the impression the 
> information is not yet used for optimization. But expect soon some 
> support (possibly something like #pragma simd, #pragma vector for 
> C/C++ and usage for DO CONCURRENT in Fortran) - but I don't know which 
> pragma and when the support will be added.
DO CONCURRENT needs a more satisfactory way to invoke omp parallel. A 
limited facility (beyond current auto-parallelization) would not appear 
in ifort until next year.  It seems too difficult to cover all 
possibilities.  Auto-vectorization works well already (in gfortran, for 
example).


-- 
Tim Prince



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