GCC and OpenACC

Mikel Mendizabal mikel.mendizabal.morentin@desy.de
Wed Jan 12 10:01:10 GMT 2022

Dear Richard,

Thanks a lot for the fast response. Here I send you a self contained example. I took it from openACC tutorial in [1]. I modified the code and added a dummy vector of size 1, lines 140-160 in the attached file (jsolvef_mikel.F90). The dummy vector (residual_v)  is the array form of the variable residual.

If the reduction clause (line 142) is commented both residual and residual_v give the same result. If in the reduction clause residual_v is given a compilation error appears, that the reduction clause does not support this variable datatype. In [2] I added the OpenACC 3.1 documentation. In section 1.12 the changes from v2.6 to v2.7 are described. More precisely, in line 612-613 the next is quoted:

"Arrays, subarrays and composite variables are now allowed in reduction clauses; see Sections 2.9.11 reduction clause and 2.5.14 reduction clause.”
I hope I could explain myself properly!
Thanks a lot :) 

Mikel Mendizabal

[1] https://github.com/OpenACCUserGroup/openacc-users-group/blob/master/Contributed_Sample_Codes/Tutorial1/solver/jsolvef.F90 <https://github.com/OpenACCUserGroup/openacc-users-group/blob/master/Contributed_Sample_Codes/Tutorial1/solver/jsolvef.F90> 
[2] https://www.openacc.org/sites/default/files/inline-images/Specification/OpenACC-3.1-final.pdf <https://www.openacc.org/sites/default/files/inline-images/Specification/OpenACC-3.1-final.pdf> 

> On 11. Jan 2022, at 14:26, Richard Biener <richard.guenther@gmail.com> wrote:
> On Tue, Jan 11, 2022 at 1:52 PM Mikel Mendizabal via Gcc
> <gcc@gcc.gnu.org <mailto:gcc@gcc.gnu.org>> wrote:
>> Dear GCC developers,
>> In the past year we started the offload of our software to GPUs. We decided to go with OpenACC. The program we are trying to offload is Millepede2 (MP2),  a tracker alignment software used to align the CMS experiment tracker at the large hadron collider.
>> We are using gcc as our main compiler. However, we found a major inconvenience with OpenACC 2.6, the REDUCTION clause does not accept arrays. Thus, it not possible for us to parallelise our largest loops due to array dependencies. We managed to offload MP2 for small datasets, we worked around the reduction issue. Nonetheless, if our alignment campaign is large the workarounds are not useful anymore.
>> I went thought the new versions of OpenACC and I found that v2.7 accepts arrays for the REDUCTION clause. I was wondering if it is in your plans to include a newer version of OpenACC for the next releases.
> Can you produce a self-contained example with an array for the
> REDUCTION clause that should be accepted with OpenACC v2.7?
> Thanks,
> Richard.
>> Sincerely,
>> Mikel Mendizabal

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