Re: OpenMP target (offloading) question

[Tobias Burnus <tobias@codesourcery.com>]

Hi Harald,

interesting code; in any case for
   !$omp target
     s = 5
's' is a scalar which is mapped by default as 'firstprivate',
i.e. it is not copied back. However, OpenMP 5.1 states:

"If a list item appears in a reduction, lastprivate or linear clause
  on a combined target construct then it is treated as if it also appears
  in a map clause with a map-type of tofrom." (2.21.7)

Your code uses: omp target teams reduction(+:s)

→ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99928

Tobias

On 31.03.21 21:50, Harald Anlauf via Fortran wrote:
> Dear experts,
>
> sorry if this is a stupid question, but I was playing with offloading for
> the nvptx-none target and found different behavior between e.g. gfortran-10
> on OpenSuse and the Nvidia compiler (nvfortran) for the attached code.
>
> With "nvfortran -mp=multicore offload-test.f90" the code prints:
>
>      2.000000        2000.000
>   s1:    1001000.
>   s2:    1001000.
>
> With "/usr/bin/gfortran-10 -fopenmp -foffload=nvptx-none offload-test.f90":
>
>     2.00000000       2000.00000
>   s1:   1001000.00
>   s2:   0.00000000
>
> The core difference between the evaluations s1 and s2 is:
>
> s1:
>
> !$omp target data map(a,s)
> !$omp target teams reduction(+:s) map(s)
>      do i = 1, n
>         s = s + a(i)
>      end do
> !$omp end target teams
> !$omp end target data
>
> s2:
>
> !$omp target data map(a,s)
> !$omp target teams reduction(+:s)
>      do i = 1, n
>         s = s + a(i)
>      end do
> !$omp end target teams
> !$omp end target data
>
> I was assuming that the map clause in the reduction should not be necessary,
> but the result seems to tell me that either I am wrong (and gfortran is right),
> or nvfortran is wrong.
>
> With OpenACC this seems to be different; at least a simple example I tried
> with the reduction within an !$acc data ... !$acc end data did not show
> unexpected behavior.
>
> Can anybody tell me that I am wrong (and point me to the right place in the
> OpenMP standard), or should I open a PR?
>
> Thanks
> Harald
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