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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