Does the OpenMP 4.5/5.0 standard support Fortran automatic arrays on GPU?

Question

I have tried compiling the following code with nvhpc/21.3 to run on an Nvidia v100, but the code bombs out. So nvhpc does not support fortran automatic arrays, but does the OpenMP standard support them?

        module test_mod
        contains
          subroutine saxpy(i,n,s)
          integer                 :: i,n
          real(8), parameter      :: p=0.5
          real(8), dimension(n,n) :: a,b,c ! automatic arrays
          real(8)                 :: s
!$omp     declare target
          a(i,:) = 1.0d0
          b(i,:) = 2.0d0
          c(i,:) = 0.0d0
          do j=1,n
                c(i,j) = a(i,j) + p*b(i,j)
          end do
          s=c(i,1)
          end subroutine
        end module
 
        program test
        use test_mod
        integer, parameter :: n=100
        integer            :: i
        real(8)            :: s
!$omp   target teams distribute parallel do map(from:s)
        do i=1,n
                call saxpy(i,n,s)
        end do
        print*,'%test_omp, ',s
        end program

How did you conclude that the compiler does not support them? What exactly happened? What was the error message? — Vladimir F Героям слава, Jun 04 '21 at 20:45
If by "nvhpc" you mean the nvidia Fortran compiler, then are you sure that that compiler supports OpenMP 4.5 GPU offload at all? What reason do you have to worry that OpenMP itself does not support declared targets as automatic arrays? — francescalus, Jun 04 '21 at 22:00
https://docs.nvidia.com/hpc-sdk/compilers/hpc-compilers-user-guide/index.html#openmp-subset might be of interest, purporting to detail " the subset of OpenMP 5.0 features that the HPC compilers support". Unfortunately the situation is a mess, I can't quickly find a definitive statement of what, if any, standard of OpenMP is supported in full. To say more please explain what you mean by "the code bombs out", as Vladimir asks — Ian Bush, Jun 05 '21 at 04:20
Also please understand that Real(8) is poor practice, not portable, may not do what you think, and might not be supported by all compilers - you may not use the nvhpc suite for ever. See https://stackoverflow.com/questions/838310/fortran-90-kind-parameter — Ian Bush, Jun 05 '21 at 04:25
We know that the compiler does not support them because the Nvidia compiler team we work with said that they don't. — rosenbe2, Jun 06 '21 at 18:36
./test %test_omp, running on GPU FATAL ERROR: FORTRAN AUTO ALLOCATION FAILED FATAL ERROR: FORTRAN AUTO ALLOCATION FAILED FATAL ERROR: FORTRAN AUTO ALLOCATION FAILED Fatal error: expression 'HX_CU_CALL_CHECK(p_cuStreamSynchronize(stream[dev]))' (value 1) is not equal to expression 'HX_SUCCESS' (value 0) — rosenbe2, Jun 06 '21 at 18:37

Jim Cownie · Answer 1 · 2021-06-08T07:48:31.507

1

does the OpenMP standard support fortran automatic arrays?

The OpenMP standard assumes underlying serial language standards, but does not require them.

The OpenMP standard itself has nothing to say about features like this which are unrelated to the changes in program behaviour specified by OpenMP. (Just as OpenMP says nothing about which format specifiers are legal :-)).

More generally, the OpenMP ARB has no tests or validation suites for OpenMP compliance, so perverse vendors could claim to support a particular level of the OpenMP standard while not really doing so... Caveat emptor!)

edited Jun 08 '21 at 07:48

answered Jun 07 '21 at 08:19

Jim Cownie

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Well, that is not completely true, I remember that several features not working with OpenMP in existing compilers where explained by them not being supported (the exac behavior defined) in OpenMP 3. Associate and similar. And once GPUs come in to play, everything gets muddier. The OpenMP 4 or 5 standard could easily forbid certain features in device code. The question is whether they do. – Vladimir F Героям слава Jun 07 '21 at 10:35
So your question is not "Does OpenMP support them?", but "Does OpenMP forbid them?" that's rather a different question, and easier to answer, since if the relevant OpenMP standard says that it handles the appropriate level of the underlying language standard and does not explicitly forbid a feature then it should be supported. – Jim Cownie Jun 08 '21 at 08:27

Does the OpenMP 4.5/5.0 standard support Fortran automatic arrays on GPU?

1 Answers1