Using OpenMP in Fortran for common array operations

Question

In Fortran, given arrays A(1000000) and B(1000000), and to compute the sum you simply write:

C = A + B

However, when I want to implement OpenMP, I have to write an explicit loop:

!$OMP PARALLEL DO
DO I = 1, 1000000
C(I) = A(I) + B(I)
END DO
!$OMP END PARALLEL DO

Is there a better way to do this? I know there is -parallel compiler flag but it gives less performance.

Answer 1

For the record, you might try putting an OpenMP worksharing region around your array operation syntax:

!$omp workshare
C=A+B

Don't forget to build with OpenMP enabled (-openmp for the Intel compilers) and to set OMP_NUM_THREADS.

Answer 2

Short answer

No, there is not a better way to do this.

Long answer

WORKSHARE is much harder to implement than explicit loop parallelism. Compilers have been known to do correct but useless (aka "no-op") implementations of WORKSHARE that map it to SINGLE. This is correct but obviously provides no performance benefit.

When compilers support WORKSHARE, they may do so selectively, e.g. Intel Fortran 15, which means that depending on it may lead to mixed results.

Even when compilers do a good job, in parallel computing in general, you will find that explicit parallelization wins, because the user is required to make good design choices up front and compilers, which are much less intelligent than humans, don't have to make important decisions.