Is it possible to do a reduction on an array with openmp?

Question

Does OpenMP natively support reduction of a variable that represents an array?

This would work something like the following...

float* a = (float*) calloc(4*sizeof(float));
omp_set_num_threads(13);
#pragma omp parallel reduction(+:a)
for(i=0;i<4;i++){
   a[i] += 1;  // Thread-local copy of a incremented by something interesting
}
// a now contains [13 13 13 13]

Ideally, there would be something similar for an omp parallel for, and if you have a large enough number of threads for it to make sense, the accumulation would happen via binary tree.

Answer 1

OpenMP cannot perform reductions on array or structure type variables (see restrictions).

You also might want to read up on private and shared clauses. private declares a variable to be private to each thread, where as shared declares a variable to be shared among all threads. I also found the answer to this question very useful with regards to OpenMP and arrays.

Answer 2

Only in Fortran in OpenMP 3.0, and probably only with certain compilers.

See the last example (Example 3) on:

http://wikis.sun.com/display/openmp/Fortran+Allocatable+Arrays