Question

I’m wondering if it is feasible to make this loop parallel using openMP.

Of coarse there is the issue with the race conditions. I’m unsure how to deal with the n in the inner loop being generated by the outerloop, and the race condition with where D=A[n]. Do you think it is practical to try and make this parallel?

for(n=0; n < 10000000; ++n) {   

    for (n2=0; n2< 100; ++n2) {
        A[n]=A[n]+B[n2][n+C[n2]+200];

        }

    D=D+A[n];

}

Answer 1

Yes, this is indeed parallelizable assuming none of the pointers are aliased.

int D = 0;  //  Or whatever the type is.

#pragma omp parallel for reduction(+:D) private(n2)
for (n=0; n < 10000000; ++n) {   

    for (n2 = 0; n2 < 100; ++n2) {
        A[n] = A[n] + B[n2][n + C[n2] + 200];
    }

    D += A[n];
}

It could actually be optimized somewhat as follows:

int D = 0;  //  Or whatever the type is.

#pragma omp parallel for reduction(+:D) private(n2)
for (n=0; n < 10000000; ++n) {   

    int tmp = A[n]
    for (n2 = 0; n2 < 100; ++n2) {
        tmp += B[n2][n + C[n2] + 200];
    }

    A[n] = tmp;
    D += tmp;
}