OpenMP reduction for each element of an array

Question

Learning openMP

    // array b

    #pragma omp parallel for // reduction(&&: b[i])?
    for (i=2; i<=N; i++)
    {
      // create local array for each thread
      int *localb;
      localb = (int*) malloc(N*sizeof(int));
      memcpy(localb, b, N*sizeof(localb));

        #pragma omp for private(j)
        for (j=i+1; j<=N; j++)
        {
            if (j%i == 0)
                localb[j] = 0;
        }

Is it possible to reduce each element in global array b using reduction(&&: b[i]) so that b[i] = localb[j] && b[i]? All data is either 0 or 1; 0 if j is divisible by i, and 1 otherwise.

Array reduction is in current standard but not supported by all implementations. Anyway it tends to exhibit limited scaling — tim18
– tim18, Commented Apr 16, 2017 at 18:24

Community · Accepted Answer · 2017-05-23 11:54:12Z

2

Yes, reduction on arrays is directly supported by the standard since OpenMP 4.5, just via reduction(&&: b).

If you implementation does not yet support that, refer to the older answers about that.

edited May 23, 2017 at 11:54

CommunityBot

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answered Apr 16, 2017 at 23:07

Zulan

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3 Comments

Guille Over a year ago

And to make sure, can the reduction take place inside the inner loop, or must I create another inner loop exclusively for reducing each element of b

Zulan Over a year ago

No, reduction always takes place at the end of a parallel region. Note that you do not need to declare a localb, just use b. The rest is done by OpenMP transparently.

Guille Over a year ago

Wouldn't there be a race condition since b is a shared resource? And to clarify again (sorry), I need to add b[i] = b[i] && b[i] after the inner loop for the reduction to take place?

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