Join array results in OpenACC

I'm writing an OpenACC code that has an array dependence. Each iteration of inner loop can update the same position of array. Here's some code:

    long unsigned int digits[d + 11];
    for (long unsigned int digit = 0; digit < d + 11; ++digit)
            digits[digit] = 0;

    for (long unsigned int i = 1; i <= n; ++i) {
            long unsigned int remainder = 1;
            for (long unsigned int digit = 0; digit < d + 11 && remainder; ++digit) {
                    long unsigned int div = remainder / i;
                    long unsigned int mod = remainder % i;
                    digits[digit] += div; // here
                    remainder = mod * 10;
            }
    }

OpenMP version was writing as follows:

    #pragma omp parallel private(i)
    {
            long unsigned int digit_local[d+11];
            for(i=0;i<d+11;i++)
                    digit_local[i] = 0;

            #pragma omp for
            for (i = 1; i <= n; ++i) {
                    long unsigned int remainder = 1;
                    for (long unsigned int digit = 0; digit < d + 11 && remainder; ++digit) {
                            long unsigned int div = remainder / i;
                            long unsigned int mod = remainder % i;
                            digit_local[digit] += div;
                            remainder = mod * 10;
                    }
            }

            #pragma omp critical
            for(long unsigned int digit = 0; digit < d+11; ++digit)
                    digits[digit] += digit_local[digit];

    }

In OpenACC the keyword private works with arrays but I have no idea in how join the private arrays with global array.

Thanks.

You would use an OpenACC "atomic update" directive.

            #pragma acc atomic update
            digits[digit] += div; // here

Alternatively, you could do something similar as your OpenMP version.

    long unsigned int digit_local[d+11][n];
    #pragma acc data create(digit_local) copyout(digits)
    {

    #pragma acc parallel loop gang vector
    for (i = 1; i <= n; ++i) {
          for(j=0;j<d+11;j++) digit_local[j][i] = 0;
            long unsigned int remainder = 1;
            for (long unsigned int digit = 0; digit < d + 11 && remainder; ++digit) {
                    long unsigned int div = remainder / i;
                    long unsigned int mod = remainder % i;
                    digit_local[digit][i] += div;
                    remainder = mod * 10;
            }
    }

    #pragma acc parallel loop gang
    for(long unsigned int digit = 0; digit < d+11; ++digit) {
          long unsigned int dsum = 0;
          #pragma acc loop vector reduction(+:dsum)
          for (i = 1; i <= n; ++i) {
            dsum += digit_local[digit][i];
          }
          digits[digit] = dsum;
    }
    }

Though, I'm not sure this will see any speed-up either.

Hope this helps, Mat