OpenMP Sample for Merge Sort

void merge_sort(int a[], int tmp_a[], int first, int last)
{
    if (first < last) {
        int middle = (first + last + 1) / 2;
        merge_sort(a, tmp_a, first, middle - 1);
        merge_sort(a, tmp_a, middle,last);
        merge(a, tmp_a, first, middle, last);
    }
}

void merge_sort_openmp(int a[], int tmp_a[], int first, int last)
{
    if (first < last) {
        int middle = (first + last + 1) / 2; // = first + (last - first + 1) / 2;
        // Splits list a[first:last] into two halves (called sublists).
        // One is [first:middle-1], and another is [middle:last].
        // For sake of performance, only when the list is big enough,
        // we create tasks with #pragma omp task.
        if (last - first < 5000) {
            merge_sort(a, tmp_a, first, middle - 1);
            merge_sort(a, tmp_a, middle, last);
        }
        else {
			#pragma omp task
            merge_sort_openmp(a, tmp_a, first, middle - 1);
			#pragma omp task
            merge_sort_openmp(a, tmp_a, middle, last);
			#pragma omp taskwait
        }
        merge(a, tmp_a, first, middle, last);
}