Implementing FOR construct (GNU libgomp)

### 13.11 Implementing FOR construct ¶

      #pragma omp parallel for
      for (i = lb; i <= ub; i++)
        body;

becomes

      void subfunction (void *data)
      {
        long _s0, _e0;
        while (GOMP_loop_static_next (&_s0, &_e0))
        {
          long _e1 = _e0, i;
          for (i = _s0; i < _e1; i++)
            body;
        }
        GOMP_loop_end_nowait ();
      }

      GOMP_parallel_loop_static (subfunction, NULL, 0, lb, ub+1, 1, 0);
      subfunction (NULL);
      GOMP_parallel_end ();

      #pragma omp for schedule(runtime)
      for (i = 0; i < n; i++)
        body;

becomes

      {
        long i, _s0, _e0;
        if (GOMP_loop_runtime_start (0, n, 1, &_s0, &_e0))
          do {
            long _e1 = _e0;
            for (i = _s0, i < _e0; i++)
              body;
          } while (GOMP_loop_runtime_next (&_s0, _&e0));
        GOMP_loop_end ();
      }

Note that while it looks like there is trickiness to propagating a non-constant STEP, there isn’t really. We’re explicitly allowed to evaluate it as many times as we want, and any variables involved should automatically be handled as PRIVATE or SHARED like any other variables. So the expression should remain evaluable in the subfunction. We can also pull it into a local variable if we like, but since its supposed to remain unchanged, we can also not if we like.

If we have SCHEDULE(STATIC), and no ORDERED, then we ought to be able to get away with no work-sharing context at all, since we can simply perform the arithmetic directly in each thread to divide up the iterations. Which would mean that we wouldn’t need to call any of these routines.

There are separate routines for handling loops with an ORDERED clause. Bookkeeping for that is non-trivial...