12.8. Nested Loop Constructs

The following example of loop construct nesting is conforming because the inner and outer loop regions bind to different parallel regions:

//%compiler: clang
//%cflags: -fopenmp

/*
* name: nested_loop.1
* type: C
*/
void work(int i, int j) {}

void good_nesting(int n)
{
  int i, j;
  #pragma omp parallel default(shared)
  {
    #pragma omp for
    for (i=0; i<n; i++) {
      #pragma omp parallel shared(i, n)
      {
        #pragma omp for
        for (j=0; j < n; j++)
          work(i, j);
      }
    }
  }
}

!!%compiler: gfortran
!!%cflags: -fopenmp

! name: nested_loop.1
! type: F-fixed
      SUBROUTINE WORK(I, J)
      INTEGER I, J
      END SUBROUTINE WORK

      SUBROUTINE GOOD_NESTING(N)
      INTEGER N

        INTEGER I
!$OMP   PARALLEL DEFAULT(SHARED)
!$OMP     DO
          DO I = 1, N
!$OMP       PARALLEL SHARED(I,N)
!$OMP         DO
              DO J = 1, N
                CALL WORK(I,J)
              END DO
!$OMP       END PARALLEL
          END DO
!$OMP   END PARALLEL
      END SUBROUTINE GOOD_NESTING

The following variation of the preceding example is also conforming:

//%compiler: clang
//%cflags: -fopenmp

/*
* name: nested_loop.2
* type: C
*/
void work(int i, int j) {}


void work1(int i, int n)
{
  int j;
  #pragma omp parallel default(shared)
  {
    #pragma omp for
    for (j=0; j<n; j++)
      work(i, j);
  }
}


void good_nesting2(int n)
{
  int i;
  #pragma omp parallel default(shared)
  {
    #pragma omp for
    for (i=0; i<n; i++)
      work1(i, n);
  }
}

!!%compiler: gfortran
!!%cflags: -fopenmp

! name: nested_loop.2
! type: F-fixed
      SUBROUTINE WORK(I, J)
      INTEGER I, J
      END SUBROUTINE WORK

      SUBROUTINE WORK1(I, N)
      INTEGER J
!$OMP PARALLEL DEFAULT(SHARED)
!$OMP DO
        DO J = 1, N
          CALL WORK(I,J)
        END DO
!$OMP END PARALLEL
      END SUBROUTINE WORK1

      SUBROUTINE GOOD_NESTING2(N)
      INTEGER N
!$OMP PARALLEL DEFAULT(SHARED)
!$OMP DO
      DO I = 1, N
         CALL WORK1(I, N)
      END DO
!$OMP END PARALLEL
      END SUBROUTINE GOOD_NESTING2