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PWR004: Declare OpenMP scoping for all variables

Issue​

Explicitly declare the scope of each variable used in a OpenMP parallel region to prevent an invalid default scoping being assigned to it.

Actions​

Declare the scope of each variable through the appropriate OpenMP clause.

Relevance​

When the scope for a variable is not specified in an OpenMP parallel region, a default scope is assigned to it. This default scope can be wrong, for instance sharing a variable that should be private, which can lead to a race condition. Furthermore, it ensures that the scope of each variable has been determined and improves code readability.

Code example​

C​

In the following code, a variable factor is used in each iteration of the loop to initialize the array result:

void example() {
  int factor = 42;
  int result[10];

  #pragma omp parallel for
  for (int i = 0; i < 10; i++) {
    result[i] = factor * i;
  }
}

Having the scope declared explicitly for each variable improves readability, since it makes explicit the scope of all the variables within the parallel region:

void example() {
  int factor = 42;
  int result[10];

  #pragma omp parallel for default(none) shared(result, factor) private(i)
  for (int i = 0; i < 10; i++) {
    result[i] = factor * i;
  }
}

Fortran​

In the following code, a variable factor is used in each iteration of the loop to initialize the array result:

subroutine example()
  integer :: factor = 42
  integer :: result(10)

  !$omp parallel do
  do i = 1, 10
    result(i) = factor * i
  end do
end subroutine example

Having the scope declared explicitly for each variable improves readability, since it makes explicit the scope of all the variables within the parallel region:

subroutine example()
  integer :: factor = 42
  integer :: result(10)

  !$omp parallel do default(none) shared(factor, result) private(i)
  do i = 1, 10
    result(i) = factor * i
  end do
end subroutine example

References​