PWD011

PWD011: Missing OpenMP lastprivate clause

Issue

A variable is being incorrectly privatized in the OpenMP datascoping clauses.

Actions

Change the data scope of the variable to lastprivate.

Relevance

In some parallel loops, variables need to be private to each thread but the value computed in the last loop iterations needs to be used afterwards. For such cases, the OpenMP private scoping is incorrect and lastprivate must be used instead.

Code example

C

In the following example, liveOut is incorrectly marked as private. When a variable is marked as private, each thread is instructed to make its own copy and the original variable is not updated during parallel execution. As a result, once the loop is completed, the original liveOut won't retain the value from the last iteration:

double example(int m, double *A, double *B, double *C) {
  double liveOut;

  // liveOut is private but used after the loop, so it should be lastprivate
  #pragma omp parallel for private(liveOut) shared(A, B, C)
  for (int i = 0; i < m; i++) {
    liveOut = A[i] * B[i];
    C[i] = C[i] + liveOut;
  }

  liveOut += 5;
  return liveOut;
}

To achieve this behavior, we can use the lastprivate clause. This allows subsequent operations on liveOut to work correctly:

double example(int m, double *A, double *B, double *C) {
  double liveOut;

  #pragma omp parallel for lastprivate(liveOut) shared(A, B, C)
  for (int i = 0; i < m; i++) {
    liveOut = A[i] * B[i];
    C[i] = C[i] + liveOut;
  }

  liveOut += 5;
  return liveOut;
}

Fortran

In the following example, liveOut is incorrectly marked as private. When a variable is marked as private, each thread is instructed to make its own copy and the original variable is not updated during parallel execution. As a result, once the loop is completed, the original liveOut won't retain the value from the last iteration:

real function example(A, B, C)
  use iso_fortran_env, only: real32
  implicit none
  real(kind=real32), intent(in) :: A(:), B(:)
  real(kind=real32), intent(inout) :: C(:)
  real(kind=real32) :: liveOut
  integer :: i

  !$omp parallel do private(i, liveOut) shared(A, B, C)
  do i = 1, size(C, 1)
    liveOut = A(i) * B(i)
    C(i) = C(i) + liveOut
  end do

  liveOut = liveOut + 5
  example = liveOut
end function example

To achieve this behavior, we can use the lastprivate clause. This allows subsequent operations on liveOut to work correctly:

real function example(A, B, C)
  use iso_fortran_env, only: real32
  implicit none
  real(kind=real32), intent(in) :: A(:), B(:)
  real(kind=real32), intent(inout) :: C(:)
  real(kind=real32) :: liveOut
  integer :: i

  !$omp parallel do lastprivate(liveOut) private(i) shared(A, B, C)
  do i = 1, size(C, 1)
    liveOut = A(i) * B(i)
    C(i) = C(i) + liveOut
  end do

  liveOut = liveOut + 5
  example = liveOut
end function example

Related resources

• PWD011 examples

References

• Data-Sharing Attribute Clauses - OPENMP API Specification: Version 5.0 November 2018 [last checked August 2021]