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PWD005: Array range copied to or from the GPU does not cover the used range

Issue​

Only a part of an array was transferred to the GPU and it does not cover the used range.

Actions​

Update the copied array range to match the actual array usage in the code.

Relevance​

Minimising data transfers is one of the main optimization points when offloading computations to the GPU. An opportunity for such optimization occurs whenever only part of an array is required in a computation. In such cases, only a part of the array may be transferred to or from the GPU. However, the developer must ensure that the copied array range includes those array positions accessed in the code. Otherwise, the result is undefined behavior, most likely causing invalid memory accesses and crashes.

Code example​

C​

The following code performs the sum of two arrays:

void foo() {
  int A[100], B[100], sum[100];
  #pragma omp target map(to: A[0:50], B[0:50]) map(from: sum[0:50])
  #pragma omp parallel for
  for (int i = 0; i < 100; i++) {
    sum[i] = A[i] + B[i];
  }
}

However, only half of the array elements have been copied to the GPU. This must be fixed by matching the transferred range with array range actually used in the loop:

void foo() {
  int A[100], B[100], sum[100];
  #pragma omp target map(to: A[0:100], B[0:100]) map(from: sum[0:100])
  #pragma omp parallel for
  for (int i = 0; i < 100; i++) {
    sum[i] = A[i] + B[i];
  }
}

Fortran​

The following code performs the sum of two arrays:

subroutine example()
  integer, dimension(100) :: A, B, sum
  integer :: i

  !$omp target map(to: A(1:50), B(1:50)) map(from: sum(1:50))
  !$omp parallel do
  do i = 1, size(sum, 1)
    sum(i) = A(i) + B(i)
  end do
  !$omp end target
end subroutine example

However, only half of the array elements have been copied to the GPU. This must be fixed by matching the transferred range with array range actually used in the loop:

subroutine example()
  integer, dimension(100) :: A, B, sum
  integer :: i

  !$omp target map(to: A, B) map(from: sum)
  !$omp parallel do
  do i = 1, size(sum, 1)
    sum(i) = A(i) + B(i)
  end do
  !$omp end target
end subroutine example

When explicit ranges are not provided, the compiler automatically determines the full size of the arrays based on their declarations.

References​