PWR013: Avoid copying unused variables to or from the GPU

Issue

Unused variables should never be copied to or from the GPU to prevent unnecessary data movements between the CPU and the GPU, which impacts on performance.

Relevance

One of the key challenges when offloading work to the GPU is minimizing the data transfers between CPU memory and GPU memory. These transfers can greatly affect performance and should be as optimized as possible. Thus, only the strictly required data should be copied to or from the GPU memory.

Action

Remove the unused variables from the data mapping clauses.

Code example

In the following example, matrix B is copied to the GPU even when it is not used:

void example(double *A, double *B, double *C) {
  #pragma omp target teams distribute parallel for schedule(auto) \
  shared(A, B) map(to: A[0:100], B[0:100]) map(tofrom: C[0:100])
  for (int i = 0; i < 100; i++) {
     C[i] += A[i];
  }
}

This can be easily corrected by removing references to B from all the clauses:

void example(double *A, double *B, double *C) {
  #pragma omp target teams distribute parallel for schedule(auto) \
  shared(A) map(to: A[0:100]) map(tofrom: C[0:100])
  for (int i = 0; i < 100; i++) {
     C[i] += A[i];
  }
}

Related resources

• PWR013 examples at GitHub

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