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I am trying to compute the average value over adjacent elements within a matrix, but am stuck getting OpenMP's vectorization to work. As I understand the second nested for-loop, the reduction clause should ensure that no race conditions occur when writing to elements of next. However, when compiling the code (I tried auto-vectorization with both GCC GCC 7.3.0 and ICC and OpenMP > 4.5) I get the report: "error: reduction variable "next" must be shared on entry to this OpenMP pragma". Why does this occur when variables are shared by default? How can I fix this issue since adding shared(next) does not seem to help?

// CODE ABOVE (...)
size_t const width = 100;
size_t const height = 100;
float * restrict next = malloc(sizeof(float)*width*height);

// INITIALIZATION OF 'next' (this works fine)
#pragma omp for simd collapse(2) 
  for(size_t j = 1; j < height-1; j++)
    for(size_t i = 1; i < width-1; i++)
      next[j*width+i] = 0.0f; 

// COMPUTE AVERAGE FOR INNER ELEMENTS
#pragma omp for simd collapse(4) reduction(+:next[0:width*height]) 
for(size_t j = 1; j < height-1; ++j){
  for(size_t i = 1; i < width-1; ++i){
    // compute average over adjacent elements
    for(size_t _j = 0; _j < 3; _j++) {
      for(size_t _i = 0; _i < 3; _i++) {
          next[j*width + i] += (1.0 / 9.0) * next[(j-1 +_j)*width + (i-1 + _i)]; 
      }
    }
  }
}
dreamcrash
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St123
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1 Answers1

5

The problem is that GCC 7.3.0 does not support

#pragma omp for simd collapse(4) reduction(+:next[0:width*height])

the use of reduction of array sections in this context.

This function is support by GCC 9 forwards:

Since GCC 9, there is initial OpenMP 5 support (essentially C/C++, only). GCC 10 added some more features, mainly for C/C++ but also for Fortran.

dreamcrash
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