Is it possible to do a reduction on an array with openmp?

Question

Does OpenMP natively support reduction of a variable that represents an array?

This would work something like the following...

float* a = (float*) c         


        

Answer 1

Only in Fortran in OpenMP 3.0, and probably only with certain compilers.

See the last example (Example 3) on:

http://wikis.sun.com/display/openmp/Fortran+Allocatable+Arrays

Answer 2

Now the latest openMP 4.5 spec has supports of reduction of C/C++ arrays. http://openmp.org/wp/2015/11/openmp-45-specs-released/

And latest GCC 6.1 also has supported this feature. http://openmp.org/wp/2016/05/gcc-61-released-supports-openmp-45/

But I didn't give it a try yet. Wish others can test this feature.

Answer 3

OpenMP can perform this operation as of OpenMP 4.5 and GCC 6.3 (and possibly lower) supports it. An example program looks as follows:

#include <vector>
#include <iostream>

int main(){
  std::vector<int> vec;

  #pragma omp declare reduction (merge : std::vector<int> : omp_out.insert(omp_out.end(), omp_in.begin(), omp_in.end()))

  #pragma omp parallel for default(none) schedule(static) reduction(merge: vec)
  for(int i=0;i<100;i++)
    vec.push_back(i);

  for(const auto x: vec)
    std::cout<<x<<"\n";

  return 0;
}

Note that omp_out and omp_in are special variables and that the type of the declare reduction must match the vector you are planning to reduce on.

Answer 4

OpenMP cannot perform reductions on array or structure type variables (see restrictions).

You also might want to read up on private and shared clauses. private declares a variable to be private to each thread, where as shared declares a variable to be shared among all threads. I also found the answer to this question very useful with regards to OpenMP and arrays.

Answer 5

Array reduction is now possible with OpenMP 4.5 for C and C++. Here's an example:

#include <iostream>

int main()
{

  int myArray[6] = {};

  #pragma omp parallel for reduction(+:myArray[:6])
  for (int i=0; i<50; ++i)
  {
    double a = 2.0; // Or something non-trivial justifying the parallelism...
    for (int n = 0; n<6; ++n)
    {
      myArray[n] += a;
    }
  }
  // Print the array elements to see them summed   
  for (int n = 0; n<6; ++n)
  {
    std::cout << myArray[n] << " " << std::endl;
  } 
}

Outputs:

100
100
100
100
100
100

I compiled this with GCC 6.2. You can see which common compiler versions support the OpenMP 4.5 features here: https://www.openmp.org/resources/openmp-compilers-tools/

Note from the comments above that while this is convenient syntax, it may invoke a lot of overheads from creating copies of each array section for each thread.