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