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Does OpenMP natively support reduction of a variable that represents an array?
This would work something like the following...
float* a = (float*) calloc(4*sizeof(float));
omp_set_num_threads(13);
#pragma omp parallel reduction(+:a)
for(i=0;i<4;i++){
a[i] += 1; // Thread-local copy of a incremented by something interesting
}
// a now contains [13 13 13 13]
Ideally, there would be something similar for an omp parallel for, and if you have a large enough number of threads for it to make sense, the accumulation would happen via binary tree.
905
The OpenMP reduction clause lets you specify one or more thread-private variables that are subject to a reduction operation at the end of the parallel region. OpenMP predefines a set of reduction operators. Each reduction variable must be a scalar (for example, int , long , and float ).
Performs a reduction on all scalar variables in list using the specified operator. Reduction variables in list are separated by commas. A private copy of each variable in list is created for each thread.
The reduction clauses are data-sharing attribute clauses that can be used to perform some forms of recurrence calculations in parallel. Reduction clauses include reduction scoping clauses and reduction participating clauses. Reduction scoping clauses define the region in which a reduction is computed.
#pragma omp parallel spawns a group of threads, while #pragma omp for divides loop iterations between the spawned threads. You can do both things at once with the fused #pragma omp parallel for directive.
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.
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