10.1.3.3 Reduction

Reduction operations reduce the elements of an array into a single value. For example, summing the elements of an array into a single variable involves updating that variable in each iteration:

   DO K = 1,N
     SUM = SUM + A(I)*B(I)
   END DO

If each processor running this loop in parallel takes some subset of the iterations, the processors will interfere with each other, overwriting the value in SUM. For this to work, each processor must execute the summation one at a time, although the order is not significant.

Certain common reduction operations are recognized and handled as special cases by the compiler.