OpenMP 4 simd vectorization for c=c+a*b

Question

I do not know if OpenMP 4 support this for loop or not. The speed with and without the pragma is the same.

#pragma omp  for simd
for (size_t i = 0; i < col; i++)
{
    C[i] += A[i]* B[i];
}

Answer 1

The reason (I guess) for the pragma do be of no effect is double:

• The code vectorises already without the simd directive; and
• The code is memory bound anyway, so adding more threads to compute it won't make much differences unless it gives you access to more memory bandwidth. See this excellent answer for more details.