Question

If there is a for-loop inside a parallel region, would for-loop be parallelized again or every thread will execute its own for-loop?

T sum;

#pragma omp parallel
{
    #pragma omp for reduction(+: sum)
    for (;;)
    {
        T priv_var;

        sum += priv_var;
    }
}

Answer 1

Yes, this code will cause OpenMP to parallelise the for loop across the threads that are spawned by the parallel region. However, I believe that your current for statement is invalid for OpenMP parallelisation. You need to explicitly provide an integer loop variable, start and end, and increment expression.

In effect, your code will then be equivalent to a single loop with #pragma omp parallel for reduction(+: sum).

More information on MDSN