OpenMP calls prevent automatic vectorization when the compiler cannot move the calls outside the loop body, such as when OpenMP calls are not invariant. To fix:

1. Split the OpenMP parallel loop directive into two directives.

   Target	Directive
   Outer	#pragma omp parallel [clause, clause, ...]
   Inner	#pragma omp for [clause, clause, ...]

2. Move the OpenMP calls outside the loop when possible.

Example (original code)

#pragma omp parallel for private(tid, nthreads)
for (int k = 0; k < N; k++)
{
    tid = omp_get_thread_num(); // this call inside loop prevents vectorization
    nthreads = omp_get_num_threads(); // this call inside loop prevents vectorization
    ...
}

#pragma omp parallel private(tid, nthreads)
{
    // Move OpenMP calls here
    tid = omp_get_thread_num();
    nthreads = omp_get_num_threads();

    #pragma omp for nowait
    for (int k = 0; k < N; k++)
    {
        ...
    }
}