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	!$OMP PARALLEL [clause[[,] clause] ... ]
   Inner	!$OMP DO [clause[[,] clause] ... ]

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

Example (original code)

!$OMP PARALLEL DO PRIVATE(tid, nthreads)
do k = 1, N
    tid = omp_get_thread_num() ! this call inside loop prevents vectorization
    nthreads = omp_get_num_threads() ! this call inside loop prevents vectorization
    ...
enddo

!$OMP PARALLEL PRIVATE(tid, nthreads)
! Move OpenMP calls here
tid = omp_get_thread_num()
nthreads = omp_get_num_threads()

!$OMP DO NOWAIT
do k = 1, N
    ...
enddo
!$OMP END PARALLEL