The loop is threaded and vectorized using the !$omp parallel for simd directive, which parallelizes the loop with both threads and SIMD instructions. Specifically, the directive divides loop iterations into chunks (subsets) and distributes the chunks among threads, then chunk iterations execute concurrently using SIMD instructions. In this case, the chunk size (number of iterations per chunk) is not a multiple of vector length. To fix: Add a schedule (simd: [kind]) modifier to the !$omp parallel for simd directive.
Guarantee a multiple of vector length.
!$omp parallel do simd schedule(simd: static)
do i = 1,1000
c(i) = a(i)*b(i)
end do
!$omp end parallel do simd