One has to keep in mind that vectorization changes the order of operations inside a loop. A so-called read-after-write statement with an explicit flow dependency cannot be vectorized. The following code
double b = 15.;
for (int i = 1; i < n; i++) {
a[i] = a[i-1] + b;
}
is an example of flow dependency and results in wrong numerical results if vectorized. For a scalar operation, the value a[i-1] computed during the iteration is loaded into the right-hand side and the results are fine. In vector mode however, with a vector length of four, the values a[0] , a[1] , a[2] and a[3] from the previous loop will be loaded into the right-hand side and produce wrong results. That is, we have
a[1] = a[0] + b;
a[2] = a[1] + b;
a[3] = a[2] + b;
a[4] = a[3] + b;
and if the two first iterations are executed at the same by the SIMD instruction, the value of say a[1] could be used by the second iteration before it has been calculated by the first iteration, leading thereby to wrong results.