Processor peak performance includes the speed-up provided by the vector instructions, but in order to exploit it you need specific programming techniques.

Auto-vectorization is the easiest and more portable way to get vectorization , but not all loops can be vectorized:

Some examples:

• Loop with dependencies between iterations

for (i=1; i<MAX; i++) {
   d[i] = e[i] – a[i-1];
   a[i] = b[i] + c[i];
}

• Complex loops

• Function calls inside the loop:

    for (int i = 0; i < N; i++)   a[i] = foo(b[i]);
    
* Loops on data that are not aligned in the memory