[froydnj]

> I was surprised that the vectorisation it mentions was not performed automatically.

The OP didn't mention what compiler was being used; GCC will certainly automatically vectorize this example (and ICC probably will as well). I used GCC 4.4 for x86 with -O3 -msse2.

Of course, there's a lot of compensation code inserted for unaligned pointers and aliased pointers and the like, but automatic vectorization is certainly doable.