restricted pointers

[Segher Boessenkool]

> void vecmult(int n, int * restrict a, int * restrict b, int * restrict 
> c)
> {
>   int i;
>   for (i=0; i<n; ++i) {
>     a[i] = b[i] * c[i];
>   }
> }

> but the times with or without restrict are the same. Why isn't it
> improving performance, am I doing something wrong?

Well, no matter what, the program will have to perform n reads of
elements of b[] and c[], and perform n writes to elements of a[].

You could try adding -O3 and/or some of the vectorisation options.

Also, you could test something like

void vecmult(int n, int * restrict a, int * restrict b, int * restrict 
c)
{
   int i;
   for (i=0; i<n; ++i) {
     a[i] = b[0] * c[i];
   }
}

which has a much bigger opportunity for optimisation (no need to load
from b[0] more than once, only if there is a restrict on b[]).


Segher