How to make it faster?

When I used gprof to see which function consumed most running time, I
identified the following one.  sz was less than 5000 on average, but
foo had been called about 1,000,000 times.  I have tried using
"register sum = 0.0" and saw some improvement. My question is how to
improve foo further to make it faster.

double foo(double *a, double *b, int sz)
{

  double sum = 0.0;
  int i;

  for (i = 0; i < sz; i++)
    sum += a[i]*b[i];

  return sum;
}