How to avoid auto-vectorization for this loop (rolls at most 3 times)

[Fang, Changpeng]

It seems  the auto-vectorizer could not recognize that this loop will roll at 
most 3 times.
And it will generate quite messy code. 

int a[1024], b[1024]; 
void foo (int n) 
{
  int i;
  for (i = (n/4)*4; i< n; i++)
    a[i] =  a[i] +  b[i];
}

How can we correctly estimate the number of iterations for this case and use 
this info for the vectorizer?

Thanks,

Changpeng