faster implementation of sum ( for Codility test )

Question

How can the following simple implementation of sum be faster?

private long sum( int [] a, int begin, int end ) {
    if( a == null   ) {
        ret         


        

Answer 1

In C# 3.0, my computer and my OS this is faster as long as you can guarantee that 4 consecutive numbers won't overflow the range of an int, probably because most additions are done using 32-bit math. However using a better algorithm usually provides higher speed up than any micro-optimization.

Time for a 100 millon elements array:

4999912596452418 -> 233ms (sum)

4999912596452418 -> 126ms (sum2)

    private static long sum2(int[] a, int begin, int end)
    {
        if (a == null) { return 0; }
        long r = 0;
        int i = begin;
        for (; i < end - 3; i+=4)
        {
            //int t = ;
            r += a[i] + a[i + 1] + a[i + 2] + a[i + 3];
        }
        for (; i < end; i++) { r += a[i]; }
        return r;
    }