Question

int bNum=3;
int aNum=3;
for (int i = 0; i < bNum; i++)
        {
            for (int j = 0; j < aNum; j++)
            {
                b[i] += a[j];
                b[i + 1] += a[j + 3];
                b[i + 2] += a[j + 6];
            }
        }

I basically want the following to happen:

b[0]=a[0]+a[1]+a[2];
b[1]=a[3]+a[4]+a[5];
b[2]=a[6]+a[7]+a[8];

What is wrong with my logic?

HAHA! OOPS! My sleepy eyes are accounting for the dumb question. I see that at every iteration it get computed multiple times.

Answer 1

whatever language it is, you need to make constant index for b, variable index for a

 for (int j = 0; j < aNum; j++)
        {
            //here is loop-unrolling for b
            b[0] += a[j];
            b[1] += a[j + 3];
            b[2] += a[j + 6];
        }



  for(k=0;k<max;k++)
  {
  for (int j = 0; j < aNum; j++)
        {
            //without loop-unrolling
            b[k] += a[j+k*3];

        }
   }

i made the “k” loop outer to let it read the a[] as continuously as possible. You can make “k” loop innerside but it makes reads of a[] jumping from here to there then coming back here again an jumping again, which makes it slow.
this is not a vector though 😛 i just used it to fill the space