Why are elementwise additions much faster in separate loops than in a combined loop?

Question

Suppose a1, b1, c1, and d1 point to heap memory and my numerical code has the following core loop.

const i         


        

Answer 1

The first loop alternates writing in each variable. The second and third ones only make small jumps of element size.

Try writing two parallel lines of 20 crosses with a pen and paper separated by 20 cm. Try once finishing one and then the other line and try another time by writting a cross in each line alternately.