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

It's because the CPU doesn't have so many cache misses (where it has to wait for the array data to come from the RAM chips). It would be interesting for you to adjust the size of the arrays continually so that you exceed the sizes of the level 1 cache (L1), and then the level 2 cache (L2), of your CPU and plot the time taken for your code to execute against the sizes of the arrays. The graph shouldn't be a straight line like you'd expect.