Question

I have the following division that I need to do often:

int index = pos / 64;

Division can be expensive in the cpu level. I am hoping there is a way to do that with bitwise shift. I would also like to understand how you can go from division to shift, in other words, I don’t want to just memorize the bitwise expression.

Answer 1

int index = pos >> 6 will do it, but this is unnecessary. Any reasonable compiler will do this sort of thing for you. Certainly the Sun/Oracle compiler will.

The general rule is that i/(2^n) can be implemented with i >> n. Similarly i*(2^n) is i << n.

You need to be concerned with negative number representation if i is signed. E.g. twos-complement produces reasonable results (if right shift is arithmetic–sign bit copied). Signed magnitude does not.