c - Rounding up to next power of 2

Question

I want to write a function that returns the nearest next power of 2 number. For example if my input is 789, the output should be 1024. Is there any way of achieving this without using any loops but just using some bitwise operators?

Answer 1

Check the Bit Twiddling Hacks. You need to get the base 2 logarithm, then add 1 to that. Example for a 32-bit value:

Round up to the next highest power of 2

unsigned int v; // compute the next highest power of 2 of 32-bit v

v--;
v |= v >> 1;
v |= v >> 2;
v |= v >> 4;
v |= v >> 8;
v |= v >> 16;
v++;

The extension to other widths should be obvious.