How to find the index of the left-most 1 bit for x=2^k? [duplicate]

Question

Possible Duplicate:
Efficient bitwise operations for counting bits or find the right|left most ones

I wrote a search algorithm. In order to optimize it, I want to use a 32bit int to mark if number 0~31 could be used. That is, if I have a state State, I could easily fetch all possible number by using:

x = State & -State
State -= x

But actually, I need to know where is the 1 in x (notice that there is only 1 in x's binary form)? For example, if x = 0000 0100, I want to know that is the third.

I know I could do that by using a for loop, but it seems that it would cost much time.

I wonder that if there is a method using bitwise operation? And would static_cast<int>(log2(x)) costs much time?

Thanks for any help.

Answer 1

Many CPUs have a native hardware instruction, something like CTZ ("count trailing zeros"). GCC ex­po­ses this through the built-in function __builtin_ctz; other compilers should have similar facilities.