C macro to get the smallest power of two greater than a given number

Question

I need a C macro to get the smallest of power two greater than a given number.

For example, FIRSTFREEBIT(0x16) (binary 1_0110 ) must be equal to 0x20 .

I am going to use it as:

#include <someheader.h> // defines SOME_X and SOME_Y
enum {
     x = SOME_X,
     y = SOME_Y,
     z = FIRSTFREEBIT(x|y),
     t = z << 1,
};

A similar, but slightly different SO question: Algorithm for finding the smallest power of two that's greater or equal to a given value

Answer 1

Here's my code, you are welcome to invent something better:

#define __OR_RSHIFT__(n,x) ((x)|(x)>>n)
#define FIRST_UNUSED_BIT(x) (1+__OR_RSHIFT__(16,__OR_RSHIFT__(8,__OR_RSHIFT__(4,__OR_RSHIFT__(2,__OR_RSHIFT__(1,x))))))

Answer 2

Look at the __builtin_clz GCC intrinsic. It will give you the number of leading zero bits, which could be used to determine the position of the first bit set. Then do a left bit shift of 1 , times the position.