C to get the smallest degree of power in excess of a given number

Question

I need a C macro to get the smallest of the two powers exceeding a given number.

For example, FIRSTFREEBIT(0x16) (binary 1_0110 ) should be 0x20 .

I will 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: An algorithm for finding the least power of two greater than or equal to a given value

Answer 1

Here is my code, you can come up with 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

Take a look at the built-in GCC __builtin_clz . It will give you the number of initial zero bits that can be used to determine the position of the first set of bits. Then shift the left bit 1 , once per position.