bitcount
Fast cross-platform bit counting using intrinsic functions.
Bitcount is a cross-platform implementation of GCC's intrinsic __builtin_clz,__builtin_ctz, and __builtin_popcount. Currently only intrinsic functions are used in VS 2012 (possibly older VS versions as well), and GCC-compatible compilers (including Clang). If Bitcount is used on a system that isn't either of these, it uses a C implementation of these functions as a fallback.
The library exposes three functions which follows the same API as the corresponing GCC intrinsics (which can be found here). Note that calls to clz and ctz with the value 0 are undefined.
/* Returns the number of leading 0-bits in x, starting at the most significant
bit position. If v is 0, the result is undefined. */
unsigned int bit_clz(unsigned int v)
/* Returns the number of trailing 0-bits in x, starting at the least significant
bit position. If v is 0, the result is undefined. */
unsigned int bit_ctz(unsigned int v)
/* Returns the number of 1-bits in v. */
unsigned int bit_popcount(unsigned int v)