#define B0 (0x55555555)
#define B1 (0x33333333)
#define B2 (0x0F0F0F0F)
#define B3 (0x001F001F)
#define B4 (0x0000003F)

int countbits(int x) {
	x = (x & B0) + ((x >>  1) & B0);
	x = (x & B1) + ((x >>  2) & B1);

	x = (x + (x >>  4)) & B2;
	x = (x + (x >>  8)) /*& B3 */
	x = (x + (x >> 16)) & B4;
	return x;
}