Bitwise Tricks

C code	Description	Obtained from
`a ^= b; b ^= a; a ^= b;`	Swap a and b around	ComputerShopper
`c & -c` or `c & (~c + 1)`	return first bit set	Eivind Eklund
`~c & (c + 1)`	return first unset bit	Eivind Eklund
`unsigned i, c = <number>; for (i = 0; c; i++) c ^= c & -c;`	i will contain the number of bits set in c	Eivind Eklund
`m = (m & 0x55555555) + ((m & 0xaaaaaaaa) >> 1); m = (m & 0x33333333) + ((m & 0xcccccccc) >> 2); m = (m & 0x0f0f0f0f) + ((m & 0xf0f0f0f0) >> 4); m = (m & 0x00ff00ff) + ((m & 0xff00ff00) >> 8); m = (m & 0x0000ffff) + ((m & 0xffff0000) >> 16);`	m will now contain number of bits set in the original m	popcnt in FreeBSD's sys/i386/i386/mp_machdep.c
`v -= ((v >> 1) & 0x55555555); v = (v & 0x33333333) + ((v >> 2) & 0x33333333); v = (v + (v >> 4)) & 0x0F0F0F0F; v = (v * 0x01010101) >> 24;`	v will now contain number of bits set in the original v	popcnt comment in FreeBSD's sys/i386/i386/mp_machdep.c
`#define BITCOUNT(x) (((BX_(x)+(BX_(x)>>4)) & 0x0F0F0F0F) % 255) #define BX_(x) ((x) - (((x)>>1)&0x77777777) \ - (((x)>>2)&0x33333333) \ - (((x)>>3)&0x11111111))`	macro to return number of bits in x (assumes 32bit integer)	BSD fortune
`n = ((n >> 1) & 0x55555555) \| ((n << 1) & 0xaaaaaaaa); n = ((n >> 2) & 0x33333333) \| ((n << 2) & 0xcccccccc); n = ((n >> 4) & 0x0f0f0f0f) \| ((n << 4) & 0xf0f0f0f0); n = ((n >> 8) & 0x00ff00ff) \| ((n << 8) & 0xff00ff00); n = ((n >> 16) & 0x0000ffff) \| ((n << 16) & 0xffff0000);`	reverse bits in a 32bit integer	BSD fortune
popcnt(x ^ (x - 1)) & 31	return first bit set in x, where popcnt returns number of bits set	Peter Wemm
ffs using a lookup table	return first bit set	Colin Percival

If you have any other cool bitwise tricks you know about, please e-mail them to John-Mark Gurney <jmg@funkthat.com>, and I will add them to the list.

A large collection of them can be found at Bit Twiddling Hacks.