fast ffs and ffsl

test.c

static char ffslut32[] = {
	32,  1, 23,  2, 29, 24, 14,  3, 30, 27, 25, 18, 20, 15, 10,  4,
	31, 22, 28, 13, 26, 17, 19,  9, 21, 12, 16,  8, 11,  7,  6,  5
};

static char ffslut64[] = {
	64,  1, 48,  2, 57, 49, 28,  3, 61, 58, 50, 42, 38, 29, 17,  4,
	62, 55, 59, 36, 53, 51, 43, 22, 45, 39, 33, 30, 24, 18, 12,  5,
	63, 47, 56, 27, 60, 41, 37, 16, 54, 35, 52, 21, 44, 32, 23, 11,
	46, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19,  9, 13,  8,  7,  6
};

static int ffs32(uint32_t mask)
{
	return mask ? ffslut32[((mask & (-mask)) * 0x0FB9AC52) >> 27] : 0;
}

static int ffs64(uint64_t mask)
{
	return mask ? ffslut64[((mask & (-mask)) * 0x07EF3AE369961512ULL) >> 58] : 0;
}

int ffs(int mask)
{
	if (sizeof(int) == 8)
		return ffs64(mask);
	if (sizeof(int) == 4)
		return ffs32(mask);
	return -1;
}

int ffsl(long mask)
{
	if (sizeof(long) == 8)
		return ffs64(mask);
	if (sizeof(long) == 4)
		return ffs32(mask);
	return -1;
}