Function highbit64

freebsd/contrib/openzfs/lib/libzpool/kernel.c:703–710

* Find highest one bit set. * Returns bit number + 1 of highest bit that is set, otherwise returns 0. * The __builtin_clzll() function is supported by both GCC and Clang. */

 * The __builtin_clzll() function is supported by both GCC and Clang.
 */
int
highbit64(uint64_t i)
{
	if (i == 0)
	return (0);

	return (NBBY * sizeof (uint64_t) - __builtin_clzll(i));
}

/*
 * Find lowest one bit set.