bitops/bits.go
func msb64(n uint64) int {
	if n <= 0 {
		return -1
	}
	var r, v uint
	if n >= 1<<32 {
		r += 32
		v = uint(n >> 32)
	} else {
		v = uint(n)
	}
	if v >= 1<<16 {
		r += 16
		v >>= 16
	}
	if v >= 1<<8 {
		r += 8
		v >>= 8
	}
	if v >= 1<<4 {
		r += 4
		v >>= 4
	}
	if v >= 1<<2 {
		r += 2
		v >>= 2
	}
	r += v >> 1
	return int(r)
}
func msb64(n uint64) int {
	if n <= 0 {
		return -1
	}
	var r, v uint
	if n >= 1<<32 {
		r += 32
		v = uint(n >> 32)
	} else {
		v = uint(n)
	}
	if v >= 1<<16 {
		r += 16
		v >>= 16
	}
	if v >= 1<<8 {
		r += 8
		v >>= 8
	}
	if v >= 1<<4 {
		r += 4
		v >>= 4
	}
	if v >= 1<<2 {
		r += 2
		v >>= 2
	}
	r += v >> 1
	return int(r)
}
// msb64 returns the number of bits required to store the value x