code: func CeilToPowerOfTwo(n int) int {
  	if n&maxintHeadBit != 0 && n > maxintHeadBit {
  		panic("argument is too large")
  	}
  	if n <= 2 {
  		return n
  	}
  	n--
  	n = fillBits(n)
  	n++
  	return n
  }
docs: // CeilToPowerOfTwo returns the least power of two integer value greater than
  // or equal to n.