title: How to find a binary logarithm very fast? (O(1) at best)
body: <p>Is there any very fast method to find a binary logarithm of an integer number? For example, given a number 
  x=52656145834278593348959013841835216159447547700274555627155488768 such algorithm must find y=log(x,2) which is 215. x is always a power of 2.</p>
  
  <p>The problem seems to be really simple. All what is required is to find the position of the most significant 1 bit. There is a well-known method FloorLog, but it is not very fast especially for the very long multi-words integers.</p>
  
  <p>What is the fastest method?</p>