floating accuracy - How to do an integer log2() in C++?

Question

In the C++ standard libraries I found only a floating point log method. Now I use log to find the level of an index in a binary tree ( floor(2log(index)) ).

Code (C++):

int targetlevel = int(log(index)/log(2));

I am afraid that for some of the edge elements (the elements with value 2^n) log will return n-1.999999999999 instead of n.0. Is this fear correct? How can I modify my statement so that it always will return a correct answer?

Answer 1

If you are on a recent-ish x86 or x86-64 platform (and you probably are), use the bsr instruction which will return the position of the highest set bit in an unsigned integer. It turns out that this is exactly the same as log2(). Here is a short C or C++ function that invokes bsr using inline ASM:

#include <stdint.h>
static inline uint32_t log2(const uint32_t x) {
  uint32_t y;
  asm ( "bsr %1, %0
"
      : "=r"(y)
      : "r" (x)
  );
  return y;
}