What is the fastest algorithm to return the power of a number which is a power of 2?

Question

Given n = 2^k, how can I find k assuming n is 32-bit integer using C/C++ bitwise?

Answer 1

GCC has __builtin_clz that translates to BSR on x86/x64, CLZ on ARM, etc. and emulates the instruction if the hardware does not implement it.
Visual C++ 2005 and up has _BitScanReverse.

Using those functions, you can get your k

Answer 2

Wikipedia writes how to do it using bitwise operators:

/**
 * Returns the floor form of binary logarithm for a 32 bit integer.
 * −1 is returned if ''n'' is 0.
 */
int floorLog2(unsigned int n) {
  if (n == 0)
    return -1;

  int pos = 0;
  if (n >= 1<<16) { n >>= 16; pos += 16; }
  if (n >= 1<< 8) { n >>=  8; pos +=  8; }
  if (n >= 1<< 4) { n >>=  4; pos +=  4; }
  if (n >= 1<< 2) { n >>=  2; pos +=  2; }
  if (n >= 1<< 1) {           pos +=  1; }
  return pos;
}

Code taken from: Wikipedia on: Binary Logarithm this page has since been altered the original version with the code sample can still be found her: Wikipedia on: Binary Logarithm (24 May 2011)