Here's a fast way to count the number of bits in an integer.  It uses a 4 bit wide lookup table and interates through each nibble to add to the total number of bits set.

Code:

int bitcount(unsigned int num)

{

     int count = 0;

     static int nibblebits[] =

          {0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4};

     for(; num != 0; num >>= 4)

          count += nibblebits[num & 0x0f];

     return count;

}

Extensions:

Sources:

http://c-faq.com/misc/bitcount.html