Count number of 1's in binary representation

Question

Efficient way to count number of 1s in the binary representation of a number in O(1) if you have enough memory to play with. This is an interview question I found on an onli

Answer 1

I came here having a great belief that I know beautiful solution for this problem. Code in C:

    short numberOfOnes(unsigned int d) {
        short count = 0;

        for (; (d != 0); d &= (d - 1))
            ++count;

        return count;
    }

But after I've taken a little research on this topic (read other answers:)) I found 5 more efficient algorithms. Love SO!

There is even a CPU instruction designed specifically for this task: popcnt. (mentioned in this answer)

Description and benchmarking of many algorithms you can find here.