Position of least significant bit that is set

Question

I am looking for an efficient way to determine the position of the least significant bit that is set in an integer, e.g. for 0x0FF0 it would be 4.

A trivial impleme

Answer 1

If you have the resources, you can sacrifice memory in order to improve the speed:

static const unsigned bitPositions[MAX_INT] = { 0, 0, 1, 0, 2, /* ... */ };

unsigned GetLowestBitPos(unsigned value)
{
    assert(value != 0); // handled separately
    return bitPositions[value];
}

Note: This table would consume at least 4 GB (16 GB if we leave the return type as unsigned). This is an example of trading one limited resource (RAM) for another (execution speed).

If your function needs to remain portable and run as fast as possible at any cost, this would be the way to go. In most real-world applications, a 4GB table is unrealistic.