Fastest implementation of log2(int) and log2(float)

Question

The question is

Are there any other (and/or faster) implementations of a basic 2log?

Applications

The log2(int) and

Answer 1

There are some integer algorithms here.

In C#:

public static uint FloorLog2(uint x)
{
    x |= (x >> 1);
    x |= (x >> 2);
    x |= (x >> 4);
    x |= (x >> 8);
    x |= (x >> 16);

    return (uint)(NumBitsSet(x) - 1);
}

public static uint CeilingLog2(uint x)
{
    int y = (int)(x & (x - 1));

    y |= -y;
    y >>= (WORDBITS - 1);
    x |= (x >> 1);
    x |= (x >> 2);
    x |= (x >> 4);
    x |= (x >> 8);
    x |= (x >> 16);

    return (uint)(NumBitsSet(x) - 1 - y);
}

public static int NumBitsSet(uint x)
{
    x -= ((x >> 1) & 0x55555555);
    x = (((x >> 2) & 0x33333333) + (x & 0x33333333));
    x = (((x >> 4) + x) & 0x0f0f0f0f);
    x += (x >> 8);
    x += (x >> 16);

    return (int)(x & 0x0000003f);
}

private const int WORDBITS = 32;

You should look at the original code on the site I linked for the context, particularly what happens with Log2(0).