What does this condition test?

Question

Came across this conditional in some uncommented Objective-C code:

if (w & (w - 1))
{
    i = 1;
    while (i < w)
    {
        i *= 2;
    }
    w = i;
}

Where w is a size_t greater than 1.

Update: Added the code contained by the conditional for context.

Answer 1

It tests whether more than one bit is set in w, i.e. whether it's not an exact power of two. See here.

Answer 2

It seems it's checking for powers of two. If w is a power of 2, the bit representation of w and w-1 have no bit in common set to 1. Example : 100 for 4 and 011 for 3. Thus the bitwise and (& in C) will give false for any w which is a power of two.