Quite Incredible Optimization

I was nicely surprised today when I tried to compile the following test function with the -g3 optimization flag.

int f(int a)
{
    bool b(true);
    b &= a;
    return b ? 5 : 255;
}

The result makes use of advance arithmetic to compute the 5 or 255 out of the 0 or 1 defined by the result define in b. As expected, though, if a is not 0 or 1, the result may not be what you'd otherwise expect.

 0: 83 e7 01        and    $0x1,%edi
 3: 83 ff 01        cmp    $0x1,%edi
 6: 19 c0           sbb    %eax,%eax
 8: 25 fa 00 00 00  and    $0xfa,%eax
 d: 83 c0 05        add    $0x5,%eax
10: c3              retq

As you can see, only bit 0 of the input (%edi) is kept for the computation. The function may need to be rewritten with:

b = b && a;