Nice work Dan. The proper way to implement the code is to force sequence points at each operation:

// x^=y^=x^=y;

x ^= y;
y ^= x;
x ^= y;

Does what you expect and doesn't give the compiler hissy fits.
Thanks Russ, for an interesting stroll down memory lane :-)

(The original code is implementation dependent because even though the result is undefined by the language, a C compiler will try to generate code that does what it thinks you meant. Usually they guess right, but there's no guarantee.)