[onlydnaq]

This is a classic trick, and as you write could be used for performance benefits in the old day.

However, the semantics differ from just using a temporary variable in that if a and b are in the same memory location then the result will be zero.

This was used in an entry for the underhanded C contest [1] if I remember correctly where for an implementation of RC4 the author defined the following macro.

    #define SWAP(x, y) do { x^=y; y^=x; x^=y; }

And used it for swapping the values in the substitution table for the cipher, e.g. SWAP(S[i], S[j]). The weakness was that since sometimes the indices are the same in RC4 the substitution table would be gradually replaced with zeroes.

[1] http://www.underhanded-c.org/_page_id_16.html