[lucozade]

> couldn't it? (Does it?)

It could of course. It can do pretty much any pattern matching it likes. But I doubt very much it would because that pattern is way less common.

As the article points out, the XOR saves 3 bytes of instructions for a really, really common pattern (to zero a register, particularly the return register).

So there's very good reason to perform the XOR preferentially and hence good reason to optimise that very common idiom.

Other approaches eg add a new "zero <reg>" instruction are basically worse as they're not backward compatible and don't really improve anything other than making the assembly a tiny bit more human readable.