They are equivalent due to zero extension. See http://stackoverflow.com/questions/11177137/why-do-most-x64-... But the encoding for most instructions using 32bit operands are shorter than 64bit operands. And shorter code is better because it fits better in caches and jumps are closer and so on.

Though I was wrong about that particular case because "xor r9d, r9d" and "xor r9, r9" are both three bytes long. But "xor eax, eax" is two bytes and "xor rax, rax" is three bytes. So that's why you should use the old GPR:s when you are able to choose.

CPUs special case xor r,r as clearing the register and breaking dependency chains. Do they do the same thing when relying on implicit zero extension? (Can't be bothered to check Agner)