Well that 0 that you are loading comes from the instruction, so it is already "there". It boils down to the fact that the instruction is sorter. In fact in the

Well that 0 that you are loading comes from the instruction, so it is already "there". It boils down to the fact that the instruction is sorter. In fact in theory the load is slower, because XOR has data dependencies on the arguments. So an out-of-order processor could be delayed. However x86 has special logic that XOR with itself doesn't carry any dependencies on the arguments.