More to the point, a change like this is very likely to be _noticed_. Both in terms of the complexity that would be required to make such a change in the micro

More to the point, a change like this is very likely to be _noticed_. Both in terms of the complexity that would be required to make such a change in the microcode, and if the logic screws up and tampers with the wrong XOR, it will cause a bug that people will search for and eventually discover that the CPU was screwing up XOR under some circumstances. Remember the firestorm over the FDIV bug?