What do you mean that cpu does something different? Isnt cpu doing what is being asked, that being xor with consequence of zeroing when given two same values.
> And, having done that it removes the operation from the execution queue - that is the xor takes zero execution cycles!1 It’s essentially optimised out by the CPU
Right, it has the same consequence, but it doesn't actually perform the stated operation. ASM is just a now just a high level language that tells the computer to "please give me the same state that a PDP-11-like computer would give me upon executing these instructions."
It's emulating the zero result when it recognizes this pattern, usually by playing clever tricks with virtual registers.
But it will not execute xor, nor will it actually zero out eax in most cases.
It'll do something similar to constant propagation with the information that whenever xor eax, eax occurs; all uses of eax go through a simpler execution path until eax is overwritten.