I would say you cannot deduce much from that micro-benchmark. On x86, loops depend on the alignment of the labels and branch instructions due to the branch pred

I would say you cannot deduce much from that micro-benchmark. On x86, loops depend on the alignment of the labels and branch instructions due to the branch prediction logic. You can actually make some loops faster by inserting nops inside the loop. Since the result of the xor is never read and immediately overwritten by the following mov, I wonder if clever register renaming could actually detect that and discard the instruction.