This nop idiom is very special however, since it isn't just about efficiency: if it wasn't a nop idiom, xchg eax, eax would not be a nop at all, it would clear

This nop idiom is very special however, since it isn't just about efficiency: if it wasn't a nop idiom, xchg eax, eax would not be a nop at all, it would clear the upper 32 bits, as xchg ebx, ebx does (or any other register other than eax).