NOPs are not generally free. It's a pretty old and well known technique: https://stackoverflow.com/questions/48046814/what-methods-can-be-used-to-efficiently-

NOPs are not generally free. It's a pretty old and well known technique: https://stackoverflow.com/questions/48046814/what-methods-can-be-used-to-efficiently-extend-instruction-length-on-modern-x86 https://stackoverflow.com/questions/48046814/what-methods-ca... Note that this technique is really only legitimate where the used prefix already has defined behavior with the given instruction ("Use of repeat prefixes and/or undefined opcodes with other Intel 64 or IA-32 instructions is reserved; such use may cause unpredictable behavior."), and of course the REX prefix has special limitations. The key is redundant, not spurious. It is not a good idea to be doing rep add for example. But otherwise, there is no issue.