You're right, but only for pre-Haswell chips, where al etc. are renamed separately. From Haswell onwards, they aren't, and depend on the old value. So movzx eax

You're right, but only for pre-Haswell chips, where al etc. are renamed separately. From Haswell onwards, they aren't, and depend on the old value. So movzx eax, al gets you mathematical independence, but not out-of-order independence. There's a bunch more detail in this Stack Overflow question (referenced in the one you linked): https://stackoverflow.com/questions/45660139/how-exactly-do-partial-registers-on-haswell-skylake-perform-writing-al-seems-to https://stackoverflow.com/questions/45660139/how-exactly-do-...