Understanding partial-register slowdowns from mov instead of movzx instruction [duplicate]

问题

This question already has answers here:

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Closed 2 years ago.

I'm very new to assembly language and trying to understand some of its working principles. I read this answer and have a question about the following wording:

it avoids performance penalties that may result from writing to only the low 8 (or 16) bits of a register

What kind of performance penalty? Very uclear why writing just the first low 8 or 16 bits of a register is slower than writing the sign-extended version?

来源：https://stackoverflow.com/questions/47052342/understanding-partial-register-slowdowns-from-mov-instead-of-movzx-instruction