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(nasm.info)Section 10.1.1

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10.1.1. NASM Generates Inefficient Code
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   We sometimes get `bug' reports about NASM generating inefficient, or
even `wrong', code on instructions such as `ADD ESP,8'. This is a
deliberate design feature, connected to predictability of output: NASM,
on seeing `ADD ESP,8', will generate the form of the instruction which
leaves room for a 32-bit offset. You need to code `ADD ESP,BYTE 8' if
you want the space-efficient form of the instruction. This isn't a bug,
it's user error: if you prefer to have NASM produce the more efficient
code automatically enable optimization with the `-On' option (see *Note
Section 2.1.16::).