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Chapter 10 x87 Floating-Point Optimizations 243
Software Optimization Guide for AMD64 Processors
25112 Rev. 3.06 September 2005
the operand-size override (66h) serves as a high-performance NOP instruction and is the
recommended choice for padding an x87 instruction without altering its behavior, as shown here:
DB 066h ; Operand-size override used as high-performance NOP instruction
This usage of the operand-size override alone as a filler byte (without an accompanying NOP
instruction) is permitted only for x87 instructions. This usage of the operand-size override can be
applied to all but four of the x87 instructions. The FLDENV, FRSTOR, FSTENV, and FSAVE
instructions and their no-wait forms behave differently when associated with an operand-size
override; therefore, these should not be padded with the operand-size override.