Andi Kleen wrote:
On AMD K7/K8 a segment register prefix is a single cycle penalty.
I couldn't find anything in the Intel optimization manuals on it, but I assume
it's also not dramatic.
All I could find was:
* avoid multiple prefixes (which was the least important guideline
in instruction selection)
* avoid using multiple segment registers (the pentium M only has one
level of segment register renaming)
* avoid prefixes which take the instruction length over 7 bytes
None of these apply to the use of %gs to access PDA.
Most of the discussion about prefixes is in avoiding the 0x66 16-bit prefix.
J
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