Messages in this thread | | | Date | Sun, 27 Aug 2006 11:27:17 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH RFC 0/6] Implement per-processor data areas for i386. |
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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.
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