Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

Jeremy Fitzhardinge
2006-08-27T18:27:22Z

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