Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task

Ingo Molnar
2007-11-29T19:44:58Z

* Andi Kleen <andi@firstfloor.org> wrote:

> For i386 iirc Jeremy/Zach did the benchmarking and they settled on %fs
> because it was faster for something (originally it was %gs too)

yep. IIRC, some CPUs only optimize %fs because that's what Windows uses
and leaves Linux with %gs out in the cold. There's also a performance
penalty for overlapping segment use, if the segment cache is single
entry only with an additional optimization for NULL [which just hides
the segment cache].

But if it's good for unification we could switch that to %gs again on
32-bit. I was one of the people who advocated the use of the 'other'
segment register, so that the hardware has less overlap, but clean and
unified code trumps this concern. It shouldnt be an issue on reasonably
modern CPUs anyway.

Ingo