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SubjectRe: [PATCH 12/17] x86-64: Use absolute displacements for per-cpu accesses.

* Ingo Molnar <mingo@elte.hu> wrote:

>
> * Brian Gerst <brgerst@gmail.com> wrote:
>
> > On Sat, Jan 17, 2009 at 11:22 PM, Tejun Heo <tj@kernel.org> wrote:
> >
> > > Hello, Brian.
> > >
> > > Brian Gerst wrote:
> > > > Accessing memory through %gs should not use rip-relative addressing.
> > > > Adding a P prefix for the argument tells gcc to not add (%rip) to
> > > > the memory references.
> > >
> > > Nice catch. I dind't know about the P prefix thing. It also is used
> > > in other places too. Hmmm... I can't find anything about the P
> > > argument prefix in the gcc info page (4.3). Any ideas where I can
> > > find some information about it? It's a bit weird that it's not a
> > > constraint prefix but an argument one.
> >
> > The only place I could confirm that it works is in the gcc source
> > itself, and even there it's not well documented.
>
> does %P support go back as far as gcc 3.2 (the earliest GCC we still
> support)?

update: Brian pointed it out off-list that switch_to() already uses %P, so
we already rely on it.

Ingo


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