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

* 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)?

Ingo


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