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

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Date:	Sun, 18 Jan 2009 17:42:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/17] x86-64: Use absolute displacements for per-cpu
	accesses.


* Brian Gerst <brgerst@...il.com> wrote:

> On Sat, Jan 17, 2009 at 11:22 PM, Tejun Heo <tj@...nel.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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