linux-kernel - Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

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Message-ID: <20080306220610.GB31833@one.firstfloor.org>
Date:	Thu, 6 Mar 2008 23:06:10 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Richard Guenther <richard.guenther@...il.com>,
	Joe Buck <Joe.Buck@...opsys.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Haley <aph@...hat.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Michael Matz <matz@...e.de>, linux-kernel@...r.kernel.org,
	gcc@....gnu.org
Subject: Re: Linux doesn't follow x86/x86-64 ABI wrt direction flag

On Thu, Mar 06, 2008 at 12:56:16PM -0800, H. Peter Anvin wrote:
> Richard Guenther wrote:
> >
> >A patched GCC IMHO makes only sense if it is always-on, yet another option
> >won't help in corner cases.  And corner cases is exactly what people seem
> >to care about.  For this reason that we have this single release, 4.3.0, 
> >that
> >behaves "bad" is already a problem.
> >
> 
> The option will help embedded vendors who can guarantee that it's not a 
> problem.

For very very low values of "help".

To be realistic it is very unlikely anybody will measure a difference
from a few more or a few less clds in a program. It's not that they're
expensive instructions and they normally don't happen in inner loops either.

"If you enable this option you will get an optimization that you cannot
measure" @)

-Andi
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