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

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Date:	Thu, 6 Mar 2008 20:56:09 -0800
From:	Chris Lattner <clattner@...le.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Richard Guenther <richard.guenther@...il.com>,
	Joe Buck <Joe.Buck@...opsys.com>,
	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 Mar 6, 2008, at 2:06 PM, Andi Kleen wrote:

> 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

They aren't?  According to http://www.agner.org/optimize/instruction_tables.pdf 
, they have a latency of 52 cycles on at least one popular x86 chip.

-Chris
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