linux-kernel - Re: i686 quirk for AMD Geode

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Message-ID: <20091110205445.GB1407@ucw.cz>
Date:	Tue, 10 Nov 2009 21:54:45 +0100
From:	Pavel Machek <pavel@....cz>
To:	Willy Tarreau <w@....eu>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Matteo Croce <technoboy85@...il.com>,
	Sven-Haegar Koch <haegar@...net.de>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: i686 quirk for AMD Geode

Hi!

> Indeed, but there is a difference between [cmpxchg, bswap, cmov, nopl]
> on one side and [sse*] on the other : distros are built assuming the
> former are always available while they are not always. And the
> distro

Well, fix the distros... 

> which make the difference have to provide an dedicated build for earlier
> systems just for compatibility. SSE*, 3dnow* etc... are only used by a
> handful of media players/converters/encoders which are able to detect
> themselves what to use and already have the necessary fallbacks because
> these instruction sets vary too much between processors and vendors.
> 
> One could argue that cmpxchg/bswap/xadd are supported by 486 and that
> implementing them for 386 is almost useless now (though it costs almost
> nothing to provide them, I did a few years ago). 
> 
> CMOV/NOPL are rarely used, thus have no reason to cause a massive
> performance drop, but are frequent enough (at least cmov) for almost

*One* CMOV in the inner loop will make your performance go down 20x.

      	      	  	     	       	    		   	Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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