linux-kernel - Re: kernel + gcc 4.1 = several problems

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Message-Id: <1167831136.3095.8.camel@laptopd505.fenrus.org>
Date:	Wed, 03 Jan 2007 05:32:16 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Alan <alan@...rguk.ukuu.org.uk>
Cc:	Grzegorz Kulewski <kangur@...com.net>,
	Mikael Pettersson <mikpe@...uu.se>, s0348365@....ed.ac.uk,
	torvalds@...l.org, 76306.1226@...puserve.com, akpm@...l.org,
	bunk@...sta.de, greg@...ah.com, linux-kernel@...r.kernel.org,
	yanmin_zhang@...ux.intel.com
Subject: Re: kernel + gcc 4.1 = several problems

On Wed, 2007-01-03 at 12:44 +0000, Alan wrote:
> > > fixed. At that point an i686 kernel would contain i686 instructions and
> > > actually run on all i686 processors ending all the i586 pain for most
> > > users and distributions.
> > 
> > Could you explain why CMOV is pointless now? Are there any benchmarks 
> > proving that?
> 
> Take a look at the recent ffmpeg bits on the mplayer list for one example
> I have to hand - P4 cmov is pretty slow. The crypto folks find the same
> things.

cmov is effectively the same cost as a compare and jump, in both cases
the cpu needs to do a prediction, and on a mispredict, restart.

the reason cmov can make sense is because it's smaller code...

-- 
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