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

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Message-ID: <459B98C9.1040309@garzik.org>
Date:	Wed, 03 Jan 2007 06:51:37 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Grzegorz Kulewski <kangur@...com.net>
CC:	Alan <alan@...rguk.ukuu.org.uk>,
	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

Grzegorz Kulewski wrote:
> On Wed, 3 Jan 2007, Alan wrote:
>> The proper fix for all of this mess is to fix the gcc compiler suite to
>> actually generate i686 code when told to use i686. CMOV is an optional
>> i686 extension which gcc uses without checking. In early PIV days it made
>> sense but on modern processors CMOV is so pointless the bug should be
>> 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?

In theory modern processors should have no trouble converting a 
test/move sequence into the same uops generated by a cmov instruction, 
for one.

	Jeff



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