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Date: Wed, 17 Aug 2005 08:54:33 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: Zachary Amsden <zach@vmware.com>, Andi Kleen <ak@suse.de>, akpm@osdl.org,
       chrisl@vmware.com, chrisw@osdl.org, Keir.Fraser@cl.cam.ac.uk,
       linux-kernel@vger.kernel.org, m+Ian.Pratt@cl.cam.ac.uk,
       mbligh@mbligh.org, pratap@vmware.com, virtualization@lists.osdl.org,
       zwame@arm.linux.org.uk
Subject: Re: [PATCH 5/14] i386 / Use early clobber to eliminate rotate in
 desc
References: <200508110454.j7B4sBDK019538@zach-dev.vmware.com>	 <20050816234514.GG27628@wotan.suse.de> <43027D20.7020907@vmware.com>	 <43027F6C.4070801@zytor.com> <1124288543.773.31.camel@localhost.localdomain>
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Alan Cox wrote:
> On Maw, 2005-08-16 at 17:06 -0700, H. Peter Anvin wrote:
> 
>>At least i386 and x86-64 gcc should recognize
>>
>>	((foo << x) + (foo >> (32-x)))
>>
>>... as a 32-bit rotate
> 
> 
> Only for  1 <= x <= 31. For the x = 0 case the code posted is undefined
> and at least in some cases gcc will do "interesting" things as a result
> such as treating >> 32 as >> 0.
> 

True, although I thought gcc considered the above an idiom and would 
generate a rotate instruction.  Perhaps it should be | rather than +.

	-hpa