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From: "H. Peter Anvin" <hpa@zytor.com>
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
Date: Wed, 17 Aug 2005 08:54:33 -0700	[thread overview]
Message-ID: <43035DB9.7020709@zytor.com> (raw)
In-Reply-To: <1124288543.773.31.camel@localhost.localdomain>

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

      reply	other threads:[~2005-08-17 15:55 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2005-08-11  4:54 [PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc zach
2005-08-16 23:45 ` Andi Kleen
2005-08-16 23:56   ` Zachary Amsden
2005-08-17  0:06     ` H. Peter Anvin
2005-08-17 14:22       ` Alan Cox
2005-08-17 15:54         ` H. Peter Anvin [this message]

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