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From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: "H. Peter Anvin" <hpa@zytor.com>
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 15:22:22 +0100	[thread overview]
Message-ID: <1124288543.773.31.camel@localhost.localdomain> (raw)
In-Reply-To: <43027F6C.4070801@zytor.com>

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.

Alan


  reply	other threads:[~2005-08-17 13:56 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 [this message]
2005-08-17 15:54         ` H. Peter Anvin

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