From: "H. Peter Anvin" <hpa@zytor.com>
To: Zachary Amsden <zach@vmware.com>
Cc: 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: Tue, 16 Aug 2005 17:06:04 -0700 [thread overview]
Message-ID: <43027F6C.4070801@zytor.com> (raw)
In-Reply-To: <43027D20.7020907@vmware.com>
Zachary Amsden wrote:
>
> This one in particular is non-optimal looking from C because the
> compiler misses the potential for rotation. But, composing into
> temporaries and then issuing two writes to memory instead of multiple
> writes within the same word could actually get you a better cycle count,
> and that is something GCC just might be able to do :)
>
At least i386 and x86-64 gcc should recognize
((foo << x) + (foo >> (32-x)))
... as a 32-bit rotate; similar for 8-, 16- and 64-bit rotates of
appropriate sized items. Also, it seems it could just be an inline
function instead of a macro.
-hpa
next prev parent reply other threads:[~2005-08-17 0:07 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 [this message]
2005-08-17 14:22 ` Alan Cox
2005-08-17 15:54 ` H. Peter Anvin
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