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Date: Wed, 04 Sep 2002 15:41:19 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
Organization: Zytor Communications
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To: Gabriel Paubert <paubert@iram.es>
CC: linux-kernel@vger.kernel.org
Subject: Re: TCP Segmentation Offloading (TSO)
References: <Pine.LNX.4.33.0209050027270.7673-100000@gra-lx1.iram.es>
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Gabriel Paubert wrote:
> On 3 Sep 2002, H. Peter Anvin wrote:
> 
> [Sorry HPA, I forgot to cc to linux-kernel the first time.]
> 
> 
>>>P.S.
>>>    Using "bswap" is little bit tricky.
>>>
>>
>>It needs to be protected by CONFIG_I486 and alternate code implemented
>>for i386 (xchg %al,%ah; rol $16,%eax, xchg %al,%ah for example.)
> 
> 
> While it would work, this sequence is overkill. Unless I'm mistaken, the
> only property of bswap which is used in this case is that it swaps even
> and odd bytes, which can be done by a simple "roll $8,%eax" (or rorl).
> 
> I believe that bswap is one byte shorter than roll. In any case, using a
> rotate might be the right thing to do on other architectures.
> 

And again, I think you'll find the rotate faster on at least some x86 cores.

	-hpa


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