netdev - Re: [TCP] IPV6 : Change a divide into a right shift in tcp_v6_send

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Date:	Fri, 21 Dec 2007 03:57:17 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	jeff@...zik.org
Cc:	yoshfuji@...ux-ipv6.org, dada1@...mosbay.com,
	netdev@...r.kernel.org
Subject: Re: [TCP] IPV6 : Change a divide into a right shift in
 tcp_v6_send_ack()

From: Jeff Garzik <jeff@...zik.org>
Date: Fri, 21 Dec 2007 06:26:48 -0500

> YOSHIFUJI Hideaki / 吉藤英明 wrote:
> > In article <476B574E.80601@...mosbay.com> (at Fri, 21 Dec 2007 07:03:58 +0100), Eric Dumazet <dada1@...mosbay.com> says:
> > 
> >> Because tot_len is signed in tcp_v6_send_ack(), tot_len/4 forces compiler
> >> to emit an integer divide, while we can help it to use a right shift,
> >> less expensive.
> > 
> > Are you really sure?
> > At least, gcc-4.1.2-20061115 (debian) does not make any difference.
> 
> Quite true -- thus it is a matter of taste to the programmer.

Not true, the code output does change, check your optimize-for-size
kernel config setting.

This was discussed and explained later in this thread, and I also
explained it to you on IRC Jeff ;-)
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