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

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Date:	Fri, 21 Dec 2007 06:26:48 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	YOSHIFUJI Hideaki / 吉藤英明 
	<yoshfuji@...ux-ipv6.org>
CC:	dada1@...mosbay.com, davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [TCP] IPV6 : Change a divide into a right shift in tcp_v6_send_ack()

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.  Constant 
folding inside the compiler ensures that "foo / 4" asm output is just as 
optimal as a shift.


> And, IMHO, because shift for signed variable is fragile, so we should
> avoid using it.

I respectfully disagree, but this is an unrelated matter.  As you say, 
"/4" is fine as-is.

	Jeff


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