From: Jeff Garzik <jeff@garzik.org>
To: "YOSHIFUJI Hideaki / 吉藤英明" <yoshfuji@linux-ipv6.org>
Cc: dada1@cosmosbay.com, davem@davemloft.net, netdev@vger.kernel.org
Subject: Re: [TCP] IPV6 : Change a divide into a right shift in tcp_v6_send_ack()
Date: Fri, 21 Dec 2007 06:26:48 -0500 [thread overview]
Message-ID: <476BA2F8.1090102@garzik.org> (raw)
In-Reply-To: <20071221.155030.131184865.yoshfuji@linux-ipv6.org>
YOSHIFUJI Hideaki / 吉藤英明 wrote:
> In article <476B574E.80601@cosmosbay.com> (at Fri, 21 Dec 2007 07:03:58 +0100), Eric Dumazet <dada1@cosmosbay.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
next prev parent reply other threads:[~2007-12-21 11:26 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-12-21 6:03 [TCP] IPV6 : Change a divide into a right shift in tcp_v6_send_ack() Eric Dumazet
2007-12-21 6:50 ` YOSHIFUJI Hideaki / 吉藤英明
2007-12-21 7:06 ` Eric Dumazet
[not found] ` <20071221.162833.82587283.yoshfuji@linux-ipv6.org>
2007-12-21 7:39 ` Eric Dumazet
2007-12-21 7:44 ` YOSHIFUJI Hideaki / 吉藤英明
2007-12-21 7:50 ` Eric Dumazet
2007-12-21 9:46 ` David Miller
2007-12-21 8:11 ` Eric Dumazet
2007-12-21 11:26 ` Jeff Garzik [this message]
2007-12-21 11:57 ` David Miller
2007-12-21 12:18 ` Jeff Garzik
2007-12-21 7:21 ` Ilpo Järvinen
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