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Date: Wed, 21 Dec 2011 14:00:31 +0800
From: Michael Wang <wangyun@...ux.vnet.ibm.com>
To: Joe Perches <joe@...ches.com>
CC: Stephen Hemminger <shemminger@...tta.com>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] Avoid extra calculation in ip_route_input_common
On 12/21/2011 01:50 PM, Joe Perches wrote:
> On Wed, 2011-12-21 at 13:39 +0800, Michael Wang wrote:
>> On 12/21/2011 01:23 PM, Joe Perches wrote:
>>> On Wed, 2011-12-21 at 13:12 +0800, Michael Wang wrote:
>>>> From: Michael Wang <wangyun@...ux.vnet.ibm.com>
>>>>
>>>> If previous condition doesn't meet, the later check will be cancelled.
>>>> So we don't need to do all the calculation.
> []
>>> commit c0b8c32b1c96afc9b32b717927330025cc1c501e
>>> Author: Stephen Hemminger <shemminger@...tta.com>
>>> Date: Thu Apr 10 04:00:28 2008 -0700
>>>
>>> IPV4: use xor rather than multiple ands for route compare
>>>
>>> The comparison in ip_route_input is a hot path, by recoding the C
>>> "and" as bit operations, fewer conditional branches get generated
>>> so the code should be faster. Maybe someday Gcc will be smart
>>> enough to do this?
>> This is what confused me, why "fewer conditional branches get generated"
>> will make code faster?
>> In this example, I think the best condition when daddr is different, we
>> only need to go to one branch do compare then quit, won't this be faster?
>
> if (a && b)
> ...
> pseudo-codes to:
> if (!a)
> goto fail;
> if (!b)
> goto fail;
> ...
> fail:
>
> Each of those conditional branches has a cost.
> Combining tests of variables in the same cache lines
> has relatively little cost compared to the conditional
> branches.
>
That make sense :)
> That's the theory anyway.
>
> If you have tests that demonstrate otherwise, please
> provide them.
>
I think the previous patch should have done such test, otherwise they
won't do this change.
Thanks for your reply, that clear my confusion.
Regards,
Michael Wang
> cheers, Joe
>
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