Subject: Re: [PATCH] Avoid extra calculation in ip_route_input_common

From: Michael Wang <[email-masked]>
Date: Wed, 21 Dec 2011 14:00:31 +0800

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 <[email-masked]>
>>>>
>>>> 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 <[email-masked]>
>>> 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