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

From: Michael Wang <[email-masked]>
Date: Wed, 21 Dec 2011 13:39:01 +0800

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.
> 
> Not sure about that.
> 

Hi, Joe

Thanks for your reply :)

>> Signed-off-by: Michael Wang <[email-masked]>
>> ---
>>  net/ipv4/route.c |    8 ++++----
>>  1 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
>> index f30112f..2872bfb 100644
>> --- a/net/ipv4/route.c
>> +++ b/net/ipv4/route.c
>> @@ -2362,10 +2362,10 @@ int ip_route_input_common(struct sk_buff *skb, __be32 daddr, __be32 saddr,
>>  
>>  	for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
>>  	     rth = rcu_dereference(rth->dst.rt_next)) {
>> -		if ((((__force u32)rth->rt_key_dst ^ (__force u32)daddr) |
>> -		     ((__force u32)rth->rt_key_src ^ (__force u32)saddr) |
>> -		     (rth->rt_route_iif ^ iif) |
>> -		     (rth->rt_key_tos ^ tos)) == 0 &&
>> +		if (((__force u32)rth->rt_key_dst ^ (__force u32)daddr) == 0 &&
>> +		    ((__force u32)rth->rt_key_src ^ (__force u32)saddr) == 0 &&
>> +		    rth->rt_route_iif == iif &&
>> +		    rth->rt_key_tos == tos &&
>>  		    rth->rt_mark == skb->mark &&
>>  		    net_eq(dev_net(rth->dst.dev), net) &&
>>  		    !rt_is_expired(rth)) {
> 
> See:
> 
> 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?

Thanks,
Michael Wang

>     
>     Signed-off-by: Stephen Hemminger <[email-masked]>
>     Acked-by: Eric Dumazet <[email-masked]>
>     Signed-off-by: David S. Miller <[email-masked]>
> 
>