Subject: Re: [PATCH 3/6] IPV4 : use xor rather than multiple ands for route compare

From: Stephen Hemminger <[email-masked]>
Date: Tue, 1 Apr 2008 13:08:42 -0700

On Tue, 01 Apr 2008 07:52:03 +0200
Eric Dumazet <[email-masked]> wrote:

> Stephen Hemminger a écrit :
> > 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?
> > 
> > Signed-off-by: Stephen Hemminger <[email-masked]>
> > 
> > --- a/net/ipv4/route.c	2008-03-31 10:57:30.000000000 -0700
> > +++ b/net/ipv4/route.c	2008-03-31 11:10:44.000000000 -0700
> > @@ -2079,14 +2079,14 @@ int ip_route_input(struct sk_buff *skb, 
> >  	rcu_read_lock();
> >  	for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
> >  	     rth = rcu_dereference(rth->u.dst.rt_next)) {
> > -		if (rth->fl.fl4_dst == daddr &&
> > -		    rth->fl.fl4_src == saddr &&
> > -		    rth->fl.iif == iif &&
> > -		    rth->fl.oif == 0 &&
> > -		    rth->fl.mark == skb->mark &&
> > -		    rth->fl.fl4_tos == tos &&
> > -		    net_eq(dev_net(rth->u.dst.dev), net) &&
> > -		    rth->rt_genid == atomic_read(&rt_genid)) {
> > +		if (((rth->fl.fl4_dst ^ daddr) |
> > +		     (rth->fl.fl4_src ^ saddr) |
> > +		     (rth->fl.iif ^ iif) |
> > +		     rth->fl.oif |
> > +		     (rth->fl.mark ^ skb->mark) |
> > +		     (rth->fl.fl4_tos ^ tos) |
> > +		     (rth->rt_genid ^ atomic_read(&rt_genid))) == 0 &&
> > +		    net_eq(dev_net(rth->u.dst.dev), net)) {
> >  			dst_use(&rth->u.dst, jiffies);
> >  			RT_CACHE_STAT_INC(in_hit);
> >  			rcu_read_unlock();
> > 
> 
> Are you sure all fields share same cache lines, on 32bit and 64bit arches ?

The flow fields are all together, and the other parameters are local variables
in registers so that compare should be in one cache line.

--- a/net/ipv4/route.c	2008-03-31 17:12:30.000000000 -0700
+++ b/net/ipv4/route.c	2008-04-01 13:05:46.000000000 -0700
@@ -2079,12 +2079,12 @@ int ip_route_input(struct sk_buff *skb, 
 	rcu_read_lock();
 	for (rth = rcu_dereference(rt_hash_table[hash].chain); rth;
 	     rth = rcu_dereference(rth->u.dst.rt_next)) {
-		if (rth->fl.fl4_dst == daddr &&
-		    rth->fl.fl4_src == saddr &&
-		    rth->fl.iif == iif &&
-		    rth->fl.oif == 0 &&
+		if (((rth->fl.fl4_dst ^ daddr) |
+		     (rth->fl.fl4_src ^ saddr) |
+		     (rth->fl.iif ^ iif) |
+		     rth->fl.oif |
+		     (rth->fl.fl4_tos ^ tos)) == 0 &&
 		    rth->fl.mark == skb->mark &&
-		    rth->fl.fl4_tos == tos &&
 		    net_eq(dev_net(rth->u.dst.dev), net) &&
 		    rth->rt_genid == atomic_read(&rt_genid)) {
 			dst_use(&rth->u.dst, jiffies);