netdev - Re: netfilter 07/41: arp_tables: unfold two critical loops in arp_packet

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Date:	Tue, 24 Mar 2009 22:23:23 +0100 (CET)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	David Miller <davem@...emloft.net>
cc:	dada1@...mosbay.com, kaber@...sh.net, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: netfilter 07/41: arp_tables: unfold two critical loops in
 arp_packet_match()


On Tuesday 2009-03-24 22:18, David Miller wrote:
>> >
>> >Arches without efficient unaligned access can still perform a loop
>> >assuming 16bit alignment in ifname_compare()
>> 
>> Allow me some skepticism, but the code looks pretty much like a
>> standard memcmp.
>
>memcmp() can't make any assumptions about alignment.
>Whereas we _know_ this thing is exactly 16-bit aligned.
>
>All of the optimized memcmp() implementations look for
>32-bit alignment and punt to byte at a time comparison
>loops if things are not aligned enough.

Yes, I seem to remember glibc doing something like

 if ((addr & 0x03) != 0) {
     // process single bytes (increment addr as you go)
     // until addr & 0x03 == 0.
 }

 /* optimized loop here. also increases addr */

 if ((addr & 0x03) != 0)
     // still bytes left after loop - process on a per-byte basis

Is the cost of testing for non-4-divisibility expensive enough
to warrant not usnig memcmp?

Irrespective of all that, I think putting the interface comparison
code should be agglomerated in a function/header so that it is
replicated across iptables, ip6tables, ebtables, arptables, etc.
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