netdev - Re: [PATCH] netfilter: unfold two critical loops in ip_packet

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Date:	Fri, 30 Jan 2009 18:27:37 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>,
	Linux Network Development list <netdev@...r.kernel.org>
Subject: Re: [PATCH] netfilter: unfold two critical loops in ip_packet_match()

Andi Kleen a écrit :
> On Fri, Jan 30, 2009 at 05:54:10PM +0100, Eric Dumazet wrote:
>> Andi Kleen a écrit :
>>> Eric Dumazet <dada1@...mosbay.com> writes:
>>>
>>>> While doing oprofile tests I noticed two loops are not properly unrolled by gcc
>>> That's because nobody passed -funroll-loops. Did you try that for
>>> that file? Likely will need -O2 too
>> I dont want to unroll all loops, only those two :)
> 
> gcc 4.4 will have a way to do that per function, but earlier
> you would need to move it to a separate file and specify
> the option only for that.
> 
> Doing so would be still a good idea compared to your
> patch because the code will be cleaner and might
> be more adaptable to future architectures
> (such manual tunings tend to outdate)

So... you suggest me to split file, and use a "-funroll-loops",
that might doing strange things on some arches / compilers...

This is far too complicated and risky imho.

Check compare_ether_addr_64bits() definition in 
include/linux/etherdevice.h for a truly unreadable code :)


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