netdev - Re: [PATCH net-next-2.6] filter: optimize sk_run

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Message-Id: <20101119.092514.183050412.davem@davemloft.net>
Date:	Fri, 19 Nov 2010 09:25:14 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	hagen@...u.net, netdev@...r.kernel.org, xiaosuo@...il.com
Subject: Re: [PATCH net-next-2.6] filter: optimize sk_run_filter

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Fri, 19 Nov 2010 18:16:24 +0100

> Le vendredi 19 novembre 2010 à 09:05 -0800, David Miller a écrit :
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Fri, 19 Nov 2010 17:55:59 +0100
>> 
>> > Unfortunatly on x86_32 it also chose that f_k was more valuable in a cpu
>> > register and accumulator A lost its register to get a stack slot
>> > instead.
>> 
>> Ok that tradeoff is terrible, but it depends upon knowledge we haven't
>> given to the compiler (yet).
>> 
>> Let me think about this a bit...
> 
> By the way, I tried the 'register' keyword, and I knew it was a stupid
> idea before even trying, since compiler ignored me : "What the hell do
> you think you can tell me how to optimize this code ?"
> 
> (I am quite sure my laptop even smiled and sent to his neighbours a
> broadcast saying "Hey buddies, this dumb guy tried the register C
> keyword, isnt it funny ?")

:-)

I wonder though, since "fentry" is const in your recent patch,
you might not even need that X86_32 ifdef thing.  GCC should
actually now be able to see that fentry->k cannot change, even
when we make external function calls via load_pointer() and such.
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