From: David Miller <davem@davemloft.net>
To: eric.dumazet@gmail.com
Cc: hagen@jauu.net, netdev@vger.kernel.org, xiaosuo@gmail.com
Subject: Re: [PATCH net-next-2.6] filter: optimize sk_run_filter
Date: Fri, 19 Nov 2010 08:21:25 -0800 (PST) [thread overview]
Message-ID: <20101119.082125.193710226.davem@davemloft.net> (raw)
In-Reply-To: <1290165472.3034.109.camel@edumazet-laptop>
From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Fri, 19 Nov 2010 12:17:52 +0100
> Le vendredi 19 novembre 2010 à 10:54 +0100, Eric Dumazet a écrit :
>
>> I believe we should revert the u32 f_k = fentry->k; part
>>
>> fentry->k as is fast as f_k if stored on stack, and avoids one
>> instruction if fentry->k is not needed.
>>
>>
>
> A revert is not good on arches with decent number of registers (x86_64
> for example).
-EFIX_THE_DAMN_COMPILER
We never make calls out of this function or touch volatile memory or
create a full memory barrier between the assignment of f_k and it's
uses.
Therefore if common sub-expression elimination is working the compiler
will be able to decide properly whether to access things via memory or
use a register for the value.
Remember this is why we have that ACCESS_ONCE() thing.
We can't have it both ways, either ACCESS_ONCE() should be removed or
we should never make changes like your's and instead should submit
compiler bug reports :-)
next prev parent reply other threads:[~2010-11-19 16:21 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-11-19 2:04 [PATCH] net: reduce the lines of code Changli Gao
2010-11-19 6:35 ` Hagen Paul Pfeifer
2010-11-19 7:17 ` Changli Gao
2010-11-19 7:51 ` Eric Dumazet
2010-11-19 7:56 ` [PATCH net-next-2.6] filter: cleanup codes[] init Eric Dumazet
2010-11-19 8:04 ` [PATCH net-next-2.6] filter: use reciprocal divide Eric Dumazet
2010-11-19 8:18 ` Changli Gao
2010-11-19 18:07 ` David Miller
2010-11-19 8:38 ` [PATCH net-next-2.6] filter: cleanup codes[] init Changli Gao
2010-11-19 9:54 ` Eric Dumazet
2010-11-19 11:17 ` [PATCH net-next-2.6] filter: optimize sk_run_filter Eric Dumazet
2010-11-19 12:32 ` Changli Gao
2010-11-19 12:57 ` Changli Gao
2010-11-19 13:16 ` [PATCH net-next-2.6 v2] " Eric Dumazet
2010-11-19 14:13 ` Changli Gao
2010-11-19 17:52 ` David Miller
2010-11-19 14:17 ` Tetsuo Handa
2010-11-19 14:35 ` Eric Dumazet
2010-11-19 16:21 ` David Miller [this message]
2010-11-19 16:55 ` [PATCH net-next-2.6] " Eric Dumazet
2010-11-19 17:05 ` David Miller
2010-11-19 17:15 ` Stephen Hemminger
2010-11-19 17:21 ` David Miller
2010-11-19 17:16 ` Eric Dumazet
2010-11-19 17:25 ` David Miller
2010-11-19 12:21 ` [PATCH net-next-2.6] filter: cleanup codes[] init Changli Gao
2010-11-19 18:07 ` David Miller
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