netdev - Re: [PATCH] net: filter: Convert the BPF VM to threaded code

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Date:	Mon, 01 Aug 2011 17:57:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	hagen@...u.net, rui314@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net: filter: Convert the BPF VM to threaded code

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 01 Aug 2011 20:37:46 +0200

> Le lundi 01 août 2011 à 20:16 +0200, Hagen Paul Pfeifer a écrit :
>> * Rui Ueyama | 2011-07-29 01:10:26 [-0700]:
>> 
>> >Convert the BPF VM to threaded code to improve performance.
>> >
>> >The BPF VM is basically a big for loop containing a switch statement.  That is
>> >slow because for each instruction it checks the for loop condition and does the
>> >conditional branch of the switch statement.
>> >
>> >This patch eliminates the conditional branch, by replacing it with jump table
>> >using GCC's labels-as-values feature. The for loop condition check can also be
>> >removed, because the filter code always end with a RET instruction.
>> 
>> With commit 01f2f3f6ef4d076c I reworked the BPF code so that gcc is in the
>> ability to generate a jump table, I double checked this. Not sure what happened
>> in the meantime.
>> 
> 
> A switch() always generates one conditional branch, catching values not
> enumerated in the "case ..." clauses.

Maybe it won't if we use an enum and make sure all enum values are handled
in the switch? :-)
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