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

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Date:	Mon, 01 Aug 2011 20:37:46 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	Rui Ueyama <rui314@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: filter: Convert the BPF VM to threaded code

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.



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