netdev - Re: [PATCH] net: optimize Berkeley Packet Filter (BPF) processing

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Message-ID: <20100619221611.784f7dbc@nehalam>
Date:	Sat, 19 Jun 2010 22:16:11 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Hagen Paul Pfeifer <hagen@...u.net>
Cc:	netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH] net: optimize Berkeley Packet Filter (BPF) processing

On Sun, 20 Jun 2010 05:05:36 +0200
Hagen Paul Pfeifer <hagen@...u.net> wrote:

> Gcc is currenlty not in the ability to optimize the switch statement in
> sk_run_filter() because of dense case labels. This patch replace the
> OR'd labels with ordered sequenced case labels. The sk_chk_filter()
> function is modified to patch/replace the original OPCODES in a
> ordered but equivalent form. gcc is now in the ability to transform the
> switch statement in sk_run_filter into a jump table of complexity O(1).
> 
> Until this patch gcc generates a sequence of conditional branches (O(n) of 567
> byte .text segment size (arch x86_64):

I don't think this works because it breaks ABI compatibility for applications tha
use older versions.
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