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

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Date:	Tue, 09 Aug 2011 10:29:00 +0200
From:	Hagen Paul Pfeifer <hagen@...u.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, <rui314@...il.com>,
	<netdev@...r.kernel.org>
Subject: Re: [PATCH] net: filter: Convert the BPF VM to threaded code

On Tue, 09 Aug 2011 07:00:27 +0200, Eric Dumazet wrote:

> I tried this idea since its already an enum and all enum values are
> handled in the switch, but all gcc versions I used still generate the
> useless compare and branch (always predicted by modern CPU, so harmless
> anyway ?)

Don't think so, the list is rather long - the CPU may not have any
indication. But strange that your gcc generate a conditional sequence of
cmp and conditional jump instruction instead of a jump table! Look at
commit 01f2f3f6ef4d076c, this is what I got after I refactored to an
_dense_ enum list.

> (But this would need to use a larger kernel_sock_filter with not an u16
> code, but the target address).

Not sure if the benefit is worth to try it. Can we avoid any cacheline
misses? I don't think so.

> +		switch ((enum bpf_inst)fentry->code) {

That should not differ! Eric, sure that this do the trick?

Hagen
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