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

[Hagen Paul Pfeifer <hagen@jauu.net>]

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