Re: [PATCH v2] net: filter: Just In Time compiler

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 14 avril 2011 à 18:41 +0300, Avi Kivity a écrit :

> I'm talking about optimizing the generated code.  For example, bpf has 
> just two registers so a complex program generates a lot of loads and 
> stores.  An optimizing compiler can use extra target registers to avoid 
> those spills, and doesn't need to keep A and X in fixed registers.
> 

Thats not exactly true.

A bpf filter also uses up to 16 mem[] 'registers'.

A risc cpu (with a lot of registers) could use registers to hold part of
the mem[] array.

> If you translate the bpf program to C and optimize that with gcc you'll 
> probably get much better machine code that the jit in the patch.
> 

Well, gcc wont optimize a lot a bpf program if you ask me.

You would better make tcpdump not generate bpf but direct C code.