From: Willy Tarreau <willy@w.ods.org>
To: Eric Dumazet <dada1@cosmosbay.com>
Cc: Harald Welte <laforge@netfilter.org>,
netdev@vger.kernel.org, netfilter-devel@lists.netfilter.org,
linux-kernel@vger.kernel.org, Andi Kleen <ak@suse.de>
Subject: Re: [PATCH 2/3] netfilter : 3 patches to boost ip_tables performance
Date: Fri, 23 Sep 2005 06:02:34 +0200 [thread overview]
Message-ID: <20050923040234.GC595@alpha.home.local> (raw)
In-Reply-To: <4332AC2E.8000607@cosmosbay.com>
On Thu, Sep 22, 2005 at 03:05:50PM +0200, Eric Dumazet wrote:
(...)
> It was necessary to get the best code with gcc-3.4.4 on i386 and
> gcc-4.0.1 on x86_64
>
> For example :
>
> bool1 = FWINV(ret != 0, IPT_INV_VIA_OUT);
> if (bool1) {
>
> gives a better code than :
>
> if (FWINV(ret != 0, IPT_INV_VIA_OUT)) {
>
> (one less conditional branch)
>
> Dont ask me why, it is shocking but true :(
I also noticed many times that gcc's optimization of "if (complex condition)"
is rather poor and it's often better to put it in a variable before. I even
remember that if you use an intermediate variable, it can often generate a
CMOV instruction on processors which support it, while it produces cond tests
and jumps without the variable. Generally speaking, if you want fast code,
you have to write it as a long sequence of small instructions, just as if
you were writing assembly. As you said, shocking but true.
BTW, cheers for your optimizations !
Regards,
Willy
next prev parent reply other threads:[~2005-09-23 4:05 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <432EF0C5.5090908@cosmosbay.com>
[not found] ` <200509191948.55333.ak@suse.de>
[not found] ` <432FDAC5.3040801@cosmosbay.com>
[not found] ` <200509201830.20689.ak@suse.de>
2005-09-20 21:45 ` [PATCH] Adds sys_set_mempolicy() in include/linux/syscalls.h , Re: [PATCH, netfilter] NUMA aware ipv4/netfilter/ip_tables.c Eric Dumazet
2005-09-20 21:46 ` [PATCH] Adds sys_set_mempolicy() in include/linux/syscalls.h Eric Dumazet
2005-09-21 21:24 ` [PATCH 0/3] netfilter : 3 patches to boost ip_tables performance Eric Dumazet
2005-09-21 22:43 ` Christoph Lameter
2005-09-22 0:34 ` David S. Miller
2005-09-22 1:44 ` Christoph Lameter
2005-09-22 12:11 ` Eric Dumazet
2005-09-22 12:49 ` Christoph Hellwig
2005-09-22 12:54 ` Andi Kleen
2005-09-22 12:58 ` Christoph Hellwig
2005-09-22 13:05 ` Andi Kleen
2005-09-22 15:37 ` Christoph Lameter
2005-09-22 15:50 ` Eric Dumazet
2005-09-22 15:55 ` Christoph Lameter
2005-09-23 17:11 ` Harald Welte
2005-09-23 17:44 ` Christoph Lameter
2005-09-23 18:04 ` Dave Hansen
2005-09-26 17:58 ` vmalloc_node Christoph Lameter
2005-09-26 18:10 ` vmalloc_node Dave Hansen
2005-09-23 17:47 ` [PATCH 0/3] netfilter : 3 patches to boost ip_tables performance Eric Dumazet
2005-09-23 18:00 ` Kyle Moffett
2005-09-22 4:18 ` James Morris
2005-09-22 5:07 ` Eric Dumazet
2005-09-22 13:03 ` Andi Kleen
2005-09-22 13:30 ` Eric Dumazet
2005-09-23 17:09 ` Harald Welte
2005-09-27 16:23 ` Andi Kleen
2005-09-28 0:25 ` Henrik Nordstrom
2005-09-28 8:32 ` Harald Welte
2005-09-28 8:37 ` Andi Kleen
2005-10-04 17:01 ` Patrick McHardy
2005-10-05 16:53 ` Andi Kleen
2005-10-07 2:38 ` Harald Welte
2005-10-06 17:59 ` Andi Kleen
2005-10-07 17:08 ` Patrick McHardy
2005-10-07 17:21 ` Andi Kleen
2005-10-07 17:50 ` Patrick McHardy
2005-09-28 10:34 ` Henrik Nordstrom
2005-09-21 21:29 ` [PATCH 1/3] " Eric Dumazet
2005-09-22 12:57 ` Harald Welte
2005-09-22 13:17 ` Eric Dumazet
2005-09-21 21:32 ` [PATCH 2/3] " Eric Dumazet
2005-09-22 12:48 ` Harald Welte
2005-09-22 13:05 ` Eric Dumazet
2005-09-23 4:02 ` Willy Tarreau [this message]
2005-09-23 5:14 ` Eric Dumazet
2005-09-23 11:33 ` Willy Tarreau
2005-09-23 14:00 ` Tim Mattox
2005-09-21 21:37 ` [PATCH 3/3] " Eric Dumazet
2005-09-22 12:50 ` Harald Welte
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