From mboxrd@z Thu Jan 1 00:00:00 1970 From: Willy Tarreau Subject: Re: [PATCH 2/3] netfilter : 3 patches to boost ip_tables performance Date: Fri, 23 Sep 2005 06:02:34 +0200 Message-ID: <20050923040234.GC595@alpha.home.local> References: <432EF0C5.5090908@cosmosbay.com> <200509191948.55333.ak@suse.de> <432FDAC5.3040801@cosmosbay.com> <200509201830.20689.ak@suse.de> <433082DE.3060308@cosmosbay.com> <43308324.70403@cosmosbay.com> <4331D168.6090604@cosmosbay.com> <20050922124803.GH26520@sunbeam.de.gnumonks.org> <4332AC2E.8000607@cosmosbay.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Harald Welte , netdev@vger.kernel.org, netfilter-devel@lists.netfilter.org, linux-kernel@vger.kernel.org, Andi Kleen Return-path: To: Eric Dumazet Content-Disposition: inline In-Reply-To: <4332AC2E.8000607@cosmosbay.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: netfilter-devel-bounces@lists.netfilter.org Errors-To: netfilter-devel-bounces@lists.netfilter.org List-Id: netdev.vger.kernel.org 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