From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by monty-python.gnu.org with tmda-scanned (Exim 4.34) id 1BTnic-0007yT-BY for qemu-devel@nongnu.org; Fri, 28 May 2004 16:12:42 -0400 Received: from mail by monty-python.gnu.org with spam-scanned (Exim 4.34) id 1BTgT7-0006xa-3Z for qemu-devel@nongnu.org; Fri, 28 May 2004 08:28:46 -0400 Received: from [212.16.62.51] (helo=mail.13thfloor.at) by monty-python.gnu.org with esmtp (Exim 4.34) id 1BTfED-0008I5-Es for qemu-devel@nongnu.org; Fri, 28 May 2004 07:08:45 -0400 Date: Fri, 28 May 2004 13:08:38 +0200 From: Herbert Poetzl Subject: Re: [Qemu-devel] Overoptimization Message-ID: <20040528110838.GA19126@MAIL.13thfloor.at> References: <40B693A7.9050006@ums.usu.ru> <20040528101351.GA18592@MAIL.13thfloor.at> <40B71B59.7080406@ums.usu.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <40B71B59.7080406@ums.usu.ru> Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Alexander E. Patrakov" Cc: qemu-devel@nongnu.org On Fri, May 28, 2004 at 04:58:33PM +0600, Alexander E. Patrakov wrote: > Herbert Poetzl wrote: > >On Fri, May 28, 2004 at 07:19:35AM +0600, Alexander E. Patrakov wrote: > > > >>The CFLAGS in Makefile.target are bad for gcc 3.3.2. This is not a my > >>problem with my gcc, because it passes the "make check" testsuite except > >>some known errors in libstdc++, which cannot affect qemu. > >> > >>The victim is the tcp_input function in slirp. When compiling with the > >>decault CFLAGS, for some reason all packets are dropped as having a > >>wrong TCP checksum. I tried debugging this by placing some debug printfs > >>in tcp_input.c file, but the problem disappeared after I inserted them. > >>I concluded that it is very strange and compiled this tcp_input.c file > >>without my debug statements, but with -O1 instead of -O2. The problem > >>went away. > > > > > >gcc isn't supposed to change the semantics of > >a program based on the optimization level. > > > >nevertheless several cases are known where this > >happened, and not seldom the bug was in gcc > > > >testing the same opts with 3.3.3 or 3.4 will > >show if gcc is to blame or not. > > Done (with gcc 3.4.0). I used a truly minimal configuration of qemu in > both cases to make sure that this is not an SDL/X/whatever else problem: > > ./configure --prefix=/usr --target-list=i386-softmmu --enable-slirp > --disable-sdl > > GCC 3.4.0, however, required one more file to be compiled with -O1 > (because of some register allocation error), and there were some "struct > timezone" errors that were trivial to fix by including where > appropriate. > > The result is the same. If I compile tcp_input.c with -O2, all tcp > packets are rejected in user-net mode because of wrong tcp checksum. If > tcp_input.c is compiled with -O1, qemu works. okay, sounds like some 'broken' struct, which might get misaligned or maybe a problem with the binutils ... I'd suggest trying -Os and -O9 just to give some hints to the developers, also the detailed error messages of the specific code with -Wall might shed some light on that ... HTH, Herbert > -- > Alexander E. Patrakov