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 1BTj39-00026e-85 for qemu-devel@nongnu.org; Fri, 28 May 2004 11:13:35 -0400 Received: from mail by monty-python.gnu.org with spam-scanned (Exim 4.34) id 1BTf3K-0006xJ-Q8 for qemu-devel@nongnu.org; Fri, 28 May 2004 06:58:02 -0400 Received: from [194.226.236.116] (helo=ums.usu.ru) by monty-python.gnu.org with esmtp (TLSv1:DES-CBC3-SHA:168) (Exim 4.34) id 1BTf3J-0006wG-Qw for qemu-devel@nongnu.org; Fri, 28 May 2004 06:57:30 -0400 Message-ID: <40B71B59.7080406@ums.usu.ru> Date: Fri, 28 May 2004 16:58:33 +0600 From: "Alexander E. Patrakov" MIME-Version: 1.0 Subject: Re: [Qemu-devel] Overoptimization References: <40B693A7.9050006@ums.usu.ru> <20040528101351.GA18592@MAIL.13thfloor.at> In-Reply-To: <20040528101351.GA18592@MAIL.13thfloor.at> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Reply-To: qemu-devel@nongnu.org List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Herbert Poetzl Cc: qemu-devel@nongnu.org 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. -- Alexander E. Patrakov