From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 1539 invoked by alias); 2 Oct 2005 01:03:38 -0000 Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Archive: List-Post: List-Help: Sender: gcc-help-owner@gcc.gnu.org Received: (qmail 1529 invoked by uid 22791); 2 Oct 2005 01:03:36 -0000 Received: from qnxmail.qnx.com (HELO nimbus.ott.qnx.com) (209.226.137.76) by sourceware.org (qpsmtpd/0.30-dev) with ESMTP; Sun, 02 Oct 2005 01:03:36 +0000 Received: by nimbus.ott.qnx.com with Internet Mail Service (5.5.2653.19) id ; Sat, 1 Oct 2005 21:03:34 -0400 Message-ID: <3518719F06577C4F85DA618E3C37AB910138920F@nimbus.ott.qnx.com> From: Ryan Mansfield To: 'Bernhard Fischer' , gcc-help@gcc.gnu.org Subject: RE: gcc-head bug? feature? Date: Sun, 02 Oct 2005 01:03:00 -0000 MIME-Version: 1.0 Content-Type: text/plain X-SW-Source: 2005-10/txt/msg00001.txt.bz2 This is a problem with your code. You are violating aliasing rules as the warning message states. See 6.5/7 of the C Standard for the exact rules. Problems arising from alias violations happen to be the #1 invalid PR filed in Bugzilla. I'd add the link but Bugzilla is currently being upgraded. This problem is also mentioned under "Casting does not work as expected when optimization is turned on. " in the non-bugs section of the bugs page: http://gcc.gnu.org/bugs.html#known With gcc 4.0, -Wstrict-aliasing=2 was added to catch more of these problems, but does not catch all cases. Regards, Ryan Mansfield -----Original Message----- From: gcc-help-owner@gcc.gnu.org [mailto:gcc-help-owner@gcc.gnu.org] On Behalf Of Bernhard Fischer Sent: Saturday, October 01, 2005 7:56 PM To: gcc-help@gcc.gnu.org Cc: Bernhard Fischer Subject: gcc-head bug? feature? [please keep me CC'd] Hi, Is this a user error (thus an optimisation feature) or a glitch in gcc? $ for o in '' -O2; do echo "# o='$o'";gcc-4.1-HEAD -W -Wall -Wextra -pedantic $o -g -Wall -I. -o bar41.$o foo.c ;./bar41.$o;done # o='' var1=1020308 var1=1020809 # o='-O2' foo.c: In function 'main': foo.c:7: warning: dereferencing type-punned pointer will break strict-aliasing rules var1=1020308 var1=1020308 with O2, the second assignment is "optimized out"? gcc-4.0.1, gcc-3.4, gcc-3.3, gcc-2.95 all yield 1020308 and 1020809 for Odefault and O2, as opposed to gcc-4.1. $ cat foo.c #include int main(void) { int var1 = 0x01020304; *(char *)&var1 = 0x08; printf("var1=%x\n", var1); *(short *)&var1 = 0x0809; printf("var1=%x\n", var1); return 0; }