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@xxxxxxxxxxx [mailto:gcc-help-owner@xxxxxxxxxxx] On Behalf Of Bernhard Fischer Sent: Saturday, October 01, 2005 7:56 PM To: gcc-help@xxxxxxxxxxx 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 <stdio.h> int main(void) { int var1 = 0x01020304; *(char *)&var1 = 0x08; printf("var1=%x\n", var1); *(short *)&var1 = 0x0809; printf("var1=%x\n", var1); return 0; }
