https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
Bug ID: 66673
Summary: swapping variables via chained xor fails
Product: gcc
Version: 4.4.6
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: joe.carnuccio at qlogic dot com
Target Milestone: ---
This is the same as 39121 which has been marked RESOLVED INVALID (to which I
strongly disagree):
this produces incorrect executable: *p ^= *q ^= *p ^= *q;
( if gcc option "-Os" is used, then that produces correct executable )
( by contrast, this always produces correct executable: a ^= b ^= a ^= b; )
root@elab305:/home/joe/test/c# cat x.c
#include <stdio.h>
int main(int argc, char **argv)
{
int a = 0x32, b = 0x45;
int *p = &a, *q = &b;
*p ^= *q ^= *p ^= *q;
printf("%x %x\n", a, b);
return 0;
}
root@elab305:/home/joe/test/c# make -B x
cc -c -o x.o x.c
cc x.o -o x
root@elab305:/home/joe/test/c# ./x
0 32 <--INCORRECT
root@elab305:/home/joe/test/c# make -B x CFLAGS+='-Os'
cc -Os -c -o x.o x.c
cc x.o -o x
root@elab305:/home/joe/test/c# ./x
45 32 <--CORRECT
root@elab305:/home/joe/test/c#
Notice that when -Os (optimize for space rather than speed) is used, the
executable produces the correct result.
Also, doing the chained xor on the integer variables a and b themselves always
produces the correct result (regardless of optimization).
root@elab305:/home/joe/test/c# gcc --version
gcc (GCC) 4.4.6 20110731 (Red Hat 4.4.6-3)
. . .
root@elab305:/home/joe/test/c# uname -a
Linux elab305 2.6.32-220.el6.x86_64 #1 SMP Wed Nov 9 08:03:13 EST 2011 x86_64
x86_64 x86_64 GNU/Linux
root@elab305:/home/joe/test/c# cat /etc/issue
Red Hat Enterprise Linux Server release 6.2 (Santiago)
Kernel \r on an \m
