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* [Bug c/66673] New: swapping variables via chained xor fails
@ 2015-06-25 19:23 joe.carnuccio at qlogic dot com
2015-06-25 19:29 ` [Bug c/66673] " mpolacek at gcc dot gnu.org
` (6 more replies)
0 siblings, 7 replies; 8+ messages in thread
From: joe.carnuccio at qlogic dot com @ 2015-06-25 19:23 UTC (permalink / raw)
To: gcc-bugs
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
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/66673] swapping variables via chained xor fails
2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
@ 2015-06-25 19:29 ` mpolacek at gcc dot gnu.org
2015-06-25 19:41 ` joe.carnuccio at qlogic dot com
` (5 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: mpolacek at gcc dot gnu.org @ 2015-06-25 19:29 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
Marek Polacek <mpolacek at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
CC| |mpolacek at gcc dot gnu.org
Resolution|--- |INVALID
--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
And again you're invoking undefined behavior. Use -Wall.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/66673] swapping variables via chained xor fails
2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
2015-06-25 19:29 ` [Bug c/66673] " mpolacek at gcc dot gnu.org
@ 2015-06-25 19:41 ` joe.carnuccio at qlogic dot com
2015-06-25 19:42 ` joe.carnuccio at qlogic dot com
` (4 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: joe.carnuccio at qlogic dot com @ 2015-06-25 19:41 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
--- Comment #2 from joe.carnuccio at qlogic dot com ---
-Wall produces no warnings...
root@elab305:/home/joe/test/c# make -B x -Wall
cc -c -o x.o x.c
cc x.o -o x
root@elab305:/home/joe/test/c# ./x
0 32
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/66673] swapping variables via chained xor fails
2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
2015-06-25 19:29 ` [Bug c/66673] " mpolacek at gcc dot gnu.org
2015-06-25 19:41 ` joe.carnuccio at qlogic dot com
@ 2015-06-25 19:42 ` joe.carnuccio at qlogic dot com
2015-06-25 19:46 ` joe.carnuccio at qlogic dot com
` (3 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: joe.carnuccio at qlogic dot com @ 2015-06-25 19:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
--- Comment #3 from joe.carnuccio at qlogic dot com ---
Sorry, I ment this:
root@elab305:/home/joe/test/c# make -B x CFLAGS+='-Wall'
cc -Wall -c -o x.o x.c
cc x.o -o x
root@elab305:/home/joe/test/c# ./x
0 32
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/66673] swapping variables via chained xor fails
2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
` (2 preceding siblings ...)
2015-06-25 19:42 ` joe.carnuccio at qlogic dot com
@ 2015-06-25 19:46 ` joe.carnuccio at qlogic dot com
2015-06-25 19:54 ` trippels at gcc dot gnu.org
` (2 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: joe.carnuccio at qlogic dot com @ 2015-06-25 19:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
--- Comment #4 from joe.carnuccio at qlogic dot com ---
if I do a ^= b ^= a ^= b it always work correctly;
doing *p ^= *q ^= *p ^= *q fails (unless -Os is used);
i.e. dereferenced pointers are being treated differently
int a = 0x32, b = 0x45;
int *p = &a, *q = &b;
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/66673] swapping variables via chained xor fails
2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
` (3 preceding siblings ...)
2015-06-25 19:46 ` joe.carnuccio at qlogic dot com
@ 2015-06-25 19:54 ` trippels at gcc dot gnu.org
2015-06-25 19:56 ` mpolacek at gcc dot gnu.org
2015-06-25 21:09 ` [Bug c/66673] Wsequence-point warning missing when swapping variables via chained xor trippels at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: trippels at gcc dot gnu.org @ 2015-06-25 19:54 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
--- Comment #5 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
https://www.google.com/?#q=nasal%20demons&lang=en
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/66673] swapping variables via chained xor fails
2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
` (4 preceding siblings ...)
2015-06-25 19:54 ` trippels at gcc dot gnu.org
@ 2015-06-25 19:56 ` mpolacek at gcc dot gnu.org
2015-06-25 21:09 ` [Bug c/66673] Wsequence-point warning missing when swapping variables via chained xor trippels at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: mpolacek at gcc dot gnu.org @ 2015-06-25 19:56 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to joe.carnuccio from comment #2)
> -Wall produces no warnings...
Oh, you're using too old GCC. Please try a newer version.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c/66673] Wsequence-point warning missing when swapping variables via chained xor
2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
` (5 preceding siblings ...)
2015-06-25 19:56 ` mpolacek at gcc dot gnu.org
@ 2015-06-25 21:09 ` trippels at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: trippels at gcc dot gnu.org @ 2015-06-25 21:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66673
Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|FIXED |INVALID
^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2015-06-25 21:09 UTC | newest]
Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
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2015-06-25 19:23 [Bug c/66673] New: swapping variables via chained xor fails joe.carnuccio at qlogic dot com
2015-06-25 19:29 ` [Bug c/66673] " mpolacek at gcc dot gnu.org
2015-06-25 19:41 ` joe.carnuccio at qlogic dot com
2015-06-25 19:42 ` joe.carnuccio at qlogic dot com
2015-06-25 19:46 ` joe.carnuccio at qlogic dot com
2015-06-25 19:54 ` trippels at gcc dot gnu.org
2015-06-25 19:56 ` mpolacek at gcc dot gnu.org
2015-06-25 21:09 ` [Bug c/66673] Wsequence-point warning missing when swapping variables via chained xor trippels at gcc dot gnu.org
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