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* [Bug c/51834] New: -Wsequence-point fails when convoluted expressions with multiple side effects are used
@ 2012-01-12 6:29 prasoonsaurav.nit at gmail dot com
2012-01-12 8:51 ` [Bug c/51834] " rguenth at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: prasoonsaurav.nit at gmail dot com @ 2012-01-12 6:29 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51834
Bug #: 51834
Summary: -Wsequence-point fails when convoluted expressions
with multiple side effects are used
Classification: Unclassified
Product: gcc
Version: 4.4.6
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: prasoonsaurav.nit@gmail.com
Consider the following example
int main()
{
int i=10;
i += (i , i++, i) + i; // also invokes UB
}
prasoon@Prasoon:~/test_code$ cat ub.c
int main()
{
int i=10;
i += (i , i++, i) + i; // also invokes UB
}
prasoon@Prasoon:~/test_code$ gcc -Wsequence-point ub.c
prasoon@Prasoon:~/test_code$
I don't get any warning like 'operation on 'i' may be undefined.
Another similar example
int main()
{
char *str;
char array[100]= "Hello";
if((str = array)[0] == 'H'){
//do something
}
}
As per my understanding (str = array)[0] also invokes UB but no warning is
given by gcc even after using that option.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/51834] -Wsequence-point fails when convoluted expressions with multiple side effects are used
2012-01-12 6:29 [Bug c/51834] New: -Wsequence-point fails when convoluted expressions with multiple side effects are used prasoonsaurav.nit at gmail dot com
@ 2012-01-12 8:51 ` rguenth at gcc dot gnu.org
2012-01-12 9:08 ` prasoonsaurav.nit at gmail dot com
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: rguenth at gcc dot gnu.org @ 2012-01-12 8:51 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51834
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-12 08:51:03 UTC ---
There is no undefined behavior in these testcases.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/51834] -Wsequence-point fails when convoluted expressions with multiple side effects are used
2012-01-12 6:29 [Bug c/51834] New: -Wsequence-point fails when convoluted expressions with multiple side effects are used prasoonsaurav.nit at gmail dot com
2012-01-12 8:51 ` [Bug c/51834] " rguenth at gcc dot gnu.org
@ 2012-01-12 9:08 ` prasoonsaurav.nit at gmail dot com
2012-01-12 9:20 ` schwab@linux-m68k.org
2012-04-19 15:07 ` vincent-gcc at vinc17 dot net
3 siblings, 0 replies; 5+ messages in thread
From: prasoonsaurav.nit at gmail dot com @ 2012-01-12 9:08 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51834
--- Comment #2 from Prasoon <prasoonsaurav.nit at gmail dot com> 2012-01-12 09:07:52 UTC ---
@Richard Guenther
Considering the expression i += (i,i++,i) +i;
(i,i++,i) involves change in the value of i, however comma introduces a
sequence point so very roughly considering it equivalent to
func(_some_side_effect_on_i)
we have i += func(_some_side_effect_on_i) + i;
Do you still think the behavior is well defined?
Another C++ example would be ++a = 10; where 'a'is of type int.
This invokes UB but there is no warning from the compiler side even after we
use this flag. Raising a similar bug for C++ because the differ may be
different in C and C++.
Thanks
Prasoon
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/51834] -Wsequence-point fails when convoluted expressions with multiple side effects are used
2012-01-12 6:29 [Bug c/51834] New: -Wsequence-point fails when convoluted expressions with multiple side effects are used prasoonsaurav.nit at gmail dot com
2012-01-12 8:51 ` [Bug c/51834] " rguenth at gcc dot gnu.org
2012-01-12 9:08 ` prasoonsaurav.nit at gmail dot com
@ 2012-01-12 9:20 ` schwab@linux-m68k.org
2012-04-19 15:07 ` vincent-gcc at vinc17 dot net
3 siblings, 0 replies; 5+ messages in thread
From: schwab@linux-m68k.org @ 2012-01-12 9:20 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51834
Andreas Schwab <schwab@linux-m68k.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |NEW
Last reconfirmed| |2012-01-12
Resolution|INVALID |
Ever Confirmed|0 |1
--- Comment #3 from Andreas Schwab <schwab@linux-m68k.org> 2012-01-12 09:20:10 UTC ---
(i++, i) + i is undefined. The sequence point only orders i++ and i inside the
parens, but not the operands of +. The third example is not undefined.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/51834] -Wsequence-point fails when convoluted expressions with multiple side effects are used
2012-01-12 6:29 [Bug c/51834] New: -Wsequence-point fails when convoluted expressions with multiple side effects are used prasoonsaurav.nit at gmail dot com
` (2 preceding siblings ...)
2012-01-12 9:20 ` schwab@linux-m68k.org
@ 2012-04-19 15:07 ` vincent-gcc at vinc17 dot net
3 siblings, 0 replies; 5+ messages in thread
From: vincent-gcc at vinc17 dot net @ 2012-04-19 15:07 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51834
--- Comment #4 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> 2012-04-19 15:06:58 UTC ---
(In reply to comment #3)
> (i++, i) + i is undefined. The sequence point only orders i++ and i inside the
> parens, but not the operands of +. The third example is not undefined.
The example is not (i++, i) + i, but (i, i++, i) + i, which is different
because there is a sequence point before and after the i++. Still, there seem
to be disagreements on how to interpret the standard.
There's a discussion "On sequence points and evaluation order" [1] in
comp.std.c in 1995-12 (though that's a bit old), from which there are arguments
to see the above expressions as UB. But "sequence points and evaluation order"
[2] in comp.lang.c in 2006-09 and a message from Keith Thompson [3] in
comp.std.c in 2010-10 both contradict it: they both say something like sin(x) +
cos(x) has defined behavior even if sin() and cos() both modify errno (and that
these functions can be implemented by a macro, as long as it has a sequence
point).
[1]
http://groups.google.com/group/comp.std.c/browse_thread/thread/d133e9c51bef572b/0b6545278c23d37f
[2]
http://groups.google.com/group/comp.lang.c/browse_thread/thread/c4bc836b783b91be/d807a3ad7202b45b
[3] http://groups.google.com/group/comp.std.c/msg/2dc8d2e8a0f4e572
What's strange is that GCC (4.4 to 4.7 at least) complains on
(i ? (j |= 1, 0) : 0) | (i ? (j |= 1, 0) : 0);
but not on
(j |= 1, 0) | (j |= 1, 0);
Contrary to GCC, I would say that the latter is UB (because from the root of
the expression, one can evaluate both j |= 1 without getting a sequence point
yet -- GCC should have output a warning, and that's bug 51562), but not the
former (similar to the errno case).
Here's a simple testcase I've used, with more tests:
int i, j;
static inline int set_flag (void)
{
j |= 1;
return 0;
}
#define FOO (i ? (j |= 1, 0) : 0)
#define BAR (i ? set_flag () : 0)
void fct (void)
{
FOO || FOO;
FOO | FOO;
BAR | BAR;
set_flag () + set_flag ();
j = (++i, j) + (j, ++i);
return;
}
GCC 4.7.0 warns only for "FOO | FOO;" (and I think that's incorrect, as said
above).
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2012-04-19 15:07 UTC | newest]
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-- links below jump to the message on this page --
2012-01-12 6:29 [Bug c/51834] New: -Wsequence-point fails when convoluted expressions with multiple side effects are used prasoonsaurav.nit at gmail dot com
2012-01-12 8:51 ` [Bug c/51834] " rguenth at gcc dot gnu.org
2012-01-12 9:08 ` prasoonsaurav.nit at gmail dot com
2012-01-12 9:20 ` schwab@linux-m68k.org
2012-04-19 15:07 ` vincent-gcc at vinc17 dot net
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