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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] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- 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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