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* [Bug c/53382] New: incorrect associativity in expressions @ 2012-05-16 23:06 miguel.barao at gmail dot com 2012-05-16 23:08 ` [Bug c/53382] " pinskia at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: miguel.barao at gmail dot com @ 2012-05-16 23:06 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53382 Bug #: 53382 Summary: incorrect associativity in expressions Classification: Unclassified Product: gcc Version: unknown Status: UNCONFIRMED Severity: critical Priority: P3 Component: c AssignedTo: unassigned@gcc.gnu.org ReportedBy: miguel.barao@gmail.com I found the following behavior while writing a small RPN calculator. The ideia is to perform a subtraction on the two top elements of a stack using push(&mystack, -pop(&mystack) + pop(&mystack)); Since the + associativity is left-to-right, this should pop the stack, negate this value, pop the second argument then add. But the order is reversed in gcc leading to a wrong result. The behavior was observed in gcc 4.4.5 (debian stable), 4.6.3 (ubuntu 12.04), and the llvm-gcc versions shipping with MacOSX. The clang frontend versions 1.1 (on debian stable), 3.0 (OSX and ubuntu 12.04) work correctly. Example: Suppose a stack contains [ 1 2 ], the top of the stack being 2. On gcc the above expression returns 1, the first pop executed is the second one. On clang the same expression returns -1, the first pop executed is the first one as should be. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/53382] incorrect associativity in expressions 2012-05-16 23:06 [Bug c/53382] New: incorrect associativity in expressions miguel.barao at gmail dot com @ 2012-05-16 23:08 ` pinskia at gcc dot gnu.org 2012-05-16 23:12 ` pinskia at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2012-05-16 23:08 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53382 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |RESOLVED Resolution| |INVALID --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-05-16 23:05:45 UTC --- -pop(&mystack) + pop(&mystack) Either pop can come first as there is no sequence point between them. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/53382] incorrect associativity in expressions 2012-05-16 23:06 [Bug c/53382] New: incorrect associativity in expressions miguel.barao at gmail dot com 2012-05-16 23:08 ` [Bug c/53382] " pinskia at gcc dot gnu.org @ 2012-05-16 23:12 ` pinskia at gcc dot gnu.org 2012-05-16 23:15 ` pinskia at gcc dot gnu.org 2012-05-17 10:19 ` miguel.barao at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2012-05-16 23:12 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53382 --- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-05-16 23:07:46 UTC --- See also http://c-faq.com/expr/confused.html . ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/53382] incorrect associativity in expressions 2012-05-16 23:06 [Bug c/53382] New: incorrect associativity in expressions miguel.barao at gmail dot com 2012-05-16 23:08 ` [Bug c/53382] " pinskia at gcc dot gnu.org 2012-05-16 23:12 ` pinskia at gcc dot gnu.org @ 2012-05-16 23:15 ` pinskia at gcc dot gnu.org 2012-05-17 10:19 ` miguel.barao at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2012-05-16 23:15 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53382 --- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-05-16 23:12:02 UTC --- Basically C does not specify which order of the two operands of + are evaluated first so both clang and GCC are correct. >Since the + associativity is left-to-right Kinda but the order of evaluation is different from associativity. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/53382] incorrect associativity in expressions 2012-05-16 23:06 [Bug c/53382] New: incorrect associativity in expressions miguel.barao at gmail dot com ` (2 preceding siblings ...) 2012-05-16 23:15 ` pinskia at gcc dot gnu.org @ 2012-05-17 10:19 ` miguel.barao at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: miguel.barao at gmail dot com @ 2012-05-17 10:19 UTC (permalink / raw) To: gcc-bugs http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53382 --- Comment #4 from Miguel Barao <miguel.barao at gmail dot com> 2012-05-17 09:42:48 UTC --- There was a misunderstanding between associativity and operator evaluation order in the original bugreport. I now understand that the language does not specify the evaluation order of an operator, and is bad a practice to depend such behavior. Still, I would like to add that expressions like - f( ) + f( ) and - ( f( ) - f( ) ) produce different evaluation orders, and therefor different results, when f( ) has side effects. ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2012-05-17 9:43 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2012-05-16 23:06 [Bug c/53382] New: incorrect associativity in expressions miguel.barao at gmail dot com 2012-05-16 23:08 ` [Bug c/53382] " pinskia at gcc dot gnu.org 2012-05-16 23:12 ` pinskia at gcc dot gnu.org 2012-05-16 23:15 ` pinskia at gcc dot gnu.org 2012-05-17 10:19 ` miguel.barao at gmail dot com
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