[Bug c++/52299] New: GCC warns on compile time division by zero erroneously

[Bug c++/52299] New: GCC warns on compile time division by zero erroneously

[joseph.h.garvin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

             Bug #: 52299
           Summary: GCC warns on compile time division by zero erroneously
    Classification: Unclassified
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: joseph.h.garvin@gmail.com


Test case:

template<std::size_t x>
struct test {
  static const std::size_t a_
  = x ? 10 / x : 10;
};

Output from compiling:

alignptrtest.cpp: In instantiation of ‘const size_t test<0ul>::a_’:
alignptrtest.cpp:11:   instantiated from here
alignptrtest.cpp:6: warning: division by zero

The warning should be suppressed when zero is explicitly checked for. At
runtime this could require fancy analysis that might be unreasonable to expect,
but since this is compile time it should be possible to know whether the
division by 0 really occurs. Especially since it's smart enough to give the
warning even when dividing by (x+1-1).

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2012-02-20
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-02-20 12:13:03 UTC ---
Your testcase does not compile.  Using

template<unsigned int x>
struct test {
      static const unsigned int a_
            = x ? 10 / x : 10;
};

instead does not reproduce the warning (not even with -Wall).

Please provide preprocessed source and the command-line you are using
to reproduce the issue.

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[joseph.h.garvin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

--- Comment #2 from Joseph Garvin <joseph.h.garvin at gmail dot com> 2012-02-20 16:45:21 UTC ---
#include <cstddef>

template<std::size_t x>
struct test {
  static const std::size_t a_
  = x ? 10 / x : 10;
};

I just forgot to copy and paste the include. My command line is just "g++
alignptrtest.cpp"

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[joseph.h.garvin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

--- Comment #3 from Joseph Garvin <joseph.h.garvin at gmail dot com> 2012-02-20 16:47:23 UTC ---
Also this is x86-64, which could be relevant since it affects the underlying
type of std::size_t.

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-02-20 16:53:03 UTC ---
Can't reproduce with that either, tried 4.4.6, 4.6.2 and current trunk, x86_64.

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-20 17:02:03 UTC ---
I can't reproduce it with 4.4.3, but then ...


(In reply to comment #0)
> 
> alignptrtest.cpp: In instantiation of ‘const size_t test<0ul>::a_’:
> alignptrtest.cpp:11:   instantiated from here
> alignptrtest.cpp:6: warning: division by zero

Your testcase doesn't have 11 lines and doesn't instantiate a template.

Please provide the right testcase!

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-20 17:11:03 UTC ---
This reproduces it:

template<unsigned x>
struct test {
  static const unsigned a_
  = x ? 10 / x : 10;
};

int i = test<0>::a_;

but next time please just provide code to reproduce the problem instead of
wasting three people's time (and however many more read the report and didn't
comment)

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[joseph.h.garvin at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

--- Comment #7 from Joseph Garvin <joseph.h.garvin at gmail dot com> 2012-02-20 17:56:20 UTC ---
Sorry, I should have copy and pasted the whole file. I was trying to save
people's time by providing a condensed snippet ;p

(In reply to comment #6)
> This reproduces it:
> 
> template<unsigned x>
> struct test {
>   static const unsigned a_
>   = x ? 10 / x : 10;
> };
> 
> int i = test<0>::a_;
> 
> but next time please just provide code to reproduce the problem instead of
> wasting three people's time (and however many more read the report and didn't
> comment)

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
      Known to fail|                            |3.4.3, 4.1.2, 4.4.3, 4.5.2,
                   |                            |4.6.2, 4.7.0

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-02-20 18:02:46 UTC ---
condensed is fine, incomplete is not

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-02-20 18:27:46 UTC ---
    case COND_EXPR:
      return build_x_conditional_expr
        (RECUR (TREE_OPERAND (t, 0)),
         RECUR (TREE_OPERAND (t, 1)),
         RECUR (TREE_OPERAND (t, 2)),
         complain);

For this we'd need to first RECUR (TREE_OPERAND (t, 0)), remember that result
in some temporary, and look if it is 0, non-zero or unknown.  If 0, maybe
increment temporarily c_inhibit_evaluation_warnings around RECUR (TREE_OPERAND
(t, 1)) (dunno about cp_unevaluated_operand or fold_*defer_overflow_warnings
()), if non-zero similarly for RECUR (TREE_OPERAND (t, 1)).

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |paolo.carlini at oracle dot
                   |gnu.org                     |com
   Target Milestone|---                         |4.8.0

--- Comment #10 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-03-12 16:59:20 UTC ---
On it.

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[paolo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

--- Comment #11 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> 2012-03-12 19:29:42 UTC ---
Author: paolo
Date: Mon Mar 12 19:29:38 2012
New Revision: 185264

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=185264
Log:
/cp
2012-03-12  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/52299
    * pt.c (tsubst_copy_and_build, case COND_EXPR): Avoid bogus
    division by zero warnings.

/testsuite
2012-03-12  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/52299
    * g++.dg/warn/Wdiv-by-zero-bogus.C: New.


Added:
    trunk/gcc/testsuite/g++.dg/warn/Wdiv-by-zero-bogus.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/52299] GCC warns on compile time division by zero erroneously

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52299

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #12 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-03-12 19:30:35 UTC ---
Fixed.