[Bug c/64440] New: -Wdiv-by-zero false negative on const variables

[Bug c/64440] New: -Wdiv-by-zero false negative on const variables

[chengniansun at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

            Bug ID: 64440
           Summary: -Wdiv-by-zero false negative on const variables
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chengniansun at gmail dot com

In the following test case, the variable "b" is constant zero. However, GCC
does not warn div-by-zero on the expression "a / b". More details are shown as
below: 



$: cat t.c
int f (int a) {
  const int b = 0;
  return a / b;
}
$: 
$: gcc-trunk -c -Wdiv-by-zero t.c 
$:
$: clang-trunk -c -Wdivision-by-zero t.c
t.c:3:12: warning: division by zero is undefined [-Wdivision-by-zero]
  return a / b;
           ^ ~
1 warning generated.
$:

[Bug c/64440] -Wdiv-by-zero false negative on const variables

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
In C, const int is not a constant expression and -Wdiv-by-zero only warns about
integer constant expressions.

[Bug c/64440] -Wdiv-by-zero false negative on const variables

[chengniansun at gmail dot com]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

--- Comment #2 from Chengnian Sun <chengniansun at gmail dot com> ---

(In reply to Andrew Pinski from comment #1)
> In C, const int is not a constant expression and -Wdiv-by-zero only warns
> about integer constant expressions.

Thanks for your reply. It seems GCC sometimes does consider "const int" for
other types of warnings (but not for -Wdiv-by-zero). See the following, with
-O3, GCC warns that the left shift count is negative.  

$: cat t.c
int f(int a) {
  const char c = -4;
  return a << c;
}
$: gcc-trunk -Wall -c t.c -O3
t.c: In function ‘f’:
t.c:3:12: warning: left shift count is negative
   return a << c;
            ^
$:
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To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/64441] New: A match_results returns an incorrect sub_match if the sub_match::matched is false
Date: Tue, 30 Dec 2014 09:32:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd441

            Bug ID: 64441
           Summary: A match_results returns an incorrect sub_match if the
                    sub_match::matched is false
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kariya_mitsuru at hotmail dot com

Created attachment 34362
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id4362&actioníit
g++ -v

Please see the following sample.

====================================== sample code
=====================================#include <iostream>
#include <regex>

int main()
{
    const char s[] = "abc";
    const std::regex re("(\\d+)|(\\w+)");

    std::cmatch m;
    std::regex_search(s, m, re);
    std::cout << std::boolalpha;
    for (size_t i = 0, n = m.size(); i < n; ++i) {
        auto&& sub = m[i];
        std::cout << i << ":" << sub.matched << ", str = '" << sub.str() << "',
"
            "range = [" << sub.first - s << ", " << sub.second - s << ")" <<
std::endl;
    }
}
===================================================================================================================== output ============================0:true, str = 'abc', range = [0, 3)
1:false, str = '', range = [-140734305427376, -140734305427376)
2:true, str = 'abc', range = [0, 3)
=================================================================
cf. http://melpon.org/wandbox/permlink/SBoMF5UKYYa38Y4N


The C++11 standard 28.10[re.results]/p.4 says, "Otherwise matched is false, and
members first and second point to the end of the sequence that was searched."

So, I think that the output should be

============================= output ============================0:true, str = 'abc', range = [0, 3)
1:false, str = '', range = [3, 3)
2:true, str = 'abc', range = [0, 3)
=================================================================

[Bug c/64440] -Wdiv-by-zero false negative on const variables

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

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

--- Comment #3 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Chengnian Sun from comment #2)
> Thanks for your reply. It seems GCC sometimes does consider "const int" for
> other types of warnings (but not for -Wdiv-by-zero). See the following, with
> -O3, GCC warns that the left shift count is negative.  
> 

I wonder how this happens? The warning is still given by the FE!

Nonetheless, clang is able to warn without optimization because it has some
constant propagation pass in the FE. GCC does not have such a thing and current
developers are not convinced that it is worth it.

I think there are several PRs open about this, and it doesn't seem useful to
have one PR for each possible warning that could be improved by this.
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50139

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |manu at gcc dot gnu.org
         Resolution|---                         |WONTFIX

--- Comment #4 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to nightstrike from comment #3)
> There's other relevant posts, but you get the idea.  Anyway, this should
> eventually be closed.

So be it. Thanks for noticing. 

(You can probably get a gcc.gnu.org bugzilla account, if you wish so).
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Subject: [Bug c/64442] -O1 modify output of a simple computation with rounding
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd442

--- Comment #1 from Mikael Pettersson <mikpelinux at gmail dot com> ---
Are you compiling on 32-bit x86?  I can reproduce on m68k, but not on x86_64
(even with -m32) or on ARMv5.  I suspect you're seeing the effects of excess
precision of the x87 FPU (much like the m68k FPU).

[Bug c/64440] -Wdiv-by-zero false negative on const variables

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Manuel López-Ibáñez from comment #3)
> (In reply to Chengnian Sun from comment #2)
> > Thanks for your reply. It seems GCC sometimes does consider "const int" for
> > other types of warnings (but not for -Wdiv-by-zero). See the following, with
> > -O3, GCC warns that the left shift count is negative.  
> > 
> 
> I wonder how this happens? The warning is still given by the FE!

By using decl_constant_value_for_optimization.
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd509

--- Comment #2 from Martien de Jong <maurits.de.jong at ericsson dot com> ---
That's just rephrasing my bugreport. The question is, should it type check
while parsing an expression that may not be realized? The entire idea of
switching on a type is to prevent and fix type errors, no?

[Bug c/64440] -Wdiv-by-zero false negative on const variables

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2015-01-06
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org
   Target Milestone|---                         |5.0
     Ever confirmed|0                           |1

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
C++ FE warns for the div-by-zero testcase, and C FE can easily too, I think. 
Ditto for %.  Mine.

[Bug c/64440] -Wdiv-by-zero false negative on const variables

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Author: mpolacek
Date: Wed Jan  7 08:21:50 2015
New Revision: 219279

URL: https://gcc.gnu.org/viewcvs?rev=219279&root=gcc&view=rev
Log:
    PR c/64440
    * c-common.c (c_fully_fold_internal): Warn for division and modulo
    if orig_op1 isn't INTEGER_CST, op1 is INTEGER_CST and is zero.

    * gcc.dg/pr64440.c: New test.
    * c-c++-common/pr56607.c: Don't limit dg-warnings to C++.

Added:
    trunk/gcc/testsuite/gcc.dg/pr64440.c
Modified:
    trunk/gcc/c-family/ChangeLog
    trunk/gcc/c-family/c-common.c
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/c-c++-common/pr56607.c

[Bug c/64440] -Wdiv-by-zero false negative on const variables

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64440

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.