[Bug c++/49409] New: some possible new warnings for strange code

[Bug c++/49409] New: some possible new warnings for strange code

[dcb314 at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

           Summary: some possible new warnings for strange code
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: dcb314@hotmail.com


Consider the following code

extern void g( int);

void f( int i)
{
    if (i == i) // same thing compared
        g( i);

    if (i != i) // same thing compared
        g( 2 * i);

    if ((i - i) > 10)   // X - X == 0, unless volatile
        g( 3 * i);

    if (i || i) // duplicates in ||
        g( 4 * i);

    if (i && i) // duplicates in &&
        g( 5 * i);
}

I ran the above code through the C++ compiler, with warning flags -Wall
-Wextra and it was surprisingly silent.

Would it be worthwhile for g++ to warn about some, all or none of
the above coding errors ?

[Bug c++/49409] some possible new warnings for strange code

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-06-14 22:34:34 UTC ---
some warnings would seem sensible to me

I tried clang++ which only warns about the first two, via
-Wtautological-compare

[Bug c++/49409] some possible new warnings for strange code

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.06.15 10:03:26
     Ever Confirmed|0                           |1

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-15 10:03:26 UTC ---
confirmed.

[Bug c++/49409] some possible new warnings for strange code

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |polacek at redhat dot com

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Maybe Marek is interested (beyond -Wtautological-compare)

[Bug c++/49409] some possible new warnings for strange code

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

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> ---
With -Wall -Wlogical-op we warn for all the cases except "if ((i - i) > 10)"
and I'm not sure if we really want a warning for this case.

[Bug c++/49409] some possible new warnings for strange code

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |jsm28 at gcc dot gnu.org

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Marek, could you please add to the audit trail which warnings work in which
version of GCC? Thanks in advance!

About the remaining case, I would be tempted to close the bug or maybe we could
first ask other maintainers...

[Bug c++/49409] some possible new warnings for strange code

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
-Wtautological-compare has been added to GCC 6.  -Wlogical-op is older, but the
part of it that warns about "i && i" is new and has only been added to GCC 6.

I agree about closing the bug now, BTW.

[Bug c++/49409] some possible new warnings for strange code

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49409

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
I meant "agree with closing".