[Bug fortran/56887] New: Test for equality of reals now flagged with a warning

[Bug fortran/56887] New: Test for equality of reals now flagged with a warning

[fkrogh#gcc at mathalacarte dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56887

             Bug #: 56887
           Summary: Test for equality of reals now flagged with a warning
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: fkrogh#gcc@mathalacarte.com


This is not a bug, but is a nuisance.  I like to compile with  -Wall -Wextra
and after starting to use 4.8.0, tests for equality between reals is flagged
with a warning.  Perhaps you consider it a bad habit to assign small integer
values (usually 0) to real variables and and occasionally test for equality on
those.  For me this warning is a nuisance, and I would like a way to turn it
off without losing the benefits of -WALL and -Wextra.  Alternatively just have
it work as it did in the 4.7 series.
Thanks,
   Fred

[Bug fortran/56887] Test for equality of reals now flagged with a warning

[tkoenig at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56887

Thomas Koenig <tkoenig at gcc dot gnu.org> changed:

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

--- Comment #1 from Thomas Koenig <tkoenig at gcc dot gnu.org> 2013-04-09 11:24:07 UTC ---
Please see the release notes:

"The -Wcompare-reals command-line option has been added. When this is set,
warnings are issued when comparing REAL or COMPLEX types for equality and
inequality; consider replacing a == b by abs(a−b) < eps with a suitable eps.
-Wcompare-reals is enabled by -Wextra."

Following the usual gcc option conventions, you can disable the -Wcompare-reals
 flag by -Wno-compare-reals.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idQ348

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |DUPLICATE

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2013-04-09 11:47:15 UTC ---
Dup.

*** This bug has been marked as a duplicate of bug 56793 ***

[Bug fortran/56887] Test for equality of reals now flagged with a warning

[burnus at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56887

Tobias Burnus <burnus at gcc dot gnu.org> changed:

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

--- Comment #2 from Tobias Burnus <burnus at gcc dot gnu.org> 2013-04-09 13:06:15 UTC ---
(In reply to comment #0)
> tests for equality between reals is flagged with a warning.

The support for the warning follows ISO/IEC TR 24772, which recommends users to
"Avoid creating a logical value from a test for equality or inequality
between two floating-point expressions." - And compiler vendors to provide such
a warning.

The problem with many constructs is that one cannot reliably detect whether
they are okay or a bug in the code. Thus, warnings are issued for those; there
are always false positives and missed bugs with warning diagnostic. Those
checks, where the false-positive rate is low and the likelihood for bugs is
high, are enabled by default; others only with -Wall or -Wextra or only with
-W<name-of-the-warning>.

In your case, using integer-valued floating-point numbers is probably fine. As
Thomas wrote, you can use -Wno-compare-reals to disable the warning. See GCC
4.8 release notes or gfortran's man page (or user manual) under -Wextra.

[Bug fortran/56887] Test for equality of reals now flagged with a warning

[fkrogh at mathalacarte dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56887

--- Comment #3 from fkrogh at mathalacarte dot com 2013-04-09 13:30:45 UTC ---
On 04/09/2013 06:06 AM, burnus at gcc dot gnu.org wrote:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56887
>
> Tobias Burnus <burnus at gcc dot gnu.org> changed:
>
>             What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                   CC|                            |burnus at gcc dot gnu.org
>
> --- Comment #2 from Tobias Burnus <burnus at gcc dot gnu.org> 2013-04-09 13:06:15 UTC ---
> (In reply to comment #0)
>> tests for equality between reals is flagged with a warning.
> The support for the warning follows ISO/IEC TR 24772, which recommends users to
> "Avoid creating a logical value from a test for equality or inequality
> between two floating-point expressions." - And compiler vendors to provide such
> a warning.
>
> The problem with many constructs is that one cannot reliably detect whether
> they are okay or a bug in the code. Thus, warnings are issued for those; there
> are always false positives and missed bugs with warning diagnostic. Those
> checks, where the false-positive rate is low and the likelihood for bugs is
> high, are enabled by default; others only with -Wall or -Wextra or only with
> -W<name-of-the-warning>.
>
> In your case, using integer-valued floating-point numbers is probably fine. As
> Thomas wrote, you can use -Wno-compare-reals to disable the warning. See GCC
> 4.8 release notes or gfortran's man page (or user manual) under -Wextra.
>

Many thanks Tobias.  That works fine for me and is just what I wanted.  
For some reason it is not in my info pages or man pages -- I really did 
look for anything like this.

On another subject, I know that you had some contact with Dr. Richard 
Hanson concerning IEEE exceptions.  He has completed his work on these 
and is free to tun them over to GNU.  He is under the impression that 
there is no interest on your part.

Thanks again,
    Fred