[Bug c++/105278] New: no warning for precise literals compared with floats

[Bug c++/105278] New: no warning for precise literals compared with floats

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

            Bug ID: 105278
           Summary: no warning for precise literals compared with floats
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dcb314 at hotmail dot com
  Target Milestone: ---

For this C++ code:

extern void g( int);

void f( float a)
{
        if (a == 0.1234)
                g( 1);
}

recent gcc trunk has nothing to say:

$ /home/dcb/gcc/results/bin/gcc -c -g -O2 -Wall -Wextra -pedantic apr14b.cc
$ 

Here is clang finding the problem:

$ /home/dcb/llvm/results/bin/clang -c -g -O2 -Wall   apr14b.cc
apr14b.cc:6:8: warning: floating-point comparison is always false; constant
cannot be represented exactly in type 'float' [-Wliteral-range]
        if (a == 0.1234)
            ~ ^  ~~~~~~~~~~~
1 warning generated.

Interestingly, if float is replaced by double, the warning goes away.

[Bug c++/105278] no warning for precise literals compared with floats

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org
           Keywords|                            |diagnostic

--- Comment #1 from Eric Gallager <egallager at gcc dot gnu.org> ---
-Wfloat-equal gets you a warning, as does -Wdouble-promotion:

$ /usr/local/bin/gcc -c -g -O2 -Wall -Wextra -pedantic -Wconversion
-Wfloat-equal -Wdouble-promotion 105278.c
105278.c: In function 'f':
105278.c:5:15: warning: comparing floating-point with '==' or '!=' is unsafe
[-Wfloat-equal]
    5 |         if (a == 0.1234)
      |               ^~
105278.c:5:15: warning: implicit conversion from 'float' to 'double' to match
other operand of binary expression [-Wdouble-promotion]
$

[Bug c++/105278] no warning for precise literals compared with floats

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

--- Comment #2 from David Binderman <dcb314 at hotmail dot com> ---
(In reply to Eric Gallager from comment #1)
> -Wfloat-equal gets you a warning, as does -Wdouble-promotion:

Thanks for that. This looks like another case where an obscure flag
really ought to be in -Wall or at least -Wextra.

I'll be adding it into my local compiler.

[Bug c++/105278] no warning for precise literals compared with floats

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to David Binderman from comment #2)
> (In reply to Eric Gallager from comment #1)
> > -Wfloat-equal gets you a warning, as does -Wdouble-promotion:
> 
> Thanks for that. This looks like another case where an obscure flag
> really ought to be in -Wall or at least -Wextra.
> 
> I'll be adding it into my local compiler.

Which one of them specifically do you want as part of -Wall or -Wextra? Because
the latter is listed explicitly in some documentation somewhere as an example
of a flag that would be bad to include in -Wall or -Wextra.

[Bug c++/105278] no warning for precise literals compared with floats

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

--- Comment #4 from David Binderman <dcb314 at hotmail dot com> ---
(In reply to Eric Gallager from comment #3)
> (In reply to David Binderman from comment #2)
> > (In reply to Eric Gallager from comment #1)
> > > -Wfloat-equal gets you a warning, as does -Wdouble-promotion:
> > 
> > Thanks for that. This looks like another case where an obscure flag
> > really ought to be in -Wall or at least -Wextra.
> > 
> > I'll be adding it into my local compiler.
> 
> Which one of them specifically do you want as part of -Wall or -Wextra?
> Because the latter is listed explicitly in some documentation somewhere as
> an example of a flag that would be bad to include in -Wall or -Wextra.

Sorry, I should have been more clear -Wfloat-equal. I haven't looked into
the pros and cons of -Wdouble-promotion. 

If you've got some spare time to pin down "some documentation somewhere",
then that would be appreciated.

[Bug c++/105278] no warning for precise literals compared with floats

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note I think GCC's -Wfloat-equal is more reasonible than Clang's
-Wliteral-range really. The reason is because even if something can be
represented exactly in floating point (e.g. 3.0 or even 0.0), you could in
theory still get a few precision mismatch due to FMA or some other slight
differences in HW implementation.

[Bug c++/105278] -Wliteral-range vs -Wfloat-equal

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|no warning for precise      |-Wliteral-range vs
                   |literals compared with      |-Wfloat-equal
                   |floats                      |

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't think clang implements -Wfloat-equal at all, at least they didn't at
the last time I looked a few years back.

[Bug c++/105278] -Wliteral-range vs -Wfloat-equal

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

--- Comment #7 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #6)
> I don't think clang implements -Wfloat-equal at all, at least they didn't at
> the last time I looked a few years back.

I just checked their diagnostics reference page and it says that they do:
https://clang.llvm.org/docs/DiagnosticsReference.html#wfloat-equal

[Bug c++/105278] -Wliteral-range vs -Wfloat-equal

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105278

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So clang emits one or the other warning for the code but not both.

You can defect the warning in clang by doing:
```
extern void g( int);
void f(float a)
{
  double b = a;
  if (b == 0.1234)
    g( 1);
}
```

LLVM actually opimizes away the comparison while GCC does not.