[Bug libstdc++/110615] New: std::abs converts integers to floats and back

[Bug libstdc++/110615] New: std::abs converts integers to floats and back

[julien.jorge@stuff-o-matic.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110615

            Bug ID: 110615
           Summary: std::abs converts integers to floats and back
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: julien.jorge@stuff-o-matic.com
  Target Milestone: ---

The implementation of std::abs in current HEAD
(a3ad2301d2f4aab2deeb286fa5bd0282260bfd0a) is as follows (in
libstdc++-v3/include/c_std/cmath):

    template<typename _Tp>
    inline typename __gnu_cxx::__enable_if<__is_integer<_Tp>::__value, 
                                           double>::__type
    abs(_Tp __x)
    { return __builtin_fabs(__x); }

The function is only used when _Tp is an integer (due to the __enable_if part),
yet it calls __builtin_fabs where, I believe, __builtin_llabs should have been
used. Unless the intent was to negate the condition in __enable_if?

I observed a lot of int <-> float conversions in the assembly of my program on
an older version of GCC with this implementation, unless I included cstdlib
which defines abs(long long) and co. The int <-> float conversions do not
happen with current HEAD but I believe it is due to a side effect of cmath
transitively including stdlib.h.

I first observed this with GCC 4.8.5. Compiler explorer shows int <-> float
conversions when cstdlib is missing with GCC up to 6.4:
https://godbolt.org/z/bWfEv1jxP

[Bug libstdc++/110615] std::abs converts integers to floats and back

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110615

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED
            Version|unknown                     |6.5.0

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is the expected behaviour for those old releases (which are no longer
supported or maintained, and so there's no point reporting bugs in them).

Prior to the resolutions of https://wg21.link/lwg2192 and
https://wg21.link/lwg2294 the integer overloads of std::abs were only declared
in <cstdlib>, so if you only include <cmath> then you only get the
floating-point overloads.

GCC 7.0 implements the resolutions to those issues, in r7-3447-g37b204de605563

[Bug libstdc++/110615] std::abs converts integers to floats and back

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110615

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Julien Jorge from comment #0)
> The int <-> float conversions
> do not happen with current HEAD but I believe it is due to a side effect of
> cmath transitively including stdlib.h.

No, it's due to the resolution of LWG 2192.

There is no transitive include of <stdlib.h> in <cmath>, instead both <cstdlib>
and <cmath> include <bits/std_abs.h> which defines all overloads for integers
and floating-point types.

This is by design, not a side effect.

[Bug libstdc++/110615] std::abs converts integers to floats and back

[julien.jorge@stuff-o-matic.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110615

--- Comment #3 from Julien Jorge <julien.jorge@stuff-o-matic.com> ---
> This is the expected behaviour for those old releases (which are no longer supported or maintained, and so there's no point reporting bugs in them).

Well, it was more an attempt to raise awareness on a behaviour I assumed to be
accidental in recent releases rather than an actual bug report for old releases
:) 

Anyway it is clear that I was mistaken! Thank you for the feedback and the
links :)