[Bug c++/96526] New: New built-in needed to implement std::strong_order total order on floating point types

[Bug c++/96526] New: New built-in needed to implement std::strong_order total order on floating point types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96526

            Bug ID: 96526
           Summary: New built-in needed to implement std::strong_order
                    total order on floating point types
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

https://wg21.link/cmp.alg#1.3 requires a total order for floating point types,
which includes ordering NaNs and infinities, and must be usable in constant
expressions.

The most robust way to do that is probably to make the compiler know how to do
it, possibly using mpfr_total_order_p from MPFR 4.1.0.

[Bug c++/96526] New built-in needed to implement std::strong_order total order on floating point types

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96526

--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
You could define __builtin_totalorder functions, but watch out for the 
change made to totalorder functions following a TS 18661-1 clarification 
request, from floating-point argument types to pointer argument types (so 
you'd need to handle all the cases of libm not having the functions, libm 
having them with the old prototypes as in glibc 2.25 through 2.30, libm 
having them with the new prototypes as in glibc 2.31 and later).

[Bug libstdc++/96526] New built-in needed to implement std::strong_order total order on floating point types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96526

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-03-01
          Component|c++                         |libstdc++
     Ever confirmed|0                           |1

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Now that we have __builtin_bit_cast I think this can be done entirely as C++
library code, so re-assigning.

[Bug libstdc++/96526] implement std::strong_order total order on floating point types

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96526

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:9805965e3551b66b5bd751d6076791d00cdeb137

commit r12-7468-g9805965e3551b66b5bd751d6076791d00cdeb137
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 3 12:34:27 2022 +0000

    libstdc++: Implement std::strong_order for floating-point types [PR96526]

    This removes a FIXME in <compare>, defining the total order for
    floating-point types. I originally opened PR96526 to request a new
    compiler built-in to implement this, but now that we have std::bit_cast
    it can be done entirely in the library.

    The implementation is based on the glibc definitions of totalorder,
    totalorderf, totalorderl etc.

    I think this works for all the types that satisfy std::floating_point
    today, and should also work for the types expected to be added by P1467
    except for std::bfloat16_t. It also supports some additional types that
    don't currently satisfy std::floating_point, such as __float80, but we
    probably do want that to satisfy the concept for non-strict modes.

    libstdc++-v3/ChangeLog:

            PR libstdc++/96526
            * libsupc++/compare (strong_order): Add missing support for
            floating-point types.
            *
testsuite/18_support/comparisons/algorithms/strong_order_floats.cc:
            New test.

[Bug libstdc++/96526] implement std::strong_order total order on floating point types

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96526

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

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

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed for GCC 12. I might backport this to gcc-11 later.