[Bug libstdc++/104858] New: ranges::minmax double dereferences first

[Bug libstdc++/104858] New: ranges::minmax double dereferences first

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104858

            Bug ID: 104858
           Summary: ranges::minmax double dereferences first
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

In
https://github.com/gcc-mirror/gcc/blob/bded0d549fdfdc1328b2c0189dc5f8593b4cbe42/libstdc%2B%2B-v3/include/bits/ranges_algo.h#L3087:

the initial result is constructed as:

minmax_result<range_value_t<_Range>> __result = {*__first, *__first};

If *__first performs actually does work (maybe it's a transformed iterator),
then this is wasteful. But more importantly, if *__first gives you an rvalue
reference, then this moves from that iterator twice.

I think the standard requires ranges::minmax(v | views::as_rvalue) to actually
be valid (assuming an as_rvalue adaptor that ensures that the reference type is
an rvalue).

[Bug libstdc++/104858] ranges::minmax double dereferences first

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104858

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-03-10
             Status|UNCONFIRMED                 |NEW
                 CC|                            |ppalka at gcc dot gnu.org
     Ever confirmed|0                           |1

[Bug libstdc++/104858] ranges::minmax double dereferences first

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104858

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

[Bug libstdc++/104858] ranges::minmax double dereferences first

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104858

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:7e76cef873342a66408c126abceaf7dbddd3f477

commit r12-8178-g7e76cef873342a66408c126abceaf7dbddd3f477
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Apr 15 14:41:14 2022 -0400

    libstdc++: Avoid double-deref of __first in ranges::minmax [PR104858]

            PR libstdc++/104858

    libstdc++-v3/ChangeLog:

            * include/bits/ranges_algo.h (__minmax_fn): Avoid dereferencing
            __first twice at the start.
            * testsuite/25_algorithms/minmax/constrained.cc (test06): New test.

[Bug libstdc++/104858] ranges::minmax double dereferences first

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104858

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.4

[Bug libstdc++/104858] ranges::minmax double dereferences first

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104858

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug libstdc++/104858] ranges::minmax double dereferences first

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104858

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|10.5                        |12.0

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixed.