[Bug c++/106276] New: Missing -Wpessimizing-move warning

[Bug c++/106276] New: Missing -Wpessimizing-move warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

            Bug ID: 106276
           Summary: Missing -Wpessimizing-move warning
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Looks like we're missing a warning on the std::move line here:

namespace std {
template <typename _Tp> _Tp &&move(_Tp &&);
}
char take_buffer;
struct label_text {
  label_text take() { return std::move(label_text(&take_buffer)); }
  label_text(char *);
};

$ xclang++ -c -Wall -W line-map.ii
line-map.ii:6:30: warning: moving a temporary object prevents copy elision
[-Wpessimizing-move]
  label_text take() { return std::move(label_text(&take_buffer)); }
                             ^
line-map.ii:6:30: note: remove std::move call here
  label_text take() { return std::move(label_text(&take_buffer)); }
                             ^~~~~~~~~~                        ~
$ xg++ -c -Wall -W line-map.ii
$

[Bug c++/106276] Missing -Wpessimizing-move warning

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-07-13
                 CC|                            |marxin at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW

[Bug c++/106276] Missing -Wpessimizing-move warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

[Bug c++/106276] Missing -Wpessimizing-move warning

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

David Binderman <dcb314 at hotmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dcb314 at hotmail dot com

--- Comment #1 from David Binderman <dcb314 at hotmail dot com> ---
Can a simpler word than "elison" be used ? Remove or delete maybe ?

This would help stop many folks, including me, reaching for their dictionaries
to discover what the compiler is warning about.

Eschew prolixity.

[Bug c++/106276] Missing -Wpessimizing-move warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
IMHO no.  "Copy elision" is a well-established term in C++ and naming it
differently would be more confusing.

[Bug c++/106276] Missing -Wpessimizing-move warning

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Agreed. Inventing new imprecise terminology for it would not be an improvement.

[Bug c++/106276] Missing -Wpessimizing-move warning

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to David Binderman from comment #1)
> This would help stop many folks, including me, reaching for their
> dictionaries
> to discover what the compiler is warning about.

Don't use a dictionary then:
https://en.cppreference.com/w/cpp/language/copy_elision
https://en.wikipedia.org/wiki/Copy_elision
https://stackoverflow.com/questions/12953127/what-are-copy-elision-and-return-value-optimization

The gcc manual also explains the warning better than looking up individual
words in a dictionary will.

> Eschew prolixity.

This is not prolixity, it's a term of art.

[Bug c++/106276] Missing -Wpessimizing-move warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=90428

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Ah, this is pretty much a dup of a half of bug 90428.

[Bug c++/106276] Missing -Wpessimizing-move warning

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:ae3459cd5956fcd29e8aa5408efb707cb7d9b14f

commit r13-2045-gae3459cd5956fcd29e8aa5408efb707cb7d9b14f
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Jul 22 18:10:30 2022 -0400

    c++: Extend -Wpessimizing-move for class prvalues [PR106276]

    We already have a warning that warns about pessimizing std::move
    in a return statement, when it prevents the NRVO:

      T fn()
      {
        T t;
        return std::move (t); // warning \o/
      }

    However, the warning doesn't warn when what we are returning is a class
    prvalue, that is, when std::move prevents the RVO:

      T fn()
      {
        T t;
        return std::move (T{}); // no warning :-(
      }

    This came up recently in GCC:
    <https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598177.html>.

    This patch fixes that.  I would like to extend the warning further, so
    that it warns in more contexts, e.g.:

      T t = std::move(T());

    or

      void foo (T);
      foo (std::move(T()));

            PR c++/106276

    gcc/cp/ChangeLog:

            * typeck.cc (can_do_rvo_p): New.
            (maybe_warn_pessimizing_move): Warn when moving a temporary object
            in a return statement prevents copy elision.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/Wpessimizing-move7.C: New test.

[Bug c++/106276] Missing -Wpessimizing-move warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106276

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Done for GCC 13.