[Bug c/106272] New: clang build: new warning ?

[Bug c/106272] New: clang build: new warning ?

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

            Bug ID: 106272
           Summary: clang build: new warning ?
           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: ---

Recent gcc builds with clang say this:

libcpp/include/line-map.h:1882:12: warning: moving a temporary object prevents
copy elision [-Wpessimizing-move]

I have little idea what that means, but the line of code is

    return std::move (label_text (buffer, true));

This new warning appears about 700 times, so it might be important.

[Bug preprocessor/106272] clang build: new warning ?

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-07-12
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to David Binderman from comment #0)
> This new warning appears about 700 times, so it might be important.

It's not. But the move is useless and shouldn't be there.

[Bug preprocessor/106272] clang build: new warning ?

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

--- Comment #2 from David Binderman <dcb314 at hotmail dot com> ---
I just noticed similar four lines earlier:

libcpp/include/line-map.h:1876:12: warning: moving a temporary object prevents
copy elision [-Wpessimizing-move]

Source code is

    return std::move (label_text (const_cast <char *> (buffer), false));

About 700 mentions of this one, as well.

[Bug preprocessor/106272] clang build: new warning ?

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

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

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

--- Comment #3 from Eric Gallager <egallager at gcc dot gnu.org> ---
Note that GCC has its own version of -Wpessimizing-move, too... any idea why
clang's version of the flag catches it, but gcc's doesn't?

[Bug preprocessor/106272] clang build: new warning ?

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Eric Gallager from comment #3)
> Note that GCC has its own version of -Wpessimizing-move, too... any idea why
> clang's version of the flag catches it, but gcc's doesn't?

No, but I'm going to reduce libcpp/line-map.ii to create a testcase and file a
bug and since it's my warning, maybe even fix it.

[Bug preprocessor/106272] clang build: new warning ?

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

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

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

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
And while at it, why don't I fix this one.

[Bug preprocessor/106272] clang build: new warning ?

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=106276

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
(In reply to Marek Polacek from comment #4)
> (In reply to Eric Gallager from comment #3)
> > Note that GCC has its own version of -Wpessimizing-move, too... any idea why
> > clang's version of the flag catches it, but gcc's doesn't?
> 
> No, but I'm going to reduce libcpp/line-map.ii to create a testcase and file
> a bug and since it's my warning, maybe even fix it.

Bug 106276.

[Bug preprocessor/106272] clang build: new warning ?

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

--- Comment #7 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:86a15b266a7284f3aa1b12494a475f31416b981d

commit r13-1689-g86a15b266a7284f3aa1b12494a475f31416b981d
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Jul 12 20:18:56 2022 -0400

    libcpp: Avoid pessimizing std::move [PR106272]

    std::move in a return statement can prevent the NRVO:
   
<https://developers.redhat.com/blog/2019/04/12/understanding-when-not-to-stdmove-in-c>

    PR106272 reports that we have two such cases in class label_text's
    member functions.  We have -Wpessimizing-move that's supposed to detect
    problematic std::move uses, but in this case it didn't trigger.  I've filed
    PR106276 to track that.

            PR preprocessor/106272

    libcpp/ChangeLog:

            * include/line-map.h (class label_text): Don't std::move in a
return
            statement.

[Bug preprocessor/106272] clang build: new warning ?

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

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

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

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed.

[Bug preprocessor/106272] clang build: new warning ?

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106272

--- Comment #9 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Thanks!