[Bug c++/51571] New: No named return value optimization while adding a dummy scope

[Bug c++/51571] New: No named return value optimization while adding a dummy scope

[prasoonsaurav.nit at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

             Bug #: 51571
           Summary: No named return value optimization while adding a
                    dummy scope
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: prasoonsaurav.nit@gmail.com


Simple code snippet

#include <iostream>
int global;
struct A
{
   A(){}
   A(const A&x){
       ++global;
   }
   ~A(){}
};
A foo()
{  
     A a;
     return a;  
}
int main()
{
   A x = foo();
   std::cout << global;
}
Output : 0

When the definition of foo is changed to

A foo()
{ 
  { 
     A a;
     return a;  
  }
}
I get 1 as the output i.e copy c-tor gets called once.

Compiler is not optimizing the call to the copy c-tor in this case.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[prasoonsaurav.nit at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Prasoon <prasoonsaurav.nit at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |critical

[Bug c++/51571] No named return value optimization while adding a dummy scope

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-12-15
     Ever Confirmed|0                           |1
      Known to fail|                            |4.2.3, 4.7.0
           Severity|critical                    |normal

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-12-15 21:59:31 UTC ---
Confirmed, this is just a missed optimization and not very critical really.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Paolo Carlini <paolo.carlini at oracle dot com> changed:

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

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-12-18 10:55:08 UTC ---
Adding Jason, I seem to remember he did NRVO on trees.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[guillaume.melquiond at inria dot fr]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Guillaume Melquiond <guillaume.melquiond at inria dot fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |guillaume.melquiond at inria dot f
                   |                            |r

--- Comment #3 from Guillaume Melquiond <guillaume.melquiond at inria dot fr> ---
I have recently encountered a similar problem, but in a much more general case.

struct A {
    A(int);
    A(A const &);
    ~A();
};

A f(bool b)
{
    if (b) return A(0);
    A a(1);
    return a;
}

All the return statements dominated by variable "a" return "a", so its
construction should happen in-place, hence eliding copy-construction and
destruction. Unfortunately, this is not what happens with g++ 4.8.0.

Interestingly enough, if one uninlines the code by hand, g++ actually generates
the optimal code, so it is possible though cumbersome to work around the missed
optimization:

inline A f2()
{
    A a(1);
    return a;
}

A f1(bool b)
{
    if (b) return A(0);
        return f2();
}

produces

A f1(bool) (bool b)
{
  <bb 2>:
  if (b_2(D) != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 3>:
  A::A (_4(D), 0);
  goto <bb 5>;

  <bb 4>:
  A::A (_4(D), 1);

  <bb 5>:
  return _4(D);
}

[Bug c++/51571] No named return value optimization while adding a dummy scope

[marc at kdab dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

marc at kdab dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marc at kdab dot com

--- Comment #4 from marc at kdab dot com ---
I would like to strongly oppose the notion that this "just a missed
optimisation and not very critical, really".

NRVO is not just "an optimisation". It's actually one that is explcitly
permitted to change observable behaviour of the program and it's extremely
powerful.

And it it _required_ for performant C++. Just try to return a std::vector by
value to see the importance of this optimisation. This is not missed
optimisation. This is premature pessimisation.

You could just as well stop all optimisation work for the C++ frontend until
this is implemented, because any other optimisation effords are dwarfed by the
overhead when NRVO is expected by the developer but not applied.

Please make this a top priority. Every C++ program will benefit both in text
size and in runtime performance - dramatically.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Marc Glisse <glisse at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |b7.10110111 at gmail dot com

--- Comment #6 from Marc Glisse <glisse at gcc dot gnu.org> ---
*** Bug 96065 has been marked as a duplicate of this bug. ***

[Bug c++/51571] No named return value optimization while adding a dummy scope

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |barry.revzin at gmail dot com

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
*** Bug 97562 has been marked as a duplicate of this bug. ***

[Bug c++/51571] No named return value optimization while adding a dummy scope

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

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

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

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
See bug 97562 for a nice testcase.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Jason Merrill <jason at gcc dot gnu.org> changed:

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

[Bug c++/51571] No named return value optimization while adding a dummy scope

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

--- Comment #9 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:7e0b65b239c3a0d68ce94896b236b03de666ffd6

commit r14-1593-g7e0b65b239c3a0d68ce94896b236b03de666ffd6
Author: Jason Merrill <jason@redhat.com>
Date:   Sun Jun 4 12:09:11 2023 -0400

    c++: enable NRVO from inner block [PR51571]

    Our implementation of the named return value optimization has been limited
    to variables declared in the outermost block of the function, to avoid
    needing to handle the case where the variable needs to be destroyed due to
    going out of scope.  PR92407 pointed out a case we were missing, where the
    variable goes out of scope due to a goto and we were failing to destroy it.

    It occurred to me that this problem is the flip side of PR33799, where we
    need to be sure to destroy the return value if a cleanup throws on return;
    here we want to avoid destroying the return value when exiting the
    variable's scope on return.  We can use the same flag to indicate to both
    cleanups that we're returning.

    This implements the guaranteed copy elision specified by P2025 (which is
not
    yet part of the draft standard).

            PR c++/51571
            PR c++/92407

    gcc/cp/ChangeLog:

            * decl.cc (finish_function): Simplify NRV handling.
            * except.cc (maybe_set_retval_sentinel): Also set if NRV.
            (maybe_splice_retval_cleanup): Don't add the cleanup region
            if we don't need it.
            * semantics.cc (nrv_data): Add simple field.
            (finalize_nrv): Set it.
            (finalize_nrv_r): Check it and retval sentinel.
            * cp-tree.h (finalize_nrv): Adjust declaration.
            * typeck.cc (check_return_expr): Remove named_labels check.

    gcc/testsuite/ChangeLog:

            * g++.dg/opt/nrv23.C: New test.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |14.0

--- Comment #10 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed for GCC 14.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

--- Comment #11 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to CVS Commits from comment #9)
>     This implements the guaranteed copy elision specified by P2025

Or not; I just noticed that P2025 also requires a fix for PR53637.

[Bug c++/51571] No named return value optimization while adding a dummy scope

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|14.0                        |---

[Bug c++/51571] No named return value optimization while adding a dummy scope

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51571

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |14.0
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #12 from Jason Merrill <jason at gcc dot gnu.org> ---
But this particular issue is indeed fixed for 14.