[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|gcc-bugs at gcc dot gnu.org        |
           Assignee|unassigned at gcc dot gnu.org      |paolo.carlini at oracle dot com
   Target Milestone|---                         |4.9.0

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Mine.

[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958

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

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

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Fixed for 4.9.0.

[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |galdralag at bk dot ru

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
*** Bug 61935 has been marked as a duplicate of this bug. ***

[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |petschy at gmail dot com

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> ---
*** Bug 63657 has been marked as a duplicate of this bug. ***

[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Sat Nov 22 02:21:35 2014
New Revision: 217957

URL: https://gcc.gnu.org/viewcvs?rev=217957&root=gcc&view=rev
Log:
    PR c++/63657
    PR c++/38958
    * call.c (set_up_extended_ref_temp): Set TREE_USED on the reference
    if the temporary has a non-trivial destructor.
    * decl.c (poplevel): Don't look through references.

Added:
    trunk/gcc/testsuite/g++.dg/warn/Wunused-var-22.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/call.c
    trunk/gcc/cp/decl.c

[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Wed Jan  7 15:18:05 2015
New Revision: 219306

URL: https://gcc.gnu.org/viewcvs?rev=219306&root=gcc&view=rev
Log:
    PR c++/63657
    PR c++/38958
    * call.c (set_up_extended_ref_temp): Set TREE_USED on the reference
    if the temporary has a non-trivial destructor.
    * decl.c (poplevel): Don't look through references.

Added:
    branches/gcc-4_9-branch/gcc/testsuite/g++.dg/warn/Wunused-var-22.C
Modified:
    branches/gcc-4_9-branch/gcc/cp/ChangeLog
    branches/gcc-4_9-branch/gcc/cp/call.c
    branches/gcc-4_9-branch/gcc/cp/decl.c

[Bug c++/38958] New: 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[fs dot open dot work at googlemail dot com]

Given a class C with destructor side-effects (as is common with RAII classes)
and a function fn which returns a rvalue of type C

   C fn()

the lifetime of the rvalue returned by fn() can be extended by holding a
reference to const C with

   C const& r = fn()

Although r is not explicitly referenced subsequently in the calling scope, upon
leaving scope the rvalue's dtor will be invoked.  So r is implicitly 'used'.

The assignment
   C c = fn()
has the same effect because of return value optimization.  Though
copy-semantics must be checked, the copy is elided.

G++ generates an 'unused variable' warning for 'r' but not for 'c'.

The following program demonstrates the warning.  The 'volatile int g' and
forced-inline noise is to compare the generated assembly to that generated when
the const& is removed.  The output in both cases is equivalent (identical at
-O2; the only difference in lower optimizations is an additional indirection
with the const& present).

----------------------------------
volatile int g; // for obvious side-effect visibility in generated assembly

struct Lock
{
   __attribute__((always_inline)) // for obvious side-effect visibility in
generated assembly

   ~Lock() { g = 0; }
};

Lock AcquireLock() { return Lock(); }

int main()
{
   Lock const& lock = AcquireLock();
   g = 1;
   g = 2;
   g = 3;
}
----------------------------------
In both cases, const& present and not present, the sequence of values moved
into g are 1,2,3,0 as desired.

This might be related to Bug 10416 but does not appear to duplicate it since
removing the const& removes the warning in this case.

This warning can be avoided by relying on return-value optimization and using
the assignment without the const& to generate equivalent code but I don't think
it should be emitted at all if it is not for the RVO version.  I would expect
it emitted for both or not emitted for either.


-- 
           Summary: 'unused variable' warning emitted when extending the
                    lifetime of a returned RAII type by holding a reference
                    to const despite delayed destructor side-effects. [dtor]
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: fs dot open dot work at googlemail dot com
 GCC build triplet: i686-pc-linux-gnu, mingw
  GCC host triplet: i686-pc-linux-gnu, mingw
GCC target triplet: i686-pc-linux-gnu, mingw, arm-xscale-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958

[Bug c++/38958] 'unused variable' warning emitted when extending the lifetime of a returned RAII type by holding a reference to const despite delayed destructor side-effects. [dtor]

[bangerth at gmail dot com]

------- Comment #1 from bangerth at gmail dot com  2009-01-30 15:23 -------
Confirmed.


-- 

bangerth at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |diagnostic
   Last reconfirmed|0000-00-00 00:00:00         |2009-01-30 15:23:42
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38958