[Bug debug/46815] New: Invalid DW_AT_location for a retval instance of a class that has a virtual function

[Bug debug/46815] New: Invalid DW_AT_location for a retval instance of a class that has a virtual function

[siddhesh.poyarekar at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46815

           Summary: Invalid DW_AT_location for a retval instance of a
                    class that has a virtual function
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: siddhesh.poyarekar@gmail.com


Created attachment 22649
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22649
Reproducer program to run under gdb

Problem Description:

When a local object is created from a class with a virtual function and that
object is also the retval for the function it is the local variable in, the
debuginfo generated for the object indicates that it is located in a register,
while in reality, its address is located in the register.

Steps to Reproduce:

$ cat > rep.cpp
class Sequence {
public:
  virtual void foo(void);
};

Sequence getSequence()
{
  Sequence data;
  return data;
}

$ g++ -g -c rep.cpp
$ objdump -W rep.o | less
...
 <3><c4>: Abbrev Number: 13 (DW_TAG_variable)
    <c5>   DW_AT_name        : (indirect string, offset: 0x43): data    
    <c9>   DW_AT_decl_file   : 1        
    <ca>   DW_AT_decl_line   : 8        
    <cb>   DW_AT_type        : <0x31>   
    <cf>   DW_AT_location    : 1 byte block: 53         (DW_OP_reg3)
...

Additional Information:

Attached reproducer fullrep.cpp can be used to demonstrate this problem as seen
in gdb.

1) Build fullrep.cpp: gcc -g fullrep.cpp
2) Run under gdb:

[siddhesh@spoyarek gcc-foo]$ gdb -q ./a.out
Reading symbols from /home/siddhesh/gcc-foo/a.out...done.
(gdb) break getSequence
Breakpoint 1 at 0x4007e0: file fullrep.cpp, line 14.
(gdb) r
Starting program: /home/siddhesh/gcc-foo/a.out 

Breakpoint 1, getSequence () at fullrep.cpp:14
14      Sequence data;
(gdb) p data
$1 = {_vptr.Sequence = 0x7fffffffdfd0, i = 4}
(gdb) p &data
Address requested for identifier "data" which is in register $rbx
(gdb) p *((Sequence *)$rbx)
$2 = {_vptr.Sequence = 0x7fffffffe0c0, i = 0}
(gdb)

[Bug debug/46815] Invalid DW_AT_location for a retval instance of a class that has a virtual function

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46815

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2010-12-06 15:39:51 UTC ---
Created attachment 22660
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22660
gcc46-pr46815.patch

Ugh.  So what happens is that we set DECL_VALUE_EXPR of the NRV optimized var
to RESULT_DECL, but then in cp_genericize as it is addressable change the
RESULT_DECL into DECL_BY_REFERENCE decl and convert_from_reference whenever it
is seen in the stmts (but don't adjust the DECL_VALUE_EXPR of the var).
Which means we end up using DW_OP_reg3 instead of DW_OP_breg3 0.

Either we can do something like this patch, i.e. replace DECL_VALUE_EXPR during
genericization, or we could do something similar, but faster by saving pointer
to the NRV optimized VAR_DECL somewhere in the lang specific stuff for the
FUNCTION_DECL, or, if we can be 100% sure by finalize_nrv time that
non-addressable RESULT_DECL isn't ever going to turn into TREE_ADDRESSABLE
RESULT_DECL, we could assume early what cp_genericize will do and set
DECL_VALUE_EXPR in finalize_nrv to INDIRECT_REF of the RESULT_DECL (which would
mean somewhat invalid tree until cp_genericize is run, but perhaps nothing
would notice), or we could teach dwarf2out.c if it uses DECL_VALUE_EXPR and it
points to DECL_BY_REFERENCE decl that it should add indirection automatically
(but then, the question is what all will actually break by that).

Jason, any preferences?

[Bug debug/46815] Invalid DW_AT_location for a retval instance of a class that has a virtual function

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46815

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> 2010-12-13 16:03:22 UTC ---
Let's go ahead with this patch.

[Bug debug/46815] Invalid DW_AT_location for a retval instance of a class that has a virtual function

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46815

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2010-12-15 17:50:40 UTC ---
Author: jakub
Date: Wed Dec 15 17:50:34 2010
New Revision: 167865

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=167865
Log:
    PR debug/46815
    * cp-gimplify.c (cp_genericize): When changing RESULT_DECL
    into invisible reference, change also DECL_VALUE_EXPR of
    NRV optimized variable.

    * g++.dg/guality/pr46815.C: New test.

Added:
    trunk/gcc/testsuite/g++.dg/guality/pr46815.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/cp-gimplify.c
    trunk/gcc/testsuite/ChangeLog

[Bug debug/46815] Invalid DW_AT_location for a retval instance of a class that has a virtual function

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46815

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

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

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2010-12-15 22:01:34 UTC ---
Fixed.