[Bug c++/11836] New: gdb cannot access variables that are subject to return value optimization

[Bug c++/11836] New: gdb cannot access variables that are subject to return value optimization

[andre dot poenitz at nokia dot com]

#include <string>

std::string foo()
{
    std::string s;
    s = "hello";
    s += "world";
    return s;
}

std::string bar()
{
    std::string s;
    s = "hello";
    s += "world";
    return "something else";
}

int main()
{
    int a = foo().size();
    int b = bar().size();
    return a + b;
}

/*

Gcc seems to apply return value optimization (even at -O0) but there is
some problem with either the generated debug information or the way
gdb handles it. 

To reproduce it, save this file here as rvo.cpp and run

    g++ -g -O0 rvo.cpp -o rvo 

    gdb -ex 'set confirm off' -ex 'file ./rvo' \
        -ex 'b rvo.cpp:8' -ex 'b rvo.cpp:16' \
        -ex 'run' -ex 'print s'  \
        -ex 'cont' -ex 'print s' \
        -ex 'quit'


The result will look like:

    Breakpoint 1, foo () at rvo.cpp:8
    8           s += "world";
    $1 = {static npos = <optimized out>, 
      _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>>
    = {<No daa fields>}, <No data fields>}, _M_p = 0x1 <Address 0x1 out of
    bounds>}}

    Breakpoint 2, bar () at rvo.cpp:16
    16          s += "world";
    $2 = {static npos = <optimized out>, 
      _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>>
    = {<No data fields>}, <No data fields>}, _M_p = 0x804b014 "hello"}}

    Program exited with code 030.

RVO seems to be applied in the first case with "dangling" debug information.
If no RVO is used (second case), all is fine.


*/

-- 
           Summary: gdb cannot access variables that are subject to return
                    value optimization
           Product: gdb
           Version: 7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at sourceware dot org
        ReportedBy: andre dot poenitz at nokia dot com
                CC: gdb-prs at sourceware dot org
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu


http://sourceware.org/bugzilla/show_bug.cgi?id=11836

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[Bug c++/11836] gdb cannot access variables that are subject to return value optimization

[andre dot poenitz at nokia dot com]

------- Additional Comments From andre dot poenitz at nokia dot com  2010-07-23 10:48 -------
Created an attachment (id=4882)
 --> (http://sourceware.org/bugzilla/attachment.cgi?id=4882&action=view)
Test code and build instructions


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[Bug c++/11836] gdb cannot access variables that are subject to return value optimization

[swagiaal at redhat dot com]

------- Additional Comments From swagiaal at redhat dot com  2010-07-23 14:54 -------
Andre,

Thanks for the nice test case. 's' seems to have a location in the debug info
but it is either wrong or gdb is reading it wrong. I'll take a look at this when
I get a chance. If anyone else is interested, in the mean while, go for it :)

Sami

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |swagiaal at redhat dot com
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1


http://sourceware.org/bugzilla/show_bug.cgi?id=11836

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[Bug c++/11836] gdb cannot access variables that are subject to return value optimization

[andre dot poenitz at nokia dot com]

------- Additional Comments From andre dot poenitz at nokia dot com  2010-07-23 16:43 -------

It turns out it is known on the gcc side: 

  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44731

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http://sourceware.org/bugzilla/show_bug.cgi?id=11836

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[Bug c++/11836] gdb cannot access variables that are subject to return value optimization

[tromey at redhat dot com]

------- Additional Comments From tromey at redhat dot com  2010-07-23 16:46 -------
Possibly this gcc bug:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44731

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tromey at redhat dot com


http://sourceware.org/bugzilla/show_bug.cgi?id=11836

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