[Bug c++/13403] New: invalid vptr after gdb prints the return valueof a virtual function member on x86_64 system

["zbojthe at urdeso dot hu" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13403

             Bug #: 13403
           Summary: invalid vptr after gdb prints the return value of a
                    virtual function member on x86_64 system
           Product: gdb
           Version: 7.3
            Status: NEW
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned@sourceware.org
        ReportedBy: zbojthe@urdeso.hu
    Classification: Unclassified


Created attachment 6051
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6051
c++ code, build/run scripts, outputs

The virtual function getMyVal() of MyClass returns a MyVal object as value, and
MyVal object size is small, and MyVal object has copy constructor.

When I use "p myClass->getMyVal()", the gdb calls the copy ctor of MyVal badly,
and copy ctor overwrite the vptr of the myClass.

When I remove the copy ctor of MyVal, or I add some dummy members to MyVal, the
gdb will work correctly.

Tested in Ubuntu 11.10, 64 bit. (On Ubuntu 11.10, 32 bit, work correctly)

See attached files.

--8<--
a.cc:
class MyVal
{
    int val;
    // when added next line, the gdb print work correctly when dimension >= 4
    // int dummy[4];
  public:
    MyVal() { val = 42; }
    MyVal(int _val) { val = _val; }
    // when remove the next copy ctor, the gdb print work correctly
    MyVal(const MyVal& other) { val = other.val; }
};

class MyClassBase
{
  public:
    virtual MyVal getMyVal() = 0;
};

class MyClass : public MyClassBase
{
    MyVal myVal;
  public:
    virtual MyVal getMyVal() { return myVal; }
};

int main(int argc, const char **argv)
{
    MyClassBase *myClass = new MyClass();
    MyVal m;
    m = myClass->getMyVal();
    m = myClass->getMyVal();
}
--8<--
debugging:
gdb a.out
start
n
n
p *myClass
# {_vptr.MyClassBase = <valid_vptr_address>}
p myClass->getMyVal()
# {val = <address of myClass>}
p *myClass
# {_vptr.MyClassBase = 0x0}
p myClass->getMyVal()
#Cannot access memory at address 0x0

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