[Bug c++/68738] New: call to overridden function segfaults

[rianquinn at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68738

            Bug ID: 68738
           Summary: call to overridden function segfaults
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rianquinn at gmail dot com
  Target Milestone: ---

Using the TARGET=elf-x86_64 compiler (OS development), I get a strange crash
with C++. The class definition is as follows:

class Blah1
{
public:
    Blah1() {}
    virtual ~Blah1() {}

    virtual int foo() { return 0; }
};

class Blah2 : public Blah1
{
public:
    Blah2() {}
    ~Blah2() {}

    int boo() { return 1; }
    int foo() override { return 1; }
};

Blah2 g_blah2;

int do_something()
{
    Blah2 *p_blah2 = &g_blah2;
    int i = p_blah2->foo();      // <----- crash here
}

The compiled assembly for this looks something like:

 c68:   48 89 45 e8             mov    %rax,-0x18(%rbp)
 c6c:   48 8b 45 e8             mov    -0x18(%rbp),%rax
 c70:   48 8b 00                mov    (%rax),%rax
 c73:   48 83 c0 10             add    $0x10,%rax
 c77:   48 8b 00                mov    (%rax),%rax
 c7a:   48 8b 55 e8             mov    -0x18(%rbp),%rdx
 c7e:   48 89 d7                mov    %rdx,%rdi
 c81:   ff d0                   callq  *%rax

What's strange to me is it's not attempting to lookup the global symbol from
the GOT. If I change the code to:

int do_something()
{
    Blah2 &p_blah2 = g_blah2;
    int i = p_blah2.foo();      // <----- works fine
}

And the compiled assembly looks like:

ca3:    e8 88 fe ff ff          callq  b30 <_ZN5Blah23fooEv@plt>

Which has the GOT lookup like you would expect. Not sure what's going on here,
but it seems like a bug with G++. 

Thanks,
- Rian