[llvm-bugs] [Bug 28997] New: "non-virtual thunk" abi_tag mangling is different from g++

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28997

            Bug ID: 28997
           Summary: "non-virtual thunk" abi_tag mangling is different from
                    g++
           Product: clang
           Version: 3.9
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: joubert.sy at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Hi,

This is an upstream bug report for the following Debian bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=834492

The new mangling behavior of clang++ for GCC abi_tag attribute seems to be
different from the g++ one on some edge cases.

Consider the following code:
$ cat classes.hpp
#include <string>

struct Mother
{
  virtual ~Mother() {};
  int bar;
};

struct Father
{
  virtual std::string get_foo()  = 0;
};

class Derived:
  public Mother,
  public Father
{
public:
  std::string get_foo();
};

$ cat classes.cpp
#include "classes.hpp"

std::string Derived::get_foo() { return ""; }

$ cat main.cpp
#include "classes.hpp"

struct Final:
  public Derived
{
};

int main()
{
  Final().get_foo();
}

When compiling the classes with clang++ and trying to link the executable with
g++ one gets the following link error:
$ clang++-3.9 -c classes.cpp && g++-5 main.cpp classes.o
/tmp/cchpctTt.o:main.cpp:vtable for Final : erreur : référence à « non-
virtual thunk to Derived::get_foo() » non définie
collect2: error: ld returned 1 exit status

And it seems this symbol is mangled differently by clang++ and g++:
$ clang++-3.9 -c classes.cpp && nm -C classes.o | grep get_foo
0000000000000000 T Derived::get_foo[abi:cxx11]()
0000000000000070 T non-virtual thunk to Derived::get_foo[abi:cxx11]()

$ g++-5 -c classes.cpp && nm -C classes.o | grep get_foo
0000000000000000 T Derived::get_foo[abi:cxx11]()
0000000000000065 T non-virtual thunk to Derived::get_foo()

$ g++-6 -c classes.cpp && nm -C classes.o | grep get_foo
0000000000000000 T Derived::get_foo[abi:cxx11]()
0000000000000065 T non-virtual thunk to Derived::get_foo()

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