VTables in C++98 versus C++11

Clang Frontend
1

I'm working on making the Clang tests tolerate a default dialect of
C++11 (currently it is C++98). I ran into a funny situation with
test/CodeGenCXX/vtable-layout.cpp, which uses -fdump-vtable-layouts to
look at, well, vtable layouts.

Consider this section of the test:
//////////
namespace Test40 {
  struct A {
    virtual void foo() = 0;
  };
  struct B : public A {
    virtual void foo();
  };
  struct C: public B {
    virtual int f(int);
    virtual int f();
    virtual int g(int);
    virtual int g();
    virtual int h(int);
    virtual int h();
    virtual int i(int);
    virtual int i();
  };
  class D : C {};
  D d;
}
//////////

There is a significant difference in what gets dumped, depending on
the dialect. First let's look at what we get with C++98:

$ clang -cc1 t.cpp -triple=x86_64-apple-darwin10 -emit-llvm-only \
  -fdump-vtable-layouts -std=c++98 | grep entries
Vtable for 'Test40::D' (11 entries).
Vtable for 'Test40::C' (11 entries).
VTable indices for 'Test40::C' (8 entries).
Vtable for 'Test40::B' (3 entries).
VTable indices for 'Test40::B' (1 entries).
Vtable for 'Test40::A' (3 entries).
VTable indices for 'Test40::A' (1 entries).

That is, a Vtable for each class, and indices for everything except the
most-derived class.

Now look at what we get with C++11:

$ clang -cc1 t.cpp -triple=x86_64-apple-darwin10 -emit-llvm-only \
  -fdump-vtable-layouts -std=c++11 | grep entries
Vtable for 'Test40::D' (11 entries).

That is, all of the base-class information is not present.

Is this a "correct" difference between C++98 and C++11?
If so, then I can just skip this test for the C++11 case.
But it seemed like a pretty weird thing to be different.

Thanks,
--paulr

2

From: cfe-dev [mailto:cfe-dev-bounces@lists.llvm.org] On Behalf Of
Robinson, Paul via cfe-dev
Sent: Monday, December 19, 2016 4:13 PM
To: cfe-dev@lists.llvm.org
Subject: [cfe-dev] VTables in C++98 versus C++11

I'm working on making the Clang tests tolerate a default dialect of
C++11 (currently it is C++98). I ran into a funny situation with
test/CodeGenCXX/vtable-layout.cpp, which uses -fdump-vtable-layouts to
look at, well, vtable layouts.

Consider this section of the test:
//////////
namespace Test40 {
  struct A {
    virtual void foo() = 0;
  };
  struct B : public A {
    virtual void foo();
  };
  struct C: public B {
    virtual int f(int);
    virtual int f();
    virtual int g(int);
    virtual int g();
    virtual int h(int);
    virtual int h();
    virtual int i(int);
    virtual int i();
  };
  class D : C {};
  D d;
}
//////////

There is a significant difference in what gets dumped, depending on
the dialect. First let's look at what we get with C++98:

$ clang -cc1 t.cpp -triple=x86_64-apple-darwin10 -emit-llvm-only \
  -fdump-vtable-layouts -std=c++98 | grep entries
Vtable for 'Test40::D' (11 entries).
Vtable for 'Test40::C' (11 entries).
VTable indices for 'Test40::C' (8 entries).
Vtable for 'Test40::B' (3 entries).
VTable indices for 'Test40::B' (1 entries).
Vtable for 'Test40::A' (3 entries).
VTable indices for 'Test40::A' (1 entries).

That is, a Vtable for each class, and indices for everything except the
most-derived class.

Now look at what we get with C++11:

$ clang -cc1 t.cpp -triple=x86_64-apple-darwin10 -emit-llvm-only \
  -fdump-vtable-layouts -std=c++11 | grep entries
Vtable for 'Test40::D' (11 entries).

That is, all of the base-class information is not present.

Is this a "correct" difference between C++98 and C++11?
If so, then I can just skip this test for the C++11 case.
But it seemed like a pretty weird thing to be different.

If I define one of C's virtual functions, then we emit the
vtable as expected for C++11 too. I guess I'll go with that.

3

I think the difference here is that D’s implicit default constructor becomes constexpr by default in C++11, so there’s no dynamic initialization.

In C++98, we would have a dynamic initializer that calls D::D(), which would call C::C(), and so on, referencing all vtables as we go. In C++11, we can statically initialize ‘d’ without calling C::C(), and without referencing C’s vtable at all.