[cfe-dev] VTables in C++98 versus C++11

[Reid Kleckner via cfe-dev]

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.

On Tue, Dec 20, 2016 at 10:48 AM, Robinson, Paul via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
>
> > -----Original Message-----
> > From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of
> > Robinson, Paul via cfe-dev
> > Sent: Monday, December 19, 2016 4:13 PM
> > To: cfe-dev at 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.
>
> >
> > Thanks,
> > --paulr
> >
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