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

[Robinson, Paul via cfe-dev]

> -----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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