[cfe-dev] A question about C++ mangling for a complicated template case

[Richard Smith]

On Wed, Oct 22, 2014 at 3:58 AM, Jiangning Liu <liujiangning1 at gmail.com>
wrote:

> Hi,
>
> I have a question about C++ name mangling.
>
> For the following small program (I failed to get it further minimized),
> clang++ and g++ generate different symbols for function fin introduced by
> the last specialization statement.
>
> clang++: AA<3>::aa BB::fin<3, AA>(AA const&)
> g++:        AA<3>::aa BB::fin<3, AA>(AA<3> const&)
>

Demangled names are not useful when talking about mangling differences. The
mangled names are:

  _ZN2BB3finILi3E2AAEENT0_IXT_EE2aaERKS2_ (from clang)
  _ZN2BB3finILi3E2AAEENT0_IXT_EE2aaERKS3_ (from gcc)

GCC's mangling appears to be correct here (modulo a bug in the ABI that
gives a mangling to an empty <prefix>). S2_ refers to T0_ from the
nested-name-specifier in the return type; S3_ refers to T0_IXT_EE, which is
what we want in the parameter's type.


> If I change the return type of fin to be void, I can see the argument
> AA<3> can be correctly generated for clang++.
>
> {code}
> template <int dim>
>
> class AA
> {
>     template <class DH> struct Iter;
>
>     template <template <int> class DH>
>     struct Iter<DH<3> >
>     {
>         typedef DH<3> AA_type;
>     };
>
>     typedef Iter<AA<dim> > IteratorSelector;
> public:
>     typedef typename IteratorSelector::AA_type aa;
> };
>
> class BB
> {
>     template <int dim, template<int> class C>
>     static typename C<dim>::aa fin (const C<dim> &container);
>
>     template <int dim>
>     static void pp (const AA<dim>& dof)
>     {
>         typename AA<dim>::aa temp = fin (dof);
>     }
> };
>
> template void BB::pp<3> (const AA<3>&);
> {code}
>
> So anybody know why?
>
> This problem will cause failure of linking object files generated by
> clang++ and g++ separately, and potentially introduce run-time bugs.
>
> Thanks,
> -Jiangning
>
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>
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