A question about C++ mangling for a complicated template case

[Jiangning Liu]

Hi Richard,

Thanks a lot for your feedback!

Now I can generate an even smaller test case, so I filed a but at
http://llvm.org/bugs/show_bug.cgi?id=21351.

The small test case is like

{code}
emplate <int dim> class AA { public: AA(){} };


class BB
{
public:
    template <template<int> class C>
    static C<3> fin (const C<3> &);
};

AA<3> f(const AA<3> &dof)
{
    return BB::fin(dof);
}
{code}

I'm not familiar with clang, so what is the term or symbol T0_IXT_EE ? I
can't find it in clang source code. Can you explain more? I want to get
this bug fixed, and it is blocking my benchmarking work.

Thanks,
-Jiangning

2014-10-23 5:20 GMT+08:00 Richard Smith <richard at metafoo.co.uk>:

> 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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>> cfe-commits at cs.uiuc.edu
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>>
>>
>
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