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[libc++] Fix extern template visibility for Windows

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On Windows, marking an `extern template class` declaration as exported
actually forces an instantiation, which is not the desired behavior.
Instead, the actual explicit instantiations need to be exported.

Differential Revision: https://reviews.llvm.org/D24679

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@281925 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Shoaib Meenai
2016-09-19 18:29:07 +00:00
parent 7d7f3cc9b1
commit e5cbce437b
5 changed files with 85 additions and 56 deletions
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20
docs/DesignDocs/VisibilityMacros.rst
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@@ -71,6 +71,26 @@ Visibility Macros
However since `_LIBCPP_TYPE_VIS_ONLY` is the same as `_LIBCPP_TYPE_VIS` the
visibility is already correct. The macro has an empty definition with GCC.
**Windows Behavior**: `extern template` and `dllexport` are fundamentally
incompatible *on a template class* on Windows; the former suppresses
instantiation, while the latter forces it. Specifying both on the same
declaration makes the template class be instantiated, which is not desirable
inside headers. This macro therefore expands to `dllimport` outside of libc++
but nothing inside of it (rather than expanding to `dllexport`); instead, the
explicit instantiations themselves are marked as exported. Note that this
applies *only* to extern template *classes*. Extern template *functions* obey
regular import/export semantics, and applying `dllexport` directly to the
extern template declaration is the correct thing to do for them.
**_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS**
Mark the member functions, typeinfo, and vtable of an explicit instantiation
of a class template as being exported by the libc++ library. This attribute
must be specified on all template class explicit instantiations.
It is only necessary to mark the explicit instantiation itself (as opposed to
the extern template declaration) as exported on Windows, as discussed above.
On all other platforms, this macro has an empty definition.
**_LIBCPP_EXTERN_TEMPLATE_INLINE_VISIBILITY**
Mark a member function of a class template as hidden and inline except when
building the libc++ library where it marks the symbol as being exported by