[PATCH] Add full semantic support for dllimport/export attributes

Nico Rieck nico.rieck at gmail.com
Fri Jul 5 18:13:55 PDT 2013

This is the first patch in a series to get full support for the dllexport/dllimport attributes and handles the semantic side.

In particular it:
- Adds support for class-wide export/import attributes (PR11170)
- Handles dllexport/dllimport on inline functions and templates
- Extends tests for globals and functions.

It deviates from MSVC's behavior in the following cases:
- If a dllimported global variable is redeclared without dllimport, the attribute is ignored with a warning. GCC does the same, MSVC makes the global dllexport.
- Out-of-line member function definitions of dllimported class templates are allowed if the definition is declared as inline. MSVC allows this for classes, but not for class templates.

In some cases a dllimport/dllexport attribute is ignored. If this happens on
a member of an exported/imported class, we need a way to drop the attribute.
This is implemented by propagating these attributes to every member during semantic analysis. To differentiate them from explicit member attributes, they have their inherited status set.
To make this work a small change (or possibly fix?) to table-gen was required so that the inherited status of an attribute is preserved when instantiating template attributes.

The semantics this patch should cover are:

1. dllexport and dllimport can be applied to functions, static data
   members and global variables. Both can also be applied to typedefs
   and enums (without effect) if -fms-extensions is specified, for all
   other cases the attributes are ignored and a warning is issued. It is
   an error to apply them to non-static data members.

2. dllexport requires a definition. The extern keyword can be used to
   force a declaration.

3. dllimport requires a declaration.

4. dllexport exposes the function or variable with its decorated name.
  a) For C++, this includes name mangling.
  b) For C or extern "C" functions, this includes platform-specific
     decoration for the calling convention.

5. dllexport takes precedence over dllimport. If both are specified,
   dllimport is ignored and a warning is issued.

6. Classes
  a) When applied to a class, dllexport or dllimport affect all static
     and non-static member functions and all static data members,
     including all non-deleted special members.
  b) Everything that is accessible to the DLL's client according to C++
     access rules is exported (including private data members referenced
     in inline functions).
  c) Members can be imported or exported selectively.
  d) Explicit dllexport/dllimport attributes on members of exported/
     imported classes are forbidden.
  e) Base classes of an exported class should be exported. Emits a
     warning otherwise. (-Wmissing-dllexport)
  f) Virtual tables and typeinfo follow the emission rules of the
     Itanium ABI, i.e. for an exported/imported dynamic class with key
     function, the needed globals are exported/imported. If there is no
     key function, they are emitted everywhere and are not exported/

7. Inline functions
  a) dllexport can be applied to inline function definitions. The
     function is always instantiated and exported, presumed to be
     imported by another program.
  b) dllimport can be applied to inline function definitions. Such
     functions can be expanded, but never instantiated.
  c) C99 inline functions cannot be exported or imported.

8. Templates
  a) If a class template is exported, explicit and implicit
     instantiations are also exported.
  b) Non-exported/imported class templates can be exported/imported by
     decorating an explicit instantiation.
  c) Base classes that are template specializations must be instantiated
     explicitly to export them.
  d) If a class is marked as dllexport, any specializations of class
     templates in the class hierarchy are implicitly marked as
     dllexport. This means that class templates are explicitly
     instantiated and the class's members must be defined.


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