Automatic generation of method export file for shared libraries

[tonerdo]

At this time when building a shared library on Windows (.dll) you have to specify the path to a handcrafted .def file. The compiler should automatically generate a def file on Windows for all methods marked with the NativeCallable attributes

Parent issue: #1285

[jkotas]

automatically generate a def file on Windows for all methods

All methods in user supplied assemblies. NativeCallable entry points used internally by the implementation should be excluded.

[tonerdo]

Working on this first

[jkotas]

BTW: The non-Windows platforms have .def file equivalent as well. It is passed to the linker using -exported_symbols_list on Mac or --version-script on Linux. E.g. we use it when building coreclr.dll to control what gets exported: https://github.com/dotnet/coreclr/blob/master/src/dlls/mscoree/coreclr/CMakeLists.txt#L42

[tonerdo]

Will take a look, thanks

[tonerdo]

Currently we don't need to specify a definition file on Unix for NativeCallable methods because all symbols are exported by default. Are you looking at exported_symbols_list and --version-script as a means to export ONLY NativeCallable methods that the developer defines?

[jkotas]

Yes, either that or making the CoreRT symbols hidden by default - emit the CoreRT symbols on Unix with the same set of flags as clang emits them with -fvisibility=hidden.

Exporting all symbols by default bloats the binary a lot. This is primarily a binary size optimization, a pretty significant one I think.