是否应该将libc ++协程的suspend_always :: await_suspend“导出”到c ++ experimental.lib？

Question

I've got this error when compile coroutine against libc++ that I built

"C:\\Program Files\\LLVM\\bin\\lld-link" ... c++.lib c++experimental.lib ...
lld-link: error: undefined symbol: __declspec(dllimport) public: void __cdecl std::experimental::coroutines_v1::suspend_always::await_suspend(class std::experimental::coroutines_v1::coroutine_handle<void>) const

Before that, I've built libc++ using -DLIBCXX_ENABLE_SHARED=YES -DLIBCXX_ENABLE_STATIC=NO -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=YES

set LIB=C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.20.27508\lib\x64;C:\Program Files (x86)\Windows Kits\10\Lib\10.0.17763.0\ucrt\x64;C:\Program Files (x86)\Windows Kits\10\Lib\10.0.17763.0\um\x64
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_MAKE_PROGRAM="C:/Program Files (x86)/Ninja/ninja.exe" -DCMAKE_SYSTEM_NAME=Windows -DCMAKE_C_COMPILER="C:/Program Files/LLVM/bin/clang-cl.exe" -DCMAKE_C_FLAGS="-fms-compatibility-version=19.20.27508 --target=x86_64-pc-windows-msvc19.20.27508" -DCMAKE_CXX_COMPILER="C:/Program Files/LLVM/bin/clang-cl.exe" -DCMAKE_CXX_FLAGS="-fms-compatibility-version=19.20.27508 --target=x86_64-pc-windows-msvc19.20.27508" -DCMAKE_C_LINK_EXECUTABLE="C:/Program Files/LLVM/bin/lld-link.exe" -DCMAKE_CXX_LINK_EXECUTABLE="C:/Program Files/LLVM/bin/lld-link.exe" -DLLVM_PATH="C:/Program Files/LLVM" -DLIBCXX_ENABLE_SHARED=YES -DLIBCXX_ENABLE_STATIC=NO -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=YES "C:/libcxx-master"
ninja -j 2

So I tried nm c++.lib and nm c++experimental.lib . I found no trace of suspend_always .

It seems suspend_always::await_suspend is missing in the dll. So I delve in some c++ macro.

In C:/libcxx-master/CMakeLists.txt , there is _LIBCPP_BUILDING_LIBRARY defined.

add_definitions(-D_LIBCPP_BUILDING_LIBRARY)

In C:/libcxx-master/include/__config , we have

#if defined(__ELF__)
#  define _LIBCPP_OBJECT_FORMAT_ELF   1
#elif defined(__MACH__)
#  define _LIBCPP_OBJECT_FORMAT_MACHO 1
#elif defined(_WIN32)
#  define _LIBCPP_OBJECT_FORMAT_COFF  1
#elif defined(__wasm__)
#  define _LIBCPP_OBJECT_FORMAT_WASM  1
#else
#  error Unknown object file format
#endif

I'm in _WIN32 so we have _LIBCPP_OBJECT_FORMAT_COFF well defined .
Further downward, we have

#if defined(_LIBCPP_OBJECT_FORMAT_COFF)

#ifdef _DLL
#  define _LIBCPP_CRT_FUNC __declspec(dllimport)
#else
#  define _LIBCPP_CRT_FUNC
#endif

#if defined(_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS)
#  define _LIBCPP_DLL_VIS
#  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
#  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
#  define _LIBCPP_OVERRIDABLE_FUNC_VIS
#  define _LIBCPP_EXPORTED_FROM_ABI
#elif defined(_LIBCPP_BUILDING_LIBRARY)
#  define _LIBCPP_DLL_VIS __declspec(dllexport)
#  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
#  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
#  define _LIBCPP_OVERRIDABLE_FUNC_VIS _LIBCPP_DLL_VIS
#  define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllexport)
#else
#  define _LIBCPP_DLL_VIS __declspec(dllimport)
#  define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
#  define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
#  define _LIBCPP_OVERRIDABLE_FUNC_VIS
#  define _LIBCPP_EXPORTED_FROM_ABI __declspec(dllimport)
#endif

#define _LIBCPP_TYPE_VIS            _LIBCPP_DLL_VIS
#define _LIBCPP_FUNC_VIS            _LIBCPP_DLL_VIS
#define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS
#define _LIBCPP_HIDDEN
#define _LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS
#define _LIBCPP_TEMPLATE_VIS
#define _LIBCPP_ENUM_VIS

#endif // defined(_LIBCPP_OBJECT_FORMAT_COFF)

This means when we are building libc++, _LIBCPP_TYPE_VIS becomes __declspec(dllexport) . So we can export concrete types to DLL

When we compile code, _LIBCPP_TYPE_VIS becomes __declspec(dllimport) . So we can import concrete types from DLL

And experimental/coroutine file includes experimental/__config which includes __config and contains this struct suspend_always definition

struct _LIBCPP_TYPE_VIS suspend_always {
  _LIBCPP_INLINE_VISIBILITY
  bool await_ready() const _NOEXCEPT { return false; }
  _LIBCPP_INLINE_VISIBILITY
  void await_suspend(coroutine_handle<>) const _NOEXCEPT {}
  _LIBCPP_INLINE_VISIBILITY
  void await_resume() const _NOEXCEPT {}
};

So when we build libc++, we have struct __declspec(dllexport) suspend_always . When we compile, we have struct __declspec(dllimport) suspend_always

I even tried hardcoding struct __declspec(dllexport) suspend_always when building libc++ , the result is still the same.

---

Then I came across an idea. I create libcxx-master/src/experimental/coroutine and just make it #include <experimental/coroutine> . Rebuild libc++.dll and it turns out that working fine this time.

My question is whether struct suspend_always (and suspend_never ) is really supposed to be exported to c++experimental.lib ?

Where can I report this to the implementors?

Answer 1

Seems like a bug. It should be fixed in r358551.