Expect DLL builds on Windows by default and require a custom __config for static

builds.

On Windows the __declspec(dllimport) and __declspec(dllexport) attributes
require linking to a DLL, not a static library. Previously these annotations
were disabled by default unless _LIBCPP_DLL was defined. However the DLL
configuration is probably the more common one, so it should be supported by
default.

This patch enables import/export attributes by default and adds a
_LIBCPP_DISABLE_DLL_IMPORT_EXPORT macro which can be used to disable this
behavior. If libc++ is built as a static library on Windows then a custom __config
header will be generated that predefines this macro.

This patch is based off work by Shoaib Meenai.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@282449 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -330,6 +330,8 @@ endif()
 # headers
 add_compile_flags_if_supported(-nostdinc++)
 
+# Let the library headers know they are currently being used to build the
+# library.
 add_definitions(-D_LIBCPP_BUILDING_LIBRARY)
 
 # Warning flags ===============================================================
@@ -455,6 +457,14 @@ config_define_if(LIBCXX_HAS_PTHREAD_API _LIBCPP_HAS_THREAD_API_PTHREAD)
 config_define_if(LIBCXX_HAS_EXTERNAL_THREAD_API _LIBCPP_HAS_THREAD_API_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 
+# By default libc++ on Windows expects to use a shared library, which requires
+# the headers to use DLL import/export semantics. However when building a
+# static library only we modify the headers to disable DLL import/export.
+if (DEFINED WIN32 AND LIBCXX_ENABLE_STATIC AND NOT LIBCXX_ENABLE_SHARED)
+  message(STATUS "Generating custom __config for non-DLL Windows build")
+  config_define(ON _LIBCPP_DISABLE_DLL_IMPORT_EXPORT)
+endif()
+
 if (LIBCXX_NEEDS_SITE_CONFIG)
   configure_file(
     include/__config_site.in

include/__config

@@ -519,20 +519,20 @@ namespace std {
 
 
 #ifdef _WIN32
-// only really useful for a DLL. _LIBCPP_DLL should be a compiler builtin define ideally...
+#if defined(_LIBCPP_DISABLE_DLL_IMPORT_EXPORT)
-#if defined(_LIBCPP_DLL) && defined(cxx_EXPORTS)
-# define _LIBCPP_DLL_VIS __declspec(dllexport)
-# define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
-# define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS _LIBCPP_DLL_VIS
-#elif defined(_LIBCPP_DLL)
-# define _LIBCPP_DLL_VIS __declspec(dllimport)
-# define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
-# define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
-#else
 # define _LIBCPP_DLL_VIS
 # define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS
 # define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
+#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
+#else
+# define _LIBCPP_DLL_VIS __declspec(dllimport)
+# define _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS _LIBCPP_DLL_VIS
+# define _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS
 #endif
+
 #define _LIBCPP_TYPE_VIS            _LIBCPP_DLL_VIS
 #define _LIBCPP_FUNC_VIS            _LIBCPP_DLL_VIS
 #define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS

include/__config_site.in

@@ -21,5 +21,6 @@
 #cmakedefine _LIBCPP_HAS_MUSL_LIBC
 #cmakedefine _LIBCPP_HAS_THREAD_API_PTHREAD
 #cmakedefine _LIBCPP_HAS_THREAD_API_EXTERNAL
+#cmakedefine _LIBCPP_DISABLE_DLL_IMPORT_EXPORT
 
 #endif // _LIBCPP_CONFIG_SITE