stdlib: be more DLL friendly on Windows

When the standard library is built dynamically on COFF targets, the public
interfaces must be decorated in order to generate a proper DLL which can be
confused by the dependent libraries.  When the exported interface is used, it
must be indirectly addressed.  This can be done manually in code or the MS
extension of `__declspec(dllimport)` may be used to indicate to the compiler
that this symbol be addressed indirectly.  This permits building more pieces of
the standard library dynamically on Windows.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -1237,6 +1237,19 @@ function(add_swift_library name)
         elseif("${sdk}" STREQUAL "WATCHOS" OR "${sdk}" STREQUAL "WATCHOS_SIMULATOR")
           list(APPEND swiftlib_swift_compile_flags_all
               ${SWIFTLIB_SWIFT_COMPILE_FLAGS_WATCHOS})
+        elseif("${sdk}" STREQUAL "WINDOWS")
+          # FIXME(SR2005) static and shared are not mutually exclusive; however
+          # since we do a single build of the sources, this doesnt work for
+          # building both simultaneously.  Effecitvely, only shared builds are
+          # supported on windows currently.
+          if(SWIFTLIB_SHARED)
+            list(APPEND swiftlib_swift_compile_flags_all -D_USRDLL)
+            if(SWIFTLIB_IS_STDLIB_CORE)
+              list(APPEND swiftlib_swift_compile_flags_all -DswiftCore_EXPORTS)
+            endif()
+          elseif(SWIFTLIB_STATIC)
+            list(APPEND swiftlib_swift_compile_flags_all -D_LIB)
+          endif()
         endif()
 
         # Add this library variant.

stdlib/public/SwiftShims/Visibility.h

@@ -63,10 +63,10 @@
 # if defined(__CYGWIN__)
 #  define SWIFT_RUNTIME_EXPORT
 # else
-#  if defined(__SWIFT_CURRENT_DYLIB)
+#  if defined(swiftCore_EXPORTS)
 #   define SWIFT_RUNTIME_EXPORT __declspec(dllexport)
 #  else
-#   define SWIFT_RUNTIME_EXPORT
+#   define SWIFT_RUNTIME_EXPORT __declspec(dllimport)
 #  endif
 # endif
 #endif

stdlib/public/runtime/CMakeLists.txt

@@ -67,7 +67,7 @@ add_swift_library(swiftRuntime OBJECT_LIBRARY TARGET_LIBRARY
   ${swift_runtime_sources}
   ${swift_runtime_objc_sources}
   ${swift_runtime_leaks_sources}
-  C_COMPILE_FLAGS ${swift_runtime_compile_flags}
+  C_COMPILE_FLAGS ${swift_runtime_compile_flags} -DswiftCore_EXPORTS
   INSTALL_IN_COMPONENT never_install)
 
 set(ELFISH_SDKS)

stdlib/public/stubs/CMakeLists.txt

@@ -22,7 +22,7 @@ add_swift_library(swiftStdlibStubs OBJECT_LIBRARY TARGET_LIBRARY
   UnicodeExtendedGraphemeClusters.cpp.gyb
   ${swift_stubs_objc_sources}
   ${swift_stubs_unicode_normalization_sources}
-  C_COMPILE_FLAGS ${SWIFT_CORE_CXX_FLAGS}
+  C_COMPILE_FLAGS ${SWIFT_CORE_CXX_FLAGS} -DswiftCore_EXPORTS
   LINK_LIBRARIES ${swift_stubs_link_libraries}
   INSTALL_IN_COMPONENT stdlib)