Fix cross-compilation for the Windows stdlib

Disable SwiftPrivate, remove -Wl,-z,defs, and use the VFS overlay for compiling the runtime.

Author: troughton

cmake/modules/AddSwift.cmake

@@ -734,6 +734,7 @@ function(_add_swift_library_single target name)
     swift_windows_generate_sdk_vfs_overlay(SWIFTLIB_SINGLE_VFS_OVERLAY_FLAGS)
     foreach(flag ${SWIFTLIB_SINGLE_VFS_OVERLAY_FLAGS})
       list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS -Xcc;${flag})
+      list(APPEND SWIFTLIB_SINGLE_C_COMPILE_FLAGS ${flag})
     endforeach()
     foreach(directory ${SWIFTLIB_INCLUDE})
       list(APPEND SWIFTLIB_SINGLE_SWIFT_COMPILE_FLAGS -Xfrontend;-I${directory})
@@ -1474,6 +1475,11 @@ function(add_swift_library name)
         continue()
       endif()
 
+      # TODO: Currently SwiftPrivate runs into linker problems on Windows. See SR-6489.
+      if ("${sdk}" STREQUAL "WINDOWS" AND "${name}" MATCHES "SwiftPrivate")
+        continue()
+      endif()
+
       set(THIN_INPUT_TARGETS)
 
       # For each architecture supported by this SDK
@@ -1613,6 +1619,18 @@ function(add_swift_library name)
          endif()
        endif()
 
+       # We unconditionally removed "-z,defs" from CMAKE_SHARED_LINKER_FLAGS in 
+       # swift_common_standalone_build_config_llvm within SwiftSharedCMakeConfig.cmake,
+       # where it was added by a call to HandleLLVMOptions. 
+       #
+       # Rather than applying it to all targets and libraries, we here add it back to 
+       # supported targets and libraries only. 
+       # Windows doesn't support the -z,defs flag and RemoteMirror needs to build 
+       # with undefined symbols.
+       if(NOT "${sdk}" STREQUAL "WINDOWS" AND NOT "${name}" STREQUAL "swiftRemoteMirror")
+          list(APPEND swiftlib_link_flags_all "-Wl,-z,defs")
+       endif()
+
         # Add this library variant.
         _add_swift_library_single(
           ${VARIANT_NAME}

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -78,9 +78,16 @@ macro(swift_common_standalone_build_config_llvm product is_cross_compiling)
   include(AddSwiftTableGen) # This imports TableGen from LLVM.
   include(HandleLLVMOptions)
 
-  # HACK: this ugly tweaking is to prevent the propagation of the flag from LLVM
-  # into swift.  The use of this flag pollutes all targets, and we are not able
-  # to remove it on a per-target basis which breaks cross-compilation.
+  # HACK: Windows doesn't support -z,defs as a linker flag. 
+  #
+  # Normally, LLVM would only add it as an option for non-Windows systems;
+  # however, due to the custom scheme Swift uses for cross-compilation, the 
+  # CMAKE_SHARED_LINKER_FLAGS are determined based on the host system and 
+  # then applied to all targets. 
+  # 
+  # To work around this, we unconditionally remove the flag here and then 
+  # selectively add it to the per-target link flags; this is currently done
+  # in add_swift_library within AddSwift.cmake.
   string(REGEX REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
 
   set(PACKAGE_VERSION "${LLVM_PACKAGE_VERSION}")

stdlib/public/SwiftRemoteMirror/CMakeLists.txt

@@ -1,6 +1,3 @@
-# HACK: Force this library to build with undefined symbols.
-string(REGEX REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
-
 # libswiftRemoteMirror.dylib should not have runtime dependencies; it's
 # always built as a shared library.
 if(SWIFT_BUILD_DYNAMIC_STDLIB OR SWIFT_BUILD_REMOTE_MIRROR)