Fix issues building libraries as more than one type with Xcode

[beanz]

Summary

CMake+Xcode doesn't seem to handle targets that only have object
sources. This patch works around that limitation by adding a dummy
soruce file to any library target that is generated by llvm_add_library
when object libraries are generated.

Object libraries are generated whenever llvm_add_library is passed more
than one library type, which is now the default case for clang static
libraries (which generate STATIC and OBJECT libraries).

[xbolva00]

llvm/CMakeLists.txt

45

Typo

[joanlluch]

This fixed bug 42528. okey to me.

llvm/CMakeLists.txt

 
 if ((CMAKE_GENERATOR MATCHES "Visual Studio") AND (CMAKE_GENERATOR_TOOLSET STREQUAL ""))
   message(WARNING "Visual Studio generators use the x86 host compiler by "
                   "default, even for 64-bit targets. This can result in linker "
                   "instability and out of memory errors. To use the 64-bit "
                   "host compiler, pass -Thost=x64 on the CMake command line.")
 endif()
 
+if (CMAKE_GENERATOR STREQUAL "Xcode" AND NOT CMAKE_OSX_ARCHITECTURES)
+  # Some CMake features like object libraries get confused if you don't
+  # explicitly speciy an architecture setting with the Xcode generator.
+  set(CMAKE_OSX_ARCHITECTURES "x86_64")
+endif()
+
 project(LLVM
   VERSION ${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}
   LANGUAGES C CXX ASM)
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
   message(STATUS "No build type selected, default to Debug")
   set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)
 endif()

llvm/cmake/modules/AddLLVM.cmake

function(llvm_add_library name)
   # Generate objlib
   if((ARG_SHARED AND ARG_STATIC) OR ARG_OBJECT)
     # Generate an obj library for both targets.
     set(obj_name "obj.${name}")
     add_library(${obj_name} OBJECT EXCLUDE_FROM_ALL
       ${ALL_FILES}
       )
     llvm_update_compile_flags(${obj_name})
-    set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>")
+    if(CMAKE_GENERATOR STREQUAL "Xcode")
+      set(DUMMY_FILE ${CMAKE_CURRENT_BINARY_DIR}/Dummy.c)
+      file(WRITE ${DUMMY_FILE} "// This file intentionally empty\n")
+      set_property(SOURCE ${DUMMY_FILE} APPEND_STRING PROPERTY COMPILE_FLAGS "-Wno-empty-translation-unit")
+    endif()
+    set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>" ${DUMMY_FILE})
 
     # Do add_dependencies(obj) later due to CMake issue 14747.
     list(APPEND objlibs ${obj_name})
 
     set_target_properties(${obj_name} PROPERTIES FOLDER "Object Libraries")
   endif()
 
   if(ARG_SHARED AND ARG_STATIC)