[CMake] Improve the clang order-file generation workflow

[beanz]

Summary

With this change generating clang order files using dtrace uses the following workflow:

Configure LLVM & Clang using any options on a system with dtrace

cmake <whatever options you want>

Builds clang and runs the perf-training data to generate the order file

ninja generate-order-file

Re-link clang with the new order file

ninja clang

This patch works by setting a default path to the order file (which can be overridden by the user). If the order file doesn't exist during configuration CMake will create an empty one.

CMake then ties up the dependencies between the clang link job and the order file, and generate-order-file overwrites CLANG_ORDER_FILE with the new order file.

[beanz]

Gate adding the -order_file linker option on whether the linker supports it.

CMakeLists.txt

 endif()
 
 option(CLANG_INCLUDE_DOCS "Generate build targets for the Clang docs."
   ${LLVM_INCLUDE_DOCS})
 if( CLANG_INCLUDE_DOCS )
   add_subdirectory(docs)
 endif()
 
-set(CLANG_ORDER_FILE "" CACHE FILEPATH
+# this line is needed as a cleanup to ensure that any CMakeCaches with the old
+# default value get updated to the new default.
+if(CLANG_ORDER_FILE STREQUAL "")
+  unset(CLANG_ORDER_FILE CACHE)
+endif()
+
+set(CLANG_ORDER_FILE ${CMAKE_CURRENT_BINARY_DIR}/clang.order CACHE FILEPATH
   "Order file to use when compiling clang in order to improve startup time.")
 
+if(NOT EXISTS ${CLANG_ORDER_FILE})
+  execute_process(COMMAND ${CMAKE_COMMAND} -E touch ${CLANG_ORDER_FILE})
+endif()
+
 if (CLANG_BUILT_STANDALONE OR CMAKE_VERSION VERSION_EQUAL 3 OR
     CMAKE_VERSION VERSION_GREATER 3)
   # Generate a list of CMake library targets so that other CMake projects can
   # link against them. LLVM calls its version of this file LLVMExports.cmake, but
   # the usual CMake convention seems to be ${Project}Targets.cmake.
   set(CLANG_INSTALL_PACKAGE_DIR lib${LLVM_LIBDIR_SUFFIX}/cmake/clang)
   set(clang_cmake_builddir "${CMAKE_BINARY_DIR}/${CLANG_INSTALL_PACKAGE_DIR}")
   get_property(CLANG_EXPORTS GLOBAL PROPERTY CLANG_EXPORTS)

tools/driver/CMakeLists.txt

if (APPLE)
   configure_file("${TOOL_INFO_PLIST}.in" "${TOOL_INFO_PLIST_OUT}" @ONLY)
   
   set(TOOL_INFO_UTI)
   set(TOOL_INFO_NAME)
   set(TOOL_INFO_VERSION)
   set(TOOL_INFO_BUILD_VERSION)
 endif()
 
-if(CLANG_ORDER_FILE)
+# The MSVC linker doesn't respect the order_file option
+if(NOT MSVC)
   target_link_libraries(clang "-Wl,-order_file,${CLANG_ORDER_FILE}")
+  set_target_properties(clang PROPERTIES LINK_DEPENDS ${CLANG_ORDER_FILE})
 endif()
 
 if(WITH_POLLY AND LINK_POLLY_INTO_TOOLS)
   target_link_libraries(clang Polly)
   if(POLLY_LINK_LIBS)
     foreach(lib ${POLLY_LINK_LIBS})
       target_link_libraries(clang ${lib})
     endforeach(lib)
   endif(POLLY_LINK_LIBS)
 endif(WITH_POLLY AND LINK_POLLY_INTO_TOOLS)

utils/perf-training/CMakeLists.txt

add_lit_testsuite(generate-dtrace-logs "Generating clang dtrace data"
     ${CMAKE_CURRENT_BINARY_DIR}/order-files/
     DEPENDS clang clear-dtrace-logs
     )
 
   add_custom_target(clear-dtrace-logs
     COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py clean ${CMAKE_CURRENT_BINARY_DIR} dtrace
     COMMENT "Clearing old dtrace data")
 
-
   add_custom_target(generate-order-file
-    COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py gen-order-file --binary $<TARGET_FILE:clang> --output ${CMAKE_CURRENT_BINARY_DIR}/clang.order ${CMAKE_CURRENT_BINARY_DIR}
+    COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/perf-helper.py gen-order-file --binary $<TARGET_FILE:clang> --output ${CLANG_ORDER_FILE} ${CMAKE_CURRENT_BINARY_DIR}
     COMMENT "Generating order file"
     DEPENDS generate-dtrace-logs)
 endif()