[lto][cmake] When building with lto on macOS, specify to the linker the path where it should put the generated LTO object file.

This is important so that the object file references in the debug info are
preserved for possible use by dsymutil.

rdar://28062659

Author: gottesmm

cmake/modules/AddSwift.cmake

@@ -250,13 +250,14 @@ function(_add_variant_swift_compile_flags
 endfunction()
 
 function(_add_variant_link_flags)
-  set(oneValueArgs SDK ARCH BUILD_TYPE ENABLE_ASSERTIONS ANALYZE_CODE_COVERAGE DEPLOYMENT_VERSION_IOS RESULT_VAR_NAME ENABLE_LTO)
+  set(oneValueArgs SDK ARCH BUILD_TYPE ENABLE_ASSERTIONS ANALYZE_CODE_COVERAGE
+  DEPLOYMENT_VERSION_IOS RESULT_VAR_NAME ENABLE_LTO LTO_OBJECT_NAME)
   cmake_parse_arguments(LFLAGS
     ""
     "${oneValueArgs}"
     ""
     ${ARGN})
-  
+
   if("${LFLAGS_SDK}" STREQUAL "")
     message(FATAL_ERROR "Should specify an SDK")
   endif()
@@ -294,6 +295,19 @@ function(_add_variant_link_flags)
         "${SWIFT_ANDROID_NDK_PATH}/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a/libc++_shared.so")
   else()
     list(APPEND result "-lobjc")
+
+    # If lto is enabled, we need to add the object path flag so that the LTO code
+    # generator leaves the intermediate object file in a place where it will not
+    # be touched. The reason why this must be done is that on OS X, debug info is
+    # left in object files. So if the object file is removed when we go to
+    # generate a dsym, the debug info is gone.
+    if (LFLAGS_ENABLE_LTO)
+      precondition(LFLAGS_LTO_OBJECT_NAME
+        MESSAGE "Should specify a unique name for the lto object")
+      set(lto_object_dir ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR})
+      set(lto_object ${lto_object_dir}/${LFLAGS_LTO_OBJECT_NAME}-lto.o)
+        list(APPEND result "-Wl,-object_path_lto,${lto_object}")
+      endif()
   endif()
 
   if(NOT "${SWIFT_${LFLAGS_SDK}_ICU_UC}" STREQUAL "")
@@ -932,6 +946,7 @@ function(_add_swift_library_single target name)
     ENABLE_ASSERTIONS "${enable_assertions}"
     ANALYZE_CODE_COVERAGE "${analyze_code_coverage}"
     ENABLE_LTO "${lto_type}"
+    LTO_OBJECT_NAME "${target}-${SWIFTLIB_SINGLE_SDK}-${SWIFTLIB_SINGLE_ARCHITECTURE}"
     DEPLOYMENT_VERSION_IOS "${SWIFTLIB_DEPLOYMENT_VERSION_IOS}"
     RESULT_VAR_NAME link_flags
       )
@@ -1641,6 +1656,7 @@ function(_add_swift_executable_single name)
     BUILD_TYPE "${CMAKE_BUILD_TYPE}"
     ENABLE_ASSERTIONS "${LLVM_ENABLE_ASSERTIONS}"
     ENABLE_LTO "${SWIFT_TOOLS_ENABLE_LTO}"
+    LTO_OBJECT_NAME "${name}-${SWIFTEXE_SINGLE_SDK}-${SWIFTEXE_SINGLE_ARCHITECTURE}"
     ANALYZE_CODE_COVERAGE "${SWIFT_ANALYZE_CODE_COVERAGE}"
     RESULT_VAR_NAME link_flags)
 

tools/SourceKit/CMakeLists.txt

@@ -83,6 +83,7 @@ function(add_sourcekit_default_compiler_flags target)
     BUILD_TYPE "${build_type}"
     ENABLE_ASSERTIONS "${enable_assertions}"
     ENABLE_LTO "${SWIFT_TOOLS_ENABLE_LTO}"
+    LTO_OBJECT_NAME "${target}-${sdk}-${arch}"
     ANALYZE_CODE_COVERAGE "${analyze_code_coverage}"
     RESULT_VAR_NAME link_flags)