[cmake] Add support for building the swift stdlib with LTO.

This is controlled with the option SWIFT_STDLIB_ENABLE_LTO=thin|full.

Author: gottesmm

stdlib/CMakeLists.txt

@@ -79,6 +79,10 @@ option(SWIFT_BUILD_TEST_SUPPORT_MODULES
        "Whether to build StdlibUnittest and other test support modules. Defaults to On when SWIFT_BUILD_SDK_OVERLAY is On, or when SWIFT_INCLUDE_TESTS is On."
        "${SWIFT_BUILD_TEST_SUPPORT_MODULES_default}")
 
+set(SWIFT_STDLIB_ENABLE_LTO OFF CACHE STRING "Build Swift stdlib with LTO. One
+    must specify the form of LTO by setting this to one of: 'full', 'thin'. This
+    option only affects the standard library and runtime, not tools.")
+
 #
 # End of user-configurable options.
 #

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -623,7 +623,8 @@ function(_add_swift_target_library_single target name)
         DARWIN_INSTALL_NAME_DIR
         SDK
         DEPLOYMENT_VERSION_MACCATALYST
-        MACCATALYST_BUILD_FLAVOR)
+        MACCATALYST_BUILD_FLAVOR
+        ENABLE_LTO)
   set(SWIFTLIB_SINGLE_multiple_parameter_options
         C_COMPILE_FLAGS
         DEPENDS
@@ -778,6 +779,7 @@ function(_add_swift_target_library_single target name)
       ${SWIFTLIB_SINGLE_IS_SDK_OVERLAY_keyword}
       ${embed_bitcode_arg}
       ${SWIFTLIB_SINGLE_STATIC_keyword}
+      ENABLE_LTO "${SWIFTLIB_SINGLE_ENABLE_LTO}"
       INSTALL_IN_COMPONENT "${SWIFTLIB_SINGLE_INSTALL_IN_COMPONENT}"
       MACCATALYST_BUILD_FLAVOR "${SWIFTLIB_SINGLE_MACCATALYST_BUILD_FLAVOR}")
   add_swift_source_group("${SWIFTLIB_SINGLE_EXTERNAL_SOURCES}")
@@ -1139,7 +1141,7 @@ function(_add_swift_target_library_single target name)
   endif()
 
   if (NOT SWIFTLIB_SINGLE_TARGET_LIBRARY)
-    set(lto_type "${SWIFT_TOOLS_ENABLE_LTO}")
+    set(lto_type "${SWIFT_STDLIB_ENABLE_LTO}")
   endif()
 
   _add_target_variant_c_compile_flags(
@@ -1988,7 +1990,7 @@ function(add_swift_target_library name)
         DEPLOYMENT_VERSION_TVOS "${SWIFTLIB_DEPLOYMENT_VERSION_TVOS}"
         DEPLOYMENT_VERSION_WATCHOS "${SWIFTLIB_DEPLOYMENT_VERSION_WATCHOS}"
         MACCATALYST_BUILD_FLAVOR "${maccatalyst_build_flavor}"
-
+        ENABLE_LTO "${SWIFT_STDLIB_ENABLE_LTO}"
         GYB_SOURCES ${SWIFTLIB_GYB_SOURCES}
       )
     if(NOT SWIFT_BUILT_STANDALONE AND NOT "${CMAKE_C_COMPILER_ID}" MATCHES "Clang")
@@ -2322,15 +2324,15 @@ function(_add_swift_target_executable_single name)
     ARCH "${SWIFTEXE_SINGLE_ARCHITECTURE}"
     BUILD_TYPE "${CMAKE_BUILD_TYPE}"
     ENABLE_ASSERTIONS "${LLVM_ENABLE_ASSERTIONS}"
-    ENABLE_LTO "${SWIFT_TOOLS_ENABLE_LTO}"
+    ENABLE_LTO "${SWIFT_STDLIB_ENABLE_LTO}"
     ANALYZE_CODE_COVERAGE "${SWIFT_ANALYZE_CODE_COVERAGE}"
     RESULT_VAR_NAME c_compile_flags)
   _add_target_variant_link_flags(
     SDK "${SWIFTEXE_SINGLE_SDK}"
     ARCH "${SWIFTEXE_SINGLE_ARCHITECTURE}"
     BUILD_TYPE "${CMAKE_BUILD_TYPE}"
     ENABLE_ASSERTIONS "${LLVM_ENABLE_ASSERTIONS}"
-    ENABLE_LTO "${SWIFT_TOOLS_ENABLE_LTO}"
+    ENABLE_LTO "${SWIFT_STDLIB_ENABLE_LTO}"
     LTO_OBJECT_NAME "${name}-${SWIFTEXE_SINGLE_SDK}-${SWIFTEXE_SINGLE_ARCHITECTURE}"
     ANALYZE_CODE_COVERAGE "${SWIFT_ANALYZE_CODE_COVERAGE}"
     RESULT_VAR_NAME link_flags
@@ -2350,6 +2352,7 @@ function(_add_swift_target_executable_single name)
       SDK ${SWIFTEXE_SINGLE_SDK}
       ARCHITECTURE ${SWIFTEXE_SINGLE_ARCHITECTURE}
       COMPILE_FLAGS ${SWIFTEXE_SINGLE_COMPILE_FLAGS}
+      ENABLE_LTO "${SWIFT_STDLIB_ENABLE_LTO}"
       IS_MAIN)
   add_swift_source_group("${SWIFTEXE_SINGLE_EXTERNAL_SOURCES}")
 

stdlib/cmake/modules/SwiftSource.cmake

@@ -1,6 +1,15 @@
 include(macCatalystUtils)
 include(SwiftUtils)
 
+function(_compute_lto_swift_flag option out_var)
+  string(TOLOWER "${option}" lowercase_option)
+  if (lowercase_option STREQUAL "full")
+    set(${out_var} "-lto=llvm-full" PARENT_SCOPE)
+  elseif (lowercase_option STREQUAL "thin")
+    set(${out_var} "-lto=llvm-thin" PARENT_SCOPE)
+  endif()
+endfunction()
+
 # Compute the library subdirectory to use for the given sdk and
 # architecture, placing the result in 'result_var_name'.
 function(compute_library_subdir result_var_name sdk arch)
@@ -42,7 +51,7 @@ function(handle_swift_sources
   cmake_parse_arguments(SWIFTSOURCES
       "IS_MAIN;IS_STDLIB;IS_STDLIB_CORE;IS_SDK_OVERLAY;EMBED_BITCODE;STATIC"
       "SDK;ARCHITECTURE;INSTALL_IN_COMPONENT;MACCATALYST_BUILD_FLAVOR"
-      "DEPENDS;COMPILE_FLAGS;MODULE_NAME"
+      "DEPENDS;COMPILE_FLAGS;MODULE_NAME;ENABLE_LTO"
       ${ARGN})
   translate_flag(${SWIFTSOURCES_IS_MAIN} "IS_MAIN" IS_MAIN_arg)
   translate_flag(${SWIFTSOURCES_IS_STDLIB} "IS_STDLIB" IS_STDLIB_arg)
@@ -107,7 +116,12 @@ function(handle_swift_sources
     if(sdk IN_LIST SWIFT_APPLE_PLATFORMS OR sdk STREQUAL "MACCATALYST")
       list(APPEND swift_compile_flags "-save-optimization-record=bitstream")
     endif()
-
+    if (SWIFTSOURCES_ENABLE_LTO)
+      _compute_lto_swift_flag("${SWIFTSOURCES_ENABLE_LTO}" _lto_flag_out)
+      if (_lto_flag_out)
+        list(APPEND swift_compile_flags "${_lto_flag_out}")
+      endif()
+    endif()
     _compile_swift_files(
         dependency_target
         module_dependency_target