buildsystem: Add an option to enable LTO

This commit adds option to enable Link Time Optimization.

Signed-off-by: Radosław Koppel <radoslaw.koppel@nordicsemi.no>

CMakeLists.txt

@@ -213,6 +213,11 @@ endif()
 # Apply the final optimization flag(s)
 zephyr_compile_options(${OPTIMIZATION_FLAG})
 
+if(CONFIG_LTO)
+  add_compile_options($<TARGET_PROPERTY:compiler,optimization_lto>)
+  add_link_options($<TARGET_PROPERTY:linker,lto_arguments>)
+endif()
+
 # @Intent: Obtain compiler specific flags related to C++ that are not influenced by kconfig
 zephyr_compile_options($<$<COMPILE_LANGUAGE:CXX>:$<TARGET_PROPERTY:compiler-cpp,required>>)
 
@@ -805,6 +810,10 @@ target_include_directories(${OFFSETS_LIB} PRIVATE
   kernel/include
   ${ARCH_DIR}/${ARCH}/include
   )
+
+# Make sure that LTO will never be enabled when compiling offsets.c
+set_source_files_properties(${OFFSETS_C_PATH} PROPERTIES COMPILE_OPTIONS $<TARGET_PROPERTY:compiler,prohibit_lto>)
+
 target_link_libraries(${OFFSETS_LIB} zephyr_interface)
 add_dependencies(zephyr_interface
   ${SYSCALL_LIST_H_TARGET}

Kconfig.zephyr

@@ -428,6 +428,13 @@ config NO_OPTIMIZATIONS
 	  default stack sizes in order to avoid stack overflows.
 endchoice
 
+config LTO
+	bool "Link Time Optimization [EXPERIMENTAL]"
+	depends on !(GEN_ISR_TABLES || GEN_IRQ_VECTOR_TABLE) || ISR_TABLES_LOCAL_DECLARATION
+	select EXPERIMENTAL
+	help
+	  This option enables Link Time Optimization.
+
 config COMPILER_WARNINGS_AS_ERRORS
 	bool "Treat warnings as errors"
 	help

cmake/bintools/gnu/target.cmake

@@ -5,8 +5,13 @@
 find_program(CMAKE_OBJCOPY ${CROSS_COMPILE}objcopy PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
 find_program(CMAKE_OBJDUMP ${CROSS_COMPILE}objdump PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
 find_program(CMAKE_AS      ${CROSS_COMPILE}as      PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
-find_program(CMAKE_AR      ${CROSS_COMPILE}ar      PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
-find_program(CMAKE_RANLIB  ${CROSS_COMPILE}ranlib  PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
+if(CONFIG_LTO)
+  find_program(CMAKE_AR      ${CROSS_COMPILE}gcc-ar      PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
+  find_program(CMAKE_RANLIB  ${CROSS_COMPILE}gcc-ranlib  PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
+else()
+  find_program(CMAKE_AR      ${CROSS_COMPILE}ar      PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
+  find_program(CMAKE_RANLIB  ${CROSS_COMPILE}ranlib  PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
+endif()
 find_program(CMAKE_READELF ${CROSS_COMPILE}readelf PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
 find_program(CMAKE_NM      ${CROSS_COMPILE}nm      PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)
 find_program(CMAKE_STRIP   ${CROSS_COMPILE}strip   PATHS ${TOOLCHAIN_HOME} NO_DEFAULT_PATH)

cmake/compiler/gcc/compiler_flags.cmake

@@ -21,6 +21,9 @@ endif()
 set_compiler_property(PROPERTY optimization_speed -O2)
 set_compiler_property(PROPERTY optimization_size  -Os)
 
+set_compiler_property(PROPERTY optimization_lto -flto)
+set_compiler_property(PROPERTY prohibit_lto -fno-lto)
+
 #######################################################
 # This section covers flags related to warning levels #
 #######################################################

cmake/linker/ld/linker_flags.cmake

@@ -12,6 +12,8 @@ endif()
 
 set_property(TARGET linker PROPERTY partial_linking "-r")
 
+set_property(TARGET linker PROPERTY lto_arguments -flto -fno-ipa-sra -ffunction-sections -fdata-sections)
+
 # Some linker flags might not be purely ld specific, but a combination of
 # linker and compiler, such as:
 # --coverage for clang