[nrf fromtree] toolchain: gcc: Add a new LTO_SINGLE_THREADED option for LTO

As described in this issue:
zephyrproject-rtos/sdk-ng#1038
`-flto=auto` fails on some (slower) Windows machines due to an issue with
the Zephyr SDK's GCC toolchain for Windows.
In order to allow users to work around this issue, introduce a new
CONFIG_LTO_SINGLE_THREADED option that switches to `-flto=1`, which
enforces a single thread when processing LTO.

Signed-off-by: Carles Cufi <[email protected]>
(cherry picked from commit d0547b6)

Author: carlescufi

CMakeLists.txt

@@ -311,8 +311,13 @@ compiler_simple_options(simple_options)
 toolchain_linker_add_compiler_options(${simple_options})
 
 if(CONFIG_LTO)
-  zephyr_compile_options($<TARGET_PROPERTY:compiler,optimization_lto>)
-  add_link_options($<TARGET_PROPERTY:linker,lto_arguments>)
+  if(CONFIG_LTO_SINGLE_THREADED)
+    zephyr_compile_options($<TARGET_PROPERTY:compiler,optimization_lto_st>)
+    add_link_options($<TARGET_PROPERTY:linker,lto_arguments_st>)
+  else()
+    zephyr_compile_options($<TARGET_PROPERTY:compiler,optimization_lto>)
+    add_link_options($<TARGET_PROPERTY:linker,lto_arguments>)
+  endif()
 endif()
 
 if(CONFIG_STD_C23)

Kconfig.zephyr

@@ -529,6 +529,14 @@ config LTO
 	help
 	  This option enables Link Time Optimization.
 
+config LTO_SINGLE_THREADED
+	bool "Single-threaded LTO"
+	depends on LTO
+	help
+	  This option instructs the linker to use a single thread to process
+	  LTO. See the following issue for more info:
+	  https://github.com/zephyrproject-rtos/sdk-ng/issues/1038
+
 config COMPILER_WARNINGS_AS_ERRORS
 	bool "Treat warnings as errors"
 	help

cmake/compiler/compiler_flags_template.cmake

@@ -17,6 +17,9 @@ set_compiler_property(PROPERTY optimization_size_aggressive)
 
 set_compiler_property(PROPERTY optimization_fast)
 
+set_compiler_property(PROPERTY optimization_lto)
+set_compiler_property(PROPERTY optimization_lto_st)
+
 #######################################################
 # This section covers flags related to warning levels #
 #######################################################

cmake/compiler/gcc/compiler_flags.cmake

@@ -25,6 +25,7 @@ set_compiler_property(PROPERTY optimization_fast -Ofast)
 
 if(CMAKE_C_COMPILER_VERSION GREATER_EQUAL "4.5.0")
   set_compiler_property(PROPERTY optimization_lto -flto=auto)
+  set_compiler_property(PROPERTY optimization_lto_st -flto=1)
   set_compiler_property(PROPERTY prohibit_lto -fno-lto)
 endif()
 

cmake/linker/ld/linker_flags.cmake

@@ -43,6 +43,7 @@ endif()
 set_property(TARGET linker PROPERTY partial_linking "-r")
 
 set_property(TARGET linker PROPERTY lto_arguments -flto=auto -fno-ipa-sra -ffunction-sections -fdata-sections)
+set_property(TARGET linker PROPERTY lto_arguments_st -flto=1 -fno-ipa-sra -ffunction-sections -fdata-sections)
 
 check_set_linker_property(TARGET linker PROPERTY no_relax ${LINKERFLAGPREFIX},--no-relax)
 

cmake/linker/linker_flags_template.cmake

@@ -50,6 +50,10 @@ set_property(TARGET linker PROPERTY no_relax)
 # Linker flag for enabling relaxation of address optimization for jump calls.
 set_property(TARGET linker PROPERTY relax)
 
+# Linker flags for Link Time Optimizations (LTO)
+set_property(TARGET linker PROPERTY lto_arguments)
+set_property(TARGET linker PROPERTY lto_arguments_st)
+
 # Linker flag for defining specs. Defined only by gcc, when gcc is used as
 # front-end for ld.
 set_compiler_property(PROPERTY specs)

cmake/linker/lld/linker_flags.cmake

@@ -14,6 +14,7 @@ endif()
 set_property(TARGET linker PROPERTY no_position_independent "${LINKERFLAGPREFIX},--no-pie")
 
 set_property(TARGET linker PROPERTY lto_arguments)
+set_property(TARGET linker PROPERTY lto_arguments_st)
 
 check_set_linker_property(TARGET linker PROPERTY sort_alignment ${LINKERFLAGPREFIX},--sort-section=alignment)