kernel: Add Kconfig option to disable LTO for kernel sources

Some SoCs require kernel code to be placed in RAM, which makes
link-time optimization (LTO) unsuitable for these files.
Disabling LTO allows the affected code to be linked as separate
objects and placed in specific memory regions.

Running kernel code from RAM can improve execution performance,
especially for timing-critical routines or context switch paths.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>

CMakeLists.txt

@@ -315,7 +315,9 @@ if(CONFIG_LTO)
     zephyr_compile_options($<TARGET_PROPERTY:compiler,optimization_lto_st>)
     add_link_options($<TARGET_PROPERTY:linker,lto_arguments_st>)
   else()
-    zephyr_compile_options($<TARGET_PROPERTY:compiler,optimization_lto>)
+    set(genex_tgt_lto "$<TARGET_PROPERTY:LTO>")
+    set(genex_lto "$<TARGET_PROPERTY:compiler,optimization_lto>")
+    zephyr_compile_options("$<$<OR:$<STREQUAL:${genex_tgt_lto},>,${genex_tgt_lto}>:${genex_lto}>")
     add_link_options($<TARGET_PROPERTY:linker,lto_arguments>)
   endif()
 endif()

kernel/CMakeLists.txt

@@ -195,6 +195,11 @@ target_link_libraries(kernel zephyr_interface)
 
 endif()
 
+# Optionally build kernel sources without LTO
+if(CONFIG_KERNEL_NO_LTO)
+  set_target_properties(kernel PROPERTIES LTO 0)
+endif()
+
 add_dependencies(kernel zephyr_generated_headers)
 
 unset(libkernel)

kernel/Kconfig

@@ -1165,6 +1165,19 @@ endif # BOOTARGS
 
 endmenu
 
+config KERNEL_NO_LTO
+	bool
+	depends on LTO
+	depends on XIP
+	help
+	  Some SoCs require kernel code to be placed in RAM, which makes link-time
+	  optimization (LTO) unsuitable for these files (-fno-lto). Disabling LTO
+	  allows the affected code to be linked as separate objects and placed in
+	  specific memory regions.
+
+	  Running kernel code from RAM can improve execution performance, especially
+	  for timing-critical routines or context switch paths.
+
 rsource "Kconfig.device"
 rsource "Kconfig.vm"
 rsource "Kconfig.init"