Add option to enable LTO easily (#5674)

* Add option to enable LTO easily and disable features that cause compiling errors with LTO

* Add documentation about LTO option

* Add to show_options

changelog.md

@@ -13,3 +13,4 @@
 04-22-2019 - Add Split RGB support  
 04-24-2019 - fix LIB_SRC and QUANTUM_LIB_SRC for ARM  
 04-24-2019 - Add RGB Split fixes and RGB Names  
+05-05-2019 - Add `LINK_TIME_OPTIMIZATION_ENABLE` to enable LTO and disable problematic features that cause LTO to fail

show_options.mk

@@ -88,7 +88,8 @@ OTHER_OPTION_NAMES = \
   RGB_MATRIX_KEYPRESSES \
   LED_MIRRORED \
   RGBLIGHT_FULL_POWER \
-  Link_Time_Optimization
+  Link_Time_Optimization \
+  LINK_TIME_OPTIMIZATION_ENABLE
 
 define NAME_ECHO
 	@echo "  $1 = $($1)		# $(origin $1)"

tmk_core/common.mk

@@ -208,6 +208,13 @@ ifeq ($(strip $(SHARED_EP_ENABLE)), yes)
     TMK_COMMON_DEFS += -DSHARED_EP_ENABLE
 endif
 
+
+ifeq ($(strip $(LINK_TIME_OPTIMIZATION_ENABLE)), yes)
+    EXTRAFLAGS += -flto
+    TMK_COMMON_DEFS += -DLINK_TIME_OPTIMIZATION_ENABLE
+    TMK_COMMON_DEFS += -DNO_ACTION_MACRO
+    TMK_COMMON_DEFS += -DNO_ACTION_FUNCTION
+endif
 # Bootloader address
 ifdef STM32_BOOTLOADER_ADDRESS
     TMK_COMMON_DEFS += -DSTM32_BOOTLOADER_ADDRESS=$(STM32_BOOTLOADER_ADDRESS)