build: support building with Link Time Optimizations
Add plumbing for building U-Boot with Link Time Optimizations.
When building with LTO, $(PLATFORM_LIBS) has to be in --whole-archive /
--no-whole-archive group, otherwise some functions declared in assembly
may not be resolved and linking may fail.
Note: clang may throw away linker list symbols it thinks are unused when
compiling with LTO. To force these symbols to be included, we refer to
them via the __ADDRESSABLE macro in a C file generated from compiled
built-in.o files before linking.
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Kbuild b/Kbuild
index 1eac091..bf52e54 100644
--- a/Kbuild
+++ b/Kbuild
@@ -10,6 +10,8 @@
always := $(generic-offsets-file)
targets := lib/asm-offsets.s
+CFLAGS_REMOVE_asm-offsets.o := $(LTO_CFLAGS)
+
$(obj)/$(generic-offsets-file): $(obj)/lib/asm-offsets.s FORCE
$(call filechk,offsets,__GENERIC_ASM_OFFSETS_H__)