build: support building with Link Time Optimizations
Add plumbing for building U-Boot with Link Time Optimizations.
When building with LTO, $(PLATFORM_LIBS) has to be in --whole-archive /
--no-whole-archive group, otherwise some functions declared in assembly
may not be resolved and linking may fail.
Note: clang may throw away linker list symbols it thinks are unused when
compiling with LTO. To force these symbols to be included, we refer to
them via the __ADDRESSABLE macro in a C file generated from compiled
built-in.o files before linking.
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 03f74ab..d2fdac7 100644
--- a/Makefile
+++ b/Makefile
@@ -676,6 +676,31 @@
KBUILD_CFLAGS += -O2
endif
+LTO_CFLAGS :=
+LTO_FINAL_LDFLAGS :=
+export LTO_CFLAGS LTO_FINAL_LDFLAGS
+ifdef CONFIG_LTO
+ ifeq ($(cc-name),clang)
+ LTO_CFLAGS += -flto
+ LTO_FINAL_LDFLAGS += -flto
+
+ AR = $(shell $(CC) -print-prog-name=llvm-ar)
+ NM = $(shell $(CC) -print-prog-name=llvm-nm)
+ else
+ NPROC := $(shell nproc 2>/dev/null || echo 1)
+ LTO_CFLAGS += -flto=$(NPROC)
+ LTO_FINAL_LDFLAGS += -fuse-linker-plugin -flto=$(NPROC)
+
+ # use plugin aware tools
+ AR = $(CROSS_COMPILE)gcc-ar
+ NM = $(CROSS_COMPILE)gcc-nm
+ endif
+
+ CFLAGS_NON_EFI += $(LTO_CFLAGS)
+
+ KBUILD_CFLAGS += $(LTO_CFLAGS)
+endif
+
ifeq ($(CONFIG_STACKPROTECTOR),y)
KBUILD_CFLAGS += $(call cc-option,-fstack-protector-strong)
CFLAGS_EFI += $(call cc-option,-fno-stack-protector)
@@ -1708,8 +1733,45 @@
ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(ARCH)/Makefile.postlink)
+# Generate linker list symbols references to force compiler to not optimize
+# them away when compiling with LTO
+ifdef CONFIG_LTO
+u-boot-keep-syms-lto := keep-syms-lto.o
+u-boot-keep-syms-lto_c := $(patsubst %.o,%.c,$(u-boot-keep-syms-lto))
+
+quiet_cmd_keep_syms_lto = KSL $@
+ cmd_keep_syms_lto = \
+ NM=$(NM) $(srctree)/scripts/gen_ll_addressable_symbols.sh $^ >$@
+
+quiet_cmd_keep_syms_lto_cc = KSLCC $@
+ cmd_keep_syms_lto_cc = \
+ $(CC) $(filter-out $(LTO_CFLAGS),$(c_flags)) -c -o $@ $<
+
+$(u-boot-keep-syms-lto_c): $(u-boot-main)
+ $(call if_changed,keep_syms_lto)
+$(u-boot-keep-syms-lto): $(u-boot-keep-syms-lto_c)
+ $(call if_changed,keep_syms_lto_cc)
+else
+u-boot-keep-syms-lto :=
+endif
+
# Rule to link u-boot
# May be overridden by arch/$(ARCH)/config.mk
+ifdef CONFIG_LTO
+quiet_cmd_u-boot__ ?= LTO $@
+ cmd_u-boot__ ?= \
+ $(CC) -nostdlib -nostartfiles \
+ $(LTO_FINAL_LDFLAGS) $(c_flags) \
+ $(KBUILD_LDFLAGS:%=-Wl,%) $(LDFLAGS_u-boot:%=-Wl,%) -o $@ \
+ -T u-boot.lds $(u-boot-init) \
+ -Wl,--whole-archive \
+ $(u-boot-main) \
+ $(u-boot-keep-syms-lto) \
+ $(PLATFORM_LIBS) \
+ -Wl,--no-whole-archive \
+ -Wl,-Map,u-boot.map; \
+ $(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
+else
quiet_cmd_u-boot__ ?= LD $@
cmd_u-boot__ ?= $(LD) $(KBUILD_LDFLAGS) $(LDFLAGS_u-boot) -o $@ \
-T u-boot.lds $(u-boot-init) \
@@ -1718,6 +1780,7 @@
--no-whole-archive \
$(PLATFORM_LIBS) -Map u-boot.map; \
$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
+endif
quiet_cmd_smap = GEN common/system_map.o
cmd_smap = \
@@ -1726,7 +1789,7 @@
$(CC) $(c_flags) -DSYSTEM_MAP="\"$${smap}\"" \
-c $(srctree)/common/system_map.c -o common/system_map.o
-u-boot: $(u-boot-init) $(u-boot-main) u-boot.lds FORCE
+u-boot: $(u-boot-init) $(u-boot-main) $(u-boot-keep-syms-lto) u-boot.lds FORCE
+$(call if_changed,u-boot__)
ifeq ($(CONFIG_KALLSYMS),y)
$(call cmd,smap)
@@ -2009,7 +2072,7 @@
boot* u-boot* MLO* SPL System.map fit-dtb.blob* \
u-boot-ivt.img.log u-boot-dtb.imx.log SPL.log u-boot.imx.log \
lpc32xx-* bl31.c bl31.elf bl31_*.bin image.map tispl.bin* \
- idbloader.img flash.bin flash.log defconfig
+ idbloader.img flash.bin flash.log defconfig keep-syms-lto.c
# Directories & files removed with 'make mrproper'
MRPROPER_DIRS += include/config include/generated spl tpl \