[PATCH u-boot v3 17/39] build: support building with Link Time Optimizations

Marek Behún
2021-03-16T12:25:47Z

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.

Signed-off-by: Marek Beh?n <marek.behun@nic.cz>
---
Kbuild | 2 ++
Kconfig | 24 ++++++++++++++++++++++++
Makefile | 37 +++++++++++++++++++++++++++++++++++++
scripts/Makefile.lib | 3 +++
scripts/Makefile.spl | 17 +++++++++++++++++
5 files changed, 83 insertions(+)

diff --git a/Kbuild b/Kbuild
index 1eac091594..bf52e54051 100644
--- a/Kbuild
+++ b/Kbuild
@@ -10,6 +10,8 @@ generic-offsets-file := include/generated/generic-asm-offsets.h
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__)

diff --git a/Kconfig b/Kconfig
index 86f0a39bb0..f8c1a77bed 100644
--- a/Kconfig
+++ b/Kconfig
@@ -85,6 +85,30 @@ config SPL_OPTIMIZE_INLINING
do what it thinks is best, which is desirable in some cases for size
reasons.

+config ARCH_SUPPORTS_LTO
+ bool
+
+config LTO
+ bool "Enable Link Time Optimizations"
+ depends on ARCH_SUPPORTS_LTO
+ default n
+ help
+ This option enables Link Time Optimization (LTO), a mechanism which
+ allows the compiler to optimize between different compilation units.
+
+ This can optimize away dead code paths, resulting in smaller binary
+ size (if CC_OPTIMIZE_FOR_SIZE is enabled).
+
+ This option is not available for every architecture and may
+ introduce bugs.
+
+ Currently, when compiling with GCC, due to a weird bug regarding
+ jobserver, the final linking will not respect make's --jobs argument.
+ Instead all available processors will be used (as reported by the
+ nproc command).
+
+ If unsure, say n.
+
config TPL_OPTIMIZE_INLINING
bool "Allow compiler to uninline functions marked 'inline' in TPL"
depends on TPL
diff --git a/Makefile b/Makefile
index c9b5fac7a9..952c165d8f 100644
--- a/Makefile
+++ b/Makefile
@@ -677,6 +677,28 @@ else
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
+ 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
+
KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
KBUILD_CFLAGS += $(call cc-option,-fno-delete-null-pointer-checks)

@@ -1746,6 +1768,20 @@ ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(ARCH)/Makefile.postlink)

# 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) \
+ $(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) \
@@ -1754,6 +1790,7 @@ quiet_cmd_u-boot__ ?= LD $@
--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 = \
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 78543c6dd1..78bbebe7e9 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -419,6 +419,9 @@ $(obj)/%_efi.so: $(obj)/%.o $(obj)/efi_crt0.o $(obj)/efi_reloc.o $(obj)/efi_free

targets += $(obj)/efi_crt0.o $(obj)/efi_reloc.o $(obj)/efi_freestanding.o

+CFLAGS_REMOVE_efi_reloc.o := $(LTO_CFLAGS)
+CFLAGS_REMOVE_efi_freestanding.o := $(LTO_CFLAGS)
+
# ACPI
# ---------------------------------------------------------------------------
#
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index 44ed6d6826..9c9d59635d 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -426,6 +426,22 @@ $(obj)/$(SPL_BIN).sym: $(obj)/$(SPL_BIN) FORCE

# Rule to link u-boot-spl
# May be overridden by arch/$(ARCH)/config.mk
+ifdef CONFIG_LTO
+quiet_cmd_u-boot-spl ?= LTO $@
+ cmd_u-boot-spl ?= \
+ ( \
+ cd $(obj) && \
+ $(CC) -nostdlib -nostartfiles $(LTO_FINAL_LDFLAGS) $(c_flags) \
+ $(KBUILD_LDFLAGS:%=-Wl,%) $(LDFLAGS_$(@F):%=-Wl,%) \
+ $(patsubst $(obj)/%,%,$(u-boot-spl-init)) \
+ -Wl,--whole-archive \
+ $(patsubst $(obj)/%,%,$(u-boot-spl-main)) \
+ $(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) \
+ $(PLATFORM_LIBS) \
+ -Wl,--no-whole-archive \
+ -Wl,-Map,$(SPL_BIN).map -o $(SPL_BIN) \
+ )
+else
quiet_cmd_u-boot-spl ?= LD $@
cmd_u-boot-spl ?= \
( \
@@ -438,6 +454,7 @@ quiet_cmd_u-boot-spl ?= LD $@
--no-whole-archive \
$(PLATFORM_LIBS) -Map $(SPL_BIN).map -o $(SPL_BIN) \
)
+endif

$(obj)/$(SPL_BIN): $(u-boot-spl-platdata) $(u-boot-spl-init) \
$(u-boot-spl-main) $(obj)/u-boot-spl.lds FORCE
--
2.26.2