[PATCH u-boot v4 14/36] build: support building with Link Time Optimizations
Marek Behún
marek.behun at nic.cz
Thu May 20 13:24:03 CEST 2021
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 at nic.cz>
---
Kbuild | 2 +
Kconfig | 24 ++++++++++
Makefile | 67 ++++++++++++++++++++++++++-
scripts/Makefile.lib | 3 ++
scripts/Makefile.spl | 44 +++++++++++++++++-
scripts/gen_ll_addressable_symbols.sh | 12 +++++
6 files changed, 149 insertions(+), 3 deletions(-)
create mode 100755 scripts/gen_ll_addressable_symbols.sh
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
$(call filechk,offsets,__GENERIC_ASM_OFFSETS_H__)
diff --git a/Kconfig b/Kconfig
index 86f0a39bb0..f8c1a77bed 100644
--- a/Kconfig
+++ b/Kconfig
@@ -85,6 +85,30 @@ config SPL_OPTIMIZE_INLINING
do what it thinks is best, which is desirable in some cases for size
reasons.
+config ARCH_SUPPORTS_LTO
+ bool
+
+config LTO
+ bool "Enable Link Time Optimizations"
+ depends on ARCH_SUPPORTS_LTO
+ default n
+ help
+ This option enables Link Time Optimization (LTO), a mechanism which
+ allows the compiler to optimize between different compilation units.
+
+ This can optimize away dead code paths, resulting in smaller binary
+ size (if CC_OPTIMIZE_FOR_SIZE is enabled).
+
+ This option is not available for every architecture and may
+ introduce bugs.
+
+ Currently, when compiling with GCC, due to a weird bug regarding
+ jobserver, the final linking will not respect make's --jobs argument.
+ Instead all available processors will be used (as reported by the
+ nproc command).
+
+ If unsure, say n.
+
config TPL_OPTIMIZE_INLINING
bool "Allow compiler to uninline functions marked 'inline' in TPL"
depends on TPL
diff --git a/Makefile b/Makefile
index 70728b3351..2329edc159 100644
--- a/Makefile
+++ b/Makefile
@@ -676,6 +676,31 @@ else
KBUILD_CFLAGS += -O2
endif
+LTO_CFLAGS :=
+LTO_FINAL_LDFLAGS :=
+export LTO_CFLAGS LTO_FINAL_LDFLAGS
+ifdef CONFIG_LTO
+ ifeq ($(cc-name),clang)
+ LTO_CFLAGS += -flto
+ LTO_FINAL_LDFLAGS += -flto
+
+ 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)
@@ -1706,8 +1731,45 @@ u-boot-swap.bin: u-boot.bin FORCE
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) \
@@ -1716,6 +1778,7 @@ 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 = \
@@ -1724,7 +1787,7 @@ cmd_smap = \
$(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)
@@ -2007,7 +2070,7 @@ CLEAN_FILES += include/bmp_logo.h include/bmp_logo_data.h tools/version.h \
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 \
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 ac2d2033ba..7872cbaabe 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -448,8 +448,48 @@ quiet_cmd_sym ?= SYM $@
$(obj)/$(SPL_BIN).sym: $(obj)/$(SPL_BIN) FORCE
$(call if_changed,sym)
+# Generate linker list symbols references to force compiler to not optimize
+# them away when compiling with LTO
+ifdef CONFIG_LTO
+u-boot-spl-keep-syms-lto := $(obj)/keep-syms-lto.o
+u-boot-spl-keep-syms-lto_c := \
+ $(patsubst $(obj)/%.o,$(obj)/%.c,$(u-boot-spl-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-spl-keep-syms-lto_c): $(u-boot-spl-main) $(u-boot-spl-platdata)
+ $(call if_changed,keep_syms_lto)
+$(u-boot-spl-keep-syms-lto): $(u-boot-spl-keep-syms-lto_c)
+ $(call if_changed,keep_syms_lto_cc)
+else
+u-boot-spl-keep-syms-lto :=
+endif
+
# Rule to link u-boot-spl
# May be overridden by arch/$(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)) \
+ $(patsubst $(obj)/%,%,$(u-boot-spl-keep-syms-lto)) \
+ $(PLATFORM_LIBS) \
+ -Wl,--no-whole-archive \
+ -Wl,-Map,$(SPL_BIN).map -o $(SPL_BIN) \
+ )
+else
quiet_cmd_u-boot-spl ?= LD $@
cmd_u-boot-spl ?= \
( \
@@ -462,9 +502,11 @@ 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
+ $(u-boot-spl-main) $(u-boot-spl-keep-syms-lto) \
+ $(obj)/u-boot-spl.lds FORCE
$(call if_changed,u-boot-spl)
$(sort $(u-boot-spl-init) $(u-boot-spl-main)): $(u-boot-spl-dirs) ;
diff --git a/scripts/gen_ll_addressable_symbols.sh b/scripts/gen_ll_addressable_symbols.sh
new file mode 100755
index 0000000000..3978a39d97
--- /dev/null
+++ b/scripts/gen_ll_addressable_symbols.sh
@@ -0,0 +1,12 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0+
+# Copyright (C) 2020 Marek Behún <marek.behun at nic.cz>
+
+# Generate __ADDRESSABLE(symbol) for every linker list entry symbol, so that LTO
+# does not optimize these symbols away
+
+set -e
+
+echo '#include <common.h>'
+$NM "$@" 2>/dev/null | grep -oe '_u_boot_list_2_[a-zA-Z0-9_]*_2_[a-zA-Z0-9_]*' | \
+ sort -u | sed -e 's/^\(.*\)/extern char \1[];\n__ADDRESSABLE(\1);/'
--
2.26.3
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