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>
This commit is contained in:
Marek Behún 2021-05-20 13:24:03 +02:00 committed by Tom Rini
parent 958f2e57ef
commit c1094987d1
6 changed files with 149 additions and 3 deletions

2
Kbuild
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@ -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__)

24
Kconfig
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@ -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

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@ -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)
@ -1708,8 +1733,45 @@ 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) \
@ -1718,6 +1780,7 @@ 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 = \
@ -1726,7 +1789,7 @@ 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)
@ -2009,7 +2072,7 @@ 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 \

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@ -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
# ---------------------------------------------------------------------------
#

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@ -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) ;

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@ -0,0 +1,12 @@
#!/bin/bash
# SPDX-License-Identifier: GPL-2.0+
# Copyright (C) 2020 Marek Behún <marek.behun@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);/'