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 (file)
--- 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 86f0a39..f8c1a77 100644 (file)
--- 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 03f74ab..d2fdac7 100644 (file)
--- 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)
@@ -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 \

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 78543c6..78bbebe 100644 (file)
--- 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 ac2d203..7872cba 100644 (file)
--- 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 (executable)
index 0000000..3978a39
--- /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@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);/'