Commit 2d4ebfb0 authored by Simon Glass's avatar Simon Glass
Browse files

Makefile: v2 Allow LTO to be disabled for a build



LTO (Link-Time Optimisation) is an very useful feature which can
significantly reduce the size of U-Boot binaries. So far it has been
made available for selected ARM boards and sandbox.

However, incremental builds are much slower when LTO is used. For example,
an incremental build of sandbox takes 2.1 seconds on my machine, but 6.7
seconds with LTO enabled.

Add a LTO_BUILD=n parameter to the build, so it can be disabled during
development if needed, for faster builds.

Add some documentation about LTO while we are here.
Signed-off-by: Simon Glass's avatarSimon Glass <sjg@chromium.org>
Series-to: u-boot
Series-cc: trini, heinrich
Series-cc: Marek Behún <marek.behun@nic.cz>
parent a958c585
......@@ -434,6 +434,9 @@ KBUILD_CFLAGS += -fshort-wchar -fno-strict-aliasing
KBUILD_AFLAGS := -D__ASSEMBLY__
KBUILD_LDFLAGS :=
# Set this to "n" use of LTO for this build, e.g. LTO_BUILD=n
LTO_BUILD ?= y
ifeq ($(cc-name),clang)
ifneq ($(CROSS_COMPILE),)
CLANG_TARGET := --target=$(notdir $(CROSS_COMPILE:%-=%))
......@@ -643,6 +646,11 @@ export CFLAGS_EFI # Compiler flags to add when building EFI app
export CFLAGS_NON_EFI # Compiler flags to remove when building EFI app
export EFI_TARGET # binutils target if EFI is natively supported
export LTO_ENABLE
# This is y if LTO is enabled for this build
LTO_ENABLE=$(if $(CONFIG_LTO),$(LTO_BUILD),)
# If board code explicitly specified LDSCRIPT or CONFIG_SYS_LDSCRIPT, use
# that (or fail if absent). Otherwise, search for a linker script in a
# standard location.
......@@ -690,16 +698,16 @@ endif
LTO_CFLAGS :=
LTO_FINAL_LDFLAGS :=
export LTO_CFLAGS LTO_FINAL_LDFLAGS
ifdef CONFIG_LTO
ifeq ($(LTO_ENABLE),y)
ifeq ($(cc-name),clang)
LTO_CFLAGS += -flto
LTO_CFLAGS += -DLTO_ENABLE -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_CFLAGS += -DLTO_ENABLE -flto=$(NPROC)
LTO_FINAL_LDFLAGS += -fuse-linker-plugin -flto=$(NPROC)
# use plugin aware tools
......@@ -1740,7 +1748,7 @@ 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
ifeq ($(LTO_ENABLE),y)
u-boot-keep-syms-lto := keep-syms-lto.o
u-boot-keep-syms-lto_c := $(patsubst %.o,%.c,$(u-boot-keep-syms-lto))
......@@ -1762,7 +1770,7 @@ endif
# Rule to link u-boot
# May be overridden by arch/$(ARCH)/config.mk
ifdef CONFIG_LTO
ifeq ($(LTO_ENABLE),y)
quiet_cmd_u-boot__ ?= LTO $@
cmd_u-boot__ ?= \
$(CC) -nostdlib -nostartfiles \
......
......@@ -15,11 +15,11 @@ CFLAGS_NON_EFI := -fno-pic -ffixed-r9 -ffunction-sections -fdata-sections \
-fstack-protector-strong
CFLAGS_EFI := -fpic -fshort-wchar
ifneq ($(CONFIG_LTO)$(CONFIG_USE_PRIVATE_LIBGCC),yy)
ifneq ($(LTO_ENABLE)$(CONFIG_USE_PRIVATE_LIBGCC),yy)
LDFLAGS_FINAL += --gc-sections
endif
ifndef CONFIG_LTO
ifneq ($(LTO_ENABLE),y)
PLATFORM_RELFLAGS += -ffunction-sections -fdata-sections
endif
......
......@@ -98,7 +98,7 @@ struct arch_global_data {
#include <asm-generic/global_data.h>
#if defined(__clang__) || defined(CONFIG_LTO)
#if defined(__clang__) || defined(LTO_ENABLE)
#define DECLARE_GLOBAL_DATA_PTR
#define gd get_gd()
......
......@@ -152,6 +152,23 @@ of dtc is new enough. It also makes sure that pylibfdt is present, if needed
Note that the :doc:`tools` are always built with the included version of libfdt
so it is not possible to build U-Boot tools with a system libfdt, at present.
Link-time optimisation (LTO)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
U-Boot supports link-time optimisation which can reduce the size of the final
U-Boot binaries, particularly with SPL.
At present this can be enabled by ARM boards by adding `CONFIG_LTO=y` into the
defconfig file. Other architectures are not supported. LTO is enabled by default
for sandbox.
This does incur a link-time penalty of several seconds. For faster incremental
builds during development, you can disable it by setting `LTO_BUILD` to `n`.
.. code-block:: bash
LTO_BUILD=n make
Other build targets
~~~~~~~~~~~~~~~~~~~
......
......@@ -472,7 +472,7 @@ endif
# Rule to link u-boot-spl
# May be overridden by arch/$(ARCH)/config.mk
ifdef CONFIG_LTO
ifeq ($(LTO_ENABLE),y)
quiet_cmd_u-boot-spl ?= LTO $@
cmd_u-boot-spl ?= \
( \
......
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