On Sun, 27 Mar 2022 at 21:27, Simon Glass sjg@chromium.org wrote:
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.
This is something that has been bothering me too. I'd resorted to adding `# CONFIG_LTO is not set` to sandbox_defconfig because I couldn't work out how to stop .config being regenerated each build. But then the diff gets in the way and is difficult to manage.
Add a LTO_BUILD=n parameter to the build, so it can be disabled during development if needed, for faster builds.
If you go the build parameter route rather than config route to address this, could the flag be consistent e.g. with NO_SDL=1, which would mean NO_LTO=1?
Add some documentation about LTO while we are here.
Signed-off-by: Simon Glass sjg@chromium.org
Makefile | 18 +++++++++++++----- arch/arm/config.mk | 4 ++-- arch/arm/include/asm/global_data.h | 2 +- doc/build/gcc.rst | 17 +++++++++++++++++ scripts/Makefile.spl | 2 +- 5 files changed, 34 insertions(+), 9 deletions(-)
diff --git a/Makefile b/Makefile index 06572ac07ee..c9585ddebfc 100644 --- a/Makefile +++ b/Makefile @@ -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__ ?= \ $(CC) -nostdlib -nostartfiles \ diff --git a/arch/arm/config.mk b/arch/arm/config.mk index b107b1af27a..065dbec4064 100644 --- a/arch/arm/config.mk +++ b/arch/arm/config.mk @@ -15,11 +15,11 @@ CFLAGS_NON_EFI := -fno-pic -ffixed-r9 -ffunction-sections -fdata-sections \ -fstack-protector-strong CFLAGS_EFI := -fpic -fshort-wchar
-ifneq ($(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
diff --git a/arch/arm/include/asm/global_data.h b/arch/arm/include/asm/global_data.h index 085e12b5d4d..b255b195aa0 100644 --- a/arch/arm/include/asm/global_data.h +++ b/arch/arm/include/asm/global_data.h @@ -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() diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst index 470a7aa3498..792be176aad 100644 --- a/doc/build/gcc.rst +++ b/doc/build/gcc.rst @@ -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
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl index 83a95ee4aa2..e3ca69c4449 100644 --- a/scripts/Makefile.spl +++ b/scripts/Makefile.spl @@ -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 ?= \ ( \ -- 2.35.1.1021.g381101b075-goog