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 NO_LTO parameter to the build, similar to NO_SDL, 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 sjg@chromium.org ---
Changes in v3: - Rework to operate like the NO_SDL flag
Makefile | 17 ++++++++++++----- 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, 33 insertions(+), 9 deletions(-)
diff --git a/Makefile b/Makefile index ff25f929748..348e2130c47 100644 --- a/Makefile +++ b/Makefile @@ -643,6 +643,13 @@ 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. See NO_LTO=1 to disable LTO +ifeq ($(NO_LTO),) +LTO_ENABLE=$(if $(CONFIG_LTO),y) +endif + # If board code explicitly specified LDSCRIPT or CONFIG_SYS_LDSCRIPT, use # that (or fail if absent). Otherwise, search for a linker script in a # standard location. @@ -702,16 +709,16 @@ endif LTO_CFLAGS := LTO_FINAL_LDFLAGS := export LTO_CFLAGS LTO_FINAL_LDFLAGS -ifdef CONFIG_LTO +ifeq ($(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 @@ -1760,7 +1767,7 @@ 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))
@@ -1782,7 +1789,7 @@ 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__ ?= \ $(CC) -nostdlib -nostartfiles \ diff --git a/arch/arm/config.mk b/arch/arm/config.mk index b3548ce2439..2065438d053 100644 --- a/arch/arm/config.mk +++ b/arch/arm/config.mk @@ -15,11 +15,11 @@ CFLAGS_NON_EFI := -fno-pic -ffixed-r9 -ffunction-sections -fdata-sections \ -fstack-protector-strong CFLAGS_EFI := -fpic -fshort-wchar
-ifneq ($(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 6ee2a767615..cd6112dfcda 100644 --- a/arch/arm/include/asm/global_data.h +++ b/arch/arm/include/asm/global_data.h @@ -101,7 +101,7 @@ 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 ee544ad87ee..a71f860a487 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 `NO_LTO` to `1`. + +.. code-block:: bash + + NO_LTO=1 make + Other build targets ~~~~~~~~~~~~~~~~~~~
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl index 1cfb8115e31..415451431b9 100644 --- a/scripts/Makefile.spl +++ b/scripts/Makefile.spl @@ -488,7 +488,7 @@ 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 ?= \ ( \