Makefile: 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 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>

diff --git a/Makefile b/Makefile
index 541e942ed515a9c92c0dbbf0f8904c49ee5255b0..27375f033f434a497793e271512507678b7e6e5d 100644 (file)
--- 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 $(CONFIG_LTO),y)
+endif
+
 # If board code explicitly specified LDSCRIPT or CONFIG_SYS_LDSCRIPT, use
 # that (or fail if absent).  Otherwise, search for a linker script in a
 # standard location.
@@ -708,16 +715,16 @@ endif
 LTO_CFLAGS :=
 LTO_FINAL_LDFLAGS :=
 export LTO_CFLAGS LTO_FINAL_LDFLAGS
-ifdef CONFIG_LTO
+ifeq ($(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
@@ -1765,7 +1772,7 @@ 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))
 
@@ -1787,7 +1794,7 @@ 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 b3548ce2439242a66bc62a7a39359fa336a283f2..2065438d0530fb89cd6d01943c1361127cd862b7 100644 (file)
--- 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 6ee2a767615823a16d042bfa29d5570201de3610..cd6112dfcda530a1c0ee9bc2e52ba3d36967a536 100644 (file)
--- 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 ee544ad87ee6522941a0f2c236c79f21dfa06606..a71f860a487e0058b784cb1ebfc11f208401e9a2 100644 (file)
--- 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 3bafeb4fe98ed4e81fc12fb41b667daecfd128b4..0b3a51da13bf5eded436d447ff0e5b6eccd7dc1f 100644 (file)
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -489,7 +489,7 @@ 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 ?= \
        (                                                                       \