[PATCH] Makefile: Allow LTO to be disabled for a build

[Simon Glass]

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 <sjg at chromium.org>
---

 Makefile                           | 18 +++++++++++++-----
 arch/arm/config.mk                 |  4 ++--
 arch/arm/include/asm/global_data.h |  2 +-
 doc/build/gcc.rst                  | 18 ++++++++++++++++++
 4 files changed, 34 insertions(+), 8 deletions(-)

diff --git a/Makefile b/Makefile
index 3014788e14e..f3f9092de06 100644
--- a/Makefile
+++ b/Makefile
@@ -430,6 +430,9 @@ 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:%-=%))
@@ -637,6 +640,11 @@ 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.
@@ -684,16 +692,16 @@ 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
@@ -1732,7 +1740,7 @@ 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))
 
@@ -1754,7 +1762,7 @@ 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 16c63e12667..956fa5ad5aa 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 0cdc307d57b..7fc5952b661 100644
--- a/doc/build/gcc.rst
+++ b/doc/build/gcc.rst
@@ -120,6 +120,24 @@ Further important build parameters are
 * O=<dir> - generate all output files in directory <dir>, including .config
 * V=1 - verbose build
 
+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
 ~~~~~~~~~~~~~~~~~~~
 
-- 
2.33.0.685.g46640cef36-goog