scripts/Makefile.lto

#
# Support for gcc link time optimization
#

DISABLE_LTO :=
LTO_CFLAGS :=

export DISABLE_LTO
export LTO_CFLAGS

ifdef CONFIG_LTO
# 4.7 works mostly, but it sometimes loses symbols on large builds
# This can be worked around by marking those symbols visible,
# but that is fairly ugly and the problem is gone with 4.8
# 4.8 was very slow
# 4.9 still had a few bugs that needed workarounds
# make it 5.0+ only for now, which is also faster again
# 5.0 has some rare issues in the lto-streamer, could make this 6.0
ifeq ($(call cc-ifversion, -ge, 0500,y),y)
# is the compiler compiled with LTO?
ifneq ($(call cc-option,${LTO_CFLAGS},n),n)
# binutils before 2.27 has various problems with plugins
ifeq ($(call ld-ifversion,-ge,227000000,y),y)

       $(info Enabling LTO)

# gcc 5.0 supports __attribute__((no_reorder)), so we don't
# need to disable reordering globally, but can only use
# it on init calls.
# Disabled for now because of some undebugged bug in 5.x
#ifeq ($(call cc-ifversion,-ge,0500,y),y)
#	DISABLE_TL_REORDER :=
#else
	DISABLE_TL_REORDER := -fno-toplevel-reorder
#endif
	LTO_CFLAGS := -flto $(DISABLE_TL_REORDER)
	LTO_FINAL_CFLAGS := -fuse-linker-plugin

# the -fno-toplevel-reorder is to preserve the order of initcalls
# everything else should tolerate reordering
	LTO_FINAL_CFLAGS += $(DISABLE_TL_REORDER)

# enable LTO and set the jobs used by the LTO phase
# this should be -flto=jobserver to coordinate with the
# parent make, but work around
# http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50639
# use as many jobs as processors are online for now
# this actually seems to be a kernel bug with the pipe code
	LTO_FINAL_CFLAGS := -flto=$(shell getconf _NPROCESSORS_ONLN)
	#LTO_FINAL_CFLAGS := -flto=jobserver

	# requires plugin ar
	LTO_CFLAGS += -fno-fat-lto-objects

# Used to disable LTO for specific files (e.g. vdso)
	DISABLE_LTO := -fno-lto

	LTO_FINAL_CFLAGS += ${LTO_CFLAGS} -fwhole-program

ifdef CONFIG_LTO_DEBUG
	LTO_FINAL_CFLAGS += -dH -fdump-ipa-cgraph -fdump-ipa-inline-details
	# useful to add to reproduce compiler crashes
	# -Wl,-plugin-save-temps -save-temps
	LTO_CFLAGS +=
endif
ifdef CONFIG_LTO_CP_CLONE
	LTO_FINAL_CFLAGS += -fipa-cp-clone
	LTO_CFLAGS += -fipa-cp-clone
endif

	KBUILD_CFLAGS += ${LTO_CFLAGS}

	# finally enable LTO for the final linking stage

	LDFINAL := ${CONFIG_SHELL} ${srctree}/scripts/gcc-ld \
                  ${LTO_FINAL_CFLAGS}

	# LTO gcc creates a lot of files in TMPDIR, and with /tmp as tmpfs
	# it's easy to drive the machine OOM. Use the object directory
	# instead.
	TMPDIR ?= $(objtree)
	export TMPDIR

	# use plugin aware tools
	AR = $(CROSS_COMPILE)gcc-ar
	NM = $(CROSS_COMPILE)gcc-nm
else
        $(warning WARNING old binutils. LTO disabled)
endif
else
        $(warning "WARNING: Compiler/Linker does not support LTO/WHOPR with linker plugin. CONFIG_LTO disabled.")
endif
else
        $(warning "WARNING: GCC $(call cc-version) too old for LTO/WHOPR. CONFIG_LTO disabled")
endif
endif