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Commit 475bdd7d authored by Sami Tolvanen's avatar Sami Tolvanen
Browse files

FROMLIST: BACKPORT: kbuild: add support for clang LTO 

This change adds the configuration option CONFIG_LTO_CLANG, and
build system support for clang's Link Time Optimization (LTO). In
preparation for LTO support for other compilers, potentially common
parts of the changes are gated behind CONFIG_LTO instead.

With -flto, instead of object files, clang produces LLVM bitcode,
which is compiled into a native object at link time, allowing the
final binary to be optimized globally. For more details, see:

  https://llvm.org/docs/LinkTimeOptimization.html

While the kernel normally uses GNU ld for linking, LLVM supports LTO
only with lld or GNU gold linkers. This patch set assumes gold will
be used with the LLVMgold plug-in to perform the LTO link step. Due
to potential incompatibilities with GNU ld, this change also adds
LDFINAL_vmlinux for using a different linker for the vmlinux_link
step, and defaults to using GNU ld.

Assuming LLVMgold.so is in LD_LIBRARY_PATH and CONFIG_LTO_CLANG has
been selected, an LTO kernel can be built simply by running make
CC=clang. LTO requires clang >= 5.0 and gold from binutils >= 2.27.

Bug: 62093296
Bug: 67506682
Change-Id: Ibcd9fc7ec501b4f30b43b4877897615645f8655f
(am from https://patchwork.kernel.org/patch/10060329/

)
Signed-off-by: default avatarSami Tolvanen <samitolvanen@google.com>
parent 331f1f5c

Makefile

+53 −1
Original line number Diff line number Diff line
@@ -348,6 +348,7 @@ include scripts/Kbuild.include 
# Make variables (CC, etc...)

AS		= $(CROSS_COMPILE)as

LD		= $(CROSS_COMPILE)ld

LDGOLD		= $(CROSS_COMPILE)ld.gold

CC		= $(CROSS_COMPILE)gcc

CPP		= $(CC) -E

AR		= $(CROSS_COMPILE)ar
@@ -623,6 +624,20 @@ CFLAGS_GCOV := -fprofile-arcs -ftest-coverage -fno-tree-loop-im $(call cc-disabl 
CFLAGS_KCOV	:= $(call cc-option,-fsanitize-coverage=trace-pc,)

export CFLAGS_GCOV CFLAGS_KCOV



# Make toolchain changes before including arch/$(SRCARCH)/Makefile to ensure

# ar/cc/ld-* macros return correct values.

ifdef CONFIG_LTO_CLANG

# use GNU gold with LLVMgold for LTO linking, and LD for vmlinux_link

LDFINAL_vmlinux := $(LD)

LD		:= $(LDGOLD)

LDFLAGS		+= -plugin LLVMgold.so

# use llvm-ar for building symbol tables from IR files, and llvm-dis instead

# of objdump for processing symbol versions and exports

LLVM_AR		:= llvm-ar

LLVM_DIS	:= llvm-dis

export LLVM_AR LLVM_DIS

endif



# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default

# values of the respective KBUILD_* variables

ARCH_CPPFLAGS :=
@@ -641,6 +656,26 @@ KBUILD_CFLAGS += $(call cc-option,-ffunction-sections,) 
KBUILD_CFLAGS	+= $(call cc-option,-fdata-sections,)

endif



ifdef CONFIG_LTO_CLANG

lto-clang-flags	:= -flto -fvisibility=hidden



# allow disabling only clang LTO where needed

DISABLE_LTO_CLANG := -fno-lto -fvisibility=default

export DISABLE_LTO_CLANG

endif



ifdef CONFIG_LTO

lto-flags	:= $(lto-clang-flags)

KBUILD_CFLAGS	+= $(lto-flags)



DISABLE_LTO	:= $(DISABLE_LTO_CLANG)

export DISABLE_LTO



# LDFINAL_vmlinux and LDFLAGS_FINAL_vmlinux can be set to override

# the linker and flags for vmlinux_link.

export LDFINAL_vmlinux LDFLAGS_FINAL_vmlinux

endif



ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE

KBUILD_CFLAGS	+= $(call cc-option,-Oz,-Os)

KBUILD_CFLAGS	+= $(call cc-disable-warning,maybe-uninitialized,)
@@ -1083,6 +1118,22 @@ prepare-objtool: $(objtool_target) 
# CC_STACKPROTECTOR_STRONG! Why did it build with _REGULAR?!")

PHONY += prepare-compiler-check

prepare-compiler-check: FORCE

# Make sure we're using a supported toolchain with LTO_CLANG

ifdef CONFIG_LTO_CLANG

  ifneq ($(call clang-ifversion, -ge, 0500, y), y)

	@echo Cannot use CONFIG_LTO_CLANG: requires clang 5.0 or later >&2 && exit 1

  endif

  ifneq ($(call gold-ifversion, -ge, 112000000, y), y)

	@echo Cannot use CONFIG_LTO_CLANG: requires GNU gold 1.12 or later >&2 && exit 1

  endif

endif

# Make sure compiler supports LTO flags

ifdef lto-flags

  ifeq ($(call cc-option, $(lto-flags)),)

	@echo Cannot use CONFIG_LTO: $(lto-flags) not supported by compiler \

		>&2 && exit 1

  endif

endif

# Make sure compiler supports requested stack protector flag.

ifdef stackp-name

  ifeq ($(call cc-option, $(stackp-flag)),)
@@ -1569,7 +1620,8 @@ clean: $(clean-dirs) 
		-o -name modules.builtin -o -name '.tmp_*.o.*' \

		-o -name '*.c.[012]*.*' \

		-o -name '*.ll' \

		-o -name '*.gcno' \) -type f -print | xargs rm -f

		-o -name '*.gcno' \

		-o -name '*.*.symversions' \) -type f -print | xargs rm -f



# Generate tags for editors

# ---------------------------------------------------------------------------

arch/Kconfig

+30 −0
Original line number Diff line number Diff line
@@ -489,6 +489,36 @@ config LD_DEAD_CODE_DATA_ELIMINATION 
	  sections (e.g., '.text.init'). Typically '.' in section names

	  is used to distinguish them from label names / C identifiers.



config LTO

	bool



config ARCH_SUPPORTS_LTO_CLANG

	bool

	help

	  An architecture should select this option it supports:

	  - compiling with clang,

	  - compiling inline assembly with clang's integrated assembler,

	  - and linking with either lld or GNU gold w/ LLVMgold.



config LTO_CLANG

	bool "Use clang Link Time Optimization (LTO) (EXPERIMENTAL)"

	depends on ARCH_SUPPORTS_LTO_CLANG

	depends on !FTRACE_MCOUNT_RECORD

	select LTO

	select THIN_ARCHIVES

	select LD_DEAD_CODE_DATA_ELIMINATION

	help

          This option enables clang's Link Time Optimization (LTO), which allows

          the compiler to optimize the kernel globally at link time. If you

          enable this option, the compiler generates LLVM IR instead of object

          files, and the actual compilation from IR occurs at the LTO link step,

          which may take several minutes.



          If you select this option, you must compile the kernel with clang >=

          5.0 (make CC=clang) and GNU gold from binutils >= 2.27, and have the

          LLVMgold plug-in in LD_LIBRARY_PATH.





config HAVE_ARCH_WITHIN_STACK_FRAMES

	bool

	help

scripts/Makefile.build

+93 −12
Original line number Diff line number Diff line
@@ -208,6 +208,23 @@ else 


cmd_cc_o_c = $(CC) $(c_flags) -c -o $(@D)/.tmp_$(@F) $<



ifdef CONFIG_LTO_CLANG

# Generate .o.symversions files for each .o with exported symbols, and link these

# to the kernel and/or modules at the end.

cmd_modversions_c =								\

	if $(OBJDUMP) -h $(@D)/.tmp_$(@F) >/dev/null 2>/dev/null; then		\

		if $(OBJDUMP) -h $(@D)/.tmp_$(@F) | grep -q __ksymtab; then	\

			$(call cmd_gensymtypes_c,$(KBUILD_SYMTYPES),$(@:.o=.symtypes)) \

			    > $(@D)/$(@F).symversions;				\

		fi;								\

	else									\

		if $(LLVM_DIS) -o=- $(@D)/.tmp_$(@F) | grep -q __ksymtab; then	\

			$(call cmd_gensymtypes_c,$(KBUILD_SYMTYPES),$(@:.o=.symtypes)) \

			    > $(@D)/$(@F).symversions;				\

		fi;								\

	fi;									\

	mv -f $(@D)/.tmp_$(@F) $@;

else

cmd_modversions_c =								\

	if $(OBJDUMP) -h $(@D)/.tmp_$(@F) | grep -q __ksymtab; then		\

		$(call cmd_gensymtypes_c,$(KBUILD_SYMTYPES),$(@:.o=.symtypes))	\
@@ -220,6 +237,7 @@ cmd_modversions_c = \ 
		mv -f $(@D)/.tmp_$(@F) $@;					\

	fi;

endif

endif



ifdef CONFIG_FTRACE_MCOUNT_RECORD

ifdef BUILD_C_RECORDMCOUNT
@@ -434,9 +452,30 @@ $(sort $(subdir-obj-y)): $(subdir-ym) ; 
#

ifdef builtin-target



ifdef CONFIG_LTO_CLANG

  ifdef CONFIG_MODVERSIONS

    # combine symversions for later processing

    update_lto_symversions =				\

	rm -f $@.symversions; 				\

	for i in $(filter-out FORCE,$^); do		\

		if [ -f $$i.symversions ]; then		\

			cat $$i.symversions 		\

				>> $@.symversions;	\

		fi;					\

	done;

  endif

  # rebuild the symbol table with llvm-ar to include IR files

  update_lto_symtable = ;				\

	mv -f $@ $@.tmp;				\

	$(LLVM_AR) rcsT$(KBUILD_ARFLAGS) $@ 		\

		$$($(AR) t $@.tmp); 			\

	rm -f $@.tmp

endif



ifdef CONFIG_THIN_ARCHIVES

  cmd_make_builtin = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS)

  cmd_make_empty_builtin = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS)

  cmd_make_builtin = $(update_lto_symversions)	\

	rm -f $@; $(AR) rcSTP$(KBUILD_ARFLAGS)

  cmd_make_empty_builtin = rm -f $@; $(AR) rcSTP$(KBUILD_ARFLAGS)

  quiet_cmd_link_o_target = AR      $@

else

  cmd_make_builtin = $(LD) $(ld_flags) -r -o
@@ -476,7 +515,11 @@ ifdef lib-target 
quiet_cmd_link_l_target = AR      $@



ifdef CONFIG_THIN_ARCHIVES

  cmd_link_l_target = rm -f $@; $(AR) rcsT$(KBUILD_ARFLAGS) $@ $(lib-y)

  cmd_link_l_target = 					\

	$(update_lto_symversions)			\

	rm -f $@; 					\

	$(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(lib-y)	\

	$(update_lto_symtable)

else

  cmd_link_l_target = rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@ $(lib-y)

endif
@@ -494,14 +537,36 @@ else 
ref_prefix = EXTERN(

endif



quiet_cmd_export_list = EXPORTS $@

cmd_export_list = $(OBJDUMP) -h $< | \

	sed -ne '/___ksymtab/{s/.*+/$(ref_prefix)/;s/ .*/)/;p}' >$(ksyms-lds);\

	rm -f $(dummy-object);\

	$(AR) rcs$(KBUILD_ARFLAGS) $(dummy-object);\

filter_export_list = sed -ne '/___ksymtab/s/.*+\([^ "]*\).*/$(ref_prefix)\1)/p'

link_export_list = rm -f $(dummy-object);\

	echo | $(CC) $(a_flags) -c -o $(dummy-object) -x assembler -;\

	$(LD) $(ld_flags) -r -o $@ -T $(ksyms-lds) $(dummy-object);\

	rm $(dummy-object) $(ksyms-lds)



quiet_cmd_export_list = EXPORTS $@



ifdef CONFIG_LTO_CLANG

# objdump doesn't understand IR files and llvm-dis doesn't support archives,

# so we'll walk through each file in the archive separately

cmd_export_list = 					\

	rm -f $(ksyms-lds);				\

	for o in $$($(AR) t $<); do			\

		if $(OBJDUMP) -h $$o >/dev/null 2>/dev/null; then \

			$(OBJDUMP) -h $$o | 		\

				$(filter_export_list) 	\

				>>$(ksyms-lds);		\

		else					\

			$(LLVM_DIS) -o=- $$o |		\

				$(filter_export_list) 	\

				>>$(ksyms-lds);		\

		fi; 					\

	done;						\

	$(link_export_list)

else

cmd_export_list = $(OBJDUMP) -h $< | $(filter_export_list) >$(ksyms-lds); \

	$(link_export_list)

endif



$(obj)/lib-ksyms.o: $(lib-target) FORCE

	$(call if_changed,export_list)
@@ -525,20 +590,36 @@ $($(subst $(obj)/,,$(@:.o=-objs))) \ 
$($(subst $(obj)/,,$(@:.o=-y)))       \

$($(subst $(obj)/,,$(@:.o=-m)))), $^)



cmd_link_multi-link = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalysis)



ifdef CONFIG_THIN_ARCHIVES

  quiet_cmd_link_multi-y = AR      $@

  cmd_link_multi-y = $(update_lto_symversions) \

	rm -f $@; $(AR) rcSTP$(KBUILD_ARFLAGS) $@ $(link_multi_deps) \

	$(update_lto_symtable)

else

  quiet_cmd_link_multi-y = LD      $@

cmd_link_multi-y = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalysis)

  cmd_link_multi-y = $(cmd_link_multi-link)

endif



quiet_cmd_link_multi-m = LD [M]  $@



ifdef CONFIG_LTO_CLANG

  # don't compile IR until needed

  cmd_link_multi-m = $(cmd_link_multi-y)

else

  cmd_link_multi-m = $(cmd_link_multi-link)

endif



$(multi-used-y): FORCE

	$(call if_changed,link_multi-y)

$(call multi_depend, $(multi-used-y), .o, -objs -y)



$(multi-used-m): FORCE

	$(call if_changed,link_multi-m)

	@{ echo $(@:.o=.ko); echo $(link_multi_deps); \

	   $(cmd_undef_syms); } > $(MODVERDIR)/$(@F:.o=.mod)



$(call multi_depend, $(multi-used-y), .o, -objs -y)

$(call multi_depend, $(multi-used-m), .o, -objs -y -m)



targets += $(multi-used-y) $(multi-used-m)

scripts/Makefile.modpost

+33 −7
Original line number Diff line number Diff line
@@ -82,12 +82,28 @@ modpost = scripts/mod/modpost \ 


MODPOST_OPT=$(subst -i,-n,$(filter -i,$(MAKEFLAGS)))



# If CONFIG_LTO_CLANG is enabled, .o files are either LLVM IR, or empty, so we

# need to link them into actual objects before passing them to modpost

modpost-ext = $(if $(CONFIG_LTO_CLANG),.lto,)



ifdef CONFIG_LTO_CLANG

quiet_cmd_cc_lto_link_modules = LD [M]  $@

cmd_cc_lto_link_modules =						\

	$(LD) $(ld_flags) -r -o $(@)					\

		$(shell [ -s $(@:$(modpost-ext).o=.o.symversions) ] &&	\

			echo -T $(@:$(modpost-ext).o=.o.symversions))	\

		--whole-archive $(filter-out FORCE,$^)



$(modules:.ko=$(modpost-ext).o): %$(modpost-ext).o: %.o FORCE

	$(call if_changed,cc_lto_link_modules)

endif



# We can go over command line length here, so be careful.

quiet_cmd_modpost = MODPOST $(words $(filter-out vmlinux FORCE, $^)) modules

      cmd_modpost = $(MODLISTCMD) | sed 's/\.ko$$/.o/' | $(modpost) $(MODPOST_OPT) -s -T -

      cmd_modpost = $(MODLISTCMD) | sed 's/\.ko$$/$(modpost-ext)\.o/' | $(modpost) $(MODPOST_OPT) -s -T -



PHONY += __modpost

__modpost: $(modules:.ko=.o) FORCE

__modpost: $(modules:.ko=$(modpost-ext).o) FORCE

	$(call cmd,modpost) $(wildcard vmlinux)



quiet_cmd_kernel-mod = MODPOST $@
@@ -98,8 +114,7 @@ vmlinux.o: FORCE 


# Declare generated files as targets for modpost

$(symverfile):         __modpost ;

$(modules:.ko=.mod.c): __modpost ;



$(modules:.ko=$(modpost-ext).mod.c): __modpost ;



# Step 5), compile all *.mod.c files
@@ -110,22 +125,33 @@ quiet_cmd_cc_o_c = CC $@ 
      cmd_cc_o_c = $(CC) $(c_flags) $(KBUILD_CFLAGS_MODULE) $(CFLAGS_MODULE) \

		   -c -o $@ $<



$(modules:.ko=.mod.o): %.mod.o: %.mod.c FORCE

$(modules:.ko=.mod.o): %.mod.o: %$(modpost-ext).mod.c FORCE

	$(call if_changed_dep,cc_o_c)



targets += $(modules:.ko=.mod.o)

targets += $(modules:.ko=$(modpost-ext).mod.o)



ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)



# Step 6), final link of the modules with optional arch pass after final link

quiet_cmd_ld_ko_o = LD [M]  $@



ifdef CONFIG_LTO_CLANG

      cmd_ld_ko_o = 							\

	$(LD) -r $(LDFLAGS)                                 		\

		 $(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) 		\

		 $(shell [ -s $(@:.ko=.o.symversions) ] &&		\

			echo -T $(@:.ko=.o.symversions))  		\

		 -o $@ --whole-archive					\

		 $(filter-out FORCE,$(^:$(modpost-ext).o=.o))

else

      cmd_ld_ko_o =                                                     \

	$(LD) -r $(LDFLAGS)                                             \

                 $(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE)             \

                 -o $@ $(filter-out FORCE,$^) ;                         \

	$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)

endif



$(modules): %.ko :%.o %.mod.o FORCE

$(modules): %.ko: %$(modpost-ext).o %.mod.o FORCE

	+$(call if_changed,ld_ko_o)



targets += $(modules)

scripts/link-vmlinux.sh

+67 −14
Original line number Diff line number Diff line
@@ -53,9 +53,40 @@ archive_builtin() 
		${AR} rcsT${KBUILD_ARFLAGS} built-in.o			\

					${KBUILD_VMLINUX_INIT}		\

					${KBUILD_VMLINUX_MAIN}



		if [ -n "${CONFIG_LTO_CLANG}" ]; then

			mv -f built-in.o built-in.o.tmp

			${LLVM_AR} rcsT${KBUILD_ARFLAGS} built-in.o $(${AR} t built-in.o.tmp)

			rm -f built-in.o.tmp

		fi

	fi

}



# If CONFIG_LTO_CLANG is selected, collect generated symbol versions into

# .tmp_symversions

modversions()

{

	if [ -z "${CONFIG_LTO_CLANG}" ]; then

		return

	fi



	if [ -z "${CONFIG_MODVERSIONS}" ]; then

		return

	fi



	rm -f .tmp_symversions



	for a in built-in.o ${KBUILD_VMLINUX_LIBS}; do

		for o in $(${AR} t $a); do

			if [ -f ${o}.symversions ]; then

				cat ${o}.symversions >> .tmp_symversions

			fi

		done

	done



	echo "-T .tmp_symversions"

}



# Link of vmlinux.o used for section mismatch analysis

# ${1} output file

modpost_link()
@@ -70,7 +101,16 @@ modpost_link() 
			${KBUILD_VMLINUX_MAIN}				\

			--end-group"

	fi

	${LD} ${LDFLAGS} -r -o ${1} ${objects}



	if [ -n "${CONFIG_LTO_CLANG}" ]; then

		# This might take a while, so indicate that we're doing

		# an LTO link

		info LTO vmlinux.o

	else

		info LD vmlinux.o

	fi



	${LD} ${LDFLAGS} -r -o ${1} $(modversions) ${objects}

}



# Link of vmlinux
@@ -82,7 +122,15 @@ vmlinux_link() 
	local objects



	if [ "${SRCARCH}" != "um" ]; then

		if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then

		local ld=${LD}

		local ldflags="${LDFLAGS} ${LDFLAGS_vmlinux}"



		if [ -n "${LDFINAL_vmlinux}" ]; then

			ld=${LDFINAL_vmlinux}

			ldflags="${LDFLAGS_FINAL_vmlinux} ${LDFLAGS_vmlinux}"

		fi



		if [[ -n "${CONFIG_THIN_ARCHIVES}" && -z "${CONFIG_LTO_CLANG}" ]]; then

			objects="--whole-archive built-in.o ${1}"

		else

			objects="${KBUILD_VMLINUX_INIT}			\
@@ -92,8 +140,7 @@ vmlinux_link() 
				${1}"

		fi



		${LD} ${LDFLAGS} ${LDFLAGS_vmlinux} -o ${2}		\

			-T ${lds} ${objects}

		${ld} ${ldflags} -o ${2} -T ${lds} ${objects}

	else

		if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then

			objects="-Wl,--whole-archive built-in.o ${1}"
@@ -113,7 +160,6 @@ vmlinux_link() 
	fi

}





# Create ${2} .o file with all symbols from the ${1} object file

kallsyms()

{
@@ -164,6 +210,7 @@ cleanup() 
	rm -f .tmp_System.map

	rm -f .tmp_kallsyms*

	rm -f .tmp_version

	rm -f .tmp_symversions

	rm -f .tmp_vmlinux*

	rm -f built-in.o

	rm -f System.map
@@ -209,15 +256,6 @@ case "${KCONFIG_CONFIG}" in 
	. "./${KCONFIG_CONFIG}"

esac



archive_builtin



#link vmlinux.o

info LD vmlinux.o

modpost_link vmlinux.o



# modpost vmlinux.o to check for section mismatches

${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o



# Update version

info GEN .version

if [ ! -r .version ]; then
@@ -228,9 +266,24 @@ else 
	expr 0$(cat .old_version) + 1 >.version;

fi;



archive_builtin



#link vmlinux.o

modpost_link vmlinux.o



# modpost vmlinux.o to check for section mismatches

${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o



# final build of init/

${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init GCC_PLUGINS_CFLAGS="${GCC_PLUGINS_CFLAGS}"



if [ -n "${CONFIG_LTO_CLANG}" ]; then

	# Re-use vmlinux.o, so we can avoid the slow LTO link step in

	# vmlinux_link

	KBUILD_VMLINUX_INIT=

	KBUILD_VMLINUX_MAIN=vmlinux.o

fi



kallsymso=""

kallsyms_vmlinux=""

if [ -n "${CONFIG_KALLSYMS}" ]; then