Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 6b12b774 authored by Sami Tolvanen's avatar Sami Tolvanen Committed by Alistair Strachan
Browse files

ANDROID: 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 lld will
be used.

Bug: 62093296
Bug: 67506682
Bug: 133186739
Change-Id: Ibcd9fc7ec501b4f30b43b4877897615645f8655f
Signed-off-by: default avatarSami Tolvanen <samitolvanen@google.com>
parent 319e1fdb

Makefile

+28 −1
Original line number Diff line number Diff line
@@ -608,6 +608,16 @@ ifdef CONFIG_FUNCTION_TRACER 
  CC_FLAGS_FTRACE := -pg

endif



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

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

ifdef CONFIG_LTO_CLANG

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

# of objdump for processing symbol versions and exports

LLVM_AR		:= llvm-ar

LLVM_NM		:= llvm-nm

export LLVM_AR LLVM_NM

endif



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

# values of the respective KBUILD_* variables

ARCH_CPPFLAGS :=
@@ -795,6 +805,22 @@ KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections 
LDFLAGS_vmlinux += --gc-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_CFLAGS	:= $(lto-clang-flags)

KBUILD_CFLAGS	+= $(LTO_CFLAGS)



DISABLE_LTO	:= $(DISABLE_LTO_CLANG)

export LTO_CFLAGS DISABLE_LTO

endif



# arch Makefile may override CC so keep this after arch Makefile is included

NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
@@ -1583,7 +1609,8 @@ 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

+38 −0
Original line number Diff line number Diff line
@@ -474,6 +474,44 @@ config STACKPROTECTOR_STRONG 
	  about 20% of all kernel functions, which increases the kernel code

	  size by about 2%.



config LTO

	def_bool n



config ARCH_SUPPORTS_LTO_CLANG

	bool

	help

	  An architecture should select this option if it supports:

	  - compiling with clang,

	  - compiling inline assembly with clang's integrated assembler,

	  - and linking with LLD.



choice

	prompt "Link-Time Optimization (LTO) (EXPERIMENTAL)"

	default LTO_NONE

	help

	  This option turns on Link-Time Optimization (LTO).



config LTO_NONE

	bool "None"



config LTO_CLANG

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

	depends on ARCH_SUPPORTS_LTO_CLANG

	depends on !FTRACE_MCOUNT_RECORD

	depends on CC_IS_CLANG && LD_IS_LLD

	select LTO

	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 and

	  LLD.



endchoice



config HAVE_ARCH_WITHIN_STACK_FRAMES

	bool

	help

include/asm-generic/vmlinux.lds.h

+1 −1
Original line number Diff line number Diff line
@@ -66,7 +66,7 @@ 
 * RODATA_MAIN is not used because existing code already defines .rodata.x

 * sections to be brought in with rodata.

 */

#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION

#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)

#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*

#define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..LPBX*

#define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]*

scripts/Makefile.build

+88 −6
Original line number Diff line number Diff line
@@ -185,6 +185,23 @@ 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_NM) $(@D)/.tmp_$(@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))	\
@@ -197,6 +214,7 @@ cmd_modversions_c = \ 
		mv -f $(@D)/.tmp_$(@F) $@;					\

	fi;

endif

endif



ifdef CONFIG_FTRACE_MCOUNT_RECORD

ifndef CC_USING_RECORD_MCOUNT
@@ -429,6 +447,26 @@ $(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler 
# To build objects in subdirs, we need to descend into the directories

$(sort $(subdir-obj-y)): $(subdir-ym) ;



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



#

# Rule to compile a set of .o files into one .o file

#
@@ -439,7 +477,8 @@ ifdef builtin-target 
# scripts/link-vmlinux.sh builds an aggregate built-in.a with a symbol

# table and index.

quiet_cmd_ar_builtin = AR      $@

      cmd_ar_builtin = rm -f $@; \

      cmd_ar_builtin = $(update_lto_symversions) \

		     rm -f $@; \

                     $(AR) rcSTP$(KBUILD_ARFLAGS) $@ $(filter $(real-obj-y), $^)



$(builtin-target): $(real-obj-y) FORCE
@@ -468,7 +507,11 @@ ifdef lib-target 
quiet_cmd_link_l_target = AR      $@



# lib target archives do get a symbol table and index

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

cmd_link_l_target = 					\

	$(update_lto_symversions)			\

	rm -f $@; 					\

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

	$(update_lto_symtable)



$(lib-target): $(lib-y) FORCE

	$(call if_changed,link_l_target)
@@ -479,13 +522,34 @@ dummy-object = $(obj)/.lib_exports.o 
ksyms-lds = $(dot-target).lds



quiet_cmd_export_list = EXPORTS $@

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

	sed -ne '/___ksymtab/s/.*+\([^ ]*\).*/EXTERN(\1)/p' >$(ksyms-lds);\

	rm -f $(dummy-object);\

filter_export_list = sed -ne '/___ksymtab/s/.*+\([^ ]*\).*/EXTERN(\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)



ifdef CONFIG_LTO_CLANG

# objdump doesn't understand IR files and llvm-nm 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_NM) $$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)
@@ -509,13 +573,31 @@ $($(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)



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)



quiet_cmd_link_multi-m = LD [M]  $@

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



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)



$(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-m)

scripts/Makefile.modpost

+30 −4
Original line number Diff line number Diff line
@@ -83,12 +83,28 @@ 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 $@
@@ -100,7 +116,6 @@ vmlinux.o: FORCE 
# Declare generated files as targets for modpost

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





# Step 5), compile all *.mod.c files



# modname is set to make c_flags define KBUILD_MODNAME
@@ -119,13 +134,24 @@ 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 $(KBUILD_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)