[PATCH v2 12/18] kbuild: add support for clang LTO

Sami Tolvanen
2017-11-15T21:35:04Z

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. Recommended versions are >= 5.0 for clang, and >= 2.27 for
binutils.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
.gitignore | 2 ++
Makefile | 54 +++++++++++++++++++++++++++-
arch/Kconfig | 32 +++++++++++++++++
scripts/Makefile.build | 66 +++++++++++++++++++++++++++++++++-
scripts/Makefile.modpost | 63 ++++++++++++++++++++++++++++-----
scripts/link-vmlinux.sh | 92 ++++++++++++++++++++++++++++++++++++++++++------
6 files changed, 288 insertions(+), 21 deletions(-)

diff --git a/.gitignore b/.gitignore
index 6c119eab5d46..ac236e2bb9b1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -11,6 +11,7 @@
#
.*
*.a
+*.a.*
*.bin
*.bz2
*.c.[012]*.*
@@ -28,6 +29,7 @@
*.lzma
*.lzo
*.mod.c
+*.modversions
*.o
*.o.*
*.order
diff --git a/Makefile b/Makefile
index f976af9525bf..8141b4c8f1bf 100644
--- a/Makefile
+++ b/Makefile
@@ -350,6 +350,7 @@ 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 +624,15 @@ endif
# Defaults to vmlinux, but the arch makefile usually adds further targets
all: vmlinux

+# Make toolchain changes before including arch/$(SRCARCH)/Makefile to ensure
+# 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_GOLD += -plugin LLVMgold.so
+endif
+
# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
# values of the respective KBUILD_* variables
ARCH_CPPFLAGS :=
@@ -789,6 +799,32 @@ 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
+export DISABLE_LTO_CLANG
+
+ifdef CONFIG_MODVERSIONS
+# llvm-dis is used instead of objdump to process LLVM IR files
+LLVM_DIS := llvm-dis
+export LLVM_DIS
+endif
+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
+
# arch Makefile may override CC so keep this after arch Makefile is included
NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
CHECKFLAGS += $(NOSTDINC_FLAGS)
@@ -1090,6 +1126,19 @@ prepare-objtool: $(objtool_target)
# CC_STACKPROTECTOR_STRONG! Why did it build with _REGULAR?!")
PHONY += prepare-compiler-check
prepare-compiler-check: FORCE
+# Make sure we're using clang with LTO_CLANG
+ifdef CONFIG_LTO_CLANG
+ ifneq ($(cc-name),clang)
+ @echo Cannot use CONFIG_LTO_CLANG without CC=clang >&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)),)
@@ -1564,7 +1613,10 @@ 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 '*.[oa].objects' \
+ -o -name '*.o.symversions' \
+ -o -name '*.modversions' \) -type f -print | xargs rm -f

# Generate tags for editors
# ---------------------------------------------------------------------------
diff --git a/arch/Kconfig b/arch/Kconfig
index 400b9e1b2f27..bb5296ecebdd 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -585,6 +585,7 @@ config CC_STACKPROTECTOR_STRONG
endchoice

config THIN_ARCHIVES
+ depends on !LTO_CLANG
def_bool y
help
Select this if the architecture wants to use thin archives
@@ -605,6 +606,37 @@ 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)"
+ depends on ARCH_SUPPORTS_LTO_CLANG
+ depends on !FTRACE_MCOUNT_RECORD
+ select LTO
+ 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
+ (make CC=clang) and have the LVMgold plug-in in LD_LIBRARY_PATH.
+
+ Using clang >= 5.0 and GNU gold from binutils >= 2.27 is recommended
+ for this option.
+
+
config HAVE_ARCH_WITHIN_STACK_FRAMES
bool
help
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index e63af4e19382..e8bf5c440612 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -210,6 +210,23 @@ 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 echo '$(c_flags)' | grep -q -- '$(DISABLE_LTO_CLANG)'; 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)) \
@@ -222,6 +239,7 @@ cmd_modversions_c = \
mv -f $(@D)/.tmp_$(@F) $@; \
fi;
endif
+endif

ifdef CONFIG_FTRACE_MCOUNT_RECORD
ifdef BUILD_C_RECORDMCOUNT
@@ -443,6 +461,13 @@ $(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
+# If LTO is enabled, we remove all intermediate linking steps and instead
+# collect a list of all objects to be linked at the end.
+cc_lto_objects = $(foreach o,$(1),\
+ [ -f ${o}.objects ] && cat ${o}.objects || echo ${o};)
+endif
+
#
# Rule to compile a set of .o files into one .o file
#
@@ -458,6 +483,16 @@ else
quiet_cmd_link_o_target = LD $@
endif

+ifdef CONFIG_LTO_CLANG
+builtin-cmds = $(call cc_lto_objects,$(filter $(obj-y), $^))
+
+quiet_cmd_update_builtin = GEN $@
+cmd_update_builtin = (cat /dev/null; $(builtin-cmds)) > $@.objects && \
+ cat /dev/null > $@
+
+$(builtin-target): $(obj-y) FORCE
+ $(call if_changed,update_builtin)
+else
# If the list of objects to link is empty, just create an empty built-in.o
cmd_link_o_target = $(if $(strip $(obj-y)),\
$(cmd_make_builtin) $@ $(filter $(obj-y), $^) \
@@ -466,6 +501,7 @@ cmd_link_o_target = $(if $(strip $(obj-y)),\

$(builtin-target): $(obj-y) FORCE
$(call if_changed,link_o_target)
+endif

targets += $(builtin-target)
endif # builtin-target
@@ -487,6 +523,16 @@ $(modorder-target): $(subdir-ym) FORCE
# Rule to compile a set of .o files into one .a file
#
ifdef lib-target
+ifdef CONFIG_LTO_CLANG
+lib-target-cmds = $(call cc_lto_objects,$(lib-y))
+
+quiet_cmd_update_lib_target = GEN $@
+cmd_update_lib_target = (cat /dev/null; $(lib-target-cmds)) > $@.objects && \
+ cat /dev/null > $@
+
+$(lib-target): $(lib-y) FORCE
+ $(call if_changed,update_lib_target)
+else
quiet_cmd_link_l_target = AR $@

ifdef CONFIG_THIN_ARCHIVES
@@ -497,6 +543,7 @@ endif

$(lib-target): $(lib-y) FORCE
$(call if_changed,link_l_target)
+endif

targets += $(lib-target)

@@ -552,14 +599,31 @@ endif
quiet_cmd_link_multi-m = LD [M] $@
cmd_link_multi-m = $(cmd_link_multi-link)

+ifdef CONFIG_LTO_CLANG
+multi-deps-cmds = $(call cc_lto_objects,$(link_multi_deps))
+
+quiet_cmd_update_multi_deps = GEN $@
+cmd_update_multi_deps = (cat /dev/null; $(multi-deps-cmds)) > $@.objects && \
+ cat /dev/null > $@
+
+$(multi-used-y): FORCE
+ $(call if_changed,update_multi_deps)
+
+$(multi-used-m): FORCE
+ $(call if_changed,update_multi_deps)
+ @{ echo $(@:.o=.ko); echo $(link_multi_deps); \
+ $(cmd_undef_syms); } > $(MODVERDIR)/$(@F:.o=.mod)
+else
$(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)
+endif
+
+$(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)
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 991db7d6e4df..cb1c040a006c 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -83,12 +83,46 @@ 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_modversions = GEN [M] $@
+cmd_cc_lto_modversions = \
+ rm -f $(@); \
+ if [ -f $(@:.modversions=.o).objects ]; then \
+ for i in `cat $(@:.modversions=.o).objects`; do \
+ [ -s $$i.symversions ] && \
+ cat $$i.symversions >> $(@); \
+ done; \
+ else \
+ [ -s $(@:.modversions=.o).symversions ] && \
+ cat $(@:.modversions=.o).symversions >> $(@); \
+ fi
+
+$(modules:.ko=.modversions): FORCE
+ $(call if_changed,cc_lto_modversions)
+
+quiet_cmd_cc_lto_link_modules = LD [M] $@
+cmd_cc_lto_link_modules = \
+ $(LD) $(ld_flags) -r -o $(@) \
+ $(shell [ -s $(@:$(modpost-ext).o=.modversions) ] && \
+ echo -T $(@:$(modpost-ext).o=.modversions)) \
+ $(shell [ -f $(@:$(modpost-ext).o=.o).objects ] && \
+ cat $(@:$(modpost-ext).o=.o).objects || \
+ echo $(@:$(modpost-ext).o=.o))
+
+$(modules:.ko=$(modpost-ext).o): %$(modpost-ext).o: %.o %.modversions 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 +132,7 @@ vmlinux.o: FORCE
$(call cmd,kernel-mod)

# Declare generated files as targets for modpost
-$(modules:.ko=.mod.c): __modpost ;
-
+$(modules:.ko=$(modpost-ext).mod.c): __modpost ;

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

@@ -110,23 +143,37 @@ 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
+lto_ko_objects = $(foreach o,$(1:$(modpost-ext).o=.o), \
+ $(shell [ -f $(o).objects ] && \
+ cat $(o).objects || echo $(o)))
+
+ cmd_ld_ko_o = \
+ $(LD) -r $(LDFLAGS) \
+ $(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \
+ $(shell [ -s $(@:.ko=.modversions) ] && \
+ echo -T $(@:.ko=.modversions)) \
+ -o $@ $(call lto_ko_objects, $(filter-out FORCE,$^))
+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

-$(modules): %.ko :%.o %.mod.o FORCE
- +$(call if_changed,ld_ko_o)
+$(modules): %.ko: %$(modpost-ext).o %.mod.o FORCE
+ $(call if_changed,ld_ko_o)

targets += $(modules)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index e6818b8e7141..fccc123b691f 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -64,6 +64,53 @@ archive_builtin()
fi
}

+# If CONFIG_LTO_CLANG is selected, the compiler produces LLVM IR files instead
+# of ELF object files. This function expands individual IR files from a list of
+# objects that would have otherwise been linked already.
+expand()
+{
+ if [ -z "${CONFIG_LTO_CLANG}" ]; then
+ echo $*
+ fi
+
+ local objs
+
+ for o in $*; do
+ if [ -f ${o}.objects ]; then
+ objs="${objs} $(xargs < ${o}.objects)"
+ else
+ objs="${objs} ${o}"
+ fi
+ done
+
+ echo "${objs}"
+}
+
+# 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 o in $(expand ${KBUILD_VMLINUX_INIT}) \
+ $(expand ${KBUILD_VMLINUX_MAIN}) \
+ $(expand ${KBUILD_VMLINUX_LIBS}); do
+ if [ -f ${o}.symversions ]; then
+ cat ${o}.symversions >> .tmp_symversions
+ fi
+ done
+
+ echo "-T .tmp_symversions"
+}
+
# Link of vmlinux.o used for section mismatch analysis
# ${1} output file
modpost_link()
@@ -78,13 +125,22 @@ modpost_link()
${KBUILD_VMLINUX_LIBS} \
--end-group"
else
- objects="${KBUILD_VMLINUX_INIT} \
+ objects="$(expand ${KBUILD_VMLINUX_INIT}) \
--start-group \
- ${KBUILD_VMLINUX_MAIN} \
- ${KBUILD_VMLINUX_LIBS} \
+ $(expand ${KBUILD_VMLINUX_MAIN}) \
+ $(expand ${KBUILD_VMLINUX_LIBS}) \
--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
@@ -96,6 +152,14 @@ vmlinux_link()
local objects

if [ "${SRCARCH}" != "um" ]; then
+ local ld=${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}" ]; then
objects="--whole-archive \
built-in.o \
@@ -105,16 +169,15 @@ vmlinux_link()
--end-group \
${1}"
else
- objects="${KBUILD_VMLINUX_INIT} \
+ objects="$(expand ${KBUILD_VMLINUX_INIT}) \
--start-group \
- ${KBUILD_VMLINUX_MAIN} \
- ${KBUILD_VMLINUX_LIBS} \
+ $(expand ${KBUILD_VMLINUX_MAIN}) \
+ $(expand ${KBUILD_VMLINUX_LIBS}) \
--end-group \
${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 \
@@ -141,7 +204,6 @@ vmlinux_link()
fi
}

-
# Create ${2} .o file with all symbols from the ${1} object file
kallsyms()
{
@@ -192,6 +254,7 @@ 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
@@ -253,12 +316,19 @@ ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init GCC_PLUGINS_CFLAGS="${GC
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

+if [ -n "${CONFIG_LTO_CLANG}" ]; then
+ # Re-use vmlinux.o, so we can avoid the slow LTO link step in
+ # vmlinux_link
+ KBUILD_VMLINUX_INIT=
+ KBUILD_VMLINUX_MAIN=vmlinux.o
+ KBUILD_VMLINUX_LIBS=
+fi
+
kallsymso=""
kallsyms_vmlinux=""
if [ -n "${CONFIG_KALLSYMS}" ]; then
--
2.15.0.448.gf294e3d99a-goog