On Wed, Nov 18, 2020 at 2:07 PM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote: > > This change adds build system support for Clang's Link Time > Optimization (LTO). With -flto, instead of ELF object files, Clang > produces LLVM bitcode, which is compiled into native code at link > time, allowing the final binary to be optimized globally. For more > details, see: > > https://llvm.org/docs/LinkTimeOptimization.html > > The Kconfig option CONFIG_LTO_CLANG is implemented as a choice, > which defaults to LTO being disabled. To use LTO, the architecture > must select ARCH_SUPPORTS_LTO_CLANG and support: > > - compiling with Clang, > - compiling inline assembly with Clang's integrated assembler, > - and linking with LLD. > > While using full LTO results in the best runtime performance, the > compilation is not scalable in time or memory. CONFIG_THINLTO > enables ThinLTO, which allows parallel optimization and faster > incremental builds. ThinLTO is used by default if the architecture > also selects ARCH_SUPPORTS_THINLTO: > > https://clang.llvm.org/docs/ThinLTO.html > > To enable LTO, LLVM tools must be used to handle bitcode files. The > easiest way is to pass the LLVM=1 option to make: > > $ make LLVM=1 defconfig > $ scripts/config -e LTO_CLANG > $ make LLVM=1 > > Alternatively, at least the following LLVM tools must be used: > > CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm > > To prepare for LTO support with other compilers, common parts are > gated behind the CONFIG_LTO option, and LTO can be disabled for > specific files by filtering out CC_FLAGS_LTO. > > Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx> > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > Makefile | 19 +++++++- > arch/Kconfig | 75 +++++++++++++++++++++++++++++++ > include/asm-generic/vmlinux.lds.h | 11 +++-- > scripts/Makefile.build | 9 +++- > scripts/Makefile.modfinal | 9 +++- > scripts/Makefile.modpost | 21 ++++++++- > scripts/link-vmlinux.sh | 32 +++++++++---- > 7 files changed, 158 insertions(+), 18 deletions(-) > > diff --git a/Makefile b/Makefile > index 8c8feb4245a6..240560e88d69 100644 > --- a/Makefile > +++ b/Makefile > @@ -893,6 +893,21 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) > export CC_FLAGS_SCS > endif > > +ifdef CONFIG_LTO_CLANG > +ifdef CONFIG_THINLTO > +CC_FLAGS_LTO += -flto=thin -fsplit-lto-unit > +KBUILD_LDFLAGS += --thinlto-cache-dir=$(extmod-prefix).thinlto-cache > +else > +CC_FLAGS_LTO += -flto > +endif > +CC_FLAGS_LTO += -fvisibility=default > +endif > + > +ifdef CONFIG_LTO > +KBUILD_CFLAGS += $(CC_FLAGS_LTO) > +export CC_FLAGS_LTO > +endif > + > ifdef CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_32B > KBUILD_CFLAGS += -falign-functions=32 > endif > @@ -1473,7 +1488,7 @@ MRPROPER_FILES += include/config include/generated \ > *.spec > > # Directories & files removed with 'make distclean' > -DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS > +DISTCLEAN_FILES += tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS .thinlto-cache > > # clean - Delete most, but leave enough to build external modules > # > @@ -1719,7 +1734,7 @@ PHONY += compile_commands.json > > clean-dirs := $(KBUILD_EXTMOD) > clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \ > - $(KBUILD_EXTMOD)/compile_commands.json > + $(KBUILD_EXTMOD)/compile_commands.json $(KBUILD_EXTMOD)/.thinlto-cache > > PHONY += help > help: > diff --git a/arch/Kconfig b/arch/Kconfig > index 56b6ccc0e32d..a41fcb3ca7c6 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -598,6 +598,81 @@ config SHADOW_CALL_STACK > reading and writing arbitrary memory may be able to locate them > and hijack control flow by modifying the stacks. > > +config LTO > + bool > + > +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. > + > +config ARCH_SUPPORTS_THINLTO > + bool > + help > + An architecture should select this option if it supports Clang's > + ThinLTO. > + > +config THINLTO > + bool "Clang ThinLTO" > + depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO > + default y > + help > + This option enables Clang's ThinLTO, which allows for parallel > + optimization and faster incremental compiles. More information > + can be found from Clang's documentation: > + > + https://clang.llvm.org/docs/ThinLTO.html > + > + If you say N here, the compiler will use full LTO, which may > + produce faster code, but building the kernel will be significantly > + slower as the linker won't efficiently utilize multiple threads. > + > + If unsure, say Y. I think the order of these new configs makes it so that ThinLTO appears above LTO in menuconfig; I don't like that, and wish it came immediately after. Does `THINLTO` have to be defined _after_ the choice for LTO_NONE/LTO_CLANG, perhaps? Secondly, I don't like how ThinLTO is a config and not a choice. If I don't set ThinLTO, what am I getting? That's a rhetorical question; I know its full LTO, and I guess the help text does talk about the tradeoffs and what you would get. I guess what's curious to me is "why does it display ThinLTO? Why not FullLTO?" I can't help but wonder if a kconfig `choice` rather than a `config` would be better here, that way it's more obvious the user is making a choice between ThinLTO vs Full LTO, rather than the current patches which look like "ThinkLTO on/off." These are cosmetic concerns, feel free to ignore. Just a thought. > + > +choice > + prompt "Link Time Optimization (LTO)" > + default LTO_NONE > + help > + This option enables Link Time Optimization (LTO), which allows the > + compiler to optimize binaries globally. > + > + If unsure, select LTO_NONE. Note that LTO is very resource-intensive > + so it's disabled by default. > + > +config LTO_NONE > + bool "None" > + > +config LTO_CLANG > + bool "Clang's Link Time Optimization (EXPERIMENTAL)" > + # Clang >= 11: https://github.com/ClangBuiltLinux/linux/issues/510 > + depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD > + depends on $(success,$(NM) --help | head -n 1 | grep -qi llvm) > + depends on $(success,$(AR) --help | head -n 1 | grep -qi llvm) > + depends on ARCH_SUPPORTS_LTO_CLANG > + depends on !FTRACE_MCOUNT_USE_RECORDMCOUNT > + depends on !KASAN > + depends on !GCOV_KERNEL > + depends on !MODVERSIONS > + select LTO > + help > + This option enables Clang's Link Time Optimization (LTO), which > + allows the compiler to optimize the kernel globally. If you enable > + this option, the compiler generates LLVM bitcode instead of ELF > + object files, and the actual compilation from bitcode happens at > + the LTO link step, which may take several minutes depending on the > + kernel configuration. More information can be found from LLVM's > + documentation: > + > + https://llvm.org/docs/LinkTimeOptimization.html > + > + To select this option, you also need to use LLVM tools to handle > + the bitcode by passing LLVM=1 to make. > + > +endchoice > + > config HAVE_ARCH_WITHIN_STACK_FRAMES > bool > help > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index b2b3d81b1535..8988a2e445d8 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -90,15 +90,18 @@ > * .data. We don't want to pull in .data..other sections, which Linux > * has defined. Same for text and bss. > * > + * With LTO_CLANG, the linker also splits sections by default, so we need > + * these macros to combine the sections during the final link. > + * > * 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 DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral* > #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]* > -#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* > -#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* > +#define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L* > +#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral* > #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]* > #else > #define TEXT_MAIN .text > diff --git a/scripts/Makefile.build b/scripts/Makefile.build > index 2175ddb1ee0c..ed74b2f986f7 100644 > --- a/scripts/Makefile.build > +++ b/scripts/Makefile.build > @@ -111,7 +111,7 @@ endif > # --------------------------------------------------------------------------- > > quiet_cmd_cc_s_c = CC $(quiet_modtag) $@ > - cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) -fverbose-asm -S -o $@ $< > + cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS) $(CC_FLAGS_LTO), $(c_flags)) -fverbose-asm -S -o $@ $< > > $(obj)/%.s: $(src)/%.c FORCE > $(call if_changed_dep,cc_s_c) > @@ -425,8 +425,15 @@ $(obj)/lib.a: $(lib-y) FORCE > # Do not replace $(filter %.o,^) with $(real-prereqs). When a single object > # module is turned into a multi object module, $^ will contain header file > # dependencies recorded in the .*.cmd file. > +ifdef CONFIG_LTO_CLANG > +quiet_cmd_link_multi-m = AR [M] $@ > +cmd_link_multi-m = \ > + rm -f $@; \ > + $(AR) cDPrsT $@ $(filter %.o,$^) > +else > quiet_cmd_link_multi-m = LD [M] $@ > cmd_link_multi-m = $(LD) $(ld_flags) -r -o $@ $(filter %.o,$^) > +endif > > $(multi-used-m): FORCE > $(call if_changed,link_multi-m) > diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal > index ae01baf96f4e..2cb9a1d88434 100644 > --- a/scripts/Makefile.modfinal > +++ b/scripts/Makefile.modfinal > @@ -6,6 +6,7 @@ > PHONY := __modfinal > __modfinal: > > +include $(objtree)/include/config/auto.conf > include $(srctree)/scripts/Kbuild.include > > # for c_flags > @@ -29,6 +30,12 @@ quiet_cmd_cc_o_c = CC [M] $@ > > ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink) > > +ifdef CONFIG_LTO_CLANG > +# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to > +# avoid a second slow LTO link > +prelink-ext := .lto > +endif > + > quiet_cmd_ld_ko_o = LD [M] $@ > cmd_ld_ko_o = \ > $(LD) -r $(KBUILD_LDFLAGS) \ > @@ -36,7 +43,7 @@ quiet_cmd_ld_ko_o = LD [M] $@ > -T scripts/module.lds -o $@ $(filter %.o, $^); \ > $(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true) > > -$(modules): %.ko: %.o %.mod.o scripts/module.lds FORCE > +$(modules): %.ko: %$(prelink-ext).o %.mod.o scripts/module.lds FORCE > +$(call if_changed,ld_ko_o) > > targets += $(modules) $(modules:.ko=.mod.o) > diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost > index f54b6ac37ac2..9ff8bfdb574d 100644 > --- a/scripts/Makefile.modpost > +++ b/scripts/Makefile.modpost > @@ -43,6 +43,9 @@ __modpost: > include include/config/auto.conf > include scripts/Kbuild.include > > +# for ld_flags > +include scripts/Makefile.lib > + > MODPOST = scripts/mod/modpost \ > $(if $(CONFIG_MODVERSIONS),-m) \ > $(if $(CONFIG_MODULE_SRCVERSION_ALL),-a) \ > @@ -102,12 +105,26 @@ $(input-symdump): > @echo >&2 'WARNING: Symbol version dump "$@" is missing.' > @echo >&2 ' Modules may not have dependencies or modversions.' > > +ifdef CONFIG_LTO_CLANG > +# With CONFIG_LTO_CLANG, .o files might be LLVM bitcode, so we need to run > +# LTO to compile them into native code before running modpost > +prelink-ext := .lto > + > +quiet_cmd_cc_lto_link_modules = LTO [M] $@ > +cmd_cc_lto_link_modules = $(LD) $(ld_flags) -r -o $@ --whole-archive $^ > + > +%.lto.o: %.o > + $(call if_changed,cc_lto_link_modules) > +endif > + > +modules := $(sort $(shell cat $(MODORDER))) > + > # Read out modules.order to pass in modpost. > # Otherwise, allmodconfig would fail with "Argument list too long". > quiet_cmd_modpost = MODPOST $@ > - cmd_modpost = sed 's/ko$$/o/' $< | $(MODPOST) -T - > + cmd_modpost = sed 's/\.ko$$/$(prelink-ext)\.o/' $< | $(MODPOST) -T - > > -$(output-symdump): $(MODORDER) $(input-symdump) FORCE > +$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(prelink-ext).o) FORCE > $(call if_changed,modpost) > > targets += $(output-symdump) > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh > index 6eded325c837..596507573a48 100755 > --- a/scripts/link-vmlinux.sh > +++ b/scripts/link-vmlinux.sh > @@ -56,6 +56,14 @@ modpost_link() > ${KBUILD_VMLINUX_LIBS} \ > --end-group" > > + if [ -n "${CONFIG_LTO_CLANG}" ]; then > + # This might take a while, so indicate that we're doing > + # an LTO link > + info LTO ${1} > + else > + info LD ${1} > + fi > + > ${LD} ${KBUILD_LDFLAGS} -r -o ${1} ${objects} > } > > @@ -103,13 +111,22 @@ vmlinux_link() > fi > > if [ "${SRCARCH}" != "um" ]; then > - objects="--whole-archive \ > - ${KBUILD_VMLINUX_OBJS} \ > - --no-whole-archive \ > - --start-group \ > - ${KBUILD_VMLINUX_LIBS} \ > - --end-group \ > - ${@}" > + if [ -n "${CONFIG_LTO_CLANG}" ]; then > + # Use vmlinux.o instead of performing the slow LTO > + # link again. > + objects="--whole-archive \ > + vmlinux.o \ > + --no-whole-archive \ > + ${@}" > + else > + objects="--whole-archive \ > + ${KBUILD_VMLINUX_OBJS} \ > + --no-whole-archive \ > + --start-group \ > + ${KBUILD_VMLINUX_LIBS} \ > + --end-group \ > + ${@}" > + fi > > ${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux} \ > ${strip_debug#-Wl,} \ > @@ -274,7 +291,6 @@ fi; > ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init need-builtin=1 > > #link vmlinux.o > -info LD vmlinux.o > modpost_link vmlinux.o > objtool_link vmlinux.o > > -- > 2.29.2.299.gdc1121823c-goog > -- Thanks, ~Nick Desaulniers
