| Message ID | 20200624203200.78870-3-samitolvanen@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | add support for Clang LTO | expand |
On Wed, Jun 24, 2020 at 1:32 PM Sami Tolvanen <samitolvanen@google.com> 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. > > Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in > follow-up patches. > > Signed-off-by: Sami Tolvanen <samitolvanen@google.com> > --- > Makefile | 16 ++++++++ > arch/Kconfig | 66 +++++++++++++++++++++++++++++++ > include/asm-generic/vmlinux.lds.h | 11 ++++-- > scripts/Makefile.build | 9 ++++- > scripts/Makefile.modfinal | 9 ++++- > scripts/Makefile.modpost | 24 ++++++++++- > scripts/link-vmlinux.sh | 32 +++++++++++---- > 7 files changed, 151 insertions(+), 16 deletions(-) > > diff --git a/Makefile b/Makefile > index ac2c61c37a73..0c7fe6fb2143 100644 > --- a/Makefile > +++ b/Makefile > @@ -886,6 +886,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) > export CC_FLAGS_SCS > endif > > +ifdef CONFIG_LTO_CLANG > +ifdef CONFIG_THINLTO > +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit) The kconfig change gates this on clang-11; do we still need the cc-option check here, or can we hardcode the use of -fsplit-lto-unit? Playing with the flag in godbolt, it looks like clang-8 had support for this flag. > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache It might be nice to have `make distclean` or even `make clean` scrub the .thinlto-cache? Also, I verified that the `.gitignore` rule for `.*` properly ignores this dir. > +else > +CC_FLAGS_LTO_CLANG := -flto > +endif > +CC_FLAGS_LTO_CLANG += -fvisibility=default > +endif > + > +ifdef CONFIG_LTO > +CC_FLAGS_LTO := $(CC_FLAGS_LTO_CLANG) > +KBUILD_CFLAGS += $(CC_FLAGS_LTO) > +export CC_FLAGS_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) > > diff --git a/arch/Kconfig b/arch/Kconfig > index 8cc35dc556c7..e00b122293f8 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -552,6 +552,72 @@ 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 > + > +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. > + > +config LTO_NONE > + bool "None" > + > +config LTO_CLANG > + bool "Clang's Link Time Optimization (EXPERIMENTAL)" > + 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_RECORD > + depends on !KASAN > + 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 db600ef218d7..78079000c05a 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -89,15 +89,18 @@ > * .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 2e8810b7e5ed..f307e708a1b7 100644 > --- a/scripts/Makefile.build > +++ b/scripts/Makefile.build > @@ -108,7 +108,7 @@ endif > # --------------------------------------------------------------------------- > > quiet_cmd_cc_s_c = CC $(quiet_modtag) $@ > - cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) $(DISABLE_LTO) -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) > @@ -424,8 +424,15 @@ $(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 $@; \ > + $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(filter %.o,$^) > +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 411c1e600e7d..1005b147abd0 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) \ > @@ -37,7 +44,7 @@ quiet_cmd_ld_ko_o = LD [M] $@ > -o $@ $(filter %.o, $^); \ > $(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true) > > -$(modules): %.ko: %.o %.mod.o $(KBUILD_LDS_MODULE) FORCE > +$(modules): %.ko: %$(prelink-ext).o %.mod.o $(KBUILD_LDS_MODULE) FORCE > +$(call if_changed,ld_ko_o) > > targets += $(modules) $(modules:.ko=.mod.o) > diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost > index 3651cbf6ad49..9ced8aecd579 100644 > --- a/scripts/Makefile.modpost > +++ b/scripts/Makefile.modpost > @@ -102,12 +102,32 @@ $(input-symdump): > @echo >&2 'WARNING: Symbol version dump "$@" 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 $(filter-out FORCE,$^) > + > +%.lto.o: %.o FORCE > + $(call if_changed,cc_lto_link_modules) > + > +PHONY += FORCE > +FORCE: > + > +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 92dd745906f4..a681b3b6722e 100755 > --- a/scripts/link-vmlinux.sh > +++ b/scripts/link-vmlinux.sh > @@ -52,6 +52,14 @@ 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} ${objects} > } > > @@ -99,13 +107,22 @@ vmlinux_link() > fi > > if [ "${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,} \ > @@ -270,7 +287,6 @@ 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 > > -- > 2.27.0.212.ge8ba1cc988-goog >
On Wed, Jun 24, 2020 at 01:53:52PM -0700, Nick Desaulniers wrote: > On Wed, Jun 24, 2020 at 1:32 PM Sami Tolvanen <samitolvanen@google.com> wrote: > > > > diff --git a/Makefile b/Makefile > > index ac2c61c37a73..0c7fe6fb2143 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -886,6 +886,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) > > export CC_FLAGS_SCS > > endif > > > > +ifdef CONFIG_LTO_CLANG > > +ifdef CONFIG_THINLTO > > +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit) > > The kconfig change gates this on clang-11; do we still need the > cc-option check here, or can we hardcode the use of -fsplit-lto-unit? > Playing with the flag in godbolt, it looks like clang-8 had support > for this flag. True, we don't need cc-option here anymore. I'll remove it, thanks. > > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache > > It might be nice to have `make distclean` or even `make clean` scrub > the .thinlto-cache? Also, I verified that the `.gitignore` rule for > `.*` properly ignores this dir. Sure, distclean sounds appropriate to me. Sami
Hi Sami, On Wed, Jun 24, 2020 at 01:31:40PM -0700, 'Sami Tolvanen' via Clang Built Linux 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. > > Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in > follow-up patches. > > Signed-off-by: Sami Tolvanen <samitolvanen@google.com> > --- > Makefile | 16 ++++++++ > arch/Kconfig | 66 +++++++++++++++++++++++++++++++ > include/asm-generic/vmlinux.lds.h | 11 ++++-- > scripts/Makefile.build | 9 ++++- > scripts/Makefile.modfinal | 9 ++++- > scripts/Makefile.modpost | 24 ++++++++++- > scripts/link-vmlinux.sh | 32 +++++++++++---- > 7 files changed, 151 insertions(+), 16 deletions(-) > > diff --git a/Makefile b/Makefile > index ac2c61c37a73..0c7fe6fb2143 100644 > --- a/Makefile > +++ b/Makefile > @@ -886,6 +886,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) > export CC_FLAGS_SCS > endif > > +ifdef CONFIG_LTO_CLANG > +ifdef CONFIG_THINLTO > +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit) > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache > +else > +CC_FLAGS_LTO_CLANG := -flto > +endif > +CC_FLAGS_LTO_CLANG += -fvisibility=default > +endif > + > +ifdef CONFIG_LTO > +CC_FLAGS_LTO := $(CC_FLAGS_LTO_CLANG) > +KBUILD_CFLAGS += $(CC_FLAGS_LTO) > +export CC_FLAGS_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) > > diff --git a/arch/Kconfig b/arch/Kconfig > index 8cc35dc556c7..e00b122293f8 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -552,6 +552,72 @@ 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 > + > +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. > + > +config LTO_NONE > + bool "None" > + > +config LTO_CLANG > + bool "Clang's Link Time Optimization (EXPERIMENTAL)" > + depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD I am curious, what is the reason for gating this at clang 11.0.0? Presumably this? https://github.com/ClangBuiltLinux/linux/issues/510 It might be nice to notate this so that we do not have to wonder :) Cheers, Nathan
On Wed, Jun 24, 2020 at 07:26:47PM -0700, Nathan Chancellor wrote: > Hi Sami, > > On Wed, Jun 24, 2020 at 01:31:40PM -0700, 'Sami Tolvanen' via Clang Built Linux 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. > > > > Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in > > follow-up patches. > > > > Signed-off-by: Sami Tolvanen <samitolvanen@google.com> > > --- > > Makefile | 16 ++++++++ > > arch/Kconfig | 66 +++++++++++++++++++++++++++++++ > > include/asm-generic/vmlinux.lds.h | 11 ++++-- > > scripts/Makefile.build | 9 ++++- > > scripts/Makefile.modfinal | 9 ++++- > > scripts/Makefile.modpost | 24 ++++++++++- > > scripts/link-vmlinux.sh | 32 +++++++++++---- > > 7 files changed, 151 insertions(+), 16 deletions(-) > > > > diff --git a/Makefile b/Makefile > > index ac2c61c37a73..0c7fe6fb2143 100644 > > --- a/Makefile > > +++ b/Makefile > > @@ -886,6 +886,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) > > export CC_FLAGS_SCS > > endif > > > > +ifdef CONFIG_LTO_CLANG > > +ifdef CONFIG_THINLTO > > +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit) > > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache > > +else > > +CC_FLAGS_LTO_CLANG := -flto > > +endif > > +CC_FLAGS_LTO_CLANG += -fvisibility=default > > +endif > > + > > +ifdef CONFIG_LTO > > +CC_FLAGS_LTO := $(CC_FLAGS_LTO_CLANG) > > +KBUILD_CFLAGS += $(CC_FLAGS_LTO) > > +export CC_FLAGS_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) > > > > diff --git a/arch/Kconfig b/arch/Kconfig > > index 8cc35dc556c7..e00b122293f8 100644 > > --- a/arch/Kconfig > > +++ b/arch/Kconfig > > @@ -552,6 +552,72 @@ 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 > > + > > +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. > > + > > +config LTO_NONE > > + bool "None" > > + > > +config LTO_CLANG > > + bool "Clang's Link Time Optimization (EXPERIMENTAL)" > > + depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD > > I am curious, what is the reason for gating this at clang 11.0.0? > > Presumably this? https://github.com/ClangBuiltLinux/linux/issues/510 > > It might be nice to notate this so that we do not have to wonder :) Yes, that's the reason. I'll add a note about it. Thanks! Sami
diff --git a/Makefile b/Makefile index ac2c61c37a73..0c7fe6fb2143 100644 --- a/Makefile +++ b/Makefile @@ -886,6 +886,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) export CC_FLAGS_SCS endif +ifdef CONFIG_LTO_CLANG +ifdef CONFIG_THINLTO +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit) +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache +else +CC_FLAGS_LTO_CLANG := -flto +endif +CC_FLAGS_LTO_CLANG += -fvisibility=default +endif + +ifdef CONFIG_LTO +CC_FLAGS_LTO := $(CC_FLAGS_LTO_CLANG) +KBUILD_CFLAGS += $(CC_FLAGS_LTO) +export CC_FLAGS_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) diff --git a/arch/Kconfig b/arch/Kconfig index 8cc35dc556c7..e00b122293f8 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -552,6 +552,72 @@ 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 + +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. + +config LTO_NONE + bool "None" + +config LTO_CLANG + bool "Clang's Link Time Optimization (EXPERIMENTAL)" + 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_RECORD + depends on !KASAN + 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 db600ef218d7..78079000c05a 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -89,15 +89,18 @@ * .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 2e8810b7e5ed..f307e708a1b7 100644 --- a/scripts/Makefile.build +++ b/scripts/Makefile.build @@ -108,7 +108,7 @@ endif # --------------------------------------------------------------------------- quiet_cmd_cc_s_c = CC $(quiet_modtag) $@ - cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS), $(c_flags)) $(DISABLE_LTO) -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) @@ -424,8 +424,15 @@ $(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 $@; \ + $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(filter %.o,$^) +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 411c1e600e7d..1005b147abd0 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) \ @@ -37,7 +44,7 @@ quiet_cmd_ld_ko_o = LD [M] $@ -o $@ $(filter %.o, $^); \ $(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true) -$(modules): %.ko: %.o %.mod.o $(KBUILD_LDS_MODULE) FORCE +$(modules): %.ko: %$(prelink-ext).o %.mod.o $(KBUILD_LDS_MODULE) FORCE +$(call if_changed,ld_ko_o) targets += $(modules) $(modules:.ko=.mod.o) diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost index 3651cbf6ad49..9ced8aecd579 100644 --- a/scripts/Makefile.modpost +++ b/scripts/Makefile.modpost @@ -102,12 +102,32 @@ $(input-symdump): @echo >&2 'WARNING: Symbol version dump "$@" 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 $(filter-out FORCE,$^) + +%.lto.o: %.o FORCE + $(call if_changed,cc_lto_link_modules) + +PHONY += FORCE +FORCE: + +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 92dd745906f4..a681b3b6722e 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -52,6 +52,14 @@ 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} ${objects} } @@ -99,13 +107,22 @@ vmlinux_link() fi if [ "${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,} \ @@ -270,7 +287,6 @@ 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
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. Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in follow-up patches. Signed-off-by: Sami Tolvanen <samitolvanen@google.com> --- Makefile | 16 ++++++++ arch/Kconfig | 66 +++++++++++++++++++++++++++++++ include/asm-generic/vmlinux.lds.h | 11 ++++-- scripts/Makefile.build | 9 ++++- scripts/Makefile.modfinal | 9 ++++- scripts/Makefile.modpost | 24 ++++++++++- scripts/link-vmlinux.sh | 32 +++++++++++---- 7 files changed, 151 insertions(+), 16 deletions(-)