Re: [PATCH v7 02/17] kbuild: add support for Clang LTO

Nick Desaulniers
2020-11-18T23:49:42Z

On Wed, Nov 18, 2020 at 2:07 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.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
> 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