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Tue, 08 Sep 2020 10:30:42 -0700 (PDT) Date: Tue, 8 Sep 2020 10:30:36 -0700 From: Sami Tolvanen To: Masahiro Yamada Subject: Re: [PATCH v2 09/28] kbuild: add support for Clang LTO Message-ID: <20200908173036.GD2743468@google.com> References: <20200624203200.78870-1-samitolvanen@google.com> <20200903203053.3411268-1-samitolvanen@google.com> <20200903203053.3411268-10-samitolvanen@google.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200908_133045_723966_5F7AC2A0 X-CRM114-Status: GOOD ( 33.71 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch , X86 ML , Kees Cook , "Paul E. McKenney" , Kernel Hardening , Peter Zijlstra , Greg Kroah-Hartman , Linux Kbuild mailing list , Nick Desaulniers , Linux Kernel Mailing List , Steven Rostedt , clang-built-linux , linux-pci@vger.kernel.org, Will Deacon , linux-arm-kernel Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Sep 08, 2020 at 12:30:14AM +0900, Masahiro Yamada wrote: > On Fri, Sep 4, 2020 at 5:31 AM Sami Tolvanen 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 > > --- > > Makefile | 18 +++++++- > > arch/Kconfig | 68 +++++++++++++++++++++++++++++++ > > 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, 154 insertions(+), 17 deletions(-) > > > > > #define TEXT_MAIN .text > > diff --git a/scripts/Makefile.build b/scripts/Makefile.build > > index 6ecf30c70ced..a5f4b5d407e6 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)) $(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) > > @@ -428,8 +428,15 @@ $(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) $@ $(filter %.o,$^) > > > KBUILD_ARFLAGS no longer exists in the mainline. > (commit 13dc8c029cabf52ba95f60c56eb104d4d95d5889) Thanks, I'll drop this in the next version. > > +ifdef CONFIG_LTO_CLANG > > +# With CONFIG_LTO_CLANG, .o files might be LLVM bitcode, > > or, .o files might be even thin archives. Right, and with LTO the thin archive might also point to a mix of bitcode and ELF to further complicate things. > For example, > > $ file net/ipv6/netfilter/nf_defrag_ipv6.o > net/ipv6/netfilter/nf_defrag_ipv6.o: thin archive with 6 symbol entries > > > Now we have 3 possibilities for .o files: > > - ELF (real .o) > - LLVM bitcode (.bc) > - Thin archive (.a) > > > Let me discuss how to proceed with this... Did you have something in mind to make this cleaner? Sami _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel