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Date: Wed, 2 Dec 2020 16:07:54 -0800
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
Will Deacon <will@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>, Kees Cook <keescook@...omium.org>,
clang-built-linux <clang-built-linux@...glegroups.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-arch <linux-arch@...r.kernel.org>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
PCI <linux-pci@...r.kernel.org>
Subject: Re: [PATCH v8 02/16] kbuild: add support for Clang LTO
On Tue, Dec 1, 2020 at 1:37 PM Sami Tolvanen <samitolvanen@...gle.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@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
--
Thanks,
~Nick Desaulniers
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