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Message-ID: <20200908170256.GA2743468@google.com>
Date: Tue, 8 Sep 2020 10:02:56 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Masahiro Yamada <masahiroy@...nel.org>, Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Paul E. McKenney" <paulmck@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
clang-built-linux@...glegroups.com,
kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
x86@...nel.org
Subject: Re: [PATCH v2 09/28] kbuild: add support for Clang LTO
On Thu, Sep 03, 2020 at 03:08:59PM -0700, Kees Cook wrote:
> On Thu, Sep 03, 2020 at 01:30:34PM -0700, 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 <samitolvanen@...gle.com>
>
> I remain crazy excited about being able to use this in upstream. :)
>
> The only suggestion I have here, if it might help with clarity, would be
> to remove DISABLE_LTO globally as a separate patch, since it's entirely
> unused in the kernel right now. This series removes it as it goes, which
> I think is fine, but it might cause some reviewers to ponder "what's
> this DISABLE_LTO thing? Don't we need that?" without realizing currently
> unused in the kernel.
Sure, that makes sense. I'll add a patch to remove DISABLE_LTO treewide
in v3.
Sami
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