linux-kernel - Re: [PATCH v2 00/18] Add support for clang LTO

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Date:   Thu, 16 Nov 2017 23:53:00 +0300
From:   Yury Norov <ynorov@...iumnetworks.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Alex Matveev <alxmtvv@...il.com>, Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [PATCH v2 00/18] Add support for clang LTO

On Wed, Nov 15, 2017 at 01:34:10PM -0800, Sami Tolvanen wrote:
> This series adds build system support for compiling the kernel with clang
> Link Time Optimization (LTO), using GNU gold with the LLVMgold plug-in
> for linking. Some background for clang's LTO support is available here:
> 
>   https://llvm.org/docs/LinkTimeOptimization.html
> 
> With -flto, clang produces LLVM bitcode instead of object files, and
> the compilation to native code happens at link time. In addition, clang
> cannot use an external assembler for inline assembly when LTO is enabled,
> which causes further compatibility issues.
> 
> The patches in this series remove intermediate linking steps when LTO is
> used, postpone processing done on object files until after the LTO link
> step, add workarounds for GNU gold incompatibilities, and address inline
> assembly incompatibilities for arm64.
> 
> These changes allow arm64 defconfig to be compiled with LTO, but other
> architectures are not enabled until compatibility issues have been
> addressed. In particular, x86 inline assembly doesn't currently compile
> with clang's integrated assembler due to this LLVM bug:
> 
>   https://bugs.llvm.org/show_bug.cgi?id=24487
> 
> Due to recent bug fixes in the toolchain, it's recommended to use clang
> 5.0 or later, and GNU gold from binutils 2.27 or later, although older
> versions may also work depending on your kernel configuration.

So, you don't guarantee that kernel will work with old compiler and
toolchain? If so, I would suggest you to add the patch that checks
their versions and disables LTO if needed.

Yury