linux-kernel - Re: [v2,12/18] kbuild: add support for clang LTO

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Message-ID: <20171129233008.GB138088@samitolvanen.mtv.corp.google.com>
Date:   Wed, 29 Nov 2017 15:30:08 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Nicholas Piggin <npiggin@...il.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>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [v2,12/18] kbuild: add support for clang LTO

On Tue, Nov 21, 2017 at 11:01:52AM +1000, Nicholas Piggin wrote:
> I just wonder are you doing this because there is some worthwhile
> performance gain? Or to enable more testing and development of LTO?
> Any clues for why a user would want to enable it.

I'm primarily interested in CFI, which with clang requires LTO; not
for the optimizations, but for source visibility. We do expect to see
performance improvements with LTO though, especially if combined with
PGO.

> Thanks, if you could. Possibly file a request with LLVMgold too, it
> seems to be that toolchain support for archives is quite strong, so it
> will be good to keep pushing for that.

It turns out LLVMgold is fine with mixed IR/object archives, but we need
to use llvm-ar to generate symbol tables for them, and there are some
compatibility issues with objdump that I had to work around. I'll send
v3 for review once I receive some feedback for the clang/gold patches we
need first.

Sami