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

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Message-ID: <20171120202152.GA89108@samitolvanen.mtv.corp.google.com>
Date:   Mon, 20 Nov 2017 12:21:52 -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 Sat, Nov 18, 2017 at 01:21:39PM +1000, Nicholas Piggin wrote:
> Do you have any kind of numbers for this, out of curiosity? Binary
> size, performance, build time?

I don't have performance numbers to share. Are there any specific
benchmarks you'd be interested in seeing? Build time typically
increases with LTO and in my experience, binary size tends to increase
by ~10-15% as well.

> Why is this needed? It would have been nice to get rid of the
> !THIN_ARCHIVES option if you can make the patches work with the thin
> archives paths.

I believe LLVMgold doesn't know how to deal with an archive of LLVM IR
files, but I can certainly use thin archives as an index and extract
the path names for linking. I'll look into it.

Sami