linux-kernel - Re: [PATCH 20/21] Kbuild, lto: Add Link Time Optimization support

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Message-ID: <20180127005554.GM26209@tassilo.jf.intel.com>
Date:   Fri, 26 Jan 2018 16:55:54 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        samitolvanen@...gle.com, alxmtvv@...il.com,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 20/21] Kbuild, lto: Add Link Time Optimization support

On Sat, Jan 27, 2018 at 01:15:49AM +0100, Arnd Bergmann wrote:
> On Mon, Nov 27, 2017 at 10:34 PM, Andi Kleen <andi@...stfloor.org> wrote:
> > From: Andi Kleen <ak@...ux.intel.com>
> > - Add a new LDFINAL variable that controls the final link
> > for vmlinux or module. In this case we call gcc-ld instead
> > of ld, to run the LTO step.
> 
> When I tried this out on allmodconfig (following the lwn article), I ran into
> a number of warnings:

Thanks for testing. Yes it's a known issue: during one module build
when modpost looks at the file it is still in LTO format, and
modpost doesn't understand the LTO symbol table. I had a patch
to teach it to it at some point, but it got lost somewhere.
The LLVM LTO patchkit has a different solution that actually
fixes the sequence to run modpost only after a LTO final link,
but I haven't gotten around to port that one.

It seems to work already for the single file modules.

-Andi