linux-kernel - Re: [RFC PATCH] x86: enable dead code and data elimination (LTO)

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Message-ID: <CAK7LNAR-VF2o3-qcCXnkath_B74KQodh_vezPRErGPhZSao2VA@mail.gmail.com>
Date:   Sun, 9 Jul 2017 22:29:37 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Paul Burton <paul.burton@...tec.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86: enable dead code and data elimination (LTO)

Hi.

2017-07-09 18:05 GMT+09:00 Ingo Molnar <mingo@...nel.org>:
>
> * Nicholas Piggin <npiggin@...il.com> wrote:
>
>> FYI, easiest way to check if you forgot to KEEP a linker table is
>> to look at `readelf -S vmlinux` differences, and to see what is
>> being trimmed, look at nm differences or use --print-gc-sections
>> LD option to see what symbols you're trimming. Linker tables,
>> boot entry, and exception entry tends to require anchoring.
>
> Could you please add a debug build target to display all discarded
> symbols/sections? Something like:
>
>         make lto-check
>
> ... or so?
>
> Thanks,
>
>         Ingo


Actually, LTO activity existed some years ago
(but not pulled in).

http://www.spinics.net/lists/linux-kbuild/msg09242.html


IIUC, this patch enables "dead code elimination",
(or "garbage collection"?),
but I think it is different from what is called LTO.


-- 
Best Regards
Masahiro Yamada