linux-kernel - Re: [PATCH] compiler: allow all arches to enable CONFIG_OPTIMIZE

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Date:   Wed, 20 Mar 2019 10:41:43 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Dave Hansen <dave@...1.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        linux-mips@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paul Burton <paul.burton@...s.com>,
        Ingo Molnar <mingo@...hat.com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] compiler: allow all arches to enable CONFIG_OPTIMIZE_INLINING

On Wed, Mar 20, 2019 at 7:21 AM Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
>
> Commit 60a3cdd06394 ("x86: add optimized inlining") introduced
> CONFIG_OPTIMIZE_INLINING, but it has been available only for x86.
>
> The idea is obviously arch-agnostic although we need some code fixups.
> This commit moves the config entry from arch/x86/Kconfig.debug to
> lib/Kconfig.debug so that all architectures (except MIPS for now) can
> benefit from it.
>
> At this moment, I added "depends on !MIPS" because fixing 0day bot reports
> for MIPS was complex to me.
>
> I tested this patch on my arm/arm64 boards.
>
> This can make a huge difference in kernel image size especially when
> CONFIG_OPTIMIZE_FOR_SIZE is enabled.
>
> For example, I got 3.5% smaller arm64 kernel image for v5.1-rc1.
>
>   dec       file
>   18983424  arch/arm64/boot/Image.before
>   18321920  arch/arm64/boot/Image.after
>
> This also slightly improves the "Kernel hacking" Kconfig menu.
> Commit e61aca5158a8 ("Merge branch 'kconfig-diet' from Dave Hansen')
> mentioned this config option would be a good fit in the "compiler option"
> menu. I did so.

I think this is a good idea in general, but it is likely to cause a lot of
new warnings. Especially the -Wmaybe-uninitialized warnings get
new false positives every time we get substantially different inlining
decisions.

I've added your patch to my randconfig test setup and will let you
know if I see anything noticeable. I'm currently testing clang-arm32,
clang-arm64 and gcc-x86.

      Arnd