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

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Message-Id: <20190321080133.GB3916@osiris>
Date:   Thu, 21 Mar 2019 09:01:33 +0100
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Ingo Molnar <mingo@...hat.com>,
        Paul Burton <paul.burton@...s.com>, linux-mips@...r.kernel.org,
        x86@...nel.org, linux-mtd@...ts.infradead.org,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, Dave Hansen <dave@...1.net>,
        linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH] compiler: allow all arches to enable
 CONFIG_OPTIMIZE_INLINING

On Wed, Mar 20, 2019 at 03:20:27PM +0900, Masahiro Yamada 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

Well, this will change, since now people (have to) start adding
__always_inline annotations on all architectures, most likely until
all have about the same amount of annotations like x86. This will
reduce the benefit.

Not sure if it's really a win that we get the inline vs
__always_inline discussion now on all architectures.