linux-kernel - RE: FYI -ffreestanding shrinks kernel by 2% on x86

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Message-ID: <3a3b9e840dbb456ab01815f7afdbb493@AcuMS.aculab.com>
Date:   Mon, 13 May 2019 09:13:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ingo Molnar' <mingo@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Subject: RE: FYI -ffreestanding shrinks kernel by 2% on x86_64

From: Ingo Molnar
> Sent: 12 May 2019 10:32
...
> Has anyone investigated by any chance where the -ffreestanding space
> savings come from mostly - is it mostly in cold paths, or does it make or
> hot codepaths more efficient as well?
> 
> If it's the latter then the kernel would be directly faster as well
> (fewer instructions executed), not just indirectly from better cache
> packing, I suppse?

My guess is that -ffreestanding stops gcc inlining memcpy() (etc).
The calls can be smaller than the inline code, but will (probably)
run more slowly.

	David

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