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

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Date:   Sun, 12 May 2019 11:32:28 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     linux-kernel@...r.kernel.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

* Alexey Dobriyan <adobriyan@...il.com> wrote:

> On Sat, May 11, 2019 at 11:02:24PM +0300, Alexey Dobriyan wrote:
> > I compiled current F29 kernel config on x86_64 (5.0.13-200.fc29.x86_64)
> > with -ffreestanding. The results are interesting :^):
> > 
> > 	add/remove: 30/22 grow/shrink: 1290/46867 up/down: 33658/-1778055 (-1744397)
> > 	Total: Before=83298859, After=81554462, chg -2.09% (!)
> > 
> > That's original config with modules compiled built-in.
> 
> Argh, it's the other way: adding -ffreestanding shrinks kernel by 2%.

This is a very interesting finding, as we've seen numerous code 
generation artifacts from GCC assuming libgcc things.

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?

Thanks,

	Ingo