From: Ingo Molnar <mingo@kernel.org>
To: Alexey Dobriyan <adobriyan@gmail.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Linus Torvalds <torvalds@linux-foundation.org>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>
Subject: Re: FYI -ffreestanding shrinks kernel by 2% on x86_64
Date: Sun, 12 May 2019 11:32:28 +0200 [thread overview]
Message-ID: <20190512093228.GA8088@gmail.com> (raw)
In-Reply-To: <20190511201344.GA11535@avx2>
* Alexey Dobriyan <adobriyan@gmail.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
next prev parent reply other threads:[~2019-05-12 9:32 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-11 20:02 FYI -ffreestanding bloats kernel by 2% on x86_64 Alexey Dobriyan
2019-05-11 20:13 ` FYI -ffreestanding shrinks " Alexey Dobriyan
2019-05-12 9:32 ` Ingo Molnar [this message]
2019-05-12 17:55 ` Alexey Dobriyan
2019-05-13 9:13 ` David Laight
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