linux-kernel - Re: PATCH][RFC][resend] CC_OPTIMIZE_FOR

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Message-ID: <20110322102741.GA4448@elte.hu>
Date:	Tue, 22 Mar 2011 11:27:41 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...nel.org>
Cc:	Jesper Juhl <jj@...osbits.net>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>,
	Eric Paris <eparis@...hat.com>,
	Roman Zippel <zippel@...ux-m68k.org>,
	linux-kbuild@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: PATCH][RFC][resend] CC_OPTIMIZE_FOR_SIZE should default to N


* Pekka Enberg <penberg@...nel.org> wrote:

> Hi Jesper,
> 
> On Mon, Mar 21, 2011 at 10:08 PM, Jesper Juhl <jj@...osbits.net> wrote:
> > I believe that the majority of systems we are built on want a -O2 compiled
> > kernel. Optimizing for size (-Os) is mainly benneficial for embedded
> > systems and systems with very small CPU caches (correct me if I'm wrong).
> 
> Please take a look at commit 0910b44 ("Expose "Optimize for size"
> option for everybody") for the reasoning behind defaulting to -Os.

If that situation has changed - if GCC has regressed in this area then a commit 
changing the default IMHO gains a lot of credibility if it is backed by careful 
measurements using perf stat --repeat or similar tools.

See the hard numbers in this upstream commit for example:

  ea7145477a46: x86: Separate out entry text section

there we were able to prove the positive effects of a pretty subtle change to 
the layout of the instruction cache, with a measurement noise in the 0.1% 
range.

Thanks,

	Ingo
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