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Date: Tue, 22 Mar 2011 11:27:41 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Pekka Enberg <penberg@kernel.org>
Cc: Jesper Juhl <jj@chaosbits.net>, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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        linux-kbuild@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: PATCH][RFC][resend] CC_OPTIMIZE_FOR_SIZE should default to N
Message-ID: <20110322102741.GA4448@elte.hu>
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* Pekka Enberg <penberg@kernel.org> wrote:

> Hi Jesper,
> 
> On Mon, Mar 21, 2011 at 10:08 PM, Jesper Juhl <jj@chaosbits.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