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Date: Wed, 22 Apr 2009 08:07:01 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Andi Kleen <andi@firstfloor.org>, Jeremy Fitzhardinge <jeremy@goop.org>,
       Mathieu Desnoyers <compudj@krystal.dyndns.org>,
       linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Andrew Morton <akpm@linux-foundation.org>,
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       Peter Zijlstra <peterz@infradead.org>,
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       Li Zefan <lizf@cn.fujitsu.com>,
       KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Masami Hiramatsu <mhiramat@redhat.com>,
       "Frank Ch. Eigler" <fche@elastic.org>, Tom Zanussi <tzanussi@gmail.com>,
       Jiaying Zhang <jiayingz@google.com>, Michael Rubin <mrubin@google.com>,
       Martin Bligh <mbligh@google.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Neil Horman <nhorman@tuxdriver.com>,
       Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>,
       Pekka@firstfloor.org
Subject: Re: [PATCH 2/8] tracing: create automated trace defines
Message-ID: <20090422060701.GB14687@one.firstfloor.org>
References: <49E6065B.7080409@goop.org> <20090416023456.GC22378@Krystal> <49E69E76.9030608@goop.org> <20090416234410.GA20513@Krystal> <87zlebpzmk.fsf@basil.nowhere.org> <20090421155106.GE3792@Krystal> <49EDFFE6.1080401@goop.org> <alpine.DEB.2.00.0904211320440.10097@gandalf.stny.rr.com> <20090421202828.GB32179@basil> <alpine.DEB.2.00.0904211711260.10097@gandalf.stny.rr.com>
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> I think it was Ingo that let out the idea, and I'm starting to like it.
> 
> Perhaps we should fork off gcc and ship Linux with its own compiler. This 
> way we can optimize it for the kernel and not worry about any userland 
> optimizations.
> 
> I would like to do something like:
> 
> 	if (unlikely(err)) {
> 		__section__(".error_sect") {


gcc already supports that, you don't need to fork anything. It's called
hot/cold partitioning. Basically it splits functions into hot and cold
and unlikely parts and all the cold/unlikely parts go into a separate 
sections.

I think it's normally not enabled by default on x86 though, probably because
it doesn't help too much.

By default (unless you specify -fno-reorder-blocks) it does the same
without sections, just moving unlikely code out of line.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.
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