From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Richard Guenther" <richard.guenther@gmail.com>
Subject: Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y impact
Date: Fri, 9 Jan 2009 19:59:22 +0100
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Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
	"Matthew Wilcox" <matthew@wil.cx>,
	"Dirk Hohndel" <hohndel@infradead.org>,
	"H. Peter Anvin" <hpa@zytor.com>, "Ingo Molnar" <mingo@elte.hu>,
	"jim owens" <jowens@hp.com>,
	"Chris Mason" <chris.mason@oracle.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Steven Rostedt" <rostedt@goodmis.org>, paulmck@linux.vnet.ibm.com,
	"Gregory Haskins" <ghaskins@novell.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Nick Piggin" <npiggin@suse.de>,
	"Peter Morreale" <pmorreale@novell.com>,
	"Sven Dietrich" <SDietrich@novell.com>, jh@suse.cz
To: "Andi Kleen" <andi@firstfloor.org>
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In-Reply-To: <20090109181952.GI26290@one.firstfloor.org>
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On Fri, Jan 9, 2009 at 7:19 PM, Andi Kleen <andi@firstfloor.org> wrote:
>> So we do have special issues. And exactly _because_ we have special issues
>> we should also expect that some compiler defaults simply won't ever really
>> be appropriate for us.
>
> I agree that the kernel needs quite different inlining heuristics
> than let's say a template heavy C++ program. I guess that is
> also where our trouble comes from -- gcc is more tuned for the
> later. Perhaps because the C++ programmers are better at working
> with the gcc developers?
>
> But it's also not inconceivable that gcc adds a -fkernel-inlining or
> similar that changes the parameters if we ask nicely. I suppose
> actually such a parameter would be useful for far more programs
> than the kernel.

I think that the kernel is a perfect target to optimize default -Os behavior for
(whereas template heavy C++ programs are a target to optimize -O2 for).
And I think we did a good job in listening to kernel developers if once in
time they tried to talk to us - GCC 4.3 should be good in compiling the
kernel with default -Os settings.  We, unfortunately, cannot retroactively
fix old versions that kernel developers happen to like and still use.

Richard.