linux-kernel - Re: [patch] measurements, numbers about CONFIG_OPTIMIZE

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Date:	Fri, 9 Jan 2009 19:59:22 +0100
From:	"Richard Guenther" <richard.guenther@...il.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Matthew Wilcox" <matthew@....cx>,
	"Dirk Hohndel" <hohndel@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, "Ingo Molnar" <mingo@...e.hu>,
	"jim owens" <jowens@...com>,
	"Chris Mason" <chris.mason@...cle.com>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Steven Rostedt" <rostedt@...dmis.org>, paulmck@...ux.vnet.ibm.com,
	"Gregory Haskins" <ghaskins@...ell.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Nick Piggin" <npiggin@...e.de>,
	"Peter Morreale" <pmorreale@...ell.com>,
	"Sven Dietrich" <SDietrich@...ell.com>, jh@...e.cz
Subject: Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y impact

On Fri, Jan 9, 2009 at 7:19 PM, Andi Kleen <andi@...stfloor.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.
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