linux-kernel - Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Tue, 20 Jan 2009 22:05:15 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Bernd Schmidt <bernds_cb1@...nline.de>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Harvey Harrison <harvey.harrison@...il.com>,
	"H. Peter Anvin" <hpa@...or.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>,
	Matthew Wilcox <matthew@....cx>,
	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>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>, jh@...e.cz
Subject: Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex:
	implement adaptive spinning


* David Woodhouse <dwmw2@...radead.org> wrote:

> On Tue, 2009-01-20 at 13:38 +0100, Ingo Molnar wrote:
> > 
> > * Nick Piggin <npiggin@...e.de> wrote:
> > 
> > > > > it seems like a nice opt-in thing that can be used where the aliases 
> > > > > are verified and the code is particularly performance critical...
> > > > 
> > > > Yes. I think we could use it in the kernel, although I'm not sure how 
> > > > many cases we would ever find where we really care.
> > > 
> > > Yeah, we don't tend to do a lot of intensive data processing, so it is 
> > > normally the cache misses that hurt most as you noted earlier.
> > > 
> > > Some places it might be appropriate, though. It might be nice if it can 
> > > bring code size down too...
> > 
> > I checked, its size effects were miniscule [0.17%] on the x86 defconfig 
> > kernel and it seems to be a clear loss in total cost as there would be an 
> > ongoing maintenance cost
> 
> They were talking about 'restrict', not strict-aliasing. Where it can be 
> used, it's going to give you optimisations that strict-aliasing can't.

the two are obviously related (just that the 'restrict' keyword can be 
used for same-type pointers so it gives even broader leeway) so i used the 
0.17% figure i already had to give a ballpark figure about what such type 
of optimizations can bring us in general.

(Different-type pointer uses are a common pattern: we have a lot of places 
where we have pointers to structures with different types so 
strict-aliasing optimization opportunities apply quite broadly already.)

	Ingo
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