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

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Date:	Tue, 20 Jan 2009 05:20:48 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Bernd Schmidt <bernds_cb1@...nline.de>,
	Andi Kleen <andi@...stfloor.org>,
	David Woodhouse <dwmw2@...radead.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

> The problem with 'restrict' is that almost nobody uses it, and it does 

Also gcc traditionally didn't do a very good job using it (this
might be better in the very latest versions). At least some of the 3.x
often discarded this information. 

> automatically. But it should work well as a way to get Fortran-like 
> performance from HPC workloads written in C - which is where most of the 
> people are who really want the alias analysis.

It's more than just HPC  -- a lot of code has critical loops.

> > 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. 

Very little I suspect. Also the optimizations that gcc does with this
often increase the code size. While that can be a win, with people
judging gcc's output apparently *ONLY* on the code size as seen
in this thread[1] it would obviously not compete well.

-Andi 

[1] although there are compilers around that generate smaller code
than gcc at its best.

-- 
ak@...ux.intel.com -- Speaking for myself only.
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