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

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Date:	Mon, 12 Jan 2009 11:22:56 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Bernd Schmidt <bernds_cb1@...nline.de>,
	David Woodhouse <dwmw2@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Harvey Harrison <harvey.harrison@...il.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>,
	Nick Piggin <npiggin@...e.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

Andi Kleen wrote:
> On Mon, Jan 12, 2009 at 11:02:17AM -0800, Linus Torvalds wrote:
>>> Something at the back of my mind said "aliasing".
>>>
>>> $ gcc linus.c -O2 -S ; grep subl linus.s
>>>         subl    $1624, %esp
>>> $ gcc linus.c -O2 -S -fno-strict-aliasing; grep subl linus.s
>>>         subl    $824, %esp
>>>
>>> That's with 4.3.2.
>> Interesting. 
>>
>> Nonsensical, but interesting.
> 
> What I find nonsensical is that -fno-strict-aliasing generates
> better code here. Normally one would expect the compiler seeing
> more aliases with that option and then be more conservative
> regarding any sharing. But it seems to be the other way round
> here.

For this to be convolved with aliasing *AT ALL* indicates this is done 
incorrectly.

This is about storage allocation, not aliases.  Storage allocation only 
depends on lifetime.

	-hpa

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