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

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Message-ID: <496B86B5.3090707@t-online.de>
Date:	Mon, 12 Jan 2009 19:06:45 +0100
From:	Bernd Schmidt <bernds_cb1@...nline.de>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	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>,
	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 Sun, Jan 11, 2009 at 04:21:03PM -0800, Linus Torvalds wrote:
>>
>> On Mon, 12 Jan 2009, Andi Kleen wrote:
>>> so at least least for this case it works. Your case also doesn't work 
>>> for me. So it looks like gcc didn't like something you did in your test 
>>> program.
>> I very intentionally used _different_ types.
>>
>> If you use the same type, gcc will apparenrly happily say "hey, I can 
>> combine two variables of the same type with different liveness into the 
>> same variable".
> 
> Confirmed.
> 
>> But that's not the interesting case.
> 
> Weird. I wonder where this strange restriction comes from.

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.


Bernd
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