linux-kernel - Re: [tip:x86/urgent] x86: Add memory modify constraints to xchg() and cmpxchg()

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Message-ID: <4C5053DA.2050909@zytor.com>
Date:	Wed, 28 Jul 2010 08:59:22 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	mingo@...hat.com, linux-kernel@...r.kernel.org, glommer@...hat.com,
	alan@...rguk.ukuu.org.uk, zamsden@...hat.com, stable@...nel.org,
	mtosatti@...hat.com, gregkh@...e.de, peter@...frader.org,
	tglx@...utronix.de, avi@...hat.com,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86: Add memory modify constraints to xchg()
 and 	cmpxchg()

On 07/28/2010 08:45 AM, Linus Torvalds wrote:
> On Tue, Jul 27, 2010 at 10:33 PM, tip-bot for H. Peter Anvin
> <hpa@...or.com> wrote:
>>
>> x86: Add memory modify constraints to xchg() and cmpxchg()
> 
> Ack. I assume this doesn't really change the code generated? At least
> not with a gcc that honors the whole memory clobber thing properly?

I didn't do a binary A:B comparison, on the assumption it would preturb
the code too much to be equal, but I guess it would be a good idea to at
least try.

> I also suspect that we can/should get rid of the __xg() thing - it was
> there just to make sure gcc didn't see the memory read as a single
> word and tried to optimize it. With the "+m" it probably doesn't
> matter any more (don't know if it ever did)

"+m" would still see as a single word, but the __xg() thing should be
obsoleted by asm volatile + "memory", neither of which were in the
1.3.11 version.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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