linux-kernel - Re: [PATCH] make atomic_t volatile on all architectures

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Date:	Sun, 12 Aug 2007 02:54:16 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	wjiang@...ilience.com, wensong@...ux-vs.org,
	heiko.carstens@...ibm.com, linux-kernel@...r.kernel.org,
	ak@...e.de, cfriesen@...tel.com, netdev@...r.kernel.org,
	horms@...ge.net.au, akpm@...ux-foundation.org,
	Chuck Ebbert <cebbert@...hat.com>, davem@...emloft.net,
	zlynx@....org, Chris Snook <csnook@...hat.com>
Subject: Re: [PATCH] make atomic_t volatile on all architectures



On Sun, 12 Aug 2007, Martin Schwidefsky wrote:
> 
> The duplication "=m" and "m" with the same constraint is rather 
> annoying.

It's not only annoying, it causes gcc to generate bad code too. At least 
certain versions of gcc will generate the address *twice*, even if there 
is obviously only one address used.

If you have problems with "+m", you are often actually better off using 
just "m" and then adding a memory clobber. But that has other code 
generation downsides.

			Linus
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