linux-kernel - Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered unnecessarily

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Date:	Wed, 25 Jul 2007 18:22:20 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Trent Piepho <xyzzy@...akeasy.org>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 6/8] i386: bitops: Don't mark memory as clobbered
 unnecessarily



On Wed, 25 Jul 2007, Linus Torvalds wrote:
> 
> Hmm. I really think you should take this up with the gcc people. That 
> looks like a gcc bug - because there really is nothing that guarantees 
> that the asm doesn't change the array that "x" points to, and the asm 
> clearly talks about clobbering memory.

Actually, I take that back. I think gcc does the right thing, and yes, 
it's explained by the memory clobber being just a blind "write to memory" 
rather than read memory. My bad.

It does leave us with very few ways of saying that an asm can *read* 
memory, and so it might be good to have it clarified that "volatile" 
implies that (at least with the memory clobber).

Your examples are good, I think.

		Linus
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