linux-kernel - Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile_

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Date:	Mon, 23 Jul 2007 23:37:51 +0530 (IST)
From:	Satyam Sharma <ssatyam@....iitk.ac.in>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__
 __volatile__

On Mon, 23 Jul 2007, Jeremy Fitzhardinge wrote:

> I'm not quite sure what your point is.

Could be a case of terminology confusion ...

> The paragraph you quoted is
> pretty explicit in saying that volatile doesn't prevent an "asm
> volatile" from being interspersed with other code, and the example just
> before that is explicit in talking about how to use dependencies to
> control the ordering of asm volatiles with respect to surrounding code.

Yes, that was the (2).

> In fact nothing in that section precludes asm volatiles from being
> reordered with respect to each other either; you just have to make sure
> your dependency constraints are all correct.

The (3) as I had originally written / meant was that multiple
instructions in a volatile asm would not get _individually_
interspersed with the rest of the code i.e. be emitted out
_consecutively_. I don't think we need any such guarantees for
the non-atomic variants of those operations, so it's good to
let the compiler have a free hand with what it wants to do,
and optimize/combine multiple bitops as necessary / possible,
which was the original intention.

Satyam
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