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From: Andreas Schwab <schwab@suse.de>
To: =?iso-8859-1?Q?J=2EA=2E_Magall=F3n?= <jamagallon@ono.com>
Cc: Arjan van de Ven <arjan@infradead.org>,
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       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
Subject: Re: [patch] spinlocks: remove 'volatile'
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X-Yow: I'm having a MID-WEEK CRISIS!
Date: Thu, 06 Jul 2006 16:26:38 +0200
In-Reply-To: <20060706153955.0740b934@werewolf.auna.net> (J. A.
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"J.A. Magallón" <jamagallon@ono.com> writes:

> The barrier prevents the compiler of translating this to:
>
> for (int i=0; i<10; i++)
> {
> 	b[i] = *inb;
> 	a[i] = *ina;
> }
>
> or even to:
>
> for (int i=0; i<10; i++)
> 	a[i] = *ina;
> for (int i=0; i<10; i++)
> 	b[i] = *inb;
>
> but does not prevent it to do this:
>
> register int tmp_a = *ina;
> register int tmp_b = *inb;
>
> for (int i=0; i<10; i++)
> {
> 	a[i] = tmp_a;
> 	b[i] = tmp_b;
> }
>
> because nor 'ina' nor 'inb' change under what the compiler sees inside
> the loop. 'volatile' prevents the compiler of do a high level cache of
> *ina or *inb.

Actually the compiler may not do any of these transformations because *ina
or *inb may alias any of a[i] or b[i].

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."