Re: [patch] spinlocks: remove 'volatile'

Andreas Schwab
2006-07-06T14:26:51Z

"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
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