Re: [patch 4/21] fix ARCH_HAS_PREFETCH

[Andrew Morton <akpm@zip.com.au>]

Alan Cox wrote:
> 
> On Sun, 2002-08-11 at 08:38, Andrew Morton wrote:
> > This information loss is unfortunate.  Examples:
> >
> >       for (i = 0; i < N; i++)
> >               prefetch(foo[i]);
> >
> >    Problem is, if `prefetch' is a no-op, the compiler will still
> >    generate an empty busy-wait loop.  Which it must do.
> 
> Why - nothing there is volatile

Because the compiler sees:

	for (i = 0; i < N; i++)
		;

and it says "ah ha.  A busy wait delay loop" and leaves it alone.

It's actually a special-case inside the compiler to not optimise
away such constructs.