Re: Linux and aliasing?

[Linus Torvalds <torvalds@transmeta.com>]

On Fri, 4 Jun 1999, Richard Henderson wrote:
> 
> Doing what you want is actually very hard for GCC right now.  Consider
> 
> 	int i;
> 	short s, *ps = (short *)&i;
> 	i = 0;
> 	s = *ps;

Note that while the kernel may contain constructs like the above, I never
meant for the "extended rule" to cover them. We'd have to fix them up.

The "pointer cast rule" was meant to allow people to know about and
override the type-based aliasing - it wasn't meant to handle every single
pointer cast automatically being non-aliased. I would consider such
behaviour to be basically (a) unimplementable and (b) too non-local.

I obviously didn't explain that very well, although I hope my later email
about the _implementation_ side explained the details more clearly.

The concept was never meant to avoid alias information on any global
scale. I think type-based alias information is important. It was meant to
be a syntactically simple way to override =specific= instances where the
programmer knows he is playing games with typing.

As an example, the above sequence obviously has a alia problem as it
stands now. My suggestion would _not_ make the above code generate
anything different at all. The only thing my suggestion really does is
give the programmer a chance to say "oh, I see: the above worked in the
original ANSI C, but it does not work with the new one, and I only care
about gcc anyway, so I can do the quick fix by just adding the cast":

	s = *(short *)ps;

Note that the cast above in C terms is a no-op: it casts a short pointer
to a short pointer, but it would be a way to tell gcc that this access
should not be aliased.

> Due to a long-ago quirk of history, GCC processes the abstract syntax
> tree one statement at a time, so the fact of the cast is long gone by
> the time we do the dereference.

I agree 100% with the concern you raise, and I'd just like to say that
that was never the intention. Having some kind of complete flow would
obviously be a very broken concept, and I fully understand the horror
people felt if they thought that was what I proposed.

		Linus