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Re: g++ and aliasing bools
- From: Daniel Berlin <dan at dberlin dot org>
- To: Robert Dewar <dewar at gnat dot com>
- Cc: gcc at gcc dot gnu dot org, <kenner at vlsi1 dot ultra dot nyu dot edu>
- Date: Fri, 25 Jan 2002 11:30:49 -0500 (EST)
- Subject: Re: g++ and aliasing bools
On Fri, 25 Jan 2002, Robert Dewar wrote:
> <<The language semantics don't necessarily give you enough to be able to
> place all types in the right sets. If it doesn't, you are back to
> undecidability.
> >>
>
> Of course it is undecidable in the dynamic sense, but there we are not
> interested in "the right sets", we are interested in sets that we can
> prove are disjoint. It is quite "right" to put everything in the same
> set, just not very efficient. We are NOT looking for an optimal solution,
> here, that's obvious to anyone that that's unattainable.
Of course.
>
> What we are looking for is improved, verifiable, principles for splitting
> the sets more finely.
>
> Sometimes I really think they should not teach anyone about undecidability.
> It always ends up with people looking at a perfectly simple problem like
> this one (simple conceptually, not simple to get good solutions to), and
> worrying about undecidability when it is a complete red herring.
Not here, however.
>
> If you propose that two items are not aliased, and it is undecidable whether
> they are aliased, that's fine, it just means they go in the same alias set,
> no big deal!
However, the important point is that the language semantics don't allow
us to get it *right*, not just deciding. We may get it *wrong*, and think
we have it *right*.
--Dan