[Bug c/22485] pointer +- integer is never NULL

["falk at debian dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From falk at debian dot org  2005-07-15 14:22 -------
(In reply to comment #13)
> Subject: Re:  pointer +- integer is never NULL
> 
> "falk at debian dot org" <gcc-bugzilla@gcc.gnu.org> writes:
> | Sorry, I cannot follow you. I'd find it massively unsurprising if
> | reinterpret_cast<int*>(0) produces a null pointer, and if I then get
> | undefined behavior for doing something with it that is undefined for a
> | null pointer.
> 
> But, if I used reinterpret_cast to turn an integer value 0 into a
> pointer, there is no reason why the compiler would assume that I do not
> know the underlying machine and what I'm doing with the pointer.

The note merely requires the result of the mapping to be unsurprising;
it does not say anything about further operations of this result. Therefore,
it is completely irrelevant here.

> | As it seems, arguing with different levels of surprisingness seems to
> | be somewhat subjective, so I don't think this leads us anywhere.
> 
> I'm not actually arguing on different level of surprisingness.  I'm
> just looking at reinterpret_cast and its implication. 

I don't see you bringing any argument here exept one based on a side note
about surprisingness, which IMHO doesn't even apply here. So I am still
convinced that nullpointer+0 is clearly undefined.

> | This is a more relevant point. I don't think this optimization would
> | break offsetof-like macros, since they'd use null pointer *constants*,
>                                                             ^^^^^^^^^^^
> 
> For the offsetof *macro*, yes
> But that is not the case for codes that uses
> reinterpret_cat<int*>(expr), where expr is an integer expression with
> value 0.  Scanning a region of memory starting from zero, is not
> exactly the kind of thing never done in practice.

Can you give a complete example where this optimization would fail, that you
would consider reasonable and realistic?

> | which we could easily avoid to tag as non-null.
> 
> so you would have to pretend that a null pointer constant is not null?
> That is even more bizarre arithmetic.

I have no trouble doing bizarre arithmetic when the user gives invalid input.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22485