[Bug c/59850] Support sparse-style pointer address spaces (type attributes)

["josh at joshtriplett dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59850

--- Comment #7 from Josh Triplett <josh at joshtriplett dot org> ---
(In reply to Tom Tromey from comment #6)
> Null pointer constants are treated specially, which makes sense,
> but only if they have type "void *" and are in address space 0.

Otherwise, they're not a null pointer constant, so they're not treated
specially. :)

A null pointer constant must be in address space 0; that's for compatibility
with the standard definition of NULL, to avoid needing unique NULL constants
for each possible address space (USER_NULL, IOMEM_NULL).  I don't think it
makes sense to treat a 0 in address space 1 as a null pointer constant; safer
to give a warning for mixing address spaces.  The goal isn't to treat 0
magically as an address in all address spaces; it's specifically to treat NULL
as the null for all address spaces.

> That is, this works:
> 
> #define NULL ((__attribute__ ((address_space (0))) void *) 0)
> __attribute__((address_space (1))) int *p = NULL;
> 
> But this gets a warning:
> 
> #define NULL ((__attribute__ ((address_space (1))) void *) 0)
> __attribute__((address_space (0))) int *p = NULL;

I can't think of a legitimate reason to have a null pointer constant in a
non-zero address space; there's already a null pointer constant, NULL,
effectively in all address spaces, so why would you want to redefine it?

And on the contrary, I can think of a very good reason to *have* this warning:
suppose you wanted to define an INVALID_FOO_POINTER in the foo address space,
and you decided to use 0 as the invalid value.  You should get a warning if you
try to use INVALID_FOO_POINTER with a non-foo pointer type; it shouldn't
magically pass silently just because the chosen value is 0.

> And so does this:
> 
> #define NULL ((int *) 0)
> __attribute__((address_space (1))) int *p = NULL;
> 
> 
> I'm not sure whether that last one ought to be an error or not.

That isn't a null pointer constant, since it isn't (void *); it can't be
converted to any other pointer type without warning, and I don't think it's
unreasonable to say it can't be converted to any other address space without
warning either.