Conversion of pointers to integers and vice versa; pointer tagging

[Marc Nieper-Wißkirchen]

Am Mi., 2. Jan. 2019 um 17:56 Uhr schrieb David Malcolm <dmalcolm@redhat.com
>:

> On Tue, 2019-01-01 at 12:00 +0100, Marc Nieper-Wißkirchen wrote:
> > Implementations of languages with garbage collection and dynamic
> > typing often employ the concept of tagged pointers. Unused bits of a
> > pointer representation are used to distinguish between pointers to
> > different types or between pointers and immediate values.
> >
> > Implementing such a language with libgccjit raises a couple of
> > questions:
> >
> > 1) libgccjit has currently only a limited set of type-casting
> > expressions. In particular, there is no type-casting expression
> > between an integer type (e.g. uintptr_t) and a pointer type defined.
> >
> > Are there technical reasons for that or is that just an area where
> > libgccjit still has to be complemented?
>
> In general, I've chosen to require the user to explicitly supply any
> casting that's needed, out of a desire to catch problems early.
>
> gcc_jit_context_new_cast only supports a few conversions.  IIRC this
> was to simplify the internal implementation; perhaps more conversions
> could be allowed.
>
> > I guess, a workaround could be to use union types à la
> >
> > union {
> >   uintptr_t i;
> >   void *p;
> > }
> >
> > everywhere.
>
> That ought to work.
>

Please excuse resurrecting this thread, but it has occurred to me that I
don't know how to create an integer type whose width is exactly the same as
the one of a pointer using libgccjit. Wouldn't it make sense to add
something like GCC_JIT_TYPE_UINTPTR_T or similar? Or am I supposed to use,
say, Autoconf, to check the sizes of the provided types and choose one
accordingly?

Thanks,

Marc