If you try to build Executor on a 64-bit machine, without telling the
compiler to use 32-bit pointers, the compilation will fail due to the
conversion being a work in progress.

This file is a rough dump of what I've been thinking about as I've
explored building Executor on a 64-bit machine.  I'm about to switch
to some other projects before I finish this exploration, and am
writing these notes primarily for myself.  There's no developer's
guide to Syn68k/ROMlib/Executor, so although it's possible for someone
else to finish the 64-bit mods up without me, it would take a bit of
figuring out what we currently do and why, something I don't currently
have the time to document.

At one point we had Executor running on an Alpha using 64-bit pointers
for all our native code, but 32-bit pointers for the Mac memory.  We
did this by altering gcc to allow us to use the ":32" bit-field
notation with pointers.  This worked OK and could possibly work again,
but there's another way we can store 32-bits of pointer in a variable
and still keep the type that the pointer points to, and that's to use
a union of a 32-bit unsigned and an array of zero of the pointers we
really want.

For example

        typedef union {
          uint32_t p_val;
          SomePointerType p_type[0];
        } PackedSomePointerType;

        PackedPointerType pp;

Now pp.p_val can be the 32-bit value thats in Mac memory, with
(typeof pp.p_type[0]) being the type of pointer that pp.p_val can expand to.

I haven't decided exactly how to make use of this type.  I've experimented with
defining two macros: MAKE_HIDDEN and PACKED_MEMBER that collectively replace
the two places we used to use ":32".  MAKE_HIDDEN replaces the way we used to
make 32-bit pointers for Handles and PACKED_MEMBER replaces the way we were
putting 32-bit pointers in structs.  We then need to rewrite some of the macros
that we use for dereferencing our 32-bit pointers.

Unfortunately, a combination of having the compiler allow us to mix
and match 32-bit pointers and 64-bit pointers along with the fact that
we didn't keep the Alpha port up to date means that the set of macros
we're using is incomplete and some macros will need to be split into
different variants for different uses.  For example, HxX dereferences
a handle and then picks up the field that is pointed to, but does not
swap the field.  We can make a HxX that does that in general, but code like
this:

  if (t_aux_c && HxX (t_aux_c, acCTable))

will not work though, since acCTable itself is a PACKED_MEMBER and so the
test for zero vs. non-zero won't work with the union.  If we introduce HxZ,
a new macro that can be used for testing zero vs. non-zero, we can change
the code to work right:

  if (t_aux_c && HxZ (t_aux_c, acCTable))

But that requires that one way or the other we determine when we need
to change HxX into HxZ.

Some of the other changes that need to be made include:

	defines like  #  define TheGDevice	(TheGDevice_H.p)
        need to be #  define TheGDevice	((typeof (TheGDevice_H.type[0]))(TheGDevice_H.pp))
        
        A bunch(all?) of MR that should be PPR

        A bunch of RM that should be RPP (and intermediate values
        that are actually unions)

        Assignments to PACKED_MEMBER fields (including things
        like CopyMacHandle)

Since the only reason to make Executor work on 64-bit machines is for fun, I
have no idea when I'll have a chance to finish these mods.  If it's something
that interests you, drop me a note on Github, where my account is "ctm".