MPS: Forwording symbols

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Ok, I've looked a bit closer, and I think we can avoid dumping the
the forwarding structs altogether.

There are 5 types of forwarding structs

 enum Lisp_Fwd_Type
   {
     Lisp_Fwd_Int,		/* Fwd to a C `int' variable.  */
     Lisp_Fwd_Bool,		/* Fwd to a C boolean var.  */
     Lisp_Fwd_Obj,		/* Fwd to a C Lisp_Object variable.  */
     Lisp_Fwd_Buffer_Obj,	/* Fwd to a Lisp_Object field of buffers.  */
     Lisp_Fwd_Kboard_Obj		/* Fwd to a Lisp_Object field of kboards.  */
   };

Four of them contain only the type and either integer offsets or
pointers to variables in Emacs' data segment. The only interesting one
is Lisp_Fwd_Buffer_Obj which looks like

  struct Lisp_Buffer_Objfwd
    {
      enum Lisp_Fwd_Type type;	/* = Lisp_Fwd_Buffer_Obj */
      int offset;
      /* One of Qnil, Qintegerp, Qsymbolp, Qstringp, Qfloatp or Qnumberp.  */
      Lisp_Object predicate;
    };

which has an additional member for the predicate. AFAICT, the comment is
true, and thus predicate is also a constant because it is always from a
DEFSYM, i.e. it is a symbol from lispsym.

Means that dumping the fwd structs is not strictly needed. (And maybe
one should replace the Lisp_Object predicate member with an enum, to
make that sure for the future.)

If we don't dump_fwd, we would have to make sure though not to overwrite
the existing values for symbols in lispsym, which happens in
dump_do_dump_relocation

    case RELOC_DUMP_TO_EMACS_PTR_RAW:
      {
        uintptr_t value = dump_read_word_from_dump (dump_base, reloc_offset);
        eassert (dump_reloc_size (reloc) == sizeof (value));
        value += emacs_basis ();
        dump_write_word_to_dump (dump_base, reloc_offset, value);
        break;
      }

The name dump_write_word_to_dump is misleading. It does a memcpy to
Emacs' data segment.

Maybe one could introduce a new RELOC_xyz type to signify that a
forwarding symbol is patched and save the fwd value around the memcpy if
needed.

Or something like that?