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/* Abstraction of GNU v3 abi.

   Contributed by Jim Blandy <jimb@redhat.com>

   Copyright (C) 2001, 2002, 2003, 2005, 2006, 2007, 2008, 2009, 2010

   Free Software Foundation, Inc.

   This file is part of GDB.

   This program is free software; you can redistribute it and/or modify

   it under the terms of the GNU General Public License as published by

   the Free Software Foundation; either version 3 of the License, or

   (at your option) any later version.

   This program is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License

   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include "defs.h"

#include "value.h"

#include "cp-abi.h"

#include "cp-support.h"

#include "demangle.h"

#include "objfiles.h"

#include "valprint.h"

#include "gdb_assert.h"

#include "gdb_string.h"

static struct cp_abi_ops gnu_v3_abi_ops;

static int

gnuv3_is_vtable_name (const char *name)

{

  return strncmp (name, "_ZTV", 4) == 0;

}

static int

gnuv3_is_operator_name (const char *name)

{

  return strncmp (name, "operator", 8) == 0;

}

/* To help us find the components of a vtable, we build ourselves a

   GDB type object representing the vtable structure.  Following the

   V3 ABI, it goes something like this:

   struct gdb_gnu_v3_abi_vtable {

     / * An array of virtual call and virtual base offsets.  The real

         length of this array depends on the class hierarchy; we use

         negative subscripts to access the elements.  Yucky, but

         better than the alternatives.  * /

     ptrdiff_t vcall_and_vbase_offsets[0];

     / * The offset from a virtual pointer referring to this table

         to the top of the complete object.  * /

     ptrdiff_t offset_to_top;

     / * The type_info pointer for this class.  This is really a

         std::type_info *, but GDB doesn't really look at the

         type_info object itself, so we don't bother to get the type

         exactly right.  * /

     void *type_info;

     / * Virtual table pointers in objects point here.  * /

     / * Virtual function pointers.  Like the vcall/vbase array, the

         real length of this table depends on the class hierarchy.  * /

     void (*virtual_functions[0]) ();

   };

   The catch, of course, is that the exact layout of this table

   depends on the ABI --- word size, endianness, alignment, etc.  So

   the GDB type object is actually a per-architecture kind of thing.

   vtable_type_gdbarch_data is a gdbarch per-architecture data pointer

   which refers to the struct type * for this structure, laid out

   appropriately for the architecture.  */

static struct gdbarch_data *vtable_type_gdbarch_data;

/* Human-readable names for the numbers of the fields above.  */

enum {

  vtable_field_vcall_and_vbase_offsets,

  vtable_field_offset_to_top,

  vtable_field_type_info,

  vtable_field_virtual_functions

};

/* Return a GDB type representing `struct gdb_gnu_v3_abi_vtable',

   described above, laid out appropriately for ARCH.

   We use this function as the gdbarch per-architecture data

   initialization function.  */

static void *

build_gdb_vtable_type (struct gdbarch *arch)

{

  struct type *t;

  struct field *field_list, *field;

  int offset;

  struct type *void_ptr_type

    = builtin_type (arch)->builtin_data_ptr;

  struct type *ptr_to_void_fn_type

    = builtin_type (arch)->builtin_func_ptr;

  /* ARCH can't give us the true ptrdiff_t type, so we guess.  */

  struct type *ptrdiff_type

    = arch_integer_type (arch, gdbarch_ptr_bit (arch), 0, "ptrdiff_t");

  /* We assume no padding is necessary, since GDB doesn't know

     anything about alignment at the moment.  If this assumption bites

     us, we should add a gdbarch method which, given a type, returns

     the alignment that type requires, and then use that here.  */

  /* Build the field list.  */

  field_list = xmalloc (sizeof (struct field [4]));

  memset (field_list, 0, sizeof (struct field [4]));

  field = &field_list[0];

  offset = 0;

  /* ptrdiff_t vcall_and_vbase_offsets[0]; */

  FIELD_NAME (*field) = "vcall_and_vbase_offsets";

  FIELD_TYPE (*field) = lookup_array_range_type (ptrdiff_type, 0, -1);

  FIELD_BITPOS (*field) = offset * TARGET_CHAR_BIT;

  offset += TYPE_LENGTH (FIELD_TYPE (*field));

  field++;

  /* ptrdiff_t offset_to_top; */

  FIELD_NAME (*field) = "offset_to_top";

  FIELD_TYPE (*field) = ptrdiff_type;

  FIELD_BITPOS (*field) = offset * TARGET_CHAR_BIT;

  offset += TYPE_LENGTH (FIELD_TYPE (*field));

  field++;

  /* void *type_info; */

  FIELD_NAME (*field) = "type_info";

  FIELD_TYPE (*field) = void_ptr_type;

  FIELD_BITPOS (*field) = offset * TARGET_CHAR_BIT;

  offset += TYPE_LENGTH (FIELD_TYPE (*field));

  field++;

  /* void (*virtual_functions[0]) (); */

  FIELD_NAME (*field) = "virtual_functions";

  FIELD_TYPE (*field) = lookup_array_range_type (ptr_to_void_fn_type, 0, -1);

  FIELD_BITPOS (*field) = offset * TARGET_CHAR_BIT;

  offset += TYPE_LENGTH (FIELD_TYPE (*field));

  field++;

  /* We assumed in the allocation above that there were four fields.  */

  gdb_assert (field == (field_list + 4));

  t = arch_type (arch, TYPE_CODE_STRUCT, offset, NULL);

  TYPE_NFIELDS (t) = field - field_list;

  TYPE_FIELDS (t) = field_list;

  TYPE_TAG_NAME (t) = "gdb_gnu_v3_abi_vtable";

  INIT_CPLUS_SPECIFIC (t);

  return t;

}

/* Return the ptrdiff_t type used in the vtable type.  */

static struct type *

vtable_ptrdiff_type (struct gdbarch *gdbarch)

{

  struct type *vtable_type = gdbarch_data (gdbarch, vtable_type_gdbarch_data);

  /* The "offset_to_top" field has the appropriate (ptrdiff_t) type.  */

  return TYPE_FIELD_TYPE (vtable_type, vtable_field_offset_to_top);

}

/* Return the offset from the start of the imaginary `struct

   gdb_gnu_v3_abi_vtable' object to the vtable's "address point"

   (i.e., where objects' virtual table pointers point).  */

static int

vtable_address_point_offset (struct gdbarch *gdbarch)

{

  struct type *vtable_type = gdbarch_data (gdbarch, vtable_type_gdbarch_data);

  return (TYPE_FIELD_BITPOS (vtable_type, vtable_field_virtual_functions)

          / TARGET_CHAR_BIT);

}

/* Determine whether structure TYPE is a dynamic class.  Cache the

   result.  */

static int

gnuv3_dynamic_class (struct type *type)

{

  int fieldnum, fieldelem;

  if (TYPE_CPLUS_DYNAMIC (type))

    return TYPE_CPLUS_DYNAMIC (type) == 1;

  ALLOCATE_CPLUS_STRUCT_TYPE (type);

  for (fieldnum = 0; fieldnum < TYPE_N_BASECLASSES (type); fieldnum++)

    if (BASETYPE_VIA_VIRTUAL (type, fieldnum)

        || gnuv3_dynamic_class (TYPE_FIELD_TYPE (type, fieldnum)))

      {

        TYPE_CPLUS_DYNAMIC (type) = 1;

        return 1;

      }

  for (fieldnum = 0; fieldnum < TYPE_NFN_FIELDS (type); fieldnum++)

    for (fieldelem = 0; fieldelem < TYPE_FN_FIELDLIST_LENGTH (type, fieldnum);

         fieldelem++)

      {

        struct fn_field *f = TYPE_FN_FIELDLIST1 (type, fieldnum);

        if (TYPE_FN_FIELD_VIRTUAL_P (f, fieldelem))

          {

            TYPE_CPLUS_DYNAMIC (type) = 1;

            return 1;

          }

      }

  TYPE_CPLUS_DYNAMIC (type) = -1;

  return 0;

}

/* Find the vtable for a value of CONTAINER_TYPE located at

   CONTAINER_ADDR.  Return a value of the correct vtable type for this

   architecture, or NULL if CONTAINER does not have a vtable.  */

static struct value *

gnuv3_get_vtable (struct gdbarch *gdbarch,

                  struct type *container_type, CORE_ADDR container_addr)

{

  struct type *vtable_type = gdbarch_data (gdbarch,

                                           vtable_type_gdbarch_data);

  struct type *vtable_pointer_type;

  struct value *vtable_pointer;

  CORE_ADDR vtable_address;

  /* If this type does not have a virtual table, don't read the first

     field.  */

  if (!gnuv3_dynamic_class (check_typedef (container_type)))

    return NULL;

  /* We do not consult the debug information to find the virtual table.

     The ABI specifies that it is always at offset zero in any class,

     and debug information may not represent it.

     We avoid using value_contents on principle, because the object might

     be large.  */

  /* Find the type "pointer to virtual table".  */

  vtable_pointer_type = lookup_pointer_type (vtable_type);

  /* Load it from the start of the class.  */

  vtable_pointer = value_at (vtable_pointer_type, container_addr);

  vtable_address = value_as_address (vtable_pointer);

  /* Correct it to point at the start of the virtual table, rather

     than the address point.  */

  return value_at_lazy (vtable_type,

                        vtable_address - vtable_address_point_offset (gdbarch));

}

static struct type *

gnuv3_rtti_type (struct value *value,

                 int *full_p, int *top_p, int *using_enc_p)

{

  struct gdbarch *gdbarch;

  struct type *values_type = check_typedef (value_type (value));

  struct value *vtable;

  struct minimal_symbol *vtable_symbol;

  const char *vtable_symbol_name;

  const char *class_name;

  struct type *run_time_type;

  LONGEST offset_to_top;

  /* We only have RTTI for class objects.  */

  if (TYPE_CODE (values_type) != TYPE_CODE_CLASS)

    return NULL;

  /* Determine architecture.  */

  gdbarch = get_type_arch (values_type);

  if (using_enc_p)

    *using_enc_p = 0;

  vtable = gnuv3_get_vtable (gdbarch, value_type (value),

                             value_as_address (value_addr (value)));

  if (vtable == NULL)

    return NULL;

  /* Find the linker symbol for this vtable.  */

  vtable_symbol

    = lookup_minimal_symbol_by_pc (value_address (vtable)

                                   + value_embedded_offset (vtable));

  if (! vtable_symbol)

    return NULL;

  /* The symbol's demangled name should be something like "vtable for

     CLASS", where CLASS is the name of the run-time type of VALUE.

     If we didn't like this approach, we could instead look in the

     type_info object itself to get the class name.  But this way

     should work just as well, and doesn't read target memory.  */

  vtable_symbol_name = SYMBOL_DEMANGLED_NAME (vtable_symbol);

  if (vtable_symbol_name == NULL

      || strncmp (vtable_symbol_name, "vtable for ", 11))

    {

      warning (_("can't find linker symbol for virtual table for `%s' value"),

               TYPE_NAME (values_type));

      if (vtable_symbol_name)

        warning (_("  found `%s' instead"), vtable_symbol_name);

      return NULL;

    }

  class_name = vtable_symbol_name + 11;

  /* Try to look up the class name as a type name.  */

  /* FIXME: chastain/2003-11-26: block=NULL is bogus.  See pr gdb/1465. */

  run_time_type = cp_lookup_rtti_type (class_name, NULL);

  if (run_time_type == NULL)

    return NULL;

  /* Get the offset from VALUE to the top of the complete object.

     NOTE: this is the reverse of the meaning of *TOP_P.  */

  offset_to_top

    = value_as_long (value_field (vtable, vtable_field_offset_to_top));

  if (full_p)

    *full_p = (- offset_to_top == value_embedded_offset (value)

               && (TYPE_LENGTH (value_enclosing_type (value))

                   >= TYPE_LENGTH (run_time_type)));

  if (top_p)

    *top_p = - offset_to_top;

  return run_time_type;

}

/* Return a function pointer for CONTAINER's VTABLE_INDEX'th virtual

   function, of type FNTYPE.  */

static struct value *

gnuv3_get_virtual_fn (struct gdbarch *gdbarch, struct value *container,

                      struct type *fntype, int vtable_index)

{

  struct value *vtable, *vfn;

  /* Every class with virtual functions must have a vtable.  */

  vtable = gnuv3_get_vtable (gdbarch, value_type (container),

                             value_as_address (value_addr (container)));

  gdb_assert (vtable != NULL);

  /* Fetch the appropriate function pointer from the vtable.  */

  vfn = value_subscript (value_field (vtable, vtable_field_virtual_functions),

                         vtable_index);

  /* If this architecture uses function descriptors directly in the vtable,

     then the address of the vtable entry is actually a "function pointer"

     (i.e. points to the descriptor).  We don't need to scale the index

     by the size of a function descriptor; GCC does that before outputing

     debug information.  */

  if (gdbarch_vtable_function_descriptors (gdbarch))

    vfn = value_addr (vfn);

  /* Cast the function pointer to the appropriate type.  */

  vfn = value_cast (lookup_pointer_type (fntype), vfn);

  return vfn;

}

/* GNU v3 implementation of value_virtual_fn_field.  See cp-abi.h

   for a description of the arguments.  */

static struct value *

gnuv3_virtual_fn_field (struct value **value_p,

                        struct fn_field *f, int j,

                        struct type *vfn_base, int offset)

{

  struct type *values_type = check_typedef (value_type (*value_p));

  struct gdbarch *gdbarch;

  /* Some simple sanity checks.  */

  if (TYPE_CODE (values_type) != TYPE_CODE_CLASS)

    error (_("Only classes can have virtual functions."));

  /* Determine architecture.  */

  gdbarch = get_type_arch (values_type);

  /* Cast our value to the base class which defines this virtual

     function.  This takes care of any necessary `this'

     adjustments.  */

  if (vfn_base != values_type)

    *value_p = value_cast (vfn_base, *value_p);

  return gnuv3_get_virtual_fn (gdbarch, *value_p, TYPE_FN_FIELD_TYPE (f, j),

                               TYPE_FN_FIELD_VOFFSET (f, j));

}

/* Compute the offset of the baseclass which is

   the INDEXth baseclass of class TYPE,

   for value at VALADDR (in host) at ADDRESS (in target).

   The result is the offset of the baseclass value relative

   to (the address of)(ARG) + OFFSET.

   -1 is returned on error. */

static int

gnuv3_baseclass_offset (struct type *type, int index, const bfd_byte *valaddr,

                        CORE_ADDR address)

{

  struct gdbarch *gdbarch;

  struct type *ptr_type;

  struct value *vtable;

  struct value *vbase_array;

  long int cur_base_offset, base_offset;

  /* Determine architecture.  */

  gdbarch = get_type_arch (type);

  ptr_type = builtin_type (gdbarch)->builtin_data_ptr;

  /* If it isn't a virtual base, this is easy.  The offset is in the

     type definition.  */

  if (!BASETYPE_VIA_VIRTUAL (type, index))

    return TYPE_BASECLASS_BITPOS (type, index) / 8;

  /* To access a virtual base, we need to use the vbase offset stored in

     our vtable.  Recent GCC versions provide this information.  If it isn't

     available, we could get what we needed from RTTI, or from drawing the

     complete inheritance graph based on the debug info.  Neither is

     worthwhile.  */

  cur_base_offset = TYPE_BASECLASS_BITPOS (type, index) / 8;

  if (cur_base_offset >= - vtable_address_point_offset (gdbarch))

    error (_("Expected a negative vbase offset (old compiler?)"));

  cur_base_offset = cur_base_offset + vtable_address_point_offset (gdbarch);

  if ((- cur_base_offset) % TYPE_LENGTH (ptr_type) != 0)

    error (_("Misaligned vbase offset."));

  cur_base_offset = cur_base_offset / ((int) TYPE_LENGTH (ptr_type));

  vtable = gnuv3_get_vtable (gdbarch, type, address);

  gdb_assert (vtable != NULL);

  vbase_array = value_field (vtable, vtable_field_vcall_and_vbase_offsets);

  base_offset = value_as_long (value_subscript (vbase_array, cur_base_offset));

  return base_offset;

}

/* Locate a virtual method in DOMAIN or its non-virtual base classes

   which has virtual table index VOFFSET.  The method has an associated

   "this" adjustment of ADJUSTMENT bytes.  */

static const char *

gnuv3_find_method_in (struct type *domain, CORE_ADDR voffset,

                      LONGEST adjustment)

{

  int i;

  const char *physname;

  /* Search this class first.  */

  physname = NULL;

  if (adjustment == 0)

    {

      int len;

      len = TYPE_NFN_FIELDS (domain);

      for (i = 0; i < len; i++)

        {

          int len2, j;

          struct fn_field *f;

          f = TYPE_FN_FIELDLIST1 (domain, i);

          len2 = TYPE_FN_FIELDLIST_LENGTH (domain, i);

          check_stub_method_group (domain, i);

          for (j = 0; j < len2; j++)

            if (TYPE_FN_FIELD_VOFFSET (f, j) == voffset)

              return TYPE_FN_FIELD_PHYSNAME (f, j);

        }

    }

  /* Next search non-virtual bases.  If it's in a virtual base,

     we're out of luck.  */

  for (i = 0; i < TYPE_N_BASECLASSES (domain); i++)

    {

      int pos;

      struct type *basetype;

      if (BASETYPE_VIA_VIRTUAL (domain, i))

        continue;

      pos = TYPE_BASECLASS_BITPOS (domain, i) / 8;

      basetype = TYPE_FIELD_TYPE (domain, i);

      /* Recurse with a modified adjustment.  We don't need to adjust

         voffset.  */

      if (adjustment >= pos && adjustment < pos + TYPE_LENGTH (basetype))

        return gnuv3_find_method_in (basetype, voffset, adjustment - pos);

    }

  return NULL;

}

/* Decode GNU v3 method pointer.  */

static int

gnuv3_decode_method_ptr (struct gdbarch *gdbarch,

                         const gdb_byte *contents,

                         CORE_ADDR *value_p,

                         LONGEST *adjustment_p)

{

  struct type *funcptr_type = builtin_type (gdbarch)->builtin_func_ptr;

  struct type *offset_type = vtable_ptrdiff_type (gdbarch);

  enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);

  CORE_ADDR ptr_value;

  LONGEST voffset, adjustment;

  int vbit;

  /* Extract the pointer to member.  The first element is either a pointer

     or a vtable offset.  For pointers, we need to use extract_typed_address

     to allow the back-end to convert the pointer to a GDB address -- but

     vtable offsets we must handle as integers.  At this point, we do not

     yet know which case we have, so we extract the value under both

     interpretations and choose the right one later on.  */

  ptr_value = extract_typed_address (contents, funcptr_type);

  voffset = extract_signed_integer (contents,

                                    TYPE_LENGTH (funcptr_type), byte_order);

  contents += TYPE_LENGTH (funcptr_type);

  adjustment = extract_signed_integer (contents,

                                       TYPE_LENGTH (offset_type), byte_order);

  if (!gdbarch_vbit_in_delta (gdbarch))

    {

      vbit = voffset & 1;

      voffset = voffset ^ vbit;

    }

  else

    {

      vbit = adjustment & 1;

      adjustment = adjustment >> 1;

    }

  *value_p = vbit? voffset : ptr_value;

  *adjustment_p = adjustment;

  return vbit;

}

/* GNU v3 implementation of cplus_print_method_ptr.  */

static void

gnuv3_print_method_ptr (const gdb_byte *contents,

                        struct type *type,

                        struct ui_file *stream)

{

  struct type *domain = TYPE_DOMAIN_TYPE (type);

  struct gdbarch *gdbarch = get_type_arch (domain);

  CORE_ADDR ptr_value;

  LONGEST adjustment;

  int vbit;

  /* Extract the pointer to member.  */

  vbit = gnuv3_decode_method_ptr (gdbarch, contents, &ptr_value, &adjustment);

  /* Check for NULL.  */

  if (ptr_value == 0 && vbit == 0)

    {

      fprintf_filtered (stream, "NULL");

      return;

    }

  /* Search for a virtual method.  */

  if (vbit)

    {

      CORE_ADDR voffset;

      const char *physname;

      /* It's a virtual table offset, maybe in this class.  Search

         for a field with the correct vtable offset.  First convert it

         to an index, as used in TYPE_FN_FIELD_VOFFSET.  */

      voffset = ptr_value / TYPE_LENGTH (vtable_ptrdiff_type (gdbarch));

      physname = gnuv3_find_method_in (domain, voffset, adjustment);

      /* If we found a method, print that.  We don't bother to disambiguate

         possible paths to the method based on the adjustment.  */

      if (physname)

        {

          char *demangled_name = cplus_demangle (physname,

                                                 DMGL_ANSI | DMGL_PARAMS);

          if (demangled_name != NULL)

            {

              fprintf_filtered (stream, "&virtual ");

              fputs_filtered (demangled_name, stream);

              xfree (demangled_name);

              return;

            }

        }

    }

  /* We didn't find it; print the raw data.  */

  if (vbit)

    {

      fprintf_filtered (stream, "&virtual table offset ");

      print_longest (stream, 'd', 1, ptr_value);

    }

  else

    print_address_demangle (gdbarch, ptr_value, stream, demangle);

  if (adjustment)

    {

      fprintf_filtered (stream, ", this adjustment ");

      print_longest (stream, 'd', 1, adjustment);

    }

}

/* GNU v3 implementation of cplus_method_ptr_size.  */

static int

gnuv3_method_ptr_size (struct type *type)

{

  struct type *domain_type = check_typedef (TYPE_DOMAIN_TYPE (type));

  struct gdbarch *gdbarch = get_type_arch (domain_type);

  return 2 * TYPE_LENGTH (builtin_type (gdbarch)->builtin_data_ptr);

}

/* GNU v3 implementation of cplus_make_method_ptr.  */

static void

gnuv3_make_method_ptr (struct type *type, gdb_byte *contents,

                       CORE_ADDR value, int is_virtual)

{

  struct type *domain_type = check_typedef (TYPE_DOMAIN_TYPE (type));

  struct gdbarch *gdbarch = get_type_arch (domain_type);

  int size = TYPE_LENGTH (builtin_type (gdbarch)->builtin_data_ptr);

  enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);

  /* FIXME drow/2006-12-24: The adjustment of "this" is currently

     always zero, since the method pointer is of the correct type.

     But if the method pointer came from a base class, this is

     incorrect - it should be the offset to the base.  The best

     fix might be to create the pointer to member pointing at the

     base class and cast it to the derived class, but that requires

     support for adjusting pointers to members when casting them -

     not currently supported by GDB.  */

  if (!gdbarch_vbit_in_delta (gdbarch))

    {

      store_unsigned_integer (contents, size, byte_order, value | is_virtual);

      store_unsigned_integer (contents + size, size, byte_order, 0);

    }

  else

    {

      store_unsigned_integer (contents, size, byte_order, value);

      store_unsigned_integer (contents + size, size, byte_order, is_virtual);

    }

}

/* GNU v3 implementation of cplus_method_ptr_to_value.  */

static struct value *

gnuv3_method_ptr_to_value (struct value **this_p, struct value *method_ptr)

{

  struct gdbarch *gdbarch;

  const gdb_byte *contents = value_contents (method_ptr);

  CORE_ADDR ptr_value;

  struct type *domain_type, *final_type, *method_type;

  LONGEST adjustment;

  int vbit;

  domain_type = TYPE_DOMAIN_TYPE (check_typedef (value_type (method_ptr)));

  final_type = lookup_pointer_type (domain_type);

  method_type = TYPE_TARGET_TYPE (check_typedef (value_type (method_ptr)));

  /* Extract the pointer to member.  */

  gdbarch = get_type_arch (domain_type);

  vbit = gnuv3_decode_method_ptr (gdbarch, contents, &ptr_value, &adjustment);

  /* First convert THIS to match the containing type of the pointer to

     member.  This cast may adjust the value of THIS.  */

  *this_p = value_cast (final_type, *this_p);

  /* Then apply whatever adjustment is necessary.  This creates a somewhat

     strange pointer: it claims to have type FINAL_TYPE, but in fact it

     might not be a valid FINAL_TYPE.  For instance, it might be a

     base class of FINAL_TYPE.  And if it's not the primary base class,

     then printing it out as a FINAL_TYPE object would produce some pretty

     garbage.

     But we don't really know the type of the first argument in

     METHOD_TYPE either, which is why this happens.  We can't

     dereference this later as a FINAL_TYPE, but once we arrive in the

     called method we'll have debugging information for the type of

     "this" - and that'll match the value we produce here.

     You can provoke this case by casting a Base::* to a Derived::*, for

     instance.  */

  *this_p = value_cast (builtin_type (gdbarch)->builtin_data_ptr, *this_p);

  *this_p = value_ptradd (*this_p, adjustment);

  *this_p = value_cast (final_type, *this_p);

  if (vbit)

    {

      LONGEST voffset;