Handle Ada extended access thick pointers

In Ada, sometimes an array is represented as a "thick" pointer -- a
structure that holds a pointer to the array data and another pointer
to the bounds structure.

A new "extended access" feature is being added to GNAT which changes
the shape of these objects.  With the new feature, the bounds are
inlined into the thick pointer.

This patch changes gdb to understand this new feature.  A test case is
provided; it is written in C to avoid requiring a newer GNAT just for
this test.

Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/ada-lang.c

@@ -1764,16 +1764,6 @@ desc_base_type (struct type *type)
     return type;
 }
 
-/* True iff TYPE indicates a "thin" array pointer type.  */
-
-static int
-is_thin_pntr (struct type *type)
-{
-  return
-    is_suffix (ada_type_name (desc_base_type (type)), "___XUT")
-    || is_suffix (ada_type_name (desc_base_type (type)), "___XUT___XVE");
-}
-
 /* The descriptor type for thin pointer type TYPE.  */
 
 static struct type *
@@ -1812,14 +1802,45 @@ thin_data_pntr (struct value *val)
     return value_from_longest (data_type, val->address ());
 }
 
-/* True iff TYPE indicates a "thick" array pointer type.  */
+/* Different kinds of Ada-specific pointers.  */
+enum class pointer_kind
+{
+  /* Not a thin or thick pointer.  */
+  NOT_SPECIAL,
+  /* A thin pointer.  */
+  THIN,
+  /* Thick pointer where the bounds are accessed via the P_BOUNDS
+     member, a pointer to the bounds.  */
+  P_BOUNDS,
+  /* Thick pointer where the bounds are accessed via the BOUNDS
+     member, which directly holds the bounds.  */
+  EXTENDED_ACCESS,
+};
 
-static int
+/* Return a pointer_kind indicating whether TYPE is a special category
-is_thick_pntr (struct type *type)
+   of pointer.  Note that TYPE may be NULL, in which case NOT_SPECIAL
+   is returned.  */
+
+static pointer_kind
+categorize_pointer (struct type *type)
 {
   type = desc_base_type (type);
-  return (type != NULL && type->code () == TYPE_CODE_STRUCT
+  if (type == nullptr)
-	  && lookup_struct_elt_type (type, "P_BOUNDS", 1) != NULL);
+    return pointer_kind::NOT_SPECIAL;
+
+  if (is_suffix (ada_type_name (type), "___XUT")
+      || is_suffix (ada_type_name (type), "___XUT___XVE"))
+    return pointer_kind::THIN;
+
+  if (type->code () == TYPE_CODE_STRUCT)
+    {
+      if (lookup_struct_elt_type (type, "P_BOUNDS", 1) != nullptr)
+	return pointer_kind::P_BOUNDS;
+      if (lookup_struct_elt_type (type, "BOUNDS", 1) != nullptr)
+	return pointer_kind::EXTENDED_ACCESS;
+    }
+
+  return pointer_kind::NOT_SPECIAL;
 }
 
 /* If TYPE is the type of an array descriptor (fat or thin pointer) or a
@@ -1830,11 +1851,9 @@ desc_bounds_type (struct type *type)
 {
   struct type *r;
 
+  pointer_kind kind = categorize_pointer (type);
   type = desc_base_type (type);
-
+  if (kind == pointer_kind::THIN)
-  if (type == NULL)
-    return NULL;
-  else if (is_thin_pntr (type))
     {
       type = thin_descriptor_type (type);
       if (type == NULL)
@@ -1843,25 +1862,36 @@ desc_bounds_type (struct type *type)
       if (r != NULL)
 	return ada_check_typedef (r);
     }
-  else if (type->code () == TYPE_CODE_STRUCT)
+  else if (kind == pointer_kind::P_BOUNDS
+	   || kind == pointer_kind::EXTENDED_ACCESS)
     {
-      r = lookup_struct_elt_type (type, "P_BOUNDS", 1);
+      const char *field_name = (kind == pointer_kind::EXTENDED_ACCESS
+				? "BOUNDS"
+				: "P_BOUNDS");
+      r = lookup_struct_elt_type (type, field_name, 1);
       if (r != NULL)
-	return ada_check_typedef (ada_check_typedef (r)->target_type ());
+	{
+	  struct type *bounds_type = ada_check_typedef (r);
+	  if (bounds_type->code () == TYPE_CODE_PTR)
+	    bounds_type = ada_check_typedef (bounds_type->target_type ());
+	  return bounds_type;
+	}
     }
   return NULL;
 }
 
 /* If ARR is an array descriptor (fat or thin pointer), or pointer to
-   one, a pointer to its bounds data.  Otherwise, throw an
+   one, return a pointer to its bounds data.  For a fat pointer, this
-   exception.  */
+   means the structure holding the bounds -- so for extended access
+   pointers in particular, it will just return the descriptor itself.
+   Otherwise, throw an exception.  */
 
 static struct value *
 desc_bounds (struct value *arr)
 {
   struct type *type = ada_check_typedef (arr->type ());
-
+  pointer_kind kind = categorize_pointer (type);
-  if (is_thin_pntr (type))
+  if (kind == pointer_kind::THIN)
     {
       struct type *bounds_type =
 	desc_bounds_type (thin_descriptor_type (type));
@@ -1882,14 +1912,18 @@ desc_bounds (struct value *arr)
 	value_from_longest (lookup_pointer_type (bounds_type),
 			    addr - bounds_type->length ());
     }
-
+  else if (kind == pointer_kind::P_BOUNDS
-  else if (is_thick_pntr (type))
+	   || kind == pointer_kind::EXTENDED_ACCESS)
     {
-      struct value *p_bounds = value_struct_elt (&arr, {}, "P_BOUNDS", NULL,
+      const char *field_name = (kind == pointer_kind::EXTENDED_ACCESS
-					       _("Bad GNAT array descriptor"));
+				? "BOUNDS"
+				: "P_BOUNDS");
+      struct value *p_bounds
+	= value_struct_elt (&arr, {}, field_name, nullptr,
+			    _("Bad GNAT array descriptor"));
       struct type *p_bounds_type = p_bounds->type ();
 
-      if (p_bounds_type
+      if (p_bounds_type != nullptr
 	  && p_bounds_type->code () == TYPE_CODE_PTR)
 	{
 	  struct type *target_type = p_bounds_type->target_type ();
@@ -1899,7 +1933,8 @@ desc_bounds (struct value *arr)
 				   (ada_check_typedef (target_type)),
 				   p_bounds);
 	}
-      else
+      else if (p_bounds_type == nullptr
+	       || kind != pointer_kind::EXTENDED_ACCESS)
 	error (_("Bad GNAT array descriptor"));
 
       return p_bounds;
@@ -1939,12 +1974,13 @@ fat_pntr_bounds_bitsize (struct type *type)
 static struct type *
 desc_data_target_type (struct type *type)
 {
+  pointer_kind kind = categorize_pointer (type);
   type = desc_base_type (type);
-
   /* NOTE: The following is bogus; see comment in desc_bounds.  */
-  if (is_thin_pntr (type))
+  if (kind == pointer_kind::THIN)
     return desc_base_type (thin_descriptor_type (type)->field (1).type ());
-  else if (is_thick_pntr (type))
+  else if (kind == pointer_kind::P_BOUNDS
+	   || kind == pointer_kind::EXTENDED_ACCESS)
     {
       struct type *data_type = lookup_struct_elt_type (type, "P_ARRAY", 1);
 
@@ -1963,10 +1999,11 @@ static struct value *
 desc_data (struct value *arr)
 {
   struct type *type = arr->type ();
-
+  pointer_kind kind = categorize_pointer (type);
-  if (is_thin_pntr (type))
+  if (kind == pointer_kind::THIN)
     return thin_data_pntr (arr);
-  else if (is_thick_pntr (type))
+  else if (kind == pointer_kind::P_BOUNDS
+	   || kind == pointer_kind::EXTENDED_ACCESS)
     return value_struct_elt (&arr, {}, "P_ARRAY", NULL,
 			     _("Bad GNAT array descriptor"));
   else
@@ -2164,7 +2201,10 @@ ada_type_of_array (struct value *arr, int bounds)
 	return ada_check_typedef (arr->type ());
 
       descriptor = desc_bounds (arr);
-      if (value_as_long (descriptor) == 0)
+      /* In the extended access case, the bounds struct is "inline" so
+	 the pointer cannot be NULL.  */
+      if (ada_check_typedef (descriptor->type ())->code () == TYPE_CODE_PTR
+	  && value_as_long (descriptor) == 0)
 	return NULL;
       while (arity > 0)
 	{
@@ -2305,7 +2345,8 @@ ada_is_unconstrained_packed_array_type (struct type *type)
   /* If we saw GNAT encodings, then the above code is sufficient.
      However, with minimal encodings, we will just have a thick
      pointer instead.  */
-  if (is_thick_pntr (type))
+  pointer_kind kind = categorize_pointer (type);
+  if (kind == pointer_kind::P_BOUNDS || kind == pointer_kind::EXTENDED_ACCESS)
     {
       type = desc_base_type (type);
       /* The structure's first field is a pointer to an array, so this
@@ -2357,7 +2398,9 @@ decode_packed_array_bitsize (struct type *type)
   tail = strstr (raw_name, "___XP");
   if (tail == nullptr)
     {
-      gdb_assert (is_thick_pntr (type));
+      pointer_kind kind = categorize_pointer (type);
+      gdb_assert (kind == pointer_kind::P_BOUNDS
+		  || kind == pointer_kind::EXTENDED_ACCESS);
       /* The structure's first field is a pointer to an array, so this
 	 fetches the array type.  */
       type = type->field (0).type ()->target_type ();
@@ -3018,8 +3061,11 @@ value_assign_to_component (struct value *container, struct value *component,
 bool
 ada_is_access_to_unconstrained_array (struct type *type)
 {
-  return (type->code () == TYPE_CODE_TYPEDEF
+  if (type->code () != TYPE_CODE_TYPEDEF)
-	  && is_thick_pntr (ada_typedef_target_type (type)));
+    return false;
+  pointer_kind kind = categorize_pointer (ada_typedef_target_type (type));
+  return (kind == pointer_kind::P_BOUNDS
+	  || kind == pointer_kind::EXTENDED_ACCESS);
 }
 
 /* The value of the element of array ARR at the ARITY indices given in IND.
@@ -11054,7 +11100,9 @@ ada_unop_ind_operation::evaluate (struct type *expect_type,
 
 	  if (arrType == NULL)
 	    error (_("Attempt to dereference null array pointer."));
-	  if (is_thick_pntr (type))
+	  pointer_kind kind = categorize_pointer (type);
+	  if (kind == pointer_kind::P_BOUNDS
+	      || kind == pointer_kind::EXTENDED_ACCESS)
 	    return arg1;
 	  return value_at_lazy (arrType, 0);
 	}

gdb/testsuite/gdb.ada/extended-access.c

@@ -0,0 +1,38 @@
+/* This test program is part of GDB, the GNU debugger.
+
+   Copyright 2025 Free Software Foundation, Inc.
+
+   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/>.  */
+
+int array_data[] = { 23, 24, 25, 26, 27 };
+
+struct bounds
+{
+  int LB0;
+  int UB0;
+};
+
+struct extended_access_ptr
+{
+  int (*P_ARRAY)[];
+  struct bounds BOUNDS;
+};
+
+struct extended_access_ptr the_array = { &array_data, { 93, 97 } };
+
+int
+main (void)
+{
+  return 0;
+}

gdb/testsuite/gdb.ada/extended-access.exp

@@ -0,0 +1,29 @@
+# Copyright 2025 Free Software Foundation, Inc.
+
+# 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/>.
+
+require allow_ada_tests
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" ${testfile} ${srcfile}]} {
+    return
+}
+
+# The test case is written in C, because it was easy to make the
+# required type there without requiring a new version of GNAT.
+gdb_test_no_output "set lang ada"
+
+gdb_test "print the_array.all" \
+    [string_to_regexp " = (93 => 23, 24, 25, 26, 27)"]