[Python] Add methods reference_value and const_value to gdb.Value

[Siva Chandra <sivachandra@google.com>]

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This will help address libstdc++/65840:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65840

gdb/ChangeLog:

2015-04-27  Siva Chandra Reddy  <sivachandra@google.com>

        * NEWS (Python Scripting): Mention the new gdb.Value methods.
        * python/py-value.c (valpy_reference_value): New function.
        (valpy_const_value): Likewise.
        (value_object_methods): Add new methods.
        * value.c (make_cv_value): New function.
        * value.h (make_cv_value): Declare.

gdb/doc/ChangeLog:

2015-04-27  Siva Chandra Reddy  <sivachandra@google.com>

        * python.texi (Values From Inferior): Add descriptions of new
        methods gdb.Value.reference_value and gdb.Value.const_value.

gdb/testsuite/ChangeLog:

2015-04-27  Siva Chandra Reddy  <sivachandra@google.com>

        * gdb.python/py-xmethods.cc: Enhance test case.
        * gdb.python/py-xmethods.exp: New tests.
        * gdb.python/py-xmethods.py (A_indexoper): New xmethod worker
        function.
        (B_indexoper): Likewise.
        (global_dm_list) : Add new xmethod worker functions.

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diff --git a/gdb/NEWS b/gdb/NEWS
index b711553..627afd0 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -52,6 +52,9 @@
      which is the name of the objfile as specified by the user,
      without, for example, resolving symlinks.
   ** You can now write frame unwinders in Python.
+  ** gdb.Value objects have new methods "reference_value" and
+     "const_value" which return a reference to the value and a
+     "const" version of the value respectively.
 
 * New commands
 
diff --git a/gdb/doc/python.texi b/gdb/doc/python.texi
index 448fa8b2..1b1079d 100644
--- a/gdb/doc/python.texi
+++ b/gdb/doc/python.texi
@@ -781,6 +781,16 @@ The @code{gdb.Value} object @code{py_val} is identical to that
 corresponding to @code{val}.
 @end defun
 
+@defun Value.reference_value ()
+Return a @code{gdb.Value} object which is a reference to the value
+encapsulated by this instance.
+@end defun
+
+@defun Value.const_value ()
+Return a @code{gdb.Value} object which is a @code{const} version of the
+value encapsulated by this instance.
+@end defun
+
 @defun Value.dynamic_cast (type)
 Like @code{Value.cast}, but works as if the C@t{++} @code{dynamic_cast}
 operator were used.  Consult a C@t{++} reference for details.
diff --git a/gdb/python/py-value.c b/gdb/python/py-value.c
index 6622d11..94ff9ba 100644
--- a/gdb/python/py-value.c
+++ b/gdb/python/py-value.c
@@ -236,6 +236,60 @@ valpy_referenced_value (PyObject *self, PyObject *args)
   return result;
 }
 
+/* Return a value which is a reference to the value.  */
+
+static PyObject *
+valpy_reference_value (PyObject *self, PyObject *args)
+{
+  PyObject *result = NULL;
+
+  TRY
+    {
+      struct value *self_val;
+      struct cleanup *cleanup = make_cleanup_value_free_to_mark (value_mark ());
+
+      self_val = ((value_object *) self)->value;
+      result = value_to_value_object (value_ref (self_val));
+
+      do_cleanups (cleanup);
+    }
+  CATCH (except, RETURN_MASK_ALL)
+    {
+      GDB_PY_HANDLE_EXCEPTION (except);
+    }
+  END_CATCH
+
+  return result;
+}
+
+/* Return a "const" qualified version of the value.  */
+
+static PyObject *
+valpy_const_value (PyObject *self, PyObject *args)
+{
+  PyObject *result = NULL;
+
+  TRY
+    {
+      struct value *self_val, *res_val;
+      struct type *const_type;
+      struct cleanup *cleanup = make_cleanup_value_free_to_mark (value_mark ());
+
+      self_val = ((value_object *) self)->value;
+      res_val = make_cv_value (1, 0, self_val);
+      result = value_to_value_object (res_val);
+
+      do_cleanups (cleanup);
+    }
+  CATCH (except, RETURN_MASK_ALL)
+    {
+      GDB_PY_HANDLE_EXCEPTION (except);
+    }
+  END_CATCH
+
+  return result;
+}
+
 /* Return "&value".  */
 static PyObject *
 valpy_get_address (PyObject *self, void *closure)
@@ -1692,6 +1746,10 @@ reinterpret_cast operator."
   { "dereference", valpy_dereference, METH_NOARGS, "Dereferences the value." },
   { "referenced_value", valpy_referenced_value, METH_NOARGS,
     "Return the value referenced by a TYPE_CODE_REF or TYPE_CODE_PTR value." },
+  { "reference_value", valpy_reference_value, METH_NOARGS,
+    "Return a value of type TYPE_CODE_REF referencing this value." },
+  { "const_value", valpy_const_value, METH_NOARGS,
+    "Return a 'const' qualied version of the same value." },
   { "lazy_string", (PyCFunction) valpy_lazy_string,
     METH_VARARGS | METH_KEYWORDS,
     "lazy_string ([encoding]  [, length]) -> lazy_string\n\
diff --git a/gdb/testsuite/gdb.python/py-xmethods.cc b/gdb/testsuite/gdb.python/py-xmethods.cc
index aedd1de..98bdb72 100644
--- a/gdb/testsuite/gdb.python/py-xmethods.cc
+++ b/gdb/testsuite/gdb.python/py-xmethods.cc
@@ -173,7 +173,7 @@ int main(void)
 
   for (int i = 0; i < 10; i++)
     {
-      a1.array[i] = a2.array[i] = i;
+      a1.array[i] = a2.array[i] = b1.array[i] = i;
     }
 
   return 0; /* Break here.  */
diff --git a/gdb/testsuite/gdb.python/py-xmethods.exp b/gdb/testsuite/gdb.python/py-xmethods.exp
index a83b14d..eacfa0c 100644
--- a/gdb/testsuite/gdb.python/py-xmethods.exp
+++ b/gdb/testsuite/gdb.python/py-xmethods.exp
@@ -100,6 +100,8 @@ gdb_test "p a1.geta()" "From Python <A_geta>.*5" "After: a1.geta()"
 gdb_test "p ++a1" "From Python <plus_plus_A>.*6" "After: ++a1"
 gdb_test "p a1.getarrayind(5)" "From Python <A_getarrayind>.*5" \
   "After: a1.getarrayind(5)"
+gdb_test "P a1\[6\]" ".*int &.*6" "After a1\[\]"
+gdb_test "P b1\[7\]" ".*const int &.*7" "After b1\[\]"
 # Note the following test.  Xmethods on dynamc types are not looked up
 # currently.  Hence, even though a_ptr points to a B object, the xmethod
 # defined for A objects is invoked.
diff --git a/gdb/testsuite/gdb.python/py-xmethods.py b/gdb/testsuite/gdb.python/py-xmethods.py
index 78935e1..3194e89 100644
--- a/gdb/testsuite/gdb.python/py-xmethods.py
+++ b/gdb/testsuite/gdb.python/py-xmethods.py
@@ -43,6 +43,12 @@ def A_getarrayind(obj, index):
   print('From Python <A_getarrayind>:')
   return obj['array'][index]
 
+def A_indexoper(obj, index):
+  return obj['array'][index].reference_value()
+
+def B_indexoper(obj, index):
+  return obj['array'][index].const_value().reference_value()
+
 
 type_A = gdb.parse_and_eval('(dop::A *) 0').type.target()
 type_B = gdb.parse_and_eval('(dop::B *) 0').type.target()
@@ -208,6 +214,16 @@ global_dm_list = [
                          '^getarrayind$',
                          A_getarrayind,
                          type_int),
+    SimpleXMethodMatcher('A_indexoper',
+                         '^dop::A$',
+                         'operator\\[\\]',
+                         A_indexoper,
+                         type_int),
+    SimpleXMethodMatcher('B_indexoper',
+                         '^dop::B$',
+                         'operator\\[\\]',
+                         B_indexoper,
+                         type_int)
 ]
 
 for matcher in global_dm_list:
diff --git a/gdb/value.c b/gdb/value.c
index cb56849..242b46c 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -1704,6 +1704,22 @@ value_copy (struct value *arg)
   return val;
 }
 
+/* Return a "const" and/or "volatile" qualified version of the value V.
+   If CNST is true, then the returned value will be qualified with
+   "const".
+   if VOLTL is true, then the returned value will be qualified with
+   "volatile".  */
+
+struct value *
+make_cv_value (int cnst, int voltl, struct value *v)
+{
+  struct value *cv_val = value_copy (v);
+
+  cv_val->type = make_cv_type (cnst, voltl, value_type (v), NULL);
+
+  return cv_val;
+}
+
 /* Return a version of ARG that is non-lvalue.  */
 
 struct value *
diff --git a/gdb/value.h b/gdb/value.h
index 21baa32..9aeaa4a 100644
--- a/gdb/value.h
+++ b/gdb/value.h
@@ -1040,6 +1040,8 @@ extern struct value *value_non_lval (struct value *);
 
 extern void value_force_lval (struct value *, CORE_ADDR);
 
+extern struct value *make_cv_value (int, int, struct value *);
+
 extern void preserve_one_value (struct value *, struct objfile *, htab_t);
 
 /* From valops.c */