gdb/python: Reuse gdb.RegisterDescriptor objects where possible

Instead of having the gdb.RegisterDescriptorIterator creating new
gdb.RegisterDescriptor objects for each regnum, instead cache
gdb.RegisterDescriptor objects on the gdbarch object and reuse these.

This means that for every gdbarch/regnum pair there is a single unique
gdb.RegisterDescriptor, this feels like a neater implementation than
the existing one.

It is possible for a user to see (in Python code) that the descriptors
are now identical, but as the descriptors are read-only this should
make no real difference.

There should be no other user visible changes.

gdb/ChangeLog:

	* python/py-registers.c (gdbpy_register_object_data): New static
	global.
	(gdbpy_register_object_data_init): New function.
	(gdbpy_new_register_descriptor): Renamed to...
	(gdbpy_get_register_descriptor): ...this, and update to reuse
	existing register descriptors where possible.
	(gdbpy_register_descriptor_iter_next): Update.
	(gdbpy_initialize_registers): Register new gdbarch data.

gdb/testsuite/ChangeLog:

	* gdb.python/py-arch-reg-names.exp: Additional tests.

1 files changed, 48 insertions, 13 deletions

diff --git a/gdb/python/py-registers.c b/gdb/python/py-registers.c
index db0fe37..8e22a919 100644
--- a/gdb/python/py-registers.c
+++ b/gdb/python/py-registers.c
@@ -24,6 +24,9 @@
 #include "reggroups.h"
 #include "python-internal.h"
 
+/* Token to access per-gdbarch data related to register descriptors.  */
+static struct gdbarch_data *gdbpy_register_object_data = NULL;
+
 /* Structure for iterator over register descriptors.  */
 typedef struct {
   PyObject_HEAD
@@ -81,6 +84,17 @@ typedef struct {
 extern PyTypeObject reggroup_object_type
     CPYCHECKER_TYPE_OBJECT_FOR_TYPEDEF ("reggroup_object");
 
+/* Associates a vector of gdb.RegisterDescriptor objects with GDBARCH as
+   gdbarch_data via the gdbarch post init registration mechanism
+   (gdbarch_data_register_post_init).  */
+
+static void *
+gdbpy_register_object_data_init (struct gdbarch *gdbarch)
+{
+  std::vector<gdbpy_ref<>> *vec = new (std::vector<gdbpy_ref<>>);
+  return (void *) vec;
+}
+
 /* Create a new gdb.RegisterGroup object wrapping REGGROUP.  */
 
 static PyObject *
@@ -117,20 +131,38 @@ gdbpy_reggroup_name (PyObject *self, void *closure)
   return gdbpy_reggroup_to_string (self);
 }
 
-/* Create an return a new gdb.RegisterDescriptor object.  */
-static PyObject *
-gdbpy_new_register_descriptor (struct gdbarch *gdbarch,
+/* Return a gdb.RegisterDescriptor object for REGNUM from GDBARCH.  For
+   each REGNUM (in GDBARCH) only one descriptor is ever created, which is
+   then cached on the GDBARCH.  */
+
+static gdbpy_ref<>
+gdbpy_get_register_descriptor (struct gdbarch *gdbarch,
 			       int regnum)
 {
-  /* Create a new object and fill in its details.  */
-  register_descriptor_object *reg
-    = PyObject_New (register_descriptor_object,
-		    &register_descriptor_object_type);
-  if (reg == NULL)
-    return NULL;
-  reg->regnum = regnum;
-  reg->gdbarch = gdbarch;
-  return (PyObject *) reg;
+  auto vec = (std::vector<gdbpy_ref<>> *) gdbarch_data
+    (gdbarch, gdbpy_register_object_data);
+
+  /* Ensure that we have enough entries in the vector.  */
+  if (vec->size () <= regnum)
+    vec->resize ((regnum + 1), nullptr);
+
+  /* If we don't already have a descriptor for REGNUM in GDBARCH then
+     create one now.  */
+  if (vec->at (regnum) == nullptr)
+    {
+      gdbpy_ref <register_descriptor_object> reg
+	(PyObject_New (register_descriptor_object,
+		       &register_descriptor_object_type));
+      if (reg == NULL)
+	return NULL;
+      reg->regnum = regnum;
+      reg->gdbarch = gdbarch;
+      vec->at (regnum) = gdbpy_ref<> ((PyObject *) reg.release ());
+    }
+
+  /* Grab the register descriptor from the vector, the reference count is
+     automatically incremented thanks to gdbpy_ref.  */
+  return vec->at (regnum);
 }
 
 /* Convert the register descriptor to a string.  */
@@ -281,7 +313,7 @@ gdbpy_register_descriptor_iter_next (PyObject *self)
       iter_obj->regnum++;
 
       if (name != nullptr && *name != '\0')
-	return gdbpy_new_register_descriptor (gdbarch, regnum);
+	return gdbpy_get_register_descriptor (gdbarch, regnum).release ();
     }
   while (true);
 }
@@ -291,6 +323,9 @@ gdbpy_register_descriptor_iter_next (PyObject *self)
 int
 gdbpy_initialize_registers ()
 {
+  gdbpy_register_object_data
+    = gdbarch_data_register_post_init (gdbpy_register_object_data_init);
+
   register_descriptor_object_type.tp_new = PyType_GenericNew;
   if (PyType_Ready (&register_descriptor_object_type) < 0)
     return -1;