Fix GC tracking error for instances of mathutils types

Mathutils types were always GC tracked even when it wasn't intended.
Not having to track objects speeds up Python execution.

In an isolated benchmark created to stress test the GC
creating 4-million vectors (re-assigning them 100 times), this gives
an overall ~2.5x speedup, see: P3221.

Details:

Since [0] (which added support for sub-classed mathutils types)
tp_alloc was called which defaults to PyType_GenericAlloc which always
GC tracked the resulting object when Py_TPFLAGS_HAVE_GC was set.

Avoid using PyType_GenericAlloc unless the type is sub-classed,
in that case the object is un-tracked.

Add asserts that the tracked state is as expected before tracking &
un-tracking, to ensure changes to object creation don't cause objects
to be tracked unintentionally.

Also assign the PyTypeObject.tp_is_gc callback so types optionally GC
track objects only do so when an object is referenced.

[0]: fbd936494495d0de54eef24a97957e000306785f

1 files changed, 14 insertions, 0 deletions

diff --git a/source/blender/python/generic/idprop_py_api.c b/source/blender/python/generic/idprop_py_api.c
index 333ab9487d1..3978f7f37cc 100644
--- a/source/blender/python/generic/idprop_py_api.c
+++ b/source/blender/python/generic/idprop_py_api.c
@@ -909,6 +909,11 @@ static int BPy_IDGroup_Iter_clear(BPy_IDGroup_Iter *self)
   return 0;
 }
 
+static int BPy_IDGroup_Iter_is_gc(BPy_IDGroup_Iter *self)
+{
+  return (self->group != NULL);
+}
+
 static bool BPy_Group_Iter_same_size_or_raise_error(BPy_IDGroup_Iter *self)
 {
   if (self->len_init == self->group->prop->len) {
@@ -1000,6 +1005,7 @@ static void IDGroup_Iter_init_type(void)
   SHARED_MEMBER_SET(tp_flags, Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC);
   SHARED_MEMBER_SET(tp_traverse, (traverseproc)BPy_IDGroup_Iter_traverse);
   SHARED_MEMBER_SET(tp_clear, (inquiry)BPy_IDGroup_Iter_clear);
+  SHARED_MEMBER_SET(tp_is_gc, (inquiry)BPy_IDGroup_Iter_is_gc);
   SHARED_MEMBER_SET(tp_iter, PyObject_SelfIter);
 
 #undef SHARED_MEMBER_SET
@@ -1015,6 +1021,7 @@ static PyObject *IDGroup_Iter_New_WithType(BPy_IDProperty *group,
   iter->group = group;
   if (group != NULL) {
     Py_INCREF(group);
+    BLI_assert(!PyObject_GC_IsTracked((PyObject *)iter));
     PyObject_GC_Track(iter);
     iter->cur = (reversed ? group->prop->data.group.last : group->prop->data.group.first);
     iter->len_init = group->prop->len;
@@ -1086,6 +1093,11 @@ static int BPy_IDGroup_View_clear(BPy_IDGroup_View *self)
   return 0;
 }
 
+static int BPy_IDGroup_View_is_gc(BPy_IDGroup_View *self)
+{
+  return (self->group != NULL);
+}
+
 /* View Specific API's (Key/Value/Items). */
 
 static PyObject *BPy_Group_ViewKeys_iter(BPy_IDGroup_View *self)
@@ -1233,6 +1245,7 @@ static void IDGroup_View_init_type(void)
   SHARED_MEMBER_SET(tp_flags, Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC);
   SHARED_MEMBER_SET(tp_traverse, (traverseproc)BPy_IDGroup_View_traverse);
   SHARED_MEMBER_SET(tp_clear, (inquiry)BPy_IDGroup_View_clear);
+  SHARED_MEMBER_SET(tp_is_gc, (inquiry)BPy_IDGroup_View_is_gc);
   SHARED_MEMBER_SET(tp_methods, BPy_IDGroup_View_methods);
 
 #undef SHARED_MEMBER_SET
@@ -2087,6 +2100,7 @@ static BPy_IDGroup_View *IDGroup_View_New_WithType(BPy_IDProperty *group, PyType
   iter->group = group;
   if (group != NULL) {
     Py_INCREF(group);
+    BLI_assert(!PyObject_GC_IsTracked((PyObject *)iter));
     PyObject_GC_Track(iter);
   }
   return iter;