We use _PyGIDefaultArgPlaceholder as a sentinel value to represent default values during function argument list construction. Right now, it's a Python type object. We make it using PyObject_New, so most of its fields end up uninitialized. The object body being uninitialized wouldn't be a problem if the placeholder object were unreachable, but the object *can* be reached during GC by traversal through frame objects. From valgrind:

==1184== Conditional jump or move depends on uninitialised value(s)
==1184==    at 0x528801: visit_decref (gcmodule.c:373)
==1184==    by 0x472165: tupletraverse (tupleobject.c:564)
==1184==    by 0x528A84: subtract_refs (gcmodule.c:398)
==1184==    by 0x528A84: collect (gcmodule.c:969)
==1184==    by 0x529708: collect_with_callback (gcmodule.c:1140)
==1184==    by 0x529708: collect_generations (gcmodule.c:1163)
==1184==    by 0x529E30: _PyObject_GC_Malloc (gcmodule.c:1738)
==1184==    by 0x529E30: _PyObject_GC_NewVar (gcmodule.c:1765)
==1184==    by 0x5EBAD9: PyFrame_New (frameobject.c:671)
==1184==    by 0x4DD633: PyEval_EvalCodeEx (ceval.c:3414)
==1184==    by 0x5EC76B: function_call (funcobject.c:632)
==1184==    by 0x428FF9: PyObject_Call (abstract.c:2040)
==1184==    by 0x5E115C: method_call (classobject.c:347)
==1184==    by 0x428FF9: PyObject_Call (abstract.c:2040)
==1184==    by 0x47B2BA: slot_tp_init (typeobject.c:6177)
==1184==  Uninitialised value was created by a heap allocation
==1184==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1184==    by 0x464E5C: _PyObject_New (object.c:247)
==1184==    by 0xA6174F6: pyglib__gi_module_create (in /usr/lib/python3/dist-packages/gi/_gi.cpython-34m-x86_64-linux-gnu.so)
==1184==    by 0x4FAF66: _PyImport_LoadDynamicModule (importdl.c:88)
==1184==    by 0x4F709C: _imp_load_dynamic_impl (import.c:2263)
==1184==    by 0x4F709C: _imp_load_dynamic (import.c:2239)
==1184==    by 0x4DAD2A: ext_do_call (ceval.c:4558)
==1184==    by 0x4DAD2A: PyEval_EvalFrameEx (ceval.c:2878)
==1184==    by 0x4DDD03: PyEval_EvalCodeEx (ceval.c:3588)
==1184==    by 0x4DAA7F: fast_function (ceval.c:4344)
==1184==    by 0x4DAA7F: call_function (ceval.c:4262)
==1184==    by 0x4DAA7F: PyEval_EvalFrameEx (ceval.c:2838)
==1184==    by 0x4DDD03: PyEval_EvalCodeEx (ceval.c:3588)
==1184==    by 0x5EC856: function_call (funcobject.c:632)
==1184==    by 0x428FF9: PyObject_Call (abstract.c:2040)
==1184==    by 0x4D67D6: ext_do_call (ceval.c:4561)
==1184==    by 0x4D67D6: PyEval_EvalFrameEx (ceval.c:2878)

Depending on the exact contents of the uninitialized memory, the GC can go on to cause other kinds of memory corruption through the process.

IMHO, the easiest fix for this problem is to just make the placeholder a simpler data structure, like a list. PyType_Type is also a pig, memory-wise: its tp_basicsize is 824 bytes here!

diff --git a/gi/gimodule.c b/gi/gimodule.c
index e14b4f6a..5f8853c8 100644
--- a/gi/gimodule.c
+++ b/gi/gimodule.c
@@ -730,7 +730,7 @@ PYGLIB_MODULE_START(_gi, "_gi")
     /* Place holder object used to fill in "from Python" argument lists
      * for values not supplied by the caller but support a GI default.
      */
-    _PyGIDefaultArgPlaceholder = PyObject_New(PyObject, &PyType_Type);
+    _PyGIDefaultArgPlaceholder = PyList_New(0);
 
     Py_INCREF (PyGIWarning);
     PyModule_AddObject (module, "PyGIWarning", PyGIWarning);