Put comment back.

Author: markshannon

Python/ceval.c

@@ -4429,7 +4429,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, InterpreterFrame *frame, int thr
 
         TARGET(LOAD_METHOD_WITH_DICT) {
             /* LOAD_METHOD, with a dict
-             Can be either a managed dict, or a tp_dictoffset  offset.*/
+             Can be either a managed dict, or a tp_dictoffset offset.*/
             assert(cframe.use_tracing == 0);
             PyObject *self = TOP();
             PyTypeObject *self_cls = Py_TYPE(self);

Python/specialize.c

@@ -1164,6 +1164,20 @@ _Py_Specialize_LoadMethod(PyObject *owner, _Py_CODEUNIT *instr, PyObject *name,
             *instr = _Py_MAKECODEUNIT(LOAD_METHOD_WITH_DICT, _Py_OPARG(*instr));
             break;
     }
+    /* `descr` is borrowed. This is safe for methods (even inherited ones from
+    *  super classes!) as long as tp_version_tag is validated for two main reasons:
+    *
+    *  1. The class will always hold a reference to the method so it will
+    *  usually not be GC-ed. Should it be deleted in Python, e.g.
+    *  `del obj.meth`, tp_version_tag will be invalidated, because of reason 2.
+    *
+    *  2. The pre-existing type method cache (MCACHE) uses the same principles
+    *  of caching a borrowed descriptor. The MCACHE infrastructure does all the
+    *  heavy lifting for us. E.g. it invalidates tp_version_tag on any MRO
+    *  modification, on any type object change along said MRO, etc. (see
+    *  PyType_Modified usages in typeobject.c). The MCACHE has been
+    *  working since Python 2.6 and it's battle-tested.
+    */
     cache1->tp_version = owner_cls->tp_version_tag;
     cache2->obj = descr;
     // Fall through.