Clarify division between pre-defined stackref indexes and others

Author: markshannon

Include/internal/pycore_stackref.h

@@ -63,11 +63,13 @@ extern void _Py_stackref_associate(PyInterpreterState *interp, PyObject *obj, _P
 
 static const _PyStackRef PyStackRef_NULL = { .index = 0 };
 
+// Use the first 3 even numbers for None, True and False.
+// Odd numbers are reserved for (tagged) integers
 #define PyStackRef_None ((_PyStackRef){ .index = 2 } )
 #define PyStackRef_False ((_PyStackRef){ .index = 4 })
 #define PyStackRef_True ((_PyStackRef){ .index = 6 })
 
-#define LAST_PREDEFINED_STACKREF_INDEX 6
+#define INITIAL_STACKREF_INDEX 8
 
 static inline int
 PyStackRef_IsNull(_PyStackRef ref)

Python/pystate.c

@@ -673,7 +673,7 @@ init_interpreter(PyInterpreterState *interp,
         interp->dtoa = (struct _dtoa_state)_dtoa_state_INIT(interp);
     }
 #if !defined(Py_GIL_DISABLED) && defined(Py_STACKREF_DEBUG)
-    interp->next_stackref = 2;
+    interp->next_stackref = INITIAL_STACKREF_INDEX;
     _Py_hashtable_allocator_t alloc = {
         .malloc = malloc,
         .free = free,

Python/stackrefs.c

@@ -70,7 +70,7 @@ _Py_stackref_close(_PyStackRef ref, const char *filename, int linenumber)
 
     }
     PyObject *obj;
-    if (ref.index <= LAST_PREDEFINED_STACKREF_INDEX) {
+    if (ref.index < INITIAL_STACKREF_INDEX) {
         if (ref.index == 0) {
             _Py_FatalErrorFormat(__func__, "Passing NULL to PyStackRef_CLOSE at %s:%d\n", filename, linenumber);
         }
@@ -128,7 +128,7 @@ _Py_stackref_create(PyObject *obj, const char *filename, int linenumber)
 void
 _Py_stackref_record_borrow(_PyStackRef ref, const char *filename, int linenumber)
 {
-    if (ref.index <= LAST_PREDEFINED_STACKREF_INDEX) {
+    if (ref.index < INITIAL_STACKREF_INDEX) {
         return;
     }
     PyInterpreterState *interp = PyInterpreterState_Get();
@@ -152,8 +152,7 @@ _Py_stackref_record_borrow(_PyStackRef ref, const char *filename, int linenumber
 void
 _Py_stackref_associate(PyInterpreterState *interp, PyObject *obj, _PyStackRef ref)
 {
-    assert(interp->next_stackref >= ref.index);
-    interp->next_stackref = ref.index+2;
+    assert(ref.index < INITIAL_STACKREF_INDEX);
     TableEntry *entry = make_table_entry(obj, "builtin-object", 0);
     if (entry == NULL) {
         Py_FatalError("No memory left for stackref debug table");