Make explicit that refcnt is 32 bits even on 64 bit machines

Author: markshannon

Include/object.h

@@ -120,9 +120,19 @@ struct _object {
     __pragma(warning(disable: 4201))
 #endif
     union {
-       Py_ssize_t ob_refcnt;
 #if SIZEOF_VOID_P > 4
-       PY_UINT32_T ob_refcnt_split[2];
+        PY_INT64_T ob_refcnt_full; /* This field is needed for efficient initialization with Clang on ARM */
+        struct {
+#  if PY_BIG_ENDIAN
+            PY_UINT32_T ob_flags;
+            PY_UINT32_T ob_refcnt;
+#  else
+            PY_UINT32_T ob_refcnt;
+            PY_UINT32_T ob_flags;
+#  endif
+        };
+#else
+        Py_ssize_t ob_refcnt;
 #endif
     };
 #ifdef _MSC_VER

Include/refcount.h

@@ -252,13 +252,13 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
         _Py_atomic_add_ssize(&op->ob_ref_shared, (1 << _Py_REF_SHARED_SHIFT));
     }
 #elif SIZEOF_VOID_P > 4
-    PY_UINT32_T cur_refcnt = op->ob_refcnt_split[PY_BIG_ENDIAN];
+    PY_UINT32_T cur_refcnt = op->ob_refcnt;
     if (((int32_t)cur_refcnt) < 0) {
         // the object is immortal
         _Py_INCREF_IMMORTAL_STAT_INC();
         return;
     }
-    op->ob_refcnt_split[PY_BIG_ENDIAN] = cur_refcnt + 1;
+    op->ob_refcnt = cur_refcnt + 1;
 #else
     if (_Py_IsImmortal(op)) {
         _Py_INCREF_IMMORTAL_STAT_INC();
@@ -354,7 +354,13 @@ static inline void Py_DECREF(PyObject *op)
 #elif defined(Py_REF_DEBUG)
 static inline void Py_DECREF(const char *filename, int lineno, PyObject *op)
 {
+#if SIZEOF_VOID_P > 4
+    /* If an object has been freed, it will have a negative full refcnt
+     * If it has not it been freed, will have a very large refcnt */
+    if (op->ob_refcnt_full <= 0 || op->ob_refcnt > (UINT32_MAX - (1<<20))) {
+#else
     if (op->ob_refcnt <= 0) {
+#endif
         _Py_NegativeRefcount(filename, lineno, op);
     }
     if (_Py_IsImmortal(op)) {

Objects/object.c

@@ -2475,7 +2475,9 @@ new_reference(PyObject *op)
 {
     // Skip the immortal object check in Py_SET_REFCNT; always set refcnt to 1
 #if !defined(Py_GIL_DISABLED)
-    op->ob_refcnt = 1;
+    op->ob_refcnt_full = 1;
+    assert(op->ob_refcnt == 1);
+    assert(op->ob_flags == 0);
 #else
     op->ob_tid = _Py_ThreadId();
     op->_padding = 0;