Add flags to object header to assess cost of maintain might-be-in-cycle flag

Author: markshannon

Include/internal/pycore_object.h

@@ -187,6 +187,7 @@ _Py_DECREF_SPECIALIZED(PyObject *op, const destructor destruct)
 #endif
     if (--op->ob_refcnt != 0) {
         assert(op->ob_refcnt > 0);
+        op->ob_flags |= _Py_MAYBE_IN_CYCLE;
     }
     else {
 #ifdef Py_TRACE_REFS
@@ -212,6 +213,7 @@ _Py_DECREF_NO_DEALLOC(PyObject *op)
         _Py_FatalRefcountError("Expected a positive remaining refcount");
     }
 #endif
+    op->ob_flags |= _Py_MAYBE_IN_CYCLE;
 }
 
 #else

Include/object.h

@@ -79,19 +79,22 @@ whose size is determined when the object is allocated.
         (type),                     \
     },
 #else
-#define PyObject_HEAD_INIT(type)    \
-    {                               \
-        { _Py_IMMORTAL_REFCNT },    \
-        (type)                      \
+
+#define PyObject_HEAD_INIT(type)           \
+    {                                      \
+        .ob_refcnt = _Py_IMMORTAL_REFCNT,  \
+        .ob_flags = 0,                     \
+        .ob_type = (type)                  \
     },
-#endif
 
 #define PyVarObject_HEAD_INIT(type, size) \
     {                                     \
         PyObject_HEAD_INIT(type)          \
         (size)                            \
     },
 
+#endif
+
 /* PyObject_VAR_HEAD defines the initial segment of all variable-size
  * container objects.  These end with a declaration of an array with 1
  * element, but enough space is malloc'ed so that the array actually
@@ -119,17 +122,31 @@ struct _object {
     __pragma(warning(push))
     __pragma(warning(disable: 4201))
 #endif
-    union {
-       Py_ssize_t ob_refcnt;
 #if SIZEOF_VOID_P > 4
-       PY_UINT32_T ob_refcnt_split[2];
+
+#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_UINT32_T ob_refcnt;
+#endif
+
 #ifdef _MSC_VER
     __pragma(warning(pop))
 #endif
 
     PyTypeObject *ob_type;
+#if SIZEOF_VOID_P <= 4
+    PY_UINT32_T ob_flags;
+#endif
+
+#define _Py_MAYBE_IN_CYCLE          (1 << 31)
+
 };
 #else
 // Objects that are not owned by any thread use a thread id (tid) of zero.

Include/refcount.h

@@ -238,14 +238,14 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
     }
 #elif SIZEOF_VOID_P > 4
     // Portable saturated add, branching on the carry flag and set low bits
-    PY_UINT32_T cur_refcnt = op->ob_refcnt_split[PY_BIG_ENDIAN];
+    PY_UINT32_T cur_refcnt = op->ob_refcnt;
     PY_UINT32_T new_refcnt = cur_refcnt + 1;
     if (new_refcnt == 0) {
         // cur_refcnt is equal to _Py_IMMORTAL_REFCNT: the object is immortal,
         // do nothing
         return;
     }
-    op->ob_refcnt_split[PY_BIG_ENDIAN] = new_refcnt;
+    op->ob_refcnt = new_refcnt;
 #else
     // Explicitly check immortality against the immortal value
     if (_Py_IsImmortal(op)) {
@@ -350,6 +350,9 @@ static inline void Py_DECREF(const char *filename, int lineno, PyObject *op)
     if (--op->ob_refcnt == 0) {
         _Py_Dealloc(op);
     }
+    else {
+        op->ob_flags |= _Py_MAYBE_IN_CYCLE;
+    }
 }
 #define Py_DECREF(op) Py_DECREF(__FILE__, __LINE__, _PyObject_CAST(op))
 
@@ -365,6 +368,9 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
     if (--op->ob_refcnt == 0) {
         _Py_Dealloc(op);
     }
+    else {
+        op->ob_flags |= _Py_MAYBE_IN_CYCLE;
+    }
 }
 #define Py_DECREF(op) Py_DECREF(_PyObject_CAST(op))
 #endif

Objects/object.c

@@ -2369,6 +2369,7 @@ 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_flags = 0;
 #else
     op->ob_tid = _Py_ThreadId();
     op->_padding = 0;
@@ -2411,6 +2412,7 @@ _Py_SetImmortalUntracked(PyObject *op)
     op->ob_ref_shared = 0;
 #else
     op->ob_refcnt = _Py_IMMORTAL_REFCNT;
+    op->ob_flags = 0;
 #endif
 }
 

Python/ceval.c

@@ -71,6 +71,9 @@
             destructor dealloc = Py_TYPE(op)->tp_dealloc; \
             (*dealloc)(op); \
         } \
+        else { \
+            op->ob_flags |= _Py_MAYBE_IN_CYCLE; \
+        } \
     } while (0)
 
 #undef Py_XDECREF
@@ -98,6 +101,9 @@
             destructor d = (destructor)(dealloc); \
             d(op); \
         } \
+        else { \
+            op->ob_flags |= _Py_MAYBE_IN_CYCLE; \
+        } \
     } while (0)
 #endif