1 files changed, 36 insertions, 7 deletions
diff --git a/Include/object.h b/Include/object.h
index 75624fe..3774f12 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -598,15 +598,44 @@ static inline void Py_DECREF(PyObject *op)
  * one of those can't cause problems -- but in part that relies on that
  * Python integers aren't currently weakly referencable.  Best practice is
  * to use Py_CLEAR() even if you can't think of a reason for why you need to.
+ *
+ * gh-98724: Use a temporary variable to only evaluate the macro argument once,
+ * to avoid the duplication of side effects if the argument has side effects.
+ *
+ * gh-99701: If the PyObject* type is used with casting arguments to PyObject*,
+ * the code can be miscompiled with strict aliasing because of type punning.
+ * With strict aliasing, a compiler considers that two pointers of different
+ * types cannot read or write the same memory which enables optimization
+ * opportunities.
+ *
+ * If available, use _Py_TYPEOF() to use the 'op' type for temporary variables,
+ * and so avoid type punning. Otherwise, use memcpy() which causes type erasure
+ * and so prevents the compiler to reuse an old cached 'op' value after
+ * Py_CLEAR().
  */
-#define Py_CLEAR(op)                            \
-    do {                                        \
-        PyObject *_py_tmp = _PyObject_CAST(op); \
-        if (_py_tmp != NULL) {                  \
-            (op) = NULL;                        \
-            Py_DECREF(_py_tmp);                 \
-        }                                       \
+#ifdef _Py_TYPEOF
+#define Py_CLEAR(op) \
+    do { \
+        _Py_TYPEOF(op)* _tmp_op_ptr = &(op); \
+        _Py_TYPEOF(op) _tmp_old_op = (*_tmp_op_ptr); \
+        if (_tmp_old_op != NULL) { \
+            *_tmp_op_ptr = _Py_NULL; \
+            Py_DECREF(_tmp_old_op); \
+        } \
     } while (0)
+#else
+#define Py_CLEAR(op) \
+    do { \
+        PyObject **_tmp_op_ptr = _Py_CAST(PyObject**, &(op)); \
+        PyObject *_tmp_old_op = (*_tmp_op_ptr); \
+        if (_tmp_old_op != NULL) { \
+            PyObject *_null_ptr = _Py_NULL; \
+            memcpy(_tmp_op_ptr, &_null_ptr, sizeof(PyObject*)); \
+            Py_DECREF(_tmp_old_op); \
+        } \
+    } while (0)
+#endif
+
 
 /* Function to use in case the object pointer can be NULL: */
 static inline void Py_XINCREF(PyObject *op)