Use a tuple to store overwritten fast locals

Author: mpage

Include/internal/pycore_frame.h

@@ -32,9 +32,9 @@ struct _frame {
        PyEval_GetLocals requires a borrowed reference so the actual reference
        is stored here */
     PyObject *f_locals_cache;
-    /* A list containing strong references to fast locals that were overwritten
+    /* A tuple containing strong references to fast locals that were overwritten
      * via f_locals. Borrowed references to these locals may exist in frames
-     * closer to the top of the stack. The references in this list act as
+     * closer to the top of the stack. The references in this tuple act as
      * "support" for the borrowed references, ensuring that they remain valid.
      */
     PyObject *f_overwritten_fast_locals;

Objects/frameobject.c

@@ -206,6 +206,29 @@ framelocalsproxy_getitem(PyObject *self, PyObject *key)
     return NULL;
 }
 
+static int
+add_overwritten_fast_local(PyFrameObject *frame, PyObject *obj)
+{
+    Py_ssize_t new_size = 1;
+    if (frame->f_overwritten_fast_locals != NULL) {
+        new_size = PyTuple_Size(frame->f_overwritten_fast_locals);
+        if (new_size == -1) {
+            return -1;
+        }
+    }
+    PyObject *new_tuple = PyTuple_New(new_size);
+    if (new_tuple == NULL) {
+        return -1;
+    }
+    for (Py_ssize_t i = 0; i < new_size - 1; i++) {
+        PyObject *o = PyTuple_GET_ITEM(frame->f_overwritten_fast_locals, i);
+        PyTuple_SET_ITEM(new_tuple, i, Py_NewRef(o));
+    }
+    PyTuple_SET_ITEM(new_tuple, new_size - 1, Py_NewRef(obj));
+    Py_XSETREF(frame->f_overwritten_fast_locals, new_tuple);
+    return 0;
+}
+
 static int
 framelocalsproxy_setitem(PyObject *self, PyObject *key, PyObject *value)
 {
@@ -246,13 +269,7 @@ framelocalsproxy_setitem(PyObject *self, PyObject *key, PyObject *value)
         } else if (value != PyStackRef_AsPyObjectBorrow(oldvalue)) {
             PyObject *old_obj = PyStackRef_AsPyObjectBorrow(fast[i]);
             if (old_obj != NULL && !_Py_IsImmortal(old_obj)) {
-                if (frame->f_overwritten_fast_locals == NULL) {
-                    frame->f_overwritten_fast_locals = PyList_New(0);
-                    if (frame->f_overwritten_fast_locals == NULL) {
-                        return -1;
-                    }
-                }
-                if (PyList_Append(frame->f_overwritten_fast_locals, old_obj) < 0) {
+                if (add_overwritten_fast_local(frame, old_obj) < 0) {
                     return -1;
                 }
                 PyStackRef_CLOSE(fast[i]);