Restrict specialization in free-threaded build

Author: markshannon

Python/bytecodes.c

@@ -3286,9 +3286,6 @@ dummy_func(
             PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
             EXIT_IF(Py_TYPE(iter_o) != &PyTuple_Type);
             assert(PyStackRef_IsTaggedInt(null_or_index));
-#ifdef Py_GIL_DISABLED
-            EXIT_IF(!_PyObject_IsUniquelyReferenced(iter_o));
-#endif
         }
 
         replaced op(_ITER_JUMP_TUPLE, (iter, null_or_index -- iter, null_or_index)) {

Python/executor_cases.c.h

@@ -2904,13 +2904,18 @@ _Py_Specialize_ForIter(_PyStackRef iter, _PyStackRef null_or_index, _Py_CODEUNIT
     assert(_PyOpcode_Caches[FOR_ITER] == INLINE_CACHE_ENTRIES_FOR_ITER);
     PyObject *iter_o = PyStackRef_AsPyObjectBorrow(iter);
     PyTypeObject *tp = Py_TYPE(iter_o);
+
+    if (PyStackRef_IsNull(null_or_index)) {
 #ifdef Py_GIL_DISABLED
-    // Only specialize for lists owned by this thread or shared
-    if (!_Py_IsOwnedByCurrentThread(iter_o) && !_PyObject_GC_IS_SHARED(iter_o)) {
-        goto failure;
-    }
+        // Only specialize for uniquely referenced iterators, so that we know
+        // they're only referenced by this one thread. This is more limiting
+        // than we need (even `it = iter(mylist); for item in it:` won't get
+        // specialized) but we don't have a way to check whether we're the only
+        // _thread_ who has access to the object.
+        if (!_PyObject_IsUniquelyReferenced(iter_o)) {
+            goto failure;
+        }
 #endif
-    if (PyStackRef_IsNull(null_or_index)) {
         if (tp == &PyRangeIter_Type) {
             specialize(instr, FOR_ITER_RANGE);
             return;
@@ -2930,6 +2935,12 @@ _Py_Specialize_ForIter(_PyStackRef iter, _PyStackRef null_or_index, _Py_CODEUNIT
     }
     else {
         if (tp == &PyList_Type) {
+#ifdef Py_GIL_DISABLED
+            // Only specialize for lists owned by this thread or shared
+            if (!_Py_IsOwnedByCurrentThread(iter_o) && !_PyObject_GC_IS_SHARED(iter_o)) {
+                goto failure;
+            }
+#endif
             specialize(instr, FOR_ITER_LIST);
             return;
         }