Avoid calling PySequence_List() in some cases if the argument is a new list already. (GH-3494)

Cython/Compiler/ExprNodes.py

@@ -7797,8 +7797,11 @@ class SequenceNode(ExprNode):
 
         starred_target.allocate(code)
         target_list = starred_target.result()
-        code.putln("%s = PySequence_List(%s); %s" % (
+        code.putln("%s = %s(%s); %s" % (
             target_list,
+            "__Pyx_PySequence_ListKeepNew" if (
+                    not iterator_temp and rhs.is_temp and rhs.type in (py_object_type, list_type))
+                else "PySequence_List",
             iterator_temp or rhs.py_result(),
             code.error_goto_if_null(target_list, self.pos)))
         starred_target.generate_gotref(code)
@@ -8531,7 +8534,9 @@ class MergedSequenceNode(ExprNode):
         else:
             code.putln("%s = %s(%s); %s" % (
                 self.result(),
-                'PySet_New' if is_set else 'PySequence_List',
+                'PySet_New' if is_set
+                    else "__Pyx_PySequence_ListKeepNew" if item.is_temp and item.type in (py_object_type, list_type)
+                    else "PySequence_List",
                 item.py_result(),
                 code.error_goto_if_null(self.result(), self.pos)))
             self.generate_gotref(code)

Cython/Compiler/Optimize.py

@@ -1753,7 +1753,11 @@ class EarlyReplaceBuiltinCalls(Visitor.EnvTransform):
             # Interestingly, PySequence_List works on a lot of non-sequence
             # things as well.
             list_node = loop_node = ExprNodes.PythonCapiCallNode(
-                node.pos, "PySequence_List", self.PySequence_List_func_type,
+                node.pos,
+                "__Pyx_PySequence_ListKeepNew"
+                    if arg.is_temp and arg.type in (PyrexTypes.py_object_type, Builtin.list_type)
+                    else "PySequence_List",
+                self.PySequence_List_func_type,
                 args=pos_args, is_temp=True)
 
         result_node = UtilNodes.ResultRefNode(
@@ -2415,8 +2419,14 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
             return node
         arg = pos_args[0]
         return ExprNodes.PythonCapiCallNode(
-            node.pos, "PySequence_List", self.PySequence_List_func_type,
-            args=pos_args, is_temp=node.is_temp)
+            node.pos,
+            "__Pyx_PySequence_ListKeepNew"
+                if node.is_temp and arg.is_temp and arg.type in (PyrexTypes.py_object_type, Builtin.list_type)
+                else "PySequence_List",
+            self.PySequence_List_func_type,
+            args=pos_args,
+            is_temp=node.is_temp,
+        )
 
     PyList_AsTuple_func_type = PyrexTypes.CFuncType(
         Builtin.tuple_type, [

Cython/Utility/ModuleSetupCode.c

@@ -760,6 +760,13 @@ static CYTHON_INLINE PyObject * __Pyx_PyDict_GetItemStrWithError(PyObject *dict,
   #define __Pyx_PyBaseString_CheckExact(obj) (PyString_CheckExact(obj) || PyUnicode_CheckExact(obj))
 #endif
 
+#if CYTHON_COMPILING_IN_CPYTHON
+  #define __Pyx_PySequence_ListKeepNew(obj) \
+    (likely(PyList_CheckExact(obj) && Py_REFCNT(obj) == 1) ? __Pyx_NewRef(obj) : PySequence_List(obj))
+#else
+  #define __Pyx_PySequence_ListKeepNew(obj)  PySequence_List(obj)
+#endif
+
 #ifndef PySet_CheckExact
   #define PySet_CheckExact(obj)        (Py_TYPE(obj) == &PySet_Type)
 #endif