add StackRoot<> and #defines for DECREF

from_cpython/Include/object.h

@@ -826,7 +826,7 @@ PyAPI_FUNC(void) _Py_AddToAllObjects(PyObject *, int force) PYSTON_NOEXCEPT;
 
 
 #define Py_INCREF(op) ((void)(op))
-#define Py_DECREF(op) ((void)(op))
+#define Py_DECREF(op) asm volatile("" : : "X"(op))
 
 /* Safely decref `op` and set `op` to NULL, especially useful in tp_clear
  * and tp_dealloc implementatons.
@@ -874,7 +874,7 @@ PyAPI_FUNC(void) _Py_AddToAllObjects(PyObject *, int force) PYSTON_NOEXCEPT;
 /* Macros to use in case the object pointer may be NULL: */
 // Pyston change: made these noops as well
 #define Py_XINCREF(op) ((void)(op))
-#define Py_XDECREF(op) ((void)(op))
+#define Py_XDECREF(op) asm volatile("" : : "X"(op))
 
 /*
 These are provided as conveniences to Python runtime embedders, so that

src/gc/roots.h

+// Copyright (c) 2014-2015 Dropbox, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef PYSTON_GC_ROOTS_H
+#define PYSTON_GC_ROOTS_H
+
+#include "core/common.h"
+#include "core/threading.h"
+
+namespace pyston {
+
+#define GC_KEEP_ALIVE(t) asm volatile("" : : "X"(t))
+
+template <class T> class StackRoot {
+public:
+    StackRoot(T* t) : t(t) {}
+    StackRoot(const StackRoot& other) : t(other.t) {}
+    ~StackRoot() { GC_KEEP_ALIVE(t); }
+
+    T& operator*() const { return *t; }
+    T* operator->() const { return t; }
+
+    operator T*() const { return t; }
+
+private:
+    T* t;
+};
+
+
+class Box;
+class BoxedString;
+typedef StackRoot<Box> RootedBox;
+typedef StackRoot<BoxedString> RootedBoxedString;
+
+//
+// the above types can be used whenever we need to explicitly root a Box subclass within some lexical scope.
+//
+// {
+//     RootedBoxedString sub(new BoxedString("hello world"));
+//     for (auto c : sub->s) {
+//       doSomethingThatCouldTriggerACollection();
+//     }
+//     // sub will be rooted conservatively until here
+// }
+//
+}
+
+#endif