diff -r 000000000000 -r 6474c204b198 js/src/jshashutil.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jshashutil.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,64 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef jshashutil_h
+#define jshashutil_h
+
+#include "jscntxt.h"
+
+namespace js {
+
+/*
+ * Used to add entries to a js::HashMap or HashSet where the key depends on a GC
+ * thing that may be moved by generational collection between the call to
+ * lookupForAdd() and relookupOrAdd().
+ */
+template <class T>
+struct DependentAddPtr
+{
+    typedef typename T::AddPtr AddPtr;
+    typedef typename T::Entry Entry;
+
+    template <class Lookup>
+    DependentAddPtr(const ExclusiveContext *cx, const T &table, const Lookup &lookup)
+      : addPtr(table.lookupForAdd(lookup))
+#ifdef JSGC_GENERATIONAL
+      , originalGcNumber(cx->zone()->gcNumber())
+#endif
+        {}
+
+    template <class KeyInput, class ValueInput>
+    bool add(const ExclusiveContext *cx, T &table, const KeyInput &key, const ValueInput &value) {
+#ifdef JSGC_GENERATIONAL
+        bool gcHappened = originalGcNumber != cx->zone()->gcNumber();
+        if (gcHappened)
+            addPtr = table.lookupForAdd(key);
+#endif
+        return table.relookupOrAdd(addPtr, key, value);
+    }
+
+    typedef void (DependentAddPtr::* ConvertibleToBool)();
+    void nonNull() {}
+
+    bool found() const                 { return addPtr.found(); }
+    operator ConvertibleToBool() const { return found() ? &DependentAddPtr::nonNull : 0; }
+    const Entry &operator*() const     { return *addPtr; }
+    const Entry *operator->() const    { return &*addPtr; }
+
+  private:
+    AddPtr addPtr ;
+#ifdef JSGC_GENERATIONAL
+    const uint64_t originalGcNumber;
+#endif
+
+    DependentAddPtr() MOZ_DELETE;
+    DependentAddPtr(const DependentAddPtr&) MOZ_DELETE;
+    DependentAddPtr& operator=(const DependentAddPtr&) MOZ_DELETE;
+};
+
+} // namespace js
+
+#endif