diff -r 000000000000 -r 6474c204b198 js/src/gc/Nursery-inl.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/gc/Nursery-inl.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,86 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=4 sw=4 et tw=79 ft=cpp:
+ *
+ * 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 gc_Nursery_inl_h
+#define gc_Nursery_inl_h
+
+#ifdef JSGC_GENERATIONAL
+
+#include "gc/Nursery.h"
+
+#include "gc/Heap.h"
+
+namespace js {
+namespace gc {
+
+/*
+ * This structure overlays a Cell in the Nursery and re-purposes its memory
+ * for managing the Nursery collection process.
+ */
+class RelocationOverlay
+{
+    friend class MinorCollectionTracer;
+
+    /* The low bit is set so this should never equal a normal pointer. */
+    static const uintptr_t Relocated = uintptr_t(0xbad0bad1);
+
+    /* Set to Relocated when moved. */
+    uintptr_t magic_;
+
+    /* The location |this| was moved to. */
+    Cell *newLocation_;
+
+    /* A list entry to track all relocated things. */
+    RelocationOverlay *next_;
+
+  public:
+    static RelocationOverlay *fromCell(Cell *cell) {
+        JS_ASSERT(!cell->isTenured());
+        return reinterpret_cast<RelocationOverlay *>(cell);
+    }
+
+    bool isForwarded() const {
+        return magic_ == Relocated;
+    }
+
+    Cell *forwardingAddress() const {
+        JS_ASSERT(isForwarded());
+        return newLocation_;
+    }
+
+    void forwardTo(Cell *cell) {
+        JS_ASSERT(!isForwarded());
+        magic_ = Relocated;
+        newLocation_ = cell;
+        next_ = nullptr;
+    }
+
+    RelocationOverlay *next() const {
+        return next_;
+    }
+};
+
+} /* namespace gc */
+} /* namespace js */
+
+template <typename T>
+MOZ_ALWAYS_INLINE bool
+js::Nursery::getForwardedPointer(T **ref)
+{
+    JS_ASSERT(ref);
+    JS_ASSERT(isInside(*ref));
+    const gc::RelocationOverlay *overlay = reinterpret_cast<const gc::RelocationOverlay *>(*ref);
+    if (!overlay->isForwarded())
+        return false;
+    /* This static cast from Cell* restricts T to valid (GC thing) types. */
+    *ref = static_cast<T *>(overlay->forwardingAddress());
+    return true;
+}
+
+#endif /* JSGC_GENERATIONAL */
+
+#endif /* gc_Nursery_inl_h */