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diff --git a/src/third_party/mozjs-60/extract/js/src/gc/RelocationOverlay.h b/src/third_party/mozjs-60/extract/js/src/gc/RelocationOverlay.h
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+/* -*- 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/. */
+
+/*
+ * GC-internal definition of relocation overlay used while moving cells.
+ */
+
+#ifndef gc_RelocationOverlay_h
+#define gc_RelocationOverlay_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/EndianUtils.h"
+
+#include <stdint.h>
+
+#include "js/HeapAPI.h"
+#include "vm/JSObject.h"
+#include "vm/Shape.h"
+
+namespace js {
+namespace gc {
+
+struct Cell;
+
+/*
+ * This structure overlays a Cell that has been moved and provides a way to find
+ * its new location. It's used during generational and compacting GC.
+ */
+class RelocationOverlay
+{
+    /* See comment in js/public/HeapAPI.h. */
+    static const uint32_t Relocated = js::gc::Relocated;
+
+
+#if MOZ_LITTLE_ENDIAN
+    /*
+     * Keep the first 32 bits untouched. Use them to distinguish strings from
+     * objects in the nursery.
+     */
+    uint32_t preserve_;
+
+    /* Set to Relocated when moved. */
+    uint32_t magic_;
+#else
+    /*
+     * On big-endian, we need to reorder to keep preserve_ lined up with the
+     * low 32 bits of the aligned group_ pointer in JSObject.
+     */
+    uint32_t magic_;
+    uint32_t preserve_;
+#endif
+
+
+
+    /* The location |this| was moved to. */
+    Cell* newLocation_;
+
+    /* A list entry to track all relocated things. */
+    RelocationOverlay* next_;
+
+  public:
+    static const RelocationOverlay* fromCell(const Cell* cell) {
+        return reinterpret_cast<const RelocationOverlay*>(cell);
+    }
+
+    static RelocationOverlay* fromCell(Cell* cell) {
+        return reinterpret_cast<RelocationOverlay*>(cell);
+    }
+
+    bool isForwarded() const {
+        (void) preserve_; // Suppress warning
+        return magic_ == Relocated;
+    }
+
+    Cell* forwardingAddress() const {
+        MOZ_ASSERT(isForwarded());
+        return newLocation_;
+    }
+
+    void forwardTo(Cell* cell);
+
+    RelocationOverlay*& nextRef() {
+        MOZ_ASSERT(isForwarded());
+        return next_;
+    }
+
+    RelocationOverlay* next() const {
+        MOZ_ASSERT(isForwarded());
+        return next_;
+    }
+
+    static bool isCellForwarded(const Cell* cell) {
+        return fromCell(cell)->isForwarded();
+    }
+};
+
+} // namespace gc
+} // namespace js
+
+#endif /* gc_RelocationOverlay_h */