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diff --git a/js/src/vm/ReceiverGuard.h b/js/src/vm/ReceiverGuard.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/. */
+
+#ifndef vm_ReceiverGuard_h
+#define vm_ReceiverGuard_h
+
+#include "vm/Shape.h"
+
+namespace js {
+
+// A ReceiverGuard encapsulates the information about an object that needs to
+// be tested to determine if it has the same 'structure' as another object.
+// The guard includes the shape and/or group of the object, and which of these
+// is tested, as well as the meaning here of 'structure', depends on the kind
+// of object being tested:
+//
+// NativeObject: The structure of a native object is determined by its shape.
+//   Two objects with the same shape have the same class, prototype, flags,
+//   and all properties except those stored in dense elements.
+//
+// ProxyObject: The structure of a proxy object is determined by its shape.
+//   Proxies with the same shape have the same class and prototype, but no
+//   other commonality is guaranteed.
+//
+// TypedObject: The structure of a typed object is determined by its group.
+//   All typed objects with the same group have the same class, prototype, and
+//   own properties.
+//
+// UnboxedPlainObject: The structure of an unboxed plain object is determined
+//   by its group and its expando object's shape, if there is one. All unboxed
+//   plain objects with the same group and expando shape have the same
+//   properties except those stored in the expando's dense elements.
+
+class HeapReceiverGuard;
+class RootedReceiverGuard;
+
+class ReceiverGuard
+{
+  public:
+    ObjectGroup* group;
+    Shape* shape;
+
+    ReceiverGuard()
+      : group(nullptr), shape(nullptr)
+    {}
+
+    inline MOZ_IMPLICIT ReceiverGuard(const HeapReceiverGuard& guard);
+    inline MOZ_IMPLICIT ReceiverGuard(const RootedReceiverGuard& guard);
+
+    explicit ReceiverGuard(JSObject* obj);
+    ReceiverGuard(ObjectGroup* group, Shape* shape);
+
+    bool operator ==(const ReceiverGuard& other) const {
+        return group == other.group && shape == other.shape;
+    }
+
+    bool operator !=(const ReceiverGuard& other) const {
+        return !(*this == other);
+    }
+
+    uintptr_t hash() const {
+        return (uintptr_t(group) >> 3) ^ (uintptr_t(shape) >> 3);
+    }
+};
+
+class HeapReceiverGuard
+{
+    GCPtrObjectGroup group_;
+    GCPtrShape shape_;
+
+  public:
+    explicit HeapReceiverGuard(const ReceiverGuard& guard)
+      : group_(guard.group), shape_(guard.shape)
+    {}
+
+    bool matches(const ReceiverGuard& guard) {
+        return group_ == guard.group && shape_ == guard.shape;
+    }
+
+    void update(const ReceiverGuard& other) {
+        group_ = other.group;
+        shape_ = other.shape;
+    }
+
+    void init(const ReceiverGuard& other) {
+        group_.init(other.group);
+        shape_.init(other.shape);
+    }
+
+    void trace(JSTracer* trc);
+
+    Shape* shape() const {
+        return shape_;
+    }
+    ObjectGroup* group() const {
+        return group_;
+    }
+
+    static size_t offsetOfShape() {
+        return offsetof(HeapReceiverGuard, shape_);
+    }
+    static size_t offsetOfGroup() {
+        return offsetof(HeapReceiverGuard, group_);
+    }
+
+    // Bits to munge into Baseline IC compiler keys when that IC has a
+    // HeapReceiverGuard. This uses at most two bits for data.
+    static int32_t keyBits(JSObject* obj);
+};
+
+class RootedReceiverGuard
+{
+  public:
+    RootedObjectGroup group;
+    RootedShape shape;
+
+    RootedReceiverGuard(JSContext* cx, const ReceiverGuard& guard)
+      : group(cx, guard.group), shape(cx, guard.shape)
+    {}
+};
+
+inline
+ReceiverGuard::ReceiverGuard(const HeapReceiverGuard& guard)
+  : group(guard.group()), shape(guard.shape())
+{}
+
+inline
+ReceiverGuard::ReceiverGuard(const RootedReceiverGuard& guard)
+  : group(guard.group), shape(guard.shape)
+{}
+
+} // namespace js
+
+#endif /* vm_ReceiverGuard_h */