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diff --git a/js/src/vm/Watchtower.h b/js/src/vm/Watchtower.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * 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_Watchtower_h
+#define vm_Watchtower_h
+
+#include "js/TypeDecls.h"
+#include "vm/NativeObject.h"
+
+namespace js {
+
+// [SMDOC] Watchtower
+//
+// Watchtower is a framework to hook into changes to certain objects. This gives
+// us the ability to, for instance, invalidate caches or purge Warp code on
+// object layout changes.
+//
+// Watchtower is only used for objects with certain ObjectFlags set on the
+// Shape. This minimizes performance overhead for most objects.
+//
+// We currently use Watchtower for:
+//
+// - Invalidating the shape teleporting optimization. See the "Shape Teleporting
+//   Optimization" SMDOC comment in CacheIR.cpp.
+//
+// - Invalidating the MegamorphicCache, a property lookup cache for megamorphic
+//   property accesses. See the SMDOC comment in vm/Caches.h.
+//
+// There's also a testing mechanism that lets us write tests for Watchtower
+// hooks. See setWatchtowerCallback and addWatchtowerTarget defined in
+// TestingFunctions.cpp.
+class Watchtower {
+  static bool watchPropertyAddSlow(JSContext* cx, Handle<NativeObject*> obj,
+                                   HandleId id);
+  static bool watchPropertyRemoveSlow(JSContext* cx, Handle<NativeObject*> obj,
+                                      HandleId id);
+  static bool watchPropertyChangeSlow(JSContext* cx, Handle<NativeObject*> obj,
+                                      HandleId id, PropertyFlags flags);
+  template <AllowGC allowGC>
+  static bool watchPropertyModificationSlow(
+      JSContext* cx,
+      typename MaybeRooted<NativeObject*, allowGC>::HandleType obj,
+      typename MaybeRooted<PropertyKey, allowGC>::HandleType id);
+  static bool watchFreezeOrSealSlow(JSContext* cx, Handle<NativeObject*> obj);
+  static bool watchProtoChangeSlow(JSContext* cx, HandleObject obj);
+  static bool watchObjectSwapSlow(JSContext* cx, HandleObject a,
+                                  HandleObject b);
+
+ public:
+  static bool watchesPropertyAdd(NativeObject* obj) {
+    return obj->hasAnyFlag(
+        {ObjectFlag::IsUsedAsPrototype, ObjectFlag::UseWatchtowerTestingLog});
+  }
+  static bool watchesPropertyRemove(NativeObject* obj) {
+    return obj->hasAnyFlag(
+        {ObjectFlag::IsUsedAsPrototype, ObjectFlag::GenerationCountedGlobal,
+         ObjectFlag::UseWatchtowerTestingLog, ObjectFlag::HasFuseProperty});
+  }
+  static bool watchesPropertyChange(NativeObject* obj) {
+    return obj->hasAnyFlag(
+        {ObjectFlag::IsUsedAsPrototype, ObjectFlag::GenerationCountedGlobal,
+         ObjectFlag::HasFuseProperty, ObjectFlag::UseWatchtowerTestingLog});
+  }
+  static bool watchesPropertyModification(NativeObject* obj) {
+    return obj->hasAnyFlag(
+        {ObjectFlag::HasFuseProperty, ObjectFlag::UseWatchtowerTestingLog});
+  }
+  static bool watchesFreezeOrSeal(NativeObject* obj) {
+    return obj->hasAnyFlag({ObjectFlag::UseWatchtowerTestingLog});
+  }
+  static bool watchesProtoChange(JSObject* obj) {
+    return obj->hasAnyFlag(
+        {ObjectFlag::IsUsedAsPrototype, ObjectFlag::UseWatchtowerTestingLog});
+  }
+  static bool watchesObjectSwap(JSObject* a, JSObject* b) {
+    auto watches = [](JSObject* obj) {
+      return obj->hasAnyFlag(
+          {ObjectFlag::IsUsedAsPrototype, ObjectFlag::UseWatchtowerTestingLog});
+    };
+    return watches(a) || watches(b);
+  }
+
+  static bool watchPropertyAdd(JSContext* cx, Handle<NativeObject*> obj,
+                               HandleId id) {
+    if (MOZ_LIKELY(!watchesPropertyAdd(obj))) {
+      return true;
+    }
+    return watchPropertyAddSlow(cx, obj, id);
+  }
+  static bool watchPropertyRemove(JSContext* cx, Handle<NativeObject*> obj,
+                                  HandleId id) {
+    if (MOZ_LIKELY(!watchesPropertyRemove(obj))) {
+      return true;
+    }
+    return watchPropertyRemoveSlow(cx, obj, id);
+  }
+  static bool watchPropertyChange(JSContext* cx, Handle<NativeObject*> obj,
+                                  HandleId id, PropertyFlags flags) {
+    if (MOZ_LIKELY(!watchesPropertyChange(obj))) {
+      return true;
+    }
+    return watchPropertyChangeSlow(cx, obj, id, flags);
+  }
+
+  // Note: We can only watch property modification for regular object slots
+  // with an id, not reserved slots.
+  template <AllowGC allowGC>
+  static bool watchPropertyModification(
+      JSContext* cx,
+      typename MaybeRooted<NativeObject*, allowGC>::HandleType obj,
+      typename MaybeRooted<PropertyKey, allowGC>::HandleType id) {
+    if (MOZ_LIKELY(!watchesPropertyModification(obj))) {
+      return true;
+    }
+    return watchPropertyModificationSlow<allowGC>(cx, obj, id);
+  }
+  static bool watchFreezeOrSeal(JSContext* cx, Handle<NativeObject*> obj) {
+    if (MOZ_LIKELY(!watchesFreezeOrSeal(obj))) {
+      return true;
+    }
+    return watchFreezeOrSealSlow(cx, obj);
+  }
+  static bool watchProtoChange(JSContext* cx, HandleObject obj) {
+    if (MOZ_LIKELY(!watchesProtoChange(obj))) {
+      return true;
+    }
+    return watchProtoChangeSlow(cx, obj);
+  }
+
+  static bool watchObjectSwap(JSContext* cx, HandleObject a, HandleObject b) {
+    if (MOZ_LIKELY(!watchesObjectSwap(a, b))) {
+      return true;
+    }
+    return watchObjectSwapSlow(cx, a, b);
+  }
+};
+
+}  // namespace js
+
+#endif /* vm_Watchtower_h */