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diff --git a/toolkit/components/addoncompat/Prefetcher.jsm b/toolkit/components/addoncompat/Prefetcher.jsm
new file mode 100644
index 000000000..2d836690c
--- /dev/null
+++ b/toolkit/components/addoncompat/Prefetcher.jsm
@@ -0,0 +1,557 @@
+// 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/.
+
+this.EXPORTED_SYMBOLS = ["Prefetcher"];
+
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "Preferences",
+                                  "resource://gre/modules/Preferences.jsm");
+
+// Rules are defined at the bottom of this file.
+var PrefetcherRules = {};
+
+/*
+ * When events that trigger in the content process are forwarded to
+ * add-ons in the chrome process, we expect the add-ons to send a lot
+ * of CPOWs to query content nodes while processing the events. To
+ * speed this up, the prefetching system anticipates which properties
+ * will be read and reads them ahead of time. The prefetched
+ * properties are passed to the chrome process along with each
+ * event. A typical scenario might work like this:
+ *
+ * 1. "load" event fires in content
+ * 2. Content process prefetches:
+ *      event.target.defaultView = <win 1>
+ *      <win 1>.location = <location obj>
+ *      event.target.getElementsByTagName("form") = [<elt 1>, <elt 2>]
+ *      <elt 1>.id = "login-form"
+ *      <elt 2>.id = "subscribe-form"
+ * 3. Content process forwards "load" event to add-on along with
+ *    prefetched data
+ * 4. Add-on reads:
+ *      event.target.defaultView (already prefetched)
+ *      event.target.getElementsByTagName("form") (already prefetched)
+ *      <elt 1>.id (already prefetched)
+ *      <elt 1>.className (not prefetched; CPOW must be sent)
+ *
+ * The amount of data to prefetch is determined based on the add-on ID
+ * and the event type. The specific data to select is determined using
+ * a set of Datalog-like rules (http://en.wikipedia.org/wiki/Datalog).
+ *
+ * Rules operate on a series of "tables" like in a database. Each
+ * table contains a set of content-process objects. When an event
+ * handler runs, it seeds some initial tables with objects of
+ * interest. For example, the Event table might start out containing
+ * the event that fired.
+ *
+ * Objects are added to tables using a set of rules of the form "if X
+ * is in table A, then add F(X) to table B", where F(X) is typically a
+ * property access or a method call. The most common functions F are:
+ *
+ * PropertyOp(destTable, sourceTable, property):
+ *   For each object X in sourceTable, add X.property to destTable.
+ * MethodOp(destTable, sourceTable, method, args):
+ *   For each object X in sourceTable, add X.method(args) to destTable.
+ * CollectionOp(destTable, sourceTable):
+ *   For each object X in sourceTable, add X[i] to destTable for
+ *   all i from 0 to X.length - 1.
+ *
+ * To generate the prefetching in the example above, the following
+ * rules would work:
+ *
+ * 1. PropertyOp("EventTarget", "Event", "target")
+ * 2. PropertyOp("Window", "EventTarget", "defaultView")
+ * 3. MethodOp("FormCollection", "EventTarget", "getElementsByTagName", "form")
+ * 4. CollectionOp("Form", "FormCollection")
+ * 5. PropertyOp(null, "Form", "id")
+ *
+ * Rules are written at the bottom of this file.
+ *
+ * When a rule runs, it will usually generate some cache entries that
+ * will be passed to the chrome process. For example, when PropertyOp
+ * prefetches obj.prop and gets the value X, it caches the value of
+ * obj and X. When the chrome process receives this data, it creates a
+ * two-level map [obj -> [prop -> X]]. When the add-on accesses a
+ * property on obj, the add-on shim code consults this map to see if
+ * the property has already been cached.
+ */
+
+const PREF_PREFETCHING_ENABLED = "extensions.interposition.prefetching";
+
+function isPrimitive(v) {
+  if (!v)
+    return true;
+  let type = typeof(v);
+  return type !== "object" && type !== "function";
+}
+
+function objAddr(obj)
+{
+/*
+  if (!isPrimitive(obj)) {
+    return String(obj) + "[" + Cu.getJSTestingFunctions().objectAddress(obj) + "]";
+  }
+  return String(obj);
+*/
+}
+
+function log(/* ...args*/)
+{
+/*
+  for (let arg of args) {
+    dump(arg);
+    dump(" ");
+  }
+  dump("\n");
+*/
+}
+
+function logPrefetch(/* kind, value1, component, value2*/)
+{
+/*
+  log("prefetching", kind, objAddr(value1) + "." + component, "=", objAddr(value2));
+*/
+}
+
+/*
+ * All the Op classes (representing Datalog rules) have the same interface:
+ *   outputTable: Table that objects generated by the rule are added to.
+ *     Note that this can be null.
+ *   inputTable: Table that the rule draws objects from.
+ *   addObject(database, obj): Called when an object is added to inputTable.
+ *     This code should take care of adding objects to outputTable.
+ *     Data to be cached should be stored by calling database.cache.
+ *   makeCacheEntry(item, cache):
+ *     Called by the chrome process to create the two-level map of
+ *     prefetched objects. |item| holds the cached data
+ *     generated by the content process. |cache| is the map to be
+ *     generated.
+ */
+
+function PropertyOp(outputTable, inputTable, prop)
+{
+  this.outputTable = outputTable;
+  this.inputTable = inputTable;
+  this.prop = prop;
+}
+
+PropertyOp.prototype.addObject = function(database, obj)
+{
+  let has = false, propValue;
+  try {
+    if (this.prop in obj) {
+      has = true;
+      propValue = obj[this.prop];
+    }
+  } catch (e) {
+    // Don't cache anything if an exception is thrown.
+    return;
+  }
+
+  logPrefetch("prop", obj, this.prop, propValue);
+  database.cache(this.index, obj, has, propValue);
+  if (has && !isPrimitive(propValue) && this.outputTable) {
+    database.add(this.outputTable, propValue);
+  }
+}
+
+PropertyOp.prototype.makeCacheEntry = function(item, cache)
+{
+  let [, obj, , propValue] = item;
+
+  let desc = { configurable: false, enumerable: true, writable: false, value: propValue };
+
+  if (!cache.has(obj)) {
+    cache.set(obj, new Map());
+  }
+  let propMap = cache.get(obj);
+  propMap.set(this.prop, desc);
+}
+
+function MethodOp(outputTable, inputTable, method, ...args)
+{
+  this.outputTable = outputTable;
+  this.inputTable = inputTable;
+  this.method = method;
+  this.args = args;
+}
+
+MethodOp.prototype.addObject = function(database, obj)
+{
+  let result;
+  try {
+    result = obj[this.method].apply(obj, this.args);
+  } catch (e) {
+    // Don't cache anything if an exception is thrown.
+    return;
+  }
+
+  logPrefetch("method", obj, this.method + "(" + this.args + ")", result);
+  database.cache(this.index, obj, result);
+  if (!isPrimitive(result) && this.outputTable) {
+    database.add(this.outputTable, result);
+  }
+}
+
+MethodOp.prototype.makeCacheEntry = function(item, cache)
+{
+  let [, obj, result] = item;
+
+  if (!cache.has(obj)) {
+    cache.set(obj, new Map());
+  }
+  let propMap = cache.get(obj);
+  let fallback = propMap.get(this.method);
+
+  let method = this.method;
+  let selfArgs = this.args;
+  let methodImpl = function(...args) {
+    if (args.length == selfArgs.length && args.every((v, i) => v === selfArgs[i])) {
+      return result;
+    }
+
+    if (fallback) {
+      return fallback.value(...args);
+    }
+    return obj[method](...args);
+  };
+
+  let desc = { configurable: false, enumerable: true, writable: false, value: methodImpl };
+  propMap.set(this.method, desc);
+}
+
+function CollectionOp(outputTable, inputTable)
+{
+  this.outputTable = outputTable;
+  this.inputTable = inputTable;
+}
+
+CollectionOp.prototype.addObject = function(database, obj)
+{
+  let elements = [];
+  try {
+    let len = obj.length;
+    for (let i = 0; i < len; i++) {
+      logPrefetch("index", obj, i, obj[i]);
+      elements.push(obj[i]);
+    }
+  } catch (e) {
+    // Don't cache anything if an exception is thrown.
+    return;
+  }
+
+  database.cache(this.index, obj, ...elements);
+  for (let i = 0; i < elements.length; i++) {
+    if (!isPrimitive(elements[i]) && this.outputTable) {
+      database.add(this.outputTable, elements[i]);
+    }
+  }
+}
+
+CollectionOp.prototype.makeCacheEntry = function(item, cache)
+{
+  let [, obj, ...elements] = item;
+
+  if (!cache.has(obj)) {
+    cache.set(obj, new Map());
+  }
+  let propMap = cache.get(obj);
+
+  let lenDesc = { configurable: false, enumerable: true, writable: false, value: elements.length };
+  propMap.set("length", lenDesc);
+
+  for (let i = 0; i < elements.length; i++) {
+    let desc = { configurable: false, enumerable: true, writable: false, value: elements[i] };
+    propMap.set(i, desc);
+  }
+}
+
+function CopyOp(outputTable, inputTable)
+{
+  this.outputTable = outputTable;
+  this.inputTable = inputTable;
+}
+
+CopyOp.prototype.addObject = function(database, obj)
+{
+  database.add(this.outputTable, obj);
+}
+
+function Database(trigger, addons)
+{
+  // Create a map of rules that apply to this specific trigger and set
+  // of add-ons. The rules are indexed based on their inputTable.
+  this.rules = new Map();
+  for (let addon of addons) {
+    let addonRules = PrefetcherRules[addon] || {};
+    let triggerRules = addonRules[trigger] || [];
+    for (let rule of triggerRules) {
+      let inTable = rule.inputTable;
+      if (!this.rules.has(inTable)) {
+        this.rules.set(inTable, new Set());
+      }
+      let set = this.rules.get(inTable);
+      set.add(rule);
+    }
+  }
+
+  // this.tables maps table names to sets of objects contained in them.
+  this.tables = new Map();
+
+  // todo is a worklist of items added to tables that have not had
+  // rules run on them yet.
+  this.todo = [];
+
+  // Cached data to be sent to the chrome process.
+  this.cached = [];
+}
+
+Database.prototype = {
+  // Add an object to a table.
+  add: function(table, obj) {
+    if (!this.tables.has(table)) {
+      this.tables.set(table, new Set());
+    }
+    let tableSet = this.tables.get(table);
+    if (tableSet.has(obj)) {
+      return;
+    }
+    tableSet.add(obj);
+
+    this.todo.push([table, obj]);
+  },
+
+  cache: function(...args) {
+    this.cached.push(args);
+  },
+
+  // Run a fixed-point iteration that adds objects to table based on
+  // this.rules until there are no more objects to add.
+  process: function() {
+    while (this.todo.length) {
+      let [table, obj] = this.todo.pop();
+      let rules = this.rules.get(table);
+      if (!rules) {
+        continue;
+      }
+      for (let rule of rules) {
+        rule.addObject(this, obj);
+      }
+    }
+  },
+};
+
+var Prefetcher = {
+  init: function() {
+    // Give an index to each rule and store it in this.ruleMap based
+    // on the index. The index is used to serialize and deserialize
+    // data from content to chrome.
+    let counter = 0;
+    this.ruleMap = new Map();
+    for (let addon in PrefetcherRules) {
+      for (let trigger in PrefetcherRules[addon]) {
+        for (let rule of PrefetcherRules[addon][trigger]) {
+          rule.index = counter++;
+          this.ruleMap.set(rule.index, rule);
+        }
+      }
+    }
+
+    this.prefetchingEnabled = Preferences.get(PREF_PREFETCHING_ENABLED, false);
+    Services.prefs.addObserver(PREF_PREFETCHING_ENABLED, this, false);
+    Services.obs.addObserver(this, "xpcom-shutdown", false);
+  },
+
+  observe: function(subject, topic, data) {
+    if (topic == "xpcom-shutdown") {
+      Services.prefs.removeObserver(PREF_PREFETCHING_ENABLED, this);
+      Services.obs.removeObserver(this, "xpcom-shutdown");
+    } else if (topic == PREF_PREFETCHING_ENABLED) {
+      this.prefetchingEnabled = Preferences.get(PREF_PREFETCHING_ENABLED, false);
+    }
+  },
+
+  // Called when an event occurs in the content process. The event is
+  // described by the trigger string. |addons| is a list of addons
+  // that have listeners installed for the event. |args| is
+  // event-specific data (such as the event object).
+  prefetch: function(trigger, addons, args) {
+    if (!this.prefetchingEnabled) {
+      return [[], []];
+    }
+
+    let db = new Database(trigger, addons);
+    for (let table in args) {
+      log("root", table, "=", objAddr(args[table]));
+      db.add(table, args[table]);
+    }
+
+    // Prefetch objects and add them to tables.
+    db.process();
+
+    // Data passed to sendAsyncMessage must be split into a JSON
+    // portion and a CPOW portion. This code splits apart db.cached
+    // into these two pieces. Any object in db.cache is added to an
+    // array of CPOWs and replaced with {cpow: <index in array>}.
+    let cpowIndexes = new Map();
+    let prefetched = [];
+    let cpows = [];
+    for (let item of db.cached) {
+      item = item.map((elt) => {
+        if (!isPrimitive(elt)) {
+          if (!cpowIndexes.has(elt)) {
+            let index = cpows.length;
+            cpows.push(elt);
+            cpowIndexes.set(elt, index);
+          }
+          return {cpow: cpowIndexes.get(elt)};
+        }
+        return elt;
+      });
+
+      prefetched.push(item);
+    }
+
+    return [prefetched, cpows];
+  },
+
+  cache: null,
+
+  // Generate a two-level mapping based on cached data received from
+  // the content process.
+  generateCache: function(prefetched, cpows) {
+    let cache = new Map();
+    for (let item of prefetched) {
+      // Replace anything of the form {cpow: <index>} with the actual
+      // object in |cpows|.
+      item = item.map((elt) => {
+        if (!isPrimitive(elt)) {
+          return cpows[elt.cpow];
+        }
+        return elt;
+      });
+
+      let index = item[0];
+      let op = this.ruleMap.get(index);
+      op.makeCacheEntry(item, cache);
+    }
+    return cache;
+  },
+
+  // Run |func|, using the prefetched data in |prefetched| and |cpows|
+  // as a cache.
+  withPrefetching: function(prefetched, cpows, func) {
+    if (!this.prefetchingEnabled) {
+      return func();
+    }
+
+    this.cache = this.generateCache(prefetched, cpows);
+
+    try {
+      log("Prefetching on");
+      return func();
+    } finally {
+      // After we return from this event handler, the content process
+      // is free to continue executing, so we invalidate our cache.
+      log("Prefetching off");
+      this.cache = null;
+    }
+  },
+
+  // Called by shim code in the chrome process to check if target.prop
+  // is cached.
+  lookupInCache: function(addon, target, prop) {
+    if (!this.cache || !Cu.isCrossProcessWrapper(target)) {
+      return null;
+    }
+
+    let propMap = this.cache.get(target);
+    if (!propMap) {
+      return null;
+    }
+
+    return propMap.get(prop);
+  },
+};
+
+var AdblockId = "{d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}";
+var AdblockRules = {
+  "ContentPolicy.shouldLoad": [
+    new MethodOp("Node", "InitNode", "QueryInterface", Ci.nsISupports),
+    new PropertyOp("Document", "Node", "ownerDocument"),
+    new PropertyOp("Window", "Node", "defaultView"),
+    new PropertyOp("Window", "Document", "defaultView"),
+    new PropertyOp("TopWindow", "Window", "top"),
+    new PropertyOp("WindowLocation", "Window", "location"),
+    new PropertyOp(null, "WindowLocation", "href"),
+    new PropertyOp("Window", "Window", "parent"),
+    new PropertyOp(null, "Window", "name"),
+    new PropertyOp("Document", "Window", "document"),
+    new PropertyOp("TopDocumentElement", "Document", "documentElement"),
+    new MethodOp(null, "TopDocumentElement", "getAttribute", "data-adblockkey"),
+  ]
+};
+PrefetcherRules[AdblockId] = AdblockRules;
+
+var LastpassId = "support@lastpass.com";
+var LastpassRules = {
+  "EventTarget.handleEvent": [
+    new PropertyOp("EventTarget", "Event", "target"),
+    new PropertyOp("EventOriginalTarget", "Event", "originalTarget"),
+    new PropertyOp("Window", "EventOriginalTarget", "defaultView"),
+
+    new CopyOp("Frame", "Window"),
+    new PropertyOp("FrameCollection", "Window", "frames"),
+    new CollectionOp("Frame", "FrameCollection"),
+    new PropertyOp("FrameCollection", "Frame", "frames"),
+    new PropertyOp("FrameDocument", "Frame", "document"),
+    new PropertyOp(null, "Frame", "window"),
+    new PropertyOp(null, "FrameDocument", "defaultView"),
+
+    new PropertyOp("FrameDocumentLocation", "FrameDocument", "location"),
+    new PropertyOp(null, "FrameDocumentLocation", "href"),
+    new PropertyOp("FrameLocation", "Frame", "location"),
+    new PropertyOp(null, "FrameLocation", "href"),
+
+    new MethodOp("FormCollection", "FrameDocument", "getElementsByTagName", "form"),
+    new MethodOp("FormCollection", "FrameDocument", "getElementsByTagName", "FORM"),
+    new CollectionOp("Form", "FormCollection"),
+    new PropertyOp("FormElementCollection", "Form", "elements"),
+    new CollectionOp("FormElement", "FormElementCollection"),
+    new PropertyOp("Style", "Form", "style"),
+
+    new PropertyOp(null, "FormElement", "type"),
+    new PropertyOp(null, "FormElement", "name"),
+    new PropertyOp(null, "FormElement", "value"),
+    new PropertyOp(null, "FormElement", "tagName"),
+    new PropertyOp(null, "FormElement", "id"),
+    new PropertyOp("Style", "FormElement", "style"),
+
+    new PropertyOp(null, "Style", "visibility"),
+
+    new MethodOp("MetaElementsCollection", "EventOriginalTarget", "getElementsByTagName", "meta"),
+    new CollectionOp("MetaElement", "MetaElementsCollection"),
+    new PropertyOp(null, "MetaElement", "httpEquiv"),
+
+    new MethodOp("InputElementCollection", "FrameDocument", "getElementsByTagName", "input"),
+    new MethodOp("InputElementCollection", "FrameDocument", "getElementsByTagName", "INPUT"),
+    new CollectionOp("InputElement", "InputElementCollection"),
+    new PropertyOp(null, "InputElement", "type"),
+    new PropertyOp(null, "InputElement", "name"),
+    new PropertyOp(null, "InputElement", "tagName"),
+    new PropertyOp(null, "InputElement", "form"),
+
+    new PropertyOp("BodyElement", "FrameDocument", "body"),
+    new PropertyOp("BodyInnerText", "BodyElement", "innerText"),
+
+    new PropertyOp("DocumentFormCollection", "FrameDocument", "forms"),
+    new CollectionOp("DocumentForm", "DocumentFormCollection"),
+  ]
+};
+PrefetcherRules[LastpassId] = LastpassRules;