Issue 235 - Speed up ElemHide.getSelectorsForDomain

lib/elemHide.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 35 matching lines @@
 var filtersByDomain = Object.create(null);
 
 /**
  * Indicates whether we are using (and maintaining) the filtersByDomain lookup.
  * (Will be false for Firefox)
  * @type Boolean
  */
 var usingFiltersByDomain = !("nsIStyleSheetService" in Ci);
 
 /**
+ * Object to be used instead when a filter has a blank domains property.
+ */
+var defaultDomains = Object.create(null);
+defaultDomains[""] = true;
+
+/**
  * Lookup table, keys are known element hiding exceptions
  * @type Object
  */
 var knownExceptions = Object.create(null);
 
 /**
  * Lookup table, lists of element hiding exceptions by selector
  * @type Object
  */
 var exceptions = Object.create(null);
@@ skipping 78 matching lines @@
       let key;
       do {
         key = Math.random().toFixed(15).substr(5);
       } while (key in filterByKey);
 
       filterByKey[key] = filter;
       keyByFilter[filter.text] = key;
 
       if (usingFiltersByDomain)
       {
-        let domainMatches = filter.domains || {"": true};

[Wladimir Palant, 2016/05/18 13:38:06]

You are iterating over object keys below but {"": true} has more properties than
just "" (unlike filter.domains which doesn't have a prototype). It's probably
best to have a global defaultDomains variable that has this object without
prototype.

[Sebastian Noack, 2016/05/18 13:48:25]

Also note that for..in loops on objects in hash-table mode cause the function to
de-deoptimized in V8. However, whether this is the case here depends a lot of
the context. So please make sure that there is no deopt. And if necessary resort
to Object.keys() + for(;;)

[kzar, 2016/05/18 14:23:00]

Done.

[kzar, 2016/05/18 14:23:00]

The only way I know to check if a function is deoptimized is to find it in the
profiler. It was a little hard to find it but finally found it and the
Elemhide.add function is not deoptimized with this change apparently.
(Unfortunately I only found it in Botttom Up view so far, finding it in Top Down
was even harder.)

+        let domainMatches = filter.domains || defaultDomains;
         for (let domain in domainMatches)
         {
-          domain = domain.toUpperCase();

[Wladimir Palant, 2016/05/18 13:38:06]

This operation is unnecessary, the domains are already upper-case.

[kzar, 2016/05/18 14:23:00]

Done.

           let filters = filtersByDomain[domain];
           if (!filters)
             filters = filtersByDomain[domain] = Object.create(null);
-          filters[filter.text] = domainMatches[domain] && filter;

[Wladimir Palant, 2016/05/18 13:38:05]

I know, Sebastian loves such constructs. Still, I consider this too non-obvious.
Could you write this out, with a proper if statement?

  if (domainMatches[domain])
    filters[filter.text] = filter;
  else
    filters[filter.text] = false;

[kzar, 2016/05/18 14:23:00]

Done.

+
+          if (domainMatches[domain])
+            filters[filter.text] = filter;
+          else
+            filters[filter.text] = false;
         }
       }
 
       ElemHide.isDirty = true;
     }
   },
 
   /**
    * Removes an element hiding filter
    * @param {ElemHideFilter} filter
@@ skipping 259 matching lines @@
       if (currentDomain == "")
         break;
 
       let nextDot = currentDomain.indexOf(".");
       currentDomain = nextDot == -1 ? "" : currentDomain.substr(nextDot + 1);
     }
 
     return selectors;
   }
 };