Issue 4071 - Speed up ElemHide.getSelectorsForDomain for domains with no filters

lib/elemHide.js

Index: lib/elemHide.js
diff --git a/lib/elemHide.js b/lib/elemHide.js
index ee309d1d4a823b798804f41693dc97c22236d598..5ba0be1508670fce8003777a5ec2430a6b232fc5 100644
--- a/lib/elemHide.js
+++ b/lib/elemHide.js
@@ -55,6 +55,12 @@ var usingGetSelectorsForDomain = !("nsIStyleSheetService" in Ci);
 var filtersByDomain = Object.create(null);
 
 /**
+ * Lookup table, count of filter exceptions by domain.
+ * @type Object
+ */
+var exceptionCountByDomain = Object.create(null);
+
+/**
  * Lookup table, filters by selector. (Only contains filters that have a
  * selector that is unconditionally matched for all domains.)
  */
@@ -135,6 +141,7 @@ var ElemHide = exports.ElemHide =
     keyByFilter = Object.create(null);
     filtersByDomain = Object.create(null);
     filtersBySelector = Object.create(null);
+    exceptionCountByDomain = Object.create(null);
     unconditionalSelectors = null;
     knownExceptions = Object.create(null);
     exceptions = Object.create(null);
@@ -153,9 +160,17 @@ var ElemHide = exports.ElemHide =
         filters = filtersByDomain[domain] = Object.create(null);
 
       if (domains[domain])
+      {
         filters[key] = filter;
+      }
       else
+      {
         filters[key] = false;
+        if (domain in exceptionCountByDomain)
+          exceptionCountByDomain[domain] += 1;
+        else
+          exceptionCountByDomain[domain] = 1;
+      }
     }
   },
 
@@ -177,6 +192,15 @@ var ElemHide = exports.ElemHide =
 
       if (usingGetSelectorsForDomain)
       {
+        let domains = filter.domains || defaultDomains;
+        for (let domain in domains)
+        {
+          if (domain in exceptionCountByDomain)
+            exceptionCountByDomain[domain] += 1;
+          else
+            exceptionCountByDomain[domain] = 1;
+        }
+
         // If this is the first exception for a previously unconditionally
         // applied element hiding selector we need to take care to update the
         // lookups.
@@ -256,6 +280,16 @@ var ElemHide = exports.ElemHide =
 
       if (usingGetSelectorsForDomain)
       {
+        let domains = filter.domains || defaultDomains;
+        for (let domain in domains)
+        {
+          let count = exceptionCountByDomain[domain];
+          if (count > 1)
+            exceptionCountByDomain[domain] -= 1;
+          else if (count)

[Sebastian Noack, 2016/05/26 11:46:43]

It seems the condition to check for non-zero count is redundant. Then you also
don't need the count variable.

However, I wonder whether we should delete properties from
exceptionCountByDomain anyway. It seems we don't do that for filtersByDomain
either.

+            delete exceptionCountByDomain[domain];
+        }
+
         let filters = filtersBySelector[filter.selector];
         if (filters)
         {
@@ -501,12 +535,31 @@ var ElemHide = exports.ElemHide =
       unconditionalSelectors = Object.keys(filtersBySelector);
     let selectors = specificOnly ? [] : unconditionalSelectors.slice();
 
+    let anyExceptions = false;
+
     let seenFilters = Object.create(null);
     let currentDomain = domain ? domain.toUpperCase() : "";
     while (true)
     {
-      if (specificOnly && currentDomain == "")
-        break;
+      if (currentDomain == "")
+      {
+        if (specificOnly)
+          break;
+
+        // For domains with no exceptions we can add all generic rules
+        if (!anyExceptions)
+        {
+          let filters = filtersByDomain[""];
+          for (let filterKey in filters)
+          {
+            let filter = filters[filterKey];
+            if (filter)

[Sebastian Noack, 2016/05/26 11:46:43]

Just for my understanding, what is the scenario when "filter" evaluates to false
here?

+              selectors.push(filters[filterKey].selector);

[Sebastian Noack, 2016/05/26 11:46:43]

Use the "filter" variable here?

+          }
+          break;
+        }
+      }
+      anyExceptions = anyExceptions || exceptionCountByDomain[currentDomain];

[Sebastian Noack, 2016/05/26 11:46:43]

I think it's preferable to write it like this:

 if (exceptionCountByDomain[currentDomain])
    anyExceptions = true;

If for nothing else, to not change the type of the "anyExceptions" variable.

 
       let filters = filtersByDomain[currentDomain];
       if (filters)