Bug 934995, Speed up querySelectorAll by doing less allocations, r=bz

--HG--
extra : rebase_source : 757985381b6413257ef2bf82e7602fc6b0298545

content/base/src/nsContentList.h

@@ -95,6 +95,10 @@ public:
   virtual JSObject* WrapObject(JSContext *cx, JS::Handle<JSObject*> scope)
     MOZ_OVERRIDE = 0;
 
+  void SetCapacity(uint32_t aCapacity)
+  {
+    mElements.SetCapacity(aCapacity);
+  }
 protected:
   /**
    * To be called from non-destructor locations (e.g. unlink) that want to

content/base/src/nsINode.cpp

@@ -2337,8 +2337,8 @@ AddScopeElements(TreeMatchContext& aMatchContext,
 // Actually find elements matching aSelectorList (which must not be
 // null) and which are descendants of aRoot and put them in aList.  If
 // onlyFirstMatch, then stop once the first one is found.
-template<bool onlyFirstMatch, class T>
-inline static nsresult
+template<bool onlyFirstMatch, class Collector, class T>
+MOZ_ALWAYS_INLINE static nsresult
 FindMatchingElements(nsINode* aRoot, const nsAString& aSelector, T &aList)
 {
 
@@ -2424,6 +2424,7 @@ FindMatchingElements(nsINode* aRoot, const nsAString& aSelector, T &aList)
     return NS_OK;
   }
 
+  Collector results;
   for (nsIContent* cur = aRoot->GetFirstChild();
        cur;
        cur = cur->GetNextNode(aRoot)) {
@@ -2431,10 +2432,19 @@ FindMatchingElements(nsINode* aRoot, const nsAString& aSelector, T &aList)
         nsCSSRuleProcessor::SelectorListMatches(cur->AsElement(),
                                                 matchingContext,
                                                 selectorList)) {
-      aList.AppendElement(cur->AsElement());
       if (onlyFirstMatch) {
+        aList.AppendElement(cur->AsElement());
         return NS_OK;
       }
+      results.AppendElement(cur->AsElement());
+    }
+  }
+
+  const uint32_t len = results.Length();
+  if (len) {
+    aList.SetCapacity(len);
+    for (uint32_t i = 0; i < len; ++i) {
+      aList.AppendElement(results.ElementAt(i));
     }
   }
 
@@ -2447,6 +2457,10 @@ struct ElementHolder {
     NS_ABORT_IF_FALSE(!mElement, "Should only get one element");
     mElement = aElement;
   }
+  void SetCapacity(uint32_t aCapacity) { MOZ_CRASH("Don't call me!"); }
+  uint32_t Length() { return 0; }
+  Element* ElementAt(uint32_t aIndex) { return nullptr; }
+
   Element* mElement;
 };
 
@@ -2454,7 +2468,7 @@ Element*
 nsINode::QuerySelector(const nsAString& aSelector, ErrorResult& aResult)
 {
   ElementHolder holder;
-  aResult = FindMatchingElements<true>(this, aSelector, holder);
+  aResult = FindMatchingElements<true, ElementHolder>(this, aSelector, holder);
 
   return holder.mElement;
 }
@@ -2464,7 +2478,10 @@ nsINode::QuerySelectorAll(const nsAString& aSelector, ErrorResult& aResult)
 {
   nsRefPtr<nsSimpleContentList> contentList = new nsSimpleContentList(this);
 
-  aResult = FindMatchingElements<false>(this, aSelector, *contentList);
+  aResult =
+    FindMatchingElements<false, nsAutoTArray<Element*, 128>>(this,
+                                                             aSelector,
+                                                             *contentList);
 
   return contentList.forget();
 }