Bug 838688 - Optimize CollectFloats by doing an early return if this block doesn't have any floats. r=bzbarsky

layout/generic/nsBlockFrame.cpp

@@ -6895,8 +6895,8 @@ nsBlockFrame::ReflowBullet(nsIFrame* aBulletFrame,
 // that are float containing blocks.  Floats that or not children of 'this'
 // are ignored (they are not added to aList).
 void
-nsBlockFrame::CollectFloats(nsIFrame* aFrame, nsFrameList& aList,
-                            bool aCollectSiblings)
+nsBlockFrame::DoCollectFloats(nsIFrame* aFrame, nsFrameList& aList,
+                              bool aCollectSiblings)
 {
   while (aFrame) {
     // Don't descend into float containing blocks.
@@ -6934,8 +6934,8 @@ nsBlockFrame::CollectFloats(nsIFrame* aFrame, nsFrameList& aList,
         // XXXmats nsInlineFrame's lazy reparenting depends on NOT doing that.
       }
 
-      CollectFloats(aFrame->GetFirstPrincipalChild(), aList, true);
-      CollectFloats(aFrame->GetFirstChild(kOverflowList), aList, true);
+      DoCollectFloats(aFrame->GetFirstPrincipalChild(), aList, true);
+      DoCollectFloats(aFrame->GetFirstChild(kOverflowList), aList, true);
     }
     if (!aCollectSiblings)
       break;

layout/generic/nsBlockFrame.h

@@ -478,6 +478,25 @@ protected:
     */
   bool DrainOverflowLines();
 
+  /**
+   * @return false iff this block does not have a float on any child list.
+   * This function is O(1).
+   */
+  bool MaybeHasFloats() const {
+    if (!mFloats.IsEmpty()) {
+      return true;
+    }
+    // XXX this could be replaced with HasPushedFloats() if we enforced
+    // removing the property when the frame list becomes empty.
+    nsFrameList* list = GetPushedFloats();
+    if (list && !list->IsEmpty()) {
+      return true;
+    }
+    // For the OverflowOutOfFlowsProperty I think we do enforce that, but it's
+    // a mix of out-of-flow frames, so that's why the method name has "Maybe".
+    return GetStateBits() & NS_BLOCK_HAS_OVERFLOW_OUT_OF_FLOWS;
+  }
+
   /** grab pushed floats from this block's prevInFlow, and splice
     * them into this block's mFloats list.
     */
@@ -505,7 +524,14 @@ protected:
   void RemoveFloat(nsIFrame* aFloat);
 
   void CollectFloats(nsIFrame* aFrame, nsFrameList& aList,
-                     bool aCollectFromSiblings);
+                     bool aCollectFromSiblings) {
+    if (MaybeHasFloats()) {
+      DoCollectFloats(aFrame, aList, aCollectFromSiblings);
+    }
+  }
+  void DoCollectFloats(nsIFrame* aFrame, nsFrameList& aList,
+                       bool aCollectFromSiblings);
+
   // Remove a float, abs, rel positioned frame from the appropriate block's list
   static void DoRemoveOutOfFlowFrame(nsIFrame* aFrame);