Document early bail in contentWasLaidOut() better.

third_party/WebKit/Source/core/layout/LayoutMultiColumnFlowThread.cpp

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 900 matching lines @@
         }
     }
     m_lastSetWorkedOn = nullptr;
 }
 
 void LayoutMultiColumnFlowThread::contentWasLaidOut(LayoutUnit logicalTopInFlowThreadAfterPagination)
 {
     // Check if we need another fragmentainer group. If we've run out of columns in the last
     // fragmentainer group (column row), we need to insert another fragmentainer group to hold more
     // columns.
-    if (!multiColumnBlockFlow()->isInsideFlowThread())
-        return; // Early bail. We're not nested, so waste no more time on this.
+
+    // First figure out if there's any chance that we're nested at all. If we can be sure that
+    // we're not, bail early. This code is run very often, and since locating a containing flow
+    // thread has some cost (depending on tree depth), avoid calling enclosingFlowThread() right
+    // away. This test may give some false positives (hence the "mayBe"), if we're in an
+    // out-of-flow subtree and have an outer multicol container that doesn't affect us, but that's
+    // okay. We'll discover that further down the road when trying to locate our enclosing flow
+    // thread for real.
+    bool mayBeNested = multiColumnBlockFlow()->isInsideFlowThread();
+    if (!mayBeNested)
+        return;
     if (!isInInitialLayoutPass()) {
         // We only insert additional fragmentainer groups in the initial layout pass. We only want
         // to balance columns in the last fragmentainer group (if we need to balance at all), so we
         // want that last fragmentainer group to be the same one in all layout passes that follow.
         return;
     }
     LayoutMultiColumnSet* columnSet = columnSetAtBlockOffset(logicalTopInFlowThreadAfterPagination);
     if (!columnSet)
         return;
     MultiColumnFragmentainerGroup& row = columnSet->fragmentainerGroupAtFlowThreadOffset(logicalTopInFlowThreadAfterPagination);
     if (!row.isLastGroup())
         return;
     appendNewFragmentainerGroupIfNeeded(logicalTopInFlowThreadAfterPagination);
 }
 
 }