Improve performance of SELECT element by optimizing Vector.

third_party/WebKit/Source/core/html/HTMLSelectElement.cpp

 /*
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2010 Google Inc. All rights reserved.
  * Copyright (C) 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
@@ skipping 633 matching lines @@
     // Cache the selection state so we can restore the old selection as the new
     // selection pivots around this anchor index.
     // Example:
     // 1. Press the mouse button on the second OPTION
     //   m_activeSelectionAnchorIndex = 1
     // 2. Drag the mouse pointer onto the fifth OPTION
     //   m_activeSelectionEndIndex = 4, options at 1-4 indices are selected.
     // 3. Drag the mouse pointer onto the fourth OPTION
     //   m_activeSelectionEndIndex = 3, options at 1-3 indices are selected.
     //   updateListBoxSelection needs to clear selection of the fifth OPTION.
-    m_cachedStateForActiveSelection.clear();
+    m_cachedStateForActiveSelection.resize(0);
     for (auto& element : listItems()) {
         m_cachedStateForActiveSelection.append(isHTMLOptionElement(*element) && toHTMLOptionElement(element)->selected());
     }
 }
 
 void HTMLSelectElement::setActiveSelectionEnd(HTMLOptionElement* option)
 {
     m_activeSelectionEnd = option;
 }
 
@@ skipping 128 matching lines @@
 
     if (layoutObject()) {
         if (AXObjectCache* cache = layoutObject()->document().existingAXObjectCache())
             cache->childrenChanged(this);
     }
 }
 
 void HTMLSelectElement::recalcListItems() const
 {
     TRACE_EVENT0("blink", "HTMLSelectElement::recalcListItems");
-    m_listItems.clear();
+    m_listItems.resize(0);

[sof, 2016/06/29 05:43:24]

Isn't there a risk of m_listItem's backing store never shrinking for a <select/>
with many and then very few <option/>s?

[tkent, 2016/06/29 06:07:46]

There is.
But I guess such scenario is not common.

 
     m_shouldRecalcListItems = false;
 
     for (Element* currentElement = ElementTraversal::firstWithin(*this); currentElement && m_listItems.size() < maxListItems; ) {
         if (!currentElement->isHTMLElement()) {
             currentElement = ElementTraversal::nextSkippingChildren(*currentElement, this);
             continue;
         }
         HTMLElement& current = toHTMLElement(*currentElement);
 
@@ skipping 1258 matching lines @@
 }
 
 void HTMLSelectElement::didMutateSubtree()
 {
     DCHECK(popupIsVisible());
     DCHECK(m_popup);
     m_popup->updateFromElement(PopupMenu::ByDOMChange);
 }
 
 } // namespace blink