[oilpan]: Avoid refcounting QualifiedName's nullQName when tracing.

Source/core/dom/QualifiedName.h

Index: Source/core/dom/QualifiedName.h
diff --git a/Source/core/dom/QualifiedName.h b/Source/core/dom/QualifiedName.h
index b589440d942e0125885430a010aa8ffbec6b18c9..9706da834453a6c0cfb59b51d871ff9e7f0bd9bb 100644
--- a/Source/core/dom/QualifiedName.h
+++ b/Source/core/dom/QualifiedName.h
@@ -67,9 +67,7 @@ public:
 
     QualifiedName(const AtomicString& prefix, const AtomicString& localName, const AtomicString& namespaceURI);
     ~QualifiedName();
-#ifdef QNAME_DEFAULT_CONSTRUCTOR
-    QualifiedName() { }
-#endif
+    QualifiedName() : m_impl(nullptr) { }
 
     QualifiedName(const QualifiedName& other) : m_impl(other.m_impl) { }
     const QualifiedName& operator=(const QualifiedName& other) { m_impl = other.m_impl; return *this; }
@@ -85,19 +83,20 @@ public:
 
     bool matchesPossiblyIgnoringCase(const QualifiedName& other, bool shouldIgnoreCase) const { return m_impl == other.m_impl || (equalPossiblyIgnoringCase(localName(), other.localName(), shouldIgnoreCase) && namespaceURI() == other.namespaceURI()); }
 
-    bool hasPrefix() const { return m_impl->m_prefix != nullAtom; }
+    bool hasPrefix() const { return m_impl ? (m_impl->m_prefix != nullAtom) : false; }

[esprehn, 2014/06/05 00:58:44]

Don't use a ternary, use &&.

return m_impl && m_impl->m_prefix != nullAtom;

     void setPrefix(const AtomicString& prefix) { *this = QualifiedName(prefix, localName(), namespaceURI()); }
 
-    const AtomicString& prefix() const { return m_impl->m_prefix; }
-    const AtomicString& localName() const { return m_impl->m_localName; }
-    const AtomicString& namespaceURI() const { return m_impl->m_namespace; }
+    const AtomicString& prefix() const { return m_impl ? m_impl->m_prefix : nullAtom; }
+    const AtomicString& localName() const { return m_impl ? m_impl->m_localName : nullAtom; }
+    const AtomicString& namespaceURI() const { return m_impl ? m_impl->m_namespace : nullAtom; }

[esprehn, 2014/06/05 00:58:44]

This code can all be really hot, we call into it all over selector checking and
other things. Are we sure we want branches inside it?

 
     // Uppercased localName, cached for efficiency
     const AtomicString& localNameUpper() const;
 
     String toString() const;
 
-    QualifiedNameImpl* impl() const { return m_impl.get(); }
+    QualifiedNameImpl* impl() const { return m_impl ? m_impl.get() : 0; }

[esprehn, 2014/06/05 00:58:44]

The ternary is not needed here.

+    bool isNull() const { return !m_impl; }
 
     // Init routine for globals
     static void init();
@@ -153,8 +152,8 @@ namespace WTF {
     };
 
     template<> struct HashTraits<WebCore::QualifiedName> : SimpleClassHashTraits<WebCore::QualifiedName> {
-        static const bool emptyValueIsZero = false;
-        static WebCore::QualifiedName emptyValue() { return WebCore::nullQName(); }
+        static const bool hasIsEmptyValueFunction = true;
+        static bool isEmptyValue(const WebCore::QualifiedName& q) { return q.isNull(); }
     };
 }