Remove QHashDummyNode and avoid undifined behavior

A lot of code in QHash is doing casting to QHashNode while the pointer
may be of type QHashDummyNode.  This is a lot of undefined behavior.
Remove QHashDummyNode and specialize QHashNode for QHashDummyValue instead.
QHashDummyValue is the only type for which QTypeInfo::isDummy is true.
Q_DUMMY and QTypeInfo::isDummy are internal API, so is QHashDummyNode, so
we can remove them.

Task-number: QTBUG-40029
Change-Id: I60c2ff0933075b9202bde89a9992746052f75133
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 17 insertions, 26 deletions

diff --git a/src/corelib/global/qtypeinfo.h b/src/corelib/global/qtypeinfo.h
index 8478e08416..70dc623e0c 100644
--- a/src/corelib/global/qtypeinfo.h
+++ b/src/corelib/global/qtypeinfo.h
@@ -64,7 +64,7 @@ public:
         isComplex = true,
         isStatic = true,
         isLarge = (sizeof(T)>sizeof(void*)),
-        isDummy = false,
+        isDummy = false, //### Qt6: remove
         sizeOf = sizeof(T)
     };
 };
diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index 1d76a2b3d1..db2d084035 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -217,17 +217,23 @@ private:
     Q_DISABLE_COPY(QHashNode)
 };
 
-template <class Key, class T>
-struct QHashDummyNode
+// Specialize for QHashDummyValue in order to save some memory
+template <class Key>
+struct QHashNode<Key, QHashDummyValue>
 {
-    QHashNode<Key, T> *next;
+    union {
+        QHashNode *next;
+        QHashDummyValue value;
+    };
     const uint h;
     const Key key;
 
-    inline QHashDummyNode(const Key &key0, uint hash, QHashNode<Key, T> *n) : next(n), h(hash), key(key0) {}
+    inline QHashNode(const Key &key0, const QHashDummyValue &, uint hash, QHashNode *n)
+        : next(n), h(hash), key(key0) {}
+    inline bool same_key(uint h0, const Key &key0) const { return h0 == h && key0 == key; }
 
 private:
-    Q_DISABLE_COPY(QHashDummyNode)
+    Q_DISABLE_COPY(QHashNode)
 };
 
 
@@ -276,7 +282,6 @@ Q_HASH_DECLARE_INT_NODES(uint);
 template <class Key, class T>
 class QHash
 {
-    typedef QHashDummyNode<Key, T> DummyNode;
     typedef QHashNode<Key, T> Node;
 
     union {
@@ -289,7 +294,6 @@ class QHash
     }
 
     static inline int alignOfNode() { return qMax<int>(sizeof(void*), Q_ALIGNOF(Node)); }
-    static inline int alignOfDummyNode() { return qMax<int>(sizeof(void*), Q_ALIGNOF(DummyNode)); }
 
 public:
     inline QHash() : d(const_cast<QHashData *>(&QHashData::shared_null)) { }
@@ -554,25 +558,14 @@ template <class Key, class T>
 Q_INLINE_TEMPLATE void QHash<Key, T>::duplicateNode(QHashData::Node *node, void *newNode)
 {
     Node *concreteNode = concrete(node);
-    if (QTypeInfo<T>::isDummy) {
-        (void) new (newNode) DummyNode(concreteNode->key, concreteNode->h, 0);
-    } else {
-        (void) new (newNode) Node(concreteNode->key, concreteNode->value, concreteNode->h, 0);
-    }
+    new (newNode) Node(concreteNode->key, concreteNode->value, concreteNode->h, 0);
 }
 
 template <class Key, class T>
 Q_INLINE_TEMPLATE typename QHash<Key, T>::Node *
 QHash<Key, T>::createNode(uint ah, const Key &akey, const T &avalue, Node **anextNode)
 {
-    Node *node;
-
-    if (QTypeInfo<T>::isDummy) {
-        node = reinterpret_cast<Node *>(new (d->allocateNode(alignOfDummyNode())) DummyNode(akey, ah, *anextNode));
-    } else {
-        node = new (d->allocateNode(alignOfNode())) Node(akey, avalue, ah, *anextNode);
-    }
-
+    Node *node = new (d->allocateNode(alignOfNode())) Node(akey, avalue, ah, *anextNode);
     *anextNode = node;
     ++d->size;
     return node;
@@ -605,9 +598,7 @@ Q_INLINE_TEMPLATE void QHash<Key, T>::clear()
 template <class Key, class T>
 Q_OUTOFLINE_TEMPLATE void QHash<Key, T>::detach_helper()
 {
-    QHashData *x = d->detach_helper(duplicateNode, deleteNode2,
-        QTypeInfo<T>::isDummy ? sizeof(DummyNode) : sizeof(Node),
-        QTypeInfo<T>::isDummy ? alignOfDummyNode() : alignOfNode());
+    QHashData *x = d->detach_helper(duplicateNode, deleteNode2, sizeof(Node), alignOfNode());
     if (!d->ref.deref())
         freeData(d);
     d = x;
@@ -789,7 +780,7 @@ Q_INLINE_TEMPLATE typename QHash<Key, T>::iterator QHash<Key, T>::insert(const K
         return iterator(createNode(h, akey, avalue, node));
     }
 
-    if (!QTypeInfo<T>::isDummy)
+    if (!QtPrivate::is_same<T, QHashDummyValue>::value)
         (*node)->value = avalue;
     return iterator(*node);
 }
@@ -958,7 +949,7 @@ Q_OUTOFLINE_TEMPLATE bool QHash<Key, T>::operator==(const QHash<Key, T> &other)
         do {
             if (it2 == other.end() || !(it2.key() == akey))
                 return false;
-            if (!QTypeInfo<T>::isDummy && !(it.value() == it2.value()))
+            if (!(it.value() == it2.value()))
                 return false;
             ++it;
             ++it2;