TreeModel/Find: Save a few cycles when searching

Calls parent() only ~two out of three times and remove some
function calls in the TreeModel::parent() implementation,
reducing the overall footprint of this bit from ~5 to ~1.5%
when searching something in the project tree (test case
was Creator-in-Creator, and searching for 'dddddddddd',
resulting in 8 complete scans of the tree.

Task-number: QTCREATORBUG-17956
Change-Id: I4e46ef0467dd2aea58a7c944e1a2ee5c01e6fbba
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/libs/utils/treemodel.cpp

@@ -919,11 +919,9 @@ QModelIndex BaseTreeModel::parent(const QModelIndex &idx) const
     if (!grandparent)
         return QModelIndex();
 
-    for (int i = 0, n = grandparent->childCount(); i < n; ++i)
-        if (grandparent->childAt(i) == parent)
-            return createIndex(i, 0, static_cast<void*>(parent));
-
-    return QModelIndex();
+    // This is on the performance-critical path for ItemViewFind.
+    const int i = grandparent->m_children.indexOf(parent);
+    return createIndex(i, 0, static_cast<void*>(parent));
 }
 
 int BaseTreeModel::rowCount(const QModelIndex &idx) const

src/libs/utils/treemodel.h

@@ -66,7 +66,7 @@ public:
 
     using const_iterator = QVector<TreeItem *>::const_iterator;
     using value_type = TreeItem *;
-    int childCount() const { return end() - begin(); }
+    int childCount() const { return m_children.size(); }
     int indexInParent() const;
     TreeItem *childAt(int index) const;
     int indexOf(const TreeItem *item) const;

src/plugins/coreplugin/find/itemviewfind.cpp

@@ -249,12 +249,13 @@ QModelIndex ItemViewFind::nextIndex(const QModelIndex &idx, bool *wrapped) const
         return model->index(0, 0);
 
     // same parent has more columns, go to next column
-    if (idx.column() + 1 < model->columnCount(idx.parent()))
-        return model->index(idx.row(), idx.column() + 1, idx.parent());
+    const QModelIndex parentIdx = idx.parent();
+    if (idx.column() + 1 < model->columnCount(parentIdx))
+        return model->index(idx.row(), idx.column() + 1, parentIdx);
 
     // tree views have their children attached to first column
     // make sure we are at first column
-    QModelIndex current = model->index(idx.row(), 0, idx.parent());
+    QModelIndex current = model->index(idx.row(), 0, parentIdx);
 
     // check for children
     if (d->m_option == FetchMoreWhileSearching && model->canFetchMore(current))