avoid double reallocations in appending operations

operator+=() and co. would first detach, and then realloc if they found
the reservation too small. in particular, appending anything to an empty
list would trigger this double reallocation (first copy shared_null,
then grow the copy).

Reviewed-by: joao
Reviewed-by: denis

1 files changed, 24 insertions, 4 deletions

diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index 8c75c985a4..1edac03100 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -73,6 +73,7 @@ struct Q_CORE_EXPORT QListData {
     enum { DataHeaderSize = sizeof(Data) - sizeof(void *) };
 
     Data *detach(int alloc);
+    Data *detach_grow(int alloc);
     Data *detach(); // remove in 5.0
     Data *detach2(); // remove in 5.0
     Data *detach3(); // remove in 5.0
@@ -121,6 +122,7 @@ public:
     inline int size() const { return p.size(); }
 
     inline void detach() { if (d->ref != 1) detach_helper(); }
+    inline void detach_grow(int by) { if (d->ref != 1) detach_helper_grow(d->end - d->begin + by); }
 
     inline void detachShared()
     {
@@ -333,6 +335,7 @@ public:
 #endif
 
 private:
+    void detach_helper_grow(int alloc);
     void detach_helper(int alloc);
     void detach_helper();
     void free(QListData::Data *d);
@@ -480,7 +483,7 @@ Q_OUTOFLINE_TEMPLATE void QList<T>::reserve(int alloc)
 template <typename T>
 Q_OUTOFLINE_TEMPLATE void QList<T>::append(const T &t)
 {
-    detach();
+    detach_grow(1);
     if (QTypeInfo<T>::isLarge || QTypeInfo<T>::isStatic) {
         Node *n = reinterpret_cast<Node *>(p.append());
         QT_TRY {
@@ -504,7 +507,7 @@ Q_OUTOFLINE_TEMPLATE void QList<T>::append(const T &t)
 template <typename T>
 inline void QList<T>::prepend(const T &t)
 {
-    detach();
+    detach_grow(1);
     if (QTypeInfo<T>::isLarge || QTypeInfo<T>::isStatic) {
         Node *n = reinterpret_cast<Node *>(p.prepend());
         QT_TRY {
@@ -528,7 +531,7 @@ inline void QList<T>::prepend(const T &t)
 template <typename T>
 inline void QList<T>::insert(int i, const T &t)
 {
-    detach();
+    detach_grow(1);
     if (QTypeInfo<T>::isLarge || QTypeInfo<T>::isStatic) {
         Node *n = reinterpret_cast<Node *>(p.insert(i));
         QT_TRY {
@@ -622,6 +625,23 @@ Q_OUTOFLINE_TEMPLATE T QList<T>::value(int i, const T& defaultValue) const
 }
 
 template <typename T>
+Q_OUTOFLINE_TEMPLATE void QList<T>::detach_helper_grow(int alloc)
+{
+    Node *n = reinterpret_cast<Node *>(p.begin());
+    QListData::Data *x = p.detach_grow(alloc);
+    QT_TRY {
+        node_copy(reinterpret_cast<Node *>(p.begin()), reinterpret_cast<Node *>(p.end()), n);
+    } QT_CATCH(...) {
+        qFree(d);
+        d = x;
+        QT_RETHROW;
+    }
+
+    if (!x->ref.deref())
+        free(x);
+}
+
+template <typename T>
 Q_OUTOFLINE_TEMPLATE void QList<T>::detach_helper(int alloc)
 {
     Node *n = reinterpret_cast<Node *>(p.begin());
@@ -730,7 +750,7 @@ Q_OUTOFLINE_TEMPLATE typename QList<T>::iterator QList<T>::erase(typename QList<
 template <typename T>
 Q_OUTOFLINE_TEMPLATE QList<T> &QList<T>::operator+=(const QList<T> &l)
 {
-    detach();
+    detach_grow(l.size());
     Node *n = reinterpret_cast<Node *>(p.append2(l.p));
     QT_TRY{
         node_copy(n, reinterpret_cast<Node *>(p.end()), reinterpret_cast<Node *>(l.p.begin()));