Quick: Only take a copy of changeListeners when there are some

In the case where there are no change listeners, there is no need to
do an ref() operation (which involves atomic loads) only to find out
that it's an empty vector (!isSharable). Better still: the whole loop
start-up can be skipped.

Change-Id: I94fd22029a321a5dbef571145007071a54f5b04b
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 88 insertions, 64 deletions

diff --git a/src/quick/items/qquickitem.cpp b/src/quick/items/qquickitem.cpp
index efb7d46efa..9e86247162 100644
--- a/src/quick/items/qquickitem.cpp
+++ b/src/quick/items/qquickitem.cpp
@@ -2308,29 +2308,31 @@ QQuickItem::~QQuickItem()
     while (!d->childItems.isEmpty())
         d->childItems.constFirst()->setParentItem(0);
 
-    const auto listeners = d->changeListeners; // NOTE: intentional copy (QTBUG-54732)
-    for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-        QQuickAnchorsPrivate *anchor = change.listener->anchorPrivate();
-        if (anchor)
-            anchor->clearItem(this);
-    }
+    if (!d->changeListeners.isEmpty()) {
+        const auto listeners = d->changeListeners; // NOTE: intentional copy (QTBUG-54732)
+        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+            QQuickAnchorsPrivate *anchor = change.listener->anchorPrivate();
+            if (anchor)
+                anchor->clearItem(this);
+        }
 
-    /*
+        /*
         update item anchors that depended on us unless they are our child (and will also be destroyed),
         or our sibling, and our parent is also being destroyed.
     */
-    for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-        QQuickAnchorsPrivate *anchor = change.listener->anchorPrivate();
-        if (anchor && anchor->item && anchor->item->parentItem() && anchor->item->parentItem() != this)
-            anchor->update();
-    }
+        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+            QQuickAnchorsPrivate *anchor = change.listener->anchorPrivate();
+            if (anchor && anchor->item && anchor->item->parentItem() && anchor->item->parentItem() != this)
+                anchor->update();
+        }
 
-    for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-        if (change.types & QQuickItemPrivate::Destroyed)
-            change.listener->itemDestroyed(this);
-    }
+        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+            if (change.types & QQuickItemPrivate::Destroyed)
+                change.listener->itemDestroyed(this);
+        }
 
-    d->changeListeners.clear();
+        d->changeListeners.clear();
+    }
 
     /*
        Remove any references our transforms have to us, in case they try to
@@ -3549,10 +3551,12 @@ QQuickAnchors *QQuickItemPrivate::anchors() const
 void QQuickItemPrivate::siblingOrderChanged()
 {
     Q_Q(QQuickItem);
-    const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-    for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-        if (change.types & QQuickItemPrivate::SiblingOrder) {
-            change.listener->itemSiblingOrderChanged(q);
+    if (!changeListeners.isEmpty()) {
+        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+            if (change.types & QQuickItemPrivate::SiblingOrder) {
+                change.listener->itemSiblingOrderChanged(q);
+            }
         }
     }
 }
@@ -3663,11 +3667,13 @@ void QQuickItem::geometryChanged(const QRectF &newGeometry, const QRectF &oldGeo
     change.setWidthChange(diff.width() != 0);
     change.setHeightChange(diff.height() != 0);
 
-    const auto listeners = d->changeListeners; // NOTE: intentional copy (QTBUG-54732)
-    for (const QQuickItemPrivate::ChangeListener &listener : listeners) {
-        if (listener.types & QQuickItemPrivate::Geometry) {
-            if (change.matches(listener.gTypes))
-                listener.listener->itemGeometryChanged(this, change, diff);
+    if (!d->changeListeners.isEmpty()) {
+        const auto listeners = d->changeListeners; // NOTE: intentional copy (QTBUG-54732)
+        for (const QQuickItemPrivate::ChangeListener &listener : listeners) {
+            if (listener.types & QQuickItemPrivate::Geometry) {
+                if (change.matches(listener.gTypes))
+                    listener.listener->itemGeometryChanged(this, change, diff);
+            }
         }
     }
 
@@ -4224,12 +4230,14 @@ void QQuickItem::setBaselineOffset(qreal offset)
 
     d->baselineOffset = offset;
 
-    const auto listeners = d->changeListeners; // NOTE: intentional copy (QTBUG-54732)
-    for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-        if (change.types & QQuickItemPrivate::Geometry) {
-            QQuickAnchorsPrivate *anchor = change.listener->anchorPrivate();
-            if (anchor)
-                anchor->updateVerticalAnchors();
+    if (!d->changeListeners.isEmpty()) {
+        const auto listeners = d->changeListeners; // NOTE: intentional copy (QTBUG-54732)
+        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+            if (change.types & QQuickItemPrivate::Geometry) {
+                QQuickAnchorsPrivate *anchor = change.listener->anchorPrivate();
+                if (anchor)
+                    anchor->updateVerticalAnchors();
+            }
         }
     }
 
@@ -5977,20 +5985,24 @@ void QQuickItemPrivate::itemChange(QQuickItem::ItemChange change, const QQuickIt
     switch (change) {
     case QQuickItem::ItemChildAddedChange: {
         q->itemChange(change, data);
-        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-            if (change.types & QQuickItemPrivate::Children) {
-                change.listener->itemChildAdded(q, data.item);
+        if (!changeListeners.isEmpty()) {
+            const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+            for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+                if (change.types & QQuickItemPrivate::Children) {
+                    change.listener->itemChildAdded(q, data.item);
+                }
             }
         }
         break;
     }
     case QQuickItem::ItemChildRemovedChange: {
         q->itemChange(change, data);
-        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-            if (change.types & QQuickItemPrivate::Children) {
-                change.listener->itemChildRemoved(q, data.item);
+        if (!changeListeners.isEmpty()) {
+            const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+            for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+                if (change.types & QQuickItemPrivate::Children) {
+                    change.listener->itemChildRemoved(q, data.item);
+                }
             }
         }
         break;
@@ -6000,30 +6012,36 @@ void QQuickItemPrivate::itemChange(QQuickItem::ItemChange change, const QQuickIt
         break;
     case QQuickItem::ItemVisibleHasChanged: {
         q->itemChange(change, data);
-        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-            if (change.types & QQuickItemPrivate::Visibility) {
-                change.listener->itemVisibilityChanged(q);
+        if (!changeListeners.isEmpty()) {
+            const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+            for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+                if (change.types & QQuickItemPrivate::Visibility) {
+                    change.listener->itemVisibilityChanged(q);
+                }
             }
         }
         break;
     }
     case QQuickItem::ItemParentHasChanged: {
         q->itemChange(change, data);
-        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-            if (change.types & QQuickItemPrivate::Parent) {
-                change.listener->itemParentChanged(q, data.item);
+        if (!changeListeners.isEmpty()) {
+            const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+            for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+                if (change.types & QQuickItemPrivate::Parent) {
+                    change.listener->itemParentChanged(q, data.item);
+                }
             }
         }
         break;
     }
     case QQuickItem::ItemOpacityHasChanged: {
         q->itemChange(change, data);
-        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-            if (change.types & QQuickItemPrivate::Opacity) {
-                change.listener->itemOpacityChanged(q);
+        if (!changeListeners.isEmpty()) {
+            const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+            for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+                if (change.types & QQuickItemPrivate::Opacity) {
+                    change.listener->itemOpacityChanged(q);
+                }
             }
         }
         break;
@@ -6033,10 +6051,12 @@ void QQuickItemPrivate::itemChange(QQuickItem::ItemChange change, const QQuickIt
         break;
     case QQuickItem::ItemRotationHasChanged: {
         q->itemChange(change, data);
-        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-            if (change.types & QQuickItemPrivate::Rotation) {
-                change.listener->itemRotationChanged(q);
+        if (!changeListeners.isEmpty()) {
+            const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+            for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+                if (change.types & QQuickItemPrivate::Rotation) {
+                    change.listener->itemRotationChanged(q);
+                }
             }
         }
         break;
@@ -6395,10 +6415,12 @@ void QQuickItem::resetWidth()
 void QQuickItemPrivate::implicitWidthChanged()
 {
     Q_Q(QQuickItem);
-    const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-    for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-        if (change.types & QQuickItemPrivate::ImplicitWidth) {
-            change.listener->itemImplicitWidthChanged(q);
+    if (!changeListeners.isEmpty()) {
+        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+            if (change.types & QQuickItemPrivate::ImplicitWidth) {
+                change.listener->itemImplicitWidthChanged(q);
+            }
         }
     }
     emit q->implicitWidthChanged();
@@ -6559,10 +6581,12 @@ void QQuickItem::resetHeight()
 void QQuickItemPrivate::implicitHeightChanged()
 {
     Q_Q(QQuickItem);
-    const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
-    for (const QQuickItemPrivate::ChangeListener &change : listeners) {
-        if (change.types & QQuickItemPrivate::ImplicitHeight) {
-            change.listener->itemImplicitHeightChanged(q);
+    if (!changeListeners.isEmpty()) {
+        const auto listeners = changeListeners; // NOTE: intentional copy (QTBUG-54732)
+        for (const QQuickItemPrivate::ChangeListener &change : listeners) {
+            if (change.types & QQuickItemPrivate::ImplicitHeight) {
+                change.listener->itemImplicitHeightChanged(q);
+            }
         }
     }
     emit q->implicitHeightChanged();