QML: Do not register static QML dependencies on every call.

QML objects can be re-parented on the fly, resulting in different
dependencies for expressions like 'parent.width'. So, because of this,
dependencies are cleared and re-calculated after every binding
evaluation.

However, dependencies on properties of the scope and context objects
cannot change, because these objects do not get changed for the
life-time of a binding. So we can permanently register them. This is
only done for bindings, not for functions, because those might be
conditionally executed.

According to valgrind, this is a reduction of ~186 instructions on x86
for every evaluation of:
Item {
    height: width
}

Change-Id: Ib095497323d4f08caf712d480007e2627a176369
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 10 insertions, 3 deletions

diff --git a/src/qml/qml/qqmljavascriptexpression_p.h b/src/qml/qml/qqmljavascriptexpression_p.h
index 4472a643ff..a5c7a80153 100644
--- a/src/qml/qml/qqmljavascriptexpression_p.h
+++ b/src/qml/qml/qqmljavascriptexpression_p.h
@@ -136,7 +136,8 @@ public:
     inline bool hasDelayedError() const;
     QQmlError error(QQmlEngine *) const;
     void clearError();
-    void clearGuards();
+    void clearActiveGuards();
+    void clearPermanentGuards();
     QQmlDelayedError *delayedError();
 
     static QV4::ReturnedValue evalFunction(QQmlContextData *ctxt, QObject *scope,
@@ -157,10 +158,12 @@ private:
     //    activeGuards:flag2  - useSharedContext
     QBiPointer<QObject, DeleteWatcher> m_scopeObject;
     QForwardFieldList<QQmlJavaScriptExpressionGuard, &QQmlJavaScriptExpressionGuard::next> activeGuards;
+    QForwardFieldList<QQmlJavaScriptExpressionGuard, &QQmlJavaScriptExpressionGuard::next> permanentGuards;
 
     QQmlContextData *m_context;
     QQmlJavaScriptExpression **m_prevExpression;
     QQmlJavaScriptExpression  *m_nextExpression;
+    bool m_permanentDependenciesRegistered = false;
 
 protected:
     QV4::PersistentValue m_function;
@@ -177,10 +180,14 @@ public:
         Q_ASSERT(errorString == 0);
     }
 
-    void captureProperty(QQmlNotifier *);
-    void captureProperty(QObject *, int, int);
+    enum Duration {
+        OnlyOnce,
+        Permanently
+    };
 
     static void registerQmlDependencies(const QV4::CompiledData::Function *compiledFunction, const QV4::Scope &scope);
+    void captureProperty(QQmlNotifier *, Duration duration = OnlyOnce);
+    void captureProperty(QObject *, int, int, Duration duration = OnlyOnce);
 
     QQmlEngine *engine;
     QQmlJavaScriptExpression *expression;