Remove bindings that have no dependencies

After the initial enabling of a binding we can quickly determine if
there is a chance whether this binding will be re-evaluated again in the
future as a consequence of dependency changes (properties or
translations). If that is not the case, then we can save memory by
removing that binding again.

One implementation consequence of this change is that such constant
bindings used with the "when" property of states require a proper
reference count increase, which was previously implicit through the
binding association with the target object.

In tst_qqmlecmascript a test that verifies that we don't create run-time
bindings for assignments of literal "null" to QObject pointer properties
now also covers the more complex case where we don't know the property
at parse time. We still evaluate the binding once though and perform one
property assignment.

Similarly on the QtQuick Designer Support API test side a binding such
as

    x: Math.max(0, 200)

will not create a persistent binding anymore and needs a tweak to
remain.

On a large scale application this optimization saved up to 5% of all
bindings on start-up (~9000 of ~180000). On Linux x86-64 one binding is
approximately 144 bytes, so the savings are in the range of ~1.2 MB of
heap, as well as reduced fragmentation.

Task-number: QTBUG-64541
Change-Id: Id3653008346fdf36611f5b4c4e82f5f78b5319aa
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/qml/qml/qqmlvaluetypeproxybinding_p.h b/src/qml/qml/qqmlvaluetypeproxybinding_p.h
index ba0d305bd9..35b54c339b 100644
--- a/src/qml/qml/qqmlvaluetypeproxybinding_p.h
+++ b/src/qml/qml/qqmlvaluetypeproxybinding_p.h
@@ -55,7 +55,7 @@
 
 QT_BEGIN_NAMESPACE
 
-class QQmlValueTypeProxyBinding : public QQmlAbstractBinding
+class Q_AUTOTEST_EXPORT QQmlValueTypeProxyBinding : public QQmlAbstractBinding
 {
 public:
     QQmlValueTypeProxyBinding(QObject *o, QQmlPropertyIndex coreIndex);