Get rid of one call into the TLS when evaluating bindings

Store a pointer to the TLS in the BingingEvaluationState. Like this,
we can save us one TLS lookup in the destructor. Shaves off a couple
of percent during binding evaluation.

Change-Id: Idc9dc5b0ea202aaeb68cdc063700b8e4968753dc
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -193,14 +193,17 @@ static thread_local BindingEvaluationState *currentBindingEvaluationState = null
 BindingEvaluationState::BindingEvaluationState(QPropertyBindingPrivate *binding)
     : binding(binding)
 {
-    previousState = currentBindingEvaluationState;
-    currentBindingEvaluationState = this;
+    // store a pointer to the currentBindingEvaluationState to avoid a TLS lookup in
+    // the destructor (as these come with a non zero cost)
+    currentState = &currentBindingEvaluationState;
+    previousState = *currentState;
+    *currentState = this;
     binding->clearDependencyObservers();
 }
 
 BindingEvaluationState::~BindingEvaluationState()
 {
-    currentBindingEvaluationState = previousState;
+    *currentState = previousState;
 }
 
 void QPropertyBase::evaluateIfDirty()

src/corelib/kernel/qproperty_p.h

@@ -112,6 +112,7 @@ struct BindingEvaluationState
     ~BindingEvaluationState();
     QPropertyBindingPrivate *binding;
     BindingEvaluationState *previousState = nullptr;
+    BindingEvaluationState **currentState = nullptr;
 };
 
 QT_END_NAMESPACE