Refactor persistent values

Use a page wise allocation mechanism for persistent
values. This significantly reduces memory consumption
of persistent values and also improves their performance
a lot.

Change-Id: I8499d2ca5bdd871e029f643ae605a94544558bb5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

4 files changed, 324 insertions, 193 deletions

diff --git a/src/qml/jsapi/qjsengine.cpp b/src/qml/jsapi/qjsengine.cpp
index 831287d0b3..d54af28f26 100644
--- a/src/qml/jsapi/qjsengine.cpp
+++ b/src/qml/jsapi/qjsengine.cpp
@@ -421,11 +421,18 @@ QJSValue QJSEngine::create(int type, const void *ptr)
 bool QJSEngine::convertV2(const QJSValue &value, int type, void *ptr)
 {
     QJSValuePrivate *vp = QJSValuePrivate::get(value);
-    if (vp->engine) {
-        QV4::Scope scope(vp->engine);
-        QV4::ScopedValue v(scope, vp->getValue(scope.engine));
+    QV4::ExecutionEngine *v4 = vp->engine();
+    QV4::Value scratch;
+    QV4::Value *val = vp->valueForData(&scratch);
+    if (v4) {
+        QV4::Scope scope(v4);
+        QV4::ScopedValue v(scope, *val);
         return scope.engine->metaTypeFromJS(v, type, ptr);
-    } else if (vp->value.isEmpty()) {
+    }
+
+    Q_ASSERT(vp->persistent.isEmpty());
+
+    if (!val) {
         if (vp->unboundData.userType() == QMetaType::QString) {
             QString string = vp->unboundData.toString();
             // have a string based value without engine. Do conversion manually
@@ -478,50 +485,52 @@ bool QJSEngine::convertV2(const QJSValue &value, int type, void *ptr)
         } else {
             return QMetaType::convert(&vp->unboundData.data_ptr(), vp->unboundData.userType(), ptr, type);
         }
-    } else {
-        switch (type) {
-            case QMetaType::Bool:
-                *reinterpret_cast<bool*>(ptr) = vp->value.toBoolean();
-                return true;
-            case QMetaType::Int:
-                *reinterpret_cast<int*>(ptr) = vp->value.toInt32();
-                return true;
-            case QMetaType::UInt:
-                *reinterpret_cast<uint*>(ptr) = vp->value.toUInt32();
-                return true;
-            case QMetaType::LongLong:
-                *reinterpret_cast<qlonglong*>(ptr) = vp->value.toInteger();
-                return true;
-            case QMetaType::ULongLong:
-                *reinterpret_cast<qulonglong*>(ptr) = vp->value.toInteger();
-                return true;
-            case QMetaType::Double:
-                *reinterpret_cast<double*>(ptr) = vp->value.toNumber();
-                return true;
-            case QMetaType::QString:
-                *reinterpret_cast<QString*>(ptr) = value.toString();
-                return true;
-            case QMetaType::Float:
-                *reinterpret_cast<float*>(ptr) = vp->value.toNumber();
-                return true;
-            case QMetaType::Short:
-                *reinterpret_cast<short*>(ptr) = vp->value.toInt32();
-                return true;
-            case QMetaType::UShort:
-                *reinterpret_cast<unsigned short*>(ptr) = vp->value.toUInt16();
-                return true;
-            case QMetaType::Char:
-                *reinterpret_cast<char*>(ptr) = vp->value.toInt32();
-                return true;
-            case QMetaType::UChar:
-                *reinterpret_cast<unsigned char*>(ptr) = vp->value.toUInt16();
-                return true;
-            case QMetaType::QChar:
-                *reinterpret_cast<QChar*>(ptr) = vp->value.toUInt16();
-                return true;
-            default:
-                return false;
-        }
+    }
+
+    Q_ASSERT(val);
+
+    switch (type) {
+        case QMetaType::Bool:
+            *reinterpret_cast<bool*>(ptr) = val->toBoolean();
+            return true;
+        case QMetaType::Int:
+            *reinterpret_cast<int*>(ptr) = val->toInt32();
+            return true;
+        case QMetaType::UInt:
+            *reinterpret_cast<uint*>(ptr) = val->toUInt32();
+            return true;
+        case QMetaType::LongLong:
+            *reinterpret_cast<qlonglong*>(ptr) = val->toInteger();
+            return true;
+        case QMetaType::ULongLong:
+            *reinterpret_cast<qulonglong*>(ptr) = val->toInteger();
+            return true;
+        case QMetaType::Double:
+            *reinterpret_cast<double*>(ptr) = val->toNumber();
+            return true;
+        case QMetaType::QString:
+            *reinterpret_cast<QString*>(ptr) = val->toQStringNoThrow();
+            return true;
+        case QMetaType::Float:
+            *reinterpret_cast<float*>(ptr) = val->toNumber();
+            return true;
+        case QMetaType::Short:
+            *reinterpret_cast<short*>(ptr) = val->toInt32();
+            return true;
+        case QMetaType::UShort:
+            *reinterpret_cast<unsigned short*>(ptr) = val->toUInt16();
+            return true;
+        case QMetaType::Char:
+            *reinterpret_cast<char*>(ptr) = val->toInt32();
+            return true;
+        case QMetaType::UChar:
+            *reinterpret_cast<unsigned char*>(ptr) = val->toUInt16();
+            return true;
+        case QMetaType::QChar:
+            *reinterpret_cast<QChar*>(ptr) = val->toUInt16();
+            return true;
+        default:
+            return false;
     }
 }
 
diff --git a/src/qml/jsapi/qjsvalue.cpp b/src/qml/jsapi/qjsvalue.cpp
index d888ea3535..a158d3ad72 100644
--- a/src/qml/jsapi/qjsvalue.cpp
+++ b/src/qml/jsapi/qjsvalue.cpp
@@ -50,24 +50,50 @@
 
 QV4::ReturnedValue QJSValuePrivate::getValue(QV4::ExecutionEngine *e)
 {
-    if (!this->engine) {
-        this->engine = e;
-    } else if (this->engine != e) {
+    QV4::ExecutionEngine *engine = persistent.engine();
+    if (!engine) {
+        persistent = QV4::PersistentValue(e, e->fromVariant(unboundData));
+        unboundData.clear();
+        engine = e;
+    }
+
+    if (engine != e) {
         qWarning("JSValue can't be reassigned to another engine.");
         return QV4::Encode::undefined();
     }
 
-    if (value.isEmpty()) {
-        value = QV4::Encode(e->fromVariant(unboundData));
-        PersistentValuePrivate **listRoot = &engine->memoryManager->m_persistentValues;
-        prev = listRoot;
-        next = *listRoot;
-        *prev = this;
-        if (next)
-            next->prev = &this->next;
-        unboundData.clear();
+    return persistent.value();
+}
+
+QV4::Value QJSValuePrivate::primitiveValueForUnboundData() const
+{
+    QV4::Value v = QV4::Primitive::emptyValue();
+    switch ((int)unboundData.type()) {
+    case QMetaType::UnknownType:
+    case QMetaType::Void:
+        v = QV4::Encode::undefined();
+        break;
+    case QMetaType::VoidStar:
+        v = QV4::Encode::null();
+        break;
+    case QMetaType::Bool:
+        v = QV4::Encode(unboundData.toBool());
+        break;
+    case QMetaType::Double:
+        v = QV4::Encode(unboundData.toDouble());
+        break;
+    case QMetaType::Int:
+    case QMetaType::Short:
+    case QMetaType::UShort:
+    case QMetaType::Char:
+    case QMetaType::UChar:
+        v = QV4::Encode(unboundData.toInt());
+        break;
+    case QMetaType::UInt:
+        v = QV4::Encode(unboundData.toUInt());
+        break;
     }
-    return value.asReturnedValue();
+    return v;
 }
 
 /*!
@@ -142,7 +168,7 @@ using namespace QV4;
   Constructs a new QJSValue with a boolean \a value.
 */
 QJSValue::QJSValue(bool value)
-    : d(new QJSValuePrivate(Encode(value)))
+    : d(new QJSValuePrivate(QVariant(value)))
 {
 }
 
@@ -155,7 +181,7 @@ QJSValue::QJSValue(QJSValuePrivate *dd)
   Constructs a new QJSValue with a number \a value.
 */
 QJSValue::QJSValue(int value)
-    : d(new QJSValuePrivate(Encode(value)))
+    : d(new QJSValuePrivate(QVariant((double)value)))
 {
 }
 
@@ -163,7 +189,7 @@ QJSValue::QJSValue(int value)
   Constructs a new QJSValue with a number \a value.
 */
 QJSValue::QJSValue(uint value)
-    : d(new QJSValuePrivate(Encode(value)))
+    : d(new QJSValuePrivate(QVariant((double)value)))
 {
 }
 
@@ -171,7 +197,7 @@ QJSValue::QJSValue(uint value)
   Constructs a new QJSValue with a number \a value.
 */
 QJSValue::QJSValue(double value)
-    : d(new QJSValuePrivate(Encode(value)))
+    : d(new QJSValuePrivate(QVariant(value)))
 {
 }
 
@@ -187,8 +213,10 @@ QJSValue::QJSValue(const QString& value)
   Constructs a new QJSValue with a special \a value.
 */
 QJSValue::QJSValue(SpecialValue value)
-    : d(new QJSValuePrivate(value == UndefinedValue ? Encode::undefined() : Encode::null()))
+    : d(new QJSValuePrivate)
 {
+    if (value == NullValue)
+        d->unboundData = QVariant(QMetaType::VoidStar, (void *)0);
 }
 
 /*!
@@ -238,7 +266,9 @@ QJSValue::~QJSValue()
 */
 bool QJSValue::isBool() const
 {
-    return d->value.isBoolean();
+    if (d->persistent.valueRef())
+        return d->persistent.valueRef()->isBoolean();
+    return d->unboundData.type() == QVariant::Bool;
 }
 
 /*!
@@ -249,7 +279,9 @@ bool QJSValue::isBool() const
 */
 bool QJSValue::isNumber() const
 {
-    return d->value.isNumber();
+    if (d->persistent.valueRef())
+        return d->persistent.valueRef()->isNumber();
+    return d->unboundData.type() == QVariant::Double;
 }
 
 /*!
@@ -258,7 +290,9 @@ bool QJSValue::isNumber() const
 */
 bool QJSValue::isNull() const
 {
-    return d->value.isNull();
+    if (d->persistent.valueRef())
+        return d->persistent.valueRef()->isNull();
+    return (int)d->unboundData.type() == QMetaType::VoidStar;
 }
 
 /*!
@@ -269,7 +303,9 @@ bool QJSValue::isNull() const
 */
 bool QJSValue::isString() const
 {
-    return d->value.isEmpty() || d->value.isString();
+    if (d->persistent.valueRef())
+        return d->persistent.valueRef()->isString();
+    return (int)d->unboundData.type() == QMetaType::QString;
 }
 
 /*!
@@ -278,7 +314,9 @@ bool QJSValue::isString() const
 */
 bool QJSValue::isUndefined() const
 {
-    return d->value.isUndefined();
+    if (d->persistent.valueRef())
+        return d->persistent.valueRef()->isUndefined();
+    return (int)d->unboundData.type() == QMetaType::UnknownType || (int)d->unboundData.type() == QMetaType::Void;
 }
 
 /*!
@@ -287,7 +325,9 @@ bool QJSValue::isUndefined() const
 */
 bool QJSValue::isError() const
 {
-    Object *o = d->value.asObject();
+    if (d->persistent.isEmpty())
+        return false;
+    Object *o = d->persistent.valueRef()->asObject();
     return o && o->asErrorObject();
 }
 
@@ -299,7 +339,9 @@ bool QJSValue::isError() const
 */
 bool QJSValue::isArray() const
 {
-    return d->value.asArrayObject();
+    if (d->persistent.isEmpty())
+        return false;
+    return d->persistent.valueRef()->asArrayObject();
 }
 
 /*!
@@ -313,7 +355,9 @@ bool QJSValue::isArray() const
 */
 bool QJSValue::isObject() const
 {
-    return d->value.asObject();
+    if (d->persistent.isEmpty())
+        return false;
+    return d->persistent.valueRef()->asObject();
 }
 
 /*!
@@ -324,7 +368,9 @@ bool QJSValue::isObject() const
 */
 bool QJSValue::isCallable() const
 {
-    return d->value.asFunctionObject();
+    if (d->persistent.isEmpty())
+        return false;
+    return d->persistent.valueRef()->asFunctionObject();
 }
 
 /*!
@@ -335,8 +381,9 @@ bool QJSValue::isCallable() const
 */
 bool QJSValue::isVariant() const
 {
-    Managed *m = d->value.asManaged();
-    return m ? m->as<QV4::VariantObject>() : 0;
+    if (d->persistent.isEmpty())
+        return false;
+    return d->persistent.valueRef()->as<QV4::VariantObject>();
 }
 
 /*!
@@ -353,7 +400,10 @@ bool QJSValue::isVariant() const
 */
 QString QJSValue::toString() const
 {
-    if (d->value.isEmpty()) {
+    QV4::Value scratch;
+    QV4::Value *val = d->valueForData(&scratch);
+
+    if (!val) {
         if (d->unboundData.type() == QVariant::Map)
             return QStringLiteral("[object Object]");
         else if (d->unboundData.type() == QVariant::List) {
@@ -368,7 +418,7 @@ QString QJSValue::toString() const
         }
         return d->unboundData.toString();
     }
-    return d->value.toQStringNoThrow();
+    return val->toQStringNoThrow();
 }
 
 /*!
@@ -385,7 +435,10 @@ QString QJSValue::toString() const
 */
 double QJSValue::toNumber() const
 {
-    if (d->value.isEmpty()) {
+    QV4::Value scratch;
+    QV4::Value *val = d->valueForData(&scratch);
+
+    if (!val) {
         if (d->unboundData.type() == QVariant::String)
             return RuntimeHelpers::stringToNumber(d->unboundData.toString());
         else if (d->unboundData.canConvert<double>())
@@ -394,9 +447,10 @@ double QJSValue::toNumber() const
             return std::numeric_limits<double>::quiet_NaN();
     }
 
-    double dbl = d->value.toNumber();
-    if (d->engine && d->engine->hasException) {
-        d->engine->catchException();
+    double dbl = val->toNumber();
+    QV4::ExecutionEngine *engine = d->persistent.engine();
+    if (engine && engine->hasException) {
+        engine->catchException();
         return 0;
     }
     return dbl;
@@ -416,16 +470,20 @@ double QJSValue::toNumber() const
 */
 bool QJSValue::toBool() const
 {
-    if (d->value.isEmpty()) {
+    QV4::Value scratch;
+    QV4::Value *val = d->valueForData(&scratch);
+
+    if (!val) {
         if (d->unboundData.userType() == QMetaType::QString)
             return d->unboundData.toString().length() > 0;
         else
             return d->unboundData.toBool();
     }
 
-    bool b = d->value.toBoolean();
-    if (d->engine && d->engine->hasException) {
-        d->engine->catchException();
+    bool b = val->toBoolean();
+    QV4::ExecutionEngine *engine = d->persistent.engine();
+    if (engine && engine->hasException) {
+        engine->catchException();
         return false;
     }
     return b;
@@ -445,16 +503,20 @@ bool QJSValue::toBool() const
 */
 qint32 QJSValue::toInt() const
 {
-    if (d->value.isEmpty()) {
+    QV4::Value scratch;
+    QV4::Value *val = d->valueForData(&scratch);
+
+    if (!val) {
         if (d->unboundData.userType() == QMetaType::QString)
             return QV4::Primitive::toInt32(RuntimeHelpers::stringToNumber(d->unboundData.toString()));
         else
             return d->unboundData.toInt();
     }
 
-    qint32 i = d->value.toInt32();
-    if (d->engine && d->engine->hasException) {
-        d->engine->catchException();
+    qint32 i = val->toInt32();
+    QV4::ExecutionEngine *engine = d->persistent.engine();
+    if (engine && engine->hasException) {
+        engine->catchException();
         return 0;
     }
     return i;
@@ -474,16 +536,20 @@ qint32 QJSValue::toInt() const
 */
 quint32 QJSValue::toUInt() const
 {
-    if (d->value.isEmpty()) {
+    QV4::Value scratch;
+    QV4::Value *val = d->valueForData(&scratch);
+
+    if (!val) {
         if (d->unboundData.userType() == QMetaType::QString)
             return QV4::Primitive::toUInt32(RuntimeHelpers::stringToNumber(d->unboundData.toString()));
         else
             return d->unboundData.toUInt();
     }
 
-    quint32 u = d->value.toUInt32();
-    if (d->engine && d->engine->hasException) {
-        d->engine->catchException();
+    quint32 u = val->toUInt32();
+    QV4::ExecutionEngine *engine = d->persistent.engine();
+    if (engine && engine->hasException) {
+        engine->catchException();
         return 0;
     }
     return u;
@@ -513,24 +579,24 @@ quint32 QJSValue::toUInt() const
 */
 QVariant QJSValue::toVariant() const
 {
-    if (d->value.isEmpty())
+    if (d->persistent.isEmpty())
         return d->unboundData;
 
-    if (Object *o = d->value.asObject())
-        return o->engine()->toVariant(d->value, /*typeHint*/ -1, /*createJSValueForObjects*/ false);
-
-    if (d->value.isString())
-        return QVariant(d->value.stringValue()->toQString());
-    if (d->value.isBoolean())
-        return QVariant(d->value.booleanValue());
-    if (d->value.isNumber()) {
-        if (d->value.isInt32())
-            return QVariant(d->value.integerValue());
-        return QVariant(d->value.asDouble());
+    if (Object *o = d->persistent.valueRef()->asObject())
+        return o->engine()->toVariant(*d->persistent.valueRef(), /*typeHint*/ -1, /*createJSValueForObjects*/ false);
+
+    if (d->persistent.valueRef()->isString())
+        return QVariant(d->persistent.valueRef()->stringValue()->toQString());
+    if (d->persistent.valueRef()->isBoolean())
+        return QVariant(d->persistent.valueRef()->booleanValue());
+    if (d->persistent.valueRef()->isNumber()) {
+        if (d->persistent.valueRef()->isInt32())
+            return QVariant(d->persistent.valueRef()->integerValue());
+        return QVariant(d->persistent.valueRef()->asDouble());
     }
-    if (d->value.isNull())
+    if (d->persistent.valueRef()->isNull())
         return QVariant(QMetaType::VoidStar, 0);
-    Q_ASSERT(d->value.isUndefined());
+    Q_ASSERT(d->persistent.valueRef()->isUndefined());
     return QVariant();
 }
 
@@ -551,11 +617,14 @@ QVariant QJSValue::toVariant() const
 */
 QJSValue QJSValue::call(const QJSValueList &args)
 {
-    FunctionObject *f = d->value.asFunctionObject();
+    if (d->persistent.isEmpty())
+        return QJSValue();
+
+    FunctionObject *f = d->persistent.valueRef()->asFunctionObject();
     if (!f)
         return QJSValue();
 
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->persistent.engine();
     Q_ASSERT(engine);
 
     Scope scope(engine);
@@ -570,8 +639,8 @@ QJSValue QJSValue::call(const QJSValueList &args)
     }
 
     ScopedValue result(scope, f->call(callData));
-    if (d->engine->hasException)
-        result = d->engine->catchException();
+    if (engine->hasException)
+        result = engine->catchException();
 
     return new QJSValuePrivate(engine, result);
 }
@@ -598,11 +667,14 @@ QJSValue QJSValue::call(const QJSValueList &args)
 */
 QJSValue QJSValue::callWithInstance(const QJSValue &instance, const QJSValueList &args)
 {
-    FunctionObject *f = d->value.asFunctionObject();
+    if (d->persistent.isEmpty())
+        return QJSValue();
+
+    FunctionObject *f = d->persistent.valueRef()->asFunctionObject();
     if (!f)
         return QJSValue();
 
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->persistent.engine();
     Q_ASSERT(engine);
     Scope scope(engine);
 
@@ -622,8 +694,8 @@ QJSValue QJSValue::callWithInstance(const QJSValue &instance, const QJSValueList
     }
 
     ScopedValue result(scope, f->call(callData));
-    if (d->engine->hasException)
-        result = d->engine->catchException();
+    if (engine->hasException)
+        result = engine->catchException();
 
     return new QJSValuePrivate(engine, result);
 }
@@ -648,11 +720,14 @@ QJSValue QJSValue::callWithInstance(const QJSValue &instance, const QJSValueList
 */
 QJSValue QJSValue::callAsConstructor(const QJSValueList &args)
 {
-    FunctionObject *f = d->value.asFunctionObject();
+    if (d->persistent.isEmpty())
+        return QJSValue();
+
+    FunctionObject *f = d->persistent.valueRef()->asFunctionObject();
     if (!f)
         return QJSValue();
 
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->persistent.engine();
     Q_ASSERT(engine);
 
     Scope scope(engine);
@@ -666,8 +741,8 @@ QJSValue QJSValue::callAsConstructor(const QJSValueList &args)
     }
 
     ScopedValue result(scope, f->construct(callData));
-    if (d->engine->hasException)
-        result = d->engine->catchException();
+    if (engine->hasException)
+        result = engine->catchException();
 
     return new QJSValuePrivate(engine, result);
 }
@@ -683,7 +758,7 @@ QJSValue QJSValue::callAsConstructor(const QJSValueList &args)
 */
 QJSEngine* QJSValue::engine() const
 {
-    QV4::ExecutionEngine *engine = d->engine;
+    QV4::ExecutionEngine *engine = d->persistent.engine();
     if (engine)
         return engine->jsEngine();
     return 0;
@@ -700,11 +775,11 @@ QJSEngine* QJSValue::engine() const
 */
 QJSValue QJSValue::prototype() const
 {
-    QV4::ExecutionEngine *engine = d->engine;
+    QV4::ExecutionEngine *engine = d->persistent.engine();
     if (!engine)
         return QJSValue();
     QV4::Scope scope(engine);
-    ScopedObject o(scope, d->value.asObject());
+    ScopedObject o(scope, d->persistent.valueRef()->asObject());
     if (!o)
         return QJSValue();
     ScopedObject p(scope, o->prototype());
@@ -727,19 +802,20 @@ QJSValue QJSValue::prototype() const
 */
 void QJSValue::setPrototype(const QJSValue& prototype)
 {
-    ExecutionEngine *v4 = d->engine;
+    ExecutionEngine *v4 = d->persistent.engine();
     if (!v4)
         return;
     Scope scope(v4);
-    ScopedObject o(scope, d->value);
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return;
-    if (prototype.d->value.isNull()) {
+    prototype.d->getValue(v4);
+    if (prototype.d->persistent.valueRef()->isNull()) {
         o->setPrototype(0);
         return;
     }
 
-    ScopedObject p(scope, prototype.d->value);
+    ScopedObject p(scope, prototype.d->persistent.value());
     if (!p)
         return;
     if (o->engine() != p->engine()) {
@@ -809,17 +885,21 @@ static bool js_equal(const QString &string, QV4::ValueRef value)
 */
 bool QJSValue::equals(const QJSValue& other) const
 {
-    if (d->value.isEmpty()) {
-        if (other.d->value.isEmpty())
+    QV4::Value s1, s2;
+    QV4::Value *v = d->valueForData(&s1);
+    QV4::Value *ov = other.d->valueForData(&s2);
+
+    if (!v) {
+        if (!ov)
             return d->unboundData == other.d->unboundData;
         if (d->unboundData.type() == QVariant::Map || d->unboundData.type() == QVariant::List)
             return false;
-        return js_equal(d->unboundData.toString(), QV4::ValueRef(other.d->value));
-    }
-    if (other.d->value.isEmpty())
+        return js_equal(d->unboundData.toString(), *ov);
+        }
+    if (!ov)
         return other.equals(*this);
 
-    return Runtime::compareEqual(QV4::ValueRef(d), QV4::ValueRef(other.d));
+    return Runtime::compareEqual(*v, *ov);
 }
 
 /*!
@@ -846,19 +926,23 @@ bool QJSValue::equals(const QJSValue& other) const
 */
 bool QJSValue::strictlyEquals(const QJSValue& other) const
 {
-    if (d->value.isEmpty()) {
-        if (other.d->value.isEmpty())
+    QV4::Value s1, s2;
+    QV4::Value *v = d->valueForData(&s1);
+    QV4::Value *ov = other.d->valueForData(&s2);
+
+    if (!v) {
+        if (!ov)
             return d->unboundData == other.d->unboundData;
         if (d->unboundData.type() == QVariant::Map || d->unboundData.type() == QVariant::List)
             return false;
-        if (other.d->value.isString())
-            return d->unboundData.toString() == other.d->value.stringValue()->toQString();
+        if (ov->isString())
+            return d->unboundData.toString() == ov->stringValue()->toQString();
         return false;
     }
-    if (other.d->value.isEmpty())
+    if (!ov)
         return other.strictlyEquals(*this);
 
-    return RuntimeHelpers::strictEqual(QV4::ValueRef(d), QV4::ValueRef(other.d));
+    return RuntimeHelpers::strictEqual(*v, *ov);
 }
 
 /*!
@@ -876,12 +960,12 @@ bool QJSValue::strictlyEquals(const QJSValue& other) const
 */
 QJSValue QJSValue::property(const QString& name) const
 {
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->engine();
     if (!engine)
         return QJSValue();
-    QV4::Scope scope(engine);
 
-    ScopedObject o(scope, d->value);
+    QV4::Scope scope(engine);
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return QJSValue();
 
@@ -892,8 +976,8 @@ QJSValue QJSValue::property(const QString& name) const
 
     s->makeIdentifier();
     QV4::ScopedValue result(scope, o->get(s));
-    if (d->engine->hasException)
-        result = d->engine->catchException();
+    if (engine->hasException)
+        result = engine->catchException();
 
     return new QJSValuePrivate(engine, result);
 }
@@ -912,18 +996,18 @@ QJSValue QJSValue::property(const QString& name) const
 */
 QJSValue QJSValue::property(quint32 arrayIndex) const
 {
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->engine();
     if (!engine)
         return QJSValue();
 
     QV4::Scope scope(engine);
-    ScopedObject o(scope, d->value);
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return QJSValue();
 
     QV4::ScopedValue result(scope, arrayIndex == UINT_MAX ? o->get(engine->id_uintMax) : o->getIndexed(arrayIndex));
-    if (d->engine->hasException)
-        d->engine->catchException();
+    if (engine->hasException)
+        engine->catchException();
     return new QJSValuePrivate(engine, result);
 }
 
@@ -940,12 +1024,12 @@ QJSValue QJSValue::property(quint32 arrayIndex) const
 */
 void QJSValue::setProperty(const QString& name, const QJSValue& value)
 {
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->engine();
     if (!engine)
         return;
     Scope scope(engine);
 
-    ScopedObject o(scope, d->value);
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return;
 
@@ -964,8 +1048,8 @@ void QJSValue::setProperty(const QString& name, const QJSValue& value)
     s->makeIdentifier();
     QV4::ScopedValue v(scope, value.d->getValue(engine));
     o->put(s, v);
-    if (d->engine->hasException)
-        d->engine->catchException();
+    if (engine->hasException)
+        engine->catchException();
 }
 
 /*!
@@ -982,12 +1066,12 @@ void QJSValue::setProperty(const QString& name, const QJSValue& value)
 */
 void QJSValue::setProperty(quint32 arrayIndex, const QJSValue& value)
 {
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->engine();
     if (!engine)
         return;
     Scope scope(engine);
 
-    ScopedObject o(scope, d->value);
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return;
 
@@ -996,8 +1080,8 @@ void QJSValue::setProperty(quint32 arrayIndex, const QJSValue& value)
         o->putIndexed(arrayIndex, v);
     else
         o->put(engine->id_uintMax, v);
-    if (d->engine->hasException)
-        d->engine->catchException();
+    if (engine->hasException)
+        engine->catchException();
 }
 
 /*!
@@ -1022,16 +1106,16 @@ void QJSValue::setProperty(quint32 arrayIndex, const QJSValue& value)
 */
 bool QJSValue::deleteProperty(const QString &name)
 {
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->engine();
     Scope scope(engine);
-    ScopedObject o(scope, d->value.asObject());
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return false;
 
     ScopedString s(scope, engine->newString(name));
     bool b = o->deleteProperty(s);
-    if (d->engine->hasException)
-        d->engine->catchException();
+    if (engine->hasException)
+        engine->catchException();
     return b;
 }
 
@@ -1043,12 +1127,12 @@ bool QJSValue::deleteProperty(const QString &name)
 */
 bool QJSValue::hasProperty(const QString &name) const
 {
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->engine();
     if (!engine)
         return false;
 
     Scope scope(engine);
-    ScopedObject o(scope, d->value);
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return false;
 
@@ -1064,12 +1148,12 @@ bool QJSValue::hasProperty(const QString &name) const
 */
 bool QJSValue::hasOwnProperty(const QString &name) const
 {
-    ExecutionEngine *engine = d->engine;
+    ExecutionEngine *engine = d->engine();
     if (!engine)
         return false;
 
     Scope scope(engine);
-    ScopedObject o(scope, d->value);
+    ScopedObject o(scope, d->persistent.value());
     if (!o)
         return false;
 
@@ -1089,10 +1173,11 @@ bool QJSValue::hasOwnProperty(const QString &name) const
  */
 QObject *QJSValue::toQObject() const
 {
-    if (!d->engine)
+    ExecutionEngine *engine = d->engine();
+    if (!engine)
         return 0;
-    QV4::Scope scope(d->engine);
-    QV4::Scoped<QV4::QObjectWrapper> wrapper(scope, d->value);
+    QV4::Scope scope(engine);
+    QV4::Scoped<QV4::QObjectWrapper> wrapper(scope, d->persistent.value());
     if (!wrapper)
         return 0;
 
@@ -1108,10 +1193,12 @@ QObject *QJSValue::toQObject() const
 */
 QDateTime QJSValue::toDateTime() const
 {
-    QV4::DateObject *date = d->value.asDateObject();
-    if (!date)
-        return QDateTime();
-    return date->toQDateTime();
+    if (!d->persistent.isEmpty()) {
+        QV4::DateObject *date = d->persistent.valueRef()->asDateObject();
+        if (date)
+            return date->toQDateTime();
+    }
+    return QDateTime();
 }
 
 /*!
@@ -1120,7 +1207,7 @@ QDateTime QJSValue::toDateTime() const
 */
 bool QJSValue::isDate() const
 {
-    return d->value.asDateObject();
+    return !d->persistent.isEmpty() && d->persistent.valueRef()->asDateObject();
 }
 
 /*!
@@ -1129,7 +1216,7 @@ bool QJSValue::isDate() const
 */
 bool QJSValue::isRegExp() const
 {
-    return d->value.as<RegExpObject>();
+    return !d->persistent.isEmpty() && d->persistent.valueRef()->as<RegExpObject>();
 }
 
 /*!
@@ -1143,7 +1230,7 @@ bool QJSValue::isRegExp() const
 */
 bool QJSValue::isQObject() const
 {
-    return d->value.as<QV4::QObjectWrapper>() != 0;
+    return !d->persistent.isEmpty() && d->persistent.valueRef()->as<QV4::QObjectWrapper>() != 0;
 }
 
 QT_END_NAMESPACE
diff --git a/src/qml/jsapi/qjsvalue_p.h b/src/qml/jsapi/qjsvalue_p.h
index 43a3a74e38..3d7afdbd75 100644
--- a/src/qml/jsapi/qjsvalue_p.h
+++ b/src/qml/jsapi/qjsvalue_p.h
@@ -52,6 +52,8 @@
 #include <private/qv4engine_p.h>
 #include <private/qv4object_p.h>
 #include <private/qflagpointer_p.h>
+#include <private/qv4mm_p.h>
+#include <private/qv4persistent_p.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -59,30 +61,60 @@ QT_BEGIN_NAMESPACE
   \internal
   \class QJSValuePrivate
 */
-class Q_QML_PRIVATE_EXPORT QJSValuePrivate : public QV4::PersistentValuePrivate
+class Q_QML_PRIVATE_EXPORT QJSValuePrivate
 {
 public:
-    QJSValuePrivate(QV4::ExecutionEngine *engine, const QV4::ValueRef v)
-        : PersistentValuePrivate(v.asReturnedValue(), engine)
+    QJSValuePrivate()
+        : refcount(1)
+    {}
+    QJSValuePrivate(QV4::ExecutionEngine *e, const QV4::ValueRef v)
+        : refcount(1),
+          persistent(e, v)
     {
-        Q_ASSERT(!value.isEmpty());
+        Q_ASSERT(!v->isEmpty());
     }
-    QJSValuePrivate(QV4::ReturnedValue v)
-        : PersistentValuePrivate(v)
+    QJSValuePrivate(QV4::ExecutionEngine *e, QV4::ReturnedValue v)
+        : refcount(1),
+          persistent(e, v)
     {
-        Q_ASSERT(!value.isEmpty());
     }
-    QJSValuePrivate(const QString &s)
-        : PersistentValuePrivate(QV4::Primitive::emptyValue().asReturnedValue()),
-          unboundData(s)
+    explicit QJSValuePrivate(const QVariant &v)
+        : refcount(1),
+          unboundData(v)
     {
     }
 
+    bool checkEngine(QV4::ExecutionEngine *e) {
+        if (persistent.isEmpty())
+            getValue(e);
+        return persistent.engine() == e;
+    }
+
     QV4::ReturnedValue getValue(QV4::ExecutionEngine *e);
 
     static QJSValuePrivate *get(const QJSValue &v) { return v.d; }
+    QV4::ExecutionEngine *engine() const { return persistent.engine(); }
+
+    void ref() { ++refcount; }
+    void deref() {
+        if (!--refcount)
+            delete this;
+    }
+
+    QV4::Value *valueForData(QV4::Value *scratch) const {
+        QV4::Value *v = persistent.valueRef();
+        if (v)
+            return v;
+        *scratch = primitiveValueForUnboundData();
+        return (scratch->isEmpty() ? 0 : scratch);
+    }
+    QV4::Value primitiveValueForUnboundData() const;
 
+    int refcount;
+    QV4::PersistentValue persistent;
     QVariant unboundData;
+private:
+    QJSValuePrivate(QV4::ReturnedValue v) Q_DECL_EQ_DELETE;
 };
 
 QT_END_NAMESPACE
diff --git a/src/qml/jsapi/qjsvalueiterator.cpp b/src/qml/jsapi/qjsvalueiterator.cpp
index c74a5d2d76..e23bd5d763 100644
--- a/src/qml/jsapi/qjsvalueiterator.cpp
+++ b/src/qml/jsapi/qjsvalueiterator.cpp
@@ -46,12 +46,12 @@ QJSValueIteratorPrivate::QJSValueIteratorPrivate(const QJSValue &v)
     , nextIndex(UINT_MAX)
 {
     QJSValuePrivate *jsp = QJSValuePrivate::get(value);
-    QV4::ExecutionEngine *e = jsp->engine;
+    QV4::ExecutionEngine *e = jsp->persistent.engine();
     if (!e)
         return;
 
     QV4::Scope scope(e);
-    QV4::ScopedObject o(scope, jsp->value);
+    QV4::ScopedObject o(scope, jsp->persistent.value());
     iterator.set(e, e->newForEachIteratorObject(o));
 
     currentName = (QV4::String *)0;
@@ -123,7 +123,8 @@ QJSValueIterator::~QJSValueIterator()
 */
 bool QJSValueIterator::hasNext() const
 {
-    if (!QJSValuePrivate::get(d_ptr->value)->value.isObject())
+    QV4::Value *val = QJSValuePrivate::get(d_ptr->value)->persistent.valueRef();
+    if (!val || !val->isObject())
         return false;
     return !!d_ptr->nextName || d_ptr->nextIndex != UINT_MAX;
 }
@@ -138,7 +139,8 @@ bool QJSValueIterator::hasNext() const
 */
 bool QJSValueIterator::next()
 {
-    if (!QJSValuePrivate::get(d_ptr->value)->value.isObject())
+    QV4::Value *val = QJSValuePrivate::get(d_ptr->value)->persistent.valueRef();
+    if (!val || !val->isObject())
         return false;
     d_ptr->currentName = d_ptr->nextName;
     d_ptr->currentIndex = d_ptr->nextIndex;
@@ -164,7 +166,8 @@ bool QJSValueIterator::next()
 */
 QString QJSValueIterator::name() const
 {
-    if (!QJSValuePrivate::get(d_ptr->value)->value.isObject())
+    QV4::Value *val = QJSValuePrivate::get(d_ptr->value)->persistent.valueRef();
+    if (!val || !val->isObject())
         return QString();
     if (!!d_ptr->currentName)
         return d_ptr->currentName->toQString();
@@ -186,7 +189,7 @@ QJSValue QJSValueIterator::value() const
     if (!engine)
         return QJSValue();
     QV4::Scope scope(engine);
-    QV4::ScopedObject obj(scope, QJSValuePrivate::get(d_ptr->value)->value);
+    QV4::ScopedObject obj(scope, QJSValuePrivate::get(d_ptr->value)->persistent.value());
     if (!obj)
         return QJSValue();
 
@@ -216,13 +219,13 @@ QJSValueIterator& QJSValueIterator::operator=(QJSValue& object)
     d_ptr->nextName = (QV4::String *)0;
     QV4::ExecutionEngine *v4 = d_ptr->iterator.engine();
     if (!v4) {
-        d_ptr->iterator.set(v4, QV4::Encode::undefined());
+        d_ptr->iterator.clear();
         return *this;
     }
 
     QJSValuePrivate *jsp = QJSValuePrivate::get(object);
     QV4::Scope scope(v4);
-    QV4::ScopedObject o(scope, jsp->value);
+    QV4::ScopedObject o(scope, jsp->persistent.value());
     d_ptr->iterator.set(v4, v4->newForEachIteratorObject(o));
     QV4::Scoped<QV4::ForEachIteratorObject> it(scope, d_ptr->iterator.value());
     it->d()->it.flags =  QV4::ObjectIterator::NoFlags;