Cleanup JS stack allocations

Avoid double writes to the stack, and use scope.alloc() for
most allocations on the stack.

Change-Id: I8b89273c1b6796d955fc8eeb72c67cff208ef786
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/qml/jsruntime/qv4context.cpp

@@ -110,7 +110,7 @@ Heap::CallContext *ExecutionContext::newCallContext(CppStackFrame *frame)
     uint nLocals = compiledFunction->nLocals;
     c->locals.size = nLocals;
     c->locals.alloc = localsAndFormals;
-    // memory allocated from the JS heap is 0 initialized, so check if undefined is 0
+    // memory allocated from the JS heap is 0 initialized, so check if empty is 0
     Q_ASSERT(Primitive::undefinedValue().asReturnedValue() == 0);
 
     Value *args = c->locals.values + nLocals;

src/qml/jsruntime/qv4engine.cpp

@@ -193,6 +193,7 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
     }
 
     exceptionValue = jsAlloca(1);
+    *exceptionValue = Encode::undefined();
     globalObject = static_cast<Object *>(jsAlloca(1));
     jsObjects = jsAlloca(NJSObjects);
     typedArrayPrototype = static_cast<Object *>(jsAlloca(NTypedArrayTypes));

src/qml/jsruntime/qv4engine_p.h

@@ -142,8 +142,6 @@ public:
     QML_NEARLY_ALWAYS_INLINE Value *jsAlloca(int nValues) {
         Value *ptr = jsStackTop;
         jsStackTop = ptr + nValues;
-        for (int i = 0; i < nValues; ++i)
-            ptr[i] = Primitive::undefinedValue();
         return ptr;
     }
 

src/qml/jsruntime/qv4functionobject.cpp

@@ -344,7 +344,7 @@ ReturnedValue FunctionPrototype::method_apply(const QV4::FunctionObject *b, cons
     uint len = arr->getLength();
 
     Scope scope(v4);
-    Value *arguments = v4->jsAlloca(len);
+    Value *arguments = scope.alloc(len);
     if (len) {
         if (ArgumentsObject::isNonStrictArgumentsObject(arr) && !arr->cast<ArgumentsObject>()->fullyCreated()) {
             QV4::ArgumentsObject *a = arr->cast<ArgumentsObject>();

src/qml/jsruntime/qv4jscall_p.h

@@ -67,7 +67,7 @@ struct JSCallData {
         if (thisObject)
             this->thisObject = const_cast<Value *>(thisObject);
         else
-            this->thisObject = scope.alloc(1);
+            this->thisObject = scope.alloc();
         if (argv)
             this->args = const_cast<Value *>(argv);
         else
@@ -80,8 +80,7 @@ struct JSCallData {
 
     CallData *callData(const FunctionObject *f = nullptr) const {
         int size = int(offsetof(QV4::CallData, args)/sizeof(QV4::Value)) + argc;
-        CallData *ptr = reinterpret_cast<CallData *>(scope.engine->jsStackTop);
+        CallData *ptr = reinterpret_cast<CallData *>(scope.alloc<Scope::Uninitialized>(size));
-        scope.engine->jsStackTop += size;
         ptr->function = Encode::undefined();
         ptr->context = Encode::undefined();
         ptr->accumulator = Encode::undefined();

src/qml/jsruntime/qv4objectiterator_p.h

@@ -92,8 +92,8 @@ struct Q_QML_EXPORT ObjectIterator: ObjectIteratorData
     ObjectIterator(Scope &scope, const Object *o, uint flags)
     {
         engine = scope.engine;
-        object = scope.alloc(1);
+        object = scope.alloc();
-        current = scope.alloc(1);
+        current = scope.alloc();
         arrayNode = nullptr;
         arrayIndex = 0;
         memberIndex = 0;

src/qml/jsruntime/qv4scopedvalue_p.h

@@ -117,8 +117,45 @@ struct Scope {
         engine->jsStackTop = mark;
     }
 
-    QML_NEARLY_ALWAYS_INLINE Value *alloc(int nValues) const {
+    enum AllocMode {
-        return engine->jsAlloca(nValues);
+        Undefined,
+        Empty,
+        /* Be careful when using Uninitialized, the stack has to be fully initialized before calling into the memory manager again */
+        Uninitialized
+    };
+    template <AllocMode mode = Undefined>
+    QML_NEARLY_ALWAYS_INLINE Value *alloc(int nValues) const
+    {
+        Value *ptr = engine->jsAlloca(nValues);
+        switch (mode) {
+        case Undefined:
+            for (int i = 0; i < nValues; ++i)
+                ptr[i] = Primitive::undefinedValue();
+            break;
+        case Empty:
+            for (int i = 0; i < nValues; ++i)
+                ptr[i] = Primitive::emptyValue();
+            break;
+        case Uninitialized:
+            break;
+        }
+        return ptr;
+    }
+    template <AllocMode mode = Undefined>
+    QML_NEARLY_ALWAYS_INLINE Value *alloc() const
+    {
+        Value *ptr = engine->jsAlloca(1);
+        switch (mode) {
+        case Undefined:
+            *ptr = Primitive::undefinedValue();
+            break;
+        case Empty:
+            *ptr = Primitive::emptyValue();
+            break;
+        case Uninitialized:
+            break;
+        }
+        return ptr;
     }
 
     bool hasException() const {
@@ -136,31 +173,31 @@ struct ScopedValue
 {
     ScopedValue(const Scope &scope)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.alloc<Scope::Uninitialized>();
         ptr->setRawValue(0);
     }
 
     ScopedValue(const Scope &scope, const Value &v)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.alloc<Scope::Uninitialized>();
         *ptr = v;
     }
 
     ScopedValue(const Scope &scope, Heap::Base *o)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.alloc<Scope::Uninitialized>();
         ptr->setM(o);
     }
 
     ScopedValue(const Scope &scope, Managed *m)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.alloc<Scope::Uninitialized>();
         ptr->setRawValue(m->asReturnedValue());
     }
 
     ScopedValue(const Scope &scope, const ReturnedValue &v)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.alloc<Scope::Uninitialized>();
         ptr->setRawValue(v);
     }
 
@@ -214,66 +251,66 @@ struct Scoped
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Undefined>();
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, const Value &v)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         setPointer(v.as<T>());
     }
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, Heap::Base *o)
     {
         Value v;
         v = o;
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         setPointer(v.as<T>());
     }
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, const ScopedValue &v)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         setPointer(v.ptr->as<T>());
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, const Value &v, ConvertType)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         ptr->setRawValue(value_convert<T>(scope.engine, v));
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, const Value *v)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         setPointer(v ? v->as<T>() : nullptr);
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, T *t)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         setPointer(t);
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, const T *t)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         setPointer(t);
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, typename T::Data *t)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         *ptr = t;
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, const ReturnedValue &v)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         setPointer(QV4::Value::fromReturnedValue(v).as<T>());
     }
 
     QML_NEARLY_ALWAYS_INLINE Scoped(const Scope &scope, const ReturnedValue &v, ConvertType)
     {
-        ptr = scope.engine->jsAlloca(1);
+        ptr = scope.alloc<Scope::Uninitialized>();
         ptr->setRawValue(value_convert<T>(scope.engine, QV4::Value::fromReturnedValue(v)));
     }
 

src/qml/qml/qqmlobjectcreator.cpp

@@ -252,7 +252,7 @@ bool QQmlObjectCreator::populateDeferredProperties(QObject *instance, QQmlData::
     Q_ASSERT(!sharedState->allJavaScriptObjects);
     sharedState->allJavaScriptObjects = valueScope.alloc(compilationUnit->totalObjectCount);
 
-    QV4::QmlContext *qmlContext = static_cast<QV4::QmlContext *>(valueScope.alloc(1));
+    QV4::QmlContext *qmlContext = static_cast<QV4::QmlContext *>(valueScope.alloc());
 
     qSwap(_qmlContext, qmlContext);
 
@@ -312,7 +312,7 @@ bool QQmlObjectCreator::populateDeferredBinding(const QQmlProperty &qmlProperty,
     if (!sharedState->allJavaScriptObjects)
         sharedState->allJavaScriptObjects = valueScope.alloc(compilationUnit->totalObjectCount);
 
-    QV4::QmlContext *qmlContext = static_cast<QV4::QmlContext *>(valueScope.alloc(1));
+    QV4::QmlContext *qmlContext = static_cast<QV4::QmlContext *>(valueScope.alloc());
 
     qSwap(_qmlContext, qmlContext);
 
@@ -1292,7 +1292,7 @@ QObject *QQmlObjectCreator::createInstance(int index, QObject *parent, bool isCo
     ++sharedState->allJavaScriptObjects;
 
     QV4::Scope valueScope(v4);
-    QV4::QmlContext *qmlContext = static_cast<QV4::QmlContext *>(valueScope.alloc(1));
+    QV4::QmlContext *qmlContext = static_cast<QV4::QmlContext *>(valueScope.alloc());
 
     qSwap(_qmlContext, qmlContext);