Avoid needless construction and destruction of return values

In most cases the AOT compiled function will successfully placement-new
the return value. Therefore, we can provide uninitialized space. Only do
the construct/destruct dance in the cases when it's already slow.

Change-Id: Ia339774fde03e459f290f167ddadd1c47a644b8e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

4 files changed, 15 insertions, 14 deletions

diff --git a/src/qml/jsruntime/qv4jscall_p.h b/src/qml/jsruntime/qv4jscall_p.h
index de4acca8a3..3f0c8ee06d 100644
--- a/src/qml/jsruntime/qv4jscall_p.h
+++ b/src/qml/jsruntime/qv4jscall_p.h
@@ -184,7 +184,6 @@ ReturnedValue convertAndCall(
     types[0] = aotFunction->returnType;
     if (const qsizetype returnSize = types[0].sizeOf()) {
         Q_ALLOCA_ASSIGN(void, returnValue, returnSize);
-        types[0].construct(returnValue);
         values[0] = returnValue;
     } else {
         values[0] = nullptr;
@@ -227,15 +226,16 @@ void convertAndCall(ExecutionEngine *engine, const Value *thisObject,
     const QMetaType resultType = types[0];
     if (scope.hasException()) {
         // Clear the return value
-        resultType.destruct(result);
-        resultType.construct(result, nullptr);
+        resultType.construct(result);
     } else {
         // When the return type is QVariant, JS objects are to be returned as
         // QJSValue wrapped in QVariant. metaTypeFromJS unwraps them, unfortunately.
-        if (resultType == QMetaType::fromType<QVariant>())
-            *static_cast<QVariant *>(result) = scope.engine->toVariant(jsResult, QMetaType {});
-        else
+        if (resultType == QMetaType::fromType<QVariant>()) {
+            new (result) QVariant(scope.engine->toVariant(jsResult, QMetaType {}));
+        } else {
+            resultType.construct(result);
             scope.engine->metaTypeFromJS(jsResult, resultType, result);
+        }
     }
 }
 
diff --git a/src/qml/jsruntime/qv4vme_moth.cpp b/src/qml/jsruntime/qv4vme_moth.cpp
index eb18ea933b..9077529277 100644
--- a/src/qml/jsruntime/qv4vme_moth.cpp
+++ b/src/qml/jsruntime/qv4vme_moth.cpp
@@ -468,13 +468,9 @@ void VME::exec(MetaTypesStackFrame *frame, ExecutionEngine *engine)
 
     const QMetaType returnType = function->aotFunction->returnType;
     Q_ALLOCA_DECLARE(void, transformedResult);
-    if (frame->returnValue()) {
-        if (returnType == frame->returnType()) {
-            returnType.destruct(frame->returnValue());
-        } else {
-            Q_ASSERT(returnType.sizeOf() > 0);
-            Q_ALLOCA_ASSIGN(void, transformedResult, returnType.sizeOf());
-        }
+    if (frame->returnValue() && returnType != frame->returnType()) {
+        Q_ASSERT(returnType.sizeOf() > 0);
+        Q_ALLOCA_ASSIGN(void, transformedResult, returnType.sizeOf());
     }
 
     QQmlPrivate::AOTCompiledContext aotContext;
@@ -491,6 +487,8 @@ void VME::exec(MetaTypesStackFrame *frame, ExecutionEngine *engine)
                 transformedArguments ? transformedArguments : frame->argv());
 
     if (transformedResult) {
+        // Convert needs a pre-constructed target.
+        frame->returnType().construct(frame->returnValue());
         QMetaType::convert(returnType, transformedResult,
                            frame->returnType(), frame->returnValue());
         returnType.destruct(transformedResult);
diff --git a/src/qml/qml/qqmlbinding.cpp b/src/qml/qml/qqmlbinding.cpp
index b9d783736c..650482b3f0 100644
--- a/src/qml/qml/qqmlbinding.cpp
+++ b/src/qml/qml/qqmlbinding.cpp
@@ -255,7 +255,6 @@ protected:
             const auto size = returnType.sizeOf();
             if (Q_LIKELY(size > 0)) {
                 Q_ALLOCA_VAR(void, result, size);
-                returnType.construct(result);
                 const bool isUndefined = !evaluate(result, returnType);
                 if (canWrite())
                     error = !write(result, returnType, isUndefined, flags);
diff --git a/src/qml/qml/qqmlvmemetaobject.cpp b/src/qml/qml/qqmlvmemetaobject.cpp
index 7580d45104..7e6e4b1ef3 100644
--- a/src/qml/qml/qqmlvmemetaobject.cpp
+++ b/src/qml/qml/qqmlvmemetaobject.cpp
@@ -1002,10 +1002,14 @@ int QQmlVMEMetaObject::metaCall(QObject *o, QMetaObject::Call c, int _id, void *
                 if (arguments && arguments->names) {
                     const auto parameterCount = arguments->names->count();
                     Q_ASSERT(parameterCount == function->formalParameterCount());
+                    if (void *result = a[0])
+                        arguments->types[0].destruct(result);
                     function->call(v4->globalObject, a, arguments->types, parameterCount);
                 } else {
                     Q_ASSERT(function->formalParameterCount() == 0);
                     const QMetaType returnType = methodData->propType();
+                    if (void *result = a[0])
+                        returnType.destruct(result);
                     function->call(v4->globalObject, a, &returnType, 0);
                 }