Allow AOT functions to signal an undefined result via the context

undefined as value returned from bindings has the special meaning of
resetting the binding. As AOT-compiled functions return the actual type
of the binding rather than a QV4::Value, we cannot always encode
undefined. Therefore, add a flag that tells us whether the result was
supposed to be undefined.

Pick-to: 6.2
Change-Id: Iac2298869dde80f6d889240dd8200b2ad83e5dc5
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

5 files changed, 17 insertions, 2 deletions

diff --git a/src/qml/jsruntime/qv4function.cpp b/src/qml/jsruntime/qv4function.cpp
index 7b5685a2a0..95a7506c80 100644
--- a/src/qml/jsruntime/qv4function.cpp
+++ b/src/qml/jsruntime/qv4function.cpp
@@ -74,7 +74,7 @@ bool Function::call(QObject *thisObject, void **a, const QMetaType *types, int a
     frame.push(engine);
     Moth::VME::exec(&frame, engine);
     frame.pop(engine);
-    return true;
+    return !frame.isReturnValueUndefined();
 }
 
 ReturnedValue Function::call(
diff --git a/src/qml/jsruntime/qv4stackframe_p.h b/src/qml/jsruntime/qv4stackframe_p.h
index 1439e9f677..9a121b48cd 100644
--- a/src/qml/jsruntime/qv4stackframe_p.h
+++ b/src/qml/jsruntime/qv4stackframe_p.h
@@ -90,6 +90,7 @@ struct Q_QML_PRIVATE_EXPORT CppStackFrameBase
             QObject *thisObject;
             const QMetaType *metaTypes;
             void **returnAndArgs;
+            bool returnValueIsUndefined;
         };
     };
 
@@ -163,11 +164,15 @@ struct Q_QML_PRIVATE_EXPORT MetaTypesStackFrame : public CppStackFrame
         CppStackFrameBase::context = context;
         CppStackFrameBase::metaTypes = metaTypes;
         CppStackFrameBase::returnAndArgs = returnAndArgs;
+        CppStackFrameBase::returnValueIsUndefined = false;
     }
 
     QMetaType returnType() const { return metaTypes[0]; }
     void *returnValue() const { return returnAndArgs[0]; }
 
+    bool isReturnValueUndefined() const { return CppStackFrameBase::returnValueIsUndefined; }
+    void setReturnValueUndefined() { CppStackFrameBase::returnValueIsUndefined = true; }
+
     const QMetaType *argTypes() const { return metaTypes + 1; }
     void **argv() const { return returnAndArgs + 1; }
 
diff --git a/src/qml/jsruntime/qv4vme_moth.cpp b/src/qml/jsruntime/qv4vme_moth.cpp
index 009cff067a..94860f0521 100644
--- a/src/qml/jsruntime/qv4vme_moth.cpp
+++ b/src/qml/jsruntime/qv4vme_moth.cpp
@@ -432,8 +432,10 @@ static bool compareEqualInt(QV4::Value &accumulator, QV4::Value lhs, int rhs)
 void VME::exec(MetaTypesStackFrame *frame, ExecutionEngine *engine)
 {
     qt_v4ResolvePendingBreakpointsHook();
-    if (engine->checkStackLimits())
+    if (engine->checkStackLimits()) {
+        frame->setReturnValueUndefined();
         return;
+    }
     ExecutionEngineCallDepthRecorder executionEngineCallDepthRecorder(engine);
 
     Function *function = frame->v4Function;
diff --git a/src/qml/qml/qqml.cpp b/src/qml/qml/qqml.cpp
index 2144893c2d..3b0000ab02 100644
--- a/src/qml/qml/qqml.cpp
+++ b/src/qml/qml/qqml.cpp
@@ -712,6 +712,13 @@ void AOTCompiledContext::setInstructionPointer(int offset) const
         frame->instructionPointer = offset;
 }
 
+void AOTCompiledContext::setReturnValueUndefined() const
+{
+    if (auto *frame = engine->handle()->currentStackFrame) {
+        Q_ASSERT(frame->isMetaTypesFrame());
+        static_cast<QV4::MetaTypesStackFrame *>(frame)->setReturnValueUndefined();
+    }
+}
 
 static void captureObjectProperty(
         QObject *object, const QQmlPropertyCache *propertyCache,
diff --git a/src/qml/qml/qqmlprivate.h b/src/qml/qml/qqmlprivate.h
index 71855df966..11c948445f 100644
--- a/src/qml/qml/qqmlprivate.h
+++ b/src/qml/qml/qqmlprivate.h
@@ -612,6 +612,7 @@ namespace QQmlPrivate
 
         QJSValue jsMetaType(int index) const;
         void setInstructionPointer(int offset) const;
+        void setReturnValueUndefined() const;
 
         // Run QQmlPropertyCapture::captureProperty() without retrieving the value.
         bool captureLookup(uint index, QObject *object) const;