Remove std::function from QProperty interface

std::function as a type is rather unfortunate for us, as its SSO buffer
makes it rather large, and we can ensure that the function is never
empty.
Considering that we do need to allocate memory for
QPropertyBindingPrivate anyway, we can get rid of the SSO buffer and
instead coalesce the allocations (similar to how std::make_shared works).
The memory looks then like
[--QPropertyBindingPrivate--][Functor]
and QPropertyBindingPrivate can get a pointer to the functor via
reinterpret_cast<std::byte>(this)+sizeof(QPropertyBindingPrivate).
To actually do anything with the functor, we do however need a "vtable"
which describes how we can call, destroy and move the functor. This is
done by creating a constexpr struct of function pointers, and storing a
pointer to it in QPropertyBindingPrivate.

As a consequence of those changes, we cannot use QESDP anymore, as we
now have to carefully deallocate the buffer we used for both the
QPropertyBindingPrivate and the functor. We introduce a custom
refcounting pointer for that. While we're at it, we make the refcount
non-atomic, as bindings do not work across threads to begin with.

Moreover, we can now make the class non-virtual, as that was only needed
to hack around limitations of QESDP in the context of exported symbols.

Change-Id: Idc5507e4c120e28df5bd5aea717fe69f15e540dc
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 44 insertions, 21 deletions

diff --git a/src/corelib/kernel/qproperty.cpp b/src/corelib/kernel/qproperty.cpp
index b55069d1ae..103c3d8482 100644
--- a/src/corelib/kernel/qproperty.cpp
+++ b/src/corelib/kernel/qproperty.cpp
@@ -47,6 +47,24 @@ QT_BEGIN_NAMESPACE
 
 using namespace QtPrivate;
 
+QPropertyBindingPrivatePtr::~QPropertyBindingPrivatePtr()
+{
+    if (d && (--d->ref == 0))
+        QPropertyBindingPrivate::destroyAndFreeMemory(static_cast<QPropertyBindingPrivate *>(d));
+}
+
+void QPropertyBindingPrivatePtr::reset(QtPrivate::RefCounted *ptr) noexcept
+{
+    if (ptr != d) {
+        if (ptr)
+            ptr->ref++;
+        auto *old = qExchange(d, ptr);
+        if (old && (--old->ref == 0))
+            QPropertyBindingPrivate::destroyAndFreeMemory(static_cast<QPropertyBindingPrivate *>(d));
+    }
+}
+
+
 void QPropertyBindingDataPointer::addObserver(QPropertyObserver *observer)
 {
     if (auto *binding = bindingPtr()) {
@@ -69,13 +87,15 @@ QPropertyBindingPrivate::~QPropertyBindingPrivate()
 {
     if (firstObserver)
         firstObserver.unlink();
+    if (vtable->size)
+        vtable->destroy(reinterpret_cast<std::byte *>(this) + sizeof(QPropertyBindingPrivate));
 }
 
 void QPropertyBindingPrivate::unlinkAndDeref()
 {
     propertyDataPtr = nullptr;
-    if (!ref.deref())
-        delete this;
+    if (--ref == 0)
+        destroyAndFreeMemory(this);
 }
 
 void QPropertyBindingPrivate::markDirtyAndNotifyObservers()
@@ -129,9 +149,9 @@ bool QPropertyBindingPrivate::evaluateIfDirtyAndReturnTrueIfValueChanged(const Q
     QUntypedPropertyData *mutable_data = const_cast<QUntypedPropertyData *>(data);
 
     if (hasBindingWrapper) {
-        changed = staticBindingWrapper(metaType, mutable_data, evaluationFunction);
+        changed = staticBindingWrapper(metaType, mutable_data, {vtable, reinterpret_cast<std::byte *>(this)+QPropertyBindingPrivate::getSizeEnsuringAlignment()});
     } else {
-        changed = evaluationFunction(metaType, mutable_data);
+        changed = vtable->call(metaType, mutable_data, reinterpret_cast<std::byte *>(this)+ QPropertyBindingPrivate::getSizeEnsuringAlignment());
     }
 
     dirty = false;
@@ -140,10 +160,12 @@ bool QPropertyBindingPrivate::evaluateIfDirtyAndReturnTrueIfValueChanged(const Q
 
 QUntypedPropertyBinding::QUntypedPropertyBinding() = default;
 
-QUntypedPropertyBinding::QUntypedPropertyBinding(QMetaType metaType, QUntypedPropertyBinding::BindingEvaluationFunction function,
+QUntypedPropertyBinding::QUntypedPropertyBinding(QMetaType metaType, const BindingFunctionVTable *vtable, void *function,
                                                  const QPropertyBindingSourceLocation &location)
-    : d(new QPropertyBindingPrivate(metaType, std::move(function), std::move(location)))
 {
+    std::byte *mem = new std::byte[QPropertyBindingPrivate::getSizeEnsuringAlignment() + vtable->size]();
+    d = new(mem) QPropertyBindingPrivate(metaType, vtable, std::move(location));
+    vtable->moveConstruct(mem+sizeof(QPropertyBindingPrivate), function);
 }
 
 QUntypedPropertyBinding::QUntypedPropertyBinding(QUntypedPropertyBinding &&other)
@@ -186,14 +208,14 @@ QPropertyBindingError QUntypedPropertyBinding::error() const
 {
     if (!d)
         return QPropertyBindingError();
-    return d->bindingError();
+    return static_cast<QPropertyBindingPrivate *>(d.get())->bindingError();
 }
 
 QMetaType QUntypedPropertyBinding::valueMetaType() const
 {
     if (!d)
         return QMetaType();
-    return d->valueMetaType();
+    return static_cast<QPropertyBindingPrivate *>(d.get())->valueMetaType();
 }
 
 QPropertyBindingData::~QPropertyBindingData()
@@ -221,28 +243,29 @@ QUntypedPropertyBinding QPropertyBindingData::setBinding(const QUntypedPropertyB
 
     if (auto *existingBinding = d.bindingPtr()) {
         if (existingBinding == newBinding.data())
-            return QUntypedPropertyBinding(oldBinding.data());
+            return QUntypedPropertyBinding(static_cast<QPropertyBindingPrivate *>(oldBinding.data()));
         oldBinding = QPropertyBindingPrivatePtr(existingBinding);
-        observer = oldBinding->takeObservers();
-        oldBinding->unlinkAndDeref();
+        observer = static_cast<QPropertyBindingPrivate *>(oldBinding.data())->takeObservers();
+        static_cast<QPropertyBindingPrivate *>(oldBinding.data())->unlinkAndDeref();
         d_ptr &= FlagMask;
     } else {
         observer = d.firstObserver();
     }
 
     if (newBinding) {
-        newBinding.data()->ref.ref();
+        newBinding.data()->addRef();
         d_ptr = (d_ptr & FlagMask) | reinterpret_cast<quintptr>(newBinding.data());
         d_ptr |= BindingBit;
-        newBinding->setDirty(true);
-        newBinding->setProperty(propertyDataPtr);
+        auto newBindingRaw = static_cast<QPropertyBindingPrivate *>(newBinding.data());
+        newBindingRaw->setDirty(true);
+        newBindingRaw->setProperty(propertyDataPtr);
         if (observer)
-            newBinding->prependObserver(observer);
-        newBinding->setStaticObserver(staticObserverCallback, guardCallback);
-        if (newBinding->requiresEagerEvaluation()) {
-            auto changed = newBinding->evaluateIfDirtyAndReturnTrueIfValueChanged(propertyDataPtr);
+            newBindingRaw->prependObserver(observer);
+        newBindingRaw->setStaticObserver(staticObserverCallback, guardCallback);
+        if (newBindingRaw->requiresEagerEvaluation()) {
+            auto changed = newBindingRaw->evaluateIfDirtyAndReturnTrueIfValueChanged(propertyDataPtr);
             if (changed)
-                observer.notify(newBinding.data(), propertyDataPtr, /*alreadyKnownToHaveChanged=*/true);
+                observer.notify(newBindingRaw, propertyDataPtr, /*alreadyKnownToHaveChanged=*/true);
         }
     } else if (observer) {
         d.setObservers(observer.ptr);
@@ -251,9 +274,9 @@ QUntypedPropertyBinding QPropertyBindingData::setBinding(const QUntypedPropertyB
     }
 
     if (oldBinding)
-        oldBinding->detachFromProperty();
+        static_cast<QPropertyBindingPrivate *>(oldBinding.data())->detachFromProperty();
 
-    return QUntypedPropertyBinding(oldBinding.data());
+    return QUntypedPropertyBinding(static_cast<QPropertyBindingPrivate *>(oldBinding.data()));
 }
 
 QPropertyBindingData::QPropertyBindingData(QPropertyBindingData &&other) : d_ptr(std::exchange(other.d_ptr, 0))