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>

4 files changed, 219 insertions, 54 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))
diff --git a/src/corelib/kernel/qproperty.h b/src/corelib/kernel/qproperty.h
index 8b400e7285..d612486e2c 100644
--- a/src/corelib/kernel/qproperty.h
+++ b/src/corelib/kernel/qproperty.h
@@ -141,10 +141,16 @@ class Q_CORE_EXPORT QUntypedPropertyBinding
 {
 public:
     // writes binding result into dataPtr
-    using BindingEvaluationFunction = QtPrivate::QPropertyBindingFunction;
+    using BindingFunctionVTable = QtPrivate::BindingFunctionVTable;
 
     QUntypedPropertyBinding();
-    QUntypedPropertyBinding(QMetaType metaType, BindingEvaluationFunction function, const QPropertyBindingSourceLocation &location);
+    QUntypedPropertyBinding(QMetaType metaType, const BindingFunctionVTable *vtable, void *function, const QPropertyBindingSourceLocation &location);
+
+    template<typename Functor>
+    QUntypedPropertyBinding(QMetaType metaType, Functor &&f, const QPropertyBindingSourceLocation &location)
+        : QUntypedPropertyBinding(metaType, &QtPrivate::bindingFunctionVTable<std::remove_reference_t<Functor>>, &f, location)
+    {}
+
     QUntypedPropertyBinding(QUntypedPropertyBinding &&other);
     QUntypedPropertyBinding(const QUntypedPropertyBinding &other);
     QUntypedPropertyBinding &operator=(const QUntypedPropertyBinding &other);
@@ -168,29 +174,13 @@ private:
 template <typename PropertyType>
 class QPropertyBinding : public QUntypedPropertyBinding
 {
-    template <typename Functor>
-    struct BindingAdaptor
-    {
-        Functor impl;
-        bool operator()(QMetaType /*metaType*/, QUntypedPropertyData *dataPtr)
-        {
-            QPropertyData<PropertyType> *propertyPtr = static_cast<QPropertyData<PropertyType> *>(dataPtr);
-            PropertyType newValue = impl();
-            if constexpr (QTypeTraits::has_operator_equal_v<PropertyType>) {
-                if (newValue == propertyPtr->valueBypassingBindings())
-                    return false;
-            }
-            propertyPtr->setValueBypassingBindings(std::move(newValue));
-            return true;
-        }
-    };
 
 public:
     QPropertyBinding() = default;
 
     template<typename Functor>
     QPropertyBinding(Functor &&f, const QPropertyBindingSourceLocation &location)
-        : QUntypedPropertyBinding(QMetaType::fromType<PropertyType>(), BindingAdaptor<Functor>{std::forward<Functor>(f)}, location)
+        : QUntypedPropertyBinding(QMetaType::fromType<PropertyType>(), &QtPrivate::bindingFunctionVTable<std::remove_reference_t<Functor>, PropertyType>, &f, location)
     {}
 
     template<typename Property, typename = typename Property::InheritsQUntypedPropertyData>
diff --git a/src/corelib/kernel/qproperty_p.h b/src/corelib/kernel/qproperty_p.h
index fdb5f14ccd..99a145c65a 100644
--- a/src/corelib/kernel/qproperty_p.h
+++ b/src/corelib/kernel/qproperty_p.h
@@ -54,11 +54,9 @@
 #include <qglobal.h>
 #include <qproperty.h>
 
-#include <qvarlengtharray.h>
 #include <qscopedpointer.h>
 #include <vector>
 
-
 QT_BEGIN_NAMESPACE
 
 // Keep all classes related to QProperty in one compilation unit. Performance of this code is crucial and
@@ -156,14 +154,15 @@ struct QBindingStatus
     QtPrivate::CurrentCompatProperty *currentCompatProperty = nullptr;
 };
 
-class Q_CORE_EXPORT QPropertyBindingPrivate : public QSharedData
+class Q_CORE_EXPORT QPropertyBindingPrivate : public QtPrivate::RefCounted
 {
 private:
     friend struct QPropertyBindingDataPointer;
+    friend class QPropertyBindingPrivatePtr;
 
     using ObserverArray = std::array<QPropertyObserver, 4>;
 
-    // QSharedData is 4 bytes. Use the padding for the bools as we need 8 byte alignment below.
+private:
 
     // a dependent property has changed, and the binding needs to be reevaluated on access
     bool dirty = false;
@@ -174,7 +173,7 @@ private:
     // used to detect binding loops for eagerly evaluated properties
     bool eagerlyUpdating:1;
 
-    QUntypedPropertyBinding::BindingEvaluationFunction evaluationFunction;
+    const QtPrivate::BindingFunctionVTable *vtable;
 
     union {
         QtPrivate::QPropertyObserverCallback staticObserverCallback = nullptr;
@@ -193,19 +192,29 @@ private:
     QMetaType metaType;
 
 public:
+    static constexpr size_t getSizeEnsuringAlignment() {
+        constexpr auto align = alignof (std::max_align_t) - 1;
+        constexpr size_t sizeEnsuringAlignment = (sizeof(QPropertyBindingPrivate) + align) & ~align;
+        static_assert (sizeEnsuringAlignment % alignof (std::max_align_t) == 0,
+                   "Required for placement new'ing the function behind it.");
+        return sizeEnsuringAlignment;
+    }
+
+
     // public because the auto-tests access it, too.
     size_t dependencyObserverCount = 0;
 
-    QPropertyBindingPrivate(QMetaType metaType, QUntypedPropertyBinding::BindingEvaluationFunction evaluationFunction,
+    QPropertyBindingPrivate(QMetaType metaType, const QtPrivate::BindingFunctionVTable *vtable,
                             const QPropertyBindingSourceLocation &location)
         : hasBindingWrapper(false)
         , eagerlyUpdating(false)
-        , evaluationFunction(std::move(evaluationFunction))
+        , vtable(vtable)
         , inlineDependencyObservers() // Explicit initialization required because of union
         , location(location)
         , metaType(metaType)
     {}
-    virtual ~QPropertyBindingPrivate();
+    ~QPropertyBindingPrivate();
+
 
     void setDirty(bool d) { dirty = d; }
     void setProperty(QUntypedPropertyData *propertyPtr) { propertyDataPtr = propertyPtr; }
@@ -277,7 +286,7 @@ public:
     bool evaluateIfDirtyAndReturnTrueIfValueChanged(const QUntypedPropertyData *data);
 
     static QPropertyBindingPrivate *get(const QUntypedPropertyBinding &binding)
-    { return binding.d.data(); }
+    { return static_cast<QPropertyBindingPrivate *>(binding.d.data()); }
 
     void setError(QPropertyBindingError &&e)
     { error = std::move(e); }
@@ -293,6 +302,17 @@ public:
     bool requiresEagerEvaluation() const { return hasBindingWrapper; }
 
     static QPropertyBindingPrivate *currentlyEvaluatingBinding();
+
+    static void destroyAndFreeMemory(QPropertyBindingPrivate *priv) {
+        if (priv->vtable->size == 0) {
+            // special hack for QQmlPropertyBinding which has a
+            // different memory layout than normal QPropertyBindings
+            priv->vtable->destroy(priv);
+        } else{
+            priv->~QPropertyBindingPrivate();
+            delete[] reinterpret_cast<std::byte *>(priv);
+        }
+    }
 };
 
 inline void QPropertyBindingDataPointer::setFirstObserver(QPropertyObserver *observer)
@@ -337,11 +357,11 @@ class QObjectCompatProperty : public QPropertyData<T>
         char *that = const_cast<char *>(reinterpret_cast<const char *>(this));
         return reinterpret_cast<Class *>(that - QtPrivate::detail::getOffset(Offset));
     }
-    static bool bindingWrapper(QMetaType type, QUntypedPropertyData *dataPtr, const QtPrivate::QPropertyBindingFunction &binding)
+    static bool bindingWrapper(QMetaType type, QUntypedPropertyData *dataPtr, QtPrivate::QPropertyBindingFunction binding)
     {
         auto *thisData = static_cast<ThisType *>(dataPtr);
         QPropertyData<T> copy;
-        binding(type, &copy);
+        binding.vtable->call(type, &copy, binding.functor);
         if constexpr (QTypeTraits::has_operator_equal_v<T>)
             if (copy.valueBypassingBindings() == thisData->valueBypassingBindings())
                 return false;
diff --git a/src/corelib/kernel/qpropertyprivate.h b/src/corelib/kernel/qpropertyprivate.h
index a0dfbd97c9..df12c1fdb3 100644
--- a/src/corelib/kernel/qpropertyprivate.h
+++ b/src/corelib/kernel/qpropertyprivate.h
@@ -60,9 +60,87 @@
 
 QT_BEGIN_NAMESPACE
 
+namespace QtPrivate {
+// QPropertyBindingPrivatePtr operates on a RefCountingMixin solely so that we can inline
+// the constructor and copy constructor
+struct RefCounted {
+    int ref = 0;
+    void addRef() {++ref;}
+    bool deref() {--ref; return ref;}
+};
+}
+
+class QPropertyBindingPrivate;
+class QPropertyBindingPrivatePtr
+{
+public:
+    using T = QtPrivate::RefCounted;
+    T &operator*() const { return *d; }
+    T *operator->() noexcept { return d; }
+    T *operator->() const noexcept { return d; }
+    explicit operator T *() { return d; }
+    explicit operator const T *() const noexcept { return d; }
+    T *data() const noexcept { return d; }
+    T *get() const noexcept { return d; }
+    const T *constData() const noexcept { return d; }
+    T *take() noexcept { T *x = d; d = nullptr; return x; }
+
+    QPropertyBindingPrivatePtr() noexcept : d(nullptr) { }
+    Q_CORE_EXPORT ~QPropertyBindingPrivatePtr();
+
+    explicit QPropertyBindingPrivatePtr(T *data) noexcept : d(data) { if (d) d->addRef(); }
+    QPropertyBindingPrivatePtr(const QPropertyBindingPrivatePtr &o) noexcept
+        : d(o.d) { if (d) d->addRef(); }
+
+    void reset(T *ptr = nullptr) noexcept;
+
+    QPropertyBindingPrivatePtr &operator=(const QPropertyBindingPrivatePtr &o) noexcept
+    {
+        reset(o.d);
+        return *this;
+    }
+    QPropertyBindingPrivatePtr &operator=(T *o) noexcept
+    {
+        reset(o);
+        return *this;
+    }
+    QPropertyBindingPrivatePtr(QPropertyBindingPrivatePtr &&o) noexcept : d(qExchange(o.d, nullptr)) {}
+    QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_MOVE_AND_SWAP(QPropertyBindingPrivatePtr)
+
+    operator bool () const noexcept { return d != nullptr; }
+    bool operator!() const noexcept { return d == nullptr; }
+
+    void swap(QPropertyBindingPrivatePtr &other) noexcept
+    { qSwap(d, other.d); }
+
+    friend bool operator==(const QPropertyBindingPrivatePtr &p1, const QPropertyBindingPrivatePtr &p2) noexcept
+    { return p1.d == p2.d; }
+    friend bool operator!=(const QPropertyBindingPrivatePtr &p1, const QPropertyBindingPrivatePtr &p2) noexcept
+    { return p1.d != p2.d; }
+    friend bool operator==(const QPropertyBindingPrivatePtr &p1, const T *ptr) noexcept
+    { return p1.d == ptr; }
+    friend bool operator!=(const QPropertyBindingPrivatePtr &p1, const T *ptr) noexcept
+    { return p1.d != ptr; }
+    friend bool operator==(const T *ptr, const QPropertyBindingPrivatePtr &p2) noexcept
+    { return ptr == p2.d; }
+    friend bool operator!=(const T *ptr, const QPropertyBindingPrivatePtr &p2) noexcept
+    { return ptr != p2.d; }
+    friend bool operator==(const QPropertyBindingPrivatePtr &p1, std::nullptr_t) noexcept
+    { return !p1; }
+    friend bool operator!=(const QPropertyBindingPrivatePtr &p1, std::nullptr_t) noexcept
+    { return p1; }
+    friend bool operator==(std::nullptr_t, const QPropertyBindingPrivatePtr &p2) noexcept
+    { return !p2; }
+    friend bool operator!=(std::nullptr_t, const QPropertyBindingPrivatePtr &p2) noexcept
+    { return p2; }
+
+private:
+    QtPrivate::RefCounted *d;
+};
+
+
 class QUntypedPropertyBinding;
 class QPropertyBindingPrivate;
-using QPropertyBindingPrivatePtr = QExplicitlySharedDataPointer<QPropertyBindingPrivate>;
 struct QPropertyBindingDataPointer;
 
 class QUntypedPropertyData
@@ -72,12 +150,66 @@ public:
     struct InheritsQUntypedPropertyData {};
 };
 
+template <typename T>
+class QPropertyData;
+
 namespace QtPrivate {
 
+struct BindingFunctionVTable
+{
+    using CallFn = bool(*)(QMetaType, QUntypedPropertyData *, void *);
+    using DtorFn = void(*)(void *);
+    using MoveCtrFn = void(*)(void *, void *);
+    const CallFn call;
+    const DtorFn destroy;
+    const MoveCtrFn moveConstruct;
+    const qsizetype size;
+
+    template<typename Callable, typename PropertyType=void>
+    static constexpr BindingFunctionVTable createFor()
+    {
+        static_assert (alignof(Callable) <= alignof(std::max_align_t), "Bindings do not support overaligned functors!");
+        return {
+            /*call=*/[](QMetaType metaType, QUntypedPropertyData *dataPtr, void *f){
+                if constexpr (!std::is_invocable_v<Callable>) {
+                    // we got an untyped callable
+                    static_assert (std::is_invocable_r_v<bool, Callable, QMetaType, QUntypedPropertyData *> );
+                    auto untypedEvaluationFunction = static_cast<Callable *>(f);
+                    return std::invoke(*untypedEvaluationFunction, metaType, dataPtr);
+                } else {
+                    QPropertyData<PropertyType> *propertyPtr = static_cast<QPropertyData<PropertyType> *>(dataPtr);
+                    // That is allowed by POSIX even if Callable is a function pointer
+                    auto evaluationFunction = static_cast<Callable *>(f);
+                    PropertyType newValue = std::invoke(*evaluationFunction);
+                    if constexpr (QTypeTraits::has_operator_equal_v<PropertyType>) {
+                        if (newValue == propertyPtr->valueBypassingBindings())
+                            return false;
+                    }
+                    propertyPtr->setValueBypassingBindings(std::move(newValue));
+                    return true;
+                }
+            },
+            /*destroy*/[](void *f){ static_cast<Callable *>(f)->~Callable(); },
+            /*moveConstruct*/[](void *addr, void *other){
+                new (addr) Callable(std::move(*static_cast<Callable *>(other)));
+            },
+            /*size*/sizeof(Callable)
+        };
+    }
+};
+
+template<typename Callable, typename PropertyType=void>
+inline constexpr BindingFunctionVTable bindingFunctionVTable = BindingFunctionVTable::createFor<Callable, PropertyType>();
+
+
 // writes binding result into dataPtr
-using QPropertyBindingFunction = std::function<bool(QMetaType metaType, QUntypedPropertyData *dataPtr)>;
+struct QPropertyBindingFunction {
+    const QtPrivate::BindingFunctionVTable *vtable;
+    void *functor;
+};
+
 using QPropertyObserverCallback = void (*)(QUntypedPropertyData *);
-using QPropertyBindingWrapper = bool(*)(QMetaType, QUntypedPropertyData *dataPtr, const QPropertyBindingFunction &);
+using QPropertyBindingWrapper = bool(*)(QMetaType, QUntypedPropertyData *dataPtr, QPropertyBindingFunction);
 
 class Q_CORE_EXPORT QPropertyBindingData
 {