QPrivateSlotBase: add empty base class optimization

There are few slots whose lambdas are empty; most will at least
capture [this]. But there are a few in Qt examples that do, e.g. []{
qApp->quit(); }. Logging is also an example. So go the extra mile and
optimize for empty functors by inheriting from them as opposed to
storing them in a member variable.

Change-Id: I3904f10db5ebe904ba889d29c08569edd804df3b
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 30 insertions, 5 deletions

diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index 539818ac1d..c119fdfe7e 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -467,10 +467,35 @@ inline void QObjectPrivate::disconnectNotify(const QMetaMethod &signal)
 }
 
 namespace QtPrivate {
-template<typename Func, typename Args, typename R> class QPrivateSlotObject : public QSlotObjectBase
+
+template <typename Func>
+struct FunctionStorageByValue
+{
+    Func f;
+    Func &func() noexcept { return f; }
+};
+
+template <typename Func>
+struct FunctionStorageEmptyBaseClassOptimization : Func
+{
+    Func &func() noexcept { return *this; }
+    using Func::Func;
+};
+
+template <typename Func>
+using FunctionStorage = typename std::conditional_t<
+        std::conjunction_v<
+            std::is_empty<Func>,
+            std::negation<std::is_final<Func>>
+        >,
+        FunctionStorageEmptyBaseClassOptimization<Func>,
+        FunctionStorageByValue<Func>
+    >;
+
+template<typename Func, typename Args, typename R>
+class QPrivateSlotObject : public QSlotObjectBase, private FunctionStorage<Func>
 {
     typedef QtPrivate::FunctionPointer<Func> FuncType;
-    Func function;
     static void impl(int which, QSlotObjectBase *this_, QObject *r, void **a, bool *ret)
     {
         switch (which) {
@@ -478,17 +503,17 @@ template<typename Func, typename Args, typename R> class QPrivateSlotObject : pu
                 delete static_cast<QPrivateSlotObject*>(this_);
                 break;
             case Call:
-                FuncType::template call<Args, R>(static_cast<QPrivateSlotObject*>(this_)->function,
+                FuncType::template call<Args, R>(static_cast<QPrivateSlotObject*>(this_)->func(),
                                                  static_cast<typename FuncType::Object *>(QObjectPrivate::get(r)), a);
                 break;
             case Compare:
-                *ret = *reinterpret_cast<Func *>(a) == static_cast<QPrivateSlotObject*>(this_)->function;
+                *ret = *reinterpret_cast<Func *>(a) == static_cast<QPrivateSlotObject*>(this_)->func();
                 break;
             case NumOperations: ;
         }
     }
 public:
-    explicit QPrivateSlotObject(Func f) : QSlotObjectBase(&impl), function(f) {}
+    explicit QPrivateSlotObject(Func f) : QSlotObjectBase(&impl), FunctionStorage<Func>{std::move(f)} {}
 };
 } //namespace QtPrivate