From c61d9873e5e30723bc5558b509b3320f2abf1da7 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@kdab.com>
Date: Wed, 26 Jun 2019 23:35:12 +0200
Subject: [PATCH] QPrivateSlotBase: add empty base class optimization
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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>
---
 src/corelib/kernel/qobject_p.h                | 35 ++++++++++++++++---
 .../corelib/kernel/qobject/tst_qobject.cpp    |  2 +-
 2 files changed, 31 insertions(+), 6 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
 
diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index 3d52478299..599f690ab2 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
@@ -5221,7 +5221,7 @@ namespace ManyArgumentNamespace {
         }
     };
 
-    struct Funct6 {
+    struct Funct6 final {
         void operator()(const QString &a, const QString &b, const QString &c, const QString&d, const QString&e, const QString&f) {
             MANYARGUMENT_COMPARE(a); MANYARGUMENT_COMPARE(b); MANYARGUMENT_COMPARE(c);
             MANYARGUMENT_COMPARE(d); MANYARGUMENT_COMPARE(e); MANYARGUMENT_COMPARE(f);