Refactor memory allocation for arguments of QMetaCallEvents

There are two cases:

In a BlockingQueuedConnection, QMetaCallEvent doesn't allocate memory
and instead passes already existing pointers through. A QSemaphore
is used to serialize data access between threads. So the constructor
taking a QSemaphore can be simplified to only accept an existing arg
array.

In a QueuedConnection, QMetaCallEvent needs to make deep copies of
the arguments, and memory needs to be allocated based on the number
of arguments. The previous code put the burden of memory allocation
on the code generating the event, while the memory was free'd by
~QMetaCallEvent. Instead, make it QMetaCallEvent's responsibility
to allocate and free the memory as needed, and adjust the code
generating QMetaCallEvents.

We can allocate the memory for types and pointers to arguments in a
single block, starting with the space for the array of void*, followed
by the space for the array of integers to avoid byte alignment issues.
By pre-allocating the space that's needed by three arguments, we can
avoid all mallocs for the majority of QMetaCallEvents.

Until this change has propagated through qt5.git, we need to keep the
old API that is still used by QtDeclarative around. Once QtDeclarative
has migrated to the new API, it can be removed.

Change-Id: Id7359ffc14897237ea9672dabae9ef199a821907
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 40 insertions, 16 deletions

diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index 95ffc1b2e8..5d0b3a8399 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -509,29 +509,53 @@ private:
 class Q_CORE_EXPORT QMetaCallEvent : public QAbstractMetaCallEvent
 {
 public:
-    QMetaCallEvent(ushort method_offset, ushort method_relative, QObjectPrivate::StaticMetaCallFunction callFunction , const QObject *sender, int signalId,
-                   int nargs = 0, int *types = nullptr, void **args = nullptr, QSemaphore *semaphore = nullptr);
-    /*! \internal
-        \a signalId is in the signal index range (see QObjectPrivate::signalIndex()).
-    */
-    QMetaCallEvent(QtPrivate::QSlotObjectBase *slotObj, const QObject *sender, int signalId,
-                   int nargs = 0, int *types = nullptr, void **args = nullptr, QSemaphore *semaphore = nullptr);
+    // kept for compatibility until QtDeclarative has moved over
+    QMetaCallEvent(ushort method_offset, ushort method_relative,
+                   QObjectPrivate::StaticMetaCallFunction callFunction,
+                   const QObject *sender, int signalId,
+                   int nargs, int *types, void **args);
+
+    // blocking queued with semaphore - args always owned by caller
+    QMetaCallEvent(ushort method_offset, ushort method_relative,
+                   QObjectPrivate::StaticMetaCallFunction callFunction,
+                   const QObject *sender, int signalId,
+                   void **args, QSemaphore *semaphore);
+    QMetaCallEvent(QtPrivate::QSlotObjectBase *slotObj,
+                   const QObject *sender, int signalId,
+                   void **args, QSemaphore *semaphore);
+
+    // queued - args allocated by event, copied by caller
+    QMetaCallEvent(ushort method_offset, ushort method_relative,
+                   QObjectPrivate::StaticMetaCallFunction callFunction,
+                   const QObject *sender, int signalId,
+                   int nargs);
+    QMetaCallEvent(QtPrivate::QSlotObjectBase *slotObj,
+                   const QObject *sender, int signalId,
+                   int nargs);
 
     ~QMetaCallEvent() override;
 
-    inline int id() const { return method_offset_ + method_relative_; }
-    inline void **args() const { return args_; }
+    inline int id() const { return d.method_offset_ + d.method_relative_; }
+    inline const void * const* args() const { return d.args_; }
+    inline void ** args() { return d.args_; }
+    inline const int *types() const { return reinterpret_cast<int*>(d.args_ + d.nargs_); }
+    inline int *types() { return reinterpret_cast<int*>(d.args_ + d.nargs_); }
 
     virtual void placeMetaCall(QObject *object) override;
 
 private:
-    QtPrivate::QSlotObjectBase *slotObj_;
-    int nargs_;
-    int *types_;
-    void **args_;
-    QObjectPrivate::StaticMetaCallFunction callFunction_;
-    ushort method_offset_;
-    ushort method_relative_;
+    inline void allocArgs();
+
+    struct Data {
+        QtPrivate::QSlotObjectBase *slotObj_;
+        void **args_;
+        QObjectPrivate::StaticMetaCallFunction callFunction_;
+        int nargs_;
+        ushort method_offset_;
+        ushort method_relative_;
+    } d;
+    // preallocate enough space for three arguments
+    char prealloc_[3*(sizeof(void*) + sizeof(int))];
 };
 
 class QBoolBlocker