QSlotObjectBase: combat virtual function "bloat"

In C++, the compiler creates extra functions and data for classes
with virtual functions. This can lead to "virtual function bloat":
  http://www.boost.org/doc/libs/1_47_0/doc/html/function/misc.html#id1382504

This is especially true when the number of instances is of the same
order of magnitute as the number of derived classes, such as is
common with type erasure techniques.

One such case is the QSlotObjectBase hierarchy, which this patch
tackles.

The mechanics of this optimisation are simple: re-implement the
virtual function call mechanism by hand, with function pointers.

But we go one step further and collapse the vtable into a single
pointer to a function that implements all three currently-defined
operations, swtching on an 'int which' argument. This even allows
us to extend this in a BC way, should that become necessary later,
by adding a new Operation and using the void** argument to
transport arguments, if any.

This approach was inspired by:
  Ulrich Drepper: How To Write Shared Libraries, Section 2.4.4
  http://www.akkadia.org/drepper/dsohowto.pdf

Also move the QSlotObjectBase hierarchy out of QObject so as not
to export all the derived classes.

This was pointed out in review by Thiago.

Results (Linux amd64, GCC 4.8-pre -O2 -std=c++11, stripped):

  size tst_qobject*

     text    data     bss     dec     hex filename
   523275   21192      48  544515   84f03 tst_qobject (old)
   507343   13984      48  521375   7f49f tst_qobject (new)

  relinfo.pl tst_qobject*

   (old) tst_qobject: 473 relocations, 0 relative (0%), 240 PLT entries, 240 for local syms (100%), 0 users
   (new) tst_qobject: 323 relocations, 0 relative (0%), 238 PLT entries, 238 for local syms (100%), 0 users

Change-Id: I40ad4744dde8c5c29ef62ed2d82d4b1ede178510
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index 02580caf41..d47426b14c 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -120,7 +120,7 @@ public:
         QObject *receiver;
         union {
             StaticMetaCallFunction callFunction;
-            QObject::QSlotObjectBase *slotObj;
+            QtPrivate::QSlotObjectBase *slotObj;
         };
         // The next pointer for the singly-linked ConnectionList
         Connection *nextConnectionList;
@@ -280,7 +280,7 @@ public:
     /*! \internal
         \a signalId is in the signal index range (see QObjectPrivate::signalIndex()).
     */
-    QMetaCallEvent(QObject::QSlotObjectBase *slotObj, const QObject *sender, int signalId,
+    QMetaCallEvent(QtPrivate::QSlotObjectBase *slotObj, const QObject *sender, int signalId,
                    int nargs = 0, int *types = 0, void **args = 0, QSemaphore *semaphore = 0);
 
     ~QMetaCallEvent();
@@ -293,7 +293,7 @@ public:
     virtual void placeMetaCall(QObject *object);
 
 private:
-    QObject::QSlotObjectBase *slotObj_;
+    QtPrivate::QSlotObjectBase *slotObj_;
     const QObject *sender_;
     int signalId_;
     int nargs_;