Call QObject::disconnectNotify() when receiver is destroyed

Store the signal index in QObjectPrivate::Connection, thereby making
it available in "implicit" disconnect contexts (i.e., receiver
deletion).

This change does not cause the size of QObjectPrivate::Connection
to grow (still 40 bytes on 32-bit Linux, 72 bytes on 64-bit Mac).

Valgrinding the new benchmark indicates that the percentage of the
time spent in the QObject destructor increased from 7.8% to 8.4%
on ia32, for that particular stress test; the increase is the
combined cost of calling metaObject(), QMetaObjectPrivate::signal(),
and disconnectNotify() for one connection. In practice, the measured
wallclock time increased by about 3ms for a 500ms run (which
repeatedly constructs, connects, and destroys an object).

Task-number: QTBUG-4844

Change-Id: I1beb01c753f31542fc0acb62edb4c6d165fcc5b4
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/tests/benchmarks/corelib/kernel/qobject/main.cpp b/tests/benchmarks/corelib/kernel/qobject/main.cpp
index a5a26c1b2a..90912d460c 100644
--- a/tests/benchmarks/corelib/kernel/qobject/main.cpp
+++ b/tests/benchmarks/corelib/kernel/qobject/main.cpp
@@ -61,6 +61,7 @@ private slots:
     void dynamic_property_benchmark();
     void connect_disconnect_benchmark_data();
     void connect_disconnect_benchmark();
+    void receiver_destroyed_benchmark();
 };
 
 struct Functor {
@@ -236,6 +237,15 @@ void QObjectBenchmark::connect_disconnect_benchmark()
     }
 }
 
+void QObjectBenchmark::receiver_destroyed_benchmark()
+{
+    Object sender;
+    QBENCHMARK {
+        Object receiver;
+        QObject::connect(&sender, &Object::signal0, &receiver, &Object::slot0);
+    }
+}
+
 QTEST_MAIN(QObjectBenchmark)
 
 #include "main.moc"