Introduce Q_TRACE_SCOPE to simplify tracing of a function entry/exit

Additionally, we also add a Q_TRACE_EXIT which runs a trace point when
the scope is exited, leveraging qScopeGuard behind the scenes.
Q_TRACE_SCOPE uses Q_TRACE_EXIT internally - the difference is that
the _SCOPE version enforces the naming scheme of _entry / _exit for the
tracepoints, whereas Q_TRACE_EXIT can be used generically.

Change-Id: I4a2f5ea09f451fcf664d07fd493b679f7527ac06
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qtrace_p.h

@@ -52,11 +52,18 @@
 //
 
 /*
- * The Qt tracepoints API consists of only three macros:
+ * The Qt tracepoints API consists of only five macros:
  *
  *     - Q_TRACE(tracepoint, args...)
  *       Fires 'tracepoint' if it is enabled.
  *
+ *     - Q_TRACE_EXIT(tracepoint, args...)
+ *       Fires 'tracepoint' if it is enabled when the current scope exists.
+ *
+ *     - Q_TRACE_SCOPE(tracepoint, args...)
+ *       Wrapper around Q_TRACE/_EXIT to trace entry and exit. First it traces
+ *       `${tracepoint}_entry` and then `${tracepoint}_exit` on scope exit.
+ *
  *     - Q_UNCONDITIONAL_TRACE(tracepoint, args...)
  *       Fires 'tracepoint' unconditionally: no check is performed to query
  *       whether 'tracepoint' is enabled.
@@ -110,15 +117,23 @@
  */
 
 #include <QtCore/qglobal.h>
+#include <QtCore/qscopeguard.h>
 
 QT_BEGIN_NAMESPACE
 
 #if defined(Q_TRACEPOINT) && !defined(QT_BOOTSTRAPPED)
 #  define Q_TRACE(x, ...) QtPrivate::trace_ ## x(__VA_ARGS__)
+#  define Q_TRACE_EXIT(x, ...) \
+    const auto qTraceExit_ ## x ## __COUNTER__ = qScopeGuard([&]() { Q_TRACE(x, __VA_ARGS__); });
+#  define Q_TRACE_SCOPE(x, ...) \
+    Q_TRACE(x ## _entry, __VA_ARGS__); \
+    Q_TRACE_EXIT(x ## _exit, __VA_ARGS__);
 #  define Q_UNCONDITIONAL_TRACE(x, ...) QtPrivate::do_trace_ ## x(__VA_ARGS__)
 #  define Q_TRACE_ENABLED(x) QtPrivate::trace_ ## x ## _enabled()
 #else
 #  define Q_TRACE(x, ...)
+#  define Q_TRACE_EXIT(x, ...)
+#  define Q_TRACE_SCOPE(x, ...)
 #  define Q_UNCONDITIONAL_TRACE(x, ...)
 #  define Q_TRACE_ENABLED(x) false
 #endif // defined(Q_TRACEPOINT) && !defined(QT_BOOTSTRAPPED)

src/corelib/kernel/qcoreapplication.cpp

@@ -778,7 +778,7 @@ QCoreApplication::QCoreApplication(int &argc, char **argv
 
 void QCoreApplicationPrivate::init()
 {
-    Q_TRACE(QCoreApplicationPrivate_init_entry);
+    Q_TRACE_SCOPE(QCoreApplicationPrivate_init);
 
 #if defined(Q_OS_MACOS)
     QMacAutoReleasePool pool;
@@ -883,8 +883,6 @@ void QCoreApplicationPrivate::init()
 #ifndef QT_NO_QOBJECT
     is_app_running = true; // No longer starting up.
 #endif
-
-    Q_TRACE(QCoreApplicationPrivate_init_exit);
 }
 
 /*!

src/gui/kernel/qguiapplication.cpp

@@ -1421,7 +1421,7 @@ void QGuiApplicationPrivate::eventDispatcherReady()
 
 void QGuiApplicationPrivate::init()
 {
-    Q_TRACE(QGuiApplicationPrivate_init_entry);
+    Q_TRACE_SCOPE(QGuiApplicationPrivate_init);
 
 #if defined(Q_OS_MACOS)
     QMacAutoReleasePool pool;
@@ -1585,8 +1585,6 @@ void QGuiApplicationPrivate::init()
     if (!QGuiApplicationPrivate::displayName)
         QObject::connect(q, &QGuiApplication::applicationNameChanged,
                          q, &QGuiApplication::applicationDisplayNameChanged);
-
-    Q_TRACE(QGuiApplicationPrivate_init_exit);
 }
 
 extern void qt_cleanupFontDatabase();
@@ -1830,7 +1828,7 @@ bool QGuiApplicationPrivate::processNativeEvent(QWindow *window, const QByteArra
 
 void QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent *e)
 {
-    Q_TRACE(QGuiApplicationPrivate_processWindowSystemEvent_entry, e->type);
+    Q_TRACE_SCOPE(QGuiApplicationPrivate_processWindowSystemEvent, e->type);
 
     switch(e->type) {
     case QWindowSystemInterfacePrivate::Mouse:
@@ -1940,8 +1938,6 @@ void QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePriv
         qWarning() << "Unknown user input event type:" << e->type;
         break;
     }
-
-    Q_TRACE(QGuiApplicationPrivate_processWindowSystemEvent_exit, e->type);
 }
 
 /*! \internal