Fix performance regression of posted events

Commit 31f1ff91028dd7f90925d5b3737e4d88b5fb07aa introduced a performance
regression by using SetTimer() to delay the next call to sendPostedEvents().
SetTimer() has a minimum resolution, around 15-16ms, which is too slow for
most uses. Fast timers and QWidget::update() use posted events to do their
job, and suffered as a result.

This commit goes away from using SetTimer() to using a GetMessage() hook that
examines the input queue as soon as message are pulled from the queue. Now
there is no large delay between calls to sendPostedEvents(), they are sent
as quickly as possible (once all other input and timer messages have been
delivered).

Reviewed-by: Prasanth Ullattil
Task-number: QTBUG-6083

2 files changed, 40 insertions, 29 deletions

diff --git a/src/corelib/kernel/qeventdispatcher_win.cpp b/src/corelib/kernel/qeventdispatcher_win.cpp
index b197b9d..c6eef5e 100644
--- a/src/corelib/kernel/qeventdispatcher_win.cpp
+++ b/src/corelib/kernel/qeventdispatcher_win.cpp
@@ -336,6 +336,7 @@ public:
 
     // internal window handle used for socketnotifiers/timers/etc
     HWND internalHwnd;
+    HHOOK getMessageHook;
 
     // for controlling when to send posted events
     QAtomicInt serialNumber;
@@ -363,7 +364,7 @@ public:
 };
 
 QEventDispatcherWin32Private::QEventDispatcherWin32Private()
-    : threadId(GetCurrentThreadId()), interrupt(false), internalHwnd(0), serialNumber(0), lastSerialNumber(0), wakeUps(0)
+    : threadId(GetCurrentThreadId()), interrupt(false), internalHwnd(0), getMessageHook(0), serialNumber(0), lastSerialNumber(0), wakeUps(0)
 {
     resolveTimerAPI();
 }
@@ -471,37 +472,11 @@ LRESULT CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp)
         }
         return 0;
     } else if (message == WM_TIMER) {    
-        if (wp == SendPostedEventsTimerId) {
-            KillTimer(d->internalHwnd, wp);
-            int localSerialNumber = d->serialNumber;
-            (void) d->wakeUps.fetchAndStoreRelease(0);
-            if (localSerialNumber != d->lastSerialNumber) {
-                PostMessage(d->internalHwnd, WM_QT_SENDPOSTEDEVENTS, 0, 0);
-            }
-        } else {
-            Q_ASSERT(d != 0);
-            d->sendTimerEvent(wp);
-        }
+        Q_ASSERT(d != 0);
+        d->sendTimerEvent(wp);
         return 0;
     } else if (message == WM_QT_SENDPOSTEDEVENTS) {
         int localSerialNumber = d->serialNumber;
-
-        if (GetQueueStatus(QS_INPUT | QS_RAWINPUT | QS_TIMER) != 0) {
-            // delay the next pass of sendPostedEvents() until we get the special
-            // WM_TIMER, which allows all pending Windows messages to be processed
-            if (SetTimer(d->internalHwnd, SendPostedEventsTimerId, 0, 0) == 0) {
-                // failed to start the timer, oops, clear wakeUps in an attempt to keep things running
-                qErrnoWarning("Qt: INTERNAL ERROR: failed to start sendPostedEvents() timer");
-                d->wakeUps.fetchAndStoreRelease(0);
-            } else {
-                // SetTimer() succeeded, nothing to do now
-                ;
-            }
-        } else {
-            // nothing pending in the queue, let sendPostedEvents go through
-            d->wakeUps.fetchAndStoreRelease(0);
-        }
-
         if (localSerialNumber != d->lastSerialNumber) {
             d->lastSerialNumber = localSerialNumber;
             QCoreApplicationPrivate::sendPostedEvents(0, 0, d->threadData);
@@ -512,6 +487,31 @@ LRESULT CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp)
     return DefWindowProc(hwnd, message, wp, lp);
 }
 
+LRESULT CALLBACK qt_GetMessageHook(int code, WPARAM wp, LPARAM lp)
+{
+    if (wp == PM_REMOVE) {
+        QEventDispatcherWin32 *q = qobject_cast<QEventDispatcherWin32 *>(QAbstractEventDispatcher::instance());
+        Q_ASSERT(q != 0);
+        if (q) {
+            QEventDispatcherWin32Private *d = q->d_func();
+            int localSerialNumber = d->serialNumber;
+            if (HIWORD(GetQueueStatus(QS_INPUT | QS_RAWINPUT | QS_TIMER)) == 0) {
+                // no more input or timer events in the message queue, we can allow posted events to be
+                // sent now
+                (void) d->wakeUps.fetchAndStoreRelease(0);
+                MSG *msg = (MSG *) lp;
+                if (localSerialNumber != d->lastSerialNumber
+                    // if this message IS the one that triggers sendPostedEvents(), no need to post it again
+                    && msg->hwnd != d->internalHwnd
+                    && msg->message != WM_QT_SENDPOSTEDEVENTS) {
+                    PostMessage(d->internalHwnd, WM_QT_SENDPOSTEDEVENTS, 0, 0);
+                }
+            }
+        }
+    }
+    return CallNextHookEx(0, code, wp, lp);
+}
+
 static HWND qt_create_internal_window(const QEventDispatcherWin32 *eventDispatcher)
 {
     // make sure that multiple Qt's can coexist in the same process
@@ -636,6 +636,12 @@ void QEventDispatcherWin32::createInternalHwnd()
         return;
     d->internalHwnd = qt_create_internal_window(this);
 
+    // setup GetMessage hook needed to drive our posted events
+    d->getMessageHook = SetWindowsHookEx(WH_GETMESSAGE, (HOOKPROC) qt_GetMessageHook, NULL, GetCurrentThreadId());
+    if (!d->getMessageHook) {
+        qFatal("Qt: INTERNALL ERROR: failed to install GetMessage hook");
+    }
+
     // register all socket notifiers
     QList<int> sockets = (d->sn_read.keys().toSet()
                           + d->sn_write.keys().toSet()
@@ -1058,6 +1064,10 @@ void QEventDispatcherWin32::closingDown()
         d->unregisterTimer(d->timerVec.at(i), true);
     d->timerVec.clear();
     d->timerDict.clear();
+
+    if (d->getMessageHook)
+        UnhookWindowsHookEx(d->getMessageHook);
+    d->getMessageHook = 0;
 }
 
 bool QEventDispatcherWin32::event(QEvent *e)
diff --git a/src/corelib/kernel/qeventdispatcher_win_p.h b/src/corelib/kernel/qeventdispatcher_win_p.h
index a5ef4d4..7f0e87d 100644
--- a/src/corelib/kernel/qeventdispatcher_win_p.h
+++ b/src/corelib/kernel/qeventdispatcher_win_p.h
@@ -102,6 +102,7 @@ public:
 
 private:
     friend LRESULT CALLBACK qt_internal_proc(HWND hwnd, UINT message, WPARAM wp, LPARAM lp);
+    friend LRESULT CALLBACK qt_GetMessageHook(int, WPARAM, LPARAM);
 };
 
 QT_END_NAMESPACE