source: trunk/src/corelib/kernel/qabstracteventdispatcher.cpp@ 967

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#include "qabstracteventdispatcher.h"
#include "qabstracteventdispatcher_p.h"

#include "qthread.h"
#include <private/qthread_p.h>
#include <private/qcoreapplication_p.h>

QT_BEGIN_NAMESPACE

// we allow for 2^24 = 8^8 = 16777216 simultaneously running timers
enum { NumberOfBuckets = 8, FirstBucketSize = 8 };

static const int BucketSize[NumberOfBuckets] =
    { 8, 64, 512, 4096, 32768, 262144, 2097152, 16777216 - 2396744 };
static const int BucketOffset[NumberOfBuckets] =
    { 0,  8,  72,  584,  4680,  37448,  299592,  2396744 };

static int FirstBucket[FirstBucketSize] = { 1, 2, 3, 4, 5, 6, 7, 8 };
static QBasicAtomicPointer<int> timerIds[NumberOfBuckets] =
    { Q_BASIC_ATOMIC_INITIALIZER(FirstBucket),
      Q_BASIC_ATOMIC_INITIALIZER(0),
      Q_BASIC_ATOMIC_INITIALIZER(0),
      Q_BASIC_ATOMIC_INITIALIZER(0),
      Q_BASIC_ATOMIC_INITIALIZER(0),
      Q_BASIC_ATOMIC_INITIALIZER(0),
      Q_BASIC_ATOMIC_INITIALIZER(0),
      Q_BASIC_ATOMIC_INITIALIZER(0) };

static void timerIdsDestructorFunction()
{
    // start at one, the first bucket is pre-allocated
    for (int i = 1; i < NumberOfBuckets; ++i)
        delete [] static_cast<int *>(timerIds[i]);
}
Q_DESTRUCTOR_FUNCTION(timerIdsDestructorFunction)

static QBasicAtomicInt nextFreeTimerId = Q_BASIC_ATOMIC_INITIALIZER(1);

static const int TimerIdMask = 0x00ffffff;
static const int TimerSerialMask = ~TimerIdMask & ~0x80000000;
static const int TimerSerialCounter = TimerIdMask + 1;

// avoid the ABA-problem by using 7 of the top 8 bits of the timerId as a serial number
static inline int prepareNewValueWithSerialNumber(int oldId, int newId)
{
    return (newId & TimerIdMask) | ((oldId + TimerSerialCounter) & TimerSerialMask);
}

static inline int bucketOffset(int timerId)
{
    for (int i = 0; i < NumberOfBuckets; ++i) {
        if (timerId < BucketSize[i])
            return i;
        timerId -= BucketSize[i];
    }
    qFatal("QAbstractEventDispatcher: INTERNAL ERROR, timer ID %d is too large", timerId);
    return -1;
}

static inline int bucketIndex(int bucket, int timerId)
{
    return timerId - BucketOffset[bucket];
}

static inline int *allocateBucket(int bucket)
{
    // allocate a new bucket
    const int size = BucketSize[bucket];
    const int offset = BucketOffset[bucket];
    int *b = new int[size];
    for (int i = 0; i != size; ++i)
        b[i] = offset + i + 1;
    return b;
}

void QAbstractEventDispatcherPrivate::init()
{
    Q_Q(QAbstractEventDispatcher);
    if (threadData->eventDispatcher != 0) {
        qWarning("QAbstractEventDispatcher: An event dispatcher has already been created for this thread");
    } else {
        threadData->eventDispatcher = q;
    }
}

// Timer IDs are implemented using a free-list;
// there's a vector initialized with:
//    X[i] = i + 1
// and nextFreeTimerId starts with 1.
//
// Allocating a timer ID involves taking the ID from
//    X[nextFreeTimerId]
// updating nextFreeTimerId to this value and returning the old value
//
// When the timer ID is allocated, its cell in the vector is unused (it's a
// free list). As an added protection, we use the cell to store an invalid
// (negative) value that we can later check for integrity.
//
// (continues below).
int QAbstractEventDispatcherPrivate::allocateTimerId()
{
    int timerId, newTimerId;
    int at, *b;
    do {
        timerId = nextFreeTimerId; //.loadAcquire(); // ### FIXME Proper memory ordering semantics

        // which bucket are we looking in?
        int which = timerId & TimerIdMask;
        int bucket = bucketOffset(which);
        at = bucketIndex(bucket, which);
        b = timerIds[bucket];

        if (!b) {
            // allocate a new bucket
            b = allocateBucket(bucket);
            if (!timerIds[bucket].testAndSetRelease(0, b)) {
                // another thread won the race to allocate the bucket
                delete [] b;
                b = timerIds[bucket];
            }
        }

        newTimerId = prepareNewValueWithSerialNumber(timerId, b[at]);
    } while (!nextFreeTimerId.testAndSetRelaxed(timerId, newTimerId));

    b[at] = -timerId;

    return timerId;
}

// Releasing a timer ID requires putting the current ID back in the vector;
// we do it by setting:
//    X[timerId] = nextFreeTimerId;
// then we update nextFreeTimerId to the timer we've just released
//
// The extra code in allocateTimerId and releaseTimerId are ABA prevention
// and bucket memory. The buckets are simply to make sure we allocate only
// the necessary number of timers. See above.
//
// ABA prevention simply adds a value to 7 of the top 8 bits when resetting
// nextFreeTimerId.
void QAbstractEventDispatcherPrivate::releaseTimerId(int timerId)
{
    int which = timerId & TimerIdMask;
    int bucket = bucketOffset(which);
    int at = bucketIndex(bucket, which);
    int *b = timerIds[bucket];

    Q_ASSERT(b[at] == -timerId);

    int freeId, newTimerId;
    do {
        freeId = nextFreeTimerId;//.loadAcquire(); // ### FIXME Proper memory ordering semantics
        b[at] = freeId & TimerIdMask;

        newTimerId = prepareNewValueWithSerialNumber(freeId, timerId);
    } while (!nextFreeTimerId.testAndSetRelease(freeId, newTimerId));
}

/*!
    \class QAbstractEventDispatcher
    \brief The QAbstractEventDispatcher class provides an interface to manage Qt's event queue.

    \ingroup events

    An event dispatcher receives events from the window system and other
    sources. It then sends them to the QCoreApplication or QApplication
    instance for processing and delivery. QAbstractEventDispatcher provides
    fine-grained control over event delivery.

    For simple control of event processing use
    QCoreApplication::processEvents().

    For finer control of the application's event loop, call
    instance() and call functions on the QAbstractEventDispatcher
    object that is returned. If you want to use your own instance of
    QAbstractEventDispatcher or of a QAbstractEventDispatcher
    subclass, you must create your instance \e before you create the
    QApplication object.

    The main event loop is started by calling
    QCoreApplication::exec(), and stopped by calling
    QCoreApplication::exit(). Local event loops can be created using
    QEventLoop.

    Programs that perform long operations can call processEvents()
    with a bitwise OR combination of various QEventLoop::ProcessEventsFlag
    values to control which events should be delivered.

    QAbstractEventDispatcher also allows the integration of an
    external event loop with the Qt event loop. For example, the
    \l{Qt Solutions}{Motif Extension Qt Solution} includes a
    reimplementation of QAbstractEventDispatcher that merges Qt and
    Motif events together.

    \sa QEventLoop, QCoreApplication
*/

/*!
    Constructs a new event dispatcher with the given \a parent.
*/
QAbstractEventDispatcher::QAbstractEventDispatcher(QObject *parent)
    : QObject(*new QAbstractEventDispatcherPrivate, parent)
{
    Q_D(QAbstractEventDispatcher);
    d->init();
}

/*!
    \internal
*/
QAbstractEventDispatcher::QAbstractEventDispatcher(QAbstractEventDispatcherPrivate &dd,
                                                   QObject *parent)
    : QObject(dd, parent)
{
    Q_D(QAbstractEventDispatcher);
    d->init();
}

/*!
    Destroys the event dispatcher.
*/
QAbstractEventDispatcher::~QAbstractEventDispatcher()
{ }

/*!
    Returns a pointer to the event dispatcher object for the specified
    \a thread. If \a thread is zero, the current thread is used. If no
    event dispatcher exists for the specified thread, this function
    returns 0.

    \bold{Note:} If Qt is built without thread support, the \a thread
    argument is ignored.
 */
QAbstractEventDispatcher *QAbstractEventDispatcher::instance(QThread *thread)
{
    QThreadData *data = thread ? QThreadData::get2(thread) : QThreadData::current();