Fix performance regression on Mac OS X when creating/destroying QMutex

This reverts commit cf17b743d2fe84ab259b7232ab07b58a1872e18e, which
changed QMutex to use a Mach semaphore_t on Mac OS X. We now use
pthread_mutex_t on Mac OS X as well. Contention performance is mostly
unchanged, but the new constructionQMutex() benchmark added in this
commit shows that creating/destroying a semaphore_t is about 20 times
slower than pthread_mutex_t.

Reviewed-by: Olivier Goffart
Reviewed-by: João Abecasis

3 files changed, 26 insertions, 51 deletions

diff --git a/src/corelib/thread/qmutex_p.h b/src/corelib/thread/qmutex_p.h
index a9923c4..d2ffd28 100644
--- a/src/corelib/thread/qmutex_p.h
+++ b/src/corelib/thread/qmutex_p.h
@@ -58,10 +58,6 @@
 #include <QtCore/qnamespace.h>
 #include <QtCore/qmutex.h>
 
-#if defined(Q_OS_MAC)
-# include <mach/semaphore.h>
-#endif
-
 #if defined(Q_OS_SYMBIAN)
 # include <e32std.h>
 #endif
@@ -83,9 +79,7 @@ public:
     Qt::HANDLE owner;
     uint count;
 
-#if defined(Q_OS_MAC)
-    semaphore_t mach_semaphore;
-#elif defined(Q_OS_UNIX) && !defined(Q_OS_LINUX) && !defined(Q_OS_SYMBIAN)
+#if defined(Q_OS_UNIX) && !defined(Q_OS_LINUX) && !defined(Q_OS_SYMBIAN)
     volatile bool wakeup;
     pthread_mutex_t mutex;
     pthread_cond_t cond;
diff --git a/src/corelib/thread/qmutex_unix.cpp b/src/corelib/thread/qmutex_unix.cpp
index 2a9d23c..790fad3 100644
--- a/src/corelib/thread/qmutex_unix.cpp
+++ b/src/corelib/thread/qmutex_unix.cpp
@@ -65,7 +65,7 @@
 
 QT_BEGIN_NAMESPACE
 
-#if !defined(Q_OS_MAC) && !defined(Q_OS_LINUX)
+#if !defined(Q_OS_LINUX)
 static void report_error(int code, const char *where, const char *what)
 {
     if (code != 0)
@@ -77,11 +77,7 @@ static void report_error(int code, const char *where, const char *what)
 QMutexPrivate::QMutexPrivate(QMutex::RecursionMode mode)
     : QMutexData(mode), maximumSpinTime(MaximumSpinTimeThreshold), averageWaitTime(0), owner(0), count(0)
 {
-#if defined(Q_OS_MAC)
-    kern_return_t r = semaphore_create(mach_task_self(), &mach_semaphore, SYNC_POLICY_FIFO, 0);
-    if (r != KERN_SUCCESS)
-        qWarning("QMutex: failed to create semaphore, error %d", r);
-#elif !defined(Q_OS_LINUX)
+#if !defined(Q_OS_LINUX)
     wakeup = false;
     report_error(pthread_mutex_init(&mutex, NULL), "QMutex", "mutex init");
     report_error(pthread_cond_init(&cond, NULL), "QMutex", "cv init");
@@ -90,47 +86,13 @@ QMutexPrivate::QMutexPrivate(QMutex::RecursionMode mode)
 
 QMutexPrivate::~QMutexPrivate()
 {
-#if defined(Q_OS_MAC)
-    kern_return_t r = semaphore_destroy(mach_task_self(), mach_semaphore);
-    if (r != KERN_SUCCESS)
-        qWarning("QMutex: failed to destroy semaphore, error %d", r);
-#elif !defined(Q_OS_LINUX)
+#if !defined(Q_OS_LINUX)
     report_error(pthread_cond_destroy(&cond), "QMutex", "cv destroy");
     report_error(pthread_mutex_destroy(&mutex), "QMutex", "mutex destroy");
 #endif
 }
 
-#if defined(Q_OS_MAC)
-
-bool QMutexPrivate::wait(int timeout)
-{
-    if (contenders.fetchAndAddAcquire(1) == 0) {
-        // lock acquired without waiting
-        return true;
-    }
-    kern_return_t r;
-    if (timeout < 0) {
-        do {
-            r = semaphore_wait(mach_semaphore);
-        } while (r == KERN_ABORTED);
-	if (r != KERN_SUCCESS)
-            qWarning("QMutex: infinite wait failed, error %d", r);
-    } else {
-        mach_timespec_t ts;
-        ts.tv_nsec = ((timeout % 1000) * 1000) * 1000;
-        ts.tv_sec = (timeout / 1000);
-        r = semaphore_timedwait(mach_semaphore, ts);
-    }
-    contenders.deref();
-    return r == KERN_SUCCESS;
-}
-
-void QMutexPrivate::wakeUp()
-{
-    semaphore_signal(mach_semaphore);
-}
-
-#elif defined(Q_OS_LINUX)
+#if defined(Q_OS_LINUX)
 
 static inline int _q_futex(volatile int *addr, int op, int val, const struct timespec *timeout, int *addr2, int val2)
 {
@@ -174,7 +136,7 @@ void QMutexPrivate::wakeUp()
     (void) _q_futex(&contenders._q_value, FUTEX_WAKE, 1, 0, 0, 0);
 }
 
-#else // !Q_OS_MAC && !Q_OS_LINUX
+#else // !Q_OS_LINUX
 
 bool QMutexPrivate::wait(int timeout)
 {
@@ -221,7 +183,7 @@ void QMutexPrivate::wakeUp()
     report_error(pthread_mutex_unlock(&mutex), "QMutex::unlock", "mutex unlock");
 }
 
-#endif // !Q_OS_MAC && !Q_OS_LINUX
+#endif // !Q_OS_LINUX
 
 QT_END_NAMESPACE
 
diff --git a/tests/benchmarks/corelib/thread/qmutex/tst_qmutex.cpp b/tests/benchmarks/corelib/thread/qmutex/tst_qmutex.cpp
index 05a1575..eca38b6 100644
--- a/tests/benchmarks/corelib/thread/qmutex/tst_qmutex.cpp
+++ b/tests/benchmarks/corelib/thread/qmutex/tst_qmutex.cpp
@@ -128,7 +128,9 @@ private slots:
     void noThread_data();
     void noThread();
 
+    void constructionNative();
     void uncontendedNative();
+    void constructionQMutex();
     void uncontendedQMutex();
     void uncontendedQMutexLocker();
 
@@ -205,6 +207,15 @@ void tst_QMutex::noThread()
     QCOMPARE(int(count), N);
 }
 
+void tst_QMutex::constructionNative()
+{
+    QBENCHMARK {
+        NativeMutexType mutex;
+        NativeMutexInitialize(&mutex);
+        NativeMutexDestroy(&mutex);
+    }
+}
+
 void tst_QMutex::uncontendedNative()
 {
     NativeMutexType mutex;
@@ -216,6 +227,14 @@ void tst_QMutex::uncontendedNative()
     NativeMutexDestroy(&mutex);
 }
 
+void tst_QMutex::constructionQMutex()
+{
+    QBENCHMARK {
+        QMutex mutex;
+        Q_UNUSED(mutex);
+    }
+}
+
 void tst_QMutex::uncontendedQMutex()
 {
     QMutex mutex;