Q{Semaphore,ReadWriteLock}Private: reorganize the members

They are now ordered so that the mutex and the wait condition are in
different cache lines, to avoid false sharing situations, if the types
are holding the actual threading primitive structures, not mere pointers
to some other structure elsewhere.

For 64-bit systems:

OS                  | mutex |  cond | Remark
--------------------+-------+-------+------------------
Darwin              |    64 |    48 |
FreeBSD             |     8 |     8 |
INTEGRITY           |     8 |     8 | QMutex & QWaitCondition
Linux               |    24 |    48 | Always uses futex
MinGW (Winpthreads) |     8 |     8 | Always uses futex
MSVC (MS STL)       |    32 |    16 | Always uses futex
NetBSD              |    48 |    40 |
OpenBSD             |     8 |     8 |
QNX                 |     ? |     ? |

Change-Id: I63b988479db546dabffcfffd176698e4f0097e90
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 23 insertions, 6 deletions

diff --git a/src/corelib/thread/qsemaphore.cpp b/src/corelib/thread/qsemaphore.cpp
index 73e88cf42c..4915bde25a 100644
--- a/src/corelib/thread/qsemaphore.cpp
+++ b/src/corelib/thread/qsemaphore.cpp
@@ -251,14 +251,31 @@ futexSemaphoreTryAcquire(QBasicAtomicInteger<quintptr> &u, int n, T timeout)
     return false;
 }
 
-class QSemaphorePrivate {
-public:
-    explicit QSemaphorePrivate(int n) : avail(n) { }
+namespace { namespace QtSemaphorePrivate {
+using namespace QtPrivate;
+struct Layout1
+{
+    alignas(IdealMutexAlignment) QtPrivate::mutex mutex;
+    qsizetype avail = 0;
+    alignas(IdealMutexAlignment) QtPrivate::condition_variable cond;
+};
+
+struct Layout2
+{
+    alignas(IdealMutexAlignment) QtPrivate::mutex mutex;
+    alignas(IdealMutexAlignment) QtPrivate::condition_variable cond;
+    qsizetype avail = 0;
+};
 
-    QtPrivate::mutex mutex;
-    QtPrivate::condition_variable cond;
+// Choose Layout1 if it is smaller than Layout2. That happens for platforms
+// where sizeof(mutex) is 64.
+using Members = std::conditional_t<sizeof(Layout1) <= sizeof(Layout2), Layout1, Layout2>;
+} } // namespace QtSemaphorePrivate
 
-    int avail;
+class QSemaphorePrivate : public QtSemaphorePrivate::Members
+{
+public:
+    explicit QSemaphorePrivate(qsizetype n) { avail = n; }
 };
 
 /*!