diff --git a/src/corelib/thread/qreadwritelock_p.h b/src/corelib/thread/qreadwritelock_p.h
index d1f887eb45b..596239e3256 100644
--- a/src/corelib/thread/qreadwritelock_p.h
+++ b/src/corelib/thread/qreadwritelock_p.h
@@ -45,9 +45,10 @@ public:
     explicit QReadWriteLockPrivate(bool isRecursive = false)
         : recursive(isRecursive) {}
 
-    QtPrivate::mutex mutex;
-    QtPrivate::condition_variable writerCond;
+    alignas(QtPrivate::IdealMutexAlignment) QtPrivate::condition_variable writerCond;
     QtPrivate::condition_variable readerCond;
+
+    alignas(QtPrivate::IdealMutexAlignment) QtPrivate::mutex mutex;
     int readerCount = 0;
     int writerCount = 0;
     int waitingReaders = 0;
diff --git a/src/corelib/thread/qsemaphore.cpp b/src/corelib/thread/qsemaphore.cpp
index 73e88cf42c8..4915bde25a9 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 {
+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;
+};
+
+// 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
+
+class QSemaphorePrivate : public QtSemaphorePrivate::Members
+{
 public:
-    explicit QSemaphorePrivate(int n) : avail(n) { }
-
-    QtPrivate::mutex mutex;
-    QtPrivate::condition_variable cond;
-
-    int avail;
+    explicit QSemaphorePrivate(qsizetype n) { avail = n; }
 };
 
 /*!
diff --git a/src/corelib/thread/qwaitcondition_p.h b/src/corelib/thread/qwaitcondition_p.h
index cfb36ca30be..f3c931f3922 100644
--- a/src/corelib/thread/qwaitcondition_p.h
+++ b/src/corelib/thread/qwaitcondition_p.h
@@ -123,6 +123,14 @@ using condition_variable = std::condition_variable;
 
 #endif // C++11 threads
 
+// Ideal alignment for mutex and condition_variable: it's the hardware
+// interference size (size of a cache line) if the types are likely to contain
+// the actual data structures, otherwise just that of a pointer.
+static constexpr quintptr IdealMutexAlignment =
+        sizeof(QtPrivate::mutex) > sizeof(void *) &&
+        sizeof(QtPrivate::condition_variable) > sizeof(void *) ?
+            64 : alignof(void*);
+
 } // namespace QtPrivate
 
 QT_END_NAMESPACE