QQmlMetaObjectPointer: optimize atomic operations [2/2]: acquires /= 2

In the destructor, as well as in the metaObject() method, the old code
used the implicit conversion from QAtomic<T> to T, which is equivalent
to load-acquire. The code performed up to two reads each, which means
two load-acquire operations where one suffices.

Cache the result of the loadAcquire() and perform further operations
on the cached value instead of the atomic variable.

Found by locally disabling QAtomic<T> -> T implicit conversion
operators.

Change-Id: I6a4ceaf462714eaad8c79fc4f26a9521525ee1c7
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 7 insertions, 5 deletions

diff --git a/src/qml/qml/qqmlpropertycache_p.h b/src/qml/qml/qqmlpropertycache_p.h
index 7f3fc5f4fc..e6a08f33c7 100644
--- a/src/qml/qml/qqmlpropertycache_p.h
+++ b/src/qml/qml/qqmlpropertycache_p.h
@@ -85,8 +85,9 @@ public:
 
     ~QQmlMetaObjectPointer()
     {
-        if (d & Shared)
-            reinterpret_cast<SharedHolder *>(d ^ Shared)->release();
+        const auto dd = d.loadAcquire();
+        if (dd & Shared)
+            reinterpret_cast<SharedHolder *>(dd ^ Shared)->release();
     }
 
     QQmlMetaObjectPointer(const QQmlMetaObjectPointer &other)
@@ -111,9 +112,10 @@ public:
 
     const QMetaObject *metaObject() const
     {
-        if (d & Shared)
-            return reinterpret_cast<SharedHolder *>(d ^ Shared)->metaObject;
-        return reinterpret_cast<const QMetaObject *>(d.loadRelaxed());
+        const auto dd = d.loadAcquire();
+        if (dd & Shared)
+            return reinterpret_cast<SharedHolder *>(dd ^ Shared)->metaObject;
+        return reinterpret_cast<const QMetaObject *>(dd);
     }
 
     bool isShared() const