QBasicAtomicInteger: Add (internal) refRelaxed

It is known that incrementing the refcount can use relaxed semantics,
compare
https://web.archive.org/web/20251016043603/https://devblogs.microsoft.com/oldnewthing/20251015-00/?p=111686

However, we can't modify QBasicAtomic::ref, because that is documented
to use "ordered" semantics.
So introduce a new (internal) refRelaxed method, which simply calls
fetchAndAddRelaxed(1) instead. Compared to ref, we also do not return
anything, as no expected user has a need for the return value (and it
only causes more work for the compiler to get rid of it again).
Our deref operation is still using acquire_release semantics, so
everything is fine.
Port QArrayData to use it as a first user so that the functionality is
tested.

Change-Id: I678870551fe85b83d9bb073ddb5947e649845264
Reviewed-by: Marc Mutz <[email protected]>

Author: Inkane

src/corelib/thread/qatomic.cpp

@@ -315,6 +315,19 @@
     \sa deref(), operator++()
 */
 
+/*!
+    \fn template <typename T> void QAtomicInteger<T>::refRelaxed()
+    \internal
+    Atomically increments the value of this QAtomicInteger.
+
+    In contrast to ref(), this uses relaxed semantics, which is
+    all that is needed for reference counting (together with deref's
+    acquire-release semantics).
+    It also doesn't return anything.
+
+    \sa deref(), operator++()
+*/
+
 /*!
     \fn template <typename T> T QAtomicInteger<T>::operator++()
     \since 5.3

src/corelib/thread/qatomic.h

@@ -46,6 +46,7 @@ class QAtomicInteger : public QBasicAtomicInteger<T>
     static constexpr bool isReferenceCountingWaitFree();
 
     bool ref();
+    void refRelaxed();
     bool deref();
 
     static constexpr bool isTestAndSetNative();

src/corelib/thread/qbasicatomic.h

@@ -46,6 +46,7 @@ class QBasicAtomicInteger
     static constexpr bool isReferenceCountingWaitFree() noexcept { return Ops::isReferenceCountingWaitFree(); }
 
     bool ref() noexcept { return Ops::ref(_q_value); }
+    void refRelaxed() noexcept { Ops::fetchAndAddRelaxed(_q_value, 1); }
     bool deref() noexcept { return Ops::deref(_q_value); }
 
     static constexpr bool isTestAndSetNative() noexcept { return Ops::isTestAndSetNative(); }

src/corelib/tools/qarraydata.h

@@ -58,7 +58,7 @@ struct QArrayData
     /// Returns true if sharing took place
     bool ref() noexcept
     {
-        ref_.ref();
+        ref_.refRelaxed(); // suffices for ref-counting
         return true;
     }