QVariant: Extract Method PrivateShared::computeOffset()

Will be re-used in the upcoming emplace() function.

No attempt is made to re-write the expression to be more
readable. That's left for another commit.

Task-number: QTBUG-112187
Change-Id: Id391b78f1477c5225beda8a32c4f6c1393dd51bb
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/kernel/qvariant.h

@@ -78,6 +78,7 @@ public:
     private:
         inline PrivateShared() : ref(1) { }
     public:
+        static int computeOffset(PrivateShared *ps, size_t align);
         static size_t computeAllocationSize(size_t size, size_t align);
         static PrivateShared *create(size_t size, size_t align);
         static void free(PrivateShared *p);

src/corelib/kernel/qvariant_p.h

@@ -34,6 +34,11 @@ customConstructShared(size_t size, size_t align, F &&construct)
     return ptr.release();
 }
 
+inline int QVariant::PrivateShared::computeOffset(PrivateShared *ps, size_t align)
+{
+    return int(((quintptr(ps) + sizeof(PrivateShared) + align - 1) & ~(align - 1)) - quintptr(ps));
+}
+
 inline size_t QVariant::PrivateShared::computeAllocationSize(size_t size, size_t align)
 {
     size += sizeof(PrivateShared);
@@ -52,7 +57,7 @@ inline QVariant::PrivateShared *QVariant::PrivateShared::create(size_t size, siz
     size = computeAllocationSize(size, align);
     void *data = operator new(size);
     auto *ps = new (data) QVariant::PrivateShared();
-    ps->offset = int(((quintptr(ps) + sizeof(PrivateShared) + align - 1) & ~(align - 1)) - quintptr(ps));
+    ps->offset = computeOffset(ps, align);
     return ps;
 }