Stop using the reference counter to store data state

Instead of using the reference count to store whether the data is
sharable and whether the header is immutable, move the settings to the
flags member. This allows us to save one comparison per deref() or
needsDetach(). It also allows for the possibility of mutable data
pointed to by a static header.

Change-Id: Ie678a2ff2bb9bce73497cb6138b431c465b0f3bb
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 17 insertions, 10 deletions

diff --git a/src/corelib/tools/qarraydata.h b/src/corelib/tools/qarraydata.h
index 183cf9f002..56d61340f0 100644
--- a/src/corelib/tools/qarraydata.h
+++ b/src/corelib/tools/qarraydata.h
@@ -56,18 +56,19 @@ struct Q_CORE_EXPORT QArrayData
         CapacityReserved     = 0x0010,  //!< the capacity was reserved by the user, try to keep it
         GrowsForward         = 0x0020,  //!< allocate with eyes towards growing through append()
         GrowsBackwards       = 0x0040,  //!< allocate with eyes towards growing through prepend()
-        Mutable              = 0x0080,  //!< the data can be changed; doesn't say anything about the header
+        MutableData          = 0x0080,  //!< the data can be changed; doesn't say anything about the header
+        ImmutableHeader      = 0x0100,  //!< the header is static, it can't be changed
 
         /// this option is used by the Q_ARRAY_LITERAL and similar macros
-        StaticDataFlags = RawDataType,
+        StaticDataFlags = RawDataType | ImmutableHeader,
         /// this option is used by the allocate() function
-        DefaultAllocationFlags = 0,
+        DefaultAllocationFlags = MutableData,
         /// this option is used by the prepareRawData() function
         DefaultRawFlags = 0
     };
     Q_DECLARE_FLAGS(ArrayOptions, ArrayOption)
 
-    QtPrivate::RefCount ref_;
+    QBasicAtomicInt ref_;
     uint flags;
     int size;
     uint alloc;
@@ -84,13 +85,19 @@ struct Q_CORE_EXPORT QArrayData
         return alloc;
     }
 
+    /// Returns true if sharing took place
     bool ref()
     {
-        return ref_.ref();
+        if (!isStatic())
+            ref_.ref();
+        return true;
     }
 
+    /// Returns false if deallocation is necessary
     bool deref()
     {
+        if (isStatic())
+            return true;
         return ref_.deref();
     }
 
@@ -113,17 +120,17 @@ struct Q_CORE_EXPORT QArrayData
     // follow COW principles.
     bool isMutable() const
     {
-        return flags & Mutable;
+        return flags & MutableData;
     }
 
     bool isStatic() const
     {
-        return ref_.isStatic();
+        return flags & ImmutableHeader;
     }
 
     bool isShared() const
     {
-        return ref_.isShared();
+        return ref_.loadRelaxed() != 1;
     }
 
     // Returns true if a detach is necessary before modifying the data
@@ -131,7 +138,7 @@ struct Q_CORE_EXPORT QArrayData
     // detaching is necessary, you should be in a non-const function already
     bool needsDetach()
     {
-        // ### optimize me -- this currently requires 3 conditionals!
+        // requires two conditionals
         return !isMutable() || isShared();
     }
 
@@ -354,7 +361,7 @@ struct QArrayDataPointerRef
 };
 
 #define Q_STATIC_ARRAY_DATA_HEADER_INITIALIZER_WITH_OFFSET(size, offset) \
-    { Q_REFCOUNT_INITIALIZE_STATIC, QArrayData::StaticDataFlags, size, 0, offset } \
+    { Q_BASIC_ATOMIC_INITIALIZER(-1), QArrayData::StaticDataFlags, size, 0, offset } \
     /**/
 
 #define Q_STATIC_ARRAY_DATA_HEADER_INITIALIZER(type, size) \