Only heap allocate binding bits storage if needed.

For samegame, this has the following change on the total bytes allocated:

Startup (main page):
    Before: 1636
     After: 1072
Difference: 564 bytes (-34%)

Actual game (single player):
    Before: 14120
     After: 10432
Difference: 3688 bytes (-26%)

Done-with: Robin Burchell <robin.burchell@viroteck.net>
Change-Id: I29b7a79400090180f405693093f266c151260fcc
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

2 files changed, 43 insertions, 15 deletions

diff --git a/src/qml/qml/qqmldata_p.h b/src/qml/qml/qqmldata_p.h
index c9bae8e774..c7654be545 100644
--- a/src/qml/qml/qqmldata_p.h
+++ b/src/qml/qml/qqmldata_p.h
@@ -123,8 +123,14 @@ public:
     quint32 parentFrozen:1;
     quint32 dummy:22;
 
+    // When bindingBitsSize < 32, we store the binding bit flags inside
+    // bindingBitsValue. When we need more than 32 bits, we allocated
+    // sufficient space and use bindingBits to point to it.
     int bindingBitsSize;
-    quint32 *bindingBits;
+    union {
+        quint32 *bindingBits;
+        quint32 bindingBitsValue;
+    };
 
     struct NotifyList {
         quint64 connectionMask;
@@ -261,19 +267,19 @@ QQmlNotifierEndpoint *QQmlData::notify(int index)
 bool QQmlData::hasBindingBit(int coreIndex) const
 {
     int bit = coreIndex * 2;
-    if (bindingBitsSize > bit)
-        return bindingBits[bit / 32] & (1 << (bit % 32));
-    else
-        return false;
+
+    return bindingBitsSize > bit &&
+           ((bindingBitsSize == 32) ? (bindingBitsValue & (1 << bit)) :
+                                      (bindingBits[bit / 32] & (1 << (bit % 32))));
 }
 
 bool QQmlData::hasPendingBindingBit(int coreIndex) const
 {
     int bit = coreIndex * 2 + 1;
-    if (bindingBitsSize > bit)
-        return bindingBits[bit / 32] & (1 << (bit % 32));
-    else
-        return false;
+
+    return bindingBitsSize > bit &&
+           ((bindingBitsSize == 32) ? (bindingBitsValue & (1 << bit)) :
+                                      (bindingBits[bit / 32] & (1 << (bit % 32))));
 }
 
 void QQmlData::flushPendingBinding(QObject *o, int coreIndex)
diff --git a/src/qml/qml/qqmlengine.cpp b/src/qml/qml/qqmlengine.cpp
index 33be4c321f..ab67d7738d 100644
--- a/src/qml/qml/qqmlengine.cpp
+++ b/src/qml/qml/qqmlengine.cpp
@@ -1685,7 +1685,7 @@ void QQmlData::destroyed(QObject *object)
         signalHandler = next;
     }
 
-    if (bindingBits)
+    if (bindingBitsSize > 32)
         free(bindingBits);
 
     if (propertyCache)
@@ -1733,14 +1733,24 @@ void QQmlData::parentChanged(QObject *object, QObject *parent)
 
 static void QQmlData_setBit(QQmlData *data, QObject *obj, int bit)
 {
+    if (data->bindingBitsSize == 0 && bit < 32) {
+        data->bindingBitsSize = 32;
+    }
+
     if (data->bindingBitsSize <= bit) {
         int props = QQmlMetaObject(obj).propertyCount();
         Q_ASSERT(bit < 2 * props);
 
         int arraySize = (2 * props + 31) / 32;
-        int oldArraySize = data->bindingBitsSize / 32;
+        Q_ASSERT(arraySize > 1);
 
-        data->bindingBits = (quint32 *)realloc(data->bindingBits,
+        // special handling for 32 here is to make sure we wipe the first byte
+        // when going from bindingBitsValue to bindingBits, and preserve the old
+        // set bits so we can restore them after the allocation
+        int oldArraySize = data->bindingBitsSize > 32 ? data->bindingBitsSize / 32 : 0;
+        quint32 oldValue = data->bindingBitsSize == 32 ? data->bindingBitsValue : 0;
+
+        data->bindingBits = (quint32 *)realloc((data->bindingBitsSize == 32) ? 0 : data->bindingBits,
                                                arraySize * sizeof(quint32));
 
         memset(data->bindingBits + oldArraySize,
@@ -1748,15 +1758,27 @@ static void QQmlData_setBit(QQmlData *data, QObject *obj, int bit)
                sizeof(quint32) * (arraySize - oldArraySize));
 
         data->bindingBitsSize = arraySize * 32;
+
+        // reinstate bindingBitsValue after we dropped it
+        if (oldValue) {
+            memcpy(data->bindingBits, &oldValue, sizeof(oldValue));
+        }
     }
 
-    data->bindingBits[bit / 32] |= (1 << (bit % 32));
+    if (data->bindingBitsSize == 32)
+        data->bindingBitsValue |= (1 << (bit % 32));
+    else
+        data->bindingBits[bit / 32] |= (1 << (bit % 32));
 }
 
 static void QQmlData_clearBit(QQmlData *data, int bit)
 {
-    if (data->bindingBitsSize > bit)
-        data->bindingBits[bit / 32] &= ~(1 << (bit % 32));
+    if (data->bindingBitsSize > bit) {
+        if (data->bindingBitsSize == 32)
+            data->bindingBitsValue &= ~(1 << (bit % 32));
+        else
+            data->bindingBits[bit / 32] &= ~(1 << (bit % 32));
+    }
 }
 
 void QQmlData::clearBindingBit(int coreIndex)