Implement a real write barrier

Implement a Steel write barrier for our objects. The barrier is
interesting as it can also be used for incremental GC runs by
simply turning the barrier on and leaving old objects marked as
black.

Change-Id: I0b273974d94a990dee3cd9298089b8b202c75bf2
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 7 insertions, 6 deletions

diff --git a/src/qml/memory/qv4mm_p.h b/src/qml/memory/qv4mm_p.h
index 6e9303acb6..3e542b0aa3 100644
--- a/src/qml/memory/qv4mm_p.h
+++ b/src/qml/memory/qv4mm_p.h
@@ -80,27 +80,28 @@ struct StackAllocator {
     StackAllocator(ChunkAllocator *chunkAlloc);
 
     T *allocate() {
-        T *m = nextFree->as<T>();
+        HeapItem *m = nextFree;
         if (Q_UNLIKELY(nextFree == lastInChunk)) {
             nextChunk();
         } else {
             nextFree += requiredSlots;
         }
-#if MM_DEBUG
+#if !defined(QT_NO_DEBUG) || defined(MM_DEBUG)
         Chunk *c = m->chunk();
         Chunk::setBit(c->objectBitmap, m - c->realBase());
 #endif
-        return m;
+        return m->as<T>();
     }
     void free() {
-#if MM_DEBUG
-        Chunk::clearBit(item->chunk()->objectBitmap, item - item->chunk()->realBase());
-#endif
         if (Q_UNLIKELY(nextFree == firstInChunk)) {
             prevChunk();
         } else {
             nextFree -= requiredSlots;
         }
+#if !defined(QT_NO_DEBUG) || defined(MM_DEBUG)
+        Chunk *c = nextFree->chunk();
+        Chunk::clearBit(c->objectBitmap, nextFree - c->realBase());
+#endif
     }
 
     void nextChunk();