Long live incremental garbage collection in QML!

The design of the garbage collector is described in
src/qml/memory/design.md.

The gc and gcdone test helpers are adjusted to drive the gc to
completion, even when in incremental mode.

Parts of tst_qv4mm and tst_qqmlqt need to run with the incremental gc
disabled, as they call gc inside QML and assumes that the GC finishes
before returning.

Initial-patch-by: Rafal Chomentowski <rafal.chomentowski@ge.com>
Task-number: QTBUG-119274
Change-Id: I1d94f41bc7a434fad67de0fd46454b6db285f2eb
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

1 files changed, 10 insertions, 2 deletions

diff --git a/src/qml/jsapi/qjsvalue_p.h b/src/qml/jsapi/qjsvalue_p.h
index b95e76e76e..4624652c93 100644
--- a/src/qml/jsapi/qjsvalue_p.h
+++ b/src/qml/jsapi/qjsvalue_p.h
@@ -150,9 +150,17 @@ public:
         case QV4::StaticValue::Integer_Type:
             return encode(qv4Value.integerValue());
         case QV4::StaticValue::Managed_Type: {
-            QV4::Value *m = qv4Value.as<QV4::Managed>()->engine()
-                    ->memoryManager->m_persistentValues->allocate();
+            auto managed = qv4Value.as<QV4::Managed>();
+            auto engine = managed->engine();
+            auto mm = engine->memoryManager;
+            QV4::Value *m = mm->m_persistentValues->allocate();
             Q_ASSERT(m);
+            // we create a new strong reference to the heap managed object
+            // to avoid having to rescan the persistent values, we mark it here
+            QV4::WriteBarrier::markCustom(engine, [&](QV4::MarkStack *stack){
+                if constexpr (QV4::WriteBarrier::isInsertionBarrier)
+                    managed->heapObject()->mark(stack);
+            });
             *m = qv4Value;
             return encodePointer(m, Kind::QV4ValuePtr);
         }