QML: Make Heap::Object and all subclasses trivial

GCC6 might dead-store-eliminate out our secret write to Base::mmdata,
because it expects all memory content to be "undefined" before
constructor calls. Clang might take the same approach if the constructor
of Heap::Object is removed.

By making these structs trivial, it also makes them memcpy-able.

Change-Id: I055b2ad28311b997fbe059849ebda4d5894eaa9b
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

2 files changed, 34 insertions, 26 deletions

diff --git a/src/qml/memory/qv4heap_p.h b/src/qml/memory/qv4heap_p.h
index d15d14e463..abbe85c04e 100644
--- a/src/qml/memory/qv4heap_p.h
+++ b/src/qml/memory/qv4heap_p.h
@@ -58,6 +58,12 @@
 // parent's init all up the inheritance chain), define QML_CHECK_INIT_DESTROY_CALLS below.
 #undef QML_CHECK_INIT_DESTROY_CALLS
 
+#if defined(_MSC_VER) && (_MSC_VER < 1900) // broken compilers:
+#  define V4_ASSERT_IS_TRIVIAL(x)
+#else // working compilers:
+#  define V4_ASSERT_IS_TRIVIAL(x) Q_STATIC_ASSERT(std::is_trivial< x >::value);
+#endif
+
 QT_BEGIN_NAMESPACE
 
 namespace QV4 {
@@ -82,6 +88,8 @@ struct VTable
 namespace Heap {
 
 struct Q_QML_EXPORT Base {
+    void *operator new(size_t) = delete;
+
     quintptr mm_data; // vtable and markbit
 
     inline ReturnedValue asReturnedValue() const;
@@ -128,13 +136,19 @@ struct Q_QML_EXPORT Base {
     void init() { setInitialized(); }
 #ifdef QML_CHECK_INIT_DESTROY_CALLS
     bool _isInitialized;
-    void _checkIsInitialized() { Q_ASSERT(_isInitialized); }
+    void _checkIsInitialized() {
+        if (!_isInitialized)
+            fprintf(stderr, "ERROR: use of object '%s' before call to init() !!\n",
+                    vtable()->className);
+        Q_ASSERT(_isInitialized);
+    }
     void setInitialized() { Q_ASSERT(!_isInitialized); _isInitialized = true; }
 #else
     Q_ALWAYS_INLINE void _checkIsInitialized() {}
     Q_ALWAYS_INLINE void setInitialized() {}
 #endif
 };
+V4_ASSERT_IS_TRIVIAL(Base)
 
 template <typename T>
 struct Pointer {
@@ -148,7 +162,7 @@ struct Pointer {
 
     T *ptr;
 };
-Q_STATIC_ASSERT(std::is_trivial<Pointer<void>>::value);
+V4_ASSERT_IS_TRIVIAL(Pointer<void>)
 
 }
 
@@ -204,7 +218,7 @@ private:
     QtSharedPointer::ExternalRefCountData *d;
     QObject *qObject;
 };
-Q_STATIC_ASSERT(std::is_trivial<QQmlQPointer<QObject>>::value);
+V4_ASSERT_IS_TRIVIAL(QQmlQPointer<QObject>)
 #endif
 
 }
diff --git a/src/qml/memory/qv4mm_p.h b/src/qml/memory/qv4mm_p.h
index cfd348e14a..e68fc07a46 100644
--- a/src/qml/memory/qv4mm_p.h
+++ b/src/qml/memory/qv4mm_p.h
@@ -108,6 +108,7 @@ public:
     template<typename ManagedType>
     inline typename ManagedType::Data *allocManaged(std::size_t size, std::size_t unmanagedSize = 0)
     {
+        V4_ASSERT_IS_TRIVIAL(typename ManagedType::Data)
         size = align(size);
         Heap::Base *o = allocData(size, unmanagedSize);
         o->setVtable(ManagedType::staticVTable());
@@ -142,7 +143,6 @@ public:
     template <typename ManagedType, typename Arg1>
     typename ManagedType::Data *allocWithStringData(std::size_t unmanagedSize, Arg1 arg1)
     {
-        Q_STATIC_ASSERT(std::is_trivial<typename ManagedType::Data>::value); // TODO: move down to allocManaged
         Scope scope(engine);
         Scoped<ManagedType> t(scope, allocManaged<ManagedType>(sizeof(typename ManagedType::Data), unmanagedSize));
         t->d_unchecked()->init(this, arg1);
@@ -154,7 +154,7 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>(ic));
-        (void)new (t->d()) typename ObjectType::Data();
+        t->d_unchecked()->init();
         return t->d();
     }
 
@@ -163,8 +163,8 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>(ic));
-        t->d()->prototype = prototype->d();
-        (void)new (t->d()) typename ObjectType::Data();
+        t->d_unchecked()->prototype = prototype->d();
+        t->d_unchecked()->init();
         return t->d();
     }
 
@@ -173,8 +173,8 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>(ic));
-        t->d()->prototype = prototype->d();
-        (void)new (t->d()) typename ObjectType::Data(arg1);
+        t->d_unchecked()->prototype = prototype->d();
+        t->d_unchecked()->init(arg1);
         return t->d();
     }
 
@@ -183,8 +183,8 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>(ic));
-        t->d()->prototype = prototype->d();
-        (void)new (t->d()) typename ObjectType::Data(arg1, arg2);
+        t->d_unchecked()->prototype = prototype->d();
+        t->d_unchecked()->init(arg1, arg2);
         return t->d();
     }
 
@@ -193,8 +193,8 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>(ic));
-        t->d()->prototype = prototype->d();
-        (void)new (t->d()) typename ObjectType::Data(arg1, arg2, arg3);
+        t->d_unchecked()->prototype = prototype->d();
+        t->d_unchecked()->init(arg1, arg2, arg3);
         return t->d();
     }
 
@@ -203,8 +203,8 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>(ic));
-        t->d()->prototype = prototype->d();
-        (void)new (t->d()) typename ObjectType::Data(arg1, arg2, arg3, arg4);
+        t->d_unchecked()->prototype = prototype->d();
+        t->d_unchecked()->init(arg1, arg2, arg3, arg4);
         return t->d();
     }
 
@@ -213,7 +213,7 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>());
-        (void)new (t->d()) typename ObjectType::Data();
+        t->d_unchecked()->init();
         return t->d();
     }
 
@@ -222,7 +222,7 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>());
-        (void)new (t->d()) typename ObjectType::Data(arg1);
+        t->d_unchecked()->init(arg1);
         return t->d();
     }
 
@@ -231,7 +231,7 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>());
-        (void)new (t->d()) typename ObjectType::Data(arg1, arg2);
+        t->d_unchecked()->init(arg1, arg2);
         return t->d();
     }
 
@@ -240,7 +240,7 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>());
-        (void)new (t->d()) typename ObjectType::Data(arg1, arg2, arg3);
+        t->d_unchecked()->init(arg1, arg2, arg3);
         return t->d();
     }
 
@@ -249,7 +249,7 @@ public:
     {
         Scope scope(engine);
         Scoped<ObjectType> t(scope, allocateObject<ObjectType>());
-        (void)new (t->d()) typename ObjectType::Data(arg1, arg2, arg3, arg4);
+        t->d_unchecked()->init(arg1, arg2, arg3, arg4);
         return t->d();
     }
 
@@ -257,7 +257,6 @@ public:
     template <typename ManagedType>
     typename ManagedType::Data *alloc()
     {
-        Q_STATIC_ASSERT(std::is_trivial<typename ManagedType::Data>::value); // TODO: move down to allocManaged
         Scope scope(engine);
         Scoped<ManagedType> t(scope, allocManaged<ManagedType>(sizeof(typename ManagedType::Data)));
         t->d_unchecked()->init();
@@ -267,7 +266,6 @@ public:
     template <typename ManagedType, typename Arg1>
     typename ManagedType::Data *alloc(Arg1 arg1)
     {
-        Q_STATIC_ASSERT(std::is_trivial<typename ManagedType::Data>::value); // TODO: move down to allocManaged
         Scope scope(engine);
         Scoped<ManagedType> t(scope, allocManaged<ManagedType>(sizeof(typename ManagedType::Data)));
         t->d_unchecked()->init(arg1);
@@ -277,7 +275,6 @@ public:
     template <typename ManagedType, typename Arg1, typename Arg2>
     typename ManagedType::Data *alloc(Arg1 arg1, Arg2 arg2)
     {
-        Q_STATIC_ASSERT(std::is_trivial<typename ManagedType::Data>::value); // TODO: move down to allocManaged
         Scope scope(engine);
         Scoped<ManagedType> t(scope, allocManaged<ManagedType>(sizeof(typename ManagedType::Data)));
         t->d_unchecked()->init(arg1, arg2);
@@ -287,7 +284,6 @@ public:
     template <typename ManagedType, typename Arg1, typename Arg2, typename Arg3>
     typename ManagedType::Data *alloc(Arg1 arg1, Arg2 arg2, Arg3 arg3)
     {
-        Q_STATIC_ASSERT(std::is_trivial<typename ManagedType::Data>::value); // TODO: move down to allocManaged
         Scope scope(engine);
         Scoped<ManagedType> t(scope, allocManaged<ManagedType>(sizeof(typename ManagedType::Data)));
         t->d_unchecked()->init(arg1, arg2, arg3);
@@ -297,7 +293,6 @@ public:
     template <typename ManagedType, typename Arg1, typename Arg2, typename Arg3, typename Arg4>
     typename ManagedType::Data *alloc(Arg1 arg1, Arg2 arg2, Arg3 arg3, Arg4 arg4)
     {
-        Q_STATIC_ASSERT(std::is_trivial<typename ManagedType::Data>::value); // TODO: move down to allocManaged
         Scope scope(engine);
         Scoped<ManagedType> t(scope, allocManaged<ManagedType>(sizeof(typename ManagedType::Data)));
         t->d_unchecked()->init(arg1, arg2, arg3, arg4);
@@ -307,7 +302,6 @@ public:
     template <typename ManagedType, typename Arg1, typename Arg2, typename Arg3, typename Arg4, typename Arg5>
     typename ManagedType::Data *alloc(Arg1 arg1, Arg2 arg2, Arg3 arg3, Arg4 arg4, Arg5 arg5)
     {
-        Q_STATIC_ASSERT(std::is_trivial<typename ManagedType::Data>::value); // TODO: move down to allocManaged
         Scope scope(engine);
         Scoped<ManagedType> t(scope, allocManaged<ManagedType>(sizeof(typename ManagedType::Data)));
         t->d_unchecked()->init(arg1, arg2, arg3, arg4, arg5);