Allocate QML defined properties without extra room

QV4::MemberData is used to allocate storage for QML defined properties.
QV4::MemberData::reallocate() is multiplying each allocation by two
as a growth strategy. This is a waste of memory for QML defined properties.

This commits extends the QV4::MemberData with an additional method
to allow exactly sized allocations.

Change-Id: I5c0a3a5a3852d39af9e1afb512380fb1400d2448
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

3 files changed, 22 insertions, 11 deletions

diff --git a/src/qml/jsruntime/qv4memberdata.cpp b/src/qml/jsruntime/qv4memberdata.cpp
index 5ec5cb1f58..20f8d9ec0f 100644
--- a/src/qml/jsruntime/qv4memberdata.cpp
+++ b/src/qml/jsruntime/qv4memberdata.cpp
@@ -46,20 +46,30 @@ void MemberData::markObjects(Heap::Base *that, ExecutionEngine *e)
         m->data[i].mark(e);
 }
 
-Heap::MemberData *MemberData::reallocate(ExecutionEngine *e, Heap::MemberData *old, uint idx)
+static Heap::MemberData *reallocateHelper(ExecutionEngine *e, Heap::MemberData *old, uint n)
 {
-    uint s = old ? old->size : 0;
-    if (idx < s)
-        return old;
-
-    int newAlloc = qMax((uint)4, 2*idx);
-    uint alloc = sizeof(Heap::MemberData) + (newAlloc)*sizeof(Value);
+    uint alloc = sizeof(Heap::MemberData) + (n)*sizeof(Value);
     Scope scope(e);
     Scoped<MemberData> newMemberData(scope, e->memoryManager->allocManaged<MemberData>(alloc));
     if (old)
-        memcpy(newMemberData->d(), old, sizeof(Heap::MemberData) + s*sizeof(Value));
+        memcpy(newMemberData->d(), old, sizeof(Heap::MemberData) + old->size * sizeof(Value));
     else
         new (newMemberData->d()) Heap::MemberData;
-    newMemberData->d()->size = newAlloc;
+    newMemberData->d()->size = n;
     return newMemberData->d();
 }
+
+Heap::MemberData *MemberData::allocate(ExecutionEngine *e, uint n)
+{
+    return reallocateHelper(e, 0, n);
+}
+
+Heap::MemberData *MemberData::reallocate(ExecutionEngine *e, Heap::MemberData *old, uint n)
+{
+    uint s = old ? old->size : 0;
+    if (n < s)
+        return old;
+
+    // n is multiplied by two to leave room for growth
+    return reallocateHelper(e, old, qMax((uint)4, 2*n));
+}
diff --git a/src/qml/jsruntime/qv4memberdata_p.h b/src/qml/jsruntime/qv4memberdata_p.h
index 4c7cfa47cb..50b8ddb3d1 100644
--- a/src/qml/jsruntime/qv4memberdata_p.h
+++ b/src/qml/jsruntime/qv4memberdata_p.h
@@ -61,7 +61,8 @@ struct MemberData : Managed
     Value *data() { return d()->data; }
     inline uint size() const { return d()->size; }
 
-    static Heap::MemberData *reallocate(QV4::ExecutionEngine *e, Heap::MemberData *old, uint idx);
+    static Heap::MemberData *allocate(QV4::ExecutionEngine *e, uint n);
+    static Heap::MemberData *reallocate(QV4::ExecutionEngine *e, Heap::MemberData *old, uint n);
 
     static void markObjects(Heap::Base *that, ExecutionEngine *e);
 };
diff --git a/src/qml/qml/qqmlvmemetaobject.cpp b/src/qml/qml/qqmlvmemetaobject.cpp
index 0ffeddbabc..4b19d1af04 100644
--- a/src/qml/qml/qqmlvmemetaobject.cpp
+++ b/src/qml/qml/qqmlvmemetaobject.cpp
@@ -1086,7 +1086,7 @@ void QQmlVMEMetaObject::allocateProperties()
 {
     Q_ASSERT(cache && cache->engine);
     QV4::ExecutionEngine *v4 = cache->engine;
-    QV4::Heap::MemberData *data = QV4::MemberData::reallocate(v4, 0, metaData->propertyCount);
+    QV4::Heap::MemberData *data = QV4::MemberData::allocate(v4, metaData->propertyCount);
     properties.set(v4, data);
     for (uint i = 0; i < data->size; ++i)
         data->data[i] = QV4::Encode::undefined();