Track up to 5 stable maps as field type.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 4a3b54252dca409223e2227572b0a247e8c40c78..4a0173b5f3a64c5b0fa12d185f88f9c7e1ce7401 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -63,15 +63,16 @@ namespace internal {
 
 Handle<HeapType> Object::OptimalType(Isolate* isolate,
                                      Representation representation) {
-  if (!FLAG_track_field_types) return HeapType::Any(isolate);
-  if (representation.IsNone()) return HeapType::None(isolate);
-  if (representation.IsHeapObject() && IsHeapObject()) {
-    // We can track only JavaScript objects with stable maps.
-    Handle<Map> map(HeapObject::cast(this)->map(), isolate);
-    if (map->is_stable() &&
-        map->instance_type() >= FIRST_NONCALLABLE_SPEC_OBJECT_TYPE &&
-        map->instance_type() <= LAST_NONCALLABLE_SPEC_OBJECT_TYPE) {
-      return HeapType::Class(map, isolate);
+  if (FLAG_track_field_types) {
+    if (representation.IsNone()) return HeapType::None(isolate);
+    if (representation.IsHeapObject() && IsHeapObject()) {
+      // We can track only JavaScript objects with stable maps.
+      Handle<Map> map(HeapObject::cast(this)->map(), isolate);
+      if (map->is_stable() &&
+          map->instance_type() >= FIRST_NONCALLABLE_SPEC_OBJECT_TYPE &&
+          map->instance_type() <= LAST_NONCALLABLE_SPEC_OBJECT_TYPE) {
+        return HeapType::Class(map, isolate);
+      }
     }
   }
   return HeapType::Any(isolate);
@@ -2531,6 +2532,14 @@ Handle<HeapType> Map::GeneralizeFieldType(Handle<HeapType> old_field_type,
                                           Isolate* isolate) {
   if (new_field_type->NowIs(old_field_type)) return old_field_type;
   if (old_field_type->NowIs(new_field_type)) return new_field_type;
+  int num_old_classes = old_field_type->NumClasses();
+  int num_new_classes = new_field_type->NumClasses();
+  if (num_old_classes > 0 &&
+      num_new_classes > 0 &&
+      num_old_classes + num_new_classes <= 5 &&

[Sven Panne, 2014/04/17 06:43:41]

Perhaps give the "5" a name...

[Benedikt Meurer, 2014/04/17 07:26:35]

Done.

+      old_field_type->NowStable() && new_field_type->NowStable()) {
+    return HeapType::Union(new_field_type, old_field_type, isolate);

[Sven Panne, 2014/04/17 06:43:41]

As discussed offline: Perhaps calculate the union first and then check its size.

[Benedikt Meurer, 2014/04/17 07:26:35]

Done.

+  }
   return HeapType::Any(isolate);
 }