Adapt Danno's Track Allocation Info idea to fast literals. When allocating a literal array,

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 1d95a5c15271ab3d955e2cbd3f2b6ad1b270cc25..1201c8b0793cec2b1a29b2bae591b36b6eea79e1 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -10637,6 +10637,32 @@ MaybeObject* JSObject::TransitionElementsKind(ElementsKind to_kind) {
 
   if (from_kind == to_kind) return this;
 
+  // Are we in new space?
+  if (FLAG_use_allocation_site_info && GetHeap()->InNewSpace(this)) {

[danno, 2012/12/26 10:32:01]

The machinery that checks/gets an AllocationSiteInfo for an Object might be
nicely encapsulated as a static method on the AllocationSiteInfo, either
returning a valid AllocationSizeInfo* or NULL/undefined if there is none.

[mvstanton, 2013/01/03 14:40:43]

Done.

+    // Are we below the top of the heap?
+    Address ptr_end = (reinterpret_cast<Address>(this) - 1) + Size();
+    if ((ptr_end + AllocationSiteInfo::kSize) <= GetHeap()->NewSpaceTop()) {
+      // There is room for allocation info. Do we have some?
+      Map** possible_allocation_map = reinterpret_cast<Map**>(ptr_end);
+      if (*possible_allocation_map == GetHeap()->allocation_site_info_map()) {
+        AllocationSiteInfo* info = AllocationSiteInfo::cast(
+            reinterpret_cast<Object*>(ptr_end + 1));
+        JSObject* payload = JSObject::cast(info->payload());
+        if (payload->GetElementsKind() != to_kind) {
+          if (IsMoreGeneralElementsKindTransition(
+                  payload->GetElementsKind(),
+                  to_kind)) {
+            PrintF("Transitioning array to %s\n",
+                   ElementsKindToString(to_kind));
+            MaybeObject* maybe_result =
+                payload->TransitionElementsKind(to_kind);
+            if (maybe_result->IsFailure()) return maybe_result;
+          }
+        }
+      }
+    }
+  }
+
   Isolate* isolate = GetIsolate();
   if (elements() == isolate->heap()->empty_fixed_array() ||
       (IsFastSmiOrObjectElementsKind(from_kind) &&