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

src/objects.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 7884 matching lines @@
   // that are a multiple of pointer size.
   Address end_of_string = address() + string_size;
   heap->CreateFillerObjectAt(end_of_string, delta);
   if (Marking::IsBlack(Marking::MarkBitFrom(this))) {
     MemoryChunk::IncrementLiveBytesFromMutator(address(), -delta);
   }
   return this;
 }
 
 
+AllocationSiteInfo* AllocationSiteInfo::FindAfterJSObject(JSObject* object) {

[danno, 2013/01/04 08:50:55]

How about "FindForJSObject?" FindAfterJSObject implies the implementation detail
of how the object is actually found, which the caller doesn't really care about.

[mvstanton, 2013/01/04 12:07:52]

Done.

+  // Are we in new space and using the allocation site info feature?

[danno, 2013/01/04 08:50:55]

nit: the comment seems a little superfluous, since the following code line does
exactly what it states. Perhaps instead a comment describing under which
circumstances such an object is found, i.e. something like "currently,
AllocationSiteInfo objects are only allocated immediately after JSArrays in
NewSpace, and detecting whether a JSArray has one involves carefully checking
the object immediately after the JSArray if there is one whether it qualifies". 

[mvstanton, 2013/01/04 12:07:52]

Way more useful comment, thx!

+  if (FLAG_use_allocation_site_info && object->GetHeap()->InNewSpace(object)) {
+    Address ptr_end = (reinterpret_cast<Address>(object) - kHeapObjectTag) +
+        object->Size();
+    if ((ptr_end + AllocationSiteInfo::kSize) <=
+        object->GetHeap()->NewSpaceTop()) {
+      // There is room in newspace for allocation info. Do we have some?
+      Map** possible_allocation_map = reinterpret_cast<Map**>(ptr_end);
+      if (*possible_allocation_map ==
+          object->GetHeap()->allocation_site_info_map()) {
+        AllocationSiteInfo* info = AllocationSiteInfo::cast(
+            reinterpret_cast<Object*>(ptr_end + 1));
+        return info;
+      }
+    }
+  }
+  return NULL;
+}
+
+
 uint32_t StringHasher::MakeArrayIndexHash(uint32_t value, int length) {
   // For array indexes mix the length into the hash as an array index could
   // be zero.
   ASSERT(length > 0);
   ASSERT(length <= String::kMaxArrayIndexSize);
   ASSERT(TenToThe(String::kMaxCachedArrayIndexLength) <
          (1 << String::kArrayIndexValueBits));
 
   value <<= String::kHashShift;
   value |= length << String::kArrayIndexHashLengthShift;
@@ skipping 2326 matching lines @@
     }
     if (convert_to_slow) {
       MaybeObject* result = NormalizeElements();
       if (result->IsFailure()) return result;
       return SetDictionaryElement(index, value, NONE, strict_mode,
                                   check_prototype);
     }
   }
   // Convert to fast double elements if appropriate.
   if (HasFastSmiElements() && !value->IsSmi() && value->IsNumber()) {
+    // Consider fixing the boilerplate as well if we have one.
+    ElementsKind to_kind = IsHoleyElementsKind(elements_kind)
+        ? FAST_HOLEY_DOUBLE_ELEMENTS
+        : FAST_DOUBLE_ELEMENTS;
+
+    MaybeObject* trans = PossiblyTransitionBoilerplate(to_kind);
+    if (trans != NULL && trans->IsFailure()) return trans;
+
     MaybeObject* maybe =
         SetFastDoubleElementsCapacityAndLength(new_capacity, array_length);
     if (maybe->IsFailure()) return maybe;
     FixedDoubleArray::cast(elements())->set(index, value->Number());
     ValidateElements();
     return value;
   }
   // Change elements kind from Smi-only to generic FAST if necessary.
   if (HasFastSmiElements() && !value->IsSmi()) {
     Map* new_map;
@@ skipping 521 matching lines @@
 
 
 Handle<Object> JSObject::TransitionElementsKind(Handle<JSObject> object,
                                                 ElementsKind to_kind) {
   CALL_HEAP_FUNCTION(object->GetIsolate(),
                      object->TransitionElementsKind(to_kind),
                      Object);
 }
 
 
+MaybeObject* JSObject::PossiblyTransitionBoilerplate(ElementsKind to_kind) {

[danno, 2013/01/04 08:50:55]

Do you want to ASSERT(IsJSArray())? Maybe make the name
PossibleTransitionArrayBoilerplate?

[mvstanton, 2013/01/04 12:07:52]

Done.

+  MaybeObject* ret = NULL;
+  // Are we in new space?

[danno, 2013/01/04 08:50:55]

nit: comment seems incorrect, the FindAfterJSObject does more.

[mvstanton, 2013/01/04 12:07:52]

Done.

+  AllocationSiteInfo* info = AllocationSiteInfo::FindAfterJSObject(this);
+  if (info != NULL) {
+    JSObject* payload = JSObject::cast(info->payload());
+    if (payload->GetElementsKind() != to_kind) {
+      if (IsMoreGeneralElementsKindTransition(payload->GetElementsKind(),
+                                              to_kind)) {
+        ret = payload->TransitionElementsKind(to_kind);
+        if (ret->IsFailure()) return ret;
+      }
+    }
+  }
+  return ret;
+}
+
+
 MaybeObject* JSObject::TransitionElementsKind(ElementsKind to_kind) {
   ASSERT(!map()->is_observed());
   ElementsKind from_kind = map()->elements_kind();
 
   if (IsFastHoleyElementsKind(from_kind)) {
     to_kind = GetHoleyElementsKind(to_kind);
   }
 
   if (from_kind == to_kind) return this;
 
+  MaybeObject* trans = PossiblyTransitionBoilerplate(to_kind);
+  if (trans != NULL && trans->IsFailure()) return trans;
+
   Isolate* isolate = GetIsolate();
   if (elements() == isolate->heap()->empty_fixed_array() ||
       (IsFastSmiOrObjectElementsKind(from_kind) &&
        IsFastSmiOrObjectElementsKind(to_kind)) ||
       (from_kind == FAST_DOUBLE_ELEMENTS &&
        to_kind == FAST_HOLEY_DOUBLE_ELEMENTS)) {
     ASSERT(from_kind != TERMINAL_FAST_ELEMENTS_KIND);
     // No change is needed to the elements() buffer, the transition
     // only requires a map change.
     MaybeObject* maybe_new_map = GetElementsTransitionMap(isolate, to_kind);
@@ skipping 3330 matching lines @@
   set_year(Smi::FromInt(year), SKIP_WRITE_BARRIER);
   set_month(Smi::FromInt(month), SKIP_WRITE_BARRIER);
   set_day(Smi::FromInt(day), SKIP_WRITE_BARRIER);
   set_weekday(Smi::FromInt(weekday), SKIP_WRITE_BARRIER);
   set_hour(Smi::FromInt(hour), SKIP_WRITE_BARRIER);
   set_min(Smi::FromInt(min), SKIP_WRITE_BARRIER);
   set_sec(Smi::FromInt(sec), SKIP_WRITE_BARRIER);
 }
 
 } }  // namespace v8::internal