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

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
diff --git a/src/x64/codegen-x64.cc b/src/x64/codegen-x64.cc
index 7954604e999160e9dff1f677855d5c8ad1f7213d..c9c298f6ecc5fdd7bb9d68b2b8e7035c7115abde 100644
--- a/src/x64/codegen-x64.cc
+++ b/src/x64/codegen-x64.cc
@@ -283,6 +283,10 @@ void ElementsTransitionGenerator::GenerateSmiToDouble(
   // The fail label is not actually used since we do not allocate.
   Label allocated, new_backing_store, only_change_map, done;
 
+  if (FLAG_use_allocation_site_info) {
+    masm->PerformAllocationSiteInfoCheck(fail);
+  }
+
   // Check for empty arrays, which only require a map transition and no changes
   // to the backing store.
   __ movq(r8, FieldOperand(rdx, JSObject::kElementsOffset));
@@ -404,6 +408,11 @@ void ElementsTransitionGenerator::GenerateDoubleToObject(
   // -----------------------------------
   Label loop, entry, convert_hole, gc_required, only_change_map;
 
+  // TODO(mvstanton): let's not pre-transition for the double->object case.
+  // if (FLAG_use_allocation_site_info) {
+  //   masm->PerformAllocationSiteInfoCheck(fail);
+  // }

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

Remove

+
   // Check for empty arrays, which only require a map transition and no changes
   // to the backing store.
   __ movq(r8, FieldOperand(rdx, JSObject::kElementsOffset));