Allocation space decisions are precisely made in hydrogen.

src/code-stubs-hydrogen.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 416 matching lines @@
   int size = JSObject::kHeaderSize + casted_stub()->length() * kPointerSize;
   HValue* boilerplate_size =
       AddInstruction(new(zone) HInstanceSize(boilerplate));
   HValue* size_in_words =
       AddInstruction(new(zone) HConstant(size >> kPointerSizeLog2));
   checker.If<HCompareNumericAndBranch>(boilerplate_size,
                                        size_in_words, Token::EQ);
   checker.Then();
 
   HValue* size_in_bytes = AddInstruction(new(zone) HConstant(size));
-  HAllocate::Flags flags = HAllocate::CAN_ALLOCATE_IN_NEW_SPACE;
+  HAllocate::Flags flags = HAllocate::DefaultFlags();
   if (isolate()->heap()->ShouldGloballyPretenure()) {
-    flags = static_cast<HAllocate::Flags>(
-       flags | HAllocate::CAN_ALLOCATE_IN_OLD_POINTER_SPACE);
+    flags = HAllocate::ALLOCATE_IN_OLD_POINTER_SPACE;
   }
 
   HInstruction* object = AddInstruction(new(zone)
       HAllocate(context(), size_in_bytes, HType::JSObject(), flags));
 
   for (int i = 0; i < size; i += kPointerSize) {
     HObjectAccess access = HObjectAccess::ForJSObjectOffset(i);
     AddStore(object, access, AddLoad(boilerplate, access));
   }
 
   environment()->Push(object);
   checker.ElseDeopt();
   checker.End();
 
   return environment()->Pop();
 }
 
 
 Handle<Code> FastCloneShallowObjectStub::GenerateCode() {
   return DoGenerateCode(this);
 }
 
 
 template <>
 HValue* CodeStubGraphBuilder<CreateAllocationSiteStub>::BuildCodeStub() {
   Zone* zone = this->zone();
 
   HValue* size = AddInstruction(new(zone) HConstant(AllocationSite::kSize));
-  HAllocate::Flags flags = HAllocate::DefaultFlags();
-  flags = static_cast<HAllocate::Flags>(
-      flags | HAllocate::CAN_ALLOCATE_IN_OLD_POINTER_SPACE);
+  HAllocate::Flags flags = HAllocate::ALLOCATE_IN_OLD_POINTER_SPACE;
   HInstruction* object = AddInstruction(new(zone)
       HAllocate(context(), size, HType::JSObject(), flags));
 
   // Store the map
   Handle<Map> allocation_site_map(isolate()->heap()->allocation_site_map(),
                                   isolate());
   AddStoreMapConstant(object, allocation_site_map);
 
   // Store the payload (smi elements kind)
   HValue* initial_elements_kind = AddInstruction(new(zone) HConstant(
@@ skipping 470 matching lines @@
   return value;
 }
 
 
 Handle<Code> ElementsTransitionAndStoreStub::GenerateCode() {
   return DoGenerateCode(this);
 }
 
 
 } }  // namespace v8::internal