text: // Copyright 2012 the V8 project authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  #ifndef V8_HEAP_HEAP_H_
  #define V8_HEAP_HEAP_H_
  
  #include <cmath>
  #include <map>
  
  #include "src/allocation.h"
  #include "src/assert-scope.h"
  #include "src/counters.h"
  #include "src/globals.h"
  #include "src/heap/gc-idle-time-handler.h"
  #include "src/heap/gc-tracer.h"
  #include "src/heap/incremental-marking.h"
  #include "src/heap/mark-compact.h"
  #include "src/heap/objects-visiting.h"
  #include "src/heap/spaces.h"
  #include "src/heap/store-buffer.h"
  #include "src/list.h"
  #include "src/splay-tree-inl.h"
  
  namespace v8 {
  namespace internal {
  
  // Defines all the roots in Heap.
  #define STRONG_ROOT_LIST(V)                                                    \
    V(Map, byte_array_map, ByteArrayMap)                                         \
    V(Map, free_space_map, FreeSpaceMap)                                         \
    V(Map, one_pointer_filler_map, OnePointerFillerMap)                          \
    V(Map, two_pointer_filler_map, TwoPointerFillerMap)                          \
    /* Cluster the most popular ones in a few cache lines here at the top.    */ \
    V(Smi, store_buffer_top, StoreBufferTop)                                     \
    V(Oddball, undefined_value, UndefinedValue)                                  \
    V(Oddball, the_hole_value, TheHoleValue)                                     \
    V(Oddball, null_value, NullValue)                                            \
    V(Oddball, true_value, TrueValue)                                            \
    V(Oddball, false_value, FalseValue)                                          \
    V(String, empty_string, empty_string)                                        \
    V(Oddball, uninitialized_value, UninitializedValue)                          \
    V(Map, cell_map, CellMap)                                                    \
    V(Map, global_property_cell_map, GlobalPropertyCellMap)                      \
    V(Map, shared_function_info_map, SharedFunctionInfoMap)                      \
    V(Map, meta_map, MetaMap)                                                    \
    V(Map, heap_number_map, HeapNumberMap)                                       \
    V(Map, mutable_heap_number_map, MutableHeapNumberMap)                        \
    V(Map, native_context_map, NativeContextMap)                                 \
    V(Map, fixed_array_map, FixedArrayMap)                                       \
    V(Map, code_map, CodeMap)                                                    \
    V(Map, scope_info_map, ScopeInfoMap)                                         \
    V(Map, fixed_cow_array_map, FixedCOWArrayMap)                                \
    V(Map, fixed_double_array_map, FixedDoubleArrayMap)                          \
    V(Map, constant_pool_array_map, ConstantPoolArrayMap)                        \
    V(Map, weak_cell_map, WeakCellMap)                                           \
    V(Map, one_byte_string_map, OneByteStringMap)                                \
    V(Map, one_byte_internalized_string_map, OneByteInternalizedStringMap)       \
    V(Map, function_context_map, FunctionContextMap)                             \
    V(FixedArray, empty_fixed_array, EmptyFixedArray)                            \
    V(ByteArray, empty_byte_array, EmptyByteArray)                               \
    V(DescriptorArray, empty_descriptor_array, EmptyDescriptorArray)             \
    V(ConstantPoolArray, empty_constant_pool_array, EmptyConstantPoolArray)      \
    /* The roots above this line should be boring from a GC point of view.    */ \
    /* This means they are never in new space and never on a page that is     */ \
    /* being compacted.                                                       */ \
    V(Oddball, no_interceptor_result_sentinel, NoInterceptorResultSentinel)      \
    V(Oddball, arguments_marker, ArgumentsMarker)                                \
    V(Oddball, exception, Exception)                                             \
    V(Oddball, termination_exception, TerminationException)                      \
    V(FixedArray, number_string_cache, NumberStringCache)                        \
    V(Object, instanceof_cache_function, InstanceofCacheFunction)                \
    V(Object, instanceof_cache_map, InstanceofCacheMap)                          \
    V(Object, instanceof_cache_answer, InstanceofCacheAnswer)                    \
    V(FixedArray, single_character_string_cache, SingleCharacterStringCache)     \
    V(FixedArray, string_split_cache, StringSplitCache)                          \
    V(FixedArray, regexp_multiple_cache, RegExpMultipleCache)                    \
    V(Smi, hash_seed, HashSeed)                                                  \
    V(Map, hash_table_map, HashTableMap)                                         \
    V(Map, ordered_hash_table_map, OrderedHashTableMap)                          \
    V(Map, symbol_map, SymbolMap)                                                \
    V(Map, string_map, StringMap)                                                \
    V(Map, cons_one_byte_string_map, ConsOneByteStringMap)                       \
    V(Map, cons_string_map, ConsStringMap)                                       \
    V(Map, sliced_string_map, SlicedStringMap)                                   \
    V(Map, sliced_one_byte_string_map, SlicedOneByteStringMap)                   \
    V(Map, external_string_map, ExternalStringMap)                               \
    V(Map, external_string_with_one_byte_data_map,                               \
      ExternalStringWithOneByteDataMap)                                          \
    V(Map, external_one_byte_string_map, ExternalOneByteStringMap)               \
    V(Map, native_source_string_map, NativeSourceStringMap)                      \
    V(Map, short_external_string_map, ShortExternalStringMap)                    \
    V(Map, short_external_string_with_one_byte_data_map,                         \
      ShortExternalStringWithOneByteDataMap)                                     \
    V(Map, internalized_string_map, InternalizedStringMap)                       \
    V(Map, external_internalized_string_map, ExternalInternalizedStringMap)      \
    V(Map, external_internalized_string_with_one_byte_data_map,                  \
      ExternalInternalizedStringWithOneByteDataMap)                              \
    V(Map, external_one_byte_internalized_string_map,                            \
      ExternalOneByteInternalizedStringMap)                                      \
    V(Map, short_external_internalized_string_map,                               \
      ShortExternalInternalizedStringMap)                                        \
    V(Map, short_external_internalized_string_with_one_byte_data_map,            \
      ShortExternalInternalizedStringWithOneByteDataMap)                         \
    V(Map, short_external_one_byte_internalized_string_map,                      \
      ShortExternalOneByteInternalizedStringMap)                                 \
    V(Map, short_external_one_byte_string_map, ShortExternalOneByteStringMap)    \
    V(Map, external_int8_array_map, ExternalInt8ArrayMap)                        \
    V(Map, external_uint8_array_map, ExternalUint8ArrayMap)                      \
    V(Map, external_int16_array_map, ExternalInt16ArrayMap)                      \
    V(Map, external_uint16_array_map, ExternalUint16ArrayMap)                    \
    V(Map, external_int32_array_map, ExternalInt32ArrayMap)                      \
    V(Map, external_uint32_array_map, ExternalUint32ArrayMap)                    \
    V(Map, external_float32_array_map, ExternalFloat32ArrayMap)                  \
    V(Map, external_float64_array_map, ExternalFloat64ArrayMap)                  \
    V(Map, external_uint8_clamped_array_map, ExternalUint8ClampedArrayMap)       \
    V(ExternalArray, empty_external_int8_array, EmptyExternalInt8Array)          \
    V(ExternalArray, empty_external_uint8_array, EmptyExternalUint8Array)        \
    V(ExternalArray, empty_external_int16_array, EmptyExternalInt16Array)        \
    V(ExternalArray, empty_external_uint16_array, EmptyExternalUint16Array)      \
    V(ExternalArray, empty_external_int32_array, EmptyExternalInt32Array)        \
    V(ExternalArray, empty_external_uint32_array, EmptyExternalUint32Array)      \
    V(ExternalArray, empty_external_float32_array, EmptyExternalFloat32Array)    \
    V(ExternalArray, empty_external_float64_array, EmptyExternalFloat64Array)    \
    V(ExternalArray, empty_external_uint8_clamped_array,                         \
      EmptyExternalUint8ClampedArray)                                            \
    V(Map, fixed_uint8_array_map, FixedUint8ArrayMap)                            \
    V(Map, fixed_int8_array_map, FixedInt8ArrayMap)                              \
    V(Map, fixed_uint16_array_map, FixedUint16ArrayMap)                          \
    V(Map, fixed_int16_array_map, FixedInt16ArrayMap)                            \
    V(Map, fixed_uint32_array_map, FixedUint32ArrayMap)                          \
    V(Map, fixed_int32_array_map, FixedInt32ArrayMap)                            \
    V(Map, fixed_float32_array_map, FixedFloat32ArrayMap)                        \
    V(Map, fixed_float64_array_map, FixedFloat64ArrayMap)                        \
    V(Map, fixed_uint8_clamped_array_map, FixedUint8ClampedArrayMap)             \
    V(FixedTypedArrayBase, empty_fixed_uint8_array, EmptyFixedUint8Array)        \
    V(FixedTypedArrayBase, empty_fixed_int8_array, EmptyFixedInt8Array)          \
    V(FixedTypedArrayBase, empty_fixed_uint16_array, EmptyFixedUint16Array)      \
    V(FixedTypedArrayBase, empty_fixed_int16_array, EmptyFixedInt16Array)        \
    V(FixedTypedArrayBase, empty_fixed_uint32_array, EmptyFixedUint32Array)      \
    V(FixedTypedArrayBase, empty_fixed_int32_array, EmptyFixedInt32Array)        \
    V(FixedTypedArrayBase, empty_fixed_float32_array, EmptyFixedFloat32Array)    \
    V(FixedTypedArrayBase, empty_fixed_float64_array, EmptyFixedFloat64Array)    \
    V(FixedTypedArrayBase, empty_fixed_uint8_clamped_array,                      \
      EmptyFixedUint8ClampedArray)                                               \
    V(Map, sloppy_arguments_elements_map, SloppyArgumentsElementsMap)            \
    V(Map, catch_context_map, CatchContextMap)                                   \
    V(Map, with_context_map, WithContextMap)                                     \
    V(Map, block_context_map, BlockContextMap)                                   \
    V(Map, module_context_map, ModuleContextMap)                                 \
    V(Map, script_context_map, ScriptContextMap)                                 \
    V(Map, script_context_table_map, ScriptContextTableMap)                      \
    V(Map, undefined_map, UndefinedMap)                                          \
    V(Map, the_hole_map, TheHoleMap)                                             \
    V(Map, null_map, NullMap)                                                    \
    V(Map, boolean_map, BooleanMap)                                              \
    V(Map, uninitialized_map, UninitializedMap)                                  \
    V(Map, arguments_marker_map, ArgumentsMarkerMap)                             \
    V(Map, no_interceptor_result_sentinel_map, NoInterceptorResultSentinelMap)   \
    V(Map, exception_map, ExceptionMap)                                          \
    V(Map, termination_exception_map, TerminationExceptionMap)                   \
    V(Map, message_object_map, JSMessageObjectMap)                               \
    V(Map, foreign_map, ForeignMap)                                              \
    V(Map, neander_map, NeanderMap)                                              \
    V(Map, external_map, ExternalMap)                                            \
    V(HeapNumber, nan_value, NanValue)                                           \
    V(HeapNumber, infinity_value, InfinityValue)                                 \
    V(HeapNumber, minus_zero_value, MinusZeroValue)                              \
    V(JSObject, message_listeners, MessageListeners)                             \
    V(UnseededNumberDictionary, code_stubs, CodeStubs)                           \
    V(UnseededNumberDictionary, non_monomorphic_cache, NonMonomorphicCache)      \
    V(PolymorphicCodeCache, polymorphic_code_cache, PolymorphicCodeCache)        \
    V(Code, js_entry_code, JsEntryCode)                                          \
    V(Code, js_construct_entry_code, JsConstructEntryCode)                       \
    V(FixedArray, natives_source_cache, NativesSourceCache)                      \
    V(FixedArray, experimental_natives_source_cache,                             \
      ExperimentalNativesSourceCache)                                            \
    V(FixedArray, extra_natives_source_cache, ExtraNativesSourceCache)           \
    V(Script, empty_script, EmptyScript)                                         \
    V(NameDictionary, intrinsic_function_names, IntrinsicFunctionNames)          \
    V(Cell, undefined_cell, UndefinedCell)                                       \
    V(JSObject, observation_state, ObservationState)                             \
    V(Object, symbol_registry, SymbolRegistry)                                   \
    V(SeededNumberDictionary, empty_slow_element_dictionary,                     \
      EmptySlowElementDictionary)                                                \
    V(FixedArray, materialized_objects, MaterializedObjects)                     \
    V(FixedArray, allocation_sites_scratchpad, AllocationSitesScratchpad)        \
    V(FixedArray, microtask_queue, MicrotaskQueue)                               \
    V(FixedArray, keyed_load_dummy_vector, KeyedLoadDummyVector)                 \
    V(FixedArray, detached_contexts, DetachedContexts)                           \
    V(ArrayList, retained_maps, RetainedMaps)                                    \
    V(WeakHashTable, weak_object_to_code_table, WeakObjectToCodeTable)           \
    V(PropertyCell, array_protector, ArrayProtector)                             \
    V(Object, weak_stack_trace_list, WeakStackTraceList)
  
  // Entries in this list are limited to Smis and are not visited during GC.
  #define SMI_ROOT_LIST(V)                                                   \
    V(Smi, stack_limit, StackLimit)                                          \
    V(Smi, real_stack_limit, RealStackLimit)                                 \
    V(Smi, last_script_id, LastScriptId)                                     \
    V(Smi, arguments_adaptor_deopt_pc_offset, ArgumentsAdaptorDeoptPCOffset) \
    V(Smi, construct_stub_deopt_pc_offset, ConstructStubDeoptPCOffset)       \
    V(Smi, getter_stub_deopt_pc_offset, GetterStubDeoptPCOffset)             \
    V(Smi, setter_stub_deopt_pc_offset, SetterStubDeoptPCOffset)
  
  
  #define ROOT_LIST(V)  \
    STRONG_ROOT_LIST(V) \
    SMI_ROOT_LIST(V)    \
    V(StringTable, string_table, StringTable)
  
  #define INTERNALIZED_STRING_LIST(V)                            \
    V(Object_string, "Object")                                   \
    V(proto_string, "__proto__")                                 \
    V(arguments_string, "arguments")                             \
    V(Arguments_string, "Arguments")                             \
    V(caller_string, "caller")                                   \
    V(boolean_string, "boolean")                                 \
    V(Boolean_string, "Boolean")                                 \
    V(callee_string, "callee")                                   \
    V(constructor_string, "constructor")                         \
    V(dot_result_string, ".result")                              \
    V(eval_string, "eval")                                       \
    V(function_string, "function")                               \
    V(Function_string, "Function")                               \
    V(length_string, "length")                                   \
    V(name_string, "name")                                       \
    V(null_string, "null")                                       \
    V(number_string, "number")                                   \
    V(Number_string, "Number")                                   \
    V(nan_string, "NaN")                                         \
    V(source_string, "source")                                   \
    V(source_url_string, "source_url")                           \
    V(source_mapping_url_string, "source_mapping_url")           \
    V(global_string, "global")                                   \
    V(ignore_case_string, "ignoreCase")                          \
    V(multiline_string, "multiline")                             \
    V(sticky_string, "sticky")                                   \
    V(unicode_string, "unicode")                                 \
    V(harmony_regexps_string, "harmony_regexps")                 \
    V(harmony_tostring_string, "harmony_tostring")               \
    V(harmony_unicode_regexps_string, "harmony_unicode_regexps") \
    V(input_string, "input")                                     \
    V(index_string, "index")                                     \
    V(last_index_string, "lastIndex")                            \
    V(object_string, "object")                                   \
    V(prototype_string, "prototype")                             \
    V(string_string, "string")                                   \
    V(String_string, "String")                                   \
    V(symbol_string, "symbol")                                   \
    V(Symbol_string, "Symbol")                                   \
    V(Map_string, "Map")                                         \
    V(Set_string, "Set")                                         \
    V(WeakMap_string, "WeakMap")                                 \
    V(WeakSet_string, "WeakSet")                                 \
    V(for_string, "for")                                         \
    V(for_api_string, "for_api")                                 \
    V(for_intern_string, "for_intern")                           \
    V(private_api_string, "private_api")                         \
    V(private_intern_string, "private_intern")                   \
    V(Date_string, "Date")                                       \
    V(char_at_string, "CharAt")                                  \
    V(undefined_string, "undefined")                             \
    V(value_of_string, "valueOf")                                \
    V(stack_string, "stack")                                     \
    V(toJSON_string, "toJSON")                                   \
    V(KeyedLoadMonomorphic_string, "KeyedLoadMonomorphic")       \
    V(KeyedStoreMonomorphic_string, "KeyedStoreMonomorphic")     \
    V(stack_overflow_string, "$stackOverflowBoilerplate")        \
    V(illegal_access_string, "illegal access")                   \
    V(cell_value_string, "%cell_value")                          \
    V(illegal_argument_string, "illegal argument")               \
    V(identity_hash_string, "v8::IdentityHash")                  \
    V(closure_string, "(closure)")                               \
    V(dot_string, ".")                                           \
    V(compare_ic_string, "==")                                   \
    V(strict_compare_ic_string, "===")                           \
    V(infinity_string, "Infinity")                               \
    V(minus_infinity_string, "-Infinity")                        \
    V(query_colon_string, "(?:)")                                \
    V(Generator_string, "Generator")                             \
    V(throw_string, "throw")                                     \
    V(done_string, "done")                                       \
    V(value_string, "value")                                     \
    V(next_string, "next")                                       \
    V(byte_length_string, "byteLength")                          \
    V(byte_offset_string, "byteOffset")                          \
    V(minus_zero_string, "-0")                                   \
    V(Array_string, "Array")                                     \
    V(Error_string, "Error")                                     \
    V(RegExp_string, "RegExp")
  
  #define PRIVATE_SYMBOL_LIST(V)      \
    V(nonextensible_symbol)           \
    V(sealed_symbol)                  \
    V(frozen_symbol)                  \
    V(nonexistent_symbol)             \
    V(elements_transition_symbol)     \
    V(observed_symbol)                \
    V(uninitialized_symbol)           \
    V(megamorphic_symbol)             \
    V(premonomorphic_symbol)          \
    V(stack_trace_symbol)             \
    V(detailed_stack_trace_symbol)    \
    V(normal_ic_symbol)               \
    V(home_object_symbol)             \
    V(intl_initialized_marker_symbol) \
    V(intl_impl_object_symbol)        \
    V(promise_debug_marker_symbol)    \
    V(promise_has_handler_symbol)     \
    V(class_script_symbol)            \
    V(class_start_position_symbol)    \
    V(class_end_position_symbol)      \
    V(error_start_pos_symbol)         \
    V(error_end_pos_symbol)           \
    V(error_script_symbol)
  
  #define PUBLIC_SYMBOL_LIST(V)                                    \
    V(has_instance_symbol, symbolHasInstance, Symbol.hasInstance)  \
    V(is_concat_spreadable_symbol, symbolIsConcatSpreadable,       \
      Symbol.isConcatSpreadable)                                   \
    V(is_regexp_symbol, symbolIsRegExp, Symbol.isRegExp)           \
    V(iterator_symbol, symbolIterator, Symbol.iterator)            \
    V(to_string_tag_symbol, symbolToStringTag, Symbol.toStringTag) \
    V(unscopables_symbol, symbolUnscopables, Symbol.unscopables)
  
  // Heap roots that are known to be immortal immovable, for which we can safely
  // skip write barriers. This list is not complete and has omissions.
  #define IMMORTAL_IMMOVABLE_ROOT_LIST(V) \
    V(ByteArrayMap)                       \
    V(FreeSpaceMap)                       \
    V(OnePointerFillerMap)                \
    V(TwoPointerFillerMap)                \
    V(UndefinedValue)                     \
    V(TheHoleValue)                       \
    V(NullValue)                          \
    V(TrueValue)                          \
    V(FalseValue)                         \
    V(UninitializedValue)                 \
    V(CellMap)                            \
    V(GlobalPropertyCellMap)              \
    V(SharedFunctionInfoMap)              \
    V(MetaMap)                            \
    V(HeapNumberMap)                      \
    V(MutableHeapNumberMap)               \
    V(NativeContextMap)                   \
    V(FixedArrayMap)                      \
    V(CodeMap)                            \
    V(ScopeInfoMap)                       \
    V(FixedCOWArrayMap)                   \
    V(FixedDoubleArrayMap)                \
    V(ConstantPoolArrayMap)               \
    V(WeakCellMap)                        \
    V(NoInterceptorResultSentinel)        \
    V(HashTableMap)                       \
    V(OrderedHashTableMap)                \
    V(EmptyFixedArray)                    \
    V(EmptyByteArray)                     \
    V(EmptyDescriptorArray)               \
    V(EmptyConstantPoolArray)             \
    V(ArgumentsMarker)                    \
    V(SymbolMap)                          \
    V(SloppyArgumentsElementsMap)         \
    V(FunctionContextMap)                 \
    V(CatchContextMap)                    \
    V(WithContextMap)                     \
    V(BlockContextMap)                    \
    V(ModuleContextMap)                   \
    V(ScriptContextMap)                   \
    V(UndefinedMap)                       \
    V(TheHoleMap)                         \
    V(NullMap)                            \
    V(BooleanMap)                         \
    V(UninitializedMap)                   \
    V(ArgumentsMarkerMap)                 \
    V(JSMessageObjectMap)                 \
    V(ForeignMap)                         \
    V(NeanderMap)                         \
    V(empty_string)                       \
    PRIVATE_SYMBOL_LIST(V)
  
  // Forward declarations.
  class HeapStats;
  class Isolate;
  class WeakObjectRetainer;
  
  
  typedef String* (*ExternalStringTableUpdaterCallback)(Heap* heap,
                                                        Object** pointer);
  
  class StoreBufferRebuilder {
   public:
    explicit StoreBufferRebuilder(StoreBuffer* store_buffer)
        : store_buffer_(store_buffer) {}
  
    void Callback(MemoryChunk* page, StoreBufferEvent event);
  
   private:
    StoreBuffer* store_buffer_;
  
    // We record in this variable how full the store buffer was when we started
    // iterating over the current page, finding pointers to new space.  If the
    // store buffer overflows again we can exempt the page from the store buffer
    // by rewinding to this point instead of having to search the store buffer.
    Object*** start_of_current_page_;
    // The current page we are scanning in the store buffer iterator.
    MemoryChunk* current_page_;
  };
  
  
  // A queue of objects promoted during scavenge. Each object is accompanied
  // by it's size to avoid dereferencing a map pointer for scanning.
  // The last page in to-space is used for the promotion queue. On conflict
  // during scavenge, the promotion queue is allocated externally and all
  // entries are copied to the external queue.
  class PromotionQueue {
   public:
    explicit PromotionQueue(Heap* heap)
        : front_(NULL),
          rear_(NULL),
          limit_(NULL),
          emergency_stack_(0),
          heap_(heap) {}
  
    void Initialize();
  
    void Destroy() {
      DCHECK(is_empty());
      delete emergency_stack_;
      emergency_stack_ = NULL;
    }
  
    Page* GetHeadPage() {
      return Page::FromAllocationTop(reinterpret_cast<Address>(rear_));
    }
  
    void SetNewLimit(Address limit) {
      // If we are already using an emergency stack, we can ignore it.
      if (emergency_stack_) return;
  
      // If the limit is not on the same page, we can ignore it.
      if (Page::FromAllocationTop(limit) != GetHeadPage()) return;
  
      limit_ = reinterpret_cast<intptr_t*>(limit);
  
      if (limit_ <= rear_) {
        return;
      }
  
      RelocateQueueHead();
    }
  
    bool IsBelowPromotionQueue(Address to_space_top) {
      // If an emergency stack is used, the to-space address cannot interfere
      // with the promotion queue.
      if (emergency_stack_) return true;
  
      // If the given to-space top pointer and the head of the promotion queue
      // are not on the same page, then the to-space objects are below the
      // promotion queue.
      if (GetHeadPage() != Page::FromAddress(to_space_top)) {
        return true;
      }
      // If the to space top pointer is smaller or equal than the promotion
      // queue head, then the to-space objects are below the promotion queue.
      return reinterpret_cast<intptr_t*>(to_space_top) <= rear_;
    }
  
    bool is_empty() {
      return (front_ == rear_) &&
             (emergency_stack_ == NULL || emergency_stack_->length() == 0);
    }
  
    inline void insert(HeapObject* target, int size);
  
    void remove(HeapObject** target, int* size) {
      DCHECK(!is_empty());
      if (front_ == rear_) {
        Entry e = emergency_stack_->RemoveLast();
        *target = e.obj_;
        *size = e.size_;
        return;
      }
  
      *target = reinterpret_cast<HeapObject*>(*(--front_));
      *size = static_cast<int>(*(--front_));
      // Assert no underflow.
      SemiSpace::AssertValidRange(reinterpret_cast<Address>(rear_),
                                  reinterpret_cast<Address>(front_));
    }
  
   private:
    // The front of the queue is higher in the memory page chain than the rear.
    intptr_t* front_;
    intptr_t* rear_;
    intptr_t* limit_;
  
    static const int kEntrySizeInWords = 2;
  
    struct Entry {
      Entry(HeapObject* obj, int size) : obj_(obj), size_(size) {}
  
      HeapObject* obj_;
      int size_;
    };
    List<Entry>* emergency_stack_;
  
    Heap* heap_;
  
    void RelocateQueueHead();
  
    DISALLOW_COPY_AND_ASSIGN(PromotionQueue);
  };
  
  
  typedef void (*ScavengingCallback)(Map* map, HeapObject** slot,
                                     HeapObject* object);
  
  
  // External strings table is a place where all external strings are
  // registered.  We need to keep track of such strings to properly
  // finalize them.
  class ExternalStringTable {
   public:
    // Registers an external string.
    inline void AddString(String* string);
  
    inline void Iterate(ObjectVisitor* v);
  
    // Restores internal invariant and gets rid of collected strings.
    // Must be called after each Iterate() that modified the strings.
    void CleanUp();
  
    // Destroys all allocated memory.
    void TearDown();
  
   private:
    explicit ExternalStringTable(Heap* heap) : heap_(heap) {}
  
    friend class Heap;
  
    inline void Verify();
  
    inline void AddOldString(String* string);
  
    // Notifies the table that only a prefix of the new list is valid.
    inline void ShrinkNewStrings(int position);
  
    // To speed up scavenge collections new space string are kept
    // separate from old space strings.
    List<Object*> new_space_strings_;
    List<Object*> old_space_strings_;
  
    Heap* heap_;
  
    DISALLOW_COPY_AND_ASSIGN(ExternalStringTable);
  };
  
  
  enum ArrayStorageAllocationMode {
    DONT_INITIALIZE_ARRAY_ELEMENTS,
    INITIALIZE_ARRAY_ELEMENTS_WITH_HOLE
  };
  
  
  class Heap {
   public:
    // Configure heap size in MB before setup. Return false if the heap has been
    // set up already.
    bool ConfigureHeap(int max_semi_space_size, int max_old_space_size,
                       int max_executable_size, size_t code_range_size);
    bool ConfigureHeapDefault();
  
    // Prepares the heap, setting up memory areas that are needed in the isolate
    // without actually creating any objects.
    bool SetUp();
  
    // Bootstraps the object heap with the core set of objects required to run.
    // Returns whether it succeeded.
    bool CreateHeapObjects();
  
    // Destroys all memory allocated by the heap.
    void TearDown();
  
    // Set the stack limit in the roots_ array.  Some architectures generate
    // code that looks here, because it is faster than loading from the static
    // jslimit_/real_jslimit_ variable in the StackGuard.
    void SetStackLimits();
  
    // Notifies the heap that is ok to start marking or other activities that
    // should not happen during deserialization.
    void NotifyDeserializationComplete();
  
    // Returns whether SetUp has been called.
    bool HasBeenSetUp();
  
    // Returns the maximum amount of memory reserved for the heap.  For
    // the young generation, we reserve 4 times the amount needed for a
    // semi space.  The young generation consists of two semi spaces and
    // we reserve twice the amount needed for those in order to ensure
    // that new space can be aligned to its size.
    intptr_t MaxReserved() {
      return 4 * reserved_semispace_size_ + max_old_generation_size_;
    }
    int MaxSemiSpaceSize() { return max_semi_space_size_; }
    int ReservedSemiSpaceSize() { return reserved_semispace_size_; }
    int InitialSemiSpaceSize() { return initial_semispace_size_; }
    int TargetSemiSpaceSize() { return target_semispace_size_; }
    intptr_t MaxOldGenerationSize() { return max_old_generation_size_; }
    intptr_t MaxExecutableSize() { return max_executable_size_; }
  
    // Returns the capacity of the heap in bytes w/o growing. Heap grows when
    // more spaces are needed until it reaches the limit.
    intptr_t Capacity();
  
    // Returns the amount of memory currently committed for the heap.
    intptr_t CommittedMemory();
  
    // Returns the amount of memory currently committed for the old space.
    intptr_t CommittedOldGenerationMemory();
  
    // Returns the amount of executable memory currently committed for the heap.
    intptr_t CommittedMemoryExecutable();
  
    // Returns the amount of phyical memory currently committed for the heap.
    size_t CommittedPhysicalMemory();
  
    // Returns the maximum amount of memory ever committed for the heap.
    intptr_t MaximumCommittedMemory() { return maximum_committed_; }
  
    // Updates the maximum committed memory for the heap. Should be called
    // whenever a space grows.
    void UpdateMaximumCommitted();
  
    // Returns the available bytes in space w/o growing.
    // Heap doesn't guarantee that it can allocate an object that requires
    // all available bytes. Check MaxHeapObjectSize() instead.
    intptr_t Available();
  
    // Returns of size of all objects residing in the heap.
    intptr_t SizeOfObjects();
  
    intptr_t old_generation_allocation_limit() const {
      return old_generation_allocation_limit_;
    }
  
    // Return the starting address and a mask for the new space.  And-masking an
    // address with the mask will result in the start address of the new space
    // for all addresses in either semispace.
    Address NewSpaceStart() { return new_space_.start(); }
    uintptr_t NewSpaceMask() { return new_space_.mask(); }
    Address NewSpaceTop() { return new_space_.top(); }
  
    NewSpace* new_space() { return &new_space_; }
    OldSpace* old_space() { return old_space_; }
    OldSpace* code_space() { return code_space_; }
    MapSpace* map_space() { return map_space_; }
    LargeObjectSpace* lo_space() { return lo_space_; }
    PagedSpace* paged_space(int idx) {
      switch (idx) {
        case OLD_SPACE:
          return old_space();
        case MAP_SPACE:
          return map_space();
        case CODE_SPACE:
          return code_space();
        case NEW_SPACE:
        case LO_SPACE:
          UNREACHABLE();
      }
      return NULL;
    }
    Space* space(int idx) {
      switch (idx) {
        case NEW_SPACE:
          return new_space();
        case LO_SPACE:
          return lo_space();
        default:
          return paged_space(idx);
      }
    }
  
    // Returns name of the space.
    const char* GetSpaceName(int idx);
  
    bool always_allocate() { return always_allocate_scope_depth_ != 0; }
    Address always_allocate_scope_depth_address() {
      return reinterpret_cast<Address>(&always_allocate_scope_depth_);
    }
  
    Address* NewSpaceAllocationTopAddress() {
      return new_space_.allocation_top_address();
    }
    Address* NewSpaceAllocationLimitAddress() {
      return new_space_.allocation_limit_address();
    }
  
    Address* OldSpaceAllocationTopAddress() {
      return old_space_->allocation_top_address();
    }
    Address* OldSpaceAllocationLimitAddress() {
      return old_space_->allocation_limit_address();
    }
  
    // TODO(hpayer): There is still a missmatch between capacity and actual
    // committed memory size.
    bool CanExpandOldGeneration(int size) {
      return (CommittedOldGenerationMemory() + size) < MaxOldGenerationSize();
    }
  
    // Returns a deep copy of the JavaScript object.
    // Properties and elements are copied too.
    // Optionally takes an AllocationSite to be appended in an AllocationMemento.
    MUST_USE_RESULT AllocationResult
        CopyJSObject(JSObject* source, AllocationSite* site = NULL);
  
    // This method assumes overallocation of one word. It will store a filler
    // before the object if the given object is not double aligned, otherwise
    // it will place the filler after the object.
    MUST_USE_RESULT HeapObject* EnsureAligned(HeapObject* object, int size,
                                              AllocationAlignment alignment);
  
    MUST_USE_RESULT HeapObject* PrecedeWithFiller(HeapObject* object);
  
    // Clear the Instanceof cache (used when a prototype changes).
    inline void ClearInstanceofCache();
  
    // Iterates the whole code space to clear all ICs of the given kind.
    void ClearAllICsByKind(Code::Kind kind);
  
    // FreeSpace objects have a null map after deserialization. Update the map.
    void RepairFreeListsAfterDeserialization();
  
    template <typename T>
    static inline bool IsOneByte(T t, int chars);
  
    // Move len elements within a given array from src_index index to dst_index
    // index.
    void MoveElements(FixedArray* array, int dst_index, int src_index, int len);
  
    // Sloppy mode arguments object size.
    static const int kSloppyArgumentsObjectSize =
        JSObject::kHeaderSize + 2 * kPointerSize;
    // Strict mode arguments has no callee so it is smaller.
    static const int kStrictArgumentsObjectSize =
        JSObject::kHeaderSize + 1 * kPointerSize;
    // Indicies for direct access into argument objects.
    static const int kArgumentsLengthIndex = 0;
    // callee is only valid in sloppy mode.
    static const int kArgumentsCalleeIndex = 1;
  
    // Finalizes an external string by deleting the associated external
    // data and clearing the resource pointer.
    inline void FinalizeExternalString(String* string);
  
    // Initialize a filler object to keep the ability to iterate over the heap
    // when introducing gaps within pages.
    void CreateFillerObjectAt(Address addr, int size);
  
    bool CanMoveObjectStart(HeapObject* object);
  
    // Indicates whether live bytes adjustment is triggered
    // - from within the GC code before sweeping started (SEQUENTIAL_TO_SWEEPER),
    // - or from within GC (CONCURRENT_TO_SWEEPER),
    // - or mutator code (CONCURRENT_TO_SWEEPER).
    enum InvocationMode { SEQUENTIAL_TO_SWEEPER, CONCURRENT_TO_SWEEPER };
  
    // Maintain consistency of live bytes during incremental marking.
    void AdjustLiveBytes(Address address, int by, InvocationMode mode);
  
    // Trim the given array from the left. Note that this relocates the object
    // start and hence is only valid if there is only a single reference to it.
    FixedArrayBase* LeftTrimFixedArray(FixedArrayBase* obj, int elements_to_trim);
  
    // Trim the given array from the right.
    template<Heap::InvocationMode mode>
    void RightTrimFixedArray(FixedArrayBase* obj, int elements_to_trim);
  
    // Converts the given boolean condition to JavaScript boolean value.
    inline Object* ToBoolean(bool condition);
  
    // Performs garbage collection operation.
    // Returns whether there is a chance that another major GC could
    // collect more garbage.
    inline bool CollectGarbage(
        AllocationSpace space, const char* gc_reason = NULL,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    static const int kNoGCFlags = 0;
    static const int kReduceMemoryFootprintMask = 1;
    static const int kAbortIncrementalMarkingMask = 2;
    static const int kFinalizeIncrementalMarkingMask = 4;
  
    // Making the heap iterable requires us to abort incremental marking.
    static const int kMakeHeapIterableMask = kAbortIncrementalMarkingMask;
  
    // Invoked when GC was requested via the stack guard.
    void HandleGCRequest();
  
    // Attempt to over-approximate the weak closure by marking object groups and
    // implicit references from global handles, but don't atomically complete
    // marking. If we continue to mark incrementally, we might have marked
    // objects that die later.
    void OverApproximateWeakClosure(const char* gc_reason);
  
    // Performs a full garbage collection.  If (flags & kMakeHeapIterableMask) is
    // non-zero, then the slower precise sweeper is used, which leaves the heap
    // in a state where we can iterate over the heap visiting all objects.
    void CollectAllGarbage(
        int flags = kFinalizeIncrementalMarkingMask, const char* gc_reason = NULL,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    // Last hope GC, should try to squeeze as much as possible.
    void CollectAllAvailableGarbage(const char* gc_reason = NULL);
  
    // Check whether the heap is currently iterable.
    bool IsHeapIterable();
  
    // Notify the heap that a context has been disposed.
    int NotifyContextDisposed(bool dependant_context);
  
    inline void increment_scan_on_scavenge_pages() {
      scan_on_scavenge_pages_++;
      if (FLAG_gc_verbose) {
        PrintF("Scan-on-scavenge pages: %d\n", scan_on_scavenge_pages_);
      }
    }
  
    inline void decrement_scan_on_scavenge_pages() {
      scan_on_scavenge_pages_--;
      if (FLAG_gc_verbose) {
        PrintF("Scan-on-scavenge pages: %d\n", scan_on_scavenge_pages_);
      }
    }
  
    PromotionQueue* promotion_queue() { return &promotion_queue_; }
  
    void AddGCPrologueCallback(v8::Isolate::GCPrologueCallback callback,
                               GCType gc_type_filter, bool pass_isolate = true);
    void RemoveGCPrologueCallback(v8::Isolate::GCPrologueCallback callback);
  
    void AddGCEpilogueCallback(v8::Isolate::GCEpilogueCallback callback,
                               GCType gc_type_filter, bool pass_isolate = true);
    void RemoveGCEpilogueCallback(v8::Isolate::GCEpilogueCallback callback);
  
  // Heap root getters.  We have versions with and without type::cast() here.
  // You can't use type::cast during GC because the assert fails.
  // TODO(1490): Try removing the unchecked accessors, now that GC marking does
  // not corrupt the map.
  #define ROOT_ACCESSOR(type, name, camel_name)                           \
    type* name() { return type::cast(roots_[k##camel_name##RootIndex]); } \
    type* raw_unchecked_##name() {                                        \
      return reinterpret_cast<type*>(roots_[k##camel_name##RootIndex]);   \
    }
    ROOT_LIST(ROOT_ACCESSOR)
  #undef ROOT_ACCESSOR
  
  // Utility type maps
  #define STRUCT_MAP_ACCESSOR(NAME, Name, name) \
    Map* name##_map() { return Map::cast(roots_[k##Name##MapRootIndex]); }
    STRUCT_LIST(STRUCT_MAP_ACCESSOR)
  #undef STRUCT_MAP_ACCESSOR
  
  #define STRING_ACCESSOR(name, str) \
    String* name() { return String::cast(roots_[k##name##RootIndex]); }
    INTERNALIZED_STRING_LIST(STRING_ACCESSOR)
  #undef STRING_ACCESSOR
  
  #define SYMBOL_ACCESSOR(name) \
    Symbol* name() { return Symbol::cast(roots_[k##name##RootIndex]); }
    PRIVATE_SYMBOL_LIST(SYMBOL_ACCESSOR)
  #undef SYMBOL_ACCESSOR
  
  #define SYMBOL_ACCESSOR(name, varname, description) \
    Symbol* name() { return Symbol::cast(roots_[k##name##RootIndex]); }
    PUBLIC_SYMBOL_LIST(SYMBOL_ACCESSOR)
  #undef SYMBOL_ACCESSOR
  
    // The hidden_string is special because it is the empty string, but does
    // not match the empty string.
    String* hidden_string() { return hidden_string_; }
  
    void set_native_contexts_list(Object* object) {
      native_contexts_list_ = object;
    }
    Object* native_contexts_list() const { return native_contexts_list_; }
  
    void set_allocation_sites_list(Object* object) {
      allocation_sites_list_ = object;
    }
    Object* allocation_sites_list() { return allocation_sites_list_; }
  
    // Used in CreateAllocationSiteStub and the (de)serializer.
    Object** allocation_sites_list_address() { return &allocation_sites_list_; }
  
    void set_encountered_weak_collections(Object* weak_collection) {
      encountered_weak_collections_ = weak_collection;
    }
    Object* encountered_weak_collections() const {
      return encountered_weak_collections_;
    }
  
    void set_encountered_weak_cells(Object* weak_cell) {
      encountered_weak_cells_ = weak_cell;
    }
    Object* encountered_weak_cells() const { return encountered_weak_cells_; }
  
    // Number of mark-sweeps.
    unsigned int ms_count() { return ms_count_; }
  
    // Iterates over all roots in the heap.
    void IterateRoots(ObjectVisitor* v, VisitMode mode);
    // Iterates over all strong roots in the heap.
    void IterateStrongRoots(ObjectVisitor* v, VisitMode mode);
    // Iterates over entries in the smi roots list.  Only interesting to the
    // serializer/deserializer, since GC does not care about smis.
    void IterateSmiRoots(ObjectVisitor* v);
    // Iterates over all the other roots in the heap.
    void IterateWeakRoots(ObjectVisitor* v, VisitMode mode);
  
    // Iterate pointers to from semispace of new space found in memory interval
    // from start to end.
    void IterateAndMarkPointersToFromSpace(bool record_slots, Address start,
                                           Address end,
                                           ObjectSlotCallback callback);
  
    // Returns whether the object resides in new space.
    inline bool InNewSpace(Object* object);
    inline bool InNewSpace(Address address);
    inline bool InNewSpacePage(Address address);
    inline bool InFromSpace(Object* object);
    inline bool InToSpace(Object* object);
  
    // Returns whether the object resides in old space.
    inline bool InOldSpace(Address address);
    inline bool InOldSpace(Object* object);
  
    // Checks whether an address/object in the heap (including auxiliary
    // area and unused area).
    bool Contains(Address addr);
    bool Contains(HeapObject* value);
  
    // Checks whether an address/object in a space.
    // Currently used by tests, serialization and heap verification only.
    bool InSpace(Address addr, AllocationSpace space);
    bool InSpace(HeapObject* value, AllocationSpace space);
  
    // Checks whether the given object is allowed to be migrated from it's
    // current space into the given destination space. Used for debugging.
    inline bool AllowedToBeMigrated(HeapObject* object, AllocationSpace dest);
  
    // Sets the stub_cache_ (only used when expanding the dictionary).
    void public_set_code_stubs(UnseededNumberDictionary* value) {
      roots_[kCodeStubsRootIndex] = value;
    }
  
    // Support for computing object sizes for old objects during GCs. Returns
    // a function that is guaranteed to be safe for computing object sizes in
    // the current GC phase.
    HeapObjectCallback GcSafeSizeOfOldObjectFunction() {
      return gc_safe_size_of_old_object_;
    }
  
    // Sets the non_monomorphic_cache_ (only used when expanding the dictionary).
    void public_set_non_monomorphic_cache(UnseededNumberDictionary* value) {
      roots_[kNonMonomorphicCacheRootIndex] = value;
    }
  
    void public_set_empty_script(Script* script) {
      roots_[kEmptyScriptRootIndex] = script;
    }
  
    void public_set_store_buffer_top(Address* top) {
      roots_[kStoreBufferTopRootIndex] = reinterpret_cast<Smi*>(top);
    }
  
    void public_set_materialized_objects(FixedArray* objects) {
      roots_[kMaterializedObjectsRootIndex] = objects;
    }
  
    // Generated code can embed this address to get access to the roots.
    Object** roots_array_start() { return roots_; }
  
    Address* store_buffer_top_address() {
      return reinterpret_cast<Address*>(&roots_[kStoreBufferTopRootIndex]);
    }
  
    static bool RootIsImmortalImmovable(int root_index);
    void CheckHandleCount();
  
  #ifdef VERIFY_HEAP
    // Verify the heap is in its normal state before or after a GC.
    void Verify();
  #endif
  
  #ifdef DEBUG
    void Print();
    void PrintHandles();
  
    // Report heap statistics.
    void ReportHeapStatistics(const char* title);
    void ReportCodeStatistics(const char* title);
  #endif
  
    // Zapping is needed for verify heap, and always done in debug builds.
    static inline bool ShouldZapGarbage() {
  #ifdef DEBUG
      return true;
  #else
  #ifdef VERIFY_HEAP
      return FLAG_verify_heap;
  #else
      return false;
  #endif
  #endif
    }
  
    // Number of "runtime allocations" done so far.
    uint32_t allocations_count() { return allocations_count_; }
  
    // Returns deterministic "time" value in ms. Works only with
    // FLAG_verify_predictable.
    double synthetic_time() { return allocations_count_ / 2.0; }
  
    // Print short heap statistics.
    void PrintShortHeapStatistics();
  
    size_t object_count_last_gc(size_t index) {
      return index < OBJECT_STATS_COUNT ? object_counts_last_time_[index] : 0;
    }
    size_t object_size_last_gc(size_t index) {
      return index < OBJECT_STATS_COUNT ? object_sizes_last_time_[index] : 0;
    }
  
    // Write barrier support for address[offset] = o.
    INLINE(void RecordWrite(Address address, int offset));
  
    // Write barrier support for address[start : start + len[ = o.
    INLINE(void RecordWrites(Address address, int start, int len));
  
    enum HeapState { NOT_IN_GC, SCAVENGE, MARK_COMPACT };
    inline HeapState gc_state() { return gc_state_; }
  
    inline bool IsInGCPostProcessing() { return gc_post_processing_depth_ > 0; }
  
  #ifdef DEBUG
    void set_allocation_timeout(int timeout) { allocation_timeout_ = timeout; }
  
    void TracePathToObjectFrom(Object* target, Object* root);
    void TracePathToObject(Object* target);
    void TracePathToGlobal();
  #endif
  
    // Callback function passed to Heap::Iterate etc.  Copies an object if
    // necessary, the object might be promoted to an old space.  The caller must
    // ensure the precondition that the object is (a) a heap object and (b) in
    // the heap's from space.
    static inline void ScavengePointer(HeapObject** p);
    static inline void ScavengeObject(HeapObject** p, HeapObject* object);
  
    enum ScratchpadSlotMode { IGNORE_SCRATCHPAD_SLOT, RECORD_SCRATCHPAD_SLOT };
  
    // If an object has an AllocationMemento trailing it, return it, otherwise
    // return NULL;
    inline AllocationMemento* FindAllocationMemento(HeapObject* object);
  
    // An object may have an AllocationSite associated with it through a trailing
    // AllocationMemento. Its feedback should be updated when objects are found
    // in the heap.
    static inline void UpdateAllocationSiteFeedback(HeapObject* object,
                                                    ScratchpadSlotMode mode);
  
    // Support for partial snapshots.  After calling this we have a linear
    // space to write objects in each space.
    struct Chunk {
      uint32_t size;
      Address start;
      Address end;
    };
  
    typedef List<Chunk> Reservation;
  
    // Returns false if not able to reserve.
    bool ReserveSpace(Reservation* reservations);
  
    //
    // Support for the API.
    //
  
    void CreateApiObjects();
  
    inline intptr_t PromotedTotalSize() {
      int64_t total = PromotedSpaceSizeOfObjects() + PromotedExternalMemorySize();
      if (total > kMaxInt) return static_cast<intptr_t>(kMaxInt);
      if (total < 0) return 0;
      return static_cast<intptr_t>(total);
    }
  
    inline intptr_t OldGenerationSpaceAvailable() {
      return old_generation_allocation_limit_ - PromotedTotalSize();
    }
  
    inline intptr_t OldGenerationCapacityAvailable() {
      return max_old_generation_size_ - PromotedTotalSize();
    }
  
    static const intptr_t kMinimumOldGenerationAllocationLimit =
        8 * (Page::kPageSize > MB ? Page::kPageSize : MB);
  
    static const int kInitalOldGenerationLimitFactor = 2;
  
  #if V8_OS_ANDROID
    // Don't apply pointer multiplier on Android since it has no swap space and
    // should instead adapt it's heap size based on available physical memory.
    static const int kPointerMultiplier = 1;
  #else
    static const int kPointerMultiplier = i::kPointerSize / 4;
  #endif
  
    // The new space size has to be a power of 2. Sizes are in MB.
    static const int kMaxSemiSpaceSizeLowMemoryDevice = 1 * kPointerMultiplier;
    static const int kMaxSemiSpaceSizeMediumMemoryDevice = 4 * kPointerMultiplier;
    static const int kMaxSemiSpaceSizeHighMemoryDevice = 8 * kPointerMultiplier;
    static const int kMaxSemiSpaceSizeHugeMemoryDevice = 8 * kPointerMultiplier;
  
    // The old space size has to be a multiple of Page::kPageSize.
    // Sizes are in MB.
    static const int kMaxOldSpaceSizeLowMemoryDevice = 128 * kPointerMultiplier;
    static const int kMaxOldSpaceSizeMediumMemoryDevice =
        256 * kPointerMultiplier;
    static const int kMaxOldSpaceSizeHighMemoryDevice = 512 * kPointerMultiplier;
    static const int kMaxOldSpaceSizeHugeMemoryDevice = 700 * kPointerMultiplier;
  
    // The executable size has to be a multiple of Page::kPageSize.
    // Sizes are in MB.
    static const int kMaxExecutableSizeLowMemoryDevice = 96 * kPointerMultiplier;
    static const int kMaxExecutableSizeMediumMemoryDevice =
        192 * kPointerMultiplier;
    static const int kMaxExecutableSizeHighMemoryDevice =
        256 * kPointerMultiplier;
    static const int kMaxExecutableSizeHugeMemoryDevice =
        256 * kPointerMultiplier;
  
    // Calculates the allocation limit based on a given growing factor and a
    // given old generation size.
    intptr_t CalculateOldGenerationAllocationLimit(double factor,
                                                   intptr_t old_gen_size);
  
    // Sets the allocation limit to trigger the next full garbage collection.
    void SetOldGenerationAllocationLimit(intptr_t old_gen_size,
                                         int freed_global_handles);
  
    // Indicates whether inline bump-pointer allocation has been disabled.
    bool inline_allocation_disabled() { return inline_allocation_disabled_; }
  
    // Switch whether inline bump-pointer allocation should be used.
    void EnableInlineAllocation();
    void DisableInlineAllocation();
  
    // Implements the corresponding V8 API function.
    bool IdleNotification(double deadline_in_seconds);
    bool IdleNotification(int idle_time_in_ms);
  
    double MonotonicallyIncreasingTimeInMs();
  
    // Declare all the root indices.  This defines the root list order.
    enum RootListIndex {
  #define ROOT_INDEX_DECLARATION(type, name, camel_name) k##camel_name##RootIndex,
      STRONG_ROOT_LIST(ROOT_INDEX_DECLARATION)
  #undef ROOT_INDEX_DECLARATION
  
  #define STRING_INDEX_DECLARATION(name, str) k##name##RootIndex,
      INTERNALIZED_STRING_LIST(STRING_INDEX_DECLARATION)
  #undef STRING_DECLARATION
  
  #define SYMBOL_INDEX_DECLARATION(name) k##name##RootIndex,
      PRIVATE_SYMBOL_LIST(SYMBOL_INDEX_DECLARATION)
  #undef SYMBOL_INDEX_DECLARATION
  
  #define SYMBOL_INDEX_DECLARATION(name, varname, description) k##name##RootIndex,
      PUBLIC_SYMBOL_LIST(SYMBOL_INDEX_DECLARATION)
  #undef SYMBOL_INDEX_DECLARATION
  
  // Utility type maps
  #define DECLARE_STRUCT_MAP(NAME, Name, name) k##Name##MapRootIndex,
      STRUCT_LIST(DECLARE_STRUCT_MAP)
  #undef DECLARE_STRUCT_MAP
      kStringTableRootIndex,
  
  #define ROOT_INDEX_DECLARATION(type, name, camel_name) k##camel_name##RootIndex,
      SMI_ROOT_LIST(ROOT_INDEX_DECLARATION)
  #undef ROOT_INDEX_DECLARATION
      kRootListLength,
      kStrongRootListLength = kStringTableRootIndex,
      kSmiRootsStart = kStringTableRootIndex + 1
    };
  
    // Get the root list index for {object} if such a root list index exists.
    bool GetRootListIndex(Handle<HeapObject> object,
                          Heap::RootListIndex* index_return);
  
    Object* root(RootListIndex index) { return roots_[index]; }
  
    STATIC_ASSERT(kUndefinedValueRootIndex ==
                  Internals::kUndefinedValueRootIndex);
    STATIC_ASSERT(kNullValueRootIndex == Internals::kNullValueRootIndex);
    STATIC_ASSERT(kTrueValueRootIndex == Internals::kTrueValueRootIndex);
    STATIC_ASSERT(kFalseValueRootIndex == Internals::kFalseValueRootIndex);
    STATIC_ASSERT(kempty_stringRootIndex == Internals::kEmptyStringRootIndex);
  
    // Generated code can embed direct references to non-writable roots if
    // they are in new space.
    static bool RootCanBeWrittenAfterInitialization(RootListIndex root_index);
    // Generated code can treat direct references to this root as constant.
    bool RootCanBeTreatedAsConstant(RootListIndex root_index);
  
    Map* MapForFixedTypedArray(ExternalArrayType array_type);
    RootListIndex RootIndexForFixedTypedArray(ExternalArrayType array_type);
  
    Map* MapForExternalArrayType(ExternalArrayType array_type);
    RootListIndex RootIndexForExternalArrayType(ExternalArrayType array_type);
  
    RootListIndex RootIndexForEmptyExternalArray(ElementsKind kind);
    RootListIndex RootIndexForEmptyFixedTypedArray(ElementsKind kind);
    ExternalArray* EmptyExternalArrayForMap(Map* map);
    FixedTypedArrayBase* EmptyFixedTypedArrayForMap(Map* map);
  
    void RecordStats(HeapStats* stats, bool take_snapshot = false);
  
    // Copy block of memory from src to dst. Size of block should be aligned
    // by pointer size.
    static inline void CopyBlock(Address dst, Address src, int byte_size);
  
    // Optimized version of memmove for blocks with pointer size aligned sizes and
    // pointer size aligned addresses.
    static inline void MoveBlock(Address dst, Address src, int byte_size);
  
    // Check new space expansion criteria and expand semispaces if it was hit.
    void CheckNewSpaceExpansionCriteria();
  
    inline void IncrementPromotedObjectsSize(int object_size) {
      DCHECK(object_size > 0);
      promoted_objects_size_ += object_size;
    }
  
    inline void IncrementSemiSpaceCopiedObjectSize(int object_size) {
      DCHECK(object_size > 0);
      semi_space_copied_object_size_ += object_size;
    }
  
    inline void IncrementNodesDiedInNewSpace() { nodes_died_in_new_space_++; }
  
    inline void IncrementNodesCopiedInNewSpace() { nodes_copied_in_new_space_++; }
  
    inline void IncrementNodesPromoted() { nodes_promoted_++; }
  
    inline void IncrementYoungSurvivorsCounter(int survived) {
      DCHECK(survived >= 0);
      survived_last_scavenge_ = survived;
      survived_since_last_expansion_ += survived;
    }
  
    inline bool HeapIsFullEnoughToStartIncrementalMarking(intptr_t limit) {
      if (FLAG_stress_compaction && (gc_count_ & 1) != 0) return true;
  
      intptr_t adjusted_allocation_limit = limit - new_space_.Capacity();
  
      if (PromotedTotalSize() >= adjusted_allocation_limit) return true;
  
      return false;
    }
  
    void UpdateNewSpaceReferencesInExternalStringTable(
        ExternalStringTableUpdaterCallback updater_func);
  
    void UpdateReferencesInExternalStringTable(
        ExternalStringTableUpdaterCallback updater_func);
  
    void ProcessAllWeakReferences(WeakObjectRetainer* retainer);
    void ProcessYoungWeakReferences(WeakObjectRetainer* retainer);
  
    void VisitExternalResources(v8::ExternalResourceVisitor* visitor);
  
    // An object should be promoted if the object has survived a
    // scavenge operation.
    inline bool ShouldBePromoted(Address old_address, int object_size);
  
    void ClearJSFunctionResultCaches();
  
    void ClearNormalizedMapCaches();
  
    GCTracer* tracer() { return &tracer_; }
  
    // Returns the size of objects residing in non new spaces.
    intptr_t PromotedSpaceSizeOfObjects();
  
    double total_regexp_code_generated() { return total_regexp_code_generated_; }
    void IncreaseTotalRegexpCodeGenerated(int size) {
      total_regexp_code_generated_ += size;
    }
  
    void IncrementCodeGeneratedBytes(bool is_crankshafted, int size) {
      if (is_crankshafted) {
        crankshaft_codegen_bytes_generated_ += size;
      } else {
        full_codegen_bytes_generated_ += size;
      }
    }
  
    // Update GC statistics that are tracked on the Heap.
    void UpdateCumulativeGCStatistics(double duration, double spent_in_mutator,
                                      double marking_time);
  
    // Returns maximum GC pause.
    double get_max_gc_pause() { return max_gc_pause_; }
  
    // Returns maximum size of objects alive after GC.
    intptr_t get_max_alive_after_gc() { return max_alive_after_gc_; }
  
    // Returns minimal interval between two subsequent collections.
    double get_min_in_mutator() { return min_in_mutator_; }
  
    void IncrementDeferredCount(v8::Isolate::UseCounterFeature feature);
  
    MarkCompactCollector* mark_compact_collector() {
      return &mark_compact_collector_;
    }
  
    StoreBuffer* store_buffer() { return &store_buffer_; }
  
    Marking* marking() { return &marking_; }
  
    IncrementalMarking* incremental_marking() { return &incremental_marking_; }
  
    ExternalStringTable* external_string_table() {
      return &external_string_table_;
    }
  
    // Returns the current sweep generation.
    int sweep_generation() { return sweep_generation_; }
  
    bool concurrent_sweeping_enabled() { return concurrent_sweeping_enabled_; }
  
    inline Isolate* isolate();
  
    void CallGCPrologueCallbacks(GCType gc_type, GCCallbackFlags flags);
    void CallGCEpilogueCallbacks(GCType gc_type, GCCallbackFlags flags);
  
    inline bool OldGenerationAllocationLimitReached();
  
    inline void DoScavengeObject(Map* map, HeapObject** slot, HeapObject* obj) {
      scavenging_visitors_table_.GetVisitor(map)(map, slot, obj);
    }
  
    void QueueMemoryChunkForFree(MemoryChunk* chunk);
    void FreeQueuedChunks();
  
    int gc_count() const { return gc_count_; }
  
    bool RecentIdleNotificationHappened();
  
    // Completely clear the Instanceof cache (to stop it keeping objects alive
    // around a GC).
    inline void CompletelyClearInstanceofCache();
  
    // The roots that have an index less than this are always in old space.
    static const int kOldSpaceRoots = 0x20;
  
    uint32_t HashSeed() {
      uint32_t seed = static_cast<uint32_t>(hash_seed()->value());
      DCHECK(FLAG_randomize_hashes || seed == 0);
      return seed;
    }
  
    Smi* NextScriptId() {
      int next_id = last_script_id()->value() + 1;
      if (!Smi::IsValid(next_id) || next_id < 0) next_id = 1;
      Smi* next_id_smi = Smi::FromInt(next_id);
      set_last_script_id(next_id_smi);
      return next_id_smi;
    }
  
    void SetArgumentsAdaptorDeoptPCOffset(int pc_offset) {
      DCHECK(arguments_adaptor_deopt_pc_offset() == Smi::FromInt(0));
      set_arguments_adaptor_deopt_pc_offset(Smi::FromInt(pc_offset));
    }
  
    void SetConstructStubDeoptPCOffset(int pc_offset) {
      DCHECK(construct_stub_deopt_pc_offset() == Smi::FromInt(0));
      set_construct_stub_deopt_pc_offset(Smi::FromInt(pc_offset));
    }
  
    void SetGetterStubDeoptPCOffset(int pc_offset) {
      DCHECK(getter_stub_deopt_pc_offset() == Smi::FromInt(0));
      set_getter_stub_deopt_pc_offset(Smi::FromInt(pc_offset));
    }
  
    void SetSetterStubDeoptPCOffset(int pc_offset) {
      DCHECK(setter_stub_deopt_pc_offset() == Smi::FromInt(0));
      set_setter_stub_deopt_pc_offset(Smi::FromInt(pc_offset));
    }
  
    // For post mortem debugging.
    void RememberUnmappedPage(Address page, bool compacted);
  
    // Global inline caching age: it is incremented on some GCs after context
    // disposal. We use it to flush inline caches.
    int global_ic_age() { return global_ic_age_; }
  
    void AgeInlineCaches() {
      global_ic_age_ = (global_ic_age_ + 1) & SharedFunctionInfo::ICAgeBits::kMax;
    }
  
    int64_t amount_of_external_allocated_memory() {
      return amount_of_external_allocated_memory_;
    }
  
    void DeoptMarkedAllocationSites();
  
    bool MaximumSizeScavenge() { return maximum_size_scavenges_ > 0; }
  
    bool DeoptMaybeTenuredAllocationSites() {
      return new_space_.IsAtMaximumCapacity() && maximum_size_scavenges_ == 0;
    }
  
    // ObjectStats are kept in two arrays, counts and sizes. Related stats are
    // stored in a contiguous linear buffer. Stats groups are stored one after
    // another.
    enum {
      FIRST_CODE_KIND_SUB_TYPE = LAST_TYPE + 1,
      FIRST_FIXED_ARRAY_SUB_TYPE =
          FIRST_CODE_KIND_SUB_TYPE + Code::NUMBER_OF_KINDS,
      FIRST_CODE_AGE_SUB_TYPE =
          FIRST_FIXED_ARRAY_SUB_TYPE + LAST_FIXED_ARRAY_SUB_TYPE + 1,
      OBJECT_STATS_COUNT = FIRST_CODE_AGE_SUB_TYPE + Code::kCodeAgeCount + 1
    };
  
    void RecordObjectStats(InstanceType type, size_t size) {
      DCHECK(type <= LAST_TYPE);
      object_counts_[type]++;
      object_sizes_[type] += size;
    }
  
    void RecordCodeSubTypeStats(int code_sub_type, int code_age, size_t size) {
      int code_sub_type_index = FIRST_CODE_KIND_SUB_TYPE + code_sub_type;
      int code_age_index =
          FIRST_CODE_AGE_SUB_TYPE + code_age - Code::kFirstCodeAge;
      DCHECK(code_sub_type_index >= FIRST_CODE_KIND_SUB_TYPE &&
             code_sub_type_index < FIRST_CODE_AGE_SUB_TYPE);
      DCHECK(code_age_index >= FIRST_CODE_AGE_SUB_TYPE &&
             code_age_index < OBJECT_STATS_COUNT);
      object_counts_[code_sub_type_index]++;
      object_sizes_[code_sub_type_index] += size;
      object_counts_[code_age_index]++;
      object_sizes_[code_age_index] += size;
    }
  
    void RecordFixedArraySubTypeStats(int array_sub_type, size_t size) {
      DCHECK(array_sub_type <= LAST_FIXED_ARRAY_SUB_TYPE);
      object_counts_[FIRST_FIXED_ARRAY_SUB_TYPE + array_sub_type]++;
      object_sizes_[FIRST_FIXED_ARRAY_SUB_TYPE + array_sub_type] += size;
    }
  
    void TraceObjectStats();
    void TraceObjectStat(const char* name, int count, int size, double time);
    void CheckpointObjectStats();
    bool GetObjectTypeName(size_t index, const char** object_type,
                           const char** object_sub_type);
  
    void RegisterStrongRoots(Object** start, Object** end);
    void UnregisterStrongRoots(Object** start);
  
    // Taking this lock prevents the GC from entering a phase that relocates
    // object references.
    class RelocationLock {
     public:
      explicit RelocationLock(Heap* heap) : heap_(heap) {
        heap_->relocation_mutex_.Lock();
      }
  
      ~RelocationLock() { heap_->relocation_mutex_.Unlock(); }
  
     private:
      Heap* heap_;
    };
  
    // An optional version of the above lock that can be used for some critical
    // sections on the mutator thread; only safe since the GC currently does not
    // do concurrent compaction.
    class OptionalRelocationLock {
     public:
      OptionalRelocationLock(Heap* heap, bool concurrent)
          : heap_(heap), concurrent_(concurrent) {
        if (concurrent_) heap_->relocation_mutex_.Lock();
      }
  
      ~OptionalRelocationLock() {
        if (concurrent_) heap_->relocation_mutex_.Unlock();
      }
  
     private:
      Heap* heap_;
      bool concurrent_;
    };
  
    void AddWeakObjectToCodeDependency(Handle<HeapObject> obj,
                                       Handle<DependentCode> dep);
  
    DependentCode* LookupWeakObjectToCodeDependency(Handle<HeapObject> obj);
  
    void AddRetainedMap(Handle<Map> map);
  
    static void FatalProcessOutOfMemory(const char* location,
                                        bool take_snapshot = false);
  
    // This event is triggered after successful allocation of a new object made
    // by runtime. Allocations of target space for object evacuation do not
    // trigger the event. In order to track ALL allocations one must turn off
    // FLAG_inline_new and FLAG_use_allocation_folding.
    inline void OnAllocationEvent(HeapObject* object, int size_in_bytes);
  
    // This event is triggered after object is moved to a new place.
    inline void OnMoveEvent(HeapObject* target, HeapObject* source,
                            int size_in_bytes);
  
    bool deserialization_complete() const { return deserialization_complete_; }
  
    void RegisterNewArrayBuffer(void* data, size_t length);
    void UnregisterArrayBuffer(void* data);
    void RegisterLiveArrayBuffer(void* data);
    void FreeDeadArrayBuffers();
  
   protected:
    // Methods made available to tests.
  
    // Allocates a JS Map in the heap.
    MUST_USE_RESULT AllocationResult
        AllocateMap(InstanceType instance_type, int instance_size,
                    ElementsKind elements_kind = TERMINAL_FAST_ELEMENTS_KIND);
  
    // Allocates and initializes a new JavaScript object based on a
    // constructor.
    // If allocation_site is non-null, then a memento is emitted after the object
    // that points to the site.
    MUST_USE_RESULT AllocationResult
        AllocateJSObject(JSFunction* constructor,
                         PretenureFlag pretenure = NOT_TENURED,
                         AllocationSite* allocation_site = NULL);
  
    // Allocates and initializes a new JavaScript object based on a map.
    // Passing an allocation site means that a memento will be created that
    // points to the site.
    MUST_USE_RESULT AllocationResult
        AllocateJSObjectFromMap(Map* map, PretenureFlag pretenure = NOT_TENURED,
                                bool alloc_props = true,
                                AllocationSite* allocation_site = NULL);
  
    // Allocated a HeapNumber from value.
    MUST_USE_RESULT AllocationResult
        AllocateHeapNumber(double value, MutableMode mode = IMMUTABLE,
                           PretenureFlag pretenure = NOT_TENURED);
  
    // Allocate a byte array of the specified length
    MUST_USE_RESULT AllocationResult
        AllocateByteArray(int length, PretenureFlag pretenure = NOT_TENURED);
  
    // Copy the code and scope info part of the code object, but insert
    // the provided data as the relocation information.
    MUST_USE_RESULT AllocationResult
        CopyCode(Code* code, Vector<byte> reloc_info);
  
    MUST_USE_RESULT AllocationResult CopyCode(Code* code);
  
    // Allocates a fixed array initialized with undefined values
    MUST_USE_RESULT AllocationResult
        AllocateFixedArray(int length, PretenureFlag pretenure = NOT_TENURED);
  
    static const int kInitialStringTableSize = 2048;
    static const int kInitialEvalCacheSize = 64;
    static const int kInitialNumberStringCacheSize = 256;
  
   private:
    Heap();
  
    // The amount of external memory registered through the API kept alive
    // by global handles
    int64_t amount_of_external_allocated_memory_;
  
    // Caches the amount of external memory registered at the last global gc.
    int64_t amount_of_external_allocated_memory_at_last_global_gc_;
  
    // This can be calculated directly from a pointer to the heap; however, it is
    // more expedient to get at the isolate directly from within Heap methods.
    Isolate* isolate_;
  
    Object* roots_[kRootListLength];
  
    size_t code_range_size_;
    int reserved_semispace_size_;
    int max_semi_space_size_;
    int initial_semispace_size_;
    int target_semispace_size_;
    intptr_t max_old_generation_size_;
    intptr_t initial_old_generation_size_;
    bool old_generation_size_configured_;
    intptr_t max_executable_size_;
    intptr_t maximum_committed_;
  
    // For keeping track of how much data has survived
    // scavenge since last new space expansion.
    int survived_since_last_expansion_;
  
    // ... and since the last scavenge.
    int survived_last_scavenge_;
  
    // For keeping track on when to flush RegExp code.
    int sweep_generation_;
  
    int always_allocate_scope_depth_;
  
    // For keeping track of context disposals.
    int contexts_disposed_;
  
    int global_ic_age_;
  
    int scan_on_scavenge_pages_;
  
    NewSpace new_space_;
    OldSpace* old_space_;
    OldSpace* code_space_;
    MapSpace* map_space_;
    LargeObjectSpace* lo_space_;
    HeapState gc_state_;
    int gc_post_processing_depth_;
    Address new_space_top_after_last_gc_;
  
    // Returns the amount of external memory registered since last global gc.
    int64_t PromotedExternalMemorySize();
  
    // How many "runtime allocations" happened.
    uint32_t allocations_count_;
  
    // Running hash over allocations performed.
    uint32_t raw_allocations_hash_;
  
    // Countdown counter, dumps allocation hash when 0.
    uint32_t dump_allocations_hash_countdown_;
  
    // How many mark-sweep collections happened.
    unsigned int ms_count_;
  
    // How many gc happened.
    unsigned int gc_count_;
  
    // For post mortem debugging.
    static const int kRememberedUnmappedPages = 128;
    int remembered_unmapped_pages_index_;
    Address remembered_unmapped_pages_[kRememberedUnmappedPages];
  
    // Total length of the strings we failed to flatten since the last GC.
    int unflattened_strings_length_;
  
  #define ROOT_ACCESSOR(type, name, camel_name)                                 \
    inline void set_##name(type* value) {                                       \
      /* The deserializer makes use of the fact that these common roots are */  \
      /* never in new space and never on a page that is being compacted.    */  \
      DCHECK(!deserialization_complete() ||                                     \
             RootCanBeWrittenAfterInitialization(k##camel_name##RootIndex));    \
      DCHECK(k##camel_name##RootIndex >= kOldSpaceRoots || !InNewSpace(value)); \
      roots_[k##camel_name##RootIndex] = value;                                 \
    }
    ROOT_LIST(ROOT_ACCESSOR)
  #undef ROOT_ACCESSOR
  
  #ifdef DEBUG
    // If the --gc-interval flag is set to a positive value, this
    // variable holds the value indicating the number of allocations
    // remain until the next failure and garbage collection.
    int allocation_timeout_;
  #endif  // DEBUG
  
    // Limit that triggers a global GC on the next (normally caused) GC.  This
    // is checked when we have already decided to do a GC to help determine
    // which collector to invoke, before expanding a paged space in the old
    // generation and on every allocation in large object space.
    intptr_t old_generation_allocation_limit_;
  
    // The allocation limit when there is >16.66ms idle time in the idle time
    // handler.
    intptr_t idle_old_generation_allocation_limit_;
  
    // Indicates that an allocation has failed in the old generation since the
    // last GC.
    bool old_gen_exhausted_;
  
    // Indicates that inline bump-pointer allocation has been globally disabled
    // for all spaces. This is used to disable allocations in generated code.
    bool inline_allocation_disabled_;
  
    // Weak list heads, threaded through the objects.
    // List heads are initialized lazily and contain the undefined_value at start.
    Object* native_contexts_list_;
    Object* allocation_sites_list_;
  
    // List of encountered weak collections (JSWeakMap and JSWeakSet) during
    // marking. It is initialized during marking, destroyed after marking and
    // contains Smi(0) while marking is not active.
    Object* encountered_weak_collections_;
  
    Object* encountered_weak_cells_;
  
    StoreBufferRebuilder store_buffer_rebuilder_;
  
    struct StringTypeTable {
      InstanceType type;
      int size;
      RootListIndex index;
    };
  
    struct ConstantStringTable {
      const char* contents;
      RootListIndex index;
    };
  
    struct StructTable {
      InstanceType type;
      int size;
      RootListIndex index;
    };
  
    static const StringTypeTable string_type_table[];
    static const ConstantStringTable constant_string_table[];
    static const StructTable struct_table[];
  
    // The special hidden string which is an empty string, but does not match
    // any string when looked up in properties.
    String* hidden_string_;
  
    // GC callback function, called before and after mark-compact GC.
    // Allocations in the callback function are disallowed.
    struct GCPrologueCallbackPair {
      GCPrologueCallbackPair(v8::Isolate::GCPrologueCallback callback,
                             GCType gc_type, bool pass_isolate)
          : callback(callback), gc_type(gc_type), pass_isolate_(pass_isolate) {}
      bool operator==(const GCPrologueCallbackPair& pair) const {
        return pair.callback == callback;
      }
      v8::Isolate::GCPrologueCallback callback;
      GCType gc_type;
      // TODO(dcarney): remove variable
      bool pass_isolate_;
    };
    List<GCPrologueCallbackPair> gc_prologue_callbacks_;
  
    struct GCEpilogueCallbackPair {
      GCEpilogueCallbackPair(v8::Isolate::GCPrologueCallback callback,
                             GCType gc_type, bool pass_isolate)
          : callback(callback), gc_type(gc_type), pass_isolate_(pass_isolate) {}
      bool operator==(const GCEpilogueCallbackPair& pair) const {
        return pair.callback == callback;
      }
      v8::Isolate::GCPrologueCallback callback;
      GCType gc_type;
      // TODO(dcarney): remove variable
      bool pass_isolate_;
    };
    List<GCEpilogueCallbackPair> gc_epilogue_callbacks_;
  
    // Support for computing object sizes during GC.
    HeapObjectCallback gc_safe_size_of_old_object_;
    static int GcSafeSizeOfOldObject(HeapObject* object);
  
    // Update the GC state. Called from the mark-compact collector.
    void MarkMapPointersAsEncoded(bool encoded) {
      DCHECK(!encoded);
      gc_safe_size_of_old_object_ = &GcSafeSizeOfOldObject;
    }
  
    // Code that should be run before and after each GC.  Includes some
    // reporting/verification activities when compiled with DEBUG set.
    void GarbageCollectionPrologue();
    void GarbageCollectionEpilogue();
  
    void PreprocessStackTraces();
  
    // Pretenuring decisions are made based on feedback collected during new
    // space evacuation. Note that between feedback collection and calling this
    // method object in old space must not move.
    // Right now we only process pretenuring feedback in high promotion mode.
    void ProcessPretenuringFeedback();
  
    // Checks whether a global GC is necessary
    GarbageCollector SelectGarbageCollector(AllocationSpace space,
                                            const char** reason);
  
    // Make sure there is a filler value behind the top of the new space
    // so that the GC does not confuse some unintialized/stale memory
    // with the allocation memento of the object at the top
    void EnsureFillerObjectAtTop();
  
    // Ensure that we have swept all spaces in such a way that we can iterate
    // over all objects.  May cause a GC.
    void MakeHeapIterable();
  
    // Performs garbage collection operation.
    // Returns whether there is a chance that another major GC could
    // collect more garbage.
    bool CollectGarbage(
        GarbageCollector collector, const char* gc_reason,
        const char* collector_reason,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    // Performs garbage collection
    // Returns whether there is a chance another major GC could
    // collect more garbage.
    bool PerformGarbageCollection(
        GarbageCollector collector,
        const GCCallbackFlags gc_callback_flags = kNoGCCallbackFlags);
  
    inline void UpdateOldSpaceLimits();
  
    // Selects the proper allocation space depending on the given object
    // size and pretenuring decision.
    static AllocationSpace SelectSpace(int object_size,
                                       PretenureFlag pretenure) {
      if (object_size > Page::kMaxRegularHeapObjectSize) return LO_SPACE;
      return (pretenure == TENURED) ? OLD_SPACE : NEW_SPACE;
    }
  
    HeapObject* DoubleAlignForDeserialization(HeapObject* object, int size);
  
    // Allocate an uninitialized object.  The memory is non-executable if the
    // hardware and OS allow.  This is the single choke-point for allocations
    // performed by the runtime and should not be bypassed (to extend this to
    // inlined allocations, use the Heap::DisableInlineAllocation() support).
    MUST_USE_RESULT inline AllocationResult AllocateRaw(
        int size_in_bytes, AllocationSpace space, AllocationSpace retry_space,
        AllocationAlignment aligment = kWordAligned);
  
    // Allocates a heap object based on the map.
    MUST_USE_RESULT AllocationResult
        Allocate(Map* map, AllocationSpace space,
                 AllocationSite* allocation_site = NULL);
  
    // Allocates a partial map for bootstrapping.
    MUST_USE_RESULT AllocationResult
        AllocatePartialMap(InstanceType instance_type, int instance_size);
  
    // Initializes a JSObject based on its map.
    void InitializeJSObjectFromMap(JSObject* obj, FixedArray* properties,
                                   Map* map);
    void InitializeAllocationMemento(AllocationMemento* memento,
                                     AllocationSite* allocation_site);
  
    // Allocate a block of memory in the given space (filled with a filler).
    // Used as a fall-back for generated code when the space is full.
    MUST_USE_RESULT AllocationResult
        AllocateFillerObject(int size, bool double_align, AllocationSpace space);
  
    // Allocate an uninitialized fixed array.
    MUST_USE_RESULT AllocationResult
        AllocateRawFixedArray(int length, PretenureFlag pretenure);
  
    // Allocate an uninitialized fixed double array.
    MUST_USE_RESULT AllocationResult
        AllocateRawFixedDoubleArray(int length, PretenureFlag pretenure);
  
    // Allocate an initialized fixed array with the given filler value.
    MUST_USE_RESULT AllocationResult
        AllocateFixedArrayWithFiller(int length, PretenureFlag pretenure,
                                     Object* filler);
  
    // Allocate and partially initializes a String.  There are two String
    // encodings: one-byte and two-byte.  These functions allocate a string of
    // the given length and set its map and length fields.  The characters of
    // the string are uninitialized.
    MUST_USE_RESULT AllocationResult
        AllocateRawOneByteString(int length, PretenureFlag pretenure);
    MUST_USE_RESULT AllocationResult
        AllocateRawTwoByteString(int length, PretenureFlag pretenure);
  
    bool CreateInitialMaps();
    void CreateInitialObjects();
  
    // Allocates an internalized string in old space based on the character
    // stream.
    MUST_USE_RESULT inline AllocationResult AllocateInternalizedStringFromUtf8(
        Vector<const char> str, int chars, uint32_t hash_field);
  
    MUST_USE_RESULT inline AllocationResult AllocateOneByteInternalizedString(
        Vector<const uint8_t> str, uint32_t hash_field);
  
    MUST_USE_RESULT inline AllocationResult AllocateTwoByteInternalizedString(
        Vector<const uc16> str, uint32_t hash_field);
  
    template <bool is_one_byte, typename T>
    MUST_USE_RESULT AllocationResult
        AllocateInternalizedStringImpl(T t, int chars, uint32_t hash_field);
  
    template <typename T>
    MUST_USE_RESULT inline AllocationResult AllocateInternalizedStringImpl(
        T t, int chars, uint32_t hash_field);
  
    // Allocates an uninitialized fixed array. It must be filled by the caller.
    MUST_USE_RESULT AllocationResult AllocateUninitializedFixedArray(int length);
  
    // Make a copy of src and return it. Returns
    // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
    MUST_USE_RESULT inline AllocationResult CopyFixedArray(FixedArray* src);
  
    // Make a copy of src, set the map, and return the copy. Returns
    // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
    MUST_USE_RESULT AllocationResult
        CopyFixedArrayWithMap(FixedArray* src, Map* map);
  
    // Make a copy of src and return it. Returns
    // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
    MUST_USE_RESULT inline AllocationResult CopyFixedDoubleArray(
        FixedDoubleArray* src);
  
    // Make a copy of src and return it. Returns
    // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
    MUST_USE_RESULT inline AllocationResult CopyConstantPoolArray(
        ConstantPoolArray* src);
  
  
    // Computes a single character string where the character has code.
    // A cache is used for one-byte (Latin1) codes.
    MUST_USE_RESULT AllocationResult
        LookupSingleCharacterStringFromCode(uint16_t code);
  
    // Allocate a symbol in old space.
    MUST_USE_RESULT AllocationResult AllocateSymbol();
  
    // Make a copy of src, set the map, and return the copy.
    MUST_USE_RESULT AllocationResult
        CopyConstantPoolArrayWithMap(ConstantPoolArray* src, Map* map);
  
    MUST_USE_RESULT AllocationResult AllocateConstantPoolArray(
        const ConstantPoolArray::NumberOfEntries& small);
  
    MUST_USE_RESULT AllocationResult AllocateExtendedConstantPoolArray(
        const ConstantPoolArray::NumberOfEntries& small,
        const ConstantPoolArray::NumberOfEntries& extended);
  
    // Allocates an external array of the specified length and type.
    MUST_USE_RESULT AllocationResult
        AllocateExternalArray(int length, ExternalArrayType array_type,
                              void* external_pointer, PretenureFlag pretenure);
  
    // Allocates a fixed typed array of the specified length and type.
    MUST_USE_RESULT AllocationResult
        AllocateFixedTypedArray(int length, ExternalArrayType array_type,
                                PretenureFlag pretenure);
  
    // Make a copy of src and return it.
    MUST_USE_RESULT AllocationResult CopyAndTenureFixedCOWArray(FixedArray* src);
  
    // Make a copy of src, set the map, and return the copy.
    MUST_USE_RESULT AllocationResult
        CopyFixedDoubleArrayWithMap(FixedDoubleArray* src, Map* map);
  
    // Allocates a fixed double array with uninitialized values. Returns
    MUST_USE_RESULT AllocationResult AllocateUninitializedFixedDoubleArray(
        int length, PretenureFlag pretenure = NOT_TENURED);
  
    // These five Create*EntryStub functions are here and forced to not be inlined
    // because of a gcc-4.4 bug that assigns wrong vtable entries.
    NO_INLINE(void CreateJSEntryStub());
    NO_INLINE(void CreateJSConstructEntryStub());
  
    void CreateFixedStubs();
  
    // Allocate empty fixed array.
    MUST_USE_RESULT AllocationResult AllocateEmptyFixedArray();
  
    // Allocate empty external array of given type.
    MUST_USE_RESULT AllocationResult
        AllocateEmptyExternalArray(ExternalArrayType array_type);
  
    // Allocate empty fixed typed array of given type.
    MUST_USE_RESULT AllocationResult
        AllocateEmptyFixedTypedArray(ExternalArrayType array_type);
  
    // Allocate empty constant pool array.
    MUST_USE_RESULT AllocationResult AllocateEmptyConstantPoolArray();
  
    // Allocate a tenured simple cell.
    MUST_USE_RESULT AllocationResult AllocateCell(Object* value);
  
    // Allocate a tenured JS global property cell initialized with the hole.
    MUST_USE_RESULT AllocationResult AllocatePropertyCell();
  
    MUST_USE_RESULT AllocationResult AllocateWeakCell(HeapObject* value);
  
    // Allocates a new utility object in the old generation.
    MUST_USE_RESULT AllocationResult AllocateStruct(InstanceType type);
  
    // Allocates a new foreign object.
    MUST_USE_RESULT AllocationResult
        AllocateForeign(Address address, PretenureFlag pretenure = NOT_TENURED);
  
    MUST_USE_RESULT AllocationResult
        AllocateCode(int object_size, bool immovable);
  
    MUST_USE_RESULT AllocationResult InternalizeStringWithKey(HashTableKey* key);
  
    MUST_USE_RESULT AllocationResult InternalizeString(String* str);
  
    // Performs a minor collection in new generation.
    void Scavenge();
  
    // Commits from space if it is uncommitted.
    void EnsureFromSpaceIsCommitted();
  
    // Uncommit unused semi space.
    bool UncommitFromSpace() { return new_space_.UncommitFromSpace(); }
  
    // Fill in bogus values in from space
    void ZapFromSpace();
  
    static String* UpdateNewSpaceReferenceInExternalStringTableEntry(
        Heap* heap, Object** pointer);
  
    Address DoScavenge(ObjectVisitor* scavenge_visitor, Address new_space_front);
    static void ScavengeStoreBufferCallback(Heap* heap, MemoryChunk* page,
                                            StoreBufferEvent event);
  
    // Performs a major collection in the whole heap.
    void MarkCompact();
  
    // Code to be run before and after mark-compact.
    void MarkCompactPrologue();
    void MarkCompactEpilogue();
  
    void ProcessNativeContexts(WeakObjectRetainer* retainer);
    void ProcessAllocationSites(WeakObjectRetainer* retainer);
  
    // Deopts all code that contains allocation instruction which are tenured or
    // not tenured. Moreover it clears the pretenuring allocation site statistics.
    void ResetAllAllocationSitesDependentCode(PretenureFlag flag);
  
    // Evaluates local pretenuring for the old space and calls
    // ResetAllTenuredAllocationSitesDependentCode if too many objects died in
    // the old space.
    void EvaluateOldSpaceLocalPretenuring(uint64_t size_of_objects_before_gc);
  
    // Called on heap tear-down.
    void TearDownArrayBuffers();
  
    // Record statistics before and after garbage collection.
    void ReportStatisticsBeforeGC();
    void ReportStatisticsAfterGC();
  
    // Slow part of scavenge object.
    static void ScavengeObjectSlow(HeapObject** p, HeapObject* object);
  
    // Total RegExp code ever generated
    double total_regexp_code_generated_;
  
    int deferred_counters_[v8::Isolate::kUseCounterFeatureCount];
  
    GCTracer tracer_;
  
    // Creates and installs the full-sized number string cache.
    int FullSizeNumberStringCacheLength();
    // Flush the number to string cache.
    void FlushNumberStringCache();
  
    // Sets used allocation sites entries to undefined.
    void FlushAllocationSitesScratchpad();
  
    // Initializes the allocation sites scratchpad with undefined values.
    void InitializeAllocationSitesScratchpad();
  
    // Adds an allocation site to the scratchpad if there is space left.
    void AddAllocationSiteToScratchpad(AllocationSite* site,
                                       ScratchpadSlotMode mode);
  
    void UpdateSurvivalStatistics(int start_new_space_size);
  
    static const int kYoungSurvivalRateHighThreshold = 90;
    static const int kYoungSurvivalRateAllowedDeviation = 15;
  
    static const int kOldSurvivalRateLowThreshold = 10;
  
    int high_survival_rate_period_length_;
    intptr_t promoted_objects_size_;
    double promotion_ratio_;
    double promotion_rate_;
    intptr_t semi_space_copied_object_size_;
    intptr_t previous_semi_space_copied_object_size_;
    double semi_space_copied_rate_;
    int nodes_died_in_new_space_;
    int nodes_copied_in_new_space_;
    int nodes_promoted_;
  
    // This is the pretenuring trigger for allocation sites that are in maybe
    // tenure state. When we switched to the maximum new space size we deoptimize
    // the code that belongs to the allocation site and derive the lifetime
    // of the allocation site.
    unsigned int maximum_size_scavenges_;
  
    // TODO(hpayer): Allocation site pretenuring may make this method obsolete.
    // Re-visit incremental marking heuristics.
    bool IsHighSurvivalRate() { return high_survival_rate_period_length_ > 0; }
  
    void ConfigureInitialOldGenerationSize();
  
    void SelectScavengingVisitorsTable();
  
    bool TryFinalizeIdleIncrementalMarking(
        double idle_time_in_ms, size_t size_of_objects,
        size_t mark_compact_speed_in_bytes_per_ms);
  
    void ClearObjectStats(bool clear_last_time_stats = false);
  
    inline void UpdateAllocationsHash(HeapObject* object);
    inline void UpdateAllocationsHash(uint32_t value);
    inline void PrintAlloctionsHash();
  
    // Object counts and used memory by InstanceType
    size_t object_counts_[OBJECT_STATS_COUNT];
    size_t object_counts_last_time_[OBJECT_STATS_COUNT];
    size_t object_sizes_[OBJECT_STATS_COUNT];
    size_t object_sizes_last_time_[OBJECT_STATS_COUNT];
  
    // Maximum GC pause.
    double max_gc_pause_;
  
    // Total time spent in GC.
    double total_gc_time_ms_;
  
    // Maximum size of objects alive after GC.
    intptr_t max_alive_after_gc_;
  
    // Minimal interval between two subsequent collections.
    double min_in_mutator_;
  
    // Cumulative GC time spent in marking
    double marking_time_;
  
    // Cumulative GC time spent in sweeping
    double sweeping_time_;
  
    // Last time an idle notification happened
    double last_idle_notification_time_;
  
    MarkCompactCollector mark_compact_collector_;
  
    StoreBuffer store_buffer_;
  
    Marking marking_;
  
    IncrementalMarking incremental_marking_;
  
    GCIdleTimeHandler gc_idle_time_handler_;
  
    unsigned int gc_count_at_last_idle_gc_;
  
    // These two counters are monotomically increasing and never reset.
    size_t full_codegen_bytes_generated_;
    size_t crankshaft_codegen_bytes_generated_;
  
    // If the --deopt_every_n_garbage_collections flag is set to a positive value,
    // this variable holds the number of garbage collections since the last
    // deoptimization triggered by garbage collection.
    int gcs_since_last_deopt_;
  
    static const int kAllocationSiteScratchpadSize = 256;
    int allocation_sites_scratchpad_length_;
  
    static const int kMaxMarkCompactsInIdleRound = 7;
    static const int kIdleScavengeThreshold = 5;
  
    // Shared state read by the scavenge collector and set by ScavengeObject.
    PromotionQueue promotion_queue_;
  
    // Flag is set when the heap has been configured.  The heap can be repeatedly
    // configured through the API until it is set up.
    bool configured_;
  
    ExternalStringTable external_string_table_;
  
    VisitorDispatchTable<ScavengingCallback> scavenging_visitors_table_;
  
    MemoryChunk* chunks_queued_for_free_;
  
    base::Mutex relocation_mutex_;
  
    int gc_callbacks_depth_;
  
    bool deserialization_complete_;
  
    bool concurrent_sweeping_enabled_;
  
    std::map<void*, size_t> live_array_buffers_;
    std::map<void*, size_t> not_yet_discovered_array_buffers_;
  
    struct StrongRootsList;
    StrongRootsList* strong_roots_list_;
  
    friend class AlwaysAllocateScope;
    friend class Deserializer;
    friend class Factory;
    friend class GCCallbacksScope;
    friend class GCTracer;
    friend class HeapIterator;
    friend class Isolate;
    friend class MarkCompactCollector;
    friend class MarkCompactMarkingVisitor;
    friend class MapCompact;
    friend class Page;
  
    DISALLOW_COPY_AND_ASSIGN(Heap);
  };
  
  
  class HeapStats {
   public:
    static const int kStartMarker = 0xDECADE00;
    static const int kEndMarker = 0xDECADE01;
  
    int* start_marker;                       //  0
    int* new_space_size;                     //  1
    int* new_space_capacity;                 //  2
    intptr_t* old_space_size;                //  3
    intptr_t* old_space_capacity;            //  4
    intptr_t* code_space_size;               //  5
    intptr_t* code_space_capacity;           //  6
    intptr_t* map_space_size;                //  7
    intptr_t* map_space_capacity;            //  8
    intptr_t* lo_space_size;                 //  9
    int* global_handle_count;                // 10
    int* weak_global_handle_count;           // 11
    int* pending_global_handle_count;        // 12
    int* near_death_global_handle_count;     // 13
    int* free_global_handle_count;           // 14
    intptr_t* memory_allocator_size;         // 15
    intptr_t* memory_allocator_capacity;     // 16
    int* objects_per_type;                   // 17
    int* size_per_type;                      // 18
    int* os_error;                           // 19
    int* end_marker;                         // 20
  };
  
  
  class AlwaysAllocateScope {
   public:
    explicit inline AlwaysAllocateScope(Isolate* isolate);
    inline ~AlwaysAllocateScope();
  
   private:
    // Implicitly disable artificial allocation failures.
    Heap* heap_;
    DisallowAllocationFailure daf_;
  };
  
  
  class GCCallbacksScope {
   public:
    explicit inline GCCallbacksScope(Heap* heap);
    inline ~GCCallbacksScope();
  
    inline bool CheckReenter();
  
   private:
    Heap* heap_;
  };
  
  
  // Visitor class to verify interior pointers in spaces that do not contain
  // or care about intergenerational references. All heap object pointers have to
  // point into the heap to a location that has a map pointer at its first word.
  // Caveat: Heap::Contains is an approximation because it can return true for
  // objects in a heap space but above the allocation pointer.
  class VerifyPointersVisitor : public ObjectVisitor {
   public:
    inline void VisitPointers(Object** start, Object** end);
  };
  
  
  // Verify that all objects are Smis.
  class VerifySmisVisitor : public ObjectVisitor {
   public:
    inline void VisitPointers(Object** start, Object** end);
  };
  
  
  // Space iterator for iterating over all spaces of the heap.  Returns each space
  // in turn, and null when it is done.
  class AllSpaces BASE_EMBEDDED {
   public:
    explicit AllSpaces(Heap* heap) : heap_(heap), counter_(FIRST_SPACE) {}
    Space* next();
  
   private:
    Heap* heap_;
    int counter_;
  };
  
  
  // Space iterator for iterating over all old spaces of the heap: Old space
  // and code space.  Returns each space in turn, and null when it is done.
  class OldSpaces BASE_EMBEDDED {
   public:
    explicit OldSpaces(Heap* heap) : heap_(heap), counter_(OLD_SPACE) {}
    OldSpace* next();
  
   private:
    Heap* heap_;
    int counter_;
  };
  
  
  // Space iterator for iterating over all the paged spaces of the heap: Map
  // space, old space, code space and cell space.  Returns
  // each space in turn, and null when it is done.
  class PagedSpaces BASE_EMBEDDED {
   public:
    explicit PagedSpaces(Heap* heap) : heap_(heap), counter_(OLD_SPACE) {}
    PagedSpace* next();
  
   private:
    Heap* heap_;
    int counter_;
  };
  
  
  // Space iterator for iterating over all spaces of the heap.
  // For each space an object iterator is provided. The deallocation of the
  // returned object iterators is handled by the space iterator.
  class SpaceIterator : public Malloced {
   public:
    explicit SpaceIterator(Heap* heap);
    SpaceIterator(Heap* heap, HeapObjectCallback size_func);
    virtual ~SpaceIterator();
  
    bool has_next();
    ObjectIterator* next();
  
   private:
    ObjectIterator* CreateIterator();
  
    Heap* heap_;
    int current_space_;         // from enum AllocationSpace.
    ObjectIterator* iterator_;  // object iterator for the current space.
    HeapObjectCallback size_func_;
  };
  
  
  // A HeapIterator provides iteration over the whole heap. It
  // aggregates the specific iterators for the different spaces as
  // these can only iterate over one space only.
  //
  // HeapIterator ensures there is no allocation during its lifetime
  // (using an embedded DisallowHeapAllocation instance).
  //
  // HeapIterator can skip free list nodes (that is, de-allocated heap
  // objects that still remain in the heap). As implementation of free
  // nodes filtering uses GC marks, it can't be used during MS/MC GC
  // phases. Also, it is forbidden to interrupt iteration in this mode,
  // as this will leave heap objects marked (and thus, unusable).
  class HeapObjectsFilter;
  
  class HeapIterator BASE_EMBEDDED {
   public:
    enum HeapObjectsFiltering { kNoFiltering, kFilterUnreachable };
  
    explicit HeapIterator(Heap* heap);
    HeapIterator(Heap* heap, HeapObjectsFiltering filtering);
    ~HeapIterator();
  
    HeapObject* next();
    void reset();
  
   private:
    struct MakeHeapIterableHelper {
      explicit MakeHeapIterableHelper(Heap* heap) { heap->MakeHeapIterable(); }
    };
  
    // Perform the initialization.
    void Init();
    // Perform all necessary shutdown (destruction) work.
    void Shutdown();
    HeapObject* NextObject();
  
    MakeHeapIterableHelper make_heap_iterable_helper_;
    DisallowHeapAllocation no_heap_allocation_;
    Heap* heap_;
    HeapObjectsFiltering filtering_;
    HeapObjectsFilter* filter_;
    // Space iterator for iterating all the spaces.
    SpaceIterator* space_iterator_;
    // Object iterator for the space currently being iterated.
    ObjectIterator* object_iterator_;
  };
  
  
  // Cache for mapping (map, property name) into field offset.
  // Cleared at startup and prior to mark sweep collection.
  class KeyedLookupCache {
   public:
    // Lookup field offset for (map, name). If absent, -1 is returned.
    int Lookup(Handle<Map> map, Handle<Name> name);
  
    // Update an element in the cache.
    void Update(Handle<Map> map, Handle<Name> name, int field_offset);
  
    // Clear the cache.
    void Clear();
  
    static const int kLength = 256;
    static const int kCapacityMask = kLength - 1;
    static const int kMapHashShift = 5;
    static const int kHashMask = -4;  // Zero the last two bits.
    static const int kEntriesPerBucket = 4;
    static const int kEntryLength = 2;
    static const int kMapIndex = 0;
    static const int kKeyIndex = 1;
    static const int kNotFound = -1;
  
    // kEntriesPerBucket should be a power of 2.
    STATIC_ASSERT((kEntriesPerBucket & (kEntriesPerBucket - 1)) == 0);
    STATIC_ASSERT(kEntriesPerBucket == -kHashMask);
  
   private:
    KeyedLookupCache() {
      for (int i = 0; i < kLength; ++i) {
        keys_[i].map = NULL;
        keys_[i].name = NULL;
        field_offsets_[i] = kNotFound;
      }
    }
  
    static inline int Hash(Handle<Map> map, Handle<Name> name);
  
    // Get the address of the keys and field_offsets arrays.  Used in
    // generated code to perform cache lookups.
    Address keys_address() { return reinterpret_cast<Address>(&keys_); }
  
    Address field_offsets_address() {
      return reinterpret_cast<Address>(&field_offsets_);
    }
  
    struct Key {
      Map* map;
      Name* name;
    };
  
    Key keys_[kLength];
    int field_offsets_[kLength];
  
    friend class ExternalReference;
    friend class Isolate;
    DISALLOW_COPY_AND_ASSIGN(KeyedLookupCache);
  };
  
  
  // Cache for mapping (map, property name) into descriptor index.
  // The cache contains both positive and negative results.
  // Descriptor index equals kNotFound means the property is absent.
  // Cleared at startup and prior to any gc.
  class DescriptorLookupCache {
   public:
    // Lookup descriptor index for (map, name).
    // If absent, kAbsent is returned.
    int Lookup(Map* source, Name* name) {
      if (!name->IsUniqueName()) return kAbsent;
      int index = Hash(source, name);
      Key& key = keys_[index];
      if ((key.source == source) && (key.name == name)) return results_[index];
      return kAbsent;
    }
  
    // Update an element in the cache.
    void Update(Map* source, Name* name, int result) {
      DCHECK(result != kAbsent);
      if (name->IsUniqueName()) {
        int index = Hash(source, name);
        Key& key = keys_[index];
        key.source = source;
        key.name = name;
        results_[index] = result;
      }
    }
  
    // Clear the cache.
    void Clear();
  
    static const int kAbsent = -2;
  
   private:
    DescriptorLookupCache() {
      for (int i = 0; i < kLength; ++i) {
        keys_[i].source = NULL;
        keys_[i].name = NULL;
        results_[i] = kAbsent;
      }
    }
  
    static int Hash(Object* source, Name* name) {
      // Uses only lower 32 bits if pointers are larger.
      uint32_t source_hash =
          static_cast<uint32_t>(reinterpret_cast<uintptr_t>(source)) >>
          kPointerSizeLog2;
      uint32_t name_hash =
          static_cast<uint32_t>(reinterpret_cast<uintptr_t>(name)) >>
          kPointerSizeLog2;
      return (source_hash ^ name_hash) % kLength;
    }
  
    static const int kLength = 64;
    struct Key {
      Map* source;
      Name* name;
    };
  
    Key keys_[kLength];
    int results_[kLength];
  
    friend class Isolate;
    DISALLOW_COPY_AND_ASSIGN(DescriptorLookupCache);
  };
  
  
  class RegExpResultsCache {
   public:
    enum ResultsCacheType { REGEXP_MULTIPLE_INDICES, STRING_SPLIT_SUBSTRINGS };
  
    // Attempt to retrieve a cached result.  On failure, 0 is returned as a Smi.
    // On success, the returned result is guaranteed to be a COW-array.
    static Object* Lookup(Heap* heap, String* key_string, Object* key_pattern,
                          ResultsCacheType type);
    // Attempt to add value_array to the cache specified by type.  On success,
    // value_array is turned into a COW-array.
    static void Enter(Isolate* isolate, Handle<String> key_string,
                      Handle<Object> key_pattern, Handle<FixedArray> value_array,
                      ResultsCacheType type);
    static void Clear(FixedArray* cache);
    static const int kRegExpResultsCacheSize = 0x100;
  
   private:
    static const int kArrayEntriesPerCacheEntry = 4;
    static const int kStringOffset = 0;
    static const int kPatternOffset = 1;
    static const int kArrayOffset = 2;
  };
  
  
  // Abstract base class for checking whether a weak object should be retained.
  class WeakObjectRetainer {
   public:
    virtual ~WeakObjectRetainer() {}
  
    // Return whether this object should be retained. If NULL is returned the
    // object has no references. Otherwise the address of the retained object
    // should be returned as in some GC situations the object has been moved.
    virtual Object* RetainAs(Object* object) = 0;
  };
  
  
  // Intrusive object marking uses least significant bit of
  // heap object's map word to mark objects.
  // Normally all map words have least significant bit set
  // because they contain tagged map pointer.
  // If the bit is not set object is marked.
  // All objects should be unmarked before resuming
  // JavaScript execution.
  class IntrusiveMarking {
   public:
    static bool IsMarked(HeapObject* object) {
      return (object->map_word().ToRawValue() & kNotMarkedBit) == 0;
    }
  
    static void ClearMark(HeapObject* object) {
      uintptr_t map_word = object->map_word().ToRawValue();
      object->set_map_word(MapWord::FromRawValue(map_word | kNotMarkedBit));
      DCHECK(!IsMarked(object));
    }
  
    static void SetMark(HeapObject* object) {
      uintptr_t map_word = object->map_word().ToRawValue();
      object->set_map_word(MapWord::FromRawValue(map_word & ~kNotMarkedBit));
      DCHECK(IsMarked(object));
    }
  
    static Map* MapOfMarkedObject(HeapObject* object) {
      uintptr_t map_word = object->map_word().ToRawValue();
      return MapWord::FromRawValue(map_word | kNotMarkedBit).ToMap();
    }
  
    static int SizeOfMarkedObject(HeapObject* object) {
      return object->SizeFromMap(MapOfMarkedObject(object));
    }
  
   private:
    static const uintptr_t kNotMarkedBit = 0x1;
    STATIC_ASSERT((kHeapObjectTag & kNotMarkedBit) != 0);  // NOLINT
  };
  
  
  #ifdef DEBUG
  // Helper class for tracing paths to a search target Object from all roots.
  // The TracePathFrom() method can be used to trace paths from a specific
  // object to the search target object.
  class PathTracer : public ObjectVisitor {
   public:
    enum WhatToFind {
      FIND_ALL,   // Will find all matches.
      FIND_FIRST  // Will stop the search after first match.
    };
  
    // Tags 0, 1, and 3 are used. Use 2 for marking visited HeapObject.
    static const int kMarkTag = 2;
  
    // For the WhatToFind arg, if FIND_FIRST is specified, tracing will stop
    // after the first match.  If FIND_ALL is specified, then tracing will be
    // done for all matches.
    PathTracer(Object* search_target, WhatToFind what_to_find,
               VisitMode visit_mode)
        : search_target_(search_target),
          found_target_(false),
          found_target_in_trace_(false),
          what_to_find_(what_to_find),
          visit_mode_(visit_mode),
          object_stack_(20),
          no_allocation() {}
  
    virtual void VisitPointers(Object** start, Object** end);
  
    void Reset();
    void TracePathFrom(Object** root);
  
    bool found() const { return found_target_; }
  
    static Object* const kAnyGlobalObject;
  
   protected:
    class MarkVisitor;
    class UnmarkVisitor;
  
    void MarkRecursively(Object** p, MarkVisitor* mark_visitor);
    void UnmarkRecursively(Object** p, UnmarkVisitor* unmark_visitor);
    virtual void ProcessResults();
  
    Object* search_target_;
    bool found_target_;
    bool found_target_in_trace_;
    WhatToFind what_to_find_;
    VisitMode visit_mode_;
    List<Object*> object_stack_;
  
    DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
  
   private:
    DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
  };
  #endif  // DEBUG
  }
  }  // namespace v8::internal
  
  #endif  // V8_HEAP_HEAP_H_
  