text: // Copyright 2006-2008 the V8 project authors. All rights reserved.
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  //       notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  //       copyright notice, this list of conditions and the following
  //       disclaimer in the documentation and/or other materials provided
  //       with the distribution.
  //     * Neither the name of Google Inc. nor the names of its
  //       contributors may be used to endorse or promote products derived
  //       from this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  #ifndef V8_HEAP_H_
  #define V8_HEAP_H_
  
  #include <math.h>
  
  #include "zone-inl.h"
  
  
  namespace v8 {
  namespace internal {
  
  // Defines all the roots in Heap.
  #define UNCONDITIONAL_STRONG_ROOT_LIST(V)                                      \
    /* Put the byte array map early.  We need it to be in place by the time   */ \
    /* the deserializer hits the next page, since it wants to put a byte      */ \
    /* array in the unused space at the end of the page.                      */ \
    V(Map, byte_array_map, ByteArrayMap)                                         \
    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, stack_limit, StackLimit)                                              \
    V(Object, undefined_value, UndefinedValue)                                   \
    V(Object, the_hole_value, TheHoleValue)                                      \
    V(Object, null_value, NullValue)                                             \
    V(Object, true_value, TrueValue)                                             \
    V(Object, false_value, FalseValue)                                           \
    V(Map, heap_number_map, HeapNumberMap)                                       \
    V(Map, global_context_map, GlobalContextMap)                                 \
    V(Map, fixed_array_map, FixedArrayMap)                                       \
    V(Object, no_interceptor_result_sentinel, NoInterceptorResultSentinel)       \
    V(Map, meta_map, MetaMap)                                                    \
    V(Object, termination_exception, TerminationException)                       \
    V(Map, hash_table_map, HashTableMap)                                         \
    V(FixedArray, empty_fixed_array, EmptyFixedArray)                            \
    V(Map, short_string_map, ShortStringMap)                                     \
    V(Map, medium_string_map, MediumStringMap)                                   \
    V(Map, long_string_map, LongStringMap)                                       \
    V(Map, short_ascii_string_map, ShortAsciiStringMap)                          \
    V(Map, medium_ascii_string_map, MediumAsciiStringMap)                        \
    V(Map, long_ascii_string_map, LongAsciiStringMap)                            \
    V(Map, short_symbol_map, ShortSymbolMap)                                     \
    V(Map, medium_symbol_map, MediumSymbolMap)                                   \
    V(Map, long_symbol_map, LongSymbolMap)                                       \
    V(Map, short_ascii_symbol_map, ShortAsciiSymbolMap)                          \
    V(Map, medium_ascii_symbol_map, MediumAsciiSymbolMap)                        \
    V(Map, long_ascii_symbol_map, LongAsciiSymbolMap)                            \
    V(Map, short_cons_symbol_map, ShortConsSymbolMap)                            \
    V(Map, medium_cons_symbol_map, MediumConsSymbolMap)                          \
    V(Map, long_cons_symbol_map, LongConsSymbolMap)                              \
    V(Map, short_cons_ascii_symbol_map, ShortConsAsciiSymbolMap)                 \
    V(Map, medium_cons_ascii_symbol_map, MediumConsAsciiSymbolMap)               \
    V(Map, long_cons_ascii_symbol_map, LongConsAsciiSymbolMap)                   \
    V(Map, short_sliced_symbol_map, ShortSlicedSymbolMap)                        \
    V(Map, medium_sliced_symbol_map, MediumSlicedSymbolMap)                      \
    V(Map, long_sliced_symbol_map, LongSlicedSymbolMap)                          \
    V(Map, short_sliced_ascii_symbol_map, ShortSlicedAsciiSymbolMap)             \
    V(Map, medium_sliced_ascii_symbol_map, MediumSlicedAsciiSymbolMap)           \
    V(Map, long_sliced_ascii_symbol_map, LongSlicedAsciiSymbolMap)               \
    V(Map, short_external_symbol_map, ShortExternalSymbolMap)                    \
    V(Map, medium_external_symbol_map, MediumExternalSymbolMap)                  \
    V(Map, long_external_symbol_map, LongExternalSymbolMap)                      \
    V(Map, short_external_ascii_symbol_map, ShortExternalAsciiSymbolMap)         \
    V(Map, medium_external_ascii_symbol_map, MediumExternalAsciiSymbolMap)       \
    V(Map, long_external_ascii_symbol_map, LongExternalAsciiSymbolMap)           \
    V(Map, short_cons_string_map, ShortConsStringMap)                            \
    V(Map, medium_cons_string_map, MediumConsStringMap)                          \
    V(Map, long_cons_string_map, LongConsStringMap)                              \
    V(Map, short_cons_ascii_string_map, ShortConsAsciiStringMap)                 \
    V(Map, medium_cons_ascii_string_map, MediumConsAsciiStringMap)               \
    V(Map, long_cons_ascii_string_map, LongConsAsciiStringMap)                   \
    V(Map, short_sliced_string_map, ShortSlicedStringMap)                        \
    V(Map, medium_sliced_string_map, MediumSlicedStringMap)                      \
    V(Map, long_sliced_string_map, LongSlicedStringMap)                          \
    V(Map, short_sliced_ascii_string_map, ShortSlicedAsciiStringMap)             \
    V(Map, medium_sliced_ascii_string_map, MediumSlicedAsciiStringMap)           \
    V(Map, long_sliced_ascii_string_map, LongSlicedAsciiStringMap)               \
    V(Map, short_external_string_map, ShortExternalStringMap)                    \
    V(Map, medium_external_string_map, MediumExternalStringMap)                  \
    V(Map, long_external_string_map, LongExternalStringMap)                      \
    V(Map, short_external_ascii_string_map, ShortExternalAsciiStringMap)         \
    V(Map, medium_external_ascii_string_map, MediumExternalAsciiStringMap)       \
    V(Map, long_external_ascii_string_map, LongExternalAsciiStringMap)           \
    V(Map, undetectable_short_string_map, UndetectableShortStringMap)            \
    V(Map, undetectable_medium_string_map, UndetectableMediumStringMap)          \
    V(Map, undetectable_long_string_map, UndetectableLongStringMap)              \
    V(Map, undetectable_short_ascii_string_map, UndetectableShortAsciiStringMap) \
    V(Map,                                                                       \
      undetectable_medium_ascii_string_map,                                      \
      UndetectableMediumAsciiStringMap)                                          \
    V(Map, undetectable_long_ascii_string_map, UndetectableLongAsciiStringMap)   \
    V(Map, pixel_array_map, PixelArrayMap)                                       \
    V(Map, external_byte_array_map, ExternalByteArrayMap)                        \
    V(Map, external_unsigned_byte_array_map, ExternalUnsignedByteArrayMap)       \
    V(Map, external_short_array_map, ExternalShortArrayMap)                      \
    V(Map, external_unsigned_short_array_map, ExternalUnsignedShortArrayMap)     \
    V(Map, external_int_array_map, ExternalIntArrayMap)                          \
    V(Map, external_unsigned_int_array_map, ExternalUnsignedIntArrayMap)         \
    V(Map, external_float_array_map, ExternalFloatArrayMap)                      \
    V(Map, context_map, ContextMap)                                              \
    V(Map, catch_context_map, CatchContextMap)                                   \
    V(Map, code_map, CodeMap)                                                    \
    V(Map, oddball_map, OddballMap)                                              \
    V(Map, global_property_cell_map, GlobalPropertyCellMap)                      \
    V(Map, boilerplate_function_map, BoilerplateFunctionMap)                     \
    V(Map, shared_function_info_map, SharedFunctionInfoMap)                      \
    V(Map, proxy_map, ProxyMap)                                                  \
    V(Object, nan_value, NanValue)                                               \
    V(Object, minus_zero_value, MinusZeroValue)                                  \
    V(String, empty_string, EmptyString)                                         \
    V(DescriptorArray, empty_descriptor_array, EmptyDescriptorArray)             \
    V(Map, neander_map, NeanderMap)                                              \
    V(JSObject, message_listeners, MessageListeners)                             \
    V(Proxy, prototype_accessors, PrototypeAccessors)                            \
    V(NumberDictionary, code_stubs, CodeStubs)                                   \
    V(NumberDictionary, non_monomorphic_cache, NonMonomorphicCache)              \
    V(Code, js_entry_code, JsEntryCode)                                          \
    V(Code, js_construct_entry_code, JsConstructEntryCode)                       \
    V(Code, c_entry_code, CEntryCode)                                            \
    V(Code, c_entry_debug_break_code, CEntryDebugBreakCode)                      \
    V(FixedArray, number_string_cache, NumberStringCache)                        \
    V(FixedArray, single_character_string_cache, SingleCharacterStringCache)     \
    V(FixedArray, natives_source_cache, NativesSourceCache)                      \
    V(Object, last_script_id, LastScriptId)                                      \
    V(Smi, real_stack_limit, RealStackLimit)                                     \
  
  #if V8_TARGET_ARCH_ARM && V8_NATIVE_REGEXP
  #define STRONG_ROOT_LIST(V)                                                    \
    UNCONDITIONAL_STRONG_ROOT_LIST(V)                                            \
    V(Code, re_c_entry_code, RegExpCEntryCode)
  #else
  #define STRONG_ROOT_LIST(V) UNCONDITIONAL_STRONG_ROOT_LIST(V)
  #endif
  
  #define ROOT_LIST(V)                                  \
    STRONG_ROOT_LIST(V)                                 \
    V(SymbolTable, symbol_table, SymbolTable)
  
  #define SYMBOL_LIST(V)                                                   \
    V(Array_symbol, "Array")                                               \
    V(Object_symbol, "Object")                                             \
    V(Proto_symbol, "__proto__")                                           \
    V(StringImpl_symbol, "StringImpl")                                     \
    V(arguments_symbol, "arguments")                                       \
    V(Arguments_symbol, "Arguments")                                       \
    V(arguments_shadow_symbol, ".arguments")                               \
    V(call_symbol, "call")                                                 \
    V(apply_symbol, "apply")                                               \
    V(caller_symbol, "caller")                                             \
    V(boolean_symbol, "boolean")                                           \
    V(Boolean_symbol, "Boolean")                                           \
    V(callee_symbol, "callee")                                             \
    V(constructor_symbol, "constructor")                                   \
    V(code_symbol, ".code")                                                \
    V(result_symbol, ".result")                                            \
    V(catch_var_symbol, ".catch-var")                                      \
    V(empty_symbol, "")                                                    \
    V(eval_symbol, "eval")                                                 \
    V(function_symbol, "function")                                         \
    V(length_symbol, "length")                                             \
    V(name_symbol, "name")                                                 \
    V(number_symbol, "number")                                             \
    V(Number_symbol, "Number")                                             \
    V(RegExp_symbol, "RegExp")                                             \
    V(object_symbol, "object")                                             \
    V(prototype_symbol, "prototype")                                       \
    V(string_symbol, "string")                                             \
    V(String_symbol, "String")                                             \
    V(Date_symbol, "Date")                                                 \
    V(this_symbol, "this")                                                 \
    V(to_string_symbol, "toString")                                        \
    V(char_at_symbol, "CharAt")                                            \
    V(undefined_symbol, "undefined")                                       \
    V(value_of_symbol, "valueOf")                                          \
    V(InitializeVarGlobal_symbol, "InitializeVarGlobal")                   \
    V(InitializeConstGlobal_symbol, "InitializeConstGlobal")               \
    V(stack_overflow_symbol, "kStackOverflowBoilerplate")                  \
    V(illegal_access_symbol, "illegal access")                             \
    V(out_of_memory_symbol, "out-of-memory")                               \
    V(illegal_execution_state_symbol, "illegal execution state")           \
    V(get_symbol, "get")                                                   \
    V(set_symbol, "set")                                                   \
    V(function_class_symbol, "Function")                                   \
    V(illegal_argument_symbol, "illegal argument")                         \
    V(MakeReferenceError_symbol, "MakeReferenceError")                     \
    V(MakeSyntaxError_symbol, "MakeSyntaxError")                           \
    V(MakeTypeError_symbol, "MakeTypeError")                               \
    V(invalid_lhs_in_assignment_symbol, "invalid_lhs_in_assignment")       \
    V(invalid_lhs_in_for_in_symbol, "invalid_lhs_in_for_in")               \
    V(invalid_lhs_in_postfix_op_symbol, "invalid_lhs_in_postfix_op")       \
    V(invalid_lhs_in_prefix_op_symbol, "invalid_lhs_in_prefix_op")         \
    V(illegal_return_symbol, "illegal_return")                             \
    V(illegal_break_symbol, "illegal_break")                               \
    V(illegal_continue_symbol, "illegal_continue")                         \
    V(unknown_label_symbol, "unknown_label")                               \
    V(redeclaration_symbol, "redeclaration")                               \
    V(failure_symbol, "<failure>")                                         \
    V(space_symbol, " ")                                                   \
    V(exec_symbol, "exec")                                                 \
    V(zero_symbol, "0")                                                    \
    V(global_eval_symbol, "GlobalEval")                                    \
    V(identity_hash_symbol, "v8::IdentityHash")                            \
    V(closure_symbol, "(closure)")
  
  
  // Forward declaration of the GCTracer class.
  class GCTracer;
  
  
  // The all static Heap captures the interface to the global object heap.
  // All JavaScript contexts by this process share the same object heap.
  
  class Heap : public AllStatic {
   public:
    // Configure heap size before setup. Return false if the heap has been
    // setup already.
    static bool ConfigureHeap(int max_semispace_size, int max_old_gen_size);
    static bool ConfigureHeapDefault();
  
    // Initializes the global object heap. If create_heap_objects is true,
    // also creates the basic non-mutable objects.
    // Returns whether it succeeded.
    static bool Setup(bool create_heap_objects);
  
    // Destroys all memory allocated by the heap.
    static 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.
    static void SetStackLimits();
  
    // Returns whether Setup has been called.
    static 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.
    static int MaxReserved() {
      return 4 * reserved_semispace_size_ + max_old_generation_size_;
    }
    static int MaxSemiSpaceSize() { return max_semispace_size_; }
    static int ReservedSemiSpaceSize() { return reserved_semispace_size_; }
    static int InitialSemiSpaceSize() { return initial_semispace_size_; }
    static int MaxOldGenerationSize() { return max_old_generation_size_; }
  
    // Returns the capacity of the heap in bytes w/o growing. Heap grows when
    // more spaces are needed until it reaches the limit.
    static int Capacity();
  
    // Returns the amount of memory currently committed for the heap.
    static int CommittedMemory();
  
    // 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.
    static int Available();
  
    // Returns the maximum object size in paged space.
    static inline int MaxObjectSizeInPagedSpace();
  
    // Returns of size of all objects residing in the heap.
    static int SizeOfObjects();
  
    // 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.
    static Address NewSpaceStart() { return new_space_.start(); }
    static uintptr_t NewSpaceMask() { return new_space_.mask(); }
    static Address NewSpaceTop() { return new_space_.top(); }
  
    static NewSpace* new_space() { return &new_space_; }
    static OldSpace* old_pointer_space() { return old_pointer_space_; }
    static OldSpace* old_data_space() { return old_data_space_; }
    static OldSpace* code_space() { return code_space_; }
    static MapSpace* map_space() { return map_space_; }
    static CellSpace* cell_space() { return cell_space_; }
    static LargeObjectSpace* lo_space() { return lo_space_; }
  
    static bool always_allocate() { return always_allocate_scope_depth_ != 0; }
    static Address always_allocate_scope_depth_address() {
      return reinterpret_cast<Address>(&always_allocate_scope_depth_);
    }
    static bool linear_allocation() {
        return linear_allocation_scope_depth_ != 0;
    }
  
    static Address* NewSpaceAllocationTopAddress() {
      return new_space_.allocation_top_address();
    }
    static Address* NewSpaceAllocationLimitAddress() {
      return new_space_.allocation_limit_address();
    }
  
    // Uncommit unused semi space.
    static bool UncommitFromSpace() { return new_space_.UncommitFromSpace(); }
  
  #ifdef ENABLE_HEAP_PROTECTION
    // Protect/unprotect the heap by marking all spaces read-only/writable.
    static void Protect();
    static void Unprotect();
  #endif
  
    // Allocates and initializes a new JavaScript object based on a
    // constructor.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateJSObject(JSFunction* constructor,
                                    PretenureFlag pretenure = NOT_TENURED);
  
    // Allocates and initializes a new global object based on a constructor.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateGlobalObject(JSFunction* constructor);
  
    // Returns a deep copy of the JavaScript object.
    // Properties and elements are copied too.
    // Returns failure if allocation failed.
    static Object* CopyJSObject(JSObject* source);
  
    // Allocates the function prototype.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateFunctionPrototype(JSFunction* function);
  
    // Reinitialize an JSGlobalProxy based on a constructor.  The object
    // must have the same size as objects allocated using the
    // constructor.  The object is reinitialized and behaves as an
    // object that has been freshly allocated using the constructor.
    static Object* ReinitializeJSGlobalProxy(JSFunction* constructor,
                                             JSGlobalProxy* global);
  
    // Allocates and initializes a new JavaScript object based on a map.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateJSObjectFromMap(Map* map,
                                           PretenureFlag pretenure = NOT_TENURED);
  
    // Allocates a heap object based on the map.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this function does not perform a garbage collection.
    static Object* Allocate(Map* map, AllocationSpace space);
  
    // Allocates a JS Map in the heap.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this function does not perform a garbage collection.
    static Object* AllocateMap(InstanceType instance_type, int instance_size);
  
    // Allocates a partial map for bootstrapping.
    static Object* AllocatePartialMap(InstanceType instance_type,
                                      int instance_size);
  
    // Allocate a map for the specified function
    static Object* AllocateInitialMap(JSFunction* fun);
  
    // Allocates and fully initializes a String.  There are two String
    // encodings: ASCII and two byte. One should choose between the three string
    // allocation functions based on the encoding of the string buffer used to
    // initialized the string.
    //   - ...FromAscii initializes the string from a buffer that is ASCII
    //     encoded (it does not check that the buffer is ASCII encoded) and the
    //     result will be ASCII encoded.
    //   - ...FromUTF8 initializes the string from a buffer that is UTF-8
    //     encoded.  If the characters are all single-byte characters, the
    //     result will be ASCII encoded, otherwise it will converted to two
    //     byte.
    //   - ...FromTwoByte initializes the string from a buffer that is two-byte
    //     encoded.  If the characters are all single-byte characters, the
    //     result will be converted to ASCII, otherwise it will be left as
    //     two-byte.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateStringFromAscii(
        Vector<const char> str,
        PretenureFlag pretenure = NOT_TENURED);
    static Object* AllocateStringFromUtf8(
        Vector<const char> str,
        PretenureFlag pretenure = NOT_TENURED);
    static Object* AllocateStringFromTwoByte(
        Vector<const uc16> str,
        PretenureFlag pretenure = NOT_TENURED);
  
    // Allocates a symbol in old space based on the character stream.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this function does not perform a garbage collection.
    static inline Object* AllocateSymbol(Vector<const char> str,
                                         int chars,
                                         uint32_t length_field);
  
    static Object* AllocateInternalSymbol(unibrow::CharacterStream* buffer,
                                          int chars,
                                          uint32_t length_field);
  
    static Object* AllocateExternalSymbol(Vector<const char> str,
                                          int chars);
  
  
    // Allocates and partially initializes a String.  There are two String
    // encodings: ASCII 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.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateRawAsciiString(
        int length,
        PretenureFlag pretenure = NOT_TENURED);
    static Object* AllocateRawTwoByteString(
        int length,
        PretenureFlag pretenure = NOT_TENURED);
  
    // Computes a single character string where the character has code.
    // A cache is used for ascii codes.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed. Please note this does not perform a garbage collection.
    static Object* LookupSingleCharacterStringFromCode(uint16_t code);
  
    // Allocate a byte array of the specified length
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateByteArray(int length, PretenureFlag pretenure);
  
    // Allocate a non-tenured byte array of the specified length
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateByteArray(int length);
  
    // Allocate a pixel array of the specified length
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocatePixelArray(int length,
                                      uint8_t* external_pointer,
                                      PretenureFlag pretenure);
  
    // Allocates an external array of the specified length and type.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateExternalArray(int length,
                                         ExternalArrayType array_type,
                                         void* external_pointer,
                                         PretenureFlag pretenure);
  
    // Allocate a tenured JS global property cell.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateJSGlobalPropertyCell(Object* value);
  
    // Allocates a fixed array initialized with undefined values
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateFixedArray(int length, PretenureFlag pretenure);
    // Allocate uninitialized, non-tenured fixed array with length elements.
    static Object* AllocateFixedArray(int length);
  
    // Make a copy of src and return it. Returns
    // Failure::RetryAfterGC(requested_bytes, space) if the allocation failed.
    static Object* CopyFixedArray(FixedArray* src);
  
    // Allocates a fixed array initialized with the hole values.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateFixedArrayWithHoles(int length);
  
    // AllocateHashTable is identical to AllocateFixedArray except
    // that the resulting object has hash_table_map as map.
    static Object* AllocateHashTable(int length);
  
    // Allocate a global (but otherwise uninitialized) context.
    static Object* AllocateGlobalContext();
  
    // Allocate a function context.
    static Object* AllocateFunctionContext(int length, JSFunction* closure);
  
    // Allocate a 'with' context.
    static Object* AllocateWithContext(Context* previous,
                                       JSObject* extension,
                                       bool is_catch_context);
  
    // Allocates a new utility object in the old generation.
    static Object* AllocateStruct(InstanceType type);
  
    // Allocates a function initialized with a shared part.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateFunction(Map* function_map,
                                    SharedFunctionInfo* shared,
                                    Object* prototype);
  
    // Indicies for direct access into argument objects.
    static const int arguments_callee_index = 0;
    static const int arguments_length_index = 1;
  
    // Allocates an arguments object - optionally with an elements array.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateArgumentsObject(Object* callee, int length);
  
    // Converts a double into either a Smi or a HeapNumber object.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* NewNumberFromDouble(double value,
                                       PretenureFlag pretenure = NOT_TENURED);
  
    // Same as NewNumberFromDouble, but may return a preallocated/immutable
    // number object (e.g., minus_zero_value_, nan_value_)
    static Object* NumberFromDouble(double value,
                                    PretenureFlag pretenure = NOT_TENURED);
  
    // Allocated a HeapNumber from value.
    static Object* AllocateHeapNumber(double value, PretenureFlag pretenure);
    static Object* AllocateHeapNumber(double value);  // pretenure = NOT_TENURED
  
    // Converts an int into either a Smi or a HeapNumber object.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static inline Object* NumberFromInt32(int32_t value);
  
    // Converts an int into either a Smi or a HeapNumber object.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static inline Object* NumberFromUint32(uint32_t value);
  
    // Allocates a new proxy object.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateProxy(Address proxy,
                                 PretenureFlag pretenure = NOT_TENURED);
  
    // Allocates a new SharedFunctionInfo object.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateSharedFunctionInfo(Object* name);
  
    // Allocates a new cons string object.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateConsString(String* first, String* second);
  
    // Allocates a new sliced string object which is a slice of an underlying
    // string buffer stretching from the index start (inclusive) to the index
    // end (exclusive).
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateSlicedString(String* buffer,
                                        int start,
                                        int end);
  
    // Allocates a new sub string object which is a substring of an underlying
    // string buffer stretching from the index start (inclusive) to the index
    // end (exclusive).
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateSubString(String* buffer,
                                     int start,
                                     int end);
  
    // Allocate a new external string object, which is backed by a string
    // resource that resides outside the V8 heap.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this does not perform a garbage collection.
    static Object* AllocateExternalStringFromAscii(
        ExternalAsciiString::Resource* resource);
    static Object* AllocateExternalStringFromTwoByte(
        ExternalTwoByteString::Resource* resource);
  
    // Allocates an uninitialized object.  The memory is non-executable if the
    // hardware and OS allow.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this function does not perform a garbage collection.
    static inline Object* AllocateRaw(int size_in_bytes,
                                      AllocationSpace space,
                                      AllocationSpace retry_space);
  
    // Initialize a filler object to keep the ability to iterate over the heap
    // when shortening objects.
    static void CreateFillerObjectAt(Address addr, int size);
  
    // Makes a new native code object
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed. On success, the pointer to the Code object is stored in the
    // self_reference. This allows generated code to reference its own Code
    // object by containing this pointer.
    // Please note this function does not perform a garbage collection.
    static Object* CreateCode(const CodeDesc& desc,
                              ZoneScopeInfo* sinfo,
                              Code::Flags flags,
                              Handle<Object> self_reference);
  
    static Object* CopyCode(Code* code);
    // Finds the symbol for string in the symbol table.
    // If not found, a new symbol is added to the table and returned.
    // Returns Failure::RetryAfterGC(requested_bytes, space) if allocation
    // failed.
    // Please note this function does not perform a garbage collection.
    static Object* LookupSymbol(Vector<const char> str);
    static Object* LookupAsciiSymbol(const char* str) {
      return LookupSymbol(CStrVector(str));
    }
    static Object* LookupSymbol(String* str);
    static bool LookupSymbolIfExists(String* str, String** symbol);
  
    // Compute the matching symbol map for a string if possible.
    // NULL is returned if string is in new space or not flattened.
    static Map* SymbolMapForString(String* str);
  
    // Converts the given boolean condition to JavaScript boolean value.
    static Object* ToBoolean(bool condition) {
      return condition ? true_value() : false_value();
    }
  
    // Code that should be run before and after each GC.  Includes some
    // reporting/verification activities when compiled with DEBUG set.
    static void GarbageCollectionPrologue();
    static void GarbageCollectionEpilogue();
  
    // Performs garbage collection operation.
    // Returns whether required_space bytes are available after the collection.
    static bool CollectGarbage(int required_space, AllocationSpace space);
  
    // Performs a full garbage collection. Force compaction if the
    // parameter is true.
    static void CollectAllGarbage(bool force_compaction);
  
    // Performs a full garbage collection if a context has been disposed
    // since the last time the check was performed.
    static void CollectAllGarbageIfContextDisposed();
  
    // Notify the heap that a context has been disposed.
    static void NotifyContextDisposed();
  
    // Utility to invoke the scavenger. This is needed in test code to
    // ensure correct callback for weak global handles.
    static void PerformScavenge();
  
  #ifdef DEBUG
    // Utility used with flag gc-greedy.
    static bool GarbageCollectionGreedyCheck();
  #endif
  
    static void SetGlobalGCPrologueCallback(GCCallback callback) {
      global_gc_prologue_callback_ = callback;
    }
    static void SetGlobalGCEpilogueCallback(GCCallback callback) {
      global_gc_epilogue_callback_ = 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.
  #define ROOT_ACCESSOR(type, name, camel_name)                                  \
    static inline type* name() {                                                 \
      return type::cast(roots_[k##camel_name##RootIndex]);                       \
    }                                                                            \
    static inline 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)                                  \
      static inline Map* name##_map() {                                          \
        return Map::cast(roots_[k##Name##MapRootIndex]);                         \
      }
    STRUCT_LIST(STRUCT_MAP_ACCESSOR)
  #undef STRUCT_MAP_ACCESSOR
  
  #define SYMBOL_ACCESSOR(name, str) static inline String* name() {              \
      return String::cast(roots_[k##name##RootIndex]);                           \
    }
    SYMBOL_LIST(SYMBOL_ACCESSOR)
  #undef SYMBOL_ACCESSOR
  
    // The hidden_symbol is special because it is the empty string, but does
    // not match the empty string.
    static String* hidden_symbol() { return hidden_symbol_; }
  
    // Iterates over all roots in the heap.
    static void IterateRoots(ObjectVisitor* v, VisitMode mode);
    // Iterates over all strong roots in the heap.
    static void IterateStrongRoots(ObjectVisitor* v, VisitMode mode);
  
    // Iterates remembered set of an old space.
    static void IterateRSet(PagedSpace* space, ObjectSlotCallback callback);
  
    // Iterates a range of remembered set addresses starting with rset_start
    // corresponding to the range of allocated pointers
    // [object_start, object_end).
    // Returns the number of bits that were set.
    static int IterateRSetRange(Address object_start,
                                Address object_end,
                                Address rset_start,
                                ObjectSlotCallback copy_object_func);
  
    // Returns whether the object resides in new space.
    static inline bool InNewSpace(Object* object);
    static inline bool InFromSpace(Object* object);
    static inline bool InToSpace(Object* object);
  
    // Checks whether an address/object in the heap (including auxiliary
    // area and unused area).
    static bool Contains(Address addr);
    static bool Contains(HeapObject* value);
  
    // Checks whether an address/object in a space.
    // Currently used by tests, serialization and heap verification only.
    static bool InSpace(Address addr, AllocationSpace space);
    static bool InSpace(HeapObject* value, AllocationSpace space);
  
    // Finds out which space an object should get promoted to based on its type.
    static inline OldSpace* TargetSpace(HeapObject* object);
    static inline AllocationSpace TargetSpaceId(InstanceType type);
  
    // Sets the stub_cache_ (only used when expanding the dictionary).
    static void public_set_code_stubs(NumberDictionary* value) {
      roots_[kCodeStubsRootIndex] = value;
    }
  
    // Sets the non_monomorphic_cache_ (only used when expanding the dictionary).
    static void public_set_non_monomorphic_cache(NumberDictionary* value) {
      roots_[kNonMonomorphicCacheRootIndex] = value;
    }
  
    // Update the next script id.
    static inline void SetLastScriptId(Object* last_script_id);
  
    // Generated code can embed this address to get access to the roots.
    static Object** roots_address() { return roots_; }
  
  #ifdef DEBUG
    static void Print();
    static void PrintHandles();
  
    // Verify the heap is in its normal state before or after a GC.
    static void Verify();
  
    // Report heap statistics.
    static void ReportHeapStatistics(const char* title);
    static void ReportCodeStatistics(const char* title);
  
    // Fill in bogus values in from space
    static void ZapFromSpace();
  #endif
  
  #if defined(ENABLE_LOGGING_AND_PROFILING)
    // Print short heap statistics.
    static void PrintShortHeapStatistics();
  #endif
  
    // Makes a new symbol object
    // Returns Failure::RetryAfterGC(requested_bytes, space) if the allocation
    // failed.
    // Please note this function does not perform a garbage collection.
    static Object* CreateSymbol(const char* str, int length, int hash);
    static Object* CreateSymbol(String* str);
  
    // Write barrier support for address[offset] = o.
    static inline void RecordWrite(Address address, int offset);
  
    // Given an address occupied by a live code object, return that object.
    static Object* FindCodeObject(Address a);
  
    // Invoke Shrink on shrinkable spaces.
    static void Shrink();
  
    enum HeapState { NOT_IN_GC, SCAVENGE, MARK_COMPACT };
    static inline HeapState gc_state() { return gc_state_; }
  
  #ifdef DEBUG
    static bool IsAllocationAllowed() { return allocation_allowed_; }
    static inline bool allow_allocation(bool enable);
  
    static bool disallow_allocation_failure() {
      return disallow_allocation_failure_;
    }
  
    static void TracePathToObject();
    static 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 void ScavengePointer(HeapObject** p);
    static inline void ScavengeObject(HeapObject** p, HeapObject* object);
  
    // Clear a range of remembered set addresses corresponding to the object
    // area address 'start' with size 'size_in_bytes', eg, when adding blocks
    // to the free list.
    static void ClearRSetRange(Address start, int size_in_bytes);
  
    // Rebuild remembered set in old and map spaces.
    static void RebuildRSets();
  
    // Commits from space if it is uncommitted.
    static void EnsureFromSpaceIsCommitted();
  
    //
    // Support for the API.
    //
  
    static bool CreateApiObjects();
  
    // Attempt to find the number in a small cache.  If we finds it, return
    // the string representation of the number.  Otherwise return undefined.
    static Object* GetNumberStringCache(Object* number);
  
    // Update the cache with a new number-string pair.
    static void SetNumberStringCache(Object* number, String* str);
  
    // Entries in the cache.  Must be a power of 2.
    static const int kNumberStringCacheSize = 64;
  
    // Adjusts the amount of registered external memory.
    // Returns the adjusted value.
    static inline int AdjustAmountOfExternalAllocatedMemory(int change_in_bytes);
  
    // Allocate unitialized fixed array (pretenure == NON_TENURE).
    static Object* AllocateRawFixedArray(int length);
  
    // True if we have reached the allocation limit in the old generation that
    // should force the next GC (caused normally) to be a full one.
    static bool OldGenerationPromotionLimitReached() {
      return (PromotedSpaceSize() + PromotedExternalMemorySize())
             > old_gen_promotion_limit_;
    }
  
    // True if we have reached the allocation limit in the old generation that
    // should artificially cause a GC right now.
    static bool OldGenerationAllocationLimitReached() {
      return (PromotedSpaceSize() + PromotedExternalMemorySize())
             > old_gen_allocation_limit_;
    }
  
    // Can be called when the embedding application is idle.
    static bool IdleNotification();
  
    // Declare all the root indices.
    enum RootListIndex {
  #define ROOT_INDEX_DECLARATION(type, name, camel_name) k##camel_name##RootIndex,
      STRONG_ROOT_LIST(ROOT_INDEX_DECLARATION)
  #undef ROOT_INDEX_DECLARATION
  
  // Utility type maps
  #define DECLARE_STRUCT_MAP(NAME, Name, name) k##Name##MapRootIndex,
    STRUCT_LIST(DECLARE_STRUCT_MAP)
  #undef DECLARE_STRUCT_MAP
  
  #define SYMBOL_INDEX_DECLARATION(name, str) k##name##RootIndex,
      SYMBOL_LIST(SYMBOL_INDEX_DECLARATION)
  #undef SYMBOL_DECLARATION
  
      kSymbolTableRootIndex,
      kStrongRootListLength = kSymbolTableRootIndex,
      kRootListLength
    };
  
    static Object* NumberToString(Object* number);
  
    static Map* MapForExternalArrayType(ExternalArrayType array_type);
    static RootListIndex RootIndexForExternalArrayType(
        ExternalArrayType array_type);
  
   private:
    static int reserved_semispace_size_;
    static int max_semispace_size_;
    static int initial_semispace_size_;
    static int max_old_generation_size_;
    static size_t code_range_size_;
  
    // For keeping track of how much data has survived
    // scavenge since last new space expansion.
    static int survived_since_last_expansion_;
  
    static int always_allocate_scope_depth_;
    static int linear_allocation_scope_depth_;
    static bool context_disposed_pending_;
  
    static const int kMaxMapSpaceSize = 8*MB;
  
  #if defined(V8_TARGET_ARCH_X64)
    static const int kMaxObjectSizeInNewSpace = 512*KB;
  #else
    static const int kMaxObjectSizeInNewSpace = 256*KB;
  #endif
  
    static NewSpace new_space_;
    static OldSpace* old_pointer_space_;
    static OldSpace* old_data_space_;
    static OldSpace* code_space_;
    static MapSpace* map_space_;
    static CellSpace* cell_space_;
    static LargeObjectSpace* lo_space_;
    static HeapState gc_state_;
  
    // Returns the size of object residing in non new spaces.
    static int PromotedSpaceSize();
  
    // Returns the amount of external memory registered since last global gc.
    static int PromotedExternalMemorySize();
  
    static int mc_count_;  // how many mark-compact collections happened
    static int gc_count_;  // how many gc happened
  
  #define ROOT_ACCESSOR(type, name, camel_name)                                  \
    static inline void set_##name(type* value) {                                 \
      roots_[k##camel_name##RootIndex] = value;                                  \
    }
    ROOT_LIST(ROOT_ACCESSOR)
  #undef ROOT_ACCESSOR
  
  #ifdef DEBUG
    static bool allocation_allowed_;
  
    // 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.
    static int allocation_timeout_;
  
    // Do we expect to be able to handle allocation failure at this
    // time?
    static bool disallow_allocation_failure_;
  #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.
    static int old_gen_promotion_limit_;
  
    // Limit that triggers a global GC as soon as is reasonable.  This is
    // checked before expanding a paged space in the old generation and on
    // every allocation in large object space.
    static int old_gen_allocation_limit_;
  
    // Limit on the amount of externally allocated memory allowed
    // between global GCs. If reached a global GC is forced.
    static int external_allocation_limit_;
  
    // The amount of external memory registered through the API kept alive
    // by global handles
    static int amount_of_external_allocated_memory_;
  
    // Caches the amount of external memory registered at the last global gc.
    static int amount_of_external_allocated_memory_at_last_global_gc_;
  
    // Indicates that an allocation has failed in the old generation since the
    // last GC.
    static int old_gen_exhausted_;
  
    static Object* roots_[kRootListLength];
  
    struct StringTypeTable {
      InstanceType type;
      int size;
      RootListIndex index;
    };
  
    struct ConstantSymbolTable {
      const char* contents;
      RootListIndex index;
    };
  
    struct StructTable {
      InstanceType type;
      int size;
      RootListIndex index;
    };
  
    static const StringTypeTable string_type_table[];
    static const ConstantSymbolTable constant_symbol_table[];
    static const StructTable struct_table[];
  
    // The special hidden symbol which is an empty string, but does not match
    // any string when looked up in properties.
    static String* hidden_symbol_;
  
    // GC callback function, called before and after mark-compact GC.
    // Allocations in the callback function are disallowed.
    static GCCallback global_gc_prologue_callback_;
    static GCCallback global_gc_epilogue_callback_;
  
    // Checks whether a global GC is necessary
    static GarbageCollector SelectGarbageCollector(AllocationSpace space);
  
    // Performs garbage collection
    static void PerformGarbageCollection(AllocationSpace space,
                                         GarbageCollector collector,
                                         GCTracer* tracer);
  
    // Returns either a Smi or a Number object from 'value'. If 'new_object'
    // is false, it may return a preallocated immutable object.
    static Object* SmiOrNumberFromDouble(double value,
                                         bool new_object,
                                         PretenureFlag pretenure = NOT_TENURED);
  
    // Allocate an uninitialized object in map space.  The behavior is identical
    // to Heap::AllocateRaw(size_in_bytes, MAP_SPACE), except that (a) it doesn't
    // have to test the allocation space argument and (b) can reduce code size
    // (since both AllocateRaw and AllocateRawMap are inlined).
    static inline Object* AllocateRawMap();
  
    // Allocate an uninitialized object in the global property cell space.
    static inline Object* AllocateRawCell();
  
    // Initializes a JSObject based on its map.
    static void InitializeJSObjectFromMap(JSObject* obj,
                                          FixedArray* properties,
                                          Map* map);
  
    static bool CreateInitialMaps();
    static bool CreateInitialObjects();
  
    // These four Create*EntryStub functions are here because of a gcc-4.4 bug
    // that assigns wrong vtable entries.
    static void CreateCEntryStub();
    static void CreateCEntryDebugBreakStub();
    static void CreateJSEntryStub();
    static void CreateJSConstructEntryStub();
    static void CreateRegExpCEntryStub();
  
    static void CreateFixedStubs();
  
    static Object* CreateOddball(Map* map,
                                 const char* to_string,
                                 Object* to_number);
  
    // Allocate empty fixed array.
    static Object* AllocateEmptyFixedArray();
  
    // Performs a minor collection in new generation.
    static void Scavenge();
  
    // Performs a major collection in the whole heap.
    static void MarkCompact(GCTracer* tracer);
  
    // Code to be run before and after mark-compact.
    static void MarkCompactPrologue(bool is_compacting);
    static void MarkCompactEpilogue(bool is_compacting);
  
    // Helper function used by CopyObject to copy a source object to an
    // allocated target object and update the forwarding pointer in the source
    // object.  Returns the target object.
    static HeapObject* MigrateObject(HeapObject* source,
                                     HeapObject* target,
                                     int size);
  
    // Helper function that governs the promotion policy from new space to
    // old.  If the object's old address lies below the new space's age
    // mark or if we've already filled the bottom 1/16th of the to space,
    // we try to promote this object.
    static inline bool ShouldBePromoted(Address old_address, int object_size);
  #if defined(DEBUG) || defined(ENABLE_LOGGING_AND_PROFILING)
    // Record the copy of an object in the NewSpace's statistics.
    static void RecordCopiedObject(HeapObject* obj);
  
    // Record statistics before and after garbage collection.
    static void ReportStatisticsBeforeGC();
    static void ReportStatisticsAfterGC();
  #endif
  
    // Update an old object's remembered set
    static int UpdateRSet(HeapObject* obj);
  
    // Rebuild remembered set in an old space.
    static void RebuildRSets(PagedSpace* space);
  
    // Rebuild remembered set in the large object space.
    static void RebuildRSets(LargeObjectSpace* space);
  
    // Slow part of scavenge object.
    static void ScavengeObjectSlow(HeapObject** p, HeapObject* object);
  
    // Copy memory from src to dst.
    static inline void CopyBlock(Object** dst, Object** src, int byte_size);
  
    // Initializes a function with a shared part and prototype.
    // Returns the function.
    // Note: this code was factored out of AllocateFunction such that
    // other parts of the VM could use it. Specifically, a function that creates
    // instances of type JS_FUNCTION_TYPE benefit from the use of this function.
    // Please note this does not perform a garbage collection.
    static inline Object* InitializeFunction(JSFunction* function,
                                             SharedFunctionInfo* shared,
                                             Object* prototype);
  
    static const int kInitialSymbolTableSize = 2048;
    static const int kInitialEvalCacheSize = 64;
  
    friend class Factory;
    friend class DisallowAllocationFailure;
    friend class AlwaysAllocateScope;
    friend class LinearAllocationScope;
  };
  
  
  class AlwaysAllocateScope {
   public:
    AlwaysAllocateScope() {
      // We shouldn't hit any nested scopes, because that requires
      // non-handle code to call handle code. The code still works but
      // performance will degrade, so we want to catch this situation
      // in debug mode.
      ASSERT(Heap::always_allocate_scope_depth_ == 0);
      Heap::always_allocate_scope_depth_++;
    }
  
    ~AlwaysAllocateScope() {
      Heap::always_allocate_scope_depth_--;
      ASSERT(Heap::always_allocate_scope_depth_ == 0);
    }
  };
  
  
  class LinearAllocationScope {
   public:
    LinearAllocationScope() {
      Heap::linear_allocation_scope_depth_++;
    }
  
    ~LinearAllocationScope() {
      Heap::linear_allocation_scope_depth_--;
      ASSERT(Heap::linear_allocation_scope_depth_ >= 0);
    }
  };
  
  
  #ifdef DEBUG
  // Visitor class to verify interior pointers that do not have remembered set
  // bits.  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:
    void VisitPointers(Object** start, Object** end) {
      for (Object** current = start; current < end; current++) {
        if ((*current)->IsHeapObject()) {
          HeapObject* object = HeapObject::cast(*current);
          ASSERT(Heap::Contains(object));
          ASSERT(object->map()->IsMap());
        }
      }
    }
  };
  
  
  // Visitor class to verify interior pointers that have remembered set bits.
  // As VerifyPointersVisitor but also checks that remembered set bits are
  // always set for pointers into new space.
  class VerifyPointersAndRSetVisitor: public ObjectVisitor {
   public:
    void VisitPointers(Object** start, Object** end) {
      for (Object** current = start; current < end; current++) {
        if ((*current)->IsHeapObject()) {
          HeapObject* object = HeapObject::cast(*current);
          ASSERT(Heap::Contains(object));
          ASSERT(object->map()->IsMap());
          if (Heap::InNewSpace(object)) {
            ASSERT(Page::IsRSetSet(reinterpret_cast<Address>(current), 0));
          }
        }
      }
    }
  };
  #endif
  
  
  // 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:
    Space* next();
    AllSpaces() { counter_ = FIRST_SPACE; }
   private:
    int counter_;
  };
  
  
  // Space iterator for iterating over all old spaces of the heap: Old pointer
  // space, old data space and code space.
  // Returns each space in turn, and null when it is done.
  class OldSpaces BASE_EMBEDDED {
   public:
    OldSpace* next();
    OldSpaces() { counter_ = OLD_POINTER_SPACE; }
   private:
    int counter_;
  };
  
  
  // Space iterator for iterating over all the paged spaces of the heap:
  // Map space, old pointer space, old data space and code space.
  // Returns each space in turn, and null when it is done.
  class PagedSpaces BASE_EMBEDDED {
   public:
    PagedSpace* next();
    PagedSpaces() { counter_ = OLD_POINTER_SPACE; }
   private:
    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:
    SpaceIterator();
    virtual ~SpaceIterator();
  
    bool has_next();
    ObjectIterator* next();
  
   private:
    ObjectIterator* CreateIterator();
  
    int current_space_;  // from enum AllocationSpace.
    ObjectIterator* iterator_;  // object iterator for the current space.
  };
  
  
  // A HeapIterator provides iteration over the whole heap It aggregates a the
  // specific iterators for the different spaces as these can only iterate over
  // one space only.
  
  class HeapIterator BASE_EMBEDDED {
   public:
    explicit HeapIterator();
    virtual ~HeapIterator();
  
    bool has_next();
    HeapObject* next();
    void reset();
  
   private:
    // Perform the initialization.
    void Init();
  
    // Perform all necessary shutdown (destruction) work.
    void Shutdown();
  
    // 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.
    static int Lookup(Map* map, String* name);
  
    // Update an element in the cache.
    static void Update(Map* map, String* name, int field_offset);
  
    // Clear the cache.
    static void Clear();
   private:
    static inline int Hash(Map* map, String* name);
    static const int kLength = 64;
    struct Key {
      Map* map;
      String* name;
    };
    static Key keys_[kLength];
    static int field_offsets_[kLength];
  };
  
  
  
  // Cache for mapping (array, 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.
    static int Lookup(DescriptorArray* array, String* name) {
      if (!StringShape(name).IsSymbol()) return kAbsent;
      int index = Hash(array, name);
      Key& key = keys_[index];
      if ((key.array == array) && (key.name == name)) return results_[index];
      return kAbsent;
    }
  
    // Update an element in the cache.
    static void Update(DescriptorArray* array, String* name, int result) {
      ASSERT(result != kAbsent);
      if (StringShape(name).IsSymbol()) {
        int index = Hash(array, name);
        Key& key = keys_[index];
        key.array = array;
        key.name = name;
        results_[index] = result;
      }
    }
  
    // Clear the cache.
    static void Clear();
  
    static const int kAbsent = -2;
   private:
    static int Hash(DescriptorArray* array, String* name) {
      // Uses only lower 32 bits if pointers are larger.
      uintptr_t array_hash =
          static_cast<uint32_t>(reinterpret_cast<uintptr_t>(array)) >> 2;
      uintptr_t name_hash =
          static_cast<uint32_t>(reinterpret_cast<uintptr_t>(name)) >> 2;
      return (array_hash ^ name_hash) % kLength;
    }
  
    static const int kLength = 64;
    struct Key {
      DescriptorArray* array;
      String* name;
    };
  
    static Key keys_[kLength];
    static int results_[kLength];
  };
  
  
  // ----------------------------------------------------------------------------
  // Marking stack for tracing live objects.
  
  class MarkingStack {
   public:
    void Initialize(Address low, Address high) {
      top_ = low_ = reinterpret_cast<HeapObject**>(low);
      high_ = reinterpret_cast<HeapObject**>(high);
      overflowed_ = false;
    }
  
    bool is_full() { return top_ >= high_; }
  
    bool is_empty() { return top_ <= low_; }
  
    bool overflowed() { return overflowed_; }
  
    void clear_overflowed() { overflowed_ = false; }
  
    // Push the (marked) object on the marking stack if there is room,
    // otherwise mark the object as overflowed and wait for a rescan of the
    // heap.
    void Push(HeapObject* object) {
      CHECK(object->IsHeapObject());
      if (is_full()) {
        object->SetOverflow();
        overflowed_ = true;
      } else {
        *(top_++) = object;
      }
    }
  
    HeapObject* Pop() {
      ASSERT(!is_empty());
      HeapObject* object = *(--top_);
      CHECK(object->IsHeapObject());
      return object;
    }
  
   private:
    HeapObject** low_;
    HeapObject** top_;
    HeapObject** high_;
    bool overflowed_;
  };
  
  
  // A helper class to document/test C++ scopes where we do not
  // expect a GC. Usage:
  //
  // /* Allocation not allowed: we cannot handle a GC in this scope. */
  // { AssertNoAllocation nogc;
  //   ...
  // }
  
  #ifdef DEBUG
  
  class DisallowAllocationFailure {
   public:
    DisallowAllocationFailure() {
      old_state_ = Heap::disallow_allocation_failure_;
      Heap::disallow_allocation_failure_ = true;
    }
    ~DisallowAllocationFailure() {
      Heap::disallow_allocation_failure_ = old_state_;
    }
   private:
    bool old_state_;
  };
  
  class AssertNoAllocation {
   public:
    AssertNoAllocation() {
      old_state_ = Heap::allow_allocation(false);
    }
  
    ~AssertNoAllocation() {
      Heap::allow_allocation(old_state_);
    }
  
   private:
    bool old_state_;
  };
  
  class DisableAssertNoAllocation {
   public:
    DisableAssertNoAllocation() {
      old_state_ = Heap::allow_allocation(true);
    }
  
    ~DisableAssertNoAllocation() {
      Heap::allow_allocation(old_state_);
    }
  
   private:
    bool old_state_;
  };
  
  #else  // ndef DEBUG
  
  class AssertNoAllocation {
   public:
    AssertNoAllocation() { }
    ~AssertNoAllocation() { }
  };
  
  class DisableAssertNoAllocation {
   public:
    DisableAssertNoAllocation() { }
    ~DisableAssertNoAllocation() { }
  };
  
  #endif
  
  // GCTracer collects and prints ONE line after each garbage collector
  // invocation IFF --trace_gc is used.
  
  class GCTracer BASE_EMBEDDED {
   public:
    GCTracer();
  
    ~GCTracer();
  
    // Sets the collector.
    void set_collector(GarbageCollector collector) { collector_ = collector; }
  
    // Sets the GC count.
    void set_gc_count(int count) { gc_count_ = count; }
  
    // Sets the full GC count.
    void set_full_gc_count(int count) { full_gc_count_ = count; }
  
    // Sets the flag that this is a compacting full GC.
    void set_is_compacting() { is_compacting_ = true; }
  
    // Increment and decrement the count of marked objects.
    void increment_marked_count() { ++marked_count_; }
    void decrement_marked_count() { --marked_count_; }
  
    int marked_count() { return marked_count_; }
  
   private:
    // Returns a string matching the collector.
    const char* CollectorString();
  
    // Returns size of object in heap (in MB).
    double SizeOfHeapObjects() {
      return (static_cast<double>(Heap::SizeOfObjects())) / MB;
    }
  
    double start_time_;  // Timestamp set in the constructor.
    double start_size_;  // Size of objects in heap set in constructor.
    GarbageCollector collector_;  // Type of collector.
  
    // A count (including this one, eg, the first collection is 1) of the
    // number of garbage collections.
    int gc_count_;
  
    // A count (including this one) of the number of full garbage collections.
    int full_gc_count_;
  
    // True if the current GC is a compacting full collection, false
    // otherwise.
    bool is_compacting_;
  
    // True if the *previous* full GC cwas a compacting collection (will be
    // false if there has not been a previous full GC).
    bool previous_has_compacted_;
  
    // On a full GC, a count of the number of marked objects.  Incremented
    // when an object is marked and decremented when an object's mark bit is
    // cleared.  Will be zero on a scavenge collection.
    int marked_count_;
  
    // The count from the end of the previous full GC.  Will be zero if there
    // was no previous full GC.
    int previous_marked_count_;
  };
  
  
  class TranscendentalCache {
   public:
    enum Type {ACOS, ASIN, ATAN, COS, EXP, LOG, SIN, TAN, kNumberOfCaches};
  
    explicit TranscendentalCache(Type t);
  
    // Returns a heap number with f(input), where f is a math function specified
    // by the 'type' argument.
    static inline Object* Get(Type type, double input) {
      TranscendentalCache* cache = caches_[type];
      if (cache == NULL) {
        caches_[type] = cache = new TranscendentalCache(type);
      }
      return cache->Get(input);
    }
  
    // The cache contains raw Object pointers.  This method disposes of
    // them before a garbage collection.
    static void Clear();
  
   private:
    inline Object* Get(double input) {
      Converter c;
      c.dbl = input;
      int hash = Hash(c);
      Element e = elements_[hash];
      if (e.in[0] == c.integers[0] &&
          e.in[1] == c.integers[1]) {
        ASSERT(e.output != NULL);
        return e.output;
      }
      double answer = Calculate(input);
      Object* heap_number = Heap::AllocateHeapNumber(answer);
      if (!heap_number->IsFailure()) {
        elements_[hash].in[0] = c.integers[0];
        elements_[hash].in[1] = c.integers[1];
        elements_[hash].output = heap_number;
      }
      return heap_number;
    }
  
    inline double Calculate(double input) {
      switch (type_) {
        case ACOS:
          return acos(input);
        case ASIN:
          return asin(input);
        case ATAN:
          return atan(input);
        case COS:
          return cos(input);
        case EXP:
          return exp(input);
        case LOG:
          return log(input);
        case SIN:
          return sin(input);
        case TAN:
          return tan(input);
        default:
          return 0.0;  // Never happens.
      }
    }
    static const int kCacheSize = 512;
    struct Element {
      uint32_t in[2];
      Object* output;
    };
    union Converter {
      double dbl;
      uint32_t integers[2];
    };
    inline static int Hash(const Converter& c) {
      uint32_t hash = (c.integers[0] ^ c.integers[1]);
      hash ^= hash >> 16;
      hash ^= hash >> 8;
      return (hash & (kCacheSize - 1));
    }
    static TranscendentalCache* caches_[kNumberOfCaches];
    Element elements_[kCacheSize];
    Type type_;
  };
  
  
  } }  // namespace v8::internal
  
  #endif  // V8_HEAP_H_
  