v8的Heap中RootObject的初始化

Heap類中有一個Object* root_[]，這個數組是在Heap::CreateHeapObjects方法中初始化的，root_數組非常重要，其中包含了各種類的Map對象，和其他非常重要的全局對象。對於root_數組的存取，Heap一以貫之的使用了宏定義的方式

1.root_數組的索引

  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

    // Utility type maps
#define DECLARE_STRUCT_MAP(NAME, Name, name) k##Name##MapRootIndex,
    STRUCT_LIST(DECLARE_STRUCT_MAP)
#undef DECLARE_STRUCT_MAP

    kStringTableRootIndex,
    kStrongRootListLength = kStringTableRootIndex,
    kRootListLength
  };

這裏定義了設置獲取root_數組所需要的index，它是一個枚舉類型

2.root_數組中對象的獲取

從上面index的定義，我們可以看到Index被分爲三種，分別由STRONG_ROOT_LIST，INTERNALIZED_STRING_LIST和STRUCT_LIST定義的。

我們看如下的宏定義：

#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)

#define STRUCT_MAP_ACCESSOR(NAME, Name, name)                                  \
    Map* name##_map() {                                                        \
      return Map::cast(roots_[k##Name##MapRootIndex]);                         \
    }

STRUCT_LIST(STRUCT_MAP_ACCESSOR)

#define STRING_ACCESSOR(name, str) String* name() {                            \
    return String::cast(roots_[k##name##RootIndex]);                           \
  }

INTERNALIZED_STRING_LIST(STRING_ACCESSOR)
這三個宏定義了getter函數，用於根據類型和名稱獲取root中相應的對象，分別對應上面的三種不同類型的index。

3.root_數組中對象的設置

#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.    */   \
    ASSERT(k##camel_name##RootIndex >= kOldSpaceRoots || !InNewSpace(value));  \
    roots_[k##camel_name##RootIndex] = value;                                  \
  }
  ROOT_LIST(ROOT_ACCESSOR)

這個宏顯然定義了setter函數，用於設置root_數組中的一些索引的內容。奇怪的是，這裏少了上面的兩個宏STRUCT_LIST，INTERNALIZED_STRING_LIST，這說明，由這兩個宏定義index的root_數組對象是不可寫的，相應的，由ROOT_LIST定義index的root_數組對象是可寫的。

4.列表宏

這三個宏的定義如下：

heap.h中定義如下兩個宏

#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(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, 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(Object, no_interceptor_result_sentinel, NoInterceptorResultSentinel)       \
  V(Map, hash_table_map, HashTableMap)                                         \
  V(FixedArray, empty_fixed_array, EmptyFixedArray)                            \
  V(ByteArray, empty_byte_array, EmptyByteArray)                               \
  V(DescriptorArray, empty_descriptor_array, EmptyDescriptorArray)             \
  V(Smi, stack_limit, StackLimit)                                              \
  V(Oddball, arguments_marker, ArgumentsMarker)                                \
  /* The first 32 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(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(Object, termination_exception, TerminationException)                       \
  V(Smi, hash_seed, HashSeed)                                                  \
  V(Map, symbol_map, SymbolMap)                                                \
  V(Map, string_map, StringMap)                                                \
  V(Map, ascii_string_map, AsciiStringMap)                                     \
  V(Map, cons_string_map, ConsStringMap)                                       \
  V(Map, cons_ascii_string_map, ConsAsciiStringMap)                            \
  V(Map, sliced_string_map, SlicedStringMap)                                   \
  V(Map, sliced_ascii_string_map, SlicedAsciiStringMap)                        \
  V(Map, external_string_map, ExternalStringMap)                               \
  V(Map, external_string_with_ascii_data_map, ExternalStringWithAsciiDataMap)  \
  V(Map, external_ascii_string_map, ExternalAsciiStringMap)                    \
  V(Map, short_external_string_map, ShortExternalStringMap)                    \
  V(Map,                                                                       \
    short_external_string_with_ascii_data_map,                                 \
    ShortExternalStringWithAsciiDataMap)                                       \
  V(Map, internalized_string_map, InternalizedStringMap)                       \
  V(Map, ascii_internalized_string_map, AsciiInternalizedStringMap)            \
  V(Map, cons_internalized_string_map, ConsInternalizedStringMap)              \
  V(Map, cons_ascii_internalized_string_map, ConsAsciiInternalizedStringMap)   \
  V(Map,                                                                       \
    external_internalized_string_map,                                          \
    ExternalInternalizedStringMap)                                             \
  V(Map,                                                                       \
    external_internalized_string_with_ascii_data_map,                          \
    ExternalInternalizedStringWithAsciiDataMap)                                \
  V(Map,                                                                       \
    external_ascii_internalized_string_map,                                    \
    ExternalAsciiInternalizedStringMap)                                        \
  V(Map,                                                                       \
    short_external_internalized_string_map,                                    \
    ShortExternalInternalizedStringMap)                                        \
  V(Map,                                                                       \
    short_external_internalized_string_with_ascii_data_map,                    \
    ShortExternalInternalizedStringWithAsciiDataMap)                           \
  V(Map,                                                                       \
    short_external_ascii_internalized_string_map,                              \
    ShortExternalAsciiInternalizedStringMap)                                   \
  V(Map, short_external_ascii_string_map, ShortExternalAsciiStringMap)         \
  V(Map, undetectable_string_map, UndetectableStringMap)                       \
  V(Map, undetectable_ascii_string_map, UndetectableAsciiStringMap)            \
  V(Map, external_pixel_array_map, ExternalPixelArrayMap)                      \
  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, external_double_array_map, ExternalDoubleArrayMap)                    \
  V(Map, non_strict_arguments_elements_map, NonStrictArgumentsElementsMap)     \
  V(Map, function_context_map, FunctionContextMap)                             \
  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, global_context_map, GlobalContextMap)                                 \
  V(Map, oddball_map, OddballMap)                                              \
  V(Map, message_object_map, JSMessageObjectMap)                               \
  V(Map, foreign_map, ForeignMap)                                              \
  V(HeapNumber, nan_value, NanValue)                                           \
  V(HeapNumber, infinity_value, InfinityValue)                                 \
  V(HeapNumber, minus_zero_value, MinusZeroValue)                              \
  V(Map, neander_map, NeanderMap)                                              \
  V(JSObject, message_listeners, MessageListeners)                             \
  V(Foreign, prototype_accessors, PrototypeAccessors)                          \
  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(Object, last_script_id, LastScriptId)                                      \
  V(Script, empty_script, EmptyScript)                                         \
  V(Smi, real_stack_limit, RealStackLimit)                                     \
  V(NameDictionary, intrinsic_function_names, IntrinsicFunctionNames)        \
  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)                 \
  V(JSObject, observation_state, ObservationState)                             \
  V(Map, external_map, ExternalMap)

#define ROOT_LIST(V)                                  \
  STRONG_ROOT_LIST(V)                                 \
  V(StringTable, string_table, StringTable)

#define INTERNALIZED_STRING_LIST(V)                                      \
  V(Array_string, "Array")                                               \
  V(Object_string, "Object")                                             \
  V(proto_string, "__proto__")                                           \
  V(StringImpl_string, "StringImpl")                                     \
  V(arguments_string, "arguments")                                       \
  V(Arguments_string, "Arguments")                                       \
  V(call_string, "call")                                                 \
  V(apply_string, "apply")                                               \
  V(caller_string, "caller")                                             \
  V(boolean_string, "boolean")                                           \
  V(Boolean_string, "Boolean")                                           \
  V(callee_string, "callee")                                             \
  V(constructor_string, "constructor")                                   \
  V(code_string, ".code")                                                \
  V(result_string, ".result")                                            \
  V(dot_for_string, ".for.")                                             \
  V(catch_var_string, ".catch-var")                                      \
  V(empty_string, "")                                                    \
  V(eval_string, "eval")                                                 \
  V(function_string, "function")                                         \
  V(length_string, "length")                                             \
  V(module_string, "module")                                             \
  V(name_string, "name")                                                 \
  V(native_string, "native")                                             \
  V(null_string, "null")                                                 \
  V(number_string, "number")                                             \
  V(Number_string, "Number")                                             \
  V(nan_string, "NaN")                                                   \
  V(RegExp_string, "RegExp")                                             \
  V(source_string, "source")                                             \
  V(global_string, "global")                                             \
  V(ignore_case_string, "ignoreCase")                                    \
  V(multiline_string, "multiline")                                       \
  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(Date_string, "Date")                                                 \
  V(this_string, "this")                                                 \
  V(to_string_string, "toString")                                        \
  V(char_at_string, "CharAt")                                            \
  V(undefined_string, "undefined")                                       \
  V(value_of_string, "valueOf")                                          \
  V(stack_string, "stack")                                               \
  V(InitializeVarGlobal_string, "InitializeVarGlobal")                   \
  V(InitializeConstGlobal_string, "InitializeConstGlobal")               \
  V(KeyedLoadElementMonomorphic_string,                                  \
    "KeyedLoadElementMonomorphic")                                       \
  V(KeyedStoreElementMonomorphic_string,                                 \
    "KeyedStoreElementMonomorphic")                                      \
  V(stack_overflow_string, "kStackOverflowBoilerplate")                  \
  V(illegal_access_string, "illegal access")                             \
  V(out_of_memory_string, "out-of-memory")                               \
  V(illegal_execution_state_string, "illegal execution state")           \
  V(get_string, "get")                                                   \
  V(set_string, "set")                                                   \
  V(map_field_string, "%map")                                            \
  V(elements_field_string, "%elements")                                  \
  V(length_field_string, "%length")                                      \
  V(function_class_string, "Function")                                   \
  V(illegal_argument_string, "illegal argument")                         \
  V(MakeReferenceError_string, "MakeReferenceError")                     \
  V(MakeSyntaxError_string, "MakeSyntaxError")                           \
  V(MakeTypeError_string, "MakeTypeError")                               \
  V(invalid_lhs_in_assignment_string, "invalid_lhs_in_assignment")       \
  V(invalid_lhs_in_for_in_string, "invalid_lhs_in_for_in")               \
  V(invalid_lhs_in_postfix_op_string, "invalid_lhs_in_postfix_op")       \
  V(invalid_lhs_in_prefix_op_string, "invalid_lhs_in_prefix_op")         \
  V(illegal_return_string, "illegal_return")                             \
  V(illegal_break_string, "illegal_break")                               \
  V(illegal_continue_string, "illegal_continue")                         \
  V(unknown_label_string, "unknown_label")                               \
  V(redeclaration_string, "redeclaration")                               \
  V(failure_string, "<failure>")                                         \
  V(space_string, " ")                                                   \
  V(exec_string, "exec")                                                 \
  V(zero_string, "0")                                                    \
  V(global_eval_string, "GlobalEval")                                    \
  V(identity_hash_string, "v8::IdentityHash")                            \
  V(closure_string, "(closure)")                                         \
  V(use_strict_string, "use strict")                                     \
  V(dot_string, ".")                                                     \
  V(anonymous_function_string, "(anonymous function)")                   \
  V(compare_ic_string, "==")                                             \
  V(strict_compare_ic_string, "===")                                     \
  V(infinity_string, "Infinity")                                         \
  V(minus_infinity_string, "-Infinity")                                  \
  V(hidden_stack_trace_string, "v8::hidden_stack_trace")                 \
  V(query_colon_string, "(?:)")

objects.h中定義如下的宏：

// A struct is a simple object a set of object-valued fields.  Including an
// object type in this causes the compiler to generate most of the boilerplate
// code for the class including allocation and garbage collection routines,
// casts and predicates.  All you need to define is the class, methods and
// object verification routines.  Easy, no?
//
// Note that for subtle reasons related to the ordering or numerical values of
// type tags, elements in this list have to be added to the INSTANCE_TYPE_LIST
// manually.
#define STRUCT_LIST_ALL(V)                                                     \
  V(DECLARED_ACCESSOR_DESCRIPTOR,                                              \
    DeclaredAccessorDescriptor,                                                \
    declared_accessor_descriptor)                                              \
  V(DECLARED_ACCESSOR_INFO, DeclaredAccessorInfo, declared_accessor_info)      \
  V(EXECUTABLE_ACCESSOR_INFO, ExecutableAccessorInfo, executable_accessor_info)\
  V(ACCESSOR_PAIR, AccessorPair, accessor_pair)                                \
  V(ACCESS_CHECK_INFO, AccessCheckInfo, access_check_info)                     \
  V(INTERCEPTOR_INFO, InterceptorInfo, interceptor_info)                       \
  V(CALL_HANDLER_INFO, CallHandlerInfo, call_handler_info)                     \
  V(FUNCTION_TEMPLATE_INFO, FunctionTemplateInfo, function_template_info)      \
  V(OBJECT_TEMPLATE_INFO, ObjectTemplateInfo, object_template_info)            \
  V(SIGNATURE_INFO, SignatureInfo, signature_info)                             \
  V(TYPE_SWITCH_INFO, TypeSwitchInfo, type_switch_info)                        \
  V(SCRIPT, Script, script)                                                    \
  V(ALLOCATION_SITE_INFO, AllocationSiteInfo, allocation_site_info)            \
  V(CODE_CACHE, CodeCache, code_cache)                                         \
  V(POLYMORPHIC_CODE_CACHE, PolymorphicCodeCache, polymorphic_code_cache)      \
  V(TYPE_FEEDBACK_INFO, TypeFeedbackInfo, type_feedback_info)                  \
  V(ALIASED_ARGUMENTS_ENTRY, AliasedArgumentsEntry, aliased_arguments_entry)

#ifdef ENABLE_DEBUGGER_SUPPORT
#define STRUCT_LIST_DEBUGGER(V)                                                \
  V(DEBUG_INFO, DebugInfo, debug_info)                                         \
  V(BREAK_POINT_INFO, BreakPointInfo, break_point_info)
#else
#define STRUCT_LIST_DEBUGGER(V)
#endif

#define STRUCT_LIST(V)                                                         \
  STRUCT_LIST_ALL(V)                                                           \
  STRUCT_LIST_DEBUGGER(V)