v8中builtins的初始化

builtins定义于builtins.h，它的实例存在于isolate中，在isolate的init函数中，调用builtins.SetUp函数初始化builtins，builtins内部有一个Object* builtins_[builtin_count]，所有的初始化操作，主要就是为了初始化这个对象指针数组，这个数组的每个指针指向的都是一个Code对象，也就是它是一段被编译过的代码。

一.builtins数组的索引

enum Name {
#define DEF_ENUM_C(name, ignore) k##name,
#define DEF_ENUM_A(name, kind, state, extra) k##name,
    BUILTIN_LIST_C(DEF_ENUM_C)
    BUILTIN_LIST_A(DEF_ENUM_A)
    BUILTIN_LIST_DEBUG_A(DEF_ENUM_A)
#undef DEF_ENUM_C
#undef DEF_ENUM_A
    builtin_count
  };

类似以前我们看到的索引定义方式，builtins数组的索引被分成了三组，分别使用BUILTIN_LIST_C，BUILTIN_LIST_A，BUILTIN_LIST_DEBUG_A定义。它们依次表示由C++实现的Code，由assemble实现的Code，由assemble实现的供debugger使用的Code。

二.Builtins::SetUp

void Builtins::SetUp(bool create_heap_objects) {
  ASSERT(!initialized_);
  Isolate* isolate = Isolate::Current();
  Heap* heap = isolate->heap();

  // Create a scope for the handles in the builtins.
  HandleScope scope(isolate);

  1)
  const BuiltinDesc* functions = builtin_function_table.functions();

  // For now we generate builtin adaptor code into a stack-allocated
  // buffer, before copying it into individual code objects. Be careful
  // with alignment, some platforms don't like unaligned code.
  union { int force_alignment; byte buffer[8*KB]; } u;

  // Traverse the list of builtins and generate an adaptor in a
  // separate code object for each one.
  for (int i = 0; i < builtin_count; i++) {
    if (create_heap_objects) {
      MacroAssembler masm(isolate, u.buffer, sizeof u.buffer);
      // Generate the code/adaptor.

      2)
      typedef void (*Generator)(MacroAssembler*, int, BuiltinExtraArguments);
      Generator g = FUNCTION_CAST<Generator>(functions[i].generator);
      // We pass all arguments to the generator, but it may not use all of
      // them.  This works because the first arguments are on top of the
      // stack.
      ASSERT(!masm.has_frame());
      g(&masm, functions[i].name, functions[i].extra_args);
      // Move the code into the object heap.

      3)
      CodeDesc desc;
      masm.GetCode(&desc);
      Code::Flags flags =  functions[i].flags;
      Object* code = NULL;
      {
        // During startup it's OK to always allocate and defer GC to later.
        // This simplifies things because we don't need to retry.
        AlwaysAllocateScope __scope__;

          4)
     &nbs