v8中builtins的初始化

最新推荐文章于 2025-05-27 14:40:14 发布

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最新推荐文章于 2025-05-27 14:40:14 发布

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分类专栏： Javascript V8

本文链接：https://blog.youkuaiyun.com/sunbxonline/article/details/20311311

本文详细介绍了V8引擎中builtins的初始化过程，包括如何通过`Builtins::SetUp`函数设置builtins数组，枚举`Name`定义的索引，以及`BuiltinDesc`结构的使用。初始化涉及C++实现的Code、由assemble实现的Code和debugger使用的Code。通过对`BuiltinDesc`数组的赋值，生成并跳转到外部引用的C函数地址，完成内置函数的初始化。

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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