1 摘要
上一篇文章中，Builtin作为先导知识，我们做了宏观概括和介绍。Builtin（Built-in function）是编译好的内置代码块（chunk），存储在snapshot_blob.bin文件中，V8启动时以反序列化方式加载，运行时可以直接调用。Builtins功能共计600多个，细分为多个子类型，涵盖了解释器、字节码、执行单元等多个V8核心功能，本文从微观角度剖析Builtins功能的源码，在不使用snapshot_blob.bin文件的情况下，详细说明Builtin创建和运行过程。
本文内容组织结构：Bultin初始化过程（章节2），Builtin子类型讲解(章节3)。
2 Builtin初始化
下面是code类，它负责管理所有Builtin功能，是builtin table的数据类型。
1. class Code : public HeapObject {
2. public:
3. NEVER_READ_ONLY_SPACE
4. // Opaque data type for encapsulating code flags like kind, inline
5. // cache state, and arguments count.
6. using Flags = uint32_t;
7. #define CODE_KIND_LIST(V) \
8. V(OPTIMIZED_FUNCTION) \
9. V(BYTECODE_HANDLER) \
10. V(STUB) \
11. V(BUILTIN) \
12. V(REGEXP) \
13. V(WASM_FUNCTION) \
14. V(WASM_TO_CAPI_FUNCTION) \
15. V(WASM_TO_JS_FUNCTION) \
16. V(JS_TO_WASM_FUNCTION) \
17. V(JS_TO_JS_FUNCTION) \
18. V(WASM_INTERPRETER_ENTRY) \
19. V(C_WASM_ENTRY)
20. enum Kind {
21. #define DEFINE_CODE_KIND_ENUM(name) name,
22. CODE_KIND_LIST(DEFINE_CODE_KIND_ENUM)
23. #undef DEFINE_CODE_KIND_ENUM
24. NUMBER_OF_KINDS
25. };
26. static const char* Kind2String(Kind kind);
27. // Layout description.
28. #define CODE_FIELDS(V) \
29. V(kRelocationInfoOffset, kTaggedSize) \
30. V(kDeoptimizationDataOffset, kTaggedSize) \
31. V(kSourcePositionTableOffset, kTaggedSize) \
32. V(kCodeDataContainerOffset, kTaggedSize) \
33. /* Data or code not directly visited by GC directly starts here. */ \
34. /* The serializer needs to copy bytes starting from here verbatim. */ \
35. /* Objects embedded into code is visited via reloc info. */ \
36. V(kDataStart, 0) \
37. V(kInstructionSizeOffset, kIntSize) \
38. V(kFlagsOffset, kIntSize) \
39. V(kSafepointTableOffsetOffset, kIntSize) \
40. V(kHandlerTableOffsetOffset, kIntSize) \
41. V(kConstantPoolOffsetOffset, \
42. FLAG_enable_embedded_constant_pool ? kIntSize : 0) \
43. V(kCodeCommentsOffsetOffset, kIntSize) \
44. V(kBuiltinIndexOffset, kIntSize) \
45. V(kUnalignedHeaderSize, 0) \
46. /* Add padding to align the instruction start following right after */ \
47. /* the Code object header. */ \
48. V(kOptionalPaddingOffset, CODE_POINTER_PADDING(kOptionalPaddingOffset)) \
49. V(kHeaderSize, 0)
50. DEFINE_FIELD_OFFSET_CONSTANTS(HeapObject::kHeaderSize, CODE_FIELDS)
51. #undef CODE_FIELDS
52. STATIC_ASSERT(FIELD_SIZE(kOptionalPaddingOffset) == kHeaderPaddingSize);
53. inline int GetUnwindingInfoSizeOffset() const;
54. class BodyDescriptor;
55. // Flags layout. BitField<type, shift, size>.
56. #define CODE_FLAGS_BIT_FIELDS(V, _) \
57. V(HasUnwindingInfoField, bool, 1, _) \
58. V(KindField, Kind, 5, _) \
59. V(IsTurbofannedField, bool, 1, _) \
60. V(StackSlotsField, int, 24, _) \
61. V(IsOffHeapTrampoline, bool, 1, _)
62. DEFINE_BIT_FIELDS(CODE_FLAGS_BIT_FIELDS)
63. #undef CODE_FLAGS_BIT_FIELDS
64. static_assert(NUMBER_OF_KINDS <= KindField::kMax, "Code::KindField size");
65. static_assert(IsOffHeapTrampoline::kLastUsedBit < 32,
66. "Code::flags field exhausted");
67. // KindSpecificFlags layout (STUB, BUILTIN and OPTIMIZED_FUNCTION)
68. #define CODE_KIND_SPECIFIC_FLAGS_BIT_FIELDS(V, _) \
69. V(MarkedForDeoptimizationField, bool, 1, _) \
70. V(EmbeddedObjectsClearedField, bool, 1, _) \
71. V(DeoptAlreadyCountedField, bool, 1, _) \
72. V(CanHaveWeakObjectsField, bool, 1, _) \
73. V(IsPromiseRejectionField, bool, 1, _) \
74. V(IsExceptionCaughtField, bool, 1, _)
75. DEFINE_BIT_FIELDS(CODE_KIND_SPECIFIC_FLAGS_BIT_FIELDS)
76. #undef CODE_KIND_SPECIFIC_FLAGS_BIT_FIELDS
77. private:
78. friend class RelocIterator;
79. bool is_promise_rejection() const;
80. bool is_exception_caught() const;
81. OBJECT_CONSTRUCTORS(Code, HeapObject);
82. };
//........................代码太长，省略很多.....................
//.............................................................
上述代码中，CODE_KIND_LIST从code角度定义了类型，在Builtin类中也定义了Builtin一共有七种子类型，这是两种不同的定义方式，但说的都是Builtin。Builtin的初始化工作由方法void Isolate::Initialize(Isolate* isolate,const v8::Isolate::CreateParams& params)统一完成，下面给出这个方法的部分代码。
0. void Isolate::Initialize(Isolate* isolate,
1. const v8::Isolate::CreateParams& params) {
2. i::Isolate* i_isolate = reinterpret_cast<i::Isolate*>(isolate);
3. CHECK_NOT_NULL(params.array_buffer_allocator);
4. i_isolate->set_array_buffer_allocator(params.array_buffer_allocator);
5. if (params.snapshot_blob != nullptr) {
6. i_isolate->set_snapshot_blob(params.snapshot_blob);
7. } else {
8. i_isolate->set_snapshot_blob(i::Snapshot::DefaultSnapshotBlob());
9. }
10. auto code_event_handler = params.code_event_handler;
11. //........................代码太长，省略很多.....................
12. if (!i::Snapshot::Initialize(i_isolate)) {
13. // If snapshot data was provided and we failed to deserialize it must
14. // have been corrupted.
15. if (i_isolate->snapshot_blob() != nullptr) {
16. FATAL(
17. "Failed to deserialize the V8 snapshot blob. This can mean that the "
18. "snapshot blob file is corrupted or missing.");
19. }
20. base::ElapsedTimer timer;
21. if (i::FLAG_profile_deserialization) timer.Start();
22. i_isolate->InitWithoutSnapshot();
23. if (i::FLAG_profile_deserialization) {
24. double ms = timer.Elapsed().InMillisecondsF();
25. i::PrintF("[Initializing isolate from scratch took %0.3f ms]\n", ms);
26. }
27. }
28. i_isolate->set_only_terminate_in_safe_scope(
29. params.only_terminate_in_safe_scope);
30. }
上述方面中进入第22行，最终进入下面的Builtin初始化方法。
1. void SetupIsolateDelegate::SetupBuiltinsInternal(Isolate* isolate) {
2. //...................删除部分代码，留下最核心功能
3. //...................删除部分代码，留下最核心功能
4. int index = 0;
5. Code code;
6. #define BUILD_CPP(Name) \
7. code = BuildAdaptor(isolate, index, FUNCTION_ADDR(Builtin_##Name), #Name); \
8. AddBuiltin(builtins, index++, code);
9. #define BUILD_TFJ(Name, Argc, ...) \
10. code = BuildWithCodeStubAssemblerJS( \
11. isolate, index, &Builtins::Generate_##Name, Argc, #Name); \
12. AddBuiltin(builtins, index++, code);
13. #define BUILD_TFC(Name, InterfaceDescriptor) \
14. /* Return size is from the provided CallInterfaceDescriptor. */ \
15. code = BuildWithCodeStubAssemblerCS( \
16. isolate, index, &Builtins::Generate_##Name, \
17. CallDescriptors::InterfaceDescriptor, #Name); \
18. AddBuiltin(builtins, index++, code);
19. #define BUILD_TFS(Name, ...) \
20. /* Return size for generic TF builtins (stub linkage) is always 1. */ \
21. code = \
22. BuildWithCodeStubAssemblerCS(isolate, index, &Builtins::Generate_##Name, \
23. CallDescriptors::Name, #Name); \
24. AddBuiltin(builtins, index++, code);
25. #define BUILD_TFH(Name, InterfaceDescriptor) \
26. /* Return size for IC builtins/handlers is always 1. */ \
27. code = BuildWithCodeStubAssemblerCS( \
28. isolate, index, &Builtins::Generate_##Name, \
29. CallDescriptors::InterfaceDescriptor, #Name); \
30. AddBuiltin(builtins, index++, code);
31. #define BUILD_BCH(Name, OperandScale, Bytecode) \
32. code = GenerateBytecodeHandler(isolate, index, OperandScale, Bytecode); \
33. AddBuiltin(builtins, index++, code);
34. #define BUILD_ASM(Name, InterfaceDescriptor) \
35. code = BuildWithMacroAssembler(isolate, index, Builtins::Generate_##Name, \
36. #Name); \
37. AddBuiltin(builtins, index++, code);
38. BUILTIN_LIST(BUILD_CPP, BUILD_TFJ, BUILD_TFC, BUILD_TFS, BUILD_TFH, BUILD_BCH,
39. BUILD_ASM);
40. //...................删除部分代码，留下最核心功能
41. //...................删除部分代码，留下最核心功能
42. }
上述代码只保留了最核心的Builtin初始化功能，初始化工作主要是生成并编译Builtin代码，并以独立功能的形式挂载到isolate上，以BuildWithCodeStubAssemblerCS()详细描述该过程。
见下面代码，第一个参数是isolate，用于保存初化完成的Builtin；第二个参数全局变量index，Builtin存储在isolate的数组成员中，index是数组下标；第三个参数generator是函数指针，该函数用于生成Builtin；第四个参数是call描述符；最后一个是函数名字。
1. // Builder for builtins implemented in TurboFan with CallStub linkage.
2. Code BuildWithCodeStubAssemblerCS(Isolate* isolate, int32_t builtin_index,
3. CodeAssemblerGenerator generator,
4. CallDescriptors::Key interface_descriptor,
5. const char* name) {
6. HandleScope scope(isolate);
7. // Canonicalize handles, so that we can share constant pool entries pointing
8. // to code targets without dereferencing their handles.
9. CanonicalHandleScope canonical(isolate);
10. Zone zone(isolate->allocator(), ZONE_NAME);
11. // The interface descriptor with given key must be initialized at this point
12. // and this construction just queries the details from the descriptors table.
13. CallInterfaceDescriptor descriptor(interface_descriptor);
14. // Ensure descriptor is already initialized.
15. DCHECK_LE(0, descriptor.GetRegisterParameterCount());
16. compiler::CodeAssemblerState state(
17. isolate, &zone, descriptor, Code::BUILTIN, name,
18. PoisoningMitigationLevel::kDontPoison, builtin_index);
19. generator(&state);
20. Handle<Code> code = compiler::CodeAssembler::GenerateCode(
21. &state, BuiltinAssemblerOptions(isolate, builtin_index));
22. return *code;
23. }
在代码中，第19行代码生成Builtin源码，以第一个Builtin为例说明generator(&state)的功能，此时generator指针代表的函数是TF_BUILTIN(RecordWrite, RecordWriteCodeStubAssembler)，下面是代码：
1. TF_BUILTIN(RecordWrite, RecordWriteCodeStubAssembler) {
2. Label generational_wb(this);
3. Label incremental_wb(this);
4. Label exit(this);
5. Node* remembered_set = Parameter(Descriptor::kRememberedSet);
6. Branch(ShouldEmitRememberSet(remembered_set), &generational_wb,
7. &incremental_wb);
8. BIND(&generational_wb);
9. {
10. Label test_old_to_young_flags(this);
11. Label store_buffer_exit(this), store_buffer_incremental_wb(this);
12. TNode<IntPtrT> slot = UncheckedCast<IntPtrT>(Parameter(Descriptor::kSlot));
13. Branch(IsMarking(), &test_old_to_young_flags, &store_buffer_exit);
14. BIND(&test_old_to_young_flags);
15. {
16. TNode<IntPtrT> value =
17. BitcastTaggedToWord(Load(MachineType::TaggedPointer(), slot));
18. TNode<BoolT> value_is_young =
19. IsPageFlagSet(value, MemoryChunk::kIsInYoungGenerationMask);
20. GotoIfNot(value_is_young, &incremental_wb);
21. TNode<IntPtrT> object =
22. BitcastTaggedToWord(Parameter(Descriptor::kObject));
23. TNode<BoolT> object_is_young =
24. IsPageFlagSet(object, MemoryChunk::kIsInYoungGenerationMask);
25. Branch(object_is_young, &incremental_wb, &store_buffer_incremental_wb);
26. }
27. BIND(&store_buffer_exit);
28. {
29. TNode<ExternalReference> isolate_constant =
30. ExternalConstant(ExternalReference::isolate_address(isolate()));
31. Node* fp_mode = Parameter(Descriptor::kFPMode);
32. InsertToStoreBufferAndGoto(isolate_constant, slot, fp_mode, &exit);
33. }
34. BIND(&store_buffer_incremental_wb);
35. {
36. TNode<ExternalReference> isolate_constant =
37. ExternalConstant(ExternalReference::isolate_address(isolate()));
38. Node* fp_mode = Parameter(Descriptor::kFPMode);
39. InsertToStoreBufferAndGoto(isolate_constant, slot, fp_mode,
40. &incremental_wb);
41. }
42. } //........................省略代码......................................
43. BIND(&exit);
44. IncrementCounter(isolate()->counters()->write_barriers(), 1);
45. Return(TrueConstant());
46. }
这个函数TF_BUILTIN(RecordWrite, RecordWriteCodeStubAssembler)是生成器，它的作用是生成写记录功能的源代码，TF_BUILTIN是宏模板，展开后可以看到它的类成员CodeAssemblerState* state保存了生成之后的源码。“用平台无关的生成器为特定平台生成源代码”是Builtin的常用做法，这样减少了工作量。函数执行完成后返回到BuildWithCodeStubAssemblerCS，生成的源代码经过处理后，最终由code表示，下面是code的数据类型。
class Code : public HeapObject {
public:
NEVER_READ_ONLY_SPACE
// Opaque data type for encapsulating code flags like kind, inline
// cache state, and arguments count.
using Flags = uint32_t;
#define CODE_KIND_LIST(V) \
V(OPTIMIZED_FUNCTION) \
V(BYTECODE_HANDLER) \
V(STUB) \
V(BUILTIN) \
V(REGEXP) \
V(WASM_FUNCTION) \
V(WASM_TO_CAPI_FUNCTION) \
V(WASM_TO_JS_FUNCTION) \
V(JS_TO_WASM_FUNCTION) \
V(JS_TO_JS_FUNCTION) \
V(WASM_INTERPRETER_ENTRY) \
V(C_WASM_ENTRY)
enum Kind {
#define DEFINE_CODE_KIND_ENUM(name) name,
CODE_KIND_LIST(DEFINE_CODE_KIND_ENUM)
#undef DEFINE_CODE_KIND_ENUM
NUMBER_OF_KINDS
};
//..................省略........................
//.............................................
上述代码中，可以看到从code的角度对Builtin进行了更详细的分类。另外code是堆对象，也就是说Builtin是由V8的堆栈进行管理，后续讲到堆栈时再详细说明这部分知识。图2给出函数调用堆栈，供读者自行复现。
在SetupBuiltinsInternal()中可以看到AddBuiltin()将生成的code代码添加到isolate中，代码如下。
void SetupIsolateDelegate::AddBuiltin(Builtins* builtins, int index,
Code code) {
DCHECK_EQ(index, code.builtin_index());
builtins->set_builtin(index, code);
}
//..............分隔线......................
void Builtins::set_builtin(int index, Code builtin) {
isolate_->heap()->set_builtin(index, builtin);
}
所有Builtin功能生成后保存在Address builtins_[Builtins::builtin_count]中，初始化方法SetupBuiltinsInternal按照BUILTIN_LIST的定义顺序依次完成所有Builtin的源码生成、编译和挂载到isolate。
2 Builtin子类型
从Builtins的功能看，它包括了：Ignition实现、字节码实现、以及ECMA规范实现等众多V8的核心功能，在BUILTIN_LIST定义中有详细注释，请读者自行查阅。前面讲过，从BUILTIN的实现角度分为七种类型，见下面代码：
#define BUILD_CPP(Name)
#define BUILD_TFJ(Name, Argc, ...)
#define BUILD_TFC(Name, InterfaceDescriptor)
#define BUILD_TFS(Name, ...)
#define BUILD_TFH(Name, InterfaceDescriptor)
#define BUILD_BCH(Name, OperandScale, Bytecode)
#define BUILD_ASM(Name, InterfaceDescriptor)
以子类型BUILD_CPP举例说明，下面是完整源代码。
1. Code BuildAdaptor(Isolate* isolate, int32_t builtin_index,
2. Address builtin_address, const char* name) {
3. HandleScope scope(isolate);
4. // Canonicalize handles, so that we can share constant pool entries pointing
5. // to code targets without dereferencing their handles.
6. CanonicalHandleScope canonical(isolate);
7. constexpr int kBufferSize = 32 * KB;
8. byte buffer[kBufferSize];
9. MacroAssembler masm(isolate, BuiltinAssemblerOptions(isolate, builtin_index),
10. CodeObjectRequired::kYes,
11. ExternalAssemblerBuffer(buffer, kBufferSize));
12. masm.set_builtin_index(builtin_index);
13. DCHECK(!masm.has_frame());
14. Builtins::Generate_Adaptor(&masm, builtin_address);
15. CodeDesc desc;
16. masm.GetCode(isolate, &desc);
17. Handle<Code> code = Factory::CodeBuilder(isolate, desc, Code::BUILTIN)
18. .set_self_reference(masm.CodeObject())
19. .set_builtin_index(builtin_index)
20. .Build();
21. return *code;
22. }
BuildAdaptor的生成功能由第13行代码实现，最终该代码的实现如下：
void Builtins::Generate_Adaptor(MacroAssembler* masm, Address address) {
__ LoadAddress(kJavaScriptCallExtraArg1Register,
ExternalReference::Create(address));
__ Jump(BUILTIN_CODE(masm->isolate(), AdaptorWithBuiltinExitFrame),
RelocInfo::CODE_TARGET);
}
}
上面两部分代码实现了第77号Builtin功能，名字是HandleApiCall，图2以char类型展示了生成的源代码。
总结：学习Builtin时，涉及很多系统结构相关的知识，本文讲解采用的是x64架构。每种Builtin的生成方式虽不相同，但分析源码的思路相同，有问题可以联系我。
好了，今天到这里，下次见。
恳请读者批评指正、提出宝贵意见
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