1 摘要
Execution是V8执行Javascript字节码的运行单元，它负责启动Ignition执行字节码。前几篇文章对Javascript的执行过程和理论知识做过介绍。本文重在实践，注重梳理Execution工作流程，从SharedFunction谈起，跟着Javascript测试用例的执行过程走，代码执行到哪，就讲到哪，详细阐述运行单元Execution的具体工作内容、动作步骤和关键节点，给读者展现一个“活动”的V8。
本文剩余内容按字节码的执行顺序讲解该过程中涉及到的V8源码，故只设置一个章节(章节2)。
关键字： Execution，SharedFunction，Context，BindToCurrentContext
2 Execution执行单元
经过GetSharedFunctionInfoForScript()之后，得到了MayHandle<SharedFunctionInfo>类型的maybe_result数据，见下段代码：
0. MaybeHandle<SharedFunctionInfo> Compiler::GetSharedFunctionInfoForScript(
1. Isolate* isolate, Handle<String> source,
2. const Compiler::ScriptDetails& script_details,
3. ScriptOriginOptions origin_options, v8::Extension* extension,
4. ScriptData* cached_data, ScriptCompiler::CompileOptions compile_options,
5. ScriptCompiler::NoCacheReason no_cache_reason, NativesFlag natives) {
6. //...................删除部分代码，留下最核心功能
7. // Do a lookup in the compilation cache but not for extensions.
8. MaybeHandle<SharedFunctionInfo> maybe_result;
9. IsCompiledScope is_compiled_scope;
10. if (extension == nullptr) {
11. bool can_consume_code_cache =
12. compile_options == ScriptCompiler::kConsumeCodeCache;
13. if (can_consume_code_cache) {
14. compile_timer.set_consuming_code_cache();
15. }
16. // First check per-isolate compilation cache.
17. maybe_result = compilation_cache->LookupScript(
18. source, script_details.name_obj, script_details.line_offset,
19. script_details.column_offset, origin_options, isolate->native_context(),
20. language_mode);
21. if (!maybe_result.is_null()) {
22. compile_timer.set_hit_isolate_cache();
23. }
24. }
25. //...................删除部分代码，留下最核心功能
26. if (maybe_result.is_null()) {
27. ParseInfo parse_info(isolate);
28. // No cache entry found compile the script.
29. NewScript(isolate, &parse_info, source, script_details, origin_options,
30. natives);
31. // Compile the function and add it to the isolate cache.
32. if (origin_options.IsModule()) parse_info.set_module();
33. parse_info.set_extension(extension);
34. parse_info.set_eager(compile_options == ScriptCompiler::kEagerCompile);
35. parse_info.set_language_mode(
36. stricter_language_mode(parse_info.language_mode(), language_mode));
37. maybe_result = CompileToplevel(&parse_info, isolate, &is_compiled_scope);
38. Handle<SharedFunctionInfo> result;
39. if (extension == nullptr && maybe_result.ToHandle(&result)) {
40. DCHECK(is_compiled_scope.is_compiled());
41. compilation_cache->PutScript(source, isolate->native_context(),
42. language_mode, result);
43. } else if (maybe_result.is_null() && natives != EXTENSION_CODE) {
44. isolate->ReportPendingMessages();
45. }
46. }
47. return maybe_result;
48. }
获取SharedFunction时先查找compilation cache，命中则直接返回结果，样例JS源码是第一次运行，第26行的maybe_result是null，执行37行的CompileToplevel()，得到编译后的结果maybe_result。进入Execution执行单元之前，还需要绑定当面上下文，代码如下：
0. Local<Script> UnboundScript::BindToCurrentContext() {
1. auto function_info =
2. i::Handle<i::SharedFunctionInfo>::cast(Utils::OpenHandle(this));
3. i::Isolate* isolate = function_info->GetIsolate();
4. i::Handle<i::JSFunction> function =
5. isolate->factory()->NewFunctionFromSharedFunctionInfo(
6. function_info, isolate->native_context());
7. return ToApiHandle<Script>(function);
8. }
完成绑定后，进入ExecuteString()，这是执行单元的入口函数，代码如下：
0. bool Shell::ExecuteString(Isolate* isolate, Local<String> source,
1. Local<Value> name, PrintResult print_result,
2. ReportExceptions report_exceptions,
3. ProcessMessageQueue process_message_queue) {
4. bool success = true;
5. {
6. PerIsolateData* data = PerIsolateData::Get(isolate);
7. Local<Context> realm =
8. Local<Context>::New(isolate, data->realms_[data->realm_current_]);
9. Context::Scope context_scope(realm);
10. MaybeLocal<Script> maybe_script;
11. Local<Context> context(isolate->GetCurrentContext());
12. ScriptOrigin origin(name);
13. if (options.compile_options == ScriptCompiler::kConsumeCodeCache) {
14. //...................删除部分代码，留下最核心功能
15. } else if (options.stress_background_compile) {
16. //...................删除部分代码，留下最核心功能
17. } else {
18. ScriptCompiler::Source script_source(source, origin);
19. maybe_script = ScriptCompiler::Compile(context, &script_source,
20. options.compile_options);
21. }
22. maybe_result = script->Run(realm);
23. //...................删除部分代码，留下最核心功能
24. if (options.code_cache_options ==
25. ShellOptions::CodeCacheOptions::kProduceCacheAfterExecute) {
26. // Serialize and store it in memory for the next execution.
27. ScriptCompiler::CachedData* cached_data =
28. ScriptCompiler::CreateCodeCache(script->GetUnboundScript());
29. StoreInCodeCache(isolate, source, cached_data);
30. delete cached_data;
31. }
32. if (process_message_queue && !EmptyMessageQueues(isolate)) success = false;
33. data->realm_current_ = data->realm_switch_;
34. }
35. DCHECK(!try_catch.HasCaught());
36. return success;
37. }
上述代码中，maybe_script变量在上述代码中被多次重复使用，在debug时可以观察到它代表的语义多次发生变化，我们只需关注this指针的结果即可。19行执行完后得到maybe_script结果，进入22行script->Run(realm)，代码如下：
0. MaybeLocal<Value> Script::Run(Local<Context> context) {
1. auto isolate = reinterpret_cast<i::Isolate*>(context->GetIsolate());
2. TRACE_EVENT_CALL_STATS_SCOPED(isolate, "v8", "V8.Execute");
3. ENTER_V8(isolate, context, Script, Run, MaybeLocal<Value>(),
4. InternalEscapableScope);
5. i::HistogramTimerScope execute_timer(isolate->counters()->execute(), true);
6. i::AggregatingHistogramTimerScope timer(isolate->counters()->compile_lazy());
7. i::TimerEventScope<i::TimerEventExecute> timer_scope(isolate);
8. auto fun = i::Handle<i::JSFunction>::cast(Utils::OpenHandle(this));
9. i::Handle<i::Object> receiver = isolate->global_proxy();
10. Local<Value> result;
11. has_pending_exception = !ToLocal<Value>(
12. i::Execution::Call(isolate, fun, receiver, 0, nullptr), &result);
13. RETURN_ON_FAILED_EXECUTION(Value);
14. RETURN_ESCAPED(result);
15. }
上述代码中，第8行auto fun = i::Handle<i::JSFunction>::cast(Utils::OpenHandle(this));将this结果转为JSFunction，得到console.log(JsPrint(6));的fun，fun是JSFuncion类型，fun中包括了字节码序列，进入12行的call()方法。在call()方法中，执行SetUpForCall()完成参数的设置，最后进入Invoke()，代码如下：
0. V8_WARN_UNUSED_RESULT MaybeHandle<Object> Invoke(Isolate* isolate,
1. const InvokeParams& params) {
2. //...................删除部分代码，留下最核心功能
3. Object value;
4. Handle<Code> code =
5. JSEntry(isolate, params.execution_target, params.is_construct);
6. {
7. SaveContext save(isolate);
8. SealHandleScope shs(isolate);
9. if (FLAG_clear_exceptions_on_js_entry) isolate->clear_pending_exception();
10. if (params.execution_target == Execution::Target::kCallable) {
11. // clang-format off
12. // {new_target}, {target}, {receiver}, return value: tagged pointers
13. // {argv}: pointer to array of tagged pointers
14. using JSEntryFunction = GeneratedCode<Address(
15. Address root_register_value, Address new_target, Address target,
16. Address receiver, intptr_t argc, Address** argv)>;
17. // clang-format on
18. JSEntryFunction stub_entry =
19. JSEntryFunction::FromAddress(isolate, code->InstructionStart());
20. Address orig_func = params.new_target->ptr();
21. Address func = params.target->ptr();
22. Address recv = params.receiver->ptr();
23. Address** argv = reinterpret_cast<Address**>(params.argv);
24. RuntimeCallTimerScope timer(isolate, RuntimeCallCounterId::kJS_Execution);
25. value = Object(stub_entry.Call(isolate->isolate_data()->isolate_root(),
26. orig_func, func, recv, params.argc, argv));
27. } else {
28. //...................删除部分代码，留下最核心功能
29. }
30. }
在Invoke()中，第18行获得入口Builtin功能，该Builtin的编号是40，生成函数是Generate_JSEntry，它的作用是为字节码执行做准备工作。第25行进入执行函数Call()，该函数中调用了函数指针fn_ptr_，该指针是第40号Builtin，测试样例的字节码序列是该函数的参数，目前是c++级别的调试跟踪，图1给出了调用堆栈，从fn_ptr_之后将进入汇编码。
这之后进入汇编执行，先执行下面的Generate_JSEntryVariant完成执行前的准备工作，然后执行样例代码。
0. void Generate_JSEntryVariant(MacroAssembler* masm, StackFrame::Type type,
1. Builtins::Name entry_trampoline) {
2. Label invoke, handler_entry, exit;
3. Label not_outermost_js, not_outermost_js_2;
4. { // NOLINT. Scope block confuses linter.
5. NoRootArrayScope uninitialized_root_register(masm);
6. // Set up frame.
7. __ pushq(rbp);
8. __ movq(rbp, rsp);
9. // Push the stack frame type.
10. __ Push(Immediate(StackFrame::TypeToMarker(type)));
11. // Reserve a slot for the context. It is filled after the root register has
12. // been set up.
13. __ AllocateStackSpace(kSystemPointerSize);
14. // Save callee-saved registers (X64/X32/Win64 calling conventions).
15. __ pushq(r12);
16. __ pushq(r13);
17. __ pushq(r14);
18. __ pushq(r15);
19. #ifdef _WIN64
20. __ pushq(rdi); // Only callee save in Win64 ABI, argument in AMD64 ABI.
21. __ pushq(rsi); // Only callee save in Win64 ABI, argument in AMD64 ABI.
22. #endif
23. __ pushq(rbx);
24. //.......................代码太长，省略很多.............................
25. __ ret(0);
26. }
27. } // namespace
28. //==================分隔线===============================
29. void Builtins::Generate_JSEntry(MacroAssembler* masm) {
30. Generate_JSEntryVariant(masm, StackFrame::ENTRY,
31. Builtins::kJSEntryTrampoline);
32. }
见代码30行，40号Builtin功能JSEntry的实现由Generate_JSEntryVariant负责，Builtin功能的分析方法参见上一篇文章，图2给出汇编代码执行的开始片段，请读者自行跟踪。
图2中上面有c++源码，下面就是汇编码了。测试样例console.log(JsPrint(6));，在V8中console是一个全局功能，它的实现如下代码：
0. #define RUNTIME_FUNCTION_RETURNS_TYPE(Type, InternalType, Convert, Name) \
1. static V8_INLINE InternalType __RT_impl_##Name(Arguments args, \
2. Isolate* isolate); \
3. \
4. V8_NOINLINE static Type Stats_##Name(int args_length, Address* args_object, \
5. Isolate* isolate) { \
6. RuntimeCallTimerScope timer(isolate, RuntimeCallCounterId::k##Name); \
7. TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.runtime"), \
8. "V8.Runtime_" #Name); \
9. Arguments args(args_length, args_object); \
10. return Convert(__RT_impl_##Name(args, isolate)); \
11. } \
12. \
13. Type Name(int args_length, Address* args_object, Isolate* isolate) { \
14. DCHECK(isolate->context().is_null() || isolate->context().IsContext()); \
15. CLOBBER_DOUBLE_REGISTERS(); \
16. if (V8_UNLIKELY(TracingFlags::is_runtime_stats_enabled())) { \
17. return Stats_##Name(args_length, args_object, isolate); \
18. } \
19. Arguments args(args_length, args_object); \
20. return Convert(__RT_impl_##Name(args, isolate)); \
21. } \
22. \
23. static InternalType __RT_impl_##Name(Arguments args, Isolate* isolate)
24. #define CONVERT_OBJECT(x) (x).ptr()
25. #define CONVERT_OBJECTPAIR(x) (x)
26. #define RUNTIME_FUNCTION(Name) \
27. RUNTIME_FUNCTION_RETURNS_TYPE(Address, Object, CONVERT_OBJECT, Name)
28. //==================分隔线===============================
29. //==================分隔线===============================
30. Object DeclareGlobals(Isolate* isolate, Handle<FixedArray> declarations,
31. int flags, Handle<JSFunction> closure) {
32. HandleScope scope(isolate);
33. Handle<JSGlobalObject> global(isolate->global_object());
34. Handle<Context> context(isolate->context(), isolate);
35. Handle<FeedbackVector> feedback_vector = Handle<FeedbackVector>::null();
36. Handle<ClosureFeedbackCellArray> closure_feedback_cell_array =
37. Handle<ClosureFeedbackCellArray>::null();
38. if (closure->has_feedback_vector()) {
39. feedback_vector =
40. Handle<FeedbackVector>(closure->feedback_vector(), isolate);
41. closure_feedback_cell_array = Handle<ClosureFeedbackCellArray>(
42. feedback_vector->closure_feedback_cell_array(), isolate);
43. } else {
44. closure_feedback_cell_array = Handle<ClosureFeedbackCellArray>(
45. closure->closure_feedback_cell_array(), isolate);
46. }
47. // Traverse the name/value pairs and set the properties.
48. int length = declarations->length();
49. FOR_WITH_HANDLE_SCOPE(isolate, int, i = 0, i, i < length, i += 4, {
50. Handle<String> name(String::cast(declarations->get(i)), isolate);
51. FeedbackSlot slot(Smi::ToInt(declarations->get(i + 1)));
52. Handle<Object> possibly_feedback_cell_slot(declarations->get(i + 2),
53. isolate);
54. Handle<Object> initial_value(declarations->get(i + 3), isolate);
55. bool is_var = initial_value->IsUndefined(isolate);
56. bool is_function = initial_value->IsSharedFunctionInfo();
57. DCHECK_NE(is_var, is_function);
58. Handle<Object> value;
59. if (is_function) {
60. DCHECK(possibly_feedback_cell_slot->IsSmi());
61. Handle<FeedbackCell> feedback_cell =
62. closure_feedback_cell_array->GetFeedbackCell(
63. Smi::ToInt(*possibly_feedback_cell_slot));
64. // Copy the function and update its context. Use it as value.
65. Handle<SharedFunctionInfo> shared =
66. Handle<SharedFunctionInfo>::cast(initial_value);
67. Handle<JSFunction> function =
68. isolate->factory()->NewFunctionFromSharedFunctionInfo(
69. shared, context, feedback_cell, AllocationType::kOld);
70. value = function;
71. } else {
72. value = isolate->factory()->undefined_value();
73. }
74. // Compute the property attributes. According to ECMA-262,
75. // the property must be non-configurable except in eval.
76. bool is_eval = DeclareGlobalsEvalFlag::decode(flags);
77. int attr = NONE;
78. if (!is_eval) attr |= DONT_DELETE;
79. // ES#sec-globaldeclarationinstantiation 5.d:
80. // If hasRestrictedGlobal is true, throw a SyntaxError exception.
81. Object result = DeclareGlobal(isolate, global, name, value,
82. static_cast<PropertyAttributes>(attr), is_var,
83. is_function, RedeclarationType::kSyntaxError,
84. feedback_vector, slot);
85. if (isolate->has_pending_exception()) return result;
86. });
87. return ReadOnlyRoots(isolate).undefined_value();
88. }
89. //==================分隔线===============================
90. //==================分隔线===============================
91. RUNTIME_FUNCTION(Runtime_DeclareGlobals) {
92. HandleScope scope(isolate);
93. DCHECK_EQ(3, args.length());
94. CONVERT_ARG_HANDLE_CHECKED(FixedArray, declarations, 0);
95. CONVERT_SMI_ARG_CHECKED(flags, 1);
96. CONVERT_ARG_HANDLE_CHECKED(JSFunction, closure, 2);
97. return DeclareGlobals(isolate, declarations, flags, closure);
98. }
RUNTIME_FUNCTION(Runtime_DeclareGlobals)是由宏模板定义的全局功能，该函数先检查参数的正确性，然后进入DeclareGlobals()完成具体功能。在我们的测试样例中，DeclareGlobals()是获得console的全局对象。得到该对象后返回到汇编码状态接着执行，获得该对象的log属性，也就是console.log，检测到它的参数JsPrint(6)是函数，然后编译此函数，得到一份字节码序列，开启一个新的执行单元，请读者自行跟踪。
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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