浅析 V8-turboFan（上）

一、简介

v8 是一种 JS 引擎的实现，它由Google开发，使用C++编写。v8 被设计用于提高网页浏览器内部 JavaScript 代码执行的性能。为了提高性能，v8 将会把 JS 代码转换为更高效的机器码，而非传统的使用解释器执行。因此 v8 引入了 JIT (Just-In-Time) 机制，该机制将会在运行时动态编译 JS 代码为机器码，以提高运行速度。
TurboFan是 v8 的优化编译器之一，它使用了 sea of nodes 这个编译器概念。

sea of nodes 不是单纯的指某个图的结点，它是一种特殊中间表示的图。

它的表示形式与一般的CFG/DFG不同，其具体内容请查阅上面的连接。

TurboFan的相关源码位于v8/compiler文件夹下。
这是笔者初次学习v8 turboFan所写下的笔记，内容包括但不限于turboFan运行参数的使用、部分OptimizationPhases的工作机理，以及拿来练手的GoogleCTF 2018(Final) Just-In-Time题题解。该笔记基于 Introduction to TurboFan 并适当拓宽了一部分内容。如果在阅读文章时发现错误或者存在不足之处，欢迎各位师傅斧正！

二、环境搭建

这里的环境搭建较为简单，首先搭配一个 v8 环境（必须，没有 v8 环境要怎么研究 v8， 2333）。这里使用的版本号是7.0.276.3。
如何搭配v8环境？请移步下拉&编译 chromium&v8 代码

这里需要补充一下，v8 的 gn args中必须加一个v8_untrusted_code_mitigations = false的标志，即最后使用的gn args如下：
```
# Set build arguments here. See `gn help buildargs`.
is_debug = true
target_cpu = "x64"
v8_enable_backtrace = true
v8_enable_slow_dchecks = true
v8_optimized_debug = false
# 加上这个
v8_untrusted_code_mitigations = false
```
具体原因将在下面讲解CheckBounds结点优化时提到。
然后安装一下 v8 的turbolizer，turbolizer将用于调试 v8 TurboFan中sea of nodes图的工具。
```
cd v8/tools/turbolizer
# 获取依赖项
npm i
# 构建
npm run-script build
# 直接在turbolizer文件夹下启动静态http服务
python -m SimpleHTTPServer
```
构建turbolizer时可能会报一些TypeScript的语法错误ERROR，这些ERROR无伤大雅，不影响turbolizer的功能使用。
turbolizer 的使用方式如下：
- 首先编写一段测试函数
```
// 目标优化函数
function opt_me(b) {
    let values = [42,1337];
    let x = 10;
    if (b == "foo")
      x = 5;

    let y = x + 2;
    y = y + 1000;
    y = y * 2;
    y = y & 10;
    y = y / 3;
    y = y & 1;
    return values[y];
}
// 必须！在优化该函数前必须先进行一次编译，以便于为该函数提供type feedback
opt_me();
// 必须! 使用v8 natives-syntax来强制优化该函数
%OptimizeFunctionOnNextCall(opt_me);
// 必须！ 不调用目标函数则无法执行优化
opt_me();
```
  一定要在执行%OptimizeFunctionOnNextCall(opt_me)之前调用一次目标函数，否则生成的graph将会因为没有type feedback而导致完全不一样的结果。
  
  需要注意的是type feedback有点玄学，在执行OptimizeFunctionOnNextCall前，如果目标函数内部存在一些边界操作（例如多次使用超过Number.MAX_SAFE_INTEGER大小的整数等），那么调用目标函数的方式可能会影响turboFan的功能，包括但不限于传入参数的不同、调用目标函数次数的不同等等等等。
  
  因此在执行%OptimizeFunctionOnNextCall前，目标函数的调用方式，必须自己把握，手动确认调用几次，传入什么参数会优化出特定的效果。
  
  若想优化一个函数，除了可以使用%OptimizeFunctionOnNextCall以外，还可以多次执行该函数（次数要大，建议上for循环）来触发优化。
- 然后使用 d8 执行，不过需要加上--trace-turbo参数。
```
$  ../../v8/v8/out.gn/x64.debug/d8 test.js --allow-natives-syntax --trace-turbo   
Concurrent recompilation has been disabled for tracing.
---------------------------------------------------
Begin compiling method opt_me using Turbofan
---------------------------------------------------
Finished compiling method opt_me using Turbofan
```
  之后本地就会生成turbo.cfg和turbo-xxx-xx.json文件。
- 使用浏览器打开127.0.0.1:8000（注意之前在turbolizer文件夹下启动了http服务）然后点击右上角的3号按钮，在文件选择窗口中选择刚刚生成的turbo-xxx-xx.json文件，之后就会显示以下信息：
  不过这里的结点只显示了控制结点，如果需要显示全部结点，则先点击一下上方的2号按钮，将结点全部展开，之后再点击1号按钮，重新排列：

三、turboFan的代码优化

我们可以使用 --trace-opt参数来追踪函数的优化信息。以下是函数opt_me被turboFan优化时所生成的信息。

$ ../../v8/v8/out.gn/x64.debug/d8 test.js --allow-natives-syntax --trace-opt 
[manually marking 0x0a7a24823759 <JSFunction opt_me (sfi = 0xa7a24823591)> for non-concurrent optimization]
[compiling method 0x0a7a24823759 <JSFunction opt_me (sfi = 0xa7a24823591)> using TurboFan]
[optimizing 0x0a7a24823759 <JSFunction opt_me (sfi = 0xa7a24823591)> - took 53.965, 19.410, 0.667 ms]

上面输出中的manually marking即我们在代码中手动设置的%OptimizeFunctionOnNextCall。

我们可以使用 v8 本地语法来查看优化前和优化后的机器码（使用%DisassembleFunction本地语法）

输出信息过长，这里只截取一部分输出。

$ ../../v8/v8/out.gn/x64.debug/d8 test.js --allow-natives-syntax  

0x2b59fe964c1: [Code]
 - map: 0x05116bd02ae9 <Map>
kind = BUILTIN
name = InterpreterEntryTrampoline
compiler = unknown
address = 0x2b59fe964c1

Instructions (size = 995)
0x2b59fe96500     0  488b5f27       REX.W movq rbx,[rdi+0x27]
0x2b59fe96504     4  488b5b07       REX.W movq rbx,[rbx+0x7]
0x2b59fe96508     8  488b4b0f       REX.W movq rcx,[rbx+0xf]
....

0x2b59ff49541: [Code]
 - map: 0x05116bd02ae9 <Map>
kind = OPTIMIZED_FUNCTION
stack_slots = 5
compiler = turbofan
address = 0x2b59ff49541

Instructions (size = 212)
0x2b59ff49580     0  488d1df9ffffff REX.W leaq rbx,[rip+0xfffffff9]
0x2b59ff49587     7  483bd9         REX.W cmpq rbx,rcx
0x2b59ff4958a     a  7418           jz 0x2b59ff495a4  <+0x24>
0x2b59ff4958c     c  48ba000000003e000000 REX.W movq rdx,0x3e00000000
...

可以看到，所生成的代码长度从原先的995，优化为212，大幅度优化了代码。

需要注意的是，即便不使用%OptimizeFunctionOnNextCall，将opt_me函数重复执行一定次数，一样可以触发TurboFan的优化。

细心的小伙伴应该可以在上面环境搭建的图中看到deoptimize反优化。为什么需要反优化？这就涉及到turboFan的优化机制。以下面这个js代码为例（注意：没有使用%OptimizeFunctionOnNextCall）

class Player{}
class Wall{}

function move(obj) {
  var tmp = obj.x + 42;
  var x = Math.random();
  x += 1;
  return tmp + x;
}

for (var i = 0; i < 0x10000; ++i) {
  move(new Player());
}

move(new Wall());
for (var i = 0; i < 0x10000; ++i) {
  move(new Wall());
}

跟踪一下该代码的opt以及deopt：

$ ../../v8/v8/out.gn/x64.debug/d8 test.js --allow-natives-syntax  --trace-opt --trace-deopt 
[marking 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> for optimized recompilation, reason: small function, ICs with typeinfo: 7/7 (100%), generic ICs: 0/7 (0%)]
[compiling method 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> using TurboFan]
[optimizing 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> - took 6.583, 2.385, 0.129 ms]
[completed optimizing 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)>]
# 分割线---------------------------------------------------------------------
[marking 0x3c72eab238e9 <JSFunction (sfi = 0x3c72eab234e9)> for optimized recompilation, reason: hot and stable, ICs with typeinfo: 7/13 (53%), generic ICs: 0/13 (0%)]
[compiling method 0x3c72eab238e9 <JSFunction (sfi = 0x3c72eab234e9)> using TurboFan OSR]
[optimizing 0x3c72eab238e9 <JSFunction (sfi = 0x3c72eab234e9)> - took 3.684, 7.337, 0.409 ms]
# 分割线---------------------------------------------------------------------
[deoptimizing (DEOPT soft): begin 0x3c72eab238e9 <JSFunction (sfi = 0x3c72eab234e9)> (opt #1) @6, FP to SP delta: 104, caller sp: 0x7ffed15d2a08]
            ;;; deoptimize at <test.js:15:6>, Insufficient type feedback for construct
  ...

[deoptimizing (soft): end 0x3c72eab238e9 <JSFunction (sfi = 0x3c72eab234e9)> @6 => node=154, pc=0x7f0d956522e0, caller sp=0x7ffed15d2a08, took 0.496 ms]
[deoptimizing (DEOPT eager): begin 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> (opt #0) @1, FP to SP delta: 24, caller sp: 0x7ffed15d2990]
            ;;; deoptimize at <test.js:5:17>, wrong map
  ...

[deoptimizing (eager): end 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> @1 => node=0, pc=0x7f0d956522e0, caller sp=0x7ffed15d2990, took 0.355 ms]
# 分割线---------------------------------------------------------------------
[marking 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> for optimized recompilation, reason: small function, ICs with typeinfo: 7/7 (100%), generic ICs: 0/7 (0%)]
[compiling method 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> using TurboFan]
[optimizing 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)> - took 1.435, 2.427, 0.159 ms]
[completed optimizing 0x3c72eab23a99 <JSFunction move (sfi = 0x3c72eab235f9)>]
[compiling method 0x3c72eab238e9 <JSFunction (sfi = 0x3c72eab234e9)> using TurboFan OSR]
[optimizing 0x3c72eab238e9 <JSFunction (sfi = 0x3c72eab234e9)> - took 3.399, 6.299, 0.239 ms]

首先，move函数被标记为可优化的(optimized recompilation)，原因是该函数为small function。然后便开始重新编译以及优化。
之后，move函数再一次被标记为可优化的，原因是hot and stable。这是因为 v8 首先生成的是 ignition bytecode。如果某个函数被重复执行多次，那么TurboFan就会重新生成一些优化后的代码。

以下是获取优化理由的的v8代码。如果该JS函数可被优化，则将在外部的v8函数中，mark该JS函数为待优化的。

OptimizationReason RuntimeProfiler::ShouldOptimize(JSFunction* function,
                                                   JavaScriptFrame* frame) {
  SharedFunctionInfo* shared = function->shared();
  int ticks = function->feedback_vector()->profiler_ticks();

  if (shared->GetBytecodeArray()->length() > kMaxBytecodeSizeForOpt) {
    return OptimizationReason::kDoNotOptimize;
  }

  int ticks_for_optimization =
      kProfilerTicksBeforeOptimization +
      (shared->GetBytecodeArray()->length() / kBytecodeSizeAllowancePerTick);
  // 如果执行次数较多，则标记为HotAndStable
  if (ticks >= ticks_for_optimization) {
    return OptimizationReason::kHotAndStable;
  // 如果函数较小，则为 small function
  } else if (!any_ic_changed_ && shared->GetBytecodeArray()->length() <
                                     kMaxBytecodeSizeForEarlyOpt) {
    // If no IC was patched since the last tick and this function is very
    // small, optimistically optimize it now.
    return OptimizationReason::kSmallFunction;
  } else if (FLAG_trace_opt_verbose) {
    PrintF("[not yet optimizing ");
    function->PrintName();
    PrintF(", not enough ticks: %d/%d and ", ticks,
           kProfilerTicksBeforeOptimization);
    if (any_ic_changed_) {
      PrintF("ICs changed]\n");
    } else {
      PrintF(" too large for small function optimization: %d/%d]\n",
             shared->GetBytecodeArray()->length(), kMaxBytecodeSizeForEarlyOpt);
    }
  }
  return OptimizationReason::kDoNotOptimize;
}

但接下来就开始deopt move函数了，原因是Insufficient type feedback for construct，目标代码是move(new Wall())中的new Wall()。这是因为turboFan的代码优化基于推测，即speculative optimizations。当我们多次执行move(new Player())时，turboFan会猜测move函数的参数总是Player对象，因此将move函数优化为更适合Player对象执行的代码，这样使得Player对象使用move函数时速度将会很快。这种猜想机制需要一种反馈来动态修改猜想，那么这种反馈就是 type feedback，Ignition instructions将利用 type feedback来帮助TurboFan的speculative optimizations。
v8源码中，JSFunction类中存在一个类型为FeedbackVector的成员变量，该FeedbackVector将在JS函数被编译后启用。

因此一旦传入的参数不再是Player类型，即刚刚所说的Wall类型，那么将会使得猜想不成立，因此立即反优化，即销毁一部分的ignition bytecode并重新生成。

需要注意的是，反优化机制(deoptimization)有着巨大的性能成本，应尽量避免反优化的产生。
下一个deopt的原因为wrong map。这里的map可以暂时理解为类型。与上一条deopt的原因类似，所生成的move优化函数只是针对于Player对象，因此一旦传入一个Wall对象，那么传入的类型就与函数中的类型不匹配，所以只能开始反优化。
如果我们在代码中来回使用Player对象和Wall对象，那么TurboFan也会综合考虑，并相应的再次优化代码。

四、turboFan的执行流程

turboFan的代码优化有多条执行流，其中最常见到的是下面这条：

从Runtime_CompileOptimized_Concurrent函数开始，设置并行编译&优化特定的JS函数

// v8\src\runtime\runtime-compiler.cc 46
RUNTIME_FUNCTION(Runtime_CompileOptimized_Concurrent) {
  HandleScope scope(isolate);
  DCHECK_EQ(1, args.length());
  CONVERT_ARG_HANDLE_CHECKED(JSFunction, function, 0);
  StackLimitCheck check(isolate);
  if (check.JsHasOverflowed(kStackSpaceRequiredForCompilation * KB)) {
    return isolate->StackOverflow();
  }
  // 设置并行模式，之后开始编译与优化
  if (!Compiler::CompileOptimized(function, ConcurrencyMode::kConcurrent)) {
    return ReadOnlyRoots(isolate).exception();
  }
  DCHECK(function->is_compiled());
  return function->code();
}

在Compiler::CompileOptimized函数中，继续执行GetOptimizedCode函数，并将可能生成的优化代码传递给JSFunction对象。

// v8\src\compiler.cc
bool Compiler::CompileOptimized(Handle<JSFunction> function,
                                ConcurrencyMode mode) {
  if (function->IsOptimized()) return true;
  Isolate* isolate = function->GetIsolate();
  DCHECK(AllowCompilation::IsAllowed(isolate));

  // Start a compilation.
  Handle<Code> code;
  if (!GetOptimizedCode(function, mode).ToHandle(&code)) {
    // Optimization failed, get unoptimized code. Unoptimized code must exist
    // already if we are optimizing.
    DCHECK(!isolate->has_pending_exception());
    DCHECK(function->shared()->is_compiled());
    DCHECK(function->shared()->IsInterpreted());
    code = BUILTIN_CODE(isolate, InterpreterEntryTrampoline);
  }

  // Install code on closure.
  function->set_code(*code);

  // Check postconditions on success.
  DCHECK(!isolate->has_pending_exception());
  DCHECK(function->shared()->is_compiled());
  DCHECK(function->is_compiled());
  DCHECK_IMPLIES(function->HasOptimizationMarker(),
                 function->IsInOptimizationQueue());
  DCHECK_IMPLIES(function->HasOptimizationMarker(),
                 function->ChecksOptimizationMarker());
  DCHECK_IMPLIES(function->IsInOptimizationQueue(),
                 mode == ConcurrencyMode::kConcurrent);
  return true;
}

GetOptimizedCode的函数代码如下：

// v8\src\compiler.cc
MaybeHandle<Code> GetOptimizedCode(Handle<JSFunction> function,
                                   ConcurrencyMode mode,
                                   BailoutId osr_offset = BailoutId::None(),
                                   JavaScriptFrame* osr_frame = nullptr) {
  Isolate* isolate = function->GetIsolate();
  Handle<SharedFunctionInfo> shared(function->shared(), isolate);

  // Make sure we clear the optimization marker on the function so that we
  // don't try to re-optimize.
  if (function->HasOptimizationMarker()) {
    function->ClearOptimizationMarker();
  }

  if (isolate->debug()->needs_check_on_function_call()) {
    // Do not optimize when debugger needs to hook into every call.
    return MaybeHandle<Code>();
  }

  Handle<Code> cached_code;
  if (GetCodeFromOptimizedCodeCache(function, osr_offset)
          .ToHandle(&cached_code)) {
    if (FLAG_trace_opt) {
      PrintF("[found optimized code for ");
      function->ShortPrint();
      if (!osr_offset.IsNone()) {
        PrintF(" at OSR AST id %d", osr_offset.ToInt());
      }
      PrintF("]\n");
    }
    return cached_code;
  }

  // Reset profiler ticks, function is no longer considered hot.
  DCHECK(shared->is_compiled());
  function->feedback_vector()->set_profiler_ticks(0);

  VMState<COMPILER> state(isolate);
  DCHECK(!isolate->has_pending_exception());
  PostponeInterruptsScope postpone(isolate);
  bool has_script = shared->script()->IsScript();
  // BUG(5946): This DCHECK is necessary to make certain that we won't
  // tolerate the lack of a script without bytecode.
  DCHECK_IMPLIES(!has_script, shared->HasBytecodeArray());
  std::unique_ptr<OptimizedCompilationJob> job(
      compiler::Pipeline::NewCompilationJob(isolate, function, has_script));
  OptimizedCompilationInfo* compilation_info = job->compilation_info();

  compilation_info->SetOptimizingForOsr(osr_offset, osr_frame);

  // Do not use TurboFan if we need to be able to set break points.
  if (compilation_info->shared_info()->HasBreakInfo()) {
    compilation_info->AbortOptimization(BailoutReason::kFunctionBeingDebugged);
    return MaybeHandle<Code>();
  }

  // Do not use TurboFan when %NeverOptimizeFunction was applied.
  if (shared->optimization_disabled() &&
      shared->disable_optimization_reason() ==
          BailoutReason::kOptimizationDisabledForTest) {
    compilation_info->AbortOptimization(
        BailoutReason::kOptimizationDisabledForTest);
    return MaybeHandle<Code>();
  }

  // Do not use TurboFan if optimization is disabled or function doesn't pass
  // turbo_filter.
  if (!FLAG_opt || !shared->PassesFilter(FLAG_turbo_filter)) {
    compilation_info->AbortOptimization(BailoutReason::kOptimizationDisabled);
    return MaybeHandle<Code>();
  }

  TimerEventScope<TimerEventOptimizeCode> optimize_code_timer(isolate);
  RuntimeCallTimerScope runtimeTimer(isolate,
                                     RuntimeCallCounterId::kOptimizeCode);
  TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.compile"), "V8.OptimizeCode");

  // In case of concurrent recompilation, all handles below this point will be
  // allocated in a deferred handle scope that is detached and handed off to
  // the background thread when we return.
  base::Optional<CompilationHandleScope> compilation;
  if (mode == ConcurrencyMode::kConcurrent) {
    compilation.emplace(isolate, compilation_info);
  }

  // All handles below will be canonicalized.
  CanonicalHandleScope canonical(isolate);

  // Reopen handles in the new CompilationHandleScope.
  compilation_info->ReopenHandlesInNewHandleScope(isolate);

  if (mode == ConcurrencyMode::kConcurrent) {
    if (GetOptimizedCodeLater(job.get(), isolate)) {
      job.release();  // The background recompile job owns this now.

      // Set the optimization marker and return a code object which checks it.
      function->SetOptimizationMarker(OptimizationMarker::kInOptimizationQueue);
      DCHECK(function->IsInterpreted() ||
             (!function->is_compiled() && function->shared()->IsInterpreted()));
      DCHECK(function->shared()->HasBytecodeArray());
      return BUILTIN_CODE(isolate, InterpreterEntryTrampoline);
    }
  } else {
    if (GetOptimizedCodeNow(job.get(), isolate))
      return compilation_info->code();
  }

  if (isolate->has_pending_exception()) isolate->clear_pending_exception();
  return MaybeHandle<Code>();
}

函数代码有点长，这里总结一下所做的操作：

如果之前该函数被mark为待优化的，则取消该mark（回想一下--trace-opt的输出）
如果debugger需要hook该函数，或者在该函数上下了断点，则不优化该函数，直接返回。
如果之前已经优化过该函数（存在OptimizedCodeCache），则直接返回之前优化后的代码。
重置当前函数的profiler ticks，使得该函数不再hot，这样做的目的是使当前函数不被重复优化。
如果设置了一些禁止优化的参数（例如%NeverOptimizeFunction，或者设置了turbo_filter），则取消当前函数的优化。
以上步骤完成后则开始优化代码，优化代码也有两种不同的方式，分别是并行优化和非并行优化。在大多数情况下执行的都是并行优化，因为速度更快。并行优化会先执行GetOptimizedCodeLater函数，在该函数中判断一些异常条件，例如任务队列已满或者内存占用过高。如果没有异常条件，则执行OptimizedCompilationJob::PrepareJob函数，并继续在更深层次的调用PipelineImpl::CreateGraph来建图。如果GetOptimizedCodeLater函数工作正常，则将会把优化任务Job放入任务队列中。任务队列将安排另一个线程执行优化操作。
另一个线程的栈帧如下，该线程将执行Job->ExecuteJob并在更深层次调用PipelineImpl::OptimizeGraph来优化之前建立的图结构：

当另一个线程在优化代码时，主线程可以继续执行其他任务：

综上我们可以得知，JIT最终的优化位于PipelineImpl类中，包括建图以及优化图等

// v8\src\compiler\pipeline.cc
class PipelineImpl final {
 public:
  explicit PipelineImpl(PipelineData* data) : data_(data) {}

  // Helpers for executing pipeline phases.
  template <typename Phase>
  void Run();
  template <typename Phase, typename Arg0>
  void Run(Arg0 arg_0);
  template <typename Phase, typename Arg0, typename Arg1>
  void Run(Arg0 arg_0, Arg1 arg_1);

  // Step A. Run the graph creation and initial optimization passes.
  bool CreateGraph();

  // B. Run the concurrent optimization passes.
  bool OptimizeGraph(Linkage* linkage);

  // Substep B.1. Produce a scheduled graph.
  void ComputeScheduledGraph();

  // Substep B.2. Select instructions from a scheduled graph.
  bool SelectInstructions(Linkage* linkage);

  // Step C. Run the code assembly pass.
  void AssembleCode(Linkage* linkage);

  // Step D. Run the code finalization pass.
  MaybeHandle<Code> FinalizeCode();

  // Step E. Install any code dependencies.
  bool CommitDependencies(Handle<Code> code);

  void VerifyGeneratedCodeIsIdempotent();
  void RunPrintAndVerify(const char* phase, bool untyped = false);
  MaybeHandle<Code> GenerateCode(CallDescriptor* call_descriptor);
  void AllocateRegisters(const RegisterConfiguration* config,
                         CallDescriptor* call_descriptor, bool run_verifier);

  OptimizedCompilationInfo* info() const;
  Isolate* isolate() const;
  CodeGenerator* code_generator() const;

 private:
  PipelineData* const data_;
};

五、初探optimization phases

1. 简介

与LLVM IR的各种Pass类似，turboFan中使用各类phases进行建图、搜集信息以及简化图。

以下是PipelineImpl::CreateGraph函数源码，其中使用了大量的Phase。这些Phase有些用于建图，有些用于优化（在建图时也会执行一部分简单的优化），还有些为接下来的优化做准备：

bool PipelineImpl::CreateGraph() {
  PipelineData* data = this->data_;

  data->BeginPhaseKind("graph creation");

  if (info()->trace_turbo_json_enabled() ||
      info()->trace_turbo_graph_enabled()) {
    CodeTracer::Scope tracing_scope(data->GetCodeTracer());
    OFStream os(tracing_scope.file());
    os << "---------------------------------------------------\n"
       << "Begin compiling method " << info()->GetDebugName().get()
       << " using Turbofan" << std::endl;
  }
  if (info()->trace_turbo_json_enabled()) {
    TurboCfgFile tcf(isolate());
    tcf << AsC1VCompilation(info());
  }

  data->source_positions()->AddDecorator();
  if (data->info()->trace_turbo_json_enabled()) {
    data->node_origins()->AddDecorator();
  }

  Run<GraphBuilderPhase>();
  RunPrintAndVerify(GraphBuilderPhase::phase_name(), true);

  // Perform function context specialization and inlining (if enabled).
  Run<InliningPhase>();
  RunPrintAndVerify(InliningPhase::phase_name(), true);

  // Remove dead->live edges from the graph.
  Run<EarlyGraphTrimmingPhase>();
  RunPrintAndVerify(EarlyGraphTrimmingPhase::phase_name(), true);

  // Run the type-sensitive lowerings and optimizations on the graph.
  {
    // Determine the Typer operation flags.
    Typer::Flags flags = Typer::kNoFlags;
    if (is_sloppy(info()->shared_info()->language_mode()) &&
        info()->shared_info()->IsUserJavaScript()) {
      // Sloppy mode functions always have an Object for this.
      flags |= Typer::kThisIsReceiver;
    }
    if (IsClassConstructor(info()->shared_info()->kind())) {
      // Class constructors cannot be [[Call]]ed.
      flags |= Typer::kNewTargetIsReceiver;
    }

    // Type the graph and keep the Typer running on newly created nodes within
    // this scope; the Typer is automatically unlinked from the Graph once we
    // leave this scope below.
    Typer typer(isolate(), data->js_heap_broker(), flags, data->graph());
    Run<TyperPhase>(&typer);
    RunPrintAndVerify(TyperPhase::phase_name());

    // Do some hacky things to prepare for the optimization phase.
    // (caching handles, etc.).
    Run<ConcurrentOptimizationPrepPhase>();

    if (FLAG_concurrent_compiler_frontend) {
      data->js_heap_broker()->SerializeStandardObjects();
      Run<CopyMetadataForConcurrentCompilePhase>();
    }

    // Lower JSOperators where we can determine types.
    Run<TypedLoweringPhase>();
    RunPrintAndVerify(TypedLoweringPhase::phase_name());
  }

  data->EndPhaseKind();

  return true;
}

PipelineImpl::OptimizeGraph函数代码如下，该函数将会对所建立的图进行优化：

bool PipelineImpl::OptimizeGraph(Linkage* linkage) {
  PipelineData* data = this->data_;

  data->BeginPhaseKind("lowering");

  if (data->info()->is_loop_peeling_enabled()) {
    Run<LoopPeelingPhase>();
    RunPrintAndVerify(LoopPeelingPhase::phase_name(), true);
  } else {
    Run<LoopExitEliminationPhase>();
    RunPrintAndVerify(LoopExitEliminationPhase::phase_name(), true);
  }

  if (FLAG_turbo_load_elimination) {
    Run<LoadEliminationPhase>();
    RunPrintAndVerify(LoadEliminationPhase::phase_name());
  }

  if (FLAG_turbo_escape) {
    Run<EscapeAnalysisPhase>();
    if (data->compilation_failed()) {
      info()->AbortOptimization(
          BailoutReason::kCyclicObjectStateDetectedInEscapeAnalysis);
      data->EndPhaseKind();
      return false;
    }
    RunPrintAndVerify(EscapeAnalysisPhase::phase_name());
  }

  // Perform simplified lowering. This has to run w/o the Typer decorator,
  // because we cannot compute meaningful types anyways, and the computed types
  // might even conflict with the representation/truncation logic.
  Run<SimplifiedLoweringPhase>();
  RunPrintAndVerify(SimplifiedLoweringPhase::phase_name(), true);

  // From now on it is invalid to look at types on the nodes, because the types
  // on the nodes might not make sense after representation selection due to the
  // way we handle truncations; if we'd want to look at types afterwards we'd
  // essentially need to re-type (large portions of) the graph.

  // In order to catch bugs related to type access after this point, we now
  // remove the types from the nodes (currently only in Debug builds).
#ifdef DEBUG
  Run<UntyperPhase>();
  RunPrintAndVerify(UntyperPhase::phase_name(), true);
#endif

  // Run generic lowering pass.
  Run<GenericLoweringPhase>();
  RunPrintAndVerify(GenericLoweringPhase::phase_name(), true);

  data->BeginPhaseKind("block building");

  // Run early optimization pass.
  Run<EarlyOptimizationPhase>();
  RunPrintAndVerify(EarlyOptimizationPhase::phase_name(), true);

  Run<EffectControlLinearizationPhase>();
  RunPrintAndVerify(EffectControlLinearizationPhase::phase_name(), true);

  if (FLAG_turbo_store_elimination) {
    Run<StoreStoreEliminationPhase>();
    RunPrintAndVerify(StoreStoreEliminationPhase::phase_name(), true);
  }

  // Optimize control flow.
  if (FLAG_turbo_cf_optimization) {
    Run<ControlFlowOptimizationPhase>();
    RunPrintAndVerify(ControlFlowOptimizationPhase::phase_name(), true);
  }

  // Optimize memory access and allocation operations.
  Run<MemoryOptimizationPhase>();
  // TODO(jarin, rossberg): Remove UNTYPED once machine typing works.
  RunPrintAndVerify(MemoryOptimizationPhase::phase_name(), true);

  // Lower changes that have been inserted before.
  Run<LateOptimizationPhase>();
  // TODO(jarin, rossberg): Remove UNTYPED once machine typing works.
  RunPrintAndVerify(LateOptimizationPhase::phase_name(), true);

  data->source_positions()->RemoveDecorator();
  if (data->info()->trace_turbo_json_enabled()) {
    data->node_origins()->RemoveDecorator();
  }

  ComputeScheduledGraph();

  return SelectInstructions(linkage);
}

由于上面两个函数涉及到的Phase众多，这里请各位自行阅读源码来了解各个Phase的具体功能。

接下来我们只介绍几个比较重要的Phases：GraphBuilderPhase、TyperPhase和SimplifiedLoweringPhase。

2. GraphBuilderPhase

GraphBuilderPhase将遍历字节码，并建一个初始的图，这个图将用于接下来Phase的处理，包括但不限于各种代码优化。
一个简单的例子

3. TyperPhase

TyperPhase将会遍历整个图的所有结点，并给每个结点设置一个Type属性，该操作将在建图完成后被执行

给每个结点设置Type的操作是不是极其类似于编译原理中的语义分析呢？ XD

bool PipelineImpl::CreateGraph() {
  // ...
  Run<GraphBuilderPhase>();
  RunPrintAndVerify(GraphBuilderPhase::phase_name(), true);
  // ...
  // Run the type-sensitive lowerings and optimizations on the graph.
  {
    // ...

    // Type the graph and keep the Typer running on newly created nodes within
    // this scope; the Typer is automatically unlinked from the Graph once we
    // leave this scope below.
    Typer typer(isolate(), data->js_heap_broker(), flags, data->graph());
    Run<TyperPhase>(&typer);
    RunPrintAndVerify(TyperPhase::phase_name());

    // ...
  }
  // ...
}

其中，具体执行的是TyperPhase::Run函数：

struct TyperPhase {
  static const char* phase_name() { return "typer"; }

  void Run(PipelineData* data, Zone* temp_zone, Typer* typer) {
    // ...
    typer->Run(roots, &induction_vars);
  }
};

在该函数中继续调用Typer::Run函数，并在GraphReducer::ReduceGraph函数中最终调用到Typer::Visitor::Reduce函数：

void Typer::Run(const NodeVector& roots,
                LoopVariableOptimizer* induction_vars) {
  // ...
  Visitor visitor(this, induction_vars);
  GraphReducer graph_reducer(zone(), graph());
  graph_reducer.AddReducer(&visitor);
  for (Node* const root : roots) graph_reducer.ReduceNode(root);
  graph_reducer.ReduceGraph();
  // ...
}

在Typer::Visitor::Reduce函数中存在一个较大的switch结构，通过该switch结构，当Visitor遍历每个node时，即可最终调用到对应的XXXTyper函数。

例如，对于一个JSCall结点，将在TyperPhase中最终调用到Typer::Visitor::JSCallTyper

这里我们简单看一下JSCallTyper函数源码，该函数中存在一个很大的switch结构，该结构将设置每个Builtin函数结点的Type属性，即函数的返回值类型。

Type Typer::Visitor::JSCallTyper(Type fun, Typer* t) {
  if (!fun.IsHeapConstant() || !fun.AsHeapConstant()->Ref().IsJSFunction()) {
    return Type::NonInternal();
  }
  JSFunctionRef function = fun.AsHeapConstant()->Ref().AsJSFunction();
  if (!function.shared().HasBuiltinFunctionId()) {
    return Type::NonInternal();
  }
  switch (function.shared().builtin_function_id()) {
    case BuiltinFunctionId::kMathRandom:
      return Type::PlainNumber();
  // ...

而对于一个常数NumberConstant类型，TyperPhase也会打上一个对应的类型

Type Typer::Visitor::TypeNumberConstant(Node* node) 
  // 注意这里使用的是double，这也就说明了为什么Number.MAX_SAFE_INTEGER = 9007199254740991
  double number = OpParameter<double>(node->op());
  return Type::NewConstant(number, zone());
}

而在Type::NewConstant函数中，我们会发现一个神奇的设计：

Type Type::NewConstant(double value, Zone* zone) {
  // 对于一个正常的整数
  if (RangeType::IsInteger(value)) {
    // 实际上所设置的Type是一个range！
    return Range(value, value, zone);
  // 否则如果是一个异常的-0,则返回对应的MinusZero
  } else if (IsMinusZero(value)) {
    return Type::MinusZero();
  // 如果是NAN，则返回NaN
  } else if (std::isnan(value)) {
    return Type::NaN();
  }

  DCHECK(OtherNumberConstantType::IsOtherNumberConstant(value));
  return OtherNumberConstant(value, zone);
}

对于JS代码中的一个NumberConstant，实际上设置的Type是一个Range，只不过这个Range的首尾范围均是该数，例如NumberConstant(3) => Range(3, 3, zone)

以下这张图可以证明TyperPhase正如预期那样执行：

与之相应的，v8采用了SSA。因此对于一个Phi结点，它将设置该节点的Type为几个可能值的Range的并集。

Type Typer::Visitor::TypePhi(Node* node) {
  int arity = node->op()->ValueInputCount();
  Type type = Operand(node, 0);
  for (int i = 1; i < arity; ++i) {
    type = Type::Union(type, Operand(node, i), zone());
  }
  return type;
}

请看以下示例：

4. SimplifiedLoweringPhase

SimplifiedLoweringPhase会遍历结点做一些处理，同时也会对图做一些优化操作。这里我们只关注该Phase优化CheckBound的细节，因为CheckBound通常是用于判断 JS数组（例如ArrayBuffer）是否越界使用所设置的结点。

首先我们可以通过以下路径来找到优化CheckBound的目标代码：

SimplifiedLoweringPhase::Run
    SimplifiedLowering::LowerAllNodes
      RepresentationSelector::Run
        RepresentationSelector::VisitNode

目标代码如下:

  // Dispatching routine for visiting the node {node} with the usage {use}.
    // Depending on the operator, propagate new usage info to the inputs.
    void VisitNode(Node* node, Truncation truncation,
                  SimplifiedLowering* lowering) {
      // Unconditionally eliminate unused pure nodes (only relevant if there's
      // a pure operation in between two effectful ones, where the last one
      // is unused).
      // Note: We must not do this for constants, as they are cached and we
      // would thus kill the cached {node} during lowering (i.e. replace all
      // uses with Dead), but at that point some node lowering might have
      // already taken the constant {node} from the cache (while it was in
      // a sane state still) and we would afterwards replace that use with
      // Dead as well.
      if (node->op()->ValueInputCount() > 0 &&
          node->op()->HasProperty(Operator::kPure)) {
        if (truncation.IsUnused()) return VisitUnused(node);
      }
      switch (node->opcode()) {
        // ...
          case IrOpcode::kCheckBounds: {
              const CheckParameters& p = CheckParametersOf(node->op());
              Type index_type = TypeOf(node->InputAt(0));
              Type length_type = TypeOf(node->InputAt(1));
              if (index_type.Is(Type::Integral32OrMinusZero())) {
                // Map -0 to 0, and the values in the [-2^31,-1] range to the
                // [2^31,2^32-1] range, which will be considered out-of-bounds
                // as well, because the {length_type} is limited to Unsigned31.
                VisitBinop(node, UseInfo::TruncatingWord32(),
                          MachineRepresentation::kWord32);
                if (lower() && lowering->poisoning_level_ ==
                                  PoisoningMitigationLevel::kDontPoison) {
                  // 可以看到，如果当前索引的最大值小于length的最小值，则表示当前索引的使用没有越界
                  if (index_type.IsNone() || length_type.IsNone() ||
                      (index_type.Min() >= 0.0 &&
                      index_type.Max() < length_type.Min())) {
                    // The bounds check is redundant if we already know that
                    // the index is within the bounds of [0.0, length[.
                    // CheckBound将会被优化
                    DeferReplacement(node, node->InputAt(0));
                  }
                }
              } else {
                VisitBinop(
                    node,
                    UseInfo::CheckedSigned32AsWord32(kIdentifyZeros, p.feedback()),
                    UseInfo::TruncatingWord32(), MachineRepresentation::kWord32);
              }
              return;
            }
      // ....
      }
      // ...
    }

可以看到，在CheckBound的优化判断逻辑中，如果当前索引的最大值小于length的最小值，则表示当前索引的使用没有越界，此时将会移除CheckBound结点以简化IR图。

需要注意NumberConstant结点的Type是一个Range类型，因此才会有最大值Max和最小值Min的概念。

这里需要解释一下环境搭配中所说的，为什么要添加一个编译参数v8_optimized_debug = false，注意看上面判断条件中的这行条件

if (lower() && lowering->poisoning_level_ ==
                                  PoisoningMitigationLevel::kDontPoison)

visitNode时有三个状态，分别是Phase::PROPAGATE（信息收集）、Phase::RETYPE（从类型反馈中获取类型）以及Phase::LOWER（开始优化）。当真正开始优化时，lower()条件自然成立，因此我们无需处理这个。

但对于下一个条件，通过动态调试可以得知，poisoning_level始终不为PoisoningMitigationLevel::kDontPoison。通过追溯lowering->poisoning_level_，我们可以发现它实际上在PipelineCompilationJob::PrepareJobImpl中被设置

PipelineCompilationJob::Status PipelineCompilationJob::PrepareJobImpl(
    Isolate* isolate) {
  // ...
// Compute and set poisoning level.
  PoisoningMitigationLevel load_poisoning =
      PoisoningMitigationLevel::kDontPoison;
  if (FLAG_branch_load_poisoning) {
    load_poisoning = PoisoningMitigationLevel::kPoisonAll;
  } else if (FLAG_untrusted_code_mitigations) {
    load_poisoning = PoisoningMitigationLevel::kPoisonCriticalOnly;
  }
  // ...
}

而FLAG_branch_load_poisoning始终为false，FLAG_untrusted_code_mitigations始终为true

编译参数v8_untrusted_code_mitigations 默认 true，使得宏DISABLE_UNTRUSTED_CODE_MITIGATIONS没有被定义，因此默认设置FLAG_untrusted_code_mitigations = true

// v8/src/flag-definitions.h

// 设置`FLAG_untrusted_code_mitigations`
#ifdef DISABLE_UNTRUSTED_CODE_MITIGATIONS
#define V8_DEFAULT_UNTRUSTED_CODE_MITIGATIONS false
#else
#define V8_DEFAULT_UNTRUSTED_CODE_MITIGATIONS true
#endif
DEFINE_BOOL(untrusted_code_mitigations, V8_DEFAULT_UNTRUSTED_CODE_MITIGATIONS,
            "Enable mitigations for executing untrusted code")
#undef V8_DEFAULT_UNTRUSTED_CODE_MITIGATIONS

// 设置`FLAG_branch_load_poisoning`
DEFINE_BOOL(branch_load_poisoning, false, "Mask loads with branch conditions.")

# BUILD.gn
declare_args() {
  # ...
  # Enable mitigations for executing untrusted code.
  # 默认为true
  v8_untrusted_code_mitigations = true
  # ...
}
# ...
if (!v8_untrusted_code_mitigations) {
    defines += [ "DISABLE_UNTRUSTED_CODE_MITIGATIONS" ]
  }
# ...

这样就会使得load_poisoning始终为PoisoningMitigationLevel::kPoisonCriticalOnly，因此始终无法执行checkBounds的优化操作。所以我们需要手动设置编译参数v8_untrusted_code_mitigations = false，以启动checkbounds的优化。

以下是一个简单checkbounds优化的例子

function f(x)
{
  const arr = new Array(1.1, 2.2, 3.3, 4.4, 5.5);
  let t = 1 + 1;
  return arr[t];
}

console.log(f(1));
%OptimizeFunctionOnNextCall(f);
console.log(f(1));

优化前发现存在一个checkBounds：

执行完SimplifiedLoweringPhase后，CheckBounds被优化了:

（完）