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Commit 3641951

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add eventloop TODO
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‎201709/runtime.md‎

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#### JavaScript 是如何执行的
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对于常见编译型语言(例如:Java)来说,编译步骤分为:词法分析->语法分析->语义检查->代码优化和字节生成。
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对于常见编译型语言(例如:Java)来说,编译步骤分为:词法分析->语法分析->语义检查->代码优化和字节码生成。
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对于解释型语言(例如 JavaScript)来说,通过词法分析 -> 语法分析 -> 语法树,就可以开始解释执行了。
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我们常说的 V8 是 Google 发布的开源 JavaScript 引擎,采用 C++ 编写。SpiderMonkey(Mozilla,基于 C)、Rhino(Mozilla,基于 Java),而 Nodejs 依赖于 V8 引擎开发,接下来的内容是 JavaScript 在 V8 引擎中的运行状态,而类似的 JavaScript 现代引擎对于这些实现大同小异。
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在本文的开头提到了编译型语言,解释型语言。JavaScript 是解释型语言,在生成 AST 之后,就开始一边解释,一边执行,但是有个弊端,当某段代码被多次执行时,它就有了可优化的空间(后面提到),而不用一次次的去重复之前的解释执行。
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在本文的开头提到了编译型语言,解释型语言。JavaScript 是解释型语言且`无类型`,在生成 AST 之后,就开始一边解释,一边执行,但是有个弊端,当某段代码被多次执行时,它就有了可优化的空间(比如类型判断优化),而不用一次次的去重复之前的解释执行。
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编译型语言如 JAVA,可以在执行前就进行优化编译,但是这会耗费大量的时间,显然不适用于 Web 交互。
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于是就有了,JIT(Just-in-time),JIT 是两种模式的混合。
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1.在 JavaScript 引擎中增加一个监视器(也叫分析器)。监视器监控着代码的运行情况,记录代码一共运行了多少次、如何运行的等信息,如果同一行代码运行了几次,这个代码段就被标记成了 "warm",如果运行了很多次,则被标记成 "hot"。
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2.(基线编译器)如果一段代码变成了 "warm",那么 JIT 就把它送到编译器去编译,并且把编译结果存储起来。比如,监视器监视到了,某行、某个变量执行同样的代码、使用了同样的变量类型,那么就会把编译后的版本,替换这一行代码的执行,并且存储。
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2.(基线编译器)如果一段代码变成了 "warm",那么 JIT 就把它送到基线编译器去编译,并且把编译结果存储起来。比如,监视器监视到了,某行、某个变量执行同样的代码、使用了同样的变量类型,那么就会把编译后的版本,替换这一行代码的执行,并且存储。
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3.(优化编译器)如果一个代码段变得 "hot",监视器会把它发送到优化编译器中。生成一个更快速和高效的代码版本出来,并且存储。例如:循环加一个对象属性时,假设它是 INT 类型,优先做 INT 类型的判断
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4.(去优化)可是对于 JavaScript 从来就没有确定这么一说,前 99 个对象属性保持着 INT 类型,可能第 100 个就没有这个属性了,那么这时候 JIT 会认为做了一个错误的假设,并且把优化代码丢掉,执行过程将会回到解释器或者基线编译器,这一过程叫做去优化。
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###### 优化代码图例:
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’hot‘ 代码
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"hot" 代码
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![](../assets/2017_09_03.jpg)
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‎201710/eventloop.md‎

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# Event Loop
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// TODO
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------
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作者:肖沐宸,[github](https://github.com/cheogo)。

‎GLOSSARY.md‎

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- [JavaScript 有哪些引擎](http://www.cnblogs.com/gdutbean/archive/2012/02/21/2362003.html)
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- [V8 Design Elements](https://my.oschina.net/sub/blog/152628)
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- [JavaScript Just-in-time (JIT) 工作原理](http://huziketang.com/blog/posts/detail?postId=58c12f36a6d8a07e449fdd22)
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- [JS特性性能缺陷及JIT的解决方案](http://www.cnblogs.com/hyddd/archive/2013/02/06/2908110.html)
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- [JS特性性能缺陷及JIT的解决方案](http://www.cnblogs.com/hyddd/archive/2013/02/06/2908110.html)
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## 参考文章-201710-eventloop
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‎README.md‎

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- [序章](https://cheogo.github.io/learn-javascript/201709/preface.html)
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- [JavaScript 起源](https://cheogo.github.io/learn-javascript/201709/origin.html)
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- [JavaScript 语法解析、AST、V8、JIT](https://cheogo.github.io/learn-javascript/201709/runtime.html)
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- [JavaScript 语法解析、AST、V8、JIT](https://cheogo.github.io/learn-javascript/201709/runtime.html)
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- [Event loop](https://cheogo.github.io/learn-javascript/201709/eventloop.html)

‎SUMMARY.md‎

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* [序章](201709/preface.md)
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* [JavaScript 起源](201709/origin.md)
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* [JavaScript 语法解析、AST、V8、JIT](201709/runtime.md)
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* [Event Loop](201709/eventloop.md)

‎docs/201709/origin.html‎

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Event Loop
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‎docs/201709/preface.html‎

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