Chrome浏览器引擎 Blink ＆ V8

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"背景"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这篇文章是我的前端技术系列文章中浏览器工作原理栏目中的第二篇。浏览器引擎（也被称作布局引擎或渲染引擎）是浏览器的重要组成部分。浏览器引擎最重要的工作就是将HTML文本和其他页面中的资源转换成可以与用户产生交互的页面。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了浏览器引擎外，布局引擎和渲染引擎是另外两个相关的概念，理论上，两个引擎可以独立实现，但在实际情况中，往往很少将二者分开实现。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了包含布局和渲染引擎外，浏览器引擎还遵循"},{"type":"link","attrs":{"href":"https://en.wikipedia.org/wiki/Content_Security_Policy","title":null},"content":[{"type":"text","text":"文档安全策略（Content Security Policy）"}]},{"type":"text","text":"以保证站点间相互独立。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在运行JavaScript代码的功能上，基本上主流的浏览器都使用独立的引擎，起初JavaScript语言只被用于在浏览器中使用，但现在JavaScript几乎可以在任何地方使用，这需要JavaScript引擎可以独立于浏览器单独使用。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而像"},{"type":"link","attrs":{"href":"https://en.wikipedia.org/wiki/Electron_(software_framework)","title":null},"content":[{"type":"text","text":"Electron framework"}]},{"type":"text","text":"这样的技术就是整合Chromium的渲染引擎和Nodejs而实现的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我想通过这篇文章把V8中的技术尽可能的详述，涵盖的内容会比较多，可反复阅读 :)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"常见的浏览器引擎"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"浏览器引擎是Web平台技术中一系列标准（HTML、CSS、ECMAScript、WebGL、Web Storage等等）的具体实现，不同的浏览器引擎在遵循同样的标准下，还实现了额外的功能。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Gecko是Mozilla的浏览器引擎，在Firefox中使用，"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#333333","name":"user"}}],"text":"SpiderMonkey是Firefox的JavaScript引擎。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Apple为Safari浏览器创造了Webkit引擎，Webkit引擎内置了"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#333333","name":"user"}}],"text":"JavaScriptCore引擎。虽然Apple允许在IOS设备上可以使用其他的浏览器代替Safari，但所用通过App Store分发的浏览器必须使用Webkit引擎。例如，Opera Mini浏览器在IOS设备上使用Webkit引擎，而在其他设备上使用Blink引擎。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Google起初使用Webkit作为Chrome浏览器的引擎，后来以Webkit引擎为基础创造了Blink引擎，所有基于Chromium开源浏览器衍生的产品都使用blink引擎。而大名鼎鼎的V8引擎就是Chromium-based浏览器的JavaScript引擎。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Microsoft维护着自己的EdgeHTML引擎，作为老的Trident引擎的替代方案。新的Edge的浏览器已经开始使用Chromium的Blink引擎了，而EdgeHTML引擎只在window 10上的"},{"type":"link","attrs":{"href":"https://en.wikipedia.org/wiki/Universal_Windows_Platform","title":null},"content":[{"type":"text","text":"Universal Windows 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WebView和Opera这样的应用中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"进程/线程架构"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"进程"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Chromium拥有一套多进程架构。Chromium有一个浏览器进程和多个带有沙盒能力的渲染进程。Blink则运行在渲染进程中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"从安全的角度考虑，让不同的站点保持相互隔离是非常重要的，这被称作"},{"type":"text","marks":[{"type":"strong"}],"text":"站点隔离（Site 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str;\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"转化成AST后的结构是这样的"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/7e/7e62d86a0d4c4277bc91ad76a679f88d.png","alt":null,"title":"","style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}},{"type":"strong"}],"text":"FunctionLiteral"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}}],"text":"代表sayHi函数，Block代表函数体。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}}],"text":"左边的"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}},{"type":"strong"}],"text":"VariableDeclaration"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}}],"text":"代表变量str的声明，"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}},{"type":"strong"}],"text":"Assignment"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}}],"text":"代表赋值，"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}},{"type":"strong"}],"text":"ReturnStatement"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}}],"text":"代表return语句。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这里的"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}},{"type":"strong"}],"text":"VariableProxy"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}}],"text":"代表对变量的引用，在目前这个阶段还不知道是引用哪个变量。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在分析这个阶段除了要生成AST，还要分析作用域。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/1a/1ac03cdfbd40f995afbd50da9ee3b5a8.png","alt":null,"title":"","style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过分析，全局作用域中包含函数sayHi的声明，而sayHi的函数作用域中包含变量str的声明。而两个"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}},{"type":"strong"}],"text":"VariableProxy"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#323232","name":"user"}}],"text":"所引用的变量也可以确定下来。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"color","attrs":{"color":"#202122","name":"user"}}],"text":"V8有两种分析器：Preparser和Full parser。Preparser分析器可以推迟那些不是立即需要分析的函数以减少代码启动需要的时间。Preparser只会处理语法分析和一些错误的检查而不会生成抽象语法树。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"V8在这个阶段对各种类型的标记的扫描有着各种的优化手段，感兴趣的同学可以继续阅读"},{"type":"link","attrs":{"href":"https://v8.dev/blog/scanner","title":null},"content":[{"type":"text","text":"Blazingly fast parsing, part 1: optimizing the scanner"}]},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在这里推荐一个用于分析抽象语法树的网站："},{"type":"link","attrs":{"href":"https://astexplorer.net/","title":null},"content":[{"type":"text","text":"AST Explorer"}]},{"type":"text","text":"。在各种lint自定义规则，babel、webpack插件等代码分析、生成的场景里都有帮助。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"解释（I"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}}],"text":"nterpreting"},{"type":"text","text":"）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"解释阶段会将AST转换成字节码（bytecode）。得益于"},{"type":"link","attrs":{"href":"https://en.wikipedia.org/wiki/Just-in-time_compilation","title":null},"content":[{"type":"text","text":"即时编译（just-in-time (JIT) compilation）"}]},{"type":"text","text":"技术，包括V8在内的现代浏览器JavaScript引擎结合了"},{"type":"text","marks":[{"type":"strong"}],"text":"提前编译（AOT）"},{"type":"text","text":"的高性能和解释的灵活性。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"起初，代码首先被编译器快速的编译成没有被优化过的机器码，在运行的同时再有选择的将需要优化的代码通过更高级的编译器进行优化再编译。这样做虽然提高了运行速度，但也浪费了资源。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其中比较显著的问题是被编译过的机器码会占用大量的内存，即使有的代码可能只会被执行一次。为了解决这些问题，V8团队提出了新的JavaScript解释器，"},{"type":"link","attrs":{"href":"https://v8.dev/docs/ignition","title":null},"content":[{"type":"text","text":"Ignition"}]},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}}],"text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"借助"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}},{"type":"strong"}],"text":"Ignition"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}}],"text":"，V8可以将AST先转化成更简洁的字节码，其大小与以往的机器码相比缩小到50%至25%的空间。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/35/355cb34cfd38811ba0c6e19ab7e7025c.png","alt":null,"title":"","style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":"","fromPaste":false,"pastePass":false}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}},{"type":"strong"}],"text":"Ignition"},{"type":"text","marks":[{"type":"color","attrs":{"color":"#000000","name":"user"}}],"text":"生成的"},{"type":"text","text":"字节码会被用于优化的编译器和调试工具当作数据源，当字节码生成以后，抽象语法树就可以被废弃掉以节省内存。在生成字节码的同时，还会在字节码上增加一些元数据，比如源代码的位置和用于执行字节码的处理函数。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"接下来我们利用d8近距离观察一下bytecode，这里我们将下面的JavaScript放在名为bytecode.js的文件中，并运行d8的调试工具。"}]},{"type":"codeblock","attrs":{"lang":"javascript"},"content":[{"type":"text","text":"function sayHi () {\n var str = 'hello world'\n return str\n}"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以得到下面的结果"}]},{"type":"codeblock","attrs":{"lang":"shell"},"content":[{"type":"text","text":"# d8 --print-bytecode bytecode.js\n[generated bytecode for function: (0x300d082d25e5 )]\nParameter count 1\nRegister count 2\nFrame size 16\n 0x300d082d26ae @ 0 : 12 00 LdaConstant [0]\n 0x300d082d26b0 @ 2 : 26 fa Star r0\n 0x300d082d26b2 @ 4 : 27 fe f9 Mov , r1\n 0x300d082d26b5 @ 7 : 62 3e 01 fa 02 CallRuntime [DeclareGlobals], r0-r1\n 0x300d082d26ba @ 12 : 0d LdaUndefined \n 0x300d082d26bb @ 13 : ab Return \nConstant pool (size = 1)\n0x300d082d2681: [FixedArray] in OldSpace\n - map: 0x300d08042201