用v8查看javascript生成的字节码

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1. 示意代码

var hello = "hello"
function display(hello) {
    console.log(hello)
    return 1
}

display(hello)

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2. 输出生成的字节码(--print-bytecode)

[generated bytecode for function:  (0x01b008292bf1 <SharedFunctionInfo>)]
Bytecode length: 30
Parameter count 1
Register count 3
Frame size 24
OSR nesting level: 0
Bytecode Age: 0
         000001B008292CBA @    0 : 13 00             LdaConstant [0]
         000001B008292CBC @    2 : c2                Star1
         000001B008292CBD @    3 : 19 fe f8          Mov <closure>, r2
         000001B008292CC0 @    6 : 64 4f 01 f9 02    CallRuntime [DeclareGlobals], r1-r2
         000001B008292CC5 @   11 : 13 01             LdaConstant [1]
         000001B008292CC7 @   13 : 23 01 00          StaGlobal [1], [0]
         000001B008292CCA @   16 : 21 02 02          LdaGlobal [2], [2]
         000001B008292CCD @   19 : c2                Star1
         000001B008292CCE @   20 : 21 01 04          LdaGlobal [1], [4]
         000001B008292CD1 @   23 : c1                Star2
         000001B008292CD2 @   24 : 61 f9 f8 06       CallUndefinedReceiver1 r1, r2, [6]
         000001B008292CD6 @   28 : c3                Star0
         000001B008292CD7 @   29 : a8                Return
Constant pool (size = 3)
000001B008292C85: [FixedArray] in OldSpace
 - map: 0x01b008002205 <Map>
 - length: 3
           0: 0x01b008292c39 <FixedArray[3]>
           1: 0x01b008292bb9 <String[5]: #hello>
           2: 0x01b008292bcd <String[7]: #display>
Handler Table (size = 0)
Source Position Table (size = 0)
[generated bytecode for function: display (0x01b008292c4d <SharedFunctionInfo display>)]
Bytecode length: 17
Parameter count 2
Register count 2
Frame size 16
OSR nesting level: 0
Bytecode Age: 0
         000001B008292E06 @    0 : 21 00 00          LdaGlobal [0], [0]
         000001B008292E09 @    3 : c2                Star1
         000001B008292E0A @    4 : 2d f9 01 02       LdaNamedProperty r1, [1], [2]
         000001B008292E0E @    8 : c3                Star0
         000001B008292E0F @    9 : 5d fa f9 03 04    CallProperty1 r0, r1, a0, [4]
         000001B008292E14 @   14 : 0d 01             LdaSmi [1]
         000001B008292E16 @   16 : a8                Return
Constant pool (size = 2)
000001B008292DD5: [FixedArray] in OldSpace
 - map: 0x01b008002205 <Map>
 - length: 2
           0: 0x01b00820a401 <String[7]: #console>
           1: 0x01b00820a475 <String[3]: #log>
Handler Table (size = 0)
Source Position Table (size = 0)
hello

Constant pool 表示常量池 LdaGlobal <name_index> 这条指令就是将常量池中的值load到累加器中 <name_inde>就是常量池中的索引 是一个FeedBackVector 可以理解为为后面TurboFan优化代码提供元信息 LdaConstant , LdaSmi 也类似后缀代表了不同的类型 StaGlobal <name_index> 指定恰好相反。表示将累加器中的值store到常量池中

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3. 变量声明

         000001B008292CBA @    0 : 13 00             LdaConstant [0]
         000001B008292CBC @    2 : c2                Star1
         000001B008292CBD @    3 : 19 fe f8          Mov <closure>, r2
         000001B008292CC0 @    6 : 64 4f 01 f9 02    CallRuntime [DeclareGlobals], r1-r2
         这一部分表示全局变量声明
         调用函数DeclareGlobals，将寄存器r1,r2作为参数传入

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4. 函数调用准备

         000001B008292CC7 @   13 : 23 01 00          StaGlobal [1], [0] // 累加器中的值写入hello变量
         000001B008292CCA @   16 : 21 02 02          LdaGlobal [2], [2] // 函数display写入累加器
         000001B008292CCD @   19 : c2                Star1              // display写入到寄存器r1
         000001B008292CCE @   20 : 21 01 04          LdaGlobal [1], [4] // 变量hello 写入寄存器
         000001B008292CD1 @   23 : c1                Star2              //变量hello写入寄存器r2

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5. 函数查找

        000001B008292CD2 @   24 : 61 f9 f8 06       CallUndefinedReceiver1 r1, r2, [6] //调用函数display

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6. 属性查找

        000001B008292E06 @    0 : 21 00 00          LdaGlobal [0], [0] // 加载console到累机器中
        000001B008292E09 @    3 : c2                Star1           // console写入到r1寄存器
        000001B008292E0A @    4 : 2d f9 01 02       LdaNamedProperty r1, [1], [2] // console对象下面log属性查找 

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7. 函数调用

         000001B008292E0E @    8 : c3                Star0 //查找对的对象值加载到寄存器r0
         000001B008292E0F @    9 : 5d fa f9 03 04    CallProperty1 r0, r1, a0, [4] // 调用函数其中a0为掺入的参数
         000001B008292E14 @   14 : 0d 01             LdaSmi [1] // 小整数1加载到累加器

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8. 函数信息

Parameter count 2 //参数个数(实际上只有一个入参)
Register count 2 // 寄存器个数