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介绍

代码生成取决于：源代码、目标系统体系结构、运行时环境
代码生成可以被划分为三步：

生成中间代码(IR Iermediate code 码)
生成某种汇编形式的代码，而不是真正的可执行代码
优化目标代码

中间代码

以三地址码(TAC Three Address code)为例

生成三地址码

运算

变量赋值

布尔值

流程控制语句

[TAC_Code3.png]
[TAC_Code4.png]

三地址码的实现

有四元组和三元组实现，一般都用四元组实现，因为利于后面进行进一步的优化

四元组实现

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区别

三元组比较高效，因为空间占用减少了，并且编译器不需要为临时变量生成名称
三元组使用指令索引来表示临时值，那么它们的位置的任何移动都变得困难。四元组更适合优化

生成三地址码的细节

综合属性

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[综合属性语义规则1.png]
[综合属性语义规则2.png]
[综合属性语义分析.png]

语言结构

If

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[布尔表达式例子.png]
流程控制与布尔表达式翻译
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while a<b do
  if c<d then
    x=y+z
  else
    x=y-z
[流程控制示例图.png]
代码优化
常见的优化
寄存器优化
通过提高程序运行时的缓存命中率来提高运行速度
冗余的操作
相同的子表达式
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213Z"})])])],-1)]))),T[78]||(T[78]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"E")])],-1))]),T[84]||(T[84]=V("的值恒为")),Q("mjx-container",Q2,[(H(),t("svg",T2,T[79]||(T[79]=[a('',1)]))),T[80]||(T[80]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mtext",null,"false")])],-1))]),T[85]||(T[85]=V(" ，那么")),Q("mjx-container",a2,[(H(),t("svg",t2,T[81]||(T[81]=[a('',1)]))),T[82]||(T[82]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mtext",null,"stmt")])],-1))]),T[86]||(T[86]=V("将永远不会执行，也就是所说的死代码"))])]),T[107]||(T[107]=a('耗时的操作
乘法与移位
把乘二优化成向左移位
常量优化
常量折叠
',5)),Q("p",null,[Q("mjx-container",H2,[(H(),t("svg",V2,T[87]||(T[87]=[a('',1)]))),T[88]||(T[88]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"a"),Q("mo",null,"="),Q("mn",null,"2"),Q("mo",null,"+"),Q("mn",null,"3"),Q("mo",{stretchy:"false"},"→"),Q("mi",null,"a"),Q("mo",null,"="),Q("mn",null,"5")])],-1))])]),T[108]||(T[108]=Q("h5",{id:"常量传播",tabindex:"-1"},[Q("a",{class:"header-anchor",href:"#常量传播"},[Q("span",null,"常量传播")])],-1)),Q("p",null,[Q("mjx-container",r2,[(H(),t("svg",n2,T[89]||(T[89]=[a('',1)]))),T[90]||(T[90]=Q("mjx-assistive-mml",{unselectable:"on",display:"inline"},[Q("math",{xmlns:"http://www.w3.org/1998/Math/MathML"},[Q("mi",null,"a"),Q("mo",null,"="),Q("mn",null,"2"),Q("mo",null,";"),Q("mi",null,"b"),Q("mo",null,"="),Q("mn",null,"3"),Q("mo",null,";"),Q("mi",null,"c"),Q("mo",null,"="),Q("mi",null,"a"),Q("mo",null,"+"),Q("mi",null,"b"),Q("mo",{stretchy:"false"},"→"),Q("mi",null,"c"),Q("mo",null,"="),Q("mn",null,"5")])],-1))])]),T[109]||(T[109]=a('内联展开和尾递归
[尾递归优化.png]
优化的时期
三地址码生成时：直接优化语义树，可能对某个子树进行删除或者替换
三地址码生成后：很多时候，从语法树上获取不到足够的信息来进行优化，这时候就要在中间代码上进行优化
目标代码生成时：有可能要对某个特定的指令集进行优化又被称为窥孔优化(peephole optimization)，在编译器理论中，窥孔优化是一种在生成的代码段中对非常小的指令集合执行的优化。该集合称为“窥孔”或“窗口”
优化的范围
局部优化(Local Optimizations)
局部优化适用于直线代码段，即没有跳入或跳出序列的代码序列。一个最大的直线代码序列被称为一个基本块(basic block)。局部优化就是被限制在一个基本块中
全局优化(Global Optimizations)
优化超出了基本块的范围，但仅限于单个过程。
循环是很重要的优化位置，尤其是内部循环。
Code motion
代码移动减少了循环中的代码量，就是把循环内的和循环无关的代码移动到循环外
Induction variables deletion
归纳变量（Induction Variable）是指在循环的每次迭代中都会增加或减少固定数量的变量，或者是另一个归纳变量的线性函数。归纳变量通常用于循环优化中，例如循环展开和代码移动。
[删除.png]
线程间优化（Interprocedural optimization)
超出单个线程的范围，对整个程序进行优化
优化的工具
Flow graph
第一个指令是一个 basic block 的开始
每个作为 jump 的目标的 label 的语句是一个新的 basic block 的开始
每个跟着有条件跳转的指令都会开启一个新的 basic block (跟 2 差不多)
DAG(directed acyclic graph)
DAG 跟踪在 basic block 中发生的计算和重新赋值
叶子节点是从这个 block 之外来的变量和值
内部节点都是对值的操作
赋值被表现为把目标变量或临时变量 attach 到这个节点上
基本块的开始标签和最后的跳转标签不被包括在 DAG 中
复制操作不会增加新的节点，誓师会把新的标签添加到被复制值的节点上
对相同值的重复使用也会被体现在 DAG 中
几种情况
对于 x=y+z，如果 y 和 z 不变，则+的内部节点不需要创建，而是执行 y+z （常量折叠）
对于 x=y+z，如果已经存在一个与 y+z 具有相同值的节点，我们不会创建新节点，而是给现有节点附加标签 x。 （局部公共子表达式消除）
如果 x 之前标记过其他节点，我们删除该标签，因为 x 的“当前”值是刚刚创建的节点（消除不必要的分配）

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\\n\\n代码生成取决于：源代码、目标系统体系结构、运行时环境
\\n代码生成可以被划分为三步：
\\n
\\n\\n生成中间代码(IR Iermediate code 码)
\\n生成某种汇编形式的代码，而不是真正的可执行代码
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\\n
\\n中间代码
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\\n
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