# 复写传播

复写(复制，赋值)传播的目的是借助复写语句，尽可能将tac中对复写左端项的引用替换成对右端项的引用。虽然表面上没有减少指令的条数，但这样做的好处在于优化后复写左端项有可能不再活跃，从而让死代码消除可以删除这条复写语句。

复写传播的实现方法与公共表达式提取是类似的，与可用表达式对应，它考虑的是**可用复写语句**。一条复写语句生成一条可用复写语句，一条带有定值的语句可以杀死与被定值的虚拟寄存器**相关**的所有复写语句(包括它出现在左端项或者右端项的)。除此之外，复写传播的数据流方向，交汇运算和初始值均与可用表达式的数据流相同。

按照老套路计算出每条语句处所有的可用复写语句后，考察一条语句使用的虚拟寄存器a，如果存在一条可用的复写语句，它的左端项是a，那么a可以被替换成它的右端项。这个过程还可以递归地进行下去：设这个右端项为b，如果存在一条可用的复写语句，它的左端项是b，则a可以进一步被替换成它的右端项c。这样就可以一步传播多层。

一个例子如下：

```
  b = a
  c = b
  d = c
  x = y + d
```

计算可用复写语句，每个语句后的方括号内的内容是这条语句执行**前**的可用复写语句集合：

```
  b = a []
  c = b [b = a]
  a = x [b = a, c = b]
  d = c [c = b, a = x] // 上一条语句杀死了b = a
  x = y + d [c = b, a = x, d = c]
```

执行转换：

```
  b = a
  c = a # 因为b = a可用，所以b被替换成a
  a = x
  d = b # 因为c = b可用，所以c被替换成b
  x = y + b # 因为d = c，c = b可用，所以d被替换成b
```


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