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Liveness Analysis

:material-circle-edit-outline: 约 205 个字 :material-clock-time-two-outline: 预计阅读时间 1 分钟

Flow Graph Terminology

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Liveness

A variable is live on an edge if:

  • this variable will be used later
  • this variable will not be re-defined before being used

Live-in: A variable is live-in at a node if it is live on any of the in-edges of that node;

Live-out: A variable is live-out at a node if it is live on any of the out-edges of the node

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Calculation of Liveness

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从后往前分析

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Interference Graphs

冲突图，用于寄存器分配，冲突主要指两个变量的生存期重叠

冲突图构建方法如下：

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  • 第一条即我定义新变量时不应使用还会活跃的变量在使用的寄存器
  • 第二条即针对进行复制操作的 MOVE 指令，我们不给 a 和 c 之间添加冲突边

此外，每条 MOVE 指令的两个变量用虚线连接，即合并边，如果已有干涉边就不用了