Anticipated:An expression is said to be anticipated at program point if all paths leading from eventually computes (from the values of ’s operands that are available at ).
Will-be-available:An expression is said to be will-be-available at program point if it is anticipated and not subsequently killed along all paths reaching .
An expression is said to be used at program point if there exists a path leading from that uses the expression before the operands are reevaluated.
已用表达式(Used Expressions)的分析方向为后向(backword)。
如图所示,从下往上看,未使用的点标记为0,直到使用的地方被标记为1。
引入这个定义主要是为了消除当前块之外未使用的临时变量赋值,计算方式: Used.out: sets of used (live) expressions at exit of b.