An expression A op B is very busy at a point p if along every path from p there is an expression A op B before a redefinition of either A or B.
If an expression is very busy at program point p, it makes sense to compute it there. This is often termed code hoisting and it saves space although it doesn't necessarily save time.
But it is interesting as a fourth type of data flow analysis problem: backwards flow and intersection confluence.
Equations for Very Busy Expressions
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