The Problem

Given a control flow structure, we want to discern which definitions of program quantities can affect which uses within the program.

Data flow problems fall into two classes. First are those which, given a point in the program, ask what can happen before control reaches that point--that is, what (past) definitions can affect computations at that point. We call these forward flow problems.

Second are those which, given a point in the program, ask what can happen after control leaves that point--that is, what (future) uses can be affected by computations at the point. We call these backward flow problems.

Forward flow problems include reaching definitions and available expressions. Backward flow problems include live variable analysis, very busy expressions, and reached uses.