9.2 Reaching Definitions
Reaching definitions determine for each basic block B and each program variable x what statement of the program could be the last statement defining x along some path from the start node to B.
A statement defines x if it assigns a value to x, for example, an assignment or read of x, or a procedure call passed x (not by value) or a procedure call that can access x.
Formally, we say that a definition, def
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def: x :=
reaches a point P in the program if there is some path from the start node to p along which def occurs and, subsequent to def, there are no other definitions of x. Figure 1 shows this pictorially.
