Live Variable Analysis

Live variable analysis is a classical data-flow analysis that computes, for every point in a program, the set of variables that are live — that is, variables whose current values will be read before being overwritten. The analysis propagates information backward through the control flow graph, starting from uses and tracing back to definitions, until a fixed point is reached. The results are essential for register allocation (determining which variables can share a register) and dead code elimination (identifying assignments that no subsequent instruction reads). Despite its simplicity in formulation, live variable analysis exemplifies how local facts about program behavior aggregate into global structure through iterative propagation over a graph. The dual analysis, reaching definitions, propagates forward from definitions to uses instead of backward from uses to definitions.