DeadCodeElimination

DeadCodeElimination (DCE) is a compiler optimization that removes instructions whose results are not needed to determine the program’s observable behavior. The purpose is to reduce code size and improve runtime performance by eliminating computations, assignments, and branches that do not affect outputs or side effects. Dead code includes unused variable assignments (dead stores), computations whose results are never read, and branches that are never taken due to constant conditions. It also removes unreachable code after statements such as return or throw. However, DCE only deletes code whose removal preserves semantics; statements with side effects must be kept.

Techniques rely on data-flow analyses. Liveness analysis determines whether a value may be read in the future; reaching-definition analysis tracks whether a definition could reach a use. Constant folding, branch elimination, and dead-store elimination are commonly combined in DCE. Interprocedural DCE extends the analysis across function boundaries, but is more challenging due to potential side effects and aliases.

Limitations include aliasing, pointers, and external calls where the compiler cannot safely assume no side effects. In languages with volatile memory, or with exceptions and concurrency, DCE must be conservative. DCE interacts with other optimizations such as inlining and alias analysis.

Applications: DCE is a standard part of modern optimizing compilers and virtual machines, such as GCC, LLVM, and Java JIT compilers. The result is smaller generated code, sometimes improved cache locality, and faster execution, though compile-time cost can increase.