Dead code

In computer programming, dead code is a section in the source code of a program which is executed but whose result is never used in any other computation. The execution of dead code wastes computation time and memory.

While the result of a dead computation may never be used, it may raise exceptions or affect some global state, thus removal of such code may change the output of the program and introduce unintended bugs. Compiler optimizations are typically conservative in their approach to dead code removal if there is any ambiguity as to whether removal of the dead code will affect the program output. The programmer may aid the compiler in this matter by making additional use of static and/or inline functions and enabling the use of link-time optimization.

In the above example, although the division of iX by iY is computed and never used, it will throw an exception when a division by zero occurs. Therefore the removal of the dead code may change the output of the program.

Dead code elimination is a form of compiler optimization in which dead code is removed from a program. Dead code analysis can be performed using live variable analysis, a form of static code analysis and data flow analysis. This is in contrast to unreachable code analysis which is based on control flow analysis.

The dead code elimination technique is in the same class of optimizations as unreachable code elimination and redundant code elimination.

In large programming projects, it is sometimes difficult to recognize and eliminate dead code, particularly when entire modules become dead. Test scaffolding can make it appear that the code is still live, and at times, contract language can require delivery of the code even when the code is no longer relevant.

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