Dead code elimination

In compiler theory, dead code elimination (also known as DCE, dead code removal, dead code stripping, or dead code strip) is a compiler optimization to remove code which does not affect the program results. Removing such code has several benefits: it shrinks program size, an important consideration in some contexts, and it allows the running program to avoid executing irrelevant operations, which reduces its running time. It can also enable further optimizations by simplifying program structure. Dead code includes code that can never be executed (unreachable code), and code that only affects dead variables (written to, but never read again), that is, irrelevant to the program.

Consider the following example written in C.

Simple analysis of the uses of values would show that the value of b after the first assignment is not used inside foo. Furthermore, b is declared as a local variable inside foo, so its value cannot be used outside foo. Thus, the variable b is dead and an optimizer can reclaim its storage space and eliminate its initialization.

Furthermore, because the first return statement is executed unconditionally, no feasible execution path reaches the second assignment to b. Thus, the assignment is unreachable and can be removed. If the procedure had a more complex control flow, such as a label after the return statement and a goto elsewhere in the procedure, then a feasible execution path might exist to the assignment to b.

Also, even though some calculations are performed in the function, their values are not stored in locations accessible outside the scope of this function. Furthermore, given the function returns a static value (96), it may be simplified to the value it returns (this simplification is called constant folding).

Most advanced compilers have options to activate dead code elimination, sometimes at varying levels. A lower level might only remove instructions that cannot be executed. A higher level might also not reserve space for unused variables. Yet a higher level might determine instructions or functions that serve no purpose and eliminate them.

...
Wikipedia