 Compiler optimization

Compiler optimization transforms a program representation to improve a chosen resource objective while preserving the behavior required by its programming language semantics. Objectives include execution time, code size and memory traffic. A transformation need not find a globally best program, and improving one objective can worsen another.

For example, constant folding replaces 2+3 by 5, while common subexpression elimination may compute a repeated expression once. Reusing a load across a store is valid only if the store cannot change the loaded location; alias analysis helps establish that condition. Similarly, replacing floating-point (a+b)+c by a+(b+c) can change rounding, so an optimizer needs an applicable permission or proof. Data-flow analysis supplies facts used by transformations, and undefined behavior affects which executions the language requires an implementation to preserve.