Program optimization [Aho, Sethi, and Ullman, Compilers: Principles, Techniques, and Tools, Addison-Wesley, 1988, Ch. 10; Marvin Schaefer, A Mathematical Theory of Global Program Optimization, Prentice-Hall, 1973.] can be defined as follows:

Given a program P, produce a program P' that produces the same output values as P for a given input, but has a lower cost.

Typical costs are execution time and program space. Time is usually more important; fortunately, the two usually go together.

Optimization is an economic activity:

It is not possible to optimize everything. The goal is to find leverage: cases where there is a large expected payoff for a small cost.

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