"The programmer using such a system will write his beautifully-structured, but possibly inefficient, program P; then he will interactively specify transformations that make it efficient. Such a system will be much more powerful and reliable than a completely automatic one. We can also imagine the system manipulating measurement statistics concerning how much of the total running time is spent in each statement, since the programmer will want to know which parts of his program deserve to be optimized, and how much effect an optimization will really have. The original program P should be retained along with the transformation specifications, so that it can be properly understood and maintained as time passes. As I say, this idea certainly isn't my own; it is so exciting I hope that everyone soon becomes aware of its possibilities."

-- Knuth, D. E. (1974). Structured Programming with go to Statements. ACM Computing Surveys ~