Global common subexpression elimination

When considering compiler optimization, there are two questions that immediately come to mind; one, why and to what extent is optimization necessary and two, to what extent is it possible. When considering the second question, one might immediately become discouraged since it is well known that the program equivalency problem is recursively unsolvable. It is, of course, clear from this that there will never be techniques for generating a completely optimum program. These unsolvability results, however, do not preclude the possibility of ad hoc techniques for program improvement or even a partial theory which produces a class of equivalent programs optimized in varying degrees. The reasons why optimization is required seem to me to fall in two major categories. The first I will call “local” and the second “global”.