Analysis
The objective, as ever, is to have the program run as swiftly as possible; the problem, as ever, is that it is not possible for a compiler to analyse a program and always correctly determine what it will do, still less what the programmer may have intended for it to do. By contrast, human programmers start at the other end with a purpose, and attempt to produce a program that will achieve it, preferably without expending a lot of thought in the process. So, the hope is that an optimising compiler will aid us by bridging the gap.
For various reasons, including readability, programs are frequently broken up into a number of procedures, which handle a few general cases. However, the generality of each procedure may result in wasted effort in specific usages. Interprocedural optimization represents an attempt at reducing this waste.
Suppose you have a procedure that evaluates F(x), and your code requests the result of F(6) and then later, F(6) again. Surely this second evaluation is unnecessary: the result could have been saved, and referred to later instead. This assumes that F is a pure function. Particularly, this simple optimization is foiled the moment that the implementation of F(x) is impure; that is, its execution involves reference to parameters other than the explicit argument 6 that have been changed between the invocations, or side effects such as printing some message to a log, counting the number of evaluations, accumulating the CPU time consumed, preparing internal tables so that subsequent invocations for related parameters will be facilitated, and so forth. Losing these side effects via non-evaluation a second time may be acceptable, or may not: a design issue beyond the scope of compilers.
More generally, aside from optimization, the second reason to use procedures is to avoid duplication of code that would be the same, or almost the same, each time the actions performed by the procedure are desired. A general approach to optimization would therefore be to reverse this: some or all invocations of a certain procedure are replaced by the respective code, with the parameters appropriately substituted. The compiler will then try to optimize the result.
Read more about this topic: Interprocedural Optimization
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