10.3 LOOP OPTIMIZATION

10.2 WHAT IS CODE OPTIMIZATION?

Code optimization refers to the techniques used by the compiler to improve the execution efficiency of the generated object code. It involves a complex analysis of the intermediate code and the performance of various transformations; but every optimizing transformation must also preserve the semantics of the program. That is, a compiler should not attempt any optimization that would lead to a change in the program's semantics.

Optimization can be machine-independent or machine-dependent . Machine-independent optimizations can be performed independently of the target machine for which the compiler is generating code; that is, the optimizations are not tied to the target machine's specific platform or language. Examples of machine-independent optimizations are: elimination of loop invariant computation, induction variable elimination, and elimination of common subexpressions .

On the other hand, machine-dependent optimization requires knowledge of the target machine. An attempt to generate object code that will utilize the target machine's registers more efficiently is an example of machine-dependent code optimization. Actually, code optimization is a misnomer; even after performing various optimizing transformations, there is no guarantee that the generated object code will be optimal. Hence, we are actually performing code improvement. When attempting any optimizing transformation, the following criteria should be applied:

1. The optimization should capture most of the potential improvements without an unreasonable amount of effort.

2. The optimization should be such that the meaning of the source program is preserved.

3. The optimization should, on average, reduce the time and space expended by the object code.