Code Optimization

Code Optimization

In recent years, most research and development in the area of compiler design has been focused on the optimization phases of the compiler. Optimization is the process of improving generated code so as to reduce its potential running time and/or reduce the space required to store it in memory. Software designers are often faced with decisions which involve a space-time tradeoff – i.e., one method will result in a faster program, another method will result in a program which requires less memory, but no method will do both. However, many optimization techniques are capable of improving the object program in both time and space, which is why they are employed in most modern compilers. This results from either the fact that much effort has been directed toward the development of optimization techniques, or from the fact that the code normally generated is very poor and easily improved.

Optimization techniques can be separated into two general classes: local and global.
• Local optimization techniques normally are concerned with transformations on small sections of code (involving only a few instructions) and generally operate on the machine language instructions which are produced by the code generator.
• Global optimization techniques are generally concerned with larger blocks of code, or even multiple blocks or modules, and will be applied to the intermediate form, atom strings, or syntax trees put out by the parser.

Peephole Optimization
Code generated by using the statement-by-statement code-generation strategy contains redundant instructions and suboptimal constructs. Therefore, to improve the quality of the target code, optimization is required. Peephole optimization is an effective technique for locally improving the target code. Short sequences of target code instructions are examined and replacement by faster sequences wherever possible. Typical optimizations that can be performed are:
• Elimination of redundant loads and stores
• Elimination of multiple jumps
• Elimination of unreachable code
• Algebraic simplifications
• Reducing for strength
• Use of machine idioms

Eliminating Redundant Loads and Stores
If the target code contains the instruction sequence:
1. MOV R, a
2. MOV a, R
We can delete the second instruction if it an unlabeled instruction. This is because the first instruction ensures that the value of a is already in the register R. If it is labeled, there is no guarantee that step 1 will always be executed before step 2.