What is machine dependent optimization?
Machine dependent optimization involves transformations that take into consideration, the properties of the target machine like registers and special machine instruction sequences available, etc. these techniques are applied on generated target code.
The machine dependent optimization is attained by the following measures:
- Allocation of adequate number of resources to enhance the execution efficiency of the program
- Use of immediate instructions wherever required
- The use of intermix instructions
Why is Machine Dependent Optimization Needed?
Generally, the statement by statement code generation strategy followed by most compilers generate a target code that consists of redundant instructions and suboptimal constructs.
To improve such code, some machine dependent transformations are applied on it.
Peephole optimization technique is simple and an effective machine dependent technique.
What is Peephole Optimization?
It is a simple but effective technique for locally improving the target code.
This technique tries to improve the performance of the target code by examining a short sequence of target instructions known as peephole, and replacing them by a shorter and faster sequence whenever possible.
What is Peephole?
It is a short sequence of target instruction. The peephole can be considered as a moving window on the target program. The instructions in the peephole need not be contiguous.
What is the characteristic of Peephole Optimization?
The characteristic of peephole optimization is, one improvement may spawn opportunities for additional improvements. Hence, to get maximum benefit from this technique repeated passes over the target program are necessary.
What are the Transformations done in Peephole Optimization?
In general peephole optimization involves the following six transformations:
Redundant instruction elimination
Unreachable code elimination
Flow of control optimization
Algebraic simplification
Reduction in strength
Use of machine idioms
Note: the techniques of peephole optimization can also be applied to the intermediate code.
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