Peephole Optimization in Compiler Design
Peephole optimization is a type of code Optimization performed on a small part of the code. It is performed on a very small set of instructions in a segment of code.
The small set of instructions or small part of code on which peephole optimization is performed is known as peephole or window.
It basically works on the theory of replacement in which a part of code is replaced by shorter and faster code without a change in output. The peephole is machine-dependent optimization.
Objectives of Peephole Optimization:
The objective of peephole optimization is as follows:
- To improve performance
- To reduce memory footprint
- To reduce code size
Peephole Optimization Techniques
A. Redundant load and store elimination: In this technique, redundancy is eliminated.
Initial code: y = x + 5; i = y; z = i; w = z * 3; Optimized code: y = x + 5; w = y * 3; //* there is no i now //* We've removed two redundant variables i & z whose value were just being copied from one another.
B. Constant folding: The code that can be simplified by the user itself, is simplified. Here simplification to be done at runtime are replaced with simplified code to avoid additional computation.
Initial code: x = 2 * 3; Optimized code: x = 6;
C. Strength Reduction: The operators that consume higher execution time are replaced by the operators consuming less execution time.
Initial code: y = x * 2; Optimized code: y = x + x; or y = x << 1; Initial code: y = x / 2; Optimized code: y = x >> 1;
D. Null sequences/ Simplify Algebraic Expressions : Useless operations are deleted.
a := a + 0; a := a * 1; a := a/1; a := a - 0;
E. Combine operations: Several operations are replaced by a single equivalent operation.
F. Deadcode Elimination: A part of the code which can never be executed, eliminating it will improve processing time and reduces set of instruction.
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