Local optimization (peephole) in Compiler Design - Step-by-Step Execution

Concept Flow - Local optimization (peephole)

Input: Sequence of instructions

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Select small window (peephole)

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Analyze instructions in window

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Apply optimization rules if pattern matches

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Replace with optimized instructions

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Move window forward

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Repeat

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Output optimized code

Local optimization scans small groups of instructions to find and replace inefficient patterns with better ones, improving code step-by-step.

Execution Sample

Compiler Design

LOAD R1, 0
ADD R1, 5
STORE R1, X

This code loads 0 into R1, adds 5, then stores R1 to X. Peephole optimization can simplify this.

Analysis Table

Step	Window Instructions	Pattern Matched?	Action	Resulting Instructions
1	LOAD R1, 0 ADD R1, 5	Yes (Add after Load 0)	Replace with LOAD R1, 5	LOAD R1, 5 STORE R1, X
2	LOAD R1, 5 STORE R1, X	No	No change	LOAD R1, 5 STORE R1, X
3	STORE R1, X	No	No change	LOAD R1, 5 STORE R1, X

💡 No more windows to analyze; optimization complete.

State Tracker

Variable	Start	After Step 1	After Step 2	Final
Instructions	LOAD R1, 0 ADD R1, 5 STORE R1, X	LOAD R1, 5 STORE R1, X	LOAD R1, 5 STORE R1, X	LOAD R1, 5 STORE R1, X

Key Insights - 2 Insights

Why does peephole optimization only look at a small window of instructions?

Because it focuses on local patterns to simplify or improve code without needing to analyze the entire program, as shown in Step 1 where only two instructions are checked.

What happens if no pattern matches in the current window?

The instructions remain unchanged and the window moves forward, as seen in Steps 2 and 3 where no changes occur.

Visual Quiz - 3 Questions

Look at the execution table at Step 1. What optimization was applied?

A Removed STORE instruction

B Added an extra ADD instruction

C Replaced LOAD 0 and ADD 5 with LOAD 5

D No optimization applied

💡 Hint

Check the 'Action' and 'Resulting Instructions' columns in Step 1.

At which step does the peephole window find no matching pattern?

A Both Step 2 and Step 3

B Step 2

C Step 3

D Step 1

💡 Hint

Look at the 'Pattern Matched?' column for Steps 2 and 3.

If the initial code had 'ADD R1, 0' instead of 'ADD R1, 5', what would the optimization likely do at Step 1?

A No change because ADD 0 does nothing

B Replace LOAD 0 and ADD 0 with LOAD 0

C Replace LOAD 0 and ADD 0 with LOAD 5

D Remove the LOAD instruction

💡 Hint

Consider how adding zero affects the value and what peephole optimization tries to simplify.

Concept Snapshot

Local optimization (peephole):
- Examines small instruction windows
- Matches patterns to simplify or improve code
- Replaces inefficient sequences with better ones
- Moves window forward and repeats
- Improves code locally without full program analysis

Full Transcript

Local optimization, also called peephole optimization, works by looking at small groups of instructions in a program. It checks if these instructions match known inefficient patterns. If they do, it replaces them with better instructions. For example, loading zero and then adding five can be replaced by loading five directly. The process repeats by moving the small window forward until the whole code is checked. If no pattern matches, the instructions stay the same. This method improves code step-by-step without analyzing the entire program at once.