Local optimization (peephole) in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is local optimization (peephole) in compiler design?

Local optimization, also called peephole optimization, is a technique where the compiler looks at a small set of instructions (a "peephole") and tries to replace them with a more efficient sequence without changing the program's behavior.

beginner

Why is peephole optimization called 'local' optimization?

Because it focuses on a small, localized part of the code—usually just a few instructions at a time—rather than analyzing the entire program or large blocks of code.

beginner

Give an example of a simple peephole optimization.

Replacing a sequence like "LOAD A; LOAD A" with just "LOAD A" because loading the same value twice is redundant.

intermediate

What kinds of improvements can peephole optimization achieve?

It can reduce the number of instructions, remove redundant operations, simplify calculations, and improve execution speed and code size.

intermediate

What is a limitation of peephole optimization?

It only looks at small instruction windows and cannot optimize across larger code blocks or consider global program behavior.

What does peephole optimization primarily focus on?

A Small sequences of instructions

B Entire program structure

C Memory allocation

D User interface design

Peephole optimization looks at small windows of instructions to improve them locally.

Which of the following is an example of peephole optimization?

A Changing variable names for clarity

B Replacing 'LOAD A; LOAD A' with 'LOAD A'

C Rearranging functions in a file

D Adding comments to code

Removing redundant loads is a classic peephole optimization.

What is a key benefit of peephole optimization?

A Improves code readability

B Increases program size

C Reduces instruction count

D Changes program logic

Peephole optimization reduces instructions to make code faster and smaller.

Which is NOT a characteristic of peephole optimization?

A Simplifies calculations

B Works on small instruction windows

C Removes redundant instructions

D Analyzes entire program flow

Peephole optimization does not analyze the entire program, only small parts.

Why might peephole optimization miss some improvements?

A It only looks at small parts of code

B It changes program logic

C It requires user input

D It only works on source code comments

Because peephole optimization focuses on small instruction sets, it can miss larger optimization opportunities.

Explain what local optimization (peephole) is and why it is useful in compiler design.

Think about how looking at just a few instructions can help make code faster or smaller.

Describe a simple example of peephole optimization and its effect on the code.

Consider removing repeated instructions or simplifying calculations.

Practice

1. What is the main goal of local optimization (peephole) in compiler design?

easy

A. To improve small sequences of instructions by removing redundancies

B. To rewrite the entire program in a different language

C. To increase the number of instructions for clarity

D. To change the program's output for optimization

Solution

1. Step 1: Understand local optimization scope

   Local optimization focuses on small instruction sequences, not the whole program.

2. Step 2: Identify the goal of peephole optimization

   Peephole optimization removes unnecessary or redundant instructions to make code faster and smaller without changing its meaning.

3. Final Answer:

   To improve small sequences of instructions by removing redundancies -> Option A

4. Quick Check:

   Local optimization = remove redundancies [OK]

Hint: Focus on small instruction sequences and redundancy removal [OK]

Common Mistakes:

• Thinking it rewrites the whole program
• Believing it changes program output
• Assuming it adds more instructions

2. Which of the following is a correct example of a peephole optimization?

easy

A. Replacing LOAD A; LOAD A with LOAD A

B. Changing variable names for readability

C. Adding comments to the code

D. Rearranging functions in the source file

Solution

1. Step 1: Identify peephole optimization examples

   Peephole optimization looks at small instruction sequences to remove redundancies.

2. Step 2: Evaluate each option

   Replacing LOAD A; LOAD A with LOAD A removes duplicate instructions, which is a classic peephole optimization. Options A, B, and D do not optimize instructions.

3. Final Answer:

   Replacing LOAD A; LOAD A with LOAD A -> Option A

4. Quick Check:

   Remove duplicate instructions = peephole optimization [OK]

Hint: Look for removing repeated instructions in short sequences [OK]

Common Mistakes:

• Confusing code readability changes with optimization
• Thinking comments affect optimization
• Assuming rearranging functions is peephole optimization

3. Consider this instruction sequence before peephole optimization:

LOAD R1, #5
ADD R1, R1, #0
STORE R1, 100(R2)

What will be the optimized sequence after applying peephole optimization?

medium

A. LOAD R1, #5
ADD R1, #0
STORE R1, 100(R2)

B. LOAD R1, #5
ADD R1, R1, #0
STORE R1, 100(R2)

C. LOAD R1, #5
STORE R1, 100(R2)

D. ADD R1, R1, #0
STORE R1, 100(R2)

Solution

1. Step 1: Identify redundant instructions

   The instruction ADD R1, R1, #0 adds zero, which does not change the value in R1, so it is redundant.

2. Step 2: Remove redundant instruction

   Removing the redundant ADD instruction leaves LOAD R1, #5 and STORE R1, 100(R2).

3. Final Answer:

   LOAD R1, #5 STORE R1, 100(R2) -> Option C

4. Quick Check:

   Remove add zero = optimized sequence [OK]

Hint: Remove instructions that do not change values (like add zero) [OK]

Common Mistakes:

• Keeping redundant add zero instruction
• Removing load instruction incorrectly
• Misunderstanding instruction effects

4. Identify the error in this peephole optimization attempt:

Original: LOAD R2, #10
SUB R2, R2, #0
STORE R2, 200(R3)

Optimized: LOAD R2, #10
SUB R2, R2, #10
STORE R2, 200(R3)

medium

A. The load instruction should be removed

B. No error, optimization is correct

C. The store instruction is missing

D. The subtraction of zero was incorrectly replaced with subtraction of ten

Solution

1. Step 1: Compare original and optimized subtraction

   Original subtracts zero, which does nothing. Optimized subtracts ten, which changes the value incorrectly.

2. Step 2: Identify the error in optimization

   Replacing subtraction by zero with subtraction by ten changes program meaning, which is incorrect for peephole optimization.

3. Final Answer:

   The subtraction of zero was incorrectly replaced with subtraction of ten -> Option D

4. Quick Check:

   Incorrect instruction change = error in peephole optimization [OK]

Hint: Ensure optimized instructions do not change program meaning [OK]

Common Mistakes:

• Changing instruction values incorrectly
• Removing necessary instructions
• Assuming all changes are optimizations

5. Given this code snippet:

LOAD R3, #0
ADD R3, R3, #5
ADD R3, R3, #0
STORE R3, 300(R4)

Which peephole optimization sequence correctly optimizes this code without changing its meaning?

hard

A. LOAD R3, #0
ADD R3, R3, #0
STORE R3, 300(R4)

B. LOAD R3, #5
STORE R3, 300(R4)

C. ADD R3, R3, #5
STORE R3, 300(R4)

D. LOAD R3, #5
ADD R3, R3, #0
STORE R3, 300(R4)

Solution

1. Step 1: Analyze the original instructions

   Loading zero into R3, then adding 5, then adding 0, is equivalent to loading 5 directly into R3.

2. Step 2: Apply peephole optimization

   Replace the sequence with a single instruction LOAD R3, #5 and then store. This removes redundant instructions without changing meaning.

3. Final Answer:

   LOAD R3, #5 STORE R3, 300(R4) -> Option B

4. Quick Check:

   Combine load and add zero to direct load [OK]

Hint: Combine load and add instructions when possible [OK]

Common Mistakes:

• Keeping redundant add zero
• Not combining load and add
• Removing load instruction incorrectly