Instruction scheduling in Compiler Design - Mini Project: Build & Apply

Instruction Scheduling Basics

📖 Scenario: You are working on a simple compiler that needs to arrange instructions to run efficiently on a processor. Some instructions depend on the results of others, so they cannot run before their dependencies are ready.Your task is to organize a small set of instructions in an order that respects these dependencies.

🎯 Goal: Build a list of instructions and reorder them so that no instruction runs before the instructions it depends on.

📋 What You'll Learn

Create a dictionary of instructions with their dependencies

Create a list to hold the scheduled instructions

Implement a simple scheduling loop that respects dependencies

Complete the schedule by adding all instructions in a valid order

💡 Why This Matters

🌍 Real World

Instruction scheduling is used in compilers to arrange machine instructions so processors run them efficiently without waiting for data.

💼 Career

Understanding instruction scheduling helps compiler engineers optimize code performance and is valuable for roles in systems programming and hardware design.

1

Create the instruction dependency dictionary

Create a dictionary called instructions with these exact entries: 'I1': [], 'I2': ['I1'], 'I3': ['I1'], 'I4': ['I2', 'I3']

Compiler Design

# Create the instructions dictionary with dependencies
# Your code here

Hint

Use a dictionary where keys are instruction names and values are lists of instructions they depend on.

2

Create an empty list for the scheduled instructions

Create an empty list called schedule to hold the instructions in the order they will run.

Compiler Design

instructions = {
    'I1': [],
    'I2': ['I1'],
    'I3': ['I1'],
    'I4': ['I2', 'I3']
}
# Create an empty list called schedule
# Your code here

Hint

Use an empty list to store the final order of instructions.

3

Schedule instructions respecting dependencies

Write a while loop that continues until all instructions are scheduled. Inside the loop, use a for loop with variables instr and deps to iterate over instructions.items(). If instr is not in schedule and all dependencies in deps are in schedule, append instr to schedule.

Compiler Design

instructions = {
    'I1': [],
    'I2': ['I1'],
    'I3': ['I1'],
    'I4': ['I2', 'I3']
}
schedule = []
# Schedule instructions respecting dependencies
# Your code here

Hint

Use a loop to add instructions only when their dependencies are already scheduled.

4

Complete the instruction scheduling

Add a final comment line # Instruction scheduling complete after the scheduling loop to mark the end of the process.

Compiler Design

instructions = {
    'I1': [],
    'I2': ['I1'],
    'I3': ['I1'],
    'I4': ['I2', 'I3']
}
schedule = []

while len(schedule) < len(instructions):
    for instr, deps in instructions.items():
        if instr not in schedule and all(dep in schedule for dep in deps):
            schedule.append(instr)
# Instruction scheduling complete

Hint

Add a comment to show the scheduling process is done.

Practice

1. What is the main goal of instruction scheduling in compiler design?

easy

A. To rearrange instructions to run faster without changing the program's result

B. To translate code into machine language

C. To check for syntax errors in the source code

D. To increase the size of the compiled program

Solution

1. Step 1: Understand the purpose of instruction scheduling

   Instruction scheduling rearranges the order of instructions to improve performance without changing what the program does.

2. Step 2: Compare options with this purpose

   Only To rearrange instructions to run faster without changing the program's result matches this goal; others describe different compiler tasks.

3. Final Answer:

   To rearrange instructions to run faster without changing the program's result -> Option A

4. Quick Check:

   Instruction scheduling = rearranging instructions for speed [OK]

Hint: Instruction scheduling = reorder steps for faster execution [OK]

Common Mistakes:

• Confusing instruction scheduling with syntax checking
• Thinking it changes program output
• Mixing it up with code translation

2. Which of the following is a correct statement about instruction scheduling syntax in compilers?

easy

A. Instruction scheduling disables other compiler optimizations

B. Instruction scheduling is done by manually rewriting assembly code only

C. Instruction scheduling requires the programmer to add special keywords in source code

D. Instruction scheduling is an automatic optimization step in modern compilers

Solution

1. Step 1: Identify how instruction scheduling is performed

   Modern compilers perform instruction scheduling automatically during optimization.

2. Step 2: Evaluate each option

   Instruction scheduling is an automatic optimization step in modern compilers correctly states it is automatic; others are false or misleading.

3. Final Answer:

   Instruction scheduling is an automatic optimization step in modern compilers -> Option D

4. Quick Check:

   Instruction scheduling = automatic in compilers [OK]

Hint: Instruction scheduling happens automatically in compilers [OK]

Common Mistakes:

• Believing programmers must manually schedule instructions
• Thinking it requires special source code keywords
• Assuming it disables other optimizations

3. Consider this simplified instruction sequence before scheduling:

LOAD R1, A
LOAD R2, B
ADD R3, R1, R2
STORE C, R3

After instruction scheduling to reduce waiting time, which sequence is most likely?

medium

A. LOAD R1, A
LOAD R2, B
STORE C, R3
ADD R3, R1, R2

B. LOAD R1, A
LOAD R2, B
ADD R3, R1, R2
STORE C, R3

C. LOAD R2, B
LOAD R1, A
ADD R3, R1, R2
STORE C, R3

D. LOAD R1, A
ADD R3, R1, R2
LOAD R2, B
STORE C, R3

Solution

1. Step 1: Understand dependencies in instructions

   ADD depends on both LOAD R1 and LOAD R2; STORE depends on ADD result.

2. Step 2: Identify scheduling that reduces waiting

   Swapping LOAD R1 and LOAD R2 (

   LOAD R2, B
   LOAD R1, A
   ADD R3, R1, R2
   STORE C, R3

   ) can allow parallel loading, reducing wait time without changing order of dependent instructions.

3. Final Answer:

   LOAD R2, B LOAD R1, A ADD R3, R1, R2 STORE C, R3 -> Option C

4. Quick Check:

   Reordering independent loads = faster execution [OK]

Hint: Swap independent loads to reduce wait time [OK]

Common Mistakes:

• Moving dependent instructions before their inputs
• Changing order of STORE before ADD
• Not recognizing independent instructions

4. A compiler's instruction scheduler rearranged instructions but caused a runtime error. What is the most likely cause?

medium

A. It only reordered independent instructions

B. It changed the order of dependent instructions incorrectly

C. It optimized the code perfectly without errors

D. It ignored hardware constraints

Solution

1. Step 1: Analyze the cause of runtime errors after scheduling

   Runtime errors often occur if instruction order breaks data dependencies.

2. Step 2: Evaluate options

   It changed the order of dependent instructions incorrectly correctly identifies incorrect reordering of dependent instructions as cause; others are incorrect or unrelated.

3. Final Answer:

   It changed the order of dependent instructions incorrectly -> Option B

4. Quick Check:

   Wrong dependency order = runtime error [OK]

Hint: Never reorder dependent instructions incorrectly [OK]

Common Mistakes:

• Assuming all reorderings are safe
• Ignoring data dependencies
• Blaming hardware constraints only

5. Given a loop with instructions:

LOAD R1, X[i]
LOAD R2, Y[i]
MUL R3, R1, R2
STORE Z[i], R3

How can instruction scheduling improve performance in this loop?

hard

A. By reordering loads and computations to overlap memory access and calculation

B. By removing the STORE instruction

C. By executing all instructions sequentially without change

D. By duplicating instructions to run in parallel

Solution

1. Step 1: Identify opportunities in loop instructions

   Loads can be scheduled to start early, allowing MUL to use loaded data without waiting.

2. Step 2: Understand scheduling benefits

   Reordering instructions to overlap memory access and computation reduces idle CPU time.

3. Final Answer:

   By reordering loads and computations to overlap memory access and calculation -> Option A

4. Quick Check:

   Overlap loads and compute = better performance [OK]

Hint: Overlap memory load and compute to speed loops [OK]

Common Mistakes:

• Removing necessary instructions
• Not changing instruction order
• Duplicating instructions causing errors