Instruction scheduling in Compiler Design - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary goal of instruction scheduling in compilers?

Instruction scheduling is a technique used by compilers. What is its main purpose?

A To translate high-level code into machine code

B To reorder instructions to reduce pipeline stalls and improve CPU performance

C To allocate memory for variables during program execution

D To optimize the size of the compiled binary file

💡 Hint

Think about how CPUs execute instructions and what can slow them down.

Instruction scheduling rearranges instructions to avoid delays caused by data hazards or resource conflicts, improving CPU pipeline efficiency.

📋 Factual

intermediate

Which hazard does instruction scheduling primarily aim to mitigate?

Instruction scheduling helps reduce which type of hazard in CPU pipelines?

A Structural hazard

B Thermal hazard

C Data hazard

D Control hazard

💡 Hint

Consider hazards caused by dependencies between instructions.

Instruction scheduling mainly targets data hazards, which occur when instructions depend on the results of previous instructions.

🔍 Analysis

advanced

Analyze the effect of instruction scheduling on a sequence with dependent instructions

Given a sequence of instructions where each depends on the previous one, how does instruction scheduling improve performance?

A It duplicates instructions to speed up execution

B It removes dependencies to allow parallel execution

C It delays all instructions to avoid hazards

D It can reorder independent instructions between dependent ones to fill pipeline slots and reduce stalls

💡 Hint

Think about how independent instructions can be used to avoid waiting times.

Instruction scheduling rearranges instructions so independent ones execute while waiting for dependent results, reducing idle CPU cycles.

❓ Comparison

advanced

Compare static and dynamic instruction scheduling

Which statement correctly distinguishes static instruction scheduling from dynamic instruction scheduling?

A Static scheduling is done by the compiler before execution; dynamic scheduling is done by hardware during execution

B Static scheduling happens at runtime; dynamic scheduling happens at compile time

C Static scheduling requires no knowledge of instruction dependencies; dynamic scheduling does

D Static scheduling is slower than dynamic scheduling

💡 Hint

Consider when and where the scheduling decisions are made.

Static scheduling is performed by the compiler before the program runs, while dynamic scheduling is handled by the CPU hardware during execution.

❓ Reasoning

expert

What is the impact of instruction scheduling on out-of-order execution?

How does instruction scheduling relate to out-of-order execution in modern CPUs?

A Instruction scheduling by the compiler complements out-of-order execution by reducing hazards before runtime

B Instruction scheduling replaces the need for out-of-order execution entirely

C Instruction scheduling conflicts with out-of-order execution and reduces CPU efficiency

D Instruction scheduling only applies to single-core processors and is unrelated to out-of-order execution

💡 Hint

Think about how compiler and hardware optimizations work together.

Compiler instruction scheduling helps reduce hazards early, making out-of-order execution more efficient by lowering runtime conflicts.

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