Instruction scheduling in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the main goal of instruction scheduling.

Compiler Design

The primary goal of instruction scheduling is to [1] the execution of instructions to improve performance.

Drag options to blanks, or click blank then click option'

Aoptimize

Bdelay

Cignore

Drandomize

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'delay' because scheduling sounds like waiting.

Choosing 'randomize' thinking it adds variety.

Instruction scheduling aims to optimize the order of instructions to make the program run faster.

2fill in blank

medium

Complete the code to describe a key factor instruction scheduling tries to avoid.

Compiler Design

Instruction scheduling tries to avoid [1] hazards that cause delays in execution.

Drag options to blanks, or click blank then click option'

Adata

Bsyntax

Clogic

Druntime

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'syntax' because it sounds like an error.

Choosing 'runtime' thinking about program crashes.

Data hazards occur when instructions depend on the results of previous instructions, causing delays.

3fill in blank

hard

Fix the error in the statement about instruction scheduling.

Compiler Design

Instruction scheduling rearranges instructions to [1] pipeline stalls and improve CPU efficiency.

Drag options to blanks, or click blank then click option'

Acreate

Breduce

Cignore

Dincrease

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'increase' because it sounds like adding something.

Choosing 'ignore' thinking stalls are not important.

The goal is to reduce pipeline stalls, not increase or create them.

4fill in blank

hard

Fill both blanks to complete the description of instruction scheduling techniques.

Compiler Design

Static scheduling is done at compile time, while [1] scheduling happens during [2].

Drag options to blanks, or click blank then click option'

Adynamic

Bruntime

Ccompile

Dlinking

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing compile time with runtime.

Choosing linking which is unrelated here.

Dynamic scheduling occurs during runtime, unlike static scheduling which is done at compile time.

5fill in blank

hard

Fill all three blanks to complete the code describing a scheduling method.

Compiler Design

In [1] scheduling, the processor uses a [2] window to reorder instructions and avoid [3] hazards.

Drag options to blanks, or click blank then click option'

Adynamic

Bsliding

Cdata

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing static instead of dynamic scheduling.

Confusing control hazards with data hazards.

Dynamic scheduling uses a sliding window to reorder instructions and avoid data hazards.

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