Backpatching technique in Compiler Design - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the purpose of backpatching in compiler design.

Compiler Design

Backpatching is used to [1] addresses in intermediate code during code generation.

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

Aignore

Bremove

Coptimize

Dfill in missing

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing backpatching with code optimization or removal.

Backpatching fills in missing jump addresses in intermediate code when they are not known initially.

2fill in blank

medium

Complete the sentence describing when backpatching is applied.

Compiler Design

Backpatching is applied during [1] when the target of a jump is not yet known.

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

Acode generation

Blexical analysis

Cparsing

Doptimization

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing parsing or lexical analysis instead of code generation.

Backpatching is used during code generation to fill in jump addresses that were unknown during earlier phases.

3fill in blank

hard

Fix the error in the statement about backpatching.

Compiler Design

Backpatching is used to [1].

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

Atrack

Ballocate memory

Cfill in jump addresses

Doptimize

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing backpatching with variable tracking or memory management.

Backpatching specifically fills in jump addresses in intermediate code, not variable values or memory allocation.

4fill in blank

hard

Fill both blanks to complete the backpatching code snippet.

Compiler Design

if condition:
    [1] = make_list(next_instr)
else:
    [2] = make_list(next_instr)

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

Atrue_list

Bfalse_list

Ctemp_var

Djump_addr

Attempts:

3 left

💡 Hint

Common Mistakes

Using generic names like temp_var or jump_addr instead of true_list and false_list.

In backpatching, true_list and false_list hold jump instructions for true and false branches respectively.

5fill in blank

hard

Fill all three blanks to complete the backpatching function.

Compiler Design

def backpatch(patch_list, [1], [2]):
    for instr in patch_list:
        [3][instr] = target_address

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

Atarget_address

Bpatch_list

Ccode

Dinstruction_list

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing parameter names or using patch_list twice.

The function backpatch updates the code at each instruction in patch_list with the target_address.

Practice

1. What is the main purpose of the backpatching technique in compiler design?

easy

A. To optimize arithmetic operations in expressions

B. To allocate memory for variables at compile time

C. To delay setting jump targets until their addresses are known

D. To parse source code into tokens

Solution

1. Step 1: Understand backpatching concept

   Backpatching is used to handle jumps in generated code when the target address is not yet known.

2. Step 2: Identify main purpose

   It delays setting jump targets until the correct address is available, ensuring correct jump instructions.

3. Final Answer:

   To delay setting jump targets until their addresses are known -> Option C

4. Quick Check:

   Backpatching = Delayed jump target setting [OK]

Hint: Backpatching fixes jumps only after target is known [OK]

Common Mistakes:

• Confusing backpatching with memory allocation
• Thinking it optimizes arithmetic operations
• Assuming it is related to lexical analysis

2. Which data structure is commonly used in backpatching to keep track of jump instructions that need their targets fixed later?

easy

A. Stack

B. List

C. Queue

D. Tree

Solution

1. Step 1: Recall backpatching tracking method

   Backpatching uses a list to store the addresses of jump instructions that need to be updated later.

2. Step 2: Identify correct data structure

   Lists allow easy appending and merging of jump addresses, making them ideal for backpatching.

3. Final Answer:

   List -> Option B

4. Quick Check:

   Backpatching uses lists for jump tracking [OK]

Hint: Backpatching uses lists to track jumps needing fixes [OK]

Common Mistakes:

• Choosing stack which is LIFO, not ideal here
• Confusing with queue or tree structures
• Assuming arrays without dynamic resizing

3. Consider the following pseudo-code snippet using backpatching:

if (x > 0) goto L1
goto L2
L1: y = 1
L2: y = 2

Which step best describes what backpatching does here?

medium

A. Delays setting the jump target of 'goto L1' until L1's address is known

B. Ignores the jump to L2 since it is unconditional

C. Sets the jump target of 'goto L1' immediately during parsing

D. Replaces all jumps with function calls

Solution

1. Step 1: Analyze jump instructions

   The jump 'goto L1' cannot be set immediately if L1's address is unknown during code generation.

2. Step 2: Understand backpatching role

   Backpatching delays setting the jump target until the label L1's address is determined, then fixes the jump.

3. Final Answer:

   Delays setting the jump target of 'goto L1' until L1's address is known -> Option A

4. Quick Check:

   Backpatching delays jump target setting [OK]

Hint: Backpatching waits for label address before fixing jumps [OK]

Common Mistakes:

• Assuming jump targets are set immediately
• Ignoring unconditional jumps
• Confusing jumps with function calls

4. A compiler uses backpatching but forgets to update the jump list after fixing a jump target. What is the most likely problem?

medium

A. The code will run slower but correctly

B. The compiler will crash immediately

C. All jumps will point to the wrong label

D. Some jumps remain with incorrect or missing targets

Solution

1. Step 1: Understand backpatching update process

   After fixing jump targets, the jump list must be cleared or updated to avoid reusing old entries.

2. Step 2: Identify consequence of not updating

   If the jump list is not updated, some jumps may remain unresolved, causing incorrect or missing jump targets.

3. Final Answer:

   Some jumps remain with incorrect or missing targets -> Option D

4. Quick Check:

   Not updating jump list = unresolved jumps [OK]

Hint: Always clear jump list after fixing targets [OK]

Common Mistakes:

• Assuming compiler crashes immediately
• Thinking all jumps point wrong instead of some
• Believing code runs slower but correct

5. In generating code for a while loop using backpatching, which of the following best describes the process?

hard

A. Backpatching is used to fix the jump from the loop condition to the loop body and from the loop body back to the condition

B. Backpatching is not needed because loops have fixed jump targets

C. Backpatching only fixes jumps after the loop ends

D. Backpatching replaces the loop with recursive function calls

Solution

1. Step 1: Understand backpatching in loops

   Backpatching helps fix jumps whose targets are not known during initial code generation, such as jumps from condition to body and body back to condition.

2. Step 2: Identify correct description

   Backpatching is used to fix the jump from the loop condition to the loop body and from the loop body back to the condition.

3. Final Answer:

   Backpatching is used to fix the jump from the loop condition to the loop body and from the loop body back to the condition -> Option A

4. Quick Check:

   Backpatching fixes condition-to-body and body-to-condition jumps [OK]

Hint: Backpatching connects loop condition and body jumps [OK]

Common Mistakes:

• Assuming jumps are fixed immediately
• Thinking backpatching replaces loops with recursion
• Ignoring backpatching role in loop jumps