Three-address code in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Three-address code in compiler design?

Three-address code is an intermediate code used in compilers where each instruction contains at most three addresses or operands. It simplifies complex expressions into simple steps for easier translation to machine code.

beginner

Why do compilers use Three-address code?

Compilers use Three-address code to break down complex operations into simple instructions. This makes optimization and translation to machine code easier and more efficient.

beginner

Give an example of a Three-address code instruction.

An example is: t1 = a + b; Here, 't1' is a temporary variable storing the result of adding 'a' and 'b'. This instruction has three addresses: t1, a, and b.

intermediate

How does Three-address code handle complex expressions?

It breaks complex expressions into multiple simple instructions using temporary variables. For example, (a + b) * c becomes: t1 = a + b; t2 = t1 * c;

beginner

What are the typical components of a Three-address code instruction?

A typical instruction has an operator and up to two operands, and a result location. For example, result = operand1 operator operand2.

What is the maximum number of addresses in a Three-address code instruction?

A Three

B Two

C Four

D One

By definition, Three-address code instructions have at most three addresses or operands.

Why are temporary variables used in Three-address code?

A To reduce memory usage

B To store final output only

C To store intermediate results

D To avoid using operators

Temporary variables hold intermediate results when breaking down complex expressions.

Which of the following is a valid Three-address code instruction?

A a = b + c + d

B t1 = a + b

C a + b = c

D t1 = a *

The instruction 't1 = a + b' has one operator and two operands, fitting the three-address code format.

How does Three-address code help in compiler optimization?

A By increasing the number of instructions

B By directly generating machine code

C By removing all temporary variables

D By simplifying complex expressions into simple steps

Simplifying expressions makes it easier for the compiler to optimize code.

Which of these is NOT a component of a Three-address code instruction?

A Loop counter

B Operands

C Result location

D Operator

Loop counters are not a required component of a three-address code instruction.

Explain what Three-address code is and why it is used in compilers.

Think about how complex expressions are broken down for easier processing.

Describe how a complex expression like (a + b) * (c - d) can be represented using Three-address code.

Focus on splitting the expression into smaller parts.

Practice

1. What is the main purpose of three-address code in compiler design?

easy

A. To execute machine code directly on hardware

B. To break complex instructions into simpler steps with at most three parts

C. To store source code in a compressed format

D. To replace high-level language syntax with natural language

Solution

1. Step 1: Understand the role of intermediate code

   Intermediate code like three-address code simplifies complex instructions for easier processing.

2. Step 2: Identify the key feature of three-address code

   It breaks instructions into steps with at most three parts (two operands and one result).

3. Final Answer:

   To break complex instructions into simpler steps with at most three parts -> Option B

4. Quick Check:

   Three-address code = simple 3-part instructions [OK]

Hint: Three-address code simplifies instructions into three parts [OK]

Common Mistakes:

• Confusing three-address code with machine code
• Thinking it compresses source code
• Believing it replaces programming languages

2. Which of the following is a correct example of a three-address code instruction?

easy

A. if a > b goto L1 else goto L2

B. while (a < b) { a = a + 1; }

C. print(a, b, c)

D. t1 = a + b

Solution

1. Step 1: Identify the format of three-address code

   It typically has one operator and up to two operands, assigning result to a temporary variable.

2. Step 2: Check each option

   t1 = a + b matches the format: one operator (+), two operands (a, b), and a result (t1). Others are control flow or high-level statements.

3. Final Answer:

   t1 = a + b -> Option D

4. Quick Check:

   Three-address code = one operator, two operands, one result [OK]

Hint: Look for one operator and two operands with a result variable [OK]

Common Mistakes:

• Choosing control flow statements as three-address code
• Confusing function calls with three-address code
• Ignoring the assignment to a temporary variable

3. Given the three-address code:
t1 = a + b
t2 = t1 * c
d = t2 - e
What is the final value assigned to d in terms of a, b, c, e?

medium

A. (a + b) * c - e

B. a + b * c - e

C. a + (b * c) - e

D. (a + b) - (c * e)

Solution

1. Step 1: Evaluate intermediate expressions step-by-step

   t1 = a + b; then t2 = t1 * c = (a + b) * c; finally d = t2 - e = (a + b) * c - e.

2. Step 2: Substitute and simplify

   The final expression for d is (a + b) * c - e, matching (a + b) * c - e.

3. Final Answer:

   (a + b) * c - e -> Option A

4. Quick Check:

   Follow code stepwise = (a + b) * c - e [OK]

Hint: Follow temporary variables stepwise to build final expression [OK]

Common Mistakes:

• Ignoring parentheses changing operation order
• Multiplying b by c before adding a
• Mixing subtraction order

4. Identify the error in this three-address code snippet:
t1 = a + b
t2 = t1 *
d = t2 - e

medium

A. Missing operand after '*' operator in second line

B. Incorrect use of temporary variables

C. Assignment to 'd' should be before 't2'

D. No error, code is correct

Solution

1. Step 1: Examine the second line syntax

   The second line ends with '*' operator but lacks the second operand after '*', which is invalid.

2. Step 2: Confirm other lines are correct

   First and third lines have proper assignments and operands; only second line has syntax error.

3. Final Answer:

   Missing operand after '*' operator in second line -> Option A

4. Quick Check:

   Operator must have two operands [OK]

Hint: Check each operator has two operands in three-address code [OK]

Common Mistakes:

• Ignoring incomplete operator usage
• Thinking variable order causes error
• Assuming code is correct without checking syntax

5. How would you represent the high-level statement if (a < b) c = a + b; else c = a - b; in three-address code?

hard

A. if a < b then c = a + b else c = a - b

B. c = a + b if a < b else c = a - b

C. if a < b goto L1c = a - bgoto L2L1: c = a + bL2:

D. t1 = a < bc = t1 ? a + b : a - b

Solution

1. Step 1: Translate conditional jump

   Use 'if a < b goto L1' to jump to true branch, else execute false branch.

2. Step 2: Write false branch, jump over true branch

   False branch: c = a - b; then jump to L2 to skip true branch.

3. Step 3: Write true branch and label

   Label L1: c = a + b; Label L2 marks end of conditional.

4. Final Answer:

   if a < b goto L1
   c = a - b
   goto L2
   L1: c = a + b
   L2: -> Option C

5. Quick Check:

   Use labels and jumps for if-else in three-address code [OK]

Hint: Use labels and jumps to represent if-else in three-address code [OK]

Common Mistakes:

• Writing high-level if-else syntax directly
• Using ternary operator not valid in three-address code
• Missing jump to skip true branch after false branch