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Intermediate Code Generation — TAC, 3-Address Code

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Intermediate Code Generation — TAC, 3-Address Code

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Intermediate Code Generation — TAC, 3-Address Code

🎯 Learning Objectives

Translate expressions to three-address code
Handle array references and pointer dereferences
Generate TAC for control flow statements
Use backpatching for boolean expressions

1. Three-Address Code (TAC)

1.1 Instruction Forms

Form	Meaning	Example
`x = y op z`	Binary operation	`t1 = a + b`
`x = op y`	Unary operation	`t2 = -t1`
`x = y`	Copy	`t3 = t2`
`goto L`	Unconditional jump	`goto L1`
`if x goto L`	Conditional jump	`if t1 < 0 goto L1`
`param x`	Parameter passing	`param a`
`call f, n`	Function call	`call sum, 2`
`x = y[i]`	Array access	`t4 = arr[t3]`
`x[i] = y`	Array store	`arr[t3] = t4`

1.2 Translation Example

Source: a = b * -c + b * -c


pseudo
t1 = -c
t2 = b * t1
t3 = -c
t4 = b * t3
t5 = t2 + t4
a = t5

Optimized (common subexpression elimination):


pseudo
t1 = -c
t2 = b * t1
t3 = t2 + t2
a = t3

2. Array Address Computation

2.1 1D Array

arr[i] → address = base + i × element_size TAC:


pseudo
t1 = i * 4       // element size = 4 bytes
t2 = arr + t1    // base address + offset
t3 = *t2         // load value

2.2 2D Array (Row-Major)

arr[i][j] → address = base + (i × num_cols + j) × element_size TAC:


pseudo
t1 = i * num_cols
t2 = t1 + j
t3 = t2 * 4
t4 = arr + t3
t5 = *t4

3. Control Flow Translation

3.1 If-Then-Else

Source:


c
if (x < 100) {
    y = x + 1;
} else {
    y = 0;
}

TAC:


pseudo
    if x < 100 goto L1
    goto L2
L1: t1 = x + 1
    y = t1
    goto L3
L2: y = 0
L3: ...

4. 📝 Practice Questions

Q1: Translate a = (b + c) * (d - e) to TAC.

Answer:
pseudo
t1 = b + c
t2 = d - e
t3 = t1 * t2
a = t3
Q2: Explain the difference between TAC and AST.

Answer: AST (Abstract Syntax Tree) is a tree representation of program syntax. TAC (Three-Address Code) is a linear sequence of simple instructions. TAC is closer to machine code, easier to optimize, and can be generated from AST by walking the tree. Q3: Generate TAC for for (i=0; i<10; i++) sum += i;

Answer:
pseudo
    i = 0
L1: if i < 10 goto L2
    goto L3
L2: t1 = sum + i
    sum = t1
    t2 = i + 1
    i = t2
    goto L1
L3: ...
Q4: What is backpatching?

Answer: Backpatching is a technique for generating TAC for boolean expressions and control flow when jump targets are unknown at generation time. It leaves the target address empty and fills it in later when the label's location is known. This avoids multiple passes over the code. Q5: Translate if (a > b) x = 1; else x = 2; to TAC.

Answer:
pseudo
    if a > b goto L1
    goto L2
L1: x = 1
    goto L3
L2: x = 2
L3: ...

5. 🔗 Cross-References

Week 9 - Optimization: Optimizing TAC
Week 11 - Code Generation: Translating TAC to assembly
Week 8 - Control Flow: Backpatching for boolean expressions Join Discord PreviousSemantic AnalysisNextCode Optimization

Intermediate Code Generation — TAC, 3-Address Code

Intermediate Code Generation — TAC, 3-Address Code

🎯 Learning Objectives

1. Three-Address Code (TAC)

1.1 Instruction Forms

1.2 Translation Example

2. Array Address Computation

2.1 1D Array

2.2 2D Array (Row-Major)

3. Control Flow Translation

3.1 If-Then-Else

4. 📝 Practice Questions

5. 🔗 Cross-References

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