Three-address code

ACompressed recap

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Three-address code has at most one operator per instruction, with temporaries for intermediate values; it is stored as quadruples (op, arg1, arg2, result), triples (results by position) or indirect triples (plus an order list). Arrays need address arithmetic: row-major base + ((i − l₁)n₂ + (j − l₂))w. Conditions become jumping code with short-circuit exits; if and while statements add labels and jumps; backpatching fills jump targets later from truelists and falselists.

BMerged rule card

4.2.1 Three-address code instruction forms; quadruples, triples, indirect triples

• TAC: ≤ 1 operator, ≤ 3 addresses per instruction; temporaries for intermediate values.
• Forms: x = y op z, x = op y, x = y, goto L, if x relop y goto L, param/call/return, x = y[i], x[i] = y, &, *.
• Quadruple: (op, arg1, arg2, result).
• Triple: (op, arg1, arg2), result = position (k).
• Indirect triples: pointer list gives order; easy reordering.

4.2.2 TAC for expressions and array accesses (address calculation)

• 1-D: base + (i − low) × w.
• Row-major 2-D: base + ((i − low₁) × n₂ + (j − low₂)) × w.
• Column-major 2-D: base + ((j − low₂) × n₁ + (i − low₁)) × w.
• k-D row-major: Horner form ((i₁ n₂ + i₂) n₃ + …) × w.
• TAC: scale indices by constants, then x = a[t] / a[t] = y with byte offset t.

4.2.3 TAC for if/while and Boolean expressions (short-circuit; backpatching brief)

• Relop: if a relop b goto B.true; goto B.false.
• ||: B₁.true = B.true, B₁.false → start of B₂. &&: B₁.true → start of B₂, B₁.false = B.false. !: swap.
• if: B; L1: S1; next. if-else: B; L1: S1; goto next; L2: S2; next. while: begin: B; L1: S; goto begin; next.
• Backpatching: truelist/falselist/nextlist; || fills B₁.falselist, && fills B₁.truelist.

CInterleaved GATE-style questions

Q1NAT2 marksAddress

int M[1..10][1..20] (4 bytes) row-major from 1000. Address of M[3][5]?

Numerical answer: ________

Q2MCQ1 markTriples

In triples, the result of the instruction at position 4 is referenced as:

• (A) t4
• (B) (4)
• (C) r4
• (D) a new variable

Q3NAT1 markInstructions

How many three-address instructions (no sharing, last op directly into x) does x = a * b + c * d − e need?

Numerical answer: ________

Q4MCQ2 marksJumping code

For B₁ && B₂ in jumping code, which is correct?

• (A) B₁.true = B.true
• (B) B₁.false = B.false
• (C) B₂.false = start of B₁
• (D) B₁.false = start of B₂

Q5MSQ1 markRepresentations

Which representations name temporaries explicitly?

• (A) quadruples
• (B) triples
• (C) indirect triples
• (D) three-address code text

Q6NAT2 marksBackpatching

Backpatching x < 100 || x > 200 && x != y from 100: what is the sum of the instruction numbers on the final falselist?

Numerical answer: ________

Q7MCQ1 markColumn-major

Fortran stores 2-D arrays in:

• (A) row-major order
• (B) column-major order
• (C) random order
• (D) diagonal order

Q8NAT1 markCount

How many instructions does the generated code for s = 0; i = 0; while (i < 10) { s = s + i; i = i + 2; } t = s * 2; have?

Numerical answer: ________