Intermediate Code Generation

Intermediate representations include syntax trees, DAGs, postfix notation and three-address code (TAC), where each instruction has at most three addresses: x = y op z.

Three-Address Code Example

Text

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

t1 = minus c
t2 = b * t1
t3 = minus c
t4 = b * t3
t5 = t2 + t4
a  = t5

Quadruples, Triples and Indirect Triples

#	op	arg1	arg2	result
0	minus	c		t1
1	*	b	t1	t2
2	minus	c		t3
3	*	b	t3	t4
4	+	t2	t4	t5
5	=	t5		a

Triples omit the result field and refer to earlier instructions by position, e.g. (1) * b (0). Indirect triples keep a separate list of pointers to triples, so instructions can be reordered easily during optimization.

Control Flow

Text

Source:  while (i < 10) { sum = sum + i; i = i + 1; }

L1: if i < 10 goto L2
    goto L3
L2: t1 = sum + i
    sum = t1
    t2 = i + 1
    i = t2
    goto L1
L3: ...

Backpatching fills in jump targets later, allowing one-pass generation of code for boolean expressions and control flow.

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