Three Address Statement Types - Compiler Construction - Lecture Slides, Slides of Compiler Construction

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Compiler

Construction

Lecture 38

2

Three-Address Statement Types

 Prior to proceeding with flow- of-control construct, here are the types of three-Address statements that we will use

4

Three-Address Statement Types

Assignment statement

x = op y

where op is a unary operation, e.g., unary minus, logical negation, shift operators

5

Three-Address Statement Types

Copy statement

x = y

where value of y is assigned to x

7

Three-Address Statement Types

Conditional jump

if x relop y goto L

where relop is <, =, >=, etc. If x stands in relation relop to y , execute statement with label L , next otherwise

8

Three-Address Statement Types

Indexed assignment

a) x = y[i] b) x[i] = y In a), set x to value in location i memory units beyond location y.

10

Array of Quadruples

Label Target Op Arg1 Arg L1 if_lt c d L2 goto L1: x (^) + y z L3 goto L2: x – y z L3: nop

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Flow-of-Control Statements

 We associate with a boolean expression E two labels; E.true and E.false

 The control flows to E.true if the expression evaluates to true, to E.false otherwise

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to E.true to E.false E.true:

E.false:

E.code

S 1 .code .......

S → if E then S 1

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Syntax-directed translation for S → if E then S (^1)

E.true = newlabel() E.false = S.next S 1 .next = S.next S.code = E.code || gen(E.true ‘:’) || S 1 .code

inherited attribute

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Flow-of-Control Statements

 Suppose E is a < b. E.code is

if a < b goto E.true goto E.false

 We will discuss semantic rules for boolean expressions shortly

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S → if E then S 1 else S 2

to E.true to E.false E.true:

E.false:

E.code S 1 .code

.......

goto S.next S 2 .code S.next:

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while E do S 1

to E.true to E.false E.true:

E.false:

E.code S 1 .code

.......

goto S.begin

S.begin:

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S → while E do S (^1)

S.begin = newlabel() E.true = newlabel() E.false = S.next S 1 .next = S.begin S.code = gen(S.begin ‘:’) || E.code || gen(E.true ‘:’) || S 1 .code || gen(‘goto’ S.begin)