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Related Content: CS606 - VU Lectures, Handouts, PPT Slides, Assignments, Quizzes, Papers & Books of Compiler Construction
Three-Address Statement Types
Prior to proceeding with flow-of-control construct, here are the types of three-Address statements that we will use
- Assignment statement
x = y op z
op is a binary arithmetic or logical operation - Copy statement
x = y
value of y is assigned to x - Unconditional jump
goto L
The three-address statement with label L is executed next - Conditional jump
if x relop y goto L
relop is <, =, >=, etc. If x stands in relation relop to y, execute statement with label L, next otherwise - Indexed assignment
- x = y[i]
- x[i] = y
In a), set x to value in location i memory units beyond location y.
In b), set contents of location i memory units beyond x to y.
We associate with a boolean expression E two labels (attributes); E.true and E.false. The
control flows to E.true if the expression evaluates to true, to E.false otherwise.
Following is syntax-directed translation for
S → if E then S1
E.true = newlabel()
E.false = S.next
S1.next = S.next
S.code = E.code || gen(E.true ‘:’) || S1.code
The attribute “next” records the label of the next statement to execute. “code” is string-valued attribute that holds the actual code generated in the form of a character string. The code can be eventually written out to a file. The || is the string concatenation operator, that is “hello”|| “ world” will yield the combined string “hello world”.
Suppose E is “a < b”. E.code would be
if a < b goto E.true
goto E.false
We will discuss semantic rules for boolean expressions shortly
The syntax-directed translation for
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