Sohail Aslam Compiler Construction Notes Set:6-108
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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
a) x = y[i]
b) 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
S
1.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”.
