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Related Content: CS606 - VU Lectures, Handouts, PPT Slides, Assignments, Quizzes, Papers & Books of Compiler Construction
Here is the bottom-up parse of the assignment statement a = b*-c + b*-c and the syntax-directed translation into three-address code.
| Parser action | attribute | Three- address code |
| id=id ∗ –id + id ∗ –id | ||
| id=E1 ∗ –id + id ∗ –id | E1.place = b | |
| id=E1 ∗ –E2 + id ∗ –id | E2.place = c | |
| id=E1 ∗ E2 + id ∗ –id | E2.place = t1 | t1 = – c |
| id=E1 + id ∗ –id | E1.place = t2 | t2 = b∗t1 |
| id=E1 + E2 ∗ –id | E2.place = b | |
| id=E1 + E2 ∗ –E3 | E3.place = c | |
| id=E1 + E2 ∗ E3 | E3.place = t3 | t3 = – c |
| id=E1 + E2 | E2.place = t4 | t4 = b∗t3 |
| id=E1 | E1.place = t5 | t5 = t2+t4 |
| S | a = t5 |
Representing Linear Codes
Three-address codes are often implemented as a set of quadruples. Each quadruple has
four fields: an operator, two operands (or sources) and a destination. In C++, for
example, one can design a quadruple class and then declare a simple array of quadruples.
This leads to the following arrangement; the index of the array element acts as the
number of quadruple generated.
| Target | Op | Arg1 | Arg2 |
| t1 | ← | 2 | |
| t2 | ← | y | |
| t3 | × | t1 | t2 |
| t4 | ← | x | |
| t5 | – | t4 | t3 |
An array of pointers to quads can be employed which leads to the following structure:
Flow-of-Control Statements
We now use the syntax-directed translation scheme for the flow-of-control statements found in most procedural programming languages.
where E is a boolean expression. Consider the statement
if c < d then
x = y + z
else
x = y – z
One possible 3-address code could be
if c < d goto L1
goto L2
L1: x = y + z
goto L3
L2: x = y – z
L3: nop
We will assume that a three-address statement can be symbolically labeled;
the function newlabel() returns a new symbolic label each time it is called
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