Introduction

This chapter is an introduction to the compiling techniques in Chapters 3 through 6 of this book. It illustrates the techniques by developing a working Java program that translates representative programming language statements into three-address code, an intermediate representation.

We start small by creating a syntax-directed translator that maps infix arithmetic expressions into postfix expressions. We then extend this translator to map code fragments as shown in Fig. 2.1 into three-address code of the form in Fig. 2.2.

TIPS: It is not easy to implement mapping infix arithmetic expressions into postfix expressions, there are some algorithms

• Shunting-yard algorithm

{
    int i; int j; float[100] a; float v; float x;
    while ( true ) {
        do i = i+1; while ( a[i] < v );
        do j = j-1; while ( a[j] > v );
        if ( i >= j ) break;
        x = a[i]; a[i] = a[j]; a[j] = x;
    }
}

Figure 2.1: A code fragment to be translated

1:	i = i + 1
2:	t1 = a [ i ]
3:	if t1 < v goto 1
4:	j = j - 1
5:	t2 = a [ j ]
6:	if t2 > v goto 4
7:	ifFalse i >= j goto 9
8:	goto 14
9:	x = a [ i ]
10:	t3 = a [ j ]
11:	a [ i ] = t3
12:	a [ j ] = x
13:	goto 1
14:

Figure 2.2: Simplified intermediate code for the program fragment in Fig. 2.1