9.4 Control Constructs

We now introduce a code generation scheme for statements:

S[if(e) s1 else s2]  =      E[e]
                            cmpl $1, %eax
                            jnz   lab1
                            S[s1;]
                            jmp   lab2
                      lab1: S[s2;]
                      lab2: …

Here, lab1 and lab2 are new labels and

S[e;] = G[e]

For a WHILE-DO loop,

S[while(e) s] =  lab1: E[e]
                       cmpl $1, %eax
                       jnz   lab2
                       S[s]
                       jmp   lab1
                 lab2: …

Here, lab1 and lab2 are new labels.

An alternative code generation rule for a WHILE-DO loop:

S[while(e) s] =         jmp lab2
                  lab1: S[s]
                  lab2: E[e]
                        cmpl $1, %eax
                        jz    lab1

The constructs of high-level languages can be implemented using sequences of machine instructions. It is important to select good target instructions for each source construct. Code Generation schemes can be used to describe the mapping between the IR and target language. To get better code quality, we should:

• Make good use of the target's instructions, registers, etc.
• Use refined compilation schemes to deal with special cases.