CSC448: AST Transformations: Overview of Today's Class [45/133]

Type checking continued.

Brief overview of runtime system.

Simplifying compilation units via source to source transformations:

• Elimination of initializers.

  int x = 1;    -->    int x;
                       x = 1;
              

• Elimination of high-level control structures; or introduction of (conditional) branching.

  if (e)        -->    if (e)
    s1                   goto l1;
  else                 else
    s2                   goto l2;
                     l1:
                       s1
                       goto l3;
                     l2:
                       s2
                       goto l3;
                     l3:
              

• Introduction of three address code (and temporary variables).

  x = e1+f(e2); -->    int t1;
                       int t2;
                       int t3;
                       t1 = e1;
                       t2 = e2;
                       t3 = f(t2);
                       x = t2 + t3;
              

• Unique renaming of labels.

  l:            -->    l1:
    if (e) {             if (e) {
    l:                   l2:
      s                    s
    }                    }
  
              

Comments:

• Preserve semantics of programs, e.g., scope of local variables unchanged.
• Efficiency not a consideration: extra temporary variables, useless jumps, etc.