Garbage collection

The support library contains an implementation⁶ of a conservative garbage collector using two algorithms: one using a mostly copying scheme[Bar88, Bar89] and one using mark-sweep.

A datatype can be defined to be garbage collectable with the qualifier collectable . In the following example, the datatype Exp is will be allocated from the a garbage collected heap instead of the default heap. Since it references another user defined garbage collectable object SymbolTable, the pointer to that symbol table is also marked as collectable.

   // SymbolTable is collectable
   class SymbolTable: public GCObject
   {
   public:
      class Id : public GCObject { ... };
      ...
   };

   datatype Exp :: collectable
      = INT int
      | VAR (collectable SymbolTable::Id, collectable SymbolTable *)
      | ADD Exp, Exp
      | SUB Exp, Exp
      | MUL Exp, Exp
      | DIV Exp, Exp
      ;

The corresponding instantiate datatype  statement for Exp will generate the appropriate tracing methods for garbage collection.

Please see appendix A for more details concerning garbage collection.