17.12 Rewriting references and assignments

References to bindings in recordised code are represented as standalone prelex structures; assignments to bindings in recordised code are represented as assign structs:

(assign ?prel ?rhs)

in which ?prel is a prelex structure and the right–hand side ?rhs is recordised code which, evaluated at run–time, will return the new binding’s value.

With respect to how references and assignments are actually implemented, there are three kinds of lexical bindings:

• Top level bindings, defined by library-letrec* core language forms; Handling of lexical top level bindings.
• Unassigned (read–only) local bindings, defined by let, letrec and letrec* core language forms and never subjected to a set! core language form; Handling of lexical local bindings.
• Assigned (read–write) local bindings, defined by let, letrec and letrec* core language forms and subjected to one or more set! core language forms; Handling of lexical local bindings.

The following bindings are exported by the library (vicare compiler).

Function: pass-rewrite-references-and-assignments input

Perform code transformations traversing the whole hierarchy in input, which must be a struct instance representing recordised code, and building a new hierarchy of recordised code; return the new hierarchy.

Transform references and assignments for top level lexical bindings and unassigned local lexical bindings according to the implementation of such operations. Remembering that the actual value of a top level binding is stored in the value field of a loc gensym, this function performs the following transformations:

• References to top level bindings are transformed into:

  (funcall (primref $symbol-value) (constant ?loc))
  

  which extracts the value from slot value of the loc gensym ?loc.

• Common assignments to top level bindings are transformed into:

  (funcall (primref $set-symbol-value!)
           (constant ?loc)
           ?rhs)
  

  which stores a new value in the slot value of the log gensym ?loc.

• Single assignments to top level bindings which also serve as binding initialisations are transformed into:

  (funcall (primref $init-symbol-value!)
           (constant ?loc)
           ?rhs)
  

  which stores a new value in the slot value of ?loc and, only if the value is recognised at run–time as being closure object, also stores value in the slot proc.

• Definitions of assigned local bindings are transformed as follows:

  (bind ((?prel ?init)) ?body)
  → (bind ((?tmp-prel ?init))
        (bind ((?prel (funcall (primref vector)
                               ?tmp-prel)))
          ?body))
  

• References to assigned local bindings are transformed from standalone prelex structs to:

  (funcall (primref $vector-ref) ?prel (constant 0))
  

• Assignments to assigned local bindings are transformed as follows:

  (assign ?prel ?rhs)
  → (funcall (primref $vector-set!)
              ?prel
              (constant 0)
              ?rhs)
  

After this compiler pass: there are no more assign structs in the returned recordised code.