guile/module/language/tree-il/analyze.scm

;;; TREE-IL -> GLIL compiler

;; Copyright (C) 2001,2008,2009 Free Software Foundation, Inc.

;;;; This library is free software; you can redistribute it and/or
;;;; modify it under the terms of the GNU Lesser General Public
;;;; License as published by the Free Software Foundation; either
;;;; version 3 of the License, or (at your option) any later version.
;;;; 
;;;; This library is distributed in the hope that it will be useful,
;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;;;; Lesser General Public License for more details.
;;;; 
;;;; You should have received a copy of the GNU Lesser General Public
;;;; License along with this library; if not, write to the Free Software
;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

;;; Code:

(define-module (language tree-il analyze)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (system base syntax)
  #:use-module (system base message)
  #:use-module (language tree-il)
  #:export (analyze-lexicals
            report-unused-variables
            report-possibly-unbound-variables))

;; Allocation is the process of assigning storage locations for lexical
;; variables. A lexical variable has a distinct "address", or storage
;; location, for each procedure in which it is referenced.
;;
;; A variable is "local", i.e., allocated on the stack, if it is
;; referenced from within the procedure that defined it. Otherwise it is
;; a "closure" variable. For example:
;;
;;    (lambda (a) a) ; a will be local
;; `a' is local to the procedure.
;;
;;    (lambda (a) (lambda () a))
;; `a' is local to the outer procedure, but a closure variable with
;; respect to the inner procedure.
;;
;; If a variable is ever assigned, it needs to be heap-allocated
;; ("boxed"). This is so that closures and continuations capture the
;; variable's identity, not just one of the values it may have over the
;; course of program execution. If the variable is never assigned, there
;; is no distinction between value and identity, so closing over its
;; identity (whether through closures or continuations) can make a copy
;; of its value instead.
;;
;; Local variables are stored on the stack within a procedure's call
;; frame. Their index into the stack is determined from their linear
;; postion within a procedure's binding path:
;; (let (0 1)
;;   (let (2 3) ...)
;;   (let (2) ...))
;;   (let (2 3 4) ...))
;; etc.
;;
;; This algorithm has the problem that variables are only allocated
;; indices at the end of the binding path. If variables bound early in
;; the path are not used in later portions of the path, their indices
;; will not be recycled. This problem is particularly egregious in the
;; expansion of `or':
;;
;;  (or x y z)
;;    -> (let ((a x)) (if a a (let ((b y)) (if b b z))))
;;
;; As you can see, the `a' binding is only used in the ephemeral `then'
;; clause of the first `if', but its index would be reserved for the
;; whole of the `or' expansion. So we have a hack for this specific
;; case. A proper solution would be some sort of liveness analysis, and
;; not our linear allocation algorithm.
;;
;; Closure variables are captured when a closure is created, and stored
;; in a vector. Each closure variable has a unique index into that
;; vector.
;;
;; There is one more complication. Procedures bound by <fix> may, in
;; some cases, be rendered inline to their parent procedure. That is to
;; say,
;;
;;  (letrec ((lp (lambda () (lp)))) (lp))
;;    => (fix ((lp (lambda () (lp)))) (lp))
;;      => goto FIX-BODY; LP: goto LP; FIX-BODY: goto LP;
;;         ^ jump over the loop  ^ the fixpoint lp ^ starting off the loop
;;
;; The upshot is that we don't have to allocate any space for the `lp'
;; closure at all, as it can be rendered inline as a loop. So there is
;; another kind of allocation, "label allocation", in which the
;; procedure is simply a label, placed at the start of the lambda body.
;; The label is the gensym under which the lambda expression is bound.
;;
;; The analyzer checks to see that the label is called with the correct
;; number of arguments. Calls to labels compile to rename + goto.
;; Lambda, the ultimate goto!
;;
;;
;; The return value of `analyze-lexicals' is a hash table, the
;; "allocation".
;;
;; The allocation maps gensyms -- recall that each lexically bound
;; variable has a unique gensym -- to storage locations ("addresses").
;; Since one gensym may have many storage locations, if it is referenced
;; in many procedures, it is a two-level map.
;;
;; The allocation also stored information on how many local variables
;; need to be allocated for each procedure, lexicals that have been
;; translated into labels, and information on what free variables to
;; capture from its lexical parent procedure.
;;
;; That is:
;;
;;  sym -> {lambda -> address}
;;  lambda -> (nlocs labels . free-locs)
;;
;; address ::= (local? boxed? . index)
;; labels ::= ((sym . lambda-vars) ...)
;; free-locs ::= ((sym0 . address0) (sym1 . address1) ...)
;; free variable addresses are relative to parent proc.

(define (make-hashq k v)
  (let ((res (make-hash-table)))
    (hashq-set! res k v)
    res))

(define (analyze-lexicals x)
  ;; bound-vars: lambda -> (sym ...)
  ;;  all identifiers bound within a lambda
  (define bound-vars (make-hash-table))
  ;; free-vars: lambda -> (sym ...)
  ;;  all identifiers referenced in a lambda, but not bound
  ;;  NB, this includes identifiers referenced by contained lambdas
  (define free-vars (make-hash-table))
  ;; assigned: sym -> #t
  ;;  variables that are assigned
  (define assigned (make-hash-table))
  ;; refcounts: sym -> count
  ;;  allows us to detect the or-expansion in O(1) time
  (define refcounts (make-hash-table))
  ;; labels: sym -> lambda-vars
  ;;  for determining if fixed-point procedures can be rendered as
  ;;  labels. lambda-vars may be an improper list.
  (define labels (make-hash-table))

  ;; returns variables referenced in expr
  (define (analyze! x proc labels-in-proc tail? tail-call-args)
    (define (step y) (analyze! y proc labels-in-proc #f #f))
    (define (step-tail y) (analyze! y proc labels-in-proc tail? #f))
    (define (step-tail-call y args) (analyze! y proc labels-in-proc #f
                                              (and tail? args)))
    (define (recur/labels x new-proc labels)
      (analyze! x new-proc (append labels labels-in-proc) #t #f))
    (define (recur x new-proc) (analyze! x new-proc '() tail? #f))
    (record-case x
      ((<application> proc args)
       (apply lset-union eq? (step-tail-call proc args)
              (map step args)))

      ((<conditional> test then else)
       (lset-union eq? (step test) (step-tail then) (step-tail else)))

      ((<lexical-ref> gensym)
       (hashq-set! refcounts gensym (1+ (hashq-ref refcounts gensym 0)))
       (if (not (and tail-call-args
                     (memq gensym labels-in-proc)
                     (let ((args (hashq-ref labels gensym)))
                       (and (list? args)
                            (= (length args) (length tail-call-args))))))
           (hashq-set! labels gensym #f))
       (list gensym))
      
      ((<lexical-set> gensym exp)
       (hashq-set! assigned gensym #t)
       (hashq-set! labels gensym #f)
       (lset-adjoin eq? (step exp) gensym))
      
      ((<module-set> exp)
       (step exp))
      
      ((<toplevel-set> exp)
       (step exp))
      
      ((<toplevel-define> exp)
       (step exp))
      
      ((<sequence> exps)
       (let lp ((exps exps) (ret '()))
         (cond ((null? exps) '())
               ((null? (cdr exps))
                (lset-union eq? ret (step-tail (car exps))))
               (else
                (lp (cdr exps) (lset-union eq? ret (step (car exps))))))))
      
      ((<lambda> vars body)
       (let ((locally-bound (let rev* ((vars vars) (out '()))
                              (cond ((null? vars) out)
                                    ((pair? vars) (rev* (cdr vars)
                                                        (cons (car vars) out)))
                                    (else (cons vars out))))))
         (hashq-set! bound-vars x locally-bound)
         (let* ((referenced (recur body x))
                (free (lset-difference eq? referenced locally-bound))
                (all-bound (reverse! (hashq-ref bound-vars x))))
           (hashq-set! bound-vars x all-bound)
           (hashq-set! free-vars x free)
           free)))
      
      ((<let> vars vals body)
       (hashq-set! bound-vars proc
                   (append (reverse vars) (hashq-ref bound-vars proc)))
       (lset-difference eq?
                        (apply lset-union eq? (step-tail body) (map step vals))
                        vars))
      
      ((<letrec> vars vals body)
       (hashq-set! bound-vars proc
                   (append (reverse vars) (hashq-ref bound-vars proc)))
       (for-each (lambda (sym) (hashq-set! assigned sym #t)) vars)
       (lset-difference eq?
                        (apply lset-union eq? (step-tail body) (map step vals))
                        vars))
      
      ((<fix> vars vals body)
       ;; Try to allocate these procedures as labels.
       (for-each (lambda (sym val) (hashq-set! labels sym (lambda-vars val)))
                 vars vals)
       (hashq-set! bound-vars proc
                   (append (reverse vars) (hashq-ref bound-vars proc)))
       ;; Step into subexpressions.
       (let* ((var-refs
               (map
                ;; Since we're trying to label-allocate the lambda,
                ;; pretend it's not a closure, and just recurse into its
                ;; body directly. (Otherwise, recursing on a closure
                ;; that references one of the fix's bound vars would
                ;; prevent label allocation.)
                (lambda (x)
                  (record-case x
                    ((<lambda> (lvars vars) body)
                     (let ((locally-bound
                            (let rev* ((lvars lvars) (out '()))
                              (cond ((null? lvars) out)
                                    ((pair? lvars) (rev* (cdr lvars)
                                                         (cons (car lvars) out)))
                                    (else (cons lvars out))))))
                       (hashq-set! bound-vars x locally-bound)
                       ;; recur/labels, the difference from the closure case
                       (let* ((referenced (recur/labels body x vars))
                              (free (lset-difference eq? referenced locally-bound))
                              (all-bound (reverse! (hashq-ref bound-vars x))))
                         (hashq-set! bound-vars x all-bound)
                         (hashq-set! free-vars x free)
                         free)))))
                vals))
              (vars-with-refs (map cons vars var-refs))
              (body-refs (recur/labels body proc vars)))
         (define (delabel-dependents! sym)
           (let ((refs (assq-ref vars-with-refs sym)))
             (if refs
                 (for-each (lambda (sym)
                             (if (hashq-ref labels sym)
                                 (begin
                                   (hashq-set! labels sym #f)
                                   (delabel-dependents! sym))))
                           refs))))
         ;; Stepping into the lambdas and the body might have made some
         ;; procedures not label-allocatable -- which might have
         ;; knock-on effects. For example:
         ;;   (fix ((a (lambda () (b)))
         ;;         (b (lambda () a)))
         ;;     (a))
         ;; As far as `a' is concerned, both `a' and `b' are
         ;; label-allocatable. But `b' references `a' not in a proc-tail
         ;; position, which makes `a' not label-allocatable. The
         ;; knock-on effect is that, when back-propagating this
         ;; information to `a', `b' will also become not
         ;; label-allocatable, as it is referenced within `a', which is
         ;; allocated as a closure. This is a transitive relationship.
         (for-each (lambda (sym)
                     (if (not (hashq-ref labels sym))
                         (delabel-dependents! sym)))
                   vars)
         ;; Now lift bound variables with label-allocated lambdas to the
         ;; parent procedure.
         (for-each
          (lambda (sym val)
            (if (hashq-ref labels sym)
                ;; Remove traces of the label-bound lambda. The free
                ;; vars will propagate up via the return val.
                (begin
                  (hashq-set! bound-vars proc
                              (append (hashq-ref bound-vars val)
                                      (hashq-ref bound-vars proc)))
                  (hashq-remove! bound-vars val)
                  (hashq-remove! free-vars val))))
          vars vals)
         (lset-difference eq?
                          (apply lset-union eq? body-refs var-refs)
                          vars)))
      
      ((<let-values> vars exp body)
       (let ((bound (let lp ((out (hashq-ref bound-vars proc)) (in vars))
                      (if (pair? in)
                          (lp (cons (car in) out) (cdr in))
                          (if (null? in) out (cons in out))))))
         (hashq-set! bound-vars proc bound)
         (lset-difference eq?
                          (lset-union eq? (step exp) (step-tail body))
                          bound)))
      
      (else '())))
  
  ;; allocation: sym -> {lambda -> address}
  ;;             lambda -> (nlocs labels . free-locs)
  (define allocation (make-hash-table))
  
  (define (allocate! x proc n)
    (define (recur y) (allocate! y proc n))
    (record-case x
      ((<application> proc args)
       (apply max (recur proc) (map recur args)))

      ((<conditional> test then else)
       (max (recur test) (recur then) (recur else)))

      ((<lexical-set> exp)
       (recur exp))
      
      ((<module-set> exp)
       (recur exp))
      
      ((<toplevel-set> exp)
       (recur exp))
      
      ((<toplevel-define> exp)
       (recur exp))
      
      ((<sequence> exps)
       (apply max (map recur exps)))
      
      ((<lambda> vars body)
       ;; allocate closure vars in order
       (let lp ((c (hashq-ref free-vars x)) (n 0))
         (if (pair? c)
             (begin
               (hashq-set! (hashq-ref allocation (car c))
                           x
                           `(#f ,(hashq-ref assigned (car c)) . ,n))
               (lp (cdr c) (1+ n)))))
      
       (let ((nlocs
              (let lp ((vars vars) (n 0))
                (if (not (null? vars))
                    ;; allocate args
                    (let ((v (if (pair? vars) (car vars) vars)))
                      (hashq-set! allocation v
                                  (make-hashq
                                   x `(#t ,(hashq-ref assigned v) . ,n)))
                      (lp (if (pair? vars) (cdr vars) '()) (1+ n)))
                    ;; allocate body, return number of additional locals
                    (- (allocate! body x n) n))))
             (free-addresses
              (map (lambda (v)
                     (hashq-ref (hashq-ref allocation v) proc))
                   (hashq-ref free-vars x)))
             (labels (filter cdr
                             (map (lambda (sym)
                                    (cons sym (hashq-ref labels sym)))
                                  (hashq-ref bound-vars x)))))
         ;; set procedure allocations
         (hashq-set! allocation x (cons* nlocs labels free-addresses)))
       n)

      ((<let> vars vals body)
       (let ((nmax (apply max (map recur vals))))
         (cond
          ;; the `or' hack
          ((and (conditional? body)
                (= (length vars) 1)
                (let ((v (car vars)))
                  (and (not (hashq-ref assigned v))
                       (= (hashq-ref refcounts v 0) 2)
                       (lexical-ref? (conditional-test body))
                       (eq? (lexical-ref-gensym (conditional-test body)) v)
                       (lexical-ref? (conditional-then body))
                       (eq? (lexical-ref-gensym (conditional-then body)) v))))
           (hashq-set! allocation (car vars)
                       (make-hashq proc `(#t #f . ,n)))
           ;; the 1+ for this var
           (max nmax (1+ n) (allocate! (conditional-else body) proc n)))
          (else
           (let lp ((vars vars) (n n))
             (if (null? vars)
                 (max nmax (allocate! body proc n))
                 (let ((v (car vars)))
                   (hashq-set!
                    allocation v
                    (make-hashq proc
                                `(#t ,(hashq-ref assigned v) . ,n)))
                   (lp (cdr vars) (1+ n)))))))))
      
      ((<letrec> vars vals body)
       (let lp ((vars vars) (n n))
         (if (null? vars)
             (let ((nmax (apply max
                                (map (lambda (x)
                                       (allocate! x proc n))
                                     vals))))
               (max nmax (allocate! body proc n)))
             (let ((v (car vars)))
               (hashq-set!
                allocation v
                (make-hashq proc
                            `(#t ,(hashq-ref assigned v) . ,n)))
               (lp (cdr vars) (1+ n))))))

      ((<fix> vars vals body)
       (let lp ((in vars) (n n))
         (if (null? in)
             (let lp ((vars vars) (vals vals) (nmax n))
               (cond
                ((null? vars)
                 (max nmax (allocate! body proc n)))
                ((hashq-ref labels (car vars))                 
                 ;; allocate label bindings & body inline to proc
                 (lp (cdr vars)
                     (cdr vals)
                     (record-case (car vals)
                       ((<lambda> vars body)
                        (let lp ((vars vars) (n n))
                          (if (not (null? vars))
                              ;; allocate bindings
                              (let ((v (if (pair? vars) (car vars) vars)))
                                (hashq-set!
                                 allocation v
                                 (make-hashq
                                  proc `(#t ,(hashq-ref assigned v) . ,n)))
                                (lp (if (pair? vars) (cdr vars) '()) (1+ n)))
                              ;; allocate body
                              (max nmax (allocate! body proc n))))))))
                (else
                 ;; allocate closure
                 (lp (cdr vars)
                     (cdr vals)
                     (max nmax (allocate! (car vals) proc n))))))
             
             (let ((v (car in)))
               (cond
                ((hashq-ref assigned v)
                 (error "fixpoint procedures may not be assigned" x))
                ((hashq-ref labels v)
                 ;; no binding, it's a label
                 (lp (cdr in) n))
                (else
                 ;; allocate closure binding
                 (hashq-set! allocation v (make-hashq proc `(#t #f . ,n)))
                 (lp (cdr in) (1+ n))))))))

      ((<let-values> vars exp body)
       (let ((nmax (recur exp)))
         (let lp ((vars vars) (n n))
           (cond
            ((null? vars)
             (max nmax (allocate! body proc n)))
            ((not (pair? vars))
             (hashq-set! allocation vars
                         (make-hashq proc
                                     `(#t ,(hashq-ref assigned vars) . ,n)))
             ;; the 1+ for this var
             (max nmax (allocate! body proc (1+ n))))
            (else               
             (let ((v (car vars)))
               (hashq-set!
                allocation v
                (make-hashq proc
                            `(#t ,(hashq-ref assigned v) . ,n)))
               (lp (cdr vars) (1+ n))))))))
      
      (else n)))

  (analyze! x #f '() #t #f)
  (allocate! x #f 0)

  allocation)


;;;
;;; Unused variable analysis.
;;;

;; <binding-info> records are used during tree traversals in
;; `report-unused-variables'.  They contain a list of the local vars
;; currently in scope, a list of locals vars that have been referenced, and a
;; "location stack" (the stack of `tree-il-src' values for each parent tree).
(define-record-type <binding-info>
  (make-binding-info vars refs locs)
  binding-info?
  (vars binding-info-vars)  ;; ((GENSYM NAME LOCATION) ...)
  (refs binding-info-refs)  ;; (GENSYM ...)
  (locs binding-info-locs)) ;; (LOCATION ...)

(define (report-unused-variables tree env)
  "Report about unused variables in TREE.  Return TREE."

  (define (dotless-list lst)
    ;; If LST is a dotted list, return a proper list equal to LST except that
    ;; the very last element is a pair; otherwise return LST.
    (let loop ((lst    lst)
               (result '()))
      (cond ((null? lst)
             (reverse result))
            ((pair? lst)
             (loop (cdr lst) (cons (car lst) result)))
            (else
             (loop '() (cons lst result))))))

  (tree-il-fold (lambda (x info)
                  ;; X is a leaf: extend INFO's refs accordingly.
                  (let ((refs (binding-info-refs info))
                        (vars (binding-info-vars info))
                        (locs (binding-info-locs info)))
                    (record-case x
                      ((<lexical-ref> gensym)
                       (make-binding-info vars (cons gensym refs) locs))
                      (else info))))

                (lambda (x info)
                  ;; Going down into X: extend INFO's variable list
                  ;; accordingly.
                  (let ((refs (binding-info-refs info))
                        (vars (binding-info-vars info))
                        (locs (binding-info-locs info))
                        (src  (tree-il-src x)))
                    (define (extend inner-vars inner-names)
                      (append (map (lambda (var name)
                                     (list var name src))
                                   inner-vars
                                   inner-names)
                              vars))
                    (record-case x
                      ((<lexical-set> gensym)
                       (make-binding-info vars (cons gensym refs)
                                          (cons src locs)))
                      ((<lambda> vars names)
                       (let ((vars  (dotless-list vars))
                             (names (dotless-list names)))
                         (make-binding-info (extend vars names) refs
                                            (cons src locs))))
                      ((<let> vars names)
                       (make-binding-info (extend vars names) refs
                                          (cons src locs)))
                      ((<letrec> vars names)
                       (make-binding-info (extend vars names) refs
                                          (cons src locs)))
                      ((<fix> vars names)
                       (make-binding-info (extend vars names) refs
                                          (cons src locs)))
                      ((<let-values> vars names)
                       (make-binding-info (extend vars names) refs
                                          (cons src locs)))
                      (else info))))

                (lambda (x info)
                  ;; Leaving X's scope: shrink INFO's variable list
                  ;; accordingly and reported unused nested variables.
                  (let ((refs (binding-info-refs info))
                        (vars (binding-info-vars info))
                        (locs (binding-info-locs info)))
                    (define (shrink inner-vars refs)
                      (for-each (lambda (var)
                                  (let ((gensym (car var)))
                                    ;; Don't report lambda parameters as
                                    ;; unused.
                                    (if (and (not (memq gensym refs))
                                             (not (and (lambda? x)
                                                       (memq gensym
                                                             inner-vars))))
                                        (let ((name (cadr var))
                                              ;; We can get approximate
                                              ;; source location by going up
                                              ;; the LOCS location stack.
                                              (loc  (or (caddr var)
                                                        (find pair? locs))))
                                          (warning 'unused-variable loc name)))))
                                (filter (lambda (var)
                                          (memq (car var) inner-vars))
                                        vars))
                      (fold alist-delete vars inner-vars))

                    ;; For simplicity, we leave REFS untouched, i.e., with
                    ;; names of variables that are now going out of scope.
                    ;; It doesn't hurt as these are unique names, it just
                    ;; makes REFS unnecessarily fat.
                    (record-case x
                      ((<lambda> vars)
                       (let ((vars (dotless-list vars)))
                         (make-binding-info (shrink vars refs) refs
                                            (cdr locs))))
                      ((<let> vars)
                       (make-binding-info (shrink vars refs) refs
                                          (cdr locs)))
                      ((<letrec> vars)
                       (make-binding-info (shrink vars refs) refs
                                          (cdr locs)))
                      ((<fix> vars)
                       (make-binding-info (shrink vars refs) refs
                                          (cdr locs)))
                      ((<let-values> vars)
                       (make-binding-info (shrink vars refs) refs
                                          (cdr locs)))
                      (else info))))
                (make-binding-info '() '() '())
                tree)
  tree)


;;;
;;; Unbound variable analysis.
;;;

;; <toplevel-info> records are used during tree traversal in search of
;; possibly unbound variable.  They contain a list of references to
;; potentially unbound top-level variables, a list of the top-level defines
;; that have been encountered, and a "location stack" (see above).
(define-record-type <toplevel-info>
  (make-toplevel-info refs defs locs)
  toplevel-info?
  (refs  toplevel-info-refs)  ;; ((VARIABLE-NAME . LOCATION) ...)
  (defs  toplevel-info-defs)  ;; (VARIABLE-NAME ...)
  (locs  toplevel-info-locs)) ;; (LOCATION ...)

(define (env-module e)
  "Return the module corresponding to E."
  ;; XXX: This is a bit of a hack since since representation of compile-time
  ;; environments is hidden in `(language scheme compile-tree-il)'.
  (cond ((pair? e)   (car e))
        ((module? e) e)
        (else        (current-module))))

;; TODO: Combine with `report-unused-variables' so we don't traverse the tree
;; once for each warning type.

(define (report-possibly-unbound-variables tree env)
  "Return possibly unbound variables in TREE.  Return TREE."
  (define toplevel
    (let ((env (env-module env)))
      (tree-il-fold (lambda (x info)
                      ;; X is a leaf: extend INFO's refs accordingly.
                      (let ((refs (toplevel-info-refs info))
                            (defs (toplevel-info-defs info))
                            (locs (toplevel-info-locs info)))
                        (define (bound? name)
                          (or (and (module? env)
                                   (module-variable env name))
                              (memq name defs)))

                        (record-case x
                          ((<toplevel-ref> name src)
                           (if (bound? name)
                               info
                               (let ((src (or src (find pair? locs))))
                                 (make-toplevel-info (alist-cons name src refs)
                                                     defs
                                                     locs))))
                          (else info))))

                    (lambda (x info)
                      ;; Going down into X.
                      (let* ((refs (toplevel-info-refs info))
                             (defs (toplevel-info-defs info))
                             (src  (tree-il-src x))
                             (locs (cons src (toplevel-info-locs info))))
                        (define (bound? name)
                          (or (and (module? env)
                                   (module-variable env name))
                              (memq name defs)))

                        (record-case x
                          ((<toplevel-set> name src)
                           (if (bound? name)
                               (make-toplevel-info refs defs locs)
                               (let ((src (find pair? locs)))
                                 (make-toplevel-info (alist-cons name src refs)
                                                     defs
                                                     locs))))
                          ((<toplevel-define> name)
                           (make-toplevel-info (alist-delete name refs eq?)
                                               (cons name defs)
                                               locs))
                          (else
                           (make-toplevel-info refs defs locs)))))

                    (lambda (x info)
                      ;; Leaving X's scope.
                      (let ((refs (toplevel-info-refs info))
                            (defs (toplevel-info-defs info))
                            (locs (toplevel-info-locs info)))
                        (make-toplevel-info refs defs (cdr locs))))

                    (make-toplevel-info '() '() '())
                    tree)))

  (for-each (lambda (name+loc)
              (let ((name (car name+loc))
                    (loc  (cdr name+loc)))
                (warning 'unbound-variable loc name)))
            (reverse (toplevel-info-refs toplevel)))

  tree)