;; Continuation-passing style (CPS) intermediate language (IL)

;; Copyright (C) 2013-2020 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

;;; Commentary:
;;;
;;; A module to assign stack slots to variables in a CPS term.
;;;
;;; Code:

(define-module (language cps slot-allocation)
  #:use-module (ice-9 control)
  #:use-module (ice-9 match)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-11)
  #:use-module (srfi srfi-26)
  #:use-module (language cps)
  #:use-module (language cps utils)
  #:use-module (language cps intmap)
  #:use-module (language cps intset)
  #:export (allocate-slots
            lookup-slot
            lookup-maybe-slot
            lookup-representation
            lookup-constant-value
            lookup-maybe-constant-value
            lookup-nlocals
            lookup-call-proc-slot
            lookup-parallel-moves
            lookup-slot-map))

(define-record-type $allocation
  (make-allocation slots representations constant-values call-allocs
                   shuffles frame-size)
  allocation?

  ;; A map of VAR to slot allocation.  A slot allocation is an integer,
  ;; if the variable has been assigned a slot.
  ;;
  (slots allocation-slots)

  ;; A map of VAR to representation.  A representation is 'scm, 'f64,
  ;; 'u64, or 's64.
  ;;
  (representations allocation-representations)

  ;; A map of VAR to constant value, for variables with constant values.
  ;;
  (constant-values allocation-constant-values)

  ;; A map of LABEL to /call allocs/, for expressions that continue to
  ;; $kreceive continuations: non-tail calls and $prompt expressions.
  ;;
  ;; A call alloc contains two pieces of information: the call's /proc
  ;; slot/ and a /dead slot map/.  The proc slot indicates the slot of a
  ;; procedure in a procedure call, or where the procedure would be in a
  ;; multiple-value return.
  ;;
  ;; The dead slot map indicates, what slots should be ignored by GC
  ;; when marking the frame.  A dead slot map is a bitfield, as an
  ;; integer.
  ;;
  (call-allocs allocation-call-allocs)

  ;; A map of LABEL to /parallel moves/.  Parallel moves shuffle locals
  ;; into position for a $call, $callk, or $values, or shuffle returned
  ;; values back into place in a $kreceive.
  ;;
  ;; A set of moves is expressed as an ordered list of (SRC . DST)
  ;; moves, where SRC and DST are slots.  This may involve a temporary
  ;; variable.
  ;;
  (shuffles allocation-shuffles)

  ;; The number of local slots needed for this function.  Because we can
  ;; contify common clause tails, we use one frame size for all clauses
  ;; to avoid having to adjust the frame size when continuing to labels
  ;; from other clauses.
  ;;
  (frame-size allocation-frame-size))

(define-record-type $call-alloc
  (make-call-alloc proc-slot slot-map)
  call-alloc?
  (proc-slot call-alloc-proc-slot)
  (slot-map call-alloc-slot-map))

(define (lookup-maybe-slot var allocation)
  (intmap-ref (allocation-slots allocation) var (lambda (_) #f)))

(define (lookup-slot var allocation)
  (intmap-ref (allocation-slots allocation) var))

(define (lookup-representation var allocation)
  (intmap-ref (allocation-representations allocation) var))

(define *absent* (list 'absent))

(define (lookup-constant-value var allocation)
  (let ((value (intmap-ref (allocation-constant-values allocation) var
                           (lambda (_) *absent*))))
    (when (eq? value *absent*)
      (error "Variable does not have constant value" var))
    value))

(define (lookup-maybe-constant-value var allocation)
  (let ((value (intmap-ref (allocation-constant-values allocation) var
                           (lambda (_) *absent*))))
    (if (eq? value *absent*)
        (values #f #f)
        (values #t value))))

(define (lookup-call-alloc k allocation)
  (intmap-ref (allocation-call-allocs allocation) k))

(define (lookup-call-proc-slot k allocation)
  (or (call-alloc-proc-slot (lookup-call-alloc k allocation))
      (error "Call has no proc slot" k)))

(define (lookup-parallel-moves k allocation)
  (intmap-ref (allocation-shuffles allocation) k))

(define (lookup-slot-map k allocation)
  (or (call-alloc-slot-map (lookup-call-alloc k allocation))
      (error "Call has no slot map" k)))

(define (lookup-nlocals allocation)
  (allocation-frame-size allocation))

(define-syntax-rule (persistent-intmap2 exp)
  (call-with-values (lambda () exp)
    (lambda (a b)
      (values (persistent-intmap a) (persistent-intmap b)))))

(define (compute-defs-and-uses cps)
  "Return two LABEL->VAR... maps indicating values defined at and used
by a label, respectively."
  (define (vars->intset vars)
    (fold (lambda (var set) (intset-add set var)) empty-intset vars))
  (persistent-intmap2
   (intmap-fold
    (lambda (label cont defs uses)
      (define (get-defs k)
        (match (intmap-ref cps k)
          (($ $kargs names vars) (vars->intset vars))
          (_ empty-intset)))
      (define (return d u)
        (values (intmap-add! defs label d)
                (intmap-add! uses label u)))
      (match cont
        (($ $kfun src meta self)
         (return (intset self) empty-intset))
        (($ $kargs _ _ ($ $continue k src exp))
         (match exp
           ((or ($ $const) ($ $closure))
            (return (get-defs k) empty-intset))
           (($ $call proc args)
            (return (get-defs k) (intset-add (vars->intset args) proc)))
           (($ $callk _ proc args)
            (return (get-defs k) (intset-add (vars->intset args) proc)))
           (($ $primcall name args)
            (return (get-defs k) (vars->intset args)))
           (($ $branch kt ($ $primcall name args))
            (return empty-intset (vars->intset args)))
           (($ $branch kt ($ $values args))
            (return empty-intset (vars->intset args)))
           (($ $values args)
            (return (get-defs k) (vars->intset args)))
           (($ $prompt escape? tag handler)
            (return empty-intset (intset tag)))))
        (($ $kclause arity body alt)
         (return (get-defs body) empty-intset))
        (($ $kreceive arity kargs)
         (return (get-defs kargs) empty-intset))
        (($ $ktail)
         (return empty-intset empty-intset))))
    cps
    empty-intmap
    empty-intmap)))

(define (compute-reverse-control-flow-order preds)
  "Return a LABEL->ORDER bijection where ORDER is a contiguous set of
integers starting from 0 and incrementing in sort order.  There is a
precondition that labels in PREDS are already renumbered in reverse post
order."
  (define (has-back-edge? preds)
    (let/ec return
      (intmap-fold (lambda (label labels)
                     (intset-fold (lambda (pred)
                                    (if (<= label pred)
                                        (return #t)
                                        (values)))
                                  labels)
                     (values))
                   preds)
      #f))
  (if (has-back-edge? preds)
      ;; This is more involved than forward control flow because not all
      ;; live labels are reachable from the tail.
      (persistent-intmap
       (fold2 (lambda (component order n)
                (intset-fold (lambda (label order n)
                               (values (intmap-add! order label n)
                                       (1+ n)))
                             component order n))
              (reverse (compute-sorted-strongly-connected-components preds))
              empty-intmap 0))
      ;; Just reverse forward control flow.
      (let ((max (intmap-prev preds)))
        (intmap-map (lambda (label labels) (- max label)) preds))))

(define* (add-prompt-control-flow-edges conts succs #:key complete?)
  "For all prompts in DFG in the range [MIN-LABEL, MIN-LABEL +
LABEL-COUNT), invoke F with arguments PROMPT, HANDLER, and BODY for each
body continuation in the prompt."
  (define (intset-filter pred set)
    (intset-fold (lambda (i set)
                   (if (pred i) set (intset-remove set i)))
                 set
                 set))
  (define (intset-any pred set)
    (intset-fold (lambda (i res)
                   (if (or res (pred i)) #t res))
                 set
                 #f))
  (define (compute-prompt-body label)
    (persistent-intset
     (let visit-cont ((label label) (level 1) (labels empty-intset))
       (cond
        ((zero? level) labels)
        ((intset-ref labels label) labels)
        (else
         (match (intmap-ref conts label)
           (($ $ktail)
            ;; Possible for bailouts; never reached and not part of
            ;; prompt body.
            labels)
           (cont
            (let ((labels (intset-add! labels label)))
              (match cont
                (($ $kreceive arity k) (visit-cont k level labels))
                (($ $kargs names syms ($ $continue k src ($ $primcall 'wind)))
                 (visit-cont k (1+ level) labels))
                (($ $kargs names syms
                           ($ $continue k src ($ $prompt escape? tag handler)))
                 (visit-cont handler level (visit-cont k (1+ level) labels)))
                (($ $kargs names syms ($ $continue k src ($ $primcall 'unwind)))
                 (visit-cont k (1- level) labels))
                (($ $kargs names syms ($ $continue k src ($ $branch kt)))
                 (visit-cont k level (visit-cont kt level labels)))
                (($ $kargs names syms ($ $continue k src exp))
                 (visit-cont k level labels)))))))))))
  (define (visit-prompt label handler succs)
    (let ((body (compute-prompt-body label)))
      (define (out-or-back-edge? label)
        ;; Most uses of visit-prompt-control-flow don't need every body
        ;; continuation, and would be happy getting called only for
        ;; continuations that postdominate the rest of the body.  Unless
        ;; you pass #:complete? #t, we only invoke F on continuations
        ;; that can leave the body, or on back-edges in loops.
        (intset-any (lambda (succ)
                      (or (not (intset-ref body succ))
                          (<= succ label)))
                    (intmap-ref succs label)))
      (intset-fold (lambda (pred succs)
                     (intmap-replace succs pred handler intset-add))
                   (if complete? body (intset-filter out-or-back-edge? body))
                   succs)))
  (intmap-fold
   (lambda (label cont succs)
     (match cont
       (($ $kargs _ _
           ($ $continue k _ ($ $prompt escape? tag handler)))
        (visit-prompt k handler succs))
       (_ succs)))
   conts
   succs))

(define (rename-keys map old->new)
  (persistent-intmap
   (intmap-fold (lambda (k v out)
                  (intmap-add! out (intmap-ref old->new k) v))
                map
                empty-intmap)))

(define (rename-intset set old->new)
  (intset-fold (lambda (old set) (intset-add set (intmap-ref old->new old)))
               set empty-intset))

(define (rename-graph graph old->new)
  (persistent-intmap
   (intmap-fold (lambda (pred succs out)
                  (intmap-add! out
                               (intmap-ref old->new pred)
                               (rename-intset succs old->new)))
                graph
                empty-intmap)))

(define (compute-live-variables cps defs uses)
  "Compute and return two values mapping LABEL->VAR..., where VAR... are
the definitions that are live before and after LABEL, as intsets."
  (let* ((succs (add-prompt-control-flow-edges cps (compute-successors cps)))
         (preds (invert-graph succs))
         (old->new (compute-reverse-control-flow-order preds))
         (init (persistent-intmap (intmap-fold
                                   (lambda (old new init)
                                     (intmap-add! init new empty-intset))
                                   old->new empty-intmap))))
    (call-with-values
        (lambda ()
          (solve-flow-equations (rename-graph preds old->new)
                                init init
                                (rename-keys defs old->new)
                                (rename-keys uses old->new)
                                intset-subtract intset-union intset-union))
      (lambda (in out)
        ;; As a reverse control-flow problem, the values flowing into a
        ;; node are actually the live values after the node executes.
        ;; Funny, innit?  So we return them in the reverse order.
        (let ((new->old (invert-bijection old->new)))
          (values (rename-keys out new->old)
                  (rename-keys in new->old)))))))

(define (compute-needs-slot cps defs uses)
  (define (get-defs k) (intmap-ref defs k))
  (define (get-uses label) (intmap-ref uses label))
  (intmap-fold
   (lambda (label cont needs-slot)
     (intset-union
      needs-slot
      (match cont
        (($ $kargs _ _ ($ $continue k src exp))
         (let ((defs (get-defs label)))
           (define (defs+* uses)
             (intset-union defs uses))
           (define (defs+ use)
             (intset-add defs use))
           (match exp
             (($ $const)
              empty-intset)
             (($ $primcall (or 'load-f64 'load-u64 'load-s64) (val))
              empty-intset)
             (($ $primcall 'free-ref (closure slot))
              (defs+ closure))
             (($ $primcall 'free-set! (closure slot value))
              (defs+* (intset closure value)))
             (($ $primcall 'cache-current-module! (mod . _))
              (defs+ mod))
             (($ $primcall 'cached-toplevel-box _)
              defs)
             (($ $primcall 'cached-module-box _)
              defs)
             (($ $primcall 'resolve (name bound?))
              (defs+ name))
             (($ $primcall 'make-vector/immediate (len init))
              (defs+ init))
             (($ $primcall 'vector-ref/immediate (v i))
              (defs+ v))
             (($ $primcall 'vector-set!/immediate (v i x))
              (defs+* (intset v x)))
             (($ $primcall 'allocate-struct/immediate (vtable nfields))
              (defs+ vtable))
             (($ $primcall 'struct-ref/immediate (s n))
              (defs+ s))
             (($ $primcall 'struct-set!/immediate (s n x))
              (defs+* (intset s x)))
             (($ $primcall (or 'add/immediate 'sub/immediate
                               'uadd/immediate 'usub/immediate 'umul/immediate
                               'ursh/immediate 'ulsh/immediate)
                 (x y))
              (defs+ x))
             (($ $primcall 'builtin-ref (idx))
              defs)
             (_
              (defs+* (get-uses label))))))
        (($ $kreceive arity k)
         ;; Only allocate results of function calls to slots if they are
         ;; used.
         empty-intset)
        (($ $kclause arity body alternate)
         (get-defs label))
        (($ $kfun src meta self)
         (intset self))
        (($ $ktail)
         empty-intset))))
   cps
   empty-intset))

(define (compute-lazy-vars cps live-in live-out defs needs-slot)
  "Compute and return a set of vars whose allocation can be delayed
until their use is seen.  These are \"lazy\" vars.  A var is lazy if its
uses are calls, it is always dead after the calls, and if the uses flow
to the definition.  A flow continues across a node iff the node kills no
values that need slots, and defines only lazy vars.  Calls also kill
flows; there's no sense in trying to juggle a pending frame while there
is an active call."
  (define (list->intset list)
    (persistent-intset
     (fold (lambda (i set) (intset-add! set i)) empty-intset list)))

  (let* ((succs (compute-successors cps))
         (gens (intmap-map
                (lambda (label cont)
                  (match cont
                    (($ $kargs _ _ ($ $continue _ _ ($ $call proc args)))
                     (intset-subtract (intset-add (list->intset args) proc)
                                      (intmap-ref live-out label)))
                    (($ $kargs _ _ ($ $continue _ _ ($ $callk _ proc args)))
                     (intset-subtract (intset-add (list->intset args) proc)
                                      (intmap-ref live-out label)))
                    (($ $kargs _ _ ($ $continue k _($ $values args)))
                     (match (intmap-ref cps k)
                       (($ $ktail) (list->intset args))
                       (_ #f)))
                    (_ #f)))
                cps))
         (kills (intmap-map
                 (lambda (label in)
                   (let* ((out (intmap-ref live-out label))
                          (killed (intset-subtract in out))
                          (killed-slots (intset-intersect killed needs-slot)))
                     (and (eq? killed-slots empty-intset)
                          ;; Kill output variables that need slots.
                          (intset-intersect (intmap-ref defs label)
                                            needs-slot))))
                 live-in))
         (preds (invert-graph succs))
         (old->new (compute-reverse-control-flow-order preds)))
    (define (subtract lazy kill)
      (cond
       ((eq? lazy empty-intset)
        lazy)
       ((not kill)
        empty-intset)
       ((and lazy (eq? empty-intset (intset-subtract kill lazy)))
        (intset-subtract lazy kill))
       (else
        empty-intset)))
    (define (add live gen) (or gen live))
    (define (meet in out)
      ;; Initial in is #f.
      (if in (intset-intersect in out) out))
    (call-with-values
        (lambda ()
          (let ((succs (rename-graph preds old->new))
                (init (persistent-intmap
                       (intmap-fold
                        (lambda (old new in)
                          (intmap-add! in new #f))
                        old->new empty-intmap)))
                (kills (rename-keys kills old->new))
                (gens (rename-keys gens old->new)))
            (solve-flow-equations succs init init kills gens
                                  subtract add meet)))
      (lambda (in out)
        ;; A variable is lazy if its uses reach its definition.
        (intmap-fold (lambda (label out lazy)
                       (match (intmap-ref cps label)
                         (($ $kargs names vars)
                          (let ((defs (list->intset vars)))
                            (intset-union lazy (intset-intersect out defs))))
                         (_ lazy)))
                     (rename-keys out (invert-bijection old->new))
                     empty-intset)))))

(define (find-first-zero n)
  ;; Naive implementation.
  (let lp ((slot 0))
    (if (logbit? slot n)
        (lp (1+ slot))
        slot)))

(define (find-first-trailing-zero n)
  (let lp ((slot (let lp ((count 2))
                   (if (< n (ash 1 (1- count)))
                       count
                       ;; Grow upper bound slower than factor 2 to avoid
                       ;; needless bignum allocation on 32-bit systems
                       ;; when there are more than 16 locals.
                       (lp (+ count (ash count -1)))))))
    (if (or (zero? slot) (logbit? (1- slot) n))
        slot
        (lp (1- slot)))))

(define (integers from count)
  (if (zero? count)
      '()
      (cons from (integers (1+ from) (1- count)))))

(define (solve-parallel-move src dst tmp)
  "Solve the parallel move problem between src and dst slot lists, which
are comparable with eqv?.  A tmp slot may be used."

  ;; This algorithm is taken from: "Tilting at windmills with Coq:
  ;; formal verification of a compilation algorithm for parallel moves"
  ;; by Laurence Rideau, Bernard Paul Serpette, and Xavier Leroy
  ;; <http://gallium.inria.fr/~xleroy/publi/parallel-move.pdf>

  (define (split-move moves reg)
    (let loop ((revhead '()) (tail moves))
      (match tail
        (((and s+d (s . d)) . rest)
         (if (eqv? s reg)
             (cons d (append-reverse revhead rest))
             (loop (cons s+d revhead) rest)))
        (_ #f))))

  (define (replace-last-source reg moves)
    (match moves
      ((moves ... (s . d))
       (append moves (list (cons reg d))))))

  (let loop ((to-move (map cons src dst))
             (being-moved '())
             (moved '())
             (last-source #f))
    ;; 'last-source' should always be equivalent to:
    ;; (and (pair? being-moved) (car (last being-moved)))
    (match being-moved
      (() (match to-move
            (() (reverse moved))
            (((and s+d (s . d)) . t1)
             (if (or (eqv? s d) ; idempotent
                     (not s))   ; src is a constant and can be loaded directly
                 (loop t1 '() moved #f)
                 (loop t1 (list s+d) moved s)))))
      (((and s+d (s . d)) . b)
       (match (split-move to-move d)
         ((r . t1) (loop t1 (acons d r being-moved) moved last-source))
         (#f (match b
               (() (loop to-move '() (cons s+d moved) #f))
               (_ (if (eqv? d last-source)
                      (loop to-move
                            (replace-last-source tmp b)
                            (cons s+d (acons d tmp moved))
                            tmp)
                      (loop to-move b (cons s+d moved) last-source))))))))))

(define-inlinable (add-live-slot slot live-slots)
  (logior live-slots (ash 1 slot)))

(define-inlinable (kill-dead-slot slot live-slots)
  (logand live-slots (lognot (ash 1 slot))))

(define-inlinable (compute-slot live-slots hint)
  (if (and hint (not (logbit? hint live-slots)))
      hint
      (find-first-zero live-slots)))

(define (compute-shuffles cps slots call-allocs live-in)
  (define (get-cont label)
    (intmap-ref cps label))

  (define (get-slot var)
    (intmap-ref slots var (lambda (_) #f)))

  (define (get-slots vars)
    (let lp ((vars vars))
      (match vars
        ((var . vars) (cons (get-slot var) (lp vars)))
        (_ '()))))

  (define (get-proc-slot label)
    (call-alloc-proc-slot (intmap-ref call-allocs label)))

  (define (compute-live-slots label)
    (intset-fold (lambda (var live)
                   (match (get-slot var)
                     (#f live)
                     (slot (add-live-slot slot live))))
                 (intmap-ref live-in label)
                 0))

  ;; Although some parallel moves may proceed without a temporary slot,
  ;; in general one is needed.  That temporary slot must not be part of
  ;; the source or destination sets, and that slot should not correspond
  ;; to a live variable.  Usually the source and destination sets are a
  ;; subset of the union of the live sets before and after the move.
  ;; However for stack slots that don't have names -- those slots that
  ;; correspond to function arguments or to function return values -- it
  ;; could be that they are out of the computed live set.  In that case
  ;; they need to be adjoined to the live set, used when choosing a
  ;; temporary slot.
  (define (compute-tmp-slot live stack-slots)
    (find-first-zero (fold add-live-slot live stack-slots)))

  (define (parallel-move src-slots dst-slots tmp-slot)
    (solve-parallel-move src-slots dst-slots tmp-slot))

  (define (compute-receive-shuffles label proc-slot)
    (match (get-cont label)
      (($ $kreceive arity kargs)
       (let* ((results (match (get-cont kargs)
                         (($ $kargs names vars) vars)))
              (value-slots (integers (1+ proc-slot) (length results)))
              (result-slots (get-slots results))
              ;; Filter out unused results.
              (value-slots (filter-map (lambda (val result) (and result val))
                                       value-slots result-slots))
              (result-slots (filter (lambda (x) x) result-slots))
              (live (compute-live-slots kargs)))
         (parallel-move value-slots
                        result-slots
                        (compute-tmp-slot live value-slots))))))
    
  (define (add-call-shuffles label k args shuffles)
    (match (get-cont k)
      (($ $ktail)
       (let* ((live (compute-live-slots label))
              (tail-slots (integers 0 (length args)))
              (moves (parallel-move (get-slots args)
                                    tail-slots
                                    (compute-tmp-slot live tail-slots))))
         (intmap-add! shuffles label moves)))
      (($ $kreceive)
       (let* ((live (compute-live-slots label))
              (proc-slot (get-proc-slot label))
              (call-slots (integers proc-slot (length args)))
              (arg-moves (parallel-move (get-slots args)
                                        call-slots
                                        (compute-tmp-slot live call-slots))))
         (intmap-add! (intmap-add! shuffles label arg-moves)
                      k (compute-receive-shuffles k proc-slot))))))
    
  (define (add-values-shuffles label k args shuffles)
    (match (get-cont k)
      (($ $ktail)
       (let* ((live (compute-live-slots label))
              (src-slots (get-slots args))
              (dst-slots (integers 1 (length args)))
              (moves (parallel-move src-slots dst-slots
                                    (compute-tmp-slot live dst-slots))))
         (intmap-add! shuffles label moves)))
      (($ $kargs _ dst-vars)
       (let* ((live (logior (compute-live-slots label)
                            (compute-live-slots k)))
              (src-slots (get-slots args))
              (dst-slots (get-slots dst-vars))
              (moves (parallel-move src-slots dst-slots
                                    (compute-tmp-slot live '()))))
         (intmap-add! shuffles label moves)))))

  (define (add-prompt-shuffles label k handler shuffles)
    (intmap-add! shuffles handler
                 (compute-receive-shuffles handler (get-proc-slot label))))

  (define (compute-shuffles label cont shuffles)
    (match cont
      (($ $kargs names vars ($ $continue k src exp))
       (match exp
         (($ $call proc args)
          (add-call-shuffles label k (cons proc args) shuffles))
         (($ $callk _ proc args)
          (add-call-shuffles label k (cons proc args) shuffles))
         (($ $values args)
          (add-values-shuffles label k args shuffles))
         (($ $prompt escape? tag handler)
          (add-prompt-shuffles label k handler shuffles))
         (_ shuffles)))
      (_ shuffles)))

  (persistent-intmap
   (intmap-fold compute-shuffles cps empty-intmap)))

(define (compute-frame-size cps slots call-allocs shuffles)
  ;; Minimum frame has one slot: the closure.
  (define minimum-frame-size 1)
  (define (get-shuffles label)
    (intmap-ref shuffles label))
  (define (get-proc-slot label)
    (match (intmap-ref call-allocs label (lambda (_) #f))
      (#f 0) ;; Tail call.
      (($ $call-alloc proc-slot) proc-slot)))
  (define (max-size var size)
    (match (intmap-ref slots var (lambda (_) #f))
      (#f size)
      (slot (max size (1+ slot)))))
  (define (max-size* vars size)
    (fold max-size size vars))
  (define (shuffle-size moves size)
    (match moves
      (() size)
      (((src . dst) . moves)
       (shuffle-size moves (max size (1+ src) (1+ dst))))))
  (define (call-size label nargs size)
    (shuffle-size (get-shuffles label)
                  (max (+ (get-proc-slot label) nargs) size)))
  (define (measure-cont label cont size)
    (match cont
      (($ $kargs names vars ($ $continue k src exp))
       (let ((size (max-size* vars size)))
         (match exp
           (($ $call proc args)
            (call-size label (1+ (length args)) size))
           (($ $callk _ proc args)
            (call-size label (1+ (length args)) size))
           (($ $values args)
            (shuffle-size (get-shuffles label) size))
           (_ size))))
      (($ $kreceive)
       (shuffle-size (get-shuffles label) size))
      (_ size)))

  (intmap-fold measure-cont cps minimum-frame-size))

(define (allocate-args cps)
  (intmap-fold (lambda (label cont slots)
                 (match cont
                   (($ $kfun src meta self)
                    (intmap-add! slots self 0))
                   (($ $kclause arity body alt)
                    (match (intmap-ref cps body)
                      (($ $kargs names vars)
                       (let lp ((vars vars) (slots slots) (n 1))
                         (match vars
                           (() slots)
                           ((var . vars)
                            (lp vars
                                (intmap-add! slots var n)
                                (1+ n))))))))
                   (_ slots)))
               cps empty-intmap))

(define (allocate-lazy-vars cps slots call-allocs live-in lazy)
  (define (compute-live-slots slots label)
    (intset-fold (lambda (var live)
                   (match (intmap-ref slots var (lambda (_) #f))
                     (#f live)
                     (slot (add-live-slot slot live))))
                 (intmap-ref live-in label)
                 0))

  (define (allocate var hint slots live)
    (match (and hint (intmap-ref slots var (lambda (_) #f)))
      (#f (if (intset-ref lazy var)
              (let ((slot (compute-slot live hint)))
                (values (intmap-add! slots var slot)
                        (add-live-slot slot live)))
              (values slots live)))
      (slot (values slots (add-live-slot slot live)))))

  (define (allocate* vars hints slots live)
    (match (vector vars hints)
      (#(() ()) slots)
      (#((var . vars) (hint . hints))
       (let-values (((slots live) (allocate var hint slots live)))
         (allocate* vars hints slots live)))))

  (define (get-proc-slot label)
    (match (intmap-ref call-allocs label (lambda (_) #f))
      (#f 0)
      (call (call-alloc-proc-slot call))))

  (define (allocate-call label args slots)
    (allocate* args (integers (get-proc-slot label) (length args))
               slots (compute-live-slots slots label)))

  (define (allocate-values label k args slots)
    (match (intmap-ref cps k)
      (($ $ktail)
       (allocate* args (integers 1 (length args))
                  slots (compute-live-slots slots label)))
      (($ $kargs names vars)
       (allocate* args
                  (map (cut intmap-ref slots <> (lambda (_) #f)) vars)
                  slots (compute-live-slots slots label)))))

  (define (allocate-lazy label cont slots)
    (match cont
      (($ $kargs names vars ($ $continue k src exp))
       (match exp
         (($ $call proc args)
          (allocate-call label (cons proc args) slots))
         (($ $callk _ proc args)
          (allocate-call label (cons proc args) slots))
         (($ $values args)
          (allocate-values label k args slots))
         (_ slots)))
      (_
       slots)))

  ;; Sweep right to left to visit uses before definitions.
  (persistent-intmap
   (intmap-fold-right allocate-lazy cps slots)))

(define (compute-var-representations cps)
  (define (get-defs k)
    (match (intmap-ref cps k)
      (($ $kargs names vars) vars)
      (_ '())))
  (intmap-fold
   (lambda (label cont representations)
     (match cont
       (($ $kargs _ _ ($ $continue k _ exp))
        (match (get-defs k)
          (() representations)
          ((var)
           (match exp
             (($ $values (arg))
              (intmap-add representations var
                          (intmap-ref representations arg)))
             (($ $primcall (or 'scm->f64 'load-f64
                               'bv-f32-ref 'bv-f64-ref
                               'fadd 'fsub 'fmul 'fdiv))
              (intmap-add representations var 'f64))
             (($ $primcall (or 'scm->u64 'scm->u64/truncate 'load-u64
                               'char->integer
                               'bv-length 'vector-length 'string-length
                               'uadd 'usub 'umul
                               'ulogand 'ulogior 'ulogxor 'ulogsub 'ursh 'ulsh
                               'uadd/immediate 'usub/immediate 'umul/immediate
                               'ursh/immediate 'ulsh/immediate
                               'bv-u8-ref 'bv-u16-ref 'bv-u32-ref 'bv-u64-ref))
              (intmap-add representations var 'u64))
             (($ $primcall (or 'scm->s64 'load-s64
                               'bv-s8-ref 'bv-s16-ref 'bv-s32-ref 'bv-s64-ref))
              (intmap-add representations var 's64))
             (_
              (intmap-add representations var 'scm))))
          (vars
           (match exp
             (($ $values args)
              (fold (lambda (arg var representations)
                      (intmap-add representations var
                                  (intmap-ref representations arg)))
                    representations args vars))))))
       (($ $kfun src meta self)
        (intmap-add representations self 'scm))
       (($ $kclause arity body alt)
        (fold1 (lambda (var representations)
                 (intmap-add representations var 'scm))
               (get-defs body) representations))
       (($ $kreceive arity kargs)
        (fold1 (lambda (var representations)
                 (intmap-add representations var 'scm))
               (get-defs kargs) representations))
       (($ $ktail) representations)))
   cps
   empty-intmap))

(define* (allocate-slots cps #:key (precolor-calls? #t))
  (let*-values (((defs uses) (compute-defs-and-uses cps))
                ((representations) (compute-var-representations cps))
                ((live-in live-out) (compute-live-variables cps defs uses))
                ((constants) (compute-constant-values cps))
                ((needs-slot) (compute-needs-slot cps defs uses))
                ((lazy) (if precolor-calls?
                            (compute-lazy-vars cps live-in live-out defs
                                               needs-slot)
                            empty-intset)))

    (define (empty-live-slots)
      #b0)

    (define (compute-call-proc-slot live-slots)
      (+ 2 (find-first-trailing-zero live-slots)))

    (define (compute-prompt-handler-proc-slot live-slots)
      (if (zero? live-slots)
          0
          (1- (find-first-trailing-zero live-slots))))

    (define (get-cont label)
      (intmap-ref cps label))

    (define (get-slot slots var)
      (intmap-ref slots var (lambda (_) #f)))

    (define (get-slots slots vars)
      (let lp ((vars vars))
        (match vars
          ((var . vars) (cons (get-slot slots var) (lp vars)))
          (_ '()))))

    (define (compute-live-slots* slots label live-vars)
      (intset-fold (lambda (var live)
                     (match (get-slot slots var)
                       (#f live)
                       (slot (add-live-slot slot live))))
                   (intmap-ref live-vars label)
                   0))

    (define (compute-live-in-slots slots label)
      (compute-live-slots* slots label live-in))

    (define (compute-live-out-slots slots label)
      (compute-live-slots* slots label live-out))

    (define slot-desc-dead 0)
    (define slot-desc-live-raw 1)
    (define slot-desc-live-scm 2)
    (define slot-desc-unused 3)

    (define (compute-slot-map slots live-vars nslots)
      (intset-fold
       (lambda (var slot-map)
         (match (get-slot slots var)
           (#f slot-map)
           (slot
            (let ((desc (match (intmap-ref representations var)
                          ((or 'u64 'f64 's64) slot-desc-live-raw)
                          ('scm slot-desc-live-scm))))
              (logior slot-map (ash desc (* 2 slot)))))))
       live-vars 0))

    (define (allocate var hint slots live)
      (cond
       ((not (intset-ref needs-slot var))
        (values slots live))
       ((get-slot slots var)
        => (lambda (slot)
             (values slots (add-live-slot slot live))))
       ((and (not hint) (intset-ref lazy var))
        (values slots live))
       (else
        (let ((slot (compute-slot live hint)))
          (values (intmap-add! slots var slot)
                  (add-live-slot slot live))))))

    (define (allocate* vars hints slots live)
      (match (vector vars hints)
        (#(() ()) (values slots live))
        (#((var . vars) (hint . hints))
         (call-with-values (lambda () (allocate var hint slots live))
           (lambda (slots live)
             (allocate* vars hints slots live))))))

    (define (allocate-defs label vars slots)
      (let ((live (compute-live-in-slots slots label))
            (live-vars (intmap-ref live-in label)))
        (let lp ((vars vars) (slots slots) (live live))
          (match vars
            (() (values slots live))
            ((var . vars)
             (call-with-values (lambda () (allocate var #f slots live))
               (lambda (slots live)
                 (lp vars slots
                     (let ((slot (get-slot slots var)))
                       (if (and slot (not (intset-ref live-vars var)))
                           (kill-dead-slot slot live)
                           live))))))))))

    ;; PRE-LIVE are the live slots coming into the term.  POST-LIVE
    ;; is the subset of PRE-LIVE that is still live after the term
    ;; uses its inputs.
    (define (allocate-call label k args slots call-allocs pre-live)
      (match (get-cont k)
        (($ $ktail)
         (let ((tail-slots (integers 0 (length args))))
           (values (allocate* args tail-slots slots pre-live)
                   call-allocs)))
        (($ $kreceive arity kargs)
         (let*-values
             (((post-live) (compute-live-out-slots slots label))
              ((proc-slot) (compute-call-proc-slot post-live))
              ((call-slots) (integers proc-slot (length args)))
              ((slots pre-live) (allocate* args call-slots slots pre-live))
              ;; Allow the first result to be hinted by its use, but
              ;; hint the remaining results to stay in place.  This
              ;; strikes a balance between avoiding shuffling,
              ;; especially for unused extra values, and avoiding frame
              ;; size growth due to sparse locals.
              ((slots result-live)
               (match (get-cont kargs)
                 (($ $kargs () ())
                  (values slots post-live))
                 (($ $kargs (_ . _) (_ . results))
                  (let ((result-slots (integers (+ proc-slot 2)
                                                (length results))))
                    (allocate* results result-slots slots post-live)))))
              ((slot-map) (compute-slot-map slots (intmap-ref live-out label)
                                            (- proc-slot 2)))
              ((call) (make-call-alloc proc-slot slot-map)))
           (values slots
                   (intmap-add! call-allocs label call))))))
    
    (define (allocate-values label k args slots call-allocs)
      (match (get-cont k)
        (($ $ktail)
         (values slots call-allocs))
        (($ $kargs (_) (dst))
         ;; When there is only one value in play, we allow the dst to be
         ;; hinted (see compute-lazy-vars).  If the src doesn't have a
         ;; slot, then the actual slot for the dst would end up being
         ;; decided by the call that args it.  Because we don't know the
         ;; slot, we can't really compute the parallel moves in that
         ;; case, so just bail and rely on the bytecode emitter to
         ;; handle the one-value case specially.
         (match args
           ((src)
            (let ((post-live (compute-live-out-slots slots label)))
              (values (allocate dst (get-slot slots src) slots post-live)
                      call-allocs)))))
        (($ $kargs _ dst-vars)
         (let ((src-slots (get-slots slots args))
               (post-live (compute-live-out-slots slots label)))
           (values (allocate* dst-vars src-slots slots post-live)
                   call-allocs)))))

    (define (allocate-prompt label k handler slots call-allocs)
      (match (get-cont handler)
        (($ $kreceive arity kargs)
         (let*-values
             (((handler-live) (compute-live-in-slots slots handler))
              ((proc-slot) (compute-prompt-handler-proc-slot handler-live))
              ((slot-map)  (compute-slot-map slots (intmap-ref live-in handler)
                                             (- proc-slot 2)))
              ((result-vars) (match (get-cont kargs)
                               (($ $kargs names vars) vars)))
              ((value-slots) (integers (1+ proc-slot) (length result-vars)))
              ((slots result-live) (allocate* result-vars value-slots
                                              slots handler-live)))
           (values slots
                   (intmap-add! call-allocs label
                                (make-call-alloc proc-slot slot-map)))))))

    (define (allocate-cont label cont slots call-allocs)
      (match cont
        (($ $kargs names vars ($ $continue k src exp))
         (let-values (((slots live) (allocate-defs label vars slots)))
           (match exp
             (($ $call proc args)
              (allocate-call label k (cons proc args) slots call-allocs live))
             (($ $callk _ proc args)
              (allocate-call label k (cons proc args) slots call-allocs live))
             (($ $values args)
              (allocate-values label k args slots call-allocs))
             (($ $prompt escape? tag handler)
              (allocate-prompt label k handler slots call-allocs))
             (_
              (values slots call-allocs)))))
        (_
         (values slots call-allocs))))

    (call-with-values (lambda ()
                        (let ((slots (allocate-args cps)))
                          (intmap-fold allocate-cont cps slots empty-intmap)))
      (lambda (slots calls)
        (let* ((slots (allocate-lazy-vars cps slots calls live-in lazy))
               (shuffles (compute-shuffles cps slots calls live-in))
               (frame-size (compute-frame-size cps slots calls shuffles)))
          (make-allocation slots representations constants calls
                           shuffles frame-size))))))