1 ;;; Continuation-passing style (CPS) intermediate language (IL)
3 ;; Copyright (C) 2013, 2014 Free Software Foundation, Inc.
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21 ;;; This is the continuation-passing style (CPS) intermediate language
24 ;;; There are two kinds of terms in CPS: terms that bind continuations,
25 ;;; and terms that call continuations.
27 ;;; $letk binds a set of mutually recursive continuations, each one an
28 ;;; instance of $cont. A $cont declares the name of a continuation, and
29 ;;; then contains as a subterm the particular continuation instance:
30 ;;; $kif for test continuations, $kargs for continuations that bind
33 ;;; $continue nodes call continuations. The expression contained in the
34 ;;; $continue node determines the value or values that are passed to the
35 ;;; target continuation: $const to pass a constant value, $values to
36 ;;; pass multiple named values, etc. $continue nodes also record the source at which
38 ;;; Additionally there is $letrec, a term that binds mutually recursive
39 ;;; functions. The contification pass will turn $letrec into $letk if
40 ;;; it can do so. Otherwise, the closure conversion pass will desugar
41 ;;; $letrec into an equivalent sequence of make-closure primcalls and
42 ;;; subsequent initializations of the captured variables of the
43 ;;; closures. You can think of $letrec as pertaining to "high CPS",
44 ;;; whereas later passes will only see "low CPS", which does not have
47 ;;; This particular formulation of CPS was inspired by Andrew Kennedy's
48 ;;; 2007 paper, "Compiling with Continuations, Continued". All Guile
49 ;;; hackers should read that excellent paper! As in Kennedy's paper,
50 ;;; continuations are second-class, and may be thought of as basic block
51 ;;; labels. All values are bound to variables using continuation calls:
54 ;;; There are some Guile-specific quirks as well:
56 ;;; - $kreceive represents a continuation that receives multiple values,
57 ;;; but which truncates them to some number of required values,
58 ;;; possibly with a rest list.
60 ;;; - $kfun labels an entry point for a $fun (a function), and
61 ;;; contains a $ktail representing the formal argument which is the
62 ;;; function's continuation.
64 ;;; - $kfun also contain a $kclause continuation, corresponding to
65 ;;; the first case-lambda clause of the function. $kclause actually
66 ;;; contains the clause body, and the subsequent clause (if any).
67 ;;; This is because the $kclause logically matches or doesn't match
68 ;;; a given set of actual arguments against a formal arity, then
69 ;;; proceeds to a "body" continuation (which is a $kargs).
71 ;;; That's to say that a $fun can be matched like this:
76 ;;; ($ $kfun src meta self ($ $cont ktail ($ $ktail))
78 ;;; ($ $cont kbody ($ $kargs names syms body))
82 ;;; A $continue to ktail is in tail position. $kfun, $kclause,
83 ;;; and $ktail will never be seen elsewhere in a CPS term.
85 ;;; - $prompt continues to the body of the prompt, having pushed on a
86 ;;; prompt whose handler will continue at its "handler"
87 ;;; continuation. The continuation of the prompt is responsible for
88 ;;; popping the prompt.
92 ;;; - $letk, $letrec, and $continue are terms.
94 ;;; - $cont is a continuation, containing a continuation body ($kargs,
97 ;;; - $continue terms contain an expression ($call, $const, $fun,
100 ;;; See (language tree-il compile-cps) for details on how Tree-IL
105 (define-module (language cps)
106 #:use-module (ice-9 match)
107 #:use-module ((srfi srfi-1) #:select (fold))
108 #:use-module (srfi srfi-9)
109 #:use-module (srfi srfi-9 gnu)
110 #:use-module (srfi srfi-11)
116 $letk $continue $letrec
121 ;; Continuation bodies.
122 $kif $kreceive $kargs $kfun $ktail $kclause
125 $void $const $prim $fun $closure
126 $call $callk $primcall $values $prompt
128 ;; First-order CPS root.
132 label-counter var-counter
133 fresh-label fresh-var
134 with-fresh-name-state compute-max-label-and-var
138 build-cps-term build-cps-cont build-cps-exp
139 rewrite-cps-term rewrite-cps-cont rewrite-cps-exp
142 parse-cps unparse-cps
143 make-global-cont-folder make-local-cont-folder
144 fold-conts fold-local-conts
145 visit-cont-successors))
147 ;; FIXME: Use SRFI-99, when Guile adds it.
148 (define-syntax define-record-type*
150 (define (id-append ctx . syms)
151 (datum->syntax ctx (apply symbol-append (map syntax->datum syms))))
154 (and (identifier? #'name) (and-map identifier? #'(field ...)))
155 (with-syntax ((cons (id-append #'name #'make- #'name))
156 (pred (id-append #'name #'name #'?))
157 ((getter ...) (map (lambda (f)
158 (id-append f #'name #'- f))
160 #'(define-record-type name
166 (define-syntax-rule (define-cps-type name field ...)
168 (define-record-type* name field ...)
169 (set-record-type-printer! name print-cps)))
171 (define (print-cps exp port)
172 (format port "#<cps ~S>" (unparse-cps exp)))
175 (define-record-type* $arity req opt rest kw allow-other-keys?)
178 (define-cps-type $letk conts body)
179 (define-cps-type $continue k src exp)
180 (define-cps-type $letrec names syms funs body) ; Higher-order.
183 (define-cps-type $cont k cont)
184 (define-cps-type $kif kt kf)
185 (define-cps-type $kreceive arity k)
186 (define-cps-type $kargs names syms body)
187 (define-cps-type $kfun src meta self tail clause)
188 (define-cps-type $ktail)
189 (define-cps-type $kclause arity cont alternate)
192 (define-cps-type $void)
193 (define-cps-type $const val)
194 (define-cps-type $prim name)
195 (define-cps-type $fun free body) ; Higher-order.
196 (define-cps-type $closure label nfree) ; First-order.
197 (define-cps-type $call proc args)
198 (define-cps-type $callk k proc args) ; First-order.
199 (define-cps-type $primcall name args)
200 (define-cps-type $values args)
201 (define-cps-type $prompt escape? tag handler)
203 ;; The root of a higher-order CPS term is $cont containing a $kfun. The
204 ;; root of a first-order CPS term is a $program.
205 (define-cps-type $program funs)
207 (define label-counter (make-parameter #f))
208 (define var-counter (make-parameter #f))
210 (define (fresh-label)
211 (let ((count (or (label-counter)
212 (error "fresh-label outside with-fresh-name-state"))))
213 (label-counter (1+ count))
217 (let ((count (or (var-counter)
218 (error "fresh-var outside with-fresh-name-state"))))
219 (var-counter (1+ count))
222 (define-syntax-rule (let-fresh (label ...) (var ...) body ...)
223 (let ((label (fresh-label)) ...
224 (var (fresh-var)) ...)
227 (define-syntax-rule (with-fresh-name-state fun body ...)
228 (call-with-values (lambda () (compute-max-label-and-var fun))
229 (lambda (max-label max-var)
230 (parameterize ((label-counter (1+ max-label))
231 (var-counter (1+ max-var)))
234 (define-syntax build-arity
235 (syntax-rules (unquote)
236 ((_ (unquote exp)) exp)
237 ((_ (req opt rest kw allow-other-keys?))
238 (make-$arity req opt rest kw allow-other-keys?))))
240 (define-syntax build-cont-body
241 (syntax-rules (unquote $kif $kreceive $kargs $kfun $ktail $kclause)
246 ((_ ($kreceive req rest kargs))
247 (make-$kreceive (make-$arity req '() rest '() #f) kargs))
248 ((_ ($kargs (name ...) (unquote syms) body))
249 (make-$kargs (list name ...) syms (build-cps-term body)))
250 ((_ ($kargs (name ...) (sym ...) body))
251 (make-$kargs (list name ...) (list sym ...) (build-cps-term body)))
252 ((_ ($kargs names syms body))
253 (make-$kargs names syms (build-cps-term body)))
254 ((_ ($kfun src meta self tail clause))
255 (make-$kfun src meta self (build-cps-cont tail) (build-cps-cont clause)))
258 ((_ ($kclause arity cont alternate))
259 (make-$kclause (build-arity arity) (build-cps-cont cont)
260 (build-cps-cont alternate)))))
262 (define-syntax build-cps-cont
263 (syntax-rules (unquote)
264 ((_ (unquote exp)) exp)
265 ((_ (k cont)) (make-$cont k (build-cont-body cont)))))
267 (define-syntax build-cps-exp
268 (syntax-rules (unquote
269 $void $const $prim $fun $closure
270 $call $callk $primcall $values $prompt)
271 ((_ (unquote exp)) exp)
272 ((_ ($void)) (make-$void))
273 ((_ ($const val)) (make-$const val))
274 ((_ ($prim name)) (make-$prim name))
275 ((_ ($fun free body)) (make-$fun free (build-cps-cont body)))
276 ((_ ($closure k nfree)) (make-$closure k nfree))
277 ((_ ($call proc (unquote args))) (make-$call proc args))
278 ((_ ($call proc (arg ...))) (make-$call proc (list arg ...)))
279 ((_ ($call proc args)) (make-$call proc args))
280 ((_ ($callk k proc (unquote args))) (make-$callk k proc args))
281 ((_ ($callk k proc (arg ...))) (make-$callk k proc (list arg ...)))
282 ((_ ($callk k proc args)) (make-$callk k proc args))
283 ((_ ($primcall name (unquote args))) (make-$primcall name args))
284 ((_ ($primcall name (arg ...))) (make-$primcall name (list arg ...)))
285 ((_ ($primcall name args)) (make-$primcall name args))
286 ((_ ($values (unquote args))) (make-$values args))
287 ((_ ($values (arg ...))) (make-$values (list arg ...)))
288 ((_ ($values args)) (make-$values args))
289 ((_ ($prompt escape? tag handler))
290 (make-$prompt escape? tag handler))))
292 (define-syntax build-cps-term
293 (syntax-rules (unquote $letk $letk* $letconst $letrec $program $continue)
296 ((_ ($letk (unquote conts) body))
297 (make-$letk conts (build-cps-term body)))
298 ((_ ($letk (cont ...) body))
299 (make-$letk (list (build-cps-cont cont) ...)
300 (build-cps-term body)))
301 ((_ ($letk* () body))
302 (build-cps-term body))
303 ((_ ($letk* (cont conts ...) body))
304 (build-cps-term ($letk (cont) ($letk* (conts ...) body))))
305 ((_ ($letconst () body))
306 (build-cps-term body))
307 ((_ ($letconst ((name sym val) tail ...) body))
308 (let-fresh (kconst) ()
310 ($letk ((kconst ($kargs (name) (sym) ($letconst (tail ...) body))))
311 ($continue kconst (let ((props (source-properties val)))
312 (and (pair? props) props))
314 ((_ ($letrec names gensyms funs body))
315 (make-$letrec names gensyms funs (build-cps-term body)))
316 ((_ ($program (unquote conts)))
317 (make-$program conts))
318 ((_ ($program (cont ...)))
319 (make-$program (list (build-cps-cont cont) ...)))
320 ((_ ($program conts))
321 (make-$program conts))
322 ((_ ($continue k src exp))
323 (make-$continue k src (build-cps-exp exp)))))
325 (define-syntax-rule (rewrite-cps-term x (pat body) ...)
327 (pat (build-cps-term body)) ...))
328 (define-syntax-rule (rewrite-cps-cont x (pat body) ...)
330 (pat (build-cps-cont body)) ...))
331 (define-syntax-rule (rewrite-cps-exp x (pat body) ...)
333 (pat (build-cps-exp body)) ...))
335 (define (parse-cps exp)
337 (let ((props (source-properties exp)))
338 (and (pair? props) props)))
341 (('letconst k (name sym c) body)
343 ($letk ((k ($kargs (name) (sym)
345 ($continue k (src exp) ($const c)))))
346 (('let k (name sym val) body)
348 ($letk ((k ($kargs (name) (sym)
351 (('letk (cont ...) body)
353 ($letk ,(map parse-cps cont) ,(parse-cps body))))
356 (sym ,(parse-cps body))))
358 (build-cont-body ($kif kt kf)))
359 (('kreceive req rest k)
360 (build-cont-body ($kreceive req rest k)))
361 (('kargs names syms body)
362 (build-cont-body ($kargs names syms ,(parse-cps body))))
363 (('kfun src meta self tail clause)
365 ($kfun (src exp) meta self ,(parse-cps tail)
366 ,(and=> clause parse-cps))))
370 (('kclause (req opt rest kw allow-other-keys?) body)
372 ($kclause (req opt rest kw allow-other-keys?)
375 (('kclause (req opt rest kw allow-other-keys?) body alternate)
377 ($kclause (req opt rest kw allow-other-keys?)
379 ,(parse-cps alternate))))
381 (build-cont-body ($kargs () () ,(parse-cps body))))
385 (build-cps-term ($continue k (src exp) ,(parse-cps exp))))
387 (build-cps-exp ($void)))
389 (build-cps-exp ($const exp)))
391 (build-cps-exp ($prim name)))
393 (build-cps-exp ($fun free ,(parse-cps body))))
395 (build-cps-exp ($closure k nfree)))
396 (('letrec ((name sym fun) ...) body)
398 ($letrec name sym (map parse-cps fun) ,(parse-cps body))))
399 (('program (cont ...))
400 (build-cps-term ($program ,(map parse-cps cont))))
401 (('call proc arg ...)
402 (build-cps-exp ($call proc arg)))
403 (('callk k proc arg ...)
404 (build-cps-exp ($callk k proc arg)))
405 (('primcall name arg ...)
406 (build-cps-exp ($primcall name arg)))
408 (build-cps-exp ($values arg)))
409 (('prompt escape? tag handler)
410 (build-cps-exp ($prompt escape? tag handler)))
412 (error "unexpected cps" exp))))
414 (define (unparse-cps exp)
417 (($ $letk (($ $cont k ($ $kargs (name) (sym) body)))
418 ($ $continue k src ($ $const c)))
419 `(letconst ,k (,name ,sym ,c)
420 ,(unparse-cps body)))
421 (($ $letk (($ $cont k ($ $kargs (name) (sym) body))) val)
422 `(let ,k (,name ,sym ,(unparse-cps val))
423 ,(unparse-cps body)))
424 (($ $letk conts body)
425 `(letk ,(map unparse-cps conts) ,(unparse-cps body)))
427 `(k ,sym ,(unparse-cps body)))
430 (($ $kreceive ($ $arity req () rest '() #f) k)
431 `(kreceive ,req ,rest ,k))
432 (($ $kargs () () body)
433 `(kseq ,(unparse-cps body)))
434 (($ $kargs names syms body)
435 `(kargs ,names ,syms ,(unparse-cps body)))
436 (($ $kfun src meta self tail clause)
437 `(kfun ,meta ,self ,(unparse-cps tail) ,(unparse-cps clause)))
440 (($ $kclause ($ $arity req opt rest kw allow-other-keys?) body alternate)
441 `(kclause (,req ,opt ,rest ,kw ,allow-other-keys?) ,(unparse-cps body)
442 . ,(if alternate (list (unparse-cps alternate)) '())))
445 (($ $continue k src exp)
446 `(continue ,k ,(unparse-cps exp)))
454 `(fun ,free ,(unparse-cps body)))
455 (($ $closure k nfree)
456 `(closure ,k ,nfree))
457 (($ $letrec names syms funs body)
458 `(letrec ,(map (lambda (name sym fun)
459 (list name sym (unparse-cps fun)))
461 ,(unparse-cps body)))
463 `(program ,(map unparse-cps conts)))
465 `(call ,proc ,@args))
466 (($ $callk k proc args)
467 `(callk ,k ,proc ,@args))
468 (($ $primcall name args)
469 `(primcall ,name ,@args))
472 (($ $prompt escape? tag handler)
473 `(prompt ,escape? ,tag ,handler))
475 (error "unexpected cps" exp))))
477 (define-syntax-rule (make-global-cont-folder seed ...)
478 (lambda (proc cont seed ...)
479 (define (fold-values proc in seed ...)
482 (let-values (((seed ...) (proc (car in) seed ...)))
483 (fold-values proc (cdr in) seed ...))))
485 (define (cont-folder cont seed ...)
488 (let-values (((seed ...) (proc k cont seed ...)))
490 (($ $kargs names syms body)
491 (term-folder body seed ...))
493 (($ $kfun src meta self tail clause)
494 (let-values (((seed ...) (cont-folder tail seed ...)))
496 (cont-folder clause seed ...)
499 (($ $kclause arity body alternate)
500 (let-values (((seed ...) (cont-folder body seed ...)))
502 (cont-folder alternate seed ...)
505 (_ (values seed ...)))))))
507 (define (fun-folder fun seed ...)
510 (cont-folder body seed ...))))
512 (define (term-folder term seed ...)
514 (($ $letk conts body)
515 (let-values (((seed ...) (term-folder body seed ...)))
516 (fold-values cont-folder conts seed ...)))
518 (($ $continue k src exp)
520 (($ $fun) (fun-folder exp seed ...))
521 (_ (values seed ...))))
523 (($ $letrec names syms funs body)
524 (let-values (((seed ...) (term-folder body seed ...)))
525 (fold-values fun-folder funs seed ...)))))
527 (cont-folder cont seed ...)))
529 (define-syntax-rule (make-local-cont-folder seed ...)
530 (lambda (proc cont seed ...)
531 (define (cont-folder cont seed ...)
533 (($ $cont k (and cont ($ $kargs names syms body)))
534 (let-values (((seed ...) (proc k cont seed ...)))
535 (term-folder body seed ...)))
537 (proc k cont seed ...))))
538 (define (term-folder term seed ...)
540 (($ $letk conts body)
541 (let-values (((seed ...) (term-folder body seed ...)))
542 (let lp ((conts conts) (seed seed) ...)
544 (() (values seed ...))
545 ((cont) (cont-folder cont seed ...))
547 (let-values (((seed ...) (cont-folder cont seed ...)))
548 (lp conts seed ...)))))))
549 (($ $letrec names syms funs body) (term-folder body seed ...))
550 (_ (values seed ...))))
551 (define (clause-folder clause seed ...)
553 (($ $cont k (and cont ($ $kclause arity body alternate)))
554 (let-values (((seed ...) (proc k cont seed ...)))
556 (let-values (((seed ...) (cont-folder body seed ...)))
557 (clause-folder alternate seed ...))
558 (cont-folder body seed ...))))))
560 (($ $cont k (and cont ($ $kfun src meta self tail clause)))
561 (let*-values (((seed ...) (proc k cont seed ...))
562 ((seed ...) (if clause
563 (clause-folder clause seed ...)
565 (cont-folder tail seed ...))))))
567 (define (compute-max-label-and-var fun)
570 ((make-global-cont-folder max-label max-var)
571 (lambda (label cont max-label max-var)
572 (values (max label max-label)
574 (($ $kargs names vars body)
575 (let lp ((body body) (max-var (fold max max-var vars)))
577 (($ $letk conts body) (lp body max-var))
578 (($ $letrec names vars funs body)
579 (lp body (fold max max-var vars)))
581 (($ $kfun src meta self)
586 (define (fold/2 proc in s0 s1)
589 (let-values (((s0 s1) (proc (car in) s0 s1)))
590 (fold/2 proc (cdr in) s0 s1))))
591 (let lp ((conts conts) (max-label -1) (max-var -1))
593 (values max-label max-var)
594 (call-with-values (lambda ()
595 ((make-local-cont-folder max-label max-var)
596 (lambda (label cont max-label max-var)
597 (values (max label max-label)
599 (($ $kargs names vars body)
600 (fold max max-var vars))
601 (($ $kfun src meta self)
604 (car conts) max-label max-var))
605 (lambda (max-label max-var)
606 (lp (cdr conts) max-label max-var))))))))
608 (define (fold-conts proc seed fun)
609 ((make-global-cont-folder seed) proc fun seed))
611 (define (fold-local-conts proc seed fun)
612 ((make-local-cont-folder seed) proc fun seed))
614 (define (visit-cont-successors proc cont)
616 (($ $kargs names syms body)
617 (let lp ((body body))
619 (($ $letk conts body) (lp body))
620 (($ $letrec names vars funs body) (lp body))
621 (($ $continue k src exp)
623 (($ $prompt escape? tag handler) (proc k handler))
626 (($ $kif kt kf) (proc kt kf))
628 (($ $kreceive arity k) (proc k))
630 (($ $kclause arity ($ $cont kbody) #f) (proc kbody))
632 (($ $kclause arity ($ $cont kbody) ($ $cont kalt)) (proc kbody kalt))
634 (($ $kfun src meta self tail ($ $cont clause)) (proc clause))
636 (($ $kfun src meta self tail #f) (proc))
638 (($ $ktail) (proc))))