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

;; Copyright (C) 2013, 2014 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:
;;;
;;; Primcalls that don't correspond to VM instructions are treated as if
;;; they are calls, and indeed the later reify-primitives pass turns
;;; them into calls.  Because no return arity checking is done for these
;;; primitives, if a later optimization pass simplifies the primcall to
;;; a VM operation, the tail of the simplification has to be a
;;; primcall to 'values.  Most of these primcalls can be elided, and
;;; that is the job of this pass.
;;;
;;; Code:

(define-module (language cps elide-values)
  #:use-module (ice-9 match)
  #:use-module (srfi srfi-26)
  #:use-module (language cps)
  #:use-module (language cps dfg)
  #:export (elide-values))

(define (elide-values* fun conts)
  (define (visit-cont cont)
    (rewrite-cps-cont cont
      (($ $cont sym ($ $kargs names syms body))
       (sym ($kargs names syms ,(visit-term body))))
      (($ $cont sym ($ $kfun src meta self tail clause))
       (sym ($kfun src meta self ,tail ,(and clause (visit-cont clause)))))
      (($ $cont sym ($ $kclause arity body alternate))
       (sym ($kclause ,arity ,(visit-cont body)
                      ,(and alternate (visit-cont alternate)))))
      (($ $cont)
       ,cont)))
  (define (visit-term term)
    (rewrite-cps-term term
      (($ $letk conts body)
       ($letk ,(map visit-cont conts)
         ,(visit-term body)))
      (($ $letrec names syms funs body)
       ($letrec names syms (map visit-fun funs)
         ,(visit-term body)))
      (($ $continue k src ($ $primcall 'values vals))
       ,(rewrite-cps-term (vector-ref conts k)
          (($ $ktail)
           ($continue k src ($values vals)))
          (($ $kreceive ($ $arity req () rest () #f) kargs)
           ,(cond
             ((and (not rest) (= (length vals) (length req)))
              (build-cps-term
                ($continue kargs src ($values vals))))
             ((and rest (>= (length vals) (length req)))
              (let-fresh (krest) (rest)
                (let ((vals* (append (list-head vals (length req))
                                     (list rest))))
                  (build-cps-term
                    ($letk ((krest ($kargs ('rest) (rest)
                                     ($continue kargs src
                                       ($values vals*)))))
                      ,(let lp ((tail (list-tail vals (length req)))
                                (k krest))
                         (match tail
                           (()
                            (build-cps-term ($continue k src
                                              ($const '()))))
                           ((v . tail)
                            (let-fresh (krest) (rest)
                              (build-cps-term
                                ($letk ((krest ($kargs ('rest) (rest)
                                                 ($continue k src
                                                   ($primcall 'cons
                                                              (v rest))))))
                                  ,(lp tail krest))))))))))))
             (else term)))
          (($ $kargs args)
           ,(if (< (length vals) (length args))
                term
                (let ((vals (list-head vals (length args))))
                  (build-cps-term
                    ($continue k src ($values vals))))))))
      (($ $continue k src (and fun ($ $fun)))
       ($continue k src ,(visit-fun fun)))
      (($ $continue)
       ,term)))
  (define (visit-fun fun)
    (rewrite-cps-exp fun
      (($ $fun free cont)
       ($fun free ,(visit-cont cont)))))

  (visit-cont fun))

(define (elide-values fun)
  (with-fresh-name-state fun
    (let ((conts (build-cont-table fun)))
      (elide-values* fun conts))))