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;;; Continuation-passing style (CPS) intermediate language (IL)
;; Copyright (C) 2021 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:
;;;
;;; If we have a $callk to a $kfun that has a $kclause, in most cases we
;;; can skip arity checks because the caller knows what arity the callee
;;; is expecting.
;;;
;;; Code:
(define-module (language cps elide-arity-checks)
#:use-module (ice-9 match)
#:use-module (language cps)
#:use-module (language cps intmap)
#:use-module (language cps utils)
#:use-module (language cps with-cps)
#:export (elide-arity-checks))
(define (arity-matches? arity self proc args)
(match arity
(($ $arity req () #f () #f)
(= (+ (length req) (if self 1 0))
(+ (length args) (if proc 1 0))))
(_ #f)))
(define (maybe-elide-arity-check cps kfun proc args)
(match (intmap-ref cps kfun)
(($ $kfun fsrc meta self ktail kentry)
(match (and kentry (intmap-ref cps kentry))
(($ $kclause (? (lambda (arity)
(arity-matches? arity self proc args))
arity)
kbody #f)
;; This is a compatible $callk to a $kfun that checks its arity
;; and has no alternate; arrange to elide the check.
(match (intmap-ref cps kbody)
(($ $kargs fnames fvars term)
(match term
(($ $continue (? (lambda (k) (eq? k ktail))) _
($ $callk kfun'
(? (lambda (proc') (eq? proc' self)))
(? (lambda (args) (equal? args fvars)))))
;; This function already trampolines out to another
;; function; forward this call there. Could recurse but
;; we shouldn't need to, and we don't so as to avoid
;; divergence.
(with-cps cps
(build-exp
($callk kfun' proc args))))
(_
;; Define a new unchecked function containing the body of
;; this function.
(let ((self' (and self (fresh-var)))
(fvars' (map (lambda (_) (fresh-var)) fvars)))
(with-cps cps
;; Entry of new kfun' is the $kargs kbody.
(letk kfun' ($kfun fsrc meta self ktail kbody))
(letk ktail' ($ktail))
(letk kbody' ($kargs fnames fvars'
($continue ktail' fsrc
($callk kfun' self' fvars'))))
(letk kentry' ($kclause ,arity kbody' #f))
(setk kfun ($kfun fsrc meta self' ktail' kentry'))
;; Dispatch source $callk to new kfun'.
(build-exp
($callk kfun' proc args)))))))))
(_
;; Either this is already a $callk to a "raw" $kfun (one that
;; doesn't check its arity), in which case we're good; or a call
;; with possibly incompatible arity, or a call to a case-lambda,
;; in which case we punt for now.
(with-cps cps
(build-exp ($callk kfun proc args))))))))
;; This transformation removes references to arity-checking $kfun's, but
;; doesn't remove them, leaving that to renumbering or DCE to fix up.
(define (elide-arity-checks cps)
(with-fresh-name-state cps
(persistent-intmap
(intmap-fold
(lambda (label cont cps)
(match cont
(($ $kargs names vars
($ $continue k src ($ $callk kfun proc args)))
(with-cps cps
(let$ exp (maybe-elide-arity-check kfun proc args))
(setk label ($kargs names vars
($continue k src ,exp)))))
(_ cps)))
(persistent-intmap cps)
(transient-intmap cps)))))
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