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;;; 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:
;;;
;;; Loop peeling "peels off" one iteration of a loop. When followed by
;;; common subexpression elimination, it has the effect of moving terms
;;; to the first peeled iteration, leaving the loop body with fewer
;;; terms.
;;;
;;; Loop peeling is complementary to loop-invariant code motion (LICM).
;;; LICM will hoist invariant terms that have no side effects, like
;;; $const, even if they are in branches that are not always taken.
;;; However LICM won't hoist expressions that might have side effects if
;;; it can't prove that they are reachable on every iteration. Peeling
;;; on the other hand arranges for the body to be dominated by one loop
;;; iteration, so any effect that is reachable on one full iteration can
;;; be hoisted and eliminated, which is a big boon when we consider
;;; &type-check effects. For example:
;;;
;;; x = cached-toplevel-box map
;;; y = box-ref x
;;; z = cached-toplevel-box foo
;;; w = box-ref z
;;; ...
;;;
;;; In this example, LICM could hoist X, possibly Y as well if it can
;;; prove that the body doesn't write to variables, but it won't hoist
;;; Z. In contrast, peeling + CSE will allow Z to be hoisted.
;;;
;;; Peeling does cause code growth. If this becomes a problem we will
;;; need to apply heuristics to limit its applicability.
;;;
;;; Implementation-wise, things are complicated by values flowing out of
;;; the loop. We actually perform this transformation only on loops
;;; that have a single exit continuation, so that we define values
;;; flowing out in one place. We rename the loop variables in two
;;; places internally: one for the peeled iteration, and another for
;;; the body. The loop variables' original names are then bound in a
;;; join continuation for use by successor code.
;;;
;;; Code:
(define-module (language cps peel-loops)
#:use-module (ice-9 match)
#:use-module ((srfi srfi-1) #:select (fold))
#:use-module (language cps)
#:use-module (language cps utils)
#:use-module (language cps intmap)
#:use-module (language cps intset)
#:export (peel-loops))
(define (intset-map f set)
(persistent-intmap
(intset-fold (lambda (i out) (intmap-add! out i (f i))) set empty-intmap)))
(define (loop-successors scc succs)
(intset-subtract (intset-fold (lambda (label exits)
(intset-union exits (intmap-ref succs label)))
scc empty-intset)
scc))
(define (find-exits scc succs)
(intset-fold (lambda (label exits)
(if (eq? empty-intset
(intset-subtract (intmap-ref succs label) scc))
exits
(intset-add exits label)))
scc
empty-intset))
(define (find-entry scc preds)
(trivial-intset (find-exits scc preds)))
(define (list->intset vars)
(persistent-intset
(fold1 (lambda (var set) (intset-add! set var)) vars empty-intset)))
(define (compute-bailouts cps labels)
(intset-fold (lambda (label bailouts)
(match (intmap-ref cps label)
(($ $kargs () () ($ $throw))
(intset-add bailouts label))
(_ bailouts)))
labels empty-intset))
(define (compute-live-variables cps entry body succs)
(let* ((succs (intset-map (lambda (label)
(intset-intersect (intmap-ref succs label) body))
body))
(init (intset-map (lambda (label) #f) body))
(kill (intset-map (lambda (label) #f) body))
(gen (intset-map (lambda (label)
(match (intmap-ref cps label)
(($ $kargs names vars) (list->intset vars))
(_ empty-intset)))
body))
(in (intmap-replace init entry (intmap-ref gen entry)))
(out init))
(define (subtract in kill) (or in empty-intset))
(define (add in gen) (if in (intset-union in gen) gen))
(define (meet in out) (if in (intset-intersect in out) out))
(call-with-values (lambda ()
(solve-flow-equations succs in out kill gen
subtract add meet
(intset entry)))
(lambda (in out)
out))))
(define (compute-out-vars cps entry body succs exit)
(let ((live (compute-live-variables cps entry body succs)))
(intset-fold-right
cons
(intset-fold (lambda (label live-out)
(if (intset-ref (intmap-ref succs label) exit)
(if live-out
(intset-intersect live-out (intmap-ref live label))
(intmap-ref live label))
live-out))
body #f)
'())))
(define (rename-cont cont fresh-labels fresh-vars)
(define (rename-label label)
(intmap-ref fresh-labels label (lambda (label) label)))
(define (rename-var var)
(intmap-ref fresh-vars var (lambda (var) var)))
(define (rename-exp exp)
(rewrite-exp exp
((or ($ $const) ($ $prim) ($ $const-fun) ($ $code) ($ $rec ())) ,exp)
(($ $values args)
($values ,(map rename-var args)))
(($ $call proc args)
($call (rename-var proc) ,(map rename-var args)))
(($ $callk k proc args)
($callk k (and proc (rename-var proc)) ,(map rename-var args)))
(($ $primcall name param args)
($primcall name param ,(map rename-var args)))))
(define (rename-term term)
(rewrite-term term
(($ $continue k src exp)
($continue (rename-label k) src ,(rename-exp exp)))
(($ $branch kf kt src op param args)
($branch (rename-label kf) (rename-label kt) src
op param ,(map rename-var args)))
(($ $switch kf kt* src arg)
($switch (rename-label kf) (map rename-label kt*) src
(rename-var arg)))
(($ $prompt k kh src escape? tag)
($prompt (rename-label k) (rename-label kh) src
escape? (rename-var tag)))
(($ $throw src op param args)
($throw src op param ,(map rename-var args)))))
(rewrite-cont cont
(($ $kargs names vars term)
($kargs names (map rename-var vars) ,(rename-term term)))
(($ $kreceive ($ $arity req () rest) kargs)
($kreceive req rest (rename-label kargs)))))
(define (add-renamed-bailout cps label new-label fresh-vars)
;; We could recognize longer bailout sequences here; for now just
;; single-term throws.
(define (rename-var var)
(intmap-ref fresh-vars var (lambda (var) var)))
;; FIXME: Perhaps avoid copying the bailout if it doesn't use any loop
;; var.
(match (intmap-ref cps label)
(($ $kargs () () ($ $throw src op param args))
(intmap-add cps new-label
(build-cont
($kargs () ()
($throw src op param ,(map rename-var args))))))))
(define (compute-var-names conts)
(persistent-intmap
(intmap-fold (lambda (label cont out)
(match cont
(($ $kargs names vars)
(fold (lambda (name var out)
(intmap-add! out var name))
out names vars))
(_ out)))
conts empty-intmap)))
(define (peel-loop cps entry body-labels succs preds bailouts)
(let* ((body-conts (intset-map (lambda (label) (intmap-ref cps label))
body-labels))
(var-names (compute-var-names body-conts))
(loop-exits (loop-successors body-labels succs))
(loop-bailouts (intset-intersect loop-exits bailouts))
;; All non-bailout loop exits branch to this label.
(exit (trivial-intset (intset-subtract loop-exits loop-bailouts)))
;; The variables that flow out of the loop, as a list.
(out-vars (compute-out-vars cps entry body-labels succs exit))
(out-names (map (lambda (var) (intmap-ref var-names var)) out-vars))
(join-label (fresh-label))
(join-cont (build-cont
($kargs out-names out-vars
($continue exit #f ($values ())))))
(trampoline-cont
;; A $values predecessor for the join, passing the out-vars
;; using their original names. These will get renamed in
;; both the peeled iteration and the body.
(build-cont
($kargs () ()
($continue join-label #f ($values out-vars)))))
(fresh-body-labels
;; Fresh labels for the body.
(intset-map (lambda (old) (fresh-label)) body-labels))
(fresh-body-vars
;; Fresh vars for the body.
(intmap-map (lambda (var name) (fresh-var)) var-names))
(fresh-body-bailout-labels
;; Fresh labels for bailouts from body.
(intset-map (lambda (old) (fresh-label)) loop-bailouts))
(fresh-body-entry
;; The name of the entry, but in the body.
(intmap-ref fresh-body-labels entry))
(fresh-peeled-vars
;; Fresh names for variables that flow out of the peeled iteration.
(fold1 (lambda (var out) (intmap-add out var (fresh-var)))
out-vars empty-intmap))
(peeled-bailout-labels
;; Fresh labels for bailouts from peeled iteration.
(intset-map (lambda (old) (fresh-label)) loop-bailouts))
(peeled-trampoline-label
;; Label for trampoline to pass values out of the peeled
;; iteration.
(fresh-label))
(peeled-trampoline-cont
;; Trampoline for the peeled iteration, ready to adjoin to
;; CPS.
(rename-cont trampoline-cont empty-intmap fresh-peeled-vars))
(peeled-labels
;; Exit goes to trampoline, back edges to body.
(intmap-add (intmap-add empty-intmap exit peeled-trampoline-label)
entry fresh-body-entry))
(peeled-iteration
;; The peeled iteration.
(intmap-map (lambda (label cont)
(rename-cont cont
(intmap-union peeled-labels
peeled-bailout-labels)
fresh-peeled-vars))
body-conts))
(body-trampoline-label
;; Label for trampoline to pass values out of the body.
(fresh-label))
(body-trampoline-cont
;; Trampoline for the body, ready to adjoin to CPS.
(rename-cont trampoline-cont empty-intmap fresh-body-vars))
(fresh-body
;; The body, renamed.
(let ((label-map (intmap-union
(intmap-add fresh-body-labels
exit body-trampoline-label)
fresh-body-bailout-labels)))
(persistent-intmap
(intmap-fold
(lambda (label new-label out)
(intmap-add! out new-label
(rename-cont (intmap-ref body-conts label)
label-map fresh-body-vars)))
fresh-body-labels empty-intmap)))))
(let* ((cps (intmap-add! cps join-label join-cont))
(cps (intmap-add! cps peeled-trampoline-label
peeled-trampoline-cont))
(cps (intmap-add! cps body-trampoline-label
body-trampoline-cont))
(cps (intmap-fold (lambda (label cont cps)
(intmap-replace! cps label cont))
peeled-iteration cps))
(cps (intmap-fold
(lambda (old-label new-label cps)
(add-renamed-bailout cps old-label new-label
fresh-peeled-vars))
peeled-bailout-labels cps))
(cps (intmap-fold (lambda (label cont cps)
(intmap-add! cps label cont))
fresh-body cps))
(cps (intmap-fold
(lambda (old-label new-label cps)
(add-renamed-bailout cps old-label new-label
fresh-body-vars))
fresh-body-bailout-labels cps)))
cps)))
(define (peel-loops-in-function kfun body cps)
(let* ((succs (compute-successors cps kfun))
(bailouts (compute-bailouts cps body))
(preds (invert-graph succs)))
;; We can peel if there is one non-bailout successor to the loop,
;; and if the loop has no nested functions. (Peeling a nested
;; function would cause exponential code growth.)
(define (can-peel? body)
(and (trivial-intset (intset-subtract (loop-successors body succs)
bailouts))
(intset-fold (lambda (label peel?)
(match (intmap-ref cps label)
(($ $kargs _ _ ($ $continue _ _ exp))
(match exp
(($ $fun) #f)
(($ $rec (_ . _)) #f)
(_ peel?)))
(_ peel?)))
body #t)))
(intmap-fold
(lambda (id scc cps)
(cond
((trivial-intset scc) cps)
((find-entry scc preds)
=> (lambda (entry)
(if (can-peel? scc)
(peel-loop cps entry scc succs preds bailouts)
cps)))
(else cps)))
(compute-strongly-connected-components succs kfun)
cps)))
(define (peel-loops cps)
(persistent-intmap
(with-fresh-name-state cps
(intmap-fold peel-loops-in-function
(compute-reachable-functions cps)
cps))))