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235 lines
10 KiB
Scheme
235 lines
10 KiB
Scheme
;;; Continuation-passing style (CPS) intermediate language (IL)
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;; Copyright (C) 2013, 2014, 2015 Free Software Foundation, Inc.
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;;;; This library is free software; you can redistribute it and/or
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;;;; modify it under the terms of the GNU Lesser General Public
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;;;; License as published by the Free Software Foundation; either
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;;;; version 3 of the License, or (at your option) any later version.
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;;;;
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;;;; This library is distributed in the hope that it will be useful,
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;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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;;;; Lesser General Public License for more details.
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;;;;
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;;;; You should have received a copy of the GNU Lesser General Public
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;;;; License along with this library; if not, write to the Free Software
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;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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;;; Commentary:
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;;;
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;;; Rotate loops so that they end with conditional jumps, if possible.
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;;; The result goes from:
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;;;
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;;; loop:
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;;; if x < 5 goto done;
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;;; x = x + 1;
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;;; goto loop;
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;;; done:
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;;;
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;;; if x < 5 goto done;
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;;; loop:
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;;; x = x + 1;
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;;; if x < 5 goto done;
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;;; done:
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;;;
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;;; It's more code but there are fewer instructions in the body. Note
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;;; that this transformation isn't guaranteed to produce a loop that
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;;; ends in a conditional jump, because usually your loop has some state
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;;; that it's shuffling around and for now that shuffle is reified with
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;;; the test, not the loop header. Alack.
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;;;
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;;; Implementation-wise, things are complicated by values flowing out of
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;;; the loop. We actually perform this transformation only on loops
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;;; that have a single exit continuation, so that we define values
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;;; flowing out in one place. We rename the loop variables in two
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;;; places internally: one for the peeled comparison, and another for
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;;; the body. The loop variables' original names are then bound in a
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;;; join continuation for use by successor code.
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;;;
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;;; Code:
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(define-module (language cps rotate-loops)
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#:use-module (ice-9 match)
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#:use-module (language cps)
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#:use-module (language cps utils)
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#:use-module (language cps intmap)
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#:use-module (language cps intset)
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#:export (rotate-loops))
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(define (loop-successors scc succs)
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(intset-subtract (intset-fold (lambda (label exits)
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(intset-union exits (intmap-ref succs label)))
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scc empty-intset)
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scc))
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(define (find-exits scc succs)
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(intset-fold (lambda (label exits)
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(if (eq? empty-intset
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(intset-subtract (intmap-ref succs label) scc))
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exits
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(intset-add exits label)))
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scc
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empty-intset))
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(define (find-entry scc preds)
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(trivial-intset (find-exits scc preds)))
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(define (rotate-loop cps entry-label body-labels succs preds back-edges)
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(match (intmap-ref cps entry-label)
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((and entry-cont
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($ $kargs entry-names entry-vars
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($ $continue entry-kf entry-src ($ $branch entry-kt entry-exp))))
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(let* ((exit-if-true? (intset-ref body-labels entry-kf))
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(loop-exits (find-exits body-labels succs))
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(exit (if exit-if-true? entry-kt entry-kf))
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(new-entry-label (if exit-if-true? entry-kf entry-kt))
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(join-label (fresh-label))
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(join-cont (build-cont
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($kargs entry-names entry-vars
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($continue exit entry-src ($values ())))))
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(cps (intmap-add! cps join-label join-cont)))
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(define (make-fresh-vars)
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(map (lambda (_) (fresh-var)) entry-vars))
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(define (make-trampoline k src values)
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(build-cont ($kargs () () ($continue k src ($values values)))))
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(define (replace-exit k trampoline)
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(if (eqv? k exit) trampoline k))
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(define (rename-exp exp vars)
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(define (rename-var var)
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(match (list-index entry-vars var)
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(#f var)
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(idx (list-ref vars idx))))
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(rewrite-exp exp
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((or ($ $const) ($ $prim) ($ $closure)) ,exp)
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(($ $values args)
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($values ,(map rename-var args)))
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(($ $call proc args)
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($call (rename-var proc) ,(map rename-var args)))
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(($ $callk k proc args)
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($callk k (rename-var proc) ,(map rename-var args)))
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(($ $branch kt ($ $values (arg)))
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($branch kt ($values ((rename-var arg)))))
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(($ $branch kt ($ $primcall name args))
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($branch kt ($primcall name ,(map rename-var args))))
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(($ $primcall name args)
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($primcall name ,(map rename-var args)))
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(($ $prompt escape? tag handler)
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($prompt escape? (rename-var tag) handler))))
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(define (attach-trampoline label src names vars args)
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(let* ((trampoline-out-label (fresh-label))
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(trampoline-out-cont
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(make-trampoline join-label src args))
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(trampoline-in-label (fresh-label))
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(trampoline-in-cont
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(make-trampoline new-entry-label src args))
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(kf (if exit-if-true? trampoline-in-label trampoline-out-label))
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(kt (if exit-if-true? trampoline-out-label trampoline-in-label))
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(cont (build-cont
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($kargs names vars
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($continue kf entry-src
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($branch kt ,(rename-exp entry-exp args))))))
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(cps (intmap-replace! cps label cont))
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(cps (intmap-add! cps trampoline-in-label trampoline-in-cont)))
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(intmap-add! cps trampoline-out-label trampoline-out-cont)))
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;; Rewrite the targets of the entry branch to go to
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;; trampolines. One will pass values out of the loop, and
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;; one will pass values into the loop.
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(let* ((pre-header-vars (make-fresh-vars))
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(body-vars (make-fresh-vars))
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(cps (attach-trampoline entry-label entry-src
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entry-names pre-header-vars
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pre-header-vars))
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(new-entry-cont (build-cont
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($kargs entry-names body-vars
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,(match (intmap-ref cps new-entry-label)
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(($ $kargs () () term) term)))))
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(cps (intmap-replace! cps new-entry-label new-entry-cont)))
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(intset-fold
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(lambda (label cps)
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(cond
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((intset-ref back-edges label)
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(match (intmap-ref cps label)
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(($ $kargs names vars ($ $continue _ src exp))
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(match (rename-exp exp body-vars)
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(($ $values args)
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(attach-trampoline label src names vars args))
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(exp
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(let* ((args (make-fresh-vars))
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(bind-label (fresh-label))
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(edge* (build-cont
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($kargs names vars
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($continue bind-label src ,exp))))
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(cps (intmap-replace! cps label edge*))
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;; attach-trampoline uses intmap-replace!.
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(cps (intmap-add! cps bind-label #f)))
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(attach-trampoline bind-label src
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entry-names args args)))))))
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((intset-ref loop-exits label)
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(match (intmap-ref cps label)
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(($ $kargs names vars
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($ $continue kf src ($ $branch kt exp)))
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(let* ((trampoline-out-label (fresh-label))
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(trampoline-out-cont
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(make-trampoline join-label src body-vars))
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(kf (if (eqv? kf exit) trampoline-out-label kf))
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(kt (if (eqv? kt exit) trampoline-out-label kt))
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(cont (build-cont
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($kargs names vars
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($continue kf src
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($branch kt ,(rename-exp exp body-vars))))))
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(cps (intmap-replace! cps label cont)))
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(intmap-add! cps trampoline-out-label trampoline-out-cont)))))
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(else
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(match (intmap-ref cps label)
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(($ $kargs names vars ($ $continue k src exp))
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(let ((cont (build-cont
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($kargs names vars
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($continue k src
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,(rename-exp exp body-vars))))))
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(intmap-replace! cps label cont)))
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(($ $kreceive) cps)))))
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(intset-remove body-labels entry-label)
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cps))))))
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(define (rotate-loops-in-function kfun body cps)
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(define (can-rotate? edges)
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(intset-fold (lambda (label rotate?)
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(match (intmap-ref cps label)
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(($ $kreceive) #f)
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(($ $kargs _ _ ($ $continue _ _ exp))
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(match exp
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(($ $branch) #f)
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(_ rotate?)))))
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edges #t))
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(let* ((succs (compute-successors cps kfun))
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(preds (invert-graph succs)))
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(intmap-fold
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(lambda (id scc cps)
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(cond
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((trivial-intset scc) cps)
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((find-entry scc preds)
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=> (lambda (entry)
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(let ((back-edges (intset-intersect scc
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(intmap-ref preds entry))))
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(if (and (can-rotate? back-edges)
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(trivial-intset
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(intset-subtract (intmap-ref succs entry) scc))
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(trivial-intset (loop-successors scc succs)))
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;; Loop header is an exit, and there is only one
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;; exit continuation. Loop header must then be a
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;; conditional branch and only one successor is an
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;; exit. The values flowing out of the loop are the
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;; loop variables.
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(rotate-loop cps entry scc succs preds back-edges)
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cps))))
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(else cps)))
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(compute-strongly-connected-components succs kfun)
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cps)))
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(define (rotate-loops cps)
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(persistent-intmap
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(with-fresh-name-state cps
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(intmap-fold rotate-loops-in-function
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(compute-reachable-functions cps)
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cps))))
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