Rotate comparisons down to loop back-edges

* module/language/cps/rotate-loops.scm: New pass.
* module/Makefile.am:
* module/language/cps/optimize.scm: Wire up the new pass.

module/Makefile.am

@@ -136,6 +136,7 @@ CPS_LANG_SOURCES =						\
   language/cps/prune-top-level-scopes.scm			\
   language/cps/reify-primitives.scm				\
   language/cps/renumber.scm					\
+  language/cps/rotate-loops.scm					\
   language/cps/optimize.scm					\
   language/cps/simplify.scm					\
   language/cps/self-references.scm				\

module/language/cps/optimize.scm

@@ -32,6 +32,7 @@
   #:use-module (language cps licm)
   #:use-module (language cps prune-top-level-scopes)
   #:use-module (language cps prune-bailouts)
+  #:use-module (language cps rotate-loops)
   #:use-module (language cps self-references)
   #:use-module (language cps simplify)
   #:use-module (language cps specialize-primcalls)
@@ -105,4 +106,5 @@
 
 (define-optimizer optimize-first-order-cps
   (eliminate-dead-code #:eliminate-dead-code? #t)
+  (rotate-loops #:rotate-loops? #t)
   (simplify #:simplify? #t))

module/language/cps/rotate-loops.scm

@@ -0,0 +1,217 @@
+;;; Continuation-passing style (CPS) intermediate language (IL)
+
+;; Copyright (C) 2013, 2014, 2015 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:
+;;;
+;;; Rotate loops so that they end with conditional jumps, if possible.
+;;; The result goes from:
+;;;
+;;;   loop:
+;;;     if x < 5 goto done;
+;;;     x = x + 1;
+;;;     goto loop;
+;;;   done:
+;;;
+;;;     if x < 5 goto done;
+;;;   loop:
+;;;     x = x + 1;
+;;;     if x < 5 goto done;
+;;;   done:
+;;;
+;;; It's more code but there are fewer instructions in the body.  Note
+;;; that this transformation isn't guaranteed to produce a loop that
+;;; ends in a conditional jump, because usually your loop has some state
+;;; that it's shuffling around and for now that shuffle is reified with
+;;; the test, not the loop header.  Alack.
+;;;
+;;; 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 comparison, 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 rotate-loops)
+  #:use-module (ice-9 match)
+  #:use-module ((srfi srfi-1) #:select (filter-map))
+  #:use-module (srfi srfi-9)
+  #:use-module (language cps)
+  #:use-module (language cps utils)
+  #:use-module (language cps intmap)
+  #:use-module (language cps intset)
+  #:export (rotate-loops))
+
+(define-record-type $loop
+  (make-loop entry exits body)
+  loop?
+  (entry loop-entry)
+  (exits loop-exits)
+  (body loop-body))
+
+(define (find-exits scc succs)
+  (intset-fold (lambda (label exits)
+                 (if (eq? empty-intset
+                          (intset-subtract (intmap-ref succs label) scc))
+                     exits
+                     (intset-add exits label)))
+               scc
+               empty-intset))
+
+(define (find-entry scc preds)
+  (trivial-intset (find-exits scc preds)))
+
+(define (rotate-loop cps entry-label body-labels succs preds back-edges)
+  (match (intmap-ref cps entry-label)
+    ((and entry-cont
+          ($ $kargs entry-names entry-vars
+             ($ $continue entry-kf entry-src ($ $branch entry-kt entry-exp))))
+     (let* ((exit-if-true? (intset-ref body-labels entry-kf))
+            (exit (if exit-if-true? entry-kt entry-kf))
+            (new-entry-label (if exit-if-true? entry-kf entry-kt))
+            (join-label (fresh-label))
+            (join-cont (build-cont
+                         ($kargs entry-names entry-vars
+                           ($continue exit entry-src ($values ())))))
+            (cps (intmap-add! cps join-label join-cont)))
+       (define (make-fresh-vars)
+         (map (lambda (_) (fresh-var)) entry-vars))
+       (define (make-trampoline k src values)
+         (build-cont ($kargs () () ($continue k src ($values values)))))
+       (define (replace-exit k trampoline)
+         (if (eqv? k exit) trampoline k))
+       (define (rename-exp exp vars)
+         (define (rename-var var)
+           (match (list-index entry-vars var)
+             (#f var)
+             (idx (list-ref vars idx))))
+         (rewrite-exp exp
+           ((or ($ $const) ($ $prim) ($ $closure)) ,exp)
+           (($ $values args)
+            ($values ,(map rename-var args)))
+           (($ $call proc args)
+            ($call (rename-var proc) ,(map rename-var args)))
+           (($ $callk k proc args)
+            ($callk k (rename-var proc) ,(map rename-var args)))
+           (($ $branch kt ($ $values (arg)))
+            ($branch kt ($values ((rename-var arg)))))
+           (($ $branch kt ($ $primcall name args))
+            ($branch kt ($primcall name ,(map rename-var args))))
+           (($ $primcall name args)
+            ($primcall name ,(map rename-var args)))
+           (($ $prompt escape? tag handler)
+            ($prompt escape? (rename-var tag) handler))))
+       (define (attach-trampoline label src names vars args)
+         (let* ((trampoline-out-label (fresh-label))
+                (trampoline-out-cont
+                 (make-trampoline join-label src args))
+                (trampoline-in-label (fresh-label))
+                (trampoline-in-cont
+                 (make-trampoline new-entry-label src args))
+                (kf (if exit-if-true? trampoline-in-label trampoline-out-label))
+                (kt (if exit-if-true? trampoline-out-label trampoline-in-label))
+                (cont (build-cont
+                        ($kargs names vars
+                          ($continue kf entry-src
+                            ($branch kt ,(rename-exp entry-exp args))))))
+                (cps (intmap-replace! cps label cont))
+                (cps (intmap-add! cps trampoline-in-label trampoline-in-cont)))
+           (intmap-add! cps trampoline-out-label trampoline-out-cont)))
+       ;; Rewrite the targets of the entry branch to go to
+       ;; trampolines.  One will pass values out of the loop, and
+       ;; one will pass values into the loop.
+       (let* ((pre-header-vars (make-fresh-vars))
+              (body-vars (make-fresh-vars))
+              (cps (attach-trampoline entry-label entry-src
+                                      entry-names pre-header-vars
+                                      pre-header-vars))
+              (new-entry-cont (build-cont
+                                ($kargs entry-names body-vars
+                                  ,(match (intmap-ref cps new-entry-label)
+                                     (($ $kargs () () term) term)))))
+              (cps (intmap-replace! cps new-entry-label new-entry-cont)))
+         (intset-fold
+          (lambda (label cps)
+            (if (intset-ref back-edges label)
+                (match (intmap-ref cps label)
+                  (($ $kargs names vars ($ $continue _ src exp))
+                   (match (rename-exp exp body-vars)
+                     (($ $values args)
+                      (attach-trampoline label src names vars args))
+                     (exp
+                      (let* ((args (make-fresh-vars))
+                             (bind-label (fresh-label))
+                             (edge* (build-cont
+                                      ($kargs names vars
+                                        ($continue bind-label src ,exp))))
+                             (cps (intmap-replace! cps label edge*))
+                             ;; attach-trampoline uses intmap-replace!.
+                             (cps (intmap-add! cps bind-label #f)))
+                        (attach-trampoline bind-label src
+                                           entry-names args args))))))
+                (match (intmap-ref cps label)
+                  (($ $kargs names vars ($ $continue k src exp))
+                   (let ((cont (build-cont
+                                 ($kargs names vars
+                                   ($continue k src
+                                     ,(rename-exp exp body-vars))))))
+                     (intmap-replace! cps label cont)))
+                  (($ $kreceive) cps))))
+          (intset-remove body-labels entry-label)
+          cps))))))
+
+(define (rotate-loops-in-function kfun body cps)
+  (define (can-rotate? edges)
+    (intset-fold (lambda (label rotate?)
+                   (match (intmap-ref cps label)
+                     (($ $kreceive) #f)
+                     (($ $kargs _ _ ($ $continue _ _ exp))
+                      (match exp
+                        (($ $branch) #f)
+                        (_ rotate?)))))
+                 edges #t))
+  (let* ((succs (compute-successors cps kfun))
+         (preds (invert-graph succs)))
+    (intmap-fold
+     (lambda (id scc cps)
+       (cond
+        ((trivial-intset scc) cps)
+        ((find-entry scc preds)
+         => (lambda (entry)
+              (let ((back-edges (intset-intersect scc
+                                                  (intmap-ref preds entry))))
+                (if (and (can-rotate? back-edges)
+                         (eqv? (trivial-intset (find-exits scc succs)) entry))
+                    ;; Loop header is the only exit.  It must be a
+                    ;; conditional branch and only one successor is an
+                    ;; exit.  The values flowing out of the loop are the
+                    ;; loop variables.
+                    (rotate-loop cps entry scc succs preds back-edges)
+                    cps))))
+        (else cps)))
+     (compute-strongly-connected-components succs kfun)
+     cps)))
+
+(define (rotate-loops cps)
+  (persistent-intmap
+   (with-fresh-name-state cps
+     (intmap-fold rotate-loops-in-function
+                  (compute-reachable-functions cps)
+                  cps))))