1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
|
;;; transformation of letrec into simpler forms
;; Copyright (C) 2009 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
(define-module (language tree-il fix-letrec)
#:use-module (system base syntax)
#:use-module (srfi srfi-1)
#:use-module (srfi srfi-11)
#:use-module (language tree-il)
#:use-module (language tree-il primitives)
#:export (fix-letrec!))
;; For a detailed discussion, see "Fixing Letrec: A Faithful Yet
;; Efficient Implementation of Scheme’s Recursive Binding Construct", by
;; Oscar Waddell, Dipanwita Sarkar, and R. Kent Dybvig.
(define fix-fold
(make-tree-il-folder unref ref set simple lambda complex))
(define (simple-expression? x bound-vars)
(record-case x
((<void>) #t)
((<const>) #t)
((<lexical-ref> gensym)
(not (memq gensym bound-vars)))
((<conditional> test then else)
(and (simple-expression? test bound-vars)
(simple-expression? then bound-vars)
(simple-expression? else bound-vars)))
((<sequence> exps)
(and-map (lambda (x) (simple-expression? x bound-vars))
exps))
((<application> proc args)
(and (primitive-ref? proc)
(effect-free-primitive? (primitive-ref-name proc))
(and-map (lambda (x) (simple-expression? x bound-vars))
args)))
(else #f)))
(define (partition-vars x)
(let-values
(((unref ref set simple lambda* complex)
(fix-fold x
(lambda (x unref ref set simple lambda* complex)
(record-case x
((<lexical-ref> gensym)
(values (delq gensym unref)
(lset-adjoin eq? ref gensym)
set
simple
lambda*
complex))
((<lexical-set> gensym)
(values unref
ref
(lset-adjoin eq? set gensym)
simple
lambda*
complex))
((<letrec> vars)
(values (append vars unref)
ref
set
simple
lambda*
complex))
((<let> vars)
(values (append vars unref)
ref
set
simple
lambda*
complex))
(else
(values unref ref set simple lambda* complex))))
(lambda (x unref ref set simple lambda* complex)
(record-case x
((<letrec> (orig-vars vars) vals)
(let lp ((vars orig-vars) (vals vals)
(s '()) (l '()) (c '()))
(cond
((null? vars)
(values unref
ref
set
(append s simple)
(append l lambda*)
(append c complex)))
((memq (car vars) unref)
(lp (cdr vars) (cdr vals)
s l c))
((memq (car vars) set)
(lp (cdr vars) (cdr vals)
s l (cons (car vars) c)))
((lambda? (car vals))
(lp (cdr vars) (cdr vals)
s (cons (car vars) l) c))
((simple-expression? (car vals) orig-vars)
(lp (cdr vars) (cdr vals)
(cons (car vars) s) l c))
(else
(lp (cdr vars) (cdr vals)
s l (cons (car vars) c))))))
((<let> (orig-vars vars) vals)
;; The point is to compile let-bound lambdas as
;; efficiently as we do letrec-bound lambdas, so
;; we use the same algorithm for analyzing the
;; vars. There is no problem recursing into the
;; bindings after the let, because all variables
;; have been renamed.
(let lp ((vars orig-vars) (vals vals)
(s '()) (l '()) (c '()))
(cond
((null? vars)
(values unref
ref
set
(append s simple)
(append l lambda*)
(append c complex)))
((memq (car vars) unref)
(lp (cdr vars) (cdr vals)
s l c))
((memq (car vars) set)
(lp (cdr vars) (cdr vals)
s l (cons (car vars) c)))
((and (lambda? (car vals))
(not (memq (car vars) set)))
(lp (cdr vars) (cdr vals)
s (cons (car vars) l) c))
;; There is no difference between simple and
;; complex, for the purposes of let. Just lump
;; them all into complex.
(else
(lp (cdr vars) (cdr vals)
s l (cons (car vars) c))))))
(else
(values unref ref set simple lambda* complex))))
'()
'()
'()
'()
'()
'())))
(values unref simple lambda* complex)))
(define (fix-letrec! x)
(let-values (((unref simple lambda* complex) (partition-vars x)))
(post-order!
(lambda (x)
(record-case x
;; Sets to unreferenced variables may be replaced by their
;; expression, called for effect.
((<lexical-set> gensym exp)
(if (memq gensym unref)
(make-sequence #f (list exp (make-void #f)))
x))
((<letrec> src names vars vals body)
(let ((binds (map list vars names vals)))
(define (lookup set)
(map (lambda (v) (assq v binds))
(lset-intersection eq? vars set)))
(let ((u (lookup unref))
(s (lookup simple))
(l (lookup lambda*))
(c (lookup complex)))
;; Bind "simple" bindings, and locations for complex
;; bindings.
(make-let
src
(append (map cadr s) (map cadr c))
(append (map car s) (map car c))
(append (map caddr s) (map (lambda (x) (make-void #f)) c))
;; Bind lambdas using the fixpoint operator.
(make-fix
src (map cadr l) (map car l) (map caddr l)
(make-sequence
src
(append
;; The right-hand-sides of the unreferenced
;; bindings, for effect.
(map caddr u)
(if (null? c)
;; No complex bindings, just emit the body.
(list body)
(list
;; Evaluate the the "complex" bindings, in a `let' to
;; indicate that order doesn't matter, and bind to
;; their variables.
(let ((tmps (map (lambda (x) (gensym)) c)))
(make-let
#f (map cadr c) tmps (map caddr c)
(make-sequence
#f
(map (lambda (x tmp)
(make-lexical-set
#f (cadr x) (car x)
(make-lexical-ref #f (cadr x) tmp)))
c tmps))))
;; Finally, the body.
body)))))))))
((<let> src names vars vals body)
(let ((binds (map list vars names vals)))
(define (lookup set)
(map (lambda (v) (assq v binds))
(lset-intersection eq? vars set)))
(let ((u (lookup unref))
(l (lookup lambda*))
(c (lookup complex)))
(make-sequence
src
(append
;; unreferenced bindings, called for effect.
(map caddr u)
(list
;; unassigned lambdas use fix.
(make-fix src (map cadr l) (map car l) (map caddr l)
;; and the "complex" bindings.
(make-let src (map cadr c) (map car c) (map caddr c)
body))))))))
(else x)))
x)))
|
