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1 | ;;; Continuation-passing style (CPS) intermediate language (IL) |
2 | ||
3 | ;; Copyright (C) 2013, 2014 Free Software Foundation, Inc. | |
4 | ||
5 | ;;;; This library is free software; you can redistribute it and/or | |
6 | ;;;; modify it under the terms of the GNU Lesser General Public | |
7 | ;;;; License as published by the Free Software Foundation; either | |
8 | ;;;; version 3 of the License, or (at your option) any later version. | |
9 | ;;;; | |
10 | ;;;; This library is distributed in the hope that it will be useful, | |
11 | ;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of | |
12 | ;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
13 | ;;;; Lesser General Public License for more details. | |
14 | ;;;; | |
15 | ;;;; You should have received a copy of the GNU Lesser General Public | |
16 | ;;;; License along with this library; if not, write to the Free Software | |
17 | ;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA | |
18 | ||
19 | ;;; Commentary: | |
20 | ;;; | |
21 | ;;; The fundamental lambda calculus reductions, like beta and eta | |
22 | ;;; reduction and so on. Pretty lame currently. | |
23 | ;;; | |
24 | ;;; Code: | |
25 | ||
26 | (define-module (language cps simplify) | |
27 | #:use-module (ice-9 match) | |
28 | #:use-module (srfi srfi-1) | |
29 | #:use-module (srfi srfi-26) | |
30 | #:use-module (language cps) | |
31 | #:use-module (language cps dfg) | |
32 | #:export (simplify)) | |
33 | ||
34 | ;; Currently we just try to bypass all $values nodes that we can. This | |
35 | ;; is eta-reduction on continuations. Then we prune unused | |
36 | ;; continuations. Note that this pruning is just a quick clean-up; for | |
37 | ;; a real fixed-point pass that can eliminate unused loops, the | |
38 | ;; dead-code elimination pass is there for you. But DCE introduces new | |
39 | ;; nullary $values nodes (as replacements for expressions whose values | |
40 | ;; aren't used), making it useful for this pass to include its own | |
41 | ;; little pruner. | |
42 | ||
43 | (define (compute-eta-reductions fun) | |
44 | (let ((table (make-hash-table))) | |
45 | (define (visit-cont cont) | |
46 | (match cont | |
47 | (($ $cont sym ($ $kargs names syms body)) | |
48 | (visit-term body sym syms)) | |
49 | (($ $cont sym ($ $kentry self tail clauses)) | |
50 | (for-each visit-cont clauses)) | |
51 | (($ $cont sym ($ $kclause arity body)) | |
52 | (visit-cont body)) | |
53 | (($ $cont sym _) #f))) | |
54 | (define (visit-term term term-k term-args) | |
55 | (match term | |
56 | (($ $letk conts body) | |
57 | (for-each visit-cont conts) | |
58 | (visit-term body term-k term-args)) | |
59 | (($ $letrec names syms funs body) | |
60 | (for-each visit-fun funs) | |
61 | (visit-term body term-k term-args)) | |
62 | (($ $continue k src ($ $values args)) | |
63 | (when (equal? term-args args) | |
64 | (hashq-set! table term-k k))) | |
65 | (($ $continue k src (and fun ($ $fun))) | |
66 | (visit-fun fun)) | |
67 | (($ $continue k src _) | |
68 | #f))) | |
69 | (define (visit-fun fun) | |
70 | (match fun | |
71 | (($ $fun src meta free body) | |
72 | (visit-cont body)))) | |
73 | (visit-fun fun) | |
74 | table)) | |
75 | ||
76 | (define (locally-prune-continuations fun dfg) | |
77 | (let ((cfa (analyze-control-flow fun dfg))) | |
78 | (define (must-visit-cont cont) | |
79 | (or (visit-cont cont) | |
80 | (error "cont must be reachable" cont))) | |
81 | (define (visit-cont cont) | |
82 | (match cont | |
83 | (($ $cont sym cont) | |
84 | (and (cfa-k-idx cfa sym #:default (lambda (k) #f)) | |
85 | (rewrite-cps-cont cont | |
86 | (($ $kargs names syms body) | |
87 | (sym ($kargs names syms ,(visit-term body)))) | |
88 | (($ $kentry self tail clauses) | |
89 | (sym ($kentry self ,tail ,(visit-conts clauses)))) | |
90 | (($ $kclause arity body) | |
91 | (sym ($kclause ,arity ,(must-visit-cont body)))) | |
36527695 | 92 | ((or ($ $kreceive) ($ $kif)) |
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93 | (sym ,cont))))))) |
94 | (define (visit-conts conts) | |
95 | (filter-map visit-cont conts)) | |
96 | (define (visit-term term) | |
97 | (match term | |
98 | (($ $letk conts body) | |
99 | (let ((body (visit-term body))) | |
100 | (match (visit-conts conts) | |
101 | (() body) | |
102 | (conts (build-cps-term ($letk ,conts ,body)))))) | |
103 | (($ $letrec names syms funs body) | |
104 | (build-cps-term | |
105 | ($letrec names syms funs ,(visit-term body)))) | |
106 | (($ $continue k src exp) | |
107 | term))) | |
108 | (rewrite-cps-exp fun | |
109 | (($ $fun src meta free body) | |
110 | ($fun src meta free ,(must-visit-cont body)))))) | |
111 | ||
112 | (define (eta-reduce fun) | |
113 | (let ((table (compute-eta-reductions fun)) | |
114 | (dfg (compute-dfg fun))) | |
115 | (define (reduce* k scope values?) | |
116 | (match (hashq-ref table k) | |
117 | (#f k) | |
118 | (k* | |
119 | (if (and (continuation-bound-in? k* scope dfg) | |
120 | (or values? | |
121 | (match (lookup-cont k* (dfg-cont-table dfg)) | |
122 | (($ $kargs) #t) | |
123 | (_ #f)))) | |
124 | (reduce* k* scope values?) | |
125 | k)))) | |
126 | (define (reduce k scope) | |
127 | (reduce* k scope #f)) | |
128 | (define (reduce-values k scope) | |
129 | (reduce* k scope #t)) | |
130 | (define (visit-cont cont scope) | |
131 | (rewrite-cps-cont cont | |
132 | (($ $cont sym ($ $kargs names syms body)) | |
133 | (sym ($kargs names syms ,(visit-term body sym)))) | |
134 | (($ $cont sym ($ $kentry self tail clauses)) | |
135 | (sym ($kentry self ,tail ,(map (cut visit-cont <> sym) clauses)))) | |
136 | (($ $cont sym ($ $kclause arity body)) | |
137 | (sym ($kclause ,arity ,(visit-cont body sym)))) | |
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138 | (($ $cont sym ($ $kreceive ($ $arity req () rest () #f) kargs)) |
139 | (sym ($kreceive req rest (reduce kargs scope)))) | |
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140 | (($ $cont sym ($ $kif kt kf)) |
141 | (sym ($kif (reduce kt scope) (reduce kf scope)))))) | |
142 | (define (visit-term term scope) | |
143 | (rewrite-cps-term term | |
144 | (($ $letk conts body) | |
145 | ($letk ,(map (cut visit-cont <> scope) conts) | |
146 | ,(visit-term body scope))) | |
147 | (($ $letrec names syms funs body) | |
148 | ($letrec names syms (map visit-fun funs) | |
149 | ,(visit-term body scope))) | |
150 | (($ $continue k src ($ $values args)) | |
151 | ($continue (reduce-values k scope) src ($values args))) | |
152 | (($ $continue k src (and fun ($ $fun))) | |
153 | ($continue (reduce k scope) src ,(visit-fun fun))) | |
154 | (($ $continue k src exp) | |
155 | ($continue (reduce k scope) src ,exp)))) | |
156 | (define (visit-fun fun) | |
157 | (locally-prune-continuations | |
158 | (rewrite-cps-exp fun | |
159 | (($ $fun src meta free body) | |
160 | ($fun src meta free ,(visit-cont body #f)))) | |
161 | dfg)) | |
162 | (visit-fun fun))) | |
163 | ||
164 | (define (compute-beta-reductions fun) | |
165 | ;; A continuation's body can be inlined in place of a $values | |
166 | ;; expression if the continuation is a $kargs. It should only be | |
167 | ;; inlined if it is used only once, and not recursively. | |
168 | (let ((table (make-hash-table)) | |
169 | (dfg (compute-dfg fun))) | |
170 | (define (visit-cont cont) | |
171 | (match cont | |
172 | (($ $cont sym ($ $kargs names syms body)) | |
173 | (visit-term body)) | |
174 | (($ $cont sym ($ $kentry self tail clauses)) | |
175 | (for-each visit-cont clauses)) | |
176 | (($ $cont sym ($ $kclause arity body)) | |
177 | (visit-cont body)) | |
36527695 | 178 | (($ $cont sym (or ($ $ktail) ($ $kreceive) ($ $kif))) |
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179 | #f))) |
180 | (define (visit-term term) | |
181 | (match term | |
182 | (($ $letk conts body) | |
183 | (for-each visit-cont conts) | |
184 | (visit-term body)) | |
185 | (($ $letrec names syms funs body) | |
186 | (for-each visit-fun funs) | |
187 | (visit-term body)) | |
188 | (($ $continue k src ($ $values args)) | |
189 | (match (lookup-cont k (dfg-cont-table dfg)) | |
190 | (($ $kargs names syms body) | |
191 | (match (lookup-predecessors k dfg) | |
192 | ((_) | |
193 | ;; There is only one use, and it is this use. We assume | |
194 | ;; it's not recursive, as there would to be some other | |
195 | ;; use for control flow to reach this loop. Store the k | |
196 | ;; -> body mapping in the table. Also store the | |
197 | ;; substitutions for the variables bound by the inlined | |
198 | ;; continuation. | |
199 | (for-each (cut hashq-set! table <> <>) syms args) | |
200 | (hashq-set! table k body)) | |
201 | (_ #f))) | |
202 | (_ #f))) | |
203 | (($ $continue k src (and fun ($ $fun))) | |
204 | (visit-fun fun)) | |
205 | (($ $continue k src _) | |
206 | #f))) | |
207 | (define (visit-fun fun) | |
208 | (match fun | |
209 | (($ $fun src meta free body) | |
210 | (visit-cont body)))) | |
211 | (visit-fun fun) | |
212 | table)) | |
213 | ||
214 | (define (beta-reduce fun) | |
215 | (let ((table (compute-beta-reductions fun))) | |
216 | (define (subst var) | |
217 | (cond ((hashq-ref table var) => subst) | |
218 | (else var))) | |
219 | (define (must-visit-cont cont) | |
220 | (or (visit-cont cont) | |
221 | (error "continuation must not be inlined" cont))) | |
222 | (define (visit-cont cont) | |
223 | (match cont | |
224 | (($ $cont sym cont) | |
225 | (and (not (hashq-ref table sym)) | |
226 | (rewrite-cps-cont cont | |
227 | (($ $kargs names syms body) | |
228 | (sym ($kargs names syms ,(visit-term body)))) | |
229 | (($ $kentry self tail clauses) | |
230 | (sym ($kentry self ,tail ,(map must-visit-cont clauses)))) | |
231 | (($ $kclause arity body) | |
232 | (sym ($kclause ,arity ,(must-visit-cont body)))) | |
36527695 | 233 | ((or ($ $kreceive) ($ $kif)) |
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234 | (sym ,cont))))))) |
235 | (define (visit-term term) | |
236 | (match term | |
237 | (($ $letk conts body) | |
238 | (match (filter-map visit-cont conts) | |
239 | (() (visit-term body)) | |
240 | (conts (build-cps-term | |
241 | ($letk ,conts ,(visit-term body)))))) | |
242 | (($ $letrec names syms funs body) | |
243 | (build-cps-term | |
244 | ($letrec names syms (map visit-fun funs) | |
245 | ,(visit-term body)))) | |
246 | (($ $continue k src exp) | |
247 | (cond | |
248 | ((hashq-ref table k) => visit-term) | |
249 | (else | |
250 | (build-cps-term | |
251 | ($continue k src | |
252 | ,(match exp | |
253 | ((or ($ $void) ($ $const) ($ $prim)) exp) | |
254 | (($ $fun) (visit-fun exp)) | |
255 | (($ $call proc args) | |
256 | (let ((args (map subst args))) | |
257 | (build-cps-exp ($call (subst proc) args)))) | |
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258 | (($ $callk k proc args) |
259 | (let ((args (map subst args))) | |
260 | (build-cps-exp ($callk k (subst proc) args)))) | |
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261 | (($ $primcall name args) |
262 | (let ((args (map subst args))) | |
263 | (build-cps-exp ($primcall name args)))) | |
264 | (($ $values args) | |
265 | (let ((args (map subst args))) | |
266 | (build-cps-exp ($values args)))) | |
267 | (($ $prompt escape? tag handler) | |
268 | (build-cps-exp ($prompt escape? (subst tag) handler))))))))))) | |
269 | (define (visit-fun fun) | |
270 | (rewrite-cps-exp fun | |
271 | (($ $fun src meta free body) | |
272 | ($fun src meta (map subst free) ,(must-visit-cont body))))) | |
273 | (visit-fun fun))) | |
274 | ||
275 | (define (simplify fun) | |
276 | (eta-reduce (beta-reduce fun))) |