summaryrefslogtreecommitdiff
path: root/module/language/tree-il/effects.scm
blob: 6db4d476d8dcfc4fba4e1abb6480d24b1a3cb28c (plain)
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
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
;;; Effects analysis on Tree-IL

;; Copyright (C) 2011, 2012, 2013 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 effects)
  #:use-module (language tree-il)
  #:use-module (language tree-il primitives)
  #:use-module (ice-9 match)
  #:export (make-effects-analyzer
            &mutable-lexical
            &toplevel
            &fluid
            &definite-bailout
            &possible-bailout
            &zero-values
            &allocation
            &mutable-data
            &type-check
            &all-effects
            effects-commute?
            exclude-effects
            effect-free?
            constant?
            depends-on-effects?
            causes-effects?))

;;;
;;; Hey, it's some effects analysis!  If you invoke
;;; `make-effects-analyzer', you get a procedure that computes the set
;;; of effects that an expression depends on and causes.  This
;;; information is useful when writing algorithms that move code around,
;;; while preserving the semantics of an input program.
;;;
;;; The effects set is represented by a bitfield, as a fixnum.  The set
;;; of possible effects is modelled rather coarsely.  For example, a
;;; toplevel reference to FOO is modelled as depending on the &toplevel
;;; effect, and causing a &type-check effect.  If any intervening code
;;; sets any toplevel variable, that will block motion of FOO.
;;;
;;; For each effect, two bits are reserved: one to indicate that an
;;; expression depends on the effect, and the other to indicate that an
;;; expression causes the effect.
;;;

(define-syntax define-effects
  (lambda (x)
    (syntax-case x ()
      ((_ all name ...)
       (with-syntax (((n ...) (iota (length #'(name ...)))))
         #'(begin
             (define-syntax name (identifier-syntax (ash 1 (* n 2))))
             ...
             (define-syntax all (identifier-syntax (logior name ...)))))))))

;; Here we define the effects, indicating the meaning of the effect.
;;
;; Effects that are described in a "depends on" sense can also be used
;; in the "causes" sense.
;;
;; Effects that are described as causing an effect are not usually used
;; in a "depends-on" sense.  Although the "depends-on" sense is used
;; when checking for the existence of the "causes" effect, the effects
;; analyzer will not associate the "depends-on" sense of these effects
;; with any expression.
;;
(define-effects &all-effects
  ;; Indicates that an expression depends on the value of a mutable
  ;; lexical variable.
  &mutable-lexical

  ;; Indicates that an expression depends on the value of a toplevel
  ;; variable.
  &toplevel

  ;; Indicates that an expression depends on the value of a fluid
  ;; variable.
  &fluid

  ;; Indicates that an expression definitely causes a non-local,
  ;; non-resumable exit -- a bailout.  Only used in the "changes" sense.
  &definite-bailout

  ;; Indicates that an expression may cause a bailout.
  &possible-bailout

  ;; Indicates than an expression may return zero values -- a "causes"
  ;; effect.
  &zero-values

  ;; Indicates that an expression may return a fresh object -- a
  ;; "causes" effect.
  &allocation

  ;; Indicates that an expression depends on the value of a mutable data
  ;; structure.
  &mutable-data

  ;; Indicates that an expression may cause a type check.  A type check,
  ;; for the purposes of this analysis, is the possibility of throwing
  ;; an exception the first time an expression is evaluated.  If the
  ;; expression did not cause an exception to be thrown, users can
  ;; assume that evaluating the expression again will not cause an
  ;; exception to be thrown.
  ;;
  ;; For example, (+ x y) might throw if X or Y are not numbers.  But if
  ;; it doesn't throw, it should be safe to elide a dominated, common
  ;; subexpression (+ x y).
  &type-check)

(define-syntax &no-effects (identifier-syntax 0))

;; Definite bailout is an oddball effect.  Since it indicates that an
;; expression definitely causes bailout, it's not in the set of effects
;; of a call to an unknown procedure.  At the same time, it's also
;; special in that a definite bailout in a subexpression doesn't always
;; cause an outer expression to include &definite-bailout in its
;; effects.  For that reason we have to treat it specially.
;;
(define-syntax &all-effects-but-bailout
  (identifier-syntax
   (logand &all-effects (lognot &definite-bailout))))

(define-inlinable (cause effect)
  (ash effect 1))

(define-inlinable (&depends-on a)
  (logand a &all-effects))
(define-inlinable (&causes a)
  (logand a (cause &all-effects)))

(define (exclude-effects effects exclude)
  (logand effects (lognot (cause exclude))))
(define (effect-free? effects)
  (zero? (&causes effects)))
(define (constant? effects)
  (zero? effects))

(define-inlinable (depends-on-effects? x effects)
  (not (zero? (logand (&depends-on x) effects))))
(define-inlinable (causes-effects? x effects)
  (not (zero? (logand (&causes x) (cause effects)))))

(define-inlinable (effects-commute? a b)
  (and (not (causes-effects? a (&depends-on b)))
       (not (causes-effects? b (&depends-on a)))))

(define (make-effects-analyzer assigned-lexical?)
  "Returns a procedure of type EXP -> EFFECTS that analyzes the effects
of an expression."

  (let ((cache (make-hash-table)))
    (define* (compute-effects exp #:optional (lookup (lambda (x) #f)))
      (define (compute-effects exp)
        (or (hashq-ref cache exp)
            (let ((effects (visit exp)))
              (hashq-set! cache exp effects)
              effects)))

      (define (accumulate-effects exps)
        (let lp ((exps exps) (out &no-effects))
          (if (null? exps)
              out
              (lp (cdr exps) (logior out (compute-effects (car exps)))))))

      (define (visit exp)
        (match exp
          (($ <const>)
           &no-effects)
          (($ <void>)
           &no-effects)
          (($ <lexical-ref> _ _ gensym)
           (if (assigned-lexical? gensym)
               &mutable-lexical
               &no-effects))
          (($ <lexical-set> _ name gensym exp)
           (logior (cause &mutable-lexical)
                   (compute-effects exp)))
          (($ <let> _ names gensyms vals body)
           (logior (if (or-map assigned-lexical? gensyms)
                       (cause &allocation)
                       &no-effects)
                   (accumulate-effects vals)
                   (compute-effects body)))
          (($ <letrec> _ in-order? names gensyms vals body)
           (logior (if (or-map assigned-lexical? gensyms)
                       (cause &allocation)
                       &no-effects)
                   (accumulate-effects vals)
                   (compute-effects body)))
          (($ <fix> _ names gensyms vals body)
           (logior (if (or-map assigned-lexical? gensyms)
                       (cause &allocation)
                       &no-effects)
                   (accumulate-effects vals)
                   (compute-effects body)))
          (($ <let-values> _ producer consumer)
           (logior (compute-effects producer)
                   (compute-effects consumer)
                   (cause &type-check)))
          (($ <dynwind> _ winder body unwinder)
           (logior (compute-effects winder)
                   (compute-effects body)
                   (compute-effects unwinder)))
          (($ <dynlet> _ fluids vals body)
           (logior (accumulate-effects fluids)
                   (accumulate-effects vals)
                   (cause &type-check)
                   (cause &fluid)
                   (compute-effects body)))
          (($ <dynref> _ fluid)
           (logior (compute-effects fluid)
                   (cause &type-check)
                   &fluid))
          (($ <dynset> _ fluid exp)
           (logior (compute-effects fluid)
                   (compute-effects exp)
                   (cause &type-check)
                   (cause &fluid)))
          (($ <toplevel-ref>)
           (logior &toplevel
                   (cause &type-check)))
          (($ <module-ref>)
           (logior &toplevel
                   (cause &type-check)))
          (($ <module-set> _ mod name public? exp)
           (logior (cause &toplevel)
                   (cause &type-check)
                   (compute-effects exp)))
          (($ <toplevel-define> _ name exp)
           (logior (cause &toplevel)
                   (compute-effects exp)))
          (($ <toplevel-set> _ name exp)
           (logior (cause &toplevel)
                   (compute-effects exp)))
          (($ <primitive-ref>)
           &no-effects)
          (($ <conditional> _ test consequent alternate)
           (let ((tfx (compute-effects test))
                 (cfx (compute-effects consequent))
                 (afx (compute-effects alternate)))
             (if (causes-effects? (logior tfx (logand afx cfx))
                                  &definite-bailout)
                 (logior tfx cfx afx)
                 (exclude-effects (logior tfx cfx afx)
                                  &definite-bailout))))

          ;; Zero values.
          (($ <application> _ ($ <primitive-ref> _ 'values) ())
           (cause &zero-values))

          ;; Effect-free primitives.
          (($ <application> _
              ($ <primitive-ref> _ (or 'values 'eq? 'eqv? 'equal?))
              args)
           (accumulate-effects args))

          (($ <application> _
              ($ <primitive-ref> _ (or 'not 'pair? 'null? 'list? 'symbol?
                                       'vector? 'struct? 'string? 'number?
                                       'char?))
              (arg))
           (compute-effects arg))

          ;; Primitives that allocate memory.
          (($ <application> _ ($ <primitive-ref> _ 'cons) (x y))
           (logior (compute-effects x) (compute-effects y)
                   (cause &allocation)))

          (($ <application> _ ($ <primitive-ref> _ (or 'list 'vector)) args)
           (logior (accumulate-effects args) (cause &allocation)))

          (($ <application> _ ($ <primitive-ref> _ 'make-prompt-tag) ())
           (cause &allocation))

          (($ <application> _ ($ <primitive-ref> _ 'make-prompt-tag) (arg))
           (logior (compute-effects arg) (cause &allocation)))

          ;; Primitives that are normally effect-free, but which might
          ;; cause type checks, allocate memory, or access mutable
          ;; memory.  FIXME: expand, to be more precise.
          (($ <application> _
              ($ <primitive-ref> _ (and name
                                        (? effect-free-primitive?)))
              args)
           (logior (accumulate-effects args)
                   (cause &type-check)
                   (if (constructor-primitive? name)
                       (cause &allocation)
                       (if (accessor-primitive? name)
                           &mutable-data
                           &no-effects))))
      
          ;; Lambda applications might throw wrong-number-of-args.
          (($ <application> _ ($ <lambda> _ _ body) args)
           (logior (accumulate-effects args)
                   (match body
                     (($ <lambda-case> _ req #f #f #f () syms body #f)
                      (logior (compute-effects body)
                              (if (= (length req) (length args))
                                  0
                                  (cause &type-check))))
                     (($ <lambda-case>)
                      (logior (compute-effects body)
                              (cause &type-check)))
                     (#f
                      ;; Calling a case-lambda with no clauses
                      ;; definitely causes bailout.
                      (logior (cause &definite-bailout)
                              (cause &possible-bailout))))))
        
          ;; Bailout primitives.
          (($ <application> src ($ <primitive-ref> _ (? bailout-primitive? name))
              args)
           (logior (accumulate-effects args)
                   (cause &definite-bailout)
                   (cause &possible-bailout)))

          ;; A call to a lexically bound procedure, perhaps labels
          ;; allocated.
          (($ <application> _ (and proc ($ <lexical-ref> _ _ sym)) args)
           (cond
            ((lookup sym)
             => (lambda (proc)
                  (compute-effects (make-application #f proc args))))
            (else
             (logior &all-effects-but-bailout
                     (cause &all-effects-but-bailout)))))

          ;; A call to an unknown procedure can do anything.
          (($ <application> _ proc args)
           (logior &all-effects-but-bailout
                   (cause &all-effects-but-bailout)))

          (($ <lambda> _ meta body)
           &no-effects)
          (($ <lambda-case> _ req opt rest kw inits gensyms body alt)
           (logior (exclude-effects (accumulate-effects inits)
                                    &definite-bailout)
                   (if (or-map assigned-lexical? gensyms)
                       (cause &allocation)
                       &no-effects)
                   (compute-effects body)
                   (if alt (compute-effects alt) &no-effects)))

          (($ <sequence> _ exps)
           (let lp ((exps exps) (effects &no-effects))
             (match exps
               ((tail)
                (logior (compute-effects tail)
                        ;; Returning zero values to a for-effect continuation is
                        ;; not observable.
                        (exclude-effects effects (cause &zero-values))))
               ((head . tail)
                (lp tail (logior (compute-effects head) effects))))))

          (($ <prompt> _ tag body handler)
           (logior (compute-effects tag)
                   (compute-effects body)
                   (compute-effects handler)))

          (($ <abort> _ tag args tail)
           (logior &all-effects-but-bailout
                   (cause &all-effects-but-bailout)))))

      (compute-effects exp))

    compute-effects))