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1 /* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
2 * vim: set ts=8 sw=4 et tw=78:
3 *
4 * ***** BEGIN LICENSE BLOCK *****
5 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
6 *
7 * The contents of this file are subject to the Mozilla Public License Version
8 * 1.1 (the "License"); you may not use this file except in compliance with
9 * the License. You may obtain a copy of the License at
10 * http://www.mozilla.org/MPL/
11 *
12 * Software distributed under the License is distributed on an "AS IS" basis,
13 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
14 * for the specific language governing rights and limitations under the
15 * License.
16 *
17 * The Original Code is Mozilla Communicator client code, released
18 * March 31, 1998.
19 *
20 * The Initial Developer of the Original Code is
21 * Netscape Communications Corporation.
22 * Portions created by the Initial Developer are Copyright (C) 1998
23 * the Initial Developer. All Rights Reserved.
24 *
25 * Contributor(s):
26 *
27 * Alternatively, the contents of this file may be used under the terms of
28 * either of the GNU General Public License Version 2 or later (the "GPL"),
29 * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
30 * in which case the provisions of the GPL or the LGPL are applicable instead
31 * of those above. If you wish to allow use of your version of this file only
32 * under the terms of either the GPL or the LGPL, and not to allow others to
33 * use your version of this file under the terms of the MPL, indicate your
34 * decision by deleting the provisions above and replace them with the notice
35 * and other provisions required by the GPL or the LGPL. If you do not delete
36 * the provisions above, a recipient may use your version of this file under
37 * the terms of any one of the MPL, the GPL or the LGPL.
38 *
39 * ***** END LICENSE BLOCK ***** */
40
41 #ifndef jsscope_h___
42 #define jsscope_h___
43 /*
44 * JS symbol tables.
45 */
46 #include "jstypes.h"
47 #include "jslock.h"
48 #include "jsobj.h"
49 #include "jsprvtd.h"
50 #include "jspubtd.h"
51
52 JS_BEGIN_EXTERN_C
53
54 /*
55 * Given P independent, non-unique properties each of size S words mapped by
56 * all scopes in a runtime, construct a property tree of N nodes each of size
57 * S+L words (L for tree linkage). A nominal L value is 2 for leftmost-child
58 * and right-sibling links. We hope that the N < P by enough that the space
59 * overhead of L, and the overhead of scope entries pointing at property tree
60 * nodes, is worth it.
61 *
62 * The tree construction goes as follows. If any empty scope in the runtime
63 * has a property X added to it, find or create a node under the tree root
64 * labeled X, and set scope->lastProp to point at that node. If any non-empty
65 * scope whose most recently added property is labeled Y has another property
66 * labeled Z added, find or create a node for Z under the node that was added
67 * for Y, and set scope->lastProp to point at that node.
68 *
69 * A property is labeled by its members' values: id, getter, setter, slot,
70 * attributes, tiny or short id, and a field telling for..in order. Note that
71 * labels are not unique in the tree, but they are unique among a node's kids
72 * (barring rare and benign multi-threaded race condition outcomes, see below)
73 * and along any ancestor line from the tree root to a given leaf node (except
74 * for the hard case of duplicate formal parameters to a function).
75 *
76 * Thus the root of the tree represents all empty scopes, and the first ply
77 * of the tree represents all scopes containing one property, etc. Each node
78 * in the tree can stand for any number of scopes having the same ordered set
79 * of properties, where that node was the last added to the scope. (We need
80 * not store the root of the tree as a node, and do not -- all we need are
81 * links to its kids.)
82 *
83 * Sidebar on for..in loop order: ECMA requires no particular order, but this
84 * implementation has promised and delivered property definition order, and
85 * compatibility is king. We could use an order number per property, which
86 * would require a sort in js_Enumerate, and an entry order generation number
87 * per scope. An order number beats a list, which should be doubly-linked for
88 * O(1) delete. An even better scheme is to use a parent link in the property
89 * tree, so that the ancestor line can be iterated from scope->lastProp when
90 * filling in a JSIdArray from back to front. This parent link also helps the
91 * GC to sweep properties iteratively.
92 *
93 * What if a property Y is deleted from a scope? If Y is the last property in
94 * the scope, we simply adjust the scope's lastProp member after we remove the
95 * scope's hash-table entry pointing at that property node. The parent link
96 * mentioned in the for..in sidebar above makes this adjustment O(1). But if
97 * Y comes between X and Z in the scope, then we might have to "fork" the tree
98 * at X, leaving X->Y->Z in case other scopes have those properties added in
99 * that order; and to finish the fork, we'd add a node labeled Z with the path
100 * X->Z, if it doesn't exist. This could lead to lots of extra nodes, and to
101 * O(n^2) growth when deleting lots of properties.
102 *
103 * Rather, for O(1) growth all around, we should share the path X->Y->Z among
104 * scopes having those three properties added in that order, and among scopes
105 * having only X->Z where Y was deleted. All such scopes have a lastProp that
106 * points to the Z child of Y. But a scope in which Y was deleted does not
107 * have a table entry for Y, and when iterating that scope by traversing the
108 * ancestor line from Z, we will have to test for a table entry for each node,
109 * skipping nodes that lack entries.
110 *
111 * What if we add Y again? X->Y->Z->Y is wrong and we'll enumerate Y twice.
112 * Therefore we must fork in such a case, if not earlier. Because delete is
113 * "bursty", we should not fork eagerly. Delaying a fork till we are at risk
114 * of adding Y after it was deleted already requires a flag in the JSScope, to
115 * wit, SCOPE_MIDDLE_DELETE.
116 *
117 * What about thread safety? If the property tree operations done by requests
118 * are find-node and insert-node, then the only hazard is duplicate insertion.
119 * This is harmless except for minor bloat. When all requests have ended or
120 * been suspended, the GC is free to sweep the tree after marking all nodes
121 * reachable from scopes, performing remove-node operations as needed.
122 *
123 * Is the property tree worth it compared to property storage in each table's
124 * entries? To decide, we must find the relation <> between the words used
125 * with a property tree and the words required without a tree.
126 *
127 * Model all scopes as one super-scope of capacity T entries (T a power of 2).
128 * Let alpha be the load factor of this double hash-table. With the property
129 * tree, each entry in the table is a word-sized pointer to a node that can be
130 * shared by many scopes. But all such pointers are overhead compared to the
131 * situation without the property tree, where the table stores property nodes
132 * directly, as entries each of size S words. With the property tree, we need
133 * L=2 extra words per node for siblings and kids pointers. Without the tree,
134 * (1-alpha)*S*T words are wasted on free or removed sentinel-entries required
135 * by double hashing.
136 *
137 * Therefore,
138 *
139 * (property tree) <> (no property tree)
140 * N*(S+L) + T <> S*T
141 * N*(S+L) + T <> P*S + (1-alpha)*S*T
142 * N*(S+L) + alpha*T + (1-alpha)*T <> P*S + (1-alpha)*S*T
143 *
144 * Note that P is alpha*T by definition, so
145 *
146 * N*(S+L) + P + (1-alpha)*T <> P*S + (1-alpha)*S*T
147 * N*(S+L) <> P*S - P + (1-alpha)*S*T - (1-alpha)*T
148 * N*(S+L) <> (P + (1-alpha)*T) * (S-1)
149 * N*(S+L) <> (P + (1-alpha)*P/alpha) * (S-1)
150 * N*(S+L) <> P * (1/alpha) * (S-1)
151 *
152 * Let N = P*beta for a compression ratio beta, beta <= 1:
153 *
154 * P*beta*(S+L) <> P * (1/alpha) * (S-1)
155 * beta*(S+L) <> (S-1)/alpha
156 * beta <> (S-1)/((S+L)*alpha)
157 *
158 * For S = 6 (32-bit architectures) and L = 2, the property tree wins iff
159 *
160 * beta < 5/(8*alpha)
161 *
162 * We ensure that alpha <= .75, so the property tree wins if beta < .83_. An
163 * average beta from recent Mozilla browser startups was around .6.
164 *
165 * Can we reduce L? Observe that the property tree degenerates into a list of
166 * lists if at most one property Y follows X in all scopes. In or near such a
167 * case, we waste a word on the right-sibling link outside of the root ply of
168 * the tree. Note also that the root ply tends to be large, so O(n^2) growth
169 * searching it is likely, indicating the need for hashing (but with increased
170 * thread safety costs).
171 *
172 * If only K out of N nodes in the property tree have more than one child, we
173 * could eliminate the sibling link and overlay a children list or hash-table
174 * pointer on the leftmost-child link (which would then be either null or an
175 * only-child link; the overlay could be tagged in the low bit of the pointer,
176 * or flagged elsewhere in the property tree node, although such a flag must
177 * not be considered when comparing node labels during tree search).
178 *
179 * For such a system, L = 1 + (K * averageChildrenTableSize) / N instead of 2.
180 * If K << N, L approaches 1 and the property tree wins if beta < .95.
181 *
182 * We observe that fan-out below the root ply of the property tree appears to
183 * have extremely low degree (see the MeterPropertyTree code that histograms
184 * child-counts in jsscope.c), so instead of a hash-table we use a linked list
185 * of child node pointer arrays ("kid chunks"). The details are isolated in
186 * jsscope.c; others must treat JSScopeProperty.kids as opaque. We leave it
187 * strongly typed for debug-ability of the common (null or one-kid) cases.
188 *
189 * One final twist (can you stand it?): the mean number of entries per scope
190 * in Mozilla is < 5, with a large standard deviation (~8). Instead of always
191 * allocating scope->table, we leave it null while initializing all the other
192 * scope members as if it were non-null and minimal-length. Until a property
193 * is added that crosses the threshold of 6 or more entries for hashing, or
194 * until a "middle delete" occurs, we use linear search from scope->lastProp
195 * to find a given id, and save on the space overhead of a hash table.
196 */
197
198 struct JSScope {
199 JSObjectMap map; /* base class state */
200 #ifdef JS_THREADSAFE
201 JSTitle title; /* lock state */
202 #endif
203 JSObject *object; /* object that owns this scope */
204 uint32 shape; /* property cache shape identifier */
205 uint8 flags; /* flags, see below */
206 int8 hashShift; /* multiplicative hash shift */
207 uint16 spare; /* reserved */
208 uint32 entryCount; /* number of entries in table */
209 uint32 removedCount; /* removed entry sentinels in table */
210 JSScopeProperty **table; /* table of ptrs to shared tree nodes */
211 JSScopeProperty *lastProp; /* pointer to last property added */
212 };
213
214 #ifdef JS_THREADSAFE
215 JS_STATIC_ASSERT(offsetof(JSScope, title) == sizeof(JSObjectMap));
216 #endif
217
218 #define JS_IS_SCOPE_LOCKED(cx, scope) JS_IS_TITLE_LOCKED(cx, &(scope)->title)
219
220 #define OBJ_SCOPE(obj) ((JSScope *)(obj)->map)
221 #define OBJ_SHAPE(obj) (OBJ_SCOPE(obj)->shape)
222
223 #define SCOPE_MAKE_UNIQUE_SHAPE(cx,scope) \
224 ((scope)->shape = js_GenerateShape((cx), JS_FALSE, NULL))
225
226 #define SCOPE_EXTEND_SHAPE(cx,scope,sprop) \
227 JS_BEGIN_MACRO \
228 if (!(scope)->lastProp || \
229 (scope)->shape == (scope)->lastProp->shape) { \
230 (scope)->shape = (sprop)->shape; \
231 } else { \
232 (scope)->shape = js_GenerateShape((cx), JS_FALSE, sprop); \
233 } \
234 JS_END_MACRO
235
236 /* By definition, hashShift = JS_DHASH_BITS - log2(capacity). */
237 #define SCOPE_CAPACITY(scope) JS_BIT(JS_DHASH_BITS-(scope)->hashShift)
238
239 /* Scope flags and some macros to hide them from other files than jsscope.c. */
240 #define SCOPE_MIDDLE_DELETE 0x0001
241 #define SCOPE_SEALED 0x0002
242 #define SCOPE_BRANDED 0x0004
243
244 #define SCOPE_HAD_MIDDLE_DELETE(scope) ((scope)->flags & SCOPE_MIDDLE_DELETE)
245 #define SCOPE_SET_MIDDLE_DELETE(scope) ((scope)->flags |= SCOPE_MIDDLE_DELETE)
246 #define SCOPE_CLR_MIDDLE_DELETE(scope) ((scope)->flags &= ~SCOPE_MIDDLE_DELETE)
247
248 #define SCOPE_IS_SEALED(scope) ((scope)->flags & SCOPE_SEALED)
249 #define SCOPE_SET_SEALED(scope) ((scope)->flags |= SCOPE_SEALED)
250 #if 0
251 /*
252 * Don't define this, it can't be done safely because JS_LOCK_OBJ will avoid
253 * taking the lock if the object owns its scope and the scope is sealed.
254 */
255 #undef SCOPE_CLR_SEALED(scope) ((scope)->flags &= ~SCOPE_SEALED)
256 #endif
257
258 /*
259 * A branded scope's object contains plain old methods (function-valued
260 * properties without magic getters and setters), and its scope->shape
261 * evolves whenever a function value changes.
262 */
263 #define SCOPE_IS_BRANDED(scope) ((scope)->flags & SCOPE_BRANDED)
264 #define SCOPE_SET_BRANDED(scope) ((scope)->flags |= SCOPE_BRANDED)
265 #define SCOPE_CLR_BRANDED(scope) ((scope)->flags &= ~SCOPE_BRANDED)
266
267 /*
268 * A little information hiding for scope->lastProp, in case it ever becomes
269 * a tagged pointer again.
270 */
271 #define SCOPE_LAST_PROP(scope) ((scope)->lastProp)
272 #define SCOPE_REMOVE_LAST_PROP(scope) ((scope)->lastProp = \
273 (scope)->lastProp->parent)
274
275 struct JSScopeProperty {
276 jsid id; /* int-tagged jsval/untagged JSAtom* */
277 JSPropertyOp getter; /* getter and setter hooks or objects */
278 JSPropertyOp setter;
279 uint32 slot; /* abstract index in object slots */
280 uint8 attrs; /* attributes, see jsapi.h JSPROP_* */
281 uint8 flags; /* flags, see below for defines */
282 int16 shortid; /* tinyid, or local arg/var index */
283 JSScopeProperty *parent; /* parent node, reverse for..in order */
284 JSScopeProperty *kids; /* null, single child, or a tagged ptr
285 to many-kids data structure */
286 uint32 shape; /* property cache shape identifier */
287 };
288
289 /* JSScopeProperty pointer tag bit indicating a collision. */
290 #define SPROP_COLLISION ((jsuword)1)
291 #define SPROP_REMOVED ((JSScopeProperty *) SPROP_COLLISION)
292
293 /* Macros to get and set sprop pointer values and collision flags. */
294 #define SPROP_IS_FREE(sprop) ((sprop) == NULL)
295 #define SPROP_IS_REMOVED(sprop) ((sprop) == SPROP_REMOVED)
296 #define SPROP_IS_LIVE(sprop) ((sprop) > SPROP_REMOVED)
297 #define SPROP_FLAG_COLLISION(spp,sprop) (*(spp) = (JSScopeProperty *) \
298 ((jsuword)(sprop) | SPROP_COLLISION))
299 #define SPROP_HAD_COLLISION(sprop) ((jsuword)(sprop) & SPROP_COLLISION)
300 #define SPROP_FETCH(spp) SPROP_CLEAR_COLLISION(*(spp))
301
302 #define SPROP_CLEAR_COLLISION(sprop) \
303 ((JSScopeProperty *) ((jsuword)(sprop) & ~SPROP_COLLISION))
304
305 #define SPROP_STORE_PRESERVING_COLLISION(spp, sprop) \
306 (*(spp) = (JSScopeProperty *) ((jsuword)(sprop) \
307 | SPROP_HAD_COLLISION(*(spp))))
308
309 /* Bits stored in sprop->flags. */
310 #define SPROP_MARK 0x01
311 #define SPROP_IS_ALIAS 0x02
312 #define SPROP_HAS_SHORTID 0x04
313 #define SPROP_FLAG_SHAPE_REGEN 0x08
314
315 /*
316 * If SPROP_HAS_SHORTID is set in sprop->flags, we use sprop->shortid rather
317 * than id when calling sprop's getter or setter.
318 */
319 #define SPROP_USERID(sprop) \
320 (((sprop)->flags & SPROP_HAS_SHORTID) ? INT_TO_JSVAL((sprop)->shortid) \
321 : ID_TO_VALUE((sprop)->id))
322
323 #define SPROP_INVALID_SLOT 0xffffffff
324
325 #define SLOT_IN_SCOPE(slot,scope) ((slot) < (scope)->map.freeslot)
326 #define SPROP_HAS_VALID_SLOT(sprop,scope) SLOT_IN_SCOPE((sprop)->slot, scope)
327
328 #define SPROP_HAS_STUB_GETTER(sprop) (!(sprop)->getter)
329 #define SPROP_HAS_STUB_SETTER(sprop) (!(sprop)->setter)
330
331 /*
332 * NB: SPROP_GET must not be called if SPROP_HAS_STUB_GETTER(sprop).
333 */
334 #define SPROP_GET(cx,sprop,obj,obj2,vp) \
335 (((sprop)->attrs & JSPROP_GETTER) \
336 ? js_InternalGetOrSet(cx, obj, (sprop)->id, \
337 OBJECT_TO_JSVAL((sprop)->getter), JSACC_READ, \
338 0, 0, vp) \
339 : (sprop)->getter(cx, OBJ_THIS_OBJECT(cx,obj), SPROP_USERID(sprop), vp))
340
341 /*
342 * NB: SPROP_SET must not be called if (SPROP_HAS_STUB_SETTER(sprop) &&
343 * !(sprop->attrs & JSPROP_GETTER)).
344 */
345 #define SPROP_SET(cx,sprop,obj,obj2,vp) \
346 (((sprop)->attrs & JSPROP_SETTER) \
347 ? js_InternalGetOrSet(cx, obj, (sprop)->id, \
348 OBJECT_TO_JSVAL((sprop)->setter), JSACC_WRITE, \
349 1, vp, vp) \
350 : ((sprop)->attrs & JSPROP_GETTER) \
351 ? (JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, \
352 JSMSG_GETTER_ONLY, NULL), JS_FALSE) \
353 : (sprop)->setter(cx, OBJ_THIS_OBJECT(cx,obj), SPROP_USERID(sprop), vp))
354
355 /* Macro for common expression to test for shared permanent attributes. */
356 #define SPROP_IS_SHARED_PERMANENT(sprop) \
357 ((~(sprop)->attrs & (JSPROP_SHARED | JSPROP_PERMANENT)) == 0)
358
359 extern JSScope *
360 js_GetMutableScope(JSContext *cx, JSObject *obj);
361
362 extern JSScope *
363 js_NewScope(JSContext *cx, jsrefcount nrefs, JSObjectOps *ops, JSClass *clasp,
364 JSObject *obj);
365
366 extern void
367 js_DestroyScope(JSContext *cx, JSScope *scope);
368
369 extern JS_FRIEND_API(JSScopeProperty **)
370 js_SearchScope(JSScope *scope, jsid id, JSBool adding);
371
372 #define SCOPE_GET_PROPERTY(scope, id) \
373 SPROP_FETCH(js_SearchScope(scope, id, JS_FALSE))
374
375 #define SCOPE_HAS_PROPERTY(scope, sprop) \
376 (SCOPE_GET_PROPERTY(scope, (sprop)->id) == (sprop))
377
378 extern JSScopeProperty *
379 js_AddScopeProperty(JSContext *cx, JSScope *scope, jsid id,
380 JSPropertyOp getter, JSPropertyOp setter, uint32 slot,
381 uintN attrs, uintN flags, intN shortid);
382
383 extern JSScopeProperty *
384 js_ChangeScopePropertyAttrs(JSContext *cx, JSScope *scope,
385 JSScopeProperty *sprop, uintN attrs, uintN mask,
386 JSPropertyOp getter, JSPropertyOp setter);
387
388 extern JSBool
389 js_RemoveScopeProperty(JSContext *cx, JSScope *scope, jsid id);
390
391 extern void
392 js_ClearScope(JSContext *cx, JSScope *scope);
393
394 /*
395 * These macros used to inline short code sequences, but they grew over time.
396 * We retain them for internal backward compatibility, and in case one or both
397 * ever shrink to inline-able size.
398 */
399 #define TRACE_ID(trc, id) js_TraceId(trc, id)
400 #define TRACE_SCOPE_PROPERTY(trc, sprop) js_TraceScopeProperty(trc, sprop)
401
402 extern void
403 js_TraceId(JSTracer *trc, jsid id);
404
405 extern void
406 js_TraceScopeProperty(JSTracer *trc, JSScopeProperty *sprop);
407
408 extern void
409 js_SweepScopeProperties(JSContext *cx);
410
411 extern JSBool
412 js_InitPropertyTree(JSRuntime *rt);
413
414 extern void
415 js_FinishPropertyTree(JSRuntime *rt);
416
417 JS_END_EXTERN_C
418
419 #endif /* jsscope_h___ */

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