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 "jsobj.h" 48 #include "jsprvtd.h" 49 #include "jspubtd.h" 50 51 #ifdef JS_THREADSAFE 52 # include "jslock.h" 53 #endif 54 55 /* 56 * Given P independent, non-unique properties each of size S words mapped by 57 * all scopes in a runtime, construct a property tree of N nodes each of size 58 * S+L words (L for tree linkage). A nominal L value is 2 for leftmost-child 59 * and right-sibling links. We hope that the N < P by enough that the space 60 * overhead of L, and the overhead of scope entries pointing at property tree 61 * nodes, is worth it. 62 * 63 * The tree construction goes as follows. If any empty scope in the runtime 64 * has a property X added to it, find or create a node under the tree root 65 * labeled X, and set scope->lastProp to point at that node. If any non-empty 66 * scope whose most recently added property is labeled Y has another property 67 * labeled Z added, find or create a node for Z under the node that was added 68 * for Y, and set scope->lastProp to point at that node. 69 * 70 * A property is labeled by its members' values: id, getter, setter, slot, 71 * attributes, tiny or short id, and a field telling for..in order. Note that 72 * labels are not unique in the tree, but they are unique among a node's kids 73 * (barring rare and benign multi-threaded race condition outcomes, see below) 74 * and along any ancestor line from the tree root to a given leaf node (except 75 * for the hard case of duplicate formal parameters to a function). 76 * 77 * Thus the root of the tree represents all empty scopes, and the first ply 78 * of the tree represents all scopes containing one property, etc. Each node 79 * in the tree can stand for any number of scopes having the same ordered set 80 * of properties, where that node was the last added to the scope. (We need 81 * not store the root of the tree as a node, and do not -- all we need are 82 * links to its kids.) 83 * 84 * Sidebar on for..in loop order: ECMA requires no particular order, but this 85 * implementation has promised and delivered property definition order, and 86 * compatibility is king. We could use an order number per property, which 87 * would require a sort in js_Enumerate, and an entry order generation number 88 * per scope. An order number beats a list, which should be doubly-linked for 89 * O(1) delete. An even better scheme is to use a parent link in the property 90 * tree, so that the ancestor line can be iterated from scope->lastProp when 91 * filling in a JSIdArray from back to front. This parent link also helps the 92 * GC to sweep properties iteratively. 93 * 94 * What if a property Y is deleted from a scope? If Y is the last property in 95 * the scope, we simply adjust the scope's lastProp member after we remove the 96 * scope's hash-table entry pointing at that property node. The parent link 97 * mentioned in the for..in sidebar above makes this adjustment O(1). But if 98 * Y comes between X and Z in the scope, then we might have to "fork" the tree 99 * at X, leaving X->Y->Z in case other scopes have those properties added in 100 * that order; and to finish the fork, we'd add a node labeled Z with the path 101 * X->Z, if it doesn't exist. This could lead to lots of extra nodes, and to 102 * O(n^2) growth when deleting lots of properties. 103 * 104 * Rather, for O(1) growth all around, we should share the path X->Y->Z among 105 * scopes having those three properties added in that order, and among scopes 106 * having only X->Z where Y was deleted. All such scopes have a lastProp that 107 * points to the Z child of Y. But a scope in which Y was deleted does not 108 * have a table entry for Y, and when iterating that scope by traversing the 109 * ancestor line from Z, we will have to test for a table entry for each node, 110 * skipping nodes that lack entries. 111 * 112 * What if we add Y again? X->Y->Z->Y is wrong and we'll enumerate Y twice. 113 * Therefore we must fork in such a case, if not earlier. Because delete is 114 * "bursty", we should not fork eagerly. Delaying a fork till we are at risk 115 * of adding Y after it was deleted already requires a flag in the JSScope, to 116 * wit, SCOPE_MIDDLE_DELETE. 117 * 118 * What about thread safety? If the property tree operations done by requests 119 * are find-node and insert-node, then the only hazard is duplicate insertion. 120 * This is harmless except for minor bloat. When all requests have ended or 121 * been suspended, the GC is free to sweep the tree after marking all nodes 122 * reachable from scopes, performing remove-node operations as needed. Note 123 * also that the stable storage of the property nodes during active requests 124 * permits the property cache (see jsinterp.h) to dereference JSScopeProperty 125 * weak references safely. 126 * 127 * Is the property tree worth it compared to property storage in each table's 128 * entries? To decide, we must find the relation <> between the words used 129 * with a property tree and the words required without a tree. 130 * 131 * Model all scopes as one super-scope of capacity T entries (T a power of 2). 132 * Let alpha be the load factor of this double hash-table. With the property 133 * tree, each entry in the table is a word-sized pointer to a node that can be 134 * shared by many scopes. But all such pointers are overhead compared to the 135 * situation without the property tree, where the table stores property nodes 136 * directly, as entries each of size S words. With the property tree, we need 137 * L=2 extra words per node for siblings and kids pointers. Without the tree, 138 * (1-alpha)*S*T words are wasted on free or removed sentinel-entries required 139 * by double hashing. 140 * 141 * Therefore, 142 * 143 * (property tree) <> (no property tree) 144 * N*(S+L) + T <> S*T 145 * N*(S+L) + T <> P*S + (1-alpha)*S*T 146 * N*(S+L) + alpha*T + (1-alpha)*T <> P*S + (1-alpha)*S*T 147 * 148 * Note that P is alpha*T by definition, so 149 * 150 * N*(S+L) + P + (1-alpha)*T <> P*S + (1-alpha)*S*T 151 * N*(S+L) <> P*S - P + (1-alpha)*S*T - (1-alpha)*T 152 * N*(S+L) <> (P + (1-alpha)*T) * (S-1) 153 * N*(S+L) <> (P + (1-alpha)*P/alpha) * (S-1) 154 * N*(S+L) <> P * (1/alpha) * (S-1) 155 * 156 * Let N = P*beta for a compression ratio beta, beta <= 1: 157 * 158 * P*beta*(S+L) <> P * (1/alpha) * (S-1) 159 * beta*(S+L) <> (S-1)/alpha 160 * beta <> (S-1)/((S+L)*alpha) 161 * 162 * For S = 6 (32-bit architectures) and L = 2, the property tree wins iff 163 * 164 * beta < 5/(8*alpha) 165 * 166 * We ensure that alpha <= .75, so the property tree wins if beta < .83_. An 167 * average beta from recent Mozilla browser startups was around .6. 168 * 169 * Can we reduce L? Observe that the property tree degenerates into a list of 170 * lists if at most one property Y follows X in all scopes. In or near such a 171 * case, we waste a word on the right-sibling link outside of the root ply of 172 * the tree. Note also that the root ply tends to be large, so O(n^2) growth 173 * searching it is likely, indicating the need for hashing (but with increased 174 * thread safety costs). 175 * 176 * If only K out of N nodes in the property tree have more than one child, we 177 * could eliminate the sibling link and overlay a children list or hash-table 178 * pointer on the leftmost-child link (which would then be either null or an 179 * only-child link; the overlay could be tagged in the low bit of the pointer, 180 * or flagged elsewhere in the property tree node, although such a flag must 181 * not be considered when comparing node labels during tree search). 182 * 183 * For such a system, L = 1 + (K * averageChildrenTableSize) / N instead of 2. 184 * If K << N, L approaches 1 and the property tree wins if beta < .95. 185 * 186 * We observe that fan-out below the root ply of the property tree appears to 187 * have extremely low degree (see the MeterPropertyTree code that histograms 188 * child-counts in jsscope.c), so instead of a hash-table we use a linked list 189 * of child node pointer arrays ("kid chunks"). The details are isolated in 190 * jsscope.c; others must treat JSScopeProperty.kids as opaque. We leave it 191 * strongly typed for debug-ability of the common (null or one-kid) cases. 192 * 193 * One final twist (can you stand it?): the mean number of entries per scope 194 * in Mozilla is < 5, with a large standard deviation (~8). Instead of always 195 * allocating scope->table, we leave it null while initializing all the other 196 * scope members as if it were non-null and minimal-length. Until a property 197 * is added that crosses the threshold of 6 or more entries for hashing, or 198 * until a "middle delete" occurs, we use linear search from scope->lastProp 199 * to find a given id, and save on the space overhead of a hash table. 200 */ 201 202 struct JSScope { 203 JSObjectMap map; /* base class state */ 204 JSObject *object; /* object that owns this scope */ 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 #ifdef JS_THREADSAFE 213 JSContext *ownercx; /* creating context, NULL if shared */ 214 JSThinLock lock; /* binary semaphore protecting scope */ 215 union { /* union lockful and lock-free state: */ 216 jsrefcount count; /* lock entry count for reentrancy */ 217 JSScope *link; /* next link in rt->scopeSharingTodo */ 218 } u; 219 #ifdef DEBUG 220 const char *file[4]; /* file where lock was (re-)taken */ 221 unsigned int line[4]; /* line where lock was (re-)taken */ 222 #endif 223 #endif 224 }; 225 226 #define OBJ_SCOPE(obj) ((JSScope *)(obj)->map) 227 228 /* By definition, hashShift = JS_DHASH_BITS - log2(capacity). */ 229 #define SCOPE_CAPACITY(scope) JS_BIT(JS_DHASH_BITS-(scope)->hashShift) 230 231 /* Scope flags and some macros to hide them from other files than jsscope.c. */ 232 #define SCOPE_MIDDLE_DELETE 0x0001 233 #define SCOPE_SEALED 0x0002 234 235 #define SCOPE_HAD_MIDDLE_DELETE(scope) ((scope)->flags & SCOPE_MIDDLE_DELETE) 236 #define SCOPE_SET_MIDDLE_DELETE(scope) ((scope)->flags |= SCOPE_MIDDLE_DELETE) 237 #define SCOPE_CLR_MIDDLE_DELETE(scope) ((scope)->flags &= ~SCOPE_MIDDLE_DELETE) 238 239 #define SCOPE_IS_SEALED(scope) ((scope)->flags & SCOPE_SEALED) 240 #define SCOPE_SET_SEALED(scope) ((scope)->flags |= SCOPE_SEALED) 241 #if 0 242 /* 243 * Don't define this, it can't be done safely because JS_LOCK_OBJ will avoid 244 * taking the lock if the object owns its scope and the scope is sealed. 245 */ 246 #define SCOPE_CLR_SEALED(scope) ((scope)->flags &= ~SCOPE_SEALED) 247 #endif 248 249 /* 250 * A little information hiding for scope->lastProp, in case it ever becomes 251 * a tagged pointer again. 252 */ 253 #define SCOPE_LAST_PROP(scope) ((scope)->lastProp) 254 #define SCOPE_REMOVE_LAST_PROP(scope) ((scope)->lastProp = \ 255 (scope)->lastProp->parent) 256 257 struct JSScopeProperty { 258 jsid id; /* int-tagged jsval/untagged JSAtom* */ 259 JSPropertyOp getter; /* getter and setter hooks or objects */ 260 JSPropertyOp setter; 261 uint32 slot; /* index in obj->slots vector */ 262 uint8 attrs; /* attributes, see jsapi.h JSPROP_* */ 263 uint8 flags; /* flags, see below for defines */ 264 int16 shortid; /* tinyid, or local arg/var index */ 265 JSScopeProperty *parent; /* parent node, reverse for..in order */ 266 JSScopeProperty *kids; /* null, single child, or a tagged ptr 267 to many-kids data structure */ 268 }; 269 270 /* JSScopeProperty pointer tag bit indicating a collision. */ 271 #define SPROP_COLLISION ((jsuword)1) 272 #define SPROP_REMOVED ((JSScopeProperty *) SPROP_COLLISION) 273 274 /* Macros to get and set sprop pointer values and collision flags. */ 275 #define SPROP_IS_FREE(sprop) ((sprop) == NULL) 276 #define SPROP_IS_REMOVED(sprop) ((sprop) == SPROP_REMOVED) 277 #define SPROP_IS_LIVE(sprop) ((sprop) > SPROP_REMOVED) 278 #define SPROP_FLAG_COLLISION(spp,sprop) (*(spp) = (JSScopeProperty *) \ 279 ((jsuword)(sprop) | SPROP_COLLISION)) 280 #define SPROP_HAD_COLLISION(sprop) ((jsuword)(sprop) & SPROP_COLLISION) 281 #define SPROP_FETCH(spp) SPROP_CLEAR_COLLISION(*(spp)) 282 283 #define SPROP_CLEAR_COLLISION(sprop) \ 284 ((JSScopeProperty *) ((jsuword)(sprop) & ~SPROP_COLLISION)) 285 286 #define SPROP_STORE_PRESERVING_COLLISION(spp, sprop) \ 287 (*(spp) = (JSScopeProperty *) ((jsuword)(sprop) \ 288 | SPROP_HAD_COLLISION(*(spp)))) 289 290 /* Bits stored in sprop->flags. */ 291 #define SPROP_MARK 0x01 292 #define SPROP_IS_DUPLICATE 0x02 293 #define SPROP_IS_ALIAS 0x04 294 #define SPROP_HAS_SHORTID 0x08 295 #define SPROP_IS_HIDDEN 0x10 /* a normally-hidden property, 296 e.g., function arg or var */ 297 298 /* 299 * If SPROP_HAS_SHORTID is set in sprop->flags, we use sprop->shortid rather 300 * than id when calling sprop's getter or setter. 301 */ 302 #define SPROP_USERID(sprop) \ 303 (((sprop)->flags & SPROP_HAS_SHORTID) ? INT_TO_JSVAL((sprop)->shortid) \ 304 : ID_TO_VALUE((sprop)->id)) 305 306 #define SPROP_INVALID_SLOT 0xffffffff 307 308 #define SLOT_IN_SCOPE(slot,scope) ((slot) < (scope)->map.freeslot) 309 #define SPROP_HAS_VALID_SLOT(sprop,scope) SLOT_IN_SCOPE((sprop)->slot, scope) 310 311 #define SPROP_HAS_STUB_GETTER(sprop) (!(sprop)->getter) 312 #define SPROP_HAS_STUB_SETTER(sprop) (!(sprop)->setter) 313 314 /* 315 * NB: SPROP_GET must not be called if SPROP_HAS_STUB_GETTER(sprop). 316 */ 317 #define SPROP_GET(cx,sprop,obj,obj2,vp) \ 318 (((sprop)->attrs & JSPROP_GETTER) \ 319 ? js_InternalGetOrSet(cx, obj, (sprop)->id, \ 320 OBJECT_TO_JSVAL((sprop)->getter), JSACC_READ, \ 321 0, 0, vp) \ 322 : (sprop)->getter(cx, OBJ_THIS_OBJECT(cx,obj), SPROP_USERID(sprop), vp)) 323 324 /* 325 * NB: SPROP_SET must not be called if (SPROP_HAS_STUB_SETTER(sprop) && 326 * !(sprop->attrs & JSPROP_GETTER)). 327 */ 328 #define SPROP_SET(cx,sprop,obj,obj2,vp) \ 329 (((sprop)->attrs & JSPROP_SETTER) \ 330 ? js_InternalGetOrSet(cx, obj, (sprop)->id, \ 331 OBJECT_TO_JSVAL((sprop)->setter), JSACC_WRITE, \ 332 1, vp, vp) \ 333 : ((sprop)->attrs & JSPROP_GETTER) \ 334 ? (JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, \ 335 JSMSG_GETTER_ONLY, NULL), JS_FALSE) \ 336 : (sprop)->setter(cx, OBJ_THIS_OBJECT(cx,obj), SPROP_USERID(sprop), vp)) 337 338 /* Macro for common expression to test for shared permanent attributes. */ 339 #define SPROP_IS_SHARED_PERMANENT(sprop) \ 340 ((~(sprop)->attrs & (JSPROP_SHARED | JSPROP_PERMANENT)) == 0) 341 342 extern JSScope * 343 js_GetMutableScope(JSContext *cx, JSObject *obj); 344 345 extern JSScope * 346 js_NewScope(JSContext *cx, jsrefcount nrefs, JSObjectOps *ops, JSClass *clasp, 347 JSObject *obj); 348 349 extern void 350 js_DestroyScope(JSContext *cx, JSScope *scope); 351 352 #define ID_TO_VALUE(id) (JSID_IS_ATOM(id) ? ATOM_JSID_TO_JSVAL(id) : \ 353 JSID_IS_OBJECT(id) ? OBJECT_JSID_TO_JSVAL(id) : \ 354 (jsval)(id)) 355 #define HASH_ID(id) (JSID_IS_ATOM(id) ? JSID_TO_ATOM(id)->number : \ 356 JSID_IS_OBJECT(id) ? (jsatomid) JSID_CLRTAG(id) : \ 357 (jsatomid) JSID_TO_INT(id)) 358 359 extern JS_FRIEND_API(JSScopeProperty **) 360 js_SearchScope(JSScope *scope, jsid id, JSBool adding); 361 362 #define SCOPE_GET_PROPERTY(scope, id) \ 363 SPROP_FETCH(js_SearchScope(scope, id, JS_FALSE)) 364 365 #define SCOPE_HAS_PROPERTY(scope, sprop) \ 366 (SCOPE_GET_PROPERTY(scope, (sprop)->id) == (sprop)) 367 368 extern JSScopeProperty * 369 js_AddScopeProperty(JSContext *cx, JSScope *scope, jsid id, 370 JSPropertyOp getter, JSPropertyOp setter, uint32 slot, 371 uintN attrs, uintN flags, intN shortid); 372 373 extern JSScopeProperty * 374 js_ChangeScopePropertyAttrs(JSContext *cx, JSScope *scope, 375 JSScopeProperty *sprop, uintN attrs, uintN mask, 376 JSPropertyOp getter, JSPropertyOp setter); 377 378 extern JSBool 379 js_RemoveScopeProperty(JSContext *cx, JSScope *scope, jsid id); 380 381 extern void 382 js_ClearScope(JSContext *cx, JSScope *scope); 383 384 /* 385 * These macros used to inline short code sequences, but they grew over time. 386 * We retain them for internal backward compatibility, and in case one or both 387 * ever shrink to inline-able size. 388 */ 389 #define MARK_ID(cx,id) js_MarkId(cx, id) 390 #define MARK_SCOPE_PROPERTY(cx,sprop) js_MarkScopeProperty(cx, sprop) 391 392 extern void 393 js_MarkId(JSContext *cx, jsid id); 394 395 extern void 396 js_MarkScopeProperty(JSContext *cx, JSScopeProperty *sprop); 397 398 extern void 399 js_SweepScopeProperties(JSRuntime *rt); 400 401 extern JSBool 402 js_InitPropertyTree(JSRuntime *rt); 403 404 extern void 405 js_FinishPropertyTree(JSRuntime *rt); 406 407 #endif /* jsscope_h___ */ 408