/* -*- Mode: C; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*- * vim: set ts=8 sw=4 et tw=78: * * ***** BEGIN LICENSE BLOCK ***** * Version: MPL 1.1/GPL 2.0/LGPL 2.1 * * The contents of this file are subject to the Mozilla Public License Version * 1.1 (the "License"); you may not use this file except in compliance with * the License. You may obtain a copy of the License at * http://www.mozilla.org/MPL/ * * Software distributed under the License is distributed on an "AS IS" basis, * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License * for the specific language governing rights and limitations under the * License. * * The Original Code is Mozilla Communicator client code, released * March 31, 1998. * * The Initial Developer of the Original Code is * Netscape Communications Corporation. * Portions created by the Initial Developer are Copyright (C) 1998 * the Initial Developer. All Rights Reserved. * * Contributor(s): * * Alternatively, the contents of this file may be used under the terms of * either of the GNU General Public License Version 2 or later (the "GPL"), * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"), * in which case the provisions of the GPL or the LGPL are applicable instead * of those above. If you wish to allow use of your version of this file only * under the terms of either the GPL or the LGPL, and not to allow others to * use your version of this file under the terms of the MPL, indicate your * decision by deleting the provisions above and replace them with the notice * and other provisions required by the GPL or the LGPL. If you do not delete * the provisions above, a recipient may use your version of this file under * the terms of any one of the MPL, the GPL or the LGPL. * * ***** END LICENSE BLOCK ***** */ #ifndef jsscope_h___ #define jsscope_h___ /* * JS symbol tables. */ #include "jstypes.h" #include "jslock.h" #include "jsobj.h" #include "jsprvtd.h" #include "jspubtd.h" JS_BEGIN_EXTERN_C /* * Given P independent, non-unique properties each of size S words mapped by * all scopes in a runtime, construct a property tree of N nodes each of size * S+L words (L for tree linkage). A nominal L value is 2 for leftmost-child * and right-sibling links. We hope that the N < P by enough that the space * overhead of L, and the overhead of scope entries pointing at property tree * nodes, is worth it. * * The tree construction goes as follows. If any empty scope in the runtime * has a property X added to it, find or create a node under the tree root * labeled X, and set scope->lastProp to point at that node. If any non-empty * scope whose most recently added property is labeled Y has another property * labeled Z added, find or create a node for Z under the node that was added * for Y, and set scope->lastProp to point at that node. * * A property is labeled by its members' values: id, getter, setter, slot, * attributes, tiny or short id, and a field telling for..in order. Note that * labels are not unique in the tree, but they are unique among a node's kids * (barring rare and benign multi-threaded race condition outcomes, see below) * and along any ancestor line from the tree root to a given leaf node (except * for the hard case of duplicate formal parameters to a function). * * Thus the root of the tree represents all empty scopes, and the first ply * of the tree represents all scopes containing one property, etc. Each node * in the tree can stand for any number of scopes having the same ordered set * of properties, where that node was the last added to the scope. (We need * not store the root of the tree as a node, and do not -- all we need are * links to its kids.) * * Sidebar on for..in loop order: ECMA requires no particular order, but this * implementation has promised and delivered property definition order, and * compatibility is king. We could use an order number per property, which * would require a sort in js_Enumerate, and an entry order generation number * per scope. An order number beats a list, which should be doubly-linked for * O(1) delete. An even better scheme is to use a parent link in the property * tree, so that the ancestor line can be iterated from scope->lastProp when * filling in a JSIdArray from back to front. This parent link also helps the * GC to sweep properties iteratively. * * What if a property Y is deleted from a scope? If Y is the last property in * the scope, we simply adjust the scope's lastProp member after we remove the * scope's hash-table entry pointing at that property node. The parent link * mentioned in the for..in sidebar above makes this adjustment O(1). But if * Y comes between X and Z in the scope, then we might have to "fork" the tree * at X, leaving X->Y->Z in case other scopes have those properties added in * that order; and to finish the fork, we'd add a node labeled Z with the path * X->Z, if it doesn't exist. This could lead to lots of extra nodes, and to * O(n^2) growth when deleting lots of properties. * * Rather, for O(1) growth all around, we should share the path X->Y->Z among * scopes having those three properties added in that order, and among scopes * having only X->Z where Y was deleted. All such scopes have a lastProp that * points to the Z child of Y. But a scope in which Y was deleted does not * have a table entry for Y, and when iterating that scope by traversing the * ancestor line from Z, we will have to test for a table entry for each node, * skipping nodes that lack entries. * * What if we add Y again? X->Y->Z->Y is wrong and we'll enumerate Y twice. * Therefore we must fork in such a case, if not earlier. Because delete is * "bursty", we should not fork eagerly. Delaying a fork till we are at risk * of adding Y after it was deleted already requires a flag in the JSScope, to * wit, SCOPE_MIDDLE_DELETE. * * What about thread safety? If the property tree operations done by requests * are find-node and insert-node, then the only hazard is duplicate insertion. * This is harmless except for minor bloat. When all requests have ended or * been suspended, the GC is free to sweep the tree after marking all nodes * reachable from scopes, performing remove-node operations as needed. * * Is the property tree worth it compared to property storage in each table's * entries? To decide, we must find the relation <> between the words used * with a property tree and the words required without a tree. * * Model all scopes as one super-scope of capacity T entries (T a power of 2). * Let alpha be the load factor of this double hash-table. With the property * tree, each entry in the table is a word-sized pointer to a node that can be * shared by many scopes. But all such pointers are overhead compared to the * situation without the property tree, where the table stores property nodes * directly, as entries each of size S words. With the property tree, we need * L=2 extra words per node for siblings and kids pointers. Without the tree, * (1-alpha)*S*T words are wasted on free or removed sentinel-entries required * by double hashing. * * Therefore, * * (property tree) <> (no property tree) * N*(S+L) + T <> S*T * N*(S+L) + T <> P*S + (1-alpha)*S*T * N*(S+L) + alpha*T + (1-alpha)*T <> P*S + (1-alpha)*S*T * * Note that P is alpha*T by definition, so * * N*(S+L) + P + (1-alpha)*T <> P*S + (1-alpha)*S*T * N*(S+L) <> P*S - P + (1-alpha)*S*T - (1-alpha)*T * N*(S+L) <> (P + (1-alpha)*T) * (S-1) * N*(S+L) <> (P + (1-alpha)*P/alpha) * (S-1) * N*(S+L) <> P * (1/alpha) * (S-1) * * Let N = P*beta for a compression ratio beta, beta <= 1: * * P*beta*(S+L) <> P * (1/alpha) * (S-1) * beta*(S+L) <> (S-1)/alpha * beta <> (S-1)/((S+L)*alpha) * * For S = 6 (32-bit architectures) and L = 2, the property tree wins iff * * beta < 5/(8*alpha) * * We ensure that alpha <= .75, so the property tree wins if beta < .83_. An * average beta from recent Mozilla browser startups was around .6. * * Can we reduce L? Observe that the property tree degenerates into a list of * lists if at most one property Y follows X in all scopes. In or near such a * case, we waste a word on the right-sibling link outside of the root ply of * the tree. Note also that the root ply tends to be large, so O(n^2) growth * searching it is likely, indicating the need for hashing (but with increased * thread safety costs). * * If only K out of N nodes in the property tree have more than one child, we * could eliminate the sibling link and overlay a children list or hash-table * pointer on the leftmost-child link (which would then be either null or an * only-child link; the overlay could be tagged in the low bit of the pointer, * or flagged elsewhere in the property tree node, although such a flag must * not be considered when comparing node labels during tree search). * * For such a system, L = 1 + (K * averageChildrenTableSize) / N instead of 2. * If K << N, L approaches 1 and the property tree wins if beta < .95. * * We observe that fan-out below the root ply of the property tree appears to * have extremely low degree (see the MeterPropertyTree code that histograms * child-counts in jsscope.c), so instead of a hash-table we use a linked list * of child node pointer arrays ("kid chunks"). The details are isolated in * jsscope.c; others must treat JSScopeProperty.kids as opaque. We leave it * strongly typed for debug-ability of the common (null or one-kid) cases. * * One final twist (can you stand it?): the mean number of entries per scope * in Mozilla is < 5, with a large standard deviation (~8). Instead of always * allocating scope->table, we leave it null while initializing all the other * scope members as if it were non-null and minimal-length. Until a property * is added that crosses the threshold of 6 or more entries for hashing, or * until a "middle delete" occurs, we use linear search from scope->lastProp * to find a given id, and save on the space overhead of a hash table. */ struct JSScope { JSObjectMap map; /* base class state */ #ifdef JS_THREADSAFE JSTitle title; /* lock state */ #endif JSObject *object; /* object that owns this scope */ uint32 shape; /* property cache shape identifier */ uint8 flags; /* flags, see below */ int8 hashShift; /* multiplicative hash shift */ uint16 spare; /* reserved */ uint32 entryCount; /* number of entries in table */ uint32 removedCount; /* removed entry sentinels in table */ JSScopeProperty **table; /* table of ptrs to shared tree nodes */ JSScopeProperty *lastProp; /* pointer to last property added */ }; #define JS_IS_SCOPE_LOCKED(cx, scope) JS_IS_TITLE_LOCKED(cx, &(scope)->title) #define OBJ_SCOPE(obj) ((JSScope *)(obj)->map) #define OBJ_SHAPE(obj) (OBJ_SCOPE(obj)->shape) #define SCOPE_MAKE_UNIQUE_SHAPE(cx,scope) \ ((scope)->shape = js_GenerateShape((cx), JS_FALSE, NULL)) #define SCOPE_EXTEND_SHAPE(cx,scope,sprop) \ JS_BEGIN_MACRO \ if (!(scope)->lastProp || \ (scope)->shape == (scope)->lastProp->shape) { \ (scope)->shape = (sprop)->shape; \ } else { \ (scope)->shape = js_GenerateShape((cx), JS_FALSE, sprop); \ } \ JS_END_MACRO /* By definition, hashShift = JS_DHASH_BITS - log2(capacity). */ #define SCOPE_CAPACITY(scope) JS_BIT(JS_DHASH_BITS-(scope)->hashShift) /* Scope flags and some macros to hide them from other files than jsscope.c. */ #define SCOPE_MIDDLE_DELETE 0x0001 #define SCOPE_SEALED 0x0002 #define SCOPE_BRANDED 0x0004 #define SCOPE_INDEXED_PROPERTIES 0x0008 #define SCOPE_HAD_MIDDLE_DELETE(scope) ((scope)->flags & SCOPE_MIDDLE_DELETE) #define SCOPE_SET_MIDDLE_DELETE(scope) ((scope)->flags |= SCOPE_MIDDLE_DELETE) #define SCOPE_CLR_MIDDLE_DELETE(scope) ((scope)->flags &= ~SCOPE_MIDDLE_DELETE) #define SCOPE_HAS_INDEXED_PROPERTIES(scope) ((scope)->flags & SCOPE_INDEXED_PROPERTIES) #define SCOPE_SET_INDEXED_PROPERTIES(scope) ((scope)->flags |= SCOPE_INDEXED_PROPERTIES) #define SCOPE_IS_SEALED(scope) ((scope)->flags & SCOPE_SEALED) #define SCOPE_SET_SEALED(scope) ((scope)->flags |= SCOPE_SEALED) #if 0 /* * Don't define this, it can't be done safely because JS_LOCK_OBJ will avoid * taking the lock if the object owns its scope and the scope is sealed. */ #undef SCOPE_CLR_SEALED(scope) ((scope)->flags &= ~SCOPE_SEALED) #endif /* * A branded scope's object contains plain old methods (function-valued * properties without magic getters and setters), and its scope->shape * evolves whenever a function value changes. */ #define SCOPE_IS_BRANDED(scope) ((scope)->flags & SCOPE_BRANDED) #define SCOPE_SET_BRANDED(scope) ((scope)->flags |= SCOPE_BRANDED) #define SCOPE_CLR_BRANDED(scope) ((scope)->flags &= ~SCOPE_BRANDED) /* * A little information hiding for scope->lastProp, in case it ever becomes * a tagged pointer again. */ #define SCOPE_LAST_PROP(scope) ((scope)->lastProp) #define SCOPE_REMOVE_LAST_PROP(scope) ((scope)->lastProp = \ (scope)->lastProp->parent) /* * Helpers for reinterpreting JSPropertyOp as JSObject* for scripted getters * and setters. */ static inline JSObject * js_CastAsObject(JSPropertyOp op) { return JS_FUNC_TO_DATA_PTR(JSObject *, op); } static inline jsval js_CastAsObjectJSVal(JSPropertyOp op) { return OBJECT_TO_JSVAL(JS_FUNC_TO_DATA_PTR(JSObject *, op)); } static inline JSPropertyOp js_CastAsPropertyOp(JSObject *object) { return JS_DATA_TO_FUNC_PTR(JSPropertyOp, object); } struct JSScopeProperty { jsid id; /* int-tagged jsval/untagged JSAtom* */ JSPropertyOp getter; /* getter and setter hooks or objects */ JSPropertyOp setter; uint32 slot; /* abstract index in object slots */ uint8 attrs; /* attributes, see jsapi.h JSPROP_* */ uint8 flags; /* flags, see below for defines */ int16 shortid; /* tinyid, or local arg/var index */ JSScopeProperty *parent; /* parent node, reverse for..in order */ JSScopeProperty *kids; /* null, single child, or a tagged ptr to many-kids data structure */ uint32 shape; /* property cache shape identifier */ }; /* JSScopeProperty pointer tag bit indicating a collision. */ #define SPROP_COLLISION ((jsuword)1) #define SPROP_REMOVED ((JSScopeProperty *) SPROP_COLLISION) /* Macros to get and set sprop pointer values and collision flags. */ #define SPROP_IS_FREE(sprop) ((sprop) == NULL) #define SPROP_IS_REMOVED(sprop) ((sprop) == SPROP_REMOVED) #define SPROP_IS_LIVE(sprop) ((sprop) > SPROP_REMOVED) #define SPROP_FLAG_COLLISION(spp,sprop) (*(spp) = (JSScopeProperty *) \ ((jsuword)(sprop) | SPROP_COLLISION)) #define SPROP_HAD_COLLISION(sprop) ((jsuword)(sprop) & SPROP_COLLISION) #define SPROP_FETCH(spp) SPROP_CLEAR_COLLISION(*(spp)) #define SPROP_CLEAR_COLLISION(sprop) \ ((JSScopeProperty *) ((jsuword)(sprop) & ~SPROP_COLLISION)) #define SPROP_STORE_PRESERVING_COLLISION(spp, sprop) \ (*(spp) = (JSScopeProperty *) ((jsuword)(sprop) \ | SPROP_HAD_COLLISION(*(spp)))) /* Bits stored in sprop->flags. */ #define SPROP_MARK 0x01 #define SPROP_IS_ALIAS 0x02 #define SPROP_HAS_SHORTID 0x04 #define SPROP_FLAG_SHAPE_REGEN 0x08 /* * If SPROP_HAS_SHORTID is set in sprop->flags, we use sprop->shortid rather * than id when calling sprop's getter or setter. */ #define SPROP_USERID(sprop) \ (((sprop)->flags & SPROP_HAS_SHORTID) ? INT_TO_JSVAL((sprop)->shortid) \ : ID_TO_VALUE((sprop)->id)) #define SPROP_INVALID_SLOT 0xffffffff #define SLOT_IN_SCOPE(slot,scope) ((slot) < (scope)->map.freeslot) #define SPROP_HAS_VALID_SLOT(sprop,scope) SLOT_IN_SCOPE((sprop)->slot, scope) #define SPROP_HAS_STUB_GETTER(sprop) (!(sprop)->getter) #define SPROP_HAS_STUB_SETTER(sprop) (!(sprop)->setter) static JS_INLINE JSBool js_GetSprop(JSContext* cx, JSScopeProperty* sprop, JSObject* obj, jsval* vp) { JS_ASSERT(!SPROP_HAS_STUB_GETTER(sprop)); if (sprop->attrs & JSPROP_GETTER) { jsval fval = js_CastAsObjectJSVal(sprop->getter); return js_InternalGetOrSet(cx, obj, sprop->id, fval, JSACC_READ, 0, 0, vp); } return sprop->getter(cx, obj, SPROP_USERID(sprop), vp); } static JS_INLINE JSBool js_SetSprop(JSContext* cx, JSScopeProperty* sprop, JSObject* obj, jsval* vp) { JS_ASSERT(!(SPROP_HAS_STUB_SETTER(sprop) && !(sprop->attrs & JSPROP_GETTER))); if (sprop->attrs & JSPROP_SETTER) { jsval fval = js_CastAsObjectJSVal(sprop->setter); return js_InternalGetOrSet(cx, obj, (sprop)->id, fval, JSACC_WRITE, 1, vp, vp); } if (sprop->attrs & JSPROP_GETTER) { JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, JSMSG_GETTER_ONLY, NULL); return JS_FALSE; } return sprop->setter(cx, obj, SPROP_USERID(sprop), vp); } /* Macro for common expression to test for shared permanent attributes. */ #define SPROP_IS_SHARED_PERMANENT(sprop) \ ((~(sprop)->attrs & (JSPROP_SHARED | JSPROP_PERMANENT)) == 0) extern JSScope * js_GetMutableScope(JSContext *cx, JSObject *obj); extern JSScope * js_NewScope(JSContext *cx, jsrefcount nrefs, JSObjectOps *ops, JSClass *clasp, JSObject *obj); extern void js_DestroyScope(JSContext *cx, JSScope *scope); extern JS_FRIEND_API(JSScopeProperty **) js_SearchScope(JSScope *scope, jsid id, JSBool adding); #define SCOPE_GET_PROPERTY(scope, id) \ SPROP_FETCH(js_SearchScope(scope, id, JS_FALSE)) #define SCOPE_HAS_PROPERTY(scope, sprop) \ (SCOPE_GET_PROPERTY(scope, (sprop)->id) == (sprop)) /* * If cacheByPrevShape is not null, *cacheByPrevShape must be false on * entrance. On exit it will point to true if this call added the property * predictably and js_FillPropertyCache can optimize for that. */ extern JSScopeProperty * js_AddScopeProperty(JSContext *cx, JSScope *scope, jsid id, JSPropertyOp getter, JSPropertyOp setter, uint32 slot, uintN attrs, uintN flags, intN shortid, JSBool *cacheByPrevShape); extern JSScopeProperty * js_ChangeScopePropertyAttrs(JSContext *cx, JSScope *scope, JSScopeProperty *sprop, uintN attrs, uintN mask, JSPropertyOp getter, JSPropertyOp setter); extern JSBool js_RemoveScopeProperty(JSContext *cx, JSScope *scope, jsid id); extern void js_ClearScope(JSContext *cx, JSScope *scope); /* * These macros used to inline short code sequences, but they grew over time. * We retain them for internal backward compatibility, and in case one or both * ever shrink to inline-able size. */ #define TRACE_ID(trc, id) js_TraceId(trc, id) #define TRACE_SCOPE_PROPERTY(trc, sprop) js_TraceScopeProperty(trc, sprop) extern void js_TraceId(JSTracer *trc, jsid id); extern void js_TraceScopeProperty(JSTracer *trc, JSScopeProperty *sprop); extern void js_SweepScopeProperties(JSContext *cx); extern JSBool js_InitPropertyTree(JSRuntime *rt); extern void js_FinishPropertyTree(JSRuntime *rt); JS_END_EXTERN_C #endif /* jsscope_h___ */