webkitgtk-2.16.5HEADwebkitgtk-2.16.5master

1 files changed, 726 insertions, 595 deletions

diff --git a/Source/JavaScriptCore/runtime/Structure.cpp b/Source/JavaScriptCore/runtime/Structure.cpp
index 8781ab007..d730254c5 100644
--- a/Source/JavaScriptCore/runtime/Structure.cpp
+++ b/Source/JavaScriptCore/runtime/Structure.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2008, 2009, 2013 Apple Inc. All rights reserved.
+ * Copyright (C) 2008, 2009, 2013-2016 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -10,10 +10,10 @@
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
- * THIS SOFTWARE IS PROVIDED BY APPLE COMPUTER, INC. ``AS IS'' AND ANY
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
- * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
@@ -28,75 +28,99 @@
 
 #include "CodeBlock.h"
 #include "DumpContext.h"
+#include "JSCInlines.h"
 #include "JSObject.h"
-#include "JSPropertyNameIterator.h"
+#include "JSPropertyNameEnumerator.h"
 #include "Lookup.h"
+#include "PropertyMapHashTable.h"
 #include "PropertyNameArray.h"
 #include "StructureChain.h"
 #include "StructureRareDataInlines.h"
+#include "WeakGCMapInlines.h"
 #include <wtf/CommaPrinter.h>
-#include <wtf/RefCountedLeakCounter.h>
+#include <wtf/NeverDestroyed.h>
+#include <wtf/ProcessID.h>
 #include <wtf/RefPtr.h>
 #include <wtf/Threading.h>
 
 #define DUMP_STRUCTURE_ID_STATISTICS 0
 
-#ifndef NDEBUG
-#define DO_PROPERTYMAP_CONSTENCY_CHECK 0
-#else
-#define DO_PROPERTYMAP_CONSTENCY_CHECK 0
-#endif
-
 using namespace std;
 using namespace WTF;
 
-#if DUMP_PROPERTYMAP_STATS
-
-int numProbes;
-int numCollisions;
-int numRehashes;
-int numRemoves;
-
-#endif
-
 namespace JSC {
 
 #if DUMP_STRUCTURE_ID_STATISTICS
 static HashSet<Structure*>& liveStructureSet = *(new HashSet<Structure*>);
 #endif
 
-bool StructureTransitionTable::contains(StringImpl* rep, unsigned attributes) const
+class SingleSlotTransitionWeakOwner final : public WeakHandleOwner {
+    void finalize(Handle<Unknown>, void* context) override
+    {
+        StructureTransitionTable* table = reinterpret_cast<StructureTransitionTable*>(context);
+        ASSERT(table->isUsingSingleSlot());
+        WeakSet::deallocate(table->weakImpl());
+        table->m_data = StructureTransitionTable::UsingSingleSlotFlag;
+    }
+};
+
+static SingleSlotTransitionWeakOwner& singleSlotTransitionWeakOwner()
+{
+    static NeverDestroyed<SingleSlotTransitionWeakOwner> owner;
+    return owner;
+}
+
+inline Structure* StructureTransitionTable::singleTransition() const
+{
+    ASSERT(isUsingSingleSlot());
+    if (WeakImpl* impl = this->weakImpl()) {
+        if (impl->state() == WeakImpl::Live)
+            return jsCast<Structure*>(impl->jsValue().asCell());
+    }
+    return nullptr;
+}
+
+inline void StructureTransitionTable::setSingleTransition(Structure* structure)
+{
+    ASSERT(isUsingSingleSlot());
+    if (WeakImpl* impl = this->weakImpl())
+        WeakSet::deallocate(impl);
+    WeakImpl* impl = WeakSet::allocate(structure, &singleSlotTransitionWeakOwner(), this);
+    m_data = reinterpret_cast<intptr_t>(impl) | UsingSingleSlotFlag;
+}
+
+bool StructureTransitionTable::contains(UniquedStringImpl* rep, unsigned attributes) const
 {
     if (isUsingSingleSlot()) {
         Structure* transition = singleTransition();
-        return transition && transition->m_nameInPrevious == rep && transition->m_attributesInPrevious == attributes;
+        return transition && transition->m_nameInPrevious == rep && transition->attributesInPrevious() == attributes;
     }
     return map()->get(std::make_pair(rep, attributes));
 }
 
-inline Structure* StructureTransitionTable::get(StringImpl* rep, unsigned attributes) const
+Structure* StructureTransitionTable::get(UniquedStringImpl* rep, unsigned attributes) const
 {
     if (isUsingSingleSlot()) {
         Structure* transition = singleTransition();
-        return (transition && transition->m_nameInPrevious == rep && transition->m_attributesInPrevious == attributes) ? transition : 0;
+        return (transition && transition->m_nameInPrevious == rep && transition->attributesInPrevious() == attributes) ? transition : 0;
     }
     return map()->get(std::make_pair(rep, attributes));
 }
 
-inline void StructureTransitionTable::add(VM& vm, Structure* structure)
+void StructureTransitionTable::add(VM& vm, Structure* structure)
 {
     if (isUsingSingleSlot()) {
         Structure* existingTransition = singleTransition();
 
         // This handles the first transition being added.
         if (!existingTransition) {
-            setSingleTransition(vm, structure);
+            setSingleTransition(structure);
             return;
         }
 
         // This handles the second transition being added
         // (or the first transition being despecified!)
-        setMap(new TransitionMap());
+        setMap(new TransitionMap(vm));
         add(vm, existingTransition);
     }
 
@@ -105,7 +129,7 @@ inline void StructureTransitionTable::add(VM& vm, Structure* structure)
     // Newer versions of the STL have an std::make_pair function that takes rvalue references.
     // When either of the parameters are bitfields, the C++ compiler will try to bind them as lvalues, which is invalid. To work around this, use unary "+" to make the parameter an rvalue.
     // See https://bugs.webkit.org/show_bug.cgi?id=59261 for more details
-    map()->set(std::make_pair(structure->m_nameInPrevious.get(), +structure->m_attributesInPrevious), structure);
+    map()->set(std::make_pair(structure->m_nameInPrevious.get(), +structure->attributesInPrevious()), structure);
 }
 
 void Structure::dumpStatistics()
@@ -133,9 +157,9 @@ void Structure::dumpStatistics()
                 break;
         }
 
-        if (structure->propertyTable()) {
+        if (PropertyTable* table = structure->propertyTableOrNull()) {
             ++numberWithPropertyMaps;
-            totalPropertyMapsSize += structure->propertyTable()->sizeInMemory();
+            totalPropertyMapsSize += table->sizeInMemory();
         }
     }
 
@@ -155,33 +179,38 @@ void Structure::dumpStatistics()
 
 Structure::Structure(VM& vm, JSGlobalObject* globalObject, JSValue prototype, const TypeInfo& typeInfo, const ClassInfo* classInfo, IndexingType indexingType, unsigned inlineCapacity)
     : JSCell(vm, vm.structureStructure.get())
+    , m_blob(vm.heap.structureIDTable().allocateID(this), indexingType, typeInfo)
+    , m_outOfLineTypeFlags(typeInfo.outOfLineTypeFlags())
     , m_globalObject(vm, this, globalObject, WriteBarrier<JSGlobalObject>::MayBeNull)
     , m_prototype(vm, this, prototype)
     , m_classInfo(classInfo)
     , m_transitionWatchpointSet(IsWatched)
     , m_offset(invalidOffset)
-    , m_typeInfo(typeInfo)
-    , m_indexingType(indexingType)
     , m_inlineCapacity(inlineCapacity)
-    , m_dictionaryKind(NoneDictionaryKind)
-    , m_isPinnedPropertyTable(false)
-    , m_hasGetterSetterProperties(classInfo->hasStaticSetterOrReadonlyProperties(vm))
-    , m_hasReadOnlyOrGetterSetterPropertiesExcludingProto(classInfo->hasStaticSetterOrReadonlyProperties(vm))
-    , m_hasNonEnumerableProperties(false)
-    , m_attributesInPrevious(0)
-    , m_specificFunctionThrashCount(0)
-    , m_preventExtensions(false)
-    , m_didTransition(false)
-    , m_staticFunctionReified(false)
+    , m_bitField(0)
 {
+    setDictionaryKind(NoneDictionaryKind);
+    setIsPinnedPropertyTable(false);
+    setHasGetterSetterProperties(classInfo->hasStaticSetterOrReadonlyProperties());
+    setHasCustomGetterSetterProperties(false);
+    setHasReadOnlyOrGetterSetterPropertiesExcludingProto(classInfo->hasStaticSetterOrReadonlyProperties());
+    setIsQuickPropertyAccessAllowedForEnumeration(true);
+    setAttributesInPrevious(0);
+    setDidPreventExtensions(false);
+    setDidTransition(false);
+    setStaticPropertiesReified(false);
+    setTransitionWatchpointIsLikelyToBeFired(false);
+    setHasBeenDictionary(false);
+    setIsAddingPropertyForTransition(false);
+ 
     ASSERT(inlineCapacity <= JSFinalObject::maxInlineCapacity());
     ASSERT(static_cast<PropertyOffset>(inlineCapacity) < firstOutOfLineOffset);
-    ASSERT(!typeInfo.structureHasRareData());
-    ASSERT(hasReadOnlyOrGetterSetterPropertiesExcludingProto() || !m_classInfo->hasStaticSetterOrReadonlyProperties(vm));
-    ASSERT(hasGetterSetterProperties() || !m_classInfo->hasStaticSetterOrReadonlyProperties(vm));
+    ASSERT(!hasRareData());
+    ASSERT(hasReadOnlyOrGetterSetterPropertiesExcludingProto() || !m_classInfo->hasStaticSetterOrReadonlyProperties());
+    ASSERT(hasGetterSetterProperties() || !m_classInfo->hasStaticSetterOrReadonlyProperties());
 }
 
-const ClassInfo Structure::s_info = { "Structure", 0, 0, 0, CREATE_METHOD_TABLE(Structure) };
+const ClassInfo Structure::s_info = { "Structure", 0, 0, CREATE_METHOD_TABLE(Structure) };
 
 Structure::Structure(VM& vm)
     : JSCell(CreatingEarlyCell)
@@ -189,54 +218,76 @@ Structure::Structure(VM& vm)
     , m_classInfo(info())
     , m_transitionWatchpointSet(IsWatched)
     , m_offset(invalidOffset)
-    , m_typeInfo(CompoundType, OverridesVisitChildren)
-    , m_indexingType(0)
     , m_inlineCapacity(0)
-    , m_dictionaryKind(NoneDictionaryKind)
-    , m_isPinnedPropertyTable(false)
-    , m_hasGetterSetterProperties(m_classInfo->hasStaticSetterOrReadonlyProperties(vm))
-    , m_hasReadOnlyOrGetterSetterPropertiesExcludingProto(m_classInfo->hasStaticSetterOrReadonlyProperties(vm))
-    , m_hasNonEnumerableProperties(false)
-    , m_attributesInPrevious(0)
-    , m_specificFunctionThrashCount(0)
-    , m_preventExtensions(false)
-    , m_didTransition(false)
-    , m_staticFunctionReified(false)
-{
-    ASSERT(hasReadOnlyOrGetterSetterPropertiesExcludingProto() || !m_classInfo->hasStaticSetterOrReadonlyProperties(vm));
-    ASSERT(hasGetterSetterProperties() || !m_classInfo->hasStaticSetterOrReadonlyProperties(vm));
-}
-
-Structure::Structure(VM& vm, const Structure* previous)
+    , m_bitField(0)
+{
+    setDictionaryKind(NoneDictionaryKind);
+    setIsPinnedPropertyTable(false);
+    setHasGetterSetterProperties(m_classInfo->hasStaticSetterOrReadonlyProperties());
+    setHasCustomGetterSetterProperties(false);
+    setHasReadOnlyOrGetterSetterPropertiesExcludingProto(m_classInfo->hasStaticSetterOrReadonlyProperties());
+    setIsQuickPropertyAccessAllowedForEnumeration(true);
+    setAttributesInPrevious(0);
+    setDidPreventExtensions(false);
+    setDidTransition(false);
+    setStaticPropertiesReified(false);
+    setTransitionWatchpointIsLikelyToBeFired(false);
+    setHasBeenDictionary(false);
+    setIsAddingPropertyForTransition(false);
+ 
+    TypeInfo typeInfo = TypeInfo(CellType, StructureFlags);
+    m_blob = StructureIDBlob(vm.heap.structureIDTable().allocateID(this), 0, typeInfo);
+    m_outOfLineTypeFlags = typeInfo.outOfLineTypeFlags();
+
+    ASSERT(hasReadOnlyOrGetterSetterPropertiesExcludingProto() || !m_classInfo->hasStaticSetterOrReadonlyProperties());
+    ASSERT(hasGetterSetterProperties() || !m_classInfo->hasStaticSetterOrReadonlyProperties());
+}
+
+Structure::Structure(VM& vm, Structure* previous, DeferredStructureTransitionWatchpointFire* deferred)
     : JSCell(vm, vm.structureStructure.get())
     , m_prototype(vm, this, previous->storedPrototype())
     , m_classInfo(previous->m_classInfo)
     , m_transitionWatchpointSet(IsWatched)
     , m_offset(invalidOffset)
-    , m_typeInfo(previous->typeInfo().type(), previous->typeInfo().flags() & ~StructureHasRareData)
-    , m_indexingType(previous->indexingTypeIncludingHistory())
     , m_inlineCapacity(previous->m_inlineCapacity)
-    , m_dictionaryKind(previous->m_dictionaryKind)
-    , m_isPinnedPropertyTable(false)
-    , m_hasGetterSetterProperties(previous->m_hasGetterSetterProperties)
-    , m_hasReadOnlyOrGetterSetterPropertiesExcludingProto(previous->m_hasReadOnlyOrGetterSetterPropertiesExcludingProto)
-    , m_hasNonEnumerableProperties(previous->m_hasNonEnumerableProperties)
-    , m_attributesInPrevious(0)
-    , m_specificFunctionThrashCount(previous->m_specificFunctionThrashCount)
-    , m_preventExtensions(previous->m_preventExtensions)
-    , m_didTransition(true)
-    , m_staticFunctionReified(previous->m_staticFunctionReified)
-{
-    if (previous->typeInfo().structureHasRareData() && previous->rareData()->needsCloning())
-        cloneRareDataFrom(vm, previous);
-    else if (previous->previousID())
-        m_previousOrRareData.set(vm, this, previous->previousID());
-
-    previous->notifyTransitionFromThisStructure();
+    , m_bitField(0)
+{
+    setDictionaryKind(previous->dictionaryKind());
+    setIsPinnedPropertyTable(previous->hasBeenFlattenedBefore());
+    setHasGetterSetterProperties(previous->hasGetterSetterProperties());
+    setHasCustomGetterSetterProperties(previous->hasCustomGetterSetterProperties());
+    setHasReadOnlyOrGetterSetterPropertiesExcludingProto(previous->hasReadOnlyOrGetterSetterPropertiesExcludingProto());
+    setIsQuickPropertyAccessAllowedForEnumeration(previous->isQuickPropertyAccessAllowedForEnumeration());
+    setAttributesInPrevious(0);
+    setDidPreventExtensions(previous->didPreventExtensions());
+    setDidTransition(true);
+    setStaticPropertiesReified(previous->staticPropertiesReified());
+    setHasBeenDictionary(previous->hasBeenDictionary());
+    setIsAddingPropertyForTransition(false);
+ 
+    TypeInfo typeInfo = previous->typeInfo();
+    m_blob = StructureIDBlob(vm.heap.structureIDTable().allocateID(this), previous->indexingTypeIncludingHistory(), typeInfo);
+    m_outOfLineTypeFlags = typeInfo.outOfLineTypeFlags();
+
+    ASSERT(!previous->typeInfo().structureIsImmortal());
+    setPreviousID(vm, previous);
+
+    previous->didTransitionFromThisStructure(deferred);
+    
+    // Copy this bit now, in case previous was being watched.
+    setTransitionWatchpointIsLikelyToBeFired(previous->transitionWatchpointIsLikelyToBeFired());
+
     if (previous->m_globalObject)
         m_globalObject.set(vm, this, previous->m_globalObject.get());
-    ASSERT(hasReadOnlyOrGetterSetterPropertiesExcludingProto() || !m_classInfo->hasStaticSetterOrReadonlyProperties(vm));
-    ASSERT(hasGetterSetterProperties() || !m_classInfo->hasStaticSetterOrReadonlyProperties(vm));
+    ASSERT(hasReadOnlyOrGetterSetterPropertiesExcludingProto() || !m_classInfo->hasStaticSetterOrReadonlyProperties());
+    ASSERT(hasGetterSetterProperties() || !m_classInfo->hasStaticSetterOrReadonlyProperties());
+}
+
+Structure::~Structure()
+{
+    if (typeInfo().structureIsImmortal())
+        return;
+    Heap::heap(this)->structureIDTable().deallocateID(this, m_blob.structureID());
 }
 
 void Structure::destroy(JSCell* cell)
@@ -252,7 +303,7 @@ void Structure::findStructuresAndMapForMaterialization(Vector<Structure*, 8>& st
     for (structure = this; structure; structure = structure->previousID()) {
         structure->m_lock.lock();
         
-        table = structure->propertyTable().get();
+        table = structure->propertyTableOrNull();
         if (table) {
             // Leave the structure locked, so that the caller can do things to it atomically
             // before it loses its property table.
@@ -267,78 +318,66 @@ void Structure::findStructuresAndMapForMaterialization(Vector<Structure*, 8>& st
     ASSERT(!table);
 }
 
-void Structure::materializePropertyMap(VM& vm)
+PropertyTable* Structure::materializePropertyTable(VM& vm, bool setPropertyTable)
 {
     ASSERT(structure()->classInfo() == info());
-    ASSERT(!propertyTable());
-
+    ASSERT(!isAddingPropertyForTransition());
+    
+    DeferGC deferGC(vm.heap);
+    
     Vector<Structure*, 8> structures;
     Structure* structure;
     PropertyTable* table;
     
     findStructuresAndMapForMaterialization(structures, structure, table);
     
+    unsigned capacity = numberOfSlotsForLastOffset(m_offset, m_inlineCapacity);
     if (table) {
-        table = table->copy(vm, structure, numberOfSlotsForLastOffset(m_offset, m_inlineCapacity));
+        table = table->copy(vm, capacity);
         structure->m_lock.unlock();
-    }
+    } else
+        table = PropertyTable::create(vm, capacity);
     
     // Must hold the lock on this structure, since we will be modifying this structure's
     // property map. We don't want getConcurrently() to see the property map in a half-baked
     // state.
-    GCSafeConcurrentJITLocker locker(m_lock, vm.heap);
-    if (!table)
-        createPropertyMap(locker, vm, numberOfSlotsForLastOffset(m_offset, m_inlineCapacity));
-    else
-        propertyTable().set(vm, this, table);
+    GCSafeConcurrentJSLocker locker(m_lock, vm.heap);
+    if (setPropertyTable)
+        this->setPropertyTable(vm, table);
+
+    InferredTypeTable* typeTable = m_inferredTypeTable.get();
 
     for (size_t i = structures.size(); i--;) {
         structure = structures[i];
         if (!structure->m_nameInPrevious)
             continue;
-        PropertyMapEntry entry(vm, this, structure->m_nameInPrevious.get(), structure->m_offset, structure->m_attributesInPrevious, structure->m_specificValueInPrevious.get());
-        propertyTable()->add(entry, m_offset, PropertyTable::PropertyOffsetMustNotChange);
+        PropertyMapEntry entry(structure->m_nameInPrevious.get(), structure->m_offset, structure->attributesInPrevious());
+        if (typeTable && typeTable->get(structure->m_nameInPrevious.get()))
+            entry.hasInferredType = true;
+        table->add(entry, m_offset, PropertyTable::PropertyOffsetMustNotChange);
     }
     
-    checkOffsetConsistency();
-}
-
-inline size_t nextOutOfLineStorageCapacity(size_t currentCapacity)
-{
-    if (!currentCapacity)
-        return initialOutOfLineCapacity;
-    return currentCapacity * outOfLineGrowthFactor;
-}
-
-size_t Structure::suggestedNewOutOfLineStorageCapacity()
-{
-    return nextOutOfLineStorageCapacity(outOfLineCapacity());
-}
- 
-void Structure::despecifyDictionaryFunction(VM& vm, PropertyName propertyName)
-{
-    StringImpl* rep = propertyName.uid();
-
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessary(vm, deferGC);
-
-    ASSERT(isDictionary());
-    ASSERT(propertyTable());
-
-    PropertyMapEntry* entry = propertyTable()->find(rep).first;
-    ASSERT(entry);
-    entry->specificValue.clear();
+    checkOffsetConsistency(
+        table,
+        [&] () {
+            dataLog("Detected in materializePropertyTable.\n");
+            dataLog("Found structure = ", RawPointer(structure), "\n");
+            dataLog("structures = ");
+            CommaPrinter comma;
+            for (Structure* structure : structures)
+                dataLog(comma, RawPointer(structure));
+            dataLog("\n");
+        });
+    
+    return table;
 }
 
-Structure* Structure::addPropertyTransitionToExistingStructureImpl(Structure* structure, StringImpl* uid, unsigned attributes, JSCell* specificValue, PropertyOffset& offset)
+Structure* Structure::addPropertyTransitionToExistingStructureImpl(Structure* structure, UniquedStringImpl* uid, unsigned attributes, PropertyOffset& offset)
 {
     ASSERT(!structure->isDictionary());
     ASSERT(structure->isObject());
 
     if (Structure* existingTransition = structure->m_transitionTable.get(uid, attributes)) {
-        JSCell* specificValueInPrevious = existingTransition->m_specificValueInPrevious.get();
-        if (specificValueInPrevious && specificValueInPrevious != specificValue)
-            return 0;
         validateOffset(existingTransition->m_offset, existingTransition->inlineCapacity());
         offset = existingTransition->m_offset;
         return existingTransition;
@@ -347,16 +386,16 @@ Structure* Structure::addPropertyTransitionToExistingStructureImpl(Structure* st
     return 0;
 }
 
-Structure* Structure::addPropertyTransitionToExistingStructure(Structure* structure, PropertyName propertyName, unsigned attributes, JSCell* specificValue, PropertyOffset& offset)
+Structure* Structure::addPropertyTransitionToExistingStructure(Structure* structure, PropertyName propertyName, unsigned attributes, PropertyOffset& offset)
 {
     ASSERT(!isCompilationThread());
-    return addPropertyTransitionToExistingStructureImpl(structure, propertyName.uid(), attributes, specificValue, offset);
+    return addPropertyTransitionToExistingStructureImpl(structure, propertyName.uid(), attributes, offset);
 }
 
-Structure* Structure::addPropertyTransitionToExistingStructureConcurrently(Structure* structure, StringImpl* uid, unsigned attributes, JSCell* specificValue, PropertyOffset& offset)
+Structure* Structure::addPropertyTransitionToExistingStructureConcurrently(Structure* structure, UniquedStringImpl* uid, unsigned attributes, PropertyOffset& offset)
 {
-    ConcurrentJITLocker locker(structure->m_lock);
-    return addPropertyTransitionToExistingStructureImpl(structure, uid, attributes, specificValue, offset);
+    ConcurrentJSLocker locker(structure->m_lock);
+    return addPropertyTransitionToExistingStructureImpl(structure, uid, attributes, offset);
 }
 
 bool Structure::anyObjectInChainMayInterceptIndexedAccesses() const
@@ -373,6 +412,30 @@ bool Structure::anyObjectInChainMayInterceptIndexedAccesses() const
     }
 }
 
+bool Structure::holesMustForwardToPrototype(VM& vm) const
+{
+    if (this->mayInterceptIndexedAccesses())
+        return true;
+
+    JSValue prototype = this->storedPrototype();
+    if (!prototype.isObject())
+        return false;
+    JSObject* object = asObject(prototype);
+
+    while (true) {
+        Structure& structure = *object->structure(vm);
+        if (hasIndexedProperties(object->indexingType()) || structure.mayInterceptIndexedAccesses())
+            return true;
+        prototype = structure.storedPrototype();
+        if (!prototype.isObject())
+            return false;
+        object = asObject(prototype);
+    }
+
+    RELEASE_ASSERT_NOT_REACHED();
+    return false;
+}
+
 bool Structure::needsSlowPutIndexing() const
 {
     return anyObjectInChainMayInterceptIndexedAccesses()
@@ -382,57 +445,74 @@ bool Structure::needsSlowPutIndexing() const
 NonPropertyTransition Structure::suggestedArrayStorageTransition() const
 {
     if (needsSlowPutIndexing())
-        return AllocateSlowPutArrayStorage;
+        return NonPropertyTransition::AllocateSlowPutArrayStorage;
     
-    return AllocateArrayStorage;
+    return NonPropertyTransition::AllocateArrayStorage;
 }
 
-Structure* Structure::addPropertyTransition(VM& vm, Structure* structure, PropertyName propertyName, unsigned attributes, JSCell* specificValue, PropertyOffset& offset, PutPropertySlot::Context context)
+Structure* Structure::addPropertyTransition(VM& vm, Structure* structure, PropertyName propertyName, unsigned attributes, PropertyOffset& offset)
 {
-    // If we have a specific function, we may have got to this point if there is
-    // already a transition with the correct property name and attributes, but
-    // specialized to a different function.  In this case we just want to give up
-    // and despecialize the transition.
-    // In this case we clear the value of specificFunction which will result
-    // in us adding a non-specific transition, and any subsequent lookup in
-    // Structure::addPropertyTransitionToExistingStructure will just use that.
-    if (specificValue && structure->m_transitionTable.contains(propertyName.uid(), attributes))
-        specificValue = 0;
+    Structure* newStructure = addPropertyTransitionToExistingStructure(
+        structure, propertyName, attributes, offset);
+    if (newStructure)
+        return newStructure;
+
+    return addNewPropertyTransition(
+        vm, structure, propertyName, attributes, offset, PutPropertySlot::UnknownContext);
+}
 
+Structure* Structure::addNewPropertyTransition(VM& vm, Structure* structure, PropertyName propertyName, unsigned attributes, PropertyOffset& offset, PutPropertySlot::Context context, DeferredStructureTransitionWatchpointFire* deferred)
+{
     ASSERT(!structure->isDictionary());
     ASSERT(structure->isObject());
-    ASSERT(!Structure::addPropertyTransitionToExistingStructure(structure, propertyName, attributes, specificValue, offset));
+    ASSERT(!Structure::addPropertyTransitionToExistingStructure(structure, propertyName, attributes, offset));
     
-    if (structure->m_specificFunctionThrashCount == maxSpecificFunctionThrashCount)
-        specificValue = 0;
-
     int maxTransitionLength;
     if (context == PutPropertySlot::PutById)
         maxTransitionLength = s_maxTransitionLengthForNonEvalPutById;
     else
         maxTransitionLength = s_maxTransitionLength;
     if (structure->transitionCount() > maxTransitionLength) {
-        Structure* transition = toCacheableDictionaryTransition(vm, structure);
+        Structure* transition = toCacheableDictionaryTransition(vm, structure, deferred);
         ASSERT(structure != transition);
-        offset = transition->putSpecificValue(vm, propertyName, attributes, specificValue);
+        offset = transition->add(vm, propertyName, attributes);
         return transition;
     }
     
-    Structure* transition = create(vm, structure);
+    Structure* transition = create(vm, structure, deferred);
 
     transition->m_cachedPrototypeChain.setMayBeNull(vm, transition, structure->m_cachedPrototypeChain.get());
-    transition->setPreviousID(vm, transition, structure);
+    
+    // While we are adding the property, rematerializing the property table is super weird: we already
+    // have a m_nameInPrevious and attributesInPrevious but the m_offset is still wrong. If the
+    // materialization algorithm runs, it'll build a property table that already has the property but
+    // at a bogus offset. Rather than try to teach the materialization code how to create a table under
+    // those conditions, we just tell the GC not to blow the table away during this period of time.
+    // Holding the lock ensures that we either do this before the GC starts scanning the structure, in
+    // which case the GC will not blow the table away, or we do it after the GC already ran in which
+    // case all is well.  If it wasn't for the lock, the GC would have TOCTOU: if could read
+    // isAddingPropertyForTransition before we set it to true, and then blow the table away after.
+    {
+        ConcurrentJSLocker locker(transition->m_lock);
+        transition->setIsAddingPropertyForTransition(true);
+    }
+    
     transition->m_nameInPrevious = propertyName.uid();
-    transition->m_attributesInPrevious = attributes;
-    transition->m_specificValueInPrevious.setMayBeNull(vm, transition, specificValue);
-    transition->propertyTable().set(vm, transition, structure->takePropertyTableOrCloneIfPinned(vm, transition));
+    transition->setAttributesInPrevious(attributes);
+    transition->setPropertyTable(vm, structure->takePropertyTableOrCloneIfPinned(vm));
     transition->m_offset = structure->m_offset;
+    transition->m_inferredTypeTable.setMayBeNull(vm, transition, structure->m_inferredTypeTable.get());
+
+    offset = transition->add(vm, propertyName, attributes);
 
-    offset = transition->putSpecificValue(vm, propertyName, attributes, specificValue);
+    // Now that everything is fine with the new structure's bookkeeping, the GC is free to blow the
+    // table away if it wants. We can now rebuild it fine.
+    WTF::storeStoreFence();
+    transition->setIsAddingPropertyForTransition(false);
 
     checkOffset(transition->m_offset, transition->inlineCapacity());
     {
-        ConcurrentJITLocker locker(structure->m_lock);
+        ConcurrentJSLocker locker(structure->m_lock);
         structure->m_transitionTable.add(vm, transition);
     }
     transition->checkOffsetConsistency();
@@ -442,6 +522,24 @@ Structure* Structure::addPropertyTransition(VM& vm, Structure* structure, Proper
 
 Structure* Structure::removePropertyTransition(VM& vm, Structure* structure, PropertyName propertyName, PropertyOffset& offset)
 {
+    // NOTE: There are some good reasons why this goes directly to uncacheable dictionary rather than
+    // caching the removal. We can fix all of these things, but we must remember to do so, if we ever try
+    // to optimize this case.
+    //
+    // - Cached transitions usually steal the property table, and assume that this is possible because they
+    //   can just rebuild the table by looking at past transitions. That code assumes that the table only
+    //   grew and never shrank. To support removals, we'd have to change the property table materialization
+    //   code to handle deletions. Also, we have logic to get the list of properties on a structure that
+    //   lacks a property table by just looking back through the set of transitions since the last
+    //   structure that had a pinned table. That logic would also have to be changed to handle cached
+    //   removals.
+    //
+    // - InferredTypeTable assumes that removal has never happened. This is important since if we could
+    //   remove a property and then re-add it later, then the "absence means top" optimization wouldn't
+    //   work anymore, unless removal also either poisoned type inference (by doing something equivalent to
+    //   hasBeenDictionary) or by strongly marking the entry as Top by ensuring that it is not absent, but
+    //   instead, has a null entry.
+    
     ASSERT(!structure->isUncacheableDictionary());
 
     Structure* transition = toUncacheableDictionaryTransition(vm, structure);
@@ -454,60 +552,32 @@ Structure* Structure::removePropertyTransition(VM& vm, Structure* structure, Pro
 
 Structure* Structure::changePrototypeTransition(VM& vm, Structure* structure, JSValue prototype)
 {
-    Structure* transition = create(vm, structure);
-
-    transition->m_prototype.set(vm, transition, prototype);
-
     DeferGC deferGC(vm.heap);
-    structure->materializePropertyMapIfNecessary(vm, deferGC);
-    transition->propertyTable().set(vm, transition, structure->copyPropertyTableForPinning(vm, transition));
-    transition->m_offset = structure->m_offset;
-    transition->pin();
-
-    transition->checkOffsetConsistency();
-    return transition;
-}
-
-Structure* Structure::despecifyFunctionTransition(VM& vm, Structure* structure, PropertyName replaceFunction)
-{
-    ASSERT(structure->m_specificFunctionThrashCount < maxSpecificFunctionThrashCount);
     Structure* transition = create(vm, structure);
 
-    ++transition->m_specificFunctionThrashCount;
+    transition->m_prototype.set(vm, transition, prototype);
 
-    DeferGC deferGC(vm.heap);
-    structure->materializePropertyMapIfNecessary(vm, deferGC);
-    transition->propertyTable().set(vm, transition, structure->copyPropertyTableForPinning(vm, transition));
+    PropertyTable* table = structure->copyPropertyTableForPinning(vm);
+    transition->pin(holdLock(transition->m_lock), vm, table);
     transition->m_offset = structure->m_offset;
-    transition->pin();
-
-    if (transition->m_specificFunctionThrashCount == maxSpecificFunctionThrashCount)
-        transition->despecifyAllFunctions(vm);
-    else {
-        bool removed = transition->despecifyFunction(vm, replaceFunction);
-        ASSERT_UNUSED(removed, removed);
-    }
-
+    
     transition->checkOffsetConsistency();
     return transition;
 }
 
 Structure* Structure::attributeChangeTransition(VM& vm, Structure* structure, PropertyName propertyName, unsigned attributes)
 {
-    DeferGC deferGC(vm.heap);
     if (!structure->isUncacheableDictionary()) {
         Structure* transition = create(vm, structure);
 
-        structure->materializePropertyMapIfNecessary(vm, deferGC);
-        transition->propertyTable().set(vm, transition, structure->copyPropertyTableForPinning(vm, transition));
+        PropertyTable* table = structure->copyPropertyTableForPinning(vm);
+        transition->pin(holdLock(transition->m_lock), vm, table);
         transition->m_offset = structure->m_offset;
-        transition->pin();
         
         structure = transition;
     }
 
-    ASSERT(structure->propertyTable());
-    PropertyMapEntry* entry = structure->propertyTable()->find(propertyName.uid()).first;
+    PropertyMapEntry* entry = structure->ensurePropertyTable(vm)->get(propertyName.uid());
     ASSERT(entry);
     entry->attributes = attributes;
 
@@ -515,26 +585,26 @@ Structure* Structure::attributeChangeTransition(VM& vm, Structure* structure, Pr
     return structure;
 }
 
-Structure* Structure::toDictionaryTransition(VM& vm, Structure* structure, DictionaryKind kind)
+Structure* Structure::toDictionaryTransition(VM& vm, Structure* structure, DictionaryKind kind, DeferredStructureTransitionWatchpointFire* deferred)
 {
     ASSERT(!structure->isUncacheableDictionary());
+    DeferGC deferGC(vm.heap);
     
-    Structure* transition = create(vm, structure);
+    Structure* transition = create(vm, structure, deferred);
 
-    DeferGC deferGC(vm.heap);
-    structure->materializePropertyMapIfNecessary(vm, deferGC);
-    transition->propertyTable().set(vm, transition, structure->copyPropertyTableForPinning(vm, transition));
+    PropertyTable* table = structure->copyPropertyTableForPinning(vm);
+    transition->pin(holdLock(transition->m_lock), vm, table);
     transition->m_offset = structure->m_offset;
-    transition->m_dictionaryKind = kind;
-    transition->pin();
-
+    transition->setDictionaryKind(kind);
+    transition->setHasBeenDictionary(true);
+    
     transition->checkOffsetConsistency();
     return transition;
 }
 
-Structure* Structure::toCacheableDictionaryTransition(VM& vm, Structure* structure)
+Structure* Structure::toCacheableDictionaryTransition(VM& vm, Structure* structure, DeferredStructureTransitionWatchpointFire* deferred)
 {
-    return toDictionaryTransition(vm, structure, CachedDictionaryKind);
+    return toDictionaryTransition(vm, structure, CachedDictionaryKind, deferred);
 }
 
 Structure* Structure::toUncacheableDictionaryTransition(VM& vm, Structure* structure)
@@ -542,109 +612,105 @@ Structure* Structure::toUncacheableDictionaryTransition(VM& vm, Structure* struc
     return toDictionaryTransition(vm, structure, UncachedDictionaryKind);
 }
 
-// In future we may want to cache this transition.
 Structure* Structure::sealTransition(VM& vm, Structure* structure)
 {
-    Structure* transition = preventExtensionsTransition(vm, structure);
-
-    if (transition->propertyTable()) {
-        PropertyTable::iterator end = transition->propertyTable()->end();
-        for (PropertyTable::iterator iter = transition->propertyTable()->begin(); iter != end; ++iter)
-            iter->attributes |= DontDelete;
-    }
-
-    transition->checkOffsetConsistency();
-    return transition;
+    return nonPropertyTransition(vm, structure, NonPropertyTransition::Seal);
 }
 
-// In future we may want to cache this transition.
 Structure* Structure::freezeTransition(VM& vm, Structure* structure)
 {
-    Structure* transition = preventExtensionsTransition(vm, structure);
-
-    if (transition->propertyTable()) {
-        PropertyTable::iterator iter = transition->propertyTable()->begin();
-        PropertyTable::iterator end = transition->propertyTable()->end();
-        if (iter != end)
-            transition->m_hasReadOnlyOrGetterSetterPropertiesExcludingProto = true;
-        for (; iter != end; ++iter)
-            iter->attributes |= iter->attributes & Accessor ? DontDelete : (DontDelete | ReadOnly);
-    }
-
-    ASSERT(transition->hasReadOnlyOrGetterSetterPropertiesExcludingProto() || !transition->classInfo()->hasStaticSetterOrReadonlyProperties(vm));
-    ASSERT(transition->hasGetterSetterProperties() || !transition->classInfo()->hasStaticSetterOrReadonlyProperties(vm));
-    transition->checkOffsetConsistency();
-    return transition;
+    return nonPropertyTransition(vm, structure, NonPropertyTransition::Freeze);
 }
 
-// In future we may want to cache this transition.
 Structure* Structure::preventExtensionsTransition(VM& vm, Structure* structure)
 {
-    Structure* transition = create(vm, structure);
-
-    // Don't set m_offset, as one can not transition to this.
-
-    DeferGC deferGC(vm.heap);
-    structure->materializePropertyMapIfNecessary(vm, deferGC);
-    transition->propertyTable().set(vm, transition, structure->copyPropertyTableForPinning(vm, transition));
-    transition->m_offset = structure->m_offset;
-    transition->m_preventExtensions = true;
-    transition->pin();
-
-    transition->checkOffsetConsistency();
-    return transition;
+    return nonPropertyTransition(vm, structure, NonPropertyTransition::PreventExtensions);
 }
 
-PropertyTable* Structure::takePropertyTableOrCloneIfPinned(VM& vm, Structure* owner)
+PropertyTable* Structure::takePropertyTableOrCloneIfPinned(VM& vm)
 {
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessaryForPinning(vm, deferGC);
-    
-    if (m_isPinnedPropertyTable)
-        return propertyTable()->copy(vm, owner, propertyTable()->size() + 1);
-    
-    // Hold the lock while stealing the table - so that getConcurrently() on another thread
-    // will either have to bypass this structure, or will get to use the property table
-    // before it is stolen.
-    ConcurrentJITLocker locker(m_lock);
-    PropertyTable* takenPropertyTable = propertyTable().get();
-    propertyTable().clear();
-    return takenPropertyTable;
+    // This must always return a property table. It can't return null.
+    PropertyTable* result = propertyTableOrNull();
+    if (result) {
+        if (isPinnedPropertyTable())
+            return result->copy(vm, result->size() + 1);
+        ConcurrentJSLocker locker(m_lock);
+        setPropertyTable(vm, nullptr);
+        return result;
+    }
+    bool setPropertyTable = false;
+    return materializePropertyTable(vm, setPropertyTable);
 }
 
 Structure* Structure::nonPropertyTransition(VM& vm, Structure* structure, NonPropertyTransition transitionKind)
 {
     unsigned attributes = toAttributes(transitionKind);
-    IndexingType indexingType = newIndexingType(structure->indexingTypeIncludingHistory(), transitionKind);
+    IndexingType indexingTypeIncludingHistory = newIndexingType(structure->indexingTypeIncludingHistory(), transitionKind);
     
-    if (JSGlobalObject* globalObject = structure->m_globalObject.get()) {
-        if (globalObject->isOriginalArrayStructure(structure)) {
-            Structure* result = globalObject->originalArrayStructureForIndexingType(indexingType);
-            if (result->indexingTypeIncludingHistory() == indexingType) {
-                structure->notifyTransitionFromThisStructure();
-                return result;
+    if (changesIndexingType(transitionKind)) {
+        if (JSGlobalObject* globalObject = structure->m_globalObject.get()) {
+            if (globalObject->isOriginalArrayStructure(structure)) {
+                Structure* result = globalObject->originalArrayStructureForIndexingType(indexingTypeIncludingHistory);
+                if (result->indexingTypeIncludingHistory() == indexingTypeIncludingHistory) {
+                    structure->didTransitionFromThisStructure();
+                    return result;
+                }
             }
         }
     }
     
-    if (Structure* existingTransition = structure->m_transitionTable.get(0, attributes)) {
-        ASSERT(existingTransition->m_attributesInPrevious == attributes);
-        ASSERT(existingTransition->indexingTypeIncludingHistory() == indexingType);
+    Structure* existingTransition;
+    if (!structure->isDictionary() && (existingTransition = structure->m_transitionTable.get(0, attributes))) {
+        ASSERT(existingTransition->attributesInPrevious() == attributes);
+        ASSERT(existingTransition->indexingTypeIncludingHistory() == indexingTypeIncludingHistory);
         return existingTransition;
     }
     
+    DeferGC deferGC(vm.heap);
+    
     Structure* transition = create(vm, structure);
-    transition->setPreviousID(vm, transition, structure);
-    transition->m_attributesInPrevious = attributes;
-    transition->m_indexingType = indexingType;
-    transition->propertyTable().set(vm, transition, structure->takePropertyTableOrCloneIfPinned(vm, transition));
-    transition->m_offset = structure->m_offset;
-    checkOffset(transition->m_offset, transition->inlineCapacity());
+    transition->setAttributesInPrevious(attributes);
+    transition->m_blob.setIndexingTypeIncludingHistory(indexingTypeIncludingHistory);
     
-    {
-        ConcurrentJITLocker locker(structure->m_lock);
+    if (preventsExtensions(transitionKind))
+        transition->setDidPreventExtensions(true);
+    
+    if (setsDontDeleteOnAllProperties(transitionKind)
+        || setsReadOnlyOnNonAccessorProperties(transitionKind)) {
+        // We pin the property table on transitions that do wholesale editing of the property
+        // table, since our logic for walking the property transition chain to rematerialize the
+        // table doesn't know how to take into account such wholesale edits.
+
+        PropertyTable* table = structure->copyPropertyTableForPinning(vm);
+        transition->pinForCaching(holdLock(transition->m_lock), vm, table);
+        transition->m_offset = structure->m_offset;
+        
+        table = transition->propertyTableOrNull();
+        RELEASE_ASSERT(table);
+        for (auto& entry : *table) {
+            if (setsDontDeleteOnAllProperties(transitionKind))
+                entry.attributes |= DontDelete;
+            if (setsReadOnlyOnNonAccessorProperties(transitionKind) && !(entry.attributes & Accessor))
+                entry.attributes |= ReadOnly;
+        }
+    } else {
+        transition->setPropertyTable(vm, structure->takePropertyTableOrCloneIfPinned(vm));
+        transition->m_offset = structure->m_offset;
+        checkOffset(transition->m_offset, transition->inlineCapacity());
+    }
+    
+    if (setsReadOnlyOnNonAccessorProperties(transitionKind)
+        && !transition->propertyTableOrNull()->isEmpty())
+        transition->setHasReadOnlyOrGetterSetterPropertiesExcludingProto(true);
+    
+    if (structure->isDictionary()) {
+        PropertyTable* table = transition->ensurePropertyTable(vm);
+        transition->pin(holdLock(transition->m_lock), vm, table);
+    } else {
+        auto locker = holdLock(structure->m_lock);
         structure->m_transitionTable.add(vm, transition);
     }
+
     transition->checkOffsetConsistency();
     return transition;
 }
@@ -652,16 +718,15 @@ Structure* Structure::nonPropertyTransition(VM& vm, Structure* structure, NonPro
 // In future we may want to cache this property.
 bool Structure::isSealed(VM& vm)
 {
-    if (isExtensible())
+    if (isStructureExtensible())
         return false;
 
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessary(vm, deferGC);
-    if (!propertyTable())
+    PropertyTable* table = ensurePropertyTableIfNotEmpty(vm);
+    if (!table)
         return true;
-
-    PropertyTable::iterator end = propertyTable()->end();
-    for (PropertyTable::iterator iter = propertyTable()->begin(); iter != end; ++iter) {
+    
+    PropertyTable::iterator end = table->end();
+    for (PropertyTable::iterator iter = table->begin(); iter != end; ++iter) {
         if ((iter->attributes & DontDelete) != DontDelete)
             return false;
     }
@@ -671,16 +736,15 @@ bool Structure::isSealed(VM& vm)
 // In future we may want to cache this property.
 bool Structure::isFrozen(VM& vm)
 {
-    if (isExtensible())
+    if (isStructureExtensible())
         return false;
 
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessary(vm, deferGC);
-    if (!propertyTable())
+    PropertyTable* table = ensurePropertyTableIfNotEmpty(vm);
+    if (!table)
         return true;
-
-    PropertyTable::iterator end = propertyTable()->end();
-    for (PropertyTable::iterator iter = propertyTable()->begin(); iter != end; ++iter) {
+    
+    PropertyTable::iterator end = table->end();
+    for (PropertyTable::iterator iter = table->begin(); iter != end; ++iter) {
         if (!(iter->attributes & DontDelete))
             return false;
         if (!(iter->attributes & (ReadOnly | Accessor)))
@@ -693,19 +757,27 @@ Structure* Structure::flattenDictionaryStructure(VM& vm, JSObject* object)
 {
     checkOffsetConsistency();
     ASSERT(isDictionary());
+    
+    GCSafeConcurrentJSLocker locker(m_lock, vm.heap);
+    
+    object->setStructureIDDirectly(nuke(id()));
+    WTF::storeStoreFence();
+
+    size_t beforeOutOfLineCapacity = this->outOfLineCapacity();
     if (isUncacheableDictionary()) {
-        ASSERT(propertyTable());
+        PropertyTable* table = propertyTableOrNull();
+        ASSERT(table);
 
-        size_t propertyCount = propertyTable()->size();
+        size_t propertyCount = table->size();
 
         // Holds our values compacted by insertion order.
         Vector<JSValue> values(propertyCount);
 
         // Copies out our values from their hashed locations, compacting property table offsets as we go.
         unsigned i = 0;
-        PropertyTable::iterator end = propertyTable()->end();
+        PropertyTable::iterator end = table->end();
         m_offset = invalidOffset;
-        for (PropertyTable::iterator iter = propertyTable()->begin(); iter != end; ++iter, ++i) {
+        for (PropertyTable::iterator iter = table->begin(); iter != end; ++iter, ++i) {
             values[i] = object->getDirect(iter->offset);
             m_offset = iter->offset = offsetForPropertyNumber(i, m_inlineCapacity);
         }
@@ -714,267 +786,285 @@ Structure* Structure::flattenDictionaryStructure(VM& vm, JSObject* object)
         for (unsigned i = 0; i < propertyCount; i++)
             object->putDirect(vm, offsetForPropertyNumber(i, m_inlineCapacity), values[i]);
 
-        propertyTable()->clearDeletedOffsets();
+        table->clearDeletedOffsets();
         checkOffsetConsistency();
     }
 
-    m_dictionaryKind = NoneDictionaryKind;
+    setDictionaryKind(NoneDictionaryKind);
+    setHasBeenFlattenedBefore(true);
+
+    size_t afterOutOfLineCapacity = this->outOfLineCapacity();
+
+    if (object->butterfly() && beforeOutOfLineCapacity != afterOutOfLineCapacity) {
+        ASSERT(beforeOutOfLineCapacity > afterOutOfLineCapacity);
+        // If the object had a Butterfly but after flattening/compacting we no longer have need of it,
+        // we need to zero it out because the collector depends on the Structure to know the size for copying.
+        if (!afterOutOfLineCapacity && !this->hasIndexingHeader(object))
+            object->setButterfly(vm, nullptr);
+        // If the object was down-sized to the point where the base of the Butterfly is no longer within the 
+        // first CopiedBlock::blockSize bytes, we'll get the wrong answer if we try to mask the base back to 
+        // the CopiedBlock header. To prevent this case we need to memmove the Butterfly down.
+        else
+            object->shiftButterflyAfterFlattening(locker, vm, this, afterOutOfLineCapacity);
+    }
+    
+    WTF::storeStoreFence();
+    object->setStructureIDDirectly(id());
 
-    // If the object had a Butterfly but after flattening/compacting we no longer have need of it,
-    // we need to zero it out because the collector depends on the Structure to know the size for copying.
-    if (object->butterfly() && !this->outOfLineCapacity() && !this->hasIndexingHeader(object))
-        object->setStructureAndButterfly(vm, this, 0);
+    // FIXME: This is probably no longer needed since we have a stronger mechanism
+    // for detecting races and rescanning an object.
+    // https://bugs.webkit.org/show_bug.cgi?id=166989
+    vm.heap.writeBarrier(object);
 
     return this;
 }
 
-PropertyOffset Structure::addPropertyWithoutTransition(VM& vm, PropertyName propertyName, unsigned attributes, JSCell* specificValue)
+void Structure::pin(const AbstractLocker&, VM& vm, PropertyTable* table)
 {
-    ASSERT(!enumerationCache());
-
-    if (m_specificFunctionThrashCount == maxSpecificFunctionThrashCount)
-        specificValue = 0;
+    setIsPinnedPropertyTable(true);
+    setPropertyTable(vm, table);
+    clearPreviousID();
+    m_nameInPrevious = nullptr;
+}
 
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessaryForPinning(vm, deferGC);
-    
-    pin();
+void Structure::pinForCaching(const AbstractLocker&, VM& vm, PropertyTable* table)
+{
+    setIsPinnedPropertyTable(true);
+    setPropertyTable(vm, table);
+    m_nameInPrevious = nullptr;
+}
 
-    return putSpecificValue(vm, propertyName, attributes, specificValue);
+void Structure::allocateRareData(VM& vm)
+{
+    ASSERT(!hasRareData());
+    StructureRareData* rareData = StructureRareData::create(vm, previousID());
+    WTF::storeStoreFence();
+    m_previousOrRareData.set(vm, this, rareData);
+    ASSERT(hasRareData());
 }
 
-PropertyOffset Structure::removePropertyWithoutTransition(VM& vm, PropertyName propertyName)
+WatchpointSet* Structure::ensurePropertyReplacementWatchpointSet(VM& vm, PropertyOffset offset)
 {
-    ASSERT(isUncacheableDictionary());
-    ASSERT(!enumerationCache());
+    ASSERT(!isUncacheableDictionary());
 
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessaryForPinning(vm, deferGC);
+    // In some places it's convenient to call this with an invalid offset. So, we do the check here.
+    if (!isValidOffset(offset))
+        return nullptr;
+    
+    if (!hasRareData())
+        allocateRareData(vm);
+    ConcurrentJSLocker locker(m_lock);
+    StructureRareData* rareData = this->rareData();
+    if (!rareData->m_replacementWatchpointSets) {
+        rareData->m_replacementWatchpointSets =
+            std::make_unique<StructureRareData::PropertyWatchpointMap>();
+        WTF::storeStoreFence();
+    }
+    auto result = rareData->m_replacementWatchpointSets->add(offset, nullptr);
+    if (result.isNewEntry)
+        result.iterator->value = adoptRef(new WatchpointSet(IsWatched));
+    return result.iterator->value.get();
+}
 
-    pin();
-    return remove(propertyName);
+void Structure::startWatchingPropertyForReplacements(VM& vm, PropertyName propertyName)
+{
+    ASSERT(!isUncacheableDictionary());
+    
+    startWatchingPropertyForReplacements(vm, get(vm, propertyName));
 }
 
-void Structure::pin()
+void Structure::didCachePropertyReplacement(VM& vm, PropertyOffset offset)
 {
-    ASSERT(propertyTable());
-    m_isPinnedPropertyTable = true;
-    clearPreviousID();
-    m_nameInPrevious.clear();
+    ensurePropertyReplacementWatchpointSet(vm, offset)->fireAll(vm, "Did cache property replacement");
 }
 
-void Structure::allocateRareData(VM& vm)
+void Structure::startWatchingInternalProperties(VM& vm)
 {
-    ASSERT(!typeInfo().structureHasRareData());
-    StructureRareData* rareData = StructureRareData::create(vm, previous());
-    m_typeInfo = TypeInfo(typeInfo().type(), typeInfo().flags() | StructureHasRareData);
-    m_previousOrRareData.set(vm, this, rareData);
+    if (!isUncacheableDictionary()) {
+        startWatchingPropertyForReplacements(vm, vm.propertyNames->toString);
+        startWatchingPropertyForReplacements(vm, vm.propertyNames->valueOf);
+    }
+    setDidWatchInternalProperties(true);
 }
 
-void Structure::cloneRareDataFrom(VM& vm, const Structure* other)
+void Structure::willStoreValueSlow(
+    VM& vm, PropertyName propertyName, JSValue value, bool shouldOptimize,
+    InferredTypeTable::StoredPropertyAge age)
 {
-    ASSERT(other->typeInfo().structureHasRareData());
-    StructureRareData* newRareData = StructureRareData::clone(vm, other->rareData());
-    m_typeInfo = TypeInfo(typeInfo().type(), typeInfo().flags() | StructureHasRareData);
-    m_previousOrRareData.set(vm, this, newRareData);
+    ASSERT(!isCompilationThread());
+    ASSERT(structure()->classInfo() == info());
+    ASSERT(!hasBeenDictionary());
+
+    // Create the inferred type table before doing anything else, so that we don't GC after we have already
+    // grabbed a pointer into the property map.
+    InferredTypeTable* table = m_inferredTypeTable.get();
+    if (!table) {
+        table = InferredTypeTable::create(vm);
+        WTF::storeStoreFence();
+        m_inferredTypeTable.set(vm, this, table);
+    }
+
+    // This only works if we've got a property table.
+    PropertyTable* propertyTable = ensurePropertyTable(vm);
+    
+    // We must be calling this after having created the given property or confirmed that it was present
+    // already, so the property must be present.
+    PropertyMapEntry* entry = propertyTable->get(propertyName.uid());
+    ASSERT(entry);
+    
+    if (shouldOptimize)
+        entry->hasInferredType = table->willStoreValue(vm, propertyName, value, age);
+    else {
+        table->makeTop(vm, propertyName, age);
+        entry->hasInferredType = false;
+    }
+    
+    propertyTable->use(); // This makes it safe to use entry above.
 }
 
 #if DUMP_PROPERTYMAP_STATS
 
+PropertyMapHashTableStats* propertyMapHashTableStats = 0;
+
 struct PropertyMapStatisticsExitLogger {
+    PropertyMapStatisticsExitLogger();
     ~PropertyMapStatisticsExitLogger();
 };
 
-static PropertyMapStatisticsExitLogger logger;
+DEFINE_GLOBAL_FOR_LOGGING(PropertyMapStatisticsExitLogger, logger, );
 
-PropertyMapStatisticsExitLogger::~PropertyMapStatisticsExitLogger()
+PropertyMapStatisticsExitLogger::PropertyMapStatisticsExitLogger()
 {
-    dataLogF("\nJSC::PropertyMap statistics\n\n");
-    dataLogF("%d probes\n", numProbes);
-    dataLogF("%d collisions (%.1f%%)\n", numCollisions, 100.0 * numCollisions / numProbes);
-    dataLogF("%d rehashes\n", numRehashes);
-    dataLogF("%d removes\n", numRemoves);
+    propertyMapHashTableStats = adoptPtr(new PropertyMapHashTableStats()).leakPtr();
 }
 
-#endif
-
-#if !DO_PROPERTYMAP_CONSTENCY_CHECK
-
-inline void Structure::checkConsistency()
+PropertyMapStatisticsExitLogger::~PropertyMapStatisticsExitLogger()
 {
-    checkOffsetConsistency();
+    unsigned finds = propertyMapHashTableStats->numFinds;
+    unsigned collisions = propertyMapHashTableStats->numCollisions;
+    dataLogF("\nJSC::PropertyMap statistics for process %d\n\n", getCurrentProcessID());
+    dataLogF("%d finds\n", finds);
+    dataLogF("%d collisions (%.1f%%)\n", collisions, 100.0 * collisions / finds);
+    dataLogF("%d lookups\n", propertyMapHashTableStats->numLookups.load());
+    dataLogF("%d lookup probings\n", propertyMapHashTableStats->numLookupProbing.load());
+    dataLogF("%d adds\n", propertyMapHashTableStats->numAdds.load());
+    dataLogF("%d removes\n", propertyMapHashTableStats->numRemoves.load());
+    dataLogF("%d rehashes\n", propertyMapHashTableStats->numRehashes.load());
+    dataLogF("%d reinserts\n", propertyMapHashTableStats->numReinserts.load());
 }
 
 #endif
 
-PropertyTable* Structure::copyPropertyTable(VM& vm, Structure* owner)
+PropertyTable* Structure::copyPropertyTableForPinning(VM& vm)
 {
-    if (!propertyTable())
-        return 0;
-    return PropertyTable::clone(vm, owner, *propertyTable().get());
+    if (PropertyTable* table = propertyTableOrNull())
+        return PropertyTable::clone(vm, *table);
+    bool setPropertyTable = false;
+    return materializePropertyTable(vm, setPropertyTable);
 }
 
-PropertyTable* Structure::copyPropertyTableForPinning(VM& vm, Structure* owner)
+PropertyOffset Structure::getConcurrently(UniquedStringImpl* uid, unsigned& attributes)
 {
-    if (propertyTable())
-        return PropertyTable::clone(vm, owner, *propertyTable().get());
-    return PropertyTable::create(vm, numberOfSlotsForLastOffset(m_offset, m_inlineCapacity));
-}
-
-PropertyOffset Structure::getConcurrently(VM&, StringImpl* uid, unsigned& attributes, JSCell*& specificValue)
-{
-    Vector<Structure*, 8> structures;
-    Structure* structure;
-    PropertyTable* table;
-    
-    findStructuresAndMapForMaterialization(structures, structure, table);
+    PropertyOffset result = invalidOffset;
     
-    if (table) {
-        PropertyMapEntry* entry = table->find(uid).first;
-        if (entry) {
-            attributes = entry->attributes;
-            specificValue = entry->specificValue.get();
-            PropertyOffset result = entry->offset;
-            structure->m_lock.unlock();
-            return result;
-        }
-        structure->m_lock.unlock();
-    }
-    
-    for (unsigned i = structures.size(); i--;) {
-        structure = structures[i];
-        if (structure->m_nameInPrevious.get() != uid)
-            continue;
-        
-        attributes = structure->m_attributesInPrevious;
-        specificValue = structure->m_specificValueInPrevious.get();
-        return structure->m_offset;
-    }
+    forEachPropertyConcurrently(
+        [&] (const PropertyMapEntry& candidate) -> bool {
+            if (candidate.key != uid)
+                return true;
+            
+            result = candidate.offset;
+            attributes = candidate.attributes;
+            return false;
+        });
     
-    return invalidOffset;
+    return result;
 }
 
-PropertyOffset Structure::get(VM& vm, PropertyName propertyName, unsigned& attributes, JSCell*& specificValue)
+Vector<PropertyMapEntry> Structure::getPropertiesConcurrently()
 {
-    ASSERT(!isCompilationThread());
-    ASSERT(structure()->classInfo() == info());
+    Vector<PropertyMapEntry> result;
 
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessary(vm, deferGC);
-    if (!propertyTable())
-        return invalidOffset;
-
-    PropertyMapEntry* entry = propertyTable()->find(propertyName.uid()).first;
-    if (!entry)
-        return invalidOffset;
-
-    attributes = entry->attributes;
-    specificValue = entry->specificValue.get();
-    return entry->offset;
+    forEachPropertyConcurrently(
+        [&] (const PropertyMapEntry& entry) -> bool {
+            result.append(entry);
+            return true;
+        });
+    
+    return result;
 }
 
-bool Structure::despecifyFunction(VM& vm, PropertyName propertyName)
+PropertyOffset Structure::add(VM& vm, PropertyName propertyName, unsigned attributes)
 {
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessary(vm, deferGC);
-    if (!propertyTable())
-        return false;
-
-    PropertyMapEntry* entry = propertyTable()->find(propertyName.uid()).first;
-    if (!entry)
-        return false;
-
-    ASSERT(entry->specificValue);
-    entry->specificValue.clear();
-    return true;
+    return add<ShouldPin::No>(
+        vm, propertyName, attributes,
+        [this] (const GCSafeConcurrentJSLocker&, PropertyOffset, PropertyOffset newLastOffset) {
+            setLastOffset(newLastOffset);
+        });
 }
 
-void Structure::despecifyAllFunctions(VM& vm)
+PropertyOffset Structure::remove(PropertyName propertyName)
 {
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessary(vm, deferGC);
-    if (!propertyTable())
-        return;
-
-    PropertyTable::iterator end = propertyTable()->end();
-    for (PropertyTable::iterator iter = propertyTable()->begin(); iter != end; ++iter)
-        iter->specificValue.clear();
+    return remove(propertyName, [] (const ConcurrentJSLocker&, PropertyOffset) { });
 }
 
-PropertyOffset Structure::putSpecificValue(VM& vm, PropertyName propertyName, unsigned attributes, JSCell* specificValue)
+void Structure::getPropertyNamesFromStructure(VM& vm, PropertyNameArray& propertyNames, EnumerationMode mode)
 {
-    GCSafeConcurrentJITLocker locker(m_lock, vm.heap);
+    PropertyTable* table = ensurePropertyTableIfNotEmpty(vm);
+    if (!table)
+        return;
     
-    ASSERT(!JSC::isValidOffset(get(vm, propertyName)));
-
-    checkConsistency();
-    if (attributes & DontEnum)
-        m_hasNonEnumerableProperties = true;
-
-    StringImpl* rep = propertyName.uid();
-
-    if (!propertyTable())
-        createPropertyMap(locker, vm);
-
-    PropertyOffset newOffset = propertyTable()->nextOffset(m_inlineCapacity);
-
-    propertyTable()->add(PropertyMapEntry(vm, this, rep, newOffset, attributes, specificValue), m_offset, PropertyTable::PropertyOffsetMayChange);
+    bool knownUnique = propertyNames.canAddKnownUniqueForStructure();
     
-    checkConsistency();
-    return newOffset;
+    PropertyTable::iterator end = table->end();
+    for (PropertyTable::iterator iter = table->begin(); iter != end; ++iter) {
+        ASSERT(!isQuickPropertyAccessAllowedForEnumeration() || !(iter->attributes & DontEnum));
+        ASSERT(!isQuickPropertyAccessAllowedForEnumeration() || !iter->key->isSymbol());
+        if (!(iter->attributes & DontEnum) || mode.includeDontEnumProperties()) {
+            if (iter->key->isSymbol() && !propertyNames.includeSymbolProperties())
+                continue;
+            if (knownUnique)
+                propertyNames.addUnchecked(iter->key);
+            else
+                propertyNames.add(iter->key);
+        }
+    }
 }
 
-PropertyOffset Structure::remove(PropertyName propertyName)
+void StructureFireDetail::dump(PrintStream& out) const
 {
-    ConcurrentJITLocker locker(m_lock);
-    
-    checkConsistency();
-
-    StringImpl* rep = propertyName.uid();
-
-    if (!propertyTable())
-        return invalidOffset;
-
-    PropertyTable::find_iterator position = propertyTable()->find(rep);
-    if (!position.first)
-        return invalidOffset;
-
-    PropertyOffset offset = position.first->offset;
-
-    propertyTable()->remove(position);
-    propertyTable()->addDeletedOffset(offset);
-
-    checkConsistency();
-    return offset;
+    out.print("Structure transition from ", *m_structure);
 }
 
-void Structure::createPropertyMap(const GCSafeConcurrentJITLocker&, VM& vm, unsigned capacity)
+DeferredStructureTransitionWatchpointFire::DeferredStructureTransitionWatchpointFire()
+    : m_structure(nullptr)
 {
-    ASSERT(!propertyTable());
-
-    checkConsistency();
-    propertyTable().set(vm, this, PropertyTable::create(vm, capacity));
 }
 
-void Structure::getPropertyNamesFromStructure(VM& vm, PropertyNameArray& propertyNames, EnumerationMode mode)
+DeferredStructureTransitionWatchpointFire::~DeferredStructureTransitionWatchpointFire()
 {
-    DeferGC deferGC(vm.heap);
-    materializePropertyMapIfNecessary(vm, deferGC);
-    if (!propertyTable())
-        return;
+    if (m_structure)
+        m_structure->transitionWatchpointSet().fireAll(*m_structure->vm(), StructureFireDetail(m_structure));
+}
 
-    bool knownUnique = !propertyNames.size();
+void DeferredStructureTransitionWatchpointFire::add(const Structure* structure)
+{
+    RELEASE_ASSERT(!m_structure);
+    RELEASE_ASSERT(structure);
+    m_structure = structure;
+}
 
-    PropertyTable::iterator end = propertyTable()->end();
-    for (PropertyTable::iterator iter = propertyTable()->begin(); iter != end; ++iter) {
-        ASSERT(m_hasNonEnumerableProperties || !(iter->attributes & DontEnum));
-        if (iter->key->isIdentifier() && (!(iter->attributes & DontEnum) || mode == IncludeDontEnumProperties)) {
-            if (knownUnique)
-                propertyNames.addKnownUnique(iter->key);
-            else
-                propertyNames.add(iter->key);
-        }
-    }
+void Structure::didTransitionFromThisStructure(DeferredStructureTransitionWatchpointFire* deferred) const
+{
+    // If the structure is being watched, and this is the kind of structure that the DFG would
+    // like to watch, then make sure to note for all future versions of this structure that it's
+    // unwise to watch it.
+    if (m_transitionWatchpointSet.isBeingWatched())
+        const_cast<Structure*>(this)->setTransitionWatchpointIsLikelyToBeFired(true);
+    
+    if (deferred)
+        deferred->add(this);
+    else
+        m_transitionWatchpointSet.fireAll(*vm(), StructureFireDetail(this));
 }
 
 JSValue Structure::prototypeForLookup(CodeBlock* codeBlock) const
@@ -986,30 +1076,54 @@ void Structure::visitChildren(JSCell* cell, SlotVisitor& visitor)
 {
     Structure* thisObject = jsCast<Structure*>(cell);
     ASSERT_GC_OBJECT_INHERITS(thisObject, info());
-    ASSERT(thisObject->structure()->typeInfo().overridesVisitChildren());
 
     JSCell::visitChildren(thisObject, visitor);
-    visitor.append(&thisObject->m_globalObject);
+    
+    ConcurrentJSLocker locker(thisObject->m_lock);
+    
+    visitor.append(thisObject->m_globalObject);
     if (!thisObject->isObject())
         thisObject->m_cachedPrototypeChain.clear();
     else {
-        visitor.append(&thisObject->m_prototype);
-        visitor.append(&thisObject->m_cachedPrototypeChain);
+        visitor.append(thisObject->m_prototype);
+        visitor.append(thisObject->m_cachedPrototypeChain);
     }
-    visitor.append(&thisObject->m_previousOrRareData);
-    visitor.append(&thisObject->m_specificValueInPrevious);
-
-    if (thisObject->m_isPinnedPropertyTable) {
-        ASSERT(thisObject->m_propertyTableUnsafe);
-        visitor.append(&thisObject->m_propertyTableUnsafe);
-    } else if (thisObject->m_propertyTableUnsafe)
+    visitor.append(thisObject->m_previousOrRareData);
+
+    if (thisObject->isPinnedPropertyTable() || thisObject->isAddingPropertyForTransition()) {
+        // NOTE: This can interleave in pin(), in which case it may see a null property table.
+        // That's fine, because then the barrier will fire and we will scan this again.
+        visitor.append(thisObject->m_propertyTableUnsafe);
+    } else if (visitor.isBuildingHeapSnapshot())
+        visitor.append(thisObject->m_propertyTableUnsafe);
+    else if (thisObject->m_propertyTableUnsafe)
         thisObject->m_propertyTableUnsafe.clear();
+
+    visitor.append(thisObject->m_inferredTypeTable);
+}
+
+bool Structure::isCheapDuringGC()
+{
+    // FIXME: We could make this even safer by returning false if this structure's property table
+    // has any large property names.
+    // https://bugs.webkit.org/show_bug.cgi?id=157334
+    
+    return (!m_globalObject || Heap::isMarkedConcurrently(m_globalObject.get()))
+        && (!storedPrototypeObject() || Heap::isMarkedConcurrently(storedPrototypeObject()));
+}
+
+bool Structure::markIfCheap(SlotVisitor& visitor)
+{
+    if (!isCheapDuringGC())
+        return Heap::isMarkedConcurrently(this);
+    
+    visitor.appendUnbarriered(this);
+    return true;
 }
 
 bool Structure::prototypeChainMayInterceptStoreTo(VM& vm, PropertyName propertyName)
 {
-    unsigned i = propertyName.asIndex();
-    if (i != PropertyName::NotAnIndex)
+    if (parseIndex(propertyName))
         return anyObjectInChainMayInterceptIndexedAccesses();
     
     for (Structure* current = this; ;) {
@@ -1017,11 +1131,10 @@ bool Structure::prototypeChainMayInterceptStoreTo(VM& vm, PropertyName propertyN
         if (prototype.isNull())
             return false;
         
-        current = prototype.asCell()->structure();
+        current = prototype.asCell()->structure(vm);
         
         unsigned attributes;
-        JSCell* specificValue;
-        PropertyOffset offset = current->get(vm, propertyName, attributes, specificValue);
+        PropertyOffset offset = current->get(vm, propertyName, attributes);
         if (!JSC::isValidOffset(offset))
             continue;
         
@@ -1032,39 +1145,83 @@ bool Structure::prototypeChainMayInterceptStoreTo(VM& vm, PropertyName propertyN
     }
 }
 
+Ref<StructureShape> Structure::toStructureShape(JSValue value)
+{
+    Ref<StructureShape> baseShape = StructureShape::create();
+    RefPtr<StructureShape> curShape = baseShape.ptr();
+    Structure* curStructure = this;
+    JSValue curValue = value;
+    while (curStructure) {
+        curStructure->forEachPropertyConcurrently(
+            [&] (const PropertyMapEntry& entry) -> bool {
+                curShape->addProperty(*entry.key);
+                return true;
+            });
+
+        if (JSObject* curObject = curValue.getObject())
+            curShape->setConstructorName(JSObject::calculatedClassName(curObject));
+        else
+            curShape->setConstructorName(curStructure->classInfo()->className);
+
+        if (curStructure->isDictionary())
+            curShape->enterDictionaryMode();
+
+        curShape->markAsFinal();
+
+        if (curStructure->storedPrototypeStructure()) {
+            auto newShape = StructureShape::create();
+            curShape->setProto(newShape.copyRef());
+            curShape = WTFMove(newShape);
+            curValue = curStructure->storedPrototype();
+        }
+
+        curStructure = curStructure->storedPrototypeStructure();
+    }
+    
+    return baseShape;
+}
+
+bool Structure::canUseForAllocationsOf(Structure* other)
+{
+    return inlineCapacity() == other->inlineCapacity()
+        && storedPrototype() == other->storedPrototype()
+        && objectInitializationBlob() == other->objectInitializationBlob();
+}
+
 void Structure::dump(PrintStream& out) const
 {
     out.print(RawPointer(this), ":[", classInfo()->className, ", {");
     
-    Vector<Structure*, 8> structures;
-    Structure* structure;
-    PropertyTable* table;
-    
-    const_cast<Structure*>(this)->findStructuresAndMapForMaterialization(
-        structures, structure, table);
-    
     CommaPrinter comma;
     
-    if (table) {
-        PropertyTable::iterator iter = table->begin();
-        PropertyTable::iterator end = table->end();
-        for (; iter != end; ++iter)
-            out.print(comma, iter->key, ":", static_cast<int>(iter->offset));
-        
-        structure->m_lock.unlock();
-    }
-    
-    for (unsigned i = structures.size(); i--;) {
-        Structure* structure = structures[i];
-        if (!structure->m_nameInPrevious)
-            continue;
-        out.print(comma, structure->m_nameInPrevious.get(), ":", static_cast<int>(structure->m_offset));
-    }
+    const_cast<Structure*>(this)->forEachPropertyConcurrently(
+        [&] (const PropertyMapEntry& entry) -> bool {
+            out.print(comma, entry.key, ":", static_cast<int>(entry.offset));
+            return true;
+        });
     
     out.print("}, ", IndexingTypeDump(indexingType()));
     
     if (m_prototype.get().isCell())
         out.print(", Proto:", RawPointer(m_prototype.get().asCell()));
+
+    switch (dictionaryKind()) {
+    case NoneDictionaryKind:
+        if (hasBeenDictionary())
+            out.print(", Has been dictionary");
+        break;
+    case CachedDictionaryKind:
+        out.print(", Dictionary");
+        break;
+    case UncachedDictionaryKind:
+        out.print(", UncacheableDictionary");
+        break;
+    }
+
+    if (transitionWatchpointSetIsStillValid())
+        out.print(", Leaf");
+    else if (transitionWatchpointIsLikelyToBeFired())
+        out.print(", Shady leaf");
     
     out.print("]");
 }
@@ -1087,92 +1244,66 @@ void Structure::dumpContextHeader(PrintStream& out)
     out.print("Structures:");
 }
 
-#if DO_PROPERTYMAP_CONSTENCY_CHECK
-
-void PropertyTable::checkConsistency()
+bool ClassInfo::hasStaticSetterOrReadonlyProperties() const
 {
-    checkOffsetConsistency();
-    ASSERT(m_indexSize >= PropertyTable::MinimumTableSize);
-    ASSERT(m_indexMask);
-    ASSERT(m_indexSize == m_indexMask + 1);
-    ASSERT(!(m_indexSize & m_indexMask));
-
-    ASSERT(m_keyCount <= m_indexSize / 2);
-    ASSERT(m_keyCount + m_deletedCount <= m_indexSize / 2);
-    ASSERT(m_deletedCount <= m_indexSize / 4);
-
-    unsigned indexCount = 0;
-    unsigned deletedIndexCount = 0;
-    for (unsigned a = 0; a != m_indexSize; ++a) {
-        unsigned entryIndex = m_index[a];
-        if (entryIndex == PropertyTable::EmptyEntryIndex)
-            continue;
-        if (entryIndex == deletedEntryIndex()) {
-            ++deletedIndexCount;
-            continue;
-        }
-        ASSERT(entryIndex < deletedEntryIndex());
-        ASSERT(entryIndex - 1 <= usedCount());
-        ++indexCount;
-
-        for (unsigned b = a + 1; b != m_indexSize; ++b)
-            ASSERT(m_index[b] != entryIndex);
-    }
-    ASSERT(indexCount == m_keyCount);
-    ASSERT(deletedIndexCount == m_deletedCount);
-
-    ASSERT(!table()[deletedEntryIndex() - 1].key);
-
-    unsigned nonEmptyEntryCount = 0;
-    for (unsigned c = 0; c < usedCount(); ++c) {
-        StringImpl* rep = table()[c].key;
-        if (rep == PROPERTY_MAP_DELETED_ENTRY_KEY)
-            continue;
-        ++nonEmptyEntryCount;
-        unsigned i = rep->existingHash();
-        unsigned k = 0;
-        unsigned entryIndex;
-        while (1) {
-            entryIndex = m_index[i & m_indexMask];
-            ASSERT(entryIndex != PropertyTable::EmptyEntryIndex);
-            if (rep == table()[entryIndex - 1].key)
-                break;
-            if (k == 0)
-                k = 1 | doubleHash(rep->existingHash());
-            i += k;
+    for (const ClassInfo* ci = this; ci; ci = ci->parentClass) {
+        if (const HashTable* table = ci->staticPropHashTable) {
+            if (table->hasSetterOrReadonlyProperties)
+                return true;
         }
-        ASSERT(entryIndex == c + 1);
     }
+    return false;
+}
 
-    ASSERT(nonEmptyEntryCount == m_keyCount);
+void Structure::setCachedPropertyNameEnumerator(VM& vm, JSPropertyNameEnumerator* enumerator)
+{
+    ASSERT(!isDictionary());
+    if (!hasRareData())
+        allocateRareData(vm);
+    rareData()->setCachedPropertyNameEnumerator(vm, enumerator);
 }
 
-void Structure::checkConsistency()
+JSPropertyNameEnumerator* Structure::cachedPropertyNameEnumerator() const
 {
-    if (!propertyTable())
-        return;
+    if (!hasRareData())
+        return nullptr;
+    return rareData()->cachedPropertyNameEnumerator();
+}
 
-    if (!m_hasNonEnumerableProperties) {
-        PropertyTable::iterator end = propertyTable()->end();
-        for (PropertyTable::iterator iter = propertyTable()->begin(); iter != end; ++iter) {
-            ASSERT(!(iter->attributes & DontEnum));
-        }
-    }
+bool Structure::canCachePropertyNameEnumerator() const
+{
+    if (isDictionary())
+        return false;
 
-    propertyTable()->checkConsistency();
-}
+    if (hasIndexedProperties(indexingType()))
+        return false;
 
-#endif // DO_PROPERTYMAP_CONSTENCY_CHECK
+    if (typeInfo().overridesGetPropertyNames())
+        return false;
 
-bool ClassInfo::hasStaticSetterOrReadonlyProperties(VM& vm) const
-{
-    for (const ClassInfo* ci = this; ci; ci = ci->parentClass) {
-        if (const HashTable* table = ci->propHashTable(vm)) {
-            if (table->hasSetterOrReadonlyProperties)
-                return true;
-        }
+    StructureChain* structureChain = m_cachedPrototypeChain.get();
+    ASSERT(structureChain);
+    WriteBarrier<Structure>* structure = structureChain->head();
+    while (true) {
+        if (!structure->get())
+            break;
+        if (structure->get()->typeInfo().overridesGetPropertyNames())
+            return false;
+        structure++;
     }
-    return false;
+    
+    return true;
+}
+    
+bool Structure::canAccessPropertiesQuicklyForEnumeration() const
+{
+    if (!isQuickPropertyAccessAllowedForEnumeration())
+        return false;
+    if (hasGetterSetterProperties())
+        return false;
+    if (isUncacheableDictionary())
+        return false;
+    return true;
 }
 
 } // namespace JSC