PrototypeMap.cpp   [plain text]

/*
 * Copyright (C) 2013, 2016 Apple Inc. All rights reserved.
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#include "config.h"
#include "PrototypeMap.h"

#include "IndexingType.h"
#include "JSGlobalObject.h"
#include "JSCInlines.h"

namespace JSC {

void PrototypeMap::addPrototype(JSObject* object)
{
    m_prototypes.set(object, object);

    // Note that this method makes the somewhat odd decision to not check if this
    // object currently has indexed accessors. We could do that check here, and if
    // indexed accessors were found, we could tell the global object to have a bad
    // time. But we avoid this, to allow the following to be always fast:
    //
    // 1) Create an object.
    // 2) Give it a setter or read-only property that happens to have a numeric name.
    // 3) Allocate objects that use this object as a prototype.
    //
    // This avoids anyone having a bad time. Even if the instance objects end up
    // having indexed storage, the creation of indexed storage leads to a prototype
    // chain walk that detects the presence of indexed setters and then does the
    // right thing. As a result, having a bad time only happens if you add an
    // indexed setter (or getter, or read-only field) to an object that is already
    // used as a prototype.
}

inline Structure* PrototypeMap::createEmptyStructure(JSObject* prototype, const TypeInfo& typeInfo, const ClassInfo* classInfo, IndexingType indexingType, unsigned inlineCapacity)
{
    auto key = std::make_pair(prototype, std::make_pair(inlineCapacity, classInfo));
    if (Structure* structure = m_structures.get(key)) {
        ASSERT(isPrototype(prototype));
        return structure;
    }

    addPrototype(prototype);
    Structure* structure = Structure::create(
        prototype->globalObject()->vm(), prototype->globalObject(), prototype, typeInfo, classInfo, indexingType, inlineCapacity);
    m_structures.set(key, Weak<Structure>(structure));
    return structure;
}

Structure* PrototypeMap::emptyStructureForPrototypeFromBaseStructure(JSObject* prototype, Structure* baseStructure)
{
    // We currently do not have inline capacity static analysis for subclasses and all internal function constructors have a default inline capacity of 0.
    IndexingType indexingType = baseStructure->indexingType();
    if (prototype->structure()->anyObjectInChainMayInterceptIndexedAccesses() && hasIndexedProperties(indexingType))
        indexingType = (indexingType & ~IndexingShapeMask) | SlowPutArrayStorageShape;

    return createEmptyStructure(prototype, baseStructure->typeInfo(), baseStructure->classInfo(), indexingType, 0);
}

Structure* PrototypeMap::emptyObjectStructureForPrototype(JSObject* prototype, unsigned inlineCapacity)
{
    return createEmptyStructure(prototype, JSFinalObject::typeInfo(), JSFinalObject::info(), JSFinalObject::defaultIndexingType, inlineCapacity);
}

void PrototypeMap::clearEmptyObjectStructureForPrototype(JSObject* object, unsigned inlineCapacity)
{
    m_structures.remove(std::make_pair(object, std::make_pair(inlineCapacity, JSFinalObject::info())));
}

} // namespace JSC