Source/JavaScriptCore/runtime/ObjectConstructorInlines.h

/*
 * Copyright (C) 2021 Apple Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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 * 1. Redistributions of source code must retain the above copyright
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 *    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 INC. AND ITS CONTRIBUTORS ``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 INC. OR ITS CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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#pragma once

#include "ObjectConstructor.h"

namespace JSC {

ALWAYS_INLINE bool canPerformFastPropertyEnumerationForObjectAssign(Structure* structure)
{
    if (structure->typeInfo().overridesGetOwnPropertySlot())
        return false;
    if (structure->typeInfo().overridesAnyFormOfGetOwnPropertyNames())
        return false;
    // FIXME: Indexed properties can be handled.
    // https://bugs.webkit.org/show_bug.cgi?id=185358
    if (hasIndexedProperties(structure->indexingType()))
        return false;
    if (structure->hasGetterSetterProperties())
        return false;
    if (structure->hasReadOnlyOrGetterSetterPropertiesExcludingProto())
        return false;
    if (structure->hasCustomGetterSetterProperties())
        return false;
    if (structure->isUncacheableDictionary())
        return false;
    // Cannot perform fast [[Put]] to |target| if the property names of the |source| contain "__proto__".
    if (structure->hasUnderscoreProtoPropertyExcludingOriginalProto())
        return false;
    return true;
}

ALWAYS_INLINE bool canPerformFastPropertyEnumerationForJSONStringify(Structure* structure)
{
    return canPerformFastPropertyEnumerationForObjectAssign(structure);
}

ALWAYS_INLINE void objectAssignFast(VM& vm, JSObject* target, JSObject* source, Vector<RefPtr<UniquedStringImpl>, 8>& properties, MarkedArgumentBuffer& values)
{
    // |source| Structure does not have any getters. And target can perform fast put.
    // So enumerating properties and putting properties are non observable.

    // FIXME: It doesn't seem like we should have to do this in two phases, but
    // we're running into crashes where it appears that source is transitioning
    // under us, and even ends up in a state where it has a null butterfly. My
    // leading hypothesis here is that we fire some value replacement watchpoint
    // that ends up transitioning the structure underneath us.
    // https://bugs.webkit.org/show_bug.cgi?id=187837

    // FIXME: This fast path is very similar to ObjectConstructor' one. But extracting it to a function caused performance
    // regression in object-assign-replace. Since the code is small and fast path, we keep both.

    // Do not clear since Vector::clear shrinks the backing store.
    properties.resize(0);
    values.clear();
    source->structure(vm)->forEachProperty(vm, [&] (const PropertyMapEntry& entry) -> bool {
        if (entry.attributes & PropertyAttribute::DontEnum)
            return true;

        PropertyName propertyName(entry.key);
        if (propertyName.isPrivateName())
            return true;

        properties.append(entry.key);
        values.appendWithCrashOnOverflow(source->getDirect(entry.offset));

        return true;
    });

    for (size_t i = 0; i < properties.size(); ++i) {
        // FIXME: We could put properties in a batching manner to accelerate Object.assign more.
        // https://bugs.webkit.org/show_bug.cgi?id=185358
        PutPropertySlot putPropertySlot(target, true);
        target->putOwnDataProperty(vm, properties[i].get(), values.at(i), putPropertySlot);
    }
}


} // namespace JSC