Source/JavaScriptCore/runtime/StructureChain.cpp

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 * Copyright (C) 2008-2021 Apple Inc. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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#include "config.h"
#include "StructureChain.h"

#include "JSCInlines.h"

namespace JSC {
    
const ClassInfo StructureChain::s_info = { "StructureChain"_s, nullptr, nullptr, nullptr, CREATE_METHOD_TABLE(StructureChain) };

StructureChain::StructureChain(VM& vm, Structure* structure, StructureID* vector)
    : Base(vm, structure)
    , m_vector(vm, this, vector)
{
}

StructureChain* StructureChain::create(VM& vm, JSObject* head)
{
    // FIXME: Make StructureChain::create fail for large chain. Caching large chain is not so profitable.
    // By making the size <= UINT16_MAX, we can store length in a high bits of auxiliary pointer.
    // https://bugs.webkit.org/show_bug.cgi?id=200290
    size_t size = 0;
    for (JSObject* current = head; current; current = current->structure()->storedPrototypeObject(current))
        ++size;
    ++size; // Sentinel nullptr.
    size_t bytes = Checked<size_t>(size) * sizeof(StructureID);
    void* vector = vm.jsValueGigacageAuxiliarySpace().allocate(vm, bytes, nullptr, AllocationFailureMode::Assert);
    static_assert(!StructureID().bits(), "Make sure the value we're going to memcpy below matches the default StructureID");
    memset(vector, 0, bytes);
    StructureChain* chain = new (NotNull, allocateCell<StructureChain>(vm)) StructureChain(vm, vm.structureChainStructure.get(), static_cast<StructureID*>(vector));
    chain->finishCreation(vm, head);
    return chain;
}

void StructureChain::finishCreation(VM& vm, JSObject* head)
{
    Base::finishCreation(vm);
    size_t i = 0;
    for (JSObject* current = head; current; current = current->structure()->storedPrototypeObject(current)) {
        Structure* structure = current->structure();
        m_vector.get()[i++] = structure->id();
        vm.writeBarrier(this);
    }
}

template<typename Visitor>
void StructureChain::visitChildrenImpl(JSCell* cell, Visitor& visitor)
{
    StructureChain* thisObject = jsCast<StructureChain*>(cell);
    ASSERT_GC_OBJECT_INHERITS(thisObject, info());
    Base::visitChildren(thisObject, visitor);
    visitor.markAuxiliary(thisObject->m_vector.get());
    for (auto* current = thisObject->m_vector.get(); *current; ++current) {
        StructureID structureID = *current;
        Structure* structure = structureID.decode();
        visitor.appendUnbarriered(structure);
    }
}

DEFINE_VISIT_CHILDREN(StructureChain);

} // namespace JSC