Source/JavaScriptCore/runtime/DisallowVMEntry.h

/*
 * Copyright (C) 2017-2020 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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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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#pragma once

#include <wtf/ForbidHeapAllocation.h>

namespace JSC {

class VM;

// The only reason we implement DisallowVMEntry as specialization of a template
// is so that we can work around having to #include VM.h, which can hurt build
// time. This defers the cost of #include'ing VM.h to only the clients that
// need it.

template<typename VMType = VM>
class DisallowVMEntryImpl {
    WTF_FORBID_HEAP_ALLOCATION;
public:
    DisallowVMEntryImpl(VMType& vm)
        : m_vm(&vm)
    {
        m_vm->disallowVMEntryCount++;
    }

    DisallowVMEntryImpl(const DisallowVMEntryImpl& other)
        : m_vm(other.m_vm)
    {
        m_vm->disallowVMEntryCount++;
    }

    ~DisallowVMEntryImpl()
    {
        RELEASE_ASSERT(m_vm->disallowVMEntryCount);
        m_vm->disallowVMEntryCount--;
        m_vm = nullptr;
    }

    DisallowVMEntryImpl& operator=(const DisallowVMEntryImpl& other)
    {
        RELEASE_ASSERT(m_vm && m_vm == other.m_vm);
        RELEASE_ASSERT(m_vm->disallowVMEntryCount);
        // Conceptually, we need to decrement the disallowVMEntryCount of the
        // old m_vm, and increment the disallowVMEntryCount of the new m_vm.
        // But since the old and the new m_vm should always be the same, the
        // decrementing and incrementing cancels out, and there's nothing more
        // to do here.
        return *this;
    }

private:
    VMType* m_vm;
};

using DisallowVMEntry = DisallowVMEntryImpl<VM>;

} // namespace JSC