Source/JavaScriptCore/wasm/WasmIndexOrName.h

/*
 * Copyright (C) 2017 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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 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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#pragma once

#include "WasmName.h"
#include <wtf/StdLibExtras.h>
#include <wtf/text/WTFString.h>

namespace JSC { namespace Wasm {

struct IndexOrName {
    typedef size_t Index;

    IndexOrName()
        : m_index(emptyTag)
    { }
    IndexOrName(Index, const Name*);
    bool isEmpty() const { return bitwise_cast<Index>(*this) & emptyTag; }
    bool isIndex() const { return bitwise_cast<Index>(*this) & indexTag; }
    bool isName() const { return !(isEmpty() || isName()); }

    friend String makeString(const IndexOrName&);

private:
    union {
        Index m_index;
        const Name* m_name;
    };

    // Use the top bits as tags. Neither pointers nor the function index space should use them.
    static constexpr Index indexTag = 1ull << (CHAR_BIT * sizeof(Index) - 1);
    static constexpr Index emptyTag = 1ull << (CHAR_BIT * sizeof(Index) - 2);
    static constexpr Index allTags = indexTag | emptyTag;
};

String makeString(const IndexOrName&);

} } // namespace JSC::Wasm