Source/JavaScriptCore/dfg/DFGDoesGCCheck.h

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

#include <cstdint>
#include <wtf/Assertions.h>

namespace JSC {

class VM;

namespace DFG {

struct DoesGCCheck {
    enum class Special {
        Uninitialized,
        DFGOSRExit,
        FTLOSRExit,
        NumberOfSpecials
    };

    DoesGCCheck()
        : m_value(encode(true, Special::Uninitialized))
    { }

    static uint32_t encode(bool expectDoesGC, unsigned nodeIndex, unsigned nodeOp)
    {
        // We know nodeOp always fits because of the static_assert in DFGDoesGCCheck.cpp.
        ASSERT((nodeIndex << nodeIndexShift) >> nodeIndexShift == nodeIndex);
        return nodeIndex << nodeIndexShift | nodeOp << nodeOpShift | bits(expectDoesGC);
    }

    static uint32_t encode(bool expectDoesGC, Special special)
    {
        return bits(special) << specialShift | isSpecialBit | bits(expectDoesGC);
    }

    void set(bool expectDoesGC, unsigned nodeIndex, unsigned nodeOp)
    {
        m_value = encode(expectDoesGC, nodeIndex, nodeOp);
    }

    void set(bool expectDoesGC, Special special)
    {
        m_value = encode(expectDoesGC, special);
    }

    bool expectDoesGC() const { return m_value & expectDoesGCBit; }
    bool isSpecial() const { return m_value & isSpecialBit; }

    Special special() { return static_cast<Special>(m_value >> specialShift); }

    unsigned nodeOp() { return (m_value >> nodeOpShift) & nodeOpMask; }
    unsigned nodeIndex() { return m_value >> nodeIndexShift; }

    JS_EXPORT_PRIVATE void verifyCanGC(VM&);

private:
    template<typename T> static uint32_t bits(T value) { return static_cast<uint32_t>(value); }

    // The value cannot be both a Special and contain node information at the
    // time. Hence, the 2 can have separate encodings. The isSpecial bit
    // determines which encoding is in use.

    static constexpr unsigned expectDoesGCBit = 1 << 0;
    static constexpr unsigned isSpecialBit = 1 << 1;
    static constexpr unsigned commonBits = 2;
    static_assert((expectDoesGCBit | isSpecialBit) == (1 << commonBits) - 1);

    static constexpr unsigned specialShift = commonBits;

    static constexpr unsigned nodeOpBits = 9;
    static constexpr unsigned nodeOpMask = (1 << nodeOpBits) - 1;
    static constexpr unsigned nodeOpShift = commonBits;

    static constexpr unsigned nodeIndexBits = 21;
    static constexpr unsigned nodeIndexShift = nodeOpShift + nodeOpBits;
    static_assert(nodeIndexShift + nodeIndexBits == 32);

    uint32_t m_value { 0 };
};

} // namespace DFG

using DFG::DoesGCCheck;

} // namespace JSC