Source/JavaScriptCore/b3/air/AirTmpWidth.h

/*
 * Copyright (C) 2015-2017 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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 *    notice, this list of conditions and the following disclaimer in the
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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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#pragma once

#if ENABLE(B3_JIT)

#include "AirArg.h"

namespace JSC { namespace B3 { namespace Air {

class Code;

class TmpWidth {
public:
    TmpWidth();
    TmpWidth(Code&);
    ~TmpWidth();

    void recompute(Code&);

    // The width of a Tmp is the number of bits that you need to be able to track without some trivial
    // recovery. A Tmp may have a "subwidth" (say, Width32 on a 64-bit system) if either of the following
    // is true:
    //
    // - The high bits are never read.
    // - The high bits are always zero.
    //
    // This doesn't tell you which of those properties holds, but you can query that using the other
    // methods.
    Width width(Tmp tmp) const
    {
        auto iter = m_width.find(tmp);
        if (iter == m_width.end())
            return minimumWidth(Arg(tmp).bank());
        return std::min(iter->value.use, iter->value.def);
    }

    // Return the minimum required width for all defs/uses of this Tmp.
    Width requiredWidth(Tmp tmp)
    {
        auto iter = m_width.find(tmp);
        if (iter == m_width.end())
            return minimumWidth(Arg(tmp).bank());
        return std::max(iter->value.use, iter->value.def);
    }

    // This indirectly tells you how much of the tmp's high bits are guaranteed to be zero. The number of
    // high bits that are zero are:
    //
    //     TotalBits - defWidth(tmp)
    //
    // Where TotalBits are the total number of bits in the register, so 64 on a 64-bit system.
    Width defWidth(Tmp tmp) const
    {
        auto iter = m_width.find(tmp);
        if (iter == m_width.end())
            return minimumWidth(Arg(tmp).bank());
        return iter->value.def;
    }

    // This tells you how much of Tmp is going to be read.
    Width useWidth(Tmp tmp) const
    {
        auto iter = m_width.find(tmp);
        if (iter == m_width.end())
            return minimumWidth(Arg(tmp).bank());
        return iter->value.use;
    }
    
private:
    struct Widths {
        Widths() { }

        Widths(Bank bank)
        {
            use = minimumWidth(bank);
            def = minimumWidth(bank);
        }

        void dump(PrintStream& out) const;
        
        Width use;
        Width def;
    };
    
    HashMap<Tmp, Widths> m_width;
};

} } } // namespace JSC::B3::Air

#endif // ENABLE(B3_JIT)