DFGVariableAccessData.h   [plain text]

/*
 * Copyright (C) 2011, 2012 Apple Inc. All rights reserved.
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#ifndef DFGVariableAccessData_h
#define DFGVariableAccessData_h

#include "DFGDoubleFormatState.h"
#include "DFGNodeFlags.h"
#include "Operands.h"
#include "PredictedType.h"
#include "VirtualRegister.h"
#include <wtf/Platform.h>
#include <wtf/UnionFind.h>
#include <wtf/Vector.h>

namespace JSC { namespace DFG {

class VariableAccessData : public UnionFind<VariableAccessData> {
public:
    enum Ballot { VoteValue, VoteDouble };

    VariableAccessData()
        : m_local(static_cast<VirtualRegister>(std::numeric_limits<int>::min()))
        , m_prediction(PredictNone)
        , m_argumentAwarePrediction(PredictNone)
        , m_flags(0)
        , m_doubleFormatState(EmptyDoubleFormatState)
    {
        clearVotes();
    }
    
    VariableAccessData(VirtualRegister local)
        : m_local(local)
        , m_prediction(PredictNone)
        , m_argumentAwarePrediction(PredictNone)
        , m_flags(0)
        , m_doubleFormatState(EmptyDoubleFormatState)
    {
        clearVotes();
    }
    
    VirtualRegister local()
    {
        ASSERT(m_local == find()->m_local);
        return m_local;
    }
    
    int operand()
    {
        return static_cast<int>(local());
    }
    
    bool predict(PredictedType prediction)
    {
        VariableAccessData* self = find();
        bool result = mergePrediction(self->m_prediction, prediction);
        if (result)
            mergePrediction(m_argumentAwarePrediction, m_prediction);
        return result;
    }
    
    PredictedType nonUnifiedPrediction()
    {
        return m_prediction;
    }
    
    PredictedType prediction()
    {
        return find()->m_prediction;
    }
    
    PredictedType argumentAwarePrediction()
    {
        return find()->m_argumentAwarePrediction;
    }
    
    bool mergeArgumentAwarePrediction(PredictedType prediction)
    {
        return mergePrediction(find()->m_argumentAwarePrediction, prediction);
    }
    
    void clearVotes()
    {
        ASSERT(find() == this);
        m_votes[VoteValue] = 0;
        m_votes[VoteDouble] = 0;
    }
    
    void vote(Ballot ballot)
    {
        ASSERT(static_cast<unsigned>(ballot) < 2);
        m_votes[ballot]++;
    }
    
    double doubleVoteRatio()
    {
        ASSERT(find() == this);
        return static_cast<double>(m_votes[VoteDouble]) / m_votes[VoteValue];
    }
    
    bool shouldUseDoubleFormatAccordingToVote()
    {
        // We don't support this facility for arguments, yet.
        // FIXME: make this work for arguments.
        if (operandIsArgument(operand()))
            return false;
        
        // If the variable is not a number prediction, then this doesn't
        // make any sense.
        if (!isNumberPrediction(prediction()))
            return false;
        
        // If the variable is predicted to hold only doubles, then it's a
        // no-brainer: it should be formatted as a double.
        if (isDoublePrediction(prediction()))
            return true;
        
        // If the variable is known to be used as an integer, then be safe -
        // don't force it to be a double.
        if (flags() & NodeUsedAsInt)
            return false;
        
        // If the variable has been voted to become a double, then make it a
        // double.
        if (doubleVoteRatio() >= Options::doubleVoteRatioForDoubleFormat)
            return true;
        
        return false;
    }
    
    DoubleFormatState doubleFormatState()
    {
        return find()->m_doubleFormatState;
    }
    
    bool shouldUseDoubleFormat()
    {
        ASSERT(isRoot());
        return m_doubleFormatState == UsingDoubleFormat;
    }
    
    bool tallyVotesForShouldUseDoubleFormat()
    {
        ASSERT(isRoot());
        
        if (m_doubleFormatState == CantUseDoubleFormat)
            return false;
        
        bool newValueOfShouldUseDoubleFormat = shouldUseDoubleFormatAccordingToVote();
        if (!newValueOfShouldUseDoubleFormat) {
            // We monotonically convert to double. Hence, if the fixpoint leads us to conclude that we should
            // switch back to int, we instead ignore this and stick with double.
            return false;
        }
        
        if (m_doubleFormatState == UsingDoubleFormat)
            return false;
        
        return DFG::mergeDoubleFormatState(m_doubleFormatState, UsingDoubleFormat);
    }
    
    bool mergeDoubleFormatState(DoubleFormatState doubleFormatState)
    {
        return DFG::mergeDoubleFormatState(find()->m_doubleFormatState, doubleFormatState);
    }
    
    bool makePredictionForDoubleFormat()
    {
        ASSERT(isRoot());
        
        if (m_doubleFormatState != UsingDoubleFormat)
            return false;
        
        return mergePrediction(m_prediction, PredictDouble);
    }
    
    NodeFlags flags() const { return m_flags; }
    
    bool mergeFlags(NodeFlags newFlags)
    {
        newFlags |= m_flags;
        if (newFlags == m_flags)
            return false;
        m_flags = newFlags;
        return true;
    }
    
private:
    // This is slightly space-inefficient, since anything we're unified with
    // will have the same operand and should have the same prediction. But
    // putting them here simplifies the code, and we don't expect DFG space
    // usage for variable access nodes do be significant.

    VirtualRegister m_local;
    PredictedType m_prediction;
    PredictedType m_argumentAwarePrediction;
    NodeFlags m_flags;
    
    float m_votes[2];
    DoubleFormatState m_doubleFormatState;
};

} } // namespace JSC::DFG

#endif // DFGVariableAccessData_h