path: root/libevmasm/GasMeter.h

/*
    This file is part of cpp-ethereum.

    cpp-ethereum is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    cpp-ethereum is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with cpp-ethereum.  If not, see <http://www.gnu.org/licenses/>.
*/
/** @file GasMeter.cpp
 * @author Christian <c@ethdev.com>
 * @date 2015
 */

#pragma once

#include <ostream>
#include <tuple>
#include <libevmasm/ExpressionClasses.h>
#include <libevmasm/AssemblyItem.h>
#include <libethcore/ChainOperationParams.h>

namespace dev
{
namespace eth
{

class KnownState;

// TODO: FIXME: HOMESTEAD: XXX: @chfast populate m_schedule from an ExtVMFace instance via ExtVMFace::evmSchedule.

/**
 * Class that helps computing the maximum gas consumption for instructions.
 * Has to be initialized with a certain known state that will be automatically updated for
 * each call to estimateMax. These calls have to supply strictly subsequent AssemblyItems.
 * A new gas meter has to be constructed (with a new state) for control flow changes.
 */
class GasMeter
{
public:
    struct GasConsumption
    {
        GasConsumption(u256 _value = 0, bool _infinite = false): value(_value), isInfinite(_infinite) {}
        static GasConsumption infinite() { return GasConsumption(0, true); }

        GasConsumption& operator+=(GasConsumption const& _other);
        bool operator<(GasConsumption const& _other) const { return this->tuple() < _other.tuple(); }

        std::tuple<bool const&, u256 const&> tuple() const { return std::tie(isInfinite, value); }

        u256 value;
        bool isInfinite;
    };

    /// Constructs a new gas meter given the current state.
    explicit GasMeter(std::shared_ptr<KnownState> const& _state, u256 const& _largestMemoryAccess = 0):
        m_state(_state), m_largestMemoryAccess(_largestMemoryAccess) {}

    /// @returns an upper bound on the gas consumed by the given instruction and updates
    /// the state.
    GasConsumption estimateMax(AssemblyItem const& _item);

    u256 const& largestMemoryAccess() const { return m_largestMemoryAccess; }

    u256 runGas(Instruction _instruction) const { return runGas(_instruction, m_schedule); }
    static u256 runGas(Instruction _instruction, EVMSchedule const& _es);

private:
    /// @returns _multiplier * (_value + 31) / 32, if _value is a known constant and infinite otherwise.
    GasConsumption wordGas(u256 const& _multiplier, ExpressionClasses::Id _value);
    /// @returns the gas needed to access the given memory position.
    /// @todo this assumes that memory was never accessed before and thus over-estimates gas usage.
    GasConsumption memoryGas(ExpressionClasses::Id _position);
    /// @returns the memory gas for accessing the memory at a specific offset for a number of bytes
    /// given as values on the stack at the given relative positions.
    GasConsumption memoryGas(int _stackPosOffset, int _stackPosSize);

    std::shared_ptr<KnownState> m_state;
    /// Largest point where memory was accessed since the creation of this object.
    u256 m_largestMemoryAccess;

    EVMSchedule m_schedule;
};

inline std::ostream& operator<<(std::ostream& _str, GasMeter::GasConsumption const& _consumption)
{
    if (_consumption.isInfinite)
        return _str << "[???]";
    else
        return _str << std::dec << _consumption.value;
}


}
}