path: root/libevmasm/PathGasMeter.h

/*
    This file is part of solidity.

    solidity 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.

    solidity 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 solidity.  If not, see <http://www.gnu.org/licenses/>.
*/
/** @file PathGasMeter.cpp
 * @author Christian <c@ethdev.com>
 * @date 2015
 */

#pragma once

#include <libevmasm/GasMeter.h>

#include <libsolidity/interface/EVMVersion.h>

#include <set>
#include <vector>
#include <memory>

namespace dev
{
namespace eth
{

class KnownState;

struct GasPath
{
    size_t index = 0;
    std::shared_ptr<KnownState> state;
    u256 largestMemoryAccess;
    GasMeter::GasConsumption gas;
    std::set<size_t> visitedJumpdests;
};

/**
 * Computes an upper bound on the gas usage of a computation starting at a certain position in
 * a list of AssemblyItems in a given state until the computation stops.
 * Can be used to estimate the gas usage of functions on any given input.
 */
class PathGasMeter
{
public:
    explicit PathGasMeter(AssemblyItems const& _items, solidity::EVMVersion _evmVersion);

    GasMeter::GasConsumption estimateMax(size_t _startIndex, std::shared_ptr<KnownState> const& _state);

    static GasMeter::GasConsumption estimateMax(
        AssemblyItems const& _items,
        solidity::EVMVersion _evmVersion,
        size_t _startIndex,
        std::shared_ptr<KnownState> const& _state
    )
    {
        return PathGasMeter(_items, _evmVersion).estimateMax(_startIndex, _state);
    }

private:
    /// Adds a new path item to the queue, but only if we do not already have
    /// a higher gas usage at that point.
    /// This is not exact as different state might influence higher gas costs at a later
    /// point in time, but it greatly reduces computational overhead.
    void queue(std::unique_ptr<GasPath>&& _newPath);
    GasMeter::GasConsumption handleQueueItem();

    /// Map of jumpdest -> gas path, so not really a queue. We only have one queued up
    /// item per jumpdest, because of the behaviour of `queue` above.
    std::map<size_t, std::unique_ptr<GasPath>> m_queue;
    std::map<size_t, GasMeter::GasConsumption> m_highestGasUsagePerJumpdest;
    std::map<u256, size_t> m_tagPositions;
    AssemblyItems const& m_items;
    solidity::EVMVersion m_evmVersion;
};

}
}